Functionally gradient multilayer coating enabled flexible sensors for lead detection in water and soil

Lead contamination is a global concern owing to its toxicity to all living beings. According to the WHO, lead poisoning kills an estimated 1 million people each year. Global focus is on the development of low-cost sensor systems capable of detecting lead at extremely low concentrations. We, for the first time, develop a novel flexible working electrode for lead ion detection that is based on the concept of functionally gradient multilayer coating, comprised of bio-inspired graphene nanocomposite (GAP), hydrogenated amorphous carbon (a-C:H), and copper (Cu). Polypropylene mesh is used as a flexible substrate (F), which is then coated with magnetron sputtered Cu and PECVD-based a-C:H layers, followed by application of GAP layer via drop casting process. The GAP nanocomposite is comprised up of alanine, polynorepinephrine, and reduced graphene oxide. To ensure uniform deposition of Cu, a-C:H, and GAP layers, structural, morphological, and electrochemical studies are performed at each step. The electrochemical response of GAP/a-C:H/Cu/F electrode is found to be three folds higher than Cu/F electrode and 1.5 fold higher than a-C:H/Cu/F with good electrochemical stability. To achieve best sensing response, the a-C:H film thickness, supporting electrolyte, and deposition time are also optimized. The developed sensor is found to have low detection limit (0.5 ppb), high sensitivity (6.42 µA/ppb) with good stability and anti-interference properties (<3.5 %). This flexible sensor has also been tested against tap water and soil samples for real-world applications. This study offers an innovative approach for developing flexible sensors that has high potential to aid in effective monitoring of heavy metal ions in the environment.