Ambient Oxygen-Doped Conjugated Polymer for pH-Activatable Aggregation-Enhanced Photoacoustic Imaging in the Second Near-Infrared Window

Activatable probes with potential for intraoperative tumor-specific fluorescence-imaging guided surgery

Owing to societal development and aging population, the impact of cancer on human health and quality of life has increased. Early detection and surgical treatment are the most effective approaches for most cancer patients. As the scope of conventional tumor resection is determined by auxiliary examination and surgeon experience, there is often insufficient recognition of tiny tumors. The ability to detect such tumors can be improved by using fluorescent tumor-specific probes for surgical navigation. This review mainly describes the design principles and mechanisms of activatable probes for the fluorescence imaging of tumors. This type of probe is nonfluorescent in normal tissue but exhibits obvious fluorescence emission upon encountering tumor-specific substrates, such as enzymes or bioactive molecules, or changes in the microenvironment, such as a low pH. In some cases, a single-factor response does not guarantee the effective fluorescence labeling of tumors. Therefore, two-factor-activatable fluorescence imaging probes that react with two specific factors in tumor cells have also been developed. Compared with single biomarker testing, the simultaneous monitoring of multiple biomarkers may provide additional insight into the role of these substances in cancer development and aid in improving the accuracy of early cancer diagnosis. Research and progress in this field can provide new methods for precision medicine and targeted therapy. The development of new approaches for early diagnosis and treatment can effectively improve the prognosis of cancer patients and help enhance their quality of life.

Tracking tumor heterogeneity and progression with near-infrared II fluorophores

Heterogeneous cells are the main feature of tumors with unique genetic and phenotypic characteristics, which can stimulate differentially the progression, metastasis, and drug resistance. Importantly, heterogeneity is pervasive in human malignant tumors, and identification of the degree of tumor heterogeneity in individual tumors and progression is a critical task for tumor treatment. However, current medical tests cannot meet these needs; in particular, the need for noninvasive visualization of single-cell heterogeneity. Near-infrared II (NIR-II, 1000-1700 nm) imaging exhibits an exciting prospect for non-invasive monitoring due to the high temporal-spatial resolution. More importantly, NIR-II imaging displays more extended tissue penetration depths and reduced tissue backgrounds because of the significantly lower photon scattering and tissue autofluorescence than traditional the near-infrared I (NIR-I) imaging. In this review, we summarize systematically the advances made in NIR-II in tumor imaging, especially in the detection of tumor heterogeneity and progression as well as in tumor treatment. As a non-invasive visual inspection modality, NIR-II imaging shows promising prospects for understanding the differences in tumor heterogeneity and progression and is envisioned to have the potential to be used clinically.

Polymerization in living organisms

Vital biomacromolecules, such as RNA, DNA, polysaccharides and proteins, are synthesized inside cells via the polymerization of small biomolecules to support and multiply life. The study of polymerization reactions in living organisms is an emerging field in which the high diversity and efficiency of chemistry as well as the flexibility and ingeniousness of physiological environment are incisively and vividly embodied. Efforts have been made to design and develop in situ intra/extracellular polymerization reactions. Many important research areas, including cell surface engineering, biocompatible polymerization, cell behavior regulation, living cell imaging, targeted bacteriostasis and precise tumor therapy, have witnessed the elegant demeanour of polymerization reactions in living organisms. In this review, recent advances in polymerization in living organisms are summarized and presented according to different polymerization methods. The inspiration from biomacromolecule synthesis in nature highlights the feasibility and uniqueness of triggering living polymerization for cell-based biological applications. A series of examples of polymerization reactions in living organisms are discussed, along with their designs, mechanisms of action, and corresponding applications. The current challenges and prospects in this lifeful field are also proposed.

Crosslinked albumin-manganese nanoaggregates with sensitized T1 relaxivity and indocyanine green loading for multimodal imaging and cancer phototherapy

Albumin-manganese-based nanocomposites (AMNs) characterized by simple preparation and good biocompatibility have been widely used for in vivo T₁-weighted magnetic resonance imaging (MRI) and cancer theranostics. Herein, an aggregation and crosslinking assembly strategy was proposed to achieve the sensitization to T₁ relaxivity of the albumin-manganese nanocomposite. At a relatively low Mn content (0.35%), the aggregation and crosslinking of bovine serum albumin-MnO₂ (BM) resulted in a dramatic increase of T₁ relaxivity from 5.49 to 67.2 mM^-1 s^-1. Upon the loading of indocyanine green (ICG) into the crosslinked BM nanoaggregates (C-BM), the T₁ relaxivity of the C-BM/ICG nanocomposite (C-BM/I) was further increased to 97.3 mM^-1 s^-1, which was much higher than those reported previously even at high Mn contents. Moreover, the presence of C-BM greatly enhanced the photoacoustic (PA) and photothermal effects of ICG at 830 and 808 nm, respectively, and the second near infrared fluorescence (NIR-II FL) of ICG also showed better stability. Therefore, the synthesized C-BM/ICG nanocomposite exhibited remarkable performance in in vivo multimodal imaging of tumors, such as T₁-weighted MRI, NIR-II FL imaging and PA imaging, and cancer phototherapy with little side effects. This work provided a highly efficient and promising multifunctional nanoprobe for breaking through the limits of cancer theranostics, and opened a new avenue for the development of high-relaxivity AMNs and multimodal imaging methodology.

Acoustic-Based Theranostic Probes Activated by Tumor Microenvironment for Accurate Tumor Diagnosis and Assisted Tumor Therapy

Acoustic-based imaging techniques, including ultrasonography and photoacoustic imaging, are powerful noninvasive approaches for tumor imaging owing to sound transmission facilitation, deep tissue penetration, and high spatiotemporal resolution. Usually, imaging modes were classified into "always-on" mode and "activatable" mode. Conventional "always-on" acoustic-based probes often have difficulty distinguishing lesion regions of interest from surrounding healthy tissues due to poor target-to-background signal ratios. As compared, activatable probes have attracted attention with improved sensitivity, which can boost or amplify imaging signals only in response to specific biomolecular recognition or interactions. The tumor microenvironment (TME) exhibits abnormal physiological conditions that can be used to identify tumor sections from normal tissues. Various types of organic dyes and biomaterials can react with TME, leading to obvious changes in their optical properties. The TME also affects the self-assembly or aggregation state of nanoparticles, which can be used to design activatable imaging probes. Moreover, acoustic-based imaging probes and therapeutic agents can be coencapsulated into one nanocarrier to develop nanotheranostic probes, achieving tumor imaging and cooperative therapy. Satisfactorily, ultrasound waves not only accelerate the release of encapsulated therapeutic agents but also activate therapeutic agents to exert or enhance their therapeutic performance. Meanwhile, various photoacoustic probes can convert photon energy into heat under irradiation, achieving photoacoustic imaging and cooperative photothermal therapy. In this review, we focus on the recently developed TME-triggered ultrasound and photoacoustic theranostic probes for precise tumor imaging and targeted tumor therapy.

Cathespin B-Initiated Cypate Nanoparticle Formation for Tumor Photoacoustic Imaging

Cathepsin B (CTSB) is a lysosomal protease that is overexpressed in the early stage of many cancer types. Precise evaluation of CTSB expression in vivo may provide a promising method for the early diagnosis of cancers. By virtue of the high-resolution PA imaging modality, a "smart" photoacoustic (PA) probe Cypate-CBT, which can self-assemble to cypate-containing nanoparticles in response to abundant GSH and CTSB inside tumor cells, was developed for the sensitive and specific detection of CTSB activity. Compared with unmodified Cypate, our probe Cypate-CBT exhibited a 4.9-fold or 4.7-fold PA signal enhancement in CTSB-overexpressing MDA-MB-231 cancer cells or tumors, respectively, revealing intracellular accumulation of the probe after CTSB-initiated self-assembly. We expect Cypate-CBT to be employed as an effective PA imaging agent for clinical diagnosis of cancer at early stages.

Activatable Second Near-Infrared Fluorescent Probes: A New Accurate Diagnosis Strategy for Diseases

Recently, second near-infrared (NIR-II) fluorescent imaging has been widely applied in biomedical diagnosis, due to its high spatiotemporal resolution and deep tissue penetration. In contrast to the "always on" NIR-II fluorescent probes, the activatable NIR-II fluorescent probes have specific targeting to biological tissues, showing a higher imaging signal-to-background ratio and a lower detection limit. Therefore, it is of great significance to utilize disease-associated endogenous stimuli (such as pH values, enzyme existence, hypoxia condition and so on) to activate the NIR-II probes and achieve switchable fluorescent signals for specific deep bioimaging. This review introduces recent strategies and mechanisms for activatable NIR-II fluorescent probes and their applications in biosensing and bioimaging. Moreover, the potential challenges and perspectives of activatable NIR-II fluorescent probes are also discussed.

Dual-Emission Carbonized Polymer Dots for Ratiometric pH Sensing, pH-Dependent Generation of Singlet Oxygen, and Imaging-Guided Dynamics Monitoring of Photodynamic Therapy

The pH environment in cancer cells has been demonstrated to display vital influences on the therapeutic effect of photodynamic therapy (PDT). It is very interesting to develop pH-responsive probes for simultaneous pH sensing and dynamics monitoring of the effects of PDT, and therefore assessing the correlation between them. In this study, a multifunctional fluorescence probe, dual-emission carbonized polymer dot (CPD) in blue and red regions, which uses ethylene imine polymer (PEI) and 4,4',4″,4‴-(porphine-5, 10, 15, 20-tetrayl) tetrakis (benzoic acid) (TCPP) as precursors through a one-step hydrothermal amide reaction, has been designed for ratiometric pH sensing, generating pH-dependent ¹O₂ for PDT of cancer cells, and investigating the dynamics effects of PDT through pH-guided imaging. The prepared CPDs were successfully used for ratiometric pH response, pH-dependent generation of ¹O₂, and dynamics monitoring PDT in HeLa cells. This study may provide an alternative strategy to prepare CPD-based theranostic integrated nanoprobes for PDT through the rational design of precursors.

Near infrared imaging of intracellular GSH by AuNCs@MnO2 core-shell nanoparticles based on the absorption competition mechanism

Dynamically monitoring intracellular glutathione (GSH), a crucial biomarker of oxidative stress, is of significance for the diagnosis and treatment of certain diseases. Although manganese dioxide (MnO2) based GSH fluorescent sensors have exhibited high sensitivity and good selectivity owing to the specific reactivity between GSH and MnO2, near-infrared (NIR) MnO2 based nanoprobes for GSH detection are scarce. Herein, we have developed a NIR activatable fluorescence nanoprobe for the imaging and determination of intracellular GSH based on a core-shell nanoparticle, consisting of NIR emitted gold nanocluster doped silica as the fluorescent core and manganese dioxide as the GSH-responsive shell (named AuNCs@MnO2). Due to the absorption competition mechanism, the outer MnO2 shell rather than the inner AuNCs core preferentially absorbed the excitation light, thus leading to fluorescence quenching of the inner AuNCs core. Upon addition of GSH, the fluorescence of the nanoprobe restored along with the reduction of MnO2 to Mn2+ because of the absorption competition disappearance-induced emission. The activatable fluorescence linearly increased upon changing the GSH concentration in the range of 2 to 5000 μM with a detection limit of 0.67 μM. The cytotoxicity test shows that the AuNCs@MnO2 nanoprobes have a good biocompatibility. After entering the cancer cells, the intracellular GSH degraded the outermost MnO2 shell and initiated the NIR fluorescence restoration of AuNCs, which can be used to monitor the dynamic change of intracellular GSH. This strategy provides an NIR-activatable way to detect GSH levels in living cells and offers a promising platform for the diagnosis and treatment of GSH-related diseases.