Bio-waste of the citrus-hybrid calamansi orange can cause toxicity in ecosystems through water contamination due to the high percentage of D-Limonene in the essential oil of the peels. An abundance of the inedible calamansi peels in landfills correlates to the low cost and popularity of this ingredient in cuisines and natural remedies. However, we aimed to repurpose the peels as a sustainable precursor for the green synthesis of carbon quantum dots (CQDs) to be used as a sensor for detecting heavy metals. We utilized hydrothermal carbonization (HTC) to catalyze the thermal conversion of calamansi peels in a hydrothermal reactor to produce biochar with CQDs. The transmittance, fluorescence, and absorbance of the CQDs were characterized through Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet Visible Spectroscopy (UV-Vis), and fluorescent spectroscopy. Once a promising quantum yield was reached, calamansi peel CQDs were incorporated into novel sensors for detecting pollutants in water. We fabricated the sensors utilizing Direct Ink Writing (DIW) 3D printing with a sustainable ink derived from naturally sourced nanocellulose fibers. This work demonstrates how biomaterials can be harnessed to create low-cost sensors for the detection of heavy metals to improve water quality.

Special acknowledgement to the National Science Foundation, Office of International Science and Engineering Award #2245405 given to University of San Diego to collaborate with Center for Research in Advanced Materials (Centro de Investigación en Materiales Avanzados, CIMAV) in Monterrey, Mexico.