Characterization of Hydrophobic Modification of Starch NanoParticle by Pyrene Fluorescence

The properties of starch have been extensively studied as starch is readily available in large quantities, biodegradable, inexpensive, and non toxic. In particular, starch has been hydrophobically modified to widen its application range. For examples, the modification of starch with fatty acid2 and butylene glycidyl ether3 has been implemented to investigate its potential use in drug delivery. In a more recent development, Starch NanoParticles (SNPs) have been produced by EcoSynthetix.4 SNPs are produced by extruding starch in the presence of a crosslinking agent. SNPs were modified with hexanoic and propionic anhydride in order to increase their hydrophobicity and turn them into amphiphilic particles. The hydrophobic groups of the amphiphilic SNPs dispersed in water were expected to aggregate and generate hydrophobic microdomains to minimize exposure to water. The formation of hydrophobic microdomains suggests that the hydrophobically modified SNPs (HM-SNPs) could be used as carriers for drug delivery. The increase in hydrophobicity of the HM-SNPs as a function of the degree of substitution (DS) and the nature of the hydrophobic modification was investigated with pyrene fluorescence. Pyrene is a hydrophobic fluorophore consisting of four aromatic rings. Formation of hydrophobic microdomains was demonstrated from changes in the ratio of the pyrene emission spectrum which responds to the polarity of the local environment5 and the lifetime of pyrene. Since a useful drug carrier should minimize interactions between the loaded drug and the solvent in order to protect the drug from a loss of functionality, quenching experiments were conducted with pyrene to determine the level of protection offered by HMSNPs. Nitromethane was used as a water soluble quencher. As the hydrophobicity of the HMSNPs increased with increasing DS, pyrene became less accessible to nitromethane due to the viscosity and hydrophobicity of the hydrophobic microdomains. The decrease in interactions between pyrene and nitromethane resulted in a decrease in kq, the rate constant for quenching of pyrene by nitromethane. Changes in kq as a function of DS were examined to determine how the hydrophobicity of the HM-SNPs would affect protection of a hydrophobic cargo with increasing DS. The bimolecular quenching rate constant, kq, was determined by using the Stern Volmer equation1 given in Equation 1 where 0 and represent the number average lifetime of pyrene without and with quencher, respectively.