e measurement of the same variable on a collection of particles provided the system considered is in a state of equilibrium. However, as Krger points out, "although diffusive processes have been investigated for the past 150 years, the principle of ergodicity has not yet been experimentally verified." This is because it has so far been possible to quantify diffusive processes only by means of ensemble measurements i.e. measurements of many particles simultaneously. One of the most informative methods for this purpose is pulsed-field gradient nuclear magnetic resonance (PFG-NMR), a technique for which Krger and his group are well known. The actual trajectory of a single particle, on the other hand, could not be observed directly. "With the development of single-molecule spectroscopy and single-molecule microscopy, we can now follow the trajectories and therefore monitor the diffusion behavior of single molecules," Bruchle explains. Optical tracking methods visualize molecules on the basis of their fluorescence, making it possible for their positions to be localized and monitored with a precision of a few nanometers.
This still leaves one problem to be solved - successful application of the two methods requires very different, indeed apparently conflicting, conditions. NMR measurements need high concentrations of molecules with large diffusion coefficients, while single-molecule spectroscopy works best with extremely dilute solutions of species with small diffusion coefficients. By using particular organic dyes with high fluorescence yields in combination with porous silicate glasses containing networks of nanometer-sized channels in which the dye molecules can diffuse, the researchers were able to create conditions that were compatible with both methods. This experimental set-up allowed them to perform single-molecule and ensemble measurements on the same system.
When the two teams compared their data, they found that the diffusion coefficientsPage: 1 2 3 Related biology news :1
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