Nanoscale steric hindrance guides size-selective adsorption of gold nanoparticles into titanium nanowells

Anodization of titanium is used to form dual scale nanoporous titanium oxide surfaces (NanoTi) that are highly stable, relatively low cost and exhibit unique optoelectronic properties. Due to their high biocompatibility, electrical conductivity and surface area, NanoTi surfaces also promise to be useful electrode materials for biosensors. In this study, we investigated the feasibility of using NanoTi surfaces as a size-selective electrode material for the detection of nano-sized objects. Gold nanoparticles ranging from 16 to 69 nm in diameter were used to model the binding behaviors of bioanalytes, such as enzymes, exosomes and viruses. A size-dependent, steric-like exclusion phenomenon appears to determine the binding location of the nanoparticles within the dual scale nanostructure present on the NanoTi surfaces. Nanoparticles smaller than the pore diameter were observed to bind to ridges between pores, while larger nanoparticles bound onto the pores. The binding events were monitored by Electrochemical Impedance Spectroscopy (EIS), which highlighted that the electrochemical signal measured was dependent on the location of the nanoparticle upon binding. On-pore binding reduced the overall system resistance, while on-ridge binding increased it. These proof-of-concept results indicate that NanoTi surfaces could be used in the development of electrochemical sensors that intrinsically promote the binding of a nano-analyte within a certain size range.