Black silicon SERS substrate: Effect of surface morphology on SERS detection and application of single algal cell analysis

• Black silicon substrate with controllable tip density was fabricated by maskless ICP–RIE etching.
• Better SERS effect relates to the higher tip density and bridged gold structure.
• Enhanced carotenoid and lipid Raman signals of single algal cell was acquired by using black silicon SERS substrate.
• Black silicon SERS substrate possesses large area and uniform SERS effect.

In this study, we have investigated the effect of the surface morphology of the black silicon substrate on surface enhanced Raman spectroscopy (SERS) and explored its application of single algal cell detection. By adjusting the O2 and SF6 flow rates in the cryogenic plasma etching process, different surface morphologies of the black silicon substrate was produced without performing the lithographic process. It was found the Raman signals were better enhanced as the tip density of the black silicon substrate increased. In addition, as the thickness of the deposited gold layer increased, the SERS effect increased as well, which could be owing to the generation of more hot spots by bridging individual silicon tips through deposition of gold layer. For the black silicon substrate with tip density of 30 tips/μm2 and covered by 400 nm deposited gold layer, the detection limit of 10 fM R6G solution concentration with uniform SERS effect across the substrate was achieved. Furthermore, detection of individual algal cell (Chlorella vulgaris) was performed at the SERS substrate as fabricated and the Raman signals of carotenoid and lipid were substantially enhanced.