Photoluminescent Silica Nanotubes and Nanodisks Prepared by the Reverse Micelle Sol−Gel Method

The reverse micelle sol−gel method was used earlier to prepare silica nanotubes, in aerosol OT/ n-heptane/water microemulsions containing FeCl3. The present communication reports the remarkable effect of the amount of water in the microemulsions on the shape, size, and spectral properties of the silica nanostructures formed. Nanotubes are formed, as expected, at lower water contents. However, for higher water contents, nanodisks form in predominance. This rather surprising observation indicates the formation of flat, disklike water pools in this medium. Notably, a phase separation occurs at higher water contents, and this appears to be essential for the formation of the disklike nanostructures. Hence, we propose that flat water pools form at the interface of the two liquid phases. The nanotubes and nanodisks exhibit blue photoluminescence. The photoluminescence of the nanotubes is more susceptible to quenching by moisture than that of the nanodisks. Luminescence is restored by heating or purging nitrogen or oxygen. Time-resolved photoluminescence studies conform to a model in which the luminescence is ascribed to a particular kind of defect center, with some contribution from surface-associated defects

Lamellar Micelles as Templates for the Preparation of Silica Nanodisks

We have reported earlier that silica nanodisks are formed by the reverse micelle sol–gel (RMSG) method, from ternary Aerosol OT (AOT)/n-heptane/water mixtures. These nanodisks are formed from the ternary mixtures with higher water contents and containing a significant concentration of FeCl3. Such mixtures undergo a phase separation. The present work seeks to identify the origin of these nanodisks. They are found to be produced from the lower, water-rich layer and not from the upper, oil-rich layer or from the interfacial region. Further, upon increasing the water content by a factor of 10, the phase separation is achieved in the absence of the salt as well. In the next step, the naodisks are observed to form from concentrated aqueous AOT solutions, with no involvement of organic solvents. Polarized optical microscopy and infrared studies reveal the occurrence of lamellar AOT micelles in these media. These micelles act as the templates for the nanodisks. This phenomenon paves the way for the soft chemical preparation of nanodisks in the aqueous phase, without the need of using salt or organic solvents

Picosecond Bessel Beam Fabricated Pure, Gold-Coated Silver Nanostructures for Trace-Level Sensing of Multiple Explosives and Hazardous Molecules

A zeroth-order, non-diffracting Bessel beam, generated by picosecond laser pulses (1064 nm, 10 Hz, 30 ps) through an axicon, was utilized to perform pulse energy-dependent (12 mJ, 16 mJ, 20 mJ, 24 mJ) laser ablation of silver (Ag) substrates in air. The fabrication resulted in finger-like Ag nanostructures (NSs) in the sub-200 nm domain and obtained structures were characterized using the FESEM and AFM techniques. Subsequently, we employed those Ag NSs in surface-enhanced Raman spectroscopy (SERS) studies achieving promising sensing results towards trace-level detection of six different hazardous materials (explosive molecules of picric acid (PA) and ammonium nitrate (AN), a pesticide thiram (TH) and the dye molecules of Methylene Blue (MB), Malachite Green (MG), and Nile Blue (NB)) along with a biomolecule (hen egg white lysozyme (HEWL)). The remarkably superior plasmonic behaviour exhibited by the AgNS corresponding to 16 mJ pulse ablation energy was further explored. To accomplish a real-time application-oriented understanding, time-dependent studies were performed utilizing the AgNS prepared with 16 mJ and TH molecule by collecting the SERS data periodically for up to 120 days. The coated AgNSs were prepared with optimized gold (Au) deposition, accomplishing a much lower trace detection in the case of thiram (~50 pM compared to ~50 nM achieved prior to the coating) as well as superior EF up to ~108 (~106 before Au coating). Additionally, these substrates have demonstrated superior stability compared to those obtained before Au coating.</jats:p

Enhanced Trapping Efficiency in Acid-Treated Silica Nanostructures

The color of the photoluminescence (PL) of silica nanostructures changes remarkably from blue to bluish green, with the addition of mineral acids. This remarkable observation is rationalized in light of an excited-state protonation of some of the trap states, as well as electrostatic assistance provided by the H3O+ ions adsorbed at the surface. A combination of these two leads to more efficient trapping. The enhanced trapping is manifested in the time-resolved PL parameters and spectra. A fast decay of PL is observed in the higher-energy region of the spectrum, while a rise, with an almost equal time constant, is observed at the lower-energy end. Time-resolved area-normalized emission spectra are dominated by a structured spectrum in the high-energy region at shorter times and a broad, red shifted spectrum at longer times

Interaction of Surface Trap States and Defect Pair of Photoluminescent Silica Nanostructures with H₂O₂ and Solvents

The photoluminescence (PL) of silica nano-structures has been found to be affected remarkably by organic solvents and hydrogen peroxide. The absorption and PL emission as well as excitation spectra show a variation in the position of the spectral maxima, with the change in the solvent polarity. The most interesting effect of solvents is observed in the PL decays. The decays, which are biexponential in polar media, are single exponential in nonpolar solvents. This observation indicates the absence or destabilization of surface related trap states in nonpolar media. On the other hand, these states seem to be stabilized in polar solvents, as the long component of the PL decay is further enhanced in the dispersions in polar solvents. Such enhancement occurs, most likely, because of hydrogen bonding between the silanol groups and the solvent molecules. PL quenching is observed upon addition of H2O2. Surprisingly, this is accompanied by an enhancement in the long component of the decay. This pair of apparently contradictory observations indicates the stabilization of surface trap states with H2O2 and at the same time, formation of new, nonluminescent defect centers by a reaction between the photoluminescent defect pair and H2O2

Picosecond Bessel Beam Fabricated Pure, Gold-Coated Silver Nanostructures for Trace-Level Sensing of Multiple Explosives and Hazardous Molecules

A zeroth-order, non-diffracting Bessel beam, generated by picosecond laser pulses (1064 nm, 10 Hz, 30 ps) through an axicon, was utilized to perform pulse energy-dependent (12 mJ, 16 mJ, 20 mJ, 24 mJ) laser ablation of silver (Ag) substrates in air. The fabrication resulted in finger-like Ag nanostructures (NSs) in the sub-200 nm domain and obtained structures were characterized using the FESEM and AFM techniques. Subsequently, we employed those Ag NSs in surface-enhanced Raman spectroscopy (SERS) studies achieving promising sensing results towards trace-level detection of six different hazardous materials (explosive molecules of picric acid (PA) and ammonium nitrate (AN), a pesticide thiram (TH) and the dye molecules of Methylene Blue (MB), Malachite Green (MG), and Nile Blue (NB)) along with a biomolecule (hen egg white lysozyme (HEWL)). The remarkably superior plasmonic behaviour exhibited by the AgNS corresponding to 16 mJ pulse ablation energy was further explored. To accomplish a real-time application-oriented understanding, time-dependent studies were performed utilizing the AgNS prepared with 16 mJ and TH molecule by collecting the SERS data periodically for up to 120 days. The coated AgNSs were prepared with optimized gold (Au) deposition, accomplishing a much lower trace detection in the case of thiram (~50 pM compared to ~50 nM achieved prior to the coating) as well as superior EF up to ~108 (~106 before Au coating). Additionally, these substrates have demonstrated superior stability compared to those obtained before Au coating