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A substrate independent approach for generation of surface gradients

A substrate independent approach for generation of surface gradients
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  A substrate independent approach for generation of surface gradients Renee V. Goreham  a , Agnieszka Mierczynska  b , Madelene Pierce  b , Robert D. Short  a , Shima Taheri  a ,Akash Bachhuka  a , Alex Cavallaro  a , Louise E. Smith  a , Krasimir Vasilev  a, ⁎ a Mawson Institute, University of South Australia, Mawson Lakes 5095, Australia b Ian Wark Research Institute, University of South Australia, Mawson Lakes 5095, Australia a b s t r a c ta r t i c l e i n f o Available online 7 November 2012 Keywords: GradientsNanoparticlesPlasma polymerizationSurface modi fi cationNanotopography Recently, surface gradients have attracted signi fi cant interest for various research and technological applica-tions. In this paper, we report a facile and versatile method for generating surface gradients of immobilizednanoparticles,nanotopography and ligandsthatisindependent fromthesubstratematerial.Themethodcon-sists of   fi rst depositing a functional polymer layer on a substrate and subsequent time controlled immersionof this functionalized substrate in solution gold nanoparticles (AuNPs), silver nanoparticles (AgNPs) or poly(styrenesulfonate) (PSS). Chemical characterization by X-ray Photoelectron Spectroscopy (XPS) andmorphological analysis by Atomic Force Microscopy (AFM) show that the density of nanoparticles and theconcentration of PSS across the surface increases in a gradient manner. As expected, time of immersiondetermines the concentration of surface bound species. We also demonstrate the generation of surfacegradients of pure nanotopography. This is achieved by depositing a 5 nm thick plasma polymer layer ontopof thenumber density gradientof nanoparticlestoachieve ahomogeneous surface chemistry. Thesurfaceindependent approach forgenerationof surface gradients presented in this paper may open opportunities fora wider use of surface gradient in research and in various technologies.© 2012 Elsevier B.V. All rights reserved. 1. Introduction Over the last decade, there has been an expanding interest indevices where surface properties such as chemistry [1], wettability[2], biomolecules [3,4] and nanoparticles [5] can be controlled in a gradient manner. Such gradients are attractive in a number of   fi eldsincluding cell biology, diagnostics and sensors, catalysis, and elec-tronics [5 – 7]. Surface gradients have also been utilized to drivephysical processes, for example, the directional movement of waterdroplets  ‘ uphill ’  following a gradient of surface energy [2]. Gradientsare also important in the  fi eld of biology as in-vitro devices that allowinvestigating biological processes driven by gradients of signalingbio-molecules or extracellular matrix properties which at present areonly poorly understood; such processes are chemotaxis, immuneresponse and cancer metastasis [7,8]. For sensing applications, surfacegradients can be designed and tailored in such a fashion that a singlesample can be used to obtain multiple data points. The bene fi ts of using gradients, instead of the traditional methods which require alarge number of samples, are reduced errors caused by sample repro-ducibility and increased speed of analysis. For example, Wells et al. [9]used surface gradient of carboxyl group concentration to determinethat the degree of embryonic stem cell spreading was critical for theirself renewal [9]. Nanoparticle density gradients have been used toexamine cell adhesion. It was demonstrated that nanoscale surfaceroughness needs to be carefully considered as the number of attachingcells decreased markedly with increased nanoparticle surface density[10,11]. Arnold et al. [12] used gradients of nanoparticles to provide anchorage points for the immobilization of cell-adhesion ligands. Thissystemallowed examiningtheeffectofliganddensity oncellsignaling,polarization and migration. Chemical gradients have also been used todetermine the effect of small functional groups on kidney stem celldifferentiation [13].Various methods for generation of surface gradients of chemistry,nanoparticlesorbiomoleculeshavebeenreportedandsummarizedinseveral excellent reviews [14,15]. However, most of these methodsinvolve the use of a speci fi c substrate to facilitate generation of self assembled monolayers (SAMs) or chemical grafting. For example,SAMs of silanes require a silica substrate while thiols require a gold fi lm.Furthermore, a naturallychargedsubstrateisrequiredforgener-ation of gradient using electrostatic binding. A method for generationofchemicalgradientsthatissubstrateindependenthasbeenreportedby Whittle et al. [16]. This method has also been extended to gradi-ents of nanoparticles, ligands and proteins [17 – 19]. However, themethod requires a special piece of equipment for plasma copolymer-ization through a mask which is currently available only at theMawson Institute of the University of South Australia. Another methodforgenerationofsurfacegradientsthatrequiresspeci fi cequipmentand Thin Solid Films 528 (2013) 106 – 110 ⁎  Correspondingauthorat:MawsonInstituteandSchoolofAdvancedManufacturing,MawsonLakesCampus,UniversityofSouthAustralia,MawsonLakesSA5095,Australia.Tel.: +61 8 83025697. E-mail address: (K. Vasilev).0040-6090/$  –  see front matter © 2012 Elsevier B.V. All rights reserved. Contents lists available at SciVerse ScienceDirect Thin Solid Films  journal homepage:  is not accessible to the wide research community is through the use of micro fl uidic devices [20].The goal of this work is to develop a versatile method for genera-tion of surface gradients that can be applied to practically any type of substrate material and used for attachment of different entities suchas nanoparticles and ligands. Substrate independent approach isimportant from a prospective of using a standardized method forfabricationof surface gradientsthatcan be comparably andreproduc-ibly used by the wider scienti fi c community and also allowing gradi-ent to be deposited on diagnostic devices or sensors from differentmaterials (gold, silicon, plastic, etc.). Plasma polymerization is proba-bly the only technique that can be used to deposit a  fi lm of nanoscalethickness independently on material type or shape. It has beenrecently demonstrated that after a coating of 2 – 3 nm thickness isdeposited the further  fi lm growth is substrate independent [21 – 23].Thus, this work aims to demonstrate that a thin plasma polymer fi lm can be utilized for generation of surface gradients of immobilizednanoparticles, pure nanotopography and ligands. 2. Experimental  2.1. Materials Allyamine (AA) (98%, Aldrich), poly (vinyl sulfonate) sodium salt(PVS) (Aldrich), silver nitrate (Aldrich), sodium borohydrate(Aldrich),hydrogentetrachloroaurate(99.9985%,ProSciTech),trisodiumcitrate(99%,BHDChemicals,AustraliaPty.Ltd.),2-mercaptosuccinicacid(97%, Aldrich), and poly(styrenesulfonate) (PSS) (Aldrich) were used asreceived. For solution preparation and glassware cleaning, high puritywater was used, produced by the sequential treatments of reverseosmosis,twostagesofmixbedionexchange,twostagesofactivecarbontreatment, and a  fi nal  fi ltering step through a 0.22  μ  m  fi lter. The  fi nalconductivity was less than 0.5  μ  S/cm with a surface tension of 72.8 mN/m at 20 °C.  2.2. Plasma polymerization Plasma polymerization was carried out in a custom-built reactordescribed elsewhere using a 13.56 MHz plasma generator [16]. Depo-sitionofallylaminewascarriedoutata fl owrateof10 sccm,powerof 10 W and time of deposition of 4 min or 2 min. These conditionsresult in nitrogen-rich  fi lms with a thickness of about 15 nm, aspreviously reported. Before deposition, all substrates were cleanedby oxygen plasma for 2 min using a power of 20 W.  2.3. Synthesis of gold nanoparticles (AuNPs) AuNPs were synthesized by citrate reduction of HAuCl 4 . Particlesof ~38 nm diameter were synthesized from 150 ml of a 0.01% boilingsolution of HAuCl 4 , to which 1.5 ml of a 1% solution of sodium citratewas added under vigorous stirring [24]. The solution was left to boilfor 20 min and then allowed to cool to ambient temperature. TheAuNPs were then surface modi fi ed with 2-mercaprosuccinic acid asin Zhu et al. [25]. The particle diameters were con fi rmed via AFMimaging.  2.4. Synthesis of silver nanoparticles (AgNPs) AgNPs were synthesized as in Vasilev et al. [26]. In a typicalsynthetic procedure 0.002 M/l of AgNO 3  was dissolved in 50 ml of Milli-Q water in an Erlenmeyer  fl ask. 0.25 mg/ml of PVS was addedunder vigorous stirring. The role of PVS is to serve as a capping andstabilizing agent. After 5 min, 0.002 M/l of NaBH 4  was added drop-wise. The color of the solution changed from colorless to darkyellow-brownish within a few seconds. The synthesis was conductedat room temperature and the AgNPs colloidal solutions were sealedand stored in dark. These nanoparticles are stable for several months.  2.5. PSS solution preparation The PSS solution was used as a 0.02 M solution in Milli-Q water.The pH of the solution was adjusted to pH=2 by hydrochloric acid.  2.6. Gradients preparation The nanoparticles were gradually absorbed onto the functional-ized amine substrates by controlled immersion into the solution of targeted species (in this work AuNPs, AgNPs and PSS). The substrateswere dipped gradually with a linear motion drive (Zaber T-LSR series),usingZabersoftware.Aftertherequiredlengthwasimmersed(in this work 10 mm) the substrate was immediately removed andthoroughly washed with Milli-Q water to remove all weakly boundspecies.  2.7. X-ray photoelectron spectroscopy XPS analysis wasusedto determine thesurfacecompositionoftheplasma polymer and the gradients of deposited AuNPs, AgNPs andPSS. XPS spectra were recorded on a Specs SAGE XPS spectrometerusing Al K α  radiation source (h ν =1253.6 eV) operated at 10 kV and 20 mA. Elements present in a sample surface were identi fi edfrom the survey spectrum recorded over the energy range of 0 – 1000 eV at a pass energy of 100 eV and a resolution of 0.5 eV.Theareasunderselectedphotoelectronpeaksina widescanspectrumwere used to calculate percentage atomic concentrations (excludinghydrogen). High-energy resolution (0.1 eV) spectra were thenrecorded for pertinent photoelectron peaks at a pass energy of 20 eV to identify the possible chemical binding environments foreach element. All the binding energies (BEs) were referenced to theC1s neutral carbon peak at 285 eV, to compensate for the effect of surface charging. The XPS analysis area was circular with a diameterof 0.7 mm. The processing and curve- fi tting of the high-energy reso-lution spectra were performed using CasaXPS software.  2.8. Atomic force microscopy (AFM) An NT-MDT NTEGRA SPM atomic force microscope (AFM) wasused in non-contact mode to provide topographical images. Siliconnitride non-contact tips coated with Au on the re fl ective side(NT-MDT,NSG03)wereusedandhadresonancefrequenciesbetween65and100 kHz.Theamplitudeofoscillationwas10 nm,andthescanrate for 4  μ  m×4  μ  m images was 0.5 Hz. The scanner used had amaximum range of 100  μ  m and was calibrated using 1.5  μ  m standardgrids with a height of 22 nm. 3. Results and discussion A schematic of our experimental approach is shown in Fig. 1. Instep one the substrate is modi fi ed with amine functional groups bydepositing a thin plasma polymer  fi lm from vapor of allylamine[26]. The plasma polymer was deposited at a  fl ow rate of 10 sccm,using power of 10 W for 4 min. The conditions used in the plasmareactor for this work provide a  fi lm thickness of about 15 nm asmeasured by ellipsometry. The resultant nitrogen containing  fi lmshave been demonstrated to contain a population of primary aminesurface groups, which have been utilized in previous studies for elec-trostatic binding of heparin [27] and covalent immobilization of poly(ethylene glycol) [17].In a second step the amine functionalized surfaces are immersedin a solution of the species that are to be immobilized on the surfacewith a controlled speed. In this paper, we demonstrate electrostatic 107 R.V. Goreham et al. / Thin Solid Films 528 (2013) 106  – 110  bindingofAuNPs,AgNPsorPSS,howeverthestrategycouldbeusedinthe samemanner for covalent immobilization through an appropriatechemistry. The AuNPs are surface modi fi ed with 2-mercaptosuccinicacid[25].Thistypeofmodi fi cationprovidescarboxylicacidfunctionalgroupsonthesurfaceoftheAuNPswhichinwatercarryanetnegativecharge. The AgNPs were stabilized with a surface layer of PVS, whichalso provides a negatively charge surface. PSS is a well knownnegatively charge polyelectrolyte extensively used in layer-by-layerassemblies [28]. The strategy of immersion with controlled speed isbased on the hypothesis that the time of interaction of the modi fi edsubstrate with the solution of targeted species (either nanoparticlesor molecules) will allow control over the surface immobilizationdensity across the substrate. In other words, the longer the time of interaction, the higher the number of attached nanoparticles onmolecules.First, we  fi rst demonstrate the surface immobilization of goldnanoparticles of 38 nm diameter in a number density gradientmanner.Theallyaminemodi fi edsubstrateisimmersedinthesolutionof nanoparticles with a speed of 1.67 mm/h. In this experiment, theimmersion speed corresponds to a surface gradient of 10 mm inlength and 6 h of immersion time for the end of the substrate thatinteracted for the longest time with the solution of nanoparticles.Fig. 2 shows that gold atomic concentration across the surface asdetermined from XPS measurements. The gradient spans betweenposition 1 mm and position 11 mm across the surface as position11 mm was immersed for the longest time of 6 h and position 1 mmhad practically no contact with the solution of nanoparticles. Asexpected, the gold atomic percentage increased from position 1 mm(0 at.%) to just above 9 at.% at position 11 mm (squares). Since thespeedofimmersionisconstant,theresultantgradientisalmostlinear.However, the  ‘ as prepared ’  number density gradient of nano-particles would not be of particular use if one wants to utilize it toexamine the effect of surface nanotopography on any physical or bio-logical phenomenon because in addition to the variation of the lateralspacing and number of the nanoparticles across the surface, there isalso a variation in the surface chemistry. Such errors have been madein the past. For example in Ref. [10] the authors used amine surfacesto generate gradient of adsorbed silica nanospheres and then usedthese gradient to examine the effect of surface nanotopography oncell adhesion. Since chemistry and topography both vary across thegradient, an obvious question to be asked is  “ what do cells  ‘ sense ’ — chemistry or topography? ” . In order to convert the number densitygradient of nanoparticles to gradient of pure surface nanotopography,we deposited on top of the nanoparticles another layer of allylaminebased plasma polymer (referred as AApp later) of 5 nm thickness. Aswe have already reported [23], this thickness is suf  fi cient to generateacontinuousplasmapolymer fi lm.Thisthicknessalsoallowspreservingthemagnitudeofsurfacenanotopography.ThecirclesinFig.2showthegold atomic concentration measured by XPS after deposition of theadditional AApp  fi lm. Gold signal is still detectable because the XPSsampling depth of about 10 nm is larger than the thickness of theAApp overlayer. However, the atomic percentage of gold is largelyreduced.To visualize the nanoparticle surface density, we imaged  fi vepoints across the gradient by AFM. Fig. 3 shows representative AFMimagesatpositions1,3,6,9,and11 mmacrossthegradient.Inagree-ment with the XPS data, the number and density of AuNPs increasesfrom position 1 mm to position 11 mm. In addition, the nanoparticlesare adsorbed as individual entities and no aggregations are present.The images shown in Fig. 3 present the gradient after deposition of an AApp  fi lm. The cross section pro fi les, visualizing the height of thenanoparticles before and after additional  fi lm deposition, are shownin the supporting material and clearly show that this 5 nm thickoverlayer does not alter the nanotopography height.In order to demonstrate the importance of preserving homoge-neous chemistry when studying the effect of surface nanotopographyon various phenomena we characterized the wettability across thenanoparticle gradients ( ‘ as prepared ’  and after additional AApp fi lm) by sessile drop water contact angle measurements. The resultsare shown in Fig. 4. The contact angle on the pure AApp end of thesurface is 68° which is consistent with earlier studies [17,19]. The ‘ as prepared ’  nanoparticle gradient shows a decrease in the contactangle towards the highest nanoparticle concentration (circles).Although one would expect the contact angle to increase with theincreasing scale of nanotopography, the contact angle reduction inthis particular case is not surprising because, as stressed above, the Fig. 1.  Schematic representation of the strategy for preparing surface gradients.(1) Functionalization of a substrate by plasma polymerization; (2) the functionalizedsubstrate is immersed intoasolution ofeither AuNPs,AgNPsor PSS and (3) anticipatedformation of a gradients of nanoparticle or ligand density by electrostatic adsorption. 0 2 4 6 8 10 120246810    A  u   C  o  n  c  e  n   t  r  a   t   i  o  n   (   A   t   %   ) Position on gradient (mm) Fig. 2.  Gold atomic percentage obtained from the XPS measurements across the aminemodi fi ed surface after immobilization of 38 nm diameter gold nanoparticles AuNPs(squares) and after adding a~5 nm allylamine plasma polymer coating (circles).Error bars are standard deviation of three measurements.108  R.V. Goreham et al. / Thin Solid Films 528 (2013) 106  – 110  surface chemistry has also changed. The nanoparticles are surfacemodi fi ed with carboxyl acid groups which have a lower contractangle than the AApp surfaces. The change in surface chemistrycould also be due to the presence of carboxyl species remainingfrom the AuNP solution (i.e. unreacted citrate used during the AuNPsynthesis) which could also have adsorbed to the surface. Once theadditional AApp overlayer is deposited, the wettability trend isreversed. Since the surface chemistry is homogeneous, the contactangle is only in fl uenced by the surface nanotopography. The squaresin Fig. 4 show that the contact angle increases with increasing thenanoparticle density across the surface.This method for generation of number density gradients of nanoparticles can be easily extended to other types of nanoparticles.For example, AgNPs (shown in Fig. 5) capped with sulfonic acidgroups [26] were used in the same fashion as the AuNPs to producea nanoparticle density gradient. XPS analysis shows that the concen-tration of Ag increases from the 1 mm to the 11 mm position acrossthe gradient. The sulfonic acid groups passivating the AgNPs carry anegative charge in water and as in the case of AuNPs leads to a largernumber of nanoparticles adsorbed onto the end of the substrate thatwas exposed to the solution for the longest time, which in this casewas 12 h.This method for generation of surface gradients is applicablenot only to nanoparticles but also to polyelectrolytes which prac-tically results in a chemical gradient across the substrate. Chemi-cal gradients have become important to understand essentialbiological processes such as cell adhesion, proliferation, differenti-ation and migration [9,13,29]. We used PSS as a demonstration forthe generation of such chemical gradients. At the PSS solutionconcentration used in this example, the adsorption of the poly-electrolyte to the amine surface is very fast so our immersionspeed was 2 mm/min. In other words, the end of the surface thathad longest immersion time interacted with the PSS solution for5 min. The atomic concentration of sulfur across the surface deter-mined from XPS measurements is shown in Fig. 6. As expected,the electrostatic binding of the negatively charged sulfonic acidgroups to the positively charged amine groups is time dependent.The sulfur atomic concentration increased linearly from 0.8 at.%position at 1 mm to 9.4 at.% at position 11 mm. The small sulfurconcentration at position 1 mm is probably due to inaccuracy of the dipping stage alignment or small displacement during theXPS measurements.The use of plasma deposition to provide a surface chemical gradi-entmakestheapproachreportedinthispaperapplicabletopracticallyanytypeofsubstrate.Furthermore,themethodusedforgenerationof amine functionalized surfaces can be extended to generation of othersurface functionalities, such as carboxyl acid, epoxy or aldehydegroups. Negatively charged carboxyl acid groups can be used for elec-trostatic binding of positively charged nanoparticles or polyelectro-lytes. Carboxyl acid, epoxy and aldehyde groups are all well suitedfor covalent immobilization of various species such as proteins,ligands or nanoparticles. In addition, the length of the gradient canbe easily controlled for the requirement of a particular application. 4. Conclusions In summary, we have developed a versatile method for the genera-tion of surface gradients of nanoparticle density, pure nanotopographyandchemicalligands.Themethodconsistsoftwosteps; fi rstthegener-ation amine functionalized substrates via plasma deposition andsecond, the subsequent time dependent electrostatic adsorption of surfacefunctionalized,negativelychargednanoparticlesorpolyelectro-lytes to an amine functionalized surface which contain a population of primary amines that readily protonate in aqueous solution at pH 7. Fig. 3.  AFM micrographs across the surface of the nanoparticle gradient at positions 1, 3, 6, 9 and 11 mm after applying an additional thin  fi lm of AApp. 135791140455055606570758085    C  o  n   t  a  c   t   A  n  g   l  e   (   d  e  g  r  e  e   ) Position on gradient (mm) Fig. 4.  Sessile drop static contact angle across the  ‘ as prepared ’  nanoparticle densitygradient (circles) and after addition of a 5 nm thick AApp  fi lm (squares). 0 2 4 6 8 10 12-    A  g  c  o  n  c  e  n   t  r  a   t   i  o  n   (   A   t   %   ) Position on gradient (mm) Fig. 5.  Silver atomic percentage obtained from XPS measurements of polyvinyl-sulfonate-stabilized silver nanoparticles adsorbed accross the amine functionalisedsurface. Error bars are standard deviation of three measurements.109 R.V. Goreham et al. / Thin Solid Films 528 (2013) 106  – 110  Chemical characterization by XPS showed that the atomic surfaceconcentration of targeted species (i.e. Ag, Au or S) increases in a linearfashion across the gradients. AFM imaging showed an increase in thenumber and density of adsorbed nanoparticles towards the end of thegradientthatwasexposedforlongesttimetothenanoparticlesolution.Importantly, the nanoparticles adsorb as separate entities withoutaggregation.Wealsodemonstratedthemethodisapplicableforgener-ation of surface concentration gradients of other species such as poly-electrolytes. We also showed that the number density gradients of nanoparticles can be extended to gradients of pure nanotopographythrough the deposition of an additional plasma polymer overlayer.The fact that the method for generation of surface gradients of nanoparticles developed in this work is surface independent, mayopen opportunities for a wider use of surface gradient in research andin various technologies.  Acknowledgments KV thanks ARC for the support through fellowship FT100100292.  Appendix A. Supplementary data Supplementary data to this article can be found online at http:// References [1] L. Tauk, A.P. Schroeder, G. Decher, N. Giuseppone, Nat. Chem. 1 (9) (2009) 7.[2] G. Fang, W.T. Li, X. Wang, G. Qiao, Langmuir 24 (20) (2008) 9.[3] S. Morgenthaler, C. Zinc, B. Stadlerm, J. voros, S. Lee, N.D. Spencer, S.G.P. Tosatti,Biointerphases 1 (4) (2006) 9.[4] K.Vasilev,Z.Poh,K.Kant,J.Chan,A.Michelmore,D.Losic,Biomaterials31(2010)9.[5] R.R. Bhat, D.A. Fischer, J. 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