Fast nanoparticle rotational and translational diffusion in synovial fluid and hyaluronic acid solutions.

Nanoparticles are under investigation as diagnostic and therapeutic agents for joint diseases, such as osteoarthritis. However, there is incomplete understanding of nanoparticle diffusion in synovial fluid, the fluid inside the joint, which consists of a mixture of the polyelectrolyte hyaluronic acid, proteins, and other components. Here, we show that rotational and translational diffusion of polymer-coated nanoparticles in quiescent synovial fluid and in hyaluronic acid solutions is well described by the Stokes-Einstein relationship, albeit with an effective medium viscosity that is much smaller than the macroscopic low shear viscosity of the fluid. This effective medium viscosity is well described by an equation for the viscosity of dilute polymer chains, where the additional viscous dissipation arises because of the presence of the polymer segments. These results shed light on the diffusive behavior of polymer-coated inorganic nanoparticles in complex and crowded biological environments, such as in the joint.

Characterization and performance evaluation of the XSPA-500k detector using synchrotron radiation.

Hybrid photon counting (HPC) detectors are widely used at both synchrotron facilities and in-house laboratories. The features of HPC detectors, such as no readout noise, high dynamic range, high frame rate, excellent point spread function, no blurring etc. along with fast data acquisition, provide a high-performance detector with a low detection limit and high sensitivity. Several HPC detector systems have been developed around the world. A number of them are commercially available and used in academia and industry. One of the important features of an HPC detector is a fast readout speed. Most HPC detectors can easily achieve over 1000 frames s-1, one or two orders of magnitude faster than conventional CCD detectors. Nevertheless, advanced scientific challenges require ever faster detectors in order to study dynamical phenomena in matter. The XSPA-500k detector can achieve 56 kframes s-1 continuously, without dead-time between frames. Using `burst mode', a special mode of the UFXC32k ASIC, the frame rate reaches 1 000 000 frames s-1. XSPA-500k was fully evaluated at the Metrology beamline at Synchrotron SOLEIL (France) and its readout speed was confirmed by tracking the synchrotron bunch time structure. The uniformity of response, modulation transfer function, linearity, energy resolution and other performance metrics were also verified either with fluorescence X-rays illuminating the full area of the detector or with the direct beam.

Evolution of structure and dynamics of thermo-reversible nanoparticle gels-A combined XPCS and rheology study.

A combined X-ray photon correlation spectroscopy and rheology study is carried out to capture the evolution of structure, fast particle-scale dynamics, and moduli (elastic and loss) at early times of gel formation near the fluid-gel boundary of a suspension of nanoparticles. The system is comprised of moderately concentrated suspensions of octadecyl silica in decalin (ϕ = 0.2) undergoing thermoreversible gelation. Near the gel boundary, the rate of gel formation is very sensitive to changes in attraction strength. However, we find that at different attraction strengths, the system goes through identical intermediate states of microscopic and macroscopic behavior, even though the absolute time needed to form a gel varies by orders of magnitude. We identify a single dimensionless time parameter, tw/tg, where tw is the wait time following the quench and tg is the rheologically determined gel time, that captures the similarity in gel formation at a range of attraction strengths. Following a temperature quench below the gel boundary, the system is initially fluidlike and forms diffusive clusters (∼8.5 times the particle diameter). After a lag-time, tL, clusters aggregate to form a network like structure which is characterized by the onset of mechanical rigidity and a rapid growth in microscopic relaxation times. At tg, the Baxter parameter obtained from adhesive hard sphere fits of the structure factor attains a constant value corresponding to the theoretical percolation boundary, thus demonstrating that gelation is percolation driven.

α-Synuclein Sterically Stabilizes Spherical Nanoparticle-Supported Lipid Bilayers.

While it is generally accepted that neuronal protein α-synuclein binds to highly curved and highly charged lipid membranes, its biological function beyond binding remains unknown despite its fundamental link to Parkinson's disease. Herein, we utilize spherical nanoparticle lipid bilayers (SSLBs) to recapitulate the charge and curvature limit of membrane organelles with which α-synuclein associates and probe how α-synuclein affects SSLB structure and dynamics as a proxy for interorganelle interactions. Small-angle X-ray scattering and X-ray photon correlation spectroscopy reveal our SSLBs form aggregates that are clearly broken up by the addition of α-synuclein, a clear indication that α-synuclein confers steric stabilization to membrane surfaces.

Sub-microsecond-resolved multi-speckle X-ray photon correlation spectroscopy with a pixel array detector.

Small-angle X-ray photon correlation spectroscopy (XPCS) measurements spanning delay times from 826 ns to 52.8 s were performed using a photon-counting pixel array detector with a dynamic range of 0-3 (2 bits). Fine resolution and a wide dynamic range of time scales was achieved by combining two modes of operation of the detector: (i) continuous mode, where data acquisition and data readout are performed in parallel with a frame acquisition time of 19.36 µs, and (ii) burst mode, where 12 frames are acquired with frame integration times of either 2.56 µs frame-1 or 826 ns frame-1 followed by 3.49 ms or 1.16 ms, respectively, for readout. The applicability of the detector for performing multi-speckle XPCS was demonstrated by measuring the Brownian dynamics of 10 nm-radius gold and 57 nm-radius silica colloids in water at room temperature. In addition, the capability of the detector to faithfully record one- and two-photon counts was examined by comparing the statistical distribution of photon counts with expected probabilities from the negative binomial distribution. It was found that in burst mode the ratio of 2 s to 1 s is markedly smaller than predicted and that this is attributable to pixel-response dead-time.

Hard-sphere-like dynamics in highly concentrated alpha-crystallin suspensions.

The dynamics of concentrated suspensions of the eye-lens protein alpha crystallin have been measured using x-ray photon correlation spectroscopy. Measurements were made at wave vectors corresponding to the first peak in the hard-sphere structure factor and volume fractions close to the critical volume fraction for the glass transition. Langevin dynamics simulations were also performed in parallel to the experiments. The intermediate scattering function f(q,τ) could be fit using a stretched exponential decay for both experiments and numerical simulations. The measured relaxation times show good agreement with simulations for polydisperse hard-sphere colloids.

Evaluation of the UFXC32k photon-counting detector for pump-probe experiments using synchrotron radiation.

This paper presents the performance of a single-photon-counting hybrid pixel X-ray detector with synchrotron radiation. The camera was evaluated with respect to time-resolved experiments, namely pump-probe-probe experiments held at SOLEIL. The UFXC camera shows very good energy resolution of around 1.5 keV and allows the minimum threshold setting to be as low as 3 keV keeping the high-count-rate capabilities. Measurements of a synchrotron characteristic filling mode prove the proper separation of an isolated bunch of photons and the usability of the detector in time-resolved experiments.

Dynamic Scaling of Colloidal Gel Formation at Intermediate Concentrations.

We have examined the formation and dissolution of gels composed of intermediate volume-fraction nanoparticles with temperature-dependent short-range attractions using small-angle x-ray scattering, x-ray photon correlation spectroscopy, and rheology to obtain nanoscale and macroscale sensitivity to structure and dynamics. Gel formation after temperature quenches to the vicinity of the rheologically determined gel temperature, T_{gel}, was characterized via the slowdown of dynamics and changes in microstructure observed in the intensity autocorrelation functions and structure factor, respectively, as a function of quench depth (ΔT=T_{quench}-T_{gel}), wave vector, and formation time t_{f}. We find the wave-vector-dependent dynamics, microstructure, and rheology at a particular ΔT and t_{f} map to those at other ΔTs and t_{f}s via an effective scaling temperature, T_{s}. A single T_{s} applies to a broad range of ΔT and t_{f} but does depend on the particle size. The rate of formation implied by the scaling is a far stronger function of ΔT than expected from the attraction strength between colloids. We interpret this strong temperature dependence in terms of cooperative bonding required to form stable gels via energetically favored, local structures.

Submillisecond X-ray photon correlation spectroscopy from a pixel array detector with fast dual gating and no readout dead-time.

Small-angle scattering X-ray photon correlation spectroscopy (XPCS) studies were performed using a novel photon-counting pixel array detector with dual counters for each pixel. Each counter can be read out independently from the other to ensure there is no readout dead-time between the neighboring frames. A maximum frame rate of 11.8 kHz was achieved. Results on test samples show good agreement with simple diffusion. The potential of extending the time resolution of XPCS beyond the limit set by the detector frame rate using dual counters is also discussed.