Monodisperse Dual‐Functional Upconversion Nanoparticles Enabled Near‐Infrared Organolead Halide Perovskite Solar Cells

Extending the spectral absorption of organolead halide perovskite solar cells from visible into near‐infrared (NIR) range renders the minimization of non‐absorption loss of solar photons with improved energy alignment. Herein, we report on, for the first time, a viable strategy of capitalizing on judiciously synthesized monodisperse NaYF₄:Yb/Er upconversion nanoparticles (UCNPs) as the mesoporous electrode for CH₃NH₃PbI₃ perovskite solar cells and more importantly confer perovskite solar cells to be operative under NIR light. Uniform NaYF₄:Yb/Er UCNPs are first crafted by employing rationally designed double hydrophilic star‐like poly(acrylic acid)‐block‐poly(ethylene oxide) (PAA‐b‐PEO) diblock copolymer as nanoreactor, imparting the solubility of UCNPs and the tunability of film porosity during the manufacturing process. The subsequent incorporation of NaYF₄:Yb/Er UCNPs as the mesoporous electrode led to a high efficiency of 17.8 %, which was further increased to 18.1 % upon NIR irradiation. The in situ integration of upconversion materials as functional components of perovskite solar cells offers the expanded flexibility for engineering the device architecture and broadening the solar spectral use.     

A "turn-off" luminescence resonance energy transfer aptamer sensor based on near-infrared upconverting NaYF4:Yb3+,Tm3+ nanoparticles as donors and gold nanorods as acceptors

Near-infrared upconverting NaYF₄:Yb^3*,Tm^3* nanophosphors modified with poly（acrylic acid） were prepared and characterized by transmission electron microscopy and luminescence spectroscopy.Based on the observed overlap between the emission spectrum of the NaYF₄:Yb^3*,Tm^3* nanophosphors and the absorption spectrum of the gold nanorods,we believe that a new "turn-off luminescence resonance energy transfer aptamer sensor was constructed for sensing thrombin in near-infrared region.     

Synthesis and Infrared Multi‐band Absorption Properties of Core‐shell NaYF4:Yb3+, Er3+@SiO2 Nanoparticles

Due to the unique size effects, nanomaterials in infrared absorption have attracted much attention for their strong absorption in the infrared region. To achieve the infrared multi‐band absorption, we propose to synthesize a core‐shell structure nanomaterial consisting of NaYF₄:Yb³⁺, Er³⁺ core and a layer of SiO₂ as shell. A series of NaYF₄:Yb³⁺, Er³⁺ nanocrystals were synthesized through hydrothermal method by adjusting the ratio of citric acid(CA)‐to‐NaOH, and the effects of CA concentration, and NaOH concentration were studied in detail. NaYF₄:Yb³⁺, Er³⁺@SiO₂ nanoparticles were synthesized by sol‐gel method using TEOS as silica source. The results show that the core‐shell NaYF₄:Yb³⁺, Er³⁺@SiO₂ nanoparticles were successfully synthesized. Up‐conversion spectra of these nanoparticles were recorded with 980 nm laser excitation under room temperature. There are no changes of the emission centers of nanoparticles before or after silica coating, but the emission intensities of nanoparticles after silica coating are weakened. Furthermore, the property of infrared multi‐band absorption was tested through ultraviolet‐visible‐near infrared spectrophotometer and infrared absorption spectra. The results illustrate that the multi‐band infrared absorption nanomaterial was successfully synthesized.     

Synthesis and characterization of nanoporous NaYF4:Yb3+, Tm3+@SiO2 nanocomposites

Novel upconversion nanocomposites with nanoporous structure were presented in this paper. Silica-coated cubic NaYF₄:Yb³⁺, Tm³⁺ nanoparticles were first prepared. After annealing, monodisperse cubic/hexagonal mixed phases NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles were obtained, and the NaYF₄:Yb³⁺, Tm³⁺ cores became nanoporous. To the best of our knowledge, the nanoporous structure in NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanocomposites was observed for the first time. They demonstrate increased upconversion emission compared with unannealed dense NaYF₄:Yb³⁺, Tm³⁺ nanoparticles due to the appearance of the hexagonal NaYF₄:Yb³⁺, Tm³⁺. The silica shell not only makes the nanocomposites possess bio-affinity but also protects the NaYF₄:Yb³⁺, Tm³⁺ cores from aggregating and growing up. Thus the upconversion, nanoporous and bio-affinity properties were combined into one single nanoparticle. The nanocomposites have been characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), small angle X-ray diffraction (SAXRD) and emission spectroscopy. These multifunctional nanocomposites are expected to find applications in biological fields, such as biolabels, drug storage and delivery.     

NIR triggered NaYF4:Yb3+,Tm3+@NaYF4/CsPb(Br1-x/Ix)3 composite for up-converted white-light emission and dual-model anti-counterfeiting applications

Assembling nanomaterials from two classes with exceptional control at the nanoscale can lead to new nanohybrids with novel properties. Here, we report the tunable up-conversion luminescence properties of CsPb(Br_1-x/I_x)₃ perovskite nanocrystals (PeNCs) sensitized by NaYF₄:Yb,Tm@NaYF₄ up-conversion nanoparticles (UCNPs) at 980 nm excitation. The up-conversion luminescence of NaYF₄:Yb³⁺,Tm³⁺@NaYF₄/CsPb(Br_1-x/I_x)₃ composite demonstrates that the radiative photon reabsorption process is accountable for the UC energy transfer from excited levels of Tm³⁺-based UCNPs to PeNCs. The long-lived Tm³⁺ states feed PeNCs carriers with intrinsic lifetimes extending from nanoseconds to microseconds. By varying the UCNPs/PeNCs concentration ratio, the NaYF₄:Yb³⁺,Tm³⁺@NaYF₄/CsPb(Br_0.55I_0.45)₃ composite generates UC white light emission. The near-infrared excited white light-emitting devices are more compatible with human tissues than blue light-excited ones. Therefore, the prototype of UC white light-emitting diode is developed by coupling the UCNPs/PeNCs composite coated glass plate onto a commercial 940 nm-light-emitting diode chip. To overcome the counterfeiting risk that arises in the case of a single fluorescence mode, we developed a simple dual-model strategy based on manipulation of UC and down-conversion luminescence in anti-counterfeiting under 980 nm and 365 nm excitation, which makes it difficult to encrypt the information. In addition, the UCNPs/PeNCs composite exhibited better photostability under near-infrared illumination, retaining 85% of initial photoluminescence intensity, solving the problem of photo-instability.     

Synthesis,properties and mechanism of photodegradation of core–shell structured upconversion luminescent NaYF4:Yb3+,Er3+@BiOCl

Microspherical bismuth oxychloride (BiOCl) can only utilize ultraviolet (UV) light to promote photocatalytic reactions. To overcome this limitation, a uniform and thin BiOCl nanosheet was synthesized with a particle size of about 200 nm. As results of UV–visible diffuse reflectance spectroscopy showed, the band gap of this nanostructure was reduced to 2.78 eV, indicating that the BiOCl nanosheet could absorb and utilize visible light. Furthermore, the upconversion material NaYF₄ doped with rare earth ions Yb³⁺ and Er³⁺ emitted visible light at 410 nm following excitation with near‐infrared (NIR) light (980 nm), which could be utilized by BiOCl to produce a photocatalytic reaction. To produce a high‐efficiency photocatalyst (NaYF₄:Yb³⁺,Er³⁺@BiOCl), BiOCl‐loaded NaYF₄:Yb³⁺,Er³⁺ was successfully synthesized via a simple two‐step hydrothermal method. The as‐synthesized material was confirmed using X‐ray diffraction, scanning electron microscopy, X‐ray photoelectron spectroscopy as well as other characterizations. The removal ratio of methylene blue by NaYF₄:Yb³⁺,Er³⁺@BiOCl was much higher than that of BiOCl alone. Recycling experiments verified the stability of NaYF₄:Yb³⁺,Er³⁺@BiOCl, which demonstrated excellent adsorption, strong visible‐light absorption and high electron–hole separation efficiency. Such properties are expected to be useful in practical applications, and a further understanding of the NIR‐light‐responsive photocatalytic mechanism of this new catalytic material would be conducive to improving its structural design and function.     

Designing Upconversion Nanocrystals Capable of 745 nm Sensitization and 803 nm Emission for Deep‐Tissue Imaging

A crystal design strategy is described that generates hexagonal‐phased NaYF₄:Nd/Yb@NaYF₄:Yb/Tm luminescent nanocrystals with the ability to emit light at 803 nm when illuminated at 745 nm. This is accomplished by taking advantage of the large absorption cross‐section of Nd³⁺ between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd³⁺→Yb³⁺→Yb³⁺→Tm³⁺. Mechanistic investigations suggest that a cascaded two‐photon energy transfer upconversion process underlies the emission mechanism. This protocol enables deep‐tissue imaging to be achieved while mitigating the attenuation effect associated with the visible emission and the overheating constraint imposed by conventional 980 nm excitation.     

Shape,Size, and Phase‐Controlled Rare‐Earth Fluoride Nanocrystals with Optical Up‐Conversion Properties

High‐quality rare‐earth fluorides, α‐NaMF₄ (M=Dy, Ho, Er, Tm, Y, Yb, and Lu) nanocrystals and β‐NaMF₄ (M=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Y, Yb, and Lu) nanoarrays, have been synthesized by using oleic acid as a stabilizing agent through a facile hydrothermal method at 130–230 °C. The phase, shape, and size of the products are varied by careful control of synthetic conditions, including hydrothermal temperature and time, and the amounts of reactants and solvents. Tuning the hydrothermal temperature, time, and the amount of NaOH can cause the transformation from the cubic α‐NaMF₄ to hexagonal phase β‐NaMF₄. Upon adjustment of the amount of NaOH, NaF, M³⁺, and ethanol, the morphologies for the β‐NaMF₄ nanoarrays can range from tube, rod, wire, and zigzagged rod, to flower‐patterned disk. Simultaneously, the size of the rare‐earth fluoride crystals is variable from 5 nm to several micrometers. A combination of “diffusion‐controlled growth” and the “organic–inorganic interface effect” is proposed to understand the formation of the nanocrystals. An ideal “1D growth” of rare‐earth fluorides is preferred at high temperatures and high ethanol contents, from which the tube‐ and rodlike nanoarrays with high aspect ratio are obtained. In contrast, the disklike β‐NaMF₄ nanoarrays with low aspect ratios are produced by decreasing the ethanol content or prolonging the reaction time, an effect probably caused by “1D/2D ripening”. Multicolor up‐conversion fluorescence is also successfully realized in the Yb³⁺/Er³⁺ (green, red) and Yb³⁺/Tm³⁺ (blue) co‐doped α‐NaYF₄ nanocrystals and β‐NaYF₄ nanoarrays by excitation in the NIR region (980 nm).     

Synthesis of Upconverting Hydrogel Nanocomposites Using Thiol‐Ene Click Chemistry: Template for the Formation of Dendrimer‐Like Gold Nanoparticle Assemblies

The synthesis of upconverting hydrogel nanocomposites by base‐catalyzed thiol‐ene click reaction between 10‐undecenoic acid capped Yb³⁺/Er³⁺‐doped NaYF₄ nanoparticles and pentaerythritol tetrakis(3‐mercaptopropionate) (PETMP) as tetrathiol monomer is reported. This synthetic strategy for nanocomposite gels is quite different from works where usually the preformed gels are mixed with the nanoparticles. Developing nanocomposites by surface modification of capping ligands would allow tuning and controlling of the separation of the nanoparticles inside the gel network. The hydrogel nanocomposites prepared by thiol‐ene click reaction show strong enhancement in luminescence intensity compared to 10‐undecenoic acid‐capped Yb³⁺/Er³⁺‐doped NaYF₄ nanoparticles through the upconversion process (under 980 nm laser excitation). The hydrogel nanocomposites display strong swelling characteristics in water resulting in porous structures. Interestingly, the resulting nanocomposite gels act as templates for the synthesis of dendrimer‐like Au nanostructures when HAuCl₄ is reduced in the presence of the nanocomposite gels.     

Upconversion nanoparticle-based fluorescence resonance energy transfer assay for Cr(III) ions in urine

A novel assay of chromium(III) ion based on upconversion fluorescence resonance energy transfer was designed and established. Lysine-capped NaYF₄:Yb/Er upconversion nanoparticles (UCNPs) and dimercaptosuccinic acid-capped gold nanoparticles (AuNPs) were used as the energy donor and acceptor, respectively. They were bound together via electrostatic interaction, resulting in the quenching of the fluorescence of UCNPs by AuNPs. Chromium(III) ions can specifically and strongly interact with dimercaptosuccinic acid that was modified on the surface of AuNPs, leading to the separation of AuNPs from UCNPs and the recovery of fluorescence of UCNPs. The fluorescence recovery of UCNPs showed a good linear response to Cr³⁺ concentration in the range of 2–500 nM with a detection limit of 0.8 nM. This method was further applied to determine the levels of Cr³⁺ in urine. Compared with other fluorescence methods, current method displayed very high sensitivity and signal-to-noise ratio because of the excitation of near-infrared that can eliminate autofluorescence, providing a promising examination of biological samples for the diagnostic purposes.     

Rare‐Earth‐Based Nanoparticles with Simultaneously Enhanced Near‐Infrared (NIR)‐Visible (Vis) and NIR‐NIR Dual‐Conversion Luminescence for Multimodal Imaging

Multifunctional NaGdF₄:Yb³⁺,Er³⁺,Nd³⁺@NaGdF₄:Nd³⁺ core–shell nanoparticles (called Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:Nd³⁺ NPs) with simultaneously enhanced near‐infrared (NIR)‐visible (Vis) and NIR‐NIR dual‐conversion (up and down) luminescence (UCL/DCL) properties were successfully synthesized. The resulting core–shell NPs simultaneously emitted enhanced UCL at 522, 540, and 660 nm and DCL at 980 and 1060 nm under the excitation of a 793 nm laser. The enhanced UCL and DCL can be explained by complex energy‐transfer processes, Nd³⁺→Yb³⁺→Er³⁺ and Nd³⁺→Yb³⁺, respectively. The effects of Nd³⁺ concentration and shell thickness on the UCL/DCL properties were systematically investigated. The UCL and DCL properties of NPs were observed under the optimal conditions: a shell Nd³⁺ content of 20 % and a shell thickness of approximately 5 nm. Moreover, the Gd:Yb³⁺,Er³⁺,Nd³⁺@Gd:20 % Nd³⁺ NPs exhibited remarkable magnetic resonance imaging (MRI) properties similar to that of a clinical agent, Omniscan. Thus, the core–shell NPs with excellent UCL/DCL/magnetic resonance imaging (MRI) properties have great potential for both in vitro and in vivo multimodal bioimaging.     

An efficient upconversion luminescence energy transfer system for determination of trace amounts of nitrite based on NaYF4:Yb3+, Er3+ as donor

Based on NaYF₄:Yb³⁺, Er³⁺ upconversion nanocrystals as donor and 4-((4-(2-aminoethylamino)naphthalen-1-yl)diazenyl) benzenesulfonic acid dihydrochloride (ANDBS) as acceptor, an efficient luminescence energy transfer (LET) system was developed for selective and sensitive determination of trace amounts of nitrite. Based on Griess Reaction, ANDBS was generated by the quantitative reaction of nitrite, sulfanilamide and N-(1-naphtyl)-ethylenediamine dihydrochloride (N1NED). The degree of the overlaps between the emission spectrum of NaYF₄:Yb³⁺, Er³⁺ and the absorption spectrum of ANDBS were effective for luminescence energy transfer. Under the optimal condition, the upconversion luminescence quenching of NaYF₄:Yb³⁺, Er³⁺ was in proportion to the trace amounts of nitrite. The detection limit for nitrite achieved is 0.0046 μg mL^?1 and the system shows high sensitivity towards nitrite at 0.008000–0.2500 μg mL^?1 range.     

Strong Single‐Band Blue Emission from Colloidal Ce3+/Tm3+‐Doped NaYF4 Nanocrystals for Light‐Emitting Applications

An intense single‐band blue emission at λ=450 nm is observed from Tm³⁺ ions through Ce³⁺ sensitization, for the first time, in colloidal Ce³⁺/Tm³⁺‐doped NaYF₄ nanocrystals. The intense Tm³⁺ emission through broad‐band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light‐emitting diodes (LEDs) to develop phosphor‐based blue LEDs.     

Lanthanide‐Doped LiLuF4 Upconversion Nanoprobes for the Detection of Disease Biomarkers

Lanthanide‐doped upconversion nanoparticles (UCNPs) have shown great promise in bioapplications. Exploring new host materials to realize efficient upconversion luminescence (UCL) output is a goal of general concern. Herein, we develop a unique strategy for the synthesis of novel LiLuF₄:Ln³⁺ core/shell UCNPs with typically high absolute upconversion quantum yields of 5.0 % and 7.6 % for Er³⁺ and Tm³⁺, respectively. Based on our customized UCL biodetection system, we demonstrate for the first time the application of LiLuF₄:Ln³⁺ core/shell UCNPs as sensitive UCL bioprobes for the detection of an important disease marker β subunit of human chorionic gonadotropin (β‐hCG) with a detection limit of 3.8 ng mL⁻¹, which is comparable to the β‐hCG level in the serum of normal humans. Furthermore, we use these UCNPs in proof‐of‐concept computed tomography imaging and UCL imaging of cancer cells, thus revealing the great potential of LiLuF₄:Ln³⁺ UCNPs as efficient nano‐bioprobes in disease diagnosis.     

Boosting Luminance Energy Transfer Efficiency in Upconversion Nanoparticles with an Energy‐Concentrating Zone

Despite the successful application of upconversion nanoparticles (UCNPs), their low energy transfer efficiency is still a bottleneck to further applications. Here we design UCNPs with a multilayer structure, including an inert NaYF₄:Gd core and an energy‐concentrating zone (ECZ), for efficient energy concentration. The ECZ is composed of an emitting layer of NaYF₄:Yb,Er and an absorption layer of NaYF₄:Nd,Yb with antenna IRDye 800CW to manipulate the energy transfer. The stable and tight packing of 800CW linked originally with a bisphosphonate 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, the ECZ UCNPs show 3600 times higher luminescence intensity with an energy transfer efficiency near 60 %. In proof‐of‐concept applications, this type of structure was employed for Hg²⁺ detection and for photodynamic therapy under hypoxic conditions.     

Unraveling Epitaxial Habits in the NaLnF4 System for Color Multiplexing at the Single‐Particle Level

We report an epitaxial growth technique for scalable production of hybrid sodium rare‐earth fluoride (NaLnF₄) microcrystals, including NaYF₄, NaYbF₄, and NaLuF₄ material systems. The single crystalline nature of the as‐synthesized products makes them strong upconversion emission. The freedom of combining a lanthanide activator (Er³⁺ or Tm³⁺) with a sensitizer (Yb³⁺) at various doping concentrations readily gives access to color multiplexing at the single‐particle level. Our kinetic and thermodynamic investigations on the epitaxial growth of core–shell microcrystals using NaLnF₄ particle seeds suggest that within a certain size regime it is plausible to exert precise control over shell thickness and growth orientation under hydrothermal conditions.     

Synthesis and surface modification of ultrasmall monodisperse NaYF4:Yb3+/Tm3+ upconversion nanoparticles

The emerging upconversion nanoparticles (UCNP) have gained substantial consideration in the field of bioanalytical as well as diagnostic applications. Therefore, great progress has been made in the synthesis and surface modification of luminescent UCNPs over the last two decades. In this paper, we have reported monodispersed and high luminescent upconversion nanoparticles NaYF₄: 20%Yb³⁺, 2%Tm³⁺ have been synthesized using a solvothermal method, followed by a coating of the NaYF₄ shell with a thin layer of SiO₂ on the surface to afford the core-shell NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles (NP@SiO₂). The prepared nanoparticles were of strong upconversion fluorescent emission intensity, hexagonal phase, and with an average size of about 8 ± 1 nm, which have been characterized by luminescence spectroscopy, powder X-ray diffraction (P-XRD), Dynamic light scattering (DLS), and Transmission electron microscopy (TEM). The results indicate that the NP@SiO₂ can be used for the conjugation of fluorescent probes for various biomolecules and can find applications in cancer cell imaging and disease diagnosis.     

Highly Efficient Visible‐to‐NIR Luminescence of Lanthanide(III) Complexes with Zwitterionic Ligands Bearing Charge‐Transfer Character: Beyond Triplet Sensitization

Two zwitterionic‐type ligands featuring π–π* and intraligand charge‐transfer (ILCT) excited states, namely 1,1′‐(2,3,5,6‐tetramethyl‐1,4‐phenylene)bis(methylene)dipyridinium‐4‐olate (TMPBPO) and 1‐dodecylpyridin‐4(1 H)‐one (DOPO), have been prepared and applied to the assembly of lanthanide coordination complexes in an effort to understand the ligand‐direction effect on the structure of the Ln complexes and the ligand sensitization effect on the luminescence of the Ln complexes. Due to the wide‐band triplet states plus additional ILCT excitation states extending into lower energy levels, broadly and strongly sensitized photoluminescence of f→f transitions from various Ln³⁺ ions were observed to cover the visible to near‐infrared (NIR) regions. Among which, the Pr, Sm, Dy, and Tm complexes simultaneously display both strong visible and NIR emissions. Based on the isostructural feature of the Ln complexes, color tuning and single‐component white light was achieved by preparation of solid solutions of the ternary systems Gd‐Eu‐Tb (for TMPBPO) and La‐Eu‐Tb and La‐Dy‐Sm (for DOPO). Moreover, the visible and NIR luminescence lifetimes of the Ln complexes with the TMPBPO ligand were investigated from 77 to 298 K, revealing a strong temperature dependence of the Tm³⁺ (³H₄) and Yb³⁺ (²F_5/2) decay dynamics, which has not been explored before for their coordination complexes.     

Graphene‐Oxide‐Modified Lanthanide Nanoprobes for Tumor‐Targeted Visible/NIR‐II Luminescence Imaging

The synthesis of hydrophilic lanthanide‐doped nanocrystals (Ln³⁺‐NCs) with molecular recognition ability for bioimaging currently remains a challenge. Herein, we present an effective strategy to circumvent this bottleneck by encapsulating Ln³⁺‐NCs in graphene oxide (NCs@GO). Monodisperse NCs@GO was prepared by optimizing GO size and core–shell structure of NaYF₄:Yb,Er@NaYF₄, thus combining the intense visible/near‐infrared II (NIR‐II) luminescence of NCs and the unique surface properties and biomedical functions of GO. Such nanostructures not only feature broad solvent dispersibility, efficient cell uptake, and excellent biocompatibility but also enable further modifications with various agents such as DNA, proteins, or nanoparticles without tedious procedures. Moreover, we demonstrate in proof‐of‐concept experiments that NCs@GO can realize simultaneous intracellular tracking and microRNA‐21 visualization, as well as highly sensitive in vivo tumor‐targeted NIR‐II imaging at 1525 nm.     

An Efficient Near‐Infrared Emissive Artificial Supramolecular Light‐Harvesting System for Imaging in the Golgi Apparatus

Light‐harvesting systems are an important way for capturing, transferring and utilizing light energy. It remains a key challenge to develop highly efficient artificial light‐harvesting systems. Herein, we report a supramolecular co‐assembly based on lower‐rim dodecyl‐modified sulfonatocalix[4]arene (SC4AD) and naphthyl‐1,8‐diphenyl pyridinium derivative (NPS) as a light‐harvesting platform. NPS as a donor shows significant aggregation induced emission enhancement (AIEE) after assembling with SC4AD. Upon introduction of Nile blue (NiB) as an acceptor into the NPS‐SC4AD co‐assembly, the light‐harvesting system becomes near‐infrared (NIR) emissive (675 nm). Importantly, the NIR emitting NPS‐SC4AD‐NiB system exhibits an ultrahigh antenna effect (33.1) at a high donor/acceptor ratio (250:1). By co‐staining PC‐3 cells with a Golgi staining reagent, NBD C₆‐ceramide, NIR imaging in the Golgi apparatus has been demonstrated using these NIR emissive nanoparticles.