M.M. Hossain et al. / Electrochemistry Communications 13 (2011) 611–614
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4
0 nM. This indicates that the OPH hydrolysis of paraoxon produces
protons, which then transfer across the polarized interface with the
support of the proton selective ligand, ETH 1778. On the other hand, a
peak shaped voltammogram was obtained due to the linear diffusion
flux while the proton transfers from the viscous organic gel phase back
to the aqueous phase. The peak current was also linearly increased as a
function of the paraoxon concentration while keeping OPH concentra-
tion at 40 nM. In order to achieve a linear response of the observed
steady-state currents versus the proton concentration generated by the
OPH reaction with paraoxon, an excess ionophore concentration with
+
+
respect to the proton concentration was maintained; D
H
L
[H ] « D [L]
[10]. It should be noted that at the excess ionophore concentration, a
small potential shift less than 10 mV towards more negative potentials
in the measured reverse peak current was observed as the proton
concentration was increased probably due to uncompensated IR drop.
The concentration of OPH also critically affects the sensor's capability
for paraxon detection. In a series of cyclic voltammetry experiments
at different OPH concentrations with paraxon fixed at 50 μM, we
observed that the amount of released protons increased linearly with
increasing OPH concentration from 20 to 40 nM (data not shown).
Above this value, the OPH reaction rate began to decrease as indicated
by a reduction in the measured proton transfer signal. This is probably
due to OPH adsorption at the interface forming an organic dielectric
bilayer, which hinders both the hydrolysis and proton transfer re-
actions [13]. Therefore, an OPH concentration of 40 nM, which shows
the highest sensing signal for paraoxon, was used for all experiments.
Also the pH of the aqueous solution affects the hydrolysis and experiments
were performed to establish that a pH of 6.5 resulted in the optimum
sensing response.
3
.2. Quantitative detection using differential pulse stripping
voltammetry
Further improvements in the sensitivity were explored by the use
of differential pulse striping voltammetry with a preconcentration of
proton in the gel phase for a certain period prior to the analysis. A
potential of 550 mV was applied for 20 s which permits the generated
protons by OPH reaction with paraoxon to transfer from the water to
the organic phase and to be preconcentrated in the organic gel layer.
After the preconcentration period, the accumulated protons in the
organic gel layer followed by the complexation with ETH 1778 are
then stripped from the organic gel to the aqueous solution and the
corresponding negative peak current response around 350 mV due
to the proton transfer back to the water phase is recorded with
respect to the difference pulsing potential step ranging from 700 mV
to 100 mV. A series of differential pulse stripping voltammograms in
Fig. 2(a) for the different paraoxon concentration show that the
negative peak currents at 350 mV increase proportionally as a
function of paraoxon concentration ranging from 0.5 μM to 100 μM.
Both a potential increment of 10 mV and 20 mV were also investigated.
The 20 mV increment data shows about 10% larger changes in the
signal than that of the 10 mV increment for all paraoxon concentra-
tions studied in Fig. 2(a). An average of peak currents versus
paraoxon concentration was plotted in Fig. 2(b) with an excellent
linear fit for a dynamic range of 0.5–100 μM paraoxon where five
different sets of strip-sensors using Cell 1 were tested. A relatively
low relative error (2.5%) with a superb reproducibility of our strip-
sensor for paraoxon was also achieved. Our detection limit of 0.5 μM
is comparable to other OPH based sensing techniques reported in
literature [4,7,14]. The detection limit was achieved by directly
comparing peak currents obtained in the presence and in the absence
of 0.5 μM paraoxon in the Cell 1 at the signal to noise ratio of 3:1.
As a final demonstration, the selectivity of our paraoxon sensor
against a series of well-known interfering species in soil and natural
water samples were investigated. No significant changes in the negative
peak current responsible for the transfer of protons activated by
Fig. 2. (a) Differential pulse stripping voltammograms (DPSV) obtained for the
detection of paraoxon at concentrations of 0.5, 5, 10, 20, 40, 60, 80 and 100 μM (dotted
lines). The scan was directed from high to low potentials to drive the assisted proton
transfer from the PVC-NPOE gel layer to the aqueous phase containing different
concentrations of paraoxon. The solid line represents the DPSV obtained in the absence
of paraoxon. A deposition potential of 550 mV for 20 s prior to analysis was applied for
the preconcentration of proton in the organic phase. Potential increment=20 mV,
pulse potential=50 mV, pulse duration=50 ms. (b) A plot of negative peak current
values for differential pulse sensing measurements at various concentrations of
paraoxon. Dotted line shows a linear fit.
paraoxon reacting with OPH were observed in the presence of 1 mM of
each NaCl, CdCl , glucose, phenol as well as 1 mg/L of n-hexane and
2
humic acid. In each interfering agent experiments, the paraoxon
concentration ranging from 0.5 to 100 μM was studied. It is worth
+
while noting that Na ions known to transfer across the ITIES facilitated
by ETH 1778 [15] exhibits little effect on our proton sensing signal; this
is probably due to the fact that the use of pH 6.5 prohibits the efficient
+
transfer of Na ion across the ITIES facilitated by ETH 1778 [16]. This
great selectivity is one of the major advantages of using our proton
selective strip-sensors over other OPH based electrochemical (EC)
detection methods; for example, a EC detection method based on the
oxidation of hydrolytic productssuchasp-nitrophenol from the reaction
with paraoxon could be affected by various electro-oxidizable interfer-
ing species such as phenol, pentachlorophenol and others [17].
4. Conclusions
We demonstrate the first realization of utilizing ITIES to create an
amperometric strip-sensor based on facilitated proton transfer aiming
Hossain, Md. Mokarrom
