In this study, we have generated a femtosecond (fs), non-diffracting Bessel beam (800 nm, 1 kHz, 50 fs) of zeroth order through an axicon (IR range, 100, AR coated). We have performed laser ablation of a bimetallic alloy (50% gold, 50 % silver) in the air engaging the generated fs Bessel beam. The high-intensity Bessel beam-matter interaction resulted in the fabrication of exotic bimetallic nanostructures. Extensive field emission scanning electron microscope and atomic force microscopy characterizations were undertaken to study the nanoscale topographical formations. The fs Bessel beam-induced ablation on the alloy target, involving the beam profile imprint on a single surface spot, followed by overlapping two ablation zones, has been meticulously explored. The central lobe ablated area, along with concentric rings-ablated exotic patterns, were thoroughly investigated in the topographical characterization. In the case of the complete raster scan ablation, ladder-like periodic surface structures (with sub ~20 nm growths on the ladder steps) were observed. Energy-dispersive X-ray mapping was performed to confirm the elemental distribution in the nanostructured areas. Subsequently, these plasmonic nanostructures were utilized as surface-enhanced Raman scattering (SERS) platforms to detect traces of real-time explosives, ammonium nitrate (AN), and Tetryl (TL). The SERS spectra of AN depicted a signature Raman peak at 1043 cm-1, whereas TL exhibited a signature peak near 1353 cm-1. The lowest possible detected traces were 10 μM and 5 μM, for AN and TL, respectively.
In this work, we report on the trace detection of an explosive molecule, picric acid (PA), and a dye molecule, malachite green (MG), using surface enhanced Raman scattering (SERS) technique. We have synthesized porous Silicon (PSi) by a simple electrochemical etching method and anisotropic gold nanostars (AuNSs) using chemical reduction of the gold salt (HAuCl4). Rough PSi acts as a suitable platform for generating SERS hotspots upon the addition of these AuNSs. The average particle size was found to be <50 nm with a strong absorption peak in the near infra-red (NIR) spectral region. PSi substrates along with AuNSs on its surface are used to explore their detection performance for PA and MG at different concentrations. Furthermore, we have compared the Raman signal intensities of Malachite Green (MG) on Si, PSi with and without Au NSs. Without Au NSs, bare PSi was found to exhibit a low Raman signal as compared to bare Si due to its hindering effect of an analyte molecule in the pore structures. However, this signal is enhanced by employing AuNSs onto the roughened porous surface. A portable Raman spectrometer (BWTEK) was used for all the SERS measurements with an excitation wavelength of 785 nm. We have achieved detection of MG at nanomolar (10-9 M) and PA at micromolar (10-6 M) concentrations using these hybrid SERS substrates. The enhancement factor was estimated to be in the range of 104-105. We believe that the optimization of porosity in PSi and sizes of AuNSs will improve the limit of detection further.
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