pH-Sensitive Fluorescence Emission of Boron/Nitrogen Co-Doped Carbon Quantum Dots

Abstract

Carbon quantum dots (CQDs) with their strong photoluminescence (PL) activity, high biocompatibility, robust stability, low cytotoxicity, and flexible surface structures have been employed in many fields including chemical sensing, biosensing, photocatalyst, energy storage, and biomedical applications. Of note, CQDs present an intrinsic pH-sensitive PL nature indicating their intense potential for pH-mediated sensing and imaging. Despite the numerous studies performed in the last two decades, the pH-sensitive PL mechanism of CQDs is still under debate and must be clarified to overcome the limitations in practical applications. Therefore, in this report, we performed a systematical study to determine the pH-sensitive PL nature of boron/nitrogen co-doped CQDs (B/N CQDs). In the first part, B/N CQDs with a strong blue emission were fabricated via a hydrothermal synthesis procedure. B/N-CQDs showed a strong blue PL emission with high quantum yield and excitation-dependent nature. Under the low pH conditions (pH 3), B/N-CQDs exhibited a robust green fluorescence emission with a significant red-shift (48 nm) and the loss of the excitation-dependent nature. The change in PL nature originated from the protonation of surface groups, a decrease in negative surface charge (from −20.6 to −1.23 eV), and finally, aggregation of the nanostructure (the size of CQDs from 4.8 to 7.5 nm). However, in the case of alkaline conditions, the deprotonation surface groups significantly enhanced the surface charge and led to the emergence of a negative ‘protective shell’ with a zeta potential of −71.3 eV. In a high pH medium (pH 13), PL spectra showed the loss of excitation-dependent features and a red-shift (35 nm) in emission peak maxima with lower intensity. This report provides significant progress in the clarification of the pH-sensitive PL mechanism of CQDs. We envision that the proposed CQDs would provide unique opportunities in the fabrication of novel pH sensor systems and fluorescence imaging where a wide range of pH sensitivity is required.

Green Synthesis of Various Heteroatom‐doped Carbon Quantum Dots from Urine, Whey, and Their Mixture: The Optimization of Synthesis and Potential Applications

“…The tunable PL emission through manipulating excitation wavelength provides unquestionable opportunities in various applications. So far, many convincing mechanisms such as the quantum confinement effect and surface traps have been proposed to elucidate the excitation‐dependent PL emission [6–8,38–41] . UV‐vis absorption spectra of U‐CQDs (Figure 1b) indicated that various transitions contributed to the final optical characteristics of the material.…”

“…The decrease in emission intensity with increasing pH can be attributed to the deprotonation of surface groups and resultant aggregation. [8] This phenomenon shows the potential of U/W-CQDs in pH sensor applications including pH monitoring in colloid systems and inside the cell. However, further structural and spectroscopic analyses are required which will be the topic of our perspective work.…”

“…We observed prominent bands at 3181, 3042, 2968, 1580, 1410, and 1117 cm À 1 which were assigned to υ (-OH), υ (À NH), υ (CÀ H), δ (C=C)/ υ (C=O), υ (CÀ H) and, υ (CÀ N), respectively. [46,47,53] While the hydrophilic groups such as υ (À OH), υ (À NH) and, υ (C=O) provide excellent solubility in water, [7,8,48,[54][55][56] the bands for δ (C=C), υ (CÀ H) and, υ (CÀ N) groups show the presence of polyaromatic characteristic of the CQDs' core. [57,58] The wide-scanning XPS spectra of W-CQDs (Figure 2d) indicated the presence of mainly C (C1s), N (N1s), and, O (O1s) at 284, 399, and 530 eV binding energies, respectively.…”

“…[2,23] Contrary to top-down strategies, through bottom-up approaches, CQDs are synthesized via the treatment of carbon-based small molecules as precursor material by various methods including microwave synthesis, pyrolysis, combustion, thermal decomposition, solvothermal, and hydrothermal synthesis. [6][7][8]24,25] Of these approaches, the hydrothermal method is the most preferred synthesis procedure due to its simplicity, low cost, high flexibility in the selection of the precursor material, and non-toxic and environmentally friendly nature which are essential for green synthesis routes. [6][7][8] Recently, there has been a lot of interest in the use of biomass waste from many sources, including food, agriculture, and domestic waste, as precursors in the manufacture of CQDs.…”

“…The requirement of high‐cost preparation devices (for example laser sources and electrochemical workstations) and harsh synthesis conditions with long durations, low product yield, and the lack of size control are major drawbacks of top‐down approaches which limit their employment for practical applications [2,23] . Contrary to top‐down strategies, through bottom‐up approaches, CQDs are synthesized via the treatment of carbon‐based small molecules as precursor material by various methods including microwave synthesis, pyrolysis, combustion, thermal decomposition, solvothermal, and hydrothermal synthesis [6–8,24,25] . Of these approaches, the hydrothermal method is the most preferred synthesis procedure due to its simplicity, low cost, high flexibility in the selection of the precursor material, and non‐toxic and environmentally friendly nature which are essential for green synthesis routes [6–8] …”