Flexible and highly selective NO2 gas sensor based on direct-ink-writing of eco-friendly graphene oxide for smart wearable application

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Nitrogen dioxide (NO 2 ) is a major cause of respiratory disorders in outdoor and indoor environments. Real-time NO 2 monitoring using nonintrusive wearable devices can save lives and provide valuable health data. This study reports a room-temperature, wearable, and flexible smart NO 2 gas sensor fabricated via cost-effective printing technology on a polyimide substrate. The sensor uses alkali lignin with edge-oxidised graphene oxide (EGO-AL) ink, demonstrating a sensitivity of 1.70% ppm⁻ 1 and a detection limit of 12.70 ppb, with excellent selectivity towards NO 2 . The high sensing properties are attributed to labile oxygen functional groups from GO and alkali lignin, offering abundant interacting sites for NO 2 adsorption and electron transfer. The sensor fully recovers to the baseline after heat treatment at 150 °C, indicating its reusability. Integration into lab coats showcased its wearable application, utilising a flexible printed circuit board to wirelessly alert the wearer via cell phone to harmful NO 2 levels (>3 ppm) in the environment. This smart sensing application underscores the potential for practical, real-time air quality monitoring, personal safety enhancement, and health management. • NO 2 gas sensor fabricated via DIW method using eco-friendly EGO-AL ink. • Sensor exhibit high sensitivity (1.70 % ppm −1 ) and low detection limit (12.70 ppb). • Excellent reusability with full recovery after heat treatment at 150 °C. • Wireless data transmission via Bluetooth/Wi-Fi for real-time monitoring. • Integrated into lab coat for smart wearable safety applications.