The Investigation of Ti Doped Zinc Oxide Sensing Membrane in Bio-Sensor Applications

Abstract

In this study, we propose a highly sensitive electrolyte–insulator–semiconductor (EIS) pH sensor witha layer of novel metal-oxide. We compared the ZnO and Ti-doped ZnO sensing membrane applied in EIS structure and investigated the characteristics of Ti-doped ZnO sensing membrane. Furthermore, we studied the sensing performance of the ZnO and Ti-doped ZnO EIS structure in different (Na+, K+ , Urea, Glucose)solution. The EIS structures of a ZnO sensing membrane were fabricated on 4-in n-type (100) Si wafers. A 50 nm ZnO film was deposited on theSi substrate by radio frequency reactive sputtering from ZnO and Ti target in the ratio of Ar/O2 = 20/5. Subsequently, samples were annealed in O2ambient for 30 s by RTA at various temperatures from 600, 700, 800. After that, the back-side contact of the Si wafer was deposited by Al film with 300nm-thick. Then, the sensing area was defined by standard photo lithography process using a photo sensitive epoxy, SU8-2005. Finally, the samples were fabricated on the copper lines of PCB by silver gel. Epoxy package was used to separate the EIS structure. Based on X-ray diffraction and atomic force microscopy data, it is found that Ti doped ZnO sensing membrane has higher surface roughness than the ZnO sensing film. High temperature annealing (700 ◦C) leads to the formation of crystalline oxide thin film with a rough surface. The pH sensing experiments of EIS with Ti doped ZnO membrane annealed at 700 ◦C shows that reference voltage is linearly dependent onsolution pH and an average sensitivity of 57.56 mV/pH. This outstanding pH sensitivity may be attributed to the high surface roughness of oxide membrane. Besides, Ti doped ZnO sensing film has lower drift rate (0.29mV/hr) and hysteresis voltage (2.79mV) than the ZnO sensing film. To study the urea and glucose sensing property of the ZnO and Ti-doped ZnO on EIS structure, we prepared the solutions of urea and glucose which concentration were controlled in a range from 1mM~32mM and 2mM~8mM. Then, we picked the conditions of ZnO sensing membrane for RTA temperature at 600oC in O2and Ti-doped ZnO sensing membrane for RTA temperature at 700oC. The sensitivity of Ti-doped ZnO in O2ambient has better quality than the ZnO, which sample in urea solution was 3.62mV/mM, the linearity was 87.36%. Besides, sample in glucose solution was 6.42mV/mM, and the linearity was 90.57%. Finally, we successfully demonstrate the use of Ti doped ZnO EIS structure with uricase-immobilized alginate film for the detection of uric acid in serum. The concentration of serum uric acid measured by EIS biosensor is comparable to that determined by commercial assay kit. This sensitive EIS based biosensorcan potentially be served as the diagnosis tool for general clinical examinations.