Abstract
Biomass-derived carbon dots (CDs) are biocompatible and have potential for a variety of applications, including bioimaging and biosensing. In this work, we use ground soybean residuals to synthesize carbon nanoparticles by hydrothermal carbonization (HTC), annealing at high temperature, and laser ablation (LA) in a NH4OH solution. The carbon nanoparticles synthesized with the HTC process (HTC-CDs) exhibit photoluminescent characteristics with strong blue emission. The annealing of the HTC-processed carbon particles in the range of 250 to 850 °C causes a loss of the photoluminescent characteristics of the CDs without any significant change in the microstructure (amorphous structure) of the carbon particles. The LA processing of the annealed HTC-processed carbon particles introduces nitrogen-containing surface-functional groups and leads to the recovery of the photoluminescent features that are different from those of the HTC-CDs and dependent on the fraction of nitrogen in the surface-functional groups. The photoluminescence of both the HTC-CDs and LA-CDs is largely due to the presence of N-containing surface-functional groups. The quantum yield of the LA-CDs is more constant than that of the HTC-CDs under continuous UV excitation and does not exhibit a significant reduction after 150 min of excitation. The methods used in this work provide a simple and green strategy to introduce N-surface-functional groups to carbon nanoparticles made from biomass and biowaste and to produce stable photoluminescent CDs with excellent water-wettability.
Highlights
Carbon-based quantum dots, which are referred to as carbon dots (CDs) [1] and are less-toxic and ecologically friendly [2,3], have attracted great interest due to their unique properties, such as high water solubility [4], high chemical stability [5], high photostability [6], tunable excitation and emission wavelength [7], and low cost [8]
All the nanoparticles are of polygonal shape (Figure 2a–c), suggesting that the annealing at high temperatures and the laser ablation did not cause any significant changes to the morphology of the nanoparticles
The EDS and X-ray photoelectron spectroscopy (XPS) analyses of the Hydrothermal carbonization (HTC)-CDs shown in Figure S1 and Table S1 in Supporting Information File 1 confirm that the main component of the HTC-CDs is carbon
Summary
Carbon-based quantum dots, which are referred to as carbon dots (CDs) [1] and are less-toxic and ecologically friendly [2,3], have attracted great interest due to their unique properties, such as high water solubility [4], high chemical stability [5], high photostability [6], tunable excitation and emission wavelength [7], and low cost [8]. Several methods are available for synthesizing CDs, including oxidation and reduction [13,14,15], laser ablation [16], microwave irradiation [9], pyrolysis [17], and hydrothermal treatment [18] Some of these methods are tedious and time consuming and use strong acids and/or surface treatment to improve their water solubility and luminescence properties. Hydrothermal carbonization (HTC), which can be considered as a “green technology”, has been used to produce photoluminescent CDs from biomass, including glucose, sucrose, citric acid [19], chitosan [20], orange juice [21], grass [22] and soy milk [10]. Liu et al [22] produced photoluminescent polymer nanodots of 3–5 nm in diameter by using grass as a precursor at 180 °C, and Zhu et al [10] synthesized bifunctional blue-emission carbon nanodots with diameters of 13–40 nm from soy milk at 180 °C
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