Torsten Mayr

Precipitation as a simple and versatile method for preparation of optical nanochemosensors.

Optical nanosensors for such important analytes as oxygen, pH, temperature, etc. are manufactured in a simple way via precipitation. Lipophilic indicators are entrapped into nanobeads based on poly(methyl methacrylate), polystyrene, polyurethanes, ethylcellulose, and other polymers. Charged groups greatly facilitate formation of the small beads and increase their stability. Sensing properties of the beads can be tuned by choosing the appropriate indicator. Nanosensors for carbon dioxide and ammonia are found to be cross-sensitive to pH if dispersed in aqueous media. These nanobeads are successfully employed to design bulk optodes. Nanochemosensors with enhanced brightness via light-harvesting and multi-functional magnetic nanosensors also are prepared.

Dual lifetime referenced optical sensor membrane for the determination of copper(II) ions

A sensor membrane has been developed for the determination of copper(II) ions that displays excellent performance due to internal referencing of luminescence intensities. The applied sensing scheme (dual lifetime referencing) makes use of the indicator lucifer yellow (LY) and an inert reference luminophore (a ruthenium complex entrapped in polyacrylonitrile beads). Both are contained in a hydrogel matrix. The copper-dependent fluorescence intensity change of LY can be converted in either a phase shift or time dependent parameter. The sensing membrane is capable of determining copper(II) with an outstanding high selectivity over a dynamic range between 1 and 1000µM in neutral or weekly acidic conditions. The advantages of the referencing method over intensity based measurementswas demonstrated by the measurement of turbid solutions. The scheme was also applied to two-dimensional measurements in the time domain. Sensor integrated microtiterplates were imaged with a CCD camera gated with square pulses in the microsecond range

Poly(styrene-block-vinylpyrrolidone) Beads as a Versatile Material for Simple Fabrication of Optical Nanosensors

A versatile platform for designing optical nanosensors is proposed. The sensing chemistries are entrapped into the poly(styrene-block-vinylpyrrolidone) nanobeads having the average size of 245 nm in aqueous media. Addressable staining into the core or the shell of the beads results in nanosensors for essential analytes such as dissolved oxygen, temperature, pH, chloride, and copper ions. Two immobilization procedures are developed: staining in the polystyrene core is performed from a tetrahydrofuran/water mixture (50:50 v/v) and staining in the poly(vinylpyrrolidone) shell is achieved by using the ethanol/water mixture (70:30 v/v). The oxygen and temperature indicators should be preferably immobilized into the core, whereas nanosensors for ions are manufactured by staining into the shell. In the case of the lipophilic pH indicators both procedures result in similar pKa values. The unique properties of the beads make them promising for sensing and imaging even in very complex media, multianalyte sensing, and monitoring of very fast processes.

Fluorescence sensors for trace monitoring of dissolved ammonia.

Even though monitoring of dissolved ammonia is acutely important for environmental studies, fish farms and for industrial surveillance, no system for the performance of online measurements at the concentrations needed exists so far. For many applications it is necessary to detect dissolved ammonia concentrations at sub mg/l-levels, because ammonia is reported to be toxic for aquatic organisms above 25 microg/l. We present new ammonia sensitive materials consisting of fluorescent pH indicators embedded into different cellulose esters. The low pK(a) value of the indicators and the high solubility of ammonia in the cellulose polymers lead to detection limits below 1 microg/l and a dynamic range between 5 and 1000 microg/l. Response times at these trace concentration levels are in the order of 20-30 min. The sensors are suitable for fresh and sea water monitoring by an additional silicon layer preventing the interference of protons and salinity. The fluorescent indicators Eosin ethylester and 2,7-dichlorofluorescein methylester (DCF) were investigated to achieve sensors with a dynamic range matching the target concentrations. Sensors with improved performance were obtained by employing cellulose ester nanospheres with incorporated Eosin ethylester. The simple sensor design has a high potential to be applied in miniaturized optical measurement system for online ammonia detection.

Highly selective optical sensing of copper(II) ions based on fluorescence quenching of immobilised Lucifer Yellow

We describe the development of an optical sensing scheme for the determination of copper(II) in drinking or waste water. It is based on static quenching of the fluorescence of Lucifer Yellow immobilised on anion exchanger particles, embedded in a hydrogel. The sensing membrane allows the determination of copper(II) in the 0.01 μM (0.63 μg l−1) to 100 μM (6300 μg l−1) concentration range with an outstanding high selectivity. The change in fluorescence on exposure to a significant concentration of 31 μM (2000 μg l−1) is −60%. The response time is concentration dependent and varies from 100 to 3 min. Selectivity was investigated by the separate solution method; mercury(II) was found to be the only interferent. The effect of pH was evaluated in the range 4.0–6.8. The application of the sensing membrane as a single shot test was demonstrated using microtitre plates for ncopper(II) determination in tap water samples.

A planar waveguide optical sensor employing simple light coupling

The novel optical sensor concept utilizes the sensing layer as light propagating layer and employs a new method to couple light into a planar waveguide.

Intraparticle concentration gradients for substrate and acidic product in immobilized cephalosporin C amidase and their dependencies on carrier characteristics and reaction parameters

Cephalosporin C amidase was covalently attached using a protein loading of 7.0–200u2009mg protein/g dry carrier on four epoxy‐activated Sepabeads differing in particle size and pore diameter. Initial‐rate kinetic analysis showed that for Sepabeads with small pore diameters (30–40u2009nm), the apparent KM of the amidase for hydrolysis of cephalosporin C at 37°C and pH 8.0 increased ∼3‐fold in response to increased particle size (∼120–400u2009µm) and increased amount of immobilized enzyme (7.0–70u2009mg protein/g dry carrier) while maximum specific activity (3.2u2009U/mg protein; 25% of free amidase) was affected only by particle size. In contrast, for Sepabeads with wide pores (150–250u2009nm), the KM was independent of the enzyme loading. Internal effectiveness factors calculated from observable Thiele modulus reflected the dependence of KM on geometrical parameters of the particles. A new method for determination of the overall intraparticle pH was developed based on luminescence lifetime measurements in the frequency domain. Sepabeads were doubly labeled using a lipophilic variant of the pH‐sensitive dye fluorescein, and Ru(II) tris(4,7‐diphenyl‐1,10‐phenantroline) whose phosphorescence properties are independent of pH. Luminescent lifetime measurements of doubly labeled particle suspensions showed superior signal‐to‐noise ratio compared to fluorescence intensity‐based measurements using singly labeled particles. The difference at apparent steady state (ΔpH) between bulk (external pH) and intraparticle pH (internal pH) was as large as ∼0.6u2009units. The ΔpH was dependent on substrate concentration, particle size, and pore diameter. Therefore, these results characterize the role of carrier characteristics and reaction parameters in the formation of concentration gradients for substrate and acidic product during hydrolysis of cephalosporin C by immobilized amidase. The strong pH dependence of the immobilized amidase underscores the importance of considering intraparticle pH gradients in the design of an efficient carrier‐bound biocatalyst. Biotechnol. Bioeng. 2010;106: 528–540.

Multi-ion imaging using fluorescent sensors in a microtiterplate array format

A novel type of sensor array destined for water analyses is described. The sensor delivers simple on/off patterns of complex ion mixtures. Fluorescent indicators for Ca2+, Na+, Mg2+, SO42-, Cl− and Hg2+ were arranged in microtiterplates and the analytical information was imaged with a CCD camera within microseconds using an intrinsically referencing method.

Filter cubes with built-in ultrabright light-emitting diodes as exchangeable excitation light sources in fluorescence microscopy.

The use of ultrabright light‐emitting diodes as a potential substitute for conventional excitation light sources in fluorescence microscopy is demonstrated. We integrated ultrabright light‐emitting diodes in the filter block of a conventional fluorescence microscope together with a collimating Fresnel lens, a holographic diffuser and emission filters. This setup enabled convenient changes between different excitation light sources and resulted in high excitation efficiencies. Quantitative comparison of image intensities of test samples revealed that light‐emitting diodes yielded intensities in the range of a mercury arc lamp depending on the wavelength. The use of ultrabright light‐emitting diodes also enabled luminescence lifetime imaging without the need for image intensification.

Microsensors for detection of ammonia at ppb-concentration levels

A fibre optic sensor for the detection of dissolved ammonia in concentrations below 100 µg L−1 is presented. The sensing principle is based on an immobilized fluorescent pH indicator which is deprotonated by ammonia. The resulting ammonium ion is stabilized by a cation trap. The components were immobilized in a cellulose ester as matrix polymer. Microsensors prepared with this sensor chemistry showed a detection limit of 0.5 µg L−1. The dynamic range of the sensor matches perfectly with the concentration levels (<25 µg L−1) that are known to be toxic for organisms. The determination of ammonia is important, because of the toxicity of this compound to organisms. Microsensors are powerful tools to establish microprofiles of high spatial resolution with the least disturbance of the ecological system. Optical microsensors for ammonia can be easily fabricated using the novel sensor chemistry. The potential interference of trimethylamine was minimized using an 18-crown-6 ether derivative as the cation trap and by the permeability properties of the polymer matrix. Cross-sensitivities towards protons and alkali ions are prevented by a Teflon coating covering the ammonia sensitive layer. This makes the sensor suitable for measurements in fresh and salt water.