Using Supramolecular Binding Motifs To Provide Precise Control over ...

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Using Supramolecular Binding Motifs To Provide Precise Control over the Ratio and Distribution of Species in Multiple Component Films Grafted on Surfaces: Demonstration Using Electrochemical Assembly from Aryl Diazonium Salts Alicia L. Gui,† Hon Man Yau,† Donald S. Thomas,‡ Muthukumar Chockalingam,† Jason B. Harper,† and J. Justin Gooding*,† †

School of Chemistry and Australian Centre for NanoMedicine, The University of New South Wales, Sydney, NSW 2052, Australia Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia

‡

S Supporting Information *

ABSTRACT: Supramolecular interactions between two surface modification species are explored to control the ratio and distribution of these species on the resultant surface. A binary mixture of aryl diazonium salts bearing oppositely charged para-substituents (either −SO3− or −N+(Me)3), which also reduce at different potentials, has been examined on glassy carbon surfaces using cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). Striking features were observed: (1) the two aryl diazonium salts in the mixed solution undergo reductive adsorption at the same potential which is distinctively less negative than the potential required for the reduction of either of the two aryl diazonium salts alone; (2) the surface ratio of the two phenyl derivatives is consistently 1:1 regardless of the ratio of the two aryl diazonium salts in the modification solutions. Homogeneous distribution of the two oppositely charged phenyl species on the modified surface has also been suggested by XPS survey spectra. Diffusion coefficient measurements by DOSY NMR and DFT based computation have indicated the association of the two aryl diazonium species in the solution, which has led to changes in the molecular orbital energies of the two species. This study highlights the potential of using intermolecular interactions to control the assembly of multicomponent thin layers.

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INTRODUCTION

where specific molecular binding motifs are employed to fabricate new materials with molecular level precision. Intermolecular forces such as van der Waals interactions between carbon chains, and hydrogen bonding, dipole−dipole interactions and electrostatic forces between tail groups have been reported to have a significant influence on the composition and homogeneity of binary mixed SAMs formed by alkanethiol chemistry.11−13 With regard to this subject, interesting observations have been made in studies carried out by Kang et al.,14,15 which examine the extent of influence of dipolar interactions on the adsorption behavior of mixed alkanethiols in different solvents. Particularly in their study of a mixed SAM formed from 4′-nitro-4-mercaptobiphenyl and 4′(dimethylamino)-4-mercaptobiphenyl in toluene, the surface ratio of the two thiols presented a consistent value near 1:1 across a wide range of varying ratios in solution.15 They have concluded the constant balanced ratio of the two alkanethiols in

The benefits of fabricating multiple functionalities through organic thin layers on surfaces have been presented in a wide range of applications, including surface wetting,1 chemical sensing and biosensing,2 cell culture,3 molecular diagnostics,4 and photosynthesis.5 There have been examples where up to 17 different molecular components are incorporated into a single interface in the literature.6 Being able to control the ratio and distribution of different functionalities is critical in tuning the chemical and physical properties of surfaces. Particularly in biosensor applications, a high degree of control over grafting multiple functional groups to the surface, sometimes even at the molecular level, is often required for the construction of a well-defined biosensing interface.7,8 Despite the need for precise control over the distribution of molecules in the interface, typically the different components are simply mixed in a solution prior to deposition onto the surface with little regard to how the different modifying molecules interact with each other.9,10 This is somewhat surprising considering the rise in molecular materials, such as metal−organic frameworks, © 2013 American Chemical Society

Received: January 28, 2013 Revised: March 12, 2013 Published: March 25, 2013 4772

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previous studies have revealed the important role played by the reductive potential of aryl diazonium salts in determining the surface ratio of different functionalities in mixed layers. However, consideration of any intermolecular interaction effect is not possible from the experiments in previous studies as the aryl diazonium salts used do not contain functionalities that strongly interact with each other. In the current work, the deposition behavior of a binary mixture of aryl diazonium salts, 4-sulfophenyl (SP) diazonium salts and 4-trimethylammoniophenyl (TMAP) diazonium salts, onto glassy carbon electrodes was investigated. The two aryl diazonium species bear oppositely charged groups and are different in reductive adsorption potentials. Hence, the current work is of fundamental importance as there are two driving forces that may determine the ratio of species on the surface: electrostatic interactions and the reductive adsorption potential. Furthermore, unlike alkanethiol SAMs, aryl diazonium salt derived layers are covalently attached to the surface, so surface mobility of the adsorbed species is precluded. Therefore, it is unclear which of these factors may dominate and why. This paper aims to explore the questions raised above, using cyclic voltammetry to interrogate the electrochemical deposition behavior of the SP/TMAP aryl diazonium salt mixture, in solutions of various SP/TMAP ratios. Surface composition of mixed phenyl derivative layers was characterized using XPS measurements and was compared to the solution composition. The effects of the reductive adsorption potential and electrostatic interaction on the ratio of SP and TMAP in grafted layers were evaluated. Particularly with regards to understanding how the electrostatic interaction influences the reductive adsorption process of SP/TMAP diazonium salts mixture, diffusion coefficient measurements using DOSY NMR and molecular orbital energy analysis with density functional theory (DFT) have been performed, to ascertain whether the interaction between the two modifying species happens either in the solution or during the reductive adsorption.

the mixed SAMs is driven by the formation of a twodimensional assembly with a zero net dipole moment. Adsorption of binary mixtures of alkanethiols each bearing balanced charged terminal groups has been used to prepare SAMs presenting zwitterionic properties by Whitesides and coworkers in their protein adsorption studies.16,17 However, detailed adsorption behavior, which might be influenced by electrostatic force, is not mentioned in their work. Later work by Ooi et al.18 aimed at understanding the mixing properties of 2-aminoethanethiol (AET) and 2-mercaptoethanesulfonic acid (MES) has shown preferential formation of 1:1 mixed SAM in a broad range of MES mole fraction in solution from 0.01 to 0.95. Chen et al.19 have also observed in their work a constant 1:1 ratio of mixed SAM formed from 11-mercapto-N,N,Ntrimethylammonium chloride (TMA) and 11-mercaptoundecylsulfonic acid (SA) with solution mole fractions of TMA varying from 0.2 to 0.9. The two studies above have concluded that electrostatic interaction between oppositely charged tail groups have driven the coassembly of the two alkanethiols, hence forming homogeneous mixed SAMs with an overall zero charge and a constant 1:1 ratio of the components. However, with the dynamic nature of alkanethiol SAMs, originating from the surface mobility of the adsorbed alkyl derivatives, as well as their adsorption and desorption kinetics,20 it is not clear whether the resultant 1:1 ratios on the surface are the consequence of association between components in solution prior to SAM formation or the result of the SAMs forming their most thermodynamically stable state. We are particularly interested in electrodeposition of aryl diazonium salts for surface functionalization, mainly because the resultant phenyl layers are robust compared with alkanethiols SAMs on gold.7 The much wider potential windows and greater long-term stability of aryl diazonium salt derived layers have made this method more favorable for the fabrication of electrochemical sensing interfaces. Furthermore, another big advantage of this system, over other fabrication approaches, is it being compatible with a surprisingly broad range of materials including all the carbon allotropes,21−24 metals,25−28 semiconductors,28,29 indium tin oxides,30,31 and polymers.32,33 Despite the fact that extensive fundamental studies have been performed to control the process of aryl diazonium salt deposition (including the monolayer strategies),34−39 the literature has been devoid of studies with respect to the adsorption of the mixture of aryl diazonium salts,9,40−42 which is essential to the control of grafting multiple functional groups to the surface. Two recent papers from Belanger’s group42 and Gooding’s group9 have investigated the deposition behavior of mixed aryl diazonium salts at the fundamental level. The former study by Belanger and coworkers investigated the adsorption of two sets of binary mixtures of para-substituted aryl diazonium salts on carbon electrode, −Br/−NO2 and −Br/−N(Et)2. The influence of the ratios of the two aryl diazonium compounds in solution on the composition of mixed layers was examined. The latter work performed by Gooding and co-workers involve examining deposition of eight sets of binary mixed aryl diazonium salts bearing the para-substituents −Br, −COOH, −SH, −NH2, and −NO2 on both carbon and gold electrodes. Both studies concluded that the adsorption of binary aryl diazonium salt mixtures is dominated by the one that is easier to reduce (less negative potential required for the reduction). That is, the mole fraction of the more easily reduced aryl diazonium salt is higher on the surface than in the corresponding solution. These

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EXPERIMENTAL SECTION

Reagents and Materials. Tetrabutylammonium tetrafluoroborate (NBu4BF4), sodium nitrite (NaNO2), potassium chloride (KCl), fluoroboric acid (HBF4), potassium ferricyanide (K3Fe(CN)6), hexamine ruthenium(III) chloride (Ru(NH3)6Cl3), sodium benzenesulfonate (C 6 H 5 SO 3 Na), trimethylphenylammonium chloride ((CH3)3N(Cl)C6H5), and acetonitrile (CH3CN, HPLC grade) were obtained from Sigma-Aldrich (Sydney, Australia). 4-Aminophenyltrimethylammonium iodide hydrochloride ((CH 3)3N(I)C6H5− NH2·HCl) was purchased from Acros Organics (Geel, Belgium). Hydrochloric acid (HCl(aq), 32%) was obtained from Ajax Finechem (Sydney, Australia). All reagents were used as received, and aqueous solutions were prepared with Milli-Q water (18 MW cm−1, Millipore, Sydney, Australia). 4-(Trimethylammonio)phenyl Diazonium Tetrafluoroborate. Synthesis of 4-(trimethylammonio)phenyl diazonium tetrafluoroborate followed the procedure from Traylor et al.43 A suspension of 1.57 g (0.005 mol) of 4-aminophenyltrimethylammonium iodide in 10 mL (0.081 mol) of fluoroboric acid was added dropwise with a 2.5 mL aqueous solution containing 0.7 g (0.01 mol) of sodium nitrite. The reaction mixture was stirred and kept at −5 °C in an ice−salt bath during this addition and for 30 min thereafter. The suspension color turned from brown to black due to the precipitation of iodine. Then vacuum filtration was immediately performed to remove precipitates. The beige-colored filtrate was then transferred into a freezer, and after 48 h, white flakelike crystals formed and were collected by filtration. The product was washed with ethanol and 2,2-dimethoxypropane. Purification of the product was carried out by recrystallization from a mixture of acetonitrile and diethyl ether. The purified product was 4773

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dried and stored over calcium chloride in a desiccator at 4 °C. 1H NMR (300 MHz, CD3CN) δ: 8.86 (d, J = 9.08 Hz, 2H), 8.45 (d, J = 9.08 Hz, 2H). Electrochemical Measurement. All electrochemical measurements were performed with a μAutoLabIII potentiostat (Metrohm AutoLab B.V. Netherlands) and a conventional three-electrode system, comprised of a GC working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl 3.0 M NaCl electrode (CH Instruments) as reference. All potentials were reported versus the Ag/AgCl reference electrode at room temperature. Data collecting and processing were performed with the operation software GPES 4.9. Preparation of Modified Glassy Carbon Electrode and Plate. Glassy carbon (GC) electrodes (CH Instruments, 3 mm diameter disks) were first polished successively in 1.0, 0.3, and 0.05 μm alumina slurries for 3 min at each grade. The polished electrodes were then thoroughly rinsed and sonicated in Milli-Q water for 5 min to remove the alumina residue. Surface derivatization of cleaned GC electrodes with 4-(trimethylammonio)phenyl was performed in solutions of 4(trimethylammonio)phenyl diazonium salt concentrations of 1, 5, and 10 mM in acetonitrile. The molecule 4-sulfophenyl was grafted in 5 mM 4-sulfophenyl diazonium salt solution in 1:4 water−acetonitrile mixed medium. SP/TMAP mixed diazonium salts grafting was conducted in the 5 mM total aryl diazonium salts concentration with the SP:TMAP ratio of 3:7, 1:1, and 7:3 in 1:4 water−acetonitrile mixed medium. All aryl diazonium salt solutions contain 0.1 M NBu4BF4, as supporting electrolyte. The grafting of different phenyl layers were conducted using cyclic voltammetry at a scan rate of 100 mV/s from 0.7 to −1.5 V for three cycles. All aryl diazonium salt solutions were deaerated with argon through bubbling for at least 30 min prior to the derivatization and were kept under a blanket of argon during reductive adsorption. Cleaning and modification of glassy carbon plates (HTW, Germany) for surface characterization by XPS was performed in the same manner as was done with the disk electrode described above. Cleaned plates were clipped on one end to the potentiostat and the other end dipped into the aryl diazonium salt solution for grafting. The modified plates were thoroughly rinsed with acetonitrile and Milli-Q water with brief sonication, and then the plates were dried and kept under argon gas before the XPS measurement. Extra steps for removing physically adsorbed aryl diazonium salts from the modified electrodes and plates was performed by rinsing with 10 M hydrochloric acid (HCl) for 30 s twice and then rinsing with a copious amount of Milli-Q water. X-ray Photoelectron Spectroscopy Measurement. XPS spectra were obtained using an EscaLab 220-IXL spectrometer with a monochromated Al Kα source (1486.6 eV), hemispherical analyzer, and multichannel detector. The spectra were accumulated at a takeoff angle of 90° with a 0.79 mm2 spot size at a pressure of less than 10−8 mbar. Survey scans (0−1100 eV) were carried out with 1.0 eV step size, 100 ms dwell time, and analyzer pass energy 100 eV. Highresolution scans (S 2p, C 1s, N 1s) were carried out with 0.1 eV step size, 100 ms dwell time, and pass energy 20 eV. Binding energies of elements was corrected with reference to graphite carbon C 1s (284.4 eV). The XPS spectrum was analyzed with the curve-fitting program XPSPEAK 4.1 and involved background subtraction using Shirley routine and a subsequent nonlinear least-squares fitting to mixed Gaussian−Lorentzian functions (with Gaussian−Lorentzian ratio of O 1s 45%, S 2p 30%, C 1s 20%, N 1s 100%). The atomic ratios of different species on the surfaces were calculated by the normalized area of peaks, which is the area under the peaks of the narrow scan spectrum (core-level spectrum) divided by the number of scans and the element sensitivity factor. For the elements considered, the sensitivity factors are S 2p 1.68, O 1s 2.93, C 1s 1.00, and N 1s 1.80. Diffusion Coefficient Measurement by DOSY NMR. The NMR spectra for diffusion ordered spectroscopy (DOSY) NMR plot were recorded on a Bruker Advance III 500.13 MHz spectrometer equipped with a 5 mm TBI gradient probe having a maximum gradient strength of 55.0 G/cm. The measurement was conducted at 298 K under nitrogen gas flow without spinning of the sample. Diffusion NMR

spectra were obtained using a stimulated echo bipolar gradient (STEBPGP 1s) pulse sequence. Sample preparation for DOSY experiments were performed by dissolving appropriate amounts of sulfophenyl (SP) diazonium tetrafluoroborate and 4-(trimethylammonio)phenyl (TMAP) diazonium tetrafluoroborate in 0.74 mL of CD3CN−D2O (3:1) mixed solvent in order to obtain (1) a 5 mM SP diazonium salt solution, (2) a 5 mM TMAP diazonium salt solution, and (3) a 10 mM (5 + 5) SP/ TMAP mixed diazonium salt solution. For each sample, one set of diffusion NMR measurements was taken. Before each set of measurements, preliminary 1D NMR measurements were performed to obtain the optimized value of the gradient pulse with δ and the diffusion delay Δ. The optimized value of δ was 2.8 ms and the optimized value of Δ was 70 ms for sample 1 (SP), 60 ms for sample 2 (TMAP), and 70 ms for sample 3 (SP/ TMAP Mix). Sixteen experiments ranging linearly from 2% to 95% of the maximum gradient strength (55.0 G/cm) and a fixed gradient recovery delay (τ = 0.2 ms) were acquired. 32 scans were obtained, each with 65 536 data points and a recycle delay (D1) of 5 s. NMR spectrum analysis and decay curve fitting for diffusion coefficient were performed by Bruker Topspin v 2.1 with internal analysis tools to generate the DOSY spectra. Computational Methods. All density functional theory (DFT) calculations were performed with Gaussian 0344 at the B3LYP level of theory using the 6-311+G(d,p) basis set, and visualization was carried out using Avogadro.45 Final gas-phase geometries were calculated using the GDIIS optimization algorithm and tight convergence criteria, which were confirmed as minima by the absence of imaginary frequencies in frequency calculations. Molecular orbital and natural bond orbital (NBO) analyses were performed using the same density functional and basis set on the respective optimized structures.

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RESULTS AND DISCUSSION Electrochemical Deposition Behavior of SP and TMAP Diazonium Salts Mixtures Investigated Using Cyclic Voltammetry and X-ray Photoelectron Spectroscopy. Prior to examining the mixed layers formed by SP and TMAP, it was necessary to first characterize the reductive adsorption behavior of the two individual salts and the mixture of the two salts. The reductive adsorption behavior of SP diazonium salt, TMAP diazonium salt, and 1:1 SP/TMAP diazonium salts mixture in their respective 5 mM solutions was studied by cyclic voltammetry. As there have not been any reports of the surface grafting using the TMAP diazonium salt, a more detailed investigation of its reductive adsorption behavior is provided in the Supporting Information, which includes the influence of concentration on the grafting of TMAP layers (Figure SI.1), the electrochemical response of surfaces modified with TMAP layers toward the soluble redox probe Ru3+(NH3)6 (Figure SI.2), and the XPS characterization (Figure SI.3). Figure 1 shows the reductive adsorption cyclic voltammograms (CVs) of the mixture and the two individual salts. In all three CVs a clear peak indicative of the reductive adsorption is observed with the peak being at a different potential in all cases. Importantly, the CV of a mixed SP/TMAP diazonium salts solution exhibits a single reductive adsorption peak at about −0.25 V. This peak position is a significant positive shift (0.28 V more positive than SP) of the reduction potential compared to either of the two individual salts (observed at around −0.53 V for SP and −0.41 V for TMAP). Additionally, despite showing a reduction peak less than half of the size of two individual aryl diazonium salts reduction peaks, the complete passivation of the electrode surface caused by deposition of phenyl derivatives in mixed solution took only one cycle. In the case of TMAP diazonium salt solution, more than one cycle is required for the complete passivation of the electrode surface. 4774

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produced that adsorb onto the electrode surface might be much higher than in single aryl diazonium salt solutions. To find out whether both or just one of the two phenyl species have been attached (particularly in what the ratio) from the mixed solution, surface characterization has been performed by XPS. To investigate the connection between the SP/TMAP ratio in the deposition solution and the ratio on the resulting modified glassy carbon surface, XPS measurements were performed on surfaces modified with SP and TMAP single component layers and mixed layers from solutions with SP/ TMAP ratio of 3:7, 1:1, and 7:3. The samples were cleaned by rinsing with copious amounts of acetonitrile and water with brief sonication right after modification. As shown in Figure 2b−d, the presence of sulfur signal in the spectra of mixed

Figure 1. Cyclic voltammogram profile comparison of reductive adsorption in solution of 5 mM mixed diazonium salts of 4-sulfophenyl and 4-(trimethylammonio)phenyl with a ratio of 1:1 (2.5 mM + 2.5 mM) (a); 5 mM 4-(trimethylammonio)phenyl diazonium salt in acetonitrile (b); 5 mM 4-sulfophenyl diazonium salt in 1:3 water− acetonitrile solution (c), with 0.1 M of NBu4BF4, at the scan rate of 100 mV/s. CV representations are first scan (black), second scan (dark gray), and third scan (gray).

Figure 2. X-ray photoelectron spectra of survey scans of carbon surfaces modified by SP diazonium salt only (a); SP/TMAP mixed aryl diazonium salts of solution ratio 3:7 (b), 1:1 (c), and 7:3 (d); TMAP diazonium salt only (e).

In SP diazonium salt solution, the surface passivation is almost complete after one cycle, but the passivation is significantly less efficient than in a solution of the TMAP/SP mixture with regards to the attachment of the phenyl radical as indicated by the much larger reductive peak for SP than the mixture. The cyclic voltammetry of reductive adsorption was also recorded in solutions of 5 mM mixed aryl diazonium salts with SP/TMAP ratio of 3:7 and 7:3. The CVs are similar to Figure 1a, again exhibiting only one reduction peak and showing only minor differences in peak potential and peak size, compared with in 1:1 solution. A comparison of the peak potentials, together with the charge passed (normalized by per unit area of electrode surface) under the reduction peak of the initial scan of CVs, in solutions of various compositions, has been provided in Table SI.2. Considering the reductive adsorption peaks for SP and TMAP diazonium salts are at different potentials, naively one would expect either evidence of two peaks for reduction of the SP/TMAP diazonium salts mixture or one broad peak that encompasses the reductive adsorption potentials of both of these two salts. However, the peak potential for the mixed TMAP/SP diazonium salts reduction is considerably more positive than either of the two single aryl diazonium salts alone. This shift in peak potential suggests that in the mixed system the energy barriers for reducing SP and TMAP diazonium salts might have been both decreased. Furthermore, the differences shown by the three CVs in Figure 1 in terms of the size of reduction peaks and the rate of electrode surface passivation suggests that, in mixed solutions, the percentage of radicals

layers provides evidence for the presence of SP on the surface. The presence of a nitrogen signal after modification with the TMAP/SP mixture however does not unequivocally show TMAP is present on the surface since there is also a nitrogen signal observed on single-component surfaces. Hence, more detailed analysis of the narrow scan of N 1s of all surfaces is needed. Importantly, no significant difference can be observed between the XPS spectra of the grafted mixed layers despite solutions with three different SP/TMAP ratios being employed. An interesting observation from the XPS survey scans of single component layers (Figure 2a,e) compared with the mixed layers is the glassy carbon modified with single component layers shows the presence of adsorbed counterions required to neutralize the surface charge. Sodium signals (from Na+) can be observed in the survey spectrum of SP layers (Figure 2a), while in the case of TMAP layers, fluorine signals are present (from BF4−, Figure 2e). However, on carbon surfaces modified from mixed solutions, neither sodium nor fluorine signals can be observed, indicating the charge neutralization is achieved by the presence of the oppositely charged sulfo and ammonium moieties. Although detailed qualitative analysis of the core-level spectra and quantitative analysis of surface bound species are needed, the fact that no counterions are present on surfaces modified from mixed solutions strongly implies that the two oppositely 4775

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Figure 3. X-ray photoelectron spectrum of mixed layers of SP and TMAP on glassy carbon surface. The S 2p, C 1s, and N 1s narrow scans of mixed layers grafted from mixed solution of SP/TMAP in the ratio of 1:1.

charged moieties are present in a ratio of 1:1. It might also suggest that the distribution of the two components to be homogeneous, or at least no significant large domains of one type of phenyl derivative are present, as if this were the case counterions to restore electrical neutrality would be expected. The above observation is not enough to exclude the possibility of an electrostatic bilayer structure of the mixed surfaces. That is, one species is attached on to the surface covalently while the other species is simply electrostatically adsorbed. In order to ensure the XPS signal only comes from the covalently attached molecules, samples modified with mixed layers were rinsed with 10 M hydrochloric acid twice, followed by copious amount of rinsing with Milli-Q water. Additional to the rinsing process, electrochemical treatment has been applied to the mixed layers grafted surfaces. The modified surfaces were scanned between −1.0 and 0.8 V in 0.1 M aqueous KCl solution for up to 60 cycles. The main purpose of this treatment is to investigate the electrochemical stability of the surface attached mixed layers.46 It is important to examine the electrochemical robustness of the grafted SP/TMAP mixed layers for its further application. Furthermore, by scanning the electrode surface between positive and negative potentials, the surface oscillates between positive and negative of the point of zero charge, hence the surface is repulsive to negatively and positively charged molecules. Therefore, this treatment is also functioning as a method of removing physically adsorbed charged molecules. The result of XPS characterization of mix layers grafted samples has shown no significant changes occurred to the grafted layers before and after the acid rinsing steps plus 60 cycles of CV scans (see Figure SI.4 for details). We believe these treatments ensure the XPS observations are derived only from the covalently attached species on the surface. Hence-

forth, the XPS data used all comes from samples treated with 10 M HCl rinsing and electrochemical scans. Because of the fact that only minor differences in the XPS spectra are observed between samples modified with solutions of different SP/TMAP ratios, only the XPS N 1s, S 2p, and C 1s core-level spectra (narrow scans) from a sample modified from a solution of ratio 1:1 are presented in Figure 3. As shown in Figure 3 in N 1s and S 2p narrow scans, the nitrogen 403.3 eV peak (peak marked as N2 in Figure 2 N 1s spectrum) and sulfur 168.0 eV peak pair (Figure 2 S 2p spectrum, peak denoted as S, is fitted with the S2p1/2 peak and S 2p3/2 peak centered at 167.7 and 168.8 eV, respectively) are the characteristic signals of TMAP47 and SP48,49 moieties in the mixed layers. The narrow scan of carbon C 1s can be fitted with three peaks centered at 284.3 eV (C1), 285.4 eV (C2), and 286.4 eV (C3). The C3 peak is assigned to the four carbon atoms that bond directly to the nitrogen in trimethylammonio group, while peak C2 is assigned to the aromatic carbon atoms including five from TMAP moieties and all six from SP moieties. The C1 peak is from the carbon signal of glassy carbon substrate. Table 1 presents the comparison of SP/TMAP surface ratio of the mixed layer grafted surfaces from solutions of three different ratios. The ratios are calculated from the normalized area of XPS peaks of TMAP (N2 in Figure 3 N 1s spectrum) and SP (Figure 3 the whole spectrum of S 2p, denoted as peak S); the definition of normalized area can be found in the Experimental Section. The comparison suggests the surface ratios of the two charged species in mixed layers are all essentially 1:1 regardless of the ratio of the two aryl diazonium salts in the modification solutions. That is, no significant difference was observed between grafted mixed layers prepared from solutions of different SP/TMAP ratios from the 4776

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Changes of the diffusion coefficient D can be very useful as an indicator of the association/dissociation between ionic species because according to Stokes−Einstein equation

Table 1. Comparison of SP/TMAP Diazonium Salts Ratios in Mixed Modification Solutions to the XPS Atomic Ratio of SP and TMAP Moieties in the Mixed Layer Grafted on Glassy Carbon Surfacesa SP/TMAP solution ratio

SP/TMAP surface ratio

3:7 1:1 7:3

1:1.01 (s = 0.05) 1:1.02 (s = 0.04) 1:0.99 (s = 0.05)

D = kT /6πηrH

where k is the Boltzmann constant, T the temperature, η the viscosity of the medium, and rH the hydrodynamic radius of the molecule; the diffusion constant D decreases as the size (hydrodynamic radius) of the molecule gets larger. Therefore, the association of molecules, resulting in formation of a larger entity, can be probed by a decrease in the diffusion coefficient. In the case of this study, a decrease in the diffusion coefficients of both SP and TMAP diazonium species should be expected if the two species associate. If the individual D of the two are very different from each other, then observation of the approximately same diffusion coefficient in mixed solutions will be a good indicator of the association. Diffusion coefficient measurements were performed using DOSY (diffusion ordered spectroscopy) NMR. The comparison of diffusion coefficients of the SP and TMAP diazonium species between the mixed solution and their separate solutions is presented in Table 3.

a

Calculation of average values and standard deviation (s) of measurements from three sets of samples. Each set contains three samples modified from mixed diazonium salt solutions of three different SP/TMAP ratios (n = 9).

perspective of the chemical composition. Table 2 presents the good correlation between the predicted molecular structures Table 2. Normalized Area Ratios of Subpeaks from XPS (Experimental Ratio) and the Correlation of the Ratios to the Molecular Structure of SP and TMAP Moieties on Mixed Layers Modified Glassy Carbon (Theoretical Ratio)a C2/C3 C3/N2 C2+C3/N2+S

exptl ratio

theor ratio

11:4.04 (s = 0.08) 4:1.021 (s = 0.003) 15:2.04 (s = 0.05)

11:4 4:1 15:2

Table 3. Result of Diffusion NMR (DOSY) Measurement, the Comparison of Diffusion Coefficients of SP and TMAP Diazonium Species between in Single Diazonium Salt Solutions and SP/TMAP Mixed Diazonium Salts Solutiona

a

Average values and standard deviation (s) were calculated from the measurements of nine samples (n = 9).

SP TMAP SP/TMAP

(shown in Figure 3) on the surface with the measured values from XPS (also calculated from normal areas). Because of the similarity of the result from samples modified in solutions of three different ratios, all nine samples from Table 1 have been used in the calculation in Table 2. The ability of the SP/TMAP mixed layers to restrict access of soluble redox species Fe(CN)63− and Ru(NH3)63+ to the underlying electrode has also been examined by CV which has also been compared with the surface modified with single component layers (shown in Figure SI.5). The mixed layers have shown no significant suppression to the electrochemistry of (Ru(NH3)63+), which carries out outer-sphere electron transfer, but present a distinctly greater barrier effect toward inner-sphere electron transfer redox species (Fe(CN)63−). Intermolecular Interaction between SP Diazonium Species and TMAP Diazonium Species in Solution. The studies on reductive adsorption behavior in the SP and TMAP diazonium salts mixture have found two unusual features which are (1) the reduction of the two aryl diazonium salts occurred at the same potential which is less negative than either of their reductive potentials alone and (2) the surface ratio of the two covalently attached phenyl derivatives SP/TMAP presented a constant value 1:1, which is independent of the ratio of the two components in the deposition solutions. The combination of the two observations strongly suggests that the two aryl diazonium species have associated to form pairs in solution prior to reductive deposition, presumably through electrostatic interactions between the two species. This association might have an impact on the electronic structures, leading to lower energy requirements for the reductive adsorption reaction. The following will include diffusion coefficients and computational studies to provide support to these suggestions.

DSP (× 10−9 m2/s)

DTMAP (× 10−9 m2/s)

2.116 (s = 0.009) NA 1.968 (s = 0.005)

NA 2.483 (s = 0.013) 1.982 (s = 0.007)

a

Average values and standard deviation (s) were calculated from multiple measurements and curve fittings (n = 6).

As shown in the Table 3, the results of diffusion coefficient measurements suggest the association of SP and TMAP diazonium species in mixed solution; this association is shown by decreased D, which is consistent with a greater hydrodynamic radius and the approximate equivalence of the diffusion constants of each species in the solution containing the mixture of the two salts. It is hypothesized that the more positive reduction potential observed in CVs of deposition in the SP/TMAP mixture, relative to either individual salt, is due to electrostatic interactions between the two salts. To provide evidence for such a hypothesis, an additional electrochemical study was performed. In this study the reductive adsorption of SP diazonium salt in the presence of benzene trimethylammonium (BTMA) cation was performed. Similarly, the reductive adsorption of TMAP diazonium salt in the presence of benzenesulfonate (BS) anion was investigated. BTMA and BS were chosen because they have similar molecular structures to TMAP diazonium salt and SP diazonium salt, respectively, but lack the diazonium moiety and hence are unable to reduce or deposit onto the electrode surface. As Figure 4 shows, when the corresponding oppositely charged substituted benzene species was present, reductive adsorption of both SP diazonium salt and TMAP diazonium salt was observed to shift slightly positive, about 20 mV (s = 3, n = 3), of the reduction peak potential for the SP diazonium salt and 45 mV (s = 5, n = 3) for the TMAP diazonium salt. In particular, the change in the shape of the reduction curve of SP diazonium salt in the presence of BTMA, shown in Figure 4a, is 4777

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Figure 5. SP and TMAP diazonium binary complex showing interactions between the HOMO of the SP diazonium component and the LUMO of the TMAP diazonium component.

Figure 4. Cyclic voltammetry initial scan of the reductive adsorption of SP diazonium salt with (gray curve) and without (black curve) BTMA (a); TMAP diazonium salt with (gray curve) and without (black curve) BS (b), in the solution of 1:3 water−acetonitrile containing 0.1 M of NBu4 BF4, at scan rate of 100 mV/s. The inset graphs are the zoom-in of reduction peaks with the peak potentials marked by the dashed lines.

electrochemical experiments. Examination of the net electron population on each aryl diazonium species in the binary complex shows that there is a net decrease in electron density on the SP diazonium species and a corresponding increase on the TMAP diazonium species as a result of the association. Particular attention was paid to the electron population on the diazonium-bearing carbon (C1) and the associated nitrogen centers (N1 and N2, N1 is the nitrogen which directly attaches to the benzene ring), presented in Table 4, as charges

an indication of the increased rate of surface passivation upon formation of phenyl layers. In general, the cyclic voltammetry changes observed are consistent with those described for mixtures of SP/TMAP and suggest that intermolecular interactions of SP and TMAP diazonium salts with the corresponding benzene species bearing oppositely charged substituent is occurring. Although the interaction has similar impact on the reductive adsorption behavior of SP diazonium salt and TMAP diazonium salt, as compared to the mixing of the two aryl diazonium salts, the extent of influence is weaker. It is of interest to consider whether molecular modeling might be used to rationalize both the interaction between the two species (already supported by DOSY NMR experiments) and how the association of the two diazonium species is related to the change in their electrochemical behavior (already demonstrated by electrochemical measurements). The following study by means of computation based on DFT (density functional theory) is to interrogate how the observation above in electrochemical and XPS experiments is related to SP and TMAP association. Geometry optimization using different starting geometries yielded the binary complex structure shown in Figure 5 as the lowest in energy, which was consequently used in all further relevant analyses. It is worth noting that the interaction observed can be attributed to substantial association between the HOMO of the SP diazonium species and LUMO of the TMAP diazonium species; the slightly bent nature of the TMAP diazonium species in the ion pair shown in Figure 5 is likely due to electrostatic interactions of the positively charged functionalities (−N+(CH3)3 and −N2+) on the TMAP with the sulfonate functionality, where the diazonium and trimethylammonium functional groups of the TMAP component were observed to deviate from the planarity of the benzene ring by 1.7° and 3.5°, respectively. Natural bond orbital analysis was performed on the isolated diazonium species, as well as the SP/TMAP binary complex, to examine whether any changes in electron population can be correlated to the shifting of the reduction potential observed in

Table 4. NPA Charges Calculated for Selected Atoms of the Isolated Aryl Diazonium Salts and the Corresponding Atoms of the Aryl Diazonium Salts in the Binary Complex NPA charges (on the specific atom) SP (isolated) TMAP (isolated) SP (in the complex) TMAP (in the complex)

C1

N1

N2

−0.012 0.027 0.005 0.032

0.124 0.138 0.137 0.152

0.191 0.382 0.304 0.333

determined from natural population analysis (NPA). Significant changes in electron distribution were observed progressing from the isolated aryl diazonium species to the corresponding aryl diazonium species in the paired system. Decreases in electron populations at each center, as indicated by increased charges, were observed in all cases with the exception of N2 on the TMAP diazonium species. Also worth noting is the substantial decrease in electron population on N2 of the SP diazonium salt. Such decreases in electron population would likely lead to significant changes in potential required for the aryl diazonium reduction. For this reason, energies were also calculated for the LUMO of each of the isolated diazonium species and the corresponding molecular orbitals in each half of the complex (Table 5). LUMO energies can, at least to a first approximation, give an indication of the one-electron affinity of a species. Initially in the separate aryl diazonium species, as presented in Table 5, the LUMO energy is substantially higher for the SP diazonium species than that of the TMAP diazonium species. This is consistent with what was observed in cyclic voltammetry, where the reduction potential required to deposit the SP diazonium species was more negative relative to the reduction potential required to deposit the TMAP diazonium species. 4778

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measurements have suggested the association of two diazonium species in solution. Gas phase modeling of SP/TMAP diazonium complex has revealed interaction between their molecular orbitals, which is induced by electrostatic attraction. The modeling has also shown there are changes in electron population and LUMO energies of SP and TMAP diazonium species due to the association, which is in general correlated to the shifting of reductive potential observed. The result of this research work highlights the potential for utilizing supramolecular interactions between modifiers in mixed solutions to alter and manipulate the adsorption behavior on a surface and hence achieve a greater control over the interfacial architecture of the modified surface. The current study has also shown a surface presenting zwitterionic properties can be simply obtained from the deposition of opposite charged aryl diazonium salts. This aryl diazonium salt derived zwitterionic surface might have protein resistance, as it has been reported by Whitesides and co-workers in previous studies of alkanethiol-based zwitterionic SAMs.16,17 The antifouling property of this surface will be investigated in our future paper.

Table 5. LUMO Energies Calculated for Each of the Aryl Diazonium Species as Isolated Species and Energies for the Corresponding Molecular Orbital Counterparts in the Binary Complex MO energies (eV) atom

SP

TMAP

isolated paired

−4.84 −9.22

−11.4 −9.80

As distinct from the separated aryl diazonium species, the binary complex shows significant changes in the energies of the molecular orbitals in the associated system. The LUMO of each component was calculated to shift such that the final energies of the relevant orbitals in the paired systems become much closer in energy. As such, the results suggest that the reduction potential required to deposit both species in the complex would be similar. However, the calculations also suggest that the reductive potential of SP/TMAP diazonium complex would be at a point in between the potentials required for reduction of the two isolated species. In summary, DFT calculations of the SP/TMAP complex and that of each of the isolated aryl diazonium salts have demonstrated favorable interactions between the SP and TMAP diazonium species in the gas phase, which results in the formation of a binary complex. This is consistent with diffusion NMR experiments and the 1:1 SP/TMAP codeposition observed in surface grafting cyclic voltammetry. Calculations have also demonstrated that pairing between the SP and TMAP diazonium species results in changes in the electron distribution through the entire structure, particularly near the diazonium functionality, and the orbital energies of the diazonium moiety of each aryl diazonium salt become very similar. The results agree with the observation in electrochemical experiments with regards to the salts interacting with each other changes in potential of the complex compared with the individual salts and that the two salts in the complex will have similar, or the same, reduction potential. There is one discrepancy between experiment and calculation regarding the LUMO energy change of TMAP diazonium species, suggesting TMAP alone would be reduced at more positive potentials than the complex. It should be pointed out that the computations above have been carried out in gas phase, while the electrochemical experiments will, by definition, be affected by solvent. However, the extent of agreement between the modeling and experimental data is very encouraging and suggests that such modeling can be used as a guide for explaining the experimental observation.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammetry of deposition of TMAP diazonium salt on GC surfaces in solutions of different salt concentration, electrochemical responses of TMAP layers modified GC electrode to Fe(CN)63− in aqueous solution, XPS characterization of TMAP layers grafted GC surfaces, comparison of cyclic voltammetry peak potential and peak size of SP, TMAP single diazonium salt deposition and SP/TMAP mixed diazonium salts deposition, XPS studies of the electrochemical stability of SP/TMAP mixed layers, electrochemical responses of SP/TMAP mixed layers grafted GC electrode to Fe(CN)63− and R3+(NH3)6 in aqueous solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Australian Research Council (LP0775216) and the Australian Government for funding. A.L.G. thanks J. Ginges for valuable suggestion in sample processing and the University of New South Wales for the scholarship.

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CONCLUSION The electrochemical deposition behavior of mixed SP/TMAP diazonium salts has been revealed by cyclic voltammetry and XPS studies with two striking features observed. First, the surface grafted mixed phenyl derivative layers have presented a constant 1:1 ratio of SP/TMAP phenyl derivatives, independent of the mixing ratio of the corresponding aryl diazonium salts in solution. Analysis of XPS survey spectra also suggests the distribution of the two phenyl derivates is likely to be homogeneous. Second, the reduction of two diazonium salts take place at the same potential in the mixed solution, which is less negative than the potential required for the reduction of either of the two alone. Furthermore, diffusion coefficient

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