Preparation,Characterization And Application Of Polyurea And Its Composites With Carbon Dots Drived From Poly (Ethylene Glycol)

With the rapid development of the petrochemical,metallurgical minerals,and textile industries,the amount of wastewater polluted by metal ions has significantly increased,constituting a serious threat to human health.As a specific polymer material,polyureas have been widely used in a variety of fields,such as waterproofing,anti-corrosion,anti-wear and surface decoration.However,as functional materials,polyurea is rarely studied in chemical adsorption.In recent years,a novel type offluorescent polymers has been gining more importance.These polymers are featured by the absence of any traditional chromophores,and consisting of only electron-rich heteroatoms（such as N,O,S,P,etc.）.Polyureas are rich in nitrogen and oxygen atoms.Previous studies have found that they are indeed fluorescence emissive.In this work,toluene diisocyanate（TDI）is used as the monomer,and porous polyurea with rich amine groups on itssurface is prepared by the reaction of TDI with water or diamine,leading to two different polyureas,denoted as PU and PU2,with the number referring to the carbon atom numbers in diamine used for the preparation of the polyurea.The adsorption of selected metal ions on PU2 is investigated.Carbon dots（CD）are known to possess abundant surface functional groups and are shown to have excellent photoluminescence properties,chemical stability,biocompatibility,and low toxicity.Nowadays,CD has attracted more and more attention,and extensive researches have been reported.In contrast,very few reports are available on polymer-CD composites.Due to its simple structure and excellent passivation ability,polyethylene glycol（PEG）is a good carbon source of low-cots to prepare CD.Nowadays,CD have attracted more and more attention,and extensive researches have been reported.In contrast,very few reports are available on polymer-CD composites.This work has been taken under these backgrounds.In the first part,toluene diisocyanate（TDI）is used as the monomer,and porous polyureawith rich amine groups on itssurface is prepared by the reaction of TDI with water（PU）ordiamine（PU2）.The adsorption performance of selected metal ions on PU2 is investigated.PEG is heated at 320℃ in order to obtain CD,which is then used to fabricate PU-CD composites.The luminescence behavior of the composite material was investigated;the luminescence mechanism and tentative applications of the composite as propbe for monitoring and detection of selected metal ion and antibioticsare discussed.The detailed contents are as follows:（1）PU2 microstructure is characterized using different instrumentation.Effects of p H,adsorption time and initial ion concentration on the adsorption are studied.The results show that PU2 has selectivity for the adsorption of copper ions,and the adsorption capacity is significantly higher than most of the reported adsorbents,though the reusability is not outstandingly good.The high Cu²⁺adsorption is interpreted through a complex coordination between Cu²⁺and functional groups on PU2,supported by XPS analysis.（2）PU2 is then modified with sodium chloroacetate to obtain carboxyl-containing PU2（PU2-M）.The desorption capacity of the adsorbed Cu²⁺from this PU2-M is significantly enhanced,and so is the reusability of the adsorbent.The results indicate that this carboxylationis an easy and effective pathway to increasing the performance of this polyurea adsorbent,with high performance for Cu²⁺adsorption and reusability superior to most of the reported adsorbents.The modification is optimized for the preparation of PU2-M,by measuring the content of amino groups on PU2 surface.The improved Cu²⁺adsorption and reusability of PU2-M are interpreted through the same complex coordination mechanism as for PU2,in association with the stability constants of acid-base equilibrium of amine and carboxyl towards Cu²⁺and protons.（3）CDs are prepared by direct heating PEG at 320℃.The formation mechanism of this CD is studied through its structure characterization.Using the interaction between carboxyl groups on the surface of CDs and amine groups on PU surface,a composite material（PU-CD）with fluorescent properties is prepared.Compared with CD and PU,the fluorescence intensity of this PU-CD was greatly enhanced.By changing the preparation conditions of CDs,it was found that,increased temperature,from 220℃up to 320℃,leads to anenhanced fluorescence emission for the composite PU-CD;Decrease in PU particle size and increase of the mass ratio of CD/PU lead also to enhancement of the fluorescence emission.The luminescence mechanisms of CD and PU-CD are also discussed.Compared to PEG,the enhancement of fluorescence emission of CD is most likely owing to the crosslink enhanced emission（CEE）effect.PEG,initially non-emissive or with feeble emission under limited conditions,has changed its energy level structure or transition barrier after treated at high temperature（e.g.320℃）,the non-radiative transition is largely inhibited in particular,which leads to the enhancement of the fluorescent emission.With regard to the enhancement of fluorescence emissionin of PU-CD composites,it is also owing to the CEE effect mentioned above.（4）In the presence of a douzen of competing metal ions,the emission of PU-CD is selectively quenched by Fe³⁺and Cr⁶⁺,because of their high standard electrode potentials,and charge transfer out from fluorescent PU-CD.Using PU-CD as sensing probe,dtection is carried out for Fe³⁺and Cr⁶⁺,resulting in detection limit（DL）of 43 nm for Fe³⁺and 120 nm for Cr⁶⁺,which are obviously lower in comparison with reported DLs obtained from a variety of probe materials.The emission of PU-CD composite is also quenched by tetracycline antibiotics,here referring to doxycycline（DOX）and tetracycline（TC）.Quite good selectivity and sensitivity are observed.Similar as for Fe³⁺ions,a DL of 12 n M is obtained for DOX and110 n M for TC.Compared with recently reported results from different probes,these DLs are better off,indicating that PU-CD is also a good sensing probe for detection and monitoring of these antibiotics.