Determination of primary aromatic amines using immobilized nanoparticles based surface-enhanced Raman spectroscopy

Primary aromatic amines(PAAs) are substances with toxicity and suspected human carcinogenicity. A facile method for highly sensitive detection of PAAs using surface-enhanced Raman spectroscopy(SERS)is reported. The immobilization of Au nanoparticles(AuNPs) on the glycidyl methacrylate–ethylene dimethacrylate(GMA-EDMA) materials makes the substrate a closely packed but not aggregated AuNP arrays which provides a prominent SERS enhancement. Four PAAs with different substituent groups,namely, p-toluidine, p-nitroaniline, benzidine and 4,4-methylene-bis-(2-chloroaniline) have been successfully identified and quantified. High sensitivity and good linear relationship between SERS signals and concentrations of PAAs are obtained for all four PAAs.     

Swellable polymer films containing Au nanoparticles for point-of-care therapeutic drug monitoring using surface-enhanced Raman spectroscopy

Large (10 × 10 cm) sheets of surface-enhanced Raman spectroscopy (SERS) active polymer have been prepared by stabilising metal nanoparticle aggregates within dry hydroxyethylcellulose (HEC) films. In these films the aggregates are protected by the polymer matrix during storage but in use they are released when aqueous analyte droplets cause the films to swell to their gel form. The fact that these “Poly-SERS” films can be prepared in bulk but then cut to size and stored in air before use means that they provide a cost effective and convenient method for routine SERS analysis. Here we have tested both Ag and Au Poly-SERS films for use in point-of-care monitoring of therapeutic drugs, using phenytoin as the test compound. Phenytoin in water could readily be detected using Ag Poly-SERS films but dissolving the compound in phosphate buffered saline (PBS) to mimic body fluid samples caused loss of the drug signal due to competition for metal surface sites from Cl⁻ ions in the buffer solution. However, with Au Poly-SERS films there was no detectable interference from Cl⁻ and these materials allowed phenytoin to be detected at 1.8 mg L⁻¹, even in PBS. The target range of detection of phenytoin in therapeutic drug monitoring is 10–20 mg L⁻¹. With the Au Poly-SERS films, the absolute signal generated by a given concentration of phenytoin was lower for the films than for the parent colloid but the SERS signals were still high enough to be used for therapeutic monitoring, so the cost in sensitivity for moving from simple aqueous colloids to films is not so large that it outweighs the advantages which the films bring for practical applications, in particular their ease of use and long shelf life.     

Interaction of soil humic acids with herbicide paraquat analyzed by surface-enhanced Raman scattering and fluorescence spectroscopy on silver plasmonic nanoparticles

A study of the interaction between paraquat (methyl viologen) and humic acids, extracted from a soil amended over 30 years with crop residues, cow slurries and cattle manure, was carried out by two emission spectroscopies based on plasmonic effects: surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). To carry out this study Ag nanoparticles were used. The complex formation was tested by analyzing the effect of the herbicide on humic acids, and by varying experimental parameters such as the pH and the laser excitation wavelength. The study of the vibrational bands led to infer information about the interaction mechanism of paraquat with humic acids and to find a correlation between this interaction and the humic acids structural modification induced by the different amendments added to soil.     

Silver–platinum core–shell nanoparticles for surface-enhanced Raman spectroscopy

Core–shell Ag@Pt nanoparticles have been synthesised by the means of seed-growth reaction including reduction of PtCl₄²⁻ with silver and replacing Ag atoms with Pt. Surface-enhanced Raman scattering (SERS) spectra of pyridine (which gives slightly different spectra when interacting with various metals) adsorbed on synthesised Ag@Pt clusters were measured. SERS measurements have revealed that deposition of the platinum layer causes near elimination of the spectral interferences from pyridine directly interacting with the silver core. The average SERS enhancement factor for pyridine adsorbed on the Ag@Pt clusters was estimated as equal to about 10³–10⁴, significantly higher than the SERS enhancement factor achievable on the pure platinum nanostructures. Using the silver core (instead of the previously used gold cores) allows for measurement of strong SERS spectra on the Pt covered nanostructures for the wider range of the excitation radiation. This procedure of platinum deposition was tested with various silver nanoparticles – produced with borohydride, citrate and citrate/borohydride methods – which substantially differ in size distribution. The application of formed Ag@Pt structures for obtaining intense Raman spectra for molecules adsorbed on only slightly modified platinum surfaces is discussed.     

Intercalator-DNA interactions revealed by NIR surface-enhanced Raman spectroscopy

Using 1064 nm excited surface-enhanced Raman spectroscopy (SERS) a well known intercalator, ethidium bromide (EB), and a structurally related compound, 4-methyl-2,7-diamino-5,10-diphenyl-4,9-diazapyrenium hydrogensulfate (ADAP), have been studied. Concentration dependent SERS spectra of both aromatic species (1 × 10⁻⁷-5 × 10⁻⁵ M) indicated existence of dimeric associates at high concentration and an equilibrium shift towards monomers with a concentration decrease. Interactions of the intercalating molecules with DNA have been studied for various intercalator/DNA (base pair) molar ratios ranging from 10/1 to 1/10. In colloidal samples containing an intercalator in excess relative to DNA binding sites (from 10/1 to 2/1) enhancement of the Raman scattering gradually weakened, indicating a decrease in a number of free molecules adsorbed on the metal surface due to binding with DNA. At the drug/DNA ratios of 1/2 and 1/5 weaker but observable SERS bands indicated insertion of the drug molecules between the base pairs (intercalation strongly diminished interaction of the drug molecules with metal surface) as well as non-intercalative binding of the drug molecules able to stay in closer contact with a metal surface. A total intercalation of EB and ADAP molecules (intercalator/DNA of 1/7 and 1/10) resulted in almost complete loss of the SERS signal. Intensity of the SERS spectra of the intercalator/DNA complexes relative to the SERS intensity of the free intercalating molecules diminished to a lesser degree for ADAP/DNA than for EB/DNA. The obtained difference was attributed to a larger aromatic surface of the ADAP molecules which, although intercalated, could be positioned near the enhancing nanoparticles, unlike the smaller EB molecules which were deeply inserted within the DNA helix.     

Highly reproducible surface-enhanced Raman scattering-active Au nanostructures prepared by simple electrodeposition: Origin of surface-enhanced Raman scattering activity and applications as electrochemical substrates

The fabrication of effective surface-enhanced Raman scattering (SERS) substrates has been the subject of intensive research because of their useful applications. In this paper, dendritic gold (Au) rod (DAR) structures prepared by simple one-step electrodeposition in a short time were examined as an effective SERS-active substrate. The SERS activity of the DAR surfaces was compared to that of other nanostructured Au surfaces with different morphologies, and its dependence on the structural variation of DAR structures was examined. These comparisonal investigations revealed that highly faceted sharp edge sites present on the DAR surfaces play a critical role in inducing a high SERS activity. The SERS enhancement factor was estimated to be greater than 10⁵, and the detection limit of rhodamine 6G at DAR surfaces was 10⁻⁸ M. The DAR surfaces exhibit excellent spot-to-spot and substrate-to-substrate SERS enhancement reproducibility, and their long-term stability is very good. It was also demonstrated that the DAR surfaces can be effectively utilized in electrochemical SERS systems, wherein a reversible SERS behavior was obtained during the cycling to cathodic potential regions. Considering the straightforward preparation of DAR substrates and the clean nature of SERS-active Au surfaces prepared in the absence of additives, we expect that DAR surfaces can be used as cost-effective SERS substrates in analytical and electrochemical applications.     

In-situ surface-enhanced Raman scattering and FT-Raman spectroscopy of black prints

The black inkjet and laser prints were analysed with regard to application in forensic analysis of questioned documents. The purpose of this work was to study spectral properties and compare the suitability of surface-enhanced Raman scattering (SERS) with Fourier transform Raman spectra of prints. This work aimed to find optimal surface-enhanced Raman spectroscopic approach for the future analysis of documents using statistical methods. In this work, we analysed eight prints of four laser and four inkjet devices. The samples were measured using two dispersive Raman devices; (DXR Raman microscope with excitation line 532 nm, Foram 685-2 spectrometer − 685 nm) and FT-Raman device (Bruker Spectrometer MultiRAM with excitation line 1064 nm). The silver nanoparticles (AgNPs) colloid for SERS experiment were synthesised and checked by UV–vis spectroscopy and scanning electron microscopy (SEM). The remarkable differences caused by centrifugation of silver colloid were observed just in the SEM images. The main contribution of this paper is to propose the novel approach achieving sufficient SERS signal intensity of black prints using the both, laser and inkjet printers. Moreover, this method is based on just a single metal colloid, and the analysis can be performed in-situ, i.e. directly on the printed sample surface. We consider the SERS could by highly promising and universal for applications in the forensic analysis of printed documents with the combination of statistical method when conventional methods are not effective.     

Advances in surface-enhanced Raman spectroscopy for analysis of pharmaceuticals: A review

Recently, Raman spectroscopy become a popular and potential analytical technique for the analysis of pharmaceuticals as a result of its advancement. The innovation of laser technology, Fourier Transform-Raman spectrometers with charge coupled device (CCD) detectors, ease of sample preparation and handling, mitigation of sub-sampling problems using different geometric laser irradiance patterns and invention of different optical components of Raman spectrometers are contributors of the advancement of Raman spectroscopy. Transmission Raman Spectroscopy is a useful tool in pharmaceutical analysis to address the problems related with sub-sampling in conventional Raman back scattering. More importantly, the development of surface-enhanced Raman scattering (SERS) has been a prominent advancement for Raman spectroscopy to be applied for pharmaceuticals analysis as it avoids the inherent insensitivity and fluorescence problems. As the active pharmaceutical ingredients (APIs) contain aromatic or conjugated domains with strong Raman scattering activity, Raman spectroscopy is an attractive alternative conventional analytical method for pharmaceuticals. Coupling of Raman spectroscopy with separation techniques is also another advancement applied to reduce or avoid possible spectral interferences. Therefore, in this review, transmission Raman spectroscopy, SERS, and SERS coupled with various separation techniques for pharmaceutical analysis are presented.     

Surface-enhanced Raman spectroscopy toward application in plasmonic photocatalysis on metal nanostructures

Among photothermal, photovoltaic and photochemical techniques, photochemistry is superior in energy storage and transportation by converting photons into chemical fuels. Recently plasmonic photocatalysis, based on localized surface plasmon resonance (LSPR) generated from noble metal nanostructures, has attracted much attention. It promotes photochemical reaction efficiency by optimizing the solar spectrum absorption and the surface reaction kinetics. The deeper understanding is in urgent need for the development of novel plasmonic photocatalysts. Surface-enhanced Raman spectroscopy (SERS), which is also originated from the LSPR effect, provides an excellent opportunity to probe and monitor plasmonic photoreactions in situ and in real-time, with a very high surface sensitivity and energy resolution. Here, fundamentals of plasmonic photocatalysis and SERS are first presented based on their connections to the LSPR effect. Following by a validity analysis, latest studies of SERS applied for the plasmon mediated photochemical reaction are reviewed, focusing on the reaction kinetics and mechanism exploration. Finally, limitations of the present study, as well as the future research directions, are briefly analyzed and discussed.     

Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips

The influence of dielectric substrates on the Raman scattering activities of Ag overlayers has been investigated. Materials with low refractive indices, such as SiO₂, SiO_x and AlF₃, were found to provide suitable supporting platforms for Ag films to give strong surface-enhanced Raman scattering for dye molecules when illuminated at 488 nm. This finding was then extended to tip-enhanced Raman scattering (TERS). Huge enhancements of 70–80×, corresponding to net enhancements of >10⁴, were observed for brilliant cresyl blue test analyte when Ag-coated tips made from or precoated with low refractive index materials were applied. The yield of fabricated tips that significantly enhance the Raman signals was found to be close to 100%. These findings provide crucial steps towards the use of TERS as a robust technique for rapid chemical imaging with nanometer spatial resolution. Figure Silver-coated dielectric tips for tip-enhanced Raman scattering (TERS) are capable of more than 10,000-fold enhancement     

A comparison of surface-enhanced infrared and surface-enhanced Raman spectra of pyrazine adsorbed on polycrystalline gold electrodes

The adsorption of pyrazine at a polycrystalline Au film electrode has been investigated using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), chronocoulometry, and ac impedance. Combining the SEIRA data and the thermodynamic data (the surface charge density of the electrode and the relative Gibbs surface excess of pyrazine), pyrazine was found to adsorb on the surface with a vertical end-on configuration via one N atom. The SEIRA spectra were compared with published surface-enhanced Raman (SER) spectra of pyrazine in order to clarify the reason for the breakdown of the Raman selection rule in the SER spectra. The activation of originally Raman-forbidden modes in the SER spectra is well explained by a photo-driven charge-transfer mechanism. The charge-transfer is deduced to be from filled metal states near the Fermi level to the first and second excited states of pyrazine. It is emphasized that the comparative SEIRA and SER studies are important for a better understanding of the electrochemical interface.     

Density functional theory and surface-enhanced Raman spectroscopy studies on endocrine-disrupting chemical,dimethyl phthalate

A method to monitor endocrine-disrupting chemical contamination phthalate esters (PAEs) by surface-enhanced Raman scattering (SERS) spectroscopy has been investigated. The molecular structure and assignment of the vibrations of dimethyl phthalate (DMP), forming short chains in PAEs, has been studied by density functional theory (DFT) calculations. The structure of DMP with the dihedral angles of 1C-6C-11C-13O and 4C-5C-18C-20O being 133.78° and −24.00°, respectively, has the lowest energy. Theoretical vibrational frequencies using B3LYP/6-31 + G(d) (after scaling) show excellent agreement with the experimental normal Raman spectrum. In the region 200–1800 cm⁻¹, SERS spectra of DMP were measured using ordered gold nanosubstrates. All except the weak signals in the normal Raman spectrum of DMP were successfully enhanced. These results demonstrate that SERS technology could be developed as a rapid method for screening of DMP.     

Focused-ion-beam-fabricated Au nanorods coupled with Ag nanoparticles used as surface-enhanced Raman scattering-active substrate for analyzing trace melamine constituents in solution

A well-ordered Au-nanorod array with a controlled tip ring diameter (Au_NRs_d) was fabricated using the focused ion beam method. Au_NRs_d was then coupled with Ag nanoparticles (Ag NPs) to bridge the gaps among Au nanorods. The effect of surface-enhanced Raman scattering (SERS) on Au_NRs_d and Ag NPs/Au_NRs_d was particularly verified using crystal violet (CV) as the molecular probe. Raman intensity obtained from a characteristic peak of CV on Au_NRs_d was estimated by an enhancement factor of ≈10⁷ in magnitude, which increased ≈10¹² in magnitude for that on Ag NPs/Au_NRs_d. A highly SERS-active Ag NPs/Au_NRs_d was furthermore applied for the detection of melamine (MEL) at very low concentrations. Raman-active peaks of MEL (10⁻³ to 10⁻¹² M) in water or milk solution upon Au_NRs_d or Ag NPs/Au_NRs_d were well distinguished. The peaks at 680 and 702 cm⁻¹ for MEL molecules were found suitable to be used as the index for sensing low-concentration MEL in a varied solution, while that at 1051 cm⁻¹ was practical to interpret MEL molecules in water or milk solution bonded with Au (i.e., Au_NRs_d) or Ag (i.e., Ag NPs/Au_NRs_d) surface. At the interface of Ag NPs/Au_NRs_d and MEL molecules in milk solution, a laser-induced electromagnetic field or hotspot effect was produced and competent to sense low-concentration MEL molecules interacting with Ag and Au surfaces. Accordingly, Ag NPs/Au_NRs_d is very promising to be used as a fast and sensitive tool for screening MEL in complex matrices such as adulteration in e.g., food and pharmaceutical products.     

SPR effect of Au nanoparticles on the visible photocatalytic RhB degradation and NO oxidation over TiO2 hollow nanoboxes

Three-dimensional (3D) TiO₂ hollow structures have attracted much attention due to their unique properties. However, the large bandgap of (3.2 eV) results in the fact that anatase TiO₂ photocatalyst can only be excited by UV light, which only accounts for 3–5% of the solar energy. On considering that nobel metallatic nanomaterials can harvest visible light due to surface plasmon resonance (SPR) effect, in this paper, three kinds of Au nanoparticles with different morphologies, namely Au nanospheres (Au-NSs), Au nanorods (Au-NRs) and Au nanopentogons (Au-NPs) were prepared and used as photosensitizers to modified TiO₂ hollow nanoboxes (TiO₂-HNBs), aiming to explore high efficient visible-light-responsive photocatalyst. The photoreacitivty of Au/TiO₂-HNBs was evaluated by photoctalytic oxidation of Rhodamine B (RhB) and NO under visible irradiation (λ > 420 nm). It was found that the visible photoreactivity of TiO₂-HNBs was greatly enhanced after modified with Au nanoparticles, and TiO₂-HNBs loaded with Au-NRs exhibit the highest visible photocatalytic activity towards both RhB degradation and NO oxidation. Upon visible irradiation, SPR effect induces the production of hot electrons from the Au nanoparticles, which can further transfer to the conduction band of TiO₂-HNBs to produce superoxide radicals (O₂⁻), resulting in an efficient separation of photo-generated electron-hole pairs. The photoreactivity of Au-NRs/TiO₂-HNBs towards RhB degradation almost keeps unchanged even after recycling used for 5 times, indicating that it is promising to be use in practical applications.     

Progress in plasmonic engineering of surface-enhanced Raman-scattering substrates toward ultra-trace analysis

This review describes advances made toward the application of surface-enhanced Raman scattering (SERS) in sensitive analysis and diagnostics. In the early sections of this review we briefly introduce the fundamentals of SERS including a discussion of SERS at the single-molecule level. Applications relevant to trace analysis, environmental monitoring, and homeland security and defense, for example high explosives and contaminant detection, are emphasized. Because the key to wider application of SERS analysis lies in the development of highly enhancing substrates, in the second half of the review we focus our attention on the extensive progress made in designing innovative soluble, supported, and ordered SERS-active nano-architectures to harness the potential of this technique toward solving current and emerging analytical tasks. No attempt or claim is made to review the field exhaustively in its entirety nor to cover all applications, but only to describe several significant milestones and progress made in these important areas and to provide some perspective on where the field is quickly moving.Dedicated to the memory of Wilhelm Fresenius     

Extremely sensitive sandwich assay of kanamycin using surface-enhanced Raman scattering of 2-mercaptobenzothiazole labeled gold@silver nanoparticles

Herein, we report the development of extremely sensitive sandwich assay of kanamycin using a combination of anti-kanamycin functionalized hybrid magnetic (Fe₃O₄) nanoparticles (MNPs) and 2-mercaptobenzothiazole labeled Au-core@Ag-shell nanoparticles as the recognition and surface-enhanced Raman scattering (SERS) substrate, respectively. The hybrid MNPs were first prepared via surface-mediated RAFT polymerization of N-acryloyl-l-glutamic acid in the presence of 2-(butylsulfanylcarbonylthiolsulfanyl) propionic acid-modified MNPs as a RAFT agent and then biofunctionalized with anti-kanamycin, which are both specific for kanamycin and can be collected via a simple magnet. After separating kanamycin from the sample matrix, they were sandwiched with the SERS substrate. According to our experimental results, the limit of detection (LOD) was determined to be 2 pg mL⁻¹, this value being about 3–7 times more than sensitive than the LOD of previously reported results, which can be explained by the higher SERS activity of silver coated gold nanoparticles. The analysis time took less than 10 min, including washing and optical detection steps. Furthermore, the sandwich assay was evaluated for investigating the kanamycin specificity on neomycin, gentamycin and streptomycin and detecting kanamycin in artificially contaminated milk.     

Stellate porous silica based surface-enhanced Raman scattering system for traceable gene delivery

Numerous nanocarriers have been currently developed for intracellular delivery. The potential cytotoxicity of these very small inorganic nanocarriers has raised great consideration. Thus, it becomes of utmost importance to conduct the intracellular trace of nanocarriers. Among many analytical techniques, surface enhanced Raman scattering(SERS) method is one of the current state-of-the-art techniques for cell visualization and trace. In this work, a novel stellate porous silica based gene delivery system has been designed for SERS trace purpose. A stellate porous silica nanoparticle modified with many small Au nanoparticles is designed to replace common metallic SERS tags. The results show that the designed system not only could deliver si RNA into cells for therapy, but also could realize SERS trace with high sensitivity and non-invasive features. The constructed delivery system has considerable potential to trace the dynamic gene delivery in living cells.     

Detection of bacteria by surface-enhanced Raman spectroscopy

The detection and identification of dilute bacterial samples by surface-enhanced Raman spectroscopy has been explored by mixing aqueous suspensions of bacteria with a suspension of nanocolloidal silver particles. An estimate of the detection limit of E. coli was obtained by varying the concentration of bacteria. By correcting the Raman spectra for the broad librational OH band of water, reproducible spectra were obtained for E. coli concentrations as low as approximately 10³ cfu/mL. To aid in the assignment of Raman bands, spectra for E. coli in D₂O are also reported. Figure Light scattering apparatus used to detect bacteria     

Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering

In this work, silver nanoparticles (AgNPs) decorated magnetic microspheres (MMs) are prepared as surface-enhanced Raman scattering (SERS) substrate for the analysis of adenine in aqueous solutions. To prepare these substrates, magnetic particles were first synthesized by coprecipitation of Fe(II) and Fe(III) with ammonium hydroxide. A thin layer of cross-linked polymer was formed on these magnetic particles by polymerization through suspension of magnetic particles into a solution of divinyl benzene/methyl methacrylate. The resulted polymer protected magnetic particles are round in shape with a size of 80 μm in diameter. To form AgNPs on these MMs, photochemical reduction method was employed and the factors in photochemical reduction method were studied and optimized for the preparation of highly sensitive and stable AgNPs on MMs substrates (abbreviated as AgMMs substrates). By dispersing the AgMMs in aqueous samples, cylindrical magnet was used to attract the AgMMs for SERS detections. The observed enhancement factor of AgMMs reached 7 orders in magnitude for detection of adenine with a detection limit approaching to few hundreds of nanomolar.     

Label-free and direct protein detection on 3D plasmonic nanovoid structures using surface-enhanced Raman scattering

In this paper we use surface-enhanced Raman spectroscopy (SERS) on 3D metallic structures for label-free detection and characterization of proteins of interest at low concentrations. The substrates are prepared via nanopatterning with latex nano/microparticles and Cr and Ag sputtering, yielding stable, tunable, and mechanically flexible plasmonic structures. The nanovoids generate a SERS signal of the proteins of interest that is background free and independent of the protein charge. Concentrations as low as 0.05 μg mL⁻¹ could be detected for 4 different proteins. The proteins also exhibit significantly different SERS spectra on these substrates, which is an important feature for future label-free direct detection schemes.