Low-density Self-assembled Monolayers Research Articles

Directing Reaction Pathways on Supported Metal Catalysts with Low-Density Self-Assembled Monolayers

Controlling reactant adsorption on catalyst surfaces is crucial to reaction activity and selectivity. One method for improving selectivity is by imposing steric constraints to bias the reactant binding orientation. In this study, thiol self-assembled monolayers (SAMs) were deposited onto Pt/Al2O3 catalysts as a method for controlling activity and selectivity via steric effects. In addition to a full monolayer, a low-density SAM-coated catalyst was employed. A number of characterization techniques demonstrated the successful deposition of homogeneous low-density SAMs on the metal surface with reduced site-blocking compared to a full high-density monolayer. Reaction kinetic studies showed increased benzyl alcohol hydrodeoxygenation (HDO) selectivity for both SAM-modified catalysts. This was attributed to the inability of the reactant to adsorb on the catalyst with the aromatic ring parallel to the surface, thus preventing decarbonylation and ring hydrogenation reaction pathways. Additionally, SAM density influenced reaction activity significantly, with the low-density-modified SAM catalyst being more active than the catalyst coated with a full monolayer. Moreover, liquid-phase hydrogenation reactions were used to investigate the relationship between SAM density and reactivity for reactant molecules of various sizes. In all cases, the low-density SAM improved reaction rates relative to dense SAMs. The effect of controlling ligand density depended on the type of reaction: high ligand densities greatly diminished ring hydrogenation, while HDO was largely unaffected, suggesting a potential strategy for size-selective reaction rate and selectivity control.

On-Demand Modulation of Bacterial Cell Fates on Multifunctional Dynamic Substrates.

This paper reports unprecedented dynamic surfaces based on zwitterionic low-density self-assembled monolayers (LDSAMs) of alkanethiolates on gold, which integrate three interconvertible states-bacteria-adherable, bactericidal, and nonfouling states-through electrical modulations. The conformations of alkanethiolates were electrically modulated to generate zwitterionic, anionic, and cationic surfaces, which responded differently to bacteria and determined the fate of bacteria. Furthermore, the reversible switching of multifunctions of the surface was realized for killing bacteria and subsequently releasing dead bacteria from the surface. For practical application of our strategy, we examined the selective antibacterial effect of our surface for eradication of mycoplasma contaminants in contaminated mammalian cell cultures.

Role of Solution and Surface Coverage on Voltage-Induced Response of Low-Density Self-Assembled Monolayers

Low-density self-assembled monolayers (LD-SAMs) exhibit voltage-induced conformational changes leading to tunable wetting properties and ionic permeability. Here we investigate the role of solution conditions (pH, salt concentration, presence of acetonitrile) and surface coverage via nonfaradaic electrochemical impedance spectroscopy and contact angle measurements. We observe a large change in receding angle (32°) in an aqueous environment for an optimal combination of solution and film composition. This voltage-induced response in wetting properties is also correlated with a large change in dielectric properties as measured from electrochemical impedance spectroscopy. Our results demonstrate that the molecular nature of this responsive surface makes it particularly sensitive to solution conditions, a feature that could be exploited for sensing or detection.

Bonding Asymmetry and Adatoms in Low-Density Self-Assembled Monolayers of Dithiols on Au(111)

The involvement of Au adatoms in the interfacial structure of monothiols on Au(111) is now established, but the same issue is virtually unexplored in the case of dithiols. To gain more insight into the latter, we have studied by scanning tunneling microscopy the self-assembly of two prototypic symmetric dithiols (1,6-hexanedithiol and biphenyl-4,4′-dimethanethiol) from dilute aqueous solutions and correlated their growth with the deconstruction of the Au(111) herringbone pattern known to produce adatoms. For both molecules, we determine an initial low-density monolayer where the molecules are lying down and paired by 0.45 A tall protrusions, assigned to Au adatoms. The other terminal group is imaged differently, revealing a strong asymmetry in the dithiol bonding. The formation of vacancy islands and, thus, the extraction of additional adatoms from terraces are detected only after substantial molecular rearrangement and loss of bonding asymmetry.

Space-Filling Trialkoxysilane: Synthesis and Self-Assembly into Low-Density Monolayers

A novel space-filling trialkoxysilane derivative was synthesized using a two-step strategy from commercially available starting materials to produce the precursor for the formation of low-density self-assembled monolayers. Self-assembled monolayers of the synthesized compound were prepared on three different substrates (Si/SiO(2), glass and ITO) and were characterized using contact angle, ellipsometry and sum-frequency generation spectroscopy. Removal of the space-filling protecting group, (2-chlorophenyl)diphenyl methanol, yields a carboxy-terminated surface. Correspondingly, the contact angle and film thickness decrease and the SFG spectra clearly indicate an increase in gauche defect concentration characteristic of a low-density disordered monolayer.

Electrochemical Stability of Low-Density Carboxylic Acid Terminated Monolayers

Using cyclic voltammetry, we studied the reductive desorption of a series of low-density self-assembled monolayers (LD-SAMs) made from mercaptohexadecanoic acid (MHA). Desorption experiments indicate that LD-SAMs are significantly less stable than a full MHA monolayer. All three LD-SAMs investigated presented a broad desorption peak in the region between −1.0 and −0.8 V vs SCE, which is in contrast to the single peak with a narrow distribution at −1.11 V observed for the full MHA monolayer. Upon backfilling the LD-SAM with a shorter-chain thiol (mercaptohexanol), the desorption of the mixed monolayer displayed a single peak, indicative of a well-mixed monolayer. Our results show that LD-SAMs are promising candidates for mixed monolayers and suggest that the broad desorption peak observed for LD-SAMs might arise due to adsorption at different binding sites.

Factors governing the reversible change in ionic permeability of a low-density monolayer

Abstract The dynamic response of low-density self-assembled monolayers (SAMs) on gold has been investigated with electrochemical impedance spectroscopy (EIS) under non-faradaic conditions and contact angle measurements. The impedance of low-density 16-mercaptohexadecanoic acid (LD-MHA) SAMs is compared with that of densely-packed MHA and tetraethylammonium–MHA (TEA–MHA) ion-pair monolayers in 0.1 M KCl (pH 8.4). The films composed of ion-pairs and loosely-packed MHA have higher capacitance and are more permeable to ions than fully-packed MHA SAMs. The impedance phase angle of the LD-MHA SAM at 0.1 Hz decreases reversibly by 10° upon scanning the applied potential from −0.1 V to +0.3 V (vs. SCE). Fitting the impedance data to an equivalent circuit correlates the decrease in phase angle with an order-of-magnitude drop in monolayer resistance (i.e. increase in the ionic permeability of the monolayer). The ion-pair and dense MHA SAMs do not exhibit an increase in permeability upon scanning the applied potential from −0.1 V to +0.3 V (vs. SCE). The capacitances of the LD-MHA and TEA–MHA films exhibit a small, but significant (15%), change with applied potential between −0.1 V and +0.3 V (vs. SCE). The impedance of the LD-MHA SAM is found to be independent of the electrolyte concentration (0.01–1 M KCl), and displays hysteresis with the direction of the potential scan. Receding contact angle measurements performed at −0.1 V and +0.3 V using the same electrolyte show that the higher ionic permeability of the LD-MHA SAM at +0.3 V is caused by a voltage-induced structural change (molecular re-organization) within the film. The agreement between EIS measurements performed at pH 3.3 and pH 8.4 shows that electrostatic attraction between a negatively-charged thiol tailgroup and oppositely-charged gold electrode is not required for this structural change to occur.

Chemical, electrochemical, and structural stability of low-density self-assembled monolayers.

The stability of low-density self-assembled monolayers of mercaptohexadecanoic acid on gold is studied under a variety of storage conditions--air at room temperature, argon at room temperature and 4 degrees C, and ethanol at room temperature. The structural monotony of the low-density monolayers was assessed by monitoring the alkyl chains of LDSAMs by grazing-angle Fourier transform infrared spectroscopy as a function of time. Independently of the storage conditions, both symmetric and asymmetric methylene stretches at 2923 and 2852 cm-1 decreased after 4 weeks to 2919 and 2849 cm-1, respectively. These data suggest an increased ordering of the alkyl chains that is distinctly different from that of conventional high-density monolayers of mercaptohexadecanoic acid included as a reference in this study. As a further extension of this observation, the electrochemical barrier properties of the low-density monolayers were assessed by electrochemical impedance spectroscopy and did not change significantly for any of the storage conditions over a period of 4 weeks. Moreover, X-ray photoelectron spectroscopy was used to assess the chemical changes in the low-density monolayers over time. The chemical composition was essentially unaltered for all storage conditions. Specifically, oxidation of the sulfur headgroup, a common cause of monolayer degradation, was excluded for all test conditions on the basis of XPS analysis. This study confirms excellent storage stability for low-density monolayers under commonly used storage conditions and bridges an important technological gap between these systems and conventional high-density systems.

Switching the electrochemical impedance of low-density self-assembled monolayers.

Because the active remodeling of biointerfaces is a paramount feature of nature, it is very likely that future, advanced biomaterials will be required to mimic at least certain aspects of the dynamic properties of natural interfaces. This need has fueled a quest for model surfaces that can undergo reversible switching upon application of external stimuli. Herein, we report the synthesis and characterization of a model system for studying reversibly switching surfaces based on low-density monolayers of mercaptohexadecanoic acid and mercaptoundecanoic acid. These monolayers were assembled on both gold and silver electrodes. When conducting electrochemical impedance spectroscopy under physiological conditions, these monolayers exhibit significant changes in their electrochemical barrier properties upon application of electrical DC potentials below +400 mV with respect to a standard calomel electrode. We further found the impedance switching to be reversible under physiological conditions. Moreover, the impedance can be fine-tuned by changing the magnitude of the applied electrical potential. Before and during impedance switching at pH 7.4 in aqueous buffer solutions, the low-density monolayers showed good stability according to grazing angle infrared spectroscopy data. We anticipate low-density monolayers to be potentially useful model surfaces when designing active biointerfaces for cell-based studies or rechargeable biosensors.

Low-Density Self-Assembled Monolayers on Gold Derived from Chelating 2-Monoalkylpropane-1,3-dithiols

Low-density self-assembled monolayers (SAMs) on gold were generated by the adsorption of a series of specifically designed 2-monoalkylpropane-1,3-dithiol derivatives, CH3(CH2)nCH[CH2SH]2, where n = 11, 13, 14. The monolayers were characterized by optical ellipsometry, X-ray photoelectron spectroscopy, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy, and sum-frequency generation. Comparison of these data to those collected on SAMs generated from normal alkanethiols, CH3(CH2)n+2SH, and 2,2-dialkylpropane-1,3-dithiol derivatives, [CH3(CH2)n]2C[CH2SH]2, of similar chain length suggests that the new “monoalkanedithiol” SAMs are the least crystalline, exposing both methyl and methylene groups at the interface due to the low density of alkyl chains. Further comparison of these low-density SAMs to those obtained on branched and linear polyethylene films suggests that the exposure of interfacial methylene groups is greater for the polymer films.