~

~ ,ndion J.Phys. 658

(5), 459-465

Carotenoids applications

'1991)

: Novel biomolecules

of potential

device

Prabir Pal and T N Misra Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta- 700032, India Abstract:

The presence of various ad ,.,b,d

goo.. In...'

tb, _Ioondue-

~~

. tion current of carotenoids. Among different carotenoids crocetin is the sensitive sensor for ethanol vapour. Thin film surface cell shows faster response and recovery. Very high photoconductive response is obtained for crocetinaldehyde and can be used for optical switching devices. Carotenoids are capable of forming charge-transfer complex with iodine. About seven order of current enhancement is observed for Iycopene-iodine CT complex. A Mg/lycopeneiodine CT complex/C solid state battery is developed. Battery parameters indicate its applicability for microgadget energizer.

Keywords: Organic semiconductors, vapour adsorption, charge-transfer complexes, current-voltage characteristics, open circuit voltage.

PACSNos: 87.10. +e, 87.15. Mi, 72.80. Le

I.

~

Introduction

Carotenoids constitute the second major pigment found in all autotropic plants. Theirpresencein plant leavesalongwith chlorophyllsand also in gr&)nand purple photosynthetic bacteria indicate their involvement in photosynthesis (Rosenberg 1961, 1966a,Goodwin1980). Freeand protein bound carotenoidsare found in certain loci of undisputed importance such as in the rod and cone layers of eye and in the olfactoryepithelium of animals and of human beings. Visual pigment rhodopsin is a carotenoid 11-cis retinal. bound to an apoprotein 'opsin by a protonated Schiff-base to the terminal amino group of lysine residue. Photoisomerisation of retinal is the basic process of our visual sensory perception. Another sensory perception in which carotenoids are involved is olfaction (Rosenberg et al 1968). In Figure 1 some commoncaroten()idsare shown. These are conjugated 1T-electronicchain molecules with alternate single and double bonds. The conjugated 1T-electronicstructure gives these compounds the properties of semiand photoconductorsin the crystallinestate. Their unique semi- and photoconductive properties lead to the possibility that these materials can be used in 'thedevelopmentof some useful devices such as gas sensors, optical switching 459

460

Prabir Pal and T N Misra

devices, organic metals and solid state battery. In this paper we report the experimentalresults of our studies in the context of device development. 0 II C-OCH3

o,c.

I

OCH3 t.

McithylbiJtin

0

CH3

2.

Astacin

CHJ COOH

,

CH3

3.

Cl"ocrtin

CH3

tHO

OHC 4.

CH3

CrocetinaidehYde

CH3 CH3

CHJ

CH3

6. p-caroton. figure I. Chemicalstructure of some linear conjugated polyenes.

~

Carotenoids: Novelbiomoleculesof poJe.nti,al ~e¥iceapplications

4~1

~. Experiment~J Highly pure caroteooid~used in this investigation wereobtained as a gift from the Hoffman-la-RocheCo., Switzerland and were used without further purification after checking the purity of the sample by absorption spectroscopy. The sample was taken in a sandwich ceIl with -stainless-steeland.SnOIl coated glass electrodes for conductivity measurement. The ceIl was placed in a suitably designed conductivity chamber (Mallik et al 1979) with a quartz window for photoconductivity studies. For current measurement, Keithley 617 programmable electrometer was used. This electrometer was also used as a voltage source to apply d.c. voltage. Temperature measuJeffi!lntwas made using a copp~r-constantan thermocouple. A 100 watt xenon lamp was used for s~{3ac:lY ~tate photoconductivity work. A Shimadzu spectrophotometer (model NQ. 21.0A) was used to run the absorption spectrum. The sandwich ceIl thickness and area were maintained at 0.005 cm and 0.25 cmll respectively. Several measurements were made in order to ensure reproducibility of results both in vacuum and also in an atmosphere of dry nitrogen. '.

-

3. Results and discussion 3.1. Gassensors: Carotenoids are semiconductors in the ~oUd state.

The interesting aspect about

the semiconductingpropertiesof the carotenoidsis that it is verysensitiveto the ambiant atmm;phere. As the ambiant gas molecules get' adsorbed on the carotenoid crystals, .its conductive properties change significantly. It has been

reported (Rosenberget al 1968) that presence of nitrogen dioxide, hydrogen sulphide, ammonia, methanol, nitric oxide, acetone etc. enhances the conductivity of ,8-caroteneby 10"-108 tim~s but only modera~eincrease is observed with methyl acetate, nitrous oxide etc. With nitrogen, hydrogen and helium,gas there is no significant change. On methyl acetate vapour adsorption vitamin A alcohol ceIl senses methyl acetate vapour very efficiently but only low response is obtained in ,8-carotenecell. Tabl.e1 shows our results on ethanol gas sensors with carotenoids. After trying a number of carotenoids, crocetin is found to be most sensitive biomaterial for alcohol sensing. . Both thin film sandwich ceIl and sur{ace ceIl have been studied. Vacuum deposi~ed th,in film surface ceIl sensor shows very fast response and recovery. The active layers used ,in our sensors have not yet been optimised. Future work to optimise sensor characteristics i~ needed I;In~low noise, IC-compatibleFETdevice structures is'to be designed an~ fabricated. The deposition conditions for our thin films are completely compatible with such structures. The unique electrical properties of the films are a result of the film morphology associated with our specific cleposition approach. 12

-.

462

Prabir Pal and T N Misra

Thus different polyenes responddifferently to different gases as regardstheir electrical conductivity. By using different polyenes, it is thus possible to developa sensorspecific to a particular gas. Table I. Ethanol sensors with carotenoids. Sensor at 25°C. Partial vapour pressure of alcohol is SO mm. 51. No. 1 2 3 4 5 6

Carotenoid

uAfuv Sensitivity

Vitamin A alcohol Vitamin A acetate

3 x 10' 2x10' 5 x 1O' 4 x 10. 1 x 10' S x 1O'

.B-Apo-S' carotenal Astacin Methylbixin Crocetin

uA=conductivity after adsorption of alcohol. uv=conductivity in dry nitrogen atmosphere,

3.2. Optical switchingdevices: The basic properties required for an optical switching device is that a high resistance material becomes conducting due to the action of light. Someof the polyenes possessthis properties to a good extent. The current-voltage characteristics of dark and photocurrent of crocetinaldehyde crystals in a sandwich cell are shown in Figure 2. If one choosesa working voltage of 40 V, it is seenthat the current increases 10'" times due to the action of light. It is possible to design the cell with larger surface area and smaller thickness to enhancethe photoconductivity of the sandwich cell by a few order of magnitude more. These materials can, therefore, be used to develop an optical switching device. 3.3. Organicconductors: The fundamental requirement of high conductivity is partially filled conduction band i.e., mixed valence state. This situation is not met in carotenoid crystals. These are semiconductors with a energy band gap, an empty conduction band and a partially filled valence band. However, this situation can bA changed by forming charge-transfer complexes. Some organic charge-transfer salts, where the donor and the acceptor molecules form segregated stackings in the lattice and the charge transfer between them is partial, are highly conducting (Torrance 1985). The other mechanism of high conductivity in organic so.lid in inter!':oliton hopping which makes doped polyacetylenes and related conjugated polymers highly conducting (Kivelson 1981). Like polyacetylenes, polyenes are"also:conjugated 1T-electroriic system with alternate single and double bonds in the main chain. One can find a carotenoid with favourable end groups or tailor a caro-

'-

Carotenoids : Novel

biomoleclilesof potential device applications

1(5

463

to"

p

, ,I

,. ,p

, ,, ,6

.d' ,,

r-;.. Q. e

~ ~ t69 w

,,

a: a:

a

,0"

-.

~ ~

,','If

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, p'

0

I0

.:I: a..

, ,,

, j!J'

~ Q

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totO. 2

1613

10 APPLIED VOLTAGE (Volt)

Figure 2. I-V characteristics (-) represents dark and

100

of crocetinaldehyde in dark and on illumination. for photoconductivity.

(-. -)represents

"'..."

t(f10

'..."

10-4

", "

"

,...

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'I

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... \0-11 z

1&1 IX IX ::I U

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115"2

].1

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10""

3-2

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10.00 -1

r-

K

Figure 3. Arrhenius plots for pure Iycopene and Iycopene-iodine complexes. Solid lines represent pure Iycopene and dashed lines represent Iycopene-iodine complexes.

-~ J

'.

464

rI .J.

Prabir Pal and 'f N Misra

tenoid suitably to make the soliton conduction possible. Polyenes, however, are known to form CT complexes. There is enormous increase in conductivity in some carotenoids when complex~d with suitable electron acceptors. In Figure3, log of current agains~ inverse temRerature plot of some polyeQes and there CT complexes with ioctine are shown. There is seven order of magnitude current enhancement in the CT complex. It, therefore, seems possible to developorganic conductors based on carotenoids. 3.4. Solidstatebattery: Working of carotenpid based solid state battery has been reported. Jain and his

collaborators(Jain et al 1988) u$edVitam!:n A acetate-II!complexas the cathodic material and ~n as the anode. Lycopene-II!complexis a better material for solid

.

state battery.

'In such batteries

no metal salt is J9hysically usee;! in the cell

assembly but it is formed during the operation of th.e battery. The battery assembly is shown in the inset of FigureA. Mg anode and Iy~opene-II!complex 2.0

l:/lP.OTEIIO'.

+

-.It Com;>!..

E ?': ~~

::;.:§ ...:.=

,M g

1.6

~ <-GRAPHITE (Inerl Electrode) t CT 5." Lay., :~i

.,,"";.

MOIZ

-:>

1.2

J, 0.. .0 « 0 0.8

'" <.:) « I-

-' 0 >

0.4

0

40

eo

120

100

200

riME (lfours) Figure 4. Discharge characteristics of.,the cell with config\.lration Mg/lycopeneiodine complex/C across a load .of 240 k,Q (discharge current ~ 4 pA). The inset shows the schematic representation of the Mg/lycopene-iodine complex/C cell.

,

in a pressed pellet form are

used as the.cathode.

graphite inert electrode.

I II is released from the .CT complex and a Mgll! salt

These -are then backed with a

layer is formed at the interface. The electrochemical current is derived from the reactipnof Mg with II!' An open circuit voltage 1.65 V obtained against theoreticallyexpected1.84V(Meier1974). The salt layer is permeable(to:tli1e 1- ion but

! I I~

of

Carotenolds : Novel blomoleculesof potential device applications

465

insulating to electron and this keeps the battery operating. The voltage-current plot is shown in Figure5. In the initial stage it is linear. At this stage the overvoltage which is evidently due to the internal resistance of the battery, is negligible. At higher current drain voltage drops more rapidly as polarisation of the electrodes sets in. The value of overvoltage increases abruptly for current

.

10.0

.....

~ UJ

C>

;! ...J t.O 0 >

0.1' I

1O

100

1001

CURRENT (JlA)

Figure 5. Current-voltage characteristics of electrochemical cell with configuration Mg/lycopene-iodlne complex/C.

about 200 pA The battery has very satisfactory discharge characteristics as shown in Figure 4. When 4 /loAis drawn, there is negligible drop of voltage upto 160 hrs. The power density of such battery is 2.3 W/kg. 4. Conclusion Carotenoid gives us the sense of vision and the sense of smell. It can also give us some useful devices. Acknowledgment Thanks are due to MIS Hoffmann-Ia Roche and Co. Ltd. for generous gift of carotenoids. References

Goodwin T W 1980 The Carotenoids2nd edn vol. I (London: Chapman and Hall) Jain K M. AbrahamSand Hundet A 1988JapanJ. Appl.Phys.27 867 KivelsonS 1981 Mol. Cryst.Liq. Cryst.77 65 MallikB. Ghosh A and MisraTN 1979 Bull.Chern.Soc.52 2091 Meier H 1974 Organic Semiconductors (Weinhelm: VerlagChemie) p 455 Rosenberg B 1961). Opt.Soc.Am.51 238 -1966a Advancesin radiationbiologyvol 2 (New York: Academic)p 193 RosenbergB. MisraT N and Switzer R 1968 Nature217 423 Torrance J B 1985 Mol. Cryst. Liq. Cryst. 12655