Multiple Hydrogen Bonding for Reversible Polymer Surface Adhesion

Multiple Hydrogen Bonding for Reversible Polymer Surface Adhesion Kalpana Viswanathan,Hayriye Ozhalici,Casey L.Elkins,Cheryl Heisey,Thomas C.Ward,*and Timothy E.LongDepartment of Chemistry,Macromolecules and Interfaces Institute,Virginia Polytechnic Institute andState Uni V ersity,Blacksburg Virginia24061Recei V ed August18,2005.In Final Form:No V ember9,2005Specific and reversible adhesion of a terminal thymine-functionalized polystyrene(PS-thymine)was demonstrated for a silicon surface with complementary adenine recognition sites.A novel adenine-containing triethoxysilane(ADPTES), which was suitable for covalent attachment to silanol containing surfaces,was synthesized in one step from adenine and3-isocyanatopropyl triethoxysilane(IPTES).1H and13C NMR spectroscopy and fast atom bombardment mass spectroscopy confirmed the chemical structure,and29Si NMR spectroscopy indicated the absence of any premature hydrolysis of the alkoxysilane derivative.X-ray photoelectron spectroscopy(XPS)and water contact angle measurements indicated the attachment of PS-thymine to silicon surfaces that were modified with a mixture of ADPTES and 3-mercaptopropyl triethoxysilane(MPTES).PS-thymine attachment to surfaces that were modified with only MPTES was not observed.The exclusive attachment of PS-thymine to an ADPTES/MPTES-modified surface confirmed hydrogen bonding-mediated adenine-thymine association to silicon surfaces containing a sufficiently low concentration of adenine recognition sites.Although PS-thymine attachment to the ADPTES/MPTES-modified surfaces was insensitive to THF rinsing,the PS-thymine was completely removed from the surface upon DMSO rinsing because of the disruption of adenine-thymine hydrogen bonding with a more polar aprotic solvent.PS-thymine was successfully reattached to the ADPTES/MPTES-modified surface following the DMSO rinse,demonstrating the solvato-reversible nature of the adenine-thymine association.IntroductionMolecular recognition,which is defined as the specific interaction between multiple molecules without the involvement of covalent bonding,forms the basis of supramolecular orga-nization in biological systems.1Noncovalent interactions such as electrostatic,hydrogen bonding,host-guest,andπ-πstacking are often of central importance in molecular recognition.2-4 Although relatively weak,these directional,multivalent interac-tions lead to the self-assembly of molecules into complex hierarchies with well-defined functional superstructures.Lehn and co-workers first developed self-assembly strategies based on molecular recognition to construct synthetic materials with unique physical properties.5The use of such molecular recognition sites on surfaces provides a platform for tuning the chemical as well as physical properties of a surface in a reversible and“on-demand”fashion.Also,molecular recognition sites on surfaces enables the preparation of functional surfaces for a myriad of applications ranging from electronics and optics to sensors and catalysis.6-10Moreover,the utility of functionalized thiols and alkoxy-or chlorosilanes will lead to surfaces modified with the desired molecular recognition groups.11-14Previous efforts have employed molecular recognition groups on surfaces including selective inclusional complexation and ionic/hydrogen bonding interactions.15Several groups have used cyclodextrin(CD)-based host-guest interactions16,17as model membrane receptors and chemical sensors.8Kitano et ed cyclic voltammetry to study the association constants(K a)for the complexation of R-and -CD SAMs on gold surfaces with a variety of guest molecules such as bisphenols,azo compounds, and phthalic acid esters.8,9,15,18Several groups have demonstrated the capability of CD-modified gold surfaces in electrochemically induced reversible sensing of ferrocene molecules for potential applications in electronics and sensors.19-21The reversible ferrocene-CD interactions were also used in the self-assembly of nanoparticles.22,23Electrostatic interactions between amines and carboxylic acids have also found use in the immobilization of nanoparticles and nanorods on gold surfaces.Sastry et al.24and Murphy et al.25 demonstrated the deposition of carboxylic acid-functionalized*Corresponding author.E-mail:tward@.(1)Ha¨ussling,L.;Ringsdorf,H.;Schmitt,F.-J.;Knoll,ngmuir1991,17, 1837.(2)Lindoy,I.F.;Atkinson,M.F.Self-Assembly in Supramolecular Systems; Royal Society of Chemistry:Cambridge,U.K.,2000.(3)Whitesides,G.M.;Grzybowski,B.Science2002,295,2418.(4)Stupp,S.I.;LeBonheur,V.;Walker,K.;Li,L.S.;Huggins,K.E.;Keser, M.;Amstutz,A.Science1997,276,384.(5)Lehn,J.-M.Angew.Chem.,Int.Ed.Engl.1988,27,89.(6)Mbindyo,J.K.N.;Reiss,B.D.;Martin,B.R.;Keating,C.D.;Natan,M. 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In the case of interaction between the acid SAMs and organic base vapors,the vapor-phase molecules readily desorbed upon N2purging,which indicated a relatively weak interaction between the sensor and probe molecules.47Kaifer et al.reported the assembly of crown ether fullerenes onto gold surfaces that were coated with ammonium-terminated SAMs.48Rogers et ed tetrapyridylporphyrins that were capable of forming multiple hydrogen bonds with carboxyl-terminated SAMs to obtain stable porphyrin-functionalized gold surfaces.49The deposition of individual nanometer-sized hydrogen-bonded assemblies of barbituric/cyanuric acid on calix[4]arene dimelamine monolayers on gold via hydrogen bonding interactions was also reported.50 Rotello et al.demonstrated recognition between gold surfaces that were patterned with diacyl-2,6-diaminopyridine(DAP)and an electroactive ferrocene-terminated uracil in solution using scanning tunneling microscopy(STM).51Competitive adsorption between ferrocene and alkyl uracil led to the reversible tuning of the electroactivity of the surface.Vansco et al.recently reported solvent and temperature effects for the dimerization of quadruple hydrogen bonding trifluromethyl/ferrocenyl-pyrimidone deriva-tives from solution to pyrimidonedisulfide SAMs on a gold surface.52It is important to note that surface immobilization enhanced the apparent association constant because the surface-confined dimers were stable for more than20h in CHCl3in contrast to only170ms for neat pyrimidinone dimers.However, the interaction on the surface was destroyed upon washing with DMSO and hot CHCl3.Polymers that exhibit such reversible interactions with a solid surface hold considerable potential as releasable coatings and smart adhesives.In addition,the capability of reversibly attaching polymers to solid surfaces enables tunable surface energetics. Polymers that were functionalized with hydrogen bonding groups were assembled onto complementary surfaces to direct nano-particle assembly and tune surface characteristics.53-57Rotello and co-workers used nanoparticles and polymers functionalized with complementary hydrogen bonding groups such as thymine and triazine to form extended nanoparticle aggregates.53,54The nature and degree of substitution of the hydrogen bonding groups controlled the morphology of the nanoparticle aggregates.Rotello and co-workers also demonstrated hydrogen bond-mediated(26)Galow,T.H.;Boal,A.K.;Rotello,V.M.Ad V.Mater.2000,12, 576.(27)Auer,F.;Scotti,M.;Ulman,A.;Jorden,R.;Sellergren,B.;Garno,J.;Liu,ngmuir2000,16,7554.(28)Brunsveld,L.;Folmer,B.J.B.;Meijer,E.W.;Sijbesma,R.P.Chem. 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P.;Rotello,V.M.Nature2000,404,746.(54)Boal,A.K.;Gray,M.;Ilhan,F.;Clavier,G.M.;Kapitzky,L.;Rotello, V.M.Tetrahedron2002,58,765.(55)Sanyal,A.;Norsten,T.B.;Oktay,U.;Rotello,ngmuir2004,20, 5958.(56)Norsten,T.B.;Jeoung,E.;Thibault,R.J.;Rotello,ngmuir2003, 19,7089.(57)Jeoung,E.;Carroll,J.B.;Rotello,mun.2002,1510.1100Langmuir,Vol.22,No.3,2006Viswanathan et al.attachment of polyhedral oligomeric silesesquioxanes(POSS) that were derivatized with DAP to thymine-functionalized gold surfaces.57In a similar study,the adsorption of a diblock PS-b-P(S-r-CH2ODAP)copolymer onto thymine-functionalized gold substrates was studied,and it was reported that the polymer adsorption decreased as the length of the DAP-containing block increased.56The same research group also reported the reversible adsorption/desorption of DAP-containing monoblock and di-block copolymers onto thymine-functionalized gold surfaces.55 Although complementary triple hydrogen bonding thymine-DAP as well as self-complementary UPy-based quadruple hydrogen bonding interactions were studied earlier,hydrogen bonding interactions on surfaces using DNA base pairs have not received significant attention.In addition to exhibiting highspecificity,these interactions also have reasonable thermal stability.Several groups have studied the influence of DNA base pairs on the properties of polymers functionalized with these groups.43,58A limited number of earlier studies have employed adenine-thymine interactions on surfaces.59A relevant manu-script described the assembly of zeolites that were functionalized with thymine groups to adenine-functionalized glass surfaces.59 This article will describe an investigation of the stability and reversibility of DNA-base-pair-mediated association between a polymer and silicon surface.In the current study,silicon surfaces were modified with adenine using a novel adenine-derivatized triethoxysilane.To our knowledge,this is the first report on the modification of silicon surfaces with alkoxysilanes that are functionalized with multiple hydrogen bonding groups.PS containing a single,terminal,complementary thymine group was obtained via a sec-butyllithium(sec-BuLi)-initiated living anionic polymerization of styrene and subsequent termination with ethylene oxide.60,61Thymine was subsequently introduced at the PS chain end using a two-step Michael addition-based synthetic strategy.43Reversible attachment of the PS-thymine to the adenine-functionalized silicon surface was probed using XPS and water contact angle measurements.Experimental SectionMaterials.3-Isocyanatopropyl triethoxysilane(IPTES,95%, Aldrich)was vacuum distilled in the absence of purification reagents (0.1mmHg,30°C).Styrene(Aldrich)was stirred over CaH2for3 to4days,distilled under vacuum(0.1mmHg,10°C)after repeated degassing and freeze-thaw cycles,and stored at-25°C.Styrene was vacuum distilled from dibutylmagnesium(DBM)under similar conditions immediately prior to polymerization.DBM(25%solution in heptane,FMC Corporation Lithium Division),sec-BuLi(1.4M in cyclohexane,Aldrich),ethylene oxide(99%,Aldrich),anhydrous DMSO(Aldrich),potassium tert-butoxide(Aldrich),methanol(EMD Chemicals),dichloromethane(EMD Chemicals),concentrated H2SO4(VWR International),NH4OH(VWR International),H2O2 (30%,EMD Chemicals),and3-mercaptopropyl triethoxysilane (MPTES,95%,Gelest,Inc.)were used as received.Cyclohexane (EMD Chemicals)was distilled from sodium immediately prior to use.Adenine and thymine(Aldrich)were dried overnight in a vacuum oven at60°lipore milli-Q water was used for surface cleaning. Silicon wafers were a generous gift from the Hewlett-Packard Company.Synthesis of Adenine-Functionalized Triethoxysilane (ADPTES).The synthesis of ADPTES involved the reaction of adenine with IPTES in anhydrous DMSO as shown in Scheme1.All glassware was rigorously cleaned and flame dried.Dry adenine(2g,14.8mmol)was dissolved in anhydrous DMSO(30mL)at145°C under nitrogen.IPTES(4mL,16.2mmol,1.1equiv)was dissolved in anhydrous DMSO(10mL)in a100mL round-bottomed flask.The adenine solution was slowly added to a room-temperaturesolution of IPTES using a double-tipped needle(cannula)over30min.The mixture was allowed to stir for an additional2h to ensurecomplete reaction.DMSO was removed under vacuum,and thesolid product was dissolved in CHCl3and passed through Celite.The solid was then recrystallized from cold toluene.The productwas isolated in about20%yield.1H NMR(400MHz,25°C inCDCl3):δ0.77(-CH2CH2C H2Si-:2H,t);1.27(-Si-(OCH2-C H3)3:9H,t);1.85(-CH2C H2CH2Si-:2H,m);3.54(-C H2N-:2H,q);3.87(-Si-(OC H2CH3)3:6H,q);5.64(-N H CON H-:2H,s);8.41(-N d C H-NH-:1H,s);8.57(-N-C H d N-:1H,s);8.97(-N d CH-N H-:1H,s).13C NMR(100MHz,25°C inCDCl3):δ7.28,17.9,22.6,42.2,58.1,119.6,138.8,147.8,148.6,152.7,155.5.m/z)382.18(theoretical:382.5)Synthesis of Acrylated-and Thymine-Functionalized Poly-styrene.Hydroxy-terminated polystyrene(PS-OH)was synthesizedvia anionic polymerization of styrene initiated with sec-BuLi incyclohexane at40°C.The living anions were end-capped withethylene oxide.60,61Acrylation of PS-OH and subsequent reactionwith thymine was performed according to a literature procedure.43The PS-thymine product was characterized using1H NMRspectroscopy.PS-OH:M n)1700g/mol,M w/M n)1.08;PS-thymine:M n)2000g/mol,M w/M n)1.05. Characterization.Size-exclusion chromatography(SEC)data was obtained using a717Autosampler system equipped with three in-line5µm PLgel MIXED-C columns,a Waters2410refractive index detector operating at880nm,a Wyatt Technology miniDawn multiple-angle laser-light scattering(MALLS)detector operating at 690nm and calibrated with polystyrene standards,and a Viscotek model270differential/light-scattering dual detector.The refractive index increment(dn/d c)was calculated online.1H,13C,and29Si NMR spectroscopy were performed on a Varian UNITY spectrometer at400,100,and79.5MHz,respectively,with CDCl3as the solvent. Fast atom bombardment mass spectrometry(FAB-MS)was carried out using a Fissons Instruments VG Quattro.Substrate Treatment.Silicon wafers were cut into1cm2samplesand sonicated for5min in dichloromethane and5min in methanol.The wafers were blown with N2and cleaned with freshly preparedPiranha solution(concentrated H2SO4/H2O2;v/v:70/30)at90°Cfor1h,rinsed with milli-Q water,and cleaned with NH3/H2O2/H2O(v/v/v:1/1/5)at60°C for15min.The wafers were then rinsed withmilli-Q water several times,blown dry with N2,and immediatelyimmersed in the alkoxysilane solution and allowed to stir for a giventime.(Caution:Piranha solution reacts V iolently with many organicmaterials and should be handled with care.)Covalent Modification of Silicon Surfaces.Freshly cleanedsilicon wafers were modified with a5mM solution of(25/75;mol/mol)ADPTES/MPTES in CHCl3for6h and subsequently sonicatedthree times in CHCl3to remove any physisorbed material.Surfacemodification exclusively with MPTES and ADPTES was carried(58)Overberger,C.G.;Inaki,Y.;Nambu,Y.J.Polym.Sci.,Part A:Polym. Chem.1979,17,1759.(59)Park,J.S.;Lee,G.S.;Lee,Y.-J.;Park,Y.S.;Yoon,K.B.J.Am.Chem. Soc.2002,124,13366.(60)Quirk,R.P.;Mathers,R.T.;Wesdemiotis, C.;Arnould,M. A. Macromolecules2002,35,2912.(61)Quirk,R.P.;Pickel,D.A.;Hasegawa,H.Macromol.Symp.2005,226, 69.Scheme1.Synthesis of ADPTES with Selective Coupling tothe SecondaryAmineHydrogen Bonding for Re V ersible Polymer Adhesion Langmuir,Vol.22,No.3,20061101out under similar conditions.Wafers were characterized after each modification step using XPS and water contact angle measurements.PS -Thymine/PS -OH Treatment.The alkoxysilane-modified surfaces were immersed in a 5mM solution of PS -thymine or PS -OH in CHCl 3for 24h and exhaustively rinsed with both CHCl 3and THF.Surface Characterization.The modified silicon surfaces were sonicated in a Branson 1200ultrasonic generator for 1h.Static water contact angles were measured in sessile drop mode using an FTA-200contact angle goniometer with a syringe-driven droplet.The values were measured 30s after dispensing the drop to obtain equilibrium values.Contact angles were measured at four to five different spots on each surface.XPS was obtained on a Perkin-Elmer model 5400instrument fitted with a Mg K R X-ray source (1253.8eV)at a takeoff angle of 30°.The anode was operated at 250W.The 285eV photoelectron peak of C 1s electrons was used as an internal XPS standard.Results and DiscussionA novel adenine-containing alkoxysilane was synthesized in a single step via the reaction of adenine with IPTES (Scheme 1).1H and 13C NMR spectroscopy and FAB-MS confirmed the chemical structure of the isolated product.Characteristic 1H NMR resonances associated with the adenine groups were observed at 5.64ppm (-N H CON H -),8.41ppm (-N d C H -NH -),8.57ppm (-N -C H d N -),and 8.97ppm (-N d CH -N H -)in addition to resonances associated with triethoxysilane,which confirmed successful derivatization (Figure 1).29Si NMR spectra for both IPTES and ADPTES exhibited a single alkoxysilane Si resonance and confirmed the absence of any alkoxysilane hydrolysis (Figure 2)because any hydrolysis or condensation of the ethoxysilane group would have resulted in a pronouncedshift in the Si resonance to lower or higher field,respectively.62A three-step synthetic procedure for the synthesis of PS -thymine was adopted from our earlier literature,43and 1H NMR spectroscopy confirmed successful functionalization.Freshly cleaned silicon surfaces were modified with a mixture of ADPTES/MPTES at a molar ratio of 1:3from a CHCl 3solution.Anhydrous CHCl 3was used to eliminate both premature hydrolysis of the ethoxysilane groups and multilayer formation.63A solution concentration of 5mM was used.MPTES was used as a diluent to lower the concentration of adenine groups on the silicon surface.Dilution of the adenine groups was considered because earlier literature has shown that the exclusive presence of molecular recognition groups on solid surfaces hinders the recognition phenomenon.1,64The XPS elemental compositions of MPTES-and ADPTES/MPTES-coated silicon surfaces are shown in Table 1.It was not possible to quantify the mercapto groups on the surface because of the susceptibility of -SH groups to oxidation under ambient conditions.65Also,it was not possible to avoid oxidation of the mercapto groups despite performing reactions and storing the samples in the absence of light.The sulfur region in the XPS spectrum showed two peaks centered at 163.4and 168.3eV(62)Kelts,L.W.;Armstrong,N.J.J.Mater.Res.1989,4,423.(63)Moon,J.H.;Shin,J.W.;Kim,S.Y.;Park,ngmuir 1996,12,4621.(64)Wang,Z.-H.;Jin,G.Colloids Surf.,B 2004,34,173.(65)Liu,J.;Hlady,V.Colloids Surf.,B 1996,8,25.Figure 1.1H NMR spectrum of ADPTES in CDCl 3(2wt %solution).Figure 2.29Si NMR spectra in 16wt %CDCl 3containing 0.06M Cr(acac)3of (a)IPTES and (b)ADPTES.Table 1.XPS Elemental Composition of the MPTES and ADPTES/MPTES-Modified Silicon Surface before and afterPS -Thymine Treatmentatomic composition (%)MPTES modified b ADPTES/MPTESmodified c XPS photoeletcronpeak abinding energy (eV)before after d before after dC 1s 285.029272033O 1s 532.335373630Si 2p 102.7;99.031313528N 1s 399.945S 2p163.4;168.35554aXPS conditions:Mg anode,takeoff angle )30°.b Surface modification with MPTES:5mM MPTES in CHCl 3,6h,extracted with CHCl 3.c Surface modification with the ADPTES/MPTES mix-ture:5mM ADPTES/MPTES (1:3molar ratio)in CHCl 3,6h,extracted with CHCl 3.d Surface modification with PS:5mM PS -thymine in CHCl 3,24h,extracted with CHCl 3/THF.1102Langmuir,Vol.22,No.3,2006Viswanathan et al.attributed to -SH groups and oxidized byproduct species,respectively.65However,the latter peak at 168.3eV was predominant,which indicated that most mercapto groups were oxidized under the experimental conditions.The S 2p photo-electron peak also indicated some residual sulfuric acid that was used in the cleaning procedure;this led to further difficulty in estimating the actual concentration of the mercapto sites on the surface.Although N was not observed on silicon surfaces that were coated with MPTES,XPS analysis of the silicon surface detected N after treatment with the ADPTES/MPTES mixture,as shown in Figure 3,which indicated successful adenine functionalization.The principal N 1s core-level peak exhibited a binding energy of 399.9eV,which was similar to literature values.66The C/N ratio was calculated on the basis of the atomic compositions given in Table 1for the surface modified with the mixture before polymer deposition and were higher than expected on the basis of the chemical composition of ADPTES,which further indicated co-deposition of MPTES on the surface.However,it was not possible to determine the actual ratio of N/S on the surface because of the above-mentioned reasons.Diluent systems with distinct XPS photoelectron peaks are currently under investigation in an attempt to determine the relative ratios of diluent/adenine on the surface.However,on the basis of a C/N ratio of 3.7and the chemical composition of the alkoxysilanes,it was estimated that there was 1ADPTES for every 4.4MPTES on the surface.This ratio is higher than the theoretical ratio based on solution composition,which would ideally give a C/N ratio of 3:1,assuming complete hydrolysis of the ethoxy groups.Preferential chemisorption of the mercapto compound 67on the surface was disregarded on the basis of the following justification.For the MPTES-coated surface,the C/S ratio that was calculated from XPS was 6,which is higher than the expected ratio of 3calculated from the chemical composition of MPTES.Such a high ratio for C/S was observed for surfaces coated with mercaptopropyl trimethoxysilane and MPTES and was attributed to the presence of carbon impurities.65,68A similar trend was observed for surfaces coated with ADPTES,where the surfaces showed a C/N ratio of 2.8compared to the expected value of 1.5.Thus,the higher than expected C/N ratio from the relative concentrations of the two silanes in solution in the case of ADPTES and the mixture was attributed to the presence of carbon contamination.The atomic compositions of the MPTES-and ADPTES/MPTES-coated surfaces following PS -thymine treatment and solvent rinse are also listed in Table 1.The atomic %C on silicon surfaces that were modified with the ADPTES/MPTES mixture increased following exposure to the PS -thymine solution,whereas the MPTES-modified surface showed no change.The %O and %Si on the ADPTES/MPTES-modified surface also decreased upon exposure to PS -thymine solution,which further confirmed the hydrogen bond-mediated attachment of PS -thymine to the ADPTES/MPTES-modified surface illustrated in Figure 4.When the surface was modified only with ADPTES,XPS atomic %C did not reveal polymer attachment as shown in Table 2,which suggested the role of steric hindrance in molecular recognition in a fashion similar to the earlier literature.1,64Water contact angle measurements further supported XPS data.Static water contact angle results for MPTES-,ADPTES-,and ADPTES/MPTES-modified surfaces before and after PS -thymine treatment are summarized in Table 3.(66)Herne,T.M.;Tarlov,M.J.J.Am.Chem.Soc.1997,119,8916.(67)Heise,A.;Stamm,M.;Rauscher,M.;Duschner,H.;Menzel,H.Thin Solid Films 1998,327-329,199.(68)Hu,M.;Noda,S.;Okubo,T.;Yamaguchi,Y.;Komiyama,H.Appl.Surf.Sci.2001,181,307.Figure 3.XPS survey spectra of silicon surfaces modified with (a)MPTES and (b)ADPTES/MPTES.Figure 4.Depiction of proposed molecular recognition between an ADPTES/MPTES-modified silicon surface and a thymine-func-tionalized polymer.Table 2.XPS Elemental Composition of the ADPTES-Modified Silicon Surface before and after PS -Thymine Treatment atomic composition (%)XPSphotoelectron peak abefore b after c C 1s 2827O 1s 3233Si 2p 3131N 1s99aXPS conditions:Mg anode,takeoff angle )30°.b Surface modification with ADPTES:5mM ADPTES in CHCl 3,6h,extracted with CHCl 3.c Surface modification with PS:5mM PS -thymine in CHCl 3,24h,extracted with CHCl 3/THF.Hydrogen Bonding for Re V ersible Polymer Adhesion Langmuir,Vol.22,No.3,20061103。