Herein, for the first time, we report the single molecule surface enhanced resonance Raman scattering (SERRS) and surface enhanced Raman scattering (SERS) spectra with high signal to noise ratio (S/N) using plasmon-active substrates fabricated by sprouted potato shaped Au-Ag bimetallic nanoparticles, prepared using a new one-step synthesis method. This particular shape of the nanoparticles has been obtained by fixing the amount of Au and carefully adjusting the amount of Ag. These nanoparticles have been characterized using scanning electron microscopy, extinction spectroscopy, and glancing angle X-ray diffraction. The single molecule sensitivity of SERS substrates has been tested with two different molecular Raman probes. The origin of the electromagnetic enhancement of single molecule Raman scattering in the presence of sprouted shape nanoparticles has been explained using quasi-static theory as well as finite element method (FEM) simulations. Moreover, the role of (i) methods for binding Raman probe molecules to the substrate, (ii) concentration of molecules, and (iii) Au-Ag ratio on the spectra of molecules has been studied in detail.
We report a two-step enhancement of Raman scattering signal (η) of a few dye molecules. In the first step, high-quality surface-enhanced Raman scattering (SERS) substrates have been used. The SERS substrates were fabricated by direct current sputtering of Au followed by thermal annealing. The role of thermal annealing of the SERS substrates and numerical aperture of Raman microscopic objective lens on the enhancement has been studied for optimizing the enhancement in the SERS technique. In the second step, the value of η obtained with conventional SERS technique has been improved significantly with the help of photonic nanojet (PNJ) of an optical microsphere (PNJ-mediated SERS technique). The signal to noise ratio and reproducibility of the experimental results have been found to be very high. Based on our theoretical simulations on PNJ, a few suitable parameters have been proposed for obtaining better enhancement using this technique. To the best of our belief, this report will enable the SERS community to improve η value with ease.
Finite element method simulations have been carried out on the photonic nanojet (PNJ) mediated surface enhanced Raman scattering (SERS) technique for the first time, and this technique has been found to provide (i) better Raman scattering enhancement of single molecules and (ii) a long working distance between the microscopic objective lens and sample, as compared with the conventional SERS technique. A PNJ mediated surface enhanced fluorescence (SEF) technique has been proposed to enhance the fluorescence of single molecules using the combination of localized surface plasmons inside nanostructures and the PNJ of a dielectric microsphere (MS), and this technique is numerically proved to be efficient as compared with a conventional SEF technique. Moreover, the generation of a PNJ from single lollipop shaped microstructures and its applications in the above mentioned techniques have been reported.
We report the enhancement of Raman scattering signal for the first time using photonic nanojet of a lollipop‐shaped microstructure (LMS) fabricated with the help of a nanosecond CO2 laser. The LMS consists of a dielectric microsphere attached to a long stem/tapered fiber. The enhancement was achieved by focusing the incident laser light on sample surface through the microsphere of LMS. The experimentally observed enhancement was found to be comparable with that obtained from commercial microspheres that are not attached to any stems. The value of enhancement was found to increase with the microsphere diameter. The dielectric stems/tapered fibers alone of the LMS are found not efficient for enhancing the Raman scattering signals. In contrast to the commercial microspheres, the fabricated LMSs are easy to handle, portable, and reusable. Also, unlike commercial microspheres, these LMSs allow the enhancement of Raman scattering without modifying the surface of substrates containing sample. Hence, these LMSs are extremely useful in the case of photonic nanojet‐mediated surface‐enhanced Raman scattering and fluorescence techniques for enhancing the Raman scattering and fluorescence signals of single/few molecules, respectively.
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