MoS2 nanosheets decorated plasmonic silicon nanowires as SERS substrates for ultra-sensitive multiple analyte detection

We report the fabrication of hybrid SERS-active substrates for ultra-sensitive detection of diverse analyte molecules such as explosives, dyes, and bio-proteins. Vertically aligned (12-µm long, 250-nm diameter) Si nanowires (SiNWs) were prepared by a facile silver-assisted electroless (SAE) etching process. Ag and Au nanoparticles (NPs) were initially decorated on SiNWs using a simple immersion bath. A hybrid SERS substrate was designed by encapsulating the 2D material (MoS2) on bi-metallic (Ag-Au nanoparticles) functionalized SiNWs. The synergetic effect from MoS2 nano-sheets and bi-metallic NPs produced superior detection of analytes down to ultra-trace levels. The versatility of hybrid substrate was tested extensively by detecting (i) malachite green (MG), (ii) melamine, (iii) naphthalene, (iv) bovine serum albumin (BSA), (v) adenine, (vi) L-Cysteine, (vii) Escherichia coli (E. coli), and (viii) tetryl (an explosive) molecules. We have observed excellent signal reproducibility with very low RSD values (3-5%). The analytic enhancement factors were estimated to be 3.6×108 for MG, 1.2×107 for melamine, and 6.4×105 for BSA, respectively. The estimated ‘limits of detection’ (LOD) for MG, melamine, naphthalene, adenine, and BSA were 216 pM, 42 nM, 78 nM, 5 nM, 7 nM, respectively. Furthermore, the durability of the substrate was studied extensively by evaluating the signal strength of 100 µM melamine for a prolonged duration in an ambient atmosphere. This established novel, robust, versatile, and potentially practical SERS-active hybrid substrates facilitate easy molecular detection in the field in conjunction with a portable or hand-held Raman spectrometer.