Direct detection of aggregates in highly turbid colloidal suspensions of polystyrene nanoparticles.

We demonstrate a total internal reflection-based method that detects, for the first time to the best of our knowledge, directly without any sample dilution or special sample preparation, the presence of aggregates in highly turbid aqueous suspensions of polystyrene nanospheres. Aggregation is induced by changing either the sample pH or ionic strength. The polystyrene mass density in our samples is two orders of magnitude higher than previously reported polystyrene aggregation studies. In cases when aggregates have formed but do not yet occupy a significant fraction of the sample volume, our sensor outperforms state of the art techniques such as dynamic light scattering in terms of sensitivity. Conversely, when the sample volume is dominated by aggregates, our sensor is not as effective.

Metabolic selectivity and growth of Clostridium thermocellum in continuous culture under elevated hydrostatic pressure.

The continuous culture of Clostridium thermocellum, a thermophilic bacterium capable of producing ethanol from cellulosic material, is demonstrated at elevated hydrostatic pressure (7.0 MPa, 17.3 MPa) and compared with cultures at atmospheric pressure. A commercial limitation of ethanol production by C. thermocellum is low ethanol yield due to the formation of organic acids (acetate, lactate). At elevated hydrostatic pressure, ethanol:acetate (E/A) ratios increased >10(2) relative to atmospheric pressure. Cell growth was inhibited by approximately 40% and 60% for incubations at 7.0 MPa and 17.3 MPa, respectively, relative to continuous culture at atmospheric pressure. A decrease in the theoretical maximum growth yield and an increase in the maintenance coefficient indicated that more cellobiose and ATP are channeled towards maintaining cellular function in pressurized cultures. Shifts in product selectivity toward ethanol are consistent with previous observations of hydrostatic pressure effects in batch cultures. The results are partially attributed to the increasing concentration of dissolved product gases (H2, CO2) with increasing pressure; and they highlight the utility of continuous culture experiments for the quantification of the complex role of dissolved gas and pressure effects on metabolic activity.

Toxicity effects of compressed and supercritical solvents on thermophilic microbial metabolism.

Selection of biocompatible solvents is critical when designing bioprocessing applications for the in situ biphasic extraction of metabolic end-products. The prediction of the biocompatibility of supercritical and compressed solvents is more complicated than for liquid solvents, because their properties can change significantly with pressure and temperature. The activity of the anaerobic thermophilic bacterium, Clostridium thermocellum, was studied when the organism was incubated in the presence of compressed nitrogen, ethane, and propane at 333 K and multiple pressures. The metabolic activity of the organisms in contact with compressed solvents was analyzed using traditional indicators of solvent biocompatibility, such as log P, interfacial tension, and solvent density. The toxicity of the compressed solvents was compared with the phase and molecular toxicity effects measured in liquid alkanes at atmospheric pressure. Inactivation increased with time in the presence of the compressed solvents, but was constant in the presence of atmospheric liquid solvents. Knowledge of molecular and phase toxicity provides a framework for the interpretation of C. thermocellum metabolism in contact with atmospheric and compressed solvents.