Biosensing applications of ZnO nanostructures

Henry and Peter proposed that the first physical basis of the model is in accordance with the oxidation of CO on the biosensor surface by chemisorbed oxygen species and subsequent emission of an electron from the chemisorbed species into the conduction band (CB) of the sensor [27]. The detection principle of enzymatic biosensors with ZnO NSs is based on the two reactions [6], as shown in Scheme 18.2: (i1) Ionosorption: This is the catalyst reaction of the biomolecule with the enzyme produces ions, which are able to increase the conductivity of the host material by providing the excess of electrons into the CB. (i2) Physisorption: In a typical situation, oxygen molecules are physorbed on the surface sites; however, while oxygen molecules move from site to site, they get ionized by extracting an electron from the CB and thus are ionosorbed on the surface as Oads-(O-or O2-depending on the energy available). This ionosorption effect leads to a decrease in the conductance of the sensor and an increase in the potential barrier at the grain boundaries (O2 + 2e-Æ 2Oads-). (ii) Oxidation/reduction of ZnO, which occurs by the absorbed oxygen and the catalytically produced excess ions. The ions (R) released from the catalytic reaction are able to react with the surface-adsorbed oxygen (Oads-), releasing the trapped electrons into the CB of the ZnO. The energy released at this moment is enough for the electrons to jump up into the conduction, which consequently increases the conductivity of the sensor (R + Oads-Æ RO + e-). Finally, the RO desorption primarily depends on the recovery time and the temperature.