| Propolis is a mixture of natural resins,mainly collected by bees from buds and exudates of some plants and processed through their maxillary glands.Propolis is rich in polyphenols components,which endows it with unique physiological and pharmacological activities such as antibacterial,anti-inflammatory,antiviral,antioxidant,immune regulation,anti-cancer functions.However,the use of propolis in food and medicine is limited because of its low solubility,strong smell and low bioavailability in water.Carboxymethyl chitosan(CMCS),a water-soluble chitosan derivative,has the advantages of pH sensitivity,bioadhesion,non-toxic degradation products and controllable release as carriers.It is widely recognized as a polymer that can be used for targeted release.Folic acid(FA)is a kind of low water-soluble vitamin that can be grafted onto hydrophilic carboxymethyl chitosan chains as a natural hydrophobic group to form amphiphilic FA-CMCS conjugates.Moreover,FA as a targeted group,can specifically identify the FA receptor overexpressed on the surface of tumor cells.In this study,FA-CMCS nanoparticles loaded with propolis were successfully prepared for the first time through pH-driven method by using the principle of propolis deprotonation under alkaline conditions.The addition of organic solvents was avoided and no special equipment was required in the whole experiment process.The operation was simple with low energy consumption.Main research conclusions of this part are as follows:(1)FA-CMCS nanoparticles prepared under optimal conditions were spherical and evenly distributed,with an average particle size of ~ 170 nm and zeta potential of ~-30 mV.The encapsulation efficiency and drug loading capacity of propolis were 73% and 16.88%,respectively.Fourier transform infrared spectroscopy(FTIR)showed that the main interaction forces of nanoparticles were hydrophobic interaction and hydrogen bond.(2)The experimental results also showed that the FA-CMCS nanoparticle delivery system loaded with propolis not only had a high encapsulation efficiency of propolis but also a good environmental stability,which could reduce the sudden release of propolis in the stomach environment and continuouslyrelease it in the intestinal and colonic fluid,especially in colonic fluid,thus improved the oral bioavailability of propolis.In addition,propolis loaded FA-CMCS nanoparticles effectively enhanced the water solubility of propolis,and in vitro antioxidant activity of the embedded propolis was significantly higher than that of the free propolis dispersed in water.FA-CMCS nanoparticles were verified to be a good delivery carrier for propolis with practical application prospect in preparing clarified propolis oral liquid.However,the isoelectric point of carboxymethyl chitosan was around pH 5.0 which limited the applied range of the system in terms of food compounding,since the pH range of most processed food systems are weak acidic.To solve this problem,we use ε-polylysine(EPL)to coat FA-CMCS nanoparticles.EPL is a cationic polypeptide with an isoelectric point around pH 9.0 and an optimum pH of 5.0-8.0.It can form polyelectrolyte complex with FA-CMCS by electrostatic interaction under weak acid condition.At pH 6.7,EPL adsorbed on the surface of the nanoparticle through electrostatic attraction to form FA-CMCS-EPL nanoparticle.The average particle size of FA-CMCS-EPL nanoparticles prepared under the optimal conditions was ~ 150 nm and the zeta potential was ~-27 mV.FTIR revealed that hydrogen bond,hydrophobic force and electrostatic force were the main interaction forces in nanoparticles formation.The coating of ε-polylysine stabilized nanoparticles under weak acid conditions of pH 4.0-7.0,and it was found that FA-CMCS-EPL nanoparticles had strong thermal stability and storage tolerance,which broadened the application range of propolis in processed food fields.Besides propolis delivery,our study also provided an innovative idea of encapsulating hydrophobic bioactive substances using amphiphilic carboxymethyl chitosan nanoparticles based on pH-driven method,such as curcumin,resveratrol and quercetin,so as to expand their application scope in food,medicine and cosmetics. |
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