Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Microwave-assisted synthesis of polyamine-functionalized carbon dots from xylan and their use for the detection of tannic acid
Graphical Abstract
Introduction
Tannic acid (TA), as a kind of natural phenolic compound exists in a variety of plants, exhibits many advantages, such as antimutagenic, antioxidant, anticarcinogenic and antitoxic activities [1]. Due to these special physiological and pharmacological activities, TA is an important component of medicine for therapy of burns, diarrhea and poison. Besides its pharmaceutical effect, TA is also used as clarifying agent in the brewing and wine industry, while the content of this food additive is also an essential standard to evaluate their quality. Most importantly, TA is one of the main persistent organic pollutants in leather industrial sewage and can cause serious problems for the environment, this pollutant is harmful to the root system of plants through toxic tannic acid-metal complex that formed easily from the interaction between TA and metal in most surface and ground water. Therefore, the determination of TA is very important for food analysis, medical studies as well as sewage detection. To date, various methods have been used for the TA determination, including spectrophotometry [2], fluorescence [3], electrochemistry [4], liquid chromatography [5], thin-layer chromatography [6], and protein precipitation [7]. However, the most of current approaches have suffered from certain disadvantages, such as poor specificity and selectivity, instability of reagents, expensive equipment or standardization difficulties. Thus, developing highly sensitive, simple and cost-efficient techniques for the estimation of TA level is of great significance.
Currently, fluorescent carbon dots (CDs), a relatively new member of carbon family, have attracted much attention due to their charming advantages, such as excellent photoluminescence (PL), high stability, low toxicity and environmental friendliness relative to semiconductor quantum dots and traditional organic dyes. Apart from the use of CDs for biological labeling [8], photocatalysis [9] and optoelectronic devices [10], CDs have been demonstrated to be suitable for sensing various heavy metal ions, including Cu2+ [11,12], Hg2+ [[13], [14], [15], [16]], Fe3+ [17,18], Cr6+ [19,20] and Au3+ [21]. Since the CDs-based sensors with high sensitivity, rapid analysis, simple operation, and excellent interference resistance ability, The functionalized CDs were also confirmed to be promising sensing platforms for detecting several molecules, such as glucose [22], dopamine [23,24], picric acid [25,26] and ascorbic acid [20,27], Thus, the functionalized CDs that highly dependent on the selection of precursors and passivant are also feasible for TA detection. To the best of our knowledge, only a handful reports have synthesized the functionalized CDs for TA detection, for instance, Ahmed et al. prepared CDs-based sensors for detecting TA by using 6-bromohexylboronic acid as carbon source [28], but certain critical problems still exist, including high cost of the starting materials, very low quantum yields (QYs) and poor PL intensity. Liu et al. based on the enhancement effect of chemiluminescence to determine TA. However, these CDs-based nanosensors have suffered from time consuming synthesis route and the involvement of complicated test procedure [29]. Therefore, it is highly desirable to develop simple and efficient CDs-based sensors for TA detection.
Although many efforts has been made to synthesize CDs in last decades, the efficient one-step synthesis following bottom-up route for the rapid production of CDs from biomass-based precursors is still challengeable. Herein, the low-cost and renewable xylan, which is rich in hemicellulose, was used as a potential carbon source for CDs production in hydrothermal condition. For further improving the fluorescent efficiency of the CDs and providing active sites on the surface of CDs to recognize analytes, the branched polyethyleneimine (BPEI) with polyamine structure was chosen as the functionalizing and capping regent to passivate the xylan-derived CDs. Considering microwave irradiation which allows for intensive, efficient and homogeneous energy distribution is more effective than the traditional hydrothermal treatment in accelerating the reaction rate in an aqueous phase, a microwave reactor that features temperature control system was used to synthesize BPEI functionalized CDs (BPEI-CDs). Due to the strong interaction of reactants with microwave irradiation and the fast heating rate can be achieved, the synthetic process can be finished in one-step within just a few minutes. These as-prepared fluorescent nanoparticles were found to be sensitive and selective probes for detecting TA (Scheme 1). In particular, the established detecting platform for TA can be directly used in real lake water and white wine samples. Moreover, the possible mechanism for the newly established detection system was also discussed. Such reliable sensors along with the efficient synthesis strategy will afford a new way for TA quantitative determination as well as improve the add-value of biomass-based CDs.
Section snippets
Materials
Xylan (85%), BPEI, quinine sulfate, TA, gallic acid, sucrose, tartaric acid, oxalic acid and citric acid were all purchased from Aladdin Co., Ltd. (Shanghai, China). Sulfate acid, sodium hydroxide and ethyl alcohol were procured from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Phosphate buffer solutions (PBS) with different pH values were obtained from Haibiao Technologies Co., Ltd. (Fuzhou, China). All other reagents were purchased from Xilong Chemical Co., Ltd. (Shanghai, China).
Morphology and Chemical Structure of the BPEI-CDs
The morphology and size of the BPEI-CDs were characterized by HRTEM. As shown in Fig. 1a, the synthesized nanostructures are almost spherical and well separated from each other. Moreover, from the HRTEM image, it can be seen that the single nanoparticle exhibits coreāshell structure containing a small carbon core and a coating layer, such composite structure could be highly relate to the BPEI passivation. As judge from about 140 individual particles, the size of BPEI-CDs is mainly distributed
Sensitivity and Selectivity of TA Detection
Considering the significant effect of TA on environment and human health, it is obviously important to develop efficient TA detection method which can monitor TA level sensitively and selectively in aqueous system. It was found that the synthesized BPEI-CDs meet well with the requirements of a fluorescent probe. Most importantly, the establishment of optimum condition for TA detection by using the BPEI-CDs is necessary for accurate determination. Several studies have confirmed that the PL
Analysis of Real Samples
To evaluate the BPEI-CDs-based sensors for TA detection in practical application, the established sensing platform in water solution and ethanol solution were applied to detect TA in the lake water and white wine, respectively. The test results are listed in Table 2. The recovery of TA by the standard addition has been obtained to evaluate the veracity of the proposed assay method. It can be seen that the spiked recovery values varied from 95.2 to 112.0% for lake water samples and 94.0 to
Conclusions
In summary, polyamine-functionalized CDs were synthesized by simple one-step microwave-assisted pyrolysis xylan in the presence of BPEI as a passivant. Besides the usage of low cost and sustainable carbon source in the synthesis method, the BPEI-CDs can be obtained at a low temperature within just 15āÆmin. These nanoparticles with excitation-independent PL properties were found to be promising sensors for detecting TA in aqueous solution and ethanol solution. Ultimately, the developed detection
Acknowledgements
This work was supported by the Jiangsu Nature Science Foundation (BK20161524), the Program for 333 Talents Project in Jiangsu Province (Grant No. BRA2016381), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0839), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Doctorate Fellowship Foundation of Nanjing Forestry
Declarations of interest
None.
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