Sensors andand Materials,Materials, Vol.
Vol. 17,17, No.
No. 77 (2005)(2005) 385– 390Sensors
MYU Tokyo

385

S & M 0615

Selectivity Control in a Sweetness Sensor
Using Lipid/Polymer Membranes
Hong Cui*, Masaaki Habara, Hidekazu Ikezaki1 and Kiyoshi Toko2
Graduate School of Systems Life Sciences, Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
1Intelligent Sensor Technology, Inc.
1800 Onna, Atugishi, Kanagawa 243-8555, Japan
2
Graduate School of Systems Information Science and Electrical Engineering, Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan(Received December 1, 2004; accepted May 9, 2005)
Key words:

taste sensor, lipid/polymer membrane, sweetness, selectivity

We studied the selectivity control in a sweetness sensor with lipid/polymer membranes.
Sweet tasting substances such as sucrose, fructose and glucose are nonelectrolytes andtherefore are susceptible to interference from electrolytes and/or adsorptive substances. In
this study, we focused on suppressing the adsorption of bitter and astringent substances ona membrane surface. The membrane was designed to be electrically neutral to avoid
interference from electrolytes. Additives used in this experiment have a hydroxyl group
and are not protonated compounds, which change the hydrophobicity character of amembrane. The results show that n-tetradecyl alcohol reduced the responses to bitter andastringent substances.

1.

Introduction

A multichannel taste sensor with several lipid/polymer membranes with different
characteristics can detect tastes in a manner similar to the human gustatory system.
Information from substances producing taste is transformed into electric signals, which areentered into a computer. The sensor output shows different patterns for chemical substances that have different taste qualities such as saltiness and bitterness, while it shows
similar patterns for chemical substances with similar taste sensations.(1)However, the electric responses for nonelectrolytes such as sucrose were one-fifth to
one-tenth of those for electrolytic with substance taste. In our previous study, wedeveloped a sweetness sensor with a lipid/polymer membrane.(2) However, the sensor does
not have enough selectivity for sweet tasting substances. The sensor responds not only tosweeteners but also to bitter, astringent and umami tastes. In this study, we focused on
suppressing the responses to bitterness and astringency. The adsorption of bitter andastringent substances on the membrane surface is generally caused by hydrophobicinteractions. We used materials containing hydroxyl groups in order to control theselectivity for sweetness.
*Corresponding author, e-mail address: sai@belab.ed.kyushu-u.ac.jp

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Sensors and Materials, Vol. 17, No. 7 (2005)

Materials and Methods

A schematic diagram of the taste-sensing system SA402B (Intelligent Sensor Technology, Inc.) is shown in Fig.1. The detector of the sensor usually consists of eight kinds ofelectrodes with different lipids/polymers. The sensor detects taste information which istransformed into electric signals that correlate with membrane potentials. Depending on
the substances to be measured, different lipid/polymer membranes are available.
Materials used to prepare a sweetness sensor in this study are listed in Table 1.(2) Lipids
were mixed with polyvinyl chloride (PVC) and a plasticizer (DOPP:dioctylphenylphosphonate) dissolved in tetrahydrofuran and then dried on a glass plate. The lipid/Fig. 1

Schematic diagram of experimental setup.

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Table 1
Materials used for membranes.
abbr.
TDAB
DOPP
PVC

composition
tetradodecylammonium bromide, 1.50 mg
dioctyl phenylphosphonate, 1.0 ml
polyvinyl chloride, 800 mg

polymer membrane is a transparent, soft film approximately 200 μm thick. Then themembrane was preconditioned with 0.05 wt% tannic acid and 10 mM KOH+30% EtOH+100
mM KCl solution.
A multichannel electrode was connected to eight channels through high-input impedance amplifiers. The electric signal from the sensor was converted to a digital code by anA/D converter and was entered into a computer. The electrode was made of an Ag wire, thesurface of which was plated with AgCl, in an internal cavity filled with 3.3 M KCl andsome excess AgCl. Then the voltage difference between the multichannel electrode andthe Ag/AgCl reference electrode was measured.
Figure 2 shows the procedure of measurement. Vc is the electric potential of thecontrol, and Vs is the potential of sample. Therefore, the sensor output should be the value(Vs–Vc). The control is composed of 30 mM KCl and 0.3 mM L(+)-tartaric acid. Aftermeasuring the sample, washing the membrane removes the adsorbed substance andrefreshes the membrane. Table 2 shows the control and the taste samples which consist of
typical taste substances used in this experiment.

3.

Results and Discussion

Sweet tasting substances such as sucrose, fructose and glucose are nonelectrolytes andare susceptible to interference by electrolytes when membrane potential is measured.
Therefore, a membrane was designed to be electrically neutrality to obtain higher sensitivity for sweeteners. Additives used in this experiment are not protonated compounds, and it
was expected that they had little interference on membrane potential. Table 3 shows thechemical formulas of lipids added to form membranes. Since they have a hydroxyl group
and are not dissociated, it is expected that they do not change the membrane potential andincrease the hydrophilicity in order to avoid hydrophobic interactions from other substances that have bitter and astringent tastes.
Initially, we added n-heptyl alcohol (C6), n-tetradecyl alcohol (C13) and stearyl
alcohol (C17) into the membrane forming materials listed in Table 1 and examined theresponse to sucrose. Each additive has a hydroxyl group, which would increase thehydrophilicity, but different lengths of carbon chain. The vertical axis (Fig. 3) shows theelectric response obtained from a taste sensor and the horizontal axis is the concentration ofadditive. As shown in Fig.3, the electric response for sucrose was changed as the amountof additive increased. From this result, 1.50 mg TDAB+100 mg C6, 1.50 mg TDAB+100mg C13, and 1.50 mg TDAB+30 mg C17 were sensitive to sucrose and were examined forsensitivity to basic tastes listed in Table 2.

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Measurement of control sample: Vc

Measurement of sample: VsCleaning
Fig. 2

Measurement procedure.

Table 2
List of sample used in this experiment.
Samplecontrolsaltinesssourness
umami
bitterness 1
bitterness 2
astringency
sweetnesscomposition
30 mM KCl+0.3 mM L(+)-tartaric acid
300 mM KCl+0.3 mM L(+)-tartaric acid
30 mM KCl+3 mM L(+)-tartaric acid
10 mM MSG+control sample
0.1 mM quinine+control sample
0.01 vol% iso α acid+control sample
0.05 wt% tannic acid+control sample
1 M sucrose+control sample

Table 3Structural formulas of additives to change the hydrophobicity of membranes.
compositionn-hexyl alcohol
n-heptyl alcohol
n-tetradecyl alcohol
stearyl alcohol

structural formulaCH3-(CH2)5-OH
CH3-(CH2)6-OH
CH3-(CH2)13-OH
CH3-(CH2)17-OH

10

Electric response (mV)addr.
C5C6C13C17–10–30–50
10Fig. 330

100

Concentration of additives (mg).

300

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Figure 4 shows the electric responses to basic tastes using membranes such as 1.50 mg
TDAB+100 mg C6, 1.50 mg TDAB+100 mg C13, and 1.50 mg TDAB+30 mg C17. By
adding C13, the sensitivity to bitterness and astringency was dramatically decreased. The
adsorption of bitter and astringenct substances on the membrane surface is generally
caused by hydrophobic interactions. From this result, the hydroxyl group and length ofcarbon chain of C13 has the optimum effect on reducing hydrophobic interactions.
Next we optimized the composition of the membrane in order to obtain a higher outputfor sucrose. We changed the volume of tetradodecylammonium bromide (TDAB) from 1.0
mg to 2.0 mg and n-tetradecyl alcohol (C13) from 10 mg to 300 mg. As shown in Fig. 5,
only the composition with 1.50 mg TDAB showed a response to sucrose, and the highest
response was for 100 mg C13.
Figure 6 is a comparison of the output of the newly developed sensor with the originalone. Although the responses to sourness and umami were increased slightly, the outputs
for bitterness and astringency were dramatically reduced. Furthermore, the sensitivity forsweetness was twice as high as for the original sensor.

Electric response (mV)40
200–20–40–60–80saltinesssourness umami

bitterness 1 bitterness 2 astringency sweetness

Fig. 4

Electric responses to basic taste substances.

Electric response (mV)

200–20–40–60
10Fig. 530100
C13 (n-tetradecyl alcohol) (mg)300

Sensitivity for sucrose with changing concentration of n-tetradecyl alcohol.

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Electric response (mV)40
200–20–40–60–80saltiness

Fig. 64.

sourness

umami

bitterness 1 bitterness 2 astringency sweetness

Electric responses to basic taste substances.

Conclusions

We studied the selectivity control for sweetness sensor with lipid/polymer membranes.
In this study, we focused on suppressing the adsorption of bitter and astringent substanceson the membrane surface. Because the adsorption of bitterness and astringency is
generally caused by hydrophobic interactions, we used materials containing a hydroxyl
group to change the hydrophobicity of the membrane. From these results, it was found thatn-tetradecyl alcohol was useful to control selectivity in measuring sweet tasting substances.

References
1 K. Toko: Biomimetic Sensor Technology (Cambridge University Press, Cambridge, 2000)
Chap. 6.
2 M. Habara, H. Ikezaki and K. Toko: Biosens. Bioelectron. 19 (2004) 1559.
3 K. Toko: Sens. Actuators B64 (2000) 205.
4 H. Ikezaki, R. Toukubo, Y. Kobayashi, Y. Naito, A. Taniguchi and K. Toko: Digest Tech.
Papers Transducers 99 (1999) 1634.