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Surface Functionalization of Metals by Alkyl Chains through a Radical Crossover Reaction Dardan Hetemi, Jérôme Médard, Philippe Decorse, Catherine Combellas, Frédéric Kanoufi, Jean Pinson, and Fetah I. Podvorica Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01557 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on June 4, 2016
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Surface Functionalization of Metals by Alkyl Chains through a Radical Crossover Reaction Dardan Hetemi,a,b Jérôme Médard,a Philippe Decorse,a Catherine Combellas,a Frédéric Kanoufi, a Jean Pinson*a and Fetah I. Podvorica*a,b a
Sorbonne Paris Cité, Univ Paris Diderot, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75013 Paris, France.
b
University of Prishtina, Chemistry Department of Natural Sciences Faculty, rr. “Nëna Tereze” nr. 5, 10000
Prishtina, Kosovo.
ABSTRACT Alkyl chains are covalently attached onto metal surfaces by indirect reduction of the bromoalkyl derivative (RBr). This indirect reaction involves the formation (by spontaneous or electrochemical reduction of the 2,6-dimethylbenzenediazonium salt) of a sterically hindered aryl radical that abstracts a Br atom from RBr but does not react with the surface. This crossover reaction furnishes an alkyl radical that reacts with the surface. Starting from 6bromohexanoic acid carboxylic functionalized gold surfaces are prepared. “Layer by Layer” (LbL) assemblies are built from these surfaces and present some ionic selectivity.
INTRODUCTION Herein we describe an original electrografting method for the attachment of alkyl chains to the surface of a metal starting from easily available bromoalkyl derivatives and using a radical crossover reaction where a radical is transferred from an aryl group to an alkyl chain. Electrografting describes the electrochemical reactions that permit to anchor organic groups on conducting substrates: carbon in its many forms, metals and semi-conductors.1,2 In most of these reactions radicals are the key species that are produced after an electron transfer and they react with the surface. The bonding is strong and in some cases the existence of a covalent bond has been demonstrated by experimental3 or theoretical methods.4 Aryl radicals can be obtained by reduction of diazonium salts,5 oxidation of hydrazines,6 and carbanions7 while alkyl radicals result from the oxidation of carboxylates8 or carbanions9 and reduction of alkyl halides.10 Since the latter compounds are electrochemically reduced at quite negative potentials, it is necessary to reduce C6H13I at -2.3 V/SCE in acetonitrile (ACN) on an Au electrode to attach the resulting radical to this surface.10 This potential is close to the 1 ACS Paragon Plus Environment
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background discharge, which could make difficult the surface modification on other substrates. In addition, primary alkyl radicals are prone to be reduced into their anions11 and many functional substituents on the alkyl chain would not withstand such negative potentials and could be reduced. These grafting conditions severely hamper the development of this method. We recently demonstrated that this problem can be circumvented: 2,6dimethylbenzenediazonium (2,6-DMBD) is easily reduced (-0.25 V/SCE) on a glassy carbon (GC) electrode to the 2,6-dimethylphenyl radical. The latter radical is sterically hindered and cannot bind to the surface but in the presence of an alkyliodide it can abstract a iodine atom to generate an alkyl radical and 1-iodo-2,6-dimethylbenzene through a radical crossover reaction; the resulting alkyl radical binds to the surface at a potential shifted positively by ~1.7 V.12,13 Herein we show that i) the reaction is not restricted to iodoalkyl derivatives but can be extended to the more available bromoalkyl compounds; ii) it is possible to use 2,6-DMBD either previously synthetized or prepared in-situ by reaction of 2,6-dimethylaniline in the presence of NOBF4; iii) such indirect grafting of alkyl bromides can be performed electrochemically or chemically under sonication; iv) the grafted alkylcarboxylic films can be used as an anchor layer to build LbL (Layer by Layer) assemblies. Scheme 1 shows the different compounds and the corresponding grafted surfaces.
Br-(CH2)7-CH3 1
CH3
Br-(CH2)5-COOH 2
N
Au -(CH2)7-CH3
+
CH3
Au -(CH2)5-COOH
N
BF4-
2,6-DMBD Au-2
Au-1
Au -(CH2)5-C(=O)-NH-CH2-Fc
NH2 +
Fe 2
Au-2-NH-CH2-Fc
-
Fc = ferrocene H2N NH2
Fc-CH2-NH2 NH
H2N
NH2
N
n
NH
NH
N N
H2N
Polyethyleneimine branched PEI
N NH2 O
OH
Polyacrylic acid PAA n
Scheme 1. The different compounds and modified surfaces.
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EXPERIMENTAL Chemicals. 1-bromooctane 1 (99 %),
2.6-dimethylaniline (99 %), hydroxylamine
hydrochloride (99 %), sodium hydroxide (98 %), sodium sulfate (99 %), lithium aluminum hydride (95 %), N-hydroxysulfosuccinimide sodium salt (98 %, NHSS), N-(3dimethylaminopropyl)-N-ethylcarbodiimide
hydrochloride
(98
%,
EDC),
ferrocencarboxaldehyde (98 %, FcCO), polyethyleneimine-ethylenediamine branched (PEI) and poly(acrylic acid) (PAA), dichloromethane (99.8 %), ferrocene methanol (FcOH), hexaammineruthenium chloride (Ru(NH3)6Cl3 ) and potassium ferricyanide (K3[Fe(CN)6]) were purchased from Sigma Aldrich, tetrahydrofuran (99 %) from Acros Organics and 6bromohexanoic acid 2 (98 %) from Alfa Aesar. They were used without further purification. The ferrocenylmethylamine (FcCH2NH2) was prepared according to a published procedure.14 The 2,6-dimethylbenzenediazonium tetrafluoroborate, +N2(CH3)2C6H3, BF4- (2,6-DMBD), was synthesized by dissolving 20 mmol of 2,6-dimethylaniline in 10 mL cold ACN (-20° C) and adding rapidly 22 mmol of nitrosonium tetrafluoroborate (NOBF4); the solution was left for 1 h at -20 °C and 2,6-DMBD was precipitated by adding cold ether (-20 °C). The product was purified by dissolution in ACN and precipitation in ether. IR-ATR: -N2+: 2269 cm-1, aromatic vibrations: 1585, 1478 cm-1, BF4-:1036 cm-1, C-H out of plane: 806 cm-1. 1H NMR (DMSOd6, Bruker 400 MHz) δ ppm: 8.02 (t, 1H), 7.65 ( d, 2H), 2.95(s, 6H). Substrates. The Au coated (100 nm) Si wafers were obtained from Sigma Aldrich. Before modification, they were rinsed in concentrated sulphuric acid, ultrasonicated in Milli-Q water for 8 min, cleaned with pure ethanol and dried under a stream of argon. The electrodes for cyclic voltammetry were Au rods (1 mm diameter) sealed in glass. They were polished with different grades of polishing papers and finally with a 0.04 µm alumina slurry on a polishing cloth (DP-Nap, Struers, Denmark), using a Presi Mecatech 234 polishing machine. Al and Cu samples were industrial samples. Before use, the plates were rinsed with Milli-Q water and sonicated for 8 min in acetone to avoid organic contaminants. Functionalization of Au Electrodes and Au Plates by Alkyl Groups. Electrografting was performed by chronoamperometry (E = -0.5 V/SCE for 1200 s in ACN + 0.1 M NBu4PF6 solutions containing 20 mM of 2,6-DMBD and 100 mM of 1 or 2. ACN was chosen as the solvent since the
starting alkyl halides are insoluble in water and ACN is not too reactive by hydrogen atom 3 ACS Paragon Plus Environment
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transfer to radicals that could compete with Br abstraction. This procedure provides, respectively, Au-1el and Au-2el. Each grafting experiments was reproduced at least five times.
The reference electrode was SCE and the counter electrode a platinum foil. After grafting, Au-1el, 2el were rinsed in acetone under sonication for 8 min and then in ethanol (puriss. ≥ 99.8 %). Chemical grafting was performed by 8 min ultrasonication of an Au plate immersed in an ACN solution containing 0.1 M NBu4PF6 + 2,6-dimethylaniline (20 mM) + 2 (100 mM) to which NOBF4 (30 mM) was rapidly added; this procedure provides Au-2ch. Peptidic Coupling of FcCH2NH2 with Au Derivatized Carboxylic Surfaces. After modification, an Au electrode surface was covered with a solution of 60 mM NHSS and 30 mM EDC in ultrapure water for 90 min, followed by rinsing in water. An amide bond was formed by reacting the activated Au-2el with a solution of 0.5 mM FcCH2NH2 in ACN to give Au-2-NHCH2Fc by overnight reaction. The modified surface was carefully rinsed ultrasonically in ACN for 3 min and then with acetone for 8 min and dried under argon. Cyclic voltammograms of immobilized Au-2-NHCH2Fc were obtained in ACN + 0.1 M NBu4PF6. Layer-by-Layer Assembly. Layer-by-layer assembled films on Au modified surface were created by sequential treatment of Au-2el plates in polyelectrolyte solutions. The solutions of PEI and PAA (5 mg/mL) were adjusted to pH 7. Adsorption of PEI or PAA occurred by placing a modified Au plate in a stirred solution of PEI or PAA for 30 min. Rinsing with water was performed for 15 min between each addition of a polyelectrolyte solution. The thickness of the films was measured by ellipsometry. Water Contact Angles. They were measured with a Kruss DSA3 instrument. A drop (3 µL) of Milli- Q water was automatically deposited on the top of the test sample placed in a horizontal position on the instrument stage. At least five measurements were made for each sample. The values of the contact angles were calculated by the tangent method using the DropShapeAnalysis software. IR Spectra. The IRRAS and ATR Spectra of modified plates were recorded using a purged (low CO2, dry air) Jasco FT/IR-6100 Fourier Transform InfraRed Spectrometer equipped with MCT (mercury-cadmium-telluride) detector. For each spectrum, 1000 scans were accumulated with a spectral resolution of 4 cm-1. The background recorded before each spectrum was that of a clean substrate. ATR spectra were recorded with a germanium ATR accessory (Jasco ATR PR0470-H).
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XPS Spectra. X-ray photoelectron spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a micro-focused, monochromatic Al Kα X-ray source (1486.6 eV) and a magnetic lens, which increases the electron acceptance angle and hence the sensitivity. The pass energy was set at 150 and 40 eV for the survey and the narrow regions, respectively. The Avantage Software, version 4.67, was used for digital acquisition and data processing. The spectra were calibrated against C1s set at 285 eV. Ellipsometry. Thicknesses of the films on Au were measured with a mono wavelength ellipsometer Sentech SE400. The following values were taken for gold ns = 0.17, ks = 3.43; they were measured on the clean surfaces before grafting. The film thicknesses were determined from the same plates after modification, taking ns = 1.46, ks = 0 for the organic layer. Electrochemical Measurements. Electrochemical experiments were performed with an EG&G 263A potentiostat/galvanostat and an Echem 4.30 version software. All experiments were carried out in ACN solutions deoxygenated with nitrogen. All potentials are referred to the SCE electrode.
RESULTS AND DISCUSSION Electrografting of 2 on Gold. 2 is reduced at negative potentials: Ep ~ -2.6 V/SCE on Au (Figure SI1 in Supporting Information, note the high current due to the background discharge). In turn, 2,6-DMBD is reduced at low potentials: Ep = -0.22 vs Ag/AgCl on a GC electrode;15 on Au in the presence of 2 the peak current of 2,6-DMBD at Ep = -0.1 V/SCE decreases upon repetitive scanning (Figure 1) although it has been shown that the reduction of 2,6-DMBD does not produce any surface modification.15 A similar voltammogram (Figure SI2 in Supporting Information) is obtained by in situ formation of the diazonium salt. Electrografting was achieved (both with synthetized and in situ prepared 2,6-DMBD) by maintaining the potential of an Au plate at E = -0.5 V/SCE for 1200 s to give Au-2el and Au2el in situ, respectively. Chemical Grafting of 2 on Gold. A gold plate was immersed into a solution of 2, (100 mM) and of 2,6-DMBD (20 mM) prepared in-situ (see experimental part) and maintained for 8 min under sonication, which allows cleaving 2,6-DMBD homolytically. The 2,6-dimethylphenyl radical, which is produced yields the alkyl radical, which in turn binds to the electrode to give Au-2ch.16 The high stability of similar films was previously demonstrated by ultrasonication in acetone, perfluorohexane, boiling toluene, and potential excursions to −1.8 V/Ag/AgCl.13 5 ACS Paragon Plus Environment
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2
c 0
b -2
A/µI
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-4 -6
a
-8 -10 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
E/V/SCE
Figure 1. Cyclic voltammetry on an Au electrode (d = 1 mm) in an ACN + 0.1 M NBu4PF6 solution of BrC5H10COOH (100 mM) + 2,6-DMBD (10 mM ). a) 1st, b) 8th scan and c) no 2,6-DMBD present . Scan rate, v = 0.1 V/s.
Water Contact Angles. The water contact angles for Au-2el (obtained from 2,6-DMBD synthetized before the experiment) and Au-2ch are, respectively, Θ = 64° ± 1 and 68° ± 1. These values are higher than that of ω-carboxylic dodecylsulfide on Au (Θ ~ 40°), which probably indicates that either the alkyl chain is more exposed on the surface or that the film is less compact.17 IR Spectra. The IRRAS spectra of the Au plates modified with 1 and 2 present the signature of the stretching of the alkyl chains in the 3000-2800 cm-1 range (Figure 2A for Au-1el and Figure 2C for Au-2el). In the case of Au-2el, the signatures of the C=O bond and the –COOH group are present, respectively, at 1725 cm-1 (Figure 2B) and as a broad band between 3600 and 3000 cm-1 (Figure SI3 in Supporting Information). The assignment of these bands is supported by comparison with the spectra of the starting compounds 1 and 2 (Figure 2). Identical spectra are obtained whether Au-2el is obtained from 2,6-DMBD previously synthetized (Figure 2B) or prepared in situ (Figure SI4b in the Supporting Information). When grafting has been carried out under sonication, the absorbance of the C=O band of Au2ch is about 30% of that of Au-2el (Figure SI4c in Supporting Information). The IRRAS spectrum of an Au plate obtained under the same conditions but in the absence of 2,6-DMBD indicates that no reaction occurs in that case (Figure SI5 in Supporting Information). Al (oxide) and Cu can also be modified in the same way (Figure SI6 in Supporting Information).
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2928
A
2926 Abs (a.u.)
Au-1el 2856
1
3050
2855
3000
2900
2800
Wavenumber/cm-1
B
1725
Abs (a.u.)
1732
Au-2el 2
1850
1800
1700
1600
Wavenumber/cm -1
C
2952 2946
3000
Au-2el 2875
Abs (a.u.)
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
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2
2867
2900
2800
Wavenumber/cm -1 Figure 2. (A) IR-ATR spectrum of 1 and IRRAS spectrum of Au-1el; (B, C) IR-ATR spectrum of 2 and IRRAS spectrum of Au-2el. Au-1 el and Au-2el obtained by chronoamperometry at -0.5 V/SCE for 1200 s ([1 or 2] = 100 mM, [2,6DMBD] = 20 mM. A-C: normalized spectra-arbitrary, absorbance units.
XPS spectra. XPS confirms the presence of the carboxyl group on the Au-2el surface (Figure 3). The survey spectrum shows the presence of Au4f (84 eV, 35%), C1s (283-291 eV, 48%), N1s (400 eV, 2%) and O1s (533 eV, 15%). The observed ratio for C1s/O1s is 48/15 ~3, which 7 ACS Paragon Plus Environment
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corresponds to the theoretical value. Deconvolution of the C1s peak reflects the grafting of the carboxylic group: C-C/C-H: 284.8 eV; CH2-C=O: 286.5 eV and O-C=O: 288.8 eV.18 For the XPS spectrum of unmodified Au see Figure SI7 in Supporting Information. 900 Au 4f
800
A
I/Counts/1000
700 600 N 1s
500 400 300
C 1s
200 100
O 1s
0 0
200
400
600
800
1000
1200
Binding energy/eV 70
B
C-C/C-H
65
I/Counts/1000
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
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60 55 CH2-C=O
50
O-C=O 45 40 282
284
286
288
290
Binding Energy/eV
Figure 3. XPS spectra of Au-2el. A) Survey spectrum. B) Deconvoluted C1s region.
Thickness of the Au-2el Film. The dry state thickness of the grafted films on Au plates is th Au-2el = 3.1 ± 0.3 or th Au-2el in situ = 2.5 ± 0.3 nm (as the average of 10 measurements for each). These values are in agreement with the presence of Au4f on the XPS spectrum of Au2el (it can be observed only with organic films thinner than 5-10 nm). Since the length of a fully extended -(CH2)5-COOH group is approximately 0.9 nm, the film would be equivalent to 3-4 monolayers. Surface Concentration. The surface concentration of -C5H10-COOH groups in Au-2el was obtained by attaching ferrocenemethylamine by amide coupling to the carboxylic group (Au2el-NHCH2Fc). The surface projected concentration of the ferrocene groups was deduced from the integration of the voltammogram of ferrocene (Figure 4): ΓFc = 5 ± 0.2x10-10 mole cm-2. We consider that the film thickness is the same on an Au electrode and plate; in the 8 ACS Paragon Plus Environment
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latter case it corresponds to 3-4 monolayers, as shown above. This surface concentration can be compared with that of the ordered monolayers obtained from thiols SAMs on Au(111). The √3 x √3 R30° alkanethiolate lattice exhibits a hexagonal symmetry with 0.5 nm distance between nearest neighbours;19 this corresponds to 7.7x10-10 mole cm-2. However, the interpretation of these similar numbers is not straightforward: i) it is not certain that the reaction with ferrocene is quantitative, but ii) ferrocene can react with carboxylic groups inside the film leading to an overestimation of the surface concentration of alkyl chains within the layer. The most likely is that the chains of the oligomers obtained herein are not straight and their surface concentration should be lower than that of thiols SAMs. Rate of Electron Transfer through the Au-2el Film. Au-2el-NHCH2Fc presents the characteristic reversible voltammogram of grafted ferrocene (Fc) since its peak height varies linearly with the scan rate for low values (Figure SI8 in Supporting Information). The rate of electron transfer was measured by recording the reversible voltammogram of Au-2elNHCH2Fc at different scan rates (Figure 4). The small negative current observed at the beginning of the curve may be due to the presence of a small quantity of gold oxide; it disappears during consecutive scans in addition to the rise of the capacitive current. The plot of the anodic and cathodic peak potentials as a function of the logarithm of the scan rate allows deducing the apparent rate of charge transfer, k, from the trumpet like curve: ks = αnFvc/RT = (1 - α) nFva/RT α is the charge transfer coefficient, n the number of electrons transferred, F the Faraday constant, vc and va are the intercepts of the cathodic and anodic straight lines with the abscissa; herein, it is assumed that α= 0.5 and n = 1.20 From the plot of Figure 4, the apparent rate of charge transfer is ks ∼ 3 s-1. This value can be compared with that of ferrocene groups attached to gold through long chain thiol SAMs. For example, Au-SH-(CH2)9-CONH-Fc (somewhat similar to Au-2el-NHCH2Fc) diluted with HO-(CH2)10-SH chains presents a rate of electron transfer ks= 6.9x103 s-1. With different chain lengths and two different links between ferrocene and the alkyl chain, the rates vary from 1.8x103 to 1.1x105 s-1.
21,22
However for low compacity films the rate of electron transfer was found to be independent of the chain length.23 When comparing the value of Γalkyl with that of a compact film, as discussed above, the first model seems most appropriate. It gives values of kinetics of electron transfer for alkyl thiols that are at least three orders of magnitude larger than in our case. In addition, by varying the chain length of alkylthiols, ln ks was shown to decrease linearly with the length (l) of the chain with a slope close to β ~ 1 Å-1.21 From the dependence of ln ks as a 9 ACS Paragon Plus Environment
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function of l, one obtains the length of an alkylthiol for which ln ks = 1.1 (l ~ 2.2 nm). Considering i) the uncertainty on the thickness measured for Au-2el (3.1 ± 0.3 nm) that does not take into account the ferrocene moieties and, ii) the exponential variation of ks with the chain length, it is not reasonable to go further into a quantitative comparison of the charge transfer in Au-2el-NHCH2Fc and in grafted alkylthiols. Qualitatively, charge transfer in Au2el-NHCH2Fc is considerably faster than in a grafted alkylthiol of the same length, which means that the ferrocene units interrogated by cyclic voltammetry have penetrated within the grafted layer; this is in agreement with the comparison of surface concentrations, as discussed above.
A 400
i / nA
200 0
-200 -400 0
200
400
600
800
E/mV vs SCE 0.55
Ep / V vs SCE
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B
0.45
0.35
0.25 -3
-1
1
log(v / V s-1)
Figure 4. (A) Cyclic voltammogram of Au-2el-NHCH2Fc obtained in ACN + 0.1 M NBu4PF6 (v = 0.1 V/s). (B) Ep = f(logv) plot obtained for the voltammograms at different scan rates.
Mechanism. The IR and XPS data for Au-1el and 2el indicate that a film is deposited on the surface; its structure corresponds to that presented in Scheme 1, it resists ultrasonic rinsing and is therefore strongly bonded to the surface. This grafting reaction takes place at the reduction potential of 2,6-DMBD, which is 2.35 V more positive than that for the direct 10 ACS Paragon Plus Environment
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reduction of 2 on Au. Primary alkyl radicals produced during the direct electrochemical reduction of RBr at such negative potentials are easily reduced to their anions.11 The mechanism of the reaction (Scheme 2) should be similar to that observed with iodo derivatives. Reduction of 2,6-DMBD leads to the 2,6-dimethylphenyl radical [R1] that does not react with the surface but abstracts a bromine atom from the bromoalkyl compound to give an alkyl radical and 2,6-dimethylbromobenzene [R2]; finally, the alkyl radical reacts with the surface [R3]. Further attack (through hydrogen atom abstraction) of the first grafted layer by an alkyl radical leads to the growth of the film [R3]. The abstraction of a hydrogen24 or iodine atom by the 2,6-dimethylphenyl radical has already been observed but since the C-Br bond of 1 is stronger than a C-I bond12 the reaction could be more difficult. The thermodynamics of this reaction was therefore examined. Consideration of bond dissociation energies indicate that reaction [R2] should be exothermic25 (BDE = 273.3 kJ mol-1 and 336.4 ± 6.3 kJ mol-1 for Br-(CH2)2COOH and bromobenzene, respectively), even if one takes in account the buttressing effect of the two methyl groups adjacent to the bromine atom (this buttressing effect for a iodine atom is ~ 6 kJ mol-1, it should be lower for a bromine atom13). Since the ∆S of [R2] should be small, one can estimate ∆G[R2] ~ - (336.4 - 6) + 273.3 < - 57.1 kJ mol-1. The formation of the alkyl radical is therefore more favorable starting from the bromo compound than from the iodo (∆G[R2iodo] ~ -35 kJ mol-1). CH3
Au +
CH3 N
+
1eN
.
Au +
CH3 CH3
.
CH3 CH3
+ Br-R
R
.
+
Br
CH3
Au + R
.
[R1]
[R2]
CH3
Au
R
Au
R
R
R
[R3]
Scheme 2. Grafting mechanism of R-Br on Au
Layer-by-layer Post Functionalization of Au-2el. The latter alkylcarboxylic films can be used to grow LbLs. Such constructs have been widely investigated and used as electrochromic thin films, solid state electrolytes, nanomechanical thin films, or for drug delivery. 26,27-34 The LbL film is required to be attached to a charged surface. For that, the negative charge can be established, for example, by i) oxidizing a carbon surface, ii) treating a metal oxide surface with a base, or iii) attaching an organothiol layer terminated by a sulfonate or carboxylate group onto gold.35 Herein, the strongly bonded alkylcarboxylate group of Au-2el was used. 11 ACS Paragon Plus Environment
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The Au-2el film was immersed in a pH 7 solution to transform the carboxylic group of 2 into a carboxylate, then PEI (the amine functions of which are protonated at pH 7) and PAA were added sequentially and repeatedly to give finally a 9-layer thick assembly. The structure of this construct was checked by IRRAS spectroscopy (Figures 5 for the assembly and SI9 and SI10 in Supporting Information for PEI and PAA). The characteristic peaks of the two components are present in the LbL assembly: 3252 cm-1 (-OH and -NH stretching; by comparison 3250 cm-1 for -OH in PAA and 3271 cm-1 for -NH in PEI), 1725 cm-1 (-C=O stretching, by comparison 1715 cm-1 for PAA) and 1565 cm-1 (-NH deformation, by comparison 1580 cm-1 for PEI).
The XPS spectra of Au-2el+(PEI+PAA)1 and Au-
2el+(PEI+PAA)4 that are presented in Figures SI11 and SI12 (Supporting Information), respectively, confirm the structure of the LbL assembly. The dry-state thickness of the film measured after each addition step presents a saw tooth pattern (Figure 6); it increases upon addition of PAA but partly decreases upon addition of PEI. Such a behavior has already been observed by ellipsometry and Electrochemical Quartz Crystal Microbalance (EQCM) and rationalized in the growth of polyallylamine-Os/Lacccase and poly(allylamine)-ferrocene / glucose oxidase LbLs.36-38 Au-2+PEI+PAA
Abs
0.028
0.014
0 1800
1600
1400
1200
1000
Wavenumber/cm-1 0.036
Abs
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Au-2+PEI+PAA
0.018
0 3600
3400
3200
3000
2800
Wavenumber/cm-1 Figure 5. IRRAS spectra of Au-2+PEI+PAA.
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25 20 Th (nm)
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15 10 5 0 0
1
2
3
4
5
6
7
8
Number of layers
9
10
▪
Figure 6. Layer-by-layer thickness as a function of successive layers deposition. ( ) thickness of Au-2el, (♦) after addition of PEI and (●) after addition of PPA. (●) the mean value after each PEI + PAA cycle.
Finally, the blocking propreties of Au-2el-(PEI-PAA)4 were tested by recording the voltammograms of three redox probes: FcOH/FcOH+, Ru(NH3)62+/3+ and Fe(CN)64-/3-. These three compounds present a reversible voltammogram on a bare Au electrode (Figure 7). Little modification is observed on Au-2el-(PEI-PAA)4 with FcOH/FcOH+ and Fe(CN)64-/3- (a ~ 25 and 66% decrease of the current, respectively) as a whole this assembly remains very permeable to both Fc/Fc+ and Fe(CN)64-/3-. On the contrary, the LbL assembly traps the Ru(NH3)62+/3+ as the cathodic voltammetric peak is more than 3 times larger on the modified than on the bare electrode. This Au-2el-(PEI-PAA)4 construct therefore presents a specific response to ions: it filters FcOH/FcOH+ and Fe(CN)64-/3- but complexes Ru(NH3)62+/3+. Similar experiments have been described where LbL films composed of poly(vinyl sulfate) (PVS) and different types of polyamines are deposited on electrodes. The LbL film-coated electrodes exhibit a redox response to the Fe(CN)64-/3- ion when the outermost surface of the LbL film is covered with the cationic poly(amine)s. Conversely, no significant response was observed on the LbL film-coated electrodes whose outermost surface was covered with PVS due to an electrostatic repulsion between Fe(CN)64-/3- ion and the negatively-charged PVS layer. These experiments point to the entrapment of the Fe(CN)64-/3- in the film.39 Fe(CN)64-/3 has also been concentrated in polysaccharide/PEI or poly(diallyldimethylammonium chloride) (PDDA) LbL films deposited on an Au electrode; the concentration was assigned to a strong binding of the Fe(CN)64-/3- ions to the positively charged sites arising from the protonated amino groups in the films.40 In addition, monovalent ion selectivity of cation exchange membranes has been induced by deposition of a PEI/Polystyrene sulfonate LbL.41 These literature results are in agreement with those reported in this paper and open interesting perspectives that we intend to explore further. 13 ACS Paragon Plus Environment
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A
4
a
3
I/µA
2 1
b
0 -1 -2 -3 0
0.1
0.2
0.3
0.4
E/V/SCE 1.8
B
I/µA
0.8 -0.2 -1.2
b
-2.2
a
-3.2 -4.2 -0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
E/V/SCE 10 5
C
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
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-5
a
-10
b
-15 -20 -25 -0.5
-0.4
-0.3
-0.2
-0.1
0
E/V/SCE Figure 7. Cyclic voltammetry in a pH 7 buffer solution of 1 mM of (A) FcOH, (B) K3[Fe(CN)6] and (C) Ru(NH3)6Cl3 on
(a) a bare Au electrode and (b) an Au-2el-(PEI-PAA)4 electrode (obtained by
chronoamperometry at -0.5 V/SCE for 1200 s ([2] = 100 mM, [2,6DMBD] = 20 mM), v=0.1 V/s).
CONCLUSION The surface of gold (and also Al and Cu) can be derivatized by indirect reduction of bromoalkyl chains. This can be accomplished electrochemically or spontaneously. The key species in the grafting reaction is the alkyl radical that binds to the surface. It is obtained through a radical crossover reaction where an aryl radical produces an alkyl radical by 14 ACS Paragon Plus Environment
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bromine atom abstraction. The modified surface was characterized and its surface concentration estimated. By using Br-(C5H10)-COOH as starting material, it is possible to obtain carboxylic terminated films. The latter group can be used to attach a Layer by Layer assembly that presents some selectivity towards ions.
ACKNOWLEDGEMENT F. I. Podvorica thanks Université Paris Diderot for inviting him in Paris.
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ASSOCIATED CONTENT Supporting Information Available: Cyclic voltammetry for 2. IR for Au-2el. Comparison of the grafting methods Blank experiments in the absence of 2,6-DMBD. Chemical grafting on Al and Cu. XPS spectrum of unmodified Au. Voltammetry peak current for Au-2-NHCH2Fc. Infrared for PAA and PEI. XPS for Au-2el+(PEI+PAA)1 and Au-2el+(PEI+PAA)4.
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TABLE OF CONTENTS
Au
COO- + PEI + - PAA n COO- + PEI + - PAA n
˙CH2(CH2)4-COO- =
COO-
polyethyleneimine = + PEI + polyacrylic acid
= - PAA -
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