Background-free fluorescence decay time sensing and imaging of pH with highly photostable diazaoxotriangulenium dyes

Novel fluorescent diazaoxatriangulenium (DAOTA) pH indicators for lifetime-based self-referenced pH sensing are reported. The DAOTA dyes were decorated with phenolic receptor groups inducing fluorescence quenching via photoinduced electron transfer mechanism. Electron-withdrawing chlorine substituents ensure response in the most relevant pH range (apparent pK'a values ~5 and 7.5 for the p,p-dichlorophenol- and the p-chlorophenol-substituted dyes, respectively). The dyes feature long fluorescence lifetime (17-20 ns), high quantum yield (~60%) and high photostability. Planar optodes are prepared upon immobilization of the dyes into polyurethane hydrogel D4. Apart from the response in the fluorescence intensity, the optodes show pH-dependent lifetime behaviour which makes them suitable for studying 2D pH distribution with help of fluorescence lifetime imaging technique. The lifetime response is particularly pronounced for the sensors with high dye concentration (0.5-1% wt. in respect to the polymer) and is attributed to efficient homo-FRET mechanism

Porphyrin based metal-organic frameworks with record sensitivity in optical oxygen sensing

The optical oxygen sensing capabilities of the porphyrin-based metal-organic frameworks, PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224 were investigated. The bimolecular quenching constants (kq) of 37000 (PCN-224), 6700 (Pd(II)PCN-224) and 3900 Pa-1s-1 (Pt(II)PCN-224) were found and reveal an exceptionally high oxygen-permeability for these materials. A fast gas transport within the network, large pore sizes, electronic and spatial isolation of the porphyrin indicator in the framework are held responsible for the unprecedentedly high kq values. PCN-224 shows 6.7 ns fluorescence lifetime and the fluorescence in air is quenched by 4.2-fold. The metal-organic frameworks based on phosphorescent Pt(II) and Pd(II) porphyrins possess significantly longer decay times of 18.6 and 390 µs, respectively, and are suited to detect oxygen in trace and ultra-trace ranges with limits of detection of 1 and 0.015 Pa, respectively. Apart from free-standing crystals, also metal-organic frameworks supported on different fibrous substrates (poly(acrylonitrile) nanofibers, glass fibres), and flat substrates (TLC silica-gel, poly(amide) filter) were prepared in order to provide oxygen sensor materials of practical use. Electrospun and thermally treated poly(acrylonitrile) nanofibers were proven to be particularly favourable and the resulting composite material exhibited the same sensitivity as the free crystals. All sensing materials show reversible cross-talk to humidity at levels up to 53 % relative humidity but demonstrate a drastic decrease of oxygen sensitivity at high humidity levels and when exposed to water

Optical Oxygen Sensors Show Reversible Cross-Talk and/or Degradation in the Presence of Nitrogen Dioxide

A variety of luminescent dyes including the most common indicators for optical oxygen sensors were investigated in regard to their stability and photophysical properties in the presence of nitrogen dioxide. The dyes were immobilized in polystyrene and subjected to NO2 concentrations from 40 to 5500 ppm. The majority of dyes show fast degradation of optical properties due to the reaction with NO2. The class of phosphorescent metalloporphyrins shows the highest resistance against nitrogen dioxide. Among them, palladium(II) and platinum(II) complexes of octasubstituted sulfonylated benzoporphyrins are identified as the most stable dyes with almost no decomposition in the presence of NO2. The phosphorescence of these dyes is reversibly quenched by nitrogen dioxide. Immobilized in various polymeric matrices, the sulfonylated Pt(II) benzoporphyrin demonstrates about one order of magnitude more efficient quenching by NO2 than by molecular oxygen. Our study demonstrates that virtually all commercially available and reported optical oxygen sensors are likely to show either irreversible decomposition in the presence of nitrogen dioxide or reversible luminescence quenching. They should be used with extreme caution if NO2 is present in relatively high concentrations or it may be generated from other species such as nitric oxide. As an important consequence of nearly anoxic systems, production of nitrogen dioxide or nitric oxide may be therefore erroneously interpreted as an increase in oxygen concentration