Alkylthiol Self-assembled Monolayers Research Articles

Organic transistors realized by an environmental friendly microcontact printing approach

Abstract A patterning method was developed that allows for combining the advantages of a low cost microcontact printing process on rigid and flexible substrates with the advantages of conventional semiconductor processing. The patterning approach combines printing of self-assembled monolayers with selective dewetting while being compatible with conventional semiconductor processes. Alkyl thiol self-assembled monolayers were printed on gold and silver films on rigid and flexible substrates. The printed regions turn hydrophobic while the bare regions remain hydrophilic. The hydrophilic regions of the surface were selectively wetted by a polymer like poly-methyl methacrylate. The selectively patterned poly-methyl methacrylate is used as etch resist while the bare regions of the gold film were patterned by a potassium iodide/iodine solution. The potassium iodide/iodine etchant is compatible with conventional semiconductor processing in contrast to the commonly used ferri/ferrocyanide etching solutions. The method allows for patterning of gold and silver films with submicron dimension on flexible and rigid substrates. Organic thin-film transistors were realized with drain and source fabricated by the proposed approach. The performance of the organic transistor is comparable to devices fabricated by photolithography.

Soft Hedgehogs on Coarse Carpets: A Molecular Simulation Study of Capped Nanocrystals Interacting with Self-Assembled Monolayers of Alkylthiols on a Gold (111) Surface

We report atomistic simulations of alkylthiol capped gold nanocrystals (NCs) adsorbed on a gold (111) surface that is covered with a self-assembled monolayer (SAM) of alkylthiols. For these systems, we have computed the potential of mean force as a function of NC−surface distance for alkylthiols of varying tail length (butanethiol−dodecanethiol), temperature (250−400 K), and NC size (1.8−2.7 nm). The length of the thiol in the SAM determines alone the equilibrium distance between the NC and the surface. The equilibrium distance does not depend on the length of the capping molecule on a NC. We explain this finding by the efficient packing of capping molecules on top of the SAM. Conversely, the depth of the potential well only depends on the length of the capping molecule on the NC, and this dependence is linear. The well depth does not depend on the length of the thiol in the SAM. The structure of the SAM and of the capping layer on the NC is described using appropriate order parameters. We also study the behavior of systems where either the capping layer of the NC or the SAM on a flat surface is absent. Unlike capped NCs, bare NCs diffuse a few angstroms into the SAM. When a SAM is not present, the capping layer does not protect a NC from sintering with the surface.

D2O Water Interaction with Textured Carboxylic Acid-Terminated Monolayer Surfaces Characterized by Temperature-Programmed Desorption and Molecular Dynamics

We have used a combination of temperature-programmed desorption (TPD) experiments and molecular dynamics (MD) simulations to characterize interactions between water and carboxylic acid-terminated surfaces. In the TPD experiments, D2O water interacts with alkylthiol self-assembled monolayers (SAMs) comprised of 3-mercaptopropionic acid (C3 acid), 15-mercaptopentadecaonic acid (C15 acid), 16-mercaptohexadenoic acid (C16 acid), or two-component monolayers of these constituents. Water TPD spectra exhibit broad, first-order desorption profiles with maximum desorption temperatures ranging from 168 K during desorption from the C16 acid to a maximum desorption temperature of 200 K when water desorbs from the C3 acid surface. For water desorption from the C15 acid and for the two-component surfaces, desorption traces adopt intermediate profiles between these two extremes. Desorption activation energies range from 42 and 50 kJ mol−1. In the MD studies, submonolayer concentrations of adsorbed water interact with sla...

Model non-equilibrium molecular dynamics simulations of heat transfer from a hot gold surface to an alkylthiolate self-assembled monolayer

Model non-equilibrium molecular dynamics (MD) simulations are presented of heat transfer from a hot Au {111} substrate to an alkylthiolate self-assembled monolayer (H-SAM) to assist in obtaining an atomic-level understanding of experiments by Wang et al. (Z. Wang, J. A. Carter, A. Lagutchev, Y. K. Koh, N.-H. Seong, D. G. Cahill, and D. D. Dlott, Science, 2007, 317, 787). Different models are considered to determine how they affect the heat transfer dynamics. They include temperature equilibrated (TE) and temperature gradient (TG) thermostat models for the Au(s) surface, and soft and stiff S/Au(s) models for bonding of the S-atoms to the Au(s) surface. A detailed analysis of the non-equilibrium heat transfer at the heterogeneous interface is presented. There is a short time temperature gradient within the top layers of the Au(s) surface. The S-atoms heat rapidly, much faster than do the C-atoms in the alkylthiolate chains. A high thermal conductivity in the H-SAM, perpendicular to the interface, results in nearly identical temperatures for the CH(2) and CH(3) groups versus time. Thermal-induced disorder is analyzed for the Au(s) substrate, the S/Au(s) interface and the H-SAM. Before heat transfer occurs from the hot Au(s) substrate to the H-SAM, there is disorder at the S/Au(s) interface and within the alkylthiolate chains arising from heat-induced disorder near the surface of hot Au(s). The short-time rapid heating of the S-atoms enhances this disorder. The increasing disorder of H-SAM chains with time results from both disorder at the Au/S interface and heat transfer to the H-SAM chains.

The ability of surface characteristics of materials to trigger leukocyte tissue factor expression

Biomaterial-induced thrombosis is usually attributed to blood coagulation initiated by contact phase and platelet-related reactions. Considering the major role of extrinsic initiation in blood coagulation in vivo, we studied the material related-induction of this pathway by investigating the relevance of surface properties for the expression of Tissue Factor (TF), a critical initiator of the extrinsic pathway of coagulation. We incubated materials with self-assembled monolayers of alkylthiols (SAMs) displaying various ratios of –CH 3, –OH, and –COOH terminations with fresh heparinized whole human blood in vitro. The transcription of TF-mRNA in leukocytes showed clear differences in relation to surface properties and increased over time. In addition, a positive correlation between TF transcription and its presence on leukocytes, granulocyte activation, and complement activation was found. Cells displaying the highest TF expression after material contact had significantly lower intracellular TF, pointing to previous TF release. Yet under the conditions of our whole blood incubation set-up within the limited time frame the observed initiation of the extrinsic pathway did not trigger blood coagulation.

Blood coagulation on biomaterials requires the combination of distinct activation processes

The rational design of hemocompatible materials requires a mechanistic understanding of activation processes induced at the blood–material interface. Binary self-assembled monolayers of alkyl thiols (SAMs) with various ratios of –CH3 and –COOH terminations were used to study the relevance of hydrophobic and negatively charged surfaces for the initiation of blood coagulation. Platelet adhesion and activation of the intrinsic coagulation pathway scaled with the surface composition: the numbers of adherent platelets were highest on the 100%-CH3 surface whereas the greatest contact activation was seen on 100%-COOH surfaces. In vitro whole blood incubation assays showed, however, that the surfaces exposing either –CH3or –COOH groups induced comparably low levels of thrombin formation while the surfaces with intermediate contents of both terminating groups had significantly higher values. These results reveal that contact activation and platelet adhesion have a strong synergistic effect on coagulation on blood-contacting materials even though these events in isolation are not sufficient to induce substantial thrombin formation. Successful surface design strategies for hemocompatible materials therefore need to carefully consider the interplay of both processes.

Characterisation of Self-Assembled Monolayers on Germanium Surfaces via NEXAFS

The formation of self-assembled monolayers (SAMs) by suitable organic molecules with appropriate anchor groups on semiconductor surfaces may be used either to probe both the chemical state and the quality of the surface or to achieve surface passivation. Molecules with thiol anchor groups are able to bond to hydrogen-terminated germanium surfaces (Ge-S bond). We have prepared SAMs of alkylthiols with trifluoroacetate head groups on germanium. The germanium surface prior to and after SAMs formation has been characterised by Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS) with synchrotron radiation in the PTB laboratory at BESSY II. We succeeded in assigning S-NEXAFS peaks to the Ge-S bond formed during self-assembly and were able to distinguish between different fluorine species via F-NEXAFS.

Dynamics of the Reaction of O(3P) Atoms with Alkylthiol Self-assembled Monolayers

We have studied the dynamics of the reactions of O((3)P) atoms with alkylthiol self-assembled monolayers (SAMs). Superthermal O((3)P) atoms, with a fairly broad distribution of laboratory-frame kinetic energies (mean = 16 kJ mol(-1), fwhm = 26 kJ mol(-1)), were generated by 355 nm photolysis of NO(2) introduced at a low pressure above the SAM surface. Nascent OH v' = 0 products were detected by laser-induced fluorescence. SAMs of two different alkyl chain lengths, C(6) and C(18), were studied. The existence of SAM layers, and their robustness under our experimental conditions during the relevant measurement period, were confirmed by scanning-tunneling microscopy (STM). Reaction at the SAM surface was verified as the authentic source of the hydroxyl radicals using a perdeuterated C(6)D(13)-SAM sample. The OH appearance profiles as a function of photolysis-probe delay, and the rotational-state distributions at their peaks, were compared with those of liquid squalane (C(30)H(62), 2,6,10,15,19,23-hexamethyltetracosane). The reactivity of the SAMs and of squalane was found to be comparable. We conclude that the O((3)P) atoms must be able to access the more reactive secondary hydrogen atoms along the alkyl chains of the SAMs. We find no perceptible differences in reactivity or product energy disposal between the two SAM chain lengths. Both produce a substantial fraction of the OH with relatively high velocities, which must result from direct, impulsive reaction. There is also a slower component, with velocities consistent with a thermal, trapping-desorption mechanism. The proportion of this component appears to be lower for SAMs than for squalane. This would be compatible with the expected greater smoothness of the SAM surface at the molecular scale. We find little evidence for significant rotational excitation of the OH products, although the details of any correlation between translational and rotational energy release require further investigation. We compare our results with the limited available prior theoretical modeling of O((3)P) + SAM systems.

A molecular dynamics study on heat transfer characteristics at the interfaces of alkanethiolate self-assembled monolayer and organic solvent

In this paper, we present molecular dynamics (MD) simulations of interfaces composed of self-assembled monolayers (SAMs) and solvents in order to investigate the heat transfer characteristics at the interface. Two typical normal alkylthiolate SAMs with different chain lengths, i.e., 1-propanethiol C(3)H(7)SH and 1-dodecanethiol (C(12)H(25)SH) chemically adsorbed on Au(111) substrate surfaces, were used, and toluene was adopted as the organic solvent. In addition to the SAM systems, an interface composed of the bare solid substrate and solvent (without SAMs) was analyzed for comparison. Nonequilibrium MD simulations, in which a temperature gradient perpendicular to the interface was imposed, were performed and the difference in thermal boundary resistance in the interface systems was discussed. We observed that the SAM interfaces have smaller thermal resistance when compared with that of the bare solid interface. In order to understand the mechanisms of the small resistance at the SAM-solvent interfaces, the vibrational character of molecules in each phase, which contacted each other at the interface was analyzed and a detailed adsorbed structure of solvent molecule in the interface region was extracted. As a result, a clear difference in these characters was found between the SAM interfaces and bare solid interface.

Preparation and Characterization of Self-Assembled Monolayers on Germanium Surfaces

The formation of self-assembled monolayers (SAMs) by specific organic molecules with appropriate anchor groups on semiconductor surfaces may be used to probe the chemical state and quality of the surface or to achieve surface passivation. Molecules with thiol anchor groups are able to bond to hydrogen-terminated germanium surfaces (Ge-S bond). We have prepared SAMs of alkylthiols with different head groups on germanium. Since the surface preparation of germanium is neither well understood nor developed, the controlled preparation of an oxide-free completely H-terminated surface which is a prerequisite for SAM formation of alkylthiols turned out to be a major challenge. Several approaches have been studied. The characterization of the germanium surface prior to and after SAMs formation has been performed by AFM, XPS, Synchrotron-TXRF and -NEXAFS.

Control of Steric Hindrance on Restriction Enzyme Reactions with Surface-Bound DNA Nanostructures

To understand better enzyme/DNA interactions and to design innovative detectors based on DNA nanoarrays, we need to study the effect of nanometric confinement on the biochemical activity of the DNA molecules. We focus on the study of the restriction enzyme reactions (DpnII) within DNA nanostructures on flat gold films by atomic force microscopy (AFM). Typically we work with a few patches of DNA self assembled monolayers (SAMs) that are hundred nm in size and are lithographically fabricated within alkylthiol SAMs by AFM nanografting. We start by nanografting a few patches of a single-stranded DNA (ssDNA) molecule of 44 base pairs (bps) with a 4 bps recognition sequence (specific for DpnII) in the middle. Afterwards, reaction-ready DNA nanopatches are obtained by hybridization with a complementary 44bps ssDNA sequence. The enzymatic reactions were carried out over nanopatches with different density. By carrying out AFM height measurements, we are able to show that the capability of the DpnII enzyme to reach and react at the recognition site is easily varied by controlling the DNA packing in the nanostructures. We have found strong evidence that inside our ordered DNA nanostructures the enzyme (that works as a dimer) can operate down to the limit in which the space between adjacent DNA molecules is equal to the size of the DNA/enzyme complex. Similar experiments were carried out with a DNA sequence without the recognition site, clearly finding that in that case the enzymatic reaction did not lead to digestion of the molecules. These findings suggest that it is possible to tune the efficiency of an enzymatic reaction on a surface by controlling the steric hindrance inside the DNA nanopatches without vary any further physical or chemical variable. These findings are opening the door to novel applications in both the fields of biosensing and fundamental biophysics.

Adsorption and release behavior of bare and DNA-wrapped-carbon nanotubes on self-assembled monolayer surface

The adsorption and release behavior of single-stranded DNA-wrapped single-walled carbon nanotubes (ssDNA-w-SWCNTs) on alkylthiol self-assembled monolayer (SAM) surface was systematically characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Fast electron transfer between bare Au electrode and redox species blocked by the alkylthiol SAM can be restored by SWCNTs or ssDNA-w-SWCNTs. The release of ssDNA-w-SWCNTs is carried out by positive or negative desorption potential. SWCNTs/SAM or ssDNA-w-SWCNTs/SAM is completely removed from Au surface at + 0.90 V or − 1.40 V (vs. 3.0 M KCl|Ag|AgCl). The controlled release of SWCNTs/SAM and ssDNA-w-SWCNTs/SAM holds great promise for gene delivering.

Fluorescence Imaging of the Oxidative Desorption of a BODIPY-Alkyl-Thiol Monolayer Coated Au Bead

The reductive and oxidative desorption of a BODIPY labeled alkylthiol self-assembled monolayer (SAM) on Au was studied using electrochemical methods coupled with fluorescence microscopy and image analysis procedures to monitor the removal of the adsorbed layer. Two SAMs were formed using two lengths of the alkyl chain (C10 and C16). The BODIPY fluorescent moiety used is known to form dimers which through donor-acceptor energy transfer results in red-shifted fluorescence. Fluorescence from the monomer and dimer were used to study the nature of the desorbed molecules during cyclic step changes in potential. The reductive desorption was observed to occur over a small potential window (0.15 V) signified by an increase in capacitance and in fluorescence. Oxidative readsorption was also observed through a decrease in capacitance and a lack of total removal of the fluorescent layer. Removal by oxidative desorption occurred at positive potentials over a broad potential range near the oxidation of the bare Au. The resulting fluorescence showed that the desorbed molecules remained near the electrode surface and were not dispersed over the 20 s waiting time. The rate of change of the fluorescence for oxidative desorption was much slower than the reductive desorption. Comparing monomer and dimer fluorescence intensities indicated that the dimer was formed on the Au surface and desorbed as a dimer, rather than forming from desorbed monomers near the electrode surface. The dimer fluorescence can only be observed through energy transfer from the excited monomer suggesting that the monomers and dimers must be in close proximity in aggregates near the electrode. The fluorescence yield for longer alkyl chain was always lower presumably due to its decreased solubility in the interfacial region resulting in a more efficient fluorescence quenching. The oxidative desorption process results in a significantly etched or roughened electrode surface suggesting the coupling of thiol oxidative removal and Au oxide formation which results in the removal of Au from the electrode.

Noncontact to contact tunneling microscopy in self-assembled monolayers of alkylthiols on gold

The mechanical interaction between a scanning tunneling microscopy (STM) probe and hexadecane (C16) alkylthiol molecules in a self-assembled monolayer was investigated by sensing the force during constant current mode STM imaging. The force regime changed from attractive to repulsive over the insulating molecule islands under feedback control of the current. The repulsive force on the molecule was strongly dependent on the setpoint value of the current during STM operation. In our experiments, the threshold for contact was found at a tunneling current of 1 pA when the sample bias is 2 V. At higher current, the apparent height of molecular islands changed logarithmically with current. In addition, the current as a function of applied load revealed a stepwise increase, indicative of discrete molecular tilting events. A tunneling decay constant beta of =0.53+/-0.02 A(-1) was obtained based on the measurement of the height of molecules and the tunneling current.

Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide−Ions with a Perfluorinated Alkylthiol Self-Assembled Monolayer Surface

Classical trajectory simulations are performed to study energy transfer in collisions of protonated diglycine, gly2-H+, and dialanine, ala2-H+, ions with a fluorinated octanethiol self-assembled monolayer (F-SAM) surface for collision energies Ei in the range of 5−70 eV and incident angles θi of 0 and 45° with respect to the surface normal. Both explicit-atom (EA) and united-atom (UA) models were used to represent the F-SAM surface. The simulations show the distribution of energy transfer to the peptide−ion’s internal degrees of freedom, ΔEint, to the surface, ΔEsurf, and in peptide−ion translation, Ef, are very similar for gly2-H+, and ala2-H+. The average percentage energy transferred to ΔEsurf and Ef increases and decreases, respectively, with an increase in Ei, while the average percentage energy transfer to ΔEint is nearly independent of Ei. Changing θi from 0 to 45° decreases and increases the percentage of energy transfer to ΔEsurf and Ef, respectively, but has little change in the transfer to ΔEint. Average percentage energy transfer to the surface is found to approximately depend on Ei according to exp(−b/Ei). Comparisons with previous simulations show that peptide-H+ collisions with the EA F-SAM model transfer approximately a factor of 2 more energy to ΔEint than do collisions with the hydrogenated SAM, that is, H-SAM. Replacing the mass of the F atoms by that of a H atom in the simulations, without changing the potential, shows that the different ΔEint energy transfer efficiencies for the F-SAM and H-SAM surfaces is a mass effect. The simulations for ala2-H+ colliding with the EA F-SAM surface give P(ΔEint) distributions in good agreement with previous experiments and an average transfer to ΔEint of 15% as compared with the experimental value of 21%. The UA F-SAM model gives energy transfer efficiencies in qualitative agreement with those of the EA model, but there are important quantitative differences.

Generation and Characterization of Super-Hydrophobic Micro- and Nano-structured Surfaces

Here we describe the use of a backside exposure method for the creation of high aspect ratio tapered microstructures with nano-porous sidewalls. These sidewalls result from the exposure and curing conditions of the SU8 matrix. The structures show ultra-hydrophobic behavior with water contact angles of more than 160°, without an additional coating of the SU8 polymer. By choosing appropriate exposure conditions, needlelike structures can be created with a high level of porosity, covering their entire surface. The contact angle hysteresis values of such structures lie in the range of 20°. After an additional deposition of smooth and thin metallic gold film and coating it with an alkyl thiol selfassembled monolayer (SAM), we were able to decrease this hysteresis to values of around 10°. By using thin and rough metallic films like wet chemically oxidized titanium oxide and a fluoralkylsilane SAM, the hysteresis values could be reduced to only 4°.

An analytical potential energy function to model protonated peptide soft-landing experiments. The CH3NH3+/CH4 interactions

To model soft-landing of peptide ions on surfaces, it is important to have accurate intermolecular potentials between these ions and surfaces. As part of this goal, ab initio calculations at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections, were performed to determine both the long-range attractive and short-range repulsive potentials for CH(4) interacting with the -NH(3)(+) group of CH(3)NH(3)(+). Potential energy curves for four different orientations between CH(4) and CH(3)NH(3)(+) were determined from the calculations to obtain accurate descriptions of the interactions between the atoms of CH(4) and those of -NH(3)(+). A universal analytic function was not found that could accurately represent both the long-range and short-range potentials for collision energies as high as those obtained in surface-induced-dissociation (SID) experiments. Instead, long-range and short-range analytic potentials were developed separately, by simultaneously fitting the four ab initio potential energy curves with a sum of two-body interactions between the atoms of CH(4) and -NH(3)(+), and then connecting these long-range and short-range two-body potentials with switching functions. Following a previous work [J. Am. Chem. Soc., 2002, 124, 1524], these two-body potentials may be used to describe the interactions of the N and H atoms of the -NH(3)(+) group of a protonated peptide ion with the H and C atoms of alkane-type surfaces such as alkyl thiol self-assembled monolayers and H-terminated diamond. Accurate short-range and long-range potentials are imperative to model protonated peptide ion soft-landing experiments. The former controls the collision energy transfer, whereas the latter describes the binding of the ion to the surface. A comparison of the ab initio potential energy curves for CH(3)NH(3)(+)/CH(4) with those for NH(4)(+)/CH(4) shows that they give nearly identical two-body interactions between the atoms of -NH(3)(+) and those of CH(4), showing that the smaller NH(4)(+)/CH(4) system may be used to obtain the two-body potentials. A comparison of the four ab initio potential energy curves reported here for CH(3)NH(3)(+)/CH(4), with those given by the AMBER and CHARMM molecular mechanical potentials, show that these latter potentials "roughly" approximate the long-range attractions, but are grossly in error for the short-range repulsions. The work reported here illustrates that high-level ab initio calculations of intermolecular potentials between small model molecules may be used to develop accurate analytical intermolecular potentials between peptide ions and surfaces.

Tunnelling conductance of vectorial porphyrin monolayers

A method for antipodal attachment of sulfur linkers to a nickel chelated porphyrin has been developed to facilitate robust chemisorption at gold electrodes. At sufficiently high molecular coverage ellipsometric, infrared and electroanalytical data are supportive of a close-to-vertical molecular orientation with respect to the substrate. Molecular conductance was assessed both within neat porphyrin monolayer films and on individual molecules isolated within an ordered saturated alkyl thiol (1-tetradecanethiol) self-assembled monolayer and then capped with chemisorbed gold nanoparticles. In the former, three distinct groups of current–voltage (I–V) curves were observed both by scanning tunnelling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM), corresponding to the conductance of one, two, and three molecules, which were quantizable as integer multiples of a fundamental molecular conductance (1.37 ± 0.25 nS). This value compares well with that obtained on isolated molecules and is significantly higher than the host matrix. In all cases I–V trends are well described by a non-resonant tunnelling model.

Chemical Dynamics Study of Intrasurface Hydrogen-Bonding Effects in Gas−Surface Energy Exchange and Accommodation

Classical chemical dynamics simulations were performed to compare the efficiency of energy transfer in collisions of Ar with 300 K CH3- and OH-terminated alkyl thiol self-assembled monolayer surfaces (i.e., H-SAM and HO-SAM) and compare with previous experiments (Anal. Chim. Acta 2003, 496, 249). The experiments show that energy is transferred less efficiently to the HO-SAM. The H-SAM has a periodic, ordered surface structure, whereas the surface of the HO-SAM has a disordered, “glassy” structure as a result of “clustering” of the terminal OH groups. The Ar atom has a much stronger physisorption interaction with the HO-SAM, arising from the strong Ar···O van der Waals interaction. Though the simulations show that physisorption is more important for Ar atoms colliding with the HO-SAM, energy transfer is less efficient to this surface. The latter results from a significant difference in the energy transfer for direct collisions with the two surfaces. More energy is deposited in the H-SAM for direct collisio...

Influence of self-assembled monolayer surface chemistry on Candida antarctica lipase B adsorption and specific activity

Immobilization of Candida antarctica B lipase was examined on gold surfaces modified with either methyl- or hydroxyl-terminated self-assembled alkylthiol monolayers (SAMs), representing hydrophobic and hydrophilic surfaces, respectively. Lipase adsorption was monitored gravimetrically using a quartz crystal microbalance. Lipase activity was determined colorimetrically by following p-nitrophenol propionate hydrolysis. Adsorbed lipase topography was examined by atomic force microscopy. The extent of lipase adsorption was nearly identical on either surface (approximately 240 ng cm −2), but its specific activity was sixfold higher on the methyl-terminated SAM, showing no activity loss upon immobilization. A uniform, 5.5 nm high, highly packed monolayer of CALB formed on the methyl-terminated SAM, while the adsorbed protein was disordered on the hydroxyl-terminated SAM. Hydrophobic surfaces thus may specifically orient the lipase in a highly active state.