Dye-sensitized lanthanide containing nanoparticles for luminescence based applications

Due to their exceptional luminescent properties, lanthanide (Ln) complexes represent a unique palette of probes in the spectroscopic toolkit. Their extremely weak brightness due to forbidden Ln electronic transitions can be overcome by indirect dye-sensitization from the antenna effect brought by organic ligands. Despite the improvement brought by the antenna effect, (bio)analytical applications with discrete Ln complexes as luminescent markers still suffers from low sensitivity as they are limited by the complex brightness. Thus, there is a need to develop nano-objects that cumulate the spectroscopic properties of multiple Ln ions. This review firstly gives a brief introduction of the spectral properties of lanthanides both in complexes and in nanoparticles (NPs). Then, the research progress of the design of Ln-doped inorganic NPs with capping antennas, Ln-complex encapsulated NPs and Ln-complex surface functionalized NPs is presented along with a summary of the various photosensitizing ligands and of the spectroscopic properties (excited-state lifetime, brightness, quantum yield). The review also emphasizes the problems and limitations encountered over the years and the solutions provided to address them. Finally, a comparison of the advantages and drawbacks of the three types of NP is provided as well as a conclusion about the remaining challenges both in the design of brighter NPs and in the luminescence based applications.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Recent Progress in Smart Polymeric Gel-Based Information Storage for Anti-Counterfeiting

Information security protection has a tremendous impact on human life, social stability and national security, leading to the rapid development of anti-counterfeiting materials and related techniques. However, the traditional stored information on hard or dry media is often static and lacks functions, which makes it challenging to deal with increasing and powerful counterfeiting technologies. Modified intelligent polymeric gels exhibit color changes and shape morphing under external stimuli, which give them great potential for applications in information storage. This paper provides an overview of the latest progress in polymeric gel-based information storage materials in relation to counterfeiting. Following a brief introduction of anti-counterfeiting materials, the preparation methods for intelligent gels with adjustable colors (e.g., chemical colors and physical colors) and various encryption/decryption modes involving dimensions and diverse colors are outlined. Finally, the challenges and prospects for information storage and anti-counterfeiting based on smart gels are discussed.

Special Issue: Rare earth luminescent materials

This special issue covers a series of cutting-edge works on exploring novel rare earth luminescent materials and their applications in lighting, display, information storage, sensing, and bioimaging as well as therapy. [Image: see text]

Highly Retentive, Anti-Interference, and Covert Individual Marking Taggant with Exceptional Skin Penetration

The development of high-performance individual marking taggants is of great significance. However, the interaction between taggant and skin is not fully understood, and a standard for marking taggants has yet to be realized. To achieve a highly retentive, anti-interference, and covert individual marking fluorescent taggant, Mn²⁺ -doped NaYF₄ :Yb/Er upconversion nanoparticles (UCNPs), are surface-functionalized with polyethyleneimine (PEI) to remarkably enhance the interaction between the amino groups and skin, and thus to facilitate the surface adhesion and chemical penetration of the taggant. Electrostatic interaction between PEI₆₀₀ -UCNPs and skin as well as remarkable penetration inside the epidermis is responsible for excellent taggant retention capability, even while faced with robust washing, vigorous wiping, and rubbing for more than 100 cycles. Good anti-interference capability and reliable marking performance in real cases are ensured by an intrinsic upconversion characteristic with a distinct red luminescent emission under 980 nm excitation. The present methodology is expected to shed light on the design of high-performance individual marking taggants from the perspective of the underlying interaction between taggant and skin, and to help advance the use of fluorescent taggants for practical application, such as special character tracking.

Heterogeneous Oxysulfide@Fluoride Core/Shell Nanocrystals for Upconversion-Based Nanothermometry

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) often suffer from weak luminescence, especially when their sizes are ultrasmall (less than 10 nm). Enhancing the upconversion luminescence (UCL) efficiency of ultrasmall UCNPs has remained a challenge that must be undertaken if any practical applications are to be envisaged. Herein, we present a Ln³⁺-doped oxysulfide@fluoride core/shell heterostructure which shows efficient UCL properties under 980 nm excitation and good stability in solution. Through epitaxial heterogeneous growth, a ∼4 nm optically inert β-NaYF₄ shell was coated onto ∼5 nm ultrasmall Gd₂O₂S:20%Yb,1%Tm. These Gd₂O₂S:20%Yb,1%Tm@NaYF₄ core/shell UCNPs exhibit a more than 800-fold increase in UCL intensity compared to the unprotected core, a 180-fold increase in luminescence decay time of the ³H₄ → ³H₆ Tm³⁺ transition from 5 to 900 μs, and an upconversion quantum yield (UCQY) of 0.76% at an excitation power density of 155 W/cm². Likewise, Gd₂O₂S:20%Yb,2%Er@NaYF₄ core/shell UCNPs show a nearly 5000-fold increase of their UCL intensity compared to the Gd₂O₂S:20%Yb,2%Er core and a maximum UCQY of 0.61%. In the Yb/Er core-shell UCNP system, the observed variation of luminescence intensity ratio seems to originate from a change in lattice strain as the temperature is elevated. For nanothermometry applications, the thermal sensitivities based on thermally coupled levels are estimated for both Yb/Tm and Yb/Er doped Gd₂O₂S@NaYF₄ core/shell UCNPs.

Upconversion Nanostructures Applied in Theranostic Systems

Upconversion (UC) nanostructures, which can upconvert near-infrared (NIR) light with low energy to visible or UV light with higher energy, are investigated for theranostic applications. The surface of lanthanide (Ln)-doped UC nanostructures can be modified with different functional groups and bioconjugated with biomolecules for therapeutic systems. On the other hand, organic molecular-based UC nanostructures, by using the triplet-triplet annihilation (TTA) UC mechanism, have high UC quantum yields and do not require high excitation power. In this review, the major UC mechanisms in different nanostructures have been introduced, including the Ln-doped UC mechanism and the TTA UC mechanism. The design and fabrication of Ln-doped UC nanostructures and TTA UC-based UC nanostructures for theranostic applications have been reviewed and discussed. In addition, the current progress in the application of UC nanostructures for diagnosis and therapy has been summarized, including tumor-targeted bioimaging and chemotherapy, image-guided diagnosis and phototherapy, NIR-triggered controlled drug releasing and bioimaging. We also provide insight into the development of emerging UC nanostructures in the field of theranostics.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

Thermal enhancing effect of upconversion luminescence in Er3+/Yb3+ co-doped Cs3BiSr(P2O7)2 phosphors

A series of Er³⁺/Yb³⁺ co-doped Cs₃BiSr(P₂O₇)₂ (CBSPO) phosphors with significant thermal enhancement were successfully prepared using the solid-state method and the thermal-enhancing effect was studied in detail. When the temperature increased from 303 to 723 K, the upconversion luminescence (UCL) emission intensities of ²H_11/2 → ⁴I_15/2 and ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 of Er³⁺ in CBSPO:0.1Er³⁺/0.2Yb³⁺ were enhanced 22.81 and 10.06 times under 980 nm laser excitation, respectively. Meanwhile, the UCL color of the sample obviously changed from orange to green with the increase in temperature. Furthermore, the UCL thermal enhancement mechanism was demonstrated, which originates from phonon-assisted transitions. In addition, the maximum absolute temperature sensitivity for CBSPO:0.1Er³⁺/0.2Yb³⁺ was calculated to be 0.01026 K^-1 at 563 K via luminescence intensity ratio (LIR) technology. All the results show that the CBSPO:Er³⁺/Yb³⁺ phosphors can be used for safety warning and temperature measurement at high temperatures.

Hollow nanoparticles synthesized via Ostwald ripening and their upconversion luminescence-mediated Boltzmann thermometry over a wide temperature range

Upconversion nanoparticles (UCNPs) with hollow structures exhibit many fascinating optical properties due to their special morphology. However, there are few reports on the exploration of hollow UCNPs and their optical applications, mainly because of the difficulty in constructing hollow structures by conventional methods. Here, we report a one-step template-free method to synthesize NaBiF₄:Yb,Er (NBFYE) hollow UCNPs via Ostwald ripening under solvothermal conditions. Moreover, we also elucidate the possible formation mechanism of hollow nanoparticles (HNPs) by studying the growth process of nanoparticles in detail. By changing the contents of polyacrylic acid and H₂O in the reaction system, the central cavity size of NBFYE nanoparticles can be adjusted. Benefiting from the structural characteristics of large internal surface area and high surface permeability, NBFYE HNPs exhibit excellent luminescence properties under 980 nm near-infrared irradiation. Importantly, NBFYE hollow UCNPs can act as self-referenced ratiometric luminescent thermometers under 980 nm laser irradiation, which are effective over a wide temperature range from 223 K to 548 K and have a maximum sensitivity value of 0.0065 K^-1 at 514 K. Our work clearly demonstrates a novel method for synthesizing HNPs and develops their applications, which provides a new idea for constructing hollow structure UCNPs and will also encourage researchers to further explore the optical applications of hollow UCNPs.

Deterministic Relation between Optical Polarization and Lattice Symmetry Revealed in Ion-Doped Single Microcrystals

Rare-earth ion doped crystals are of great significance for microsensing and quantum information, while the ions in the crystals emit light with spontaneous partial polarization, which is, though believed to be originated from the crystal lattice structure, still lacking a deterministic explanation that can be tested with quantitative accuracy. We report experimental evidence showing the profound physical relation between the polarization degree of light emitted by the doped ion and the lattice symmetry by demonstrating, with high precision, that the lattice constant ratio c/a directly quantifies the macroscopic effective polar angle of the electric and magnetic dipoles, which essentially determines the linear polarization degree of the emission. Based on this result, we further propose a pure optical technology to identify the three-dimensional orientation of a rod-shaped single microcrystal using the polarization-resolved microspectroscopy. Our results, demonstrating the physical origin of light polarization in ion-doped crystals, allow work toward on-demand polarization control with crystallography and provide a versatile platform for polarization-based microscale sensing in dynamical systems.

Recent Progress in Lanthanide-Doped Inorganic Perovskite Nanocrystals and Nanoheterostructures: A Future Vision of Bioimaging

All-inorganic lead halide perovskite nanocrystals have great potential in optoelectronics and photovoltaics. However, their biological applications have not been explored much owing to their poor stability and shallow penetration depth of ultraviolet (UV) excitation light into tissues. Interestingly, the combination of all-inorganic halide perovskite nanocrystals (IHP NCs) with nanoparticles consisting of lanthanide-doped matrix (Ln NPs, such as NaYF4:Yb,Er NPs) is stable, near-infrared (NIR) excitable and emission tuneable (up-shifting emission), all of them desirable properties for biological applications. In addition, luminescence in inorganic perovskite nanomaterials has recently been sensitized via lanthanide doping. In this review, we discuss the progress of various Ln-doped all-inorganic halide perovskites (LnIHP). The unique properties of nanoheterostructures based on the interaction between IHP NCs and Ln NPs as well as those of LnIHP NCs are also detailed. Moreover, a systematic discussion of basic principles and mechanisms as well as of the recent advancements in bio-imaging based on these materials are presented. Finally, the challenges and future perspectives of bio-imaging based on NIR-triggered sensitized luminescence of IHP NCs are discussed.

Achieving Multicolor Upconversion Emissions without Changing Compositions

It is widely recognized that a proper way of adjusting fluorescence color is meaningful for pushing forward upconversion materials to be utilized in anti-counterfeiting, display and solid-state lightning applications. Traditional routes that apply different host materials and/or doping categories to adjust fluorescence color have shown large color region tunability yet have to rely on complex synthesis process accompanied with time and raw material consumption. In this work, in order to get a wide luminous color gamut without depending on reciprocating synthesis, we desinged and provided a high-sensitizer-concentration upconversion crystals, hexagonal NaLuF₄:Yb³⁺/Er³⁺ (50/2 mol%), whose red-to-green emission intensity ratio can be conveniently tuned from 2.69 to 4.96 by simply modulating excitation power densities. The promoted three-photon-population progress of red emission achieved by using an intensive excitation laser is considered to be responsible for the facile upconversion modulation. The results may provide new ideas for emission color control that based on external parameters in identical host and the greatly amplified excitation power-sensitivity of NaLuF₄:Yb³⁺/Er³⁺ (50/2 mol%) is highly potential for fluorescence anti-fake and colorful display applications.

Tether-free photothermal deep-brain stimulation in freely behaving mice via wide-field illumination in the near-infrared-II window

Neural circuitry is typically modulated via invasive brain implants and tethered optical fibres in restrained animals. Here we show that wide-field illumination in the second near-infrared spectral window (NIR-II) enables implant-and-tether-free deep-brain stimulation in freely behaving mice with stereotactically injected macromolecular photothermal transducers activating neurons ectopically expressing the temperature-sensitive transient receptor potential cation channel subfamily V member 1 (TRPV1). The macromolecular transducers, ~40 nm in size and consisting of a semiconducting polymer core and an amphiphilic polymer shell, have a photothermal conversion efficiency of 71% at 1,064 nm, the wavelength at which light attenuation by brain tissue is minimized (within the 400-1,800 nm spectral window). TRPV1-expressing neurons in the hippocampus, motor cortex and ventral tegmental area of mice can be activated with minimal thermal damage on wide-field NIR-II illumination from a light source placed at distances higher than 50 cm above the animal's head and at an incident power density of 10 mW mm-2. Deep-brain stimulation via wide-field NIR-II illumination may open up opportunities for social behavioural studies in small animals.

Tunable and Enhanced NIR-II Luminescence from Heavily Doped Rare-Earth Nanoparticles for In Vivo Bioimaging

The last decade has witnessed the booming development of optical imaging in the second near-infrared (NIR-II, 1000-1700 nm) window for disease screening and image-guided surgical interventions, due to the merits of multi-color observations and high spatio-temporal resolution in deep tissue. Therefore, bright and multispectral NIR-II probes are required and play a key role. Here, we report the synthesis of a set of bright rare-earth based NIR-II downshifting nanoparticles (DSNPs) with hexagonal phase (β phase). As compared with the widely reported DSNPs (β-NaYF4@NaYF₄:20Yb/(0.5-2)A@NaYF₄; A = Ho, Pr, Tm or Er) previously, we reveal that the concentrations of both sensitizers and activators can be further highly doped, not limited by the concentration quenching effect. Our results demonstrate that the optimized formula in the heavily doped DSNPs (β-NaYF₄@NaYbF₄:A@NaYF₄, A = 20Ho, 3Pr, 4Tm or 10Er) leads to 1.2- to 4.2-folds NIR-II luminescence enhancement. Especially for the heavily Er-doped DSNPs with long-wavelength photons extending to the NIR-IIb window (1500-1700 nm), we can further boost their luminescence through introducing a beneficial cross-relaxation and host matrix with higher phonon energy (cubic phase NaYF₄@NaYbF₄:10Er/5Ce@NaYF₄), leading to a total of ∼11.4-fold enhancement. The resulting biocompatible, bright NIR-II emitting DSNPs enable us to in vivo monitor the cerebral vessels through the intact scalp and skull, as well as two-color dynamic tumor imaging with high spatial resolution. This work suggests the potential of the heavily doped DSNPs for multiplexed imaging in cerebrovascular abnormalities toward the diagnosis and therapy of the cerebral diseases.

Modification-Free Fluorescent Biosensor for CEA Based on Polydopamine-Coated Upconversion Nanoparticles

Upconversion nanoparticles (UCNPs) have achieved considerable success in protein sensing in vitro. And aptamer is one of the most frequently used biomolecules to modify the nanoparticles for protein assay. However, the complicated process of modifying UCNPs with DNA and the susceptibility of the phosphate groups of DNA backbone to adsorb on the surface of UCNPs have limited their practical applications. To overcome these limitations, a modification-free fluorescent biosensor based on polydopamine-coated upconversion nanoparticles (UCNPs@PDA) is proposed. It consists of UCNPs@PDA and CEA aptamer-functionalized AuNPs (AuNPs-CEA aptamer). The CEA aptamer on AuNPs can be adsorbed onto the surface of UCNPs@PDA due to the interactions of π-π stacking and hydrogen bonding, triggering the process of fluorescence resonance energy transfer (FRET) from UCNPs@PDA to AuNPs-CEA aptamer. In the presence of CEA, the AuNPs-CEA aptamer departs from UCNPs@PDA due to the stronger affinity of CEA with its aptamer. Therefore, the recovery of upconversion fluorescence can sensitively quantify the concentration of CEA. This biosensor provides a linear range from 0.1 to 100 ng/mL for CEA with a LOD of 0.031 ng/mL in an aqueous solution. In spiked human serum samples, the same linear range is acquired with a slightly higher LOD of 0.055 ng/mL, demonstrating the great potential of the biosensor in practical application.

A Self-Made Optical Tweezers Integrated Upconversion Luminescence Confocal Scanning Instrument Enables Stable and Noninvasive Long-Term In Situ Imaging a Single Suspension Cell Under Exceptionally Efficient Luminescent Resonance Energy Transfer Sensing

It is necessary to explore labeling probes with worthy optical properties and a noninvasive fluorescence imaging manner for stable long-term in situ measuring a single suspension cell. In response to these goals, we herein make a breakthrough on two fronts. On one hand, a co-sensitizer-induced efficient 808 nm near-infrared light-excited luminescence-confined upconversion nanoparticle with a low thermal effect is fabricated by employing a layer-by-layer seed growing approach to develop a sandwich structure, under which the luminescence domain is vastly restricted into an extremely thin inner shell (∼ 2.77 nm) to finally bring about a high-efficiency luminescent resonance energy transfer (LRET) sensing behavior. On the other hand, a self-made optical tweezers integrated upconversion luminescence confocal scanning instrument is applied to enhance the imaging accuracy, after which the liquid viscous force is sufficiently overcome by the resulting single beam gradient force and the analyzed suspension cell is always immobilized to the focal plane to ensure a constant luminescence excitation condition. By making use of a metal ion-dependent DNAzyme and a hairpin DNA strand to design a corresponding LRET sensing system, our nanoprobe shows satisfactory assay performance for two model biomolecules (Ca²⁺ and TK1 messenger RNA). Following the optical trapping-assisted imaging, this exceptional measurement method is capable of effectively monitoring the intracellular target changes in different physiological states, endowing a powerful toolbox for single cell analysis.

Rare earth nanoparticles for sprayed and intravenous NIR II imaging and photodynamic therapy of tongue cancer

In this research, rare earth nanoparticles coupled with dihydroartemisinin (DHA) and a targeted antibody (RENP-DHA-Cap) for sprayed NIR II imaging and photodynamic therapy (PDT) of tongue cancer were designed. Genetic algorithms combined with combinatorial chemistry were proposed and successfully achieved in a single optimized luminescent phosphor with enhanced NIR II and high upconversion luminescence (UCL) under a NIR laser of wavelength 980 nm or/and 808 nm. In particular, T1 magnetic resonance imaging (MRI) signals can be adjusted with the Gd ion concentration. In combination with the targeted antibody of capmatinib (Cap), precise NIR II imaging for in situ tongue cancer by a simple spray method can be achieved. Most importantly, NIR II imaging and PDT treatment can be realized with RENP-DHA-capmatinib injected intravenously. This orthogonal theranostic mode with precise diagnosis under 808 nm and targeted effective treatment under 980 nm may promote tongue cancer theranostics.

Excited-State Dynamics of Crossing-Controlled Energy Transfer in Europium Complexes

Photosensitized energy transfer (EnT) phenomena occur frequently in a variety of photophysical and photochemical processes and have traditionally been treated with the donor-acceptor distance-dependent Förster and Dexter models. However, incorrect arguments and formulae were employed by ignoring energy resonance conditions and the selection rules of the state-to-state transition in special cases, especially for the sensitive intramolecular EnT of lanthanide complexes. Herein, we proposed an innovative model of energy-degeneracy-crossing-controlled EnT, which can be experimentally confirmed by time-resolved two-dimensional photoluminescence measurements. The computationally determined energy resonance region provides the most effective channel to achieve metal-to-ligand EnT beyond the distance-dependent model and sensitively bifurcates into symmetry-allowed or -forbidden channels for some representative europium antenna complexes. The outcomes of the multidisciplinary treatment contribute to a complementary EnT model that can be tuned by introducing a phosphorescence modulator and altering the antenna-related parameters of the ligand-centered energy level of the ³ππ* state and its spin-orbit coupling for the ³ππ* → S₀ ^* transition through mechanism-guided crystal engineering and should motivate further development of mechanistic models and applications.

Amino acid derivative-based Ln-metallohydrogels with multi-stimuli responsiveness and applications

Metallohydrogels and lanthanide (Ln) fluorescent materials have gained much attention recently. In this study, we designed and synthesized a facile gelator of a phenylalanine-based derivative containing an indazole group (namely IZF). It was found that IZF can self-assemble to form hydrogel at pH ≤ 7. Meanwhile, IZF and Tb³⁺/Eu³⁺ can co-assemble to generate IZF-Tb and IZF-Eu metallohydrogels with green and red fluorescence, respectively, at pH 8-11, with excellent multi-stimuli responsiveness. The bimetallic hydrogels of IZF-Tb/Eu exhibit different colors under UV light by adjusting the ratio of Tb³⁺ and Eu³⁺. Moreover, white light emission was achieved with IZF-Tb/Eu bimetallic gels through doping carbon dots (CDs) by tailoring the stoichiometric ratio of Ln-complex and CDs. Remarkably, IZF-Tb and IZF-Eu could be used as fluorescent inks with excellent stability. This study indicates that the amino acid derivative-based Ln-metallohydrogels are excellent candidates for constructing information storage and multiple anti-counterfeiting materials.

Eu3+ Doped LaF3 -based Inorganic-organic Hybrid Nanostructured Materials for Broad-spectrum Excitation and Strong Photoluminescence

Inorganic-organic hybrid nanoparticles formed by lanthanide-doped nanostructures and organic ligands have been intensively studied, which could greatly increase their photoluminescence performance as a result of the energy transfer process from organic ligands to Ln³⁺ ions. However, the photoluminescence intensity and excitation spectral width are still quite limited on coordinating with a single type of organic ligand. In this work, Eu³⁺ -doped LaF₃ (LaF₃ :Eu³⁺ ) nanoparticles were prepared using hydrothermal method, which was then hybridized with benzoic acid and thenoyltrifluoroacetone to form the hybrid nanostructures. After that, the hybrid nanostructures were mixed with 2,2'-azobisisobutyronitrile and methyl methacrylate to prepare the composites. The sample obtained by hybridization and composite doping with 5% Eu³⁺ exhibited the best photoluminescence performance. The excitation peak width and luminescence intensity of the hybrid nanostructures were significantly increased. The excitation spectral width of the inorganic-organic mixed hybrid nanostructures was particularly enhanced, which covers the whole ultraviolet (UV) band region of solar light on earth. The prepared composites exhibited good optical properties.

Tailoring of upconversion luminescence of Al3+engineered titanate phosphor for non-invasive thermometry

The ability of rare-earth-doped ferroelectric oxides to achieve outstanding upconversion (UC) performances under NIR irradiation despite possessing intrinsic electric properties drives researchers all over the globe to work in this field. The structural and spectroscopic characteristics of the Bi4Ti3O12 phosphor integrated with Er3+, Yb3+, and Al3+ have been thoroughly investigated in this study. The considerable increase in UC emission ~three times caused by the addition of Al3+ ions has been observed and discussed. The processes connected with the UC emission related to the pump power variation have been realized using the rate law equation. Aside from having high sensitivity of 0.011 K-1 at room temperature, the prepared phosphor possesses excellent thermal stability, i.e., it retains ~ 73% of its initial intensity with the addition of Al3+ ions.

Simultaneous ultraviolet-C and near-infrared enhancement in heterogeneous lanthanide nanocrystals

Lanthanide-doped nanocrystals that simultaneously convert near-infrared (NIR) irradiation into emission of shorter (ultraviolet-C, UVC) and longer wavelengths (NIR) offer many exciting opportunities for application in drug release, photodynamic therapy, deep-tissue bioimaging, and solid-state lasing. However, a formidable challenge is the development of lanthanide-doped nanocrystals with efficient UVC and NIR emissions simultaneously due to their low conversion efficiency. Here, we report a dye-sensitized heterogeneous core-multishell architecture with enhanced UVC emission and NIR emission under 793 nm excitation. This nanocrystal design efficiently suppresses energy trapping induced by interior lattice defects and promotes upconverted UVC emission from Gd³⁺. Moreover, a significant downshifting emission from Yb³⁺ at 980 nm was also observed owing to an efficient energy transfer from Nd³⁺ to Yb³⁺. Furthermore, by taking advantage of ICG sensitization, we realized a largely enhanced emission from the UVC to NIR spectral region. This study provides a mechanistic understanding of the upconversion and downshifting processes within a heterogeneous architecture while offering exciting opportunities for important biological and energy applications.

Optical Properties of Mn-Doped CuGa(In)S-ZnS Nanocrystals (NCs): Effects of Host NC and Mn Concentration

Time-gated fluorescence measurement (TGFM) using long-life fluorescence probes is a highly sensitive fluorescence-measurement technology due to the inherently high signal-to-background ratio. Although many probes for TGFM such as luminescent-metal-complex probes and lanthanide-doped nanoparticles are in development, they generally need sophisticated/expensive instruments for biosensing/imaging applications. Probes possessing high brightness, low-energy (visible light) excitation, and long lifetimes up to milliseconds of luminescence, are highly desired in order to simplify the optical and electronic design of time-gated instruments (e.g., adopting non-UV-grade optics or low-speed electronics), lower the instrument complexity and cost, and facilitate broader applications of TGFM. In this work, we developed Mn-doped CuGa(In)S-ZnS nanocrystals (NCs) using simple and standard synthetic steps to achieve all the desired optical features in order to investigate how the optical properties (fluorescence/absorption spectra, brightness, and lifetimes) of the Mn-doped NCs are affected by different host NCs and Mn concentrations in host NCs. With optimal synthetic conditions, a library of Mn-doped NCs was achieved that possessed high brightness (up to 47% quantum yield), low-energy excitation (by 405 nm visible light), and long lifetimes (up to 3.67 ms). Additionally, the time-domain fluorescence characteristics of optimal Mn-doped NCs were measured under pulsed 405 nm laser excitation and bandpass-filter-based emission collection. The measurement results indicate the feasibility of these optimal Mn-doped NCs in TGFM-based biosensing/imaging.

Study of synthesis temperature effect on β-NaGdF4: Yb3+, Er3+upconversion luminescence efficiency and decay time using maximum entropy method

Upconversion materials have several advantages for many applications due to their great potential in converting infrared light to visible. For practical use, it is necessary to achieve high intensity of UC luminescence, so the studies of the optimal synthesis parameters for upconversion nanoparticles are still going on. In the present work, we analyzed the synthesis temperature effect on the efficiency and luminescence decay of β-NaGd0.78Yb0.20Er0.02F4 (15-25 nm) upconversion nanoparticles with hexagonal crystal structure synthesized by anhydrous solvothermal technique. The synthesis temperature was varied in the 290-320°C range. The synthesis temperature was shown to have a significant influence on the upconversion luminescence efficiency and decay time. The coherent scattering domain linearly depended on the synthesis temperature and was in the range 13.1-22.3 nm, while the efficiency of the upconversion luminescence increases exponentially from 0.02 to 0.10% under 1 W/cm2 excitation. For a fundamental analysis of the reasons for the upconversion luminescence intensity dependence on the synthesis temperature, it was proposed to use the maximum entropy method for luminescence decay kinetics processing. This method does not require a preliminary setting of the number of exponents and, due to this, makes it possible to estimate additional components in the luminescence decay kinetics, which are attributed to different populations of rare-earth ions in different conditions. Two components in the green luminescence and one component in the red luminescence decay kinetics were revealed for nanoparticles prepared at 290-300°C. An intense short and a weak long component in green luminescence decay kinetics could be associated with two different populations of ions in the surface quenching layer and the crystal core volume. With an increase in the synthesis temperature, the second component disappears, and the decay time increases due to an increase in the number of ions in the crystal core volume and a more uniform distribution of dopants.

Tuning Luminescence of Lanthanide-Doped Upconversion Nanoparticles through Simultaneous Binary Cation Exchange

Dual-mode luminescent nanomaterials have outstanding performance in biosensing and multistage anticounterfeiting. Herein, we report the tuning of optical attributes of lanthanide-doped nanoparticles (NPs) via simultaneous binary cation exchange. We show that cation exchange of NaYF₄:Yb/Er (18/2 mol %)@NaLnF₄ (Ln = Y and Gd) NPs with a combination of Ce³⁺ and Tb³⁺ enables the resultant nanoparticles to exhibit both upconversion and downshifting emissions upon excitation at 980 and 254 nm, respectively. We find that in addition to introducing downshifting emission attributes, the use of Tb³⁺ ions allows conservation of the integrity of the parent core@shell NPs by decreasing the dissociation tendency caused by Ce³⁺ ions during the cation exchange. The upconversion color output can be tuned from green to red and blue by changing lanthanide combinations in the core NPs. This work not only provides an effective strategy for the optical tuning of lanthanide-doped NPs but also builds a platform for probing the difference in the reactivity nature of lanthanides.

Self-Assembly of Upconversion Nanoparticles Based Materials and Their Emerging Applications

In the past few decades, significant progress of the conventional upconversion nanoparticles (UCNPs) based nanoplatform has been achieved in many fields, and with the development of nanoscience and nanotechnology, more and more complex situations need a UCNPs based nanoplatform having multifunctions for specific multimodal or multiplexed applications. Through self-assembly, different UCNPs or UCNPs with other materials could be combined together within an entity. It is more like an ideal UCNPs nanoplatform, a unique system with the properties defined by its individual components as well as by the morphology of the composite. Various designs can show their different desired properties depending on the application situation. This review provides a complete summary on the optimization of the synthesis method for the recently designed UCNPs assemblies and summarizes various applications, including dual-modality cell imaging, molecular delivery, detection, and programmed control therapy. The challenges and limitations the UCNPs assembly faces and the potential solutions in this field are also presented.

Ultrasensitive Pressure-Induced Optical Materials: Europium-Doped Hafnium Silicates with a Khibinskite Structure for Optical Pressure Sensors and WLEDs

Ultrasensitive pressure-induced optical materials are of great importance owing to their potential applications in optical pressure sensors. However, the lack of outstanding pressure sensitivity, observable color evolution, and structure reliability limits their further development in both practical applications and luminescence theory. To overcome the above problems, an enlightening methodology is proposed to explore the high sensitivity and phase stability of hafnium silicate K₂HfSi₂O₇ (KHSO) phosphor with a Khibinskite structure. By employing X-ray diffraction (XRD) Rietveld refinement, cryogenic spectroscopy, and ancillary calculations, information on Eu²⁺ ion occupation is completely obtained at atmospheric pressure. The remarkable pressure sensitivity (dλ/dP = 3.25 nm/GPa^-1) and excellent phase stability up to 20 GPa, along with the reproducible color hue variation, exhibit unprecedented superiority when used in optical pressure sensors. These advantages can be assigned to the pressure-induced Eu²⁺-selective occupation and the unique properties of 5d-4f transition (Stokes shift, nephelauxetic effect, and intense crystal field strength), which are clearly proved by measuring the XRD patterns, Raman spectra, and Gaussian fitting spectra under compression and decompression processes. The excellent luminescence property manifests that KHSO/Eu²⁺ can be considered as a potential luminescent material for solid-state lighting and optical pressure sensors.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

Shedding light on neurons: optical approaches for neuromodulation

Today's optical neuromodulation techniques are rapidly evolving, benefiting from advances in photonics, genetics and materials science. In this review, we provide an up-to-date overview of the latest optical approaches for neuromodulation. We begin with the physical principles and constraints underlying the interaction between light and neural tissue. We then present advances in optical neurotechnologies in seven modules: conventional optical fibers, multifunctional fibers, optical waveguides, light-emitting diodes, upconversion nanoparticles, optical neuromodulation based on the secondary effects of light, and unconventional light sources facilitated by ultrasound and magnetic fields. We conclude our review with an outlook on new methods and mechanisms that afford optical neuromodulation with minimal invasiveness and footprint.

Influence of the synthesis route on the spectroscopic, cytotoxic, and temperature-sensing properties of oleate-capped and ligand-free core/shell nanoparticles

The right choice of synthesis route for upconverting nanoparticles (UCNPs) is crucial for obtaining a well-defined product with a specific application capability. Thus we decided to compare the physicochemical, cytotoxic, and temperature-sensing properties of UCNPs obtained from different rare earth (RE) ions, which has been made for the first time in a single study. The core/shell NaYF₄:Yb³⁺,Er³⁺/NaYF₄ UCNPs were obtained by reaction in a mixture of oleic acid and octadecene, and their highly stable water colloids were prepared using the ligand-free modification method. Both oleate-capped and ligand-free UCNPs exhibited a bright upconversion emission upon 975 nm excitation. Moreover, slope values, emission quantum yields, and luminescence lifetimes confirmed an effective energy transfer between the Yb³⁺ and Er³⁺ ions. Additionally, the water colloids of the UCNPs showed temperature-sensing properties with a good thermal sensitivity level, higher than 1 % K^-1 at 358 K. Evaluation of the cytotoxicity profiles of the obtained products indicated that cell viability was decreased in a dose-dependent manner in the analyzed concentration range.

A review on colorimetric assays for DNA virus detection

Early detection is one of the ways to deal with DNA virus widespread prevalence, and it is necessary to know new diagnostic methods and techniques. Colorimetric assays are one of the most advantageous methods in detecting viruses. These methods are based on color change, which can be seen either with the naked eye or with special devices. The aim of this study is to introduce and evaluate effective colorimetric methods based on amplification, nanoparticle, CRISPR/Cas, and Lateral flow in the diagnosis of DNA viruses and to discuss the effectiveness of each of the updated methods. Compared to the other methods, colorimetric assays are preferred for faster detection, high efficiency, cheaper cost, and high sensitivity and specificity. It is expected that the spread of these viruses can be prevented by identifying and developing new methods.

Glucose Sensing in Human Whole Blood Based on Near-Infrared Phosphors and Outlier Treatment with the Programming Language "R"

A near-infrared paper-based analytical device (NIR-PAD) for glucose detection in whole blood was based on iridium(III) metal complexes embedded in a three-dimensional (3D) enzyme gel. These complexes emit NIR luminescence, can avoid interference from the color of blood, and increase the sensitivity of sensing glucose. The glucose reaction behaviors of another two different iridium(III) and platinum(II) complexes were also tested. When the glucose solution was added to the device, the oxidation of glucose by glucose oxidase caused oxygen consumption and increased the intensity of the phosphorescence emission. To the best of our knowledge, this is the first time that data have been treated with the programming language "R", which uses Tukey's test to identify the outliers in the data and calculate a median for establishing a calibration curve, in order to improve the accuracy of NIR-PADs for sensing glucose. Compared with other published devices, NIR-PADs exhibit a wider linear range (1-30 mM, [relative emission intensity] = 0.0250[glucose] + 0.0451, and R 2 = 0.9984), a low detection limit (0.7 mM), a short response time (<2 s), and a small sample volume (2 μL). Finally, blood specimens were obtained from 19 patients enrolled in Taipei Veterans General Hospital under an approved IRB protocol (Taiwan; 2017-12-002CC). The sensors exhibited remarkable characteristics for glucose detection in comparison with other methods, including the clinical method in hospitals as well as those without blood sample pretreatment or a dilution factor. The above results confirm that NIR-PAD sensors can be put to practical use for glucose detection.

Turn-on fluorescence ferrous ions detection based on MnO2 nanosheets modified upconverion nanoparticles

A turn on upconversion fluorescence probe based on the combination of ~32 nm NaYF₄: Yb/Tm nanoparticles and MnO₂ nanosheets has been established for rapid, sensitive detection of Fe²⁺ ions levels in aqueous solutions and serum. X-ray diffraction (XRD), transmission electron microscopy (TEM), absorption and emission spectra have been used to characterize the crystal structure, morphology and optical properties of the samples. MnO₂ nanosheets on the surface of UCNPs act as a fluorescence quencher, resulting in the quenching of the blue fluorescence (with excitation/emission maximum of 980/476 nm) via fluorescence resonance energy transfer from upconversion nanoparticles to MnO₂ nanosheets. With the adding of Fe²⁺, upconversion fluorescence of the nanocomposites recovers due to the reduction of MnO₂ to Mn²⁺. Because of the low background of the probe offered by upconversion fluorescence, this probe can be used for detecting Fe²⁺ in aqueous solutions in the range of 0.1-22 μM with detection limit of 0.113 μM. The developed method has also been applied to detect 10 μM Fe²⁺ ions in serum with recoveries ranging from 97.6 to 105.3% for the five serum samples. Significantly, the probe shows fast response and stable signal, which is beneficial for long-time dynamic sensing. Thus, the proposed strategy holds great potential for disease diagnosis and treatment.

Mn-Doped AgZnInS/ZnS Nanocrystals (NCs): Effects of Zn Etching on the NC Optical Properties

Mn-doped I(II)-III-VI NCs (e.g., Mn-doped AgZnInS/ZnS NCs) possessing low-energy excitation, high brightness and long fluorescence lifetimes have been desired for time-gated fluorescence biosensing/imaging. In this type of NCs, their optical properties are significantly affected by the microscopic interactions between Mn and Mn and between Mn and host NC, the compositions of NCs, and the defects in NCs. On the other hand, it is known that Zn etching to core I(II)-III-VI NCs in NC synthesis can significantly enhance the NC brightness because Zn can exchange surface atoms (e.g., Ag and In) in NCs to minimize NC surface-defects. But for Mn-doped I(II)-III-VI NCs, Zn etching could etch out not only surface-atoms of host NCs (e.g., Ag and In) but also Mn in NCs. As a result, it could significantly affect the NC compositions and the microscopic interactions between Mn and Mn as well as between Mn and host NC, and thus the optical properties of NCs (like lifetime and absorption/emission spectra). Therefore, it is needed to investigate how Zn etching would affect the optical properties of such Mn-doped NCs. In this study, a series of Mn-doped AgZnInS NCs with different Mn doping levels were prepared through nucleation doping, and then Zn etching was applied to etch these core NCs. To identify the effects of Zn etching on NC optical properties, ZnS coating (a different ZnS shelling approach by injecting Zn precursor and S precursor alternately in synthesis) was performed on the same Mn:AgZnInS NCs, and the optical properties of NCs with these two different ZnS shelling approaches were compared. Experimental results showed that under appropriate Mn doping levels in synthesis, Zn etching instead of ZnS coating can produce low-energy excitable NCs with higher QYs and longer lifetimes, which would further facilitate the use of such NCs in time-gated fluorescence measurement. To understand the reasons for the different optical properties under different ZnS shelling approaches, the material characteristics of the prepared NCs were further measured/analyzed and the possible fluorescence mechanisms were discussed.

A quantitative single-nanowire study on the plasmonic enhancement for the upconversion photoluminescence of rare-earth-doped nanoparticles

The enhancement effects of the upconversion photoluminescence of rare-earth-doped nanoparticles by the localized surface plasmon resonance of a single Ag nanowire are quantified on a single-nanowire scale assisted by selective etching treatment.

Aptasensors for the detection of infectious pathogens: design strategies and point-of-care testing

The epidemic of infectious diseases caused by contagious pathogens is a life-threatening hazard to the entire human population worldwide. A timely and accurate diagnosis is the critical link in the fight against infectious diseases. Aptamer-based biosensors, the so-called aptasensors, employ nucleic acid aptamers as bio-receptors for the recognition of target pathogens of interest. This review focuses on the design strategies as well as state-of-the-art technologies of aptasensor-based diagnostics for infectious pathogens (mainly bacteria and viruses), covering the utilization of three major signal transducers, the employment of aptamers as recognition moieties, the construction of versatile biosensing platforms (mostly micro and nanomaterial-based), innovated reporting mechanisms, and signal enhancement approaches. Advanced point-of-care testing (POCT) for infectious disease diagnostics are also discussed highlighting some representative ready-to-use devices to address the urgent needs of currently prevalent coronavirus disease 2019 (COVID-19). Pressing issues in aptamer-based technology and some future perspectives of aptasensors are provided for the implementation of aptasensor-based diagnostics into practical application.

Thermal Enhancement of Upconversion in Lanthanide-doped Gd2Ti2O7 Crystals via Fast Evaporation in sol-gel Procedure

Thermal quenching effect caused by the increased multi-phonon assisted non-radiative relaxation possibility greatly restricts the application of luminescent materials. Herein, a modified sol-gel method where the gels are achieved by...

Responsive Sensors of Upconversion Nanoparticles

Upconversion nanoparticles are a class of luminescent materials that convert longer-wavelength near-infrared photons into visible and ultraviolet emissions. They can respond to various external stimuli, which underpins many opportunities for developing the next generation of sensing technologies. In this perspective, the unique stimuli-responsive properties of upconverting nanoparticles are introduced, and their recent implementations in sensing are summarized. Promising material development strategies for enhancing the key sensing merits, including intrinsic sensitivity, biocompatibility and modality, are identified and discussed. The outlooks on future technological developments, novel sensing concepts, and applications of nanoscale upconversion sensors are provided.

Boosting and Activating NIR-IIb Luminescence of Ag2Te Quantum Dots with a Molecular Trigger

Near-infrared (NIR) excited and NIR-IIb emissive Ag₂Te quantum dots (QDs) display significant advantages in luminescence bioimaging and biosensing due to their unique photophysical properties. However, the poor luminescence intensity and limited strategy for constructing activatable probes severely restrict the wide bioapplications of Ag₂Te QDs. Herein, we proposed a NIR dye-sensitization strategy to solve these two problems. First, we used IR-780 as the antenna for Ag₂Te QDs to improve the ability of harvesting excitation light, obtaining 21-fold luminescence enhancement at 1620 nm under an 808 nm laser irradiation. Subsequently, by further functionalizing the heptamethine cyanine with a recognition unit of glutathione (GSH), Cy-GSH with target-triggered emission was yielded, which served as the potential sensitizer for Ag₂Te QDs to fabricate an activatable ratiometric NIR-IIb nanoprobe for visualizing GSH in vivo with high contrast. This new strategy is expected as a powerful tool to promote the bioapplication of NIR-IIb QDs in the future.