A highly efficient multifunctional nanoplatform for simultaneous upconversion luminescence (UCL) imaging and photodynamic therapy has been developed on the basis of selective energy transfer from multicolor luminescent NaYF(4):Yb(3+),Er(3+) upconversion nanoparticles (UCNPs) to photosensitizers (PS). Different from popular approaches based on electrostatic or hydrophobic interactions, over 100 photosensitizing molecules were covalently bonded to every 20 nm UCNP, which significantly strengthened the UCNP-PS linkage and reduced the probability of leakage/desorption of the PS. Over 80% UCL was transferred to PS, and the singlet oxygen production was readily detected by its feature emission at 1270 nm. Tests performed on JAR choriocarcinoma and NIH 3T3 fibroblast cells verified the efficient endocytosis and photodynamic effect of the nanoplatform with 980 nm irradiation specific to JAR cancer cells. Our work highlights the promise of using UCNPs for potential image-guided cancer photodynamic therapy.

Recent efforts and progress in unraveling the fundamental mechanism of excitation energy migration dynamics in upconversion nanomaterials are covered in this review, including short-and long-term interactions and other interactions in homogeneous and heterogeneous nanostructures. Comprehension of the role of spatial confinement in excitation energy migration processes is updated. Problems and challenges are also addressed. Key learning points(1) Energy migration dynamics and interactions in upconversion nanosystems.(2) Excitation energy loss channels in upconversion nanomaterials. (3) Challenges in acquiring a high efficiency and broad excitation spectrum of upconversion emission in nanomaterials.

Strong red upconversion luminescence of rare-earth ions doped in nanocrystals is desirable for the biological/biomedical applications. In this paper, we describe the great enhancement of red upconversion emission (4F9/2 --> I15/2 transition of Er3+ ion) in NaYF4:Yb3+, Er3+ nanocrystals at low doping level, which is ascribed to the effectiveness of the multiphonon relaxation process due to the existence of citrate as a chelator and cross relaxation between Er3+ ions. The dissolution-recrystallization transformation, governing both the intrinsic crystalline phase (cubic and/or hexagonal phase) and the growth regime (thermodynamic vs kinetic), is responsible for the phase control of the NaYF4 crystals. The possible formation mechanism of the NaYF4 crystals and the role of trisodium citrate which acts as a chelating agent and shape modifier are discussed in detail. It is also found that the alpha --> beta phase transition is favored by the high molar ratio of fluoride to lanthanide and high hydrothermal temperature as well as long hydrothermal time.

Coating a homogeneous layer outside the core nanoparticles has become a common method to improve the optical properties of nanoparticles. For rare earth ion-doped nanoparticles, although the homogeneous coating is found to enhance the upconversion luminescence, a large deviation in the reported enhancement amplitude, however, demonstrates the lack of a complete picture of the enhancement mechanism. In this work, we have performed steady-state and time-resolved spectroscopic studies on one of the most efficient upconversion nanosystemsβ-NaYF 4 :Yb 3+ ,Er 3+ and β-NaYF 4 :Yb 3+ ,Er 3+ @β-NaYF 4 core/shell nanoparticles. Roles of the surface quenching centers, typically the high-frequency vibrational modes provided by the organic surfactants in the upconversion luminescence, are studied in detail. Our results show that excitation power density, once over a threshold, say ∼150 W/cm 2 in this case, does have a non-negligible annealing effect, which may even lead to high luminescence upconversion intensity of the core nanoparticles compared to the shell-coated nanoparticles. The surface-related high-frequency vibrational modes play an important role in the upconversion process and in the laser annealing process, and the latter manifests itself in the difference of the laser annealing effect between the core and core/shell nanoparticles. From the upconversion luminescence kinetics analysis, it turns out that the luminescent centers of the core nanoparticle are severely quenched but homogeneous coating can effectively reduce the quenching, enhancing the upconversion luminescence. It is concluded that the upconversion emission spectrum, or more specifically the ratio between the red and green emissions, can be greatly altered by excitation power density for core nanoparticles but not for core/shell nanoparticles.

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3-Mercaptopropionic acid stabilized CdTe/CdS core/shell quantum dots (QDs) were prepared in an aqueous solution following the synthetic route of successive ion layer adsorption and reaction. The photoluminescence quantum yield of the CdTe QDs could reach 40%, from 8% of the bare core, via the control of the shell thickness. The CdTe/CdS QDs exhibited also a significant red shift of emission and excitation peaks when the shell layer grew. The experiments revealed that the CdTe/CdS QDs evolved from type I to type II core/shell structures with the increase of the shell thickness, and the evolution process is affected by the core size, shell thickness, surface quality of the core and shell, as unraveled by steady-state and time-resolved spectroscopy. The lack of photoluminescence lifetime lengthening was ascribed to the surface influence of the shell.

UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl) General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). UvA-DARE (Digital AcademicRepository Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The in vivo biological applications of upconversion nanoparticles (UCNPs) prefer excitation at 700-850 nm, instead of 980 nm, due to the absorption of water. Recent approaches in constructing robust Nd 3+ doped UCNPs with 808 nm excitation properties rely on a thick Nd 3+ sensitized shell.However, for the very important and popular Förster resonance energy transfer (FRET)-based applications, such as photodynamic therapy (PDT) or switchable biosensors, this type of structure has restrictions resulting in a poor energy transfer. In this work, we have designed a NaYF 4 :Yb/Ho@NaYF 4 :Nd@NaYF 4 core-shell-shell nanostructure. We have proven that this optimal structure balances the robustness of the upconversion emission and the FRET efficiency for FRET-based bioapplications. A proof of the concept was demonstrated for photodynamic therapy and simultaneous fluorescence imaging of HeLa cells triggered by 808 nm light, where low heating and a high PDT efficacy were achieved.