A convenient fluorescence sensor of tetracycline based on B, N codoped carbon dots/polymer composite film
Introduction
Since first discovery in 2004 (Xu et al., 2004), carbon dots (CDs) as competitive candidates among carbon-based nanomaterials have received considerable attentions. As zero-dimensional carbon nanomaterials with almost spherical morphology, they own universal size less than 20 nm with abundant functional groups on their surface (Zhu et al., 2015, Zhu et al., 2017). The multiple groups can help CDs to be modified easily with many organic or polymeric molecules (Yang, Jiang, Qiu, Jiang, & Zuo, 2015), and to construct multifarious sensors. The carbon atoms in the core of CDs consist of sp2/sp3 electronic structures and display amorphous carbon form or graphite lattice (Bhattacharya et al., 2016, Zhang et al., 2016). By comparison with some traditional cadmium-based quantum dots; CDs exhibit higher photoluminescence (PL) stability, better biocompatibility, as well as lower toxicity and cost (Xia et al., 2019, Yuan et al., 2016). These properties afford CDs economical promise in practical applications.
Despite the rapid advancements, there are challenges in the development of CDs, one of which is the synthesis of CDs with high PL quantum yield (QY). The extra surface passivation and composition of CDs are often required for optical property tuning, such as organic molecules, heteroatoms doping, as well as polymers (Peng, Ji, Zhou, Zhao, & Leblanc, 2020). Polymers are ideal materials for compounding with CDs because polymer can provide mechanical support and CDs can disperse well in polymer's structure. In this case, the dispersion of CDs can prevent their PL quenching (Du, Wang, Wu, & Chen, 2021). Polyvinyl alcohol (PVA) has repeating CH2CH(OH) units. PVA is generally considered a nontoxic material. Due to its low toxicity, excellent biocompatibility, and water absorption, PVA is widely used in tissue engineering and medical applications (Hu et al., 2016, Pathan et al., 2015). PVA has also been used as an immobilization carrier for reinforced composites to improve its mechanical performance by complex with other materials such as graphene (Zhao, Zhang, Chen, & Lu, 2010) and graphene oxide (Kashyap et al., 2016, Qi et al., 2013). Since CDs have plenty of functional groups including graphene oxide on their surface, they can also interact strongly with PVA chain and improve the mechanical properties of PVA hydrogel (Kashyap et al., 2016). More importantly, incorporating CDs into PVA hydrogel can expand the application range in fluorescence sensors for PVA. Furthermore, the immobilization of CDs in PVA carrier can stabilize the fluorescence of CDs inhibiting the fluorescence quench deriving from the aggregation of CDs with time.
Tetracyclines (TCs) are a group of antibiotics derived from Streptomyces spp. with a broad-spectrum activity. In veterinary medicine the most commonly used TCs are tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC) and doxycycline (DC). TC is a broad-spectrum antimicrobial drug for treatment of bacterial infections in humans or animals (Feng et al., 2015, Wang et al., 2018). TC is also used for feed additive in the agriculture sector (Jia et al., 2019, Li et al., 2019). The excessive use of TC caused global concern due to its many side effects. The long-time intake of TC in food stuffs may cause some serious risks; such as drug resistance of microbial strains, allergic or toxic reactions (Gong, Li, Qing, Zhang, & Feng, 2019). Thus, it is necessary to explore simple and effective means for the detection of TC in food products. Different analytical approaches for the detection of TC have been developed, including chromatography (Al-Afy, Sereshti, Hijazi, & Rashidi, 2018), electrophoresis (Zhou, Deng, Shi, & Zhou, 2017), electrochemistry (Kim et al., 2010, Tang et al., 2018), and enzyme-linked immunosorbent assay (ELISA) (Gong et al., 2019). These techniques are time consuming and need expensive apparatus and sophisticate skills. The optical approaches, by contrast, are superior due to their excellent sensitivity and selectivity, easy operation, and fast response (Li et al., 2019, Raizada et al., 2017, Zhang et al., 2018). The fluorescence sensing methods based on CDs are widely used for the determination of TCs such as N, S co-doped CDs (Zhao et al., 2018, Zhao et al., 2020), nitrogen doped CDs (Yan et al., 2019), and carbon nitride quantum dots (Hu et al., 2020). All of these assays are carried out in aqueous media, and they are not strict sensors.
In this work, a novel sensor of TC was designed using CDs/PVA composite film based on B, N co-doped CDs. PVA was selected as a supporting matrix due to its good comprehensive properties including strong mechanical strength and thermal stability and excellent carrier property (Lim et al., 2015). Since there are many strong hydrogen bonds among PVA chains, the fabrication of PVA-based matrix was mostly conducted in solutions. Herein, urea was added to the PVA system to obtain strong mechanical strength. CDs were prepared from 3-aminophenylboronic acid and citric acid using a hydrothermal method. CDs were treated with PVA to form a polymer composite film with bright blue fluorescence. Since the fluorescence of CDs/PVA can be quenched easily due to the inner filter effect (IFE), a convenient, sensitive, selective, as well as portable sensor was constructed to determine TC in real samples. The preparation of CDs and assay process of TC were illustrated in a schematic diagram (Scheme 1).
Section snippets
Apparatus
CDs were prepared by a Teflon-sealed autoclave reactor in a vacuum drying oven. An LS-55 spectrofluorometer was used to obtain fluorescence spectra and intensity. A Lambda 750 spectrophotometer was employed to measure absorption spectra. A JEM 2100 transmission electron microscope (TEM) was selected to study the morphology and size of CDs. TEM worked with 200 kV acceleration voltage, and a copper grid coated with carbon was used for sample suspension. A thermo Fisher Scientific FIB-SEM GX4 was
Design strategy
Fluorescence quenching means any process that decreases the quantum yield or the fluorescence intensity from a fluorophore. Various molecular interactions can lead to fluorescence quenching. These include energy transfer, molecular rearrangements, collisional quenching and so on. The intrinsical issue of fluorimetric procedures is the absorption of exciting and/or emission by dissolved materials, including the fluorophore itself. This procedure is called the inner filter effect (IFE) (
Conclusions
In this work, a sensitive, specific, and convenient fluorescence sensor was constructed for TC assay. It was achieved by incorporating CDs into PVA film. The immobilization of CDs in PVA carrier can stabilize the fluorescence of CDs inhibiting the fluorescence quench deriving from the aggregation of CDs with time. B, N-codoped CDs was synthesized by a solvothermal method using citric acid and 3-aminophenylboronic acid as precursors. CDs exhibit bright blue fluorescence. Due to the internal
CRediT authorship contribution statement
Yingying Hu: Data curation, Writing – original draft, Methodology, Investigation. Rentian Guan: Formal analysis. Shuai Zhang: Formal analysis. Xiaoyu Fan: Formal analysis. Wenjing Liu: Formal analysis. Keying Zhang: Formal analysis. Xiaodong Shao: Visualization. Xia Li: Formal analysis. Qiaoli Yue: Funding acquisition, Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported financially by Natural Science Foundation of China (91543206) and research foundation of Liaocheng University (318050022).
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