Preparation Of Functionalized ZIF-8 For Adsorption Of Antioxidant Peptides And Water/methanol

Zeolitic Imidazolate Framework-8,as a typical MOFs materials,has been widely used in many fields,such as embedding and releasing biomacromolecules and adsorpting gas molecules because of its mild synthesis conditions and stable structure.In this paper,a new functional ZIF-8 material is prepared through further modification of ZIF-8.The main work is divided into two parts as follows:（1）Based on the strategy of amino acid molecular imprinting,histidine（His）,tryptophan（Trp）and tyrosine（Tyr）were used as imprinted molecules to increase the pore size.Taking antioxidant peptides as the object of investigation,the functional units were built in ZIF-8 skeleton in order to improve the adsorption ability of antioxidant peptides in silkworm pupae protein hydrolysate.The results show that because His and 2-methylimidazole had similar molecular structure,so it can be imprinted into ZIF-8 to form 15-28（?）defective pores.However,the indolyl of Trp and hydroxyphenyl of Tyr has no coordination ability compared with 2-methylimidazole so can not be imprinted into ZIF-8,leading to inhibit the growth of ZIF-8.Compared with ZIF-8,the adsorption performance of ZIF-His improve significantly.After 30minutes of adsorption,the EC₅₀ value of DPPH·free radical increases to2.5 times,the EC₅₀ value of ABTS·⁺free radical increases to 1.5 times and the reduction ability of Fe³⁺decrease nearly 1.0 time.However,ZIF-Trp and ZIF-Tyr has little selective adsorption capacity of antioxidant peptides.In addition,the elution rates of antioxidant peptides of the four materials reach more than 95%.After five cycles,the adsorption rates of ZIF-8 and ZIF-His only decrease by 2～3%and 4～5%,and the desorption rates only decrease 3～4%and 5～6%.At the same time,through nitrogen physical adsorption-desorption and amino acid composition analysis,it can be seen that the surface area and volume of mesopores decrease by about 68%after ZIF-His adsorption,while the specific surface area and volume of micropores decrease only by about27%.The His content in the eluent increase by about 60%,the Trp content increase by about 50%,and the Tyr and Phe content increase by about 20%.Therefore,it can be concluded that the imprinted defect pores in ZIF-His can conducive the diffusion of peptide molecules into the pore channel,and the imidazolyl group in the backbone can be combined with His,Trp,Tyr and Phe through theπ-πeffect.In addition,the uncoordinated carboxyl and amino groups in His can further enhance the interaction with antioxidant polypeptides through hydrogen bonding to achieve specific adsorption of antioxidant polypeptides.（2）Based on the strategy of functional ligand post-replacement,3-methyl-1,2,4-triazole（TZ-CH）,1H-1,2,4-triazole-3-carboxylicacid（TZ-CO）and 3-amino-1,2,4-triazole（TZ-NH）were used as the polar ligands to replace 2-methylimidazole,the hydrophobic ligand of ZIF-8 in order to enhance the polarity of ZIF-8 skeleton.Taking water and methanol molecules as the objects of investigation,the polar functional units were constructed in the hydrophobic ZIF-8 framework to improve the polarity of the ZIF-8 structure.The experimental results show that ZIF-TZ-CH and ZIF-TZ-CO still maintain high porosity and structural stability after modification.Compared with ZIF-8,the inflexion points of ZIF-TZ-CH and ZIF-TZ-CO on water decrease from P/P₀=0.96 to 0.30 and 0.60,as well as the saturated adsorption capacities of water increase nearly 21 and18 times,respectively.Moreover,the inflexion points of ZIF-TZ-CH and ZIF-TZ-CO on methanol decrease from P/P₀=0.15 to 0.04 and 0.08,respectively.Then,the isotherm adsorption lines of ZIF-8,ZIF-TZ-CH and ZIF-TZ-CO of water and methanol at different temperatures were analyzed.It is found that the adsorption heat and the absolute values of Gibbs free energy are arranged in the following orde between three materials and water as well as methanol:ZIF-TZ-CH>ZIF-TZ-CO>ZIF-8.It is concluded that that the adsorption force of ZIF-TZ-CH with water and methanol is higher than ZIF-TZ-CO and ZIF-8.After five cycles,the saturated adsorption capacities of water and methanol on ZIF-TZ-CH only decrease by 3～4%and 4～5%.Furthmore,it is found that the adsorption inflection point and adsorption capacity of ZIF-TZ-CH are as good as the polarity ZIF-90,which prove that ZIF-TZ-CH also has the potential ability to become an excellent polarity ZIFs.