【文章內(nèi)容簡介】 solution, for example [10–13]. In this study BR is immobilized in dried thin films which are studied as biomaterialbased sensors capable of converting light into an electric signal. This study is related to the development of an intelligent artificial retina based on BR [9], and continues the research on optical properties of BR[14–19] changing the point of view to photoelectrical properties.2. Optoelectronic sensorThe thin film elements of our study were prepared by bining wildtype BR with polyvinylalcohol (PVA) to form a dry film. The size of these BR–PVA elements was about 20 20 mm. The purple membrane (PM) fragments consisting of BR molecules and lipids were isolated from the archaeal cells using a slightly modified version of the standard method [20]. The concentration of BR after isolation was mg/ml. 300 ll of BR sample was mixed with 750 ll of 20% PVA. The BR suspension was neutralized using 3 ll of 1 M phosphate buffer. The suspension was pipeted on conductive glass with SnO2as the conductive layer. We used the gravity coating method to evaporate water and produce thin films which are easy to handle. A thin layer of gold was sputtered on top of the film to function as the counter electrode. The output impedance of the BR–PVA element is several megaohms, thus impedance matching is needed when the element is connected to an amplifier. An operational amplifier (TL082MJG) was used as a voltage follower which was connected to the element as closely as possible with silver paint. The BR–PVA element and the voltage follower were installed into an aluminum housing to reduce electromagnetic interference from the environment. The window to illuminate the BR–PVA element was covered by a metallic mesh with a transmittance of 49%. The voltage follower was connected to a passive RC filter (3 dB cutoff frequency 7 Hz) to remove the DC from the response. Next, three cascaded secondorder filters(3 dB cutoff frequency 5 kHz) were used for high frequency noise cancellation since the fastest ponents of the photoelectric response are not needed in our implementation. Last, the response was amplified using an operational amplifier (TL084AC) for which the amplification coefficient was 56. The signal conditioning circuits were installed into another aluminum housing.3. Measurements The photoelectrical properties of the BR–PVA elements including the photoelectric response in time, the variance of the response when measured from a set of elements, and the area, intensityand wavelength dependence of the response were measured. Performing the measurements is not astraightforward task because of two reasons. First, BR is a moderately large molecule, a folded protein for which the charge transfer process is not exactly known. Second, the BR molecules in a thin film are embedded into the lipid bilayer and the PM fragments are surrounded by the PVA matrix,thus the results are subject to some modelling uncertainty. The photoelectric responses of BR in solution and in dried films has been widely studied, forexample [10–13,21–23]. When BR is incorporated into an artificial membrane, illumination by continuous light generates a proton gradient across the membrane (DC photoelectric effect). Additionally, a fast photoelectric response can be registered if a short pulse of light is used to excite the BR molecules (AC photoelectric effect) [11]. We use the latter phenomenon to register the changes in the illumination. The photoelectric response from the BR–PVA element illuminated by the pulsed Oriel series Q flashlamp is shown in Fig. 1. The flashlamp has a considerably shorter pulse than the camera flash used in our earlier measurements ( us vs. ~1ms). The discharge energy of the light source was 160mJ, and the frequency of light pulses was Hz. No saturation effects were noticed even when the interval between the light pulses was decreased. The peaktopeak voltage of the response after amplification was V, and the signaltonoise ratio dB. The time constant of the L ! M transition in the photocycle, about 20 ms, is three orders of magnitude slower than and the return back to the basic state BR is parable to the results obtained by others studying BR–PVA thin films [21]. Even with the cascaded lowpass filters,it was possible to observe that the response consists of several ponents related to the intermediate states of the photocycle. Ambient light during the measurement affects the response because it changes the proportion of dark and light adapted BR molecules in the element and causes some molec