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Anal. Chem. 1990, 62, 2054-2055
Ion-Selective Optrode Using Hexadecyl-Acridine Orange Attached on Poly(viny1 chloride) Membrane Sir: We have recently reported a potassium ion optrode using dodecyl-acridine orange (dodecyl-AO+) attached on a poly(viny1 chloride) (PVC) membrane (I). The positively charged chromophore (AO+) of dodecyl-AO+ is known to fluoresce more strongly in a nonpolar solvent than in a polar solvent (2). The chromophore moves toward the sample solution by extraction of a potassium ion into the membrane. The polarity change around the chromophore causes a sensor response; i.e. the fluorescence intensity decreases as the polarity increases. The potassium ion is selectively extracted into the membrane with an ionophore of valinomycin. Then, the sensor response is selective to the potassium ion. It is worth mentioning that the sensor response is reversible. This is due to a long alkyl chain in dodecyl-A0+, which retains the chromophore at the boundary between the membrane and the sample solution. However, the fluorescence intensity of dodecyl-AO+ gradually decreases with time (-18% /h) since dodecyl-A0+ is slightly soluble in water. T o overcome this problem, a more lipophilic acridine orange (3,6-bis(dimethylamino)-IO-hexadecylacridinium ion, hexadecyl-A0+) is attached on the PVC membrane. The stability of the sensor response is substantially improved, providing a more stable response. Many ionophores are reported to extract a specific cation into an organic phase (3). In the proposed mechanism herein, it is possible to construct various kinds of optrodes sensitive to many cations by using these ionophores instead of valinomycin. The aim of this paper is to show that this is a general approach to construct a cation-selective optrode. Polynactin is well-known as an ionophore to an ammonium ion and has been used in the electrochemical ( 4 ) and optical (5) detection of the ammonium ion. So, polynactin might be used to construct an ammonium ion optrode. The sensor response to anions is also investigated in this study. The selectivity and the response mechanism of the sensor are discussed. EXPERIMENTAL SECTION Apparatus. The experimental apparatus is almost identical with that previously reported (I). However, the output power of an argon ion laser used as an excitation source was reduced to 4 pW, since the fluorescence intensity was sufficiently strong and photobleaching of hexadecyl-A0+ was avoided by reduction of the laser power. Chemical Reagents. Acridine orange, cetyl bromide (CI6HSBr), NaH2P04,KCl, and NaCl were obtained from Wako Pure Chemical. Bis(2-ethylhexyl) sebacate, Na2HP04,NH4Cl, and NH4SCN were purchased from Kishida Chemical. A reagent of NaC104was obtained from Katayama Chemical. Polynactin was supplied from Chugai Pharmaceutical as a mixture of dinactin, trinactin, and tetranactin (molecularratio, 1:45). Other chemicals were the same as those used in the previous study ( I ) . A buffer solution (pH 7.0) containing 40 mM of NaH,P04 and 27 mM of Na2HP04was used for preparation of the sample solution throughout this experiment. Hexadecyl-AO+Br-. A lipophilic dye of hexadecyl-AO+Brwas synthesized from acridine orange and cetyl bromide, according to refs 6 and 7. The product was identified by a 'H NMR spectrometer (Hitachi, R-90). The purity was ascertained to be better than 96%. Since hexadecyl-AO+Br-was insoluble in water, it was first M. This dye dissolved in ethanol at a concentration of 4 X solution (0.05mL) was added to 5 mL of bis(2-ethylhexyl)sebacate used as a membrane solvent or to 5 mL of buffer solution used for preparation of the sample solution. The fluorescence spectra for these solutions were measured by a commercial fluorescence
spectrophotometer (Hitachi, MPF-4). Membrane Preparation. Sodium tetrakis[3,5-bis(trifluoromethy1)phenyljborate (Na+TFPB-,1 mg) was first dissolved in tetrahydrofuran (2.5 mL). Polynactin (2.5 mg), bis(2-ethylhexyl) sebacate (0.5 g), and tetrahydrofuran (47 mL) were added to the Sa'TFPB- solution. PVC (0.2 g) was further dissolved in this solution. A distal end of an optical fiber was dipped into the mixed solution, and tetrahydrofuran was vaporized to form a PVC membrane. The thickness of the membrane was 2 gm, which was calculated from the droplet volume attached on the distal end of the optical fiber. The hexadecyl-AO'Br- solution (8X lo4 M) prepared in ethanol was diluted 50 times with water. The PVC membrane attached on the distal end of the optical fiber was immersed into the hexadecyl-AO'Br- solution for 10 s, and hexadecyl-A0+was ion exchanged with a sodium ion in the PVC membrane. The sensor was washed with a copious amount of buffer solution.
RESULTS AND DISCUSSION Fluorescence of Hexadecyl-AO'. The excitation and emission spectra for hexadecyl-A0+ were quite similar to those for dodecyl-A0+ ( 1 ) . The fluorescence intensity of hexadecyl-A0' in bis(2-ethylhexyl) sebacate was 40 times larger than that in the buffer solution. Thus hexadecyl-A0+ was ascertained to be useful as a hydrophobic probe for evaluation of the polarity around the probe molecule. Sensor Response. A response curve of the sensor to an ammonium ion is shown in Figure 1. The counteranion used is a chlorine ion. The sensor response is reversible to the ammonium ion concentration, and the time required for (1 - I / e) response is 30 s. The detection limit of the ammonium ion is 0.02 mM, which is determined by fluctuation of the signal intensity (1.4%) mainly due to the unstable output power of the argon ion laser (*0.5%). Hexadecyl-AO+is more lipophilic than dodecyl-AO+ and is much insoluble in water; leaching of hexadecyl-A0+ into water is negligibly small. Thus no degradation of the sensor response is observed in the time period demonstrated, which is in contrast to the case for dodecyl-A0+. From the theoretical derivations described in the previous paper (Z), the fluoresence intensity of the chromophore should decrease linearly with increase of the ammonium ion concentration when the concentration of free polynactin in the membrane is constant. However, the sensor response, shown in Figure 1, is not linear. The sensor response is determined by the concentration of hexadecyl-A0+ in the membrane, which decreases by ion exchange with the ammonium ion; the ammonium ion is selectively extracted into the membrane as an ammonium ion-polynactin complex. The concentration of the complex in the membrane was calculated at various ammonium ion concentrations. The initial concentration of polynactin used in the calculation was 6 x M, the extraction constant of the complex being lo3( 5 ) . Due to rather high concentrations of the ammonium ion in the sample solution, most polynactin in the membrane forms a complex with the ammonium ion. Then, the concentration of free polynactin decreases, causing signal saturation. The observed sensor response agrees quite well with the complex concentration calculated from the above mechanism. Thus the concave calibration curve is ascribed to decrease of free polynactin in the membrane. Selectivity. The constructed sensor was immersed into the potassium (0.01 M) and sodium (0.1 M) ion solutions, and the signal changes were compared with that for the ammonium ion. The counteranion in the sample solution was a chlorine
0003-2700/90/0362-2054$02.50/0 C 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62,
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Figure 3. Response mechanism for anions.
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Time/min Figure 1. Response curve of ammonium ion optrode.
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NO. 18, SEPTEMBER 15, 1990 2055
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chromophore and is easily extracted into the membrane. The polarity around the chromophore decreases, which increases the fluorescence intensity of the chromophore. Thus the concentration of TFPB- in the membrane should be carefully optimized to reduce the interference by anions and to maintain the selectivity between cations. Anion-Sensitive Optrode. As described above, the sensor response is interfered by a bulky anion. It implies that the sensor is sensitive to anions. When the sensor was immersed into the 10-3-10-2 M NaC104 solution, the fluorescence intensity increased 1.2-1.5 times. The sensor response was reversible. Interference by a sodium ion a t present concentration levels was negligible, so that this sensor response was ascribed to a perchlorate ion involved. From the present results, the sensitivity for anions apparently increases in the order of C1- < SCN- < Clod-, i.e. according to the Hofmeister series (8). This fact implies that the response mechanism described above is plausible, and the selectivity sequence is determined by solubility of individual anions into the membrane.
LITERATURE CITED
Figure 2. Interference by counteranion.
ion in both the experiments. The selectivity coefficient is defined by the ratio of the equivalent ammonium ion concentration calculated from the signal intensity in the response curve to the concentration of the specific alkali-metal ion dissolved. The selectivity coefficients obtained for the potassium and sodium ions were 0.3 and 5 X loa, respectively. These values are comparable to those reported for the electrochemical ( 4 ) and optical (5) ammonium ion sensors. Interference by Anions. Figure 2 shows the calibration curves for the ammonium ion, in which chlorine and thiocyanate ions are used as counteranions. The calibration curve for the NH4SCN solution deviates from that of the NH4Cl solution as the sample concentration increases. Above M, the fluorescence intensity for the NH4SCN solution becomes much higher than that of the initial fluorescence intensity; i.e. the fluorescence enhancement effect is oppositely observed. The response mechanism for anions is illustrated in Figure 3. The chromophore of hexadecyl-A0+ is first moved toward the sample solution. The anion exchanger of TFPB- plays an important role to exclude anions from the membrane. However, the concentration of TFPB- in the membrane (2 X M) should be reduced as much as possible to improve the selectivity between cations (1). A bulky anion such as a thiocyanate ion is ion-paired with the charged
(1) Kawabata, Y.; Kamichika, T.; Imasaka, T.; Ishibashi, N. Anal. Chern. 1990, 62, 1528-1531. (2) Kubota, Y.; Kodama, M.; Miura, M. Bull. Chem. SOC.Jpn. 1973, 46, 100
(3) D&ich, B. J . Chem. Educ. 1985, 62, 954-964. (4) Scholer, R. P.; Simon, W. Chimle 1970, 2 4 , 372. (5) Seiler, K.; Morf, W. E.; Rusterholz, B.; Simon, W. Anal. Sci. 1989, 5 , 557-561. (6) Miethke, E.; Zanker, V. 2. Phys. Chem. (frankfurt am Main) 1958, 18. 375-390. (7) Yamagishi, A.; Masui, T.; Watanabe, F. J . Phys. Chem. 1981, 85, 281-285. (8) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247-260. Author to whom correspondence should be addressed.
Yuji Kawabata Toshito Kamichika Totaro Imasaka Nobuhiko Ishibashi* Faculty of Engineering Kyushu University Hakozaki, Fukuoka 812 Japan
RECEIVED for review May 21,1990. Accepted June 26,1990. This research was supported by Grant-in-Aid for Scientific Research from the Ministry of Education of Japan and from The Salt Science Research Foundation.
