On 22. August 2019 Sigrid Selberg will defend her doctoral thesis „Synthesis and properties of lipophilic phosphazene-based indicator molecules“).
Professor Ivo Leito (PhD), UT
Senior research associate Borislav Kovačević (PhD), Ruđer Bošković Institute, Zagreb, Croatia
The aim of this thesis was to design and synthesize a family of novel lipophilic phosphazene-based indicators – active both via visible absorption and fluorescence – and to create an original method for quantifying acidity/basicity of lipophilic molecules in practically water-immiscible solvents (biphasic pKa values). It is all about the purpose to offer an alternative pH determination method to the electrochemical pH sensing, since there are some drawbacks related to the electrochemical method. The appropriate alternative pH determination method is optical pH sensing. Optical pH sensor requires one or several different “work-horses”, to be more specific, the indicator molecules. There are many different pH indicators known to mankind, common examples are phenolphthalein, methyl orange, thymol blue, alizarin yellow etc. There is one big “but”: these known indicators are usually hydrophilic and have localized charges in their protonated/deprotonated forms. Therefore these indicators are not perfect for designing an optical pH sensor, since indicators are generally embedded in a non-polar polymer media. To put it simply, hydrophilic molecules do not sit very happily in a non-polar media and they leave it as soon as possible, leaving the sensor without its “work-horse”.
16 novel phosphazene-bases have been synthesized and fully characterized in this work. It is shown that these molecules have all the desired and suitable properties to employ them in the design of optical pH sensors. These molecules are sensitive to the pH, suitable to use in environments with pH ˃ 7, the both forms are highly lipophilic and can act as "silent spectators" in monitoring the acidity/basicity of different low-polarity systems.