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.     

Fabrication of flower-like silver nanoparticles for surface-enhanced Raman scattering

The flower-like silver nanoparticles have been synthesized by reducing silver nitrate (AgNO₃) with ascorbic acid (AA) as the reductant and polyvinyl pyrrolidone (PVP) as the capping agent under vigorous stirring. Such flower-like nanoparticles are aggregates of small nanoplates and nanorods. They were tested as substrates for the surface-enhanced Raman scattering (SERS), showing high sensitivity for detecting Rhodamine 6G (R6G) at a concentration as low as 10^-7 mol/L. It has been found that replacing mechanical stirring with ultrasound sonication would drastically change the particle morphology, from flower-like nanoparticles to well-dispersed smaller nanoparticles. Furthermore, when trace amounts of NaCl were added into the reagents, well-dispersed Ag nanoparticles formed even in vigorous stirring. These phenomena can be explained with the diffusion and reactant supply during nucleation and growth of Ag nanoparticles.     

Cuttlebone-derived organic matrix as a scaffold for assembly of silver nanoparticles and application of the composite films in surface-enhanced Raman scattering

Biologically derived materials provide a rich variety of approaches toward new functional materials because of their fascinating structures and environment-friendly features, which is currently a topic of research interest. In this paper, we show that the cuttlebone-derived organic matrix (CDOM) is an excellent scaffold for the one-step synthesis and assembly of silver nanoparticles (AgNPs), which can be further used as substrate for surface-enhanced Raman scattering (SERS). Formation of AgNPs–CDOM composite was accomplished by the reaction of CDOM with AgNO₃ and NH₃·H₂O solution at 80 °C without using any other stabilizer and reducing agents. UV–vis spectra and TEM were utilized to characterize the AgNPs and investigate their formation process. Results demonstrate that the size and distribution of AgNPs can be partly regulated by changing incubation time; the concentration of NH₃·H₂O is critical to the formation rate of AgNPs. As a proof of principle, we show that the AgNPs–CDOM composite can be employed in trace analysis using SERS.     

Biocompatible core-shell nanoparticle-based surface-enhanced Raman scattering probes for detection of DNA related to HIV gene using silica-coated magnetic nanoparticles as separation tools

A novel, highly selective DNA hybridization assay has been developed based on surface-enhanced Raman scattering (SERS) for DNA sequences related to HIV. This strategy employs the Ag/SiO₂ core-shell nanoparticle-based Raman tags and the amino group modified silica-coated magnetic nanoparticles as immobilization matrix and separation tool. The hybridization reaction was performed between Raman tags functionalized with 3′-amino-labeled oligonucleotides as detection probes and the amino group modified silica-coated magnetic nanoparticles functionalized with 5′-amino-labeled oligonucleotides as capture probes. The Raman spectra of Raman tags can be used to monitor the presence of target oligonucleotides. The utilization of silica-coated magnetic nanoparticles not only avoided time-consuming washing, but also amplified the signal of hybridization assay. Additionally, the results of control experiments show that no or very low signal would be obtained if the hybridization assay is conducted in the presence of DNA sequences other than complementary oligonucleotides related to HIV gene such as non-complementary oligonucleotides, four bases mismatch oligonucleotides, two bases mismatch oligonucleotides and even single base mismatch oligonucleotides. It was demonstrated that the method developed in this work has high selectivity and sensitivity for DNA detection related to HIV gene.     

Improved surface-enhanced Raman scattering on optimum electrochemically roughened silver substrates

In this work, the effects of preparation conditions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) were first investigated. The optimum roughening conditions for obtaining strongest SERS of Rhodamine 6G (R6G) are as follows. Ag electrodes were cycled in deoxygenated aqueous solutions containing 0.1 M NaCl from −0.3 to +0.2 V versus Ag/AgCl at 25 mV s⁻¹ for five scans. The SERS of R6G adsorbed on this optimum procedure-prepared roughened Ag substrate exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a normally roughened Ag substrate.     

Improved surface-enhanced Raman scattering on electrochemically roughened silver substrates prepared in bielectrolyte solutions

In this work, the effect of supplemental LiClO₄ electrolytes in KCl solutions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) was first investigated. To prepare SERS-active substrates by ORC procedures, electrolytes of KCl were generally employed. In contrast, LiClO₄ ones were unsuitable for producing SERS-active substrates. Encouragingly, SERS of Rhodamine 6G (R6G) adsorbed on the roughened Ag substrate prepared in an aqueous solution containing KCl and LiClO₄ electrolytes exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a roughened Ag substrate prepared in a solution only containing KCl. Further investigations indicate that the oxidation state of Cl on the roughened Ag substrate demonstrates decided effects on this improved SERS.     

Surface-enhanced Raman scattering for perchlorate detection using cystamine-modified gold nanoparticles

Perchlorate (ClO₄⁻) has recently emerged as a widespread environmental contaminant found in groundwater and surface water, and there is a great need for rapid detection and monitoring of this contaminant. This study presents a new technique using cystamine-modified gold nanoparticles as a substrate for surface-enhanced Raman scattering (SERS) detection of perchlorate at low concentrations. A detection limit of 5 × 10⁻⁶ M (0.5 mg/L) has been achieved using this method without sample preconcentration. This result was attributed to a strong plasmon enhancement by gold metal surfaces and the electrostatic attraction of ClO₄⁻ onto positively charged, cystamine-modified gold nanoparticles at a low pH. The methodology also was found to be reproducible, quantitative, and not susceptible to significant interference from the presence of anions such as sulfate, phosphate, nitrate and chloride at concentrations <1 mM, making it potentially suitable for rapid screening and routine analysis of perchlorate in environmental samples.     

A simple approach for ultrasensitive detection of bisphenols by multiplexed surface-enhanced Raman scattering

Bisphenol A (BPA) is well known for its use in plastic manufacture and thermal paper production despite its risk of health toxicity as an endocrine disruptor in humans. Since the publication of new legislation regarding the use of BPA, manufacturers have begun to replace BPA with other phenolic molecules such as bisphenol F (BPF) and bisphenol B (BPB), but there are no guarantees regarding the health safety of these compounds at this time. In this context, a very simple, cheap and fast surface-enhanced Raman scattering (SERS) method was developed for the sensitive detection of these molecules in spiked tap water solutions. Silver nanoparticles were used as SERS substrates. An original strategy was employed to circumvent the issue of the affinity of bisphenols for metallic surfaces and the silver nanoparticles surface was functionalized using pyridine in order to improve again the sensitivity of the detection. Semi-quantitative detections were performed in tap water solutions at a concentrations range from 0.25 to 20 μg L⁻¹ for BPA and BPB and from 5 to 100 μg L⁻¹ for BPF. Moreover, a feasibility study for performing a multiplex-SERS detection of these molecules was also performed before successfully implementing the developed SERS method on real samples.     

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.     

Sonoelectrochemical synthesis of spike-like gold-silver alloy nanoparticles from bulk substrates and the application on surface-enhanced Raman scattering

The synthesis of non-spherical spike-like gold-silver alloy nanoparticles on platinum substrates was first developed by sonoelectrochemical methods in this study. First, a silver substrate was roughened by a triangular-wave oxidation-reduction cycle (ORC) in an aqueous solution containing 0.1 M HCl. Silver-containing complexes were found in the solution after the ORC treatment. Then a gold substrate was subsequently roughened by the similar ORC treatment in the same silver complexes-containing solution. After this procedure, Au- and Ag-containing complexes were left in the solution. Subsequently, the Au working electrode was immediately replaced by a Pt electrode. A cathodic overpotential was applied under controlled sonication and slight stirring to synthesize Au-Ag alloy nanoparticles on the Pt substrate. Encouragingly, the surface-enhanced Raman scattering (SERS) of Rhodamine 6G on the Au-Ag alloy nanoparticles-deposited Pt substrate exhibits a higher intensity by eight-fold of magnitude and a better resolution, as compared to that obtained on the Au nanoparticles-deposited Pt substrate.     

A colorimetric and surface-enhanced Raman scattering dual-signal sensor for Hg based on Bismuthiol II-capped gold nanoparticles

The addition of Bismuthiol II to the gold nanoparticles (AuNPs) solution led to the aggregation of AuNPs with a color change from red to blue. As a result, hot spots were formed and strong surface-enhanced Raman scattering (SERS) signal of Bismuthiol II was observed. However, the Bismuthiol II-induced aggregation of AuNPs could be reversed by Hg²⁺ in the system, accompanied by a remarkable color change from blue to red. As evidenced by UV–vis and SERS spectroscopy, the variation in absorption band and SERS intensity was strongly dependent on the concentration of Hg²⁺, suggesting a colorimetric and SERS dual-signal sensor for Hg²⁺. The sensor had a high sensitivity, low detection limits of 2 nM and 30 nM could be achieved by UV–vis spectroscopy and by SERS spectroscopy, respectively. Other environmentally relevant metal ions did not interfere with the detection of Hg²⁺. The method was successfully applied to detect Hg²⁺ in water samples. It was simple, rapid and cost-effective without any modifying or labeling procedure.     

Explosive and chemical threat detection by surface-enhanced Raman scattering: A review

Acts of terror and warfare threats are challenging tasks for defense agencies around the world and of growing importance to security conscious policy makers and the general public. Explosives and chemical warfare agents are two of the major concerns in this context, as illustrated by the recent Boston Marathon bombing and nerve gas attacks on civilians in the Middle East. To prevent such tragic disasters, security personnel must be able to find, identify and deactivate the threats at multiple locations and levels. This involves major technical and practical challenges, such as detection of ultra-low quantities of hazardous compounds at remote locations for anti-terror purposes and monitoring of environmental sanitation of dumped or left behind toxic substances and explosives. Surface-enhanced Raman scattering (SERS) is one of todays most interesting and rapidly developing methods for label-free ultrasensitive vibrational “fingerprinting” of a variety of molecular compounds. Performance highlights include attomolar detection of TNT and DNT explosives, a sensitivity that few, if any, other technique can compete with. Moreover, instrumentation needed for SERS analysis are becoming progressively better, smaller and cheaper, and can today be acquired for a retail price close to 10,000 US$. This contribution aims to give a comprehensive overview of SERS as a technique for detection of explosives and chemical threats. We discuss the prospects of SERS becoming a major tool for convenient in-situ threat identification and we summarize existing SERS detection methods and substrates with particular focus on ultra-sensitive real-time detection. General concepts, detection capabilities and perspectives are discussed in order to guide potential users of the technique for homeland security and anti-warfare purposes.     

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.     

Planar monolithic porous polymer layers functionalized with gold nanoparticles as large-area substrates for sensitive surface-enhanced Raman scattering sensing of bacteria

For the first time, large-area surface-enhanced Raman scattering sensing active substrates using porous polymer monolithic layers have been successfully prepared. Our approach includes a simple photoinitiated polymerization process using glycidyl methacrylate and ethylene dimethacrylate in a glass mold, followed by a chemical reaction of the epoxy functionalities leading to thiols, and the attachment of preformed gold nanoparticles. We demonstrated that this very simple process produced uniform and reproducible large area surfaces that significantly enhance sensitivity of Raman spectroscopy. Experiments were also carried out that confirmed preferential adsorption of living bacteria Escherichia coli from a very dilute solution on the surface of the monolithic layer, and immediate detection of the captured microorganisms using the SERS spectrum.     

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.     

Partition layer-modified substrates for reversible surface-enhanced Raman scattering detection of polycyclic aromatic hydrocarbons

Herein, we present progress towards an analytical sensor for polycyclic aromatic hydrocarbons (PAHs) using surface-enhanced Raman scattering (SERS) on partition layer-modified nanostructured substrates. Specifically, a 1-decanethiol monolayer has been assembled on a silver film over nanospheres substrate to concentrate PAHs within the zone of SERS detection. Both anthracene and pyrene were detected with limits of detection at 300 and 700 pM, respectively. The measured SERS spectra allowed for easy distinction of the two PAH compounds, due to varying peak locations, and insight into the partitioning mechanism. Additionally, exposure to a common environmental interferant, Suwannee River fulvic acid, did not impede the measurement of the PAHs, and the sensor is reusable after a short exposure to 1-octanol. Finally, the utility of this sensing platform for PAH detection was compared to that achievable for other classes of organic pollutants such as polychlorinated biphenyls and polybrominated diphenyl ethers. Figure SERS detection of polycyclic aromatic hydrocarbons facilitated via partition layer modified substrates.

Surface-enhanced Raman scattering detection of cholinesterase inhibitors

A new sensitive surface-enhanced Raman scattering (SERS) assay for detection of cholinesterase inhibitors such as organophosphorous pesticides using silver colloidal nanoparticles was developed and optimized. Acetylcholinesterase (AChE) mediated the hydrolysis of acetylthiocholine to produce thiocholine, which interacted with the silver nanoparticles to give a specific SERS spectrum. Variation in enzyme activity due to inhibition was measured from changes in intensity of a characteristic peak (772 cm⁻¹) of the SERS spectrum that was directly correlated with the concentration of produced thiocholine. The method was demonstrated for the detection of paraoxon as reference AChE inhibitor. Limit of detection of paraoxon for 5 min incubation at 25 °C was 1.8 × 10⁻⁸ M. This assay can be utilized for the detection of trace amounts of any AChE inhibitor.     

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.     

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.