03 Nov (Macro)molecules with anchoring moieties: a versatile chemistry to tune the surface properties

The modification of surfaces by the deposition and attachment of specific (macro)molecules is a key strategy to tune the properties of substrates, thus providing an access to a wide range of characteristics for numerous fields of application. Different chemistries can be used to coat the surfaces, which involve various modes of reaction and structures, and thus result in a variety of applicative properties. Three anchoring groups will be detailed here, namely silanes, phosphonates and catechols.

Silanes

Reactions of organosilanes with surfaces bearing hydroxyl groups (e.g. silica, aluminium oxide, zinc oxide) have found an interest to modify metallic substrates and inorganic nanoparticles or to impart specific properties to sol-gel networks. The main advantage of using silanes for surface modification is the rapid formation of a covalent siloxane linkage with the substrate by hydrolysis and condensation of the alkoxysilane groups (sol-gel process) (Figure 1). Excellent substrates for modification are surfaces containing silanol (Si-OH) groups, but metal oxides are also good candidates.²

Silane Coupling Agents (SCAs)

Silane coupling agents (SCAs) constitute a special group of compounds that have the ability to form stable bonds between both organic and inorganic materials due to the presence of (i) an alkoxysilyl group, which is prone to hydrolysis and bonding to a mineral filler, and (ii) an organic function with affinity to a polymer matrix. SCAs are commonly applied in material chemistry as adhesion promoters, polymer cross-linking agents and additives in reinforced composites.³

SCAs are generally synthesized by a two-step process from hetero-bifunctional molecules involving (i) the introduction of the alkoxysilane group, usually by hydrosilylation of a C=C double bond, and (ii) a substitution reaction on the other reactive moiety. The substitution reaction can be used to prepare polymerizable SCAs with (meth)acrylic or (meth)acrylamide groups³ and functional SCAs with various other reactive groups.

By changing the structure and functionality of SCAs, a wide range of properties and applications can be reached. For example, (meth)acrylic SCAs can be applied in the development UV-curable acrylate composites,⁴ and cyclocarbonate SCAs can be employed in the conception of hybrid non-isocyanate polyurethanes (NIPU) for high-performance adhesives.^{5, 6}

Fluoroalkylsilanes for hydrophobic coatings

Recently, the introduction of fluoroalkylsilanes (FASs) directly into sol-gel networks was reported as a convenient method to obtain water-repellent silica films. This one-step co-condensation technique was proved to exhibit similar properties to a classic multi-step surface silanization, but easier to perform (one-pot and mild conditions) and in accordance with industrial standards.⁷

Si-PEO for hydrophilic coatings

Polyethylene oxide (PEO) macromolecules have been used in the manufacture of anti-fogging layers due to their hydrophilic character which endow coatings with water absorption capacity. For example, anti-fogging coatings were made from PEO hybridized by end-chain functionalization with (3-isocyanatopropyl)triethoxysilane, allowing a high transparency of coated glass following exposure to boiling water.^{8, 9} More recently, Maeda et al. have investigated the effect of introducing PEO bis triethoxysilane into a polysilsesquioxane (PSQ) antifogging film, and showed that the hydrophilic and flexible PEO chains were effective to improve the durability while maintaining the antifogging property and mechanical strength.¹⁰

Silane-PEG have also found many uses in the biomedical field for reducing protein adsorption of silicone-based biomaterials or for improving antifouling properties of stainless steel used in orthopedic implants and cardiovascular stents.^{11, 12}

Phosphonic acids for anticorrosion

Phosphonic acids possess exceptional binding properties on metals and metal oxides, and owing to the hydrolytic stability of the P-O-metal linkages, these monolayers are more resistant to hydrolysis than silane or carboxylic acid-derived monolayers, and almost comparable to catechol-based monolayers. These advantages have led to their use as anticorrosion coatings that prevent oxygen diffusion towards a metal surface.²

Phosphonic acids for biomedical applications

Phosphonate-based surface modifications are also very interesting for biomedical applications, for instance for the development of new contrast agents based on magnetic inorganic nanoparticles modified with radiolabeled bisphosphonates to obtain simultaneous in vivo magnetic resonance imaging (MRI) and optical imaging. In addition, surface modification of stainless steel or titanium medical implants by (bis)phosphonate anchoring is an attractive route to improve the adhesion to bone tissue and cells, enhance long-term stability and prevent infections by suppressing bacterial adhesion.²

Catechols

A biomimetic mode of attachment

The biomimetic mode of binding of catechol-derived films has been studied intensely over the last few years and was found to be highly complex. As shown in Figure 4, catechol groups undergo various types of interactions (both non-covalent forces and chemical bonding) with different substrate surfaces.

Figure 4. Adhesive interactions between catechol groups and various substrates

For instance, catechol groups have a strong affinity for hydrophilic surfaces due to their capacity to establish hydrogen bonds. Thanks to the presence of two neighboring hydroxyl groups, catechols can anchor to inorganic surfaces (silica, metal oxides, etc.) by means of hydrogen bonding or coordination. These interactions, even non-covalent ones, are strong enough to displace pre-adsorbed water molecules, allowing the catechol to adhere in wet environments. On the other hand, the catechol benzene ring allows to establish π–π stacking interactions with substrates rich in aromatic systems (e.g. graphene, CNTs or polymers containing aromatic groups). Finally, catechol groups can form covalent bonding with organic substrates. In this case, catechols must be oxidized to the quinone form to favor the nucleophilic attack of groups such as amines or thiols. It is worthy to note that in most cases, the coexistence of different adhesive modes occurs.^{18, 20}

Catechol-based polymers for various applications

Antifouling coatings

Adhesives

The remarkable underwater adhesion with self-healing properties in mussel-adhesive proteins have been the subject of intensive scientific research over the past few decades. Many studies describing synthetic catechol-based polymeric adhesives with recognizable biomimetic features have been published in the last ten years.²²

For example, Zhao et al. have developed smart wet adhesives by combining an adhesive DOPA-copolymer with a thermoresponsive polymer matrix based on poly(N-isopropylacrylamide) (pNIPAM), a polymer well-known for its reversible conformational transition as the temperature changes around its lower critical solution temperature (LCST). By taking advantage of the synergistic cooperation of catechol chemistry and responsive wettability, the authors were able to create a robust and smart underwater adhesive which endows a tunable and reversible modulation, and can be deposited on a wide range of substrates.²³

Water treatment

Catechol-based materials have also been interestingly studied for environmental applications such as water treatment. In a recent paper, catechol was investigated to prepare a catechol-based hypercrosslinked polymer (HCP) as a solid adsorbent for iron (Fe) removal from water. Catechol-HCP was synthesized by Friedel–Crafts alkylation and obtained as a black powder having a good compatibility with water. Due to the good interaction between catechol moieties and Fe, Catechol-HCP was proved to adsorb high amount of Fe, with an adsorption efficiency of 94%. Moreover, the material can be reused without losing its efficiency for at least four cycles.²⁴