Rapid pyrolysis of Cu²⁺-polluted eggshell membrane into a functional Cu²⁺-Cu⁺/biochar for ultrasensitive electrochemical detection of nitrite in water

Bioremediation is one of efficient methods to solve the issues of water or soil contaminated by metal ions. However, the harvested biowaste is often troublesome to handle owing to the second pollution. Herein, the waste eggshell membrane was used to adsorb Cu²⁺ in wastewater, which was then converted into biochar containing copper ions (Cu²⁺-Cu⁺/Biochar) via a rapid pyrolysis. By integrating the collective advantages of eggshell membrane and Cu²⁺-Cu⁺, such as superior electrical conductivity, enlarged electrochemically active surface area, unique three-dimensional porous network characteristics, and fast charge transport, the Cu²⁺-Cu⁺/Biochar system can be used as a self-supporting sensor for detection of nitrite (NO₂ ⁻). The sensor demonstrated superior electrochemical sensing abilities accompanied by a broad linear range (1–300 μM), ultralow detection limit (0.63 μM), and high sensitivity (30.0 μA·mM⁻¹·cm⁻²). In addition, the fabricated electrochemical sensor has excellent stability, good reproducibility, and strong anti-interference performance. More importantly, the sensor has a high recovery rate when it is used to detect nitrite in tap water, mineral water, and sausage, indicating the feasibility of using this sensor in practical applications. This study provides a green and sustainable approach for simultaneous treatment of biomass waste eggshell membrane, remedy of heavy metals, and electrochemical detection of nitrite.