Aleksandra Lobnik

pH optical sensors based on sol–gels: Chemical doping versus covalent immobilization

Abstract pH sensitive fluorescent sol–gels were obtained by both covalent immobilization of aminofluorescein (AF) via isocyanate or epoxy groups, and by co-condensation of tetramethoxysilane (TMOS) and phenyltrimethoxysilane (Ph-TriMOS) in the presence of AF. The gel precursors were deposited on glass supports, cured, and characterized in terms of response to pH, p K a values, effects of ionic strength, response time, leaching, long-term stability, and photostability. The addition of Ph-TriMOS is found to exhibit a pronounced effect on the performance of the materials. Sensor layers based on TMOS doped with AF were found to be most appropriate for purposes of sensing pH in giving large relative signal changes and displaying rapid response times over the pH range 4–9.

Sol-gel based optical sensor for dissolved ammonia

Abstract Ormosils were prepared in different ratios of tetramethoxysilane and organically modified sol-gel precursors of type R′Si(OR) 3 and R′ 2 Si(OR) 2 . The pH indicator aminofluorescein was incorporated into various ormosils and the resulting materials were tested for their relative response to pH and dissolved ammonia at constant pH. R′ 2 Si(OR) 2 -based layers were recognized as the most promising materials because of their high permeability to ammonia, impermeability to ions (protons), long operational lifetime and good photostability.

Detection of nanomolar concentrations of copper(II) with a Tb-quinoline-2-one probe using luminescence quenching or luminescence decay time.

We present a time-resolved (gated) luminescence-based method for determination of Cu2+ ions in microtiterplate format in the nanomolar concentration range using the novel long-lived terbium-[1-methyl-4-hydroxy-3-(N-2-ethyl-5-aminothiadiazolyl-)-carbamoyl-quinoline-2-one] (TbL) complex. The probe works best in Tb:L = 1:2 stoichiometry at neutral pH. The dynamic range is from 10 to 300 nmol L(-1) of Cu2+ and the limit of detection is 4.3 nmol L(-1). This is the lowest limit of detection achieved so far for luminescent lanthanide-based probes for copper. It is shown that gating can efficiently suppress intense, short decaying background fluorescence e.g. that of Rhodamine 6G. The assay can be performed by measurement of luminescence decay time, as well. Stern-Volmer studies indicate that static quenching dominates over dynamic quenching. TbL2 was tested for the effect of some relevant interferents and the assay was applied to the determination of copper in tap water samples. The results achieved were in good agreement with those of a reference method.

Probing the Polarity of Sol-Gels and Ormosils via the Absorption of Nile Red

Conventional sol-gels are rather hydrophilic. A more hydrophobic material is obtained preparing organically modified siloxanes (ormosils). The polarity-sensitive probe Nile Red (NR) was doped in various sol-gels to probe their micro-polarity. The experiments show that the NR is an excellent probe and sensitive to the polarity of its microenvironment. Spectroscopic studies reveal remarkable changes in the absorption band positions and intensities as a function of the polarity of the sol-gel, which depends on the different precursors used. Furthermore, sol aging, gelation and temporal stability as a function of different ormosils have been investigated.

Fluorescent potential-sensitive dyes for use in solid state sensors for potassium ion

Fluorescent potential-sensitive dyes (PSDs) were screened for use in potassium-sensitive optical sensor membranes. A potassium-sensitive plasticized PVC layer containing valinomycin as an ion carrier, lipophilic borate salt as an anionic additive, and a PSD was chosen as a model for evaluating the applicability of PSDs. A commercial carbocyanine dye (3,3′-dihexadecyloxacarbocyanine perchlorate) was found to exhibit the best properties in terms of signal changes, photostability and operational life-time. The sensor membrane responds reversibly to potassium ion, with fluorescence intensity changes exceeding 50% and response times being of the order of 1 min. The response to potassium is slightly pH dependent. Typically, an 8% change in intensity is observed over the range pH 5–8. The new sensor membrane exhibits significantly improved signal changes compared to previous optodes based on related sensing schemes. We also report the effects of different plasticizers and lipophilic additives on the response of the layers. The response mechanism is discussed with respect to morphology of the membrane.

Removal of Pb(II) ions from aqueous systems using thiol-functionalized cobalt-ferrite magnetic nanoparticles

We have investigated the removal of Pb2+ ions from water using thiol-functionalized, cobalt-ferrite, magnetic nanoparticles. These magnetic nanoparticles were prepared using the co-precipitation method and their surfaces were modified with tetraethoxy silane and 3-mercaptopropyl trimethoxysilane in order to provide a sufficient surface concentration of the thiol (–SH) functional groups. The adsorption of the Pb2+ ions from the aqueous solutions onto the thiol-functionalized, cobalt-ferrite, magnetic nanoparticles was studied. The investigated parameters include the pH value of the model water, the concentration of the adsorbent, the contact time and the temperature of adsorption. The removal of the Pb2+ was found to be greater at the higher pH values and increasing the temperature was also found to increase the removal of Pb2+ ions.

Polarity studies on ormosils using a solvatochromic fluorescent probe

Ormosils (organically modified siloxanes) are a relatively new family of materials, prepared by the sol–gel method, with properties that are intermediate between those of glasses and polymers. To probe the micropolarity of various solvents and ormosils, a ketocyanine dye (KC) with unique solvatochromic properties in fluorescence was used. KC is an excellent probe and is sensitive to the polarity of its microenvironment. Fluorescence studies revealed remarkable changes in the fluorescence band positions or intensities as a function of the polarity of the ormosil, which depend on the different ormosil precursors used. Storage stability was also investigated.

Optical Chemical Sensors:Design and Applications

Optical sensors, or opt(r)odes, represent a group of chemical sensors in which electromagnetic (EM) radiation is used to generate the analytical signal in a transduction element. The interaction of this radiation with the sample is evaluated from the change of a particular optical parameter and is related to the concentration of the analyte (Blum, 1997). Typically, an optical chemical sensor consists of a chemical recognition phase (sensing element or receptor) coupled with a transduction element (Fig. 1). The receptor identifies a parameter, e.g., the concentration of a given compound, pH, etc., and provides an optical signal proportional to the magnitude of this parameter. The function of the receptor is fulfilled in many cases by a thin layer that is able to interact with the analyte molecules, catalyse a reaction selectively, or participate in a chemical equilibrium together with the analyte. The transducer translates the optical signal produced by the receptor into a measurable signal that is suitable for processing by amplification, filtering, recording, display, etc. (Grundler, 2007; Nagl & Wolfbeis, 2008).

Microtiterplate phosphate assay based on luminescence quenching of a terbium complex amenable to decay time detection

We describe a terbium-ligand complex (TbL) for a microtiterplate assay for phosphate (P) in the 0.3-100 micromol L(-1) range based on luminescence quenching. As the pH optimum is at neutral pH (7.4) the probe is quenched by both, primary (H(2)PO(4)(-)) and secondary phosphate (HPO(4)(2-)). The LOD is 110 nmol L(-1). A Stern-Volmer study revealed that quenching is mostly static. Due to the ms-decay time of TbL, the first luminescence lifetime assay for phosphate could also be developed. The lifetime-based calibration plot is linear between 0.5 and 5 micromol L(-1) of P. The effect of various surfactants on assay performance and a study on interferents are presented. The probe was successfully applied to determination of P in commercial plant fertilizers and validated against the molybdenum blue test. The probe is the most sensitive lanthanide-based probe for phosphate.

Reduction of Biocide-Polluted Wastewater Using O3 and H2O2/O3 Oxidation Processes

The objective of the presented study was to test various oxidation processes with the aim being to reduce the concentration and toxicity of biocide wastewater from a Slovenian phytopharmaceutical factory. Laboratory-scale experiments employing two AOP processes – ozonation (O3) and peroxone (H2O2/O3) – were applied to reduce the concentration of the active components involved, i.e., methylisothiazolone (MI), chloromethylisothiazolone (CMI) and dichloromethylisothiazolone (DCMI). The reduction of the biocide wastewater load for the performed oxidation processes was evaluated using ecological parameters. The H2O2/O3 oxidation procedure using an O3 flow rate of 1g/L h, at a pH value of 10 and with the addition of 5 ml of H2O2 (0.3 M) proved to be the most effective treatment. The toxicity of the biocide-load wastewater with an initial EC50 = 0.38%, decreased to EC50 (24h) >100% and EC50 (48h) = 76%.