A three‐dimensional surface‐enhanced Raman scattering substrate: Au nanoparticle supramolecular self‐assembly in anodic aluminum oxide template

A three‐dimensional surface‐enhanced Raman scattering (SERS) substrate via the self‐assembly of properly sized Au nanoparticles in anodic aluminum oxide templates was designed and prepared. Au nanoparticles first underwent hydrophobic surface modification. Then, the hydrophobic Au nanoparticles self‐assembled, aggregated and formed many hot spots in the anodic aluminum oxide templates through a supramolecular interaction. We chose thiophenol as a probe molecule to evaluate the SERS enhancement ability of this three‐dimensional substrate. The enhancement factor was calculated to be 4.6 × 10⁶ under the radiation of a 785‐nm laser. By further comparing SERS signals from different points on the same substrate, we confirmed that this substrate possessed good reproducibility and could be applied for SERS detection. Copyright © 2011 John Wiley & Sons, Ltd.     

Trace analysis of polycyclic aromatic hydrocarbons using calixarene layered gold colloid film as substrates for surface‐enhanced Raman scattering

A surface‐enhanced Raman scattering (SERS) active substrate for the detection of polycyclic aromatic hydrocarbons (PAHs) was developed, which used 25, 27‐dimercaptoacetic acid‐26, 28‐dihydroxy‐4‐terbutyl calix[4]arene (DMCX) to functionalize a gold colloid film. This SERS‐active substrate prepared by self‐assembly method exhibits a high sensitivity, especially for the detection of PAHs. With the use of this SERS‐active substrate and with the application of the shifted excitation Raman difference spectroscopy (SERDS) technique, Raman signals of pyrene and anthracene in aqueous solutions at low concentration level (500 pM) can be obtained. Moreover, because PAHs are blocked from being directly adsorbed on gold colloid by DMCX and the photochemical reactions of adsorbates are avoided, the Raman bands of PAHs adsorbed on DMCX‐fuctionalized gold colloid film can be one‐to‐one correspondence with those of solid PAHs, and additionally, this SERS‐active substrate can be easily cleaned and reused. The obtained results demonstrate that the DMCX‐functionalized gold colloid films prepared by self‐assembly method have great potential to be developed to an in situ PAHs detection substrate. Copyright © 2012 John Wiley & Sons, Ltd.     

An improved self‐assembly gold colloid film as surface‐enhanced Raman substrate for detection of trace‐level polycyclic aromatic hydrocarbons in aqueous solution

To detect trace‐level polycyclic aromatic hydrocarbons, some investigations of an improved self‐assembly method are carried out using gold colloid films for the preparation of the surface‐enhanced Raman scattering (SERS)‐active substrate. Extinction spectra and scanning electron microscopy images reveal that controllable surface plasmonic metal substrates can be obtained by increasing the temperature of (3‐aminopropyl)trimethoxysilane solution up to 64.5 °C. The SERS‐active substrates have a high enhancement factor, and they can be both easily prepared and reproducible. With the use of these substrates, different concentrations of pyrene and anthracene in aqueous solutions were detected by SERS. A further enhancement can be supported by shifted excitation Raman difference spectroscopy. Raman signals of pyrene and anthracene adsorbed on gold colloid substrates up to limits of detection at 5 and 1 nmol/l, respectively, can be obtained. The quantitative analysis shows the possibility of in situ detection of polycyclic aromatic hydrocarbons while such gold colloid film serves as a SERS‐active substrate. Copyright © 2012 John Wiley & Sons, Ltd.     

Self‐assembled silver nanoparticle monolayer on glassy carbon: an approach to SERS substrate

In this paper, the fabrication of an active surface‐enhanced Raman scattering (SERS) substrate by self‐assembled silver nanoparticles on a monolayer of 4‐aminophenyl‐group‐modified glassy carbon (GC) is reported. Silver nanoparticles are attached to the substrate through the electrostatic force between the negatively charged silver nanoparticles and the positively charged 4‐aminophenyl groups on GC. The active SERS substrate has been characterized by means of tapping‐mode atomic force microscopy (AFM), indicating that large quantities of silver nanoparticles are uniformly coated on the substrate. Rhodamine 6G (R6G) and p‐aminothiophenol (p‐ATP) are used as the probe molecules for SERS, resulting in high sensitivity to the SERS response, with the detection limit reaching as low as 10⁻⁹ M . This approach is easily controlled and reproducible, and more importantly, can extend the range of usable substrates to carbon‐based materials for SERS with high sensitivity. Copyright © 2007 John Wiley & Sons, Ltd.     

Rapid atto‐molar level detection of surface‐enhanced Raman spectroscopy technique based on glycidyl methacrylate–ethylene dimethacrylate (GMA–EDMA) porous material

A novel miniature device for rapid ultra‐sensitive surface‐enhanced Raman scattering (SERS) detection was developed in the present study. The device was made of a syringe, a piece of filter, and a Teflon tube. Therefore, it was with advantages of simplicity, miniaturization, and easy operability. The tube was filled in advance with the glycidyl methacrylate‐ethylene dimethacrylate powder porous material which has been proved to increase the sensitivity of normal SERS dramatically, then the mixture solution containing the analyte, silver colloid, and NaCl solution passed through the porous material by the action of the syringe. SERS signals were collected from the surface of the material. Rhodamine 6G (R6G), p‐aminothiophenol (PATP), and thiabendazole (TBZ) were employed as the probe molecules in the present work. R6G at microlitre‐scale can be detected at an extremely low concentration of 10^–18 mol/l, and the relative standard deviation of spot to spot is 14.16% at the intensity of the band at 609 cm⁻¹. The concentrations of PATP and TBZ that can be detected with the method are 10⁻¹¹ mol/l and 1.3 × 10⁻⁶ mol/l, respectively. This method not only has achieved the ultra‐sensitive detection of dye and pesticide but also realized the simple, rapid, and small sample quantity requirement detection, and it is of great potential use for lots of analytes. Copyright © 2013 John Wiley & Sons, Ltd.     

Multiscale electromagnetic SERS enhancement on self‐assembled micropatterned gold nanoparticle films

In this work, we demonstrate a cascaded, multiplicative electromagnetic enhancement effect in surface‐enhanced Raman scattering (SERS) on periodically micropatterned films made of colloidal gold nanoparticles, prepared by a self‐assembly approach, without implying lithography procedures. The multiplicative enhancement effect is obtained by combining surface plasmon near‐field enhancement due to nanoscale features with far‐field photonic coupling by periodic microscale features. The effect is observed for both internal Raman reporters (molecules attached to the Au colloids before their assembly) and external Raman probes (molecules adsorbed on the samples after film assembly). The ability of the patterned films for far‐field light coupling is supported by reflectivity spectra, which present minima/maxima in the visible spectral range. Finite‐difference time‐domain computer simulations of the electric field distribution also support this interpretation. The fabricated dual‐scale SERS substrates exhibit a good spot‐to‐spot reproducibility and time stability, as proved by the SERS response over a time scale longer than 1 month. The experimental demonstration of this cascaded electromagnetic enhancement effect contributes to a better understanding of SERS and can affect future design of SERS substrates. Moreover, such dual‐scale colloidal films prepared by convective self‐assembly can be of general interest for the broader field of nanoparticle‐based devices. Copyright © 2014 John Wiley & Sons, Ltd.     

On‐line surface enhanced Raman spectroscopic detection in a recyclable Au@SiO2 modified glass capillary

A facile method was developed to fabricate a high sensitive, reproducible and recyclable surface enhanced Raman spectroscopy (SERS) active glass capillary. The Au nanoparticles were synthesized through a seed‐mediated growth approach and then self‐assembled onto the inner wall of glass capillaries. The attached Au nanoparticles were homogeneously coated with thin silica shell by using the silane coupling agent to functionalize the Au surface. By using thiophenol (TP) as SERS probe molecules, the substrate exhibited robust SERS effects. The adsorbed SERS probe molecules could be rapidly and completely removed away by flowing sodium borohydride solution and thus to obtain a refresh Au@SiO₂ film‐coated substrate for the cyclic detection on different species. The on‐line detection of TP and malachite green (MG) with different concentrations was performed in the flowing system. The intensities of SERS signals were dependent on concentrations of the detected molecules. The results indicated that the SERS‐active substrate has potential applications on the on‐line qualitative and quasi‐quantitative analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanoporous Ag–GaN thin films prepared by metal‐assisted electroless etching and deposition as three‐dimensional substrates for surface‐enhanced Raman scattering

Three‐dimensional (3D) nanoporous gallium nitride (PGaN) scaffolds are fabricated by Pt‐assisted electroless hydrofluoric acid (HF) etching of crystalline GaN followed by in situ electroless deposition of Ag nanostructures onto the interior surfaces of the nanopores, yielding a large surface area substrate for surface‐enhanced Raman scattering (SERS). The resulting 3D SERS‐active substrates have been optimized by varying reaction parameters and starting material concentration, exhibiting enhanced Raman signals 10–100× more intense than either (1) sputtered Ag‐coated porous GaN or (2) Ag‐coated planar GaN. The increase in SERS signal is attributed to a combination of the large surface area and the inherent transparency of PGaN in the visible spectral region. Overall, Ag‐decorated PGaN is a promising platform for high sensitivity SERS detection and chemical analysis, particularly for reaction and metabolic products that can be trapped inside the highly anisotropic nanoscale pores of PGaN. The potential of this sampling mode is illustrated by the ability to acquire Raman spectra of adenine down to 5 fmol. Additionally, correlated SERS and laser desorption/ionization mass spectrometry spectra can be acquired from same sample spot without further preparation, opening new possibilities for the investigation of surface‐bound molecules with substantially enhanced information content. Copyright © 2012 John Wiley & Sons, Ltd.     

Complex concentration dependence of SERS and UV–Vis absorption of glycine/Ag‐substrates because of glycine‐mediated Ag‐nanostructure modifications

Complex concentration‐dependence of surface‐enhanced Raman scattering (SERS) and UV–Vis absorption of Ag‐nanoparticles (AgNPs) mixed with Gly has been observed. Surprisingly, with decreasing Gly concentration, a new band in UV–Vis absorption of AgNPs/Gly mixtures is found to red‐shift with increasing intensity, until a turning point at a critical concentration. Further diluting Gly, the new band blue‐shifts with decreasing intensity. Similarly, the SERS intensities of Gly bands at 615 and 905 cm^–1 consistently increase with decreasing Gly concentrations, reaching maxima at the critical concentration. This agrees consistently with the variation in position and intensity of the new developing plasmon absorption band. Interestingly, transmission electron microscopy (TEM) revealed Gly‐induced modifications of AgNPs, including a reassembling and increasing aspect ratio with deceasing Gly concentration. The concentration‐dependent behavior of UV–Vis absorption, SERS, and TEM of AgNPs/Gly mixtures could be due to the complex nature of Gly‐AgNPs interaction depending on the molecular density, as supported by TEM images. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering‐based approach for DNA detection at low concentrations via polyvinyl alcohol‐protected silver grasslike patterns

A simple method is demonstrated to detect DNA at low concentrations on the basis of surface‐enhanced Raman scattering (SERS) via polyvinyl alcohol‐protected silver grasslike patterns (PVA‐Ag GPs) grown on the surface of the common Al substrate. By the SERS measurements of sodium citrate and thymine, the PVA‐Ag GPs are shown to be an excellent SERS substrate with good activity, stability and reproducibility. With the use of the tested molecule of thymine, the enhancement factor of the PVA‐Ag GPs is up to ~1.4 × 10⁸. The PVA‐Ag GPs are also shown to be an excellent SERS substrate with good biocompatibility for DNA detection, and the detection limit is down to ~10⁻⁵ mg/g. Meanwhile, the assignations of the Raman bands and the adsorption behaviors of the DNA molecules are also analyzed. In this work, the geometry optimization and the wavenumber analysis of adenine–Ag and guanine–Ag complexes for the ground states are performed using density functional theory, B3LYP functional and the LanL2DZ basis set. The transition energies and the oscillator strengths of adenine–Ag and guanine–Ag for the lowest six singlet excited states were calculated by using the time‐dependent density functional theory method with the same functional and basis set. The results show that the charge transfer in the adenine–Ag and guanine–Ag complexes should be the chemical factor for the SERS of the DNA molecules. Lastly, this method may be employed in large‐scale preparation of substrates that have been widely applied in the Raman analysis of DNA because the fabrication process is simple and inexpensive. Copyright © 2011 John Wiley & Sons, Ltd.     

The role of the dielectric environment in surface‐enhanced Raman scattering on the detection of a 4‐nitrothiophenol monolayer

Nanotechnology enables the generation and characterization of novel surface‐enhanced Raman scattering (SERS) substrates. In this study, we focus on the impact of the carrier material of the SERS active layer and hence the dielectric environment to the enhancement. Therefore, a self‐assembled monolayer of 4‐nitrothiophenol is immobilized on silver and gold particles substrates on a quartz carrier. The detection of the monolayer occurs through the quartz carrier and through air. For the former, an increase of the intensity of the SERS bands in the spectrum is observed compared to the latter. The magnitude of the increase is larger for gold than for silver. Calculations according to the theoretical model of the electromagnetic enhancement agree with our experimental data. The presented detection mode will stimulate the fabrication of novel SERS sensors. Copyright © 2013 John Wiley & Sons, Ltd.     

Graphenic Aerogels Decorated with Ag Nanoparticles as 3D SERS Substrates for Biosensing

A versatile and efficient surface-enhanced Raman scattering (SERS) substrate based on a hybrid aerogel composed of reduced graphene oxide (rGO) decorated with silver nanoparticles (AgNPs), suitable for highly sensitive label-free detection of chemical and biological species, is presented. The simple and low-cost one-pot hydrothermal synthesis allows obtaining of a 3D nanostructured spongy-like matrix that shows good spatial distribution of Ag nanoparticles in intimate contact with rGO flakes, characterized by means of several morphological, structural, and compositional techniques. The nanostructured material, tested by SERS analysis with both rhodamine 6G (R6G) and 4-mercaptobenzoic acid (MBA), shows a satisfying SERS efficiency, quantified in terms of minimum detectable concentration of 10⁻¹⁰ and 10⁻⁷ m , corresponding to on- and off-resonant excitation, respectively. The versatility of chemical/biochemical functionalization is successfully demonstrated by exploiting different routes, by immobilizing both protoporphyrin IX (PRPIX) and hemin (H) that take advantage of π−π non-covalent bonding with the graphene layers, as well as thiol-ended oligonucleotides (DNA probes/aptamers) directly grafted on the AgNPs. Finally, after the successful integration of the hybrid aerogel into a microfluidic chip, the biorecognition of miR222 is obtained demonstrating the reliability of the aerogel substrate as SERS platform for biosensing.     

A facile method for the synthesis of large‐size Ag nanoparticles as efficient SERS substrates

Silver nanoparticles (Ag NPs) enjoy a reputation as an ultrasensitive substrate for surface‐enhanced Raman spectroscopy (SERS). However, large‐scale synthesis of Ag NPs in a controlled manner is a challenging task for a long period of time. Here, we reported a simple seed‐mediated method to synthesize Ag NPs with controllable sizes from 50 to 300 nm, which were characterized by scanning electron microscopy (SEM) and UV–Vis spectroscopy. SERS spectra of Rhodamine 6G (R6G) from the as‐prepared Ag NPs substrates indicate that the enhancement capability of Ag NPs varies with different excitation wavelengths. The Ag NPs with average sizes of ~150, ~175, and ~225 nm show the highest SERS activities for 532, 633, and 785‐nm excitation, respectively. Significantly, 150‐nm Ag NPs exhibit an enhancement factor exceeding 10⁸ for pyridine (Py) molecules in electrochemical SERS (EC‐SERS) measurements. Furthermore, finite‐difference time‐domain (FDTD) calculation is employed to explain the size‐dependent SERS activity. Finally, the potential of the as‐prepared SERS substrates is demonstrated with the detection of malachite green. Copyright © 2016 John Wiley & Sons, Ltd.     

Photochemical decoration of silver nanoparticles on ZnO nanowires as a three‐dimensional substrate for surface‐enhanced Raman scattering measurement

To increase the sensitivity in surface‐enhanced Raman scattering (SERS) measurement, a three‐dimensional (3D) SERS substrate was prepared by the decoration of silver nanoparticles (AgNPs) on the side walls of ZnO nanowires. The prepared 3D SERS substrates provide the advantages of highly loaded density of AgNPs, with a large specific surface area to interact with analytes, and the ease for the analytes to access the surfaces of AgNPs. To prepare the substrates, ZnO nanowires were first grown on a glass plate by wet chemical method. By treating SnCl₂ on the surfaces of ZnO nanowires, Ag seeds could be formed on the side wall of the ZnO nanowires, which were further grown to a suitable size for SERS measurements via photochemical reduction. To optimize and understand the influences of the parameters used in preparation of the substrates, the reaction conditions were systematically adjusted and examined. Results indicated that AgNPs could be successfully decorated on the side wall of the ZnO nanowires only by the assistances of SnCl₂. The size and density of AgNPs were affected by both the concentration of silver nitrate and the irradiation time. With optimized condition, the prepared 3D substrates provided an enhancement factor approaching 7 orders of magnitude compared with conventional Raman intensity. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering sensing on black silicon

Reactive ion etching was used to fabricate black‐Si over the entire surface area of 4‐inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300–1000 nm spectral range. The spikes of the black‐Si substrates were coated by gold, resulting in an island film for surface‐enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10^?6 M (at count rate > 10² s^?1 . mW^?1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ～ 100‐nm‐thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile‐on‐immobile platform for SERS sensing is introduced by using dog‐bone Au nanoparticles on the Au/black‐Si substrate. The SERS intensity shows a non‐linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications.     

Analysis of self‐repair mechanisms of Phaseolus vulgaris var. saxa using near‐infrared surface‐enhanced Raman spectroscopy

We used surface‐enhanced Raman spectroscopy (SERS) to investigate ultrastructural changes in cell‐wall composition during the self‐repair of lacerated hypocotyls of Phaseolus vulgaris var. saxa. A detailed study of self‐repair mechanisms requires localized information about cell‐wall structure and morphology in addition to the chemical cell‐wall composition. Characteristic Raman and SER spectra yielded two‐dimensional maps of cross sections of P. vulgaris var. saxa visualizing chemical compositions in the walls of different cell types and during various repair phases. SERS substrate particles were produced by the reduction of gold chloride on the plant tissue surface and characterized with absorption spectroscopy, scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The SERS results were compared with stained cross sections of the same plant using dark‐field microscopy with focus on lignin and suberin contents in repairing cells. In addition, SERS measurements revealed Au cyanide compounds on the cell surface, indicating the formation of hydrogen cyanide during the self‐repair phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Improved performance of surface‐enhanced Raman scattering on silver substrates prepared in nitric acid solutions

In this work, we propose a new electrochemical method to prepare surface‐enhanced Raman scattering (SERS)‐active silver substrates in nitric acid solutions. Experimental results indicate that the SERS intensity of adsorbed Rhodamine 6G (R6G) can be significantly increased, as compared with that of R6G adsorbed on a SERS‐active Ag substrate prepared by an electrochemical method in a chloride‐containing solution, which was generally employed in the literature. Moreover, the SERS of R6G on the newly developed substrate (prepared in a nitric acid solution) still performs well at a high temperature of 250 °C. However, the enhancement capability of the SERS‐active substrate prepared in a chloride‐containing solution is seriously destroyed at temperatures higher than 150 °C. Further investigations indicate that the oxidation states of roughened Ag substrates prepared in nitric acid solutions under different experiment conditions have less influence on the corresponding SERS performances. Instead, different surface morphologies of roughened Ag substrates and different contents of nitrogen‐containing dopping ions on the roughened Ag substrates demonstrate significant effects on the corresponding SERS performances. Copyright © 2011 John Wiley & Sons, Ltd.     

Improved performances on surface‐enhanced Raman scattering based on electrochemically roughened gold substrates modified with SiO2 nanoparticles

In this work, we use electrochemical oxidation–reduction cycles (ORC) methods to prepare surface‐enhanced Raman scattering (SERS)‐active gold substrates modified with SiO₂ nanoparticles to improve the corresponding SERS performances. Based on the modified substrates, the SERS of Rhodamine 6G (R6G) exhibits a higher intensity by 3‐fold of magnitude, as compared with that of R6G adsorbed on a SERS‐active Au substrate without the modification of SiO₂ nanoparticles. Moreover, the SERS enhancement capabilities of the modified and the unmodified Au substrates are seriously destroyed at temperatures higher than 250 and 200 °C, respectively. These results indicate that the modification of SiO₂ nanoparticles can improve the thermal stability of SERS‐active substrates. The aging in SERS intensity is also depressed on this modified Au substrate due to the contribution of SiO₂ nanoparticles to SERS effects. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of the potential tumor marker of AFP using surface‐enhanced Raman scattering‐based immunoassay

The development of rapid, highly sensitive detection methods for α‐fetoprotein (AFP) is very important. As hepatocellular carcinoma is closely related to the level of AFP in the blood, it is necessary to maintain an AFP concentration below the safety limit. In this paper, we propose a universal, rapid, sensitive, and highly specific immunoassay system utilizing gold nanoparticles (AuNPs) and surface‐enhanced Raman scattering (SERS). This new system features a sandwich structure combining mercaptobenzoic acid‐labeled immunogold nanoparticles with the antigen and the antibody atop a pre‐designed substrate made of a glass slide modified with AuNPs. This SERS‐based immunoassay can detect AFP concentrations as low as 100 pg/ml, which is a significant improvement on the capabilities of the enzyme‐linked immunosorbent assay method. A good linear relationship between the SERS peak intensity and the logarithm of antigen concentrations (from 1 ng/ml to 100 ng/ml) was observed. This technique provides an effective model for the detection of biomarkers in medical diagnostics, criminal investigation, and other fields. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy for trace‐level detection of explosives

The detection of explosives and their associated compounds for security screening is an active area of research and a wide variety of detection methods are involved in this very challenging area. Surface‐enhanced Raman scattering (SERS) spectroscopy is one of the most sensitive tools for the detection of molecules adsorbed on nano‐scale roughened metal surface. Moreover, SERS combines high sensitivity with the observation of vibrational spectra of species, giving complete information on the molecular structure of material under study. In this paper, SERS was applied to the detection of very small quantities of explosives adsorbed on industrially made substrates. The spectra were acquired with a compact Raman spectrometer. Usually, a high signal‐to‐noise (S/N) spectrum, suitable for identification of explosive molecules down to few hundreds of picograms, was achieved within 30 s. Our measurements suggest that it is possible to exploit SERS using a practical detection instrument for routine analysis. Copyright © 2010 John Wiley & Sons, Ltd.