Vincent Ball &Doriane Del Frari &Marc Michel &Markus J.Buehler &Valérie Toniazzo &
Manoj K.Singh &Jose Gracio &David Ruch
Published online:21December 2011
#Springer Science+Business Media,LLC 2011
Abstract Polydopamine films have been introduced by Messersmith et al.as a possible “versatile ”surface function-alization method allowing to coat the surface of almost all known materials even superhydrophobic surfaces.These new kinds of coatings also confer a plethora of functional-ities to the coated materials owing to the complex chemistry of the catechol quinone moieties present on the surface of polydopamine.These coatings may hence become an inter-esting alternative to established surface coatings like self-assembled monolayers and polyelectrolyte multilayered films.In this review,we describe the knowledge acquired in the last 3years about the deposition mechanisms of polydopamine films,their properties,and various applications in surface science at the nanoscale.
Keywords Surface functionalization .Polydopamine .Melanin .Reaction mechanisms .Biocompatible films
1Introduction
In modern technology,there is an urgent need to improve the coating of the surfaces of materials to achieve better film homogeneity and stability and enhanced functionality and versatility of the deposition method.Material coatings are aimed not only to protect the underlying materials from external agents such as strong oxidants but also to confer them with new functionalities.Indeed,the first interaction a material establishes with its environment is through its sur-face,and a stable and tight monolayer of molecules is sufficient to completely change the surface chemistry of a material.This is exemplified by the value of its surface tension [1].Biomaterials are a marvelous example of this rule;when put in a living organism,their fate depends on their biocompatibility (no release of toxic ions or molecules)and the nature of the first molecules that interact with their surface.These “first adsorbing ”molecules are most often proteins [2].In case where these proteins present strong interactions with the cells from the host organism,one can expect a safe recolonization of living tissue around the implanted material.On the other hand,when some bacteria remain adherent on the surface of the implanted biomaterial,an inflammatory cascade may lead to the rejection of the implant with a lot of complications for the patient [3].
More generally,self-assembled structures from living organisms offer wonderful examples of optimized materials with respect to adhesion [4]or mechanical properties [5,6]and for the design of functionalized surfaces.Among the most fascinating biomaterials owing to their hierarchical structure,one can distinguish nacre [7],biosilica [8],silk [9–11],and melanin.Melanin is the natural photoprotectant of the human skin and plays a major role in many pathological processes.Natural melanin,either the black eumelanin or the
V .Ball (*):D.Del Frari :M.Michel :V .Toniazzo :D.Ruch Department for Advanced Materials and Structures,Centre de Recherche Public Henri Tudor,66rue de Luxembourg,
4002Esch-sur-Alzette,Luxembourg e-mail:vincent.ball@tudor.lu
M.J.Buehler
Laboratory for Atomistic and Molecular Mechanics (LAMM),Department of Civil and Environmental Engineering,Massachusetts Institute of Technology,Cambridge,MA 02139,USA
M.K.Singh :J.Gracio
Center for Mechanical Technology and Automation,University of Aveiro,
Campus Universitário de Santiago,3810-193Aveiro,Portugal
BioNanoSci.(2012)2:16–34DOI 10.1007/s12668-011-0032-3
The most fascinating features of melanin lie in its elec-tronic and optical properties.It was assumed to behave like an amorphous semi-conductor[16],and its direct current conductivity values are reported to fluctuate between10−5 and10−13S cm−1.There is increasingly strong evidence that such a large range of conductivity values,which are tem-perature dependent,are related to hydration effects.To clarify this point,the adsorption isotherm of water on mel-anin has only been investigated very recently.Water adsorbs on melanin grains owing to the hydrophilicity of the chem-ical groups present at its surface[17].From the point of view of the optical properties,the most salient features of melanins are the broadband and monotonous absorption spectrum(with very small contribution from light scattering [18,19]),the very weak emission quantum yield[20],and the violation of the mirror rule between the absorption and emission spectra[14].
The aim of the present article is to review the most recent advances in melanin and melanin-inspired materials used for applications as biologically inspired surface coatings.The interest in using catechol amine-derived materials in surface chemistry comes from the important fraction(up to40mol%) of3,4dihydroxyphenyl-L-analine in the amino acid sequence of the Mytilus edulis foot protein(MEFP)5which allows for the strong adhesion of mussels to solid surfaces in the pres-ence of high shear stresses[21].
2The Need for a Universal Surface Functionalization Method
Since ancient times,the coating of the material surfaces has been a major concern not only for esthetic reasons but also to improve the stability and the longevity of materials. Protection against corrosion is emblematic for the need to coat metallic surfaces with films impermeable to corrosive agents.Painting has been and still is a major coating tech-nology and can afford robust coatings provided good adhe-sion between the material and the painting is attained. However,the need for thin films of controllable thickness and adjustable functionality appeared during the twentieth century.Among these technologies allowing to reach such requirements,one can cite electropolymerization[22],the deposition of Langmuir–Blodgett films[23,24],self-assembled monolayers[25],the deposition of films in a layer-by-layer fashion[26–28],and plasma deposition[29].
All these methods have specific advantages and draw-backs but share a certain surface specificity.For instance, self-assembled monolayers can only be deposited on the surface of noble metals(Pt,Ag,Au)using molecules carrying thiol groups or on the surface of oxides using alkylsilanes. Layer-by-layer deposition of polyelectrolyte multilayer (PEM)films requires the use of charged surfaces:The depo-sition of PEMs on poly(tetrafluoroethylene)is possible only after priming with a plasma deposited allylamine[30],where-as chemical(physical or vapor)deposition techniques work under vacuum.
From this perspective,there is a clear need to develop universal coating technologies allowing to functionalize the surface of almost,if not all,classes of materials at the material’s surface solvent interface.The types of glues used by living organisms can be a marvelous source of inspira-tion to develop such universal coatings.Mussels are a par-ticularly well-suited example:They adhere strongly to wood or stones in conditions of high shear stresses,the rate of water flow being as high as10ms−1.The adhesion of these organisms to solid surfaces is possible through specific proteins called M.edulis foot proteins at the end of the mussel’s byssus[31].A close look at the amino acid se-quence of such proteins shows that they are enriched,up to 40%in molar fraction,in3,4-dihydroxy-L-phenylanaline,a hydroxylated version of the natural amino acid tyrosine [32].Polymers carrying catechol groups allow for impres-sive bond strength with surfaces and allow to mimic the bond strength afforded by MEFP5[33].
These findings have allowed the development of new surface functionalization methods based on molecules car-rying catechol groups.These methods have been recently reviewed elsewhere[34].
In this review,we will mainly focus on films made through oxidation processes in the presence of catechol amine solutions(as dopamine or norepinephrine,whose structure is depicted in Scheme1).The feasibility and the universality of such coatings with respect to the nature of the substrate to be coated has been demonstrated by Lee et al. [35,36].
O
H
O
H
NH
2
a
H
H
2
b
Scheme1Chemical formula of dopamine(a)and norepinephrine(b), the two most common catechol amines used to produce polydopamine films on solid surfaces
As we will see in the next paragraph,polydopamine coat-ings produced through an oxidation process allow us to control the deposition rate of the films.There are,however,other methods that allow one to produce coatings with similar prop-erties using natural or synthetic melanin particles.Among such methods,one can cite spray coating [37],spin coating [38],solvent casting [39],or electrochemical deposition from basic melanin suspensions [40].In general,however,solid-state melanin is a hard to process material owing to its insolubility in most organic solvents.Fortunately,it is soluble in strongly alkaline aqueous solutions.This makes melanin a material hard to characterize and to understand,which is mandatory owing to its fascinating properties [13,14].The possibility to produce melanin-like coatings on many different substrates is not only a formidable opportunity in surface science but also an ideal platform to characterize the properties of the material since many analytical techniques like X-ray photoelectron spectros-copy,atomic force microscopy,etc.are well suited for the analysis of planar surfaces.
3Mechanism of Polydopamine Film Formation Eumelanins are fascinating materials owing to their bio-optoelectronic properties but also challenging owing to their structural complexity [41].A recent and complete review of the physicochemical properties of eumelanin includes their optical,electrical,and mechanical properties [42].The basic knowledge and the known properties of melanin,in solution,are summarized in Fig.1.
The most important aspects with respect to polydopamine films are their optical and electrical properties.Of even more
fundamental importance is the question of polydopamine ’s structure:Is it a polymer or a hierarchical supramolecular and amorphous aggregate?In case where polydopamine is a poly-mer (the name spontaneously implies a polymeric nature,but to our knowledge no experimental proof is available at pres-ent),this would perfectly justify the employed denomination for the films produced during oxidation of dopamine solutions in the presence of the substrate to be coated.There is,however,strong evidence that polydopamine is a eumelanin-like mate-rial for which the most probable model is that of a hierarchical aggregate of oligomers (the so-called stacked protomolecule model)made from 5,6-dihydroxyindole (Fig.1).This model relies mostly on investigations performed using scanning tun-neling microscopy [43].Recent calculations performed at the density functional theory level suggest that the most stable aggregate of 5,6-dihydroxyindole is a tetramer having the planar geometry of a phtalocyanine (Fig.2)[44].
The “stacked protomolecule model ”allows us also to explain the X-ray diffraction studies performed by Thathachari and Blois [45]as well as the atomic force microscopy data of Clancy and Simon [46]and the matrix-assisted laser desorp-tion mass spectra data of Pezzella et al.[47].Unfortunately,in the case of polydopamine films produced according to the method proposed by Lee et al.[35],there is not yet a definitive proof that the obtained coatings display the characteristic fea-tures of eumelanin.In the case where the used catecholamine was norepinephrine,the authors called the obtained coating “polynorepinephrine ”[36].Since there is no proof that these coatings are different from those made from dopamine,we call all the coatings obtained from catecholamines in the presence of oxidants,polydopamine coatings in a systematic manner.There is now accumulating evidence that the polydopamine
Oligomers of 5,6-dihydroxyindole
First level aggregation
Photoprotectant (human skin)
(Parkinson’s disease, skin cancer)
Structural material (peacock feathers,
butterfly wings, jaws of Glycera )
Fig.1Multistep self-assembly of oligomers made from 5,6-dihydroxyindole to yield a hierarchical and multifunctional material,melanin.The scale in the TEM micrograph corre-sponds to 100nm;personal data of V .Ball
coatings share a lot of common features with eumelanin,namely:&Their composition,based on X-ray photoelectron spec-troscopy,is similar to that of eumelanin [48].
&The solution from which these coatings are made con-taining grains whose solid-state NMR spectrum is very similar to that of eumelanin [48].
&
The surface morphology (Fig.3)of the obtained coat-ings is made from platelets a few hundred nanometers in diameter,very similar to that found for the eumelanin grains found in natural organisms like in the Glycera jaws [49].
&
The UV –vis [48](Fig.4)and infrared spectra of the coatings are similar but not identical to those of synthetic eumelanin [50].
The influence of the used oxidant has also been investi-gated:It appears that oxidants other than oxygen are effi-cient in the deposition of polydopamine films.It has been shown that dopamine solutions containing ammonium per-sulfate,sodium periodate,and sodium perchlorate allow for the deposition of films having the composition of polydop-amine [51].Of the highest interest is that the deposition of polydopamine occurs at a pH of about 4when ammonium persulfate is the oxidant.The same finding holds true when copper (II)sulfate is the oxidant [52].These findings show that some fundamental investigations about the oxidation mechanism of dopamine into dopamine quinone,which is known to be a pH-dependent equilibrium,are necessary (Scheme 2).
This equilibrium is shifted to the reactant when the pH decreases,and hence,the deposition of polydopamine should not occur in acidic conditions.This is indeed the case when the dopamine solutions are aerated:The deposition rate of the film vanishes to zero when the pH is below 5.5(V .Ball et al.,to be submitted for publication).However,in the presence of persulfate anions or Cu 2+cations,the deposition occurs even at pH 4.More interesting is that the deposition of the coating occurs up to thicknesses higher than the critical value of 45nm obtained when O 2is the oxidant (Fig.5a )[52].
The films produced through oxidation in the presence of Cu 2+contain 2%(in molar fraction)of Cu 2+cations and
not
Fig.2Structural model of the most stable tetramer of 5,6-dihydrox-yindole.Reproduced with permission from [44
]
Fig.3Surface topography of polydopamine films obtained by expos-ing silicon substrates to a dopamine solution (2mg mL −1in 50mM Tris buffer at pH 8.5)during 24h.The surface topography has been obtained in the dry state by atomic force microscopy operated in the contact mode.The polydopamine film has been needle-scratched to allow for film thickness measurement.The right part of the image corresponds to the naked silicon substrate.The right panel represents the average surface profile in the region corresponding to the hatched rectangle of the left image .Adapted from [80]with permission from Elsevier
metallic copper as expected (indeed copper is obtained through reduction of Cu 2+).The incorporation of Cu 2+in the film may well be related to the well-known affinity of melanin to metallic cations [53–55].The very peculiar growth rate of the film ’s thickness may well be related to the presence of Cu 2+cations in solution which could play the role of a glue allowing the agglomeration of small oligomers of polydopamine.
The presence of copper II cations in the films also man-ifests in spectral changes of the film:Whereas the films produced from oxygenated solutions display the monoto-nous decrease in absorbance,as expected for melanin in the
framework of the “chemical disorder model ”[56],those produced from deoxygenated solutions in the presence of Cu 2+display some marked peaks at 370and at 315nm (Fig.4b ).This may well be related to the influence of Cu 2+on the self-assembly of the thin polydopamine films.Indeed,it has been shown by means of small angle X-ray and neutron scattering that Cu 2+cations affect the aggregation pathway of synthetic melanin (obtained from tyrosine)in solution [57].These points remain to be explored in more details to under-stand the film deposition kinetics in the presence of Cu 2+as an oxidant.Anyway,the fact that polydopamine films can be deposited onto substrates even in acidic conditions consider-ably broadens the potentiality of the obtained films particular-ly when the substrate to be coated is pH sensitive.
It has also been found that at a given pH value of 8.5,the nature of the used buffer has a marked influence on the polydopamine film deposition process.In the presence of Tris buffer (50mM,pH 08.5),the polydopamine film rea-ches a final and constant thickness of about 45nm whereas in the presence of phosphate buffer,the film continues to growth up to 100nm without measurable incorporation of phosphates in the polydopamine film [52].
Complementary to the solution oxidation methods de-scribed up to know,it is also possible to produce polydop-amine like coatings through electropolymerization in a pH and in an electrolyte-dependent manner [58].Electropoly-merization performed in deoxygenated solutions offers the advantage to allow the restriction of the reaction on the surface of the electrode to be coated,but it has the drawback to be limited on the surface of electrically conducting mate-rials.Another advantage of the electropolymerization is that the maximal film thickness can be reached in about 100cycles of cyclic voltammetry (scan rate of 10mV s −1
wavelength (nm)
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a b s o r b a n c e
0.02
0.040.060.080.100.12
0.14Fig.4UV –vis spectra (two independent experiments)of polydop-amine films produced by exposure of a quartz slide to eight freshly prepared dopamine solutions (2mg mL −1in the presence of 50mM Tris buffer at pH 8.5).Each exposure lasted over 5min without rinse with buffer solutions between two deposition steps.From [48]with authorization from the American Chemical
Society
Scheme 2Reaction pathway leading to the production of 5,6-indolequinone from dopamine
between −0.5and +0.4V versus Ag/AgCl)and in a reaction time of about 3.5h whereas the same thickness of 45nm by solution oxidation is only reached in about 15h [35].
In the case where the polydopamine film is deposited by electropolymerization,the film thickness reaches also a final constant value after which the dopamine in solution cannot further be oxidized.We will explain the origin of this film growth cessation in Section 4.Interestingly this limiting thickness is of 45nm,the same maximal value reached when the substrate to be coated is put in the presence of an oxygenated dopamine solution at pH 8.5.In this latter case,the maximal film thickness is reached even if there remains a lot of polydopamine grains in solution.This implies that the polydopamine grains do not adhere to the deposited film probably due to electrostatic repulsions.We wondered if the maximal thickness of 45nm is an absolute limitation of melanin films.Indeed,if polydopamine is a universal,substrate-independent coating methodology,one should be able to deposit polydopamine on a substrate already coated with the same film.Indeed,this works (Fig.6)provided that the dopamine solution is regularly refreshed [52].The film can then grow with maximal thick-ness increments of about 45nm in each deposition step.These experiments show the importance of dopamine and/or its small oligomers in the deposition process.
A very interesting finding was that dopamine is able to reduce graphene oxide and to stabilize the obtained gra-phene in solution.This is possible because polydopamine coats the graphene sheets and hence modifies its interactions
with water [59].Up to now the mechanism by which poly-dopamine grows from the surface is not well-known.It might well be that the reaction mechanism is similar to that of the deposition of polyaniline,implying the adsorption of radicalar species from which the growth process occurs.Whatever the details of the deposition mechanism,it is clear
b
a
/ nm
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Fig.5a Thickness evolution with time of “polydopamine ”films produced from dopamine solutions (2mg mL −1)in the presence of oxygenated Tris buffer (50mM at pH 8.5)(white circle )and in the presence of the same buffer but deoxygenated and in the presence of 30mM copper sulfate (blue circle ).b UV –vis spectra of
“polydopamine ”films deposited on a quartz slide from oxygenated (black line )or from deoxygenated solutions containing 30mM copper sulfate (blue line ).Adapted from [52]with authorization from the American Chemical Society
total immersion time / h
t o t a l m e l a n i n t h i c k n e s s / n m
50100150200250Fig.6Thickness of polydopamine coatings on silicon substrates as measured by ellipsometry in the dry state after a single injection of dopamine (2mg mL −1in the presence of 50mM Tris buffer at 50mM)(red square )and after multiple injections of dopamine in the same conditions (blue circle ,black triangle ,green inverted triangle ).Each new injection of a freshly dissolved dopamine solution corresponds to a vertical arrow .Figure modified from [52]with authorization from the American Chemical Society
that small molecular species like dopamine and its oligomers play an important role in the deposition process: When a dopamine solution is oxygenated during4h,it is already black but no film deposition is observed when a silicon slide is put in this solution[48].
It should also be pointed out that the molecular details of the self-assembly of melanin in solution are not well-known yet.The basic building block,5,6-indolequinone (Scheme2),undergoes dimerization,but there are several isomers or the dimers,three of which being stable enough to contribute to the formation of trimers[60].This highlights the structural complexity of the obtained melanin.The re-activity of dopamine[61]and of5,6-dihydroxyindole in solution is a very active research field[62–65]and correla-tions should be made between this fundamental research in organic chemistry and the surface science of polydopamine deposition.Many theoretical investigations have been per-formed on the structure of melanin in solution[66],but to our knowledge,not much research has been done toward understanding polydopamine deposition on surfaces.In or-der to correlate polydopamine formation in solution and on surface,an important point is to know if the basic building blocks are identical in both phases.Owing to the composi-tion and UV–vis spectra(Fig.4)of the polydopamine films produced in the presence of dioxygen,this is a reasonable assumption.
Quite surprisingly,polydopamine films have not yet been produced from solvents other than water.Based on the results of Lorite et al.[67],it is not unexpected that films produced from dopamine solutions in organic solvents could have other morphologies and structures than those produced in water.Indeed,Lorite et al.evidenced the importance of the presence of water on the film morphology after solvent casting of synthetic melanin.Melanin has been synthesized either in water,in dimethyl sulfoxide(DMSO),or in N,N dimethylformamide(DMF).The root mean squared rough-ness of the films scaled as a power law of the image size L with a scaling powerα.αwas found to be much larger (about0.6)when melanin was synthesized in water than for melanin prepared in DMSO or DMF(α∼0.4–0.45)[67].
As a last point,one may wonder if the interaction strength of polydopamine coatings with the substrate is independent or not from the surface chemistry of the latter. It is well established that the final film thickness obtained in a one-step deposition process(i.e.,no fresh dopamine sol-utions are put in contact with the substrate,see Fig.5)is almost independent from the substrate[35].But it would be highly surprising if the adhesion strength of the polydop-amine films would not depend on the surface chemistry of the substrate.Indeed,there are recent qualitative experi-ments relating some spontaneous delamination of the poly-dopamine films from silicon substrates(Del Frari et al., submitted for publication).4Properties of Polydopamine Films
Owing to their composition and heterogeneous structure, polydopamine films should offer a plethora of possible applications in engineering the surface of materials or to modify the surface chemistry of colloids and nanoparticles. In addition,the presence of quinone or catechol groups offers an interesting platform for secondary surface func-tionalization as was already illustrated in the first papers dealing with the deposition of polydopamine from dopa-mine solutions[35].In the last3years,the promises of polydopamine coatings have been widely illustrated,as will be shown in the paragraphs below.
4.1Stability of Polydopamine Films
Polydopamine films deposited on silica were stable in the presence of a physiological buffer for at least4days as evaluated by means of ellipsometry measurements.The stability was also excellent at pH1,and only14%of film erosion was found within54h.But the films spontaneously delaminated within15min of exposure to sodium hydroxide solutions at pH13,most probably due to the pH induced dissolution of the silicon oxide layer covering the silicon slide,but also in relation to the increased solubility of “melanin-like materials”in basic media[48].
4.2Surface Energy of Polydopamine Films
The most evident consequence of the deposition of a film having a given constant composition(N/C∼0.125,the same ratio as for dopamine[35,48])is the change in the surface energy of the coated material.Indeed,static contact angle measurements on polydopamine films with water as the solvent mostly point to contact angle values of about50–55°when polydopamine is deposited on hydrophilic sub-strates[35,48,68].On copper coated with polydopamine, the static contact angle of water also decreased to50°[69]. On gold,platinum,and indium tin oxide,the static contact angle of polydopamine coatings for water reaches also a constant value of46–51°after3h of contact with a dopa-mine solution[70].However,on polyethylene,the contact angle decreases from127±6°to only92.8±1.2°after func-tionalization with polydopamine and on polytetrafluoro-ethylene the contact angle decreases from124±3.4°to 80.6±5.0°[71].
When the polydopamine coatings are produced from norepinephrine(Scheme1),the static contact angle for water is close to62°on the surface of almost all the coated materials.This may well originate from the influence of the precursor molecule on the composition of the obtained film [36].Indeed,norepinephrine contains one hydroxyl group more than dopamine(Scheme1).Polydopamine films canalso be deposited on superhydrophobic surfaces,anodic alu-minum oxide covered with fluorosilanes,allowing for a re-duction in the static water contact angle from158.5±2.8°to 37.3±2.6°[72].
The surface energy of polydopamine using the contact angles of droplets made from different solvents(water, diiodomethane,and formamide)has been evaluated[73]. The polar component and apolar components of the surface energy amount to42.6±1.0and18.1±0.9mJ m−2for poly (L-lactic acid)(PLA)treated with a dopamine solution dur-ing24h(2mg mL−1dopamine in10mM Tris buffer at pH8.5),showing that polydopamine coatings are essentially polar and hydrophilic in nature[73].
4.3Optical Properties of Polydopamine Films
The melanin films produced through spin coating of synthetic melanin from ammonium hydroxide solutions had an extinc-tion coefficient of3×106m−1at5nm,taking the scattering losses into account through an integrating sphere.The films prepared by oxidation of dopamine using O2as the oxidant had an extinction coefficient of(2.6±0.4)×106m−1[74]at 5nm in close agreement with the values obtained by the group of Bothma et al.[38].The extinction coefficient of the films could be reduced by a factor of two when melanin particles(also produced by oxygen mediated oxidation of a 2-mg mL−1buffered dopamine solution)were deposited on the quartz substrate in a layer-by-layer approach using poly (diallyldimethyl ammonium chloride)as the polycation[74].
Photopyroelectric spectroscopy was performed on elec-tropolymerized melanin films.These experiments allowed to calculate the optical absorption of melanin in the solid state and to estimate an optical gap at1.70eV[75].
4.4Electrochemical Properties of Polydopamine Films The impedance spectra of polydopamine films produced by successive immersions in dopamine solutions(in a closed vessel)have been measured as a function of the number of the immersion steps of the substrate in the freshly prepared dopamine solutions(2mg mL−1in the presence of50mM Tris buffer at pH08.5)[76].From the resistance to electron transfer and taking the film thickness into account,it was found that the conductivity in the direction perpendicular to the melanin films lies between0.25and1.3×10−10S cm−1. This range of values is below those determined for synthetic 3,4-dihydroxy-L-phenylalanine(DOPA)–melanin in the presence of100%relative humidity,namely10−5S cm−1 [38,77].However,the measurement method used by Jastr-zebska et al.[77]was totally different from that used by Ball,namely electrochemical impedance spectroscopy[76]. In addition,the nature of the samples was totally different in these two studies:Jastrzebska et al.[77]used pellets from melanin powder whereas Ball[76]made his measurements on polydopamine films.
The permeability of polydopamine films to ions depends on the pH and hence on the surface charge of the film,but also on its porosity.Positron annihilation spectroscopy per-formed on polydopamine films allowed to show that the concentration in organic material was higher at the sub-strate/film than at the film/solution interface[78].To our knowledge,no quantitative investigation of the pore size distribution has been performed on polydopamine coatings.
The polydopamine films display a pH-dependent perm-selectivity:They are positively charged at pH below about4 [76]and hence display some permeability for anions.Above the isoelectric point,they are permeable to cations[79].
For a given electrochemical probe,the permeability of polydopamine coatings depends markedly on the preparation method.For a given film thickness,the relative oxidation current is higher for films made by electropolymerization than for those made by oxygen bubbling(Fig.7)[80].
4.5Electromagnetic Properties of Polydopamine Films Melanin films produced by spin coating of synthetic melanin from a ammonium hydroxide solution have a conductivity of 2.5×10−5S m−1at ambient temperature and in the presence of 100%relative humidity[38].The melanin films produced by spray coating display a decreasing resistance when the tem-perature increases,a typical behavior of a semi-conductor[37
].
Fig.7Relative peak currents of Fe(CN)−(1mM in presence of the Tris–NaNO3buffer at pH08.5)versus thickness of melanin deposits on the working electrode obtained by successive immersion in freshly prepared dopamine solution(circles),by continuous oxygenation of a single dopamine solution(triangles),by oxygenation with cupper sulfate(squares),or by electropolymerization(diamonds).Each point corresponds to an experiment performed on an independent electrode modified by a polydopamine film.Reproduced from[80]with autho-rization from Science DirectMelanin grains display a characteristic signal in electron spin resonance(ESR)due to the presence of semiquinone-like radicals[81,82].An almost identical signal is found for polydopamine films prepared from oxygenated dopamine solutions at pH8.5[83].It has been estimated,based on the ESR signal intensity,the film density,and its thickness, that about one over26indole quinone group is present in the form of a radical.The strong analogy between the ESR spectra of polydopamine films and those of bulk melanin is an additional strong argument to those proposed in the previous section that polydopamine films may indeed be melanin coatings.
5Applications of Thin Polydopamine Films
5.1Polydopamine as Coatings for the Inhibition
of Corrosion
The compact and homogeneous nature of polydopamine coatings as well as their permselectivity makes them natural candidates as anti-corrosion coatings.Good resistance against corrosion was observed on copper after coating with polydopamine and its subsequent reaction with1-dodecanethiol.This two step functionalization of copper made the surface hydrophobic with a static contact for water of120°.A great part of the corrosion inhibition in the presence of3.5%NaCl(w/v)solutions may hence be due to a lack of direct contact between water and copper[69].In a similar manner,silicon[84]and aluminum[85]were modified with(3-mercaptopropyl)trimethoxysilane before deposition of the polydopamine film and further modifica-tion with tetradecanoyl chloride.Significant improvement in the inhibition of corrosion was found on the surface of these materials.
More research is needed to demonstrate the intrinsic ability of polydopamine coatings to protect the substrates onto which they adhere against corrosion.A step toward this goal will be to investigate the adhesion strength,the stability in time,and the oxygen/hydronium ion permeability of melanin films on variety of different substrates.Up to now,the only studies in which different substrates were compared for the thickness and composition of polydop-amine films were reported by Lee et al.[35,36].
5.2Polydopamine for Adhesion and Lubrication Improvement
A three-layer coating comprising an intermediate polydop-amine layer and an upper layer of stearoyl chloride allowed to obtain excellent tribological behavior on silicon.The intermediate polydopamine layer was assembled on a mono-layer of3-aminopropyltriethoxysilane(APTES)to improve its adhesion to the underlying silicon substrate[86].Indeed, strong adhesion between the lubricant layer and the sub-strate is mandatory to ensure stability of the coating under wear.The stearoyl chloride interacted with polydopamine to form amide bonds and the C18chains made the film hydro-phobic in order to improve the tribological properties of the coating.The relative friction coefficient obtained from AFM measurements was decreased by a factor of about2for the APTES–polydopamine–stearoyl chloride trilayer with re-spect to the APTES-functionalized silicon.In a macroscopic ball-on-plate test,the friction coefficient of the APTES–olydopamine–stearoyl chloride trilayer was found equal to 0.2whereas it was of0.1on the APTES–stearoyl chloride bilayer.However,the anti-wear lifetime of the bilayer(i.e., without polydopamine)was only of50s whereas it was higher than3,600s for the trilayer(i.e.,in the presence of the polydopamine interlayer),showing that polydopamine stabilizes the coating[86].
Polydopamine allowed to increase the strength of com-posite fibers made from PLA platelets and vegetal fibers. The Young’s modulus of the composite made from20% PLA was increased by5%when PLA was coated with polydopamine before blending with the fibers[73].
5.3Polydopamine Films for Grafting of Biomolecules Melanin extracted from living organisms is always found in the presence of proteins[49]owing to the intrinsic ability of catechol and quinone containing materials to form covalent bonds with nucleophilic molecules.The possibility to an-chor proteins via their amino groups on the surface of polydopamine films is illustrated in Scheme3.The reaction between amino groups of proteins and the catechol groups immobilized at the surface,or at the surface of melanin grains in natural melanin,offers the advantage to be a one-step reaction.
The first example of protein binding to polydopamine films has been given by Lee et al.[87].Their findings were further confirmed by Bernsmann et al.[88]who additionally estimated the specific binding capacity of polydopamine grains produced through the oxidation of dopamine solu-tions(2mg mL−1dopamine in the presence of50mM Tris buffer and in the presence of oxygen during24h)(Fig.8).
OH
OH
R-NH
OH
OH
R-NH
Scheme3The reaction between proteins via free amino groups to the surface of polydopamine functionalized substrates.Inspired from[87]
Lee et al.showed that the polydopamine films made from norepinephrine were able to bind trypsin and that the bound enzyme was active to hydrolyze N -α-benzoyl-D ,L -arginine p -nitroanilide [36].Polyethylene glycol with terminal amino groups could be grafted on polydopamine films in a similar manner to coat the inner wall of capillaries.Such coated capillaries where non-adhesive for proteins allowing to in-crease the efficiency of their separation in capillary electro-phoresis [].In a different manner,a polydopamine coating can be used as an adhesive layer for open tubular electro-chromatography [90].
Molecules containing thiol functionalities also bind to polydopamine films.This is expected on the basis of the catechol and quinone groups available on the surface of polydopamine [35].
5.4Composites Based on Polydopamine
The reducing ability of polydopamine coatings allows for the spontaneous reduction of metallic cations,among them silver and iron,as was already demonstrated in the funding paper of Lee et al.[35].Silver-decorated polydopamine layers constitute seeds for silver platting,and the obtained silver deposit can be sintered at room temperature by simple and rapid contact with electrolyte solutions (CaCl 2,NaCl,MgSO 4)[91].The resistivity of the coatings decreases importantly upon treatment with the electrolyte solution,highlighting the efficiency of this sintering method.It has been attributed to coordination of the cations from the electrolyte by the benzoquinone ligands covering the depos-ited silver nanoparticles,inducing the desorption of the insulating benzoquinone from the surface of the nanoparticles.
The deposition of polydopamine on Fe 3O 4nanoparticles allowed for further deposition of gold nanoparticles (Fig.9)and their subsequent functionalization with self-assembled
monolayers ending with iminodiacetic –cupper complexes.These core –shell particles were used as specific ligands to fish bovine hemoglobin from complex protein mixtures [92].
Melanin films containing magnetite nanoparticles have also been deposited on Au(111)through electrodeposition of melanin grains (in the presence of 0.1M NaOH and at −1.0V versus the saturated calomel electrode).These films allowed for an electrocatalytic reduction of hydrogen per-oxide.The electrocatalytic behavior is due to the iron bound to melanin and not to Fe 3O 4,since the electrocatalytic activity was conserved in neutral or acidic solutions or in the presence of ethylene diaminetetraacetic acid which dis-solve magnetite [93].
Graphene sheets modified with polydopamine could eas-ily be decorated with silver particles through polydopamine induced reduction of silver cations [59].In an identical
initial PTEA amount (mol/g)
a m i n o
b i n d i n g
c a p a c i t y (m o l /g
)
4.0e-4
6.0e-48.0e-41.0e-31.2e-31.4e-3 1.6e-3N S
S
H N
OH OH
R +HS
OH
H N
S
+HS
H N
OH
OH
R
+2
HO S
S
OH
Fig.8Use of 2-(2pyridyl dithiol)ethylamine to estimate the binding capacity of melanin grains for small amino groups.Modified from [88]with authorization from Science Direct
Au
Fig.9TEM micrograph of a Fe 3O 4–polydopamine –Au core –shell particle prepared at Fe 3O 4–polydopamine/HAuCl 4mass ratio of 100/3.Modified from [92]with authorization from the Royal Chemical Society
Many attempts have been made to produce composite materials by associating polydopamine films with other materials to confer the huge potentiality of polydopamine to the obtained composites.The mechanical properties of composites made from vegetal fibers and PLA platelets are improved when PLA is covered with the catechol-rich film [73].
Polyelectrolyte multilayer films made from hyaluronic acid and poly-L-lysine(PLL)were put in contact with buffered dopamine solutions(2mg mL−1in50mM Tris buffer at pH8.5)and could be detached as free standing black films after oxidation of dopamine and its trans-formation into polydopamine.Fluorescence recovery af-ter photobleaching allowed to show that the mobility of the PLL chains labeled with fluorescein isothiocyanate was drastically reduced after the formation of polydop-amine.The rigidification of the film is ascribed to the formation of covalent bonds between the amino groups of PLL and the cathechol groups of polydopamine[96].
Polymers containing DOPA,lysine(Lys),and poly(eth-ylene glycol)(PEG)were also deposited in a layer-by-layer manner with sodium montmorillonite to obtain rigid mem-branes having an elastic modulus of6.8±0.9GPa and an ultimate stress of200±28MPa[97].The idea behind this investigation was for an improvement of the interaction strength between the polymer and the clay nanosheets to improve the load transfer in the nanocomposite.The poly-mer carrying functionalities similar to those of the MEFP5 in mussel was successful to realize this aim.Indeed,refer-ence experiments were performed with the polymer carrying only Lys and PEG:The composite multilayer films had a Young modulus40%lower than that of the polymer carry-ing the3,4-dihydroxy-L-phenylalanine moieties.More spec-tacular was the four-fold increase in toughness upon the incorporation of DOPA[94].The crosslinking of the DOPA containing composites was made through a combination of DOPA-Fe3+complexation and autoxidation of DOPA in alkaline and oxygenated media.
Of great interest is the paper by Jaber and Lambert who found that the oxidation of DOPA in the presence of laponite and produces an exfoliation of the clay[98].In addition,the presence of the clay accelerates the oxidation–aggregation mechanism of DOPA,and at the end,a composite material is produced as shown by thermogravimetric analysis,trans-mission electron microscopy,and nuclear magnetic reso-nance spectroscopy[98].This work may present fascinating possibilities for the production of a new gener-ation of nanocomposites.5.5Antibacterial Coatings
When silver is deposited on a polydopamine coating depos-ited on a cotton fabric,this textile acquires antibacterial properties[99].In addition,silver nanoparticles(about50–100nm in diameter)remain on the surface of the modified fabric for a small number of washing steps.We recently confirmed these results,but we also showed that silver is almost quantitatively desorbed from the polydopamine sur-face in contact with the Escherichia coli containing medium [83].We then tried,unsuccessfully,to reinitiate the deposi-tion of silver nanoparticles from a silver nitrate solution.The origin of the electrons able to reduce silver cations was asked in this contribution.Electron spin resonance spectros-copy allowed to show that the concentration of free radicals was not affected during silver reduction,suggesting that the paramagnetic species of the polydopamine coating are not at the origin of its reducing ability[83].
Layer-by-layer films made from poly(ethylene imine) modified with catechol groups(PEI-C)and from hyaluronic acid modified with the same moieties can be deposited on substrates like polyethylene or poly(tetrafluoroethylene) which are usually not well suited for electrostatic adsorp-tion.In addition,when such films are put in contact with silver nitrate solutions,silver cations are reduced in metallic silver.This reduction process confers some antimicrobial properties against E.coli.[100].
Unfortunately there are no available investigations on the antibacterial and immunological response of pristine poly-dopamine films.Such investigations should be made urgent-ly owing to the implication of melanin,the probable material of the polydopamine films,in microbial pathogen-esis[101].
5.6Membranes Decorated with Polydopamine—Applications for Separations and in Energy Conversion Processes
The possibility to have a quasi-universal functionalization method should allow to solve one of the major challenges in separation membranes,namely to obtain a robust coating of an active layer,selective enough to afford efficient separa-tions and thin enough not to decrease the permeation flux too markedly.Polydopamine coatings seem to be an ideal candidate to solve this problem.
Indeed,porous polysulfone membranes can be easily modified with polydopamine(Fig.10).The polydopamine coatings confer interesting permselectivity to the micropo-rous polysulfone membranes.The enrichment factor of thio-phene,defined as the ratio between the weight fractions of this solute in the permeate and in the feed compartments, was found to depend on the concentration of the dopamine solution,on the pH of the solution used during the
deposition [78].It was found that the permeation flux in-creased with the initial pH value of the dopamine solution and that the enrichment factor of thiophene decreased with pH in the range from 7.5to 9.5.These interesting findings are in line with positron annihilation spectroscopy which allowed to show that the porosity of the polydopamine coatings increases with the pH of the dopamine solution used to produce the film [78].A Nafion membrane used in fuel cells and coated with polydopamine enhances the meth-anol barrier properties without hindering the proton trans-port,a property which is mandatory to enhance the efficiency of the methanol fuel cells [102].
5.7Sensing and Biosensing on Polydopamine-Coated Substrates
The possibility to immobilize proteins on the surface of polydopamine coatings [87,88]offers numerous advantages as biosensing platforms.It has to be noted that protein
binding not only occurs through covalent binding via Shiff base reaction but that electrostatic interactions also play a role [88].
For instance,the immobilization of antibodies directed against sulfate-reducing bacteria (SRB)on a polydopamine layer allows for the specific detection of theses bacteria which reduce sulfate anions into highly toxic and corrosive sulfides [103].In addition,when the polydopamine film was deposited on a working electrode,the resistance to the transfer of electrons,obtained from the analysis of electro-chemical impedance spectra,was a linear function of the number of colony forming units of SRB [103].
Anti-human IgG could be immobilized directly on polydopamine films produced through electropolymeri-zation from a dopamine and anti-human IgG containing solution.These antigens could be recognized by human IgGs put in contact with the film [104].Moreover,the immobilization of anti-human IgG into polydopamine films was compared to its immobilization in polypyrrole films,and it appeared that polydopamine coatings dis-play a higher density of binding sites to human IgGs than their polypyrrole counterparts [104].
The same group reported the possibility to incorpo-rate active glucose oxidase into electropolymerized pol-ydopamine films.Very interesting was the finding that preoxidized dopamine,due to the presence of potassium ferricyanide in solution,allowed for more efficient im-mobilization of the enzyme [105].Nicotine was also imprinted in polydopamine films deposited by electro-polymerization in order to produce capacitive sensors specific for nicotine [106].
Concanavalin A (conA)bound to the surface of a polydopamine film can be used to recognize specifically the α-mannose carrying glycoprotein ribonuclease B whereas the α-mannose-deficient ribonuclease A was not bound to the conA-modified polydopamine film.The interaction between the bound lectin and the ribo-nucleases was investigated in real time by means of surface plasmon resonance spectroscopy and surface-enhanced laser desorption/ionization mass spectrometry [107].Polydop-amine films deposited on indium tin oxide electrodes and decorated with flowerlike gold nanoparticles can be used as sensing platforms for the ultrasensitive detection of analytes (detection limit of 10−12M for rhodamine 6G)in surface-enhanced Raman scattering [108].
Gold nanoparticles/polyaniline/polydopamine conjugates could be immobilized on electrodes and allowed to electro-catalyze the oxidation of ascorbic acid.The device dis-played a linear response for ascorbic acid concentrations varying from 1.0×10−6to 1.9×10−3M [109].Finally,com-posites between halosite clay nanotubes and polydopamine can interact with Ru(bpy)32+cations to yield an electro-chemiluminescent sensing platform [110
].
Fig.10Scanning electron micrograph of the cross section of a poly-sulfone microporous membrane coated with a single layer (a )and a double layer of polydopamine (b ).From [78]with the authorization from the American Chemical Society
5.8Polydopamine Films as Substrates to Initiate Polymerization
Lee et al.demonstrated that polydopamine layers produced from norepinephrine containing solutions can be used to initiate ring opening polymerization of polycaprolactone.This was possible owing to the presence of alkyl hydroxyl groups on the surface of the films [36].
Zhu and Edmodson demonstrated that it is possible to use polydopamine coatings made from 2-bromoisobutyril bromide-modified dopamine to grow poly(methylmethacry-late)(PMMA)through an oxygen tolerant variant of atom transfer radical polymerization.PMMA brushes reached a thickness of 240nm in 24h [111].
The surface of carbon nanotubes precoated with polydop-amine could also be modified with polydimethyaminoethyl-methacrylate (PDMAEMA)brushes through surface-initiated atom transfer radical polymerization (Fig.11)[112].The obtained brushes were quaternized with iodo-methane and loaded with PdCl 42−complex anions through an anion exchange process.The metallic cations were final-ly reduced into Pd nanoparticles (10nm in average diame-ter)to obtain an electroactive composite allowing to oxidize
methanol.
Fig.11a Modification of
single-walled carbon nanotubes with polydopamine followed with the grafting of polymer brushes.b Influence of the PDMAEMA brushes on the thermal stability of the com-posite nanotubes coated with polydopamine.c TEM micro-graph of a nanotube coated with polydopamine,quaternized PDMAEMA followed by re-duction of PdCl 42−.From [112]with authorization from Science Direct
The fact that polydopamine is negatively charged at physi-ological pH and its ability to interact with metallic cations [53,54]should offer perspectives in biomineralization pro-cesses.Indeed,polydopamine coatings facilitate the precip-itation of calcium carbonate from supersaturated solutions. More interestingly,it is the most unstable polymorph of calcium carbonate,namely vaterite,that is deposited and does not reprecipitate in more stable aragonite or calcite [113].The crystallization of hydroxyapatite on polydop-amine films has also been investigated[114].The addition of biocompatible minerals to polydopamine could offer interesting perspectives for the obtention of fully biocom-patible nanocomposites.
5.10Production of Hollow Polydopamine Capsules
Polymer films have been coated with polydopamine yielding to core–shell composites.When the core is dissolved with a specific solvent,one obtains hollow polydopamine capsules [115–118].
In this kind of capsules,the shell can be deposited in one step which is a major advantage with respect to hollow capsules made through layer-by-layer coating of the core. When the silica particles are put in a basic dopamine solu-tion,the wall thickness reaches20nm,significantly lower than the polydopamine film thickness reached on flat sub-strates(about45nm[35]).The obtained capsules did not affect the viability of LIM1215cells.
Ochs et al.have produced biodegradable capsules by coat-ing silica particles with poly-L-glutamic acid(PGA)modified with dopamine groups.The degradability of the obtained hollow capsules(after dissolution of the silica core)depends markedly on the percentage of dopamine grafting on the PGA chains[118].At least15%of the glutamic acid functionalities need to be modified with dopamine to obtain stable capsules after the silica removal.The wall thickness of the obtained shell increases from7.7nm when PGA carries15%of dopa-mine to14.7nm when the dopamine fraction is increased to 25%.Multiple deposition steps of PGA carrying15%of dopamine groups were possible and allowed for an increase in the capsule thickness.These capsules can be degraded in contact with proteases to release their encapsulated cargo [118].However,when the degree of PGA modification is equal to25%,the capsules cannot be degraded anymore.
Porous calcium carbonate colloids were loaded with a first enzyme(αamylase),the composite was covered with polydopamine,and a second enzyme was grafted on the external wall of the polydopamine coating(glucose oxi-dase).After dissolution of the CaCO3core with EDTA,the enzymes kept their activity and allowed to create an enzy-matic cascade[119].5.11Polydopamine Films as Substrates for Cell Adhesion The biological applications of polydopamine films have been illustrated in the first original paper describing this surface functionalization method[35].Since a complete review of this application field is now available[120],we will only focus on the most important findings.
Patterned polydopamine arrays produced by injecting do-pamine solutions in poly(dimethylsiloxane)micro-channels allowed the adhesion of various kinds of cells as fibrosarcoma HT1080,mouse preosteoblasts MC3T3-E1,and mouse fibro-blasts NIH-3T3[121].The blood compatibility of nylon, cellulose,and polyethersulfone decorated with polydopamine films was demonstrated by means of platelet adhesion[122].
The cytotoxicity of core/shell magnetite–polydopamine particles,defined as the ratio between the number of dead cells to the total number of cells,was evaluated and found to be excellent[123]:This finding opens interesting perspec-tives for the targeting of magnetic particles to specific tis-sues,the specificity being provided by the biomolecules bound to polydopamine and the toxic Fe3O4being engulfed in the impermeable and stable polydopamine shell.
In the interesting paper by Lynge et al.,it was shown that the fluorescently labeled lipids of liposomes covered with a polydopamine film can be internalized by myoblasts adher-ing on the composite liposome–polydopamine layer[124]. The adhesion of neurons onto polydopamine films is a hot topic owing to the possible implication of melanin in Par-kinson’s disease[125].Melanin films made by solvent casting of melanin grains were used to investigate the adhe-sion of Schwann cells as well as PC12cells.The obtained coatings had a DC conductivity of(7.00±1.10)×10−5S cm−1in the presence of100%relative humidity,a conductivity value very close to that obtained by the group of Bothma et al.[38].On melanin films,the growth of Schwann cells as well as the neurite extension was enhanced with respect to reference collagen films[126].
The viability of hippocampal neurons was evaluated on polydopamine films produced from oxygenated dopamine solutions by the live/dead test.The polydopamine coatings were modified with adsorption/covalent binding of poly-L-lysine to improve the cell adhesion.[70].On the surface of platinum,indium tin oxide,and glass,the cell viability was higher than30%,a value similar to that obtained on the reference,a glass substrate coated with poly-L-lysine.It was also demonstrated that the polydopamine coating was stable in a biological environment(phosphate buffer at pH7.4,5% CO2at37°C)for at least25days[70].The most interesting finding from this investigation was that the adherent hippo-campal cells could be stimulated through the polydopamine layer from a gold electrode.
The adhesion strength between Saccharomyces cerevisiae, Bacillus subtilis,and E.coli with polydopamine films has
been investigated by means of atomic force microscopy [127].In the field of biotechnology,yeast cell s (S.cerevisiae )were encapsulated in a polydopamine coating.They preserved their viability and the cell cycle depended on the thickness of the coating [128].Such an approach will allow to protect cells from external stresses in a “bacterial spore ”-like manner.Interestingly,the polydopamine shell did not prevent the yeast cells from the dividing:It just increased the quiescent phase before cell division.The lag phase preceding cell division depended on the thickness of the polydopamine layer.In addition,polydopamine retarded the digestion of yeast by lyticase [128].The polydopamine-coated yeast could be easily coated with avidin allowing for a specific adhesion of the cells
on patterned surfaces modified with biotin on specific loca-tions [128].All these possible applications of polydopamine films are summarized in Table 1and Scheme 4.
6Conclusions and Outlook
Polydopamine coatings (certainly made of melanin)con-stitute a fascinating new surface functionalization meth-od.The polydopamine film deposition can be a one-step process (but it can also be extended to multiple depo-sition steps,allowing to produce very thick coatings)performed in environmentally friendly conditions and
Table 1Overview of the main applications of thin polydopamine films Application field Comments and main results
References Inhibition of corrosion
Corrosion inhibition is only efficient if the polydopamine coating is post-modified with a self-assembled monolayer [69,84,85]Improvement of tribological properties Polydopamine is part of a three-layer architecture [85,86]Nanocomposites
Coating of nanoparticles,carbon nanotubes
[92–94]Insertion of polydopamine into clays
[98]
Grafting of biomolecules and substrates to initiate polymerization Molecules containing amino and thiol groups bind covalently to polydopamine films in a one-step reaction
[36,87–90]Polymerization initiators can be bound to polydopamine films to perform RAFT
[36,111,112]Antibacterial coatings
Silver ions are reduced in contact with polydopamine and form silver nanoparticles.However,silver is leached in the bacterial suspension and a second reduction step of Ag +cations in Ag is not possible [83,99]Membranes for separations and fuel cells
Modification of a polysulfone membrane to make it permselective
[78]The permselectivity of polydopamine is pH dependent and also depends on the synthesis method
[79,80]Improvement of methanol barrier in fuel cells
[102]
Films for sensing
Highly selective sensing of an analyte can be obtained by imprinting the molecule of interest in the polydopamine film during its formation [104–106]Coating of particles for the production of hollow capsules
Hollow capsules can be obtained in a single-step reaction followed by a removal step of the sacrificial core
[115–119]Controlled adhesion of cells
[70,125–127
]
Inhibition of corrosion
Composite and
multifunctional films
Coatings for tribological properties
Biocompatible coatings
Hollow and rigid capsules
Redox active
and films for sensing platforms
Antibacterial coatings
and for fuel cells
«polydopamine »
coatings
Scheme 4Survey of the already explored application domains of polydopamine films.The picture corresponds to a polydopamine film
deposited on a quartz slide from a 2-mg mL −1dopamine solution in the presence of 50mM Tris buffer at pH 8.5.Air was bub-bled in this solution during 24h
Many efforts have to be devoted in the future to make benefit from the optical properties of polydopamine and to combine them with electroactive and photoactive materials like electrochromic and photochromic materials.Such re-search effort would allow to produce biocompatible compo-sites of high added value and stimuli sensitive coatings.
The ability to produce films having a composition close to that of the Substantia nigra of the central nervous system may offer interesting possibilities to investigate the fate of neurons in conditions near to pathological ones.We also anticipate that polydopamine coatings could offer nice op-portunities when coupled to stimuli sensitive polymers as well as for the obtention of a new generation of nanocomposites.
Nevertheless to reach such goals,much effort is neces-sary to explain the deposition mechanism of polydopamine. The most challenging questions will be to understand:
1.The first steps of the deposition mechanism,whose in
which the first oligomers of5,6-dihydroxyindole bind to the substrate
2.Why the thickness of the polydopamine film reaches a
limiting value of45nm when oxygen is the oxidant and why different thickness growth rates and limiting thick-ness values are reached when different oxidant(Cu2+for instance)are used
3.Why polydopamine deposition can occur in acidic me-
dia in the presence of oxidants like periodate or persulfate
Great progress in the surface science of polydopamine films will be obtained if this community will collaborate closely with researches implied in the fundamental mecha-nisms of oxidation–aggregation processes of catechols as well as with biologists implied in the processes of melano-genesis.A combined experimental–computational approach could be a powerful method to advance this field.Acknowledgments V.Ball acknowledges all his collaborators at AMS for their help during the preparation of the manuscript.The FEDER “Compétitivitérégionale et emploi”2007-2013is acknowledged for financial support under Chaptochem project N°2009-02-039-35.
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