depositacion electroforetica dentro de campos electricos modulados

Electrophoretic deposition under modulated electric fields a review - RSC Advances (RSC Publishing) DOI101039C2RA01342H

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DOI 101039C2RA01342H (Review Article) RSC Adv 2012 2 7633-7646

Electrophoretic deposition under modulated electric fields a review

Malika Ammam Faculty of Science University of Ontario Institute of Technology 2000 Simcoe Street North Oshawa ON L1H 7K4 CANADA E-mail m78ammamyahoofr MalikaAmmamuoitca Fax 905-721-3304 Tel (905)721-8668 ext 3625

Received 21st December 2011 Accepted 16th May 2012

First published on the web 21st May 2012

Classical electrophoretic deposition (EPD) relies on continuous direct current

(CDC) to deposit charged particles on electrodes In recent decades modulated electric fields such as pulsed direct current (PDC) and alternating current (AC) have been investigated This paper reviews EPD under these

modulated electric fields and major applications of the deposited microstructures The paper starts with a short overview of EPD principals

such as the electrical double layer of the charged particle electrophoretic mobility and main suspension parameters including zeta potential particle size conductivity viscosity and stability of the suspension The EPD

mechanisms from the earliest model reported by Hamaker and Verwey to latest models including Sarkar and Nicholson model and influence of the electrohydrodynamics and electroosmosis as well as electrode surface and its

electrochemical double layer on the deposition process have been briefly discussed Two categories of modulated electric fields PDC and AC fields

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have been addressed with their advantages and disadvantages It is found that compared to CDC PDC offers the advantage of i) reducing the coalescence between gas bubbles induced by water electrolysis from aqueous suspensions

hence yielding deposition of smooth and uniform coatings ii) reducing aggregation and disaggregation of nanometer sized particles leading to formation of uniform and homogenous deposits and iii) PDC generates low change in pH near the electrode thus it is convenient for deposition of biochemical and biological species in their highly active states The main disadvantage of PDC over CDC lies in the decrease of the deposition yield The latter can be more pronounced if low time-pulses are used Various categories of AC signals including symmetrical fields with no net DC component and asymmetrical AC signals without and with net DC component have been discussed Overall the deposition rate under AC fields increases with polarization time and amplitude With respect to frequency the deposition rate increases with frequency up to certain value then drops at elevated frequencies It is noted that deposition under AC signals offers the possibility to produce superior quality coatings from aqueous suspensions because electrolysis of water as well as particle orientation during the

deposition could be controlled From the application standpoint PDC and AC offers new application perspectives such as in biotechnology Because under modulated electric fields EPD can now be accomplished from aqueous

suspensions with low water electrolysis rates a variety of biochemical and

biological species can be deposited to yield highly active layers suitable for a wide range of applications including biosensors biofuel cells and bioreactors



Malika Ammam

Malika Ammam received her MS degree from the University of Pierre et Marie CURIE in July 2002 and her PhD from the University of Paris Sud XI France in September 2005 From 2006 to 2007 she worked as a research fellow at the University of Kansas in collaboration with Pinnacle Technology Inc (US) From 2007 to 2010 she worked as a research associate at KU Leuven Belgium and she is currently working at the University of Ontario Institute of Technology in collaboration with Alcohol Countermeasure Systems Corporation Canada Electrophoretic deposition electrochemistry materials sensing and energy are among her research projects and interests

1 Introduction

Electrophoretic deposition (EPD) is an important technology for colloidal coating processes Fig

1 illustrates a schematic representation of an EPD cell Under the influence of direct current (DC)

electric field the charged colloids or particles suspended or dispersed in a fluid move towards the

electrode and deposit1ndash4 Depending on surface charge of the suspended particles two kind of EPD can be defined If the particles are positively charged (Fig 1) they will move towards the

electrode with the opposite sign the cathode and this process is called cathodic EPD By

contrast if the dispersed particles are negatively charged they will be attracted by the positively charged electrode the anode and this process is named anodic EPD



Fig 1 Schematic diagram of electrophoretic deposition of

charged particles on the anode of an EPD cell with planar electrodes

In EPD upon application of a potential a current will flow between the parallel plate electrodes

(Fig 1) The magnitude of the current that flows is influenced by a variety of system parameters

including the cell constant the solution conductivity and interfacial electrochemical kinetics The electric field strength in the suspension (E) is related to the current density (J) and the conductivity of the suspension (k) which is nothing more than the microscopic formulation of the Ohms law (J = kE)Basic EPD is accomplished from organic suspensions that have several advantages including low

conductivity and good chemical stability of the suspension and absence of the electrochemical reactions and Joule heating at the electrodes This leads to formation of high quality coatings Nevertheless the use of organic solvents is associated with many problems including the cost

volatile toxicity most of the time and flammability Furthermore because organic solvents have

low dielectric constant (dissociation power) this induces a limited particle charge Thus high electric fields strengths are required to move the suspended particles towards the electrodeEPD from aqueous suspensions is a good alternative for solving many problems associated with

the organic solvents such as the cost and the environmental load Furthermore because the

dissociation power of water is high (dielectric constant 801) this results in a buildup of a

charge of the particle Consequently low electric fields strengths can be used The main



disadvantage of EPD from aqueous suspensions is water electrolysis Above the thermodynamic

voltages of water oxidation and reduction5 electrochemical reactions occur at the electrodes and

generate gases O2 at the anode and H2 at the cathode (details on the reactions can be found in

section 311) The formed gas bubbles can be incorporated in the deposit and yields damaged and poor quality coatings To overcome this problem several approaches have been madeThe first attempted approach was to carry out the EPD process at voltages below the thermodynamic values of water electrolysis This provided successful production of alumina

microstructures6 However the low applied voltages yielded only low deposition rates that are limited to small wall thicknesses Other concepts based on sacrificial electrodes and competitive

reactions have been explored For example Chronberg and Haumlndle7 employed zinc (Zn) rollers as deposition electrodes to compete the oxidation reaction of water with that of Zn a more facile

reaction Nonetheless low deposition rates as well as contamination of the coatings with Zn limited the process Materials that store gases such as palladium (Pd) have also been investigated

as cathodes for H2 storage8 Unfortunately only small amount of H2 can be stored within the

material Finally chemical additives such as hydroquinone were found to be useful for

suppressing O2 evolved at the anode910 But the concept is only helpful under particular

conditions such as alkaline pHOne promising method to deal with electrolysis of water during the EPD process from aqueous

suspensions is by using membranes11ndash15 A porous and ion-permeable membrane is placed between the anode and the cathode hence dividing the EPD cell into two chambers The deposition occurs on the membrane itself but the ions can pass through the membrane pores to recombine at the electrodes and form gases The sufficient distance which separates the membrane from the electrodes allows formation of free-bubble deposits The membrane based process was successfully used to deposit different materials such as alumina silica glass and

zirconia

Another efficient means to decrease the amount of the evolved gas bubbles at the electrodes and allow formation of homogenous deposits from aqueous suspensions is to carry out the EPD process under modulated electric fields such as pulsed direct current (PDC) and alternating current (AC) Some examples of these signals are shown in Fig 2 The basic difference between

continuous direct current (CDC) shown in Fig 2A and PDC of Fig 2B is that the voltage of a

CDC is roughly constant whereas the voltage of a PDC wave continually varies but like a DC wave the sign of the voltage is constant In AC the voltage continually varies between positive and negative values It is worth noting that the rectangular waveform of Fig 2 constitutes only



one example of a theoretical infinite number of waveforms In AC if voltage-time of the positive and negative half cycles is equal the waveform is symmetrical with theoretically no net DC component (Fig 2C) However if voltage-time of the two half cycles is different the AC wave is

asymmetrical and depending on whether the surface areas of both half-cycles are equal or not the wave can be asymmetrical with net DC component (Fig 2D) or asymmetrical with no net

DC component (Fig 2E)

Fig 2 Schematic representation of some electrical signals (A) continuous direct current (CDC) (B) pulsed direct current (PDC)

(C) symmetrical alternating current (AC) with no net DC component (D) asymmetrical AC signal with net DC component

and (E) asymmetrical AC wave with no net DC component In recent decades these modulated electric fields have been investigated not only for EPD of

ceramic and polymer particles but also for biochemical and biological species that are more

sensitive to pH shifts or electrochemical reactions products formed at the electrodes Because



modulated electric fields generate a low rate of water electrolysis biochemical and biological

species can be deposited in their highly active states For instance EPD of bacterial cells and

enzymes from aqueous solutions under CDC has been reported to some extent16 However it is not clear from the text whether the activity of the species after CDC that generates a high amount of electrolysis was examined or notThis paper reviews some selected literature dealing with EPD under modulated electric fields

PDC and AC and the main applications of the deposited microstructures However first some necessary reminders and updates about EPD principles should be addressed

2 EPD principals

21 Electrical double layer and electrophoretic mobility

Charged particles in a fluid are surrounded by a cloud of ions Ions of opposite charge (counterions) are attracted towards the surface of the charged particle while ions of similar charge are pushed away from that surface This yields a net electrical charge on one side of the interface and a charge of opposite sign on the other side of the interface giving rise to what is called the electrical double layer of the particle The widely accepted model for the particle

double layer is the Stern model17 A schematic representation of the particle electrical double layer with distribution of charged species and potential drop across the double layer in accordance with Stern model is shown in Fig 3 If we assume a negatively charged particle the

ions of opposite charge closest to surface of the particle form what is known as a Stern layer and the rest of the ions which are distributed more broadly form what is called the diffuse double layer (Fig 3) The particle surface at the plane of shear between Stern and the diffuse layer is

characterized by a potential called zeta potential ζ which is involved in nearly all

electrokinetics1819 Application of an external electric field can shear away some of the ions outside the Stern layer and yields a drift and migration of the particle towards the electrode of opposite charge (Fig 1)



Fig 3 Schematic representation of the electric double layer of the charged particle and potential drop across the double layer (a)

Surface charge (b) stern layer (c) diffuse layers of counter-ions Image reproduced from ref 1 with permission of Elsevier

The main electrophoretic characteristic of the particle under the influence of the electric field is called electrophoretic mobility μeph which can be defined as the coefficient of proportionality

between the electric field strength E and the particle velocity Veph19ndash21

Veph = μephE (1)

In turn the electrophoretic mobility increases with the particle zeta potential ζ and decreases with the viscosity of the media η The coefficient of correlation between μ and ζ depends on the size of the particles The electrophoretic mobility of small particles defined as having a radius r much smaller than the Debye length of the counterionic atmosphere (r ≪ 1k) is described by the Huckel equation

μeph = 2εε0ζ3η (2)

where ε and ε0 are the dielectric permittivity of the media and vacuum respectively

The electrophoretic mobility of particles of size much greater than the Debye length of the counterionic layer (r ≫ 1k) is given by the HelmholtzndashSmoluchowski equation



μeph = εε0ζη (3)

The displacement of a particle for a finite time Δt under uniform DC electrophoresis can

reasonably be defined as21

xDC = μephEDCΔt + x0 (4)

where the constant x0 determines the initial location of the particle

22 Key suspension parameters

EPD is a two-step mechanism First the particles must migrate to the deposition electrode under

the action of an electric field and second the particles must coagulate and deposit on the electrode (Fig 1) There are a number of suspension parameters that influence the EPD process

as well as the quality of the deposited coating Some of the important ones are discussed below

221 Zeta potential Zeta potential is a key parameter The dispersed particles are required to have an elevated and a uniform surface charge which will determine their stability in the suspension The stability of the charged particles is essentially governed by the sum of the attractive and the repulsive forces between the particles mainly electrostatic and van der Waals in nature To prevent agglomeration between particles high particle charge is required to create high electrostatic repulsion By contrast if the particle charge is low the particles will coagulate

The particle charge or zeta potential can be controlled by various charging agents such as acids

bases or specifically adsorbed polyelectrolytes22 However before using charging agents to increase the charge of the suspended particles the first common step in suspension preparation is powder washing This step allows the removal of any residual impurities that are incorporated with the powder and may affect the stability of the suspension thus the deposition characteristics

Basu et al23 reported that unwashed powder led to an unstable suspension which in turn led to lower deposition yields and a decrease in green density of the deposit by about 15ndash25 The problem was solved by repetitive washing of the powder with deionized water Also during the

washing process significant reduction in the conductivity of the supernatant was observed23

Hence it can be concluded that the washing process is an important factor in suspension stabilityWhile the washing process can easily be accomplished with inorganic materials such as ceramic



particles23 for biomolecules and biological cells this step might be difficult Because enzymes and biological cells usually contain salts for electroneutrality their dispersion even in ultrapure water gives rise to high conductivity suspensions which in turn may affect their zeta potential as

well as the suspension stability24ndash26

The charge on a biomolecule such as enzymes proteins or biological cells arises from the

ionization of the functional groups present within the biomolecule In proteins or enzymes these

groups carried out by essentially the amino acids residues give rise to either positive or negative charges In acidic pH the net charge of the enzymes is usually positive whereas in alkaline pH the charge is negative At a certain pH enzymes will have no net charge called isoionic point which is around pH 4 for glucose oxidase for example At neutral pH enzymes such as glucose

oxidase are negatively charged and the zeta potential of enzymes depends largely on the

dissolving media Matsumoto et al27 measured the zeta potential of glucose oxidase dissolved in

high ionic strength phosphate buffer solution as minus04 mV This led to practically no

electrophoretic mobility of the enzyme This problem can be solved if the enzyme is dissolved in

ultrapure water Ammam and Fransaer24 measured the zeta potential of glucose oxidase dissolved

in ultrapure water containing low amount of NaOH as charging agent as minus107 mV The latter led

to a net electrophoretic mobility of the enzyme when asymmetrical AC fields are applied to form thick deposits on the electrodeFor biological cells the washing procedure is easier than for enzymes and proteins because cells

can be dispensed in water and filtered off to remove the excess salts Consequently adequate zeta

potentials of cells suspended in water have been recorded162829 A typical example on how zeta

potential varies with pH of the suspension is shown in Fig 4 for E Coli microorganism and

inclusion bodies16 The isoionic point of these species is located at pH 3ndash4 At higher pH the species are negatively charged whereas at lower pH the species are positively charged The charge or zeta potential of the species increases when pH is moved away from the isoionic point

Dependence of zeta potential on pH for a suspension of pure E Coli bacteria and a suspension of pure inclusion bodies Image reproduced from ref 16 with permission of Elsevier



Fig 4 Dependence of zeta potential on pH for a suspension of pure E Coli bacteria and a suspension of pure inclusion bodies Image reproduced from ref 16 with permission of

Elsevier

222 Particle size Particle size is an important parameter in the EPD process For stable suspension it is important that the dispersed particles remain well dispersed and stable to yield homogenous and uniform deposits For ceramic particles it is reported that the optimal range of

particle size lies between 1ndash20 μm30 The problem with larger sizes is that the particles tend to sediment under the gravitational forces leading in vertical cells to thick deposits on the bottom and thin films on top Likewise very small particles tend to aggregate and thus yield inhomogeneous deposits

223 Conductivity and viscosity of the suspension In EPD the amount of the deposited

particles is related to the strength of the electric field The applied electric field between the anode and cathode is not only carried by the charged particles but mostly by the free coexisting ions (electrolyte) On the other hand the amount of the coexisting ions determines the conductivity of the suspension an important factor in EPD If the conductivity of the suspension

is too high ie the amount of the free ions is considerable the particle mobility decreases

because the zeta potential is reduced26 By contrast if the conductivity is too low the suspension

will be resistive leading to loss in stability25

In turn the conductivity is related to dielectric constant or dissociation power of the solvent It is

reported that optimal deposition rates are obtained with liquids having dielectric constant in the

range of 12ndash2531 specifically associated with organic solvents With liquids having high

dissociation power such as water (dielectric constant 801 at 20 degC) the amount of ions in the

suspension must be very low in order to yield stable suspension hence high deposition rate Also viscosity accounts in EPD process The amount of the dispersed particles and the pH affect the

viscosity of the suspension2532 Low viscosities are required for successful EPD and optimal

dispersant concentration can experimentally be determined32

224 Suspension stability A stable suspension with well-dispersed and controlled surface

charge is essential for successful EPD25 The stability of the suspension is characterized by the



rate and tendency of the charged particles to undergo or avoid flocculation The suspension stability depends on several factors such as size of the particles their zeta potential Debye length of the solution as well as the presence of adsorbed species Average sized particles with uniform surface charge tend to stay suspended for long periods due to Brownian motions Bigger sized particles tend to sediment because of the strong gravitational forces thus require continuous hydrodynamic agitation to stay in the suspension Stable suspension shows no flocculation or sedimentation of the particles However if the suspension is too stable meaning that the repulsive forces between the particles are too strong the electric field will not be able to overcome these forces and move the particles towards the electrode thus there will be no deposition Near the electrode if the suspension remains stable there will be no deposition Fortunately presence of other phenomena such as electrochemical reactions electroosmosis and electrohydrodynamic flows induce instability near the electrode and cause the particles to fluctuate and precipitate

23 EPD mechanisms

After migration of the charged particles from the bulk to the electrode of opposite charge the next step in EPD process is the coagulation and deposition Although EPD is known since the

19th century the deposition mechanisms are not fully understood and still under debate Several theories have been proposed

The earliest dates back to 1940s Hamaker and Verwey33 suggested that deposition is due to accumulation of the charged particles on the electrode under the action of the applied electric field Nevertheless the proposed mechanism is more useful for deposition of particles on a membrane placed between two electrodes rather than for deposition on the electrode

In 1992 Grillon et al34 suggested that EPD occurs by particle charge neutralization The charged

particle approaching the electrode neutralizes upon contact with the opposite charge of the electrode and precipitate The drawback of this theory is that the deposition is limited to a monolayer and this cannot explain the formation of thick coatings

Few years later Koelmans35 and later De and Nicholson36 proposed that deposition is governed by electrochemical particle coagulation Due to electrochemical reactions at the electrodes during the EPD process the ionic strength near the electrode surface would increase compared to bulk This induces a reduction of the repulsive forces between particles near the electrode which in turn lowers zeta potential and induces precipitation This theory is useful to explain deposition of

particles from suspensions containing water but not EPD from organic suspensions since no

electrochemical reactions occur



To overcome the problem dealing with coagulation in absence of the electrochemical reactions

Sarkar and Nicholson17 suggested that deposition occurs through the double layer distortion and thinning followed by coagulation of the particles on the electrode As illustrated in Fig 5 three

main successive steps take placei) When the charged particle that is surrounded by the diffuse double layer is subjected to an electric field it undergoes a distortion and becomes thinner ahead and wider behindii) During the migration of the charged particle ions of similar charge will move in same direction of the particle and can react with the counter ions accompanying the charged particle The latter induces a thinning of the double layer of the particleiii) Because the double layer of the particle is now thinner it can easily approach another particle with also a thinner double layer and if the contact is close enough to favor van der Waals attractive forces the deposition occursSo far no experimental data have yet been presented to support this theory However thanks to

imagery techniques experimental data are now available on how the particles aggregate on the

electrode The aggregation is modeled by various mechanisms such as electrohydrodynamics37

and electroosmosis3839 The action of an electric field on charge of the particle diffuse layer gives rise to the electroosmotic flow while action of an electric field on charge induced by the electric field on the electrode gives rise to the electrohydrodynamic flow For electroosmosis thickness of the electrical double layer of the particle particularly the diffuse layer is a key determinant of the electroosmotic forces A thick diffuse layer yields significant electroosmotic forces while a

thin diffuse layer gives rise to negligible electroosmotic forces39



Fig 5 Electrical double layer distortion and thinning mechanism for electrophoretic deposition Image reproduced from ref 17

with permission of Wiley amp Sons Also the electrode surface and its electrochemical double layer play an important role in the

deposition process540 Polarizable electrodes create a substantial potential drop in the electrochemical double layer of the electrode thus most of the current is capacitive Consequently the electric field strength reduces in the bulk of the suspension and induces instability By contrast non-polarisable electrodes generate minimum drop current and most of the current is Faradaic This induces an effective electric strength in the bulk of the suspension

Prieve et al40 have recently reviewed 2D assembly of particles under DC and AC electric fields



and emphasized the role of the electrochemical double layer of the electrode in the deposition process

3 EPD under modulated electric fields

31 PDC fields

The advantages of PDC shown in Fig 2B over CDC of Fig 2A for EPD of charged particles can

be summarized as i) PDC significantly reduces the coalescence between gas bubbles induced by water electrolysis

Thus yields deposition of smooth and uniform coatingsii) PDC reduces aggregation between nanometer sized particles leading to formation of uniform and homogenous depositsiii) PDC generates low change in pH near the electrode Hence is convenient for deposition of biochemical and biological species in their highly active states

311 Reduction in coalescence between gas bubbles PDC has been investigated for EPD of

ceramic particles41ndash46 polymers47 carbon nanotubes48 and enzymes49 In all cases low rates of

gas bubbles have been noticed when PDC is used Fig 6 depicts typical topographies of alumina

deposits produced from aqueous suspensions using CDC and PDC with various time-pulses It can be seen that under CDC the deposits are porous and inhomogeneous This is due to water

electrolysis which forms gases O2 at the anode and H2 at the cathode following the reactions I

to IV5



Fig 6 Surface morphology of deposits obtained by PDC EPD at

constant voltage mode of 20 V (A) and at constant current mode of 0004 A (B) Image reproduced from ref 46 with permission of

Elsevier Anode 2H2O = O2 + 4H+ + 4eminus (Eo = minus123 V vs NHE) (I)

4OHminus = O2 + 2H2O + 4eminus (Eo = minus040 V vs NHE) (II)

Cathode 2H+ + 2eminus = H2 (Eo = 0 V vs NHE) (III)

2H2O + 2eminus = H2 + 2OHminus (Eo = minus083 V vs NHE) (IV)

The dominant reaction would of course depend on whether the aqueous suspension contains sufficient amounts of H+ OHminus or only water For example if the solution contains a sufficient

amount of OHminus OHminus would be the first species to be oxidized into O2 at the anode because the

reaction requires less voltage (minus04 V) than that of water oxidation (minus123 V) Similarly if H+

were added to the aqueous suspension H+ would be the first species to be reduced into H2 at the

cathode because the reaction requires 0 V compared to reduction of water molecules (minus083 V)

However if pure water is used as a fluid suspension after the reduction and oxidation of the

small amount of H+ and OHminus present at the concentration of 10minus7 M water molecule will be the

main species to be oxidized at the anode and reduced at the cathode with voltages of minus123 V and minus083 V respectivelyDuring EPD process the generated gas bubbles can be incorporated in the deposit and results in



porous morphologies (CDC of Fig 6) By comparison under PDC the number and size of the

pores can be reduced if time-pulses are decreased Theoretically once the thermodynamic voltages of water electrolysis are reached water starts to decompose into O2 and H2

Nevertheless depending on the reaction conditions such as nature of the electrode material nature of the electrolyte and its concentration etc the potentials may shift from the thermodynamic values The latter defines the kinetics of water electrolysis reactions

Besra et al46 measured larger shifts in pH near the electrode using CDC compared to PDC This may suggest that PDC induces low rates of electrolysis compared to CDC However taking into account the total polarization time (ON + OFF) used for PDC that is higher than that used for CDC since CDC has no OFF-time it is possible to propose that using PDC the reaction products formed during the ON-time diffuse towards the bulk solution during the OFF-time inducing less change in pH near the electrode Thus it maybe concluded that unless extremely low time-pulses

are used50 PDC probably do not generate low rates of electrolysis but more appropriately it minimizes coalescence between gas bubbles thus preventing formation of larger onesFig 7 supports this claim It shows a comparison between SEM images of polypyrrole (PPy)

coatings on glassy carbon (GC) substrates produced by electropolymerization of pyrrole

monomer from low conductivity solutions using CDC (Fig 7A 7B and 7C) and PDC (Fig 7D)51

The same deposition time is used for all coatings In addition 10 s is used as time-pulse for PDC that is large enough to induce formation of gas bubbles since micro-meter sized bubbles were

observed even with pulses of 01 ms50Fig 7A 7B and 7C clearly depict rough and

inhomogeneous topographies of PPy deposits under CDC This is due to the large amount of gas bubbles incorporated in the coatings More importantly Fig 7C clearly shows that under CDC

the gas bubbles coalesce and when maximum volumes are reached the bubbles explode and form patterns that can be described as ldquoopen flowersrdquo By comparison the topography of PPy coating obtained by means of PDC is smoother and homogenous (Fig 7D) Interruption of the field

during OFF-time for PDC breaks apart most of the tiny formed bubbles Hence prevent their coalescence during the next ON-time polarization



Fig 7 SEM images of electrodeposited polypyrrole (PPy) from

pyrrole aqueous solutions using CDC (A B C) and PDC (D) at 4 V

vs AgAgCl for equal ON-time deposition mode Part of the image reproduced from ref 51 with permission of Elsevier

312 Reduction of aggregations between particles The second advantage of PDC over CDC is that PDC prevents the particles near the electrode from aggregation Fig 8A and 8B depict

SEM images of 2D polystyrene particles deposited by means of PDC and CDC respectively47 While aggregation between particles can be observed with CDC (Fig 8B) the monolayer formed

by means of PDC depicts more independent particles (Fig 8A) Under PDC the decrease in

aggregation between the particles can be assigned to

i) Reduction in the electroosmotic flow that is induced near the electrode (Fig 8C) In other

words when an electric field is applied between the anode and the cathode the charged particles move towards the electrode of opposite charge As the particles approach the electrode the electroosmotic forces induced by the electric field interfere within the Debye length range and bend the electrophoretic forces This causes their aggregation in CDC Under PDC the electroosmotic flow stops during the OFF-time and when the next ON-time starts the particles near the electrode move under electrophoresis without significant interference from

electroosmosis and thus deposit independently47



ii) Reduction in coalescence between the formed gas bubbles In other words under CDC

electrolysis of water induces large-sized bubbles that may generate a pressure on the surrounding

particles and push them away As a result the particles will be gathered in between the formed gas bubbles and aggregate as in Fig 8B By comparison because PDC prevent gas bubbles from

coalescence and reduce their size the pressure applied by the formed bubbles should also be

reduced This yields deposition of independent particles at 2D (Fig 8A) and 3D43

Single-size deposit particles by PDC (A) and CDC polarisation (B) All particles are deposited at 33 V cmminus1 for 2 min (C) Illustration of on-going deposit particles during EPD The effect of electrokinetic phenomena is to induce the on-going particles as they approach the substrate The off-mode condition reduces the effect of electroosmosis on an incoming deposit particle (particle 2) Image reproduced from ref 47 with permission of Elsevier

Fig 8 Single-size deposit particles by PDC (A) and CDC polarisation (B) All particles are

deposited at 33 V cmminus1 for 2 min (C) Illustration of on-going deposit particles during EPD The effect of electrokinetic phenomena is

to induce the on-going particles as they approach the substrate The off-mode condition

reduces the effect of electroosmosis on an incoming deposit particle (particle 2) Image

reproduced from ref 47 with permission of

Elsevier

313 Preservation of the activity of deposited biomolecules PDC is advantageous over CDC



for EPD of biomolecules such as enzymes49 Because PDC yields low change in pH near the

electrode46 consequently the activity of the deposited enzyme is preserved Ammam and

Fransaer49 noticed a decrease in activity of the deposited enzyme by 25 when they switched from PDC to CDC under the same conditions

314 Disadvantages of PDC over CDC The main disadvantage of PDC over CDC is the decrease in the deposition yield The latter can be more pronounced if low time-pulses are used as

depicted in Fig 64246 Although the deposition mechanisms under PDC still under investigation

if sufficient time-pulses are employed accumulation of the particles on the electrode is attributed to the strong inertia forces induced by the PDC field This results in migration of the particles near the electrode for a few more seconds following the OFF-time mode to reach the electrode

and deposit without bubbles46 The reduction in the deposition yield using PDC is attributed to

the low change in pH near the electrode which slow down the charged particles located near the

electrode to reach their isoelectric point and precipitate353646

32 AC fields

While in DC fields the electric charge flows only in one direction in AC fields the movement of the electric charge periodically reverses direction between the positive and negative (Fig 2) As a

consequence under an AC field the dynamic of particles are much more complicated than in DC Thus analysis of the experimental observations is often difficult because there are a number of phenomena and forces that arise from the interaction of the AC field with the suspended particle and this is influenced by a range of parameters including frequency particle size electrolyte conductivity electric field distribution and whether the current passes through the capacitance of

the electrode double layer or through the electrochemical reaction4052ndash60 Some of the forces are

well-studied and documented in literature such as electrophoresis61ndash63 dielectrophoresis64ndash67

electrorotation6869 and travelling-wave dielectrophoresis70 The basic difference between these

forces is schematically depicted in Fig 9



Fig 9 Schematic representation of a charged particle under the influence of electrophoresis (A) dielectrophoresis (B)

electrorotation (C) and travelling wave dielectrophoresis (D)

Electrophoresis In electrophoresis (Fig 9A) a dispersed charged particle in a fluid moves and

migrates under the action of a uniform electric field towards the electrode of opposite charge

Dielectrophoresis If the electric field is not uniform as illustrated in Fig 9B the interaction

between the induced dipole and the non-uniform electric field generates a force Due to presence of the electric field gradient these forces are not equal in magnitude and generate net movement of the charged particle under dielectrophoresis

Electrorotation If the electric field is rotating as depicted in Fig 9C a dipole is induced across

the particle which will simultaneously rotate with the electric field If the angular velocity of the rotating field is large enough this will require certain time to form a dipole on the particle which will lag behind the field The non-zero angle between the field and the dipole induces the particle to rotate in the field and gives rise to electrorotation

Travelling wave dielectrophoresis Travelling wave dielectrophoresis is a linear form of



electrorotation As displayed in Fig 9D instead of a circular arrangement the electrodes are

arranged in a linear manner with 90deg phase shift from each other An electric field wave is produced between the electrodes and interaction between this wave and the polarizable particle induces a dipole that will move with the electric field If the field wave is strong enough the dipole will lag behind the field just like in electrorotation However because the electric wave is travelling in linear instead of circular manner a force rather than a torque will be induced thus leading to a drift of the particle from one electrode to another

Therefore as one can imagine under AC fields various phenomena could occur simultaneously and the deposition mechanisms are often unclear For example at low frequencies the interaction between the charged particle and the field gives rise to electrophoresis AC field also produces a

frequency dependent dipole on a polarisable suspended particle and depending on the field distribution interaction between the dipole and the electric field can generate dielectrophoresis

electrorotation and travelling wave dielectrophoresis (Fig 9) Furthermore if the AC field is

strong enough it induces a fluid flow and heating at the electrodes The AC field can interact with the fluid to produce electrohydrodynamic and electroosmotic forces as well as electrothermal due to Joule heating The resulting fluid flow may induce a drag force on the particle and thus a motion Depending on the frequency and the ionic conductivity (electrolyte) the electrohydrodynamic flow may induce aggregation or separation between the suspended particles near the electrode At high frequencies electrothermal fluid flow competes with the dielectrophoretic forces and electrohydrodynamics account At low frequencies AC electroosmotic flow dominates and this is strong even at low potentials At intermediate frequencies both electrohydrodymics and electrosmosis are active On top of all the applied forces particlendashparticle interaction and Brownian motion occur continuouslyBecause of the complicated dynamics of particles under AC fields the formulas describing the displacement of the charged particles in the AC field are also complicated if compared to the simple DC formula shown in equation (4) The displacement of charged particles in AC fields will depend essentially on nature of the AC waveforms (sine triangular rectangular etc) as well

as the symmetry or asymmetry of the signal which will define the mathematical formulas2021 For example in case of an harmonically oscillating electric field the amplitude of the peak-to-

peak displacement xAC of the charged particle is reported as21

xAC = minus[μephEACsin(ωt)]ω + x1 (5)



where EAC is the peak to peak amplitude ω is the applied angular frequency μeph is the

electrophoretic mobility and x1 determines the center of the trajectory

321 Symmetrical AC fields with no net DC component Symmetrical AC-electric fields have been used since the 19th century for EPD of charged particles This can be found in a large

number of patents typically for industrial applications such as electro-coating of paint based

polymers and copolymers71 A typical example of symmetrical AC waveform that generates no

net DC component is shown in Fig 2C What theoretically can be predicted using symmetrical

and uniform AC waveforms is that at low frequencies and during the first half-cycle of the period the charged particle moves a certain distance d During the other half-cycle the particle changes its direction back to its original position minusd Consequently the particle oscillates around

a fixed spot in the fluid between the two electrodes and its net migration becomes zero2072 Therefore it can be concluded that symmetrical and uniform AC signals would not be appropriate for an efficient EPD Nevertheless a large number of scientists have succeeded to deposit thin and even thick coatings using symmetrical waves In view of this in 1979 Ulberg et

al73 deposited weakly charged submicron oligomeric epoxy resin particles Later Hirata et al74 reported EPD of alumina particles under symmetrical sine waves from aqueous suspensions at

voltage of 6 V and frequencies of 012 1 and 10 kHz In the past decade Poortinga et al75 investigated symmetrical AC signals for EPD of spherical and rod-shaped bacterial cells and

noticed that the bacterial cells adhere and deposit on the electrode if sufficiently high currents are used

Although the deposition mechanisms under these signals are not fully understood the formed

monolayers have been attributed by Ulberg et al73 and Hirata et al74 to diffusion (inertia free) since the particles were observed to coagulate on the electrode instantly On the other hand other phenomena such as electrohydrodynamic and electroosmotic forces may account in the

deposition process In this context Sides54 reported that when the frequency changes the interaction strength between the particles near the electrode can reach its maximum and it can even reverse attractive to repulsive forces and vice versa leading to aggregation of the particles

on the electrode54

So far when uniform and symmetrical AC signals are used low deposition rates that are limited

to monolayers were usually depicted287475 There are some reported contradictory observations where using symmetrical or uniform AC signals thick deposits were obtained For example

Gardeshzadeh et al76 deposited thick films of multiple walled carbon nanotubes using square and



sine waves at frequencies of 1ndash1000 Hz as well as thick films of SnO277 Heidari et al7879

followed the procedure by Gardeshzadeh and obtained thick films Also Riahifar et al7280 employed similar category of electrodes as Heidari and deposited thick TiO2 films

According to theoretical predictions these symmetrical signals should not form thick deposits The latter might be related to the setup used to carry out the deposition Non-planar and non-parallel electrodes have been employed An example of the electrodes configuration is illustrated in Fig 10 This configuration suggest generation of non-uniform electric fields which will give

rise to not only electrophoresis but to eventually other forces such as dielectrophoresis and

travelling wave dielectrophoresis of the particles from one electrode to another

Fig 10 Optical microscopy image of the electrode configuration

used by Heidari et al79to deposit WO3 particles Because space

between the electrodes is only hundreds of μm the particles have connected the electrodes together Image reproduced from ref 79

with permission of Springer The main characteristics of EPD under symmetrical AC waveforms is that the deposition rate

increases by reducing the frequency727375 and increasing the voltage7273 Also the deposition



yield augments with the polarization time then reaches a saturation plateau7377

322 Asymmetrical AC field with net DC component A typical example of asymmetrical AC wave with net DC component is shown is Fig 3D Because the surface area voltagendashtime (Endasht) of

the positive and negative half-cycles are not equal a DC component is induced This leads to a net drift of the charged particle towards the electrode of opposite charge Furthermore under this category of signals inertia forces could also be an important driving force for migration of the particles because during the fast voltage change from the positive to the negative signal and vice versa the particles are accelerated and they cannot slow down open the reverse signal The latter

has been experimentally observed by Nold and Clasen81 who recorded at the output a supplementary peak every third peak that has initially not been programmed

The advantage of the asymmetrical AC signal with net DC over pure DC consists of producing superior quality coatings because electrolysis as well as particle orientation during deposition

could be controlled to some extent In view of this Shindo et al82 employed AC signals with controlled reverse current to lt 10 to form coatings with superior corrosion resistance Nold and

Clasen81 used asymmetric AC square wave with negative net DC component to deposit alumina

particles coatings from aqueous suspensions The obtained coatings were smooth and bubble-

free Yue et al83 coupled a weak DC to a strong AC field to deposit Ag-sheathed Bi2Sr2CaCu2Oδ

tapes with improved microstructuresThe main characteristics of EPD under asymmetrical AC signals with net DC component are

i) In order to prevent formation of bubbles if the applied current density increased the frequency

should also be increased81

ii) Compared to pure DC the obtained deposition rates are low but the quality is better since the coatings have no incorporated gas bubblesiii) If the employed AC field is stronger than the DC component the produced microstructures

should have high probability of orientation83 thus a better quality

323 Asymmetrical AC field with no net DC component Fig 2E depicts an example of the

asymmetrical AC waveform with no net DC component It will be noted that surface areas of the positive and negative half-cycles are equal Thus theoretically no net DC component is induced (intEdt = 0 over one period) to cause migration of the charged particle Nevertheless the difference in potential height and time duration (Endasht) between the two half-cycles could cause the particle to migrate greater distance just like in DC fields if the applied voltage is sufficiently high



For low electric field strengths traditional electrophoresis theory assumes a linear dependence

between the velocity of the particle motion Veph and the electric field strength E as illustrated in

eqn (1) If the voltage is sufficiently high this theory becomes non-linear and can be described by

adding a second term with the cubic dependence on the electric field strength20

Veph = μephE + μeph3E3 (6)

In other words in the signal of Fig 2E during the weak voltage-pulse the nonlinear component

would be absent and only the linear one is presentThe theory of non-linear or non-uniform electrophoresis was observed and theoretically described

in many papers in the course of the past 35 years2084ndash94 and the reason behind it is to do with the

electrical double layer of the suspended charged particle Stotz86 suggested that if the electric field is sufficiently strong the electric double layer would completely be stripped off Thus the particle charge can directly be calculated by equating the Coulomb and Stokes forcesRecently the theory of nonlinear electrophoresis was tested experimentally and found to

successfully produce thick deposits of inorganic9596 organicndashinorganic particles97 and enzymes

and biological cells24282998ndash100103 Using asymmetrical triangular waves Neirinck et al9596 deposited thick and uniform alumina coatings with higher green density from low aqueous

conductivity suspensions at 50 Hz and 500 Vpndashp Green density can be defined as the ratio of the

ceramic powder volume to the external volume of the deposit If the green density is low cracking during drying andor sintering of the deposit occur because of the large change in the volume Fig 11 illustrates the shape of the triangular wave used for EPD and the resulting

alumina deposit in comparison with a coating obtained using CDC While using CDC from aqueous suspensions a porous coating is formed the deposit produced under asymmetrical AC waveform is non-porous and homogenous The reason for that is to do with water electrolysis



Fig 11 (A) One period of the applied asymmetric triangular AC signal and alumina deposits formed using 100 V DC for 1200 s (B)

and 50 Hz and 500 Vpndashp asymmetric AC field (C) Image

reproduced from ref 95 with permission of Elsevier According to theoretical predictions electrochemical reactions are suppressed using symmetric AC signals (Fig 2C) if the frequency is sufficiently high because most of the electrochemical

reactions are slow20 Moreover even when the frequency lies below the cut-off frequency of the electrochemical reaction reaction products formed during the positive (or negative) part of the period are partly consumed when the signal is reversed during the rest of the period thus lowering the reaction products By contrast when asymmetrical AC signals such as the triangular wave of Fig 11 are used of which the integral over one period is zero the difference in the

voltage height and time duration (Endasht) between the positive and the negative half-cycles will produce water electrolysis The rate of electrolysis increases by lowering the frequency elevating

the voltage and the ionic conductivity of the suspension101 However compared to the electrolysis rates obtained by CDC the rates under asymmetrical AC without net DC component are significantly low to yield large gas bubbles and damage of the deposited coatingIt turns out that the conditions from which EPD is accomplished under asymmetrical AC without

net DC component including aqueous solvents and low electrolysis rates are conditions suitable

to deposit biomolecules such as enzymes and biological cells Ammam and Fransaer24 demonstrated that using asymmetrical triangular waveform of Fig 11 thick and active layers of

glucose oxidase (more than 10 μm) could be deposited at 30 Hz and 160 Vpndashp Such thicknesses

cannot be achieved by classical electrodeposition from either buffers27 or low conductivity

solutions49 This suggests that under asymmetrical waves the enzymes migrate greater distance to



reach the electrode and deposit Also 89 μm thick layers of biological cells S Cerevisiae were deposited under the applied conditions of 30 Hz and 200 Vpndashp for 30 min (Fig 12B) By

comparison using symmetrical triangular AC waves without net DC component only a monolayer of S Cerevisiae can be formed (Fig 12A)

Fig 12 SEM micrographs showing the morphology of deposited S Cerevisiae cells layers using symmetrical triangular waveform for

30 min at 30 Hz and 200 Vpndashp (A) and using asymmetrical triangular

wave of Fig 11(A) for 30 min at 30 Hz and 200 Vpndashp (B) Part of

the image reproduced from ref 28 with permission of Elsevier The main characteristics of EPD under asymmetrical AC waves without net DC component are

i) The deposition yield increases linearly with the polarization time for both inorganic ceramic

materials9596 and biochemical and biological species2428

ii) The deposition rates vary quasi-linearly with the applied amplitude then reach a saturation

plateau2495 confirming in some sort that nonlinear mode of electrophoresis takes place at high

voltages (eqn (6))

iii) With respect to frequency the deposition yield increases with the frequency from 0ndash50 Hz

reaches a plateau95 then drops at elevated frequencies24 This is understandable since if the frequency is changing faster the particles will no longer follow the AC field because of their inertia As a consequence the particles oscillate in a single place and their net migration is significantly reduced

4 Applications of films by EPD under PDC and AC

In recent years there has been a rising interest in nano and micro-structured materials because they usually exhibit novel and improved properties suitable for various applications Compared to



other deposition processes such as lithography self-assembly dip coating and spin coating EPD

technique offers the advantage of being versatile cost saving and more importantly allows deposition of high rates of particles in a controlled structural manner Although applications of films by EPD under CDC fields are wide and can be found in advanced

ceramic materials coatings for thin and thick films multilayered composites functionally graded materials hybrid materials self-supported components micro-patterned assemblies and

nanotechnology1 EPD under modulated electric fields PDC and AC offers new application

perspectives such as in biotechnology Because EPD can be accomplished from aqueous

suspensions with low electrolysis rates under modulated electric fields a variety of biochemical and biological species can be deposited to yield highly active layers suitable for a range of

applications including biosensors biofuel cells and bioreactors2428297598ndash104

Because EPD is an automated process a variety of parameters such as voltage frequency

concentration of the dispersed species and deposition time can be controlled to yield a better response under the optimized conditions With regard to this large number of biosensors with improved characteristics have recently been designed and manufactured by means of EPD under

PDC and AC electric fields for detection of various analytes including glucose244999 lactose98

glutamate100 and H2O2103 Moreover it is found that some biosensors are even dependable for

determination of the analyte in real samples such as in milk and in blood9899 The latter is

interesting considering the challenging aspects of the behavior of biosensors in real samples105 By adding redox mediators these electrodes have also been demonstrated to be useful for the

manufacturing of glucoseO2 biofuel cells104

Various microorganisms such as E coli and S Serevisiae cells have also been successfully

deposited under modulated electric fields to form active monolayers2975102 and multilayers28 In addition some studies demonstrated that the viability of the deposited cells is even superior with

respect to similarly treated free cells thus improved bioreactors28

Another important aspect of modulated electric fields over the classical CDC in EPD process is

the control over the orientation of the particle during the deposition The latter yields better ordered nano and micro-structures In view of this controlled zeolite microstructures can be

formed on glassy carbon under PDC44 and under AC fields rod-shaped bacteria A naeslundii75

as well as Bi2Sr2CaCu2Oδ tapes83 are observed to align parallel to the AC field The well-

organized and oriented deposited structures give rise to improved properties which is observed in

many applications including superconductors83 supercapacitors97 field emission display devices



based carbon nanotubes48 and sensitive ceramic gas sensors for CO and NO2 detection77ndash79

Because PDC and AC generate low rate of water electrolysis and prevent coalescence between

gas bubbles high quality coatings useful for large range of applications such as ceramics and

polymers can be produced41ndash47717280819596 The smooth and non-porous texture of the coatings

yields better anticorrosion properties

5 Conclusions

The reviewed literature revealed that EPD under modulated electric fields PDC and AC is a very

promising process for the deposition of high quality coatings From environmentally friendly aqueous solvents it is shown that a variety of materials including inorganic organicndashinorganic

and biochemical and biological species can be deposited in controlled fashions The formed deposits displayed improved characteristics compared to what can be obtained by classical CDC fields including smooth and bubble-free films well oriented microstructures and high activity biochemical and biological deposited species In addition because EPD under PDC and AC is

automated this leads to high reproducibility The latter have been proven to lead to new applications particularly in biotechnology such as in biosensors (glucose glutamate hydrogen

peroxide) biofuel cells and cell based bioreactors with improved characteristics With respect to

this particular attention should be paid to deposition of biochemical and biological charged particles such as enzymes and biological cells because the conditions under which EPD can be

performed are now suitable for these species Given the great potential of EPD under modulated

electric fields for manipulation and assembly of particles further research and development of

these processes will allow the understanding of the mechanisms behind the assemblies Also design and fabrication of 2D and 3D dimensional microstructures for novel generation of future advanced applications in nano and biotechnology

Acknowledgements

The author would like to thank KU Leuven (GOA08007) and the Belgian Federal Science Policy Office (BELSPO) through the IUAP project INANOMAT (contract P617 Belgium) and Natural Sciences and Engineering Research Council of Canada and the University of Ontario Institute of Technology (Canada) for their support

References



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deposition of nanosized powders proceeding of the 27th Annual Cocoa Beach Conference on Advanced Ceramics and Composites A ed W M Kriven and H T Lin Ceramic Engineering and Science Proceedings 2008 vol 24 pp 69ndash74 Search PubMed

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electrohydrodynamic flow kinetic experiments and scaling analysis Phys Rev 2004 E69 0214051ndash0214058 Search PubMed (b) M G Song K J M Bishop A O Pinchuk B Kowalczyk and B A Grzybowski Formation of dense nanoparticle monolayer mediated by alternating current electric fields and electrohydrodynamic flows



J Phys Chem C 2010 114 8800ndash8805 CrossRef CAS Search PubMed 38 Y Solomentsev M Bohmer and J L Anderson Particle clustering and pattern formation

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(c) T Sunamori S Obana Electrophoretic Coating Process Using Alternating Current JP 51123243 A 19761027 Search PubMed (d) T Sunamori S Obana Electrophoretic Coating of Substrates by Alternating Currents 1976 JP 51055340 A 19760515 Search PubMed (e) T Sunamori Y Obana Alternating Current Electrophoretic Coating of Metal with Resins 1976 JP 51086544 A 19760729 Search PubMed (f) T K Kokai Electrophoretic Coating of Anodized Aluminum Substrates by Alternating Current 1980 JP 55054596 A 19800421 Search PubMed



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numbers Electrophoresis 2002 23 2012ndash2022 CrossRef CAS Search PubMed 92 S S Dukhin Electrokinetic phenomena of the second kind and their applications Adv

Colloid Interface Sci 1991 35 173ndash196 CrossRef CAS Search PubMed 93 N A Mishchuk S S Dukhin in Interfacial Electrokinetics and Electrophoresis ed A

Delgado Marcel Decker New YorkBasel 2002 pp 241ndash275 Search PubMed 94 A S Dukhin Biospecific mechanism of double layer formation and peculiarities of cell

electrophoresis Colloids Surf A 1993 73 29ndash48 CrossRef Search PubMed 95 B Neirinck J Fransaer O Van der Biest and J Vleugels Aqueous electrophoretic

deposition in asymmetric AC electric fields (AC-EPD) Electrochem Commun 2009 11 57ndash60 CrossRef CAS Search PubMed

96 B Neirinck J Fransaer J Vleugels and O Van der Biest Aqueous Electrophoretic Deposition at High Electric Fields Electrophoretic deposition Fundamentals and



applications III Book Series Key Eng Mater 2009 vol 412 pp 33ndash38 Search PubMed 97 M Ammam and J Fransaer Ionic liquid-heteropolyacid Synthesis characterization and

supercapacitor study of films deposited by electrophoresis J Electrochem Soc 2010 158 A14ndashA21 CrossRef Search PubMed

98 M Ammam and J Fransaer Two-enzyme lactose biosensor based on β-galactosidase and glucose oxidase deposited by AC-electrophoresis Characteristics and performance for lactose determination in milk Sens Actuators B 2010 B148 583ndash589 CrossRef CAS Search PubMed

99 M Ammam and J Fransaer Glucose microbiosensor based on glucose oxidase immobilized by AC-EPD Characteristics and performance in human serum and in blood of critically ill rabbits Sens Actuators B 2010 B145 46ndash53 CrossRef CAS Search PubMed

100 M Ammam and J Fransaer Highly sensitive and selective glutamate microbiosensor

based on cast polyurethaneAC-electrophoresis deposited multiwalled carbon nanotubes and then glutamate oxidaseelectrosynthesized polypyrrolePt electrode Biosens Bioelectron 2010 25 1597ndash1602 CrossRef CAS Search PubMed

101 M Ammam and J Fransaer Effects of AC-electrolysis on the enzymatic activity of glucose oxidase Electroanalysis 2011 23 755ndash763 CrossRef CAS Search PubMed

102 V Brisson and R D Tilton Self-assembly and two-dimensional patterning of cell arrays by electrophoretic deposition Biotechnol Bioeng 2002 77 290ndash295 CrossRef CAS Search PubMed

103 M Ammam and J Fransaer AC-electrophoretic deposition of metalloenzymes Catalase as a case study for the sensitive and selective detection of H2O2 Sens Actuators B 2011

160 1063ndash1069 CrossRef CAS Search PubMed 104 M Ammam and J Fransaer GlucoseO2 biofuel cell based on enzymes redox mediators

and Multiple-walled carbon nanotubes deposited by AC-electrophoresis then stabilized by electropolymerized polypyrrole Biotechnol Bioeng 2012 109 1601 DOI101002bit24438

105 G S Wilson and M Ammam In vivo Biosensors FEBS J 2007 274 5452ndash

546 CrossRef CAS Search PubMed

This journal is copy The Royal Society of Chemistry 2012


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have been addressed with their advantages and disadvantages It is found that compared to CDC PDC offers the advantage of i) reducing the coalescence between gas bubbles induced by water electrolysis from aqueous suspensions

hence yielding deposition of smooth and uniform coatings ii) reducing aggregation and disaggregation of nanometer sized particles leading to formation of uniform and homogenous deposits and iii) PDC generates low change in pH near the electrode thus it is convenient for deposition of biochemical and biological species in their highly active states The main disadvantage of PDC over CDC lies in the decrease of the deposition yield The latter can be more pronounced if low time-pulses are used Various categories of AC signals including symmetrical fields with no net DC component and asymmetrical AC signals without and with net DC component have been discussed Overall the deposition rate under AC fields increases with polarization time and amplitude With respect to frequency the deposition rate increases with frequency up to certain value then drops at elevated frequencies It is noted that deposition under AC signals offers the possibility to produce superior quality coatings from aqueous suspensions because electrolysis of water as well as particle orientation during the

deposition could be controlled From the application standpoint PDC and AC offers new application perspectives such as in biotechnology Because under modulated electric fields EPD can now be accomplished from aqueous

suspensions with low water electrolysis rates a variety of biochemical and

biological species can be deposited to yield highly active layers suitable for a wide range of applications including biosensors biofuel cells and bioreactors



Malika Ammam


1 Introduction


































zirconia




















2 EPD principals













Veph = μephE (1)












































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































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Malika Ammam


1 Introduction


































zirconia




















2 EPD principals













Veph = μephE (1)












































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

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f2 off



























zirconia




















2 EPD principals













Veph = μephE (1)












































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

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2 EPD principals













Veph = μephE (1)












































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



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Veph = μephE (1)












































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

f1 off

f2 off







































Elsevier




















23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

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23 EPD mechanisms




























31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

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31 PDC fields









to IV5
























































Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































rscorg



form1

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Elsevier













32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































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32 AC fields













































on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































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on the electrode54











































































voltages (eqn (6))





































5 Conclusions










Acknowledgements


References



















































































































































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voltages (eqn (6))





































5 Conclusions










Acknowledgements


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5 Conclusions










Acknowledgements


References



















































































































































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depositacion electroforetica dentro de campos electricos modulados

Education

deposition of biochemical

deposition rate

deposition process

deposition yield

ac fields http

symmetrical fields

yielding deposition

current ac