Showing papers by "L. Andrew Lyon published in 2007"

Au nanoparticle templated synthesis of pNIPAm nanogels

Abstract: We have explored a synthetic route toward poly(N-isopropylacrylamide) (pNIPAm) nanogels by growing a pNIPAm shell onto a metal nanoparticle seed. Nuclei compatible with precipitation polymerization of thermoresponsive polymers were formed by adsorption of NH2-terminated pNIPAm on Au nanoparticles. The adsorbed pNIPAm layer, when heated above the LCST, collapses onto the Au nanoparticle surface. This polymer layer thus serves as a hydrophobic nucleus for growing pNIPAm oligoradicals during polymer synthesis, resulting in the formation of a pNIPAm shell. Etching of the Au core from the polymer-coated particles with KCN results in hollow hydrogel nanoparticles. The rate of Au core etching as studied by the decrease in Au nanoparticle plasmon absorbance was shown to depend on polymer shell composition, shell thickness, and the thermosensitivity of the polymer. Efficient quenching of fluorophores incorporated in the polymer shell by the core Au nanoparticles was observed; however, the shell fluorescence was re...

In-situ AFM studies of the phase-transition behavior of single thermoresponsive hydrogel particles.

Abstract: The volume phase transition (VPT) behavior of individual thermally responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) hydrogel microparticles was studied by in-situ dynamic mode atomic force microscopy (AFM) and force spectroscopy during heating and cooling cycles. Hydrogel samples were prepared by electrostatic immobilization of microparticles to amine-modified gold surfaces. The AFM studies of particle deswelling were performed by varying the force applied on the particles during imaging as a function of the geometry and material of the AFM probe. Aluminum-coated silicon cantilevers were found to influence substantially the behavior of the particles during the VPT, leading to a significant shape change. Low force impact magnetic excitation of the AFM probe (MAC mode) during dynamic mode measurements resulted in an undisturbed deswelling behavior enabling observation of the expected volume changes of the particles without significant tip−sample interaction. Hence, MAC-mode AFM was det...

Self-Assembly of “Paint-On” Colloidal Crystals Using Poly(styrene-co-N-isopropylacrylamide) Spheres

Abstract: We report on the synthesis of monodispersed, spherical colloidal particles composed of poly(styrene-co-N-isopropylacrylamide) (pS-co-NIPAm) that rapidly self-assemble via simple solvent evaporation to form 3-D colloidal crystals that are both robust and uniformly diffractive. As a suspension of the pS-co-NIPAm particles dries, the interactions of their soft, flexible peripheries assist in particle self-assembly to form a close-packed ordered structure. Interparticle adhesion occurs at these contacts as the final amount of solvent evaporates, enhancing the durability of the assembly. The dense core provides stability of the ordered structure subsequent to drying. These dried, crystalline films exhibit Bragg diffraction, which can be modified by changing the particle size through varying the monomer feed concentrations. The facile drying process used for crystalline assembly presents a unique approach to photonic inks that can be used as “paint-on” (i.e., processable) dispersions, which are advantageous for...

Colloidal Crystals of Thermosensitive, Core/Shell Hybrid Microgels

Abstract: We present the assembly of colloidal crystals composed of various thermoresponsive core/shell microgels and their hybrid microgel counterparts that contain localized Au nanoparticles. To obtain nanostructured microgels, we conducted a three-stage polymerization. First, core microgels composed of cross-linked poly(N-isopropylacrylamide) (pNIPAm) were synthesized by precipitation polymerization. Using these microgels as cores or “seeds”, a copolymer shell of pNIPAm was added to these core particles using a functional comonomer. Finally, a second shell consisting solely of cross-linked pNIPAm was added onto the preformed core/shell microgels by using this seeded polymerization technique. With use of these core/shell/shell microgels as templates, Au nanoparticles were synthesized in situ, using the cationic sites in the inner shell to nucleate particle growth. To control the optical properties of the Au nanoparticles, electroless Au plating was carried out with use of the preformed Au nanoparticles in the mic...

Crystallization behavior of soft, attractive microgels.

Abstract: The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.

Phase behavior in highly concentrated assemblies of microgels with soft repulsive interaction potentials.

Abstract: Microgel particles with a soft repulsive interaction potential are investigated with particle tracking methods to study the phase behavior of soft-sphere systems. The use of poly(N-isopropylacrylamide) particles allows the effective volume fraction of a sample to be tuned via thermal modulation without altering the particle number density. This allows for investigation of the phase behavior of an assembly as a function of its initial packing density. In particular, we have elucidated the influence of soft colloid “overpacking” on the freezing effective volume fraction (φeff,f). These studies thereby illustrate the interplay between energetics/packing forces occurring at the colloidal and polymer chain length scales.

Bimodal swelling responses in microgel thin films.

Abstract: A series of studies on microgel thin films is described, wherein quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy (AFM) have been used to probe the properties of microstructured polymer thin films as a function of film architecture and solution pH. Thin films composed of pNIPAm-co-AAc microgels were constructed by using spin-coating layer-by-layer (scLbL) assembly with poly(allylamine hydrochloride) (PAH) as a polycationic "glue". Our findings suggest that the interaction between the negatively charged microgels and the positively charged PAH has a significant impact on the pH responsivity of the film. These effects are observable in both the optical and mechanical behaviors of the films. The most significant changes in behavior are observed when the motional resistance of a quartz oscillator is monitored via QCM experiments. Slight changes to the film architecture and alternating the pH of the environment significantly changes the QCM and SPR responses, suggesting a pH-dependent swelling that is dependent on both particle swelling and polyelectrolyte de-complexation. Together, these studies allow for a deeper understanding of the morphological changes that take place in environmentally responsive microgel-based thin films.

Displacement-Induced Switching Rates of Bioresponsive Hydrogel Microlenses

Abstract: We report investigations of the response rates of bioresponsive hydrogel microlenses in order to gain a deeper understanding of their potential utility as a new label-free biosensing construct. Bioresponsive microgels were prepared from multi-responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgels after conjugation with biotin (as a bound target) and aminobenzophenone (ABP) (as a photoaffinity label) via carbodiimide chemistry. Bioresponsive hydrogel microlenses were then formed from individual microgels via Coulombic assembly of the anionic microgels onto a positively charged glass substrate. Incubation with polyclonal anti-biotin resulted in antibody-cross-linked microlenses. The antibodies were then photoligated to the hydrogel network via UV irradiation, making the microlenses reversibly sensitive to free biocytin by disruption of biotin−antibiotin binding. The response rates of the microlenses to biocytin binding (bound antibody−antigen displacement) were studied by monitoring t...