In this report, we present the results of our investigations into the elucidation of the chemical structure of moieties responsible for the blue and green luminescence of CDs derived from the microwave-assisted pyrolysis of citric acid in the presence of urea. The molecular fluorophore that forms during the synthesis of green fluorescing CDs is 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT).

Carbon dots (CDs), an emerging class of nanomaterials, have attracted considerable attention due to their intriguing photophysical properties. Despite their indisputable potential of utilization in many fascinating areas of research and life, some fundamental aspects concerning their structure and the origin of their photoluminescence (PL) properties still await clarification. The mechanism of PL emission of CDs is associated with their structure, which is dependent on the carbonization process. At the initial stages of CD synthesis via a bottom-up approach, molecular fluorophores are considered to dominate the optical characteristics of the resulting nanomaterials. In this review, the recent progress in the use of molecular state theory for explanation of the structure–property relationship in CDs is summarized. This review focuses exclusively on the molecular fluorophores existing in nanomaterials prepared from citric acid (CA) as one of the most frequent carbon sources reported for the bottom-up synthesis of CDs. Consequently, the most relevant transformations of CA and the history of molecular fluorophores derived from it are described, followed by an in-depth discussion on their relevance in understanding the specific photophysical properties of blue-, green-, and red-emitting CDs. Finally, the challenging issues and future perspectives of molecular state PL mechanism exploration in CDs are highlighted.

Citric acid-based carbon dots (CDs) as nano-photoinitiating catalysts for 3D printing.

The mechanism of the molecular photoluminescence of carbon dots (CDs) has gained much attention recently, due to several reports of formation of small fluorophore species at reaction conditions applied in CD synthesis. In this work, two novel green‐emitting species are discovered in postsynthetic mixtures of citric acid (CA)–urea CDs and identified as 3‐(4‐hydroxy‐1,3,6‐trioxo‐5H‐pyrrolo[3,4‐c]pyridine‐7‐yl)‐2,6‐dioxo‐piperidine‐4‐carboxyamide (HTPPC) and 7‐[(2,5‐dioxopyrrolidin‐3‐yl)methyl]‐4‐hydroxy‐5H‐pyrrolo[3,4‐c]pyridine‐1,3,6‐trione (DMHPPT). The less‐considered covalent character of interactions between the molecular fluorophores and the CD structure is revealed during the first steps of formation of the frequently studied CA–urea CDs. Thus, specific reaction sites in the HPPT fluorophore (4‐hydroxy‐1H‐pyrrolo[3,4‐c]pyridine‐1,3,6(2H,5H)‐trione), involved in the formation of covalent bonds with CDs, are identified; the origin of the reactive species in the synthesis mixture is revealed; and the spectroscopic consequences of this binding are demonstrated using steady‐state and time‐resolved spectroscopy methods accompanied by quantum chemical modeling. The hypothesis suggesting the formation of CDs at the expense of molecular fluorophores is validated experimentally. The covalent nature of bonding of molecular fluorophores to the CDs involving a reaction with double bond functionalities of CDs is successfully expanded to citrazinic acid fluorophores, suggesting that a similar mechanism can be present in other carbon dot systems.

Herein, a novel fluorescent method for the determination of d-panthenol (DP) level in solutions with no separate hydrolysis step has been revealed based on the utilization of citric acid (CA) as a derivatizing agent. Consequently, the essential parameters of the derivatization process were established, resulting in the development of sensitive, repeatable, and accurate determination of panthenol. The method was approved, and its usefulness in characterizing the concentration of DP in pharmaceutical formulations and selectivity in the determination of DP were validated. The chemical structure of the new fluorophore formulating in the reaction in DP with CA, i.e., 6-oxo-3,4-dihydro-2H,6H-pyrido[2,1-b][1,3]oxazine-8-carboxylic acid (ODPC), was elucidated using detailed NMR experiments: one-dimensional (¹H, ¹³C) as well as two-dimensional NMR spectra (¹H-¹H COSY, ¹H-¹³C HSQC, ¹H-¹³C HMBC, ¹H-¹⁵N HSQC, ¹H-¹⁵N HMBC).