Dual-Mode Upconversion Nanoprobe Enables Broad-Range Thermometry from Cryogenic to Room Temperature

Towards core-shell engineering for efficient luminescence and temperature sensing

Research on the core-shell design of rare earth-doped nanoparticles has recently gained significant attention, particularly in exploring the synergistic effects of combining active and inert shell layers. In this study, we successfully synthesized 8 types of spherical core-shell Na-based nanoparticles to enhance the efficiency of core-shell design in upconversion luminescence and temperature sensing through the strategic arrangement of inert and active layers. The most effective upconversion luminescence was observed under 980 nm and 808 nm laser excitation using NaYF4 inert shell NaYF4:Yb3+, Er3+@ NaYF4 and NaYF4@ NaYF4:Yb3+, Nd3+ core-shell nanostructures. Moreover, the incorporation of the NaYbF4 active shell structure led to a significant increase in relative sensitivity in ratio luminescence thermometry. Notably, the NaYF4:Yb3+, Nd3+, Er3+@ NaYbF4 core-shell structure demonstrated the highest relative sensitivity of 1.12 %K-1. This research underscores the crucial role of inert shell layers in enhancing upconversion luminescence in core-shell structure design, while active layers play a key role in achieving high-sensitivity temperature detection capabilities.

Quantitative Fluorescent Lateral Flow Strip Sensor for Myocardial Infarction Using Purity-Color Upconversion Nanoparticles

Acute myocardial infarction is a serious cardiovascular disease and poses significant risks to human health. Its early diagnosis and real-time detection are of great importance. Herein, we design a low-cost device that has a high sensitivity of cTnT and cTnI detection. Dual-color upconversion nanoparticles (UCNPs) are prepared as probes, which not only have high-purity red upconversion luminescence (UCL) under 980 or 808 nm excitation but also achieve good temperature sensing. Temperature-dependent multicolor emission excitation is obtained, and the color turns from white to orange and red with increasing temperature. In particular, the maximum SR and SA values based on nonthermally coupled levels are 4.76% K-1 and 8.6% K-1, which are higher than those based on thermally coupled levels. With the UCNPs-based lateral flow strip (LFS), the specific detection of cTnI and cTnT antigens in samples is achieved with a detection limit of 0.001 ng/mL, which is 1 order of magnitude lower than that of their clinical cutoff. The UCNPs-LFS device has a low-cost laser diode and a simplified laser and permits a mobile-phone camera to collect the results, which has an important influence on the field of biomarker sensing.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Multimode Luminescence Tailoring and Improvement of Cs2 NaHoCl6 Cryolite Crystals via Sb3+ /Yb3+ Alloying for Versatile Photoelectric Applications

Lead-free halide double perovskites are currently gaining significant attention owing to their exceptional environmental friendliness, structural adjustability as well as self-trapped exciton emission. However, stable and efficient double perovskite with multimode luminescence and tunable spectra are still urgently needed for multifunctional photoelectric application. Herein, holmium based cryolite materials (Cs2 NaHoCl6 ) with anti-thermal quenching and multimode photoluminescence were successfully synthesized. By the further alloying of Sb3+ (s-p transitions) and Yb3+ (f-f transitions) ions, its luminescence properties can be well modulated, originating from tailoring band gap structure and enriching electron transition channels. Upon Sb3+ substitution in Cs2 NaHoCl6 , additional absorption peaking at 334 nm results in the tremendous increase of photoluminescence quantum yield (PLQY). Meanwhile, not only the typical NIR emission around 980 nm of Ho3+ is enhanced, but also the red and NIR emissions show a diverse range of anti-thermal quenching photoluminescence behaviors. Furthermore, through designing Yb3+ doping, the up-conversion photoluminescence can be triggered by changing excitation laser power density (yellow-to-orange) and Yb3+ doping concentration (red-to-green). Through a combined experimental-theoretical approach, the related luminescence mechanism is revealed. In general, by alloying Sb3+ /Yb3+ in Cs2 NaHoCl6 , abundant energy level ladders are constructed and more luminescence modes are derived, demonstrating great potential in multifunctional photoelectric applications.

Luminescence Thermometry with Nanoparticles: A Review

Luminescence thermometry has emerged as a very versatile optical technique for remote temperature measurements, exhibiting a wide range of applicability spanning from cryogenic temperatures to 2000 K. This technology has found extensive utilization across many disciplines. In the last thirty years, there has been significant growth in the field of luminous thermometry. This growth has been accompanied by the development of temperature read-out procedures, the creation of luminescent materials for very sensitive temperature probes, and advancements in theoretical understanding. This review article primarily centers on luminescent nanoparticles employed in the field of luminescence thermometry. In this paper, we provide a comprehensive survey of the recent literature pertaining to the utilization of lanthanide and transition metal nanophosphors, semiconductor quantum dots, polymer nanoparticles, carbon dots, and nanodiamonds for luminescence thermometry. In addition, we engage in a discussion regarding the benefits and limitations of nanoparticles in comparison with conventional, microsized probes for their application in luminescent thermometry.

Toward Magneto-Optical Cryogenic Thermometers with High Sensitivity: A Magnetic Circular Dichroism Based Thermometric Approach

Remote temperature probing at the cryogenic range is of utmost importance for the advancement of future quantum technologies. Despite the notable achievements in luminescent thermometers, accurately measuring temperatures below 10 K remains a challenging endeavor. In this study, we propose a novel magneto-optical thermometric approach based on the magnetic-circular dichroism (MCD) technique, which offers unprecedented capabilities for meticulous temperature variation analysis at cryogenic temperatures. The inherent temperature sensitivity of the MCD C-term, in conjunction with both positive and negative signals, enables highly sensitive magneto-optical temperature probing. Additionally, a groundbreaking relative thermal sensitivity value of 95.3 % K-1 at 2.54 K can be achieved using a mononuclear lanthanide complex, [[Ho(acac)3 (phen)], in the presence of a 0.25 T applied magnetic field and using a combination of multiparametric thermal read-out with multiple regression. These results unequivocally demonstrate the viability and effectiveness of our methodology for cryogenic temperature sensing.

Luminescence Temperature Sensing and First Principles Calculation of Photoelectric Properties in C12A7 Co-Doped Eu3+ Ions

Developing high-resolution, high-accuracy fluorescent thermometers is challenging. In this study, the optical properties and thermal sensing of Yb-, Tm-, and Eu-co-doped C12A7 (C12A7:Yb/Eu/Tm), with flower-like structure upconversion microparticles, were studied. Eu³⁺ doping induced an approximately 6-fold change in the upconversion luminescence (UCL) output in comparison with C12A7:Yb/Tm microparticles. The maximum relative temperature sensitivity (S) of C12A7:Yb/Eu/Tm reached 3.0% K^-1, representing an approximately 5-fold difference compared with the value of C12A7:Yb/Tm. In particular, the multicolor upconversion emission of C12A7:Yb/Eu/Tm can easily change from blue to white UCL with increasing temperature. Moreover, the band structure, total density, and optical coefficient of C12A7:Yb/Eu/Tm were investigated via density functional theory. The total density of O atoms increased in comparison with the total density of pure C12A7, indicating that substitution of Ca²⁺ by Yb/Eu/Tm produced positive vacancies on the cage structure. The optical coefficient of C12A7 was improved by the Yb/Eu/Tm dopant. The thermally regulated multicolor characteristics and thermally coupled energy levels of Tm³⁺ provide "dual adjustment temperature sensing", which is a promising strategy for realizing accurate and effective temperature sensors.

Self-optimized single-nanowire photoluminescence thermometry

Nanomaterials-based photoluminescence thermometry (PLT) is a new contact-free photonic approach for temperature sensing, important for applications ranging from quantum technology to biomedical imaging and diagnostics. Even though numerous new materials have been explored, great challenges and deficiencies remain that hamper many applications. In contrast to most of the existing approaches that use large ensembles of rare-earth-doped nanomaterials with large volumes and unavoidable inhomogeneity, we demonstrate the ultimate size reduction and simplicity of PLT by using only a single erbium-chloride-silicate (ECS) nanowire. Importantly, we propose and demonstrate a novel strategy that contains a self-optimization or "smart" procedure to automatically identify the best PL intensity ratio for temperature sensing. The automated procedure is used to self-optimize key sensing metrics, such as sensitivity, precision, or resolution to achieve an all-around superior PLT including several record-setting metrics including the first sensitivity exceeding 100% K^-1 (~138% K^-1), the highest resolution of 0.01 K, and the largest range of sensible temperatures 4-500 K operating completely within 1500-1800 nm (an important biological window). The high-quality ECS nanowire enables the use of well-resolved Stark-sublevels to construct a series of PL intensity ratios for optimization in infrared, allowing the completely Boltzmann-based sensing at cryogenic temperature for the first time. Our single-nanowire PLT and the proposed optimization strategy overcome many existing challenges and could fundamentally impact PL nano-thermometry and related applications such as single-cell thermometry.

ZnTe Crystal Multimode Cryogenic Thermometry Using Raman and Luminescence Spectroscopy

In this study, ZnTe crystal was applied to provide precise thermal sensing for cryogenic temperatures. Multiple techniques, namely Raman and photoluminescence spectroscopies, were used to broaden the operating temperature range and improve the reliability of the proposed thermometers. Raman-based temperature sensing could be applied in the range of 20-100 K, while luminescence-based thermometry could be utilized in a narrower range of 20-70 K. However, the latter strategy provides better relative thermal sensitivity and temperature resolution. The best thermal performances based on a single temperature-dependent parameter attain S_r = 3.82% K^-1 and ΔT = 0.12 K at T = 50 K. The synergy between multiple linear regression and multiparametric thermal sensing demonstrated for Raman-based thermometry results in a ten-fold improvement of _Sr and a two-fold enhancement of ΔT. All studies performed testify that the ZnTe crystal is a promising multimode contactless optical sensor for cryogenic thermometry.

2D Thermal Maps Using Hyperspectral Scanning of Single Upconverting Microcrystals: Experimental Artifacts and Image Processing

Whereas lanthanide-based upconverting particles are promising candidates for several micro- and nanothermometry applications, understanding spatially varying effects related to their internal dynamics and interactions with the environment near the surface remains challenging. To separate the bulk from the surface response, this work proposes and performs hyperspectral sample-scanning experiments to obtain spatially resolved thermometric measurements on single microparticles of NaYF₄: Yb³⁺,Er³⁺. Our results showed that the particle's thermometric response depends on the excitation laser incidence position, which may directly affect the temperature readout. Furthermore, it was noticed that even minor temperature changes (<1 K) caused by room temperature variations at the spectrometer CCD sensor used to record the luminescence signal may significantly modify the measurements. This work also provides some suggestions for building 2D thermal maps that shall be helpful for understanding surface-related effects in micro- and nanothermometers using hyperspectral techniques. Therefore, the results presented herein may impact applications of lanthanide-based nanothermometers, as in the understanding of energy-transfer processes inside systems such as nanoelectronic devices or living cells.

Charge Transfer-Triggered Bi3+ Near-Infrared Emission in Y2Ti2O7 for Dual-Mode Temperature Sensing

Trivalent bismuth is a popular main group ion showing versatile luminescent behaviors in a broad spectral range from ultraviolet to visible, but barely in the near-infrared (NIR) region. In this study, we have observed unexpected NIR emission at ∼744 nm in a Bi³⁺-doped pyrochlore, Y₂Ti₂O₇ (YTOB). Our first-principles electronic structure calculation and analysis of the Bi local structure via extended X-ray absorption fine structure indicate that only Bi³⁺ species appears in YTOB and it has a similar local environment to that of Y³⁺. The NIR emission is assigned to a Ti⁴⁺ → Bi³⁺ metal-to-metal charge transfer process. Moreover, we have demonstrated dual-mode luminescence thermometry based on the luminescence intensity ratio (LIR) and lifetime (τ) in 0.5% Bi³⁺ and 0.5% Pr³⁺ co-doped Y₂Ti₂O₇ (YTOB0.5P0.5). It exhibits high thermometric sensitivity simultaneously in the cryogenic temperature range from 78 to 298 K based on τ of the NIR emission of Bi³⁺ at 748 nm and in the temperature range of 278-378 K based on the LIR of Bi³⁺ to Pr³⁺ emissions (I₇₄₈/I₆₁₅). As a novel LIR-τ dual-mode thermometric material over a wide temperature range, the maximum relative sensitivities of the YTOB0.5P0.5 reach 3.53% K^-1 at 298 K from the τ mode and 3.52% K^-1 at 318 K based on the LIR mode. The dual-mode luminescence thermometry with high responsivity from our Bi³⁺-based pyrochlore Y₂Ti₂O₇ phosphor opens a new avenue for more luminescent materials toward multi-mode thermometry applied in complex temperature-sensing conditions.

Activators Confined Upconversion Nanoprobe with Near-Unity Förster Resonance Energy Transfer Efficiency for Ultrasensitive Detection

Lanthanide-doped upconversion nanoparticles (UCNPs) as energy donors for Förster resonance energy transfer (FRET) are promising in biosensing, bioimaging, and therapeutic applications. However, traditional FRET-based UC nanoprobes show low efficiency and poor sensitivity because only partial activators in UCNPs possessing suitable distance with energy acceptors (<10 nm) can activate the FRET process. Herein, a novel excited-state energy distribution-modulated upconversion nanostructure is explored for highly efficient FRET. Integration of the optimal 4% Er³⁺ doped shell and 100% Yb³⁺ core achieves ∼4.5-fold UC enhancement compared with commonly used NaYF₄:20%Yb³⁺,2%Er³⁺ nanoparticles, enabling maximum donation of excitation energy to an acceptor. The spatial confinement strategy shortens significantly the energy-transfer distance (∼4.5 nm) and thus demonstrates experimentally a 91.9% FRET efficiency inside the neutral red (NR)-conjugated NaYbF₄@NaYF₄:20%Yb³⁺,4%Er³⁺ nanoprobe, which greatly outperforms the NaYbF₄@NaYF₄:20%Yb³⁺,4%Er³⁺@SiO₂@NR nanoprobe (27.7% efficiency). Theoretical FRET efficiency calculation and in situ single-nanoparticle FRET measurement further confirm the excellent energy-transfer behavior. The well-designed nanoprobe shows a much lower detection limit of 0.6 ng/mL and higher sensitivity and is superior to the reported NO₂^- probes. Our work provides a feasible strategy to exploit highly efficient FRET-based luminescence nanoprobes for ultrasensitive detection of analytes.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

Lanthanide-doped upconversion nanoparticles (UCNPs), characterized by converting low-energy excitation to high-energy emission, have attracted considerable interest due to their inherent advantages of large anti-Stokes shifts, sharp and narrow multicolor emissions, negligible autofluorescence background interference, and excellent chemical- and photo-stability. These features make them promising luminophores for sensing applications. In this review, we give a comprehensive overview of lanthanide-doped upconversion nanophosphors including the fundamental principle for the construction of UCNPs with efficient upconversion luminescence (UCL), followed by state-of-the-art strategies for the synthesis and surface modification of UCNPs, and finally describing current advances in the sensing application of upconversion-based probes for the quantitative analysis of various analytes including pH, ions, molecules, bacteria, reactive species, temperature, and pressure. In addition, emerging sensing applications like photodetection, velocimetry, electromagnetic field, and voltage sensing are highlighted.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Dual-functional ratiometric fluorescent sensor based on mixed-lanthanide metal-organic frameworks for detection of trace water and temperature

The rapid-response ratiometric sensors are promising materials to detect trace water and temperature. However, the accurately visualized water assay in very narrow-range still remains a challenge. Herein, a novel dual-functional...

Influence of the surrounding medium on the luminescence-based thermometric properties of single Yb3+/Er3+ codoped yttria nanocrystals

While temperature measurements with nanometric spatial resolution can provide valuable information in several fields, most of the current literature using rare-earth based nanothermometers report ensemble-averaged data. Neglecting individual characteristics of each nanocrystal (NC) may lead to important inaccuracies in the temperature measurements. In this work, individual Yb³⁺/Er³⁺ codoped yttria NCs are characterized as nanothermometers when embedded in different environments (air, water and ethylene glycol) using the same 5 NCs in all measurements, applying the luminescence intensity ratio technique. The obtained results show that the nanothermometric behavior of each NC in water is equivalent to that in air, up to an overall brightness reduction related to a decrease in collected light. Also, it was observed that the thermometric parameters from each NC can be much more precisely determined than those from the "ensemble" equivalent to the set of 5 single NCs. The "ensemble" parameters have increased uncertainties mainly due to NC size-related variations, which we associate to differences in the surface/volume ratio. Besides the reduced parameter uncertainty, it was also noticed that the single-NC thermometric parameters are directly correlated to the NC brightness, with a dependence that is consistent with the expected variation in the surface/volume ratio. The relevance of surface effects also became evident when the NCs were embedded in ethylene glycol, for which a molecular vibrational mode can resonantly interact with the Er³⁺ ions electronic excited states used in the present experiments. The methods discussed herein are suitable for contactless on-site calibration of the NCs thermometric response. Therefore, this work can also be useful in the development of measurement and calibration protocols for several lanthanide-based nanothermometric systems.

Cryogenic enabled multicolor upconversion luminescence of KLa(MoO4)2:Yb3+/Ho3+ for dual-mode anti-counterfeiting

The rational development of multicolor upconversion (UC) luminescent materials is particularly promising for achieving high-tech anti-counterfeiting and security applications. Here, an Ho³⁺ and Yb³⁺ ion co-doped KLa(MoO₄)₂ material can achieve multicolored UC luminescence by thermally manipulating the electron transition process, which could be developed to execute advanced optical anti-counterfeiting applications. The emission color of this material turns from bright green to deep orange with the temperature controlled from 85 K to 240 K in a cryogenic environment. The maximum absolute sensitivity and relative sensitivity of this temperature-sensing material based on non-thermally coupled levels of Ho³⁺ ions reached 0.049 K^-1 and 4.6% K^-1. And utilizing the thermochromic luminescence properties and high sensitivity for low temperature of the KLa(MoO₄)₂:Yb³⁺/Ho³⁺ UC material, we created KLa(MoO₄)₂:Yb³⁺/Ho³⁺ fluorescent security inks and UC photonic barcodes to realize novel visual reading and digital recognition dual-mode anti-counterfeiting in a secure manner. These results may provide useful enlightenment for the design and modulation of high-sensitivity temperature-sensing materials for high-level anti-counterfeiting applications.

Upconversion nanoparticles modified by Cu2S for photothermal therapy along with real-time optical thermometry

Highly effective photothermal conversion performance coupled with high resolution temperature detection in real time is urgently needed for photothermal therapy (PTT). Herein, ultra-small Cu₂S nanoparticles (NPs) were designed to absorb on the surface of NaScF₄: Yb³⁺/Er³⁺/Mn²⁺@NaScF₄@SiO₂ NPs to form a central-satellite system, in which the Cu₂S NPs play the role of providing significant light-to-heat conversion ability and the Er³⁺ ions in the NaScF₄: Yb³⁺/Er³⁺/Mn²⁺ cores act as a thermometric probe based on the fluorescence intensity ratio (FIR) technology operating in the biological windows. A wavelength of 915 nm is used instead of the conventional 980 nm excitation wavelength to eliminate the laser induced overheating effect for the bio-tissues, by which Yb³⁺ can also be effectively excited. The temperature resolution of the FIR-based optical thermometer is determined to be better than 0.08 K over the biophysical temperature range with a minimal value of 0.06 K at 298 K, perfectly satisfying the requirements of biomedicine. Under the radiation of 915 nm light, the Cu₂S NPs exhibit remarkable light-to-heat conversion capacity, which is proved by photothermal ablation testing of E. coli. The results reveal the enormous potential of the present NPs for PTT integrated with real-time temperature sensing with high resolution.

Optical Temperature Sensing of YbNbO4:Er3+ Phosphors Synthesized by Hydrothermal Method

The novel YbNbO4:Er3+ phosphors were firstly synthesized through the hydrothermal method by adding LiOH·H2O as flux in the H2O/EG system. YbNbO4:Er3+ phosphors showed the agglomerated irregular polygons coexisting with some tiny grains. XRD and Raman spectra were measured to understand the phase structure and the crystal growth mechanism of YbNbO4:Er3+ phosphors. The upconversion (UC) emission spectra, the pump power dependency and UC mechanism were studied under 980 nm excitation. Based on the fluorescence intensity ratio technique, YbNbO4:Er3+ exhibited the maximum sensor sensitivity of 0.00712 K−1 at 220 K, providing a promising application in optical low-temperature sensors.

Design of a bi-functional NaScF4: Yb3+/Er3+ nanoparticles for deep-tissue bioimaging and optical thermometry through Mn2+ doping

An approximately monochromatic red upconversion (UC) emission is successfully realized in NaScF₄: Yb³⁺/Er³⁺ nanoparticles (NPs) through Mn²⁺ ions doping without phase transition. The Mn²⁺ ions play a role of bridge during the energy transfer process from green emission state ²H_11/2/⁴S_3/2 of Er³⁺ to red emission state ⁴F_9/2 of Er³⁺, which significantly accelerates the red UC enhancement. The strongest red luminescence is observed in the sample containing 10% Mn²⁺ ions (Mn-10) with an enhancement factor of 7.5 times. Meanwhile, an ultrasensitive optical thermometry in the physiological temperature region can be realized by utilizing the fluorescence intensity ratio (FIR) between two thermally coupled Stark transitions of Er³⁺: ⁴I_13/2 → ⁴I_15/2, locating in the near-infrared (NIR) long wavelength region of the second biological window. Its relative sensitivity S_R can be expressed by 340/T², which is much higher than most optical thermometers based on thermally coupled Stark sublevels reported by the previous papers. Beyond that, an ex vivo experiment is designed to evaluate the penetration depth of the red and NIR emission of Mn-10 in the biological tissues, revealing that they can reach depth of at least 3 mm and 5 mm respectively. More importantly, the increasing tissue thickness has almost no effect on the FIR values. All the results show that the present sample is a promising bi-functional nano probe which can be used for bioimaging and temperature sensing in the deep tissues through the strong red UC emission and ultrasensitive NIR optical thermometer, respectively.

Improving the temperature-dependent sensitivity of Ca9Y(PO4)7:Ce3+,Mn2+ and g-C3N4 composite phosphors by mechanical mixing

Luminescent properties of CYPO:Ce³⁺,Mn²⁺/g-C₃N₄ at different temperatures under 318 nm excitation.

Highly Sensitive Upconverting Nanoplatform for Luminescent Thermometry from Ambient to Cryogenic Temperature

Precise assessment of temperature is crucial in many physical, technological, and biological applications where optical thermometry has attracted considerable attention primarily due to fast response, contactless measurement route, and electromagnetic passivity. Rare-earth-doped thermographic phosphors that rely on ratiometric sensing are very efficient near and above room temperature. However, being dependent on the thermally-assisted migration of carriers to higher excited states, they are largely limited by the quenching of the activation mechanism at low temperatures. In this paper, we demonstrate a strategy to pass through this bottleneck by designing a linear colorimetric thermometer by which we could estimate down to 4 K. The change in perceptual color fidelity metric provides an accurate measure for the sensitivity of the thermometer that attains a maximum value of 0.86 K^-1 . Thermally coupled states in Er³⁺ are also used as a ratiometric sensor from room temperature to ∼140 K. The results obtained in this work clearly show that Yb³⁺ -Er³⁺ co-doped NaGdF₄ microcrystals are a promising system that enables reliable bimodal thermometry in a very wide temperature range from ultralow (4 K) to ambient (290 K) conditions.