Anal. Chem. 1995,67, 288-291
Surface-Modified Cobalt-Based Sensor as a Phosphate=SensitiveElectrode Dan Xiao, Hong-Yan Yuan, Jun Li, and Ru-Qin Yu* Department of Chemistty and Chemical Engineeflng, Hunan University, Changsha 4 10082, China
Anew phosphate ion-sensitive electrode based on a cobalt matrix has been prepared. The electrode showed a relalively selective potentiometric response toward which seems to originate from the COOlayer covering the electrode surface. The response mechanism of the electrode is discussed in terms of the host-guest chemistry of a nonstoichiometric compound of COO. The design and characterization of new materials for fabrication of solid state ion-sensitive electrodes have been the subject of a number of recent investigations. Ion implantation was used to create Na+-1SFET and chloride-sensitive solid state electrodes with some new characteristics.1.2 Tungsten bronze was used for the preparation of a cation ~ e n s o r . Palladium ~ hydrides4 and a-zirconium hydrogen phosphate crystals5 were utilized for pH measurements. Since the first attempt to prepare a phosphate ion-sensitive electrode about 30 years ago,6 the design of a practically useful phosphate ion sensor remained an unsolved problem in sensor studies. Grabner et aL7 tried to use glassy carbon electrodes modified by Bi/BiPO1 as phosphate ion sensors. Later these authors claimed*that the experiment with Bi/BiPO4 was difficult to reproduce and suggested chemically modified electrodes with Ag/Ag3PO4 for the same purpose. The use of enzymesgJ0 seemed to be an effective approach for phosphate assay, through enzyme probes are generally difficult to commercialize for wide application. The use of organotin compounds as ion-exchanger-type carriers for phosphatel1-l4 seemed quite promising; the lifetime of this kind of phosphate sensors was, unfortunately, rather limited. As (1) Ito, T.;Inagaki, H.; Igarashi, I. IEEE Trans. Electron Devices 1 9 8 8 , ED35, 56-64. (2) Glass, R S.; Musket, R G.; Hong, IC C. Anal. Chem. 1991, 63, 22032206. (3) Dobson, J. V.; Comer, J.J.Electroanal. Chem. Intetfacial Electrochem. 1987, 220(2), 225-234. (4) Kihara, S.; Yoshida, 2.; Matsui, M. Bull. Inst. Chem. Res. Kyoto Univ. 1986, 64(4), 207-217. (5) Palombari, R; Casciola, M. J. Electroanal. Chem. Intevfacial Electrochem. 1987,216(1-2), 283-288. (6) Pungor, E.; Toth, IC; Havas, J. Mikrochem. Acta 1966(4-5), 689-698. (7)Grabner, E. W.; Vermes, I.; Koenig, IC H. J. Electroanal. Chem. Intetfacial Electrochem. 1986,214, 135-140. (8) Vermes, I.; Grabner, E. W. J. Electroanal. Chem. Intetfacial Electrochem. 1990,284,351-321. (9) d'Urso, E. M.; Coulet, P. R Anal. Chim.Acta 1990, 239, 1-5. (10) Wollenberger, U.; Schubert F.; Scheller, F. W. Sens.Actuntols 1992,B7(13), 412-415. (11) Glaszier, S. A; Arnold, M. A Anal. Chem. 1988,60, 2540-2542. (12) Glaszier, S. A; Arnold, M. A Anal. Le#. 1989,22, 1075-1088. (13) Glaszier, S. A; Arnold, M. A Anal. Chem. 1 9 9 1 , 63, 754-759. (14) Chaniotakis, N. A; Jurkschat, IC; Ruhlemann, A Anal. Chim.Acta 1993, 282, 345-352.
288 Analytical Chemistry, Vol. 67,No. 2, January 15, 1995
we have just reported in a preliminary comm~nication,'~ a sensor based on a cobalt matrix is an excellent phosphate ion sensor which shows a potentiometric response in the range of 10-jmol/L of phosphate with a slope of monovalent anion (55 mV/pC) . The sensor possessed excellent potentiometric selectivity for dihydrogen phosphate ion with respect to many common anions, such as sulfate, chloride, nitrate, and acetate. In this paper, the detailed potentiometric response characteristics of this cobaltbased sensor are reported, and the mechanism of the electrochemical response is discussed in terms of the interpretation of XPS measurements. A mechanism involving nonstoichiometric compound COOformation on the electrode surface is proposed. It is postulated that cobalt oxide provides cavities as the host, accepting H2PO4- as the most favorite guest, which is responsible for the phosphate function of the sensor. The newly proposed phosphate ion sensor has the evident advantage of simplicity, excellent selectivity,low cost, long-term electrochemicalstability, and long lifetime. The electrode can also be used for determination of phosphate contained in adenosine 5'-triphosphate and adenosine 5'-diphosphate. EXPERIMENTAL SECTION Reagents. Disodium adenosine 5'-triphosphate (ATP), disodium adenosine 5'-diphosphate (ADP), and disodium adenosine 5'-monophosphate (AMP) were biochemical reagents (Dongfong Biochemistry Reagents, Shanghai, China). Cobalt rod (99.99%,5 mm) was obtained from Johnson Matthey (London, U.K.). All other chemicals were of analytical grade. Deionized or distilled water was used throughout. Fabrication of the Electrodes. The electrode was prepared according to the following procedure. A section of about 2 cm of the cobalt rod was cut and coated with Teflon. The surface of one end was polished with fine emery paper (no. 6), and the other end was welded to a copper wire. The electrode was then pretreated by immersing it in water with a double junction saturated calomel electrode with 1 mol/L KNO3 in the outer compartment, and the potential was monitored. After a steady state potential was obtained, the electrode was removed from water and immersed in 0.025 mol/L potassium acid phthalate buffer solution for about 20 min. A new steady state potential should be established as an indication of completion of the pretreatment process. The electrodes used for measuring ATP, ADP,and AMP were further conditioned for about 20 min in the ATP, ADP, and A M P solutions, respectively, at the activity level of mol/L prepared with 0.015 mol/L potassium acid phthalate buffer solution. (15) Dan, X.; Ru-Qin,Y.; Jun, L.; Kong-Quan, Y. GaodengXuexiao Huaxue Xuebao 1994, 15(2), 193-194. 0003-2700/95/0367-0288$9.00/0 0 1995 American Chemical Society
-520
-480
-440
-400
I binding energy
( e ~ )
Figure 1. XPS spectra of (a) the newly polished cobalt rod surface; (b) the pretreated electrode surface, and (c) the electrode surface after 10 h of continuous measurement.
Apparatus. The potential was measured with an Orion SA720 ion meter and recorded wiht a "-204 recorder (Dahua Instruments,Shanghai, China), All solutions were stirred continuously using a magnetic stimng bar during the potential measurements, and the temperature was kept at 30 f 1 "C. XPS Experiment. The X-ray electron spectroscopic experiment was carried out with MICROLAB MKll (VG SIC, Sussex, U.K) for the determination of the composition of the electrode surface layer. An AI K a X-ray source was used to provide the primary X-ray. Before XPS was performed, the newly polished cobalt surface and the pretreatment or used electrode surface were washed sequentially with deionized water and acetone. The ac Impedance Experiments. The ac impedance of an electrode pretreated in water and 0.025 mol/L potassium acid phthalate buffer solution in sequence was recorded with the PARM 368-2 system @G&G Princeton Applied Research, Princeton, NJ). The frequency range used was 105-10-3 Hz (at 30 OC). RESULTS AND DISCUSSION
Figure 1 shows the XPS spectra of the newly polished cobalt rod surface and the electrode surface after pretreatment and after continuous use in KH2P04 solutions buffered with potassium acid phthalate for 10 h. Curve a is the XPS pattern of the newly polished cobalt rod surface; the binding energy peaks of C02p3/2 and C o ~ pof~ cobalt / ~ appeared at 777.9 and 793.0 eV, respectively, while the peaks of C02p3/2and Co~p'/~ of COO appeared at 780.0 and 795.5 eV, respectively. The COOfound on the newly polished cobalt surface might form during polishing by adsorption of oxygen and oxidation of the surface. After pretreatment of the electrode in water and 0.025 mol/L potassium acid phthalate buffer solution in sequence, a decrease of the Co2p312and Co2p1I2peaks of cobalt on the electrode surface was observed,while the Co2p312 and Co~p'/~ peaks of COOincreased (curve b). Only the C02p3/2 and Co2p1I2peaks of COOwere observed from the XPS after the electrode was used for continuous measurement in the phosphate solution buffered by potassium acid phthalate for 10 h (curve c), and the C0~p3/~ and C02p112 peaks of cobalt disappeared. The XPS results demonstrate that a layer of COOcovering the electrodes surface was formed during the electrode pretreatment. After the electrode was used to continuously measure dihydrogen phosphate in potassium acid phthalate buffer solution for about 10 h, the COOlayer on the electrode surface grew to such a thickness that the X-rays provided by an Al Ka X-ray source could not reach the depth of about 50-500 nm, and the Co2p3I2and C 0 2 p ~signals /~ of the cobalt matrix could not be detected.
-360
-320
7
S
6
3
4.
2
1
-log a Figure 2. Response curves of electrodes. The symbols
0 refer to three individual electrodes prepared separately. Table 1. Selectivity Coefficients, Anions.
c12
x
sod2-
N038x
1
x
q&, of Common
AcO1
x
.,A, and
1
Br-
H~As04-
x
4x
Mixed solution method.
After pretreatment in water and buffer solution in sequence, the electrode exhibited a potentiometric response toward dihydrogen phosphate ion. The potentiometric response toward dihydrogen phosphate with 0.025 mol/L potassium acid phthalate as buffer solution at pH 4.0 is shown in Figure 2, with the activities obtained using the activity coefficients calculated in the buffer solutions. The potentiometric response showed a linear activity range from 10-2 to 10-5 mol/L, with a detection limit of 5 x 10-6 mol/L obtained according to the IUPAC recommended method. The slope was about 55 mV/decade change of activity. The response time, zgo, defined as the time required for the potential to change by 90% of the difference between the initial and final potential, was about two min for mol/L and below 1min for mol/L. The response time was of the order of seconds for an activity level above mol/L. The potentiometric selectivity coefficientsfor the electrode are shown in Table 1. One can notice that the electrode has excellent selectivityfor dihyrogen phosphate with respect to many common anions. This is an outstanding feature of the proposed dihydrogen phosphate sensor in comparison with the numerous phosphate electrodes reported so far. The selectivity coefficient of dihydrogen arsenate is about 4 x It seems that the selective response of the electrode toward HzPO4- is not associated with some specific structure of dihydrogen phosphate ion (such as the two hydrogen bonds and one negatively charged site, 0-),as H&04- has a similar structure. In order to investigate the reproducibility of the electrodes, the potentiometric responses of three individual electrode preAnalytical Chemistry, Vol. 67, No. 2, January 15, 1995
289
Table 2. Reproducibility of the Potentiometric Response (mV) of the Electrode
activity level (mol/L) day
1 2 3
4 5
10-5
10-4
-323 -328 -324 -329 -322
-377 -382 -378 -379 -380
-433 -434 -434 -435 -436
-325 f 3
-379 f 2
-434
(-02'
0 2 -
(-02'
02-
c02+ 0 2 -
C02'
0 2 -
0
02.
-440
*1
co2+
02.
co2+ 0 2 -
pared separately were compared as shown in Figure 2; the potentiometric response of one electrode was recorded for 5 days, as shown in Table 2. The drift of the response potential of the electrode was about 2-3 mV at an activity level above mol/L during 5 days. The reproducibility of the potential measurement indicates that the potential-determining process at the electrodesolution interface is well defined. It has been postulated that the COO layer formed at the electrode surface serves as the sensitive membrane responding toward dihydrogen phosphate ion. The response mechanism could be interpreted in terms of the host-guest chemistry of the nonstoichiometric compound.16-18COOis a well-known nonstoichiometric compound with Co2+ imperfections and exhibits a j-type semiconductor character (Figure 3). The nonstoichiometric oxide of Col-,O can act as an oxygen sensor,16with the electric resistance changing with the 02 partial pressure. The mechanism of such an oxygen sensor was thought as17 (1) the binding of an oxygen atom by the surface, (2) two of the inner Co2+each transferring an electron to the oxygen atom, and (3) Co2+transferring to the surface, forming a hole (0).The reaction can be expressed as follows:
(1)
When the electrode is immersed in an aqueous solution containing dissolved oxygen, the latter tended to react with the COO layer formed during the process of polishing: 1/20,(dissolved)
K=
+ 2C02+ = 0,- + 2C03+ + 0
[Ol [Co3+12[02-l [O,(dissolved) I 1/2[Co2+12
(2) (3)
As shown in Figure 3, [Co2+land IO2-] are constant, and for each (16) Oehlig, J. J.; Jamet, A; Duquesnoy, A. C. R. Acad. Sci., Ser. C 1972,274, 1021-1024. (17) Lapedes, D. N., Ed. McGrawHill Encyclopedia of Science & Technology,4th ed.; McGraw-HillBook Co.: London, 1977;Chinese translation by An-Bang, D.; et al. Science Publishing House: Beijing, 1980; Vol. 7, pp 81-88. (18) Mandelcorn, L., Ed. Non-stoichiometric Compounds; Academic Press Inc.: New York, 1964; Chapter 6.
290 Analytical Chemistry, Vol. 67,No. 2, January 15, 1995
2min
t
coz
+ 2c02+ = 02-+ 2c03+ +
.
Figure 4. Effect of oxygen on the potential response.
Figure 3. Col-aO structure. The charge of each loosing Co2+ (0) is balanced by two Co3+ appearing in its neighborhood.
1/20,(g)
02
O2
I
co2+ c03+ 0 2 .
02.
I
10-4
I
C02'
02.
co3+
J-
I
-2801
co2+ 0 2 .
02.
tI
.36 -520
10-3
hole (0)formed, two Co3+ions are produced in the neighborhood, according to eq 2. So
K=
[mi3 [O, (dissolved) ] ' I 2
(4)
where the hole might play the role of the host, accepting the HzPO4- ion as the guest species. Therefore,
The potential of the electrode is expressed as
(7)
Combining eqs 4 and 7, the potential of the electrode can be described by the following equation:
RT
E = P - - l nF
[H,PO,-] [O, (dissolved) I 'I6 [DH2P04-1
(8)
When the dissolved oxygen is constant, one can write
E = E - RT -In[H,PO,-l F
(9)
According to eq 8, the electrode potential change depends not only on the change of activity of HzPO4- but also on the change of the amount of dissolved oxygen. This was experimentally veri6ed by bubbling oxygen into the measurement system. Bubbling oxygen into the solution showed an effect equivalent to the decrease of HzPO4- activity (Figure 4), while bubbling nitrogen or argon into the solution, expelling dissolved oxygen, was equivalent to the increase of HzP04- activity (not shown in Figure 4). This process is a reversible one, though the phenomenon, unfortunately, can only be qualitatively observed, as the detailed
z
1
10
15
20
25
30
35
40
Figure 5. The ac impedance plot.
process of the reaction between the dissolved oxygen and the electrode surface might be rather complicated. When the activity mol/L, only a minor effect of the change of H2P04- was over of the amount of dissolved oxygen on the potentiometric response was observed. It seemed that in this case most of the holes of the Col-,O layer were covered by HzP04- species, prohibiting the oxygen atoms from penetrating into the deep inner Col-,O layer. The concentration of buffer solution (potassium acid phthalate) was quite critical. In potassium acid phthalate solutions of concentrations above or below 0.025 mol/L, a slight superNemstian response of the electrode was observed. Therefore, the potassium acid phthalate concentration should be kept at the level of 0.025 mol/L as an optimum buffer medium. This concentration might be different for other phosphate-containing compounds, such as ATP and ADP (vide infra). The ac impedance plot of the electrode pretreated as mentioned in the Experimental Section and conditioned in potassium acid phthalate buffer is shown in Figure 5. Two semicircles were observed. The left semicircle refers to the capacitance of the Col-,O/Co-solution double layer, and the right semicircle seemed to originate from the adsorption of charged species of HzP04- on the Co1-,0/Co surface as the consequence of the host-guest reaction of the holes with HzPOd-. A preliminary interesting application of the phosphate sensor is the assay of ATP and related compounds. After preliminary conditioning in W4 mol/L disodium ATP or ADP solutions prepared with 0.015 mol/L potassium acid phthalate as buffer solution, an ATP- or ADP-sensitive sensor can thus be realized. As shown in Figure 6, the ATP-sensitive sensor shows potentiometric response toward disodium ATP in 0.015 mol/L potassium acid phthalate buffer, with a linear range of 10-6-10-3 mol/L and a slope of 34-36 mV/decade. The ADP-sensitiveelectrode shows a linear potentiometric response of 10-6-10-3 mol/L toward disodium ADP in the same buffer solution, with a slope of 48-50
6
5
4
3
- l0gC
Figure 6. Response curves of phosphate-containingcompounds (a) ATP, (b) ADP, and (c) AMP.
mV/decade. When disodium AMP solution was used as the conditioning agent, the electrode could not be transformed into a practically usable AMP sensor, as only about 5-8 mV potential change was observed for the electrode thus prepared for the concentration change from 10-6 to mol/L disodium AMP. It seemed that the cavity of the COOlayer as the host did not accept the phosphate binding with adenosine as guest but accepted the phosphate binding with a second phosphate. It was also noticed that the electrode for ATP had a potentiometric response slope of 34-36 mV/decade, while the electrode for ADP had a potentiometric response slope of 48-50 mV/decade. The major charge form for ATP and ADP is ATP- and ADP-, respectively, in the buffer solutions at pH 4.0.19This seemed to correspond to the fact that the number of “additional” pieces of phosphate binding with the boundary phosphate as the “guest” was two for ATP (ca. 35 mV/decade) and only one for ADP (ca. 50 mV/ decade). ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China and Electroanalytical Laboratory of Changchun Institute of Applied Chemistry, Academia Sinica. Received for review February 9, 1994. Accepted October 6,1994.@ AC940145M
(19)Dean, J. A, Ed. Lunge’s Handbook ofChemisty, 13th ed.; McGraw-Hill Book Co.: London, 1985; Tables 5-8.
@Abstractpublished in Advance ACS Abstracts, November 15, 1994.
Analylical Chemistry, Vol. 67, No. 2, January 15, 1995
291
