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

Luca Malfatti,Stefania Mura,Luigi Stagi,Plinio Innocenzi,Robert Ludmerczki

doi:10.3390/ma13163654

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.

Highlights

Carbon dots (C-dots) are luminescent materials whose potential applications span different fields: lasing, photocatalysis, chemical and biological sensing and bioimaging [1,2]
In a recent work [10], we have reported the high fluorescence tunability C-dots synthesized by carbonization of citric acid and urea
The optical response of C-dots prepared from citric acid and urea in acid and alkaline solutions have been systematically studied to reveal the dependence of fluorescence properties on the variations

Summary

Introduction

Carbon dots (C-dots) are luminescent materials whose potential applications span different fields: lasing, photocatalysis, chemical and biological sensing and bioimaging [1,2]. Several syntheses employ citric acid as a precursor [4,5], which has the advantage of being highly biocompatible [6]. The quantum yield (QY) of the product is in general in the range of 5–15%, which is quite low in comparison to C-dots obtained with other precursors [7]. A better QY is achieved by adding a nitrogen-source as a precursor, such as urea [8,9]. In a recent work [10], we have reported the high fluorescence tunability C-dots synthesized by carbonization of citric acid and urea. A marked blue to green shift has been obtained as a function of the precursors molar ratio

Methods

Results

Conclusion