Gerhard J. Mohr

Optical sensors for a wide pH range based on azo dyes immobilized on a novel support

Abstract New reactive azo dyes are described which are useful for optical determination of pH. The dyes are covalently linked (via ethylsulfonyl groups) to a novel type of transparent solid films consisting of a thin film of cellulose on top of a polyester support. The resulting membranes exhibit distinct color change when changing the pH and, in most cases, are compatible with LED light sources. Various membranes have been developed with p K a values ranging from 0.5 to 11.28, thus practically covering the whole pH range. Because of the stability of the dyes, the covalent bond and the solid support, the new membranes exhibit storage stability over years, and operational lifetimes of weeks, good reproducibility and a fast (30–60 s) response to pH.

On the Design of Fluorescent Ratiometric Nanosensors

Advances in nanoparticle technology have recently offered new tools to the bioanalytical field of research. In particular, new nanoparticle-based sensors have appeared able to give quantitative information about different species (ions, metabolites, biomolecules) in biosamples through ratiometric measurements. This article describes the methodologies developed so far in the design of such nanosensors. In particular, the different approaches to immobilize fluorescent chemosensor dyes to nanoparticles are presented. Concept designs of ratiometric nanosensors in terms of composition and architecture are also described and illustrated with examples taken from the literature.

A chromoreactand for the selective detection of HSO3− based on the reversible bisulfite addition reaction in polymer membranes

A new chromoreactand for the reversible optical detection of bisulfite anion is presented with a sensitivity in the range from 0.1 to 10 mM while no absorbance changes are observed for anions such as sulfate, chloride, phosphate or hydroxide.

Biocompatible fluorescent nanoparticles for pH-sensoring

Dialysis of a mixture of fluorescein and sulforhodamine B marked dextran derivatives yields biocompatible and tuneable nanosensors that can be used for ratiometric pH measurements.

Ratiometric pH-nanosensors based on rhodamine-doped silica nanoparticles functionalized with a naphthalimide derivative.

This paper describes the preparation of two-dye-doped silica nanoparticles for ratiometric pH measurements in the biologically relevant pH-range. While a rhodamine derivative is embedded in a silica core and used as the reference, a pH-sensitive naphthalimide dye is immobilized on the previously amino-functionalized core through two different approaches. Either the naphthalimides carboxylic group is activated to a succinimidyl-ester to form an amide bond or the system can be built up via solid-phase organic synthesis in only two steps. Both types of nanosensors are characterized in terms of morphology (dynamic light scattering, transmission electron microscopy) and optical properties (steady-state fluorescence spectroscopy). In terms of application, e.g. reproducibility and handling of the synthesis, the first approach gave very good results with respect to size and size distribution and a pK(a) value of 6.55 was found that is comparable to the free indicator dye in solution. The solid-phase organic synthesis method proves the possibility of covalent immobilization of naphthalimides to amino-functionalized surfaces, showing the stability of the polymeric substrate and achieving comparable results for pH sensing.

Star-Shaped Tripodal Chemosensors for the Detection of Aliphatic Amines

In this work, three new tripodal triphenylamine dyes are presented that are capable of reversibly binding amines and diamines to form hemiaminals through a covalent bond. The dyes were synthesized by the Heck reaction and possess stilbene units with one, two, or three trifluoroacetyl groups as receptor moieties. Their interaction with amines and diamines led to changes in their absorption and emission properties, which were detected by UV/Vis and fluorescence spectroscopy. The influence of the number of trifluoroacetyl receptor moieties on the selectivity and sensitivity of the dyes was studied. Enhanced sensitivity and selectivity for diaminoalkanes was found for the dye we have labeled Tripod-1, with three chemically reactive trifluoroacetyl groups, related to only one or two trifluoroacetyl groups in the dye molecule.

Chemosensors Based on Molecularly Imprinted Polymers

A sensor is a device, which responds to a physical or chemical stimulus in order to produce a measurable detection signal or to control another operation [1]. Sensors are encountered in innumerable applications and have become an integral part of our day-to-day life. Examples of everyday use of sensors include a thermocouple, which responds to the change in temperature by an output voltage, or a touch-sensitive sensor of an interactive monitor screen. Basically, a sensor can respond, that is change its signal, to a single factor being sensed, i.e. either to the change of temperature or pressure in the above examples.

Development and critical evaluation of fluorescent chloride nanosensors.

In this study, we describe the preparation and evaluation of new fluorescent sensor nanoparticles for the ratiometric measurement of chloride concentrations. Both a chloride-sensitive dye (lucigenin) and a reference dye (sulforhodamine derivative) were incorporated into polyacrylamide nanoparticles via inverse microemulsion polymerization and investigated for their response to chloride ions in buffered suspension as well as in living cells. The fluorescence intensity of lucigenin reversibly decreased in the presence of chloride ions due to a collisional quenching process, which can be described with the Stern-Volmer equation. The determined Stern-Volmer constant K SV for the quenching of lucigenin incorporated into particles was found to be 53 M (-1) and is considerably smaller than the Stern-Volmer constant for quenching of free lucigenin ( K SV = 250 M (-1)) under the same conditions. To test the nanosensors in living cells, we incorporated them into Chinese hamster ovary cells and mouse fibroblasts by using the conventional lipofectamin technique and monitored the response to changing chloride concentrations in the cell.

Development of chromogenic reactands for optical sensing of alcohols

Abstract A new class of chromogenic reactands (or ‘chromoreactands’) has been synthesised that reversibly interact with alcohols resulting in a change in absorbance. When embedded in plasticised PVC membranes, 4-(N,N-dioctylamino)-4′-trifluoroacetyl-azobenzene (ETHT 4001) shows a significant signal change on exposure to aqueous ethanol solution with a decrease in absorbance at around 490 nm and an increase in absorbance at around 430 nm wavelength. The sensor layer exhibits a dynamic range from 2 to 50 vol% ethanol with highest sensitivity in the 5–35 vol% range. The limit of detection is 1.5 vol%. The absorbance of the sensor membrane is virtually insensitive to changes in pH, however, the magnitude of the relative signal change between plain buffer and buffer containing ethanol is pH dependent. A similar response is observed for 1-(N,N-dioctylamino)-4-(4-trifluoroacetylphenylazo)-naphthalene (ETHT 4002), however, with the absorbance shifted to longer wavelengths. The behaviour of the reactands dissolved in alcohols correlates with the selectivity of the dyes in the sensor membranes.

New chromoreactands for the detection of aldehydes, amines and alcohols

Abstract Two indicator dyes are presented that perform reversible chemical reactions with neutral analytes in thin layers of plasticised poly vinyl chloride (PVC). Since these dyes combine the properties of a selective ligand with those of a chemical reagent, they are termed chromoreactands. The lipophilic decyl derivative of α-Naphthyl Red (CR-418) reversibly reacts with formaldehyde shifting the absorbance maximum in plasticised PVC from 438 to 449xa0nm. The sensor membrane exhibits highest sensitivity to formaldehyde in the 1–100xa0mM range with a detection limit of 0.3xa0mM. A bisazo dye with a reactive trifluoroacetyl group (CR-573) reversibly forms a hemiaminal with amphetamine in thin layers of plasticised PVC and changes its colour from blue to red (corresponding to a shift in absorbance maximum from 575 to 548xa0nm). The sensitivity of the dye in the polymer layer covers the range from 0.3 to 30xa0mM amphetamine with a detection limit of 0.1xa0mM. The same dye also detects ethanol when the catalyst tridodecylmethylammonium chloride is used to accelerate the chemical reaction. The sensor layer exhibits a sensitive range from 1 to 40% ( v/v) ethanol and a detection limit of 0.8% (v/v). Maximum signal changes are observed at 630xa0nm making CR-573 compatible with cheap light sources and detectors.