Physical Structure and Electrochemical Response of Diamond Graphite Nanoplatelets: from CVD Synthesis to Label-Free Biosensors

Hybrid diamond-graphite nanoplatelet (DGNP) thin films are produced and applied to label-free impedimetric biosensors for the first time, using avidin detection as a proof of concept. The DGNPs are synthesized by microwave plasma chemical vapor deposition through H2/CH4/N2 gas mixtures in a reproducible and rapid single-step process. The material building unit consists of an inner two-dimensional-like nanodiamond with preferential vertical alignment covered by and covalently bound to nanocrystalline graphite grains, exhibiting {111}diamond||{0002}graphite epitaxy. The DGNP films' morphostructural aspects are of interest for electrochemical transduction, in general, and for Faradaic impedimetric biosensors, in particular, combining enhanced surface area for biorecognition element loading and facile Faradaic charge transfer. Charge transfer rate constants in phosphate buffer saline/[Fe(CN)6]4- solution are shown to increase up to 5.6 × 10-3 cm s-1 upon N2 addition to DGNP synthesis. For the impedimetric detection of avidin, biotin molecules are covalently bound as avidin specific recognition elements on (3-aminopropyl)triethoxysilane-functionalized DGNP surfaces. Avidin quantification is attained within the 10-1000 μg mL-1 range following a logarithmic dependency. The limits of detection and of quantitation are 1.3 and 6.4 μg mL-1 (19 and 93 nM), respectively, and 2.3 and 13.8 μg mL-1 (33 and 200 nM) when considering the nonspecific response of the sensors.