Photosensor Based on an FET Utilizing a Biocomponent of

Sep 4, 2009 - Kazuo Nakazato ... Science, Faculty of Science and Technology, The Tokyo University of Science, 2641 Yamazaki, Noda-shi,Chiba. 278-8510 ...
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Photosensor Based on an FET Utilizing a Biocomponent of Photosystem I for Use in Imaging Devices Nao Terasaki,*,† Noritaka Yamamoto,‡ Mineyuki Hattori,‡ Nobutaka Tanigaki,‡ Takashi Hiraga,‡ Kohsuke Ito,§ Masae Konno,§ Masako Iwai,§ Yasunori Inoue,§ Sigeyasu Uno,^ and Kazuo Nakazato^ † National Institute of Advanced Industrial Science and Technology (AIST), Measurement Solution Research Center,807-1 Shuku-machi, Tosu, Saga 841-0052, Japan, ‡National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan, §Department of Applied Biological Science, Faculty of Science and Technology, The Tokyo University of Science, 2641 Yamazaki, Noda-shi,Chiba 278-8510, Japan, and ^Department of Electrical Engineering and Computer Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Received March 29, 2009. Revised Manuscript Received August 13, 2009 We have investigated a photosensor that consists of a field emission transistor (FET) utilizing the biocomponent of the photosystem I (PSI) protein complex for use in an imaging device. The PSI was immobilized on a gold electrode via the self-assembling monolayer (SAM) of 3-mercapto-1-propanesulfonic acid sodium salt to obtain a PSI-modified gold electrode. As for the PSI-modified gold electrode, the basic photoresponses originating from the excitation of PSI, including the photocurrent (106 nA) and the photoresponse of the open-circuit voltage (photo-Voc: 28.6 mV), were characterized. Then, the PSI-modified gold electrode was linked to the gate of the FET using a lead line, and the device was successfully driven by the photoelectric signals from the PSI like a voltage follower circuit. Further, we successfully demonstrated that the PSI-based FET acts as a photosensor in imaging devices.

1. Introduction Biocomponents from living body are very attractive nanomaterials because they have already achieved ultrahigh and ultimate performance as the result of many cycles of natural selection and mutation. Especially in the case of photosynthesis, it is wellknown that the quantum yield of the photo-electroconversion in the photosynthesis reaction center is almost unity.1,2 The high performance of biocomponents can be attributed to well-designed spatial configurations (position, direction, etc.) and the environmental control of the functional molecules in the biocomponents. Many scientists have made efforts to mimic and model biofunctions, and only some of them have developed materials with performance identical to or exceeding that of the original biocomponents.3-8 On the other hand, materials with ultimate and ultrahigh performance already exist in nature. From these viewpoints, we have proposed a new concept in which biocomponents are employed as vital constituents of the artificial devices; for this purpose, we designed a biophotosensor system consisting of a photosystem I (PSI) protein complex and an ion-sensitized *Corresponding author: Tel þ81 942 81 4038; fax þ81 942 81 3690, e-mail [email protected]. (1) Emerson, R.; Chalmers, R. V.; Cederstand, C.; Brody, M. Science 1956, 123, 673–674. (2) Emerson, R.; Chalmers, R. V.; Cederstand Proc. Natl. Acad. Sci. U.S.A. 1957, 43, 133–143. (3) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89–112. (4) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009–1020. (5) Pedersen, C. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1021–1027. (6) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Nature 1995, 375, 345– 347. (7) Baim, C. D.; Whitesides, G. M. Science 1988, 240, 62–63. (8) Gust, D.; Moore, T. A. Science 1989, 244, 35–41. (9) Terasaki, N.; Yamamoto, N.; Tamada, K.; Hattori, M.; Hiraga, T.; Tohri, A.; Sato, I.; Iwai, M.; Taguchi, S.; Enami, I.; Inoue, Y.; Yamanoi, Y.; Yonezawa, T.; Mizuno, K.; Murata, M.; Nishihara, H.; Yoneyama, S.; Minakata, M.; Ohmori, T.; Sakai, M.; Fujii, M. Biochim. Biophys. Acta 2007, 1767, 653–659.

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field emitting transistor (ISFET).9 As the first step in our experiments, we selected the ISFET as the photoelectric-signal detection device, as it is commercially available and can be easily used and handled; further, it is often used for the demonstration of functionalized sensors modified by original molecules even in aqueous environment.10-15 However, since the chemical composition of the ion-sensitized gate film of the ISFET has not been determined and opened, understanding the direct correlation between the properties of the functional molecules adsorbed on the gate film and the FET performance becomes difficult. Further, as is evident from the results of our previous experiment,9 although the signal from the PSI protein could be measured well reproducibly, the measurements were carried out under feedback of the applied potential between gate and source of FET (Vgs) to maintain the current between source and drain of FET (Ids) zero (0). The researchers in the field of practicable integrated semiconductor devices have also pointed out the strangeness; the 0 value of the Ids region is not used as usual in the operation of the FET. Therefore, previous studies have only demonstrated the possibility of detecting signals from the PSI protein and have not focused on the development and possibility of practical biophotosensors based on PSI proteins and FETs. On the other hand, from the viewpoint of practicality, FETs equipped with voltage follower circuits are indispensable, particularly in the case of sensors.16 In this study, we have investigated (10) Sato, I.; Karube, I.; Suzuki, S.; Aikawa, K. Anal. Chim. Acta 1979, 106, 369–372. (11) Wakida, S. Sens. Technol. 1992, 12, 33–40. (12) Bergveld, P.; van Hal, R. E. G. Biosens. Bioelectron. 1995, 10, 405–414. (13) Lahav, M.; Kharitonov, A. B.; Katz, O.; Kunitake, T.; Willner, I. Anal. Chem. 2001, 73, 720–723. (14) Kharitonov, A. B.; Wasserman, J.; Katz, E.; Willner, I. J. Phys. Chem. B 2001, 105, 4205–4213. (15) Martinoia, S.; Massobrio, P. Biosens. Bioelectron. 2004, 19, 1487–1496. (16) Nakazato, K.; Ohura, M.; Uno, S. IEICE Trans. 2008, 9, 1505–1515.

Published on Web 09/04/2009

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Figure 2. Energy diagram of the functional molecules in the PSI20 and electron mediators for photocurrent generation.9,24

Figure 1. Concept underlying the use of biocomponent-based FET in photosensing devices.

the use of photosensors in imaging devices; the photosensor consists of an FET that utilizes the PSI biocomponent (Figure 1). On the basis of previous knowledge, we have investigated precisely signals detection from the PSI by using an FET like a voltage follower circuit as the major milestone for the practical use. Moreover, we also have evaluated the FET performance when the desired level of current flows between the source and the drain. The advantage of the current system is that it helps measure the charge and voltage corresponding to the FET gate film accurately. Further, a high signal-to-noise ratio (SNR) can be achieved, and this system is suitable for micronization and integration.16 In addition, the gate film is made of gold, and orientation of the molecules adsorbed on it can be easily controlled in molecular order; thus, it is possible to understand the relationship between the molecular characteristics and the FET performance. Our investigations revealed that PSI-based FETs are successfully operated by the photoelectric signals from the PSI from the viewpoint of voltage follower. Further, we demonstrated that PSI-based FETs can be used as photosensors in imaging devices.

2. Experimental Section PSI, isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus, was used to obtain the photosynthetic biocomponent17 because it is highly thermostable. Many studies on this biocomponent involve structural analysis,18-20 investigation of the electron transfer reaction,21 etc. We prepared a stable biocomponent of PSI by applying the His-tag method on Thermosynechococcus elongatus BP-1. Six histidine residues and residues for thrombin recognition (LVPRGSHHHHHH) were attached to the carboxyl terminus of the PsaF subunit of PSI. The PSI complex was isolated from thylakoids by means of Ni2þ affinity column chromatography. Thylakoid membranes were extracted by means of osmotic shock from 4- to 5-day-old cells treated with lysozyme.22 The membranes were suspended in a mixture of 30 mM HEPES-NaOH (pH 7.0), 10 mM MgCl2, and 25% glycerol and were solubilized with 1.0% n-dodecyl-β-D-maltoside (β-DM) for 30 min in the (17) Iwaki, M.; Ito, S. FBS Lett. 1989, 256, 11–16. (18) Jordan, P.; Fromme, P.; Witt, H. T.; Klukas, O.; Saenger, W.; Krauβ, N. Nature 2001, 411, 909–917. (19) Fromme, P.; Jordan, P.; Krauβ, N. Biochim. Biophys. Acta 2001, 1507, 5– 31. (20) Bibby, T. S.; Nield, J.; Barber, J. Nature 2001, 412, 743–745. (21) Brettel, K. Biochim. Biophys. Acta 1997, 1318, 322–373. (22) Kamiya, N.; Shen, J. R. Proc. Natl Acad. Sci. U.S.A. 2003, 100, 98–103.

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dark at 4 C. The suspension was centrifuged at 80 000g for 1 h at 4 C; the supernatant was loaded at a flow rate of 1 mL/min onto an Ni-affinity column (HisTrap HP; 5 mL; GE Healthcare UK Ltd.) preequilibrated with 40 mM MES-NaOH (pH 6.5), 100 mM NaCl, 15 mM CaCl2, 15 mM MgCl2, 0.03% β-DM, and 10% glycerol (buffer A). The PSI complexes were washed with buffer A supplemented with 60 mM imidazole and then eluted with 300 mM imidazole in buffer A at a flow rate of 0.5 mL/min. Poly(ethylene glycol) 6000 (purity: 50%; Wako Pure Chemical Industries, Ltd., Japan) solution was added to the eluted PSI fraction, and the PSI complex was precipitated by centrifugation. Pellets of the PSI complex were resuspended in 20 mM MESNaOH (pH 6.4) and 0.02% β-DM. The His-tag sequence was removed by treating the PSI complex with 3 U/mg of thrombin (product no. 27-0846-01; GE Healthcare UK Ltd.) for 16-20 h at 4 C. The removed His-tag sequence and thrombin were separated from the PSI complex by using ultrafiltration (Amicom Ultra-15, 10 000 NMWL, MILLIPORE). We confirmed the removal of the His-tag sequence from the PSI complex by using SDS-PAGE and immunoblot analysis with anti-His antibodies. The photoexcitation of a chlorophyll heterodimer (P700) induces its oxidation and a series of efficient electron transfer steps, as shown in Figure 2: P700 f chlorophyll a (A0) f phylloquinone (A1) f iron-sulfur (Fe-S) clusters (FX, FA, and FB). The preservation of the bioactivity of purified PSI was ascertained by monitoring the methyl viologen coupled oxygen consumption detected by an Clerk-type electrode.23 It was found that purified PSI has a photosynthetic activity of ∼150 μmol of O2/(mg 3 Chl h) and that this value remains almost constant for at least 20 days. For this experiment, purified PSI (20 μg of Chl/mL) was used along with 0.02% β-DM in MES-NaOH (pH 6.4) buffer solution with 16 mM L(þ)-ascorbic acid sodium salt (NaAsc) acting as the reducing reagent, 0.08 mM 2,6-dichlorophenolindophenol (DCIP) acting as the electron donor, and 160 μM methyl viologen acting as the electron acceptor (Figure 3). The PSI-modified gold electrode was prepared according to Scheme 1,24 from the viewpoint of avoidance of unnecessary treatment that lead extraction of chlorophyll molecules and loss of absorbance,9 in other words, from the viewpoint of stable photoresponse. It is known that the modification of the surface of the gold electrode by PSI with almost controlled orientation can be accomplished mainly via electrostatic interaction between a negatively charged self-assembling monolayer (SAM) and a positively charged ferredoxin-binding site around the Fe-S clusters (FX, FA, and FB) of PSI.24-26 Thus, the SAM of 3-mercapto-1-propanesulfonic acid sodium salt (MPS; Aldrich) was first assembled on the gold electrode surface. Then, the (23) Link, S.; El-Sayed, M. A. Int. Rev. Phys. Chem. 2000, 19, 409–453. (24) Terasaki, N.; Yamamoto, N.; Hiraga, T.; Sato, I.; Inoue, Y.; Yamada, S. Thin Solid Films 2006, 499, 153–156. (25) Lee, I.; Lee, J. W.; Greenbaum, E. Phys. Rev. Lett. 1997, 79, 3294–3297. (26) Ko, B. S.; Babcock, B.; Jennings, G. K.; Tilden, S. G.; Peterson, R. R.; Cliffel, D. E. Langmuir 2004, 20, 4033–4038.

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Figure 4. Atomic force micrograph (AFM) images of (a) the bare gold electrode and (b) PSI-modified gold electrode. Figure 3. Dependence of oxygen consumption on storage time as the photosynthetic activity of PSI proceeds. Measurement solution: purified PSI (10 μg of Chl/mL), 0.02% β-DM in MES-NaOH (pH 6.4) buffer solution with 16 mM NaAsc, 0.08 mM DCIP, and 160 μM methyl viologen. Scheme 1. Procedure for the Preparation of PSI-Modified Gold Electrodes

recorded as the photocurrent by using a potentiostat (FUSO, HECS318C). The Voc photoresponses were measured and recoded as the photo-Voc by using an electrochemical analyzer ALS420a (BAS. Inc.). The photoresponse of the source-drain current of the FET device (photo-Ids) as the FET performance was measured using 5 V of applied source-drain potential (Vds) and 1.5 V of applied gate-source potential (Vgs), and it was recorded by using a picoammeter/voltage source (model 6487, Keithley Inc.). For the imaging experiment performed using the PSI-based FET, a famous traditional Japanese picture was used as the original picture, as shown in Figure 9c.9 Light powers of 16 scales corresponding to the concentration of gray color were emitted from a blue LED light at 420 nm and used as the input signal; the input signal was controlled and radiated onto the PSI-based FET.

3. Results and Discussion MPS-modified gold electrode was immersed in 2.8 mg/mL of PSI suspended in MES-NaOH buffer solution (pH 6.4) for 4 days to obtain PSI-modified gold electrode. The immobilization of PSI on the gold surface was evaluated by performing atomic force micrograph (AFM, SPI3800; SII) shown in Figure 4 and quartz crystal microbalance (QCM) measurements (ALS420a; BAS Inc.). In order to prove a drive of a FET in the viewpoint of voltage follower, it is indispensable to demonstrate that a value of charge generated on the gate film of the FET is consistent with the sensing value of the FET obtained as the shifted value of current-voltage curve (I-V curve). For the demonstration, the following fabrication and experiments on photoresponses were carried out. In order to fabricate the PSI-based FET, commercial discrete FET parts (silicon N-channel MOS-type; JEDEC; 2SK941) were used, and the PSI-modified gold electrode was linked to the gate of the FET using a lead line. The experimental workflow for evaluating the series of photoresponses is described here. First, the photoresponse of the current (photocurrent) in the three-electrodes mode [counter electrode (CE): Pt wire; reference electrode (RE): Ag/AgCl (saturated KClaq)] and the open-circuit voltage (photo-Voc; CE: Pt wire) of the PSI-modified gold electrode was evaluated by using electrochemical cell. Further, after leading off the PSI-modified gold electrode to the gate of the FET, the photoresponse of the sourcedrain current of the PSI-based FET (photo-Ids) was measured. These experiments of the photoresponse (photocurrent, photoVoc, and photo-Ids) were carried out in the presence of 0.25 M NaAsc as a sacrificial regent and 0.1 M sodium perchlorate (NaClO4) aqueous MES-NaOH buffer solution at room temperature.9,24,27 Light emitted from a xenon lamp (LAX-Cute, Asahi Spectra Co.) was allowed to pass through an optical filter and then irradiated onto the modified (working) electrode (light intensity: 1.8 mW at 680 nm; irradiation area: 0.16 cm2). The current responses to the light irradiation were measured and (27) Terasaki, N.; Yamamoto, N.; Hiraga, T. Thin Solid Films, accepted.

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The AFM image of the bare gold electrode (Figure 4a) shows some atomically flat grains (size: several hundred nanometers). The step height at the grain boundaries indicated by the arrow point was estimated to be 0.32 nm and corresponded to the thickness of the monatomic layer of gold. On the other hand, the closely packed spherical structures on the PSI immobilized gold electrode can be observed in Figure 4b, and the heights of ca. 11 nm are almost identical to the monomer size of PSI.18 The amount of PSI immobilized on the gold electrode was estimated by performing QCM measurement to be 7.7  10-13 mol cm-2, which is reasonable as monolayer coverage based on the size of PSI.9 The photocurrent, photo-Voc, and photo-Ids were measured in the presence of 0.25 M NaAsc acting as the sacrificial regent with monochromatic light irradiation (1.8 mW at 680 nm).9,24,27 When the light was radiated onto the PSI-modified gold electrode, photoresponse could be clearly observed in all measurements (shown in Figure 5). It can be observed that the response of the photocurrent was quick; on the other hand, the photoresponses of photo-Voc and photo-Ids were similar and not very quick. The quick response of the photocurrent was probably due to specific spatial adsorption at ferredoxin binding site around the Fe-S clusters (FX, FA, and FB), the end of the electron relay system of the PSI, and smooth electron injection from the PSI to the gold electrode. On the other hand, the low photo-Voc value could be attributed to direct electron transfer from the bare portion of the gold electrode (resembling a pinhole) to the electroactive species in the electrolyte solution.27 The backward electron transfer from the pinhole resulted in loss of electrons accumulated at the gold electrode. In other words, the values of photo-Voc and photo-Ids were affected by the low and slow photo-Voc response. In order to evaluate the change in photocurrent activity with time, the dependence of the photocurrent and dark current of the PSI-modified gold electrode on the storage time was measured DOI: 10.1021/la901091e

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Figure 5. Photoresponse of (a) on photocurrent, (b) photo-Voc, and (c) photo-Ids upon illumination with stable light at 680 nm (1.8 mW). Electrolyte solution: 0.25 M sodium NaAsc, 2.5  10-3 M DCIP, and 0.1 M NaClO4 in aqueous MES-NaOH buffer solution.

Figure 6. Dependence of the photocurrent (b) and dark current (O) of PSI-modified gold electrode on the storage time. λ = 680 nm (1.8 mW). Electrolyte solution: 0.25 M NaAsc, 2.5  10-3 M DCIP, and 0.1 M NaClO4 in aqueous MES-NaOH buffer solution.

(Figure 6). As the storage time increased beyond 2 days, the photocurrent (b) gradually decreased, although the PSI-modified gold electrode was stored in the dark at 4 C in the MES-NaOH (pH 6.4) buffer solution and β-DM. The storage conditions were identical to those used when measuring oxygen consumption, as shown in Figure 3. Therefore, the photosynthetic activity of PSI should be consistent with that observed during the measurement of oxygen consumption. Unlike the photocurrent, the dark current (O) gradually increased with the storage time. It has been found that dark current is significantly affected by the surface covered by PSI on the gold electrode (PSI coverage) and that it decreases with an increase in the PSI coverage because of the preservation of direct electron transfer between the electrolytes in solution and the gold electrode by the bulk PSI protein.24 These results imply that the PSI adsorbed on the gold electrode leaves the surface and dissolves in the buffer solution. Therefore, all the experiments were carried out within 2 days after the PSI modification process. The photocurrent action spectrum of the PSI-modified gold electrodes (0) is shown in Figure 7a. This action spectrum has four peaks located at approximately 430, 590, 630, and 680 nm, and these peak positions are well consistent with the absorption spectrum of PSI (solid line). The action spectra of photo-Voc of the PSI-modified gold electrode (O; Figure 7b) and photo-Ids of the PSI-based FET device (b; Figure 7b) also coincide with the absorption spectrum of 11972 DOI: 10.1021/la901091e

Figure 7. Action spectra of the photoresponses of (a) photocurrent (0), (b) photo-Voc (O), and photo-Ids (b). Solid lines in both figures mean absorption spectrum of 10 mg/mL PSI MES-NaOH aqueous solution. Electrolyte solution: 0.25 M NaAsc, 2.5  10-3 M DCIP, and 0.1 M NaClO4 in aqueous MES-NaOH buffer solution.

PSI. These results clearly indicate that these photoresponses (photocurrent, photo-Voc, and photo-Ids) originate from the photoexcitation of PSI. On the other hand, the slightly blue shift of action spectra, ∼10 nm, seems to be observed; this reflects that the shorter peak wavelength of absorption spectrum of PSI immobilized on gold electrode surface (672 nm, data not shown) than the case of PSI aqueous solution (679 nm). The blue shift was probably due to the structural and environmental changes of PSI, especially at around chlorophyll molecules, accompanied by the immobilization of PSI on the electrode surface. Figure 8a shows the Ids-Vgs curves of the PSI-based FET measured in the dark (9) and under irradiation at 680 nm (b); it was found that Ids increased exponentially with Vgs. In order to clarify the relationship between the Ids-Vgs curves measured in the dark and under illumination, the Ids-Vgs curve measured in the dark was perpendicularly shifted by 28.6 mV along the Vgs axis. The shifted Ids-Vgs curve (0) was well consistent with the Ids-Vgs curve obtained under illumination (b), as shown in Figure 8b. The photo-Voc value of the illuminated PSI-modified gold electrode (28.6 mV) and the value by which the Ids-Vgs curve was perpendicularly shifted along the Vgs axis were identical. This clearly indicates that the FET was successfully driven like as the voltage follower circuit.16 The similarity in the photoresponse property of photo-Vgs and photo-Ids, as shown in Figure 5, can be attributed to the manner in which the FET is driven. Such photoresponses could be generated via the mechanism described below. At first, P700 and other chlorophyll molecules are excited by the incident light, and an electron is Langmuir 2009, 25(19), 11969–11974

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Figure 8. (a) Ids-Vgs curves of the PSI-based FET in the dark (9) and under illumination at 680 nm (b). (b) Comparison of the perpendicularly shifted Ids-Vgs curve measured in the dark (0) and the original Ids-Vgs curve measured under illumination at 680 nm (b). Electrolyte solution: 0.25 M NaAsc, 2.5  10-3 M DCIP and 0.1 M NaClO4 in aqueous MES-NaOH buffer solution.

transferred to the FB molecule through the efficient electron relay system in PSI; the oxidized P700 is reduced by the sacrificial regent (NaAsc) dissolved in the measurement solution.9,21,24 Simultaneously, the electron on FB at the end of the electron relay system in PSI is injected into the gold electrode, and the photocurrent and photo-Voc of the electrode are recorded (Figure 2). Finally, the light-induced electric signal of the gold electrode is led to the gate of the discrete FET along the lead line, and the current between the source and the drain is varied with respect to the photo-Ids values. Figure 9 shows the results of the demonstrative experiments on the PSI-based FET used in an imaging device. Both the incident light power from the blue LED light at 420 nm (]: Figure 9a) and the recorded photo-Ids values (b: Figure 9b) were perfectly proportional to the input signal of 16 scales. For the imaging experiment performed using the PSI-based FET, a famous traditional Japanese picture was used as the original picture, as shown in Figure 9c.9 Light powers of 16 scales corresponding to the concentration of gray color were emitted from a blue LED light at 420 nm and used as the input signal; the input signal was controlled and radiated onto the PSI-based FET. The input signals from the picture at around Mt. Fuji (the square area) divided into 21  17 pixels (Figure 9d) were successfully regenerated as the output picture with the same picture pattern by using the PSI-based FET (Figure 9e). This is the first demonstration of a photosensor that consists of the biocomponent of PSI and an FET operated like the voltage follower circuit and that can be used in an imaging device. Langmuir 2009, 25(19), 11969–11974

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Figure 9. Correlation between (a) the radiated light power from the blue LED light at 420 nm (]) and (b) the recorded photo-Ids values (b) with the input signal of 16 scales. (c) Original black and white picture used for the imaging experiment, (d) input image incident as radiated light power using the blue LED light at 420 nm, and (e) output image regenerated from the photo-Ids values of the PSI-based FET. Electrolyte solution: 0.25 M NaAsc, 2.5  10-3 M DCIP, and 0.1 M NaClO4 in aqueous MES-NaOH buffer solution.

4. Conclusion We investigated the use of a photosensor consisting of an FET utilizing the biocomponent of the PSI protein complex in an imaging device. The PSI-modified gold electrode was linked to the gate of the FET using a lead line, and the device was successfully driven by the photoelectric signals from the PSI like a voltage follower circuit. Further, we successfully demonstrated that the PSI-based FET acted as a sensor in the imaging device. In this study, because we focused on the providing the first demonstration of the use of a PSI-based FET from the viewpoint of voltage follower and its applicability in imaging devices, we prepared the PSI-modified gold electrode, which acted as the gate film in the FET, via an easy method. However, the current device is not stable over a period of days. Thus, durable PSI-modified gold electrodes obtained using more sophisticated immobilization methods9,28-34 should be indispensable in practical photosensing and imaging devices from the viewpoints of lifetime and performance. Further, a solid or quasi-solid sensor and a sensor array on (28) Katz, E. J. Electroanal. Chem. 1994, 365, 157–164. (29) Das, P.; Kiley, P. J.; Segal, M.; Norville, J.; Yu, A. A.; Wang, L.; Trammell, S. A.; Reddick, L. E.; Kumar, R.; Stellacci, F.; Lebedev, N.; Schnur, J.; Bruce, D. B.; Zhang, S.; Baldo, M. Nano Lett. 2004, 4, 1079–1083. (30) Frolov, L.; Rosenwaks, Y.; Carmeli, C.; Carmeli, I. Adv. Mater. 2005, 20, 2434–2437. (31) Terasaki, N.; Iwai, M.; Yamamoto, N.; Hiraga, T.; Yamada, S.; Inoue, Y. Thin Solid Films 2008, 516, 2553–2557. (32) Frolov, L.; Wilner, O.; Carmeli, C.; Carmeli, I. Adv. Mater. 2008, 20, 263– 266. (33) Faulkner, C. J.; Lees, S. Peter; Ciesielski, N.; Cliffel, D. E.; Jennings, G. K. Langmuir 2008, 84, 8409–8412. (34) Terasaki, N.; Yamamoto, N.; Hiraga, T.; Yamanoi, Y.; Yonezawa, T.; Nishihara, H.; Ohmori, T.; Sakai, M.; Fujii, M.; Tohri, A.; Iwai, M.; Inoue, Y.; Yoneyama, S.; Minakata, M.; Enami, I. Angew. Chem., Int. Ed. Engl. 2009, 48, 1585–1587.

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a small tip are promising practical devices. In future studies, we will attempt to develop such devices. Acknowledgment. This research was financially supported by Development of Systems and Technology for Advanced

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Measurement and Analysis (JST). We much appreciate Prof. Hiroshi Nishihara, Prof. Masaaki Fujii, Dr. Tetsu Yonezawa, Dr. Makoto Sakai, Dr. Yasunori Yamanoi, and Dr. Tsutomu Omori for their cooperation. I thank Ms. Shinobu Sano for her help with the measurements of photoresponses.

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