Rational Design of a Small Molecular Near-Infrared Fluorophore for Improved In Vivo Fluorescence Imaging.

The near-infrared (NIR) fluorescence imaging modality has great potential for application in biomedical imaging research owing to its unique characteristics, such as low tissue autofluorescence and noninvasive visualization with high spatial resolution. Although a variety of NIR fluorophores are continuously reported, the commercially available NIR fluorophores are still limited, owing to complex synthetic processes and poor physicochemical properties. To address this issue, a small molecular NIR fluorophore (SMF800) was designed and developed in the present work to improve in vivo target-specific fluorescence imaging. After conjugation with pamidronate (PAM) and bovine serum albumin (BSA), the SMF800 conjugates exhibited successful in vivo targeting in bone and tumor tissues with low background uptake, respectively. The improved in vivo performance of the SMF800 conjugate demonstrated that the small molecular NIR fluorophore SMF800 can be widely used in a much broader range of imaging applications. The structure of SMF800, which was developed by considering two important physicochemical properties, water solubility and conjugatability, is first introduced. Therefore, this work suggests a simple and rational approach to design small, hydrophilic, and conjugatable NIR fluorophores for targeted bioimaging.

Anti-Quenching NIR-II J-Aggregates of Benzo[c]thiophene Fluorophore for Highly Efficient Bioimaging and Phototheranostics.

Molecular fluorophores with the second near-infrared (NIR-II) emission hold great potential for deep-tissue bioimaging owing to their excellent biocompatibility and high resolution. Recently, J-aggregates are used to construct long-wavelength NIR-II emitters as their optical bands show remarkable red shifts upon forming water-dispersible nano-aggregates. However, their wide applications in the NIR-II fluorescence imaging are impeded by the limited varieties of J-type backbone and serious fluorescence quenching. Herein, a bright benzo[c]thiophene (BT) J-aggregate fluorophore (BT6) with anti-quenching effect is reported for highly efficient NIR-II bioimaging and phototheranostics. The BT fluorophores are manipulated to have Stokes shift over 400 nm and aggregation-induced emission (AIE) property for conquering the self-quenching issue of the J-type fluorophores. Upon forming BT6 assemblies in an aqueous environment, the absorption over 800 nm and NIR-II emission over 1000 nm are boosted for more than 41 and 26 folds, respectively. In vivo visualization of the whole-body blood vessel and imaging-guided phototherapy results verify that BT6 NPs are excellent agent for NIR-II fluorescence imaging and cancer phototheranostics. This work develops a strategy to construct bright NIR-II J-aggregates with precisely manipulated anti-quenching properties for highly efficient biomedical applications.

A Precipitation-Enhanced Emission (PEE) Strategy for Increasing the Brightness and Reducing the Liver Retention of NIR-II Fluorophores.

The lack of organic fluorophores with high quantum yields (QYs) and low liver retention in the second near-infrared (NIR-II) window has become a bottleneck in the bioimaging field. An approach to address these problems is proposed by encapsulating phosphorylated fluorescent dyes into biodegradable calcium phosphate nanoparticles. First, an NIR-II molecule, LJ-2P, is designed with increased water solubility by introducing two phosphate groups. Meanwhile, LJ-2P co-precipitates with calcium ions to form LJ-2P nanoparticles (NPs). The QYs of LJ-2P NPs in aqueous solution is increased by 36.57-fold to 5.12% compared with that of LJ-2P. This unique phenomenon is named as precipitation-enhanced emission (PEE), whose detailed mechanism is explored by femtosecond transient absorption. It is demonstrated that co-precipitation of LJ-2P with calcium ions changes the micro-environment, which restricts the molecular rotation and reduces the interaction of water molecules, especially the excited-state proton transfer. In addition, due to the pH-sensitive nature, more than 80% of the LJ-2P NPs are metabolized in the liver within 24 h. Based on the excellent optical properties and good biocompatibility, high-contrast vascular visualization and breast tumor detecting are achieved. This strategy can apply to other NIR-II fluorophores to achieve high QYs and low liver retention.

Small Molecular NIR-II Fluorophores for Cancer Phototheranostics.

Phototheranostics integrates deep-tissue imaging with phototherapy (containing photothermal therapy and photodynamic therapy), holding great promise in early diagnosis and precision treatment of cancers. Recently, second near-infrared (NIR-II) fluorescence imaging exhibits the merits of high accuracy and specificity, as well as real-time detection. Among the NIR-II fluorophores, organic small molecular fluorophores have shown superior properties in the biocompatibility, variable structure, and tunable emission wavelength than the inorganic NIR-II materials. What's more, some small molecular fluorophores also display excellent cytotoxicity when illuminated with the NIR laser. This review summarizes the progress of small molecular NIR-II fluorophores with different central cores for cancer phototheranostics in the past few years, focusing on the molecular structures and phototheranostic performances. Furthermore, challenges and prospects of future development toward clinical translation are discussed.