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Structural Characterization of Self-Assembled Polypeptide Films on Titanium and Glass Surfaces by Atomic Force Microscopy

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Structural Characterization of Self-Assembled Polypeptide Films on Titanium and Glass Surfaces by Atomic Force Microscopy
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   Physical Chemistry Chemical Physics   This paper is published as part of a PCCP Themed Issue on: Interfacial Systems Chemistry: Out of the Vacuum, Through the Liquid, Into the Cell  Guest Editors: Professor Armin Gölzhäuser (Bielefeld) & Professor Christof Wöll (Karlsruhe) Editorial Interfacial systems chemistry: out of the vacuum—through the liquid—into the cell   Phys. Chem. Chem. Phys. , 2010 DOI:  10.1039/c004746p  Perspective The role of inert surface chemistry in marine biofouling prevention   Axel Rosenhahn, Sören Schilp, Hans Jürgen Kreuzer and Michael Grunze, Phys. Chem. Chem. Phys. , 2010 DOI:  10.1039/c001968m  Communication Self-assembled monolayers of polar molecules on Au(111) surfaces: distributing the dipoles  David A. Egger, Ferdinand Rissner, Gerold M. Rangger, Oliver T. Hofmann, Lukas Wittwer, Georg Heimel and Egbert Zojer, Phys. Chem. Chem. 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Phys. , 2010 DOI:  10.1039/c000304m   Structural characterization of self-assembled monolayersof pyridine-terminated thiolates on gold Jinxuan Liu, a Bjo ¨rn Schu ¨pbach, b Asif Bashir, c Osama Shekhah, d  Alexei Nefedov, d  Martin Kind, b Andreas Terfort* b and Christof Wo ¨ll* d  Received 17th November 2009, Accepted 8th February 2010First published as an Advance Article on the web 4th March 2010 DOI: 10.1039/b924246p Self-assembled monolayers (SAMs) fabricated on Au(111) substrates from a homologous seriesof pyridine-terminated organothiols have been investigated using ultra high vacuum infraredreflection adsorption spectroscopy (UHV-IRRAS), X-ray photoelectron spectroscopy (XPS),scanning tunnelling microscopy (STM) and near-edge X-ray absorption fine structure (NEXAFS)spectroscopy. A total of 4 different pyridine-based organothiols have been investigated, consistingof a pyridine unit, one or two phenyl units, a spacer of between one and three methylene unitsand, finally, a thiol unit. For all pyridine-terminated thiols the immersion of Au-substrates in thecorresponding ethanolic solutions was found to result in the formation of highly ordered anddensely packed SAMs. For an even number of the methylene spacers between the SH group andthe aromatic moieties, the SAM unit-cell is rather large,  ð 5  ffiffiffi  3 p    3 Þ rect, whereas in case of an oddnumber of methylene units a smaller unit cell is adopted,  ð 2  ffiffiffi  3 p    ffiffiffi  3 p  Þ R 30  . The tilt angle of themolecules amounts to 15 1 . In contrast to expectation, the pyridine-terminated organic surfacesexposed by the corresponding SAMs showed a surprisingly strong resistance with regard toprotonation. Introduction Although self-assembled monolayers (SAMs) preparedfrom thiols on gold have been investigated for more thantwenty-five years, this field is still developing quickly becauseof the diversity and numerous potential applications of theseorganic thin films. 1–6 Whereas in earlier work on SAMs thefocus was on  n -alkanethiolates on gold to unravel funda-mental aspects of film formation, structure and properties 1,3 in later years aromatic thiolates have attracted an increasingamount of attention. This interest results from the higherrigidity of the molecular backbones which in many cases haveallowed for a better control of the monolayer structure. 7–14 Today, SAMs gain an increasing importance with regardto the generation of organic surfaces exposing predefinedfunctionalities. 15 Attaching an appropriate function,  e.g.  –COOH, 16–19  –SH, 20–23  –NH 224–26 or –OH 18,27 at the o -position of the organothiol allows to tailor the wettabilityand the reactivity of the organic surfaces exposed by theSAMs, which have numerous potential applications inmolecular electronics, 28–31 electrochemistry, 32–36 or bio-chemistry. 37–42 A particularly exciting new field of SAMapplication is interface-based supramolecular chemistry,where organic monolayers are used as substrates to anchorand grow highly complex materials like metal–organic frame-works (MOFs). 43–51 Pyridine-terminated SAMs represent aparticularly interesting type of organic substrates. Thenitrogen lone pair electrons of the pyridine unit exposed atthe surface can act as Lewis base. This functionality has  e.g. been used for the complexation of Pd salts, which were thenreduced electrochemically to yield metallic Pd particles. 52,53 Pyridine-terminated surfaces have also been used to enhancethe rate of heterogeneous electron transfer between electrodesand the solution phase of biological species. 54 Recentstudies have revealed that the chemical activity of pyridine-terminated SAMs,  e.g.  with regard to protonation 55,56 or interaction with water 57 is quite complicated. Theproperties cannot be predicted in a straightforward fashionfrom the properties of pyridine in solution, the specialproperties of pyridine-terminated surfaces clearly need furtherinvestigations.The first studies of pyridine-terminated SAMs were carriedout using 4-mercapto-pyridine. 52,54,58,59 More recently, SAMsformed from other pyridine-functionalized thiols or disulfideshave been investigated 55–57 to improve the understanding of factors that influence film growth, the reactivity of this class of monolayers and their applicability as anchoring layer for  e.g. ,metal–organic frameworks. 46,50 Here, we present a compre-hensive study of SAMs prepared from a series of four relatedpyridine-terminated thiols with backbones comprising of bothaliphatic and aromatic parts. Fig. 1 shows schematic drawingsof SAMs formed from (4-(4-pyridyl)phenyl)methanethiol(PP1), (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol (PPP1),2-(4-(4-(4-pyridyl)phenyl)phenyl)ethanethiol (PPP2) and 3-(4-(4-(4-pyridyl)phenyl)phenyl)propanethiol (PPP3) on Au(111). a Lehrstuhl fu ¨ r Physikalische Chemie I, Ruhr-Universita ¨ t Bochum,44780 Bochum, Germany b Institut fu ¨ r Anorganische und Analytische Chemie,Goethe-Universita ¨ t Frankfurt am Main, 60325 Frankfurt, Germany c Interface Chemistry and Surface Engineering, Max-Planck-Institut fu ¨ r Eisenforschung, 40237 Du ¨ sseldorf, Germany d  Institute of Functional Interfaces, Karlsruhe Institute of Technology,74800 Karlsruhe, Germany This journal is   c  the Owner Societies 2010  Phys. Chem. Chem. Phys.,  2010,  12 , 4459–4472  |  4459 PAPER www.rsc.org/pccp  |  Physical Chemistry Chemical Physics  These films were thoroughly characterized employing a varietyof surface-analytical techniques. Experimental Synthesis of pyridine-terminated Organothiols The pyridine-terminated organothiols were obtained employinga newly established synthesis route which will be describedelsewhere. 60 Preparation of the self-assembled monolayers The SAMs made from PP1, PPP1, PPP2 and PPP3 wereprepared by immersing Au substrates into 20  m M ethanolicsolutions of the corresponding pyridine-terminated organo-thiols for 20–24 h. After removal of the samples from solution,they were rinsed with ethanol and dried in a stream of N 2 .For spectroscopicstudies(UHV-IRRAS,XPSandNEXAFS),the following method to obtain substrates with Au(111)surfaces was employed: a 150 nm gold (Chempur, 99.99%)layer at a rate of 1 nm s  1 was deposited onto a Si(100) wafer(Wacker). An 8 nm titanium (Chempur, 99.8%) layer wasdeposited at a rate of 0.15 nm s  1 as an adhesion layer betweenthe Si substrate and the Au layer. Metal deposition was carriedout using a commercial vaporisator (Leybold Univex 300).The deposition rate was monitored using a quartz crystalmicrobalance.For the STM measurements, freshly cleaved mica sheets(Mahlwerk Neubauer-Friedrich Geffers) were heated to280  1 C for about two days inside the evaporation chamberto remove residual water and other contaminations from theambient. Subsequently, a 140 nm gold layer (99.995%Chempur) was deposited by thermal evaporation at a sub-strate temperature of 280  1 C and a pressure of  B 10  7 mbarusing the above-mentioned vaporisator. The substrate wascooled down to room temperature in the evaporation chamberafter deposition and was then flame annealed using a butane– oxygen flame directly before SAM preparation. Using thisprocedure, Au substrates with well-defined terraces exhibitinga (111) surface orientation are obtained routinely. 61,62 For the protonation experiments, freshly prepared Au/Sisubstrates covered with pyridine-terminated SAMs wereimmersed into (1) 0.5 M sulfuric acid (H 2 SO 4 ) aqueoussolution for about 40 min, followed by rinsing with dimethyl-formamide and drying with N 2  before characterization by IRspectroscopyat ambient conditions (2) 10 mM trifluoromethane-sulfonic acid (TfOH) solution in a 9:1 mixture of CCl 4  andCH 3 CN as solvent for about 5 min. Subsequently the sampleswere stored in the load lock chamber of a UHV apparatus(see below) and kept for about one hour at a pressure of  B 10  7 mbar before recording of IR spectra under UHVconditions. Infrared (IR) spectroscopy Bulk spectra of the organothiols investigated here for KBrpellets were obtained using a dry-air purged BioRad ExcaliburFTS-3000 FTIR-spectrometer equipped with a DTGSdetector. IRRA spectra of the SAMs were recorded with aUHV apparatus (Prevac) with an attached FTIR spectro-meter (Bruker VERTEX 80v) which has been describedelsewhere. 63–66 The base pressure of the measurement chamberamounted to 2  10  10 mbar. All spectra were acquired with aresolution of 2 cm  1 . Some additional IRRA spectra weretaken under ambient conditions with the BioRad spectrometer.All IRRA spectra were recorded in grazing incidencereflection mode at an angle of incidence amounting to 80 1 relative to the surface normal using liquid nitrogen cooledmercury cadmium telluride (MCT) narrow band detectors.Perdeuterated hexadecanethiol-SAMs on Au/Si were used forreference measurements. XPS The X-ray photoelectron spectroscopy (XPS) measurementswere performed in a UHV apparatus based on a modifiedLeybold XPS system with a double-anode X-ray source. Forthe measurements reported here, an Al K a  X-ray source withan energy resolution of about 0.8 eV was used at normalincidence. The base pressure of the apparatus was below3    10  10 mbar. The energy scales of all spectra werereferenced to the Au 4f  7/2  peak located at a binding energy Fig. 1  Schematic drawings of the pyridine-terminated SAMs. 4460  |  Phys. Chem. Chem. Phys.,  2010,  12 , 4459–4472 This journal is   c  the Owner Societies 2010  of 84.0 eV. To estimate the layer thickness of the investigatedSAMs, they were mounted on the sample holder together witha reference sample covered with a SAM of a known thickness(n-decanethiolate SAM). By this means, for both pyridine-terminated samples and reference sample the geometricconditions ( i.e. , distance and angles of X-ray gun and energyanalyzer toward the sample) were identical. STM STM micrographs were recorded under ambient conditionsemploying a Jeol JSPM 4210 microscope using tips preparedmechanically by cutting a 0.25 mm Pt/Ir (80:20) wire(Goodfellow). The tunneling current with respect to thesample varied from 0.1 to 0.4 nA and the sample bias from  200 to 600 mV. No tip-induced changes were observed forthese tunneling conditions. NEXAFS The NEXAFS measurements were performed at the dipolebeamline HE-SGM of the synchrotron storage ring BESSY IIin Berlin (Germany). All NEXAFS measurements werecarried out with linearly polarized radiation (polarizationfactor  P E 82% 67 ) with an energy resolution of better than350 meV. NEXAFS spectra were recorded at the C K-edgeand the N K-edge in the partial electron yield mode with aretarding voltage of    150 V at the C K-edge and   250 V atthe N K-edge, respectively. In the partial electron yield mode,retarding potentials are applied to assure that only near-surface electrons are detected. 91 The NEXAFS raw data werenormalized in a multi step procedure by considering theincident photon flux, which was monitored by the photo-current on the gold grid, and using the background signal of the clean Au substrate. A carbon contamination of a gold gridwith a characteristic peak at 284.81 eV was registeredsimultaneously with each spectrum and served as a referencefor photon energy calibration. To determine the molecularorientation from the linear dichroism, spectra were recordedfor 5 different incidence-angles  y  of the synchrotron radiation( y  = 20, 30, 55, 70 and 90 1  with respect to the surface). Theoretical calculations of IR and NEXAFS spectra Theoretical values of the vibrational frequencies of the isolatedmolecules have been performed by employing quantum-chemical DFT calculations using the Gaussian 03 programpackage. 68 The employed approach (functional, basis sets) wasthe same as used in a previous publication 55 on a relatedsystem (B3LYP/cc pvDZ). The computed IR-frequencieshave been scaled with the same factor of 0.967. 55 Thecomputational results were used to aid the assignment of thevibrational bands and to estimate the directions of the corres-ponding transition dipole moments (TDMs).In order to provide a reliable basis for the assignment of thefeatures in the experimental NEXAFS data, in particular withregard to understanding the srcin of the splitting in the C 1s p * resonance, and in order to gain more insight into theconformation of the chemisorbed molecules within the SAMs,a series of calculations with the quantum chemistry programpackage StoBe 69 were carried out. StoBe can deal with ratherlarge molecules and clusters and has specific implementationsto reliably describe inner-shell spectroscopies. 70,71 Results IR spectroscopy Like other organic substrates with a reactive termination( e.g. , OH, 27 COOH 16 ), the surfaces of pyridine-terminatedSAMs are prone to adsorption of water 57 at ambientconditions, which might give rise to protonation. 55 To avoidsuch contaminations of the organic surfaces exposed by theSAMs by water and other molecules at ambient conditions,the acquisition of IR-spectra for the SAMs studied here hasbeen carried out under ultra high vacuum conditions using theUHV-IR apparatus mentioned above. No further annealingwas applied after transfer of the samples into UHV.Fig. 2–5 display the UHV-IRRA spectra recorded for PP1-,PPP1-, PPP2-, and PPP3-SAMs (panels a) together withadditional bulk IR spectra recorded for KBr pellets (panels b)and the results of   ab initio  calculations (panels c). The assign-ment of the vibrational features as listed in Table 1was carried out using these theoretical results as well asassignments provided in previous work. 72–74 Generally, a verysatisfying agreement between the theoretical and experimentalband positions is observed. Comparison of the SAM spectraand the bulk (KBr) spectra confirms that organic thin layers of PP1, PPP1, PPP2, and PPP3 have formed upon immersion of the Au-substrates into the respective ethanolic solutions. AllIR-bands observed for the SAMs also appear in the respectivebulk spectra. Some IR bands, however, are only present in theKBr data ( e.g.  the SH stretching mode  6  at about 2570 cm  1 )or are markedly attenuated in the UHV-IRRA spectra withregard to the bulk data. While the disappearance of the SHstretching mode can be explained with the cleavage of the SHbond (and subsequent formation of a S–Au bond), theattenuation of several other bands result from the so-calledsurface selection rule governing IR-spectroscopy onmetals. 75,76 According to this rule, vibrational modes with a Fig. 2  Experimental and calculated spectra of PP1-species. Panel a:UHV-IRRA spectrum of the PP1-SAM, panel b: bulk spectrum of PP1 taken from KBr pellets, panel c: calculated spectra of the isolatedPP1 molecule. Calculated spectra are given in arbitrary units of absorption. This journal is   c  the Owner Societies 2010  Phys. Chem. Chem. Phys.,  2010,  12 , 4459–4472  |  4461
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