Electrically Tunable Ultra-specific Zinc Oxide Biosensor
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Electrically Tunable Ultra-specific Zinc Oxide Biosensor Rujuta D.Munje1, Andi Wangzhou 1, Anjan Panneer Selvam 1 ,Sriram Muthukumar2 and Shalini Prasad1 1 Department of Bioengineering, University of Texas at Dallas, 800 W. Campbell Road, EC 39, Richardson, TX 75080, USA 2 Department of Materials Science and Engineering, University of Texas at Dallas, 800 W. Campbell Road, EC 39, Richardson, TX 75080, USA
ABSTRACT Zinc oxide surface states can be utilized for ultra-specific detection of biomolecules. The major challenges in using ZnO for bio-sensing are attaining enhanced sensitivity and specificity. In this study, we explore the functionalization of zinc in ZnO through utilizing the thiol bond. The purpose of this study is to demonstrate that the ZnO based sensor is capable of achieving high specificity in presence of competitive surface binding through the thiol bond. The final goal is to design an ultra-specific biosensor to detect low occurring biomolecules. In this study, we have selected cortisol as a stress marker to demonstrate quantification and detection from synthetic sweat. In order to demonstrate ultra-specificity, we have used two competitive thiol based molecules binding to zinc, a linker Dithiobis succinimidyl propionate (DSP) and reducing agent of DSP, Dithiothreitol (DTT). Electrochemical impedance spectroscopy (EIS) is used to quantify the signal obtained through various ratiometric concentrations of DSP and DTT. To validate the EIS study results, inherent fluorescence studies are done by mapping changes in green emission spectrum of ZnO before and after linker functionalization. The optimal combination in terms of highest signal is identified to be of 25mM DTT and 50mM DSP. This is implemented in the experiments performed to calibrate the cortisol concentration in synthetic sweat. This study demonstrates the detection of cortisol antigen in synthetic sweat present within the physiological levels of 8 ng/mL to 140 ng/mL.
INTRODUCTION Zinc oxide (ZnO) is a bio-compatible semiconductor material with a wide direct band gap of 3.37eV and tunable electrical properties. It is an appealing material for biosensing based on protein, enzymatic, nucleic acid conjugation. Sputtered ZnO thin films have been used for detecting a wide range of biomolecules such as cholesterol, glucose in recent applications [1, 2]. The challenge has been the absence of reliable and specific bio-chemical sensors for achieving ultra-sensitive multiplexed detection. Particular deposition conditions can be used to achieve specific surface terminations of ZnO, thus giving us control over the bio-functionalization of ZnO surface [3]. Dithiobis succinimidyl propionate (DSP) binds to Zn2+ in ZnO through thiol bonding. The dissociation constant Kd of Zn-S bond is high and hence DSP is valuable for achieving specificity. The reducing agent of DSP, Dithiothreitol (DTT) cleaves the disulfide bond in DSP [4]. This cleaving can benefit in enhancing the linker coverage on ZnO surface thus
leading to amplified signal obtained through bindin
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