Anatomy of the dielectric behavior of methyl-m-toluate glasses during and after vapor deposition.

Glassy films of methyl-m-toluate have been vapor deposited onto a substrate equipped with interdigitated electrodes, facilitating in situ dielectric relaxation measurements during and after deposition. Samples of 200 nm thickness have been deposited at rates of 0.1 nm/s at a variety of deposition temperatures between 40 K and Tg = 170 K. With increasing depth below the surface, the dielectric loss changes gradually from a value reflecting a mobile surface layer to that of the kinetically stable glass. The thickness of this more mobile layer varies from below 1 to beyond 10 nm as the deposition temperature is increased, and its average fictive temperature is near Tg for all deposition temperatures. Judged by the dielectric loss, the liquid-like portion of the surface layer exceeds a thickness of 1 nm only for deposition temperatures above 0.8Tg, where near-equilibrium glassy states are obtained. After deposition, the dielectric loss of the material positioned about 5-30 nm below the surface decreases for thousands of seconds of annealing time, whereas the bulk of the film remains unchanged.

Unusual Transformation of Mixed Isomer Decahydroisoquinoline Stable Glasses.

Dielectric relaxation was used to characterize the ability of vapor-deposited mixtures of cis- and trans-decahydroisoquinoline (DHIQ) to form glasses with a high kinetic stability. Vapor-deposited mixtures are technologically relevant, and the effect of mixing on glass stability is a relatively unexplored area. Mixed isomers and pure trans-DHIQ form highly stable glasses that isothermally transform in approximately 104 τα (where τα is the structural relaxation time of the supercooled liquid). Isomeric composition of the glasses does not play a significant role in the maximum kinetic stability of the resulting films. Secondary relaxations in DHIQ are associated with an intramolecular conformational change and are suppressed to a significant extent in highly stable glasses. During isothermal annealing experiments, stable glasses were found to transform initially via a growth front mechanism that transitions to a homogeneous bulk mechanism. Surprisingly, the time dependence of the bulk transformation is different from that reported for other stable glasses and cannot be interpreted in terms of a simple nucleation and growth model.

A liquid with distinct metastable structures: Supercooled butyronitrile.

The dielectric relaxation behavior of the molecular glass former butyronitrile is revisited by measuring both bulk samples cooled from the melt and samples obtained by physical vapor deposition. We find that the dielectric constant in the viscous regime of the bulk liquid is much higher than reported previously, reaching εs = 63 at T = 103 K, i.e., just above the glass transition temperature Tg = 97 K. By contrast, varying the deposition temperature and rate of vapor-deposited samples leads to dielectric constants in a range between 4.5 and 63 at T = 103 K. Values much below εs = 63 persist for thousands of seconds, where the dielectric relaxation time is about 0.1 s. The observations can be interpreted by the formation of clusters in which pair-wise anti-parallel dipole orientation is the preferred state at temperatures well below the glass transition. These non-crystalline clusters are long-lived even above Tg, where the remaining volume fraction is in the state of the equilibrium polar liquid.

In situ observation of fast surface dynamics during the vapor-deposition of a stable organic glass.

By measuring the increments of dielectric capacitance (ΔC) and dissipation (Δtan δ) during physical vapor deposition of a 110 nm film of a molecular glass former, we provide direct evidence of the mobile surface layer that is made responsible for the extraordinary properties of vapor deposited glasses. Depositing at a rate of 0.1 nm s-1 onto a substrate at Tdep = 75 K = 0.82Tg, we observe a 2.5 nm thick surface layer with an average relaxation time of 0.1 s, while the glass growing underneath has a high kinetic stability. The level of Δtan δ continues to decrease for thousands of seconds after terminating the deposition process, indicating a slow aging-like increase in packing density near the surface. At very low deposition temperatures, 32 and 42 K, the surface layer thicknesses and mobilities are reduced, as are the kinetic stabilities.

Physical vapor deposition of a polyamorphic system: Triphenyl phosphite.

In situ AC nanocalorimetry and dielectric spectroscopy were used to analyze films of vapor-deposited triphenyl phosphite. The goal of this work was to investigate the properties of vapor-deposited glasses of this known polyamorphic system and to determine which liquid is formed when the glass is heated. We find that triphenyl phosphite forms a kinetically stable glass when prepared at substrate temperatures of 0.75-0.95Tg, where Tg is the glass transition temperature. Regardless of the substrate temperature utilized during deposition of triphenyl phosphite, heating a vapor-deposited glass always forms the ordinary supercooled liquid (liquid 1). The identity of liquid 1 was confirmed by both the calorimetric signal and the shape and position of the dielectric spectra. For the purposes of comparison, the glacial phase of triphenyl phosphite (liquid 2) was prepared by the conventional method of annealing liquid 1. We speculate that these new results and previous work on vapor deposition of other polyamorphic systems can be explained by the free surface structure being similar to one polyamorph even in a temperature regime where the other polyamorph is more thermodynamically stable in the bulk.

Dielectric properties of vapor-deposited propylbenzenes.

Dielectric susceptibility data of vapor-deposited films of iso-propylbenzene (IPB) and n-propylbenzene (NPB) have been recorded across a wide range of deposition temperatures, Tdep, mostly below the glass transition temperature, Tg. The results for the real and imaginary components of dielectric susceptibility are compared with recently published results for 2-methyltetrahydrofuran (MTHF). Common to all three systems are the following: (i) increased kinetic stability seen as higher onset temperature for the transformation to the liquid state for Tdep ≈ 0.85Tg; (ii) the reduction of the dielectric loss (χ″) for as-deposited glasses, a signature of increased packing density that is maximal for Tdep ≈ 0.85Tg; and (iii) a reduced level of the storage component (χ') for as-deposited glasses, an effect that is almost deposition temperature invariant for Tdep < Tg. Material specific behavior is observed when heating the as-deposited films to 1.2Tg: IPB and NPB transform directly into the ordinary liquid state if judged on the basis of dielectric susceptibility, whereas MTHF has been reported to enter an unusual liquid state prior to a liquid-liquid transition at higher temperatures. These results are discussed in the context of the curious scattering results reported by Ishii et al. for some benzene derivatives, which hint at a liquid-liquid transformation.

Relationship between aged and vapor-deposited organic glasses: Secondary relaxations in methyl-m-toluate.

In situ interdigitated electrode broadband dielectric spectroscopy was used to characterize the excess wing relaxations in vapor-deposited and aged glasses of methyl-m-toluate (MMT, Tg = 170 K). MMT displays typical excess wing relaxations in dielectric spectra of its supercooled liquid and glasses. Physical vapor deposition produced glasses with degrees of suppression of the excess wing relaxation that varied systematically with deposition conditions, up to a maximum suppression of more than a factor of 3. The glass deposited at a relatively high temperature, 0.96 Tg (163 K), showed the same amount of suppression as that of a liquid-cooled glass aged to equilibrium at this temperature. The suppression of the excess wing relaxation was strongly correlated with the kinetic stability of the vapor-deposited glasses. Comparisons with aged MMT glasses allowed an estimate of the structural relaxation time of the vapor-deposited glasses. The dependence of the estimated structural relaxation times upon the substrate temperature was found to be stronger than Arrhenius but weaker than Vogel-Fulcher-Tammann dependence predicted from extrapolation of relaxation times in the supercooled liquid. Additionally, this work provides the first example of the separation of primary and secondary relaxations using physical vapor deposition.

Ultrastable and polyamorphic states of vapor-deposited 2-methyltetrahydrofuran.

This work reports results gained from dielectric spectroscopy on the organic molecular glass-former 2-methyltetrahydrofuran (MTHF), which was deposited onto an interdigitated electrode device by physical vapor deposition. By a suitable selection of preparation parameters (deposition temperature, deposition rate, and annealing conditions), various states of MTHF could be created: ultrastable glass, a liquid state with unusual dielectric properties, or the ordinary liquid state as obtained by supercooling. Observations on kinetic stability as well as on the suppression of dielectric loss in the ultrastable state resemble previous findings for other molecular glass-formers. Remarkably, after annealing just above Tg, all vapor-deposited films of MTHF display a static dielectric constant in the liquid state (εs) that is up to a factor of two below that of the ordinary bulk liquid. A structural transition to the ordinary liquid-cooled state of MTHF occurs at temperatures far above its conventional Tg, indicative of polyamorphism: the formation of an unusual structure that is achieved by physical vapor deposition and that differs from the ordinary liquid state obtained by supercooling. The present results also reveal that the dielectric constant of the as deposited glass (ε∞) is reduced to practically the value of the squared refractive index, n2.

Modifying hydrogen-bonded structures by physical vapor deposition: 4-methyl-3-heptanol.

We prepared films of 4-methyl-3-heptanol by vapor depositing onto substrates held at temperatures between Tdep = 0.6Tg and Tg, where Tg is the glass transition temperature. Using deposition rates between 0.9 and 6.0 nm/s, we prepared films about 5 µm thick and measured the dielectric properties via an interdigitated electrode cell onto which films were deposited. Samples prepared at Tdep = Tg display the dielectric behavior of the ordinary supercooled liquid. Films deposited at lower deposition temperatures show a high dielectric loss upon heating toward Tg, which decreases by a factor of about 12 by annealing at Tg = 162 K. This change is consistent with either a drop of the Kirkwood correlation factor, gk, by a factor of about 10, or an increase in the dielectric relaxation times, both being indicative of changes toward ring-like hydrogen-bonded structure characteristic of the ordinary liquid. We rationalize the high dielectric relaxation amplitude in the vapor deposited glass by suggesting that depositions at low temperature provide insufficient time for molecules to form ring-like supramolecular structures for which dipole moments cancel. Surprisingly, above Tg of the ordinary liquid, these vapor deposited films fail to completely recover the dielectric properties of the liquid obtained by supercooling. Instead, the dielectric relaxation remains slower and its amplitude much higher than that of the equilibrium liquid state, indicative of a structure that differs from the equilibrium liquid up to at least Tg + 40 K.

Ultrasonic processing of SbSI nanowires for their application to gas sensors.

Ultrasonic processing has been applied to create durable electrical contacts between antimony sulfoiodide (SbSI) nanowires and Au microelectrodes on glass substrate. After DC electric field alignment of SbSI nanowires between the microelectrodes, the sample was irradiated with ultrasound using chromium copper alloy sonotrode ended with silicon carbide (SiC) single crystal. The SEM and AFM investigations have showed that the ends of SbSI nanowires have been well compacted and bonded with microelectrodes. Ultrasonic processing has caused 420% increase of DC electric conductance of the junctions between Au microelectrodes and SbSI nanowires. The fabricated structures of SbSI nanowires bonded to Au microelectrodes are useful e.g. as nitrous oxide (N2O) gas sensors. These low power devices can operate at room temperature and do not require heating system for recovery.

Suppression of ß Relaxation in Vapor-Deposited Ultrastable Glasses.

Glassy materials display numerous important properties which relate to the presence and intensity of the secondary (ß) relaxations that dominate the dynamics below the glass transition temperature. However, experimental protocols such as annealing allow little control over the ß relaxation for most glasses. Here we report on the ß relaxation of toluene in highly stable glasses prepared by physical vapor deposition. At conditions that generate the highest kinetic stability, about 70% of the ß relaxation intensity is suppressed, indicating the proximity of this state to the long-sought "ideal glass." While preparing such a state via deposition takes less than an hour, it would require ~3500 years of annealing an ordinary glass to obtain similarly suppressed dynamics.

Peptide conjugated chitosan foam as a novel approach for capture-purification and rapid detection of hapten--example of ochratoxin A.

A novel bioassay for the detection and monitoring of Ochratoxin A (OTA), a natural carcinogenic mycotoxin produced by Aspergillus and Penicillium fungi, has been developed and applied for the screening of red wine. Here we report the immobilization and orientation of NOF4, a synthetic peptide, onto 3-D porous chitosan supports using a N-terminal histidine tag to allow binding to M(++) ions that were previously adsorbed onto the high surface area biopolymer. Three divalent cations (M(++)=Zn(++), Co(++), Ni(++)) were evaluated and were found to adsorb via a Langmuir model and to have binding capacities in the order Zn(++)>Co(++)>Ni(++). Following Zn(++) saturation and washing, C-terminus vs. the N-terminus His-tagged NOF4 was evaluated. At 1000 µg L(-1) OTA the N-terminus immobilization was more efficient (2.5 times) in the capture of OTA. HRP labeled OTA was added to the antigen solutions (standards or samples) and together competitively incubated on biospecific chitosan foam. The chemiluminescence substrate luminol was then added and after 5 min of enzymatic reaction, light emission signals (λmax=425 nm) were analyzed. Calibration curves of %B/B0 vs. OTA concentration in PBS showed that half-inhibition occurred at 1.17 µg L(-1), allowing a range of discrimination of 0.25 and 25 µg L(-1). In red wine, the minimum concentration of OTA that the system can detect was 0.5 µg L(-1) and could detect up to 5 µg L(-1). Assay validation was performed against immunoaffinity column (IAC) tandem reversed-phase high pressure liquid chromatography with fluorescence detection (HPLC-FLD) and provided quite good agreement. The association of chitosan foam and specific peptide represents a new approach with potential for both purification-concentration and detection of small molecules. In the future this assay will be implemented in a solid-sate bioelectronic format.

Role of fragility in the formation of highly stable organic glasses.

In situ dielectric spectroscopy has been used to characterize vapor-deposited glasses of methyl-m-toluate (MMT), an organic glass former with low fragility (m = 60). Deposition near 0.84T(g) produces glasses of very high kinetic stability; these materials are comparable in stability to the most stable glasses produced from more fragile glass formers. Highly stable glasses of MMT, when annealed above T(g), transform into the supercooled liquid by a heterogeneous mechanism. A constant velocity propagating front is initiated at the free surface and controls the transformation of thin films. The transition to a bulk-dominated transformation process occurs at 5 µm, the largest length scale reported for any glass. Contrary to recent conclusions, we find that physical vapor deposition can form highly stable organic glasses across the entire range of liquid fragilities.

Protein immobilization on 3C-SiC (100) as a substrate for detecting the onset of acute myocardial infarction (AMI).

Silicon Carbide (SiC), has been shown to be a bio- and hema-compatible substrate that could potentially be used in biosensor applications. The development of a viable biorecognition interface using SiC as the substrate material for bio-detection is described. Surface modification with 3-aminopropyltriethoxysilane (APTES) and immobilization via covalent conjugation of antimyoglobin (anti-Myo) on the modified surfaces is achieved, which are initial steps for immunosensing based devices. Successful formation of APTES layers and antibody immobilization were identified with surface water contact angle (SWCA), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).

Potentiometric monitoring DNA hybridization.

The usual procedure to monitor the ion exchange of small ions utilizes a potentiometer with a selective membrane as part of the working electrode. As the next step, we have applied polyaniline electrodes to the monitoring the activity macromolecular ions during DNA hybridization. Single-strand oligonucleotide (ssODN) probes were immobilized using a nucleophilic substitution reaction of the thiolated ssODN molecules with polyaniline. The anionic phosphate groups of the probe molecules also interacted with the cationic-doped polyaniline surface. Three useful findings were observed with the potentiometric experiments. First, the binding of the complimentary target molecules with the immobilized probes revealed a substantial potential change. Further, potential change was observed neither with the non-complimentary targets nor with the samples with a mutation in the sequence. The last two experiments were important for the future evaluation of the impact of medium and potential interfering compounds: anionic groups and hydrogen bonding groups in the non-complimentary samples did not cause any interactions.

A disattenuated correlation estimate when variables are measured with error: illustration estimating cross-platform correlations.

Previous cross-platform reproducibility studies have compared consistency of intensities as well as consistency of fold changes across different platforms using Pearson's correlation coefficient. In this study, we propose the use of measurement error models for estimating gene-specific correlations. Additionally, gene-specific reliability estimates are shown to be useful in prioritizing clones for sequence verification rather than selecting clones using a simple random sample. The proposed 'disattenuated' correlation may prove useful in a wide variety of studies when both X and Y are measured with error, such as in confirmation studies of microarray gene expression values, wherein more reliable laboratory assays such as real-time polymerase chain reaction are used.

Enzyme microgels in packed-bed bioreactors with downstream amperometric detection using microfabricated interdigitated microsensor electrode arrays.

In this article, we describe the use of pH- responsive hydrogels as matrices for the immobilization of two enzymes, glucose oxidase (GOx) and glutamate oxidase (GlutOx). Spherical hydrogel beads were prepared by inverse suspension polymerization and the enzymes were immobilized by either physical entrapment or covalent immobilization within or on the hydrogel surface. Packed-bed bioreactors were prepared containing the bioactive hydrogels and these incorporated into flow injection (FI) systems for the quantitation of glucose and monosodium glutamate (MSG) respectively. The FI amperometric detector comprised a microfabricated interdigitated array within a thin-layer flow cell. For the FI manifold incorporating immobilized GOx, glucose response curves were found to be linear over the concentration range 1.8-280 mg dL(-1) (0.1-15.5 mM) with a detection limit of 1.4 mg dL(-1) (0.08 mM). Up to 20 samples can be manually analyzed per hour, with the hydrogel-GOx bioreactor exhibiting good within-day (0.19%) precision. The optimized FI manifold for MSG quantitation yielded a linear response range of up to 135 mg dL(-1) (8 mM) with a detection limit of 3.38 mg dL(-1) (0.2 mM) and a throughput of 30 samples h(-1). Analysis of commercially produced soup samples gave a within-day precision of 3.6%. Bioreactors containing these two physically entrapped enzymes retained > 60% of their initial activities after a storage period of up to 1 year.

New developments in microarray technology.

Microarrays have emerged as indispensable research tools for gene expression profiling and mutation analysis. New classification of cancer subtypes, dissecting the yeast metabolism and large-scale genotyping of human single nucleotide polymorphisms are important results being obtained with this technique. Realizing the microsphere-based massively parallel signature sequencing technique as fluid microarrays, building new types of protein arrays and constructing miniaturized flow-through systems, which can potentially take this technology from the research bench into industrial, clinical and other routine applications, exemplify the intense developments that are now ongoing in this field.