Fluorescent sensors phosphates

The design of fluorescent sensors is of major importance because of the high demand in analytical chemistry, clinical biochemistry, medicine, the environment, etc. Numerous chemical and biochemical analytes can be detected by fluorescence methods cations (H+, Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Pb2+, Al3+, Cd2+, etc.), anions (halide ions, citrates, carboxylates, phosphates, ATP, etc.), neutral molecules (sugars, e.g. glucose, etc.) and gases (O2, CO2, NO, etc.). There is already a wide choice of fluorescent molecular sensors for particular applications and many of them are commercially available. However, there is still a need for sensors with improved selectivity and minimum perturbation of the microenvironment to be probed. Moreover, there is the potential for progress in the development of fluorescent sensors for biochemical analytes (amino acids, coenzymes, carbohydrates, nucleosides, nucleotides, etc.). 

Another important target for fluorescent sensors are highly toxic phosphates, found in various pesticides and chemical warfare agents, commonly known as nerve gases (Figure 16.16) [47]. There are two main families of these chemical warfare agents G and V family. Hie former contains electrophilic substituent at phosphorus atom... 

Yoon, Kim and co-workers have reported a highly effective fluorescent sensor for dUiydrogen phosphate based on a 1,8-disubstituted-anthracene-dimer macrocycle bridged by two imidazolium subimits (68) [81]. 

The first fluorescent sensors for saccharides were based on fluorophore appended boronic acids. Czarnik showed that 2- and 9- anthrylboronic acid [36, 37] (1 and 2) could be used to detect saccharides (Figure 12.2). With these systems, the negatively charged boronate has a lower fluorescence than the neutral boronic acid. Since the pFC of a boronic acid is lowered on saccharide binding, the fluorescence of these systems at a fixed pH decreases when saccharides are added. The observed stability constant (f pp) for 1 was 270 with D-fructose at pH 7.4 (phosphate buffer). 

A binuclear system (94) has been found to selectively recognise PPi (pyrophosphate, 20 ) over structurally similar phosphate ATP and other anions by creating a strong binding in the complex (95), leading to a fluorescent sensor at pH 7.4 in water (Scheme 24). 

The carrier used for this purpose consisted of a 0.1 M phosphate buffer of pH 7. The appearance of the sensing microzone is shown in Fig. 5.5.B. The oxygen optrode used was based on a 10-pm silicone rubber film containing dissolved decacyclene as indicator (S) that was fixed on a 110-pm thick polyester support (PS). A 9-pm black PTFE membrane (I) was used for optical insulation. The dye fluorescence was found to be markedly dependent on the concentration of oxygen, which exerted a quenching effect on it. The enzyme (glutamate oxidase) was immobilized on a 150-pm thick immunoaffmity membrane (E). The sensor was prepared similarly as reported by Trettnak et al. [7]. 

OF optical fibers, IOS integrated optical sensors, A absorbance, R reflectance, F fluorescence, ev evanescent wave, ISP isopropyl alcohol, DOS bis(2-ethylhexyl)sebacate, o - NPOE ortho-nitrophenyl octyl ether, TOP tris(2-ethylhexyl)phosphate... 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

Figure 6. Cumulative Release of HPTS from EVA Microparticles. Sensors were fabricated and soaked in pH 7.8 phosphate buffer. The release of HPTS from microparticles entrapped in polyacrylamide on sensor tips was monitored by measuring the increase in fluorescence intensity over 300 hours.

The sensor developed by Gawley and co-workers is based on de Silva s modular approach comprises an azacrown ether linked to a fluorophore by a short link. The molecule combines the aza[18]crown-6 recognition element with a fluorescent coumaryl group attached by a methylene spacer. In tests it was shown to bind to saxitoxin with a binding constant on the order of 105 M 1, even in a phosphate buffer at physiological pH and concentrations of sodium and potassium, and could detect levels of the toxin down to 1(T7 M. Subsequent modification of the sensor allowed it to be linked to the surface of a quartz slide to allow the fluorescent response to concentrations of saxitoxin between 10-4 and 10-6 M to be detected via a fibre optic system. This level of sensitivity is comparable to the current mouse bioassay that requires the inoculation of a large number of animals to determine the concentration of saxitoxin present in the test sample [25],... 

Figure 16,12 Molecular structures of selected PET (photoinitiated electron transfer)-based fluorescent phosphate sensors...

Figure 16.13 Structures of Ru(bpy)3-based fluorescent phosphate and phosphoric acid ester sensors...

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