Hydroxyl-terminated Monolayers Research Articles

Survey of oligoethylene glycol-based self-assembled monolayers on electrochemical aptamer-based sensor in biological fluids

The ability to monitor levels of endogenous markers and clearance profiles of drugs and their metabolites can improve the quality of biomedical research and precision with which therapies are individualized. Towards this end, electrochemical aptamer-based (EAB) sensors have been developed that support the real-time monitoring of specific analytes in vivo with clinically relevant specificity and sensitivity. A challenge associated with the in vivo deployment of EAB sensors, however, is how to manage the signal drift which, although correctable, ultimately leads to unacceptably low signal-to-noise ratios, limiting the measurement duration. Motivated by the correction of signal drift, in this paper, we have explored the use of oligoethylene glycol (OEG), a widely employed antifouling coating, to reduce the signal drift in EAB sensors. Counter to expectations, however, when challenged in 37 °C whole blood in vitro, EAB sensors employing OEG-modified self-assembled monolayers exhibit both greater drift and reduced signal gain, compared with those employ a simple, hydroxyl-terminated monolayer. On the other hand, when EAB sensor was prepared with a mix monolayer using MCH and lipoamido OEG 2 alcohol, reduced signal noise was observed compared to the same sensor prepared with MCH presumably due to improved SAM construction. These results suggest broader exploration of antifouling materials will be required to improve the signal drift of EAB sensors.

Degradable Polycaprolactone and Polylactide Homopolymer and Block Copolymer Brushes Prepared by Surface-Initiated Polymerization with Triazabicyclodecene and Zirconium Catalysts.

Surface-initiated ring-opening polymerization (SI-ROP) of polycaprolactone (PCL) and polylactide (PLA) polymer brushes with controlled degradation rates were prepared on oxide substrates. PCL brushes were polymerized from hydroxyl-terminated monolayers utilizing triazabicyclodecene (TBD) as the polymerization catalyst. A consistent brush thickness of 40 nm could be achieved with a reproducible unique crystalline morphology. The organocatalyzed PCL brushes were chain extended using lactide in the presence of zirconium n-butoxide to successfully grow PCL/PLA block copolymer (PCL-b-PLA) brushes with a final thickness of 55 nm. The degradation properties of "grafted from" PCL brush and the PCL-b-PLA brush were compared to "grafted to" PCL brushes, and we observed that the brush density plays a major role in degradation kinetics. Solutions of methanol/water at pH 14 were used to better solvate the brushes and increase the kinetics of degradation. This framework enables a control of degradation that allows for the precise removal of these coatings.

Spatially Selective Formation of Hydrocarbon, Fluorocarbon, and Hydroxyl-Terminated Monolayers on a Microelectrode Array

A protection-deprotection strategy, using gold oxide as a passivating layer, was used to direct the self-assembly of monolayers (SAMs) selectively at individual gold microelectrodes in an array. This approach allowed the formation of hydroxyl-terminated monolayers, without side reactions, in addition to hydrocarbon and fluorocarbon SAMs. Fluorescence microscopy was used to visualize selective dewetting of hydrophobic monolayers by an aqueous dye solution, and spatially resolved X-ray photoelectron spectroscopy was used to demonstrate a lack of cross-contamination on neighboring microelectrodes in the array.

Examining the frictional forces between mixed hydrophobic – hydrophilic alkylsilane monolayers

Monolayers presenting methyl-terminated (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces on silica have been studied by molecular dynamics simulation and the effects of hydrogen bonding, chain length, and chain mixing on the frictional properties determined. The hydroxyl-terminated monolayers were found to show large adhesion zones as a result of strong interfacial interlayer hydrogen bonds; the interfacial sliding forces observed in the hydroxyl-terminated monolayers being one order of magnitude higher than the interfacial forces for the hydrophobic surfaces at the characteristic point of zero-load. Mixed hydroxyl- and methyl-terminated monolayers of equal length were found to exhibit intermediate shear stress values between those observed for pure monolayers, with the magnitude of the shear stress depending on the surface content of the hydroxyl-terminated chains. For mixed monolayers of unequal chain lengths, at high loads a maximum in the magnitude of the shear stress as a function of the length of the methyl-terminated chain was observed due to the creation of a buffer zone between the hydroxyl-terminated chains that produces strong hydrogen-bonding interactions. The effect of a constant normal load or constant separation simulation ensemble on the results has also been studied and in general found to have minimal influence on the observed behavior, although some differences are observed for the shear stress at intermediate normal loads due to the formation of stronger hydrogen bond networks at constant load compared to constant separation.

Direct Photochemical Functionalization of Si(111) with Undecenol

Direct UV photochemical functionalization of H-terminated Si(111) with bifunctional 10-undecen-1-ol was achieved with selective attachment via its vinyl end, resulting in the formation of a compact monolayer with free terminal alcohol groups. This is due to the faster radical propagation mechanism in hydrosilylation with alkene compared to the nucleophilic attack mechanism of alcohol, which is impeded by intermolecular hydrogen bonding present at room temperature. Evidence from X-ray photoelectron spectroscopy, infrared spectroscopy, and resistance to fluoride etching shows that Si-C is the interfacial bond, and atomic force microscopy shows the presence of a smooth, uniform monolayer conforming to the atomic terraces of the Si(111) surface. The application of such a hydroxyl-terminated monolayer was demonstrated by tethering a bromoinitiator through surface esterification and thereafter subjecting the surface to the surface-initiated atom-transfer radical polymerization of butyl methacrylate. The poly(butyl methacrylate) brushes formed were found to be smooth (R(a) < 0.3 nm) and uniform even for a thin film of 4.0 nm.

Fabrication of Hierarchical CaCO3 Mesoporous Spheres: Particle-Mediated Self-Organization Induced by Biphase Interfaces and SAMs

Highly ordered hierarchical calcium carbonate is an important phase involved in calcification by a wide variety of invertebrate organisms, and its formation is of technological interest in the development of functional materials. In this article, porous CaCO(3) hierarchical microspheres with a hedgehoglike appearance have been fabricated on the flexible substrate under mild conditions. There are two points that play important roles in the regular organization of the terminal products: one is the biphase interfaces, which are generated by organic solvent n-hexane and an aqueous saturated solution of Ca(OH)(2), and the other is hydroxyl-terminated monolayers assembled on the flexible PET (poly(ethylene terephthalate)) substrate. The SEM images show that novel CaCO(3) hierarchical microspheres consist of densely stacked "shuttles" by the oriented self-organization of CaCO(3) nanoparticles. The IR and XRD spectra indicate that the as-synthesized products are composed of a calcite phase obtained by an ACC (amorphous calcium carbonate)-to-calcite transformation. In view of the results, a nanoparticle-mediated self-organization process induced by biphase interfaces and SAMs template is proposed for the integration of functional materials and nanodevices.

Surface and Frictional Properties of Two-Component Alkylsilane Monolayers and Hydroxyl-Terminated Monolayers on Silicon

To investigate the effect of chain mobility and functional group exposure on friction, the tribological properties of one-component (pure) and two-component (mixed) alkylsilane monolayers on silicon, with variance in the ratios of long- and short-chain molecules, are presented. The tribological properties of these monolayers were measured with a ball-on-flat tribometer at 9.8 mN load and a speed of 0.1 mm/s. The molecular precursors investigated were n-hexyltrichlorosilane (C6), n-dodecyltrichlorosilane (C12), n-hexadecyltrichlorosilane (C16), n-octadecyltrichlorosilane (C18), and n-docosyltrichlorosilane (C22). Created from these molecules were the following four mixed monolayers: C6/C18, C12/C18, C16/C18, and C12/C22 in which the ratio of long- and short-chain molecules was varied in order to create different levels of mobility and average chain cant in the upper liquidlike region of the film. For the two-component monolayers, once a critical thickness is reached, the tribological properties were indistinguishable from the pure monolayers although the oleophilicity was much higher for the mixed films. Altering the terminal group of the monolayer to a hydroxyl, however, resulted in an increased coefficient of friction and significant adhesive forces between the monolayer and the stainless steel probe of the microtribometer. This tribological behavior can be explained by the low surface energies of the methyl and methylene groups and the increased adhesion that occurs with hydroxyl-terminated monolayers due to stronger interfacial interactions. Collectively, the results indicate that monolayer thickness and surface energy greatly affect the tribological properties of a monolayer but that molecular mobility within the film and interfacial oleophilicity are less important at these loads and speeds.

Square wave voltammetric detection of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene on a gold electrode modified with self-assembled monolayers

Abstract A rotating disc gold electrode, either bare or modified with an alkanethiol self-assembled monolayer, was used to measure the simultaneous square wave voltammetric differential current response of the electrochemical reduction of the two ordnance-related compounds (ORCs) 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT). On bare electrodes, background currents limited sensitivity to ppm levels. In order to suppress these interfering currents, the gold electrodes were modified with alkanethiols of various chain lengths, from 3 to 12 carbons, with hydrophilic (hydroxyl) or hydrophobic (methyl) end groups presented to the electrolyte. Gold electrodes endowed with hydroxyl-terminated monolayers enabled clear detection of TNT and DNT from mixtures of these two compounds. In addition, longer alkanethiols allowed extension of the working potential range to more negative values than achieved with shorter alkanethiols.

Electrochemistry and photoelectrochemistry of photosystem I adsorbed on hydroxyl-terminated monolayers

Direct electrochemistry studies on Photosystem I (PSI) were performed using cyclic voltammetry and square wave voltammetry. PSI centers stabilized in aqueous solution by Triton X-100 surfactant were adsorbed on hydroxyl-terminated hexanethiol modified gold electrodes. We have identified the electron donor, P700, and the electron acceptor sites, F A / F B , based on the previously reported preferred orientation for P700 on hydroxyl-terminated self-assembled monolayers. The reported potential values ( E P700 = +0.51 V vs. NHE; E FA/FB = −0.36 V vs. NHE) correlate very well with the established literature for P700, while the weaker signal for F A / F B lies within previous literature values. We were able to identify both redox centers on the same voltammogram. The P700 center clearly shows reversible electrochemical behavior. The expected F A / F B reduction is small in comparison, reflecting the dominant orientation of PSI with the F A / F B centers farther away and the P700 center nearer the electrode surface. As a molecular diode, PSI does not permit reverse direction conductivity to the F B so the small F A / F B peaks reflect other orientations besides the predominant one. PSI adsorbed on a hydroxyl-terminated hexanethiol modified gold substrate displays a photoenhanced reduction current for the P700 + center in the presence of light and an electron acceptor, methyl viologen. This photoelectrochemical response demonstrates protein functionality after adsorption.

Photoinduced Deprotection and ZnO Patterning of Hydroxyl-Terminated Siloxane-Based Monolayers

Trimethoxy-[11-(2-nitrobenzyloxy)undecyl]silane (1) and trimethoxy-[17-(2-nitrobenzyloxy)heptadecyl]silane (2) have been used for the covalent assembly of siloxane-based photopatternable monolayers. Exposing the monolayers to UV light (312 +/- 10 nm) results in the generation of reactive hydroxyl-terminated monolayers without affecting the film quality. The new monolayers, deprotection chemistry, and the effect of photoinduced headgroup lift-off on the monolayer microstructure have been studied in detail by a full complement of physicochemical techniques, including optical (UV-vis) spectroscopy, ellipsometry, aqueous contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), synchrotron X-ray reflectivity (XRR), and atomic force microscopy (AFM and AFM-force spectroscopy). AFM-force spectroscopy was used to analyze hydrogen-bond interactions as a function of the nature of the solid-liquid interface. AFM-force spectroscopy indicates a hydrogen-bond energy for photodeprotected monolayers of 8.2 kJ mol(-1) (approximately 2 kcal mol(-1)). Scanning electron microscopy (SEM) revealed that treatment of photopatterned monolayers with ZnEt2 solutions resulted in well-defined approximately 2 microm x 2 microm features of 10 A thick ZnO layers.

Leukocyte adhesion on model surfaces under flow: effects of surface chemistry, protein adsorption, and shear rate.

The effect of specific chemical functionalities on the adhesion of polymorphonuclear leukocytes (PMNs) under flow was investigated using a set of well-characterized, chemically functionalized surfaces prepared by self-assembly of alkanethiolate monolayers on gold surfaces. Terminal functionalities included CH(3), CH(2)OH, COOH, and (OCH(2)CH(2))(3)OH groups. A new surface modification was used to incorporate a phosphorylcholine moiety on the hydroxyl-terminated monolayer. Surface modification was verified using contact-angle measurements, ellipsometry, and X-ray photoelectron spectroscopy. Adhesion on the surfaces was studied in the presence and absence of pre-adsorbed fibrinogen. Fibrinogen adsorption on self-assembled monolayers (SAMs) was quantified using radioisotope detection. PMN adhesion was found to be dependent on the monolayer's terminal functionality. Adhesion was higher on the hydrophobic CH(3) surface and the polar COOH monolayer. Leukocyte adhesion was least on the phosphorylcholine-rich surface, followed by the ethylene-oxide-containing monolayer. Cell adhesion also was low on the hydrophilic OH monolayer. Attachment was decreased with increasing shear rate, exhibiting a three-fold decrease between 20 and 100 s(-1). Fibrinogen adsorption was higher on the CH(3) monolayer but comparable for the other four SAMs. Preincubation of the surfaces with fibrinogen decreased adhesion on all SAMs examined.

Interfacial Reactivity of Hydroxyl-Terminated Monolayers in the Absence of Solvents

We compare the interfacial reactivity between vapor- or liquid-phase heptanoyl chloride (C6COCl) and hydroxyl-terminated monolayers prepared from 4-hydroxythiophenol (HTP), 11-mercaptoundecanol (MUD), and a hydroxyl-terminated, fourth-generation poly(amidoamine) (PAMAM) dendrimer (G4-OH). Fourier transform infrared−external reflection spectroscopy indicates that both vapor- and liquid-phase C6COCl reacts with all three hydroxyl-terminated self-assembled monolayers (SAMs) to yield ester-coupled bilayers. The spectroscopic data were confirmed by contact-angle goniometry, which revealed advancing contact-angle increases of between 60 and 75° after vapor-phase reaction, and ellipsometric measurements, which indicated that the films increased in thickness by 0.4−0.6 nm. Similar results were obtained when the ester-coupling reaction was carried out in liquid-phase CH2Cl2. Real-time, quantitative surface acoustic wave (SAW)-based nanogravimetry indicates that the vapor-phase reactions go to completion in <1 min. The order of reactivity of the monolayers decreases in the order G4-OH > MUD > HTP. This is interpreted in terms of monolayer structure and the intrinsic properties of the particular coupling reaction studied.

Functionalization of carbon nanotube AFM probes using tip-activated gases

Multiwalled carbon nanotube (MWNT) probe microscopy tips have been functionalized with gases, activated in a transient arc produced at the tip ends. Adhesion measurements between these tips and hydroxyl-terminated monolayers versus pH reveal that MWNT tips reacted in O 2, H 2 and N 2 display acidic, pH-independent and basic properties, respectively. MWNT tips derivatized in O 2/N 2 and H 2/N 2 mixtures demonstrate the greater reactivity of carbon nanotubes towards O 2 and H 2, respectively. Moreover, the chemical reactivity of tips functionalized in N 2 suggests termination in aromatic nitrogen heterocycles. Tip-activated gas functionalization of MWNTs provides facile generation of chemically sensitive nanoprobes.

Modification of Silicon Nitride Tips with Trichlorosilane Self-Assembled Monolayers (SAMs) for Chemical Force Microscopy

Chemical modification of Si3N4 atomic force microscopy tips was performed by a mild pretreatment using ozone and alkaline/acid solutions and subsequent formation of self-assembled monolayers (SAMs) with two kinds of ω-alkenyltrichlorosilanes (CH2CH(CH2)nSiCl3, n = 4, 9; HTS and UTS, respectively). Hydroboration of their vinyl groups gave hydroxyl-terminated SAMs. The extent of SAM formation was assessed by determining the adhesion between Si3N4 tips on one hand and bare mica or mica substrates covered with UTS SAMs on the other hand. All adhesion forces were determined with the samples immersed in water to eliminate effects of capillary forces. Contact angles on analogously treated planar Si3N4 substrates were measured for comparison. Proper conditions for SAM formation on Si3N4 tips were found to be similar to those for formation of SAMs on Si3N4 substrates, as shown by the good temporal correlation between the adhesion forces observed between UTS-modified tips and mica and the contact angles of water on planar Si3N4 substrates modified under identical conditions. Due to hydrophobic interactions, strong adhesion between tips modified with HTS or UTS SAMs on one hand and UTS SAMs on mica on the other hand was observed. The adhesion of tips modified with HTS to hydrophobic surfaces decreased only slightly upon hydroboration of the vinyl groups. Furthermore, hydroxyl-terminated tips derived from HTS adhered more strongly to vinyl-terminated monolayers on mica than to hydroxyl-terminated monolayers. This suggests substantial disorder in the HTS-type SAMs formed on Si3N4 tips, resulting in strong hydrophobic interactions between exposed alkyl chains on the tips and the hydrophobic surfaces. This seems to conceal the specific interactions between the terminal functional groups. Evidence for such disorder is also given by FTIR spectra and by the larger friction forces obtained for HTS monolayers on Si3N4 than for UTS monolayers on Si3N4. On the other hand, conversion of the terminal vinyl groups of the UTS SAMs on the Si3N4 tips to hydroxyl groups led to a large decrease in the adhesion to hydrophobic UTS monolayers and a small increase in the adhesion to hydroxyl-terminated monolayers derived from UTS. This shows that formation of SAMs of trichlorosilanes with long alkyl chains on Si3N4 tips and subsequent functional group conversion is a promising method for the preparation of modified tips for chemical force microscopy.

Bacterial adhesion to hydroxyl- and methyl-terminated alkanethiol self-assembled monolayers

The attachment of bacteria to solid surfaces is influenced by substratum chemistry, but to determine the mechanistic basis of this relationship, homogeneous, well-defined substrata are required. Self-assembled monolayers (SAMs) were constructed from alkanethiols to produce a range of substrata with different exposed functional groups, i.e., methyl and hydroxyl groups and a series of mixtures of the two. Percentages of hydroxyl groups in the SAMs and substratum wettability were measured by X-ray photoelectron spectroscopy and contact angles of water and hexadecane, respectively. SAMs exhibited various substratum compositions and wettabilities, ranging from hydrophilic, hydroxyl-terminated monolayers to hydrophobic, methyl-terminated monolayers. The kinetics of attachment of an estuarine bacterium to these surfaces in a laminar flow chamber were measured over periods of 120 min. The initial rate of net adhesion, the number of cells attached after 120 min, the percentage of attached cells that adsorbed or desorbed between successive measurements, and the residence times of attached cells were quantified by phase-contrast microscopy and digital image processing. The greatest numbers of attached cells occurred on hydrophobic surfaces, because (i) the initial rates of adhesion and the mean numbers of cells that attached after 120 min increased with the methyl content of the SAM and the contact angle of water and (ii) the percentage of cells that desorbed between successive measurements (ca. 2 min) decreased with increasing substratum hydrophobicity. With all surfaces, 60 to 80% of the cells that desorbed during the 120-min exposure period had residence times of less than 10 min, suggesting that establishment of firm adhesion occurred quickly on all of the test surfaces.

Surface plasmon microscopy of biotin-streptavidin binding reactions on UV-photopatterned alkanethiol self-assembled monolayers

We report surface plasmon imaging of streptavidin binding to photopatterned biotinylated alkanethiol self-assembled monolayers (SAMs) on gold. Micrometer-scale patterns of a mixed biotin- and hydroxyl-terminated monolayer were formed in an inert, hydroxy-terminated alkanethiol monolayer using a UV-photopatterning procedure. Using surface plasmon microscopy, contrast is readily observed between the mixed biotin- and hydroxy-terminated SAM region after specific binding of streptavidin has occurred and the pure hydroxy-terminated region where nonspecific binding of streptavidin is negligible. Surface plasmon microscopy was also able to monitor in situ and in real time the binding of streptavidin to the patterned SAMs. The ability of surface plasmon microscopy to detect and spatially resolve 2-dimensional monolayer binding events may prove useful in diagnostic applications involving the parallel interrogation at surface biomolecular arrays.