Surface enhanced Raman scattering as a sensing technique using silicon nanowires and plasmonic nanoparticles

Abstract

To overcome the classic sensitivity vs selectivity trade-off often associated with sensors used in diagnostic applications, signature spectroscopic information that is characteristic of the molecules to be sensed can be exploited. Raman spectroscopy offers such information and is suitable for biological fluids. It is considered a label-free sensing method that inherently has excellent specificity. Sensitivity on the other hand is generally low unless amplification of the generally weak Raman signal is achieved. Surface enhanced Raman Scattering (SERS) employs localized surface plasmons on metallic nanoparticles to amplify this signal by several order of magnitude. In this work, SERS substrates were prepared by growing silver nanoparticles using electrodeposition on silicon nanowires that were prepared using metal assisted chemical etching. Experimental results agree with finite difference time domain (FDTD) simulation results. Using pyridine as a probe molecule, Raman signal intensity was found to correlate well with the pyridine concentration in the range 10-6 M to 10-9 M, indicating its applicability as a quantitative sensor. Very low concentration of pyridine, 10-11 M, was detected although at this low concentration the detection is only qualitative. The enhancement factor was calculated to reach 1011. Spot-to-spot, sample-to-sample, and batch-to-batch variation was studied to ensure repeatability, which had been a long-standing issue of low-cost SERS substrates. In addition, experiments over several days highlight the robustness of these SERS substrates. This work bolsters the use of SERS as a low cost sensing method with good sensitivity and specificity for a plethora of applications without compromising on repeatability or robustness.