Construction And Properties Of Polyethyleneimine Modified Cellulose Fluorescent Materials

The pollution of heavy metals in industry,agriculture and human activities has become increasingly prominent,causing serious harm to the natural environment and human life safety.For example,when the concentration of Fe3+exceeds 5 ppm in the solvent recovery system of Lyocell fiber,it can lead to the violent decomposition of N-methylmorpholine-N-oxide and even explosion.Therefore,it is very important to realize the efficient detection of Fe3+.Aiming at the key problems of metal-ions detection materials,such as unclear electron transfer mechanisms between recognition units and Fe3+,a series of polyethyleneimine(PEI)modified cellulose iron ion probes were prepared in this paper by using cellulose or filter paper with the advantages of low cost,environmentally friendly and good biocompatibility as matrix,and by introducing polyethylenimine,graphene oxide and rhodamine 6G groups to enhance the coordination abilities between cellulose materials and Fe3+.The main contents are as follows:(1)Using microcrystalline cellulose(MCC),polyethyleneimine,salicylaldehyde(SA)as raw materials,the C=N,O-H and N-H units were anchored on the surface of MCC by NaIO4 oxidation and Schiff’s base reactions,and the sensor CE-PEI-SA for Fe3+ detection was prepared.It was found that the oxidation time had a great influence on the aldehyde content in the dialdehyde microcrystalline cellulose(DAC).After chemical modifications of DAC with polyethyleneimine/salicylaldehyde,the original crystal structure would be destroyed,the surface became rougher and the reactive sites increased.After the chelation reaction between CE-PEi-SA and Fe3+,the mixed system was changed from yellow to black brown within 1 min and the fluorescence was obviously quenched,which suggested that CE-PEI-SA showed excellent selectivity,naked-eye detection for Fe3+and had obvious anti-interference performance for others metal ions such as Ca2+,Mg2+ or Fe2+.There was a good linear relationship between the UV absorption value at 501 nm of CE-PEI-SA and Fe3+concentrations(4～20 ppm),and the detection limit 0.01 ppm was obtained.In addition,CE-PEI-SA had good recoveries(97.22%～102.15%)and low relative standard deviations(less than 3%)for the detection of Fe3+ in tap waters and swimming pool waters.The results of density functional theory(DFT)showed that the introduction of SA unit into CE-PEI can improve the electron cloud density and the reactivity with Fe3+.However,the particles of CE-PEI-SA are relatively large and unevenly dispersed,and the chelation process with Fe3+ only occurred on its surface,the detection performance of Fe3+ can still be improved.Therefore,the development of cellulose sensors with high dispersity is very important.(2)A probe CNC-PEI-R6G for detecting Fe3+ was prepared by grafting oxidized dialdehyde cellulose nanocrystals(D-CNC)with rhodamine 6G-polyethyleneimine.It was found that oxidation time had a great impact on the aldehyde content in D-CNC.TEM results showed that CNC,D-CNC and CNC-PEI-R6G were rod-like structures and all of them were well dispersed in water.After these modifications,the lengths of the modified CNC decreased.After CNC-PEI-R6G interacted with Fe3+,the mixed solutions were changed from colorless to pink with the increase of UV absorption at 530 nm and bright fluorescence emission at 566 nm,indicated that the probe CNC-PEI-R6G can be used to detect Fe3+ in aqueous mediums by naked eyes and fluorescence spectroscopy.The fluorescence intensity at 566 nm of CNC-PEI-R6G exhibited a good linear relationship with Fe3+concentrations(0～8 ppm),and the detection limit was 0.03 ppm.In addition,the probe CNC-PEI-R6G can also be applied to detect Fe3+ in tap waters and swimming pool waters with good recoveries(97.00%～101.83%)and low relative standard deviations(0.99%～2.63%).(3)In order to explore the detection performance of the smaller-sized cellulose fluorescent probe for Fe3+,nitrogen-doped carbon dots(N-CDs)were prepared by hydrothermal method using MCC as the carbon source and PEI as the nitrogen source.It was found that the addition ratio of MCC to PEI had a great influence on the fluorescence quantum yield of N-CDs.The results of TEM and XPS showed that N-CDs 2 had spherical structure and a wide size distribution ranging from 1 to 9 um.The surface of N-CDs 2 contained abundant hydrophilic groups such as-NH2 and-OH,which made it have good water-solubility.The fluorescence properties of N-CDs 2 depended on the excitation wavelengths(300～430 nm).When the excitation wavelength was 340 nm,N-CDs 2 exhibited the strongest fluorescence emission at 466 nm.It was also found that solvent,temperature and pH had significant influences on the fluorescence intensity of N-CDs 2,while the irradiation time of UV lamp had little effect.With addition of Fe3+ions,the fluorescence of N-CDs 2 was greatly quenched and the quenching degree was reached to 82.84%.The fluorescence intensity of N-CDs 2 displayed a good linear equation with Fe3+concentrations(0～14 ppm),and the detection limit 0.21 ppm was achieved.In addition,N-CDs 2 can also be successfully applied to detect Fe3+ in real water samples with good recoveries(98.25%～102.75%)and low relative standard deviations(less than 3%).According to the fluorescence lifetime data,the quenching process of N-CDs 2 by Fe3+may due to static fluorescence quenching.(4)In order to realize the characteristics of easy portability and easy operation of the above-mentioned iron ion sensors,the paper-based sensor FP/GO/R6G-PEI was prepared by physical modifications of filter paper(FP)with graphene oxide(GO)and rhodamine 6G-polyethyleneimine,which realized the selective and qualitative detection of Fe3+in water.It was found that the gap size between the fibers of the filter paper was significantly reduced,but the surface smoothness was improved after the filter paper was modified by GO/R6G-PEI.The crystallinities of FP,FP/GO and FP/GO/R6G-PEI changed little,but the whiteness and air permeability decreased and tensile strength increased.After interaction with Fe3+,FP/GO/R6G-PEI was changed from white to pink and the fluorescence intensity at 566 nm increased significantly.Taking(Structure 3)in R6G-PEI as a simplified model,the energy gap(0.53 eV)between Structure 3 and Fe3+was significantly smaller than that of Structure 3(3.12 eV),indicating that the mixed system of Structure 3 with Fe3+was more stable.In this paper,a variety of novel cellulose fluorescent materials were prepared,which not only realized the efficient detection of Fe3+in water,but also provided a new idea for the high value utilization of cellulose materials.