Results and discussions

5.3 Results and discussions

Figures 5-2 (a) and (b) show the pH and pNa sensitivities of the sole ZrO2 membrane and with Nafion coating, respectively. The pH and pNa sensitivities of the ZrO2 membrane with and without Nafion coating are the same. According to the study of Gorchkov [5.24], the pH response of an ISFET would be influenced by the natures of the polymeric membranes, as the consequence of the “equivalent membrane charge.” Therefore, the slightly higher sensitivity of the ZrO2 membrane with the Nafion coating could be explained as extra cations stored in the Nafion membrane that leads to a surface potential increase. However, the effect is limited. Meanwhile, Nafion is a perfluorinated polymer that can be divided into three parts: a hydrophobic fluorocarbon backbone C–F, an interfacial region of relatively large fractional void volume, and clustered regions where the majority of the ionic exchange sites, counter ions, and absorbed water exist [5.25, 5.26].

Accordingly, the Nafion membrane was a low impedance film, which unblocks the hydrogen and sodium ions; those ions from the electrolyte can pass through the Nafion membrane with a low resist and an established potential on the ZrO surface. The pNa

sensitivity was much lower than the pH sensitivity. The result reveals that it was mainly dominated by the selectivity property of the ZrO2 film, though the fact that the mobility of H⁺ was about 5 times that of Na⁺ in the Nafion membrane [5.27]. The selectivity coefficient can be determined by the methods based on the Nicolsky–Eisenman equation [5.28]. Meanwhile, the linearity of the pNa response was not as good as the pH response.

The difference might be that the Nafion membrane has a slightly higher affinity for Na⁺ than for H⁺ [5.27], which affects the interface potential between the Nafion membrane and the ZrO2 film. As shown in Figure 5-2 (c), the IDS-VGS curves were similar for both membranes except the threshold voltage shift; it revealed that ZrO2 films with or without Nafion coating have identical electrical characteristics. The REFETs produced by additional membrane coating on the top of the ISFETs should maintain the original electrical properties, such as transconductance, for differential measurements.

Table 5-1 and Figure 5-3 show the measurement results of pH and pNa sensitivities for the tested conditions. Sensitivities to the hydrogen ions for all tested polymer-based REFETs were below 10 mV/pH except P3HT without HMDS pretreatment. However, pH sensitivities of all the REFETs were reduced. The reasons for the dramatic pH sensitivity decrease can be explained by employing a site-binding model [5.29, 5.30]. The pH sensitivity of an ISFET is expressed in Equations 4-1 and 4-2. According to the equations, large values of C_dif /β_int would result in a lower pH sensitivity. This means that surfaces with small buffer capacity (small value of surface reactive sites and of surface charges) show low pH sensitivity. For the double-layer polymer/Nafion as shown in Figure 5-1 (a), the sensing layers were PR and P3HT, whose surface site densities are smaller than ZrO2; therefore, the pH sensitivities were low. For the single-layer PR/Nafion composite, as shown in Figure 5-1 (b), it was long chain hydrocarbon molecules which have the characteristic of a small buffer capacity and result in a dramatic decrease in the pH

sensitivities. For the single-layer P3HT/Nafion composite, because P3HT is the conductive polymer that provides extra paths for electrons to pass through the membrane, the pH sensitivity was higher than that of the PR/Nafion composite.

HMDS is an adhesion promoter for hydrophobic polymers that attempt to attach on hydrophilic inorganic films. By comparing the pH sensitivity of the tested composite films 2 and 3, 4 and 5, and 6 and 7, all films with the HMDS layer have less sensitivity variation than those without the HMDS layer. It shows that the additional deposition of HMDS improved the stability of test films. Polymerbased HMDS also reduced the pH sensitivity.

Figure 5-6 shows the Ids-Vgs curves of the HMDS coated ZrO2 membrane. The sensitivity was decreased from 57.89 to 37.22 mV/pH, and the coating of HMDS caused more than a 35% reduction in sensitivity. Accordingly, the insensitivity character of REFET was not only dominated by the entrapped hydrophobic polymers but also contributed by HMDS. It was evident by comparing films 2 and 3, 4 and 5, and 6 and 7 that the films with the HMDS layer obtained lower pH sensitivity than those without the HMDS layer.

Among all the test films, the combination of Nafion-PR composite/HMDS has the lowest pH sensitivity, and the similar pNa response as the ZrO2 film, as shown in Figure 5-5, demonstrated the capability to serve as a REFET. Comparing with Figure 5-2 (b), the linearity became worse for the pH response but became better for pNa. The reasons for both cases are different. For pH, it is because the measured potential range was highly reduced that the measurement error becomes more significant. For pNa, a possible reason is that the property of the Nafion film was altered by the mixed composition, which restricted the transport of Na+ and the potential variation and, therefore, became more stable.

Figure 5-6 shows the differential result of the pH and pNa measurements for the ISFET/REFET arrangement; responses with more than 50 mV/pH sensitivity and less than 5 mV/pNa interference can be obtained in the range of pH 1–13. Polymers entrapped in

Nafion play the role of blocking the most paths of ion exchange to achieve a low ion sensitivity requirement for REFET. Nevertheless, the induced impedance change by the alteration of the overall membrane composition should be considered. Figure 5-7 describes the transconductance (gm) curves of the ISFET, the proposed REFET, and the REFET with epoxy coating membranes. The gm curve with epoxy-REFET shows different behaviors from the other two. On the other hand, the curves of the proposed REFET and ISFET show similar curvatures in the linear region (Vgs-Vth: 0 – 1.2 V). The gm curves represent the signal transformation characteristics of the device. An effective differential measurement requires similar gm curvatures for ISFET and REFET in pair. Otherwise, mismatch issues would occur. Because the epoxy is an ion-blocking membrane that causes a polarizable interface whose potential difference changes as a consequence of the potential difference across the whole system, it would alter the original electrical properties of an ISFET.

Consequently, the electrical mismatch of the ISFET/REFET differential pair would result in the common mode noises, which cannot be entirely eliminated. On the contrary, an ion-unblocking REFET as proposed causes a nonpolarizable interface, whose potential difference across it is virtually fixed. Meanwhile, it maintains the electrical properties of the original film and makes the ISFET/REFET differential pair match, which can easily eliminate the common mode noises with simple readout circuits. The gm of the ZrO2

membrane was slightly altered due to the addition of PR; however, the electrical characteristic in most linear regions was not altered accordingly, which provided an operating region for the ISFET/REFET pair in wide dynamic pH measurements.

In this experiment, it was found that the measurement could not be conducted without the Nafion protection because P3HT and PR alone would dissolve in the buffer solution within a very short time period. The drift performances of the Nafion-PR composite/HMDS film were 3.5 mV/h and less than 1 mV/h for the first and second 4 h tests, as shown in

Figure 5-8. It demonstrated that the incorporation of the Nafion enhanced the stability of the proposed REFET.