Development of Miniaturized Microneedle Potentiometric Sensors for Real-Time Label-Free pH Detection using AgIO3/Ag/PTFE/WO3/W

This work presents a new design of miniaturized microneedle pH sensors based on AgIO 3 /Ag/PTFE/WO 3 /W, which can be used to measure the real-time label-free pH of various micro-sized cells and small-volume bodily fluids. The fabrication process comprises multiple steps, including creating W microneedles and depositing AgIO 3 /Ag/PTFE/WO 3 layers. A needle-like structure was created at the end of a cleaved W rod using electrolytic polishing. The sensing layer (working electrode, WE) of WO 3 films was developed onto W microneedles using the thermal oxidation method. A polytetrafluoroethylene (PTFE) coating was applied as an insulating layer between the working electrode and the reference electrode (RE, AgIO 3 /Ag thin film). The PTFE and AgIO 3 /Ag layers were fabricated using the hand spray and RF sputtering methods, respectively. The microstructural and compositional features of miniaturized microneedle sensors (AgIO 3 /Ag/PTFE/WO 3 /W) were analyzed using FESEM, EDX, XRD, and Raman techniques. The miniaturized microneedle sensors exhibited excellent analytical performance, including a Nernstian slope of -49.36 mV/pH and a linear response range from pH 4.01 to pH 9.18, as verified by testing with a pH standard solution. Ex vivo pH testing results of fish eggs demonstrated the ability of the fabricated miniature microneedle pH sensor to penetrate biological tissue and accurately measure pH values.