group of the indicator dye. The resulting sensor layers exhibit
good operational and shelf life, especially if compared to
enzymatic sensors. Although a chemical reaction is responsible
for the selective recognition process, the response is relatively
fast. These findings are generic in that other analytically
relevant anions such as cyanide or nitrite may also be detected
via chemical reactions in polymer layers and these are under
current investigation in our laboratory.
This work was supported by a Marie Curie Fellowship of the
European Community programme Improving Human Research
Potential under contract number HPMP-CT-2001-01350. This
support is gratefully acknowledged. I also would like to thank
the referees for stimulating comments.
Fig. 3 Calibration plot of the sensor layer M1 measured at 540 nm upon
exposure to different concentrations of bisulfite, all at pH 4.9.
Notes and references
†
4-N,N-Dioctylamino-4A-formyl-2A-nitroazobenzene (CR-514) was ob-
tained by diazo-coupling 4-formyl-2-nitroaniline6 to N,N-dioctylaniline5 in
acetic acid as the solvent at 10 °C for 3 h. The resulting lipophilic formylazo
dye CR-514 was precipitated by adding aqueous sodium acetate, extracted
into dichloromethane and dried over magnesium sulfate. The azo dye was
then purified on silica gel using dichloromethane/hexane (4+1) as the eluent.
1H NMR (250 MHz, CDCl3) of CR-514, d (ppm): 10.08 (s, 1H), 8.29 (s,
1H), 8.08 (d, 1H), 7.86 (m, 3H), 6.68 (d, 2H), 3.40 (t, 4H), 1.65 (m, 4H),
1.31 (m, 20H), 0.88 (t, 6H). Analysis: Calculated for C29H42N4O3 (494.68):
C, 70.41; H, 8.56; N, 11.33. Found: C, 70.28; H, 8.44; N, 11.24%; mp 78U80
°C. lmax (MeCN): 514 nm, e514 = 39200 M21 cm21
.
Membrane M1 was obtained by dissolving 1.0 mg of the chromoreactand
CR-514 together with 1.2 mg of dioctadecylmethylamine, 40 mg of the
plasticizer 2-nitrophenyloctyl ether and 80 mg of PVC in 0.75 ml of
tetrahydrofuran, and spin-coating 0.3 ml of the solution on a rotating glass
plate (serving as a mechanical support for the sensor layer). The resulting
sensor layer with a thickness of 3U5 mm was placed in ambient air for
drying. Then, the sensor layer was fixed in a home-made flow-through cell.5
The measurements were performed by placing the flow-cell in the
spectrometer and pumping the sample solutions through the cell at a flow
rate of 1.7 ml min21 using a peristaltic pump. The absorbance spectra were
recorded on a Lambda 16 UV-VIS spectrometer (Perkin Elmer) at 25 ± 2
°C.
M2 was prepared by dissolving 1.0 mg of CR-514, 1.2 mg of
dioctadecylmethylamine, 16 mg of PVC and 32 mg of the plasticiser in 0.3
ml of tetrahydrofuran, and pipetting 0.05 ml on a glass disk to give a layer
of 1 cm diameter. The layer was peeled off and fixed in the FT-IR
photometer. Anion solutions were obtained by dissolving the appropriate
amount of each analyte in 0.1 M citrate buffer adjusted to pH 4.9 by addition
of sodium hydroxide solution. Due to the pH of 4.9, the investigated sodium
sulfite was present in the chemically reactive bisulfite form. Bisulfite
solutions were prepared freshly and used within 6 h.
Fig. 4 Response and reversibility of the sensor layer M1 measured at 540
nm upon exposure to: a, plain buffer; b, 1.0 mM bisulfite; c, 6.0 mM
bisulfite; d, 30.0 mM bisulfite, all at pH 4.9.
co-extraction of the bisulfite anion with a proton into the layer
is necessary in order to provide electroneutrality within the
polymer layer, a mechanism frequently encountered for ion
diffusion into plasticised polymers.2 The layer M1 exhibited
high selectivity over other anions such as sulfate, phosphate or
chloride in that only a signal change for bisulfite (due to the
selective interaction) was observed and none at all for 50 mM
concentrations of the other anions at pH 4.9. The layer did not
show cross-sensitivity to 50 mM 1-propylamine and to ethanol
in concentrations as high as 16 vol% at pH 4.9. The layer also
did not change colour upon exposure to 0.1 M sodium
hydroxide solution and only a small increase in absorbance of
4% at 524 nm upon exposure to 0.1 M hydrochloric acid was
visible, which was fully reversed upon exposure to plain buffer.
However, the layer was cross-sensitive to cyanide concentra-
tions higher than 1 mM at pH 4.9, but such high amounts of
hydrogen cyanide are not expected to be present in beverages.
In order to elucidate the mechanism responsible for the
colour changes of the chromogenic aldehyde with bisulfite, a
membrane layer with enhanced amount of CR-514 in plasti-
cised PVC (M2) was investigated via FT-IR spectroscopy. The
layer was fixed in the photometer and measured before and after
exposure to aqueous bisulfite solutions. The spectra showed that
in the case of 30 mM buffered bisulfite, the carbonyl vibration
vanished almost completely at 1699 cm21 and was recovered
again upon exposure to plain buffer, clearly indicating the
reversibility of the chemical interaction.
1 A. Stangelmayer, I. Klimant and O. S. Wolfbeis, Fresenius J. Anal.
Chem., 1998, 362, 73; G. J. Mohr, T. Werner, I. Oehme, C. Preininger, I.
Klimant, B. Kovacs and O. S. Wolfbeis, Adv. Mater., 1997, 14, 1108.
2 I. H. A. Badr, A. Plata, P. Molina, M. Alajarin, A. Vidal and L. G. Bachas,
Anal. Chim. Acta, 1999, 388, 63; M. Kuratli and E. Pretsch, Anal. Chem.,
1994, 66, 85.
3 T. M. A. Razek, M. J. Miller, S. S. M. Hassan and M. A. Arnold, Talanta,
1999, 50, 491; D. Papkovsky, M. A. Uskova, G. V. Ponomarev, T.
Korpela, S. Kulmala and G. G. Guilbault, Anal. Chim. Acta, 1998, 374,
1.
4 X. Xie, Z. Shakhsher, A. A. Suleiman, G. G. Guilbault, Z. Yang and Z.
Sun, Talanta, 1994, 41, 317; R. Keil, R. Hampp and H. Ziegler, Anal.
Chem., 1989, 61, 1755.
5 G. J. Mohr, D. Citterio and U. E. Spichiger-Keller, Sens. Actuat. B, 1998,
49, 226; G. J. Mohr, C. Demuth and U. E. Spichiger-Keller, Anal. Chem.,
1998, 70, 3868; G. J. Mohr, U. E. Spichiger, W. Jona and H. Langhals,
Anal. Chem., 2000, 72, 1084.
In summary, chromoreactands are known to offer the
possibility for detecting electrically neutral analytes with
significant absorbance changes in the visible spectral range.
However, the present paper shows that chromoreactands can
also be used for the detection of ionic species such as bisulfite
that performs a reversible chemical reaction with the formyl
6 K. A. Bello and J. Griffiths, Dyes Pigm., 1989, 10, 65.
7 D. P. Kjell, B. J. Slattery and M. J. Semo, J. Org. Chem., 1999, 64, 5722;
A. K. Mitra, A. De and N. Karchaudhuri, J Chem Res. (S), 1999, 9, 560;
A. I. Vogel, Practical Organic Chemistry, Longmans, London, 4th edn.,
1978.
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