Quantitative carbon detector (QCD) for calibration-free, high-resolution characterization of complex mixtures.

Current research of complex chemical systems, including biomass pyrolysis, petroleum refining, and wastewater remediation requires analysis of large analyte mixtures (>100 compounds). Quantification of each carbon-containing analyte by existing methods (flame ionization detection) requires extensive identification and calibration. In this work, we describe an integrated microreactor system called the Quantitative Carbon Detector (QCD) for use with current gas chromatography techniques for calibration-free quantitation of analyte mixtures. Combined heating, catalytic combustion, methanation and gas co-reactant mixing within a single modular reactor fully converts all analytes to methane (>99.9%) within a thermodynamic operable regime. Residence time distribution of the QCD reveals negligible loss in chromatographic resolution consistent with fine separation of complex mixtures including cellulose pyrolysis products.

A portable time-domain LED fluorimeter for nanosecond fluorescence lifetime measurements.

Fluorescence lifetime measurements are becoming increasingly important in chemical and biological research. Time-domain lifetime measurements offer fluorescence multiplexing and improved handling of interferers compared with the frequency-domain technique. In this paper, an all solid-state, filterless, and highly portable light-emitting-diode based time-domain fluorimeter (LED TDF) is reported for the measurement of nanosecond fluorescence lifetimes. LED based excitation provides more wavelengths options compared to laser diode based excitation, but the excitation is less effective due to the uncollimated beam, less optical power, and longer latency in state transition. Pulse triggering and pre-bias techniques were implemented in our LED TDF to improve the peak optical power to over 100 mW. The proposed pulsing circuit achieved an excitation light fall time of less than 2 ns. Electrical resetting technique realized a time-gated photo-detector to remove the interference of the excitation light with fluorescence. These techniques allow the LED fluorimeter to accurately measure the fluorescence lifetime of fluorescein down to concentration of 0.5 µM. In addition, all filters required in traditional instruments are eliminated for the non-attenuated excitation/emission light power. These achievements make the reported device attractive to biochemical laboratories seeking for highly portable lifetime detection devices for developing sensors based on fluorescence lifetime changes. The device was initially validated by measuring the lifetimes of three commercial fluorophores and comparing them with reported lifetime data. It was subsequently used to characterize a ZnSe quantum dot based DNA sensor.

Homogeneous immunoassays based on fluorescence emission intensity variations of zinc selenide quantum dot sensors.

The fluorescence emission intensity of ZnSe quantum dots (QDs) conjugated to proteins to form QD-based biomolecular sensors increases significantly upon binding of the sensors to target proteins in solution. This phenomenon enables the development of homogeneous, separation-free immunoassays for rapid quantitative detection of proteins in solution. Proof-of-principle assays were developed by dosing a solution containing a biomolecular target with a solution containing the corresponding QD-based sensor and monitoring the changes in the peak fluorescence emission intensity of the QDs. Direct immunoassays for detecting basic fibroblast growth factor (bFGF) and prostate-specific antigen (PSA) in solution were demonstrated using QD-anti-bFGF and QD-anti-PSA sensors. A competitive immunoassay for detecting human serum albumin (HSA) was also demonstrated by dosing samples containing HSA with QD-HSA sensors and free anti-HSA antibodies. The QD-HSA sensors were tested in 1000× diluted human serum and found to be unaffected by interference from other proteins. The lower limit of detection of the assays was equal to the lowest sensor concentration in the solution that can be unambiguously detected, typically less than 1 nM. The dynamic range of the assays was determined by identifying the sensor concentration above which optical interference between QDs affected adversely the observed fluorescence emission intensity. The upper limit of this concentration was 2.5 µM for 4 nm QDs. The ZnSe QD-based sensors were stable and preserved ~80% of their initial peak emission intensity after two months in refrigerated storage. These biosensors have potential applications in rapid sensing of target proteins for emergency and point-of-care diagnostic applications.

Lattice Monte Carlo simulation of semiconductor nanocrystal synthesis in microemulsion droplets.

A lattice Monte Carlo model has been developed to describe the formation of a single semiconductor nanocrystal (quantum dot) inside a droplet of a microemulsion. The motivation stems from the need to understand the kinetics of quantum dot formation in microemulsion templates with minimal droplet-droplet coalescence. In these systems, a fixed amount of a reactant is dissolved in each droplet, and another reactant is supplied by diffusion through the interface. Nucleation is facilitated by a spontaneous reaction between the precursors at the droplet interface, and the coalescence of nuclei and clusters ultimately leads to the formation of a single particle. The size of the final particle is controlled by the concentration of the first reactant. A hard-sphere potential is used to describe cluster-cluster interactions. The overall particle formation time initially increases with final particle size, quickly passes through a maximum, and subsequently decreases due to the formation of large intermediate clusters apparently acting as effective collision partners to smaller ones. Studies of the evolution of intermediate cluster sizes provided mechanistic details of the final particle formation through cluster-cluster coalescence. A generalized dimensionless equation is obtained that relates the formation time of the final particle to its size for various droplet sizes and diffusivities of the first reactant and clusters. A parametric study reveals that the final particle formation time is more sensitive to changes in the cluster-cluster coalescence probability than in the probability of nucleation.

Effects of ligand coordination number and surface curvature on the stability of gold nanoparticles in aqueous solutions.

The colloidal stability of gold nanoparticles (AuNPs) cap-exchanged with either monothiol- or dithiolane-terminated PEG-OCH(3) ligands was investigated. Three distinct aspects were explored: (1) effects of excess salt concentration; (2) ligation competition by dithiothreitol (DTT); and (3) resistance to sodium cyanide digestion. We found that overall ligands presenting higher coordination numbers (dithiolane) exhibit much better stability to excess added salt and against competition from DTT compared to their monodentate counterparts. Resistance to NaCN digestion indicated that there is a balance between coordination number and density of ligand packing on the NP surface. For smaller NPs, where a larger surface curvature reduces the ligand packing density, a higher coordination number is clearly beneficial. In comparison, a higher ligand density allowed by the smaller curvature for larger nanocrystals makes monothiol-PEG-capped NPs more resistant to cyanide digestion. The present study indicates that balance between the coordination number and surface packing density is crucial to enhancing the colloidal stability of AuNPs.

Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films.

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of adsorbed SiH(3) radical precursor dissociation on surfaces of MD-grown a-Si:H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55 eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20 eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si:H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50 eV. The results are consistent with experimental measurements of a-Si:H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si:H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH((s)), becomes increasingly more dominant as the temperature is increased.

Templated synthesis of ZnSe nanostructures using lyotropic liquid crystals.

We report a technique for controlled synthesis of zero-, one-, and two-dimensional compound semiconductor nanostructures by using cubic, hexagonal, and lamellar lyotropic liquid crystals as templates, respectively. The liquid crystals were formed by self-assembly in a ternary system consisting of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer as the surfactant, heptane as the non-polar dispersed phase, and formamide as the polar continuous phase. ZnSe quantum dots and nanowires with diameters smaller than 10 nm, as well as free-standing, disc-like quantum wells, were grown inside the spherical, cylindrical, and planar nanodomains, respectively, by reacting diethylzinc that was dissolved in the heptane domains with hydrogen selenide gas that was brought into contact with the liquid crystal in a sealed chamber at room temperature and atmospheric pressure. The shape and size of the resulting nanostructures can be manipulated by selecting the templating phase of the liquid crystal, the size of the dispersed nanodomains that is controlled by the composition of the template, and the concentration of diethylzinc in them.

Synthesis and size control of luminescent ZnSe nanocrystals by a microemulsion-gas contacting technique.

A scalable method for controlled synthesis of luminescent compound semiconductor nanocrystals (quantum dots) using microemulsion-gas contacting at room temperature is reported. The technique exploits the dispersed phase of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were synthesized by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The ZnSe nanocrystals exhibit size-dependent luminescence and excellent photostability.

Transport and kinetic processes underlying biomolecular interactions in the BIACORE optical biosensor.

The transport and kinetic processes describing biomolecular interactions in the BIACORE optical biosensor have been studied with the help of a mathematical model. In comparison to previous models, the model presented here couples, for the first time, transport phenomena in the flow channel with hindered diffusive transport and reactions inside the hydrogel. Simulated experiments based on this model, and two simpler models extant in the literature, are used to identify cases under which the detailed model is essential for accurate prediction of kinetic parameters. It is shown that this model can substantially improve the accuracy of kinetic parameter estimation when transport limitations in the flow channel and/or the hydrogel significantly influence the observed instrument response curves. The model can extend the range of the instrument's applicability to higher concentrations of immobilized species within the hydrogel. It can also be used for accurate design of experiments with the purpose of minimizing errors in the estimation of the kinetic parameters.