Protocol for the Preparation of Stimuli-Responsive Gold Nanoparticles Capped with Elastin-Based Pentapeptides
Stimuli-responsive materials are playing an increasingly important role in a wide range of applications such as drug delivery, diagnostics, sensors, and tissue engineering. Among them, gold nanoparticles responding to changes in their surrounding environm
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Introduction Recently, nanomaterials sensitive to changes in their environment have been at the core of many research programs in the materials science community. Among this class of materials, gold nanoparticles (Au NPs) with surface coatings responsive to external stimuli such as light exposure and variations in pH or temperature have received much attention. This is not surprising since they are promising candidates in the development of many new applications in the field of sensory sciences and for the controlled release of active agents (1–6). To date, the primed strategy for the preparation of thermosensitive particles is the grafting of linear thermally responsive polymers onto the surface of Au NPs (1–4). By far the most commonly used polymer is poly(N-isopropylacrylamide) (pNIPAM). When heated at around 32°C, aqueous solutions of pNIPAM undergo a phase transition leading to the precipitation of the polymeric chains (7). However, even on gold surfaces, this lower critical solution temperature (LCST) can only be varied within a 10°C range, limiting the number of potential applications (5). In order to broaden the scope of thermally
Volkmar Weissig et al. (eds.), Cellular and Subcellular Nanotechnology: Methods and Protocols, Methods in Molecular Biology, vol. 991, DOI 10.1007/978-1-62703-336-7_32, © Springer Science+Business Media New York 2013
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responsive nanoparticles, novel strategies are required to obtain nanoparticles with flexibly tunable transition temperatures. This could be achieved by interfacing nanoparticles with stimuli-responsive polypeptides/proteins such as elastin-like polypeptides (ELPs). ELPs are synthetic polypeptides derived from the structural protein elastin, whose most prominent amino acid sequence is VPGVG (where V = valine, P = proline, and G = glycine) (8). When heated, synthetic polymers made of the VPGVG sequence, poly(VPGVG), undergo a transition from a hydrophilic random coil conformation to a hydrophobic b-spiral; the ELPs then aggregate and precipitate (9). Linear poly(VPGVG) is not the only ELP that exhibits this LCST behavior. The phenomenon has also been observed with polymers having VPGVG side chains (10–13) and even with single repeats (14) of the pentapeptide. Moreover, the transition temperature of these elastin-based structures can be tuned within a wide temperature range by varying various parameters such as the molecular weight, concentration, and pH (15, 16). Replacing the fourth residue with any other amino acid (VPGXG), except proline, also has an important influence on the LCST (15). The idea of coating gold nanostructures with ELPs has been exploited in the past (17, 18). Thermally and optically responsive gold nanoassemblies have been prepared using cysteine-containing ELPs. However, the preparation of such polypeptides involves genetically encoded recombinant methods and is nontrivial to most chemists. We recently described a novel strategy to prepare thermally responsive materials based on the use of gold nanoparticles
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