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.

Interplay Between Silicon Nanocrystal Size and Local Environment Within Porous Silicon on the Analyte-Dependent Photoluminescence Response.

Porous silicon (pSi) exhibits strong photoluminescence (PL) and its PL is often exploited for chemical sensor development. However, the sensor response is not uniform across a pSi specimen. We use co-localized confocal PL and Raman scattering mapping to establish a relationship between the analyte-induced PL response and the silicon nanocrystallite size, size distribution, and amorphous silicon (aSi) contribution across a pSi specimen. Using toluene as a model analyte, high analyte-induced PL response is associated with areas within the specimen that have (i) low aSi content, (ii) silicon nanocrystallites having diameters between 2 and 5 nm, and (iii) silicon nanocrystallites that exhibit a narrow size distributions (≤1% relative standard deviation).

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.

Gallium indium eutectic masking prior to porous silicon formation creates unique spatially-dependent chemistries.

We demonstrate that gallium indium (GaIn) eutectic can be used to create interesting crystalline Si/porous silicon (cSi/pSi) platforms that exhibit unique analyte- and spatially-dependent photoluminescence (PL) responses. Here we characterize these cSi/pSi regions by using profilometry, scanning electron microscopy (SEM), wide-field PL microscopy, and Fourier transform infrared (FTIR) microscopy. As we move along a vector from the cSi/pSi interface out into "bulk" pSi, the: (i) analyte-dependent, PL-based response initially increases and then decreases; (ii) total PL emission intensity, in the absence of analyte, increases; (iii) pSi thickness increases; and (iv) relative O2Si-H to Si-H band amplitude ratio decreases. Thus, the analyte-dependent PL response magnitude is correlated to the extent of pSi oxidation; which can be easily controlled by using GaIn eutectic as a mask during the pSi fabrication process.

Exploiting the 3-Aminopropyltriethoxysilane (APTES) autocatalytic nature to create bioconjugated microarrays on hydrogen-passivated porous silicon.

Porous silicon (pSi) based microarrays are attractive because pSi: (i) can be modified in many ways, (ii) possesses a high surface area, and (iii) exhibits strong photoluminescence (PL). These characteristics make pSi-based microarrays candidates for a host of applications including sensing, optoelectronic devices, and photodetectors. Microarray fabrication requires a high-throughput approach to produce chemically modified, spatially isolated spots on a particular substrate. The most stable platforms are characterized by covalent attachment to the substrate. In this paper we exploit the autocatalytic nature of 3-aminopropyltriethoxysilane (APTES) to contact pin-print APTES directly onto as prepared, H-passivated pSi (ap-pSi) without the need for a formal oxidation step. We assess the APTES-derived spots by using PL and Fourier transform infrared spectroscopy (FT-IR) imaging and determine the spot size and spatial homogeneity. All APTES-derived spots exhibited two distinct regions; a silanized core surrounded by an oxidized halo. By decreasing the APTES concentration and increasing the acid concentration, the oxidized halo size decreased by 60%; however, the silanized core diameter remains APTES and acid concentration independent. Bioconjugation can be achieved to all APTES-derived features; however, the highest biomolecule loading was realized by using pure APTES. Together these experiments demonstrate an easy and simple strategy for creating protein microarrays on pSi.

Origin of Analyte-Induced Porous Silicon Photoluminescence Quenching.

We report on gaseous analyte-induced photoluminescence (PL) quenching of porous silicon, as-prepared (ap-pSi) and oxidized (ox-pSi). By using steady-state and emission wavelength-dependent time-resolved intensity luminescence measurements in concert with a global analysis scheme, we find that the analyte-induced quenching is best described by a three-component static quenching model. In the model, there are blue, green, and red emitters (associated with the nanocrystallite core and surface trap states) that each exhibit unique analyte-emitter association constants and these association constants are a consequence of differences in the pSi surface chemistries.

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.

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.

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.

Instrumentation for Reliably Determining Porous Silicon Photoluminescence Responses to Gaseous Analyte Vapors.

We report new instrumentation for rapidly and reliably measuring the temperature-dependent photoluminescence response from porous silicon as a function of analyte vapor concentration. The new system maintains the porous silicon under inert conditions and it allows on-the-fly steady-state and time-resolved photoluminescence intensity and hyper-spectral measurements between 293 K and 450 K. The new system yields reliable data at least 100-fold faster in comparison to previous instrument platforms.

Contact Pin-Printing onto Porous Silicon for Creating Microarrays with High Chemical Diversity.

We explore the size and spatial microheterogeneity of contact pin-printed spots formed on porous silicon (pSi). Glycerol was contact printed at room temperature onto as-prepared, hydrogen-passivated pSi (ap-pSi) using 50 or 200 µm diameter solid pins. The pSi was then subjected to a strong oxidizing environment (gaseous O3) and washed to remove the glycerol masks. The glycerol-free regions were converted to oxidized pSi (ox-pSi); the glycerol-coated regions were protected from O3, but not entirely. The final array is described as circularly shaped "ap-pSi" regions on a field of ox-pSi. When comparing the areas outside and inside the glycerol-masked pSi spots, one finds dramatic differences in the Si-O-Si, SiHx (x = 1-3) and OySiHx (y, x = 1-3) levels with a spatially dependent continuum of compositions across the spot diameter. Experimental conditions could be adjusted to tune the final ap-pSi spot diameter and edge widths from 90 µm to 520 µm and 20 µm to 130 µm, respectively. The resulting ap-pSi spot diameter is explained by using molecular kinetic theory and time-dependent glycerol imbibement into the pSi within a one-dimensional Darcy's law model.

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.

Spectroscopic characteristics of carbon dots (C-dots) derived from carbon fibers and conversion to sulfur-bridged C-dots nanosheets.

We synthesized sub-10 nm carbon nanoparticles (CNPs) consistent with photoluminescent carbon dots (C-dots) from carbon fiber starting material. The production of different C-dots fractions was monitored over seven days. During the course of the reaction, one fraction of C-dots species with relatively high photoluminescence was short-lived, emerging during the first hour of reaction but disappearing after one day of reaction. Isolation of this species during the first hour of the reaction was crucial to obtaining higher-luminescent C-dots species. When the reaction proceeded for one week, the appearance of larger nanostructures was observed over time, with lateral dimensions approaching 200 nm. The experimental evidence suggests that these larger species are formed from small C-dot nanoparticles bridged together by sulfur-based moieties between the C-dot edge groups, as if the C-dots polymerized by cross-linking the edge groups through sulfur bridges. Their size can be tailored by controlling the reaction time. Our results highlight the variety of CNP products, from sub-10 nm C-dots to ~200 nm sulfur-containing carbon nanostructures, that can be produced over time during the oxidation reaction of the graphenic starting material. Our work provides a clear understanding of when to stop the oxidation reaction during the top-down production of C-dots to obtain highly photoluminescent species or a target average particle size.

Effects of Polyhexamethylene Biguanide and Polyquaternium-1 on Phospholipid Bilayer Structure and Dynamics.

Multipurpose solutions (MPS) are a single solution that functions to simultaneously rinse, disinfect, clean, and store soft contact lenses. Several commercial MPS products contain polyhexamethylene biguanide (PHMB) and/or polyquaternium-1 (PQ-1) as antimicrobial agents. In this paper we have created an in vitro small unilamellar vesicle (SUV) model of the corneal epithelial surface, and we have assessed the interactions of PHMB and PQ-1 with several model biomembranes by using fluorescence spectroscopy, dynamic light scattering (DLS), and liquid chromatography-mass spectrometry (LC-MS). Steady-state and time-resolved fluorescence were used to assess the membrane acyl chain and polar headgroup region local microenvironment as a function of added PHMB or PQ-1. DLS was used to detect and quantify SUV aggregation induced by PHMB and PQ-1. LC-MS was used to determine the liposomal composition from any precipitated materials in comparison to the as-prepared SUVs. The results are consistent with PHMB adsorbing onto and PQ-1 intercalating into the biomembrane structure. The differences between the two interaction mechanisms have substantial impacts on the biomembrane dynamics and stability.

Ratiometric, filter-free optical sensor based on a complementary metal oxide semiconductor buried double junction photodiode.

We report a complementary metal oxide semiconductor integrated circuit (CMOS IC) with a buried double junction (BDJ) photodiode that (i) provides a real-time output signal that is related to the intensity ratio at two emission wavelengths and (ii) simultaneously eliminates the need for an optical filter to block Rayleigh scatter. We demonstrate the BDJ platform performance for gaseous NH3 and aqueous pH detection. We also compare the BDJ performance to parallel results obtained by using a slew scanned fluorimeter (SSF). The BDJ results are functionally equivalent to the SSF results without the need for any wavelength filtering or monochromators and the BDJ platform is not prone to errors associated with source intensity fluctuations or sensor signal drift.

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).

Rapid, nondestructive denim fiber bundle characterization using luminescence hyperspectral image analysis.

An investigation into the performance of luminescence-based hyperspectral imaging (LHSI) for denim fiber bundle discrimination has been conducted. We also explore the potential of nitromethane (CH3NO2) -based quenching to improve discrimination, and we determine the quenching mechanism. The luminescence spectra (450-850 nm) and images from the denim fiber bundles were obtained with excitation at 325 or 405 nm. LHSI data were recorded in less than 5 s and subsequently assessed by principal component analysis or rendered as red, green, blue (RGB) component histograms. The results show that LHSI data can be used to rapidly and uniquely discriminate between all the fiber bundle types studied in this research. These non-destructive techniques eliminate extensive sample preparation and allow for rapid hyperspectral image collection, analysis, and assessment. The quenching data also revealed that the dye molecules within the individual fiber bundles exhibit dramatically different accessibilities to CH3NO2.

Extremely strong tubular stacking of aromatic oligoamide macrocycles.

As the third-generation rigid macrocycles evolved from progenitor 1, cyclic aromatic oligoamides 3, with a backbone of reduced constraint, exhibit extremely strong stacking with an astoundingly high affinity (estimated lower limit of Kdimer > 1013 M-1 in CHCl3), which leads to dispersed tubular stacks that undergo further assembly in solution. Computational study reveals a very large binding energy (-49.77 kcal mol-1) and indicates highly cooperative local dipole interactions that account for the observed strength and directionality for the stacking of 3. In the solid-state, X-ray diffraction (XRD) confirms that the aggregation of 3 results in well-aligned tubular stacks. The persistent tubular assemblies of 3, with their non-deformable sub-nm pore, are expected to possess many interesting functions. One such function, transmembrane ion transport, is observed for 3.

Enhanced performance from a hybrid quenchometric deoxyribonucleic acid (DNA) silica xerogel gaseous oxygen sensing platform.

A complex of salmon milt deoxyribonucleic acid (DNA) and the cationic surfactant cetyltrimethylammonium (CTMA) forms an organic-soluble biomaterial that can be readily incorporated within an organically modified silane-based xerogel. The photoluminescence (PL) intensity and excited-state luminescence lifetime of tris(4,7'-diphenyl-1,10'-phenanathroline) ruthenium(II) [(Ru(dpp)3](2+), a common O2 responsive luminophore, increases in the presence of DNA-CTMA within the xerogel. The increase in the [Ru(dpp)3](2+)excited-state lifetime in the presence of DNA-CTMA arises from DNA intercalation that attenuates one or more non-radiative processes, leading to an increase in the [Ru(dpp)3](2+) excited-state lifetime. Prospects for the use of these materials in an oxygen sensor are demonstrated.