Semiconducting polymer nanoparticles as photoacoustic molecular imaging probes in living mice

Nanomaterial-based contrast agents

Medical imaging, which empowers the detection of physiological and pathological processes within living subjects, has a vital role in both preclinical and clinical diagnostics. Contrast agents are often needed to accompany anatomical data with functional information or to provide phenotyping of the disease in question. Many newly emerging contrast agents are based on nanomaterials as their high payloads, unique physicochemical properties, improved sensitivity and multimodality capacity are highly desired for many advanced forms of bioimaging techniques and applications. Here, we review the developments in the field of nanomaterial-based contrast agents. We outline important nanomaterial design considerations and discuss the effect on their physicochemical attributes, contrast properties and biological behaviour. We also describe commonly used approaches for formulating, functionalizing and characterizing these nanomaterials. Key applications are highlighted by categorizing nanomaterials on the basis of their X-ray, magnetic, nuclear, optical and/or photoacoustic contrast properties. Finally, we offer our perspectives on current challenges and emerging research topics as well as expectations for future advancements in the field.

Biomimetic Molybdenum Sulfide-Catalyzed Tumor Ferroptosis and Bioimaging

The chemical generation of singlet oxygen (¹ O₂ ) by the MoO₄ ^2- -catalyzed disproportionation of hydrogen peroxide (H₂ O₂ ) has been widely applied in numerous catalytic processes; however, such molybdate ions cannot be administered for redox-based cancer therapeutics. This work reports the albumin-mediated biomimetic synthesis of highly active molybdenum sulfide (denoted MoB) nanocatalysts that mediate the simultaneous generation of ¹ O₂ and superoxide anion (O₂ ^•- ) from H₂ O₂ , which is relatively abundant in malignant tumors. The MoB-catalyzed reactive oxygen species (ROS) are able to activate the ferroptosis pathway and cause lipid peroxidation for efficient cancer therapy. Furthermore, for the first time, the catalytic activity of MoB is visualized in situ. Moreover, a catalytic imaging modality based on MoB is developed for specific imaging of inflammation diseases without background interference. Therefore, this study presents a biomimetic strategy toward Mo-based nanocatalysts for ROS-facilitated tumor ferroptosis and catalytic imaging.

Tumor-targeting semiconducting polymer nanoparticles: efficient adjuvant photothermal therapy using ultra-low laser power inhibits recurrences after breast-conserving surgery

The need for adjuvant therapy to inhibit local recurrence after breast-conserving surgery with minimal side effects is great. Adjuvant photothermal therapy (aPTT) has the potential to replace radiotherapy and eliminates its inherent damage to healthy tissues. Herein, we functionalized semiconducting polymer nanoparticles (SPNs) with cRGD-peptide and silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (NIR775) to target breast cancer and perform aPTT under an ultra-low laser power (0.2 W cm^-2) after breast-conserving surgery (BCS). The synthesized RGD-SPN_NIR775 showed an excellent photothermal conversion efficiency and biocompatibility and was demonstrated to accumulate in tumors specifically. The BCS could be performed with confidence under the guidance of preoperative and postoperative fluorescence imaging. Notably, the aPTT completely inhibited the local recurrence after the BCS without compromising the cosmetic effect of the BCS. These results indicate the prospect of RGD-SPN_NIR775 as a theranostic nanoplatform for efficient aPTT using an ultra-low laser power to control recurrence after BCS.

Biomimetic Approach to Promote Cellular Uptake and Enhance Photoacoustic Properties of Tumor-Seeking Dyes

The attachment of glucose to drugs and imaging agents enables cancer cell targeting via interactions with GLUT1 overexpressed on the cell surface. While an added benefit of this modification is the solubilizing effect of carbohydrates, in the context of imaging agents, aqueous solubility does not guarantee decreased π-stacking or aggregation. The resulting broadening of the absorbance spectrum is a detriment to photoacoustic (PA) imaging since the signal intensity, accuracy, and image quality all rely on reliable spectral unmixing. To address this major limitation and further enhance the tumor-targeting ability of imaging agents, we have taken a biomimetic approach to design a multivalent glucose moiety (mvGlu). We showcase the utility of this new group by developing aza-BODIPY-based contrast agents boasting a significant PA signal enhancement greater than 11-fold after spectral unmixing. Moreover, when applied to targeting cancer cells, effective staining could be achieved with ultra-low dye concentrations (50 nM) and compared to a non-targeted analogue, the signal intensity was >1000-fold higher. Lastly, we employed the mvGlu technology to develop a logic-gated acoustogenic probe to detect intratumoral copper (i.e., Cu(I)), which is an emerging cancer biomarker, in a murine model of breast cancer. This exciting application was not possible using other acoustogenic probes previously developed for copper sensing.

Xanthene-Based Nitric Oxide-Responsive Nanosensor for Photoacoustic Imaging in the SWIR Window

Shortwave infrared (SWIR) dyes are characterized by their ability to absorb light from 900 to 1400 nm, which is ideal for deep tissue imaging owing to minimized light scattering and interference from endogenous pigments. An approach to access such molecules is to tune the photophysical properties of known near-infrared dyes. Herein, we report the development of a series of easily accessible (three steps) SWIR xanthene dyes based on a dibenzazepine donor conjugated to thiophene (SCR-1), thienothiophene (SCR-2), or bithiophene (SCR-3). We leverage the fact that SCR-1 undergoes a bathochromic shift when aggregated for in vivo studies by developing a ratiometric nanoparticle for NO (rNP-NO), which we employed to successfully visualize pathological levels of nitric oxide in a drug-induced liver injury model via deep tissue SWIR photoacoustic (PA) imaging. Our work demonstrates how easily this dye series can be utilized as a component in nanosensor designs for imaging studies.

Near-Infrared Phototheranostic Iron Pyrite Nanocrystals Simultaneously Induce Dual Cell Death Pathways via Enhanced Fenton Reactions in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is considered more aggressive with a poorer prognosis than other breast cancer subtypes. Through systemic bioinformatic analyses, we established the ferroptosis potential index (FPI) based on the expression profile of ferroptosis regulatory genes and found that TNBC has a higher FPI than non-TNBC in human BC cell lines and tumor tissues. To exploit this finding for potential patient stratification, we developed biologically amenable phototheranostic iron pyrite FeS2 nanocrystals (NCs) that efficiently harness near-infrared (NIR) light, as in photovoltaics, for multispectral optoacoustic tomography (MSOT) and photothermal ablation with a high photothermal conversion efficiency (PCE) of 63.1%. Upon NIR irradiation that thermodynamically enhances Fenton reactions, dual death pathways of apoptosis and ferroptosis are simultaneously triggered in TNBC cells, comprehensively limiting primary and metastatic TNBC by regulating p53, FoxO, and HIF-1 signaling pathways and attenuating a series of metabolic processes, including glutathione and amino acids. As a unitary phototheranostic agent with a safe toxicological profile, the nanocrystal represents an effective way to circumvent the lack of therapeutic targets and the propensity of multisite metastatic progression in TNBC in a streamlined workflow of cancer management with an integrated image-guided intervention.

Non-invasive molecular imaging for precision diagnosis of metastatic lymph nodes: opportunities from preclinical to clinical applications

Lymph node metastasis is an indicator of the invasiveness and aggressiveness of cancer. It is a vital prognostic factor in clinical staging of the disease and therapeutic decision-making. Patients with positive metastatic lymph nodes are likely to develop recurrent disease, distant metastasis, and succumb to death in the coming few years. Lymph node dissection and histological analysis are needed to detect whether regional lymph nodes have been infiltrated by cancer cells and determine the likely outcome of treatment and the patient's chances of survival. However, these procedures are invasive, and tissue biopsies are prone to sampling error. In recent years, advanced molecular imaging with novel imaging probes has provided new technologies that are contributing to comprehensive management of cancer, including non-invasive investigation of lymphatic drainage from tumors, identifying metastatic lymph nodes, and guiding surgeons to operate efficiently in patients with complex lesions. In this review, first, we outline the current status of different molecular imaging modalities applied for lymph node metastasis management. Second, we summarize the multi-functional imaging probes applied with the different imaging modalities as well as applications of cancer lymph node metastasis from preclinical studies to clinical translations. Third, we describe the limitations that must be considered in the field of molecular imaging for improved detection of lymph node metastasis. Finally, we propose future directions for molecular imaging technology that will allow more personalized treatment plans for patients with lymph node metastasis.

A Spiky Silver-Iron Oxide Nanoparticle for Highly Efficient Targeted Photothermal Therapy and Multimodal Imaging of Thrombosis

Thrombosis and its complications are responsible for 30% of annual deaths. Limitations of methods for diagnosing and treating thrombosis highlight the need for improvements. Agents that provide simultaneous diagnostic and therapeutic activities (theranostics) are paramount for an accurate diagnosis and rapid treatment. In this study, silver-iron oxide nanoparticles (AgIONPs) are developed for highly efficient targeted photothermal therapy and imaging of thrombosis. Small iron oxide nanoparticles are employed as seeding agents for the generation of a new class of spiky silver nanoparticles with strong absorbance in the near-infrared range. The AgIONPs are biofunctionalized with binding ligands for targeting thrombi. Photoacoustic and fluorescence imaging demonstrate the highly specific binding of AgIONPs to the thrombus when functionalized with a single chain antibody targeting activated platelets. Photothermal thrombolysis in vivo shows an increase in the temperature of thrombi and a full restoration of blood flow for targeted group but not in the non-targeted group. Thrombolysis from targeted groups is significantly improved (p < 0.0001) in comparison to the standard thrombolytic used in the clinic. Assays show no apparent side effects of AgIONPs. Altogether, this work suggests that AgIONPs are potential theranostic agents for thrombosis.

Recent progress in cancer cell membrane-based nanoparticles for biomedical applications

Immune clearance and insufficient targeting have limited the efficacy of existing therapeutic strategies for cancer. Toxic side effects and individual differences in response to treatment have further limited the benefits of clinical treatment for patients. Biomimetic cancer cell membrane-based nanotechnology has provided a new approach for biomedicine to overcome these obstacles. Biomimetic nanoparticles exhibit various effects (e.g., homotypic targeting, prolonging drug circulation, regulating the immune system, and penetrating biological barriers) after encapsulation by cancer cell membranes. The sensitivity and specificity of diagnostic methods will also be improved by utilizing the properties of cancer cell membranes. In this review, different properties and functions of cancer cell membranes are presented. Utilizing these advantages, nanoparticles can exhibit unique therapeutic capabilities in various types of diseases, such as solid tumors, hematological malignancies, immune system diseases, and cardiovascular diseases. Furthermore, cancer cell membrane-encapsulated nanoparticles show improved effectiveness and efficiency in combination with current diagnostic and therapeutic methods, which will contribute to the development of individualized treatments. This strategy has promising clinical translation prospects, and the associated challenges are discussed.

An autocatalytically-activatable hydrogen peroxide photoacoustic sensor for in situ visualization precise diagnosis and drug intervention tracing in diabetes syndrome

In situ visualization for the diagnosis of diabetic syndrome and visual monitoring the response to drug treatment is a challenge. Herein, we designed and prepared an autocatalytically-activatable hydrogen peroxide photoacoustic (PA) sensor. We first prepared the FeMoO_x nanoparticle with catalase activity, then combined it to 2,2'-azino-bis(3-ethylbenzothi-azoline-6-sulfonic acid) (ABTS) and distearoylphos-phoethanola-mine-polyethylene-glycol (DSPE-PEG) to construct a autocatalytically-activatable PA sensor (FeMoO_x@ABTS@DSPE-PEG). In its presence, ABTS can be converted into oxidized ABTS·⁺ by H₂O₂. ABTS·⁺ exhibits strong light absorption in the near-infrared region, and can serve as an ideal contrast agent for PA imaging. H₂O₂ as a biomarker of oxidative stress response is closely related to the occurrence and development of diabetes mellitus and its complications. Therefore, FeMoO_x@ABTS@DSPE-PEG was used as a PA sensor of H₂O₂ for visual monitoring of the progression of diabetes-induced liver injury and metformin-mediated treatment of diabetes. The autocatalytically-activatable PA sensor developed in this study provides a promising platform for in situ visual diagnosis of diabetes and its syndrome and monitoring the response to therapy.

Liver Injury Traceability: Spatiotemporally Monitoring Oxidative Stress Processes by Unit-Emitting Carbon Dots

Exploring the etiology of liver injury is critical to fundamental science and precise treatment, which has not yet been achieved by molecule imaging techniques. Herein, we manage to conquer this challenge by spatiotemporally monitoring oxidative stress processes using the proposed unit-emitting carbon dots (UE-C-dots) as fluorescent probes. We discover and reveal that the UE-C-dots can specifically determine hypochlorous acid (HClO) molecules, one of the important reactive oxygen/nitrogen species (ROS/RNS) in liver injury, by an excited state oxidation mechanism. Other ROS/RNS do not interfere with the assay even if their concentrations are 1000 times higher than that of HClO due to the lowest unoccupied molecular orbital level mismatch. Real-time tomographic imaging demonstrates that different stimuli cause distinctly different HClO bursts in both temporal and spatial dimensionalities. Therefore, the measurement and analysis of temporal information substantially extend our understanding on the relationships of hepatic oxidative stress and corresponding physiological/pathological behaviors.

In situ synthesized nanozyme for photoacoustic-imaging-guided photothermal therapy and tumor hypoxia relief

Nanozymes have attracted extensive research interest due to their ideal enzymatic catalytic performance; however, uncontrollable activities and nonspecific accumulation limit their further clinical application. To overcome these obstacles, we proposed in situ synthesized nanozyme, and realized the concept through an intelligent nanosystem (ISSzyme) based on Prussian blue (PB) precursor. PB nanozyme was synthesized at the tumor sites through the interaction of ISSzyme with glutathione, which was demonstrated by comparing with conventional PB nanozyme. ISSzyme is capable of tumor-specific photoacoustic imaging (PAI) and photothermal therapy (PTT), reducing the false-positive signals of PAI and the treatment side effects of PTT. ISSzyme has catalase-like activities, resulting in tumor hypoxia relief and metastasis inhibition. More importantly, the in situ synthesized PB nanozyme has the favorable property of minimal liver accumulation. Considering the above advantages, ISSzyme is expected to shed light on the design of the next-generation artificial enzymes, with many new biomedical applications.

Four-Photon Absorption Iron Complex for Magnetic Resonance/Photoacoustic Dual-Model Imaging and an Enhanced Ferroptosis Process

Four-photon absorption (4PA) multimodal therapeutic agent applied to tumor ferroptosis process tracking is rarely reported. In this paper, two functionalized terpyridine iron complexes (TD-FeCl₃, TD-Fe-TD) with four-photon absorption properties were designed and synthesized. The four-photon absorption cross sections of TD-FeCl₃ reached 6.87 × 10^-74cm⁸·s³·photon^-3. Due to its strong near-infrared absorption, TD-FeCl₃ has excellent photoacoustic imaging (PAI) capability for accurate PA imaging. TD-FeCl₃ has an efficient longitudinal electron relaxation rate (r₁ = 2.26 mM^-1 s^-1) and high spatial resolution, which can be applied as T₁-weighted magnetic resonance imaging (MRI) contrast agent for tumor imaging in vivo. In addition, Fe³⁺ as a natural ferroptosis tracer, TD-FeCl₃, is able to deplete glutathione (GSH) effectively, which can further enhance the ferroptosis process. We found that the series of cheap transition metal complexes has four-photon absorption activity and can be used as multimodal (MRI/PAI) diagnostic agents for tumor tracing processes.

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

Semiconducting Polymer Dots for Point-of-Care Biosensing and In Vivo Bioimaging: A Concise Review

In recent years, semiconducting polymer dots (Pdots) have attracted much attention due to their excellent photophysical properties and applicability, such as large absorption cross section, high brightness, tunable fluorescence emission, excellent photostability, good biocompatibility, facile modification and regulation. Therefore, Pdots have been widely used in various types of sensing and imaging in biological medicine. More importantly, the recent development of Pdots for point-of-care biosensing and in vivo imaging has emerged as a promising class of optical diagnostic technologies for clinical applications. In this review, we briefly outline strategies for the preparation and modification of Pdots and summarize the recent progress in the development of Pdots-based optical probes for analytical detection and biomedical imaging. Finally, challenges and future developments of Pdots for biomedical applications are given.

Recent Trend of Ultrasound-Mediated Nanoparticle Delivery for Brain Imaging and Treatment

In view of the fact that the blood-brain barrier (BBB) prevents the transport of imaging probes and therapeutic agents to the brain and thus hinders the diagnosis and treatment of brain-related disorders, methods of circumventing this problem (e.g., ultrasound-mediated nanoparticle delivery) have drawn much attention. Among the related techniques, focused ultrasound (FUS) is a favorite means of enhancing drug delivery via transient BBB opening. Photoacoustic brain imaging relies on the conversion of light into heat and the detection of ultrasound signals from contrast agents, offering the benefits of high resolution and large penetration depth. The extensive versatility and adjustable physicochemical properties of nanoparticles make them promising therapeutic agents and imaging probes, allowing for successful brain imaging and treatment through the combined action of ultrasound and nanoparticulate agents. FUS-induced BBB opening enables nanoparticle-based drug delivery systems to efficiently access the brain. Moreover, photoacoustic brain imaging using nanoparticle-based contrast agents effectively visualizes brain morphologies or diseases. Herein, we review the progress in the simultaneous use of nanoparticles and ultrasound in brain research, revealing the potential of ultrasound-mediated nanoparticle delivery for the effective diagnosis and treatment of brain disorders.

Rational Design of Asymmetric Polymethines to Attain NIR(II) Bioimaging at >1100 nm

Organic molecules having emission in the NIR(II) region are emergent and receiving enormous attention. Unfortunately, attaining accountable organic emission intensity around the NIR(II) region is hampered by the dominant internal conversion operated by the energy gap law, where the emission energy gap and the associated internal reorganization energy λ_int play key roles. Up to the current stage, the majority of the reported organic NIR(II) emitters belong to those polymethines terminated by two symmetric chromophores. Such a design has proved to have a small λ_int that greatly suppresses the internal conversion. However, the imposition of symmetric chromophores is stringent, limiting further development of organic NIR(II) dyes in diversity and versatility. Here, we propose a new concept where as far as the emissive state of the any asymmetric polymethines contains more or less equally transition density between two terminated chromophores, λ_int can be as small as that of the symmetric polymethines. To prove the concept, we synthesize a series of new polymethines terminated by xanthen-9-yl-benzoic acid and 2,4-diphenylthiopyrylium derivatives, yielding AJBF1112 and AEBF1119 that reveal emission peak wavelength at 1112 and 1119 nm, respectively. The quantum yield is higher than all synthesized symmetric polymethines of 2,4-diphenylthiopyrylium derivatives (SC1162, 1182, 1185, and 1230) in this study. λ_int were calculated to be as small as 6.2 and 7.3 kcal/mol for AJBF1112 and AEBF1119, respectively, proving the concept. AEBF1119 was further prepared as a polymer dot to demonstrate its in vitro specific cellular imaging and in vivo tumor/bone targeting in the NIR(II) region.

Implications of hydrogen sulfide in colorectal cancer: Mechanistic insights and diagnostic and therapeutic strategies

Hydrogen sulfide (H₂S) is an important signaling molecule in colorectal cancer (CRC). It is produced in the colon by the catalytic synthesis of the colonocytes' enzymatic systems and the release of intestinal microbes, and is oxidatively metabolized in the colonocytes' mitochondria. Both endogenous H₂S in colonic epithelial cells and exogenous H₂S in intestinal lumen contribute to the onset and progression of CRC. The up-regulation of endogenous synthetases is thought to be the cause of the elevated H₂S levels in CRC cells. Different diagnostic probes and combination therapies, as well as tumor treatment approaches through H₂S modulation, have been developed in recent years and have become active area of investigation for the diagnosis and treatment of CRC. In this review, we focus on the specific mechanisms of H₂S production and oxidative metabolism as well as the function of H₂S in the occurrence, progression, diagnosis, and treatment of CRC. We also discuss the present challenges and provide insights into the future research of this burgeoning field.

Bio-distribution of Carbon Nanoparticles Studied by Photoacoustic Measurements

Carbon-based nanomaterials are promising for a wide range of biomedical applications, i.e. drug delivery, therapy, and imaging including photoacoustic tomography, where they can serve as contrast agents, biocompatibility and biodistribution of which should be assessed before clinical setting. In this paper, localization of carbon flurooxide nanoparticles, carbon nanodots from β-alanine, carbon nanodots from urea and citric acid and glucose-ethylenediamine nanoparticles (NPs) in organs of Wistar rats were studied by photoacoustic measurements after 24 h of their intravenous injection. 16 ns light pulse from a Q-switched Nd:YAG laser with 1064 nm wavelength was used as an excitation source. The laser-induced photoacoustic signals were recorded with a ring piezoelectric detector. Light absorption by carbon NPs resulted in noticeable enhancement of the photoacoustic amplitude in the tissues where the NPs were accumulated. The NPs were preferably accumulated in liver, kidneys and spleen, and to a lesser extent in heart and gastrocnemius muscles. Together with remarkable fluorescent properties of the studied carbon nanomaterials, their photoacoustic responses allow their application for bi-modal fluorescence-photoacoustic bio-imaging.

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.

Nanostructured Graphdiyne: Synthesis and Biomedical Applications

Graphdiyne (GDY) is a 2D carbon allotrope that features a one-atom-thick network of sp- and sp²-hybridized carbon atoms with high degrees of π conjugation. Due to its distinct electronic, chemical, mechanical, and magnetic properties, GDY has attracted great attention and shown great potential in various fields, such as catalysis, energy storage, and the environment. Preparation of GDY with various nanostructures, including 0D quantum dots, 1D nanotubes/nanowires/nanoribbons, 2D nanosheets/nanowalls/ordered stripe arrays, and 3D nanospheres, greatly improves its function and has propelled its applications forward. High biocompatibility and stability make GDY a promising candidate for biomedical applications. This review introduces the latest developments in fabrication of GDY-based nanomaterials with various morphologies and summarizes their propective use in the biomedical domain, specifically focusing on their potential advantages and applications for biosensing, cancer diagnosis and therapy, radiation protection, and tissue engineering.

Nanoparticles for super-resolution microscopy: intracellular delivery and molecular targeting

Following an overview of the approaches and techniques used to acheive super-resolution microscopy, this review presents the advantages supplied by nanoparticle based probes for these applications. The various clases of nanoparticles that have been developed toward these goals are then critically described and these discussions are illustrated with a variety of examples from the recent literature.

Sound out the impaired perfusion: Photoacoustic imaging in preclinical ischemic stroke

Acoustically detecting the optical absorption contrast, photoacoustic imaging (PAI) is a highly versatile imaging modality that can provide anatomical, functional, molecular, and metabolic information of biological tissues. PAI is highly scalable and can probe the same biological process at various length scales ranging from single cells (microscopic) to the whole organ (macroscopic). Using hemoglobin as the endogenous contrast, PAI is capable of label-free imaging of blood vessels in the brain and mapping hemodynamic functions such as blood oxygenation and blood flow. These imaging merits make PAI a great tool for studying ischemic stroke, particularly for probing into hemodynamic changes and impaired cerebral blood perfusion as a consequence of stroke. In this narrative review, we aim to summarize the scientific progresses in the past decade by using PAI to monitor cerebral blood vessel impairment and restoration after ischemic stroke, mostly in the preclinical setting. We also outline and discuss the major technological barriers and challenges that need to be overcome so that PAI can play a more significant role in preclinical stroke research, and more importantly, accelerate its translation to be a useful clinical diagnosis and management tool for human strokes.

Organic persistent luminescence imaging for biomedical applications

Persistent luminescence is a unique visual phenomenon that occurs after cessation of excitation light irradiation or following oxidization of luminescent molecules. The energy stored within the molecule is released in a delayed manner, resulting in luminescence that can be maintained for seconds, minutes, hours, or even days. Organic persistent luminescence materials (OPLMs) are highly robust and their facile modification and assembly into biocompatible nanostructures makes them attractive tools for in vivo bioimaging, whilst offering an alternative to conventional fluorescence imaging materials for biomedical applications. In this review, we give attention to the existing limitations of each class of OPLM-based molecular bioimaging probes based on their luminescence mechanisms, and how recent research progress has driven efforts to circumvent their shortcomings. We discuss the multifunctionality-focused design strategies, and the broad biological application prospects of these molecular probes. Furthermore, we provide insights into the next generation of OPLMs being developed for bioimaging techniques.

Aggregation-induced emission nanoparticles with improved optical absorption for boosting fluorescence signal of tumors in vivo

Nanomaterial development has been extensively investigated for several decades to realize sensitive and accurate imaging of tumors in vivo. The manufacturing of nanoparticles with highly efficient tumor targeting and excellent optical properties is still an important research topic. The structure and composition ratio of materials that decisively contribute to the brightness and size of nanoparticles have a great influence on image sensitivity and tumor targeting efficiency. In this study, we developed aggregation-induced emission (AIE) nanoparticles with a widened light absorption window (nanoPMeOCN/BDP) to enable sensitive in vivo tumor imaging. The signal of nanoparticles is enhanced by integrating a high-density AIE polymer (PMeOCN) and light-absorbing fluorescent dye (BDP) in a nanoscopic space. BDP not only improves the light absorption of particles but also enhances the fluorescence signal of particles by effectively transferring absorbed energy to PMeOCN. The physically blended nanoPMeOCN/BDP show strong light absorption and improved sensitivity for the imaging of biological tissues because of their excellent optical performance compared to nanoPMeOCN of similar nanosizes (∼19 nm in size). In vivo imaging results further confirm that nanoPMeOCN/BDP can provide amplified signals with the successful accumulation of tumor tissue through the enhanced permeability and retention effect. We expect that the design strategy of nanoparticles with improved light absorption will provide a simple and general method for improving the accuracy of disease diagnosis.

Dual-targeting nanozyme for tumor activatable photo-chemodynamic theranostics

Abstract

Tumor phototheranostics holds a great promise on account of its high spatiotemporal resolution, tumor-specificity, and noninvasiveness. However, physical limitation of light penetration and “always on” properties of conventional photothermal-conversion agents usually cause difficulty in accurate diagnosis and completely elimination of tumor. Meanwhile, nanozymes mediated Fenton reactions can well utilize the tumor microenvironment (TME) to generate hydroxyl radicals for chemodynamic therapy (CDT), but limited by the concentration of H₂O₂ in TME and the delivery efficiency of nanozymes. To overcome these problems, a dual-targeting nanozyme (FTRNPs) is developed for tumor-specific in situ theranostics, based upon the assembling of ultrasmall Fe₃O₄ nanoparticles, 3,3’,5,5’-tetrameth-ylbenzidine (TMB) and the RGD peptide. The FTRNPs after H₂O₂ treatment exhibits superior photothermal stability and high photothermal conversion efficiency (η = 50.9%). FTRNPs shows extraordinary accumulation and retention in the tumor site by biological/physical dual-targeting, which is 3.54-fold higher than that without active targeting. Cascade-dual-response to TME for nanozymes mediated Fenton reactions and TMB oxidation further improves the accuracy of both photoacoustic imaging and photothermal therapy (PTT). The tumor inhibition rate of photo-chemodynamic therapy is ~ 97.76%, which is ~ 4-fold higher than that of PTT or CDT only. Thus, the combination of CDT and PTT to construct “turn on” nanoplatform is of great significance to overcome their respective limitations. Considering its optimized “all-in-one” performance, this new nanoplatform is expected to provide an advanced theranostic strategy for the future treatment of cancers.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01662-9.

Real-time optical oximetry during FLASH radiotherapy using a phosphorescent nanoprobe

The rapid depletion of oxygen during irradiation at ultra-high dose rate calls for tissue oximeters capable of high temporal resolution. This study demonstrates a water-soluble phosphorescent nanoprobe and fiber-coupled instrument, which together are used to measure the kinetics of oxygen depletion at 200 Hz during irradiation of in vitro solutions.

Optimized Reconstruction Procedure of Photoacoustic Imaging for Reflection Artifacts Reduction

Photoacoustic (PA) imaging technology is of some value in medical diagnoses such as breast cancer detection, vasculature imaging, and surgery navigating. While as most imaging objects are bounded, the received RF signals consist of the direct-arrived signals (DAS) from the PA sources and the boundary-reflected signals (BRS). The undesired BRS will severely impair the quality during the image reconstruction. They will bring in many artifacts and confuse the actual shape and location of the PA sources. We improved the reconstruction procedure by removing the BRS before the regular reconstruction process to suppress those artifacts. To verify our proposed method, we compared the results of the conventional and optimized procedures experimentally. In terms of qualitative observation, the reconstructed images by the optimized procedure illustrate fewer artifacts and more accurate shapes of the PA sources. To quantitatively evaluate the traditional and the optimized imaging procedure, we calculated the Distribution Relative Error (DRE) between each experiment result and its standard drawing of the phantoms. For both phantoms and the ex-vivo sample, the DREs of reconstruction result by the optimized reconstruction procedure decrease significantly. The results suggest that the optimized reconstruction process can effectively suppress the reflection artifacts and improve the shape accuracy of the PA sources.

Photoacoustic Imaging Probes for Theranostic Applications

Photoacoustic imaging (PAI), an emerging biomedical imaging technology, capitalizes on a wide range of endogenous chromophores and exogenous contrast agents to offer detailed information related to the functional and molecular content of diseased biological tissues. Compared with traditional imaging technologies, PAI offers outstanding advantages, such as a higher spatial resolution, deeper penetrability in biological tissues, and improved imaging contrast. Based on nanomaterials and small molecular organic dyes, a huge number of contrast agents have recently been developed as PAI probes for disease diagnosis and treatment. Herein, we report the recent advances in the development of nanomaterials and organic dye-based PAI probes. The current challenges in the field and future research directions for the designing and fabrication of PAI probes are proposed.

Conjugated Polymer Nanoparticles as a Universal High-Affinity Probe for the Selective Detection of Microplastics

Microplastic (MP) pollution is a global challenge that requires immediate mitigation practices. Monitoring is crucial for quantifying MPs, but their mitigation remains very challenging due to several factors, including the lack of selective materials to specific polymers, and the low sensitivity of the current detection techniques. In this work, we introduce a novel design for the selective detection of MPs through fluorescence spectroscopy by exploiting conjugated polymer nanoparticles (CPNs). Fluorescent diketopyrrolopyrrole nanoparticles were prepared by nanoprecipitation to incorporate peripheral hyaluronic acid to increase their affinity for various plastics. The affinity of the new ligand for various types of MPs was examined through several characterization techniques, including fluorescence spectroscopy and microscopy, nanoparticle tracking analysis and computational studies. The new CPN were shown to be highly fluorescent in the presence of typically abundant MPs, achieving very strong binding constants in the picomolar range. This very strong affinity for a broad family of plastics was found to be the results of cooperative supramolecular effects and topographical affinity, as probed by advanced microscopy and in silico studies. Furthermore, the new affinity probes were shown to be highly selective for MPs, allowing for their detection in heterogeneous samples, including soil debris and other organic contaminants. The new materials design introduced in this work constitute a promising platform for the development of novel MP detection devices directly useable at the point of collection. Moreover, it opens new avenue for the mitigation of this environmental hazard through tailorable materials.

New Degradable Semiconducting Polymers for Photoacoustic Imaging of λ-Carrageenan-Induced Arthritis Mouse Model

Semiconducting polymer has a high extinction coefficient and a long band absorption and can be used as a photoacoustic imaging contrast agent. However, nonbiodegradable semiconducting polymers may cause biosafety issues due to being retained in the body. Therefore, developing degradable semiconducting polymers is necessary for in vivo imaging. Herein, we developed three degradable semiconducting polymers with unique optical properties. We adjusted the optical properties of semiconducting polymers by designing the molecular structure of semiconducting polymers. Polymers with a donor-π-acceptor structure could easily improve the optical properties through adjusting the donor or acceptor units. Through adjusting the electron-donor and -acceptor units, three diketopyrrolopyrrole derivative polymers (DPPTz, DPPQu, and DPPWu) were synthesized and converted into nanosize particles. By introducing the degradable chemical groups in the main chain structure of semiconducting polymers, diketopyrrolopyrrole polymers could be degraded by ClO^-. Among these nanosize particles, DPPTz NPs and DPPQu NPs were used to achieve the in vivo photoacoustic imaging of λ-carrageenan-induced arthritis mouse model. This work provides a novel design idea for the designing of red-shifted semiconducting polymer with degradable properties.

NIR-II AIEgens with Photodynamic Effect for Advanced Theranostics

Phototheranostics that concurrently integrates accurate diagnosis (e.g., fluorescence and photoacoustic (PA) imaging) and in situ therapy (e.g., photodynamic therapy (PDT) and photothermal therapy (PTT)) into one platform represents an attractive approach for accelerating personalized and precision medicine. The second near-infrared window (NIR-II, 1000-1700 nm) has attracted considerable attention from both the scientific community and clinical doctors for improved penetration depth and excellent spatial resolution. NIR-II agents with a PDT property as well as other functions are recently emerging as a powerful tool for boosting the phototheranostic outcome. In this minireview, we summarize the recent advances of photodynamic NIR-II aggregation-induced emission luminogens (AIEgens) for biomedical applications. The molecular design strategies for tuning the electronic bandgaps and photophysical energy transformation processes are discussed. We also highlight the biomedical applications, such as image-guided therapy of both subcutaneous and orthotopic tumors, and multifunctional theranostics in combination with other treatment methods, including chemotherapy and immunotherapy; and the precise treatment of both tumor and bacterial infection. This review aims to provide guidance for PDT agents with long-wavelength emissions to improve the imaging precision and treatment efficacy. We hope it will provide a comprehensive understanding about the chemical structure-photophysical property-biomedical application relationship of NIR-II luminogens.

The sound of drug delivery: Optoacoustic imaging in pharmacology

Optoacoustic (photoacoustic) imaging offers unique opportunities for visualizing biological function in vivo by achieving high-resolution images of optical contrast much deeper than any other optical technique. The method detects ultrasound waves that are generated inside tissue by thermo-elastic expansion, i.e., the conversion of light absorption by tissue structures to ultrasound when the tissue is illuminated by the light of varying intensity. Listening instead of looking to light offers the major advantage of image formation with a resolution that obeys ultrasonic diffraction and not photon diffusion laws. While the technique has been widely used to explore contrast from endogenous photo-absorbing molecules, such as hemoglobin or melanin, the use of exogenous agents can extend applications to a larger range of biological and possible clinical applications, such as image-guided surgery, disease monitoring, and the evaluation of drug delivery, biodistribution, and kinetics. This review summarizes recent developments in optoacoustic agents, and highlights new functions visualized and potent pharmacology applications enabled with the use of external contrast agents.

The role of imaging in targeted delivery of nanomedicine for cancer therapy

Nanomedicines overcome the pharmacokinetic limitations of traditional drug formulations and have promising prospect in cancer treatment. However, nanomedicine delivery in vivo is still facing challenges from the complex physiological environment. For the purpose of effective tumor therapy, they should be designed to guarantee the five features principle, including long blood circulation, efficient tumor accumulation, deep matrix penetration, enhanced cell internalization and accurate drug release. To ensure the excellent performance of the designed nanomedicine, it would be better to monitor the drug delivery process as well as the therapeutic effects by real-time imaging. In this review, we summarize strategies in developing nanomedicines for efficiently meeting the five features of drug delivery, and the role of several imaging modalities (fluorescent imaging (FL), magnetic resonance imaging (MRI), computed tomography (CT), photoacoustic imaging (PAI), positron emission tomography (PET), and electron microscopy) in tracing drug delivery and therapeutic effect in vivo based on five features principle.

Recent advances in small molecule dye-based nanotheranostics for NIR-II photoacoustic imaging-guided cancer therapy

Photoacoustic (PA) imaging in the second near-infrared (NIR-II) window has gained more and more attention in recent years and showed great potential in the field of bioimaging. Until now, numerous materials have been developed as contrast agents for NIR-II PA imaging. Among them, small molecule dyes hold unique advantages such as definite structures and capability of fast clearance from body. By virtue of these advantages, small molecule dyes-constructed nanoparticles have relatively small size and show promise in the clinical translation. Thus, in this minireview, we summarize recent advances in small molecule dyes-based nanotheranostics for NIR-II PA imaging and cancer therapy. Studies about NIR-II PA imaging-guided phototherapy are first introduced. Then, NIR-II PA imaging-guided phototherapy-based combination therapeutic systems are reviewed. Finally, the conclusion and perspectives of this field are summarized and discussed.

Biocompatible and Photostable Photoacoustic Contrast Agents as Nanoparticles Based on Bodipy Scaffold and Polylactide Polymers: Synthesis, Formulation, and In Vivo Evaluation

We have designed a new Bodipy scaffold for efficient in vivo photoacoustic (PA) imaging of nanoparticles commonly used as drug nanovectors. The new dye has an optimized absorption band in the near-infrared window in biological tissue and a low fluorescence quantum yield that leads to a good photoacoustic generation efficiency. After Bodipy-initiated ring-opening polymerization of lactide, the polylactide-Bodipy was formulated into PEGylated nanoparticles (NPs) by mixing with PLA-PEG at different concentrations. Formulated NPs around 100 nm exhibit excellent PA properties: an absorption band at 760 nm and a molar absorption coefficient in between that of molecular PA absorbers and gold NPs. Highly improved photostability compared to cyanine-labeled PLA NPs as well as innocuity in cultured macrophages were demonstrated. After intravenous injection in healthy animals, NPs were easily detected using a commercial PA imaging system and spectral unmixing, opening the way to their use as theranostic agents.

Reactive Species-Activatable AIEgens for Biomedical Applications

Precision medicine requires highly sensitive and specific diagnostic strategies with high spatiotemporal resolution. Accurate detection and monitoring of endogenously generated biomarkers at the very early disease stage is of extensive importance for precise diagnosis and treatment. Aggregation-induced emission luminogens (AIEgens) have emerged as a new type of excellent optical agents, which show great promise for numerous biomedical applications. In this review, we highlight the recent advances of AIE-based probes for detecting reactive species (including reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), and reactive carbonyl species (RCS)) and related biomedical applications. The molecular design strategies for increasing the sensitivity, tuning the response wavelength, and realizing afterglow imaging are summarized, and theranostic applications in reactive species-related major diseases such as cancer, inflammation, and vascular diseases are reviewed. The challenges and outlooks for the reactive species-activatable AIE systems for disease diagnostics and therapeutics are also discussed. This review aims to offer guidance for designing AIE-based specifically activatable optical agents for biomedical applications, as well as providing a comprehensive understanding about the structure-property application relationships. We hope it will inspire more interesting researches about reactive species-activatable probes and advance clinical translations.

Impairing proliferation of glioblastoma multiforme with CD44+ selective conjugated polymer nanoparticles

Glioblastoma is one of the most aggressive types of cancer with success of therapy being hampered by the existence of treatment resistant populations of stem-like Tumour Initiating Cells (TICs) and poor blood-brain barrier drug penetration. Therapies capable of effectively targeting the TIC population are in high demand. Here, we synthesize spherical diketopyrrolopyrrole-based Conjugated Polymer Nanoparticles (CPNs) with an average diameter of 109 nm. CPNs were designed to include fluorescein-conjugated Hyaluronic Acid (HA), a ligand for the CD44 receptor present on one population of TICs. We demonstrate blood-brain barrier permeability of this system and concentration and cell cycle phase-dependent selective uptake of HA-CPNs in CD44 positive GBM-patient derived cultures. Interestingly, we found that uptake alone regulated the levels and signaling activity of the CD44 receptor, decreasing stemness, invasive properties and proliferation of the CD44-TIC populations in vitro and in a patient-derived xenograft zebrafish model. This work proposes a novel, CPN- based, and surface moiety-driven selective way of targeting of TIC populations in brain cancer.