Lattice Proton Intercalation to Regulate WO₃‐Based Solid‐Contact Wearable pH Sensor for Sweat Analysis

Abstract

Sweat pH monitoring is a routine indicator in wearable biotechnology. The state-of-the-art wearable pH sensors mostly rely on organic materials but face the risk of biological toxicity. WO 3 is a typical H + -sensitive inorganic material with chemical stability, biocompatibility, and low cost but low sensitivity and slow response. Lattice H + intercalation is herein proposed as an efficient approach that can greatly improve the sensitivity and selectivity of WO 3based pH sensors. Specifically, lattice H + intercalation can promote WO 3 from the monoclinic phase to cubic phase, which enhances the ion exchange capacity between WO 3 and H + . The resistance decreases more than two orders of magnitudes, which improves the interfacial charge transport. The occupancy of lattice H + leads to ion exchange only with H + , thus increasing the H + recognition. The intercalated H x WO 3 exhibits much improved sensitivity, reversibility, and response time. Additionally, the H x WO 3 is integrated with a solid reference electrode on a miniaturized chip for wearable sweat pH monitoring. The pH sensor exhibits good potential response even at curving over 270°. On-body sweat pH measurments show high accuracy compared with ex situ analyses. This work emphasizes the concept of lattice proton intercalation to regulate the H + recognition of solid contacts.

Recent Advances in Wearable Potentiometric pH Sensors

“…According to our recent study, the pH sensitivity of sputtered WO 3 films has a strong correlation with thickness [ 109 ]. The sensitivity decreases with the thickness of WO 3 , as shown in Figure 5 D. In addition, the response pH range either decreases with the thickness.…”

“…However, the sensor had a large response time of 23–28 s, which was unfavorable for the timeliness of the data in the wearable device requirements. Very recently, we proposed a flexible wearable pH sensor based on lattice proton intercalation with WO 3 [ 109 ] ( Figure 6 J). The intercalated H x WO 3 had a high reversible response sensitivity of −52.5 and −51.1 mV/pH, excellent selectivity and resistance to light and gas.…”

“…However, intrinsic WO 3 usually exhibits a sub–Nernst response and a slower pH response. The proton intercalation methodology offers a strategy for improving the sensitivity [ 109 ]. The question of how to further enhance the comprehensive performance of MO x –based pH sensors is deserving of further attention.…”

“…( G ) Complete hydration by the proton intercalation of WO 3 . Reprinted with permission from [ 109 ], Copyright (2022) John Wiley and Sons publications.…”

“…The image on the right shows a schematic of the insertion of a proton into the lattice of WO 3 and the photograph on the left shows the flexible electrode after proton intercalation. Reprinted with permission from [ 109 ], Copyright (2022) John Wiley and Sons publications.…”