Nanostructured CuO Thin-Film-Based Conductometric Sensors for Real-Time Tracking of Sweat Loss

Abstract

Enhanced sweat sensors lead to real-time, sustained, noninvasive tracking of sweat loss, ensure insight into individual health conditions at the molecular level, and have obtained prominent interest for their hopeful implementations in customized health tracking. Metal-oxide-based nanostructured electrochemical amperometric sensing materials are the best selection for continuous sweat monitoring devices owing to their high stability, high-sensing capacity, cost-effectiveness, miniaturization, and wide applicability. In this research, CuO thin films have been fabricated by successive ionic layer adsorption and reaction technique (SILAR) with and without the addition of Lawsonia inermis L. (Henna, (LiL)) leaf extract (C₁₀H₆O₃, 2-hydroxy-1,4-naphthoquinone) with a high-sensitive and rapid response for sweat solution. Despite the pristine film being responsive to the 65.50 mM sweat solution (S = 2.66), the response characteristic improves to 3.95 for the 1.0% LiL-implemented CuO film. Unmodified, 1.0% LiL and 3.0% LiL-substituted thin-film materials assure considerable linearity with linear regression ranges, R², of 0.989, 0.997, and 0.998, respectively. It is noteworthy here that this research aims to determine an enhanced system that could potentially be implemented in real-life sweat-tracking administrations. Real-time sweat loss tracking capabilities of CuO samples was found to be promising. Derived from these outcomes, we concluded that the fabricated nanostructured CuO-based sensing system is a useful application for the continuous observation of sweat loss as a biological argument and compatibility with other microelectronic technologies.

Figure 1

FESEM images of CuO thin films synthesized with various Lawsonia inermis leaf extract concentrations at different magnifications. Dandelion-like nanostructures transform into elliptical-like nanostructures with the LiL addition.

Figure 2

EDX peaks, EDX-mapping images, and elemental analysis result of CuO thin film.

Figure 3

2D and 3D AFM topography images (10 × 10 μm² scan area) of CuO thin films synthesized with various Lawsonia inermis leaf extract concentrations. Surface roughness decreased with the increasing LiL concentration.

Figure 4

Typical XRD patterns of the CuO samples synthesized with various Lawsonia inermis leaf extract concentrations. The peak intensities of the obtained CuO films decrease with increasing LiL percentage.

Figure 5

FT-IR absorption spectra of CuO samples synthesized with various Lawsonia inermis leaf extract concentrations. Inset shows the lawsone molecule.

Figure 6

Photograph experimental setup of conductometric continuous sweat loss tracking sensor based on nanostructured CuO thin films functionalized with Lawsonia inermis leaf extract.

Figure 7

Transient response of the pristine and 1.0% LiL-substituted devices to the sweat solution (65.50 mM) droplets. It is apparent that the sensing response quality severely improves with 1.0% LiL substituting in the CuO nanostructure. It can be clearly seen, when artificial sweat is performed, the conductance value of the device is increased and then the steady state is acquired.

Figure 8

Photographs of the water contact angle CuO and 3.0% LiL:CuO synthesized with Lawsonia inermis leaf extract. Measured water contact angle values of thin films (right–left side and mean contact angle).

Figure 9

Transient response of the 3.0% LiL-substituted device to the sweat solution (262.00 mM) droplets. It becomes evident that 3.0% LiL-substituted CuO sample exhibits good stability for the high concentration of artificial sweat implementation.

Figure 10

Sweat solution-sensing response for unsubstituted, 1.0 and 3.0% LiL-substituted nanostructured CuO thin-film-based devices for sweat ingredients from 14.41 to 262.00 mM. It is conspicuous that LiL concentration increase in the growth bath the sensing-response values of the fabricated devices notably, which is over definite for higher components of sweat solution.

Figure 11

Possible two-electron reduction (in basic media) and two-electron one proton reduction mechanism in lawsone molecule (in aqueous buffer). Radicals stabilized with the Cu ions.