This report describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected Au clusters (MPCs) dissolved in electrolyte solution depend on the alkanethiolate chain length (C4 to C16). The double-layer capacitances of individual MPCs (C(CLU)) are sufficiently small (sub-attoFarad, aF) that their metal core potentials change by >0.1 V increments for single electron transfers at the electrode/solution interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis varies with C(CLU). Differential pulse voltammetric measurements of C(CLU) in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simple theoretical model, which considers the capacitance as governed by the thickness of a dielectric material (the monolayer, whose chain length is varied) between concentric spheres of conductors (the Au core and the electrolyte solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron microscopy and laser ionization-desorption mass spectrometry, as a tool for evaluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexanethiolate monolayers.

Colloidal crystals assembled from stimuli-sensitive hydrogel particles composed largely of the thermoresponsive polymer poly(N-isopropylacrylamide) display unusual phase behavior because of the inherent “softness” of their interaction potentials as well as the particle thermoresponsivity. In this contribution, we review results from our group that illustrate the use of such soft-sphere building blocks in the construction of colloidal crystals. First, we describe the utility of temperature-induced volume phase transitions in controlling the crystallization and melting of the colloidal crystals. These methods then enable the study of the complex phase behavior of certain types of microgels. For example, it is proposed that multiple weak attractive interactions between particles can drive crystallization at particle concentrations well below the hard-sphere freezing point. Finally, the utility of soft-sphere crystals in the development of new photonic materials is presented in examples of laser direct writing and photopatterning of colloidal crystals based on a photothermally directed crystallization method.

The tunability of the lattice spacing of hydrogel colloidal crystals has been achieved by simply varying the water content of the microgel pellet before annealing and crystallization. The manipulation of the lattice constant of the crystals and therefore of the wavelength of the resultant Bragg peak, i.e., the color of the crystal, leads to a myriad of photonic applications.

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Thermoresponsive poly(N-isopropylacrylamide) (pNIPAm) microgel particles cross-linked with various concentrations of PEG diacrylates of 3 different PEG chain lengths were synthesized via free-radical precipitation polymerization in order to investigate the phase transition and protein adsorption behavior as the hydrophilicity of the network is increased. Photon correlation spectroscopy (PCS) reveals that, as the concentration of PEG cross-linker incorporated into the particles is increased, an increase in the temperature and breadth of the phase transition occurs. Qualitative differences in particle density using isopycnic centrifugation confirm that higher PEG concentrations result in denser networks. The efficient incorporation of PEG cross-linker was confirmed with (1)H NMR, and variable temperature NMR studies suggest that, in the deswollen state, the longer PEG cross-links protrude from the dense globular network. This behavior apparently manifests itself as a decrease in nonspecific protein adsorption with increasing PEG length and content. Furthermore, when electrostatically attached to a glass surface, the particles containing the longer chain lengths exhibited enhanced nonfouling behavior and were resistant to cell adhesion in serum-containing media. The excellent performance of these particulate films and the simplicity with which they are assembled suggests that they may be applicable in a wide range of applications where nonfouling coatings are required.