Understanding the performance of a paper-based UV exposure sensor: The photodegradation mechanism of brilliant blue FCF in the presence of TiO2 photocatalysts in both the solid state and solution.

Abstract

The decolouration of brilliant blue FCF by the action of titanium dioxide (TiO2 ) 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. The UV-initiated photodegradation of brilliant blue FCF in the presence of TiO2 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. After UV radiation exposure, the brilliant blue FCF base peak [M1 + NH4 ]+ (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. 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 TiO2 -binding catechol-like derivatives, which are then removed upon filtration.