Electrochemical metal deposition on top of an organic monolayer

Electrochemical deposition of metals (platinum or gold) only on top of an organothiolate, 1,4-benzenedimethanethiol (BDMT) or hexanedithiol (HDT), self-assembled monolayer (SAM) on a Au(111) substrate was achieved by electrochemical reduction of PtCl(4)(2-) or AuCl(4)(-) ion, which was preadsorbed on one free thiol end group of the dithiol SAM formed on a Au surface, in a metal-ion-free sulfuric acid solution at potentials more negative than the reduction potential of the metal ion. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) measurement after the reduction of preadsorbed PtCl(4)(2-) ion on BDMT/Au(111) electrode showed the presence of Pt not underneath but on top of the BDMT SAM. After a negative potential scan of the Pt/BDMT/Au(111) electrode to -1.30 V in 0.1 M KOH solution, a typical cyclic voltammogram of a clean Au(111) electrode was obtained, showing that the BDMT SAM with a Pt layer was reductively desorbed. These results proved that a Pt-BDMT SAM-Au substrate sandwich structure without a short circuit between the two metals was successfully constructed by this technique. Furthermore, a decanethiol (DT) monolayer was constructed on a Au layer, which was formed by the reduction of preadsorbed AuCl(4)(-) ion on HDT/Au(111) electrode. The formation of DT/Au/HDT/Au(111) structure was confirmed as two cathodic peaks corresponding to reductive desorption of DT from Au on top of the HDT/Au(111) at -0.97 V and that of Au/ HDT from Au(111) at -1.12 V were observed when potential was scanned negatively to -1.35 V.     

Adhesion of spherical polyelectrolyte brushes on mica: an in situ AFM investigation

We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.     

Selective reductive desorption of a SAM-coated gold electrode revealed using fluorescence microscopy

The reductive desorption of a self-assembled monolayer (SAM) of a fluorescent thiol molecule (BodipyC10SH) from Au was characterized using electrochemistry and epi-fluorescence microscopy. Molecular luminescence is quenched near a metal surface, so fluorescence was only observed for molecules reductively desorbed and then separated from the electrode surface. Fluorescence imaging showed that reductive desorption was selective, with desorption occurring from different regions of the Au electrode depending on the extent of the negative potential excursion. When desorbed, the molecules were sufficiently mobile, diffusing away from the electrode surface, thereby preventing oxidative readsorption. At sufficiently negative desorption potentials, all of the thiol was desorbed from the electrode surface, resulting in fluorescence at the air/solution interface. The selective removal of the thiol monolayer from distinct regions was correlated to features on the electrode surface and was explained through potential-dependent interfacial energies. This in situ electrofluorescence microscopy technique may be useful in sensor development.     

The kinetic study of specific adsorption of phosphate species on Pt(111) in acidic solutions

The study of the adsorption/desorption mechanism of phosphate anions at Pt(111) in acidic solution of pH 4.3 and 0.8 was performed by the potential step method in order to reveal the kinetics of anion adsorption. The current-time curve due to phosphate adsorption/desorption showed various decay features, being dependent on the potential region. The rate of current decay depended on pH, being faster in a lower pH solution. Specific adsorption processes were analyzed by the Langmuir and Elovich adsorption equations and also in terms of a two-dimensional nucleation-growth mechanism in different adsorption/desorption regions. In the case of adsorption in 0.3M phosphate buffer solution of pH 4.3, random adsorption without interaction following the Langmuir adsorption, takes place at low coverage, while random adsorption with repulsive force was observed at high coverage. In the desorption process, random desorption with repulsive force takes place at high coverage, and the repulsive force disappears where random adsorption without interaction takes place at medium coverage. When the surface coverage becomes further lower, the desorption mechanism changes dramatically into a two-dimensional nucleation-growth type, suggesting that an ordered adsorbate structure is formed after a rapid discharge process of anion adsorption.     

Using the adhesive interaction between atomic force microscopy tips and polymer surfaces to measure the elastic modulus of compliant samples

An atomic force microscope (AFM) method for measuring surface elasticity based on the adhesive interactions between an AFM tip and sample surfaces is introduced. The method is particularly useful when there is a large adhesion between the tip and soft samples, when the indentation method would be less accurate. For thin and soft samples, this method will have much less interference from the substrate than is found using the indentation method because there is only passive indentation induced by tip-sample adhesion; in contrast, a large indentation with a sharp tip in the sample may break its stress-strain linearity, or even make it fracture. For the case where it is difficult to accurately locate the tip-sample contact point, which is problematic for the indentation method, the method based on adhesive interactions is helpful because it does not require locating the tip-sample contact point when fitting the whole retraction force curve. The model is tested on PDMS polymers with different degrees of cross-linking.     

Nanolithographic write, read, and erase via reversible nanotemplated nanostructure electrodeposition on alkanethiol-modified Au(111) in an aqueous solution

A write, read, and erase nanolithographic method, combining in situ electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal of HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape, and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of the deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.     

化学力显微镜对自组装单分子膜的力滴定研究

The concept of force titration was firstly proposed based on the technique of Chemical Force Microscopy (CFM). Self-Assembled Monolayer (SAM) on substrate surface can be titrated with buffer solutions at a nanometer scale by measuring the adhesion force between the SAM-modified substrate and probe tip. The plot of adhesion force vs pH value was termed as force titration curve. As an example, the titration behavior of w-mercaptoundecanoic acid monolayer on gold has been studied. It was found that there is a big hump around pH 5~6 in its force titration curve. Taking the contact angle titration result together, an interaction model for the monolayer was suggested from the chemical hysteresis point of view.     

Measurement of the charge number per adsorbed molecule and packing densities of self-assembled long-chain monolayers of thiols

We have applied a recently developed method (Langmuir 2006, 22, 5509-5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number.     

Determination of interaction strength between corrole and phenol derivatives in aqueous media using atomic force microscopy

Atomic force microscopy (AFM) was used to measure single interaction forces between corrole (host) and phenol derivatives (guests) in aqueous media. A gold tip was modified with thiol derivatives of corrole via the Au–S covalent bond. Such a tip was used to measure adhesion forces with a planar gold substrate modified with thiol derivatives of phenol and ortho-nitrophenol in aqueous solutions. The mean force between the corrole and ortho-nitrophenol was higher than that between corrole and phenol, probably reflecting stronger hydrogen bond interaction in the former complex. In the presence of a supporting electrolyte (0.1 M K₂SO₄), the mean force increased, suggesting that electrostatic and π–π interactions play an essential role in the adhesion force. In addition, the adhesion force measured at pH 6.0 was larger than that at pH 10, reflecting the electrostatic repulsion at the higher pH. These behaviours are consistent with the potentiometric responses of a liquid membrane based on corrole to phenolic compounds. Also, the values of forces for the interaction between corrole and phenol derivatives showed the same tendency as energy calculated for these complexes. The Poisson method was used for the calculation of the single force of the chemical bond between the corrole host and the phenolic guests.     

Electroreduction of oxygen on octadecylmercaptan self-assembled monolayers

The reduction of oxygen has been studied on octadecylmercaptan self-assembled monolayers adsorbed on gold substrates in borate buffer solutions with a rotating disc electrode. A great inhibition of the oxygen reduction and other electrochemical reactions by these monolayers has been found. However, after polarisation at -0.80 V(SHE) the protecting properties of the film against electron transfer reactions are lost, and a behaviour similar to bare gold is observed. Ex situ XPS indicates that the thiol monolayer has not been desorbed to a large extent during oxygen reduction. Disorders of the monolayer structure and desorption of thiol molecules are proposed as the main reasons for the accessibility of electrochemical reactions to the surface.     

Feeling Inter‐ or Intramolecular Interactions with the Polymer Chain as Probe: Recent Progress in SMFS Studies on Macromolecular Interactions

Single‐molecule force spectroscopy (SMFS) opens new avenues for elucidating the structures and functions of large coiled molecules such as synthetic and biopolymers at the single‐molecule level. In addition, some of the features in the force–extension curves (i.e. force spectra) are closely related to primary/secondary structures of the molecules being stretched. For example, the long force plateau in the DNA stretching curve is related to the double‐helix structure. These features can be regarded as the force fingerprints of individual macromolecules. These force fingerprints can therefore be used as indicators/criteria of single‐molecule manipulation during the measurement of some unknown intra‐ or intermolecular interactions. By comparing the force spectra of a single polymer chain before and after interaction with other molecules, the mode/strength of such molecular interactions can be derived. This Review focuses on recent advances in AFM‐based SMFS studies on molecular interactions in both synthetic and biopolymer systems using a single macromolecular chain as probe, including interactions between nucleic acids and proteins, mechanochemistry of covalent bonds, conformation‐regulated enzymatic reactions, adsorption and desorption of biopolymers on a flat surface or from the nanopore of a carbon nanotube, and polymer interactions in the condensed state.     

Competitive adsorption of sodium dodecyl sulfate and polyethylene oxide at the air/water interface

Geoinspired synthetic chrysotile, which represents an ideal asbestos reference standard, has been utilized to investigate homomolecular exchange of bovine serum albumin (BSA), the major plasma protein, between the adsorbed and dissolved state at the interface between asbestos fibers and biological medium. FTIR spectroscopy has been used to quantify BSA structural modifications due to surface adhesion on chrysotile fibers as a function of the surface coating extent. Circular dichroism spectroscopy has been used to investigate the adsorption/desorption equilibrium through analysis of the BSA structural perturbations after protein desorption from chrysotile surface. Data results show clearly that in the solid state BSA modifications are driven by surface interaction with the substrate, following a bimodal adsorption evidenced by two different binding constants. On the other hand, BSA desorbed in solution is able to rearrange, in the lack of substrate, although keeping irreversible modifications with respect to the native species. The lack of regaining its native structure certainly affects albumin interaction with biological environment. The present investigation on the stoichiometric synthetic geoinspired chrysotile nanocrystals is the first approach toward a deeper attempt to use standard synthetic chrysotile reference samples in mimicking the behavior of asbestos fibers and allows to better understand their interaction with a biological environment.     

Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical force titration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasma polymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance the variation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates. The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic nature of the maleic acid groups clearly emerges from the force titration curves. The surface pK(a) values (pK(a1) = 3.5 +/- 0.5 and pK(a2) = 9.5 +/- 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The values are shifted toward higher pK(a) values when compared to maleic acid in solution. The first pK(a) appears in the titration force curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations. The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrate as reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation of counterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH, one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substrate distance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length, attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K(+) by the divalent cation Ca(2+) leads to significant changes in the force titration curve at high pH where the dicarboxylic groups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increases with pH, an effect that is explained by Ca(2+) ions bridging between two carboxylate groups.     

AFM observation of cation complexation of dibenzocrown ethers adsorbed on highly oriented pyrolytic graphite

The cation complexation behavior of dibenzocrown ethers adsorbed on highly oriented pyrolytic graphite substrates was investigated by means of atomic force microscopy using probe tips modified chemically with ammonium ion by silane coupling. The specific adhesion force based on the intermolecular force between dibenzocrown ether and ammonium ion was observed via force curve measurements in ethanol at the interface between the substrate and tip. The observed specific force decreased in the presence of the alkali metal ion in solution, indicating that the cation in solution interferes with the complexation of the crown ethers adsorbed on the substrate with the ammonium ion immobilized on the tip. The blocking effect of metal ions in solution on the observed force depended on the sizes of both the blocking cation and crown ether ring, suggesting that the surface-adsorbed dibenzocrown ethers possess a selective cation-complexing ability similar to that in their bulk state and that the adhesion force measurements using cation-modified tips allow evaluation of the cation-complexing ability of crown ethers under cation-competitive conditions.     

On the adhesion between fine particles and nanocontacts: an atomic force microscope study

In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.     

Protein adhesion on dental surfaces—a combined surface analytical approach

Protein adsorption is a field of huge interest in a number of application fields. Information on protein adhesion is accessible by a variety of methods. However, the results obtained are significantly influenced by the applied technique. The objective of this work was to understand the role of adhesion forces (obtained by scanning force spectroscopy, SFS) in the process of protein adsorption and desorption. In SFS, the protein is forced to and retracted from the surface, even under unfavorable conditions, in contrast to the natural situation. Furthermore, adhesion forces are correlated with adhesion energies, neglecting the entropic part in the Gibbs enthalpy. In this context, dynamic contact angle (DCA) measurements were performed to identify the potential of this method to complement SFS data. In DCA measurements, the protein diffuses voluntarily to the surface and information on surface coverage and reversibility of adsorption is obtained, including entropic effects (conformational changes and hydrophobic effect). It could be shown that the surface coverage (by DCA) of bovine serum albumin on dental materials correlates well with the adhesion forces (by SFS) if no hydrophobic surface is involved. On those, the entropic hydrophobic effect plays a major role. As a second task, the reversibility of the protein adsorption, i.e., the voluntary desorption as studied by DCA, was compared to the adhesion forces. Here, a correlation between low adhesion forces and good reversibility could be found as long as no covalent bonds were involved. The comparative study of DCA and SFS, thus, leads to a more detailed picture of the complete adsorption/desorption cycle.     

Nanoscale aggregate structures of trisiloxane surfactants at the solid-liquid interface

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.     

Nanoadhesion on Rigid Methyl‐Terminated Biphenyl Thiol Monolayers: A High‐Rate Dynamic Force Spectroscopy Study

Nanoadhesion on a self‐assembled monolayer of 4‐methyl‐4′‐mercaptobiphenyl is measured using a modified atomic force microscope. The dependence of the adhesion force on the loading rate is analyzed with the Dudko–Hummer–Szabo model, and the kinetic and interaction potential parameters for a single terminal group are extracted. The energy and location of the activation barrier suggest that the adhesion is dominated by van der Waals dispersion forces. The humidity effect on the nanoadhesion is also studied. The results are compared with previously measured values for methyl‐terminated alkane thiols and the influence of the thiol rigidity on the adhesion force is discussed.     

超声场下苯酚吸附相平衡和均相催化剂氧化降解实验研究

测定了常规条件和超声场条件下苯酚在活性炭上的吸附等温线,并研究了超声再生活性炭和均相催化剂氧化降解苯酚实验.研究结果表明,超声场作用下苯酚在活性炭上的吸附等温线降低,平衡吸附量减少,被脱附下来的苯酚及时被Cu2 和Fe3 型催化剂氧化降解,吸附剂得到更彻底再生,避免了二次污染,而且再生后的活性炭均具有较高的二次吸附能力,对于处理低浓度含酚废水具有实际意义.     

Adsorption of ions onto polymer surfaces and its influence on zeta potential and adhesion phenomena

 The adhesion behavior that governs many technologically and biologically relevant polymer properties can be investigated by zeta potential measurements with varied electrolyte concentration or pH. In a previous work [1] it was found that the difference of the adsorption free energies of Cl^- and K⁺ ions correlates with the adhesion force caused by van der Waals interactions, and that the decrease of adhesion strength by adsorption layers can be elucidated by zeta potential measurements. In order to confirm these interrelations, zeta potential measurements were combined with atomic force microscopy (AFM) measurements. Force–distance curves between poly(ether ether ketone) and fluorpolymers, respectively, and the Si₃N₄ tip of the AFM device in different electrolyte solutions were measured and analysed. The adsorption free energy of anions calculated from the Stern model correlates with their ability to prevent the adhesion between the polymer surface and the Si₃N₄ tip of the AFM device. These results demonstrate the influence of adsorption phenomena on the adhesion behavior of solids. The results obtained by AFM confirm the thesis that the electrical double layer of solid polymers in electrolyte solutions is governed by ion adsorption probably due to van der Waals interactions and that therefore van der Waals forces can be detected by zeta potential measurements. Received: 18 November 1997 Accepted: 19 January 1998