- Email: [email protected]
Chemical image scanner based on FDM-LAPS
Sensors and Actuators B 137 (2009) 533–538
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Chemical image scanner based on FDM-LAPS夽 K. Miyamoto a,∗ , Yohei Kuwabara a , Shin’ichiro Kanoh a , Tatsuo Yoshinobu a,b , Torsten Wagner b,c,d,e , Michael J. Schöning d,e a
Department of Electronic Engineering, Tohoku University, 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Department of Biomedical Engineering, Tohoku University, 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan c Postdoctral fellow, Japan Society for the Promotion of Science, Japan d Institute of Nano- and Biotechnologies, Aachen University of Applied Sciences, Ginsterweg 1, 52428 Jülich, Germany e Institute of Bio- and Nanosystems (IBN-2), Research Centre Jülich, 52425 Jülich, Germany b
a r t i c l e
i n f o
Article history: Received 15 August 2008 Received in revised form 21 November 2008 Accepted 3 December 2008 Available online 14 December 2008
a b s t r a c t The chemical imaging sensor is a semiconductor-based chemical sensor that can visualize the spatial distribution of chemical species. Although it has been applied to visualization of various specimens, its slow scan rate has been a problem. In this study, a new “chemical image scanner” system based on a linear LED array and frequency division multiplex (FDM) is proposed. The scan time to obtain an ion concentration image at a resolution of 16 pixels × 128 lines was reduced to 6.4 s with the new system. © 2008 Elsevier B.V. All rights reserved.
Keywords: Chemical image scanner Chemical imaging sensor Light-addressable potentiometric sensor LAPS
1. Introduction In the field of semiconductor-based chemical sensors, three types of field-effect chemical sensors with the electrolyte –insulator–semiconductor (EIS) structure [1–3] have been studied so far; (1) Ion-sensitive field-effect transistor (IS-FET) [4,5] is a well-known semiconductor-based chemical sensor, which has been already applied for pH meters. (2) EIS capacitance sensor [6] and (3) light-addressable potentiometric sensor (LAPS) [7–9] are other examples of chemical sensors based on the EIS structure. They are capable of label-free measurement of pH or ions, and 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. In the LAPS measurement, the rear surface of the Si substrate is illuminated with a modulated light and the amplitude of the induced photocurrent is measured as the sensor signal, which varies with the ion concentration of the solution in contact with the sensing surface. Here, the measurement area is defined by illumination. Based on the addressability of LAPS, Lundström et al. obtained
夽 Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-12), July 13–16, 2008, Columbus, OH, USA. ∗ Corresponding author. Tel.: +81 22 795 7076; fax: +81 22 795 7076. E-mail address: [email protected] (K. Miyamoto). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.008
artificial olfactory images with three types of catalytic metals on the sensing surface for the discrimination of gas species [10]. On the other hand, we employed a homogeneous sensing surface for two-dimensional mapping of ion concentration by using a scanning laser beam, and named it the “chemical imaging sensor” [11,12]. The chemical imaging sensor has been applied, for example, to visualization of metabolic activity of Escherichia coli [13] and electrochemical systems [14]. The chemical imaging sensor system demonstrated its effectiveness of the label-free chemical imaging based on LAPS. Due to the measurement in a pixel-by-pixel manner, however, the chemical imaging sensor requires a relatively long time to obtain a chemical image. A typical scan time at a resolution of 128 × 128 pixels is about 3 min when 10 ms of sampling time is spent for each pixel. Such scanning time is too long to observe fast chemical phenomena, thus, the development of a rapid imaging system has been strongly desired. One of the approaches to shorten the scan time is the use of multiple light sources [15,16] and frequency division multiplex (FDM) [17]. When the multiple light sources are modulated at different frequencies, the obtained signal is a superposition of each frequency component, which can be individually extracted by Fourier analysis. Zhang et al. demonstrated a multi-light LAPS using 3 light sources and parallel measurements at 3 different positions on the sensor plate [17]. Wagner et al. reported a platform of multi-light LAPS using 16 light sources and 16 parallel measurements [18], and Hu et al. also reported 3 parallel measurements using an optical chopper
534
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
Fig. 1. A schematic view of the chemical image scanner system based on FDM-LAPS.
blade [19]. These studies suggest the possibility of rapid chemical imaging with multiple light sources. In this study, we propose a new “chemical image scanner” system based on a linear LED array and the principle of FDM-LAPS. Fig. 1 schematically shows the idea of the chemical image scanner. The configuration of the system, which is similar to that of a flat-bedtype image scanner, is suitable for rapid scanning, easy operation and use of large-area sensors. The sample to be measured will be directly put on the sensing surface and the linear LED array moves in the direction perpendicular to the array to scan the sensing area. Using a linear array of 16 LEDs and multi-channel oscillators, the scan time was reduced to 6.4 s at a resolution of 16 pixels × 128 lines. 2. Experimental Fig. 1 shows the system configuration of the chemical image scanner system. The system consists of a sensor plate, a linear array of LEDs as a scanner and a control PC. Fig. 2 shows the details of the scanning system. 2.1. Sensor plate The sensor plate we used in this study is an n-type Si wafer (10–20 cm, 500 m in thickness, 4 in. in diameter, see Fig. 2(a)). The top surface of the sensor plate was covered with 100-nm silicon nitride as a pH-sensitive layer on the insulating layer of 50-nm silicon oxide. The silicon nitride layer was deposited by low-pressure chemical vapor deposition (LP-CVD) following the thermal oxidization of the silicon substrate. Stripe electrodes of gold thin layer (containing 0.5% antimony) were deposited on the polished back surface of silicon substrate to form ohmic contacts. The electrodes were deposited near the perimeters of the sensor plate, so that the measurement area would not be covered. The top surface of the sensor plate is insulated with the thin silicon dioxide and silicon nitride layers. A thin ohmic electrode was deposited on the backside of the sensor plate.
Fig. 2. (a) System configuration of the chemical image scanner. (b) 16-channel oscillator array. The inset figure shows an oscillator circuit board for a single channel.
with an interval of 3.6 mm. These LEDs are surface-mounted-type (Cat. E1S02-4G0A7, TOYODA GOSEI Co., Ltd.) and 1.5 mm × 3 mm in size. The emission from the LED is restricted within 20◦ by a small lens with a diameter of 1 mm. The wavelength and the intensity of illumination are 520–530 nm (half width = 35 nm) and 475 mcd, respectively. The linear LED array is mounted on a one-dimensional scan stage, which moves perpendicular to the array. The driving current of each LED was modulated at each individual frequency by the 16-channel programmable oscillator. The modulated lights from the linear LED array illuminated the backside of the sensor plate during the scan. Fig. 2(b) shows the 16-channel programmable oscillator. The inset figure in Fig. 2(b) shows the oscillator circuit board for a single channel. The circuit includes a programmable waveform generator (AD9833, Analog Device Co.), which generates a sinusoidal wave with high accuracy. The frequency can be programmed with 28-bit resolution, which corresponds to a resolution of 0.055 Hz in case of a 14.7 MHz master clock supplied by a quartz oscillator. The frequency of each channel can be programmed independently by the control PC.
2.2. Linear LED array 2.3. PC and software Instead of the scanning laser beam, a linear LED array is used as the light source in the new chemical image scanner system. As shown in Fig. 2(a), the linear LED array consists of 16 LEDs placed
When the 16 LEDs simultaneously illuminate different positions of the sensor plate, the photocurrent signal is a superposition of 16
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
frequency components;
535
3.2. Rapid scanning of chemical image
16
Iphoto (t) =
an sin(2fn t + n ),
(1)
n=1
where fn and n are the modulation frequency and the phase delay, respectively, of the nth LED. The amplitude an at the frequency fn can be determined as:
an =
2 Tm
2
Tm
Iphoto (t) sin(2fn t) dt 0
+
2 Tm
A chemical image is obtained as a two-dimensional map of the shift of the I–V curve at each pixel in the sensing area. In the chemical imaging sensor, however, as it is too time-consuming to carry out the measurement of I–V curve at each pixel, the bias voltage is fixed at a constant value during the scan and the photocurrent
2
Tm
Iphoto (t) cos(2fn t) dt
,
where Tm is the measurement time. When the interval of the modulation frequency is f, the measurement time Tm must be at least longer than 1/f for the separation of signals. In a typical case, the modulation frequencies of 2000, 2100, . . ., 3500 Hz were chosen with an interval of f = 100 Hz and Tm was set to be 5/f or 50 ms. In our system, the photocurrent signal Iphoto (t) is digitized at a sampling frequency fs = 100 kHz, and the amplitude of each component is calculated by the control PC as follows;
is recorded as a function of the position. The variation of the photocurrent can be converted to the shift of the I–V curve by using the slope of the I–V curve. Even without measuring the I–V curve at each pixel, the chemical imaging sensor is still too slow due to its pixel-by-pixel measurement. In the new system, the multiple points on the scan line are simultaneously illuminated and measured with the linear LED array,
2 N−1 2 N−1 2 2 an = + Iphoto (m t) sin(2fn m t) Iphoto (m t) cos(2fn m t) , N
m=0
(2)
0
N
(3)
m=0
where t is 1/fs and N is the total number of samples. The amplitudes corresponding to 16 modulation frequencies were extracted from the measured photocurrent by the program developed with LabVIEW (National Instruments Corp.). The program sets the modulation frequencies of the LEDs, controls the bias voltage, moves the scanning stage, measures the photocurrent signal with a data acquisition board, and processes the data to display the chemical image. The program has two measurement modes; (1) the I–V measurement mode which measures the I–V curves at 16 different points simultaneously, and (2) the imaging mode which acquires the twodimensional image of ion distribution by scanning the sensing area with the linear LED array. 3. Results and discussion 3.1. Multi-point I–V measurement By using LEDs modulated at different frequencies, ion concentrations can be simultaneously measured at different positions on the sensing surface of a single sensor plate. For the test of simultaneous measurement, a sample well made of PDMS, 60 mm × 38 mm in size, was prepared on the sensing surface, and was filled with pH 7 buffer solution. The bias voltage was applied between the Si substrate and a reference electrode set in the sample well. The outputs of 16 LEDs were modulated at 2000–3500 Hz with an interval of 100 Hz, and the photocurrent signal was measured and analyzed. Fig. 3(a) shows an example of FFT spectrum of the photocurrent signal. Peaks corresponding to the modulation frequencies are observed. The amplitude of the photocurrent decreased with the modulation frequency, which can be explained in terms of the low-pass filtering effect of carrier diffusion in the semiconductor layer. Fluctuation of the peak heights observed in Fig. 3(a) should be attributed to the distribution of light intensities of LEDs. As indicated by the FFT spectrum of the photocurrent, the amplitudes corresponding to the individual LED can be extracted by Fourier analysis. By repeating the measurement of photocurrent and Fourier analysis during a voltage scan in the I–V measurement mode, the I–V curves at different positions on the sensing surface can be simultaneously obtained. Fig. 3(b) shows an example of simultaneously obtained I–V curves. The bias voltage was swept from −3.0 to 0.0 V and the photocurrent was measured at every 10.0 mV.
which moves in the perpendicular direction. By determining the values of ion concentrations at multiple points at once, the total scan time is drastically reduced. In the present setup, the 16 LEDs were placed with an interval of 3.6 mm to cover a scan width of 54 mm. The typical measurement time per line was 50 ms and the typical scan speed of the linear LED array was 12.5 mm/s. In this setup, the total scan time to obtain a chemical image of 54 mm × 80 mm at a resolution of 16 pixels × 128 line was 6.4 s. Fig. 3(c) is an example of photocurrent image obtained with the new system. The bias voltage was fixed at −1.2 V during the scan. A clear photocurrent image was obtained, and a bright area corresponding to the sample well can be distinguished. In this way, a two-dimensional map of ion concentration at a resolution of 16 pixels × 128 lines was obtained in 6.4 s. It should be noted that the total scan time does not change even if the number of LED on the linear array is increased, so far as the measurement time per line and the number of lines remain the same. The number of light sources can be increased from 16 to 80 without increasing the measurement time per line, if Tm is reduced from 50 ms to 10 ms. W.J. Parak et al. reported that the lateral resolution of LAPS measurement can be better than 100 m using a focused laser beam [20]. Their sensor plate was 0.5 mm thick and 10–15 cm in resistivity, similar to our experiment. The spatial resolution depends both on the size of light spot and on the diffusion length of minority carriers. In our system, the light spot is estimated to be 1.4 mm in diameter, which is much larger than the diffusion length of minority carriers, and therefore, the spatial resolution is limited by the former. The vertical stripe pattern in Fig. 3(c) corresponds to the distribution of the peak heights observed in Fig. 3(a), which should be attributed to the difference of light intensities of LEDs and the frequency dependence of carrier diffusion. To remove the stripes from the obtained image, the amplitudes of photocurrent must be equalized for all LEDs by adjusting the driving currents as a calibration step prior to the acquisition of images. 3.3. Calibration Fig. 4(a) shows an example of I–V curves collected when each LED was driven with the same current. The I–V curves were measured for a pH 7 buffer solution in the PDMS well of 60 mm × 38 mm
536
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
ied by changing the gain of the channel. The calibration program compares the output photocurrent of each channel and adjusts the driving current of LEDs until the output photocurrents are equalized within the specified tolerance range. Fig. 5(a) shows the I–V curves of a pH 7 buffer solution after executing the calibration program for the sample in Fig. 4(a). The output photocurrent was equalized at the bias voltage of −1.2 V, and the values of photocurrent at this voltage fell in the range of 0.015–0.021 A after the calibration step. Fig. 5(b) shows a chemical image measured after equalizing the photocurrent for each LED. It is clearly observed that the difference of brightness between the rows is reduced. The residual nonuniformity of brightness observed in Fig. 5(b) should be attributed to the non-uniformity of the sensor plate, such as the distribution of the flat-band voltage and the distribution of the minority carrier life time. An image correction method to compensate for the nonuniformity of the sensor plate will be proposed in our latest paper [21]. 3.4. Example of pH image scan To test the pH image scan, a series of photocurrent images of buffer solutions with pH 4–10 were measured. Fig. 6(a) shows the obtained photocurrent images, in which the photocurrent increased with the pH value. (In this case, the PDMS well was 35 mm × 30 mm in size.) Fig. 6(b) shows a plot of averaged photocurrent at the center of the image versus the pH value. A linear dependence of the photocurrent on the pH value was observed with a slope of about 7 nA per pH. The result indicates that our chemical image scanner is capable of imaging correct pH values in the range of pH 4–10.
Fig. 3. (a) FFT spectrum of the photocurrent. (b) Simultaneous measurement of 16 I–V curves. (c) Chemical image obtained by the new system. (pH 7 buffer solution).
in size. The amplitudes of photocurrent varied depending on the modulation frequency and the characteristics of each individual LED as was observed in Fig. 3(a). The values of photocurrents at the bias voltage of −1.2 V varied in the range of 0.017–0.057 A. Fig. 4(b) is a chemical image of a homogeneous pH 7 buffer solution measured at a bias voltage of −1.2 V. The position of the LED modulated at the lowest frequency of 2000 Hz corresponds to the left end of pixel row, and the modulation frequency increases from left to right. It should be noted that the brightness of the image is higher on the left side, and lower on the right side. To compensate for this non-uniformity, an additional circuit and a calibration program were developed for automatic equalization of the photocurrent. A 16-channel programmable-gain amplifier array was developed and was inserted between the oscillator array and the LED array. The driving current of each LED can be independently var-
Fig. 4. (a) Typical I–V characteristics before the calibration, and (b) corresponding photocurrent image. (pH 7 buffer solution).
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
537
4. Conclusion In this study, we proposed the chemical image scanner system based on FDM-LAPS. The system has a linear LED array, and is capable of simultaneous measurement at 16 different positions on the sensing surface. A total scan time of 6.4 s for an ion concentration image at a resolution of 16 pixels × 128 lines was achieved with the new system. The number of pixels can be increased by increasing the number of LEDs without the cost of the total scan time. A calibration protocol was developed to equalize the photocurrent for each channel. A series of photocurrent images for pH buffer solutions was measured with the new system, and a linear dependence of the photocurrent on the pH value was demonstrated. Acknowledgment This work was supported by Grants-in-Aid for Scientific Research (contract No. 19350036) from the Japan Society for the Promotion of Science (JSPS). T.Y. acknowledges the financial support of Takahashi Industrial and Economic Research Foundation. K.M. acknowledges the financial support from Tateisi Science and Technology Foundation. References
Fig. 5. (a) I–V characteristics after the calibration, and (b) corresponding chemical photocurrent image. (pH 7 buffer solution).
Fig. 6. (a) Photocurrent images corresponding to pH 4–10. (b) A plot of averaged photocurrent as a function of pH.
[1] M.J. Schöning, A. Poghossian, BioFEDs (field-effect devices): state-of-the-art and new directions, Electroanalysis 18 (19–20) (2006) 1893–1900. [2] A. Poghossian, M.J. Schöning, Silicon-based chemical and biological field-effect sensors, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 9, American Scientific Publishers, Stevenson Ranch, USA, 2006, pp. 463–534, ISBN 1-58883-065-9. [3] A. Poghossian, M.J. Schöning, Chemical and biological field-effect sensors for liquids—a status report, in: R.S. Marks, D.C. Cullen, I. Karube, C.R. Lowe, H.H. Weetall (Eds.), Handbook of Biosensors and Biochips, John Wiley & Sons, Weinheim, Germany, 2007, pp. 1–17 (Chapter 24), ISBN 978-0-470-01905-4. [4] P. Bergveld, Development of an ion-sensitive solid-state device for neurophysical measurements, IEEE Trans. Biomed. Eng. BME-17 (1970) 70. [5] T. Matsuo, K.D. Wise, An integrated field-effect electrode for biopotential recording, IEEE Trans. Biomed. Eng. BME-21 (1974) 485–487. [6] M.J. Schöning, A. Poghossian, T. Yoshinobu, H. Lüth, Semiconductor-based fieldeffect structures for chemical sensing, SPIE Proc. 4205 (2001) 188–198. [7] D.G. Hafeman, J.W. Parce, H.M. McConnel, Light-addressable potentiometric sensor for biochemical systems, Science 240 (1988) 1182–1185. [8] J.C. Owicki, L. Bousse, D. Hafeman, G.L. Kirk, J.D. Olson, H.G. Wada, J.W. Parce, The light-addressable potentiometric sensor: principles and biological applications, Annu. Rev. Biophys. Biomol. Struct. 23 (1994) 87–113. [9] T. Wagner, M.J. Schöning, Light-addressable potentiometric sensors (LAPS): recent trends and applications, in: S. Alegret, A. Merkoci (Eds.), Electrochemical Sensor Analysis, vol. 49, Elesevier, Amsterdam, The Netherlands, 2007, pp. 87–128, ISBN 978-0-444-53053-0. [10] I. Lundström, R. Erlandsson, U. Frykman, E. Hedborg, A. Spetz, H. Sundgren, S. Welin, F. Winquist, Artificial ‘olfactory’ images from a chemical sensor using a light-pulse technique, Nature 352 (1991) 47–50. [11] M. Nakao, T. Yoshinobu, H. Iwasaki, Scanning-laser-beam semiconductor pHimaging sensor, Sens. Actuators B 20 (1994) 119–123. [12] T. Yoshinobu, H. Iwasaki, Y. Ui, K. Furuichi, Yu. Ermolenko, Yu. Mourzina, T. Wagner, N. Näther, M.J. Schöning, The light-addressable potentiometric sensor for multi-ion sensing and imaging, Methods 37 (2005) 94–102. [13] T. Yoshinobu, H. Iwasaki, M. Nakao, S. Nomura, T. Nakanishi, S. Takamatsu, K. Tomita, Visualization of pH change of E. coli with a novel pH imaging microscope, Bioimages 5 (1997) 143–147. [14] T. Yoshinobu, T. Harada, H. Iwasaki, Application of the pH-imaging sensor to determining the diffusion coefficients of ions in electrolytic solutions, Jpn. J. Appl. Phys. 39 (2000) L318–L420. [15] Y. Kanai, Y. Sasai, H. Uchida, T. Katsube, SPV ion (taste) sensor with a LED array, in: Proceedings of the 18th Chemical Sensor Symposium, Supplement A, vol. 10, 1994, p. 38. [16] B. Stein, M. George, H.E. Gaub, J.C. Behrends, W.J. Parak, Spatially resolved monitoring of the cellular metabolic activity with a semiconductor-based biosensor, Biosens. Bioelectron. 18 (2003) 31–41. [17] Q. Zhang, P. Wang, W.J. Parak, M. George, G. Zhang, A novel design of multi-light LAPS based on digital compensation of frequency domain, Sens. Actuators B 73 (2001) 152–156. [18] T. Wagner, R. Molina, T. Yoshinobu, J.P. Kloock, M. Biselli, M. Canzoneri, T. Schnitzler, M.J. Schöning, Handheld multi-channel LAPS device as a transducer platform for possible biological and chemical multi-sensor applications, Electrochim. Acta 53 (2007) 305–311.
538
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
[19] W. Hu, H. Cai, J. Fu, P. Wang, G. Yang, Line-scanning LAPS array for measurement of heavy metal ions with micro-lens array based on MEMS, Sens. Actuators B 129 (2008) 397–403. [20] W.J. Parak, U.G. Hofmann, H.E. Gaub, J.C. Owicki, Lateral resolution of light addressable potentiometric sensors: an experimental and theoretical investigation, Sens. Actuators A 63 (1997) 47–57. [21] K. Miyamoto, Y. Sugawara, S. Kanoh, T. Yoshinobu, T. Wagner, M.J. Schöning, Image correction method for the chemical imaging sensor, Sens. Actuators B, in press.
Tatsuo Yoshinobu was born in Kyoto, Japan, in 1964. He received BE, ME, and PhD degrees in electrical engineering from Kyoto University in 1987, 1989, and 1992, respectively, for his study on gas source molecular beam epitaxy of silicon carbide. In 1992, he joined the Institute of Scientific and Industrial Research, Osaka University, where he started the development of silicon-based chemical sensors. From 1999 to 2000, he was a guest scientist at the Research Centre Jülich, Germany. Since 2005, he is a professor for electronic engineering at Tohoku University, Sendai, Japan. Since 2008, he is also a professor at the Graduate School of Biomedical Engineering, Tohoku University.
Biographies
Torsten Wagner was born in Mönchengladbach, Germany, in 1978. He received his diploma in 2003 in electrical engineering from the University of Applied Sciences Aachen, his master of science in 2003 in model simulation and control from the Coventry University in UK, and his doctoral degree (Ph.D.) in 2008 from the Philips University Marburg in cooperation with the University of Applied Sciences Aachen. His research subjects concern chemical sensors, especially the light-addressable potentiometric sensor and sensor-signal processing.
Ko-ichiro Miyamoto was born in Yamaguchi, Japan, in 1979. He received BE, ME, and PhD degrees from Tohoku University in 2002, 2004 and 2006, respectively. His PhD degree is for his study on the bio-molecular sensing using infrared absorption spectroscopy. Since 2006, he is an assistant professor in the Department of Electronic Engineering, Tohoku University. His research subject is on the application of siliconbased chemical sensors for bio-molecular sensing. Yohei Kuwabara was born in Niigata, Japan, in 1982. He received BE in 2005 from Department of Electronic Engineering, Tohoku University. And he received ME in 2008 from the Department of Electronic Engineering, Tohoku University, for his study on the development of chemical image scanner based on the light-addressable potentiometric sensor. Shin’ichiro Kanoh was born in Kanagawa, Japan, in 1968. He received BE, ME, and PhD degrees in electric and telecommunication engineering from Tohoku University in 1991, 1993 and 1996, respectively. Since 1996, he is an assistant professor for electronic engineering at the Tohoku University, Sendai, Japan. His research subjects concern measurement and analysis of brain activities, biosignal measurement in general, and development of brain-computer interface.
Michael J. Schöning received his diploma degree in electrical engineering (1989) and his PhD in the field of semiconductor-based microsensors for the detection of ions in liquids (1993), both from the Karlsruhe University of Technology. In 1989, he jointed the Institute of Radiochemistry at the Research Centre Karlsruhe. Since 1993, he has been with the Institute of Thin Films and Interfaces (now, Institute of Bioand Nanosystems) at the Research Centre Jülich, and since 1999 he was appointed as full Professor at Aachen University of Applied Sciences, Campus Jülich. Since 2006, he serves as a director of the Institute of Nano- and Biotechnologies (INB) at the Aachen University of Applied Sciences. His main research subjects concern siliconbased chemical and biological sensors, thin-film technologies, solid-state physics, microsystem and nano(bio-)technology.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Chemical image scanner based on FDM-LAPS夽 K. Miyamoto a,∗ , Yohei Kuwabara a , Shin’ichiro Kanoh a , Tatsuo Yoshinobu a,b , Torsten Wagner b,c,d,e , Michael J. Schöning d,e a
Department of Electronic Engineering, Tohoku University, 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Department of Biomedical Engineering, Tohoku University, 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan c Postdoctral fellow, Japan Society for the Promotion of Science, Japan d Institute of Nano- and Biotechnologies, Aachen University of Applied Sciences, Ginsterweg 1, 52428 Jülich, Germany e Institute of Bio- and Nanosystems (IBN-2), Research Centre Jülich, 52425 Jülich, Germany b
a r t i c l e
i n f o
Article history: Received 15 August 2008 Received in revised form 21 November 2008 Accepted 3 December 2008 Available online 14 December 2008
a b s t r a c t The chemical imaging sensor is a semiconductor-based chemical sensor that can visualize the spatial distribution of chemical species. Although it has been applied to visualization of various specimens, its slow scan rate has been a problem. In this study, a new “chemical image scanner” system based on a linear LED array and frequency division multiplex (FDM) is proposed. The scan time to obtain an ion concentration image at a resolution of 16 pixels × 128 lines was reduced to 6.4 s with the new system. © 2008 Elsevier B.V. All rights reserved.
Keywords: Chemical image scanner Chemical imaging sensor Light-addressable potentiometric sensor LAPS
1. Introduction In the field of semiconductor-based chemical sensors, three types of field-effect chemical sensors with the electrolyte –insulator–semiconductor (EIS) structure [1–3] have been studied so far; (1) Ion-sensitive field-effect transistor (IS-FET) [4,5] is a well-known semiconductor-based chemical sensor, which has been already applied for pH meters. (2) EIS capacitance sensor [6] and (3) light-addressable potentiometric sensor (LAPS) [7–9] are other examples of chemical sensors based on the EIS structure. They are capable of label-free measurement of pH or ions, and 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. In the LAPS measurement, the rear surface of the Si substrate is illuminated with a modulated light and the amplitude of the induced photocurrent is measured as the sensor signal, which varies with the ion concentration of the solution in contact with the sensing surface. Here, the measurement area is defined by illumination. Based on the addressability of LAPS, Lundström et al. obtained
夽 Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-12), July 13–16, 2008, Columbus, OH, USA. ∗ Corresponding author. Tel.: +81 22 795 7076; fax: +81 22 795 7076. E-mail address: [email protected] (K. Miyamoto). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.008
artificial olfactory images with three types of catalytic metals on the sensing surface for the discrimination of gas species [10]. On the other hand, we employed a homogeneous sensing surface for two-dimensional mapping of ion concentration by using a scanning laser beam, and named it the “chemical imaging sensor” [11,12]. The chemical imaging sensor has been applied, for example, to visualization of metabolic activity of Escherichia coli [13] and electrochemical systems [14]. The chemical imaging sensor system demonstrated its effectiveness of the label-free chemical imaging based on LAPS. Due to the measurement in a pixel-by-pixel manner, however, the chemical imaging sensor requires a relatively long time to obtain a chemical image. A typical scan time at a resolution of 128 × 128 pixels is about 3 min when 10 ms of sampling time is spent for each pixel. Such scanning time is too long to observe fast chemical phenomena, thus, the development of a rapid imaging system has been strongly desired. One of the approaches to shorten the scan time is the use of multiple light sources [15,16] and frequency division multiplex (FDM) [17]. When the multiple light sources are modulated at different frequencies, the obtained signal is a superposition of each frequency component, which can be individually extracted by Fourier analysis. Zhang et al. demonstrated a multi-light LAPS using 3 light sources and parallel measurements at 3 different positions on the sensor plate [17]. Wagner et al. reported a platform of multi-light LAPS using 16 light sources and 16 parallel measurements [18], and Hu et al. also reported 3 parallel measurements using an optical chopper
534
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
Fig. 1. A schematic view of the chemical image scanner system based on FDM-LAPS.
blade [19]. These studies suggest the possibility of rapid chemical imaging with multiple light sources. In this study, we propose a new “chemical image scanner” system based on a linear LED array and the principle of FDM-LAPS. Fig. 1 schematically shows the idea of the chemical image scanner. The configuration of the system, which is similar to that of a flat-bedtype image scanner, is suitable for rapid scanning, easy operation and use of large-area sensors. The sample to be measured will be directly put on the sensing surface and the linear LED array moves in the direction perpendicular to the array to scan the sensing area. Using a linear array of 16 LEDs and multi-channel oscillators, the scan time was reduced to 6.4 s at a resolution of 16 pixels × 128 lines. 2. Experimental Fig. 1 shows the system configuration of the chemical image scanner system. The system consists of a sensor plate, a linear array of LEDs as a scanner and a control PC. Fig. 2 shows the details of the scanning system. 2.1. Sensor plate The sensor plate we used in this study is an n-type Si wafer (10–20 cm, 500 m in thickness, 4 in. in diameter, see Fig. 2(a)). The top surface of the sensor plate was covered with 100-nm silicon nitride as a pH-sensitive layer on the insulating layer of 50-nm silicon oxide. The silicon nitride layer was deposited by low-pressure chemical vapor deposition (LP-CVD) following the thermal oxidization of the silicon substrate. Stripe electrodes of gold thin layer (containing 0.5% antimony) were deposited on the polished back surface of silicon substrate to form ohmic contacts. The electrodes were deposited near the perimeters of the sensor plate, so that the measurement area would not be covered. The top surface of the sensor plate is insulated with the thin silicon dioxide and silicon nitride layers. A thin ohmic electrode was deposited on the backside of the sensor plate.
Fig. 2. (a) System configuration of the chemical image scanner. (b) 16-channel oscillator array. The inset figure shows an oscillator circuit board for a single channel.
with an interval of 3.6 mm. These LEDs are surface-mounted-type (Cat. E1S02-4G0A7, TOYODA GOSEI Co., Ltd.) and 1.5 mm × 3 mm in size. The emission from the LED is restricted within 20◦ by a small lens with a diameter of 1 mm. The wavelength and the intensity of illumination are 520–530 nm (half width = 35 nm) and 475 mcd, respectively. The linear LED array is mounted on a one-dimensional scan stage, which moves perpendicular to the array. The driving current of each LED was modulated at each individual frequency by the 16-channel programmable oscillator. The modulated lights from the linear LED array illuminated the backside of the sensor plate during the scan. Fig. 2(b) shows the 16-channel programmable oscillator. The inset figure in Fig. 2(b) shows the oscillator circuit board for a single channel. The circuit includes a programmable waveform generator (AD9833, Analog Device Co.), which generates a sinusoidal wave with high accuracy. The frequency can be programmed with 28-bit resolution, which corresponds to a resolution of 0.055 Hz in case of a 14.7 MHz master clock supplied by a quartz oscillator. The frequency of each channel can be programmed independently by the control PC.
2.2. Linear LED array 2.3. PC and software Instead of the scanning laser beam, a linear LED array is used as the light source in the new chemical image scanner system. As shown in Fig. 2(a), the linear LED array consists of 16 LEDs placed
When the 16 LEDs simultaneously illuminate different positions of the sensor plate, the photocurrent signal is a superposition of 16
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
frequency components;
535
3.2. Rapid scanning of chemical image
16
Iphoto (t) =
an sin(2fn t + n ),
(1)
n=1
where fn and n are the modulation frequency and the phase delay, respectively, of the nth LED. The amplitude an at the frequency fn can be determined as:
an =
2 Tm
2
Tm
Iphoto (t) sin(2fn t) dt 0
+
2 Tm
A chemical image is obtained as a two-dimensional map of the shift of the I–V curve at each pixel in the sensing area. In the chemical imaging sensor, however, as it is too time-consuming to carry out the measurement of I–V curve at each pixel, the bias voltage is fixed at a constant value during the scan and the photocurrent
2
Tm
Iphoto (t) cos(2fn t) dt
,
where Tm is the measurement time. When the interval of the modulation frequency is f, the measurement time Tm must be at least longer than 1/f for the separation of signals. In a typical case, the modulation frequencies of 2000, 2100, . . ., 3500 Hz were chosen with an interval of f = 100 Hz and Tm was set to be 5/f or 50 ms. In our system, the photocurrent signal Iphoto (t) is digitized at a sampling frequency fs = 100 kHz, and the amplitude of each component is calculated by the control PC as follows;
is recorded as a function of the position. The variation of the photocurrent can be converted to the shift of the I–V curve by using the slope of the I–V curve. Even without measuring the I–V curve at each pixel, the chemical imaging sensor is still too slow due to its pixel-by-pixel measurement. In the new system, the multiple points on the scan line are simultaneously illuminated and measured with the linear LED array,
2 N−1 2 N−1 2 2 an = + Iphoto (m t) sin(2fn m t) Iphoto (m t) cos(2fn m t) , N
m=0
(2)
0
N
(3)
m=0
where t is 1/fs and N is the total number of samples. The amplitudes corresponding to 16 modulation frequencies were extracted from the measured photocurrent by the program developed with LabVIEW (National Instruments Corp.). The program sets the modulation frequencies of the LEDs, controls the bias voltage, moves the scanning stage, measures the photocurrent signal with a data acquisition board, and processes the data to display the chemical image. The program has two measurement modes; (1) the I–V measurement mode which measures the I–V curves at 16 different points simultaneously, and (2) the imaging mode which acquires the twodimensional image of ion distribution by scanning the sensing area with the linear LED array. 3. Results and discussion 3.1. Multi-point I–V measurement By using LEDs modulated at different frequencies, ion concentrations can be simultaneously measured at different positions on the sensing surface of a single sensor plate. For the test of simultaneous measurement, a sample well made of PDMS, 60 mm × 38 mm in size, was prepared on the sensing surface, and was filled with pH 7 buffer solution. The bias voltage was applied between the Si substrate and a reference electrode set in the sample well. The outputs of 16 LEDs were modulated at 2000–3500 Hz with an interval of 100 Hz, and the photocurrent signal was measured and analyzed. Fig. 3(a) shows an example of FFT spectrum of the photocurrent signal. Peaks corresponding to the modulation frequencies are observed. The amplitude of the photocurrent decreased with the modulation frequency, which can be explained in terms of the low-pass filtering effect of carrier diffusion in the semiconductor layer. Fluctuation of the peak heights observed in Fig. 3(a) should be attributed to the distribution of light intensities of LEDs. As indicated by the FFT spectrum of the photocurrent, the amplitudes corresponding to the individual LED can be extracted by Fourier analysis. By repeating the measurement of photocurrent and Fourier analysis during a voltage scan in the I–V measurement mode, the I–V curves at different positions on the sensing surface can be simultaneously obtained. Fig. 3(b) shows an example of simultaneously obtained I–V curves. The bias voltage was swept from −3.0 to 0.0 V and the photocurrent was measured at every 10.0 mV.
which moves in the perpendicular direction. By determining the values of ion concentrations at multiple points at once, the total scan time is drastically reduced. In the present setup, the 16 LEDs were placed with an interval of 3.6 mm to cover a scan width of 54 mm. The typical measurement time per line was 50 ms and the typical scan speed of the linear LED array was 12.5 mm/s. In this setup, the total scan time to obtain a chemical image of 54 mm × 80 mm at a resolution of 16 pixels × 128 line was 6.4 s. Fig. 3(c) is an example of photocurrent image obtained with the new system. The bias voltage was fixed at −1.2 V during the scan. A clear photocurrent image was obtained, and a bright area corresponding to the sample well can be distinguished. In this way, a two-dimensional map of ion concentration at a resolution of 16 pixels × 128 lines was obtained in 6.4 s. It should be noted that the total scan time does not change even if the number of LED on the linear array is increased, so far as the measurement time per line and the number of lines remain the same. The number of light sources can be increased from 16 to 80 without increasing the measurement time per line, if Tm is reduced from 50 ms to 10 ms. W.J. Parak et al. reported that the lateral resolution of LAPS measurement can be better than 100 m using a focused laser beam [20]. Their sensor plate was 0.5 mm thick and 10–15 cm in resistivity, similar to our experiment. The spatial resolution depends both on the size of light spot and on the diffusion length of minority carriers. In our system, the light spot is estimated to be 1.4 mm in diameter, which is much larger than the diffusion length of minority carriers, and therefore, the spatial resolution is limited by the former. The vertical stripe pattern in Fig. 3(c) corresponds to the distribution of the peak heights observed in Fig. 3(a), which should be attributed to the difference of light intensities of LEDs and the frequency dependence of carrier diffusion. To remove the stripes from the obtained image, the amplitudes of photocurrent must be equalized for all LEDs by adjusting the driving currents as a calibration step prior to the acquisition of images. 3.3. Calibration Fig. 4(a) shows an example of I–V curves collected when each LED was driven with the same current. The I–V curves were measured for a pH 7 buffer solution in the PDMS well of 60 mm × 38 mm
536
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
ied by changing the gain of the channel. The calibration program compares the output photocurrent of each channel and adjusts the driving current of LEDs until the output photocurrents are equalized within the specified tolerance range. Fig. 5(a) shows the I–V curves of a pH 7 buffer solution after executing the calibration program for the sample in Fig. 4(a). The output photocurrent was equalized at the bias voltage of −1.2 V, and the values of photocurrent at this voltage fell in the range of 0.015–0.021 A after the calibration step. Fig. 5(b) shows a chemical image measured after equalizing the photocurrent for each LED. It is clearly observed that the difference of brightness between the rows is reduced. The residual nonuniformity of brightness observed in Fig. 5(b) should be attributed to the non-uniformity of the sensor plate, such as the distribution of the flat-band voltage and the distribution of the minority carrier life time. An image correction method to compensate for the nonuniformity of the sensor plate will be proposed in our latest paper [21]. 3.4. Example of pH image scan To test the pH image scan, a series of photocurrent images of buffer solutions with pH 4–10 were measured. Fig. 6(a) shows the obtained photocurrent images, in which the photocurrent increased with the pH value. (In this case, the PDMS well was 35 mm × 30 mm in size.) Fig. 6(b) shows a plot of averaged photocurrent at the center of the image versus the pH value. A linear dependence of the photocurrent on the pH value was observed with a slope of about 7 nA per pH. The result indicates that our chemical image scanner is capable of imaging correct pH values in the range of pH 4–10.
Fig. 3. (a) FFT spectrum of the photocurrent. (b) Simultaneous measurement of 16 I–V curves. (c) Chemical image obtained by the new system. (pH 7 buffer solution).
in size. The amplitudes of photocurrent varied depending on the modulation frequency and the characteristics of each individual LED as was observed in Fig. 3(a). The values of photocurrents at the bias voltage of −1.2 V varied in the range of 0.017–0.057 A. Fig. 4(b) is a chemical image of a homogeneous pH 7 buffer solution measured at a bias voltage of −1.2 V. The position of the LED modulated at the lowest frequency of 2000 Hz corresponds to the left end of pixel row, and the modulation frequency increases from left to right. It should be noted that the brightness of the image is higher on the left side, and lower on the right side. To compensate for this non-uniformity, an additional circuit and a calibration program were developed for automatic equalization of the photocurrent. A 16-channel programmable-gain amplifier array was developed and was inserted between the oscillator array and the LED array. The driving current of each LED can be independently var-
Fig. 4. (a) Typical I–V characteristics before the calibration, and (b) corresponding photocurrent image. (pH 7 buffer solution).
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
537
4. Conclusion In this study, we proposed the chemical image scanner system based on FDM-LAPS. The system has a linear LED array, and is capable of simultaneous measurement at 16 different positions on the sensing surface. A total scan time of 6.4 s for an ion concentration image at a resolution of 16 pixels × 128 lines was achieved with the new system. The number of pixels can be increased by increasing the number of LEDs without the cost of the total scan time. A calibration protocol was developed to equalize the photocurrent for each channel. A series of photocurrent images for pH buffer solutions was measured with the new system, and a linear dependence of the photocurrent on the pH value was demonstrated. Acknowledgment This work was supported by Grants-in-Aid for Scientific Research (contract No. 19350036) from the Japan Society for the Promotion of Science (JSPS). T.Y. acknowledges the financial support of Takahashi Industrial and Economic Research Foundation. K.M. acknowledges the financial support from Tateisi Science and Technology Foundation. References
Fig. 5. (a) I–V characteristics after the calibration, and (b) corresponding chemical photocurrent image. (pH 7 buffer solution).
Fig. 6. (a) Photocurrent images corresponding to pH 4–10. (b) A plot of averaged photocurrent as a function of pH.
[1] M.J. Schöning, A. Poghossian, BioFEDs (field-effect devices): state-of-the-art and new directions, Electroanalysis 18 (19–20) (2006) 1893–1900. [2] A. Poghossian, M.J. Schöning, Silicon-based chemical and biological field-effect sensors, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 9, American Scientific Publishers, Stevenson Ranch, USA, 2006, pp. 463–534, ISBN 1-58883-065-9. [3] A. Poghossian, M.J. Schöning, Chemical and biological field-effect sensors for liquids—a status report, in: R.S. Marks, D.C. Cullen, I. Karube, C.R. Lowe, H.H. Weetall (Eds.), Handbook of Biosensors and Biochips, John Wiley & Sons, Weinheim, Germany, 2007, pp. 1–17 (Chapter 24), ISBN 978-0-470-01905-4. [4] P. Bergveld, Development of an ion-sensitive solid-state device for neurophysical measurements, IEEE Trans. Biomed. Eng. BME-17 (1970) 70. [5] T. Matsuo, K.D. Wise, An integrated field-effect electrode for biopotential recording, IEEE Trans. Biomed. Eng. BME-21 (1974) 485–487. [6] M.J. Schöning, A. Poghossian, T. Yoshinobu, H. Lüth, Semiconductor-based fieldeffect structures for chemical sensing, SPIE Proc. 4205 (2001) 188–198. [7] D.G. Hafeman, J.W. Parce, H.M. McConnel, Light-addressable potentiometric sensor for biochemical systems, Science 240 (1988) 1182–1185. [8] J.C. Owicki, L. Bousse, D. Hafeman, G.L. Kirk, J.D. Olson, H.G. Wada, J.W. Parce, The light-addressable potentiometric sensor: principles and biological applications, Annu. Rev. Biophys. Biomol. Struct. 23 (1994) 87–113. [9] T. Wagner, M.J. Schöning, Light-addressable potentiometric sensors (LAPS): recent trends and applications, in: S. Alegret, A. Merkoci (Eds.), Electrochemical Sensor Analysis, vol. 49, Elesevier, Amsterdam, The Netherlands, 2007, pp. 87–128, ISBN 978-0-444-53053-0. [10] I. Lundström, R. Erlandsson, U. Frykman, E. Hedborg, A. Spetz, H. Sundgren, S. Welin, F. Winquist, Artificial ‘olfactory’ images from a chemical sensor using a light-pulse technique, Nature 352 (1991) 47–50. [11] M. Nakao, T. Yoshinobu, H. Iwasaki, Scanning-laser-beam semiconductor pHimaging sensor, Sens. Actuators B 20 (1994) 119–123. [12] T. Yoshinobu, H. Iwasaki, Y. Ui, K. Furuichi, Yu. Ermolenko, Yu. Mourzina, T. Wagner, N. Näther, M.J. Schöning, The light-addressable potentiometric sensor for multi-ion sensing and imaging, Methods 37 (2005) 94–102. [13] T. Yoshinobu, H. Iwasaki, M. Nakao, S. Nomura, T. Nakanishi, S. Takamatsu, K. Tomita, Visualization of pH change of E. coli with a novel pH imaging microscope, Bioimages 5 (1997) 143–147. [14] T. Yoshinobu, T. Harada, H. Iwasaki, Application of the pH-imaging sensor to determining the diffusion coefficients of ions in electrolytic solutions, Jpn. J. Appl. Phys. 39 (2000) L318–L420. [15] Y. Kanai, Y. Sasai, H. Uchida, T. Katsube, SPV ion (taste) sensor with a LED array, in: Proceedings of the 18th Chemical Sensor Symposium, Supplement A, vol. 10, 1994, p. 38. [16] B. Stein, M. George, H.E. Gaub, J.C. Behrends, W.J. Parak, Spatially resolved monitoring of the cellular metabolic activity with a semiconductor-based biosensor, Biosens. Bioelectron. 18 (2003) 31–41. [17] Q. Zhang, P. Wang, W.J. Parak, M. George, G. Zhang, A novel design of multi-light LAPS based on digital compensation of frequency domain, Sens. Actuators B 73 (2001) 152–156. [18] T. Wagner, R. Molina, T. Yoshinobu, J.P. Kloock, M. Biselli, M. Canzoneri, T. Schnitzler, M.J. Schöning, Handheld multi-channel LAPS device as a transducer platform for possible biological and chemical multi-sensor applications, Electrochim. Acta 53 (2007) 305–311.
538
K. Miyamoto et al. / Sensors and Actuators B 137 (2009) 533–538
[19] W. Hu, H. Cai, J. Fu, P. Wang, G. Yang, Line-scanning LAPS array for measurement of heavy metal ions with micro-lens array based on MEMS, Sens. Actuators B 129 (2008) 397–403. [20] W.J. Parak, U.G. Hofmann, H.E. Gaub, J.C. Owicki, Lateral resolution of light addressable potentiometric sensors: an experimental and theoretical investigation, Sens. Actuators A 63 (1997) 47–57. [21] K. Miyamoto, Y. Sugawara, S. Kanoh, T. Yoshinobu, T. Wagner, M.J. Schöning, Image correction method for the chemical imaging sensor, Sens. Actuators B, in press.
Tatsuo Yoshinobu was born in Kyoto, Japan, in 1964. He received BE, ME, and PhD degrees in electrical engineering from Kyoto University in 1987, 1989, and 1992, respectively, for his study on gas source molecular beam epitaxy of silicon carbide. In 1992, he joined the Institute of Scientific and Industrial Research, Osaka University, where he started the development of silicon-based chemical sensors. From 1999 to 2000, he was a guest scientist at the Research Centre Jülich, Germany. Since 2005, he is a professor for electronic engineering at Tohoku University, Sendai, Japan. Since 2008, he is also a professor at the Graduate School of Biomedical Engineering, Tohoku University.
Biographies
Torsten Wagner was born in Mönchengladbach, Germany, in 1978. He received his diploma in 2003 in electrical engineering from the University of Applied Sciences Aachen, his master of science in 2003 in model simulation and control from the Coventry University in UK, and his doctoral degree (Ph.D.) in 2008 from the Philips University Marburg in cooperation with the University of Applied Sciences Aachen. His research subjects concern chemical sensors, especially the light-addressable potentiometric sensor and sensor-signal processing.
Ko-ichiro Miyamoto was born in Yamaguchi, Japan, in 1979. He received BE, ME, and PhD degrees from Tohoku University in 2002, 2004 and 2006, respectively. His PhD degree is for his study on the bio-molecular sensing using infrared absorption spectroscopy. Since 2006, he is an assistant professor in the Department of Electronic Engineering, Tohoku University. His research subject is on the application of siliconbased chemical sensors for bio-molecular sensing. Yohei Kuwabara was born in Niigata, Japan, in 1982. He received BE in 2005 from Department of Electronic Engineering, Tohoku University. And he received ME in 2008 from the Department of Electronic Engineering, Tohoku University, for his study on the development of chemical image scanner based on the light-addressable potentiometric sensor. Shin’ichiro Kanoh was born in Kanagawa, Japan, in 1968. He received BE, ME, and PhD degrees in electric and telecommunication engineering from Tohoku University in 1991, 1993 and 1996, respectively. Since 1996, he is an assistant professor for electronic engineering at the Tohoku University, Sendai, Japan. His research subjects concern measurement and analysis of brain activities, biosignal measurement in general, and development of brain-computer interface.
Michael J. Schöning received his diploma degree in electrical engineering (1989) and his PhD in the field of semiconductor-based microsensors for the detection of ions in liquids (1993), both from the Karlsruhe University of Technology. In 1989, he jointed the Institute of Radiochemistry at the Research Centre Karlsruhe. Since 1993, he has been with the Institute of Thin Films and Interfaces (now, Institute of Bioand Nanosystems) at the Research Centre Jülich, and since 1999 he was appointed as full Professor at Aachen University of Applied Sciences, Campus Jülich. Since 2006, he serves as a director of the Institute of Nano- and Biotechnologies (INB) at the Aachen University of Applied Sciences. His main research subjects concern siliconbased chemical and biological sensors, thin-film technologies, solid-state physics, microsystem and nano(bio-)technology.