Facile synthesis of gelatin‐protected silver nanoparticles for SERS applications

Gelatin‐protected silver nanoparticles have been synthesized by a one‐pot, green method for surface‐enhanced Raman scattering (SERS) applications using gelatin as the reducing and stabilizing agent. The gelatin protection on silver nanoparticle surface helps improve its stability greatly and water dispersibility, while retaining high SERS activity of silver nanoparticles. The gelatin‐protected silver nanoparticles showed SERS signals as low as 100 nM of the typical Raman reporter molecules, RuBPY and R6G and 10 μM of other molecules of interest, melamine and folic acid. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical probing and imaging of live cells using SERS labels

During surface‐enhanced Raman scattering (SERS), molecules exhibit a significant increase in their Raman signals when attached, or in very close vicinity, to gold or silver nanostructures. This effect is exploited as the basis of a new class of optical labels. Here we demonstrate robust and sensitive SERS labels as probes for imaging live cells. These hybrid labels consist of gold nanoparticles with Rose Bengal or Crystal Violet attached as reporter molecules. These new labels are stable and nontoxic, do not suffer from photobleaching, and can be excited at any excitation wavelength, even in the near infrared. SERS labels can be detected and imaged through the specific Raman signatures of the reporters. In addition, surface‐enhanced Raman spectroscopy in the local optical fields of the gold nanoparticles also provides sensitive information on the immediate molecular environment of the label in the cell and allows imaging of the native constituents of the cell. This is demonstrated by images based on a characteristic Raman line of the reporter as well as by displaying lipids based on the SERS signal of the C H deformation/bending modes at ∼1470 cm⁻¹. Copyright © 2008 John Wiley & Sons, Ltd.     

荧光/表面增强拉曼散射双模式纳米光学探针的构建及性能研究

提出一种新型的荧光及表面增强拉曼散射（SERS）双模式光学纳米探针。首先,通过再沉淀-包覆法合成二氧化硅包覆香豆素6的纳米颗粒,再在二氧化硅表面静电吸附多聚赖氨酸分子形成包覆层,随后通过原位还原的方法在多聚赖氨酸壳层复合银纳米颗粒,最后在银纳米颗粒表面吸附拉曼分子即形成双模式纳米探针。该探针通过二氧化硅包覆的荧光分子产生荧光信号,以多聚赖氨酸表面的银纳米颗粒作为SERS增强基底,利用拉曼分子获得SERS信号,实现了荧光及SERS双模式成像。荧光与表面增强拉曼散射相结合的双模式分析技术可同时发挥二者的优点,提高成像的分辨率和灵敏度,在生物医学领域具有重要的应用价值。     

Surface enhanced Raman scattering of molecules related to highly ordered gold cavities

We presented a controlled particles‐in‐cavity (PIC) pattern for surface‐enhanced Raman scattering (SERS) detection. The periodic gold cavity array was fabricated by electrodeposition using highly ordered polystyrene spheres as a template. The as‐prepared gold cavities can be used as a SERS active substrate with significant spectral enhancement and reproducibility, which was evaluated by SERS signals using 4‐mercaptobenzoic acid (4‐MBA) as probe molecules. The surface of these gold cavities was further functionalized with cetyltrimethylammonium bromide molecules, which may immobilize the 4‐MBA‐modified silver nanoparticles in the gold cavity to form a PIC structure via the electrostatic interaction. We have demonstrated that there exists a pH window for the immobilization of the nanoparticles inside cavities. Therefore, the silver nanoparticles can be selectively immobilized into the functionalized gold cavities under the optimized pH value of the media. Further enhancement of the Raman scattering of the labeled molecules can be achieved due to the interconnection between the silver nanoparticles and gold cavity. Copyright © 2013 John Wiley & Sons, Ltd.     

Thiol‐immobilized silver nanoparticle aggregate films for surface enhanced Raman scattering

We report a novel method for the fabrication of films of silver nanoparticle aggregates that are strongly attached to Si substrates (Thiol‐immobilized silver nanoparticle aggregates or TISNA). The attachment is achieved by chemically modifying the surface of a Si(100) surface in order to provide SH groups covalently linked to the substrate and then aggregating silver nanoparticles on these thiol covered surfaces. The transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) characterization show a high coverage with single nanoparticles or small clusters and a partial coverage with fractal aggregates that provide potential hot spots for surface enhanced Raman scattering (SERS). We have confirmed the SERS activity of these films by adsorbing rhodamine 6G and recording the Raman spectra at several concentrations. By using the silver‐chloride stretching band as an internal standard, the adsorbate bands can be normalized in order to correct for the effects of focusing and aggregate size, which determine the number of SERS active sites in the focal area. This allows a quantitative use of SERS to be done. The adsorption–desorption of rhodamine 6G on TISNA films is reversible. These features make our TISNA films potential candidates for their use in chemical sensors based on the SERS effect. Copyright © 2008 John Wiley & Sons, Ltd.     

Visible laser–induced photoreduction of silver 4‐nitrobenzenethiolate revealed by Raman scattering spectroscopy

We have investigated the photochemical characteristics of silver 4‐nitrobenzenethiolate (Ag‐4NBT) by means of Raman spectroscopy. When Ag‐4NBT is irradiated with an argon ion laser at 514.5 nm, its Raman spectrum changes over time, resulting in the production of 4NBT‐capped silver nanoparticles. The surface‐enhanced Raman scattering (SERS) spectrum of 4NBT adsorbed on those Ag nanoparticles is subsequently converted to that of 4‐aminobenzenethiol (4ABT). These surface‐induced photoreduction characteristics were investigated by monitoring the growth of Raman peaks of 4ABT as a function of the laser exposure time. Water vapor or ambient conditions were more effective than vacuum conditions for the photoreduction of 4NBT to 4ABT. Nonetheless, the occurrence of photolysis even under vacuum conditions suggests that the benzene ring hydrogen atoms might be the H‐atom source of the nitro‐to‐amine group conversion although in ambient conditions water or solvent molecules trapped inside the Ag‐4NBT should be the primary H‐atom source and facilitate the transfer of electrons, as well as the diffusion of Ag atoms to form highly SERS‐active nanoaggregates. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman evidence for Rhodamine 6 G and its derivative with different adsorption geometry to colloidal silver nanoparticle

Some high‐affinity functional groups or resonant molecules were often used as probe molecules adsorbed on silver nanoparticles for Surface‐enhanced Raman scattering (SERS). However, it is still unclear how the attached molecules interact with the silver nanoparticles' surface, and how the anchoring groups affect the optical and electronic properties of molecules. Here, we report that surface‐enhanced Raman studies of two organic compounds; rhodamine 6G (R6G) and its aminated derivative (R‐NH₂) have very different functional groups for surface binding but nearly identical SERS spectroscopic properties at pH = 7 and UV–vis at pH = 3, respectively. A surprise was found that under the same experimental conditions, the SERS signal intensity for R6G is nearly 50‐fold higher than that of R‐NH₂. Furthermore, the pH‐dependent study reveals that the structure of R6G is irreversibly stabilized or ‘locked’ in its form and no longer responsive to pH changes. In contrast, R‐NH₂ is still sensitive to pH, and can be switched between its open‐ring and closed‐ring structures. Copyright © 2013 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.     

Surface‐enhanced Raman scattering on colloid gels originated from low molecular weight gelator

Surface‐enhanced Raman scattering (SERS) on silver and gold colloid gels formed by a low molecular weight organic gelator, bis‐(S‐phenylalanine) oxalyl amide, was obtained. Strong Raman signals dominate in the SERS spectra of hydrogels containing silver nanoparticles prepared by citrate and borohydride reduction methods, whereas broad bands of low intensity are detected in the spectra of gold colloid gels. Resemblance between Raman spectrum of the crystalline substance and the SERS spectra of the silver nanoparticle–hydrogel composites implies the electromagnetic nature of the signal enhancement. A change in Raman intensity of the benzene and amide II bands caused by an increase in temperature and concentration indicates that the gelling molecules are strongly attached through the benzene moieties to the metal nanoparticles while participating in gel formation by intermolecular hydrogen bonding between the adjacent oxalyl amide groups. Transmission electron microscopy reveals a dense gel structure in the close vicinity of the enhancing metal particles for both silver colloid gels. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of calixarene to high active surface‐enhanced Raman scattering (SERS) substrates suitable for in situ detection of polycyclic aromatic hydrocarbons (PAHs) in seawater

The characteristics of the sol–gel matrix embedding Ag nanoparticles functionalized with 25,27‐dimercaptoacetic acid‐26,28‐dihydroxy‐4‐tert‐butylcalix[4]arene (DMCX) suitable for the in situ detection of polycyclic aromatic hydrocarbons (PAHs) in seawater is presented. The DMCX‐functionalized silver nanoparticles were produced by the thermal reduction method in xerogel film. The silver colloid blocks were formed in the sol–gel matrix, with a diameter ranging from 50 to 120 nm. DMCX forming the monolayer on the silver nanoparticle surface contributes to the surface‐enhanced Raman scattering (SERS) activity due to the aggregation of silver nanoparticles and the preconcentration of PAH molecules within the zone of electromagnetic enhancement. When selected, PAH molecules e.g. pyrene and naphthalene were adsorbed onto the SERS substrate; Raman band positions of PAH were slightly shifted. A calibration procedure reveals that this type of SERS substrate has a limit of detection of 3 × 10⁻¹⁰ mol/l for pyrene and 13 × 10⁻⁹ mol/l for naphthalene in artificial seawater. The Raman signal response on a pyrene concentration change in artificial seawater was evaluated using a 671‐nm Raman setup with a flow‐through cell. This type of SERS substrate will be suitable for the in situ trace detection of pollutant chemicals in seawater. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman of indigo on a silver surface. Raman and theoretical characterization of indigo deposited on silicon dioxide-coated and uncoated silver nanoparticles

Raman, surface-enhanced Raman scattering, and shell isolated nanoparticles-enhanced Raman scattering techniques were used to study the indigo–nanoparticle interaction nature. Silver nanoparticles were employed with and without a silicon dioxide spacer inert layer. The SERS spectral profile, obtained using silver nanoparticles, is different from the Raman one, which led to the proposition that the indigo–silver interaction is in the range of intermolecular interactions. SERS spectral reproducibility suggests identical organization and orientation of the analyte on the metal surface. The shell isolated nanoparticles enhanced Raman scattering spectrum of indigo, obtained by using silicon dioxide coated silver nanoparticles resulted similar to its Raman spectrum. This result indicates that the indigo structure is chemically unmodified by the silicon dioxide-coated silver surface. From the shell-isolated nanoparticles-enhanced Raman scattering experiments, the electromagnetic mechanism is proposed as the reason for the spectral enhancement. Theoretical calculations allow one to infer both the indigo–silver surface interaction nature and the orientation of indigo on the surface.     

SERS studies of DNA nucleobases using new silver poly(methyl methacrylate) nanocomposites as analytical platforms

The implementation of polymer‐based composites provides a plausible alternative to develop efficient, handy and scalable substrates for surface‐enhanced Raman spectroscopy (SERS) aiming the widespread use of this technique for chemical analysis and molecular sensing. In this research, new poly(methylmethacrylate) based nanocomposites for SERS were prepared by in situ miniemulsion polymerization in the presence of organically capped metal silver nanoparticles. The ensuing composites have been investigated as analytical platforms for SERS detection of DNA constituents for variable analytical conditions. Finally, we show that in special cases, selective detection of DNA bases by SERS can be possible by varying the pH of the solution under analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Ag nanoparticles obtained by pulsed laser ablation in water: surface properties and SERS activity

We studied the surface properties and reactivity of silver nanoparticles obtained by picosecond or nanosecond pulsed laser ablation in water and with 1064‐nm wavelength. Ultraviolet–visible spectroscopy results and subsequent modelling by Mie theory indicated the presence of an oxide layer on the nanoparticle surface, which favours the colloidal stability, but reduces the interaction with the environment. The oxide layer is also responsible for the reduced surface enhanced Raman spectroscopy (SERS) activity of these colloids with respect to those obtained by chemical reduction. However, SERS activation can be efficiently obtained by addition of chloride ions to the colloids, leading to SERS enhancement factors that are comparable with those of the chemically prepared counterparts. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication and characterization of SERS‐active silver clusters on glassy carbon

In this article, a novel technique for the fabrication of surface enhanced Raman scattering (SERS) active silver clusters on glassy carbon (GC) has been proposed. It was found that silver clusters could be formed on a layer of positively charged poly(diallyldimethylammonium) (PDDA) anchored to a carbon surface by 4‐aminobenzoic acid when a drop containing silver nanoparticles was deposited on it. The characteristics of the obtained silver clusters have been investigated by atomic force microscopy (AFM), SERS and an SERS‐based Raman mapping technique in the form of line scanning. The AFM image shows that the silver clusters consist of several silver nanoparticles and the size of the clusters is in the range 80–100 nm. The SERS spectra of different concentrations of rhodamine 6G (R6G) on the silver clusters were obtained and compared with those from a silver colloid. The apparent enhancement factor (AEF) was estimated to be as large as 3.1 × 10⁴ relative to silver colloid, which might have resulted from the presence of ‘hot‐spots’ at the silver clusters, providing a highly localized electromagnetic field for the large enhancement of the SERS spectra of R6G. The minimum electromagnetic enhancement factor (EEF) is estimated to be 5.4 × 10⁷ by comparison with the SERS spectra of R6G on the silver clusters and on the bare GC surface. SERS‐based Raman mapping technique in the form of line scanning further illustrates the good SERS activity and reproducibility on the silver clusters. Finally, 4‐mercaptopyridine (4‐Mpy) was chosen as an analyte and the lowest detected concentration was investigated by the SERS‐active silver clusters. A concentration of 1.6 × 10⁻¹⁰ M 4‐Mpy could be detected with the SERS‐active silver clusters, showing the great potential of the technique in practical applications of microanalysis with high sensitivity. Copyright © 2006 John Wiley & Sons, Ltd.     

Visible light response of silver 4‐aminobenzenethiolate and silver 4‐dimethylaminobenzenethiolate probed by Raman scattering spectroscopy

Silver thiolate is a layered compound with a Raman spectrum that is known to change with time, becoming the same as the surface‐enhanced Raman scattering (SERS) spectrum of the parent thiol molecule adsorbed on Ag nanoparticles. On this basis, the Raman scattering characteristics of silver 4‐aminobenzenethiolate (Ag‐4ABT) compounds were investigated to determine whether certain peaks that are identifiable in the SERS spectrum of 4‐aminobenzenethiol (4‐ABT) but absent in its normal Raman spectrum were also apparent in the Ag salt spectrum. For comparative purposes, the Raman scattering characteristics of silver 4‐dimethylaminobenzenethiolate (Ag‐4MABT) were also examined. Raman spectra acquired while spinning the sample were typified by only a₁‐type vibrational bands of Ag‐4ABT and Ag‐4MABT, whereas in the static condition, several non‐a₁‐type bands were identified. The spectral patterns acquired in the static condition were similar to the intrinsic SERS spectra of 4‐ABT or 4‐dimethylaminobenzenethiol (4‐MABT) adsorbed on pure Ag nanoparticles. Notably, the CH₃ group vibrational bands were observable for Ag‐4MABT irrespective of the sample rotation. In addition, no decrease in intensity during irradiation with a visible laser was observed for any of the bands, suggesting that no chemical conversion actually took place in either 4‐ABT or 4‐MABT. The preponderance of evidence led to the conclusion that the non‐a₁‐type bands observable in the SERS spectra must be associated with the chemical enhancement mechanism acting on the Ag nanoparticles. The chemical enhancement effect was more profound at 514.5 nm than at 632.8 nm, and was more favorable for 4‐ABT than 4‐MABT at both wavelengths. Copyright © 2012 John Wiley & Sons, Ltd.     

Fiber‐optic SERS sensor with optimized geometry: testing and optimization

A unique, geometry‐optimized, surface‐enhanced Raman scattering (SERS) fiber‐optic sensor has been recently developed and built. Though this class of sensors can be very useful in many applications, their use is greatly hindered by the fact that their reusability can hardly be achieved because of the irreversible adsorption of the analyte molecules on the SERS‐active substrate. Different substrates have been tested on our sensor with the purpose of increasing its reusability by means of cleaning procedures or good reproducibility in manufacturing the sensor, keeping, however, the same enhancement. We show that a partial reusability of the sensor is possible using SERS‐active substrates prepared by a standard process of immobilization of silver nanoparticles with 3‐aminopropyltrimethoxysilane. We also show that a fairly good reproducibility can be achieved with a low‐cost substrate realized in a short time by depositing a layer of polyvinyl alcohol (PVA) containing silver nanoparticles on the etched fiber tip. We prove as well that measurements are possible even with nanoparticles dispersed in the analyte solution instead of using a substrate directly made on the sensor tip. Finally, we have successfully tested our sensor with some molecules cited in EFSA (European Food Safety Authority) and FDA (Food and Drug Administration) reports as molecules for which new detection methods are necessary. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy in living plant using triplex Au?Ag?C core–shell nanoparticles

This paper presents, for the first time, noninvasive imaging of a livingplant using biocompatible carbon‐encapsulated Au Ag nanoparticles (NPs) using micro‐Raman spectroscopy (MRS). A convenient and controllable hydrothermal synthetic route was developed to synthesize the layer‐by‐layer triplex Au Ag C core–shell NPs, which can incorporate the reporter molecule 4‐mercapto benzoic acid (4‐MBA). A unique approach was devised to deliver the carbon‐encapsulated surface‐enhanced Raman scattering (SERS) tags into the leaf of Nicotiana benthamiana. In vivo SERS mapping was subsequently performed to monitor the distribution of tags inside the leaf, which successfully avoided interference of autofluorescence from plant tissue. The imaging modality reported here and further the bio‐functionalized carbon‐encapsulated SERS NPshold significant potential as a strategy forbiochemical imaging in living plantsin a noninvasive and nontoxic manner, whichmight open up exciting opportunities for plant sciences. Copyright © 2010 John Wiley & Sons, Ltd.     

Bacterial mixture identification using Raman and surface‐enhanced Raman chemical imaging

The ability of normal Raman and surface‐enhanced Raman scattering (SERS) to identify and detect bacteria has shown great success in recent studies. The addition of silver nanoparticles to bacterial samples not only results in an enhanced Raman signal, but it also suppresses the native fluorescence associated with biological material. In this report, Raman chemical imaging (RCI) was used to analyze individual bacteria and complex mixtures of spores and vegetative cells. RCI uses every pixel or a binned pixel group (BPG) of the Raman camera as an independent Raman spectrograph, allowing collection of spatially resolved Raman spectra. The advantage of this technique resides primarily in the analysis of samples in complex backgrounds without the need for physically isolating or purifying the sample. Using a chemical imaging Raman microscope, we compare normal RCI to SERS‐assisted chemical imaging of mixtures of bacteria. In both cases, we are able to differentiate single bacterium in the Raman microscope's field of view, with a 60‐fold reduction in image acquisition time and a factor of 10 increase in the signal‐to‐noise ratio for SERS chemical imaging over normal RCI. Copyright © 2010 John Wiley & Sons, Ltd.     

Piperidine adsorption on two different silver electrodes: A combined surface enhanced Raman spectroscopy and density functional theory study

The surface enhanced Raman scattering (SERS) spectra of piperidine in silver colloid solution, on roughened silver electrode and on roughened silver electrode modified with silver nanoparticles were studied, and the high-quality SERS spectra of piperidine on roughened silver electrode modified with silver nanoparticles were obtained for the first time. Surface selection rules derived from the EM enhancement model were employed to deduce piperidine orientations on the different surfaces. On the basis of this, two models of piperidine adsorbed on the surface of the silver nanoparticles were built, and DFT-B3PW91/LanL2dz was applied to calculate the Raman frequencies. It proves that, at higher potential values, the piperidine is perpendicularly standing on the roughened silver electrode surface though its lone-electron pair, but in silver colloid solution and on the silver nanoparticles modified silver electrode the piperidine molecular lies flat on the silver surface. In the meantime, the potential dependent SERS of piperidine on the modified electrode were studied.     

The utilization of silver salts of aromatic thiols as core materials of SERS‐based molecular sensors

Silver salts of aromatic thiols are one class of organic–inorganic heterostructured materials, showing peculiar photoreaction characteristics. When an argon ion laser is exposed to silver benzenethiolate (AgBT), for instance, its Raman spectrum changes over time, eventually becoming the same as the surface‐enhanced Raman scattering (SERS) spectrum of benzenethiol on a roughened Ag substrate. AgBT and its analogs can thus be used as a core material of molecular sensors operating via SERS; we demonstrate this specifically, by monitoring the SERS peaks of BT, in which biotinylated AgBT selectively recognizes streptavidin molecules down to concentrations of 10⁻¹¹ g ml⁻¹ (i.e. ∼0.2 pM ). Since numerous silver thiolates can be used as the core material, multiple bioassays are readily accomplished using the present methodology. Copyright © 2008 John Wiley & Sons, Ltd.