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François OZANAM Laboratoire de Physique de la Matière Condensée,

Quantitative methods for controlling the amount of grafted molecules. François OZANAM Laboratoire de Physique de la Matière Condensée, Ecole polytechnique, 91128 Palaiseau,France francois.ozanam@polytechnique.fr. NaS-ERA workshop – Algiers 21-23 May 2012. -.

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François OZANAM Laboratoire de Physique de la Matière Condensée,

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  1. Quantitative methods for controlling the amount of grafted molecules François OZANAM Laboratoire de Physique de la Matière Condensée, Ecole polytechnique, 91128 Palaiseau,France francois.ozanam@polytechnique.fr NaS-ERA workshop – Algiers 21-23 May 2012

  2. - is not able to distinguish related but distinct configurations - overlooks critical surface concentrations required to efficiently achieve a given function COOH COOH - does not allow for proper understanding of surface reactions COOH COOH COOH COOH H H H H Si Si Si Si Si Si Si Si WHICH NEED TO BE QUANTITATIVE? Qualitative identification of species grafted at the surface is needed, but e.g., vs. e.g., minimum concentration required for surface protection or resistance against non specific adsorption, or maximum concentration for bulky bioprobe surface immobilisation e.g., surface acid titration, or multi-step molecular layer modifications Therefore, quantification is an effort often cumbersome, but required for accessing the full potential of functional molecular layers

  3. OUTLINE Electrochemical method for quantitative evaluation of grafted monolayers Methods for quantitative evaluation of grafted monolayers using XPS Methods for quantitative evaluation of grafted monolayers using FTIR Pushing quantitative evaluation to boundaries: the case study of acid surfaces

  4. + + e- Quantitative characterization of monolayers grafted on silicon using an electrochemical probe A reversible redox marker can be quantitatively dosed through reduction/oxidation cycles Ferrocene modified monolayer in acetonitrile + TBAP the marker can be part of the layer … 1 V/s 0.6 V/s 0.4 V/s 0.2 V/s 0.1 V/s NHS/EDC Voltammetric wave integration … or attached to it monolayer coverage B. Fabre and F. Hauquier, JPCB 110 (2006) 6848

  5. Checking the origin of the electrochemical response Surface bound species : Voltammetric peaks linearly shift with scan rate Higher scan rates yield broader peaks but coulombic charge remains constant

  6. Practical determination of the marker surface coverage 1.9  1014cm-2 2.0  1014cm-2 0.33  1014cm-2 0.16  1014cm-2 In principle, a high sensitivity can be achieved Practically, other contributions (e.g. substrate contribution associated with monolayer defects) limit the accuracy S. Ciampi et al., Langmuir 25 (2009) 2530

  7. Electrochemical method for quantitative evaluation of grafted monolayers EN RÉSUMÉ : • - Simple and accurate method • Possible substrate interferences in the presence of defects • Easy check of the consistency • - Generally needs the attachment of a specific probe  not performed on the final target layer Quantitative evaluation of grafted monolayers using XPS

  8. Si2p C1s Characterization of monolayers grafted on silicon using XPS Decyl layer Si-C bonding with the substrate no substrate oxidation low contamination level no easy surface bonding signature

  9. Quantitative analysis of XPS spectra of grafted monolayers DIFFR- ACTION ! Diffraction on "perfect"   substrates Average on all values of f is required X. Wallart et al., JACS 127 (2005) 7871

  10. Quantitative determination of the ML coverage from the Si2p signal X hn organic layer q Si experimental determination of d / lML Monolater coverage ?

  11. Extracting coverage from the monolayer thickness (empirically determined on SAM/Au) Compare with "perfect" SAMs on Au Commercially available molecular modelling 11.8 Å Shorter chains are slightly more densely packed X. Wallart et al., JACS 127 (2005) 7871

  12. Quantitative determination of the ML coverage from the C1s signal Use a reference (HOPG) sample to quantitatively determine carbon amount in the layer with direct determination of C density Alternative approach: use the angular dependence (as for Si2p) d / lML Slope More convenient plot

  13. Quantitative determination of the ML coverage from the C1s signal Use a reference (HOPG) sample to quantitatively determine carbon amount in the layer with direct determination of C density Alternative approach: use the angular dependence (as for Si2p) d / lML Slope Consistent results ! X. Wallart et al., JACS 127 (2005) 7871

  14. C1s Quantitative determination of the ML coverage from the C1s lineshape "surface" "bulk" Properly take into account the attenuation of the two components d* = dCHAIN-dSi-C 9.9 Å q d X. Wallart et al., JACS 127 (2005) 7871

  15. Quantitative evaluation of grafted monolayers using XPS EN RÉSUMÉ : • - Several (consistent) methods • Needs to average on azimuthal angles to be quantitative • C fine structure  needs minimum contamination • Most methods actually determine the layer thickness  needs to convert in a coverage by empirical comparison with SAMS/Au Quantitative evaluation of grafted monolayers using FTIR

  16. 1.0 H-Si Absorbance ln(I/I0)  1000 0.0 per reflection C=O C-OH CH2 CH2 -1.0 1000 1500 2000 2500 3000 3500 Wavenumber (cm-1) Characterization of monolayers grafted on silicon using ATR-FTIR COOH COOH COOH H H H Si Si Si Si Si Si Si Si Si Si Si Si E Si d ABSORBANCE per reflection: Z IR electric field profile I after grafting I0 before grafting

  17. Si Si IR IR Calibration of infrared intensities D d l : a few µm f 45° d : a few 100 nm experiment : d << d calibration : D >> d • unknown molecule concentration • known IR intensity • known model molecule concentration • integrate according to IR intensity A. Faucheux et al., Langmuir 22 (2006) 153

  18. IR electric field intensity at an interface IR z Si x y air or liquid Fresnel reflection coefficients

  19. 1 1 d d d Molecular layer absorption grafted molecule absorption IR z Si y x molecular layer d air 3-layer problem : Fresnel equations d << l (linearisation) and constant electric field Y. Chabal, Surf. Sci. Rep. 8 (1988) 211

  20. 1 1 d d Dissolved molecule absorption IR dissolved molecule absorption z Si y x d liquid with dissolved molecules Quantitative expression : Fresnel equations k is measured

  21. Volumic density of independent vibrational degree of freedom Imaginary part of the dielectric constant d d d d Relating the absorption of dissolved and grafted molecules Assumptions on molecular layer : homogeneous and isotropic dielectric medium no preferential absorption direction in the layer plane Choice of the liquid for calibration : Comparison experiment / calibration : along z direction in the layer along x or y direction in the layer along any direction in the liquid imaginary part of the liquid dielectric constant and A. Faucheux et al., Langmuir 22 (2006) 153

  22. ref Practical hints N needs to be accurately known ABSORBANCE per reflection: L IR beam from spectrometer to detector silicon e q f = q High value of Si refractive index: Beam aperture does not matter Incidence angle is constrained to be q, the bevel angle on the beam-entrance side qneeds to be accurately measured for each prism (typ. +/- 0.5°) together with e and L

  23. D D’ < D IR spectroscopy at a liquid/solid interface for the calibration D ABSORBANCE per reflection: I liquid with dissolved molecules I0 neat liquid Usual flaws: Bubbles shield part of the surface Vertical adjustment of the IR beam

  24. Calibration procedure absorbance of 1 monolayer ~ absorbance of a 10 mM solution Perform the measurement at various concentrations Sodium pentanoate absorbance at various concentrations Linear plot: no solubility problems Plot goes through the origin: no sizeable adsorption at the prism surface Similar test molecules give the same results

  25. One vibration in a composite band Alkyl CHs Linear baseline + 5 Voigt profiles Frequency, Width, Intensity, Integr. Intens., shape 2854 cm-1, 14.6 cm-1 ,0.076, 1.6 cm-1, 66% Lorentz 2873 cm-1, 13.5 cm-1 ,0.020, 0.42 cm-1, 100% Lorentz 2898 cm-1, 17.8 cm-1 ,0.012, 0.23 cm-1, 0% Lorentz 2923 cm-1, 22.2 cm-1 ,0.14, 4.9 cm-1, 97% Lorentz 2959 cm-1, 15.8 cm-1 ,0.041, 0.80 cm-1, 33% Lorentz Check the absence of unphysical broadening (fit artefact) Choose the most intense AND isolated peak Practical determination of infrared intensities Simple case: one isolated vibration Ester CO Linear baseline + 1 Voigt profile Frequency, Width, Intensity, Integr. Intens., shape 1743 cm-1, 9.7 cm-1 ,0.0024, 0.0036 cm-1, 92% Lorentz Use integrated intensity (insensitive to broadening, equations also hold)

  26. Suc palmytate in THF * Grafted suc ester * acid band at 1717 cm-1(not shown) also included in the fit Iteration ensures natural linewidth constant Simultaneous analysis of the infrared of several lines A. Moraillon et al., J. Phys. Chem. C 112 (2008) 7158

  27. CH2=CH-(CH2)p-2-(OCH2CH2)-OCH3 A0 = A × (2n+2) + B × (p-3) In solution After grafting Case of overlapping bands Take the integrated absorption of the whole band

  28. Quantitative evaluation of grafted monolayers using FTIR EN RÉSUMÉ : • - No "surface selection rule" in ATR • Needs to record both s and p polarization • Needs calibration against a model molecule in solution • Direct determination of surface coverage • Some pending questions regarding the dielectric environment of the vibrators • Practical determination of IR intensities  specific procedures to yield reliable results Pushing quantitative evaluation to limits: the case study of acid surfaces

  29. 0.4 3 total 0.3 2 acid AREAL CONCENTRATION OF GRAFTED SPECIES (1014 cm-2) 0.2 chains COVERAGES n/n0 1 0.1 decyl chains 0 0 0 0.5 1 ACID FRACTION IN SOLUTION A typical application of quantitative IR spectroscopy Absolute values from the calibration of C=O and CH absorption Monolayers grafted from undecylenic acid / decene mixtures IR determination of acid stoichiometry and concentration at the surface Acid preferentially grafts to silicon surface Acid incorporation somewhat lowers the layer density A. Faucheux et al., Langmuir 22 (2006) 153

  30. in-situ IR titration (100 % acid monolayer) parallel COOH decrease and COO- increase upon increasing pH

  31. shift and spreading of the titration acid ionization difficult quantitative analysis of IR signals conversion of COOH into COO- is reversible but not total up to pH = 11

  32. microscopic segregation and domain formation spreading even at low coverage acid group dispersion vs spreading of the titration Electrostatic interactions impede the ionization of acid groups BUT spreading similar at high and low dilution D. Aureau et al., Langmuir 24 (2008) 9440

  33. cooperative dipoles G. D. Mahan and A. A. Lucas, J. Chem. Phys. 1978 M. Scheffler, Surf. Sci. 1979 Dn Dn counteracting counteracting + cooperative probing molecular layer organization dipole coupling between vibrators at a surface counteracting dipoles

  34. - - Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si 70° HO HO HO O O O O O C C C Still sizeable H H H sCO asCO orientation of acid groups

  35. direct grafting: segregation and domain formation Acid layers through ester hydrolysis: homogeneous dispersion Organization in acid layers grafted acids grafted esters after hydrolysis Simulation: no measurable effect for island radius > 10 lattice sites Domain size > 10 nm

  36. O O O O N N O O O O C C O C N N H H O O O O O O C C C N N N N C C O O HO HO C C + NHS Si + EDC FAST N N anhydride O O NH NH CH3 N N O O C C EDC = CH3-CH2-N=C=N-(CH2)3-N Kinetics are exponential, but electrostatic interactions slow down the reaction significantly CH3 O HO N NHS = O Reaction yield higher in domains S. Sam et al., Langmuir 26 (2010) 809 Si Si Si Si A. Moraillon et al., J. Phys. Chem. C 112 (2008) 7158 L. Touahir et al., J. Phys. Chem. C 115 (2011) 6782 Why should we take care of ?

  37. CONCLUSIONS Electrochemical quantitative characterization is fine, but often requires specific chemical reactions to couple the marker and cannot be generally performed on the final layer XPS requires the comparison with a specific "SAM/Au" standard in order to convert the derived thickness in a coverage FTIR requires a specific comparison with a related molecule in solution. It provides Molecular coverage directly and in some favorable cases the molecule orientation Pushing the boundaries of a specific technique for the study of a specific system often brings new, sometimes unexpected, information on the system Crossing techniques at the best (therefore quantitative) level is also often a useful source of new highlights on a specific system

  38. THANKS TO In Ecole Polytechnique Philippe Allongue Jean-Noël Chazalviel Anne Chantal Gouget-Laemmel Catherine Henry de Villeneuve Khalid Lahlil Anne Moraillon Damien Aureau Carine Douarche Anne Faucheux Emmanuel Perez Juliana Salvador Andresa Larbi Touahir In Alger Noureddine Gabouze Sabrina Sam In Versailles Arnaud Etcheberry Jacky Vigneron In Lille Rabah Boukherroub Xavier Wallart FOR THEIR CONTRIBUTIONS TO THE IN-HOUSE WORK PRESENTED HERE

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