1171
Langmuir 1987,3, 1171-1172
Olfactory Reception on a Multibilayer-Coated Piezoelectric Crystal in a Gas Phase1 Yoshio Okahata* and Osamu Shimizu Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received March 6, 1987. In Final Form: July 28, 1987 Various odorants and perfumes could be detected reversibly in gas phases at 10-nglevels by the adsorption onto multibilayer-coated piezoelectric crystals whose vibration frequency was decreased proportionally with the adsorption amount. Odorants and perfumes were found to adsorb specifically to lipid bilayer matrices in the solid state below their phase-transition temperature. There was a good correlation between partition coefficients of odorants or perfumes to the multibilayer-coated crystal and the olfactory threshold or the perfume intensity in humans, respectively.
Among various chemical receptions in our bodies, olfactory receptions of odorants or perfumes are not well understood.2 The structure of odorants is extremely diverse, and it is difficult to find a chemical structure common to these substrates. Odorant molecules are lipophilic, and this suggests that the interaction of odorant molecules with a lipid bilayer matrix is important in olfactory reception.2 Koyama and Kurihara reported that the surface pressure of lipid monolayers from bovine olfactory epithelium was increased selectivelyby the addition of various odorants in a water phase.3 The responses to odorants are seen not only in olfactory cells but also in nonolfactory systems such as the Helix giant neuron: the turtle trigeminal and frog taste cells. These results suggest that odorants may be detected by direct adsorption to the lipid bilayer matrix without specific receptor proteins, which causes the membrane potential and generates the impulse in olfactory cells and nerve systems.2b Many approaches to elucidate the olfactory reception have been done using biological systems in an aqueous phase, although odorants are actually interacted with the olfactory receptor membrane in a gas phase. In this communication, we report that the simple, synthetic multibilayer film cast on a piezoelectric crystal can detect various odorants and perfumes on the order of 10 ng by the change of their vibrational frequency in a gas phase. Polymeric bilayer-forming amphiphiles of dioctadecyldimethylammonium poly(styrenesulfonate) (2C18N+2C1/ PSS-)' were dissolved in chloroform and cast on both sides of the silver electrode (each 24 mm2) deposited piezoelectric crystals (9 MHz, AT-cut, 8 X 8
(1)Synthetic Chemoreceptive Membranes. 3. For part 2, see: Okahata, Y.; En-na, G. J. Chem. SOC.,Chem. Commun., in press. (2) For a review, see: (a) Price, S. In Biochemistry of Taste and Olfaction; Cagan, R. H., Kare, M. R., Eds.; Academic: New York, 1981; pp 69-84. (b) Kurihara, K.; Yoshii, K.; Kashiwayanagi, M. Comp. Biochem. Physiol. A 1986,85A,1. (3) Koyama, K.; Kurihara, K.; Nature (London) 1972,236, 402. (4) Arvanitake, A.; Takeuchi, H.; Chalazonitio, N. Olfaction and Taste; Hayashi, T., Ed.; Pergamon: Oxford 1967; Vol. 2, p 573. ( 5 ) Tucker, D. In Handbook of Sensory Physiology; Beidler, L. M. Ed.; Springer-Verlag: Berlin, 1971; Vol. IV-1,p 151. (6) Dethier, V. G. Proc. Natl. Acad. Sci. U.S.A. 1972,69,2189. (7) Okahata, Y.; Taguchi, K.; Seki, T. J. Chem. Soc., Chem. Commun. 1986,1122. Okahata, Y.;Fujita, S.; Iizuka, N. Angew. Chem., Int. Ed. Engl. 1986,98,723.
0743-7463/87/2403-ll71$01.50/0
Table 1. Responses of Piezoelectric Crystals to @-Ionone"
.
frequency adsorbed changes amounts, partition crystals (AF), Hz ng coefficientsb -8 f 2 8f2 0.0014 uncoated 2C18N+2Cl/PSS- bilayers -720 f 10 760 f 10 0.12 -70 f 4 74 f 4 0.012 polystyrene poly(viny1 alcohol) -25 f 3 26 f 3 0.0042 42 f 5 0.0069 poly(methy1 L-glutamate) -40 f 5 0.0069 -30 f 5 32 f 5 bovine plasma albumin "[@-Ionone] = 6.12 pg in 60 mL of air. The cast amount of coatings was 20 2 pg on the electrode of the crystal. Containing f5% of experimental errors.
*
The cast f i b (2ClBN+2Cl/PSS-) was estimated to be 0.47 f 0.03 pm thick (20 f 1 pg/48 mm2) from a scanning electron mi~rograph.~ X-ray diffraction analyses showed that 2CI8N+2C1amphiphiles form extended multilamellar structures of a lipid bilayer (38 8, thick) parallel to the film plane in a polyion complex with PSS-.' The multibilayer film on the crystal showed an endothermic peak near 40 "C from differential scanning calorimetry (DSC), which means the phase transition from solid to liquid crystalline state of multibilayer membranes. The piezoelectric quartz crystal was driven a t 5-V dc, and the frequency of the vibrating crystal was measured by a Iwatsu frequency counter (SL 7201 Model) attached to the microcomputer system (NEC, PC 8801 Model). Figure 1 shows frequency changes of the multibilayercoated crystal in a gas phase at 25 "Cwhen the crystal was set in a 60-mL cloeed vessel which was saturated with the vapor of 8-ionone as a typical odorant. The concentration of 8-ionone was calculated to be 6.12 pg/60 mL of air from the saturated vapor pressure (9.9 X mmHg a t 25 "C). The frequency of the crystal was immediately decreased (M= 720 f 10 Hz) within 5 min after setting the crystal in the vapor-saturated vessel and reverted to the original value after the crystal was removed from the vessel to the atmosphere. The amount of adsorbed odorants can be calculated from the following equation:8 -AF = KAm where Am is the amount of substances adsorbed on the electrode of the crystal and K is a constant (9.5 X lo8 Hz g-l) depending on characteristics of piezoelectric crystals. The partition coefficient (E') of odorants to the lipid bilayer matrix was obtained by dividing the adsorbed amount by the weight of odorants in the vapor-saturated atmosphere. (8) Nageh-Ngwainbi, J.; Foley, P. H.; Kuan, S. S.; Guilbault, G. G. J. Am. Chem. SOC.1986,108,5444.
0 1987 American Chemical Society
Letters
1172 Langmuir, Vol. 3, No. 6,1987 I
z; '
I
.a
-600
-
m
ly
-800
0
10
5
15
20
Timelmin
m,
Figure 1. Typical piezoelectric response of the multihilayercoated crystal to @-iononeas an odorant in a gas phase at 25 OC. The crystal was set in the saturated vapor of @-ionone(6.12 K/60 mL of air) at arrow a and brought in the atmosphere at arrow h.
=5 9 C
IO-
E
2! 8
11-
E
12-
E
D
0
13-
I
benzyl acelate
-2
-1
log P In bllayers
Figure 3. Relation between partition coefficients of perfumes for the hilayer-coatedcrystal and their intensity in humans, which was corrected by partial vapor pressure?O
8c m
-
-3
/
I -5
-4
-3
-2
-1
0
log P In bilayers
Figure 2. Relation between partition coefficients of odorants to the bilayer-coated crystal and their threshold concentration in humans, obtained from ref 3. The adsorbed amount and the partition coefficient of pionone to crystals at 25 "C are summarized in Table I. 8-Ionone hardly adsorbed onto the uncoated crystal. In contrast, &ionone adsorbed specifically onto the lipid bilayer matrix of 2CI8N+2C1/PSS-films compared with the hydrophobic or the hydrophilic polymer-coated and albumin (protein)-coated crystals. The specific adsorption to bilayer films was similarly observed for varioUa odorants (l-odanol, isoamyl acetate, and ether) other than 8-ionone. The response of the 2C,,N+2C,/PSS--coated crystal depended on temperatures in the atmosphere: partition coefficients were largely decreased at temperatures above 45 "C (in the liquid crystalline state) compared with those below 35 "C (in the solid state of bilayers). These results indicate that odorants interact and adsorb specifically into the rigid solid state of lipid bilayer matrices in gas phases. Partition coefficients of various odorants (6-ionone, coumarin, citral, 1--01, isoamyl acetate, methyl acetate, and diethyl ether) for the 2C18N+2C,/PSS- multihilayercoated crystal at 25 "C (below T,)were obtained and plotted against the threshold concentration (molecules mL-' in air) of the same odorants in humans3 in Figure 2 (both in logarithmic scales). Data for seven odorants fell
on a straight line: the odorant having the lower olfactory threshold value in humans showed the higher partition to the hilayer-coated crystal. The adsorption experiments of various perfumes were also studied in gas phases a t 25 OC (below T,)in the same manner. The logarithms of partiion coefficients of six typical perfumes to the hilayer-coated crystalwere plotted against the perfume intensity obtained by Appellgin Figure 3. The perfume intensity was obtained by taking a smell of vapors from perfume solutions and comparing that smell with a standard intensity of citral, corrected hy partial vapor pressure of perfumes?O There was a good correlation between the partition coefficient for lipid bilayers and the intensity of perfumes obtained by smelling. In conclusion, odorants and perfumes could be detected hy adsorptions to the lipid bilayer matrix on a piezoelectric crystal in a gas phase. This agrees with the proposal%that the first step of olfactory reception takes place by adsorption of odorants at the lipid bilayer matrix without specific receptor proteins in biological systems. The hilayer-coated piezoelectric crystal will become a simple sensor system for general odorants and perfumes. This is the first report of the quantitative detection of various odorants or perfumes in gas phases. Acknowledgment. We thank Takasago Perfume Co., Tokyo, for samples of perfumes and helpful comments. Registry No. 2ClsN+2Cl/PSS-, 99742-69-5; polystyrene, 9003-53-6; ply(viny1alcohol),900289-5; ply(methy1L-glutamate), 25086162; poly(methy1bglutamate) SRU,2503643-5; @-ionone, 79-77-6; coumarin, 91-64-5; citral, 5392-40-5; 1-octanol, 111-87-5; amyl acetate, 628-63-7;methyl acetate, 79-20-9; diethyl ether, 60-29-7; 1-undecanol,112-42-5;p-anisaldehyde, 123-11-5;anisole, 100-66-3;phenethyl acetate, 103-45-7;benzyl acetate, 140-11-4. (9) Appell, L. Am. Perfum. Cosmet. 1964, 79.29; 1967,82,35; 1969,
a,&.
(10) Since Appell's intensitiesDwere not considered the true concentration of perfumes in the air, we corrected Appell's values by dividing them by the partial vapor pressure of each perfume.
