Quantification of Antiviral Drug Tenofovir (TFV) by Surface-Enhanced Raman Spectroscopy (SERS) Using Cumulative Distribution Functions (CDFs)

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive spectroscopic technique that generates signal-enhanced fingerprint vibrational spectra of small molecules. However, without rigorous control of SERS substrate active sites, geometry, surface area, or surface functionality, SERS is notoriously irreproducible, complicating the consistent quantitative analysis of small molecules. While evaporatively prepared samples yield significant SERS enhancement resulting in lower detection limits, the distribution of these enhancements along the SERS surface is inherently stochastic. Acquiring spatially resolved SERS spectra of these dried surfaces, we have shown that this enhancement is governed by a power law as a function of analyte concentration. Consequently, by definition, there is no true mean of SERS enhancement, requiring an alternative approach to achieve reproducible quantitative results. In this study, we introduce a new method of analysis of SERS data using a cumulative distribution function (CDF). The antiviral drug tenofovir (TFV) in an aqueous matrix was quantified down to a clinically relevant concentration of 25 ng/mL using hydroxylamine-reduced silver colloids evaporated to dryness. The data presented in this study provide a rationale for the benefits of combining a novel statistical approach using CDFs with simple and inexpensive experimental techniques to increase the precision, accuracy, and analytical sensitivity of aqueous TFV quantification by SERS. TFV calibration curves generated using CDF analysis showed higher analytical sensitivity (in the form of a normalized calibration curve average slope increase of 0.25) compared to traditional SERS intensity calculations. A second aliquot of nanoparticles and analyte dried on the SERS surface followed by CDF analysis showed further analytical sensitivity with a normalized calibration curve slope increase of 0.23 and decreased variation among replicates represented by an average standard deviation decrease of 0.02 with a second aliquot. The quantitative analysis of SERS data using CDFs presented here shows promise to be a reproducible method for quantitative analysis of SERS data, a significant step toward implementing SERS as an analytical method in clinical and industrial settings.

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.

High-Yield Electrochemical Synthesis of Silver Nanoparticles by Enzyme-Modified Boron-Doped Diamond Electrodes

We report a novel electrochemical approach for synthesizing colloidal silver in an aqueous phase by a hemoglobin-modified boron-doped diamond electrode. The resulting Ag nanoparticles are within 10 nm in size and highly monodisperse with minimal electrode deposition. We also introduce a method for measuring the yield of synthesized nanoparticles using square-wave voltammetry as an alternative to UV-vis spectroscopy. More than 50% of the transferred electrons contributed directly to the formation of silver nanoparticles. This high yield indicates that such electrochemical synthesis is an efficient one-pot method for producing colloidal silver free of toxic reagents and offers a path toward green metal nanoparticle synthesis in solution. A comparative study using alternative electrodes, modifiers, and surfactants suggests a mechanism for the formation of silver nanoparticles mediated by hemoglobin-modified boron-doped diamond electrodes.

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.

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.

Reduction of CuO butterfly wing scales generates Cu SERS substrates for DNA base detection

We prepare three-dimensional Cu plasmonic structures via a reduction of CuO photonic crystals replicated from butterfly wing scales. These Cu superstructures with high purity provide surface-enhanced Raman scattering (SERS) substrates for the label-free detection of DNA bases down to a micromolar level, which is achieved for the first time on Cu and even comparable to the detection-sensitivity for DNA bases on some Ag substrates. The generation of such superstructures has provided a substantial step for the biotemplated SERS substrates with high sensitivity, high reproducibility, and ultra-low cost to detect biomolecules, and presented affordable high-quality routine SERS consumables for corresponding biolaboratories.

Probing the plasmonic near-field by one- and two-photon excited surface enhanced Raman scattering

Strongly enhanced and spatially confined near-fields in the vicinity of plasmonic nanostructures open up exciting new capabilities for photon-driven processes and particularly also for optical spectroscopy. Surface enhanced Raman signatures of single molecules can provide us with important information about the optical near-field. We discuss one- and two-photon excited surface enhanced Raman scattering at the level of single molecules as a tool for probing the plasmonic near-field of silver nanoaggregates. The experiments reveal enhancement factors of local fields in the hottest hot spots of the near-field and their dependence on the photon energy. Also, the number of the hottest spots and their approximate geometrical size are found. Near-field amplitudes in the hottest spots can be enhanced by three orders of magnitudes. Nanoaggregates of 100 nm dimensions provide one hot spot on this highest enhancement level where the enhancement is confined within less than 1nm dimension. The near-field enhancement in the hottest spots increases with decreasing photon energy.

Sensitive cylindrical SERS substrate array for rapid microanalysis of nucleobases

In this work, a cylindrical-substrate array for surface-enhanced Raman scattering (SERS) measurements was developed to enable analysis of nucleobases in a few microliters of liquid. To eliminate uncertainties associated with SERS detection of aqueous samples, a new type of cylindrical SERS substrate was designed to confine the aqueous sample at the tip of the SERS probe. Poly(methyl methacrylate) (PMMA) optical fibers in a series of different diameters were used as the basic substrate. A solution of poly(vinylidene fluoride)/dimethylformamide (PVDF/DMF) was used to coat the tip of each fiber to increase the surface roughness and facilitate adsorption of silver nanoparticles (AgNPs) for enhancing Raman signals. A chemical reduction method was used to form AgNPs in and on the PVDF coating layer. The reagents and reaction conditions were systematically examined with the aim of estimating the optimum parameters. Unlike the spreading of aqueous sample on most SERS substrates, particularly flat ones, the new SERS substrates showed enough hydrophobicity to restrict aqueous sample to the tip area, thus enabling quantitative analysis. The required volume of sample could be as low as 1 μL with no need for a drying step in the procedure. By aligning the cylindrical SERS substrates into a solid holder, an array of cylindrical substrates was produced for mass analysis of aqueous samples. This new substrate improves both reproducibility and sensitivity for detection in aqueous samples. The enhancement factor approaches 7 orders in magnitude with a relative standard error close to 8%. Using the optimized conditions, nucleobases of adenine, cytosine, thymine, and uracil could be detected with limits approaching a few hundreds nanomolar in only a few microliters of solution.

Sensitive and selective SERS probe for trivalent chromium detection using citrate attached gold nanoparticles

In this article, we have demonstrated a sensitive and selective surface enhanced Raman spectroscopy (SERS) probe, based on citrate-capped gold nanoparticles (AuNPs), for trivalent chromium (Cr(3+)) detection. After introducing Tween 20 to a solution of citrate-capped AuNPs, the as-prepared Tween 20/citrate-AuNP probe could recognize Cr(3+) at a 50 × 10(-9) M level in an aqueous medium at a pH of 6.0. Tween 20 can stabilize the citrate-capped AuNPs against conditions of high ionic strength. Due to the chelation between Cr(3+) and citrate ions, AuNPs undergo aggregation. As a result, it formed several hot spots and provided a significant enhancement of the Raman signal intensity through electromagnetic (EM) field enhancements. A detailed mechanism for tremendous SERS intensity change had been discussed. The selectivity of this system toward Cr(3+) was 400-fold, remarkably greater than other metal ions.

SERS detection of low-concentration adenine by a patterned silver structure immersion plated on a silicon nanoporous pillar array

A patterned Ag structure was grown on a Si nanoporous pillar array (Si-NPA) by an immersion plating method, and its surface-enhanced Raman scattering (SERS) activity toward adenine was studied. It was shown that two kinds of Ag structures were grown on Si-NPA, a continuous film covering the Si-NPA substrate and composed of Ag nanocrystallites (nc-Ag), and a quasi-regular, interconnected network composed of loop-chains of sub-micron Ag crystallites surrounding the porous Si pillars. The SERS detection of low-concentration adenine solution was performed by using Ag/Si-NPA as active substrates, in which significantly enhanced Raman signals were observed. The SERS enhancement was attributed to the active spacing sites formed between the Ag particles and the nc-Ag which met the optimal size for causing a SERS effect. Based on the measured SERS spectra, the adsorption mode of adenine molecules on Ag particles was deduced. These results indicated that Ag/Si-NPA might be a promising active substrate for SERS detection of low-concentration bio-molecules.

Imaging the cell wall of living single yeast cells using surface-enhanced Raman spectroscopy

The surface of a living yeast cell (Saccharomyces cerevisiae strain W303-1A) has been labeled with silver (Ag) nanoparticles that can form nanoaggregates which have been shown to have surface-enhanced Raman scattering (SERS) activity. The cell wall of a single living yeast cell has been imaged by use of a Raman microspectroscope. The SERS spectra measured from different Ag nanoaggregates were found to be different. This can be explained on the basis of detailed spectral interpretation. The SERS spectral response originates from mannoproteins which cover the outermost regions of the yeast cell wall. Analysis of SERS spectra from the cell wall and the extracted mannoproteins from the yeast has been performed for the clarification of variation in SERS spectra.

Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists' canvas

This perspective presents recent surface-enhanced Raman spectroscopy (SERS) studies of dyes, with applications to the fields of single-molecule spectroscopy and art conservation. First we describe the development and outlook of single-molecule SERS (SMSERS). Rather than providing an exhaustive review of the literature, SMSERS experiments that we consider essential for its future development are emphasized. Shifting from single-molecule to ensemble-averaged experiments, we describe recent efforts toward SERS analysis of colorants in precious artworks. Our intention is to illustrate through these examples that the forward development of SERS is dependent upon both fundamental (e.g., SMSERS) and applied (e.g., on-the-specimen SERS of historical art objects) investigations and that the future of SERS is very bright indeed.

Plasma reduction of silver compounds for fabrication of surface-enhanced Raman scattering substrates

Several silver compounds were reduced by low-pressure air plasma to produce porous nanostructured surfaces as surface-enhanced Raman scattering (SERS) substrates. This method is advantageous because substrates are easy to prepare and the silver metal surface is inherently clean without spectroscopic background. Silver compounds were melted into 1-2 mm slugs on quartz slides and plasma treated for different lengths of time. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.

Convective assembly of bacteria for surface-enhanced Raman scattering

A sample preparation method based on convective assembly for "whole-microorganism" identification using surface-enhanced Raman scattering (SERS) is developed. With this technique, a uniform sample can easily be prepared with silver nanoparticles. During the deposition process, bacteria and nanoparticles are assembled to form a unique well-ordered structure with great reproducibility. The SERS spectra acquired from the samples prepared with this technique have better quality and improved reproducibility for SERS spectra obtained from the same sample and limited variation due to the consistent sample preparation. E. coli, a Gram-negative bacilli, and Staphylococcus cohnii, a Gram-positive coccus, are studied as model bacteria.

Observation of a small number of molecules at a metal nanogap arrayed on a solid surface using surface-enhanced Raman scattering

In situ Raman spectroscopic measurements with 785 nm excitation were carried out in aqueous solutions containing bipyridine derivatives. Intense Raman signals were observed when the Ag dimer structure was optimized. The SERS activity was dependent upon on the structure of the Ag dimer with a distinct gap distance, suggesting that the intense SERS originates from the gap part of the dimer. Characteristic time-dependent spectral changes were observed. Not only a spectrum which was the superposition of two bipyridine spectra but also spectra which can be assigned to one of the bipyridine derivatives were frequently observed. Observation using solutions with different concentrations proved that the spectra originated from very small numbers of molecules at the active SERS site of the dimer.

Electromagnetic and chemical interaction between Ag nanoparticles and adsorbed rhodamine molecules in surface-enhanced Raman scattering

The critical importance of the junction between touching or closely adjacent Ag nanoparticles associated with single-molecule sensitivity (SMS) in surface-enhanced Raman scattering (SERS) was confirmed via the following observations: (1) an additional peak is observed in elastic scattering only for the SERS-active state, which originated from absorption of adsorbates, (2) local- and far-field evaluation using a finite difference time domain method could reproduce this extra peak and anticipate the significantly enhanced field even inside the adsorbates sitting at the junction through an increased coupling of the localized surface plasmons, and (3) in addition to enhanced fluorescence of adsorbed dye, an inelastic scattering peak was observed and attributed to the metal surface electron. Concerning the chemical enhancement in SERS, Cl- anions activate the Ag-Cl-R6G (rhodamine) samples by inducing intrinsic electronic interaction between Ag and R6G molecules. This electronic interaction is irreversibly quenched by the addition of thiosulfate anions which dissolve Ag+ cations while the electromagnetic (EM) effect remains intact.

Self-Assembly of Silver Nanoparticles: Synthesis, Stabilization, Optical Properties, and Application in Surface-Enhanced Raman Scattering

Silver nanoparticle aggregates were synthesized in large scale using resorcinol under alkaline condition to obtain an assembly of silver clusters. Stable dispersion of the cluster in aqueous medium has been examined out of resorcinol-capped silver nanoparticle assemblies. The UV-vis spectroscopy during the particle evolution has been studied in detail. From the high-resolution TEM (HRTEM) image and XRD pattern it was confirmed that the particles are made of pure silver only. The capping action of resorcinol has been authenticated from the FTIR spectra. UV-vis spectroscopy and TEM images reveal that the temperature, effect of vibrational energy, heat shock, and time-dependent particle evolution have unique bearing on the stability and surface properties of the clusters. The concentrations of silver nitrate, resorcinol, and NaOH have important influence on the particle evolution and its size. TEM images incite us to examine the aggregates to capitulate surface-enhanced Raman scattering (SERS) to the single molecular level using crystal violet (CV) and cresyl fast violet (CFV) as molecular probes. The SERS intensity of CV increases with increasing the size of the silver aggregate.

Structural analysis of biological aliphatic compounds using surface-enhanced Fourier transform Raman spectroscopy

The surface-enhanced Raman scattering (SERS) technique for Fourier transform Raman spectrometry is employed to reveal the chemical structure of biological aliphatic compounds consisting of folded, long aliphatic chains. The structural analysis is performed via the measurements of the accordion-vibration modes generated in the ordered, long aliphatic chain. The SERS spectra after subtraction of a background spectrum give segment lengths that are almost perfectly consistent with the chemical structures studied by mass spectrometry. The agreement of the SERS results with those of mass spectrometry suggests the positions of kinks in the long hydrocarbon chain. The combination technique of SERS and mass spectrometry is useful to discuss the structure of folded, long biological lipids.