International Journal of Advances in Applied Sciences (IJAAS) Vol. 13, No. 1, March 2024, pp. 84~92 ISSN: 2252-8814, DOI: 10.11591/ijaas.v13.i1.pp84-92  84 Journal homepage: http://ijaas.iaescore.com Green conversion of red snapper fish scale-derived carbon dots and its absorption properties for solar thermal desalination Dolfie Paulus Pandara, Gerald Hendrik Tamuntuan, Maria Daurina Bobanto, Ferdy Ferdy Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sam Ratulangi, Manado, Indonesia Article Info ABSTRACT Article history: Received Aug 8, 2023 Revised Nov 18, 2023 Accepted Dec 14, 2023 Fisheries wastes have been used as precursors for the synthesis of carbon dots (CDs). These wastes are often converted using hydrothermal methods which require high temperature and pressure, leading to high production costs, especially for large-scale production. This study aims to innovate a low-cost synthesis method with the potential for large-scale production. Green conversion of CDs from red snapper fish scale waste was carried out using a combination of immersion method and ultrasonic wave treatment. The results showed that the products had an absorption peak at a wavelength of 404 nm and an energy bandgap of 2.7 eV. Excitation at 404 nm was associated with non-bonding n orbital to antibonding orbital * electronic transitions due to the presence of free electron pairs and related with carbon- nitrogen (C-N) bonds. The 2.7 eV energy bandgap was associated with the state of amine groups containing free electron pairs located on the surface of CDs as well as blue light emission at a wavelength of 460 nm when CDs were illuminated with ultraviolet light. Red snapper fish scale-derived CDs showed fluorescence characteristics and the presence of nitrogen elements, making them potential photothermal materials for solar-powered seawater desalination processes. Keywords: Bandgap energy Excitation Photoluminescence Photothermal Seawater evaporation This is an open access article under the CC BY-SA license. Corresponding Author: Dolfie Paulus Pandara Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sam Ratulangi Campuss Street, Bahu, Manado 95115, North Sulawesi, Indonesia Email: dpandara_fisika@unsrat.ac.id 1. INTRODUCTION Carbon dots (CDs) are materials with great potential due to their manifold applications, particularly in heavy metal monitoring (ferric ion) [1], photovoltaic materials [2], biomedical [3], and biosensors [4]. The remarkable fluorescence capability of CDs is closely associated with the quantum effects arising from the size difference, which is an important optical characteristic for both absorption and emission [5]. Fluorescence is a form of photoluminescence where CDs or carbon quantum dots, emit different colors of light with lower energy compared to the absorbed light, depending on their bandgap size [6]. Fish scales are one of the waste products obtained from fishing industry activities, seafood restaurants, and local fish markets. These materials have been reported to have a high potential as a carbon source for the production of CDs [7]. Furthermore, fish scales account for 2-4% of approximately 20-80% of the total waste generated during fish processing [8], [9]. As fish production continues to increase, the amount of fish scale produced is also expected to rise. In 2017, fishery production in Indonesia reached 23.19 million tons, thereby significantly increasing the volume of waste [10]. This increased waste production can cause environmental pollution and negative effects on human health without proper management and optimal use [11], [12]. Fish scales are biocomposite materials that can be extracted into organic components, such as chitin, lecithin, guanine, and collagen fibers, as well as inorganic components, including hydroxyapatite