Günter Mistlberger

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.

Reversible Photodynamic Chloride-Selective Sensor Based on Photochromic Spiropyran

We report here for the first time on a reversible photodynamic bulk optode sensor based on the photoswitching of a spiropyran derivative (Sp). The photoswitching of Sp induces a large basicity increase in the polymeric phase, which triggers the extraction of Cl– and H+. Cl– is stabilized by a lipophilic chloride-selective ionophore inside the membrane, while H+ binds with the open form of Sp and induces a spectral change, hence providing the sensor signal. The system was studied with spectroscopic and electrochemical methods.

Ultrasmall fluorescent ion-exchanging nanospheres containing selective ionophores.

We present a convenient precipitation procedure to fabricate ultrasmall fluorescent ion-selective nanosensors that operate on the basis of bulk ion-exchange sensing principles. The nanosphere matrix is composed of bis(2-ethylhexyl) sebacate (DOS) and a triblock copolymer Pluronic(®) F-127, which also functions as a surfactant to stabilize the nanoparticle. The particles can be prepared easily in large quantity without resorting to further complicated purification. Dynamic light scattering shows that these particles have a monodisperse size distribution with an average diameter of ∼40 nm, suggesting that the nanoparticles are among the smallest ionophore-based ion-selective nanosensors reported to date. A newly reported oxazinoindoline (Ox) as well as a Nile blue derivative (chromoionophore I) was used as a chromoionophore. Na(+)- and H(+)-selective nanospheres were characterized by absorbance and fluorescence spectroscopy. Owing to the very small size of the nanospheres, the suspension containing the particles is transparent. In the additional presence of the pH indicator HPTS, spectroscopic interrogation of pH and Na(+) in the same sample was demonstrated. As an example, the nanospheres were used to measure the Na(+) level in commercial mineral waters, and the results showed good agreement with atomic absorption spectroscopy (AAS).

Potentiometric sensors with ion-exchange Donnan exclusion membranes.

Potentiometric sensors that exhibit a non-Hofmeister selectivity sequence are normally designed by selective chemical recognition elements in the membrane. In other situations, when used as detectors in separation science, for example, membranes that respond equally to most ions are preferred. With so-called liquid membranes, a low selectivity is difficult to accomplish since these membranes are intrinsically responsive to lipophilic species. Instead, the high solubility of sample lipids in an ionophore-free sensing matrix results in a deterioration of the response. We explore here potentiometric sensors on the basis of ion-exchange membranes commonly used in fuel cell applications and electrodialysis, which have so far not found their way into the field of ion-selective electrodes. These membranes act as Donnan exclusion membranes as the ions are not stripped of their hydration shell as they interact with the membrane. Because of this, lipophilic ions are no longer preferred over hydrophilic ones, making them promising candidates for the detection of abundant ions in the presence of lipophilic ones or as detectors in separation science. Two types of cation-exchanger membranes and one anion-exchange membrane were characterized, and potentiometric measuring ranges were found to be Nernstian over a wide range down to about 10 μM concentrations. Depending on the specific membrane, lipophilic ions gave equal response to hydrophilic ones or were even somewhat discriminated. The medium and long-term stability and reproducibility of the electrode signals were found to be promising when evaluated in synthetic and whole blood samples.

Photoresponsive Ion Extraction/Release Systems: Dynamic Ion Optodes for Calcium and Sodium Based on Photochromic Spiropyran

Photoresponsive ion extraction/release systems (PRIONERS) represent a highly interesting tool for the localized and time-controlled chemical perturbation of biological materials. We report here on our first results on phototriggered calcium and sodium exchanging materials. Such materials exist in two distinct states (“on” and “off”), depending on the wavelength of illumination. We used a combination of spectroscopic and electrochemical methods to obtain a better understanding of the dynamic processes involved in the triggered ion-exchange reaction upon activation of the photoactive compound. The driving force for the ion exchange is the light-induced acidity change of the chromoionophore. Activation with UV light generates a species in the membrane with an increased pKa. Protons are pulled into the membrane, and at the same time, ions are expelled. The selectivity of the system is determined by the employed ionophore. In contrast to photoresponsive ionophore-based systems, the concept presented here is applicable for virtually any ion of interest for which an ionophore exists.

Magnetic sensor particles: an optimized magnetic separator with an optical window

Magnetically separable optical sensor particles represent a good alternative for conventional electrochemical and fibre-optical oxygen and pH sensors. Further improving the suitability of magnetically separable optical sensor particles, we reconstructed the separation adapters. Computer simulations of magnetic fields indicated that an assembly of radially magnetized rings is preferred. We investigated different materials and dimensions of adapters and improved commercially available, axially magnetized rings using a magnetically soft iron cone together with the rings. Adapters simulating a radially magnetized ring were constructed out of several block magnets arranged like a star around an optical fibre. These adapters collected the sensor particles exactly in the field of view ensuring high efficiency and, on top of that, showed a higher resistance of the resulting sensor spot against shear forces in stirred liquid. All adapters were characterized in a stirred flask recording signal intensity versus stirrer speed plots.

In situ surface functionalization of plasticized poly(vinyl chloride) membranes by ‘click chemistry’

We report here for the first time a universal method to achieve a covalent surface modification of plasticized poly(vinyl chloride) (PVC). A copper(I)-catalyzed azide–alkyne cycloaddition (‘click chemistry’) is performed on plasticized PVC containing partial azide substitutions. This surface modification is performed under mild conditions after membrane casting and is likely to be generally applicable to electrochemical and optical sensors. The concept is illustrated by attaching fluorescein and sulfonated Nile blue derivatives, as well as tetraethylene glycol to the membrane surface. Characterization by confocal microscopy, ATR-IR, QCM, UV/Vis spectroscopy and pulsed chronopotentiometry supports the surface modification procedure. As an initial example of practical utility, tetraethylene glycol modification is shown to significantly reduce surface adsorption by albumin, as evidenced by QCM and electrochemical experiments.

Insights in the determination of saxitoxin with fluorogenic crown ethers in water

In this contribution, we present new insights and a critical discussion in the optical detection of saxitoxin using fluorophores with crown ethers. Fluorescence enhancement is caused by the reduction of photoinduced electron transfer upon complexation with the analyte. Our attempts to improve this detection method neither did yield a functioning sensor nor were the attempts to reproduce published data in this area successful. Due to the fact that only low concentrations of saxitoxin are available, multiple surrogates were investigated at high concentrations. However, no turn on response was observed. Moreover, a fluorescent decomposition product of saxitoxin that forms under UV light was discovered which was in our opinion misinterpreted as a sensor response by previous publications.Graphical abstract

Compact and Low-Cost Fluorescence Based Flow-Through Analyzer for Early-Stage Classification of Potentially Toxic Algae and in Situ Semiquantification

The occurrence and intensity of (harmful) algal blooms (HABs) have increased through the years due to rapidly changing environmental conditions. At the same time, the demand for low-cost instrumentation has increased substantially, enabling the real-time monitoring and early-stage detection of HABs. To meet this challenge, we have developed a compact multi-wavelength fluorometer for less than 400 USD. This is possible by using readily available and low-cost optical and electronic components. Its modular design results in a highly versatile and flexible monitoring tool. The algae detection module enables a continuous identification and control of relevant algal groups based on their spectral characteristics with a detection limit of 10 cells per liter. Besides its usage as a benchtop module in the laboratory, the algae module has been integrated into submersible housings and applied in coastal environments. During its first in situ application in the Port of Genoa, seawater samples of mixed algal composition were used to demonstrate the successful discrimination of cyanobacteria and dinophytes as well-known toxin producing classes. Fabrication, operation, and performance as well as its first in situ application are addressed.

Magnetic Optical Sensor Particles for pH Measurement

Magnetic optical sensor particles (MOSePs) in nanometer scale for pH measurement were synthesized and characterized. The measurement of pH is based on a ratiometric evaluation of the fluorescence of a pH‐sensitive indicator dye (N‐fluorescein‐acrylamide) and a reference dye (a rhodamine derivative) which is not affected by pH. Measurement of pH in aqueous media in the range of pH 5 to 9 is reversibly possible. As the particles can be collected in front of fiber optics with a specially designed separator, signal enhancement is easily achieved. Therefore, a low concentration of MOSePs is sufficient for investigations. Using the here reported MOSePs, the read out of pH is noninvasive, i.e., no dip probe needs to be inserted into the medium. In addition, no further preparation steps are required, pH‐sensitive MOSePs can be simply pipetted into the medium prior to measurement, magnetically collected at the side and read out with a fiber optic device through the glass wall.