Laser-enhanced ionization of mercury atoms in an inert atmosphere with avalanche amplification of the signal

A new method for laser-enhanced ionization detection of mercury atoms in an inert gas atmosphere is described. The method, which is based on the avalanche amplification of the signal resulting from the ionization from a selected Rydberg level reached by a three-step laser excitation of mercury vapor in a simple quartz cell, can be applied to the determination of this element in various matrices by the use of conventional cold atomization techniques. The overall (collisional + photo) ionization efficiency is investigated at different temperatures, and the avalanche amplification effect is reported for Ar and P-10 gases at atmospheric pressure. It is shown that the amplified signal is related to the number of charges produced in the laser-irradiated volume. Under amplifier noise-limited conditions, a detection limit of ～15 Hg atoms/laser pulse in the interaction region is estimated.     

Laser-enhanced ionization detection of Pb in seawater by flow injection analysis with on-line preconcentration and separation

The flame laser-enhanced ionization (LEI) technique is coupled with the flow injection analysis system to measure the trace lead amounts in aqueous solution and in seawater. The flow injection (FI) manifold is incorporated with a microcolumn packed with a C18 bonded silica. The chelating agent DDPA is used to form the Pb-DDPA complex, which may be sorbed in the microcolumn and then eluted with methanol. The preconcentrated Pb is then detected by the LEI technique with either single-step or two-step excitation. At 5- and 15-mL volume-fixed sample loading, the detection limits of 0.011 and 0.0033 ng/mL (11 and 3.3 ppt) and enrichment factors of 16 and 48 are achieved, respectively, using a two-step FI-LEI. The sensitivity of the current system proves to be better by at least 1 order of magnitude than that of conventional LEI method. The FI-LEI also increases the tolerance of matrix interference. The LEI signal is slightly reduced to 80% intensity as 10,000 micrograms/mL (ppm) Na and K matrixes are mixed in the lead solution. The resistance to the alkali matrixes is enhanced approximately 4 times that reported previously using a similar water-immersed probe as a LEI collector. Finally, the FI-LEI is for the first time applied to detect the Pb content in seawater, achieving a result of 0.0112 +/- 0.0006 ng/mL (ppb) consistent with the certified value of 0.013 +/- 0.005 ng/mL (ppb).     

Detection and confirmation of staphylococcal enterotoxin B in apple juice and milk using piezoelectric-excited millimeter-sized cantilever sensors at 2.5 fg/mL

A sensitive and reliable method for the detection of a model toxin, staphylococcal enterotoxin B (SEB) in buffer, apple juice, and milk is shown using piezoelectric-excited, millimeter-sized cantilever (PEMC) sensors. Limit of detection in spiked milk and apple juice samples is 10 and 100 fg, respectively. PEMC sensors (2 mm(2)) are prepared by immobilizing a polyclonal antibody specific to SEB, which was exposed to 1 mL of 1% milk and apple juice containing 10 fg-10 ng. Sensor response to 100 fg, 1 pg, and 10 pg of SEB in apple juice resulted in resonance frequency decreases of 113 +/- 18 (n = 4), 308 +/- 24 (n = 4), and 521 +/- 20 (n = 2) Hz, respectively. In milk, 10 fg, 100 fg, 1 pg, and 10 pg of SEB resulted in resonance frequency decreases of 126 +/- 18 (n = 2), 143 +/- 35 (n = 4), 310 +/- 32 (n = 5), and 557 +/- 25 (n = 2) Hz, respectively. Positive detection of SEB in the sample solution was observed within the first 20 min. The responses of the sensor to positive (SEB present, but no antibody on sensor), negative (SEB absent, antibody on sensor), and buffer (SEB absent, antibody on sensor) controls were -17 +/- 10 (n = 3), -9 +/- 5 (n = 3), and -6 +/- 12 (n = 18) Hz, respectively. Positive verification of SEB detection was confirmed by two methods: (1) low-pH buffer release caused increase in resonance frequency, and (2) second antibody binding to SEB attached to sensor that caused further resonance frequency decrease. The significance of these results is that PEMC sensors can reliably detect SEB at 10-100 fg (effective concentration of 2.5 and 25 fg/mL) in complex fluids without sample preparation or the use of labeled reagents.     

Timing properties of transmission avalanche counters at moderate specific ionization

The subject of investigation is a large-area parallel plate avalanche counter (PPAC). The counter timing properties when detecting α-particles of low energy are analyzed at n-heptane vapour pressures ranging from 5 to 30 Torr. Under typical measurement conditions, a time resolution of 380 ps has been achieved at these pressures; there is no evidence for further improvement at higher pressures. Considered are the TOF test setup efficiency and the electron drift velocity in n-heptane under relatively high electric fields. Besides, the variability of the basic physical processes contributing to the detection is evaluated and the effect of the fine energy structure of the divergently emitted α-particles upon the PPAC actual time resolution is investigated. A large dynamic range of the PPAC energy resolution was noticed; the contributions of individual side effects to the detector timing properties are evaluated.The empirical relations resulting from the investigation lead to a safe method of estimating the plateau width on the time resolution curve of transmission avalanche counters.     

Multigate single-photon detection and timing discrimination with an InGaAs/lnP avalanche photodiode

Multigate detection of single photons at 1550 nm is achieved by using capacitor-balanced InGaAs/InP avalanche photodiodes, with which we experimentally demonstrate the efficient discrimination of single-photon timing by counting single-photon clicks and the corresponding afterpulses within the multiple gates. Results show that the technique of multigate detection is a practical method for the single-photon timing information process.     

Photon counting at telecom wavelengths with commercial InGaAs/InP avalanche photodiodes: Current performance

Abstract

InGaAs/InP avalanche photodiodes operated in the so-called Geiger mode currently represent the best solution to detect single-photon beyond 900nm. They cover the 1100–1650nm wavelength interval, which includes in particular the two windows used for optical communications (1310 and 1550nm). A detection efficiency at 1550nm of 10% with a dark count probability of 10^?5 ns^?1 is common, although significant variations can be encountered. At this efficiency, a FWHM temporal response of 300 ps can be achieved. Afterpulses caused by charges trapped by defects in the high field region of the junction constitute the main performance impairment phenomenon. They enhance the dark count probability and reduce out-of-gate detector blindness. These photon counting detectors can be used in optical time-domain reflectometry to improve the spatial resolution and reduce dead-zone effects. Quantum key distribution over metropolitan area networks also constitutes an important application.     

Determination of perchlorate at trace levels in drinking water by ion-pair extraction with electrospray ionization mass spectrometry

Perchlorate has been added to the U.S. Environmental Protection Agency's Drinking Water Contaminant Candidate List (CCL). The present work describes the analysis of perchlorate in water by liquid-liquid extraction followed by flow injection electrospray mass spectrometry (ESI/MS). Cationic surfactants, mostly alkyltrimethyl-ammonium salts, are used to ion-pair aqueous perchlorate, forming extractable ion pairs. The cationic surfactant associates with the perchlorate ion to form a complex detectable by ESI/MS. The selectivity of the extraction and the mass spectrometric detection increases confidence in the identification of perchlorate. The method detection limit for perchlorate based on 3.14 sigma n-1 of seven replicate injections was 100 ng L-1 (parts per trillion). Standard addition was used to quantitate perchlorate in a drinking water sample from a contaminated source, and the concentration determined agreed within experimental error with the concentration determined by ion chromatography.     

Efficient driver drowsiness detection at moderate levels of drowsiness

Previous research on driver drowsiness detection has focused primarily on lane deviation metrics and high levels of fatigue. The present research sought to develop a method for detecting driver drowsiness at more moderate levels of fatigue, well before accident risk is imminent. Eighty-seven different driver drowsiness detection metrics proposed in the literature were evaluated in two simulated shift work studies with high-fidelity simulator driving in a controlled laboratory environment. Twenty-nine participants were subjected to a night shift condition, which resulted in moderate levels of fatigue; 12 participants were in a day shift condition, which served as control. Ten simulated work days in the study design each included four 30-min driving sessions, during which participants drove a standardized scenario of rural highways. Ten straight and uneventful road segments in each driving session were designated to extract the 87 different driving metrics being evaluated. The dimensionality of the overall data set across all participants, all driving sessions and all road segments was reduced with principal component analysis, which revealed that there were two dominant dimensions: measures of steering wheel variability and measures of lateral lane position variability. The latter correlated most with an independent measure of fatigue, namely performance on a psychomotor vigilance test administered prior to each drive. We replicated our findings across eight curved road segments used for validation in each driving session. Furthermore, we showed that lateral lane position variability could be derived from measured changes in steering wheel angle through a transfer function, reflecting how steering wheel movements change vehicle heading in accordance with the forces acting on the vehicle and the road. This is important given that traditional video-based lane tracking technology is prone to data loss when lane markers are missing, when weather conditions are bad, or in darkness. Our research findings indicated that steering wheel variability provides a basis for developing a cost-effective and easy-to-install alternative technology for in-vehicle driver drowsiness detection at moderate levels of fatigue.     

Amperometric detection of nucleic acid at femtomolar levels with a nucleic acid/electrochemical activator bilayer on gold electrode

Cationic redox polymers containing osmium-bipyridine complexes strongly interact with anionic enzymes, such as glucose oxidase and peroxidases, and electrochemically "activate" the enzymes. On the basis of these observations, attempts were made to develop an ultrasensitive nucleic acid biosensor. A mixed monolayer of single-stranded oligonucleotide capture probe and 16-mercaptohexadecanoic acid was formed on a gold electrode through self-assembly. Following hybridization with a complementary nucleic acid and glucose oxidase labeled oligonucleotide detection probe, a cationic redox polymer (electrochemical activator) overcoating was applied to the electrode through layer-by-layer electrostatic self-assembly. The formation of an anionic-cationic bilayer brought the glucose oxidase in electrical contact with the redox polymer, making the bilayer an electrocatalyst for the oxidation of glucose. Thus, nucleic acid molecules were quantified amperometrically at femtomolar levels. The effect of experimental variables on the amperometric response was investigated and optimized to maximize the sensitivity and speed up the assay time. A detection limit of 1.0 fmol/L in 1.0-microL droplets and a linear current-concentration relationship up to 800 fmol/L were attained following a 30-min hybridization. The biosensor was applied to the detection of the 16S gene in a mixture of Escherichia coli 16S + 32S rRNA and a full-length rat housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), of a RT-PCR product.     

Highly sensitive detection of protein with aptamer-based target-triggering two-stage amplification

Highly sensitive detection of proteins is essential to biomedical research as well as clinical diagnosis. However, so far most detection methods rely on antibody-based assays and are usually laborious and time-consuming with poor sensitivity. Here, we develop a simple and sensitive method for the detection of a biomarker protein, platelet-derived growth factor BB (PDGF-BB), based on aptamer-based target-triggering two-stage amplification. With the involvement of an aptamer-based probe and an exponential amplification reaction (EXPAR) template, our method combines strand displacement amplification (SDA) and EXPAR, transforming the probe conformational change induced by target binding into two-stage amplification and distinct fluorescence signal. This detection method exhibits excellent specificity and high sensitivity with a detection limit of 9.04 × 10(-13) M and a detection range of more than 5 orders of magnitude, which is comparable with or even superior to most currently used approaches for PDGF-BB detection. Moreover, this detection method has significant advantages of isothermal conditions required, simple and rapid without multiple separation and washing steps, low-cost without the need of any labeled DNA probes. Furthermore, this method might be extended to sensitive detection of a variety of biomolecules whose aptamers undergo similar conformational changes.     

Immunoassays with protein misfolding cycle amplification: a platform for ultrasensitive detection of antigen

Protein misfolding cycle amplification (PMCA), a novel technology on amplifying cyclically misfolded proteins in vitro, is conceptually analogous to DNA amplification by polymerase chain reaction (PCR) and has tremendous implications for the researches and diagnosis. Here we first introduce the protein amplification technology into the classic immunoassay and develop a PMCA-based immunoassay (immuno-PMCA) for highly sensitive detection of antigen. This method takes advantage of sandwich binding of two affinity aptamers for increased specificity, magnetic nanoparticles for fast magnetic separation, PMCA for signal amplification, and conjugated polyelectrolytes for visual detection, allowing the detection limit of antigen by colorimetry down to femtomolar level with a wide linear range from 10 to 10(4) fM. More importantly, no specialized facilities or enzymes are needed either in the amplification reaction or the evaluation of results, which indicates its great potential application in immunological research and clinical diagnostics.     

Low dose heparin lock (1000 U/mL) maintains tunnelled hemodialysis catheter patency when compared with high dose heparin (5000 U/mL): A randomised controlled trial

Introduction Heparin is commonly used after hemodialysis treatments as a locking solution to prevent catheter thrombosis. The comparative efficacy and safety of different heparin concentrations to maintain catheter patency has been previously reported in retrospective studies. We conducted a prospective, randomised, controlled study of 1000 U/mL heparin (low dose) versus 5000 U/mL heparin (high dose) locking solution to maintain patency of tunnelled catheters. Methods One hundred patients receiving chronic, unit‐based hemodialysis with newly placed tunnelled hemodialysis catheters (less than 1 week) were randomly assigned to either a low dose (n = 48) or high dose heparin (n=52). The primary intention‐to‐treat analysis examined time to malfunction in both groups over a 90 day period. A secondary analysis compared baseline patient characteristics in relation to catheter malfunction. Findings Overall rate of catheter patency loss was 32% of catheters by 90 days. There was no significant difference in time to malfunction of catheters locked with low dose or high dose heparin (P = 0.5770). Time to catheter malfunction was not associated with diabetic, hypertensive or smoking status. There was no difference in mean delivered blood flow rate, venous and arterial pressure, and dialysis adequacy between low dose and high dose groups. No patient suffered a hemorrhagic complication requiring hospitalisation during the study period. Discussion Low dose heparin is adequate to maintain tunnelled hemodialysis catheter patency when compared with high dose heparin. The study also suggests that there is no relationship between catheter malfunction and diabetic, hypertensive or smoking status.     

Sensitive detection of microRNA with isothermal amplification and a single-quantum-dot-based nanosensor

MicroRNAs (miRNAs) play important roles in a wide range of biological processes, and their aberrant expressions are associated with various diseases. Here we develop a rapid, highly sensitive, and specific miRNA assay based on the two-stage exponential amplification reaction (EXPAR) and a single-quantum-dot (QD)-based nanosensor. The two-stage EXPAR involves two templates and two-stage amplification reactions under isothermal conditions. The first template enables the amplification of miRNA, and the second template enables the conversion of miRNA to the reporter oligonucleotide. Importantly, different miRNAs can be converted to the same reporter oligonucleotides, which can hybridize with the same set of capture and reporter probes to form sandwich hybrids. These sandwich hybrids can be assembled on the surface of 605 nm emission QDs (605QDs) to form the 605QD/reporter oligonucleotide/Cy5 complexes, where the 605QD functions as both a fluorescence resonance energy transfer donor and a target concentrator. Upon excitation with a wavelength of 488 nm, distinct Cy5 signals can be observed in the presence of target miRNA. This assay is highly sensitive and specific with a detection limit of 0.1 aM and can even discriminate single-nucleotide differences between miRNA family members. Moreover, in combination with the specific templates, this method can be applied for multiplex miRNA assay by simply using the same set of capture and reporter probes. This highly sensitive and specific assay has potential to become a promising miRNA quantification method in biomedical research and clinical diagnosis.     

Flow injection analysis of L-lactate with enzyme amplification and amperometric detection

A flow injection analysis method for the determination of the lactate anion with enzyme amplification and amperometric detection is described. The system utilizes an oxygen electrode for measurement of changes in the oxygen concentration in the flow stream. Two enzymes, lactate oxidase and lactate dehydrogenase, were randomly coimmobilized on aminopropyl controlled-pore glass (AMP-CPG) and packed into a reactor. beta-NADH was used as a coenzyme for the regeneration of lactate from pyruvate. The experimental conditions for the determination of the lactate anion were studied for this system by the simplex and the univariant methods. The results obtained under these two conditions were compared. The simplex experimental condition yielded a calibration curve whose linear portion had a slope that was 1.2 times greater than that of the linear portion of the curve obtained under univariant conditions. The limit of detection under simplex condition was 1.19 x 10(-7) M vs 3.29 x 10(-7) M lactate under univariant conditions. The relative standard deviation obtained for this system at 6 x 10(-6) M lactate (n = 10) was about 2.5% under simplex conditions and 3.6% under univariant maximization conditions.     

Sub-femtomolar electrochemical detection of DNA hybridization based on latex/gold nanoparticle-assisted signal amplification

We report a relatively simple electrostatic method for modifying submicrometer-size latex spheres with gold nanoparticles (AuNPs) based on layer-by-layer modification of the latex by polyelectrolytes. The AuNP coverages for 343- and 501-nm-diameter spheres were 4.0 x 10 (10) +/- 1.3 x 10 (10) and 8.2 x 10 (10) +/- 2.7 x 10 (10) particles cm (-2), respectively, which is an increase of 1 order of magnitude on the previously reported coverage at latex-AuNPs using streptavidin-biotin binding (Kawde, A.N.; Wang, J. Electroanalysis 2004, 16, 101-107). Due to the fact that the AuNPs used here are also of a larger size (mean diameter 15.5 +/- 1.6 nm, cf. 5 nm), this represents an increase of 2 orders of magnitude in the number of Au atoms delivered per sphere. The spheres were attached to DNA probes specific to E. coli and used to detect probe hybridization by dissolution of the AuNPs, followed by measurement of Au (3+) ions by anodic stripping voltammetry (ASV). Use of differential pulse voltammetry for the stripping step, along with optimization of the ASV conditions, enabled a detection limit of 0.5 fM, which is, to the best of our knowledge, equal or lower than previous voltammetric nanoparticle methods for detection of DNA hybridization.     

Trace detection of explosives with low vapor emissions by laser surface photofragmentation-fragment detection spectroscopy with an improved ionization probe

Trace explosive residues are measured in real time by surface laser photofragmentation-fragment detection (SPF-FD) spectroscopy at ambient conditions. A 248-nm laser photofragments the target residue on a substrate, and a 226-nm laser ionizes the resulting NO fragment by resonance-enhanced multiphoton ionization by means of its A-X (0, 0) transitions near 226 nm. We tested two probes on selected explosives and modeled their electric field in the presence of a substrate with an ion optics simulation program. The limits of detection range from 1 to 15 ng/cm2 (signal-to-noise ratio of 3) at 1 atm and 298 K and depend on the electrode orientation and mechanism for NO formation.     

Aptamer-based rolling circle amplification: a platform for electrochemical detection of protein

Aptamer-based rolling circle amplification (aptamer-RCA) was developed as a novel versatile electrochemical platform for ultrasensitive detection of protein. This method utilized antibodies immobilized on the electrode surface to capture the protein target, and the surface-captured protein was then sandwiched by an aptamer-primer complex. The aptamer-primer sequence mediated an in situ RCA reaction that generated hundreds of copies of a circular DNA template. Detection of the amplified copies via enzymatic silver deposition then allowed enormous sensitivity enhancement in the assay of target protein. This novel aptamer-primer design circumvented time-consuming preparation of the antibody-DNA conjugate for the common immuno-RCA assay. Moreover, the detection strategy based on enzymatic silver deposition enabled a highly efficient readout of the RCA product as compared to a redox-labeled probe based procedure that might exhibit low detection efficiency due to RCA product distance from the electrode. With the platelet-derived growth factor B-chain (PDGF-BB) as a model target, it was demonstrated that the presented method was highly sensitive and specific with a wide detection range of 4 orders of magnitude and a detection limit as low as 10 fM. Because of the wide availability of aptamers for numerous proteins, this platform holds great promise in ultrasensitive immunoassay.     

Collection of peptides released from single neurons with particle-embedded monolithic capillaries followed by detection with matrix-assisted laser desorption/ionization mass spectrometry

Characterization of the stimulated release of neuropeptides from brain slices and individual cultured neurons requires efficient collection of the releasate from relatively large volumes of physiological saline. Here, several collection approaches are optimized using particle-embedded monolithic capillaries (PEMCs) with poly(stearyl methacrylate-co-ethylene glycol dimethacrylate) monolith acting as a "glue". Two distinct extraction particles, with either pyrrolidone (PY) or ethylenediamine (EDA) as the functional group on polystyrene backbone, have been embedded into capillaries having an inner diameter of 250 μm. The capillaries act as collection devices for sampling neuropeptide release; the collection protocols are described, and the extraction efficiency of the probes are characterized. Specifically, the binding of angiotensin II from a peptide mixture onto the PY and EDA columns was 16 and 28 pmol, respectively, in a volume of 20 μL of saline. The peptides released from these columns have been characterized via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with low femtomole detection limits. When the PEMC columns were positioned in close proximity to individual neurons and 50 mM KCl was used as the secretagogue, peptides released from individual identified cultured neurons isolated from Aplysia californica were collected and characterized.