SERRS labelled beads for multiplex detection

Multiplexing potential of NIR resonant and non-resonant Raman reporters for bio-imaging applications

Multiplexed imaging, which allows for the interrogation of multiple molecular features simultaneously, is vital for addressing numerous challenges across biomedicine. Optically unique surface-enhanced Raman scattering (SERS) nanoparticles (NPs) have the potential to serve as a vehicle to achieve highly multiplexed imaging in a single acquisition, which is non-destructive, quantitative, and simple to execute. When using laser excitation at 785 nm, which allows for a lower background from biological tissues, near infrared (NIR) dyes can be used as Raman reporters to provide high Raman signal intensity due to the resonance effect. This class of imaging agents are known as surface-enhanced resonance Raman scattering (SERRS) NPs. Investigators have predominantly utilized two classes of Raman reporters in their nanoparticle constructs for use in biomedical applications: NIR-resonant and non-resonant Raman reporters. Herein, we investigate the multiplexing potential of five non-resonant SERS: BPE, 44DP, PTT, PODT, and BMMBP, and five NIR resonant SERRS NP flavors with heptamethine cyanine dyes: DTTC, IR-770, IR-780, IR-792, and IR-797, which have been extensively used for biomedical imaging applications. Although SERRS NPs display high Raman intensities, due to their resonance properties, we observed that non-resonant SERS NP concentrations can be quantitated by the intensity of their unique emissions with higher accuracy. Spectral unmixing of five-plex mixtures revealed that the studied non-resonant SERS NPs maintain their detection limits more robustly as compared to the NIR resonant SERRS NP flavors when introducing more components into a mixture.

Exploring Sensitive Label-Free Multiplex Analysis with Raman-Coded Microbeads and SERS-Coded Reporters

Suspension microsphere immunoassays are rapidly gaining attention in multiplex bioassays. Accurate detection of multiple analytes from a single measurement is critical in modern bioanalysis, which always requires complex encoding systems. In this study, a novel bioassay with Raman-coded antibody supports (polymer microbeads with different Raman signatures) and surface-enhanced Raman scattering (SERS)-coded nanotags (organic thiols on a gold nanoparticle surface with different SERS signatures) was developed as a model fluorescent, label-free, bead-based multiplex immunoassay system. The developed homogeneous immunoassays included two surface-functionalized monodisperse Raman-coded microbeads of polystyrene and poly(4-tert-butylstyrene) as the immune solid supports, and two epitope modified nanotags (self-assembled 4-mercaptobenzoic acid or 3-mercaptopropionic acid on gold nanoparticles) as the SERS-coded reporters. Such multiplex Raman/SERS-based microsphere immunoassays could selectively identify specific paratope–epitope interactions from one mixture sample solution under a single laser illumination, and thus hold great promise in future suspension multiplex analysis for diverse biomedical applications.

Development and validation of a robust multiplex serological assay to quantify antibodies specific to pertussis antigens

Despite wide spread vaccination, the public health burden of pertussis remains substantial. Current acellular pertussis vaccines comprise upto five Bordetella pertussis (Bp) antigens. Performing an ELISA to quantify antibody for each antigen is laborious and challenging to apply to pediatric samples where serum volume may be limited. We developed a microsphere based multiplex antibody capture assay (MMACA) to quantify antibodies to five pertussis antigens; pertussis toxin, pertactin, filamentous hemagglutinin and fimbrial antigens 2/3, and adenylate cyclase toxin in a single reaction (5-plex) with a calibrated reference standard, QC reagents and SAS^® based data analysis program. The goodness of fit (R²) of the standard curves for five analytes was ≥0.99, LLOQ 0.04-0.15 IU or AU/mL, accuracy 1.9%-23.8% (%E), dilutional linearity slopes 0.93-1.02 and regression coefficients r² = 0.91-0.99. MMACA had acceptable precision within a median CV of 16.0%-22.8%. Critical reagents, antigen conjugated microsphere and reporter antibody exhibited acceptable (<12.3%) lot-lot variation. MMACA can be completed in <3 h, requires low serum volume (5μL/multiplex assay) and has fast data turnaround time (<1 min). MMACA has been successfully developed and validated as a sensitive, specific, robust and rugged method suitable for simultaneous quantification of anti-Bp antibodies in serum, plasma and DBS.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

Endoscopic imaging using surface-enhanced Raman scattering

In this review, we assessed endoscopic imaging using surface-enhanced Raman scattering (SERS). As white-light endoscopy, the current standard for gastrointestinal endoscopy, is limited to morphology, Raman endoscopy using surface-enhanced Raman scattering nanoparticles (SERS endoscopy) was introduced as one of the novel functional modalities. SERS endoscopy has multiplex capability and high sensitivity with low autofluorescence and photobleaching. As a result, multiple molecular characteristics of the lesion can be accurately evaluated in real time while performing endoscopy using SERS probes and appropriate instrumentation. Especially, recently developed dual modality of fluorescence and SERS endoscopy offers easy localization with identification of multiple target molecules. For clinical use of SERS endoscopy in the future, problems of limited field of view and cytotoxicity should be addressed by fusion imaging, topical administration, and non-toxic coating of nanoparticles. We expect SERS endoscopic imaging would be an essential endoscopic technique for diagnosis of cancerous lesions, assessment of resection margins and evaluation of therapeutic responses.

On-line SERS detection of single bacterium using novel SERS nanoprobes and a microfluidic dielectrophoresis device

The integration of novel surface-enhanced Raman scattering (SERS) nanoprobes and a microfluidic dielectrophoresis (DEP) device is developed for rapid on-line SERS detection of Salmonella enterica serotype Choleraesuis and Neisseria lactamica. The SERS nanoprobes are prepared by immobilization of specific antibody onto the surface of nanoaggregate-embedded beads (NAEBs), which are silica-coated, dye-induced aggregates of a small number of gold nanoparticles (AuNPs). Each NAEB gives highly enhanced Raman signals owing to the presence of well-defined plasmonic hot spots at junctions between AuNPs. Herein, the on-line SERS detection and accurate identification of suspended bacteria with a detection capability down to a single bacterium has been realized by the NAEB-DEP-Raman spectroscopy biosensing strategy. The practical detection limit with a measurement time of 10 min is estimated to be 70 CFU mL(-1) . In comparison with whole-cell enzyme-linked immunosorbent assay (ELISA), the SERS-nanoprobe-based biosensing method provides advantages of higher sensitivity and requiring lower amount of antibody in the assay (100-fold less). The total assay time including sample pretreatment is less than 2 h. Hence, this sensing strategy is promising for faster and effective on-line multiplex detection of single pathogenic bacterium by using different bioconjugated SERS nanoprobes.

Rational design and synthesis of SERS labels

SERS labels are a new class of nanotags for optical detection based on Raman scattering. Central advantages include their spectral multiplexing capacity due to the small line width of vibrational Raman bands, quantification based on spectral intensities, high photostability, minimization of autofluorescence from biological specimens via red to near-infrared (NIR) excitation, and the need for only a single laser excitation line. Current concepts for the rational design and synthesis of SERS labels are summarized in this review. Chemical constituents of SERS labels are the plasmonically active metal colloids for signal enhancement upon resonant laser excitation, organic Raman reporter molecules for adsorption onto the metal surface for identification, and an optional protective shell. Different chemical approaches towards the synthesis of rationally designed SERS labels are highlighted, including also their subsequent bioconjugation.

Chemical imaging of live fibroblasts by SERS effective nanofilm

Label-free Raman imaging of live single NIH3T3 fibroblast produced by SERS effective SiO₂@Ag–PAA nanoshells in real time.

Recent developments and future directions in SERS for bioanalysis

The ability to develop new and sensitive methods of biomolecule detection is crucial to the advancement of pre-clinical disease diagnosis and effective patient specific treatment. Surface enhanced Raman scattering (SERS) is an optical spectroscopy amenable to this goal, as it is capable of extremely sensitive biomolecule detection and multiplexed analysis. This perspective highlights where SERS has been successfully used to detect target biomolecules, specifically DNA and proteins, and where in vivo analysis has been successfully utilised. The future of SERS development is discussed and emphasis is placed on the steps required to transport this novel technique from the research laboratory to a clinical setting for medical diagnostics.

Single-molecule surface-enhanced Raman spectroscopy

A general overview of the field of single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) as it stands today is provided. After years of debates on the basic aspects of SM-SERS, the technique is emerging as a well-established subfield of spectroscopy and SERS. SM-SERS is allowing the observation of subtle spectroscopic phenomena that were not hitherto accessible. Examples of the latter are natural isotopic substitutions in single molecules, observation of the true homogeneous broadening of Raman peaks, Raman excitation profiles of individual molecules, and SM electrochemistry. With background examples of the contributions produced by our group, properly interleaved with results by other practitioners in the field, we present some of the latest developments and promising new leads in this new field of spectroscopy.

Surface-enhanced Raman scattering-active nanostructures and strategies for bioassays

Surface-enhanced Raman scattering (SERS) techniques offer a number of advantages in molecular detection and analysis, particularly in terms of the multiplex detection of biomolecules. So far, many new SERS-based substrates and analytical techniques have been reported. For easy understanding, various SERS techniques are classified into the following four categories: adsorption-mediated direct detection; antibody- or ligand-mediated direct detection; binding catalyzed indirect detection; and tag-based indirect detection. Among these, recent successes of SERS tagging/encoding (nano/micro) materials and detection methods are highlighted, including our recent works. Some novel SERS-based strategies for the detection of several biological molecules are also introduced.

A binary functional substrate for enrichment and ultrasensitive SERS spectroscopic detection of folic acid using graphene oxide/Ag nanoparticle hybrids

Herein graphene oxide/Ag nanoparticle hybrids (GO/PDDA/AgNPs) were fabricated according to a self-assembly procedure. Using the obtained GO/PDDA/AgNPs as SERS substrates, an ultrasensitive and label-free detection of folic acid in water and serum was demonstrated based on the inherent SERS spectra of folic acid. The modified graphene oxide exhibited strong enrichment of folic acid due to the electrostatic interaction, and the self-assembled Ag nanoparticles greatly enhanced the SERS spectra of folic acid, both of which led to an ultrahigh sensitivity. Therefore, although the SERS enhancement of p-ATP on GO/PDDA/AgNPs was weaker than that on Ag nanoparticles, the SERS signals of folic acid on GO/PDDA/AgNPs were much stronger than that on Ag nanoparticles. To improve the detection, the concentration of GO/PDDA/AgNPs was optimized to reduce background of the graphene oxide. The SERS spectra of the folic acid showed that the minimum detected concentration of folic acid in water was as low as 9 nM with a linear response range from 9 to 180 nM. To estimate the feasibility of the detection method based on GO/PDDA/AgNPs for the practical applications, diluted serum containing different concentrations of folic acid was taken as real samples. It was established that the sensitivity and the linear range for the folic acid in serum were comparable to that in water. This ultrasensitive and label-free SERS detection of folic acid based on GO/PDDA/AgNPs offers great potential for practical applications of medicine and biotechnology.

DNA-gold nanoparticle reversible networks grown on cell surface marker sites: application in diagnostics

Effective identification of breast cancer stem cells (CSC) benefits from a multiplexed approach to detect cell surface markers that can distinguish this subpopulation, which can invade and proliferate at sites of metastasis. We present a new approach for dual-mode sensing based on targeting using pointer and signal enhancement using enhancer particle networks for detection by surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS). We demonstrate our concept to detect cell surface markers, CD44 and CD24, in three breast cancer cell lines to identify a CD44+/CD24- subpopulation of CSCs. The designed network structure can be well-controlled and has improved sensitivity compared to conventional approaches with ability to detect a single target on the membrane of a living cell. We have also developed a fractal approach to model the dimension of the network structure and developed an empirical relationship to estimate the number of particles in the network and its size. The empirical equation was validated with experiments and finite-difference time-domain simulations, and the cell phenotyping results were found to be in good agreement with published data from conventional sorting by flow cytometry.

High-resolution spectral analysis of individual SERS-active nanoparticles in flow

Nanoparticle spectroscopic tags based on surface enhanced Raman scattering (SERS) are playing an increasingly important role in bioassay and imaging applications. The ability to rapidly characterize large populations of such tags spectroscopically in a high-throughput flow-based platform will open new areas for their application and provide new tools for advancing their development. We demonstrate here a high-resolution spectral flow cytometer capable of acquiring Raman spectra of individual SERS-tags at flow rates of hundreds of particles per second, while maintaining the spectral resolution required to make full use of the detailed information encoded in the Raman signature for advanced multiplexing needs. The approach allows multiple optical parameters to be acquired simultaneously over thousands of individual nanoparticle tags. Characteristics such as tag size, brightness, and spectral uniformity are correlated on a per-particle basis. The tags evaluated here display highly uniform spectral signatures, but with greater variability in brightness. Subpopulations in the SERS response, not apparent in ensemble measurements, are also shown to exist. Relating tag variability to synthesis parameters makes flow-based spectral characterization a powerful tool for advancing particle development through its ability to provide rapid feedback on strategies aimed at constraining desired tag properties. Evidence for single-tag signal saturation at high excitation power densities is also shown, suggesting a role for high-throughput investigation of fundamental properties of the SERS tags as well.

Bioconjugation and characterisation of gold colloid-labelled proteins

Colloidal metal particles, in particular gold, have found many biological applications often as probes in light and electron microscopy, and more recently since the 1980s in membrane-based rapid immunoaffinity tests. The surface plasmon resonance absorbance properties in the visible spectroscopy region of gold colloids make them useful tools in medical devices, as the colloids are directly visible to the naked eye. Despite the relative ease with which gold-protein conjugates can be prepared a major issue is the manufacture of poor-quality and poorly characterised bioconjugates that can result in the under performance of subsequent diagnostic tests. This paper describes the preparation of good-quality conjugates for use in immunoassays by optimising the adsorption of antibodies onto the surface of gold colloids, followed by their subsequent characterisation. The conjugates were characterized for size, aggregation and quality using a range of techniques: UV-visible (UV/Vis) absorption spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). The biological activities of the conjugated products were also assessed using an immunoassay format and electrochemical measurements. By utilising a number of measurement techniques we aimed to gain a better understanding of the extent of particle aggregation, and the resulting stability and activity of the biological molecule on the surfaces of nanoparticles. The tools developed will enable researchers and companies to ensure the sensitivity, quality and reproducibility of batches of nanoparticle bio-conjugates.

Mixed monolayers on gold nanoparticle labels for multiplexed surface-enhanced Raman scattering based immunoassays

This paper describes a new approach, based on self-assembled mixed monolayers, to the design and preparation of extrinsic Raman labels (ERLs). ERLs function as spectroscopic tags for the readout of sandwich-type immunoassays using surface-enhanced Raman scattering (SERS). They are created by coating gold nanoparticles with Raman reporter molecules and antibodies specific for the target analyte. Mixed-monolayer ERLs are formed by covering gold nanoparticles with a mixture of two different thiolates. One thiolate serves to covalently bind antibodies to the particles, imparting biospecificity to the ERLs, while the other thiolate produces a strong Raman signal. Mixed-monolayer ERLs can be prepared in a few relatively simple steps using readily available materials. The SERS intensity of each type of ERL can be tuned to match other ERLs by adjusting the mixed monolayer composition, greatly facilitating the generation of sets of ERLs for multiplexed applications. The work herein not only describes the new pathway for ERL production, but also demonstrates the simultaneous qualitative and quantitative multiplexed detection using a set of four mixed-monolayer ERLs.

Multifunctional silver-embedded magnetic nanoparticles as SERS nanoprobes and their applications

In this study, surface-enhanced Raman spectroscopy (SERS)-encoded magnetic nanoparticles (NPs) are prepared and utilized as a multifunctional tagging material for cancer-cell targeting and separation. First, silver-embedded magnetic NPs are prepared, composed of an 18-nm magnetic core and a 16-nm-thick silica shell with silver NPs formed on the surface. After simple aromatic compounds are adsorbed on the silver-embedded magnetic NPs, they are coated with silica to provide them with chemical and physical stability. The resulting silica-encapsulated magnetic NPs (M-SERS dots) produce strong SERS signals and have magnetic properties. In a model application as a tagging material, the M-SERS dots are successfully utilized for targeting breast-cancer cells (SKBR3) and floating leukemia cells (SP2/O). The targeted cancer cells can be easily separated from the untargeted cells using an external magnetic field. The separated targeted cancer cells exhibit a Raman signal originating from the M-SERS dots. This system proves to be an efficient tool for separating targeted cells. Additionally, the magnetic-field-induced hot spots, which can provide a 1000-times-stronger SERS intensity due to aggregation of the NPs, are studied.

Polyvinylpyrrolidone- (PVP-) coated silver aggregates for high performance surface-enhanced Raman scattering in living cells

A biocompatible and stable surface-enhanced Raman scattering (SERS) probe has been successfully synthesized through a simple route with silver aggregates. Polyvinylpyrrolidone (PVP), a biocompatible polymer, was utilized to control the aggregation process and improve the chemical stability of the aggregates. Extinction spectroscopy and TEM results show the aggregation degree and core-shell structure of the probe. It is found that when we employ 4-mercaptobenzoic acid (4MBA), crystal violet (CV), Rhodamine 6G (R6G) or 4,4'-bipyridine molecules as Raman reporters, the SERS signal from the proposed probe can remain at a high level under aggressive chemical environments, even after being incorporated into living cells. In comparison with the traditional probes without the PVP shell, the new ones exhibit strong surface-enhanced effects and low toxicity towards living cells. We demonstrate that the PVP-coated silver aggregates are highly SERS effective, for which the fabrication protocol is advantageous in its simplicity and reproducibility.

Silver-coated silica beads applicable as core materials of dual-tagging sensors operating via SERS and MEF

We have developed dual-tagging sensors, operating via both surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF), composed of silver-coated silica beads onto which were deposited SERS markers and dye-grafted polyelectrolytes, for multiplex immunoassays. Initially, a very simple electroless-plating method was applied to prepare Ag-coated silica beads. The Raman markers were then assembled onto the Ag-coated silica beads, after which they were brought to stabilization by the layer-by-layer deposition of anionic and cationic polyelectrolytes including a dye-grafted polyelectrolyte. In the final stage, the dual-tagging sensors were assembled onto them with specific antibodies (antihuman-IgG or antirabbit-IgG) to detect target antigens (human-IgG or rabbit-IgG). The MEF signal was used as an immediate indicator of molecular recognition, while the SERS signals were subsequently used as the signature of specific molecular interactions. For this reason, these materials should find wide application, especially in the areas of biological sensing and recognition that rely heavily on optical and spectroscopic properties.

Functionalized nanoparticles for bioanalysis by SERRS

Metallic nanoparticles can be used as basic materials for a wide variety of purposes including building blocks for nanoassemblies, substrates for enhanced spectroscopies such as fluorescence and Raman and as labels for biomolecules. In the present paper, we report how silver and gold nanoparticles can be functionalized with specific biomolecular probes to interact in a specific manner with a target molecule to provide a change in the properties of the nanoparticles which can be measured to indicate the molecular recognition event. Examples of this approach include DNA hybridization to switch on SERRS (surface-enhanced resonance Raman scattering) when a specific target sequence is present, the use of nanoparticles for in vivo SERRS imaging and the use of nanoparticles functionalized with antibodies to provide a new type of immunoassay. These examples indicate how nanoparticles can be used to provide highly sensitive and informative data from a variety of biological systems when used in combination with SERRS.

Multiplex detection of breast cancer biomarkers using plasmonic molecular sentinel nanoprobes

We have demonstrated for the first time the feasibility of multiplex detection using the surface-enhanced Raman scattering-based molecular sentinel (MS) technology in a homogeneous solution. Two MS nanoprobes tagged with different Raman labels were used to detect the presence of the erbB-2 and ki-67 breast cancer biomarkers. The multiplexing capability of the MS technique was demonstrated by mixing the two MS nanoprobes and tested in the presence of single or multiple DNA targets.

Spectral analysis of multiplex Raman probe signatures

Raman nanoparticle probes are an emerging new class of optical labels for interrogation of physiological and pathological processes in bioassays, cells, and tissues. Although their unique emission signatures are ideal for multiplexing, the full potential of these probes has not been realized because conventional analysis methods are inadequate. We report a novel spectral fitting method that exploits the entire spectral signature to quantitatively extract individual probe signals from multiplex spectra. We evaluate the method in a series of multiplex assays using unconjugated and antibody-conjugated composite organic-inorganic nanoparticles (COINs). Results show sensitive multiplex detection of small signals (<2% of total signal) and similar detection limits in corresponding 4-plex and singlet plate binding assays. In a triplex assay on formalin-fixed human prostate tissue, two antibody-conjugated COINs and a conventional fluorophore are used to image expression of prostate-specific antigen, cytokeratin-18, and DNA. The spectral analysis method effectively removes tissue autofluorescence and other unknown background, allowing accurate and reproducible imaging (area under ROC curve 0.89 +/- 0.03) at subcellular spatial resolution. In all assay systems, the error attributable to spectral analysis constitutes <or=2% of total signal. The spectral fitting method provides (1) quantification of signals from multiplex spectra with overlapping peaks, (2) robust spot-by-spot removal of unknown background, (3) the opportunity to quantitatively assess the analysis error, (4) elimination of operator bias, and (5) simple automation appropriate for high-throughput analysis. The simple implementation and universal applicability of this approach significantly expands the potential of Raman probes for quantitative in vivo and ex vivo multiplex analysis.

Rationally designed nanostructures for surface-enhanced Raman spectroscopy

Research on surface-enhanced Raman spectroscopy (SERS) is an area of intense interest because the technique allows one to probe small collections of, and in certain cases, individual molecules using relatively straightforward spectroscopic techniques and nanostructured substrates. Researchers in this area have attempted to develop many new technological innovations including high sensitivity chemical and biological detection systems, labeling schemes for authentication and tracking purposes, and dual scanning-probe/spectroscopic techniques that simultaneously provide topographical and spectroscopic information about an underlying surface or nanostructure. However, progress has been hampered by the inability of researchers to fabricate substrates with the high sensitivity, tunability, robustness, and reproducibility necessary for truly practical and successful SERS-based systems. These limitations have been due in part to a relative lack of control over the nanoscale features of Raman substrates that are responsible for the enhancement. With the advent of nanotechnology, new approaches are being developed to overcome these issues and produce substrates with higher sensitivity, stability, and reproducibility. This tutorial review focuses on recent progress in the design and fabrication of substrates for surface-enhanced Raman spectroscopy, with an emphasis on the influence of nanotechnology.

Expanding generality of surface-enhanced Raman spectroscopy with borrowing SERS activity strategy

Surface-enhanced Raman scattering (SERS) was discovered three decades ago and has gone through a tortuous pathway to develop into a powerful diagnostic technique. Recently, the lack of substrate, surface and molecular generalities of SERS has been circumvented to a large extent by devising and utilizing various nanostructures by many groups including ours. This article aims to present our recent approaches of utilizing the borrowing SERS activity strategy mainly through constructing two types of nanostructures. The first nanostructure is chemically synthesized Au nanoparticles coated with ultra-thin shells (ca. one to ten atomic layers) of various transition metals, e.g., Pt, Pd, Ni and Co, respectively. Boosted by the long-range effect of the enhanced electromagnetic (EM) field generated by the highly SERS-active Au core, the originally low surface enhancement of the transition metal can be substantially improved giving total enhancement factors up to 10(4)-10(5). It allows us to obtain the Raman spectra of surface water, having small Raman cross-section, on several transition metals for the first time. To expand the surface generality of SERS, tip-enhanced Raman spectroscopy (TERS) has been employed. With TERS, a nanogap can be formed controllably between an atomically flat metal surface and the tip with an optimized shape, within which the enhanced EM field from the tip can be coupled (borrowed) effectively. Therefore, one can obtain surface Raman signals (TERS signals) from adsorbed species at Au(110), Au(111) and, more importantly, Pt(l10) surfaces. The enhancement factor achieved on these single crystal surfaces can be up to 106, especially with a very high spatial resolution down to about 14 nm. To fully accomplish the borrowing strategy from different nanostructures and to explain the experimental observations, a three-dimensional finite-difference time-domain method was used to calculate and evaluate the local EM field on the core-shell nanoparticle surfaces and the TERS tips. Finally, prospects and further developments of this valuable strategy are briefly discussed with emphasis on the emerging experimental methodologies.

Raman nanoparticle probes for antibody-based protein detection in tissues

Surface-enhanced Raman scattering (SERS) nanoparticles are emerging as a new approach for optical detection of biomolecules. In a model assay in formalin-fixed paraffin-embedded (FFPE) prostate tissue sections, we detect prostate-specific antigen (PSA) using antibody (Ab) conjugated to composite organic-inorganic nanoparticles (COINs), and we use identical staining protocols to compare COIN-Ab and Alexa-Ab conjugates in adjacent tissue sections. Spectral analysis illustrates the fundamental difference between fluorescence and Raman signatures and accurately extracts COIN probe signals from background autofluorescence. Probe signals are used to generate images of PSA expression on the tissue, and quality measures are presented to characterize the performance of the COIN assay in comparison to Alexa. Staining accuracy (ability to correctly identify PSA expression in epithelial cells) is somewhat less for COIN than Alexa, which is attributed to an elevated false negative rate of the COIN. However, COIN provided signal intensities comparable to Alexa, and good intra-, inter-, and lot-to-lot consistencies. Overall, COIN and Alexa detection reagents possess similar performance with FFPE tissues, supporting the further development of Raman probes for this application. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

SERS nanoparticles: a new optical detection modality for cancer diagnosis

Surface-enhanced Raman scattering (SERS) is an optical detection technique that offers advantages over traditional assay detection technologies, such as fluorescence and chemiluminescence. These advantages include sensitivity, high levels of multiplexing, robustness and ability to perform detection in blood and other biological matrices. Here, we report on the growing field of SERS-active nanoparticles as a novel method for detection, with special emphasis on their use in the field of oncology. We discuss examples of SERS-active nanoparticles used in an assay for PSA, BRCA1 and Her-2, along with examples of nucleic-acid detection. We present data on a novel homogeneous, single-tube, rapid assay for nucleic acid detection and show how it will benefit the oncology community.

Silver-particle-based surface-enhanced resonance Raman scattering spectroscopy for biomolecular sensing and recognition

We demonstrate in this work that 2-microm-sized Ag (microAg) powders can be used as a core material for constructing biomolecular sensing/recognition units operating via surface-enhanced resonance Raman scattering (SERRS). This is possible because microAg powders are very efficient substrates for both the diffuse reflectance IR and the surface-enhanced Raman scattering-SERRS spectroscopic characterization of molecular adsorbates prepared in a similar manner on silver surfaces. Besides, the agglomeration of microAg particles in a buffer solution can be prevented by the layer-by-layer deposition of cationic and anionic polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). In this particular study, we used rhodamine B isothiocyanate (RhBITC) as a SERRS marker molecule, and microAg powders adsorbed consecutively with RhBITC and PAH-PAA bilayers were finally derivatized with biotinylated poly(L-lysine). On the basis of the nature of the SERRS peaks of RhBITC, those microAg powders were confirmed to selectively recognize streptavidin molecules down to concentrations of 10(-10) g mL-1. Since a number of different molecules can be used as SERS-SERRS marker molecules, the present method proves to be an invaluable tool for multiplex biomolecular sensing/recognition via SERS and SERRS.

Composite organic-inorganic nanoparticles as Raman labels for tissue analysis

Composite organic-inorganic nanoparticles (COINs) are novel optical labels for detection of biomolecules. We have previously developed methods to encapsulate COINs and to functionalize them with antibodies. Here we report the first steps toward application of COINs to the detection of proteins in human tissues. Two analytes, PSA and CK18, are detected simultaneously using two different COINs in a direct binding assay, and two different COINs are shown to simultaneously label PSA in tissue samples.