Abstract
Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility. Sensorics is an area where COPs and COFs have demonstrated immense potential and achieved high levels of sensitivity and selectivity on account of their tunable structures. In this review, we focus on those covalent polymeric systems that use fluorescence spectroscopy as a method of detection. After briefly reviewing the physical basis of fluorescence-based sensors, we delve into various kinds of analytes that have been explored with COPs and COFs, namely, heavy metal ions, explosives, biological molecules, amines, pH, volatile organic compounds and solvents, iodine, enantiomers, gases, and anions. Throughout this work, we discuss the mechanisms involved in each sensing application and aim to quantify the potency of the discussed sensors by providing limits of detection and quenching constants when available. This review concludes with a summary of the surveyed literature and raises a few concerns that should be addressed in the future development of COP and COF fluorescence-based sensors.
Highlights
Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility
We detail the various fluorescence mechanisms that are found in the existing fluorescence-based COP and COF sensors, including internal charge transfer (ICT), resonance energy transfer (RET), photoinduced electron transfer (PET), and aggregationinduced emission (AIE)
We focus on various analytes, for instance explosives, metal cations, biological molecules, gases, amines, iodine, solvents, volatile organic compounds (VOCs), enantiomeric compounds, and anions
Summary
The very first manuscript on fluorescent COFs for metal ion detection was published by Wang et al in 2016, and it triggered the development of diverse COF-based detection systems. Sulfide-bridged polytriazine nanospheres NOP-28 were fully reversible and retained specificity among 11 different metal cations.[16] The metal ion detection was based on fluorescence quenching at 415 nm ascribed to the electron transfer from the COP to the formed guest metal complexes, or metal ioninduced polymer chain aggregation Their LOD in ethanol was still rather high at 12 ppb. Similar to Hg2+ sensors, Fe3+ sensors commonly incorporate metal cation chelation sites that lower the LODs.[33] For example, Yamamoto cross-coupling produced a series of COPs (COP6−9) with LODs in the mM range These materials were interesting for the absorption competition quenching (ACQ) mechanism for Fe3+ sensing.[34] In DMF, both the COP and the Fe3+ ions absorb light in the 250−400 nm window, so upon excitation the two species compete for the light energy.
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