Nanorod-based piezo-electrocatalytic device shows promise for non-invasive use
Uric acid (UA) is typically described as a waste product; a chemical produced by the human body as it metabolises substances called purines, and is mostly expelled in urine. In recent years, UA has been shown to have additional properties, including acting as a neurostimulant and an inducer of inflammation. As such, abnormal levels of UA are increasingly viewed as an alarm signal for numerous physiological and psychological conditions that collectively affect millions of people around the world. Clinical measurement of UA involves taking blood samples, which is slow and invasive. If UA could instead be monitored via sweat, this would provide opportunities for non-invasive, continuous, wearable and remote analysis of this biomarker.
Doing this would require sensors with superior limits of detection (LOD), as the UA level in the sweat of a healthy human is an order of magnitude lower than it is in blood (24.6 µM vs. 240–520 µM, respectively). Writing in the latest issue of Nano Energy [DOI: 10.1016/j.nanoen.2023.108978] researchers from Purdue University report on their contribution to this effort – a highly-sensitive and flexible piezo-electrocatalytic uric acid sensor.
Their device, which they call EPICS, consists of ZnO nanorods grown on an indium tin oxide-coated PET (ITO-PET) substrate, with drop-cast reduced graphene oxide (rGO). ZnO was chosen because it is nontoxic and biocompatible, with wide electrochemical potential windows, and good chemical stability. In addition, it is piezoelectric, which can promote the performance of catalytic processes under mechanical strain. Raman spectroscopy confirmed that the synthesis of the ZnO nanorods enhanced the electrical conductivity of the underlying rGO, promoting more efficient charge transfer between the layers. Cyclic voltammetry measurements were carried out on the device at various stages of its fabrication. This confirmed that the electrochemically-active surface area was boosted by the presence of the nanorods; it was four times higher on the final device than on the rGO-ITO-PET stack. Measurement of the electron transfer kinetics revealed a similar trend.
In a piezo-electrocatalytic device, the target molecule (in this case, UA) is oxidised at the anode (ZnO nanorods), while reduction reactions happen at the cathode. To evaluate the process here, the sensor was fixed at one end at the bottom of a testing beaker filled with a buffer containing 50 µM of UA. A platinum sheet was used as the anode and an Ag/AgCl electrode acted as the reference. A linear motor modulated the bending, changing the applied strain. The shift of the oxidation peak and the increase in current density that resulted from bending confirmed the piezoelectric effect in the ZnO nanorods. It was accompanied by a reduction in electrical resistance and enhanced charge transfer, further promoting the oxidation reaction.
Differential pulse voltammetry was then used to measure the catalytic sensing performance of the device. The team found that their device was almost four times more sensitive to UA when it experienced a small compressive strain (-0.9%) than when the device was undeformed. The maximum sensitivity achieved was 3.91 µA µM-1 cm-2, with a limit of detection of 0.086 µM, “outperforming all reported flexible electrochemical UA sensors.”
The authors suggest that their as-fabricated sensor could be “…. integrated into naturally curved surfaces in the human body, such as nose pads on glasses,” and that their design “allows the possibility of non-invasive monitoring of UA with a boosted performance by otherwise wasted mechanical energy, such as that from the human body.”
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Jing Jiang, Ruifang Zhang, Meng Hao Lee, Wenzhuo Wu. “Flexible piezo-electrocatalytic uric acid sensor,” Nano Energy 118 Part A (2023) 108978. DOI: 10.1016/j.nanoen.2023.108978