2 CHAPTER Development of an electrode-membrane-electrode system for selective faradaic response towards charged redox species 2.1 INTRODUCTION Development of highly sensitive and selective sensors is urgently needed in many applications including environmental studies, health and biomedical fields, to respond to recent global environmental and impending health related issues [1]. Electrochemical sensors, in particular, are highly promising due to their simple designs, low costs and possibility for miniaturization and portability [2]. However, it should be noted that the electrochemical sensors are either highly specific such as immunosensors [3] or provide indiscriminate sensing response towards a wide range of analyte species such as flow-injection sensor [4]. Development of an electrochemical sensor capable of specific identification of wide range of compounds remains a challenge. Several methods of modifying electrodes with catalytic properties to give 38 resolved peaks of analytes with thermodynamically similar redox properties, by either changing their kinetic or thermodynamic behaviours via interaction with the modifier layer, were reported [5]. Other methods include using charged polyelectrolytes to change the surface properties of electrochemical sensors, hence help differentiate between differently charged species by varying their transport across the charged modifying films [6]. It was reported that surface charges along nanochannel walls facilitated differential transport of proteins [7-9], amino acids and charged redox species [10] across membrane structures. Additionally, several recent works by White and co-workers demonstrated the use of electrostatic and photochemical controls for specific transport of charged species [11, 12]. Yamaguchi et al. found that the diffusivities of ferrocenes inside silica-surfactant nanochannels formed within a porous alumina membrane template [13] were influenced by electrostatic interaction between the charged species and the ionic charges along the nanochannel walls. Early works by Martin et al. and Stroeve et al. have reported the use of transmembrane potentials to produce the driving force for electrophoretic separation of proteins across gold-plated nanotube membranes, using an externally applied electrical field placed some distance 39 away from the membranes [7, 14]. Herein, anodic nanoporous alumina membrane (Whatman) was employed as the separation membrane. Our approach involves constructing an electrode-membrane-electrode system by sputter coating a 60µm thick alumina membrane with metal on both sides. The electrical field is applied directly across the channels within the membrane. This has the advantage of achieving high field strengths of ca. 30 kVm−1 but with very low applied potentials of ca. 2V between the platinum-coated layers [15]. The platinum coatings function as working electrodes by connection to a bipotentiostat and reference/auxiliary electrodes in the cell solution. The membrane electrodes system has been used to study electrophoretic mobility of gold nanoparticles with different sizes [16] transport of proteins and selective separation of proteins of different charges [15]. In this work, the mobilities of charged species moving within the nanochannels of the alumina membrane electrodes system were measured. In addition, simultaneous electrochemical sensing towards three ferrocene species, ferrocenemethannol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl)ferrocene (FcN), can be readily achieved under the same conditions which control the 40 species mobilities when the electrical potential field applied across the membrane was varied. 2.1.1 Ferrocene and derivatives Ferrocene is an organometallic sandwich compound consisting of a Fe (II) center and two symmetrically bound cyclopentadienide ligands. Ferrocene can be oxidized to a ferrocenium cation Fe(III) under appropriate oxidation potential. Because of facile electron transfer of ferrocene during electrochemical reactions, ferrocene and its substituted derivatives are commonly used as redox probes and mediators. Furthermore, by replacing the hydrogen atoms with electron-donating or electron-accepting groups, the reversible potential can be shifted to more negative or positive potential, respectively. One such ferrocene derivative is ferrocenemethanol which is soluble in aqueous solutions. At neutral solution pH 7, FcMeOH is uncharged. In contrast, another water-soluble derivative, the ferrocenecarboxylic acid (with pKa = 4.2) loses proton and assumes negative charge at pH 7. The third ferrocene derivative used in this work is (dimethylaminomethyl)ferrocene (pKa=9.84 )which has a positive charge at pH 7. Structures of the three ferrocene derivatives used in this work are shown in Table 2.1. 41 Table 2.1 Structures of (A)FcMeOH, (B) FcCOOH, (C)FcN and their charge in phosphate pH buffer. O NH Ferrocences HO Fe Charge 2.1.2 O Fe Neutral Negative Fe Positive Collection experiment Collection experiment is a commonly used technique for rotating ring-disk electrode. In collection experiment, the disk generated species can be observed at the ring electrode surrounding the centrally placed disk electrode. In general, collection experiment is carried out by keeping the disk electrode potential at ED such that a redox reaction takes place O + ne- → R which produces the cathodic current iD. At the same time, the ring electrode is held at sufficiently positive potential so that any R produced at the disk electrode reaches the ring will be oxidized immediately R → O + ne- and produces the anodic current iR. Ideally, the concentration of R at the ring electrode is kept at zero so that quantitative information concerning the amount of disk-generated product collected at the ring can be derived. The collection efficiency, N, can 42 be obtained from the ratio of ring current to the disk current: N= −iR iD Fig. 2.1A shows the ideal ring and disk currents obtained from a redox couple during a collection experiment in which the disk electrode potential ED, is swept cathodically, while keeping the ring potential ER sufficiently more positive than the formal potential at diffusion limiting condition. Fig. 2.1 (A)Voltammogram showing iD vs. ED and iR vs. ED with ER = E1 during a typical collection experiment (B) iR vs. ER, iD=0 (ED=E1) and iR vs. ER (ED=E2) 43 2.1.3 Shielding experiment The shielding experiment is also used for the rotating ring-disk electrode. While collection experiment sweep the disk electrode potential and keep ring potential constant, in the shielding experiment, the ring potential is scanned while disk potential is held at open circuit potential and other constant potentials. In this way, one can measures the ring current as function of ring potential and study how much of this ring current is shielded when the disk potential is held constant at different values (Fig. 2.1B). In detail, at first instance, the current at the ring electrode ( iR0 ) is generated by sweeping the ring potential to reduce O to R while the disk is kept at open-circuit condition. During subsequent sweep of ring potentials, the disk potential is held at other constant potentials such that the flux of O to the ring will be reduced (iR). The amount of shielding of the ring current can be correlated to NiD, the flux of the stable product R to the ring during the collection experiment (Fig. 2.1B): iR = iR0 − NiD Some dual electrode systems that achieve steady-state conditions can similarly show the shielding and collection effects, such as in microelectrodes, microarrays and scanning electrochemical microscopy (SECM). In our 44 electrode-membrane-electrode system, the collection and shielding experiments were employed to study the transport of different redox charged species from one of the membrane-coated porous Pt electrode layers to the other, as the redox species traverse through the membrane nanochannels. The significance of these experiments is to demonstrate the possibility of carrying out simultaneous selective resolution and quantitation of differently charged analytes using the electrode-membrane-electrode system. 2.2 2.2.1 EXPERIMENTAL Materials and Reagents Ferrocenemethanol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl)ferrocene (FcN) were obtained from Sigma-Aldrich. All ferrocenes were prepared in 0.1 M phosphate pH buffer. 25 mm diameter nanoporous alumina membranes (Anopore) were obtained from Whatman (Maidstone, Kent, UK). The membrane had a thickness of 60 µm and nominal pore size of 100 nm with a porosity of 25 to 50%. All membranes were washed and pre-treated with 35% hydrogen peroxide (Scharlau) and subsequently sputtered with platinum (99.99% purity). 45 2.2.2 Instruments Bipotentiostat CE WE2    WE1    RE Nanochannels within alumina membrane Receiver Feed Membrane electrodes Membrane electrode at receiver face Fig. 2.2 Schematic of membrane electrodes cell and detail of membrane electrode at receiver face. The reference and auxiliary electrodes were placed within the feed solution (for collection experiments) or receiver solution (for shielding experiments). The membrane electrodes were connected to working potentials and of the bipotentiostat. Solutions were stirred throughout the experiment. All transport experiments were performed using a membrane cell with two compartments (feed and receiver) (Fig. 2.2). The metal coated membrane was clamped between the two half cells using silicon O-rings as sealants. A bipotentiostat (CHI 900) with four electrode system was employed for all experiments. Platinum coated feed face and receiver face of the alumina membrane were used as two working electrodes. Potential was applied to the 46 membrane through two aluminum tape attached to the membrane electrodes. Platinum wire and Ag/AgCl/KCl (saturated) were used as auxiliary and reference electrodes respectively. 2.2.3 Experimental procedure The metal-coated membrane was left in contact with the solutions for ca. before the start of experiment. All experiments were carried out at room temperature. For cyclic voltammetry experiments, the ferrocene solutions contained 2.5 mM ferrocene species in 0.1 M phosphate pH 7.0 buffer and only one of the membrane electrodes is monitored to derive the current-voltage curves. 2.2.3.1 Collection Experiments Collection experiments were carried out in the membrane cell where the feed compartment contained 2.5 mM ferrocenes in 0.1 M phosphate pH 7.0 buffer and the receiver compartment contained the same buffer solution. Both reference and auxiliary electrodes were placed in the feed solution. The potential at the feed electrode, Ef, was swept at slow scan rate of 10 mV s-1 in the range of 0.3 to 1.0 V versus Ag/AgCl/KCl(sat). The receiver electrode was 47 microscopy studies, the electrode surface morphology closely resembled the regularly spaced porous alumina structure, but with pore sizes reduced by ca. 40% [15]. The sigmoidal shape of the voltammograms is attributed to the rapid mass transfer of the redox species to electrode surface with regular arrays of sub-micrometer dimensions [17, 18]. At the potentials where the currents reached limiting values, the faradaic currents varied linearly with concentrations of ferrocenes (Fig. 2.3B), as expected for mass transfer limited behaviours. The slope of the linear curve presents diffusion coefficient of the ferrocene. Diffusion coefficients of FcMeOH and FcCOOH are similar and larger than that of FcN. It may be due to the bigger size of FcN than that of two other ferrocenes. The background capacitance and water reaction could occur at the large surface electrode system and cause the current when the concentrations of three ferrocenes are zero. The formal potentials of ferrocenes obtained at the membrane electrodes system differed slightly from those obtained at a disk platinum electrode due to the large surface area of membrane electrodes system (1.13 cm2), giving larger iRsoln drop in the bulk solution. Compensation of iRsoln drop during or after cyclic voltammetry experiments gave the expected formal potentials of all three ferrocenes. 49 A 1.5 Current (mA) 0.5 FcMeOH -0.5 FcCOOH -1 FcN -1.5 -0.2 0.2 0.4 E vs Ag/AgCl (V) 0.6 0.8 B 0.7 0.6 Current (mA) 0.5 0.4 0.3 R = 0.9552 0.2 FcCOOH FcN FcMeOH R = 0.9764 R = 0.9766 0.1 0.0 0.0 0.1 0.1 0.2 0.2 0.3 Concentration (mM) Fig. 2.3 (A) Cyclic voltamograms of 2.5 mM ferrocene species at scan rate of 100 mV s-1 at the membrane electrode. (B) Linear response of the membrane electrode derived from the limiting current values for ferrocenemethannol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl) ferrocene (FcN). Conditions: 0.1 M phosphate buffer (pH 7.0), T= 298K. 2.3.2 Collection experiments In voltammetric techniques, migrations of redox species are generally minimized by addition of electrolyte salts in order to simplify the limiting 50 mass transfer process to diffusion, besides several other obvious advantages. Under this situation, a significant proportion of the potential drop is confined to the double layer at the working electrode, at which Faradaic processes occur. Large potential drop in the diffuse region between the working electrode and reference electrode may occurs when the working and reference electrodes are placed some distances apart or in solutions of low conductivities. In addition, it was reported that the diffusitivities of metal ions and neutral species through the nanosized channels within the nanoporous alumina membrane are significantly lower by to orders of magnitude, compared to those of bulk solutions [13]. These have been ascribed to attractive interactions between the diffusing molecules and the channel walls which slow down mass transport within the nanosized channels. Therefore, it is expected that the electrolyte solution within the nanoporous alumina membrane gives lower conductivity compared to bulk solution, since movement of ions is lower within the nanosized channels. In this work, we placed the nanoporous membrane between the working and reference electrodes to introduce an uncompensated resistance which gave rise to a potential drop across the membrane during electrochemical reactions. We are interested to use this uncompensated 51 potential drop to influence the transport of charged species within the nanochannels. This situation is readily achieved by placing the reference electrode within one compartment of the membrane cell while the membrane electrode facing the second compartment is connected to the working potential of a potentiostat. To evaluate the magnitude of the resistance within the membrane, cyclic voltammetries of ferrocenemethanol at a membrane electrode were carried out in 0.1 M pH 7.0 buffer solution with the reference electrode placed in the feed or receiver solution of the membrane cell. Comparison of the voltammograms using digital compensation of iRu drop gives the uncompensated resistance within the nanoporous membrane, Ru,m. Average Ru,m derived from several membrane electrodes is ca. 110 Ω . This gives a solution conductivity of 5×10-5 S cm-1 (membrane thickness = 60 μm; area = 1.13 cm2) which is ca. 2-3 orders of magnitude lower than the conductivity of a 0.1 M phosphate buffer solution. Collection experiments were carried out in the membrane cell for the three differently charged ferrocenes (Fig. 2.4A). The ferrocene in the bulk solution of the feed compartment was first oxidized at the feed electrode, moved across the membrane and was reduced at the receiver electrode held at 52 reducing potential of V. The collection efficiency N measures the fraction of oxidized ferrocene species arriving at the receiver electrode and is calculated using the ratio of ir/if (Fig. 2.4B). The current detected at the receiver electrode, ir, arises from the reduction of oxidized ferrocene species which have moved through the nanosized channels under the influence of electrical potential gradient ( ∂φ ). The current at the receiver electrode can be described by the ∂x following Nernst-Planck migration-diffusion equation [19]: ir = z FAuC ∂φ ∂c + nFAD ∂x ∂x Eqn. 2.1 where ir is the receiver current, u is ion mobility within the nanochannel, z is charge of the charged redox species, C is concentration of the oxidized species, D is diffusion coefficient of the species within the nanochannels, ∂c is the ∂x concentration gradient at the receiver electrode and n, F, A have the usual meanings. 53 1.0 FcMeOH A FcCOOH 0.8 FcN Current (mA) 0.6 if Best fit data 0.4 0.2 0.0 ir -0.2 -0.4 -0.6 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 E f vs Ag/AgCl (V) Collection Efficiency, N 0.7 0.6 +FcCOO- B +FcMeOH +FcN+ 0.5 0.4 0.3 0.2 0.1 0.5 0.6 E f vs Ag/AgCl (V) 0.7 Fig. 2.4 (A) Collection experiments carried out with ferrocenemethannol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl) -ferrocene (FcN). Potential at feed electrode, Ef, is swept at a slow scan rate of 10 mV s-1 while potential at receiver electrode, Er is held constant at the reducing potential of V. Reference and auxiliary electrodes are placed in the feed solutions. Experimental data of receiver currents ir obtained at Ef = 0.5-1.0 V are fitted to Eqn. 2.2 (points). For neutral species +FcCOO-, the second diffusion term in Eqn. 2.2 applies and l is set to lm. See text for best fitted parameter values of u, D and l. (B) Collection efficiency derived from (A), is calculated from the ratio of receiver current to feed current, ir/if. 54 For a membrane placed between the working and reference electrodes, the electrical potential difference (ΔE) across the membrane which influences the migration rate is derived from the uncompensated resistance (Ru,m) within the membrane which could be assumed to drop linearly across the membrane thickness (lm). ΔE relates to the measured current ir as ΔE = irRu,m. In contrast, the concentration gradient is not linear across the membrane and is more appropriately represented by C where l varies, depending on the influence l of both migration and diffusion mass transport. Thus, the current at the receiver electrode is simplified to: ir = z FAuC ΔE C + nFAD lm l Eqn. 2.2 During collection experiments, the ferrocene species were oxidized at the feed electrode to +FcMeOH, +FcCOO- and +FcN+ with +1, and +2 overall charges, respectively. The feed and receiver solutions were constantly stirred during the experiments to maintain uniform concentrations in the area close to the membrane electrode surfaces. Fig. 2.4A shows the non-linear curve fit data of the experimental results derived from the collection experiments at potentials 0.5-1.0 V and theoretical values calculated from Eqn. 2.2 for the 55 oxidized ferrocenes. A factor of if,fc species if,+FcCOO - is added to Eqn. 2.2 to offset the receiver current with respect to the feed current of the neutral species + FcCOO-, since different ferrocene species gave different responses at the membrane electrodes system (see the various if in Fig. 2.4A). The comparison of Eqn.2.2 with the experimental ir was carried out at the higher feed potentials, since these yielded limiting feed currents where concentrations of the oxidized species generated at the feed electrode was similar to bulk concentrations of the reduced species. The fitted results (points in Fig. 2.4A) gives ion mobility values of (6.3 + 0.4)×10-5 cm2 s-1 V-1 and (2.9 + 0.2)×10-5 cm2 s-1 V-1 for single charged + FcMeOH and double charged + FcN+, respectively; and diffusion coefficient value of (1.7 + 0.1)×10-7 cm2 s-1 for neutral +FcCOO-. The corresponding diffusion coefficients of +FcMeOH and FcN+ derived from the relation u = z FD RT [19] are (1.6 + 0.1)×10-6 and (3.8 + 0.3)×10-7 cm2 s-1 respectively. These diffusion coefficient values are to 15 times lower than reported values of the reduced forms [13]. The best fitted l values for +FcMeOH and +FcN+ are 60 + µm and 20 + µm, respectively, which indicate a thinner diffusion layer for the doubly charged +FcN+, due to faster mass transport under the influence of the favourable electrical field 56 gradient. There are two interesting results derived from the collection experiments. First, the feed currents of the unoxidized ferrocenes FcN+ and FcMeOH show increasing currents at potentials between 0.6 V and 1.0 V (Fig. 2.4A). In contrast, steady-state voltammetry carried out at the same membrane electrode system in the absence of a reducing potential at the receiver electrode gives fairly constant currents in the same potential range (Fig. 2.3A). The difference in current response under the two conditions is attributed to a regeneration of reduced species at the receiver electrode when a reduction potential is applied at the receiver electrode. This adds to the flux of reduced ferrocene species at the feed electrode, giving rise to feedback currents. Thus, a factor of if,fc species if,+FcCOO - is added to Eqn. 2.2 in order to offset this effect of feedback currents at both the feed and receiver electrodes. Second, the significantly different collection efficiencies obtained for different ferrocenes indicate the membrane electrodes system is able to discriminate the positively charged +FcN+ from the neutral + FcCOO- by a large selectivity factor of ca. 16 times. Herein, the selectivity factor is referred to as the ratio of collection efficiencies of charged species 57 (+FcN+ or +FcMeOH) with respect to neutral species (+FcCOO-). Our previous work using the alumina membrane system has shown that nanoparticles of different size-to-charge ratio could be differentiated based on electrophoretic transport rates under the influence of variable electrical potential gradient applied directly across the membrane electrodes system [15, 20]. We further carried out the following shielding experiment to measure the collection efficiencies of the ferrocene species under the condition where the receiver electrode was held at higher potentials relative to the feed electrode. 2.3.3 Shielding experiments Fig. 2.5 shows the results of the shielding experiments for the three ferrocene species. When the feed electrode was held at 0.7 V, the receiver currents were lower compared to those obtained at open circuit potential of the feed electrode. This is due to the arrival of some oxidized ferrocenes which could not be oxidized further at the receiver electrode. Thus the difference in the two currents is a measure of the fraction of oxidized ferrocenes collected at the receiver electrode, Nif,lim, where if,lim refers to the limiting oxidation current at the feed electrode held at 0.7 V. Collection efficiency is calculated from the 58 ratio of this current difference and the limiting current at the feed electrode held at 0.7 V (Nif,lim/if,lim). Fig. 2.6 shows the collection efficiencies for the three oxidized ferrocene species, derived from the shielding experiments. Unlike the collection experiments where the receiver electrode was held at more negative potentials relative to the feed electrode, in the shielding experiments, the reverse potential gradient condition was applied. The collection efficiencies of oxidized ferrocene species remained somewhat unchanged at potentials less than 0.6 V. At the high potentials of 0.7-1.0 V, the collection efficiencies of +FcN+ and +FcMeOH decreased with potential due to the influence of an unfavourable electrical potential field. In contrast, the collection efficiency of +FcCOO- increased from 0.6 V to 0.8 V and remained relatively constant from 0.9 to 1.0 V. This is likely due to some favourable electrostatic interactions between the COO- group of the bipolar +FcCOOmolecule and the electrical potential field. Overall, the membrane electrodes system achieved to times higher selectivity for +FcCOO- compared to + FcN+ and +FcMeOH when the receiver electrode was held at positive potentials relative to the feed electrode. 59 0.25 mA Current FcMeOH FcCOO- FcN+ 0.2 0.4 0.6 0.8 E r vs Ag/AgCl (V) Fig. 2.5 Shielding experiments for the three differently charged ferrocene species. Potential at the receiver electrode, Er, is swept at slow scan rate of 10 mV s-1 while potential at the feed electrode, Ef , is held at open circuit, o.c. (⎯) and at + 0.7 V (_ _ _) which gives limiting feed current. Reference and auxiliary electrodes are placed in the receiver solutions. 0.4 Collection Efficiency, N +FcCOO+FcMeOH +FcN+ 0.3 0.2 0.1 0.0 0.6 0.7 0.8 E r vs Ag/AgCl(V) 0.9 1.0 Fig. 2.6 Collection efficiency derived from shielding experiments. Collection efficiency is calculated from the ratio of {ir(o.c.)-ir(0.7V)}/if(0.7V). 60 2.4 CONCLUSIONS A method to measure the mobility of charged redox species moving through nanochannels within a membrane electrodes system is described. The method employed the uncompensated resistance within the membrane due to slow mass transfer of ions, in order to introduce a significant electrical potential drop across the membrane. Collection and shielding experiments carried out within a two-compartment membrane cell showed selective mass transport of the ferrocene species moving across the membrane, based on species charges. 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Toh, Bioinspiration & Biomimetics, 2008. 3(3): p. 035008-035013. 64 [...]... 19 (21 ): p 22 34 -22 42 6 Chiun-Jye Yuan, C.-L.H., Shih-Chang Wang, Ku-Shang Chang,, Electroanalysis, 20 05 17 (24 ): p 22 39 -22 45 7 Chun, K.Y., S Mafe, P Ramirez, and P Stroeve, Chemical Physics Letters, 20 06 418(4-6): p 561-564 8 Ku, J.R and P Stroeve, Langmuir, 20 04 20 (5): p 20 30 -20 32 9 Ku, J.R., S.M Lai, N Ileri, P Ramirez, S Mafe, and P Stroeve, Journal of Physical Chemistry C, 20 07 111(7): p 29 65 -29 73... solution of the membrane cell Comparison of the 2 voltammograms using digital compensation of iRu drop gives the uncompensated resistance within the nanoporous membrane, Ru,m Average Ru,m derived from several membrane electrodes is ca 110 Ω This gives a solution conductivity of 5×10-5 S cm-1 (membrane thickness = 60 μm; area = 1.13 cm2) which is ca 2- 3 orders of magnitude lower than the conductivity of. .. Nature, 20 08 4 52( 7185): p 301-310 2 Halliwell, C.M., E Simon, C.-S Toh, P.N Bartlett, and A.E.G Cass, Biosensors and Bioelectronics, 20 02 17(11- 12) : p 965-9 72 3 Koh, G., S Agarwal, P.-S Cheow, and C.-S Toh, Electrochimica Acta, 20 07 53 (2) : p 803-810 4 Hsieh, B.C., H.Y Hsiao, T.J Cheng, and R.L.C Chen, Analytica Chimica Acta, 20 08 623 (2) : p 157-1 62 5 Saeed Shahrokhian, Hamid R.Z.-M., Electroanalysis, 20 07... area of membrane electrodes system (1.13 cm2), giving larger iRsoln drop in the bulk solution Compensation of iRsoln drop during or after cyclic voltammetry experiments gave the expected formal potentials of all three ferrocenes 49 A 1.5 Current (mA) 1 0.5 0 FcMeOH -0.5 FcCOOH -1 FcN -1.5 -0 .2 0 0 .2 0.4 E vs Ag/AgCl (V) 0.6 0.8 B 0.7 0.6 Current (mA) 0.5 0.4 0.3 2 R = 0.95 52 FcCOOH 2 FcN 2 0 .2 FcMeOH... 0.1 0 .2 0 .2 0.3 Concentration (mM) Fig 2. 3 (A) Cyclic voltamograms of 2. 5 mM ferrocene species at scan rate of 100 mV s-1 at the membrane electrode (B) Linear response of the membrane electrode derived from the limiting current values for ferrocenemethannol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl) ferrocene (FcN) Conditions: 0.1 M phosphate buffer (pH 7.0), T= 29 8K 2. 3 .2 Collection... Suzuki, K Morita, and N Teramae, Journal of Physical Chemistry B, 20 08 1 12( 7): p 20 24 -20 30 14 Yu, S., S.B Lee, and C.R Martin, Analytical Chemistry, 20 03 75(6): p 123 9- 124 4 15 Cheow, P.S., E Zhi, C Ting, M.Q Tan, and C.S Toh, Electrochimica Acta, 20 08 53(14): p 4669-4673 16 Yuan, H., P.S Cheow, J Ong, and C.S Toh, Sensors and Actuators: B Chemical, 20 08 134(1): p 127 -1 32 17 Yagi, I., T Ishida, and K Uosaki,... similar to bulk concentrations of the reduced species The fitted results (points in Fig 2. 4A) gives ion mobility values of (6.3 + 0.4)×10-5 cm2 s-1 V-1 and (2. 9 + 0 .2) ×10-5 cm2 s-1 V-1 for single charged + FcMeOH and double charged + FcN+, respectively; and diffusion coefficient value of (1.7 + 0.1)×10-7 cm2 s-1 for neutral +FcCOO- The corresponding diffusion coefficients of +FcMeOH and FcN+ derived from... and K Uosaki, Electrochemistry Communications, 20 04 6(8): p 773-779 18 Dickinson, E.J.F., I Streeter, and R.G Compton, Journal of Physical 63 Chemistry B, 20 08 1 12( 13): p 4059-4066 19 Prashar J, S.P., Scarffe M and Cornell B, Journal of Materials Research, 20 07 22 (8): p 21 89 -21 94 20 Nguyen, B.T.T., E.Z.C Ting, and C.-S Toh, Bioinspiration & Biomimetics, 20 08 3(3): p 035008-035013 64 ... experiments to maintain uniform concentrations in the area close to the membrane electrode surfaces Fig 2. 4A shows the non-linear curve fit data of the experimental results derived from the collection experiments at potentials 0.5-1.0 V and theoretical values calculated from Eqn 2. 2 for the 55 oxidized ferrocenes A factor of if,fc species if,+FcCOO - is added to Eqn 2. 2 to offset the receiver current... solutions Experimental data of receiver currents ir obtained at Ef = 0.5-1.0 V are fitted to Eqn 2. 2 (points) For neutral species +FcCOO-, the second diffusion term in Eqn 2. 2 applies and l is set to lm See text for best fitted parameter values of u, D and l (B) Collection efficiency derived from (A), is calculated from the ratio of receiver current to feed current, ir/if 54 For a membrane placed between . 2 CHAPTER 2 Development of an electrode-membrane-electrode system for selective faradaic response towards charged redox species 2. 1 INTRODUCTION Development of highly sensitive. expected formal potentials of all three ferrocenes. 49 -1.5 -1 -0.5 0 0.5 1 1.5 -0 .2 0 0 .2 0.4 0.6 0.8 E vs Ag/AgCl (V) Current (mA) FcMeOH FcCOOH FcN A R 2 = 0.95 52 R 2 = 0.9764 R 2 = 0.9766 0.0 0.1 0 .2 0.3 0.4 0.5 0.6 0.7 0.00.10.10 .20 .20 . Concentration. magnitude of the receiver current i r under these conditions relates directly to the amount of oxidized ferrocene moving through the nanochannels of the membrane electrodes. 2. 2.3 .2 Shielding