Recent Advances in Bimetallic Nanoporous Gold Electrodes for Electrochemical Sensing.

Bimetallic nanocomposites and nanoparticles have received tremendous interest recently because they often exhibit better properties than single-component materials. Improved electron transfer rates and the synergistic interactions between individual metals are two of the most beneficial attributes of these materials. In this review, we focus on bimetallic nanoporous gold (NPG) because of its importance in the field of electrochemical sensing coupled with the ease with which it can be made. NPG is a particularly important scaffold because of its unique properties, including biofouling resistance and ease of modification. In this review, several different methods to synthesize NPG, along with varying modification approaches are described. These include the use of ternary alloys, immersion-reduction (chemical, electrochemical, hybrid), co-electrodeposition-annealing, and under-potential deposition coupled with surface-limited redox replacement of NPG with different metal nanoparticles (e.g., Pt, Cu, Pd, Ni, Co, Fe, etc.). The review also describes the importance of fully characterizing these bimetallic nanocomposites and critically analyzing their structure, surface morphology, surface composition, and application in electrochemical sensing of chemical and biochemical species. The authors attempt to highlight the most recent and advanced techniques for designing non-enzymatic bimetallic electrochemical nanosensors. The review opens up a window for readers to obtain detailed knowledge about the formation and structure of bimetallic electrodes and their applications in electrochemical sensing.

Single Molecule Spectroscopy Studies of Acid-Base Chemical Gradients Using Nile Red as a Probe of Local Surface Acidity.

Single molecule spectroscopy studies of local acidity along bifunctional acid-base gradients are reported. Gradients are prepared by directional vapor phase diffusion and subsequent reaction of 3-aminopropyl-trimethoxysilane with a uniform silica film. Gradient formation is confirmed by spectroscopic ellipsometry and by static water contact angle measurements. X-ray photoelectron spectroscopy is used to characterize the nitrogen content and degree of nitrogen protonation along the gradient. Nile Red is employed as the probe dye in single molecule spectroscopy studies of these gradients. While Nile Red is well-known for its solvent sensitivity, it is used here, for the first time, to sense the acid/base properties of the film in two-color wide-field fluorescence imaging experiments. The data reveal broad bimodal distributions of Nile Red emission spectra that vary along the gradient direction. The single molecule results are consistent with solution phase ensemble acid/base studies of the dye. The former reveal a gradual transition from a surface dominated by basic aminosilane sites at the high-amine end of the gradient to one dominated by acidic silanol sites at the low-amine end. The sub-diffraction-limited spatial resolution afforded by superlocalization of the single molecules reveals spatial correlations in the acid/base properties of the gradient over ∼200 nm distances. These studies provide data relevant to the use of aminosilane-modified silica in bifunctional, cooperative chemical catalysis.

Using redox potential as a feasible marker for banked blood quality and the state of oxidative stress in stored red blood cells.

BACKGROUND: Stored red blood cells (RBCs) may undergo oxidative stress over time, with functional changes affecting oxygen delivery. Central to these changes are oxidation-reduction (redox) reactions and redox potential (RP) that must be maintained for cell function. RP imbalance can lead to oxidative stress that may contribute to storage lesions. This study's purpose was to identify changes in RP over time in banked RBCs, and among RBCs of similar age. METHODS: Multiple random RBC segments from RBC units were tested (n = 32), ranging in age from 5 to 40 days, at 5-day intervals. RP was recorded by measuring open circuit potential of RBCs using nanoporous gold electrodes with Ag/AgCl reference. RP measures were also performed on peripheral venous blood from 10 healthy volunteers. RP measures were compared between RBC groups, and with volunteer blood. RESULTS: Stored RBCs show time-dependent RP increases. There were significant differences in Day 5 RP compared to all other groups (p ≤ 0.005), Day 10-15 vs. ages ≥ Day 20 (p ≤ 0.025), Day 20-25 vs. Day 40 (p = 0.039), and all groups compared to healthy volunteers. RP became more positive over time suggesting ongoing oxidation as RBCs age; however, storage time alone was not predictive of RP measured in a particular unit/segment. CONCLUSIONS: There are significant differences in RP between freshly stored RBCs and all others, with RP becoming more positive over time. However, storage time alone does not predict RP, indicating RP screening may be an important measure of RBC oxidative stress and serve as an RBC quality marker.

Potentiometric Biosensing of Ascorbic Acid, Uric Acid, and Cysteine in Microliter Volumes Using Miniaturized Nanoporous Gold Electrodes.

Potentiometric redox sensing is a relatively inexpensive and passive approach to evaluate the overall redox state of complex biological and environmental solutions. The ability to make such measurements in ultra-small volumes using high surface area, nanoporous electrodes is of particular importance as such electrodes can improve the rates of electron transfer and reduce the effects of biofouling on the electrochemical signal. This work focuses on the fabrication of miniaturized nanoporous gold (NPG) electrodes with a high surface area and a small footprint for the potentiometric redox sensing of three biologically relevant redox molecules (ascorbic acid, uric acid, and cysteine) in microliter volumes. The NPG electrodes were inexpensively made by attaching a nanoporous gold leaf prepared by dealloying 12K gold in nitric acid to a modified glass capillary (1.5 mm id) and establishing an electrode connection with copper tape. The surface area of the electrodes was ~1.5 cm2, providing a roughness factor of ~16 relative to the geometric area of 0.09 cm2. Scanning electron microscopy confirmed the nanoporous framework. A linear dependence between the open-circuit potential (OCP) and the logarithm of concentration (e.g., Nernstian-like behavior) was obtained for all three redox molecules in 100 µL buffered solutions. As a first step towards understanding a real system, the response associated with changing the concentration of one redox species in the presence of the other two was examined. These results show that at NPG, the redox potential of a solution containing biologically relevant concentrations of ascorbic acid, uric acid, and cysteine is strongly influenced by ascorbic acid. Such information is important for the measurement of redox potentials in complex biological solutions.

On the Importance of Silane Infusion Order on the Microscopic and Macroscopic Properties of Multifunctional Charge Gradients.

Four multicomponent charge gradients containing acidic and basic functionalities were prepared via sol-gel processes and the controlled-rate infusion (CRI) method to more clearly understand how preparation conditions influence macroscopic properties. CRI is used to form gradients by infusing reactive alkoxysilanes into a glass vial housing a vertically oriented modified silicon wafer. The concentration and time of infusion of the silane solutions were kept constant. Only the sequence of infusion of the silane solutions was changed. The first set of samples was prepared by initially infusing a solution containing 3-aminopropyltriethoxysilane (APTES) followed by a mercaptopropyltrimethoxysilane (MPTMS) solution. The individual gradients were formed either in an aligned or opposed fashion with respect to the initial gradient. The second set of samples was prepared by infusing the MPTMS solution first followed by the APTES solution, again in either an aligned or opposed fashion. To create charge gradients (NH3 +, SO3 -), the samples were immersed into H2O2. The extent of modification, the degree of protonation of the amine, and the thicknesses of the individual layers were examined by X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The wettability of the individual gradients was assessed via static contact angle measurements. The results demonstrate the importance of infusion order and how it influences the macroscopic and microscopic properties of gradient surfaces including the surface concentration, packing density, degree of protonation, and ultimately wettability. When the gradient materials are prepared via infusion of the APTES sol first, it results in increased deposition of both the amine and thiol groups as evidenced by XPS. Interestingly, the total thickness evaluated from ellipsometry was independent of the infusion order for the aligned gradients, indicative of significant differences in the film density. For the opposed gradients, however, the infusion of APTES first leads to a significantly thicker composite film. Furthermore, it also leads to a more pronounced gradient in the protonation of the amine, which introduces a very different surface wettability. The use of aminosilanes provides a viable approach to create gradient surfaces with different functional group distributions. These studies demonstrate that the controlled placement of functional groups on a surface can provide a new route to prepare gradient materials with improved performance.

Simulation of elution profiles in liquid chromatography - IV: Experimental characterization and modeling of solute injection profiles from a modulation valve used in two-dimensional liquid chromatography.

Simulation software for liquid chromatography can accelerate method development capabilities. In two-dimensional chromatography this is particularly attractive because there are more method variables to consider, provided simulations can account for the effects of injecting effluent from the first dimension separation into the second dimension column. In this paper we describe the adaptation of a previously described model (the Forssén model) to enable prediction of the profile of an injection pulse as it exits an Active Solvent Modulation (ASM) valve and enters the second dimension column under a variety of flow rate and sample loop size conditions (a global model). Experimentally measured injection profiles were used to train empirical models capable of generating injection profiles as a function of sample loop volume and flow rate. The resulting parameters were then used to generate an injection profile for a loop volume not used in the training set. The resulting profile agreed well with the experimentally obtained profile for this sample loop. Finally, chromatograms were simulated using previously developed simulation software incorporating the injection profile models developed in this work. Chromatographic peaks were simulated for valerophenone on an Agilent Zorbax Stablebond C18 stationary phase with an acetonitrile/water mobile phase gradient. Results of simulations based on experimental injection profiles, profiles predicted using the Forssén or global models, and rectangular injection profiles were compared. Comparison of the resulting chromatographic peaks revealed good agreement between those produced using experimental profiles or the Forssén or global models, with less than ± 0.3% deviations for retention times and less than ± 10% deviations for the peak widths (expressed as σ).

Fabrication and Characterization of a Reversed-Phase/Strong Cation Exchange Stationary Phase Gradient.

Continuous stationary phase gradients for liquid chromatography (LC) have been recently shown to be a promising method of altering selectivity. In this work, we present the first multicomponent continuous stationary phase gradient for separations involving both reversed-phase (RP) and strong cation exchange (SCX) mechanisms. These columns are fabricated using a two-step methodology based on controlled rate infusion (CRI). First, destructive CRI creates a gradient of excess silanol groups along a uniform C18 column. Next, these columns are infused with 3-mercaptopropyltrimethoxysilane (MPTMOS), which bonds to the excess silanol groups. The terminal thiols of the MPTMOS ligands are oxidized with H2O2 to create the sulfonate functionality (SO3-) needed for SCX separations. The success of the modification procedure is characterized by thermogravimetric analysis and X-ray photoelectron spectroscopy. The stability of the C18-SO3- gradients were found to have less than 5 % retention loss and the column-to-column reproducibility had a relative standard deviation under 9 %. The peak asymmetry factors for seven biogenic amines were found to be between 1.03 ± 0.04 to 1.30 ± 0.02, which illustrates minimal peak tailing due to poor packing and residual silanol groups. Characterization of the gradient columns using an isocratic mobile phase showed a unique elution order compared to a uniform C18 and SO3- columns. At lower counterion concentrations, more than 48 % of the overall retention on the gradient stationary phase is due to a SCX mechanism. Meanwhile, the RP mechanism was shown to predominate at higher counterion concentrations on the gradient columns (SCX retention contribution less than 40 %). Coupling the stationary phase gradient to a salt gradient in the mobile phase showed that the gradient phase provides a unique, intermediate selectivity to the uniform C18 and SO3- columns. Under an ACN mobile phase gradient, a significant increase (p < 0.003) in the retention times of three biogenic amines (15 - 16 %) was observed when the multicomponent gradient was oriented to have a high SO3- ligand density near the detector. This work serves as a proof-of-concept design for a multicomponent stationary phase gradient to continue fundamental studies into retention and encourage novel applications.

Experimental- and simulation-based investigations of coupling a mobile phase gradient with a continuous stationary phase gradient.

This work seeks to explore and understand the effects of column orientation and degree of modification of continuous stationary phase gradient columns under a mobile phase gradient using both simulations and experiments. Peak parameters such as retention times, peak widths and resolution are obtained for five phenolic compounds on a C18-silica gradient stationary phase. Simulations show that peak widths for the solutes are dependent upon the fractional composition of C18 and orientation of the stationary phase gradient when coupled to a mobile phase gradient. Also, when compared to a simulated uniform mixed-mode column, peak widths reach a minimum on the gradient column with a coverage higher than 50% C18 where the column is oriented to have the C18 dense region at the end. Experimentally, continuous stationary phase gradients were fabricated to have a total C18 composition of 78% of the original uniform column with an exponential profile using a previously described destructive controlled rate infusion method. Under gradient mobile phase conditions, experimental retention times for the gradient column showed a significant increase compared to the original 100% C18 column. Simulations with a similar C18 composition, however, predicted decreased retention times from the original C18 column. A statistical increase in the retention time of protocatechuic acid and decrease in the peak width of tyrosol, caffeic acid, and coumaric acid were noted when the gradient column was oriented to have the C18 dense region located near the detector. Collectively, combining gradients in both the mobile and stationary phases can yield interesting neighboring ligand effects and peak broadening/focusing effects.

Redox Potential Measurements in Red Blood Cell Packets Using Nanoporous Gold Electrodes.

The redox potential of packed red blood cells (RBCs) was measured over a 56-day storage period using a newly developed potentiometric methodology consisting of a nanoporous gold electrode and a silver chloride coated silver reference electrode. Both milliliter- and microliter-sized volumes were separately evaluated. The addition of Vitamin C (VitC) in differing doses to the packed RBCs was also assessed as a means to improve redox stability and prolong storage duration. For RBCs containing only saline, the open-circuit potential (OCP) was ∼ -80 mV vs Ag/AgCl and drifted slightly with time; greater differences were also noted between different electrodes. The addition of exogenous VitC to the RBC shifts the OCP to more negative values, stabilizes the redox potential, and improves reproducibly between different electrodes due to the poising of blood. Over the 56-day storage period, the redox potential of the RBCs increased slightly, which can be attributed to change in pH and/or increasing oxidative stress during storage. Cyclic voltammograms acquired after open-circuit potential measurements showed a characteristic peak attributed to the oxidation of VitC. This peak decreased during storage with a time constant of 20.8 days. Likewise, the intercellular concentration of VitC increased with a time constant of 20.2 days as measured using a fluorescence assay. Collectively, these results demonstrate the usefulness of electrochemical measurements in the study of stored blood products.

Destructive stationary phase gradients for reversed-phase/hydrophilic interaction liquid chromatography.

The use of stationary phase gradients for liquid chromatography (LC) is a promising new strategy to allow for specific control over the selectivity of a separation by having a gradual change in the ligand density along the length of the column. Unfortunately, there have been very few, if any, methods to prepare continuous stationary phase gradients on traditional packed LC columns. In this work, destructive methodologies are used to create stationary phase gradients on commercial C18 columns by infusing trifluoroacetic acid (TFA) onto the column through controlled rate of infusion (CRI). The introduction of TFA via CRI while the column is heated at 80 °C promotes acid hydrolysis of the alkylsilane ligand in a gradient fashion. Characterization with scanning electron microscopy and Barrett-Joyner-Halenda pore size distributions of the stationary phase after fabrication of the destructive gradient establishes that the chromatographic support was not damaged during the procedure. The shape of the gradient was examined using thermogravimetric analysis (TGA) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. TGA and ATR-FTIR showed an increase in the percent carbon loss along the length of the column, indicating that there was an increase in the C18 ligand from the front to the end of the column. Two selectivity tests demonstrated a decrease in the hydrophobicity and increase in the silanol activity of the stationary phase gradient from the uniform C18 counterpart. Additionally, the fabrication of the destructive stationary phase gradient resulted in two different surface functionalities allowing hydrophobic and hydrophilic interactions with analyte species depending on the mobile phase composition. Plots of the log of retention factor versus percent acetonitrile illustrated that these stationary phase gradients have two separation mechanisms: reversed-phase (RP) and hydrophilic interaction. Coupling the stationary phase gradient with a mobile phase gradient shows differences in the peak widths and the resolution of phenolic compounds, indicating that the orientation of the stationary phase gradient has the potential to enhance the resolution of a separation. With this methodology, stationary phase gradients can be fabricated on previously used RP columns, allowing for these columns to be repurposed.

Vapor-Phase Plotting of Organosilane Chemical Gradients.

Vapor-phase plotting of organosilane-based self-assembled monolayer (SAM) gradients is demonstrated for the first time. Patterned SAMs are formed by delivering gas-phase organotrichlorosilane precursors to a reactive silica surface using a heated glass capillary. The capillary is attached via a short flexible tube to a reservoir containing the precursor dissolved in toluene. The proximal end of the capillary is positioned at an experimentally optimized distance of 30 µm above the substrate during film deposition. The capillary is mounted to a stepper-motor-driven X, Y plotter for raster scanning above the surface. Two different organotrichlorosilane precursors are employed in this initial demonstration: n-octyltrichlorosilane and 3-cyanopropyltrichlorosilane. The dependence of SAM deposition on ambient relative humidity, capillary-substrate separation, raster-scanning speed, and solvent viscosity and volatility is explored and optimum deposition conditions are identified. The optimized procedures are used to plot uniformly modified square "pads" and gradients of the silanes. Film formation is verified and the gradient profiles are obtained by sessile drop water contact angle measurements, spectroscopic ellipsometry measurements of film thickness, and X-ray photoelectron spectroscopy mapping. The resolution of the plotting process is currently in the millimeter range and depends on capillary diameter and distance from the substrate surface. Vapor-phase plotting affords a unique direct-write method for producing patterned and chemically graded SAMS that may find applications in microfluidic devices, planar chromatography, and optical and electronic devices.

Biofouling-Resistant Platinum Bimetallic Alloys.

A new electrosynthetic approach for the fabrication of three-dimensional bicontinuous nanoporous platinum-based (3D-BC-NP-Pt(Au)) electrodes is described. Binary Pt-Ag alloys are first electrodeposited on gold substrates from appropriately formulated plating solutions. Following annealing and dealloying, a new family of nanoporous platinum-based electrodes emerges whose morphology, porosity, and chemical compositions depend on electrodeposition parameters and plating solution composition. Scanning electron microscopy images reveal an interesting and distinctive nanoporous gold-like microstructure with pores and ligaments in the 10-30 nm range arranged in a bicontinuous fashion throughout the thickness of the film. X-ray photoelectron spectroscopy (XPS) confirms that the as-formed electrodeposited films are silver-rich platinum binary alloys. Interestingly, XPS also reveals that after annealing and dealloying, the electrodes are actually ternary alloys containing platinum, gold, and a small amount of residual silver that remains after dealloying. Electrochemical measurements are consistent with this result and disclose a high surface area with roughness factors of 15-24. The ability to successfully conduct electrochemical measurements in biofouling solutions via a unique biosieving-like mechanism is demonstrated by exposure of the unique 3D bicontinuous nanoporous platinum-based electrode to fibrinogen in phosphate buffer and in a solution containing red blood cells. The work described herein has the potential to enrich the fields of electrochemical sensing and biosensing via the introduction of new 3D bicontinuous nanostructured porous platinum-based electrodes that can be easily and reliably fabricated.

Probing the Local Dielectric Constant of Plasmid DNA in Solution and Adsorbed on Chemically Graded Aminosilane Surfaces.

Nile Red dye was used to determine the dielectric constant, ε, of nonpolar microenvironments in double stranded DNA (ds-DNA) both in aqueous buffer solution and when adsorbed on amine-modified surfaces. The value of ε within the DNA decreased with increasing buffer concentration. Values of ε ∼ 6.75 and ∼3.00 were obtained in 0.1 mM phosphate buffered saline (PBS) and in 10 mM PBS, respectively. Similar effects were observed upon adsorption to chemically graded amine-modified silica surfaces. Under 1 mM buffer, ε was measured to be ∼2.84 and ∼1.90 at the low amine (high silica) and high amine (low silica) ends of the gradient, respectively. An increase in the buffer concentration again led to a decrease in ε, but only at the low amine end. It is concluded that high buffer concentrations and binding to an amine surface cause a condensation of the DNA, forming less polar microenvironments. These results provide important knowledge of the factors governing the polarity of DNA microenvironments to which intercalators bind.

pH and Surface Charge Switchability on Bifunctional Charge Gradients.

Multifunctionalized pH-sensitive silica gradients containing acidic and basic functional groups have been prepared to evaluate how the spatial arrangement of active sites on a surface influences the surface charge and pH switchability. The gradient surfaces were prepared using controlled rate infusion in such a manner that the individual gradients in the strong acid (sulfonic acid) and in the weak base (propylamine) align, whereas a gradient in the weakly acidic silanol groups opposes them. The relative amounts of the three species were varied by controlling the composition of the deposition solution, whereas the hydrophobicity of the underlying surface was set by using base layer-coated substrates prepared from either tetramethoxysilane or tetramethoxysilane/octyltrimethoxysilane mixtures. Results from X-ray photoelectron spectroscopy confirm that aligned gradients are formed in both amine and sulfonic acid groups, and the relative amounts bound to the surface follow that expected from the solution composition. Water contact angle measurements show a 40°-50° change across the length of the gradient, the exact values being dependent on the hydrophobicity of the base layer. Zeta potential measurements on gradient mimics reveal that there is a pH where the net charge on the gradient surface is predicted to have a constant but nonzero value. Static contact angle measurements and modeling confirm this prediction. At a pH acidic of this value, the gradient in charge runs in one direction, whereas at a pH basic of this value, the gradient in charge runs in the other direction. This point can be strategically moved from acidic values to basic values by changing the relative amounts of acidic and basic functionalities on the surface. The origin of this unique pH switchability can be found in acid-base chemistry. By modeling the charge along the gradient surface using a simple equilibrium model, a distribution of pKa values were noted in these materials.

Bacteria assisted protein imprinting in sol-gel derived films.

A hierarchical imprinting strategy was used to create protein imprints in a silicate film with a high binding capacity as well as selectivity toward the imprint protein and little specificity towards other proteins. In the first part of this work, rod-shaped bacteria were used as templates to create imprints in silica films of various thicknesses to open up the silica framework and increase the surface area exposed to solution. In the second part, the protein (e.g., cytochrome c (CYC) or green fluorescent protein (GFP)) was covalently attached to the surface of Bacillus subtilis and this protein-bacteria complex served as the imprint moiety. Atomic force microscopy and scanning electron microscopy were used to image the micron-size rod-shaped bacteria imprints formed on the silica surface. Fluorescence microscopy, which was used to follow the fabrication process with GFP as the representative protein, clearly demonstrated protein imprinting, protein removal and protein rebinding as well as protein specificity. Visible absorption spectroscopy using CYC as the imprint protein demonstrated relatively fast uptake kinetics and also good specificity against other proteins including bovine serum albumin (BSA), horseradish peroxidase (HRP), glucose oxidase (GOD), and lysozyme (LYZ). Collectively this work demonstrates a new surface bio-imprinting approach that generates recognition sites for proteins and provides a viable means to increase the binding capacity of such imprinted thin films.

Organosilane Chemical Gradients: Progress, Properties, and Promise.

Chemical gradients play an important role in nature, driving many different phenomena critical to life, including the transport of chemical species across membranes and the transport, attachment, and assembly of cells. Taking a cue from these natural processes, scientists and engineers are now working to develop synthetic chemical gradients for use in a broad range of applications, such as in high-throughput investigations of surface properties, as means to guide the motions and/or assembly of liquid droplets, vesicles, nanoparticles, and cells and as new media for stationary-phase-gradient chemical separations. Our groups have been working to develop new methods for preparing chemical gradients from organoalkoxysilane and organochlorosilane precursors and to obtain a better understanding of their properties on macroscopic to microscopic length scales. This review highlights our recent work on the development of controlled-rate infusion and infusion-withdrawal dip-coating methods for the preparation of gradients on planar glass and silicon substrates, on thin-layer chromatography plates, and in capillaries and monoliths for liquid chromatography. We also cover the new knowledge gained from the characterization of our gradients using sessile drop and Wilhelmy plate dynamic water contact angle measurements, X-ray photoelectron spectroscopy mapping, and single-molecule tracking and spectroscopy. Our studies reveal important evidence of phase separation and cooperative interactions occurring along multicomponent gradients. Emerging concepts and new directions in the preparation and characterization of organosilane-based chemical gradients are also discussed.

Base Layer Influence on Protonated Aminosilane Gradient Wettability.

Protonated amine gradients have been prepared on silicon wafers via programmed controlled rate infusion (CRI) with varying degrees of hydrophobicity and characterized by X-ray photoelectron spectroscopy (XPS) and static and Wilhelmy plate dynamic contact angle measurements. Initially, base layers were spin coated from sols containing tetramethoxysilane (TMOS) and either phenyltrimethoxysilane (PTMOS), dimethyldimethoxysilane (DMDMOS), or octyltrimethoxysilane (OTMOS, C8). Amine gradients were then prepared from 3-aminopropyltriethoxysilane (APTEOS) via CRI. Gradients were exposed to concentrated HCl vapor for amine protonation. XPS showed that NH2 functional groups were distributed in a gradient fashion as a result of CRI controlling the time of exposure to APTEOS. Interestingly, the overall extent of N modification depended on the type of base layer used for gradient formation. The C8-derived base layer had about half the amount of nitrogen on the surface as compared to those prepared from TMOS, which was attributed to a reduction in the number and accessibility of surface silanol groups. The wettability and contact angle (CA) hysteresis were also dependent on the base layer and varied along the length of the gradient. The greatest CA change across the length of the gradient was observed on the gradient formed on the C8-derived base layer. Likewise, the CA hysteresis was approximately 2 times larger on the C8-modified surfaces, indicative of greater chemical inhomogeneity. In contrast to uniformly modified substrates, Wilhelmy plate CA analysis that involves the immersion of samples gave a unique S-shaped CA distance curve for the gradients. The three curve segments correspond to hydrophilic, hydrophobic, and a middle connecting region. Importantly, these curves give precise CAs along the gradient that reflect the surface chemistry and coverage defined by programmed CRI processing.

Single Molecule Catch and Release: Potential-Dependent Plasmid DNA Adsorption along Chemically Graded Electrode Surfaces.

Single molecule detection methods were employed to study the potential dependent adsorption and desorption of dye labeled plasmid DNA along chemical gradients prepared on indium tin oxide (ITO) electrodes. Gradients were formed over silica-base-layer-coated ITO surfaces by exposing them in a directional fashion to aminopropyltrimethoxysilane from the vapor phase. Sessile drop water contact angle measurements, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy were used to verify that a gradient was formed and to characterize its wettability, thickness, and composition as a function of position. The gradient-coated ITO electrode served as both the working electrode and a window into the electrochemical cell used to manipulate DNA adsorption. For single molecule studies, the electrochemical cell was filled with buffer solution containing YOYO-1-labeled plasmid DNA. Fluorescence videos acquired along the gradients depicted clear position-, potential-, and pH-dependent variations in DNA adsorption and desorption. The results demonstrate that DNA adsorption was largely independent of applied potential and irreversible at high amine coverage (i.e., multilayers), under pH ∼ 6 buffer. DNA adsorption became more reversible as the amine coverage decreased and the solution pH increased. Potential dependent control over DNA adsorption and desorption was best achieved at monolayer-to-submonolayer aminosilane coverage under pH ∼ 8 buffer. The knowledge gained in these studies will aid in the development of electrochemical methods for the capture and release of DNA and other polyelectrolytes at electrode surfaces.

Molecular Combing of λ-DNA using Self-Propelled Water Droplets on Wettability Gradient Surfaces.

Surface wettability gradients were used to elongate and align double stranded λ-DNA. Gradients were prepared by vapor phase deposition of octyltrichlorosilane (C8-silane) and fluorinated octyltrichlorosilane (F-silane) precursors. Gradient formation was confirmed by water contact angle and ellipsometric film thickness measurements. Placement of a droplet of aqueous DNA solution on the hydrophobic end of each gradient led to spontaneous motion of the droplet toward the hydrophilic end and deposition of the DNA. Fluorescence imaging of surface-adsorbed YOYO-1 labeled DNA molecules revealed that they are elongated and aligned perpendicular to the droplet-surface contact line at all positions along the gradient, consistent with a dominant role played by surface tension forces in elongating the DNA. The density of adsorbed DNA was found to be greatest on the C8-silane gradient at its hydrophobic end. DNA density decreased toward the hydrophilic end, while the length of the elongated DNA was less dependent on position. The elongation of DNA molecules by spontaneous droplet motion on chemical gradient surfaces has possible applications in DNA barcoding and studies of DNA-protein interactions.

Mesoporous Hybrid Polypyrrole-Silica Nanocomposite Films with a Strata-Like Structure.

Using a single-potential-step coelectrodeposition route, Ppy-SiO2 nanocomposite films characterized by a multimodal porous structure were cathodically deposited from ethanolic solutions on oxidizable and nonoxidizable substrates for the first time. The materials produced have an interesting and unique strata-like pore structure along their depth. With the exception of a silica-rich inner region, the nanocomposite films are homogeneous in composition. Because the region closest to the electrode surface is silica-rich, the fabrication of Ppy-SiO2 and Ppy free-standing films become possible using a multistep etching strategy. Such films can be captured on a variety of different supports depending on the application, and they maintain their conductivity when interfaced with an electrode surface. These mesoporous composite films form through a unique mechanism that involves the production of two catalysts, OH(-) and NO(+). Through the process of understanding the reaction mechanism, we highlighted the effect of two simultaneous competing redox reactions occurring at the electrode interface on the morphology of the electrodeposited Ppy nanocomposite films and how solvent can influence the Ppy electropolymerization reaction mechanism and hence control the morphology of the final material. In an ethanolic solvent system, the pyrrole monomers undergo a step-growth polymerization, and particulate-like nanostructured films were obtained even upon changing the monomer or acid concentration. In an aqueous-based system, nanowire-like structures were produced, which is consistent with a chain-growth mechanism. Such materials are promising candidates for a wide range of applications including electrochemical sensing, energy storage, and catalysis.