Review of 2D MnO2 Nanosheets as FRET-Based Nanodot Fluorescence Quenchers in Chemosensing Applications

In the past decade, a variety of MnO2 nanostructures have been used in chemosensing applications because of their unique redox properties, structural diversity, and broad absorption band, which makes them good energy acceptors. The most common strategy in Förster (or fluorescence) resonance energy transfer (FRET)-based chemosensing assays is to use low-dimensional fluorescent dots like carbon-based dots, metal dots, and even fluorescent dyes to form donor–acceptor pairs with 2D MnO2 nanosheets (NSs). This review is a collection of the current library on fluorescent nanodots and MnO2 NSs-based FRET conjugates as sensing platforms with a focus on highlighting the strategies involved in the detection of various analytes. Commonly, an analyte-triggered redox reaction results in the decomposition of MnO2 NSs, leading to the release of fluorescent nanodots in solution for the turn-on detection of various reducing species. We intend to elaborately discuss the systematic upgradation of detection strategies involving fluorescent dots@2D MnO2 NSs. Recently, a cascade of redox reactions was judiciously employed to target several bioanalytes involving a fluorescent dots@MnO2 NSs FRET conjugate in a selective and sensitive manner. These sensing assays are categorized by the use of different fluorescent nanodots [carbon dots (CDs) and graphene quantum dots (GQDs), polymer dots, metal dots, etc.] with 2D MnO2 NSs as FRET pairs in different sensing applications. Further, the analytes were categorized as reducing species, oxidants, enzymes, microbes, etc. The focus was paid to the sensing efficiency of the analytical tool. As a part of the study, a brief account of the synthesis of nanodots and MnO2 NSs, their properties, and the FRET mechanism is also included. In a few cases, the sensing mechanism follows an inner filter effect without a prerequisite of the distance between the fluorophore and quencher units in the sensing of analytes, which are also covered in this collection. The available literature indicates a bright future for these FRET-based contemporary sensing strategies as environmentally benign, cost-effective, label-free techniques, which are discussed in the end.