Darkroom-Free pH Planar Optode via Image Color Correction for Portable Imaging of Extreme Acidity

Abstract Classical pH planar optodes (POs) rely on a darkroom to reduce analytical interference from ambient light fluctuations, a requirement that severely limits portability and field applicability. To overcome this, we report a darkroom-free PO strategy integrating three key components: (i) a pixel-level image color correction (CC) technique to compensate for lighting and device variations; (ii) a robust quantitative parameter Huev, which outperforms conventional RGB ratios in stability against exposure and photobleaching; and (iii) a Congo red-nylon sensing film with a multicolor reflective-light response, tailored to the requirements of both the CC workflow and the Huev parameter. The as-prepared PO-sensing film exhibits a blue-to-red color transition in the extremely acidic pH range of 0.5−1.5. While a pH imaging method covering this range was previously reported, our method shows markedly improved sensitivity in this range. Following CC, the mean pH analytical error for aqueous solutions under varying light sources and imaging devices was reduced to 0.087 ± 0.055, compared with 0.504 ± 0.420 without CC. In soil pH imaging, the corrected pH maps acquired under multiple lighting and camera conditions exhibit high consistency, with maximal deviations in pH depth profiles of ∼0.2 pH units. Only a smartphone, a color card, and a white light source (e.g., sunlight) are required to construct the imaging system, dramatically lowering the cost and technical barriers for field deployment. This work not only achieves high-sensitivity PO for extremely acidity detection but also provides a generalizable strategy for developing darkroom-free PO for other pH intervals and target analytes.