_applied sciences_  
_Article_  
Development and Characterization of N/S-Carbon Quantum  
Dots by Valorizing Greek Crayfish Food Waste  
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and high photoluminescence (PL) intensity, which is dependent on the wavelength of the exciting radiation [6; 7; 8; 9]. Although Carbon QDs mostly consist of carbon, they have high aqueous solubility [5; 7], which can be enhanced by the presence of nitrogen-containing functional groups on the surface of the dots [10]. The versatility of Carbon QDs' properties renders them as potential candidate materials in a plethora of applications. Firstly, their fluorescent abilities make them ideal for use in electrochemical luminescence, optronics, and sensors, as well as in photocatalysis [5; 8]. In addition, they can be applied in the field of biosciences and specifically in biosensing, bioimaging, and biological labeling [5; 6; 8; 11]. Alternatively, they can be also applied as nanocarriers for biomolecules or drugs [5; 8; 11], as well as antibacterial agents [9]. One of their most interesting applications concerns the chemical sensing of metal ions and is based on their fluorescence properties. The sensing of Hg\({}^{2+}\) ions can be considered as the prime example of this utilization of Carbon QDs [11], which is very important considering the toxicity of Hg to humans and the environment. CQD-based chemical sensing can be also applied for other ions, such as Fe\({}^{3+}\)[12], Ag\({}^{+}\)[13], and Cu\({}^{2+}\)[14]. Recently, Carbon QDs have been suggested as excellent candidates for food preservation applications due to their low price, low toxicity, and significant antioxidant and antibacterial properties [15; 16]. Carbon QDs, due to their water solubility, can be applied directly as food preservatives or incorporated in water-soluble polymers and biopolymers [17; 18]. Water-soluble polymers, such as poly-vinyl alcohol, and water-soluble biopolymers, such as gelatin and chitosan, are ideal matrices to incorporate carbon QDs in active films and coatings for the preservation of various foods [7; 18; 19; 20]. In the field of carbon QDs synthesis, a variety of preparation methods are used, which can be divided in two categories: (i) top-down and (ii) bottom-up [7; 21]. Top-down methods require higher temperatures and pressures, whilst bottom-up methods are performed in milder conditions. Top-down methods operate by breaking down larger carbon structures to form the dots [22], while bottom-up methods focus on converting small, carbon-rich biomolecules into Carbon QDs [7; 22]. Top-down preparation approaches includes arc discharge and laser ablation, which are quick, but result in a low quantum yield (QY) and poor size control. Additionally, electrochemical carbonization is performed in a single step and provides good size control, but is not suitable for many precursors [5]. Some commonly known bottom-up synthetic routes are microwave irradiation, which is quick and cost-effective but provides little control over the dots' sizes; ultrasound; conventional heating; and hydrothermal/solvothermal treatment [7]. In this research, the hydrothermal route was followed because of the low cost, eco-friendliness, and non-toxicity, so it can be easily used on biomass-based raw materials such as crayfish shell. Previously synthesized Carbon QDs using this method have a diameter of 1.5 to 4.5 nm [5]. The properties of Carbon QDs can be enhanced by doping with a variety of elements including nitrogen, sulfur, phosphorus, and calcium [5; 23]. Doping Carbon QDs with nitrogen is a very effective way to upgrade their photoluminescence characteristics and to increase their QY, while metal ion sensing is performed more efficiently [23]. Phosphorus and nitrogen co-doped dots also display improved PL properties and higher efficiency in electrocatalysis [5; 23].