Surface engineering of bromine-based plasma polymer films: a step

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Surface engineering of bromine-based plasma polymer films: a step toward high thiol density-containing organic coatings Damien Thiry, Matthias Pouyanne, Damien Cossement, Axel Hemberg, and Rony Snyders Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01045 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Surface engineering of bromine-based plasma polymer films: a step toward high thiol densitycontaining organic coatings Damien Thiry,1* Matthias Pouyanne,1 Damien Cossement,2Axel Hemberg2 and Rony Snyders1,2 1

Chimie des Interactions Plasma-Surface (ChIPS), CIRMAP, Université de Mons, 20 Place du Parc, B-7000 Mons, Belgium 2

Materia-Nova Research Center, Parc Initialis, B-7000 Mons, Belgium

1 ACS Paragon Plus Environment

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Abstract Nowadays, the development of synthetic methods regarding the fabrication of –SH containing organic coatings continues to attract a considerable attention. Among the potential techniques, the plasma polymerization appears as one of the most promising but the difficulty to control the chemical composition of the layers is highly limiting. In this context, in this work, we report on an original method combining dry and wet chemistry approaches in view of selectively incorporating –SH functions in organic coatings. Our strategy is based on the (i) synthesis of a bromine containing plasma polymer film (Br-PPF) followed by (ii) a selective grafting of dithiol-based molecule on C-Br bond. Investigating the plasma polymerization process has revealed that, in our experimental window, the load of energy in the discharge has little influence on the chemical composition as well as on the cross-linking degree of the layers. This behavior is explained by considering the concomitant influence of the gas phase reactions and the supply of energy to the growing film through ion bombardment. With regard to the functionalization strategy, based on comparative XPS measurements, it has been unambiguously demonstrated that a selective reaction between propanedithiol and the C-Br bond acting as reactive center takes place resulting in the removing of the bromine atom and the incorporation of –SH groups in the PPF. Depending on the grafting reaction duration, the relative proportion of carbon bearing the –SH group is found to evolve from 4 to 6%. On the other hand, the dissolution of unbounded bromine-based species in the liquid medium during the grafting procedure is also evidenced. The whole set of our results clearly demonstrates the attractiveness of our strategy paving the way for new development in the fabrication of –SH rich containing organic thin films.


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Introduction The synthesis of organic thin films containing/supporting thiol groups (–SH) has rapidly emerged as an important field rising a considerable attention from many research communities.1-8 This interest toward this class of material arises from the “attractive” reactivity of the –SH function paving the way for surface engineering process in several important technological sectors. For instance, the –SH groups could serve as surface reactive sites for specifically binding biomolecules such as DNA appealing for disease diagnosis or genome analysis.1,9 On the other hand, the great affinity between –SH group and gold opens up the possibility for using the material as a stabilizing platform for gold nanoparticles finding application in the catalysis area.5,10 Traditionally, the strategy for synthesizing –SH supported surfaces is based on selfassembling methods. Nevertheless, it has been reported that this approach suffers of several drawbacks. For instance, the efficiency of this grafting procedure strongly depends on the surface chemistry of the substrate therefore limiting the range of materials that can be treated.11 Furthermore, in some cases, the terminated thiol function is obtained after subsequent surface reactions reducing the overall yield of the functionalization and leading to a poor control of the surface chemistry.1,12 Moreover, the occurrence of several side reactions between two neighboring –SH terminated groups could also lead to unpredictable results.2-3,12 On this basis, over the past decade, as an alternative method, the plasma polymerization from a thiol containing organic precursor was studied.11,13-20 The inherent substrate independent nature of the plasma polymerization process, the outstanding properties of the formed layers (i.e. high thermal and mechanical stability, insolubility in most of solvents), the low environmental impact of the process (i.e. no use of solvents), the broad control over the physico-chemical properties of the layers by adjusting the process parameters,… justify the popularity gained by the plasma polymerization method in this context. 3 ACS Paragon Plus Environment

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The growth mechanism of a plasma polymer film (PPF) is based on the activation in the plasma of an organic precursor and includes a multitude of gas phase and surface reactions channels.21-23 This highly complex mechanism induces the main drawback of the plasma polymerization process, namely the low chemical selectivity of the formed layers even when using a monofunctional precursor. In this context, since 1980, improving and controlling the retention of the chemical group hosted by the precursor has rapidly become the object of intensive fundamental researches in the plasma polymerization field.21-22,24-33 Although today, several strategies (i.e. use of low energetic conditions, pulsed mode, clever choice of the precursor,…) have allowed to significantly improve the control of the chemical composition of PPF, an irregular structure predominates for several plasma polymer families including – SH based ones. In order to overcome this problem, Friedrich et al. have developed an original method combining plasma polymerization and wet chemistry.31,34-36 The idea behind this approach is, in a first step, to synthesize a plasma polymer from a bromine-based precursor. Due to its electronic configuration, the bromine atom can only form one chemical bond resulting in the formation of monofunctionalized plasma polymer films containing mainly C-Br bonds. In a further stage, the C-Br chemical groups are employed as reaction center sites for a subsequent grafting procedure of molecules bearing the desired functionality. This strategy has revealed its efficiency in view of the fabrication of organic coatings with a well-defined chemical composition and containing a high density of alcohol, primary amine or azide basedfunctionalities.31,34,37-38 In this work, a similar concept is applied for the first time in view of selectively introducing –SH chemical function in a plasma polymer. Our approach (schematically described in scheme 1) involves at first the formation of a bromine containing plasma polymer film from bromopropane (Br-PPF). In a second step, the C-Br chemical groups 4 ACS Paragon Plus Environment

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present in the material are exploited for inducing a selective grafting reaction with a dithiolbased molecule which would result in the formation of a terminal –SH function (see scheme 1).

Scheme 1. Overall strategy developed in this work consisting in a first step in the plasma polymerization of 1-bromopropane followed by the grafting of propanedithiol through a chemical reaction with the C-Br bond.


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Experimental part Reagents and materials. 1-bromopropane (99% purity), N-ethylmaleimide (98% purity), ethanol (96% purity), triethylamine (> 99.5 % purity) and1,3 dipropanethiol (99% purity) were purchased from Sigma Aldrich. The phosphate buffer (KH2PO4/Na2HPO4, pH = 7) was obtained from Chem Lab. The substrates used in this work were 1x1 cm2 Si wafers (110). Before their introduction into the chamber, the substrates were cleaned with methanol. Plasma Polymerization. The depositions of bromine containing plasma polymer films from bromopropane were carried out in a metallic vacuum chamber: 65 cm in length and 35 cm in diameter. The reactor was pumped through a combination of a turbomolecular and primary pumps allowing to reach a residual pressure lower than 2 X 10-6 Torr. In this work, the working pressure, fixed at 40 mTorr, is controlled by a throttle valve. The precursor flow rate was fixed at 10 sccm (standard cm3 per minute). The regulation of the precursor flow rate is achieved by an ad hoc system made up of a container (200 cm3) filled with the precursor, a mass flow controller and a heating unit (Omicron Technologies). The plasma is capacitively coupled to the discharge using a magnetron cathode covered by a graphite target (99% purity, 3” in diameter) connected through a matching network to a Radio-Frequency power supply (Advanced Energy) operating at 13.56 MHZ. For all the experiments, the distance between the substrate and the cathode is fixed at 10 cm whereas the substrate is kept at the floating potential. The influence of the RF power applied to the cathode (PRF) was studied in the range 20-100 W. Surface Characterization. XPS measurements were performed using a PHI 5000 VersaProbe apparatus with an Al Kα monochromatized radiation source (1486 eV). If not mentioned, photoelectrons were collected at a takeoff angle of 45° from the surface normal.


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All spectra were charge corrected with respect to the hydrocarbon component of the C1s peak at 285 eV. For spectral curve fitting of C1s, Br3d and S2p photoelectrons peaks using Casa software, a full width at half-maximum of 1.1-1.5 eV and a Gauss-Lorentz function (70 % Gauss) were applied. Regarding the fitting procedure of the S2p and Br3d peaks, a spin-orbit splitting of 1.2 eV and 1.05 eV were set, respectively. The respective intensities ratio for both doublets were fixed at 2:1 (S2p3/2:S2p1/2) and 3:2 (Br3d5/2 and Br3d3/2). Static ToF-SIMS measurements were acquired using ToF-SIMS IV instrument from ION TOF GmbH. A 25keV Ga+ ion beam at a current of 0.8pA rasterred over a scan area of 200 X 200 µm2 for 150 s. The spectra were acquired both in positive and negative mode. The peak intensity of the secondary ions was normalized with respect to the total ion count. Fabrication of thiol-terminated plasma polymer. For incorporating –SH functions in the Br-PPF, the coatings were immersed in ethanol solution containing 10-1 M of propanedithiol and thriethylamine acting as catalyst and thus favoring the occurrence of the reaction.

39

The

reaction was carried out at 60 °C and the duration varied from 1h to 48h. After immersion, the PPF was rinsed in ethanol solution in order to eliminate the unreacted molecules and dried under nitrogen flow before analysis. For quantifying the –SH content in the functionalized plasma polymer film, a derivatization procedure using N-ethylmaleimide as a labelling molecule is employed (see scheme 2). The derivatization reaction was performed according to a procedure previously established, i.e. in a phosphate buffer at pH = 7 containing 10-1M N-ethylmaleimide and for a reaction duration of 86 h.17 After the reaction, the samples were rinsed in distilled water and dried under nitrogen flow.


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RSH

+

O

N

O

N-ethylmaleimide

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O

N

O

RS

Scheme 2. Schematic description of the reaction occurring between an –SH chemical group and N-ethylmaleimide. Results and discussion In a first attempt, the investigation of the plasma polymerization of bromopropane which has received little attention in the literature is undertaken. The objective is to improve our understanding on the growth mechanism of this plasma polymer family. The deposition rate is found to evolve from ~ 6 nm/min to ~ 25 nm/min when increasing PRF (Figure 1). Giving our specific experimental configuration, the growth of the layer could, in principle, come from two different contributions: (i) the condensation of the sputtered particles from the graphite target and the (ii) adsorption of reactive species (i.e. ions and mainly radicals) resulting from the precursor dissociation. Nevertheless, it has been reported that for pure organic discharge (i.e. no Ar), the growth of the layer can mainly be ascribed to the chemisorption of the film-forming species formed in the gas phase.40 Thus, the technique operates in the so-called pure “Plasma Enhanced Chemical Vapor Deposition” (PECVD) method. The expected evolution of the deposition rate with PRF is attributed to an increase in the electron density and hence the concentration of film-forming species generated through fragmentations reactions of the organic precursor.21,41


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Figure 1. Evolution of the deposition rate as a function of PRF. With regard to the chemical composition of the coatings, the analysis of the XPS survey reveals the presence of bromine, carbon and oxygen elements (Figure 2a). The presence of oxygen (typically ranging from 4 to 8 at. %) is attributed to the well-known postoxidation of the PPF occurring during the transfer of the samples to the XPS apparatus.42-43 To obtain a more detailed knowledge on the chemical functionality of the coatings, the C1s and Br3d are fitted (Figure 3). The spectral curve fitting of the C1s peak reveals the presence of four components: C-C/H at 285 eV, C-Br at 285.9 eV, C-OR (R: H or C) at 286.7 eV and C=O at 288 eV (Figure 3a).44 As previously discussed, the two latter components result from the post-oxidation reactions. On the other hand, the Br3d spectrum consists of only one doublet for which the position (i.e. Br3d5/2 at 69.5 eV) is consistent with C-Br bonds (Figure 3b).44-45 As shown in Figure 4, the bromine to the carbon content ratio (Br/C) is nearly constant (i.e. ~ 0.32) within the confidence intervals whatever PRF. This point will be discussed in more details hereafter.


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Figure 2. Typical XPS survey of (a) as-deposited PPF-Br for PRF = 60 W (in blue), (b) after the functionalization procedure using dipropanedithiol (in red) and (c) after the derivatization procedure using N-ethylmalemide as a labelling molecule.

Figure 3. Typical spectral curve fitting of the high resolution (a) C1s carbon and (b) Br3d bromine photoelectron peaks for as deposited Br-PPF synthesized at PRF = 60 W.

In order to thoroughly characterize the physico-chemical properties of the Br-PPF, ToF-SIMS experiments are carried out. In the context of the chemical characterization of plasma polymers, in comparison with the XPS method, this technique has been proved to provide additional information about the physico-chemical properties of the layers in term of cross-linking/branching degree.18,46-48 Figure S1 represents typical ToF-SIMS spectra recorded in positive mode of Br-PPF as a function of PRF. The variation of relative intensities of peaks regarding the experimental conditions reflects different surface properties in terms of chemical composition or cross-linking degree. However, as usually encountered with regard 10 ACS Paragon Plus Environment

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to the analysis of plasma polymers, the mass spectra contain numerous peaks (i.e. > 150), making highly challenging to establish the link between the surface fragmentation pattern and the physico-chemical properties of the coatings.

In this context, by systematically

investigating the ToF-SIMS spectra of several plasma and conventional polymers families, Oran et al. have highlighted the correlation between the sum of intensity of a group of specific peaks with the chemical composition and cross-linking density of the layers.49-52 In this work, a similar strategy is applied for the treatment of the ToF-SIMS spectra. At first, the sum of the relative intensities of bromine-containing peaks (∑ [Br]) are studied (Figure 5a). It has to be mentioned that only for this specific case, the mass spectra recorded in negative mode have been considered owing to the high tendency for brominebased sputtered fragments to form negative ions. The nearly constant evolution of ∑ [Br] reveals that the bromine content in the layers is practically constant whatever PRF, thus correlating the XPS data. On the other hand, the cross-linking degree of PPF was found to be inversely correlated with the total secondary ions intensity (∑Secondary

ions)

for spectra recorded in

positive mode.51 In our case, considering the confidence intervals, ∑Secondary ions is not affected by PRF indicating a similar cross-linking density of the coatings (Figure 5b). This is further supported by evaluating the ratio ∑(C6-C8)/∑(C2-C8) (i.e. total yield of C6Hx-C8Hx hydrocarbon secondary positive ion cluster to the total yield of C2Hx-C8Hx clusters) also related to the cross-linking and branching character of the layers (Figure 5c).49,52 From the ToF-SIMS and XPS data, one can conclude that the Br-PPF contain a similar bromine content and cross-linking degree whatever the load of energy applied in the discharge. To tentatively explain this behaviour, let us consider the overall growth mechanism of plasma polymers.21 Briefly, reactive species including ions and mainly radicals are generated in the plasma through electron-impact dissociation reactions of the precursor. Then, 11 ACS Paragon Plus Environment

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the adsorption of these species at the growing film interface gives rise to the growth of the organic layer. Simultaneously, energy is also supplied at the growing film interface through ionic bombardment inducing additional surface reactions including the creation of dangling bonds, densification of the layer or ion-activating etching reactions. The chemical composition and cross-linking degree therefore results from the complex interplay between the gas phase and surface reactions. It has been reported for oxygen and nitrogen-based PPF that, for a certain range of energy supplied in the plasma, the concentration of the heteroeolement (i.e. oxygen or nitrogen) is constant.53-55 This is understood by considering that, in this regime, the chemical fragmentation pathway of the precursor is identical. In other words, the increase in the load of energy in the discharge only results in an increase in the concentration of film-forming species resulting in an increase in the deposition rate; the chemical nature of the condensing moieties being not affected. Based on these considerations and our experimental observations (i.e. increase in the deposition rate and similar concentration of bromine whatever PRF), it can be assumed that a similar situation takes place for the bromopropane plasma in our experimental window. On the other hand, for several plasma polymer families, a linear correlation has been highlighted between the energy dissipated in the film during growth (ε) defined according to eq. 1 and the density of the layer directly correlated to the cross-linking degree:41

ε=

i Γi R

(1)

where i and Γi represent the mean energy and flux of bombarding ions, respectively. R states for the deposition rate. Increasing PRF likely results in the increase in the energy brought through ionic bombardment toward the growing film.56 However, this effect is compensated by the increase 12 ACS Paragon Plus Environment

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in the deposition rate as shown in Figure 1. Both phenomena might therefore result in a nearly constant value of ε explaining the similar cross-linked density of the Br-PPF whatever PRF. It has to be mentioned that further clarification about the growth mechanism could be obtained by performing plasma diagnostic experiments. This point will be addressed in a forthcoming work. Based on the above conclusions, for the rest of the work, the Br-PPF synthesized for an intermediate PRF value of 60 W is considered.

Figure 4. Evolution of the Br/C ratio of Br-PPF as a function of PRF.


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Figure 5. Evolution of (a) ∑ [Br], (b) ∑Secondary ions and (c) ∑(C6-C8)/(C2-C8) deduced from the ToF-SIMS spectra of Br-PPF as a function of PRF. See the text for more details. In view of incorporating –SH functions in the material taking advantage of the reactivity of the C-Br bond, a chemical reaction is induced between Br-PPF and propanedithiol. The choice of the grafting reaction is inspired from the study of Byström et al. investigating the nucleophilic reaction between the C-Br bond and a thiol-terminated glycidyl methacrylate telomers.39 It has been demonstrated that the reaction proceeds through the nucleophilic attack on the C-Br bond of the thiol-terminal group of the grafting molecule resulting, in fine, in the formation of the thioether bond (i.e. C-S-C) and the removing of bromine atom in the form of HBr. Since, in this work, the objective is to incorporate –SH terminated groups at the interface, in comparison with the strategy employed by Byström et al., a molecule containing an –SH function at each extremity is consequently employed. This approach, employed to the best of our knowledge for the first time, would therefore result in 14 ACS Paragon Plus Environment

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the grafting of one –SH group through a reaction with the C-Br bond and then the incorporation of the terminal –SH function in the PPF (see scheme 1). The comparison between the XPS survey before and after the grafting reaction unambiguously reveals the incorporation of sulfur after the procedure (Figure 2b). Furthermore, bromine is still identified after the reaction indicating that all the available C-Br bonds have not reacted with the dithiol-based molecule. This point will be discussed in more details later. To obtain more information on the chemical reactions occurring during the functionalization step, high resolution C1s, Br3d and S2p are examined (Figure 6). From the spectral curve fitting of the C1s, no additional component is observed after the grafting reaction (Figure 6a) in comparison with the analysis of the as-deposited material (Figure 3a). Note that given the very close chemical shift associated to the C-Br and the C-S functions, both chemical groups can not be discriminated. Regarding the Br3d envelope, an additional component is unambiguously identified and attributed to remaining physisorbed HBr as also evidenced for the bromination of carbon black (Figure 6b).44 The presence of HBr can be understood considering the grafting reaction mechanism involving the formation of HBr as a leaving molecule. With regard to the sulfur photoelectron peak, only one chemical function is identified (Figure 6c). The position of the S2p3/2 (i.e. at. 163.5 eV) is consistent with sulfur bonded to carbon and/or hydrogen further confirming the expected mechanism of the grafting reaction (scheme 1).57


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Figure 6. Typical spectral curve fitting of the high resolution (a) C1s carbon, (b) Br3d bromine and (c) S2p sulfur photoelectron peaks for Br-PPF after the functionalization procedure using dipropanedithiol for a reaction duration of 7h.

The kinetics of the grafting reaction is evaluated by following the evolution of the S/C and Br/C ratio as a function of the reaction duration (Figure 7a,b). The S/C ratio strongly increases before reaching a plateau (i.e. ~ 0.12) for a reaction duration of ~ 7 h suggesting that the reaction has reached its equilibrium within the analysis depth of the XPS estimated to be ~ 7 nm.17 The observed trend could be explained by a fast surface reaction involving at first the reaction of highly accessible C-Br bonds followed by a diffusion-limited reaction in the bulk of the Br-PPF.17 As expected, the Br/C ratio follows an opposite trend, namely, a sudden decrease before a stabilization to a value of about ~ 0.03. As previously evoked, even when the reaction reaches its equilibrium, bromine is still present indicating that the functionalisation procedure is not fully quantitative. To unambiguously confirm this interpretation, the depth dependence of the functionalization reaction is probed by modifying the angle of the XPS analysis. Indeed, an alternative explanation of the presence of residual bromine is a limited diffusion pathway of the propanedithiol molecules inside the polymer matrix preventing them to reach bromine function present under the surface but still within the analysis depth of the XPS. For a reaction duration of 7h, the S/C ratios evaluated for different analysis angles with reference to the sample surface (θ) are 0.101 ± 0.006 (i.e. for θ = 25°), 0.104 ± 0.006 (i.e. for θ = 45°) and 0.112 ± 0.006 (i.e. for θ = 90°). The consistency


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between the values taking into account the confidence intervals clearly indicates that the XPS analysis depth is smaller than the diffusion pathway of the grafting molecules. Taking into account that the grafting reaction introduces two sulfur atoms per C-Br link and assuming that the reaction mechanism proceeds as the one described in scheme 1, the relative proportion of carbon bearing the –SH group ([SH]theo.) can be calculated according to eq. 2 : SH theo. =

S

2

C

.100 (%) (2)

Where [S] and [C] represent the atomic content of sulfur and carbon measured by XPS after the grafting reaction, respectively.

Figure 7. Evolution of the (a) S/C ratio, (b) Br/C ratio and (c) [SH]theo. as a function of the grafting reaction duration. It can be learned from Figure 7c that [SH]theo. evolves from ~ 4 to ~ 6 % with the reaction duration. Subtracting the amount of unreacted C-Br bonds (i.e. ~ 3 C-Br per 100 17 ACS Paragon Plus Environment

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carbon atoms) to the initial proportion present in the as-deposited Br-PPF (i.e. ~ 30 C-Br per 100 carbon atoms), around 27 C-Br per 100 carbon atoms were a priori available for reacting with propanedithiol. In this case, taking into account the stoichiometry of the grafting reaction (i.e. two sulfur atoms introduced per C-Br bond), a proportion of carbon bearing a –SH group of ~ 13 % is expected in contrast to the maximum value deduced from the XPS data after the functionalization procedure (i.e.~ 6%). Therefore, it seems that an additional phenomenon takes place during the immersion of the Br-PPF. A potential explanation is the release of unbounded bromine-based species (e.g. CH3Br) from the PPF into the solution as frequently encountered for several plasma polymer families.18,58-61 To validate this hypothesis, the Br/C ratio is evaluated before and after immersion in pure ethanol solution. As shown in Figure 8, a large decrease in the Br/C ratio (from ~ 0.3 to ~0.08) occurs likely due to release of the bromine-based species in the solution in line with our explanation.

Figure 8. Evolution of the Br/C ratio for as deposited Br-PPF and after immersion in pure ethanol solution for 48 h. Finally, It is worth noting that due to the reactivity of the –SH function with C-Br chemical group, the grafting of both –SH groups present at each extremity of the propanedithiol molecule with two C-Br neighbouring functions can not be totally excluded. The occurrence of this side reaction would therefore reduce the amount of –SH functions incorporated in the PPF. In this context, a derivatization reaction using N-ethylmaleimide as a


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labelling molecule aiming to quantify the –SH groups in the PPF following the functionalisation procedure is applied. It has been reported that N-ethylmaleimide reacts selectively and quantitatively with –SH functions.17 The considered reactions introduces nitrogen which is further quantified by XPS to calculate the “real” relative proportion of carbon hosting an –SH function ([SH]real) following eq. 3:

SH = . 100 (%) (3)

Where [N] and [C] represent the atomic carbon and nitrogen concentration measured by XPS after the derivatization procedure, respectively. The term 6[N] is related to the amount of carbon introduced through the considered reaction (see scheme 2). As depicted in Figure 2c, nitrogen is clearly identified after the derivatization reaction indicating the presence of –SH functions in the PPF. Moreover, this is also further supported by the fitting procedure of the C1s photoelectron peak revealing the incorporation of the imide function (i.e. N-C=O) present in the labelling molecule (Figure S2). Through the quantification of nitrogen, the calculation of [SH]real gives a value of ~ 6.53 ± 0.5 % in comparison with ~ 6 % after the grafting reaction. The full consistency between both values confirm the efficiency of the intended functionalization reaction for introducing, in a selective way, thiol groups from Br-PPF according to the mechanism described in scheme 1. It is important to note that in comparison to other functionalization strategies involving thiol-based grafting molecule,62-63 the reaction established in this work proceeds only in one step. Furthermore, the reactivity of incorporated –SH group toward N-ethylmaleimide paves the way for using the material in the biotechnology field since most of immobilization procedures involving thiol-terminated support employ modified biomolecules with a maleimide-based extremity.1 It should also be noted that for such a kind of application, the presence of residual


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bromine (~ 3 C-Br per 100 carbon atoms) would not affect the behaviour of the material since the reaction between maleimide-based molecule and thiol is highly specific.15 With regard to conventional plasma polymerization process involving the use of a single molecule precursor (e.g. propanethiol)18 or hydrocarbon/ H2S gas mixture,13,20 a maximum density of carbon bearing the –SH group of around 4 % is obtained in comparison to ~ 6% here clearly revealing the attractiveness of our method. Conclusions In this work, a novel approach aiming to selectively incorporate thiol functions from bromine containing plasma polymer films is established. Our overall strategy is based on exploiting the reactivity of C-Br bonds contained in the organic coating by inducing a selective reaction with a dithiol-based molecule giving rise to the introduction of –SH function in the material. Regarding the plasma polymerization of bromopropane, it has been shown that the chemical composition as well as the cross-linking degree are not strongly affected by the energy dissipated in the plasma. This behavior is ascribed to a similar (i) chemical fragmentation pathway of the organic precursor and (ii) density of energy provided to the growing film by ions with respect to the total amount of matter deposited. Comparative XPS measurements clearly demonstrate the efficiency of the intended grafting reaction of propanedithiol on C-Br labile bonds resulting in the introduction of –SH function in the plasma polymer. The grafting reaction yields to the incorporation at maximum of 6 –SH groups per 100 carbon atoms which in comparison to conventional plasma polymerization process is ~50 % higher. Furthermore, due to the intrinsic good adhesion properties of plasma polymers, this method could potentially be applied to almost all kind of


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substrates (e.g. glass, polymers, metals,..) even with complex geometries (e.g. nanoparticles, nanotubes). Finally, it has to be mentioned that the thiol density could, a priori, be significantly increased since not all the C-Br functions contained in the as-deposited material react with the grafting molecule. Indeed, a fraction (~ 25%) is lost during the functionalization step. This phenomenon has likely for origin the presence of unbounded bromine-based species in the freshly synthesized plasma polymer, which are dissolved in the solution during the functionalization procedure. Therefore, increasing the amount of chemically attached bromine-based species in the plasma polymer network would enable to improve the thiol functionalization level. This point will be discussed in a forthcoming work. ASSOCIATED CONTENT Supporting information Typical ToF-SIMS spectra recorded in positive mode; Spectral curve fitting of the high resolution C1s carbon peak after the derivatization procedure. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions All the authors contribute equally to the manuscript. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT D. Thiry thanks the “Région Wallone” through the “CLEANAIR” project for financially assisting this research.


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ABBREVIATIONS Br-PPF, plasma polymer film synthesized from 1-bromopropane; PPF, plasma polymer film; PRF, RF power applied to the cathode; ToF-SIMS, Time of Flight Secondary Ion Mass Spectrometry; XPS, X-Ray Photoelectron Spectroscopy.


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Table of Content Graphic


Surface engineering of bromine-based plasma polymer films: a step

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