Adenine is an important molecule for biomedical and agricultural research and applications. The detection of low concentration adenine molecules is thus desirable. Surface-enhanced Raman scattering (SERS) is a promising label-free detection and fingerprinting technique for molecules of significance. A novel SERS sensor made of clusters of silver nanostructures deposited on copper bumps in valleys of an etched silicon substrate was previously reported to exhibit a low and reproducible detection limit for a 10-11 M neutral adenine aqueous solution. Reflection of laser illumination from the silicon surface surrounding a valley provides additional directions of laser excitation to adenine molecules adsorbing on a silver surface for the generation of enhanced SERS signal strength leading to a low detection limit. This paper further reports a concentration dependent shift of the ring-breathing mode SERS adenine peak towards 760 cm-1 with decreasing concentration and its pH-dependent SERS signal strength. For applications, where the pH value can vary, reproducible detection of 10-12 M adenine in a pH 9 aqueous solution is feasible, making the novel SERS structure a desirable pico-molar adenine sensor.
Keywords: Raman scattering; SERS; adenine; copper; pH; plasmon; silver.