Thermal decomposition of citric acid is one of the most common synthesis methods for fluorescent carbon dots; the reaction pathway is, however, quite complex and the details are still far from being understood. For instance, several intermediates form during the process and they also give rise to fluorescent species. In the present work, the formation of fluorescent C‐dots from citric acid has been studied as a function of reaction time by coupling infrared analysis, X‐ray photoelectron spectroscopy, liquid chromatography/mass spectroscopy (LC/MS) with the change of the optical properties, absorption and emission. The reaction intermediates, which have been identified at different stages, produce two main emissive species, in the green and blue, as also indicated by the decay time analysis. C‐dots formed from the intermediates have also been synthesised by thermal decomposition, which gave an emission maximum around 450 nm. The citric acid C‐dots in water show short temporal stability, but their functionalisation with 3‐aminopropyltriethoxysilane reduces the quenching. The understanding of the citric acid thermal decomposition reaction is expected to improve the control and reproducibility of C‐dots synthesis.

Highly fluorescent blue and green-emitting carbon dots have been designed to be integrated into solgel processing of hybrid organic-inorganic materials through surface modification with an organosilane, 3-(aminopropyl)triethoxysilane (APTES). The carbon dots have been synthesised using citric acid and urea as precursors; the intense fluorescence exhibited by the nanoparticles, among the highest reported in the scientific literature, has been stabilised against quenching by APTES. When the modification is carried out in an aqueous solution, it leads to the formation of silica around the C-dots and an increase of luminescence, but also to the formation of large clusters which do not allow the deposition of optically transparent films. On the contrary, when the C-dots are modified in ethanol, the APTES improves the stability in the precursor sol even if any passivating thin silica shell does not form. Hybrid films containing APTES-functionalized C-dots are transparent with no traces of C-dots aggregation and show an intense luminescence in the blue and green range.Carbon dots (C-dots) are fluorescent nanomaterials with optical properties comparable to semiconductor quantum dots. C-dots, however, have a much lower cost and environmental impact, which make them a hot topic of research 1,2 . A major advantage is the possibility to produce C-dots from an almost endless variety of precursors and methods. On the other hand, strict control of the properties through the process is still challenging to achieve, and the main efforts are now dedicated to obtaining reliable and reproducible synthesis.The citric acid (CA) alone or in combination with other compounds is one of the most popular precursors for C-dots 3 . CA-based C-dots have on their surface different carboxy-groups, which increase the solubility and allow surface passivation or functionalization with organic molecules 4-6 or polymers 7 . In general, pure CA C-dots, without any modification, show a weak emission, and doping 1 with B, N, S, Si and P atoms is a possible solution to improve their quantum yield. Most of the CA C-dots are doped with nitrogen that enhances the luminescence by producing azo-compounds through the reaction between the carboxylic and amino groups; after carbonisation, they form water-dispersible and highly emitting C-dots 8 . Different amines have been used for this purpose, such as ethylenediamine (EDA) 2,5,9 , hexamethylenetetramine 6 , o-phenylenediamine (o-PD) 10 , triethylenetetramine 11 , hexadecylamine (HDA) 8 , and triethanolamine 6 . Quantum yields (QY) under 8% for most of the amines, with the exception of EDA 6 which gives a QY of 86%, have been obtained. Urea, because of the high nitrogen content, can be used for doping CA C-dots 1,5,12-17 and different methods have been developed so far. Hydrothermal treatment in an autoclave and microwave exposure are simple synthesis for producing luminescent C-dots from citric acid and urea. Low QY (16%) 1 have been obtained by processing the dots via oven treatments, with even lower QY value...

Understanding the luminescence of carbon dots is a highly challenging task because of the complex reactions involved in the synthesis process. Several by-products form at different reaction stages and become possible sources of emission. Citrazinic acid and its derivatives, in particular, have been identified as intermediates that give rise to blue fluorescence. Full comprehension of the optical properties of citrazinic acid itself is, however, still lacking. In particular, citrazinic acid has the property of forming different tautomers and aggregates such as dimers. However, the nature of these chemical species and the correlation with their relative optical properties have been only partially explored. In the present work, we have used a combination of spectroscopic techniques, UV−visible and fluorescence spectroscopy, time-resolved photoluminescence and computational simulation, to study the different species which citrazinic acid forms in water as a function of CZA molarity. A monomer-todimer transformation and a hypsochromic shift are observed with concentration. The monomer is in the keto structure and does not form other tautomers while the dimers are fluorescent J-type aggregates. The formation of aggregates strongly modulates the optical properties of citrazinic acid.