Phase Dependent Electrochemical Properties of Polar Self-Assembled Monolayers (SAMs) Modified via the Fusion of Mixed Phospholipid Vesicles

Unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and varying quantities of either 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol) (sodium salt) (DMPG) or 1,2-dimyristoyl-3-trimethylammonium-propane (chloride salt) (DMTAP) were used to deposit lipid bilayer assemblies on self-assembled monolayers (SAMs) on gold. The supporting SAMs in turn were composed of ferrocene-functionalized hexadecanethiol chains (FcC16SH) diluted to low coverage in 1-hydroxylhexadecanethiol (HOC16SH) or a single-component monolayer phase of the latter. The mass coverages of the DMPC/DMPG layers deposited in this way were measured using surface plasmon resonance (SPR) and found to decrease with an increasing content of DMPG in the vesicles. The SPR data show that the lipid assembly, while stable with respect to gentle rinsing in aqueous buffer, is reversible and the lipid adlayer is removable by immersion in a solvent such as ethanol. The effects of the adsorbed lipid layer on the electrochemical interactions of the hybrid lipid/SAM with several redox probes [e.g., K4Fe(CN)6, Ru(NH3)6Cl3, and C5H5Fe[(C5H4CH2N+H(CH3)2] were characterized using cyclic voltammetry (CV). At a composition of 5% DMPG in DMPC, the permeabilities of the probes through the lipid layer were affected significantly relative to that observed with a pure DMPC layer. These effects include a striking observation of an enhanced, ionic-charge-specific molecular discrimination of the electrochemical probes. At higher concentrations of the DMPG, significant permeation of the lipid adlayer was seen for all the probes. These latter changes are also attended by a significant increase in the capacitive currents measured in CV experiments as compared to those observed for either a pure SAM or one modified by only DMPC. This effect likely results from the influence of the charged lipid on the diffuse Gouy−Chapman electrolyte layer at the SAM interface. In contrast to the behaviors seen with DMPG, the incorporation of DMTAP into the adsorbed DMPC had no impact on the permeation of the adlayer by soluble redox probes as judged by the observed electrochemistry, a result that appears to correlate with a less ideal mixing of lipids in the DMPC/DMTAP system relative to that of a DMPC/DMPG mixture

Molecular Recognition at Model Organic Interfaces: Electrochemical Discrimination Using Self-Assembled Monolayers (SAMs) Modified via the Fusion of Phospholipid Vesicles

Supported lipid layers were formed via the fusion of large unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) to mixed self-assembled monolayers (SAMs) on gold comprised of ferrocene-functionalized hexadecanethiol chains (FcCO2C16SH) diluted in either hexadecanethiol (C16SH) or 1-hydroxylhexadecanethiol (HOC16SH). For the former case, the DMPC adsorbs predominantly as a single layer to form a hybrid bilayer membrane (HBM). The structures obtained in this way were characterized by methods that include electrochemical measurements, ellipsometry, and surface plasmon resonance (SPR). Cyclic voltammetry (CV) reveals that the electrochemistry of the ferrocene groups present in the SAM is strongly perturbed by the adlayer structure. The electrochemical behaviors of the ferrocene groups incorporated into a mixed SAM prepared using the more polar hydroxyl terminated thiol are quite different. The adsorption of DMPC via vesicle fusion in this case leads to the adsorption of bilayer assemblies of the lipid on top of the SAM. The coverages of the DMPC suggested by the SPR data lie between the values expected for fusion processes depositing either one or two bilayers of the lipid on top of the SAM. The electrochemical properties of the ferrocene moieties present in this structure were found to be largely unperturbed following the DMPC adsorption. Subsequent studies revealed that the adsorbed DMPC strongly influences the interactions of the tethered ferrocene groups with secondary aqueous molecular redox probes present in the electrolyte solution; permselective properties are seen in this adlayer structure. The varying degrees of electrochemical rectification seen in CV surveys demonstrated that probes such as K4Fe(CN)6, C5H5Fe(C5H4CH2N(CH3)3)PF6, and Ru(NH3)6Cl3 appear to penetrate the DMPC layer while species such as C5H5Fe[C5H4CH2N+H(CH3)2], C5H5Fe(C5H4CH2OH), and C5H5Fe(C5H4COO-) do not. We believe that molecular scale defect structures present in the adsorbed DMPC layers confer the molecular discrimination properties seen. A qualitative structural model is proposed

Fabrication of Patterned Multicomponent Protein Gradients and Gradient Arrays Using Microfluidic Depletion

We demonstrate that depletion effects in the fluids used to fill a poly(dimethylsiloxane) microfluidic device can be used in conjunction with its design rules to generate patterned protein gradients. The linear portions of these structures can be designed to present gradients of bound protein coveragevarying from near-saturation to effectively zeroover distances ranging from a few hundred micrometers to more than 1 cm by design. Such patterns can be developed in a simple, single-channel form as well as in a multichannel gradient array of more complex design. The patterning protocols also support the use of multiple protein sources, and we demonstrate an assembly process mediated by a protein that inhibits adsorption to generate a gradient array in pixel form. We describe examples of multiple protein gradient patterns along with simple immunoassays to illustrate the scope of the methodology, the activity of the patterned proteins, and their recognition in gradient form on a surface. These gradients should prove useful to studies in biosensor and bioassay development and as substrates for cell culture to study growth and motility

Driven Pattern Formation in Organic Thin Film Materials: Complex Mesoscopic Organization in Microcontact Printing on Si/SiO₂ via the Spontaneous Dewetting of a Functionalized Perfluoropolyether Ink^†

Polyfluoropolyether (PFPE) films have long been used as lubricant coatings for magnetic recording media. In this paper, we demonstrate that the unique wetting properties of PFPEsmore specifically, the dynamical organizations that result from spontaneous dewettingcan be harnessed to generate mesoscopically patterned features of these materials on SiO2. In this work, a functionalized PFPE amphiphile with the formula CF3CF2CF2O(CF(CF3)CF2O)nCF(CF3)CONHCH2CH2CH2Si(OCH3)3 (Krytox SA, DuPont) was deposited on a SiO2 surface by both spin-casting and contact printing. Both methods produce complex surface structures comprised of beaded domains and depletion regions (and in the case of spin-casting, also thin films) that result from dewetting processes. Spontaneous dewetting was used to generate self-organizing PFPE bead patterns by microcontact printing. The wetting transitions in this latter case occur directly on the printing tool and, via the bias provide by the topography of the stamp, provide a means for generating and transferring complex organizations of adsorbate domains to the substrate. The combination of contact printing with spontaneous dewetting of the PFPE enabled us to produce high-fidelity patterns of discrete, micron-scale beads from printing tools with continuous line shapes without any alteration to the original mask pattern. The patterned beads typically had radii and characteristic separation lengths on the micron-scale, and heights on the nanoscale. These length scales appear to be governed by the combined influences of solvent-mediated nucleation processes and coarsening. Characterization was performed by optical microscopy, atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). These data suggest general design strategies that can be exploited to control the contours and structural profiles that result from the dewetting of the PFPE ink on a stamp used for contact printing

Collision-Induced Desorption and Reaction on Hydrogen-Covered Al(111) Single Crystals: Hydrogen <i>in</i> Aluminum?

We examine the recombination and desorption of hydrogen from an aluminum(111) surface focusing on desorption processes that lead to the formation of dihydrogen and aluminum hydride (presumably alane). In addition to simple temperature-programmed reaction spectroscopy (TPRS), we examine the perturbations which occur to the desorption kinetics of these species as a result of the energy transfer due to collisions of a xenon beam at 1.6, 2.8, and 3.6 eV with a hydrogen covered surface. Whereas the recombinative desorption of dihydrogen from an Al(111) surface nominally follows an unusual zero-order rate law, bombardment of a hydrogen-covered surface with an energetic xenon atom leads to a kinetic profile more closely modeled by a higher reaction order. It also was found that upon exposure to the beam, the peak area (and thus the desorption yield) for the H2 desorption was reduced 5−25% depending on the length of exposure whereas for the aluminum hydride the percent reduction ranged from 10−80%. This suggests that both a stimulated etching and a change in state result from the beam exposure. We present evidence that suggests that the initial state of the bound hydrogen may involve at least in part a subsurface occupation

Microfluidic Device for the Discrimination of Single-Nucleotide Polymorphisms in DNA Oligomers Using Electrochemically Actuated Alkaline Dehybridization

This work describes an integrated microfluidic (μ-fl) device that can be used to effect separations that discriminate single-nucleotide polymorphisms (SNP) based on kinetic differences in the lability of perfectly matched (PM) and mismatched (MM) DNA duplexes during alkaline dehybridization. For this purpose a 21-base single-stranded DNA (ssDNA) probe sequence was immobilized on agarose-coated magnetic beads, that in turn can be localized within the channels of a poly(dimethylsiloxane) microfluidic device using an embedded magnetic separator. The PM and MM ssDNA targets were hybridized with the probe to form a mixture of PM and MM DNA duplexes using standard protocols, and the hydroxide ions necessary for mediating the dehybridization were generated electrochemically in situ by performing the oxygen reduction reaction (ORR) using O2 that passively permeates the device at a Pt working electrode (Pt-WE) embedded within the microfluidic channel system. The alkaline DNA dehybridization process was followed using fluorescence microscopy. The results of this study show that the two duplexes exhibit different kinetics of dehybridization, rate profiles that can be manipulated as a function of both the amount of the hydroxide ions generated and the mass-transfer characteristics of their transport within the device. This system is shown to function as a durable platform for effecting hybridization/dehybridization cycles using a nonthermal, electrochemical actuation mechanism, one that may enable new designs for lab-on-a-chip devices used in DNA analysis

Quantitative Imaging of Protein Adsorption on Patterned Organic Thin-Film Arrays Using Secondary Electron Emission

Secondary electron emission is developed as a means to quantify and image protein binding to Au surfaces modified with patterned organic thin-film arrays. Alkane thiols were patterned via microcontact printing on gold, and their effects on the secondary electron (SE) yield of the surface, systematically quantified. We show that a self-assembled monolayer (SAM) of hexadecane thiol significantly increases the SE yield over the native gold surface, a yield that increases as a function of alkane chain length (C8−C16). This effect is linearly correlated with the surface potentials and wetting properties of these SAMs. Surface layers comprised of poly(ethylene glycol) (PEG) grafted polyacrylamide polymers behave differently, affecting the SE yield by attenuation according to the polymer thickness. These results demonstrate the relative contributions of factors related to the adsorbate molecular structures that serve to strongly mediate the SE yield, providing a foundation for exploiting them as a quantitative electron imaging probe. The latter capability is demonstrated using a model microfluidic assay in which a series of proteins was spatially addressed to a SAM-based pixel array. The gray scale contrasts seen with protein adsorption are directly correlated with both protein molecular weight and mass coverage. These methods are used in two model protein assay experiments:  (1) the measurement of the concentration dependent adsorption isotherm for a model protein (fibrinogen); and (2) the selective recognition of a biotinylated protein layer by avidin. These results demonstrate a unique approach to imaging protein binding processes on surfaces with both high analytical and spatial sensitivity

Catalytic Amplification of Patterning via Surface-Confined Ring-Opening Metathesis Polymerization on Mixed Primer Layers Formed by Contact Printing

Thin films of polymers synthesized from strained olefins were formed on Si(100) via ring-opening metathesis polymerization (ROMP) initiated by a Ru−alkylidene catalyst bound to the surface by means of the alkylidene ligands. Specifically, a mixture of 7-octenyltrichlorosilane (3) and octyltrichlorosilane (4) was deposited onto Si/SiO2 surfaces via contact printing, and this assembly was subsequently treated with RuCl2(PCy3)2(CHPh) (1). The resulting surface-tethered Ru alkylidene complex reacts with norbornene derivatives to give thin polymer films. The growth of the polymer film, as monitored by ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), is strongly influenced by the initial concentration of the catalyst at the surface and by the monomer reactivity. The catalyst concentration at the Si/SiO2 surface was controlled by adjusting the relative ratio of 3 and 4 in the “primer” used in the contact-printing step. These studies revealed that the success of the catalytic amplification step required that the linker be present on the surface in very low coverage. The optimal concentration of 3 in the mixed primer was 20−40% with a total mass coverage of the mixed self-assembled monolayer (SAM) of approximately one-third that of a full monolayer coverage. Possible causes of the inefficient polymerization at high coverages of 3 are discussed; a leading candidate is that the catalytic Ru centers are deactivated by reactions with neighboring olefinic linker sites or with other Ru centers. In a representative class of polymerization, reticulated films of poly(2,2,2-trifluoroethyl bicyclo[2.2.1]hept-2-ene-5-carboxylate) (5) with an average thickness of ∼100 Å were obtained after ∼3 h of reaction at room temperature. The polymerization can be completely described by a rate law involving coupled step-growth and competing unimolecular termination reactions. The polymerization process can be used to amplify catalytically (that is, to increase the mass coverages of) latent images present in SAMs that have been patterned via microcontact printing (μCP). To do so, a patterned resist SAM of octadecyltrichlorosilane (OTS) was deposited first via μCP. A second SAM based on the mixed phase of 3 and 4 was then orthogonally assembled in a self-registering printing step carried out using an unpatterned poly(dimethylsiloxane) (PDMS) stamp. Activation of this composite SAM via a treatment first with 1 and then with 5 resulted in an additive deposition of the polymer pattern

The Size-Dependent Structural Phase Behaviors of Supported Bimetallic (Pt−Ru) Nanoparticles

We describe in this report the preparation, structural characterization, and phase behaviors exhibited by supported metallic and bimetallic nanoparticles. Homometallic nanoparticles of either Pt or Ru were synthesized by the reduction of various precursors ((CH3)2Pt(COD), H2PtCl6, and RuCl3) onto different carbon supports:  Vulcan XC-72 (VXC) and Shawinigan Acetylene Black (SAB). The choice of precursor has a large structural influence on the reductive condensation of the Pt metal particles. All of the various precursors and supports produced particles with very similar size distributions, with the exception of (CH3)2Pt(COD), which formed a complex distribution of small (20 Å) and large (>50 Å) particles. The centerpiece of this study is the characterization of the growth behaviors seen in the synthesis of binary Pt−Ru nanoparticles. These heterometallic particles were synthesized via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal; the latter serve as nucleating sites for the growth of the binary phase. As shown via data from X-ray photoelectron spectroscopy (XPS), electron microscopy, and energy-dispersive X-ray analysis (EDX), this growth technique yields fully alloyed metallic nanoparticles, albeit ones of varying size and compositional distributions depending on the specific conditions used. Generally we found that the particles had a wide composition distribution. The nature of this distribution and the correlations between the nanoparticle sizes, compositions, and structures embedded in it were characterized in depth by scanning transmission electron microscopy (STEM). These results are used to establish an apparent size correlated binary phase diagram of the bimetallic (Pt−Ru) nanoparticles. The structural properties of the supported bimetallic nanoclusters are different from that of the bulk, as evidenced by the presence of strongly persistent metastable structures that are not found in the bulk phase diagram

Surface-Mediated Segregation and Transport Processes in Mixed Hydrocarbon Multilayer Assemblies

The transport and structural phase dynamics exhibited by multilayer assemblies comprised of cyclic and linear alkanes are analyzed with reflection absorption infrared (RAIR) and temperature-programmed desorption spectroscopies. Infrared spectroscopy reveals that methyl group substitutions have a significant effect on the nature of the mode softening seen in the C−H stretching region for surface-bound cyclohexanes. The magnitude of the red-shifts seen increases with the degree of methyl substitution. The energetics of the surface binding do not correlate in a simple way with the magnitude of red-shifts seen in the RAIR spectra, however. We find instead that the strengths of the surface interactions are more directly correlated with both the size and shape of the molecule (with the latter presumably reflecting its ability to form a densely packed structure). We also find that the diffusion of molecules in a mixed hydrocarbon multilayer assembly is weakly activated, with substantial interlayer mixing being seen at temperatures significantly below the threshold for the desorption of the multilayer. The mixing, while driven by mass action, shows a pronounced bias for the surface binding of n-alkanes over cycloalkanes of similar molecular weight (e.g., n-octane is more strongly bound on Pt(111) than is cis-1,3-dimethylcyclohexane). The data strongly suggest that attractive lateral interactions in the adsorbed layers lead to the biases seen in this surface-induced segregation. Thermal desorption spectra confirm this sensitivity and interestingly show multiple monolayer desorption features for cyclic alkane adsorbates when mixed in a monolayer assembly with an appropriate linear n-alkane. We suggest that the attractive lateral interactions in the monolayer lead to the formation of island domains and that the desorption kinetics appear to sensitively reflect this underlying rate/structure sensitivity