The superposition of the second crystal oscillation

The superposition of the second crystal oscillation during physical contact between the sphere and surface is more pronounced for lower modulus surface, such as PTFE as demonstrated in Fig. 6. For example, the impact of a polypropylene sphere against the PTFE surface, the second crystal oscillation peak is very evident at around 160μs, which also is approximately the time that the sphere rebounds off of the surface according to the HSD theory. For the case of a PTFE sphere and a PTFE surface, the crystal appears to nearly complete the second oscillation before the sphere leaves the surface, which creates a very broad, but nearly symmetric transient signal. The estimated HSD and KV model parameters obtained by fitting the measured signals are listed in Table 3. The average residual error of the fits is greater for the HSD model compared to the KV model. Moreover, the residual error of the fits for both models typically increases as the average modulus of the surface and sphere decreases. It is interesting to note that the regression values of the crystal parameters appear physically plausible based on literature for quartz dynamic load cells. Using the mean values for the model parameters, the modulus and Poisson\'s ratio for a series of sphere and surface combinations is estimated from the experimental data, and are listed in Tables 4 and 5, for the HSD and KV models, respectively. Comparing the predicted Young\'s elastic modulus in Tables 5, and 4, with the literature values in Tables 1 and 2 it is evident that the Young\'s elastic modulus predicted by the KV model is more accurate than the HSD model. Moreover, the R2 correlation coefficient between measurement and theory, is much greater for the HSD model (0.48) compared to the KV model (0.86). The KV model also results in less variation in the prediction of the sphere modulus for a given surface compared to the HSD model. For example, the predicted modulus for the acetal spheres against the four surfaces ranges from 4.18GPa to 6.85GPa using the HSD model, and from 3.16GPa to 3.91GPa for the KV model. Similarly, the KV model has less prediction variability than the HSD model for a particular surface using different spheres.
Conclusions
Introduction Reduction reactions of enzymes and other biological macromolecules at an electrode are usually very slow because of slow order thapsigargin transfer [1]. This problem can be overcome by using small molecule as an electron transfer mediator with rapid and reversible reduction capability. The reduced form of a mediator reacts with the enzyme rapidly to reduce it while the mediator is oxidized. Mediators enhance the electron transfer rate due to their higher mobility. Many electrochemical systems have been developed with various mediators. Biosensors, especially amperometric sensors, which consist of oxidase and peroxidase enzymes, have gained great interest in recent years. In many amperometric biosensors, hydroquinone (HQ) is often used as one of the most common mediators. It is oxidized to benzoquinone while reducing enzymes. HQ facilitates the transfer of electrons that are generated as a result of the redox reaction [9,15] between the redox center of horseradish peroxidase (HRP) and the electrode Although redox mediators show good electron transfer properties for electrochemical sensors, it should be noted that they are electroactive compounds and therefore may interfere with the redox reactions of interest and affect the response with noise. This interference compromises the lower detection limit of sensors. In addition, mediators may contaminate the samples and may be toxic to the sensing elements. Therefore, immobilization of enzyme on an electrode modified with conductive materials has attracted much attention over the past years. These systems facilitate direct electron transfer between the electrode and enzyme without the use of chemical mediators suspended in a liquid phase. Coche-Guérente et al. immobilized horseradish peroxidase (HRP), glucose oxidase (GOD) and graphite nanoparticles in a silica gel matrix to detect hydrogen peroxide [2]. Dispersed graphite particles in the matrix provided a connection between the electroactive site of HRP and the electrode surface. Ivanova et al. [7] used a carbon paste electrode containing ruthenium complexes as mediator for electron transfer from enzymes which were immobilized in three different ways: inside the carbon paste, at the surface and inside a Nafion film. They used this sensor architecture for amperometric oxidation of nicotinamide adenine dinucleotide and showed that electrochemical properties of the sensor strongly depended on the method of immobilization. Liu et al. also developed a mediatorless biosensor using HRP immobilized on a colloidal gold-modified carbon paste electrode for detection of hydrogen peroxide. They reported fast response and acceptable reproducibility of the sensor [10]. Recently Gu et al. used a mixture of HRP, DNA, chitosan and iron oxide magnetic nanoparticles that were deposited on a glassy carbon electrode. Their result showed that the mixture exhibited high sensitivity towards hydrogen peroxide without mediators [5].