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.     

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.     

Adsorption of self-assembled monolayers of mercaptan on gold

XPS and AES are suitable techniques for studying organic monolayers on metals if radiation doses are kept low. The adsorption of self-assembled (SA) mercaptan monolayers on gold is a process in two stages. The adsorption to near completeness is very rapid. However, the process of orientation of the carbon chains, which is responsible for the blocking of electrochemical reactions takes much longer, as could be shown by ARXPS (angle resolved X-ray photo electron spectroscopy). Adsorption under potential control allows electrochemical experiments during the adsorption process as e.g. the measurement of the capacity of the electric double layer. Furthermore the control of the potential guarantees sure that the metal/liquid interface is well defined during the adsorption process.     

Adsorption of self-assembled monolayers of mercaptan on gold

XPS and AES are suitable techniques for studying organic monolayers on metals if radiation doses are kept low. The adsorption of self-assembled (SA) mercaptan monolayers on gold is a process in two stages. The adsorption to near completeness is very rapid. However, the process of orientation of the carbon chains, which is responsible for the blocking of electrochemical reactions takes much longer, as could be shown by ARXPS (angle resolved X-ray photo electron spectroscopy). Adsorption under potential control allows electrochemical experiments during the adsorption process as e.g. the measurement of the capacity of the electric double layer. Furthermore the control of the potential guarantees sure that the metal/liquid interface is well defined during the adsorption process.     

Determination of the surface pK of carboxylic- and amine-terminated alkanethiols using surface plasmon resonance spectroscopy

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.     

Imaging photoelectron transmission through self-assembled monolayers: the work-function of alkanethiols coated gold

In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolayers. Using this method, we measure the relative work-function of alkanethiols of different length on gold substrate, and we are able to follow the changes occurring when the surface coverage is varied. We also computed the work-function of model alkanethiols using a plane-wave density functional theory approach, in order to demonstrate the correlation between changes in the work-function with the monolayer organization and density.     

Protein interactions with self-assembled monolayers presenting multimodal ligands: a surface plasmon resonance study

This paper describes the use of surface plasmon resonance (SPR) spectroscopy and self-assembled monolayers (SAMs) to understand the characteristics of surfaces that promote the adsorption of proteins at high ionic strengths (high-salt conditions). We synthesized SAMs presenting different multimodal ligands and determined the influence of surface composition, solution composition, and the nature of the protein on the extent of protein adsorption onto the SAMs. Our results confirm that hydrophobic interactions can contribute significantly to protein adsorption under high-salt conditions. In particular, the extent of protein adsorption under high-salt conditions increased with increasing surface hydrophobicity. The extent of protein adsorption was also influenced by the solution composition and decreased with an increase in the chaotropicity of the anion. The combination of SPR and SAMs is well-suited for studying the interaction of proteins with complex surfaces of relevance to chromatography.     

Binary self-assembled monolayers of alkanethiols on gold: deposition from solution versus microcontact printing and the study of surface nanobubbles

The coadsorption of alkanethiols on noble metals has been recognized for a long time as a suitable means of affording surfaces with systematically varied wettability and other properties. In this article, we report on a comparative study of the composition of the mixed self-assembled monolayers (SAMs) obtained (i) by the coadsorption of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) from ethanol and chloroform onto gold substrates and (ii) by microcontact printing using poly(dimethyl siloxane) (PDMS) stamps. SAMs prepared by coadsorption from solution showed a preferential adsorption of ODT for both solvents, but this trend was reversed in microcontact-printed SAMs when using chloroform as a solvent, as evidenced by contact angle and Fourier transform infrared (FTIR) spectroscopy measurements. An approximately linear relationship between the static contact angle and the degree of swelling with different solvents was observed, which suggests that the surface composition can be controlled by the interaction of the solvent and the PDMS elastomer. The altered preference is attributed to the different partitioning of the two thiols into solvent-swelled PDMS, as shown by (1)H NMR spectroscopy. Finally, molecularly mixed binary SAMs on ODT and MHDA on template-stripped gold were applied to study the effect of surface nanobubbles on wettability by atomic force microscopy (AFM). With a decreasing macroscopic contact angle measured through water, the nanoscopic contact angle was found to decrease as well.     

Cholesterol biosensor based on amino-undecanethiol self-assembled monolayer using surface plasmon resonance technique

Cholesterol oxidase has been covalently immobilized onto 11-amino-1-undecanethiol hydrochloride (AUT) self-assembled monolayer (SAM) fabricated on gold (Au) substrates using glutaraldehyde as a cross-linker. These ChOx/AUT/Au bioelectrodes characterized using contact angle (CA) measurements; electrochemical technique and atomic force microscopy (AFM) have been utilized for the estimation of cholesterol in solution using the surface plasmon resonance (SPR) technique. These biosensing electrodes exhibiting linearity from 50 to 500 mg/dL of cholesterol in solution and sensitivity of 1.23 m0/(mg dL), can be used more than 20 times and have a shelf life of about 10 weeks when stored at 4 degrees C.     

Multichannel surface plasmon resonance imaging and analysis of micropatterned self-assembled monolayers and protein affinity interactions

Multichannel images of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol self-assembled monolayers together with a biospecific interferon-gamma (IFN-gamma)/anti-IFN-gamma antibody immunoaffinity interaction were observed by the two-dimensional surface plasmon resonance (2D-SPR) imaging system. With the fabricated 2D-SPR imaging system, adopting a white light source in combination with a narrow band-pass filter, sharp images were resolved, minimizing the diffraction patterns on the resulting images. Micropatterning of self-assembled monolayers was acheived by exploiting the UV photolysis of thiol bonding, instead of conventional photolithography. The line profile calibration of the image contrast with ellipsometric analysis enabled us to discriminate the change in monolayer thickness within a sub-nanometer scale. For the protein interactions on the surface, the biospecific affinity recognition reaction of IFN-gamma antigen with surface-immobilized antibody was analyzed. Through the signal amplification strategy based on the enzyme-catalyzed precipitation reaction in a sandwich-type immunoassay, biospecific antigen binding was found detectable down to a concentration of 1 ng/mL.     

In situ surface plasmon resonance measurements of self-assembled monolayers of ferrocenylalkylthiols under constant potentials.

A commercial system for surface plasmon resonance (SPR) possessing a batch-type flow channel has been simply modified so as to conduct in situ SPR measurements under polarization of an Au sensor chip at constant potentials. The modified instrument can monitor electrochemical reactions of monolayer materials with high stability and high reproducibility. The redox reaction of a self-assembled monolayer (SAM) of 6-ferrocenyl-1-hexanethiol (FcHT) induced the resonance angle shifts, the magnitudes of which accorded with the Nernst equation. The measurements in electrolyte solutions containing different electrolyte anions revealed that the SPR measurements detected ion pairing of electrolyte anions with oxidized FcHT. In cases of measurements in alkylsulfonic acid solution, simulation of the results based on the N-layer model has clarified that alkylsulfonate anions make an assembled layer on the FcHT SAM.     

Parameters important in fabricating enzyme electrodes using self-assembled monolayers of alkanethiols.

The fabrication of enzyme electrodes using self-assembled monolayers (SAMs) has attracted considerable interest because of the spatial control over the enzyme immobilization. A model system of glucose oxidase covalently bound to a gold electrode modified with a SAM of 3-mercaptopropionic acid was investigated with regard to the effect of fabrication variables such as the surface topography of the underlying gold electrode, the conditions during covalent attachment of the enzyme and the buffer used. The resultant monolayer enzyme electrodes have excellent sensitivity and dynamic range which can easily be adjusted by controlling the amount of enzyme immobilized. The major drawback of such electrodes is the response which is limited by the kinetics of the enzyme rather than mass transport of substrates. Approaches to bringing such enzyme electrodes into the mass transport limiting regime by exploiting direct electron transfer between the enzyme and the electrode are outlined.     

Chemical force microscopy of mixed self-assembled monolayers of alkanethiols on gold: evidence for phase separation

Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.     

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.     

Electrochemical surface plasmon resonance investigation of dodecyl sulfate adsorption to electroactive self-assembled monolayers via ion-pairing interactions

The redox-induced assembly of amphiphilic molecules and macromolecules at electrode surfaces is a potentially attractive means of electrochemically modulating the organization of materials and nanostructures on solid substrates via ion-pairing interactions or charge-transfer complexation. In this regard, we have investigated the potential-induced adsorption and aggregation of dodecyl sulfate, a common anionic surfactant, at a ferrocenylundecanethiolate (FcC11SAu) self-assembled monolayer (SAM)/aqueous solution interface by electrochemical surface plasmon resonance (ESPR) spectroscopy. The surfactant anions adsorb onto the electroactive SAM by specific ion-pairing interactions with the oxidized ferricinium species. The ferricinium charge density (QFc+) obtained by cyclic voltammetry and surface coverage measured by SPR indicate that the dodecyl sulfate forms an interdigitated monolayer, where half of the surfactant molecules have their sulfate headgroups paired to the surface and half have their headgroups exposed to the aqueous solution. The surface coverage of dodecyl sulfate was found to depend on both the ferricinium surface concentration and the surfactant aggregation state in solution. A maximum coverage of dodecyl sulfate on the ferricinium surface is obtained below the critical micelle concentration (cmc), in contrast to dodecyl sulfate adsorption to SAM surfaces of static positive charge. This marked difference in adsorption behavior is attributed to the dynamic generation of ferricinium by potential cycling and the specific nature of the ion-pairing interactions versus pure electrostatic ones. The results presented point to a new way of organizing molecules via electrical stimulus.     

Sensitive detection of tuberculosis using nanoparticle-enhanced surface plasmon resonance

We show that the antigen CFP-10 (found in tissue fluids of tuberculosis patients) can be used as a marker protein in a surface-plasmon resonance (SPR) based method for early and simplified diagnosis of tuberculosis. A sandwich SPR immunosensor was constructed by immobilizing the CFP-10 antibody on a self-assembled monolayer on a gold surface, this followed by blocking it with bovine serum albumin. Following exposure of the sensor surface to a sample containing CFP-10, secondary antibody immobilized on nickel oxide nanoparticles are injected which causes a large SPR signal change. The method has a dynamic range from 0.1 to around 150 ng per mL of CFP-10, and a detection limit as low as 0.1 ng per mL. This is assumed to be due to the high amplification power of the NiO nanoparticles.

Adsorption behavior of plasmid DNA on binary self-assembled monolayers modified gold substrates

Gold is known to have good biocompatibility because of its inert activity and the surface property can be easily tailored with self-assembled monolayers (SAMs). In previous works, gold surfaces were tailored with homogeneously mixed amine and carboxylic acid functional groups to generate surfaces with a series of isoelectronic points (IEPs). In other words, by tailoring the chemical composition in binary SAMs, different surface potentials can be obtained under controlled pH environments. To understand how the surface potentials affect the interaction at the interface, a binary-SAMs-modified Au electrode on a quartz crystal microbalance with dissipation detection (QCM-D) was used owing to the high weight sensitivity of QCM-D. In QCM-D, the frequency shift and the energy dissipation are monitored simultaneously to determine the adsorption behaviors of the plasmid DNA to surfaces of various potentials in Tris-buffered NaCl solutions of different pH. The results revealed that the plasmid DNA can be adsorbed on the SAM-modified surfaces electrostatically; thus, in general, the amount of adsorbed plasmid DNA decreased with increasing environmental pH and the decreasing ratio of the amine functional groups on the surfaces owing to weaker positive potentials on the surface. For the high amine-containing surfaces, due to the strong electrostatic attraction, denser films were observed, and thus, the apparent thickness decreased slightly. The negatively charged carboxylic acid surfaces can still adsorb the negatively charged plasmid DNA at some conditions. In other words, the electrostatic model cannot explain the adsorption behavior completely, and the induced dipole (Debye) interaction between the charged and polarizable molecules needs to be considered as well.     

Detection of ozone gas using gold nanoislands and surface plasmon resonance

Gold nanoislands interact with gaseous ozone to produce a surface plasmon resonance shift, similarly to the interaction of ozone and gold nanoparticles in water. Gold nanoislands are produced by sputtering, which significantly simplifies the synthesis and produces controlled size for the gold nanoislands. The shift of surface plasmon resonance peak was monitored while gold nanoislands were exposed to variable concentration of gaseous ozone. The shift was then correlated with ozone concentration. Our current results indicate sensing gaseous ozone at concentration of as low as 20 μg/L is achievable. Gold nanoislands were reversed to their original wavelength and were able to cycle between the wavelengths as ozone was introduced and removed. Potentially, this system can be useful as a sensor that identifies the presence of ozone at low part-per-billion concentrations of ozone in gaseous media.     

Dielectric properties and frequency response of self-assembled monolayers of alkanethiols

This paper presents dielectric properties of alkanethiol self-assembled monolayers (SAMs) under an ac electric field. Using a Hg-SAM/SAM-Hg junction, we measured the ac impedance of alkanethiol SAMs using a sinusoidal perturbation of 30 mV (peak-to-peak) with frequency ranging from 1 Hz to 1 MHz at zero bias. Semicircles at higher frequencies and at middle frequencies along with Warburg lines at lower frequencies were observed in complex plane impedance plots, that is, Nyquist plots. The frequency response of SAMs was analyzed by modeling the junction using an equivalent circuit and fitting the Nyquist plots. The semicircles at higher frequencies are attributed to the effect of the SAM/SAM interfaces, and the ones at middle frequencies are attributed to the effect of alkanethiol SAMs. The comparison in the plots of the imaginary part of the impedance Z against frequency for the bare Hg electrodes (in pure ethanal) and the SAM-covered Hg electrodes (in alkanethiol solution) supports the analysis. The Warburg lines are attributed to a certain ionic impurity. The dielectric loss spectra are further analyzed. Chain-length-dependent peaks, which correspond to different relaxation mechanisms, at higher frequencies and middle frequencies were observed in the spectra of the dissipation factor (tan delta vs frequency). The peaks move to small frequency with the increase of chain length of alkanethiols. Using a correlation of peak position with the chain length, we then derived active energies of 39-99 meV for alkanethiol SAMs of C7-C18 under an ac electric field.     

Solid-phase synthesis of alkanethiols for the preparation of self-assembled monolayers

Self-assembled monolayers (SAMs) of alkanethiols (ATs) on gold can be used to fabricate surfaces for nanoscience and biology. The chemical structure of the interface can be tailored simply by modifying the AT headgroup. To streamline access to different precursor ATs, we developed a general solid-phase synthetic route. A key feature of this route is the use of a modified resin containing an AT linker ("AT resin") because it minimizes purification steps. The precursor to the AT resin was prepared in five steps, and all of the synthetic intermediates are stable solids that can be purified by crystallization. Accordingly, the AT resin can be prepared on a multigram scale. The utility of the AT resin was evaluated by using it to generate a variety of ATs. For example, ATs presenting different types of integrin-binding ligands (linear and cyclic RGD derivatives) were prepared and used to form arrays of SAMs that support cell adhesion. Additionally, the AT resin also provides a starting point for the synthesis of ATs presenting reactive groups (e.g., an amine-reactive AT or a maleimide-containing alkanedisulfide) or protein immobilization tags (e.g., biotin-AT). Thus, our synthetic strategy provides a convenient and flexible means for the synthesis of the necessary building blocks for custom SAMs and SAM arrays.