Preparation, Stabilization and Surface Functionalization of Gold Nanoparticles
3.5. Interaction of the surface functionalized gold nanoparticles with proteins (BSA and bovine serum)
For a further evaluation of the suitability of the particles for intravenous administration, the stability of the particles was investigated in presence of different proteins, which are present in body fluids. The interaction of the prepared gold nanoparticles either coated or only citrate-stabilized with bovine serum albumin (BSA), the most abundant protein in the plasma, and bovine serum was studied. Citrate-stabilized GNPs used for this investigation rapidly aggregated after addition of bovine serum albumin (BSA) and serum, which is indicated by the disappearance of the characteristic surface plasmon resonance peak in the UV spectrum, in case of BSA, (Figure 11) or by appearance of a new wide absorption band at wave lengths above 650 nm as in case of serum (Figure 12). The large size increase of the citrate-stabilized GNPs induced by the addition of BSA and serum, as shown by photon correlation spectroscopy measurements (Figure 9) furthermore confirms the instability. The observations could be attributed to the interaction of cysteine groups of the added proteins with the GNPs, or the shielding of the negative surface charges by extensive protein adsorption on the particle surface, which then also may induce the further formation of aggregates in the dispersion. In contrast to the bare particles, the spectra of polymer-protected
GNPs are either not affected (in case of mPEG-AlkSH coated GNPs) or show only a slight increase in the size (in case of NH2-PEG-AlkSH-coated GNPs) by addition of BSA and serum. The slight size increase of NH2PEG-AlkSH-coated GNPs can be attributed to the occurring interaction of the exposed protonated amino groups with negatively charged carboxylate groups on the added proteins, which may lead to a partial protein coating of the particles [47]. The protein resistance (i.e. the stability) of PEG-coated GNPs is a consequence of the present interfacial water layer, which prevents a direct contact between the lipophilic particle surface and the added proteins [48].
Figure 11: Effect of BSA on the UV-Vis spectra of mPEG-AlkSH-coated GNPs (a), NH2PEG-AlkSH-coated GNPs (b) and citrate-stabilized GNPs (c)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
400 500 600 700 800 900 1000
Wave length (nm)
Absorbance
a
b
c
Figure 12: Effect of bovine serum on the UV-Vis spectra of mPEG-AlkSH-coated GNPs (a), NH2PEG-AlkSH-coated GNPs (b) and citrate-stabilized GNPs (c)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
400 500 600 700 800 900 1000
Wave length (nm)
Absorbance a
b c
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
400 500 600 700 800 900 1000
Wave length (nm)
Absorbance a
b c
4. Conclusion
Uniform gold nanoparticles were prepared in aqueous solutions by sodium citrate reduction of HAuCl4.3H2O. The obtained sizes and size distribution as well as the measured surface plasmon resonance of the prepared nanoparticles was strongly influenced by the gold/citrate ratio and could be optimized to produce reproducibly sufficiently small particles.
The surface functionalization and further stabilization of the gold nanoparticles was achieved using the heterobifunctional thioalkylated poly(ethylene glycol) derivatives, which were already described in this thesis and which are well known to form well ordered and packed self assembled monolayers on gold surface. The size of the accordingly modified particles slightly increased, while there was also a pronounced increase of the zeta potential observed after the PEGylation of the particles. The finally obtained PEG-coated gold nanoparticles showed a significantly enhanced stability in the presence of electrolyte and proteins.
In conclusion the here presented nanoparticles provide a valuable tool for further modifications on the incorporated terminal amine group, which allows further binding or attachment of biomolecules or other moieties to the particles. The obtained particles then can be used for in vitro or in vivo studies due to their excellent stability in biological fluids.
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