| Agar is a hydrophilic plant polysaccharide with natural,non-toxic,and biocompatible characteristics,which is widely used in food,medicine,and bioengineering.However,native agar is single in nature and can be modified to provide unique properties to broaden its application.This study used three long-chain anhydrides with different carbon chain lengths and three short-chain anhydrides with other molecular structures as esterifying agents to prepare agar derivatives with good emulsification and encapsulation properties.Then,the enduring influence of the chemical structures of the anhydrides on the agar’s physicochemical properties,structural characterization,and application properties was investigated.The results provide a theoretical basis for speculating the conformational relationships.The primary studies and experimental results are as follows.Firstiy,agar was chemically modified using long-chain anhydride of C12–C18.The effects of different degrees of substitution(DS≈0.02,0.04,0.06 and 0.10)and anhydride carbon chain length on esterified agar’s physicochemical and emulsification properties were investigated.The results showed that the gel properties(gel strength,solidification temperature,melting temperature)of long-chain anhydride esterified agar decreased with increasing DS and anhydride carbon chain length.At the same time,introducing long hydrophobic chains and carboxyl groups of the anhydride decreased the surface and interfacial tension of the long-chain anhydride esterified agar.It increased the contact angle and surface charge.The results of the emulsification property measurements showed that the improved agar particle properties contributed to the formation of a dense layer and physical barrier at the oil droplet interface,reduced the particle size of the agar-based emulsion(108.10 μm → 60.53 μm),and improved emulsion stability(storage emulsion index of 62.96 ± 3.25%).Then,the influence of long-chain anhydride DS and carbon chain length on the structural characteristics of agar was analyzed by structural characterization,and the conformational relationship between the structure and emulsification properties of long-chain anhydride esterified agar was initially explored.Fourier infrared spectroscopy(FTIR)results showed that the carboxyl groups and long hydrophobic chains of the long-chain anhydride affect the OH···O and OH···OH hydrogen bonding interactions between the agars.Nuclear magnetic resonance spectroscopy(NMR)and molecular weight tests showed that the basic structure of the longchain anhydride esterified agar remained unchanged except for the specific anhydride groups.Scanning electron microscopy(SEM)showed that esterified agar has larger pores and looser network structure than natural agar.TG-DSC showed that esterified agar has strong electrostatic repulsion and molecular chain flexibility,which makes its thermal stability poor.Rheological characterization showed that the esterified agar disrupted the gel network of the agar due to the electrostatic repulsion of the carboxyl groups and the hydrophobic effect of the long carbon chains,resulting in a gradual decrease in gel strength,gel temperature,and melt temperature with increasing DS and anhydride carbon chain length,and a gradual reduction in gel properties.The agar structure-activity relationship demonstrated that the DS and anhydride carbon chain length of long-chain esterified agar could be adjusted to provide appropriate hydrophobicity and electrostatic interactions,thus improving the emulsion stability of agar-based emulsions.Next,three different structures of short-chain anhydride were introduced into the agar molecule to investigate the effect of different DS(0.02,0.04,0.06 and 0.10)and anhydride structures on its physicochemical properties.The results showed that the gel properties(gel strength,solidification temperature,melting temperature)of the short-chain anhydride esterified agar were significantly reduced.Still,its water retention increased from 300% to 1730% and its swellability increased by a factor of 3.5.Secondly,agar microspheres’ drug encapsulation rate and in vitro release behavior were investigated using curcumin(CUR)as the hydrophobic active ingredient.The results showed that the excellent swelling and hydrophobicity of esterified agar contributed to the encapsulation and delivery of CUR(70.3% encapsulation rate);the in vitro release of CUR followed the Korsmeyer-Peppas release model,and the release process was controlled by p H,with acidic conditions making the release of CUR more difficult and weak alkaline conditions facilitating its slow release(51% release rate).Finally,the effects of short-chain anhydride DS,carbon chain length,and saturation on the structural characteristics of agar were analyzed by structural characterization,and the structureactivity relationship between the structure and drug delivery ability of short-chain anhydride esterified agar was preliminarily explored.FTIR and TG-DSC results showed that increasing the carbon chain length and saturation of DS and anhydride would affect the hydrophobic interaction and hydrogen bonding of short-chain anhydride esterified agar,change the stable structure of agar,reduce thermal stability,and thus affect its gel properties.LF-NMR showed that the access of the anhydride group increased the immobilised water trapped in the gel network,facilitating the entry and uptake of water molecules.SEM results showed that the hydrophilic carboxyl group and porous network structure in esterified agar could provide more binding sites for the adsorption of water molecules,so the water retention performance was improved.The ability of agar microspheres to encapsulate CUR could be explained by the pore structure,swelling properties,and carboxyl binding of agar.The hydrophobic pore structure and excellent swelling properties of esterified agar improved the bioavailability of CUR.The structure-activity relationship between the structural characteristics and delivery capacity of short-chain anhydride esterified agar showed the application potential of hydrogel microspheres in loading hydrophobic active ingredients and sustained release,which provided the possibility for the application of agar in drug delivery systems. |
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