Ultrasensitive detection of crystal violet using a molybdenum sulfide-silver nanostructure-based sensing platform: roles of the adsorbing semiconductor in SERS signal enhancement

Crystal violet (CV) is an organic dye that is stabilized by the extensive resonance delocalization of electrons over three electron-donating amine groups. This prevents the molecule from being linked to a metal surface, and therefore, reduces the sensitivity of surface-enhanced Raman scattering (SERS) sensors for this toxic dye. In this work, we improved the sensing performance of a silver-based SERS sensor for CV detection by modifying the active substrate. Molybdenum sulfide (MoS2) nanosheets were employed as a scaffold for anchoring electrochemically synthesized silver nanoparticles (e-AgNPs) through a single step of ultrasonication, leading to the formation of MoS2/Ag nanocomposites. As an excellent adsorbent, MoS2 promoted the adsorption of CV onto the surface of the substrate, allowing more CV molecules to be able to experience the SERS effect originating from the e-AgNPs. Hence, the SERS signal of CV was significantly enhanced. In addition, the effects of the MoS2 content of the nanocomposites on their SERS performance were also taken into account. Using MoS2/Ag with the most optimal MoS2 content of 10%, the SERS sensor exhibited the best enhancement of the SERS signal of CV with an impressive detection limit of 1.17 × 10-11 M in standard water and 10-9 M in tap water thanks to an enhancement factor of 2.9 × 106, which was 11.2 times higher than that using pure e-AgNPs.

Stimuli-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps

Incorporating stimuli-responsive block copolymers to hierarchical metallic nanoparticles (MNPs) is of particular interest due to their tunable plasmonic properties responding to environmental stimuli. We herein report thermo-responsive plasmonic core-satellite hybrid nanostructures with tunable nanogaps as surface-enhanced Raman scattering (SERS) nanotags. Two different diblock copolymers with opposite charges, poly(acrylic acid-b-N-isopropylacrylamide) (p(AAc-b-NIPAM)) and poly(N,N-dimethylaminoethyl methacrylate-b-N-isopropylacrylamide) (p(DMAEMA-b-NIPAM)), were synthesized. The negatively charged p(AAc-b-NIPAM)s were bound to gold nanospheres (GNSs), while the positively charged p(DMAEMA-b-NIPAM)s were conjugated to gold nanorods (GNRs) via gold-sulfur bonds. When p(AAc-b-NIPAM)-GNSs and p(DMAEMA-b-NIPAM)-GNRs were electrostatically complexed, plasmonic hybrid nanostructures consisting of both GNS satellites and a GNR core were formed. Dynamic tuning of electromagnetic coupling of their nanogaps was achieved via a temperature-triggered conformational change of p(NIPAM) blocks. Furthermore, a sandwich-type immunoassay for the detection of immunoglobulin G was performed to demonstrate these core-satellites as potential SERS nanotags. Our results showed that these plasmonic core-satellites with stimuli-responsiveness are promising for SERS-based biosensing applications.

Optimization of surface enhanced Raman scattering performance based on Ag nanoparticle-modified vanadium-titanium nanorods with tunable nanogaps

The combination of new noble metal nanomaterials and surface enhanced Raman scattering (SERS) technology has become a new strategy to solve the problem of low sensitivity in the detection of traditional Chinese medicine. In this work, taking natural cicada wing (C.w.) as a template, by optimizing the magnetron sputtering experimental parameters for the growth of Ag nanoparticles (NPs) on vanadium-titanium (V-Ti) nanorods, the nanogaps between the nanorods were effectively regulated and the Raman signal intensity of the Ag₁₅/V-Ti₂₀/C.w. substrate was improved. The proposed homogeneous nanostructure exhibited high SERS activity through the synergistic effect of the electromagnetic enhancement mechanism at the nanogaps between the Ag NPs modified V-Ti nanorods. The analytical enhancement factor (AEF) value was as high as 1.819 × 10⁸, and the limit of detection (LOD) was 1 × 10^-11 M for R6G. The large-scale distribution of regular electromagnetic enhancement "hot spots" ensured the good reproducibility with the relative standard deviation (RSD) value less than 7.31%. More importantly, the active compound of Artemisinin corresponded the pharmacological effect of Artemisia annua was screened out by SERS technology, and achieved a LOD of 0.01 mg/l. This reliable preparation technology was practically applicable to produce SERS-active substrates in detection of pharmacodynamic substance in traditional Chinese medicine.

Recent Progress of SERS Nanoprobe for pH Detecting and Its Application in Biological Imaging

As pH value almost affects the function of cells and organisms in all aspects, in biology, biochemical and many other research fields, it is necessary to apply simple, intuitive, sensitive, stable detection of pH and base characteristics inside and outside the cell. Therefore, many research groups have explored the design and application of pH probes based on surface enhanced Raman scattering (SERS). In this review article, we discussed the basic theoretical background of explaining the working mechanism of pH SERS sensors, and also briefly described the significance of cell pH measurement, and simply classified and summarized the factors that affected the performance of pH SERS probes. Some applications of pH probes based on surface enhanced Raman scattering in intracellular and extracellular pH imaging and the combination of other analytical detection techniques are described. Finally, the development prospect of this field is presented.

Sandwiching analytes with structurally diverse plasmonic nanoparticles on paper substrates for surface enhanced Raman spectroscopy

This report describes the systematic combination of structurally diverse plasmonic metal nanoparticles (AgNPs, AuNPs, Ag core-Au shell NPs, and anisotropic AuNPs) on flexible paper-based materials to induce signal-enhancing environments for surface enhanced Raman spectroscopy (SERS) applications. The anisotropic AuNP-modified paper exhibits the highest SERS response due to the surface area and the nature of the broad surface plasmon resonance (SPR) neighboring the Raman excitation wavelength. The subsequent addition of a second layer with these four NPs (e.g., sandwich arrangement) leads to the notable increase of the SERS signals by inducing a high probability of electromagnetic field environments associated with the interparticle SPR coupling and hot spots. After examining sixteen total combinations, the highest SERS response is obtained from the second layer with AgNPs on the anisotropic AuNP paper substrate, which allows for a higher calibration sensitivity and wider dynamic range than those of typical AuNP-AuNP arrangement. The variation of the SERS signals is also found to be below 20% based on multiple measurements (both intra-sample and inter-sample). Furthermore, the degree of SERS signal reductions for the sandwiched analytes is notably slow, indicating their increased long-term stability. The optimized combination is then employed in the detection of let-7f microRNA to demonstrate their practicability as SERS substrates. Precisely introducing interparticle coupling and hot spots with readily available plasmonic NPs still allows for the design of inexpensive and practical signal enhancing substrates that are capable of increasing the calibration sensitivity, extending the dynamic range, and lowering the detection limit of various organic and biological molecules.

Electrostatic Self-Assembly of Ti3C2Tx MXene and Gold Nanorods as an Efficient Surface-Enhanced Raman Scattering Platform for Reliable and High-Sensitivity Determination of Organic Pollutants

A reliable surface-enhanced Raman scattering (SERS) substrate composed of two-dimensional (2D) MXene (Ti₃C₂T_x) nanosheets and gold nanorods (AuNRs) is designed and fabricated for sensitive detection of organic pollutants. The AuNRs are uniformly distributed on the surface of the 2D MXene nanosheets because of the strong electrostatic interactions, forming abundant SERS hot spots. The MXene/AuNR SERS substrate exhibits high sensitivity and excellent reproducibility in the determination of common organic dyes such as rhodamine 6G, crystal violet, and malachite green. The detection limits are 1 × 10^-12, 1 × 10^-12, and 1 × 10^-10 M, and relative standard deviations determined from 13 areas on each sample are 18.1, 10.1, and 15.6%, respectively. In the determination of more complex organic pesticides and pollutants, the substrate also shows excellent sensitivity and quantitative detection, and the detection limits for thiram and diquat of 1 × 10^-10 and 1 × 10^-8 M, respectively, are much lower than the contaminant levels stipulated by the US Environmental Protection Agency. The MXene/AuNR composite constitutes an efficient SERS platform for reliable and high-sensitivity environmental analysis and food safety monitoring.

Synthesis and Characterization of Tunable, pH-Responsive Nanoparticle-Microgel Composites for Surface-Enhanced Raman Scattering Detection

The synthesis of microgels with pH-tunable swelling leads to adjustable and pH-responsive substrates for surface-enhanced Raman scattering (SERS)-active nanoparticles (NPs). Sterically stabilized and cross-linked latexes were synthesized from random copolymers of styrene (S) and 2-vinylpyridine (2VP). The pH-dependent latex-to-microgel transition and swellability were tuned based on their hydrophobic-to-hydrophilic content established by the S/2VP ratio. The electrostatic loading of polystyrene/poly(2-vinylpyridine) microgels [PS _x P2VP _y (M)] with anions such as tetrachloroaurate (AuCl₄ ^-) and borate-capped Ag NPs was quantified. The PS _x P2VP _y (M) can load ∼0.3 equiv of AuCl₄ ^- and the subsequent photoreduction results in Au NP-loaded PS _x P2VP _y (M) with NPs located throughout the structure. Loading PS _x P2VP _y (M) with borate-capped Ag NPs produces PS _x P2VP _y (M) with NPs located on the surface of the microgels, where the Ag content is set by S/2VP. The pH-responsive SERS activity is also reported for these Ag NP-loaded microgels. Analytical enhancement factors for dissolved crystal violet are high (i.e., 10⁹ to 10¹⁰) and are set by S/2VP. The Ag NP-loaded microgels with ∼80 wt % 2VP exhibited the most stable pH dependent response.