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

R. M. TRUJILLO; L. D. SAPPIA; I. LORITE; R. MADRID

Workshop; International Workshop of Impedance Spectroscopy 2016; 2016

Nanostructured ZnO with its unique propertiescould provide a suitable substrate for the immobilization ofenzymes, or bio recognition of other molecular elements, whileretaining their biological activity. This leads to the constructionof different biosensors with enhanced analytical performance [1].Nanomaterials based on ZnO have been widely used to buildglucose sensors due to its high isoelectric point. It also createsa biologically friendly environment to preserve the activity ofthe enzyme. We have reported that nanostructured ZnO thinfilms (TF) present a molecular influence of different solutionson its transport properties. The composition of the liquid, bymeans of the charge of the ions, produces strong changes inthe transport properties of the TF. These effects were evaluatedby impedance spectroscopy [2]. Glucose biosensors have beenintensively investigated due to their importance in the foodindustry and in the determination of glucose concentrationin blood when diagnosing and control-ling people affected bydiabetes. In an effort to develop more sensitive and reliableglucose biosensors, nanotechnology is being widely applied [1] [3].Nevertheless ZnO nanostructures started to be used as glucosebiosensors less than ten years ago [4]. First generation glucosebiosensor needs a bias voltage polarization in order to transducethe signal [5] [6] [7], second generation uses a chemical mediatorsuch as ferro/ferricyanide, [8], while third generation biosensortransduces the analytical signal directly from the biorecognitionevent [9]. Another interesting application of ZnO nanostructuresis in the non-enzymatic detection of glucose. The uses of ZnO TFsis mainly widespread as gas sensors [10] [11] [12] [13]. On theother hand, the reported works on ZnO TFs as biosensors usedthis material submerging it in a liquid containing the desiredanalyte. Nevertheless, we believe that there are no publishedreports about the study of ZnO TFs in the way we proposehere, where a drop of a solution is deposited on the TF. Inparticular, thin films with polycrystalline structure are interestingdue to the possible variation of properties related to interfacesor grain boundaries. The last one plays an important role in theelectrical transport since a significant amount of ad-atoms canbe accumulated directly at the grain boundary or within a rangeof a few atomic layers close to it, apart from lattice defects inthe ?bulk? of the crystallites. Due to the junction between thedifferent grains, the so-called grain boundary, a double Schottkybarrier is formed. Its height depends on the defects or dopantswhich lay mainly at the inter-grain. It can provide differentpathways of conduction from grain to grain boundary or acontribution of both of them [14]. For investigation of these effectsand to clarify the possible impacts of defects on the conductivityproperties, impedance spectroscopy is a useful technique becauseit can reveal the contribution of different electrical conductionpaths in these structures [15]. In a previous paper we examinedthe variability of the impedance for TFs samples obtained fromdifferent regions of the same thin film, under different conditionsand solutions [2] . In this work we present the influence of theTF?s growth temperature in its sensitivity to changes producedby the GOx activity when glucose is drop casted onto the TF.Impedance spectroscopy at zero bias polarization was used toevaluate the response of the substrate to different glucose andother solutions concentrations. We may have found that the lowerthe growth temperature, the more sensitive the response of theTF to different concentrations of the analyte. Special attention isdrawn towards the fact that we observed a significant change onlywhen 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 usingbias polarization voltage or chemical mediator. These results canhelp to design a new concept in biosensing