The Profile of Payload Release from Gold Nanoparticles Modified with a BODIPY®/PEG Mixed Monolayer

Gold nanoparticles with an average diameter of similar to 15.5 nm as measured via TEM were functionalised with various amounts of thiol-terminated 5000MW poly (ethylene glycol) (0-37.75 mu g/ml), or with a mixed monolayer of thiol-terminated BODIPY and poly (ethylene glycol) (PEG) in various ratios. BODIPY is a fluorescing molecule which was used here as a model payload, while PEG was added to the surface in order to increase nanoparticle stability and biocompatibility. It was observed that the gold nanoparticle surface saturates with PEG at a loading rate of similar to 15 wt%. Glutathione (GSH) is an antioxidant that occurs as high concentrations inside cells and this can be used to trigger therapeutic payload release therein. The release of BODIPY from nanoparticles functionalised with a mixed monolayer of PEG and BODIPY was investigated at typical intracellular glutathione concentrations. The release profiles were fitted using zero, first order, Higuchi and Reciprocal Powered Time (RPT) models. It was observed that BODIPY release from the surface of nanoparticles capped only with BODIPY and PEG was best described by the RPT and Higuchi models, suggesting diffusion and diffusion-dissolution controlled release. However, the release profile of the nanoparticles capped only with BODIPY did not fit the expected profile due to agglomeration effects. The ratio of BOBIPY to PEG on the surface had a dramatic effect on the release rate. The predicted time to release 50% of the payload compound from the gold nanoparticles displayed a similar to 500 fold increase for a mixed monolayer prepared with 0.5-18.75 mu g/ml of BODIPY-PEG compared to nanoparticle samples prepared with 1.5-6.25 mu g/ml of BODIPY-PEG.