Harnessing PEI-Templated Aggregation of Bisindolyl Methane Derivatives for Dual-Mode Ratiometric Detection of Ascorbic Acid in Fruit Samples and LbL-Assembled Paper Strips

Owing to the high amine density and functional versatility of PEI, we synthesized a bisindolyl methane (BIM) based PEI composite (probe 1-PEI). This biobased smart material was employed as a ratiometric dual-mode colorimetric and fluorescent sensor for ascorbic acid (AA) in aqueous media. Initially, we elucidated the systematic assembly of luminescent BIM chromophores onto the PEI backbone, resulting in templated aggregates, meticulously regulated by non-covalent interactions. The probe 1-PEI composite displayed enhanced fluorescence (∼9.2-fold) upon exposure to AA, whereas the free probe exhibited a negligible response. This enhancement in sensitivity is attributed to the formation of highly ordered, long-range aggregates within the polymer composite, facilitating amplified fluorescence response, while the free probe forms spontaneous aggregates, thereby hindering the effective interaction with AA. Mechanistic investigations suggest that hydrogen bonding and electrostatic interactions between the probe 1-PEI composite and AA induce a change in the nature of the resulting aggregate, accompanied by changes in the microenvironment and charge transfer state of the probe, which collectively drive the observed spectral responses. Furthermore, to demonstrate real-world applicability, the probe 1-PEI composite was incorporated into a Layer-by-Layer (LbL)-coated cellulose paper-based sensor (sustainable approach), enabling on-site AA detection. This eco-friendly sensor remained stable for over a week and exhibited high selectivity against competing analytes, with clear visual detection. By integrating biocompatible components and eliminating the need for synthetic dyes or toxic reagents, this work underscores the potential of the probe 1-PEI composite as a sustainable, highly sensitive, and selective AA sensor (LOD = 0.18 μM) for environmental and analytical applications.