| Mercury is widely spread in nature and can be bioaccumulated through the food chain.It can cause great damage to the nervous system even at very low concentrations in human body and is considered to be a highly toxic and dangerous element to human beings.Therefore,it is very desirable and essential to design and explore new chemosensors with high selectivity and sensitivity for the detection of Hg2+ ion.The traditional recognizition for Hg2+ is mainly based on non-covalent interaction which is hard to achieve specificity.In this paper,we design and synthesize five kinds of Hg2+ fluorescent chemosensors which based on the specific catalysis characteristics of Hg2+ towards certain chemical reactions.Irreversible chemical reactions between Hg2+ and these chemosensors form a new covalent bond and thus cause changes in the spectral properties of the chemosensors molecule,which enables the selective recognition of Hg2+.In addition,hydrogen sulfide(H2S)is widespread in all kinds of geological structures.It is produce by natural decomposition of organic matters in anoxic.Although pungent to eyes and throat,H2S can quickly impair the sense of smell at a low concentrations,and can even lead to death at a high concentrations.Recent studies indicate that H2S can also be produced by the metabolism of the organism,and it is considered as a third gasotransmitters following the nitric oxide(NO)and carbon monoxide(CO).However,abnormal levels of H2S in the body can cause Alzheimer's disease and Down's syndrome and other diseases.Therefore,it is highly demanded to achieve a real-time and sensitive detection of H2S in both external environment and within the body.In this paper,two 2-styrylquinoline-based fluorescent chemosensors for the selective detection of H2S based on the reduction of hydroxylamine and azide by H2S were designed and synthesized.The main contents of this paper are summarized as follows:Three novel fluorescence enhancing chemosensors 2a-c using 1,8-naphthalimidesa as fluorophore and based on the specific mercury-promoted desulfurization reaction were designed and synthesized.Upon the addition of Hg2+ to the solution of 2c in mixed CH3OH-HEPES(0.02 M,pH=7.01)(7/3,v/v),the maximum absorption wavelength at 394 nm in the UV-visible absorption spectrum was blue shifted to 382 nm and also an obvious 53.6-fold enhancement of the emission intensity at 520 nm was discovered.The detection of 2c towards Hg2+ has a short response time,high selectivity and a wide applicable pH value range.The desulfurization reaction of 2c with Hg2+ converts thiourea into urea leading to a color change from dark to bright green under an UV light.The sensing mechanism was promoted by 1H NMR,13C NMR and HRMS analysis.We also designed and synthesized another new Hg2+ fluorescent chemosensor based on a tetraphenylethylene fluorophore for the detection of Hg2+ in water via an Hg2+-triggered aldehyde recovery reaction.Copound 3 can form a 1:1 complex with Fe3+ in pure water,which contributes an 8.71-fold decrease in fluorescence intensity.Interestingly,this complex can selectively recognize Hg2+ in its water solution.Upon the addition of 2 equivalents of Hg2+,the thioacetal is deprotected to give according aldehyde while a 5.86-fold increase of emission intensity was discovered.Therefore,compound 3 performed as a typical"ON-OFF-ON" probe for Fe3+ and Hg2+.The deprotection reaction can be completed within 5 minutes and the minimum concentration that can be detected is 2 ppb.The complexation and deprotection mechanism was finally confirmed by 1H NMR.A new ratio fluorescent chemosensor for Hg2+ was synthesized by combining thiourethane with 2-styrylquinoline fluorophore.The desulfurization and hydrolysis reaction of 4a in CH3CN-HEPES(0.02 M,pH=7.01)(1/1,v/v)upon the addition of Hg2+ lead to ratio change in both the UV-visible absorption and the fluorescence spectra.A remarkable red-shifted from 352 nm to 406 nm in the UV-vis spectrum and an obvious 122 nm red-shifted in the emission maximum wavelength from 446 nm to 568 nm in the fluorescence spectra were observed.The solution color change from colorless to pale yellow under visible light and from blue to bright yellow under an UV light,which allows the detection of Hg2+ can be realized both by the naked eye and fluorescence spectro.Thiourea formed by reacting mono-substituted ethylene diamine with an isothiocyanate could be converted to imidazoline via an Hg2+-triggered desulfation and cyclization reaction.Based on this specific reaction,another new ratio Hg2+ chemosensor incorporating a 2-styrylquinoline fluorophore moiety was synthesized.The detection process of compound 5a was acted through the intramolecular charge transfer(ICT)mechanism.Upon the addition of Hg2+ to the solution of 5a in CH3CN-HEPES(0.02 M,pH=7.01)(1/1,v/v),a remarkable blue-shifted from 384 nm to 326 nm in the UV-vis spectrum and an obvious 78 nm blue-shifted in the emission maximum wavelength from 526 nm to 448 nm in the fluorescence spectra were observed.The color of the solution changes from pale yellow to colorless under visible light and from bright yellow to light blue under an UV light.The detection process can be completed within 1.5 minutes according to a 1:1 chemical reaction ratio.The reaction products were separated and characterized by 1H NMR to promote its recognition mechanism.Two new rhodamine derivatives compound 6a and 6b were synthesized,among which 6b performed as a NIR fluorescent chemosensor for Hg2+ through sulfamide desulfurization while 6a performed as a comparative compound.Upon the addition of Hg2+ to the solution of 6b in CH3CN-HEPES(0.02 M,pH=7.0)(1/1,v/v),the tautomer of 6b reacted with Hg2+,desulfurization and finally converted to an open-ring form of rhodamine while a 106-fold increase of emission intensity was discovered.The solution color change from colorless to purple under visible light and dark to red under an UV light,which allows the detection of Hg2+ both by the naked eye and fluorescence spectrometer.Its recognition mechanism was promoted by ESI-MS and IR analysis of the reaction product.Two 2-styrylquinoline-based fluorescent chemosensor 7a and 7b for H2S based on the reduction characteristics of H2S was synthesized,the action sites of which was hydroxylamine(7a)and azide(7b),respectively.At room temperature,upon the addition of 5 equivalents of H2S to the solution of 7a in CH3CN-HEPES(0.02 M,pH=7.01)(1/4,v/v),the.hydroxylamine in the molecular structure of 7a is reduced to an amino group,which leads to a 16.6-fold increase in fluorescence intensity after 10 mins reaction.The detection of 7a towards H2S has high selectivity and an wide applicable pH value range.However,upon the addition of 5 equivalents of H2S to the solution of 7b under the same conditions,there is only a 3-foldincrease in fluorescence intensity due to its poor water solubility.Compound 7b did not show as effective detection of H2S as 7a.In summary,seven fluorescent chemosensors for Hg2+ based on Hg2+-induced specific chemical reactions and two fluorescent chemosensors for H2S based on the reduction characteristics of H2S were synthesized,respectively.All of these fluorescent chemosensors which based on special chemical reaction can selectively recognize target molecule and have potential applications for the real-time detection of small molecules in the external environment or within the body. |
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