Simple frequency-based sensing of viscosity and dielectric properties of a liquid using acoustic resonators.

The effects of a finite polarizability of a liquid sample onto the series resonance frequency of a piezoelectric resonator are described within the small-load approximation. It is found that the sample's electrical capacitance (the ratio of surface potential and surface polarization at the crystal surface) enters the motional branch of the equivalent circuit, thereby shifting the series resonance frequency. This effect is caused by piezoelectric stiffening. Using a conventional quartz crystal with small hole in the front electrode and exposing this crystal to a variety of different liquids, we demonstrate that derived equation describes the experiment reasonably well. Electric and dielectric effects are particularly strong for torsional resonators, which operate at a frequency of around 56 kHz. These are in commercial use for determination of the viscosity of engine oils. We propose a simple method to in situ switch the strength of piezoelectric stiffening. It allows separation of the viscoelastic parameters from the electric polarizability. It could, for instance, be used to differentiate between gasoline and water being accidentally admitted to the lubricant reservoir. The instrument requires a single resonator and is based on the determination of only 2 frequency shifts.

Electrophoretic properties of BSA-coated quantum dots.

Low toxic InP/ZnS quantum dots (QDs), ZnS:Mn(2+)/ZnS nanocrystals and CdSe/ZnS nanoparticles were rendered water-dispersible by different ligand-exchange methods. Eventually, they were coated with bovine serum albumin (BSA) as a model protein. All particles were characterised by isotachophoresis (ITP), laser Doppler velocimetry (LDV) and agarose gel electrophoresis. It was found that the electrophoretic mobility and colloidal stability of ZnS:Mn(2+)/ZnS and CdSe/ZnS nanoparticles, which bore short-chain surface ligands, was primarily governed by charges on the nanoparticles, whereas InP/ZnS nanocrystals were not charged per se. BSA-coated nanoparticles showed lower electrophoretic mobility, which was attributed to their larger size and smaller overall charge. However, these particles were colloidally stable. This stability was probably caused by steric stabilisation of the BSA coating.

Calibration-free concentration determination of charged colloidal nanoparticles and determination of effective charges by capillary isotachophoresis.

Although colloidal nanoparticles show an electrophoretic heterogeneity under the conditions of capillary electrophoresis, which can be either due to the particle-size distribution and/or the particle shape distribution and/or the zeta-potential distribution, they can form correct isotachophoretic zones with sharp-moving boundaries. Therefore, the technique of isotachophoresis permits to generate plugs in which the co-ions and counter ions of the original colloidal solution are removed and replaced by the buffering counter ions of the leading electrolyte. It is shown that analytical isotachophoresis can be used to measure directly, without calibration, the molar (particle) concentration of dispersed ionic colloids provided that the transference number and the mean effective charge number of the particles (within the isotachophoretic zone) can be determined with adequate accuracy. The method can also be used to measure directly the effective charge number of biomacromolecules or colloidal particles, if solutions with known molar (particle) concentration can be prepared. The validity of the approach was confirmed for a model solution containing a known molar concentration of bovine serum albumin.

Determination of defect states in semiconductor nanocrystals by cyclic voltammetry.

Colloidal, monodisperse CdSe nanocrystals were homogeneously dispersed in an ionic liquid and investigated by means of cyclic voltammetry. Almost all known defect states in semiconductor nanocrystal were quantitatively measured with this nonoptical method (including nonradiative defect states). Variation of the illumination and temperature resulted in excitation of defect-trapped electrons into the conducting band. Thus, we succeeded for the first time to correlate defect states in nanocrystals with those in the corresponding bulk crystals.