green synthesis of carbon dots from agricultural waste through ultrasonication: the utilization of tofu byproducts

The problem of food industry waste, specifically tofu solid waste (tsw), represents an environmental challenge and an opportunity for optimal utilization of untapped resources. therefore, this study aims to investigate the green synthesis of tsw-based carbon dots (c-dots) using the ultrasonication method to produce products with superior functional characteristics. tsw was first dried, ground, extracted, and mixed with citric acid and urea. the synthesis was conducted for 1 and 2 hours using ultrasonic waves at a frequency of 40 khz. the c-dots obtained were characterized using various techniques, including yield, photoluminescence, uv–vis spectroscopy, ftir, zeta potential, color, and x-ray diffraction (xrd). the results showed that the highest yield of 97% was achieved with a synthesis time of 1 hour. moreover, the strong bluish fluorescence of c-dots under uv light indicated successful carbonization and doping. the uv–vis spectrum also exhibited absorbance peaks at 322 nm and 328 nm, suggesting the presence of π–π* and n–π* transitions. the ftir spectrum showed the presence of hydroxyl groups and aromatic structures, with two main peaks at wavenumbers 1629 cm⁻¹ and 3353 cm⁻¹. furthermore, positive zeta potential values of 1.67 mv and 1.13 mv indicated good dispersion stability in liquid media, and xrd data showed a high crystallinity index of 78.73% with a crystallite size of more than 68 nm. this study confirmed that tsw could be an effective and sustainable carbon source for c-dot synthesis. the results also contribute to the development of waste-based biomaterial technology and support the application of circular economy principles in the management of renewable resources.