Post-synthetic modification of a metal-organic framework with fluorescent-tag for dual naked-eye sensing in aqueous medium

The existing metal-organic framework (MOF) called UiO-66-NH2 (1; UiO = University of Oslo) was post-synthetically modified by condensation with 1-pyrenecarboxaldehyde. The pyrene-tagged MOF (1′) exhibited ∼3-fold enhancement in fluorescence intensity over the un-functionalized one with a new emission peak at 470 nm due to the formation pyrene excimer within the framework of the MOF. 1′ showed fast response time, excellent selectivity and sensitivity for sensing of biologically active anions like F− and H2PO4− in pure aqueous medium via fluorescence ‘turn-on’ mechanism with detection limits of 8.2 × 10−7 M and 7.3 × 10−7 M, respectively. The fluorescence enhancement and the distinctive blue shifts in the emission bands of 1′ in presence of the anions are attributed to strong hydrogen bonding interactions and static pyrene excimer formation, respectively. 1′ also displayed rapid, selective and sensitive detection of 2,4,6-trinitrophenol (TNP) in aqueous medium via fluorescence ‘turn-off’ mechanism with a detection limit of 2.1 × 10−7 M. The highly selective fluorescence quenching is ascribed to electron and energy transfer processes as well as molecular interactions (π-π and electrostatic) of the MOF with TNP. The excellent detection performance of 1′ in aqueous medium makes it a promising dual sensor material for real-field applications.