Photostable upconverting and downconverting pH sensors based on combination of a colorimetric NIR indicator and stable inorganic phosphors as secondary emitters

New robust optical pH sensors rely on a combination of a photochemically stable NIR colorimetric aza-BODIPY dye and inorganic phosphors. Inner-filter effect is utilized to convert the colorimetric changes into a luminescence response. Chromium(III)-activated gadolinium aluminium borate and Egyptian Blue were used as inert reference phosphors, enabling two-wavelength ratiometric read-out in the emission and excitation mode, respectively. Additionally, we synthetized dually-emitting upconverting particles (UCPs) based on lanthanide-doped molybdates (NaCaY_1−xLa_x(MoO₄)₃, La = Yb(III), Ho(III) and Tm(III)) featuring anti-Stokes emission peaks in the red/NIR spectral region (645/800 nm). Upon NIR excitation (980 nm) the NIR emission of the upconverting particles is modulated by the pH indicator, whereas the red emission serves as a reference. Importantly, the spectral properties of the pH sensor based on UCPs are fully compatible with the biological optical window.