Detection of melamine in liquid milk using surface‐enhanced Raman scattering spectroscopy

Melamine, a nitrogen‐rich chemical, has recently caused enormous economic losses to the food industry due to the cases of milk products adulterated by melamine. This has led to an urgent need of rapid and reliable methods for detection of melamine in food. In this study, surface‐enhanced Raman scattering (SERS) spectroscopy was used to detect melamine in liquid milk. The sample preparation with liquid milk is very easy; it has to be only diluted with double‐distilled water followed by centrifugation. By using a silver colloid, at least a 10⁵‐fold enhancement of the Raman signal was achieved for the measurement of melamine. The limit of detection by this method was 0.01 µg ml⁻¹ for melamine standard samples. Based on the intensity of the Raman vibrational bands normalised to that of the band at 928 cm⁻¹ (CH₂), an external standard method was employed for quantitative analysis. The linear regression square (R²) of the curve was 0.9998; the limit of quantitation using this approach was 0.5 µg ml⁻¹ of melamine in liquid milk; the relative standard deviation was ≤10%; and recoveries were from 93 to 109%. The test results for SERS were very precise and as good as those obtained by liquid chromatography/tandem mass spectrometry. The method was simple, fast(only needs about 3 min), cost effective, and sensitive for the detection of melamine in liquid milk samples. Therefore, it is more suitable for the field detection of melamine in liquid milk. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of periodical Ag–Au compound nanostructure films with controllable Ag nanoparticle aggregate patterns: a study on surface‐enhanced Raman scattering

The Ag–Au compound nanostructure films with controllable patterns of Ag nanoparticle (NP) aggregates were fabricated. A strategy of two‐step synthesis was employed toward the target products. Firstly, the precursor Au NP (17 nm) films were synthesized as templates. Secondly, the Ag NPs (45 nm) were deposited on the precursor films. Three types of Ag NP aggregates were obtained including discrete Ag NPs (discrete type), necklace‐like Ag NP aggregates (necklace type), and huddle‐like Ag NP aggregates (huddle type). The surface‐enhanced Raman scattering (SERS) property was studied on these nanostructures by using the probing molecule of rhodamine 6G under the excitation laser of 514.5 nm. Interestingly, the different types of samples showed different enhancement abilities. A statistical method was employed to assess the enhancement. The relative enhancement factor for each Ag NP was estimated quantitatively under the ratio of 1 : 25 : 18 for the discrete‐type, necklace‐type, and huddle‐type samples at the given concentration of 10⁻⁸ mol/l. This research shows that the enhancement ability of each Ag NP is dependent on the aggregate morphology. Moreover, the different enhancement abilities displayed different limit detection concentrations up to 10⁻⁸, 10⁻¹¹, and 10⁻⁹ mol/l, separately. The understanding of the relationship between the defined nanostructures and the SERS enhancement is very meaningful for the design of new SERS substrates with better performance. Copyright © 2015 John Wiley & Sons, Ltd.     

Silica‐overcoated substrates for detection of proteins by surface‐enhanced Raman spectroscopy

Ag film over nanosphere (AgFON) substrates for surface‐enhanced Raman spectroscopy (SERS) are shown to be ineffective for the detection of proteins in phosphate buffer solution (PBS) because of the decomposition of the substrate resulting in a total loss of SERS activity. However, modification of these substrates with SiO₂ overlayers overcomes this problem. The SiO₂ overlayers are produced by filtered arc deposition (FAD) and are characterised by atomic force microscopy (AFM). Their porosity is examined using Raman spectroscopy and the detection of cytochrome c and bovine serum albumin in PBS is successfully demonstrated. These findings show promise for the detection of proteins in biologically relevant conditions using Ag‐based SERS substrates. Copyright © 2008 John Wiley & Sons, Ltd.     

Detection and quantitative analysis of carbendazim herbicide on Ag nanoparticles via surface‐enhanced Raman scattering

Carbendazim (MBC) is a fungicide widely used in agriculture, and there are serious concerns regarding the health risks that could be caused by this fungicide. Here, we explore its ultrasensitive detection by surface‐enhanced Raman scattering (SERS). First, to obtain maximum SERS signal, the adsorption of the target molecule onto metallic surface is essential. Therefore, we study the adsorption of the MBC onto the nanoparticle surface by SERS under different experimental conditions, such as different synthesis methods of nanoparticle, variable excitation wavelength, and fungicide concentration with the aim to detect MBC at low concentrations. Experiments are carried out with three kinds of colloidal nanoparticles: Ag and Au reduced by citrate and Ag reduced by hydroxylamine. However, mainly Ag colloids are highly efficient in the SERS detection of MBC. In addition, theoretical calculations of MBC Raman spectrum and that of the surface complex are used to help with the understanding the mechanisms responsible for the interaction between MBC and Ag. Ultraviolet–visible absorption spectroscopy showed displacement to the red of the plasmon resonance of Ag colloid in the presence of MBC. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication of surface‐enhanced Raman scattering‐active ZnO/Ag composite microspheres

We show in this paper how zinc oxide (ZnO)/silver (Ag) composite microspheres can be prepared by the reduction of Ag(NH₃)₂⁺ with the reducing agent formaldehyde in aqueous solution on the surface of ZnO microspheres. During the preparation, Sn²⁺ was absorbed on the surface of ZnO microspheres for sensitization and activation, and then Ag(NH₃)₂⁺ was reduced to Ag nanoparticles by the reducing agent to obtain ZnO/Ag composite microspheres. SEM and TEM images revealed silver nanoparticles with a diameter ranging from tens to 100 nm. X‐Ray photoelectron spectra (XPS), X‐ray diffraction (XRD) patterns and UV‐vis spectra were used to characterize the structure of the ZnO/Ag composite microspheres. The origin of the surface‐enhanced Raman scattering properties was traced to the surface of the ZnO/Ag composite microspheres. The enhancement factor was estimated in detail, and the enhancement mechanism for the SERS effect was also investigated. Copyright © 2007 John Wiley & Sons, Ltd.     

Label‐free optical detection of type II diabetes based on surface‐enhanced Raman spectroscopy and multivariate analysis

Surface‐enhanced Raman scattering (SERS) spectroscopy was first employed to detect oxyhemoglobin (OxyHb, the common type of hemoglobin) variation in type II diabetic development without using exogenous reagents. Using silver nanoparticles as SERS‐active substrate, high‐quality SERS spectra are obtained from blood OxyHb samples of 49 diabetic patients and 40 healthy volunteers. Tentative assignment of the observed SERS bands indicates specific structural changes of OxyHb molecule in diabetes, including heme transformation and globin variation. Furthermore, partial least squares and principal component analysis combined with linear discriminate analysis diagnostic algorithms are employed to analyze and classify the SERS spectra acquired from diabetic and healthy OxyHb, yielding the diagnostic accuracies of 90.0% and 95.5%, respectively. This exploratory work suggests that the silver nanoparticles‐based OxyHb SERS method in combination with multivariate statistical analysis has great potential for the label‐free detection of type II diabetes. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanogaps in 2D Ag‐nanocap arrays for surface‐enhanced Raman scattering

The surface‐enhanced Raman scattering substrate of Ag–Ag nanocap arrays are prepared by depositing Ag film onto two‐dimensional (2D) polystyrene colloidal nanosphere templates. When the original colloidal arrays are used as the substrate for Ag deposition, surface‐enhanced Raman scattering (SERS) enhancements show the strong size‐dependence behaviours. When O₂‐plasma etched 2D polystyrene templates are used as the substrate for Ag deposition to form nanogaps, the gap sizes between adjacent Ag nanocaps from 5 to 20 nm generate even greater SERS enhancements. When SiO₂ coverage is deposited to isolate the Ag nanocaps from the neighbours, the SERS signals are enhanced more. The significant SERS effects are due to the coupling between Ag nanocaps controlled by the distance, which enhances the local electric‐field intensity. 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.     

Identification of berberine in ancient and historical textiles by surface‐enhanced Raman scattering

The highly fluorescent natural dye berberine can be easily identified in microscopic textile samples by surface‐enhanced Raman spectroscopy employing citrate‐reduced Ag colloid. The ordinary Raman (OR) and SERS spectra of berberine are presented and discussed in the light of a DFT calculation. Using FT‐Raman and FT‐SERS we could reliably compare relative intensity shifts and investigate the adsorption geometry of berberine on Ag nanoparticles. The significant enhancement in the FT‐SERS spectrum of the out‐of‐plane ring system bending deformation mode at 729 cm⁻¹ relative to a group of in‐plane vibrations at around 1500 cm⁻¹ was interpreted as evidence of a ‘flat‐on’ adsorption geometry. SERS was successfully used to identify berberine in silk fiber samples coated with colloidal Ag following a pretreatment with HCl vapor. The SERS method allowed us to detect berberine in a microscopic sample of a single silk fiber from a severely degraded and soiled 17th Century Chinese textile fragment. Copyright © 2007 John Wiley & Sons, Ltd.     

Characterization of polycyclic aromatic hydrocarbons using Raman and surface‐enhanced Raman spectroscopy

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous atmospheric pollutants and food contaminants, which exhibit potent carcinogenicity, mutagenicity, and teratogenicity. Vibrational spectroscopy techniques, especially Raman spectroscopy and surface‐enhanced Raman spectroscopy (SERS), can be potentially used as an alternative technique to liquid and gas chromatography in PAH analysis. However, there is limited information on the intrinsic Raman and SERS fingerprints of PAHs. In this study, we have acquired the Raman and SERS spectra of seven PAH compounds and compared their experimental spectra with theoretical Raman spectra calculated by density function theory (DFT). The vibrational modes corresponding to the Raman peaks have also been assigned using DFT. Characteristic Raman and SERS peaks have been identified for five PAH compounds, and the limits of detection were estimated. Such information could be useful for developing SERS assays for simple and rapid PAH identification. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental and statistical analysis methods for peptide detection using surface‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy (SERS) has the potential to make a significant impact in biology research due to its ability to provide information orthogonal to that obtained by traditional techniques such as mass spectrometry (MS). While SERS has been well studied for its use in chemical applications, detailed investigations with biological molecules are less common. In addition, a clear understanding of how methodology and molecular characteristics impact the intensity, the number of peaks, and the signal‐to‐noise of SERS spectra is largely missing. By varying the concentration and order of addition of the SERS‐enhancer salt (LiCl) with colloidal silver, we were able to evaluate the impact of these variables on peptide spectra using a quantitative measure of spectra quality based on the number of peaks and peak intensity. The LiCl concentration and order of addition that produced the best SERS spectra were applied to a panel of synthetic peptides with a range of charges and isoelectric points (pIs) where the pI was directly correlated with higher spectral quality. Those peptides with moderate to high pIs and spectra quality scores were differentiated from each other using the improved method and a hierarchical clustering algorithm. In addition, the same method and algorithm was applied to a set of highly similar phosphorylated peptides, and it was possible to successfully classify the majority of peptides on the basis of species‐specific peak differences. Copyright © 2008 John Wiley & Sons, Ltd.     

Silver‐coated inverse opals formed from polystyrene spheres for surface‐enhanced Raman scattering

We have developed a new substrate for surface‐enhanced Raman scattering (SERS) measurements involving a thin silver layer deposited over an ion‐etched TiO₂ inverse opal. The latter is formed by chemically infiltrating a polystyrene opal array with TiO₂ followed by a thermal decomposition of the spheres. The SERS response of the these substrates is examined for several sphere sizes and lasers wavelengths; the results show that such substrates yield high enhance factors, comparable to substrates involving a silver layer deposited directly on a polystyrene opal array. Copyright © 2010 John Wiley & Sons, Ltd.     

Multianalyte immunoassay based on surface‐enhanced Raman spectroscopy

In this paper, two immunoassay methods based on SERS are developed for multiplex analysis, both of which stemmed from the concept of forming a sandwich structure ‘capture antibody substrate/antigen/Raman‐reporter‐labeled immuno‐nanoparticles’. They are two‐molecule labeled one‐nanoparticle and one‐molecule labeled two‐nanoparticle methods. In both the methods, two different antibodies covalently bound to a solid substrate can specifically capture two different antigens from a sample. The captured antigens in turn bind selectively to their corresponding antibodies immobilized on Raman‐reporter‐labeled nanoparticles. Multianalyte immunoassay is successfully demonstrated by the detection of characteristic Raman bands of the probe molecules only when the antigen and antibody are matched. Copyright © 2007 John Wiley & Sons, Ltd.     

Direct observation of surface‐enhanced Raman scattering in ZnO nanocrystals

We have been able to observe the surface‐enhanced Raman scattering (SERS) from 4‐mercaptopyridine (4‐Mpy) molecules adsorbed on ZnO nanocrystals, which display 10³ enhancement factors (EFs). An excitation wavelength‐dependent behavior is clearly observed. Another molecule BVPP is also observed to have surface‐enhanced Raman signals. The chemical enhancement is most likely responsible for the observed enhancement, since plasmon resonances are ruled out. The research is important not only for a better understanding of the SERS mechanism, but also for extension of the application of Raman spectroscopy to a variety of adsorption problems on a semiconductor surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Silver nanoparticles synthesized by direct photoreduction of metal salts. Application in surface‐enhanced Raman spectroscopy

A simple synthesis method of silver nanoparticles and its application as an active surface‐enhanced Raman spectroscopy (SERS) colloid are presented in this work. The photoreduction of AgNO₃ in presence of sodium citrate (NaCit) was carried out by irradiation with different light sources (UV, white, blue, cyan, green, and orange) at room temperature. The evaluation of silver nanoparticles obtained as a function of irradiation time (1–24 h) and light source was followed by UV‐visible absorption spectroscopy. This light‐modification process results in a colloid with distinctive optical properties that can be related to the size and shape of the particles. The Ag colloids, as prepared, were employed as active colloids in SERS. Pyridine and caffeine were used as test molecules. Copyright © 2008 John Wiley & Sons, Ltd.     

Microstructured polymer‐based substrates with broadband absorption for surface‐enhanced Raman scattering

Large area (3 × 3 cm²) substrates for surface‐enhanced Raman scattering were fabricated by combining femtosecond laser microstructuring and soft lithography techniques. The fabrication procedure is as follows: (i) femtosecond laser machining is used to create a silicon master copy, (ii) replicates from polydimethylsiloxane are made, and (iii) a 50‐nm‐thick gold film is deposited on the surface of the replicates. The resulting substrates exhibit strongly enhanced absorption in the spectral region of 350 ∼ 1000 nm and generate enhanced Raman signal with enhancement factor of the order of 10⁷ for 10^{‐ 6} M rhodamine 6G. The main advantages of our substrates are low cost, large active area, and possibility for mass replication. Copyright © 2013 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.     

Revealing the invisible: using surface‐enhanced Raman spectroscopy to identify minute remnants of color in Winslow Homer's colorless skies

The fading of pigments in items of importance to cultural heritage, such as paintings, works of art on paper, and textiles, is a ubiquitous problem. Tools currently available that can detect and identify organic colorants in severely degraded works of art are rare, given the heavy deterioration and restricted availability of the sample. Recently, however, surface‐enhanced Raman scattering (SERS) spectroscopy has shown great promise in detecting and identifying mass‐limited samples. The art conservation field has seized upon the opportunity opened up by this powerful analytical technique to enable the identification of microscopic amounts of organic molecules used as artists' colorants in complex matrices, such as biomaterials (i.e. dyed natural textiles, linseed oil biofilms present in oil paintings, etc.), a possibility that was previously precluded due to interfering fluorescence and small sample size. Here, we report SERS spectra recorded directly on single particles of red lake pigments from an important historical watercolor by the American master Winslow Homer (1836–1910) that suffered some degree of fading. The accurate colorant identification provided by SERS, enhanced by comparison with reference samples of historical watercolors, has thus enabled important discoveries regarding the materials and intended meanings behind artworks from one of the most influential American watercolor painters. Copyright © 2011 John Wiley & Sons, Ltd.     

Quick detection of contaminants leaching from polypropylene centrifuge tubes with surface‐enhanced Raman spectroscopy and ultraviolet absorption spectroscopy

Anomalous surface‐enhanced Raman scattering (SERS) peaks were identified for liquid sample stored in polypropylene (PP) centrifuge tubes for months. We observed unexpected Raman peaks during experiments with thiamine hydrochloride aqueous solutions stored in PP tubes for 2 months. In order to identify the contaminants, we have performed SERS experiments on deionized (DI) water stored in PP centrifuge tubes for 2 months and compared them with those from fresh DI water sample. We have also carried out ultraviolet (UV) absorption spectra for both fresh and contaminated water. We believe that the water is contaminated because of chemicals leaching from the PP tube. From the gas chromatography‐mass spectrometry data, the main contaminants were found to be phthalic acid (PA) and its derivatives. Further SERS and UV absorption experiment for PA correlated well with the anomalous peaks identified earlier. We qualitatively confirmed the identification and quantitatively estimated the concentration of the suspect contaminants as between 1 and 10 µM with both SERS and UV absorption spectroscopy. With UV absorption spectroscopy, we precisely estimated the concentration as 2.1 µM . We have shown that the sample in PP tube can be contaminated by the leaching chemicals upon long‐term storage, and suggest SERS and UV absorption spectroscopy as two quick and simple techniques to detect the contamination. Copyright © 2011 John Wiley & Sons, Ltd.     

Ultrathin silver‐coated gold nanoparticles as suitable substrate for surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) is an extremely powerful tool for the analysis of the composition of bimetallic nanoparticle (BNP) surfaces because of the different adsorption schemes adopted by several molecules on different metals, such as Au and Ag. The preparation of BNPs normally implies a change in the plasmonic properties of the core metal. However, for technological applications it could be interesting to synthesize core–shell structures preserving these original plasmonic properties. In this work, we present a facile method for coating colloidal gold nanoparticles (NPs) in solution with a very thin shell of silver. The resulting bimetallic Au@Ag system maintains the optical properties of gold but shows the chemical surface affinity of silver. The effectiveness of the coating method, as well as the progressive silver enrichment of the outermost part of the Au NPs, has been monitored through the SERS spectra of several species (chloride, luteolin, thiophenol and lucigenin), which show different behaviors on gold and silver surfaces. A growth mechanism of the Ag shell is proposed on the basis of the spectroscopic and microscopic data consisting in the formation and deposit of Ag clusters on the Au NP surface. Copyright © 2009 John Wiley & Sons, Ltd.