Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

Advances in engineering near-infrared luminescent materials

Near-infrared (NIR) luminescent materials have emerged as a growing field of interest, particularly for imaging and optics applications in biology, chemistry, and physics. However, the development of materials for this and other use cases has been hindered by a range of issues that prevents their widespread use beyond benchtop research. This review explores emerging trends in some of the most promising NIR materials and their applications. In particular, we focus on how a more comprehensive understanding of intrinsic NIR material properties might allow researchers to better leverage these traits for innovative and robust applications in biological and physical sciences.

Lanthanide clusters of phenanthroline containing a pyridine-pyrazole based ligand: magnetism and cell imaging

In this contribution, we report the synthesis, characterization and luminescence-magnetic properties of Ln-clusters (Ln = Gd3+, Eu3+ and Tb3+) using a new pyridine-pyrazole functionalized ligand fitted with a chromophoric phenanthroline backbone. The unorthodox N-rich ligand forms isostructural trinuclear lanthanide complexes with a topology that closely resembles two interdigitating hairpins. The clusters crystallize in chiral space groups and also exhibit chirality for bulk samples, which were further confirmed using solid state CD spectra. Magnetic studies on the complexes reveal their interesting features while the Gd cluster shows a significant cryogenic magnetic cooling behaviour with a moderately high magnetic entropy change of -23.42 J kg-1 K-1 at 7 T and 2 K. On the other hand, Eu and Tb complexes exhibit interesting fluorescence properties. The compounds were subsequently used as fluorescent probes for the imaging of human breast adenocarcinoma (MCF7) cells. Live cell confocal microscopy images show that the complexes penetrate beyond the usual cytoplasm region and can be useful in imaging the nucleus region of MCF7 cells.

Oligonucleotide sensor based on magnetic capture and photoligation of upconverting nanoparticles in solid surfaces

In this work, we present a luminescence platform that can be used as point of care system for determining the presence and concentration of specific oligonucleotide sequences. This sensor exhibited a limit of detection as low as 50 fM by means of: (i) the use of single-stranded DNA (ssDNA) functionalized magnetic microparticles that captured and concentrated ssDNA-upconverting nanoparticles (ssDNA-UCNPs) on a solid support, when the target sequence (miR-21-5p DNA-analogue) was in the sample, and (ii) a photoligation reaction that covalently linked the ssDNA-UCNPs and the ssDNA magnetic microparticles, allowing stringent washes. The presented sensor showed a similar limit of detection when the assays were conducted in samples containing total miRNA extracted from human serum, demonstrating its suitability for detecting small specific oligonucleotide sequences under real-like conditions. The strategy of combining UCNPs, magnetic microparticles, and a photoligation reaction provides new insight into low-cost, rapid, and ultra-sensitive detection of oligonucleotide sequences.

Upconversion nanoparticles coated with molecularly imprinted polymers for specific sensing

The development of fluorescent sensors based on lanthanide-doped luminescent nanoparticles has increased their application in biomarker detection. Lanthanide-doped upconversion nanoparticles (UCNPs) have been explored as one of the most promising sensors owing to their merits such as excellent photostability, zero background auto-fluorescence, and reduced side effects of near-infrared triggered treatments. However, traditional upconversion luminescence assay based on direct Fluorescence Resonance Energy Transfer (FRET) between the target molecules and surface of UCNPs encounters low detection accuracy due to superficial adsorption interactions. In this work, we use a molecularly imprinting technique to achieve the specific interaction between UCNPs and molecules for accurate sensing. We demonstrate this by synthesizing a nanostructure with a molecularly imprinted polymer at the surface of UCNPs, in which the imprinted cavities can specifically capture the target molecule of rhodamine B. The upconversion signal changes in relation to the molecule concentration due to FRET. Quantitative analysis shows that the fluorescence-quenching rate is consistent with the Stern-Volmer equation, resulting in a limit of detection of 6.27 μg mL^-1. Our fluorescence sensing approach integrates the advantages of both nonlinear upconversion and molecular imprinting technologies, showing great potential for the detection of specific molecules.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln³⁺ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu³⁺ , Tb³⁺ ) and Cu²⁺ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

Excitation efficiency determines the upconversion luminescence intensity of β-NaYF4:Er3+,Yb3+ nanoparticles in magnetic fields up to 70 T

Lanthanide-doped nanoparticles enable conversion of near-infrared photons to visible ones. This property is envisioned as a basis of a broad range of applications: from optoelectronics, via energy conversion, to bio-sensing and phototherapy. The spectrum of applications can be extended if magnetooptical properties of lanthanide dopants are well understood. However, at present, there are many conflicting reports on the influence of the magnetic field on the upconverted luminescence. In this work, we resolve this discrepancy by performing a comprehensive study of β-NaYF4:Er3+,Yb3+ nanoparticles. Crucially, we show that the magnetic field impacts the luminescence only via a Zeeman-driven detuning between the excitation laser and the absorption transition. On the other hand, the energy transfer and multiphonon relaxation rates are unaffected. We propose a phenomenological model, which qualitatively reproduces the experimental results. The presented results are expected to lead to design of novel, dual-mode opto-magnetic upconverting nanomaterials.

Temperature Sensing in the Short-Wave Infrared Spectral Region Using Core-Shell NaGdF4:Yb3+, Ho3+, Er3+@NaYF4 Nanothermometers

The short-wave infrared region (SWIR) is promising for deep-tissue visualization and temperature sensing due to higher penetration depth and reduced scattering of radiation. However, the strong quenching of luminescence in biological media and low thermal sensitivity of nanothermometers in this region are major drawbacks that limit their practical application. Nanoparticles doped with rare-earth ions are widely used as thermal sensors operating in the SWIR region through the luminescence intensity ratio (LIR) approach. In this study, the effect of the shell on the sensitivity of temperature determination using NaGdF₄ nanoparticles doped with rare-earth ions (REI) Yb³⁺, Ho³⁺, and Er³⁺ coated with an inert NaYF₄ shell was investigated. We found that coating the nanoparticles with a shell significantly increases the intensity of luminescence in the SWIR range, prevents water from quenching luminescence, and decreases the temperature of laser-induced heating. Thermometry in the SWIR spectral region was demonstrated using synthesized nanoparticles in dry powder and in water. The core-shell nanoparticles obtained had intense luminescence and made it possible to determine temperatures in the range of 20–40 °C. The relative thermal sensitivity of core-shell NPs was 0.68% °C⁻¹ in water and 4.2% °C⁻¹ in dry powder.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+)

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K-1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb3+-Tm3+ codoped YVO4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm3+ emissions associated with the 3F2,3 and 3H4 thermally coupled levels, that is, 3F2,3 → 3H6/3H4 → 3H6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb3+ (2F5/2 → 2F7/2) and Tm3+ (3H4 → 3H6). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

Video-rate upconversion display from optimized lanthanide ion doped upconversion nanoparticles

Volumetric displays that create bright image points within a transparent bulk are one of the most attractive technologies in everyday life. Lanthanide ion doped upconversion nanoparticles (UCNPs) are promising luminescent nanomaterials for background free, full-colour volumetric displays of transparent bulk materials. However, video-rate display using UCNPs has been limited by their low emission intensity. Herein, we developed a video-rate upconversion display system with much enhanced brightness. The integral emission intensity of the single UCNPs was fully employed for video-rate display. It was maximized by optimizing the emitter concentration and, more importantly, by temporally synchronizing the scanning time of the excitation light to the the raised emission time of the single UCNPs. The excitation power dependent emission response and emission time decay curves were systematically characterized for the single UCNPs with various emitter concentrations from 0.5% to 6%. 1%Tm³⁺ doped UCNPs presented the highest integral emission intensity. By embedding this UCNPs into a polyvinyl acetate (PVA) film, we achieved a two-dimensional (2D) upconversion display with a frame rate of 29 Hz for 35 by 50 pixels. This work demonstrates that the temporal response as well as the integral emission intensity enable video-rate upconversion display.

Lanthanide Upconverted Luminescence for Simultaneous Contactless Optical Thermometry and Manometry-Sensing under Extreme Conditions of Pressure and Temperature

The growing interest in the miniaturization of various devices and conducting experiments under extreme conditions of pressure and temperature causes the need for the development of small, contactless, precise, and accurate optical sensors without any electrical connections. In this work, YF₃:Yb³⁺-Er³⁺ upconverting microparticles are used as a bifunctional luminescence sensor for simultaneous temperature and pressure measurements. Different changes in the properties of Er³⁺ green and red upconverted luminescence, after excitation of Yb³⁺ ions in the near-infrared at ∼975 nm, are used to calibrate pressure and/or temperature inside the hydrostatic chamber of a diamond anvil cell (DAC). For temperature sensing, changes in the relative intensities of the Er³⁺ green upconverted luminescence of ²H_11/2 and ⁴S_3/2 thermally coupled multiplets to the ⁴I_15/2 ground state, whose relative populations follow a Boltzmann distribution, are calibrated. For pressure sensing, the spectral shift of the Er³⁺ upconverted red emission peak at ∼665 nm, between the Stark sublevels of the ⁴F_9/2 → ⁴I_15/2 transition, is used. Experiments performed under simultaneous extreme conditions of pressure, up to ∼8 GPa, and temperature, up to ∼473 K, confirm the possibility of remote optical pressure and temperature sensing.

Nonlinear photoresponse of NaYF4:Yb,Er@NaYF4 nanocrystals under green CW excitation: a comprehensive study

One of the efficient and well-known upconverting nanomaterials is NaYF₄:Yb,Er@NaYF₄, which emits photoluminescence at 545 nm and 660 nm under an excitation of 980 nm. Here, the nonlinearity of β-NaYF₄:Yb,Er@NaYF₄ at 532 nm is investigated using three nonlinear approaches. For the first time, the nonlinear optical conjugation of NaYF₄:Yb,Er@NaYF₄ nanocrystals is observed using the degenerate four-wave mixing method. In the optical bistability study, the optical hysteresis of NaYF₄:Yb,Er@NaYF₄ is measured using the Mach–Zehnder interferometer nonlinear ring cavity, and the results of bistability loops show different behaviors at different power regimes. Finally, the Z-scan technique is used for determining the nonlinear absorption and refraction coefficients, which are calculated in the order of 10⁻⁴ (cm W⁻¹) and 10⁻⁸ (cm² W⁻¹), respectively. The results indicate that by increasing incident powers, optical behaviour changes in both optical bistability and Z-scan. Therefore, the results exhibit that the β-NaYF₄:Yb,Er@NaYF₄ nanocrystals have nonlinear photoresponses at both 980 and 532 nm, which could be promising for photonic devices based on NIR light and visible light.

NaYF₄:Yb,Er@NaYF₄ is an efficient and well-known upconverting nanomaterials at 980 nm, also it has strong optical nonlinearity at 532 nm related to energy states of the Yb/Er system which is determined by a unique approach.

Upconversion nanotubes with tunable fluorescence properties based on Gd2O2S:Ln3+ (Ln3+ = Yb3+, Er3+) and derivatives for photodynamic therapy

In this study, Gd2O2S:Ln3+ (Ln3+ = Yb3+, Er3+) upconversion nanotubes (UCNTs) were synthesised by using Gd(OH)3:Ln3+ (Ln3+ = Yb3+, Er3+) nanotubes as the template. The luminescent and biological properties of Gd2O2S:Ln3+ (Ln3+ = Yb3+, Er3+) UCNTs, along with photodynamic therapy (PDT) applications of the Gd2O2S:8%Yb3+,2%Er3+ UCNT-Ce6 (chlorin e6) nanocomposites, were systematically studied. The resultant UCNTs showed excellent biocompatibility with human retinal pigment cells (ARPE-19) even after a prolonged incubation time of 72 h, and could be used as luminescent probes. Microscopic imaging revealed that the UCNTs existed mainly in cytoplasm. PDT studies on the Gd2O2S:8%Yb3+,2%Er3+ UCNT-Ce6 nanocomposites indicate that the growth of the tumour (cell) could be inhibited dramatically when it was injected (incubated) with Gd2O2S:8%Yb3+,2%Er3+ UCNT-Ce6 nanocomposites under the irradiation of 980 nm laser.

Structural, optical and magnetic properties of Y3-0.02-xEr0.02Yb x Al5O12 (0 < x < 0.20) nanocrystals: effect of Yb content

The paramagnetic Y_3-0.02-x Er_0.02Yb _x Al₅O₁₂ (x = 0.02, 0.06, 0.10, 0.12, 0.18, 0.20) nanocrystals (NCs) were synthesized by the microwave-induced solution combustion method. The XRD, TEM and SEM techniques were applied to determine the NCs' structures and sizes. The XRD patterns confirmed that the NCs have for the most part a regular structure of the Y₃Al₅O₁₂ (YAG) phase. The changes of the distance between donor Yb³⁺ (sensitizer) and acceptor Er³⁺ (activator) were realized by changing the donor's concentration with a constant amount of acceptor. Under 980 nm excitation, at room temperature, the NCs exhibited strong red emission near 660 and 675 nm, and green upconversion emission at 550 nm, corresponding to the intra 4f transitions of Er³⁺ (⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺ (⁴I_15/2). The strongest emission was observed in a sample containing 18% Yb³⁺ ions. The red and green emission intensities are respectively about 5 and 12 times higher as compared to NCs doped with 2% of Yb³⁺. In order to prove that the main factor responsible for the increase of the upconversion luminescence efficiency is reduction of the distance between Yb³⁺ and Er³⁺, we examined, for the first time the influence of hydrostatic pressure on luminescence and luminescence decay time of the radiative transitions inside donor ion. The decrease of both luminescence intensity and luminescence decay times, with increasing hydrostatic pressure was observed. After applying hydrostatic pressure to samples with e.g. 2% and 6% Yb³⁺, the distance between the donor and acceptor decreases. However, for higher concentrations of the donor, this distance is smaller, and this leads to the effective energy transfer to Er³⁺ ions. With increasing pressure, the maximum intensity of near infrared emission is observed at 1029, 1038 and 1047 nm, what corresponds to ²F_5/2 → ²F_7/2 transition of Yb³⁺.

Efficient sub-15 nm cubic-phase core/shell upconversion nanoparticles as reporters for ensemble and single particle studies

Single particle imaging of upconversion nanoparticles (UCNPs) has typically been realized using hexagonal (β) phase lanthanide-doped sodium yttrium fluoride (NaYF₄) materials, the upconversion luminescence (UCL) of which saturates at power densities (P) of several hundred W cm^-2 under 980 nm near-infrared (NIR) excitation. Cubic (α) phase UCNPs have been mostly neglected because of their commonly observed lower UCL efficiency at comparable P in ensemble level studies. Here, we describe a set of sub-15 nm ytterbium-enriched α-NaYbF₄:Er³⁺@CaF₂ core/shell UCNPs doped with varying Er³⁺ concentrations (5-25%), studied over a wide P range of ∼8-10⁵ W cm^-2, which emit intense UCL even at a low P of 10 W cm^-2 and also saturate at relatively low P. The highest upconversion quantum yield (Φ_UC) and the highest particle brightness were obtained for an Er³⁺ dopant concentration of 12%, reaching the highest Φ_UC of 0.77% at a saturation power density (P_sat) of 110 W cm^-2. These 12%Er³⁺-doped core/shell UCNPs were also the brightest UCNPs among this series under microscopic conditions at high P of ∼10²-10⁵ W cm^-2 as demonstrated by imaging studies at the single particle level. Our results underline the potential applicability of the described sub-15 nm cubic-phase core/shell UCNPs for ensemble- and single particle-level bioimaging.

Near-infrared control and real-time detection of osteogenic differentiation in mesenchymal stem cells by multifunctional upconversion nanoparticles

Finding a method to control and detect the differentiation of stem cells in real time remains a challenge for regenerative medicine. Here we developed the multifunctional upconversion nanoparticle (UCNP) approach for both near-infrared (NIR) control and the real-time detection of osteogenic differentiation in mesenchymal stem cells (MSCs). We first synthesized Tm/Er doped core-shell UCNPs (NaYF4:Yb/Tm/Er@NaYF4), and the core-shell UCNPs were coated with mesoporous silica for drug loading and installing photomechanical azobenzene (azo). Then the Arg-Gly-Asp (RGD) peptide and the matrix metalloproteinase 13 (MMP13) sensitive peptide-black hole quencher-3 group (CGPLGVRGK-BHQ-3) were conjugated on the surface of UCNPs for cell targeting and detection of cell differentiation. The final multifunctional UCNPs are called UCNP@mSiO2-azo-peptide-BHQ-3. The drug icariin (ICA), which can induce the osteogenic differentiation of MSCs, was loaded into UCNP@mSiO2-azo-peptide-BHQ-3 to form the UCNP nanocomplexes. ICA could be released from UCNP nanocomplexes in a NIR-controlled manner that is based on the transformation of the trans-isomer of azo into the cis isomer under the upconverted UV and visible light. Meanwhile, UCNP@mSiO2-azo-peptide-BHQ-3 could also be used as a nanoprobe to detect the activity of the MMP13 enzyme by enzyme digestion and UCNP fluorescence recovery. By detecting MMP13, which is produced by osteogenic differentiation, a real-time detection of cell differentiation in living differentiated MSCs could be achieved using UCNP nanoprobes. Thus, the multifunctional UCNPs combined the control of cell differentiation with the real-time detection of cell differentiation in MSCs, which makes them a powerful tool for regulating and detecting the differentiation of MSCs in regenerative medicine.

Recent Advances in Porphyrin-Based Inorganic Nanoparticles for Cancer Treatment

The application of porphyrins and their derivatives have been investigated extensively over the past years for phototherapy cancer treatment. Phototherapeutic Porphyrins have the ability to generate high levels of reactive oxygen with a low dark toxicity and these properties have made them robust photosensitizing agents. In recent years, Porphyrins have been combined with various nanomaterials in order to improve their bio-distribution. These combinations allow for nanoparticles to enhance photodynamic therapy (PDT) cancer treatment and adding additional nanotheranostics (photothermal therapy-PTT) as well as enhance photodiagnosis (PDD) to the reaction. This review examines various porphyrin-based inorganic nanoparticles developed for phototherapy nanotheranostic cancer treatment over the last three years (2017 to 2020). Furthermore, current challenges in the development and future perspectives of porphyrin-based nanomedicines for cancer treatment are also highlighted.

Enhancing multiphoton upconversion through interfacial energy transfer in multilayered nanoparticles

Photon upconversion in lanthanide-doped upconversion nanoparticles offers a wide variety of applications including deep-tissue biophotonics. However, the upconversion luminescence and efficiency, especially involving multiple photons, is still limited by the concentration quenching effect. Here, we demonstrate a multilayered core-shell-shell structure for lanthanide doped NaYF₄, where Er³⁺ activators and Yb³⁺ sensitizers are spatially separated, which can enhance the multiphoton emission from Er³⁺ by 100-fold compared with the multiphoton emission from canonical core-shell nanocrystals. This difference is due to the excitation energy transfer at the interface between activator core and sensitizer shell being unexpectedly efficient, as revealed by the structural and temperature dependence of the multiphoton upconversion luminescence. Therefore, the concentration quenching is suppressed via alleviation of cross-relaxation between the activator and the sensitizer, resulting in a high quantum yield of up to 6.34% for this layered structure. These findings will enable versatile design of multiphoton upconverting nanoparticles overcoming the conventional limitation.

Combating Concentration Quenching in Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) are a special class of luminescent nanomaterials that convert multiwavelength near-infrared (NIR) excitation into tunable emissions spanning the deep ultraviolet (UV) to NIR regions. In addition to large anti-Stokes shift, UCNPs also feature a sharp emission bandwidth, long excited-state lifetime, as well as high resistance to optical blinking and photobleaching. Therefore, UCNPs have been identified as promising candidates to solve many challenging problems in fields ranging from biological imaging and therapeutics to photovoltaics and photonics. Nevertheless, the progress of utilizing an upconversion process is being hindered by the limited emission intensity, principally due to low oscillator strength in these nanoparticles. UCNPs essentially resemble the optical characteristics of their bulk counterparts, which take advantage of electronic transition within the 4f configuration of the lanthanide dopants to realize photon energy conversions. In general, a high dopant concentration promotes upconversion luminescence by providing a high density of optical centers to collect and to sustain the energy of the excitation light. However, an increase in dopant concentration induces self-quenching processes that offset the emission gain and may eventually result in attenuation of the overall emission intensity. This phenomenon known as concentration quenching represents a major obstacle to constructing bright UCNPs. In recent years, advances in nanoparticle research have led to the emergence of several strategies for mitigating energy loss at elevated dopant concentrations. In consequence, doping high levels of lanthanide ions in UCNPs has become a viable solution to boosting the emission intensity of photon upconversion. On account of extensive energy exchange interaction in heavily doped UCNPs, the spectrum tunability of photon upconversion is also greatly enhanced. These advances have largely expanded the scope of upconversion research. To provide guidelines for enhancing upconversion through heavy doping, we attempt to review recent advances in the understanding and control of concentration quenching in UCNPs. With significant advancements made in the chemical synthesis, we are now able to exquisitely control the doping of lanthanide ions in various nanoparticles of well-defined size, morphology, and core-shell structure. We show that, by confining energy transfer in nanostructured host materials in conjunction with innovative excitation schemes, concentration quenching of upconversion luminescence is largely alleviated. As a result, unusually high dopant concentrations can be used to construct UCNPs displaying high brightness and large anti-Stokes shift. We demonstrate that the development of heavily doped UCNPs enables advanced bioimaging and photonic applications that can hardly be fulfilled by conventional UCNPs comprising low concentrations of lanthanide dopants.

UV-Vis-NIR absorption spectra of lanthanide oxides and fluorides

Absorption spectra of inorganic lanthanide fluorides and oxides.

Lanthanide nanoparticles with efficient near-infrared-II emission for biological applications

We discuss designing efficient NIR-II-emitting lanthanide NPs and summarize their recent progress in bioimaging, therapy, and biosensing, as well as their limitations and future opportunities.

Multi-mode optical coded patterns enabled by upconversion nanoparticles and photonic crystals

In this study, we designed the luminescent enhanced composites by combining upconversion nanoparticles (UCNPs) and photonic crystals (PCs), and prepared multi-mode coding patterns through utilizing the optical properties of both materials, respectively. We prepared UCNPs with different luminescence and the composite materials can be quickly obtained by spin-coating the UCNPs on the surface of the PCs. We discover that the composite materials have significant luminescent enhancement and the enhancement of upconversion is attributed to the Bragg reflection of the photonic band gap. We fabricated upconversion luminescent lattices based on the surface of the PCs by dropping and endowed with the meaning of the Morse code so that it can have more information in different modes. In addition, we further obtained the cipher table pattern according to the structural color of the PCs and the luminescence of the UCNPs. The results reveal great potential applications in the field of multi-mode optical imaging and anti-counterfeiting.

Design, synthesis and photocytotoxicity of upconversion nanoparticles: Potential applications for near-infrared photodynamic and photothermal therapy

Photodynamic therapy (PDT) and photothermal therapy (PTT) are emerging modalities for the treatment of tumors and nonmalignant conditions, based on the use of photosensitizers to generate singlet oxygen or heat, respectively, upon light (laser) irradiation. They have potential advantages over conventional treatments, being minimally invasive with precise spatial-temporal selectivity and reduced side effects. However, most clinically employed PDT agents are activated at visible (vis) wavelengths for which the tissue penetration and, hence, effective treatment depth are compromised. In addition, the lipophilicity of near-infrared (NIR) photothermal agents limits their use and efficiency. To achieve combined PDT/PTT effects, both excitation wavelengths need to be tuned into the NIR spectral window of biological tissues. This paper reports the synthesis of neodymium-doped upconversion nanoparticles (NaYF₄ :Yb,Er,Nd@NaYF₄ :Nd) that convert 800 nm light into vis wavelengths, which can then activate conventional photosensitizers on the nanoparticle surface for PDT. Covalently bonded IR-780 dyes can readily be activated by 800 nm laser irradiation. The PEGylated nanoplatform exhibited a narrow size distribution, good stability and efficient generation of singlet oxygen under laser irradiation. The in vitro photocytotoxicity of this engineered nanoplatform as either a PDT or PTT agent in HeLa cells is demonstrated, while fluorescence microscopy in nanoplatform-incubated cells highlights its potential for bioimaging.

Remote Light-Responsive Nanocarriers for Controlled Drug Delivery: Advances and Perspectives

Engineering of smart photoactivated nanomaterials for targeted drug delivery systems (DDS) has recently attracted considerable research interest as light enables precise and accurate controlled release of drug molecules in specific diseased cells and/or tissues in a highly spatial and temporal manner. In general, the development of appropriate light-triggered DDS relies on processes of photolysis, photoisomerization, photo-cross-linking/un-cross-linking, and photoreduction, which are normally sensitive to ultraviolet (UV) or visible (Vis) light irradiation. Considering the issues of poor tissue penetration and high phototoxicity of these high-energy photons of UV/Vis light, recently nanocarriers have been developed based on light-response to low-energy photon irradiation, in particular for the light wavelengths located in the near infrared (NIR) range. NIR light-triggered drug release systems are normally achieved by using two-photon absorption and photon upconversion processes. Herein, recent advances of light-responsive nanoplatforms for controlled drug release are reviewed, covering the mechanism of light responsive small molecules and polymers, UV and Vis light responsive nanocarriers, and NIR light responsive nanocarriers. NIR-light triggered drug delivery by two-photon excitation and upconversion luminescence strategies is also included. In addition, the challenges and future perspectives for the development of light triggered DDS are highlighted.

Imaging dopant distribution across complete phase transformation by TEM and upconversion emission

Correlating dopant distribution to its optical response represents a complex challenge for nanomaterials science. Differentiating the "true" clustering nature from dopant pairs formed in statistical distribution complicates even more the elucidation of doping-functionality relationship. The present study associates lanthanide dopant distribution, including all significant events (enrichment, depletion and surface segregation), to its optical response in upconversion (UPC) at the ensemble and single-nanoparticle level. A small deviation from the Er nominal concentration of a few percent is able to induce clear differences in Er UPC emission color, intensity, excited-state dynamics and ultimately, UPC mechanisms, across tetragonal to monoclinic phase transformation in rationally designed Er doped ZrO2 nanoparticles. Rare evidence of a heterogeneous dopant distribution leading to the coexistence of two polymorphs in a single nanoparticle is revealed by Z- and phase contrast transmission electron microscopy (TEM). Despite their spatial proximity, Er in the two polymorphs are spectroscopically isolated, i.e. they do not communicate by energy transfer. Segregated Er, which is well imaged in TEM, is absent in UPC, while the minor phase content overlooked by X-ray diffraction and TEM is revealed by UPC. The outstanding sensitivity of combined TEM and UPC emission to subtle deviations from uniform doping in the diluted concentration regime renders such an approach relevant for various functional oxides supporting lanthanide dopants as emitters.

Guar gum modified upconversion nanocomposites for colorectal cancer treatment through enzyme-responsive drug release and NIR-triggered photodynamic therapy

Multimodal therapeutic approach towards colorectal cancer (CRC) holds great promise. There is, however, no convincing strategy reported to date that employs a multimodal strategy in CRC treatment. The present study reports an intense green-emitting core-shell photoluminescent upconversion (CSGU) nanocrystal engineered to synergistically perform photodynamic and enzyme-triggered delivery of the chemotherapeutic agent for an enhanced therapeutic outcome on HT-29 colon carcinoma cells in vitro. The photodynamic activity is achieved by the energy transfer between CSGU and the chemically conjugated Rose Bengal (RB) molecules that are further protected by a mesoporous silica (MS) layer. The chemical assay demonstrates a remarkable FRET mediated generation of ¹O₂ under NIR (980 nm) excitation. The outermost MS layer of the nanoplatform is utilized for the loading of the 5FU anticancer drug, which is further capped with a guar gum (GG) polysaccharide polymer. The release of the 5FU is specifically triggered by the degradation of the GG cap by specific enzymes secreted from colonic microflora, which otherwise showed 'zero-release behavior' in the absence of any enzymatic trigger in various simulated gastro-intestinal (GI) conditions. Furthermore, the enhanced therapeutic efficacy of the nanoplatform (CSGUR-MSGG/5FU) was evaluated through in vitro studies using HT-29 CRC cell lines by various biochemical and microscopic assays by the simultaneous triggering effect of colonic enzyme and 980 nm laser excitation. In addition, the strong visible emission from the nanoplatform has been utilized for NIR-induced cellular bioimaging.

Structure-Property Relationships in Lanthanide-Doped Upconverting Nanocrystals: Recent Advances in Understanding Core-Shell Structures

The production of upconverting nanostructures with tailored optical properties is of major technological interest, and rapid progress toward the realization of such production has been made in recent years. Ultimately, accurate understanding of nanostructure organization will lead to design rules for accurately tailoring optical properties. Here, the context of open questions still of general importance to the upconversion and nanocrystal communities is presented, with a particular emphasis on the structure-property relationships of core-shell upconverting nanocrystals. Although the optical properties of the latter have been thoroughly investigated, little is known regarding their atomic-scale organization. Indeed, solving the atomic-scale structure of such nanomaterials is challenging because of their intrinsic nonperiodic nature. Familiar concepts of crystallography are no longer appropriate; chemical and structural modulation waves must be introduced. To reveal the exact core-shell structures, innovative characterization techniques need to be applied and developed, as discussed herein. The continued development and application of structural characterization techniques will be vital to consolidate the currently incomplete link between atomic-scale structure and upconversion properties. This will ultimately provide a valuable contribution to the emerging detailed guidelines on how to better design upconverting nanostructures to achieve given optical properties in terms of efficiency, absorption, spectral emission, and dynamics.

Near-Infrared-Triggered Photodynamic, Photothermal, and on Demand Chemotherapy by Multifunctional Upconversion Nanocomposite

In an attempt to integrate photodynamic therapy (PDT) with photothermal therapy and chemotherapy for enhanced anticancer activity, we have rationally synthesized a multifunctional upconversion nanoplatform using NaYF₄:Yb/Tm/Er/Fe nanoparticles (NPs) as the core and NaYbF₄:1% Tm as a shell. The as-synthesized core-shell upconversion (CSU) NPs exhibited diverse and enhanced photoluminescence emissions in a wide range (UV to NIR) consequent upon Fe³⁺ doping in the core and fabrication of an active shell. Subsequently, CSU was first decorated with titania NPs as photosensitizers. Next, the mesoporous silica (MS) shell loaded with doxorubicin (DOX) via a photocleavable Ru complex as the gating molecule was developed around titania-containing CSU. Finally, gold nanorods (GNRs) with localized surface plasmon resonance (LSPR) at 800 nm were incorporated around the MS layer to obtain the multifunctional nanoplatform. We demonstrated that the UV, blue, and NIR emissions from the CSU produced ROS-mediated PDT through titania activation, induced DOX release through photocleavage of the Ru complex, and generated hyperthermia by LSPR activity of GNRs, respectively, upon a single NIR excitation through FRET. The therapeutic efficacy was validated on HeLa cell lines in vitro by various microscopic and biochemical studies under a significantly milder NIR irradiation and lower dosage of the nanoplatforms, which have been further demonstrated as diagnostic nanoprobes for cell imaging.

Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: β-NaYF4:Yb3+- Er3+@SiO2 Temperature Sensor

Upconverting core@shell type β-NaYF₄:Yb³⁺-Er³⁺@SiO₂ nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λ_ex = 975 nm), i.e., upconversion luminescence of Er³⁺ and down-shifting emission of Yb³⁺, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er³⁺ and Yb³⁺ observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er³⁺ levels (²H_11/2 → ⁴I_15/2/⁴S_3/2 → ⁴I_15/2) and the nonthermally coupled Yb³⁺/Er³⁺ levels (²F_5/2 → ²F_7/2/⁴I_9/2 → ⁴I_15/2 or ⁴F_9/2 → ⁴I_15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er³⁺ 525/545 nm (max S_r = 1.31% K^-1) and Yb³⁺/Er³⁺ 1010/810 nm (1.64% K^-1) or 1010/660 nm (0.96% K^-1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb³⁺/Er³⁺ bands, vs. the typically used Er³⁺ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis-NIR optical, contactless sensors of temperature.

Upconversion Nanocomposite Materials With Designed Thermal Response for Optoelectronic Devices

Upconversion is a non-linear optical phenomenon by which low energy photons stimulate the emission of higher energy ones. Applications of upconversion materials are wide and cover diverse areas such as bio-imaging, solar cells, optical thermometry, displays, and anti-counterfeiting technologies, among others. When these materials are synthesized in the form of nanoparticles, the effect of temperature on the optical emissions depends critically on their size, creating new opportunities for innovation. However, it remains a challenge to achieve upconversion materials that can be easily processed for their direct application or for the manufacture of optoelectronic devices. In this work, we developed nanocomposite materials based on upconversion nanoparticles (UCNPs) dispersed in a polymer matrix of either polylactic acid or poly(methyl methacrylate). These materials can be processed from solution to form thin film multilayers, which can be patterned by applying soft-lithography techniques to produce the desired features in the micro-scale, and luminescent tracks when used as nanocomposite inks. The high homogeneity of the films, the uniform distribution of the UCNPs and the easygoing deposition process are the distinctive features of such an approach. Furthermore, the size-dependent thermal properties of UCNPs can be exploited by a proper formulation of the nanocomposites in order to develop materials with high thermal sensitivity and a thermochromic response. Here, we thus present different strategies for designing optical devices through patterning techniques, ink dispensing and multilayer stacking. By applying upconverting nanocomposites with unique thermal responses, local heating effects in designed nanostructures were observed.

In situ reversible color variation of a ready-made upconversion material using the designed component of a three-state fluorescence switching system

In recent years, upconversion materials have attracted considerable attention because of their unique physicochemical features. Numerous studies have focused on the synthesis of upconversion materials with different colors. However, an easier way to vary the upconversion colors without changing the materials' components has not been extensively studied. In this study, we realized the in situ color variation of the designed upconversion material with the help of a three-state fluorescence switching hybrid device. The device was composed of Prussian blue and upconversion materials; the former element functioned as a fluorescence resonance energy transfer acceptor and the latter acted as a donor. Smartly applying the RGB color model guaranteed multicolor of the device. Moreover, the highest fluorescence contrast of the three-state fluorescence switching system was 86% (larger than the result of a previous study), and the three-state reversibility was remarkable; this was probably owing to the unique layer-by-layer dripping/electrodepositing assembly method. To the best of our knowledge, the in situ reversible color variation of the ready-made upconversion material has been demonstrated for the first time.

Controlling upconversion luminescence patterns in space with red emission enhancement from a single fluoride microcrystal by tuning the excitation mode

The ability to control the upconversion (UC) luminescence patterns in space from lanthanide-doped UC luminescence materials is very important for many applications including three-dimensional color displays, optical waveguides and optical communication. In this work, the fascinating UC luminescence patterns could be adjusted from the blue or green flower-like emission pattern to the red flame-like irradiation pattern with a red luminescence enhancement from a single β-NaYF₄:Yb³⁺/(Tm³⁺ or Er³⁺) microcrystal (MC) by varying the excitation position. The red-to-blue (R/B) and the red-to-green (R/G) emission intensity ratios from the single MC particle that the focal point position of excitation NIR laser is on the side lace are much stronger than that in the case of the focused laser beam on the center. We think that the physical mechanism from the changes in the luminescence pattern and the emission intensity ratio is explained by the optical waveguide effect based on the total reflection effect. These results provide a new strategy for facilitating fundamental investigations of the UC micro/nano-materials, which will lead to promising applications in three-dimensional color display, optical waveguides and optical communication.

The UC luminescence patterns can be controlled from the flower-like emission pattern to the red flame-like irradiation pattern with a red luminescence enhancement for a single lanthanide-doped fluoride material by varying the excitation position.

Upconversion metal (Zr, Hf, and Ta) oxide aerogels

We report here a new feasible approach to produce upconversion luminescent metal oxide aerogels with high textural characteristics. Monolithic aerogels show upconversion luminescence converting near-infra red excitation into visible light emission.

The effect of low- and high-penetration light on localized cancer therapy

The design of a delivery system allowing targeted and controlled drug release has been considered one of the main strategies used to provide individualized cancer therapy, to improve survival statistics, and to enhance quality-of-life. External stimuli including low- and high-penetration light have been shown to have the ability to turn drug delivery on and off in a non-invasive remotely-controlled fashion. The success of this approach has been closely related to the development of a variety of drug delivery systems - from photosensitive liposomes to gold nanocages - and relies on multiple mechanisms of drug release activation. In this review, we make reference to the two extremes of the light spectrum and their potential as triggers for the delivery of antitumor drugs, along with the most recent achievements in preclinical trials and the challenges to an efficient translation of this technology to the clinical setting.

Near infrared emission properties of Er doped cubic sesquioxides in the second/third biological windows

In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800-1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900-1700 nm) emission of significant member of cubic sesquioxides, Er-Lu2O3 nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu2O3 are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu2O3 and Er-Y2O3 induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.

Impact of Lanthanide Nanomaterials on Photonic Devices and Smart Applications

Half a century after its initial emergence, lanthanide photonics is facing a profound remodeling induced by the upsurge of nanomaterials. Lanthanide-doped nanomaterials hold promise for bioapplications and photonic devices because they ally the unmatched advantages of lanthanide photophysical properties with those arising from large surface-to-volume ratios and quantum confinement that are typical of nanoobjects. Cutting-edge technologies and devices have recently arisen from this association and are in turn promoting nanophotonic materials as essential tools for a deeper understanding of biological mechanisms and related medical diagnosis and therapy, and as crucial building blocks for next-generation photonic devices. Here, the recent progress in the development of nanomaterials, nanotechnologies, and nanodevices for clinical uses and commercial exploitation is reviewed. The candidate nanomaterials with mature synthesis protocols and compelling optical uniqueness are surveyed. The specific fields that are directly driven by lanthanide doped nanomaterials are emphasized, spanning from in vivo imaging and theranostics, micro-/nanoscopic techniques, point-of-care medical testing, forensic fingerprints detection, to micro-LED devices.

Heavy doping of ceria by wet impregnation: a viable alternative to bulk doping approaches

To avoid the deleterious effects of dopant segregation, synthesis methods that facilitate a homogenous dopant distribution in the ceria lattice were employed. Though doping ceria by wet impregnation was also credited to induce a homogeneous solid solution even in the heavy regime (concentration ≥20%, A. Corma, P. Atienzar, H. Garcia and J. Chane-Ching, Nat. Mater., 2004, 3, 394-397), no follow up investigation has been reported. Herein, we investigated ceria nanoparticles (1%Tm-CeO2 and 1%Eu-CeO2) wet-impregnated with trivalent rare-earth (Yb, 20%), bivalent (Ca, 20%) and isovalent (Zr, 30%) metals, followed by annealing in air. Homogeneity of the solid solutions of Yb-impregnated ceria was confirmed by a two-feature characterization toolbox that included X-ray diffraction, Raman spectroscopy, transmission electron microscopy, as well as up-conversion emission as a probe tool. Since the up-conversion emission of Tm was not detectable in the absence of Yb while its efficiency depends on the average distance between Yb and Tm ions, the Yb incorporation and its migration from the surface to the lattice bulk sites in wet-impregnated ceria can be "visualized" and compared with that of the Yb bulk-doped counterpart. The use of Eu luminescence as a local probe confirmed the homogeneity of solid solutions of Ca and Zr-impregnated ceria and also sustained the opposite roles of Ca and Zr as the repeller and the scavenger of oxygen vacancies, respectively. All these results suggested that heavy doping of ceria by wet impregnation with metals with +2, +3 and +4 valencies represent a facile alternative to conventional doping approaches. Therefore, the effects of the amount and the type of metal dopant on the structural properties of CeO2 could be investigated in a more systematic and probably a more reproducible manner, which would significantly increase the potential of ceria in catalysis and other applications.

Lanthanide Doped Near Infrared Active Upconversion Nanophosphors: Fundamental Concepts, Synthesis Strategies, and Technological Applications

Near infrared (NIR) light utilization in a range of current technologies has gained huge significance due to its abundance in nature and nondestructive properties. NIR active lanthanide (Ln) doped upconversion nanomaterials synthesized in controlled shape, size, and surface functionality can be combined with various pertinent materials for extensive applications in diverse fields. Upconversion nanophosphors (UCNP) possess unique abilities, such as deep tissue penetration, enhanced photostability, low toxicity, sharp emission peaks, long anti-Stokes shift, etc., which have bestowed them with prodigious advantages over other conventional luminescent materials. As new generation fluorophores, UCNP have found a wide range of applications in various fields. In this Review, a comprehensive overview of lanthanide doped NIR active UCNP is provided by discussing the fundamental concepts including the different mechanisms proposed for explaining the upconversion processes, followed by the different strategies employed for the synthesis of these materials, and finally the technological applications of UCNP, mainly in the fields of bioimaging, drug delivery, sensing, and photocatalysis by highlighting the recent works in these areas. In addition, a brief note on the applications of UCNP in other fields is also provided along with the summary and future perspectives of these materials.

Time-dependent luminescence loss for individual upconversion nanoparticles upon dilution in aqueous solution

Single-particle luminescence microscopy is a powerful method to extract information on biological systems that is not accessible by ensemble-level methods. Upconversion nanoparticles (UCNPs) are a particularly promising luminophore for single-particle microscopy as they provide stable, non-blinking luminescence and allow the avoidance of biological autofluorescence by their anti-Stokes emission. Recently, ensemble measurements of diluted aqueous dispersions of UCNPs have shown the instability of luminescence over time due to particle dissolution-related effects. This can be especially detrimental for single-particle experiments. However, this effect has never been estimated at the individual particle level. Here, the luminescence response of individual UCNPs under aqueous conditions is investigated by quantitative wide-field microscopy. The particles exhibit a rapid luminescence loss, accompanied by large changes in spectral response, leading to a considerable heterogeneity in their luminescence and band intensity ratio. Moreover, the dissolution-caused intensity loss is not correlated with the initial particle intensity or band ratio, which makes it virtually unpredictable. These effects and the subsequent development of their heterogeneity can be largely slowed down by adding millimolar concentrations of sodium fluoride in buffer. As a consequence, the presented data indicate that microscopy experiments employing UCNPs in an aqueous environment should be performed under conditions that carefully prevent these effects.

Rational Surface Design of Upconversion Nanoparticles with Polyethylenimine Coating for Biomedical Applications: Better Safe than Brighter?

Upconversion nanoparticles (UCNPs) coated with polyethylenimine (PEI) are popular background-free optical contrast probes and efficient drug and gene delivery agents attracting attention in science, industry, and medicine. Their unique optical properties are especially useful for subsurface nanotheranostics applications, in particular, in skin. However, high cytotoxicity of PEI limits safe use of UCNP@PEI, and this represents a major barrier for clinical translation of UCNP@PEI-based technologies. Our study aims to address this problem by exploring additional surface modifications to UCNP@PEI to create less toxic and functional nanotheranostic materials. We designed and synthesized six types of layered polymer coatings that envelop the original UCNP@PEI surface, five of which reduced the cytotoxicity to human skin keratinocytes under acute (24 h) and subacute (120 h) exposure. In parallel, we examined the photoluminescence spectra and lifetime of the surface-modified UCNP@PEI. To quantify their brightness, we developed original methodology to precisely measure the colloidal concentration to normalize the photoluminescence signal using a nondigesting mass spectrometry protocol. Our results, specified for the individual coatings, show that, despite decreasing the cytotoxicity, the external polymer coatings of UCNP@PEI quench the upconversion photoluminescence in biologically relevant aqueous environments. This trade-off between cytotoxicity and brightness for surface-coated UCNPs emphasizes the need for the combined assessment of the viability of normal cells exposed to the nanoparticles and the photophysical properties of postmodification UCNPs. We present an optimized methodology for rational surface design of UCNP@PEI in biologically relevant conditions, which is essential to facilitate the translation of such nanoparticles to the clinical applications.