One-Pot Electrografting of Mixed Monolayers with Controlled

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One-Pot Electrografting of Mixed Mono-Layers with Controlled Composition Luis Miguel Santos, Alice Mattiuzzi, Ivan Jabin, Nicolas Vandencasteele, Francois Reniers, Olivia Reinaud, Philippe Hapiot, Sébastien Lhenry, Yann R Leroux, and Corinne Lagrost J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5052003 • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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The Journal of Physical Chemistry

One-Pot Electrografting of Mixed Mono-Layers with Controlled Composition. Luis Santos,† Alice Mattiuzzi,‡ Ivan Jabin,*, ‡ Nicolas Vandencasteele,┴ François Reniers,┴ Olivia Reinaud,*,§ Philippe Hapiot,† Sébastien Lhenry,† Yann Leroux,† and Corinne Lagrost*,†

†

Institut des Sciences Chimiques de Rennes UMR n° 6226, Université de Rennes 1 and CNRS Equipe MaCSE, Campus de Beaulieu, 35042 Rennes cedex, France

‡

Laboratoire de Chimie Organique, Université Libre de Bruxelles (ULB), CP 160/06, 50 avenue F.D. Roosevelt, 1050 Brussels, Belgium ┴

Chimie Analytique et Chimie des Interfaces, Université Libre de Bruxelles (ULB), CP 255, Campus de la Plaine, boulevard du Triomphe, 1050 Brussels, Belgium

§

Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques UMR n°8601, PRES

Sorbonne Paris Cité, Université Paris Descartes and CNRS, 45 rue des Saints-Pères, 75006 Paris, France

ACS Paragon Plus Environment

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ABSTRACT: Surface functionalization with ultra-thin layers exhibiting highly robust interface is of paramount importance to design materials with tailored properties or with operating functions, without modifying drastically the materials bulk structures. A fine tuning of the surface composition obtained for instance from binary mixed layers is also a key issue for developing high value-added applications like efficient sensors. Herein, binary mixtures of calix[4]arene-tetra-diazonium salts in situ generated from their corresponding calix[4]tetraanilines are electrografted to form covalently-bound monolayers onto substrates for yielding versatile functionalizable molecular platforms. Wettability studies, X-ray photoelectron spectroscopy (XPS) analyses and Scanning Electrochemical Microscopy (SECM) show the formation of homogeneous mixed monolayers. The distribution of the two calixarenes on the surface is directed by their relative molar fraction in the deposition solution. The strategy allows the control of the composition of mixed monolayers in a one-step approach. Postfunctionalization of the mixed layers with ferrocene centers is performed to exemplify the benefit of a dilution procedure when functional groups are introduced at the calix[4]arene small rim. This study contributes to highlight the potential of the diazonium salts electrografting as a competitive alternative to chemisorption strategy like self-assembled monolayers (SAMs) of alkyl thiols in the field of surface functionalization.

Keywords: diazonium electrografting, calixarene, binary monolayer, surface functionalization, controlled interfaces


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1. INTRODUCTION The electrografting of organic structures from the reduction of aryldiazonium salts has become a recognized method for surface functionalization.1,2,3,4,5 Contrary to the organothiols selfassembling, this method can be applied to a wide range of conducting (graphite, glassy carbon, Fe, Co, Cu, Zn, Pt, Au…) or semi-conducting (SiH, SiO2, SiOC…) materials and provides organic layers that are generally highly stable, resistant to heat, to chemical degradation and to ultrasonication. The method is also easy to process and fast (deposition time of a few seconds to minutes). The electrografting process consists in the transient formation of aryls radicals that attack the electrode surface, allowing a robust attachment of the organic layers at the surface. The efficiency of the procedure is due to the high reactivity of the aryl radicals produced at the vicinity of the electrode surface. This reactivity is paradoxically at the origin of a major drawback of the method. The electrogenerated aryl radicals also react with already-grafted aryl species, leading to the formation of disordered polyaryl multilayers. This lack of control of the layers thickness and organization precludes any tight control of the interfacial properties of functionalized surfaces and may be clearly a strong disadvantage. In this context, considerable efforts have recently been made to produce monolayers or ultrathin layers with diazonium chemistry. Beyond the empirical control of experimental conditions that permits the preparation of ultra-thin layers, several strategies inspired from chemical engineering have been proposed to form monolayer films with high reproducibility.6,7,8,9,10,11,12 The most efficient approach developed for obtaining post-functionalisable monolayers is the design of diazonium derivatives with a sterically-hindered or an electrostatically-shielded cleavable protective group.7,8,9 After the film formation, the protective group is removed to expose a newly


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activated monolayer to a post-functionalization step directed to the introduction of functional molecules. A clever approach to form monolayers has been also reported more recently, employing a radical scavenger during the electrografting process to prevent the polymerization of the electrogenerated radicals on a carbon surface.13 In this regard, we have proposed a different strategy based on the use of pre-organized macrocycles, i.e. calix[4]arenes molecules with four aryldiazonium functions at their large rim (Scheme 1).14,15 These molecules form closely-packed monolayers with a very robust interface. The small rim of the immobilized calixarenes can be decorated with various reactive appended arms (up to four) to introduce functional entities onto the surface. This strategy provides a fine spatial control of the postfunctionalization reactions as it is imposed by the geometry of the small rim and the number of the appended arms.14

Scheme 1. Representation of the strategy involving the electrografting of calix[4]arenetetradiazonium, the structure of calix[4]tetra-anilines 1-4. PPF is pyrolized photoresist film, GC glassy carbon.


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Another fundamental issue that enhances the attractiveness of the aryldiazonium grafting is the possibility of designing mixed monolayers. Such mixed systems have many potential useful applications, particularly those where it is of fundamental importance to control the areal density of functional groups or to tune the chemical or physical properties of the monolayers by changing the ratio between the molecular components. Tailoring the electrode surface with different functionalities organized as monolayers is particularly attractive in the fabrication of sensors or biosensors as the design of the interface is crucial for the sensing performance. The dilution of the functional species helps in increasing the sensor sensitivity with a better accessibility of the active species and/or in limiting non-specific adsorption of proteins in the recognition process.16,17,18 Thus, mixed monolayers of thiols derivatives adsorbed on gold have been extensively studied, being prepared either by co-adsorption or exchange reactions.19,20 The co-adsorption of two different thiols from a mixed solution often leads to fractional coverage that is different from the molecular composition in solution, due to a kinetic competition between the adsorption of the two components.19,21,22 In addition, it is often observed that the two molecular components can segregate upon self-assembly leading to phase separation of the two molecules on the surface.19,23,24 Unlike thiol derivatives, the aryl groups grafted from electrochemical reduction of aryldiazonium salts are unlikely to become phase separated because of their covalent binding to the surface that precludes any surface mobility. However, mixed layers derived

from

the

electrografting

of

aryldiazonium

are

more

rarely

described,16,17,18,25,26,27,28,29,30,31,32,33,34 and even scarcer if mixed monolayers are considered.35 The control of the surface composition with the electroreduction of a mixture of aryldiazonium salts is extremely difficult to achieve in a one-step procedure because of the high and unselective reactivity of the electrogenerated radicals. The surface concentration of the component for which


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the diazonium cation is the easiest to reduce is generally found to be higher than its mole fraction in the solution.28,30 The formation of multilayers where the electrogenerated aryl radicals are grafted over the primary layer is another limitation for the preparation of homogeneous mixed layers with diazonium electrochemistry. Thus, procedures with sequential grafting have been preferentially developed to prepare binary layers with adjusted morphology and surface composition.26,33,34,35 Nonetheless, examples of one-step formation of binary layers have been reported in case of aryldiazonium cations with close reduction potentials,31 or, more recently, with a mixture of two aryldiazonium salts bearing oppositely charged para-substituents.27,32 Intermolecular interactions between the two charged substituted aryldiazonium compounds (SO3- and N+(Me)3 groups) allows an homogeneous distribution of the phenyl species on the surface with a constant 1:1 ratio whatever the molar ratio of the two diazonium salts in the solution.32 A drawback of this strategy is the impossibility to tune the surface molar ratio of the two components. The present work reports a first investigation on the one-step formation of binary monolayers from the electrografting of a mixture of two different calix[4]arene-tetradiazonium salts in situ generated. These aryldiazonium species exhibit a common calix[4]arene scaffold with different substituents anchored on the calix small rim. The modified surfaces were characterized by contact angle measurements, XPS spectroscopy and SECM. A first objective of our study is to compare the surface composition of the resulting interfaces to that of the corresponding grafting solution in order to explore the possibility of controlling the preparation of binary mixed monolayers with the calixarene-based strategy. This question is of crucial importance for obtaining multifunctional surfaces that are tunable at will, but it also takes on a fundamental importance to evaluate how covalent grafting is able to compete with the chemisorption


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strategies like SAMs of alkyl thiols on gold, which is a chemical route widely developed for materials integration.

2. EXPERIMENTAL SECTION 2.1. Chemicals. The reagents and solvents used for surface chemistry were of high purity grade (Sigma-Aldrich, SDS, Alfa Aesar). The calix[4]tetra-anilines were synthesized as previously described.14 Ferrocenepentylamine (Fc-C5H10-NH2) was synthesized from ferrocene (Aldrich) and 5chlorovaleryl chloride (TCI) in five steps by adapting previously published procedures.36,37 An orange oil was obtained in an overall 51 % yield, 1H- NMR (CDCl3, 300 MHz, 298K) δ (ppm): 3.97-4.02 (m, 9H, Cp), 2.62 (t, 2H, J = 6 Hz, CH2-NH2), 2.26 (t, 2H, J = 6 Hz, Fc-CH2), 1.42 (m, 6H, NH2CH2-(CH2)3), 1.19 (s, 2H, NH2). 2.2. Surface modification procedures. Except for post-functionalization experiments where gold disk electrodes of 2 mm diameter (IJ Cambria) were employed, the surfaces used were gold substrates (gold layer onto Si substrates) purchased from Aldrich. The surfaces were thoroughly cleaned before surface functionalization (piranha solution, concentrated H2SO4 and ethanol). Caution: The concentrated H2SO4/H2O (aq) piranha solution is very dangerous, particularly in contact with organic materials, and should be handled very carefully. Surface modification was carried out in an ice bath. The diazonium cations were produced in situ. NaNO2 (4 eq.) was added to 0.5 M HCl aqueous solution containing 1 mM (total concentration) of calix[4]tetra-anilines (whether pure or mixture of two compounds). After 5 min under argon bubbling, the electrografting was achieved by applying a potential equal to -0.5 V vs SCE for 5 min. After each experiment, the samples were ultrasonicated for 5 minutes in four


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solvents with different hydrophobicity (i.e., water, ethanol, dichloromethane and toluene), and dried under argon. 2.3. Electrochemistry. Cyclic voltammograms were recorded with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/galvanostat from Eco Chemie B.V.) in a three electrodes setup with a SCE reference electrode and a platinum foil as counter electrode. The surface coverage of electroactive ferrocene is obtained from Faraday’s law, Γ = Q/nFA where Q is the charge obtained from the integration of the area under the voltammetric peaks, n is the number of exchanged electrons (n = 1), F is the Faraday constant and A the geometric area of the electrode. 2.4. SECM analyses. Measurements were performed using a homemade three-electrode setup similar to that described in ref

38,39,40

. The reference electrode was an Ag quasi-reference

electrode and a Pt wire was used as a counter electrode. The SECM setup is equipped with adjustable stage for tilt angle correction and is controlled by the SECMx software.39 The applied potential at the microelectrode tip is chosen as being sufficiently positive to ensure a fast electron transfer at the tip (diffusion plateau of the mediator). The microelectrode tip was a commercial 5 µm radius platinum disk (IJ Cambria) with a typical RG around 10 (RG is the ratio of the total electrode radius including the glass insulator over tip radius). The tip parameters (a and RG) were characterized independently from approach curves of an insulator and a conductor sample. The fitting curves were calculated with the MIRA software package40 and provided the dimensionless constant κ = kel a/D where kel is the apparent charge transfer constant at the sample surface, D the diffusion coefficient of the mediator and a the tip radius. All SECM experiments (two series of samples) were performed at room temperature in unbiased conditions (the substrates are not electrically connected). Approach curves were recorded at three different spots randomly distributed on the surface for each samples. The diffusion coefficient of the mediators


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were measured at a Pt microelectrode in the same media used for SECM studies and was found equal to 9.3 10-6 cm2 s-1 for dopamine and 4.6 10-6 cm2 s-1 for ferrocyanide, respectively. kel was determined at bare gold samples for dopamine and ferrocyanide, being respectively 0.13 and 0.02 cm.s-1. 2.4. Contact angle measurements. The static contact angles of 2 µL ultrapure water drops were measured on four different spots on the surfaces with an easy drop goniometer (Krüss). The contact angles were determined using a tangent 2 or circle fitting models. 2.5. XPS measurements. XPS measurements were performed on a PHI 5600 instrument. All spectra were acquired using the Mg Kα X-ray source (hν = 1253.6 eV) operating at 300 W with a 45° take-off angle (TOA). Here the photoelectron TOA is defined as the angle between the surface normal and the axis of the analyzer lens. Survey spectra (0-1000 eV) were acquired with an analyzer pass energy of 187.85 eV (dwell time = 0.05s, 1eV/step, #scan = 4), high resolution C1s spectra were acquired at 23.5 eV of pass energy (dwell time = 0.1 s, 0.1 eV/step, #scan = 10). Binding energies were referenced to the Au4f7/2 peak at 84.0 eV. The spot size was set to a diameter of 400 µm. The atomic concentration for surface composition was estimated using the integrated peaks areas, the peaks area where normalized by the manufacturer supplied sensitivity factor. The composition was calculated using the average value of 3 measurements on individual spot for each sample. The core level C1s spectra were peak-fitted using the CasaXPS software (Casa Sofware, Ltd. version 2.3.15).

3. RESULTS AND DISCUSSION 3.1. Preparation and studies of the binary mixed layers. Binary mixtures were prepared by using calix[4]tetra-anilines that are functionalized with different groups at the small rim (Scheme


9


1). As preliminary experiments, the electrochemical behaviour of their corresponding diazonium cations was briefly investigated by cyclic voltammetry. Typical cyclic voltammograms recorded upon reduction of the isolated calix[4]arene-tetradiazonium tetrafluoroborate in acetonitrile solution are displayed in Figure 1.41 Irreversible cathodic peaks are observed during the first sweep and disappear during the subsequent scans, leaving a small current. This behaviour indicates the formation of a grafted layer onto the electrode surface.1 Remarkably, close reductive peak potentials values are observed for the different diazonium cations, at about 0.04 V/SCE, 0.02 V/SCE, 0.06 V/SCE and -0.06 V/SCE for calixarene-tetradiazonium derived from compounds 1-4, respectively. This observation is explained by the presence of a common scaffold (+N2-Ar-OCH2-) that displays very similar reactivity whatever the R1 or R2 substitution pattern. This result is obviously crucial for developing mixed layers of controlled composition with diazonium chemistry.

3 0 i (µA)

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-3 -6

a)

-9 -12

-0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

E / V vs SCE


10

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i (µA)

0 -1 -2 -3

b) -0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

E / V vs SCE

2 1

i (µA)

0 -1 -2 -3

c)

-4 -5 -6

-0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

E / V vs SCE 2 1 0 i (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 -2

d)

-3 -4

-0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

E / V vs SCE

Figure 1. Cyclic voltammograms at a gold disk electrode of electrochemical reduction of calix[4]arene-tetradiazonium cations (1-3 mM) isolated from compounds 1 (a), 2 (b), 3 (c) and 4 (d) in CH3CN containing 0.1 M NBu4PF6 at a scan rate of 0.1 V s-1. 42 Solid lines correspond to the first scan. Dashed lines are the following 2-5th scans.


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For the preparation of the mixed monolayers, the electrografting process was conducted with diazonium cations in situ generated,

43

by using a starting grafting solution containing two

distinct calix[4]tetra-anilines (Scheme 1) with various composition. The addition of NaNO2 to aqueous acidic solution containing the calix[4]tetra-anilines 1-4 allows the production of the corresponding diazonium salts, which, after electroreduction, generate densely packed and robust monolayers of calix[4]arenes on gold or carbon surfaces.14 Mixtures of compounds 2 and 4 with different ratios, i.e. 0/100, 50/50, 90/10, 100/0, were generally used to exemplify the formation of the mixed monolayers. Gold electrode surfaces were modified by the application of a reductive single step potential (-0.5 V/SCE) during 5 minutes. After thorough rinsing under ultrasonication with a series of solvents (water, ethanol, dichloromethane, toluene) and drying under a stream of argon, water contact angle measurements were performed on the modified surfaces using the sessile drop technique. Figure 2 shows the images of the water droplets in contact with the different surfaces together with the obtained contact angle values. The static contact angle value decreases as the proportion of 4 in the deposition solution increases. This observation suggests that the surface composition incorporates more hydrophilic moieties when the deposition solution contains larger concentrations of 4 with respect to compound 2, in agreement with the hydrophilic character of 4 vs the hydrophobic nature of 2.


12

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Figure 2. Profile of a water droplet (2 µL) in contact with a gold substrate modified from the electrografting of diazonium cations in situ generated from different 2/4 ratios in the deposition solution a) 0/100 b) 50/50 c) 90/10 d) 100/0.

In the case of binary mixed assembly of alkanethiols, contact angle measurements have been widely used for compositional analysis.19,21,44 The equilibrium contact angles of a chemically heterogeneous surface can be calculated from phenomenological Cassie (eq 1)45 or Israelachvili equations (eq 2).46 A A 1 A B 1 A 1 A 1 A 1 B

(1) (2)

where θ, θA, θB are the contact angle of a liquid in contact with binary layers composed of A and B, with pure homogeneous layer of A and with pure homogeneous layer of B, respectively; fA denotes the fractional surface coverage of A, the fractional coverage of B, fB, being equal to 1 - f A. The Cassie Equation is applicable for modelling surfaces with separate, discrete chemical patches while the Israelachvili equation is more suitable when the size of the patches approaches molecular to atomic dimensions.46 These equations have been often used to establish a correlation between the surface composition and the solution composition.


13


By identifying the fractional surface coverage to the molar fraction in solution, we plotted the variation of cosine θ against the molar fraction of compound 4 according the Cassie and Israelachvili models (Figure 3). Linear relationships were obtained, and the contact angle data are observed to better fit the Israelachvili equation (R= 0.991) than the Cassie model (R = 0.986). These results suggest that the binary layers are mixed on very small length scales and that the molar fraction in solution and the fractional surface coverage are close to each other.

2.0

0.4 linear fit (R=0.986)

linear fit(R=0.991)

1.8 (1+ cos theta )2

0.3 cos theta

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0.2

A

0.1

1.6 1.4 1.2

B

1.0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

molar fraction of 4

0.4

0.6

0.8

1.0

molar fraction of 2

Figure 3. Variation of the cosine of the water contact angle of the modified surfaces as a function of molar fraction of compound 4 in the solution deposition according the Cassie‘s law (A) or the Israelachvili’s law (B). R is the linear correlation coefficient.

The modified surfaces were further analysed by X-Ray photoelectron spectroscopy (XPS). On the whole, the survey spectra showed main peaks corresponding to the elemental species C, N, O, Au and F (Figure S2). All survey spectra display the characteristic gold peaks with main ones at 84 and 88 eV assigned to Au4f7/2 and Au4f5/2, respectively. An overall attenuation of the Au photoelectron peaks and a significant increase in the C1s (285 eV) and O1s (533 eV) peak heights, as compared with a bare gold sample,47 indicates the formation of a thin organic layer


14

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onto the gold substrate. It can be noted the absence of a N1s peak attributed to the diazonium functions (at 404 eV), indicating the complete transformation of the diazonium cations. However, low intensity signals at 400 eV could be observed in some samples, probably due to a very small proportion of azo groups (-N=N-) formed from diazonium salts as already reported in the literature.43,48,49,50,51,52 As expected, F1s photoelectron peak at 689 eV is observed in samples prepared with a grafting solution containing the compound 2 in ratios (2/4) 50/50, 90/10 and 100/0 but is absent in the case of the sample that was prepared with deposition solution only containing compound 4 (ratio 0/100). Several components could be identified in the high resolution C1s core level spectra corresponding to the four different modified surfaces (Figure 4). A main component centered at 285 eV could be mostly assigned to aliphatic and aromatic carbons of calix[4]arenes. Two other components at 287 and 289 eV denote the presence of C-O and C=O containing species, respectively. They correspond partly to the C-O-C/C-O and – COOH environments present in the calix[4]arene frame, the contribution of the –COOH group being connected to the compound 4 in the grafting solution (Figure 4a,b,c). These two components also originate from contamination species since they could be observed as low intensity signals in bare gold samples (Figure S3) and since the -COOH-type component is also present on the spectrum corresponding to the surface that was prepared from the electrografting of pure solution of 2 (Figure 4d). Another component centered at 293 eV, which is characteristic to the CF3 group, shows up as the proportion of 2 increases in the grafting solution.


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Figure 4. High-resolution C1s core level spectra (and the corresponding fitting decomposition) of gold surfaces modified by electrografting diazonium cations in situ generated from different 2/4 ratios in the deposition solution a) 0/100 b) 50/50 c) 90/10 d) 100/0.

Evolution of the atomic percentage of C, O and F is presented in Table 1 as a function of the 2/4 ratios in solution. Interestingly, an increase of the oxygen atomic percentage is observed when the deposition solution contains larger concentration of compound 4 whereas an increase of the fluorine atomic percentage is detected when the deposition solution contains larger concentration of compound 2. However, it is difficult to extract the composition of the surface with great accuracy with the XPS data because they are spoiled with the presence of contaminants species, namely those corresponding to oxidized carbon. This point is illustrated in Table 1 by comparing the atomic ratios determined from the analyses of the high-resolution C1s


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core level spectra with calculated “theoretically-expected” corresponding ratios. Some discrepancies are observed, especially when the C in COOH/ C in CF3 ratio (see for instance values obtained for composition mixtures corresponding to 90/10 and 100/0) is considered. The experimental ratios C in CF3/C in C-C or C=C fit the corresponding theoretical ones in a better fashion. Nonetheless, the XPS data unambiguously demonstrate the formation of mixed layers grafted onto the gold electrodes. In addition, the results suggest that the surface amount of each calix[4]arene component could be related to the concentration of the corresponding diazonium in the grafting solution.

Table 1. Atomic ratios for C, F, O present at the modified surfaces evaluated from the XPS data as a function of the binary mixture composition in solution. 2/4

ratio

in

solution

0/0

0/100

50/50

90/10

100/0

68.1 ±

61.7 ±

58.5 ±

58.3 ±

(Bare gold)

%C

57.1

±

1.7 %F

1.9

n.d.

n.d.

2.4 6.7

0.9 ±

2.4 %O

10.0 2.9

C in COOH a

±

18.5 ± 0.9

12.4 ± 0.3

8.4

1.9 ±

0.4 8.5 0.3

9.8

±

0.6 ±

7.9

±

0.5

-

1.96

1.63

0.96

-

1

0.11

0

-

0.070

0.075

0.114

C in CF3

Theoreticalb C in CF3

a

C in C-C/CC


17


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Theoreticalb a

-

0.072

0.115

Page 18 of 35

0.125

calculated from the decomposition in C1s high-resolution core level spectra (see Table S1). b the theoretical

ratios are calculated from the chemical structure of the corresponding calix[4]arenes, assuming that the fractional surface coverage is equal to the corresponding molar fraction in the deposition solution.

Scanning electrochemical microscopy (SECM) in “Feedback mode” was used to characterize the calix[4]arene-modified gold surfaces with two redox mediators (dopamine and ferrocyanide). In principle, SECM allows local investigations (at the micrometer scale) of the interaction of the substrate under consideration with a redox species (the mediator) that is electrochemically produced at an ultramicroelectrode (the tip).53 This interaction is followed through the analysis of the electrochemical current flowing at the tip when it is approached down to the substrate. Typical experiments consist in recording approach curves where the normalized current I = i/iinf is examined as a function of the normalized distance d/a. i is the current at the tip localized at a distance d from the substrate, iinf is the steady state current (when the tip is at infinite distance of the substrate) iinf = 4 n F D C a, n is the number of electrons exchanged, F the Faraday constant, D and C are the diffusion coefficient and the bulk concentration of the mediator, respectively, and a is the radius of the ultramicroelectrode. By varying the redox mediators, this electrochemical technique provides information about charge transfer occurring at a grafted layer, probing the redox reactivity of redox-active molecules immobilized in the layers and/or the permeation/tunneling of the mediator through the layer. The charge transfer properties of the grafted layers towards the oxidized mediators could be quantified through the apparent kinetic charge transfer constant kel, which is obtained from the numerical fitting of the approach curves.53,54,55 Figure 5 displays typical approach curves recorded with the mix layers deposited on gold as substrates. Considering dopamine as redox mediator, the normalized current I


18

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diminishes as the tip approaches the substrate, for all substrates. This behaviour corresponds to a negative feedback and could be fitted in a good agreement with the theoretical curves expected for insulating surfaces. Whatever the mixtures used in the deposition solution, the resulting layers exhibit a strong blocking character towards dopamine (Table 2). The reversible oxidation of dopamine to the corresponding o-quinone in aqueous acidic solution is particularly sensitive to surface modification since the molecule needs to reach the substrate to transfer its charge.56 Indeed, dopamine used as a redox mediator probe in a SECM analysis has permitted the detection of pinholes/defects within an ultrathin layer.57 Herein, the SECM curves indicate that the mediator can neither tunnel nor diffuse through the layers to exchange charge with the underlying gold surface. This shows that the mixed layers remain dense and compact and that possible pinholes/defects/empty spaces have a smaller size than that of dopamine. The SECM approach curves over the same modified gold surfaces but interrogated with ferrocyanide as mediator probe displays a different behavior and the charge transfer was not totally inhibited by the calixarene layer (Table 2, Figure 5B). As a matter of fact, similar results from electrochemical impedance spectroscopy studies have been recently reported with monolayers or very thin layers of phenyl groups that were electrografted from diazonium salts.13,58 Under these conditions, the electronic charges are probably transmitted through the layer via a tunneling mechanism.57 A remarkable feature of the approach curves recorded with ferrocyanide is that the “negative feedback“ character progressively decreases as the molar fraction of compound 2 becomes larger in the solution deposition. The apparent charge transfer rate constants kel are found to vary between 9.2 10-4 < kel < 1.1 10-2 cm s-1 (Table 2). The slower electron transfer is detected over the layer prepared with the pure compound 4 (Figure 5B, black curve) whereas the faster is obtained over the pure O-butylCF3 terminated layers (Figure 5B, red curve). Layers


19


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Page 20 of 35

issued from 50/50 and 90/10 2/4 ratios show intermediate charge transfer kinetics (Figure 5B, green and blue curves). In PBS buffer, at pH 7, the COOH functions present at the calix[4]arene small rim must be deprotonated, hence giving rise to anionic carboxylate termini. Then, an electrostatic repulsion between the COO- terminal groups and the ferricyanide may occur, increasing the blocking properties of the layers containing more –COO- functional groups and then preventing the electronic regeneration of the redox mediator.48b

A

B

Figure 5. Approach curves recorded on a 5µm radius disk Pt tip electrode using dopamine (1mM in 0.1 M H2SO4 aqueous solution) (A) and [Fe(CN)6]4- (1 mM in phosphate buffer (pH = 7) containing 0.1 M KCl) (B) as redox mediators. The probed gold substrates were prepared from the electro-grafting of diazonium cations in situ generated from different 2/4 ratios in the deposition solution: 0/100 (square, black curve); 50/50 (star, green curve); 90/10 (triangle, blue curve); 100/0 (circle, red curve). Lines are the corresponding theoretical curves (See Table 2).

These SECM analyses confirm the conclusions obtained by contact angle measurements and XPS characterizations. They strongly support the formation of mixed layers from calix[4]arenes molecules. The surface composition varies with the molar fractional composition of the corresponding calix[4]tetra-anilines in the starting grafting solution.


20

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Furthermore, approach curves were recorded at different places of the substrates. For all the surfaces studied, a good reproducibility (shape of the approach curves, kel) was obtained. These observations show the homogeneity of the modified surfaces, strengthening the idea that the binary layers are mixed on small length scales.

Table 2. Apparent charge transfer rate constant determined from SECM measurements using dopamine and ferrocyanide as mediators.

2/4 ratio

0/100

50/50

90/10

100/0

9.3 ± 0.7

7.4 ± 1.5

3.7 ± 0.3

kel (cm.s-1) dopamine as

0 10-4

redox

10-4

10-4

mediator

kel (cm.s-1) ferrocyanide as

1.2 ± 0.3 10-3

1.65 ± 0.2 10-3

4.2 ± 0.9 10-3

9.6 ± 2 103

redox mediator

Post-functionalization of the mixed layers. As evoked in the introduction, the preparation of mixed layers allows the optimization of the accessibility of the functional groups by using a diluent compound. In order to evaluate whether dilution could maximize the number of


21


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Page 22 of 35

functional groups available for surface chemical coupling, the previous examples of modified gold surfaces with COOH as appended arms were considered in post-functionalization reactions. Chemical coupling with ferrocenepentylamine (Fc-C5H10-NH2) was performed on dilute layers obtained from grafting solution with the 2/4 ratios equal to 0/100, 50/50 and 90/10.59 The redoxactive ferrocene centres are covalently coupled to the calix[4]arene platforms through an amide bond using acyl chloride activation (Figure 6).59

Figure 6. Chemical post-functionalization of mixed monolayers with ferrocenepentylamine.

Surface modified with a pure solution of compound 3 was also post-functionalized to have a reference experiment. After the chemical post-functionalization, the modified gold electrodes were analysed by cyclic voltammetry in CH2Cl2 containing 0.2 M Bu4NPF6 at different scan rates. A well-defined redox system corresponding to the immobilised ferrocene/ferrocenium couple was observed at 0.35 V vs SCE for all surfaces (Figure 7). Accordingly, the peak currents were found to vary linearly with the scan rates (Figure S4) as expected for surface-confined electroactive species. Integration of the voltammetric peaks allows the estimation of the surface concentration of ferrocene moieties. The surface concentration corresponding to a surface modified with diazonium cations issued from pure solution of 3 is found to be Γa = 8.8 ± 0.5 10-


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11

mol.cm-2. This value could be considered as a reference experimental value accounting for the

single decoration of the calixarene platform. Note that the maximal theoretical value, assuming that the calixarenes are arranged as closest-packing monolayers and that the postfunctionalization reaction takes place with 100 % yield, is 9.8 10-11 mol.cm-2.14 In the case of a surface modified with diazonium cations issued from a pure solution of 4, the surface concentration of ferrocene moities was found to be Γb = 2.1 ± 0.7 10-10 mol.cm-2. By comparison with the Γa value, it can be estimated that more than two carboxyl groups were coupled to a ferrocenyl compound per calix[4]arene platform.14 The surface concentrations related to the postfunctionalization of the layers prepared from the 50/50 and 90/10 mixtures of 2 and 4 were found to be Γc = 1.01 ± 0.4 10-10 mol.cm-2 and Γd = 5.1 ± 0.4 10-11 mol.cm-2, respectively. The Γc value corresponds to half of the Γb values, in good agreement with the 50 % fractional molar composition of 4 in the deposition solution. In contrast, the Γd value appears to be somewhat greater than the value that could be expected from 10% of Γb i.e. 2.1 10-11 mol.cm-2. This result suggests that even more ferrocenyl compounds per calix[4]arene platform could be introduced when the functionalizable calixarenes are strongly diluted. Taking Γa as reference value for single functionalization and assuming that 3 or 4 carboxyl groups are functionalized, respectively, it could be deduced from Γd, surface fractional compositions of 19 % and 14.5 % for the COOH terminated calix[4]arene (compound 4), respectively. Such values remain quite close to the molar fraction used in the deposition solution and are in fairly good agreement with the results obtained in the characterization studies, namely the contact angle measurements. Finally, these results showed that the mixed calix[4]arene-tetradiazonium strategy allows the control of areal density of functional group in the layers, which is of critical importance to many applications.


23


80 60 40

20 V s 1Vs

-1

60

a)

10 V s-1

40

-1

i (µA)

i (µA)

20 0 -20

b)

0.2 V s-1

20 0 -20 -40

-40

-60

-60

-80 0.0

0.2 0.4 E / V vs SCE

0.6

0.0

0.8

0.2 0.4 E / V vs SCE

0.6

0.8

80 60 20 V s-1 1 V s-1

40 20

c) i(µA)

i (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

0 -20 -40

40

20 V s-1

20

2 V s-1

d)

0 -20

-60 -80

-40

0.0

0.2 0.4 0.6 E / V vs SCE

0.8

0.0

0.2 0.4 E / V vs SCE

0.6

0.8

Figure 7. Cyclic voltammetry in CH2Cl2 containing 0.2 M Bu4NPF6 of gold electrodes modified with calix[4]arenes and post-functionalized with Fc-C5H10-NH2. The modified surfaces were obtained from the electrografting of diazonium cations in situ generated from a) a pure solution of 3; b) a pure solution of 4; c) a 50/50 mix solution of 2 and 4 and d) a 90/10 mix solution of 2 and 4.


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4. CONCLUSION By using a calix-macrocycle strategy, mixed binary monolayers were prepared in a single step from the electrografting of binary mixtures of diazonium salts. The diazonium cations were in situ produced from the corresponding calix[4]tetra-anilines. An interesting property of the calixtetradiazonium salts for developing such mixed layers is the fact that they exhibit very close reductive potentials because they share a common scaffold. Different and complementary analyses demonstrate the formation of binary monolayers that are mixed on very small length scales. It was shown that their surface composition could be directed by the molar composition of the calix[4]tetra-anilines in the starting grafting solution, suggesting a fairly good control of the mixing of the two components onto the surface from their relative concentration in the solution deposition. Dilution of a functionalizable component with a non-functionalizable one allows the coupling of larger amount of Fc-C5H10-NH2 per calix[4]arene platform, indicating that mixed calix layers could permit an optimization of the accessibility of the functionalizable groups present at the calixarene small rim. In future work, these versatile mixed calix[4]arene monolayers will be employed for the anchoring of biological systems like redox proteins or DNA, promoting the development of robust biodevices.

AUTHOR INFORMATION Corresponding authors *E-mail:

[email protected],

E-mail:

[email protected],

E-mail:

[email protected] Notes The authors declare no competing financial interest.


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Page 26 of 35

ACKNOWLEDGEMENTS This work was supported by the Agence Nationale de la Recherche (ANR10-BLAN-0714 Cavity-zyme(Cu) project). The authors thank Dr J.L. Fillaut (ISCR, Rennes) for his advices in the synthesis of Fc-C5H10-NH2. AM is grateful to Région Wallonne for FSO grant.

ASSOCIATED CONTENT Supporting Information Profile of water contact angle on a bare gold surface, XPS survey spectra, High resolution XPS C1s core level spectra of a bare gold electrode, Atomic percentage of each component in the C1s core level spectra, Variation of the peak current as a function of scan rate corresponding to the voltammograms recorded for post-functionalized surfaces. This information is available free of charge via the Internet at http://pubs.acs.org.

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One-Pot Electrografting of Mixed Monolayers with Controlled

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