Study On The Recognition Performance Of Cucurbit/dye Supramolecular Fluorescent Probes For Ions Or Small Molecules

Cucurbit[n]urils（Q[n]）is a macrocyclic synthetic compound with negatively charged carbonyl ports and neutral hydrophobic cavities.Not only can Q[n]contain guest molecules through hydrophobic cavities,but also occurs carbonyl ports coordination through oxygen and metal ions.When a host-guest complex is formed by cucurbiturils and dyes,the acid-base ionization constant and photophysical and chemical properties of the dye molecule can be changed.When the target analyte is added to a cucurbit[n]uril-dye and is bound to the cucurbit[n]uril by means of competitive substitution.If the photophysical and chemical properties of the dyes in the encapsulation and free state are different,the target analyte will be accompanied by a change in the fluorescence signal after binding to the cucurbit[n]urils,forming an"off-on"or"on-off"fluorescent probe.So,the ions or the small molecules can be identified.In this paper,cucurbit[n]urils（cucurbit[7]urils（Q[7]）and trans cucurbit[7]urils（i Q[7]））and two biological dyes（Hoechst33258（H33258）,4-（4-dimethylaminostyryl）quinoline（DSQ））can form host-guest complexes at different pH conditions,constructing cucurbit[n]urils/dye supramolecular fluorescent probes（H33258@i Q[7],H33258@Q[7]and DSQ@i Q[7]）.We use fluorescent probes to study the recognition properties of metal ions,anions and amino acids,and also explore possible mechanisms of action.The main research contents and conclusions are as follows:First,the interaction between trans i Q[7]and H33258 dye was studied in HCl solution of pH 2 and Na OH solution of pH 11,and dumbbell-shaped H33258@i Q[7]supramolecular fluorescence probes were successfully designed at two pH conditions.The probes show strong fluorescence at pH 2,and weak fluorescence at pH 11.In addition,the probes were used to study the recognition performance of 36 metal cations（including alkali metals,alkaline earth metals,transition metals,and lanthanide metals）.The results show that,the H33258@i Q[7]supramolecular fluorescent probe has a unique selective recognition of Hg²⁺and has a significant fluorescence quenching phenomenon at pH 2,and its detection limit is 5.45×10^-7 mol/L.It is possible that Hg²⁺interacts with the nitrogen atom of H33258 to change the electron cloud distribution of H33258 dye.It is possible that internal charge transfer（ICT）is the dominant process that quenches the fluorescence of H33258@i Q[7]probe.whereas the probe showed selective recognition of Al³⁺,Fe³⁺and Cr³⁺,especially the high selective recognition of lanthanum Ce³⁺,all of which have obvious fluorescence enhancement at pH=11,and their detection limits are:1.35×10^-6 mol/L,2.73×10^-6 mol/L,1.79×10^-6 mol/L,and4.09×10^-6 mol/L.Al³⁺may be mainly located at the port of i Q[7],which affects the electron distribution of the aromatic ring in the H33258 guest.Ce³⁺may mainly interact with the nitrogen atom in the H33258 guest,which may affect the photoinduced electron transfer（PET）process of H33258;for Fe³⁺and Cr³⁺,these two effects may occur simultaneously.The H33258@i Q[7]probe establish a new research platform for expanding the supramolecular fluorescence recognition metal ions of the Q[n]-based host-guest chemistry.Secondly,the interaction between the Q[7]and the H33258 dye was studied under two pH conditions（pH 2 and pH 11）.It was found that a Q[7]included a part of piperazine and the benzimidazole near part of piperazine of H33258 and the other Q[7]include the phenol part of H33258,so the H33258@Q[7]inclusion complex supramolecular fluorescent probe was designed,which has stronger fluorescence at pH2,and the fluorescence is weak at pH 11.In addition,the recognition properties of H33258@Q[7]inclusion complexes for 15 tetrabutylammonium anions at pH 2 and pH11 were investigated.The results showed that,the inclusion complex of H33258@Q[7]（1:2）showed unique recognition ability for HPO₄^2-at pH 11,its fluorescence was enhanced,and its detection limit was 7.50×10^-6 mol/L.Most transition cations have a significant effect on the recognition of HPO₄^2-by the inclusion complex of H33258@Q[7],while the effects of alkali metal cations,alkaline earth metal cations and anions are not significant.It is possible that HPO₄^2-with tetrahedral structure has-OH group and-O^-group,interacts with prominent imidazole moiety through hydrogen bonding,and the effect of the presence of HPO₄^2-on H33258@Q[7]（especially the phenol moiety）,Which may cause the charge density of the H33258 guest to increase,thereby increasing the fluorescence intensity.More importantly,the probe could open a new platform in anion recognition by based Q[n]-chemistry.Finally,the interaction of i Q[7]with DSQ dye was studied under three different pH conditions（pH 4.5,pH 5.5 and pH 7.1）.It was found that i Q[7]interacted with DSQ at different pHs to cause fluorescence enhancement of different degrees and form DSQ@i Q[7]complexes of different colors.Therefore,we proposed a DSQ@i Q[7]supramolecular fluorescence array sensor for the determination of L-α-amino acids in aqueous solutions.When 13 L-α-amino acids are added to DSQ@i Q[7]with different pH conditions（pH=4.5,pH=5.5 and pH=7.1）,they will produce different color and fluorescence changes.Through principal component analysis（PCA）,we can identify structurally similar L-α-amino acids.It is possible that a hydrogen bond was formed between the hydrogen atom of ammonium in the L-α-amino acid molecule and the nitrogen atom of dimethylamine in the DSQ molecule.The DSQ@i Q[7]sensor has different affinity for amino acids.This Cucurbit[n]uril/dye supramolecular and amino acid chemical sensing helps to detect amino acids in different side chains and provides the possibility to track protein structure.