Abstract:

Objective Carbon dots (CDs), as a type of carbon nanomaterial, possess inherent fluorescence as a characteristic property. They also hold significant potential as flame retardants in polymer materials. However, the current cost of preparing carbon dots is high. The objective of this study is to explore new precursor materials, potentially with lower cost, for carbon dot synthesis, enhance the carbon dot synthesis process, reduce production costs, improve production efficiency, and investigate the application of modified carbon dots in PET flame retardancy and fluorescence anti-counterfeiting.
Method This research employs low-cost and non-toxic gelatin and urea as precursors and utilizes a high-efficiency pyrolysis method to synthesize gelatin-based nitrogen-doped carbon dots (NCDs). The study also optimizes the synthesis conditions, including precursor-to-feed ratio, reaction temperature, and reaction time. Following the purification of NCDs synthesized under the optimal conditions, their morphology, structure, and fluorescence properties are characterized and tested. Furthermore, NCDs are blended with PET to produce PET-NCDs flame-retardant modified composite materials, and the thermal stability, flame-retardant performance, and combustion behavior of PET-NCDs are examined. NCDs are also employed as fluorescent agents to prepare fluorescent ink, and their anti-counterfeiting effectiveness is assessed.
Results The test results show that the optimal preparation conditions for synthesizing NCDs using gelatin and urea as precursors are as follows: a mass ratio of gelatin to urea of 5:1.1, a reaction temperature of 260 ℃, and a reaction time of 12 hours (Fig. 1). TEM characterization reveals that NCDs have a quasi-spherical structure, with particle sizes ranging from 2.03 to 4.62 nm and an interplanar spacing of 0.21 nm (Fig. 2). Characterization using XPS, FT-IR, and NMR H indicates the presence of functional groups such as hydroxyl and amino groups on the surface of NCDs (Figs. 3 - 5). The UV-vis spectrum of NCDs in ethanol solution exhibits an absorption peak at 241 nm, attributed to the π-π* transition of sp² bonds, and the solution displays prominent blue fluorescence under a 365 nm UV lamp (Fig. 6(a)). Fluorescence emission spectra of NCDs under different excitation wavelengths demonstrate significant changes in fluorescence intensity, indicating typical excitation wavelength dependency (Fig. 6(b)). Using an integrating sphere, the fluorescence quantum yield of NCDs is determined to be 47.12% under optimal conditions (Fig. 6(c)). The fluorescence lifetime of NCDs is measured using a fluorescence spectrometer, excited with 360 nm light, and the decay curve at 430 nm is fitted using an exponential function (exponential). Analysis reveals two fluorescence emission centers with lifetimes of 2.37 ns and 6.70 ns, resulting in an average lifetime of 4.92 ns (Fig. 6(d)). In the characterization of NCDs-PET with a 5% NCDs doping level, the thermal stability of PET-NCDs is significantly improved compared to pure PET, as evidenced by TG and DTG curves, with a notable reduction in the maximum mass loss rate and increased residual char content at 700 ℃ (Tab. 1). Flame-retardant performance has an elevated limiting oxygen index (LOI) of 26% and its vertical combustion test (UL-94) is rated V-2 (Tab. 2). Cone calorimetry testing demonstrates a 42% reduction in the peak heat release rate for PET-NCDs compared to PET, with lower overall heat release rates and the formation of an evident expanded char layer after combustion (Fig. 8 and Fig. 9). Additionally, the fluorescent ink containing NCDs produces patterns that are invisible under natural light but visible under UV light, thus demonstrating its anti-counterfeiting effect (Fig. 10).
Conclusion NCDs synthesized through the pyrolysis method using gelatin and urea as precursors exhibit excellent fluorescence properties and a high fluorescence quantum yield. When incorporated into PET through blending, they demonstrate effective flame-retardant properties, making them suitable for the production of fluorescent ink with potential applications in fluorescence-based anti-counterfeiting.

Key words: carbon dot, gelatin, nitrogen doping, pyrolysis, flame retardant, fluorescent anti-counterfeiting