Chemically non-perturbing SERS detection of catalytic reaction with black silicon

All-dielectric resonant micro- and nano-structures made of the high-index dielectrics recently emerge as a promising SERS platform which can complement or potentially replace the metal-based counterparts in routine sensing measurements. These unique structures combine the highly-tunable optical response and high field enhancement with the non-invasiveness, i.e., chemically non-perturbing the analyte, simple chemical modification and recyclability. Meanwhile, the commercially competitive fabrication technologies for mass production of such structures are still missing. Here, we attest a chemically inert black silicon (b-Si) substrate consisting of randomly-arranged spiky Mie resonators for a true non-invasive SERS identification of the molecular fingerprints at low concentrations. Based on comparative in-situ SERS tracking of the para-aminothiophenol -to-4,4` dimercaptoazobenzene catalytic conversion on the bare and metal-coated b-Si, we justify applicability of the metal-free b-Si for the ultra-sensitive non-invasive SERS detection at concentration level as low as 10^-6 M. We perform finite-difference time-domain calculations to reveal the electromagnetic enhancement provided by an isolated spiky Si resonator in the visible spectral range. Additional comparative SERS studies of the PATP-to-DMAB conversion performed with a chemically active bare black copper oxide as well as SERS detection of the slow daylight-driven PATP-to-DAMP catalytic conversion in the aqueous methanol solution loaded with colloidal silver nanoparticles confirm the non-invasive SERS performance of the all-dielectric crystalline b-Si substrate. Proposed SERS substrate can be fabricated using simple scalable technology of plasma etching amenable on large substrate areas making such inexpensive all-dielectric substrates promising for routine SERS applications, where the non-invasiveness is of mandatory importance.