Shpol'skii spectroscopy at the interface of two non-polar microenvironments: a novel approach for the analysis of polycylic aromatic compounds

This article presents a thorough investigation of quantitative parameters for the analysis of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzofurans (PDBFs) partitioning between an octadecyl silica extraction membrane and microliters of an n-alkane solvent spiked on the surface of the membrane. Shpol'skii spectrometry is directly performed on the surface of the membrane with the aid of a cryogenic fiber optic probe. The analyte concentration in the layer of Shpol'skii solvent follows a linear relationship with the analyte concentration in the water sample and the same is observed for the phosphorescence signal of the cryogenic probe. The accuracy and precision needed for quantitative analysis in aqueous samples is demonstrated. The analytical figures of merit show the feasibility to determine organic pollutants at the parts-per-trillion level with minimum solvent consumption.     

Short communication controlling LC-SPE-NMR systems

There are several stages of the LC-SPE-NMR process that should be monitored closely to ensure an efficient isolation and concentration of the target analyte, for instance analyte break-through and compound transfer from the LC-SPE to the NMR probe. In this study, analyte break-through monitoring was performed with a UV detector and a mass spectrometer placed after the SPE unit. Easy break-through was a problem when attempting multiple trapping of various compounds using C18 SPE cartridges with the original commercial system. However, on lowering the flow rate over the SPE system and using SPE cartridges packed with porous carbon, the number of trappings possible increased five-fold. To increase control over the on-line SPE-NMR transfer, a gradient pump-UV system was used to elute compounds trapped on an SPE to an NMR probe. The analyte band was placed in the active volume of the probe by a stop-flow mechanism. The modified LC-SPE system was also coupled with off-line NMR analysis for determination of a degradation product of the insecticide monuron, present in the low ppm range.     

Combination of a Mid-infrared Hollow Waveguide Gas Sensor with a Supported Capillary Membrane Sampler for the Detection of Organic Compounds in Water

A Fourier transform infrared (FT-IR) spectroscopy based gas sensor for continuous analysis of liquid phase samples has been developed, coupling a short hollow waveguide (HWG) gas cell with a supported capillary membrane sampler (SCMS) probe. Passing an inert carrier gas through the thin-walled tubular silicon membrane enables the permeation of volatile organic compounds (VOCs) present in aqueous solution and facilitates their continuous and quantitative detection in the infrared hollow fiber by multiple internal reflection spectroscopy. The sensitivity of the sensor system has been determined at the ppb (μg/L) concentration level and the response time ranges from few minutes to 30 min, depending on the analyte and the permeation properties of the sampling membrane.

The experimental set-up consists of Bruker Vector 22 FT-IR spectrometer with an externally aligned 50 cm long silica HWG coupled to the SCMS, which is immersed into a glass flask filled with analyte solution and kept under constant stirring.

Aqueous solutions of benzene, toluene, xylene isomers and chloroform were qualitatively and quantitatively analyzed confirming the feasibility of this sensor approach for environmental analysis.     

A Review of Microdialysis Sampling Systems

The most important aspects of microdialysis are a theoretical understanding of the process, the microdialysis membrane and the design of the microdialysis probe including the inner cannula dimensions. Several efforts have been made to theoretically account for the processes that take place during microdialysis. These have been employed to develop optimal sampling conditions so as to increase the applicability of the technique for in situ sampling and as a sample clean-up technique prior to chromatography. On the occasion of Prof. Lo Gorton’s 60th birthday, this review highlights the challenge presented by low analyte recoveries that is the major bottleneck in the wider use of microdialysis. The discussion concludes by considering how to increase analyte recovery through a multiple probe approach or by an increase in recovery in the light of the advantages of nanotechnology. Both approaches could impact on the use of microdialysis as a sampling and sample clean-up technique for liquid chromatography.     

Performance of an ion trap mass spectrometer modified to accept a direct insertion membrane probe in analysis of low level pollutants in water

Modifications to a Finnigan ITS40 ion trap mass spectrometer are described which allow its use with a direct insertion probe. Details are given of the fabrication of a membrane probe for such an instrument. The membrane probe, which includes facilities for heating the fluid, employs a tubular membrane which is located just outside the electrode structure of the ion trap. Direct analysis of organic compounds in aqueous solution is demonstrated using a silicone membrane, with compounds such as benzene, chlorobenzene and dichloroethene being studied below the 1 ppb level. The effects of operating parameters including probe temperature, ion trap temperature, solution flow rate, mass spectrometer scan speed, and instrument tune procedures are explored in detail. Optimum performance characteristics are identified and trace level detection of eight organic compounds in the parts per trillion range is demonstrated. In seven of the eight cases studied, detection limits are below the EPA practical limit of quantitation levels. It is shown that the most sensitive mode of operation is when steady state passage of the analyte across the membrane is achieved, however, the time required for this is long in the case of some samples, and a dynamic flow injection analysis procedure is then favored. Use of the modified inlet system for solid sample introduction via a standard solids probe is also demonstrated.     

Evaluation of a dialysis probe for continuous sampling in fermentors and in complex media

A dialysis probe is described for continuous sampling from complex solutions, such as fermentation broth, milk or waste water, to yield samples suitable for liquid chromatography, flow injection analysis, enzyme calorimetry, etc. The analyte is transferred to a flow stream separated from the sample by a dialysis membrane that is protected from fouling by a strong tangential flow of the sample solution. This flow is accomplished by placing a magnetic stirring bar close to the membrane surface. The device is constructed of materials permitting the probe to be steam-sterilized when mounted inside a fermentor.     

Membrane-controlled reagent-delivery systems--a new approach for the continuous production of reagent and standard solutions

A new simple and robust system for the production of standard solutions, based on the mass-transfer of analytes through membranes, is described. The device consists of a cone-shaped reservoir vessel, filled with a concentrated solution of the analyte and separated from a liquid acceptor stream by a membrane. Mass-flow from donor to acceptor solution is controlled by the mass-transfer-affecting properties of the active membrane area, which is determined by the hole in a template (diameter 0.8 mm) placed between the membrane and the acceptor-channel. Using nitrate as model analyte and a track-etched membrane filter (pore size 0.1 microm) dilution factors up to 2,400,000 with long-term reproducible accuracy of < 2% have been achieved. Adjustment of a requested concentration is possible by varying either the flow rate of the acceptor stream or the concentration of the reservoir solution.     

Membrane-controlled reagent-delivery systems – a new approach for the continuous production of reagent and standard solutions

A new simple and robust system for the production of standard solutions, based on the mass-transfer of analytes through membranes, is described. The device consists of a cone-shaped reservoir vessel, filled with a concentrated solution of the analyte and separated from a liquid acceptor stream by a membrane. Mass-flow from donor to acceptor solution is controlled by the mass-transfer-affecting properties of the active membrane area, which is determined by the hole in a template (diameter 0.8 mm) placed between the membrane and the acceptor-channel. Using nitrate as model analyte and a track-etched membrane filter (pore size 0.1 μm) dilution factors up to 2,400,000 with long-term reproducible accuracy of < 2% have been achieved. Adjustment of a requested concentration is possible by varying either the flow rate of the acceptor stream or the concentration of the reservoir solution. Received: 22 November 2000 / Revised: 5 March 2001 / Accepted: 7 March 2001     

Hollow fiber membrane concentrator for on-line preconcentration

Solvent removal by membrane permeation is presented as a method for on-line preconcentration. Experiments were carried out to develop a one-step, on-line, concentration process using a microporous composite hydrophobic membrane, or a polar solvent-permeable Nafion membrane depending on the solvent. Both polar and non-polar hollow fiber membranes were found to be effective in concentrating trace analytes. A large increase in analyte enrichment factors was found for both concentrator modules. Enrichment factors as high as 18.9 were observed. Residence time and operating temperature were found to be important parameters. Several different model compounds were preconcentrated. Further, in a Nafion membrane (polar solvent-permeable), analyte interaction with membrane bound sulfonic acid residues resulted in the loss of reactive analytes such as atrazine (ATZ). All analytes were successfully concentrated and detected using a polypropylene-siloxane composite membrane system when hexane was used as the solvent.     

Signal amplification and detection via a supramolecular allosteric catalyst

The design of a supramolecular allosteric catalyst system for catalytic signal amplification and detection is presented. The catalyst was switched "on" by the introduction of an analyte that also behaves as an allosteric activator. Concentrations of Cl- ions as low as 800 nM were catalytically amplified and detected. The signal was transduced via a pH-sensitive fluorescent probe and observed visually using a laboratory, handheld UV lamp and by spectrophotometry. Furthermore, the allosteric effect was quantified using gas chromatography for a range of Cl- concentrations. This three-part detection scheme involving analyte binding, allosteric catalyst activation, and signal transduction represents a new approach to small-molecule detection.     

A unique paradigm for a Turn-ON near-infrared cyanine-based probe: noninvasive intravital optical imaging of hydrogen peroxide

The development of highly sensitive fluorescent probes in combination with innovative optical techniques is a promising strategy for intravital noninvasive quantitative imaging. Cyanine fluorochromes belong to a superfamily of dyes that have attracted substantial attention in probe design for molecular imaging. We have developed a novel paradigm to introduce a Turn-ON mechanism in cyanine molecules, based on a distinctive change in their π-electrons system. Our new cyanine fluorochrome is synthesized through a simple two-step procedure and has an unprecedented high fluorescence quantum yield of 16% and large extinction coefficient of 52,000 M(-1)cm(-1). The synthetic strategy allows one to prepare probes for various analytes by introducing a specific triggering group on the probe molecule. The probe was equipped with a corresponding trigger and demonstrated efficient imaging of endogenous hydrogen peroxide, produced in an acute lipopolysaccharide-induced inflammation model in mice. This approach provides, for the first time, an available methodology to prepare modular molecular Turn-ON probes that can release an active cyanine fluorophore upon reaction with specific analyte.     

Non aggregated colloidal silver nanoparticles for surface enhanced resonance Raman spectroscopy

Silver nanoparticles with a tuneable λ max were produced as colloids by heterogeneous nucleation. The synthesis process is both fast and repeatable, producing stable PVA capped nanoparticles. The colloid's effectiveness in the SERRS system was investigated using Rhodamine 6G, R6G, Crystal Violet, CV, and Malachite Green, MG, as probe molecules. A clear sensing trend was observed, where the Raman signal emitted was significantly enhanced by the addition of silver nanoparticles. A build up of signal intensity is observed until an optimum ratio is achieved, followed by a decline in signal intensity as the concentration of nanoparticles is further increased. The sensing trend appeared to be dependant on the structure of these model molecules with similarly structured compounds exhibiting similar trends. Thus a maximum enhancement with the Ag: analyte molar ratio of ～ 5.56: 1, was seen for CV and MG whereas R6G had a maximum enhancement at the Ag: analyte molar ratio of ～ 2.25: 1.     

Label-free electrochemical detection of nanomolar adenosine based on target-induced aptamer displacement

In this work, a label-free electrochemical sensor based on target-induced displacement is reported with adenosine as the model analyte. The sensing substrate is prepared using a gold electrode modified with a self-assembled monolayer of 1,6-hexanedithiol that mediates the assembly of a gold nanoparticle film, which can increase the surface loading of capture probe and enhance the signal. An aptamer for adenosine is applied to hybridizing with the capture probe, yielding a double-stranded complex of the aptamer and the capture probe on the surface. The interaction of adenosine with the aptamer displaces the aptamer sequence and causes it to dissociate from the interface. This results in a decrease in the amount of aptamer/capture probe duplex form, and, accordingly, the desorption of methylene blue, an electroactive indicator bound to the duplex, from the electrode. Then, the redox current of the indicator can reflect the concentration of the analyte. The fabricated sensor is shown to exhibit high sensitivity, desirable selectivity and a three-decade wide linear range.     

Augmentation of enzyme electrode sensitivity using biocatalytic preconcentration

A method for increasing the sensitivity of enzyme sensors based on biocatalytic accumulation of an intermediate product was investigated using a biospecific electrode consisting of an immobilized glucose dehydrogenase-lactate dehydrogenase-lactate monooxygenase membrane and an electrochemical oxygen probe. Addition of the analyte (glucose) and an excess of NAD⁺ to the background solution permits NADH to be biocatalytically preconcentrated in the enzyme membrane. When this reaction has approached equilibrium, the sensor signal is generated by injection of an excess of pyruvate, thus starting oxygen consumption catalysed by the sequential lactate dehydrogenase-lactate monooxygenase reaction. Glucose can be determined at concentrations between 10 and 100 μM. Compared with operation of the sensor without NADH preconcentration, the increase in the sensitivity to glucose is 18-fold in the current-time mode and 40-fold in the derivative current-time mode. The measuring regime permits interferences from the sample solution to be avoided.     

A long-lived luminescence and EPR bimodal lanthanide-based probe for free radicals

We developed a novel spin-labeled terbium complex Tb(3+)/cs124-DTPA-TEMPO (1) by covalently labeling a nitroxide radical on the terbium complex for monitoring free radicals of various areas. This lanthanide complex probe shows a high EPR signal which resulted from the nitroxide radical moiety, and is weakly luminescent which resulted from the intramolecular quenching effect of the nitroxide radical on sensitised terbium luminescence. The intensity of both the EPR and luminescence can be modulated by eliminating the paramagnetism of the nitroxide radical through recognition of a carbon-centered radical analyte and thus gives a quantification of the analyte. We have preliminarily applied this probe in the luminescent detection of model carbon-centered radicals and hydroxyl radicals (·OH). This probe is water-soluble and contains lanthanide-luminescence properties, favorable for the time-resolved luminescence technique. The investigation of the intramolecular quenching process has showed that the labeled nitroxide radical quenches multiple excited states of the terbium complex, resulting in highly efficient quenching of terbium luminescence. This probe is the first example of intramolecular modulation of lanthanide luminescence by a nitroxide radical.     

An electrospray membrane probe for the analysis of volatile and semi-volatile organic compounds in water

A new membrane probe incorporating electrospray ionization (ESI) was designed, built and coupled to an ion trap mass spectrometer to detect low levels of semi-volatile organic compounds (SVOCs) in water. Similar to other membrane introduction mass spectrometry (MIMS) systems, the probe contains a capillary polydimethylsiloxane (PDMS) membrane to allow for the preferential permeation of small molecules but, in contrast, the interface uses a liquid/membrane/liquid interface rather than liquid/membrane/gas. The ESI source allows the probe to be operated at atmospheric pressure in positive or negative ionization mode and the lack of fragmentation in ESI allows for the simultaneous screening of many analytes with high sensitivity. The interface allows for the addition of additives to both the external and the internal liquid mobile phases to selectively improve permeation and/or the ionization efficiency of various classes of compounds. Characterization of the probe with methanol as the internal mobile phase showed that the signal for aniline optimized at 60 degrees C and an internal flow rate between 2-5 microL/min. The transfer of analyte through the membrane from water to methanol ensures a strong signal and robust electrospray for both positive and negative ion mode which is not typical when spraying pure water. Detection limits for aniline, pyridine and pentachlorophenol, and for the herbicides alachlor, atrazine, butachlor, metolachlor and simazine, were in the ppb to pptr range.     

A Water Soluble 2-Phenyl-5-(pyridin-3-yl)-1,3,4-oxadiazole Based Probe: Antimicrobial Activity and Colorimetric/Fluorescence pH Response

The growing demand of responsive tools for biological and biomedical applications pushes towards new low-cost probes easy to synthesize and versatile. Current optical probes are theranostic tools simultaneously responsive to biological parameters/analyte and therapeutically operating. Among the optical methods for pH monitoring, simple small organic molecules including multifunctional probes for simultaneous biological activity being highly desired by scientists and technicians. Here, we present a novel pH-responsive probe with a three-ring heteroaromatic pattern and a flexible cationic chain. The novel molecule shows real-time naked-eye colorimetric and fluorescence response in the slightly acidic pH range besides its excellent solubility both in the organic phase and in water. In addition, the small probe shows significant antibacterial activity, particularly against Escherichia coli. Single-crystal X-ray study and density functional theory (DFT) calculations rationalize the molecule spectroscopic response. Finally, molecular dynamics (MD) elucidate the interactions between the probe and a model cell membrane.     

Evaluation of an overdetermined system based on multiple ion-selective electrodes of the same type

Possible advantages of using multiple ion-selective electrodes of the same type for the determination of analyte ions in multicomponent mixtures are investigated. Specifically, potassium and ammonium ions are determined in model multicomponent mixtures with a combination of four valinomycin and four nonactin polymeric membrane electrodes. The electrode potentials are monitored by a specially designed data-acquisition system, and conventional matrix operations are used to calculate the ion concentrations. The results indicate that greater confidence in the analyses can be obtained from an overdetermined system of this type.     

Use of a multi-tube Nafion® membrane dryer for desolvation with thermospray sample introduction to inductively coupled plasma-atomic emission spectrometry

A multi-tube Nafion^® membrane dryer used as a part of a desolvation system in conjunction with thermospray nebulization was optimized and characterized with inductively coupled plasma-atomic emission spectrometry (ICP-AES). Either argon or nitrogen could be used as the sweep gas, and optimum conditions were found to be at low temperature and low sweep gas flow rate. Analyte sensitivity was not significantly affected by placing the membrane between the plasma and the nebulizer, although about 20% of the analyte entering the dryer is lost within the dryer. A dual role of the membrane dryer was demonstrated. As a secondary step within the desolvation system, it enabled a high desolvation efficiency of 99.94% for aerosols from 1% (v/v) nitric acid. Plasma solvent load could be reduced to 0.9 mg min⁻¹ with a tap water cooled condenser combined with the membrane dryer, compared to 21 mg min⁻¹ with the normal chilled condenser desolvation system. Meanwhile, the membrane was also found to act as a pulse dampener, eliminating the plasma pulsation in the central channel caused by thermospray nebulization and thus improving the analytical performance of the system. The average relative standard deviations (RSD) with the optimized membrane/thermospray system were 0.83% and 0.60% for the background and analyte signals, respectively, which were reduced by a factor of 1.9 and 2.7 for the background and analyte signals, respectively, compared to thermospray without the membrane desolvation, and were essentially identical to those obtained with pneumatic nebulization sample introduction. The improvements in detection limits with the membrane/thermospray system were 1.2–3.0 times with an average factor of 1.8 compared to thermospray without the membrane dryer, and 18–68 times with an average factor of 39 compared to the standard pneumatic nebulization sample introduction system without a desolvation unit. The detection limits for Mn, Mg, Cr and Cd with the present thermospray/membrane system were comparable to those reported for pneumatic nebulization ICP mass spectrometry.     

Quinoprotein glucose dehydrogenase modified thick-film electrodes for the amperometric detection of phenolic compounds in flow injection analysis

The use of thick-film electrodes as basic transducers for highly sensitive amperometric biosensors using PQQ (pyrroloquinoline quinone) dependent glucose dehydrogenase (GDH) with short response times is described. The enzyme is embedded in a polyurethane matrix on top of a platinum based thick film electrode and its ability to reduce oxidized phenolic compounds is exploited. The electrochemical amplification is based on the oxidation of the analyte on the surface of the electrode followed by its enzymatic reduction. Different parameters of the glucose dehydrogenase electrode system using dopamine as a model analyte were optimized, e.g., membrane thickness, pH value, buffer system, flow rate and storage conditions. Using optimized parameters the sensitivity and detection limits for various phenolic compounds were evaluated. The comparison of electrodes from the identical as well as from different batches shows the ability to produce a number of well reproducible sensors showing remarkably small differences with respect to parameters as sensitivity, response times and measuring range.