Visualization of Ion Distribution by the Chemical Imaging Sensor

For the visualization of chemical species in the solution, markers and labels such as fluorescent dyes are often utilized. For certain applications in biology, however, toxicity of such dyes may be a problem, especially in the case of cells and tissues in culture. In this study, a label-free method of ion imaging based on a semiconductor device is developed. Chemical sensors based on semiconductor devices are advantageous for miniaturization, integration with the peripheral circuits and fabrication of various structures on their surfaces with the help of microfabrication techniques such as photolithography. A well-known semiconductor-based chemical sensor is the ion-sensitive field-effect transistor (ISFET) [1,2], which has the field effect structure or the electrolyteinsulator-semiconductor (EIS) structure. The ISFET is put to practical use, for example, in a portable pH meter. The EIS capacitive sensor [3] and the lightaddressable potentiometric sensor (LAPS) [4] are other examples of chemical sensors based on the same EIS structure. Figure 1 schematically compares the structures of these three semiconductor-based chemical sensors. Chemical sensors based on the EIS structure detect the change of the distribution of carriers in the semiconductor layer through the field effect, which responds to the change of the ion concentration of the solution in contact with the sensing surface of the insulating layer. The ISFET detects the change of the conductance of the channel between the source and the drain electrodes, whereas the EIS capacitive sensor and the LAPS detect the change of the capacitance of the depletion layer at the semiconductor-insulator interface. In the case of p-type semiconductor, the depletion layer grows thicker and the capacitance of the depletion layer becomes smaller when the sensing surface is more positively charged. In the LAPS measurement, a photocurrent is generated to detect the change of the capacitance of the depletion layer. The back surface of the sensor plate is illuminated with a light beam modulated at a frequency of several kHz, and the amplitude of the ac photocurrent is measured as a function of the bias voltage applied to the EIS system. Figure 2(a) shows typical current-voltage characteristics of a pH-sensitive LAPS with a Ta2O5 film as the insulating layer. A positive bias (applied to the solution with respect to the semiconductor substrate) in Fig. 2(a) corresponds to the depletion and inversion states of the semiconductor layer, and a negative bias corresponds to the accumulation state. In Fig. 2(b), the bias voltage at the inflection point of the current-voltage curve is plotted as a function of the pH value, showing the pH sensitivity of 57.9 mV/pH. The measuring area of a LAPS sensor is restricted to the illuminated area, and therefore, many measuring spots can be defined on the sensing surface of a single sensor plate.