Kasprzyk Wiktor, Romańczyk Piotr P, Kurek Stefan S, Świergosz Tomasz
Biotechnology and Physical Chemistry, Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland.
Nanoscale. 2024 Sep 19;16(36):17079-17089. doi: 10.1039/d4nr02517b.
New molecular fluorophores are constantly being discovered in post-synthetic mixtures of carbon dots (CDs), prompting researchers to elucidate their role in the optical properties of these nanomaterials. It has been reported that the green-emitting fluorophore that forms during the synthesis of popular citric acid/urea CDs is HPPT (4-hydroxy-1-pyrrolo[3,4-]pyridine-1,3,6(2,5)-trione). However, due to the low concentration of HPPT-like molecules within the structure of CDs, their actual binding and contribution to the optical properties of CDs has not so far been convincingly confirmed. In this joint experimental and quantum chemical study, we show that HPPT is a strong acid and only its anionic form, HPPT, present in solutions of pH 0-10, is emissive. Next, its fluorescence can be switched off rapidly in strongly alkaline environments as a result of HPPT hydrolysis, leading to the opening of its pyrrole ring and formation of CDPC (3-carbamoyl-2,6-dihydroxypyridine-4-carboxylic acid), existing as the CDPC dianion under these conditions. Eventually, we found that the ring opening hydrolysis is reversible and the green emission may be restored in acidic environments. The kinetics and mechanism of this hydrolysis were also revealed. The optical features of citric acid (CA)-urea CDs under various conditions were compared with a simpler CD system prepared by treating the CDs obtained from CA solely with HPPT (HPPT@CDs). Our results indicate the feasibility of the post-synthetic modification of HPPT present in the structures of CA-urea CDs and HPPT@CDs. Without HPPT they emit blue fluorescence only. Thus, this makes the nanosystem switch the PL emission colour reversibly from green to blue owing to the opening and closing of the pyrrole ring in HPPT-like molecules. More importantly, the latter process may be considered a first step toward genuine fine tuning of the PL emission colour from CDs. These findings are of general importance to the further development of citric acid-based CDs with tailored properties.
在碳点（CDs）的合成后混合物中，新的分子荧光团不断被发现，这促使研究人员去阐明它们在这些纳米材料光学性质中所起的作用。据报道，在常见的柠檬酸/尿素CDs合成过程中形成的绿色发射荧光团是HPPT（4-羟基-1-吡咯并[3,4-]吡啶-1,3,6(2,5)-三酮）。然而，由于CDs结构中类HPPT分子的浓度较低，其实际结合情况以及对CDs光学性质的贡献至今尚未得到令人信服的证实。在这项联合实验和量子化学研究中，我们表明HPPT是一种强酸，只有其阴离子形式HPPT⁻，存在于pH值为0 - 10的溶液中时才具有发射性。接下来，由于HPPT水解，其荧光在强碱性环境中会迅速熄灭，导致其吡咯环打开并形成CDPC（3-氨基甲酰基-2,6-二羟基吡啶-4-羧酸），在这些条件下以CDPC²⁻形式存在。最终，我们发现开环水解是可逆的，并且在酸性环境中绿色发射可能会恢复。还揭示了这种水解的动力学和机制。将各种条件下柠檬酸（CA）-尿素CDs的光学特征与通过仅用HPPT处理由CA得到的CDs制备的更简单的CD系统（HPPT@CDs）进行了比较。我们的结果表明对CA-尿素CDs和HPPT@CDs结构中存在的HPPT进行合成后修饰是可行的。没有HPPT时它们仅发射蓝色荧光。因此，由于类HPPT分子中吡咯环的打开和关闭，这个纳米系统能够使PL发射颜色从绿色可逆地切换到蓝色。更重要的是，后一过程可被视为朝着真正精细调节CDs的PL发射颜色迈出的第一步。这些发现对于进一步开发具有定制性质的基于柠檬酸的CDs具有普遍重要性。
