Ravi Kant Soni

We report on the detection of molecules at the trace level using deep‐UV surface‐enhanced Raman spectroscopy (SERS) on shape‐anisotropic rhodium (Rh) nanoparticles. Rh nanoparticles of three distinct morphologies, triangular nanoplates, rectangular nanoplates, and concave nanocubes, that exhibit strong plasmon resonances in the UV range were exploited for deep‐UV SERS. With an enhancement factor greater than 10⁵, Rh concave nanocubes demonstrate the highest SERS activity, which is well corroborated by electric field distribution maps calculated using finite‐difference time‐domain (FDTD) simulations. The deep‐UV SERS substrate based on Rh concave nanocubes enables the detection of trace levels of explosive molecules such as p‐nitrobenzoic acid and 2,4‐dinitrotoluene with detection limits of 10⁻¹⁰ M and 10⁻⁷ M, respectively, and ammonium nitrate with a detection limit of 10⁻¹² M. Deep‐UV SERS detection of explosive molecules on a low‐cost and simple‐to‐fabricate Rh nanoparticles substrate holds tremendous potential for the SERS‐based identification of hazardous materials.

Multibranched silver nanostructures (AgNSts) have attracted much attention as promising candidates for surface‐enhanced Raman spectroscopy (SERS) due to their unique plasmonic properties. We have chemically synthesized silver nanodendrites (AgNDs) and silver stars (AgSs) and investigated their SERS performance for trace‐level molecule detection. High enhancement factor (EF) ~10¹⁰ and attomolar detection limits were obtained with multibranched nanostructure‐based SERS substrates for methylene blue, thiram, and phosmet. The confinement of the local fields at the sharp tips and in the intrabranch and interbranch nanogaps of AgNDs offer highly dense three‐dimensional (3D) SERS “hot spots” and large signal enhancements of ~10⁹. Further, the as‐prepared AgND‐based substrates were utilized for the ultrasensitive identification of explosive molecules 2,4‐DNT, PNBA, and PA with limit‐of‐detection (LOD) down to ~5.3 × 10⁻¹⁶, 2.9 × 10⁻¹⁶, and 3.8 × 10⁻¹² M concentrations, respectively. The spectral modifications and appearance of new SERS peaks in the low wavenumber region indicate the metal–molecule complex formation and confirm the contribution of the chemical enhancement mechanism. The narrow spectral widths of Raman peaks allow the highly selective multiplexed detection of explosive molecules from the two‐component (2‐plex) mixture of dyes with different concentrations. Further, the density functional theory (DFT)‐based Raman spectrum calculations of the molecules exhibit a reasonably good correspondence with the experimental results. Therefore, the molecularly specific and distinguishably sharp Raman features enable the ultrasensitive and multiplexed detection of analytes molecules with our AgNSt‐based SERS substrates.

We report a facile synthesis strategy of bimetallic ultra-thin shell wrapped gold-palladium core-shell nanoparticles employing liquid-assisted pulsed laser ablation (LA-PLA) technique. The bimetallic nanoparticles were prepared by double-step successive and single-step simultaneous laser ablation in the presence of precursor solutions. The columbic attraction of the metal ions in the solution with the ablated metal nanoparticles ensures the uniform wrapping of ultrathin shall layer through the electrostatic self-assembly. The APTMS-functionalized nanoparticles-based SERS substrates exhibit greatly amplified signals along with excellent signal reproducibility and repeatability with an RSD value of less than 20% for multiple substrates. The prepared core-shell NPs depict significantly improved SERS activity as compared to their corresponding monometallic samples for the MB molecules down to the ~1.38×10-12 M detectability. Owing to the ultra-thin Pd shell formation around the central Au core, the superior SERS activity was achieved with Au@Pd core-shell NPs through borrowing SERS activity for the trace-level detection of explosives ammonium nitrate (AN) and picric acid (PA) exhibiting detectability of ~1.9×10-7 M and 3.49×10-8 M concentrations, respectively. To determine the influence of Au in the Pd@Au and Au@Pd core-shell NPs, numerical simulations were performed to explore the plasmonic response and electric-field distributions of bimetallic NPs and compare them to monometallic PdNPs. Further, the Au@PdNPs-based substrates were utilized for the simultaneous identification of different concentrations of explosives from the fixed concentration MB dye molecules with high specificity. The single-step laser ablation provides ultra-thin shell-coated Au@PdNPs with significantly improved and reproducible SERS activity compared to PdNPs.

Schematic representation of SERS and self-cleaning features of biosynthetically grown ZA nano-hybrids.

We report a facile synthesis procedure of multi-spiked star-shaped silver structures with sharp morphological features and broad plasmonic absorption for the fabrication of surface-enhanced Raman scattering (SERS) substrates. The multi-branched morphology of silver stars (AgSs) provides copious hot spots on SERS-active substrates to detect ultra-low concentrations of probe molecules. Significant Raman intensity enhancement factors of the order of ∼10 ⁹ were obtained for the different analytes. The AgSs-based SERS-substrates show ultra-sensitive SERS performance for the detection of probe molecules rhodamine B, methylene blue, thiram, and phosmet with detectabilities 2.6 × 10 ⁻¹⁶ , 1.97 × 10 ⁻¹⁷ , 1.78 × 10 ⁻¹⁸ , and 4.08 × 10 ⁻¹⁴ M concentration, respectively. The improved SERS sensitivity originated from the additional contribution of chemical enhancement, corroborated by the appearance of new Raman bands in the SERS spectrum. For practical applications, SERS based molecule detection requires high sensitivity, specificity, uniformity, and reproducibility of the SERS signal. We demonstrate the simultaneous detection of multiple analytes, with high specificity, from the complex mixture of molecules with different compositions. The fabricated SERS substrates provide excellent signal uniformity and reproducibility with ∼10% relative standard deviation for the multiple substrates. The real-life application of AgSs-based SERS substrates with the spiked apple juice and ITO-coated PET films without pre-processing is also demonstrated.

2D-layered nanostructure-based advanced coatings are the most recent advancement in the field of advanced oxidation processes. Effective charge separation is one of the key parameters in enhancing the photodegradation ability of 2D-layered nanostructures. We report the cost-effective hydrothermal synthesis of MoS ₂ thin films on conductive ITO substrates with tunable morphology. Scanning electron microscopy studies manifested the modulation in the morphology of MoS ₂ thin films and confirmed the presence of spherical, thread, and sheet-like nanostructures under the evolution of precursor concentration. UV–visible absorption spectroscopy revealed the modulation in the optical absorption and effective bandgap narrowing with the changes in the surface morphology. The estimated bandgap value of MoS ₂ thin film samples varies from 1.82 to 1.69 eV. PL spectroscopy and electrochemical studies display the morphology-dependent charge separation behavior and confirm that the lowest recombination rate is attained by thread-like MoS ₂ nanostructures. Raman spectroscopy and x-ray photoelectron spectroscopy confirms the 2H phase of MoS ₂ thin films. MoS ₂ thin films with thread-like structures are found to be the most efficient for sunlight-induced photodegradation activity as compared to other MoS ₂ samples. Thread-like MoS ₂ structures containing MoS ₂ thin film with the highest charge transport properties decompose 85% of 5 µ M methylene blue molecules and 90.4% of 5 µ M rhodamine B molecule solution in 40 min and 80 min, respectively, under natural sunlight. The prominent charge separation effects on the enhanced photodegradation capabilities of MoS ₂ thin films due to distinct variations in morphology, which have not been reported till now, are explored precisely.