Luminescence Thermometry on the Route of the Mobile-Based Internet of Things (IoT): How Smart QR Codes Make It Real

Thermal properties of nanofluids using hydrophilic and hydrophobic LiYF4:Yb/Er upconverting nanoparticles

Luminescent nanoparticles have shown great potential for thermal sensing in bio-applications. Nonetheless, these materials lack water dispersibility that can be overcome by modifying their surface properties with water dispersible molecules such as cysteine. Herein, we employ LiYF4:Er3+/Yb3+ upconverting nanoparticles (UCNPs) capped with oleate or modified with cysteine dispersed in cyclohexane or in water, respectively, as thermal probes. Upconversion emission was used to sense temperature with a relative thermal sensitivity of ∼1.24% K-1 (at 300 K) and a temperature uncertainty of 0.8 K for the oleate capped and of 0.5 K for cysteine modified NPs. To study the effect of the cysteine modification in the heat transfer processes, the thermal conductivity of the nanofluids was determined, yielding 0.123(6) W m-1 K-1 for the oleate capped UCNPs dispersed in cyclohexane and 0.50(7) W m-1 K-1 for the cysteine modified UCNPs dispersed in water. Moreover, through the heating curves, the nanofluids' thermal resistances were estimated, showing that the cysteine modification partially prevents heat transfer.

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.

Communicating Supraparticles to Enable Perceptual, Information-Providing Matter

Materials are the fundament of the physical world, whereas information and its exchange are the centerpieces of the digital world. Their fruitful synergy offers countless opportunities for realizing desired digital transformation processes in the physical world of materials. Yet, to date, a perfect connection between these worlds is missing. From the perspective, this can be achieved by overcoming the paradigm of considering materials as passive objects and turning them into perceptual, information-providing matter. This matter is capable of communicating associated digitally stored information, for example, its origin, fate, and material type as well as its intactness on demand. Herein, the concept of realizing perceptual, information-providing matter by integrating customizable (sub-)micrometer-sized communicating supraparticles (CSPs) is presented. They are assembled from individual nanoparticulate and/or (macro)molecular building blocks with spectrally differentiable signals that are either robust or stimuli-susceptible. Their combination yields functional signal characteristics that provide an identification signature and one or multiple stimuli-recorder features. This enables CSPs to communicate associated digital information on the tagged material and its encountered stimuli histories upon signal readout anywhere across its life cycle. Ultimately, CSPs link the materials and digital worlds with numerous use cases thereof, in particular fostering the transition into an age of sustainability.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Luminescent Single-Molecule Magnets as Dual Magneto-Optical Molecular Thermometers

Luminescent thermometry allows the remote detection of the temperature and holds great potential in future technological applications in which conventional systems could not operate. Complementary approaches to measuring the temperature aiming to enhance the thermal sensitivity would however represent a decisive step forward. For the first time, we demonstrate the proof-of-concept that luminescence thermometry could be associated with a complementary temperature readout related to a different property. Namely, we propose to take advantage of the temperature dependence of both magnetic (canonical susceptibility and relaxation time) and luminescence features (emission intensity) found in Single-Molecule Magnets (SMM) to develop original dual magneto-optical molecular thermometers to conciliate high-performance SMM and Boltzmann-type luminescence thermometry. We highlight this integrative approach to concurrent luminescent and magnetic thermometry using an air-stable benchmark SMM [Dy(bbpen)Cl] (H2 bbpen=N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethyl-enediamine)) exhibiting Dy3+ luminescence. The synergy between multiparametric magneto-optical readouts and multiple linear regression makes possible a 10-fold improvement in the relative thermal sensitivity of the thermometer over the whole temperature range, compared with the values obtained with the single optical or magnetic devices.

Human-Machine Interaction via Dual Modes of Voice and Gesture Enabled by Triboelectric Nanogenerator and Machine Learning

With the development of science and technology, human-machine interaction has brought great benefits to the society. Here, we design a voice and gesture signal translator (VGST), which can translate natural actions into electrical signals and realize efficient communication in human-machine interface. By spraying silk protein on the copper of the device, the VGST can achieve improved output and a wide frequency response of 20-2000 Hz with a high sensitivity of 167 mV/dB, and the resolution of frequency detection can reach 0.1 Hz. By designing its internal structure, its resonant frequency and output voltage can be adjusted. The VGST can be used as a high-fidelity platform to effectively recover recorded music and can also be combined with machine learning algorithms to realize the function of speech recognition with a high accuracy rate of 97%. It also has good antinoise performance to recognize speech correctly even in noisy environments. Meanwhile, in gesture recognition, the triboelectric translator is able to recognize simple hand gestures and to judge the distance between hand and the VGST based on the principle of electrostatic induction. This work demonstrates that triboelectric nanogenerator (TENG) technology can have great application prospects and significant advantages in human-machine interaction and high-fidelity platforms.

Machine Learning Enabled Image Analysis of Time-Temperature Sensing Colloidal Arrays

Smart, responsive materials are required in various advanced applications ranging from anti-counterfeiting to autonomous sensing. Colloidal crystals are a versatile material class for optically based sensing applications owing to their photonic stopband. A careful combination of materials synthesis and colloidal mesostructure rendered such systems helpful in responding to stimuli such as gases, humidity, or temperature. Here, an approach is demonstrated to simultaneously and independently measure the time and temperature solely based on the inherent material properties of complex colloidal crystal mixtures. An array of colloidal crystals, each featuring unique film formation kinetics, is fabricated. Combined with machine learning-enabled image analysis, the colloidal crystal arrays can autonomously record isothermal heating events - readout proceeds by acquiring photographs of the applied sensor using a standard smartphone camera. The concept shows how the progressing use of machine learning in materials science has the potential to allow non-classical forms of data acquisition and evaluation. This can provide novel insights into multiparameter systems and simplify applications of novel materials.

A Self-Powered, Highly Embedded and Sensitive Tribo-Label-Sensor for the Fast and Stable Label Printer

Label-sensor is an essential component of the label printer which is becoming a most significant tool for the development of Internet of Things (IoT). However, some drawbacks of the traditional infrared label-sensor make the printer fail to realize the high-speed recognition of labels as well as stable printing. Herein, we propose a self-powered and highly sensitive tribo-label-sensor (TLS) for accurate label identification, positioning and counting by embedding triboelectric nanogenerator into the indispensable roller structure of a label printer. The sensing mechanism, device parameters and deep comparison with infrared sensor are systematically studied both in theory and experiment. As the results, TLS delivers 6 times higher signal magnitude than traditional one. Moreover, TLS is immune to label jitter and temperature variation during fast printing and can also be used for transparent label directly and shows long-term robustness. This work may provide an alternative toolkit with outstanding advantages to improve current label printer and further promote the development of IoT.

Ultrasensitive Optical Thermometry via Inhibiting the Energy Transfer in Zero-Dimensional Lead-Free Metal Halide Single Crystals

Self-referencing optical thermometry based on the fluorescence intensity ratio (FIR) have drawn extensive attention as a result of their high sensitivity and non-invasively fast response to temperature. However, it is a great challenge for luminescent materials to achieve simultaneously high absolute and relative temperature sensitivity based on the FIR technique. Herein, we developed a novel optical thermometer by designing hybrid lead-free metal halide (TTPhP)₂MnCl₄:Sb³⁺ (TTPhP⁺ = tetraphenylphosphonium cation) single crystals with multimodal photoluminescence (PL). The large TTPhP⁺ organic chain resulted in isolated [MnCl₄]^2- and [SbCl₅]^2- in the single crystal, which leads to a negligible energy trasfer process within them. Therefore, the two PL bands (band 1 from [MnCl₄]^2-) with a peak at 518 nm and band 2 (from [SbCl₅]²) with a peak at 640 nm exhibit different thermal-quenching effects, which resulted in excellent temperature sensitivity, with the maximum absolute and relative sensitivities reaching 0.236 K^-1 and 3.77% K^-1 in a temperature range from 300 to 400 K. Both the absolute and relative sensitivities are among the highest values for luminescence thermometry.

Multiplexed Covalent Patterns on Double-Reactive Porous Coating

We have conceptualized and demonstrated an approach based on the combination of hydrophobicity, a substrate‐independent dip coating as porous material with double residual chemical reactivities for implementing multiplexed, miniaturized and unclonable bulk‐infused patterns of different fluorophores following distinct reaction pathways. The embedded hydrophobicity (∼102°) restricted the unwanted spreading of beaded aqueous ink on the coating. The constructions of micropatterns on porous dip‐coating via ink‐jet printing or microchannel cantilever spotting offered orthogonal read‐out and remained readable even after removal of the exterior of the coating.

Tensor Network-Encrypted Physical Anti-counterfeiting Passport for Digital Twin Authentication

The trend of digitalization has produced rapidly increasing data interaction and authentication demand in today's internet of things ecosystem. To face the challenge, we demonstrated a micro-scale label by direct laser writing to perform as a passport between the physical and digital worlds. On this label, the user information is encrypted into three-dimensional geometric structures by a tensor network and then authenticated through the decryption system based on computer vision. A two-step printing methodology is applied to code the randomly distributed fluorescence from doped quantum dots, which achieved physical unclonable functions (PUFs) of the passport. The 10⁵ bits/mm² data storage density enables abundant encrypted information from physical worlds, for example, the biometric data of human users. This passport guarantees the strong correlation between the user's privacy data and the PUF-assisted codes, successfully overcoming the illegal transfer of authentication information. Due to its ultra-high security level and convenience, the printed passport has enormous potential in future digital twin authentication anytime anywhere, including personal identity, valuable certificates, and car networking.

1D-Zigzag Eu3+/Tb3+ Coordination Chains as Luminescent Ratiometric Thermometers Endowed with Multicolor Emission

Two homometallic Coordination Polymers (CPs) with composition [Ln(hfac)₃bipy]_n (Ln³⁺ = Eu³⁺, 1, and Tb³⁺, 2; hfac = hexafluoroacetylacetonato, bipy = 4,4′-bipyridine) were used to develop a family of ratiometric luminescent thermometers containing Eu³⁺ and Tb³⁺ as red and green emitters, respectively. The thermometric properties of pure CPs and of their mixtures having an Eu³⁺/Tb³⁺ molar ratio of 1:1, 1:3, 1:5, and 1:10 (samples: Eu1Tb1, Eu1Tb3, Eu1Tb5, and Eu1Tb10) were studied in the 83–383 K temperature range. Irrespective of the chemical composition, we observed similar thermometric responses characterized by broad applicative temperature ranges (from 100 to 165 K wide), and high relative thermal sensitivity values (S_r), up to 2.40% K⁻¹, in the physiological temperature range (298–318 K). All samples showed emissions endowed with peculiar and continuous color variation from green (83 K) to red (383 K) that can be exploited to develop a colorimetric temperature indicator. At fixed temperature, the color of the emitted light can be tuned by varying composition and excitation wavelength.

Thermochromic Luminescent Nanomaterials Based on Mn4+/Tb3+ Codoping for Temperature Imaging with Digital Cameras

A new thermographic nanocrystalline Sr4Al14O25:Mn4+,Tb3+ phosphor was developed, and the concentrations of both dopants and the synthesis conditions were optimized. The combination of the thermally quenched luminescence from the Mn4+ ions to the almost temperature-independent emission from Tb3+ provides a sensitive luminescent thermometer (SR = 2.8%/°C at 150 °C) with strong emission color variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150 °C SR(x) = 0.6%/°C and SR(y) = 0.4%/°C, respectively. Noncontact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images, thereby confirming the high application potential of Sr4Al14O25:Mn4+,Tb3+ nanocrystals for thermal sensing and mapping.

A study on temperature sensing performance based on the luminescence of Eu3+ and Er3+ co-doped YNbO4

Eu3+ and Er3+ co-doped YNbO4 powder phosphors were synthesized by a traditional high-temperature solid-state reaction method. A laser of 487.6 nm wavelength was selected to be the excitation source which can pump Eu3+ ions from its thermally populated low-lying 7F2 ground state to the excited state 5D2 and lift the Er3+ ions from their 4I15/2 to 4F7/2 states. Fluorescence intensity ratio (FIR) between the 5D0 → 7FJ emissions of Eu3+ and 4S3/2 → 4I15/2 emissions of Er3+ ions is remarkably dependent on temperature because of the dramatic increase of Eu3+ luminescence against a slight quenching of Er3+ luminescence with the rise of temperature. This temperature sensitive FIR can be favorably employed for temperature sensing, owing to this novel scheme of 5D2 excitation, instead of 5D0, from 7F2 and utilizing Er3+ luminescence as a reference for FIR measurements. This sample is also prominent for its excellent signal-to-noise ratio and is a promising candidate for an optical temperature sensor.

A solid-state colorimetric fluorescence Pb2+-sensing scheme: mechanically-driven CsPbBr3 nanocrystallization in glass

Highly toxic Pb2+ poses a great threat to the health of human beings and ecosystems, urgently calling for an efficient Pb2+ detection method. Herein, we demonstrated a brand-new solid-state fluorescence Pb2+-sensing scheme based on a type of Pb2+-responsive borate glass powder that is able to precipitate CsPbBr3 nanocrystals on the glass surface upon grinding with Pb2+ sources, following a mechanically driven glass crystallization mechanism. Pb2+ sensing is achieved via the Pb2+ concentration-dependent green emission of CsPbBr3 as an indicator signal and independent red emission of Eu3+ as a reference signal. Under UV light irradiation, the obvious emissive color variation from red to green as Pb2+ concentration varies enables the intuitive Pb2+-sensing by naked eyes. With the aid of a spectrometer and smartphone, Pb2+ concentration can be quantitatively determined with the detection limit as low as ∼70 ppm and ∼400 ppm, respectively. The semi-quantitative Pb2+ detection is also possible by comparing the emissive color with the calibrated color card. Hopefully, the proposed solid-state fluorescence Pb2+-sensing strategy with high selectivity can be used for portable and quick Pb2+ analysis in daily life.

High-sensitivity dual UV/NIR-excited luminescence thermometry by rare earth vanadate nanoparticles

High-crystallinity Ln³⁺-doped YVO₄ nanoparticles combine multiple emissions under dual UV/NIR excitation, promoting high performance self-referenced luminescence thermometry.