Electrical Contact of Metals at the Nanoscale Overcomes the Oxidative Susceptibility of Silver-Based Nanobiosensors

Silver (Ag) is one of the most sensitive noble metals for nanoplasmonic applications. However, Ag is prone to oxidation in wet environments (e.g., aqueous and organic solvents), which usually leads to poor adhesion between Ag and dielectrics, thereby limiting its use in biosensing applications. To address this challenge, we propose a new design principle where one metal corrodes preferentially when in electrical contact with the other metal, in the presence of an electrolyte. Specifically, by using a simple two-step thermal dewetting protocol, we fabricate bimetallic Ag and titanium nanoislands (Ag/Ti NI) on top of SiO2 surfaces, where Ti suppresses the Ag from its oxidation in wet environments. As the number of free electrons in valence band increases from 1 in Ag to 5 in Ag/Ti, our bimetallic Ag/Ti NI achieve a superior refractive index sensitivity of 112 nm/RIU in comparison to Ag-based plasmonic materials with similar sizes (20-40 nm) and morphologies (i.e., nanoisland shapes), with high stability (100 days). We further demonstrate the use of Ag/Ti NI substrates as a generic refractive index sensor and for the detection of human C-reactive protein with a limit of detection of 80 fM. Our work presents a very promising synthesis strategy to prevent oxidation issues of metals at the nanoscale, which is crucial in developing nanomaterial based biosensors with a long shelf life.