Dye-sensitized upconversion nanocomposites for ratiometric semi-quantitative detection of hypochlorite in vivo

Luminescent photon-upconversion nanoparticles with advanced functionalization for smart sensing and imaging

Photon-upconversion nanoparticles (UCNP) have already been established as labels for affinity assays in analog and digital formats. Here, advanced, or smart, systems based on UCNPs coated with active shells, fluorescent dyes, and metal and semiconductor nanoparticles participating in energy transfer reactions are reviewed. In addition, switching elements can be embedded in such assemblies and provide temporal and spatial control of action, which is important for intracellular imaging and monitoring activities. Demonstration and critical comments on representative approaches demonstrating the progress in the use of such UCNPs in bioanalytical assays, imaging, and monitoring of target molecules in cells are reported, including particular examples in the field of cancer theranostics.

Recent progress of UCNPs-MoS2 nanocomposites as a platform for biological applications

Composite materials can take advantages of the functional benefits of multiple pure nanomaterials to a greater degree than single nanomaterials alone. The UCNPs-MoS2 composite is a nano-application platform that combines upconversion luminescence and photothermal properties. Upconversion nanoparticles (UCNPs) are inorganic nanomaterials with long-wavelength excitation and short-wavelength tunable emission capabilities, and are able to effectively convert near-infrared (NIR) light into visible light for increased photostability. However, UCNPs have a low capacity for absorbing visible light, whereas MoS2 shows better absorption in the ultraviolet and visible regions. By integrating the benefits of UCNPs and MoS2, UCNPs-MoS2 nanocomposites can convert NIR light with a higher depth of detection into visible light for application with MoS2 through fluorescence resonance energy transfer (FRET), which compensates for the issues of MoS2's low tissue penetration light-absorbing wavelengths and expands its potential biological applications. Therefore, starting from the construction of UCNPs-MoS2 nanoplatforms, herein, we review the research progress in biological applications, including biosensing, phototherapy, bioimaging, and targeted drug delivery. Additionally, the current challenges and future development trends of UCNPs-MoS2 nanocomposites for biological applications are also discussed.

An NIR-IIb emissive transmembrane voltage nano-indicator for the optical monitoring of electrophysiological activities in vivo

In vivo transmembrane-voltage detection reflected the electrophysiological activities of the biological system, which is crucial for the diagnosis of neuronal disease. Traditional implanted electrodes can only monitor limited regions and induce relatively large tissue damage. Despite emerging monitoring methods based on optical imaging have access to signal recording in a larger area, the recording wavelength of less than 1000 nm seriously weakens the detection depth and resolution in vivo. Herein, a Förster resonance energy transfer (FRET)-based nano-indicator, NaYbF4:Er@NaYF4@Cy7.5@DPPC (Cy7.5-ErNP) with emission in the near-infrared IIb biological window (NIR-IIb, 1500-1700 nm) is developed for transmembrane-voltage detection. Cy7.5 dye is found to be voltage-sensitive and is employed as the energy donor for the energy transfer to the lanthanide nanoparticle, NaYbF4:Er@NaYF4 (ErNP), which works as the acceptor to achieve electrophysiological signal responsive NIR-IIb luminescence. Benefiting from the high penetration and low scattering of NIR-IIb luminescence, the Cy7.5-ErNP enables both the visualization of action potential in vitro and monitoring of Mesial Temporal lobe epilepsy (mTLE) disease in vivo. This work presents a concept for leveraging the lanthanide luminescent nanoprobes to visualize electrophysiological activity in vivo, which facilitates the development of an optical nano-indicator for the diagnosis of neurological disorders.

Boosting Dye-Sensitized Luminescence by Enhanced Short-Range Triplet Energy Transfer

Dye-sensitization can enhance lanthanide-based upconversion luminescence, but is hindered by interfacial energy transfer from organic dye to lanthanide ion Yb3+ . To overcome these limitations, modifying coordination sites on dye conjugated structures and minimizing the distance between fluorescence cores and Yb3+ in upconversion nanoparticles (UCNPs) are proposed. The specially designed near-infrared (NIR) dye, disulfo-indocyanine green (disulfo-ICG), acts as the antenna molecule and exhibits a 2413-fold increase in luminescence under 808 nm excitation compared to UCNPs alone using 980 nm irradiation. The significant improvement is attributed to the high energy transfer efficiency of 72.1% from disulfo-ICG to Yb3+ in UCNPs, with majority of energy originating from triplet state (T1 ) of disulfo-ICG. Shortening the distance between the dye and lanthanide ions increases the probability of energy transfer and strengthens the heavy atom effect, leading to enhanced T1 generation and improved dye-triplet sensitization upconversion. Importantly, this approach also applies to 730 nm excitation Cy7-SO3 sensitization system, overcoming the spectral mismatch between Cy7 and Yb3+ and achieving a 52-fold enhancement in luminescence. Furthermore, the enhancement of upconversion at single particle level through dye-sensitization is demonstrated. This strategy expands the range of NIR dyes for sensitization and opens new avenues for highly efficient dye-sensitized upconversion systems.

A dye-quenched/sensitized switching upconversion nanoprobe for high-contrast mapping of the pH-related tumor microenvironment

Nanoprobes based on lanthanide-doped upconversion nanoparticles (UCNPs) exhibit promising potential in bioimaging and biosensing due to their unique optical properties. However, conventional UCNP nanoprobes based on the dye quenching effect are still limited in biosensing due to their low upconversion efficiency. The advent of dye-sensitized upconversion has resulted in nanoprobes with significantly enhanced efficiency; however, these still suffer from a high initial emissive background. In view of this, herein, we have constructed a dye-quenched/sensitized switching upconversion nanoprobe for high-contrast imaging of the pH-related tumor microenvironment. Under normal conditions, the luminescence of the nanoprobe at 540 nm (UCL540) was significantly quenched by the employed dye. Upon being triggered by an acid, the dye would switch to its fluorescent form to sensitize the luminescence of UCNPs, affording a significant enhancement of UCL540. The switching from dye-quenched UCL to dye-sensitized UCL jointly enables the detection of a high signal-to-background ratio (high up to 50), allowing for high-contrast mapping of the tumor specific acidic microenvironment in vivo. We believe that this nanoplatform holds considerable promise for acid-related sensing.

Dye-Triplet-Sensitized Downshifting Nanoprobes with Ratiometric Dual-NIR-IIb Emission for Accurate In Vivo Detection

Despite the emerging near-infrared-IIb (NIR-IIb, 1500-1700 nm) bioimaging significantly improving the in vivo penetration depth and resolution, quantitative detection with accuracy remains challenging due to its inhomogeneous fluorescence signal attenuation in biological tissue. Here, ratiometric dual-NIR-IIb in vivo detection with excitation wavelengths of 808 and 980 nm is presented using analyte-responsive dye-triplet-sensitized downshifting nanoprobes (DSNPs). NIR cyanine dye IR-808, a recognizer of biomarker hypochlorite (ClO-), is introduced to trigger a triplet energy transfer process from the dye to Er3+ ions of DSNPs under 808 nm excitation, facilitating the formation of an analyte-responsive 1525 nm NIR-IIb assay channel. Meanwhile, DSNPs also enable emitting intrinsic nonanalyte-dependent downshifting fluorescence at the same NIR-IIb window under 980 nm excitation, serving as a self-calibrated signal to alleviate the interference from the probe amount and depth. Due to the two detected emissions sharing identical light propagation and scattering, the ratiometric NIR-IIb signal is demonstrated to ignore the depth of penetration in biotissue. The arthritis lesions are distinguished from normal tissue using ratiometric probes, and the amount of ClO- can be accurately output by the established detection curves.

Activatable fluorescent probes for imaging and diagnosis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease that is primarily manifested as synovitis and polyarticular opacity and typically leads to serious joint damage and irreversible disability, thus adversely affecting locomotion ability and life quality. Consequently, good prognosis heavily relies on the early diagnosis and effective therapeutic monitoring of RA. Activatable fluorescent probes play vital roles in the detection and imaging of biomarkers for disease diagnosis and in vivo imaging. Herein, we review the fluorescent probes developed for the detection and imaging of RA biomarkers, namely reactive oxygen/nitrogen species (hypochlorous acid, peroxynitrite, hydroxyl radical, nitroxyl), pH, and cysteine, and address the related challenges and prospects to inspire the design of novel fluorescent probes and the improvement of their performance in RA studies.

Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

In terms of the combined advantages of upconversion luminescence (UCL) properties and dual-signal ratiometric outputs toward specific targets, the ratiometric UCL nanoprobes exhibit significant applications. This review summarizes and discusses the recent advances in ratiometric UCL nanoprobes, mainly including the construction of nanoprobe systems for sensing, imaging, and phototherapeutics. First, the construction strategies are introduced, involving different types of nanoprobe systems, construction methods, and ratiometric dual-signal modes. Then, the sensing applications are summarized, involving types of targets, sensing mechanisms, sensing targets, and naked-eye visual detection of UCL colors. Afterward, the phototherapeutic applications are discussed, including bio-toxicity, bio-distribution, biosensing, and bioimaging at the level of living cells and small animals, and biomedicine therapy. Particularly, each section is commented on by discussing the state-of-the-art relevant studies on ratiometric UCL nanoprobe systems. Moreover, the current status, challenges, and perspectives in the forthcoming studies are discussed. This review facilitates the exploration of functionally luminescent nanoprobes for excellent sensing, imaging, biomedicine, and multiple applications in significant fields.

A Comprehensive Review on Upconversion Nanomaterials-Based Fluorescent Sensor for Environment, Biology, Food and Medicine Applications

Near-infrared-excited upconversion nanoparticles (UCNPs) have multicolor emissions, a low auto-fluorescence background, a high chemical stability, and a long fluorescence lifetime. The fluorescent probes based on UCNPs have achieved great success in the analysis of different samples. Here, we presented the research results of UCNPs probes utilized in analytical applications including environment, biology, food and medicine in the last five years; we also introduced the design and construction of upconversion optical sensing platforms. Future trends and challenges of the UCNPs used in the analytical field have also been discussed with particular emphasis.

Applications of upconversion nanoparticles in analytical and biomedical sciences: a review

Lanthanide-doped upconversion nanoparticles (UCNPs) have gained more attention from researchers due to their unique properties of photon conversion from an excitation/incident wavelength to a more suitable emission wavelength at a designated site, thus improving the scope in the life sciences field. Due to their fascinating and unique optical properties, UCNPs offer attractive opportunities in theranostics for early diagnostics and treatment of deadly diseases such as cancer. Also, several efforts have been made on emerging approaches for the fabrication and surface functionalization of luminescent UCNPs in optical biosensing applications using various infrared excitation wavelengths. In this review, we discussed the recent advancements of UCNP-based analytical chemistry approaches for sensing and theranostics using a 980 nm laser as the excitation source. The key analytical merits of UNCP-integrated fluorescence analytical approaches for assaying a wide variety of target analytes are discussed. We have described the mechanisms of the upconversion (UC) process, and the application of surface-modified UCNPs for in vitro/in vivo bioimaging, photodynamic therapy (PDT), and photothermal therapy (PTT). Based on the latest scientific achievements, the advantages and disadvantages of UCNPs in biomedical and optical applications are also discussed to overcome the shortcomings and to improve the future study directions. This review delivers beneficial practical information of UCNPs in the past few years, and insights into their research in various fields are also discussed precisely.

Upconversion nanomaterials and delivery systems for smart photonic medicines and healthcare devices

In the past decade, upconversion (UC) nanomaterials have been extensively investigated for the applications to photomedicines with their unique features including biocompatibility, near-infrared (NIR) to visible conversion, photostability, controllable emission bands, and facile multi-functionality. These characteristics of UC nanomaterials enable versatile light delivery for deep tissue biophotonic applications. Among various stimuli-responsive delivery systems, the light-responsive delivery process has been greatly advantageous to develop spatiotemporally controllable on-demand "smart" photonic medicines. UC nanomaterials are classified largely to two groups depending on the photon UC pathway and compositions: inorganic lanthanide-doped UC nanoparticles and organic triplet-triplet annihilation UC (TTA-UC) nanomaterials. Here, we review the current-state-of-art inorganic and organic UC nanomaterials for photo-medicinal applications including photothermal therapy (PTT), photodynamic therapy (PDT), photo-triggered chemo and gene therapy, multimodal immunotherapy, NIR mediated neuromodulations, and photochemical tissue bonding (PTB). We also discuss the future research direction of this field and the challenges for further clinical development.

Micellar Ratiometric Fluorescent Blood pH Probe Based on Triplet-Sensitized Upconversion and Energy-Transfer Behaviors

The measurement of pH is greatly significant in monitoring physiological and biochemical states. In this work, a novel micellar ratiometric fluorescent probe featuring sophisticated energy-transfer (ET) behaviors with p-nitrophenol (PNP) as the energy acceptor and a triplet-triplet annihilation upconversion (TTA-UC) system as the energy donor was designed. The pH-induced molecular configuration of PNP determined the process for the transfer of energy from TTA-UC to PNP. The introduction of the TTA-UC system enabled probe excitation under a long wavelength and afforded a ratiometric signal for pH detection with excellent reliability over diverse interfering factors. This TTA-UC/ET pH probe demonstrated a high sensitivity to hydronium below nanomolar concentrations and an excellent anti-interference ability in serum samples, which provided a novel significant strategy for rapid and accurate detection of blood pH in vitro.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

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

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

Lanthanide-doped upconversion nanoparticles have attracted extensive research interest due to their promising applications in various fields.

Advanced NIR ratiometric probes for intravital biomedical imaging

Near-infrared (NIR) fluorescence imaging technology (NIR-I region, 650-950 nm and NIR-II region, 1000-1700 nm), with deeper tissue penetration and less disturbance from auto-fluorescence than that in visible region (400-650 nm), is playing a more and more extensive role in the field of biomedical imaging. With the development of precise medicine, intelligent NIR fluorescent probes have been meticulously designed to provide more sensitive, specific and accurate feedback on detection. Especially, recently developed ratiometric fluorescent probes have been devoted to quantify physiological and pathological parameters with a combination of responsive fluorescence changes and self-calibration. Herein, we systemically introduced the construction strategies of NIR ratiometric fluorescent probes and their applications in biological imagingin vivo, such as molecular detection, pH and temperature measurement, drug delivery monitoring and treatment evaluation. We further summarized possible optimization on the design of ratiometric probes for quantitative analysis with NIR fluorescence, and prospected the broader optical applications of ratiometric probes in life science and clinical translation.

Enhancing Dye-Triplet-Sensitized Upconversion Emission Through the Heavy-Atom Effect in CsLu2 F7 :Yb/Er Nanoprobes

Lanthanide (Ln³⁺ )-doped upconversion (UC) nanoprobes, which have drawn extensive attention for various bioapplications, usually suffer from small absorption cross-sections and weak luminescence intensity of Ln³⁺ ions. Herein, we report the controlled synthesis of a new class of Ln³⁺ -doped UC nanoprobes based on CsLu₂ F₇ :Yb/Er nanocrystals (NCs), which can effectively increase the intersystem crossing (ISC) efficiency from singlet excited state to triplet excited state of IR808 up to 99.3 % through the heavy atom effect. By virtue of the efficient triplet sensitization of IR808, the optimal UC luminescence (UCL) intensity of IR808-modified CsLu₂ F₇ :Yb/Er NCs is enhanced by 1309 times upon excitation at 808 nm. Benefiting from the intense dye-triplet-sensitized UCL, the nanoprobes are demonstrated for sensitive assay of extracellular and intracellular hypochlorite with an 808-nm/980-nm dual excited ratiometric strategy.

Sensors/nanosensors based on upconversion materials for the determination of pharmaceuticals and biomolecules: An overview

Upconversion materials have been the focus of a large body of research in analytical and clinical fields in the last two decades owing to their ability to convert light between various spectral regions and their particular photophysical features. They emit efficient and sharp ultraviolet (UV) or visible luminescence after excitation with near-infrared (NIR) light. These features overcome some of the disadvantages reported for conventional fluorescent materials and provide opportunities for high sensitivity chemo-and bio-sensing. Here, we review studies that used upconversion materials as sensors for the determination of pharmaceuticals and biomolecules in the last two decades. The articles included in this review were retrieved from the SCOPUS database using the search phrases: "upconversion nanoparticles for determination of pharmaceutical compounds", and "upconversion nanoparticles for determination of biomolecules". Details of each developed upconversion nanoparticles based sensor along with their relevant analytical parameters are reported and carefully explained.

Lanthanide-Based Nanocomposites for Photothermal Therapy under Near-Infrared Laser: Relationship between Light and Heat, Biostability, and Reaction Temperature

In this research, typical organic/inorganic photothermal therapy (PTT) agents were designed with a combination of upconversion luminescent (UCL) or near-infrared (NIR) II imaging rare-earth nanomaterials for photo-acoustic (PA)/UCL/NIR II imaging-guided PTT under NIR laser irradiation. The results show the following: (1) The PTT effect mainly comes from NIR absorption and partly from UCL light conversion. (2) Visible UCL emission is mainly quenched by NIR absorption of the coated PTT agent and partly quenched by visible absorption, indicating that excitation may play a more important role than in the UCL emission process. (3) The biostability of the composite might be decided by the synthesis reaction temperature. Among the five inorganic/organic nanocomposites, UCNP@MnO₂ is the most suitable candidate for cancer diagnosis and treatment because of its stimuli-response ability to the micro-acid environment of tumor cells and highest biostability. The composites generate heat for PTT after entering the tumor cells, and then, the visible light emission gradually regains as MnO₂ is reduced to colorless Mn²⁺ ions, thereby illuminating the cancer cells after the therapy.

Strategies for Constructing Upconversion Luminescence Nanoprobes to Improve Signal Contrast

Lanthanide-doped upconversion nanoparticles (UCNPs) can convert two or more lower-energy near-infrared photons to a single photon with higher energy, which makes them particularly suitable for constructing nanoprobes with large imaging depth and minimal interference of autofluorescence and light scattering from biosamples. Furthermore, they feature excellent photostability, sharp and narrow emissions, and large anti-Stokes shift, which confer them the capability of long-period bioimaging and real-time tracking. In recent years, UCNPs-based nanoprobes (UC-nanoprobes) have been attracting increasing interest in biological and medical research. Signal contrast, the ratio of signal intensity after and before the reaction of the probe and target, is the determinant factor of the sensitivity of all reaction-based probes. This progress report presents the methods of constructing UC-nanoprobes, with a focus fixed on recent strategies to improve the signal contrast, which have kept on promoting the bioapplication of this type of probe.

A Strategy of NIR Dual-Excitation Upconversion for Ratiometric Intracellular Detection

Intracellular detection is highly desirable for biological research and clinical diagnosis, yet its quantitative analysis with noninvasivity, sensitivity, and accuracy remains challenging. Herein, a near-infrared (NIR) dual-excitation strategy is reported for ratiometric intracellular detection through the design of dye-sensitized upconversion probes and employment of a purpose-built NIR dual-laser confocal microscope. NIR dye IR808, a recognizer of intracellular analyte hypochlorite, is introduced as energy donor and Yb,Er-doped NaGdF4 upconversion nanoparticles are adopted as energy acceptor in the as-designed nanoprobes. The efficient analyte-dependent energy transfer and low background luminescence endow the nanoprobes with ultrahigh sensitivity. In addition, with the nonanalyte-dependent upconversion luminescence (UCL) excited by 980 nm as a self-calibrated signal, the interference from environmental fluctuation can be alleviated. Furthermore, the dual 808/980 nm excited ratiometric UCL is demonstrated for the quantification of the level of intracellular hypochlorite. Particularly, the intrinsic hypochlorite with only nanomolar concentration in live MCF-7 cells in the absence of exogenous stimuli is determined. Such an NIR dual-excitation ratiometric strategy based on dye-sensitized UCL probes can be easily extended to detect various intracellular analytes through tailoring the reactive NIR dyes, which provides a promising tool for probing biochemical processes in live cells and diagnosing diseases.

Comparative investigation of the optical spectroscopic and thermal effect in Nd3+-doped nanoparticles

Nd3+-doped nanoparticles involving 808 nm excitation hold great promise in various biomedical applications, such as bioimaging, biodetection, theranostics and optogenetics. Here we present the synthesis and characterization of core-multishell Nd3+-doped nanoparticles displaying excellent optical properties. We systematically studied the influence of doping concentration, nanostructure design, excitation wavelength and size effect on the upconversion luminescence of Nd3+-doped nanoparticles. Remarkably, the emission intensity of optimized nanoparticles with 808 nm excitation is three times higher than the emission intensity of those with 980 nm excitation. Surprisingly, the optical profiles of Nd3+-doped nanoparticles strongly depend on the excitation wavelengths. The dominant effect responsible for the emission intensity difference and the energy transfer mechanism upon different excitation wavelengths are investigated. Interestingly, the heavily Nd3+-doped nanoparticles not only display efficient upconversion luminescence, but also are able to convert the excitation source to heat under a single 808 nm excitation source. Importantly, these efforts will lead to Nd3+-doped nanoparticles with unprecedented optical and thermal properties that will have broad utility in fundamental research and technological applications.