controlled-potential pulse electrolysis can be performed with
reference electrodes, which can be settled by inserting the
backside of the electrolysis columns.
The dual electrolysis was performed by using two potentiostats,
HECS 311B, with a timer unit (Huso Co. Ltd., Kawasaki, Japan)
designed especially for the present purpose. That is, the timer
unit made it possible to set the electrolysis time and the
electrolysis current (or potential) independently for each elec-
trolysis line, under the conditions that the dual pulses finished at
the same time. Hence, the solutions of ion radicals can be mixed
effectively just after the pulse electrolysis, which is useful for
observation of the ECL reactions of ion radicals with various
stabilities.
Immediately after imposing the pulses onto the dual-column
electrodes, the electrolyzed solutions were mixed rapidly in the
double two jet mixer (JM) and delivered into an optical cell (OC).
Figure 3A shows the optical arrangement for the absorption
measurement with two optical fibers and as an optical cell with a
light path of 2.0 mm. UV-visible measurements were utilized
for identification of the generation of ion radicals and for
optimization of the electrolysis conditions to perform the quantita-
tive electrolysis.
Fig u re 4 . Absorption spectra of (A) DPA•+ and (B) DPA•- observed
at the optimized electrolysis conditions after the electrolysis was
performed only for single-electrolysis column. Sample solution: [DPA]
1.0 mM and [TBAP] 0.1 M in AN. Electrolysis time, 1.0 s. Electrolysis
current: (A) 5.0 mA; (B) -7.0 mA.
RESULTS AND DISCUSSION
Fundamental Evaluation of Dual-Electrolysis Stopped-
Flow Method for ECL Observation. We first examined how
For the ECL observation, while the ECL reactions could be
observed using the mixer and the optical cell in Figure 3A, we
refined the optical cell part to collect the ECL emission as much
as possible. As a consequence, we adopted a jet mixing optical
cell (JO) as shown in Figure 3B. This cell was designed to collect
the emission light directly from the point where the two solutions
were mixed, so that high-sensitivity measurement could be
performed compared with the measurement using the jet mixer
and the optical cell.
The spectrophotometer used was a USP-501 (Unisoku Co. Ltd.,
Hirakata, Japan), which was used for the observation of UV-
visible spectra and ECL emission spectra. For observation of ECL
emission, we used both a photon-counting system and an image-
intensified multichannel photodiode array detector (II + MCPD).
The former was used for detecting the time course of the amount
of the emitting light without diffracting the light. The latter was
used for observing ECL spectra with a spectrophotometer.
Both detection systems, the electrolysis cell, and the stopped-
flow apparatus were constructed with the help of Unisoku Co.
Ltd. The whole system was controlled by a microcomputer (PC-
ECL reactions can be observed in the developed dual-electrolysis
stopped-flow apparatus by measuring the reactions between DPA•+
•-
and DPA . Before ECL measurements, electrolysis conditions
were optimized by measuring the UV-visible absorption spectra.
Parts A and B of Figure 4 show the absorption spectra observed
at optimized electrolysis conditions for the generation of DPA•+
•-
and DPA , respectively. These spectra were observed after
electrolysis was performed only for a single electrolysis line. In
these measurements, the sample solution contained 1.0 mM DPA
and 0.1 M TBAP in AN, and the electrolysis time was 1.0 s. For
maximum generation, the electrolysis current was 5.0 mA for
•
+
•-
DPA , and -7.0 mA for DPA . The difference in the current
values presumably reflects the difference in the residual currents,
of which the reduction is larger than the oxidation in the
electrolysis of DPA. For other ECL reaction systems, the
conditions for maximum generation of ion radicals were optimized
before the ECL measurements.
Figure 5 shows the time course of the absorbance and the
ECL observation. Parts A and B of Figure 5 are the results
measured after electrolysis with a single column as in the above
measurement of absorption spectra. These show that DPA•+ and
DPA•- were delivered into the optical cell by a piston drive and
remained in the cell without decaying. Figure 5C shows the result
of photon-counting measurement after generating both solutions
of DPA•+ and DPA , and then they were mixed together in the
optical cell. The emission was clearly detected on the time
domains when the solutions were flowing into the optical cell.
Since the ECL reactions are so fast as to be described by the
diffusion-controlled reactions, it is inferred that the emission
disappeared immediately after the solutions were stopped. How-
ever, even for such a fast reaction, owing to the high concentration
generation of homogeneous ion radical solutions using the column
electrolysis method, the proposed method was found to be utilized
for the observation of ECL reactions.
9
801 FA, NEC).
In the ECL measurement, in particular, special attention was
paid to degassing the solutions. For each solution, using a flask
with two stopcocks, the oxygen was removed and replaced with
nitrogen with a vacuum pump. The solutions were directly
delivered to the reservoirs of the stopped-flow apparatus without
contacting air.
Reagents. Rubrene (RU, Nacalai tesque), 9,10-diphenyl-
anthracene (DPA, Nacalai tesque), pyrene (PY, Wako chemicals),
and thianthrene (TH, Wako chemicals) were used as received.
Synthetic and purification methods for 9-chloro-10-phenylan-
•-
thracenes (XPA) and 9,10-dihalogenoanthracenes (DXA) were
described previously.6
,10
The purification methods of acetonitrile
(
AN) and tetrabutylammonium perchlorate (TBAP) were also
described previously.6
By preliminary measurements using photon-counting detection,
the optical part was refined to achieve the effective ECL observa-
(
10) Oyama, M.; Okazaki, S. J. Electroanal. Chem. 1 9 9 1 , 297, 557-563.
Analytical Chemistry, Vol. 70, No. 23, December 1, 1998 5081