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First Direct Imaging of Swollen Microgel Particles Print

Microgels are soft-material particles consisting of cross-linked polymer networks, 100 nm to 1 μm in diameter, dispersed in a continuous medium such as water. A useful feature of certain types of microgel particles is that they can swell or shrink with changes in external triggers such as pH and temperature. Thus microgel particles can act like "nanosponges" and offer many potential applications in medicine, environmental science, and industry. Because microgels are usually employed in their swollen state, in situ characterization of these particles under such conditions is desirable for understanding their behavior. However, optical microscopy is inadequate to this task, being limited in resolution and by the very small difference in refractive index (i.e., contrast) between the swollen particles and the continuous phase. Now, an international team of researchers from the U.S. and U.K. have obtained the first images of swollen microgel particles directly in aqueous solution using x-ray microscopy at the ALS, which, together with spectroscopic determination of their chemical state, provides insight into the underlying swelling mechanism.

Loopy but Useful

It shouldn't be surprising that polymers—giant molecules formed by chaining together countless numbers of simpler molecules—might curl back on themselves repeatedly until they resemble a loopy ball of tangled yarn. More surprising is the fact that the tangle can have unusual properties that, when controlled, would prove highly useful. For example, the polymer balls found in microgels are known to balloon in size when exposed to certain controllable conditions. Such particles hold the promise of a number of potential applications, including heavy-metal sequestration, dynamically tunable microlenses, and templates for the synthesis of inorganic nanoparticles.

In particular, microgels, which are biocompatible, are of great interest as drug-delivery vehicles. Numerous targets for therapeutic drugs, such as tumors and inflammatory tissues, exist at acidic conditions, and many protein-loaded microgels have already been synthesized and investigated. Thus, microgel particles stimulated by an acidic environment should be able to expand on demand to deliver their contents where needed. Realization of that promise, however, requires careful study of the mechanisms involved. Scanning transmission x-ray microscopy at the ALS provides the resolution and contrast necessary to untangle exactly what is going on.

The special properties of microgel particles are due to the presence of covalent bonds between different parts of the polymer chains (i.e., "cross-linking") and the presence of active functional groups. They allow, for example, the polymer network to retain water and the microgels to exhibit interesting physical properties not seen with common polymer latex microspheres. In this study, the researchers investigated lightly cross-linked poly(4-vinylpyridine)-silica (P4VP-SiO2) nanocomposite microgel particles synthesized in aqueous solution. Below a critical pH value of around 3.7, the 4-vinylpyridine residues become fully protonated, leading to significant swelling. The hydrodynamic diameter measured by dynamic light scattering (DLS) changes from around 230 nm (pH 8.8) to 620 nm (pH 3), which indicates a volumetric swelling factor of more than an order of magnitude.

Variation of hydrodynamic diameter with solution pH for lightly cross-linked P4VP-SiO2 nanocomposite microgel particles. The shaded region indicates the pH range in which flocculation (aggregation) was observed. The midpoint of this region corresponds approximately to the isoelectric point (where the particles don't move in an electric field). The digital photographs indicate the visual appearance of this nanocomposite microgel dispersion at pH 3 and pH 10.

The x-ray microscopy characterization was carried out using the polymer scanning transmission x-ray microscopy (polymer-STXM) endstation at ALS Beamline 5.3.2. STXM provides both high spatial resolution imaging (better than 50 nm) with zone-plate focusing and good chemical sensitivity based on near-edge x-ray fine structure (NEXAFS) spectroscopy. Using the "water window" photon energy region between the carbon and oxygen 1s absorption, the microgel particles can be imaged in their swollen state directly in aqueous solution through the use of a so-called wet cell, which consists of two thin silicon nitride membranes that are sealed together.

The investigation focused on nitrogen 1s rather than carbon 1s NEXAFS for both STXM imaging and spectroscopic studies. This was partly owing to the ease of handling multicomponent complex samples in wet cells at this particular energy but also because the pH-sensitive chemical environment of the nitrogen atom is of particular interest in this system. The researchers first obtained the nitrogen 1s NEXAFS spectra for both neutral and protonated linear P4VP homopolymer. These well-resolved lowest-photon-energy peaks served as excellent markers for the protonated and neutral states of the pyridine rings, and the corresponding two photon energies provided sufficient chemical contrast between the microgel particles at high and low pH (neutral form: 398.9 eV, protonated form: 400.4 eV).

STXM optical density images of aqueous dispersions of nanocomposite microgel particles in their (a) nonswollen (pH 10) and (b) swollen (pH 2.5) states.

The STXM images showed the well-dispersed swollen microgel particles at low pH in contrast to the nonswollen particles at high pH. The average size data of each particle was in good agreement with the DLS data. Subsequently, nitrogen 1s NEXAFS spectra were acquired from the individual hydrated microgel particles to estimate their degree of protonation. The resulting spectra at both low and high pH have sharp peaks at the same photon energies as the corresponding reference spectra. From this result, the researchers concluded that the nitrogen atoms of these P4VP-based cationic microgel particles are completely protonated at low pH. The researchers expect that these results will lead to the systematic investigation of a range of microgel particles in the future.

Nitrogen 1s NEXAFS spectra of (a) P4VP-SiO2 nanocomposite microgel particles in the wet cell at pH 10 and 2.5, and (b) linear P4VP homopolymer dried in either its neutral or fully protonated form on Si3N4.

Research conducted by T. Araki and H. Ade (North Carolina State University) and S. Fujii and S.P. Armes (University of Sheffield, U.K.).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); Royal Society/Wolfson Research Merit Award; and Engineering and Physical Sciences Research Council (EPSRC). Operation of the ALS is supported by BES.

Publication about this research: S. Fujii, S.P. Armes, T. Araki, and H. Ade, "Direct imaging and spectroscopic characterization of stimulus-responsive microgels," J. Am. Chem. Soc. 127, 16808 (2005).