Advancements in Glucose Monitoring: A Thin Film ZnO-Nanoflakes Based Highly Sensitive Wearable Biosensor for Noninvasive Sweat-Based Point-of-Care Monitoring for Diabetes
Authors: Hossain G M Mehedi , Jalal Ahmed Hasnain , Pala Nezih , Alam Fahmida 
Published: August 2024

In the contemporary global landscape, diabetes is a prevalent and concerning presence that contributes to noteworthy mortality rates. The etiology of this condition involves a subtle interaction between insulin deficiency and elevated blood sugar levels which results in a complex metabolic disorder. The harmful consequences of this physiological abnormality become evident through the increased probability of kidney failure, heart attack, chronic liver diseases, and a pernicious ocular affliction leading to blindness. Against this backdrop, the necessity for prompt, accurate, and continuous monitoring of glucose concentrations with any form of flexible sensor becomes notably evident for type I and type II diabetes patients, pregnant women with diabetes, athletes, and high-risk persons. Sweat, an often-overlooked physiological excretion, is a reservoir of vital biological information due to its accessibility and analyzability. It is an advantageous matrix for glucose monitoring, surpassing blood, saliva, and tears. Its non-invasive nature offers a patient-friendly alternative to conventional blood sampling. The continuous, real-time nature of sweat extraction facilitates dynamic monitoring without the discomfort associated with invasive methods. This inherent advantage positions sweat as a promising matrix for glucose sensing, underscoring its potential as a valuable tool in diabetes management. It overcomes the limitations posed by other bodily fluids like saliva and tears in terms of accessibility, patient comfort, and reflective accuracy. The assimilation of nanomaterials into biosensing technologies heralds a paradigm shift, conferring heightened sensitivity upon these mechanisms. Noteworthy among these innovations are flexible wearable biosensors leveraging zinc oxide (ZnO) thin films, offering promising avenues for the discernment of glucose levels within sweat for healthcare monitoring. ZnO nanoflakes (ZnO-NFs) distinguish themselves by endowing flexible biosensors with unparalleled sensitivity, reliability, continuous monitoring capabilities, and non-invasiveness from zinc oxide nanoparticles incorporated graphene-carbon nanotube hybrid (GR-CNT-ZnO) and ergocalciferol-multi walled carbon nanotubes based (ERGOc-MWCNTd/Nf) traditional glucose sensors. The distinctive advantages of ZnO-NFs over bulk materials are multifaceted. Notably, the polarized (0001) plane orientation inherent in ZnO-NFs amplifies enzyme immobilization, thereby augmenting sensing performance. The elevated isoelectric point (9.5) of ZnO-NFs facilitates the immobilization of biomolecules, obviating the need for an ancillary binding layer. A single-step and efficacious sonochemical approach has been used to synthesize a thin layer of ZnO-NFs virtually on any substrate. The electrochemical flexible glucose biosensor was fabricated through the judicious immobilization of glucose oxidase (GOx) on ZnO nanoflakes, possessing a mere 20nm thickness and synthesized adeptly on an Au-coated stretchable polyethylene terephthalate (PET). The sensitivity of the flexible biosensor was determined by 29.97 μA/decade/cm² and the minimum detection limit is 100 μM. The coefficient of determination of the designed flexible electrode is found 97.76%, which is significantly linear. The designed PET/Au/ZnO-NFS/GOx flexible biosensors showed good repeatability for different sample concentrations. The enzymatic flexible sensor also showed good repeatability for three sample concentrations. The reproducibility data for two identical flexible sensors were found within the LoA of the Bland-Altman plot with a statistically significant p-value of 0.08 derived from the student t-test. Moreover, the flexible sensor exhibited notable repeatability, reproducibility, and remarkable stability conducive to the precise quantification of glucose. The application of this flexible wearable sensor has been validated through its utilization in human sweat samples, yielding results that align favorably with conventional methodology.