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ZetaView® – Nanoparticle Tracking Analyzer

ZetaView® – Nanoparticle Tracking Analyzer for hydrodynamic Particle Size, Zeta Potential and Concentration

ZetaView

First principal electrophoresis zeta potential and Brownian diffusion size

With ZetaView® individual particle tracking, classical micro-electrophoresis and Brownian motion have a modern analysis tool. Auto-alignment and auto-focussing make the “Seeing is Believing” principle alive. Highly resolved zeta potential- and size- histograms as well as profiles are derived from thousands of particles in a routine way. In addition, low particle concentrations can be determined. A separate model with a short wavelength laser is available for discrimination of fluorescent particles. The visualisation in general contributes to analysis certainty and to new discoveries.

The main features of the ZetaView®

Automation – In the past, micro-electrophoresis arrangements were tedious to work with. Alignment often exceeded measurement time. With ZetaView® Laser Scattering Video Microscope, the benefits of individual particle tracking can be extended and fully appreciated.

Passive stability elements – A patented 5 point support of the cell assembly guarantees excellent image sharpness stability. Unsymmetrical cell coating by sedimentation is avoided by the vertical orientation of those cell walls facing the microscope (figure 1).

Impressions on nano-systems – Particles < 10 nm do not scatter enough light to measure electrophoresis. However, as the few agglomerates of them tend to have the same zeta potential, but are not abundant enough for conventional LDE Laser Doppler Electrophoresis, the ZetaView® shows its qualities in this situation. Furthermore, in some applications as milk, it may be useful to discriminate between spherical, irregular or outranging particles.

Theory – The Smoluchowski formula is applied to calculate the zeta potential histogram from the measured electrophoretic spectrum. A provision is made for other than Smoluchoski’s relation. The diffusion constant is calculated from the direct observation of Brownian motion and transferred into a size histogram via the Einstein Stokes relation between diffusion constant and particle size.

Range – Depending on the model, the direct tracking of particles is possible in a size range starting at 10 nm for silver or gold and correspondingly higher for particles with less scattering power. As long as particles do not sediment or float they can be tracked for analysis. For electrophoresis the upper size limit can be 50 µm. For diffusion size the upper limit is 1-3 µm.

Concentration and particle counting – As a minimum concentration, 106 particles per cm³ are needed. The maximum is 1010 p/cm³. For higher concentrations, dilution or optical filtering is required. In volume concentration, up to 1000 ppm of 200 nm sized particles have been measured.

Accuracy and precision – For zeta potential, the accuracy is 4 mV, the precision 2 mV, instrument to instrument repeatability 2 mV. In size determination of a 100 nm standard latex suspension accuracy is 6 nm in number, precision 2 nm, instrument to instrument repeatability 4 nm. At a concentration of 10 Million of 100 nm particles per mL the  accuracy is 1 Mio / mL, the precision 1 Mio / mL. The repeatability from instrument to instrument is 1 Mio / mL. All given data are valid with provision of correct camera settings.

 

The Method

The fine laser beam illuminates the particles making them scatter light. As we can see stars due to their emitted light, we see particles with size 100 times below the diffraction limit.

ZetaView Measurement Principle
Fig. 1: Optical Layout
Schematics of the laser light scattering microscope set-up with the electrophoresis cell in cross section, the microscope objective and the video camera.

On the left side of figure1, the cross section of the cell is seen  in  direction  of  the  electrodes.  The  image  of  the particles is focused onto the video camera. From measuring the particle velocity and direction under an applied electric field, the electrophoretic mobility and polarity of charge are determined. With no field taken into account, only Brownian motion  is  determined.  Electric  field,  temperature  and conductivity are measured at each experiment. By scanning through the cell a profile measurement may be taken at 11 different  locations  The  results  in  nanoparticle  size, concentration and zeta potential are averaged. Cleaning of the electrodes is rarely necessary, if at all.

Applications zeta potential

Wall coating – From the curvature of the electro-phoresis velocity profile, the amount of ionic coating on the cell walls and the polarity is caluculated in most cases for the purpose of measurement quality (Fig.2). The correctness of the electrophoresis zeta potential determination is not influenced by the wall coating at all.

ZetaView wall coating
Fig. 2:
Dependence of the curvature of the velocity (combined electroosmosis and electrophoresis mobility) from the ionic state of the cell walls. Green: +50 mV Al2O3, walls are cationically coated. Red: Anionic polysterene, -25 mV zeta potential; the walls are neutral. Blue: polysterene -40 mV, cell walls are uncoated and anionic. The profiles are taken automatically.

High resolution zeta potential – Individual particle tracking offers high resolution. In general, chemically homogeneous samples reveal a narrow zeta potential distribution. Otherwise, broader distributions or side peaks may show up (Fig. 3).

ZetaView - high resolution
Fig. 3: 150 nm polystyrene sample. Blue: pH=3.3; red: pH= 7.0; green: pH=10.2. First of all, the sample is strongly pH dependent; second, the side peak present in the sour region up to pH=7 has disappeared at pH = 10.2.

Diffusion size distribution

From Brownian motion the diffusion constant is tracked for every particle and calculated into size by respecting the temperature and viscosity of the fluid. Like for electrophoresis, the temperature is constantly monitored. Size distributions of silver and gold nano-particles are shown in figure 4.

ZetaView Diffusion Size Distribution
Fig. 4: Size distribution from highly diluted gold and silver colloid, showing agglomerates. Gold 50 nm was measured with a 650 nm red laser, silver 1.7 nm (verified with 180° DLS Nanotrac® in original concentration) with a 405 nm blue laser.

 

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