COOH-terminated Self-assembled Monolayers Research Articles

Characterization of COOH-Terminated Self-Assembled Monolayers and Adsorption Efficiency of DNA Molecules Studied by X-ray Photoelectron Spectroscopy and Near-Edge X-ray Absorption Fine Structure Spectroscopy

Self-assembled monolayers (SAMs) of three thiol derivatives with a COOH group at the terminus of the molecule were prepared on Au(111) substrates as templates for immobilization of DNA molecules. The adsorption states and the structures were examined by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. S 2p XPS results showed that these molecules are mostly chemisorbed on the Au(111) surface through S–Au bonds. The molecular orientation in these SAMs was elucidated using the polarization dependence of the C K-edge NEXAFS spectra. These results suggested that the film structure and the molecular packing differ among these SAMs. We compared the amounts of the amino-terminated DNA molecules adsorbed on these SAMs estimated from the peak intensity of the N 1s XP spectra. These results show that the adsorption efficiency of the DNA molecules depends on the structures of the COOH-terminated SAMs.

Surface Structures and Adsorption States of Self-Assembled Monolayers Formed by Various COOH-Terminated Alkanethiols on Au(111)

Surface structures and adsorption states of self-assembled monolayers (SAMs) formed by COOH-terminated alkanethiols (ATs) with different alkyl chain lengths were examined by scanning tunneling microscopy (STM), contact angle measurement, and X-ray photoelectron spectroscopy (XPS). STM imaging reveals that COOH-terminated ATs with longer alkyl chains [HS(CH2)nCOOH, n≥7] can easily form well-ordered SAMs, while COOH-terminated ATs with shorter alkyl chains hardly form ordered SAMs [HS(CH2)nCOOH, n≤5]. From contact angle and XPS measurements, however, we found that the most surface characteristics, such as surface wettability as well as the adsorption state and the amount of adsorbed molecules, are independent of alkyl chain length. In this study, we elucidate that the intermolecular interaction between alkyl chains is a very important factor in obtaining two-dimensional well-ordered COOH-terminated SAMs.

Fabrication of COOH-terminated self-assembled monolayers for DNA sensors

Surface-functionalization of the self-assembled monolayer (SAM) can be achieved by introducing a functional group in a molecule at the terminal. To immobilize biomolecules such as nucleotides and proteins on a gold substrate, COOH-terminated thiol SAMs are preferably employed. Here, we fabricated and characterized several kinds of SAMs of the COOH-terminated dithiol compounds by surface plasmon resonance (SPR), X-ray photoemission spectroscopy (XPS). Amino-terminated oligonucleotide was chemically bound on the SAM via the amide coupling using a condensing agent. Moreover, hybridization the DNA-immobilized SAM with target DNA was in situ monitored by SPR. S2p XPS spectra of the SAMs showed that the S atoms are chemically bound on the Au surface, suggesting the formation of the SAM on Au (1 1 1) substrate. The efficiency of DNA immobilization on the SAMs was evaluated by Nls XPS. The efficient hybridization by the SAM was observed by SPR compared with a COOH-terminated normal alkane thiol SAM system.

Deposition of TTF derivative on carboxyl terminated self-assembled monolayers

Oxygen-substituted tetrathiafulvalene (TTF) derivatives of bis(ethlenedioxy)tetrathiafulvalene (BEDO-TTF) were deposited on COOH-terminated alkylsilane self-assembled monolayers (SAMs). The BEDO-TTF molecules were effectively attached on the COOH-terminated SAMs. A formation of mixed valence state (MVS), which is a characteristic conducting phase of TTF, was observed by Fourier transform infrared spectroscopy. Such MVS formation is estimated to be brought about by an appearance of charge–transfer complex of (BEDO-TTF) 2(COO⋯H⋯OOC) through a chemical reaction with COOH group of the surface. Atomic force microscopy images revealed the existence of many small islands on the surface. It is expected that the first layer of BEDO-TTF molecules are absorbed onto COOH group of the surface through the reaction, then the first layer stimulates a crystallization of BEDO-TTF above it, giving rise to the islands formation.

Temperature-dependent intermolecular force measurement of poly( N-isopropylacrylamide) grafted surface with protein

We have investigated the temperature dependence of the intermolecular force between poly( N-isopropylacrylamide) (PNiPAM) grafted surface and bovine serum albumin (BSA) in phosphate buffer (pH 7.4) using atomic force microscopy at the nanonewton scale. These observations show that the interaction force is nearly zero below the phase transition temperature of PNiPAM and that it increases steeply during the phase transition. Since the PNiPAM chains are grafted onto the aminosilane ( γ-aminopropyltriethoxysilane)-treated silicon wafer, we measured the force–distance curve of BSA-immobilized tips for the bare and the aminosilane-treated silicon wafer. These surfaces show no temperature dependence and their values are different from those of the PNiPAM-grafted surfaces at 30 °C. The results indicate that the measured adhesion force is between the PNiPAM-grafted surface and the BSA-immobilized tip. Our studies on the intermolecular force between other surfaces (CH 3- and COOH-terminated self-assembled monolayers) and the BSA-immobilized tip indicate that the variation in the intermolecular force between the PNiPAM surface and BSA with temperature can be attributed to the changes in the properties of the PNiPAM chains. From consideration of the PNiPAM phase transition mechanism, it is speculated that the intermolecular force between the PNiPAM-grafted surface and BSA would be affected by changes in the arrangement of the bound water molecules around the PNiPAM chain and by changes in the conformation (i.e., in the chain mobility) of the PNiPAM chain during the phase transition.

Deciphering the interactions between liquid crystals and chemically functionalized surfaces: Role of hydrogen bonding on orientations of liquid crystals

This paper reports an investigation of the influence of interfacial hydrogen bonding on the orientations of liquid crystals at surfaces. The orientations of 10 liquid crystals were examined on carboxylic acid- or methyl-terminated self-assembled monolayers (SAMs) supported on obliquely deposited gold films. The mesogens comprising the liquid crystals each possessed different hydrogen bond acceptors. The azimuthal orientations (in-plane orientation of director relative to the direction of the deposition of gold films) of the liquid crystals were found to depend on the nature of the hydrogen bond acceptors in the mesogens when the SAMs presented carboxylic acid groups but were independent of the hydrogen bond acceptors when the SAMs presented methyl groups. Polarization modulated infrared reflection–absorption spectroscopy (PM-IRRAS) revealed that liquid crystals containing nitrogen atoms form hydrogen bonds with the carboxylic acid groups of the SAMs, and thereby disrupt the hydrogen bonds amongst carboxylic acid groups within the SAMs. Furthermore, liquid crystals that disrupt the interfacial hydrogen bonds within the SAMs assume an azimuthal orientation that is orthogonal to the orientation of liquid crystals that do not disrupt the hydrogen bonds within the SAMs. We conclude that the azimuthal orientations of the liquid crystals reflect the relative strength of hydrogen bonding between the mesogens and the COOH-terminated SAMs.

Grafting Nitrilotriacetic Groups onto Carboxylic Acid-Terminated Self-Assembled Monolayers on Gold Surfaces for Immobilization of Histidine-Tagged Proteins

In this paper, we report a common intermediate method to present nitrilotriacetic acid (NTA) groups on gold surfaces for immobilizing His-tagged proteins onto the surfaces, and a full characterization of self-assembled monolayers (SAMs) terminating in carboxylic acids [HS(CH2)15COOH (C15-COOH), HS(CH2)11(OCH2CH2)3OCH2COOH (EG3-COOH), and HS(CH2)11(OCH2CH2)5OCH2COOH (EG5-COOH)] and coupling reactions of an NTA-containing primary amine [(1S)-N-(5-amino-1-carboxypentyl)iminodiacetic acid; NTA−NH2] with the carboxylic acid on surfaces. The lateral packing densities of the COOH-terminated SAMs were calculated to be 4.32 (for C15-COOH), 3.49 (for EG3-COOH), and 2.65 (for EG5-COOH) molecules/nm2. The packing densities were decreased by incorporating a relatively flexible ethylene glycol (EG) group into the backbone of alkanethiols and increasing the number of the EG groups in the backbone of alkanethiols. The NTA group was then attached by coupling NTA−NH2 with the COOH group on the surfaces, followed by a Ni(II...

Influence of buried hydrogen-bonding groups within monolayer films on gas-surface energy exchange and accommodation.

Self-assembled monolayers (SAMs) of carbonyl-containing alkanethiols on gold are employed to explore the influence of hydrogen-bonding interactions on gas-surface energy exchange and accommodation. H-bonding, COOH-terminated SAMs are found to produce more impulsive scattering and less thermal accommodation than non-H-bonding, COOCH3-terminated monolayers. For carbamate-functionalized SAMs of the form Au/S(CH2)16OCONH(CH2)(n-1)CH3, impulsive scattering decreases and accommodation increases as the H-bonding group is positioned farther below the terminal CH3.

Electrokinetic Potentials of Binary Self-Assembled Monolayers on Gold: Acid−Base Reactions and Double Layer Structure

The electrokinetic charge density and the dissociation behavior of the surface carboxylic acid groups in binary self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA)/11-mercapto-undecanol (MUOH), and 16-mercaptohexadecanoic acid (MHA)/hexadecanethiol (HDT) on planar gold surfaces were studied by streaming potential and streaming current measurements in aqueous electrolyte solutions. For both two-component SAMs, the isoelectric point decreased with lower surface fractions (<0.25) of the carboxy-terminated alkanethiols, indicating that electrostatic interactions of the acid anions cause the significantly attenuated acidity of high coverage COOH-terminated SAMs. This agrees well with theoretical considerations on the influence of electrostatic energy on the interfacial acid-base equilibrium. Diffuse layer charge densities calculated from zeta potentials in monovalent electrolyte solutions show that the countercharge in the diffuse layer increases with decreasing surface charge but is due to inner layer counterion adsorption still significantly lower than predicted by the standard Gouy-Chapman theory. In the case of bivalent counterions, ion binding models successfully describe the observed electrokinetic potentials.

Adsorption of polyacrylic acid on self-assembled monolayers investigated by single-molecule force spectroscopy

Single-molecule force spectroscopy was used to study the adsorption and desorption processes of individual polyacrylic acid (PAA) chains from self-assembled monolayers (SAMs) with different terminal groups (CH3, OH, COOH). PAA was grafted to the tip of an AFM cantilever and all measurements were recorded with the same tip. For the OH- and the COOH-SAM, the desorption force was found to be independent of the salt concentration for NaCl, as well as for CaSO4. On CH3-SAM, a repulsive electrostatic interaction was found with NaCl in solution whereas a slightly attractive electrostatic interaction occurred with CaSO4 in solution. On the OH- and the COOH-terminated SAM, the non-electrostatic contributions to the desorption forces are of comparable magnitude (∼50–65 pN) whereas the non-electrostatic contribution for the CH3-terminated SAM is significantly higher (∼80–90 pN). Additional experimental findings concerning the length, the total number of adsorption events and the number of multiple polymer strands adsorbed in parallel, support the assumption that this difference in adsorption force is due to a different quality of the solvent for the polyacrylic acid chains at the SAM–water interface. Comparative surface pressure measurements at the air–water interface revealed a surface energy for polyacrylic acid of 30 mJ m2 which would correspond to a force contribution of ∼10 pN to the desorption force. This is in good agreement with the experimental results on hydrophobic SAMs. For further characterization of the surfaces the pH was varied from 3 to 11 at constant salt concentration (for a NaCl concentration of 100 mM and a CaSO4 concentration of 5 mM, respectively). In the presence of NaCl, the desorption force was found to be independent of the pH for all three substrates. For CaSO4, the desorption force was pH-independent for the OH- and the CH3-SAM. In the case of the COOH-SAM, a jump from 60 to 90 pN was recorded at pH 8, which we attribute to the formation of salt bridges between the polyacrylic acid chain and the COOH-SAM at higher pH.

Use of AFM to probe the adsorption strength and time-dependent changes of albumin on self-assembled monolayers.

The adsorption kinetics of human serum albumin (HSA) on CH3- and COOH-terminated self-assembled monolayers (SAMs) has been investigated using radioassays and atomic force microscopy (AFM). On both surfaces, the amount of HSA adsorbed reached a plateau after 30 min. The plateau level was higher on the CH3 compared to the COOH surface. The adhesion force (Fadh), measured using Si3N4 AFM tips in water, decreased with time of contact with the HSA solution on the CH3 surface. This time-dependent change in the adhesiveness of the adsorbed protein is best explained by a change in the conformation or orientation. In contrast, Fadh was independent of the time of contact with the HSA solution on the COOH surface, indicating that once adsorbed, the HSA molecules do not undergo further conformation or orientation changes. The perturbation induced by scanning with the AFM in water on the adsorbed HSA layers was greater on CH3 surfaces than on COOH surfaces, suggesting a weaker protein-substratum interaction on the CH3-terminated SAMs. This was further confirmed by a stronger desorption of HSA following sodium dodecyl sulfate (SDS) treatment on the CH3 surface compared to the COOH surface. Taken together, these data suggest that for COOH SAMs, (1) there is a strong interaction between HSA and the substratum; (2) there is an absence of reorientation with time; and (3) there is a smaller amount of adsorbed protein at 24 h, possibly due to increased but rapid spreading/denaturation of the protein. On the CH3 surface, less deformation of HSA occurs and the molecules maintain a higher mobility at short adsorption times. AFM measurements performed after aging of an adsorbed HSA layer in buffer suggests the role played by HSA in solution in determining the time-dependent conformation and/or orientation changes.

Chemical Effects on the Adhesion and Friction between Alkanethiol Monolayers: Molecular Dynamics Simulations

The chemical effects on the adhesion and friction between alkanethiol self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. Alkanethiol SAMs (S(CH2)nX) are investigated as a function of the terminal group (X = CH3, OH, COOH) and chain length n. The adhesive force is calculated as a function of the separation between two SAMs with the same terminal group. The maximum attraction for methyl-terminated SAMs is much weaker than for the OH- and COOH-terminated SAMs which form interlayer hydrogen bonds. The COOH-terminated SAMs have the strongest attraction. For the COOH- and OH-terminated SAMs, the adhesion−separation curves depend strongly on n. In both cases the change in the position of the attractive minimum is large as a function of n. The magnitude of the attractive minimum varies little for OH-terminated SAMs, and the variation is also small for COOH-terminated SAMs once the odd:even effect is considered. The source of the n dependence is structural changes within the...

The effect of hydrogen-bonding and terminal group structure on the dynamics of Ar collisions with self-assembled monolayers

Gas–surface scattering experiments are used to probe the dynamics of energy exchange in collisions of 80 kJ/mol Ar atoms with self-assembled monolayers (SAMs) composed of HS(CH 2) 11OH, HS(CH 2) 10COOH, HS(CH 2) 11CH 3, and HS(CH 2) 9CHCH 2 alkanethiols on gold. Strong similarities in scattering from the methyl and alkene-terminated SAMs indicate that end-group torsional motion and vibration of the terminal C–C bond play only a minor role in the gas–surface energy transfer dynamics. Both of the purely hydrocarbon monolayers proved to be highly effective at dissipating the Ar incident energy. In scattering experiments performed at a specular angle of 30°, over 60% of the incident Ar atoms reached thermal or near-thermal equilibrium before desorbing from the CH 3- and CH 2-terminated monolayers. For those atoms that do scatter directly from the surfaces after an impulsive collision, less than 13% of their incident energy is retained. In comparison, the OH- and COOH-terminated SAMs also yield nearly identical scattering results, but they appear to be much more rigid than the hydrocarbon monolayers. For the OH and COOH surfaces, only 43% of the atoms scatter at 30° off the surface normal after approaching thermal equilibrium and the atoms that scatter impulsively from the surface retain approximately 17% of their collision energy. Since these energy exchange characteristics do not appear to be the result of alterations in the structure or mass of the four monolayers, and the energy modes in the terminus of the monolayers have only minor influences on energy exchange, the differences in the scattering dynamics are attributed to the hydrogen-bonding nature of the COOH- and OH-terminated alkanethiol self-assembled monolayers.

Substrate-Determined Shape of Free Surface Profiles in Spin-Cast Polymer Blend Films

Sectional views of thin films of symmetric polystyrene/polyisoprene (PS/PI) blends spin- cast from toluene (C6H5CH3) onto CH3- and COOH-terminated self-assembled monolayers (CH3-SAM and COOH-SAM) show concave- (sharp-edged) and convex-shaped (round) protrusions, respectively, while other morphology features are identical. A 3-dimensional phase domain arrangement was determined with spectroscopic techniques (profiling and mapping mode of dynamic secondary ion mass spectrometry, dSIMS, and X-ray photoelectron spectroscopy). Surface topography was examined with atomic force microscopy and monitored, during the dSIMS analysis, with secondary electrons. In addition, solvent evaporation from PS, PI, and PS/PI layers cast on CH3-SAM and COOH-SAM was determined. The collected data were used to put forward a model of morphology formation and to elucidate the role of evaporation speed dependent on substrate chemistry in this process, demonstrated here for the first time.

Electron-transfer dynamics of cytochrome C: a change in the reaction mechanism with distance.

Directly linking a gold electrode coated with pyridine-terminated self-assembled monolayers (SAMs) to the redox-active heme unit of cytochrome c has enabled the electron-transfer mechanism to be studied as a function of distance. Comparison of the kinetic data for this system (see graph) with earlier data on COOH-terminated SAMs requires a change in the electron-transfer mechanism with distance from the electrode surface that does not involve large-amplitude conformational rearrangement.

Highly Selective Protein Patterning on Gold−Silicon Substrates for Biosensor Applications

Proteins were precisely patterned on 2D sensor surfaces using photolithography and chemical selectivity. Microarrays of gold squares were fabricated on silicon substrates. The gold regions were modified with mixed COOH-terminated self-assembled monolayers (SAMs) to have a high affinity for the desired proteins or peptides. The silicon regions were modified with polyethylene glycol (PEG) by silanization to provide a high resistivity to protein adsorption. Protein surface coverage was visualized by fluorescence microscopy and atomic force microscopy (AFM). AFM was also used for studying protein morphology to understand the interaction of proteins with SAMs at the molecular level. Proteins and peptides immobilized on SAMs were examined by Fourier transform infrared (FTIR) spectroscopy. Contact angle measurements for surface wettability were conducted to confirm the success of the surface modification reactions. Protein resistance by the PEGs immobilized on bare silicon substrates and on the silicon regions of gold-patterned silicon substrates was compared, and it was found that the latter has a higher resistivity to protein adsorption. Both fluorescence and high-resolution AFM images indicated that bovine serum albumin (BSA) and fibronectin molecules formed a densely packed layer on the gold regions of the patterned substrates, while the immunoglobulin's (IgG) coverage was low. Specific antigen−antibody binding (BSA−anti-BSA) was studied using the surface plasmon resonance (SPR) technique for characterizing the bioactivity of the antigen attached to the gold substrates. The SPR results showed that the BSA proteins bound covalently to the gold surfaces have a much better bioactivity than those bound physically. This study suggests that protein or peptide, molecular structures, and the immobilization technique influence the coverage, morphology, and bioactivity of the attached proteins on the substrates which is crucial to the operational behavior of biosensors.