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Article

Establishment of a derivatization method to quantifythiol function in sulfur-containing plasma polymer films

Damien Thiry, Remy Francq, Damien Cossement, David Guerin, Dominique Vuillaume, and Rony SnydersLangmuir, Just Accepted Manuscript • DOI: 10.1021/la402891t • Publication Date (Web): 25 Sep 2013

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Establishment of a derivatization method to quantify

thiol function in sulfur-containing plasma polymer

films

Damien Thiry1*, Remy Francq1,2, Damien Cossement2, David Guerin3, Dominique Vuillaume3 and

Rony Snyders1,2

(1) Chimie des Interactions Plasma Surface (ChIPS), CIRMAP, Université de Mons, 23 Place du Parc, B-7000

Mons, Belgium

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

(3) Molecular Nanostructures & Devices” group, Institut d'Electronique, Microélectronique et

Nanotechnologie (IEMN), Centre National de la Recherche Scientifique (CNRS), BP60069, avenue

Poincaré, F-59652 cedex, Villeneuve d'Ascq, France

*Corresponding author: Tel: +32 (0) 65 55 49 45, Fax: +32 (0) 65 55 49 41, E-mail:

damien.thiry@umons.ac.be

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Abstract

Thiol-supported surfaces draw more and more interest in numerous fields of

applications from biotechnology to catalysis. Among the various strategies to generate such

surfaces, the plasma polymerization of a thiol-containing molecule appears to be one of the

ideal candidates. Nevertheless, considering such an approach, a careful characterization of the

material surface chemistry is necessary. In this work, an original chemical derivatization

method aiming to quantitatively probe the –SH functions in plasma polymers was established

using N-ethylmaleimide as a labeling molecule. The method was qualitatively and

quantitatively validated on Self Assembled Monolayers of 3-mercaptopropyl-trimethoxysilane

exhibiting a –SH terminated group used as “model” surface. For a quantitative determination

of the –SH content in propanethiol plasma polymers, the kinetic of the reaction was

investigated. The latter is described as a two steps mechanism, namely a fast surface reaction

followed by a diffusion limited one. The density of –SH groups deduced from the

derivatization method (~4%) is in good agreement with typical values measured in some other

plasma polymer families. The whole set of our data opens up new possibilities for optimizing

the –SH content in thiol-based plasma polymer films.

Keywords

Plasma Polymer, thiol, chemical derivatization, N-ethylmaleimide

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Introduction

Plasma polymerization from a thiol(-SH)-based precursor is a promising technique to

grow –SH supported organic thin films1 that could be used in many applications such as (i) a

platform for DNA immobilization2,3, (ii) interlayer for promoting gold adhesion4 or as (iii) a

support for gold nanoparticles.5 An often reported limitation of plasma polymerization is the

low chemical specificity of the synthesized films, even when using a mono-functional

precursor.6-8 This phenomenon is mainly attributed to the numerous precursor fragmentations

pathways occurring in the plasma.6

The evaluation of the surface density of particular chemical groups (in our case –SH

group) on synthesized films is therefore of crucial importance in view of the future

applications. Most of the time, this evaluation is performed using surface sensitive analytical

techniques such as X-Ray Photoelectron spectroscopy (XPS) and, in a lesser extent, Time-of-

Flight Secondary Ion Mass Spectrometry (ToF-SIMS).9-13 However, for both techniques,

considering that most of the time the surface chemistry is complex, the discrimination and the

quantification of specific chemical groups is a tricky task.11-13 This is also the case for SH-

based plasma polymer films (SH-PPF). For example, concerning the XPS measurements, the

chemical shift associated to the different sulfur-based chemical functionalities (e.g C-SH, C-

S-C, C=S, C-S-S) are too low compared to the XPS resolution for allowing an accurate

spectral curve fitting of both the C1s and the S2p photoelectron peaks.14,15 On the other hand,

considering ToF-SIMS analysis, owing to the complexity of the surface fragmentation pattern,

the extraction of information about the concentration of a particular chemical group is quite

complex.

To address these problems, the combination of analytical techniques along with

chemical derivatization is generally employed.12,13 This approach consists of inducing a

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surface reaction between the probed chemical group and a tag molecule containing a specific

element or function which can be easily detected by the analytical instrumental technique.

This strategy is commonly and successfully employed for the quantification of primary amine

(C-NH2)9,16, carboxylic acid (C-COOH)8, alcohol (C-OH)17 and ketone (C=O)10

functionalities.

To date, likely due to the low number of studies devoted to SH-based plasma polymer

films (SH-PPF), no method was developed to enable the quantification of the –SH groups by

chemical derivatization. However, as the impact of such films becomes more and more

significant in numerous fields of applications, the development of such a method turns out to

be a necessity.

Numerous studies using MALDI-ToF mass spectrometry18, fluorescence, absorption and

infrared spectroscopy2,19,20, cyclic voltametry21 and XPS22 pointed out that N-ethylmaleimide

reacts selectively and quantitatively with –SH functions.23 The reaction mechanism consists

of a nucleophilic addition between the sulfur atom and the double bond contained in the

maleimide structure following the formation of a stable thioether link as described in

Equation 1.23 As a consequence, a new chemical element (N) and new chemical

functionalities (OCN, CNR) are generated.

RSHN

OO

NOO

RS+

N-ethylmaleimide

In this work, we report, for the first time, the use of N-ethylmaleimide as a labeling

molecule for the quantitative determination of –SH functionalities by chemical derivatization

in SH-PPF synthesized using propanethiol as a chemical precursor. Aiming to validate our

approach, the considered derivatization reaction was previously evaluated on Self Assembled

Equation 1

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Monolayers of 3-mercaptopropyl-trimethoxysilane (MPTS-SAM) on silicon substrate. This

surface exhibiting a free –SH terminated chemical group as schematically shown in Scheme 1

can therefore potentially undergo a chemical reaction with the N-ethylmaleimide.24 This

strategy allowed to independently studying the considered reaction from qualitative and

quantitative points of view before its application to plasma polymer layer.

Scheme 1: Schematic description of a MPTS-SAM on silicon substrate

Experimental part

Thiol-terminated SAM by silanization with MPTS. The reaction was carried out by a

vapor-phase deposition technique in a schlenk flask connected to a vacuum/N2 line. The

silicon substrate previously cleaned following a procedure described elsewhere 25 was treated

in the presence of saturated MPTS vapors. The sample was placed at a distance of 5 cm of a

small cup containing 100 µL of MPTS. The schlenk flask was purged several times by

vacuum/N2 cycles, evacuated to a pressure of 0.2 Torr and then sealed at this pressure for

about 18 h at room temperature to allow the grafting of the SAM. The functionalized substrate

was cleaned 2 min in ethanol under ultrasounds then it was dried under nitrogen stream.

Plasma Polymerization from propanethiol. The propanethiol plasma polymers (Pr-

PPF) were deposited from 1-propanethiol (99%, Sigma Aldrich) on previously cleaned

1x1 cm2 silicon wafers following a procedure described elsewhere.26 The depositions were

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carried out in a metallic vacuum chamber (65 cm length, 35cm diameter), pumped by a

combination of turbomolecular and primary pumps allowing to reach a residual pressure

lower than 2 x 10-6 Torr. During the process, the working pressure fixed at 40 mTorr, was

controlled by a throttle valve connected to a capacitive gauge. The plasma was sustained by a

one-turn inductive Cu coil (10 cm in diameter, located 10 cm away from the substrate)

connected to an Advanced Energy radiofrequency (13,56 MHz) power supply via a matching

network. For all the experiments, the precursor flow rate was kept constant at 10 sccm and

the substrate was at the floating potential. Pr-PPFs were synthesized varying the mean power

dissipated in the plasma (<P>) ranging from 14 to 100W. <P> was adjusted by the modulation

of the “off-time” (0-3.6 ms) while the plasma “on-time” and the peak power were kept

constant at 0.5 ms and 100W, respectively. It should be noted that at this power value, the

discharge is inductively coupled.1,27

XPS. XPS measurements on the as-deposited and derivatized Pr-PPF/MPTS-SAM were

performed using a PHI 5000 VersaProbe apparatus with an Al Kα monochromatized radiation

source (1486 eV). The XPS instrument is directly connected to the plasma deposition

chamber. Hence, in the case of the as-deposited Pr-PPF, the samples were analysed without

exposure to the air. The pressure in the analysis chamber was typically 3.10-7 Pa.

Photoelectrons were collected at a take-off angle of 45° from the surface normal. All spectra

were charge corrected with respect to the hydrocarbon component of the C 1s peak at 285 eV.

The XPS survey were acquired using a pass energy of 117.4eV. Concerning the high

resolution peaks of each element, a pass energy of 23.5 eV was employed. For spectral curve

fitting of the carbon photoelectron peak using PHI Multipak Software, a full width at half-

maximum of 1-1.3 eV and a Gauss-Lorentz function (70-85% Gauss) were applied. The high

resolution C1s photoelectrons peaks were acquired with energy step of 0.2 eV and 0.05 eV for

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the Pr-PPF and the MPTS-SAM, respectively. The XPS resolution of our apparatus is

estimated to be 0.5 eV.28

ToF-SIMS. Static ToF-SIMS measurements were acquired using a ToF-SIMS IV

instrument from ION TOF GmbH. A 25 keV Ga+ ion beam at a current of 0.8 pA rasterred

over a scan area of 200*200 µm2 during 150 sec. Owing to the tendency for sulfur, oxygen

and nitrogen to form negative ions under primary ions bombardment during ToF-SIMS

experiments, the measurements were acquired in negative mode.29 In the spectra, the peak

intensity of the secondary ions was normalized with respect to the total ion count.

Ellipsometry. Spectroscopic ellipsometry data in the visible range was obtained using a

UVISEL by Jobin Yvon Horiba Spectroscopic Ellipsometer equipped with a DeltaPsi 2 data

analysis software. The system acquired a spectrum ranging from 2 to 4.5 eV (corresponding

to 300-750 nm) with 0.05 eV (or 7.5 nm) intervals. To assess the thickness of the as-deposited

and the derivatized MPTS-SAM, a 3-layer model was used: Si/SiO2 (Native oxide) /SAM.

The optical properties of Si and SiO2 are found in the software library and integrated to the

model. The SiO2 thickness was measured independently and estimated to 12 Å. Concerning

the SAM, a refractive index of 1.5 was used. More details about the methodology can be

found elsewhere.25 The accuracy of the SAM thickness measurements is estimated to be ±2

Å.

Derivatization reaction. The derivatization reactions were carried out in a phosphate

buffer (KH2PO4/Na2HPO4, Chem Lab) at pH = 7 for kinetic considerations and stability of N-

ethylmaleimide (99%, Sigma Aldrich) in aqueous solution.20,22 The N-ethylmaleimide

concentration was fixed at10-1 M. The reaction duration varied from 3h to 200h for Pr-PPF

and was fixed to 18h in the case of MPTS-SAM. After immersion, the Pr-PFF/MPTS-SAM

were rinsed in the buffer solution (without N-ethylmaleimide) for 5 min to eliminate the

unreacted molecules and dried under nitrogen flow before analysis.

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Results and discussion

In order to evaluate the efficiency of the grafting of MPTS-SAM on silicon substrate,

thickness measurements using ellipsometry are performed. Table 1 summarizes the

thicknesses of the SAMs and provides a comparison with the expected value for a “near-

perfect” densely packed SAM. The expected thickness corresponds to the length of the

molecules, as given by PM3 conformation optimization with the CS-MOPAC software30

assuming that the main axis of the molecule is perpendicular to the surface substrate. A good

agreement between the measured and the expected values is observed suggesting that alkyl

chains are densely packed and that end groups are directed away from the silicon surface.

After the chemical derivatization (CD) reaction, an increase in the thickness is observed (from

6.7Å to 12Å, See table1). The obtained thickness is consistent with the expected theoretical

value considering the grafting of N-ethylmaleimide molecule at the –SH terminal group

through the reaction described in the Equation 1 (see Figure S1 for a schematic description).

Table 1. Thickness of the as-deposited MPTS-SAM and after the derivatization reaction.

To assess the variation of the chemical composition of the surface after CD, the XPS

survey measured before and after CD are compared (Figure 1). Concerning the as-deposited

MPTS-SAM, the data reveal the presence of sulfur and carbon illustrating again the grafting

efficiency (Figure 1a). After the CD procedure, nitrogen, present in the labeling molecule, is

clearly identified in the survey (Figure 1b). Following the reaction described in Equation 1,

new carbon-based functionalities are introduced. Therefore, the high resolution C1s peaks

were compared before and after the mentioned reaction in order to get information on the

chemical reactions occurring during immersion (Figure 2). All components used in this work

Monolayer Thickness ( Å )

Theoretical Measured MPTS-SAM 7.2 6.7 ± 2

MPTS-SAM after CD 11.8 12 ± 2

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for the spectral curve fitting are summarized in Table 2. According to the chemical

composition of the as-deposited MPTS-SAM, three components are used for the fitting

procedure: C1, C2 and C3 associated to C-Si, C-S, C-C/C-H, respectively (Figure 2a). After

CD , two additional components, namely C4 (C-N) and C5 (O=C-N) are identified (Figure

2b). Both these functionalities are present in the N-ethylmaleimide structure and therefore

likely result from the incorporation of the labeling molecule through the derivatization

reaction described in the Equation 1. In addition, it should be mentioned that the position of

the N1s signal (not shown) at 400.5 eV is consistent with the expected value of a nitrogen

atom involved in a maleimide structure.31

Figure 1: XPS survey of the (a) as-deposited MPTS-SAM and (b) after the derivatization reaction.

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Figure 2: C1s photoelectron peak fitting corresponding to (a) a as-deposited MPTS-SAM and (b) after the derivatization reaction.

Table 2. Labeling of all components used in this work for the spectral curve fitting of the C1s photoelectron peak shown in Figures 2 and 5.

Components

Attribution

References MPTS-SAM

MPTS-SAM (After CD)

Pr-PPF Pr-PPF

(After CD)

C1 C-Si

(284.6 eV)

C-Si (284.4eV)

/ /

32

C2 C-C/C-H (285 eV)

C-C/C-H (284.9 eV)

C-C/C-H (284.8 eV)

C-C/C-H (284.8 eV)

31

C3 C-S

(285.6 eV) C-S

(285.6 eV) C-S

(285.6 eV) C-S

(285.6 eV) 31

C4 / C-N

(286.5 eV) / C-O/C-N

(286.4 eV) 33,34

C5 / O=C-N

(288. 6 eV) / O=C-N

(288. 5 eV) 31,34

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As a complement to the XPS data, ToF-SIMS analysis have been performed on the same

samples (Figure 3). After CD, a new peak at m/z = 158 is clearly observed in the mass

spectra (Figure 3b). Based on the CD mechanism (Equation 1), this negative ion could be

assigned to the [C6H8NO2S]- fragment (exact m/z = 158.028, observed m/z = 158.021). The

assumed fragmentation pathway leading to the emission of this fragment is schematically

represented in Figure 4a. The detection of this anion, characteristic of the probe molecule,

constitutes an additional evidence of a reaction between the –SH groups of the MPTS-SAM

and N-ethylmaleimide.

In order to get more quantitative information, the yield of the derivatization reaction on

MPTS-SAM is calculated considering the nitrogen to sulfur ratio (N/S) measured by XPS.

Although the CD reaction is considered as nearly quantitative in the literature, a value of 0.48

was found meaning that only ~50% of –SH functionalities have reacted with the labeling

molecule. This result can be explained taking into account some steric effects considering the

space between two neighboring chains in MPTS-SAM and the size of the labeling molecule.

Indeed, the area per molecule in a MPTS-SAM is estimated to 23 Å2 giving a space between

chains of 5.4 Å.24 On the other hand, based on MOPAC calculation data (See Figure S1), the

diameter of the labeling molecule can be estimated to 6.3 Å. If we assimilate the molecules to

cylinder, the space between two chains in MPTS-SAM is too low to allow a reaction with two

neighboring –SH functions (See a schematic diagram in Figure S2). Considering this steric

effect, only one –SH group over two can react with the labeling molecule which is consistent

with our experimental data. Therefore, the derivatization reaction can be considered as nearly

quantitative.

The MPTS-SAM situation is obviously quite different than the one encountered in PPF

for which the density of –SH groups is likely dramatically lower. Based on our theoretical

estimation, a minimal distance of ~ 6Å is required for the reaction occurring at two

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neighboring –SH groups. Considering simple carbon-carbon bond (~1.54Å), this minimal

space corresponds to a density of –SH group of around 40%. The latter value corresponds

therefore to the maximum –SH density which can be reached using our method. Nevertheless,

it is accepted that for plasma polymers the density of a particular chemical group is most of

the time much lower than 40% due to the high number of fragmentations/rearrangements

reactions occurring in the plasma and at the growing film interface.8,16,35 Therefore, for this

class of materials, the steric effect induced by the derivatization molecule would not represent

a limiting factor for a quantitative determination of the –SH density.

Figure 3: Partial normalized negative ToF-SIMS spectra of a (a) as-deposited MPTS-SAM and (b) after the derivatization reaction.

Figure 4: Possible fragmentation pathways providing secondary ions at (a) m/z = 158 ([C6H8NO2S]-) and (b) at m/z = 142 ([C6H8NO3]

- ) observed in the ToF-SIMS spectra.

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Concerning the Pr-PPF, for all deposition conditions, the XPS survey spectra (not

shown) reveal the presence of nitrogen after the chemical derivatization (CD) procedure

whatever the reaction duration while, obviously, it is absent from the as-deposited Pr-PPF

XPS spectrum. The elemental composition of the Pr-PPF synthesized at different <P> at each

fabrication step is summarized in the Table S1 (Supporting Information). In order to get more

information about the reactions occurring during CD, the high resolution C1s envelope was

examined before and after derivatization (Figure 5). Figure 5a, corresponding to the as-

deposited Pr-PPF, is composed of two components (C2 and C3) referring to aliphatic bonds

and carbon/sulfur bond (C-SR, C-SH,C=S), respectively (See Table 2 for component

labeling).31 Figure 5b shows that after CD similarly to the derivatized MPTS-SAM, new

chemical functionalities are introduced in the Pr-PFF as revealed by the appearance of two

additional components: C4 and C5 associated to C-NR/C-OR and O=C-N bonds, respectively

(see Table 2).

The presence of the O=C-N and C-N bonds is attributed to the incorporation of N-

ethylmaleimide at the Pr-PPF surface most likely by following the reaction depicted in

Equation 1. Concerning the C4 component, it should be noted that in addition to C-N bonds,

a contribution of C-OR has also to be taken into account. Indeed, the presence of C-OR bonds

after CD results from the reaction between the trapped Pr-PFF radicals and the ambient

oxygen or water present in the CD solution as frequently encountered in plasma

polymerization.36

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Figure 5: C1s photoelectron peak fitting corresponding to the analysis of a Pr-PPF synthesized at <P> = 100W (a) as-deposited and (b) after the derivatization reaction during

86h.

In addition, ToF-SIMS analyses were performed on the same samples (Figure 6). After

CD, a new peak at m/z = 158 as previously observed in derivatized MPTS-SAM and

attributed to [C6H8NO2S]- fragment (exact m/z = 158.028, observed m/z = 158.018) is clearly

observed in the mass spectra (Figure 6b). This additionally confirms the grafting of N-

ethylmaleimide in Pr-PPF. In a lesser extent, another signal at m/z = 142 is also identified

(Figure 6b). The latter is assigned to [C6H8NO3]- (exact m/z = 142.055, observed m/z =

142.046) fragment for which a probable structure is represented in Figure 4b. This ion likely

results from a reaction between –OH functionalities incorporated in the Pr-PFF by post-

oxidation reaction and N-ethylmaleimide. Based on organic chemistry literature, this reaction

is unexpected since it should be catalyzed under basic or acidic conditions to occur

efficiently.37 Nevertheless, in their work, Jin et al. compared the reactivity of -OH/-SH

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terminated oligonucleotides towards maleimide moieties supported on a surface and showed

that a potential reaction between –OH and maleimide species is possible.38 Based on

comparative fluorescence spectroscopy measurements, the reaction yield between –OH

functions and the maleimide surface is estimated to be approximately 10% if we consider a

complete reaction with –SH groups. In a first approach, this side reaction can therefore be

neglected as proved by Shen et al. through XPS measurements.22 In our case, although only

~4 at.% of oxygen is introduced through post-oxidation of the Pr-PPF, this fragment is likely

observed because of the extreme sensitivity of ToF-SIMS for the detection of oxygen-based

anions.29

Figure 6: Partial normalized negative ToF-SIMS spectra of a Pr-PFF synthesized for <P> = 100 W (a) as-deposited and (b) after the derivatization reaction during 86h.

In order to complete our understanding, the kinetics of the N-ethylmaleimide CD of

the Pr-PPF has been evaluated. Figure 7 reports the –SH density as a function of the reaction

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duration for Pr-PPF synthesized for <P> = 14W, 38W and 100W. Assuming a selective

reaction between N-ethylmaleimide and the –SH functions, the concentration of carbon

bearing the –SH groups [SH] is calculated using Equation 2.16

[N][SH] . 100 (%)

[C] 6[N]=

−

Equation 2

Where [N] and [C] represent the atomic carbon and nitrogen concentration measured by XPS,

respectively. The term 6[N] is related to the amount of carbon introduced through the CD

reaction (See Equation 1).

Independently of <P>, [SH] increases strongly with the reaction duration and then,

reaches a plateau suggesting that all the –SH functions have reacted with N-ethylmaleimide

within the analysis depth of the XPS, which is estimated to be 7 nm.39 This trend is explained

by a fast surface reaction 2 followed by a diffusion limited reaction in the bulk of the Pr-PPF.

In our range of <P>, it can be learned that, for all power conditions, the plateau appears

for similar reaction duration (~40h). This suggests that the diffusion of the N-ethylmaleimide

molecule, often linked to the plasma polymer crosslinking, is not affected by <P>. In addition,

[SH] measured in the plateau region is similar whatever the <P>. This is also supported by

ToF-SIMS data related to the relative intensity of the [C6H8NO2S]- fragment (See Figure S3

in Supporting Information ). These observations consistently suggest that, in our range of

power, the plasma polymerization process is not significantly affected by <P>. Indeed, in

plasma polymerization, it is generally accepted that the retention of the functionality hosted

by the precursor molecule (-SH function in our case) and the crosslinking of the plasma

polymer evolve with inversely proportional trends as a function of <P>.26 However, in a

recent work of Hegemann et al, it has been showed that the retention of a particular function

as well as the cross-linking degree of the layers strongly depend on the “momentum flux” per

deposition rate to the growing film interface.40 This latter parameter is directly proportional to

the energy and the flow of bombarding ions. In our case, it can be reasonably assumed that the

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growth of the layer occurs mainly during ton as our precursor does not contain any

unsaturation (double, triple bond) limiting therefore its grafting at the interface during toff.

Therefore, the energetic conditions trough ionic bombardment to the growing film should be

equivalent for all <P> as ton and the peak power were fixed. Based on these considerations, we

expect an equivalent momentum flux per deposition rate and consequently a nearly constant

[SH] and cross-linking degree with <P> explaining the data recorded in Figure 7. Hence,

considering the modulation of <P> only through the variation of toff, the peak power seems to

be the major factor controlling the layer properties in case of –SH-based plasma polymer

films.

It should be mentioned that the [SH] density measured in this work (varying from 3.5 to

5% taking into account the error bar) are in good agreement with data reported for other types

of PPF for similar conditions.6,16

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Figure 7: Evolution of the [SH] (calculated using equation 2) as a function of the reaction duration for <P> = 100W (a), 38 W (b) and 14 W (c). The errors bars correspond to the standard deviations values calculated from measurements using different areas on the

sample’s surface. The line is drawn as a visual guide.

Conclusions

A chemical derivatization method has been established to quantitatively probe the –SH

function in propanethiol plasma polymers using N-ethylmaleimide as a labeling molecule.

The derivatization reaction was first evaluated on a “model” surface, namely 3-

mercaptopropyl-trimethoxysilane self-assembled monolayers exhibiting a –SH terminated

function. The combination of ellipsometry, XPS and ToF-SIMS data clearly demonstrate the

grafting of N-ethylmaleimide through a chemical reaction with –SH group. Considering the

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steric effect induced by the grafting of N-ethylmaleimide, it was shown that the derivatization

reaction is quantitative.

Concerning the propanethiol plasma polymers, it has been shown that the reaction

between the –SH function and N-ethylmaleimide is not fully selective and that the –OH

function can also react with the derivative agent but to a significantly lower extent than the –

SH groups. The study of the kinetics of the reaction reveal first a fast surface reaction

followed by a diffusion limited step.

We believe that these results pave the way for optimizing the surface concentration of –

SH groups in sulfur-based plasma polymers.

Acknowledgements

The authors thank F.R.I.A grant of the Communauté de Française de Belgique and the

Belgian Government through the «Pôle d’Attraction Interuniversitaire» (PAI, P7/34, “Plasma-

Surface Interaction”, Ψ) for financial support.

Supporting Information

Schematic description of MPTS-SAM exhibiting a N-ethylmaleimide grafted at the

sulfur extremity. Comparison of the thiol density measured by XPS and ToF-SIMS.

Elemental composition measured by XPS for Pr-PPF as deposited and after the chemical

derivatization reaction. This material is available free of charge via the Internet

http://pubs.acs.org.

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Table of Contents Graphic

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