The effect of surface coating on energy migration-mediated upconversion

Yb3+-sensitized upconversion and downshifting luminescence in Nd3+ ions through energy migration

A core-shell-shell nanostructure composed of NaGdF₄:Yb/Tm@NaGdF₄:Nd@NaYF₄ is developed to realize Yb³⁺-sensitized upconversion and downshifting luminescence in Nd³⁺ ions. The unusual photon conversion property stems from a gadolinium sublattice mediated Yb³⁺→ Tm³⁺→ Gd³⁺→ Nd³⁺ energy transfer pathway. The energy transfer processes are investigated by varying the dopant concentration and distribution, in conjunction with time decay measurements.

808 nm excited energy migration upconversion nanoparticles driven by a Nd3+-Trinity system with color-tunability and superior luminescence properties

We have developed energy migration upconversion (EMU) nanoparticles (UCNPs) with optimal Nd³⁺-sensitization under excitation of an 808 nm laser to avoid over-heating effects caused by a 980 nm laser while maximizing the excitation efficiency. To realize efficient 808 nm sensitization, a "Nd³⁺-Trinity system" was implemented in the energy migration upconversion (EMU) cores (NaGdF₄:Yb,Tm@NaGdF₄:Yb,X, X = Eu/Tb), resulting in a core-multishell structure of EMU cores (accumulation layer@activation layer)@transition layer@harvest layer@activation layer. The spatially separated dopants and optimized Yb³⁺/Nd³⁺ content effectively prevented severe quenching events in the UCNPs and their Nd³⁺-sensitized EMU-based photoluminescence mechanism was studied under 808 nm excitation. These Nd³⁺-Trinity EMU system UCNPs presented enhanced upconversion luminescence and prolonged lifetime compared to the 980 nm excited UCNPs of the EMU system. It is proposed that 975 nm and 1056 nm NIR photons induced from the Nd³⁺ → Yb³⁺ energy transfer facilitate the Tm³⁺ accumulation process due to the matched energy gaps, which contributes to the extended lifetimes. More importantly, the synthesized UCNPs had a small average size of sub-15 nm and they not only exhibited color-tunability via Eu³⁺/Tb³⁺ activators, but also released a larger portion of Tm³⁺ red emission at 647 nm and had better penetration ability in water under 808 nm excitation, which are favorable for bioimaging applications.

NIR/blue light emission optimization of NaY1−(x+y)YbxF4:Tmy upconversion nanoparticles via Yb3+/Tm3+ dopant balancing

A data driven approach provides better understanding of the role of dopant balancing in the upconversion process and presents an effective strategy to enhance the optical properties of upconversion nanoparticles.

Facile ex situ NaF size/morphology tuning strategy for highly monodisperse sub-5 nm β-NaGdF4:Yb/Er

A facile ex situ NaF size/morphology tuning strategy for NaF release rate regulation was presented and successfully used to achieve time-saving controlled solvothermal synthesis of highly monodisperse/crystalline sub-5 nm β-NaGdF₄:Yb/Er at a high growth temperature of 300 °C.

Energy transfer-based biodetection using optical nanomaterials

This review focuses on recent progress in the development of FRET probes and the applications of FRET-based sensing systems.

Fabrication of pH-responsive PLGA(UCNPs/DOX) nanocapsules with upconversion luminescence for drug delivery

The integration of anticancer drugs and inorganic nanocrystals in polymer nanocapsules is a widely used strategy to improve their functionality, stability and sustained release. However, the complexity in the preparation of functional nanocapsules and their reproducibility still challenge these promising drug carriers in clinical application. Here we introduce a simple one-step self-assembly strategy to prepare multifunctional nanocapsules based on simultaneous poly (DL-lactic-co-glycolic acid) (PLGA) encapsulation of antitumor drug doxorubicin hydrochloride (DOX) and NaYF4:Yb,Er@NaGdF4 upconversion nanoparticles (UCNPs) for cancer cell imaging and drug delivery. The obtained PLGA(UCNPs/DOX) nanocapsules with a small size of ≈150 nm possessed bright upconversion fluorescence and could act as T 1- weighted contrast agents for magnetic resonance imaging (MRI). Moreover, the PLGA(UCNPs/DOX) nanocapsules exhibited pH-responsive drug releasing behavior, causing the loaded DOX easily releasing at cancer cells, and an obvious cytotoxicity via MTT assay. The endocytosis process of PLGA (UCNPs/DOX) nanocapsules is evaluated using optical microscopy and upconversion fluorescence microscopy. These results demonstrated that the developed PLGA nanocapsules could serve as multifunctional drug delivery systems for cancer imaging and therapy.

Unravelling the energy transfer of Er3+-self-sensitized upconversion in Er3+-Yb3+-Er3+ clustered core@shell nanoparticles

Unravelling upconversion (UC) energy transfer mechanisms is significant for designing novel efficient anti-Stokes phosphors. We have studied the correlation of different lanthanide dopants within Er³⁺-self-sensitized core@shell upconversion nanoparticles (UCNPs). Here, our focus will be on high-concentration dopants that are able to sufficiently produce the clustering effect, especially within the interplay between Er³⁺ and Yb³⁺. We demonstrate that whatever the amount of the self-sensitizer (e.g., Er³⁺), abnormal absorption enhancement will occur as long as Yb³⁺ clusters are present. This effect originates from the substantial energy transfer between Yb³⁺-Yb³⁺ clusters despite the increased energy transfer from Yb³⁺ to Er³⁺. Therefore, the energy transfer efficiency is still constrained. However, we conversely used one of the aforementioned quench-paths of UC energy transfer to easily transfer the energy from the in-shell shell layer to the in-core area with the assistance of the energy potential reservoir, which was given by the homogeneous core@shell band offset at the interface region. Indirectly, we actualize the Er³⁺ UC luminescence with self-sensitization through an extended energy transfer path. This work provides a solid support and analytic theory for unraveling the energy transfer mechanism from recent works on Er³⁺ self-sensitized UC luminescence.

A novel upconversion luminescence derived photoelectrochemical immunoassay: ultrasensitive detection to alpha-fetoprotein

Fabrication of near-infrared light-triggered photoelectrochemical (PEC) sensors based on the upconversion nanophosphors (UCNPs) is a novel approach, which exhibits the advantages of low background signal and non-damage to the biological substance as well as high sensitivity and improved electric detection in PEC sensors. Herein we demonstrate the preparation of novel and high-quality ZnO inverse opal photonic crystals (IOPCs)/Ag/NaYF₄:Yb,Tm hybrid films by different advanced film techniques, including colloidal self-assembling, vapor phase deposition and pulsed laser deposition and its application to sensitive detection of alpha-fetoprotein (AFP). In the complex device, ZnO IOPCs and surface plasmon resonance (SPR) of silver had ability to largely enhance local excitation electromagnetic field of NaYF₄:Yb,Tm, resulting in efficient near-infrared to visible/ultraviolet upconversion luminescence (UCL). The ultraviolet light emitted by NaYF₄:Yb,Tm could b further reabsorbed by ZnO, generating PEC responses. Furthermore, because of the high specific surface area of ZnO IOPCs and the conductivity of Ag films, ZnO IOPCs/Ag/NaYF₄:Yb,Tm hybrid films based near-infrared light-triggered PEC sensors showed ultrasensitive detection of AFP with a linear range from 0.05 ng mL^-1 to 100 ng mL^-1 and a low detection limit of ∼0.04 ng mL^-1 (40 pg mL^-1). Such an advanced device also shows promise in detection of other cancer markers in clinical and biological analysis.

Energy Migration Upconversion in Ce(III)-Doped Heterogeneous Core-Shell-Shell Nanoparticles

One major challenge in upconversion research is to develop new materials and structures to expand the emission spectrum. Herein, a heterogeneous core-shell-shell nanostructure of NaYbF₄ :Gd/Tm@NaGdF₄ @CaF₂ :Ce is developed to realize efficient photon upconversion in Ce³⁺ ions through a Gd-mediated energy migration process. The design takes advantage of CaF₂ host that reduces the 4f-5d excitation frequency of Ce³⁺ to match the emission line of Gd³⁺ . Meanwhile, CaF₂ is isostructural with NaGdF₄ and can form a continuous crystalline lattice with the core layer. As a result, effective Yb³⁺ → Tm³⁺ → Gd³⁺ → Ce³⁺ energy transfer can be established in a single nanoparticle. This effect enables efficient ultraviolet emission of Ce³⁺ following near infrared excitation into the core layer. The Ce³⁺ upconversion emission achieved in the core-shell-shell nanoparticles features broad bandwidth and long lifetime, which offers exciting opportunities of realizing tunable lasing emissions in the ultraviolet spectral region.

Broadband Ce(III)-Sensitized Quantum Cutting in Core-Shell Nanoparticles: Mechanistic Investigation and Photovoltaic Application

Quantum cutting in lanthanide-doped luminescent materials is promising for applications such as solar cells, mercury-free lamps, and plasma panel displays because of the ability to emit multiple photons for each absorbed higher-energy photon. Herein, a broadband Ce³⁺-sensitized quantum cutting process in Nd³⁺ ions is reported though gadolinium sublattice-mediated energy migration in a NaGdF₄:Ce@NaGdF₄:Nd@NaYF₄ nanostructure. The Nd³⁺ ions show downconversion of one ultraviolet photon through two successive energy transitions, resulting in one visible photon and one near-infrared (NIR) photon. A class of NaGdF₄:Ce@NaGdF₄:Nd/Yb@NaYF₄ nanoparticles is further developed to expand the spectrum of quantum cutting in the NIR. When the quantum cutting nanoparticles are incorporated into a hybrid crystalline silicon (c-Si) solar cell, a 1.2-fold increase in short-circuit current and a 1.4-fold increase in power conversion efficiency is demonstrated under short-wavelength ultraviolet irradiation. These insights should enhance our ability to control and utilize spectral downconversion with lanthanide ions.

An assembly and interaction of upconversion and plasmonic nanoparticles on organometallic nanofibers: enhanced multicolor upconversion, downshifting emission and the plasmonic effect

We present novel inorganic-organic hybrid nanoparticles (HNPs) constituting inorganic NPs, NaY_0.78Er_0.02Yb_0.2F₄, and organometallic nanofiber, Tb(ASA)₃Phen (TAP). X-ray diffraction, Fourier transform infrared absorption and transmission electron microscopy analyses reveal that prepared ultrafine upconversion NPs (UCNPs (5-8 nm)) are dispersed on the surface of the TAP nanofibers. We observe that the addition of TAP in UCNPs effectively limits the surface quenching to boost the upconversion (UC) intensity and enables tuning of UC emission from the green to the red region by controlling the phonon frequency around the Er³⁺ ion. On the other hand, TAP is an excellent source of green emission under ultraviolet exposure. Therefore prepared HNPs not only give enhanced and tunable UC but also emit a strong green color in the downshifting (DS) process. To further enhance the dual-mode emission of HNPs, silver NPs (AgNPs) are introduced. The emission intensity of UC as well as DS emission is found to be strongly modulated in the presence of AgNPs. It is found that AgNPs enhance red UC emission. The possible mechanism involved in enhanced emission intensity and color output is investigated in detail. The important optical properties of these nano-hybrid materials provide a great opportunity in the fields of biological imaging, drug delivery and energy devices.

Boosting the down-shifting luminescence of rare-earth nanocrystals for biological imaging beyond 1500 nm

In vivo fluorescence imaging in the near-infrared region between 1500–1700 nm (NIR-IIb window) affords high spatial resolution, deep-tissue penetration, and diminished auto-fluorescence due to the suppressed scattering of long-wavelength photons and large fluorophore Stokes shifts. However, very few NIR-IIb fluorescent probes exist currently. Here, we report the synthesis of a down-conversion luminescent rare-earth nanocrystal with cerium doping (Er/Ce co-doped NaYbF₄ nanocrystal core with an inert NaYF₄ shell). Ce doping is found to suppress the up-conversion pathway while boosting down-conversion by ~9-fold to produce bright 1550 nm luminescence under 980 nm excitation. Optimization of the inert shell coating surrounding the core and hydrophilic surface functionalization minimize the luminescence quenching effect by water. The resulting biocompatible, bright 1550 nm emitting nanoparticles enable fast in vivo imaging of blood vasculature in the mouse brain and hindlimb in the NIR-IIb window with short exposure time of 20 ms for rare-earth based probes.

Fluorescence imaging in the near-infrared window between 1500–1700 nm (NIR-IIb window) offers superior spatial resolution and tissue penetration depth, but few NIR-IIb probes exist. Here, the authors synthesize rare earth down-converting nanocrystals as promising fluorescent probes for in vivo imaging in this spectral region.

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide-doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF₄ :Yb³⁺ /Er³⁺ /Mn²⁺ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm-diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near-field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time-resolved measurements and finite-difference time-domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.

Confining Excitation Energy in Er3+ -Sensitized Upconversion Nanocrystals through Tm3+ -Mediated Transient Energy Trapping

A new class of lanthanide-doped upconversion nanoparticles are presented that are without Yb³⁺ or Nd³⁺ sensitizers in the host lattice. In erbium-enriched core-shell NaErF₄ :Tm (0.5 mol %)@NaYF₄ nanoparticles, a high degree of energy migration between Er³⁺ ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb³⁺ -Er³⁺ system, the Er³⁺ ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm³⁺ has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700-fold enhancement) and near-infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red-emitting upconversion nanoprobes for biological applications.

Cooperative and non-cooperative sensitization upconversion in lanthanide-doped LiYbF4 nanoparticles

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) have attracted tremendous interest owing to their potential bioapplications. However, the intrinsic photophysics responsible for upconversion (UC) especially the cooperative sensitization UC (CSU) in colloidal Ln³⁺-doped UCNPs has remained untouched so far. Herein, we report a unique strategy for the synthesis of high-quality LiYbF₄:Ln³⁺ core-only and core/shell UCNPs with tunable particle sizes and shell thicknesses. Energy transfer UC from Er³⁺, Ho³⁺ and Tm³⁺ and CSU from Tb³⁺ were comprehensively surveyed under 980 nm excitation. Through surface passivation, we achieved efficient non-cooperative sensitization UC with absolute UC quantum yields (QYs) of 3.36%, 0.69% and 0.81% for Er³⁺, Ho³⁺ and Tm³⁺, respectively. Particularly, we for the first time quantitatively determined the CSU efficiency for Tb³⁺ with an absolute QY of 0.0085% under excitation at a power density of 70 W cm^-2. By means of temperature-dependent steady-state and transient UC spectroscopy, we unraveled the dominant mechanisms of phonon-assisted cooperative energy transfer (T > 100 K) and sequential dimer ground-state absorption/excited-state absorption (T < 100 K) for the CSU process in LiYbF₄:Tb³⁺ UCNPs.

Tuning the upconversion light emission by bandgap engineering in bismuth oxide-based upconverting nanoparticles

In the field of novel applications involving upconverting processes, the determination of new strategies for realizing emission-tunable nanomaterials is a challenge. In this work the design of Y³⁺ and Er³⁺ codoped bismuth oxide-based upconverting nanoparticles is presented, evidencing that the active role of the matrix allows for the emission selectivity with chromaticity control. The bandgap of the bismuth oxide-based host can be manipulated in the range of 0.65 eV, consequently leading to upconversion emission color tunability from red to yellow-greenish. The resulting fine control of the nanoparticle chromaticity through accurate host bandgap engineering reveals a novel concept for the development of a new generation of upconverting nanophosphors.

Flexible transparent displays based on core/shell upconversion nanophosphor-incorporated polymer waveguides

Core/shell (C/S)-structured upconversion nanophosphor (UCNP)-incorporated polymer waveguide-based flexible transparent displays are demonstrated. Bright green- and blue-emitting Li(Gd,Y)F₄:Yb,Er and Li(Gd,Y)F₄:Yb,Tm UCNPs are synthesized via solution chemical route. Their upconversion luminescence (UCL) intensities are enhanced by the formation of C/S structure with LiYF₄ shell. The Li(Gd,Y)F₄:Yb,Er/LiYF₄ and Li(Gd,Y)F₄:Yb,Tm/LiYF₄ C/S UCNPs exhibit 3.3 and 2.0 times higher UCL intensities than core counterparts, respectively. In addition, NaGdF₄:Yb,Tm/NaGdF₄:Eu C/S UCNPs are synthesized and they show red emission via energy transfer and migration of Yb³⁺ → Tm³⁺ → Gd³⁺ → Eu³⁺. The C/S UCNPs are incorporated into bisphenol A ethoxylate diacrylate which is used as a core material of polymer waveguides. The fabricated stripe-type polymer waveguides are highly flexible and transparent (transmittance > 90% in spectral range of 443–900 nm). The polymer waveguides exhibit bright blue, green, and red luminescence, depending on the incorporated UCNPs into the polymer core, under coupling with a near infrared (NIR) laser. Moreover, patterned polymer waveguide-based display devices are fabricated by reactive ion etching process and they realize bright blue-, green-, and red-colored characters under coupling with an NIR laser.

Ultrasmall-Superbright Neodymium-Upconversion Nanoparticles via Energy Migration Manipulation and Lattice Modification: 808 nm-Activated Drug Release

Nd³⁺-sensitized upconversion nanoparticles are among the most promising emerging fluorescent nanotransducers. They are activated by 808 nm irradiation, which features merits such as limited tissue overheating and deeper penetration depth, and hence are attractive for diagnostic and therapeutic applications. Recent studies indicate that ultrasmall nanoparticles (<10 nm) are potentially more suitable for clinical application due to their favorable biodistribution and safety profiles. However, upconversion nanoparticles in the sub-10 nm range suffer from poor luminescence due to their ultrasmall size and greater proportion of lattice defects. To reconcile these opposing traits, we adopt a combinatorial strategy of energy migration manipulation and crystal lattice modification, creating ultrasmall-superbright Nd³⁺-sensitized nanoparticles with 2 orders of magnitude enhancement in upconversion luminescence. Specifically, we configure a sandwich-type nanostructure with a Yb³⁺-enriched intermediate layer [Nd³⁺]-[Yb³⁺-Yb³⁺]-[Yb³⁺-Tm³⁺] to form a positively reinforced energy migration system, while introducing Ca²⁺ into the crystal lattice to reduce lattice defects. Furthermore, we apply the nanoparticles to 808 nm light-mediated drug release. The results indicate time-dependent cancer cells killing and better antitumor activities. These ultrasmall-superbright dots have unraveled more opportunities in upconversion photomedicine with the promise of potentially safer and more effective therapy.

Sequential Growth of NaYF4:Yb/Er@NaGdF4 Nanodumbbells for Dual-Modality Fluorescence and Magnetic Resonance Imaging

Upconversional core-shell nanostructures have gained considerable attention due to their distinct enhanced fluorescence efficiency, multifunctionality, and specific applications. Recently, we have developed a sequential growth process to fabricate unique upconversion core-shell nanoparticles. Time evolution of morphology for the NaYF₄:Yb/Er@NaGdF₄ nanodumbbells has been extensively investigated. An Ostwald ripening growth mechanism has been proposed to illustrate the formation of NaYF₄:Yb/Er@NaGdF₄ nanodumbbells. The hydrophilic NaYF₄:Yb/Er@NaGdF₄ core-shell nanodumbbells exhibited strong upconversion fluorescence and showed higher magnetic resonance longitudinal relaxivity (r₁ = 7.81 mM^-1 s^-1) than commercial contrast agents (Gd-DTPA). NaYF₄:Yb/Er@NaGdF₄ nanodumbbells can serve as good candidates for high efficiency fluorescence and magnetic resonance imaging.

Emerging ≈800 nm Excited Lanthanide-Doped Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles can tune near-infrared light to visible or even ultra-violet light in emissions. Due to their unique photophysical and photochemical properties, as well as their promising bioapplications, there has been a great deal of enthusiastic research performed to study the properties of lanthanide-doped upconversion nanoparticles in the past few years. Despite the considerable progress in this area, numerous challenges associated with the nanoparticles, such as a low upconversion efficiency, limited host materials, and a confined excitation wavelength, still remain, thus hindering further development with respect to their applications and in fundamental science. Recently, innovative strategies that utilize alternative sensitizers have been designed in order to engineer the excitation wavelengths of upconversion nanoparticles. Here, focusing on the excitation wavelength at ≈800 nm, recent advances in the design, property tuning, and applications of ≈800 nm excited upconversion nanoparticles are summarized. Benefiting from the unique features of ≈800 nm light, including deep tissue penetration depth and low photothermal effect, the ≈800 nm excited upconversion nanoparticles exhibit superior potential for biosensing, bioimaging, drug delivery, therapy, and three dimensional displays. The critical aspects of such emerging nanoparticles with regards to meeting the ever-changing needs of future development are also discussed.

Enhancing Multiphoton Upconversion from NaYF4:Yb/Tm@NaYF4 Core-Shell Nanoparticles via the Use of Laser Cavity

We discover that emission efficiency of Tm³⁺-doped upconversion nanoparticles can be enhanced through the use of a laser cavity. With suitable control of the lasing conditions, the population of the intermediate excited states of the Tm³⁺ can be clamped at a required value above the excitation threshold. As a result, upconversion efficiency for the 300-620 nm emission band of the Tm³⁺-doped nanoparticles under 976 nm excitation can be enhanced by an order of magnitude over the case without a laser cavity. This is because the intrinsic recombination process of the intermediate excited states is suppressed and the surplus of excitation power directly contributes to the enhancement of multiphoton upconversion. Furthermore, our theoretical investigation has shown that the improvement of upconversion emission efficiency is mainly dependent on the cavity loss, so that this strategy can also be extended to other lanthanide-doped systems.

Resonance Energy Transfer in Upconversion Nanoplatforms for Selective Biodetection

Resonance energy transfer (RET) describes the process that energy is transferred from an excited donor to an acceptor molecule, leading to a reduction in the fluorescence emission intensity of the donor and an increase in that of the acceptor. By this technique, measurements with the good sensitivity can be made about distance within 1 to 10 nm under physiological conditions. For this reason, the RET technique has been widely used in polymer science, biochemistry, and structural biology. Recently, a number of RET systems incorporated with nanoparticles, such as quantum dots, gold nanoparticles, and upconversion nanoparticles, have been developed. These nanocrystals retain their optical superiority and can act as either a donor or a quencher, thereby enhancing the performance of RET systems and providing more opportunities in excitation wavelength selection. Notably, lanthanide-doped upconversion nanophosphors (UCNPs) have attracted considerable attention due to their inherent advantages of large anti-Stoke shifts, long luminescence lifetimes, and absence of autofluorescence under low energy near-infrared (NIR) light excitation. These nanoparticles are promising for the biodetection of various types of analytes. Undoubtedly, the developments of those applications usually rely on resonance energy transfer, which could be regarded as a flexible technology to mediate energy transfer from upconversion phosphor to acceptor for the design of luminescent functional nanoplatforms. Currently, researchers have developed many RET-based upconversion nanosystems (RET-UCNP) that respond to specific changes in the biological environments. Specifically, small organic molecules, biological molecules, metal-organic complexes, or inorganic nanoparticles were carefully selected and bound to the surface of upconversion nanoparticles for the preparation of RET-UCNP nanosystems. Benefiting from the advantage and versatility offered by this technology, the research of RET-based upconversion nanomaterials should have significant implications for advanced biomedical applications. It should be noted that energy transfer in a UCNP based nanosystem is most often related to resonance energy transfer but that reabsorption (and maybe other energy transfer processes) may also play an important role and that more studies regarding the fundamental aspects for energy transfer with UCNPs is necessary. In this Account, we present an overview of recent advances in RET-based upconversion nanocomposites for biodetection with a particular focus on our own work. We have designed a series of upconversion nanoplatforms with remarkably high versatility for different applications. The experience gained from our strategic design and experimental investigations will allow for the construction of next-generation luminescent nanoplatform with marked improvements in their performance. The key aspects of this Account include fundamental principles, design and preparation strategies, biodetection in vitro and in vivo, future opportunities, and challenges of RET-UCNP nanosystems.

High performance magneto-fluorescent nanoparticles assembled from terbium and gadolinium 1,3-diketones

Polyelectrolyte-coated nanoparticles consisting of terbium and gadolinium complexes with calix[4]arene tetra-diketone ligand were first synthesized. The antenna effect of the ligand on Tb(III) green luminescence and the presence of water molecules in the coordination sphere of Gd(III) bring strong luminescent and magnetic performance to the core-shell nanoparticles. The size and the core-shell morphology of the colloids were studied using transmission electron microscopy and dynamic light scattering. The correlation between photophysical and magnetic properties of the nanoparticles and their core composition was highlighted. The core composition was optimized for the longitudinal relaxivity to be greater than that of the commercial magnetic resonance imaging (MRI) contrast agents together with high level of Tb(III)-centered luminescence. The tuning of both magnetic and luminescent output of nanoparticles is obtained via the simple variation of lanthanide chelates concentrations in the initial synthetic solution. The exposure of the pheochromocytoma 12 (PC 12) tumor cells and periphery human blood lymphocytes to nanoparticles results in negligible effect on cell viability, decreased platelet aggregation and bright coloring, indicating the nanoparticles as promising candidates for dual magneto-fluorescent bioimaging.

Optical–magnetic bifunctional properties and mechanistic insights on upconversion of NaYF4:Yb,Ho,Tm@NaGdF4 with a tunable nanodumbbell morphology

Optical–magnetic bifunctional upconversion of core–shell particles of NaYF₄:Yb,Ho,Tm@NaGdF₄ with a nanodumbbell-shaped morphology.

Comprehensive studies of the Li+ effect on NaYF4:Yb/Er nanocrystals: morphology, structure, and upconversion luminescence

Impurity doping plays a critical role in altering the properties of target nanomaterials in terms of designed morphologies, crystal structures, and functionalities.

NIR-driven water splitting by layered bismuth oxyhalide sheets for effective photodynamic therapy

Two major issues of finding the appropriate photosensitizer and raising the penetration depth of irradiation light exist in further developing of photodynamic therapy (PDT).

Hexagonal β-Na(Y,Yb)F4 based core/shell nanorods: epitaxial growth, enhanced and tailored up-conversion emission

Rare earth ions-doped hexagonal β-Na(Y,Yb)F₄ nanorods can be coated perfectly with either optically active or inert shells to improve and/or tailor the upconversion emission through a one-step epitaxial growth method from α-phased nanoparticles.

Lanthanide-doped bismuth oxobromide nanosheets for self-activated photodynamic therapy

Low tissue penetration depth of the excited light and complicated synthetic procedures greatly hinder the clinical application of photodynamic therapy (PDT).

Current Advances in Lanthanide-Doped Upconversion Nanostructures for Detection and Bioapplication

Along with the development of science and technology, lanthanide-doped upconversion nanostructures as a new type of materials have taken their place in the field of nanomaterials. Upconversion luminescence is a nonlinear optical phenomenon, which absorbs two or more photons and emits one photon. Compared with traditional luminescence materials, upconversion nanostructures have many advantages, such as weak background interference, long lifetime, low excitation energy, and strong tissue penetration. These interesting nanostructures can be applied in anticounterfeit, solar cell, detection, bioimaging, therapy, and so on. This review is focused on the current advances in lanthanide-doped upconversion nanostructures, covering not only basic luminescence mechanism, synthesis, and modification methods but also the design and fabrication of upconversion nanostructures, like core-shell nanoparticles or nanocomposites. At last, this review emphasizes the application of upconversion nanostructure in detection and bioimaging and therapy. Learning more about the advances of upconversion nanostructures can help us better exploit their excellent performance and use them in practice.

Near-Infrared Upconversion Chemodosimeter for In Vivo Detection of Cu(2+) in Wilson Disease

Near-infrared upconversion chemodosimetry is a promising detection method by virtue of the frequency upconversion technique, which shows very high sensitivity and selectivity for the detection of Cu(2+) ions in vitro and in vivo. This method offers a new opportunity for noninvasive diagnosis of Wilson disease associated with Cu(2+) detection in clinical medicine.

Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

Upconversion photoluminescence is a nonlinear effect where multiple lower energy excitation photons produce higher energy emission photons. This fundamentally interesting process has many applications in biomedical imaging, light source and display technology, and solar energy harvesting. In this review we discuss the underlying physical principles and their modelling using rate equations. We discuss how the understanding of photophysical processes enabled a strategic influence over the optical properties of upconversion especially in rationally designed materials. We subsequently present an overview of recent experimental strategies to control and optimize the optical properties of upconversion nanoparticles, focussing on their emission spectral properties and brightness.

Lanthanide-Doped Upconversion Nanoparticles: Emerging Intelligent Light-Activated Drug Delivery Systems

The development of drug delivery systems (DDSs) using near infrared (NIR) light and upconversion nanoparticles (UCNPs) has generated intensive interest over the past five years. These NIR-initiated DDSs not only offer a high degree of spatial and temporal determination of therapeutic release but also provide precise control over the released dosage. Furthermore, these nanoplatforms confer several advantages over conventional light-based DDSs-NIR offers better tissue penetration depth and a reduced risk of cellular photo-damage caused by exposure to light at high-energy wavelengths (e.g., ultraviolet light, <400 nm). The development of DDSs that can be activated by low intensity NIR illumination is highly desirable to avoid exposing living tissues to excessive heat that can limit the in vivo application of these DDSs. This encompasses research in three directions: (i) enhancing the quantum yield of the UCNPs; (ii) incorporation of photo-responsive materials with red-shifted absorptions into the UCNPs; and (iii) tuning the UCNPs excitation wavelength. This review focuses on recent advances in the development of NIR-initiated DDS, with emphasis on the use of photo-responsive compounds and polymeric materials conjugated onto UCNPs. The challenges that limit UCNPs clinical applications, alongside with the aforementioned techniques that have emerged to overcome these limitations, are highlighted.

Direct observation of the core/double-shell architecture of intense dual-mode luminescent tetragonal bipyramidal nanophosphors

Highly efficient downconversion (DC) green-emitting LiYF4:Ce,Tb nanophosphors have been synthesized for bright dual-mode upconversion (UC) and DC green-emitting core/double-shell (C/D-S) nanophosphors-Li(Gd,Y)F4:Yb(18%),Er(2%)/LiYF4:Ce(15%),Tb(15%)/LiYF4-and the C/D-S structure has been proved by extensive scanning transmission electron microscopy (STEM) analysis. Colloidal LiYF4:Ce,Tb nanophosphors with a tetragonal bipyramidal shape are synthesized for the first time and they show intense DC green light via energy transfer from Ce(3+) to Tb(3+) under illumination with ultraviolet (UV) light. The LiYF4:Ce,Tb nanophosphors show 65 times higher photoluminescence intensity than LiYF4:Tb nanophosphors under illumination with UV light and the LiYF4:Ce,Tb is adapted into a luminescent shell of the tetragonal bipyramidal C/D-S nanophosphors. The formation of the DC shell on the core significantly enhances UC luminescence from the UC core under irradiation of near infrared light and concurrently generates DC luminescence from the core/shell nanophosphors under UV light. Coating with an inert inorganic shell further enhances the UC-DC dual-mode luminescence by suppressing the surface quenching effect. The C/D-S nanophosphors show 3.8% UC quantum efficiency (QE) at 239 W cm(-2) and 73.0 ± 0.1% DC QE. The designed C/D-S architecture in tetragonal bipyramidal nanophosphors is rigorously verified by an energy dispersive X-ray spectroscopy (EDX) analysis, with the assistance of line profile simulation, using an aberration-corrected scanning transmission electron microscope equipped with a high-efficiency EDX. The feasibility of these C/D-S nanophosphors for transparent display devices is also considered.

A Single Excitation-Duplexed Imaging Strategy for Profiling Cell Surface Protein-Specific Glycoforms

This work develops a site-specific duplexed luminescence resonance energy transfer system on cell surface for simultaneous imaging of two kinds of monosaccharides on a specific protein by single near-infrared excitation. The single excitation-duplexed imaging system utilizes aptamer modified upconversion luminescent nanoparticles as an energy donor to target the protein, and two fluorescent dye acceptors to tag two kinds of cell surface monosaccharides by a dual metabolic labeling technique. Upon excitation at 980 nm, only the dyes linked to protein-specific glycans can be lit up by the donor by two parallel energy transfer processes, for in situ duplexed imaging of glycoforms on specific protein. Using MUC1 as the model, this strategy can visualize distinct glycoforms of MUC1 on various cell types and quantitatively track terminal monosaccharide pattern. This approach provides a versatile platform for profiling protein-specific glycoforms, thus contributing to the study of the regulation mechanisms of protein functions by glycosylation.

Polydopamine Grafted Porous Graphene as Biocompatible Nanoreactor for Efficient Identification of Membrane Proteins

Functional nanomaterials, used as nanoreactors, have shown great advantages in a variety of applications in biomedical fields. Herein, we designed a novel nanoreactor system toward the application in membrane proteomics by using polydopamine-coated nanoporous graphene foams (NGFs-PD) prepared by a facile in situ oxidative polymerization. Taking advantage of the unique 3-D structure and surface functionalization, NGFs-PD can quickly adsorb a large amount of hydrophobic membrane proteins dissolved in sodium dodecyl sulfonate (SDS)/methanol and hydrophilic trypsin in aqueous solution, and then confine the proteolysis in the nanoscale domains to fasten the reaction rate. Therefore, the current nanoreactor system combines the multifunctions of highly efficient solubilization, immobilization, and proteolysis of membrane proteins. With the nanoreactor, digestion of standard membrane proteins can be finished in 10 min. 893 membrane proteins were identified from human glioma cells (U251). All these superiorities indicate that the biocompatible NGFs-PD nanoreactor system is of great promise to facilitate high-throughput membrane proteomic analysis.

Engineering of Lanthanide-Doped Upconversion Nanoparticles for Optical Encoding

Lanthanide-doped upconversion nanoparticles (UCNPs) are an emerging class of luminescent materials that emit UV or visible light under near infra-red (NIR) excitations, thereby possessing a large anti-Stokes shift property. Due to their sharp excitation and emission bands, excellent photo- and chemical stability, low autofluorescence, and high tissue penetration depth of the NIR light used for excitation, UCNPs have surpassed conventional fluorophores in many bioapplications. A better understanding of the mechanism of upconversion, as well as the development of better approaches to preparing UCNPs, have provided more opportunities to explore their use for optical encoding, which has the potential for applications in multiplex detection and imaging. With the current ability to precisely control the microstructure and properties of UCNPs to produce particles of tunable emission, excitation, luminescence lifetime, and size, various strategies for optical encoding based on UCNPs can now be developed. These optical properties of UCNPs (such as emission and excitation wavelengths, ratiometric intensity, luminescence lifetime, and multicolor patterns), and the strategies employed to engineer these properties for optical encoding of UCNPs through homogeneous ion doping, heterogeneous structure fabrication and microbead encapsulation are reviewed. The challenges and potential solutions faced by UCNP optical encoding are also discussed.