Renal‐clearable Molecular Semiconductor for Second Near‐Infrared Fluorescence Imaging of Kidney Dysfunction

Development of a novel chitosan-based two-photon fluorescent nanoprobe with enhanced stability for the specific detection of endogenous H2O2

Hydrogen peroxide (H₂O₂) acts as an important reactive oxygen species (ROS) and maintains the redox equilibrium in organisms. Imbalance of H₂O₂ concentration is associated with the development of many diseases. Traditional small molecular based fluorescent probes often show drawbacks of cytotoxicity and easily metabolic clearance. Herein, a chitosan-based two-photon fluorescent nanoprobe (DC-BI) was constructed and applied for H₂O₂ detection in live organisms. DC-BI was composed by chitosan nanoparticles and a two-photon fluorophore of naphthalimide analogues (BI) with H₂O₂-responsive property. The structure of DC-BI was characterized by NMR, FTIR, XPS, XRD, DLS and MLS analyses. As study shown, the nanoprobe DC-BI exhibited improved distribution stability and smaller cytotoxicity. In the presence of H₂O₂, both the absorption and emission spectra show dramatic changes, the fluorescence intensity at 580 nm obviously enhanced. Furthermore, fluorescence imaging results indicate that DC-BI is capable of imaging endogenous H₂O₂ in cells and zebrafish. The design and development of chitosan-based nanoprobe DC-BI has provided a general example of nanoprobe construction with excellent distribution stability, two-photon property, and biocompatibility.

An Activatable NIR-II Fluorescent Reporter for In Vivo Imaging of Amyloid-β Plaques

Fluorescence imaging in the second near-infrared (NIR-II) window holds great promise for in vivo visualization of amyloid-β (Aβ) pathology, which can facilitate characterization and deep understanding of Alzheimer's disease (AD); however, it has been rarely exploited. Herein, we report the development of NIR-II fluorescent reporters with a donor-π-acceptor (D-π-A) architecture for specific detection of Aβ plaques in AD-model mice. Among all the designed probes, DMP2 exhibits the highest affinity to Aβ fibrils and can specifically activate its NIR-II fluorescence after binding to Aβ fibrils via suppressed twisted intramolecular charge transfer (TICT) effect. With suitable lipophilicity for ideal blood-brain barrier (BBB) penetrability and deep-tissue penetration of NIR-II fluorescence, DMP2 possesses specific detection of Aβ plaques in in vivo AD-model mice. Thus, this study presents a potential agent for non-invasive imaging of Aβ plaques and deep deciphering of AD progression.

Increasing Molecular Planarity through Donor/Side-Chain Engineering for Improved NIR-IIa Fluorescence Imaging and NIR-II Photothermal Therapy under 1064 nm

Developing conjugated small molecules (CSM) with intense NIR-II (1000-1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high-performance NIR-II CSM for phototheranostic by tailoring molecular planarity. A series of CSM show bathochromic absorption extended to the NIR-II region upon the increasing thiophene number, but an excessive number of thiophene results in decreased NIR-IIa (1300-1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR-II absorption coefficient, NIR-IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to molecular planarity stemming from the collective contribution of donor/side-chain engineering. This finding directs the design of NIR-II CSM by rational manipulating molecular planarity to perform 1064 nm mediated phototheranostic at high efficiency.

A Renal-Clearable Activatable Molecular Probe for Fluoro-Photacoustic and Radioactive Imaging of Cancer Biomarkers

In vivo simultaneous visualization of multiple biomarkers is critical to accurately diagnose disease and decipher fundamental processes at a certain pathological evolution, which however is rarely exploited. Herein, a multimodal activatable imaging probe (P-¹²⁵ I) is reported with activatable fluoro-photoacoustic and radioactive signal for in vivo imaging of biomarkers (i.e., hepsin and prostate-specific membrane antigen (PSMA)) associated with prostate cancer diagnosis and prognosis. P-¹²⁵ I contains a near-infrared (NIR) dye that is caged with a hepsin-cleavable peptide sequence and linked with a radiolabeled PSMA-targeted ligand (PSMAL). After systemic administration, P-¹²⁵ I actively targets the tumor site via specific recognition between PSMA and PSMAL moiety and in-situ generates of activated fluoro-photoacoustic signal after reacting with hepsin to release the free dye (uncaged state). P-¹²⁵ I achieves precisely early detection of prostate cancer and renal clearance to alleviate toxicity issues. In addition, the accumulated radioactive and activated photoacoustic signal of probe correlates well with the respective expression level of PSMA and hepsin, which provides valuable foreseeability for cancer progression and prognosis. Thus, this study presents a multimodal activatable probe for early detection and in-depth deciphering of prostate cancer.

A Bright, Renal-Clearable NIR-II Brush Macromolecular Probe with Long Blood Circulation Time for Kidney Disease Bioimaging

Early detection of kidney disease is of vital importance due to its current prevalence worldwide. Fluorescence imaging, especially in the second near-infrared window (NIR-II) has been regarded as a promising technique for the early diagnosis of kidney disease due to the superior resolution and sensitivity. However, the reported NIR-II organic renal-clearable probes are hampered by their low brightness (ϵmax Φf>1000 nm <10 M-1  cm-1 ) and limited blood circulation time (t1/2 <2 h), which impede the targeted imaging performance. Herein, we develop the aza-boron-dipyrromethene (aza-BODIPY) brush macromolecular probes (Fudan BDIPY Probes (FBP 912)) with high brightness (ϵmax Φf>1000 nm ≈60 M-1  cm-1 ), which is about 10-fold higher than that of previously reported NIR-II renal-clearable organic probes. FBP 912 exhibits an average diameter of ≈4 nm and high renal clearance efficiency (≈65 % excretion through the kidney within 12 h), showing superior performance for non-invasively diagnosis of renal ischemia-reperfusion injury (RIR) earlier than clinical serum-based protocols. Additionally, the high molecular weight polymer brush enables FBP 912 with prolonged circulation time (t1/2 ≈6.1 h) and higher brightness than traditional PEGylated renal-clearable control fluorophores (t1/2 <2 h), facilitating for 4T1 tumor passive targeted imaging and renal cell carcinoma active targeted imaging with higher signal-to-noise ratio and extended retention time.

Donor/π-Bridge Manipulation for Constructing a Stable NIR-II Aggregation-Induced Emission Luminogen with Balanced Phototheranostic Performance*

Owing to their versatile functionality and tunable energy dissipation, aggregation-induced emission luminogens (AIEgens) have emerged as a potential platform for multimodal theranostics. Nevertheless, the construction of AIE-active phototheranostic agents in the second near-infrared window (NIR-II, 1000-1700 nm), which allows superior resolution and minimized photodamage, is still a formidable challenge. Herein, benzo[c]thiophene serves as an electron-rich and bulky donor (D)/π-bridge, which can enlarge the conjugation length and distort the backbone of an AIEgen. By precise D/π-bridge engineering, highly stable NIR-II AIEgen DPBTA-DPTQ nanoparticles are obtained with acceptable NIR-II fluorescence quantum yield and excellent photothermal conversion efficiency. In addition, the spatial conformation of DPBTA-DPTQ is determined for the first time by X-ray single crystal diffraction and theoretical simulations. DPBTA-DPTQ NPs have good biocompatibility and show efficient photothermal therapeutic effects in in vitro tests. Furthermore, DPBTA-DPTQ NPs were used in fluorescence-photoacoustic-photothermal trimodal imaging-guided photothermal eradication of tumors in HepG2 and B16-F10 tumor-xenografted mice.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

NIR-II Fluorescent Brightness Promoted by "Ring Fusion" for the Detection of Intestinal Inflammation

Fluorophores with emission in the second near-infrared window (NIR-II) have displayed salient advantages for biomedical applications. However, the common strategy of reducing the energy bandgap of fluorophores so as to achieve red-shifted wavelengths always leads to compromised fluorescent brightness. Herein, we propose a molecular design concept of "ring-fusion" to modify the acceptor of AIEgen that can extend the luminous wavelength from NIR-I to NIR-II. The fused-acceptor-containing fluorophore yielded, TTQP, has an enhanced absorption coefficient with a higher brightness in nanoparticle formation compared to its NIR-I emissive counterpart (TTQ-DP) with a non-fused acceptor. Theoretical calculation further confirms that the ring fusion can efficiently promote the rigidity and planarity of the electron-deficient core, leading to a lower reorganization energy and nonradiative decay. The TTQP NPs yielded thus allow sensitive NIR-II fluorescence imaging of vasculature and intestinal inflammation in mice models. Therefore, we anticipate that our work will provide a promising molecular-engineering strategy to enrich the library and broaden the application scope of NIR-II fluorophores.

Azide-Dye Unexpected Bone Targeting for Near-Infrared Window II Osteoporosis Imaging

Azide is an important chemical functional group and has been widely used in chemical biology. However, the impact of azide on the in vivo behaviors of compounds has been rarely studied. Herein, azide was introduced into a fluorescent dye for the near-infrared window two (NIR-II) bone imaging. Specifically, we designed and synthesized the small-molecule NIR-II dyes, N₃-FEP-4T capped with azide and FEP-4T without azide capping. In vitro assays revealed that N₃-FEP-4T showed 5- and 5.6- times higher hydroxyapatite accumulation and macrophage uptake than those of FEP-4T, respectively. Moreover, N₃-FEP-4T displayed higher bone uptakes and much better bone NIR-II imaging quality, demonstrating the specific bone-targeting ability of the azide-containing probe. N₃-FEP-4T was then further successfully used for osteoporosis NIR-II imaging. Overall, our study provides insights into the impact of azide on the in vivo behavior of azide-containing compounds and opens a new window for biological application of azide.

Enhanced Ultrasound Contrast of Renal-Clearable Luminescent Gold Nanoparticles

Renal-clearable nanoparticles are typically fast eliminated through the free glomerular filtration, which show weak interaction with the renal compartments and negligible ultrasound signals, raising challenges in direct imaging of kidney diseases. Here, we report the ultrasmall renal-clearable luminescent gold nanoparticles (AuNPs) with both pH-induced charge reversal and aggregation properties, and discover that enhanced ultrasound contrast could be facilely acquired through the increased tubular reabsorption and in situ aggregation of AuNPs in renal tubule cells in injured kidneys. The tuning elimination pathway of the renal-clearable luminescent AuNPs is further demonstrated to provide a synergistical fluorescence and ultrasound imaging strategy for diagnosing early kidney injury with precise anatomical information.

An NIR-II-Emissive Photosensitizer for Hypoxia-Tolerant Photodynamic Theranostics

As a common feature in a majority of malignant tumors, hypoxia has become the Achilles' heel of photodynamic therapy (PDT). The development of type-I photosensitizers that show hypoxia-tolerant PDT efficiency provides a straightforward way to address this issue. However, type-I PDT materials have rarely been discovered. Herein, a π-conjugated molecule with A-D-A configuration, COi6-4Cl, is reported. The H₂ O-dispersible nanoparticle of COi6-4Cl can be activated by an 880 nm laser, and displays hypoxia-tolerant type I/II combined PDT capability, and more notably, a high NIR-II fluorescence with a quantum yield over 5%. Moreover, COi6-4Cl shows a negligible photothermal conversion effect. The non-radiative decay of COi6-4Cl is suppressed in the dispersed and aggregated state due to the restricted molecular vibrations and distinct intermolecular steric hindrance induced by its four bulky side chains. These features make COi6-4Cl a distinguished single-NIR-wavelength-activated phototheranostic material, which performs well in NIR-II fluorescence-guided PDT treatment and shows an enhanced in vivo anti-tumor efficiency over the clinically approved Chlorin e6, by the equal stresses on hypoxia-tolerant anti-tumor therapy and deep-penetration imaging. Therefore, the great potential of COi6-4Cl in precise PDT cancer therapy against hypoxia challenges is demonstrated.

Activatable Molecular Probes for Second Near-Infrared Fluorescence, Chemiluminescence, and Photoacoustic Imaging

Optical imaging plays a crucial role in biomedicine. However, due to strong light scattering and autofluorescence in biological tissue between 650-900 nm, conventional optical imaging often has a poor signal-to-background ratio and shallow penetration depth, which limits its ability in deep-tissue in vivo imaging. Second near-infrared fluorescence, chemiluminescence, and photoacoustic imaging modalities mitigate these issues by their respective advantages of minimized light scattering, eliminated external excitation, and ultrasound detection. To enable disease detection, activatable molecular probes (AMPs) with the ability to change their second near-infrared fluorescence, chemiluminescence, or photoacoustic signals in response to a biomarker have been developed. This Minireview summarizes the molecular design strategies, sensing mechanisms, and imaging applications of AMPs. The potential challenges and perspectives of AMPs in deep-tissue imaging are also discussed.

A Renal-Clearable Macromolecular Reporter for Near-Infrared Fluorescence Imaging of Bladder Cancer

Bladder cancer (BC) is a prevalent disease with high morbidity and mortality; however, in vivo optical imaging of BC remains challenging because of the lack of cancer-specific optical agents with high renal clearance. Herein, a macromolecular reporter (CyP1) was synthesized for real-time near-infrared fluorescence (NIRF) imaging and urinalysis of BC in living mice. Because of the high renal clearance (ca. 94 % of the injection dosage at 24 h post-injection) and its cancer biomarker (APN=aminopeptidase N) specificity, CyP1 can be efficiently transported to the bladder and specially turn on its NIRF signal to report the detection of BC in living mice. Moreover, CyP1 can be used for optical urinalysis, permitting the ex vivo tracking of tumor progression for therapeutic evaluation and easy translation of CyP2 as an in vitro diagnostic assay. This study not only provides new opportunities for non-invasive diagnosis of BC, but also reveals useful guidelines for the development of molecular reporters for the detection of bladder diseases.