The last decade of research in the physical sciences has seen a dramatic increase in the study of nanoscale materials. Today, "nanoscience" has emerged as a multidisciplinary effort, wherein obtaining a fundamental understanding of the optical, electrical, magnetic, and mechanical properties of nanostructures promises to deliver the next generation of functional materials for a wide range of applications. While this range of efforts is extremely broad, much of the work has focused on "hard" materials, such as Buckyballs, carbon nanotubes, metals, semiconductors, and organic or inorganic dielectrics. Meanwhile, the soft materials of current interest typically include conducting or emissive polymers for "plastic electronics" applications. Despite the continued interest in these established areas of nanoscience, new classes of soft nanomaterials are being developed from more traditional polymeric constructs. Specifically, nanostructured hydrogels are emerging as a promising group of materials for multiple biotechnology applications as the need for advanced materials in the post-genomic era grows. This review will present some of the recent advances in the marriage between water-swellable networks and nanoscience.

We describe the design of fluorescent, thermoresponsive microgels surface-functionalized with folic acid. Incubation of these particles with KB cells grown in folate-free medium results in efficient endocytosis of the particles via a receptor-mediated pathway. Laser scanning confocal microscopy and flow cytometry show efficient uptake of folate-modified particles over cationic control particles. Staining of the cells with Lysotracker red, followed by confocal imaging, shows anticorrelation between the particle and endosome fluorescence, which is taken as evidence of particle escape from the endosomes to the cytosol. Finally, the strong dependence of particle swelling on temperature was used to induce particle collapse and aggregation following uptake, which causes significant cytotoxicity. Thus, we have developed polymeric nanoparticles that may display antitumor activity, as they effectively target cancer cells and undergo endosomal escape to the cytosol, and they can then be triggered to cause cell death.

We report investigations of bioresponsive hydrogel microlenses as a new protein detection technology. Stimuli-responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels have been synthesized via free-radical precipitation polymerization. These hydrogel microparticles were then functionalized with biotin via EDC coupling. Hydrogel microlenses were prepared from the particles via Coulombic assembly onto a silane-modified glass substrate. Arrays containing both pNIPAm-co-AAc microgels (as an internal control) and biotinylated pNIPAm-co-AAc microgels were then used to detect multivalent binding of both avidin and polyclonal anti-biotin. Protein binding was determined by monitoring the optical properties of the microlenses using a brightfield optical microscopy technique. The microlens method is shown to be very specific for the target protein, with no detectable interference from nonspecific protein binding. Finally, the reversibility of the hydrogel microlens assay has been studied in the case of anti-biotin to determine the potential application of the microlens assay technology in a displacement-type assay. These results suggest that the microlens method may be an appropriate one for label-free detection of proteins or small molecules via displacement of tethered protein--ligand pairs.

Thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm) microgels possessing a hollow structure have been synthesized from core-shell nanoparticles upon oxidation of the particle core, followed by removal of the produced polymer segments by centrifugation. N,N'-(1,2-dihydroxyethylene)bisacrylamide (DHEA) is used as a cross-linker for preparing the degradable core, whereas N,N'-methylenebis(acrylamide) (BIS) is used as a cross-linker to add a nondegradable pNIPAm shell. Addition of NaIO(4) to a suspension of these particles in water leads to controlled degradation of the particle core by cleavage of the 1,2-glycol bond in DHEA. Fluorescence spectroscopy, UV/Vis spectroscopy, and photon correlation spectroscopy are used to characterize the hollow particles produced.

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Photoresponsive microgels have been prepared by precipitation polymerization of the thermoresponsive polymer poly(N-isopropylacrylamide) followed by covalent conjugation of the temperature-jump dye malachite green. The photoresponsivity of these dye-labeled microgels was characterized by a pump-probe optical setup. A HeNe laser is used for exciting the dye molecules and a near-IR-diode laser is used to simultaneously measure the turbidity of the colloidal dispersion. Irradiation of malachite green increases the temperature of the sample through rapid nonradiative decay, thereby causing the polymer chains to aggregate. On deswelling, a decrease in the intensity of transmitted light is observed due to scattering. It is also observed that the photoresponsive behavior of the microgels is dependent on the concentration of the dye, intensity of the laser, and bath temperature.

Sifting through the surface: Permselective core/shell microgels have been prepared by using a labile cross‐linker, which can be cleaved stoichiometrically to control the porosity of the shell. Proteins that are smaller than the pore size are allowed to permeate through the shell to bind with the core‐bound ligand (arrows; see picture).

We report on the deswelling behavior of microcomposite hydrogel films composed of poly(N-isopropylacrylamide) (pNIPAm). These films were synthesized first by formation of pNIPAm microgel particles via precipitation polymerization, followed by polymerization of these particles into a pNIPAm thin film gel matrix immobilized on a glass slide. The deswelling behavior of these composite films was studied by temperature-dependent light scattering (i.e., turbidity). In conventional (not microstructured) films the scattering increases and plateaus as the temperature is increased beyond the lower critical solution temperature (LCST) of pNIPAm. In contrast to these conventional films, microgel composite films display an increase in turbidity until the LCST, followed by a rapid turbidity decrease beyond the LCST. It is also observed that the rate at which scattering decreases depends on the concentration of the hydrogel particles in the film. Together, these results offer insight into the morphological changes occurring during the deswelling processes in composite hydrogel thin films.