We report the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core-shell morphologies. Core particles composed of cross-linked poly(Nisopropylacrylamide) (p-NIPAm) or poly(NIPAm-co-acrylic acid) (p-NIPAm-AAc) were synthesized via precipitation polymerization and then used as nuclei for subsequent polymerization of p-NIPAm-AAc and p-NIPAm, respectively. The presence of a core-shell morphology was confirmed by transmission electron microscopy (TEM). Thermally initiated volume phase transitions were interrogated via temperature-programmed photon correlation spectroscopy (TP-PCS) as a function of solution pH. The p-NIPAm-AAc core hydrogel displays both a strong temperature and pH dependence on swelling. However, both p-NIPAm-AAc (core)/p-NIPAm (shell) and p-NIPAm (core)/p-NIPAm-AAc (shell) particles display a more complex pH dependence than the homogeneous particles. Specifically, a multistep volume phase transition appears when the AAc component becomes highly charged at a high pH. It is apparent from the measured deswelling curves that a portion of the particle swelling behavior is dominated by p-NIPAm, regardless of its location in the particle. However, deswelling behavior that is due to a mixture of p-NIPAm-AAc and p-NIPAm is evident, as well as a regime that is largely attributed to p-NIPAm-AAc alone. Small differences in the effect of pH on the two core-shell particles indicate that the influence of p-NIPAm is somewhat greater when it is localized in the shell.

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.

Surface plasmon resonance (SPR) biosensing using colloidal Au enhancement is reported. Immobilization of approximately 11-nm-diameter colloidal Au to an evaporated Au film results in a large shift in plasmon angle, a broadened plasmon resonance, and an increase in minimum reflectance. The incorporation of colloidal Au into SPR biosensing results in increased SPR sensitivity to protein-protein interactions when a Au film-immobilized antibody and an antigen-colloidal Au conjugate comprise the binding pair. A highly specific particle-enhanced analogue of a sandwich immunoassay is also demonstrated by complexing the Au particle to a secondary antibody. A tremendous signal amplification is observed, as addition of the antibody-Au colloid conjugate results in a 25-fold larger signal than that due to addition of a free antibody solution that is 6 orders of magnitude more concentrated. Picomolar detection of human immunoglobulin G has been realized using particle enhancement, with the theoretical limits for the technique being much lower. Finally, a quasi-linear relationship between particle coverage and plasmon angle shift is presented, thereby providing for a direct correlation between plasmon shift and solution antigen concentration. Together, these results represent significant advances in the generality and sensitivity of SPR as it is applied to biosensing.

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)­functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

We report the observation of unidirectional plasmon propagation in metallic nanowires over distances >10 µm. Through control of the incident excitation wavelength and rod composition, we demonstrate the selective coupling of photons into the plasmon mode of a 20 nm diameter nanowire. This mode then propagates in a nonemissive fashion down the wire length before being emitted as an elastically scattered photon at the distal end. As expected from previous studies of plasmon excitation in nanoparticles and thin films, we observe a strong wavelength and material dependence of this phenomenon. This metal-dependent plasmon propagation is exploited to produce a wire through which plasmons propagate unidirectionally. A bimetallic wire with a sharp Au/Ag heterojunction is shown to display both wavelength dependence and unidirectionality with respect to plasmon propagation across the heterojunction. It is expected that these results will contribute to the growing interest in optical energy transport in molecular-level and nanoscale devices.

A reflectance method has been used to assess conduction band edge energies (E cb) for nanocrystalline TiO2(anatase) electrodes in contact with aqueous electrolytes. The measurements, which were made over a range of nearly 40 pH units, reveal a Nernstian dependence of E cb upon pH over most of this range, i.e., a −64 mV shift per unit decrease in log(proton activity) between H0 = −8 and H- = +23. Electrochemical quartz crystal microbalance (EQCM) measurements have established that charge compensating proton uptake occurs at potentials negative of E cb. Uptake occurs over the entire EQCM-accessible pH range (H0 = −5 to pH = +11). The combined findings are inconsistent with E cb control solely via surface protonation and deprotonation reactions, whose pK a's occur in the vicinity of pH 4 and 10. They are consistent, however, with a mechanism whereby:  (a) electrochemical generation of Ti(III) trap sites, in the log(proton activity) range from H0 = −8 to H- = +23, is accompanied quantitatively by proton intercalation, (b) conversion of the trap sites back to oxidation state IV is accompanied quantitatively by proton expulsion, and (c) the conduction band edge energy is controlled by the pH-dependent trap-based Ti(III/IV) couple. The pH independence found for E cb above H- = +23 and below H0 = −8 is ascribed to an eventual decoupling of proton intercalation from electron addition.

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This manuscript describes the stepwise, ligand-directed assembly, characterization, and prospective applications of three-dimensional Au and Ag nanoparticle, multlilayered films. Films were prepared by successive treatments of a Au nanoparticle monolayer with a bifunctional cross-linker and colloidal Au or Ag solutions. Changes in film electrical and optical properties are reported for a series of bifunctional cross-linkers of varying molecular lengths. Interestingly, these films exhibit Beer's law behavior despite the presence of strong interparticle optical coupling. Multilayer films with greater than six exposures to 2-mercaptoethylamine and Au colloid were highly conductive and resembled bulk Au in appearance. In contrast, films of similar particle coverage generated using a longer cross-linker (1,6-hexanedithiol) exhibited higher transmission in the near-infrared region and exhibited a reduced conductivity. Measurement of the multilayer morphology with atomic force microscopy , electrostatic force microscopy, and field emission scanning electron microscopy revealed a porous, discontinuous morphology composed of large, continuous regions of aggregated nanoparticles. This, in turn, results in a surface roughness contribution to surface plasmon scattering and surface-enhanced Raman scattering observed for Au, Au/Ag, and Ag colloid multilayers. Particulate multilayer films made using horseradish peroxidase as a cross-linker remained enzymatically active, even beneath three layers of colloidal Au. Multilayers could also be prepared on surfaces patterned by microcontact printing. These data show how Au colloid multilayers grown in solution are a viable alternative to evaporated metal films for a number of applications.