Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.

Ultra sensitive label free surface enhanced Raman spectroscopy method for the detection of biomolecules

We present a proof of concept for a novel nanosensor for the detection of ultra-trace amounts of bio-active molecules in complex matrices. The nanosensor is comprised of gold nanoparticles with an ultra-thin silica shell and antibody surface attachment, which allows for the immobilization and direct detection of bio-active molecules by surface enhanced Raman spectroscopy (SERS) without requiring a Raman label. The ultra-thin passive layer (~1.3 nm thickness) prevents competing molecules from binding non-selectively to the gold surface without compromising the signal enhancement. The antibodies attached on the surface of the nanoparticles selectively bind to the target molecule with high affinity. The interaction between the nanosensor and the target analyte result in conformational rearrangements of the antibody binding sites, leading to significant changes in the surface enhanced Raman spectra of the nanoparticles when compared to the spectra of the un-reacted nanoparticles. Nanosensors of this design targeting the bio-active compounds erythropoietin and caffeine were able to detect ultra-trace amounts the analyte to the lower quantification limits of 3.5×10−13 M and 1×10−9 M, respectively.

Rapid detection of TNT in aqueous media by selective label free surface enhanced Raman spectroscopy

We report rapid and ultra-sensitive detection system for 2,4,6-trinitrotoluene (TNT) using unmodified gold nanoparticles and surface-enhanced Raman spectroscopy (SERS). First, Meisenheimer complex has been formed in aqueous solution between TNT and cysteamine in less than 15 min of mixing. The complex formation is confirmed by the development of a pink colour and a new UV–vis absorption band around 520 nm. Second, the developed Meisenheimer complex is spontaneously self-assembled onto unmodified gold nanoparticles through a stable Au–S bond between the cysteamine moiety and the gold surface. The developed mono layer of cysteamine-TNT is then screened by SERS to detect and quantify TNT. Our experimental results demonstrate that the SERS-based assay provide an ultra-sensitive approach for the detection of TNT down to 22.7 ng/L. The unambiguous fingerprint identification of TNT by SERS represents a key advantage for our proposed method. The new method provides high selectivity towards TNT over 2,4 DNT and picric acid. Therefore it satisfies the practical requirements for the rapid screening of TNT in real life samples where the interim 24-h average allowable concentration of TNT in waste water is 0.04 mg/L.

Density Functional Theoretical Modeling, Electrostatic Surface Potential and Surface Enhanced Raman Spectroscopic Studies on Biosynthesized Silver Nanoparticles: Observation of 400 pM Sensitivity to Explosives

Interaction of adsorbate on charged surfaces, orientation of the analyte on the surface, and surface enhancement aspects have been studied. These aspects have been explored in details to explain the surface-enhanced Raman spectroscopic (SERS) spectra of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW or CL-20), a well-known explosive, and 2,4,6-trinitrotoluene (TNT) using one-pot synthesis of silver nanoparticles via biosynthetic route using natural precursor extracts of clove and pepper. The biosynthesized silver nanoparticles (bio Ag Nps) have been characterized using UV-vis spectroscopy, scanning electron microscopy and atomic force microscopy. SERS studies conducted using bio Ag Nps on different water insoluble analytes, such as CL-20 and TNT, lead to SERS signals at concentration levels of 400 pM. The experimental findings have been corroborated with density functional computational results, electrostatic surface potential calculations, Fukui functions and potential measurements.

Quantitative surface-enhanced Raman spectroscopy of dipicolinic acid - towards rapid anthrax endospore detection

Dipicolinic acid (DPA) is an excellent marker compound for bacterial spores, including those of Bacillus anthracis ( anthrax). Surface-enhanced Raman spectroscopy (SERS) potentially has the sensitivity and discrimination needed for trace DPA analysis, but mixing DPA solutions with citrate-reduced silver colloid only yielded measurable SERS spectra at much higher (> 80 ppm) concentrations than would be desirable for anthrax detection. Aggregation of the colloid with halide salts eliminated even these small DPA bands but aggregation with Na2SO4(aq) resulted in a remarkable increase in the DPA signals. With sulfate aggregation even 1 ppm solutions gave detectable signals with 10 s accumulation times, which is in the sensitivity range required. Addition of CNS- as an internal standard allowed quantitative DPA analysis, plotting the intensity of the strong DPA 1010 cm(-1) band (normalised to the ca. 2120 cm(-1) CNS- band) against DPA concentration gave a linear calibration (R-2 = 0.986) over the range 0 - 50 ppm DPA. The inclusion of thiocyanate also allows false negatives due to accidental deactivation of the enhancing medium to be detected.

Combined antenna and localized plasmon resonance in Raman scattering from Random arrays of silver-coated, vertically aligned multiwalled carbon nanotubes

The electric field enhancement associated with detailed structure within novel optical antenna nanostructures is modeled using the surface integral equation technique in the context of surface-enhanced Raman scattering (SERS). The antennae comprise random arrays of vertically aligned, multi-walled carbon nanotubes dressed with highly granular Ag. Different types of "hot-spot" underpinning the SERS are identified, but contrasting characteristics are revealed. Those at the outer edges of the Ag grains are antenna driven with field enhancement amplified in antenna antinodes while intergrain hotspots are largely independent of antenna activity. Hot-spots between the tops of antennae leaning towards each other also appear to benefit from antenna amplification.

RAMAN-SCATTERING AND SCANNING ELECTRON-TUNNELING STUDIES OF METAL LIQUID-LIKE FILMS PRODUCED FROM SILVER AND GOLD SOLS

Surface-enhanced Raman scattering (SERS) excited at several visible wavelengths and recorded using a cooled charged-coupled device detector is reported from the mobile, interfacial, liquid-like metal films (MELLFs) formed when solutions of metal complexes or pyridine in chlorocarbon solvents are mixed with aqueous sols of silver or gold. MELLF formation has not previously been reported for gold sols or for pyridine as stabilizer. Comparison of the spectra for the MELLFs formed from individual metal complexes and from 50:50 mixtures show that the spectral patterns observed for the latter are distinctive and are not generally equivalent to the sum of the spectra associated with the individual complexes, in contrast to the situation observed for sols where the individual spectra do appear to be additive. Raman scattering from both gold and silver MELLFs is readily observed at excitation wavelengths in the red, around 750 nm, but at 514 nm only that from silver films is detectable. These findings are considered in terms of particle size and absorption band intensities. A preliminary study of the film surface topography and particle size was carried out by scanning tunnelling electron microscopy (STM) of Ag MELLFs deposited on gold-coated mica substrates. Computer-processed images of the STM data show the presence on the film surface of finger-like bars, 200-400 nm long with approximately square cross-section, 40-60 nm side, together with other smaller cuboid features. The implications of these findings in relation to SERS are briefly considered.

Preaggregated Ag Nanoparticles in Dry Swellable Gel Films for Off-the-Shelf Surface-Enhanced Raman Spectroscopy

Large, thin (50 mu m) dry polymer sheets containing numerous surface-enhanced Raman spectroscopy (SERS) active Ag nanopartide aggregates have been prepared by drying aqueous mixtures of hydroxyethylcelloulose (HEC) and preaggregated Ag colloid in 10 x 10 cm molds. In these dry films, the particle aggregates are protected from the environment during storage and are easy to handle; for example, they can be cut to size with scissors. When in use, the highly swellable HEC polymer allowed the films to rapidly absorb aqueous analyte solutions while simultaneously releasing the Ag nanoparticle aggregates to interact with the analyte and generate large SERS signals. Either the films could be immersed in the analyte solution or 5 mu L droplets were applied to the surface; in the latter method, the local swelling caused the active area to dome upward, but the swollen film remained physically robust and could be handled as required. Importantly, encapsulation and release did not significantly compromise the SERS performance of the colloid; the signals given by the swollen films were similar to the very high signals obtained from the parent citrate-reduced colloid and were an order of magnitude larger than a commercially available nanoparticle substrate. These "Poly-SERS" films retained 70% of their SERS activity after being stored for 1 year in air. The films were sufficiently homogeneous to give a standard deviation of 3.2% in the absolute signal levels obtained from a test analyte, primarily due to the films' ability to suppress "coffee ring" drying marks, which meant that quantitative analysis without an internal standard was possible. The majority of the work used aqueous thiophenol as the test analyte; however, preliminary studies showed that the Poly-SERS films could also be used with nonaqueous solvents and for a range of other analytes including theophylline, a therapeutic drug, at a concentration as low as 1.0 x 10(-5) mol dm(-3) (1.8 mg/dm(3)), well below the sensitivity required for theophylline monitoring where the target range is 10-20 mg/dm(3).

Analysis of 5-hydroxyisoflavones by surface-enhanced Raman spectroscopy : genistein and methoxy derivatives

A series of 5-hydroxy-isoflavones—genistein, biochanin A, prunetin, and 4′,7-dimethoxygenistein—have been studied by surface-enhanced Raman spectroscopy (SERS). Citrate reduced silver colloids were employed as a standard technique to measure SER spectra over a range of pH and concentrations. Density functional theory calculations were used to assist in determining the mode of interaction of isoflavones with the silver nanoparticles. It is revealed that biochanin A and prunetin interact with the silver nanoparticles upon deprotonation of the 7- and the 4′-OH groups, respectively, to show SERS activity. Correlations of their spectra with SERS of genistein strongly support the presence of multiple interaction modes involving both of the OH groups in genistein, in a similar manner to daidzein. Surprisingly, however, under these conditions, the 5-OH group was found to be noninteractive as revealed by attempts to measure SERS of 4′,7-dimethoxygenistein. This was attributed partly to the low solubility and, more importantly, to the influence of steric hindrance, caused by the position of the pendant phenyl ring, which prevented interaction with the Ag colloid surface. These results complement recent work on daidzein and formononetin and provide further insight into understanding the SER spectra of isoflavones.

Development of a surface-enhanced raman spectroscopy method for the detection of benzodiazepines in urine

Benzodiazepines are among the most prescribed compounds for anti-anxiety and are present in many toxicological screens. These drugs are also prominent in the commission of drug facilitated sexual assaults due their effects on the central nervous system. Due to their potency, a low dose of these compounds is often administered to victims; therefore, the target detection limit for these compounds in biological samples is 10 ng/mL. Currently these compounds are predominantly analyzed using immunoassay techniques; however more specific screening methods are needed. ^ The goal of this dissertation was to develop a rapid, specific screening technique for benzodiazepines in urine samples utilizing surface-enhanced Raman spectroscopy (SERS), which has previously been shown be capable of to detect trace quantities of pharmaceutical compounds in aqueous solutions. Surface enhanced Raman spectroscopy has the advantage of overcoming the low sensitivity and fluorescence effects seen with conventional Raman spectroscopy. The spectra are obtained by applying an analyte onto a SERS-active metal substrate such as colloidal metal particles. SERS signals can be further increased with the addition of aggregate solutions. These agents cause the nanoparticles to amass and form hot-spots which increase the signal intensity. ^ In this work, the colloidal particles are spherical gold nanoparticles in aqueous solution with an average size of approximately 30 nm. The optimum aggregating agent for the detection of benzodiazepines was determined to be 16.7 mM MgCl2, providing the highest signal intensities at the lowest drug concentrations with limits of detection between 0.5 and 127 ng/mL. A supported liquid extraction technique was utilized as a rapid clean extraction for benzodiazepines from urine at a pH of 5.0, allowing for clean extraction with limits of detection between 6 and 640 ng/mL. It was shown that at this pH other drugs that are prevalent in urine samples can be removed providing the selective detection of the benzodiazepine of interest. ^ This technique has been shown to provide rapid (less than twenty minutes), sensitive, and specific detection of benzodiazepines at low concentrations in urine. It provides the forensic community with a sensitive and specific screening technique for the detection of benzodiazepines in drug facilitated assault cases.^

Development of a Surface-Enhanced Raman Spectroscopy Method for the Detection of Benzodiazepines in Urine

Benzodiazepines are among the most prescribed compounds for anti-anxiety and are present in many toxicological screens. These drugs are also prominent in the commission of drug facilitated sexual assaults due their effects on the central nervous system. Due to their potency, a low dose of these compounds is often administered to victims; therefore, the target detection limit for these compounds in biological samples is 10 ng/mL. Currently these compounds are predominantly analyzed using immunoassay techniques; however more specific screening methods are needed. The goal of this dissertation was to develop a rapid, specific screening technique for benzodiazepines in urine samples utilizing surface-enhanced Raman spectroscopy (SERS), which has previously been shown be capable of to detect trace quantities of pharmaceutical compounds in aqueous solutions. Surface enhanced Raman spectroscopy has the advantage of overcoming the low sensitivity and fluorescence effects seen with conventional Raman spectroscopy. The spectra are obtained by applying an analyte onto a SERS-active metal substrate such as colloidal metal particles. SERS signals can be further increased with the addition of aggregate solutions. These agents cause the nanoparticles to amass and form hot-spots which increase the signal intensity. In this work, the colloidal particles are spherical gold nanoparticles in aqueous solution with an average size of approximately 30 nm. The optimum aggregating agent for the detection of benzodiazepines was determined to be 16.7 mM MgCl2, providing the highest signal intensities at the lowest drug concentrations with limits of detection between 0.5 and 127 ng/mL. A supported liquid extraction technique was utilized as a rapid clean extraction for benzodiazepines from urine at a pH of 5.0, allowing for clean extraction with limits of detection between 6 and 640 ng/mL. It was shown that at this pH other drugs that are prevalent in urine samples can be removed providing the selective detection of the benzodiazepine of interest. This technique has been shown to provide rapid (less than twenty minutes), sensitive, and specific detection of benzodiazepines at low concentrations in urine. It provides the forensic community with a sensitive and specific screening technique for the detection of benzodiazepines in drug facilitated assault cases.

Surface-Enhanced Raman Spectroscopy as a Probe of the Surface Chemistry of Nanostructured Materials

Surface-enhanced Raman spectroscopy (SERS) is now widely used as a rapid and inexpensive tool for chemical/biochemical analysis. The method can give enormous increases in the intensities of the Raman signals of low-concentration molecular targets if they are adsorbed on suitable enhancing substrates, which are typically composed of nanostructured Ag or Au. However, the features of SERS that allow it to be used as a chemical sensor also mean that it can be used as a powerful probe of the surface chemistry of any nanostructured material that can provide SERS enhancement. This is important because it is the surface chemistry that controls how these materials interact with their local environment and, in real applications, this interaction can be more important than more commonly measured properties such as morphology or plasmonic absorption. Here, the opportunity that this approach to SERS provides is illustrated with examples where the surface chemistry is both characterized and controlled in order to create functional nanomaterials.

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.

Boron nitride nanosheets improve sensitivity and reusability of surface-enhanced raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) is a useful multidisciplinary analytic technique. However, it is still a challenge to produce SERS substrates that are highly sensitive, reproducible, stable, reusable, and scalable. Herein, we demonstrate that atomically thin boron nitride (BN) nanosheets have many unique and desirable properties to help solve this challenge. The synergic effect of the atomic thickness, high flexibility, stronger surface adsorption capability, electrical insulation, impermeability, high thermal and chemical stability of BN nanosheets can increase the Raman sensitivity by up to two orders, and in the meantime attain long-term stability and extraordinary reusability not achievable by other materials. These advances will greatly facilitate the wider use of SERS in many fields.

Detection of DNA mutations using novel SERS (Surface-Enhanced Raman Spectroscopy) diagnostic platform

This article describes the detection of DNA mutations using novel Au-Ag coated GaN substrate as SERS (surface-enhanced Raman spectroscopy) diagnostic platform. Oligonucleotide sequences corresponding to the BCR-ABL (breakpoint cluster region-Abelson) gene responsible for development of chronic myelogenous leukemia were used as a model system to demonstrate the discrimination between the wild type and Met244Val mutations. The thiolated ssDNA (single-strand DNA) was immobilized on the SERS-active surface and then hybridized to a labeled target sequence from solution. An intense SERS signal of the reporter molecule MGITC was detected from the complementary target due to formation of double helix. The SERS signal was either not observed, or decreased dramatically for a negative control sample consisting of labeled DNA that was not complementary to the DNA probe. The results indicate that our SERS substrate offers an opportunity for the development of novel diagnostic assays.