Introduction of thiol moieties, including their thiol–ene reactions and air oxidation, onto polyelectrolyte multilayer substrates

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Abstract

We describe the derivatization of uncross-linked and cross-linked layer-by-layer (LbL) assemblies of polyelectrolytes (polyallylamine hydrochloride and polyacrylic acid) with sulfydryl groups via Traut’s reagent (2-iminothiolane). This thiolation was optimized with regards to temperature, concentration, and pH. The stability of the resulting –SH groups in the air was determined by X-ray photoelectron spectroscopy (XPS). This air oxidation has obvious implications for the use of thiol–ene reactions in materials chemistry, and there appears to be little on this topic in the literature. Three main S 2s signals were observed by XPS: at 231.5 eV (oxidized sulfur), 227.6 eV (thiol groups), and 225.4 eV (thiolate groups). Due to their rapid oxidation, we recommend that thiolated surfaces be used immediately after they are prepared. As driven by 254 nm UV light, thiol groups on polyelectrolyte multilayers react with 1,2-polybutadiene (PBd), and residual carbon–carbon double bonds on adsorbed PBd similarly react with another thiol. In the case of a fluorinated thiol, surfaces with high water contact angles (ca. 120°) are obtained. Modest exposures to light result in derivatization, while longer exposures damage the assemblies. Polyelectrolyte–thiol–PBd–thiol assemblies delaminate from their substrates when immersed for long periods of time in water. Surface silanization with an amino silane prevents this delamination and leads to stable assemblies. These assemblies withstand various stability tests. Techniques used to analyze the materials in this study include X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and contact angle goniometry.

Introduction

Surface modification continues to be a very active area of research in modern science because of the many potential applications of functionalized surfaces in biosensors, fuel cells, photovoltaics, bioconjugate chemistry, molecular electronics, adhesion, microfluidics, etc. [1]. Some of the basic building blocks of such advanced materials are self-assembled monolayers on gold [2], [3], organosilane monolayers on hydroxylated surfaces [4], [5], [6], [7], [8], alkene and alkyne monolayers on hydrogen-terminated, nanoparticulate, and scribed silicon [9], [10], [11], [12], [13], [14], [15], phosphonates on alumina, zirconia, and related substrates [16], [17], [18], [19], [20], the many reactions of bioconjugate chemistry [21], the layer-by-layer assembly of polyelectrolytes and other charged species [22], [23], [24], [25], thiol–ene chemistry [26], [27], [28], [29], [30], and carbon nanotubes [31], [32], [33]. Complex molecular assemblies on surfaces may be prepared using combinations of different chemistries that are compatible with a specific substrate and each other.

Recent developments in thiol–ene chemistry have created a powerful set of tools for materials development, surface modification, and nanotechnology [34], [35]. This chemistry is based on the facile reaction between carbon–carbon double bonds and sulfhydryls (–SH groups). The resulting C–S linkages show good robustness. In the past few years we have employed this chemistry in two ways. First, taking advantage of the well-known reaction between surface Si–H groups and carbon–carbon double bonds, which is obviously analogous to thiol–ene chemistry, we derivatized hydrogen-terminated silicon with 1,2-polybutadiene (PBd) [30]. The residual, unreacted carbon–carbon double bonds on this surface were then reacted with various thiols. Second, we created thiol monolayers on gold from an α,ω – dithiol [26]. The resulting, pendant thiol groups were reacted with PBd and then derivatized with other thiols. This approach yielded a much more stable assembly compared to a traditional monolayer on gold.

In this contribution we continue to explore these reactions and possibilities. In particular, we create uncross-linked and thermally cross-linked layer-by-layer (LbL) assemblies of polyelectrolytes, polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA), and then derivatize them with sulfydryl groups using 2-iminothiolane (Traut’s reagent) (see Fig. 1). Bruening et al. and Rubner et al. have previously studied the thermal cross-linking of polyelectrolytes, which increases the stability of the films [36], [37], [38], [39]. We then determine, by X-ray photoelectron spectroscopy (XPS), the stability of the resulting –SH groups in the air. These results are particularly relevant to the application of the thiol–ene reaction to materials chemistry because there appears to be little in the literature on this topic. We then confirm that, as driven by UV light, surface –SH groups on polyelectrolyte multilayers react with PBd, and that residual carbon–carbon double bonds on the PBd will react with another thiol. Interestingly, modest exposure to the light results in derivatization, while longer exposures result in damage to the assemblies. Finally, we observe that polyelectrolyte-thiol-PBd-thiol assemblies delaminate from their substrates when immersed in water for significant periods of time. Deposition of an amino silane onto the silicon prior to the LbL of the polyelectrolytes results in robust assemblies. The assemblies are also stable against swabbing and the Scotch tape test. Techniques used to confirm and analyze film growth and composition herein include: XPS, time-of-flight secondary ion mass spectrometry (ToF-SIMS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and contact angle goniometry.

Section snippets

Materials

Polyallylamine hydrochloride (PAH) (Mw  58 kg mol−1), poly(acrylic acid) (PAA) (Mw  10 kg mol−1), 1,2-polybutadiene (PBd) (approx. 90% 1,2-vinyl), triethanolamine hydrochloride (TEA, ⩾99.0%) and 1H,1H,2H,2H-perfluorodecanethiol (PDT, 99.0%) were purchased from Sigma–Aldrich (St. Louis, MO). Traut’s reagent was from ProteoChem (Denver, CO). Ethanol (200 proof) was from Decon Laboratories (King of Prussia, PA). Stabilized THF was from Mallinckrodt, Phillipsburg, NJ. Water (resistivity 18  cm) for

LbL Films of PAH and PAA

The LbL deposition of PAH and PAA was explored with solutions of these polyelectrolytes at ca. pH 5.0 and 5.7, respectively [36], [40]. A series of assemblies was also prepared at ca. pH 8.0 (PAH solution) and 8.7 (PAA solution). Details regarding their growth, thermal cross-linking, and stability is given in the Supporting Information.

Thiol termination of PAH-terminated PAH/PAA LbL assemblies

PAH-terminated LbL assemblies of PAH and PAA ought to possess unreacted amine groups at their surfaces, i.e., this should certainly be the case for

Conclusions

A complex molecular assembly was prepared on a silicon surface using a combination of uncross-linked or thermally cross-linked polyelectrolyte multilayers, bioconjugate chemistry with Traut’s reagent (2-iminothiolane), thiol–ene chemistry with PBd, thiol–ene chemistry with a fluorinated thioalkane, and silane chemistry. The oxidation of thiols on assemblies exposed to the air was monitored by XPS, which showed them to be relatively unstable. Thiol-terminated assemblies were reactive with PBd,

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources

We gratefully acknowledge funding from the National Science Foundation (CBET-0708347).

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