Inner-filter-induced bias in fluorescence-based hydroxyl radical dosimetry using terephthalic acid.

Fluorescence-based terephthalic acid (TA) dosimetry is widely employed for the quantitative determination of hydroxyl radicals (OH) in aqueous systems. The method implicitly assumes that the optical properties of the sample remain constant during analysis and that the detected fluorescence originates exclusively from 2-hydroxyterephthalic acid (2-hTA). Here, we demonstrate that this assumption can break down under plasma-liquid interaction conditions, leading to substantial analytical bias. We show that plasma exposure of TA solutions generates light-absorbing by-products with low fluorescence quantum yield, which induce pronounced primary and secondary inner filter effects. These effects attenuate excitation at 310 nm and reabsorb emission at 425 nm, causing systematic suppression of the fluorescence signal used for OH quantification. As a result, OH radical yields are significantly underestimated even during early reaction stages commonly assumed to be free from optical artifacts. Spectroscopic analysis combined with excitation-emission mapping and high-resolution transmission electron microscopy confirms that plasma-generated carbonaceous nanostructures contribute significantly to the UV-visible absorption of the system and represent a confirmed class of interferents responsible for the observed distortion. These findings reveal a fundamental limitation of fluorescence-based TA dosimetry in plasma-treated systems and highlight the necessity of accounting for inner filter effects or employing complementary diagnostic strategies to ensure reliable quantitative interpretation of fluorescence-based radical assays.