Anal. Chem. 1994,66,1853-1859
Differential Cyclic Voltfluorometry and Chronofluorometry of the Transfer of Fluorescent Ions across the 1,2=Dichloroethane-Water Interface Takashi Kakluchl' and Yoko Takasut Department of Physical Chemistry, Yokohama National University, Yokohama 240, Japan
The time derivative of fluorescence vs time curves recorded with voltage-scanfluorometry (Kakiuchi, T.; Takasu, Y Senda, M. Anal. Cbem. 1992,64,3096-3100) for the transfer of the divalent anionic form of Eosin B (EB) across the 1,2dichloroethane-water interface gives curves isomorphic to the cyclic voltammograms of ion transfer of EB when the concentration of EB in the aqueous phase is in the micromolar range or lower. From the differential cyclic voltfluorograms obtained, electrochemical properties of ion transfer, e.g., the Gibbs energy of transfer of fluorescent ions and the reversibility of ion transfer, can be estimated at the nanomolar level, which is -5 orders of magnitude more sensitive than conventional cyclic voltammetry of the ion transfer. Chronofluorometry, Le., the potential-step transient of the fluorescence for the transfer of fluorescent ions, is demonstrated to be utilized as a counterpart of the chronocoulometry of ion transfer in the same low concentration range.
.;
Recent development of the electrochemistry at liquid-liquid interfaces has revealed intriguing features of charge-transfer processes at the interface, which are in many ways relevant to various analytical techniques, e.g., solvent extraction and selective detection of ions with ion-selective electrodes.IJ In most electrochemical studies of charge transfer at the interfaces, dc or ac amperometry with controlled voltage and potentiometry with controlled current has been employed. A relatively large double-layer capacitance at liquid-liquid interfaces3 and residual current due to nonideal polarizability of the interface435 usually set the lower detection limit of these standard electrochemical approaches to the order of 0.1 mM. Voltage-scan fluorometry of ion transfer at the I '2-dichloroethane (DCE)-water interface that we proposed recently can detect nanomolar levels of fluorescent ions in the aqueous phase.6 A theoretical treatment based on a reversible ion transfer across the liquid-liquid interface in combination with the Lambert-Beer's law explained well the properties of voltfluorograms in a higher concentration range of Rose Bengal of millimolar order.6 However, in the previous study, the shape of voltfluorograms recorded at lower concentrations deviated from the theoretical prediction.6 Although the On leave from Department of Agricultural Chemistry, Kyoto University, Japan (1) Girault, H. H.; Schiffrin, D. J. In Electroanalytical Chemistry; Bard, A. J.,
Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp 1-144. (2) Senda, M.; Kakiuchi, T.; Osakai, T. Electrochim. Acta 1991, 36, 253-262. (3) Samec, Z. Chem. Rev. 1988, 88, 617-632. (4) Kakiuchi, T.; Senda, M. Bull. Chem. Soc. Jpn. 1983, 56, 1322-1326. (5) Kakiuchi, T.; Senda, M. Collecf.Czech. Chem. Commun. 1991.56, 112-129. (6) Kakiuchi, T.; Takasu, Y.; Senda, M. Anal. Chem. 1992,64,3096-3100; Anal. Chem. 1993,65, 1123. 0003-2700/94/0366-1853$04.50/0
0 1994 American Chemical Society
observed linearity between the peak height of the voltfluorograms and theconcentration suffices for a sensitive detection of fluorescent ions at the nanomolar level, this discrepancy limited the method in quantitative electrochemical characterization of charge-transfer processes at the nanomolar level. Since insusceptibility to double-layer charging current and the negligible effect of iR drop at lower concentrations of analyte ions, another significant feature of the voltagecontrolled fluorometry, make the method promising in studying charge-transfer kinetics, it is worthwhile studying the behavior of the method in detail. Further development of voltagecontrolled fluorometry is also important in view of the potential usefulness of the method in studying the mechanism of ionionophore complex formation7-13using fluorescent ionophores and of photoinduced electron-transfer reactions at liquidliquid interface^.^"^' In this paper, we report that voltfluorograms in harmony with the previously proposed model6 can be obtained for the transfer of Eosin B (EB) at the DCE-water interface by minimizing the convective motion of solutions in the vicinity of the interface. Further, we will show that the differentiation of the voltfluorograms with respect to time gives curves isomorphic to cyclic voltammograms of EB transfer, when the concentration of EB in the aqueous phase is lower than micromolar. From differential cyclic voltfluorograms (DCVFs), electrochemical properties of the ion transfer, e.g., half-wave potential and reversibility of ion transfer, can be deduced in the nanomolar range of fluorescent ions. In the second part, we report the potential-step chronofluorometry for the transfer of Eosin Y (EY) across the DCE-water interface. ~~
~
(7) Yoshida, Z.; Freiser, H. J. Electroanal. Chem.Interfacial Electrochem. 1984, 179,
31-39.
(8) Homolka, D.; Wendt, H. Ber. Bunrenges. Phys. Chem. 1988,89,1075-1082. (9) Kakutani, T.; Nishiwaki, Y.; Osakai, T.; Senda, M. Bull. Chem. Soc. Jpn. 1986.59,
781-788.
(IO) Sinru, L.; Freiser, H. Anal. Chem. 1987, 59, 2834-2838. (1 . 1). Alemu, H.; Hundhamer, B.; Solomon, T. J. Electroanal. Chem. Interfacial
Electrochem. 1990, 294, 165-177. (12) Matsuda, H.; Yamada, Y.; Kanamori, K.; Kudo, Y.; Takeda, Y. Bull. Chem. Soc. Jpn. 1991,64, 1497-1508. (13) Kakiuchi, T. J. Colloid Interface Sci. 1993, 156, 406-414. (14) Thomson, F. L.; Yellowless, L. J.;Girault, H. J. Chem.Soc.,Chem. Commun. 1988, 1547-1549. (15) MareEek, V.; De Armond, A. H.; De Armond. M. K. J. Am. Chem.Soc. 1989, 111, 2561-2564. (16) Samec,Z.;Brown,A.R.;Yellowlees,L.Y.;Girault,H.;Bab,K. J.Electroana1. Chem. Interfacial Electrochem. 1989, 259, 309-3 13. (1 7) Kotov, N. A,; Kuzmin, M. G. J. Electroanal. Chem. Interfacial Electrochem. 1990, 285, 223-240.
Analytlcai Chemistry, Voi. 66, No. 11, June 1, 1994
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THEORETICAL CONSIDERATION Under the simplified assumptions that the excitation light is impinged with the angle normal to the interface and is not reflected at the interface and that the concentration quenching of fluorescence in DCE is negligible, the fluorescence intensity from the DCE phase, P(t), is given by6
p(t)= K o @ o Z o ~ o m c ~ ( x , t ) [ exx opci ~o~(f,t) _ K d o ] dx
(1)
where KO is the absorption coefficient, @O is the quantum efficiency of the fluorescence in DCE, ZO is the intensity of excitation light in the infinity (x = +-) in the DCE phase, and co(x,t) is the concentration of fluorophore ion in DCE. t is time, x is the distance normal to the interface at which x is taken as zero, and f is the integral variable. The superscript, 0, on K,a, and ci denotes that these quantities are for the DCE phase. When Koco(x,t) is sufficiently small so that the exponential term in the right-hand side (rhs) of eq 1 becomes unity, eq 1 reduces to
p(t)= Ko@oZo~omc~(x,t) dx This approximation holds in the concentration range where the fluorescence intensity is proportional to the concentration of the fluorophore: typically, the concentration range lower than micromolar at a scan rate of the order of tens of millivolts per seconde6 As is frequently employed in conventional electroanalytical chemistry,I8 we consider the case when the initial condition, co(x,t=O) = 0 and cw(x,t=O) = is satisfied, where ' c y is the bulk concentration of the fluorescent ion in the aqueous phase. Equation 2 in this case implies that the fluorescence intensity increase linearly with the bulk concentration of the fluorophore in the aqueous phase1*and, in the linear scan mode of applied potential, with the reciprocal of the square root of the scan rate.6 The total amount of the fluorescent ion in DCE at a given time is in this case equal to the charge transferred into the DCE phase and we see that
b~y,
Jomco(x,t) dx = - s1r i ( s ) ziFA 0
d7
(3)
where i ( t ) is the current across the interface carried by the fluorescent ion, zi is the charge on the ion in signed unit, F is the Faraday constant, and A is the area of the interface. From eqs 2 and 3, we can write
(4) Differentiation of both sides of eq 4 with respect to t directly leads to (5)
Thus the derivative of a voltfluorogram with respect to time is proportional to the current across the interface carried by the fluorescent ion. In the case of the cyclic voltammetric mode of applied potential, a DCVF should have the same
shape as that of a cyclicvoltammogram of ion transfer, though the low concentration of fluorescent ion precludes the measurement of the voltammogram with conventional electrochemical techniques. On the other hand, eq 4 indicates that the fluorescence intensity is proportional to the total charge of transferred fluorescent ions, QO. The potential-step chronofluorometry is in this approximation a counterpart of potential-step chronocoulometry, except that the factor related to fluorescence, KO@OZO, is multiplied on the rhs of eq 4. Substituting the Cottrell equation18 for QO, we have the expression for the potential-step chronofluorometry of reversible charge transfer in the absence of specific adsorption:
where 5 = (Dw/Do)lI2 and p = exp[(ziF/RT)(E - E:)]. D" (a= 0 or W) is the diffusion coefficient in a,F is the Faraday constant, R is the gas constant, Tis the temperature, E is the potential drop across the interface referred to appropriate reference electrodes, E: is the standard ion-transfer potential on the same scale. Similarly, for the quasi-reversible case,19
where X = z/(Dw)1/2 + g/(Do)Il2, and being the rate constant of ion transfer from W to 0 and 0 to W, respectively. As is the case of chronocoulometry,lgfor Xt1/2 > 5, eq 7 takes the limiting form:
Thus from the intercept at P(t)= 0, t1112, obtained by extrapolating the linear portion of the P(t)vs tl/* plot, we can estimate A. Unlike chronocoulometry, the slope of the P ( t ) vs t1l2 plot contains the quantities associated with fluorescence and cannot be correlated directly with the iontransfer process per se. The cross-sectional area of the beam of excitation light is in reality not necessarily equal to or greater than the crosssectional area of the interface. When only a part of the ions transferred is excited by light, a certain factor should be multiplied on the rhs of eqs 2 and 4-8. According to a more rigorous treatment of voltage-controlled fluorometry,20P(t) is proportional to the total charge transferred even when the angle of incidence of excitation light is not zero and the beamed area is smaller than the area of the DCE-water interface.
EXPERIMENTAL SECTION Materials. Sorbitan monooleate (Nakarai Chem. Ltd.), Eosin B (Nakarai tesque), Eosin Y (Wako Pure Chem. Ind.), Amaranth (Acid Red 27, Wako Pure Chem. Ind.), and other chemicals used were of reagent grade. Sorbitan monooleate was added to the DCE phase to suppress the convective motion (19) Christie, J. H.; Lauer,G.;Osteryoung, R.A.;Anson, F. C. Anal. Chem. 1963,
(IS) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; Chapters 5 and 6.
1854
Analytical Chernistty, Vol. 66,No. 11, June 1, 1994
35, 1979. (20) Kakiuchi, T.; Takasu, Y., in preparation.
of the solutions in the vicinity of the interface. Sorbitan monooleate lowers the differential capacitance in the entire range of potential studied, indicating its adsorption at the interface in this potential range.21 No facilitated transfer of ions by sorbitan monooleate was observed. In chronofluorometry measurements, Amaranth, a nonfluorescent trivalent anion having a strong absorbance, was added to the aqueous phase to suppress the contribution of fluorescence from the aqueous phase due to the excitation of fluorescent ions in the aqueous phase by evanescent wave and stray light. Amaranth was found to stay in the aqueous phase within the potential range studied, except at the negative extreme of the potential, where the foot of the wave of Amaranth ion transfer was discernible. Methods. The cell is represented as a
5 mM TPnACI
.AgCI
0 01
,
M MgCh
(W)
0 02 \I TPnATPB 05
1
m M sorbitan
M EB 0.01
20
3
. .Y
10
-
k Q
0-
or EY
M LiCl
0.01 M Phosphate-
monooleate
Borate buffer(p8 8)
PCE)
(Y
AgCl
pH of the aqueous phase was adjusted to 8, so that EB was in dianionic form. The measuring cell and the methods for fluorescence and electrochemical measurements have been described elsewhere.6 The polarized DCE-water interface formed at the orifice of a glass tube was adjusted to have a flat surface. The geometrical area of the interface was 0.166 cm2. We replaced a xenon lamp with a 15-mW Ar+ laser at 488 nm (Spectra-Physics, 161B). The 0.65-mm-diameter laser beam was reflected twice with mirrors and was led to the cell to shine on the interface from the DCE side of the interface. The angle of incidence was adjusted to -75'. In voltage-scan experiments, time-dependent fluorescence was monitored at 2-mV intervals at the maximum emission of EB, 547 nm, in the DCE phase. In potential-step experiments, time-dependent fluorescence was monitored at the maximum emission of EY in the DCE phase, 550 nm. The time interval of the fluorescence measurement was set at 3.57 ms. The solution resistance was compensated for by a positive feedback method. In chronofluorometry, the time constant of the cell is decisive to determine the response time of the interface against the programed potential. In the present study at low dye concentrations, -99% compensation of the total solution resistance of -20 kQ was usually employed. The double-layer capacitance of the polarized interface in the present cell, -3 pF, combined with the uncompensated resistance gives the time constant of -0.6 ms: one-sixth of the sampling interval. When the aqueous phase containing EB or EY was brought intocontact with the DCE phase with an opencircuit, a portion of the dyes spontaneously dissolved into the DCE phase. By holding the potential at a sufficiently positive value for several minutes, the dye in the vicinity of the DCE side of the interface came back to the aqueous phase. However, not all the dye in DCE was recovered by diffusion within a limited time. The (21) Kakiuchi, T.; Teranishi, Y.;Niki, K., in preparation.
remaining contamination of the DCE phase, together with the fluorescence from the aqueous phase, gave rise to background fluorescence, which was subtracted in data analysis. The cell for fluorescence measurements was also used for cyclic voltammetry. All measurements were made at room temperature, 20 f 2 OC.
RESULTS AND DISCUSSION Differential Cyclic Volffluorometry. Figure 1 shows the voltfluorograms for the transfer of EB at six different concentrations of EB, 'cFB, from 2 to 100 nM, when the potential was scanned linearly from E = 0.5 to 0.1 V and then from 0.1 to 0.5 V at 10 mV s-l. The background fluorescence was subtracted in voltfluorograms in Figure 1. The peaks in the voltfluorograms appeared after the direction of the scanning voltage was reversed, as predicted by the theoretical treatment proposed previously.6 In the absence of sorbitan monooleate in the DCE phase, the peak appeared around the switching potential, as was reported previously. Abnormal currents associated with convective motion of solutions and its suppression by surface-active substances have been reported for ion transfers across liquid-liquid interface^.^^.^^ The concentration dependence of the peak height, AF,, is shown in Figure 2. As shown in the inset in the figure, AFp is proportional to 'cFB up to ' c g = 100 nM. In the higher concentration range, the plot gradually deviated from the straight line owing to the attenuation of excitation light in the proximity of the interface.6 The effect of scan rate on a voltfluorogram is shown in Figure 3 at 'crB = 1 pM. AF, was inversely proportional to the square root of the scan rate (Figure 4). Figures 2 and 4 indicate that the approximation introduced in deriving eq 2 is applicable in analyzing voltfluorograms obtained at a concentration lower than 1 pM and at a scan rate greater than 10 mV s - ~ . ~ A cyclic voltammogram (CV) of the ion transfer conventionally recorded at 'crB = 0.5 mM is compared in Figure 5 (22) Koryta, J.; Vanfsck, P.; Bfezina, M. J . Electroanal. Chem. Interfacial Electrochem. 1976, 67, 263-266. (23) Kakiuchi, T. J . Electroanal. Chem. Interfacial Electrochem. 1993,345,191203.
Analytical Chemlstty, Vol. 66, No. 1 I, June 1, 1994
1855
innn -
A"""
30 I
~
I
I
1
I
l a
/
.
I
-30
1 I
v
0
20
10 b
~
I fLM w
-0.2 ~
~
....
J I
,
I
I
1
1
I
1
Flgure 2. Peak height vs curve. The inset shows the magnified view at lower values. Scan rate IO mV s-1.
E I mV 100
500
500
100 I
I
0.4
m
e
. Y
E
0.0
600
400
E I mV Flgure 5. (a) Cyclic voltammogram at = 0.5 mM at a scan rate of 200 mV s-I, after the base current at the same scan rate was subtracted. (b) Differential cyclic voltfluorogram at = (1) 10, (2) (20),and (3) 50 nM. Scan rate 10 mV s-I. The inset shows the corresponding curve at = 5 nM.
'G
-
0'
I
200
'qB
4020
'
0
1
0.2
0.4
0.6
0.8
v t x 10-3 / m V Figure 3. Effect of scan rate on voltfluorogram at rate: 5 (top) 10, 50, and 100 (bottom) mV s-l.
'gB = 1 pM. Scan
150
'G
1.0
1
Table 1. Peak Potentials in Forward and Reverse Scans, Elp and Em,and Midpoint Potential, €, for the Transfer of EB a c r w the DCE-W Interface Measured by CV and DCVF
cv
DCVF
av
0.2 0.01 0.01 0.01
5 X los 10 20 50
0.177 0.191 0.179 0.184 0.185
0.252 0.241 0.247 0.245 0.244
0.214 0.216 0.213 0.214 0.214
-0.032 -0.030 -0.031 -0.032 -0.032
Relative to the half-wavepotential of Clod- ion transfer recorded with the same cell. 50 -
;
;
00 0
02
v 0.4 112 /
mv-l/2 0.6 s1/2 0.8
1.0
Figure 4. Dependenceof peak height of voltfluorograms on scan rate. Data were taken from Figure 3.
with DCVFs obtained by differentiating voltfluorograms with respect to t by using the Savitzky-Golay algorithm.24 The DCVFs at 'cFB = 10,20, and 50 nM are shown in Figure 5b. These curves are similar in shape to the CV in Figure 5a, in which the CV after subtracting the base current is represented. The inset of Figure 5b is a DCVF at 'cFB = 5 nM, which still retains the shape of the CV. Thus, curves isomorphic to a CV corresponding to the transfer of the same ion at a much higher (24) Savitzky, A.; Golay, M. J . E. Anal. Chem. 1964, 36, 1627-1639.
1856
Analytical Chemistry, Vol. 66,No. 11, June 1, 1994
concentration can be obtained from voltage-scan fluorometry at nanomolar concentration. The peak potential and midpoint potential of DCVFs at 'cFB = 10,20, and 50 nM and those from CV at 0.5 mM are compared in Table 1. The peak potentials for the forward and reverse scans are 0.185 and 0.244 V in DCVF and 0.177 and 0.252 V in CV. The midpoint potential for DCVF, 0.214 V, agreed well with 0.215 V for CV. Thewider peakseparation in CV, 75 mV, compared with that of DCVF, 60 mV, would be attributable in the former to an uncompensated iR drop. The addition of sorbitan monooleate considerably diminished the convection motion of the solutions in the vicinity of the interface. However, when 'c& was relatively high, a part of the EB transferred to DCE was frequently carried away irreversibly into the bulk of the DCE phase from the edge of the interface, resulting in a gradual increase in the background level of fluorescence in the DCE phase. This persistent irregular motion of the solution is likely to be responsible for
6
~
10 t
. 5W
0 0
10
5 "l/Z
/
,v1/2
51/2
Figure 8. Dependenceof the peak height in the forward scan of DCVF on scan rate. = 1p ~ .
'CB
the observed wider peak separation even in DCVF, in comparison with a theoretical prediction of 30 mV (at 25 "C) for zi = -2. In fact, potential-step chronofluorometry measurements of EY transfer clearly indicate that the current is carried by divalent anions (See below.). Figure 6 shows the effect of the scan rate on the peak height in the forward scan of DCVF at 'crB = 1 pM. The proportionality between the peak height and the square root of the scan rate holds also in DCVF, as is the case of CV of EB transfer across the DCE-water interface. No appreciable dependence of peak separation on the scan rate was observed in this range of scan rate. It is seen that the electrochemical properties equivalent to those from CV can be obtained from DCVF in the nanomolar range of fluorescent ions. One disadvantage of DCVF is that the diffusion coefficient cannot be estimated from the fluorescenceintensity, since the absolute magnitudes of Io and a0 are difficult to estimate precisely. The ratio of the fluorescence collected by the photomultiplier to the total fluorescence from the DCE phase is also unknown. On the other hand, DCVF has two distinct advantages over CV. In DCVF, the double-layer charging does not in principle affect the shapeof thevoltfluorograms; thecharging is covered by nonfluorescent supporting electrolyte ions in both phases. Figure 5b shows that the current in the beginning of a forward scan was close to zero up to 0.3 V, irrespective of the scan rate. In this potential range, the scan-rate-dependent charging current inescapably appears in cyclic voltammetry and often necessitates the subtraction of the base current from the total current to estimate the current due to the transfer of analyte ions, which leaves a certain obscurity in interpreting the CV data obtained. Moreover, the minute magnitude of the current caused by the transfer of fluorescent ions frees us from applying a stringent iR compensation. In determining kinetic parameters of charge transfer relying on the measurement of electric current passing through the interface, a small error in the iR compensation or in the evaluation of a solution resistance by a certain method can cause a significant error, in particular, in fast charge-transfer p r o c e ~ s e s . In ~ ~the * ~present ~ method, (25) Milner, D. F.; Weaver, M. J. J . Electroanal. Chem. Interfacial Electrochem. 1987, 222, 21-33. (26) Kakiuchi, T.; Noguchi, J.; Kotani, M.; Senda, M. J . Electroanal. Chem. Interfacial Electrochem. 1990, 296. 517-535.
the main component of the current is the residual current due to the transfer of supporting electrolyte ions. A typical value of a solution resistance in the cell used in the present study was 30 kO and the residual current in the middle of the potential window was typically 1 pA at a scan rate of 100 mV s-I; 99% compensation of the solution resistance is easily achieved with a positive feedback method.26 An uncompensated resistance of 300 i2 then results in an iR drop of 0.3 mV, which is negligible as an error in the dc component of the applied potential. On the other hand, this order of uncompensated resistance in a CV measurement for 0.5 mM EB brings about at least several millivolts of iR drop. The situation is much worse in an uc voltammetry; the uncompensated resistance of 300 i2 rules out the extraction of kinetic parameters from uc data, since the value of the ion-transfer impedance could be 1 order of magnitude lower.26 On the contrary, DCVF at higher scan rates will therefore enable, in principle, the determination of kinetic parameters using the Nicholson-Shain method?' Higher sensitivity and time resolution of fluorescence detection, in addition to a stable liquid-liquid interface regarding both irregular motion of solutions near the interface and a gradual change in shape of the interface during the measurements, are required in further development of the method for this purpose. Chronofluorometry. The fluorescence transient after the potential was stepped from the initial potential Ei, 300 mV, where the transfer of EY from W to 0 was negligible, to a final potential, Ef, and then stepped back to E l , was recorded. Figure 7a shows two examples of fluorescence-time transients at a concentration of EY in the aqueous phase, of 1 pM, when E f = 105 (lower curve) and 80 mV (upper curve). The latter potential is close to the half-wave potential of EY transfer. The start and end of the potential steps are indicated by two vertical lines in Figure 7a. In both cases, residual fluorescence before the application of the step potential was subtracted from the measured fluorescence intensity to obtain ion-transfer fluorescence, AF. Since Amaranth was added to the aqueous phase, the change in fluorescence in the aqueous side of the interface during the polarization of the interface was supposed to make no significant contribution to the observed fluorescence intensity. It was therefore assumed that the observed change in fluorescence intensity was from the DCE phase. Similar measurements were made at varied potentials between Ef = 150 and 0 mV. The AFis replotted in Figure 7b as a function of the square root o f t . Straight lines obtained by least-squares fitting of each of the curves are shown in Figure 7b as dashed lines. It is seen that in both cases the experimental curves are well represented by straight lines; that is, the experimental curves follow the theoretical prediction given by eq 7. In other words, when 'cry is as low as 1 pM and the time scale is at least shorter than 0.5 s, the approximate expression, eq 2, is valid in the present system. From a chronofluorogram of the type shown in Figure 7a, we can then infer the properties of ion transfer by using eq 7. While the curves for E = 105 mV in Figure 7b cuts the t ' f 2 axis at A F at its origin, the curve for E = 80 mV has a
b~ry,
(27) Nicholson, R. S.;Shain, I. Anal. Chem. 1964, 36, 706-723.
AnalyHcel Chemistry. Vol. 66, No. 71, June 7, 1994
1857
I
8
. 2
. &
E4
d
200;
k
d
o
0 0
0
I
' 0.5
I
0
i
0-
0
1
0O 0
%o
,
-100
1
"o""oO
111
a
100
0
100
4 200
E i mV
1
t l s
4/
9' 0
100
200
E I mV -100 0.00
0.50
0.25 tu2
/
1 0.75
= 1 p M at 1
sl/z
Figure 7. (a) Fluorescencetransients for = 1 p M when potential Is stepped from 300 to 105 mV (lower curve) and from 300 to 80 mV (upper curve). The vertlcal broken Ilnes show the onset of potential step and step back. (b) Fluorescence intenslty vs ti/* plots corresponding to the curves in (a). (a) Fluorescence Intensity vs € plot for = 1 p M at t = 0.5 s. (b) Logarithmic analysis of the plot in (a).
nonzero value of the intercept. This seems to be reasonable in view of the dependence of X on E; X takes a maximum at the half-wave potential,'* leading to the largest intercept in a AF vs t1/2 plot at this potential. The sampling interval of fluorescence employed in the present study, 3.57 ms, is probably too large to quantitatively estimate the rate constant of EY transfer from the intercept, tI1l2= 0.058 s112. It is, however, highly probable that EY transfer across the DCE-W interface is dc quasi-reversible in this order of time scale and that the standard rate constant is less than cm s-*. This order of magnitude is consistent with Stoke's law; EY is much larger in size than tetramethyl- and tetraethylammonium ions whose standard rate constants are 1 order of magnitude higher at the nitrobenzene-water interface.26.28 Another important feature of the plots in Figure 7b is that the effect of double-layer charging and the specific adsorption of EY at the interface are neglible. Both factors should raise the intercept of the plot with AF axis above zero.29 In contrast to chronocoulometry, in which a higher concentration of analyte is preferred to surmount the contribution of doublelayer charging,18 the concentration of analyte ion can be very (28) Wandlowski, T.; MareEek, V.; Holub, K.; Samec, Z. J . Phys. Chem. 1989,93, 8204-82 12. (29) Christie, J. H.; Lauer, G.;Osteryoung, R . A. J . Electroanal. Chem.Interfacial Electrochem. 1964, 7, 6C-72.
1858
Figure 8. (a) Fluorescence intensity vs E plot for = 0.5 s. (b) Logarithmic analysis of the plot in (a).
Analytical Chemistry, Vol. 66,No. 11, June 1, 1994
small, i.e., at the nanomolar level, and hence, chronofluorometry is sensitive in detecting the specific adsorption of analyte ions at the interface. AF at t = 0.5 s is plotted against E for 'cFY = 1 micromolar in Figure 8a. As is shown in eqs 6 and 7, a P(t) vs E curve at a given value o f t has a shape that resembles a sampled-current voltammogram in chronoamperometry. When t = 0.5 s and ko N cm s-l, the second term in the bracket on the rhs of eq 8 is negligible and eq 8 reduces to the reversible case, eq 6 . Hence, the half-wave potential of the P(t)vs E curve in this case should correspond to the reversible halfwave potential. The half-wave potential estimated from this plot, 82 mV, agreed fairly well with the midpoint potential of a cyclic voltammogram recorded at 'cZy = 0.5 mM, 75 f 6 mV .3O The logarithmic analysis of the AF vs E curve is shown in Figure 8b. The plot gave a straight line. From the slope of this plot, the charge carrying the current across the interface was found to be -1.7, which is close to -2, indicating that EY crosses the interface as a divalent ionic form. This demonstrates another advantage of fluorometry over voltammetry, in that cyclic voltammetry of ion transfer across liquid-liquid interfaces often gives larger peak separation than expected, particularly when a two-electrode system is employed.23 As is the case of the CV of EB described above, CV of EY transfer also gave a peak separation of the order of 7 0 mV, which might be interpreted as suggesting unity for zi. The observed value of -1.7 for zi obtained without applying stringent iR compensation thus underlines the advantage of voltagecontrolled fluorometry over voltammetry to circumvent high solution resistancee intrinsic to liquid-liquid interfaces. (30)Kakiuchi, T.; Takasu, Y . ;Ono, K.; Niki,K., in preparation
CONCLUSIONS The quantitativeness of voltage-controlled fluorometry and DCVF predicts a possible new electrochemistry resorting not to electrical current but to fluorescence measurements in the study of charge transfer at liquid-liquid interfaces. DCVF is 5 orders of magnitude more sensitive than conventional electrochemical techniques and is promising in determining electrochemical properties of the transfer of a trace amount of fluorescent ions. Moreover, DCVF is insensitive to doublelayer charging and iR drop. The present method offers three degrees of freedom for selective detection of ions: excitation and emission wavelengths and potential drop across the interfa~e.~'The advantage of chronofluorometry over conventional electrochemical techniques with respect to the direct effect of double-layer charging endows chronofluorometry
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(31) Kakiuchi,T.;Takasu,Y. J . Elecrroanal. Chem. Interfacial Electrochem. 1994, 36.5, 293-297. (32) Shao, Y.; Girault, H. H. J . Electroanal. Chem.Interfacial Electrochem. 1990, 2a2,59-72. (33) Roe, D. T. In Laboratory Techniquesin Electroanalytical Chemistry;Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984.
with particular attractiveness in studying the kinetics of ion transfer, for which so far reliable data have been difficult to obtain by current measuring technique^.^^.^^.^^ Further improvement of the method in two aspects is decisive for this purpose: a much shorter sampling interval and more rigorous iR compensation for fast charging of the electrical double layer.33 The present methods can be extended to the study of nonfluorescent ion transfer by using fluorescent ionophores and also to the study of electron transfer as well as photoinduced charge-transfer processes.
ACKNOWLEDGMENT This work has been supported in part by a Grant-in-Aid for Scientific Research (05640568) from the Ministry of Education, Science and Culture, Japan. We thank SpectraPhysics K.K. for giving us an opportunity to use an Ar+ laser. Received for revlew September 21, 1993. Accepted March 4, 1994." *Abstract published in Advance ACS Abstracts, April 1, 1994.
Analytical Chemistry, Vol. 66, No. 11, June 1, 1994
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