Eco-friendly and efficient synthesis of nitrogen-doped carbon quantum dots for pH sensing applications

Nitrogen-doped carbon quantum dots (N-doped CQDs) were prepared using a one-step sintering technique, with ammonium citrate as the precursor. These were compared to carbon quantum dots (CQDs) synthesized from citric acid. A range of characterization techniques, including x-ray diffraction analysis (XRD), raman spectroscopy, transmission electron microscopes (TEM), x-ray photoelectron spectroscopy (XPS), ultraviolet–visible absorption spectroscopy (UV–Vis absorption spectra), and fourier transform infrared spectroscopy (FT-IR spectra), were utilized to analyze the structures of two types of CQDs. The results revealed that nitrogen doping profoundly alters the carbon structure, promoting a shift from diamond-like carbon (sp3 C) to graphite carbon (sp2 C). This alteration greatly hastens the graphitization process of the CQDs, thereby broadening fluorescence emission specturm. Moreover, the fluorescenet properties of N-doped CQDs showed a pronounced sensitivity to pH variations. Specifically, the fluorescent properties of N-doped CQDs demonstrated notable sensitivity to pH variations. It was quantitatively represented by two distinct linear relationships: in acidic conditions, it follows y=0.28+1.08∗x\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$y=0.28+1.08*x$$\end{document}, and in alkaline conditions,y=15.06-x\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$y=15.06-x$$\end{document}. (where y is the normalized fluorescence intensity and x is the pH value). For original CQDs, the change is less pronounced, with a linear relationship of y=6.62-0.022∗x\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$y=6.62-0.022*x$$\end{document}. Therefore, the N-doped CQDs exhibit a strong correlation between the fluorescence intensity and pH value in both acidic and alkaline environments, while also demonstrating the ability to change fluorescence color, indicating its potential applications in rapid response and convenient pH detection.