Surface-confined energy transfer in mixed self-assembled monolayers

We have fabricated a set of self-assembled monolayers consisting of naphthalene and dansyl derivatives in a range of surface loading ratios for the purpose of examining excitation transport in mixed self-assembled monolayer systems. Both tethered chromophores were immobilized on an epoxide-terminated adlayer on silica via an identical spacer, where the linking chemistry produced an amide linkage. X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle measurements were used to characterize these chromophore-containing layers. The excitation transfer behavior of these monolayers has been examined using steady-state and time-resolved fluorescence spectroscopy. Steady-state fluorescence measurements show that excitation transfer from the naphthalene to dansyl chromophores occurs, with the efficiency of excitation transport scaling with chromophore surface loading densities, as expected. The donor lifetimes decrease with increasing acceptor loading density, and the functional form of the acceptor decay was independent of the donor/acceptor ratio. Our findings are not consistent with a homogeneous adlayer, but do provide information on the structural heterogeneity that is characteristic of these interfaces.     

Nanoscale chemical patterns fabricated by using colloidal lithography and self-assembled monolayers

A method for preparing surfaces with well-defined nanoscale chemical patterns is described. The fabrication strategy involves creating nanoscale Au pits surrounded by a TiO2 matrix, or vice versa, using colloidal lithography, followed by selective functionalization of the Au areas by CH3-terminated alkanethiols. Using AFM force spectroscopy with chemically modified tips (OH, CH3), we show that the nanopatterned surfaces display strong chemical contrast, in the form of hydrophobic CH3 nanopatches surrounded by a hydrophilic TiO2 surface, or vice versa. The nanofabrication approach presented here offers several advantages over existing patterning technologies, among which are easiness (no sophisticated instrumentation is required), versatility (patterns with a range of surface functionalities can be prepared), and the possibility to produce patterns over large areas at low cost.     

Fabrication of self-assembled monolayers exhibiting a thiol-terminated surface

We compare two different strategies for fabricating self-assembled monolayers (SAMs) exhibiting a thiol-terminated organic surface, which recently has attracted considerable interest with regard to the binding of metal, and in particular gold particles, to these organic surfaces. Results obtained by IR spectroscopy and scanning tunneling microscopy (STM) reveal that the straightforward approach, namely, using the corresponding organodithiols, typically leads to the formation of disordered, ill-defined surfaces. We demonstrate that high-quality organothiolate adlayers exhibiting an SH-terminated surface can be prepared using dithiols where one of the thiol groups is protected by a thioester group. After formation of the organothiolate adlayer, the protecting group can be removed by immersion into NaOH solution. IR spectra recorded for the deprotected organothiolate adlayer reveal the presence of a highly oriented organothiolate adlayer.     

Chemical surface modification of self-assembled monolayers by radical nitroxide exchange reactions

This article describes the application of nitroxide exchange reactions of surface-bound alkoxyamines as a tool for reversible chemical modification of self-assembled monolayers (SAMs). This approach is based on radical chemistry, which allows for introduction of various functional groups and can be used to reversibly introduce functionalities at surfaces. To investigate the scope of this surface chemistry, alkoxyamines with different functionalities were synthesized and were then applied to the immobilization of, for example, dyes, sugars, or biotin. Surface analysis was carried out by contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy measurements. The results show that this reaction is highly efficient, reversible, and mild and allows for immobilization of various sensitive functional groups. In addition, Langmuir-Blodgett lithography was used to generate structured SAMs. Site-selective immobilization of a fluorescent dye could be achieved by nitroxide exchange reactions.     

Tetraferrocenylporphyrins as active components of self-assembled monolayers on gold surface

Novel tetraferrocenylporphyrins-containing self-assembled monolayers were prepared employing two different approaches. Self-assembled monolayers were characterized using UV-Vis spectroscopy and cyclic voltammetry (CV) whereas their photoelectrochemical properties were investigated by photocurrent generation (PG) experiments.     

Adsorption kinetics of p-nitrobenzenethiol self-assembled monolayers on a gold surface

The adsorption kinetics of octadecanethiol (ODT) and p-nitrobenzenethiol (NBT) from ethanol solutions has been studied by means of contact angle, optical ellipsometry, angle-resolved X-ray photoelectron spectroscopy (ARXPS), and grazing angle attenuated total reflection Fourier transform infrared (FTIR) measurements. ODT data were used as a reference for the kinetics studies of film growth. The growth of self-assembled monolayers from dilute solutions follows Langmuir isotherm adsorption kinetics. A saturated film is formed within 5 h after immersion in solutions of concentrations ranging from 0.0005 to 0.01 mM. The density of the monolayer depends on the concentration of the solution.     

Dendrimer-functionalized self-assembled monolayers as a surface plasmon resonance sensor surface

We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects.     

Direct patterning of self-assembled monolayers on gold using a laser beam

The development of a methodology to manipulate surface properties of a self-assembled monolayer (SAM) of alkanethiol on a gold film using direct laser patterning is the objective of this paper. The present study demonstrates proof of the concept for the feasibility of laser patterning monolayers and outlines theoretical modeling of the process to predict the resulting feature size. This approach is unique in that it eliminates the need for photolithography, is noncontact, and can be extended to other systems such as SAMs on silicon wafers or potentially polymeric substrates. A homogeneous SAM made of 1-hexadecanethiol is formed on a 300-A sputtered film of gold (supported by a soda lime glass substrate). Localized regions are then desorbed by scanning the focal spot of a 488-nm continuous-wave argon ion laser beam under a nitrogen atmosphere. The desorption occurs as a result of a high substrate temperature produced by the moving laser beam with a Gaussian spatial profile at a constant speed of 200 microm/s. After completing the scans, the sample is dipped into a dilute solution of 16-mercaptohexadecanoic acid and a hydrophilic monolayer self-assembles along the previously irradiated regions. The resultant lines are viewed, and line widths are measured using both wetting with tridecane under a light microscope and scanning electron microscopy. Using the direct laser patterning method, we have produced straight line patterns with widths of 28-170 microm. A thermal model was constructed to predict the line width of the desorbed monolayer. The effect of the laser power, beam waist, and temperature dependence of the substrate conductivity on the theoretical predictions is considered. It is shown that the theoretical predictions are in good agreement with the experimental results, and, thus, the model can effectively be used to predict experimental results.     

Structures and dynamics of self-assembled surface monolayers observed by ultrafast electron crystallography

In this communication, we report our first study of self-assembled adsorbates on metal surfaces. Specifically, we studied single-crystal clean surfaces of Au(111) with and without a monolayer of reaction involving the assembly of 2-mercaptoacetic acid from 2,2'-dithiodiacetic acid. We also studied monolayers of iron hemes. With ultrafast electron crystallography, we are able to observe and isolate structural dynamics of the substrate (gold) and adsorbate(s) following an ultrafast temperature jump.     

Effects of surface hydrophobization on the growth of self-assembled monolayers on silicon

The growth of self-assembled monolayers from octadecyltrichlorosilane (OTS) on modified silicon surfaces has been investigated. The influence of different immersion times in a deactivation reagent on the growth mechanism and the ordering of the films has been studied. Characterization of the films and the submonolayer coverage has been performed with tapping mode atomic force microscopy, ellipsometry, and infrared spectroscopy. We found that a deactivation of active sites led to a higher mobility of adsorbed molecules on the surface resulting in circular islands of highly ordered alkylsiloxane. However, upon prolonged immersion in OTS these ordered islands did not continue to grow and full monolayer coverage could not be obtained. Instead, an exchange reaction with the deactivation reagent leading to a disordered film between the ordered islands was observed. This was confirmed by external reflection infrared spectroscopy.     

Negative surface potential produced by self-assembled monolayers of helix peptides oriented vertically to a surface

Self-assembled monolayers (SAMs) of helix peptides oriented vertically to a gold surface were prepared and the surface potential measured using the Kelvin technique up to 140°C. Negative surface potentials of a few hundred millivolts were observed for the helix peptide SAMs, indicating the occurrence of the large dipole moment of the helices directing toward the surface. The longer the helix peptide, the larger was the negative surface potential obtained. The absolute value of the surface potential decreased with increase in temperature due to thermal perturbation in the helical structure. However, Fourier transform infrared reflection–absorption spectroscopy revealed that perturbation is not significant and the α-helical conformation is stable even at 140°C.     

Second-harmonic spectroscopy of surface immobilized gold nanospheres above a gold surface supported by self-assembled monolayers

We have investigated linear and nonlinear optical properties of surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembled monolayers (SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be 3 x 10(5) for the SIGN system of nanospheres 100 nm in diameter with a gap distance of 0.8 nm. The corresponding susceptibility was estimated to be chi((2))=750 pmV (1.8 x 10(-6) esu). In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large chi((2)) is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the gold nanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.     

Amperometric glucose enzyme electrode by immobilizing glucose oxidase in multilayers on self-assembled monolayers surface

A novel and robust amperometric enzyme electrode for the determination of glucose was constructed by immobilizing glucose oxidase (GOD) and Os(bpy)(2)Cl-poly(4-vinyl)pyridine (Os-PVP) complex multilayers on thiol self-assembled monolayers surface. The apparent Michaelis-Menton constant K(m)' increased with increasing the number of Os-PVP/GOD multilayers. The concentration range of linear response and detection limit were 0.1-10 and 0.05 mM, the interference of ascorbic acid and uric acid were eliminated by the presence of SAMs and the enzyme electrodes were stable over 3 weeks. The preparation technique may be useful for controlling the performance of multilayer enzyme electrodes by changing the enzyme content.     

Photoluminescence studies of oligothiophene self-assembled monolayers at low excitation energy

Photoluminescence spectroscopy studies have been performed on self-assembled monolayers (SAMs) on Au(111) of thiophene oligomers with the number of thiophen rings N=3 and N=4. Photoluminescence spectra of SAMs reveal excitonic behavior with different band resolution and temperature dependence. These differences are attributed to different SAMs structure (degree of ordering).     

Functionalized self-assembled monolayers for measuring single molecule lectin carbohydrate interactions

The specific interactions between sugar-binding proteins (lectins) and their complementary carbohydrates mediate several complex biological functions. There is a great deal of interest in uncovering the molecular basis of these interactions. In this study, we demonstrate the use of an efficient one-step amination reaction strategy to fabricate carbohydrate arrays based on mixed self-assembled monolayers. These allow specific lectin carbohydrate interactions to be interrogated at the single molecule level via AFM. The force required to directly rupture the multivalent bonds between Concanavalin A (Con A) and mannose were subsequently determined by chemical force microscopy. The mixed self-assembled monolayer provides a versatile platform with active groups to attach a 1-amino-1-deoxy sugar or a protein (Con A) while minimizing non-specific adhesion enabling quick and reliable detection of rupture forces. By altering the pH of the environment, the aggregation state of Con A was regulated, resulting in different dominant rupture forces, corresponding to di-, tri- and multiple unbinding events. We estimate the value of the rupture force for a single Con A-mannose bond to be 95 ± 10 pN. The rupture force is consistent even when the positions of the binding molecules are switched. We show that this synthesis strategy in conjunction with a mixed SAM allows determination of single molecules bond with high specificity, and may be used to investigate lectin carbohydrate interactions in the form of carbohydrate arrays as well as lectin arrays.     

Conglomerate crystallization on self-assembled monolayers

In this communication, we demonstrate that chiral self-assembled monolayers can be used for polymorphism control of chiral crystals. We studied the crystallization of DL-glutamic acid on chiral self-assembled monolayers and showed that crystallization of DL-glutamic acid on the chiral SAMs resulted in stabilization of the metastable conglomerate form.     

Imidazolium ion-terminated self-assembled monolayers on Au: effects of counteranions on surface wettability

Self-assembled monolayers presenting imidazolium ions at the tail ends (SAMIMs) having different counteranions have been prepared on Au, and the measurement of water contact angles of the surfaces proved to be an extremely valuable simple technique for quantifying the effects of counteranions on hydrophilicity and hydrophobicity of SAMIMs, which will be extrapolated to the water miscibility of the related ionic liquids.     

Tuning the hydrophobic properties of silica particles by surface silanization using mixed self-assembled monolayers

Here we describe a novel method of preparing hydrophobic silica particles (100-150 nm; water contact angle of dropcasted film ranging from 60 degrees to 168 degrees) by surface functionalization using different alkyltrichlorosilanes. During their preparation, the molecular surface roughness is also concurrently engineered facilitating a change in both the surface chemical composition and the geometrical microstructure to generate hierarchical structures. The water contact angle has been measured on drop-cast film surface. The enhancement in the water contact angle on 3D (curved) SAMs in comparison to that on 2D (planar) surface is discussed using the Cassie-Baxter equation. These silica particles can be utilized for many potential applications including selective adsorbents and catalysts, chromatographic supports and separators in microfluidic devices.