X-ray Photoelectron Spectroscopy and Differential Capacitance Study

Sep 28, 2004 - Polymeric molecules containing multiple thiol groups (polythiols) provide tenacious attachment to metal surfaces such as gold. Polythio...
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Langmuir 2004, 20, 9621-9627

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X-ray Photoelectron Spectroscopy and Differential Capacitance Study of Thiol-Functional Polysiloxane Films on Gold Supports Patrick A. Johnson and Rastislav Levicky* Department of Chemical Engineering, Columbia University, New York, New York 10027 Received June 22, 2004. In Final Form: August 18, 2004 Polymeric molecules containing multiple thiol groups (polythiols) provide tenacious attachment to metal surfaces such as gold. Polythiol films are also well suited for subsequent derivatization with biomacromolecules through remnant free thiol groups of the film. In this study, 1-3 nm thick layers of a commercial polythiol, poly((mercaptopropyl)methylsiloxane) (PMPMS), are investigated with X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy. XPS is used to reveal the surface coverage of thiolate-Au bonds between the polythiol and the metal support, which is found to be approximately 30% lower than that in alkanethiol self-assembled monolayers. The surface density of thiolate-Au bonds did not depend on film thickness provided sufficient PMPMS material was present. Differential capacitance measurements show that the effective dielectric barrier presented by PMPMS films under aqueous environments corresponds closely to their physical thickness, with even ∼1 nm films remaining impermeable to electrolyte species. Modification of the films with an oligoethylene glycol compound was also examined, in anticipation of future applications in label-free, impedance-based biomolecular diagnostics.

Introduction The chemistry of noble metal surfaces is often tailored using self-assembled monolayers (SAMs) of organosulfur compounds.1 The classical example is provided by SAMs of linear alkanethiols or disulfides.2-4 These immensely useful and popular systems continue to be broadly exploited in fundamental studies of interfacial phenomena as well as applications, for example, involving electron transfer and nanolithographic fabrication.5,6 Less understood are self-assembled films of polymeric thiol compounds.7-12 Polythiols consist of oligomeric or polymeric chains containing thiol-functional monomer residues. One motivation for using polythiols is enhanced stability provided by cooperative binding through multiple chainsurface bonds.7,9 The reported dimensions of polythiol films are typically molecular, ranging from one to a few nanometers.8,9,11,12 An additional important aspect of polythiol films is the presence of remnant (unbound) thiol moieties which provide reactive sites for modification with organic or biological molecules, including proteins and nucleic acids.8,9 These features render polythiol films highly pragmatic for applications that require production of stable, reproducible biomolecular films. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457. (6) Rampi, M. A.; Whitesides, G. M. Chem. Phys. 2002, 281, 373. (7) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (8) Sun, F.; Grainger, D. W.; Castner, D. G.; Leach-Scampavia, D. K. Macromolecules 1994, 27, 3053. (9) Johnson, P. A.; Levicky, R. Langmuir 2003, 19, 10288. (10) Wink, T.; de Beer, J.; Hennink, W. E.; Bult, A.; van Bennekom, W. P. Anal. Chem. 1999, 71, 801. (11) Xia, N.; Hu, Y.; Grainger, D. W.; Castner, D. G. Langmuir 2002, 18, 3255. (12) Tsao, M.-W.; Pfeifer, K.-H.; Rabolt, J. F.; Castner, D. G.; Haussling, L.; Ringsdorf, H. Macromolecules 1997, 30, 5913.

In this report, films of a commercial polythiol compound, poly((mercaptopropyl)methylsiloxane) (PMPMS), on gold are studied and compared with alkanethiol SAMs. Because thiol groups on a PMPMS chain are interconnected through chemical bonds, their organization at the gold surface is subject to constraints that do not arise in alkanethiol SAMs. In particular, we find that the coverage of bound sulfur atoms, determined using X-ray photoelectron spectroscopy (XPS), is about 30% lower than in alkanethiol SAMs and that this coverage is independent of film thickness. A quantitative XPS model, corrected for surface roughness by calibration against octanethiol and dodecanethiol SAMs, is used for the calculations. The capacitance of PMPMS films of variable thickness is determined using electrochemical impedance spectroscopy (EIS). Despite the amorphous, liquid state of this polymer under ambient conditions, capacitance measurements indicate that chemisorbed PMPMS films act as welldefined dielectric barriers that inhibit ion penetration. Modification of PMPMS films with an oligoethylene oxide (EO) compound, of interest for passivating the surface against adsorption of biomolecules or other species, only modestly increases the effective dielectric barrier. These studies provide key information in support of future applications of polythiol films in electrochemical biodiagnostics measurements. Experimental Methods Sample Preparation. Glass slides were cleaned by immersion in hot “piranha” (70/30 mixture of concentrated H2SO4 and H2O2 (30%)) for a minimum of 20 min, rinsed thoroughly with deionized water, and dried with a nitrogen stream. WARNING: piranha solution is extremely oxidizing and should never be stored in tightly capped containers on account of gas evolution. Cleaned slides were coated with a 20 nm Cr adhesion sublayer and a 300 nm Au top layer in a thermal evaporator. Before depositing PMPMS or alkanethiols, thus prepared gold surfaces were placed for 20 min in a UV-ozone cleaner (Jelight Co., model 342) followed by 30 min of immersion in neat ethanol to reduce gold oxide that may have formed.13 The surfaces were washed with neat toluene solvent and transferred, without drying, into solutions of PMPMS (degree of polymerization ∼ 40; Gelest Inc.) in toluene. PMPMS

10.1021/la048458s CCC: $27.50 © 2004 American Chemical Society Published on Web 09/28/2004

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concentrations ranged from 0.001 to 10 mM monomer residues/ volume, with typical immersion times of 1 min. Following adsorption of PMPMS, slides were rinsed extensively with toluene and dried under a nitrogen stream. For preparation of alkanethiol SAMs, after UV-ozone cleaning and ethanol immersion supports were placed in 1 mM dodecanethiol (98+%, Aldrich) or octanethiol (98.5%, Aldrich) solutions in ethanol for 1 h. The surfaces were then washed with ethanol and dried. PMPMS-coated Au supports were either characterized directly or modified further by reaction with bis-maleimidotetraethylene glycol (BM(PEO)4; Pierce Biotechnology). BM(PEO)4-modified samples, denoted PMPMS-EO, were prepared by immersing PMPMS-coated supports in 1 mg/mL solutions of BM(PEO)4 in SSC1M buffer (0.015 M sodium citrate, 1 M NaCl, pH 7.0) for 1 h. This step reacts maleimide moieties of BM(PEO)4 with free PMPMS thiols. Prior to characterization with XPS or EIS, PMPMS-EO samples were washed with deionized 18.2 MΩ cm water and dried. Water for rinsing and buffer preparation was provided by a Millipore Biocell system. XPS Characterization and Analysis. XPS measurements were performed on a Physical Electronics PHI 5500 instrument equipped with an Al X-ray monochromatic source (Al KR line, 1486.6 eV) and a spherical capacitor energy analyzer. Elemental scans were carried out for Au 4f, C 1s, Si 2p, Si 2s, O 1s, S 2p, and N 1s at a 45° takeoff angle. Typical integration times were 30 s for Au and O, 1.5 min for C and Si, and 3 min for N using a pass energy of 58.70 eV. S 2p traces were obtained at high resolution with 11.75 eV pass energy and integration times of 80 min. Deconvolution of S 2p signals was used to determine the coverage of sulfur atoms participating in Au-thiolate bonds. This procedure exploits the dependence of S 2p binding energy on the sulfur’s chemical state. S 2p3/2 emission is close to 162 eV for bound thiolate but increases to near 164 eV for unbound sulfur.14,15 S 2p emissions, both for thiolate and unbound sulfur atoms, were modeled as a doublet of 2p3/2 and 2p1/2 contributions. The photoionization cross section of S 2p3/2 was constrained to be twice that of S 2p1/2, with a binding energy lower by 1.2 eV.15,16 Signals were deconvoluted with the program XPSPeak, with baselines represented by a combination of Shirley and linear functions. Peak shapes were analyzed as a sum of Gaussian and Lorentzian functions with 80% Gaussian component; this particular choice was found to well represent observed peak shapes under the instrumental settings used. The data analysis follows previously reported procedures.9,17 The following expressions18 were used for quantitative interpretation of XPS data: Intensity Ik from a solid support covered by an overlayer of thickness t:

[

Ik ) R(θ)kFkXkΛSk exp

]

-t ΛΟ k sin θ

(1)

Intensity Ik from a homogeneous overlayer of thickness t:

[

(

Ik ) R(θ)kFkXkΛO k 1 - exp

-t ΛΟ k sin θ

])

(2)

Intensity Ik from a monolayer of atoms covered by an overlayer of thickness t:

[

Ik ) R(θ)krσkXk exp

]

-t 1 sin θ ΛΟ sin θ k

(3)

In the above expressions, R(θ)k is the instrument response function at takeoff angle θ for spectral line k, Fk is the number (13) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. (14) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (15) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (16) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129. (17) Petrovykh, D. Y.; Kimura-Suda, H.; Whitman, L. J.; Tarlov, M. J. J. Am. Chem. Soc. 2003, 125, 5219. (18) Fadley, C. S. Prog. Surf. Sci. 1984, 16, 275.

density of atoms that emit into line k, σk is the surface density of emitting atoms (atoms/area), r is a roughness factor approximately equal to the ratio of true to geometric area (r g 1), Xk is the differential photoionization cross section, ΛSk and ΛO k are effective attenuation lengths (EALs) for photoelectrons in the solid support and overlayer, and t is the overlayer (e.g., PMPMS) thickness. The product rσk represents the effective surface density of atoms as seen by the instrument’s analyzer. A roughness correction is therefore required before the true surface coverage σk can be calculated from eq 3. The takeoff (grazing) angle θ is defined between the path of detected photoelectrons and the sample surface. Intensities I refer to baseline-corrected peak areas. EALs were approximated by photoelectron inelastic mean free paths (IMFPs). This approximation was checked against the NIST Electron Effective-Attenuation-Length Database19 by calculating corrections due to elastic scattering.20 For all experimental conditions of interest, the differences between elastic-scatteringcorrected EALs from the database and the corresponding IMFPs were less than 5%; for simplicity, IMFPs were used. IMFPs were estimated from quantitative structure-property relationships O O ) 5.1 nm, ΛSi2s ) (QSPRs) derived by Cumpson,21 yielding ΛAu4f O O 5.0 nm, ΛS2p ) 4.9 nm, and ΛC1s ) 4.6 nm. These values are for photoelectrons traveling through PMPMS. Number densities were obtained from mass densities (PMPMS, 0.97 g/cm3; Au, 19.3 g/cm3) and, in the case of PMPMS, a stoichiometry of C4H10OSSi per monomeric residue. The EAL of Au 4f photoS , was taken as 1.74 nm.19 electrons in the gold support, ΛAu4f Instrumental software was used to scale line intensities to the response function R and photoionization cross sections X. The thickness of PMPMS films was calculated from the experimentally determined ratio of Si 2s intensity from PMPMS to that of Au 4f from the substrate. This ratio, ISi2s/IAu4f, is given by dividing eq 2 by eq 1. The resultant expression was solved for PMPMS thickness t. For comparison, thicknesses were also calculated using the ratio IC1s/IAu4f. The C 1s derived t values exceeded by ∼0.3 nm those obtained from Si 2s intensities. This difference suggests the presence of carbonaceous contamination, as was also indicated by ∼25% excess of C over Si compared to the expected PMPMS stoichiometry of 4:1 C/Si. Because of the greater specificity of the Si signal for PMPMS, Si 2s derived thicknesses were employed. The thinnest films examined were discontinuous; for these samples, reported thickness values correspond to a hypothetical continuous layer containing the same amount of PMPMS. For alkanethiol SAMs, in the absence of a more specific XPS signature, IC1s/IAu4f intensity ratios were used to calculate t. The coverage rσB of Au-thiolate bonds was estimated from deconvoluted S 2p traces. Peak areas corresponding to bound Bound Unbound ) atoms were ratioed. The coverage (IS2p ) and unbound (IS2p Bound Unbound rσB was obtained by setting the IS2p /IS2p ratio equal to eq 3 divided by eq 2. This model treats bound sulfurs as lying within a plane at the gold/PMPMS interface. Unbound atoms were assumed to be homogeneously distributed within the PMPMS at a number density FU reduced to reflect atomic conservation, FU ) δPMPMS/m - rσB/t. Here, δPMPMS is the mass density of PMPMS, m is the mass of a monomeric residue, and t is the PMPMS film thickness. SAM rσ values were obtained from experimental IS2p/IAu4f ratios, using eq 3 divided by eq 1. For both types of SAM, only bound thiolate was observed. For consistency with PMPMS analysis, IMFPs for the SAMs were also taken from QSPRs,21 O O O yielding ΛAu4f ) 4.6 nm, ΛS2p ) 4.4 nm, and ΛC1s ) 4.0 nm. These IMFPs exceed by ∼10% those reported by Bain and Whitesides.22 It was numerically confirmed that (10% changes in IMFPs caused less than 3% difference in calculated thickness and thiolate coverages, both for PMPMS and SAM specimens. This weak dependence reflects cancellation of effects in the intensity ratios used for calculations. (19) Powell, C. J.; Jablonski, A. NIST Electron Effective-AttenuationLength Database, Version 1.0; National Institute of Standards and Technology: Gaithersburg, MD, 2001. (20) Jablonski, A.; Powell, C. J. Surf. Sci. Rep. 2002, 47, 33. (21) Cumpson, P. J. Surf. Interface Anal. 2001, 31, 23. (22) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670.

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EIS Measurement and Analysis. EIS measurements were performed under nonfaradaic conditions in SSC1M buffer as the background electrolyte. A three-electrode configuration was used, with the PMPMS- or alkanethiol-modified Au electrode (1.13 cm2 area) as the working surface against which a Viton O-ring was sealed using a cylindrical glass joint. A platinum coiled wire auxiliary electrode and an Ag/AgCl/3 M NaCl reference electrode were inserted through the open end of the glass joint. Impedance spectra were collected using a nominal 5 mV ac amplitude over a frequency range of 1-200 000 Hz and for dc offsets of -150, 0, and 150 mV versus the reference. Measurements were performed on a Parstat 2263 potentiostat/galvanostat/frequency response analyzer operated by PowerSuite software (Princeton Applied Research). EIS measured impedance was interpreted in the context of a circuit consisting of a resistance RB, representing the impedance of bulk electrolyte, in series with a constant phase element23-25 (CPE) representing the interfacial impedance. The impedance of a CPE is given by 1/((iω)nQ), where ω is the applied frequency (rad/s) and n (0 e n e 1) and Q are parameters that characterize the CPE. For an interface behaving as an ideal capacitor, n ) 1 and Q reduces to the interfacial capacitance with an impedance of 1/(iωQ). The physical origins of CPE behavior are most often attributed to the presence of microscale irregularities reflecting physical roughness or chemical heterogeneity of the interface. The arguments of Scheider demonstrate that CPE-like behavior can arise from a process in which interfacial charging progresses from local centers of ion accumulation (e.g., protuberances) through a lateral, branched spreading process of ions within the double layer.26 For an interface characterized by CPE behavior, an equivalent capacitance CE per area A is calculated from23

CE ) (RB1-nQ)1/n/A

(4)

For the PMPMS-covered Au electrodes of the present study, the exponent n was generally greater than 0.97, corresponding to differences between CE and Q of 25% or less. The exception was for discontinuous (thinnest) films for which n approached 0.95. The parameters n and Q were determined by first subtracting RB, obtained from the high-frequency asymptote of the magnitude of total impedance |ZTOT| (Figure 1a), from ZTOT(ω) to yield ZS(ω), the impedance attributed to the interface. The real Re(ZS) and imaginary Im(ZS) components of ZS are related to Q and n through eqs 5 and 6:

[

]

(5)

Q ) -sin(nπ/2)/(ωn Im(ZS))

(6)

n ) (2/π) arctan

-Im(ZS) Re(ZS)

From RB, n, and Q, CE was determined using eq 4 as a function of frequency ω. This approach, as opposed to fitting average values of n and Q to the entire frequency range, was used to allow for the possibility of frequency dispersion. Figure 2 plots typical data for PMPMS and PMPMS-EO films. For both types of film, the observed variation of CE with frequency is fairly slight. However, at lowest frequencies, a gradual downward curving was often evident, indicating that experimental response was not fully accounted for by CPE-like interfacial charging. Such a trend is commonly observed in EIS measurements and is usually attributed to slow electron-transfer processes that become noticeable at these frequencies.25,27,28 In the following, CE values are reported at that frequency for which the impedance response was most capacitive as given by a (23) Brug, G. J.; van den Eeden, A. L. G.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1984, 176, 275. (24) Kerner, Z.; Pajkossy, T. Electrochim. Acta 2000, 46, 207. (25) Protsailo, L. V.; Fawcett, W. R. Electrochim. Acta 2000, 45, 3497. (26) Scheider, W. J. Phys. Chem. 1975, 79, 127. (27) Sur, U. K.; Lakshminarayanan, V. J. Colloid Interface Sci. 2002, 254, 410. (28) Hinnen, C.; Nguyen Van Huong, C.; Rousseau, A.; Dalbera, J. P. J. Electroanal. Chem. 1979, 95, 131.

Figure 1. (a) Magnitude and (b) phase of ZTOT for Au electrodes covered with 0.26, 1.3, 1.5, 1.7, and 3.0 nm of PMPMS (the arrow indicates increasing PMPMS amount). The 0.26 nm film is discontinuous (see below). Electrolyte: 0.015 sodium citrate, 1 M NaCl, pH 7.0; electrode area: 1.13 cm2; ac amplitude: 5 mV; dc offset vs Ag/AgCl/3 M NaCl: 0 mV. The EIS spectra of PMPMS-EO films were similar.

Figure 2. Traces of capacitance CE (per geometric area) vs frequency as calculated from eq 4: filled symbols, unmodified PMPMS; open symbols, PMPMS-EO films. The XPS-determined amount of PMPMS in these samples was 0.26 nm (b, O), 1.3 nm (9, 0), and 1.7 nm (2, 4). The vertical scale differs by a factor of 20 before and after the break. Measurement conditions were as in Figure 1. minimum in the phase Φ ) arctan(Im(ZTOT)/Re(ZTOT)), typically around 10 Hz. Electrochemical Determination of Surface Roughness. Roughness r, representing the ratio of true to geometric area, was estimated from the capacitance of bare Au surfaces. The measured capacitance Cmeas was divided by Csmooth, the capacitance of a smooth Au surface,29 to yield r ) Cmeas/Csmooth. Cmeas was evaluated under following conditions: 0.05 M NaF, -0.6 V vs Ag/AgCl/3 M NaCl, 5 mV rms, 25 Hz. At these negative biases, the capacitance of Au surfaces under nonadsorbing electrolytes is only weakly dependent on the crystallographic orientation of (29) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1237.

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Johnson and Levicky Table 1. XPS-Derived Film Thickness and Thiolate Coverage Valuesa preparation conditions PMPMSb 0.001 mM, 1 min 0.01 mM, 1 min 0.1 mM, 10 s 0.1 mM, 1 min 0.1 mM, 1 min 0.1 mM, 1 min 1 mM, 1 min 1 mM, 5 min 10 mM, 1 min 10 mM, 1 min average octanethiolf dodecanethiolf

tS (nm)

tSi (nm)

0.36 1.2 1.2 1.2 1.5 1.3 1.7 2.1 2.1 2.5

0.36 1.3 1.2 1.4 1.5 1.5 1.7 2.2 2.3 2.5

N/A N/A

1.5 ( 0.1d 2.0 ( 0.2d

rσ (nm-2)

σ (nm-2)

1.6 4.8 4.0 4.1 5.0 4.0 4.5 4.4 4.2 4.7 4.4 ( 0.4 6.0 ( 0.2 6.0 ( 0.6

1.3c 3.8 3.1 3.2 3.9 3.1 3.5 3.4 3.3 3.7 3.4 ( 0.3 4.67e 4.67e

a Takeoff angle ) 45°. b Concentration of monomer residues, immersion time. c Unbound sulfur below detection. d Calculated using C 1s emissions. e Taken from ref 32. f Average of two samples.

Figure 3. S 2p photoelectron traces from 1.4 and 2.3 nm thick PMPMS films: filled circles, raw data; long-dashed lines, fitted peak doublet of thiolate sulfur; short-dashed lines, fitted peak doublet of unbound sulfur; red solid lines, total (summed) fit; “noisy” black solid lines, residual difference between raw data and calculated fit. The dotted vertical lines indicate the positions of bound and unbound S 2p3/2 emissions. the Au support and the electrolyte ionic strength.30,31 Csmooth was approximated from the literature as Csmooth ) 20 µF/cm2.28,29,31 The electrode area was 1.13 cm2. PMPMS Dielectric Constant. The dielectric constant of PMPMS, PMPMS, was determined from bulk capacitance measurements over the frequency range 10 Hz to 10 MHz (Material Sensing and Instrumentation, Inc.), yielding PMPMS ) 6.0.

Results and Discussion XPS Studies. The formation of Au-thiolate bonds drives the assembly of PMPMS films and underpins their stability. The surface density of these bonds was investigated as a function of layer thickness. Figure 3 compares S 2p spectra from 1.4 and 2.3 nm thick films. From the figure, it is evident that an increase in film thickness is accompanied by growth of the nonchemisorbed S intensity (S 2p3/2 binding energy ∼ 163.7 eV) relative to the bound signal (S 2p3/2 binding energy ∼ 162 eV).14,15 Figure 3 also illustrates the results of peak shape analysis, showing decomposition into contributions from bound thiolate and unchemisorbed sulfur atoms. Calculation of bound sulfur coverage from XPS data requires the thickness t of the PMPMS layer as input. The PMPMS thickness was calculated using a model that treats the layer as compositionally homogeneous and uniform in thickness. These assumptions are questionable, especially since compositional variations are expected to be present due to surface segregation effects, most notably chemisorption of sulfur at the gold interface. To estimate (30) Clavilier, J.; Nguyen Van Huong, C. C. R. Acad. Sci. (Paris) 1969, C269, 736. (31) Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 234, 93.

the effect of such compositional variations on the calculation of film thickness, the film thickness was estimated from both Si 2s and S 2p intensities (ratioed to Au 4f signals) as described in Experimental Methods. The depth profiles of S and Si atoms are expected to differ from one another. As listed in columns 2 and 3 of Table 1, it is reassuring that both methods produced fairly similar thickness values. These results suggest that compositional variations, while certainly present, do not strongly influence the estimation of t. Nevertheless, there was a tendency for tSi to exceed tS, an observation attributed to attenuation of the S signal due to preferential localization of a fraction of S atoms at the Au surface. Reasoning that backbone silicon should be more uniformly distributed than chemisorbing sulfurs, tSi was judged to be more representative of true film thickness and was the value used in subsequent calculations. The bottom two rows in Table 1 show thickness values for octanethiol and dodecanethiol monolayers. For these monolayers, a thickness can be estimated based on bond geometries following Bain et al.,4 assigning 0.15 nm to bound S, 0.18 nm to C-S, 0.11 nm to C-H, and 0.15 nm to C-C bonds, and correcting for sp3 carbon hybridization and a mean tilt of 30° between the molecular axis and surface normal.3 This estimate yields 1.2 nm for octanethiol and 1.6 nm for dodecanethiol. The measured XPS values exceed these estimates by 0.3-0.4 nm. As noted previously, this difference may reflect the presence of adsorbed contaminants that increased measured C 1s intensities. Uncertainty in attenuation lengths and tilt of the alkanethiol molecules represent additional sources of potential error. Roughness-uncorrected coverages rσ of bound S atoms, calculated as outlined in Experimental Methods, are listed in column 4 of Table 1. No systematic trend of rσ with film thickness is observed in the range of 1.3-2.5 nm, indicating that for these films packing of the mercaptopropyl side chains at the Au support is not a significant function of film thickness. A thin, ∼0.36 nm thick sample is also included. Only bound sulfur is observed in the S 2p trace of this sample (Figure 4). Modeling this sample as a 0.36 nm thick continuous film resulted in rσ of 1.6 nm-2. However, since such thin films are expected to be discontinuous (see below), a second calculation method was also used. In the second approach, the thiolate coverage is estimated from eq 3 on the presumption that attenuation of the S 2p signal by the PMPMS overlayer is negligible (thus setting t ) 0 in eq 3) since PMPMS

Polythiol Films on Gold

Figure 4. S 2p trace from a 0.36 nm thick PMPMS film. Data from an octanethiol SAM are shown for comparison.

coverage is low. Using an experimentally measured response function R,33 this method yielded rσ ) 1.7 nm-2. By dividing rσ values by the roughness factor r, it is possible to derive the true coverage σ of thiolate-gold bonds. The roughness factor r was calibrated against alkanethiol SAMs, since σ is known independently for these systems from diffraction studies.32,34 Using σ ) 4.67 nm-2 reported by Strong and Whitesides for docosanethiol monolayers32 leads to r ) 6.0/4.67 ) 1.28. For comparison, interfacial capacitance measurements on similarly prepared electrodes resulted in r ) 1.40 ( 0.12 (three independent samples). Roughness-corrected coverages σ (using r ) 1.28) are listed in column 5 of Table 1. The average (excluding the thinnest, discontinuous film) is σ ) 3.4 ( 0.3 nm-2, 27% lower than in alkanethiol SAMs. This decrease is attributed to constraints in the relative positioning of sulfur atoms due to their interconnection along the siloxane backbone. Such hindrances do not arise in alkanethiol monolayers. Significantly, a coverage of 3.4 thiolate-gold bonds per nm2 requires at least a 0.8 nm thick film. Films below this thickness will not be able to cover the surface at full coverage and are expected to be discontinuous. Previously, Tsao et al. analyzed a very similar film of PMPMS on Au.12 The sample investigated by these authors was 2.5 nm thick, for which they reported an intensity Unbound ≈ 1 at a takeoff angle of 90°. ratio of IBound S2p /IS2p Following the same analysis as for the present samples, Tsao et al.’s results translate to rσ ) 6.2 nm-2. However, since the roughness of the Au surface was not reported it is not possible to directly compare σ values. While binding of all thiol moieties on a PMPMS chain to the support is expected to maximize attachment stability, it also implies that functionalization (e.g., with biopolymers) that relies on availability of free thiol moieties might be hindered. Therefore, films somewhat thicker than 0.8 nm are expected to provide optimal surfaces for further chemical or biological modification. (32) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (33) R at the position of interest (e.g., S 2p, N 1s) was determined by interpolation from its values at Si 2p (100 eV), Si 2s (151 eV), and O 1s (531 eV). At these positions, R was measured from fused silica slides under identical instrumental settings. The slides were cleaned in the XPS chamber with an Ar ion beam until C 1s emission was negligible to achieve a clean, overlayer-free surface, the absolute emission intensity ISi or IO was measured, and eq 1 was applied to calculate R (with the overlayer thickness set to zero). (34) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421.

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Figure 5. Capacitance CE (expressed per true area using r ) 1.28) as a function of dc bias Vdc for bare Au and two PMPMS films of different thicknesses. The 0.26 nm film is discontinuous, whereas the 1.3 nm sample represents a continuous layer. Electrolyte: 0.015 sodium citrate, 1 M NaCl, pH 7.0; electrode area: 1.13 cm2; ac amplitude: 5 mV.

EIS Studies. EIS is attractive as a label-free biodetection technique.35-40 In such applications, it is essential to understand the EIS response of the nonbiological elements of the sensing interface in order to separate it from the signal of interest, which presumably derives from biomolecular interactions. For example, if a PMPMS layer is used to anchor the biological molecules, it is crucial that its impedance not overly dominate the total response. Conceptualizing the PMPMS film as a homogeneous layer with a dielectric constant  and thickness d, the capacitance C per area is

C ) 0/d

(7)

where 0 ) 8.85 × 10-12 F/m is the permittivity constant. As higher capacitance implies decreased impedance, for EIS-based biosensing it would be beneficial to keep d as thin as possible. Nonfaradaic EIS experiments were carried out to determine how capacitance varies with PMPMS film thickness, with CE calculated as described in Experimental Methods. Figure 5 compares capacitances CE from tSi ) 0.26 and 1.3 nm thick films for three different dc biases (Vdc) under SSC1M buffer. Based on the results of the previous section, a continuous PMPMS monolayer is expected to require a minimum thickness of 0.8 nm. Thus, the thinner film is expected to be discontinuous. The much greater capacitance of this sample is comparable to that of bare Au and is consistent with a discontinuous morphology, in which the absence of a PMPMS barrier over portions of the surface allows solution ions to approach close to the metal. Over such bare patches, capacitance will be dominated by double-layer properties. As organization of the double layer is a strong function of surface potential,41 for a discontinuous film one expects the capacitance to change in response to Vdc. Indeed, this is borne out in Figure 5. In contrast, such a dependence is (35) Lee, T.-Y.; Shim, Y.-B. Anal. Chem. 2001, 73, 5629. (36) Berggren, C.; Stalhandske, P.; Brundell, J.; Johansson, G. Electroanalysis 1999, 11, 156. (37) Berggren, C.; Bjarnason, B.; Johansson, G. Electroanalysis 2001, 13, 173. (38) Berney, H.; West, J.; Haefele, E.; Alderman, J.; Lane, W.; Collins, J. K. Sens. Actuators, B 2000, 68, 100. (39) Vagin, M. Y.; Karyakina, E. E.; Hianik, T.; Karyakin, A. A. Biosens. Bioelectron. 2003, 18, 1031. (40) Gheorghe, M.; Guiseppi-Elie, A. Biosens. Bioelectron. 2003, 19, 95. (41) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley & Sons: New York, 2000.

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Langmuir, Vol. 20, No. 22, 2004

Johnson and Levicky

Figure 7. N 1s traces from PMPMS-EO (b) and PMPMS (O) films. Table 2. Comparison of XPS- and EIS-Derived Film Thickness Values PMPMS

PMPMS-EO

octanethiol

dodecanethiol

tSia (nm)

db (nm)

db (nm)

tCa (nm)

db (nm)

tCa (nm)

db (nm)

1.3 1.5 1.7 2.8

1.3 1.4 2.0 2.8

1.5 1.9 2.2 2.7

1.5

1.1

2.0

2.0

a Calculated as in Table 1. b Calculated from eq 7, with  PMPMS ) 6.0 or SAM ) 2.3 (ref 3); Vdc ) 0 mV.

Figure 6. Modification of a PMPMS film (top) with BM(PEO)4 to produce a PMPMS-EO layer (bottom).

not evident for the 1.3 nm film, which is expected to be continuous and dominated by the capacitance of the PMPMS layer rather than that of the double layer. Of course, even for continuous films, a double layer is expected at the film-electrolyte interface. This implies that a Vdc-sensitive double-layer capacitance is in series with that of the PMPMS film. Under 1 M ionic strength, as in the present experiments, the double layer is expected to be collapsed to a few angstroms and its capacitance will therefore be high. Since the overall capacitance of two capacitors in series is dominated by the lower capacitance of the two, for surfaces covered by continuous PMPMS films CE may be expected to predominantly reflect the lower capacitance of the polymer film. This is confirmed by Figure 5, which shows that CE for the continuous film does not exhibit the sensitivity to Vdc expected for a double layer. In the following discussion, pertaining to continuous films, CE will therefore be taken as reflective of PMPMS capacitance with negligible double-layer contributions. The capacitive response of PMPMS films was investigated as a function of layer thickness, using samples with tSi of 1.3, 1.5, 1.7, and 2.8 nm. Each sample was first measured by EIS to determine CE. A portion of the sample was then reacted with the bis-maleimide cross-linker BM(PEO)4 to produce an ethylene oxide modified PMPMS-EO surface (Figure 6). It is expected that some BM(PEO)4 linkers attach through both imide termini to produce short EO loops (Figure 6), while others react through one end only. The CE was next measured from the modified, PMPMS-EO region. Finally, the samples were washed with deionized water, dried, and characterized by XPS on both unmodified PMPMS as well as PMPMS-EO regions. Figure 7 shows N 1s traces from PMPMS-EO and PMPMS films; as expected, a nitrogen signature consistent with the presence of maleimide residues is observed for PMPMS-EO.

Modification of PMPMS layers with BM(PEO)4 is motivated by two considerations: first, to cross-link individual PMPMS chains into a two-dimensional network, to further enhance layer stability, and second, to endow the surface with resistance to nonspecific adsorption of biomolecules. Surfaces modified with poly(ethylene oxide) are known to suppress adsorption of proteins as well as nucleic acids.11,42-44 The coverage of BM(PEO)4 residues was estimated from absolute N 1s intensities using eq 3 under the assumption of no attenuation (t ) 0) and an interpolated value for R.33 The determined coverage of 2.4 ( 0.5 × 1014 residues cm-2 is believed to be a lower limit since some attrition of the N 1s signal is expected. EIS-derived thicknesses d were calculated from CE values using eq 7 and the bulk dielectric constant of PMPMS, PMPMS ) 6.0, and compared to XPS-derived thicknesses tSi. The results for Vdc ) 0 mV are listed in Table 2. These values were within 6% of those for Vdc of -150 and 150 mV. Close agreement is observed between tSi and d for PMPMS films (Table 2, columns 1 and 2), indicating that the PMPMS layer acts as an impermeable barrier to solution species. Its higher capacitance when contrasted with alkanethiol SAMs of comparable physical thickness, noted previously,9 is seen to derive from the higher dielectric constant of PMPMS: PMPMS ) 6.0 versus SAM ) 2.3. Here, SAM has been approximated by bulk polyethylene.3 The effect of BM(PEO)4 modification on CE and hence d is difficult to predict in advance. While addition of BM(PEO)4 adds material to the interface, an effect expected to thicken the dielectric barrier, there is also the possibility that the water-soluble BM(PEO)4 residues help partially solvate the hydrophobic PMPMS layer. Such (42) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (43) Lee, J. H.; Kopecek, J.; Andrade, J. D. J. Biomed. Mater. Res. 1989, 23, 351. (44) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044.

Polythiol Films on Gold

solvation would be expected to assist permeation of water and ions into PMPMS-EO films, bringing about an increase in CE (decrease in d). The data in Table 2, column 3, indicate that, at least for the thinner films, BM(PEO)4 modification thickens the effective dielectric barrier. It is important to note that for PMPMS-EO films the listed d values do not reflect a physical thickness. Rather, these values correspond to the thickness of a PMPMS film that would reproduce the measured PMPMS-EO capacitance, as they were calculated by assuming the dielectric constant of PMPMS. Given a molar mass of BM(PEO)4 of 352 g/gmol, assuming a density of 1 g/cm3, and using the aboveestimated coverage of 2.4 × 1014 linker residues/cm2, the increase in physical thickness attributable to BM(PEO)4 modification is ∼1.4 nm. As the EO backbone is water soluble, it is not surprising that this increase is not fully reflected in the dielectric barrier d since the added BM(PEO)4 is expected to be permeable to electrolyte. The CPE n parameter increased monotonically with film thickness from 0.979 to 0.985 for PMPMS and from 0.979 to 0.987 for PMPMS-EO surfaces. Thus thicker films were closer to ideal capacitive behavior. In comparison, under equivalent conditions, octanethiol and dodecanethiol monolayers exhibited n of 0.999 and 1.00, respectively. These high n values presumably reflect the greater order of epitaxially self-assembled alkanethiol films compared to the amorphous PMPMS. The greater discrepancy between tC and d for octanethiol compared to dodecanethiol seen in Table 2 is in agreement with earlier work3 and is attributed, as in those studies, to the morepermeable, liquidlike ordering of the octanethiol monolayer. Finally, it is noted that impedance measurements on hydrophobic, methyl-terminated alkanethiol films on gold have indicated the presence of a hydrophobic gap at the film/electrolyte interface.27 The gap significantly decreased interfacial capacitance, and it is relevant to consider whether similar effects arise in this study. Importantly, a hydrophobic gap was only observed when SAMs were formed from neat alkanethiols27 and was absent when SAMs were assembled from 1 mM solutions as in the present work. These differences were explained by organization of the hydrophobic “surface”; in particular, disorder and defects inherent in films assembled from dilute solutions prevent formation of the gap. For a dodecanethiol film with a hydrophobic gap, Sur et al.

Langmuir, Vol. 20, No. 22, 2004 9627

reported a roughness-corrected capacitance of 0.34 µF/cm2.27 In comparison, the present study finds a roughness-corrected capacitance of 1.04 µF/cm2. The higher capacitance indicates that a hydrophobic gap was not observed for the monolayers studied here, consistent with Sur et al.’s conclusions regarding the importance of sample preparation conditions. In the case of PMPMS films, close agreement between XPS- and EIS-derived thicknesses suggests that, also in this case, a hydrophobic gap was not present. Conclusions. XPS measurements show that selfassembled films of the polythiol compound PMPMS on gold are characterized by a thiolate-Au bond density of 3.4 ( 0.3 nm-2. This value is approximately 30% less than that for alkanethiol SAMs, reflecting conformational constraints imposed on the thiol groups by polymer connectivity. Films thinner than about 0.8 nm cannot maintain this surface coverage and are therefore expected to be discontinuous. Electrochemical impedance spectroscopy measurements show that continuous PMPMS films behave as impermeable barriers with thickness equivalent to their physical dimension. Modification of PMPMS films with the cross-linker bis-maleimidotetraethyleneglycol decreases the interfacial capacitance only slightly. Therefore, this modification presents a potential approach to cross-link and thus to stabilize PMPMS layers as well as to passivate them against nonspecific adsorption of solution species, with only a modest penalty in terms of increased interfacial impedance. For exploitation of PMPMS films in impedance-based biodiagnostics, it will be necessary to monitor changes in a biomolecular layer anchored to the PMPMS. The present studies provide a starting point for pursuit of such applications. Acknowledgment. This work was supported by a National Science Foundation (NSF) CAREER award (DMR-00-93758) and has used shared experimental facilities supported primarily by the MRSEC Program of the NSF (DMR-02-13574) and by the New York State Office of Science, Technology and Academic Research (NYSTAR). Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. LA048458S