Thiol-Yne Adsorbates for Stable, Low-Density, Self-AssembledMonolayers on GoldChristopher A. Stevens,†,‡ Leila Safazadeh,† and Brad J. Berron*

Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046, United States

*S Supporting Information

ABSTRACT: We present a novel approach toward carboxylate-terminated, low-density monolayers on gold, which providesexceptional adsorbate stability and conformational freedom ofinterfacial functional groups. Adsorbates are synthesized throughthe thiol-yne addition of two thiol-containing head groups to analkyne-containing tail group. The resulting monolayers have twodistinct phases: a highly crystalline head phase adjacent to the goldsubstrate, and a reduced density tail phase, which is in contact withthe environment. The ellipsometric thickness of 27 Å is consistentwith the proposed structure, where a densely packed decanedithiolmonolayer is capped with an 11 carbon long, second layer at 50% lateral chain density. The Fourier transform infrared peak at1710 cm−1 supports the presence of the carbonyl group. Further, the peaks associated with asymmetric and symmetric methylenestretching are shifted toward higher wavenumbers compared to those of well-packed self-assembled monolayers (SAMs), whichshows a lower average crystallinity of the thiol-yne monolayers compared to a typical monolayer. Contact angle measurementsindicate an intermediate surface energy for the thiol-yne monolayer surface, owing to the contribution of exposed methylenefunctionality at the surface in addition to the carbonyl terminal group. The conformational freedom at the surface wasdemonstrated through remodeling the thiol-yne surface under an applied potential. Changes in the receding contact angle inresponse to an external potential support the capacity for reorientation of the surface presenting groups. Despite the low packingat the solution interface, thiol-yne monolayers are resistant to water and ion transport (Rf ∼ 105), supporting the presence of adensely structured layer at the gold surface. Further, the electrochemical stability of the thiol-yne adsorbates exceeded that ofwell-packed SAMs, requiring a more reductive potential to desorb the thiol-yne monolayers from the gold surface. The thiol-ynemonolayer approach is not limited to carboxylate functionality and is readily adapted for low-density monolayers of variedfunctionality.

■ INTRODUCTION

Self-assembled monolayers (SAMs) are commonly used totailor the physical and chemical properties of a surface, and theyare fabricated simply through the immersion of the substrate ina solvated adsorbate.1−3 SAMs are formed from individualmolecules, which adsorb to the substrate, and are stabilized by abalance of forces into a coating, which is largely free ofdefects.4−6 Thiolate/gold systems are among the mostcommonly employed monolayers, owing to a combination ofthe inert character of gold and the versatility of the thiolchemistry. Gold substrates have additional advantages of beingelectrically conductive and active to surface plasmon sensing,where thiol/gold monolayers have been particularly effective inbiological sensing7 or stimulation8 approaches. With thepopularity of thiol monolayers, the commercial availability offunctional thiol molecules has dramatically increased. Adsor-bates typically contain methylene chain regions between 6 and16 carbons long to enable stabilization through van der Waalsinteractions. Importantly, the lateral adsorbate density of linearalkane systems is typically dense and dictated by the chainpacking in the linear methylene region.9

Traditional monolayer techniques offer little capacity fortuning of functional group density. Low-density self-assembledmonolayers (LD-SAMs) have emerged as an approach towardincreased conformational freedom of functional groups overtraditional SAMs.10−12 In LD-SAMs, the interchain spacing ofthe molecular adsorbates is increased, leaving the resultingmonolayer lacking in the crystalline structure which ischaracteristic of traditional SAMs.13 The increased chainspacing of LD-SAMs provides a uniquely disordered environ-ment for physical and chemical processes. These coatings offerimproved binding of some adsorbates through intercalation ofligands into the monolayer,14−16 reduced confinement forchemical reactions within the monolayer,17,18 and molecularrearrangement in response to external stimuli.10,11 Theformation of LD-SAMs is challenging, owing to the exchangeprocess which typically produces defect free dense SAMs.16

Approaches involving short monolayer deposition time yieldislanded structures with little conformational freedom apartfrom edge sites.4 Coadsorption of long and short chain

Received: December 24, 2013Published: February 10, 2014

Article

pubs.acs.org/Langmuir

© 2014 American Chemical Society 1949 dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−1956

molecules yet again will result in islanded regions of long chainmolecules of high crystallinity.9,19

LD-SAMs on gold substrates are typically created through atwo-step process involving the adsorption of a SAM with alabile bulky tail group and the subsequent cleavage of the tailgroup, yielding a monolayer of lower packing density.11,13,16

This basic approach was first demonstrated by Lahann and co-workers in the cleavage of a polyaromatic group frommercapto-hexadecanoic acid, yielding carboxylate-terminatedLD-SAMs on gold.11,20,21 To date, the most convenientsynthetic approach is from the Frechette group,13 wherecarboxylate anions are paired with bulky cations prior toimmersion, and subsequent cleavage of the ion pair results in acarboxylate LD-SAM of similar structure to that of Lahann.10

The Jennings lab demonstrated the hydrolysis of an ester-bound fluorocarbon group to yield a hydroxyl terminated LD-SAM.16 Other groups have employed complex chemicalsynthesis to generate adsorbates with bulky head groups atthe thiol/gold interface, but these approaches have been limitedto methyl-terminated LD-SAMs.22 To date, the diversity of tailgroup functionalities available in LD-SAMs on gold is minimalwhen compared to their densely packed analogues.22−24

The increased interchain spacing of LD-SAMs comes at thecost of a drastic reduction in van der Waals interactions, amajor force in the stability of SAMs. The reduction instabilization was previously supported electrochemically, wherethe reductive potential required to cleave the adsorbed thiolatewas lower for the LD-SAMs than in classical SAMs of identicalchemical composition.25 The role of van der Waals forces inthis stabilization was also demonstrated by backfilling a thiolateinto an LD-SAM, which partially restored the electrochemicalstability of the LD-SAM to that of a well-packed SAM. Theinstability of LD-SAMs is also observed through restructuringof the monolayer over time. Previous work studied theinfluence of extended storage on the low density structuringof cleavage-based LD-SAMs, where monolayers stored inambient conditions showed dramatic rearrangement of theLD-SAM adsorbates over several weeks into islanded regions ofhigh adsorbate density.26 This surface migration process ishypothesized to be driven by an opportunity to increase in vander Waals interactions. This also demonstrates a possibility foreven backfilled LD-SAMs to migrate and phase separateprovided sufficient intermolecular driving forces.Here, we propose an adaptable approach toward LD-SAMs

of varied functionality and high structural stability with respectto time, temperature, and solvent. The described monolayeraccomplishes both stability and low interfacial density throughtwo distinct phases: a headgroup phase adjacent to thesubstrate, and a tail group which interfaces with theenvironment (Figure 1). Each adsorbed molecule has aninverted “Y” shape, where each chain in the tail phase is boundto two chains from the head phase, leading to formation of aloosely packed tail phase. The high density of the head phaselimits rearrangement of chains, preventing monolayer re-structuring and loss of tail group spacing. We expect thecovalent double binding of the head phase to the substrate andintermolecular forces between the adsorbates in the highlypacked head phase to provide these monolayers improvedstability even over that of traditional SAMs (Figure 1b).We utilize thiol-yne chemistry as a simple approach to

synthesizing branched adsorbates. Radical-mediated, thiol-ynereactions proceed rapidly in an orthogonal fashion toquantitatively yield the 1,2-addition product. While thiol-yne

reactions have been well studied in organometallic catalysis,27,28

application of thiol-yne click chemistry was limited in materialschemistry until very recently29−33 and is virtually unexplored inmonolayer chemistry. Thiol-yne reactions consist of a two-stepreaction in which individual thiols are added sequentially. First,an activated thiol (thiyl radical) is added to an alkyne, resultingin a vinyl sulfide, and the radical is transferred to a second thiolgroup.34 In the second step, a thiyl radical is added to the vinylsulfide product, yielding a 1,2-disubstituted product. The netreaction is chain transfer limited, as indicated by a near firstorder (0.8 power) dependency on thiol and a near zero order(0.1 power) dependency on alkyne.35

The Hoyle group demonstrated the orthogonality of thiol-yne grafting reactions as a powerful tool in functionalizeddendrimer synthesis.34 Their addition of functionalized thiols tofunctional alkynes (2:1 stoichiometry) resulted in quantitativeconversion and an absence of side products in a 10 min,solution phase reaction. Additionally, thiol-yne chemistry ishighly selective in the presence of acid, alcohol, silane, andother functional groups.34 Here, we demonstrate and character-ize a highly stable, low density monolayer that is acidterminated. The orthogonally of thiol-yne chemistry supportsthe extension of this general thiol-yne adsorbate strategy to awide range of other functional groups and applications.

■ EXPERIMENTAL SECTIONMaterials. 1,10-Decanedithiol (98%) was obtained from TCI

America. n-Hexane (>95%), 10-Undecynoic acid (95%), ethanol(>99.5%), dichloromethane (>99.8%), potassium hexacyanoferrate-(III) (>99%), potassium hexacyanoferrate(II) (>99.99%), sodiumsulfate (>99%),11-mercaptoundecanoic acid (MUA; 95%), and silicawere purchased from Sigma Aldrich (St. Louis, MO) and were used as

Figure 1. Design of thiol-yne adsorbates for low interfacial density andhigh monolayer stability. (a) Solution phase synthesis of a bifunctionaladsorbate through click-chemistry and subsequent adsorption. (b)Description of stabilizing forces in thiol-yne SAMs.

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561950

received. Irgacure-184 was used as received from Ciba SpecialtyChemicals. Deionized water was produced using an 18 MΩ Milliporewater purification system. Silicon wafers (P/Boron ⟨1−0−0⟩), 150 ±0.2 mm diameter, with thickness of 600−650 μm and resistivity of<0.4, were obtained from WRS Materials.Gold Substrate Preparation. Gold-coated silicon wafers with

chromium adhesion layers (100 Å Cr, 500 Å Au) were prepared usingHummer 8.1 DC sputter. Silicon wafers were plasma cleaned and thenplaced into the sputter system chamber, where chromium (100 nm)and gold (500 nm) were sequentially deposited onto silicon wafers.The gold substrates were typically cut to 1 × 3 cm, rinsed with ethanol,and dried under a stream of N2 prior to use.Synthesis of 10,11-Bis(10-mercaptodecylthio)undecanoic

Acid. The 10,11-bis(10-mercaptodecylthio)undecanoic acid adsorbatewas synthesized by mixing 1-undecynoic acid and 1,10-decanedithiol ata molar ratio of 1:4 in dichloromethane. Excess 1,10-decanedithiol wasused to ensure the reaction would go to completion and limitcyclization. Irgacure-184 photoinitiator was added at 3% the weight ofthe reagents. Just enough solvent was used to dissolve the reagents andthe photoinitiator. The solution was then exposed to 365 nm light withthe intensity of 10 mW/cm2 for 1 h at 25 °C and then solvent wasevaporated under a N2 environment, leaving only the oily product andexcess reactants. Separation was achieved using column chromatog-raphy with silica as the stationary phase and a 25% v/v solution ofacetone in hexane as the mobile phase, and eluted materials weremonitored by thin layer chromatography. The purified productsolution was then placed under a N2 environment to evaporate thesolvent. 1H NMR (CDCl3): δ 1.2−1.5 (36 H), 1.5−1.8 (12H), 2.34(2H), 2.45 (8H), 2.5−2.8 (3H). 13C NMR confirmed the presence ofthe branched point with a peak at 46 ppm. The use of an HSQC pulsesequence linked this 46 ppm carbon shift with the proton shift of 2.64ppm.Prepara t ion of Monolayers . The 10 , 11 -b i s (10 -

mercaptodecylthio)undecanoic acid product was reconstituted inpure hexane to a concentration of 1 mM. Gold substrates wereimmersed in this solution for 24 h at room temperature to form SAMs.The samples were then rinsed in 1 mM aqueous dithiothreitol for 5min. Samples were rinsed with ethanol, followed by a brief rinse withdeionized water and pure ethanol, and then dried with a stream ofnitrogen gas prior to measurement. Results shown are averaged fromat least three measurements on at least four samples. The reportedvalues are the average ± standard deviation.Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR

spectrometer used for analysis of the SAM samples is an Agilent 680with an MCT detector, which is equipped with a universal samplingaccessory for grazing angle analysis of thin organic coatings on metalsurfaces. For SAM measurements, spectra were collected using 100scans at an incident angle of 80° from the surface normal using aplasma cleaned gold substrate as a background.Electrochemical Impedance Spectroscopy (EIS). Electro-

chemical measurements were performed using a standard three-electrode electrochemical cell, with the SAM sample as the workingelectrode, an Ag/AgCl/saturated KCl reference electrode, and a baregold substrate as counter electrode. A flat cell (Princeton appliedresearch, model K0235) was used to expose 1 cm2 of the SAM coatedworking electrode to an electrolyte solution of 1 mM K3[Fe(CN)6], 1mM K4[Fe(CN)6], and 0.1 M Na2SO4. A Gamry potentiostat(Reference 600) was used to collect the EIS measurements. Datawere taken between 10−1 and 104 HZ and fit to a Randles equivalentcircuit to determine resistance and capacitance values.Spectroscopic Ellipsometry. SAM thicknesses were measured

with a spectroscopic ellipsometer (M-2000, J.A.Woollam Co.Inc.).Measurements were taken from 245 to 725 nm at incident angles of65°−75° with 2° increments. Coating thickness and optical constantswere fit to experimental data using a standard Cauchy model.Static Contact Angle Goniometry. Static contact angles of

deionized water on monolayer surfaces were manually measured with acontact angle goniometer (Rame-Hart model 100). Advancing andreceding contact angle measurements were taken for one side of adrop volume of approximately 5 μL.

Dynamic Contact Angle Goniometry. A standard three-electrode arrangement was used to apply a potential to themonolayer-coated gold substrate within a 5 μL, 0.1 M KCl drop(adjusted to pH 11 with KOH). The monolayer-coated gold substrateacted as the working electrode, a Ag/AgCl wire was used as a pseudoreference, and a platinum wire was used as the counter electrode(Figure 2). The potential of the gold substrate, relative to the Ag/AgCl

wire, was controlled using a Reference 600 potentiostat (GamryInstruments). The potential was switched between −0.1 V (reportedas negative potential in the text) and 0.290 V (reported as positivepotential in the text) with respect to the pseudo Ag/AgCl referenceelectrode. This applied potential is within the range of stability ofstudied monolayers on gold.11,36 The potential of the pseudo referenceelectrode was measured to be 0.20 ± 0.05 V with respect to a standardAg/AgCl reference electrode.

Reductive Desorption. Cyclic voltammetry measurements weretaken using the same apparatus as for EIS, but using a nitrogen purged,0.5 M KOH electrolyte solution. Current was measured while thepotential was cycled starting from 0.345 V to −1.545 V (vs Ag/AgCl),at scan rate of 100 mV/s, with at least two cycles. The position andarea of desorption peaks of the first sweep were measured. The valueof the density of alkanethiolate monolayers at the gold-monolayerinterface ΓAu‑SR is calculated as

Γ =−‐Q

nFAAu SRAu SR

(1)

where QAu‑SR is the total charge in the desorption peak, n is thenumber of electrons involved in the electron-transfer process (n = 1for this reaction), F is the Faraday constant, and A is the electrodesurface area exposed to the alkaline solution. The values of QBMUA andQMUA are determined by integration of the area under the current−potential curves obtained in 0.5 M KOH after compensating forcharging current and dividing by the sweep rate. Averages andstandard deviations are based on the analysis of at least nine samples ofeach monolayer.

■ RESULTS AND DISCUSSIONStructure of 10,11-Bis(10-mercaptodecylthio)-

undecanoic Acid Adsorbed on Gold. Analysis of nanoscalecoatings requires complementary analyses to support theproposed physical and chemical structure of the coating.Grazing angle FTIR is commonly used to provide insight intoboth chemical composition and structuring of thin films. 10,11-Bis(10-mercaptodecylthio)undecanoic acid monolayers on goldexhibit distinct peaks associated with asymmetric (∼2929cm−1) and symmetric (∼2854 cm−1) methylene stretching, aswell as the carbonyl-stretching peak (∼1714 cm−1) associatedwith the carboxylic acid tail group (Figure 3). We contrast thisstructure with that of a traditional, well-packed mercaptounde-canoic acid SAM, where the asymmetric and symmetricmethylene stretching peak positions of LD-BMUA are red-

Figure 2. Schematic illustration of advancing and receding contactangle measurement setup.

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561951

shifted from those seen in typical acid-terminated monolayers(2917 and 2848 cm−1, respectively).16 This red shift iscommonly interpreted as a decrease in overall crystallinity forthe methylene regions of materials, where the LD-BMUAmonolayer is more disordered on average than a standard wellpacked monolayer. This finding is consistent with other reportsof low-density monolayers, where asymmetric stretching ispreviously observed at 2931 cm−1 and symmetric stretching isobserved at 2860 cm−1.37 We further elucidate the structure ofthe LD-BMUA monolayer by analyzing the chain packing in a1,10-decanedithiol monolayer of similar expected structuring tothe lower region of the LD-BMUA monolayer. The methylenepacking in the dithiol monolayer is relatively crystalline, withasymmetric and symmetric peaks at 2923 and 2848 cm−1,respectively. When considering this well-packed base layer andthe overall low crystallinity of the LD-BMUA monolayer, themajority of the disordered character of this monolayer isexpected to be in the upper (environment interfacing) portionof the monolayer.Analysis of the ellipsometric thickness further supports the

proposed monolayer structure. The lower layer of themonolayer structure is expected to be of comparable thicknessas a standard 1,10-decanedithiol monolayer (∼19 Å). Theupper phase is expected to have one chain per every two chainsof the lower phase, resulting in 50% of the density of a well-packed monolayer of comparable chain length. As such, thenonsolvated monolayer thickness of the upper layer should behalf of the measured thickness of a mercaptoundecanoic acidmonolayer (monolayer ∼ 16 Å). Combining expected layerthickness for the upper (8 Å) and lower (19 Å) layers, wecalculate an expected thickness of ∼27 Å, which is in closeagreement with our measured value of 27 ± 2 Å for the LD-BMUA monolayer (Table 1).

For further refinement of the LD-BMUA monolayerstructure, we break the investigation into two discussions: (1)the monolayer’s upper layer interaction with the environment,and (2) the monolayer’s lower layer interaction with the goldsurface.

Monolayer Interaction with the Environment. Contactangle goniometry is a convenient means of exclusively probingthe outer 1 nm of a material’s structure. Here, we expect acid-terminated methylene chains at 50% of the surface coverage ofa well-packed mercaptoundecanoic acid monolayer. Priorreports of low-density acid monolayer surfaces with ∼50%surface coverage prepared by alternative methods have yieldedintermediate surface energies (θA ∼ 68°)38 which arecomparable to our new LD-BMUA surfaces (θA ∼ 70°).The Cassie equation16 provides an estimate of fractional

composition of a smooth surface by quantifying the relativecontribution of functional groups to the overall surfaceenergy.39 Here, we have adapted the Cassie equation todescribe the expected surface energy of our LD-BMUAmonolayers by examining the relative contribution of theterminal acid groups and the partially exposed methylenefunctionality of the underlying chains.

θ φ θ

φ θ

=

+ −‐cos( ) cos( )

(1 ) cos( )

LD BMUA COOH COOH

COOH CH2 (2)

where φCOOH denotes the fraction of the surface exposingcarbonyl functionality, θLD‑BMUA is the contact angle of themixed functionality surface, θCOOH is the contact angle of a purecarbonyl surface, and θCH2

is the contact angle on apolyethylene surface. When experimental values for the acidsurface (36°), the LD-BMUA surface (70°), and polyethylene(101°, from literature)40 (Table 2), are used in eq 2, we

estimate that ∼48% of the surface is covered by carbonylgroups and the remainder by methylene groups, furthersupporting the reduced surface density of the environmentinterfacing chains.The low density of the surface region of LD-BMUA coatings

is also evidenced by evaluation of the surface’s capacity forremodeling in response to external stimuli. The enhancedconformational freedom of low-density, acid terminated SAMshas been previously exploited to dynamically control surfaceenergy in response to an applied voltage, where densely packed

Figure 3. Representative FTIR spectra for the product low-density monolayer, 11-mercaptoundecanoic acid monolayer, and 1,10-decanedithiol self-assembled monolayer. The spectra have been offset vertically for clarity. (a) Methylene stretching region, υas(CH2) and υs(CH2); (b) carbonylstretching region.

Table 1. Ellipsometric Thickness of Monolayers

monolayer thickness (Å)

11-mercaptoundecanoic acid 16 ± 31,10-decanedithiol 19 ± 3LD-BMUA 27 ± 2

Table 2. Water Advancing and Receding Contact Angles forMonolayers on Gold

monolayer θA θR

11-mercaptoundecanoic acid 36 ± 4 13 ± 3polyethylene (PE)40 101 ± 3 no dataLD-BMUA 70 ± 3 35 ± 3

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561952

surface do not exhibit a dynamic response.25,41 Thus, a changein surface wettability will support our claim of a reduced chaindensity at the surface. The largest potential-dependent changein contact angle is typically observed in the receding angle,where it is postulated that a surface will restructure only in theregion of applied potential (wetted area), which will have agreater impact on the receding angle.41

Advancing and receding contact angle values of aqueous 0.1M KCl (pH 11) on either an LD-BMUA or an MUAmonolayer were measured at different applied potentials. Fivesubsequent cycles between −0.1 and 0.290 V (vs Ag/AgCl)were measured and are provided in Figure 4. In each applied

potential, the mercaptoundecanoic acid monolayer showedidentical receding contact angles, indicating little capacity forthe standard acid-terminated SAM to reconfigure in response toan applied potential. The receding contact angle of the LD-BMUA monolayer shows a repeatable voltage dependency of∼10° over five cycles (Figure 4). The magnitude of this changeis comparable to that of other low-density monolayers. In theLD-BMUA monolayer, the chain spacing is dictated by a 2:1ratio of chains in the lower phase to chains in the upper phase.This 50% chain density of carboxylate groups is directlycomparable to stimuli-responsive monolayers reported by theFrechette group (50%), where an ∼10° change in recedingcontact angle was observed under similar conditions.11,13,42

Monolayer Structure at the Gold Interface. Theproposed structure of the lower, gold-interfacing portion ofthe LD-BMUA monolayer is expected to be consistent withother well-packed monolayers in both resistance to water andion transport and in packing density at the surface. We usedelectrochemical impedance spectroscopy to gain insight intomolecular structuring and barrier properties of monolayer.Based on the fits of impedance spectra by the Randles model,estimates of the monolayer’s resistance and capacitance weredetermined and are compiled in Table 3 and Figure 5. The filmresistance of the LD-BMUA monolayer is expected to bedictated by the well-packed lower phase, as the upper, low-density layer should provide little barrier to charge transfer. Weestimate the resistance of a well-packed base layer through theuse of an analogous 1,10-decanedithiol monolayer (Rf ∼ 104.8 Ωcm2). We determined a comparable film resistance for the LD-BMUA monolayer (∼105.0 Ω cm2), supporting the presence of

a well-packed base layer. The capacitance, however, is expectedto roughly scale with the inverse of coating thickness forsimilarly structured monolayers, and the relatively thicker LD-BMUA monolayer has a lower capacitance (∼1.5 μF) than thedecanedithiol (∼2.6 μF) or the mercaptoundecanoic acid (∼3.4μF) monolayers.Reductive desorption was used to quantitatively determine

the density of chains at the gold surface through theelectrochemical reduction of the sulfur−gold bond. Using amonolayer-coated gold substrate as the working electrode, thepotential was swept at 100 mV/s from 0.345 to −1.545 V vsAg/AgCl in 0.5 M KOH while measuring the current (Figure6). The total charge required to desorb the monolayer is thendetermined and converted into a density of thiol−gold bonds(see Experimental Section).43,44 The surface coverages of thestudied monolayers are summarized in Table 4. The densitiesof chains at the gold surface of both our LD-BMUA (5.7 ± 0.6chains/nm2) and conventional, well-packed MUA monolayers(5.6 ± 0.4 chains/nm2) are equivalent. This well-packed

Figure 4. Cosine of receding and advancing contact angles whileapplying either −0.1 or +0.29 mV vs Ag/AgCl to a monolayer coatedgold electrode. Left side (a, c) is data taken using the LD-BMUAmonolayer on gold. Right side (b, d) is data taken using the well-packed MUA monolayer on gold. Error bars are smaller than symbols,typically ±3°.

Table 3. Values for Monolayer Capacitance and Resistance

monolayer log(Rf) (Ω cm2) Cf (μF/cm2)

1,10-decanedithiol 4.81 ± 0.61 2.55 ± 0.20MUA 4.81 ± 0.20 3.40 ± 0.31LD-BMUA 4.95 ± 0.24 1.50 ± 0.60

Figure 5. Electrochemical impedance spectra obtained in 1 mMK3Fe(CN)6 and 1 mM K4Fe(CN)6 in 0.1 M Na2SO4(aq) formonolayers on gold. Spectra are shown for LD-BMUA, densely packedMUA, and densely packed 1,10-decanedithiol. Experimental data areshown as symbols, where lines are fits of circuit models to the data.

Figure 6. Cyclic voltammograms of well-packed self-assembledmonolayer of 11-mercaptoundecanoic acid and LD-BMUA. Potentialcontinuously cycled between −1.545 and +0.345 V at a sweep rate of0.1 V s−1. Spectra are offset vertically for clarity.

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561953

structure at the gold interface is expected to provide exceptionalstability for this low interfacial density product monolayer andmitigate the rearrangement of the chains over time observed inother low-density monolayer systems.Stability of 10,11-Bis(10-mercaptodecylthio)-

undecanoic Acid Low Density Monolayers. Severalprevious studies examined the stability of low-densitymonolayers and have compared that to their well-packedcounterparts. Peng and Lahann21 examined the temporalstability of the low-density monolayers (LD-SAMs) bymonitoring the alkyl chains of LD-SAMs by grazing-angleFourier transform infrared spectroscopy as a function of time.Independent of storage conditions, their data suggested anincreased ordering of the alkyl chains for LD-SAMs and acorresponding localized loss in alkyl chain spacing.Luo and Frechette42 investigated the electrochemical stability

of LD-BMUA made from mercaptohexadecanoic acid (MHA).The position of the cathodic peak for the reductive desorptionprovides quantitative information on the stability of mono-layers.44 For instance, it was previously shown that a longeralkyl chain shift the reductive desorption peak to a morenegative potential, showing the stronger van der Waalstabilization among the adsorbed chains.43,45,46 Desorptionexperiments by Luo and Frechette indicate that LD-SAMs aresignificantly less stable than a full MHA monolayer, which iscaused by the relatively weak intermolecular interactions of theLD-SAMs.25

We expect the bifunctional nature of the adsorbate moleculeto work synergistically with the highly ordered packing of thehead phase to provide LD-SAMs improved monolayer stabilityover that of traditional well-packed monolayers. We comparedthe electrochemical stabilities of our LD-BMUA against that ofa well-packed MUA SAM on gold by reductive desorption.When the potential was scanned in the negative direction from+0.345 V, cathodic peaks corresponding to the reductivedesorption of the product LD-BMUA and MUA SAM wereobserved at −1.05 and −0.88 V, respectively (Table 4). Theposition of the cathodic peak, corresponding to desorption ofthe thiolate SAM, shifted negatively for the product LD-BMUAcompared to the MUA SAM, reflecting an increase in thestability of the product LD-BMUA compared to that of MUASAM. This higher stability further supports the proposedstructure of the product monolayer. The gold-interfacing phaseof the low-density product monolayer, having a highlycrystalline structure similar to MUA SAM, is stabilized by vander Waals interactions as well as covalent bonding between thethiol and gold substrate. Additionally, the two thiol-goldlinkages per adsorbate further increase the stability of the LD-BMUA over that of a traditional well-packed SAM. Thisstability is particularly interesting, given the typical instability oflow-density monolayers. The present work reverses the trend,enabling exceptional adsorbate stability for a low density ofenvironment-interfacing functional groups.

■ CONCLUSIONSA new synthetic approach was developed to create highlystable, carboxylate-terminated, low-density self-assembledmonolayers on gold. Adsorbate molecules were formed throughthiol-yne click-chemistry and were subsequently self-assembledon a gold substrate. FTIR, contact angle goniometry,spectroscopic ellipsometry, and electrochemical impedancespectroscopy measurements, and comparison of those toresults from well-packed monolayers of mercaptoundecanoicacid and decanedithiol, support the proposed structure for thethiol-yne SAMs. The lower crystallinity, enhanced thickness,intermediate surface energy, and interfacial restructuring underpotential all support the enhanced conformational freedom ofthe thiol-yne SAM interface. Cyclic voltammetry measurementssupport an enhancement in stability for the thiol-yne SAMsover well-packed MUA SAMs. Overall, these thiol-ynestructures offer exceptional stability while providing a uniformlyreduced interfacial chain density. Further, the specificity andsimplicity of the adsorbate synthesis makes this approachattractive for the synthesis of coatings with a low functionalgroup density for application in stimuli-responsive coatings andspecialized interfacial binding and reaction studies.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed description of surface density calculations fromreductive desorption peak area. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: brad.berron@uky.edu.Present Address‡C.A.S.: McKetta Department of Chemical Engineering, TheUniversity of Texas at Austin, Austin, TX 78712-1589.Author Contributions†C.A.S. and L.S. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSAcknowledgment is made to the Donors of the AmericanChemical Society Petroleum Research Fund (52743-DNI5) forpartial support of this research. The authors thank John Leytonat the UK NMR facility for assistance in analysis of the LD-BMUA product. The authors also appreciate the assistance withgold wafer fabrication from Brian Wajdyk and Jacob Hempel atthe UK Center for Nanoscale Science and Engineering.

■ ABBREVIATIONSSAM, self-assembled monolayer; LD-SAM, low-density mono-layer; LD-BMUA, low density surface composed of 10,11-bis(10-mercaptodecylthio)undecanoic acid; MUA, mercaptoun-decanoic acid

■ REFERENCES(1) Chaudhury, M. K.; Whitesides, G. M. How to Make Water RunUphill. Science 1992, 256, 1539−1541.(2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh,A. N.; Nuzzo, R. G. Comparison of the Structures and WettingProperties of Self-Assembled Monolayers of Normal-Alkanethiols on

Table 4. Reductive Desorption Analysis of Surface ChainDensity and Stability

monolayerQAu‑SR

(μC/cm2)density of chains at Au

(nm2/molecule)peak position

(V)

MUA 89.4 ± 6.2 0.18 ± 0.01 −0.88 ± 0.05LD-BMUA 91.1 ± 9.1 0.18 ± 0.02 −1.05 ± 0.01

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561954

the Coinage Metal-Surfaces, Cu, Ag, Au. J. Am. Chem. Soc. 1991, 113,7152−7167.(3) Whitesides, G. M.; Laibinis, P. E. Wet Chemical Approaches tothe Characterization of Organic-Surfaces - Self-Assembled Monolayers,Wetting, and the Physical Organic-Chemistry of the Solid LiquidInterface. Langmuir 1990, 6, 87−96.(4) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G. Y.;Jennings, G. K.; Yong, T. H.; Laibinis, P. E. In Situ Studies of ThiolSelf-Assembly on Gold from Solution Using Atomic ForceMicroscopy. J. Chem. Phys. 1998, 108, 5002−5012.(5) Jennings, G. K.; Munro, J. C.; Yong, T. H.; Laibinis, P. E. Effect ofChain Length on the Protection of Copper by n-Alkanethiols.Langmuir 1998, 14, 6130−6139.(6) Jennings, G. K. Stability, Structure, and Barrier Properties of Self-Assembled Films on Metal Supports; Massachusetts Institute ofTechnology: Cambridge, MA, 1998.(7) Prodromidis, M. I. Impedimetric Immunosensors-A Review.Electrochim. Acta 2010, 55, 4227−4233.(8) Khan, S.; Newaz, G. A Comprehensive Review of SurfaceModification for Neural Cell Adhesion and Patterning. J. Biomed.Mater. Res., Part A 2010, 93A, 1209−1224.(9) Bain, C. D.; Whitesides, G. M. Formation of Monolayers by theCoadsorption of Thiols on Gold - Variation in the Length of the AlkylChain. J. Am. Chem. Soc. 1989, 111, 7164−7175.(10) Olivier, G. K.; Shin, D.; Gilbert, J. B.; Frechette, J. DynamicResponse of Ion-Pair Monolayers. Abstr. Pap. Am. Chem. Soci. 2009,237.(11) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.;Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. A ReversiblySwitching Surface. Science 2003, 299, 371−374.(12) Cornell, W. D.; Cieplak, P.; Christopher, I. B.; Ian, R. G.;Kenneth, M. M.; David, M. F.; David, C. S.; Thomas, F.; James, W. C.;Peter, A. K. A Second Generation Force Field for the Simulation ofProteins, Nucleic Acids, and Organic Molecules. J. Am. Chem. Soc.1995, 117.(13) Olivier, G. K.; Shin, D.; Gilbert, J. B.; Monzon, L. A. A.;Frechette, J. Supramolecular Ion-Pair Interactions To ControlMonolayer Assembly. Langmuir 2009, 25, 2159−2165.(14) Choi, E. J.; Foster, M. D.; Daly, S.; Tilton, R.; Przybycien, T.;Majkrzak, C. F.; Witte, P.; Menzel, H. Effect of Flow on Human SerumAlbumin Adsorption to Self-Assembled Monolayers of VaryingPacking Density. Langmuir 2003, 19, 5464−5474.(15) Choi, E. J.; Foster, M. D. The Role of Specific Binding inHuman Serum Albumin Adsorption to Self-Assembled Monolayers.Langmuir 2002, 18, 557−561.(16) Berron, B.; Jennings, G. K. Loosely Packed Hydroxyl-Terminated SAMS on Gold. Langmuir 2006, 22, 7235−7240.(17) Schonherr, H.; Feng, C. L.; Shovsky, A. Interfacial Reactions inConfinement: Kinetics and Temperature Dependence of Reactions inSelf-Assembled Monolayers Compared to Ultrathin Polymer Films.Langmuir 2003, 19, 10843−10851.(18) Dordi, B.; Schonherr, H.; Vancso, G. J. Reactivity in theConfinement of Self-Assembled Monolayers: Chain Length Effects onthe Hydrolysis of N-Hydroxysuccinimide Ester Disulfides on Gold.Langmuir 2003, 19, 5780−5786.(19) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides,G. M.; Nuzzo, R. G. Formation of Monolayer Films by theSpontaneous Assembly of Organic Thiols from Solution onto Gold.J. Am. Chem. Soc. 1989, 111, 321−335.(20) Peng, D. K.; Ahmadi, A. A.; Lahann, J. A Synthetic Surface thatUndergoes Spatiotemporal Remodeling. Nano Lett. 2008, 8, 3336−3340.(21) Peng, D. K.; Lahann, J. Chemical, Electrochemical, AndStructural Stability of Low-Density Self-Assembled Monolayers.Langmuir 2007, 23, 10184−10189.(22) Garg, N.; Lee, T. R. Self-Assembled Monolayers Based onChelating Aromatic Dithiols on Gold. Langmuir 1998, 14, 3815−3819.(23) Shon, Y. S.; Lee, S.; Colorado, R.; Perry, S. S.; Lee, T. R.Spiroalkanedithiol-Based SAMs Reveal Unique Insight into the

Wettabilities and Frictional Properties of Organic Thin Films. J. Am.Chem. Soc. 2000, 122, 7556−7563.(24) Lee, S.; Shon, Y. S.; Colorado, R.; Guenard, R. L.; Lee, T. R.;Perry, S. S. The Influence of Packing Densities and Surface Order onthe Frictional Properties of Alkanethiol Self-Assembled Monolayers(SAMs) on Gold: A Comparison of SAMS Derived from Normal andSpiroalkanedithiols. Langmuir 2000, 16, 2220−2224.(25) Luo, M.; Frechette, J. Electrochemical Stability of Low-DensityCarboxylic Acid Terminated Monolayers. J. Phys. Chem. C 2010, 114,20167−20172.(26) Peng, D.; Lahann, J. Chemical, Electrochemical, and StructuralStability of Low-Density Self-Assembled Monolayers. Langmuir 2007,23, 10184−10189.(27) Yadav, J. S.; Reddy, B. V. S.; Raju, A.; Ravindar, K.; Baishya, G.Hydrothiolation of Unactivated Alkynes Catalyzed by Indium(III)Bromide. Chem. Lett. 2007, 36, 1474−1475.(28) Ogawa, A.; Ikeda, T.; Kimura, K.; Hirao, T. Highly Regio- AndStereocontrolled Synthesis of Vinyl Sulfides via Transition-Metal-Catalyzed Hydrothiolation of Alkynes with Thiols. J. Am. Chem. Soc.1999, 121, 5108−5114.(29) Lowe, A. B.; Hoyle, C. E.; Bowman, C. N. Thiol-Yne ClickChemistry: A Powerful and Versatile Methodology for MaterialsSynthesis. J. Mater. Chem. 2010, 20, 4745−4750.(30) Hoyle, C. E.; Lowe, A. B.; Bowman, C. N. Thiol-ClickChemistry: A Multifaceted Toolbox for Small Molecule and PolymerSynthesis. Chem. Soc. Rev. 2010, 39, 1355−1387.(31) Fairbanks, B. D.; Scott, T. F.; Kloxin, C. J.; Anseth, K. S.;Bowman, C. N. Thiol-Yne Photopolymerizations: Novel Mechanism,Kinetics, and Step-Growth Formation of Highly Cross-LinkedNetworks. Macromolecules 2009, 42, 211−217.(32) Park, H. Y.; Kloxin, C. J.; Scott, T. F.; Bowman, C. N. StressRelaxation by Addition-Fragmentation Chain Transfer in HighlyCross-Linked Thiol-Yne Networks. Macromolecules 2010, 43, 10188−10190.(33) Chan, J. W.; Shin, J.; Hoyle, C. E.; Bowman, C. N.; Lowe, A. B.Synthesis, Thiol-Yne “Click” Photopolymerization, and PhysicalProperties of Networks Derived from Novel Multifunctional Alkynes.Macromolecules 2010, 43, 4937−4942.(34) Chan, J. W.; Hoyle, C. E.; Lowe, A. B. Sequential Phosphine-Catalyzed, Nucleophilic Thiol−Ene/Radical-Mediated Thiol−YneReactions and the Facile Orthogonal Synthesis of PolyfunctionalMaterials. J. Am. Chem. Soc. 2009, 131, 5751−5753.(35) Fairbanks, B. D.; Sims, E. A.; Anseth, K. S.; Bowman, C. N.Reaction Rates and Mechanisms for Radical, Photoinitated Addition ofThiols to Alkynes, and Implications for Thiol-Yne Photopolymeriza-tions and Click Reactions. Macromolecules 2010, 43, 4113−4119.(36) Everett, W. R.; Ingrid, F.-F. Factors That Influence the Stabilityof Self-Assembled Organothiols on Gold under ElectrochemicalConditions. Anal. Chim. Acta 1995, 307.(37) Berron, B.; Jennings, G. Loosely Packed Hydroxyl-TerminatedSAMs on Gold. Langmuir 2006, 22, 7235−7240.(38) Olivier, G.; Shin, D.; Gilbert, J.; Monzon, L.; Frechette, J.Supramolecular ion-Pair Interactions to Control Monolayer Assembly.Langmuir 2009, 25, 2159−2165.(39) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Self-AssembledMonolayers of Alkanethiols on Gold: Comparisons of MonolayersContaining Mixtures of Short- And Long-Chain Constituents withMethyl and Hydroxymethyl Terminal Groups. Langmuir 1992, 8.(40) Lu, X.; Zhang, C.; Han, Y. Low-Density PolyethyleneSuperhydrophobic Surface by Control of Its Crystallization Behavior.Macromol. Rapid Commun. 2004, 25, 1606−1610.(41) Luo, M.; Gupta, R.; Frechette, J. Modulating Contact AngleHysteresis to Direct Fluid Droplets along a Homogenous Surface. ACSAppl. Mater. Interfaces 2012, 4, 890−896.(42) Luo, M.; Frechette, J. Electrochemical Stability of Low-DensityCarboxylic Acid Terminated Monolayers. J. Phys. Chem. C 2010, 114.(43) Sumi, T.; Uosaki, K. Electrochemical Oxidative Formation andReductive Desorption of a Self-Assembled Monolayer of Decanethiol

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561955

on a Au(111) Surface in KOH Ethanol Solution. J. Phys. Chem. B2004, 108, 6422−6428.(44) Sumi, T.; Wano, H.; Uosaki, K. Electrochemical OxidativeAdsorption and Reductive Desorption of a Self-Assembled Monolayerof Decanethiol on the Au(111) Surface in KOH+ Ethanol Solution. J.Electroanal. Chem. 2003, 550, 321−325.(45) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. Thermodynami-cally Controlled Electrochemical Formation of Thiolate Monolayers atGold: Characterization and Comparison to Self-Assembled Analogs. J.Am. Chem. Soc. 1992, 114.(46) Brett, C. M. A.; Kresak, S.; Hianik, T.; Oliveira Brett, A. M.Studies on Self-Assembled Alkanethiol Monolayers Formed at AppliedPotential on Polycrystalline Gold Electrodes. Electroanalysis 2003, 15,557−565.

Langmuir Article

dx.doi.org/10.1021/la404940q | Langmuir 2014, 30, 1949−19561956