Organic frameworks-based optical sensors for uranyl ions: Unveiling mechanisms and applications

Abstract

Uranyl ions (UO₂²⁺), significant nuclear contaminants, pose severe risks to ecosystems and human health. Although conventional detection techniques such as radiochemical analysis and instrumental methods provide high accuracy, they are often hindered by large equipment size and prolonged analysis time, limiting their suitability for rapid environmental screening and emergency scenarios. In response, optical sensing technologies have attracted significant attention due to their high sensitivity and signal visibility. Among them, organic frameworks—including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs)—offer suitable platforms for constructing high-performance UO₂²⁺ optical sensors, owing to their high surface areas, tunable pores, and tailorable optical properties. This review systematically summarizes advances from 2021 to 2025 in UO₂²⁺ optical sensors based on such materials, with a focus on underlying sensing mechanisms. In fluorescence sensing, “turn-off” designs based on photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), inner filter effect (IFE), and charge transfer (CT) mechanisms are detailed, along with refined “turn-on” and self-calibrating ratiometric sensors. Beyond fluorescence, the review also addresses colorimetric sensing via nanozyme activity, electrochemiluminescence (ECL) sensing using organic frameworks as co-reaction promoters, and surface-enhanced Raman spectroscopy (SERS) and X-ray fluorescence (XRF) techniques enhanced by substrate preconcentration. While laboratory-scale detection has reached high sensitivity and selectivity, real-world applications remain challenging due to material instability in complex media, slow mass transfer, and difficulties in device integration. Future development should prioritize stable composites, multimodal sensing platforms, and AI-assisted systems to enable intelligent, on-site, real-time UO₂²⁺ monitoring.