Orange peels-derived hydrochar for chemical sensing applications

Hydrochar (HC) samples were prepared from orange peels waste (OPW) via hydrothermal carbonization (HTC) at different temperatures, from 180 to 300 °C. The complete characterization of hydrochar samples, carried out by various complementary techniques such as TGA, XRD, SEM-EDX, FT-IR, and BET surface area, highlighted their different morphological and microstructural characteristics. These modifications were also accomplished by variations of the electrical and electrochemical properties, which have been here exploited, for the first time, for the development of high performances chemical sensors based on HC as sensing element. Hydrochar derived from OPW treatment at 300 °C (HC300) showed the best characteristics, and thereby was used for the fabrication of a conductometric NO2 and electrochemical dopamine sensors. The conductometric HC300-based sensor was demonstrated to be sensitive up to a 50 ppb of NO2 in air 100 °C. Dopamine at nanomolar concentration was detected with good performances (large linear detection range from 0 to 1000 μM, and low limit of detection (LOD) of 180 nM) on the HC300-based electrochemical sensor. This work might open up the opportunity to use the waste materials from the citrus industry for the green production of hydrochar with outstanding characteristics in advanced research fields so far no reported.