Colourimetric redox-polyaniline nanoindicator for in situ vesicular trafficking of intracellular transport

Vesicular pH modulates the function of many organelles and plays a pivotal role in cell metabolism processes such as proliferation and apoptosis. Here, we introduce a simple colorimetric redox-polyaniline nanoindicator, which can detect and quantify a broader biogenic pH range with superior sensitivity compared to pre-established trafficking agents employing one-dimensional turn-on of the fluorescence resonance-energy-transfer (FRET) signal. We fabricated polyaniline-based nanoprobes, which exhibited convertible transition states according to the proton levels, as an in situ indicator of vesicular transport pH. Silica-coated Fe₃O₄-MnO heterometal nanoparticles were synthesised and utilised as a metal oxidant to polymerise the aniline monomer. Finally, silica-coated polyaniline nanoparticles with adsorbed cyanine dye fluorophores Cy3 and Cy7 (FPSNI_Cy3 and FPSNI_Cy7) were fabricated as proton-sensitive nanoindicators. Owing to the selective quenching induced by the local pH variations of vesicular transport, FPSNI_Cy3 and FPSNI_Cy7 demonstrated excellent intracellular trafficking and provided sensitive optical indication of minute proton levels.