Browsing by Subject "water"

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Open Access
Kinetic Parameters from Detection Probability in Single Molecule Force Spectroscopy
(2010) Ray, Chad; Guo, Senli; Brown, Jason; Li, Nan; Akhremitchev, Boris B
The detection probability of rupture events in A FM force spectroscopy measurements presents a viable alternative to standard methods for extracting kinetic parameters of dissociation. The detection probability has a maximum as a function of the probe velocity where (1) the probability to form a molecular bond is independent of the probe velocity and (2) the detection of rupture events is limited by noise and performed with a constant density of data points per distance of the probe displacement. This newly developed model indicates that the optimal detection velocity is independent of dissociation rate and depends on the distance to the barrier kinetic parameter. Therefore, the kinetic parameters of bond dissociation can be extracted from the dependence of detection probability on probe velocity and the detection threshold. This approach is sensitive to low rupture forces and therefore is complementary to the common most probable force data analysis approach. The developed approach is tested using rupture forces measured with specific bonds between biotin and streptavidin and with nonspecific bonds between linear alkalies in water. Results for the analysis of specific bonds rupture are consistent with the previous measurements, suggesting that rupture forces spanning a wide range of values originate from the same binding potential. Kinetic parameters obtained for linear alkalies are significantly different from previous measurements suggesting possible heterogeneity of the bound state.
Open Access
Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State
(2010) Hotze, Ernest M; Bottero, Jean-Yves; Wiesner, Mark
Theory is developed that relates the reactivity of nanoparticles to the structure of aggregates they may form in suspensions. This theory is applied to consider the case of reactive oxygen species (ROS) generation by photosensitization of C-60 fullerenes. Variations in aggregate structure and size appear to account for an apparent paradox in ROS generation as calculated using values for the photochemical kinetics of fullerene (C-60) and its hydroxylated derivative, fullerol (C-60(OH)(22-24)) and assuming that structure varies between compact and fractal objects. A region of aggregation-suppressed ROS production is identified where interactions between the particles in compact aggregates dominate the singlet oxygen production. Intrinsic kinetic properties dominate when aggregates are small and/or are characterized by low fractal dimensions. Pseudoglobal sensitivity analysis of model input variables verifies that fractal dimension, and by extension aggregation state, is the most sensitive model parameter when kinetics are well-known. This theoretical framework qualitatively predicts ROS production by fullerol suspensions 2 orders of magnitude higher compared with aggregates of largely undifferentiated C-60 despite nearly an order of magnitude higher quantum yield for the undifferentiated C-60 based on measurements for single molecules. Similar to C-60, other primary nanoparticles will exist as aggregates in many environmental and laboratory suspensions. This work provides a theoretical basis for understanding how the structure of nanoparticle aggregates may affect their reactivity.