Journal of Colloid and Interface Science 342 (2010) 499–504

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Journal of Colloid and Interface Science

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Potential driven deposition of polyelectrolytes onto the surface of cysteinemonolayers assembled on gold

Wesley Sanders a, Mark R. Anderson b,*

a Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060-0212, United Statesb University of Colorado Denver, Department of Chemistry, Campus Box 194, PO Box 173364, Denver, CO 80217-3364, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 September 2009Accepted 15 October 2009Available online 21 October 2009

Keywords:Self-assembled monolayersElectrostatic depositionImpedance spectroscopy

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.10.033

* Corresponding author. Fax: +1 303 556 4776.E-mail address: mark.anderson@ucdenver.edu (M.

Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used toexamine the ability of potential applied to a substrate to create, in situ, conditions favorable for the elec-trostatic deposition of polyelectrolytes onto a gold substrate modified by the self-assembly of cysteine.Cysteine is a zwitterionic compound that, when confined to a substrate, has the ability to establish eithera net positive or a net negative interfacial charge, depending on the conditions. As such, cysteine modifiedinterfaces could possibly be used as a versatile substrate for deposition of either cationic or anionic poly-electrolytes. The potential of zero charge of a gold electrode modified by self-assembly with cysteine inthe presence of 0.10 mol L�1 KCl and buffered at pH 5 is found by differential capacitance measurementto be �0.12(±0.02) V vs. Ag/AgCl. When �0.05 V vs. Ag/AgCl is applied to the substrate (a potential posi-tive of the PZC) in the presence of different polyelectrolytes, both impedance spectroscopy and quartzcrystal microbalance data suggest the accumulation of anionic poly(sodium styrenesulfonate) alongthe cysteine modified interface. Conversely, when �0.40 V vs. Ag/AgCl is applied to the substrate(a potential negative of the PZC), experimental results suggest the accumulation of cationic poly(dially-dimethylammonium chloride).

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1. Introduction

Electrostatic and molecular self-assembly are relatively simplemethods that have been widely applied to create chemically mod-ified interfaces. Each of these methods take advantage of thethrough-space interactions that exist between molecules to drivetheir assembly onto solid substrates. This suggests that experimen-tal manipulation of these interactions may provide some measureof control over the assembly process, and potentially can be usedto control the structure and/or properties of the modified interface.

Several research groups have looked at the role that differentintermolecular interactions play in establishing the ensemblestructure and properties of self-assembled monolayers. Bain andWhitesides generated modified interfaces by mixing two differentmercaptans in the adsorption solution [1–3]. They found that thesurface composition did not linearly track the composition of theadsorption solution; rather, differences in the extent of thethrough-space interactions among the molecules played a signif-icant role in determining the composition of the interfacial layer.Subsequent studies by Hobara et al. [4–6] show that often withthese mixed monolayers, the different components of the adsorp-tion solution will phase segregate when they adsorb to the sur-

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R. Anderson).

face. They propose that the phase segregation can be used tocreate two-dimensional molecular patterns along the interface.These results show that the strength of lateral through-spaceinteractions help to establish the structure of self-assemblinginterfacial systems.

Others have effectively used through-space interactions tocreate three-dimensional interfacial structures by depositing poly-meric materials on top of the initial monolayer. Decher et al. takeadvantage of electrostatic interactions to deposit alternating layersof polyelectrolytes onto surfaces [7–10]. Electrostatic deposition isa versatile, robust method that has been applied for the depositionof a variety of ionic materials, including proteins and enzymes[11–13]. Hammond et al. leverage both lateral interactions andlayer-by-layer deposition by combining contact printing withelectrostatic deposition to create complex three-dimensionalstructures at interfaces [14–16].

Others show that experimentally modulating the through-spaceinteractions along an interface can change the structure and alterthe properties of the modified interface. For example, Willneret al. created a monolayer in which access to the substrate is mod-ulated by experimentally inducing an isomerization of the mole-cules that comprise the monolayer [17–20]. Upon isomerization,different steric interactions exist at the interface that alter thephysical properties (e.g. the permeability of electroactive speciesto the substrate surface) of the modified interface.

500 W. Sanders, M.R. Anderson / Journal of Colloid and Interface Science 342 (2010) 499–504

Our interest is in creating a modified interface and thenmanipulating the properties of the interface so that the structurecan be subsequently modified in a controlled, reproducible fash-ion [21,22]. We previously demonstrated that potential appliedto a substrate modified with a 3-mercaptopropionic acid mono-layer could be used to modulate the properties of the modifiedinterface to drive the electrostatic assembly of polycationicpoly(diallydimethyl ammonium chloride), PDDA, onto the modi-fied substrate [21]. In this application, potential applied to thesubstrate alters the interfacial interactions from conditions thatdo not favor polyelectrolyte deposition to conditions where poly-cation deposition is favored. We found that when potentials posi-tive of the potential of zero charge (PZC) were applied to thesubstrate, the cationic polymer PDDA deposits onto the modifiedsubstrate, while PDDA does not adsorb if potentials negative ofthe PZC are applied. This result was interpreted as being due tothe pH local to the interface becoming more basic when poten-tials more positive than the PZC are applied to the substrate,and the pH change causing the deprotonation of the terminal acidgroups. Once deprotonated, the anionic charge of the interfacecreates conditions favorable for the deposition of a cationicpolymer.

A cysteine monolayer has the ability to have either a netpositive or a net negative charge depending on the solution pH.We recently demonstrated that this property could be leveragedto electrostatically deposit either polycationic or polyanionic poly-mers onto a cysteine monolayer by adjusting the bulk solution pH[22]. In this manuscript, we explore using applied potential todrive the adsorption of either polycationic PDDA or polyanionicpoly(sodium styrenesulfonate), PSS, onto a substrate modified withcysteine.

2. Experimental

2.1. Chemicals

Cysteine, poly(diallyldimethylammonium chloride) and poly(sodium styrene sulfonate) were purchased from the AldrichChemical Company (Milwaukee, WI). Potassium hexacyanoferrate(II) trihydrate, potassium hexacyanoferrate (III), potassium hydro-gen phosphate monohydrate, potassium dihydrogen phosphatemonohydrate, phosphoric acid and potassium hydroxide were pur-chased from Fischer Scientific company. Poly(diallyldimethylam-monium chloride), PDDA, was low molecular weight, 20% inwater. Poly (sodium styrene sulfonate), PSS, was 20 wt.% in water.All chemicals were analytical grade and were used without furtherpurification. Unless given otherwise, all solutions were preparedwith water deionized with an 18 MX Milli-Q ion exchange filterfrom Millipore Incorporated.

2.2. Monolayer preparation

Two millimeter gold disk working electrodes (CH Instruments,Austin, TX) were polished with 0.05 lm alumina followed by son-ication in deionized water. The electrodes were then electrochem-ically cleaned in 0.5 mol L�1 sulfuric acid by cycling the potentialbetween 2.0 and �0.8 V vs. Ag–AgCl at 0.050 V/s for 25 completecycles. Following the electrochemical cleaning, the electrodes wererinsed with deionized water, dried in a stream of nitrogen, andthen immersed in a solution of hexane containing 0.005 mol L�1

cysteine for 15 min. After cysteine immobilization, the electrodewas rinsed with ethanol and deionized water. Reductive desorp-tion measurements were conducted with electrodes modified bythis method, and the surface coverage was found to be4.8(±0.9) � 10�10 mol cm�2.

2.3. Measurements

Electrochemical impedance measurements were conductedusing a CH Instruments (Austin, TX) model 604B electrochemicalanalyzer. The impedance measurements were performed in a stan-dard three electrode cell with the modified gold electrode servingas the working electrode, a platinum wire as the auxiliary elec-trode, and a Ag/AgCl reference electrode. The supporting electro-lyte consisted of 0.005 mol L�1 FeðCNÞ3�=4�

6 dissolved in 0.10mol L�1 KCl and a phosphate buffer system containing a total phos-phate concentration of 0.050 mol L�1 and adjusted to pH 5. Imped-ance data was obtained at frequencies ranging from 100,000 to0.1 Hz using a 5 mV amplitude sinusoidal potential modulationthat is centered about the formal potential of the redox couple.Quantitative estimates of the charge-transfer resistance are ob-tained by fitting the experimental data to the Randles equivalentcircuit using the nonlinear least squares fitting routines of the soft-ware package LEVM 7.0 (available from Solartron, www.solar-tronanalytical.com).

For the potential induced polyelectrolyte deposition experi-ments, cysteine modified electrodes were placed in an aqueousphosphate buffer solution (pH 5) that contains 0.10 mol L�1 KCland either 1.8 � 10�4 mol L�1 PDDA or 1.8 � 10�4 mol L�1 PSS (theconcentration is determined with respect to the monomer formulaweight) and subject to applied potentials that are either positive ornegative of the potential of zero charge for 60 s. The potential wasthen returned to the open circuit value and the substrate was thenremoved from this solution and placed into an electrolyte solutioncomposed of 0.005 mol L�1 FeðCNÞ3�=4�

6 , 0.10 mol L�1 KCl, and pH 5phosphate buffer for the impedance measurements. These experi-mental conditions were chosen to be consistent with similar exper-iments conducted previously with a gold substrate modified with a3-mercaptopropionic acid monolayer [21].

Quartz crystal microbalance measurements were performedwith an in-lab constructed QCM oscillator connected to an HPmodel 5334B frequency analyzer [23]. Quartz crystals from Inter-national Crystal Manufacturing (Oklahoma City, OK) having a1.3 cm diameter gold electrode and 5 MHz resonant frequencywere used as the substrate. Prior to cysteine immobilization ontothe crystal’s gold electrodes, the quartz crystals were immersedin piranha solution (3:1 concentrated H2SO4:30% H2O2) for lessthan one minute to clean the surface. They were then rinsed withwater, dried in a stream of N2, and immersed in a hexane solutioncontaining 0.005 mol L�1 cysteine. The quartz crystal frequencieswere measured in air for 15 min before and after the monolayermodified crystals were exposed to the different experimental con-ditions (described above). During the measurement, the crystal fre-quency was sampled at 1 min intervals. For each trial, themeasured frequency is the average of these 15 samples. The fre-quency changes reported are an average of measurements con-ducted for three different modified crystals.

3. Results

For all measurements, the solution is buffered at pH 5, a valueclose to the isoelectric point for cysteine [24]. At this pH, themonolayer should be net neutral and provide no driving force forthe electrostatic deposition of polyelectrolyte. Differential capaci-tance measurements are used to determine the potential of zerocharge for the cysteine monolayer modified Au substrate [25,26].From these measurements, the PZC is found to be �0.12(±0.05) Vvs. Ag/AgCl. This value is consistent with experimental PZC’s mea-sured with other monolayer modified Au electrodes [25,26].

Analogous to the observed behavior of 3-mercaptopropionicacid monolayers [21], by applying potential positive or negative

W. Sanders, M.R. Anderson / Journal of Colloid and Interface Science 342 (2010) 499–504 501

of the PZC we anticipated creating conditions that favor either anet negative or a net positive charge, respectively, of the cysteinemonolayer due to the potential induced changes to the interfacialpH. We previously demonstrated that altering the net charge ofthe cysteine monolayer by adjusting the bulk solution pH in theabsence of applied potential, the electrostatic deposition of eitherPDDA (when using basic pH) or PSS (when using acidic pH) couldbe accomplished [22]. Others have also shown that changes to bulkpH create electrostatic interactions at an ionizable interface capa-ble of driving the deposition of polyelectrolytes [8,16]. By analogy,we anticipate that interfacial pH changes brought about bychanges to the substrate potential should allow the electrostaticdeposition of an appropriately charged polyelectrolyte. Changesto the interfacial properties brought about by the deposition of apolyelectrolyte are monitored by impedance spectroscopy.

Fig. 1 shows the impedance response of a cysteine monolayermodified interface before (open squares) and after (closed squares)exposure of the interface to a solution containing 1.8 � 10�4

mol L�1 PDDA and a potential positive of the PZC (Eapplied = �0.05 Vvs. Ag–AgCl) applied to the substrate. Several research groups showthat electrostatic interactions between a Redox probe and themonolayer modified interface influence the measured impedance[27,28]. Under these experimental conditions, the impedance de-creases slightly after exposure of the interface to PDDA and posi-tive potential applied to the substrate. This result suggests thatthere is a small change in the properties of the interface after theapplication of potential; however, the observed impedance changeis not nearly as large as that found when conducting similar exper-iments with a 3-mercaptopropionic acid monolayer [29], nor is itas large as the impedance change found before and after exposureof the cysteine monolayer modified interface to a 0.10 mol L�1

NaOH solution containing 1.8 � 10�4 mol L�1 PDDA [22]. The mag-nitude of the impedance change found with the cysteine mono-layer when subjected to positive potentials in the presence ofPDDA suggests that only a small amount of the PDDA is deposited.

Potentials positive of the PZC were also applied to cysteine-modified substrates in the presence of the anionic polyelectrolytePSS, and the impedance subsequently measured (Fig. 1). Underthese conditions, the impedance is found to have a larger changeafter the application of potential than the impedance decrease seenwith exposure to PDDA. The increased impedance is characteristicof electrostatic repulsion between the FeðCNÞ4�=3�

6 redox probe andthe modified interface, and suggests that PSS deposition occurredwhen the positive potential was applied to the substrate. This

Fig. 1. Representative complex impedance plots obtained by electrochemicalimpedance spectroscopy measured with a gold electrode modified with a layer ofcysteine exposed to a 0.10 mol L�1 KCl solution buffered at pH 5 containing0.005 mol L�1 FeðCNÞ�4

6 =0:005 mol L�1 FeðCNÞ�36 (i) before (open squares) and after

(solid squares) application of a potential positive of the potential of zero charge inthe presence of 1.8 � 10�4 mol L�1 PDDA and (ii) before (open circles) and after(solid circles) application of a potential positive of the potential of zero charge in thepresence of 1.8 � 10�4 mol L�1 PSS.

was an unexpected outcome as we anticipated that the positive po-tential applied to the substrate would establish a pH local to theinterface that is basic of the bulk pH (buffered at pH 5) and createconditions that favor a net negative charge of the cysteine mono-layer. If the interface had a net negative charge, those conditionswould inhibit deposition of the polyanionic PSS, the oppositebehavior of what is observed experimentally.

The magnitude of the impedance change when measured withthese modified substrates is characteristic of either the amountof material deposited onto the monolayer, the charge density ofthe interface, or to some combination of these two parameters[27–29]. Quartz crystal microbalance measurements were con-ducted using these same experimental conditions to estimate theamount of PDDA and PSS deposition under this experimental con-dition. After a potential positive of the PZC is applied to the cys-teine-modified substrate in the presence of 1.8 � 10�4 mol L�1

PDDA, a frequency decrease of 1.4(±0.6) Hz is measured. This fre-quency change corresponds to deposition of 32(±14) ng of PDDApolymer deposited. In contrast, when the QCM experiment is con-ducted in the presence of 1.8 � 10�4 mol L�1 PSS, the frequency de-creases by 5.3(±0.4) Hz. This frequency decrease corresponds to adeposition of 124(±9) ng of PSS polymer. The results of the imped-ance and QCM experiments show that, when potentials positive ofthe PZC are applied to the substrate, deposition of the anionic PSSis favored relative to that of cationic PDDA. This behavior, com-bined with the impedance data, suggests that when potentials po-sitive of the PZC are applied to the cysteine monolayer modifedsubstrate, a positive charge along the monolayer/solution interfacedevelops.

The experimental evidence that PSS deposition is favored whenpotentials positive of the PZC are applied to the substrate suggeststhat a mechanism other than the applied potential influencing thesolution pH local to the interface (which in turn influences the netcharge of the monolayer) is at work. If this is the case, then thebehavior when potentials negative of the PZC are applied shouldalso be different from that expected if the local pH were controllingthe net charge of the cysteine monolayer. Fig. 2 shows the imped-ance of the cysteine modified interface before and after a potentialnegative of the PZC is applied to the substrate in the presence of1.8 � 10�4 mol L�1 PDDA. Under these conditions, the impedancemeasured with an anionic redox probe decreases after the applica-tion of the negative potential, consistent with an electrostaticattraction between the FeðCNÞ4�=3�

6 redox probe and the monolayermodified substrate. This EIS result is characteristic of an increase in

Fig. 2. Representative complex impedance plots obtained by electrochemicalimpedance spectroscopy measured with a gold electrode modified with a layer ofcysteine exposed to a 0.10 mol L�1 KCl solution buffered at pH 5 containing0.005 mol L�1 FeðCNÞ�4

6 =0:005 mol L�1 FeðCNÞ�36 (i) before (open squares) and after

(solid squares) application of a potential negative of the potential of zero charge inthe presence of 1.8 � 10�4 mol L�1 PDDA and (ii) before (open circles) and after(solid circles) application of a potential positive of the potential of zero charge in thepresence of 1.8 � 10�4 mol L�1 PSS.

502 W. Sanders, M.R. Anderson / Journal of Colloid and Interface Science 342 (2010) 499–504

the net positive charge of the interface, and suggests that the PDDAadsorbs to the substrate under these conditions. In contrast, whenpotential negative of the PZC is applied to the cysteine-modifiedsubstrate in the presence of anionic PSS (Fig. 2, open, before the po-tential is applied, and closed circles, after the potential is applied)the impedance does not significantly change.

The impedance results suggest that, when a potential negativeof the PZC is applied to the substrate, conditions that favor selec-tive adsorption of cationic polyelectrolyte are generated; and,when potentials positive of the PZC are applied, conditions that fa-vor selective adsorption of anionic polyelectrolytes are established.These impedance results are opposite of that expected if the poten-tial were only creating local pH conditions that change the netcharge of the interface by changing the relative ionization of theamine or carboxylic acid group of the confined cysteine molecules.

QCM experiments were again conducted to confirm the imped-ance measurements. When the modified quartz crystal is exposedto potentials negative of the PZC in the presence of PDDA, thefrequency decreases by 4.7(±0.6) Hz, corresponding to depositionof 110(±4) ng of PDDA. Under the same experimental conditions,except that the cysteine interface is exposed to anionic PSS, afrequency decrease of only 1.0(±0.5) Hz is measured, correspond-ing to deposition of 23(±12) ng.

The impedance and QCM results obtained when applyingpotential to the substrate of a cysteine modified interface in thepresence of cationic PDDA and anionic PSS are opposite of thatexpected by analogy to the behavior of 3-mercaptopropionic acidmonolayers when subjected to applied potential, and cannot beattributed to changes in the pH local to the interface altering thenet charge of the monolayer. Several reports show that potentialapplied to a substrate can be used to change the orientation of mol-ecules containing charged groups that are part of a modified inter-face [30–33]. Kong et al. propose that potential applied to thesubstrate changes the conformation of the 16-mercaptohexade-canoate molecules that make-up a low-density monolayer [30].In this study, they show that at positive applied potentials, the neg-ative carboxylate group is attracted to the substrate, and this reori-entation of the molecules that comprise the monolayer influencethe adsorption of proteins to the monolayer modified interface.Somorjai et al. also show by surface energy and second harmonicgeneration measurements that potential applied to the substratereorients the 16-mercaptohexadecanoate acid molecules of themonolayer [31]. In this application, they use potential to controlthe surface energy of the modified interface.

Reorientation of cysteine and homocysteine molecules that areconfined to an interface has also been reported by Brolo et al. [32]and Zhang et al. [33] In these reports, potential applied to the sub-strate created conditions that attracted either the cationic ammo-nium group or the anionic carboxylate group.

Because cysteine is zwitterionic over a wide pH range, smallchanges in the solution pH adjacent to the interface will only havea small influence on the net charge of the monolayer. Potentialapplied to the substrate, however, may create electrostatic condi-tions that can attract either the cationic ammonium group or theanionic carboxylate group toward the substrate as suggested by

Scheme 1. Schematic representation of the proposed influence of substrate

previous studies. If this reorientation were to occur, the oppositelycharged group of the confined amino acid will be oriented towardthe solution side of the interface (Scheme 1). For example, when apotential positive of the PZC is applied to the substrate, the carbox-ylate group is attracted to the substrate and the cationic ammo-nium group repelled by the interface. This reorientation causesthe cationic ammonium group to be exposed to the adjacent solu-tion. The presence of the exposed ammonium group at the inter-face creates conditions that potentially favor deposition of theanionic PSS, and inhibits deposition of the cationic PDDA. Con-versely, when potentials negative of the PZC are applied to the sub-strate, conditions that favor attraction of the positively chargedammonium group are created, and the cysteine molecules reorientand expose the negatively charged carboxyl group to the solution.These conditions then favor deposition of the cationic PDDA andinhibit deposition of the anionic PSS.

This description is consistent with our experimental data andwith reports in the literature. In order for the molecules presentat the interface to reorient with applied potential, the individualmolecules need sufficient space to change conformation. Reductivedesorption measurements for the cysteine monolayer, shown inFig. 3, yield a coverage of 4.8(±0.9) � 10�10 mol cm�2. This cover-age is lower than the value found for saturation coverage of ann-alkanethiol monolayer (7.8 � 10�10 mol cm�2), or the coveragefound by reductive desorption for monolayers prepared with 3-mercaptopropionic acid (7.1(±0.6) � 10�10 mol cm�2) [29,34]. Atthis coverage, each cysteine molecule occupies �0.34 nm2. Thisfootprint is smaller than the 0.56 nm2 required for the reorienta-tion of 16-mercaptohexadecanoic acid molecules to create a mea-surable surface energy change, as determined by Somorjai et al.[31]. Cysteine reorientation, however, does not require as muchstructural change as 16-mercaptohexadecanoic acid molecules tocreate a different net charge exposed at the monolayer–solutioninterface.

Comparison of the QCM results obtained with potential induceddeposition of polyelectrolyte onto the cysteine monolayer modi-fied interface to results obtained by adjusting the interfacial chargewith solution pH changes (in the absence of an applied potential)show that the mass of polyelectrolyte deposited by potentialinduced deposition is much smaller than the mass deposited whenadjusting the bulk solution pH (Table 1). The differences in themass of polyelectrolyte deposited under the two experimental con-ditions suggest that the cysteine monolayer interfaces are not thesame. This is contrasted with the behavior of a 3-mercaptopropi-onic acid monolayer evaluated under similar experimental condi-tions in which the applied potential and the bulk solution pHhad nearly identical impact on the interface [21]. The electrostaticdriving force for polyelectrolyte deposition created when potentialis applied to a substrate modified with a cysteine monolayer isconsiderably smaller than that caused by changes to the solutionpH. This result is consistent with the applied potential inducingthe confined cysteine molecules to reorient at the interface, ratherthan the applied potential significantly altering the net charge ofthe monolayer by influencing the solution pH local to the interface.Potential induced monolayer reorientation will not change the net

applied potential on the orientation of the confined cysteine molecules.

Fig. 3. Representative current vs. voltage curve for the reductive desorption of a cysteine monolayer self-assembled on a gold substrate. The reductive desorption experimentwas conducted in an 0.5 mol L�1 KOH solution.

Table 1Comparison of the mass deposited onto a cysteine monolayer modified gold substrate under different experimental conditions.

Experimental conditions Mass deposited (ng)

E = �0.40 V vs. Ag–AgCl, 1.8 � 10-4 mol L�1 PDDA 110(±14)E = �0.05 V vs. Ag–AgCl, 1.8 � 10�4 mol L�1 PSS 124(±9)Open circuit potential, 0.10 mol L�1 HCl, 1.8 � 10�4 mol L�1 PSS [21] 700(±20)Open circuit potential, 0.10 mol L�1 NaOH, 1.8 � 10�4 mol L�1 PDDA [21] 770(±40)

W. Sanders, M.R. Anderson / Journal of Colloid and Interface Science 342 (2010) 499–504 503

charge of the interface; rather, the structure change will reorientthe dipole of the cysteine molecules that comprise the monolayerand that will create conditions favorable for selective deposition ofeither polycationic or polyanionic species.

Examination of the impedance and QCM data also show thatsmall changes in the QCM frequency and the EIS impedance occureven for the conditions that do not favor the polyelectrolyteadsorption. This suggests that some amount of polyelectrolyte ad-sorbs to the interface even if the potential applied to the substratedoes not favor the electrostatic adsorption of that particular poly-electrolyte. This result also suggests that the orientation of thecharged groups on the cysteine modified interface does not provideabsolute selectivity for the deposition of either polycationic PDDAor polyanionic PSS. Rather, potential applied to the cysteine-mod-ified substrate establishes conditions for the preferential adsorp-tion of different polyelectrolytes.

4. Summary

In this research, we demonstrated that potential applied to thesubstrate of a cysteine modified interface can create conditionsthat favor the deposition of either polycations or polyanions,dependent on the value of the applied potential relative to thePZC of the modified substrate. The mechanism that creates theseconditions, however, is different from that proposed when apply-ing potential to monolayers of 3-mercaptoproprionic acid. Withthe cysteine monolayers, the electrostatic attraction between thecharged ammonium or carboxylate functional groups of the con-fined zwitterionic cysteine molecules and the substrate excesscharge creates interfacial conditions that favor the reorientation

of the cysteine molecules at the interface. The reorientation ar-ranges the molecules at the interface so that one of these func-tional groups is attracted to the substrate and the other isoriented away from the substrate, toward the adjacent solution.This electrostatic ordering of the interface then apparently favorsthe deposition of a polyelectrolyte whose charge is opposite thatof the group extending away from the surface. Although theamount of polyelectrolyte deposited is less than found when theinterfacial charge is adjusted by bulk solution pH changes, poten-tial applied to the cysteine-modified substrate clearly shows aselectivity for deposition of one charged polyelectrolyte over theoppositely charged polyelectrolyte.

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