Amanda J. Haes

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Triangular silver nanoparticles ( approximately 100 nm wide and 50 nm high) have remarkable optical properties. In particular, the peak extinction wavelength, lambda(max) of their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive to nanoparticle size, shape, and local ( approximately 10-30 nm) external dielectric environment. This sensitivity of the LSPR lambda(max) to the nanoenvironment has allowed us to develop a new class of nanoscale affinity biosensors. The essential characteristics and operational principles of these LSPR nanobiosensors will be illustrated using the well-studied biotin-streptavidin system. Exposure of biotin-functionalized Ag nanotriangles to 100 nM streptavidin (SA) caused a 27.0 nm red-shift in the LSPR lambda(max). The LSPR lambda(max) shift, DeltaR/DeltaR(max), versus [SA] response curve was measured over the concentration range 10(-)(15) M < [SA] < 10(-)(6) M. Comparison of the data with the theoretical normalized response expected for 1:1 binding of a ligand to a multivalent receptor with different sites but invariant affinities yielded approximate values for the saturation response, DeltaR(max) = 26.5 nm, and the surface-confined thermodynamic binding constant K(a,surf) = 10(11) M(-)(1). At present, the limit of detection (LOD) for the LSPR nanobiosensor is found to be in the low-picomolar to high-femtomolar region. A strategy to amplify the response of the LSPR nanobiosensor using biotinylated Au colloids and thereby further improve the LOD is demonstrated. Several control experiments were performed to define the LSPR nanobiosensor's response to nonspecific binding as well as to demonstrate its response to the specific binding of another protein. These include the following: (1) electrostatic binding of SA to a nonbiotinylated surface, (2) nonspecific interactions of prebiotinylated SA to a biotinylated surface, (3) nonspecific interactions of bovine serum albumin to a biotinylated surface, and (4) specific binding of anti-biotin to a biotinylated surface. The LSPR nanobiosensor provides a pathway to ultrasensitive biodetection experiments with extremely simple, small, light, robust, low-cost instrumentation that will greatly facilitate field-portable environmental or point-of-service medical diagnostic applications.

A nanoscale optical biosensor based on localized surface plasmon resonance (LSPR) spectroscopy has been developed to monitor the interaction between the antigen, amyloid-beta derived diffusible ligands (ADDLs), and specific anti-ADDL antibodies. Using the sandwich assay format, this nanosensor provides quantitative binding information for both antigen and second antibody detection that permits the determination of ADDL concentration and offers the unique analysis of the aggregation mechanisms of this putative Alzheimer's disease pathogen at physiologically relevant monomer concentrations. Monitoring the LSPR-induced shifts from both ADDLs and a second polyclonal anti-ADDL antibody as a function of ADDL concentration reveals two ADDL epitopes that have binding constants to the specific anti-ADDL antibodies of 7.3 x 10(12) M(-1) and 9.5 x 10(8) M(-1). The analysis of human brain extract and cerebrospinal fluid samples from control and Alzheimer's disease patients reveals that the LSPR nanosensor provides new information relevant to the understanding and possible diagnosis of Alzheimer's disease.

The localized surface plasmon resonance (LSPR) nanosensor based on the optical properties of Ag nanotriangles is shown to aid in the understanding of the interaction between amyloid β-derived diffusible ligands (ADDL) and the anti-ADDL antibody, molecules possibly involved in the development of Alzheimer's disease. By varying the concentration of anti-ADDL antibody, a surface confined binding constant of 3.0 × 107 M-1 for the interaction of ADDLs and anti-ADDLs was measured. Influences of Cr, the nanoparticle adhesion layer, will be shown to be the limiting factor in the sensitivity of this assay. This is the first nonmodel application of the LSPR nanosensor.

The elucidation of the long range distance dependence of the localized surface plasmon resonance (LSPR) of surface-confined noble metal nanoparticles is the aim of this work. It was suspected that the linear distance dependence found in CH3(CH2)xSH self-assembled monolayer (SAM) formation was the thin shell limit of a longer range, nonlinear dependence. To verify this, multilayer SAM shells based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined noble metal nanoparticles and were monitored using UV−visible spectroscopy. Measurement of the LSPR extinction peak shift versus number of layers and adsorbate thickness is nonlinear and has a sensing range that is dependent on the composition, shape, in-plane width, and out-of-plane height of the nanoparticles. Theoretical calculations based on an accurate electrodynamics description of the metal nanoparticle plus surrounding layered material indicate plasmon resonance wavelength shifts that are in excellent agreement with the measurements. The calculations show that the sensing range is determined by falloff of the average induced electric field around the nanoparticle. This detailed set of experiments coupled with an excellent theory versus experiment comparison prove that the sensing capabilities of noble metal nanoparticles can be size tuned to match the dimensions of biological and chemical analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will significantly improve an already sensitive nanoparticle-based sensor.

Silver and gold nanotriangles were fabricated by nanosphere lithography (NSL) and their localized surface plasmon resonance (LSPR) spectra were measured by UV−vis extinction spectroscopy. It is demonstrated that the short range (viz., 0−2 nm) distance dependence of the electromagnetic fields that surround these nanoparticles when resonantly excited can be systematically tuned by changing their size, structure, and composition. This is accomplished by measuring the shift in the peak wavelength, λmax, of their LSPR spectra caused by the adsorption of hexadecanethiol as a function of nanoparticle size (in-plane width, out-of-plane height, and aspect ratio), shape (truncated tetrahedron versus hemisphere), and composition (silver versus gold). We find that the hexadecanethiol-induced LSPR shift for Ag triangles decreases when in-plane width is increased at fixed out-of-plane height or when height is increased at fixed width. These trends are the opposite to what was seen in an earlier study of the long range distance dependence in which 30 nm thick layers were examined (Haes et al. J. Phys. Chem. B 2004, 108, 109), but both the long and short range results are confirmed by a theoretical analysis based on finite element electrodynamics. The theory results also indicate that the short range results are primarily sensitive to hot spots (regions of high induced electric field) near the tips of the triangles, so this provides an example where enhanced local fields play an important role in extinction spectra. Our measurements further show that the hexadecanethiol-induced LSPR peak shift is larger for nanotriangles than for hemispheres with equal volumes and is larger for Ag nanotriangles than for Au nanotriangles with the same in-plane widths and out-of-plane heights. The dependence of the alkanethiol-induced LSPR peak shift on chain length for Ag nanotriangles is approximately size-independent. We anticipate that the improved understanding of the short range dependence of the adsorbate-induced LSPR peak shift on nanoparticle structure and composition reported here will translate to significant improvements in the sensitivity of refractive-index-based nanoparticle nanosensors.

In this review, the most recent progress in the development of noble metal nano-optical sensors based on localized surface plasmon resonance (LSPR) spectroscopy is summarized. The sensing principle relies on the LSPR spectral shifts caused by the surrounding dielectric environmental change in a binding event. Nanosphere lithography, an inexpensive and simple nanofabrication technique, has been used to fabricate the nanoparticles as the LSPR sensing platforms. As an example of the biosensing applications, the LSPR detection for a biomarker of Alzheimer's disease, amyloid-derived diffusable ligands, in human brain extract and cerebrospinal fluid samples is highlighted. Furthermore, the LSPR sensing method can be modified easily and used in a variety of applications. More specifically, a LSPR chip capable of multiplex sensing, a combined electrochemical and LSPR protocol and a fabrication method of solution-phase nanotriangles are presented here.

The shift in the extinction maximum, λmax, of the localized surface plasmon resonance (LSPR) spectrum of triangular Ag nanoparticles (∼90 nm wide and 50 nm high) is used to probe the interaction between a surface-confined antigen, biotin (B), and a solution-phase antibody, anti-biotin (AB). Exposure of biotin-functionalized Ag nanotriangles to 7 × 10-7 M < [AB] < 7 × 10-6 M caused a ∼38 nm red-shift in the LSPR λmax. The experimental normalized response of the LSPR λmax shift, (ΔR/ΔRmax), versus [AB] was measured over the concentration range 7 × 10-10 M < [AB] < 7 × 10-6 M. Comparison of the experimental data with the theoretical normalized response for a 1:1 binding model yielded values for the saturation response, ΔRmax = 38.0 nm, the surface-confined thermodynamic binding constant, Ka,surf = 4.5 × 107 M-1, and the limit of detection (LOD) < 7 × 10-10 M. The experimental saturation response was interpreted in terms of a closest-packed structural model for the surface B−AB complex in which the long axis of AB, lAB = 15 nm, is oriented horizontally and the short axis, hAB = 4 nm is oriented vertically to the nanoparticle surface. This model yields a quantitative response for the saturation response, ΔRmax = 40.6 nm, in good agreement with experiment, ΔRmax = 38.0 nm. An atomic force microscopy (AFM) study supports this interpretation. In addition, major improvements in the LSPR nanobiosensor are reported. The LSPR nanobiosensor substrate was changed from glass to mica, and a surfactant, Triton X-100, was used in the nanosphere lithography fabrication procedure. These changes increased the adhesion of the Ag nanotriangles by a factor of 9 as determined by AFM normal force studies. The improved adhesion of Ag nanotriangles now enables the study of the B−AB immunoassay in a physiologically relevant fluid environment as well as in real-time. These results represent important new steps in the development of the LSPR nanosensor for applications in medical diagnostics, biomedical research, and environmental science.

The term “nanoparticle stability” is widely used to describe the preservation of a particular nanostructure property ranging from aggregation, composition, crystallinity, shape, size, and surface chemistry. As a result, this catch-all term has various meanings, which depend on the specific nanoparticle property of interest and/or application. In this Feature Article, we provide an answer to the question, “What does nanoparticle stability mean?”. Broadly speaking, the definition of nanoparticle stability depends on the targeted size-dependent property that is exploited and can only exist for a finite period of time given all nanostructures are inherently thermodynamically and energetically unfavorable relative to bulk states. To answer this question specifically, however, the relationship between nanoparticle stability and the physical/chemical properties of metal/metal oxide nanoparticles is discussed. Specific definitions are explored in terms of aggregation state, core composition, shape, size, and surface chemistry. Next, mechanisms of promoting nanoparticle stability are defined and related to these same nanoparticle properties. Metrics involving both kinetics and thermodynamics are considered. Methods that provide quantitative metrics for measuring and modeling nanoparticle stability in terms of core composition, shape, size, and surface chemistry are outlined. The stability of solution-phase nanoparticles are also impacted by aggregation state. Thus, collision and DLVO theories are discussed. Finally, challenges and opportunities in understanding what nanoparticle stability means are addressed to facilitate further studies with this important class of materials.

Researchers and industrialists have taken advantage of the unusual optical, magnetic, electronic, catalytic, and mechanical properties of nanomaterials. Nanoparticles and nanoscale materials have proven to be useful for biological uses. Nanoscale materials hold a particular interest to those in the biological sciences because they are on the same size scale as biological macromolecules, proteins and nucleic acids. The interactions between biomolecules and nanomaterials have formed the basis for a number of applications including detection, biosensing, cellular and in situ hybridisation labelling, cell tagging and sorting, point-of-care diagnostics, kinetic and binding studies, imaging enhancers, and even as potential therapeutic agents. Noble metal nanoparticles are especially interesting because of their unusual optical properties which arise from their ability to support surface plasmons. In this review the authors focus on biological applications and technologies that utilise two types of related plasmonic phenomonae: localised surface plasmon resonance (LSPR) spectroscopy and surface-enhanced Raman spectroscopy (SERS). The background necessary to understand the application of LSPR and SERS to biological problems is presented and illustrative examples of resonant Rayleigh scattering, refractive index sensing, and SERS-based detection and labelling are discussed.

The peak location of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, new phenomena are revealed by exploring the influence of interacting molecular resonances and nanoparticle resonances. The LSPR peak shift and line shape induced by a resonant molecule vary with wavelength. In most instances, the oscillatory dependence of the peak shift on wavelength tracks with the wavelength dependence of the real part of the refractive index, as determined by a Kramers-Kronig transformation of the molecular resonance absorption spectrum. A quantitative assessment of this shift based on discrete dipole approximation calculations shows that the Kramers-Kronig index must be scaled in order to match experiment.

Reproducible detection of a target molecule is demonstrated using temporally stable solution-phase silica-void-gold nanoparticles and surface-enhanced Raman scattering (SERS). These composite nanostructures are homogeneous (diameter = 45 +/- 4 nm) and entrap single 13 nm gold nanoparticle cores inside porous silica membranes which prevent electromagnetic coupling and aggregation between adjacent nanoparticles. The optical properties of the gold nanoparticle cores and structural changes of the composite nanostructures are characterized using extinction spectroscopy and transmission electron microscopy, respectively, and both techniques are used to monitor the formation of the silica membrane. The resulting nanostructures exhibit temporally stable optical properties in the presence of salt and 2-naphthalenethiol. Similar SERS spectral features are observed when 2-naphthalenethiol is incubated with both bare and membrane-encapsulated gold nanoparticles. Disappearance of the S-H Raman vibrational band centered at 2566 cm(-1) with the composite nanoparticles indicates that the target molecule is binding directly to the metal surface. Furthermore, these nanostructures exhibit reproducible SERS signals for at least a 2 h period. This first demonstration of utilizing solution-phase silica-void-gold nanoparticles as reproducible SERS substrates will allow for future fundamental studies in understanding the mechanisms of SERS using solution-phase nanostructures as well as for applications that involve the direct and reproducible detection of biological and environmental molecules.

Miniature optical sensors that specifically identify low concentrations of environmental and biological substances are in high demand. Currently, there is no optical sensor that provides identification of the aforementioned species without amplification techniques at naturally occurring concentrations. Recently, it has been demonstrated that triangular silver nanoparticles have remarkable optical properties and that their enhanced sensitivity to their nanoenvironment has been used to develop a new class of optical sensors using localized surface plasmon resonance spectroscopy. The examination of both model and nonmodel biological assays using localized surface plasmon resonance spectroscopy will be presented in this review. It will be demonstrated that the use of a localized surface plasmon resonance nanosensor rivals the sensitivity and selectivity of, and provides a low-cost alternative to, commercially available sensors.

Covalently functionalized gold nanoparticles influence capillary electrophoresis separations of neurotransmitters in a concentration- and surface-chemistry-dependent manner. Gold nanoparticles with either primarily covalently functionalized carboxylic acid (Au@COOH) or amine (Au@NH(2)) surface groups are characterized using extinction spectroscopy, transmission electron microscopy, and zeta potential measurements. The impact of the presence of nanoparticles and their surface chemistry is investigated, and at least three nanoparticle-specific mechanisms are found to effect separations. First, the degree of nanoparticle-nanoparticle interactions is quantified using a new parameter termed the critical nanoparticle concentration (CNC). CNC is defined as the lowest concentration of nanoparticles that induces predominant nanoparticle aggregation under specific buffer conditions and is determined using dual-wavelength photodiode array detection. Once the CNC has been exceeded, reproducible separations are no longer observed. Second, nanoparticle-analyte interactions are dictated by electrostatic interactions which depend on the pK(a) of the analyte and surface charge of the nanoparticle. Finally, nanoparticle-capillary interactions occur in a surface-chemistry-dependent manner. Run buffer viscosity is influenced by the formation of a nanoparticle steady-state pseudostationary phase along the capillary wall. Despite differences in buffer viscosity leading to changes in neurotransmitter mobilities, no significant changes in electroosmotic flow were observed. As a result of these three nanoparticle-specific interactions, Au@NH(2) nanoparticles increase the mobility of the neurotransmitters while a smaller opposite effect is observed for Au@COOH nanoparticles. Understanding nanoparticle behavior in the presence of an electric field will have significant impacts in separation science where nanoparticles can serve to improve either the mobility or detection sensitivity of target molecules.

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

The zwitterion, N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES), facilitates the formation and stability of gold nanostars; however, little is known about how this molecule interacts with the metal postsynthesis. Herein, restructuring of gold nanostar morphology is induced upon acidification, an effect that depends on both pH and acid composition as well as on the protonation state of HEPES. Changes in molecular protonation are measured using zeta potential and modeled using DFT. The surface-sensitive technique, surface-enhanced Raman scattering (SERS), reveals that pH variations induce reversible activation of the amine and sulfonate groups in HEPES and that electron redistribution weakens its affinity to the metal thus promoting the adsorption and SERS detection of benzene. By selecting a molecule that does not induce significant desorption of the stabilizing agent, binding energies of benzene to gold are measured even though only weak London dispersion and π–π interactions promote adsorption. All in all, this molecular-level insight is expected to facilitate new applications of these nanostructures in ways that have not been possible to date.

Surface-enhanced Raman scattering (SERS), a powerful technique for trace molecular detection, depends on chemical and electromagnetic enhancements. While recent advances in instrumentation and substrate design have expanded the utility, reproducibility, and quantitative capabilities of SERS, some challenges persist. In this review, advances in quantitative SERS detection are discussed as they relate to intermolecular interactions, surface selection rules, and target molecule solubility and accessibility. After a brief introduction to Raman scattering and SERS, impacts of surface selection rules and enhancement mechanisms are discussed as they relate to the observation of activation and deactivation of normal Raman modes in SERS. Next, experimental conditions that can be used to tune molecular affinity to and density near SERS substrates are summarized and considered while tuning these parameters is conveyed. Finally, successful examples of quantitative SERS detection are discussed, and future opportunities are outlined.

In this work, silver film over silica nanosphere (AgFON) surfaces are shown to be thermally stable, SERS-active substrates that are suitable for use in ultrahigh vacuum (UHV) conditions. The metal FON surface is a materials general, cost-effective, and highly SERS-active surface. The SERS activity and thermal stability were investigated by adsorbing benzene, pyridine, and C60 onto the AgFON surface. We chose these adsorbates for the following reasons: (1) vibrational spectroscopy and temperature-programmed desorption (TPD) behavior of benzene adsorbed onto metal surfaces has been widely investigated and is a simple system to study, respectively; (2) characteristics of pyridine adsorption on the AgFON surface can be compared to a large body of previous studies; and (3) high-temperature studies of C60 adsorption can be performed. TPD demonstrates that the AgFON surface has two classes of adsorption sites: (1) those that mimic the behavior of single crystal surfaces and (2) defect sites with higher adsorbate binding energies. Room temperature annealing does not irreversibly destroy the SERS enhancement capability of this surface, thereby permitting for repeated use in UHV experiments. The AgFON surface morphology and localized surface plasmon resonance frequencies, as monitored by UV-vis extinction, change as the AgFON surface temperatures increases from 300 to 548 K, and the SERS activity corresponds with these changes. Because the AgFON surface is thermally stable at room temperature and retains high SERS-activity following temperature annealing to 573 K, it is unlikely that adatoms or adatom clusters play a significant role as adsorption sites supporting the chemical enhancement mechanism. Rather, one can conclude that the electromagnetic enhancement mechanism is the most likely origin of the SER spectra from benzene, pyridine, and C60 adsorbed on AgFON surfaces.

A novel method to produce solution-phase triangular silver nanoparticles is presented. Ag nanoparticles are prepared by nanosphere lithography and are subsequently released into solution. The resulting nanoparticles are asymmetrically functionalized to produce either single isolated nanoparticles or dimer pairs. The structural and optical properties of Ag nanoparticles have been characterized. Mie theory and the Discrete Dipole Approximation method (DDA) have been used to model and interpret the optical properties of the released Ag nanoparticles.

Self-assembled monolayer (SAM) modification is a widely used method to improve the functionality and stability of bulk and nanoscale materials. For instance, the chemical compatibility and utility of solution-phase nanoparticles are often improved using covalently bound SAMs. Herein, solution-phase gold nanoparticles are modified with thioctic acid SAMs in the presence and absence of salt. Molecular packing density on the nanoparticle surfaces is estimated using X-ray photoelectron spectroscopy and increases by ∼20% when molecular self-assembly occurs in the presence versus the absence of salt. We hypothesize that as the ionic strength of the solution increases, pinhole and collapsed-site defects in the SAM are more easily accessible as the electrostatic interaction energy between adjacent molecules decreases, thereby facilitating the subsequent assembly of additional thioctic acid molecules. Significantly, increased SAM packing densities increase the stability of functionalized gold nanoparticles by a factor of 2 relative to nanoparticles functionalized in the absence of salt. These results are expected to improve the reproducible functionalization of solution-phase nanomaterials for various applications.

Biosensors possess recognition elements that bind to target molecules which lead to detectable signals. Incorporation of noble metal nanomaterials into biosensors allows for rapid and simple biomolecule detection. Herein, recent developments in affinity nanosensors will be discussed. These sensors often include naturally occurring recognition elements such as antibodies and DNA. As samples become more complex, new recognition elements are sought. For instance, plastic antibodies provide alternative and more environmentally stable recognition elements than traditional antibodies. Molecular imprinted polymers, a class of plastic antibodies, promote biomolecule recognition and detection. The incorporation of noble metal nanomaterials into molecular imprinted polymer biosensors for real world applications will be explored. Further improvements in the design of artificial recognition agents are envisioned to facilitate new methods for complex biological and chemical analyses.

The radius of curvature of gold (Au) nanostar tips but not the overall particle dimensions can be used for understanding the large and quantitative surface-enhanced Raman scattering (SERS) signal of the uranyl (UO2)(2+) moiety. The engineered roughness of the Au nanostar architecture and the distance between the gold surface and uranyl cations are promoted using carboxylic acid terminated alkanethiols containing 2, 5, and 10 methylene groups. By systematically varying the self-assembled monolayer (SAM) thickness with these molecules, the localized surface plasmon resonance (LSPR) spectral properties are used to quantify the SAM layer thickness and to promote uranyl coordination to the Au nanostars in neutral aqueous solutions. Successful uranyl detection is demonstrated for all three functionalized Au nanostar samples as indicated by enhanced signals and red-shifts in the symmetric U(vi)-O stretch. Quantitative uranyl detection is achieved by evaluating the integrated area of these bands in the uranyl fingerprint window. By varying the concentration of uranyl, similar free energies of adsorption are observed for the three carboxylic acid terminated functionalized Au nanostar samples indicating similar coordination to uranyl, but the SERS signals scale inversely with the alkanethiol layer thickness. This distance dependence follows previously established models assuming that roughness features associated with the radius of curvature of the tips are considered. These results indicate that SERS signals using functionalized Au nanostar substrates can provide quantitative detection of small molecules and that the tip architecture plays an important role in understanding the resulting SERS intensities.

ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTPromoting Intra- and Intermolecular Interactions in Surface-Enhanced Raman ScatteringWenjing Xi, Binaya K. Shrestha, and Amanda J. Haes*View Author Information Department of Chemistry, University of Iowa, Iowa City, Iowa, 55242 United States*E-mail: [email protected]. Tel: 319-384-3695.Cite this: Anal. Chem. 2018, 90, 1, 128–143Publication Date (Web):October 23, 2017Publication History Published online1 November 2017Published inissue 2 January 2018https://doi.org/10.1021/acs.analchem.7b04225Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views2575Altmetric-Citations52LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (5 MB) Get e-AlertsSUBJECTS:Adsorption,Metals,Molecular interactions,Molecules,Raman spectroscopy Get e-Alerts

Surface chemistry variations on alkanethiol-modified gold nanoparticles (i.e., packing density, tilt angle, and composition) influence their function in nanotechnology-based applications. Accurate theoretical predictions of the stability of functionalized nanoparticles enable guided design of their properties but are often limited by the accuracy of the parameters used as model inputs. These parametrization limitations for the extended Derjaguin, Landau, Verwey, and Overbeek (xDLVO) theory are overcome using a size-dependent Hamaker constant for gold, interfacial surface potentials, and the tilt angles of self-assembled monolayers (SAMs), which improve the predictive power of xDLVO theory for modeling nanoparticle stability. Measurements of the electrical properties of gold nanoparticles functionalized with a series of thiolated acids of differing ligand lengths and SAM tilt angles validate the predictions of xDLVO theory using these new parametrizations, illustrating the potential for this approach to improve the design and control of the properties of functionalized gold nanoparticles in various applications.

The distance and orientation dependence of the heterogeneous electron-transfer reaction between ferrocytochrome c (Fe2+Cc) and a silver film over nanosphere (AgFON) electrode is examined in detail using electrochemical surface-enhanced resonance Raman spectroscopy (SERRS) as a molecularly specific and structurally sensitive probe. The distance between the Fe2+ redox center and the electrode surface is controlled by varying the chain length x of an intervening carboxylic acid terminated alkanethiol, HS(CH2)xCOOH, self-assembled monolayer (SAM). The orientation of the heme in Fe2+Cc with respect to the AgFON/S(CH2)xCOOH electrode surface is controlled by its binding motif. Electrostatic binding of Fe2+Cc to AgFON/S(CH2)xCOOH yields a highly oriented redox system with the heme edge directed toward the electrode surface. The binding constants were determined to be K = 5.0 × 106 M-1 and 1.1 × 106 M-1, respectively, for the x = 5 and x = 10 SAMs. In contrast, covalent binding of Fe2+Cc yields a randomly oriented redox system with no preferred direction between the heme edge and the electrode surface. SERRS detected electrochemistry demonstrates that Fe2+Cc electrostatically bound to the x = 5 AgFON/S(CH2)xCOOH surface exhibits reversible oxidation to ferricytochrome c, whereas Fe2+Cc electrostatically bound to the x = 10 surface exhibits irreversible oxidation. In comparison, Fe2+Cc covalently bound to the x = 5 and x = 10 surfaces both exhibit oxidation with an intermediate degree of reversibility. In addition to these primary results, the work presented here shows that AgFON/S(CH2)xCOOH surfaces (1) are biocompatible − Fe2+Cc is observed in its native state and (2) are stable to supporting electrolyte changes spanning a wide range of ionic strength and pH thus enabling, for the first time, SERRS studies of these variables in a manner not accessible with either the widely used colloid or electrochemically roughened SERS-active surfaces.

Bridging knowledge gaps in NanoEHS by identifying current fundamental science challenges and research needs.

Unwanted nanoparticle aggregation and/or agglomeration may occur when anisotropic nanoparticles are dispersed in various solvents and matrices. While extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been successfully applied to predict nanoparticle stability in solution, this model fails to accurately predict the physical stability of anisotropic nanostructures; thus limiting its applicability in practice. Herein, DLVO theory was used to accurately predict gold nanostar stability in solution by investigating how the choice of the nanostar dimension considered in calculations influences the calculated attractive and repulsive interactions between nanostructures. The use of the average radius of curvature of the nanostar tips instead of the average radius as the nanostar dimension of interest increases the accuracy with which experimentally observed nanoparticle behavior can be modeled theoretically. This prediction was validated by measuring time-dependent localized surface plasmon resonance (LSPR) spectra of gold nanostars suspended in solutions with different ionic strengths. Minimum energy barriers calculated from collision theory as a function of nanoparticle concentration were utilized to make kinetic predictions. All in all, these studies suggest that choosing the appropriate gold nanostar dimension is crucial to fully understanding and accurately predicting the stability of anisotropic nanostructures such as gold nanostars; i.e., whether the nanostructures remain stable and can be used reproducibly, or whether they aggregate and exhibit inconsistent results. Thus, the present work provides a deeper understanding of internanoparticle interactions in solution and is expected to lead to more consistent and efficient analytical and bioanalytical applications of these important materials in the future. Graphical abstract ᅟ.

Surface-enhanced Raman scattering (SERS) is a surface sensitive technique that reveals information regarding molecular adsorption driving forces at nanoparticles surfaces. While the plasmonic properties of SERS substrates provide the largest signal enhancements, chemical enhancement mechanisms are more sensitive to molecular adsorption and intermolecular interactions. Herein, gold coated silver nanoparticles that are stabilized inside microporous silica membranes are used for monitoring short-range chemical enhancement effects. First, the silica membrane provides plasmonic stability while also facilitating kinetic measurements so that impacts of molecular protonation, molecule–molecule interactions, molecule–silica interactions, and molecule–Au interactions can be identified. To do this, the vibrational frequencies of 4-mercaptobenzoic acid (4-MBA) are monitored as a function of time and pH. Applying Fick's second law to time-dependent responses reveals that molecular flux decreases with increasing pH. SERS spectra suggest that the kinetics of this phenomenon depend on the protonation state of 4-MBA and, hence, the energy required for the molecules to pass through the negatively charged silica membrane. Namely, repulsive electrostatic interactions between deprotonated molecules (R-COO–) and the silica shell increase the energy required for transport, which subsequently decreases the flux of molecules through the silica shell and subsequent adsorption to the metal surface. As pH approaches neutral conditions, the fraction of deprotonated 4-MBA increases. These molecules, which have a higher electron density in the aromatic rings versus protonated ones, favor selective chemical enhancement of the asymmetric versus symmetric C–C stretching modes. In addition, increasing intermolecular interactions between adsorbed molecules promote electron delocalization from aromatic rings to the carboxylate groups of 4-MBA. This response causes the pKa of the carboxylate to gradually increase from 4.8 (in solution) to 7.7 (on nanoparticle surfaces). Consequently, SERS signals for this molecule can be understood with respect to molecular protonation state, flux, and intermolecular interactions using these electromagnetically stable plasmonic nanostructures.

The impact of tunable morphologies and plasmonic properties of gold nanostars are evaluated for the surface enhanced Raman scattering (SERS) detection of uranyl. To do so, gold nanostars are synthesized with varying concentrations of the Good's buffer reagent, 2-[4-(2-hydroxyethyl)-1-piperazinyl]propanesulfonic acid (EPPS). EPPS plays three roles including as a reducing agent for nanostar nucleation and growth, as a nanostar-stabilizing agent for solution phase stability, and as a coordinating ligand for the capture of uranyl. The resulting nanostructures exhibit localized surface plasmon resonance (LSPR) spectra that contain two visible and one near-infrared plasmonic modes. All three optical features arise from synergistic coupling between the nanostar core and branches. The tunability of these optical resonances are correlated with nanostar morphology through careful transmission electron microscopy (TEM) analysis. As the EPPS concentration used during synthesis increases, both the length and aspect ratio of the branches increase. This causes the two lower energy extinction features to grow in magnitude and become ideal for the SERS detection of uranyl. Finally, uranyl binds to the gold nanostar surface directly and via sulfonate coordination. Changes in the uranyl signal are directly correlated to the plasmonic properties associated with the nanostar branches. Overall, this work highlights the synergistic importance of nanostar morphology and plasmonic properties for the SERS detection of small molecules.

The ability to detect sub-nanomolar concentrations of ricin using fluorescently tagged RNA aptamers is demonstrated. Aptamers rival the specificity of antibodies and have the power to simplify immunoassays using capillary electrophoresis. Under nonequilibrium conditions, a dissociation constant, Kd, of 134 nM has been monitored between the RNA aptamer and ricin A-chain. With use of this free-solution assay, the detection of 500 pM (∼14 ng/mL) or 7.1 amol of ricin is demonstrated. The presence of interfering proteins such as bovine serum albumin and casein do not inhibit this interaction at sub-nanomolar concentrations. When spiked with RNAse A, ricin can still be detected down to 1 nM concentrations despite severe aptamer degradation. This approach offers a promising method for the rapid, selective, and sensitive detection of biowarfare agents.

Increasing the citrate concentration during the seeded growth synthesis of gold nanoparticles yields materials with decreased aspect ratios and increased defect densities. The stability of these nanoparticles is attributed to variations in their overall Gibb's free energy.

Control over the composition, shape, size, stability, and local dielectric environment of solution-phase metallic substrates is vital to consistent surface-enhanced Raman scattering (SERS) signals. Because of their inherent instability, solution-phase nanoparticles can undergo uncontrolled aggregation when target molecules are added. Here, we demonstrate that both molecular surface coverage of the Raman active molecule, 2-naphthalenethiol (2-NT), and nanoparticle concentration are critical parameters for obtaining reproducible SERS signals using solution-phase gold nanoparticles. Both gold nanoparticle and 2-naphthalenethiol concentrations are varied, and the extinction of the nanoparticle substrate and the SERS intensity of the target molecule are monitored as a function of time. These results indicate that extinction and SERS spectral intensities increase predictably below full monolayer surface coverage. When excess molecules are added, uncontrolled and irreproducible nanoparticle aggregation leads to optimal overlap between the plasmonic properties of the nanoparticles and the SERS excitation wavelength. Importantly, this is the first report which correlates solution-phase nanoparticle concentration and stability to molecular surface coverage for simultaneous localized surface plasmon resonance (LSPR) and SERS spectroscopic measurements. As a result, these data should facilitate the experimental design and use of solution-phase SERS substrates for more predictable molecular detection.

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straightforward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal-to-noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all, this user's guide is expected to provide a user-friendly, straightforward approach for uranium species identification using Raman spectroscopy.

Solution-phase nanoparticles are extensively used as surface enhanced Raman scattering (SERS) substrates, but signal intensities depend on dynamic nanoparticle optical properties and stabilities as well as molecular identity and orientation. To evaluate how these contributions influence the detection of aromatic thiols, internally etched silica encapsulated gold-coated silver (IE Ag@Au@SiO2) nanoparticles are used. First, localized surface plasmon resonance (LSPR) spectroscopy is implemented to estimate molecular tilt angle. Different tilt angles are then related to functional group induced surface density differences. Next, evaluation of SERS intensities and vibrational modes suggest that molecular tilt angle and surface selection rules govern the behavior observed in SERS intensities. Finally, concentration-dependent SERS signals are modeled using the Langmuir adsorption model. Equilibrium constants and free energies associated with adsorption are consistent with differences from London dispersion force stabilization between the molecules and the metal surface. These studies suggest that the SERS intensities observed for these thiolated ligands are highly sensitive to adsorbate tilt angle relative to the nanoparticle surface, which are easily estimated because of the optical stability and controlled adsorbate interactions with IE Ag@Au@SiO2 nanoparticles and could be extended to other molecules in the future to better understand and evaluate reproducible applications using SERS.

Reproducible detection of uranyl, an important biological and environmental contaminant, from complex matrixes by surface-enhanced Raman scattering (SERS) is successfully achieved using amidoximated-polyacrylonitrile (AO-PAN) mats and carboxylated gold (Au) nanostars. SERS detection of small molecules from a sample mixture is traditionally limited by nonspecific adsorption of nontarget species to the metal nanostructures and subsequent variations in both the vibrational frequencies and intensities. Herein, this challenge is overcome using AO-PAN mats to extract uranyl from matrixes ranging in complexity including HEPES buffer, Ca(NO3)2 and NaHCO3 solutions, and synthetic urine. Subsequently, Au nanostars functionalized with carboxyl-terminated alkanethiols are used to enhance the uranyl signal. The detected SERS signals scale with uranyl uptake as confirmed using liquid scintillation counting. SERS vibrational frequencies of uranyl on both hydrated and lyophilized polymer mats are largely independent of sample matrix, indicating less complexity in the uranyl species bound to the surface of the mats vs in solution. These results suggest that matrix effects, which commonly limit the use of SERS for complex sample analysis, are minimized for uranyl detection. The presented synergistic approach for isolating uranyl from complex sample matrixes and enhancing the signal using SERS is promising for real-world sample detection and eliminates the need of radioactive tracers and extensive sample pretreatment steps.

This review focuses on the integration of noble metal nanoparticle aggregates as tags and transport vessels in cellular applications. The natural tendency of nanoparticles to aggregate can be reduced through surface modification; however, this stabilization is often compromised in the cellular environment. The degree of nanoparticle aggregation has both positive and negative consequences. Nanoparticle aggregates are more efficiently removed by the organism compared with single nanoparticles, preventing delivery to their cellular target. In addition, these aggregates are recognized by cells in different ways versus isolated nanoparticles. Despite these negatives, aggregates exhibit enhancement for many detection and treatment techniques in comparison with single nanoparticles. In coming years, the role of aggregates and better control over the degree of aggregation in cellular studies will be required for the realization of medical applications.

Silica coated gold nanospheres are purified using traditional centrifugation steps and/or anion exchange chromatography and their resulting surface-enhanced Raman activities compared. Partially silica-coated gold nanostructures are retained on the column while fully coated nanostructures elute. As a result, SERS activity becomes less erratic and follows distance dependence models. This simple chromatographic step adds a quality control measure to nanoparticle preparation which could be extended to other solution-phase nanoparticles for more predictable function in future applications.

Electrically driven separations which contain nanoparticles offer detection and separation advantages but are often difficult to reproduce. To address possible sources of separation inconsistencies, anionic functionalized gold nanoparticles are thoroughly characterized and subsequently included in continuous full filling capillary electrophoresis separations of varying concentrations of three small molecules. Citrate stabilized gold nanospheres are functionalized with 11-mercaptoundecanoic acid, 6-mercaptohexanoic acid, or thioctic acid self-assembled monolayers (SAMs) and characterized using dynamic light scattering, extinction spectroscopy, zeta potential, and X-ray photoelectron spectroscopy prior to use in capillary electrophoresis. Several important trends are noted. First, the stability of these anionic nanoparticles in the capillary improves with increased ligand packing density as indicated by a ratio of absorbance collected at 520 to 600 nm. Second, increasing nanoparticle concentration from 0 to 2 nM (0-0.002(5)%, w/w) minimally impacts analyte migration times; however, when higher nanoparticle concentrations are included within the capillary, nanoparticle aggregation occurs which induces separation inconsistencies. Third, analyte peak areas are most significantly impacted as their concentration decreases. These trends are attributed to both sample enrichment and electrostatic interactions between the anionic carboxylic acid functionalized gold nanoparticles and sample. These important findings suggest that sample concentration-induced conductivity differences between the sample matrix and separation buffer as well as SAM packing density are important parameters to both characterize and consider when nanoparticles are used during continuous full filling separations and their subsequent use to enhance spectroscopic signals to improve in-capillary analyte detection limits.

A microchip-based, displacement immunoassay for the sensitive laser-induced fluorescence detection of staphylococcal enterotoxin B is presented. The glass microchip device consists of a microchannel that contains a double weir structure for supporting antibody-functionalized microbeads. After a 30-min sample preparation step, the displacement assay was performed without user intervention and produced quantitative results in an additional 20 min. Linear detection responses were observed over 6 orders of magnitude and provided detection limits down to 1 fM (28.5 fg/mL). The surprisingly low detection limits are hypothesized to arise from field-based enrichment analogous to field-amplified stacking, chromatographic effects, and limited diffusion lengths in the microbead bed. The assay was challenged with bovine serum albumin, casein, and milk sample matrixes. This system has the potential to provide highly sensitive detection capabilities for target biomolecules.

Centrifugation is widely used in the synthesis and handling of solution-phase nanoparticles to improve their purity and to change the composition of the solvent. Herein, we couple the optical properties of citrate-stabilized gold nanoparticles and silica encapsulation to investigate how centrifugation impacts the formation of stabilized nanoparticle clusters in solution without the use of linker molecules or asymmetric functionalization. Gold nanoparticles preconcentrated using a high (9,400) g force result in linear assemblies of gold cores that are spaced by ∼1−4 nm within Aun@SiO2 structures (n = number of gold nanoparticle cores per silica shell) with ∼30% monomers, 30% dimers, 20% trimers, and 10% 4−7mers. In comparison, nanoparticles preconcentrated using (stirred) ultrafiltration and low (23) g force centrifugation have statistically identical cluster distributions (90% monomers, 9% dimers, and 1% trimers) whereas nanoparticles that are not preconcentrated always exhibit 100% monomers using the same silica coating procedure. We hypothesize that under high g force, the electrical double layer surrounding the gold nanoparticles is slightly polarized thereby increasing the attraction between nanoparticles and the formation of stable clusters. The conductivity of the solution plays an important role in this stabilization. This novel demonstration of linear cluster formation of gold nanoparticles using centrifugation suggests that this commonly used preparative tool can both positively or negatively impact the fundamental properties of these materials and their use in various applications.

The Ag nanoparticle based localized surface plasmon resonance (LSPR) nanosensor yields ultrasensitive biodetection with extremely simple, small, light, robust, and low-cost instrumentation. Using LSPR spectroscopy, the model system, biotinylated surface-confined Ag nanotriangles, was used to detect less than one picomolar up to micromolar concentrations of streptavidin. Additionally, the monitoring of anti-biotin binding to biotinylated Ag nanotriangles exhibited that the system could be used as a solution immunoassay. The system was rigorously tested for nonspecific binding interactions and was found to display virtually no adverse results. These results represent important new steps in the development of the LSPR nanobiosensor for applications in medical diagnostics, biomedical research, and environmental science.

Aggregates or clusters of primary metal nanoparticles in solution are one of the most widely used platforms for surface-enhanced Raman scattering (SERS) measurements because these nanostructures induce strong electric fields or hot spots between nanoparticles and as a result, SERS signals. While SERS signals are observed to vary with time, the impact of cluster formation mechanisms on SERS activity has been less studied. Herein, variations in time-dependent SERS signals from gold nanosphere clusters and aggregates are considered both experimentally and theoretically. An excess of the Raman reporter molecule, 2-naphthalenethiol, is added to induce rapid monolayer formation on the nanoparticles. In this diffusion-limited regime, clusters form as loosely packed fractals and the ligands help control nanoparticle separation distances once clusters form. By systematically varying gold nanosphere concentration and diameter, the reaction kinetics and dynamics associated with cluster formation can be studied. Dynamic light scattering (DLS), localized surface plasmon resonance (LSPR) spectroscopy, and SERS reveal that aggregates form reproducibly in the diffusion-limited regime and follow a self-limiting cluster size model. The rate of cluster formation during this same reaction window is explained using interaction pair potential calculations and collision theory. Diffusion-limited reaction conditions are limited by sedimentation only if sedimentation velocities exceed diffusion velocities of the clusters or via plasmon damping through radiation or scattering losses. These radiative loses are only significant when the extinction magnitude near the excitation wavelength exceeds 1.5. By evaluating these responses as a function of both nanosphere radius and concentration, time-dependent SERS signals are revealed to follow collision theory and be predictable when both nanosphere concentration and size are considered.

Tailored surface chemistry impacts nanomaterial function and stability in applications including in various capillary electrophoresis (CE) modes. Although colloidal nanoparticles were first integrated as colouring agents in artwork and pottery over 2000 years ago, recent developments in nanoparticle synthesis and surface modification increased their usefulness and incorporation in separation science. For instance, precise control of surface chemistry is critically important in modulating nanoparticle functionality and stability in dynamic environments. Herein, recent developments in nanomaterial pseudostationary and stationary phases will be summarized. First, nanomaterial core and surface chemistry compositions will be classified. Next, characterization methods will be described and related to nanomaterial function in various CE modes. Third, methods and implications of nanomaterial incorporation into CE will be discussed. Finally, nanoparticle-specific mechanisms likely involved in CE will be related to nanomaterial surface chemistry. Better understanding of surface chemistry will improve nanoparticle design for the integration into separation techniques.

Capillary electrophoresis based separations of the hypothesized Parkinson's disease biomarkers dopamine, epinephrine, pyrocatechol, L-3,4-dihydroxyphenylalanine (L-DOPA), glutathione, and uric acid are performed in the presence of a 1 nM 11-mercaptoundecanoic acid functionalized gold (Au@MUA) nanoparticle pseudostationary phase plug. Au@MUA nanoparticles are monitored in the capillary and remain stable in the presence of electrically-driven flow. Migration times, peak areas, and relative velocity changes (vs. no pseudostationary) are monitored upon varying (1) the Au@MUA nanoparticle pseudostationary phase plug length at a fixed separation voltage and (2) the separation voltage for a fixed Au@MUA nanoparticle pseudostationary phase plug length. For instance, the migration times of positively charged dopamine and epinephrine increase slightly as the nanoparticle pseudostationary phase plug length increases with concomitant decreases in peak areas and relative velocities as a result of attractive forces between the positively charged analytes and the negatively charged nanoparticles. Migration times for neutral pyrocatechol and slightly negative L-DOPA did not exhibit significant changes with increasing nanoparticle pseudostationary plug length; however, reduction in peak areas for these two molecules were evident and attributed to non-specific interactions (i.e.hydrogen bonding and van der Waals interactions) between the biomarkers and nanoparticles. Moreover, negatively charged uric acid and glutathione displayed progressively decreasing migration times and peak areas and as a result, increased relative velocities with increasing nanoparticle pseudostationary phase plug length. These trends are attributed to partitioning and exchanging with 11-mercaptoundecanoic acid on nanoparticle surfaces for uric acid and glutathione, respectively. Similar trends are observed when the separation voltage decreased thereby suggesting that nanoparticle-biomarker interaction time dictates these trends. Understanding these analyte migration time, peak area, and velocity trends will expand our insight for incorporating nanoparticles in separations.

Solution-phase nanoparticles are widely used in surface-enhanced Raman scattering (SERS) but provide limited quantitative information because of their dynamic optical properties. To overcome this problem, silica membrane stabilized solution-phase nanoparticles composed of silver cores and gold shells (Ag@Au@SiO2) are synthesized, characterized, and used for predictable and quantitative detection of 4-aminothiophenol using SERS. Two key parameters including local effective refractive index and void volume near the metal cores are correlated to SERS activity. First, the effective local refractive index and void volumes formed near the metal surface are characterized using the localized surface plasmon resonance of the Ag@Au nanoparticles and semi-empirical dielectric modeling for the porous silica membrane. The characteristic electromagnetic field decay length is estimated at 11 nm while both linear and nonlinear refractive index sensitivities are found to be 170 and 360 nm/RIU, respectively. The internally etched silica membrane stabilized Ag@Au@SiO2 nanoparticles can be engineered to exhibit effective local refractive indices ranging from 1.366 to 1.458. Second, SERS signals associated with these nanomaterials and 4-aminothiophenol are shown to indirectly depend on the effective local refractive index, which directly correlates to increasing void volume in the silica near the metal particle. This effect is attributed to well-controlled molecule-accessible volumes near the metal surface where the local electric field strength is largest. Finally, small variations in the effective refractive index (±0.01), nanoparticle concentration, and nanoparticle to molecule concentrations influence the magnitude of the SERS signal. As such, these findings are expected to improve the design and surface modification of solution-phase SERS-active substrates for quantitative and reproducible SERS detection.

Microporous silica membranes facilitate plasmonic stability of Ag@Au nanoparticles against variations in pH, ionic strength, and temperature for SERS sensing.

Aim: miRNAs have been shown to improve the restoration of craniofacial bone defects. This work aimed to enhance transfection efficiency and miR-200c-induced bone formation in alveolar bone defects via plasmid DNA encoding miR-200c delivery from CaCO 3 nanoparticles. Materials &amp; methods: The CaCO 3 / miR-200c delivery system was evaluated in vitro (microscopy, transfection efficiency, biocompatibility) and miR-200c-induced in vivo alveolar bone formation was assessed via micro-computed tomography and histology. Results: CaCO 3 nanoparticles significantly enhanced the transfection of plasmid DNA encoding miR-200c without inflammatory effects and sustained miR-200c expression. CaCO 3 / miR-200c treatment in vivo significantly increased bone formation in rat alveolar bone defects. Conclusion: CaCO 3 nanoparticles enhance miR-200c delivery to accelerate alveolar bone formation, thereby demonstrating the application of CaCO 3 / miR-200c to craniofacial bone defects.

Dentin biomodification is a promising approach to enhance dental tissue biomechanics and biostability for restorative and reparative therapies. One of the most active dentin tissue biomodifiers is proanthocyanidin (PAC)-rich natural extracts, which are used in the dental bonding procedure in combination with resin-based adhesives (RBAs). This study aimed to investigate the use of mesoporous silica nanoparticles (MSNs) for the sustained delivery of PACs for dentin biomodification as a novel drug-delivery system for dental applications. The effects of the incorporation of MSN functionalized with 3-aminopropyltriethoxysilane (APTES) and loaded with PAC into an experimental RBA were assessed by characterizing the material mechanical properties. In addition, the immediate and long-term bonding performance of an experimental resin-based primer (RBP) containing MSN-APTES loaded with PAC was also evaluated. For that, different formulations of RBA and RBP were prepared containing 20% w/v MSN-APTES loaded with PAC before or after functionalization (MSN-PAC-APTES and MSN-APTES-PAC, respectively). The incorporation of MSN-APTES-PAC did not negatively impact the degree of conversion or the overall mechanical properties of the RBA. However, adding MSN-PAC-APTES resulted in inferior mechanical properties of the experimental RBA. In the adhesion studies, APTES-functionalized MSN was successfully added to an experimental RBP for drug-delivery purposes without compromising the bond strength to the dentin or the failure mode. Interestingly, the sequence of surface functionalization with APTES resulted in differences in the bonding performance, with better long-term results for RBP containing MSN loaded with PAC after functionalization.

Abstract A short 16‐amino acid peptide has been used in place of an antibody to selectively detect the specific Anthrax biomarker, protective antigen (PA), using surface‐enhanced Raman scattering (SERS). Peptides are more stable than antibodies under various biological conditions and are easily synthesized for a specific target. A peptide that has high affinity to PA was conjugated onto gold nanoparticles along with a Raman reporter and then incubated in various concentrations of PA. Parallel studies in which the peptide sequence was replaced with an antibody were performed to compare the performance of the two methodologies. Both the peptide and antibody functionalized nanoparticles were able to specifically detect PA concentrations down to 6.1 f M . These results demonstrate that these short, robust peptides can be used in the place of traditional antibodies to specifically recognize target biomarkers in the field for the potential diagnosis of disease. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The intense color of noble metal nanoparticles has inspired artists and fascinated scientists for hundreds of years. These rich hues are due to the interaction of light with the nanostructure's localized surface plasmon (LSPR). Here, we describe three optical sensing modalities that are dependant on the effects of the LSPR. Specifically, we will demonstrate the use of LSPR supporting particles as analogues to fluorescent probes and labels for multiplex detection, sensing based on observation of changes in the LSPR spectrum caused by alteration of the local refractive index upon analyte binding, and the spectroscopic labeling of cells and tissues with Surface Enhanced Raman Scatting (SERS) active nanoparticles probes.

The development of biosensors for the diagnosis and monitoring of diseases, drug discovery, proteomics, and the environmental detection of pollutants and/or biological agents is an extremely significant problem.1 Fundamentally, a biosensor is derived from the coupling of a ligand-receptor binding reaction to a signal transducer. A variety of biosensor signal transduction methods exist, including optical, radioactive, electrochemical, piezoelectric, magnetic, micromechanical, and mass spectrometric.

Recently, nanoparticles have become the platform for many sensing schemes. In particular, the utilization of the optical response of nanoparticles to changes in their nanoenvironment has served as a signal transduction mechanism for these sensing events. For example, silver nanoparticle arrays synthesized using nanosphere lithography have served as an ultrasensitive detection platform for small molecules, proteins, and antibodies with the detection limit of 60,000 and less than 25 molecules/nanoparticle for hexadecanethiol and antibodies, respectively. While this approach is low cost and highly portable, one limitation of the array platform is that the signal arises from approximately 1x10⁶ nanoparticles. A method to improve the overall number of molecules detected would be to decrease the number of nanoparticles probed. Recently, single nanoparticle sensing has been accomplished using dark-field microscopy. A 40 nm shift in the localized surface plasmon resonance induced from less than 60,000 small-molecule adsorbates has been monitored from a single Ag nanoparticle. Additionally, streptavidin sensing has also been demonstrated using a single Ag nanoparticle. Detection platforms based on nanoparticle arrays and single nanoparticles will be discussed and compared.

The enzyme activity of xanthine oxidase (XO) is influenced by several environmental factors including solution conditions, storage conditions, inhibitors, other enzymes, and activators. For instance, the metabolic reaction involving XO and the pro-drug 6-mercaptopurine, a drug used in the treatment maintenance of acute lymphatic leukemia, Crohn's disease, and ulcerative colitis, is often modified through the use of inhibitors, which varies the kinetic parameters associated with this reaction. Methods that provide fast and accurate determination of these kinetic constants can help in understanding the mechanism of these reactions. Herein, sequential and time-delayed electrokinetic injections of unpurified and unquenched samples containing xanthine oxidase, 6-mercaptopurine, and the inhibitor allopurinol are evaluated using capillary electrophoresis (CE). Using progress curve analysis, the Michaelis constant, apparent Michaelis constant, and inhibition constant are estimated to be 43.8 ± 2.0 μM, 143.0 ± 3.7 μM and 13.2 ± 1.4 μM, respectively. In addition, a turnover number of 7.9 ± 0.2 min−1 is quantified. These values are consistent with some previously published values but were obtained without user intervention for reaction monitoring. This unique application of CE enzyme assays offers substantial advantages over traditional methods by determining kinetic parameters for enzymatic reactions with minimal (nL) sample volumes, short (<30 min) reaction analysis times, without any sample quenching or purification, and minimal user intervention.

Local refractive index sensitivity modelling using the plasmonic properties of gold nanospheres assists in the elucidation of the nanoparticle-rattle formation as a function of sample age and storage conditions.

MicroRNA (miR)-200c suppresses the initiation and progression of oral squamous cell carcinoma (OSCC), the most prevalent head and neck cancer with high recurrence, metastasis, and mortality rates. However, miR-200c-based gene therapy to inhibit OSCC growth has yet to be reported. To develop an miR-based gene therapy to improve the outcomes of OSCC treatment, this study investigates the feasibility of plasmid DNA (pDNA) encoding miR-200c delivered via nonviral CaCO3-based nanoparticles to inhibit OSCC tumor growth. CaCO3-based nanoparticles with various ratios of CaCO3 and protamine sulfate (PS) were used to transfect pDNA encoding miR-200c into OSCC cells, and the efficiency of these nanoparticles was evaluated. The proliferation, migration, and associated oncogene production, as well as in vivo tumor growth for OSCC cells overexpressing miR-200c, were also quantified. It was observed that, while CaCO3-based nanoparticles improve transfection efficiencies of pDNA miR-200c, the ratio of CaCO3 to PS significantly influences the transfection efficiency. Overexpression of miR-200c significantly reduced proliferation, migration, and oncogene expression of OSCC cells, as well as the tumor size of cell line-derived xenografts (CDX) in mice. In addition, a local administration of pDNA miR-200c using CaCO3 delivery significantly enhanced miR-200c transfection and suppressed tumor growth of CDX in mice. These results strongly indicate that the nanocomplexes of CaCO3/pDNA miR-200c may potentially be used to reduce oral cancer recurrence and improve clinical outcomes in OSCC treatment, while more comprehensive examinations to confirm the safety and efficacy of the CaCO3/pDNA miR-200c system using various preclinical models are needed.

The plasmonic properties of carboxylated gold nanostars distributed on amidoximated polyacrylonitrile (AO PAN) electrospun polymer films scale with surface-enhanced Raman scattering (SERS) intensities for coordinated uranium(VI) oxide (uranyl) species. This two-step plasmonic sensor first isolates uranyl from solution using functionalized polymers; then carboxylated gold nanostars are subsequently deposited for SERS. Spatially resolved localized surface plasmon resonance (LSPR) and SERS facilitate correlated nanostar optical density and uranyl quantification. To reduce sampling bias, gold nanostars are deposited in an inverted drop-coating geometry and measurements are conducted inside resulting nanoparticle coffee rings that form on the polymer substrates. This approach naturally preserves the plasmonic properties of gold nanostars while reducing the deposition of nanoparticle aggregates in active sensing regions, thereby maximizing both the accuracy and the precision of SERS measurements. Several advances are made. First, second-derivative analysis of LSPR spectra facilitates the quantification of local nanostar density across large regions of the sensor substrate by reducing background variations caused by the polymeric and gold materials. Second, local nanostar densities ranging from 140 to 200 pM·cm are shown to result in uranyl signals that are independent of nanostar concentration. Third, the Gibbs free energy of uranyl adsorption to carboxylated nanostars is estimated at 8.4 ± 0.2 kcal/mol. Finally, a linear dynamic range from ∼0.3 to 3.4 μg U/mg polymer is demonstrated. Signals vary by 10% or less. As such, the uniformity of the plasmonic activity of distributed gold nanostars and the employment of spatially resolved spectroscopic measurements on the composite nanomaterial sensor interface facilitate the quantitative detection of uranyl while also reducing the dependence on user expertise and the selected sampling region. These important advances are critical for the development of a user-friendly SERS-based sensor for uranyl.

Abstract Silver nanoparticles (AgNPs) have been proposed to combat oral infection due to their efficient ionic silver (Ag + ) release. However, concentrations required for antimicrobial efficacy may not be therapeutically viable. In this work, platinum‐doped silver nanoparticles (Pt‐AgNPs) were explored to evaluate their potential for enhanced Ag + release, which could lead to enhanced antimicrobial efficacy against S . aureus , P . aeruginosa , and E . coli . AgNPs doped with 0.5, 1, and 2 mol% platinum (Pt 0.5 ‐AgNPs, Pt 1 ‐AgNPs, and Pt 2 ‐AgNPs) were synthesized by a chemical reduction method. Transmission electron microscopy revealed mixed morphologies of spherical, oval, and ribbon‐like nanostructures. Surface‐enhanced Raman scattering revealed that the surface of Pt‐AgNPs was covered with up to 93% Pt. The amount of Ag + released increased 16.3‐fold for Pt 2 ‐AgNPs, compared to AgNPs. The initial lag phase in bacterial growth curve was prolonged for Pt‐AgNPs. This is consistent with a Ag + release profile that exhibited an initial burst followed by sustained release. Doping AgNPs with platinum significantly increased the antimicrobial efficacy against all species. Pt 2 ‐AgNPs exhibited the lowest minimum inhibitory concentrations, followed by Pt 1 ‐AgNPs, Pt 0.5 ‐AgNPs, and AgNPs, respectively. Doping AgNPs with a small amount of platinum promoted the release of Ag + , based on the sacrificial anodic effect, and subsequently enhanced their antimicrobial efficacy.

Raman spectral vibrational frequencies are used to probe the local chemical environment surrounding molecules in solution and adsorbed to gold nanostars. Herein, the impacts of functional group protonation on monosubstituted benzene derivatives with amine, carboxylic acid, or hydroxide are evaluated. Changes in binding affinity and orientation are apparent by evaluating systematic variations in vibrational frequencies. Notably, the electron donating abilities of these functional groups influence the vibrational frequency of the ring breathing mode, thus leading to improved spectral interpretation. Furthermore, gold nanostars are used to investigate the impact of molecular protonation on the adsorption of benzoic acid/benzoate to gold. The changes in molecular protonation are measured using zeta potential and the surface-sensitive technique, surface-enhanced Raman scattering. These methods reveal that pH variations induce carboxylate protonation and electron redistribution that weaken molecular affinity, thereby causing the molecule to adopt a perpendicular to parallel orientation with respect to the nanostar surface. Functional group identity influences the ring breathing mode frequency as a function of changes in electron donation from the functional group to the ring in solution as well as molecular affinity to and orientation on gold. This exploitation of vibrational frequencies facilitates the elucidation of molecule behavior in complex systems.

We previously observed that phosphonate functionalized electrospun nanofibers can uptake U(VI), making them promising materials for sensing and water treatment applications. Here, we investigate the optimal fabrication of these materials and their mechanism of U(VI) binding under the influence of environmentally relevant ions (e.g., Ca2+ and CO32-). We found that U(VI) uptake was greatest on polyacrylonitrile (PAN) functionalized with longer-chain phosphonate surfactants (e.g., hexa- and octadecyl phosphonate; HDPA and ODPA, respectively), which were better retained in the nanofiber after surface segregation. Subsequent uptake experiments to better understand specific solid-liquid interfacial interactions were carried out using 5 mg of HDPA-functionalized PAN mats with 10 μM U at pH 6.8 in four systems with different combinations of solutions containing 5 mM calcium (Ca2+) and 5 mM bicarbonate (HCO3-). U uptake was similar in control solutions containing no Ca2+ and HCO3- (resulting in 19 ± 3% U uptake), and in those containing only 5 mM Ca2+ (resulting in 20 ± 3% U uptake). A decrease in U uptake (10 ± 4% U uptake) was observed in experiments with HCO3-, indicating that UO2-CO3 complexes may increase uranium solubility. Results from shell-by-shell EXAFS fitting, aqueous extractions, and surface-enhanced Raman scattering (SERS) indicate that U is bound to phosphonate as a monodentate inner sphere surface complex to one of the hydroxyls in the phosphonate functional groups. New knowledge derived from this study on material fabrication and solid-liquid interfacial interactions will help to advance technologies for use in the in-situ detection and treatment of U in water.

The inherent sensitivity of molecular vibrational frequencies to their local chemical environment allows for the investigation of how small molecules interact within engineered cavities in molecular imprinted polymers (MIPs). These interactions arise via weak yet collective intermolecular interaction between the polymer and small molecule. Herein, intermolecular interactions between methacrylic acid-based MIPs and acetaminophen, aspirin, and caffeine are evaluated using shifts in the vibrational frequencies and changes in bandwidths of Raman-active modes. Recognition between these materials is measured experimentally and compared to modeled binding energies. Upon evaluation of Raman signals for the analgesics, intermolecular interactions such as hydrogen bonding and other weak interactions between the molecules and polymer backbone are quantified. Finally, dissociation constants and imprinting efficiencies are estimated for selectivity evaluation. This exploitation of the sensitivity of Raman-active vibrational band frequencies to collective intermolecular interactions for binding studies could facilitate the development and assessment of MIPs for small molecule recognition.

Abstract MicroRNA (miR)-200c suppresses the initiation and progression of oral squamous cell carcinoma (OSCC), the most prevalent head and neck cancer with high recurrence, metastasis, and mortality rates. However, miR-200c -based gene therapy to inhibit OSCC growth and metastasis has yet to be reported. To develop an miR-based gene therapy to improve the outcomes of OSCC treatment, this study investigates the feasibility of plasmid DNA encoding miR-200c delivered via non-viral CaCO 3 -based nanoparticles to inhibit OSCC tumor growth. CaCO 3 -based nanoparticles with various ratios of CaCO 3 and protamine sulfate (PS) were utilized to transfect pDNA encoding miR-200c into OSCC cells and the efficiency of these nanoparticles was evaluated. The proliferation, migration, and associated oncogene production, as well as in vivo tumor growth for OSCC cells overexpressing miR-200c were also quantified. It was observed that, while CaCO 3 -based nanoparticles improve transfection efficiencies of pDNA miR-200c , the ratio of CaCO 3 to PS significantly influences the transfection efficiency. Overexpression of miR-200c significantly reduced proliferation, migration, and oncogene expression of OSCC cells, as well as the tumor size of cell line-derived xenografts (CDX) in mice. In addition, a local administration of pDNA miR-200c using CaCO 3 delivery significantly enhanced miR-200c transfection and suppressed tumor growth of CDX in mice. These results strongly indicate that the nanocomplexes of CaCO 3 /pDNA miR-200c may potentially be used to reduce oral cancer recurrence and metastasis and improve clinical outcomes in OSCC treatment. (227 words)

Localized surface plasmon resonance (LSPR) is one of the signature optical properties of noble metal nanoparticles. Since the LSPR wavelength &#955;_max is extremely sensitive to the local environment, it allows us to develop nanoparticle-based LSPR chemical and biological sensors. In this work, we tuned the LSPR peaks of Ag nanotriangles and explored the wavelength-dependent LSPR shift upon the adsorption of some resonant molecules. The induced LSPR peak shifts (&#916;&#955;_max) vary with wavelength and the line shape of the LSPR shift is closely related to the absorption features of the resonant molecules. When the LSPR of the nanoparticles directly overlaps with the molecular resonance, a very small LSPR shift was observed. An amplified LSPR shift is found when LSPR of the nanoparticles is at a slightly longer wavelength than the molecular resonance of the adsorbates. Furthermore, we apply the "amplified" LSPR shift to detect the substrate binding of camphor to the heme-containing cytochrome P450cam protiens (CYP101). CYP101 absorb light in the visible region. When a small substrate molecule binds to CYP101, the spin state of the molecule is converted to its low spin state. By fabricating nanoparticles with the LSPR close to the molecular resonance of CYP101 proteins, the LSPR response as large as ~60 nm caused by the binding of small substrate has been demonstrated.

We previously observed that phosphonate functionalized electrospun nanofibers can uptake U(VI), making them promising materials for sensing and water treatment applications. Here, we investigate the optimal fabrication of these materials and their mechanism of U(VI) binding under the influence of environmentally relevant ions (e.g., Ca2+ and CO32-). We found that U(VI) uptake was greatest on polyacrylonitrile (PAN) functionalized with longer-chain phosphonate surfactants (e.g., hexa- and octadecyl phosphonate; HDPA and ODPA, respectively), which were better retained in the nanofiber after surface segregation. Subsequent uptake experiments to better understand specific solid-liquid interfacial interactions were carried out using 5 mg of HDPA-functionalized PAN mats with 10 mM U at pH 6.8 in four systems with different combinations of solutions containing 5 mM calcium (Ca2+) and 5 mM bicarbonate (HCO3-). U uptake was similar in control solutions containing no Ca2+ and HCO3- (resulting in 19 ± 3% U uptake), and in those containing only 5 mM Ca2+ (resulting in 20 ± 3% U uptake). A decrease in U uptake (10 ± 4% U uptake) was observed in experiments with HCO3-, indicating that UO2-CO3 complexes may increase uranium solubility. Results from shell-by-shell EXAFS fitting, aqueous extractions, and surface-enhanced Raman scattering (SERS) indicate that U is bound to phosphonate as a monodentate inner sphere surface complex to one of the hydroxyls in the phosphonate functional groups. New knowledge derived from this study on material fabrication and solid-liquid interfacial interactions will help to advance technologies for use in the in-situ detection and treatment of U in water.

There has been significant interest in the adaptation of lab on a chip devices for the separation and detection of chemical and biological toxins. Potential toxic and/or hazardous analytes of concern in our program include BWA protein toxins,e.g., SEB and ricin, ingestible CW toxins, e.g. alkaloids and rat poisons, and nitroaromatic explosives indicative of IED's. Discussion will center around our efforts to enhance sensitivity and selectivity on a microchip by incorporation of micro-solid-phase extraction, microchip bubble-cell long pathlength UV detection, organically-modified sol-gel materials for electrochromatography, protein recognition aptamers, and microfluidic-based displacement immunoassays

The chemical mechanisms of silver diamine fluoride, a caries-arresting and caries-preventing agent, are presented using Raman spectroscopy. Findings included that SDF works as a caries-arresting agent by forming Ag3PO4 and as a caries-preventing agent by forming fluorapatite (FAP), that the time needed for SDF to work as a caries-preventive agent is shorter than that needed to work as a caries-arresting agent, and the presence of thiol group (caries) delayed the formation of Ag3PO4 and FAP.

The development of nanodevices, including nanosensors that are highly sensitive and selective (give low false positives, low false negatives) have the potential to provide a major improvement over current technologies for disease understanding, treatment, and monitoring. Nanoscale sensors consume less sample volume than conventional instruments because their inherently small size scale in comparison to standard macroscale devices permits straightforward integration with microfluidic devices. Additionally, nanoscale systems often exhibit behavior that is markedly different from their macroscale counterparts, thereby providing alternative pathways for obtaining new information.

Gold nanoparticles that are protected by permeable silica membranes preserve the optical properties of the nanoparticle core in harsh environments without sacrificing enhanced spectroscopic detection. Reproducible and quantitative detection using SERS will be demonstrated.

Please forward book reviews to the Book Review Editor, Alexander Scheeline, Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL 61801.

Uranium, a radioactive material with a long half-life, accumulates in the environment in its oxidative form uranyl, which can contaminate soil and water. Importantly, uranyl forms complexes with anions and cations thereby influencing solubility, toxicity, and fate of these heavy metal species. Furthermore, uranyl speciation varies with pH. The resulting complex speciation complicates detection and/or requires significant sample pretreatment. As such, methods that are capable of identifying trace uranyl species in complex samples are needed. Herein, localized surface plasmon resonance spectroscopy (LSPR), Raman spectroscopy, and surface enhanced Raman scattering (SERS) serve as label-free and near real-time methods for identifying uranium species in complex aqueous solutions. A straight-forward protocol for spectral analysis will be shown using Raman spectroscopy and aqueous uranium samples. Raman excitation wavelength, pH, and coordinating ions are systematically varied. The spectral analysis results are rigorously validated using uranyl speciation models. Next, plasmonic nanomaterials are used to enhance the Raman signals for trace detection of low (and high) abundant species. Finally, an approach that promotes the reproducible detection of uranyl using SERS will be shown. All in all, the developed protocol provides an accurate and routine analysis of Raman spectra for uranyl species identification and relative abundance elucidation. These advances are expected to provide a straight-forward approach for uranium species identified using LSPR spectroscopy, Raman spectroscopy, and SERS.

The extinction maximum of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, the effect that molecular resonances have on the intensity, LSPR peak width, and LSPR shift of the LSPR of Ag nanoparticles is monitored. By systematically tuning the LSPR extinction maxima of Ag nanoparticles versus molecular resonances, new phenomena are revealed. First, the LSPR peak shift induced by a resonant molecule varies with wavelength. In most instances, the trends in this data qualitatively track with the Kramer's-Kronig transformation of the molecular resonance spectrum; however, the magnitude of the response is severely underestimated. This was verified from both experimental data and theoretical calculations. Because this phenomenon is revealed to be electronic transition dependent, it is hypothesized that the coupling between the molecular and plasmon resonances is responsible for this wavelength dependent observation. These results will have implications in molecular enhanced LSPR sensing and in the understanding of surface-enhanced spectroscopy.

Introduction Reproducible detection of uranyl, an important biological and environmental contaminant, from complex matrices by surface-enhanced Raman scattering (SERS) is successfully achieved using hybrid plasmonic nanoparticles. Traditionally, non-specific binding of interfering species limits detects of molecules such as uranyl. Herein, this is overcome using materials design and rigorous sample analysis workflow design. Synergistic approaches for uranyl isolation and SERS detection is promising for real-world sample detection and eliminates the need of radioactive tracers and extensive sample pretreatment steps. Uranium, a radioactive material with a long half-life, accumulates in the environment in its oxidative form uranyl, which can contaminate soil and water [1]. Importantly, uranyl forms complexes with anions and cations thereby influencing solubility, toxicity, and fate of these heavy metal species. Furthermore, uranyl speciation varies with pH. The resulting complex speciation complicates detection and/or requires significant sample pretreatment. As such, methods that are capable of identifying trace uranyl species in complex samples are needed. Herein, hybrid plasmonic nanomaterials and experimental workflow implementation will be used to establish a rigorous protocol for uranyl detection [2,3]. Namely, localized surface plasmon resonance spectroscopy (LSPR), Raman spectroscopy [4], and surface enhanced Raman scattering (SERS) [5,6] serve as label-free and near real-time methods for identifying uranium species in complex aqueous solutions. Experimental Approaches A rigorous protocol for spectral analysis will be shown using Raman spectroscopy and aqueous uranium samples. Raman excitation wavelength, pH, and coordinating ions are systematically varied. The spectral analysis results are rigorously validated using uranyl speciation models. Next, plasmonic nanomaterials are used to enhance the Raman signals for trace detection of low (and high) abundant species. Finally, an approach that promotes the reproducible detection of uranyl using SERS will be shown. All in all, the developed protocol provides an accurate and routine analysis of Raman spectra for uranyl species identification and relative abundance elucidation. These advances are expected to provide a straight-forward approach for uranium species identified using hybrid plasmonic nanomaterials. Results and Conclusions Plasmonic nanomaterials offer many advantages over traditional methodologies in applications ranging from catalysis, sensing, and imaging. Despite these strengths, challenges arise including for both direct and indirect SERS detection of uranyl include nanoparticle stability, SERS spectral complexity, and variations in SERS intensities and vibrational frequencies. All of these challenges depend on intra- and intermolecular interactions at the plasmonic metal interface as well as the plasmonic nanomaterial stability in complex matrices. Uranyl, unfortunately, exhibits poor affinity to traditional SERS substrates. Methods to promote molecule-metal interactions often lead to nanoparticle instability. Herein, gold nanostars and gold coated silver nanospheres are used. Implications of nanoparticle architecture are investigated for maximizing SERS responses and detectability of uranyl. Nanoparticle architecture and surface potential drive these interactions. To promote detectability in complex matrices, both polymer and silica surface chemistries will be evaluated to initially “screen” unwanted interfering species for adsorbing to the metal surface while also permitted the adsorption and detection of uranyl for SERS detection. Finally, solution conditions are used to identify key features that rationally promote uranyl-surface interactions without compromising the physical stability of the nanostructures. Solution composition induces changes in nanoparticle architecture and/or adsorption processes, yet systematic responses are observed. As a result, we expect these studies will broaden the scope of SERS and plasmonic-based assays as small molecules with weak affinity to metal nanostructures will be more readily detected. While uranyl is a difficult molecule to detect in complex matrices, hybrid nanomaterial design and carefully designed experimental approaches facilitate detectability in a reproducible manner. By using polymers and silica hybrid materials, influences from interfering species are minimized and solution impacts reduced. Future advances in further reducing detection limits and integrating these materials with rapid sampling platforms have the potential to lead to a revolutionary sensor for heavy metal detection. Acknowledgments Research reported in this publication was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under award number R01ES027145 and the National Science Foundation, (CHE-1707859). References 1. Lu, A.J. Haes, T.Z. Forbes, Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy, Coordination Chemistry Reviews 374: 314−344, 2018. 2. T. Phan, A.J. Haes, Impacts of pH and intermolecular interactions of surface-enhanced Raman scattering of chemical enhancements, Journal of Physical Chemistry C , 122, 14846-14856, 2018. 3. Lu, B.K. Shrestha, A.J. Haes, Importance of tilt angles of adsorbed aromatic molecules on nanoparticle rattle SERS substrates, Journal of Physical Chemistry C, 120, 20759-20767, 2016. 4. Lu, T.Z. Forbes, A.J. Haes, Evaluating best practices in Raman spectral analysis for uranium speciation and relative abundance in aqueous solution, Analytical Chemistry, 88, 773-780, 2016. 5. Lu, T.Z. Forbes, A.J. Haes, SERS detection of uranyl using functionalized gold nanostars promoted by nanoparticle shape and size, Analyst, 141, 5137-5143, 2016. 6. Lu, A.J. Johns, B. Neupane, H.T. Phan, D.M. Cwiertny, T.Z. Forbes, A.J. Haes, Matrix-independent surface-enhanced Raman scattering detection of uranyl using electrospun amidoximated polyacrylonitrile mats and gold nanostars, Analytical Chemistry , 90, 6766-5772, 2018.