A Study on the Morphology of the Silver Nanoparticles Deposited on the n-Type Porous Silicon Prepared Under Different Illumination Types

In this study, the influence of n-type porous Si (n-PS) morphology properties and the performance of Ag nanoparticles (AgNPs)/n-PS Raman substrate were investigated. Two kinds of n-PS morphology (macro n-PS and mud n-PS) structures were fabricated by laser-assisted etching (LAE) process and ordinary light-assisted etching (OLAE) process, respectively. A simple and cost-effective immersion plating process of n-PS in 0.01 M concentration of AgNO₃ for 16-min immersion time was used to synthesize AgNPs. The morphological properties of the deposited AgNPs on the macro n-PS layer showed that the deposition process is concentrated on the pore wall with a little density, while for mud n-PS, the AgNP layer is mainly composed of high-density uniformly distributed spherical particles located over the mud surface. Surface-enhanced Raman scattering (SERS) process of AgNPs/n-PS revealed strong dependence on the morphology and the density of AgNPs. Enhancement factor (EF) of Raman signal of AgNPs/mud n-PS substrate is three orders of magnitude higher than that of AgNPs/macro n-PS substrate of about 1.6 × 10¹¹ and 8.2 × 10⁸, respectively.     

Tapered Fiber Probe Modified by Ag Nanoparticles for SERS Detection

A tapered optical fiber fabricated by a simple chemical etching method and modified with Ag nanoparticles (AgNPs) by chemical deposition was evaluated for surface-enhanced Raman scattering (SERS). The fiber probe was used for SERS measurements in both direct and remote scattering modes, yielding desired performance in both scattering configurations. The state of the obtained AgNPs made a significant contribution to the high sensitivity of SERS to Rhodamine 6G (R6G) molecules (down to a concentration of 10^?7 M), and the substrate had an analyst enhancement factor (AEF) on the order of ～10⁸. Meanwhile, the SERS intensity during the evaporation process was investigated, showing a good stability at the later stage of the evaporation process. The fiber SERS probes demonstrated good reproducibility with the average relative standard deviation (RSD) values being less than 0.2 for the major Raman peaks.     

Modifying and Fine Controlling of Silver Nanoparticle Nucleation Sites and SERS Performance by Double Silicon Etching Process

Different forms of modified and well-controlled plasmonic silver nanoparticles (AgNPs) were synthesized by silver ion reduction process of porous silicon (PS). Fine control of PS surface morphology was accomplished by employing two etching processes: light-induced etching (LIE) and photo electrochemical etching (PECE). The idea was to prepare excellent and reproducible surface-enhanced Raman scattering (SERS) substrates with high enhancement performance. PS surface modification was employed to create efficient and nearly uniformly distributed AgNP hotspot regions with very high specific surface areas. Reproducibility deviation of no more than 5% and enhancement factor of 1.2 × 10¹⁴ were obtained by SERS measurements at very low, rhodamine 6G (R6G) dye, concentration 10^?15 M. The PS morphology SERS substrate was well discussed and analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and Raman measurements.     

A sensitive surface‐enhanced Raman scattering method for chondroitin sulfate with Victoria blue 4R molecular probes in nanogold sol substrate

Using silver nanoparticles (AgNPs) as the nanocatalyst, l ‐cysteine rapidly reduced HAuCl₄ to make a stable gold nanoparticle sol (Ag/AuNP) that had a high surface‐enhanced Raman scattering (SERS) activity in the presence of Victoria blue 4R (VB4r) molecular probes. Under the selected conditions, chondroitin sulfate (Chs) reacted with the VB4r probes to form associated complexes that caused the SERS effect to decrease to 1618 cm^?1. The decreased SERS intensity was linear to the Chs concentration in the range 3.1–500 ng/ml, with a detection limit of 1.0 ng/ml Chs. Accordingly, we established a simple and sensitive SERS quantitative analysis method to determine Chs in real samples, with a relative standard deviation of 1.47–3.16% and a recovery rate of 97.6–104.2%.     

Silver Nanoparticle Arrays on a DVD-Derived Template: An easy&cheap SERS Substrate

Commercially available digital versatile discs (DVDs) contain a silver-coated spiral distribution of rectangular-shaped grooves (AgDVD): for the first time, they have been used to produce surface-enhanced Raman scattering (SERS) substrates by electrochemical deposition of silver nanoparticles (AgNPs@AgDVD). The overall procedure only requires cheap and widely available materials and can be easily accomplished. Scanning electron microscopy images of AgNPs@AgDVD revealed that small AgNPs (average diameter about 15 nm) are present within the valleys of AgDVD, whereas over the ridges, the AgNPs are bigger, more densely packed and with a dendrite-like morphology somewhere. The SERS properties of these substrates have been studied in terms of the enhancement factor (EF), point-to-point reproducibility and sample-to-sample repeatability. It turned out that high SERS EF and good reproducibility requirements are both fulfilled. As for repeatability, remarkably better results than typical literature values have been achieved. Such an easy&cheap preparation along with efficient SERS properties make DVD-derived SERS substrates very good candidates for the development of convenient and disposable sensing platforms.     

Direct Electrodeposition of Hollowed Ag Nanostructures on ITO Glass for Reproducible SERS Application

Hollowed Ag nanostructures are, for the first time, electrodeposited on ITO glass without use of surfactant. The hollowed Ag nanostructure was investigated via a collaboration of scanning electron microscopy (SEM), XRD, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), XRD, and UV-vis. Results exhibited that the formation of the hollowed Ag nanostructure can be interpreted as the synergy effect of twin defect and low nucleation driving force. Surface-enhanced Raman scattering (SERS) spectra of rhodamine 6G and adenine molecules adsorbed on the surface of these Ag nanostructures were recorded. The smallest RSD of 1651 cm^?1 Raman bands of rhodamine 6G was 14.7 %, indicating that the hollowed Ag nanostructures can be utilized for reproducible SERS application. Through comparison, it was found the good crystallinity was beneficial for SERS.     

The Use of Polarization-dependent SERS from Scratched Gold Films to Selectively Eliminate Solution-phase Interference

Polarization-dependent surface-enhanced Raman scattering (SERS) was studied for oxazine 720 molecules adsorbed on a scratched gold surface placed in situ and under electrochemical control. A quantitative method for evaluating the observed polarization dependence will be introduced. This method takes into account the polarization artifacts caused by optical elements in the light microscope used for Raman microscopy. Intensity of the SERS obtained from oxazine 720 adsorbed on scratches in gold showed a polarization dependence after correction was made for these artifacts. In contrast, intensity of the ordinary Raman signal obtained from perchlorate ions in the solution above a scratched gold surface was found to be polarization-independent. Therefore, polarization effects can be used to selectively remove solution-phase interference signals from the SERS spectrum of an adsorbed analyte. These polarization effects were found to be independent of the applied potential, meaning the methodology is applicable to electrochemical SERS studies.     

Raman scattering from nucleic acids adsorbed at a silver electrode

Adsorption of nucleic acids at a silver electrode polarized to -0.6 to -0.1 V (vs. Ag/AgCl) was investigated by means of surface enhanced Raman scattering (SERS) spectroscopy. Single-stranded polyriboadenylic acid and thermally denaturated DNA adsorbed at the silver electrode yield two intense bands at 734 and 1335 cm-1 on the SERS spectra. These bands, assigned to the vibrations of adenine residue rings, were much less intense if the SERS spectra were recorded for double-helical complex polyadenylic X polyuridylic acid and native DNA. Moreover, the courses of alkaline denaturation of DNA and its digestion by deoxyribonuclease I were observed by SERS spectroscopy. The results were interpreted as support for the view that intact double-helical segments of nucleic acids are not denatured or destabilized due to their adsorption at the positively charged and roughened surface.     

The poly dA strand of poly dA.poly dT adopts an A-form in solution: a UV resonance Raman study.

The study by resonance Raman spectroscopy with a 257 nm excitation wave-length of adenine in two single-stranded polynucleotides, poly rA and poly dA, and in three double-stranded polynucleotides, poly dA.poly dT, poly(dA-dT).poly(dA-dT) and poly rA.poly rU, allows one to characterize the A-genus conformation of polynucleotides containing adenine and thymine bases. The characteristic spectrum of the A-form of the adenine strand is observed, except small differences, for poly rA, poly rA.poly rU and poly dA.poly dT. Our results prove that it is the adenine strand which adopts the A-family conformation in poly dA.poly dT.     

Therapeutic prognosis of prostate cancer using surface-enhanced Raman scattering of patient urine and multivariate statistical analysis

Surface-enhanced Raman scattering (SERS) is highly sensitive and label-free analytical technique based on Raman spectroscopy aided by field-multiplying plasmonic nanostructures. We report the use of SERS measurements of patient urine in conjunction with biostatistical algorithms to assess the treatment response of prostate cancer (PCa) in 12 recurrent (Re) and 63 nonrecurrent (NRe) patient cohorts. Multiple Raman spectra are collected from each urine sample using monodisperse silver nanoparticles (AgNPs) for Raman signal enhancement. Genetic algorithms-partial least squares-linear discriminant analysis (GA-PLS-LDA) was employed to analyze the Raman spectra. Comprehensive GA-PLS-LDA analyses of these Raman spectral features (p = 3.50 × 10⁻¹⁶ ) yield an accuracy of 86.6%, sensitivity of 86.0%, and specificity 87.1% in differentiating the Re and NRe cohorts. Our study suggests that SERS combined with multivariate GA-PLS-LDA algorithm can potentially be used to detect and monitor the risk of PCa relapse and to aid with decision-making for optimal intermediate secondary therapy to recurred patients.     

Pressure-tuning infrared and Raman microscopy study of the DNA bases: adenine,guanine, cytosine,and thymine

Diamond-anvil cell, pressure-tuning infrared (IR), and Raman microspectroscopic measurements have been undertaken to examine the effects of high pressures up to about 45?kbar on the vibrational spectra of the four DNA bases, adenine, cytosine, guanine, and thymine. Small structural changes were evident for all the four bases, viz., for adenine and cytosine at 28–31?kbar; for guanine at 16–19?kbar; and for thymine at 25–26?kbar. These changes are most likely associated with alterations in the intermolecular hydrogen-bonding interactions. The pressure dependences of the main peaks observed in the IR spectra of the two phases of guanine lie in the ?0.07–0.66 (low-pressure phase) and 0.06–0.91 (high-pressure phase) cm^?1/kbar ranges. Also, in the Raman spectra of this nucleoside base, the dν/dP values range from ?0.07–0.31 (low-pressure phase) to 0.08–0.50 (high-pressure phase) cm^?1/kbar. Similar ranges of dν/dP values were obtained for the other three nucleoside bases.     

Single-Molecule Imaging and Spectroscopy Using Fluorescence and Surface-Enhanced Raman Scattering

We extended single molecule fluorescence imaging and time-resolved fluorometry from the green to the violet-excitation regime to find feasibility of detecting and identifying fluorescent analogs of nucleic-acid bases at the single-molecule level. Using violet excitation, we observed fluorescent spotsfrom single complexes composed of a nucleotide analogue and the Klenow fragmentof DNA polymerase I. Also, we implemented Raman imaging and spectroscopy of adenine molecules adsorbed on Ag colloidal nanoparticles to find feasibility of identifying nucleic-acid bases at the single-molecule level. Surface enhanced Raman scattering (SERS) of adenine molecules showed an intermittent on-and-off behavior called blinking. The observation of blinking provides substantial evidence for detecting single adenine molecules.     

Highly reproducible immunoassay of cancer markers on a gold-patterned microarray chip using surface-enhanced Raman scattering imaging

This paper reports a highly reproducible immunoassay of cancer markers using surface-enhanced Raman scattering (SERS) imaging. SERS is a highly sensitive detection method but it is limited in its ability to achieve reproducible signal enhancement because of the difficulty with precisely controlling the uniform distribution of hot junctions. Consequently, inconsistent enhancement prevents the wide exploitation of SERS detection as a bio-detection tool for quantitative analysis. To resolve this problem, we explored the use of a SERS imaging-based immunoassay. For this purpose, Raman reporter-labeled hollow gold nanospheres (HGNs), were manufactured and antibodies were immobilized onto their surfaces for targeting specific antigens. After the formation of sandwich immunocomplexes using these functional HGNs on the surfaces of gold patterned wells, the SERS mapping images were measured. For target protein markers, 12×9 pixels were imaged using a Raman mapping technique in the 0-10(-4) g/mL concentration range, and the SERS signals for 66 pixels were averaged. Here, the SERS imaging-based assay shows much better correlations between concentration and intensity than does the conventional point-based assay. The limits of detection were determined to be 0.1 pg/mL and 1.0 pg/mL for angiogenin (ANG) and alpha-fetoprotein (AFP), respectively. This detection sensitivity is increased by three or four orders of magnitude over that of conventional ELISA method. The detectable dynamic range for SERS imaging (10(-4)-10(-12) g/mL) is also much wider than that for ELISA (10(-6)-10(-9) g/mL).     

Analysis of intracellular state based on controlled 3D nanostructures mediated surface enhanced Raman scattering

Near-infrared surface-enhanced Raman spectroscopy (SERS) is a powerful technique for analyzing the chemical composition within a single living cell at unprecedented resolution. However, current SERS methods employing uncontrollable colloidal metal particles or non-uniformly distributed metal particles on a substrate as SERS-active sites show relatively low reliability and reproducibility. Here, we report a highly-ordered SERS-active surface that is provided by a gold nano-dots array based on thermal evaporation of gold onto an ITO surface through a nanoporous alumina mask. This new combined technique showed a broader distribution of hot spots and a higher signal-to-noise ratio than current SERS techniques due to the highly reproducible and uniform geometrical structures over a large area. This SERS-active surface was applied as cell culture system to study living cells in situ within their culture environment without any external preparation processes. We applied this newly developed method to cell-based research to differentiate cell lines, cells at different cell cycle stages, and live/dead cells. The enhanced Raman signals achieved from each cell, which represent the changes in biochemical compositions, enabled differentiation of each state and the conditions of the cells. This SERS technique employing a tightly controlled nanostructure array can potentially be applied to single cell analysis, early cancer diagnosis and cell physiology research.     

Raman and surface enhanced Raman spectroscopy of 2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy-3-carboxamide labeled proteins: bovine serum albumin and cytochrome c

2,2,5,5-Tetramethyl-3-pyrrolin-1-yloxy-3-carboxamide (tempyo) labeled bovine serum albumin and cytochrome c at different pH values were prepared and investigated using Raman-resonance Raman (RR) spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy. The Raman spectra of tempyo labeled proteins in the pH 6.7-11 range were compared to those of the corresponding free species. The SERS spectra were interpreted in terms of the structural changes of the tempyo labeled proteins adsorbed on the silver colloidal surface. The tempyo spin label was found to be inactive in the Raman-RR and SERS spectra of the proteins. The alpha-helix conformation was concluded to be more favorable as the SERS binding site of bovine serum albumin. In the cytochrome c the enhancement of the bands assigned to the porphyrin macrocycle stretching mode allowed the supposition of the N-adsorption onto the colloidal surface.     

Dual-mode probe based on mesoporous silica coated gold nanorods for targeting cancer cells

A dual-mode imaging probe for targeting cancer cells has been fabricated based on mesoporous silica coated gold nanorods (MS-GNRs) for the first time. In this probe, fluorescence and surface enhanced Raman scattering (SERS) signals can be generated independently by using different excitation wavelengths. To investigate the targeting performance of the probe, folic acid (FA) is conjugated on the outer surfaces of MS-GNRs as a targeting ligand and HeLa cells were used as model cancer cells because they overexpress folate receptors (FRs). The endocytosis mechanism was verified by competing experiments with free FA through both fluorescence images and SERS mappings. Moreover, the cytotoxicity of the probe was remarkably reduced in comparison with the GNRs without the silica shell as proved by the results of MTT assay. Compared with traditional imaging probes, this new type of nanoprobe has great potential for multiplexed imaging in living cells, which can be easily realized by using fluorescence and SERS signals.     

Interface-Induced Ag Monolayer Film for Surface-Enhanced Raman Scattering Detection of Water-Insoluble Enrofloxacin

Water-insoluble molecules usually show poor surface-enhanced Raman scattering (SERS) signals, because they are hardly adsorbed on the surface of most commonly used SERS substrates, such as aqueous Ag or Au colloids. In this work, a highly sensitive and reproducible Ag monolayer film (Ag MLF) SERS substrate prepared by self-assembly of Ag nanoparticles (Ag NPs) on water/oil interface can realize the trace SERS detection of water-insoluble enrofloxacin. The positively charged phase transfer catalyst can transfer the negatively charged Ag nanoparticles in aqueous solution to the water/oil interface. At the same time, the water-insoluble enrofloxacin can also be attracted to the interface because of its lipophilic group. The type/volume of the oil phase and phase transfer catalyst and the vortex mixing time were all optimized to maximize the SERS effect of Ag MLF. Results showed that trace water-insoluble enrofloxacin can be identified by Ag MLF and its detection sensitivity was significantly improved. The proposed novel Ag MLF can be further applied to detect other water-insoluble molecules in SERS.

     

Ultra‐low background Raman sensing using a negative‐curvature fibre and no distal optics

Measuring Raman spectra through an optical fibre is usually complicated by the high intrinsic Raman scatter of the fibre material. Common solutions such as the use of multiple fibres and distal optics are complex and bulky. We demonstrate the use of single novel hollow‐core negative‐curvature fibres (NCFs) for Raman and surface‐enhanced Raman spectroscopy (SERS) sensing using no distal optics. The background Raman emission from the silica in the NCF was at least 1000× smaller than in a conventional solid fibre, while maintaining the same collection efficiency. We transmitted pump light from a 785‐nm laser through the NCF, and we collected back the weak Raman spectra of different distal samples, demonstrating the fibre probe can be used for measurements of weak Raman and SERS signals that would otherwise overlap spectrally with the silica background. The lack of distal optics and consequent small probe diameter (<0.25 mm) enable applications that were not previously possible.      

A new protein A assay based on Raman reporter labeled immunogold nanoparticles

A unique, sensitive, highly specific, and photobleaching-resistant immunoassay system utilizing gold nanoparticles and surface-enhanced Raman scattering (SERS) is described. This new system, featuring a capability of bifunctional analysis, is manufactured by chemisorption of antibody immunoglobulin G (IgG) on gold nanoparticles (AuNP), followed by coupling the Raman-active reporter molecule, 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) to the surface of IgG-AuNP. The adsorbed DTNB molecules exhibit strong Raman signals via both electromagnetic and chemical enhancement. The narrow spectral widths and high photostability assure the system to be an excellent detection label. This SERS-based immunoassay is applied to the detection of protein A, which is a specific surface antigen of Staphylococcus aureus. A working curve is obtained by plotting the intensity of the SERS signal of symmetric NO(2) stretching of DTNB at 1333cm(-1) versus the concentration of the analyte (antigen). A dynamic range of two to three orders of magnitude and a detection limit of 1pg/mL of protein A are achieved.     

Characterization of multilayer-enhanced surface-enhanced raman scattering (SERS) substrates and their potential for SERS nanoimaging

Multilayer gold surface-enhanced Raman scattering (SERS) substrates, which consist of continuous gold films that are separated by self-assembled monolayers (SAMs) and cast over 430-nm diameter silica nanospheres on a glass slide, have been evaluated as a means of further enhancing the SERS signals produced from conventional metal film over nanostructure substrates. Evaluation of the effect of various SAMs, with different terminal functional groups, on the SERS enhancement factor were measured and compared to conventional single-layer gold film over nanostructure substrates, revealing relative enhancements as great as 22.4-fold in the case of 2-mercapto-ethanol spacer layers. In addition to evaluation of the effect of different terminal functionalities, the effect of spacer length was also investigated, revealing that the shorter chain length alcohols provided the greatest signals. Employing the optimal SERS multilayer geometry, SERS nanoimaging probes were fabricated and the SERS enhancement factor and variability in enhancement factor were measured over the SERS active imaging area, providing absolute enhancements similar to previous silver-based SERS nanoimaging probes (i.e., 1.2 × 10⁸). Varying the size of the multilayer gold islands that were deposited on the tip of the SERS active nanoimaging probe, it is possible to tune the optimal SERS excitation wavelength accurately and predictably over the range of approximately 450 to 600 nm, without coating the entire surface of the probe and significantly reducing the transmission and resulting signal-to-noise ratio of the images obtained.