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Development of Reaction-based Fluorescent Probes for Metal Species

Development of Reaction-based Fluorescent Probes for Metal Species
Mithun Santra
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Section I. “Turn-on” fluorescent probes for mercury and methylmercury Species Mercury is a highly poisonous element and widespread pollutant, occurring from natural and anthropogenic sources. Organic forms of mercury, typically, methylmercury (CH3HgX, X = halides, etc) are much more toxic than inorganic mercury species (HgX2). Because of their lipid solubility, methylmercury readily crosses the blood-brain barrier, accumulates in the brain, and causes damage in the central nervous system in addition to other organs. The epidemics of Minamata Bay in Japan manifested the fatal threat of methylmercury to human health. Therefore, there is a great imputes to develop sophisticated/efficient analytical tools to detect and quantify the mercury ions. Recently, fluorescent detection technique has become quite popular compare to other analytical techniques because of its operationally simplicity and cost-effective detection method together with high selectivity and sensitivity. Therefore development of highly selective, sensitive fluorescent chemosensors are demanded to detect mercury ions as well as methyl mercury ions in living cells in presence of other metal ions. We have devised a structurally simple yet efficient fluorescent probe for organomercury as well as inorganic mercury ions. The vinyl ether probe shows specific response to the mercury species and turn-on fluorescence with high sensitivity. Here the vinyl ether undergoes mercury ion-promoted hydrolysis (oxymercuration reaction) to give the strongly fluorescent compound. With this probe, the fluorescent monitoring of mammalian cells and organisms incubated with organomercury species has been demonstrated for the first time. Section II. Reactive ratiometric probes for mercury ions Our continuing efforts to develop versatile molecular probes with several useful features such as environmental monitoring, and easy-to-use quantification of mercury species have led us to investigate a ratiometric version of the reactive probe for mercury species. Ratiometric method measuring the ratio of fluorescence intensities at two wavelengths provides an alternative approach, which can overcome the drawbacks of intensity-based measurements. It can increase the selectivity and sensitivity of a measurement and eliminate most or all of the possible variability because the ratio of the fluorescent intensities at two wavelengths is independent of the concentration of the sensor, the fluctuation of source light intensity, and the sensitivity of the instrument. Herein, we have developed a ratiometric fluorescent probe for mercury species based on the metal-promoted hydrolysis of a vinyl ether derivative of 2-(benzothiazol-2-yl)phenol in a buffer solution. Vinyl-ether N-(3-(benzo[d]thiazol-2-yl)-4-(vinyloxy)phenyl)acetamide 1 was found as an optimal probe for mercury species as it shows better reactivity, solubility, and photophysical properties compared with the others derivative of 2-(benzothiazol-2-yl)phenol. The probe responses selectively to mercury species over various other metal ions with a marked fluorescence change from blue to cyan through the excited state intramolecular proton transfer (ESIPT) process. Section III. “Turn-on” fluorescent probes for palladium species In chemical transformations, palladium plays a key role as a catalyst in the synthesis of various molecules including drugs. It is also widely used to prepare dental materials, electric equipments, automobile exhaust catalysts, and fine jewellery. However, the palladium metal complexes influence our health and environment in an adverse way because palladium ions can bind to thiol-containing amino acids, proteins (casein, silk fibroin, and many enzymes), DNA or other macromolecules (vitamin B6) and thereby may disturb a variety of cellular processes. Therefore, it is desirable to develop efficient methods for detecting the presence of palladium species. Palladium complexes exhibit oxidation states of 0, +2, and +4 as in the typical cases of Pd(PPh3)4, PdCl2 and PdCl62– respectively. Among all the palladium species, PdCl2 is the most toxic one. We have devised a fluorescein-based propargyl ether derivative 1 which is found to detect palladium species selectively at room temperature and shows a turn-on type fluorescent response. Here the propargyl ether derivative undergoes palladium catalyzed hydration pathway. Significantly, the probe senses palladium species in all the typical oxidation states (0, +2, and +4) without adding any additional reagents. With this probe we have also monitored the accumulated palladium species in living organisms. The results demonstrate for the first time that probe 1 is potentially useful for monitoring palladium species in living organisms. Part II. Development of Organoboron Fluorescent Complexes To date, significant endeavor has been committed to the development of new fluorescent dyes for applications as labels in biomedical analysis, molecular sensors, and light emitting devices, nanoscience, biological chemistry, and solar energy conversion. Some of these dyes show efficient photoluminescence and ambipolar charge transport properties. An ideal fluorescent compound should satisfy several criteria
high absorption coefficients and quantum yields, a large stokes shift, tunable absorption/emission profiles, and high chemical and photochemical stability, solubility in various solvents without forming nonemissive aggregates, low toxicity if in vivo analysis is planned. Few compounds, or families of compounds, satisfy all of these categories and thus, there are many dyes available for researchers to choose from. Despite of this abundance, new dyes with a precise set of properties are still of considerable interest to the researcher. BODIPY is one of the families of dye that has been used extensively, is the dipyrrins rigidified by boron difluoride. The dye has outstanding optical properties, extraordinary chemical versatility, and therefore, it has been used extensively in various purposes. Herein, we have devised the boron complexes based on hydroxyl benzothiazole using boron trifluoride in presence of triethyl amine. The complex 1 shows red fluorescent property, whereas complex 2 emits in the blue region. The photophysical properties for both the complexes have been measured in solid state and also in solution state. Blue fluorescent complex 2 shows high quantum yield (0.98) in solution. Both the complexes are finally characterized by X-ray crystal diffraction. We have carried out theoretical calculation using Sparton 08 software. Their experimental observation (photophysical properties) and theoretical values are relatively same.
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