Thermoresponsive materials are generating significant interest on account of the sharp and tunable temperature deswelling transition of the polymer chain. Such materials have shown promise in drug delivery devices, sensing systems, and self-assembly. Incorporation of nanoparticles (NPs), typically through covalent attachment of the polymer chains to the NP surface, can add additional functionality and tunability to such hybrid materials. The versatility of these thermoresponsive polymer/nanoparticle materials has been shown previously; however, significant and important differences exist in the published literature between virtually identical materials. Here we use poly(N-isopropylacrylamide) (PNIPAm)-AuNPs as a model system to understand the aggregation behavior of thermoresponsive polymer-coated nanoparticles in pure water, made by either grafting-to or grafting-from methods. We show that, contrary to popular belief, the aggregation of PNIPAm-coated AuNPs, and likely other such materials, relies on the size and concentration of unbound “free” PNIPAm in solution. It is this unbound polymer that also leads to an increase in solution turbidity, a characteristic that is typically used to prove nanoparticle aggregation. The size of PNIPAm used to coat the AuNPs, as well as the concentration of the resultant polymer–AuNP composites, is shown to have little effect on aggregation. Without free PNIPAm, contraction of the polymer corona in response to increasing temperature is observed, instead of nanoparticle aggregation, and is accompanied by no change in solution turbidity or color. We develop an alternative method for removing all traces of excess free polymer and develop an approach for analyzing the aggregation behavior of such materials, which truly allows for heat-triggered aggregation to be studied.
The dynamic three-dimensional structures of enzymes are dictated by secondary bonding interactions and play a crucial role in both molecular recognition and allosteric regulation. Controlled crosslinking of single polymer chains in isolation, that can be seen as a mimic of the self-organization of enzymes, has previously been realized in organic solvents through crosslinking of multivalent polymer chains under highly dilute conditions. [1,2] In this instance, crosslinking must be specifically intramolecular to form these "self-collapsed" single-chain polymeric entities, which have been reported as discrete, spherical nanoparticulate structures. [3][4][5][6][7][8][9] Whilst a few of the above systems are documented in the literature, where novel applications for such systems have been realized, [10,11] only a small number are shown to be reversible and only one example exists in water. Moreover, the controlled folding and unfolding of a single polymer chain in water has not yet been realized. A completely reversible form of this system would be beneficial for many reasons, especially in light of one notable property of these nanoparticles (NPs), which is their ability to produce non-Einsteinian reductions in viscosity. [12] Supramolecular crosslinking motifs exploit well-established non-covalent interactions and their incorporation into molecular constructs has led to the formation of materials with novel properties. [13][14][15][16][17] Notable examples of such materials predominantly include gelating entities where intermolecular crosslinking leads to gel formation. [18] This strategy has been particularly successful for systems that consist of polymeric subunits which are able to gel through multivalent functionality. [19] Cucurbit[8]uril (CB[8]), a macrocylic host molecule capable of binding two aromatic guest molecules simultaneously, is a suitable candidate for such reversible crosslinking on account of the variety of guests available for binding. [20] This allows for the use of guests with a range of orthogonal stimuli where guest binding can be controlled through simple external conditions (e.g. temperature, pH, light, competing guests), thus allowing for reversibility to be easily achieved. As a result, a variety of systems have already been produced bearing this reversible CB[8]-based crosslinking motif. [21][22][23] Herein we document a CB[8]-mediated system for the preparation of metastable single-chain polymer nanoparticles. These nanoparticles are shown to form rapidly, are highly tunable and reversible and do not require protection chemistries (Figure 1).High molecular weight and water soluble poly(Nhydroxyethylacrylamide) precursors were synthesised using recent advancements in atom transfer radical polymerization (ATRP) of acrylamides (Table S1 in the Supporting Information). [24] The pendant hydroxyl functionality afforded by the polymer thus allowed for the conjugation of isocyanate functional guests for CB[8] to the polymer backbone in a random fashion (Figure 2). [25] Guest loadings were chose...
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