The comparison of OWLS and QCM methods

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Supervisor:
Dr. Ender Ferenc
Department of Electron Devices

My thesis is about the building and measurement of polyelectrolite layers. My goal is to make comparsion monitoring with two measurement methods popular in the industry and research. Comparing the results of multiple measurement methods of the polyelectrolite stratification was dealt with by numerous research groups and is an actively investigated field of study at present. My work in this semester is also supposed to demonstrate this by OWS and QCM methods, which methods have not been compared yet with polyeletcrolites.

My work has started with demonstrating the literature of the two methods, furthermore I gained knowledge from the published experiments. This knowledge was the basis for my experiments, which I have accomplished with solutions made from PLL and PSS polyelectrolites.

Theory

OWLS

Optical waveguide spectroscopy is based on grating-assisted coupling of light into and guidance within an optical waveguide layer. By varying the angle of the incident light beam, different guided transverse electric (TE) and transverse magnetic (TM) modes can be excited. If the waveguide is coupled to a liquid cell designed for adsorption studies, the in-coupling angle of the incident light is sensitive to the refractive index and the thickness of the adsorbed

film. Provided that the refractive index of the polyelectrolyte film is known, the film thickness can be calculated.

QCM

The method is based on the piezoelectric properties of quartz crystals. If there is AC voltage applied to the conductor layers between the two sides of the crystal chip, the crystal vibrates. The frequency of the vibration depends on the mass of the crystal. If adsorption occours on the surface, the change of mass cause a change of the resonance frequency. The connection between the time dependency of the mass of the adherent layer and the frequency is described by the Sauerbre-equation:

Δm=C/n*Δf

Δm change of mass

Experimental

6 layers of PLL and PSS were built up on ITO coated OWLS and on Cr/Au SiO2 QCM sensor with PLL as the first layer. Both polyelectrolyte were dissolved in 10 mM HEPES to a concentration of 0.1-0.5 g/L. Also 10mM HEPES was used as rising buffer between each deposition. A flow rate of 50 L/min was used for all steps in the measurements.

Result and discussion

After reaching the plateau, PLL and PSS solutions were injected every 10 minutes. As Figure 2 shows, by the end of the OWLs and QCM measurements, the polyelectrolyte stratification, the adsorption had happened. By 2a, the polyelectrolyte steps can be seen, under 0.3 g/L concentration. By booth adhesion, differences can be observed. After the injections the quantity of the PLL is 120 ng, while the PSS is 135 ng. Therefore the built PSS layer is thicker than the PLL. By the other concentrations the measured values were similar. Concentration difference can not be seen. The result of the QCM measurement can be seen on figure 2b. Contrary to the previous methods, the PLL adhesion was more significant, 1061 ng, while the PSS only 858 ng. The ongoing dissipation was also modelled. The quantity depends on the monomentary resonance frequency and on the full width at half maximum.

Conclusion

The injection of PLL and PSS solution were observed. By the built layers, it can be seen, according to the theory, that the amount of adherent mass is less by OWLS measurements than by QCM. The reason of the phenomenon is that by OWLS, the amount of dry mass, and by QCM, the amount of wet mass is measured

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