Synthesis and Properties of Nitrogen-Doped Carbon Quantum Dots Using Lactic Acid as Carbon Source.

Kaixin Chang,Liyan Qi,Mingwei Guo,Woming Gao,Qianjin Zhu,Qinwei Gao

doi:10.3390/ma15020466

Abstract

Nitrogen-doped carbon quantum dots (N-CQDs) were synthesized in a one-step hydrothermal technique utilizing L-lactic acid as that of the source of carbon and ethylenediamine as that of the source of nitrogen, and were characterized using dynamic light scattering, X-ray photoelectron spectroscopy ultraviolet-visible spectrum, Fourier-transformed infrared spectrum, high-resolution transmission electron microscopy, and fluorescence spectrum. The generated N-CQDs have a spherical structure and overall diameters ranging from 1–4 nm, and their surface comprises specific functional groups such as amino, carboxyl, and hydroxyl, resulting in greater water solubility and fluorescence. The quantum yield of N-CQDs (being 46%) is significantly higher than that of the CQDs synthesized from other biomass in literatures. Its fluorescence intensity is dependent on the excitation wavelength, and N-CQDs release blue light at 365 nm under ultraviolet light. The pH values may impact the protonation of N-CQDs surface functional groups and lead to significant fluorescence quenching of N-CQDs. Therefore, the fluorescence intensity of N-CQDs is the highest at pH 7.0, but it decreases with pH as pH values being either more than or less than pH 7.0. The N-CQDs exhibit high sensitivity to Fe3+ ions, for Fe3+ ions would decrease the fluorescence intensity of N-CQDs by 99.6%, and the influence of Fe3+ ions on N-CQDs fluorescence quenching is slightly affected by other metal ions. Moreover, the fluorescence quenching efficiency of Fe3+ ions displays an obvious linear relationship to Fe3+ concentrations in a wide range of concentrations (up to 200 µM) and with a detection limit of 1.89 µM. Therefore, the generated N-CQDs may be utilized as a robust fluorescence sensor for detecting pH and Fe3+ ions.

Highlights

Carbon quantum dots (CQDs) are regarded as a novel class of fluorescence nanomaterials
The High-resolution transmission electron microscopy (HRTEM) images of Nitrogen-doped carbon quantum dots (N-CQDs) (Figure 1a) display that N-CQDs are spherical in morphology with diameters ranging from 1–4 nm (Figure 1b), which is consistent with prior literatures [36]
The N-CQDs created are nanospheres with outstanding fluorescence and good solubility in water, while the quantum yields of N-CQDs are significantly higher than that of the CQDs reported in literatures

Summary

Introduction

Carbon quantum dots (CQDs) are regarded as a novel class of fluorescence nanomaterials. Many synthetic methods of CQDs have been reported, such as arc-discharge [14], laser ablation [16], electrochemical oxidation [17], microwave method [18], and hydrothermal treatment [19]. These synthetic methods have two main purposes: one purpose is to crack different carbon source materials, and the other is the carbonization of small molecules or polymers [20].

Methods

Results

Conclusion