Detection of Low-Concentration Contaminants in Solution by Exploiting Chemical Derivatization in Surface-Enhanced Raman Spectroscopy

Rapid and Quantitative Detection of Aflatoxin B1 in Grain by Portable Raman Spectrometer

Many foodstuffs are extremely susceptible to contamination with aflatoxins, in which aflatoxin B₁ is highly toxic and carcinogenic. Therefore, it is crucial to develop a rapid and effective analytical method for detecting and monitoring aflatoxin B₁ in food. Herein, a surface-enhanced Raman spectroscopic (SERS) method combined with QuEChERS (quick, easy, cheap-effective, rugged, safe) sample pretreatment technique was used to detect aflatoxin B₁. Sample preparation was optimized into a one-step extraction method using an Au nanoparticle-based solution (Au sol) as the SERS detection substrate. An affordable portable Raman spectrometer was then used for rapid, label-free, quantitative detection of aflatoxin B₁ levels in foodstuffs. This method showed a good linear log relationship between the Raman signal intensity of aflatoxin B₁ in the 1-1000 µg L^-1 concentration range with a limit of detection of 0.85 µg kg^-1 and a correlation coefficient of 0.9836. Rapid aflatoxin B₁ detection times of ∼10 min for wheat, corn, and protein feed powder samples were also achieved. This method has high sensitivity, strong specificity, excellent stability, is simple to use, economical, and is suitable for on-site detection, with good prospects for practical application in the field of food safety.

Detection of Chloroalkanes by Surface-Enhanced Raman Spectroscopy in Microfluidic Chips

Optofluidics, a research discipline combining optics with microfluidics, currently aspires to revolutionize the analysis of biological and chemical samples, e.g., for medicine, pharmacology, or molecular biology. In order to detect low concentrations of analytes in water, we have developed an optofluidic device containing a nanostructured substrate for surface enhanced Raman spectroscopy (SERS). The geometry of the gold surface allows localized plasmon oscillations to give rise to the SERS effect, in which the Raman spectral lines are intensified by the interaction of the plasmonic field with the electrons in the molecular bonds. The SERS substrate was enclosed in a microfluidic system, which allowed transport and precise mixing of the analyzed fluids, while preventing contamination or abrasion of the highly sensitive substrate. To illustrate its practical use, we employed the device for quantitative detection of persistent environmental pollutant 1,2,3-trichloropropane in water in submillimolar concentrations. The developed sensor allows fast and simple quantification of halogenated compounds and it will contribute towards the environmental monitoring and enzymology experiments with engineered haloalkane dehalogenase enzymes.

Ultrathin polydopamine film coated gold nanoparticles: a sensitive, uniform, and stable SHINERS substrate for detection of benzotriazole

In shell-isolated nanoparticle (NP)-enhanced Raman spectroscopy (SHINERS), traditional metal oxide-based shells have inferior chemical inertness, they require strict preparation conditions, and lack specific groups, which lead to their poor selectivity toward target molecules. In this study, ultrathin and compact gold (Au)@polydopamine (PDA) SHINERS NPs were successfully fabricated by a simple self-polymerization technique. High wrapping tendency of PDA, a multifunctional biopolymer, favored the fabrication process. Au@PDA NPs exhibited a typical shell-isolated effect, i.e., Au@PDA NPs with a thick shell (more than 2.3 nm) showed a lower SERS activity, while those with an ultrathin (1.3 nm) shell exhibited higher SERS activity compared to uncoated Au NPs. The Au@PDA SHINERS substrate shows high performance in terms of sensitivity, uniformity, and stability. The relative standard deviations (RSDs) of SERS intensities from ten positions on the same substrate were less than 4%. Their Raman intensities dropped by only 15% over two months. More importantly, the Au@PDA (1.3 nm) SHINERS substrate exhibited high SERS activity for label-free and quantitative detection of benzotriazole (BTA), an important corrosion inhibitor, through utilizing a presumed π-π stacking interaction. A broad linear range from 10^-4 to 10^-8 M was achieved with a low detection limit (LOD) of 1 nM (0.119 μg L^-1). The LOD was not only significantly lower than the maximum allowable level (20 μg L^-1) of the Australian government water guide, but also lower than that of some modern methods such as fluorescence, liquid chromatography, and gas chromatography coupled with mass spectrometry. Furthermore, the substrate showed excellent discrimination against other compounds with a single aromatic ring. It is expected that the Au@PDA SHINERS substrate will offer great potential for analysis application in a complicated environmental system.

Origin of enhancement in Raman scattering from Ag-dressed carbon-nanotube antennas: experiment and modelling

The D- and G-band Raman signals from random arrays of vertically aligned, multi-walled carbon nanotubes are significantly enhanced (up to ∼14×) while the signal from the underlying Si substrate is simultaneously attenuated (up to ∼6×) when the nanotubes are dressed, either capped or coated, with Ag. These Ag-induced counter-changes originate with the difference in geometry of the nanotubes and planar Si substrate and contrast in the Ag depositions on the substrate (essentially thin film) and the nanotube (nano-particulate). The surface integral equation technique is used to perform detailed modelling of the electromagnetic response of the system in a computationally efficient manner. Within the modelling the overall antenna response of the Ag-dressed nanotubes is shown to underpin the main contribution to enhancement of the nanotube Raman signal with hot-spots between the Ag nanoparticles making a subsidiary contribution on account of their relatively weak penetration into the nanotube walls. Although additional hot-spot activity likely accounts for a shortfall in modelling relative to experiment it is nonetheless the case that the significant antenna-driven enhancement stands in marked contrast to the hot-spot dominated enhancement of the Raman spectra from molecules adsorbed on the same Ag-dressed structures. The Ag-dressing procedure for amplifying the nanotube Raman output not only allows for ready characterisation of individual nanotubes, but also evidences a small peak at ∼1150 cm^-1 (not visible for the bare, undressed nanotube) which is suggested to be due to the presence of trans-polyacetylene in the structures.

Selectivity/Specificity Improvement Strategies in Surface-Enhanced Raman Spectroscopy Analysis

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for the discrimination, identification, and potential quantification of certain compounds/organisms. However, its real application is challenging due to the multiple interference from the complicated detection matrix. Therefore, selective/specific detection is crucial for the real application of SERS technique. We summarize in this review five selective/specific detection techniques (chemical reaction, antibody, aptamer, molecularly imprinted polymers and microfluidics), which can be applied for the rapid and reliable selective/specific detection when coupled with SERS technique.

Large-volume constant-concentration sampling technique coupling with surface-enhanced Raman spectroscopy for rapid on-site gas analysis

In this work, a portable large-volume constant-concentration (LVCC) sampling technique coupling with surface-enhanced Raman spectroscopy (SERS) was developed for the rapid on-site gas analysis based on suitable derivatization methods. LVCC sampling technique mainly consisted of a specially designed sampling cell including the rigid sample container and flexible sampling bag, and an absorption-derivatization module with a portable pump and a gas flowmeter. LVCC sampling technique allowed large, alterable and well-controlled sampling volume, which kept the concentration of gas target in headspace phase constant during the entire sampling process and made the sampling result more representative. Moreover, absorption and derivatization of gas target during LVCC sampling process were efficiently merged in one step using bromine-thiourea and OPA-NH₄⁺ strategy for ethylene and SO₂ respectively, which made LVCC sampling technique conveniently adapted to consequent SERS analysis. Finally, a new LVCC sampling-SERS method was developed and successfully applied for rapid analysis of trace ethylene and SO₂ from fruits. It was satisfied that trace ethylene and SO₂ from real fruit samples could be actually and accurately quantified by this method. The minor concentration fluctuations of ethylene and SO₂ during the entire LVCC sampling process were proved to be <4.3% and 2.1% respectively. Good recoveries for ethylene and sulfur dioxide from fruit samples were achieved in range of 95.0-101% and 97.0-104% respectively. It is expected that portable LVCC sampling technique would pave the way for rapid on-site analysis of accurate concentrations of trace gas targets from real samples by SERS.

Fast assembling microarrays of superparamagnetic Fe3O4@Au nanoparticle clusters as reproducible substrates for surface-enhanced Raman scattering

It is currently a very active research area to develop new types of substrates which integrate various nanomaterials for surface-enhanced Raman scattering (SERS) techniques. Here we report a unique approach to prepare SERS substrates with reproducible performance. It features silicon mold-assisted magnetic assembling of superparamagnetic Fe3O4@Au nanoparticle clusters (NCs) into arrayed microstructures on a wafer scale. This approach enables the fabrication of both silicon-based and hydrogel-based substrates in a sequential manner. We have demonstrated that strong SERS signals can be harvested from these substrates due to an efficient coupling effect between Fe3O4@Au NCs, with enhancement factors >10(6). These substrates have been confirmed to provide reproducible SERS signals, with low variations in different locations or batches of samples. We investigate the spatial distributions of electromagnetic field enhancement around Fe3O4@Au NCs assemblies using finite-difference-time-domain (FDTD) simulations. The procedure to prepare the substrates is straightforward and fast. The silicon mold can be easily cleaned out and refilled with Fe3O4@Au NCs assisted by a magnet, therefore being re-useable for many cycles. Our approach has integrated microarray technologies and provided a platform for thousands of independently addressable SERS detection, in order to meet the requirements of a rapid, robust, and high throughput performance.

Large-scale synthesis of gold dendritic nanostructures for surface enhanced Raman scattering

A precisely controlled FIB technique is used to fabricate Au dendritic nanostructures on a large scale. These Au dendritic nanostructures exhibit excellent SERS properties with a low concentration of RhB as the target molecule and the enhancement factor of SERS reaches 10⁷.