Journal of Carbon Research C Review Citric Acid Derived Carbon Dots, the Challenge of Understanding the Synthesis-Structure Relationship Junkai Ren , Luca Malfatti and Plinio Innocenzi *   Citation: Ren, J.; Malfatti, L.; Inno- cenzi, P. Citric Acid Derived Carbon Dots, the Challenge of Understand- ing the Synthesis-Structure Relation- ship. C 2021, 7, 2. https://dx.doi.org/ 10.3390/c7010002 Received: 2 December 2020 Accepted: 17 December 2020 Published: 22 December 2020 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2020 by the authors. Li- censee 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/). Laboratory of Materials Science and Nanotechnology (LMNT), Department of Chemistry and Pharmacy, University of Sassari, CR-INSTM, Via Vienna 2, 07100 Sassari, Italy; j.ren@studenti.uniss.it (J.R.); luca.malfatti@uniss.it (L.M.) * Correspondence: plinio@uniss.it Abstract: Carbon dots (CDs) are highly-emissive nanoparticles obtained through fast and cheap syntheses. The understanding of CDs’ luminescence, however, is still far from being comprehensive. The intense photoluminescence can have different origins: molecular mechanisms, oxidation of polyaromatic graphene-like layers, and core-shell interactions of carbonaceous nanoparticles. The citric acid (CA) is one of the most common precursors for CD preparation because of its high biocompatibility, and this review is mainly focused on CA-based CDs. The different parameters that control the synthesis, such as the temperature, the reaction time, and the choice of solvents, were critically described. Particular attention was devoted to the CDs’ optical properties, such as tunable emission and quantum yields, in light of functional applications. The survey of the literature allowed correlating the preparation methods with the structures and the properties of CA-based CDs. Some basic rules to fabricate highly luminescent nanoparticles were selected by the metanalysis of the current literature in the field. In some cases, these findings can be generalized to other types of CDs prepared via liquid phase. Keywords: carbon dots; citric acid; nanoparticles; photoluminescence 1. Introduction Since the discovery of fluorescent carbon nanotubes fragments [1], carbon dots (CDs) have been extensively studied as an emerging generation of optoelectronic materials. The reason for such interest is due to both technological and basic science issues. The tunable photoluminescence and the high quantum yield (QY) combined with low-cost preparation [2–5] are undoubtedly attractive properties for material scientists looking for new light-emitting nanostructures. As a metal-free optical material, CDs are also envi- ronmentally friendly and represent a promising alternative to conventional metal-based semiconductors and rare elements. CDs could, therefore, potentially provide outstanding advantages in various fields, such as sensing [6,7], bioimaging [8], catalysis [9], lighting and displaying [10,11], lasering [12–14], etc. At the same time, CDs allow revolutionizing the actual perspective on the classical chemistry classifications. CDs are 0-D emissive spheroidal carbon-based nanostructures with a size smaller than 20 nm [2,5]. The CDs, in fact, stand in between organic (polymers) and inorganic materials (black carbon), macromolecules, and nanoparticle, between bottom- up (polycyclic aromatic compounds) and top-down synthesis (laser ablation of graphene, etc.). However, it is still a challenge to understand the CDs’ correlation between nano- structure and bright fluorescence [15]. The number of publications about CDs has rapidly increased in recent years (see Figure 1a), reflecting the utmost simplicity in preparing the materials but also the galaxy of slightly different results that small changes in the synthesis can produce. An abundance of precursors, in fact, can be used for the preparation of CDs via bottom-up methods, ranging from simple and natural molecules up to complex and expensive compounds [16]. Among C 2021, 7, 2. https://dx.doi.org/10.3390/c7010002 https://www.mdpi.com/journal/carbon