Facile synthesis and performance of pH/temperature dual-response hydrogel containing lignin-based carbon dots
Introduction
Lignin is the second abundant and renewable biomass resource on the earth [1]. Although plants can produce up to 150 billion tons of lignin on earth every year through photosynthesis, only less than 5% was used in different applications, such as concrete water reducer [2], cement grinding aid [3], pesticide dispersant [4] and dye dispersant [5]. In order to make sustainable use of lignin further, researchers have utilized lignin to product valuable carbon materials, such as activated carbons [6], templated carbons [7], mesoporous carbon monoliths [8], carbon nanofibers [9] and carbon dots [10]. Carbon quantum dots, as one of the youngest members in the carbon material family [11], have attracted increasing attention of researchers since it was initially found in 2004 [12]. Carbon dots have excellent water dispersibility, biocompatibility, light stability, non-flickering photoluminescence, and adjustable excitation and emission wavelengths, which are the advantages that the traditional fluorescent organic dyes and semiconductor quantum dots do not have. Therefore, carbon dots have great potential to be used in biological imaging [13], drug delivery [14,15], sensors [16], photocatalysis [17], and nanoelectronic devices [18].
Generally, carbon dots are fluorescent nano‑carbon materials with a particles size smaller than 10 nm [19]. The preparation strategies of carbon dots reported in literature can be simply classified into two types. One is “top-down” strategy, and the other is “bottom-up” strategy. Arc discharge [20], laser ablation [21], plasma synthesis [22], template [23] and ultrasonic methods [24] belong to the “top-down” strategy. Electrochemical [25], microwave, ultrasonic synthesis [26] and hydrothermal methods fall into the “bottom-up” strategy. Among these methods, the hydrothermal synthesis is a simple method without special equipment requirements and thereby a promising and competitive one for the synthesis of carbon dots.
In the past ten years, various substances have been used as the precursors of carbon dots, such as o-phenylenediamine [27], rhodamine [28], and thiourea [29]. With the advantages of non-toxicity, renewability and good biocompatibility, natural materials have been used in more and more studies as precursor of carbon dots, such as spider silk [30], apple juice [31], hair [32], cellulose [33], and even solid wastes including grass [34], waste onion skin [35], and paper ash [36].
With high carbon content, aromatic conjugate structure, endogenous heteroatom doping and chemical modification, lignin is an ideal carbon source for carbon dots. Shi et al. [37] prepared lignin-based carbon dots with the fluorescent quantum yield of 8.1% using alkali lignin, ethylenediamine and formaldehyde as the raw materials in a nitrogen atmosphere at 300 °C. Jiang et al. [38] prepared lignin carbon dots with a fluorescent quantum yield of 7.95% from lignin, betaine, and lactic acid when heated to 300 °C. Li et al. [36] prepared carbon dots from purified lignin with a yield of 14.24% through hydrothermal reaction at 200 °C. Chen et al. [39] prepared carbon dots with the particle size of 2–10 nm from lignin and H2O2. It is apparent that the fluorescent quantum yield of reported lignin-based carbon dots was rather low, and functionalization and valorization of the lignin-based carbon dots need further investigation.
Hydrogel is a water-soluble polymer with the network cross-linked structure. Its hydrophilic groups are bound to water molecules through and the water molecules are wrapped in the network, while hydrophobic groups expand with water. Hydrogels have been widely applied in biomedical engineering fields such as tissue engineering scaffolds, drug carriers, and so on. The research of fluorescence hydrogels related to carbon quantum dots mainly focused on the specific response of metal ions and other fields. Cheng et al. [40] added PEI-CD into the mixture of microcrystalline cellulose, sodium hydroxide and urea. After stirring for 1 h, the fluorescent intelligent hydrogels can be prepared by dropping ethanol. In the range of Fe3+ from 10 μmol/L to 100 μmol/L, the relative fluorescence intensity of the hydrogels showed a good linear relationship with the Fe3+ concentration in the solution, and its fluorescence color gradually changed from bright blue to light blue with the increase of Fe3+ concentration. Using lanthanide ions, silk fibroin derived carbon dots and Pectin/PVA hydrogel (PPH), Karen [41] synthesized a tough LPPH hydrogels, which can response to both pH and metal ions. The PL intensity ratios of the hydrogel at 473 nm and 617 nm showed a significant change at different pH values, indicating that pH response can be quantified in high sensitivity. In addition, they had different chromic responses to metal ions like Fe2+ and Fe3 + .
In this study, we developed a new and simple process for the synthesis of lignin-based carbon dots with high fluorescent quantum yield and pH-responsibility first. Lignin, citric acid, and ethylenediamine were used as raw materials. The synthesis process was optimized to maximize the fluorescence lifetime. The structure and the pH-responsibility of the lignin-based carbon dots were investigated. The present study provided a new high-value application for the lignin-based carbon dots. Besides, we established a simple synthesis method for the pH/temperature dual responsive hydrogel. L-CDs, NIPAM and PVA were used as the main raw materials, the fluorescent hydrogels with pH/temperature double response were rapidly synthesized by free radical polymerization. The effects of PVA usage on the functional groups, temperature responsiveness, rheological properties and micromorphology of hydrogels were studied. Then, the pH response of the composite hydrogel was constructed by using the prepared lignin-based carbon dots as the pH fluorescence probe, and the fluorescence intensity of the composite hydrogel under different pH was studied. This synthetic method provides a new direction for the preparation of responsive multifunctional hydrogels, and also presents a new idea for widening the application scope of hydrogels.
Section snippets
Materials
Lignin was isolated from corncob using the p-toluenesulfonic acid method, as previously described [42]. Citric acid monohydrate (CA) was purchased from Damao Chemical Reagent Factory (Tianjin, China). Ethylene Diamine (EDA) was purchased from Jiangsu Yonghua Fine Chemical Co., Ltd. (Suzhou, China). Potassium bromide (KBr) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). All chemicals were analytical grade and used as received. Deionized (DI) water was used for preparation of all
Morphology of L-CDs
The transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images of L-CDs are shown in Fig. 1(a) and (b). From the TEM image, it was clearly observed that the particle size of L-CDs was about 2–5 nm. L-CDs were uniformly dispersed in water in the form of monodisperse nanoparticles. The HRTEM image showed more detailed structures of L-CDs. L-CDs had a crystalline structure with a lattice spacing of 0.21 nm, suggesting that they had aromatic rings or graphene-like crystalline
Conclusions
In this study, lignin-based carbon dots (L-CDs) were successfully synthesized from low-cost and renewable lignin as a carbon source using a facile method. The L-CDs showed graphene-like crystalline structure and were spherical nanoparticles with an average diameter of 2–5 nm. They were well dispersible in water and had an excitation dependent behavior. The fluorescence lifetimes of the L-CDs were longer than 10 ns, which were much higher than those reported in the literature. Under the optimal
CRediT authorship contribution statement
Lan Sun: Analysis, Writing.
Zhenye Mo: Experiment implement.
Qiong Li: Measurement, Data curation.
Dafeng Zheng: Conception.
Xueqing Qiu: Supervision.
Xuejun Pan: Experiment design.
Acknowledgements
The authors thank the financial support by the National Natural Science Foundation of China (No. 21776107, No. 21690083), Key Research and Development Program of Guangdong, China (No. 2020B1111380002). China Scholarship Council (201706155098) supported Dr. Dafeng Zheng to conduct visiting research at University of Wisconsin-Madison.
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