At first, the C-V curves of EIS structures were measured in various standard pH buffer solutions ranging from pH 2 to pH 12. The real pH value was determined using a commercial pH electrode (S120C, Sensorex, Garden Grove, CA, USA) and a pH meter (HTC-201U, HOTEC, Newton, MA, USA) before measurements. The pH sensitivity was calculated from the slope of output voltage, which is obtained at the 0.6 Cmax of the normalized C-V curves. The dependences of the calculated pH sensitivity and linearity of the ALD-HfO2
-EIS structures with different thicknesses of HfO2
layers are exhibited in Figure . For the thickness lower than 10 nm, the pH sensitivity is around 40 to 45 mV/pH, and for the 3.5-nm-thick ALD-HfO2
-EIS structure, the available pH range is only from pH 4 to pH 12. Figure shows the normalized C-V curves of 3.5-nm-thick ALD-HfO2
-EIS structures, which were measured at pH 2 to pH 12. In this case, the C-V curve measured at pH 2 represents an unstable response in the accumulation region. It could be the result of the leakage current due to its flimsy thickness. The pH sensitivity is high enough (54 mV/pH) and stable when the thickness of HfO2
layers is higher than 15 nm. As compared to our previous study, the drift coefficient of the ALD-HfO2
-EIS is stable and quite low (< 0.2 mV/h) when the thickness of the ALD-HfO2
film decreases. However, for the sputtered HfO2
-EIS, the drift coefficient increases when the thickness of the sputtered HfO2
film decreases [24
]. It could be that the thin HfO2
film prepared by ALD was much denser than that deposited by sputtering [25
Figure 2 pH Sensitivity and linearity characteristics and normalized C-V curves. (a) pH sensitivity and linearity characteristics of the ALD-HfO2-EIS devices with various HfO2 thicknesses. (b) Normalized C-V curves for the ALD-HfO2 and 3.5-nm-thick ALD-HfO2-EIS (more ...)
Considering the application on biomedical sensors, the stacked structure of 15-nm-thick HfO2
/Si EIS was used. After the urease was immobilized on the surface of HfO2
layers with NH3
plasma post-treatment or the conventional silanization method, the HfO2
-EIS structures were immersed into the PB solutions with different concentrations of urea. As shown in Figure , the output voltage of the HfO2
-EIS structure with plasma treatment is similar to the response of the samples with chemical procedures, where the urea sensitivity are 105 ± 15 and 117 ± 9 mV/pUrea, respectively. The sensitivity value was the average value of five results. The C-V curves and the voltage shift in a linear range of these two methods are almost the same. The linearity of the calibration curves for both output voltages are very high and very suitable for physiological detection [15
]. Based on these results, the chemical silanization method for urease immobilization is successfully replaced by remote NH3
plasma treatment, which has advantages of improving process safety, reducing environmental pollution, and lessening the process time. In addition, comparing the two methods, processing time can be reduced by almost 24 h by remote NH3
The urea detection of ALD-HfO2-EIS structure with chemical silanization and NH3 plasma treatment.
Moreover, the replacement of silanization procedure using NH3 plasma was also performed on the immobilization process of anti-BSA. Figure shows that the response signal of the EIS membrane without any modification is 6.4 mV. The detection responses of chemical silanization and NH3 plasma treatment are 16.8 and 19.9 mV, respectively. The result indicates that the response of remote NH3 plasma is much higher than that of without plasma treatment. The results show that the NH3 plasma treatment is suitable and attractive for bio-sensing application.
The BSA detection response of ALD-HfO2-EIS structure. (a) Without any treatment, (b) with chemical silanization treatment, and (c) with NH3 plasma treatment.