Confinement-Assisted Specific Fluorescence Detection of Fe³⁺ in Aqueous Solution by Styrylpyrene-Based Microgels

In recent years, fluorescent sensors have played a compelling role in the precise detection of metal ions. However, most fluorophores are hydrophobic and cannot be used to detect aqueous samples directly. Researchers have thus addressed this problem by combining fluorophores with hydrophilic polymer carriers. Nevertheless, most studies have focused only on the role of fluorophores in the recognition of metal ions, ignoring the effect of polymers and their aggregation states on the detection. In this work, microgels with pyrene-based fluorescent cross-linked structures were prepared using a post-cross-linking strategy and were found to specifically detect Fe3+ directly in aqueous solutions without interference from other metal ions (i.e., Al3+, Ba2+, Cd2+, Co2+, Cr3+, Cu2+, Ni2+, Pb2+, Zn2+, Fe2+, Hg2+, Na+, K+, Ca2+, or Mg2+). The detection limit is 2.3 μM, which is lower than the U.S. Environmental Protection Agency’s safety limit for drinking water. The fluorescence quenching of microgels by Fe3+ was effective by both mechanisms (dynamic and static), mainly the static one. Nanoscale microgels allow Fe3+ to be confined around the pyrene of their fluorescent cross-linked structures, which induces nonradiative energy transfer and leads to fluorescence quenching of the microgels. Such a detection method utilizing the confinement effect of microgels to synergize fluorescence quenching provides an idea for the development of fluorescent sensors.