Silver nanoparticles decorated g-C3N4: An efficient SERS substrate for monitoring catalytic reduction and selective Hg2+ions detection

Graphitic carbon nitride supported Ag NPs(AgNPs@g-C₃N₄) were synthesized by an in-situ chemical reduction using a green reducing agent, tannic acid. They were characterized by UV-Vis, FTIR, XPS, XRD, FESEM, EDAX and HRTEM. They were very much SERS sensitive, and capable of detecting methylene blue and 4-aminothiophenol at 1 × 10^-12 M and 1 × 10^-10 M, respectively with the corresponding SERS enhancement factor of 1.4 × 10⁸ and 4.7 × 10⁷. Apart from its high SERS sensitivity, it exhibited high catalytic activity for the reduction of MB with NaBH₄. So, their SERS activity and catalytic activity were combined successfully to monitor catalytic reduction of MB by SERS technique. Further, the SERS activity towards MB was also employed for the detection/quantification of free Hg²⁺ ions in aqueous solution. The SERS intensity of MB drastically decreased in the presence of Hg²⁺ ions, and hence it provides novel route to detect and quantify the latter. Presence of Ca²⁺, Mg²⁺, Cu²⁺ and Cd²⁺ions showed zero interference for it. So, this study proves that Ag NPs@g-C₃N₄ as a unique substrate for multiple SERS applications.

Nonlinear Plasmonic Sensing for Label-Free and Selective Detection of Mercury at Picomolar Level

We present the concept of a nonlinear plasmonic sensing approach for rapid, sensitive, and label-free detection of mercury. Nonlinear plasmonic sensing of mercury relies on a systematic combination of nonlinear optics (NLO) with well-known concepts of amalgamation chemistry and plasmonic properties of gold nanorods. Exploiting the extreme sensitivity of the NLO process toward Hg-induced change in the local electric field of plasmonic nanorods, we succeed in improving the limit of detection (LOD) of mercury by 2-3 orders of magnitude as compared to the commonly used linear localized surface plasmon resonance (LSPR) based sensing. Using our method, an LOD of as low as 58 pM (11 ppt) has been achieved with high selectivity. Nonlinear plasmonic sensing aproach is found to work excellently for detecting mercury in real samples like blood plasma.

Plasmonic detection of mercury via amalgam formation on surface-immobilized single Au nanorods

Au nanorods were used as plasmonic transducers for investigation of mercury detection through a mechanism of amalgam formation at the nanorod surfaces. Marked scattering color transitions and associated blue shifts of the surface plasmon resonance peak wavelengths (λ_max) were measured in individual nanorods by darkfield microscopy upon chemical reduction of Hg(II). Such changes were related to compositional changes occurring as a result of Hg-Au amalgam formation as well as morphological changes in the nanorods' aspect ratios. The plot of λ_max shifts vs. Hg(II) concentration showed a linear response in the 10-100 nM concentration range. The sensitivity of the system was ascribed to the narrow width of single nanorod scattering spectra, which allowed accurate determination of peak shifts. The system displayed good selectivity as the optical response obtained for mercury was one order of magnitude higher than the response obtained with competitor ions. Analysis of mercury content in river and tap water were also performed and highlighted both the potential and limitation of the developed method for real sensing applications.

Real-Time Plasmonic Monitoring of Single Gold Amalgam Nanoalloy Electrochemical Formation and Stripping

Direct electrodeposition of mercury onto gold nanorods on an ITO substrate, without reducing agents, is reported. The growth of single gold amalgam nanoalloy particles and subsequent stripping was monitored in real-time monitoring by plasmonic effects and single-nanoparticle dark-field spectroelectrochemistry techniques. Time-dependent scattering spectral information conferred insight into the growth and stripping mechanism of a single nanoalloy particle. Four critical stages were observed: First, rapid deposition of Hg atoms onto Au nanorods; second, slow diffusion of Hg atoms into Au nanorods; third, prompt stripping of Hg atoms from Au nanorods; fourth, moderate diffusion from the inner core of Au nanorods. Under high Hg(2+) concentrations, homogeneous spherical gold amalgam nanoalloys were obtained. These results demonstrate that the morphology and composition of individual gold amalgam nanoalloys can be precisely regulated electrochemically. Moreover, gold amalgam nanoalloys with intriguing optical properties, such as modulated plasmonic lifetimes and quality factor Q, could be obtained. This may offer opportunities to extend applications in photovoltaic energy conversion and chemical sensing.

A silver electrode based surface acoustic wave (SAW) mercury vapor sensor: a physio-chemical and analytical investigation

In this study, a surface acoustic wave based Hg⁰ vapour sensor was developed where Ag IDT electrodes were employed as lone sensing elements.