Kinetic or thermodynamic control has been employed to guide the selective synthesis of conventional organic compounds, and it should be a powerful tool as well for accessing unusual inorganic nanocrystals, particularly when a series of members with similar chemical compositions and phase structures exist. Indeed, a comprehensive mapping of the energy barrier distribution of each nanocrystal in a predefined reaction system will enable not only the precise synthesis of nanocrystals with expected sizes, morphologies, phase structures, and ultimately functionalities, but also disclosure of the evolution details of nanocrystals from one structure to another. Using ScFx:Ln (x = 2.76, 3) series as a proof-of-concept, we have successfully mapped out the energy barriers that correspond to each of the ScFx:Ln nanocrystals, unraveled suitable temperatures for each type of nanocrystal formation, recorded their phase transition procedures, and also discovered the relationships of the products at each reaction stage. To testify how this approach allows one to tailor the structure-related optical properties, different lanthanide-doped ScFx nanocrystals were synthesized and a wide-range of luminescence fine-tuning was achieved, which not only showcases high quality of the nanocrystals, but also provides more candidates for various luminescence applications, especially when single-particle upconversion emission is required.

Impurity doping plays a critical role in altering the properties of target nanomaterials in terms of designed morphologies, crystal structures, and functionalities. In this work, we have performed a comprehensive investigation of the effect of Li+ doping on the morphology, crystal structure, and upconversion luminescence of NaYF4:Yb/Er nanocrystals. Different Li+ sources, e.g., LiOA and LiOH, were used and the Li+ doping concentration varied from 0 to 100 mol%. The final product changes from hexagonal NaYF4:Yb/Er to the mixture of cubic NaYF4:Yb/Er and tetragonal LiYF4:Yb/Er, and finally to pure tetragonal LiYF4:Yb/Er. More importantly, at an ultra-low concentration of 0.5 mol% Li+ doping, as high as 34 times green and 101 times red emission enhancements are achieved.

Topological control of nanostructures plays a crucial role in understanding the crystal growth process at the nanometer length scale. Here, the scalable synthesis of upconversion materials with distinct hedgehog-like morphologies by a seed-mediated synthetic procedure is reported. It is demonstrated that a close match in the crystal lattice between the core and shell components is essential for synthesizing such hierarchical nanostructures. These optical nanomaterials also enable the development of a single-particle-based platform for high-sensitivity molecular sensing.

Although multifunctional upconversion imaging probes have recently attracted considerable interest in biomedical research, there are currently few methods for stabilizing these luminescent nanoprobes with oligonucleotides in biological systems. Herein, a method to robustly disperse upconversion nanoprobes in physiological buffers based on rational design and synthesis of nanoconjugates comprising hairpin-DNA-modified gold nanoparticles is presented. This approach imparts the upconversion nanoprobes with excellent biocompatibility and circumvents the problem of particle agglomeration. By combining single-band anti-Stokes near-infrared emission and the photothermal effect mediated by the coupling of gold to upconversion nanoparticles, a simple, versatile nanoparticulate system for simultaneous deep-tissue imaging and drug molecule release in vivo is demonstrated.

AbstractAn ultrastrong and broadband chiroptical response is key but remains challenging for many device applications. A simple and cost-effective bottom-up method is introduced to fabricate large-area long-range ordered chiral ultrathin films with the Langmuir–Schaeffer technique using gold nanowires as building blocks. Significantly, as-prepared ultrathin films display giant optical activity across a broad wavelength range covering visible and near infrared regions with an anisotropic factor of up to 0.285, which is the record value for bottom-up techniques. Detailed experimental result and theoretical analysis disclose that such remarkable optical activity originates from birefringence and dichroism of the well-aligned Au nanowire layers in the ultrathin films. The universality of this facile strategy for constructing chiral ultrathin films is further demonstrated with many other one-dimensional nanomaterials.

AbstractA 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.

Lanthanide doped KRE2F7 (RE: rare earth) nanocrystals have recently emerged as a new type of upconversion host material due to their distinct crystal structure and tailorable upconversion emissions. However, the controlled synthesis and upconversion fine-tuning through host lattice manipulation have not been well explored. Herein, via Li+ doping, we have systematically studied the morphology and upconversion luminescence evolution of KSc2F7:Yb/Er nanocrystals. An ∼21 times luminescence enhancement and a red to green ratio of ∼22 were obtained, which was caused by Li+ induced local crystal field variation around the doped lanthanide ions. The understanding of Li+-induced KSc2F7:Yb/Er nanocrystal manipulation has paved a new avenue for functionality improvement via high content impurity doping, which should inspire even wider exploration of nanocrystal engineering, especially luminescent nanocrystals.

Despite recent progress in 2D nanomaterials-based biosensing, it remains challenging to achieve sensitive and high selective detection. This study develops few-layer graphdiyne (GD) nanosheets (NSs) that are used as novel sensing platforms for a variety of fluorophores real-time detection of DNA with low background and high signal-to-noise ratio, which show a distinguished fluorescence quenching ability and different affinities toward single-stranded DNA and double-stranded DNA. Importantly, for the first time, a few-layer GD NSs-based multiplexed DNA sensor is developed.

Although proton conductors derived from metal–organic frameworks (MOFs) are highly anticipated for various applications including solid-state electrolytes, H2 sensors, and ammonia synthesis, they are facing serious challenges such as poor water stability, fastidious working conditions, and low proton conductivity. Herein, we report two lanthanide–oxalate MOFs that are highly water stable, with so far the highest room-temperature proton conductivity (3.42 × 10−3 S cm−1) under 100% relative humidity (RH) among lanthanide-based MOFs and, most importantly, luminescent. Moreover, the simultaneous response of both the proton conductivity and luminescence intensity to RH allows the linkage of proton conductivity with luminescence intensity. This way, the electric signal of proton conductivity variation versus RH will be readily translated to optical signal of luminescence intensity, which can be directly visualized by the naked eye. If proper lanthanide ions or even transition-metal ions are used, the working wavelengths of luminescence emissions can be further extended from visible to near infrared light for even wider-range applications.

The capability to selectively obtain expected nanocrystals from a series of possibilities with similar chemical compositions has been a long-term challenge. β-LaF3 and β-NaLaF4 nanocrystals are a kind of host material with excellent physical and chemical stability as well as low phonon energy. Since β-NaLaF4 nanocrystals are difficult to form due to the very strong La3+–F− ionic bond, investigations of the doping effect of different lanthanide ions on the formation of the final product were carried out. The experimental results have indicated that Ce3+ and Pr3+ doping has no obvious effect on the product compared to pure β-LaF3 nanocrystals without doping, and doping of Nd3+, Sm3+, Eu3+, Gd3+, and Tb3+ leads to the formation of pure β-NaLaF4 nanocrystals while doping of Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+ only results in a mixture of both. What is more, proper lanthanide ion doping can also help in obtaining enhanced up- and downconversion luminescence. A theoretical model from the nanocrystal growth free energy point of view has been developed to explain the above phenomenon.

Enhancing upconversion emission is critical for small-sized lanthanide doped upconversion nanocrystals. A promising way is increasing the doping concentration of excitation energy absorbers, the Yb3+ sensitizer. However, it is still a challenge to obtain small-sized hexagonal NaLnF4 (Ln: lanthanide) upconversion nanocrystals with a high Yb3+ concentration due to the fast growth of NaYbF4 crystals, which hinders their applications particularly in biology. We here demonstrate a highly repeatable and controllable method for tuning the size of hexagonal NaYbF4 nanocrystals, down to ∼7 nm, without the assistance of additional impurity doping. By monitoring the reaction process, we found that ultrasmall hexagonal NaYbF4 nanocrystals were formed through an in situ transformation of their cubic counterparts. We observed an enhanced upconversion emission of NaYbF4:Tm nanocrystals when compared to that of NaYbF4:Y/Tm nanocrystals with less Yb3+ doping. After coating a thin layer of a NaYF4 shell on NaYbF4:Tm nanocrystals, a ∼100 times upconversion emission enhancement with over 800 times stronger emission in the ultraviolet and blue ranges was observed. This versatile method, together with the strong upconversion emission of the as-prepared ultrasmall nanocrystals, should facilitate the future applications of upconversion nanocrystals.

The synthesis of REF3 (RE = La–Lu, Y) nanocrystals with controlled phase structures has so far remained a challenge. Herein we have developed a one-for-all synthetic procedure that allows the successful synthesis of REF3 nanocrystals in a controlled manner. Experimental results showed that the radius of RE ions determines the phase structure: pure hexagonal REF3 (RE = La–Eu), a mixture of hexagonal and orthorhombic REF3 (RE = Gd), and pure orthorhombic REF3 (RE = Tb–Lu, Y) nanocrystals are obtained along with the decrease of the ionic radius. As Gd is positioned exactly in the middle of the lanthanides row, GdF3 nanocrystals were used as a model to further investigate how the molar ratio of F− : Gd3+, the doping of RE ions with different ionic radii, and the doping concentration of certain RE ions affects the crystal structure of the final product.

The synthesis of REF3 (RE = La–Lu, Y) nanocrystals with controlled phase structures has so far remained a challenge. Herein we have developed a one-for-all synthetic procedure that allows the successful synthesis of REF3 nanocrystals in a controlled manner. Experimental results showed that the radius of RE ions determines the phase structure: pure hexagonal REF3 (RE = La–Eu), a mixture of hexagonal and orthorhombic REF3 (RE = Gd), and pure orthorhombic REF3 (RE = Tb–Lu, Y) nanocrystals are obtained along with the decrease of the ionic radius. As Gd is positioned exactly in the middle of the lanthanides row, GdF3 nanocrystals were used as a model to further investigate how the molar ratio of F− : Gd3+, the doping of RE ions with different ionic radii, and the doping concentration of certain RE ions affects the crystal structure of the final product.

The increasingly wide application of lanthanide-based upconversion nanocrystals has generated an urgent need to control and manipulate both the morphological and optical properties of these materials. In this work, Sc3+ ions were utilized to modulate the morphology, phase structure, and upconversion luminescence properties of YF3:Yb/Er nanocrystals. At a Sc3+ doping concentration of 0–25 mol% the YF3:Yb/Er nanocrystals gradually changed from ∼16 nm rhombic nanodots to ∼57 nm zigzag-shaped nanorods, and the upconversion luminescence was enhanced by 5 times. The co-existence of the YF3:Yb/Er and ScF3:Yb/Er nanocrystals was then seen at 25–60 mol% Sc3+ doping, and finally the ScF3:Yb/Er nanocrystals became the only product at >60 mol% Sc3+ doping. A mechanism of oriented attachment growth has been proposed to explain the morphology evolution, and the upconversion luminescence details have been discussed from the crystal structure point of view.

Lanthanide-doped upconversion nanoparticles have received growing attention in the development of low-background, highly sensitive and selective sensors. Here, we report a water probe based on ligand-free NaYF4:Yb/Er nanoparticles, utilizing their intrinsically nonlinear upconversion process. The water molecule sensing was realized by monitoring the upconversion emission quenching, which is mainly attributed to efficient energy transfer between upconversion nanoparticles and water molecules as well as water-absorption-induced excitation energy attenuation. The nonlinear upconversion process, together with power function relationship between upconversion emission intensity and excitation power density, offers a sensitive detection of water content down to 0.008 vol % (80 ppm) in an organic solvent. As an added benefit, we show that noncontact detection of water can be achieved just by using water attenuation effect. Moreover, these upconversion nanoparticle based recyclable probes should be particularly suitable for real-time and long-term water monitoring, due to their superior chemical and physical stability. These results could provide insights into the design of upconversion nanoparticle based sensors.Keywords: lanthanide-doped nanoparticles; luminescence; sensor; upconversion; water

Herein we report the effect of surfactant molecules, i.e., trisodium citrate, on the morphology, size evolution, as well as the growth mechanism of ScPO4·2H2O:Ln (Ln = Ce/Tb/Eu, Yb/Er) microparticles, which are synthesized via a one-pot hydrothermal method. The up- and down-conversion photoluminescence and dynamics of ScPO4·2H2O:Ln microparticles, including the decay time, quantum efficiency, and the energy transfer mechanism with different dopants, are further investigated. Finally, a potential application of these materials as luminescent display inks is demonstrated.

Synthesis of lanthanide-doped upconversion nanocrystals (LDUNs) with controlled morphology and luminescence has long been desired in order to fulfill various application requirements. In this work, we have investigated the effect of the NaF:Ln3+ molar ratio, in the range of 1 to 20, on the morphology, crystal structure, and upconversion properties of NaxScF3+x:Yb/Er nanocrystals that are reported to possess different upconversion properties from those of NaYF4:Yb/Er nanocrystals. The experimental results prove that the NaF:Ln3+ molar ratio influences significantly the growth process of the nanocrystals, i.e. a low NaF:Ln3+ molar ratio results in hexagonal NaScF4 nanocrystals, while a high NaF:Ln3+ molar ratio favors monoclinic Na3ScF6 nanocrystals. When the NaF:Ln3+ molar ratio is as high as 6 or above, phase separation is found and hexagonal NaYbF4 nanocrystals showed up for the first time. Simply by adjusting the NaF:Ln3+ molar ratio, we have successfully achieved the simultaneous control of the shape, size, as well as the crystallographic phase of the NaxScF3+x:Yb/Er nanocrystals, which give different red to green (R/G) ratios (integral area), leading to a multicolor upconversion luminescence from orange-red to green. This study provides a vivid example to track and interpret the formation mechanisms and growth processes of NaxScF3+x:Yb/Er nanocrystals, which shall be instructive for guiding the controlled synthesis of other LDUNs and extending their according applications in optical communication, color display, anti-counterfeiting, bioimaging, and so on.

NaxScF3+x nanocrystals with controllable shapes were synthesized in the oleic acid/1-octadecene (OA/OD) coprecipitation reaction system. By adjusting the volume ratio of the solvents, well-defined NaxScF3+x nanopolyhedrons, nanoplates, nanorods, and nanospheres with different sizes and phases could be selectively obtained. The as-prepared NaxScF3+x nanocrystals were well characterized and investigated. Based on the results obtained, it was found that the solvents influenced significantly the growth process of the NaxScF3+x nanocrystals. A change in the nucleus formation rate and the responsible crystal planes leads to different morphologies of the resulting nanocrystals. Further investigation on the upconversion (UC) luminescence of the Yb/Er codoped NaxScF3+x nanocrystals showed that NaxScF3+x nanostructures could serve as host matrices to give strong UC luminescence. In addition, their different phases and morphologies were responsible for the diverse luminescence intensity.

•The design principles for luminescent probes based on lanthanide complexes are summarized.•Recognition mechanisms of luminescent probes based on lanthanide complexes for various analytes are described.•The effects of the types of lanthanide nanoparticles on the luminescence sensing behaviors are highlighted.Lanthanide-based luminescent probes have attracted increasing attention due to their unique optical properties, such as large Stokes and/or anti-Stokes shifts, long luminescence lifetimes (up to milliseconds), and narrow and compound-independent emission bands, making them widely employed in detection, diagnosis, and bioimaging. This review focuses on the recent developments of lanthanide-based luminescent probes including lanthanide complexes and lanthanide nanoparticles for probing pH, anions, metal ions, reactive oxygen species, and biomolecules (amino acids, proteins, nucleobases, and nucleic acids). The design principles and recognition mechanisms of luminescent probes based on lanthanide ions for various analytes are elaborated in detail. In the end, future research directions with great potentials and the according challenges of lanthanide-based luminescent probes are also discussed.

Rare earth (RE)-based phosphors demonstrate sharp emission lines, long lifetimes and high luminescence quantum yields; thus, they have been employed in various photoelectric devices, such as light-emitting diodes (LEDs) and solar spectral converters. However, their applications are largely confined by their narrow excitation bands and small absorption cross sections of 4f–4f transitions. In this paper, we demonstrate a novel strategy to improve and expand the visible excitation bands of Eu3+ ions through the interface energy transfer (ET) from CdTe quantum dots (QDs) to YVO4:Eu3+ inverse opal photonic crystals (IOPCs). The significant effects observed in the CdTe QDs/YVO4:Eu3+ IOPCs composites were that the excitation of Eu3+ ions was continuously extended from 450 to 590 nm and that the emission intensity of the 5D0–7FJ transitions was enhanced ∼20-fold, corresponding to the intrinsic 7F1–5D1 excitation at 538 nm. Furthermore, in the IOPC network, the ET efficiency from the QDs to YVO4:Eu3+ was greatly improved because of the suppression of energy migration among the CdTe QDs, which gave an optimum ET efficiency as high as 47%. Besides, the modulation of photonic stop bands (PSBs) on the radiative transition rates of the QDs and Eu3+ ions was studied, which showed that the decay lifetime constants for Eu3+ ions were independent of PSBs, while those of QDs demonstrated a suppression in the PSBs. Their physical nature was explained theoretically.

High quality and monodispersed ScVO4 microcrystals were successfully synthesized via a mild hydrothermal route using NH4VO3 as vanadium source. The X-ray power diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) results indicated that the size, shape, and phase formation of the ScVO4 microcrystals could be tuned by altering the reaction temperature, reaction time, and pH value of the initial solution. Furthermore, the down- and upconversion luminescence of ScVO4:Ln (Ln=Eu, Dy, Sm, Yb/Ho, Yb/Er, and Yb/Tm) microcrystals were characterized and the respective energy transfer processes were also discussed. The experimental results demonstrated that reactions at 200 °C, 24 h, and pH=6 could generate perfect ScVO4 microcrystals, which were then used for luminescence studies.Morphology evolution of ScVO4 microcrystals at different pH valuesDownload high-res image (167KB)Download full-size image

•Silk cocoon is a versatile natural protein for the fabrication of pseudographitic carbon materials with tunable flexibility.•Microbial fuel cells equipped with carbon materials derived from silk cocoon exhibit high anode performance.•Pseudographitic carbon derived from silk cocoon features enriched nitrogen content, hierarchical pores, good biocompatibility and high capacitance.Microbial fuel cells (MFCs), promising for converting biomass energy into electricity, have attracted much research enthusiasm. However, high performance anode materials for MFC, particularly with tunable flexibility for diverse cell configurations, are still limited. In this study, through a simple one-step carbonization of a versatile protein precursor, silk cocoon, both freestanding and flexible bioanode materials, with enriched nitrogen contents and hierarchical pores, can be easily fabricated. Importantly, the carbonized silk cocoon, as a freestanding MFC anode, and flexible carbon fiber, as a flexible MFC anode, exhibit high performance in electricity generation, yielding about 2.5-fold and 3.1-fold maximum gravimetric power density than that of MFCs with carbon cloth anode, respectively. We attribute the improved anode performance of these flexibility tunable carbon materials to their good biocompatibility, reduced electron transfer resistance and high capacitance. This study will not only offer great opportunities for the fabrication of high-performance MFC anode with varied designs and 3-dimensional architectures, but also shed light on the future development of MFC and proper utilization of the abundant “green” natural resources.

Herein we report the effect of surfactant molecules, i.e., trisodium citrate, on the morphology, size evolution, as well as the growth mechanism of ScPO4·2H2O:Ln (Ln = Ce/Tb/Eu, Yb/Er) microparticles, which are synthesized via a one-pot hydrothermal method. The up- and down-conversion photoluminescence and dynamics of ScPO4·2H2O:Ln microparticles, including the decay time, quantum efficiency, and the energy transfer mechanism with different dopants, are further investigated. Finally, a potential application of these materials as luminescent display inks is demonstrated.

Lanthanide doped KRE2F7 (RE: rare earth) nanocrystals have recently emerged as a new type of upconversion host material due to their distinct crystal structure and tailorable upconversion emissions. However, the controlled synthesis and upconversion fine-tuning through host lattice manipulation have not been well explored. Herein, via Li+ doping, we have systematically studied the morphology and upconversion luminescence evolution of KSc2F7:Yb/Er nanocrystals. An ∼21 times luminescence enhancement and a red to green ratio of ∼22 were obtained, which was caused by Li+ induced local crystal field variation around the doped lanthanide ions. The understanding of Li+-induced KSc2F7:Yb/Er nanocrystal manipulation has paved a new avenue for functionality improvement via high content impurity doping, which should inspire even wider exploration of nanocrystal engineering, especially luminescent nanocrystals.

The increasingly wide application of lanthanide-based upconversion nanocrystals has generated an urgent need to control and manipulate both the morphological and optical properties of these materials. In this work, Sc3+ ions were utilized to modulate the morphology, phase structure, and upconversion luminescence properties of YF3:Yb/Er nanocrystals. At a Sc3+ doping concentration of 0–25 mol% the YF3:Yb/Er nanocrystals gradually changed from ∼16 nm rhombic nanodots to ∼57 nm zigzag-shaped nanorods, and the upconversion luminescence was enhanced by 5 times. The co-existence of the YF3:Yb/Er and ScF3:Yb/Er nanocrystals was then seen at 25–60 mol% Sc3+ doping, and finally the ScF3:Yb/Er nanocrystals became the only product at >60 mol% Sc3+ doping. A mechanism of oriented attachment growth has been proposed to explain the morphology evolution, and the upconversion luminescence details have been discussed from the crystal structure point of view.

Impurity doping plays a critical role in altering the properties of target nanomaterials in terms of designed morphologies, crystal structures, and functionalities. In this work, we have performed a comprehensive investigation of the effect of Li+ doping on the morphology, crystal structure, and upconversion luminescence of NaYF4:Yb/Er nanocrystals. Different Li+ sources, e.g., LiOA and LiOH, were used and the Li+ doping concentration varied from 0 to 100 mol%. The final product changes from hexagonal NaYF4:Yb/Er to the mixture of cubic NaYF4:Yb/Er and tetragonal LiYF4:Yb/Er, and finally to pure tetragonal LiYF4:Yb/Er. More importantly, at an ultra-low concentration of 0.5 mol% Li+ doping, as high as 34 times green and 101 times red emission enhancements are achieved.