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In biomedical and optical applications, multifunctional upconverting nanoparticles (UCNPs) play an essential role where non‐invasive temperature sensing and imaging are necessary. UCNPs smaller than 20 nm, which can be excited under 808 nm wavelength, are particularly promising in this area and can be implemented in humans or other mammals. However, new versatile nanoprobes are still needed for biology, especially for challenging studies of small aquatic invertebrates. Such tools allow better monitoring and understanding of their physiology, biochemistry, and ecological responses, which is crucial due to the growing pollution of water reservoirs and climate change. Herein, multifunctional NaYF4:Yb3+, Er3+@NaNdF4:Yb3+ core@shell NPs (15 nm), forming stable aqueous colloids, exhibiting intense emissions under excitation in the first biological window (808 nm), and presenting high thermal sensitivity and resolution related to the thermally coupled energy levels of Er3+ ions, are designed and synthesized. Such properties of UCNPs are further utilized for optical imaging of aquatic invertebrates (Daphnia magna) and temperature detection inside their bodies under 808 nm excitation. This pioneering application of NaYF4:Yb3+, Er3+@NaNdF4:Yb3+ demonstrates the high potential of developed UCNPs for multifunctional applications, especially for bioimaging and temperature sensing within whole organisms. [ABSTRACT FROM AUTHOR]

A biocompatible and versatile silk‐based functional ink with hydrophilic upconversion nanoparticles (UCNPs) that can be printed or painted on arbitrary substrates, including biological surfaces is demonstrated. Hydrophilic UCNPs, ready to be dispersed in aqueous silk fibroin solution, are synthesized in large quantities via facile hydrothermal synthesis using citric acid. Based on a systematic study of dopant control to achieve robust red emission from citrate‐capped UCNPs, three primary colors are obtained, which serve as the basis for the expansion of the color palette. The synthesized UCNPs are homogeneously dispersed in regenerated silk fibroin solutions derived from Bombyx mori cocoons to produce water‐based functional inks. Cyan, magenta, and yellow (CMY) inks are created by combining red, green, and blue (RGB) inks. Demonstrator devices, where a variety of colorful patterns and codes are revealed only under near‐infrared irradiation, are realized through stencil printing or brush painting on flexible polymer films, fruit, leaves, silkworm cocoons, and pig skin. The patterns and codes are highly heat resistant (≈230 °C) and can be easily washed off with water after use. The printable/paintable inks and functions proposed here will provide new opportunities for applications where sustainability, biocompatibility, and intentional luminescence are simultaneously required. [ABSTRACT FROM AUTHOR]

In order to apply near infrared (NIR) light‐activated upconversion nanocrystals (UCNCs) in photochemistry, extremely bright luminescence is needed to excite the ultraviolet (UV)‐visible light absorbing photocatalyst/substrate. In this study, a multilayered UCNC consisting of a β‐NaYF4:Yb0.2, Tm0.005 core, heavily Yb3+‐doped β‐NaYbF4:Y0.09, Tm0.01 intermediate shell, and inert β‐NaYF4 outer shell is prepared. By compartmentalizing the sensitizer (Yb3+)‐activator (Tm3+) pair in the core and intermediate shell with different concentration ratios, a cooperative luminescence effect is discovered. Apart from core emission, energy harvesting followed by sequential energy transfer from a high density of neighboring Yb3+ to Tm3+ in the intermediate shell induces a large population of excited emitter ions. Furthermore, after back energy transfer from the core to intermediate shell, the energy is not quenched by intrinsic trap sites but is instead transferred to Tm3+, further boosting emission in the UV, blue, and NIR regions. The exceptionally bright UCNC is then used to drive NIR‐light‐activated one‐pot multicomponent reactions involving the upconversion energy transfer from UCNC to UV‐visible light absorbing photocatalyst/substrate, Lewis acid catalytic activity of the lanthanide ions (Y3+) on the surface, and photothermal property of the UCNC. The reactions studied include the deacetalization‐Knoevenagel condensation reaction, photooxidation‐cyanation reaction, and hydrodehalogenation‐cyanosilylation reaction. [ABSTRACT FROM AUTHOR]

The near‐infrared (NIR) light polarization metrology is crucial in a broad area of applications, such as optical sensing, imaging, and communications. The conventional measurement of NIR light polarization requires bulky polarization optics by the spatial or temporal splitting of the input light beam with expensive infrared detectors. Here, an intelligent fiber‐integrated NIR polarimeter based on upconversion nanoparticles (UCNPs) is demonstrated. Er‐doped UCNPs are synthesized in core–shell structures for bright upconversion luminescence (UCL), and the tapered multimode fiber (MMF) is designed for random Rayleigh‐distributed speckles. It is demonstrated for the first time that spectra‐polarization‐dependent upconverted speckle patterns captured by a cost‐effective visible image sensor can be accurately decoded via a deep neural network (ResNet50). The experimental results show that ResNet50 has a recognition accuracy of over 99.5% for the full‐Stokes parameters of NIR light polarization, along with 100% accuracy for the light wavelength ranging from 1535 nm to 1565 nm. The developed intelligent optical sensors provide a new design paradigm for fiber‐integrated multi‐dimensional NIR light field detection and can be applied in fiber‐optic sensors and communications networks. [ABSTRACT FROM AUTHOR]

Artificial chiral nanoparticles exhibit unique optical and (photo)physical properties. They can be all‐inorganic or organic–inorganic materials that are usually confined to the nanoscale by anchoring organic ligands to their surface. The ligands can also possess distinctive optical and/or (photo)physical properties, thereby adding functionality to the whole nanosystem and constituting an organic–inorganic nanohybrid. Development of luminescent chiral nanoparticles and nanohybrids is pertinent to many applications, such as (bio)sensing, therapeutics, photocatalysis, and optoelectronics, among others. This review deals with different photoactive nanomaterials, displaying either up‐shifting emission (nanoparticles consisting of a photoactive or inert matrix doped with lanthanides, Ln3+, upconversion nanoparticles, UCNPs) or down‐shifting emission, such as coinage metal nanoclusters (e.g., gold nanoclusters, AuNCs) and lead halide perovskite nanoparticles (HP NPs), as well as very low emissive nanoparticles, such as plasmonic nanoparticles (such as gold nanoparticles, AuNPs). Special attention is paid to the origin of the chiroptical response of these materials, showing that it is varied and can be alike for some of them. [ABSTRACT FROM AUTHOR]

Upconverted circularly polarized luminescence (UC‐CPL) which is coined by integrating two concepts of photon upconversion and circularly polarized luminescence, has aroused enormous attention from scientists owing to the great performance and potential applications in various fields. To date, although various types of UC‐CPL systems are reported, constructing a UC‐CPL system with multicolor emission remains a big challenge. In this work, by incorporating lanthanide‐doped upconversion nanoparticles (UCNPs) into chiral nematic liquid crystal (N*LC), multicolor UC‐CPL emission can be realized upon two kinds of modulations: the regulation of photonic bandgap (PBG) and adjustment of excitation wavelength of the doped UCNPs. More importantly, this kind of multicolor UC‐CPL system can be used as internal materials for constructing polymer‐shell microcapsules. The PBG effect can still be well preserved in the microcapsules, providing a kind of easy processable luminescent material with multicolor UC‐CPL properties, which can be used as an advanced anti‐counterfeiting material. [ABSTRACT FROM AUTHOR]

The growing crisis of antibiotic resistance calls for advanced and sustainable antibacterial strategies. In the past few years, lanthanide‐doped upconversion nanoparticles (UCNPs) with unique photophysical properties have been innovatively explored for bacterial detection and therapy, and hold promise for alleviating challenges faced in the post‐antibiotic era. In this review, the interface engineering for the bacteria‐targeted UCNP nanoplatforms is highlighted, the principles and merits of various upconversion detection strategies are discussed, and the latest progress in UCNP‐based antibacterial applications are summarized, especially in deep‐tissue infection imaging and therapy. Finally, the remaining concerns and future efforts for practical applications are put forward. This review is expected to provide comprehensive guidance for diagnosing and combating bacterial infections based on advanced UCNP nanoplatforms. [ABSTRACT FROM AUTHOR]

Integrating biocompatible metal–organic frameworks (MOFs) with upconversion nanoparticles (UCNPs), capable of converting near‐infrared (NIR) light into visible or ultraviolet emissions, affords unique UCNPs@MOFs heterostructures for nanomedicine. Alongside inherited advantages from parent UCNPs on imaging and parent MOFs with porous structure on chemodrugs delivery, upconversion Förster resonance energy transfer (FRET) from UCNPs to molecular moieties in MOFs offers unique abilities to probe or modulate intracellular and tumor microenvironments at high spatiotemporal precision. As a result, UCNPs@MOFs heterostructures allow to detect hint number of biomolecules, regulate intracellular ion concentrations for tumor cell killing and lamellipodia‐induced cell metastasis, on‐demand release NO gas molecules for antibacterial therapy and neurotherapy, and perform tumor microenvironment‐responsive multimodal therapy, all achieved possible using tissue‐penetrating NIR light stimulation of low photon flux. In this work, recent progress on the construction and applications of UCNPs@MOFs heterostructures toward these regards is surveyed. An emphasis is placed on the classification of heterostructures (Janus, core@satellite, core@shell) with distinct geometric shapes as well as the fundamental upconversion FRET mechanism underpinning all their theranostic applications. Cutting‐edge applications on combined therapies, drug delivery, multimodal imaging, and biosensing are also highlighted, along with provided perspectives on the future development. [ABSTRACT FROM AUTHOR]

Chemo‐photodynamic therapy nanodelivery systems provide a potential strategy to enhance anticancer effect. However, the treatment efficiency of stimuli‐responsive nanotheranostics remains a challenge owing to off‐target properties, poor tissue penetration, and slow drugs release. Herein, di‐selenide‐bridged mesoporous silica (Se‐Se‐mSiO2) is introduced into a near‐infrared (NIR) light triggered and tumor microenvironment (TME)‐activated intelligent nanoplatform (UCNPs@mSiO2‐Ce6@Se‐Se‐mSiO2(DOX)@HA (UCSSDH)) realizing burst drug release on demand. Typically, the UCSSDH actively targets cancer cells and accumulates in tumor site through hyaluronic acid (HA). Then, under the irradiation of 980 nm NIR light, the upconverted 650 nm emission light activates photosensitizer Ce6 to produce the singlet oxygen (1O2) for photodynamic therapy (PDT). More importantly, 1O2 further cleaves di‐selenide bonds which are sensitive to reactive oxygen species (ROS), driving the rapid degradation of Se‐Se‐mSiO2 accompanied by the burst release of doxorubicin (DOX). Consequently, UCSSDH achieves efficient chemo‐photodynamic therapy of tumors. Last but not least, the upconversion nanoparticles (UCNPs) perform upconversion luminescence (UCL) imaging to monitor the drugs delivery. Collectively, an intelligent drug delivery system integrating therapeutic, monitoring, and targeting is exploited for NIR light‐triggered synergistic tumor therapy. This study broadens fresh vision into the development of controlled drug release and provides a momentous thought for potential clinical cancer therapy. [ABSTRACT FROM AUTHOR]

A versatile strategy to print hybrid upconversion/perovskite nanoparticle clusters based on nanoxerography has been developed. The individual hybrid cluster can be tuned by different upconversion nanoparticles (UCNPs), perovskite quantum dots (QDs), and their combinations to visualize multicolor. Based on the precise positioning of the array system, the structural and optical properties of each cluster in the array can be in situ and simultaneously characterized. This approach provides a straightforward and robust platform for high‐throughput preparation and characterization of the hybrid clusters, and the ability to position functional nanoparticles at a desired area with nanoscale accuracy exhibits great potential in fabricating advanced nanodevices. [ABSTRACT FROM AUTHOR]

Förster resonance energy transfer (FRET)‐based devices have been extensively researched as potential biosensors due to their highly localized responsivity. In particular, dye‐conjugated upconverting nanoparticles (UCNPs) are among the most promising FRET‐based sensor candidates. UCNPs have a multi‐modal emission profile that allows for ratiometric sensing, and by conjugating a biosensitive dye to their surface, this profile can be used to measure localized variations in biological parameters. However, the complex nature of the UCNP energy profile as well as reabsorption of emitted photons must be taken into account in order to properly sense the target parameters. To the authors' knowledge, no proposed UCNP‐based sensor has accurately taken care of these intricacies. In this article, the authors account for these complexities by creating a FRET‐based sensor that measures pH. This sensor utilizes Thulium (Tm3+)‐doped UCNPs and the fluorescent dye fluorescein isothiocyanate (FITC). It is first demonstrated that photon reabsorption is a serious issue for the 475 nm Tm3+ emission, thereby limiting its use in FRET‐based sensing. It is then shown that by taking the ratio of the 646 and 800 nm emissions rather than the more popular 475 nm one, it is possible to measure pH exclusively through FRET. [ABSTRACT FROM AUTHOR]

Endowing advanced functional three-dimensional (3D) nanostructures with remote and dynamic controllability is currently in the spotlight of soft-matter photonics research. Herein, the near-infrared (NIR) light-driven reversible and dynamic photonic bandgap modulation of a soft photonic crystal, i.e., blue phase liquid crystals (BPLCs) with 3D nanostructures, is demonstrated through the integration of judiciously designed core-shell upconversion nanoparticles (UCNPs) and a novel axially-chiral azobenzene-based molecular photoswitch. The UCNPs exhibit high UV/vis emissions upon irradiations with 808 nm NIR light, and the intensity ratio of UV/vis luminescence can be facilely modulated through changing the excitation power. With the UCNPs as NIR-to-UV/vis light nanotransducers, the chiral molecular photoswitch shows reversible photoisomerization upon modulating the 808 nm NIR light intensity. Furthermore, NIR light-driven BPLCs loaded with the photoswitch and UCNPs are demonstrated, in which the photonic reflection can red-shift toward a longer wavelength under 808 nm light irradiation with high-power density and the blue-shift took place at lower power density with the same wavelength of light. This work may provide new insights into the large-scale fabrication of 3D photonic nanostructures with tailorable functionalities toward emerging applications of soft-matter photonics and future integrated communication technologies. [ABSTRACT FROM AUTHOR]

Advanced fluorescent anticounterfeiting technology has attracted widespread attention and encourages people to develop reliable luminescent materials with multiple colors and tunable luminescence. Here, novel nanocomposites composed of perovskite quantum dots (PeQDs) and upconversion nanoparticles (UCNPs) are successfully designed and served as efficient anticounterfeiting inks. NaYF4:Yb/Tm@NaYF4:Yb core–shell UCNPs are formed first and then ultrasmall CsPbX3 (X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) PeQDs linked on the surface of UCNPs by seeded‐growth strategy. UCNPs‐CsPbX3 nanocomposites can not only provide multicolor downconversion luminescence (DCL) under ultraviolet (UV) excitation, but also produce tunable upconversion luminescence (UCL) under 980 nm continuous‐wave laser excitation. Finally, UCX3 nanocomposites are mixed with prepolystyrene (PS) and the resulting UCX3/PS with excellent water stability and photostability can be employed as multicolor fluorescent ink for both writing and printing. These findings not only present a general method to fabricate novel nanocomposites combining both advantages of UCNPs and PeQDs, but also provide insight for the practical application in the field of anticounterfeiting. [ABSTRACT FROM AUTHOR]

Photochromic molecule‐incorporated optical devices offer desirable properties for photocontrollable optical systems, including advanced optical data storage and super‐resolution imaging. However, these molecules require multiple illumination sources, such as UV and visible light, for reversible photochemical reactions, which restricts their potential for advanced application. This study reports an effective strategy for modulating photoisomerization via a single near‐infrared light source assisted by plasmonically enhanced photoswitchable upconversion photoluminescence (UCPL). The proposed quasi‐periodic metal nanostructures to facilitate the resonance modes in the broadband region enable the substitution of the detrimental high‐energy light source (i.e., UV light) with near‐infrared stimuli, which is associated with UCPL enhancement of over two orders with spectrum orthogonality. To validate this concept, the accelerated reversible‐photoisomerization kinetics is experimentally confirmed by three‐ and tenfold amplification of the PL intensities of the photochromic disulfonyldiarylethene derivatives. Further validation of the proposed strategy is performed using photodynamic imaging, which reveals accelerated photoisomerization, high photocyclization stability, and high spatial resolution. [ABSTRACT FROM AUTHOR]

Understanding the upconversion pathways of a rare‐earth dopant is crucial to furthering the use of that material, either toward applications in imaging or elsewhere. This work outlines a new analysis approach that consists of using two synchronized widely‐tunable laser sources to explore the properties of upconverting materials. By examining sensitizer‐free rare‐earth nanoparticles based on a matrix of hexagonal sodium yttrium tetrafluoride (β‐NaYF4) doped with praseodymium but no ytterbium sensitizer, a "non‐degenerate" two‐color upconversion fluorescence at a combined excitation of 1020–850 nm is shown. This insight demonstrates the ability of this technique to locate and interrogate novel upconversion pathways. The dopant level of the nanoparticles could be modified without altering other factors, such as the particle's shape or size, that would also change optical properties and this allows investigation of the dopant‐level dependency of the optical properties. The approach also allows exploration of the time delay domain between the arrival times of the two non‐degenerate excitation pulses, which allows modulation of the brightness from the visible light emissions. This work opens up the parameter space for the systematic synthesis and characterization of new materials with non‐degenerate upconversion emission. [ABSTRACT FROM AUTHOR]

Chemical/biological sensing by means of luminescence still requires improvement of signal contrast and depth of sample imaging. Though there is a considerable number of emissive molecular and quantum dot probes, most of them absorb in the UV–vis range and emit in the visible and exhibit emission lifetime in the nanoscale. Optical bioimaging in the near infrared window based on these dyes offers reduced light scattering, lower competitive absorption, and reduced background autofluorescence, but it requires a high‐power pulsed laser. An approach to lengthening the emission lifetime of common fluorophores/probes is presented. It consists of coupling them to lanthanide‐based upconversion nanoparticles (UCNPs) via an easy, jigsaw‐like strategy. As a proof of concept, nanohybrids comprising UCNPs attached to fluorescein (a well‐known pH sensor) are prepared by connecting the key pieces through ionic interactions, i.e., without using any other reagents. Fluorescence imaging of the nanohybrids clearly shows the co‐occurrence of the UCNP and the chemical probe and as a consequence the long‐lived fluorescence of the probe due to resonance energy transfer from the UCNP. This strategy opens up endless possibilities to prepare new nanohybrids by selecting the key jigsaw pieces to exploit the color emission of common probes. [ABSTRACT FROM AUTHOR]

The millisecond‐timescale, high‐efficiency modulation of upconversion luminescence via ultraviolet laser‐induced photochemical reduction of graphene oxide is reported. The luminescence tuning is achieved by combining a thin film of graphene oxide with lanthanide‐doped upconversion nanoparticles. The recovery of the graphene‐like structure through the photochemical reduction of graphene oxide is accompanied by a variation in the absorption coefficient of the as‐synthesized nanocomposite, which enables super‐quenching of the luminescence emission from the upconversion nanoparticles under near‐infrared laser excitation with values of up to ≈90%. Further, the instantaneous reduction in the emission intensity upon ultraviolet laser irradiation allows a 106‐fold decrease of the time for fast modulation of upconversion luminescence from up to tens of minutes down to milliseconds at microwatt‐level laser power. Optical patterning is successfully produced in the nanocomposite and individual pixels are retrieved distinctly with high spatial resolution through upconversion luminescence emission quenching by reduced graphene oxide. This method offers a superior avenue for controlling upconversion luminescence with high contrast, fast reaction speed, and low power consumption. [ABSTRACT FROM AUTHOR]

Upconversion nanoparticles (UCNPs) with exceptional optical properties have emerged as a new paradigm for data encryption through multicolor coding, lifetime tuning, and excitation light phasing. However, in addition to low internal quantum efficiency, the utility of UCNPs for data encryption is largely hampered by the requirement of multiple sets of UCNPs of complex compositions with specifically tailored optical properties. Herein, it is found that metal–insulator–metal (MIM) configurations are capable of regulating upconversion emission of UCNPs through programmable geometry‐dependent plasmonic features of laser‐splashed submicron cavities. Furthermore, a ratiometric encryption strategy by regulating the ratio of dual‐band emission of a single set of UCNPs of fixed composition placed within the plasmonic MIM configuration is demonstrated. High‐throughput encryption and decryption through the luminescence ratiometric strategy can be achieved in a facile laser writing fashion with upscalability and sub‐micrometer‐scale precision. This demonstration opens a new route to luminescence modulation with large encoding capacity and underpins potential applications of UCNPs in multiplexed data storage and information encryption. [ABSTRACT FROM AUTHOR]

Near‐infrared photoacoustic (PA) imaging is developing rapidly as an alternative to optical imaging and promises benefits in terms of deeper tissue penetration and higher spatial resolution. This paper proposes a strategy to enhance the PA intensity of upconverted nanoparticles (UCNPs) by functionalized photoswitchable azobenzene‐containing poly (acrylic acid) (UCNPs@PAA‐Azo) to yield a PA signal that is up to six times higher than that of UCNPs without a decay in NIR‐II emission. The mechanism of enhanced photoacoustic activity of the UCNPs@PAA‐Azo can be attributed to the decay of UV and blue light as well as vibrations of the azobenzene group. This technology delivers high resolution of the hind limb vasculature using dual NIR‐II and PA imaging. Using this nanoprobe, NIR‐II imaging provides real‐time means for the precise resection of the lymph node (LN) during surgery, where PA imaging allows for quicker (up to 30 min) detection of LNs than that through NIR‐II imaging. Combining the advantages of dual NIR‐II and PA modalities allows us to achieve complementary information and improve the accuracy of disease diagnosis. This work is critical to the synthesis of dual NIR‐II and PA modal probes and is utilized in small animals imaging with high resolution and a high signal‐to‐noise ratio. [ABSTRACT FROM AUTHOR]

The polarization‐dependent photon upconversion luminescence properties of large‐scale orientationally ordered soft matter systems formed by colloidal nanorods dispersed in an isotropic solvent are studied. The electrostatically charged photon‐upconverting nanorods form an isotropic dispersion at low concentrations, whereas orientational order and a nematic phase emerge at high concentrations. When an alternating electric field is applied, particles align in the direction of the electric field in both nematic and isotropic phases, though the nature of this electric switching is different in these two phases. Owing to the long‐range orientational order in the nematic phase, the upconversion luminescence from the particles is polarized without an external field. Polarization dependence of these properties can also be electrically induced in an isotropic phase of the colloidal nanorods. Further, the dynamics of switching of photon upconversion luminescence in both nematic and isotropic dispersions are explored and their potential technological uses are dicussed. [ABSTRACT FROM AUTHOR]