ABSTRACT
ZnO NSs as an immobilization matrix for electrochemical biosensors improve several sensor properties, such as high sensitivity, good selectivity, broad dynamic range, fast response time, low detection limit (depending on S/N), reversibility, reliability, reproducibility, stability, and a long shell life (Tables 18.1 and 18.2). The most important results regarding the properties mentioned above are going to be discussed in the next paragraphs. 18.4.1 Sensitivity and Detection Limit
It is believed that the active surface area of ZnO NSs is proportional to the sensitivity and detection limit [24]. Therefore, the higher the effective surface area is, the better it is for biosensing. GOD-(C-ZnO NWs)-Ti bioelectrodes [2] have the highest sensitivity (35.5 mA cm-2mM-1) and the lowest detection limit (1 mM) among other ZnO NSs (nanocombs [17], NRs [19, 4], and TPSP structures [23]) for the detection of glucose. Titanium as an electrode forms a good electrical contact with the ZnO NW, and the carbon nanolayer (12 nm) plays an important role in the direct electrochemistry because of its high electron conductivity, decreasing the electron transfer resistance from ~400 to ~85 Ω. The larger specific surface area of the C-ZnO NWs (55.4 m2 g-1) compared with that of the bare ZnO NWs (10.4 m2g-1) also can be ascribed as a reason for the improved electrocatalytic activity and small detection limit. Excellent absorption and good biocompatibility of the ZnO are other determinant factors of the high sensitivity and small detection limit. Besides, well-crystallized flower-shaped ZnO NSs [24] for cholesterol analysis show the highest sensitivity (61.7 mA cm-2 mM-1) and the lowest detection limit (12 nM) among other ZnO NSs (nanoporous ZnO thin films [14], ZnO NPs [sensitivity: 23.7 mA cm-2 mM-1 and detection limit: 0.37 nM] [11], and ZnO NP-CHIT composites [sensitivity: 14.1 × 10-4 mA cm-2 mM-1 and detection limit: 0.13 mM] [12]). Such high sensitivity is ascribed to the spherical morphologies of the flowershaped ZnO NSs, which contain hexagonal-shaped NRs attached in the three dimensions, as shown in Fig. 18.1d, exhibiting a large
active surface area, and allows a large number of enzyme molecules are immobilized on the surface. Therefore, the active surface area is believed to be directly proportional to the sensitivity [24]. The detection of phenolic compounds has been improved due to the BDND thin film with ZnO NRs. Different sensitivities have been reached using tyrosinase because of the different affinities toward phenolic compounds [18]: p-cresol > 4-chlorophenol > phenol with sensitivities of 576.2, 339.3, and 287.1 mA cm-2 mM-1,respectively. The detection limit can be calculated according to the criteria 3sb/m, where sb and m are the standard deviation of the background current and the slope of the calibration graph [18]. H2O2 detection with porous nanosheet-based ZnO microspheres (shown in Fig. 18.1i) [9] has been reported with the highest sensitivity (137 mA cm-2 mM-1), which can be explained from the large surface area available for protein binding and the decrease of the diffusion distance for the substrate to access the immobilized enzyme provided by the porous nanosheet. The lowest detection limit for H2O2 has been reported with ZnO NPs with CHIT (0.097 mM) [8], indicating the higher catalytic activity of the hemoglobulin-(ZnO-CHIT)-Au film toward the reduction of H2O2. ZnO NR electrodes with a multilayer approach of PSS and HRP for H2O2 detection [7] have an increasing sensitivity level from one layer (36.28 mA mM-1) to five layers (58.15 mA mM-1), which is in accordance with the trend of the surface concentration of electroactive HRP (inversely proportional to the effective surface area). However, the detection limit reach the lowest value (1.9 mM) with three layers, which can be explained from the lowest MichaelisMenten constant (10.2 mM), showing the better affinity of HRP to H2O2 reduction. ZnO NS-based DNA electrochemical biosensors exhibit a remarkably small detection limit. For example, the ZnO nanoporous thin film has been functionalized with 20 mer thiolated oliglonucleotide physisorbed probe, specific to N. gonorrhoeae and in the presence of MB (an electroactive redox indicator able to bind specifically to the guanine bases in DNA sequence), showing an extraordinarily small detection limit of 0.704 amol [15], as shown in Fig. 18.2A, since the increased loading of the ssDNA probes can enhance the signal response of the MB.
