Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions

In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, theta(loc) ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 x square root(3)) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular pi-pi ring interactions, resulting in higher local coverages.     

Complex surface chemistry of 4-mercaptopyridine self-assembled monolayers on Au(111)

The adsorption of 4-mercaptopyridine on Au(111) from aqueous or ethanolic solutions is studied by different surface characterization techniques and density functional theory calculations (DFT) including van der Waals interactions. X-ray photoelectron spectroscopy and electrochemical data indicate that self-assembly from 4-mercaptopyridine-containing aqueous 0.1 M NaOH solutions for short immersion times (few minutes) results in a 4-mercaptopyridine (PyS) self-assembled monolayer (SAM) with surface coverage 0.2. Scanning tunneling microscopy images show an island-covered Au surface. The increase in the immersion time from minutes to hours results in a complete SAM degradation yielding adsorbed sulfur and a heavily pitted Au surface. Adsorbed sulfur is also the main product when the self-assembly process is made in ethanolic solutions irrespective of the immersion time. We demonstrate for the first time that a surface reaction is involved in PyS SAM decomposition in ethanol, a surface process not favored in water. DFT calculations suggest that the surface reaction takes place via disulfide formation driven by the higher stability of the S-Au(111) system. Other reactions that contribute to sulfidization are also detected and discussed.     

Mixed self-assembled monolayers of semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)

Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)-that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)--on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions. Holes were made with the AFM tip establishing tilt angles of 30-50 degrees with respect to the surface normal for all SAMs. STM imaging showed well-ordered, line-shaped packing patterns with molecular resolution for the SAM of 2. Similar patterned structures were not observed for 1 and 3. Mixed SAMs were prepared by exposing a SAM of 1 to ethanol solutions of either 2 or 3. STM imaging revealed that domains of molecules of 2 or 3 amidst a monolayer of 1 are formed in both cases. Whereas in the mixed SAM of 1 and 2 the domains are irregularly shaped, circular islands of uniform size are found in the mixed SAM of 1 and 3.     

Significance of local density of states in the scanning tunneling microscopy imaging of alkanethiol self-assembled monolayers

A systematic scanning tunneling microscopy (STM) study of alkanethiol self-assembled monolayers (SAMs) is presented as a function of the bias voltage, tunneling current, and tip-termini separation. Stable and etch-pit free SAMs of close-packed undecanethiol/Au(111) were obtained after annealing in ultrahigh vacuum. STM revealed two distinct c(4x2) structures with four nonequivalent molecules per unit cell. For both structures, reversible contrast variations occur upon systematically tuning the bias voltage, the current, and the tip-termini distance. These contrast transitions originate from probing the corresponding local density of states (LDOS) of each molecule and not from the reorientation of the alkanethiol chains. The STM contrast is particularly sensitive to the tip-termini separation in the range of 0.5-2.5 A, reflecting the distance-dependence of LDOS. At a fixed tip elevation, the STM contrast is less sensitive to changes in bias within 0.1-1.2 V. For the first time, we demonstrate that LDOS may override the physical height variations in the STM topographic contrast for alkanethiol SAM systems.     

Adsorption and desorption processes of self-assembled monolayers studied by surface-sensitive microscopy and spectroscopy

Adsorption and desorption processes of self-assembled monolayers (SAMs) have been studied on an Au(111) surface by scanning tunnelling microscopy (STM), atomic force microscopy (AFM), X-ray photo-electron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At the initial growth stage, the ordered nucleation of SAM located at the herringbone turns of the Au(111) − (22 × √3) surface reconstruction and diffusion-controlled domain formation have been imaged by STM and AFM. Details of the oxidation process in UV desorption were also investigated by XPS. In addition, the dimerization reaction during desorption was confirmed by TDS for the first time in the alkanethiol SAM system.     

Formation of ordered self-assembled monolayers by adsorption of octylthiocyanates on Au(111)

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of octythiocyanate (OTC) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature and 50 degrees C. The surface structures and adsorption characteristics of the OTC SAMs on Au(111) were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The STM observation showed that OTC SAMs formed in solution at room temperature have unique surface structures including the formation of ordered and disordered domains, vacancy islands, and structural defects. Moreover, we revealed for the first time that the adsorption of OTC on Au(111) in solution at 50 degrees C led to the formation of SAMs containing small ordered domains, whereas the SAMs formed by vapor deposition at 50 degrees C had long-range ordered domains, which can be described as (radical3 x 2 radical19)R5 degrees structures. XPS measurements of the peaks in the S 2p and N 1s regions for the OTC SAMs showed that vapor deposition is the more effective method as compared to solution deposition for obtaining high-quality SAMs by adsorption of OTC on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.     

Covalent attachment of a transition metal coordination complex to functionalized oligo(phenylene-ethynylene) self-assembled monolayers

We have investigated the reaction of tetrakis(dimethylamido)titanium, Ti[N(CH(3))(2)](4), with N-isopropyl-N-[4-(thien-3-ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl]ethynyl}phenyl) amine self-assembled monolayers (SAMs), on polycrystalline Au substrates. The structure of the SAMs themselves has also been investigated. Both molecules form SAMs on polycrystalline Au bound by the thiophene group. The longer-molecular-backbone molecule forms a denser SAM, with molecules characterized by a smaller tilt angle. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS have been employed to examine the kinetics of adsorption, the spatial extent of reaction, and the stoichiometry of reaction. For both the SAMs, adsorption is described well by first-order Langmuirian kinetics, and adsorption is self-limiting from T(s) = -50 to 30 degrees C. The use of angle-resolved XPS clearly demonstrates that the Ti[N(CH(3))(2)](4) reacts exclusively with the isopropylamine end group via ligand exchange, and there is no penetration of the SAM, followed by reaction at the SAM-Au interface. Moreover, the SAM molecules remain bound to the Au surface via their thiopene functionalites. From XPS, we have found that, in both cases, approximately one Ti[N(CH(3))(2)](4) is adsorbed per two SAM molecules.     

Substrate effects in poly(ethylene glycol) self-assembled monolayers on granular and flame-annealed gold

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH3O(CH2CH2O)17NHCO(CH2)2SH (PEG(17)) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG(17) SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted "soft" and "hard" due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG(17), the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG(17) SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG(17) SAM.     

Comparative electrochemical scanning tunneling microscopy study of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100): a potential-induced structural transition

We investigate the structure of nonionic fluorosurfactant zonyl FSN self-assembled monolayers on Au(111) and Au(100) in 0.05 M H(2)SO(4) as a function of the electrode potential by electrochemical scanning tunneling microscopy (ECSTM). On Au(111), a (3(1/2) × 3(1/2))R30° arrangement of the FSN SAMs is observed, which remains unchanged in the potential range where the redox reaction of FSN molecules does not occur. On Au(100), some parallel corrugations of the FSN SAMs are observed, which originate from the smaller distance and the repulsive interaction between FSN molecules to make the FSN molecules deviate from the bridging sites, and ECSTM reveals a potential-induced structural transition of the FSN SAMs. The experimental observations are rationalized by the effect of the intermolecular interaction. The smaller distance between molecules on Au(100) results in the repulsive force, which increases the probability of structural change induced by external factors (i.e., the electrode potential). The appropriate distance and interactions of FSN molecules account for the stable structure of FSN SAMs on Au(111). Surface crystallography may influence the intermolecular interaction through changing the molecular arrangements of the SAMs. The results benefit the molecular-scale understanding of the behavior of the FSN SAMs under electrochemical potential control.     

Characterization of self-assembled monolayers of thiols on Au(111)

Self-assembled monolayers (SAMs) of n-butanethiol, n-dodecanethiol and their equimolar mixture on Au(111) were prepared and characterized by ellipsometry, contact angle measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results revealed that these SAMs are oriented ultrathin films with the thickness of nanometer scale, and the SAMs were influenced by the molecular chain length, the lattice orientation and cleanliness of the substrates. The surface of the longer chain SAM is hydrophobic. The thicknesses of three SAMs of n-butanethiol, n-dodecanethiol and their mixture revealed by ellipsometry and XPS are about 0.59 - 0.67nm, 1.60- 1.69 nm and 1.23 - 1.32nm, respectively. AFM images further demonstrated that the SAM formed by the mixture has some microdomains with two different thicknesses.     

Ultrafast shock compression of self-assembled monolayers: a molecular picture

Simulations of self-assembled monolayers (SAMs) are performed to interpret experimental measurements of ultrafast approximately 1 GPa (volume compression deltaV approximately 0.1) planar shock compression dynamics probed by vibrational sum-frequency generation (SFG) spectroscopy (Lagutchev, A. S.; Patterson, J. E.; Huang, W.; Dlott, D. D. J. Phys. Chem. B 2005, 109, XXXX). The SAMs investigated are octadecanethiol (ODT) and pentadecanethiol (PDT) on Au(111) and Ag(111) substrates, and benzyl mercaptan (BMT) on Au(111). In the alkane SAMs, SFG is sensitive to the instantaneous orientation of the terminal methyl; in BMT it is sensitive to the phenyl orientation. Computed structures of alkane SAMs are in good agreement with experiment. In alkanes, the energies of gauche defects increase with increasing number and depth below the methyl plane, with the exception of ODT/Au where both single and double gauche defects at the two uppermost dihedrals have similar energies. Simulations of isothermal uniaxial compression of SAM lattices show that chain and methyl tilting is predominant in PDT/Au, ODT/Ag and PDT/Ag, whereas single and double gauche defect formation is predominant in ODT/Au. Time-resolved shock data showing transient SFG signal loss of ODT/Au and PDT/Au are fit by calculations of the terminal group orientations as a function of deltaV and their contributions to the SFG hyperpolarizability. The highly elastic response of PDT/Au results from shock-generated methyl and chain tilting. The viscoelastic response of ODT/Au results from shock generation of single and double gauche defects. Isothermal compression simulations help explain and fit the time dependence of shock spectra but generally underestimate the magnitude of SFG signal loss because they do not include effects of high-strain-rate dynamics and shock front and surface irregularities.     

Microelectrochemical modulation of micropatterned cellular environments

Patterned cell cultures obtained by microcontact printing have been modified in situ by a microelectrochemical technique. It relies on lifting cell-repellent properties of oligo(ethylene glycol)-terminated self-assembled monolayers (SAMs) by Br2, which is produced locally by an ultramicroelectrode of a scanning electrochemical microscope (SECM). After Br2 treatment the SAM shows increased permeability and terminal hydrophobicity as characterized by SECM approach curves and contact angle measurements, respectively. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopic (PM FTIRRAS) studies on macroscopic samples show that the Br2 treatment removes the oligo(ethelyene glycol) part of the monolayer within a second time scale while the alkyl part of the SAM degrades with a much slower rate. The lateral extension of the modification can be limited because heterogeneous electron transfer from the gold support destroys part of the electrogenerated Br2 once the monolayer is locally damaged in a SECM feedback configuration. This effect has been reproduced and analyzed by exposing SAM-modified samples to Br2 in the galvanic cell Au|SAM|5 microM Br2 + 0.1 M Na2SO4||10 microM KBr + 0.1 M Na2SO4|Au followed by an PM FTIRRAS characterization of the changes in the monolayer system.     

STM study of mixed alkanethiol/biphenylthiol self-assembled monolayers on Au(111)

A method is presented for depositing mixed self-assembled monolayers (SAMs) of dodecanethiol (C12) and 4'-methyl-1,1'-biphenyl-4-butane (H3C-C6H4-C6H4-(CH2)4-SH, BP4) by insertion of BP4 into a closely packed SAM of dodecanethiol on Au(111). Insertion takes place at defect sites such as domain boundaries or etch pits in the gold surface that are characteristic of C12 monolayers on gold. With a lower probability, insertion also occurs beside defect sites inside dodecanethiol domains. Insertion at defect sites results in domains of BP4, whereas insertion into C12 domains leads to isolated BP4 molecules. The isolated BP4 molecules are shown not to move at room temperature. By comparing the apparent height of the isolated BP4 molecules and BP4 domains, it is proposed that the isolated molecules have the same conformation as in the full-coverage phase. A simple two-layer model is proposed to characterize the current transport through BP4. The decay constant beta for the phenylene groups is deduced from the apparent STM heights of the inserted BP4 islands compared to the STM heights of the C12 closely packed monolayers.     

Surface characterization and platelet compatibility evaluation of the binary mixed self-assembled monolayers

This report describes a technique that used mixed self-assembled monolayer (SAM) as a model surface to evaluate the effect of steric hindrance on the SAM packing quality and its platelet compatibility. Two series of binary mixed SAMs were formed by mixing the bulky terminated alkanethiol (HS(CH2)10PO3H2) with a smaller terminated one (HS(CH2)9CH3 and HS(CH2)11OH) respectively. Surface characterization results showed the hydrophilicity on these two series of mixed SAMs changed with the solution mole fraction of PO3H2 terminated thiol, chi(PO3H2,soln), and reached to a nearly constant value as chi(PO3H2,soln) was 0.6 for PO3H2+CH3 SAM and 0.4 for PO3H2+OH SAM. This finding should be due to the gradual saturation of surface PO3H2 functionality on these mixed SAMs. The XPS analysis indicated the addition of the CH3 and OH terminated thiol could reduce the steric hindrance effect of PO3H2 functionality on monolayer formation and, henceforth, improve the SAM packing quality. In vitro platelet adhesion assay revealed the platelet compatibility on the PO3H2+OH SAMs was better than that on the PO3H2+CH3 and the pure PO3H2 ones. Moreover, the PO3H2+OH SAM with a low chi(PO3H2,soln) value exhibited the least platelet activating property of these two mixed SAM systems. These findings suggested that material's surface wettability and surface charge density should act collectively in affecting its platelet compatibility.     

Modification and stability of aromatic self-assembled monolayers upon irradiation with energetic particles

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.     

Stability of self-assembled monolayers on titanium and gold

Methyl- and hydroxyl-terminated phosphonic acid self-assembled monolayers (SAMs) were coated on Ti from aqueous solution. Dodecyl phosphate and dodecyltrichlorosilane SAMs were also coated on Ti using solution-phase deposition. The stability of SAMs on Ti was investigated in Tris-buffered saline (TBS) at 37 degrees C using X-ray photoelectron spectroscopy, contact angle goniometry, and atomic force microscopy. For comparison purposes, a hydroxyl-terminated thiol SAM was coated on Au, and its stability was also investigated under similar conditions. In TBS, a significant proportion of phosphonic acid or phosphate molecules were desorbed from the Ti surface within 1 day, while the trichlorosilane SAM on Ti or thiol SAM on Au was stable for up to 7 days under similar conditions. The stability of hydroxyl-terminated phosphonic acid SAM coated Ti and thiol SAM coated Au was investigated in ambient air and ultraviolet (UV) light. In ambient air, the phosphonic acid SAM on Ti was stable for up to 14 days, while the thiol SAM on Au was not stable for 1 day. Under UV-radiation exposure, the alkyl chains of the phosphonic acid SAM were decomposed, leaving only the phosphonate groups on the Ti surface after 12 h. Under similar conditions, decomposition of alkyl chains of the thiol SAM was observed on the Au surface accompanied by oxidation of thiolates.     

Substrate changes associated with the chemistry of self-assembled monolayers on silicon

Alkylsiloxane self-assembled monolayers (SAMs) are used in the semiconductor industry and, more recently, as proxies for organics adsorbed on airborne mineral dust and on buildings and construction materials. A number of methods have been used for removing the SAM from the substrate after reaction or use, particularly plasmas or piranha (H2SO4/H2O2) solution. However, when the substrates are reused to make new SAMs, the impact of the cleaning methods on the chemistry of subsequently formed SAMs on the surface is not known. Here we report atomic force microscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and Fourier transform infrared studies of changes in a silicon substrate upon repetitive deposition and removal of SAMs by these two methods. It is shown that a thicker layer of silicon oxide is formed, and the surface becomes irregular and roughened, particularly after the piranha treatment. This layer of silica impacts the structure of the SAMs attached to it and can serve as a reservoir for trace gases that adsorb on it, potentially contributing to the subsequent reactions of the SAM. The implications for the use of such surfaces as a proxy for reactions of organics on airborne dust particles and on structures in the boundary layer are discussed.     

Ion scattering and X-ray photoelectron spectroscopy of copper overlayers vacuum deposited onto mercaptohexadecanoic acid self-assembled monolayers

Metal overlayers deposited in vacuum onto self-assembled monolayer (SAM) systems serve as models for more complex metalized polymers. Metals (M) deposited onto SAMs with different organic functional end groups exhibit a wide range of behavior, ranging from strong chemical interactions with the end group to complete penetration of the metal through the SAM. In this work, we have characterized the interactions of Cu with the ---COOH of mercaptohexadecanoic acid (MHA, HOOC(CH₂)₁₅SH) SAMs self assembled on gold films by using X-ray photoelectron spectroscopy (XPS) to examine the chemical interactions, and a combination of XPS and ion scattering spectroscopy (ISS) to deduce the growth mode and penetration rate of the deposited Cu. We found that submonolayer amounts of Cu react with HOOC, whereas the rest of the Cu remains metallic and penetrates beneath the SAM surface to the SAM  Au interface. Considerable amounts of Cu (5 nm or more) will penetrate beneath the SAM layer, which is ca. 2.5 nm thick, despite the submonolayer presence of Cu at the SAM surface. The penetration rate depends strongly on the Cu deposition rate. Depositing copper onto MHA at 220 K or less, or using faster Cu deposition rates, results in slower or even completely suppressed penetration of the Cu through the SAM layer, whereas exposure to X-rays greatly enhances the penetration rate of large amounts of Cu through the SAM layer. The reacted copper is, based on the XPS 2p and LMM peaks, in the +2 oxidation state, but cannot be identified with a simple, stoichiometric oxide such as Cu₂O, CuO, or Cu (OH)₂.     

Structures and displacement of 1-adamantanethiol self-assembled monolayers on Au{111}

We have designed monolayers with weak intermolecular interactions for use as placeholders in intelligent self- and directed-assembly. We have shown that these 1-adamantanethiolate monolayers are labile with respect to displacement by exposing them to dilute solutions of alkanethiols. These self-assembled monolayers (SAMs) of 1-adamantanethiol on Au{111} were probed using ambient scanning tunneling microscopy (STM), and their assembled order was determined. Solution deposition of the molecules results in a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbor distance of 6.9 +/- 0.4 A. The SAMs exhibit several rotational domains, but lack the protruding domain boundaries typical of alkanethiolate SAMs, and are similarly stable at room temperature. Co-deposition of alkanethiol and 1-adamantanethiol from solution results in alkanethiolate SAMs, except when using extremely low alkanethiol to 1-adamantanethiol concentration ratios. Facile displacement of low interaction strength SAMs can be exploited to enhance patterning using soft nanolithography.     

Improving the surface biocompatibility with the use of mixed zwitterionic self-assembled monolayers prepared by a proper solvent

In this study, the mixed self-assembled monolayers (SAMs) containing the mixture of long-chain alkanethiol, SH(CH(2))(11)NH(2) and SH(CH(2))(10)SO(3)H, was prepared as a model surface to examine the interaction between the biological environment and artificial surface. The 10% (v/v) NH(4)OH ethanolic solution and DMSO were chosen as the solvents for the preparation of these mixed SAMs and the "solvent effect" was discussed. X-ray photoelectron spectroscopy (XPS) has indicated that -SO(3)H/-NH(2) mixed SAMs formed from 10% (v/v) NH(4)OH ethanolic solution were surface "-SO(3)H poor", while a nearly equivalent amount of surface -SO(3)H functionality was presented on the mixed SAMs formed from DMSO. This has resulted from the different solvation capability between solvent molecules and the alkanethiol. Such solvent effects were also reflected in various surface properties such as surface wettability and surface zeta potential. The mixed SAMs formed from DMSO were more surface hydrophilic and less negatively surface charged than from 10% (v/v) NH(4)OH ethanolic solution. In addition, these mixed SAMs formed from DMSO exhibited the least amount of protein adsorbed as well as a better platelet compatibility than its counterpart from 10% (v/v) NH(4)OH ethanolic solution. These findings indicated that choosing a proper solvent for mixed zwitterionic SAM can greatly affect its surface properties and biocompatibility, such as to form a surface with near neutrality for reducing protein adsorption and subsequent platelet adhesion and activation.