Langmuir 1997, 13, 5799-5801
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Study of the Improvement of Photoelectric Response and Stability on a 9-cis-Retinal Langmuir-Blodgett Film Containing Ultrafine Gold Particles Y. H. Sun, J. R. Li, B. F. Li, and L. Jiang* Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, China Received May 19, 1997. In Final Form: August 7, 1997X In order to improve the photoelectric response and stability of 9-cis-retinal Langmuir-Blodgett (LB) films, this article investigated the effect of nanometer-sized inorganic particles on organic functional membrane. The LB film containing 9-cis-retinal protonated Schiff base and mixed with hydrophilic gold sol containing different sizes of gold particles (from 10 to 50 nm) prepared by reducing HAuCl4 with sodium citrate was deposited in a ITO electrode. After the ITO electrode was immersed in the gold sol, 8-10 layers of LB membrane of samples containing 9-cis-retinal PSB were deposited on it, and the photoresponse and stability with and without ultrafine gold particles were measured. After added ultrafine gold particles, it was found that the photocurrent and stability of 9-cis-retinal LB films could be increased greatly. The quartz crystal microbalance, cyclic voltammetry, and absorption spectrum methods have been used to study the mechanism of this effect.
1. Introduction Bacteriorhodopsin (bR), existing in the purple membrane of Halobacterium halobium, is the best known naturally occurring retinal-protein complex. The remarkably robust nature of bR, coupled with its ability to reversibly change color upon illumination and its high cyclicity of ground-to-photoinduced state transitions, makes bR an outstanding biological material for optical information processing and energy transducing. The highvolume three-dimensional memory is an important aspect for bR to be used in the future.1,2 There are two methods to use bR as information memory material: one is to prolong the middle state (“M” state) in its photocycle, and the other is to find a new middle statesa “Q” state.2 The three-dimensional memory based on a “Q” state is simpler and more stable, so it will be one of the main research subjects in the future. Recently, it is showed that the photoreaction product of the “O” state-“P” state (490 nm) of bR in glycerol undergoes a slow thermal reaction to a further blue-shifted product named a “Q” state with an absorption maximum at 380 nm on which free 9-cis-retinal might exist in the protein binding site at pH 6.5.3 Our previous research has proved that 9-cis-retinal and filmforming agent digitonin could combine to form a mimic bR film with apparent photocurrent response.4 Nevertheless, the stability and photoelectric response of this kind of membrane were not very satisfactory. In this paper, we sought to improve the properties of 9-cis-retinal Langmuir-Blodgett (LB) films by adding inorganic ultrafine Au particles and reported the results.13 The goal of this work is to make a foundation for acquiring stable a “Q” state. X
Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) Birge, R. R. Am. Sci. 1994, 82, 348-355. (2) Birge, R. R. Sci. Am. 1995, March, 90-95. (3) Popp, A.; Wolperdinger, M.; Hampp, N. Biophys. J. 1993, 65, 1449-1459. (4) Li, J.; Wang, J.; Jiang, L. Biosens. Bioelectron. 1994, 9, 147-150. (5) Long, J. Chin. Chem. Lett. 1991, 2 (12), 921-924. (6) Frens, G. Nature (London), Phys. Sci. 1973, 20, 291. (7) Siebrands, T.; Giersig, M. Langmuir 1993, 9, 2297-2300. (8) Chen, Z.; Govender, D.; Gross, R.; Birge, R. Biosystems 1995, 35, 145-151. (9) Vsevolodov, N.; Dyukova, T. Trends Biotechnol. 1994, 12, 81-88. (10) Okahata, Y.; Kimura, K.; Ariga, K. J. Chem. Soc., Chem. Commun. 1987, 1535. (11) Sauerbrey, G. Z. Phyzik 1959, 155, 206. (12) Hong, F. T. IEEE Eng. Med. Biol. 1994, 13, 25-32.
S0743-7463(97)00513-1 CCC: $14.00
Figure 1. The photocurrent response of LB film consisting of a mixture of 9-cis-retinal PSB and digitonin under 50 mW/cm2 white light: (a) not containing Au; (b) containing 10 nm Au particles.
Figure 2. Influence of ultrafine gold particles on the stability of 9-cis-retinal PSB LB films.
2. Experimental Methods 9-cis-Retinal, ethanolamine, and digitonin were bought from Sigma. N-2-(Hydroxyethyl)piperaazine-N′-2-ethanesulfonic acid (HEPES) as biological buffer was bought from the Institute of Biochemistry, Shanghai, China. The 9-cis-retinal Schiff base were synthesized by reaction of 6 mL of 1 mM retinal in CHCl3 with 1 mL of ethanolamine (4.1 M in ethanol or 3.4 M butylamine at room temperature in the dark for 30 min). An absorption (13) Koyama, K.; Yamaguchi, N.; Miyasaka, T. Science 1994, 265, 762-765.
© 1997 American Chemical Society
5800 Langmuir, Vol. 13, No. 22, 1997
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Figure 3. Influence of gold particle size on the photocurrent responses.
Figure 4. The change of cyclic voltammetry of PSB after adding 10 nm gold particles.
Figure 5. Absorption spectrum of PSB with and without Au particles. peak at 363 nm appeared, showing a Schiff base was formed. Then a solution of 6 M HCl was added and an absorption peak at 443 nm was found which showed that a protonated Schiff base (PSB) was formed.5 These peaks coincided with the literature
Figure 6. data.14 Doubly distilled water was used for the preparation of the aqueous subphase. All experiments using retinal and its Schiff base were performed in dim red light. In this experiment, (14) Rabinovitch, B. Photochem. Photobio. 1978, 29, 567-574.
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Langmuir, Vol. 13, No. 22, 1997 5801
mixtures of 9-cis-retinal PSB in chloroform (1.5 × 10-3 mol/L) and digitonin in ethanol (1.5 × 10-3 mol/L) in a molar ratio of 2:1 were spread on the surface of the HEPES solution (5 × 10-3 mol/L, pH 6.8) to get 9-cis-retina PSB monolayers.4,15 Ultrafine gold particles were made by the method of reducing 0.01% HAuCl4 with 5.36% sodium citrate.6,7 The size of gold particles could be controlled by varying the content of sodium citrate. After analyzing with scanning electron microscopy (SEM), we chose three kinds of gold particles with different diameters: 10, 30, and 50 nm. The ITO conductive glasses (1 × 3 cm2 in size, specific resistance 5 × 10 Ω‚cm) were immersed in the newly made gold sol for about 1 h, and then were desiccated for dipping LB films. The LB film was a Z-type multilayer, built up by the vertical dipping method on ITO conductive glass at a surface pressure of 16 mN/m. The transfer of LB film was carried out in a face membrane balance (HBM-SS, Japan) with a transfer ratio of 0.8. The modified electrode (8-10 layers) was used as a working electrode and a platinum wire as a counter electrode. Two electrodes were set in an electrochemical cell containing a support electrolyte of 1 × 10-3 mol/L FeCl3 and 0.1 mol/L KCl (pH ) 2.1). Photoresponse electrical current measurements were carried out without bias by using a osillograph (Goldstar 20M, Korea), under illumination of a 400 W xenon lamp filtered through a heat shelter cuvette containing 15 cm of water. The size distribution figures were measured from the transmission electron microscopy (TEM) photos of the 10, 30, and 50 nm gold particles, and there were about 1000 particles which had been counted. The quartz crystal microbalance (QCM) measurement was performed by the frequency sensor and quartz crystal made by Sharp. The cyclic voltammetry was measured at a 250 mV/s scan velocity. The absorption spectrum was carried out on a Hitachi 557 UV spectrum instrument.
3. Results and Discussion The QCM measurement10 was obtained after the film was soaked in a 10 nm gold sol for 1 h. The frequency of the quartz crystal changed from 9.139 × 105 Hz to 8.758 × 105 Hz. The change of mass could be worked out by the equation offered by Sauerbrey:11 ∆m ) -0.781 × 10-9∆f. The mass addition of the quartz crystal was 29.7 mg. The area of the quartz crystal was 0.3 cm2 and the area of the ITO electrode was 5 cm2, so about 0.495 mg of gold particles could be deposited on the ITO surface. Figure 1 shows the photoresponses of LB films on two ITO electrodes consisting of a mixture of 9-cis-retinal and digitonin (molar ratio 2:1). An ammeter with sensitivity of 1 nm was used to measure the photocurrent caused by the irradiation of white light (50 mW/cm2) onto the modified electrode. The illuminating time was taken for 2 s everytime, and the next illumination was 1 min later. The induced photocurrent reached its maximum in about 2 s and then decayed exponentially to the background level. It was shown that after addition of ultrafine gold particles with diameters of 10-20 nm in 9-cis-retinal PSB film, the photocurrent could be enhanced for about 60%, from 60 to 100 nA at first, and the decay of photocurrent was much slower. After 10 illumination cycles, the LB film containing gold particles still has a photocurrent of (15) Palings, I.; Pardoen, J. A. Biochemistry 1987, 26, 2544-2556.
about 70 nA, whereas the LB film without gold particles decayed to only 30 nA. Figure 2 shows the influence of ultrafine gold particles on the stability of 9-cis-retinal PSB. We measured the two electrodes after 1, 3, and 5 days at room temperature. The size of gold particle we selected was about 10-20 nm. The photoresponse of PSB film without gold was almost zero after 5 days, but the PSB film with gold particles still showed a photocurrent of about 35 nA. So after addition of ultrafine gold particles, the stability of 9-cis-retinal PSB could be improved greatly. Figure 3 shows the influence of different size gold particles from 10 to 50 nm on the photocurrent responses. Three kinds of particle size were selected: 10, 30, and 50 nm. From the figure, it shows that with the enlarging of gold particles, the photoresponse current became smaller. The 10 nm gold particles could most improve the photocurrent. Figure 4 shows the change of cyclic voltammetry of 9-cisretinal PSB after adding 10 nm gold particles. Figure 5 shows the absorption spectrum shift of 9-cis-retinal PSB after adding 10 nm gold particles. Both results of the two experiments showed that ultrafine gold particles and 9-cisretinal PSB could form an unknown complex which makes the PSB LB film more stable. Because of the new complex of gold and PSB, the reduction peak of voltammetry shifted to the negative direction and the peak of the absorption spectrum shifted to a longer wavelength. Figure 6 shows the size contribution of 10, 30, and 50 nm gold particles. Here we just reported these very interesting effects of nanoparticles of gold on the photoresponse LB film behavior of a mimic retinal film. The mechanism of this effect still remains obscure, and a more detailed investigation is in process. 4. Conclusion The results described here show that the addition of ultrafine gold particles could improve the stability and photoresponse of 9-cis-retinal PSB film greatly. The photocurrent could be enhanced 60-80%, and the decay time could be prolonged substantially. Experimental results proved that the smaller gold particles (10-20 nm) showed more apparent effects. After 10 nm gold particles were added, the reduction peak of cyclic voltammetry shifted to the negative direction and the peak of the absorption spectrum shifted to the longer wavelength. We concluded from these effects that there may have been a chemical absorption and a new complex was formed. Because the reduction potential was more negative, the 9-cis-retinal PSB was not easily reduced and its stability was enhanced. Acknowledgment. The financial support of Academia Sinica and the National Science Foundation of China are greatly acknowledged. LA970513S
