Motion-sensitive positionsensor using bacteriorhodopsin
Kenji Fukuzawa
A concept of motion-sensitive position sensing that uses a film of bacteriorhodopsin (bR) treated with ahigh-pH buffer is proposed. The polarity of photo-emf, which depends on the excited intermediates ofthe bR molecule, can be used to determine movement direction. The M intermediate state with a longlifetime can be used to record the initial position. The bR-based position sensor can sense two positionsof a moving object at two different times.
Bacteriorhodopsin (bR) is the only protein in thepurple membrane (PM), a cellular membrane of thebacterium Halobacterium salinarium.1 The bR inthe PM is highly durable in ambient conditions.Recently Shen et al.2 reported that dried bR canretain its native structure in temperatures as high as140 C. In addition Miyasaka and Koyama3 esti-mated that bR remains stable in direct sunlight foryears. Because bR in PM has unique optoelectriccharacteristics as well as high durability, many groupshave intensively explored its possible electrical oroptical applications. 6 When excited by light, thebR molecule, a protein analog of the retinal pig-ment,1 6 transports protons associated with molecularcyclic transformations. It thus acts as an organicoptic-to-electric conversion material. The optoelec-tric properties of bR are quite different from those ofexisting optoelectric materials such as semiconduc-tors, because the bR molecule includes a retinalchromophore that plays a central role in light-excitedproton pumping.' The optoelectric characteristicsof bR can be modified rather easily by physical andchemical processes,7-9 and its artificial variants havebeen studied for use in holographic recording materi-als.1 0
Many groups have reported novel photodetectorswith the unique photoelectric properties of bR.6 "-'4
The author is with the Interdisciplinary Research Laboratories,Nippon Telegraph and Telephone Corporation, 3-9-11, Midoricho,Mushashino-shi, Tokyo 180, Japan.
Lewis's group' 1 2 recently proposed a method formodifying bR by treating PM films with a high-pHaqueous buffer solution and presented an image thatcan be recorded in the high-pH-treated PM films.In addition they proposed several applications ofthese treated films: a neural-network architectureand an edge-detection device.12 To our knowledge,however, bR-based position sensing has not yet beenreported, even though a position sensor is one of themost useful optical sensors. A motion-sensitive posi-tion sensor is especially useful in many applications,e.g., in robot vision.
Miyasaka et al. as well as Chen and Birge haveproposed different types of motion-sensitive artificialretina.3 131 4 These two bR-based sensors can sensean object's motion and direction of movement; how-ever, their sensors were not sensitive to displacementof a moving object but rather to its velocity, becausethey used differential light responsivity of bR atneutral pH. In this paper the author, using theimage-recording capability of bR at high pH, proposesa sensor that is sensitive to the object's displacementfor a certain time and position. This sensor is able tosense an object's position at a certain earlier time andits current position without the use of an externalmemory device. The advantage in size and simplic-ity over other solid-state position-sensitive devices isensured because this sensor is based on biologicalfunctional materials.
Position-Sensing Methods
Although the cycle of a high-pH-treated PM filmincludes several intermediate states, the intermedi-ate states other than the M state can be neglectedbecause of their short lifetimes. The photochemicalcycle of a high-pH-treated PM film can thus be
Fig. 1. Photochemical cycle of bR treated with a high-pH buffer.Although the cycle of bR includes several intermediate states, theintermediate states other than the M state can be neglectedbecause of their short lifetimes.
approximated by a two-state model that consists ofthe resting state (bR) and M intermediate state (Fig.1). On illumination with green light (X 570 nm),bR molecules are converted to the M intermediatestate, and the M state is quickly returned to bR whenilluminated by blue light (X - 410 nm). Both chemi-cal transitions are associated with photoinducedcharge movements.
Two features of the high-pH-treated bR are note-worthy for the fabrication of the motion-sensitiveposition sensor. First, thermal vibrations also re-turn the M intermediate to bR after more than 10 s,whereas the bR -> M transition time is only 40 [is.This relatively stable M state can be used for record-ing an object image. Second, the polarities of thephoto-emf's in bR -> M and M -> bR transitions areopposite. The charge movement associated with thebR -> M transition is from the cytoplasmic side of themembrane to the exterior side, and the movementassociated with the M -> bR transition is in theopposite direction. These opposite polarity photo-emf's can be used to determine the direction in whichan object moves. These two features can be used tosense two positions at two different times when anobject is moving.
In this paper the one-dimensional position sensorwhose schematic cell structure is shown in Fig. 2 isdiscussed. The PM film was isolated from Halobac-
Electrode 1SnO2 -coated
. glass substrate
Light stripe at t=t
Fig. 2. Top view of the bR-based position sensor. L and H arethe length and width of the electrode. W denotes the width of the
stripe. The moving light stripe is at positions X0 and X, at timest = 0 andt = t,.
terium salinarium S9 by a procedure modified fromthat of Oesterhelt and Stoeckenius15 (Consortium furElectrochemische Industrie in MUnchen, Germany).The standard electrodeposition method16 was used toproduce a highly oriented PM film on a transparentand conducting SnO2-coated glass. In Fig. 2 L and Hare the length and width of the electrode, which was
15 mm long and 4 mm wide, respectively. Thethickness of the PM films was a few micrometers.The deposition electric field applied was 45 V/cm, andthe deposition time was 25 s. After the film wasdried, a drop of pH = 9 borate buffer was added to thefilm, and it was dried again according to the proce-dure in Refs. 11 and 12. Two triangular tin-leadalloy films (6 [um thick) were pressed onto the PM filmas symmetrical top electrodes. The photocurrentthat flowed between the SnO2 conductive layer andthese triangular electrodes was detected by an opera-tional amplifier-based current-to-voltage (I-V) con-verter with a feedback resistance of 1 Gfl. As shownin Fig. 2 one I-V converter was attached to eachelectrode, and photocurrents from electrodes 1 and 2were detected separately. The photocurrent re-sponse patterns obtained by each electrode weresimilar to those in Ref. 11 because the sensor cell hada similar structure.
A narrow light stripe parallel to the shorter side ofthe electrode moved over the PM film in a directionparallel to the longer side. In Fig. 2 W denotes thewidth of the stripe. Green light was used as the lightstripe so that I could record the image of the stripe byinducing the bR -> M transition. The moving lightstripe was at positions X0 and X, at times t = 0 andt = tS, respectively (Fig. 2). Figure 3 shows the threesteps for sensing the positions of X0 and X, fromphotocurrents at t,.
(1) Recording the stripe image. At t = 0 a pulseof green light is used to record the light stripe imageatX0 by the bR -> M transition [hatched trapezoids A,and A2 in Fig. 3(a)].
(2) Sensing the two positions. At the sensingtime t, the green stripe is at position X1. The bluelight, which illuminates the entire sensor cell, in-duces the M -> bR transition at X = X0 [A, and A2 inFig. 3(b)], whereas the green light pulse induces thebR -> M transition at X = Xi [B1 and B2 in Fig. 3(b)].Note that the photoinduced charge movements accom-panying these transitions have opposite polarities.
(3) Erasing the stripe image. At t = te the entirecell is illuminated with blue light to return all themolecules to the bR state [Fig. 3(c)]. Because thelight-excited transition times of bR -> M and M -> bRare 40 gus and 200 ns, respectively, the minimumdetection time is of the order of 100 lis.
Let us discuss the relationship between the outputphotocurrents at t, and positions X0 and Xl. First, ina more general case, the relationship between aphotocurrent and light-illuminated area is discussed.The sensor cell consists of a sandwich cell as shown in
the inset of Fig. 4. In this case a light spot illumi-nates the cell, and photo-emf is generated in this partof the cell. The photocurrent is detected with theI-V converter. The equivalent circuit is assumed tobe as shown in Fig. 4. The terms Rp and Cp are theresistance and capacitance of the light-spot-illumi-nated part of the PM film. Rm and Cm are theresistance and capacitance of the rest of the cell. Rf
Sensor cell I-V converter
g. 3. Schematic timing chart of the light pulses. The bR statese also shown: bR and M denote the resting and M states.I The recording of the stripe image is depicted. The green lightripe is located atX0. In the stripe-illuminated part ofthe PM (Alad A2 ), the bR- M transition occurs. (b) The sensing of twositions is shown. The green light stripe is located at Xl. Blue, ht illuminates the entire cell. Blue light induces an M -> bRensition in Al and A2 at X0, whereas green light induces a bR -> Mmnsition in B and B2 at X1. Photocurrents I and 2 aretected. (c) The erasure of the stripe image is shown. Thetire bR is returned to the bR state by illumination with blueght.
and e are the feedback resistances of the I-V con-verter and the photo-emf of the bR molecules. Thecurrents i, ip, and im flow into point A in Fig. 4, and i isthe output current. They are defined by the follow-ing equations:
d(V- e) V- e
dt Rp
dV V
m m -(it Rm,
i = -(ip + W,)
(1)
(2)
(3)
where Vis the voltage at point A. Because point A isa virtual ground, V is always zero. Therefore theoutput current i can be written as
i = C de edt Rp
Light illuminated Rest of PM PM filmpart of PM
Fig. 4. Equivalent circuit of the detection system. The cell to beconsidered is shown in the inset. Rp and Cp are the resistance andcapacitance of the light-stripe-illuminated part of the PM filmshown in the inset. R and Cm are the resistance and capacitanceof the rest of the sensor cell. Rf and es are the feedback resistanceof the I-V converter and the photo-emf of the bR. Currents i, ip,and im flow into point A.
(4)
Because Rp is very large (- 1 G), the second term inEq. (4) is much smaller than the first term, which isthe displacement component of the photocurrent.The output photocurrent is therefore almost propor-tional to the product of d(ei)/dt and Cp. Thereforethe photocurrent is proportional to the square of thephoto-emf-generated part of the area (Apg in Fig. 4)because Cp and ej are proportional to Apg.
Applying the above discussion to the position sen-
sor cell, we can obtain the relationship between theoutput photocurrents at t and positions X0 and X1.I, (the photocurrent detected with I-V converter 1) isthe sum of the photocurrents generated in A1 and B1in Fig. 3(b), and 2 (the photocurrent detected withI-V converter 2) is the sum of the photocurrentsgenerated in A2 and B2 in Fig. 3(b). Considering theelectrode configuration shown in Fig. 2, we can obtainphotocurrents I and 2 at t with the followingequations:
In this case experimental conditions such as the pulseintensities or pulse-illumination intervals are re-stricted to the a + = 0 condition, whereas thedetectable area is not restricted.
(2) a + 13 • 0. Solving Eqs. (5) and (6) simulta-neously yields
X= (A-D) +2(oa + ,)L
±[a1(2AS - A2 - D2)]1/2 W
2ua(u + )L 2
(10)
H2w2 (W 21=a 2 + 2
H 2 W12 + 2L 2 (x 2
(5)
(A - D)= 2(a + 13)L
±[a13(2AS - A 2 - D2)]1/2
21(a + )L
H2W2 W 2I2 = a LZ2 L - X - 2
H 2W2 L XW 2 (6)bL2 L X1 2 6
where a and b are the photo-emf generations per thesquare of the area for M -> bR and bR -* Mtransitions, respectively. For simplicity let a =a(H 2 W 2)/L 2 and = b(H2W2)/L 2 in the followingdiscussions. Note that the signs of a and (3 areopposite. Current that flows from the exterior sideto the cytoplasmic side is defined here as positivecurrent, so the sign of a is a plus whereas that of 13 is aminus. According to Eqs. (5) and (6), the positions ofX0 and X1 are obtained from the photocurrent de-tected through electrodes 1 (I,) and 2 (I2) at t5.
When one is simultaneously solving Eqs. (5) and(6), the solutions depend on the conditions of thecoefficients a- and 1; that is, (1) a + 13 = 0 and (2) a- +p3 • 0. In general a + 1 is not equal to zero, andthese coefficients depend on the experimental setup.It is clear that the a + 13 = 0 condition is neversatisfied if a and 13 take positive values, as they do forconventional optoelectric materials. Let us first solvefor the special case of (1) and then for the moregeneral case of (2).
(1) a + 13 = 0. The solution to Eqs. (5) and (6) isobtained uniquely and can be written in the followingform:
D L S WXO +40L 2 1D) 2
D Ll S W4u-L 2 D1 2
(7)
(8)
where S = I + I2, D = 2 - I, and L and W are thelength and width of the triangular electrodes asshown in Fig. 2. The displacement can be written inthe following form:
W
2
(11)
where A = (a + 13)L2, S = I1 + I2, D = I2 - I1, and thesigns chosen for the second term in both equationsmust be identical. Although Eqs. (10) and (11) arerather complicated forms, they always have realsolutions, because the terms in the square roots ofEqs. (10) and (11) are given by the following equation,which is always either positive or zero:
A = 4a2122L2(X1 - XO)2. (12)
To select the correct pair of solutions to Eqs. (10) and(11), let us focus on the output difference D =(I2 - I,), which is related to the direction of themovement. The displacement X1 - X0 is given by
X1 - TX = -±[a1(2AS - A2
-
2u13L. (13)
The correct sign in Eq. (13) can be determined if the(X1 - X0) sign is obtained. For convenience let theX1 - X0 sign be given by Eq. (9), although Eq. (9) is forthe case in which a + 3 = 0. The X1 - X0 sign istherefore the same as that of the output difference D.For example, the plus sign in Eqs. (10) and (11) isselected if D is plus. According to this assumption,Eqs. (10) and (11) can be written as
X0 = (A - D) D/ ID I [x(2AS - A2 - D2)]1/2
2(oa + ,P)L 2a(a + P)L
W-, -s-(14)
(A -D) D/ ID |[otP(2AS - 2 D 2)]1/2
X 2(u + ,p)L 21(a + )L
w (1
2.5)
It is necessary to discuss the error in this assumption.According to Eqs. (5) and (6), the term D is also givenby
Fig. 5. Error region in which a + , • 0. AXe = Xe + W/2 - L/2and AX = X + W/2 - L/2.
An error occurs in two cases: X - X 0 > 0 when D <0 or X1 - X0 < 0 when D > 0. In Fig. 5 isschematically illustrated the region where the errorsoccur in the case in which a/,8 = -0.19. In Fig. 5AX0 = X0 + W/2 - L/2 and AX1 =AX1 + W/2 - L/2.The origin is therefore close to the middle of theelectrode in the figure, which shows that the errorregion is determined by the ratio I a/8 1. The closerthe ratio is to 1, the narrower the error region. Inthis case the experimental conditions have no limita-tion, but the detectable area is restricted as shown inFig. 5.
Results and Discussion
The feasibility of this bR-based position sensing wasexperimentally confirmed by the experimental setupschematically shown in Fig. 6. Green light and bluelight were produced with the glass filters and 150-Wxenon lamps. The transmittance maxima of theseglass filters (Melles Griot Corporation) were 530 and390 nm. The green stripe of a 1-mm-wide slit wasimaged on the PM film with an imaging lens. Bymoving mirror 1 or the imaging lens, the light stripewas moved over the PM film. The green and bluelight pulses shown in Fig. 3 were produced withphotographic mechanical shutters, pulse illuminationtiming being controlled with an 8112A pulse genera-tor and 8016A word generator (Hewlett-Packard Cor-poration). The blue light pulse illuminates the en-tire sensor cell. The pulse-illumination conditionsare listed in Table 1. To measure the displacementcurrents, output photocurrents of the I-V converterwere monitored with an ac-coupled oscilloscope. Inthe experiment a rather long green pulse width wasused to record the stripe image.
Table 1. Pulse-illumination Conditionsa
(a) For Position (b) For PositionConditions Sensing (s) Detection (s)
Recording time 0 0Recording GL pulse width 1.8 1.2
Sensing time t5 3.3 2.2GL sensing pulse width at t 0.3 0.2BL sensing pulse width at t 0.3 0.2
Erasing time te 4.2 2.8BL erase pulse width at te 0.6 0.4
aGL, green pulse; BL, blue pulse.
The coefficients at and 13 are obtained from therelationship between the output photocurrent andposition. Figure 7 shows an example of the relation-ship between the photocurrent I, at t and the light-stripe position; it also shows the dependence of thepositions of Xl and X0 whereas X0 and X, respec-tively, were fixed at 0.0 mm. The light-pulse condi-tions of Table 1(a) were used in generating these data.The curve in Fig. 7 can be approximated by thesecond-order polynomial equations, which indicatesthat Eqs. (5) and (6) are reasonable approximations.
Figure 8 shows the results of position sensing whena + = 0. The pulse-illumination conditions ofTable 1(a) were used in generating the data. Asshown in Fig. 8(a) the intensity of the blue light wasadjusted with neutral-density filters to obtain the a +1 = 0 condition, whereas the intensity of the greenlight was fixed. Figure 8(a) shows that the values ofa are roughly proportional to the intensity of the bluelight, whereas the value of is almost constant.Note that the a + = 0 condition is satisfied at thenormalized intensities from 0.15 to 0.20. Theneutral-density filters were therefore combined toadjust the intensity of blue light to 0.16. In thisexperiment ax and - were 4.9 pA/mm 2. Figures8(b) and 8(c) show the results of sensing positions X0and X1, and Fig. 8(d) shows the obtained displacementX1 - X0. Each figure shows the relationships be-tween the actual values and the values obtained withbR-based position sensing. From the output photo-currents at t (I, and I2), the positions X1 at t and X 0 att = t - 1.5 s were obtained with Eqs. (7) and (8).The positions measured for the moving 1-mm-wide
8001
bR-based position sensor
filter
Shutter Slit Xe lamp
filter
Fig. 6. Schematic experimental setup. One can move the green-light stripe by moving either mirror 1 or the imaging lens.
Fig. 8. Results of position sensing when a + 13 = 0. From the output photocurrents at t,, positions X1 at t, and X8 at t = t, - 1.5 s are
obtained with Eqs. (7) and (8). Dotted lines denote a ± 1-mm error. (a) The relationship between coefficients a and , and the blue-light
intensity (normalized). (b) The relationship between the measured and actual positions of X0. (c) The relationship between the measured
and actual position of X1. (d) The relationship between the measured and actual displacement Xl - X0.
stripe agreed with the actual positions to within 1mm. The measured displacementAXT - X0 also agreedwell with the actual displacement. These resultsdemonstrate the feasibility of bR-based position sens-ing.
According to the above discussion, when a + 13 = 0,the detectable area has no restriction in principle.In the experiment, however, there was a practicalrestriction. When IX - X0 I was equal to or lessthan the width of the stripe (W = 1 mm), the errorsobtained in the X0 and XA values were much largerthan they were when IXT - X0 was more than 1 mm.For example, the obtained values X0 and XA were 9.0and 8.4 mm even though the actual values were 3.5and 2.5 mm. When the displacement is small, the
green stripe overlaps the recorded image, and thecoefficients a and 13 are changed. In Figs. 8(b) and8(c) the data generated with displacements equal to orless than the stripe width are not shown. As can beseen from Fig. 8(d), however, the measured displace-ment XA - X0 did not show large errors in theseregions. The reason for this discrepancy is not yetclear. To summarize the results when a + 1 = 0, wesee that the measured positions agreed with theactual ones except when XI~ - X0 < W, whereas themeasured displacements agreed with the actual onesregardless of the value of W. This error becomessmaller when the stripe is narrower.
Figure 9 shows the results of position detectionwhen a + 13 • 0. The pulse-illumination conditions
of Table 1(b) were used to generate the data. Thecoefficients a and were 0.5 and -2.4 pA/mm2 ,respectively. From the output photocurrents at t, (I,and I2), positions X at t5 and X0 at t = t - 1.0 (s) wereobtained with Eqs. (14) and (15). The measuredpositions agreed with the actual positions. The mea-sured displacement X1 - X0 also showed good agree-ment with the actual displacement. These resultsshow the feasibility of bR-based position sensingwhen a + 13 • 0 as well as when a. + = 0.
In this case the detectable area is in principlerestricted (Fig. 5). However, another experimentalrestriction caused by the interference error as men-tioned above was dominant. When I - X 0I < W(W = 1.4 mm), Eqs. (5) and (6) did not have solutionsbecause A in Eq. (12) was negative, although theyalways have real solutions theoretically. In Fig. 9the data from these small-displacement cases are notshown. To summarize the results for a + 13 • 0, it isshown that the measured positions and displacementagreed with the actual ones except when I - X0I <W. The interference error becomes small if thestripe is narrow as when a + 13 = 0.
Conclusion
A protein-based motion-sensitive position sensor thatimmobilizes PM films treated with a high-pH buffer isproposed, and its feasibility is experimentally con-firmed. This bR-based sensor can sense two posi-tions, the current position of a moving light stripeand the stripe's position - 1 s earlier. The sensorperformance was investigated in two sets of experi-mental conditions for a + 1 = 0 and for a + 13 • 0 [seeEqs. (5) and (6)]. When a + = 0, positions can bemeasured in principle in all regions, although illumi-nation conditions must be adjusted. When a + 0the detectable area is restricted, whereas the illumina-tion conditions are not. In addition to these theoreti-cal errors, there is a practical interference error inboth sets of experimental conditions when the dis-placement distance is equal to or less than the widthof the light stripe.
I am indebted to H. Kuwano for promotion of thisresearch, and I thank T. Majima and T. Katsura fortheir guidance in biomaterial handling.
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