Chemical sensors respond to the presence of a specific analyte in a variety of ways. One of the most convenient is a change in optical properties, and in particular a visually perceptible colour change. Here we report the preparation of a material that changes colour in response to a chemical signal by means of a change in diffraction (rather than absorption) properties. Our material is a crystalline colloidal array of polymer spheres (roughly 100 nm diameter) polymerized within a hydrogel that swells and shrinks reversibly in the presence of certain analytes (here metal ions and glucose). The crystalline colloidal array diffracts light at (visible) wavelengths determined by the lattice spacing, which gives rise to an intense colour. The hydrogel contains either a molecular-recognition group that binds the analyte selectively (crown ethers for metal ions), or a molecular-recognition agent that reacts with the analyte selectively. These recognition events cause the gel to swell owing to an increased osmotic pressure, which increases the mean separation between the colloidal spheres and so shifts the Bragg peak of the diffracted light to longer wavelengths. We anticipate that this strategy can be used to prepare 'intelligent' materials responsive to a wide range of analytes, including viruses.

Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials

An electrohydrodynamic methodology has been developed that makes possible the precise assembly of two- and three-dimensional colloidal crystals on electrode surfaces. Electrophoretically deposited colloidal particles were observed to move toward one another over very large distances (greater than five particle diameters) to form two-dimensional colloidal crystals for both micrometer- and nanometer-size particles. This coalescence of particles with the same charge is opposite to what is expected from electrostatic considerations and appears to result from electrohydrodynamic fluid flow arising from an ionic current flowing through the solution. The ability to modulate this "lateral attraction" between particles, by adjusting field strength or frequency, facilitates the reversible formation of two-dimensional fluid and crystalline colloidal states on the electrode surface. Further manipulation allows controlled structures to be assembled.

http://science.sciencemag.org/content/sci/272/5262/706.full.pdf

Field-Induced Layering of Colloidal Crystals

Rheological properties and the equilibrium colloidal phase behavior of concentrated dispersions of a temperature sensitive microgel were investigated. The temperature sensitive hydrogel particles consist of poly (N-isopropylacrylamid) (PNiPAM) chemically crosslinked with N,N′ methylenbisacrylamid (BIS). With increasing temperature the microgel particles decrease in size (hydrodynamic radius 142 nm at 10 °C and 58 nm at 35 °C) and with it the effective volume fraction, which leads to dramatic changes in rheology—vanishing yield stress and decreasing viscosity and elastic properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted vs the effective volume fraction. The monodisperse hydrogel particles form colloidal crystals and glasses in concentrated solution but at higher effective volume fractions as compared to model hard sphere suspensions. Comparison of the experimental freezing point with soft sphere computer simulations indica...

Temperature sensitive microgel suspensions: Colloidal phase behavior and rheology of soft spheres

As one increases the concentration of a colloidal suspension, the system exhibits a dramatic increase in viscosity. Beyond a certain concentration, the system is said to be a colloidal glass; structurally, the system resembles a liquid, yet motions within the suspension are slow enough that it can be considered essentially frozen. For several decades, colloids have served as a valuable model system for understanding the glass transition in molecular systems. The spatial and temporal scales involved allow these systems to be studied by a wide variety of experimental techniques. The focus of this review is the current state of understanding of the colloidal glass transition, with an emphasis on experimental observations. A brief introduction is given to important experimental techniques used to study the glass transition in colloids. We describe features of colloidal systems near and in glassy states, including increases in viscosity and relaxation times, dynamical heterogeneity and ageing, among others. We also compare and contrast the glass transition in colloids to that in molecular liquids. Other glassy systems are briefly discussed, as well as recently developed synthesis techniques that will keep these systems rich with interesting physics for years to come.

/pdf/the-physics-of-the-colloidal-glass-transition-41jncqk0al.pdf

The Physics of the Colloidal Glass Transition

We describe a theory for a new type of colloid behavior whereby particles deposited on a surface by electrophoresis are manipulated to form two-dimensional crystals. Since the particles are equally charged, the clustering is opposite that expected from electrostatic considerations. Such behavior is consistent with migration due to electrohydrodynamic flows associated with polarization layers and ion currents. Provided colloid stability is maintained, the assembly processes take place with both dc and ac fields and may be modulated by adjusting the field strength or frequency. No migration is present at frequencies above 1 MHz. Two-dimensional fluid and crystalline states can be formed on the electrode surface. Experiments with patterned electrodes demonstrate the presence of the electrohydrodynamic flow. A mathematical model of the electrohydrodynamics provides insight into the assembly process.

https://www.princeton.edu/~cml/assets/pdf/9713trau_colloids.pdf

Assembly of Colloidal Crystals at Electrode Interfaces

bcc crystalline colloidal microsphere suspensions are shear melted into the metastable liquid phase. Recrystallization occurs via nucleation and growth of single crystallites at dilute sites. The nearly spherical growing crystals have rough interfaces with a time-independent interface velocity, $v$. The velocity is found to be consistent with an equation for normal growth with a limiting velocity determined by free-particle diffusion, ${v}_{0}\ensuremath{\sim}\frac{{D}_{0}}{\ensuremath{\xi}}$, where ${D}_{0}$ is the Stokes diffusion constant of independent noninteracting spheres, and $\ensuremath{\xi}$ is a length comparable to the particle spacing.

Nucleation and Growth of Colloidal Crystals

Small angle neutron scattering(SANS) is used to investigate the local order in aqueous, charge stabilized suspensions of 103 A diameter polystyrene latexspheres at ∼14 wt. % solids. These samples which evidence a close packed close packed structure in equilibrium are observed to undergo a transition to an amorphous order when sustaining a sufficiently large and steady shear. The shear melting phenomonology is different from that observed by light scattering for very dilute (0.16 wt. % solids) suspensions of polystyrene particles which form bcc lattices in equilibrium. Furthermore, neutron scattering reveals more detail than light scattering for concentrated suspensions and indicates a larger degree of three‐dimensional ordering under steady shear flow than is implied by sliding layer models for similar systems.

Neutron scattering from charge stabilized suspensions undergoing shear

Inelastic light scattering from particle number density fluctuations was carried out on a suspension of 0.109 μ diam sulfonated polystyrene microspheres exhibiting coexisting colloidal liquid and body‐centered‐cubic single crystalline phases. Comparison of the first cumulants of the decay of the density–density correlation function in the two phases reveals that the wave vector dependence in the liquid exhibits many features in common with that for longitudinal 110 lattice vibrations in the crystal.

L. K. Cotter

Papers

Nucleation and Growth of Colloidal Crystals

Neutron scattering from charge stabilized suspensions undergoing shear

Density fluctuation dynamics in a screened Coulomb colloid: Comparison of the liquid and bcc crystal phases

Erratum: Nucleation and growth of colloidal crystals [Phys. Rev. Lett. 57, 1733 (1986)]