Upconversion Research Articles

Significantly Boosted Upconversion Emission in Cryogenic Er@Yb@Y Core–Shell–Shell Nanostructures

AbstractRecent advances reveal that due to the cross‐relaxation restriction, impressive upconversion (UC) enhancement (≈100‐folds) can be achieved in cryogenic Er3+‐rich core‐inert shell nanostructures (e.g., NaErF4@NaYF4), which opens up exciting opportunities in diverse frontier applications. However, further promotion of UC intensity is still highly desired, in which the rational design of nanostructures can play a key role. Herein, it is demonstrated that adopting an active shell design will constantly benefit the UC within a wide temperature range (40–300 K). Specifically, through constructing the luminescent core@active shell@inert shell sandwich nanostructure (e.g., NaErF4@NaYbF4@NaYF4), 8.3–73‐folds UC enhancement will be achieved (taking the corresponding core@inert shell structures as competitors). Moreover, from spectral‐domain and time‐domain spectroscopic experiments, the relevant UC enhancement is convincingly attributed to a temperature‐dependent energy injection process (from the active shell to the luminescent core). More interestingly, the unique property of the material makes a temperature‐induced high‐level encryption application possible, which is obtained by employing the nanomaterials on a quick response (QR) code. These results not only deepen the UC mechanism in multi‐layer nanostructures, but also introduce an expanded dimension (via low temperatures) in information security.

Impact of varying Li+ concentration on the upconversion emission and temperature sensing characteristics of YVO4: Tm3+/Yb3+ phosphor

A series of Li+ codoped YVO4: Tm3+/Yb3+ phosphors were prepared by Pechini sol–gel reaction technique. Structural study of the material confirms the crystal structure of the prepared samples and also gives information about the crystallite size and phase purity with the variation of Li+ ion concentrations. Emission peaks around ∼ 476, and ∼ 799 nm corresponds to the 1G4→3H6, and 3H4→3H6 transitions, which could be seen under irradiation of NIR (980 nm) laser light. Li+ (10 mol%) codoped sample shows maximum upconversion. Li+ doping significantly increases the up-conversion emission intensities by more than 5 times for emission peaks around 476 nm and 799 nm.The possible explanation for the increase in UC (Upconversion) emission intensity is attributed to both improved crystallinity and crystal field modification around Tm3+ ions. The variation of UC emission spectra with respect to pump power was observed to investigate the underlying UC luminescent mechanism in YVO4: Tm3+/Yb3+ codoped with Li+ (10 mol%). The multiphoton involvement is observed in the power-dependent study of upconverted blue and NIR emission bands. Widely used fluorescence intensity ratio (FIR) technique was used to evaluate the temperature sensing capability of the optimized Li+ codoped phosphor. The FIR study of Stark components 3F2, and 3H4 levels indicates that this material is an efficient material for temperature sensing. The optimized sample, doped with Li+, exhibits relative sensitivity of 0.78% K−1 at a temperature of 548 K. This material have utility in both light-emitting diodes and as a temperature-sensing probe.

Thermally boosted upconversion luminescence and high-performance thermometry in ScF3:Yb3+/Tm3+ nanorods with negative thermal expansion

Luminescence nanothermometers featuring high spatial resolution and thermal resolution have aroused extensive interest in biological and nanomedical fields. However, the challenge of thermal quenching in upconversion (UC) nanocrystals hampers achieving elevated relative temperature sensitivity (Sr). In this study, the quasi cubic nanorods of ScF3:Yb3+/Tm3+ with negative thermal expansion were prepared, and thermally boosted luminescence was found to derive from the phonon-assisted energy transfer, resulting in the emission band of 3F2,3 → 3H6 of Tm3+ ions enhanced monotonously. While the emission bands of (1G4)1D2→3F4 of Tm3+ ions first increased slightly and then decreased seriously because of the competition between phonon-assisted and cross-relaxation (CR) processes. Relying on this high-contrast thermal dependent feature, the maximum relative sensitivity of 1.60% K−1 at 523 K was achieved in the proposed luminescence intensity ratio (LIR) thermometry. These findings would open a new avenue for designing thermally boosted upconversion nanocrystals and reliable optical thermometers by using Tm3+ activated luminescent materials.

A Review on the Applicability of Luminescent Phosphors and Effective Positioning in Dye‐Sensitized Solar Cells for Enhanced Performance and Stability

The narrow range of solar light absorption facilitated by the photoactive organic dyes has been restricting the significant development, and large‐scale applications of dye‐sensitized solar cells (DSSC). The major problem that limits the power conversion efficiency (PCE) of DSSC is the limitations of the organic dyes (light absorbers in DSSC) to absorb beyond the visible region of solar spectrum that resulting in lower PCE (≈13%) as compared with the commercialized technologies. Hence, the focus on developing efficient strategies to harvest the ultraviolet and infrared light regions of the solar spectrum and effective conversion into the absorption limit of dyes and thereby increasing the device performance and stability is the emerging technological development in DSSCs. Rare earth ion‐doped upconversion (UC) and downconversion (DC) phosphors are the most promising solutions for the prevalent issues of the liquid junction solar cells. This review will focus on the emerging strategies for the PCE enhancement of DSSC with UC and DC materials. An exclusive review on the facile approach of simultaneous usage of both the UC and DC phosphors and the effective positioning of these spectral conversion phosphors, to achieve highly stable DSSCs with superior PCE is provided.

Effect of Mg2+ and Zn2+ addition on photoluminescent and photoacoustic spectroscopy of La2O3:Er3+/Yb3+ phosphors

Photoluminescence (PL) and photoacoustic spectroscopy (PAS) are important techniques for materials characterization. The La2O3: Er3+/Yb3+ phosphors samples were synthesized by combustion method and annealed at 800–1200 °C for 2 h for improved photoluminescence properties. XRD data confirmed that all samples have pure hexagonal phase and FE-SEM shows irregular granular-like morphological structures and particle size that improved with annealed temperature. Then the photoacoustic (PA) amplitude and upconversion (UC) emission intensity were successively measured by a homemade PA cell under 980 nm laser excitation. A comparative relationship was made between PA and UC varying excitation power. Then, the authors showed the effect of codoped Mg2+ and Zn2+ ions on the radiative and non-radiative properties. It is observed that UC emission intensity for Zn2+ codoped La2O3: Er3+/Yb3+ phosphor has maximum, and also observed prominent absorption peaks at 521, 658, and 955 nm. These results indicate its high potential for application in photo-thermal therapy and upconversion imaging.

Reengineering the thermometric behaviors of Er3+/Yb3+-codoped Gd2Mo3O12 microparticles via dual-mode luminescence manipulation

Series of Er3+/Yb3+-codoped Gd2Mo3O12 (GMO:Er3+/2xYb3+) microparticles with dual-mode luminescence were fabricated with the assistance of sol-gel method. The phase structure, morphological information, upconversion (UC) and down-shifting luminescence properties of the final products were systematically investigated. At 980 nm light irradiation, strong UC emissions rise in all the products and their intensities depend on the dopant concentration, where the optimum status occurs at x = 0.25 and the two-photon capture procedure results in the UC emission mechanism. Besides, excited at 378 nm, the down-shifting photoluminescence is also gained in resulting samples. By means of the fluorescence intensity ratio technique, the thermometric properties of the studied samples were investigated through analyzing the diverse thermal quenching features of the green emissions from thermally coupled levels of Er3+. When the UC emission is selected, the maximum absolute and relative sensitivities of the GMO:Er3+/2xYb3+ microparticles are 0.0076 and 1.15% K−1, respectively, whereas they shift to 0.0036 and 0.89% K−1, respectively, when the down-shifting photoluminescence is adopted. Note that, the sensor sensitivities of developed microparticles are related to the luminescence mode, while they do not depend on Yb3+ content. These results suggest that the GMO:Er3+/2xYb3+ microparticles are promising thermosensitive luminescent materials for optical thermometry and their thermometric features can be tuned through changing luminescence mode.

Pure-green upconversion emission and high-sensitivity optical thermometry of Er3+-doped stoichiometric NaYb(MoO4)2

The optical temperature sensor based on fluorescence intensity ratio (FIR) offers significant advantages for rapid and non-contact temperature measurements. However, the application of optical temperature sensors is limited by the challenge of identifying advanced materials with high response sensitivity. In this study, phosphor and single-crystal states stoichiometric NaYb(MoO4)2 with Er3+ doping were prepared by solid-state reaction method and Czochralski method, respectively, for promising optical thermometry applications. The structure of the phosphor and single-crystal state samples were investigated. High-purity upconversion (UC) emissions were achieved from stoichiometric NaYb(MoO4)2:Er3+ samples with a maximum green-to-red ratio up to 70.3, which opens up numerous possibilities for their application as pure-green color converters. Besides, the UC emission spectra were monitored with respect to temperature, and the fluorescence intensity ratios (FIRs) of the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+ ions were investigated to evaluate the optical temperature sensing behaviors of NaYb(MoO4)2:Er3+. Importantly, the distinctions of optical temperature sensing behaviors between phosphor and single-crystal state samples were analyzed. The results suggest that stoichiometric NaYb(MoO4)2:Er3+, especially for single-crystal state NaYb(MoO4)2:Er3+, hold great promise as FIR-based optical temperature sensors. In addition, the results can also open up the research interests for employing stoichiometric materials and single crystal materials as optical thermometry applications.

Near-infrared optical nanothermometry via upconversion of Ho3+-sensitized nanoparticles

Recently, materials revealing the upconversion (UC) phenomenon, which is a conversion of low-energy photons to higher-energy ones, have attracted considerable attention in luminescence thermometry due to the possibility of precise and remote optical thermal sensing. The most widely studied type of luminescent thermometry uses a ratiometric approach based on changes in the UC luminescence intensity, mainly of lanthanide ions’ thermally coupled energy levels. In this work, NaYF4:Ho3+@NaYF4, and NaYF4:Ho3+, Er3+@NaYF4 nanoparticles (NPs) were synthesized by the controlled reaction in oleic acid and octadecene at 573 K. The obtained nanoparticles had hexagonal structures, oval shapes, and average sizes of 22.5 ± 2.2 nm and 22.2 ± 2.0 nm, respectively. The spectroscopic properties of the products were investigated by measurements of the UC emission under 1151 nm laser excitation in the temperature range between 295 to 378 K. The sample doped with Ho3+ and Er3+ ions showed unique behavior of enhancing emission intensity with the temperature. The relative sensitivity determined for the NPs containing Ho3+ and Er3+ ions, reached the maximum value of 1.80%/K at 378 K. Here, we prove that the NaYF4:Ho3+, Er3+@NaYF4 system presents unique and excellent optical temperature sensing properties based on the luminescence intensity ratios of the near-infrared bands of both Ho3+ and Er3+ ions.

Hydrothermal synthesis and upconversion luminescence of cubic-shaped LiNbO3:Yb3+/Er3+ nanocrystals

We exploited a simple hydrothermal route without adding organic additives or hard templates to synthesize cubic-shaped LiNbO3:Yb3+/Er3+ with the size of 400–500 nm. Based on the time-dependent investigation on the crystal structure and morphology, Yb(NO3)3 and Er(NO3)3 not only could emit green and red upconversion (UC) emission but also could be used as an etchant to increase dissolution rate of the Nb2O5 raw material in the hydrothermal synthesis route. Without addition of Yb3+/Er3+ ions, LiNbO3 phase could not be formed at even hydrothermal time of 48 h. The time-dependent Raman spectra showed the Raman peaks of LiNbO3 were occurred at the reaction time of 48 h, and the impurity Nb2O5 phase was disappeared with lengthening hydrothermal time. After calcining at 1050 °C for 2 h, the new Raman peaks at 440/487/531/552 cm−1 and the disappeared OH– absorption peaks at 3429 and 1628 cm−1 favored to produce the bright UC emissions in LiNbO3:Yb3+/Er3+ nanocrystals. The pump power dependence and UC mechanism were studied under 980 nm excitation.

Absorption bandwidth broadening of photon upconversion solid-solution organic crystals by co-dissolution of multiple sensitizers

Photon upconversion (UC) enhances the utilization efficiency of solar energy. However, UC materials generally exhibit a narrow optical absorption bandwidth of the sensitizing molecules that create excited triplet states, including the case of solid-state UC materials. Here, we demonstrate the concept of simultaneous dissolution of multiple sensitizers into crystals of fluorescent molecules to broaden the absorption bandwidth, by using an example of combining two sensitizing porphyrins. Consequently, we appreciably enhanced the UC emission intensity under simulated sunlight. In the air, the developed solid-solution crystals exhibited excellent photostability over 50 h and a low excitation threshold of <1-Sun intensity.

Up-Conversion Luminescence and Optical Temperature Sensing Behaviour of Y2O3:Ho3+, Yb3+ Phosphors

The up-conversion (UC) and temperature sensing behaviours of Y2O3:Ho3+, Yb3+ phosphors were investigated. A series of Y2O3:Ho3+, Yb3+ phosphors were synthesized using a solution combustion method. The cubic structure of the Y2O3 with an Ia3¯ space group was analysed by using X-ray powder diffraction. Scanning electron microscopy was conducted to study the surface morphologies of the UC phosphors. Under 980 nm excitation, the UC emissions of Ho3+ from the 5S2 → 5I8, 5F5 → 5I8 and 5S2 → 5I7 transitions were observed, which occurred through UC energy transfer (ET) processes. The Yb3+ ion concentration severely affected the UC emission. The sensing behaviour of the phosphor was investigated through the green (5F4, 5S2 → 5I8) to red (5F5 → 5I8) fluorescence intensity ratio (FIR). The maximum absolute and relative sensitivity values of SA = 0.08 K−1 and SR = 0.64% K−1 were obtained. The results revealed that the prepared Y2O3:Ho3+, Yb3+ phosphor is suitable for optical sensing at high temperatures.

NIR dual-wavelengths excited upconversion photoluminescence of the core–shell luminescent material for anti-counterfeiting recognition

NaGdF4:Yb,Er,Nd@NaGdF4@NaYF4:Yb,Tm core–shell upconversion (UC) luminescent materials were prepared by solvothermal method. With XRD, TEM, XPS and other characterization methods, the composition, morphology and luminescent properties of the prepared UC luminescent materials were characterized. Under the excitation of 980 nm laser and 808 nm laser, the prepared UC luminescent materials can emit blue light and red light respectively. Compared with the uncoated NaGdF4:Yb,Er,Nd materials, the photoluminescence (PL) intensity of the materials with active-shell structure could be enhanced by several times. Furthermore, the influence of Tm3+ doping concentration and other factors on the PL intensity were investigated. Furthermore, the resultant UC luminescent materials were used as the raw material of anti-counterfeiting ink. The anti-counterfeit patterns printed on the glass can emit different colors luminescence under NIR dual-wavelength excitation. The experimental results indicate that the prepared NaGdF4:Yb,Er,Nd@NaGdF4@NaYF4:Yb,Tm UC luminescent materials can be applied as screen-printing materials for anti-counterfeiting recognition labels.

NIR activated Er3+/Yb3+ doped LiYF4 upconverting nanocrystals for photothermal treatment, optical thermometry, latent fingerprint detection and security ink applications

This work reports the upconversion (UC) emission studies on Er3+/Yb3+ doped LiYF4 nanoparticles for radiation-to-heat conversion, optical thermometry using thermally coupled and uncoupled levels, latent fingerprint detection and security ink applications. LiYF4:Er3+/Yb3+ nanoparticles have been synthesized using the thermal decomposition reaction method using oleic acid and 1-octadecane as reaction medium. Crystal phase, crystallinity and size have been investigated using XRD and TEM techniques. UC emission ability of the prepared phosphor was tested using a CW laser source having 980 nm excitation. Under 980 nm excitation, synthesized nanoparticles have shown emission bands around 522 nm, 550 nm and 668 nm corresponding to transitions from 2H11/2, 4S3/2, and 4F9/2 levels to 4I15/2 level of Er3+ ions, respectively. Further, the number of photons involved in the UC process has been calculated using a pump power UC study. Decay time analysis of the most intense green (550 nm) and red (668 nm) emission bands is performed to study the emission dynamics. A study on the radiation-to-heat conversion phenomenon has revealed photothermal conversion efficiency of 29.5% which is important for photothermal therapy. The green and red UC emissions from the sample under stimulation at very low pump power (35 mW) are used to study the optical temperature sensing characteristics. The findings of this study suggest that the Er3+/Yb3+ co-doped LiYF4 phosphor may be particularly useful for creating NIR to visible upconversion and temperature sensing. Moreover, latent fingerprint detection and security ink for anti-counterfeiting applications are also demonstrated.

Simultaneously Achieving Multicolor Emission of Down‐Shifting and Up‐Conversion in Yb3+, Er3+ ‐Codoped Cs2NaGdCl6 Double Perovskites

AbstractUltraviolet (UV) excited down‐shifting (DS) double perovskites with broad emission have potential applications in anti‐counterfeiting. However, discovering new materials and achieving multimode luminescence present significant challenges. In this work, a novel double perovskite, Cs2NaGdCl6 is synthesized, with sky‐blue self‐trapped exciton (STE) emission under UV excitation using a solvothermal method. By incorporating Er3+ ions into Cs2NaGdCl6, green DS and green up‐conversion (UC) luminescence are achieved under UV and near‐infrared (NIR) excitation. The addition of the sensitizer Yb3+ ion led to a significant enhancement of 21.5‐fold in DS red luminescence and 7.5‐fold in UC green luminescence. The color of both DS and UC luminescence in Cs2NaGdCl6:Yb3+, Er3+ is further adjusted from green‐dominated to red‐dominated, due to the cross‐relaxation process between Er3+ and Yb3+, Er3+ and Er3+, respectively. The combination of UC and DS luminescence, along with the ability to modulate emission color in rare‐earth‐based materials, opens new possibilities for high security anti‐counterfeiting applications.

Transmissive fluorescent temperature sensor based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber for real-time thermal monitoring of motors using the FIR technique.

In this paper, we fabricate a transmissive fluorescent temperature sensor (TFTS) that based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber, which has the advantages of compactness and simplicity, corrosion resistance, high stability and anti-electromagnetic interference. The doping of Mo6+ ions will enhance the up-conversion (UC) fluorescence emission efficiency of Er3+ ions, thus improving the signal-to-noise ratio of TFTS. Using the fluorescence intensity ratio (FIR) technique, the real-time thermal monitoring performance of TFTS is evaluated experimentally. Apart from good stability, its maximum relative sensitivity is 0.01068 K-1 at 274 K in the measured temperature range. In addition, it is successfully used to monitor the temperature variation of the stator core and stator winding of the motor in actual operation. The results show that the maximum error between the FIR-demodulated temperature and the reference temperature is less than 1.2 K, which fully confirms the effectiveness of the TFTS for temperature monitoring. Finally, the FIR-based TFTS in this work is expected to provide a new solution for accurate and real-time thermal monitoring of motors and the like.

Recent Progress in Photonic Upconversion Materials for Organic Lanthanide Complexes.

Organic lanthanide complexes have garnered significant attention in various fields due to their intriguing energy transfer mechanism, enabling the upconversion (UC) of two or more low-energy photons into high-energy photons. In comparison to lanthanide-doped inorganic nanoparticles, organic UC complexes hold great promise for biological delivery applications due to their advantageous properties of controllable size and composition. This review aims to provide a summary of the fundamental concept and recent developments of organic lanthanide-based UC materials based on different mechanisms. Furthermore, we also detail recent applications in the fields of bioimaging and solar cells. The developments and forthcoming challenges in organic lanthanide-based UC offer readers valuable insights and opportunities to engage in further research endeavors.

Photoluminescence of Eu3+ in silicate-based phosphors by near-infrared and near-ultraviolet excitation for multifunctional applications

In this work, a series of Eu3+/Yb3+ doped La1.55SiO4.33 (LSO) phosphors were prepared by solid state reaction method for multifunctional applications. The phase purity and particle morphology were characterized with XRD and SEM technique, respectively. Upon ultraviolet light excitation, the downshifting luminescence of Eu3+ was observed in the red region, and the strongest emission peak located at 612 nm is attributed to the forced electric dipole 5D0→7F2 transition. The optimal Eu3+ doping concentration was determined to be 20 mol%, for which the internal quantum efficiency of the phosphor reaches 75.1%. The temperature-dependent emission spectra of LSO:20%Eu3+ revealed that the quenching temperature T0.5 is 483 K. For the upconversion (UC) luminescence, the Yb3+-Eu3+ codoped LSO phosphors were designed, where the Yb3+ ion was used as a sensitizer. The UC mechanism for a cooperative energy transfer was learnt from the pump-power-dependent emission spectra. By excitation at various temperatures, the intensities of emissions from different excited states showed different changes with temperature. All the above spectral characteristics indicate that the LSO:Eu3+/Yb3+ phosphors can show potential applications in LEDs, anticounterfeiting and optical thermometer.

Ultrapure single-band red upconversion luminescence in Er3+ doped sensitizer-rich ytterbium oxide transparent ceramics for solid-state lighting and temperature sensing.

Achieving single-band upconversion (UC) is a challenging but rewarding approach to attain optimal performance in diverse applications. In this paper, we successfully achieved single-band red UC luminescence in Yb2O3: Er transparent ceramics (TCs) through the utilization of a sensitizer-rich design. The Yb2O3 host, which has a maximum host lattice occupancy by Yb3+ sensitizers, facilitates the utilization of excitation light and enhances energy transfer to activators, resulting in improved UC luminescence. Specifically, by shortening the ionic spacing between sensitizer and activator, the energy back transfer and the cross-relaxation process are promoted, resulting in weakening of green energy level 4S3/2 and 2H11/2 emission and enhancement of red energy level 4F9/2 emission. The prepared Yb2O3: Er TCs exhibited superior optical properties with in-line transmittance over 80% at 600 nm. Notably, in the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. To the best of our knowledge, this is the first demonstration of pure red UC luminescence in TCs. Furthermore, the luminescent intensity ratio (LIR) technique was utilized to apply this pure red-emitting TCs for temperature sensing. The absolute sensitivity of Yb2O3: Er TCs was calculated to be 0.319% K-1 at 304 K, which is the highest level of optical thermometry based on 4F9/2 levels splitting of Er3+ known so far. The integration between pure red UC luminescence and temperature sensing performance opens up new possibilities for the development of multi-functional smart windows.

Utilizing Energy Transfer in Mn2+/Ho3+/Yb3+ Tri-doped ZnAl2O4 Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry.

Lanthanide (Ln3+)-doped upconversion (UC) phosphors converting near-infrared (NIR) light to visible light hold very high promise toward biomedical applications. The scientific findings on luminescent thermometers revealed their superiority for noninvasive thermal sensing. However, only few reports showcase their potential for applications in extreme conditions (temperatures below -70 °C) restricted by low thermal sensitivity. Here, we demonstrate the tailoring of luminescence properties via introducing Ho3+-Mn2+ energy transfer (ET) routes with judicious codoping of Mn2+ ions in ZnAl2O4/Ho3+,Yb3+ phosphor. Preferentially, a singular red UC emission is required to improve the bioimaging sensitivity and minimize tissue damage. We could attain UC emission with 94% red component by a two-photon UC process. Higher temperature annealing brings the color coordinates to the green domain, highlighting the potential for color-tunable luminescence switch. Moreover, this work investigates the thermometric properties of ZnAl2O4/Yb3+, Ho3+ in the range of 80-300 K and influence of inducing extra ET pathways by Mn2+ codoping. Interestingly, the luminescence intensities for nonthermally coupled (5F4,5S2) and the 5F5 radiative transitions of Ho3+ ions display opposite behavior at 80 and 300 K, which revealed competition between temperature-sensitive decay pathways. The codoping of Mn2+ ions is fruitful in causing a fourfold increase of absolute sensitivity. Notably, the color tunability from green through yellow to red is helpful in rough temperature estimation by naked eyes. The maximum relative (Sr) and absolute sensitivities (Sa) were estimated to be 1.89% K-1 (140 K) and 0.0734 K-1 (300 K), respectively. Even at 80 K, a Sa of 0.00447 K-1 and Sr of 0.6025% K-1 were achievable in our case, which are higher than most of the other Ln3+-based systems. The above-mentioned results demonstrate the potential of ZnAl2O4/Yb3+,Ho3+ for cryogenic optical thermometry and a strategy to design new Ln3+-based UC thermometers by taking advantage of ET routes.

Recent advances in rare earth ion-doped upconversion nanomaterials: From design to their applications in food safety analysis.

The misuse of chemicals in agricultural systems and food production leads to an increase in contaminants in food, which ultimately has adverse effects on human health. This situation has prompted a demand for sophisticated detection technologies with rapid and sensitive features, as concerns over food safety and quality have grown around the globe. The rare earth ion-doped upconversion nanoparticle (UCNP)-based sensor has emerged as an innovative and promising approach for detecting and analyzing food contaminants due to its superior photophysical properties, including low autofluorescence background, deep penetration of light, low toxicity, and minimal photodamage to the biological samples. The aim of this review was to discuss an outline of the applications of UCNPs to detect contaminants in food matrices, with particular attention on the determination of heavy metals, pesticides, pathogenic bacteria, mycotoxins, and antibiotics. The review briefly discusses the mechanism of upconversion (UC) luminescence, the synthesis, modification, functionality of UCNPs, as well as the detection principles for the design of UC biosensors. Furthermore, because current UCNP research on food safety detection is still at an early stage, this review identifies several bottlenecks that must be overcome in UCNPs and discusses the future prospects for its application in the field of food analysis.