Cadmium sulphide quantum dots with tunable electronic properties by bacterial precipitation.

We present a new method to fabricate semiconducting, transition metal nanoparticles (NPs) with tunable bandgap energies using engineered Escherichia coli. These bacteria overexpress the Treponema denticola cysteine desulfhydrase gene to facilitate precipitation of cadmium sulphide (CdS) NPs. Analysis with transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy reveal that the bacterially precipitated NPs are agglomerates of mostly quantum dots, with diameters that can range from 3 to 15 nm, embedded in a carbon-rich matrix. Additionally, conditions for bacterial CdS precipitation can be tuned to produce NPs with bandgap energies that range from quantum-confined to bulk CdS. Furthermore, inducing precipitation at different stages of bacterial growth allows for control over whether the precipitation occurs intra- or extracellularly. This control can be critically important in utilizing bacterial precipitation for the environmentally-friendly fabrication of functional, electronic and catalytic materials. Notably, the measured photoelectrochemical current generated by these NPs is comparable to values reported in the literature and higher than that of synthesized chemical bath deposited CdS NPs. This suggests that bacterially precipitated CdS NPs have potential for applications ranging from photovoltaics to photocatalysis in hydrogen evolution.

Micromechanical mapping of early osteoarthritic changes in the pericellular matrix of human articular cartilage.

OBJECTIVE: Osteoarthritis (OA) is a degenerative joint disease characterized by the progressive loss of articular cartilage. While macroscale degradation of the cartilage extracellular matrix (ECM) has been extensively studied, microscale changes in the chondrocyte pericellular matrix (PCM) and immediate microenvironment with OA are not fully understood. The objective of this study was to quantify osteoarthritic changes in the micromechanical properties of the ECM and PCM of human articular cartilage in situ using atomic force microscopy (AFM). METHOD: AFM elastic mapping was performed on cryosections of human cartilage harvested from both condyles of macroscopically normal and osteoarthritic knee joints. This method was used to test the hypotheses that both ECM and PCM regions exhibit a loss of mechanical properties with OA and that the size of the PCM is enlarged in OA cartilage as compared to normal tissue. RESULTS: Significant decreases were observed in both ECM and PCM moduli of 45% and 30%, respectively, on the medial condyle of OA knee joints as compared to cartilage from macroscopically normal joints. Enlargement of the PCM, as measured biomechanically, was also observed in medial condyle OA cartilage, reflecting the underlying distribution of type VI collagen in the region. No significant differences were observed in elastic moduli or their spatial distribution on the lateral condyle between normal and OA joints. CONCLUSION: Our findings provide new evidence of significant site-specific degenerative changes in the chondrocyte micromechanical environment with OA.

Viscoelastic properties of zonal articular chondrocytes measured by atomic force microscopy.

OBJECTIVE: Articular chondrocytes respond to chemical and mechanical signals depending on their zone of origin with respect to distance from the tissue surface. However, little is known of the zonal variations in cellular mechanical properties in cartilage. The goal of this study was to determine the zonal variations in the elastic and viscoelastic properties of porcine chondrocytes using atomic force microscopy (AFM), and to validate this method against micropipette aspiration. METHODS: A theoretical solution for stress relaxation of a viscoelastic, incompressible, isotropic surface indented with a hard, spherical indenter (5 microm diameter) was derived and fit to experimental stress-relaxation data for AFM indentation of chondrocytes isolated from the superficial or middle/deep zones of cartilage. RESULTS: The instantaneous moduli of chondrocytes were 0.55+/-0.23 kPa for superficial cells (S) and 0.29+/-0.14 kPa for middle/deep cells (M/D) (P<0.0001), and the relaxed moduli were 0.31+/-0.15 kPa (S) and 0.17+/-0.09 kPa (M/D) (P<0.0001). The apparent viscosities were 1.15+/-0.66 kPas (S) and 0.61+/-0.69 kPa-s (M/D) (P<0.0001). Results from the micropipette aspiration test showed similar cell moduli but higher apparent viscosities, indicating that mechanical properties measured by these two techniques are similar. CONCLUSION: Our findings suggest that chondrocyte biomechanical properties differ significantly with the zone of origin, consistent with previous studies showing zonal differences in chondrocyte biosynthetic activity and gene expression. Given the versatility and dynamic testing capabilities of AFM, the ability to conduct stress-relaxation measurements using this technique may provide further insight into the viscoelastic properties of isolated cells.

Normal Forces between Cellulose Surfaces Measured with Colloidal Probe Microscopy.

Colloidal probe microscopy was employed to study interactions between cellulose surfaces in aqueous solutions. Hydrodynamic forces must be accounted for in data analysis. Long-range interactions betweeen cellulose surfaces are governed by double-layer forces and, once surfaces contact, by osmotic repulsive forces and viscoelasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage. The conformation of adsorbed polyelectrolytes depends on the polymer concentration. Copyright 2000 Academic Press.