Development of membrane based electrodes for electroanalytical applications 2

Table 2.1 Structures of AFcMeOH, B FcCOOH, CFcN and their charge In general, collection experiment is carried out by keeping the disk electrode potential at ED such that a redox reaction

2 CHAPTER 2

Development of an electrode-membrane-electrode system for selective faradaic response towards charged redox species

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

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

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

species mobilities when the electrical potential field applied across the membrane was varied

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

Table 2.1 Structures of (A)FcMeOH, (B) FcCOOH, (C)FcN and their charge

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

be obtained from the ratio of ring current to the disk current:

R D

i

−

=

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)

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 ( ) 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 (i

0

iR

D

R) 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):

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

Ferrocenemethanol (FcMeOH), ferrocenecarboxylic acid (FcCOOH) and (dimethylaminomethyl)ferrocene (FcN) were obtained from Sigma-Aldrich All ferrocenes were prepared in 0.1 M phosphate pH 7 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)

2.2.2 Instruments

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 1 and 2 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

Membrane electrode at receiver face

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

The metal-coated membrane was left in contact with the solutions for ca

5 min 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

maintained at constant potential of 0 V (Er), any oxidized species reaching the

received face was reduced The magnitude of the receiver current ir under these conditions relates directly to the amount of oxidized ferrocene moving through the nanochannels of the membrane electrodes

2.2.3.2 Shielding Experiments

In the shielding experiment, the reference and auxiliary electrodes were placed within the receiver solution The feed compartment contained 2.5 mM ferrocene species in 0.1 M phosphate pH 7.0 buffer and the receiver compartment contained the same buffer solution The feed electrode was held

at open circuit or at 0.7 V to achieve limiting feed current values, while the receiver electrode was scanned from 0.3 to 1.0 V

system

Fig 2.3A shows the cyclic voltammograms obtained for the three ferrocene species At the scan rate of 100 mV s-1, sigmoidal shape voltammograms were obtained for all three ferrocenes From the previous scanning electron

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 iR soln drop during or after cyclic voltammetry experiments gave the expected formal potentials of all three ferrocenes

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

In voltammetric techniques, migrations of redox species are generally minimized by addition of electrolyte salts in order to simplify the limiting

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 1 to 2 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

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 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 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