Evaluation of Zinc Oxide Nano-Microtetrapods for Biomolecule Sensing Applications

Abstract: Zinc oxide (ZnO) is a well-known II-VI semiconductor material that has gained renewed interest in the past decade due to the developments of growth technologies and the availability of high-quality ZnO bulk single crystals. Owing to a wide direct band gap (3.37 eV), large exciton binding energy (60 meV), and high electron mobility (440 cm2 V-1 s-1), ZnO has been used for applications including actuators, optoelectronics, and sensors. ZnO nanoparticles can be synthesized in a broad variety of morphologies, such as nanotetrapods, nanotubes, and nanowires. Among these nanostructures, the tetrapods have attracted significant attention due to their unique morphology consisting of four legs connected together in a tetrahedral symmetry. Recently, it has been reported that nano-microstructured ZnO tetrapods (ZnO-Ts) can be synthesized by flame transport synthesis (FTS) in a rapid and up-scalable approach. Compared to conventional ZnO nanoparticles, the nano-microstructured ZnO-Ts can reduce cellular uptake, while still exhibiting specific nanomaterial properties due to the nanoscale tips. Moreover, the anisotropic ZnO-Ts have the advantages of multiple electron transfer paths, chemical stability, and biocompatibility, which make the ZnO-Ts promising candidates for biomolecule sensing applications. This work herein reports a systematical study on the structural, optical and electrochemical properties of the ZnO-Ts, which were synthesized by FTS using precursor Zn microparticles. The morphology of the ZnO-Ts was confirmed by scanning electron microscopy (SEM) as joint structures of four single crystalline legs, of which the diameter of each leg is 0.7-2.2 μm in average from the tip to the stem. The ZnO-Ts were dispersed in glucose solutions to study the photoluminescence as well as photocatalytic activity in a mimicked biological environment. The photoluminescence (PL) intensity in the ultraviolet (UV) region decreased with linear dependence on the glucose concentration up to 4 mM. The ZnO-Ts were also attached with glucose oxidase (GOx) and over coated with Nafion® to form the active media for electrochemical glucose sensing. The active layers were confirmed by Fourier transform infrared spectroscopy (FT-IR). Furthermore, the current response of the active layers to glucose was studied by cyclic voltammetry (CV) in various glucose concentration conditions. Stable current response to glucose was detected with linear dependence on the glucose concentration up to 12 mM, which confirms the potential of ZnO-Ts for biomolecule sensing applications.