COOH-terminated Self-assembled Monolayer Research Articles

Iron-containing metal-organic framework thin film as a drug delivery system

Selective bulk metal-organic frameworks (MOFs) have exhibited great potential in biomedical applications. However, topical treatments and drug elution coatings will require uniform films as drug delivery systems. This work studies the use of surface supportive MOF thin films for drug loading and releasing. More specifically, we focus on an iron-containing MOF, MIL-88B(Fe), on a COOH-terminated self-assembled monolayer (SAM) modified Au surface for encapsulating ibuprofen as a model drug. A combined experimental and computational approach was employed to study the fabrication of MIL-88B(Fe) film on functionalized Au surfaces. We used several surface characterization techniques, including infrared spectroscopy and scanning electron microscopy, to confirm the chemical composition and morphological changes of the surface after each modification step. The resulting MIL-88B(Fe) thin film was found capable of loading 8.7 wt% of ibuprofen using quartz crystal microbalance analysis. Moreover, we applied cluster simulations to study the binding mechanisms of MIL-88B(Fe) and its interactions with ibuprofen based on the density functional theory (DFT). The unsaturated Fe site was confirmed kinetically more favorable to bind to the COOH-end group on the SAM. Hydrogen bonding and π-CH interactions between ibuprofen and MIL-88B(Fe) promote ibuprofen being retained inside of the cages of MIL-88B(Fe).

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Model Asphaltenes Adsorbed onto Methyl- and COOH-Terminated SAMs on Gold.

Petroleum asphaltenes are surface-active compounds found in crude oils, and their interactions with surfaces and interfaces have huge implications for many facets of reservoir exploitation, including production, transportation, and oil-water separation. The asphaltene fraction in oil, found in the highest boiling-point range, is composed of many different molecules that vary in size, functionality, and polarity. Studies done on asphaltene fractions have suggested that they interact via polyaromatic and heteroaromatic ring structures and functional groups containing nitrogen, sulfur, and oxygen. However, isolating a single pure chemical structure of asphaltene in abundance is challenging and often not possible, which impairs the molecular-level study of asphaltenes of various architectures on surfaces. Thus, to further the molecular fundamental understanding, we chose to use functionalized model asphaltenes (AcChol-Th, AcChol-Ph, and 1,6-DiEtPy[Bu-Carb]) and model self-assembled monolayer (SAM) surfaces with precisely known chemical structures, whereby the hydrophobicity of the model surface is controlled. We applied solutions of asphaltenes to these SAM surfaces and then analyzed them with surface-sensitive techniques of near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). We observe no adsorption of asphaltenes to the hydrophobic surface. On the hydrophilic surface, AcChol-Ph penetrates into the SAM with a preferential orientation parallel to the surface; AcChol-Th adsorbs in a similar manner, and 1,6-DiEtPy[Bu-Carb] binds the surface with a bent binding geometry. Overall, this study demonstrates the need for studying pure and fractionated asphaltenes at the molecular level, as even within a family of asphaltene congeners, very different surface interactions can occur.

Application of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in an in-situ analysis of the stability of the surface-supported metal-organic framework HKUST-1 in water, methanol and pyridine atmospheres

Surface-supported metal-organic frameworks HKUST-1 (Hong Kong University of Science and Technology) were used as a model system for a development of a near ambient pressure (NAP) XPS based approach to investigate interaction with atmospheres of water, methanol or pyridine at pressures ranging from 1 to 4 mbar. The films were grown on a gold substrate functionalized with a COOH-terminated self-assembled monolayer using liquid-phase epitaxy in a step-by-step fashion. Measurement protocols were developed and optimised for different gases in order to obtain spectra of similar quality in terms of signal intensity, noise and shape. Peak shapes were found to depend on the efficiency of charge compensation. Reference measurements in argon proved to be a useful strategy not only for the evaluation of the Cu(II)-fraction in pristine samples, but also to identify the contributions by the respective gas atmosphere to the C 1s and O 1s photoelectron spectra. Reduced copper was found during the exposition of HKUST-1 to water vapour and pyridine, but this effect was not observed in case of methanol. Additionally, it was established that there are no changes in relative Cu(II) percentage with increasing exposure time. This indicates that saturation was reached already at the lowest time of gas exposure. A detailed elucidation of the mechanism of Cu(II) reduction to Cu(I) in HKUST-1 mediated by water and pyridine is part of ongoing work and not in the scope of the present paper.

Secondary Structures of Ubiquitin Ions Soft-Landed onto Self-Assembled Monolayer Surfaces.

The secondary structures of multiply charged ubiquitin ions soft-landed onto self-assembled monolayer (SAM) surfaces were studied using in situ infrared reflection-absorption spectroscopy (IRRAS). Two charge states of ubiquitin, 5+ and 13+, were mass selected separately from a mixture of different charge states produced by electrospray ionization (ESI). The low 5+ charge state represents a nativelike folded state of ubiquitin, while the high 13+ charge state assumes an extended, almost linear conformation. Each of the two charge states was soft-landed onto a CH3- and COOH-terminated SAM of alkanethiols on gold (HSAM and COOH-SAM). HSAM is a hydrophobic surface known to stabilize helical conformations of soft-landed protonated peptides, whereas COOH-SAM is a hydrophilic surface that preferentially stabilizes β-sheet conformations. IRRAS spectra of the soft-landed ubiquitin ions were acquired as a function of time during and after ion soft-landing. Similar to smaller peptide ions, helical conformations of ubiquitin are found to be more abundant on HSAM, while the relative abundance of β-sheet conformations increases on COOH-SAM. The initial charge state of ubiquitin also has a pronounced effect on its conformation on the surface. Specifically, on both surfaces, a higher relative abundance of helical conformations and a lower relative abundance of β-sheet conformations are observed for the 13+ charge state compared to the 5+ charge state. Time-resolved experiments indicate that the α-helical band in the spectrum of the 13+ charge state slowly increases with time on the HSAM surface and decreases in the spectrum of the 13+ charge state on COOH-SAM. These results further support the preference of the hydrophobic HSAM surface toward helical conformations and demonstrate that soft-landed protein ions may undergo slow conformational changes during and after deposition.

Neutron reflectometry studies of the adsorbed structure of the amelogenin, LRAP.

Amelogenins make up over 90% of the protein present during enamel formation and have been demonstrated to be critical in proper enamel development, but the mechanism governing this control is not well understood. Leucine-rich amelogenin peptide (LRAP) is a 59-residue splice variant of amelogenin and contains the charged regions from the full protein thought to control crystal regulation. In this work, we utilized neutron reflectivity (NR) to investigate the structure and orientation of LRAP adsorbed from solutions onto molecularly smooth COOH-terminated self-assembled monolayer (SAM) surfaces. Sedimentation velocity (SV) experiments revealed that LRAP is primarily a monomer in saturated calcium phosphate (SCP) solutions (0.15 M NaCl) at pH 7.4. LRAP adsorbed as ∼32 Å thick layers at ∼70% coverage as determined by NR. Rosetta simulations of the dimensions of LRAP in solution (37 Å diameter) indicate that the NR determined z dimension is consistent with an LRAP monomer. SV experiments and Rosetta simulations show that the LRAP monomer has an extended, asymmetric shape in solution. The NR data suggests that the protein is not completely extended on the surface, having some degree of structure away from the surface. A protein orientation with the C-terminal and inner N-terminal regions (residues ∼8-24) located near the surface is consistent with the higher scattering length density (SLD) found near the surface by NR. This work presents new information on the tertiary and quaternary structure of LRAP in solution and adsorbed onto surfaces. It also presents further evidence that the monomeric species may be an important functional form of amelogenin proteins.

Epifluorescence microscopic detection of photolithographically micropatterned aldehyde- and carboxy-terminated self-assembled monolayer

An aldehyde (CHO)-terminated self-assembled monolayer (SAM) was prepared on silicon substrates covered with native oxide (SiO2/Si) by chemical vapor deposition of triethoxysilylundecanal (TESUD). The CHO-terminated SAM was subsequently converted to the COOH-terminated SAM by photooxidation using 172-nm vacuum UV light through photomasks with square windows of 10 × 10 and 2.5 × 2.5 μm2. The chemical reactivity of each microspot composed of the COOH-terminated SAM was detected with high resolution by epifluorescence microscopy using Qdot® 655. These results demonstrated that the microspots (spot size: 2.5 × 2.5 μm2) composed of active COOH-terminated SAM were successfully fabricated on the SiO2/Si substrate at a high density of 8 × 104 spots/cm2, which was four times higher than that of previously reported miscrospots.

Surface plasmon resonance protein sensor using Vroman effect

We report a new surface plasmon resonance (SPR) protein sensor using the Vroman effect for real-time, sensitive and selective detection of protein. The sensor relies on the competitive nature of protein adsorption onto the surface, directly depending upon protein's molecular weight. The sensor uses SPR for highly sensitive biomolecular interactions detection and the Vroman effect for highly selective detection. By using the Vroman effect we bypass having to rely on bio-receptors and their attachment to transducers, a process known to be complex and time-consuming. The protein sensor is microfabricated to perform real-time protein detection using four different proteins including aprotinin (0.65 kDa), lysozyme (14.7 kDa), streptavidine (53 kDa), and isolectin (114 kDa) on three different surfaces, namely a bare-gold surface and two others modified by OH- and COOH-terminated self-assembled monolayer (SAM). The real-time adsorption and displacement of the proteins are observed by SPR and evaluated using an atomic force microscope (AFM). The sensor can distinguish proteins of at least 14.05 kDa in molecular weight and demonstrate a very low false positive rate. The protein detector can be integrated with microfluidic systems to provide extremely sensitive and selective analytical capability.

Impact of Surface Immobilization and Solution Ionic Strength on the Formal Potential of Immobilized Cytochrome c

Four different self-assembled monolayer (SAM) electrode systems were examined electrochemically in order to better understand surface charge effects on the redox thermodynamics of immobilized horse heart cytochrome c (cyt c). Neutralization of protein surface charge upon adsorption on anionic COOH-terminated SAMs was found to cause substantial changes in the formal potential, as determined by cyclic voltammetry. For cyt c immobilized on negatively charged surfaces, the formal potential shifted to more negative values as the ionic strength was decreased, which is opposite to the trend displayed by solution cyt c. In contrast, immobilization to uncharged interfaces resulted in an ionic strength dependence for cyt c that is similar to its solution behavior. The results provide insight into the importance of surface charge on the formal potential of cyt c.

Degradation of functionalized alkanethiolate monolayers by 0–18 eV electrons

Electron stimulated desorption of neutral molecular fragments is used to study degradation of ordered organic thin films under low-energy (0–18 eV) electron impact, and total electron doses ranging between 180–550 μC/cm2. Different saturated linear thiols HS(CH2)nX (n=2 or 15, and X=CH3 or COOH) are adsorbed from solution onto a gold surface to produce a self-assembled monolayer (SAM). Here, we present yield function measurements for electron stimulated desorption of moities such as H2, CH3, CH3CH2, CH3CH2CH2, CO, and CO2 from such thin chemisorbed films. For CH3-terminated SAMs, neutral fragment desorption thresholds lie between 5–7 eV, whereas for COOH-terminated SAMs, desorption thresholds as low as 0.2 and 3–5 eV are observed. The results suggest that the incident electrons interact with functional groups localized at the film–vacuum interface, which then leads to predominantly methyl group C–H, and C–COOH bond cleavage. In addition to nonresonant degradation mechanisms, which vary monotonically from threshold with increasing incident electron energy, structures in the neutral fragment desorption yield functions are related to resonant electron attachment. Particularly for Au–S(CH2)15COOH monolayers, this mechanism leads to a desorption peak of CO fragments at incident electron energies near 1.0 eV.

Surface-Initiated Free Radical Polymerization of Polystyrene Micropatterns on a Self-Assembled Monolayer on Gold

We describe the in situ synthesis of nanometer thick films of polystyrene (PS) on a self-assembled monolayer (SAM) on gold by surface-initiated free radical polymerization and further demonstrate that three-dimensional polymer structures with micrometer lateral resolution and nanometer vertical resolution can be fabricated by combining microcontact printing (μCP) with surface-initiated polymerization (SIP). We implemented SIP onto a COOH-terminated SAM on gold using a sequential approach to couple an amine-terminated free radical initiator to the terminal COOH groups presented by the SAM, followed by free radical polymerization of styrene. Each step of SIP was characterized by X-ray photoelectron spectroscopy and ellipsometry, and the PS film and micropattern were also characterized by atomic force microscopy. We typically obtained PS films with a thickness of 10−20 nm for a polymerization time of 12−24 h and with a root-mean-square roughness of ≤0.5 nm. PS micropatterns were also successfully fabricated ...

Atomic Force Microscopic Study of Specific Antigen/Antibody Binding

Using atomic force microscopy (AFM), we systematically studied the binding of three pairs of specific antigen/antibody systems: bovine serum albumin, tobacco etch virus capsid protein, and tobacco mosaic virus capsid protein and their respective specific antibodies. Our goals were to find a substrate for antigen immobilization, characterize individual antigen/antibody complexes, and investigate the antigen/antibody binding process. We found that the antigen protein can be immobilized on a −COOH-terminated self-assembled monolayer surface. Individual antigens and antigen/antibody complexes are easily identified from AFM images taken in liquid or under ambient laboratory conditions. The in situ studies suggest that antibody−antigen reactions occur in less than 4 min in buffer and that the reaction complexes are stable adsorbates once formed. As control experiments, nonspecific antibodies of equal and higher concentrations than those of specific antibodies have been used. There was no binding between nonspe...

Metal overlayers on organic functional groups of self-assembled monolayers. VI. X-ray photoelectron spectroscopy of Cr/COOH on 16-mercaptohexadecanoic acid

The interaction of Cr with the COOH groups of a self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid [HS(CH2)15COOH] on gold was studied by x-ray photoelectron spectroscopy (XPS). For each Cr overlayer thickness, a freshly prepared SAM was exposed to the evaporative deposition of Cr at ≤1×10−8 Torr and then transferred immediately in situ to a surface analysis chamber held at ≤5×10−10 Torr. Metallic Cr is deposited as evidenced by a Cr 2p3/2–2p1/2 splitting of 9.2 eV for 1 nm thick Cr films on Au. The C 1s and O 1s binding energies and intensities for the initial COOH-terminated SAM are consistent with the presence of the O=C—OH end group at the surface. For Cr overlayers of 0.04–1 nm average thickness, we identify Cr–COOH interactions primarily by changes in the O 1s level. For 0.04 nm Cr/COOH, the O 1s peak is narrowed and fitted by a single component at 532.9 eV that is between the O—C (533.9 eV) and O=C (532.5 eV) components of the bare SAM. The narrow width of this peak is evidence that the two oxygens of a single COOH end group interacting with Cr are indistinguishable by our instrument. For 0.07–1 nm average overlayer thickness, a low binding energy component (531.0–530.3 eV), assigned to a Cr(III) oxide, is of gradually increasing intensity. For coverages below 0.6 nm, analysis of XPS peak intensities indicates that adsorption of an adventitious oxygen-containing species has little effect on the results.