Controlling Microarray Feature Spreading and Response Stability on Porous Silicon Platforms by Using Alkene-Terminal Ionic Liquids and UV Hydrosilylation.

In an attempt to develop reversible sensors based on ionic liquid/porous silicon (IL/pSi) platforms, we introduce an approach using task-specific, alkene-terminal ILs (AT-ILs) for direct grafting to the hydrogen-passivated as prepared-pSi (ap-pSi) surface via UV-hydrosilylation to address previous shortcomings associated with IL pattern impermanence (i.e., spread). By employing photoluminescence emission (PLE) and Fourier-transform infrared (FT-IR) imaging measurements, we demonstrate that the covalent grafting of AT-ILs onto the ap-pSi surface via photochemical hydrosilylation not only mitigates such feature spreading but also greatly improves PLE pattern stability. Significantly, we have discovered that, upon hydrosilylation, the resulting contact pin printed IL features remain stable to repeated challenges by toluene vapors, demonstrating the utility of AT-IL hydrosilylation for producing high-fidelity microarray features on pSi toward robust optical sensory microarrays.

Effects of Acetone Vapor on the Exciton Band Photoluminescence Emission from Single- and Few-Layer WS2 on Template-Stripped Gold.

Two-dimensional (2D) materials are being used widely for chemical sensing applications due to their large surface-to-volume ratio and photoluminescence (PL) emission and emission exciton band tunability. To better understand how the analyte affects the PL response for a model 2D platform, we used atomic force microscopy (AFM) and co-localized photoluminescence (PL) and Raman mapping to characterize tungsten disulfide (WS2) flakes on template-stripped gold (TSG) under acetone challenge. We determined the PL-based response from single- and few-layer WS2 arises from three excitons (neutral, A0; biexciton, AA; and the trion, A-). The A0 exciton PL emission is the most strongly quenched by acetone whereas the A- PL emission exhibits an enhancement. We find the PL behavior is also WS2 layer number dependent.

Three-Dimensional pH Mapping within Model Hybrid Xerogel Thin Films.

When xerogel films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS) or 3-aminopropyltriethoxysilane (APTES), n-octyltriethoxysilane (C8), and TEOS are formed on Al2O3 they exhibit chemically segregated domains with unique chemistries and topographies. These characteristics are important for marine antifouling. By using the ratiometric fluorescent probe 5 (and 6)-carboxy SNARF-1 (C.SNARF-1) in concert with confocal fluorescence microscopy, we determine the pH in three dimensions within these hybrid films. For the COE/TEOS film, 4-5 µm diameter dendritically shaped features form, and they extend ∼100 nm above the film base. These dendritic features are acidic (pH < 7) in comparison to the film base. Their average diameter decreases as we progress from the solution-film interface toward the film-Al2O3 interface. Planes located at the solution-film interface, film center, and film-Al2O3 interface exhibit acidic surface areas that are 20% below, 50% above, and 70% below the average COE mole fraction used to create the film. In the APTES/C8/TEOS films, 1-3 µm diameter mesa-shaped features form, and they extend up to 450 nm above the film base. These mesa features are basic (pH > 7) in comparison to the film base and are columnar in shape, extending without change in diameter throughout the entire film. From the solution-film interface the planes located within the first 3/4 of the film exhibit basic surface areas that are equivalent to the average APTES mole fraction used to create the film. However, as one approaches the film-Al2O3 interface, many new 100-200 nm basic subsurface regions appear. The basic surface area in those film planes within 400-500 nm of the film-Al2O3 interface are enriched in APTES by up to 500% above the average APTES mole fraction used to create the film.

Ionic Liquids Can Permanently Modify Porous Silicon Surface Chemistry.

To develop ionic liquid/porous silicon (IL/pSi) microarrays we have contact pin-printed 20 hydrophobic and hydrophilic ionic liquids onto as-prepared, hydrogen-passivated porous silicon (ap-pSi) and then determined the individual IL spot size, shape and associated pSi surface chemistry. The results reveal that the hydrophobic ionic liquids oxidize the ap-pSi slightly. In contrast, the hydrophilic ionic liquids lead to heavily oxidized pSi (i.e., ox-pSi). The strong oxidation arises from residual water within the hydrophilic ILs that is delivered from these ILs into the ap-pSi matrix causing oxidation. This phenomenon is less of an issue in the hydrophobic ILs because their water solubility is substantially lower.

Hybrid Sol-Gel-Derived Films That Spontaneously Form Complex Surface Topographies.

Surface patterns over multiple length scales are known to influence various biological processes. Here we report the synthesis and characterization of new, two-component xerogel thin films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS). Atomic force microscopy (AFM) reveals films surface with branched and hyper branched architectures that are ∼2 to 30 µm in diameter, that extend ∼3 to 1300 nm above the film base plane with surface densities that range from 2 to 77% surface area coverage. Colocalized AFM and Raman spectroscopy show that these branched structures are COE-rich domains, which are slightly stiffer (as shown from phase AFM imaging) and exhibit lower capacitive force in comparison with film base plane. Raman mapping reveals there are also discrete domains (≤300 nm in diameter) that are rich in COE dimers and densified TEOS, which do not appear to correspond with any surface structure seen by AFM.

Environmentally benign sol-gel antifouling and foul-releasing coatings.

Biofouling on ships and boats, characterized by aquatic bacteria and small organisms attaching to the hull, is an important global issue, since over 80000 tons of antifouling paint is used annually. This biofilm, which can form in as little as 48 hours depending on water temperature, increases drag on watercraft, which greatly reduces their fuel efficiency. In addition, biofouling can lead to microbially induced corrosion (MIC) due to H2S formed by the bacteria, especially sulfate-reducing bacteria. When the International Maritime Organization (IMO) international convention banned the use of effective but environmentally damaging coatings containing tributyl tin in 2008, the development of clean and effective antifouling systems became more important than ever. New nonbiocidal coatings are now in high demand. Scientists have developed new polymers, materials, and biocides, including new elastomeric coatings that they have obtained by improving the original silicone (polydimethylsiloxane) formulation patented in 1975. However, the high cost of silicones, especially of fluoropolymer-modified silicones, has generally prevented their large-scale diffusion. In 2009, traditional antifouling coatings using cuprous oxide formulated in copolymer paints still represented 95% of the global market volume of anti-fouling paints. The sol-gel nanochemistry approach to functional materials has emerged as an attractive candidate for creating low fouling surfaces due to the unique structure and properties of silica-based coatings and of hybrid inorganic-organic silicas in particular. Sol-gel formulations easily bind to all types of surfaces, such as steel, fiberglass, aluminum, and wood. In addition, they can cure at room temperature and form thin glassy coatings that are markedly different from thick silicone elastomeric foul-releasing coatings. Good to excellent performance against biofouling, low cure temperatures, enhanced and prolonged chemical and physical stability, ease of application, and the waterborne nature of sol-gel coatings all support the diffusion of these paints to efficiently reduce the accumulation of fouling layers on valued surfaces immersed in marine or fluvial waters. Furthermore, sol-gel glassy coatings are transparent and can be effectively applied to optical devices, windows, and solar panels used in lake, fluvial, or marine environments. Sol-gel technology is eminently versatile, and the first generation sol-gel paints have already shown good performance. Even so, vast opportunities still exist for chemists to develop novel sol-gel derived coatings to both prevent biofouling and enhance the hydrodynamic properties of boat and ship hulls. Moreover, researchers have prepared and applied multifunctional sol-gel coatings providing protection against both biofouling and corrosion. They have tested these in the marine environment with good preliminary results. In this Account, we discuss some of our new strategies for the controlled functionalization of surfaces for the development of efficient antifouling and foul-releasing systems and summarize the main achievements with biocidal and nonbiocidal sol-gel coatings. We conclude by giving insight into the marine coatings and sol-gel products markets, providing arguments to justify our conclusion that the sol-gel coatings technology is now a mature platform for the development of economically viable and environmentally friendly antifouling and foul-release formulations of enhanced performance.

Optimizing Pin-Printed and Hydrosilylated Microarray Spot Density on Porous Silicon Platforms.

Microarrays of spatially isolated chemistries on planar surfaces are powerful tools. An important factor in microarray technology is the density of chemically unique spots that can be formed per unit area. In this paper, we use contact pin-printing and evaluate how to decrease contact pin-printed spot diameters on porous silicon (pSi) platforms. Using hydrosilylation chemistry to covalently attach chemistries to the pSi surface, the variables studied included pSi porosity and surface polarity, active agent viscosity, and pin diameter. The spot characteristics were assessed by Fourier transform infrared spectroscopy (FT-IR) microscopy and X-ray photoelectron spectroscopy (XPS). Spot size decreased as pSi porosity increased in accordance with molecular kinetic theory and Darcy's law of imbibition. Increasing active agent viscosity and pin diameter (volume of printed agent) led to larger spot diameters in accordance with molecular kinetic theory and Darcy's law. Oxidizing the pSi with H2O2 increased the surface polarity but had no detectable impact on the spot size. This is consistent with formation of an oxide layer atop an unoxidized pSi sublayer.

Probing nanoscale chemical segregation and surface properties of antifouling hybrid xerogel films.

Over the past decade there has been significant development in hybrid polymer coatings exhibiting tunable surface morphology, surface charge, and chemical segregation-all believed to be key properties in antifouling (AF) coating performance. While a large body of research exists on these materials, there have yet to be studies on all the aforementioned properties in a colocalized manner with nanoscale spatial resolution. Here, we report colocalized atomic force microscopy, scanning Kelvin probe microscopy, and confocal Raman microscopy on a model AF xerogel film composed of 1:9:9 (mol:mol:mol) 3-aminopropyltriethoxysilane (APTES), n-octyltriethoxysilane (C8), and tetraethoxysilane (TEOS) formed on Al2O3. This AF film is found to consist of three regions that are chemically and physically unique in 2D and 3D across multiple length scales: (i) a 1.5 µm thick base layer derived from all three precursors; (ii) 2-4 µm diameter mesa-like features that are enriched in free amine (from APTES), depleted in the other species and that extend 150-400 nm above the base layer; and (iii) 1-2 µm diameter subsurface inclusions within the base layer that are enriched in hydrogen-bonded amine (from APTES) and depleted in the other species.

Creating diversified response profiles from a single quenchometric sensor element by using phase-resolved luminescence.

We report a new strategy for generating a continuum of response profiles from a single luminescence-based sensor element by using phase-resolved detection. This strategy yields reliable responses that depend in a predictable manner on changes in the luminescent reporter lifetime in the presence of the target analyte, the excitation modulation frequency, and the detector (lock-in amplifier) phase angle. In the traditional steady-state mode, the sensor that we evaluate exhibits a linear, positive going response to changes in the target analyte concentration. Under phase-resolved conditions the analyte-dependent response profiles: (i) can become highly non-linear; (ii) yield negative going responses; (iii) can be biphasic; and (iv) can exhibit super sensitivity (e.g., sensitivities up to 300 fold greater in comparison to steady-state conditions).

Aqueous self-assembly of giant bottlebrush block copolymer surfactants as shape-tunable building blocks.

Programmed self-assembly of well-defined molecular building blocks enables the fabrication of precisely structured nanomaterials. In this work, we explore a new class of giant polymeric surfactants (Mn = (0.7-4.4) × 10(6) g/mol) with bottlebrush architecture and show that their persistent molecular shape leads to the formation of uniform aggregates in a predictable manner. Amphiphilic bottlebrush block copolymers containing polylactide (PLA) and poly(ethylene oxide) (PEO) side chains were synthesized by a grafting-from method, and their self-assembly in aqueous environment was studied by cryogenic transmission electron microscopy. The produced micelle structures with varying interfacial curvatures and core radii (19-55 nm) boasted rod-like hydrophilic PEO brushes protruding from the hydrophobic PLA cores normal to the interface. Highly uniform spherical micelles with low dispersities were obtained from bottlebrush amphiphiles with packing parameters of ∼0.3, estimated from the polymer structural data. Long cylindrical micelles and other nonspherical aggregates were observed for the first time for compositionally less asymmetric bottlebrush surfactants. Critical micelle concentration values of 1 nM, measured for PEO-rich bottlebrush amphiphiles, indicated an enhanced thermodynamic stability of the produced micelle aggregates. Shape-dependent assembly of bottlebrush surfactants allows for the rational fabrication of a range of micelle structures in narrow morphological windows.

pH-dependent spectroscopy of tetracycline and its analogs.

Tetracyclines (TCs), broad spectrum antibiotics widely used in the prevention and treatment of infectious diseases, are amphoteric molecules containing several ionizable functional groups that exist predominantly as zwitterions at a given pH value. TCs are reported to undergo a wide variety of reactions at different pH values i.e. TCs form to anhydrotetracyclines at low pH, 4-epitetracyclines at pH 3-5 and isotetracyclines at high pH values. The pH-dependent absorbance and emission properties of tetracycline and its 10 analogs (4-epitetracycline, doxycyline, oxytetracycline, chlortetracycline, 4-epichlortetracycline, isochlortetracycline, methacycline, rolitetracycline, minocycline, and demeclocycline) were investigated and reported in this paper. The main focus of the study was on the pH dependent transformation of epichlortetracycline, chlortetracycline and isotetracycline at basic pH. Absorption, emission and time resolved spectroscopy were used to determine the behavior of the three TC derivatives at this condition. Increasing the buffer's ionic concentration leads to faster transformation to iCTC. A pH dependent transformation of CTC to iCTC was observed and the lifetimes of CTC and iCTC were determined to be 3.0 and 5.89 ns respectively. The distribution factor of CTC to iCTC at basic pH was also reported for the first time.

Filterless optical oxygen sensor based on a CMOS buried double junction photodiode.

We present a custom CMOS IC with a buried double junction (BDJ) photodiode to detect and process the optical signal, eliminating the need for any off-chip optical filters. The on-chip signal processing circuitry improves the desired signal extraction from the optical background noise. Since the IC is manufactured using standard commercial fabrication processes with no post-processing necessary, the system can ultimately be low cost to fabricate. Additionally, because of the CMOS integration, it will consume little power when operating, and even less during stand-by.

Binding affinities of CRBPI and CRBPII for 9-cis-retinoids.

BACKGROUND: Cellular retinol binding-protein I (CRBPI) and cellular retinol binding-protein II (CRBPII) serve as intracellular retinoid chaperones that bind retinol and retinal with high affinity and facilitate substrate delivery to select enzymes that catalyze retinoic acid (RA) and retinyl ester biosynthesis. Recently, 9-cis-RA has been identified in vivo in the pancreas, where it contributes to regulating glucose-stimulated insulin secretion. In vitro, 9-cis-RA activates RXR (retinoid × receptors), which serve as therapeutic targets for treating cancer and metabolic diseases. Binding affinities and structure-function relationships have been well characterized for CRBPI and CRBPII with all-trans-retinoids, but not for 9-cis-retinoids. This study extended current knowledge by establishing binding affinities for CRBPI and CRBPII with 9-cis-retinoids. METHODS: We have determined apparent dissociation constants, K'(d), through monitoring binding of 9-cis-retinol, 9-cis-retinal, and 9-cis-RA with CRBPI and CRBPII by fluorescence spectroscopy, and analyzing the data with non-linear regression. We compared these data to the data we obtained for all-trans- and 13-cis-retinoids under identical conditions. RESULTS: CRBPI and CRBPII, respectively, bind 9-cis-retinol (K'(d), 11nM and 68nM) and 9-cis-retinal (K'(d), 8nM and 5nM) with high affinity. No significant 9-cis-RA binding was observed with CRBPI or CRBPII. CONCLUSIONS: CRBPI and CRBPII bind 9-cis-retinol and 9-cis-retinal with high affinities, albeit with affinities somewhat lower than for all-trans-retinol and all-trans-retinal. GENERAL SIGNIFICANCE: These data provide further insight into structure-binding relationships of cellular retinol binding-proteins and are consistent with a model of 9-cis-RA biosynthesis that involves chaperoned delivery of 9-cis-retinoids to enzymes that recognize retinoid binding-proteins.

Hybrid oxygen-responsive reflective Bragg grating platforms.

Oxygen responsive sensor platforms were fabricated by pin printing tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ([Ru(dpp)(3)](2+)) doped sols onto wavelength tuned reflective Bragg gratings. In an epi-luminescence configuration, these Bragg gratings (Gr) were designed to selectively reflect the O(2) responsive [Ru(dpp)(3)](2+) emission toward the detector to enhance the detected signal magnitude. The xerogel based sensors were formed onto (i) glass (XGl), (ii) directly on top of the grating (XGrGl), or (iii) on the glass substrate opposite the grating (XGlGr). The results show that all sensors exhibit linear, statistically equivalent O(2) sensitivities, and the XGrGl platform yields up to an 8-fold increase in relative detected analytical signal (RDAS) in comparison to the control (XGl) platform.

A comparison of the antifouling/foul-release characteristics of non-biocidal xerogel and commercial coatings toward micro- and macrofouling organisms.

Five non-biocidal xerogel coatings were compared to two commercial non-biocidal coatings and a silicone standard with respect to antifouling (AF)/fouling-release (FR) characteristics. The formation and release of biofilm of the marine bacterium Cellulophaga lytica, the attachment and release of the microalga Navicula incerta, and the fraction removal and critical removal stress of reattached adult barnacles of Amphibalanus amphitrite were evaluated in laboratory assays. Correlations of AF/FR performance with surface characteristics such as wettability, surface energy, elastic modulus, and surface roughness were examined. Several of the xerogel coating compositions performed well against both microfouling organisms while the commercial coatings performed less well toward the removal of microalgae. Reattached barnacle adhesion as measured by critical removal stress was significantly lower on the commercial coatings when compared to the xerogel coatings. However, two xerogel compositions showed release of 89-100% of reattached barnacles. These two formulations were also tested in the field and showed similar results.

Spontaneous multiscale phase separation within fluorinated xerogel coatings for fouling-release surfaces.

Four-component xerogel films consisting of 1 mole-% n-octadecyltrimethoxysilane (C18) and 50 mole-% tetraethoxysilane (TEOS) in combination with 1-24 mole-% tridecafluoro-1,1,2,2-tetrahydrooctyltriethoxysilane (TDF) and 25-48 mole-% n-octyltriethoxysilane (C8) and a 1:49:50 mole-% C18/TDF/TEOS were prepared. Settlement of barnacle cyprids and removal of juvenile barnacles, settlement of zoospores of the alga Ulva linza, and strength of attachment of 7-day sporelings (young plants) of Ulva were compared amongst the xerogel formulations. Several of the xerogel formulations were comparable to poly(dimethylsiloxane) elastomer with respect to removal of juvenile barnacles and removal of sporeling biomass. The 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogels displayed some phase segregation by atomic force microscopy (AFM) pre- and post-immersion in water. Imaging reflectance infrared microscopy showed the formation of islands of alkane-rich and perfluoroalkane-rich regions in these same xerogels both pre- and post-immersion in water. Surface energies were unchanged upon immersion in water for 48 h amongst the TDF-containing xerogel coatings. AFM measurements demonstrated that surface roughness on the 1:4:45:50 and 1:14:35:50 C18/TDF/C8/TEOS xerogel coatings decreased upon immersion in water.

Tetracycline speciation during molecular imprinting in xerogels results in class-selective binding.

The creation of tetracycline (TC) responsive molecularly imprinted xerogels (MIXs) was investigated using electronic absorbance, liquid chromatography-ion-trap mass spectrometry (LC-ITMS), and first-principles theory. Experimental results show that the template molecule converts to its epimer, 4-epitetracycline (ETC), during the imprinting process. Additionally, end capping of the MIX surface silanols transforms TC into anhydrotetracycline (ATC) and 4-epianhydrotetracycline (EATC). Hence, despite aiming to imprint for a single analyte (TC), one simultaneously imprints for up to four analogs (TC, ETC, EATC and ATC) within a MIX. Binding studies using LC-MS showed the binding of the prepared xerogels with the four analogs. In some formulations, preferential uptake of ETC, EATC and ATC relative to the template molecule (TC) was observed. Computations of the interaction energies between silane monomers and the four analogs reveal that ETC, EATC and ATC have higher interaction energies and are more likely to be imprinted in comparison to TC.

Fluorescence energy transfer efficiency in labeled yeast cytochrome c: a rapid screen for ion biocompatibility in aqueous ionic liquids.

A fluorescence energy transfer "de-quenching" assay was implemented to follow the equilibrium unfolding behaviour of site-specific tetramethylrhodamine-labelled yeast cytochrome c in aqueous ionic liquid solutions; additionally, this approach offers the prospect of naked eye screening for biocompatible ion combinations in hydrated ionic liquids.

Contact CMOS imaging of gaseous oxygen sensor array.

We describe a compact luminescent gaseous oxygen (O2) sensor microsystem based on the direct integration of sensor elements with a polymeric optical filter and placed on a low power complementary metal-oxide semiconductor (CMOS) imager integrated circuit (IC). The sensor operates on the measurement of excited-state emission intensity of O2-sensitive luminophore molecules tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) ([Ru(dpp)3]2+) encapsulated within sol-gel derived xerogel thin films. The polymeric optical filter is made with polydimethylsiloxane (PDMS) that is mixed with a dye (Sudan-II). The PDMS membrane surface is molded to incorporate arrays of trapezoidal microstructures that serve to focus the optical sensor signals on to the imager pixels. The molded PDMS membrane is then attached with the PDMS color filter. The xerogel sensor arrays are contact printed on top of the PDMS trapezoidal lens-like microstructures. The CMOS imager uses a 32 × 32 (1024 elements) array of active pixel sensors and each pixel includes a high-gain phototransistor to convert the detected optical signals into electrical currents. Correlated double sampling circuit, pixel address, digital control and signal integration circuits are also implemented on-chip. The CMOS imager data is read out as a serial coded signal. The CMOS imager consumes a static power of 320 µW and an average dynamic power of 625 µW when operating at 100 Hz sampling frequency and 1.8 V DC. This CMOS sensor system provides a useful platform for the development of miniaturized optical chemical gas sensors.

The control of marine biofouling on xerogel surfaces with nanometer-scale topography.

Mixtures of n-octadecyltrimethoxysilane (C18, 1-5 mole-%), n-octyltriethoxysilane (C8) and tetraethoxysilane (TEOS) gave xerogel surfaces of varying topography. The 1:49:50 C18/C8/TEOS xerogel formed 100-400-nm-wide, 2-7-nm deep pores by AFM while coatings with ≥3% C18 were free of such features. Segregation of the coating into alkane-rich and alkane-deficient regions in the 1:49:50 C18/C8/TEOS xerogel was observed by IR microscopy. Immersion in ASW for 48 h gave no statistical difference in surface energy for the 1:49:50 C18/C8/TEOS xerogel and a significant increase for the 50:50 C8/TEOS xerogel. Settlement of barnacle cyprids and removal of juvenile barnacles, settlement of zoospores of the alga Ulva linza, and strength of attachment of 7-day sporelings were compared amongst the xerogel formulations. Settlement of barnacle cyprids was significantly lower in comparison to glass and polystyrene standards. The 1:49:50 and 3:47:50 C18/C8/TEOS xerogels were comparable to PDMSE with respect to removal of juvenile barnacles and sporeling biomass, respectively.