Vibrational Stark Effect of the Electric-Field Reporter 4-Mercaptobenzonitrile as a Tool for Investigating Electrostatics at Electrode/SAM/Solution Interfaces
4-mercaptobenzonitrile (MBN) in self-assembled monolayers (SAMs) on Au and Ag electrodes was studied by surface enhanced infrared absorption and Raman spectroscopy, to correlate the nitrile stretching frequency with the local electric field exploiting the vibrational Stark effect (VSE). Using MBN SAMs in different metal/SAM interfaces, we sorted out the main factors controlling the nitrile stretching frequency, which comprise, in addition to external electric fields, the metal-MBN bond, the surface potential, and hydrogen bond interactions. On the basis of the linear relationships between the nitrile stretching and the electrode potential, an electrostatic description of the interfacial potential distribution is presented that allows for determining the electric field strengths on the SAM surface, as well as the effective potential of zero-charge of the SAM-coated metal. Comparing this latter quantity with calculated values derived from literature data, we note a very good agreement for Au/MBN but distinct deviations for Ag/MBN which may reflect either the approximations and simplifications of the model or the uncertainty in reported structural parameters for Ag/MBN. The present electrostatic model consistently explains the electric field strengths for MBN SAMs on Ag and Au as well as for thiophenol and mercaptohexanoic acid SAMs with MBN incorporated as a VSE reporter.
References
[1]
Love, J.C.; Estroff, L.A.; Kriebel, J.K.; Nuzzo, R.G.; Whitesides, G.M. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev 2005, 105, 1103–1169.
[2]
Samanta, D.; Sarkar, A. Immobilization of bio-macromolecules on self-assembled monolayers: Methods and sensor applications. Chem. Soc. Rev 2011, 40, 2567–2592.
[3]
Mbindyo, J.K.N.; Mallouk, T.E.; Mattzela, J.B.; Kratochvilova, I.; Razavi, B.; Jackson, T.N.; Mayer, T.S. Template synthesis of metal nanowires containing monolayer molecular junctions. J. Am. Chem. Soc 2002, 124, 4020–4026.
[4]
Smith, C.P.; White, H.S. Theory of the interfacial potential distribution and reversible voltammetric response of electrodes coated with electroactive molecular films. Anal. Chem 1992, 64, 2398–2405.
[5]
Ramírez, P.; Granero, A.; Andreu, R.; Cuest, A.; Mulder, W.H.; Calvente, J.J. Potential of zero charge as a sensitive probe for the titration of ionizable self-assembled monolayers. Electrochem. Commun 2008, 10, 1548–1550.
[6]
Oklejas, V.; Sjostrom, C.; Harris, J.M. SERS detection of the vibrational stark effect from nitrile-terminated SAMs to probe electric fields in the diffuse double-layer. J. Am. Chem. Soc 2002, 124, 2408–2409.
[7]
Oklejas, V.; Harris, J.M. In-situ investigation of binary-component self-assembled monolayers: A SERS-based spectroelectrochemical study of the effects of monolayer composition on interfacial structure. Langmuir 2003, 19, 5794–5801.
[8]
Suydam, I.T.; Snow, C.D.; Pande, V.S.; Boxer, S.G. Electric fields at the active site of an enzyme: Direct comparison of experiment with theory. Science 2006, 313, 200–204.
[9]
Fafarman, A.T.; Sigala, P.A.; Herschlag, D.; Boxer, S.G. Nitrile bonds as infrared probes of electrostatics in ribonuclease S. J. Am. Chem. Soc 2010, 132, 12811–12813.
[10]
Jo, H.; Culik, R.M.; Korendovych, I.V.; DeGrado, W.F.; Gai, F. Selective incorporation of nitrile-based infrared probes into proteins via cysteine alkylation. Biochemistry 2010, 49, 10354–10356.
[11]
Schkolnik, G.; Utesch, T.; Salewski, J.; Tenger, K.; Millo, D.; Kranich, A.; Zebger, I.; Schulz, C.; Zimányi, L.; Rákhely, G.; et al. Mapping local electric fields in proteins at biomimetic interfaces. Chem. Commun 2012, 48, 70–72.
[12]
Aschaffenburg, D.J.; Moog, R.S. Probing hydrogen bonding environments: Solvatochromic effects on the CN vibration of benzonitrile. J. Phys. Chem. B 2009, 113, 12736–12743.
[13]
Ghosh, A.; Remorino, A.; Tucker, M.J.; Hochstrasser, R.M. 2D IR photon echo spectroscopy reveals hydrogen bond dynamics of aromatic nitriles. Chem. Phys. Lett 2009, 469, 325–330.
[14]
Murgida, D.H.; Hildebrandt, P. Electron-transfer processes of cytochrome c at interfaces. New insights by surface-enhanced resonance raman spectroscopy. J. Phys. Chem. B 2001, 105, 1578–1586.
[15]
Lecomte, S.; Hildebrandt, P.; Soulimane, T. Dynamics of the heterogeneous electron-transfer reaction of cytochrome c552 from thermus thermophilus. A time-resolved surface-enhanced resonance raman spectroscopic study. J. Phys. Chem. B 1999, 103, 10053–10064.
[16]
Fafarman, A.T.; Sigala, P.A.; Schwans, J.P.; Fenn, T.D.; Herschlag, D.; Boxer, S.G. Quantitative, directional measurement of electric field heterogeneity in the active site of ketosteroid isomerase. Proc. Natl. Acad. Sci. USA 2012, 109, E299–E308.
[17]
Stein, S.E. Infrared Spectra. NIST Chemistry WebBook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, USA, 2003. Available online: http://webbook.nist.gov , accessed on 6 May 2012.
[18]
Suydam, I.T.; Snow, C.D.; Pande, V.S.; Boxer, S.G. Electric fields at the active site of an enzyme: Direct comparison of experiment with theory. Science 2006, 313, 200–204.
[19]
Natan, A.; Kuritz, N.; Kronik, L. Polarizability, susceptibility, and dielectric constant of nanometer-scale molecular films: A microscopic view. Adv. Funct. Mater 2010, 20, 2077–2084.
[20]
Trasatti, S. Work function, electronegativity, and electrochemical behaviour of metals. J. Electroanal. Chem 1971, 33, 351–378.
[21]
Valette, G. Hydrophilicity of metal surfaces, silver, gold and copper electrodes. J. Electroanal. Chem 1982, 139, 285–301.
[22]
Wang, L.; Rangger, G.M.; Romaner, L.; Heimel, G.; Bu？ko, T.; Ma, Z.; Li, Q.; Shuai, Z.; Zojer, E. Electronic structure of self-assembled monolayers on Au(111) surfaces: The impact of backbone polarizability. Adv. Funct. Mater 2009, 19, 3766–3775.
[23]
Heimel, G.; Romaner, L.; Zojer, E.; Brédas, J.L. Toward control of the metal-organic interfacial electronic structure in molecular electronics: A first-principles study on self-assembled monolayers of π-conjugated molecules on noble metals. Nano Lett 2007, 7, 932–940.
[24]
Ballav, N.; Schüpbach, B.; Dethloff, O.; Feulner, P.; Terfort, A.; Zharnikov, M. Direct probing molecular twist and tilt in aromatic self-assembled monolayers. J. Am. Chem. Soc 2007, 129, 15416–15417.
[25]
Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M. Structure of thioaromatic self-assembled monolayers on gold and silver. Langmuir 2001, 17, 2408–2415.
[26]
K？fer, D.; Bashir, A.; Witte, G. Interplay of anchoring and ordering in aromatic self-assembled monolayers. J. Phys. Chem. C 2007, 111, 10546–10551.
[27]
Han, S.W.; Lee, S.J.; Kim, K. Self-assembled monolayers of aromatic thiol and selenol on silver: Comparative study of adsorptivity and stability. Langmuir 2001, 17, 6981–6987.
[28]
Schalnat, M.C.; Pemberton, J.E. Comparison of a fluorinated aryl thiol self-assembled monolayer with its hydrogenated counterpart on polycrystalline Ag substrates. Langmuir 2010, 26, 11862–11869.
[29]
Shaw, S.K.; Lagutchev, A.; Dlott, D.D.; Gewirth, A.A. Sum-frequency spectroscopy of molecular adsorbates on low-index Ag surfaces: Effects of azimuthal rotation. Anal. Chem 2009, 81, 1154–1161.
[30]
Akinaga, Y.; Nakajima, T.; Hirao, K. A density functional study on the adsorption of methanethiolate on the (111) surfaces of noble metals. J. Chem. Phys 2001, 114, 8555–8564.
[31]
Ehler, T.T.; Malmberg, N.; Noe, L.J. Characterization of self-assembled alkanethiol monolayers on silver and gold using surface plasmon spectroscopy. J. Phys. Chem. B 1997, 101, 1268–1272.
[32]
Gershevitz, O.; Sukenik, C.N. In situ FTIR-ATR analysis and titration of carboxylic acid-terminated SAMs. J. Am. Chem. Soc 2004, 126, 482–483.
[33]
Burris, S.C.; Zhou, Y.; Maupin, W.A.; Ebelhar, A.J.; Daugherty, M.W. The effect of surface preparation on apparent surface pKa’s of ω-mercaptocarboxylic acid self-assembled monolayers on polycrystalline gold. J. Phys. Chem. C 2008, 112, 6811–6815.
[34]
Emets, V.V.; Damaski, B.B. The relation between the potential of zero charge and work function for sp-metals. Russ. J. Electrochem 2009, 45, 45–57.
[35]
Trasatti, S. The concept and physical meaning of absolute electrode potential: A reassessment. J. Electroanal. Chem 1982, 172, 1–13.
[36]
Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.L. The interface energetics of self-assembled monolayers on metals. Acc. Chem. Res 2008, 41, 721–729.
[37]
Natan, A.; Kronik, L.; Haick, H.; Tung, R.T. Electrostatic properties of ideal and non-ideal polar organic monolayers: Implications for electronic devices. Adv. Mater 2007, 19, 4103–4117.
[38]
Alloway, D.M.; Graham, A.L.; Yang, X.; Mudalige, A.; Colorado, R., Jr; Wysocki, V.H.; Pemberton, J.E.; Lee, T.R.; Wysocki, R.J.; Armstrong, N.R. Tuning the effective work function of gold and silver using ω-functionalized alkanethiols: Varying surface composition through dilution and choice of terminal groups. J. Phys. Chem C 2009, 113, 20328–20334.
[39]
Wisitruangsakul, N.; Zebger, I.; Ly, K.H.; Murgida, D.H.; Egkasit, S.; Hildebrandt, P. Redox-linked protein dynamics of cytochrome C probed by time-resolved surface enhanced infrared absorption spectroscopy. Phys. Chem. Chem. Phys 2008, 10, 5276–5286.
[40]
Wackerbarth, H.; Klar, U.; Gunther, W.; Hildebrandt, P. Novel time-resolved surface-enhanced (resonance) raman spectroscopic technique for studying the dynamics of interfacial processes: Application to the electron transfer reaction of cytochrome c at a silver electrode. Appl. Spectrosc 1999, 53, 283–291.
[41]
Ly, K.H.; Wisitruangsakul, N.; Sezer, M.; Feng, J.J.; Kranich, A.; Weidinger, I.; Zebger, I.; Murgida, D.H.; Hildebrandt, P. Electric field effects on the interfacial electron transfer and protein dynamics of cytochrome c. J. Electroanal. Chem. 2011, 660, 367–376.
[42]
Murgida, D.H.; Hildebrandt, P. Redox and redox-coupled processes of heme proteins and enzymes at electrochemical interfaces. Phys. Chem. Chem. Phys 2005, 7, 3773–3784.
[43]
Ly, H.K.; Sezer, M.; Wisitruangsakul, N.; Feng, J.J.; Kranich, A.; Millo, D.; Weidinger, I.M.; Zebger, I.; Murgida, D.H.; Hildebrandt, P. Surface-enhanced vibrational spectroscopy for probing transient interactions of proteins with biomimetic interfaces: Electric field effects on structure, dynamics and function of cytochrome c. FEBS J 2011, 278, 1382–1390.
