Ferrocenylalkylthiolates as a probe of heterogeneity in binary self-assembled monolayers on gold

The oft-cited complexity of tethered ferrocene electrochemistry in single component (FcRS-) or binary (FcRS-/CH3R'S-) self-assembled monolayers (SAMs) on gold has been investigated. The complex voltammetry is shown to be linked to local electrostatics caused by the formation of the ferrocenium ion. This conclusion is reached by studying model effects in binary SAMs, where a cationic alkylthiolate (H3N+ C11S-Au) is mixed with FcC12S-Au. A fitting procedure involving both a Gaussian and a Lorentzian distribution is used for deconvolution of the two peaks which are consistently observed in the SAMs when chi(Fc)surf > or = 0.2. The lower-potential (E degrees ' = 250 mV) and higher-potential (E degrees ' = 350 mV) voltammetric peaks are assigned to Fc moieties in "isolated" and "clustered" states, respectively. Use of this method to better understand SAM structure is demonstrated by distinguishing the degree of homogeneity in two binary SAMs of similar composition.     

Preparation and electrochemistry of azobenzene self-assembled monolayers on gold--long range tunneling and end-group hydrogen bonding effect

A series of thiol-functionalied azobenzene derivatives (RAzoCnSH: R=H for n=3-6, abbreviated as AzoCnSH; R=CH(3)CONH for n=4, abbreviated as aaAzoC4SH) on gold electrodes were prepared and their self-assembly and electrochemical properties were studied by cyclic voltammetry. They all formed uniform and reproducible self-assembled monolayers (SAMs) on gold and showed well-behaved voltammetric responses in aqueous solution. Both the length of the alkyl chain spacer and the H-bonding of the end acetamino group had effects on the stability and the electrochemical kinetics of the SAMs, and the effect of the H-bonding was dominant. The surface coverage of the SAMs (AzoCnSH) is gradually increased with an increase of the alkyl chain spacer length, whereas the presence of the terminal acetamino group leads to a greater increase of the surface coverage. At a low scan rate, voltammetric responses corresponding to an irreversible two-electron, two-proton reduction/oxidation of the trans-azobenzene redox center were obtained in the range of +300 mV and -800 mV, which exhibited very large peak-to-peak splitting. At a high scan rate of 500 mV/s, two steps of reversible one-electron, one-proton reduction/oxidation corresponding to the cis-isomer in azobenzene-thiol SAMs (n is odd) was clearly observed between +300 and -200 mV. The apparent electron-transfer rate is decreased with increasing distance between the azobenzene redox center and the gold electrode. The existence of the end acetamino group which restricted the conformational change during the redox process also led to a decrease of the standard rate constant, and this restriction effect is more predominant than the distance effect.     

Effect of amino,hydroxyl, and carboxyl terminal groups of alkyl chains of self-assembled monolayers on the adsorption pattern of gold nanoparticles

The adsorption pattern of gold nanoparticles (AuNPs) on functionalized self-assembled monolayers (SAMs) produced on a Au(111) surface was characterized. The Au(111) was modified with 11-amino-1-undecanethiol hydrochloride (AUT), 11-mercapto-1-undecanol (MUT), or 11-mercaptoundecanoic acid (MUA) at an elevated temperature and pressure. The AuNPs aggregated on the AUT-SAM surface, whereas they were well dispersed on the MUT-SAM surface and localized on the MUA-SAM surface. The results suggest that interactions between AuNPs differ according to the degree of peeling of citrate-layer-capped AuNPs. The degree of peeling, which is related to both the surface randomness of the SAMs and the functional characteristics of the terminal group of each SAM, was discussed on the basis of scanning tunneling microscopy observations, X-ray photoelectron spectroscopic analyses, and contact angle measurements. Our study shows that AuNP patterns can be controlled by changing the terminal group of the alkyl thiol SAM on a Au(111) surface.     

Spectroscopic fingerprints of amine and imide functional groups in self-assembled monolayers.

The electronic structures of naphthalene tetracarboxylic diimide (NTCDI) and 1,4-bis(4,6-diamino-1,3,5-triazin-2-yl)benzene (BDG) monolayer assemblies grown on Au(111) are investigated by photoemission spectroscopy, X-ray absorption, and density functional theory. The different spectroscopic features in the absorption and core-level photoemission spectra are understood in terms of contributions from different core and molecular levels at N- and O-atom sites. This study provides clear spectroscopic fingerprints for amine and imide functional end groups, which drive the self-organization process in a number of planar, pi-conjugated molecular structures.     

Effect of hydrogen bonding and cooperativity on stretching force constants of formamide

Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The C?O and N? H stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The C?O and C? N stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.     

Effect of tip size on force measurement in atomic force microscopy

An atomic force microscope (AFM) has been used to study solvation forces at the solid-liquid interface between highly oriented pyrolytic graphite (HOPG) and the liquids octamethylcyclotetrasiloxane (OMCTS), n-hexadecane (n-C16H34), and n-dodecanol (n-C11H23CH2OH). Oscillatory solvation forces (F) are observed for various measured tip radii (Rtip=15-100 nm). It is found that the normalized force data, F/Rtip, differ between AFM tips with a clear trend of decreasing F/Rtip with increasing Rtip.     

Influence of solvent environment and tip chemistry on the contact mechanics of tip-sample interactions in friction force microscopy of self-assembled monolayers of mercaptoundecanoic Acid and dodecanethiol

Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.     

Structure and electrochemistry of 4,4'-dithiodipyridine self-assembled monolayers in comparison with 4-mercaptopyridine self-assembled monolayers on Au(111)

4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.     

LZnX complexes of tripodal ligands with intramolecular RN-H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.     

Contact angles of surface nanobubbles on mixed self-assembled monolayers with systematically varied macroscopic wettability by atomic force microscopy

The dependence of the properties of so-called "surface nanobubbles" at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (θ(macro)) of the SAMs spanned the range between 107° ± 1° and 15° ± 3°. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles θ(nano) < 86° were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of θ(nano) calculated from these data were found to change from 167° ± 3° to 33° ± 58°, when θ(macro) decreased from 107° ± 1° to 37° ± 3°. While the values for θ(nano) significantly exceeded those of θ(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.     

Ferric ion immobilized on three-dimensional nanoporous gold films modified with self-assembled monolayers for electrochemical detection of hydrogen peroxide

A fast, simple square wave potential method is developed for the fabrication of a three-dimensional (3D) nanoporous gold (NPG) film. The nanostructures are characterized and confirmed by scanning electronic microscopy (SEM) and cyclic voltammetry (CV). The nanostructures modified with self-assembled monolayers (SAMs) are employed as an electrode substrate to immobilize inorganic iron(III) ion. After immobilization, iron(III) ion undergoes an effective direct electron transfer reaction with a pair of well-defined redox peak at -256 ± 10 mV (pH 7.0). The iron(III) ion modified electrode displays the excellent electrocatalytic performance for reduction of hydrogen peroxide, and thus can be used as an electrochemical sensor for detecting hydrogen peroxide with a low detection limit (1.0 × 10(-9) M), a wide linear range (9.0 × 10(-7)~5.0 × 10(-4) M), as well as good stability, selectivity and reproducibility.     

Fluorescence lifetime of Nile Red as a probe for the hydrogen bonding strength with its microenvironment

The fluorescence lifetime of Nile Red (NR) is not sensitive to dielectric solvent–solute interactions but markedly decreases with the increase of the hydrogen bond donating ability in alcohols because vibrations associated with hydrogen bonding are involved in the deactivation process. The negligible viscosity effect indicates that twisting of the diethylamino moiety of NR does not play significant role in the dissipation of the excitation energy.     

Effect of concentration on the photo-orientation and relaxation dynamics of self-assembled monolayers of mixtures of an azobenzene-based triethoxysilane with octyltriethoxysilane

Self-assembled monolayers (SAMs) were prepared from solutions with different proportions of a photoactive, azobenzene-based, silanized derivative of disperse red one (dDR1), and octyltriethoxysilane (OTE), a shorter, nonphotoactive molecule. The in-plane photoinduced orientational ordering of the resulting two component monolayers was monitored via precision measurement of in-plane birefringence using a dedicated high-extinction polarimeter. Measurements of contact angle, absorption, and birefringence show that introduction of OTE into the dDR1 deposition solution produces a continuous reduction of the surface density of dDR1 in the SAM, enabling the study of photowriting and relaxation dynamics in monolayers ranging from 100% dDR1 to samples where the dDR1 coverage is about 35%. The orientational dynamics depend strongly on the areal density of dDR1. As the fractional area of dDR1 is reduced, the rates of photowriting, photoerasing, and thermal relaxation increase, and the local orientational confinement of the molecules becomes more heterogeneous.     

Surface-assisted transient displacement charge technique. II. Effect of gases on photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was used to study charge transfer in self-assembled monolayers of 7-diethylaminocoumarin covalently linked to an oxide surface in the atmosphere of different gases. The dipole signal was found to be opposite to that in solution and dependent on the nature of the gas and its pressure. The results were explained by collision-induced relaxation that impedes uninhibited tilting of molecules onto the surface. Collisions with paramagnetic oxygen induce intersystem crossing to long-lived triplet dipolar states of coumarin with the rate close to half of that for the collision rate.     

Effect of hydrogen bonds on intermolecular crosslinking reaction by introduction of carboxyl groups in free-radical crosslinking monomethacrylate/dimethacrylate copolymerizations

The effect of physical interaction through hydrogen bonds on the intermolecular crosslinking reaction leading to the promoted gelation in free-radical crosslinking monovinyl/divinyl copolymerizations was discussed from the standpoint of the control of network formation. The solution copolymerizations of benzyl methacrylate (BzMA) with 2 mol% of 1,6-hexanediol dimethacrylate in t-butylbenzene were conducted in the absence and presence of different amounts of mono(2-methacryloyloxyethyl) succinate (MMOES). Gelation was promoted by the addition of MMOES and the ratio of the actual gel point to the theoretical one became smaller; this would be related to the formation of hydrogen bonds between carboxyl groups introduced into prepolymer and growing polymer radical. As an extension of the above discussion, we treated the effect of hydrogen bonds on the gelation in the crosslinking BzMA/triicosaethylene glycol dimethacrylate copolymerization. The addition of MMOES obviously promoted the gelation. The ratio of the actual gel point to the theoretical one calculated according to Stockmayer's equation [J. Chem. Phys. 12 (1944) 125] was obtained as 1.9, very close to unity.     

Effect of hydrogen bonding on properties of hexafluorosilicates with heterocyclic cations

The known data regarding the effects of interionic hydrogen bonding on properties of onium hexafluorosilicates with heterocyclic cations are summarized. Thermal stability parameters and water solubilities for this type of salts are shown to correlate with the number of strong and medium-strength hydrogen bonds or the number of hydrogen donors in the salt structure.     

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.