Liquid | Liquid Ion-Transfer Processes at the Dioctylphosphoric Acid

Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52 ... Robert W. French , Yohan Chan , Philip C. Bulman-Page , Frank Mar...
19 downloads 0 Views 283KB Size
Download PDF
Anal. Chem. 2004, 76, 5364-5369

Liquid | Liquid Ion-Transfer Processes at the Dioctylphosphoric Acid (N,N-didodecyl-N′,N′-diethylphenylenediamine) | Water (Electrolyte) Interface at Graphite and Mesoporous TiO2 Substrates Susan J. Stott, Katy J. McKenzie, Roger J. Mortimer, Colin M. Hayman, Benjamin R. Buckley, Philip C. Bulman Page, and Frank Marken*

Department of Chemistry, Loughborough University, Loughborough, Leicestershire, LE11 3TU, U.K. Galyna Shul and Marcin Opallo

Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland

Biphasic electrode systems are studied for the case of the oxidation of the water-insoluble liquid N,N-didodecylN′,N′-diethylphenylenediamine (DDPD) neat and dissolved in bis(2-ethylhexyl) phosphate (HDOP) and immersed in aqueous electrolyte media. The oxidation process in the absence of HDOP is accompanied by transfer of the anion (perchlorate or phosphate) from the water into the organic phase. However, in the presence of HDOP, oxidation is accompanied by proton exchange instead. This electrochemically driven proton exchange process occurs over a wide pH range. Organic microdroplet deposits of DDPD in HDOP at basal plane pyrolytic graphite electrodes are studied by voltammetric techniques and compared in their behavior to organic microphase deposits in mesoporous TiO2 thin films. The mesoporous TiO2 thin film acts as a host for the organic liquid and provides an alternative biphasic electrode system compared to the random microdroplet/graphite system. Two types of mesoporous TiO2 thin-film electrodes, (i) a 300-400-nm film on ITO and (ii) a 300-400-nm film on ITO sputter-coated with a 20-nm porous gold layer, are investigated. Biphasic electrochemical processes are of interest in a wide range of contexts including ion partitioning and sensing, biological membrane processes, electrosynthesis, and phase-transfer catalysis.1 The development of novel biphasic electrode systems is important, and only recently have the benefits of electrodes based on single droplets2,3 or arrays of microdroplets4 of water-insoluble liquids been reported. Microdroplet phases containing a femtoliter volume of redox-active material each can be deposited in the form * To whom correspondence should be addressed. Tel: 01509 22 2551. Fax: 01509 22 3925. E-mail: [email protected]. (1) See, for example: Girault, H. H. J.; Schiffrin, D. J. Electroanal. Chem. 1989, 15, 1. (2) Tasakorn, P.; Chen, J. Y.; Aoki, K. J. Electroanal. Chem. 2002, 533, 119. (3) Donten, M.; Stojek, Z.; Scholz, F. Electrochem. Commun. 2002, 4, 324.

5364 Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

of random arrays onto suitable electrode surfaces and polarized electrochemically.5 In contrast to conventional modified electrodes, these microdroplet array-modified electrodes provide a liquid | liquid-phase boundary and a reaction space separate from the aqueous phase. Initial studies on microdroplet redox systems based on tetraalkylphenylenediamines6 were reported in 1997, and since then, the electrochemical properties of a wider range of systems have been investigated.7 Microdroplet systems have been proposed for applications in electroanalysis,8 in electrocatalysis,9 and for photoelectrochemical processes in microenvironments.10 The detection of liquid | liquid ion-transfer processes at the triplephase boundary electrode | oil | water (electrolyte) has attracted attention for the determination of Gibbs free energies11 with applications in particular in pharmaceutical research.12,13 In search for novel electrode materials for microdroplet redox systems, the immobilization of electrochemically active liquids within hydrophobic silicate-carbon matrixes14-18 and in hydrophobic silicate (4) See, for example: Wadhawan, J. D.; Wain, A. J.; Kirkham, A. N.; Walton, D. J.; Wood, B.; France, R. R.; Bull, S. D.; Compton, R. G. J. Am. Chem. Soc. 2003, 125, 11418. (5) Wain, A. J.; Wadhawan, J. D.; France, R. R.; Compton, R. G. Phys. Chem. Chem. Phys. 2004, 6, 836. (6) Marken, F.; Webster, R. D.; Bull, S. D.; Davies, S. G. J. Electroanal. Chem. 1997, 437, 209. (7) Banks, C. E.; Davies, T. J.; Evans, R. G.; Hignett, G.; Wain, A. J.; Lawrence, N. S.; Wadhawan, J. D.; Marken, F.; Compton, R. G. Phys. Chem. Chem. Phys. 2003, 5, 4053. (8) Marken, F.; Blythe, A.; Compton, R. G.; Bull, S. D.; Davies, S. G. Chem. Commun. 1999, 1823. (9) Wadhawan, J. D.; Evans, R. G.; Banks, C. E.; Wilkins, S. J.; France, R. R.; Oldham, N. J.; Fairbanks, A. J.; Wood, B.; Walton, D. J.; Schro¨der, U.; Compton, R. G. J. Phys. Chem. B, 2002, 106, 9619. (10) Wadhawan, J. D.; Compton, R. G.; Marken, F.; Bull, S. D.; Davies, S. G. J. Solid State Electrochem. 2001, 5, 301. (11) Scholz, F.; Gulaboski, R.; Caban, K. Electrochem. Commun. 2003, 5, 929. (12) Bouchard, G.; Galland, A.; Carrupt, P. A.; Gulaboski, R.; Mirceski, V.; Scholz, F.; Girault, H. H. Phys. Chem. Chem. Phys. 2003, 5, 3748. (13) Gulaboski, R.; Scholz, F. J. Phys. Chem. B 2003, 107, 5650. (14) Opallo, M.; Saczek-Maj, M. Electrochem. Commun. 2001, 3, 306. (15) Saczek-Maj, M.; Opallo, M. Electroanalysis 2002, 14, 605. (16) Opallo, M.; Saczek-Maj, M. Chem. Commun. 2002, 448. 10.1021/ac049317y CCC: $27.50

© 2004 American Chemical Society Published on Web 07/29/2004

thin films19 has been reported. Porous host materials have been shown to be suitable substrates. In this report, the electrochemical behavior of microdroplet and thin-film deposits of N,N-didodecyl-N′,N′′-diethylbenzene-1,4diamine (DDPD) on basal plane pyrolytic graphite and on mesoporous TiO2 film, respectively, is investigated. It is shown that the oxidation and rereduction of DDPD are associated with the exchange of anions between aqueous and organic phases. The addition of the hydrophobic liquid dioctylphosphoric acid (HDOP) causes the oxidation process to become associated with proton exchange and is sensitive to proton activity over a wide pH range. As a liquid, HDOP can be employed directly as an acidic solvent and this simplifies the chemical system considerably. The selectivity for proton exchange is high over a wide pH range. Despite the high concentration of acid in the organic phase, the HDOP protons are believed to remain undissociated within the bulk phase and the strong interaction of the proton with the redox-active diamine DDPD is believed to be mainly responsible for the pHdependent voltammetric signal. HDOP has been used previously as a component in membrane systems20 and as metal cation extractant21 in organic-aqueous solvent mixtures. Finally, it is demonstrated that for both electrochemically driven anion and cation exchange processes a novel mesoporous titanium oxide-based electrode gives results comparable to those obtained with a random array of microdroplets on graphite. The use of a mesoporous TiO2 host material for biphasic systems is new and an important development toward well-defined biphasic electrode systems. Titanium dioxide has been chosen as a host material because (i) thin films are readily prepared via a layerby-layer dip-coating procedure,22 (ii) the oxide is contributing to some extent to charge transport due to its inherent electrical conductivity,23 and (iii) in the potential range studied here TiO2 may be regarded as an innocent electrochemically inactive host material. In future, mesoporous host materials, both innocent and noninnocent, could provide adsorption or redox-active sites in biphasic electrode systems to further broaden the potential for applications. EXPERIMENTAL SECTION Reagents. KCl, KOH, H3PO4, bis(2-ethylhexyl) phosphate, NaClO4, phytic acid dodecasodium salt, and anhydrous acetonitrile were obtained commercially and in analytical grade. Titania sol (anatase, ∼6-nm diameter, 30-35% in aqueous HNO3, pH 1-2) was obtained from Tayca Corp. (Osaka, Japan) and diluted 100fold with deionized water. Demineralized water was obtained from an Elgastat purification system (Elga, High Wycombe, Bucks, U.K.) with a resistivity not less than 18 MΩ cm. Instrumentation. Electrochemical experiments were conducted with a µ-Autolab system (Eco Chemie) in a three-electrode (17) Opallo, M.; Kukulka-Walkiewicz, J.; Saczek-Maj, M. J. Sol-Gel Sci. Technol. 2003, 26, 1045. (18) Saczek-Maj, M.; Opallo, M. Electroanalysis 2003, 15, 566. (19) Niedziolka, J.; Opallo, M. Electrochem. Commun. 2004, 6, 475. (20) Benavente, J.; Oleinikova, M.; Munoz, M.; Valiente, M. J. Electroanal. Chem. 1998, 451, 173. (21) See, for example: Golovanov, V. I. Zh. Neoorg. Khim. 1998, 43, 1575. (22) McKenzie, K. J.; Marken, F.; Oyama, M.; Gardner, C. E.; Macpherson, J. V. Electroanalysis 2004, 16, 89. (23) See, for example: Garcia-Belmonte, G.; Kytin, V.; Dittrich, T.; Bisquert, J. J. Appl. Phys. 2003, 94, 5261.

cell. The carbon working electrode was made from a 4.9-mmdiameter basal plane pyrolytic graphite disk (Le Carbon), and the reference and counter electrodes were a saturated calomel and a platinum foil, respectively. The mesoporous TiO2 working electrode was prepared from tin-doped indium oxide (ITO) coated glass (10 mm × 60 mm, resistivity 20 Ω per square) obtained from Image Optics Components Ltd. (Basildon, Essex, U.K.). The ITO electrode surface was modified with a porous titanium oxide film (see below) and gold coated in a Polaron sputter unit. An Elite tube furnace system was used for cleaning ITO electrode surfaces (at 500 °C in air) and for calcining TiO2 phytate films (at 500 °C in air). Prior to electrochemical experiments, solutions were deaerated with high-purity argon (BOC). SEM images were obtained with a Leo 1530 field emission gun scanning electron microscope (FEGSEM) system. Preparation of N,N-Didodecyl-N′,N′′-diethylbenzene-1,4diamine. DDPD was prepared following a literature method.25 A suspension of N,N-diethylbenzene-1,4-diamine (5.0 g, 30 mmol), 1-bromododecane (20.2 g, 81 mmol), and sodium bicarbonate (5.5 g, 66 mmol) in dimethylformamide (100 mL) was heated at reflux for 3 days. The mixture was cooled and filtered through a sinteredglass funnel and the filtrate concentrated in vacuo. Flash chromatography (silica, gradient elution of 1-3% ethyl acetate in petroleum ether) gave 9.9 g (66%) of the title compound as a light brown oil: 1H NMR (400 MHz, CDCl3) δ 0.86 (6 H, m, 2 N(CH2)11CH3), 1.10 (6 H, m, 2 NCH2CH3), 1.28 (36 H, m, 2 N(CH2)2(CH2)9CH3), 1.50 (4 H, m, 2 NCH2CH2(C10H21), 3.22 (8 H, br s, 2 NCH2(C11H23), 2 NCH2CH3), 6.73 (4 H, br s, ArH); 13C NMR (100 MHz, CDCl3) δ 14.1, 14.3, 22.6, 22.7, 27.3, 29.1, 29.2, 29.4, 32.0, 41.4, 114.0, 119.0, 120.2, 132.4; m/z (EI) 500 (M+, 100%); C34H64N2 requires 500.50695, found 500.50721; IR v/cm-1 (neat) 3044, 2921, 2852, 1611, 1515, 1467, 1371. Working Electrode Preparation. The majority of experimental work was carried out using a 4.9-mm-diameter basal plane pyrolytic graphite electrode. This was renewed by polishing the electrode surface on a fine (P1000) grade carborundum paper (see Figure 1A). For experiments using ITO glass electrodes, mesoporous films of TiO2 phytate were deposited following a layer-bylayer dip-coating method described previously.24 A clean ITO surface (washed with ethanol and water, dried, and 30-min heat treatment at 500 °C in air) is dipped into a solution of TiO2 nanoparticles followed by rinsing. By dipping the resulting nanoparticle deposit into a solution of phytic acid (40 mM in pH 3 aqueous solution) and rinsing, it is possible to reverse the surface charge. The dipping process was undertaken using a robotic Nima dip-coating carousel (DSG Carousel, Nima Technology, Coventry, U.K.) and repeated to give multilayer deposits of mesoporous TiO2 phytate.24 After the deposition of 15 layers, the TiO2 phytate film is dried and calcined (see Figure 1B) before a 20-nm porous gold layer is sputter coated on the surface (see Figure 1C). RESULTS AND DISCUSSION Oxidation of DDPD Deposited as Microdroplets onto Basal Plane Pyrolytic Graphite Electrodes. DDPD is deposited in the form of microdroplets onto the surface of basal plane (24) McKenzie, K. J.; Marken, F.; Hyde, M.; Compton, R. G. New J. Chem. 2002, 26, 625. (25) Marken, F.; Hayman, C. M.; Page, P. C. B. Electroanalysis 2002, 14, 172.

Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

5365

Figure 2. Cyclic voltammograms obtained for the oxidation and rereduction of microdroplets of 1.3 µg of DDPD (2.6 nmol) on (A) a 4.9-mm-diameter basal plane pyrolytic graphite electrode (i-iv: scan rates 10, 20, 50, and 100 mV s-1), (B) a 15-layer TiO2 film on ITO with 20-nm gold coating (i-iv: scan rates 10, 20, 50, and 100 mV s-1), and (C) a 15-layer TiO2 film on ITO (i-iii: scan rates 10, 20, and 50 mV s-1) immersed in aqueous 0.1 M NaClO4.

[DDPD+ClO4-](oil) + e- (1)

Voltammograms obtained for this process at a basal plane pyrolytic graphite electrode are shown in Figure 2A. A sharp and well-defined voltammetric response is detected with a midpoint potential, Emid ) 0.12 V versus SCE, which is consistent with earlier reports for the related tetraoctylphenylenediamine system.25 The process is chemically highly reversible, and the potential for the process, Emid, is affected by the concentration of perchlorate in the aqueous medium as expected for Nernstian behavior.26 Changing the scan rate causes an approximately linear increase in peak current (Figure 2A) consistent with the absence of diffusion control. For a deposit of 1.3 µg (2.6 nmol) of DDPD, a charge under the oxidation peak of ∼180 µC is detected, which suggests a conversion of ∼70% of the deposit. It is possible that this is due to some DDPD being trapped within the structure of the relatively rough graphite electrode. Oxidation of DDPD Deposited as a Liquid Film into Mesoporous TiO2. Next, to develop an alternative type of biphasic electrode system where an extended triple interface region is formed without the need for a random array of microdroplets,

The product formed after completion of the oxidation process may be regarded as a microdroplet deposit of an ionic liquid.6

(26) Schro ¨der, U.; Wadhawan, J. D.; Evans, R. G.; Compton, R. G.; Wood, B.; Walton, D. J.; France, R. R.; Marken, F.; Page, P. C. B.; Hayman, C. M. J. Phys. Chem. B 2002, 106, 8697.

Figure 1. FEGSEM images for (A) a basal plane pyrolytic graphite electrode, (B) a deposit of mesoporous TiO2 on an ITO substrate, and (C) a deposit of mesoporous TiO2 on an ITO substrate sputter coated with ∼20-nm gold. The highly porous nature of the gold film can be seen.

pyrolytic graphite electrodes by evaporation of an acetonitrile solution. The resulting microdroplet array deposit is highly water insoluble and when immersed in aqueous 0.1 M NaClO4 allows the oxidation of the phenylenediamine derivative coupled to ClO4transfer from the aqueous to the oil phase to be observed (eq 1).

process I:

DDPD(oil) + ClO4-(aq) f

5366 Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

Scheme 1

a novel mesoporous TiO2 thin film is employed. The film is composed of TiO2 nanoparticles of ∼6-nm diameter deposited as a film of 300-400-nm thickness onto ITO electrodes (see Experimental Section). Figure 1B shows an SEM image of a typical film deposit after scratching the surface of the film with a scalpel. DDPD deposited onto a clean ITO electrode and immersed in aqueous 0.1 M NaClO4 does not give well-defined voltammetric responses (not shown). In the presence of the mesoporous TiO2 film, the oxidation of DDPD (process I) and the rereduction are clearly detected. Figure 2C shows the oxidation of 1.3 µg of DDPD (2.6 nmol) deposited onto a 10 mm × 10 mm TiO2-coated ITO electrode. The charge under the voltammetric response, ∼50 µC, corresponds to a conversion of only ∼20%, which is low compared to the result observed at a graphite electrode. The increased peakto-peak separation is indicative of a slow kinetic step. However, the novel mesoporous TiO2-based electrode is clearly working and suitable for biphasic processes. It is likely that the efficiency of this type of electrode can be further improved by controlling the amount of deposit and by optimizing the TiO2 film thickness. The electrode process can be interpreted based on Scheme 1. Mesoporous TiO2 is known to conduct electricity to some extent,27,28 and therefore, an electron transfer between TiO2 and the liquid | liquid system followed by electron conduction is proposed. The increased peak-to-peak separation (see Figure 2C) suggests that part of the process (possibly electron conduction) is slow. Next, an electrode coated with a mesoporous TiO2 film and sputter-coated with a 20-nm gold film is investigated. Figure 1C (27) See, for example: Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2002, 106, 2967. (28) McKenzie, K. J.; Marken, F.; Opallo, M. Bioelectrochemistry, in press.

shows a typical SEM image of the electrode surface (after scratching with a scalpel). As reported earlier,28 the sputter-coated gold film is highly porous due to the underlying oxide film topography. The voltammetric response of a deposit of 1.3 µg (2.6 nmol) of DDPD on this type of electrode (see Figure 2B) is substantially improved when compared to the voltammetric response obtained on bare TiO2 (see Figure 2C). The conversion reaches 64%, and it is larger than that observed on only TiO2 filmmodified electrodes. This indicates that the “porotrode” configuration of the electrode is superior from the point of view of contact of redox liquid with the conductive substrate. The redox conversion can probably be significantly further improved by systematically varying the membrane thickness, hydrophobicity, or amount of deposited oil phase. Oxidation of DDPD within Microdroplets of HDOP Deposited as Microdroplets onto Basal Plane Pyrolytic Graphite Electrodes. It has been noted that phenylenediamine derivatives are readily protonated in the presence of hydrophobic anions.29 A transition from anion exchange (see process I) to proton exchange occurred as a function of anion hydrophobicity and pH. Here it is shown that the limiting case of a water-insoluble acid can be chosen to switch the anion transfer (process I) entirely to proton transfer with a working range spanning over a wide pH range. Combining the essentially water-insoluble liquid HDOP with the liquid DDPD and deposition of mixed microdroplets dramatically changes the voltammetric characteristics. HDOP may be regarded as a highly hydrophobic acid, and the interaction of this acid with DDPD leads to a novel liquid acid-base complex (see structure).

Deposition of microdroplets of this acid-base complex onto a basal plane pyrolytic graphite electrode surface and immersion in aqueous 0.1 M NaClO4 results in a new voltammetric oxidation process at a more positive potential, Emid ) 0.34 V versus SCE (see Figure 3B). Under these conditions process I is not observed despite occurring at a more negative electrode potential. From the voltammetric characteristics (vide infra), the new process can be identified as a one-electron oxidation of the HDOP-DDPD complex accompanied by proton expulsion (eq 2) into the

process II:

[HDOP-DDPD](oil) f [DPO--DDPD+](oil) + H+(aq) + e- (2)

unbuffered aqueous solution (aqueous 0.1 M NaClO4). For a 1:1 acid-to-base ratio in the deposit, the voltammetric response associated with process I is diminished and process II dominates. From the more positive peak potential observed for (29) Schro¨der, U.; Compton, R. G.; Marken, F.; Bull, S. D.; Davies, S. G.; Gilmour, S. J. Phys. Chem. B 2001, 105, 1344.

Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

5367

Figure 3. Cyclic voltammograms (scan rate 100 mV s-1) obtained for the oxidation and rereduction of microdroplets of (A) 1.3 µg of DDPD (2.6 nmol) and (B) 1.3 µg of DDPD together with 43 µg of HDOP on a 4.9-mm-diameter basal plane pyrolytic graphite electrode and immersed in aqueous 0.1 M NaClO4.

process II compared to that for process I, the absence of the free DDPD and the presence of the new acid-base complex can be inferred. However, during repeated potential cycling, depletion of HDOP and a reemerging signal for process I are observed, presumably due to loss or slow hydrolysis of the organic phosphate at the liquid | liquid phase boundary. Experiments with a higher acid-to-base ratio have been conducted. Increasing the amount of HDOP does not significantly change the electrochemical process, although it does systematically shift the voltammetric response to more positive potentials (vide infra). To record voltammograms in buffered solution systems, experiments were conducted next in aqueous phosphate buffer. Figure 4 shows typical voltammetric responses for the oxidation of microdroplets containing HDOP and DDPD deposited onto a basal plane pyrolytic graphite electrode and immersed in aqueous 0.1 M phosphate buffer solution. In voltammograms obtained in the absence of bis(2-ethylhexyl) phosphate, shown in Figure 4A(i), the phosphate electroinsertion reaction (process III) is detected at Emid ) 0.35 V versus SCE. This process is more complex and here tentatively (and in agreement with the literature30) assigned to a process associated with simultaneous potassium coinsertion (eq 3).

process III:

DDPD(oil) + KHPO4-(aq) f [DDPD+KHPO4-](oil) + e- (3)

In the presence of HDOP, the voltammetric response changes dramatically. Figure 4A(ii-iv) shows voltammograms obtained for HDOP to DDPD ratios of 2:1, 4:1, and 16:1. The voltammetric response for process II emerges, and a slight shift of the midpoint potential to more negative values upon increasing the HDOP concentration is again observed. The trend is consistent with that predicted based on the appropriate Nernst equation derived for process II (eq 4). In this equation, R denotes the gas constant, T

E ) E0′ +

+ RT [DPO DDPD ](oil) RT ln - 2.303 pH F F [HDOPDDPD](oil)

(4)

the absolute temperature, and F the Faraday constant, and the (30) Marken, F.; Hayman, C. M.; Page, P. C. B. Electrochem. Commun. 2002, 4, 462.

5368 Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

Figure 4. Cyclic voltammograms (scan rate 10 mV s-1) obtained for (A) the oxidation and rereduction of microdroplets of 0.5 µg of DDPD (1 nmol) with (i) 0, (ii) 1, (iii) 2, and (iv) 8 µg of HDOP deposited onto a 4.9-mm-diameter basal plane pyrolytic graphite electrode immersed in aqueous 0.1 M phosphate buffer (pH 7) and (B) the oxidation and rereduction of microdroplets of 0.5 µg of DDPD with 8 µg of HDOP deposited onto a 4.9-mm-diameter basal plane pyrolytic graphite electrode immersed in aqueous 0.1 M phosphate buffer at various proton activities. The inset shows a plot of midpoint potential versus pH.

reversible half-cell potential is related to activities in both the aqueous and the organic phases. Perhaps surprisingly, the charge under the voltammetric oxidation response increases with addition of more HDOP but decreases if the amount of HDOP is further increased. Integration of the current under the oxidation peak in Figure 4A(iv) gives a total charge of ∼30 µC. This result is consistent with a one-electron electrolysis of ∼30% of the 1 nmol of DDPD present at the electrode surface (process II). From the voltammetric characteristics, it can be concluded that the addition of HDOP improves mass transport via diffusion in the organic oil phase. Addition of further HDOP increases the droplet size and decreases the concentration of DDPD and therefore leads to reduced currents and conversion. Next, the effect of proton activity on the voltammetric response was investigated. Figure 4B shows voltammograms obtained for the oxidation of microdroplets containing a 1:16 mixture of DDPD to HDOP deposited onto a basal plane pyrolytic graphite electrode and immersed in aqueous 0.1 M phosphate buffer media. The midpoint potential for the voltammogram is systematically shifted to more positive potentials upon increasing the proton activity (see inset in Figure 4). Changing the pH of the phosphate buffer solution causes a well-defined but sub-Nernstian shift in midpoint potential of 41 ( 6 mV per pH unit. This deviation from the value predicted by the Nernst equation, 59 mV (see eq 4), can be tentatively attributed to a weak coupling of the proton activities in the aqueous and oil phases, e.g., by a pH-dependent surface potential of the reduced form of the microdroplets. Oxidation of DDPD within a Liquid Film of HDOP Deposited into Mesoporous TiO2. Finally, voltammetric responses are recorded for the oxidation of 1.5 µg (3.0 nmol) of DDPD in HDOP (1:38) deposited onto a ITO electrode with a

Figure 5. Cyclic voltammograms (scan rate 100 mV s-1) obtained for the oxidation and rereduction of microdroplets of 1.5 µg of DDPD (3.0 nmol) together with 114 µg of HDOP deposited onto an electrode with 15 layers of TiO2 on ITO and gold sputter-coated (∼20 nm) immersed in aqueous 0.1 M phosphate buffer (pH 7). The inset shows a plot of midpoint potential versus pH.

300-400-nm mesoporous TiO2 film and sputter-coated with 20nm gold. Figure 5 shows typical voltammetric responses obtained at pH 7 similar to those obtained at basal plane pyrolytic graphite electrodes. The voltammetric response is stable, and the charge under the voltammetric peak is consistent with ∼20% conversion. The capacitive background current in this experiment is increased due to the higher surface area of the porous gold, and a Faradaic background response is observed commencing at 0.5 V versus SCE due to the oxidation of gold. However, the pH-dependent signal for the oxidation of DDPD accompanied by proton exchange (process II) is clearly observed. A shift of this signal of ∼53 ( 6 mV per pH unit is observed. CONCLUSIONS The ability of the microdroplet redox systems to respond to proton activities and the activities of anions based on a mechanism

of liquid | liquid ion transfer has been demonstrated for two different types of electrodes: a basal plane pyrolytic graphite electrode and a novel electrode based on a thin film of mesoporous TiO2. The main achievements in this work are (i) creating a DDPD/HDOP proton-selective biphasic electrode system and (ii) for the DDPD model system, to demonstrate the use of mesoporous titania host films. Biphasic electrodes are believed to be insensitive to surface blocking, e.g., in colloidal or proteincontaining media, and may therefore show potential for future applications. The ability of mesoporous TiO2 to conduct electrons in the presence of aqueous media28 and the in comparison to carbon materials’ low capacitive background current could further contribute to the overall beneficial characteristics. However, results presented here are preliminary in nature and mechanistic details remain to be studied. More experimental work with films of variable thickness and different loadings will be required to explain and optimize the processes in the mesoporous TiO2. Surface tension effects, which can be affected by surface-active modifiers, are believed to be important and will be considered for the development of better electrodes for biphasic redox systems. ACKNOWLEDGMENT F.M. thanks the Royal Society for the award of a University Research Fellowship. K.J.M. and S.J.S. thank the RSC and the EPSRC for a DTA and an Analytical Science Studentship. We are grateful to the British Council and the Committee for Scientific Research (Project WAR/341/248) for financial support of a collaborative project between the institutes in Poland and in the U.K. Tayca Corp. is gratefully acknowledged for donating titania sol.

Received for review May 9, 2004. Accepted June 23, 2004. AC049317Y

Analytical Chemistry, Vol. 76, No. 18, September 15, 2004

5369