Chem Soc Rev View Article Online
TUTORIAL REVIEW
View Journal | View Issue
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Surface functionalisation of polymers Cite this: Chem. Soc. Rev., 2017, 46, 5701
Dardan Hetemi
ab
and Jean Pinson
*a
Many applications of polymers require the functionalisation of their surface for use in sensors, composite materials, membranes, microfluidic and biomedical devices and many others. Such surface modifications endow the surface with new properties independent of those of the bulk polymer. This tutorial review describes the different methods, based on very diverse principles, that are available to perform this surface functionalisation, including plasma and UV irradiation, atomic layer deposition, Received 1st March 2017
electrochemistry, oxidation, reduction, hydrolysis, the use of radicals and grafting ‘‘on’’ or ‘‘from’’
DOI: 10.1039/c7cs00150a
polymers. The principles of the different methods are briefly described and many examples are given to highlight the possibilities of the methods and the possible applications. A section is devoted to the
rsc.li/chem-soc-rev
surface modification of polymeric nanoparticles.
Key learning points (1) (2) (3) (4) (5)
What is the interest in modifying the surface of polymers? The many methods available for the surface modification of polymers The possible applications of surface-modified polymers The attachment of polymers on polymers The surface modification of polymers in the biomedical field
Introduction Surface modification is used for two main purposes, either to protect a material that does not resist properly under usual environmental conditions or to add specific properties to the surface. Concerning the former, the best example is given by cars, whose steel bodies would rust under ambient conditions but are protected by sophisticated processes that also provide the aesthetic aspect. Concerning the latter, drugs can be attached to the surface of a material, as in drug-releasing arterial prostheses (stents) that are used to reopen clogged arteries; a drug is imbedded in a polymer on the stainless steel surface and is released slowly to prevent restenosis of the artery. Many sensors are based on surface modifications; they can specifically recognize and quantitate an organic or bio-molecule through the surface attachment of a chemical group able to recognize the target. Glucose sensors, commercially available for diabetic patients, are based on glucose oxidase and water-soluble polycationic polymers, with tethered complexes of Os2+/3+ polymers. a
Univ Paris Diderot, Sorbonne Paris Cite´, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baı¨f, 75013 Paris Cedex 13, France. E-mail:
[email protected] b Pharmacy Department, Medical Faculty, University of Prishtina ‘‘Hasan Prishtina’’, Rr. ‘‘De¨shmore¨t e Kombit’’ p.n., 10000 Prishtina, Kosovo
This journal is © The Royal Society of Chemistry 2017
Polymers are present everywhere in our daily life; their success is due to their different properties, such as low cost, interesting mechanical properties (polypropylene (PP) is used for car bumpers, and polyamide 66 for gears), high stability under environmental conditions, and for some of them clarity (polymethylmethacrylate (PMMA) is used for large windows of aquariums and viewing ports of submersibles) or a high resistance to chemicals (polyethylene (PE) and polytetrafluoroethylene (PTFE) Teflont are used for containers of aggressive chemicals). There are a near-infinity of methods to tune their properties by choosing the starting monomer(s) and the polymerisation methods. However, their surface properties are correlated to their bulk properties, and most polymers have inert surfaces under ambient conditions. It would be interesting to tailor the surface properties of a polymer independent of its structure and bulk properties. Most polymers are poorly reactive, and for this reason the surface modification of polymers with different chemical groups (including bioactive groups) has gained sustained interest in such fields as biomedicine, electronics, textiles, packaging processes, protective coatings, friction and wear, composites, and thin-film technology. From this quite large list, one can understand the importance of this field. As the surface of most polymers is not very reactive, most of the methods involve rather harsh conditions that will break
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5701
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Tutorial Review
Chem Soc Rev
surface bonds with the help of radiation, reactive radicals, and aggressive reagents; other methods involve specially designed polymers with reactive surface groups. We will now describe the different methods, physical, chemical, in the gas phase and in solution, that can be used for the modification of the surface of polymers.1–3 This tutorial review will concentrate on the chemical aspects of the surface modification of polymers, the possible applications and the chemical mechanisms. We will try to compare the different methods but we will not detail the characterisation of the layers, nor of, for example, plasma production. Many examples will illustrate the variety of processes used for the surface modification of polymers.
Different methods for the surface modification of polymers Plasmas A plasma is a partly ionized gas containing free electrons, ions, radicals, and neutral particles (atoms and molecules). Some of these particles may be in an excited state; they can return to their ground state by photon emission which produces typical plasma light emission. Plasmas are typically obtained when gases are excited into energetic states by radio-frequency, microwaves, or electrons from a hot filament discharge.4 Several methods are available for the generation of plasmas, and the reader is referred to the original literature for more details. However, the plasmas predominantly used for surface treatment are cold non-equilibrium plasmas where the temperature of the electrons (related to their translational energy, 1–10 eV) is higher than that of ions and molecules or radicals (B0.025 eV, 298 K), and where only a few percent of the molecules are ionized. Due to their low temperature they do not destroy surfaces.4 The energy of most electrons is in the range of 1–5 eV, similar to that required to break simple organic chemical bonds. Plasmas are certainly the most widely used method for surface modification of polymers (B20 000 references under the heading ‘‘Plasma Polymers’’ in the Web of Science). Corona
Dardan Hetemi and Jean Pinson
5702 | Chem. Soc. Rev., 2017, 46, 5701--5713
discharge, a related method, involves the ionization of a gas surrounding a conductor maintained at high potential generating a cold plasma (industrial applications are similar to those of plasmas). This method has been used, for example, to create plasma gradients on polydimethylsiloxane (PDMS).5 When a surface is in contact with a plasma two types of modifications are possible: (i) simple gases (Ar, H2O, O2, N2. . .) produce reactive species that react with the polymer surface; for example, oxygen plasma permit to hydroxylate the surface of polymers, (ii) if organic molecules (such as saturated or unsaturated hydrocarbons) are used to generate a plasma, polymeric films are grown from the surface. Surface modification with plasma from inorganic gases. PE and PP that are devoid of functional groups have been treated with a water plasma to give hydrophilic surfaces; the hydroxyl groups formed on the surface have been detected by the decreased water contact angle from 971 to 541 and by the appearance of an O1s XPS peak (C–OH/–C = 13%).6 There are many examples of the modification of filtration membranes by plasma processing. O2/NH3 plasmas have been used to modify microporous polyether–polysulfone membranes; such treatments alter the surface chemistry and create permanently hydrophilic surfaces.7 The water contact angle of the membranes decreased from 661 to 01 and the hydrophilicity was maintained for more than 12 months. S-Nitrosated poly(lactic-co-glycolic acid)-cysteine (PLGHcysteine), a polymer based on biodegradable poly(lactic-co-glycolic acid), has been modified by covalent attachment of thiol groups, that were subsequently nitrosated (Fig. 1). This polymer has the ability to release NO, a molecule that has demonstrated excellent antithrombogenic (that prevents the formation of blood clots within a blood vessel) and antimicrobial properties. In order to increase the hydrophilic character of this polymer it was subjected, under carefully controlled conditions, to an H2O plasma. As a result, hydroxyl and carbonyl functional groups were implanted on the polymer surface and the water contact angle changed from 1161 to B01, i.e., from highly hydrophobic to highly hydrophilic. Surprisingly, the NO-releasing capability of the surface was little modified by this treatment.8
Dardan Hetemi is currently a teaching assistant at Pharmacy Department, Medical Faculty at the University of Prishtina – ‘‘Hasan Prishtina’’. He graduated from the same university in 2008 and received his degree of European Master in Quality in Analytical Laboratories (EMQAL) in 2012 from University of Barcelona and University of Bergen. He obtained his PhD degree in 2016 from Universite´ Paris Diderot working with Emeritus Professor Jean Pinson, Catherine Combellas and Fetah Podvorica. His research interests lie in the modification of surfaces, biopolymers, nanomaterials and drug carriers. Jean Pinson is an Emeritus Professor at Universite´ Paris Diderot after a full career in the same University, but also spent five years in a startup company Alchimer, now Aveni. He is interested in electrochemistry and surface chemistry. He developed the modification of surfaces with amines, diazonium salts, and alkyl halides.
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Chem Soc Rev
Tutorial Review
Fig. 3 PMMA becomes superhydrophobic after plasma treatment and silanisation of the OH functions with 1H,1H,2H,2H-perfluorododecyl groups (RF).
Fig. 1 Modification of the surface of a polymer by an H2O plasma without disturbing the active NO groups.
These examples show that the surfaces of polymers can be modified with simple functional (OH, NH2,. . .) groups, but there are many applications where more complex molecules must be attached to the surface; in these cases, a post-modification of the surface must be performed. The goal of the following investigation was to develop a fast and cost-effective 3D material for water decontamination.9 A chitosan (a polysugar) scaffold was constructed and irradiated with an Ar plasma. Radicals formed on the surface of the scaffold persist for a short time. These surface radicals are used to induce the polymerisation (in supercritical CO2) of 2-isopropenyl-2oxazoline. Oligo 2-oxazolines present antimicrobial activities against fungi and a broad spectrum of bacteria; they exhibit short killing times and are believed not to generate resistant bacteria. Further polymerisation with different oxazolines completed the preparation of the material that efficiently and within minutes killed Staphylococcus aureus and Escherichia coli cells upon direct contact (Fig. 2). Flat surfaces of PMMA or poly ethyletherketone (PEEK) were treated for a few minutes by low-pressure air plasma to produce oxygen-containing groups on the surface (–OH, –CHO, and –COOH); these groups were immediately reacted with a cyclohexane solution of perfluorododecyltrichlorosilane. The water contact
Fig. 2 Use of surface radicals for post-modification of the surface of a chitosan polymer.
This journal is © The Royal Society of Chemistry 2017
angle of PMMA (601) decreased to 401 after plasma treatment and increased to 1251 after silanisation. If the surface is previously plasma-textured to give hierarchical randomly rough surfaces of PMMA, the surface becomes superhydrophobic with a water contact angle of 1671 (Fig. 3).10 Plasma treatments of polymers are widely used in industry; a number of companies offer plasma treatments for plastics, for example Dyne Technology, Acxys, Lectrotreat, TriStar Plastics, Keol, Plasma Etch, and Alma Plasma. Such treatments are used for cleaning, improving surface adhesion, binding to metals (treatment of the inner surface of needle hubs prior to binding a stainless-steel needle) or plastic materials, printing on plastics (syringe barrels, see Openair; cables, see Dyne), painting, varnishing, protecting the surface with anti-scratch silica-like films, and modifying the surface of textiles.11 Plasma polymer films. The second type of modification involves a specific type of plasma-enhanced chemical-vapour deposition (PECVD), a polymerisation that provides plasma polymer films (PPF) on surfaces. A surface is introduced into a plasma chamber, filled with a low pressure (o1 Torr) of an organic compound. After plasma irradiation, a highly reticulated polymeric film without repeating units, as in conventional polymers, is formed on the surface. Electrons with a kinetic energy centered at 1–5 eV are able to break chemical bonds into a variety of chemically unstable species: ions, free radicals, activated fragments, excited states, electrons, and photons.1,12 Radicals are the dominant species (103–105 higher than the ion density) and responsible for the formation of the films through radical–radical and radical–molecule reactions that provide highly disordered polymers. But, ions also contribute to the formation of the PPF film, as explained thereafter. For insulated surfaces (such as polymers), due to the difference in thermal velocities between electrons and ions, the surface acquires a negative charge. At equilibrium, the ion flow perfectly counterbalances the electron flow, leading to a stationary electrical potential, named the floating potential (Vf), and a region called a sheath is created with a net positive charge. When positive ions enter the sheath, they are accelerated and strike the surface with a kinetic energy ranging typically from 10 to 30 eV.12 A recent study has investigated the films obtained on glass (the same should occur with a polymer) with plasma generated from different amines.13 At the beginning of the irradiation, bonds are broken on the surface by the impact of high-energy
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5703
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Tutorial Review
Chem Soc Rev
Fig. 4 Mechanism of plasma polymer formation on a substrate.
electrons (etching) and radicals are created. The radical sites can then react with plasma-phase species with the formation of a covalent bond between the film and the surface. After this initial stage, plasma polymers grow via three main mechanisms: direct ionic deposition, radical termination and radical propagation (if the molecule contains double bonds) (Fig. 4). Therefore, these polymeric films are covalently bonded to the surface, highly reticulated and disordered. Polyethylene terephthalate (PET) has been endowed with antithrombogenic properties by plasma treatment with acrylic acid (AA) that produces a surface polymer containing multiple COOH groups (Fig. 5). This polymer has a disordered structure very different from that of polyacrylic acid which has acrylic acid repeating units. Successive attachment of polyethylene glycol and heparin by EDC (a soft method for coupling an acid and an amine) furnishes the final antithrombogenic surface.14 Polyethylene glycol is known to prevent the adhesion of cells on the surface and heparin is a blood thinner used to treat and prevent deep vein thrombosis. The modified PET surface showed improved platelet adhesion and protein-adsorptive resistance after grafting of heparin (Fig. 5). PTFE is known for its very low reactivity which strongly restricts the possibility of binding of this polymer to other materials. However, the surface of expanded PTFE can be
Fig. 6
Fabrication of a Janus textile by PECVD.
modified by irradiation with an ethylene plasma that produces a thin layer of polyethylene. This polyethylene film contains a number of radicals that induce the polymerisation of acrylic acid (AA). With such a treatment, the water contact angle decreases from B1201 to B701 and the adhesion (measured as the lap shear strength with a PTFE specimen, stuck between two stainless steel surfaces by an epoxy resin) increases 3 times after 4 hours of AA polymerisation.15 Janus textiles (this term applies to a material the two sides of which have opposite properties, derived from the Greek god with two opposite faces) with different wetting properties on the two sides have been prepared. Surface functionalization by PECVD permits obtaining a porous textile that is superhydrophobic and superoleophobic on one side, while the other side becomes superhydrophilic. A thin fluorinated polymer was deposited on one side by plasma polymerisation of 1H,1H,2H,2H-perfluorooctylacrylate, while the other side was functionalized with a polymer coating made of maleic anhydride, subsequently hydrolysed to produce carboxylic acid groups on the surface. Static contact angles up to 1691 with water and 1621 with hexadecane were obtained on the fluorinated side of the fabric, while on the hydrophilic side they were too low to be measured (Fig. 6).16 Irradiation by UV rays, c-rays, and ions
Fig. 5 PET is endowed with antithrombogenic properties after grafting heparin on its surface.
5704 | Chem. Soc. Rev., 2017, 46, 5701--5713
Different modifications of polymers surfaces can be achieved by UV rays: (i) 200–400 nm radiation is mainly used for surface functionalisation but penetrates the bulk of the polymer (to more than 500 nm) and can, therefore, damage the structure of the polymer; (ii) a shorter wavelength (o200 nm) in the presence of different gases (O2, NH3) allows incorporating oxygen or nitrogen groups in the surface of the polymer, leading to changes
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Chem Soc Rev
in wettability;17 (iii) even shorter, extreme ultraviolet radiation (10–120 nm) induces surface ablation18 and carbonisation of the polymer. By UV irradiation. Bond cleavage in the polymer. The surface modification of polymers is possible by simple UV irradiation in the 200–400 nm range. For example, the thermoplastic polymer bisphenol A polycarbonate (BAPC: [OC6H4C(CH3)2C6H4OCO]n) is used as an important engineering plastic because of its low cost, good thermostability, excellent optical transparence, excellent mechanical properties, and high electrical insulation, with however a low surface reactivity. By irradiation with a high-speed scanning 355 nm laser it is possible to write on this polymer: the laser energy is sufficient to break C–C and C–O bonds, giving rise to radicals that react with ambient dioxygen to generate oxygen-containing groups (e.g. C–O, CQO, and COO!) (Fig. 7). Note that these reactions are very fast, as the laser pulse width ranges from 10 ns to 60 ns.19 A so-called ‘‘oxide nanoskin’’ of SiO2 has been grafted onto the surface of PMMA. After irradiation at 172 nm with a Xe excimer lamp, the surface of PMMA becomes hydrophilic (the water contact angle decreases from 801 to 251); this hydroxylated surface is reacted at 80 1C with tetraethoxysilane (TEOS) and the modified PMMA is irradiated again as before. As a result, a stable 3 nm thin film of SiO2 without cracks or aggregates is formed on the surface of PMMA (Fig. 8).20 Oxidation of the polymer surface. Lysozyme is an antimicrobial enzyme with food packaging applications; in order to prevent its migration it can be attached to ethylene vinyl alcohol (EVOH), a food-compatible polymer.21 The polymer is irradiated at 254 nm in the presence of air to give surface –C(QO)O! groups by oxidation of –OH groups. These carboxylic groups can then be coupled to lysozyme through classical peptidic coupling. Formation of radicals. Upon UV irradiation of polybutadiene in the presence of liquid ethyldimethylsilane (C2H5)(CH3)2Si–H and gaseous trimethylsilane (CH3)3Si–H with a medium-pressure Hg lamp (maximum emission at 254 nm) or a 193 nm excimer laser,22 the polymer is modified with silyl groups. Both of these light sources are capable of performing the homolytic cleavage of a Si–H bond to give a Si" radical that reacts with the double bond of butadiene, forming a new carbon radical that abstracts a hydrogen atom from –Si–H to give the modified polymer. Coupling of two radicals should also be possible, although disfavored by concentration effects (Fig. 9).
Fig. 7 SEM images of the BAPC surface before (a) and after (b) laser writing.
This journal is © The Royal Society of Chemistry 2017
Tutorial Review
Fig. 8 Formation of a thin silica film on the surface of PMMA.
By creating a radical on the surface of a polymer, it is possible to start a polymerisation, a reaction known as ‘‘grafting from’’, many examples of which have been described. The PEEK structure contains a benzophenone motif (photoinitiator); under UV irradiation (300–400 nm), the benzophenone group is excited to the singlet state, and then further transformed into a triplet biradical that abstracts a H atom from a H donor, to give a surface radical. This radical can initiate a ‘‘grafting from’’ polymerisation23 as shown in Fig. 10. Analogous reactions can be performed by simple deposition of the photoinitiator on the surface of a polymer such as PE.
Fig. 9
Addition of photochemically generated silyl radicals to polybutadiene.
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5705
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Tutorial Review
Chem Soc Rev
Fig. 10 ‘‘Grafting from’’ reaction of polystyrene on PEEK.
Among other interesting properties, poly(2-chloro-p-xylylene)24 (Parylene C) presents oxygen barrier properties, and has been used to protect stents, implantable electrodes and electronic circuitry from oxidation. For the coating of stents, Parylene C does not present sufficient anti-biofouling properties and is quite hydrophobic. In order to prevent the adhesion of cells on Parylene C-coated stents, this polymer has been modified with biocompatible phosphorylcholine groups (the head group of phospholipids) by a ‘‘grafting from’’ reaction (Fig. 11). Upon UV irradiation, benzophenone gives a biradical that can abstract a hydrogen atom from the methylene group of Parylene C leading to a !CH" radical; this carbon radical adds to the vinylic monomer substituted by a phosphorylcholine group. After polymerization, the water contact angle decreases from 951 to 471 and protein adhesion by approximately 75% (Fig. 11). Aryl radicals are produced by irradiation of a diazonium salt in the presence of a photoinitiator (eosin Y)25 and react with the surface of polyvinylchloride (PVC). An acetonitrile (ACN) solution of 4-nitrobenzenediazonium and eosin Y is deposited on the surface of PVC and irradiated with a cool-light LED bulb. The diazonium salt is reduced by the excited state of eosin Y providing an aryl radical and dinitrogen upon homolytic cleavage. A hydrogen atom is abstracted by the radical to give a polymer surface radical that couples with another aryl radical to give the modified polymer surface, as shown in Fig. 12.26 By gamma-rays. These very high-energy ray (4100 keV) can be used for the surface modification of polymers. Porous hydrophobic PP membranes have been modified by g-ray irradiation in the presence of oxygen.27 A very high concentration (2.8 # 10!7 mol cm!2) of peroxides is obtained on the membrane; they are decomposed at 60 1C to radicals that react with hydrophilic 2-hydroxyethyl methacrylate (HEMA), leading to graft copolymerisation. The water contact angle decreased from 1131 to 621 after modification, and the adsorption of bovine serum albumin was decreased. Atomic layer deposition In this method, a surface is placed in a heated reactor in which a gas precursor is introduced; this gas reacts with the surface in a self-limiting manner, i.e. the precursor only reacts with the active sites of the surface, giving at most a monolayer. The
5706 | Chem. Soc. Rev., 2017, 46, 5701--5713
Fig. 11 Modification of Parylene C with phosphorylcholine groups.
Fig. 12 Surface modification of PVC by aryl groups under photochemical irradiation.
precursor is then pumped out of the reactor and a second gas is introduced. In most atomic layer deposition (ALD) reactions, the second precursor reacts with the first one to give a surfacedeposited molecule. Al(CH3)3 is a typical precursor, and is followed by water as the second precursor. The process is repeated several times to give a thin Al2O3 film on the surface.28
This journal is © The Royal Society of Chemistry 2017
View Article Online
Chem Soc Rev
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Fig. 13 Al2O3.
Tutorial Review
Superhydrophobic wool fabric obtained by ALD deposition of
Fig. 14 A thin Pt film (B20 nm) deposited on a photoresist polymer (a) after removal of the polymer support (b).
This ALD process is illustrated in Fig. 13; to obtain superhydrophobic wool fabrics, aluminum oxide was deposited onto the surface using alternating pulses of trimethylaluminum as the precursor and water at 80 1C as the reacting gas. Common household liquids formed droplets on the ALD-treated wool, while stains were obtained on the untreated wool.29 A distinct characteristic of ALD is the high conformity of the film; Fig. 14 presents a Pt thin film deposited on a patterned photoresist (SU-8).30 To achieve this structure, the polymer was patterned by interference lithography and a Pt film was deposited on top by ALD: the precursor is a Pt complex (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) and the reactant gas is ozone that decomposes the complex to Pt1. The platinum growth rate is about 0.45 Å per cycle, and 420 cycles were repeated to deposit around 20 nm of metallic Pt. Interestingly, if the polymer is removed by heating, the Pt film collapses (Fig. 14b) to give a nanoaccordion that can find applications in flexible electronics. In order to promote osteointegration between cells from the bone and the highly porous poly(styrene–divinylbenzene) scaffold, Al2O3 and TiO2 have been integrated in the polymer.31 This was achieved by ALD: alumina films were first deposited by the reaction of trimethylaluminum and H2O at 33 1C and then titania films from titanium tetrachloride and H2O2 at 100 1C; the process was repeated 50 times. The deposition of alumina and titania films inside the porous network can be observed by field emission scanning electrochemical microscopy (FESEM) and energy dispersion spectroscopy (EDS) (Fig. 15). The accelerated formation of hydroxyapatite (a biocompatible compound structurally similar to bone mineral) is then possible on these modified polymers due to the negatively charged surface provided by the ultrathin ceramic interface.
Fig. 15 Cross-sectional FESEM and EDS images of a TiO2-coated porous polymer particle: (a) FESEM image of the cross-sectioned surface of a TiO2-coated porous polymer particle after 50 cycles and (b) the titanium EDS signal of the same cross-sectioned surface.
other methods, by electrochemistry.32 The first step involves the reduction of a small area of PTFE by an electrode (Pt for example) either in contact with or very close to the surface (B10 mm). In this last case, which corresponds to a scanning electrochemical microscopy (SECM) configuration, the reduction of PTFE is achieved by the reduced form of a mediator generated at the electrode. The mediator benzonitrile, as an example, presents an electrochemically reversible redox couple (benzonitrile/benzonitrile radical anion, E1 = !2.05 V/Ag/AgCl in aprotic medium). The radical anion is able to reduce the substrate: upon electron transfer the C–F bond is cleaved and the radical anion is reoxidised to benzonitrile that diffuses to the electrode where it is reduced back (Fig. 16). Reduction of fluorinated polymers is also possible with a solution of solvated electrons, e! , prepared in liquid ammonia by s;Mg2þ electrochemistry. These methods result in an n-doped polymeric carbon surface that is able to react in different ways to give: (i) unsaturated and oxygen functions (CQC, COH, CQO, COOH) if the surface comes into contact with air, (ii) a bonded polyacrylic acid film in the presence of acrylic acid, (iii) nitro or bromophenyl groups attached to the carbon in the presence of 4-nitro or
Electrochemistry Polytetrafluoroethylene (PTFE) is a particularly inert polymer but its surface functionalisation has been achieved, among
This journal is © The Royal Society of Chemistry 2017
Fig. 16 Electrochemical functionalisation of the surface of PTFE.
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5707
View Article Online
Tutorial Review
Chem Soc Rev
4-bromobenzenediazonium and (iv) Au, Ag or Cu metallic deposits in the presence of the corresponding ions.
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Chemical oxidation Strong oxidizing agents can create oxygen groups on the surface of polymers. For example, oxidation of PE by chromic acid and piranha solutions was compared with ultraviolet irradiation and oxygen plasma (surface roughness, water contract angles and surface concentration of carboxylate groups). The advancing water contact angle decreased from 991 for untreated polyethylene to 711 after treatment with chromic acid, while the roughness increased from 54 to 120 nm. Altogether the best combination of minimal roughness, high hydrophilicity, and the formation of reactive oxygenated functional groups was obtained by UV irradiation.33 Oxidation34 of polystyrene-b-poly(ethylene-cobutylene)-b-polystyrene (SEBS) by KMnO4 or KMnO4/H2SO4 provided predominantly surface O–C and O–CQO functions along with a small fraction of O–H and Mn–O groups, and the water contact angle decreased from 1061 to 451. Ozone is often used as an oxidant for the surface modification of polymers; peroxides are formed both on the surface and inside the polymer due to the diffusion of ozone in the polymer. On polyurethane, a total concentration of 2.5 # 10!9 mol cm!2 has been obtained, out of which less than 15% is located on the surface.35 Porous anodized aluminum membranes with B200 nmdiameter pores were used as molds in which PE and PTFE were cast by heating. After dissolution of the Al membrane, a structured polymeric surface was obtained. The structured PE was superhydrophobic with a water contact angle of 1721; but after oxidation with KClO4/H2SO4 the surface became superhydrophilic with a contact angle of 01 (Fig. 17).36 Food packaging materials with antioxidant activity should protect the contents from degradation, but the surface-active molecules must be strongly bonded to the surface so that they do not migrate into the food. Upon ozone treatment, peroxides are formed on the surface of PE; the radicals obtained by
Fig. 17 Water contact (a) and (c) and SEM images (b) and (d) of structured PE before (a) and (c) and after (c) and (d) surface oxidation.
5708 | Chem. Soc. Rev., 2017, 46, 5701--5713
Fig. 18 Ozone treatment of polyethylene for the attachment of active groups.
decomposition of the peroxides react with polyethylene glycol (PEG) diacrylate to give PEG-modified PE that can be reacted with ethylenediamine (as a model of a bioactive molecule) through a Michael addition reaction (Fig. 18).37 Chemical reduction The surface of polymers containing ester or ketone functions in the chain can be modified by reduction. For example, small particles of PEEK have been reduced by NaBH4 in DMSO at 120 1C.38 The OH functions obtained on the surface can be further modified (Fig. 19) to give soluble particles for potential applications in biocompatible systems.39 It is also possible to introduce sulfonic groups on the surface by reactions starting from the amino derivative shown in Fig. 19. In a similar way, PET could be reduced by LiAlH440 and the surface of structured PTFE (similar to PE of Fig. 17) could be reduced with sodium naphthenide; the water contact angle decreased from 174 to 01.36 Hydrolysis, aminolysis Polymers with ester groups such as PET can be hydrolysed in 2N NaOH at 70 1C to give surface –COO! or –COOH terminal groups, depending on further reprotonation with acetic acid.40,41 Melt-blown fiber mats of poly(butylene terephthalate) (PBT) have been hydrolysed in concentrated NaOH; after this treatment the mat becomes superhydrophilic, and superhydrophobic after further reaction with a perfluoroamine. However, these harsh treatments not only modify the chemistry of the surface but degrade the polymer that undergoes up to 60% weight loss after 30 min (Fig. 20).42
Fig. 19 Surface reduction and further modification of PEEK. (i) NaBH4, DMSO, 1201, 3 h, (ii) amide, H2SO4, AcOH, 20 1C, 48 to 72 h, (iii) RQO–Ph, LiOH, H2O–CH3CN, 501, 3 h, (iv) 1,3-propanesultone, toluene, 80 1C.
This journal is © The Royal Society of Chemistry 2017
View Article Online
Chem Soc Rev
Tutorial Review
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Fig. 20 Hydrolysis and perfluoroalkylation of a PBT fiber mat.
Biodegradable poly(lactic-co-glycolic acid) (PLGA) is a popular material for the manufacture of tissue engineering scaffolds. The surface of this polymer (thin film o2 mm) can be easily modified by aminolysis; the reaction with ethylenediamine (0.05 M) terminates after less than 20 min and hydroxyl and amino groups are formed on the surface (Fig. 21).43 PET reacts with the amino group of 3-aminopropyltriethoxysilane (APTES) to give PET-CONH(CH2)3Si(OH)3; by condensing tetraethyl orthosilicate (TEOS) on this surface, a PET supported thin silica film (PET-(SiO2)X-OH) can be obtained. Both of these surfaces react with a large number of silane coupling agents in a fashion similar to oxidized silicon wafers.44 In a similar way, surface-modified poly(ethylene naphthalate) (PEN) films were prepared with APTES in an organic solvent. These films had a high concentration of silanol groups on the surface and, upon contact, formed strong adhesive Si–O–Si bonds with each other or with a glass surface.45 Reaction with radicals, carbenes, and nitrenes Radicals. Radicals can be obtained by homolytic dediazonation of diazonium salts. For example, PMMA has been modified with perfluoroalkyl groups in one step starting from the amine NH2–C6H4–C10F21 deposited on the surface of PMMA in isopentyl nitrite at 60 1C. This permits the formation of the perfluoroalkyl diazonium salt that decomposes to the perfluoroaryl radical. This last species binds to the PMMA surface that becomes hydrophobic (water contact angle 1081).46 Alkyl radicals can also be produced by a radical crossover reaction from a sterically hindered aryl radical that cannot react with surfaces but abstracts an iodine atom from an alkyl iodide.47 These radicals react on the surface of PE and PMMA (Fig. 22). The carboxylic groups can be further modified by esterification or amidation with PEG, anthraquinone groups or a dye (neutral red).
Fig. 21 Aminolysis of PGLA.
This journal is © The Royal Society of Chemistry 2017
Fig. 22 Surface modification of PE by alkyl radicals and an image of PE modified with valeric acid groups locally post-modified with neutral red dye.
Carbenes. Carbenes can be obtained from the thermal decomposition of bis(aryldiazomethane) at 120 1C; they react with polystyrene or polyacrylate beads to give modified beads48 (Fig. 23). Further reaction with diazonium salts leads to a highly coloured material. This is an example of the complex structures that can be constructed on the surface of polymers. Nitrene. A polymer (PMPAz, Fig. 24) was synthesised that contains both phosphorylcholine and azido groups (that can be cleaved photochemically).49 PMPAz was deposited on the surface of the substrate polymer to be modified; upon irradiation at 254 nm, the azido group was cleaved into a very reactive nitrene that inserted into a carbon–hydrogen covalent bond of the substrate polymer. This ‘‘grafting on’’ reaction makes it possible to obtain polymer surfaces (PE, polystyrene (PS), PMMA,. . .) modified with phosphorylcholine groups that present a better wettability than the starting polymers (the water contact angle of PE decreases from B1001 to B651) and prevent cell adhesion on the surface of the polymer (Fig. 24). ‘‘Grafting on’’ and ‘‘grafting from’’ reactions Examples of photochemical ‘‘grafting from’’ reactions are presented in Fig. 9 and 10. We now show an example of surface initiated ATRP (atom transfer radical polymerisation), a widely used reaction for growing polymers from surfaces.50 Nanoporous polybutadiene (PB) is modified by two different methods, either by reacting the terminal double bonds of PB to obtain finally C–Br groups on the surface or by the photochemical reaction of a benzophenone modified with a C–Br bond. This group allows an ATRP reaction to be started with poly(ethylene glycol)methacrylate (PEGMA) (Fig. 25).
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5709
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Tutorial Review
Chem Soc Rev
Fig. 25 An example of a ‘‘grafting from’’ reaction: ATRP polymerisation of a polyethylene glycol monomer (MPEG) on the surface of polybutadiene (PB) by two different methods: A, by modification of the terminal double bonds (i) meta-chloroperbenzoic acid, toluene, r.t, 6 h, (ii) diisobutylaluminum hydride, r.t, 72 h, (iii) trimethylamine, dimethylaminopyridine, THF, r.t., 18 h; and B, by photografting of a benzophenone derivative, (iv) toluene, UV, 321, 1 h; (v) ATRP conditions: CuBr, bipyridine, MeOH, r.t. 1—2 h. Fig. 23 Surface modification of polystyrene and polyacrylate beads by reaction with carbenes.
Fig. 26 An example of a ‘‘grafting on’’ reaction: attachment of an alkyneterminated MPEG polymer to the surface of polybutadiene modified with azido groups, (i) m-chloroperbenzoic acid, toluene, r.t., 6 h, (ii) NaN3, MeOH/H2O, and (iii) CuBr, pentamethyldiethylenetriamine, DMF.
Fig. 24 A polymer that can be photografted through the azido group to various substrate polymers.
The same paper presents an example of a ‘‘grafting on’’ reaction where pending double bonds are modified to obtain terminal azido groups followed by a ‘‘click’’ reaction between these groups and alkyne end-capped MPEG (Fig. 26). Polymeric nanoparticles As part of the rapidly expanding field of nanomedicine, polymeric nanoparticles have been widely investigated as carriers for drug delivery; a drug loaded in a nanoparticle51 should be released in a specific site such as a tumor cell, an organ or a tissue. Before reaching the target, the nanoparticle must remain in the blood circulation at high concentration without aggregation and opsonization (the nanoparticle is marked by proteins before elimination by the phagocytic system). This is favored by a hydrophilic surface. The nanoparticle must then be transferred
5710 | Chem. Soc. Rev., 2017, 46, 5701--5713
inside the cell by interaction with the cellular membrane to deliver its payload into the cytoplasm. Therefore, the surface of polymeric nanoparticles and particularly their surface must be engineered in a very precise manner.52 In this section, we will not describe the preparation of polymeric nanoparticles; in most examples, a given surface is produced during the formation of the nanoparticle by choosing a defined preparation method and specially designed polymers. We will only give some examples of surface modification applied to nanoparticles for biomedical and analytical applications. Surface modified polymeric nanoparticles have been prepared to target liver cancer cells.53 The nanoparticle core is a polymer of lactic acid (PLA) modified with TPGS, a water soluble derivative of vitamin E. PLA is a widely used, biodegradable, FDA approved polymer and TPGS, besides having other interesting properties, is an emulsifier. This nanoparticle core is loaded with docetaxel (DTX) added during the polymerization of TPGS-PLA. DTX is one of the most active anticancer drugs for solid tumors. In order to recognize a given target, the surface of the loaded nanoparticles are then modified with specific ligands as described thereafter. First, dopamine is oxidatively polymerized in slightly basic medium
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Chem Soc Rev
in the presence of the nanoparticles, in this way polydopamine (a polymer with a complex structure) is added to the surface of the loaded nanoparticles (structure 2, Fig. 27). Galactosamine is finally attached to dopamine through Michael addition and/or Schiff base formation (structure 3, Fig. 27). The galactose group is recognized by the hepatic carcinoma cells through specific receptors located on the surface of the cell. The nanoparticles are then characterized: the final size is 209 % 5 nm, they are negatively charged due to the phenolic groups of polydopamine, the IR and XPS spectra are in agreement with the structure. DTX is released rapidly in a pH 7.4 buffer during the first two days (30%) and up to 54% after 14 days. In vivo experiments indicated that the nanoparticles are targeted to the liver cancer cells and injecting these surface engineered DTX loaded nanoparticles reduced the tumor size most significantly in hepatoma-bearing nude mice. Surface modified polymeric nanoparticles can be used for analysis inside a cell. Hydrogen peroxide is a by-product of aerobic oxidation and overproduction can trigger oxidative stress and produce biological disorders like those observed in neurodegenerative diseases, therefore new diagnostic methods are needed to detect and quantify endogenous H2O2 production. Surface modified fluorescent polyacrylonitrile nanoparticles (PAN) that can specifically detect H2O2 have been prepared. The surface modification involves the reaction of the surface cyano groups with glutaraldehyde (step 1, Fig. 28) and further reaction with 3-aminopyridine-5-boronic acid pinacol ester to give BPAN nanoparticles (step 2, Fig. 28). Due to the conjugation
Fig. 27
Surface modified nanoparticles for targeting liver cancer cells.
This journal is © The Royal Society of Chemistry 2017
Tutorial Review
Fig. 28 Surface modified PAN nanoparticles for the detection of hydrogen peroxide. (1) Glutaraldehyde, (2) 2-aminopyridine 5-boronic acid pinacolate ester, and (3) hydrogen peroxide.
between the Schiff base and pyridine boronic acid pinacol ester group, a photoinduced electron transfer takes place and the BPAN nanoparticles are six times more fluorescent than their PAN counterparts. Upon reaction with H2O2, BPAN nanoparticles are converted to HPAN nanoparticles (step 3, Fig. 28) and a new and less intense fluorescence spectrum is observed. This change in fluorescence is specific to H2O2 and does not occur with other reactive oxygen species (ROS) such as hypochlorite, tert-butyl hydroperoxide, hydroxyl radicals, and tertbutoxy radicals. The BPAN nanoparticles are sensitive enough to image and quantitate H2O2 produced in macrophages.54 In molecular imprinted polymers (MIP), a molecule (template) is imbedded in the polymer network during the polymerization, and then extracted leaving a vacant space. These vacant spaces can then trap specifically the same template molecule in a solution to be analyzed. With proteins as template molecules, the problem is more complicated: proteins are difficult to extract or trap through the polymeric network because their diffusion is limited by their large size. A solution to this problem has been proposed that makes the use of polymeric nanoparticles modified with a thin polymeric shell in which a protein (bovine hemoglobin (BHb)) is imbedded. Due to the small thickness of the shell, the protein can be extracted leaving a vacant space that can recognize the protein. Polymeric nanoparticles are prepared with a moderately hydrophilic surface (that does not denature or modify the conformation of the proteins) by copolymerization of styrene, vinylbenzyl chloride and acrylamide. The surface of these 300 nm nanoparticles is then modified by the reaction of the benzyl chloride groups present on the surface with an iniferter living polymerization initiator (step 1, Fig. 29) (G = 2.5 groups nm!2). BHb is then adsorbed on to the nanoparticles (step 2, Fig. 29); a copolymer of acrylamide, acrylic acid and a reticulating agent are grafted from the iniferter initiators located on the surface, under light irradiation, to give the final construct (step 3, Fig. 29). The polymeric shell of controlled thickness (5 nm) surrounds the adsorbed BHb proteins (size of BHb: 6.4 # 5.5 # 50 nm) that can be extracted
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5711
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Tutorial Review
Fig. 29 Molecular imprinted polymer (MIP) constructed on the modified surface of a polymeric nanoparticle for the detection of proteins.
(step 4, Fig. 29). These structures exhibit high efficiency in recognition and selective capture of BHb.55
Conclusion Surface modification of polymers is dominated by plasma methods that have reached the industrial stage. However, only simple organic groups such as OH, COOH, and NH or complex polymers without repeating units can be obtained from inorganic gases and organic molecules, respectively. Therefore, attachment of more complex molecules requires post-modification. The same occurs in the oxidation, reduction or hydrolysis of polymers. Somewhat higher molecular weight groups can be attached to the surface by electro- and photochemistry. ‘‘Grafting on’’ and ‘‘grafting from’’ methods can be used to bind a surface polymer to another polymer. Although most papers dealing with the surface modification of polymers only describe the use of a single method and even of a single procedure within a method, the different available methods should be compared when a particular application is sought. For the optimal design of devices such as sensors, ‘‘smart’’ surfaces, microfluidic systems, self-cleaning and biocompatible surfaces, one should compare the different methods available that create different chemical groups, surface concentrations, surface roughnesses, thicknesses of the film. . . It is hoped that this review will be of some help in the choice of the method.
Acknowledgements ´ Paris This work was funded by the C. N. R. S. and the Universite Diderot. D. H. gratefully acknowledges a PhD fellowship from the French Embassy in Kosovo. John Lomas (DR CNRS) is acknowledged for revising the English language.
References 1 F. Khelifa, S. Ershov, Y. Habibi, R. Snyders and P. Dubois, Chem. Rev., 2016, 116, 3975–4005, and references therein.
5712 | Chem. Soc. Rev., 2017, 46, 5701--5713
Chem Soc Rev
2 S. Yoshida, K. Hagiwara, T. Hasebean and A. Hotta, Surf. Coat. Technol., 2013, 233, 99–107, and references therein. 3 R. Wolf, Plastic Surface Modification. Surface Treatment and Adhesion, Carl Hanser Verlag, Munich, 2010, and references therein. 4 M. A. Lieberman and A. J. Lichtenberg, Principles of Plasma Discharge and Materials Processing, Wiley, New York, 2005. 5 A. Eifert, J. Petit, T. Baier, E. Bonaccurso and S. Hardt, Appl. Surf. Sci., 2015, 330, 104–110. 6 J. H. Lee, J. W. Park and H. B. Lee, Biomaterials, 1991, 12, 443–448. 7 K. R. Kull, M. L. Steen and E. R. Fisher, J. Membr. Sci., 2005, 246, 203–215. 8 A. Pegalajar-Jurado, J. M. Joslin, M. J. Hawker, M. M. Reynolds and E. R. Fisher, ACS Appl. Mater. Interfaces, 2014, 6, 12307–12320. 9 V. G. Correia, A. M. Ferraria, M. G. Pinho and A. AguiarRicardo, Biomacromolecules, 2015, 16, 3904–3915. 10 K. Ellinas, S. P. Pujari, D. A. Dragatogiannis, C. A. Charitidis, A. Tserepi, H. Zuilhof and E. Gogolides, ACS Appl. Mater. Interfaces, 2014, 6, 6510–6524. 11 A. Zille, F. R. Oliveira and A. P. Souto, Plasma Processes Polym., 2015, 12, 98–131. 12 D. Thiry, S. Konstantinidis, J. Cornil and R. Snyders, Thin Solid Films, 2016, 606, 19–44. 13 J. Ryssy, E. Prioste-Amaral, D. F. N. Assuncao, N. Rogers, G. T. S. Kirby, L. E. Smith and A. Michelmore, Phys. Chem. Chem. Phys., 2016, 18, 4496–4504. 14 K. N. Pandiyaraj, V. Selvarajan, Y. H. Rhee, H. W. Kim and S. Ismat, Mater. Sci. Eng., C, 2009, 29, 796–805. 15 Y. W. Chen-Yang, C. W. Chen, S. C. Tseng, J. Huang and Y. Z. Wu, Surf. Coat. Technol., 2004, 176, 148–156. 16 A. Airoudj, F. Bally-Le Gall and V. Roucoules, J. Phys. Chem. C, 2016, 120, 29162–29172. 17 K. Young-Jong, T. Yoshinao, M. Kuniaki, T. Yoshihiro and S. Hiroyuki, Appl. Surf. Sci., 2009, 255, 3648–3654. 18 I. U. Ahad, A. Bartnik, H. Fiedorowicz, J. Kostecki, B. Korczyc, T. Ciach and D. Brabazon, J. Biomed. Mater. Res., Part A, 2014, 102A, 3298–3310. 19 M. Jiang, J. G. Liu, S. H. Wang, M. Lv and X. Y. Zeng, Surf. Coat. Technol., 2014, 254, 423–428. 20 A. Hozumi, T. Masuda, H. Sugimura and T. Kameyama, Langmuir, 2003, 19, 7573–7579. 21 V. Muriel-Galet, J. N. Talbert, P. Hernandez-Munoz, R. Gavara and J. M. Goddard, J. Agric. Food Chem., 2013, 61, 6720–6727. 22 J. Spanring, C. Buchgraber, M. F. Ebel, R. Svagera and W. Kern, Polymer, 2006, 47, 156–165. 23 A. Yousaf, A. Farrukh, Z. Oluz, E. Tuncel, H. Duran, ˘an, T. Tekinay, H. ur Rehman and B. Yameen, S. Y. Dog React. Funct. Polym., 2014, 83, 70–75. 24 T. Goda, T. Konno, M. Takai and K. Ishihara, Colloids Surf., B, 2007, 54, 67–73. 25 H. D. Prasad and K. Burkhard, Chem. Commun., 2014, 50, 6688–6699. 26 M. Bouriga, M. M. Chehimi, C. Combellas, P. Decorse, F. Kanoufi, A. Deronzier and J. Pinson, Chem. Mater., 2013, 25, 90–97.
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 02 August 2017. Downloaded by INIST - CNRS on 03/12/2017 16:57:56.
Chem Soc Rev
27 J. K. Shim, H. S. Na, Y. M. Lee, H. Huh and Y. C. Nho, J. Membr. Sci., 2001, 190, 215–226. 28 G. N. Parsons, S. E. Atanasov, E. C. Dandley, C. K. Devine, B. Gong, J. S. Jur, K. Lee, C. J. Oldham, Q. Peng, J. C. Spagnola and P. S. Williams, Chem. Rev., 2013, 257, 3323–3331, and references therein. 29 X. Xingfang, C. Genyang, C. Fengxiang, T. Yunrong, L. Xin and X. Weilin, Appl. Surf. Sci., 2015, 349, 876–879. 30 J.-H. Min, A. Bagal, J. Z. Mundy, C. J. Oldham, B.-I. Wu, G. N. Parsons and C.-H. Chang, Nanoscale, 2016, 8, 4984–4990. 31 X. Liang, A. D. Lynn, D. M. King, S. J. Bryant and A. W. Weimer, ACS Appl. Mater. Interfaces, 2009, 1, 1988–1995. 32 C. Combellas, A. Fuchs, F. Kanoufi, D. Mazouzi and S. Nunige, Polymer, 2004, 45, 4669–4675, and references therein. 33 J. A. Barish and J. M. Goddard, J. Appl. Polym. Sci., 2011, 120, 2863–2871. 34 X. Han, Y. Zhou, J. Hu and H. Liu, J. Polym. Sci., Part B: Polym. Phys., 2010, 48, 2262–2273. 35 K. Fujimoto, Y. Takebayashi, H. Inoue and Y. Ikada, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 1035–1043. 36 J. A. Lee and T. J. McCarthy, Macromol., 2007, 40, 3965–3969. 37 J. A. Barish and J. M. Goddard, J. Food Sci., 2011, 76, E586–E591. ´mez, Macromol., 38 A. M. Dı´ez-Pascual, G. Martı´nez and M. A. Go 2009, 42, 6885–6892. `re, C. Pringalle, 39 C. Henneuse-Boxus, T. Boxus, E. Dulie L. Tesolina, Y. Adriaensen and J. Marchand-Brynaert, Polymer, 1998, 3, 5359–5369. 40 W. Chen and T. J. McCarthy, Macromol., 1998, 31, 3648–3655.
This journal is © The Royal Society of Chemistry 2017
Tutorial Review
41 S. Roux and S. Demoustier-Champagne, J. Polym. Sci., Part A: Polym. Chem., 2003, 41, 1347–1359. 42 Z. Wang, C. W. Macosko and F. S. Bates, ACS Appl. Mater. Interfaces, 2014, 6, 11640–11648. 43 T. I. Croll, A. J. O’Connor, G. W. Stevens and J. J. CooperWhite, Biomacromolecules, 2004, 5, 463–473. 44 A. Y. Fadeev and T. J. McCarthy, Langmuir, 1998, 14, 5586–5593. 45 B. Hu, J. A. Siddiqui and R. M. Ottenbrite, Chem. Phys., 2002, 203, 1631–1635. 46 M. M. Chehimi, A. Lamouri, M. Picot and J. Pinson, J. Mater. Chem. C, 2014, 2, 356–363, and quoting references. ´dard, F. Kanoufi, C. Combellas, J. Pinson 47 D. Hetemi, J. Me and F. I. Podvorica, Langmuir, 2016, 32, 512–518. 48 P. Yangab and M. G. Moloney, RSC Adv., 2016, 6, 111276–111297. 49 K. Fukazawa and K. Ishihara, ACS Appl. Mater. Interfaces, 2013, 5, 6832–6836. 50 F. Guo, K. Jankova, L. Schulte, M. E. Vigild and S. Ndoni, Langmuir, 2010, 26, 2008–2013. 51 A. Kumari, S. K. Yadav and S. C. Yadav, Colloids Surf., B, 2010, 75, 1–18. 52 M. Elsabahy and K. L. Wooley, Chem. Soc. Rev., 2012, 41, 2545–2561. 53 D. Zhu, W. Tao, H. Zhang, G. Liu, T. Wang, L. Zhang, X. Zeng and L. Mei, Acta Biomater., 2016, 30, 144–154. 54 W.-K. Oh, Y. S. Jeong, S. Kim and J. Jang, ACS Nano, 2012, 6, 8516–8524. 55 H. Ugajin, N. Oka, T. Okamoto and H. Kawaguchi, Colloid Polym. Sci., 2013, 291, 109–115.
Chem. Soc. Rev., 2017, 46, 5701--5713 | 5713
