Hypochlorite (ClO^–) is widely used for sterilization, disinfection, and bleaching due to its strong oxidative properties. However, prolonged exposure to ClO^– can pose health risks, and excessive discharge can lead to environmental pollution.

Calcium hypochlorite, which is used to produce potassium or sodium perchlorate for improvised explosives, underscores the urgency for rapid and sensitive identification of ClO^– in public health and environmental monitoring.

Current sensing methods for ClO^– face several challenges, including low molar absorption coefficients, narrow Stokes shifts, and poor sensitivity, which limit their effectiveness in complex environments.

Chemodosimeters, which detect analytes through irreversible chemical reactions, are gaining popularity because of their selectivity, high sensitivity, rapid response, and simple design. Molecules with a D-π-A structure are particularly promising, as they can enhance optical sensing signals by modifying their π conjugation or electronic properties, thus improving visualization in the detection of ClO^–.

To tackle the ClO^– sensing issue, a research team led by Prof. Dou Xincun from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (CAS) has developed a strategy for a triple-standard hypochlorite quantitative array, enabled by precise modulation of the Stokes shift in D-π-A chemodosimeters.

Their findings, published in Analytical Chemistry, highlight the importance of tuning the electron-releasing capability of the D-π-A chemodosimeters to enhance the reactivity of the recognition site, improve Stokes shifts, and increase the response speed and sensitivity towards the target analyte.

In this study, the researchers synthesized a series of D-π-A fluorescent chemodosimeters (PA-TCF, DPA-TCF, TPA-TCF) based on the Claisen−Schmidt coupling reaction.

They utilized 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (TCF) as the electron-withdrawing group, precisely modulating the electron-releasing strength (−PA > −DPA > −TPA) to generate an unsaturated double bond as the ClO^– recognition site.

It was found that as the electron-releasing capability decreased, the fluorescence intensity of the chemodosimeters diminished due to π electron delocalization and π–π stacking within the conjugated system.

The electrophilicity of the recognition site increased by 1.449 kcal/mol, resulting in a change in the sensing mode from fluorescence quenching to ratiometric fluorescence and eventually to fluorescence enhancement. The Stokes shift of the chemodosimeter was improved to 201 nm, enhancing the resolution of optical signal changes visible to the naked eye.

Furthermore, all three D-π-A fluorescent chemodosimeters demonstrated superior sensing performance for ClO^–, including low limits of detection (LOD) at 37.0, 5.1, and 1.0 nM, rapid response times of less than five seconds, and excellent selectivity in the presence of 16 different interferents.

Additionally, considering the varying performance of the chemodosimeters, the researchers developed a portable triple-standard quantitative array detection platform to validate the practical applicability of the chemodosimeter modulation strategy, enabling rapid, on-site, and quantitative detection of ClO^– in solution, with error rates ranging from 9.5% to 13.75%.

This innovative design and regulation strategy for chemodosimeters is expected to provide new solutions for enhancing sensitivity and rapid identification of oxidants, as well as advanced methodologies for detecting trace hazards.