Sampling Error: Impact on the Quantitative Analysis of Nanoparticle-Based Surface-Enhanced Raman Scattering Immunoassays

iSERS: from nanotag design to protein assays and ex vivo imaging

Proteins are an eminently important class of ubiquitous biomacromolecules with diverse biological functions, and numerous techniques for their detection, quantification, and localisation have been developed. Many of these methods exploit the selectivity arising from molecular recognition of proteins/antigens by immunoglobulins. The combination of surface-enhanced Raman scattering (SERS) with such "immuno"-techniques to immuno-SERS (iSERS) is the central topic of this review, which is focused on colloidal SERS nanotags, i.e., molecularly functionalised noble metal nanoparticles conjugated to antibodies, for their use in protein assays and ex vivo imaging. After contrasting the fundamental differences between label-free SERS and iSERS, including a balanced description of the advantages and drawbacks of the latter, we describe the usual workflow of iSERS experiments. Milestones in the development of the iSERS technology are summarised from a historical perspective. By highlighting selected examples from the literature, we illustrate the conceptual progress that has been achieved in the fields of iSERS-based protein assays and ex vivo imaging. Finally, we attempt to predict what is necessary to fully exploit the transformative potential of the iSERS technology by stimulating the transition from research in academic labs into applications for the benefit of our society.

Immunoassays: Analytical and Clinical Performance, Challenges, and Perspectives of SERS Detection in Comparison with Fluorescent Spectroscopic Detection

Immunoassays (IAs) with fluorescence-based detection are already well-established commercialized biosensing methods, such as enzyme-linked immunosorbent assay (ELISA) and lateral flow immunoassay (LFIA). Immunoassays with surface-enhanced Raman spectroscopy (SERS) detection have received significant attention from the research community for at least two decades, but so far they still lack a wide clinical commercial application. This review, unlike any other review that we have seen, performs a three-dimensional performance comparison of SERS IAs vs. fluorescence IAs. First, we compared the limit of detection (LOD) as a key performance parameter for 30 fluorescence and 30 SERS-based immunoassays reported in the literature. We also compared the clinical performances of a smaller number of available reports for SERS vs. fluorescence immunoassays (FIAs). We found that the median and geometric average LODs are about 1.5-2 orders of magnitude lower for SERS-based immunoassays in comparison to fluorescence-based immunoassays. For instance, the median LOD for SERS IA is 4.3 × 10-13 M, whereas for FIA, it is 1.5 × 10-11 M. However, there is no significant difference in average relative standard deviation (RSD)-both are about 5-6%. The analysis of sensitivity, selectivity, and accuracy reported for a limited number of the published clinical studies with SERS IA and FIA demonstrates an advantage of SERS IA over FIA, at least in terms of the median value for all three of those parameters. We discussed common and specific challenges to the performances of both SERS IA and FIA, while proposing some solutions to mitigate those challenges for both techniques. These challenges include non-specific protein binding, non-specific interactions in the immunoassays, sometimes insufficient reproducibility, relatively long assay times, photobleaching, etc. Overall, this review may be useful for a large number of researchers who would like to use immunoassays, but particularly for those who would like to make improvements and move forward in both SERS-based IAs and fluorescence-based IAs.

Surface-enhanced Raman database of 24 metabolites: Stable measurement of spectra, extraction and analysis of the main features

Surface-enhanced Raman spectroscopy (SERS) has been used in Raman-based metabolomics to provide abundant molecular fingerprint information in situ with extremely high sensitivity, without damaging the sample. However, poor reproducibility, caused by the randomness of the adsorption sites, and the short-range effect of SERS have hindered the development of SERS in metabolomics, resulting in very few SERS reference databases for small-molecule metabolites. In this work, our previously proposed large laser spot-swift mapping SERS method was adopted for the measurement of 24 commercially available metabolite standards, to provide reproducible and reliable references for Raman-based metabolomics study. Among these 24 metabolites, 22 contained no Raman data in PubChem. Other than the SERS spectra data, we extracted and explained the molecular vibration information of these metabolites, and combined with the density functional theory (DFT) calculations, we provided a new possibility for the fast Raman recognition of small-molecule metabolites. Accordingly, a large laser spot-swift mapping SERS database of metabolites in human serum was initially established, which contained not only the original spectral data but also other detailed feature information regarding the Raman peaks. With continuous accumulation, this database could play a promising role in Raman-based metabolomics and other Raman-related research.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

Design of Raman reporter-embedded magnetic/plasmonic hybrid nanostirrers for reliable microfluidic SERS biosensors

Magnetic-based microfluidic SERS biosensors hold great potential in various biological analyses due to their integrated advantages including easy manipulation, miniaturization and ultrasensitivity. However, it remains challenging to collect reliable SERS nanoprobe signals for quantitative analysis due to the irregular aggregation of magnetic carriers in a microfluidic chamber. Here, magnetic/plasmonic hybrid nanostirrers embedded with a Raman reporter are developed as capture carriers to improve the reliability of microfluidic SERS biosensors. Experimental results revealed that SERS signals from magnetic hybrid nanostirrers could serve as microenvironment beacons of their irregular aggregation, and a signal filtering method was proposed through exploring the relationship between the intensity range of beacons and the signal reproducibility of SERS nanoprobes using interleukin 6 as a model target analyte. Using the signal filtering method, reliable SERS nanoprobe signals with high reproducibility could be picked out from similar microenvironments according to their beacon intensity, and then the influence of irregular aggregation of magnetic carriers on the SERS nanoprobe could be eliminated. The filtered SERS nanoprobe signals also exhibited excellent repeatability from independent tests, which lay a solid foundation for a reliable working curve and subsequent accurate bioassay. This study provides a simple but promising route for reliable microfluidic SERS biosensors, which will further promote their practical application in biological analysis.

Aluminum Foil vs. Gold Film: Cost-Effective Substrate in Sandwich SERS Immunoassays of Biomarkers Reveals Potential for Selectivity Improvement

The first application of aluminum foil (Al F) as a low-cost/high-availability substrate for sandwich immunoassay using surface-enhanced Raman spectroscopy (SERS) is reported. Untreated and unmodified Al F and gold film are used as substrates for sandwich SERS immunoassay to detect tuberculosis biomarker MPT64 and human immunoglobulin (hIgG) in less than 24 h. The limits of detection (LODs) for tuberculosis (TB) biomarker MPT64 on Al foil, obtained with commercial antibodies, are about 1.8–1.9 ng/mL, which is comparable to the best LOD (2.1 ng/mL) reported in the literature for sandwich ELISA, made with fresh in-house antibodies. Not only is Al foil competitive with traditional SERS substrate gold for the sandwich SERS immunoassay in terms of LOD, which is in the range 18–30 pM or less than 1 pmol of human IgG, but it also has a large cost/availability advantage over gold film. Moreover, human IgG assays on Al foil and Si showed better selectivity (by about 30–70% on Al foil and at least eightfold on Si) and a nonspecific response to rat or rabbit IgG, in comparison to the selectivity in assays using gold film.

Nanoconfinement-Enhanced Electrochemiluminescence for in Situ Imaging of Single Biomolecules

Direct imaging of electrochemical reactions at the single-molecule level is of potential interest in materials, diagnostic, and catalysis applications. Electrochemiluminescence (ECL) offers the opportunity to convert redox events into photons. However, it is challenging to capture single photons emitted from a single-molecule ECL reaction at a specific location, thus limiting high-quality imaging applications. We developed the nanoreactors based on Ru(bpy)₃²⁺-doped nanoporous zeolite nanoparticles (Ru@zeolite) for direct visualization of nanoconfinement-enhanced ECL reactions. Each nanoreactor not only acts as a matrix to host Ru(bpy)₃²⁺ molecules but also provides a nanoconfined environment for the collision reactions of Ru(bpy)₃²⁺ and co-reactant radicals to realize efficient in situ ECL reactions. The nanoscale confinement resulted in enhanced ECL. Using such nanoreactors as ECL probes, a dual-signal sensing protocol for visual tracking of a single biomolecule was performed. High-resolution imaging of single membrane proteins on heterogeneous cells was effectively addressed with near-zero backgrounds. This could provide a more sensitive tool for imaging individual biomolecules and significantly advance ECL imaging in biological applications.

Real-Time Underwater Nanoplastic Detection beyond the Diffusion Limit and Low Raman Scattering Cross-Section via Electro-Photonic Tweezers

Emerging as substantial concerns in the ecosystem, submicron plastics have attracted much attention for their considerable hazards. However, their effect and even amount in the environment remain unclear. Establishing a substantive analytic platform is essential to expand the understanding of nanoplastics. However, the issues of diffusion and detection limit that arise from ultradiluted concentration and extremely small scales of nanoplastics leave significant technical hurdles to analyze the nanoplastic pollutants. In this study, we obtain effective Raman signals in real time from underwater nanoplastics with ultralow concentrations via AC electro-osmotic flows and dielectrophoretic tweezing. This enables the field-induced active collection of nanoplastics toward the optical sensing area from remote areas in a rapid manner, integrating conventional technical skills of preconcentration, separation, and identification in a single process. A step further, synergetic combination with plasmonic nanorods, accomplishes the highest on-site detection performance so far.

Electrochemically assisted wide area Raman with standard curved surface quantification method

Reproducibility is still a great challenge for surface-enhanced Raman scattering (SERS), because the uncontrollable fabrication of SERS substrates or the uneven distribution of samples on the substrate result in the signal fluctuation with or between the substrates. Herein, a novel SERS quantitative method with good reproducibility was proposed. It is based on the basic principle that the SERS signal intensity is not only related to electromagnetic enhancement and the concentration of sample, but also related to the specific surface area of the substrate. The surface area information of the substrate is obtained through electrochemical technology, and then introduced into the standard curve with the linear relationship of concentration of sample and SERS intensity as a new variable to obtain a 3D standard curved surface, which effectively corrects the signal difference between the substrates, and combines the wide area Raman method to reduce the difference within the substrate, thereby improving the reproducibility of SERS quantitative detection. Using malachite green (MG) as the probe molecule and using cyclic voltammetry to calculate the substrate area fitted plane model (CV-standard curved surface), the root mean square error (RMSE) of the predicted result is 0.26 and the relative error (RE) is 0.25. It shows that the detection error significantly reduces comparing with the traditional standard curve method. Also, the proposed method can be used in other SERS quantitative detection and has potential application prospects.

Toward Quantitative Surface-Enhanced Raman Scattering with Plasmonic Nanoparticles: Multiscale View on Heterogeneities in Particle Morphology, Surface Modification, Interface, and Analytical Protocols

Surface-enhanced Raman scattering (SERS) provides significantly enhanced Raman scattering signals from molecules adsorbed on plasmonic nanostructures, as well as the molecules' vibrational fingerprints. Plasmonic nanoparticle systems are particularly powerful for SERS substrates as they provide a wide range of structural features and plasmonic couplings to boost the enhancement, often up to >10⁸-10¹⁰. Nevertheless, nanoparticle-based SERS is not widely utilized as a means for reliable quantitative measurement of molecules largely due to limited controllability, uniformity, and scalability of plasmonic nanoparticles, poor molecular modification chemistry, and a lack of widely used analytical protocols for SERS. Furthermore, multiscale issues with plasmonic nanoparticle systems that range from atomic and molecular scales to assembled nanostructure scale are difficult to simultaneously control, analyze, and address. In this perspective, we introduce and discuss the design principles and key issues in preparing SERS nanoparticle substrates and the recent studies on the uniform and controllable synthesis and newly emerging machine learning-based analysis of plasmonic nanoparticle systems for quantitative SERS. Specifically, the multiscale point of view with plasmonic nanoparticle systems toward quantitative SERS is provided throughout this perspective. Furthermore, issues with correctly estimating and comparing SERS enhancement factors are discussed, and newly emerging statistical and artificial intelligence approaches for analyzing complex SERS systems are introduced and scrutinized to address challenges that cannot be fully resolved through synthetic improvements.

Exploring Intensity Distributions and Sampling in SERS-Based Immunoassays

Surface-enhanced Raman scattering (SERS) is a sensitive, widely used spectroscopic technique. However, SERS is perceived as poorly reproducible and insufficiently robust for standard applications in analytical chemistry. Here, we demonstrated that reliable SERS immunoassay quantification at low concentrations (pM range) can be achieved by careful experimental design and appropriate data analysis statistics. A SERS-based immunoassay for IgG in human serum (3.1-50.0 ng mL^-1 or 20.6-333 pM) was developed as a proof of concept. Calibration curves were created using the population median of the band area, centered at 592 cm^-1, of a SERS reporter (Nile Blue A). Histograms of 7200 SERS spectra show lognormal distributions. SEM images of the sensor platform confirm a correlation between the number of SERS probes (ERLs) at the surface and the SERS intensity response. The IgG immunosensor reported here presented a limit of detection of 1.11 ng mL^-1 or 7.39 pM and a limit of quantification of 9.04 ng mL^-1 or 60.30 pM, within a 95% confidence level. The % error of the predicted versus the actual response of a quality control (QC) sample was 0.13%. The percent error of the QC sample decreases exponentially with the number of measurements. Randomly selected spatially separated measurements provided lower QC % error than a larger number of measurements that were closely spaced. We propose that it is necessary to describe the measured populations using an appropriate sample size for good statistics and consider the interrogation of sufficiently large and well-separated areas of the sensor surface to achieve a reliable sampling.

Periodic Surface-Enhanced Raman Scattering-Encoded Magnetic Beads for Reliable Quantitative Surface-Enhanced Raman Scattering-Based Multiplex Bioassay

Surface-enhanced Raman scattering (SERS)-based immunoassay on encoded beads is highly attractive with the advantages of ultrasensitivity, multiplex and high throughput. However, it was a great challenge to screen out in-focus signals of the immunoconjugated SERS nanoprobes on spherical bead conveniently. Here, periodic SERS-encoded magnetic beads (PSE-MBs) were developed through droplet optofluidic technique by using monodisperse SERS-encoded magnetic nanospheres as building blocks. The designed PSE-MBs not only exhibit huge coding capacity, but also provide the strongest and reproducible SERS coding signals as "in-focus beacons". When PSE-MBs are used as capture carriers in SERS-based immunoassay, both multiple target analytes and in-focus signals of SERS nanoprobes could be easily identified according to the collected SERS coding signals. Thus, reliable quantitative analysis of multiple target analytes could be conveniently achieved by such detection protocol. Additionally, the magnetic ingredient in PSE-MBs made the operation easily during the bioassay. The multiple advantages of PSE-MBs including large coding capacity, in-focus beacons and magnetic operation endorse them to be robust capture carriers in reliable quantitative SERS-based multiplex immunoassay.

What Digital Immunoassays Can Learn from Ambient Analyte Theory: A Perspective

Immunoassays are important tools for clinical diagnosis as well as environmental and food analysis because they enable highly sensitive and quantitative measurements of analyte concentrations. In the 1980s, Roger Ekins suggested to improve the sensitivity of immunoassays by employing microspot assays, which are carried out under ambient analyte conditions and do not change the bulk analyte concentration of a sample during a measurement. More recently, the measurement of single analyte molecules has additionally attracted wide research interest. Although the ability to detect a single analyte molecule is not synonymous with the highest analytical sensitivity, single-molecule detection makes new routes accessible to avoiding background noise. This perspective follows the development of solid-phase immunoassays from the design of label techniques to single-molecule (digital) assays against the backdrop of Ekins's fundamental work on immunoassay theory. The essential aspects of both ambient analyte and digital assay approaches are presented as a guideline to finding a balance between the speed, sensitivity, and precision of immunoassays.

Application of Surface-Enhanced Raman Spectroscopy to Guide Therapy for Advanced Prostate Cancer Patients

A critical unmet need for advanced prostate cancer (PCa) patients is optimizing systemic treatments to maximize the benefit for individuals. The response of patients with metastatic castration-resistant prostate cancer (mCRPC) to androgen receptor (AR)-directed hormonal treatments (i.e., enzalutamide and abiraterone) is mediated by the expression of a molecular variant of the androgen receptor called androgen receptor variant 7 (AR-V7). Detection and measurement of AR-V7 in mCRPC patients will lead to more informed PCa treatment. Herein, we demonstrate a quantitative nanoparticle-enhanced sandwich antibody assay for the successful ex vivo measurement of AR-V7 protein in serum from mCRPC patients. The nanoparticles are constructed as extrinsic Raman spectroscopy labels (ERLs), and surface-enhanced Raman spectroscopy (SERS) is used for assay readout. Our approach does not require specialized specimen collection materials, circulating tumor cell enrichment, or pretreatment of serum. Calibration of our assay is accomplished by expressing AR-V7 in an appropriate cell line as AR-V7 is not commercially available. We demonstrate a linear calibration curve from cell lysate and correlate lysate protein with mRNA from cultured prostate cancer cells. Finally, we demonstrate a novel pilot-scale application for clinical use by quantitatively measuring AR-V7 in serum of seven advanced PCa patients. Distinct separation of PCa patients by AR-V7 status (positive or negative) was observed. Together, the presence and amount of AR-V7 in serum offer predictive and prognostic value to inform selection between two classes of systemic treatments (i.e., hormones or taxanes). Triaging patients that are AR-V7-positive to other systemic treatments (e.g., taxane-based chemotherapy) can improve progression-free survival and overall survival.

Biosensing Using SERS Active Gold Nanostructures

Surface-enhanced Raman spectroscopy (SERS) has become a powerful tool for biosensing applications owing to its fingerprint recognition, high sensitivity, multiplex detection, and biocompatibility. This review provides an overview of the most significant aspects of SERS for biomedical and biosensing applications. We first introduced the mechanisms at the basis of the SERS amplifications: electromagnetic and chemical enhancement. We then illustrated several types of substrates and fabrication methods, with a focus on gold-based nanostructures. We further analyzed the relevant factors for the characterization of the SERS sensor performances, including sensitivity, reproducibility, stability, sensor configuration (direct or indirect), and nanotoxicity. Finally, a representative selection of applications in the biomedical field is provided.

Targeting improved reproducibility in surface-enhanced Raman spectroscopy with planar substrates using 3D printed alignment holders

Drop-casting is frequently used to deliver a sample for surface-enhanced Raman spectroscopy (SERS) and can result in inhomogeneous sample distribution during solvent evaporation. While soaking can provide better analyte homogeneity, it may require more sample than is available. Failure to optically sample analyte-rich substrate locations can compromise measurement outcomes. We developed and tested 3D printed SERS substrate holders that provided spatial registry of the dried sample droplet center for subsequent optical measurements. We found that deliberate and controlled spatial offsets (0-900 µm) between the analyte drop center and the laser excitation prevented signal intensity drops of as much as ∼3× and improved reproducibility. Thus, the use of offset-controlled 3D printed holders provided a quick and inexpensive way to improve the reliability of SERS measurements when using the convenient and popular choice of sample drop-casting.

Highly Biocompatible Plasmonically Encoded Raman Scattering Nanoparticles Aid Ultrabright and Accurate Bioimaging

Plasmonically engineered nanomaterials based on Au-Ag for surface-enhanced Raman scattering (SERS)-based biomedicine is of great importance but is still far behind clinical needs because of the poor compatibility between sensitivity and safety. Here, robust plasmonically encoded Raman scattering nanoparticles, named Au core-Raman-active molecule-Ag shell-Au shell nanoparticles (CMSS NPs), were synthesized. The as-developed CMSS NPs possess a unique exterior ultrathin Au shell (∼2.2 nm thickness) that plays double key roles as an effective wrapping layer as well as a plasmonic enhancing layer, thereby showing not only extraordinary stability against oxidative damages and bioerosion but also outstanding SERS sensitivity because of the stronger in-built electromagnetic field, achieving a significant SERS enhancement factor of 3.3 × 108. The results confirm that the individual CMSS NPs show ultrahigh brightness, reproducibility, selectivity, and biocompatibility in single-cell Raman imaging. Moreover, ultrabright in vivo tumor imaging with 1 × 1 mm2 area can be quickly achieved within 35 s under open-air condition. Furthermore, by secondary plasmonic encoding, the CMSS NPs flexibly serve as nanobeacon to monitor single-cell autophagy with improved accuracy. The CMSS NPs are expected as versatile SERS probes that enable ultrabright, fast, and precise Raman-based bioimaging and clinical bioapplications.

Biomarkers Determination Based on Surface-Enhanced Raman Scattering

An overview of noteworthy new methods of biomarker determination based on surface-enhanced Raman scattering (SERS) is presented. Biomarkers can be used to identify the occurrence and development of diseases, which furthers the understanding of biological processes in the body. Accurate detection of a disease-specific biomarker is helpful for the identification, early diagnosis and prevention of a disease and for monitoring during treatment. The search for and discovery of valuable biomarkers have become important research hotspots. Different diseases have different biomarkers, some of which are involved in metabolic processes. Therefore, the fingerprint characteristics and band intensities in SERS spectra have been used to identify metabolites and analyze markers. As a promising technique, SERS has been widely used for the quantitative and qualitative determination of different types of biomarkers for different diseases. SERS techniques provide new technologies for the diagnosis of disease-related markers and determining the basis for clinical treatment. Herein, several SERS-based methods with excellent sensitivity and selectivity for the determination of biomarkers for tumors, viruses, Alzheimer’s disease, cardiac muscle tissue injury, and cell activity are highlighted.

In Vivo Spectral Distortions of Infrared Luminescent Nanothermometers Compromise Their Reliability

Luminescence nanothermometry has emerged over the past decade as an exciting field of research due to its potential applications where conventional methods have demonstrated to be ineffective. Preclinical research has been one of the areas that have benefited the most from the innovations proposed in the field. Nevertheless, certain questions concerning the reliability of the technique under in vivo conditions have been continuously overlooked by most of the scientific community. In this proof-of-concept, hyperspectral in vivo imaging is used to explain how unverified assumptions about the thermal dependence of the optical transmittance of biological tissues in the so-called biological windows can lead to erroneous measurements of temperature. Furthermore, the natural steps that should be taken in the future for a reliable in vivo luminescence nanothermometry are discussed together with a perspective view of the field after the findings here reported.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

Present and Future of Surface-Enhanced Raman Scattering

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.

An Open Source, Iterative Dual-Tree Wavelet Background Subtraction Method Extended from Automated Diffraction Pattern Analysis to Optical Spectroscopy

Background subtraction is a general problem in spectroscopy often addressed with application-specific techniques, or methods that introduce a variety of implementation barriers such as having to specify peak-free regions of the spectrum. An iterative dual-tree complex wavelet transform-based background subtraction method (DTCWT-IA) was recently developed for the analysis of ultrafast electron diffraction patterns. The method was designed to require minimal user intervention, to support streamlined analysis of many diffraction patterns with complex overlapping peaks and time-varying backgrounds, and is implemented in an open-source computer program. We examined the performance of DTCWT-IA for the analysis of spectra acquired by a range of optical spectroscopies including ultraviolet-visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman spectroscopy (SERS). A key benefit of the method is that the user need not specify regions of the spectrum where no peaks are expected to occur. SER spectra were used to investigate the robustness of DTCWT-IA to signal-to-noise levels in the spectrum and to user operation, specifically to two of the algorithm parameter settings: decomposition level and iteration number. The single, general DTCWT-IA implementation performs well in comparison to the different conventional approaches to background subtraction for UV-Vis, XPS, and SERS, while requiring minimal input. The method thus holds the same potential for optical spectroscopy as for ultrafast electron diffraction, namely streamlined analysis of spectra with complex distributions of peaks and varying signal levels, thus supporting real-time spectral analysis or the analysis of data acquired from different sources.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.

Spatially Engineered Janus Hybrid Nanozyme toward SERS Liquid Biopsy at Nano/Microscales

Nanomaterials with intrinsic enzyme-mimicking properties (nanozymes) have been widely considered as artificial enzymes in biomedicine. However, manipulating inorganic nanozymes for multivariant targeted bioanalysis is still challenging because of the insufficient catalytic efficiency and biological blocking effect. Here, we rationally designed a spatially engineered hollow Janus hybrid nanozyme vector (h-JHNzyme) based on the bifacial modulation of Ag-Au nanocages. The silver face inside the h-JHNzyme served as an interior gate to promote the enzymatic activity of the Ag-Au nanozyme, whereas two-dimensional DNAzyme-motif nanobrushes deposited on the exterior surface of the h-JHNzyme endowed it with the targeting function and tremendously enhanced the peroxidase-mimicking activity. We demonstrated that the spatially separated modulation of the h-JHNzyme propelled it as a powerful "all-in-one" enzymatic vector with excellent biocompatibility, specific vectorization, remarkable enzymatic performance, and clinical practicability. Further, we programmed it into a stringent catalytic surface-enhanced Raman scattering (SERS) liquid biopsy platform to trace multidimensional tumor-related biomarkers, such as microRNAs and circulating tumor cells, with a limit of detection of fM and single cell level, respectively. The developed enzymatic platform showed great potential in facilitating reliable quantitative SERS liquid biopsy for on-demand clinical diagnosis.

Surface-enhanced Raman scattering II: concluding remarks

Surface-enhanced Raman scattering (SERS) enables the detection of a large number of different adsorbates at extraordinarily low levels. This plasmonics-based technology has undergone a number of remarkable advances since its discovery over 40 years ago, and has emerged from being an investigative tool confined largely to the research laboratory into a much more usable tool across a broad range of investigative studies, both within the laboratory and beyond. The purpose of this Concluding remarks manuscript is to capture, at least in part, the developments in this area since the first Faraday discussion of SERS over a decade ago. It begins with a brief contextual overview and then moves into describing a few of the many highlights from the meeting. Along the way, we have added a few comments and perspectives as a means to more fully stage where the different areas of research with SERS stand today. An addendum is included that collects a few of the recent perspectives on the original work and activities in this area.

Investigation of Issues for the Accurate and Precise Measurement of an Analyte Using Surface-Enhanced Raman Scattering (SERS)

This paper builds on an earlier examination of the influence of sampling size and analyte surface density on the accuracy and precision of measurements using surface-enhanced Raman scattering (SERS) to read out heterogeneous immunoassays. Quantitation using SERS typically relies on interrogating a small area on the sample surface by using a micrometer-sized laser spot for signal generation. The information obtained using such a small portion of sample is then projected as being representative of the much larger sample, which can compromise the accuracy and precision of the measurement due to undersampling. For a heterogeneous immunoassay interrogated by SERS, quantitation is, therefore, sensitive to the size of the analyzed area and the surface density of the measured analyte. To identify conditions in which sampling error poses a threat to accuracy and precision, a simulation of a SERS immunoassay was developed and compared to experimental results. The simulation randomly distributes adsorbates across the capture surface and then measures the density of adsorbates inside areas of analysis of different sizes. This approach mimics the analysis of a heterogeneous immunoassay when using a Raman microscope with different laser spot sizes. The results of the simulations, which were confirmed experimentally by comparison to an immunoassay of human immunoglobulin G (IgG) show that the accuracy and precision of the measurement improved with larger analysis areas and higher analyte concentrations due to the increased apparent homogeneity of the analyte within the area of analysis. By imposing a threshold on precision (5%), we also begin to establish a framework for the parameters necessary to achieve reliable quantitative measurements (e.g., laser spot size, analyte concentration, and sample volume).

Intensity Fluctuations in Single-Molecule Surface-Enhanced Raman Scattering

Around 20 years ago, the first reports of single-molecule surface-enhanced Raman scattering (SM-SERS) caused a revolution in nanotechnology. Several researchers were quick to recognize the importance of a technique that can provide molecular vibrational fingerprinting at the SM level. Since then, a large amount of work has been devoted to the development of nanostructures capable of SM-SERS detection. A great effort has also been geared toward elucidating the different mechanisms that contribute to the effect. The understanding of the concept of plasmonic SERS hotspots, the role of chemical effects, and the dynamics of atomic and cluster rearrangements in nanometric domains has significantly advanced, driven by new computational and experimental methods used to study SM-SERS. In particular, SERS intensity fluctuations (SIFs) are now recognized as a hallmark of SM-SERS. Interpretation of SM-SERS data must take into consideration temporal and spatial variations as a natural consequence of the extreme localization inherent to surface plasmon resonances. Further analysis of variations in spectral signature, due to either molecular reorientation or photo (or thermal) processes, pointed to a new area that combines the power of SERS fingerprinting at the SM level to modern concepts of catalysis, such as hot-electrons-driven chemistry. This large body of work on the fundamental characteristics of the SM-SERS effect paved the way to the interpretation of other related phenomena, such as tip-enhanced Raman scattering (TERS). Despite all the fundamental progress, there are still very few examples of real applications of SM-SERS. In recent years, our research group has been studying SIFs, focused on different ways to use SM-SERS. The obvious application of SM-SERS is in analytical chemistry, particularly for quantification at ultralow concentrations (below 1 nM). However, quantification using SM-SERS faces a fundamental sampling problem: the analytes (adsorbed in very small amounts, i.e., low surface coverage) must find rare SERS hotspots (areas with intense electric field localization that yields SERS). This limitation leads to strong temporal and spatial variations in SERS intensities, which translates into very large error bars in an experimental calibration curve. We tackled this problem by introducing the concept of "digital SERS". This approach provided a roadmap for SERS quantification at ultralow concentrations and a potential pathway for a better understanding of the "reproducibility problem" associated with SERS. In this Account, we discuss not only the analytical applications but also other implementations of SM-SERS demonstrated by our group. These include the use of SM-SERS as a tool to probe colloidal aggregation, to evaluate the efficiency of SERS substrates, and to characterize the energy of localized resonances. SERS involves a series of random processes: hotspots are rare; surfaces/clusters constantly reconstruct; and molecules diffuse, adsorb, and desorb. All these pathways contribute to strong fluctuations in SERS intensities. Our work indicates that a statistical view of the effect can lead to interesting insights and the potential to fulfill the promise of this SM technique for real-world applications.

Reliability of rare-earth-doped infrared luminescent nanothermometers

The use of infrared-emitting rare-earth-doped luminescent nanoparticles as nanothermometers has attracted great attention during the last few years. The scientific community has identified rare-earth-doped luminescent nanoparticles as one of the most sensitive and versatile systems for contactless local temperature sensing in a great variety of fields, but especially in nanomedicine. Researchers are nowadays focused on the design and development of multifunctional nanothermometers with new spectral operation ranges, outstanding brightness, and enhanced sensitivities. However, no attention has been paid to the assessment of the actual reliability of the measurements provided by rare-earth-doped luminescent nanothermometers. In fact, it is assumed that they are ideal temperature sensors. Nevertheless, this is far from being true. In this work we demonstrate that the emission spectra of rare-earth-doped nanothermometers can be affected by numerous environmental and experimental factors. These include the numerical aperture of the optical elements used for their optical excitation and luminescence collection, the local concentration of nanothermometers, optical length variations, self-absorption of the luminescence by the nanothermometers themselves, and solvent optical absorption. This work concludes that rare-earth-doped luminescent nanothermometers are not as reliable as thought and, consequently, special care has to be taken when extracting temperature estimations from the variation of their emission spectra.

Acoustic Sensing Based on Density Shift of Microspheres by Surface Binding of Gold Nanoparticles

Herein, we propose a concept for sensing based on density changes of microparticles (MPs) caused by a biochemical reaction. The MPs are levitated by a combined acoustic-gravitational force at a position determined by the density and compressibility. Importantly, the levitation is independent of the MPs sizes. When gold nanoparticles (AuNPs) are bound on the surface of polymer MPs through a reaction, the density of the MPs dramatically increases, and their levitation position in the acoustic-gravitational field is lowered. Because the shift of the levitation position is proportional to the number of AuNPs bound on one MP, we can determine the number of molecules involved in the reaction. The avidin-biotin reaction is used to demonstrate the effectiveness of this concept. The number of molecules involved in the reaction is very small because the reaction space is small for an MP; thus, the method has potential for highly sensitive detection.

Single Molecule Fluorescent Colocalization of Split Aptamers for Ultrasensitive Detection of Biomolecules

Single-molecule fluorescence imaging is a promising strategy for biomolecule detection. However, the accuracy of single-molecule method is often compromised by the false-positive events at the ultralow sample levels that are caused by the nonspecific adsorption of the fluorescent labeled probe and other fluorescent impurities on the imaging surface. Here, we demonstrate an ultrasensitive single molecule detection assay based on dual-color fluorescent colocalization of spilt aptamers that was implemented to the measurement of adenosine triphosphate (ATP). The ATP aptamer was split into two fragments and labeled with green and red dye molecules, respectively. When the two probes of split aptamers were brought together by the target ATP molecule, the two colors of fluorescence of two probes were simultaneously detected through two channels and projected to the correlated locations in the two halves of image. The colocalizaiton imaging of two split apatamer probes greatly excluded the false detection of biomolecules that was usually caused by the fluorescent noise of single nonbound aptamer probes and impurities, and further improved the accuracy of measurement. The assay showed excellent selectivity and high sensitivity for ATP detection with linear range of 1 pM to 5 nM and a detection limit of 100 fM. This versatile protocol of single molecule colocalization of split apatamer can be widely applied to the ultrasensitive and highly accurate detection of many types of biomolecules in basic research and biomedical applications.

Surface-Enhanced Raman Spectroscopy for Bioanalysis: Reliability and Challenges

Surface-enhanced Raman spectroscopy (SERS) inherits the rich chemical fingerprint information on Raman spectroscopy and gains sensitivity by plasmon-enhanced excitation and scattering. In particular, most Raman peaks have a narrow width suitable for multiplex analysis, and the measurements can be conveniently made under ambient and aqueous conditions. These merits make SERS a very promising technique for studying complex biological systems, and SERS has attracted increasing interest in biorelated analysis. However, there are still great challenges that need to be addressed until it can be widely accepted by the biorelated communities, answer interesting biological questions, and solve fatal clinical problems. SERS applications in bioanalysis involve the complex interactions of plasmonic nanomaterials with biological systems and their environments. The reliability becomes the key issue of bioanalytical SERS in order to extract meaningful information from SERS data. This review provides a comprehensive overview of bioanalytical SERS with the main focus on the reliability issue. We first introduce the mechanism of SERS to guide the design of reliable SERS experiments with high detection sensitivity. We then introduce the current understanding of the interaction of nanomaterials with biological systems, mainly living cells, to guide the design of functionalized SERS nanoparticles for target detection. We further introduce the current status of label-free (direct) and labeled (indirect) SERS detections, for systems from biomolecules, to pathogens, to living cells, and we discuss the potential interferences from experimental design, measurement conditions, and data analysis. In the end, we give an outlook of the key challenges in bioanalytical SERS, including reproducibility, sensitivity, and spatial and time resolution.

Detection of the tuberculosis antigenic marker mannose-capped lipoarabinomannan in pretreated serum by surface-enhanced Raman scattering

The ability to detect tuberculosis (TB) continues to be a global health care priority. This paper describes the development and preliminary assessment of the clinical accuracy of a heterogeneous immunoassay that integrates a serum pretreatment process with readout by surface-enhanced Raman scattering (SERS) for the low-level detection of mannose-capped lipoarabinomannan (ManLAM). ManLAM is a major virulence factor in the infectious pathology of Mycobacterium tuberculosis (Mtb) that has been found in the serum and other body fluids of infected patients. The effectiveness of ManLAM as a TB diagnostic marker, however, remains unproven for reasons not yet well understood. As reported herein, we have found that (1) ManLAM complexes with proteins and possibly other components in serum; (2) these complexes have a strongly detrimental impact on the ability to detect ManLAM using an immunoassay; (3) a simple pretreatment step can disrupt this complexation; and (4) disruption by pretreatment improves detection by 250×. We also describe the results from a preliminary assessment on the utility of serum pretreatment by running immunoassays on archived specimens from 24 TB-positive patients and 10 healthy controls. ManLAM was measurable in 21 of the 24 TB-positive specimens, but not in any of the 10 control specimens. These findings, albeit for a very small specimen set, translate to a clinical sensitivity of 87.5% and a clinical specificity of 100%. Together, these results both provide much needed evidence for the clinical utility of ManLAM as a TB marker, and demonstrate the potential utility of our overall approach to serve as a new strategy for the development of diagnostic tests for this disease.

Dual-reporter SERS-based biomolecular assay with reduced false-positive signals

We present a sensitive and quantitative protein detection assay that can efficiently distinguish between specific and nonspecific target binding. Our technique combines dual affinity reagents with surface-enhanced Raman spectroscopy (SERS) and chemometric analysis. We link one Raman reporter-tagged affinity reagent to gold nanoparticles and another to a gold film, such that protein-binding events create a "hot spot" with strong SERS spectra from both Raman reporter molecules. Any signal generated in this context is indicative of recognition by both affinity labels, whereas signals generated by nonspecific binding lack one or the other label, enabling us to efficiently distinguish true from false positives. We show that the number of hot spots per unit area of our substrate offers a quantitative measure of analyte concentration and demonstrate that this dual-label, SERS-linked aptasensor assay can sensitively and selectively detect human α-thrombin in 1% human serum with a limit of detection of 86 pM.

Quantitative and multiplex dot-immunoassay using gap-enhanced Raman tags

A highly specific, quantitative, and multiplex dot immunoassay has been developed. The immunoassay utilizes functionalized plasmonic gap-enhanced Raman tags (GERTs) as labels and nitrocellulose membrane as a substrate.

The mechanism of an enzymatic reaction-induced SERS transformation for the study of enzyme–molecule interfacial interactions

The vibrational frequencies and spectral intensity of enzyme-conjugated SERS-active reporter molecules (4-MBA) shift and change regularly as a function of the concentration of glucose.