Au nanostars of different sizes and shapes prepared using a simple method and their applications.

Surface roughness of metal nanoparticles (NPs) is wellknown to play an important role in the quantitative analysis and performance of their surface-enhanced Raman scattering (SERS), but its analogous role in catalysis is not given the due credit yet. In the present work, we systematically investigated this by utilizing two significantly relevant gold (Au) nanostructures, hollow and star-shaped, in a comprehensive manner. For catalysis, the universal model reaction, the reduction of p-nitroaniline to phenylenediamine using NaBH 4 , was engaged and SERS measurements were performed by employing methylene blue as the standard analyte molecule. At the outset, the predominantly fascinating role of the nanoparticle surface area in conjunction with surface roughness in catalysis was carefully evaluated for our robustly synthesized Au hollow nanoparticles (NPs), possessing an inherent thin inner core layer of Ag, and star-shaped Au NPs along with two different-sized solid spherical Au NPs. As expected, improved performances were observed in all of the above nanoparticles, but with subtle differences. Even though the observed catalytic reaction rate constant was higher in the smaller-sized solid Au NPs, it evidently followed the first-order reaction kinetics throughout, whereas in the case of star-shaped Au NPs, the rate constant was relatively higher, yet the reaction kinetics was found to be of first order in nature, essentially envisaging the fact that increasing the catalytic surface area along with roughness solely enhances the reaction rate, but not the kinetics. Nevertheless, when hollow Au NPs were used, not only the reaction completed in a much lesser time but also the reaction behavior followed a drastically different path, i.e., zeroth-order kinetics, extensively attributed to the presence of interior cavity in the case of hollow Au NPs, thereby substantiating the increased surface area. A complementary trend was observed in the SERS applications, wherein the solid spherical and hollow Au NPs demonstrated detection up to nanomolar concentration of methylene blue dye molecules (corresponding enhancement factor of ∼10 6 ), in direct contrast with the starshaped Au NPs, which meticulously detected even <25 pM concentration (corresponding enhancement factor of ∼10 8 ) with ease, owing to its higher surface roughness, manifesting itself as the crucial parameter, thus emphasizing the fact that such a simple, yet hitherto unexplored/neglected, comparative study dictates the application potentials of a wide variety of nanostructures in general.

No abstract

A dipole emitter (analogous to organic molecules) placed near any dielectric or metal nanoparticles, based on their spatially constrained location and orientation, vividly emulate optical nanoantennas. Herein, we have extensively simulated the single dipole emitter interaction with spherical homodimers of noble metal gold and silver nanoparticles for different positional dependent orientations, exemplifying systematically the hybridization (Rabi splitting), quenching and diffuse non‐radiative energy transfer effects. The blue‐shifted scattering spectra of the coupled dipole emitter with individual Ag and Au homodimers quantify and corroborate the corresponding quantum yield/efficiency estimations. Such spatially oriented emitter interaction further distinctly reveals the enhanced near‐field and far‐field effects of the plasmonic dimer system in terms of dipolar and quadrupolar resonances, arising due to the induced polarization by respective incident electric and opto‐magnetic field components. Understanding the surface‐enhanced behavior of organic molecules that are either incorporated or randomly attached (experimentally) on the surface of metal nanoparticles will benefit greatly from the present comprehensive data analysis.