The concept of an optical chemical sensor based on an immobilised solvatochromic dye has been demonstrated and a laboratory sensor for water in organic solvents has been produced.

Solvatochromic dyes change colour according to the polarity of the liquid in which they are dissolved. They have been proposed as indicators of solvent composition, but in order to make a practical sensor such a dye must be immobilised in such a way as to retain solvatochromic behaviour, and its colour measured in a reliable manner. Reichardt's dye is a phenolbetaine which shows very strong negative solvatochromism, with the structure (R1-R5 = phenyl):

In this project solvatochromic dyes of this class have been synthesised, covalently immobilised onto silica or polystyrene, and changes in colour in response to changes in solvent or to impurities in solvent have been measured. The measurements have been made using chromatic modulation, in which the signals from multiple (in this case three) detectors of broad but overlapping spectral response are used to generate a unique chromaticity coordinate for each colour measured. Better resolution of small changes in colour is possible with this approach than with measurement of l_max by diffuse reflectance spectroscopy.

Immobilisation has been achieved through a link from the 4 position of R3 to the solid support, and the spectral response of the dye has been modified by varying the substituents R1,R2,R3, and R4.

A prototype sensor has been constructed and its characteristics for water measurement in butyl acetate, fluorobenzene and ethyl benzene investigated. The results demonstrate the possibility of an in-line continuous monitor to replace discrete measurements by Karl Fischer titration.

Reference: Russell, PC, Jones, GR, Crowther, D, Jones, M, Ahmed, SU and Huggett, P (1996) "A chromatic sensor for detecting water in organic solvents" Proc. Opto '96, 145-150. ACS Orgs. GMBH, Wunstorf, Germany.

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