Citation: Gulati, S.; Baul, A.; Amar, A.; Wadhwa, R.; Kumar, S.; Varma, R.S. Eco-Friendly and Sustainable Pathways to Photoluminescent Carbon Quantum Dots (CQDs). Nanomaterials 2023, 13, 554. https:// doi.org/10.3390/nano13030554 Academic Editor: Christophe Donnet Received: 10 January 2023 Revised: 24 January 2023 Accepted: 26 January 2023 Published: 30 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Review Eco-Friendly and Sustainable Pathways to Photoluminescent Carbon Quantum Dots (CQDs) Shikha Gulati 1, *, Arikta Baul 1 , Anoushka Amar 1 , Rachit Wadhwa 1 , Sanjay Kumar 1 and Rajender S. Varma 2, * 1 Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India 2 Institute for Nanomaterials, Advanced Technologies, and Innovation (CxI), Technical University of Liberec (TUL), Studentská 1402/2, 461 17 Liberec, Czech Republic * Correspondence: shikha2gulati@gmail.com (S.G.); varma.rajender@epa.gov (R.S.V.) Abstract: Carbon quantum dots (CQDs), a new family of photoluminescent 0D NPs, have recently received a lot of attention. They have enormous future potential due to their unique properties, which include low toxicity, high conductivity, and biocompatibility and accordingly can be used as a feasible replacement for conventional materials deployed in various optoelectronic, biomedical, and energy applications. The most recent trends and advancements in the synthesizing and setup of photoluminescent CQDs using environmentally friendly methods are thoroughly discussed in this review. The eco-friendly synthetic processes are emphasized, with a focus on biomass-derived precursors. Modification possibilities for creating newer physicochemical properties among different CQDs are also presented, along with a brief conceptual overview. The extensive amount of writings on them found in the literature explains their exceptional competence in a variety of fields, making these nanomaterials promising alternatives for real-world applications. Furthermore, the benefits, drawbacks, and opportunities for CQDs are discussed, with an emphasis on their future prospects in this emerging research field. Keywords: carbon quantum dots (CQDs); photoluminescent; bottom-up approach; top-down approach; green synthesis 1. Introduction Carbon-based nanomaterials have gained tremendous global attention from researchers, owing to their unique properties, namely high conductivity, low toxicity, biocompatibility, and easy functionalization [1–8]. This consideration can be attributed to their harmless effect on the environment when compared to other commonly deployed energy sources emanating from fossil fuels [9–13]. Since the discovery of fullerenes in the 1980s, material science and engineering related to carbon materials have piqued the public’s interest. Thus, these carbon-based substances, such as graphene quantum dots (GQDs), carbon nanodots, polymer dots, and other carbon-derived dots, are explored widely for their promising applications in carbon fixation, gas storage, adsorbents, and other fields [14,15]. Perhaps, out of this carbon family, the most commonly reported member is GQDs, which are made out of graphene, with lateral dimensions less than 100 nm, and possess some common properties of graphene and C-dots [16]. To this point, GQDs are often confused with carbon dots (C-dots), but they possess certain differences; GQD framework consists of sp 2 -hybridized carbon, whereas C-dots possess sp 3 -hybridized carbon [17]. Furthermore, C-dots are quasi-spherical carbon nanoparticles that have graphene and turbostratic carbon mixed in different ratios [18]. Realizing the potential of GQDs, researchers across the globe have suggested a plethora of techniques for the preparation of GQDs, but many of them involve harsh and non-environmentally friendly approaches, thus necessitating the search for eco-friendly methods. Over a few decades, industrial chemical research has turned its focus towards the use of renewable sources and waste-reduction approaches, leading to the evolvement of a new concept, termed green chemistry, which, endowed with its Nanomaterials 2023, 13, 554. https://doi.org/10.3390/nano13030554 https://www.mdpi.com/journal/nanomaterials