Electrochemical Identification of Molecular Heterogeneity in Binary

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Electrochemical Identification of Molecular Heterogeneity in Binary Redox Self-Assembled Monolayers on Gold Huihui Tian,†,‡ Debo Xiang,†,‡ Huibo Shao,*,† and Hua-Zhong Yu*,‡ †

Key Laboratory of Cluster Science (Ministry of Education of China) and Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 100081, Peope’s Republic of China ‡ Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *

ABSTRACT: Ferrocenylalkanethiols are excellent probes to study the structure and properties of mixed self-assembled monolayers (SAMs) on gold; in this paper, the molecular heterogeneity in binary redox-active SAMs on gold prepared via postassembly exchange and coadsorption processes is revealed electrochemically. The exchange process of single-component 11ferrocenyl-1-undecanethiolate SAMs on gold (FcC11S−Au) with 1-undecanethiol (C11SH) is first investigated; it is shown that a single pair of redox peaks can be obtained upon prolonged immersion in C11SH/ethanol solution. For the coadsorption of FcC11SH and C11SH on gold, the splitting of the redox peak diminishes when the molar ratio FcC11SH decreases to 0, fwhm < 90.6 mV, attraction forces dominate among the adsorbed Fc molecules; when νgθT < 0, fwhm > 90.6 mV, repulsion forces predominate. In our system, the fitted theoretical i−E curves of the CVs are shown in Figure 9b and c. It is surprising to notice that the anodic peak of the CVs in fact consists of a main peak and a shoulder peak (with less than 10% of total surface concentration) in both cases. Here, we are going to focus our discussion on the main peaks for which the fitted interaction parameters and formal potentials are listed in Table 1 for direct comparison.

Figure 10. Absorbance−reflectance IR spectra of SAMs: (a) C18S− Au; (b) C11S−Au; (c) FcC11S−/C11S−Au SAM prepared by the postassembly exchange method; (d) FcC11S−/C11S−Au SAM prepared by the coadsorption method; (e) single-component FcC11S−Au SAM. p-Polarized light was incident at 80° for all the spectra displayed.

of these spectra are listed in Table 2. The peak positions of the distinct CH3 and CH2 bands of the reference sample (C18S− Au) are in good agreement with the values of a highly ordered and closely packed monolayer.54,55 Comparing the IR spectra of the C11S−Au SAM and the FcC11S−Au SAM, we noticed that the peak positions of the νa (CH2) and νs (CH2) bands do not differ from each other, indicating that these two SAMs have the same level of molecular ordering.48 Because the FcC11S−Au SAM was terminated with ferrocene groups, no CH3 stretching bands were observed. Instead, we were able to observe a band at 3104 cm−1, which corresponds to the C−H stretching of ferrocene ring ν(C−H)Fc.56 It is important to compare the IR spectra of binary FcC11S−/C11S−Au monolayers prepared by postassembly exchange and by coadsorption methods. First, the intensity, shape, and position of the CH2 bands in these two systems are similar to each other (Table 2), indicating that the average packing density and overall orientation in the two monolayers are not significantly different from each other. As of the low density of ferrocene groups, the bands at 3104 cm −1 corresponding to ν(C−H)Fc are hardly noticeable in both cases. Second, the bands for the CH2 stretching modes of the mixed monolayers have shifted to higher values compared to those of the single-component SAMs suggesting that the alkyl chains in these systems are not as ordered. Although the IR spectra did not provide additional information on the molecular heterogeneity of the mixed monolayers, these results validated the electrochemical observations in terms of the overall molecular ordering and packing density. These results also suggest that the traditional electrochemical methods (e.g., cyclic voltammetry) are even more sensitive to the molecular ordering/packing in SAMs than IR spectroscopy. Considering both the electrochemical and IR spectroscopic observations, we propose a hypothetic picture of the mixed monolayers prepared by two different processes: postassembly exchange versus coadsorption (Scheme 2). In the binary redox SAMs prepared by coadsorption, the ferrocene groups are

Table 1. Fitting Results of the Anodic Peaks Shown in Figure 9a preparation method

ΓFc (10−11 mol/cm2)

E°′ (mV)

νgθT

fwhm

coadsorption exchange

5.5 5.7

179 251

−0.40 −0.98

112 151

a

All values reported are within relative uncertainties of ±5%.

The intermolecular interaction parameter of the monolayer prepared by coadsorption is −0.4, and the fwhm value is 112 mV, both close to the ideal situation (νgθT = 0 and fwhm = 90.6 mV); for the monolayer prepared by the exchange process, the fitted result deviates further (νgθT = −0.98 and fwhm = 151 mV). The fitted results of the two binary redox SAMs indicate that the repulsion force among the ferrocenylalkanethiolates in the monolayer obtained by the exchange method is much stronger than that in the coadsorbed system; this implies that the local surface density of redox centers in an exchanged monolayer is significantly higher (i.e., not as uniformly distributed) than that prepared by the coadsorption process. To provide a better understanding of the monolayer structure, especially for the two binary Fc SAMs prepared by the postassembly exchange and the coadsorption methods, reflectance absorption IR measurements were carried out. Figure 10 shows the IR spectra in the C−H stretching region for both the mixed Fc SAMs and the single-component FcC11−Au monolayers. Band assignments and peak positions 13739

dx.doi.org/10.1021/jp5040745 | J. Phys. Chem. C 2014, 118, 13733−13742

The Journal of Physical Chemistry C

Article

Table 2. Peak Assignments and Positions for Typical C−H Stretching Modes in the IR Spectra of Both Single-Component and Binary Fc SAMsa peak position (cm−1) structural group

C−H stretching mode

C18S−Au (ref)

C18S−Au

C11S−Au

FcC11S−/C11S−Au (exchange)

FcC11S−/C11S−Au (coadsorption)

FcC11S−Au

−CH2−

νa νs νa (ip) νa (op) νs (FR) νs (FR) ν (C−H)Fc

2917 2850 2965 n.o. 2938 2878 n.o.

2917 2850 2963 n.o. 2937 2878 n.o.

2921 2849 2964 n.o. 2937 2878 n.o.

2926 2857 2963 n.o. n.o. n.o. n.o.

2926 2856 2965 n.o. n.o. n.o. n.o.

2922 2949 n.o. n.o. n.o. n.o. 3104

CH3−

−CH− a

The data of C18S−Au SAMs from ref 54 is also listed for comparison.

standing of the molecular heterogeneity in binary FcC11S−/ C11S−Au SAMs, it is expected that the surface composition and molecular packing are also influenced by the alkyl chain lengths of both electroactive ferrocenylalkanethiols and diluting alkanethiols. We are currently carrying out extended experiments to tackle the above challenges, which are certainly beyond the scope of the present work.

Scheme 2. Structure of Diluted Binary Redox SAMs (e.g., FcC11S−/C11S−Au Monolayer) by (a) Coadsorption and (b) Postassembly Exchange Processes, Respectively

4. CONCLUSION It has been shown that the two diluted binary FcC11S−/ C11S−Au mixed monolayers formed by coadsorption and postassembly exchange have different redox properties. The coadsorption of FcC11SH and C11SH at a low mole ratio indeed yields a near-ideal redox-active monolayer, where the intermolecular interaction parameter approaches zero and the peak width is close to 90.6 mV. The postassembly exchange of a single-component FcC11S−Au monolayer with C11SH, on the other hand, produces a monolayer with strong intermolecular repulsion and a higher formal potential. The electrochemical behavior of the SAM suggests different microstructures (even distribution vs cluster formation) that are dictated by the respective kinetically versus thermodynamically controlled assembly processes.

isolated from each other, while those from postassembly are in the form of clusters. This is in agreement with the model proposed by Rowe and Creager for the alkanethiol assembly process.30 We believe that the FcC11S−/C11S−Au binary monolayer prepared by coadsorption is kinetically controlled: the adsorption of FcC11SH and C11SH onto the gold surface is closely related to the diffusion of the two thiols in the bulk solution, and the final composition of the monolayer correlates with their ratio (Figure 8a). The surface-tethered ferrocene redox centers are separated from each other by the alkyl chains, and thus, the repulsion force among the Fc groups is not significant (Scheme 2a). In contrast to the coadsorbed monolayer, the monolayer prepared by postassembly exchange of the single-component FcC11S−Au monolayer with C11SH is thermodynamic controlled and related to the domainlike structure and the gold−sulfur bonding strength (as shown in Scheme 2b): the exchange process happens in the FcC11S−Au domains that are bonded more weakly to the substrate, and the C11SH molecules in bulk solution refill these sites. Such substitution happens as clustered behavior; therefore, the final state of FcC11S− is distributed as clustered domains on gold surface (Scheme 2b). In this type of FcC11S−/C11S−Au monolayers, the repulsive interactions are expected to be high among the clustered ferrocenylthiolates. When one ferrocene group is oxidized to ferrocenium, the oxidation of an adjacent ferrocene will be more difficult because of the repulsion among the ferrocenium cations, so that the formal potential also shifts positively. The novelty of this work does not rely on either the characterization techniques or the monolayer systems; for the first time, we have directly compared the structure and redox properties of diluted binary SAMs prepared by coadsorption and postassembly exchange approaches. We were able to identify the appropriate method to prepae binary SAMs with uniformly distributed redox centers; this is critical for their further applications in both fundamental electron transfer studies and the development of redox-labeled biosensing devices. While we have provided a comprehensive under-

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ASSOCIATED CONTENT

* Supporting Information S

Additional experimental data including the CVs of FcC11S−/ C11S−Au monolayers upon immersion in ethanol for different periods of time, replicated measurements of these SAMs when treated with 1.0 mM C11SH/ethanol solution, and CV analysis/fitting. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Natural Science and Engineering Research Council (NSERC) of Canada, the National Science Foundation of China (Project 21173023), and the National 111 Project (B07012) of China. H.T. thanks the State Scholarship Foundation of China for 13740

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supporting her stay as a visiting student at Simon Fraser University.

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