1. SYNTHESIS AND PH-SENSING PROPERTIES OF NOVEL DIHYDROXY-PHENYLHYDRAZONE
Awad Saida,b, Nikolai Georgieva, Vladimir Bojinova
aDepartment of Organic synthesis, University of Chemical Technology and Metallurgy, Kl. Ohridski Blvd. 8, 1756 Sofiq, Bulgaria
bDepartment of Chemistry, Facultyu of Science. Assiut University, Assiut, Egypt
Assiut university
Introduction
The determination of pH, although being a classical instrumental method, is still one of the challenging problems in chemical laboratories, industry, medicine, ecology, etc. pH is a key parameter in clinical analysis, food production, biotechnological process, waste water treatment procedures, environmental and life science, etc. Recent reports discussed a relation between abnormal pH values and inappropriate cell function, growth, and division observed in some common disease types such as cancer 1 and Alzheimer’s.2 Although electrochemical pH sensors are well-established and can be used as reliable tools for a large number of analytical tasks, optical pH sensors offer unmatched advantages in many other challenging applications in particular for applications where minimal contact to the sample is preferable, where a high degree of miniaturisation is required. A number of fluorescent pH sensors have already been established in which derivatives of 8-hydroxypyrene-1,3,6-trisulfonate(HPTS) 3, fluoresceins 4 and benzo[g]xanthene dyes 5 have been the most common pH-sensitive indicator dyes but the limitation of these sensors is the complex procedures for their preparation. The fluorescence molecular sensors are designed following three basic approaches; intramolecular charge transfer (ICT) 6, photo-induced electron transfer (PET) 7 and energy transfer (ET) 8.
Experimental
Unlike the reported pH sensors, our novel compound was prepared simply and quantitively (95%) in one step by condensation between 2,4-dihydroxylacetophenone and hydrazine, Scheme 1. The synthesized compound was characterized and identified by TLC (Rf = 0.28 in a solvent system petroleum ether: ethyl acetate 1:2), UV-VIS, fluorescence, FT-IR and 1H NMR spectroscopy. The pH titration was performed in dist. water.
The sensitivity of the sensor only to higher pH’s (more than 8) was interpreted by the presence of hydrogen bonding between the phenolic OH and NH2 group that make a strong base is required to the deprotonation of the sensor.
Scheme 2
Scheme 1
Results
The Structure of the product was conformed from the 1H-NMR by δ 2.89 ppm (s, 3H, CH3), 1.97 ppm (s, 2H, NH2) 6.32 ppm (d, J 2.44 Hz, 1H, H3) 6.4 ppm (dd, J 2.45; 8.88 Hz, 1H, H5) 7.6 ppm (d, J 2.8 Hz, 1H, H6) ppm (s, 1H, OH) ppm (s, 1H, OH), Figure 1.
Figure 4
Figure 5
Effect of Ionic Strength on
Fluorescence spectrum
Effect of pH on absorption spectrum
Figure 1
The flouresence pH titation of the sensor was performed at different concentration of NaCl (Ionic Strength), Figure 6. The Fluorescence of the sensor was quenched by increasing the ionic strength. The quenching was attributed to the decrease of pKa of the deprotonation of the sensor by increasing the ionic strength.
As shown in Figure 2, the UV–vis spectrum of sensor at higher pH (higher than pH 8) exhibits an absorption maximum at 420 nm which can be assigned into the ICT (intramolecular charge transfer) band of the sensor, this band was decreased sharply, accompanied by the blue-shift to 370 nm at pH 8, after then, no spectroscopic change was observed by decreasing the pH.
Figure 6
Conclusion
Figure 2
Effect of pH on Fluorescence spectrum
The novel senor achieved many advantages, it can be prepared easily and quantitively and can be used to detect selectively only basic media (˃8) by naked eye under UV lamp. Furthermore, the ability of its using in pure water that is environmentally acceptable.
Only in basic media the sensor gave a green fluorescence under U.V. lamp, so, this sensor can be used to detect basic media by naked eye, Figure 3
References
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Figure 3
The fluorescence enhancement in basic media is attributed to the deprotonation of the compound to give the deprotonated form (form B) that has a stronger intramolecular charge transfer (ICT), Scheme 2. The fluorescence of the sensor decrease sharply at pH 8, Figures 4;5, where the sensor is present predominately in the non-flourescent form A.