Environment-dependent guest exchange in supramolecular hosts

Abstract

Dynamic exchange of guest molecules, encapsulated in host assemblies, is a phenomenon in supramolecular chemistry that has important implications in several applications. While the mechanism of exchange in micellar assemblies has been previously investigated, the effect of host and guest environment upon the guest-exchange dynamics has received little attention, if any. In this paper, we study the guest-exchange mechanism in pH-sensitive nanogels along with pH-insensitive nanogels as a control. By systematically comparing the behavior of these nanogels, we show that size, concentration, and hydrophobicity can all play a critical role in guest-exchange dynamics. More importantly, these studies reveal that the dominant mechanism of guest exchange can intimately depend on environmental factors.

Figure 1

Schematic representation of two possible mechanisms for guest exchange between cross-linked nanogels: (a) the collision–exchange–separation mechanism, also called the collision-based mechanism, and (b) the exit–re-entry mechanism, also called the diffusion-based mechanism.

Scheme 1. Structure of Polymers P1 and P2, Nanogels NG-C and NG, and the Dye Molecules DiO and DiI

Figure 2

Size distribution of nanogel at different pH values via DLS measurements: (a) NG with 30% cross-linking density and (b) NG-C with 20% cross-linking density.

Figure 3

Fluorescence evolution when NG-C respectively containing DiI and DiO are mixed in solutions at pH 7.1: (a) 0.05 mg/mL, (b) 0.10 mg/mL, and (c) 0.15 mg/mL. (d) Comparison of the dynamics of guest exchange between NG-C with different concentrations at pH 7.1.

Figure 4

Fluorescence evolution when two NG solutions independently containing DiI and DiO dye molecule as guest are mixed in solutions at different pH values: (a) pH 8.0, (b) pH 6.4, and (c) pH 5.1. (d) Guest-exchange rates between NG at different pH values. The concentration of each nanogel was 0.10 mg/mL.

Figure 5

Comparison of the dynamics of guest exchange between NG-C with 0.10 mg/mL at different pH values.

Figure 6

Effect of pH on the hydrophobicity. (a) Fluorescence emission spectrum measured for nanogel NG-C loading 2 wt % pyrene and (b) calculated I₁/I₃ ratios for nanogels at different pH values (black, NG; red, NG-C).

Figure 7

Comparison of the dynamics of guest exchange between NG of different concentrations (0.05, 0.10, and 0.15 mg/mL) at (a) pH 8.0, (b) pH 7.1, (c) pH 6.1, and (d) pH 5.1.