Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer
ARTICLE
https://www.360docs.net/doc/fe11818538.html,/Langmuir
Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer Films
Jinhong Jiang,Liping Zhu,*Lijing Zhu,Baoku Zhu,and Youyi Xu
MOE Key Laboratory of Macromolecule Synthesis and Functionalization,Department of Polymer Science and Engineering,and The Engineering Research Center of Membrane and Water Treatment Technology,Ministry of Education,Zhejiang University,Hangzhou 310027,PR China
b
Supporting Information
ABSTRACT:
This study aims to explore the fundamental surface characteristics of polydopamine (pDA)-coated hydrophobic polymer ?lms.A poly(vinylidene ?uoride)(PVDF)?lm was surface modi ?ed by dip coating in an aqueous solution of dopamine on the basis of its self-polymerization and strong adhesion feature.The self-polymerization and deposition rates of dopamine on ?lm surfaces increased with increasing temperature as evaluated by both spectroscopic ellipsometry and scanning electronic microscopy (SEM).Changes in the surface morphologies of pDA-coated ?lms as well as the size and shape of pDA particles in the solution were also investigated by SEM,atomic force microscopy (AFM),and transmission electron microscopy (TEM).The surface roughness and surface free energy of pDA-modi ?ed ?lms were mainly a ?ected by the reaction temperature and showed only a slight dependence on the reaction time and concentration of the dopamine solution.Additionally,three other typical hydrophobic polymer ?lms of polytetra ?uoroethylene (PTFE),poly(ethylene terephthalate)(PET),and polyimide (PI)were also modi ?ed by the same procedure.The lyophilicity (liquid a ?nity)and surface free energy of these polymer ?lms were enhanced signi ?cantly after being coated with pDA,as were those of PVDF ?lms.It is indicated that the deposition behavior of pDA is not strongly dependent on the nature of the substrates.This information provides us with not only a better understanding of biologically inspired surface chemistry for pDA coatings but also e ?ective strategies for exploiting the properties of dopamine to create novel functional polymer materials.
1.INTRODUCTION
Surface chemical characteristics are of prime importance to polymer materials in various applications.Polymer materials,as is well known,always exhibit distinctive physical and chemical properties such as excellent mechanical strength,impact resistance,insulation performance,and corrosion resistance as well as low cost.Recently,polymer materials have been used more and more in packaging and printing,membrane separation,magnetic materials,optical functional materials,composites,biomedical ?elds,and so forth.However,most polymer materials are inherently inert and have low surface energy,which is usually undesirable for use in industrial and academic areas,as previously reported.Therefore,surface modi ?cation is a pressing need in overcoming these shortcom-ings and obtaining surface properties as expected.
Surface modi ?cation methods for polymers include surface grafting,1layer-by-layer deposition,2self-assembled monolayers,3plasma treatment,4and so forth.Nevertheless,these methods are somewhat complex and strict under the reaction conditions in some cases and lack a general applicability to diverse polymer materials.Fortunately,a facile and versatile method for the surface modi ?cation of solid materials by dip coating in dopamine solution was proposed by Lee et al.5on the basis of basic research on bionics in recent years.Dopamine is a kind of biological neuro-transmitter that widely exists in living organisms.Dopamine and its catecholic derivatives are able to undergo oxidative polymerization
in the presence of oxygen as an oxidant under alkaline conditions.During the polymerization of dopamine,a tightly adherent poly-dopamine (pDA)layer is created on the surface of a substrate that is immersed in the dopamine solution for a certain period of time.The interactions between the pDA layer and the substrate include covalent and noncovalent interactions such as the hydrogen bonding interaction,πàπinteraction,and electro-static interaction.6Actually,the pDA coating on polymer surfaces has good stability and durability in various environments,except in strongly alkaline solution (pH >13).5,7,8Such novel means for the surface modi ?cation of polymers has been widely applied in hydrophilic modi ?cations of polymer membranes,7,9the modi ?-cation of nonwetting surfaces for cell adhesion,10second treat-ments,and surface functionalization by using adherent pDA as an intermediate layer.5,11,12
In our previous work,hydrophobic polymer membranes includ-ing polyethylene (PE),PVDF,and PTFE were surface modi ?ed by dopamine in aqueous solution with success.7The active groups in the adherent pDA layer on the membrane surface allow the covalent immobilization of biomolecules such as heparin and bovine serum albumin (BSA),thus improving the hydrophilicity
Received:July 25,2011Revised:
October 13,2011
and biocompatibility of the resultant membranes.13,14However,the surface characteristics of the pDA coating on membranes have not been investigated in detail.This basic information is important in tailoring a polymer surface with desirable properties.In this work,four typical polymer ?lms were used as hydrophobic samples for surface modi ?cation to avoid the possible disturbance of the pore structure with respect to the experimental results of the surface characteristics.Four polymer ?lms —PVDF,PTFE,PET,and PI —were modi ?ed by the pDA surface-modi ?cation technique men-tioned above,and changes in the surface free energy of pDA-coated polymer ?lms were studied.Surface energy,an important parameter of surface properties,is often used to estimate the surface adhesivity,wettability,paintability,printability,biocompatibility,and so forth.Determinations of the surface energy by contact angle measure-ments are able to provide a better understanding of the material compatibility.Moreover,the polymerization behavior of dopamine in solution and in the deposition process on various substrates under di ?erent reaction conditions was investigated as well.
2.EXPERIMENTAL SECTION
2.1.Materials and Reagents.3,4-Dihydroxyphenethylamine
(dopamine)was purchased from Sigma-Aldrich and used as received.
Tris(hydroxymethyl)aminomethane (Tris)was supplied by Sinopharm Chemical Reagent Co.,Ltd.PVDF resin (solef 1015,M n =238000g/mol,M w =573000g/mol)was obtained from Solvey Co.,Ltd.Other chemicals were of commercial analytical grade and used without further purification.PVDF films with different surface roughnesses were prepared following a reported procedure.15,16A 10wt %PVDF solution in N ,N 0-dimethyl-acetylamide (DMAc)was cast on a glass plate at room temperature.PVDF 1,PVDF 2,and PVDF 3films were obtained by evaporating the solvent from the casting films at different temperatures and rinsing in deionized water to remove the residual solvent thoroughly.
2.2.Thickness Determination.Monocrystalline silicon chips (pretreated with H 2SO 4/H 2O 2)were used for the thickness determina-tion of the deposited pDA layer.A 1wt %PVDF solution in DMAc was spin coated onto the surface of the pretreated silicon wafer for comparison.The thickness of the spin-coated PVDF film measured by spectroscopic ellipsometry was 12.1(2.4nm.A 2.0g/L dopamine solution was prepared in advance by using a Tris-HCl buffer solution (10mM,pH 8.5)as the solvent.Then the Si and PVDF spin-coated Si substrates mentioned above were immersed vertically in the freshly prepared dopamine solution in an open vessel,in continuous contact with atmospheric oxygen.The thickness of the pDA layer deposited on these two substrates was measured by a spectroscopic ellipsometer (M-2000,U.S.).The ellipsometry measurements were performed at a continuous wavelength ranging from 190to 1700nm and angle of incidence of both of 65and 70°.Δand Ψvalues measured at a wavelength of 600à1700nm were chosen for data analysis.Mono-crystalline silicon with thickness of 1mm was used as the substrate,and the Cauchy model was used to determine the thickness of the deposited pDA layer.The A n and B n parameters of the Cauchy layer were set as 1.45and 0.01,respectively,as fit parameters.Then the Cauchy para-meters and film thickness that best fit the pDA coating could be automatically obtained after computer calculation and fitting analysis.The thickness of the pDA layer deposited on the PVDF-spin-coated Si
Table 1.Surface-Energy Components of Probe Liquids a
surface-energy components (mN/m)
liquids σl σl d σl p water 72.119.952.2diiodomethane
50.0
47.4
2.6
a
Here,σl represents the total surface energy of the liquid,and σl d and σl p
represent its disperse and polar parts,respectively.
Figure 1.(a)Thickness evolution of the pDA layer deposited on Si and the PVDF spin-coated Si substrates.(b)ATR-FTIR spectra for the near surface of the PVDF ?lm and the pDA-coated PVDF ?lm.FTIR spectra of dopamine and pDA deposition.(c)Water contact angles of original and pDA-coated substrates.(C =2.0g/L,t =24
h).
substrate was calculated by subtracting the thickness of the PVDF coating layer from the measured value.
2.3.Characterization of the Product in Dopamine Solution.
A freshly prepared2.0g/L dopamine solution was stirred in the reaction temperature range from20to60°C for24h in an open vessel and was in continuous contact with atmospheric oxygen.The products from the solution were collected by centrifugation(5000rpm)for5min.The shapes and particle sizes of pDA particles and their aggregates in the supernatant were detected by a transmission electron microscopy (TEM,JEM-1230EX,Japan).The morphologies of the deposition were observed by field-emitting scanning electron microscopy(FESEM, Hitachi S-4800,Japan).The chemical composition of the deposition was analyzed by Fourier transform infrared spectra(FT-IR,VECTOR22, Germany).
2.4.Surface Modification of Hydrophobic Films.Hydro-phobic polymer films of PTFE,PET,PI,and PVDF were ultrasonically cleaned in methanol,acetone,and deionized water for15min in sequence prior to use.The aforementioned polymer films were immersed in a freshly prepared2.0g/L dopamine solution in an open vessel and were stirred and in contact with air continuously.After reacting for a period of time(t),the resultant films were removed and washed with ethanol and deionized water alternately to remove nonfirmly adsorbed pDA parti-cles.Then the films were dried to constant weight in a vacuum oven at 40°C and used for characterization.
2.5.Film Characterization.The changes in the chemical compo-sition in the near surfaces of hydrophobic polymer films after modifica-tion were analyzed by attenuated total reflectance Fourier transform infrared spectra(ATR-FTIR,Nicolet6700,U.S.).The surface morpho-logies of films were observed by a field-emitting scanning electron microscope(FESEM,Hitachi S-4800,Japan).The surface topographies of the modified films were analyzed by an atomic force microscope (AFM,SPI-3800N,Japan).The AFM images were obtained in tapping mode.The root mean square(rms)was used to evaluate the surface roughness of the polymer films on the basis of a1.0μm?1.0μm scan area.The reported rms value was an average of five measurements. The surface hydrophilicity and surface energy(SE)of polymer?lms were characterized by water contact angle measurements(CA,Data-physics OCA20,Germany).Five measurements were performed for each?lm at25°C and70%relative humidity,and the mean value was taken as the reported result.The SE value was determined according to the OWRK method by using two kinds of test liquids(Table1).Polar and disperse contributions to the surface energy are combined by forming
the sum of both parts.The calculative process of SE and its components is described in detail in the Supporting Information.
3.RESULTS AND DISCUSSION
3.1.Thickness and Chemical Compositions of Deposited Polydopamine.The thickness evolution for the deposited poly-dopamine(pDA)layer is shown in Figure1A.An increase in the
thickness of the deposited layer with increasing time and tem-perature was observed.The growth of deposited pDA was nearly linear with reaction time during the initial10h,and then the growth rate decreased after15h and the thickness of the deposited layer gradually reached a constant value.Deposited thicknesses of23.9à30.4nm for the Si substrate and48.9à66.8nm for the PVDF spin-coated Si(Si/PVDF)substrate were obtained in the temperature range of20à45°C after24h of reaction.It was revealed that an elevated reaction temperature resulted in a higher reaction rate for the dopamine monomer,thus leading to a thicker deposited layer attached to the substrate.Moreover,the different surface chemistries between Si and Si/PVDF substrates made an obvious difference in the deposition behavior under the Figure2.Morphologies of the products in dopamine solution.(a)Photo-graphs of dopamine solution before and after centrifugation.(b)TEM imagines of pDA particles in the supernatant.(c)SEM images of pDA deposition.(C=2.0g/L,t=24h).
Table2.Surface Energy and Its Components in the Original Polymer Film a
contact angle(deg)surface-energy components(mN/m) samples water diiodomethaneσs0σs0dσs0p PTFE124.2(0.898.4(0.79.39.20.1
PI82.0(0.738.9(1.339.835.9 3.9 PET85.8(1.535.2(1.641.339.0 2.2 PVDF191.6(2.651.3(1.533.031.0 2.0 PVDF294.0(2.455.5(1.330.728.9 1.7 PVDF396.8(1.460.1(1.928.126.6 1.5
a Here,σ
s0
represents the total surface energy of the original poly-mer?lm,andσs0d andσs0p stand for its disperse and polar parts,
respectively.
same reaction conditions.More dopamine oligomers and poly-mers adhered to the surface of the Si/PVDF substrate probably because of the increased surface roughness after the coating with PVDF and the stronger interactions between dopamine and the PVDF coating layer.17,18
Much research has suggested that the properties of pDA are compatible with those of eumelanins,including the chemical composition and physical morphology,5,19,20and it was also con-?rmed by our experimental results discussed below.As seen in Figure 1B,there was a remarkable change in dopamine in the FTIR spectra after oxidation and https://www.360docs.net/doc/fe11818538.html,pared with dopamine,the absorption peaks of pDA broadened and became unseparated from each other,which was almost identical to the reported FTIR spectrum of eumelanin.19,21For the ATR-FTIR spectra of PVDF ?lms in Figure 1B,several new absorption signals appeared after surface modi ?cation by dopamine solu-tion.A broad absorbance between 3600and 3100cm à1was ascribed to N àH/O àH stretching vibrations.The peaks at 1600and 1510cm à1were attributed to the overlap of the C d C resonance vibrations in the aromatic ring and the N àH bending vibrations,respectively.These results proved the incorporation of the pDA composite layer on the surface of the PVDF ?lm after dopamine modi ?cation.
The water contact angle (CA)measurement is a common method of characterizing the surface relative hydrophilicity and wetting properties.The CA of pDA-coated substrates is shown in
Figure 1C.Despite the obviously di ?erent wetting behaviors of the original substrates,all CA values of the pDA-coated sub-strates were centralized around ～45à65°,which approached the theoretical value of the pDA ?lm as reported by others.5,22The CA values of the modi ?ed substrates decreased with increasing reaction temperature,probably in relation to the thicker pDA layer over the substrates.As a matter of fact,an elevated tem-perature could accelerate the deposition of pDA on the substrates,creating a rougher pDA layer than that formed at lower tem-perature.Therefore,a more hydrophilic surface was obtained by elevating the reaction temperature from 20to 45°C.
3.2.Reaction Products in Dopamine Solution.A series of experiments were performed at 20,30,45,and 60°C to investigate the effect of temperature on the polymerization behavior of dopamine.During the self-polymerization and cross-linking reaction of dopamine,the color of the solution gradually changed from colorless to dark brown and then to black,finally returning to light yellowish brown on standing.A mass of macroscopic black particles was formed and was mostly deposited at the bottom of the vessel (Figure 2A).The color of the supernatant lightened with increasing temperature and became almost color-less when the reaction temperature reached 60°C.
The changes in the color (Figure 2A)and TEM images (Figure 2B)of the supernatant suggested that the pDA particle sizes and number declined as the temperature https://www.360docs.net/doc/fe11818538.html,rge pDA particles and their aggregates were unstable in water and
Figure 3.(a)AFM topographies and (b)surface SEM images of original PVDF ?lms.
Table 3.E ?ect of Reaction Temperature on the Surface Energy of a pDA-Deposited PVDF Film (C =2.0g/L,t =24h)a
contact angle (deg)
surface-energy components (mN/m)sample temperature (°C)
water diiodomethane σs σs d σs p (σs àσs0)/σs0
PVDF 1/pDA
2067.5(1.842.5(3.542.430.112.30.283064.7(2.438.8(1.344.831.413.40.364554.8(3.431.4(4.351.532.718.80.5660
50.4(1.1
26.2(4.0
54.8
33.8
21.0
0.66
a
Here,σs represents the total surface energy of a pDA-coated polymer ?lm,and σs d and σs p
stand for its disperse and polar parts,respectively.σs àσs0/σs0
is de ?ned to evaluate the changes in σs of a pDA-coated ?lm in comparison to that of the original ones.A high value of σs àσs0/σs0corresponds to a large increase in the total surface energy of the polymer ?lm after the coating with
pDA.
settled down by centrifugation easily,whereas small particles (<50nm)remained and were dispersed well in the supernatant. In the supernatant of the dopamine solution reacting at60°C, fewer small spherical particles could be detected,indicating that most free dopamine molecules in the solution were consumed to produce large pDA particles that subsided during the cen-trifugation step.Besides,the particles became smaller with rising temperature as found by others,19which was possibly due to the acceleration of the self-polymerization of dopamine and spherical nuclei formation by elevating the reaction temperature.How-ever,in the supernatant of the dopamine solution reacting at 20°C,quantities of tiny particles appeared and aggregated to form shapeless clusters,which would link randomly to generate larger spherical particles.It seemed that the polymerization of dopamine proceeded more thoroughly at a higher temperature.
A comparison of the SEM morphologies for pDA depositions is presented in Figure2C.No signi?cant changes were found in the morphologies of pDA depositions obtained at di?erent temperatures.The pDA depositions consisted of aggregates containing large particles with a broad size distribution ranging from～100to500nm.These particles were mainly spherical and linked to each other completely at random and then gave rise to aggregates whose sizes were～0.5à5μm(Figure S1).Dynamic light scattering(DLS)measurements also con?rmed the existence of the very broad size distribution(data not shown).
3.3.Modification of Polymer Films via Polydopamine. Four kinds of membrane materials—PTFE,PET,PI,and PVDF films—were chosen as hydrophobic samples for surface mod-ification via self-polymerization and composite of dopamine. The surface morphologies of PVDF films with different surface roughnesses and the three other films are shown in Figures3and S2,respectively.Contact angle(CA)and surface energy(SE) values of the original polymer films mentioned above are listed in Table2.It was found that the highest rms(6.6(0.5nm)belongs to the PVDF3film,which has the lowest total SE(28.1mN/m). That means that a rougher surface could extend the contact area with the probe liquids,thus increasing the CA value and reducing the total SE of the hydrophobic film.
As discussed above,the reaction temperature had a signi?cant in?uence on the polymerization process of dopamine in aqueous
solution.The same experiment was performed to investigate the deposition behavior of dopamine on the polymer?lm surface under di?erent temperatures.Figure4showed the changes in the surface morphologies of PVDF1?lms after being coated with pDA.For pDA-coated PVDF1?lms(PVDF1/pDA?lms),the surface morphologies were strikingly altered and the rms values were all elevated in comparison to that of the original PVDF1?lm.The pDA coating on the PVDF1?lm was made up of a uniform pDA layer and some covalently bonding pDA particles and aggregates.Actually,the pDA coating became thicker with increasing temperature,which was also con?rmed by the increas-ing pDA particles and aggregates appearing on the surfaces of PVDF1/pDA?lms.The deposition of pDA made the PVDF1?lm surface rougher and more nonuniform,especially under a high reaction temperature of60°C.The data of correlative SE are listed in Table3.The SE of PVDF1/pDA?lms accreted as the temperature increased and had a positive correlation with the https://www.360docs.net/doc/fe11818538.html,pared to the original PVDF1?lm,the total SE of the PVDF1/pDA?lm reached its maximum(54.8mN/m,(σsàσs0)/σs0=0.66)at a reaction temperature of60°C,which was mainly contributed by the notably increased polar share after the incorporation of the pDA layer.
The surface roughness was thought to play a role in the surface properties of polymer?lms,such as the wettability,antifouling ability,and adhesion behavior of organic substances on the?lm surface.PVDF?lms with di?erent surface roughnesses were modi?ed by dopamine solution,and the SE data of resultant PVDF/pDA?lms versus the reaction time are listed in Table4. All of the SE values of PVDF?lms apparently increased after the coating with pDA compared to the original ones,owing to the stronger polar interactions in the pDA layer.The total SE of the PVDF/pDA?lms increased within the initial12h and then hardly varied or slightly decreased after24h.The mild decline in SE was probably caused by the decrease in surface roughness during the random deposition process of pDA when prolonging the reaction time.Besides,the PVDF3/pDA?lm had the most wettable surface and the largest SE after pDA modi?cation. Possibly more pDA was deposited on the PVDF3?lm than on the other two?lms because of it had the largest surface roughness and the largest contact area with the dopamine solution.However, the PVDF3?lm kept its surface the roughest during the modi?cation procedure(Figure S3).A greater surface roughness was commonly considered to make a hydrophilic surface more Figure4.(a)AFM topographies and(b)surface SEM images of pDA-coated PVDF1?lms(C=2.0g/L,t=24
h).
hydrophilic;therefore,the PVDF3/pDA?lm exhibited the most wettable properties.On the contrary,it could make a hydrophobic surface more hydrophobic,which was consistent with the mea-surement results of the original PVDF?lms as discussed above. Dopamine is able to form a pDA layer that?rmly adheres to almost any substrate in aqueous solution,including classically adhesion-resistant materials such as PTFE.5Here we used di?erent polymer?lms as hydrophobic materials for surface modi?cation via the self-polymerization composite of dopamine.The chem-ical compositions of the modi?ed and unmodi?ed polymer?lms are presented in Figure S4.The variation of SE for pDA-coated polymer?lms with the concentration of dopamine solution and the reaction time is summarized in Tables5and6.The total SE of all of these polymer?lms increased after pDA modi?cation, especially for PTFE?lms((σsàσs0)/σs0≈3).The increased part of SE mostly lay in the polar component because the dispersed component changed little during the surface-modi?ca-tion process.The deposited pDA layer introduced stronger polar interactions onto the polymer?lm surface,containing coulombic interactions between permanent dipoles and the ones between permanent and induced dipoles.These implied that the mod-i?cation e?ect was noticeable when modifying a surface with a nonwettable,nonpolar nature by using this method.Moreover, the total SE value was quite close for each kind of polymer?lm after being coating with pDA,showing only a slight dependence on the nature of the substrate.In fact,dopamine in aqueous solution with a concentration of0.5à2.0g/L could form a continuous?lm on the substrate surface in a short time.8,17The pDA layer over the substrate surface became thicker with increasing time and increasing concentration of dopamine solu-tion.Similar chemical compositions in the surfaces of di?erent substrates could be obtained by incorporating such a pDA composite layer with a thickness ranging from several nanome-ters to several dozen nanometers during the surface modi?cation process.However,the potential in?uence of the substrate on the surface characteristics after pDA modi?cation could not be neglected
contact angle(deg)surface-energy components(mN/m) samples coating time(h)water diiodomethaneσsσs dσs p(σsàσs0)/σs0
PVDF1/pDA669.3(3.141.3(1.542.131.210.90.28 1266.5(1.339.8(2.343.731.312.40.32
2464.7(2.438.8(1.344.831.413.40.36
3666.6(2.038.7(1.444.031.912.10.33 PVDF2/pDA668.8(1.444.3(1.241.329.411.90.35 1263.1(3.038.9(2.945.531.014.50.48
2461.8(2.541.0(1.645.529.615.90.48
3667.6(2.540.7(2.343.031.111.90.40 PVDF3/pDA667.2(3.637.0(4.644.332.911.40.58 1262.2(2.833.2(1.447.633.614.00.69
2462.9(2.735.3(2.746.632.713.90.66
3664.9(2.138.2(1.144.931.813.10.60
Table5.Surface-Energy Components of pDA-Coated Polymer Films(t=6h,T=30°C)
contact angle(deg)surface-energy components(mN/m)
samples concentration of dopamine
solution(g/L)water diiodomethaneσsσs dσs p(σsàσs0)/σs0
PTFE/pDA0.586.9(3.745.6(1.736.233.3 2.9 2.89
1.083.1(1.144.7(3.037.13
2.9 4.2 2.99
1.577.0(
2.144.0(4.038.631.67.0
3.15
2.074.7(4.642.8(1.739.731.78.0
3.27 PVDF1/pDA0.575.1(3.543.4(1.939.431.57.90.19
1.067.9(0.839.4(1.743.331.911.40.31
1.568.0(1.340.4(
2.842.931.411.60.30
2.069.3(
3.141.3(1.542.131.210.90.28 PI/pDA0.563.3(1.436.4(1.246.232.313.90.16
1.061.5(0.735.8(0.847.13
2.115.00.18
1.561.9(
2.136.1(1.046.932.114.80.18
2.062.1(0.937.3(0.846.431.514.90.17 PET/pDA0.576.9(
3.033.2(0.643.037.5 5.50.04
1.077.1(
2.831.9(0.64
3.438.2 5.20.05
1.571.1(
2.528.0(2.446.038.27.80.11
2.070.8(2.929.5(1.245.637.58.10.10
because of the cracking of the pDA layer during the drying process.8As a result,the total SE of the pDA-coated polymer ?lm was a ?ected by both surface properties and dopamine coatings and reached its maximum at the proper reaction time and concentration of the reaction solution.
3.4.Possible Deposition Process of Polydopamine on a Film Surface.The polymerization mechanism and deposition behavior of dopamine on various substrates are in dispute and not yet clearly known.The possible structural evolution of dopamine in aqueous solution is shown in Figure 5A according to the published literature.5,17,23Dopamine is easily oxidized by dissolved oxygen under alkaline conditions,creating 5,6-dihydroxyindole (1)and
5,6-indolequinone (2)via intramolecular cyclization,oxidation,and rearrangement.After a multistep reaction proceeding by 1and 2,a mass of melanin-like pDA particles and aggregates was generated in the solution 8,19and a tightly adherent pDA layer was formed on the surfaces of the substrates simultaneously.
In aqueous solution,the branching reaction via 1and 2at positions 2,3,4,and 7is able to produce oligomers made of four to eight 5,6-dihydroxyindole units in a stepwise fashion as proposed in refs 11and 24.The oligomers assemble in an orderly manner through πstacking to form nanoaggregates (approximately 2à20nm)with a supramolecular architecture.The random linking between nanoaggregates ?nally generates melanin-like
contact angle (deg)
surface-energy components (mN/m)
samples concentration of dopamine
solution (g/L)
water diiodomethane σs σs d σs p (σs àσs0)/σs0
PTFE/pDA
0.573.9(1.942.0(1.840.332.08.3 3.331.071.9(2.341.6(2.641.131.79.4 3.421.579.3(3.943.3(2.438.432.7 5.7 3.132.0
81.3(2.343.4(0.937.933.1 4.8 3.08PVDF 1/pDA
0.570.4(2.642.6(2.541.330.810.50.251.064.0(0.936.9(1.945.732.213.50.381.567.2(2.439.3(2.843.631.811.90.322.0
66.5(1.339.8(2.343.731.312.40.32PI/pDA
0.558.2(2.725.5(2.351.335.915.40.291.058.8(1.329.4(0.750.134.515.60.261.564.2(2.630.1(1.047.635.512.10.202.0
64.9(1.231.5(1.546.935.011.90.18PET/pDA
0.565.4(0.730.0(0.947.235.811.40.141.062.7(2.828.8(0.448.635.712.90.181.559.7(2.428.1(0.550.035.214.80.212.0
59.8(2.8
27.3(0.7
50.2
35.6
14.6
0.22
Figure 5.(a)Structural evolution of dopamine in aqueous solution according to refs 5,17,and 23.(b)Possible deposition process of pDA on the substrate
surface.
pDA particles(approximately20à500nm)that contain very high molecular weight polymers.These particles are mainly spherical and can link to each other completely at random,giving rise to pDA aggregates(approximately0.5à5μm).The random linking between nanoaggregates as well as the aggregation process of pDA particles can also be observed in Figures2A and S1.
A working hypothesis was put forward by Bernsmann,8who assumed that the deposition of pDA was triggered by the absorption of monomeric species and small oligomers on the substrate surface. In this case,dopamine is assumed to deposit on a substrate surface in the same manner that polymerization in solution occurs.The adsorbed radicals initiate the polymerization reaction,leading to oligomers and their nanoaggregates and a subsequent high molec-ular weight polymer,which forms a continuous?lm and fully covers the substrate surface.Monomers,nanoaggregates(approximately 2à20nm),and a relatively few small pDA particles(approximately 20à50nm)coming from the solution are also capable of being incorporated into the deposited pDA layer by covalent bonding,πstacking,and other noncovalent interactions.As a result,the deposited pDA layer thickens gradually with the prolonged polymerization time(Figure5B).However,large pDA particles and their pDA aggregates cannot deposit and adhere to the substrate because they are easy to remove by rinsing in deionized water after the pDA deposition process.
4.CONCLUSIONS
We demonstrated that dopamine could self-polymerize and form a pDA layer?rmly adhered to the surfaces of various polymer substrates in the temperature range of20à60°C.The thickness of the deposited pDA layer increased with increasing reaction time or temperature and was also related to the surface roughness and surface chemistry of the substrate.Besides,organic substrates were more favorable to the deposition and adhesion of pDA by comparison with inorganic substrates.The total surface energy of polymer?lms was remarkably enhanced after the coating with pDA,which was mainly ascribed to the striking increase in the polar share.The polymerization and deposition behavior of dopamine was similar to the formation of natural eumelanins,and the pDA coatings had a similar physical morphology and chemical composi-tion to those of eumelanins.These results suggest that the surface modi?cation of solid materials by the self-polymerization compo-site of dopamine in aqueous solution is a facile and versatile method that can be performed under mild conditions.The pDA coating endows surface-modi?ed solid materials with biomimetic functions, which allows the extension of the applications of materials to various?elds and to the development of novel functional materials based on the inspiration of melanin-like pDA.
’ASSOCIATED CONTENT
b Supporting Information.TEM images of melanin-like pDA particles and their aggregates in a reaction solution.AFM topographies of pure polymer?lms and pDA-coated PVDF?lms (C=2.0g/L,t=24h,T=30°C).ATR-FTIR spectra of the near surface of pure polymer?lms and pDA-coated polymer?lms (C=2.0g/L,t=24h).More experimental details.This material is available free of charge via the Internet at https://www.360docs.net/doc/fe11818538.html,.’AUTHOR INFORMATION
Corresponding Author
*E-mail:lpzhu@https://www.360docs.net/doc/fe11818538.html,.Tel and Fax:+8657187953011.’ACKNOWLEDGMENT
We are grateful for the?nancial support of the National Nature Science Foundation of China(grant no.50803054),the Zhejiang Provincial Nature Science Foundation of China(grant no.Y4100204), the National Basic Research Program of China(973Program of China,grant no.2009CB623402),the Fundamental Research Funds for the Central Universities(MOE Engineering Research Center of Membrane and Water Treatment Technology,Zhejiang University),and the Key Innovation Team for Science and Technology of Zhejiang Province,China(2009R50047).
’REFERENCES
(1)Kato,K.;Uchida,E.;Kang,E.T.;Uyama,Y.;Ikada,Y.Prog. Polym.Sci.2003,28,209–259.
(2)Chen,W.;Thomas,J.M.Macromolecules1997,30,78–86.
(3)Luo,M.L.;Zhao,J.Q.;Tang,W.;Pu,C.S.Appl.Surf.Sci.2005, 249,76–84.
(4)Hegemann,D.;Brunner,H.;Oehr,C.Nucl.Instrum.Methods Phys.Res.,Sect.B2003,208,281–286.
(5)Lee,H.;Dellatore,S.M.;Mille,W.M.;Messersmith,P.B.Science 2007,318,426–430.
(6)Lee,H.;Scherer,N.F.;Messersmith,P.B.Proc.Natl.Acad.Sci. U.S.A.2006,103,12999–13003.
(7)Xi,Z.Y.;Xu,Y.Y.;Zhu,L.P.;Wang,Y.;Zhu,B.K.J.Membr.Sci. 2009,327,244–253.
(8)Bernsmann,F.;Ponche,A.;Ringwald,C.;Hemmerle,J.;Raya,J.; Bechinger,B.;Voegel,J.C.;Schaaf,P.;Ball,V.J.Phys.Chem.C2009, 113,8234–8242.
(9)McCloskey,B.D.;Park,H.B.;Ju,H.;Rowe,B.W.;Miller,D.J.; Chun,B.J.;Kin,K.;Freeman,B.D.Polymer2010,51,3472–3485.
(10)Ku,S.H.;Ryu,J.;Hong,S.K.;Lee,H.;Park,C.B.Biomaterials 2010,31,2535–2541.
(11)Waite,J.H.Nat.Mater.2008,7,8–9.
(12)Lee,H.;Rho,J.;Messersmith,P.B.Adv.Mater.2009,21, 431–434.
(13)Jiang,J.H.;Zhu,L.P.;Li,X.L.;Xu,Y.Y.;Zhu,B.K.J.Membr. Sci.2010,364,194–202.
(14)Zhu,L.P.;Jiang,J.H.;Zhu,B.K.;Xu,Y.Y.Colloids Surf.,B 2011,86,111–118.
(15)Bottino,A.;Capannelli,G.;Monticelli,O.;Piaggio,P.J.Membr. Sci.2000,166,23–29.
(16)Boccaccio,T.;Bottino,A.;Capannelli,G.;Piaggio,P.J.Membr. Sci.2002,210,315–329.
(17)Bernsmann,F.;Ball,V.;Addiego,F.;Ponche,A.;Michel,M.; Gracio,https://www.360docs.net/doc/fe11818538.html,ngmuir2011,27,2819–2825.
(18)Ryu,J.;Ku,S.H.;Lee,H.;Park,C.B.Adv.Funct.Mater.2010, 20,2132–2139.
(19)Ju,K.Y.;Lee,Y.;Lee,S.;Park,S.B.;Lee,J.K.Biomacromolecules 2011,12,625–632.
(20)El-Ayaan,U.;Herlinger,E.;Jameson,R.F.;Linert,W.J.Chem. Soc.,Dalton Trans.1997,2813–2818.
(21)Centeno,S.A.;Shamir,J.J.Mol.Struct.2008,873,149–159.
(22)Wei,Q.;Zhang,F.L.;Li,J.;Li,B.J.;Zhao,C.S.Polym.Chem. 2010,1,1430–1433.
(23)Li,Y.L.;Liu,M.L.;Xiang,C.H.;Xie,Q.J.;Yao,S.Z.Thin Solid Films2006,497,270–278.
(24)d’Ischia,M.;Napolitano,A.;Pezzella,A.;Meredith,P.;Sarna,T. Angew.Chem.,Int.Ed.2009,48,3914–3921.