Damena D. AgonaferDepartment of Mechanical Science

and Engineering,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801

e-mail: agonafer@illinois.edu

Edward ChainaniDepartment of Chemistry,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801

Muhammed E. OrucDepartment of Chemical and Biomolecular

Engineering,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801

Ki Sung LeeDepartment of Mechanical Science

and Engineering,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801

Mark A. ShannonDepartment of Mechanical Science

and Engineering,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801;

Department of Chemical and Biomolecular

Engineering,

University of Illinois at Urbana-Champaign,

Urbana, IL 61801

Study of Insulating Propertiesof Alkanethiol Self-AssembledMonolayers Formed UnderProlonged Incubation UsingElectrochemical ImpedanceSpectroscopyThe electrochemical interfacial properties of a well-ordered self-assembled monolayer(SAM) of 1-undecanethiol (UDT) on evaporated gold surface have been investigated byelectrochemical impedance spectroscopy (EIS) in electrolytes without a redox couple.Using a constant-phase element (CPE) series resistance model, prolonged incubationtimes (up to 120 h) show decreasing monolayer capacitance approaching the theoreticalvalue for 1-undecanethiol. Using the CPE exponent a as a measure of ideality, it wasfound that the monolayer approaches an ideal dielectric (a¼ 0.992) under prolongedincubation, which is attributed to the reduction of pinholes and defects in the monolayerduring coalescence and annealing of SAM chains. The SAMs behave as insulators until acritical potential, Vc, is exceeded in both cathodic and anodic regimes, where electrolyteions are believed to penetrate the monolayers. Using a Randles circuit model for thesecases, the variation of the capacitance and charge transfer resistance with applied dcpotential shows decreased permeability to ionic species with prolonged incubation time.The EIS data show that UDT (methylene chain length n¼ 10), incubated for 120 h, formsa monolayer whose critical voltage range extends from �0.3 to 0.5 V versus Ag/AgCl,previously attained only for alkanethiol at n¼ 15. At low frequencies where ion diffusionoccurs, almost pure capacitive phase (�89 deg) was attained with lengthy incubation.[DOI: 10.1115/1.4007698]

1 Introduction

Interfacial transport at the micro and nanoscale plays a crucialrole due to high surface area to volume ratio of micro and nanosy-tems [1]. SAMs provide a unique way to control and functionalizethe interfacial properties at a solid liquid/interface. There havebeen numerous studies on the quality of organic SAMs as a block-ing mechanism for prevention of ion adsorption [2–7], with appli-cations ranging from biosensors to nanoscale transport. Studies oncharge selectivity have been done on electroless gold-coated poly-carbontate track etched membranes (PCTE) [8]. An alkanethiolwas used to prevent anion (e.g., chloride ions) adsorption to thegold surface. The membrane was immersed in 1 mM thiol solutionfor 24 h. However, it was observed that an irreversibility of chargeselectivity occurred due to the poor quality of the monolayer overthe gold-coated membrane surface. In addition to prevention ofion adsorption, the diffuse-layer potential at the monolayer sur-face can act as a screening mechanism for nanoscale transportapplications [9]. The model of an SAM-covered electrode–electrolyte interface includes the following features: a potential ofthe metal (um), a potential drop across the monolayer (assumed tobe linear), and a potential at the surface of the monolayer (us), apotential drop in the diffuse layer, all relative to some remotepoint in the solution (uref) [5]. For a smooth gold substrate,the zeta potential should be close to, if not coincident with the

diffusive double-layer potential [10]. As the ionic strength isincreased, the diffuse-layer potential becomes a linear function ofthe applied potential. A calculation of diffuse-layer potential forHO(CH2)14SH monolayer coated Au electrode in 1 mM electro-lyte resulted in a range of roughly 640 mV results from 6300 mVof applied potential [5]. Additional studies have shown the surfacecharge density of alkane monolayers to be dependent on the pH ofthe electrolyte solution [11,12]. The diffuse-layer potentialdecreases for the same applied potential as the ionic strength isincreased, due to the compaction of the diffuse layer.

The applications of SAMs are often limited by the quality ofthe monolayer. In the biosensor arena, SAMs on a metal electrodeallow the direct electron transfer of an enzyme immobilized onthe monolayer, facilitating the investigation of enzyme kineticsusing electrochemical techniques [13–18]. Ideally, the catalyticcurrent through the enzyme is detected as increased tunneling cur-rent through the monolayer at the enzyme’s Nernst potential, butconduction through pinholes (areas where metal is exposed to so-lution due to absence of monolayer) and collapsed sites may affectthe outcome. Studies have been performed to characterize howthe temperature of the thiol solution can affect the growth of themonolayer on a gold substrate. Using STM measurements, it hasbeen demonstrated that the vacancy island area does indeedchange over a wide range of temperatures, but the overall defectfraction area does not change [19,20]. As the temperature isincreased, the number of vacancy islands decreases, but the areaof each vacancy island increases. This is a result of the Ostwaldripening process, in which larger vacancy islands grow at theexpense of smaller gold islands [21,22]. Some of the contributions

Manuscript received April 16, 2012; final manuscript received September 12,2012; published online January 18, 2013. Assoc. Editor: Debjyoti Banerjee.

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that dictate how well a monolayer is grown can be determined bythe roughness of the SAM on the substrate.

Douglass et al. studied the roughness factor for electrodes thathad dodecanethiol grown on a gold substrate [23]. It was deter-mined that the macroroughness (�10 nm) of the substrate influ-enced the capacitive nature of the system, while themicroroughness (1–10 nm) had a larger effect on the quality andstructure of the SAM. In addition to monolayers containing defectsites (pinholes), monolayers may include collapsed sites that arepresent on the substrate. Studies have been done to try and distin-guish the properties of SAMs when defect or collapsed sites arepresent. The electron rate constant for a monolayer with a col-lapsed sites was found to be 2 orders of magnitude smaller thanthat for a defect site [24]. This is evidence that tunneling effectsoccur for regions within the monolayer that contain collapsedsites. The investigation of pH on the variation of charge transferresistance (Rct) and the apparent rate constant has been done usingEIS [25]. Furthermore, EIS measurements that are done over arange of potentials give insight to the permeability of the mono-layer to penetration by solution ions. Ion penetration into themonolayer manifests as a parallel conduction path from solutionto the electrode, necessitating a shift in the model from the seriesCPE-resistor to a Randles circuit composed of two resistors, oneof which is in parallel with a capacitor. The positive and negativepotential limits where the shifts occur are termed as the criticalvoltages Vc. Studies have shown that the critical voltage range canbe dependent on the length of the alkane chain [26]. Boubour andLennox studied the critical voltages for linear chain alkane thiols[26], where it was found that the critical voltage range was chainlength dependent. For an alkanethiol of chain lengths n¼ 7–15,the Vc ranged from �0.15 V to 0.3 V, but Vc for chain lengthsbelow n¼ 15 were all less than �0.3 V. The greater impermeabil-ity of longer chains may be ascribed to increased chain–chaininteractions [27]. These attractive lateral interactions, driven byvan der Waals forces, generally increase with chain length [28].The use of EIS to study the quality of a monolayer for differentimmersion times has been done to explore the evolution of SAMsover different time scales. Diao et al. studied self-assembledmonolayers of octadecanethiol (ODT) on gold by EIS with a re-dox couple in order to make a quantitative analysis of the changeof the fractional coverage, pinhole size, and separation as a func-tion of incubation time [29]. The range of incubation times in thatstudy was from 5 s to 24 h. Their results showed that the defectivemonolayers exhibited Warburg impedances at low frequencieswith a redox couple in solution, an indication that the ions weremass transport limited while diffusing through the defect sites.Monolayer formation kinetics were characterized by an initial faststep (chemisorption) followed by a slow step of surface coverage(through lateral bonding), and the size of the pinholes remains thesame, but the number density decreases over time. However,because redox species were used, it is difficult to understand theinsulative behavior of a monolayer when charge transfer resist-ance is always present due to tunneling effects. In another investi-gation utilizing EIS, experiments were performed without a redoxcouple in the electrolyte to study the quality of a monolayer fordifferent incubation times of a gold electrode immersed in dodec-anethiol (CH3(CH2)11SH) solution [30]. A minimum of 40 h wasfound necessary to achieve phase shifts u��88 deg. For an idealinsulating (capacitive) monolayer, u¼�90 deg is expected. TheEIS measurements for the monolayers incubated at various timeswere done at a single applied potential of 0 V versus Ag/AgCl.Because ion penetration is exacerbated at anodic and cathodicpotentials, it is important to understand how the monolayer incu-bation time can affect the insulative properties of a monolayerover a range of voltages [2,3]. Furthermore, as the applied poten-tial approaches the potential of zero charge (PZC), a field caninduce a torque on the headgroup, resulting in conformationalchanges in the SAM [31]. These conformational changes can leadto a lower barrier for ion penetration through the monolayer tooccur. It is therefore imperative to understand the incubation time

of an SAM. This work explores how incubation time on growingthe monolayer can affect the critical voltage range.

2 Experimental Techniques

2.1 Electrochemical Instrumentation and Measurements.Electrochemical measurements were performed using a GamryReference 600 (Gamry Instruments, Warminster, PA) employinga three-electrode cell: The monolayer-coated gold surface acted asthe working electrode, an Ag/AgCl wire in 3 M NaCl (Bioanalyti-cal Instruments, West Lafayette, IN) as the reference, and a goldwire as the counter electrode. Gold wire counter electrodes (AlfaAesar, Ward Hill, MA) were used for the impedance measure-ments. Gold-coated microscope slides were mounted in a custom-designed cell. The gold layer was used as the working electrode.All potentials are reported with respect to the Ag/AgCl electrode.Phosphate buffer solutions (PB) were prepared as a ratio of mono-and dihydrogen potassium phosphate salts (Sigma-Aldrich, St.Louis, MO) in DI water serves as electrolyte. The total concentra-tion and pH of the electrolyte are reported. The electrolyte solu-tions were bubbled for 15 min with nitrogen to remove oxygenprior to electrochemical characterization. The pH of the electro-lyte solution was monitored before and after the experiments.Because each experiment was performed in less than 10 min, thepH of the solution did not change within the small time interval.

2.2 Preparation of Monolayer Surfaces. 1-undecanethiolwas purchased from Sigma Aldrich and dissolved in absolute etha-nol (Pharmaco-Aaper, Shelbyville, KY) at a concentration of1 mM. SPR-quality glass slides (EMF Corp., Ithaca, NY) coatedwith evaporated titanium (5 nm) and gold (100 nm) thin filmswere washed in a heated SC-1 bath (100 ml of DI water/25 ml ofH2O2/2 ml of NH4OH) and rinsed thoroughly with DI water (resis-tivity 18 MX cm) and ethanol. The clean gold surfaces were incu-bated in a sealed Petri dish for immersion times ranging from 3 h to5 days. The monolayer gold-coated substrates were first electro-chemically polished. Electrochemical polishing was done asfollows: In 0.1 M NaOH was deaerated with nitrogen gas for15 min, with two cycles being required. The first was a wide andfast sweep from �1.2 toþ 1.5 V versus Ag/AgCl at 1 V/s(21 segments were required to obtain a stable voltammogram). Thevoltage range applied was based on previous studies, which ensurea monolayer formation of gold oxide from whose electrochemicalsurface area is determined from the reduction current [32,33]. Sec-ond, without removing the electrode from solution, another (narrowand slow) sweep from �0.8 to 0.6 V versus Ag/AgCl at 0.05 V/s(21 segments) was performed. Cycling through these potentialsensures removal and formation of an oxide layer, which smoothensout the surface by removing an atomic layer of gold atoms. Thegold surface is subsequently rinsed with DI water and absoluteethanol. The Au substrates were then incubated in thiol solutions inthe manner described above. All monolayers were grown at roomtemperature. The monolayer-coated gold was rinsed with absoluteethanol and DI water and blow-dried with N2 before use.

2.3 Surface Characterization. AFM measurements of baregold substrates were made under ambient conditions using anAsylum MFP-3D (Santa Barbara, CA). AFM probe tips used werefrom BudgetSensors (Sofia, Bulgaria). Images were obtained inintermittent contact mode. Roughness was calculated using thebuilt-in software, IGOR PRO from Wavemetrics (Portland, OR).Samples were prepared by SC-1 cleaning, followed by electro-chemical polishing.

3 Results and Discussion

3.1 Determination of Electrochemical Surface Area. Thearea of the gold substrate was measured by oxygen adsorption in0.1 M NaOH, cyclic voltammetry (CV) at 50 mV s�1. The amount

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of charge necessary for complete surface coverage was reportedto be 677 lC cm�2 for polycrystalline gold [32]. The Au substratewas SC-1 cleaned for 30 min with 100:20:1 of DI water, H2O2,and NH4OH. The solution was deaerated with nitrogen for 15 min.Two CV scans were done between �0.061 V and 1.239 V (versusAg/AgCl) with a scan rate of 50 mV s�1. The total charge wasthen calculated by integrating the oxide peak area. The electro-chemical area was calculated to be 0.59 cm2.

3.2 AFM Measurements. The AFM measurements providedinformation on surface topography of the gold substrate. The mainparameter used in quantitative analysis of the surface is the rmsroughness. The rms roughness was measured to be 1.7 nm forelectrochemical polished substrates. The peak-to-peak values(maximum height difference) were 14 nm shown in Fig. 1. Thesevalues are consistent with those found in previous studies forgold-coated glass slides with a titanium adhesion layer [34],which were also used as substrates for monolayer formation.

3.3 EIS and CV Measurements. EIS and CV experimentswere done on a 1-undecanethiol monolayer that was immersed ina 1 mM thiol solution from 3 to 120 h. A gold-coated microscopeslide was used as the working electrode. The system was purgedwith nitrogen for 15 min before experiments were started. To ruleout the possibility of other redox species, several CV scans weredone which showed the absence of faradaic current with scan ratesfrom 100 mV s�1 to 1 V s�1 and voltage ranges from �0.5 V to0.5 V with only phosphate buffer as the electrolyte solution. Theionic strength (I) of the buffer solution was I¼ 0.0316 M.

Self-assembled monolayers of alkanethiols on gold have beenshown to strongly block electrochemical oxidation of gold as wellas electron transfer with redox couples in solution. The behaviorof the SAMs has been ascribed to their ability to limit access ofsolution-phase molecules (water, electrolyte ions, and redox mole-cules) to the electrode surface. The insulating abilities of thiolmonolayers on gold were demonstrated in studies that usedmethyl-terminated alkanethiols (11, 13, 15, and 17 methyl groups)could suppress gold oxide anodic stripping by 3–5 orders of mag-nitude [4,24]. The present study has confirmed that cyclic voltam-metry of bare gold surfaces in electrolyte containing no redoxspecies shows the peaks where gold oxidation and reduction ofthe oxide occur, as well as the double-layer region where there isonly charging current. The CVs compare well with bulk gold andgold sputtered on glass microscope slides. Upon the adsorption ofthiol monolayers on the gold surface, gold oxide formation andreduction are suppressed, as evidenced by the lack of peaks in theCV seen in Fig. 2, which compares two CV experiments between

a bare gold polycrystalline surface and of a gold substrate with amonolayer grown for several days.

Cyclic voltammetry of alkanethiol monolayer-coated electrodesin electrolyte solutions shows that the charging current drops dra-matically in comparison to the bare metal electrode and becomesconstant with electrode potential. This constant potential is con-sistent with the presence of a low dielectric constant between elec-trode and electrolyte. Equation (1) is an expression for thedifferential capacitance Cd, which is modeled by a series combi-nation of two capacitances, the monolayer capacitance Cm (whichreplaces the Helmholtz capacitance CH of the bare metal), and thediffuse-layer capacitance CD [35]. Monolayer capacitance is aproperty of the spacer length, while the diffuse-layer capacitanceis a function of the electrolyte concentration and applied potentialwith a minimum at the potential of zero-charge (Epzc), such that

1

Cd

¼ 1

CD

þ 1

Cm

(1)

To account for the Faradaic currents, investigators postulated thatelectron transfer could occur at pinholes (sites exposed to electro-lyte due to the absence of adsorbate) in the monolayers. Pinholesare thought to occur mainly at grain boundaries, steps, and kinksin polycrystalline gold. Even then, at these sites, the gold surfacemay be partially protected by a collapse of the monolayer, produc-ing a defect in the monolayer where molecules and ions canapproach the electrode surface at a distance shorter than the dis-tance of the SAM. The presence of pinholes and defect sites isevidenced by departure from perfect blocking behavior. Potentialdependent background currents are observed for gold-monolayer-electrolyte systems, even in the absence of redox species. Suchcurrents are explained by ion migration through pinhole defects inthe monolayer film or charge transport, through the monolayerphase [2]. Even in the absence of pinholes and defects, tunnelingthrough the full width of the monolayer contributes to a current.However, it is not easy to assess quantitatively the fraction of thesurface that is composed of pinholes or defects [7]. Electrochemi-cal impedance spectroscopy is used to determine the resistive andcapacitive nature of a system over a set of frequencies. It has beenused to characterize ionic transport through nanoporous mem-branes [36]. EIS has been applied to the detailed analysis ofthrough-film and at-defect electron transport paths in thiol-modified gold electrode in a solution containing a diffusing redoxspecies [3,6]. Several studies used EIS to show that a defect-freemonolayer obeys the Helmholtz ideal capacitor exhibiting a phaseshift �80 deg [28,37–42]. Further, the impedance spectra can be

Fig. 1 AFM measurement of bare gold substrate obtained inintermittent contact mode. The scan area shown is 1 lm by1 lm.

Fig. 2 Cyclic voltammograms of a bare polycrystalline goldsurface scanned (!) and an Au substrate with a “defect- free”thiol monolayer (n), which was grown for 5 days. The support-ing electrolyte used was 10 mM (I 5 0.0316 M) phosphate buffersolution, pH 7, and scan rate 1 V s21.

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fitted to an equivalent circuit of a solution resistance Rs in serieswith a CPE, which accounts for double-layer capacitanceand SAM capacitance. The impedance of the CPE model isexpressed as

ZCPE ¼1

Yo jwað Þ (2)

where Yo is the capacitance (F), x is the frequency of the appliedelectric field, and a is the constant which represents the ideality ofthe capacitor. The CPE is analogous to a distributed capacitance,with a being a measure of variation of the dielectric coating thick-ness, since we assume that the monolayer formation has no effecton gold electrode surface roughness [31]. Several theoretical mod-els have demonstrated that the roughness, or the degree of disor-der of a monolayer, can be estimated by the CPE exponent a[43–47]. The proof of the presence of the SAM can be observedfrom Table 1. The capacitance of the bare gold substrate isapproximately 1.4–60 times greater than the capacitances forSAMs grown from 3 h to 120 h. In addition, as shown in Table 1,the a value increases as incubation time of monolayer growthincreases. An ideal capacitor has a¼ 1, while a> 0.88 suggests anadequately smooth gold surface. As mentioned above, a CPEmodel fits better than a simple series RC model. For alkanethiolSAMs with n¼ 9 (1-dodecanethiol), a CPE of 1.36 to 1.67 lF/cm2

was reported over the range �0.2 to 0.4 V (versus Ag/AgCl) [26].Above 10�2 M electrolyte concentration the diffuse-layer capaci-tance is greater than 10 lF/cm2, so Cm dominates the interfacialcapacitance.

Figure 3 is a Bode phase plot that shows the insulative behaviorof monolayers for two different immersion times. For the mono-layer grown for 48 h, the EIS data show that the phase angle peaks

between 1 and 10 Hz at �68 to �75 deg for the electrolyte concen-trations of 1 mM and 10 mM. The dependency is due to the contri-bution of the diffuse-layer capacitance, which is strongly dependenton concentration. The model used here is from Gouy–Chapmantheory, where the capacitance of the space-charge between the elec-trolyte and the electrode is at a minimum at the potential of zero-charge and rises at higher and lower potentials. At the minimum, at25 �C with a z:z electrolyte, this capacitance is

CGC lF=cm�2� �

¼ 228 z c�1=2 (3)

where c* is bulk electrolyte concentration in mol L�1 [35]. For a1:1 electrolyte at 50 mM concentration, the double-layer capaci-tance is calculated to be 50 lF cm�2. This relatively high capaci-tance is characteristic of the gold-solution interface when it is notblocked from electrolyte by a monolayer, which corresponds topinhole areas in an SAM.

The monolayer-covered gold/solution interface forms a secondkind of interfacial capacitance. This capacitance (neglectingcharge on the head group) is dominated by the monolayer capaci-tance Cm, determined by the relative permittivity of the alkane-thiol chain and the film thickness d of the monolayer, such that

Cm ¼�o�r

d(4)

For a well-ordered monolayer, characterized by a permittivityvalue of around 2 expected for hydrocarbon chains and thicknessd� 2 nm, Cm values in the range of 1–1.8 lF cm�2 are expected,with longer-chain thiols having lower capacitance due to thehigher thickness [48]. Since Cm � CGC in the series capacitancemodel of the Guoy–Chapman–Stern model, the impedance of thesmaller capacitance Cm dominates as the frequency approacheszero (dc), and the measured CPE can be ascribed to Cm.

Collapse sites where the monolayer thickness is lower than thatof the well-ordered monolayer form a third type of capacitor.Since the electrolyte ions are blocked from approaching the goldsurface, but can approach the electrode much closer than the fullwidth of the monolayer, the measured CPE here would then belower than that of a bare electrode/electrolyte but higher than thatof the well-ordered monolayer. In the case of incomplete mono-layer coverage, with the possibility of pinholes, these threecapacitances coexist. With increased incubation time, it is reason-able to assume that with reorganization of the monolayer resultsin reduction of coverage by defects. In this case, we can expect adecrease in capacitance, as well as an increase in a values withincubation time implying that the monolayer thickness is becom-ing more uniform.

Therefore, for defect-free monolayers, the phase shift should beindependent of concentration due to the monolayer capacitanceexhibiting behavior similar to the Helmholtz capacitance, which isindependent of electrolyte concentration. A similar gold surfaceincubated for 5 days exhibited an �88 deg phase shift at 1 Hzshown in Fig. 3. At this frequency, the phase angle varied less than1.2% between 1 and 10 mM for the Au substrate immersed for sev-eral days. This is evidence of a monolayer that has minimal pin-holes and defects, due to the improved ordering of the monolayerafforded by prolonged incubation in thiol solution. It has beenreported that the structure of SAMs does not change significantlywhen exposed to a 1 mM solution for 12–18 h [28]. However, evi-dence has shown that immersion times of 7–10 days can reduce thenumber of pinholes in the SAM and the conformational defects inthe alkane chain to decrease [28]. Thus, for applications such as thestudy of electron transport by tunneling through monolayers orpreparation of surfaces that need to be well insulated from solutionredox species, longer immersion times for improved monolayerregularity and stability are needed. It is also important to note thatan inductance was present at high frequencies above 10 kHz for theEIS experiments with 1 mM electrolyte concentration. A survey of

Table 1 Model data for applied potential of 0.0 V versusAg/AgCl at various incubation times

Immersion time (h) Rs (X cm2) CPE (lS sa cm�2) a

0 (bare gold) 282 95 6 0.002 0.940 6 0.0093 269 68 6 0.008 0.944 6 0.00115 277 1.73 6 0.02 0.986 6 0.00448 280 1.6 6 0.02 0.992 6 0.004120 281 1.58 6 0.02 0.995 6 0.004

Fig. 3 Phase angle plot of undecanethiol SAMs formed at dif-ferent immersion times, with an applied potential of 20.2 V. Thephase angle at low frequencies of the 48-h grown monolayerexhibits a phase angle less than 288 deg, for 1 mM and 10 mMconcentrations. The substrate Agonafer 23 immersed in thiolsolution for 5 days shows a phase angle of 288 deg, an indica-tion of a monolayer without defects for 1 mM and 10 mMconcentrations.

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the literature was made as to find the causes of inductance in a cir-cuit with only capacitances and resistances. These effects havebeen divided into inductances that appear at low frequencies(<10 Hz) and those that appear at high frequencies (>100 kHz).For low frequencies, these have been ascribed to adsorbed inter-mediates [49] or relaxation of adsorbed anions on a metal surface[50]. However, these inductances appear at frequencies (1 and0.1 Hz) much lower than what we have observed (10 kHz). At highfrequencies, the usual suspects are stray inductances from wireleads. Indeed at these frequencies (>100 kHz), we do see an induc-tive phase shift. However, neither of the high or low frequencyexplanations account for the fact that the inductive phase in ourdata appears at 10 kHz, a frequency intermediate between low andhigh frequency situations above. Dinh et al. have suggested thatthis may be partly due to the potentiostat itself (capacitance at thereference input in conjunction with high capacitance and low resist-ance at the working electrode) at frequencies greater than about10 kHz [51]. Using various model circuits with the above character-istics, we were unable to recreate the peaked, positive phase shift at10 kHz by using model circuits; thus, we are continuing to investi-gate the matter. One other possibility is the coupling of the capaci-tance of the electrodes and wires to the Faraday cage where theexperiments on the monolayers were carried out [52].

To develop models for categorizing SAMs into those that aredefect-free or those having induced defects, the corresponding im-pedance data were fitted to equivalent circuits by using the algo-rithms built into the ECHEM ANALYST software package (Gamry,Inc.). Models were constructed using the built-in MODEL EDITOR.The Levenberg–Marquardt method was used to fit the data to theCPE-resistor circuit shown in Fig. 4(a). The results are shown inTable 1. The 1-Hz frequency was chosen for measurements due tothe relaxation times in which diffusion related phenomena occurfor alkane SAMs. This has been consistent with previous studies,which have characterized SAM layers on gold using EIS [30,38].

For a monolayer that does not exhibit purely capacitive behav-ior, a Randles circuit was used. Figure 4(b) is a Randles circuit,which includes a solution resistor in series with an RC circuit. TheRC circuit includes the double-layer capacitance of the surfaceand the charge transfer resistance, due in part to the ionic transportthrough the monolayer.

Taken as a whole, the results show that the CPE valuedecreases, while the a parameter approaches unity as the incuba-tion time increases. The high CPE (although lower than the baregold CPE value) at 3 h suggests the presence of defect collapsedsites, implying a thinner dielectric. The presence of pinholes lead-ing to a¼ 0.944 means that the electrolyte has access to the metal,further adding to the overall capacitance. A low a could alsoimply low uniformity of dielectric thickness. Increasing incuba-tion time anneals the alkanethiolate into a sufficiently uniform,very close to defect-free, blocking monolayer as evidenced by the

low CPE value and a approaching unity for 120 h. The schematicbelow (Fig. 5) shows the evolution of the SAM surface with incu-bation time. This is consistent with the results of kinetic studies ofalkanethiol adsorption as reviewed by Ulman [53] and Schreiber[27] where two distinct steps in the absorption kinetics wereobserved: a very fast step and a second slow step, which can bedescribed as a surface crystallization process. The kinetics of thesecond step is related to chain disorder, chain–chain interaction(van der Waals), and mobility of the surface chains. As previouslysuggested, the pinhole size remains invariant, while the number ofpinholes decreases with increasing immersion time in thiol solu-tion [29]. More importantly, however, the present study demon-strates that beyond 24 h, an improvement of the insulating

Fig. 4 Equivalent circuits of the thiol monolayer: (a) CPE in se-ries with a solution resistance, indicative of a “defect-free”SAM and (b) an equivalent Randle circuit of an SAM with poten-tial induced defects

Fig. 5 Schematic of alkanethiolate SAM evolution on a goldsurface over a range of incubation times

Fig. 6 Bode plot of undecanethiol applied at 0 V (versusAg/AgCl) for different incubation times

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character of the SAM is possible; and EIS is a sensitive tool formonitoring the state of the monolayer. Figure 6 shows a Bode plotof each monolayer at different incubation times. At 3 h, it is appa-rent that the number of defects and pinholes greatly affects the im-pedance at frequencies in the 1–10 Hz range in terms of bothmagnitude and phase.

Table 2 is a summary of results for incubation times of voltagesranging from �0.4 to 0.5 V (versus Ag/AgCl). It can be observedthat we were able to extend the critical voltage range to �0.3 Vfor an incubation time of 120 h. This was not observed in previouswork for alkane chains n< 15. The results of Table 2 also showthat beyond the critical voltage, the charge transfer resistanceincreases as the incubation time of the monolayer increased.Table 2 demonstrated that the critical voltage is dependent on notonly chain length but also the amount of time that the monolayeris grown. For monolayers grown for 120 h, the critical voltagewas extended to larger anodic and cathodic potentials, which inthis case is �0.3 V to þ0.5 V versus Ag/AgCl. The critical voltagebehavior of this shorter chain alkanethiol (n¼ 10) was previouslyachieved only for longer alkane chain lengths (n¼ 15) [26].

Figure 7 is a plot of the CPE and a values from the fit of theEIS data obtained for various thiol incubation times (15, 24, and120 h), and recorded at different dc potentials. The voltage rangesvary for each SAM, since SAMs grown for lower incubation timeshave lower resistances for ion conduction to occur through theSAM. The motivation behind these two plots is to show the poten-tial in which the SAM behaves as an ideal capacitor. Beyond thecritical voltage for each SAM, the CPE model is no longer valid.As incubation time is increased, the critical potentials areextended from �0.1 to þ0.2 V at 15 h and �0.3 to þ0.5 V at

120 h. This extends the useful range of the Au/SAM electrode foruse in biosensor applications where applied potentials are neededto induce signal transduction currents. Figure 8 shows the phaseshift from EIS measurements performed at a range of voltages. Ascan be seen, the monolayer grown for 15 h exhibits a small voltagerange before ion transport through the monolayer becomes rele-vant. The 48 h monolayer exhibits a larger potential range up to�0.2 V (versus Ag/AgCl). It can be seen that the phase shift is>�80 deg for cathodic potentials beyond �0.2 V (versusAg/AgCl). However, the monolayer grown over 5 days exhibits aphase shift ��80 for voltages ranging from �0.3 to 0.5 V (versusAg/AgCl). The authors of this study consider this to be evidencethat the number of pinholes within the monolayer has beenreduced, thus increasing the resistance for ions to transportthrough the monolayer.

The bimodal shape of the CPE curve for 120-h incubation is in-triguing, as it suggests two different PZC values. This behaviorimplies two sites governed by different charge environments. Themore negative of the two (�0.1 V) probably corresponds to thealkanethiol PZC; however, this is expected to be at more negativepotentials. The PZC of an n¼ 11 alkanethiol was reported to bein the vicinity of �0.37 V versus SCE [54], to �0.49 versusAg/AgCl [31,55] for UDT SAMs on Au(111). More studies arerequired to understand the reason why a shift in PZC is observed.The minimum at þ0.2 V probably corresponds to PZC of pinholesor defects where electrolyte can access small areas of bare gold.The PZC of Au(111) was measured by cyclic voltammetry in theabsence of specific adsorption (HClO4 and NaClO4, 0.1 M) to beabout 0.23 V versus SCE [38] or 0.275 V versus Ag/AgCl. Due tospecific adsorption of phosphate anions, we expect a shift toward

Table 2 Fitting parameters for the impedance measurements of SAMs incubated at differ-ent times, which are calculated using the Levenberg–Marquardt method

Immersiontime

E/V(versus Ag/AgCl)

CPE in series with resistorequivalent circuit bCPE (lS � sa � cm�2) a

Randles equivalentcircuit b

Rion (MX/cm2) Cdl (lF/cm2)

15 h 0.5 0.721 6 0.025 1.60 6 0.070.4 1.28 6 0.07 1.56 6 0.070.3 1.47 6 0.010 1.53 6 0.050.2 1.69 6 0.02 0.984 6 0.004 —a —a

0.1 1.64 6 0.02 0.988 6 0.004 —a —a

0 1.73 6 0.02 0.986 6 0.004 —a —a

�0.1 1.69 6 0.02 0.986 6 0.004 —a —a

�0.2 2.73 6 0.02 1.34 6 0.07�0.3 0.788 6 0.03 1.5 6 0.07�0.4 0.456 6 0.02 1.59 6 0.07

48 h 0.5 1.88 6 0.02 0.972 6 0.004 —a —a

0.4 1.73 6 0.02 0.977 6 0.004 —a —a

0.3 1.74 6 0.02 0.976 6 0.004 —a —a

0.2 1.68 6 0.02 0.983 6 0.004 —a —a

0.1 1.61 6 0.02 0.991 6 0.004 —a —a

0 1.60 6 0.02 0.992 6 0.004 —a —a

�0.1 1.55 6 0.02 0.992 6 0.004 —a —a

�0.2 1.56 6 0.02 0.988 6 0.004 —a —a

�0.3 1.26 6 0.02 1.46 6 0.07�0.4 0.573 6 0.02 1.57 6 0.05

5 days 0.5 1.73 6 0.02 0.988 6 0.004 —a —a

0.4 1.63 6 0.02 0.991 6 0.004 —a —a

0.3 1.59 6 0.02 0.992 6 0.002 —a —a

0.2 1.54 6 0.02 0.994 6 0.004 —a —a

0.1 1.55 6 0.02 0.994 6 0.004 —a —a

0 1.58 6 0.02 0.995 6 0.004 —a —a

�0.1 1.53 6 0.02 0.994 6 0.004 —a —a

�0.2 1.55 6 0.02 0.987 6 0.004 —a —a

�0.3 1.63 6 0.02 0.973 6 0.004 —a —a

�0.4 1.26 6 0.002 1.37 6 0.01

Note: Two equivalent circuit modes were used to describe the insulative behavior of the SAMs dependingon the potential applied to each SAM.aA CPE model (Fig. 4(a)) is used due to the insulative behavior of the SAM.

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more negative PZC as seen in the plot, where a minimum is seenat 0.1 V for 15 and 48 h incubations. It is interesting to note thatunlike the rest of the monolayers, the 120 h incubation positivePZC has shifted from 0.1 V to 0.2 V (versus Ag/AgCl). At thispoint, further comment is not warranted without additional data.The measured CPE corresponding to this PZC, however, is not anindication of the surface concentration of pinholes, as its capaci-tance is expected to be much higher than that of a full monolayer.

The high a values and the low CPE, as well as the ion flux for120 h incubated sample, are an indication of a crystalline, uniformmonolayer that has the lowest number of defects across the vari-ous incubation times.

4 Conclusion, Recommendations, and Further Work

We have shown that EIS is a sensitive probe of monolayer con-dition. Defects in the monolayer are readily detected as deviationsfrom capacitive behavior with the appearance of charge transferresistance and critical voltage behavior, and are evident even atthe usual incubation times reported in the literature. These resultshave ramifications for investigation and application of electro-chemical sensors based on immobilized enzymes on SAMs. EIS isa facile method of screening SAM electrodes for defects, espe-cially since the same equipment (potentiostat in CV or CA mode)used in the enzyme electrode studies is also utilized to check themonolayer (potentiostat in EIS mode). Short-chain thiols (n< 7)generally do not form well-ordered monolayers and sensors builton these thiols are expected to have high double-layer chargingfrom pinholes and background currents from redox species in so-lution, competing with enzyme catalytic current, and thus reduc-ing the sensitivity of the method [18,56]. Sensors made withlonger-chain thiols that were not allowed sufficient time to annealwill likewise suffer from background currents from pinhole con-duction [16]. Although the tunneling barrier at long-chain thiols(n� 14) limits the transduction current, the reduction in electrontransport rate is not as severe at medium chains and the ability toform highly ordered layers resulting in decreased background cur-rent is advantageous [17]. The larger range of working potentialsafforded by well-ordered monolayer resulting from lengthy incu-bation times allows a larger repertoire of enzymes with a widerrange of E0. For transport studies utilizing an SAM on a gold-coated nanoporous membrane, EIS can be used similarly as aprobe of the insulating property. Insulation is especially crucialfor short length scales found in microchannels and nanopores. Awider range of electrode potentials can be applied when the mono-layer is highly ordered, allowing a greater range of availablediffuse-layer potentials. Permeation of ions into a well-orderedmonolayer is reduced, extending the usable life of the device. Thepresent work has laid the groundwork for finding the limits ofapplied potentials on a blocking monolayer with minimal faradaiccurrent. In future work, studies will be done to control the diffuselayer potential at a monolayer-gold-coated nanocapillary arraymembrane (NCAM) interface, in order to achieve ion selectivity.

Acknowledgment

This work was supported by the NSF center WaterCAMPWS(The Center of Advanced Materials for the Purification of Waterwith Systems). The authors would like to thank Professor AlexScheeline (University of Illinois, Chemistry) and Kathleen Motse-good (Beckman Institute) for use of materials and facilities, andare grateful to Professor Andrzej Wieckowski and Professor JacekLipkowski for the helpful discussions. The authors would alsolike to thank Francis Limpoco, Scott Mclaren, and William Wil-son for support in AFM imaging at the Frederick Seitz MaterialsResearch Laboratory (MRL).

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