Molecular Engineering of NIR‐II Fluorophores for Improved Biomedical Detection

A near-infrared fluorescent probe based on purine for glyphosate detection in real sample, living cells and zebrafish

In this study, we developed a novel NIR purine-based fluorescent probe, EPNA, that operates on an on-off-on fluorescence mechanism. EPNA exhibits high selectivity for Cu2+ ions, with fluorescence quenching and a detection limit of 129.2 nM. Job's plot and Density Functional Theory (DFT) calculation confirm the formation of a stable 1:1 complex between EPNA and Cu2+. The EPNA-Cu2+ complex is highly sensitive to glyphosate, with fluorescence restoration upon exposure to glyphosate, achieving a detection limit of 157.4 nM. The detection of Cu2+ and glyphosate by the probe EPNA can also be observed with the naked eye under visible light, offering an easy and intuitive method for detection. The probe successfully detects glyphosate in environmental samples, living cells and zebrafish. Key advantages of the EPNA-Cu2+ system include low detection limits, exceptional selectivity, rapid response times (30 s for Cu2+ and 20 s for glyphosate), and strong resistance to interference, making it an efficient tool for glyphosate detection.

Multi-dimensional donor engineering of NIR-II AIEgens for multimodal phototheranostics of orthotopic breast cancer

"One-for-all" multimodal phototheranostic agents, which integrate multiple photodiagnostic and phototherapeutic functionalities into a single component, have emerged as promising platforms for advancing cancer treatment. Among these, agents featuring second near-infrared (NIR-II) emission are particularly appealing due to their superior tissue penetration depth and high signal-to-background ratio (SBR). However, most reported NIR-II fluorophores suffer from severely imbalanced radiative and non-radiative excited-state energy dissipation in biological environments, resulting in extremely low fluorescence quantum yields (QYs) and limited diagnostic efficacy. This highlights the urgent need for innovative molecular design strategies to develop high-performance NIR-II "one-for-all" multimodal phototheranostic agents. Herein, we present, for the first time, a multi-dimensional donor engineering protocol that optimizes donor design at the molecular, aggregated, and solvent-interaction levels. By introducing 2,4,4-trimethylpentan-2-yl groups into the diphenylamine indeno[1,2-b]thiophene donor unit, we developed a donor-acceptor-donor (D-A-D) type NIR-II aggregation-induced emission-active luminogen (AIEgen), i.e. OPITBT. When formulated into nanoparticles (NPs), OPITBT NPs exhibited a 16-fold enhancement in fluorescence QY compared to OPITBT in tetrahydrofuran, along with excellent photothermal conversion efficiency (PCE) and acceptable type-I reactive oxygen species (ROS) generation. When further fabricated into tumor-targeting NPs, the resulted OPITBT-R NPs effectively eliminated orthotopic breast cancer through fluorescence-photoacoustic-photothermal multimodal imaging-guided photodynamic-photothermal synergistic therapy under single 808 nm laser irradiation. Notably, the exceptional NIR-II fluorescence brightness of OPITBT-R NPs enables high-resolution NIR-IIb whole-body vascular imaging in living mice. This work provides a versatile strategy to enhance radiative dissipation of NIR-II fluorophores for balanced phototheranostic performance and advances the development of "one-for-all" phototheranostic systems.

An all-in-one PEGylated NIR-II conjugated polymer for high-resolution blood circulation imaging and photothermal immunotherapy

Fluorescence imaging in the second near-infrared window (NIR-II) has shown tremendous potential for in vivo monitoring of biological processes, offering high spatial resolution and real-time imaging capabilities. Conjugated polymers, commonly used as photothermal agents (PTAs) in photothermal therapy, have emerged as promising candidates for NIR-II imaging. However, their imaging efficiency is compromised by aggregation, which arises from strong π-π stacking interactions between their extended π-conjugated backbones. In this work, we designed a novel conjugated polymer (CP) and developed an integrated nanoplatform (CPN-PEGnk, n = 2 or 5) through PEGylation. Notably, CPN-PEG5k exhibited a red-shift in NIR absorption along with a marked increase in NIR-II fluorescence intensity (2.97 folds greater) compared to physically encapsulated nanoparticles (F127@CPN). Furthermore, CPN-PEG5k retained a remarkable photothermal conversion efficiency of up to 58.6%. The exceptional NIR-II imaging performance of CPN-PEG5k was validated in detailed blood circulation imaging in mice, with a signal-to-background ratio of 8.9. In addition, in a breast cancer mouse model, CPN-PEG5k successfully eradicated tumors and stimulated immune responses, effectively suppressing tumor progression and metastasis. These findings underscore the potential of CPN-PEG5k in advancing conjugated polymer applications for NIR-II imaging.

Bioengineered nanomaterials for dynamic diagnostics in vivo

In vivo diagnostics obtains real-time physiological information directly from the site of interest in a patient's body, providing more accurate disease diagnosis compared with ex vivo diagnostics. Particularly, in vivo dynamic diagnostics allows the continuous monitoring of physiological signals over a period of time, offering deeper insights into disease pathogenesis and progression. However, achieving in situ dynamic diagnostics in deep tissues presents challenges related to energy and signal penetration as well as dynamic monitoring. Bioengineered nanomaterials serve as an ideal platform for in vivo dynamic diagnostics, leveraging energy conversion and biofunctionalization to enable continuous acquisition of physiological information across temporal and spatial scales. In this review, with reference to the studies from the last five years, we summarize the fundamental components that are essential for dynamic diagnosis in vivo. Firstly, an input energy source with high tissue penetration is needed, such as near-infrared (NIR) light, X-rays, magnetic field and ultrasound. Secondly, a nanomaterial class that is responsive to such an energy source to provide a readable output signal is chosen. Thirdly, bioengineered nanoprobes are designed to exhibit spatial, temporal or spatiotemporal changes in the output signal. Finally, different methods are used to analyse the output signal of nanoprobes, such as detecting changes in optical, radiation, magnetic and ultrasound signals. This review also discusses the obstacles and potential solutions for advancing these bioengineered nanomaterials toward clinical translational applications.

Understanding the Properties of Donor-Acceptor Substituted Boron Difluoride 3-Cyanoformazanate Dyes

π-Conjugated materials offer attractive traits including semiconductivity, low-energy light absorption/photoluminescence, and solution processability that render them ubiquitous within the organic electronics field. Among many strategies for property tuning, the creation of asymmetric electronic structures through the installation of donor and acceptor substituents commonly results in low-energy absorption/photoluminescence bands. Boron difluoride formazanate dyes are readily synthesized, can be asymmetrically substituted with donor and acceptor groups, and have unexpectedly low-energy absorption/photoluminescence bands that extend into the near-infrared. In this study, we prepared a series of donor-acceptor substituted boron difluoride 3-cyanoformazanate dyes and compared their properties to symmetric analogues. Our findings suggest that donor-acceptor derivatives are highly delocalized with properties intermediate of their symmetric counterparts. Furthermore, the data obtained suggest that the N-aryl substituents act as donors to the strongly accepting boron difluoride formazanate core, regardless of the functional groups appended to them. These properties were reproduced computationally, and while the frontier orbitals calculated for donor-acceptor dyes were modestly asymmetric, there was no evidence of charge-transfer character. This work provides significant insight into the unexpected properties of boron difluoride formazanates and reveals that their strongly accepting nature circumvents the predicted augmentation of electronic structure commonly observed for donor-accepter substituted dyes.

Quantitative analysis of safflower seed oil adulteration based on near-infrared spectroscopy combined with improved sparrow algorithm optimization model ISSA-ELM

Safflower seed oil is expensive, and there are issues in the market such as the adulteration with cheaper edible oils, leading to inferior products. Near-infrared spectroscopy (NIR) is a non-destructive analytical method with the advantages of being fast, non-destructive, and pollution-free. However, in the case of low-concentration adulterated oil, the main components and their contents are nearly identical to those in pure oil, and the spectral feature peaks of the samples are similar, making it difficult for conventional analysis methods to select effective characteristic variables. Extreme Learning Machine (ELM), as a single-hidden-layer feedforward neural network model, possesses strong feature extraction and model representation capabilities. However, its random initialization of weights and biases leads to the issue of blind training. The Sparrow Search Algorithm (SSA) can effectively optimize the random initialization problem in ELM, but it is prone to issues such as being trapped in local optima and slower convergence. This study proposes a novel quantitative adulteration analysis model for safflower seed oil (ISSA-ELM), which combines ELM with an improved Sparrow Search Algorithm (ISSA). In the experiment, safflower seed oil was used as the base oil, and peanut oil, corn oil, and soybean oil were gradually added to prepare adulterated oil samples. To ensure the resolution of experimental samples and accurately capture spectral variations in the low-concentration range, a concentration gradient of 2% was used for adulteration levels between 2% and 70%, while a 5% gradient was applied for concentrations exceeding 70%. Each type of adulterant had 42 gradient concentrations, with 6 samples per concentration, totaling 252 samples. Original spectral data were collected, and the samples were randomly divided into a training set (70%) and a testing set (30%). Different preprocessing methods and feature extraction techniques were combined to discuss the results of three adulteration analysis models: PLS, SSA-ELM and ISSA-ELM. The final experimental results show that compared to the safflower seed oil adulteration prediction model based on SSA-ELM (R2 = 0.8938, RMSE = 0.0835, RPD = 2.9018)and PLS(RP2 = 0.9015, RMSEP = 0.0876, RPD = 3.0128), the model based on ISSA-ELM (RP2 = 0.9934, RMSEP = 0.0207, RPD = 10.1457) offers higher prediction accuracy and stability, and the error detection limit of ISSA-ELM can reach 2%. The ISSA optimized the SSA's tendency to reduce population diversity, slow convergence, and get stuck in local optima in the later stages of iteration, greatly enhancing the global optimization ability of the algorithm. Therefore, ISSA-ELM can effectively identify adulterated safflower seed oil, providing a technological pathway and basis for research into the adulteration of safflower seed oil.

Morphology evolution and NIR-II luminescence enhancement in LaF3:Er3+ nanoparticles

Luminescence imaging in the second near-infrared (NIR-II) window (1000-1700 nm) has attracted extensive attention in recent years owing to its superior tissue penetration ability and weak spontaneous fluorescence effects in biological tissues. However, an important bottleneck restricting the development of NIR-II luminescence imaging technology is the lack of nanoprobes with suitable morphology, particle size and high luminescence intensity. In this work, NIR-II emitted LaF3:Er3+ nanoparticles (NPs) were synthesized by a simple chemical precipitation method, where the size and morphology of LaF3:Er3+ NPs were precisely controlled through the regulation of reaction conditions. The NIR-II luminescence of Er3+ was effectively enhanced for 8.4 times by Zr4+ doping. The results provides useful reference for the research of NIR-II luminescent imaging nanoprobes.

Ultra-sensitive, versatile and portable detection of hydrazine in eco-environmental systems using a smartphone-integrated ratiometric fluorescent sensor

Hydrazine (N2H4), a highly reactive specie widely used in industrial processes, poses significant ecological risks. Accurate detection of N2H4 is essential for safeguarding public health, yet developing a robust tool for its global detection remains a significant challenge. Herein, we developed a ratiometric fluorescent sensor, DIPOT, designed for ultra-sensitive and portable detection of N2H4 in eco-environmental systems. DIPOT exhibited excellent ratiometric fluorescence performance for N2H4 in aqueous solution, with a detection limit as low as 4.5 nM and a substantial 156 nm blue shift, transitioning from red to green fluorescence. We integrated it into portable test strips for on-site quantitative detection and analysis of N2H4 vapor and solution via a smartphone application. DIPOT and its portable platform have been successfully applied to monitor ultra-trace levels of N2H4 in 20 diverse eco-environmental samples, including water, soil, crops, food and living organisms, showcasing its versatility. Furthermore, DIPOT facilitates real-time ratiometric bioimaging of N2H4 in living plants, cells and zebrafish. Our findings provide a robust and eco-friendly approach for global tracking of N2H4, representing a significant advancement in environmental sensing technology.

A Bright Organic Fluorophore for Accurate Measurement of the Relative Quantum Yield in the NIR-II Window

Organic dyes with photoluminescence in the second near-infrared window (NIR-II, 1000-1700 nm) are promising for bioimaging and optoelectronic devices. Photoluminescence quantum yield (PLQY) is a direct measure of their performance. Integrating sphere technology is effective in determining the absolute PLQY. However, the low PLQY values of most NIR-II organic fluorophores lead to significant measurement errors. Therefore, the most common method for PLQY determination is a relative approach using a photoluminescence spectrometer and a standard reference like IR-26. Although the relative method enables precise calculation of the PLQY ratio between the sample and the reference, the specific PLQY value of IR-26 is not clearly defined and is reported to range from 0.05% to 0.50%. Such a deviation can cause significant errors in relative PLQY measurements. In this study, it is reported that a bright organic fluorophore called TPE-BBT exhibits a high PLQY of 3.94% in THF, which can be accurately measured using a commercially available integrating sphere. Using TPE-BBT as a standard, the PLQY values of IR-26 in 1,2-dichloroethane and IR-1061 in dichloromethane are accurately determined to be 0.0284% and 0.182%, respectively. It is hoped that using this reliable standard will unify the evaluation criteria for NIR-II organic fluorophores.

Small organic fluorophores with SWIR emission detectable beyond 1300 nm

3,6-Dimethylamino fluorenone was functionalized with substituents to achieve an absorption maximum at 1012 nm and emission >1300 nm. TD-DFT calculations confirmed that the substituent orbitals contribute to narrowing the HOMO-LUMO energy gap. Imaging with an InGaAs-based SWIR camera and various longpass filters confirmed detection >1300 nm.

SERS/Fluorescence Dual-Modal Imaging Bioprobe for Accurate Diagnosis of Breast Cancer

Early diagnosis and precise identification of breast cancer subtypes are vital. However, current detection methods are often hindered by high costs and complexity. This study aims to develop an efficient and noninvasive method to realize efficient breast cancer detection. First, hexoctahedral gold nanoparticles (Au HNPs) are constructed, which detect molecules with concentrations as low as 10-12 M, and the EF value is ∼3.8 × 108. Then, two optical bioprobes with a surface-enhanced Raman scattering (SERS)-fluorescence (FL) dual-modal function for breast cancer cell detection and subtype identification are designed. These bioprobes exhibit excellent SERS stability since the spectral relative standard deviation (RSD) of the SERS-FL bioprobe achieves a good level of ∼10.4%. Additionally, the clear distinction between breast cancer cells and white blood cells (WBCs) under a fluorescence microscope showed that bioprobes have a good fluorescence imaging ability. More importantly, by creatively stitching the SERS spectra of the two bioprobes, a "symphonic SERS spectra" is constructed, and a linear discriminant analysis (LDA) machine learning algorithm is employed, enabling high-precision classification of breast cancer subtypes with an accuracy of 94%. This study proposes an innovative strategy combined with SERS and FL technology, which provides the possibility for rapid and accurate detection of breast cancer subtypes.

Designing small organic molecular NIR-II fluorophores by ring strain modulation

Developing small molecular organic fluorophores in the second near-infrared window (NIR-II, 1000-1700 nm) with excellent photophysical properties is an ongoing pursuit for in vivo bioimaging and biosensing. Herein, we report a strategy to modulate the optical properties of xanthene-based fluorophores by manipulating their ring strain through fine-tuning the ring size, which strongly influences the rigidity and planarity of the conjugated structure, thereby impacting their optical characteristics. Additionally, the ring strain imparts varying responsiveness to the fluorophores by affecting the pKcycl values through spirocyclization.

Achieving Ultrabright NIR-II Nanofluorophore for In Vivo Imaging by Inhibiting H-Aggregates Formation

Small molecules with an acceptor-donor-acceptor (A-D-A) structure, featuring a fused-ring core as the donor and two electron-withdrawing end groups as acceptor units, represent a potential option for NIR-II fluorophores, benefiting from their narrow bandgaps, superior light-harvesting capabilities, and exceptional photostabilities. However, their planar conformations predispose them to forming H-aggregates during self-assembly, leading to significantly reduced fluorescence quantum yield (QY) of the resulting nanofluorophores. Herein, we report a small molecule, PF8CN, with a terminal unit-A-D-A-terminal unit structure. The terminal units of 3,5-bis(octyloxy)phenyl group result in a twisted conformation for PF8CN, preventing face-to-face stacking and thereby inhibiting the formation of H-aggregates. Consequently, the NIR-II fluorescence QY of PF8CN NPs is 3.8 times that of the model nanofluorophore (F8CN NPs), which contains a substantial amount of H-aggregates. The NIR-II brightness of PF8CN NPs is 5.3- and 14.9-times that of F8CN NPs and ICG/FBS, respectively, at an equal molar concentration. Such ultrahigh NIR-II brightness of PF8CN NPs allows us to perform long-term and real-time NIR-II fluorescence imaging of cerebral and hindlimb vessels, as well as the thrombolytic process. This work provides an effective method for producing nanofluorophores with ultrahigh NIR-II brightness, positioning PF8CN NPs as a strong contender in the field of NIR-II nanofluorophores.

A Brain-Targeting NIR-II Polymeric Phototheranostic Nanoplatform toward Orthotopic Drug-Resistant Glioblastoma

Glioblastoma is the most common and devastating brain tumor owing to its high invasiveness and high-frequency drug resistance. Near infrared-II (NIR-II) imaging-guided phototherapy based on polymer luminogens provides a promising remedy against drug-resistant glioma, but it is difficult to maximize photoenergy utilization. Herein, we designed a series of semiconducting polymers to boost the visualization and ablation of glioblastoma. By subtly engineering the side chains or substituents on the phenothiazine and thiophene moieties, an NIR-II polymer luminogen with high-quality fluorescence performance, good solubility, superior photothermal conversion, and balanced reactive oxygen species generation is achieved. The optimal polymer possesses a branched alkyl chain and tetraphenylethylene pendant to manipulate the equilibrium between the radiative and nonradiative energy-dissipating channels. High-sensitivity NIR-II imaging was used to monitor the blood-brain barrier penetration and glioma cell targeting of apolipoprotein E-modified polymer nanoparticles. The NIR irradiation triggers and maximizes the photon utilization in prominent photodynamic/photothermal synergistic therapy in orthotopic drug-resistant glioblastoma.

Luminescence Fingerprint of Intracellular NIR-II Gold Nanocluster Transformation: Implications for Sensing and Imaging

Gold nanoclusters emitting in the second biological window (NIR-II-AuNCs) have gained significant interest for their potential in deep-tissue bioimaging and biosensing applications due to the partial transparency and reduced autofluorescence of tissues in this spectral range. However, the limited understanding of how the biological environment affects their luminescent properties might hinder their use in bioimaging and biosensing. In this study, we investigated the emission properties of NIR-II-AuNCs when interacting and internalizing into live cells including macrophages, fibroblasts, and cancer cell lines, revealing substantial alterations in their luminescence. A systematic comparison between control and in vitro experiments concluded that the disruption of surface ligands is the main factor responsible for these alterations. NIR-II-AuNCs within cellular environments may also be influenced by other interactions, including aggregation or complexation with proteins. Furthermore, we also corroborated these spectroscopic modifications at the in vivo level, providing additional evidence of the environmental sensitivity of NIR-II-AuNCs. The results obtained in this study contribute to a deeper understanding of the luminescence mechanisms of NIR-II-AuNCs in biological environments in cells and in living tissues and are crucial for their optimization as reliable tools in biological environment for in vitro and in vivo imaging and diagnostics.

Alleviating NIR-II emission quenching in ring-fused fluorophore via manipulating dimer populations for superior fluorescence imaging

Emission quenching resulting from fluorophore aggregation has long been a significant challenge in optimizing emission-based technologies, such as fluorescence imaging and optoelectronic devices. Alleviating this quenching in aggregates is crucial, yet progress is impeded by the limited understanding of the nature and impact of aggregates on emission. Here, we elucidate the critical role of dimeric aggregate (dimer) in alleviating second near-infrared (NIR-II, 900-1700 nm) emission quenching from ring-fused fluorophore 4F for superior fluorescence imaging. Spectral decomposition and molecular dynamics simulations demonstrate the predominance of dimer populations in 4F aggregates. Notably, dimers exhibit significantly weaker emission but intense intermolecular nonradiative (interNR) decay compared to monomers, as demonstrated by ultrafast spectra and quantum calculation. Therefore, the predominant population of dimers with weak emission and pronounced interNR feature underlies the emission quenching in 4F aggregates. This discovery guides the preparation of ultrabright NIR-II 4F nanofluorophore (4F NP3s) by decreasing dimer populations, which show 5-fold greater NIR-II brightness than indocyanine green, enabling superior resolution in visualizing blood vessels. This work offers valuable insights into aggregation-caused quenching, with broad implications extending far beyond NIR-II fluorescence imaging.

From X- To J-Aggregation: Subtly Managing Intermolecular Interactions for Superior Phototheranostics with Precise 1064 nm Excitation

The stacking mode in aggregate state results from a delicate balance of supramolecular interactions, which closely affects the optoelectronic properties of organic π-conjugated systems. Then, managing these interactions is crucial for advancing phototheranostics, yet remains challenging. A subtle strategy involving peripheral phenyl groups is debuted herein to transform X-aggregated SQ-H into J-aggregated SQ-Ph, reorienting intermolecular dipole interactions while rationally modulating π-π interactions. Co-assembled with liposomes (DSPE-PEG2000), SQ-Ph nanoparticles (NPs) exhibit low toxicity, superior biocompatibility, and a bathochromic shift to the 1064 nm match-excited NIR-II region, with a fluorescence brightness (ε1064 nm ΦNIR-II) of 4129 M-1 cm-1 and a photothermal conversion efficiency (PCE) of 48.3%. Preliminary in vivo experiments demonstrate that SQ-Ph NPs achieve a signal-to-background ratio (SBR) of up to 14.29 in NIR-II fluorescence imaging (FLI), enabling highly efficient photothermal therapy (PTT) of tumors guided by combined photoacoustic imaging (PAI). This study not only enriches the J-aggregation library but also provides a paradigm for optimizing photosensitizers at the supramolecular level.

The latest developments of near-infrared fluorescent probes from NIR-I to NIR-II for bioimaging

Near-infrared I (NIR-I: 650-950 nm) fluorescence imaging is a powerful tool for deep-tissue biological imaging, addressing the limitations of photon penetration depth in the visible-light region. Over the past decade, NIR imaging has extended to the near-infrared II (NIR-II: 1000-1700 nm) region, offering high-resolution and low background imaging by suppressing light scattering, and autofluorescence of tissues. Near-infrared fluorescent probes from NIR-I to NIR-II, with diverse functionalities, are increasingly utilized across biological fields to meet various detection needs and to explore physiological events in real time and spatial dimensions. This review discusses recent advancements in small-molecule NIR fluorescent dyes and probes, particularly those based on cyanine and rhodamine scaffolds, highlighting examples of their applications in bioimaging.

Xanthene-based NIR organic phototheranostics agents: design strategies and biomedical applications

Fluorescence imaging and phototherapy in the near-infrared window (NIR, 650-1700 nm) have attracted great attention for biomedical applications due to their minimal invasiveness, ultra-low photon scattering and high spatial-temporal precision. Among NIR emitting/absorbing organic dyes, xanthene derivatives with controllable molecular structures and optical properties, excellent fluorescence quantum yields, high molar absorption coefficients and remarkable chemical stability have been extensively studied and explored in the field of biological theranostics. The present study was aimed at providing a comprehensive summary of the progress in the development and design strategies of xanthene derivative fluorophores for advanced biological phototheranostics. This study elucidated several representative controllable strategies, including electronic programming strategies, extension of conjugated backbones, and strategic establishment of activatable fluorophores, which enhance the NIR fluorescence of xanthene backbones. Subsequently, the development of xanthene nanoplatforms based on NIR fluorescence for biological applications was detailed. Overall, this work outlines future efforts and directions for improving NIR xanthene derivatives to meet evolving clinical needs. It is anticipated that this contribution could provide a viable reference for the strategic design of organic NIR fluorophores, thereby enhancing their potential clinical practice in future.

Design strategies and biomedical applications of organic NIR-IIb fluorophores

The introduction of fluorescence imaging (FLI) in near-infrared II sub-channels (NIR-IIb, 1500-1700 nm) has revolutionized the ability to explore complex patho-physiological settings in vivo. Despite the transformative potentials, the development of organic NIR IIb dyes encounters considerable difficulties, and only a limited number of such fluorophores have been developed so far. This review systematically introduces design strategies of organic NIR-IIb fluorophores classified by molecular scaffolds, mainly including cyanine dyes and D-A-D small molecule dyes. The design strategies of cyanine dyes involve repurposing of the existing NIR dyes, conjugate reinforcement and regulation of the aggregation state. For D-A-D small molecule dyes, strategies mainly incorporate the extension of the conjugate skeleton, introduction of shielding units, and acceptor/donor engineering. We further describe recent biomedical applications including biomedical imaging and imaging-guided therapy, and conclude by clarifying the current challenges and prospects of NIR-IIb FLI.

Less Is More: Donor Engineering of a Stable Molecular Dye for Bioimaging in the NIR-IIb Window

Fluorescence molecular imaging aims to enhance clarity in the region of interest, particularly in the near-infrared IIb window (NIR-IIb, 1500-1700 nm). To achieve this, we developed a novel small-molecule dye, named DA-5, based on classic cyanine dyes (heptamethine or pentamethine is essential for wavelengths beyond 1000 nm). By reducing excessive polymethine to a single methine and disrupting symmetry to form an asymmetric donor-π-acceptor (D-π-A) architecture, we enhanced the donor's electron-donating capability, yielding emission at 1088 nm. DA-5 exhibits superior properties, including excellent chemo- and photostability, resistance against solvatochromism-caused quenching, and antiaggregation in aqueous solution. With a large Stokes shift (241 nm) and high brightness (321 M-1 cm-1), DA-5 enables high-performance imaging of the lymphatic system, intestinal vessels, whole-body angiography, and cerebral and hindlimb microvasculature in NIR-IIb. This molecular design strategy offers a promising platform for advancing in vivo biophotonics.

NIR-II upconversion nanomaterials for biomedical applications

As a nonlinear optical phenomenon, upconversion (UC) occurs when two or more low-energy excitation photons are sequentially absorbed and emitted. Upconversion nanomaterials exhibit superior photostability, non-invasiveness, a unique near-infrared anti-Stokes shift, and enhanced tissue penetration capability. However, general upconversion nanomaterials typically utilize visible light (400-700 nm) for excitation, leading to limited tissue penetration, background signal interference, limited excitation efficiency and imaging quality issues due to tissue absorption and scattering. The increasing use of upconversion nanomaterials in the near-infrared one-region (NIR-I) window (700-900 nm) offers benefits such as enhanced penetration into biological tissues, relatively improved imaging resolution, and lower spontaneous luminescence, although these materials are still susceptible to background signals, limiting their effectiveness in high signal-to-noise ratio imaging. This distinctive wavelength conversion endows upconversion nanomaterials in the NIR-II region with extraordinary potential for diverse applications. Biomedical research has primarily focused on biomedical imaging for disease diagnosis and treatment, as well as biomarker detection. Nonetheless, studies specifically targeting the NIR-II window remain limited. This paper summarizes the latest research progress on upconversion nanomaterials in the NIR-II region. It begins by introducing the preparation methods for these materials in the NIR-II, followed by their applications in imaging and biological contexts. Lastly, it discusses the primary challenges and future prospects of upconversion materials in NIR-II, aiming to promote their development.

Isomerization enhanced fluorescence brightness of benzobisthiadiazole-based NIR-II fluorophores for highly efficient fluorescence imaging: A theoretical perspective

As a cutting-edge technique, fluorescence imaging in the second near-infrared window (NIR-II) is vital for both biomedical research and clinical applications. However, its intravital imaging capacity has been restricted by the extremely limited brightness of NIR-II fluorophores. To address this challenge, we elucidated the inner mechanism of constructing high-performance NIR-II chromophores based on molecular isomer engineering from detailed computational investigations. Herein, three pairs of cis-trans isomers (cis-1, 2, 3 and trans-1, 2, 3) are designed by attaching amino, methoxyl and nitro moieties to different positions on the donor-acceptor-donor molecular skeleton with benzobisthiadiazole as the acceptor and triphenylamine as the donor. All the compounds feature efficient NIR-II emission ranging in 1000-1164 nm, and the photophysical characterizations are regulated by molecular isomer manipulation. Interestingly, fluorescence quantum yields of cis-isomers are higher than those of their trans-counterparts. These enhancements can be attributed to the significant reduction in non-radiative transition, as evidenced by the non-adiabatic excitation energy, non-adiabatic electron coupling and electron-vibration coupling. Meanwhile, fluorophores with nitro terminal group exhibit superior performance facilitated by the prominently intramolecular charge transfer. As a result, cis-3 achieves an optimal brightness maxima of 196.36 M-1 cm-1 at 632 nm. Notably, the energy gap and the hole-electron related H index are respectively identified as strongly relevant to the emission wavelength and brightness, making them capable of evaluating the feasibility of fluorophores as effective NIR-II candidates. These findings highlight the correlations between molecular geometry and luminescent properties, which will inspire more insights into the development of highly efficient NIR-II fluorophores through rational isomer engineering for biomedical applications.

NIRFluor: A Deep Learning Platform for Rapid Screening of Small Molecule Near-Infrared Fluorophores with Desired Optical Properties

Small molecule near-infrared (NIR) fluorophores play a critical role in disease diagnosis and early detection of various markers in living organisms. To accelerate their development and design, a deep learning platform, NIRFluor, was established to rapidly screen small molecule NIR fluorophores with the desired optical properties. The core component of NIRFluor is a state-of-the-art deep learning model trained on 5179 experimental big data. First, novel hybrid fingerprints including Morgan fingerprints, physicochemical properties, and solvent properties were proposed. Then, a powerful deep learning model, multitask fingerprint-enhanced graph convolutional network (MT-FinGCN), was designed, which combines fingerprint information and molecule graph structure information to achieve accurate prediction of six properties (absorption wavelength, emission wavelength, Stokes shift, extinction coefficient, photoluminescence quantum yield, and lifetime) of different small molecule NIR fluorophores in different solvents. Furthermore, the "black-box" of the GCN model was opened through interpretability studies. Finally, the well-trained models were placed on the web platform NIRFluor for free use (https://nirfluor.aicbsc.com).

Activatable Molecular Probes With Clinical Promise for NIR-II Fluorescent Imaging

The second near-infrared window (NIR-II) fluorescence imaging has been widely adopted in basic scientific research and preclinical applications due to its exceptional spatiotemporal resolution and deep tissue penetration. Among the various fluorescent agents, organic small-molecule fluorophores are considered the most promising candidates for clinical translation, owing to their well-defined chemical structures, tunable optical properties, and excellent biocompatibility. However, many currently available NIR-II fluorophores exhibit an "always-on" fluorescence signal, which leads to background noise and compromises diagnostic accuracy during disease detection. Developing NIR-II activatable organic small-molecule fluorescent probes (AOSFPs) for accurately reporting pathological changes is key to advancing NIR-II fluorescence imaging toward clinical application. This review summarizes the rational design strategies for NIR-II AOSFPs based on four core structures (cyanine, hemicyanine, xanthene, and BODIPY). These NIR-II AOSFPs hold substantial potential for clinical translation. Furthermore, the recent advances in NIR-II AOSFPs for NIR-II bioimaging are comprehensively reviewed, offering clear guidance and direction for their further development. Finally, the prospective efforts to advance NIR-II AOSFPs for clinical applications are outlined.

Molecular Engineering of 2', 7'-Dichlorofluorescein to Unlock Efficient Superoxide Anion NIR-II Fluorescent Imaging and Tumor Photothermal Therapy

Although classical fluorescent dyes feature advantages of high quantum yield, tunable "OFF-ON" fluorescence, and modifiable chemical structures, etc., their bio-applications in deep tissue remains challenging due to their excessively short emission wavelength (that may lead to superficial tissue penetration depth). Therefore, there is a pressing need for pushing the wavelength of classical dyes from visible region to NIR-II window. As a representative classical dye, the 2',7'-Dichlorofluorescein (DCF), a derivative of Fluorescein, is selected and rationally engineered to develop a novel NIR-II platform, CR-OH, which exhibits a substantial red-shift in the wavelength from the visible region to the NIR-II region. This achievement is attributed to molecular modification strategies that include extending π-conjugation, enhancing molecular rigidity, and incorporating strong electron-withdrawing groups. Furthermore, based on this developed NIR-II platform, a NIR-II fluorescence probe and a photothermal nanoagent are successfully constructed to unlock its bio-application in the NIR-II fluorescence imaging of endogenous O2 ·- fluctuations in a CIRI model for the first time, as well as effective photothermal therapy for 4T1 tumors with a high photothermal conversion efficiency (44.0%). Significantly, this work overcomes the wavelength limitation of classical dyes, effectively unlocking their applications for the diagnosis and treatment of early disease in the NIR-II window.

Integration of Motion and Stillness: A Paradigm Shift in Constructing Nearly Planar NIR-II AIEgen with Ultrahigh Molar Absorptivity and Photothermal Effect for Multimodal Phototheranostics

The two contradictory entities in nature often follow the principle of unity of opposites, leading to optimal overall performance. Particularly, aggregation-induced emission luminogens (AIEgens) with donor-acceptor (D-A) structures exhibit tunable optical properties and versatile functionalities, offering significant potential to revolutionize cancer treatment. However, trapped by low molar absorptivity (ε) owing to the distorted configurations, the ceilings of their photon-harvesting capability and the corresponding phototheranostic performance still fall short. Therefore, a research paradigm from twisted configuration to near-planar structure featuring a high ε is urgently needed for AIEgens development. Herein, by introducing the strategy of "motion and stillness" into a highly planar A-D-A skeleton, we successfully developed a near-infrared (NIR)-II AIEgen of Y5-2BO-2BTF, which boasts an impressive ε of 1.06 × 105 M-1 cm-1 and a photothermal conversion efficiency (PCE) of 77.8%. The modification of steric hindrance on the benzene ring in the acceptor unit of the aggregation-caused quenching counterpart Y5-2BO, to a meta-CF3-substituted naphthyl, leads to reversely staggered packing and various intermolecular noncovalent conformational locks in Y5-2BO-2BTF ("stillness"). Furthermore, the -CF3 moiety acted as a flexible motion unit with an ultralow energy barrier, significantly facilitating the photothermal process in loose Y5-2BO-2BTF aggregates ("motion"). Accordingly, Y5-2BO-2BTF nanoparticles enabled tumor eradication and pulmonary metastasis inhibition through NIR-II fluorescence-photoacoustic-photothermal imaging-navigated type I photodynamic-photothermal therapy. This work provides the first evidence that the highly planar conformation with a reversely staggered stacking arrangement could serve as a novel molecular design direction for AIEgens, shedding new light on constructing superior phototheranostic agents for bioimaging and cancer therapy.

Design, Synthesis, and In Vivo Imaging of a Stable Xanthene-Based Dye with NIR-II Emission up to 1450 nm

The development of long-wavelength near-infrared II (NIR-II, 900-1700 nm) dyes is highly desirable but challenging. To achieve both red-shifted absorption/emission and superior in vivo imaging capabilities, a donor-acceptor-donor (D-A-D) xanthene core was strategically modified by extending π-conjugated double bonds and enhancing electron-donating properties. Two dyes named VIX-1250 and VIX-1450 were synthesized and exhibited notably red-shifted absorption/emission peaks at 942/1250 and 1098/1450 nm, respectively. Among them, VIX-1450 demonstrated superior chemo- and photostability even at such long wavelengths. Fluorescent angiography using VIX-1450 micelles enabled high-clarity blood vessel imaging with a remarkable signal-to-noise ratio (SNR), underscoring that the dye's large Stokes shift (352 nm), good brightness (13 M-1 cm-1), and long wavelength served as key factors for high-quality in vivo biosensing. Additionally, VIX-1450 combined with ICG for dual-color imaging achieved near-zero optical cross talk, enabling different organ labeling. This study provides a new direction for the design of long-wavelength organic dyes.

Near-Infrared-Emitting Helically Twisted Conjugated Frameworks Consisting of Alternant Donor-π-Acceptor Units and Multiple Boron Atoms

A novel design strategy to construct bright and narrow near-infrared (NIR) emission materials with suppressed shoulder peaks can significantly enhance their performance in various applications. Herein, we have successfully synthesized a series of helically twisted D-π-A conjugated systems bridged by boron atoms, achieving bright red to near-infrared (NIR) emissions with notably narrow full-width at half-maximum (FWHM) values of 35 nm (0.08 eV) and photoluminescence quantum yields (PLQY) up to 80 %. These compounds display red-shifted emissions up to 753 nm in higher concentrations. Cis/trans configurational isomers of multi-boron-bridged molecule BN3 exhibit similar photophysical properties. The unique combination of boron-induced coordination-enhanced charge transfer (CE-CT) and the helically twisted conjugated framework is pivotal in achieving the red-shifted, narrowband emission. X-ray crystallographic analysis of BN2 and BN3-a reveals that the extension of boron-bridged D-π-A skeletons significantly increases the distortion of the skeleton. Systematic theoretical calculations show how the boron CE-CT mechanism, in conjunction with the helical twist, leads to the narrowing of emission bands while simultaneously red-shifting them into the NIR region. This work could open new avenues for the development of advanced materials with tailored optical properties, particularly in the challenging and highly sought-after NIR spectrum.

Frontiers in fluorescence imaging: tools for the in situ sensing of disease biomarkers

Fluorescence imaging has been recognized as a powerful tool for the real-time detection and specific imaging of biomarkers within living systems, which is crucial for early diagnosis and treatment evaluation of major diseases. Over the years, significant advancements in this field have been achieved, particularly with the development of novel fluorescent probes and advanced imaging technologies such as NIR-II imaging, super-resolution imaging, and 3D imaging. These technologies have enabled deeper tissue penetration, higher image contrast, and more accurate detection of disease-related biomarkers. Despite these advancements, challenges such as improving probe specificity, enhancing imaging depth and resolution, and optimizing signal-to-noise ratios still remain. The emergence of artificial intelligence (AI) has injected new vitality into the designs and performances of fluorescent probes, offering new tools for more precise disease diagnosis. This review will not only discuss chemical modifications of classic fluorophores and in situ visualization of various biomarkers including metal ions, reactive species, and enzymes, but also share some breakthroughs in AI-driven fluorescence imaging, aiming to provide a comprehensive understanding of these advancements. Future prospects of fluorescence imaging for biomarkers including the potential impact of AI in this rapidly evolving field are also highlighted.

Perylene-Embedded Helical Nanographenes with Emission up to 1010 nm: Synthesis, Structures, and Chiroptical Properties

Near-infrared (NIR) circularly polarized absorbing or emitting materials offer distinct advantages over their visible-light counterparts and have attracted considerable interest across various fields. Materials exhibiting NIR chiroptical properties with high fluorescence quantum yields (ΦF) are particularly rare. In this study, we report the synthesis of a series of helical nanographenes (1, 2, 3, and 4), where perylene is fused with one to four hexa-peri-hexabenzocoronene (sub) units by a strategy involving Diels-Alder cycloaddition followed by a Scholl reaction. X-ray crystallographic analysis confirmed their structures, revealing helicene moieties integrated into a highly contorted framework. Benefiting from a similar distribution pattern of frontier molecular orbitals to perylene and extended π-conjugation, compounds 1-4 demonstrate respectable ΦF values of 31.9 %, 15.0 %, 13.7 %, and 6.5 %, respectively, with emission maxima reaching up to 1010 nm. Their enantiopure forms, isolated by preparative chiral HPLC, exhibit distinct circular dichroism signals and circularly polarized luminescence across a broad spectral range, extending from the ultraviolet to the NIR.

Aggregation control of anionic pentamethine cyanine enabling excitation wavelength selective NIR-II fluorescence imaging-guided photodynamic therapy

Near-infrared (NIR)-II fluorescence imaging-guided photodynamic therapy (PDT) has shown great potential for precise diagnosis and treatment of tumors in deep tissues; however, its performance is severely limited by the undesired aggregation of photosensitizers and the competitive relationship between fluorescence emission and reactive oxygen species (ROS) generation. Herein, we report an example of an anionic pentamethine cyanine (C5T) photosensitizer for high-performance NIR-II fluorescence imaging-guided PDT. Through the counterion engineering approach, a triphenylphosphine cation (Pco) modified with oligoethylene glycol chain is synthesized and adopted as the counterion of C5T, which can effectively suppress the excessive and disordered aggregation of the resulting C5T-Pco by optimizing the dye amphipathicity and enhancing the cyanine-counterion interactions. Dynamic tuning of fluorescence characteristics and ROS generation is achieved at the aggregate level, resulting in an impressive type I ROS generation under 760 nm light irradiation, accompanied by efficient NIR-II fluorescence emission excited at 808 nm. As a result, excitation wavelength selective NIR-II fluorescence imaging-guided PDT has been successfully demonstrated for tumor diagnosis and therapeutics of female mice.

Recent Advances in Indocyanine Green-Based Probes for Second Near-Infrared Fluorescence Imaging and Therapy

Fluorescence imaging, a highly sensitive molecular imaging modality, is being increasingly integrated into clinical practice. Imaging within the second near-infrared biological window (NIR-II; 1,000 to 1,700 nm), also referred to as shortwave infrared, has received substantial attention because of its markedly reduced autofluorescence, deeper tissue penetration, and enhanced spatiotemporal resolution as compared to traditional near-infrared (NIR) imaging. Indocyanine green (ICG), a US Food and Drug Administration-approved NIR fluorophore, has long been used in clinical applications, including blood vessel angiography, vascular perfusion monitoring, and tumor detection. Recent advancements in NIR-II imaging technology have revitalized interest in ICG, revealing its extended tail fluorescence beyond 1,000 nm and reaffirming its potential as a clinically translatable NIR-II fluorophore for in vivo imaging and theranostic applications for diagnosing various diseases. This review emphasizes the notable advances in the use of ICG and its derivatives for NIR-II imaging and image-guided therapy from both fundamental and clinical perspectives. We also provide a concise conclusion and discuss the challenges and future opportunities with NIR-II imaging using clinically approved fluorophores.

Emission Library and Applications of 2,1,3-Benzothiadiazole and Its Derivative-Based Luminescent Metal-Organic Frameworks

Organic linker-based luminescent metal-organic frameworks (LMOFs) have received extensive attention due to their promising applications in chemical sensing, energy transfer, solid-state-lighting and heterogeneous catalysis. Benefiting from the virtually unlimited emissive organic linkers and the intrinsic advantages of MOFs, significant progress has been made in constructing LMOFs with specific emission behaviors and outstanding performances. Among these reported organic linkers, 2,1,3-benzothiadiazole and its derivatives, as unique building units with tunable electron-withdrawing abilities, can be used to synthesize numerous emissive linkers with a donor-bridge-acceptor-bridge-donor type structure. These linkers were utilized to coordinate with different metal nodes, forming LMOFs with diverse underlying nets and optical properties. In this Minireview, 2,1,3-benzothiadiazole and its derivative-based organic linkers and their corresponding LMOFs are summarized with which an emission library is built between the linker structures and the emission behaviors of constructed LMOFs. In particular, the preparation of LMOFs with customized emission properties ranging from deep-blue to near-infrared and sizes from dozens to hundreds of nanometers is discussed in detail. The applications of these LMOFs, including chemical sensing, energy harvesting and transfer, and catalysis, are then highlighted. Key perspectives and challenges for the future development of LMOFs are also addressed.

Highly Efficient Near-Infrared Luminescent Radicals with Emission Peaks over 750 nm

Purely organic molecules exhibiting near-infrared (NIR) emission possess considerable potential for applications in both biological and optoelectronic technological domains, owing to their inherent advantages such as cost-effectiveness, biocompatibility, and facile chemical modifiability. However, the repertoire of such molecules with emission peaks exceeding 750 nm and concurrently demonstrating high photoluminescence quantum efficiency (PLQE) remains relatively scarce due to the energy gap law. Herein, we report two open-shell NIR radical emitters, denoted as DMNA-Cz-BTM and DMNA-PyID-BTM, achieved through the strategic integration of a donor group (DMNA) onto the Cz-BTM and PyID-BTM frameworks, respectively. We found that the donor-acceptor molecular structure allows the two designed radical emitters to exhibit a charge-transfer excited state and spatially separated electron and hole levels with non-bonding characteristics. Thus, the high-frequency vibrations are effectively suppressed. Besides, the reduction of low-frequency vibrations is observed. Collectively, the non-radiative decay channel is significantly suppressed, leading to exceptional NIR PLQE values. Specifically, DMNA-Cz-BTM manifests an emission peak at 758 nm alongside a PLQE of 55 %, whereas DMNA-PyID-BTM exhibits an emission peak at 778 nm with a PLQE of 66 %. Notably, these represent the pinnacle of PLQE among metal-free organic NIR emitters with emission peaks surpassing 750 nm.

Research Progress of Cyanine-Based Near-Infrared Fluorescent Probes for Biological Application

Cyanine-based near-infrared (NIR) fluorescent probes have played vital roles in biological application due to their low interference from background fluorescence, deep tissue penetration, high sensitivity, and minimal photodamage to biological samples. They are widely utilized in molecular recognition, medical diagnosis, biomolecular detection, and biological imaging. Herein, we provide a review of recent advancements in cyanine-based NIR fluorescent probes for the detection of pH, cells, tumor as well as their application in photothermal therapy (PTT) and photodynamic therapy (PDT).

Brightness Strategies toward NIR-II Emissive Conjugated Materials: Molecular Design, Application, and Future Prospects

Recent advances have been made in second near-infrared (NIR-II) fluorescence bioimaging and many related applications because of its advantages of deep penetration, high resolution, minimal invasiveness, and good dynamic visualization. To achieve high-performance NIR-II fluorescence bioimaging, various materials and probes with bright NIR-II emission have been extensively explored in the past few years. Among these NIR-II emissive materials, conjugated polymers and conjugated small molecules have attracted wide interest due to their native biosafety and tunable optical performance. This review summarizes the brightness strategies available for NIR-II emissive conjugated materials and highlights the recent developments in NIR-II fluorescence bioimaging. A concise, detailed overview of the molecular design and regulatory approaches is provided in terms of their high brightness, long wavelengths, and superior imaging performance. Then, various typical cases in which bright conjugated materials are used as NIR-II probes are introduced by providing step-by-step examples. Finally, the current problems and challenges associated with accessing NIR-II emissive conjugated materials for bright NIR-II fluorescence bioimaging are briefly discussed, and the significance and future prospects of these materials are proposed to offer helpful guidance for the development of NIR-II emissive materials.

Turn-On NIR-II Polymer Dots with Large Stokes Shift for In Vivo Visualizing Dynamical Brain Zinc in Alzheimer's Disease Mouse

It is a critical and broad prospect to evaluate ion levels and monitor their dynamic changes in the brain for early diagnosis, in-depth mechanism investigation, and accurate staging of neurodegenerative diseases including Alzheimer's disease (AD). It is still a great challenge to in vivo track Zn2+ levels in the brain by fluorescence imaging due to the drawbacks including short emission wavelength, poor selectivity and sensitivity, and unfavorable penetration across the blood-brain barrier (BBB) for currently developed fluorescent probes. We herein engineer a fluorescent probe with a large Stokes shift of 256 nm, NNDPTQ Pdots, which display substantial Zn2+-specific turn-on response in the NIR II region with the longest emission of 1064 nm up to now. The probe shows a fast response within seconds, high selectivity, low-nanomolar affinity of 6 nM, low detection limit of 3.4 nM, and efficient BBB-permeability efficacy of 37%. The results of brain imaging demonstrate that brain Zn2+ level in AD mice is substantially higher than normal mice and also is elevated with the prolonging of AD-bearing time. This study may provide a promising fluorescent indicator for in vivo tracing of brain Zn2+ levels to reveal AD pathogenesis.

Engineering of Kidney-Targeting Fluorophores with Tunable Emission from NIR-I to NIR-II for Early Diagnosis of Kidney Disease

The development of rapidly distributed and retained probes within the kidneys is important for accurately diagnosing kidney diseases. Although molecular imaging shows the potential for non-intrusively interrogating kidney disease-related biomarkers, the limited kidney contrast of many fluorophores, owing to their relatively low distribution in the kidney, hinders their effectiveness for kidney disease detection. Herein, for the first time, an amino-functionalization strategy is proposed to construct a library of kidney-targeting fluorophores NHcy with tunable emissions from NIR-I to NIR-II. Among these, NHcy-8 is the first small-molecule NIR-II dye without a renal clearance moiety, designed specifically for kidney-targeting imaging. Building on this class of NIR-II fluorophore, the first NIR-II small-molecule kidney-targeting pH probe NIR-II-pH is developed, which exhibits a desirable kidney distribution after intravenous injection and is fluorescent only after activation by acidosis. NIR-II in vivo fluorescence/photoacoustic imaging of kidney disease models induced by cisplatin and renal I/R injury using NIR-II-pH reveals increasingly severe metabolic acidosis as the disease progressed, enabling sensitive detection of the onset of acidosis 36 h (cisplatin group) earlier than clinical methods. Thus, this study introduces a practical NIR-II kidney-targeting probe and provides a useful molecular blueprint for guiding kidney-targeting NIR-II fluorophores as diagnostic aids for kidney diseases.

Recent Advances of Organic-Inorganic Hybrid Fluorescent Hyperbranched Polymer: Synthesis, Performance Regulation Strategies and Applications

The organic-inorganic hybrid fluorescent hyperbranched polymer, including hyperbranched polysiloxane and hyperbranched polyborate, have attracted much attention due to their excellent optical properties and wide range of applications. Hyperbranched polysiloxane and polyborates, prepared by introducing Si or B elements into organic polymer chains at the molecular level through rational molecular design and novel synthesis methods, exhibit outstanding photophysical properties as an indispensable branch of organic-inorganic hybrid fluorescent materials. Herein, this review highlights the recent research progress on hyperbranched polysiloxanes and hyperbranched polyborates, including strategies for regulating their emission wavelengths, quantum yields, and fluorescence lifetimes, potential emission mechanisms, and various applications. Finally, some challenges and promising future directions in the field of organic-inorganic hybrid fluorescent polymers are summarized.

Nanoprobe Based on Novel NIR-II Quinolinium Cyanine for Multimodal Imaging

NIR-II imaging has the advantages of high sensitivity, spatiotemporal resolution, and high penetration depth, thereby serving as a potential alternative to conventional imaging methods. Herein, a novel NIR-II dye IR-1010 (λex/λem = 1010/1058 nm) is reported with high quantum yield (3.08%) and good stability, by incorporating p-methoxyphenyl groups into a quinolinium cyanine dye. Then a multifunctional nanoprobe, termed IUFP NPs, is developed by the incorporation of upconversion (UC) nanoparticles (NPs), perfluoro-15-crown-5-ether (PFCE), and IR-1010, to display the novel performance of multimodal imaging. Under the single-wavelength excitation (980 nm), IUFP NPs simultaneously emit the NIR-II fluorescence of IR-1010 and visible UC luminescence of UCNPs, and thus realize the UC imaging for cells, and NIR-II fluorescence/photoacoustic/19F magnetic resonance imaging for blood vessels, lymph nodes and tumor in mice. This work affords a novel approach to NIR-II dyes and a general strategy for the design of multimodal imaging probes.

Suppressing ROS Production of AIE Nanoprobes by Simple Matrices Optimization for CNS Cell Observation and Minimized Influence of Cytoskeleton Morphology

The visualization of the central nervous system (CNS) has proposed stringent criteria for fluorescent probes, as the inevitable production of reactive oxygen species (ROS) or heat generated from most photoluminescent probes upon excitation can disturb the normal status of relatively delicate CNS cells. In this work, a red-emitting fluorogen with aggregation-induced emission (AIE) characteristics, known as DTF, was chosen as the model fluorogen to investigate whether the side effects of ROS and heat could be suppressed through easy-to-operate processes. Specifically, DTF was encapsulated with different amphiphilic matrices to yield AIE nanoprobes, and their photoluminescent properties, ROS production, and photothermal conversion rates were examined. BSA@DTF NPs possessed 1.3-fold brightness compared to that of DSPE-PEG@DTF NPs and F127@DTF NPs but its ROS generation efficiency is markedly decreased to only 2.4% of that produced by F127@DTF NPs. Meanwhile, BSA@DTF NPs showed a negligible photothermal effect. These features make BSA@DTF NPs favorable for long-term live cell imaging, particularly for fluorescent imaging of CNS cells. BSA@DTF NPs were able to sustain the normal state of HT-22 neuronal cells with continuous illumination for at least 25 min, and they also preserved the cytoskeleton of microglia BV-2 cells as the untreated control group. This work represents a successful but easy-to-operate process to suppress the ROS generation of red-emissive AIEgen, and it highlights the importance of minimizing the ROS generation of the fluorescent probes, particularly in the application of long-term imaging of CNS cells.

Planar-Twisted Molecular Engineering for Modulating the Fluorescence Brightness of NIR-II Fluorophores with a Donor-Acceptor-Donor Skeleton

Organic molecular fluorophores have been extensively utilized for biological imaging in the visible and the first near-infrared windows. However, their applications in the second near-infrared (NIR-II) window remain constrained, primarily due to the insufficient fluorescence brightness. Herein, we employ a theoretical protocol combining the thermal vibration correlation function with the time-dependent density functional theory method to investigate the mechanism of the planar-twisted strategy for developing fluorophores with balanced NIR-II emission and fluorescence brightness. Based on a planar donor-acceptor-donor molecular skeleton, various ortho-positioned alkyl side chains with steric hindrances are tactfully incorporated into the backbone to construct a series of twisted fluorophores. Photophysical characterizations of the studied fluorophores demonstrate that the emission spectra located in the NIR-II region exhibited a hypsochromic shift with the structural distortion. Notably, conformational twisting significantly accelerated the radiative decay rate while simultaneously suppressing the nonradiative decay rate, resulting in an improved fluorescence quantum efficiency (FQE). This enhancement can be mainly attributed to both the enlarged adiabatic excitation energy and reduced nonadiabatic electronic coupling between the first excited state and the ground state. Compared with the planar fluorophore, the twisted structures possessed a more than fivefold increase in FQE. In particular, the optimal twisted fluorophore BBTD-4 demonstrated a desirable fluorescence brightness (16.59 M-1 cm-1) on the premise of typical NIR-II emission (980 nm), making it a promising candidate for NIR-II fluorescence imaging in biomedical applications. The findings in this study elucidate the available experimental observations on the analogues, highlighting a feasible approach to modulating the photophysical performances of NIR-II chromophores for developing more highly efficient fluorophores toward optical imaging applications.

Light/X-ray/ultrasound activated delayed photon emission of organic molecular probes for optical imaging: mechanisms, design strategies, and biomedical applications

Conventional optical imaging, particularly fluorescence imaging, often encounters significant background noise due to tissue autofluorescence under real-time light excitation. To address this issue, a novel optical imaging strategy that captures optical signals after light excitation has been developed. This approach relies on molecular probes designed to store photoenergy and release it gradually as photons, resulting in delayed photon emission that minimizes background noise during signal acquisition. These molecular probes undergo various photophysical processes to facilitate delayed photon emission, including (1) charge separation and recombination, (2) generation, stabilization, and conversion of the triplet excitons, and (3) generation and decomposition of chemical traps. Another challenge in optical imaging is the limited tissue penetration depth of light, which severely restricts the efficiency of energy delivery, leading to a reduced penetration depth for delayed photon emission. In contrast, X-ray and ultrasound serve as deep-tissue energy sources that facilitate the conversion of high-energy photons or mechanical waves into the potential energy of excitons or the chemical energy of intermediates. This review highlights recent advancements in organic molecular probes designed for delayed photon emission using various energy sources. We discuss distinct mechanisms, and molecular design strategies, and offer insights into the future development of organic molecular probes for enhanced delayed photon emission.

Synthesis of D-A-type groups modified aza-BODIPY fluorescent dye encapsulated by amphiphilic polypeptide nanoparticles for NIR-II phototheranostics

An innovative organic small molecule with a D-A structure was synthesized by connecting triphenylamine to BODIPY via a thiophene bridge. Triphenylamine and thiophene units ingeniously modulate the balance between steric hindrance and π-π interactions around the flat aza-BODIPY core. The molecule exhibits near-infrared fluorescence absorption and emits at roughly 1100 nm, featuring a significant Stokes shift. Both the molecule and its nanoparticles demonstrate high stability and achieve a remarkable 35 % photothermal conversion efficiency when conjugated with the P(OEGMA)20-P(Asp)14 copolymer. In vitro assessments show low dark toxicity and outstanding biocompatibility. Moreover, in vivo studies and photothermal therapy in mice indicate substantial tumor shrinkage and reduced recurrence, confirming its potential in cancer treatment. These results highlight the promise of this organic molecule and its nanoparticles for NIR-II imaging-guided photothermal therapy, introducing a novel approach to phototheranostic applications for cancer management.

Short-Wave Infrared Upconverting Nanoparticles

Optical technologies enable real-time, noninvasive analysis of complex systems but are limited to discrete regions of the optical spectrum. While wavelengths in the short-wave infrared (SWIR) window (typically, 1700-3000 nm) should enable deep subsurface penetration and reduced photodamage, there are few luminescent probes that can be excited in this region. Here, we report the discovery of lanthanide-based upconverting nanoparticles (UCNPs) that efficiently convert 1740 or 1950 nm excitation to wavelengths compatible with conventional silicon detectors. Screening of Ln3+ ion combinations by differential rate equation modeling identifies Ho3+/Tm3+ or Tm3+ dopants with strong visible or NIR-I emission following SWIR excitation. Experimental upconverted photoluminescence excitation (U-PLE) spectra find that 10% Tm3+-doped NaYF4 core/shell UCNPs have the strongest 800 nm emission from SWIR wavelengths, while UCNPs with an added 2% or 10% Ho3+ show the strongest red emission when excited at 1740 or 1950 nm. Mechanistic modeling shows that addition of a low percentage of Ho3+ to Tm3+-doped UCNPs shifts their emission from 800 to 652 nm by acting as a hub of efficient SWIR energy acceptance and redistribution up to visible emission manifolds. Parallel experimental and computational analysis shows rate equation models are able to predict compositions for specific wavelengths of both excitation and emission. These SWIR-responsive probes open a new IR bioimaging window, and are responsive at wavelengths important for vision technologies.

Reengineering of Donor-Acceptor-Donor Structured Near-Infrared II Aggregation-Induced Emission Luminogens for Starving-Photothermal Antitumor and Inhibition of Lung Metastasis

Electron acceptor possessing strong electron-withdrawing ability and exceptional stability is crucial for developing donor-acceptor-donor (D-A-D) structured aggregation-induced emission luminogens (AIEgens) with second near-infrared (NIR-II) emission. Although 6,7-diphenyl-[1,2,5] thiadiazolo [3,4-g] quinoxaline (PTQ) and benzobisthiadiazole (BBT) are widely employed as NIR-II building blocks, they still suffer from limited electron-withdrawing capacity or inadequate chemo-stability under alkaline conditions. Herein, a boron difluoride formazanate (BFF) acceptor is utilized to construct NIR-II AIEgen, which exhibits a better overall performance in terms of NIR-II emission and chemo-stability compared to the PTQ- and BBT-derived fluorophores. With finely tuned intramolecular motions and strong D-A interaction strength, TPE-BFF simultaneously exhibits high molar extinction coefficient (ε= 4.31 × 104 M-1cm-1), strong NIR-II emission (Φ = 0.49%) and photothermal effect (η = 58.5%), as well as high stability. Thanks to these merits, the thermosensitive nanoparticles constructed by integrating TPE-BFF and the antiglycolytic agent 2-deoxy-d-glucose (2DG) are successfully utilized for imaging-guided photothermal antitumor lung metastasis by regulating glycolysis and reducing ATP-dependent heat shock proteins. Combining experimental results and theoretical calculations, BFF proves to be an outstanding electron acceptor for the design of versatile NIR-II AIEgens. Overall, this study offers a promising alternative for developing multifunctional NIR-II AIEgens in biomedical applications.

Facilely Achieving Near-Infrared-II J-Aggregates through Molecular Bending on a Donor-Acceptor Fluorophore for High-Performance Tumor Phototheranostics

Constructing J-aggregated organic dyes represents a promising strategy for obtaining biomedical second near-infrared (NIR-II) emissive materials, as they exhibit red-shifted spectroscopic properties upon assembly into nanoparticles (NPs) in aqueous environments. However, currently available NIR-II J-aggregates primarily rely on specific molecular backbones with intricate design strategies and are susceptible to fluorescence quenching during assembly. A facile approach for constructing bright NIR-II J-aggregates using prevalent donor-acceptor (D-A) molecules is still lacking. In this study, we present a facile method that transforms D-A molecules into J-aggregates by simply bending the molecule through introducing a methyl group, enabling high-performance NIR-II phototheranostics. The TAA-BT-CN molecule exhibits hypsochromic-shift absorption upon forming H-aggregated NPs, while the designed mTAA-BT-CN with a bent structure demonstrates a bathochromic shift of over 100 nm in absorption upon forming J-aggregated NPs, leading to much enhanced NIR-II emission beyond 1100 nm. With respect to its H-aggregated counterpart with the aggregation-caused quenching (ACQ) phenomenon, the J-aggregated mTAA-BT-CN NPs exhibit a 7-fold increase in NIR-II fluorescence owing to their aggregation-induced emission (AIE) property as well as efficient generation of heat and reactive oxygen species under 808 nm light excitation. Finally, the mTAA-BT-CN NPs are employed for whole-body blood vessel imaging using NIR-II technology as well as imaging-guided tumor phototherapies. This study will facilitate the flourishing advancement of J-aggregates based on prevalent D-A-type molecules.

Raising Near-Infrared Photoluminescence Quantum Yield of Au42 Quantum Rod to 50% in Solutions and 75% in Films

Highly emissive gold nanoclusters (NCs) in the near-infrared (NIR) region are of wide interest, but challenges arise from the excessive nonradiative dissipation. Here, we demonstrate an effective suppression of the motions of surface motifs on the Au42(PET)32 rod (PET = 2-phenylethanethiolate) by noncoordinative interactions with amide molecules and accordingly raise the NIR emission (875/1045 nm peaks) quantum yield (QY) from 18% to 50% in deaerated solution at room temperature, which is rare in Au NCs. Cryogenic photoluminescence measurements indicate that amide molecules effectively suppress the vibrations associated with the Au-S staple motifs on Au42 and also enhance the radiative relaxation, both of which lead to stronger emission. When Au42 NCs are embedded in a polystyrene film containing amide molecules, the PLQY is further boosted to 75%. This research not only produces a highly emissive material but also provides crucial insights for the rational design of NIR emitters and advances the potential of atomically precise Au NCs for diverse applications.

Recent advances in biomaterials based near-infrared mild photothermal therapy for biomedical application: A review

Mild photothermal therapy (MPTT) generates heat therapeutic effect at the temperature below 45 °C under near-infrared (NIR) irradiation, which has the advantages of controllable treatment efficacy, lower hyperthermia temperatures, reduced dosage, and minimized damage to surrounding tissues. Despite significant progress has been achieved in MPTT, it remains primarily in the stage of basic and clinical research and has not yet seen widespread clinical adoption. Herein, a comprehensive overview of the recent NIR MPTT development was provided, aiming to emphasize the mechanism and obstacles, summarize the used photothermal agents, and introduce various biomedical applications such as anti-tumor, wound healing, and vascular disease treatment. The challenges of MPTT were proposed with potential solutions, and the future development direction in MPTT was outlooked to enhance the prospects for clinical translation.