Novel fluorescent hydrogel for the adsorption and detection of Fe (III)
Graphical abstract
Introduction
Heavy metal ions have been a global problem to harm the ecological cycle and human public health [1], and could cause numerous diseases by accumulating in organisms [2]. Among them, Fe3+ is very common in all industrial wastewater with high toxicity, and its accumulation in the human body can cause vomiting, diarrhea and intestinal damage. Therefore, the detection and removal of Fe3+ has attracted the attention of many researchers [3]. Currently, many materials have been proposed to remove heavy metal ions from the environment, such as magnetic nanoparticles, membranes, and hydrogels [[4], [5], [6]]. Among the various removal methods of Fe3+, some smart materials have attracted people's attention due to many functional groups, stimulus response, low cost, high efficiency, and environmental friendliness [7]. It is worth noting that these new materials can only be used to adsorb metal ions, but cannot detect ions in solution. In order to efficiently detect Fe3+, fluorescent probe have attracted extensive attention because of their fluorescence properties [8]. For the detection of heavy metal ions, purely aliphatic materials are more acceptable than the conventional aromatic fluorescent materials because of their less susceptible to photodegadation and aging when they produce fluorescence [9,10]. Manas et.al [11] designed and prepared nonconventional multi-functional aliphatic terpolymers with high performance for remove Cu(II).
As a new type of carbon-based fluorescent nanomaterials, carbon dots(CDs) have developed rapidly in recent years in the detection of heavy metal ions, biosensors [12], drug delivery and biological imaging [13], and so on, ascribing to its biocompatibility, low toxicity and fluorescence properties [8]. The CDs which was synthesized by one-step hydrothermal method from citric acid and ethylenediamine was proved suitable for environmental and biomedical applications due to the harmless to the human. One of the most promising applications of CDs is for the heavy metal ions fluorescent detection [14]. Lu et.al [15] designed a nitrogen-doped multifunctional CDs probe that detects the presence of Fe3+ ions through two-photon excitation.
CDs can be used as probes to directly detect metal ions in the environmental sewage, which is not conducive fast on-site identification for the timely fluorescence measurement in practical application [16]. The organic solution present in the actual wastewater makes the hydrophilic CDs easy to aggregate, resulting in its self-absorption and weakened PL intensity [17]. Most hydrophilic CDs are soluble in water and difficult to remove from the water, causing secondary pollution. In order to overcome these problems, there must be an appropriate supporter to achieve CDs dispersion and fixation [18].
Among numerous adsorption materials, the hydrogels have received increasing attention in terms of heavy metal removal due to their porous three-dimensional polymer network structure, easy loading of metal ions, reusability and large swelling ability. Recently, Gogoi [19] have prepared chitosan based hybrid CDs hydrogel membrane, which was used as solid response platform detecting heavy metal ions by hydrogen bonding. It has showed the capability of optical detecting to the heavy metal ions, and it also has a certain ability to remove these metal ions. The hybrid CDs hydrogel membrane shows the optical detection ability for heavy metal ions, and also has the ability to remove these metal ions. Generally, the CDs in the hybrid CDs hydrogel binds to the carrier through weak interaction, which is unstable in the water system. Once the prepared CDs hydrogel is put into water, it will cause secondary pollution due to the shedding of CDs. Therefore, it is necessary to prepare a suitable new material, which can make CDs exist stably in the hydrogel through covalent bond interaction. Moreover, modifiers that are covalently bonded to the surface of the substrate also have longer chemical stability than physical blending.
In this work, the PNIPAAm-CD hydrogel with good adsorption property and stable fluorescence detection for Fe3+ was prepared by copolymerizing PCDs with NIPAAm monomers. The effect PCDs on the structure, surface composition and thermosensitive properties of PNIPAAm-CD hydrogel were investigated. The fluorescence quenching effect of PCDs at 370 nm with different metal ions (K+, Ni2+, Al3+, Zn2+, Cu2+, Mg2+, Fe3+, Ag+, Pb2+, Fe2+, Co2+, Ca2+ and Hg2+) was investigated. The results showed that Fe3+ can efficiently quench the fluorescence of PCDs. The results showed that Fe3+ can efficient quench the fluorescence of PCDs. Furthermore, the fluorescence quenching effect of Fe3+ on the prepared CDs, PCDs and PNIPAAm-CD hydrogel was investigated. In addition, the adsorption kinetics modules and isotherm models of the PNIPAAm-CD hydrogel were studied. All of the results showed that PNIPAAm-CD hydrogel has potential applications as an effective, portable platform with bifunctional of visual detection and adsorption of Fe3+.
Section snippets
Materials
1, 2-Ethylenediamine (EDA), Citric acid monohydrate, glycidyl methacrylate (GMA), dimethylformamide (DMF), ethanol (EtOH) and acetone were provided from Kermel. Quinine sulfate (98 %) was obtained from Aladdin. N,N,N',N'-tetramethylenediamine (TEMED), ammonium persulfate (APS), and N, N-methylenebisacrylamide (MBAA) were purchased by Aldrich. N-isopropyl acrylamide (NIPAAm) was recrystallized from a 50:50 cyclohexane–toluene mixture before use. Fe3+ stock solution was obtained from chlorate
Synthesis of PNIPAAm-CD hydrogels
The fabrication diagram of the new PNIPAAm-CDs hydrogel combining modified CDs and responsive polymer is shown in Fig. 1. By mixing citric acid and ethylenediamine, the CDs with uniform size and high fluorescence quantum yield are prepared by hydrothermal method. Due to the presence of an epoxy ring that can interact with ethylenediamine and a carbon-carbon double bond that can react with NIPAAm in the side chain, glycidyl methacrylate (GMA) was selected as the surfactant to further modify the
Conclusions
In summary, the PNIPAAm-CD hydrogel with a three-dimensional structure was successfully synthesized by copolymerizing PCDs with NIPAAm monomers. The PNIPAAm-CD hydrogel can disperse and immobilize CDs through the strong chemical bond, which avoid leaked out under water. Therefore, the hydrogels were stable and showed the higher adsorption property for Fe3+ than the pure PNIPAAm hydrogel. In addition, the functionalization of the amine groups on the surface of the PCDs enables PNIPAAm-CD
Formatting of funding sources
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
CRediT authorship contribution statement
Dongdong Zhang: Conceptualization, Methodology, Investigation, Data curation, Revise, Calculation model. Xuetao Tian: Validation, Formal analysis. Huanhuan Li: Validation, Formal analysis. Yiping Zhao: Resources, Writing - review & editing, Supervision, Data curation. Li Chen: 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.
Acknowledgements
This work was financially sponsored by the National Natural Science Foundation of China (grant number.21374078), the Science and Technology Plans of Tianjin (grant numbers15JCYBJC17900, 15PTSYJC00250 and 17YFZCSF01230), Program for Innovative Research Team in University of Ministry of Education of China (grant number. IRT-17R80) and Program for Innovative Research Team in University of Tianjin (grant number. TD13-5044).
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