Directed crystallisation of zinc oxide on patterned surfaces

Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 mum and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.     

Construction of mixed mercaptopropionic acid/alkanethiol monolayers of controlled composition by structural control of a gold substrate with underpotentially deposited lead atoms.

Mixed monolayers of 3-mercaptopropionic acid (MPA) and alkanethiols of various chain lengths have been constructed on Au based on a novel concept, namely, control of the composition of the component thiols in mixed monolayers by controlling the surface structure of the substrate. The Au substrate surface was first modified with underpotentially deposited Pb (UPD Pb) atoms, followed by the formation of a self-assembled monolayer (SAM) of alkanethiol. The UPD Pb atoms were then oxidatively stripped from the surface to create vacant site, on which MPA was adsorbed to finally form the mixed monolayers. The surface coverages of Pb, alkanethiol and MPA, and the total numbers of thiols were determined using an electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and reductive desorption voltammetry. These results demonstrate that the surface coverage of MPA in the mixed monolayers is determined by the initial coverage of UPD Pb. Fourier transform infrared spectra also support this conclusion. The observed single peak in the cyclic voltammogram for the reductive desorption shows that MPA and alkanethiol do not form their single-component domains. Scanning tunneling microscopy revealed the single-row pinstripe structure for all the thiol adlayers formed during each step of the preparation. This shows that the surface structure of the mixed monolayers is determined by the structure of the initially formed SAM on Au partially covered with UPD Pb.     

Optical method for predicting the composition of self-assembled monolayers of mixed thiols on surfaces coated with silver nanoparticles

With a simple optical method, based on UV-vis absorption spectra on glass slides, it is possible to predict the composition of self-assembled monolayers of mixed thiols, grafted on monolayers of silver nanoparticles. Glass slides are modified with the layer-by-layer technique, first forming a monolayer of mercaptopropyltrimethoxysilane, then grafting a monolayer of silver nanoparticles on it. These surfaces are further coated by single or mixed thiol monolayers, by dipping the slides in toluene solutions of the chosen thiols. Exchange constants are calculated for the competitive deposition between the colorless 1-dodecanethiol or PEG5000 thiol and BDP-SH, with the latter being a thiol-bearing molecule containing the strongly absorbing BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) moiety, synthesized on purpose. The constants are calculated by determining the fraction of BDP-SH deposited on the surface from a solution with a given molar fraction, directly measuring the absorption spectra of BDP-SH on the slides. Then, the exchange constant for the competitive deposition between 1-dodecanethiol and PEG5000 thiol is calculated by combining their exchange constants with BDP-SH. This allows to predict the fraction of the two colorless thiols coating the silver nanoparticles slides obtained from a toluene solution with a given molar fraction, for example, of PEG5000 thiol. The correctness of the calculated surface fraction is verified by studying the coating competition between 1-dodecanethiol and a PEG5000 thiol remotely modified with a strongly absorbing fluorescein fragment.     

A versatile synthetic strategy for nanoporous gold–organic hybrid materials for electrochemistry and photocatalysis

Nanoporous gold (npAu) was employed as high surface area substrate for immobilization of redox- and photooxidative-active organic molecules. A two-step synthetic routine is demonstrated as a versatile and robust method for immobilization of various molecules. First, self-assembled monolayers (SAMs) of thiols containing an azide moiety were prepared on npAu substrates. Then, alkyne-modified ferrocene, tetrathiafulvalene, and zinc(II)phthalocyanine derivatives were covalently bound via the click reaction to this linker. Following the provided synthetic procedures high performance composite materials are generated for electrochemistry and photochemistry. The robust bonding between the organic functional group and the gold support provides stability even under strongly oxidizing conditions (applied potential or singlet oxygen).     

Modification of Ag nanoparticles with mixed thiols for improved SERS detection of poorly adsorbing target molecules: detection of MDMA

Here we report an example of a mixed thiol monolayer on the surface of Ag nanoparticles which promotes adsorption and quantitative SERS detection of 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy"); the thiols in the mixed monolayers act synergistically since MDMA does not adsorb onto colloids modified with either of the thiols separately.     

Scanning tunneling microscopy of self-assembled phenylene ethynylene oligomers on Au(111) substrates

In this paper, we report the self-assembly, electrical characterization, and surface modification of dithiolated phenylene-ethynylene oligomer monolayers on a Au(111) surface. The self-assembly was accomplished by thiol bonding the molecules from solution to a Au(111) surface. We have confirmed the formation of self-assembled monolayers by scanning tunneling microscopy (STM) and optical ellipsometry, and have studied the kinetics of film growth. We suggest that self-assembled phenylene ethynylene oligomers on Au(111) surfaces grow as thiols rather than as thiolates. Using low-temperature STM, we collected local current-voltage spectra showing negative differential resistance at 6 K.     

Molecular adsorption on ZnO(1010) single-crystal surfaces: morphology and charge transfer

While ZnO has excellent electrical properties, it has not been widely used for dye-sensitized solar cells, in part because ZnO is chemically less stable than widely used TiO(2). The functional groups typically used for surface passivation and for attaching dye molecules either bind weakly or etch the ZnO surface. We have compared the formation of molecular layers from alkane molecules with terminal carboxylic acid, alcohol, amine, phosphonic acid, or thiol functional groups on single-crystal zinc oxide (1010) surfaces. Atomic force microscopy (AFM) images show that alkyl carboxylic acids etch the surface whereas alkyl amine and alkyl alcohols bind only weakly on the ZnO(1010) surface. Phosphonic acid-terminated molecules were found to bind to the surface in a heterogeneous manner, forming clusters of molecules. Alkanethiols were found to bind to the surface, forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution. Monolayers of hexadecylphosphonic acid and octadecanethiol were further analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. AFM scratching shows that thiols were bound strongly to the ZnO surface, suggesting the formation of strong Zn-S covalent bonds. Surprisingly, the tridentate phosphonic acids adhered much more weakly than the monodentate thiol. The influence of organic grafting on the charge transfer to ZnO was studied by time-resolved surface photovoltage measurements and electrochemical impedance measurements. Our results show that the grafting of thiols to ZnO leads to robust surfaces and reduces the surface band bending due to midgap surface states.     

Surface potential studies of alkyl-thiol monolayers adsorbed on gold

The surface potential (contact potential difference) of monolayers of alkyl thiols adsorbed on gold have been measured using the Kelvin technique. The potential has been found to vary linearly with increasing chain length, n, for 6n22. The variation is discussed in terms of the changing nature of the permittivity of the hydrocarbon portion of the monolayer, a factor often ignored in studies self-assembled monolayers.     

Structural characterization of organic multilayers on silicon(111) formed by immobilization of molecular films on functionalized Si-C linked monolayers

Silicon(111)-H surfaces were derivatized with omega-functionalized alkenes in UV-mediated and thermal hydrosilylation reactions to give Si-C linked monolayers. Additional molecular layers of organic compounds were coupled either directly or via linker molecules to the functionalized alkyl monolayers. In the first instance, amino-terminated monolayers were prepared from a tert-butoxycarbonyl-protected omega-aminoalkene followed by removal of the protecting group. Various thiols were coupled to the monolayer using a heterobifunctional linker, which introduced maleimide groups onto the surface. In the second system, N-hydroxysuccinimide (NHS) ester-terminated monolayers were formed by reaction of Si-H with N-succinimidyl undecenoate. The reactivity of the NHS ester groups was confirmed by further modification of the monolayer. The stepwise assembly of these multilayer structures was characterized by X-ray reflectometry and X-ray photoelectron spectroscopy.     

Morphology of dry solid-supported protein monolayers dependent on the substrate and protein surface properties

The morphologies of dry MrgA protein monolayers on different solid substrates prepared by a three-step procedure (adsorption from an incubation solution, rinsing to remove excess salt and protein, and drying) were investigated using atomic force microscopy. MrgA is a dodecameric iron-storage protein which can form hexagonal, two-dimensional (2D) crystalline monolayers on hydrophilic surfaces at low supersaturation. The formation of such two-dimensional crystals is heavily dependent on the pH and the salinity of the incubation solution as well as on the surface properties. Correlation of surface coverage with substrate charge, ionic strength, and pH indicates the dominance of electrostatic effects in adsorption, with the balance shifting between intermolecular repulsion and protein-substrate attraction. Close to the isoelectric point (pI) of MrgA, adsorption to the surface and the formation of 2D crystals are favored. By preparation of self-assembled monolayers of thiols with different end groups on template-stripped gold, the surface properties can be varied easily from high to very low protein affinity. The resulting patterns of the crystalline protein structures are novel and could be a starting point for further scientific study, e.g., solid-supported cocrystallization with DNA, and indeed developments with technological applications, such as mesostructured deposition of MrgA-caged nanoparticles.     

Formation of alkanethiol monolayer on Ge(111).

Alkanethiols, CH3(CH2)(n-1)SH, are shown to react readily with HF-treated Ge(111) surface at room temperature to form a high-quality monolayer. The resulting films are characterized by using contact angle analysis (CAA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution electron energy loss spectroscopy (HREELS). The octanethiol and octadecanethiol films on Ge(111) both exhibit 101 degrees and 40 degrees water and hexadecane contact angles, respectively. These values indicate that the thiol surface coverage is relatively high, and that the films possess a high degree of orientational ordering. The angle-resolved XPS analysis supports that thiols are bound to the Ge surface by Ge-S bonds at the monolayer/Ge interface. The film thickness values obtained by XPS and SE agree well with the earlier reported values on alkyl monolayers on Ge(111) prepared by Grignard reaction. On the basis of HREEL spectra taken after thermal annealing steps, the monolayers are found to be thermally stable up to 450 K. The thermal stability provides further evidence that thiols are covalently bonded to Ge(111).     

The role of surface free energy on the formation of hybrid bilayer membranes

The interaction of small phospholipid vesicles with well-characterized surfaces has been studied to assess the effect of the surface free energy of the underlying monolayer on the formation of phospholipid/alkanethiol hybrid bilayer membranes (HBMs). The surface free energy was changed in a systematic manner using single-component alkanethiol monolayers and monolayers of binary mixtures of thiols. The binary surfaces were prepared on gold by self-assembly from binary solutions of the thiols HS-(CH(2))(n)()-X (n = 11, X = CH(3) or OH) in THF. Surface plasmon resonance (SPR), electrical capacitance, and atomic force microscopy (AFM) measurements were used to characterize the interaction of palmitoyl,oleoyl-phosphatidylcholine (POPC) vesicles with the surfaces. For all surfaces examined, it appears that the polar part of surface energy influences the nature of the POPC assembly that associates with the surface. Comparison of optical, capacitance, and AFM data suggests that vesicles can remain intact or partially intact even at surfaces with a contact angle with water of close to 100 degrees. In addition, comparison of the alkanethiols of different chain lengths and the fluorinated compound HS-(CH(2))(2)-(CF(2))(8)-CF(3) that characterize with a low value of the polar part of the surface energy suggests that the quality of the underlying monolayer in terms of number of defects has a significant influence on the packing density of the resulting HBM layer.     

Diverse redox-active molecules bearing O-, S-, or Se-terminated tethers for attachment to silicon in studies of molecular information storage

A molecular approach to information storage employs redox-active molecules tethered to an electroactive surface. Attachment of the molecules to electroactive surfaces requires control over the nature of the tether (linker and surface attachment group). We have synthesized a collection of redox-active molecules bearing different linkers and surface anchor groups in free or protected form (hydroxy, mercapto, S-acetylthio, and Se-acetylseleno) for attachment to surfaces such as silicon, germanium, and gold. The molecules exhibit a number of cationic oxidation states, including one (ferrocene), two [zinc(II)porphyrin], three [cobalt(II)porphyrin], or four (lanthanide triple-decker sandwich compound). Electrochemical studies of monolayers of a variety of the redox-active molecules attached to Si(100) electrodes indicate that molecules exhibit a regular mode of attachment (via a Si-X bond, X = O, S, or Se), relatively homogeneous surface organization, and robust reversible electrochemical behavior. The acetyl protecting group undergoes cleavage during the surface deposition process, enabling attachment to silicon via thio or seleno groups without handling free thiols or selenols.     

Strong resistance of oligo(phosphorylcholine) self-assembled monolayers to protein adsorption

In this work, we demonstrate the strong resistance of oligo(phosphorylcholine) (OPC) self-assembled monolayers (SAMs) to protein adsorption and cell adhesion. OPC SAMs were characterized using X-ray photoelectron spectroscopy (XPS), and protein adsorption was measured using a surface plasmon resonance (SPR) sensor. Results are compared with those of phosphorylcholine (PC) SAMs. Despite the existence of negative charge on OPC SAMs and the simple synthesis procedure of OPC thiols, OPC SAMs resist protein adsorption as effectively as or better than PC SAMs formed from highly purified PC thiols. The ease of their preparation and the effectiveness of their function make OPC SAMs an attractive alternative for creating nonfouling surfaces.     

Sum frequency generation microscopy of microcontact-printed mixed self-assembled monolayers

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.     

Self-assembled functional organic monolayers on oxide-free copper

The preparation and characterization of self-assembled monolayers on copper with n-alkyl and functional thiols was investigated. Well-ordered monolayers were obtained, while the copper remained oxide-free. Direct attachment of N-succinimidyl mercaptoundecanoate (NHS-MUA) onto the copper surface allowed for the successful attachment of biomolecules, such as β-d-glucosamine, the tripeptide glutathione, and biotin. Notably, the copper surfaces remained oxide-free even after two reaction steps. All monolayers were characterized by static water contact angle measurements, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. In addition, the biotinylated copper surfaces were employed in the immobilization of biomolecules such as streptavidin.     

Self-Assembled Monolayers of Nitron: Self-Activated and Chemically Addressable N-Heterocyclic Carbene Monolayers with Triazolone Structural Motif

N-heterocyclic carbenes (NHCs) have emerged as a unique molecular platform for the formation of self-assembled monolayers (SAMs) on various surfaces. However, active carbene formation requires deprotonation of imidazolium salt precursors, which is mostly facilitated by exposure of the salt to exogenous base. Base residues were found to be adsorbed on the metal surface and hindered the formation of well-ordered carbene-based monolayers. Herein, we show that nitron, a triazolone-based compound that freely tautomerizes to a carbene, can spontaneously self-assemble into monolayers on Pt and Au surfaces, which obviates the necessity for base-induced deprotonation for active carbene formation. SAMs of nitron were found to be thermally stable and could not be displaced by thiols, and thus their high chemical stability was demonstrated. The amino group in surface-anchored nitron was shown to be chemically available for S_N2 reactions, and makes surface-anchored nitron a chemically addressable cross-linking reagent for surface modifications.     

Spontaneous assembly of organic thiocyanates on gold sufaces. Alternative precursors for gold thiolate assemblies

Thiolate self-assembly on gold has proven to be a valuable technique for assembling monolayers on a wide variety of substrates. However, the oxidative instability of the thiols, especially aromatic thiols and alpha,omega-dithiols, presents several difficulties. Shown here is that thiocyanates, easily synthesized stable thiol derivatives, can be directly assembled on gold surfaces with no auxiliary reagents required. Assembly is complete in 24 h and leaves a similar gold thiolate structure as seen in typical thiol self-assembled monolayers.     

Electrochemical detection of nanoscale phase separation in binary self-assembled monolayers

Developing methods to probe the nature and structure of nanoscale environments continues to be a challenge in nanoscience. We report a cyclic voltammetry investigation of the internal, hydrogen-bond-driven phase separation of amide-containing thiols and alkanethiols. Amide-containing thiols with a terminal ferrocene carboxylate functional group were investigated in two binary monolayers, one homogeneously mixed and the other phase separated. The electrochemical response of the ferrocene probe was used to monitor adsorbate coverage, environment, and phase separation within each of these monolayers. The results demonstrate that the behavior of ferrocene-containing monolayers can be used to probe nanoscale organization.     

Mixed azide-terminated monolayers: a platform for modifying electrode surfaces

We have prepared and characterized mixed self-assembled monolayers (SAM) on gold electrodes from azido alkane thiols and various omega-functionalized alkane thiols. In the presence of copper(I) catalysts, these azide-modified surfaces are shown to react rapidly and quantitatively with terminal acetylenes forming 1,2,3-triazoles, via "click" chemistry. The initial azide substituents can be identified and monitored using both grazing-angle infrared (IR) and X-ray photoelectron spectrosopies. Acetylenes possessing redox-active ferrocene substituents react with the azide-terminated mixed SAMs and electrochemical measurements of the ferrocene-modified SAM electrodes have been used to quantify the redox centers attached to these platforms. Time-resolved electrochemical measurements have enabled us to follow the formation of these ferrocene centers and thus to measure the rate of the surface "click" reaction. Under optimal conditions this well-behaved second-order reaction takes place with a rate constant of 1 x 10(3) M(-)(1) s(-)(1). Typical reaction times of several minutes were realized using micromolar concentrations of acetylene. These techniques have been used to construct well-characterized, covalently modified monolayers that can be employed as functional electrode surfaces.