Despite the successful application of upconversion nanoparticles (UCNPs), their lowenergy transfer efficiency is still ab ottleneckt of urther applications.H ere we design UCNPs with am ultilayer structure,i ncluding an inert NaYF 4 :Gd core and an energy-concentrating zone (ECZ), for efficient energy concentration. The ECZ is composed of an emitting layer of NaYF 4 :Yb,Er and an absorption layer of NaYF 4 :Nd,Yb with antenna IRDye 800CW to manipulate the energy transfer.The stable and tight packingof800CW linked originally with ab isphosphonate ligand improves greatly the transfer efficiency.The proximity of the emitting layer to both surface antenna and accepter also decreases energy depletion. Compared to classical UCNPs,t he ECZ UCNPs show3 600 times higher luminescence intensity with an energy transfer efficiency near 60 %. In proof-of-concept applications,t his type of structure was employed for Hg 2+ detection and for photodynamic therapyunder hypoxic conditions. Photon upconversion indicates ap rocess of absorbing lowenergy near-infrared (NIR) photons to emit higher energy photons. [1] This process can be performed with upconversion nanoparticles (UCNPs), ac lass of nanomaterials doped with lanthanide ions, [2] which provides alarge anti-Stokes shift with sharp emission to act as favorable energy donors in luminescence resonance energy transfer (LRET). [3] Although the LRET of UCNPs has been extensively applied in biosensing and biomedicine,the low energy transfer efficiency due to the relatively low extinction coefficients of lanthanide dopants [4] and the constraint in nanoparticle dimension [5] of UCNPs is still abottleneck to further applications.Several multilayer UCNPs have been proposed to enhance the transfer efficiency. [6][7][8][9][10] However,a ssembling as pacer layer [7] or an inert shell [8] on UCNPs increases the distance from the emitter to the surface LRET acceptor,while some approaches to reduce particle size [9] increase surface quenching and lower the quantum yield. [1b] To overcome the distance effect, at hin shell of NaYbF 4 :Tm has been constructed on UCNPs for confining energy in the shell to enhance the upconversion luminescence. [10] Unfortunately the total luminescence intensity of UCNPs is still limited due to the low light absorption capability of rare-earth elements. Hence,s ome NIR dyes with large optical cross-section have been attached to UCNPs as antenna for improving the light absorption. [11] Them odification is generally performed with poly(acrylic acid), polyethyleneimine (PEI), or silica coating as the surface linker, [11e-g] which extends energy transfer distance again, and thus causes energy depletion during migration. [12] To boost efficiently the energy transfer efficiency in UCNPs,w ep ropose herein the new concept of an energyconcentrating zone (ECZ) and report on an energy concentration mechanism in which an inert core is used to confine energy in athin layer close to UCNPs surface and abisphosphonate ligand is used to link IRDye 800CW as the lightabsorbing...