Co-funcationalised gold nanoparticles for drug delivery applications

Abstract

Turkevich and modified Frens method are the oldest and most widely used protocols for the synthesis of gold nanoparticles (AuNPs). Colloidal gold (NPs) has been widely used in different biomedical applications including drug and gene delivery. The aim of this study was to use gold nanoparticles as nanocarriers for targeted drug delivery because of their known inert, non-toxic nature.

In this study, AuNPs were synthesized by reducing chloroauric acid solution (200 ml or 100 ml) with sodium citrate and a Design Expert (DoE) approach was used to investigate the effect of varying process conditions on the physical properties of AuNPs; namely size, size distribution, zeta potential and UV-Vis characteristics. The analysis showed that Au/citrate molar ratio had a significant effect on size, size distribution and the number of peaks by DLS and lambda max value in absorbance spectra by UV-Visible spectroscopy. By changing the amount of citrate during the synthesis of AuNPs the diameter could be controlled from 17 to 80 nm. A low concentration of citrate resulted in a broad size distribution with two distinct particle sizes observed by DLS and TEM.

The DoE approach helped in selecting the synthesis parameters for preparing gold nanoparticles in the size range of 15-20 nm, for subsequent experiments. The surface of the as-prepared AuNPs was modified with thiolated PEG-S, which is commonly used to improve the stability and the in vivo circulation time of gold nanoparticles. The thiol group at the terminal end, having a high affinity for gold, helps in replacing adsorbed citrate ions on the surface. The AuNPs surface was modified with 0, 0.17,3.35, 8.39, 16.78, 25.18 and 37.75 µg/ml of PEG-S, thereby influencing the hydrodynamic diameter, stability and surface charge. The results showed that the lambda max value increased from 518 to 521 nm with increasing PEG-S concentration from 0 - 37.75 µg/ml, while the size by DLS increased in order from18 nm to ~43 nm and the zeta potential values changed from -45 mV to -5 mV. TGA showed a maximum loading capacity of ~ 15 wt% with the PEG-S molecules adopting an extended chain configuration. The size of AuNPs-citrate and AuNPs-PEG-S with 25 µg/ml of PEG-S, as measured by TEM was 15.5 ±1.53 nm and 17.4± 1.26 nm, respectively (Figure 5.8).

For the final part of the research project the surface of gold nanoparticles was co-functionalised with BODIPY FL L-Cystine and PEG-S in 20 ml volume batches. The amount of BODIPY-C varied from 0.5 - 2.0 µg/ml and for BODIPY-C-PEG-S formulations the concentrations used were 0.5-18.75, 1.0-12.5 and 1.5-6.25 µg/ml. The analysis of UV-Visible spectra showed a red shift (increase in size) in lambda max with an increased concentration of BODIPY-C; with 1.72 µg/ml of BODIPY-C equating to the maximum loading. The experiments also concluded that PEG-S co-functionalisation was required for the stability in GSH. An increase in hydrodynamic diameter was also confirmed by DLS data. The size of AuNP-BODIPY-C (0.5 and2.0 µg/ml) as measured from TEM analysis was 15.8 ±1.39 nm and 16.2 ± 1.28 nm, respectively, whereas that for AuNP-BODIPY-C-PEG-S (0.5-18.75 and 1.5-6.25µg/ml) showed the average size of 17.0 ± 1.15 nm and 15.89 ± 1.48 nm, respectively.

The study for the release of BODIPY-C in the presence of typical intercellular levels of GSH using photoluminescence showed that PEG-S co-functionalisation plays acritical role in the payload release. It was observed that the addition of PEG-S to the AuNPs surface inhibits agglomeration and the burst release of BODIPY-C, which was otherwise observed in AuNPs functionalised with BODIPY-C. However, the addition of PEG-S reduced the total amount of BODIPY-C released, especially at the lower GSH concentration of 5 mM. It was observed that up to about 83% of the attached BODIPY-C was released from the co-functionalized gold nanoparticles in the presence of 10 mM GSH. The study confirms the role of PEG-S in stabilizing gold nanoparticles and describes the payload release behaviour of gold nanoparticles capped with a mixed monolayer. Varying the amount of PEG-S functionalization and GSH concentration enables control of the loading capacity and the release profile of BODIPY-C.

Date of Award	Oct 2012
Original language	English
Sponsors	Department for Employment and Learning
Supervisor	Dorian Dixon (Supervisor) & Brian Meenan (Supervisor)