0.1 m phosphate buffer solution

All the chemicals were procured from Sigma Aldrich with product numbers provided below. Gluteraldehyde (2.5%, Sigma Aldrich, St. Louis, MO, USA, #G5882), osmium tetroxide (1%, Sigma Aldrich, #75633), 0.1 M phosphate buffer solution (Sigma Aldrich, #P5244), ethanol (Sigma Aldrich, #E7023), propylene oxide (Sigma Aldrich, #82320), epoxy embedding medium 812 substitute (Sigma Aldrich, #45345), epoxy embedding medium hardener DDSA (Sigma Aldrich, #45346), epoxy embedding medium hardener MNA (Sigma Aldrich, #45347), epoxy embedding medium accelerator DMP 30 (Sigma Aldrich, #45348), oil, polycaprolactone (Sigma-Aldrich, #440744), dichloromethane (Sigma Aldrich, #270997), N-N-Dimethylformamide (Sigma Aldrich #319937).

The molecule 2,6-naphthalene dicarboxylic acid (NDCA) used for surface functionalization, a 0.1 M phosphate buffer solution (at pH 7.4), and hydrofluoric acid (HF) for Si₃N₄ etching were purchased from Sigma Aldrich (Milan, Italy). The Si₃N₄ substrates were fabricated by low-pressure chemical vapour deposition (LPCVD) on Si wafers [24 (link),25 (link)] with thicknesses of about 60 nm (areas of 2 cm²). A thrombin binding aptamer (TBA) used for fluorescence labelling was purchased from IDT Integrated DNA Technologies (Leuven, Belgium). The HPLC-purified DNA-sequence of the TBA molecule was 5’-NH₂-(CH₂)₁₂-GGT TGG TGT GGT TGG-(CH₂)₃-fluoresceinammine-3’, hereafter called TBA-FAM. The crosslinkers 1-ethyl-3-(-3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and water-soluble N-hydroxysuccinimide (sulfo-NHS) as well as Triton^TM X-100 (TX-100) were purchased from ThermoFisher Scientific Inc, whereas 6-aminofluorescein (AMF) was purchased from Sigma-Aldrich (Milan, Italy).