Interaction mechanism between TiO2 nanostructures and bovine leukemia virus proteins in photoluminescence-based immunosensors

Abstract

In this research a mechanism of interaction between a semiconducting TiO₂ layer and bovine leukemia virus protein gp51, applied in the design of photoluminescence-based immunosensors, is proposed and discussed. Protein gp51 was adsorbed on the surface of a nanostructured TiO₂ thin film, formed on glass substrates (TiO₂/glass). A photoluminescence (PL) peak shift from 517 nm to 499 nm was observed after modification of the TiO₂/glass by adsorbed gp51 (gp51/TiO₂/glass). After incubation of the gp51/TiO₂/glass in a solution containing anti-gp51, a new structure (anti-gp51/gp51/TiO₂/glass) was formed and the PL peak shifted backwards from 499 nm to 516 nm. The above-mentioned PL shifts are attributed to the variations in the self-trapped exciton energy level, which were induced by the changes of electrostatic interaction between the adsorbed gp51 and the negatively charged TiO₂ surface. The strength of the electric field affecting the photoluminescence centers, was determined from variations between the PL-spectra of TiO₂/glass, gp51/TiO₂/glass and anti-gp51/gp51/TiO₂/glass. The principle of how these electric field variations are induced has been predicted. The highlighted origin of the changes in the photoluminescence spectra of TiO₂ after its protein modification reveals an understanding of the interaction mechanism between TiO₂ and proteins that is the key issue responsible for biosensor performance.

Fig. 1. Energetic levels of pristine TiO₂ (TiO₂/glass structure). E_c, E_v – conduction and valence band of TiO₂ respectively. E_dn – electron demarcation level.

Fig. 2. Photoluminescence spectra of TiO₂/glass nanoparticles: (a) before and after the immobilization of gp51 on the TiO₂/glass surface and after BSA deposition; (b) photoluminescence spectra of gp51/TiO₂/glass based immunosensor after the interaction with analyte (anti-gp51), which is present in the aliquots at different concentrations.

Fig. 3. Energetic levels and the model based on ‘imaginary flat capacitor’, which represents averaged interaction between charges in gp51/TiO₂/glass structure: L₁ – ‘relative distance’ between ‘plates’ area of imaginary capacitor, D₂ – diameter of imaginary capacitor, which is determined by surface area of gp51 and can be used for the calculation of relative surface area of imaginary capacitor; φ₁ – potential barrier value for surface of TiO₂ interphase with air (air//TiO₂/glass); φ₂ – potential barrier value for gp51/TiO₂ structure at interphase with air (air//gp51/TiO₂/glass); Δφ_gp51 – difference of potential barriers between air//TiO₂/glass and (air//gp51/TiO₂/glass) caused by immobilization of gp51.

Fig. 4. Energetic levels and the model based on ‘imaginary flat capacitor’, which represents averaged interaction between charges in anti-gp51/gp51/TiO₂/glass structure: L₂ – ‘relative distance’ between ‘plates’ area of imaginary capacitor, D₂ – diameter of imaginary capacitor, which is determined by surface area of gp51 and can be used for the calculation of relative surface area of imaginary capacitor, φ₃ – potential barrier value for anti-gp51/gp51/TiO₂/glass structure at interphase with air (air//anti-gp51/gp51/TiO₂/glass). Δφ_{anti-gp51/gp51} – difference of potential barriers between air//TiO₂/glass and air//anti-gp51gp51/TiO₂/glass.