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Eutrophication of inland lakes,reservoirs,estuaries,and coastal waters,is becoming a frequent phenomenon,which has received increasing attention due to its harmful impacts on the aquatic environment.1,2 Cyanobacterial blooms,as one of distinct types of eutrophication,have posed a serious threat to both the environment and humans,particularly under blooming conditions.1,3 Phycocyanin(PC),an accessory cyanobacteria-specific pigment emitting red light with a maximum at about 650 nm,can provide effective indication to assess cyanobacterial blooms.4 Hence,identifying and quantifying PC by developing cost effective,rapid,facile and applicable methods is highly crucial.Herein,as shown in Figure 1,a novel and simple molecular imprinting ratiometric fluorescence probe was developed for highly selective and sensitive detection of phycocyanin(PC)on the basis of fluorescence resonance energy transfer(FRET).5 The fluorescent dye molecule nitrobenzoxadiazole(NBD)covalently bound to the SiO2 nanoparticles core was used as donor and 3-aminopropyltriethoxy silane(APTES),linked on the particle surface,as the functional monomer,along with PC as the template molecule,allowed the formation of imprinting shell-layers via a sol-gel process.The presence of PC could lead to the production of a complex between PC and APTES,which could strongly suppress the fluorescence of the NBD while enhance that of the complexes through FRET,and thereby PC could be detected by fluorescence ratio.As a result,a fast response time for PC was within 6 min,and accordingly excellent recognition specificity for PC over its analogues and high sensitivity with a low detection limit of 0.2 nM were exhibited(ref.Figure 2).Furthermore,high recoveries were attained in the range of 98.8-109.4%at three spiking levels of PC,with precisions below 4.6%in seawater and lake water samples.The imprinting ratiometric fluorescence probe demonstrated simple,rapid,highly selective recognition and highly sensitive detection and can thus push forward ultratrace analysis of complicated matrices.