Reversible Aggregation of Molecular-Like Fluorophores Driven by Extreme pH in Carbon Dots

Abstract

The origin of carbon-dots (C‑dots) fluorescence and its correlation with the dots structure still lack a comprehensive model. In particular, the core-shell model does not always fit with the experimental results, which, in some cases, suggest a molecular origin of the fluorescence. To gain a better insight, we have studied the response of molecular-like fluorophores contained in the C‑dots at extreme pH conditions. Citric acid and urea have been employed to synthesize blue and green‑emitting C‑dots. They show a different emission as a function of the pH of the dispersing media. The photoluminescence has been attributed to molecular-like fluorophores: citrazinic acid and 4‑hydroxy‑1H‑pyrrolo[3,4‑c]‑pyridine‑1,3,6‑(2H,5H)-trione. 3D and time‑resolved photoluminescence, ultraviolet–visible (UV–vis) spectroscopy, and dynamic light scattering have been used to determine the aggregation state, quantum yield and emission properties of the C-dots. The dependence of the C-dots blue and green components on the chemical environment indicates that the origin of fluorescence is due to molecular-like fluorophores.

Carbon Nanodots from an In Silico Perspective

“…For example, it has been demonstrated that CA and EDA form fluorescent species in the early stages of hydrothermal synthesis and sp 2 domains arise within minutes (Figure g). As shown in Figure h, the UV–vis absorption is characterized by two strong bands at 250 and 350 nm due to π → π* and n → π* of 2-pyridone molecular moieties. , Whether and how the fluorophores interact in the synthesis process of graphitic domains is still unknown since in the process, particularly under critical conditions such as those of solvothermal treatment or pyrolysis, several mechanisms and precursors could be involved.…”

“…As shown in Figure 7h, the UV−vis absorption is characterized by two strong bands at 250 and 350 nm due to π → π* and n → π* of 2-pyridone molecular moieties. 395,396 Whether and how the fluorophores interact in the synthesis process of graphitic domains is still unknown since in the process, particularly under critical conditions such as those of solvothermal treatment or pyrolysis, several mechanisms and precursors could be involved. The presence of molecular fluorophores is indirectly demonstrated by comparing the optical properties (UV−vis and fluorescence) of single molecules in solution with the ones in CNDs.…”

Citric Acid Derived Carbon Dots, the Challenge of Understanding the Synthesis-Structure Relationship

“…CDs made of CA and polyethylenimine (PEI) or 2,3-diaminopyridine (DAP) have shown a reduction of the QYs from ≈ 40-50% to less than 9% when dissolved in an aqueous solution of pH ≈ 1 and to 21% when the pH has been increased to 12 [88]. The peculiar behavior of some CD's fluorophores formed in situ during CDs carbonization, such as citrazinic acid, has also been recently studied at extreme pH conditions [89,90]. The same fluorophores dissolved in solution have also proved to drastically change their optical properties as a function of the concentration undergoing a transition from monomer to dimer [91].…”