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

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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 (NaCaY1−xLax(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.

Original languageEnglish
Pages (from-to)972-979
Number of pages8
JournalSensors and Actuators, B: Chemical
Publication statusPublished - 2017



  • Inner-filter effect
  • Inorganic phosphor
  • Luminescence
  • Optical sensor
  • pH
  • Upconversion

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Instrumentation
  • Condensed Matter Physics
  • Surfaces, Coatings and Films
  • Metals and Alloys
  • Materials Chemistry
  • Electrical and Electronic Engineering

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