Ion recognition properties of self-assembled monolayers (SAMs)

In the search for new sensors, self-assembled monolayers (SAMs) have gained intensive interest due to their nanometre size, highly-ordered structures, and molecular recognition properties. This article presents an overview of ion recognition at SAM-modified surface/solution interfaces, and brings up to date the most notable examples for the sensing of cations and anions. Sensing is achieved with SAMs containing redox active and inactive receptors using techniques such as fluorescence spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy.     

AFM forces measured between gold surfaces coated with self-assembled monolayers of 1-hexadecanethiol

An atomic force microscope (AFM) was used to measure the forces between gold surfaces with and without hydrophobizing them by the self-assembly of 1-hexadecanethiol. The forces measured between bare gold surfaces were fitted to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant of 1.2 x 10 (-20) J, which was close to the value determined using the methylene iodide contact angle method but was lower than that calculated using the Lifshitz theory. When the surfaces were hydrophobized in a 0.01 mM thiol-in-ethanol solution for 10 min, the measured forces exhibited a long-range force with a decay length of 35 nm. Despite its high water contact angle (105 degrees ), the force curve was smooth and exhibited no steps. When the surfaces were hydrophobized in a 1 mM thiol solution for longer than 6 h, however, the force curves exhibited steps, indicating that the long-range attractions were caused by bridging bubbles. When the measurements were conducted after washing the substrates with organic solvents, the steps disappeared and long-range attractive forces appeared. In the presence of ethanol, the water contact angle decreased to below 90 degrees , the attraction became weaker, and the force curves became smooth. On the basis of the results obtained in the present work, possible mechanisms for the long-range attractions are discussed.     

Self-assembled monolayers (SAMs) with photo-functionalities

Self-assembled monolayers (SAMs) of alkylthiol on metals, especially on gold, with photo-functionalities, such as photo-induced electron transfer, control of photo-electrochemical properties, control of electron transfer by photoisomerization, luminescence, and photo-patterning, are reviewed.     

Concentration dependence in microcontact printing of self-assembled monolayers (SAMs) of alkanethiols

This communication discusses the electrochemical assessment of self-assembled monolayers (SAMs) formed via microcontact printing with various concentrations of 1-hexadecanethiol (HDT) ink. At concentrations above 20 mM, the printed SAMs are shown to have very similar qualities to those formed from solution using much longer preparation procedures.     

Disorganised self-assembled monolayers (SAMs): the incorporation of amphiphilic molecules.

A new approach for designing a voltammetric selective electrode is presented. The approach is based on the formation of a disorganised inert self-assembled monolayer (SAM), in which an amphiphilic molecule is incorporated. The latter serves as the selectivity factor, which extracts the analyte. The purpose of these experiments is to study the parameters that affect the capability of a monolayer to host amphiphiles. As model systems we focused on the incorporation of simple amphiphilic molecules (quaternary alkyl ammonium salts), electroactive amphiphiles (dialkylviologens) and a macrocycle ligand (tetramethylcyclam) into octadecyl silane monolayers formed on indium tin oxide (ITO) and purposely made disorganised alkanethiols on gold. We find that basically, the incorporation of amphiphiles into a hydrophobic inert SAM resembles a reversed stationary phase in liquid chromatography and this configuration can be used for designing selective electrodes.     

UV photooxidation and photopatterning of alkanethiolate self-assembled monolayers (SAMs) on GaAs (001)

We have investigated the photooxidation of alkanethiolate self-assembled monoalyers (SAMs) adsorbed on GaAs (001) using time-of-flight secondary ion mass spectrometry. Both -CH3- and -COOH-terminated SAMs undergo photoreaction to form sulfonated species upon exposure to UV light from a 500 W Hg arc lamp (lambda = 280-440 nm) in the presence of oxygen. In contrast to SAMs adsorbed on metals, the photooxidation of octadecanethiol adsorbed on GaAs can be fit to two first-order reactions: a fast initial reaction followed by a second slower reaction ( approximately 6 times slower). For SAMs with shorter alkyl chain lengths, the photooxidation process is can be fit to a single first-order reaction. Using the optimal photooxidation time, we also demonstrate that SAMs can be successfully UV photopatterned on GaAs substrates producing sharp, well-defined patterns.     

Fabrication of self-assembled monolayers (SAMs) and inorganic micropattern on flexible polymer substrate

By grafting (aminopropyl)triethoxysilane (APTES) as the buffer layer on poly(ethylene terephthalate) (PET) surface, the SAMs ofoctadecyltrichlorosilane (OTS), phenyltrichlorosilane (PTCS), vinyltrichlorosilane (VTCS), andp-tolyltrichlorosilane (TTCS) were fabricated on the flexible polymer substrate. The properties of SAMs were accurately controlled by adjusting the immersing time of substrates in the solutions and the concentration of the solutions. The SAMs acted as templates, and TiO2 micropattern was successfully deposited on OTS, TTCS, and PTCS SAMs.     

Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B(10)H(13)] 1, [2-(HS)-nido-B(10)H(13)] 2, and [1,2-(HS)(2)-nido-B(10)H(12)] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B(18)H(22)·C(6)H(6) solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B(10)H(13)(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol (1)H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe(2)(PMe(2)Ph)(2)] reacts with the exposed and acidic B-H-B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe(2)Ph)(2)-nido-7-PtB(10)H(11)] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.     

Mixed self-assembled monolayers (SAMs) consisting of methoxy-tri(ethylene glycol)-terminated and alkyl-terminated dimethylchlorosilanes control the non-specific adsorption of proteins at oxidic surfaces

Monolayers from the newly synthesized compound methoxy-tri(ethylene glycol)-undecenyldimethylchlorosilane (CH3O(CH2CH2O)3(CH2)11Si(CH3)2Cl, MeO(EG)3C11DMS) and dodecyldimethylchlorosilane (DDMS), both pure and mixed, were prepared by self-assembly from organic solution in the presence of an organic base. The films obtained were characterized by advancing and receding contact angle measurements and ellipsometry to confirm the formation of self-assembled monolayers (SAMs). The resulting data on the covalently attached dimethylsilanes were compared to known oligo(ethylene glycol) (OEG)-terminated SAM systems based on terminal alkenes, thiolates or trihydrolyzable silanes. The composition of the mixed SAMs was found to depend directly and linearly on the composition of the silanization solution. Enhanced protein repellent properties were found for the SAMs using a variety of proteins, including the Ras Binding Domain (RBD), a protein with high relevance for cancer diagnostics. Roughly a RBD protein monolayer amount was adsorbed to silicon oxide surfaces silanized with DDMS or non-silanized silicon wafers, and in contrast, no RBD was adsorbed to surfaces silanized with MeO(EG)3C11DMS or to mixed monolayers consisting of DDMS and MeO(EG)3C11DMS if the content of OEG-silane overcame a critical content of X(EG) approximately 0.9.     

Self-assembled monolayers (SAMs) for electrochemical sensing

The past, present, and future of the application of self-assembled monolayers (SAMs) in electroanalytical chemistry is reviewed. SAMs for electroanalytical applications have been introduced in the early 1990s and since then have been exploited for the detection of different species ranging from metal ions to biomolecules and microorganisms. This review describes the different types of monolayers, surfaces on which they have been assembled, the various analytes, which were determined, and the various electrochemical techniques employed. The prospective and perspectives of this topic are discussed.     

Phosphonate self-assembled monolayers on aluminum surfaces

Substrates of aluminum (Al) deposited by physical vapor deposition onto Si substrates and then chemically reacted with perfluorodecylphosphonic acid (PFDPAlSi), decylphosphonic acid (DPAlSi), and octadecylphosphonic acid (ODPAlSi) were studied by x-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and friction force microscopy, a derivative of AFM, to characterize their surface chemical composition, roughness, and micro-/nanotribological properties. XPS analysis confirmed the presence of perfluorinated and nonperfluorinated alkylphosphonate molecules on the PFDPAlSi, DPAlSi, and ODPAlSi. The sessile drop static contact angle of pure water on PFDPAlSi was typically more than 130 degrees and on DPAlSi and ODPAlSi typically more than 125 degrees indicating that all phosphonic acid reacted AlSi samples were very hydrophobic. The surface roughness for PFDPAlSi, DPAlSi, ODPAlSi, and bare AlSi was approximately 35 nm as determined by AFM. The surface energy for PFDPAlSi was determined to be approximately 11 mNm by the Zisman plot method compared to 21 and 20 mNm for DPAlSi and ODPAlSi, respectively. Tribology involves the measure of lateral forces due to friction and adhesion between two surfaces. Friction, adhesion, and wear play important roles in the performance of micro-/nanoelectromechanical systems. PFDPAlSi gave the lowest adhesion and coefficient of friction values while bare AlSi gave the highest. The adhesion and coefficient of friction values for DPAlSi and ODPAlSi were comparable.     

The first observation of four-electron reduction in [60]fullerene-metal cluster self-assembled monolayers (SAMs)

Self-assembled monolayers (SAMs) of a mu 3-eta 2:eta 2:eta 2-C60 triosmium cluster complex Os3(CO)8(CN(CH2)3Si(OEt)3)(mu 3-eta 2:eta 2:eta 2-C60) (2) on ITO or Au surface exhibit ideal, well-defined electrochemical responses and remarkable electrochemical stability being reducible up to tetranionic species in their cyclic voltammograms.     

Ion recognition at the interface of self-assembled monolayers (SAMs) of bis-thioctic ester derivatives of oligo(ethyleneglycols)

Self-assembled monolayers (SAMs) of bis-thioctic ester derivatives of oligoethylene glycols were prepared. When the number of (-CH2-CH2-O-)n units in these podands was either five or six, the corresponding SAMs showed ion binding properties and selectivities similar to those exhibited by 15-crown-5 or 18-crown-6 in aqueous solution, respectively. Impedance data for the SAMs as a function of metal ion concentrations were fitted by using a Langmuir isotherm to determine the association constants (Ka) with the different ions. The SAM derived from the n = 5 compound is selective for Na+ while that with n = 6 is selective for K+. Results presented here confirm the formation of ion recognition domains during self-assembly of acyclic polyethylene glycol derivatives on gold surfaces; this suggests that surface-confined pseudocrown ether structures are formed.     

Commensurate solid-solid phase transitions in self-assembled monolayers of alkylthiolates lying on metal surfaces

A temperature-induced commensurate solid-solid phase transition in self-assembled monolayers (SAMs) of alkylthiolates lying on Pt(111) is predicted from molecular dynamics simulations based on ab initio potential energy surfaces. As the system cools down from room temperature to low enough temperature, SAMs of alkylthiolates with more than ～12 carbon atoms undergo an abrupt change of orientation from a nearly upright to a tilted configuration. As the initial hexagonal arrangement of the sulfur headgroups is kept fixed during the simulations, the phase transition is entirely governed by chain-chain interactions. Similar commensurate phase transitions are predicted for hexagonally arranged SAMs with lattice spacings of the order of 4.7-4.9 ?, which, among others, excludes the well-known cases of densely packed SAMs of alkylthiolates on Au(111) and Ag(111). These findings could be relevant for the design of novel electronic or optical devices controllable by temperature.     

Engineering silicon oxide surfaces using self-assembled monolayers

Although a molecular monolayer is only a few nanometers thick it can completely change the properties of a surface. Molecular monolayers can be readily prepared using the Langmuir-Blodgett methodology or by chemisorption on metal and oxide surfaces. This Review focuses on the use of chemisorbed self-assembled monolayers (SAMs) as a platform for the functionalization of silicon oxide surfaces. The controlled organization of molecules and molecular assemblies on silicon oxide will have a prominent place in "bottom-up" nanofabrication, which could revolutionize fields such as nanoelectronics and biotechnology in the near future. In recent years, self-assembled monolayers on silicon oxide have reached a high level of sophistication and have been combined with various lithographic patterning methods to develop new nanofabrication protocols and biological arrays. Nanoscale control over surface properties is of paramount importance to advance from 2D patterning to 3D fabrication.     

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.     

Ionic strength mediated hydrophobic force switching of CF3-terminated ethylene glycol self-assembled monolayers (SAMs) on gold

We have synthesised novel oligo(ethylene glycol), CF3-terminated switching self-assembled monolayers, which allow the force experienced by a hydrophobic object to be controlled via the ionic strength of the environment.     

Phase dependent electrochemical properties of polar self-assembled monolayers (SAMs) modified via the fusion of mixed phospholipid vesicles

Unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and varying quantities of either 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol) (sodium salt) (DMPG) or 1,2-dimyristoyl-3-trimethylammonium-propane (chloride salt) (DMTAP) were used to deposit lipid bilayer assemblies on self-assembled monolayers (SAMs) on gold. The supporting SAMs in turn were composed of ferrocene-functionalized hexadecanethiol chains (FcC16SH) diluted to low coverage in 1-hydroxylhexadecanethiol (HOC16SH) or a single-component monolayer phase of the latter. The mass coverages of the DMPC/DMPG layers deposited in this way were measured using surface plasmon resonance (SPR) and found to decrease with an increasing content of DMPG in the vesicles. The SPR data show that the lipid assembly, while stable with respect to gentle rinsing in aqueous buffer, is reversible and the lipid adlayer is removable by immersion in a solvent such as ethanol. The effects of the adsorbed lipid layer on the electrochemical interactions of the hybrid lipid/SAM with several redox probes [e.g., K4Fe(CN)6, Ru(NH3)6Cl3, and CsHsFe-[(C5H4CH2N+H(CH3)2] were characterized using cyclic voltammetry (CV). At a composition of 5% DMPG in DMPC, the permeabilities of the probes through the lipid layer were affected significantly relative to that observed with a pure DMPC layer. These effects include a striking observation of an enhanced, ionic-charge-specific molecular discrimination of the electrochemical probes. At higher concentrations of the DMPG, significant permeation of the lipid adlayer was seen for all the probes. These latter changes are also attended by a significant increase in the capacitive currents measured in CV experiments as compared to those observed for either a pure SAM or one modified by only DMPC. This effect likely results from the influence of the charged lipid on the diffuse Gouy-Chapman electrolyte layer at the SAM interface. In contrast to the behaviors seen with DMPG, the incorporation of DMTAP into the adsorbed DMPC had no impact on the permeation of the adlayer by soluble redox probes as judged by the observed electrochemistry, a result that appears to correlate with a less ideal mixing of lipids in the DMPC/DMTAP system relative to that of a DMPC/DMPG mixture.     

Surface modification of surface sol-gel derived titanium oxide films by self-assembled monolayers (SAMs) and non-specific protein adsorption studies

Biological events occurring at the implant-host interface, including protein adsorption are mainly influenced by surface properties of the implant. Titanium alloys, one of the most widely used implants, has shown good biocompatibility primarily through its surface oxide. In this study, a surface sol-gel process based on the surface reaction of metal alkoxides with a hydroxylated surface was used to prepare ultrathin titanium oxide (TiOx) coatings on silicon wafers. The oxide deposited on the surface was then modified by self-assembled monolayers (SAMs) of silanes with different functional groups. Interesting surface morphology trends and protein adhesion properties of the modified titanium oxide surfaces were observed as studied by non-specific protein binding of serum albumin. The surface properties were investigated systematically using water contact angle, ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurements. Results showed that the surface sol-gel process predominantly formed homogeneous, but rough and porous titanium oxide layers. The protein adsorption was dependent primarily on the silane chemistry, packing of the alkyl chains (extent of van der Waals interaction), morphology (porosity and roughness), and wettability of the sol-gel oxide. Comparison was made with a thermally evaporated TiOx-Ti/Si-wafer substrate (control). This method further extends the functionalization of surface sol-gel derived TiOx layers for possible titanium alloy bioimplant surface modification.