Biothiols-activated near-infrared frequency up-conversion luminescence probe for early evaluation of drug-induced hepatotoxicity

A novel biothiols-sensitive near-infrared (NIR) fluorescent probe RhDN based on a rhodamine skeleton was developed for early detection of drug-induced hepatotoxicity in living mice. RhDN can be used not only as a conventional large stokes shift fluorescent (FL) probe, but also as a kind of anti-Stokes frequency upconversion luminescence (FUCL) molecular probe, which represents a long wavelength excitation (808 nm) to short wavelength emission (760 nm), and response to Cys/Hcy/GSH with high sensitivity. Compared with traditional FL methods, the FUCL method exhibited a lower detection limit of Cys, Hcy, and GSH in 75.1 nM, 101.8 nM, and 84.9 nM, respectively. We exemplify RhDN for tracking endogenously biothiols distribution in living cells and further realize real-time in vivo bioimaging of biothiols activity in mice with dual-mode luminescence system. Moreover, RhDN has been successfully applied to visualize the detection of drug-induced hepatotoxicity in living mice. Overall, this report presents a unique approach to the development of large stokes shift NIR FUCL molecular probes for in vitro and in vivo biothiols biosensing.

Mitochondria-specific near-infrared photoactivation of peroxynitrite upconversion luminescent nanogenerator for precision cancer gas therapy

Gas therapy is emerging as a highly promising therapeutic strategy for cancer treatment. However, there are limitations, including the lack of targeted subcellular organelle accuracy and spatiotemporal release precision, associated with gas therapy. In this study, we developed a series of photoactivatable nitric oxide (NO) donors NRh-R-NO (R = Me, Et, Bn, iPr, and Ph) based on an N-nitrosated upconversion luminescent rhodamine scaffold. Under the irradiation of 808 nm light, only NRh-Ph-NO could effectively release NO and NRh-Ph with a significant turn-on frequency upconversion luminescence (FUCL) signal at 740 nm, ascribed to lower N-N bond dissociation energy. We also investigated the involved multistage near-infrared-controlled cascade release of gas therapy, including the NO released from NRh-Ph-NO along with one NRh-Ph molecule generation, the superoxide anion O2⋅- produced by the photodynamic therapy (PDT) effect of NRh-Ph, and highly toxic peroxynitrite anion (ONOO‒) generated from the co-existence of NO and O2⋅-. After mild nano-modification, the nanogenerator (NRh-Ph-NO NPs) empowered with superior biocompatibility could target mitochondria. Under an 808 nm laser irradiation, NRh-Ph-NO NPs could induce NO/ROS to generate RNS, causing a decrease in the mitochondrial membrane potential and initiating apoptosis by caspase-3 activation, which further induced tumor immunogenic cell death (ICD). In vivo therapeutic results of NRh-Ph-NO NPs showed augmented RNS-potentiated gas therapy, demonstrating excellent biocompatibility and effective tumor inhibition guided by real-time FUCL imaging. Collectively, this versatile strategy defines the targeted RNS-mediated cancer therapy.

Anti-Stokes Luminescence in Multi-Resonance-Type Thermally-Activated Delayed Fluorescence Molecules

Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti-Stokes photoluminescence (ASPL) derived from hot-band absorption in a series of multi-resonance-type thermally-activated delayed fluorescence (MR-TADF) molecules. The MR-TADF molecules exhibited an anti-Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti-Stokes shift corresponded well to the 1-0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR-TADF molecules have become a novel class of ASPL materials for various future applications, such as light-driving cooling systems.

Photoactivatable Carbo- and Silicon-Rhodamines and Their Application in MINFLUX Nanoscopy

New photoactivatable fluorescent dyes (rhodamine, carbo- and silicon-rhodamines [SiR]) with emission ranging from green to far red have been prepared, and their photophysical properties studied. The photocleavable 2-nitrobenzyloxycarbonyl unit with an alpha-carboxyl group as a branching point and additional functionality was attached to a polycyclic and lipophilic fluorescent dye. The photoactivatable probes having the HaloTagTM amine (O2) ligand bound with a dye core were obtained and applied for live-cell staining in stable cell lines incorporating Vimentin (VIM) or Nuclear Pore Complex Protein NUP96 fused with the HaloTag. The probes were applied in 2D (VIM, NUP96) and 3D (VIM) MINFLUX nanoscopy, as well as in superresolution fluorescence microscopy with single fluorophore activation (VIM, live-cell labeling). Images of VIM and NUPs labeled with different dyes were acquired and their apparent dimensions and shapes assessed on a lower single-digit nanometer scale. Applicability and performance of the photoactivatable dye derivatives were evaluated in terms of photoactivation rate, labeling and detection efficiency, number of detected photons per molecule and other parameters related to MINFLUX nanoscopy.

Albumin-based near-infrared phototheranostics for frequency upconversion luminescence/photoacoustic dual-modal imaging-guided photothermal therapy

Engineering versatile phototheranostics for multimodal diagnostic imaging and effective therapy has great potential in cancer treatment. However, developing an inherently versatile molecule is a huge challenge. In this work, a near-infrared organic dye (NRh) was synthesized and further bound with bovine serum albumin (BSA) to construct facile "one-for-all" phototheranostics (NRh-BSA NPs), which exhibited enhanced frequency upconversion luminescence (FUCL, λ_ex/em = 850/825 nm) and excellent photoacoustic (PA) and photothermal properties (λ^'_ex = 808 nm). Additionally, the BSA-modified phototheranostics NRh-BSA NPs showed specific accumulation in the tumor region through passive targeting. Based on the FUCL/PA dual modal imaging-guidance, the NRh-BSA NPs not only can guarantee the accuracy of imaging of the U87MG tumor sites, but also can improve the therapeutic effect on ablating tumors without recurrence by photothermal therapy (PTT). Collectively, our work proposed a novel strategy to construct versatile phototheranostics with the unique FUCL/PA imaging-guided technique for accurate cancer theranostics.

Colorimetric and near-infrared spectrophotometric monitoring of bisulfite using glyoxal modified chromenylium-cyanine chemosensor: Smartphone and paper strip applications for on-site food and beverages control

Detection of bisulfite (HSO₃^-) in food and beverages has vital importance because the excessive amount leads to ill effects on the human body. Colorimetric and fluorometric chromenylium-cyanine-based chemosensor CyR was synthesized and applied for high selective and sensitive analysis of HSO₃^- in red wine, rose wine and, granulated sugar with high recovery ranges and very fast response time without any interference from other competitive species. The limits of detection (LOD) for the UV-Vis and fluorescence titrations were found as 11.5 μM and 3.77 μM, respectively. The on-site and very rapid methods based on paper strips and smartphone which depend on the color changes from yellow to green have been successfully developed to analyze HSO₃^- concentration (10^-5-10^-1 M for paper strip and 163-1205 μM for smartphone). CyR and the bisulfite-adduct formed in the nucleophilic addition reaction with HSO₃^- were verified by FT-IR, ¹H NMR, and MALDI-TOF results as well as Single-Crystal X-Ray Crystallography for CyR.

Self-Assembled BODIPY Nanoparticles for Near-Infrared Fluorescence Bioimaging

In vivo optical imaging is an important application value in disease diagnosis. However, near-infrared nanoprobes with excellent luminescent properties are still scarce. Herein, two boron–dipyrromethene (BODIPY) molecules (BDP-A and BDP-B) were designed and synthesized. The BODIPY emission was tuned to the near-infrared (NIR) region by regulating the electron-donating ability of the substituents on its core structure. In addition, the introduction of polyethylene glycol (PEG) chains on BODIPY enabled the formation of self-assembled nanoparticles (NPs) to form optical nanoprobes. The self-assembled BODIPY NPs present several advantages, including NIR emission, large Stokes shifts, and high fluorescence quantum efficiency, which can increase water dispersibility and signal-to-noise ratio to decrease the interference by the biological background fluorescence. The in vitro studies revealed that these NPs can enter tumor cells and illuminate the cytoplasm through fluorescence imaging. Then, BDP-B NPs were selected for use in vivo imaging due to their unique NIR emission. BDP-B was enriched in the tumor and effectively illuminated it via an enhanced penetrability and retention effect (EPR) after being injected into the tail vein of mice. The organic nanoparticles were metabolized through the liver and kidney. Thus, the BODIPY-based nanomicelles with NIR fluorescence emission provide an effective research basis for the development of optical nanoprobes in vivo.

Anti-stokes luminescent organic nanoparticles for frequency upconversion biomedical imaging

Frequency upconversion optical imaging has attracted great attention due to its remarkable advantages over traditional down-conversion optical imaging. However, the development of frequency upconversion optical imaging is extremely limited. Herein, five derivatives with BODIPY structure (B1-B5) were developed to investigate its frequency upconversion luminescence (FUCL) performance by introducing electron-donating and electron-withdrawing groups. Except for the nitro group decorated derivative, the other derivatives have strong and stable FUCL around 520 nm under 635 nm light excitation. More importantly, B5 retains FUCL ability after self-assembly. When applied to FUCL imaging of cells, B5 nanoparticles can be enriched in the cytoplasm and show a good signal-to-noise ratio. Meanwhile, FUCL tumor imaging can be achieved after 1 h of injection. This study not only provides a potential agent for FUCL biomedical imaging but also develops a new strategy for designing FUCL agents that exhibit excellent performance.

Hot-band absorption assisted single-photon frequency upconversion luminescent nanophotosensitizer for 808 nm light triggered photodynamic immunotherapy of cancer

Frequency upconversion luminescence (FUCL) based on hot-band absorption has attracted considerable attention in bioimaging and phototherapy fields for deep-seated cancer treatment. Photoimmunotherapy, a promising therapeutic approach induced by photodynamic therapy (PDT), can selectively kill cancer cells, reverse the immunosuppressive system, boost host immune response, and elicit durable antitumor immunity. To date, few near-infrared organic photosensitizers for photodynamic immunotherapy have been reported based on hot-band absorption. Herein, we report an upconversion luminescent phthalocyanine photosensitizer PdPc(OBu)₈ with anti-Stokes emission at 748 nm and highly efficient singlet oxygen generation with hot-band absorption at 808 nm. Taking advantage of nanoliposomes, FUCL phthalocyanine nano-photosensitizers (PdPc NPs) were obtained to reduce the aggregation-caused quenching and improve water solubility and biocompatibility. PdPc NPs could be effectively accumulated in tumor tissues through intravenous administration, causing FUCL-induced PDT under 808 nm irradiation. Considering its finite immune responses and tumor ablation after PDT, a combination of PdPc NP-based PDT with checkpoint inhibitors (anti-PD-L1) for near-infrared photoimmunotherapy has been used to potentiate the antitumor efficacy that could simultaneously ablate primary tumors and inhibit the progression of distant tumors. This study can promote the development of upconversion-based PDT combined with immunotherapy for tumor precision therapy.

A ratiometric nanoprobe for the in vivo bioimaging of hypochlorous acid to detect drug-damaged liver and kidneys

As the organs responsible for toxin transformation and excretion in the body, damage to the liver and kidneys induced by inevitable drug toxicity is the main cause of acute liver and kidney injury. P-Acetamidophenol overdose leads hypochlorous acid (HClO) to accumulate in the mitochondria of tissues, ultimately resulting in acute liver and kidney injury in humans, despite its clinical use as an antipyretic medicine. Herein, we report an HClO-activatable self-assembling ratiometric nanoprobe NRH-800-PEG for screening the upregulation of HClO by colocalization in mitochondria while monitoring the changes in the endogenous HClO levels in cells with ratiometric signals. Furthermore, NRH-800-PEG was constructed to evaluate injury by fluorescence ratio imaging in the tissues of inflammatory mice. Our strategy offers a novel tool for assessing disease progression during drug-induced liver and kidney injury.

Rational design of pH-activated upconversion luminescent nanoprobes for bioimaging of tumor acidic microenvironment and the enhancement of photothermal therapy

The development of effective and safe tumor photothermal therapeutic strategies has attracted considerable attention. Herein, we synthesized tumor microenvironment (TME)-activatable self-assembling organic nanotheranostics (NRhD-PEG-X NPs (X = 1, 2, 3, and 4)) for precise tumor targeting and upconversion image-guided photothermal therapy (PTT). The amphiphilic polymer NRhD-PEG-X consisted of upconversion luminescent probes (NRhD) modified with polyethylene glycol (PEG) of various lengths. The continuous external irradiation-free photothermal NRhD-PEG-4 NPs with pKa 6.70 displayed high sensitivity and selectivity to protons, resulting in the turn-on upconversion luminescence and enhanced photothermal properties in the acidic TME without asynchronous therapy and side effects. This nanotheranostic offers acidic activatability, tumor targetability, and PTT enhancement, thus allowing autofluorescence-free upconversion luminescent imaging-guided precision PTT. Our strategy affords a paradigm to develop activatable theranostic nanoplatforms for precision medicine. STATEMENT OF SIGNIFICANCE: As a hyperthermia-based treatment, activatable photothermal therapy (PTT) is highly significant in tumor treatment. Herein, we develop acidic tumor microenvironment-activatable nanotheranostics for upconversion luminescent imaging-guided diagnosis and precision tumor-targeted PTT. PEGylation of upconversion dyes not only could self-assemble to yield organic nanoparticles in water, but it could also significantly improve biocompatibility, stability, and circulation time and tune significantly the pKa value of nanoparticles. In an acidic tumor microenvironment, NRhD-PEG-4 NPs with pKa 6.70 show high sensitivity to release NRhDH⁺-PEG-4 NPs, which exhibit good upconversion luminescence and enhanced photothermal effect. Therefore, upconversion luminescence imaging-guided precision PTT has high potential to enhance cancer diagnostic and therapeutic efficiency.

Mitochondria-Targeted Ratiometric Chemdosimeter to Detect Hypochlorite Acid for Monitoring the Drug-Damaged Liver and Kidney

Liver and kidney injury caused by drug toxicity is a serious threat to human health. Acetaminophenol (APAP), as a common antipyretic and analgesic drug, inevitably causes injury. When it is overused, hypochlorous acid (HClO) is excessively generated due to metabolic abnormalities, resulting in the accumulation of HClO in the mitochondria of liver and kidney tissues and causing damage. In this study, we designed a series of HClO responsive ratiometric chemdosimeter NRH-X (NRH-O, NRH-S, and NRH-C) to evaluate liver and kidney injury, and found that NRH-O has a specific sensitive response to HClO. NRH-O can not only monitor the variations of endogenous HClO content of living cells by fluorescence ratio changes in the mitochondria but also detect the upregulation of HClO induced by APAP. In addition, NRH-O can also be used for anatomic diagnosis of liver and kidney injury by fluorescence ratio imaging of HClO in the tissues of inflammatory mice.

Hot-Band-Absorption-Induced Anti-Stokes Fluorescence of Aggregation-Induced Emission Dots and the Influence on the Nonlinear Optical Effect

Hot-band absorption (HBA)-induced anti-Stokes fluorescence (ASF) with longer-wavelength excitation is one effective pathway to deep penetration and low autofluorescence in intravital fluorescence imaging, raising demands for fluorophores with broad spectra, high absorption, and strong emission. However, typical fluorescent dyes display some emission quenching when their concentration is increased in order to obtain brighter fluorescence. In this work, the HBA-induced ASF of aggregation-induced emission (AIE) dots is reported. BPN-BBTD dots were synthesized and confirmed with a fluorescence enhancement and a considerable ASF intensity. In addition, the mechanism of ASF and the HBA process of BPN-BBTD dots were carefully validated and discussed. To obtain the full advantages of the long-wavelength excitation and the short fluorescence lifetime in deep-tissue bioimaging, a large-depth ASF confocal microscopic imaging of in vivo cerebral vasculature was conducted under the excitation of a 980 nm continuous wave laser after intravenous injection of BPN-BBTD dots. Meanwhile, the 3D structure of the cerebrovascular network was successfully reconstructed.

Emerging Design Principle of Near-Infrared Upconversion Sensitizer Based on Mitochondria-Targeted Organic Dye for Enhanced Photodynamic Therapy

Upconversion luminescent (UCL) triggered photodynamic therapy (PDT) affords superior outcome for cancer treatment. However, conventional UCL materials which all work by a multiphoton absorption (MPA) process inevitably need extremely high power density far over the maximum permissible exposure (MPE) to laser. Here, a one-photon absorption molecular upconversion sensitizer Cy5.5-Br based on frequency upconversion luminescent (FUCL) is designed for PDT. The unusual super heavy atom effect (SHAE) in Cy5.5-Br strongly enhances its spin-orbit coupling (0.23 cm^-1 ), triplet quantum yield (11.1 %) and triplet state lifetime (18.8 μs) while the potential hot-band absorption of Cy5.5-Br is well maintained. Importantly, Cy5.5-Br can efficiently target the tumour site and kill cancer cells by destroying mitochondria under a biosafety MPE to 808 nm laser. The photostability and antitumor results are obviously superior to that of a Stokes process. This work provides a design criterion for FUCL dyes to realize effective PDT upon a biosafety optical density, possibly bringing more clinical benefits than conventional MPA materials.

Near-infrared frequency upconversion probe for revealing the relationship between glutathione S-transferase and drug-resistance

Drug resistance poses an enormous challenge for successful chemotherapy. Glutathione S-transferase (GST) has been confirmed to be involved in the progression of drug resistance to some anticancer drugs, thus revealing that the role of GST in anticancer drug resistance is necessary. Herein, by taking advantage of frequency upconversion luminescence (FUCL) technology, we reported an FUCL probe (NRh-NDs) that can detect GST based on a rhodamine derivative structure decorated with a 2,4-dinitrobenzenesulfonyl group (NDs). The NRh-NDs showed excellent sensitivity and high selectivity for GST and released the emissive dye NRh-NH₂, which showed emission and excitation wavelengths in vitro of 820 nm and 850 nm, respectively. The NRh-NDs probe successfully tested endogenic GST in U87, MCF-7 and A549 cells. The cell data showed that the increased levels of GST were positively related to cisplatin resistance but not to 5-fluorouracil resistance. These results suggested that the probe could be used as a visual tool to reveal the cause of drug resistance for cisplatin resistance in cancer treatment. Furthermore, it may serve as an effective tool to confirm the mechanism of antitumor drug resistance.

Hot-band absorption of indocyanine green for advanced anti-stokes fluorescence bioimaging

Bright anti-Stokes fluorescence (ASF) in the first near-infrared spectral region (NIR-I, 800 nm-900 nm) under the excitation of a 915 nm continuous wave (CW) laser, is observed in Indocyanine Green (ICG), a dye approved by the Food and Drug Administration for clinical use. The dependence of fluorescence intensity on excitation light power and temperature, together with fluorescence lifetime measurement, establish this ASF to be originated from absorption from a thermally excited vibrational level (hot-band absorption), as shown in our experiments, which is stronger than the upconversion fluorescence from widely-used rare-earth ion doped nanoparticles. To test the utility of this ASF NIR-I probe for advanced bioimaging, we successively apply it for biothermal sensing, cerebral blood vessel tomography and blood stream velocimetry. Moreover, in combination with L1057 nanoparticles, which absorb the ASF of ICG and emit beyond 1100 nm, these two probes generate multi-mode images in two fluorescent channels under the excitation of a single 915 nm CW laser. One channel is used to monitor two overlapping organs, urinary system & blood vessel of a live mouse, while the other shows urinary system only. Using in intraoperative real-time monitoring, such multi-mode imaging method can be beneficial for visual guiding in anatomy of the urinary system to avoid any accidental injury to the surrounding blood vessels during surgery.

Photofluorochromic water-dispersible nanoparticles for single-photon-absorption upconversion cell imaging

Photofluorochromic diarylethene (DAE) molecules have been widely investigated due to their excellent fatigue resistance and thermal stability. However, the poor water solubility of DAEs limits their biological applications to some extent. Herein, we reported two kinds of water-dispersible DAE nanoparticles (DAEI-NPs and DAEB-NPs), in which DAE molecules were stabilized by the amphiphilic polymer DSPE-mPEG2000 using the nanoprecipitation approach. The fabricated nanoparticles retain well-controlled luminescence and fluorescence photoswitching properties in aqueous solution, which could be reversibly switched on and off under the alternating irradiation of ultraviolet (UV) and visible light. In addition, the closed-ring isomers of DAEB-NPs performed hot-band-absorption-based photon upconversion when excited by a 593.5 nm laser. Bearing excellent photophysical properties and low cytotoxicity, DAEB-NPs were applicable for upconversion cell imaging without high-excitation power density and free from oxygen removal. Additionally, the imaging process could be switched on by regulating the photofluorochromic nanoparticles.

Mitochondria targeted near-infrared chemodosimeter for upconversion luminescence bioimaging of hypoxia

We report a mitochondria-targeted near-infrared probe (NRh-O) for frequency upconversion luminescence (FUCL) imaging of hypoxia. Under hypoxic conditions, NRh-O rapidly responds to release the FUCL product NRh (λ_ex/em = 850/825 nm) with high sensitivity and selectivity in mitochondria. This highlights the potential application of a hypoxia-responsive probe in early clinical diagnosis.

Highly efficient BODIPY-doped upconversion nanoparticles for deep-red luminescence bioimaging in vivo

We demonstrate a 3,5-di(p-oxethyl)styryl conjugated BODIPY showing deep-red upconversion luminescence with a high efficiency of 16.6%. Furthermore, water-soluble BODIPY-doped upconversion nanoparticles with efficiency up to 6.9% under low excitation power density (∼1 mW cm-2) are developed and enable high-performance bioimaging in vivo.

Recent progress in photosensitizers for overcoming the challenges of photodynamic therapy: from molecular design to application

Photodynamic therapy (PDT), a therapeutic mode involving light triggering, has been recognized as an attractive oncotherapy treatment. However, nonnegligible challenges remain for its further clinical use, including finite tumor suppression, poor tumor targeting, and limited therapeutic depth. The photosensitizer (PS), being the most important element of PDT, plays a decisive role in PDT treatment. This review summarizes recent progress made in the development of PSs for overcoming the above challenges. This progress has included PSs developed to display enhanced tolerance of the tumor microenvironment, improved tumor-specific selectivity, and feasibility of use in deep tissue. Based on their molecular photophysical properties and design directions, the PSs are classified by parent structures, which are discussed in detail from the molecular design to application. Finally, a brief summary of current strategies for designing PSs and future perspectives are also presented. We expect the information provided in this review to spur the further design of PSs and the clinical development of PDT-mediated cancer treatments.

Luminescent molecules towards precise cellular event regulation

A unique lanthanide complex which responds to near-infrared (NIR) stimulation was developed for remote regulation of cellular events. This molecule can be localized specifically on the cell surface. Upon NIR stimulation, strong emission of the complex can successfully modulate the activities of light-gated membrane channels and regulate the ion flux in vivo.

Triplet-Triplet Annihilation Upconversion Based Nanosensors for Fluorescence Detection of Potassium

Typical ionophore-based nanosensors use Nile blue derived indicators called chromoionophores, which must contend with strong background absorption, autofluorescence, and scattering in biological samples that limit their usefulness. Here, we demonstrate potassium-selective nanosensors that utilize triplet-triplet annihilation upconversion to minimize potential optical interference in biological media and a pH-sensitive quencher molecule to modulate the upconversion intensity in response to changes in analyte concentration. A triplet-triplet annihilation dye pair (platinum(II) octaethylporphyrin and 9,10-diphenylanthracene) was integrated into nanosensors containing an analyte binding ligand (ionophore), charge-balancing additive, and a pH indicator quencher. The nanosensor response to potassium was shown to be reversible and stable for 3 days. In addition, the nanosensors are selective against sodium, calcium, and magnesium (selectivity coefficients in log₁₀ units of -2.2 for calcium, -2.0 for sodium, and -2.4 for magnesium), three interfering ions found in biological samples. The lack of signal overlap between the upconversion nanosensors and GFP, a common biological fluorescent indicator, is demonstrated in confocal microscope images of sensors embedded in a bacterial biofilm.

Development of a novel anti-tumor theranostic platform: a near-infrared molecular upconversion sensitizer for deep-seated cancer photodynamic therapy

Upconversion-based photon-initiated therapeutic modalities, photodynamic therapy (PDT) in particular, have shown significant clinical potential in deep-seated tumor treatment. However, traditional multiphoton upconversion materials involving lanthanide (ion)-doped upconversion nanoparticles (UCNPs) and two-photon absorption (TPA) dyes often suffer from lots of inherent problems such as unknown systematic toxicity, low reproducibility, and extremely high irradiation intensity for realization of multiphoton upconversion excitation. Herein, for the first time, we report a one-photon excitation molecular photosensitizer (FUCP-1) based on a frequency upconversion luminescence (FUCL) mechanism. Under anti-Stokes (808 nm) excitation, FUCP-1 showed excellent photostability and outstanding upconversion luminescence quantum yield (up to 12.6%) for imaging-guided PDT. In vitro cellular toxicity evaluation presented outstanding inhibition of 4T1 cells by FUCP-1 with 808 nm laser irradiation (the half maximal inhibitory concentration was as low as 2.06 μM). After intravenous injection, FUCP-1 could specifically accumulate at tumor sites and obviously suppress the growth of deep-seated tumors during PDT. More importantly, FUCP-1 could be fully metabolized from the body within 24 h, thus dramatically minimizing systemic toxicity. This study might pave a new way for upconversion-based deep-seated cancer PDT.

Near-Infrared Upconversion Luminescence and Bioimaging In Vivo Based on Quantum Dots

Recently, upconversion luminescence (UCL) has been widely applied in bioimaging due to its low autofluorescence and high contrast. However, a relatively high power density is still needed in conventional UCL bioimaging. In the present study, an ultralow power density light, as low as 0.06 mW cm-2, is applied as an excitation source for UCL bioimaging with PbS/CdS/ZnS quantum dots (UCL-QDs) as probes. The speculated UCL mechanism is a phonon-assisted single-photon process, and the relative quantum yield is up to 4.6%. As determined by continuous irradiation with a 980 nm laser, the UCL-QDs show excellent photostability. Furthermore, UCL-QDs-based probe is applied in tumor, blood vessel, and lymph node bioimaging excited with an eye-safe low-power light-emitting diode light in a nude mouse with few heat effects.

Application of a bodipy-C70 dyad in triplet-triplet annihilation upconversion of perylene as a metal-free photosensitizer

A bodipy-C70 dyad was synthesized and applied in triplet-triplet annihilation (TTA) upconversion of perylene as a novel metal-free organic photosensitizer. The photophysical processes were investigated by the methods of steady-state UV-Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The bodipy-C70 dyad showed an increased molar extinction coefficient up to 82 300 mol-1 cm-1 at 518 nm compared with the C70 monomer. With photo-excitation of the bodipy moiety at 532 nm, the intramolecular singlet-singlet energy transfer between bodipy and C70 units was efficient with a quantum yield of nearly 100%, and the lowest triplet state of the dyad was subsequently populated via ISC of the C70 moiety, with a lifetime of ca. 80 μs in toluene. Electrochemical investigation suggested that the intramolecular electron transfer of the excited dyad was thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. With the presence of perylene in solution as the triplet energy acceptor and emitter, the TTA upconverted fluorescence was observed with a maximum quantum yield of 10.3%. The overall upconversion capability of 4417 M-1 cm-1 exceeded that of C70 approximately two-fold. Moreover, the bodipy-C70 dyad also exhibited an enhanced optical stability under intense irradiation. All data indicated that the dyad was another ideal photosensitizer for TTA upconversion of perylene in the fullerene derivative family.

Ratiometric Photoacoustic Nanoprobe for Bioimaging of Cu2

Aberrant copper contents implicate numerous diseases including Alzheimer's disease and Wilson's disease. Conventional copper detection technologies are difficult to offer non-invasive and accurate deep tissue detection of copper. Here, we report a photoacoustic (PA) nanoprobe (NRh-IR-NMs) for ratiometric PA imaging of Cu²⁺. The nanoprobe consists of a selective Cu²⁺-responsive probe (NRh) as the indicator and a nonresponsive dye (IR) as the internal reference. In the presence of Cu²⁺, a selective Cu²⁺-induced structure change of NRh would take place, resulting in the increase of PA signal intensity increment at 716 nm (ΔPA716). However, the ΔPA834 which attributes to IR shows negligible change. Therefore, the ratiometric PA signal (ΔPA716/ΔPA834) could be used as an indicator for Cu²⁺ detection. This ratiometric PA detection method offers a noninvasive technology with high selectivity and tissue penetration depth, which is a promising tool for deep-tissue detection of Cu²⁺ in living organisms.

Bringing upconversion down to the molecular scale

After bulk solids and nanoparticles, examples of upconversion are now emerging at the discrete molecular scale in solution.

A reversible frequency upconversion probe for real-time intracellular lysosome-pH detection and subcellular imaging

The probe NRH-Lyso shows an FUCL response to acidic pH and is a promising candidate for lysosome imaging in living cells.

Fluorescein-Inspired Near-Infrared Chemodosimeter for Luminescence Bioimaging

Luminescence bioimaging is widely used for noninvasive monitoring of biological targets in real-time with high temporal and spatial resolution. For efficient bioimaging in vivo, it is essential to develop smart organic dye platforms. Fluorescein (FL), a traditional dye, has been widely used in the biological and clinical studies. However, visible excitation and emission limited their further application for in vivo bioimaging. Nearinfrared (NIR) dyes display advantages of bioimaging because of their minimum absorption and photo-damage to biological samples, as well as deep tissue penetration and low auto-luminescence from background in the living system. Thus, some great developments of near-infrared fluorescein-inspired dyes have emerged for bioapplication in vitro and in vivo. In this review, we highlight the advances in the development of the near-infrared chemodosimeters for detection and bioimaging based on the modification of fluoresceininspired dyes naphtho-fluorescein (NPF) and cyanine-fluorescein (Cy-FL).

Luminescent Probes for Sensitive Detection of pH Changes in Live Cells through Two Near-Infrared Luminescence Channels

Two water-soluble near-infrared luminescent probes, which possess both conventional intense Stokes fluorescence and unique single-photon frequency upconversion luminescence (FUCL), were developed for sensitive and selective detection of pH changes in live cells. The water solubility and biocompatibility of these probes were achieved by introducing mannose residues through 2,2'-(ethylenedioxy)diethylamine tethered spacers to a near-infrared conventional fluorescence (CF) and FUCL organic fluorophore. At a pH higher than 7.4, the probes have ring-closed spirocyclic lactam structures, thus are colorless and nonfluorescent. Nevertheless, they sensitively respond to acidic pH values, with a drastic structural change to ring-opened spirocyclic lactam forms, which cause significant absorbance increases at 714 nm. Correspondingly, their near-infrared CF and FUCL intensities at 740 nm are also significantly enhanced when excited by 690 and 808 nm, respectively. The probes hold a variety of advantages such as high sensitivity, excellent reversibility and selectivity to pH over metal ions, low cellular autofluorescence background interference, good cell membrane permeability and photostability, as well as low cytotoxicity. Our results have successfully proven that these probes can visualize intracellular lysosomal pH changes in live cells by monitoring both near-infrared CF and FUCL changes.

Anti-Stokes shift luminescent materials for bio-applications

This review presents comprehensive discussions about three types of anti-Stokes luminescent materials and summarizes recent advances in their bioapplications.

A phenazine-barbituric acid based colorimetric and ratiometric near-infrared fluorescent probe for sensitively differentiating biothiols and its application in TiO2 sensor devices

A colorimetric and ratiometric NIR fluorescent probe based on phenazine-barbituric acid was developed for biothiols and applied to detect in cells and construct TiO₂ sensor devices.

Solvent-Induced Luminescence Variation of Upconversion Nanoparticles

Solvent plays a vital role in the syntheses, purifications, and broad applications of upconversion nanoparticles (UCNPs). In this work, the effect of various dispersive solvents, including single solvents and mixed solvents, on the luminescence properties of NaYF₄:Yb³⁺, Er³⁺ UCNPs was studied systematically. The differences in both upconversion luminescence (UCL) intensities and color outputs of the nanoparticles were observed when dispersing the UCNPs in deuterium oxide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, or water. The attenuation of the excitation and emission light of the UCNPs caused by absorption of the solvents, as well as the high-frequency vibrational groups of the solvents, such as -OH, -CH₂, and -CH₃ groups, are responsible for the decrease in UCL intensities and increase in the red to green emission intensity ratios (RGR). The changes in water or OH^- ion contents of ethanol/water mixed solvent triggered similar changes in UCL properties. Interestingly, the quenching of the solvents for the UCL cannot be fully eliminated by changing the dispersive solvents once the UCNPs have touched the solvents containing high-frequency vibrational groups. Our work will facilitate the comprehension of the solvent induced luminescence variations of the nanoparticles and provide guidance for their applications.

Reactive oxygen species generating systems meeting challenges of photodynamic cancer therapy

The reactive oxygen species (ROS)-mediated mechanism is the major cause underlying the efficacy of photodynamic therapy (PDT). The PDT procedure is based on the cascade of synergistic effects between light, a photosensitizer (PS) and oxygen, which greatly favors the spatiotemporal control of the treatment. This procedure has also evoked several unresolved challenges at different levels including (i) the limited penetration depth of light, which restricts traditional PDT to superficial tumours; (ii) oxygen reliance does not allow PDT treatment of hypoxic tumours; (iii) light can complicate the phototherapeutic outcomes because of the concurrent heat generation; (iv) specific delivery of PSs to sub-cellular organelles for exerting effective toxicity remains an issue; and (v) side effects from undesirable white-light activation and self-catalysation of traditional PSs. Recent advances in nanotechnology and nanomedicine have provided new opportunities to develop ROS-generating systems through photodynamic or non-photodynamic procedures while tackling the challenges of the current PDT approaches. In this review, we summarize the current status and discuss the possible opportunities for ROS generation for cancer therapy. We hope this review will spur pre-clinical research and clinical practice for ROS-mediated tumour treatments.