Self-Assembly of Alkanoic Acids on Gold Surfaces Modified by

J. Phys. Chem. B 2001, 105, 4951-4955

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Self-Assembly of Alkanoic Acids on Gold Surfaces Modified by Underpotential Deposition Shu-Yi Lin and Chun-hsien Chen*,† Department of Chemistry, National Tsing Hua UniVersity, 101, Section 2 Kuang Fu Road, Hsinchu, Taiwan 300, R.O.C.

Yang-Chiang Chan, Chia-Mei Lin, and Hsiu-Wei Chen*,‡ Department of Chemistry, National Sun Yat-Sen UniVersity, 70 Lien Hai Road, Kaohsiung, Taiwan 80424, R.O.C. ReceiVed: NoVember 29, 2000; In Final Form: February 14, 2001

We present an alternative to self-assembled monolayers (SAMs) of ω-functionalized alkanethiols on gold. Substrates utilized here are gold modified by electrochemical deposition of silver or copper monolayers. The metal adlayers promote anchoring of carboxylate headgroups and assembly of ω-alkanoic acids, which would otherwise exhibit no chemisorption on bare gold. Infrared reflectance absorption spectroscopy shows that the films exhibit general characteristics of SAMs. The binding scheme is different from SAMs of alkanethiols on gold and alkanoic acids on silver or copper surfaces. Wetability results obtained from contact angle measurements indicate that such films are more reproducible than SAMs of alkanoic acids on bulk silver and copper.

Introduction Self-assembled monolayers (SAMs) of ω-functionalized alkanethiols on gold have enormous application prospects because tailoring chain termini by further heterogeneous synthesis makes possible surface engineering1,2 of nanolithography and patterning,3,4 biomolecule immobilization,5-11 and physical and chemical sensing properties.10-15 In principle, similarly functionalized surfaces can be prepared by spontaneous adsorption of corresponding alkanoic acids onto aluminum, copper, and silver, which, in contrast to the inertness of gold, are oxidized readily under ambient conditions. The adsorbatesubstrate anchoring is believed to occur via reversible ionic interactions between carboxylates and metal oxides.1,2,16-19 In comparison with alkanethiols, alkanoic acids do not release unpleasant odors and commercially available alkanoic compounds have a wider variety of terminal functional groups and a significantly lower cost. However, these active substrates are prone to contamination and oxidation, which is uncontrollable under SAM preparation conditions, and are believed to be the origin of the poor reproducibility of alkanoic acid16,17 and alkanethiol SAMs.20-23 Therefore, despite the aforementioned advantages of alkanoic acids over alkanethiols and the fact that spontaneous adsorption of n-alkanoic acids on native oxide surfaces has been developed for decades,1,2,16-19,24 systems of metal oxide/alkanoic acids are not as widely accepted as alkanethiols on gold for SAM applications. Reported here is a method of preparation of alkanoic acid SAMs by employing better defined substrates where the surface metal oxide is no thicker than one monolayer and is less subject to oxidation.25-27 The substrates are monolayers of silver or copper electrochemically deposited on gold by an underpotential * Corresponding authors. † E-mail: [email protected]. Phone: +886-3-5737009. Fax: +886-3-5711082. ‡ E-mail: [email protected]. Fax: +886-7-5253909.

deposition process (upd),28-30 in which only a monolayer or submonolayer of adatoms is deposited onto foreign substrates at potentials positive from the onset of bulk reduction taking place. The gold substrate is stable under ambient conditions and can be stored in sealed containers, in absolute ethanol, or cleaned with piranha solution or plasma cleaning before use. Before upd modification, the gold surface is believed to be free of oxide, as opposed to active substrates with a thick layer of metal oxide, such as copper and aluminum. Because the thickness of the upd metal is no more than one full monolayer, the Au/Ag(upd) and Au/Cu(upd) will contain at most one atomic layer of oxide, which is better defined than their corresponding bulk materials and appears to facilitate the reproducibility of monolayer assembly. Measurements from X-ray photoelectron spectroscopy (XPS) suggest that Au/Ag(upd) is not prone to oxidation in air.25 However, it is worth noticing that XPS data indicate a possibility of the presence of surface oxide on Au/Cu(upd).25,31 For the applications of electrochemically active SAMs, because the upd redox takes place at a potential positive from that of the bulk material, the potential window of the upd modified surface extends more positively than those of bare Ag or Cu electrodes. In fact, as demonstrated by several groups, the alkanethiolate SAM and the upd adlayer remain intact at potentials even more positive than where the upd adatoms are electrodeposited.25,31 This increase in stability of the upd adlayer has been attributed to the presence of the SAM25,31 whose hydrophobicity resists the penetration of ions from the aqueous solution and retards the depletion of the upd adatoms. In this paper, we show that the affinity of carboxylate toward surfaces of Au/Ag(upd) and Au/Cu(upd) assembles alkanoic acid monolayers on gold substrate. In comparison with alkanoic acid SAMs on bulk Ag and Cu, this preparation scheme improves the reproducibility in film properties. We also show IRAS (infrared reflectance absorption spectroscopy) spectra of amineterminated alkanoic acid SAMs. Further derivatization of the

10.1021/jp004329i CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

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functionalized surface demonstrates that this method is an alternative to SAMs of ω-functionalized alkanethiols on gold. Experimental Section Preparation of Alkanoic SAMs. The substrates were 200nm gold films with a 10-nm Cr underlayer prepared by thermal evaporation (5 × 10-6 Torr, KV-301, KEY High Vacuum, Co., Nesconset, NY) onto piranha-cleaned glass slides. The piranha solution is a 1:3 (v/v) mixture of 30% H2O2 and concentrated H2SO4. This solution reacts Violently with organic materials and should be handled with great care. Underpotential deposition process modification was carried on a PAR VersaStat (EG&G Instruments Corp., Princeton, NJ) or a CHI604 potentiostat (CH Instruments Inc., Austin, TX). The upd solutions were prepared from reagent-grade chemicals and Millipore-Q purified water. The concentrations and makeup of the solutions were 0.6 mM Ag2SO4 (Fisher Scientific) and 1 mM CuSO4 (May & Baker) for Ag and Cu upd, respectively, dissolved in 0.1 M H2SO4. The reference electrode for Ag upd was a high-purity silver wire flame-annealed before experiments. A high-purity copper wire (Aldrich) was used as the reference electrode for Cu upd and was cleaned by dipping into diluted nitric acid for a short period of time prior to each use. The potential of the gold substrates was cycled from 600 mV (vs EAg+/0) and from 500 mV (vs ECu2+/0) through the upd peaks and parked at 350 mV and 0 mV, respectively, for Ag and Cu upd. The modified substrates were subsequently removed from the electrochemical solutions under potential control, rinsed with copious ethanol, blown dry in a stream of nitrogen, and transferred through air into alkanoic acid-containing solutions.25-27,31-33 The upd potentials were selected because oleophobic surfaces of alkanoic acid SAMs formed most rapidly on the upd surface modified at these potentials. SAMs of nonadecanoic acid (CH3(CH2)17CO2H) and tridecanoic acid (CH3(CH2)11CO2H) were prepared in n-hexadecane. SAMs of 12-aminododecanoic acid (NH2(CH2)11CO2H) were prepared in ethylene glycol due to its poor solubility in n-hexadecane. Further derivatization of the aminododecanoate SAM was performed by dipping the substrate into a 10% (v/v) aqueous solution of glutaraldehyde (Merck) for 20 min and then rinsing thoroughly with water. The fatty acids (Tokyo Chemical Industry, Co.) were used as received. n-Hexadecane (Sigma) was percolated through neutral alumina (Activity 1 from Merck) twice. All substrates were kept in the solution for at least 3 h. After removal from the solutions of alkanoic acids, the SAMmodified substrates were rinsed in sequence with hexane, ethanol, water, and blown dry with nitrogen. Reflectance Infrared Spectroscopy. Absorption spectroscopy was carried out with a Bio-Rad FTS-175 or a Perkin-Elmer System 2000 infrared spectrometer both equipped with an MCT detector cooled with liquid nitrogen. The measurement scheme was a single reflection mode and the p-polarized light was incident at 85° from the surface normal with a grazing angle accessory (FT-85, Spectra-Tech, Shelton, CT). The light path, detector, and sample chambers were purged with dry nitrogen. A total of 1024 scans of both the sample and the reference were collected at 2-cm-1 resolution for signal averaging. Contact Angle Measurements. Advancing contact angles were measured on static drops of water or n-hexadecane with a Contact Angle Measuring System G10 (Kruss Gmbh, Hamburg, Germany). The needle tip of the syringe remained in the drop during measurement.

Figure 1. Cyclic voltammograms of (A) Ag upd and (B) Cu upd on thermally evaporated gold films. Scan rate was 20 mV/s. The supporting electrolyte was 0.1 M H2SO4. Voltammograms of Cu upd were obtained in N2-saturated solution.

Results and Discussion Typical voltammetric curves for Ag and Cu upd in 0.1 M H2SO4 are presented in Figure 1. The negative limits of the voltammogram represent the finish of distinct upd processes and the initial stage of overpotential deposition. The number and sharpness of peaks are more distinct than those of upd on polycrystalline gold but are not as well defined as peaks on Au(111),34,35 suggesting that the gold films used in this study were polycrystalline in nature but preferentially (111)-orientated. IRAS is a convenient and well-established method of examining a SAM for molecular packing structure1,16-18,20,36-39 because ν(CH2) vibrations are very sensitive to the presence of gauche defects. For example, the position of νa(CH2) can be found at ∼ 2918 cm-1 for a high-quality SAM, and at ∼2926 cm-1 for a heavily disordered, liquidlike SAM. Displayed in Figures 2A and 2B are IRAS spectra of nonadecanoic acid (CH3(CH2)17COOH) self-assembled on Au/Ag(upd) and Au/ Cu(upd), respectively. The frequencies of νa(CH2) and νs(CH2) are centered, respectively, at 2917 and 2850 cm-1, indicating that the hydrocarbon chains in the SAMs are highly crystalline and primarily trans-zigzag extended.17,18,36,40 The all-trans conformation of the nonadecanoic acid SAMs is further confirmed by the series of bands exhibited between 1380 and 1200 cm-1. Such features are assigned to the progression of polymethylene wagging modes (ω(CH2)) and are characteristic of an all-trans conformation, which has been observed in bulk solid alkanoic acids and SAMs of alkanoic acid on silver.17,18 The narrow line width of the relatively small but distinct CH2 scissors deformation (δ(CH2)) around 1470 cm-1 is consistent with a crystalline-like environment. The facts that the δ(CH2) mode is vertical to the molecular chain axis and that IRAS is insensitive to the dipole moment vertical to surface normal indicate a tilted orientation of the chain. For SAMs of n-alkanoic acid on native oxide surfaces, the binding geometry of the carboxylate headgroup on silver17,18 is different from that on Cu.18,19 The two oxygen atoms of the carboxylate bind symmetrically to the Ag substrate.17,18 While on the surface of Cu, only one of the oxygen atoms binds to the surface and the IRAS spectra exhibit asymmetric stretch νa(COO-) at ∼ 1552 cm-1.18,19 Interestingly, both Figures 2A and 2B show the νs(COO-) bands at ∼1400 cm-1 and the absence of νa(COO-) around 1550 cm-1. These features suggest symmetrical binding schemes of the carboxylates chemisorbed onto the upd modified surfaces,17,18 distinctly different from that observed on bulk copper.18,19

Self-Assembly of Alkanoic Acids on Gold Surfaces

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Figure 2. Grazing incidence polarized infrared spectra for SAMs of alkanoic acids: (A) Au/Ag(upd)/CH3(CH2)17COOH, (B) Au/Cu(upd)/CH3(CH2)17COOH, (C) Au/Ag(upd)/CH3(CH2)11COOH, (D) Au/Cu(upd)/CH3(CH2)11COOH.

Figure 3. Grazing incidence polarized infrared spectra for SAMs of alkanoic acids: (A) Au/Ag(upd)/NH2(CH2)11COOH, (B) Au/Cu(upd)/NH2(CH2)11COOH, (C) surface reaction of Figure 3A with glutaraldehyde, and (D) surface reaction of Figure 3D with glutaraldehyde.

For n-alkanethiol SAMs on gold, the structure becomes increasingly disordered as the number of methylene chains decreases due to reduced interchain van der Waals interactions.1,2,36 Monolayer spectra of relatively short n-alkanoic acids (tridecanoic acid, CH3(CH2)11COOH) on Au/Ag(upd) and on Au/Cu(upd) are presented in Figures 2C and 2D, respectively. The spectral characteristics of the short-chain SAMs resemble qualitatively those of longer chain SAMs. Due to a decrease in number of methylene chains, the peak ratio of intensity of νa(CH2) to that of νa(CH3) is smaller than those in Figures 2A and 2B. The number of progression bands at 1380 ∼ 1200 cm-1 decreases also. The band at ∼2921 cm-1 has been assigned to νa(CH2), at ∼2852 cm-1 to νs(CH2), and at 1400(Au/Ag(upd))1409(Au/Cu(upd)) cm-1 to νs(COO-). The small but significant (up to 4-6 cm-1) red shifts for the methylene group vibration compared to disordered films20,36,41 correspond to relatively ordered chains. Figure 2 manifests the effect of Ag and Cu upd on selfassembly of alkanoic acids, which would otherwise exhibit no chemisorption on bare gold.42 The spectra are distinct from those of alkanoic acid SAMs on bulk silver and copper18,19 in line shapes and dichroic ratios for methylene absorption modes (νa(CH2)/νs(CH2)), suggesting that the structures are significantly different.20 A comprehensive structural characterization is beyond the scope of this manuscript and is currently underway.

ω-Alkanoic acid functionalized surfaces are exemplified in Figure 3A and 3B by SAMs of 12-aminododecanoic acid (NH2(CH2)11COOH) on Au/Ag(upd) and Au/Cu(upd), respectively. Although adsorption of amine groups onto Ag and Cu substrates is possible, the lack of ν(CdO) vibration suggests that the molecules are dominantly bound through the carboxylate with the amine group extending away from the upd surface. The shift of the νa(CH2) peaks to 2930 cm-1 indicates that the methylene chains are not as crystalline as those of methylterminated SAMs (Figures 2C and 2D). This disorder is a result of repulsion between neighboring amine groups protonated during the rinsing of the SAMs with water. Scheme 1 illustrates that the amine-terminated surfaces can be further derivatized with glutaraldehyde in water. The spectra of aldehyde-functionalized SAMs on Au/Ag(upd) and Au/ Cu(upd) are shown in Figures 3C and 3D, respectively. New bands at ∼1640 and 1725 cm-1 demonstrate the formation of imine (NdC) and covalently attached aldehyde, respectively. At the high-frequency region, the peaks of νa(CH2) shift slightly from 2930 to 2931 cm-1 and to 2933 cm-1, respectively, for SAMs on Ag(upd) and Cu(upd). The change in peak intensity is also minute. The insignificant changes suggest that the conformation of methylene chains is essentially not affected by the derivatization reactions. In fact, the condensation reaction of imine formation is facile and the conditions are mild. The derivatization simply involves 20-minute immersion of the

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SCHEME 1

TABLE 1: Static Advancing Contact Angles of Water on Nonadecanoic Acid SAMs Prepared on Fresh and One-Day-Old Substratesa freshly prepared substrate average (n >30) min ∼ max standard deviation a

Ag(bulk)

Au/Ag(upd)

115.6

114.6

110 ∼122 2.2

113 ∼116 0.8

Cu(bulk)

one-day-old substrate Au/Cu(upd)

Ag(bulk)

Au/Ag(upd)

Cu(bulk)

Au/Cu(upd)

119.0

115.2

105.5

115.6

115.8

114.2

115 ∼ 122 1.6

114 ∼ 116 0.6

94 ∼ 116 6.6

115 ∼ 116 0.5

97 ∼ 122 5.5

112 ∼ 115 0.5

The substrates were carefully stored in a sealed container under ambient conditions.

ω-alkanoic acid SAMs in an aqueous solution containing glutaraldehyde. Therefore, ω-alkanoic acid molecules remain intact after derivatization reactions. As confirmed by a SAM study of thermal stability by Sung et al.,23 contact angle measurements correlate reasonably well with XPS results. Therefore, variation in wetabilities of replicate samples is suitable for the evaluation of reproducibility in SAM preparation. Table 1 summarizes water advancing contact angles (θH2O) measured on nonadecanoic acid monolayers. The substrates are either freshly evaporated or carefully stored in sealed containers for 1 day. SAMs on upd substrates exhibit a narrower span of θH2O and smaller standard deviation than those prepared on bulk silver and copper, suggesting relative ease in preparing samples with reproducible properties. Thermal stability of nonadecanoic acid SAMs is examined by monitoring the static contact angle as a function of temperature. An abrupt decrease of θH2O, indicating the desorption temperature of nonadecanoic acid, is found around 155 ∼ 162 °C for SAMs on both freshly prepared bulk Ag and Au/Ag(upd), and around 162 ∼ 167 °C for SAMs on corresponding Cu substrates. Although the updmodified substrates do not show apparent improvement in thermal stability against freshly prepared bulk materials, the standard deviations of θH2O are relatively small. For one-monthold SAMs, the desorption temperature for those prepared on upd-modified gold remains the same, but that for those on bulk substrates decreases by ∼10 °C. The poor reproducibility of the alkanethiolate SAMs on copper has been associated with the susceptibility to oxidation upon exposure to air.20-23 Upd has been ascribed to a result of difference in work function between the adatom and the substrate, with the former being smaller.28-30 From the aspect of work function, it is reasonable to infer that the adlayer exhibits varying degrees of electron deficiency during the course of upd. While the preparation procedures of SAMs inevitably involve exposure of the substrates to the air, the electron-deficient adlayer suppresses the activity toward dioxygen reduction in air. Therefore, the upd adlayer is less subject to formation of surface oxide. Also noticed is the relevant observation reported by Smith and Porter17 that native oxide surfaces, such as Cu and Ag, are subject to corrosion by alkanoic acids in ethanolic solutions. Corrosion of copper oxides by alkanethiols has been attributed to the poor reproducibility of SAMs.22,23 Because upd takes place at a potential less negative than that for bulk

deposition, the formation of the adatom-substrate bond is thermodynamically more favorable and stronger than the adatom-adatom bond. It has been shown that the adatomsubstrate interactions successfully inhibit the etching mechanism that occurs during the assembly of alkanethiols on gold.25,33 Similarly, in the case of alkanoic acid SAMs, the surface morphology is preserved by the upd adlayer. Therefore, the substrate is better defined than those with native oxides and thus contributes to the improvement in SAM reproducibility. In summary, we have illustrated upd-promoted self-assembly of alkanoic acids on gold. Examples show that such SAMs exhibit reasonable thermal stability and can be prepared reproducibly, suggesting that this approach is a viable and attractive alternative to SAMs of ω-functionalized alkanethiols on gold. Acknowledgment. The authors gratefully acknowledge the Chemistry Department of National Sun Yat-Sen University and National Tsing Hua University for a generous research support. S.-Y.L. thanks NSC for a graduate research fellowship. Financial support for this project was from National Science Council, R.O.C. References and Notes (1) An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Ulman, A., Ed.; Academic Press: Boston, 1991. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823-1848. (4) Black, A. J.; Paul, K. E.; Aizenberg, J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 8356-8365. (5) Dong, Y.; Shannon, C. Anal. Chem. 2000, 72, 2371-2376. (6) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem. 1999, 71, 777-790. (7) Rao, J.; Yan, L.; Xu, B.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 2629-2630. (8) Franchina, J. G.; Lackowski, W. M.; Dermody, D. L.; Crooks, R. M.; Bergbreiter, D. E.; Sirkar, K.; Russell, R. J.; Pishko, M. V. Anal. Chem. 1999, 71, 3133-3139. (9) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044-8051. (10) Ruan, C.; Yang, F.; Lei, C.; Deng, J. Anal. Chem. 1998, 70, 17211725. (11) Willner, I.; Heleg-Shabtai, V.; Blonder, R.; Katz, E.; Tao, G.; Buckmann, A. F.; Heller, A. J. Am. Chem. Soc. 1996, 118, 10321-10322. (12) Sirkar, K.; Revzin, A.; Pishko, M. V. Anal. Chem. 2000, 72, 29302936.

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