We have designed different ELPs with inorganic nanoparticle binding properties to achieve the growth or binding of metal-based nanocrystals (NCs) as a way to form nanoparticles. The design consists in a hydrophilic ELP block followed by a more hydrophobic block with small peptide sequence at the end which is capable to specifically bind gold surfaces. We performed the assembly of ELP nanoparticles through the binding to Au nanoclusters, which can stabilize the ELP NPs assembly upon dilution and temperatures below the transition temperature. This approach is based on the generation of pristine Au nanoclusters (without functionalization of the surface) and further mixing with the diblock ELPs equipped with a Au peptide binding motif. Since ELP nanoparticle assembly is dependent on both the concentration and temperature, a set of experiments was designed to observe the AuNC-ELP hybrid behavior and understand its characteristics. In summary, the results showed that it is possible to generate stable hybrid nanoparticles with different amounts of AuNKNCs and different AuELP concentrations. The assemblies are notably sensitive to the protocol used, where the route in which the assembly is done by slow heating showed the most reliable size measurements over time. The stability of the hybrids upon dilution and cooling below transition temperature supports the hypothesis in which AuNCs can act as crosslinking points to hold the NPs together. Finally, the absence (or smoothness) of the transition temperature of the ELP hydrophobic block and the drop of the transition temperature of the hydrophilic block could reveal that the metal binding event brings ELP chains in close proximity causing the assembly of the particles even at temperatures below the transition temperature. TEM images confirmed the formation of nanoparticles with 200 nm in size that contain gold nanoclusters.
In a second approach, we made use of a hydrophilic block ELP together with a redox block able to reduce inorganic salts and achieve the formation of the ELP hybrid nanoparticles. We performed different synthetic routes to achieve the reduction of potassium permanganate to form manganese oxide nanoparticles, which can be dissolved in biologically relevant concentrations of glutathione (GHS) and hydrogen peroxide. The results showed that the synthesis route determined the long-term stability of manganese oxide-ELP hybrids, where a co-solvent synthesis has demonstrated the best stability in physiological conditions. The particles generated kept the thermoresponsive behavior of the hydrophilic block, which proves that tha surface of the particles is stabilized by this block. In addition, the formation of the maganese oxide and its dissolution was monitored, showing the possibility of triggering the disassembly of the particles by cancer cells. The hybrid particles were tested for their competence in magnetic resonance imaging, showing a successful performance as a MRI contrast agents.
