Networks of cross-linked and bundled actin filaments are ubiquitous in the cellular cytoskeleton, but their elasticity remains poorly understood. We show that these networks exhibit exceptional elastic behavior that reflects the mechanical properties of individual filaments. There are two distinct regimes of elasticity, one reflecting bending of single filaments and a second reflecting stretching of entropic fluctuations of filament length. The mechanical stiffness can vary by several decades with small changes in cross-link concentration, and can increase markedly upon application of external stress. We parameterize the full range of behavior in a state diagram and elucidate its origin with a robust model.

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While the important role of electrostatic interactions in aqueous colloidal suspensions is widely known and reasonably well-understood, their relevance to nonpolar suspensions remains mysterious. We measure the interaction potentials of colloidal particles in a nonpolar solvent with reverse micelles. We find surprisingly strong electrostatic interactions characterized by surface potentials, |ezeta|, from 2.0 to 4.4 k(B)T and screening lengths, kappa(-1), from 0.2 to 1.4 microm. Interactions depend on the concentration of reverse micelles and the degree of confinement. Furthermore, when the particles are weakly confined, the values of |ezeta| and kappa extracted from interaction measurements are consistent with bulk measurements of conductivity and electrophoretic mobility. A simple thermodynamic model, relating the structure of the micelles to the equilibrium ionic strength, is in good agreement with both conductivity and interaction measurements. Since dissociated ions are solubilized by reverse micelles, the entropic incentive to charge a particle surface is qualitatively changed from aqueous systems, and surface entropy plays an important role.

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The enhancements of normal Raman scattering, resonance Raman scattering, and fluorescence from molecules adsorbed on identical, well-characterized, silver-island films are reported. The enhancement arises from the electromagnetic interaction between the molecules and the electronic plasma resonance of the silver islands. A hierarchy of enhancement ratios is found, with typical values of 105 for RS, 103 for RRS and 10−1 to 10 for fluorescence, depending on the quantum yield of the molecular fluorescence. A model, developed on heuristic grounds and substantiated using the density matrix formalism, describes the light scattering processes and the effects of the plasma resonance. This model presents a unified picture of the surface-induced enhancement effects and is consistent with the experimental values. The comparison of all the forms of optical scattering leads to a complete determination of the role of the plasma resonances in the various portions of the scattering process. The excitation of the electronic plasma resonance results in an increased local field at the molecules leading to an increased excitation or absorption rate. Similarly, the excitation of the plasma resonance by the molecular emission dipole results in an increase in the radiative decay rate. However, the electromagnetic coupling of the molecule to the plasma resonance also adds an additional damping channel which can result in a reduction of the absorption or excitation rate as well as the emission yield. The resultant balance of these processes leads to the hierarchy in the measured enhancements. The hierarchy of enhancements is also shown to have important spectroscopic consequences.

Characterization of the properties of complex biomaterials using microrheological techniques has the promise of providing fundamental insights into their biomechanical functions; however, precise interpretations of such measurements are hindered by inadequate characterization of the interactions between tracers and the networks they probe. We here show that colloid surface chemistry can profoundly affect multiple particle tracking measurements of networks of fibrin, entangled F-actin solutions, and networks of cross-linked F-actin. We present a simple protocol to render the surface of colloidal probe particles protein-resistant by grafting short amine-terminated methoxy-poly(ethylene glycol) to the surface of carboxylated microspheres. We demonstrate that these poly(ethylene glycol)-coated tracers adsorb significantly less protein than particles coated with bovine serum albumin or unmodified probe particles. We establish that varying particle surface chemistry selectively tunes the sensitivity of the particles to different physical properties of their microenvironments. Specifically, particles that are weakly bound to a heterogeneous network are sensitive to changes in network stiffness, whereas protein-resistant tracers measure changes in the viscosity of the fluid and in the network microstructure. We demonstrate experimentally that two-particle microrheology analysis significantly reduces differences arising from tracer surface chemistry, indicating that modifications of network properties near the particle do not introduce large-scale heterogeneities. Our results establish that controlling colloid-protein interactions is crucial to the successful application of multiple particle tracking techniques to reconstituted protein networks, cytoplasm, and cells.

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