Evaluation of growth temperature influence on the transport properties of ZnO thin films by impedance spectroscopy. Preliminary results

TRUJILLO, MATÍAS; SAPPIA, LUCIANO; LORITE, ISRAEL; MADRID, ROSSANA; TIRADO, MONICA; COMEDI, DAVID; ESQUINAZI, PABLO

At Chemnitz

Workshop; International Workshop on Impedance Spectroscopy: IWIS 2016; 2016

IWIS 2016

Nanostructured ZnO with its unique properties could provide a suitable substrate for the immobilization of enzymes, or bio recognition of other molecular elements, while retaining their biological activity. This leads to the construction of different biosensors with enhanced analytical performance [1].Nanomaterials based on ZnO have been widely used to build glucose sensors due to its high isoelectric point. It also creates a biologically friendly environment to preserve the activity of the enzyme. We have reported that nanostructured ZnO thin films (TF) present a molecular influence of different solutions on its transport properties. The composition of the liquid, by means of the charge of the ions, produces strong changes in the transport properties of the TF. These effects were evaluated by impedance spectroscopy [2]. Glucose biosensors have been intensively investigated due to their importance in the food industry and in the determination of glucose concentration in blood when diagnosing and control-ling people affected by diabetes. In an effort to develop more sensitive and reliable glucose biosensors, nanotechnology is being widely applied [1] [3].Nevertheless ZnO nanostructures started to be used as glucose biosensors less than ten years ago [4]. First generation glucose biosensor needs a bias voltage polarization in order to transduce the signal [5] [6] [7], second generation uses a chemical mediator such as ferro/ferricyanide, [8], while third generation biosensor transduces the analytical signal directly from the biorecognitionevent [9]. Another interesting application of ZnO nanostructures is in the non-enzymatic detection of glucose. The uses of ZnO TFs is mainly widespread as gas sensors [10] [11] [12] [13]. On the other hand, the reported works on ZnO TFs as biosensors used this material submerging it in a liquid containing the desiredanalyte. Nevertheless, we believe that there are no published reports about the study of ZnO TFs in the way we propose here, where a drop of a solution is deposited on the TF. In particular, thin films with polycrystalline structure are interesting due to the possible variation of properties related to interfaces or grain boundaries. The last one plays an important role in theelectrical transport since a significant amount of ad-atoms can be accumulated directly at the grain boundary or within a range of a few atomic layers close to it, apart from lattice defects in the ?bulk? of the crystallites. Due to the junction between the different grains, the so-called grain boundary, a double Schottky barrier is formed. Its height depends on the defects or dopantswhich lay mainly at the inter-grain. It can provide different pathways of conduction from grain to grain boundary or a contribution of both of them [14]. For investigation of these effects and to clarify the possible impacts of defects on the conductivity properties, impedance spectroscopy is a useful technique because it can reveal the contribution of different electrical conduction paths in these structures [15]. In a previous paper we examinedthe variability of the impedance for TFs samples obtained from different regions of the same thin film, under different conditions and solutions [2] . In this work we present the influence of the TF?s growth temperature in its sensitivity to changes produced by the GOx activity when glucose is drop casted onto the TF.Impedance spectroscopy at zero bias polarization was used to evaluate the response of the substrate to different glucose and other solutions concentrations. We may have found that the lower the growth temperature, the more sensitive the response of the TF to different concentrations of the analyte. Special attention is drawn towards the fact that we observed a significant change only when glucose is placed over the TF, grown at low temperature, with GOx on top of it (TF-GOx). This may infer the capabilityof the TF for detecting GOx activity without the need for using bias polarization voltage or chemical mediator. These results can help to design a new concept in biosensing.