Understanding the performance of a paper-based UV exposure sensor: The photodegradation mechanism of brilliant blue FCF in the presence of TiO photocatalysts in both the solid state and solution.

Rationale: The decolouration of brilliant blue FCF by the action of titanium dioxide (TiO ) under ultraviolet (UV) exposure has been recently reported as the basis of a paper-based sensor for monitoring UV sun exposure. The mechanism of brilliant blue FCF photodegradation in the presence of the photocatalyst and the resulting photoproducts are thus far unknown.

Methods: The UV-initiated photodegradation of brilliant blue FCF in the presence of TiO for both the aqueous and the solid state was investigated. Degradation in the solid state was observed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). Decomposition of the dye in the aqueous state was analyzed using liquid chromatography/mass spectrometry (LC/MS) and ultraviolet-visible (UV-Vis) spectroscopy.

Results: After UV radiation exposure, the brilliant blue FCF base peak [M1 + NH ] (m/z calc. 766.194 found 766.194) decreased in the LC/MS chromatogram with a concomitant appearance of BB-FCF decomposition products involving the sequential loss of the N-ethyl and N-methylbenzene sulfonate (MBSA) groups, assigned as [M2 + H] (-MBSA, calc. 579.163 found 579.162), [M3 + H] (-MBSA, -Et, calc. 551.131 found 551.131), [M4 + H] (-2MBSA, calc. 409.158 found 409.158), [M5 + H] (-2MBSA, -Et, calc. 381.127 found 381.127). Ions [M2 + H] and [M3 + H] were also identified in the photodegradation products using MALDI-MS. Observation by UV-Vis indicated a decrease in the solution absorbance maxima and an associated blue-shift upon UV exposure in solution.

Conclusions: The LC/MS analysis indicated two main oxidation processes. The most obvious was attack of the N-methylene, eliminating either ethyl or MBSA groups. The presence of the hydroxylated decomposition product M13 ([M13 + H] , calc. 595.157 found 595.157) supported this assignment. In addition, the detection of photoproduct M8, proposed to be 3-((ethylamino)methyl)benzenesulfonic acid ([M8 + H] , calc. 216.069 found 216.069), indicates an aryl-oxidative elimination. The absence of the aryl-hydroxy products normally expected to accompany the formation of M8 is proposed to be due to TiO -binding catechol-like derivatives, which are then removed upon filtration.