The Importance of Excess Poly(N-isopropylacrylamide) for the Aggregation of Poly(N-isopropylacrylamide)-Coated Gold Nanoparticles

Abstract

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.

Keywords: LCST; N-isopropylacrylamide; NIPAm; aggregate; aggregation; gold; nanoparticle.

Figure 1

Schematic representation of the two reported outcomes of heating PNIPAm-coated AuNPs (center) above their LCST. Left: Polymer layer collapse. Right: Aggregation of the PNIPAm-coated AuNPs.

Figure 2

Dynamic light scattering (DLS) data showing the change in hydrodynamic radius (R_h) as the temperature is increased for (a) graft-from PNIPAm AuNPs and (b) graft-to PNIPAm-AuNPs before (blue circles) and after (red squares) the addition of 40 μmol (2.4 mg) of free 60k H-PNIPAm.

Figure 3

Images of 14 nm 60k-PNIPAm-coated AuNPs (1 mg/mL) taken above and below the LCST, red and blue, respectively. (a) Pure PNIPAm-AuNPs, (b) PNIPAm-AuNPs after the addition of 1.6 mg of H-PNIPAm, and (c) PNIPAm-AuNPs after the addition of a further 1.8 mg of H-PNIPAm.

Figure 4

Absorbance of 14 nm AuNPs coated with 60k PNIPAm, (a) 14 nm AuNPs coated with 15k, 30k, and 60k PNIPAm, mixed with aliquots of 60k H-PNIPAm and (b) mixed with aliquots of 15k, 30k, and 60k H-PNIPAm, after heating above the LCST, and large aggregates have been removed via heated centrifugation at low g.

Figure 5

60k PNIPAm-coated AuNPs (14 nm) with 3.4 mg of added H-PNIPAm (left vial) and P[NIPAM-co-HEAm] (right vial) taken above and below the LCST of the free polymer, right and left, respectively.