High-Resolution 3D NIR-II Photoacoustic Imaging of Cerebral and Tumor Vasculatures Using Conjugated Polymer Nanoparticles as Contrast Agent

Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part I: Materials, devices and selected applications

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part I of this two part review, firstly, active and passive nanomaterials and nanostructures for acoustics are presented. Following that, representative applications of nanoacoustics including material property characterization, nanomaterial/nanostructure manipulation, and sensing, are discussed in detail. Finally, a summary is presented with point of views on the current challenges and potential solutions in this burgeoning field.

NIR-II Fluorescent Biodegradable Nanoprobes for Precise Acute Kidney/Liver Injury Imaging and Therapy

NIR-II fluorescent nanoprobes based on inorganic materials, including rare-earth-doped nanoparticles, single-walled carbon nanotubes, CdS quantum dots (QDs), gold nanoclusters, etc., have gained growing interest in bioimaging applications. However, these nanoprobes are usually not biodegradable and lack therapeutic functions. Herein, we developed novel NIR-II fluorescence (FL) imaging and therapeutic nanoprobes based on black phosphorus QDs (BPQDs), which exhibited excellent biodegradability and high tunability of size-dependent optical properties. By adjusting the size of nanoparticles, BPQDs can specifically accumulate in the kidney or liver. Importantly, a low dosage of BPQDs can effectively protect tissues from reactive oxygen species (ROS)-mediated damage in acute kidney and liver injury, which was real-time monitored by responsive NIR-II fluorescence imaging. Overall, we developed novel NIR-II emitting and therapeutic BPQDs with excellent biodegradability vivo, providing a promising candidate for NIR-II FL imaging and ROS scavenging.

Dual-model wearable photoacoustic microscopy and electroencephalograph: study of neurovascular coupling in anesthetized and freely moving rats

Observing microscale neurovascular dynamics under different physiological conditions is of great importance to understanding brain functions and disorders. Here, we report a dual-model wearable device and an auxiliary data processing algorithm to derive neurovascular dynamics. The device integrates high-resolution photoacoustic microscopy and electroencephalography (EEG), which allows observing capillary-level hemodynamics and neural activities in anesthesia and freely moving rats. By using the developed algorithm, multiple photoacoustic/EEG parameters extracted and correlated enables investigation of the interplay between neural and vascular activities. We employed this platform to study the neurovascular coupling during different types of seizures in rats under various physiological conditions. We observed cerebral vascular vasodilation/constriction corresponding well to the seizure on/off in rats under regular anesthesia conditions, showing a strong neurovascular coupling coefficient. In rats under weak anesthesia and freely moving conditions, more intense cerebral hemodynamics and neural activities occurred with a weaker neurovascular coupling coefficient. The comprehensively quantitative analyses suggest that anesthesia has a dominant impact on the seizure onset and affect the neurovascular coupling correlation in the current drug-induced localized seizure model. Our study reveals that the designed platform has the potential to support studies on brain functions and disorders in diseased rodent models in various physiological states.

Conjugated Polymers: Optical Toolbox for Bioimaging and Cancer Therapy

Conjugated polymers (CPs) are capable of coordinating the electron coupling phenomenon to bestow powerful optoelectronic features. The light-harvesting and light-amplifying properties of CPs are extensively used in figuring out the biomedical issues with special emphasis on accurate diagnosis, effective treatment, and precise theranostics. This review summarizes the recent progress of CP materials in bioimaging, cancer therapeutics, and introduces the design strategies by rationally tuning the optical properties. The recent advances of CPs in bioimaging applications are first summarized and the challenges to clear the future directions of CPs in the respective area are discussed. In the following sections, the focus is on the burgeoning applications of CPs in phototherapy of the tumor, and illustrates the underlying photo-transforming mechanism for further molecular designing. Besides, the recent progress in the CPs-assistant drug therapy, mainly including drug delivery, gene therapeutic, the optical-activated reversion of tumor resistance, and synergistic therapy has also been discussed elaborately. In the end, the potential challenges and future developments of CPs on cancer diagnosis and therapy are also illuminated for the improvement of optical functionalization and the promotion of clinical translation.

Optical Imaging in the Second Near Infrared Window for Vascular Bioimaging

Optical imaging in the second near infrared region (NIR-II, 1000-1700 nm) provides higher resolution and deeper penetration depth for accurate and real-time vascular anatomy, blood dynamics, and function information, effectively contributing to the early diagnosis and curative effect assessment of vascular anomalies. Currently, NIR-II optical imaging demonstrates encouraging results including long-term monitoring of vascular injury and regeneration, real-time feedback of blood perfusion, tracking of lymphatic metastases, and imaging-guided surgery. This review summarizes the latest progresses of NIR-II optical imaging for angiography including fluorescence imaging, photoacoustic (PA) imaging, and optical coherence tomography (OCT). The development of current NIR-II fluorescence, PA, and OCT probes (i.e., single-walled carbon nanotubes, quantum dots, rare earth doped nanoparticles, noble metal-based nanostructures, organic dye-based probes, and semiconductor polymer nanoparticles), highlighting probe optimization regarding high brightness, longwave emission, and biocompatibility through chemical modification or nanotechnology, is first introduced. The application of NIR-II probes in angiography based on the classification of peripheral vascular, cerebrovascular, tumor vessel, and cardiovascular, is then reviewed. Major challenges and opportunities in the NIR-II optical imaging for vascular imaging are finally discussed.

Multimodal Contrast Agents for Optoacoustic Brain Imaging in Small Animals

Optoacoustic (photoacoustic) imaging has demonstrated versatile applications in biomedical research, visualizing the disease pathophysiology and monitoring the treatment effect in an animal model, as well as toward applications in the clinical setting. Given the complex disease mechanism, multimodal imaging provides important etiological insights with different molecular, structural, and functional readouts in vivo. Various multimodal optoacoustic molecular imaging approaches have been applied in preclinical brain imaging studies, including optoacoustic/fluorescence imaging, optoacoustic imaging/magnetic resonance imaging (MRI), optoacoustic imaging/MRI/Raman, optoacoustic imaging/positron emission tomography, and optoacoustic/computed tomography. There is a rapid development in molecular imaging contrast agents employing a multimodal imaging strategy for pathological targets involved in brain diseases. Many chemical dyes for optoacoustic imaging have fluorescence properties and have been applied in hybrid optoacoustic/fluorescence imaging. Nanoparticles are widely used as hybrid contrast agents for their capability to incorporate different imaging components, tunable spectrum, and photostability. In this review, we summarize contrast agents including chemical dyes and nanoparticles applied in multimodal optoacoustic brain imaging integrated with other modalities in small animals, and provide outlook for further research.

Second near-infrared (NIR-II) imaging: a novel diagnostic technique for brain diseases

Imaging in the second near-infrared II (NIR-II) window, a kind of biomedical imaging technology with characteristics of high sensitivity, high resolution, and real-time imaging, is commonly used in the diagnosis of brain diseases. Compared with the conventional visible light (400-750 nm) and NIR-I (750-900 nm) imaging, the NIR-II has a longer wavelength of 1000-1700 nm. Notably, the superiorities of NIR-II can minimize the light scattering and autofluorescence of biological tissue with the depth of brain tissue penetration up to 7.4 mm. Herein, we summarized the main principles of NIR-II in animal models of traumatic brain injury, cerebrovascular visualization, brain tumor, inflammation, and stroke. Simultaneously, we encapsulated the in vivo process of NIR-II probes and their in vivo and in vitro toxic effects. We further dissected its limitations and following optimization measures.

Monitoring Metabolites Using an NAD(P)H-sensitive Polymer Dot and a Metabolite-Specific Enzyme

We introduce an NAD(P)H-sensitive polymer dot (Pdot) biosensor for point-of-care monitoring of metabolites. The Pdot is combined with a metabolite-specific NAD(P)H-dependent enzyme that catalyzes the oxidation of the metabolite, generating NAD(P)H. Upon UV illumination, the NAD(P)H quenches the fluorescence emission of Pdot at 627 nm via electron transfer, and also fluoresces at 458 nm, resulting in a shift from red to blue emission at higher NAD(P)H concentrations. Metabolite concentration is quantified ratiometrically-based on the ratio of blue-to-red channel emission intensities, with a digital camera-with high sensitivity and specificity. We demonstrate phenylalanine biosensing in human plasma for a phenylketonuria screening test, quantifying several other disease-related metabolites (lactate, glucose, glutamate, and β-hydroxybutyrate), and a paper-based assay with smartphore imaging for point-of-care use.

Nanoprobe-mediated precise imaging and therapy of glioma

Gliomas are the most common primary brain tumors in adults, accounting for 80% of primary intracranial tumors. Due to the heterogeneous and infiltrating nature of malignant gliomas and the hindrance of the blood-brain barrier (BBB), it is very difficult to accurately image and differentiate the malignancy grade of gliomas, thus significantly influencing the diagnostic accuracy and subsequent surgery or therapy. In recent years, the rapid development of emerging nanoprobes has provided a promising opportunity for the diagnosis and treatment of gliomas. After rational component regulation and surface modification, functional nanoprobes could efficiently cross the BBB, target gliomas, and realize single-modal or multimodal imaging of gliomas with high clarity. Moreover, these contrast nanoagents could also be conjugated with therapeutic drugs and cure cancerous tissues at the same time. Herein, we focus on the design strategies of nanoprobes for effective crossing of the BBB, and introduce the recent advances in the precise imaging and therapy of gliomas using functional nanoprobes. Finally, we also discuss the challenges and future directions of nanoprobe-based diagnosis and treatment of gliomas.

A high-contrast photoacoustic agent with near-infrared emission

Benzobisthiadiazole as a typical electron acceptor, has been widely used to design fluorescent dyes and photoacoustic (PA) agents. With the strategy of constructing donor-acceptor-donor (D-A-D) type of electron characteristics, benzobisthiadiazole derivatives tend to behave stable in near-infrared absorption and emission, which is beneficial to PA imaging. In this chapter, two molecular design strategies are combined to improve the photoacoustic imaging effects of new PA contrast agent IR-1302 NPs, by installing strengthened conjugated bridges and electron donors. The nanoparticles exhibit high-contrast noninvasive photoacoustic imaging in tumor models with longer wavelength absorption and emission and show potential as a clinic contrast agent.

Nanoparticles for Stem Cell Tracking and the Potential Treatment of Cardiovascular Diseases

Stem cell-based therapies have been shown potential in regenerative medicine. In these cells, mesenchymal stem cells (MSCs) have the ability of self-renewal and being differentiated into different types of cells, such as cardiovascular cells. Moreover, MSCs have low immunogenicity and immunomodulatory properties, and can protect the myocardium, which are ideal qualities for cardiovascular repair. Transplanting mesenchymal stem cells has demonstrated improved outcomes for treating cardiovascular diseases in preclinical trials. However, there still are some challenges, such as their low rate of migration to the ischemic myocardium, low tissue retention, and low survival rate after the transplantation. To solve these problems, an ideal method should be developed to precisely and quantitatively monitor the viability of the transplanted cells in vivo for providing the guidance of clinical translation. Cell imaging is an ideal method, but requires a suitable contrast agent to label and track the cells. This article reviews the uses of nanoparticles as contrast agents for tracking MSCs and the challenges of clinical use of MSCs in the potential treatment of cardiovascular diseases.

NIR-II Absorbing Semiconducting Polymer-Triggered Gene-Directed Enzyme Prodrug Therapy for Cancer Treatment

Exploration of facile strategies for precise regulation of target gene expression remains highly challenging in the development of gene therapies. Especially, a stimuli-responsive nanocarrier integrated with ability of noninvasive remote control for treating wide types of cancers is rarely developed. Herein, a NIR-II absorbing semiconducting polymer (PBDTQ) is employed to remotely activate the heat-inducible heat-shock protein 70 (HSP70) promoter under laser irradiation, further realizing regulation of gene-directed enzyme prodrug therapy (GDEPT) for cancer treatment in mild hyperthermia. In this multifunctional nanocomposite, the PBDTQ and double suicide gene plasmid (pSG) based on HSP70 promoter are incorporated into a lipid complex. Upon NIR-II laser excitation, the mild photothermal effect (≈43 °C) generated from PBDTQ can cause the release of pSG and activation of HSP70 promoter, and then upregulate suicide gene expression triggered by the HSP70 promoter which can further convert the nontoxic prodrug into its cytotoxic metabolites. Therefore, this work demonstrates a universal NIR-II laser-triggered GDEPT using semiconducting polymers as the photothermal generator for cancer treatment with minimized collateral damage and nontargeted side effects.

Efficient and precise delivery of microRNA by photoacoustic force generated from semiconducting polymer-based nanocarriers

One major challenge in miRNA-based therapy is to explore facile delivery strategies, which can facilitate the efficient and precise accumulation of intrinsically instable microRNAs (miRNAs) at targeted tumor sites. To address this critical issue, for the first time we demonstrate that a near-infrared (NIR) pulse laser can guide efficient delivery of miRNAs mediated by a NIR-absorbing and photoacoustic active semiconducting polymer (SP) nanocarrier, which can generate photoacoustic radiation force to intravascularly overcome the endothelial barriers. Importantly, we demonstrate an ultrafast delivery of miRNA (miR-7) to tumor tissues under the irradiation of pulse laser in 20 min, showing a 5-fold boosted efficiency in comparison to the traditional passive targeting strategy. The delivered miR-7 acts as a sensitizer of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and synergizes with TRAIL-inducing compound (TIC), leading to sustained TRAIL upregulation for effective tumor suppression in mice. As such, our results indicate that the NIR-absorbing semiconducting polymer-mediated nanocarrier platform can significantly enhance the targeted delivery efficiency of therapeutic miRNAs to tumors, resulting in potent tumor growth inhibition.

Reversible Ratiometric NADH Sensing Using Semiconducting Polymer Dots

Reduced nicotinamide adenine dinucleotide (NADH) is a key coenzyme in living cells due to its role as an electron carrier in redox reactions, and its concentration is an important indicator of cell metabolic state. Abnormal NADH levels are associated with age-related metabolic diseases and neurodegenerative disorders, creating a demand for a simple, rapid analytical method for point-of-care NADH sensing. Here we develop a series of NADH-sensitive semiconducting polymer dots (Pdots) as nanoprobes for NADH measurement, and test their performance in vitro and in vivo. NADH sensing is based on electron transfer from semiconducting polymer chains in the Pdot to NADH upon UV excitation, quenching Pdot fluorescence emission. In polyfluorene-based Pdots, this mechanism resulted in an on-off NADH sensor; in DPA-CNPPV Pdots, UV excitation resulted in NADH-sensitive emission at two wavelengths, enabling ratiometric detection. Ratiometric NADH detection using DPA-CNPPV Pdots exhibits high sensitivity (3.1 μM limit of detection), excellent selectivity versus other analytes, reversibility, and a fast response (less than 5 s). We demonstrate applications of the ratiometric NADH-sensing Pdots including smartphone-based NADH imaging for point-of-care use.

Recent advances in near-infrared II imaging technology for biological detection

Molecular imaging technology enables us to observe the physiological or pathological processes in living tissue at the molecular level to accurately diagnose diseases at an early stage. Optical imaging can be employed to achieve the dynamic monitoring of tissue and pathological processes and has promising applications in biomedicine. The traditional first near-infrared (NIR-I) window (NIR-I, range from 700 to 900 nm) imaging technique has been available for more than two decades and has been extensively utilized in clinical diagnosis, treatment and scientific research. Compared with NIR-I, the second NIR window optical imaging (NIR-II, range from 1000 to 1700 nm) technology has low autofluorescence, a high signal-to-noise ratio, a high tissue penetration depth and a large Stokes shift. Recently, this technology has attracted significant attention and has also become a heavily researched topic in biomedicine. In this study, the optical characteristics of different fluorescence nanoprobes and the latest reports regarding the application of NIR-II nanoprobes in different biological tissues will be described. Furthermore, the existing problems and future application perspectives of NIR-II optical imaging probes will also be discussed.

High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror

Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an off-axis parabolic acoustic mirror with a conical bore. Quantitative simulation and experiments demonstrated that this combiner can provide better acoustic focusing performance and detection sensitivity. Moreover, OR-PAM is based on the combiner suffer low optical disorders, which guarantees the good resolution. In vivo experiments of the mouse brain and the iris were also conducted to show the practicability of the combiner in biomedicine. This proposed optical-acoustic combiner realizes a high-quality optical-acoustic confocal alignment with minimal optical disorders and acoustic insertion loss, strong acoustic focusing, and easy implementation. These characteristics might be useful for improving the performance of OR-PAM.

In vivo Multi-scale Photoacoustic Imaging Guided Photothermal Therapy of Cervical Cancer based on Customized Laser System and Targeted Nanoparticles

BACKGROUND

Effective treatment strategy for cervical carcinoma is subject to the limitation of its anatomical location and histological characteristics. Comprehensive imaging before cervical carcinoma treatment is of great significance for the patients. Current imaging methods cannot meet the requirements of high resolution, deep imaging depth and non-invasive imaging at the same time. Fortunately, Photoacoustic imaging (PAI) is a novel imaging method that combines rich optical contrast, high ultrasonic spatial resolution, and deep penetration depth in a single modality. Moreover, PAI-guided photothermal therapy (PTT) by aid of targeting nanoparticles is an emerging and effective cancer treatment in recent years.

METHODS

Here, strong near-infrared region (NIR) absorption-conjugated polymer PIIGDTS (PD) nanoparticles with folic acid (FA) modification (namely, PD-FA) that targeted at Hela cell were specifically designed as cervical tumor imaging contrast agents and photothermal agents.

RESULTS

The obtained PD-FA nanoparticles exhibited admirable photoacoustic contrast-enhancing ability and desirable PTT behavior with the photothermal conversion efficiency as high as 62.6% in vitro. Furthermore, the PAI performance and PTT efficiency were tested in HeLa tumor-bearing nude mice after injection of PD-FA nanoparticles. In vivo multi-scale, PAI provided B-san and 3D dimension imaging for intuitive and comprehensive information of Hela tumor. Moreover, the Hela tumor can be completely eliminated within 18 days after PTT, with no toxicity and side effects.

CONCLUSION

In summary, PD-FA injection combined with PAI and PTT systems provides a novel powerful tool for early diagnosis and precise treatment of cervical cancer.

Smart On-Site Immobilizable Near-Infrared II Fluorescent Nanoprobes for Ultra-Long-Term Imaging-Guided Tumor Surgery and Photothermal Therapy

Accurate diagnosis and efficient treatment of tumors are highly significant in battling cancer. Near-infrared II (NIR-II) fluorescence imaging shows big promise for deep tumor visualization in living systems due to high temporal and spatial resolution and deep tissue penetration capability, whereas the development of efficient NIR-II probes for tumor theranostics still faces a huge challenge. Herein, we have designed and constructed intelligent mPEG₅₀₀₀-PCL₃₀₀₀-encapsulated NIR-II nanoprobe ZM1068-NPs that showed great chemical stability and excellent biocompatibility. With the merits of the strong fluorescence in the NIR-II region and prominent optical-thermal conversion efficiency, this probe was successfully used for NIR-II imaging-guided surgery and photothermal therapy of breast carcinoma in living mice. More notably, it was for the first time found that ZM1068 dyes could be covalently on-site-immobilized within tumors through the thiol-chlor nucleophilic substitution reaction, resulting in improved tumor accumulation and retention time. We thus envision that this probe may provide an attractive means for precise cancer diagnosis and treatment.

Acoustic impact of the human skull on transcranial photoacoustic imaging

With balanced spatial resolution, imaging depth, and functional sensitivity, photoacoustic tomography (PAT) hold great promise for human brain imaging. However, the strong acoustic attenuation and aberration of the human skull (∼8 mm thick) are longstanding technical challenges for PAT of the human brain. In this work, we numerically investigated the impacts of the stratified human skull on photoacoustic wave propagation (i.e., the forward model) and PAT image formation (i.e., the inverse model). We simulated two representative transcranial PAT implementations: photoacoustic computed tomography (PACT) and photoacoustic macroscopy (PAMac). In the forward model, we simulated the detailed photoacoustic wave propagation from a point or line source through a digital human skull. The wave attenuation, refraction, mode conversation, and reverberation were thoroughly investigated. In the inverse model, we reconstructed the transcranial PACT and PAMac images of a point or line target enclosed by the human skull. Our results demonstrate that transcranial PAMac suffers mainly from wave reverberation within the skull, leading to prolonged signal duration and reduced axial resolution. Transcranial PACT is more susceptible to the skull's acoustic distortion, mode conversion, and reverberation, which collectively lead to strong image artifacts and deteriorated spatial resolutions. We also found that PACT with a ring-shaped transducer array shows more tolerance of the skull's adverse impacts and can provide more accurate image reconstruction. Our results suggest that incorporating the skull's geometry and acoustic properties can improve transcranial PAT image reconstruction. We expect that our results have provided a more comprehensive understanding of the acoustic impact of the human skull on transcranial PAT.

Multi-scale optoacoustic molecular imaging of brain diseases

The ability to non-invasively visualize endogenous chromophores and exogenous probes and sensors across the entire rodent brain with the high spatial and temporal resolution has empowered optoacoustic imaging modalities with unprecedented capacities for interrogating the brain under physiological and diseased conditions. This has rapidly transformed optoacoustic microscopy (OAM) and multi-spectral optoacoustic tomography (MSOT) into emerging research tools to study animal models of brain diseases. In this review, we describe the principles of optoacoustic imaging and showcase recent technical advances that enable high-resolution real-time brain observations in preclinical models. In addition, advanced molecular probe designs allow for efficient visualization of pathophysiological processes playing a central role in a variety of neurodegenerative diseases, brain tumors, and stroke. We describe outstanding challenges in optoacoustic imaging methodologies and propose a future outlook.

Real-Time Quantification of Cartilage Degeneration by GAG-Targeted Cationic Nanoparticles for Efficient Therapeutic Monitoring in Living Mice

One of the characterizations of degenerative cartilage disease is the progressive loss of glycosaminoglycans (GAGs). The real-time imaging method to quantify GAGs is of great significance for the biochemical analysis of cartilage and diagnosis and therapeutic monitoring of cartilage degeneration in vivo. To this end, a cationic photoacoustic (PA) contrast agent, poly-l-lysine melanin nanoparticles (PLL-MNPs), specifically targeting anionic GAGs was developed in this study to investigate whether it can image cartilage degeneration. PLL-MNP assessed GAG depletion by Chondroitinase ABC in vitro rat cartilage and intact ex vivo mouse knee joint. A papain-induced cartilage degenerative mice model was used for in vivo photoacoustic imaging (PAI). Oral cartilage supplement glucosamine sulfate was intragastrically administered for mice cartilage repair and the therapeutic efficacy was monitored by PLL-MNP-enhanced PAI. Histologic findings were used to further confirm PAI results. In vitro results revealed that the PLL-MNPs not only had a high binding ability with GAGs but also sensitively monitored GAG content changes by PAI. The PA signal was gradually weakened along with the depletion of GAGs in cartilage. Particularly, PLL-MNPs depicted the cartilage structure and the distribution of GAGs was demonstrated in PA images in ex vivo joints. Compared with the normal joint, a lower signal intensity was detected from degenerative joint at 3 weeks after papain injection, suggesting an early diagnosis of cartilage lesion by PLL-MNPs. Importantly, this PA-enhanced nanoprobe was suitable for monitoring in vivo efficacy of glucosamine sulfate, which effectively blocked cartilage degradation in a high dose manner. In vivo imaging findings correlated well with histological examinations. PLL-MNPs provided sensitive visualization of cartilage degeneration and promising monitoring of therapeutic response in living subjects.

Photoacoustic Imaging and Photothermal Therapy of Semiconducting Polymer Nanoparticles: Signal Amplification and Second Near-Infrared Construction

Photoacoustic (PA) imaging and photothermal therapy (PTT) have attracted extensive attention in disease diagnosis and treatment. Although many exogenous contrast agents have been developed for PA imaging and PTT, the design guidelines to amplify their imaging and therapy performances remain challenging and are highly demanded. Semiconducting polymer nanoparticles (SPNs) composed of polymers with π-electron delocalized backbones can be designed to amplify their PA imaging and PTT performance, because of their clear structure-property relation and versatility in modifying their molecular structures to tune their photophysical properties. This review summarizes the recent advances in the photoacoustic imaging and photothermal therapy applications of semiconducting polymer nanoparticles with a focus on signal amplification and second near-infrared (NIR-II, 1000-1700 nm) construction. The strategies such as structure-property screening, fluorescence quenching, accelerated heat dissipation, and size-dependent heat dissipation are first discussed to amplify the PA brightness of SPNs for in vivo PA. The molecular approaches to shifting the absorption of SPNs for NIR-II PA imaging and PTT are then introduced so as to improve the tissue penetration depth for diagnosis and therapy. At last, current challenges and perspectives of SPNs in the field of imaging and therapy are discussed.

Deep Learning Enables Superior Photoacoustic Imaging at Ultralow Laser Dosages

Optical-resolution photoacoustic microscopy (OR-PAM) is an excellent modality for in vivo biomedical imaging as it noninvasively provides high-resolution morphologic and functional information without the need for exogenous contrast agents. However, the high excitation laser dosage, limited imaging speed, and imperfect image quality still hinder the use of OR-PAM in clinical applications. The laser dosage, imaging speed, and image quality are mutually restrained by each other, and thus far, no methods have been proposed to resolve this challenge. Here, a deep learning method called the multitask residual dense network is proposed to overcome this challenge. This method utilizes an innovative strategy of integrating multisupervised learning, dual-channel sample collection, and a reasonable weight distribution. The proposed deep learning method is combined with an application-targeted modified OR-PAM system. Superior images under ultralow laser dosage (32-fold reduced dosage) are obtained for the first time in this study. Using this new technique, a high-quality, high-speed OR-PAM system that meets clinical requirements is now conceivable.

All-in-one mitochondria-targeted NIR-II fluorophores for cancer therapy and imaging

Small-molecule subcellular organelle-targeting theranostic probes are crucial for early disease diagnosis and treatment. The imaging window of these molecules is mainly focused on the visible and near-infrared region (below ∼900 nm) which limits the tissue penetration depth and therapeutic effects. Herein, a novel NIR-II small-molecule probe H4–PEG-Glu with a thiopyrylium cation was synthesized. H4–PEG-Glu not only can quickly and effectively image mitochondria in acute myeloid leukemia (AML) cells, and induce G₀/G₁ phase arrest by the intrinsic mitochondrial apoptosis pathway w/o irradiation, but also exhibit moderate cytotoxicity against AML cancer cells in a dose dependent-manner without laser irradiation. The THP-1 cells treated with H4–PEG-Glu upon NIR laser irradiation showed enhanced chemo- and photothermal therapy (CPTT) with 93.07% ± 6.43 apoptosis by Annexin V staining. Meanwhile, H4–PEG-Glu displayed high synergistic CPTT effects in vivo, as well as specific NIR-II tumor imaging in AML patient derived PDX mouse models for the first time. Our work lays down a solid foundation for designing small-molecule NIR-II mitochondria-selective theranostic probes.

Small-molecule subcellular organelle-targeting theranostic probes are crucial for early disease diagnosis and treatment.

Visualising brain capillaries in magnetic resonance images via supramolecular self-assembly

We report the supramolecular self-assembly of one fluorescein and three Gd-chelate conjugated 8-arm polyethylene glycols (8-arm PEG-FGd₃) for visualising the capillaries of the brain in magnetic resonance imaging (MRI).

Current trends in pyrrole and porphyrin-derived nanoscale materials for biomedical applications

This article is written to provide an up-to-date review of pyrrole-based biomedical materials. Porphyrins and other tetrapyrrolic molecules possess unique magnetic, optical and other photophysical properties that make them useful for bioimaging and therapy. This review touches briefly on some of the synthetic strategies to obtain porphyrin- and tetrapyrrole-based nanoparticles, as well as the variety of applications in which crosslinked, self-assembled, porphyrin-coated and other nanoparticles are utilized. We explore examples of these nanoparticles' applications in photothermal therapy, drug delivery, photodynamic therapy, stimuli response, fluorescence imaging, photoacoustic imaging, magnetic resonance imaging, computed tomography and positron emission tomography. We anticipate that this review will provide a comprehensive summary of pyrrole-derived nanoparticles and provide a guideline for their further development.

Fluorination Enhances NIR-II Fluorescence of Polymer Dots for Quantitative Brain Tumor Imaging

Here, we describe a fluorination strategy for semiconducting polymers for the development of highly bright second near-infrared region (NIR-II) probes. Tetrafluorination yielded a fluorescence QY of 3.2 % for the polymer dots (Pdots), over a 3-fold enhancement compared to non-fluorinated counterparts. The fluorescence enhancement was attributable to a nanoscale fluorous effect in the Pdots that maintained the molecular planarity and minimized the structure distortion between the excited state and ground state, thus reducing the nonradiative relaxations. By performing through-skull and through-scalp imaging of the brain vasculature of live mice, we quantitatively analyzed the vascular morphology of transgenic brain tumors in terms of the vessel lengths, vessel branches, and vessel symmetry, which showed statistically significant differences from the wild type animals. The bright NIR-II Pdots obtained through fluorination chemistry provide insightful information for precise diagnosis of the malignancy of the brain tumor.

Tocilizumab-Conjugated Polymer Nanoparticles for NIR-II Photoacoustic-Imaging-Guided Therapy of Rheumatoid Arthritis

The progressive debilitating nature of rheumatoid arthritis (RA) combined with its unknown etiology and initial similarity to other inflammatory diseases makes early diagnosis a significant challenge. Early recognition and treatment of RA is essential for achieving effective therapeutic outcome. NIR-II photoacoustic (PA) molecular imaging (PMI) is emerging as a promising new strategy for effective diagnosis and treatment guidance of RA, owing to its high sensitivity and specificity at large penetration depth. Herein, an antirheumatic targeted drug tocilizumab (TCZ) is conjugated to polymer nanoparticles (PNPs) to develop the first NIR-II theranostic nanoplatform, named TCZ-PNPs, for PA-imaging-guided therapy of RA. The TCZ-PNPs are demonstrated to have strong NIR-II extinction coefficient, high photostability and excellent biocompatibility. NIR-II PMI results reveal the excellent targeting abilities of TCZ-PNPs for the effective noninvasive diagnosis of RA joint tissue with a high signal-to noise ratio (SNR) of 35.8 dB in 3D PA tomography images. Remarkably, one-month treatment and PA monitoring using TCZ-PNPs shows RA is significantly suppressed. In addition, the therapeutic evaluation of RA mice by NIR-II PMI is shown to be consistent with clinical micro-CT and histological analysis. The TCZ-PNPs-assisted NIR-II PMI provides a new strategy for RA theranostics, therapeutic monitoring and the beyond.

Nanoparticles of Organic Electronic Materials for Biomedical Applications

Organic electronic materials play important roles in modern electronic devices such as light-emitting diodes, solar cells, and transistors. Upon interaction with light, these optically active materials can undergo different photophysical and photochemical pathways, providing unique opportunities for optimization of light emission via radiative decay, heat generation via nonradiative decay, and singlet oxygen production or phosphorescence emission via intersystem crossing, all of which open alternative opportunities for their applications in sensing, imaging, and therapy. In this Perspective, we discuss all of the pathways that determine the optical properties of high-performance organic electronic materials, focusing on the optimization of each pathway for photogeneration and relaxation of electronic excited states. We also examine nanoparticle (NP) fabrication techniques tailored to macromolecules and small molecules to render them into NPs with optimized size and distribution for biomedical applications and endow organic electronic materials with water dispersibility and biocompatibility. Lastly, we illustrate the in vitro and in vivo applications of some representative organic electronic materials after optimization of each relaxation pathway.

Self-Assembled Organic Nanomaterials for Drug Delivery, Bioimaging, and Cancer Therapy

Over the past few decades, tremendous progress has been made in the development of engineering nanomaterials, which opened new horizons in the field of diagnosis and treatment of various diseases. In particular, self-assembled organic nanomaterials with intriguing features including delicate structure tailoring, facile processability, low cost, and excellent biocompatibility have shown outstanding potential in biomedical applications because of the enhanced permeability and retention (EPR) effect and multifunctional properties. In this review, we briefly introduce distinctive merits of self-assembled organic nanomaterials for biomedical applications. The main focus will be placed on summarizing recent advances in self-assembled organic nanomedicine for drug delivery, bioimaging, and cancer phototherapy, followed by highlighting a critical perspective on further development of self-assembled organic nanomaterials for future clinical translation. We believe that the above themes will appeal to researchers from different fields, including material, chemical, and biological sciences, as well as pharmaceutics.

Multiscale high-speed photoacoustic microscopy based on free-space light transmission and a MEMS scanning mirror

The conventional photoacoustic microscopy (PAM) system allows trade-offs between lateral resolution and imaging depth, limiting its applications in biological imaging in vivo. Here we present an integrated optical-resolution (OR) and acoustic-resolution (AR) multiscale PAM based on free-space light transmission and fast microelectromechanical systems (MEMS) scanning. The lateral resolution for OR is 4.9 µm, and the lateral resolution for AR is 114.5 µm. The maximum imaging depth for OR is 0.7 mm, and the maximum imaging depth for AR is 4.1 mm. The imaging speed can reach 50 k Alines per second. The high signal-to-noise ratios and wavelength throughput are achieved by delivering light via free-space, and the high speed is achieved by a MEMS scanning mirror. The blood vasculature from superficial skin to the deep tissue of a mouse leg was imaged in vivo using two different resolutions to demonstrate the multiscale imaging capability.

Grafted semiconducting polymer amphiphiles for multimodal optical imaging and combination phototherapy

Semiconducting polymer nanoparticles (SPNs) have gained growing attention in biomedical applications. However, the preparation of SPNs is usually limited to nanoprecipitation in the presence of amphiphilic copolymers, which encounters the issue of dissociation. As an alternative to SPNs, grafted semiconducting polymer amphiphiles (SPAs) composed of a semiconducting polymer (SP) backbone and hydrophilic side chains show increased physiological stability and improved optical properties. This review summarizes recent advances in SPAs for cancer imaging and combination phototherapy. The applications of SPAs in optical imaging including fluorescence, photoacoustic, multimodal and activatable imaging are first described, followed by the discussion of applications in imaging-guided phototherapy and combination therapy, light-triggered drug delivery and gene regulation. At last, the conclusion and future prospects in this field are discussed.

Perylene Diimide Nanoprobes for In Vivo Tracking of Mesenchymal Stromal Cells Using Photoacoustic Imaging

Noninvasive bioimaging techniques are critical for assessing the biodistribution of cellular therapies longitudinally. Among them, photoacoustic imaging (PAI) can generate high-resolution images with a tissue penetration depth of ∼4 cm. However, it is essential and still highly challenging to develop stable and efficient near-infrared (NIR) probes with low toxicity for PAI. We report here the preparation and use of perylene diimide derivative (PDI) with NIR absorbance (around 700 nm) as nanoprobes for tracking mesenchymal stromal cells (MSCs) in mice. Employing an in-house synthesized star hyperbranched polymer as a stabilizer is the key to the formation of stable PDI nanoparticles with low toxicity and high uptake by the MSCs. The PDI nanoparticles remain within the MSCs as demonstrated by in vitro and in vivo assessments. The PDI-labeled MSCs injected subcutaneously on the flanks of the mice are clearly visualized with PAI up to 11 days postadministration. Furthermore, bioluminescence imaging of PDI-labeled luciferase-expressing MSCs confirms that the administered cells remain viable for the duration of the experiment. These PDI nanoprobes thus have good potential for tracking administered cells in vivo using PAI.

Organic NIR-II molecule with long blood half-life for in vivo dynamic vascular imaging

Real-time monitoring of vessel dysfunction is of great significance in preclinical research. Optical bioimaging in the second near-infrared (NIR-II) window provides advantages including high resolution and fast feedback. However, the reported molecular dyes are hampered by limited blood circulation time (~ 5-60 min) and short absorption and emission wavelength, which impede the accurate long-term monitoring. Here, we report a NIR-II molecule (LZ-1105) with absorption and emission beyond 1000 nm. Thanks to the long blood circulation time (half-life of 3.2 h), the fluorophore is used for continuous real-time monitoring of dynamic vascular processes, including ischemic reperfusion in hindlimbs, thrombolysis in carotid artery and opening and recovery of the blood brain barrier (BBB). LZ-1105 provides an approach for researchers to assess vessel dysfunction due to the long excitation and emission wavelength and long-term blood circulation properties.

Recent Developments in Microfluidic Technologies for Central Nervous System Targeted Studies

Neurodegenerative diseases (NDs) bear a lot of weight in public health. By studying the properties of the blood-brain barrier (BBB) and its fundamental interactions with the central nervous system (CNS), it is possible to improve the understanding of the pathological mechanisms behind these disorders and create new and better strategies to improve bioavailability and therapeutic efficiency, such as nanocarriers. Microfluidics is an intersectional field with many applications. Microfluidic systems can be an invaluable tool to accurately simulate the BBB microenvironment, as well as develop, in a reproducible manner, drug delivery systems with well-defined physicochemical characteristics. This review provides an overview of the most recent advances on microfluidic devices for CNS-targeted studies. Firstly, the importance of the BBB will be addressed, and different experimental BBB models will be briefly discussed. Subsequently, microfluidic-integrated BBB models (BBB/brain-on-a-chip) are introduced and the state of the art reviewed, with special emphasis on their use to study NDs. Additionally, the microfluidic preparation of nanocarriers and other compounds for CNS delivery has been covered. The last section focuses on current challenges and future perspectives of microfluidic experimentation.

Rational design of semiconducting polymer brushes as cancer theranostics

Photonic theranostics (PTs) generally contain optical agents for the optical sensing of biomolecules and therapeutic components for converting light into heat or chemical energy. Semiconducting polymer nanoparticles (SPNs) as advanced PTs possessing good biocompatibility, stable photophysical properties, and sensitive and tunable optical responses from the ultraviolet to near-infrared (NIR) II window (300-1700 nm) have recently aroused great interest. Although semiconducting polymers (SPs) with various building blocks have been synthesized and developed to meet the demands of biophotonic applications, most of the SPNs were made by a nanoprecipitation method that used amphiphilic surfactants to encapsulate SPs. Such binary SP micelles usually exhibit weakened photophysical properties of SPs and undergo dissociation in vivo. SP brushes (SPBs) are products of functional post-modification of SP backbones, which endows unique features to SPNs (e.g. enhanced optical properties and multiple chemical reaction sites for the conjunction of organic/inorganic imaging agents and therapeutics). Furthermore, the SPB-based SPNs can be highly stable due to supramolecular self-assembly and/or chemical crosslinking. In this review, we highlight the recent progress in the development of SPBs for advanced theranostics.

A simple POM clusters for in vivo NIR-II photoacoustic imaging-guided NIR-II photothermal therapy

Photoacoustic (PA) imaging in the second near-infrared (NIR-II) window exhibits enhanced deep-tissue imaging capability. Likely, cancer therapy in the NIR-II window could provide deeper penetration depth and higher exposure to laser over NIR-I. However, the traditional application of excitation light is still in the NIR-I window. In view of the excellent imaging and therapeutic capabilities of NIR-II window, we have demonstrated a simple polyoxometalate (POM) clusters (molecular formula: (Na)_n(PMo₁₂O₄₀) or (NH₄⁺)_n(PMo₁₂O₄₀)), which integrates NIR-II photoacoustic imaging and NIR-II photothermal therapy into an "all-in-one" theranostic nanoplatform, and could be used for PA imaging-guided photothermal therapy in the NIR-II window. In vivo experiments demonstrate that the POM clusters with good water solubility and biocompatibility were effective to kill tumor without recurrence and metastasis under 1064 nm laser illumination.

Functionalized NIR-II Semiconducting Polymer Nanoparticles for Single-cell to Whole-Organ Imaging of PSMA-Positive Prostate Cancer

Development of molecular probes holds great promise for early diagnosis of aggressive prostate cancer. Here, 2-[3-(1,3-dicarboxypropyl) ureido] pentanedioic acid (DUPA)-conjugated ligand and bis-isoindigo-based polymer (BTII) are synthesized to formulate semiconducting polymer nanoparticles (BTII-DUPA SPN) as a prostate-specific membrane antigen (PSMA)-targeted probe for prostate cancer imaging in the NIR-II window. Insights into the interaction of the imaging probes with the biological targets from single cell to whole organ are obtained by transient absorption (TA) microscopy and photoacoustic (PA) tomography. At single-cell level, TA microscopy reveals the targeting efficiency, kinetics, and specificity of BTII-DUPA SPN to PSMA-positive prostate cancer. At organ level, PA tomographic imaging of BTII-DUPA SPN in the NIR-II window demonstrates superior imaging depth and contrast. By intravenous administration, BTII-DUPA SPN demonstrates selective accumulation and retention in the PSMA-positive tumor, allowing noninvasive PA detection of PSMA overexpressing prostate tumors in vivo. The distribution of nanoparticles inside the tumor tissue is further analyzed through TA microscopy. These results collectively demonstrate BTII-DUPA SPN as a promising probe for prostate cancer diagnosis by PA tomography.

A novel αvβ3 integrin-targeted NIR-II nanoprobe for multimodal imaging-guided photothermal therapy of tumors in vivo

Developing novel small-molecule-based probes with both deep tissue imaging and therapeutic functions is highly significant in cancer diagnosis and treatment. Herein, we report a novel second near-infrared (NIR-II) fluorescent probe QT-RGD constructed with a NIR-II emissive organic fluorophore and two cyclic-(arginine-glycine-aspartic acid) (cRGD) peptides that can specifically bind to the tumor-associated αvβ3 integrin for accurate tumor diagnosis and targeting therapy. The isotopic 125I-labeled probe exhibited great tumor targeting ability and emitted intensive NIR-II/photoacoustic (PA)/single-photon emission computed tomography (SPECT) signals, which allows specific and sensitive multimodal visualization of tumors in vivo. More notably, this probe could also be applied for effective imaging-guided photothermal therapy (PTT) of tumors in mouse models owing to its prominent photothermal conversion efficiency and excellent photothermal stability. We thus envision that our work which unveils a combination of NIR-II/PA/SPECT imaging and PTT would offer a valuable means of improving tumor diagnostic accuracy as well as therapeutic efficacy.

NIR-II fluorescence microscopic imaging of cortical vasculature in non-human primates

Vasculature architecture of the brain can provide revealing information about mental and neurological function and disease. Fluorescence imaging in the second near-infrared (NIR-II) regime with less light scattering is a more promising method for detecting cortical vessels than traditional visible and NIR-I modes. Methods: Clinically approved dye indocyanine green (ICG) was used for NIR-II fluorescence imaging. Here, for the first time, we developed two NIR-II fluorescence microscopy systems for brain vasculature imaging in macaque monkeys. The first is a wide-field microscope with high temporal resolution for measuring blood flow velocity and cardiac impulse period, while the second is a high spatial resolution confocal microscope producing three-dimensional maps of the cortical microvascular network. Both were designed with flexibility to image various cortical locations on the head. Results: Here, ICG was proved to have high brightness in NIR-II region and an 8-fold QY increase in serum than in water. We achieved cerebrovascular functional imaging of monkey with high temporal resolution (25 frames/second) with wide-field microscope. The blood flow velocity of capillaries can be precisely calculated and the cardiac impulse period can be monitored as well. In vivo structural imaging of cerebrovasculature was accomplished with both high spatial lateral resolution (~8 µm) and high signal to background ratio (SBR). Vivid 3D reconstructed NIR-II fluorescence confocal microscopic images up to depth of 470 μm were also realized. Conclusion: This work comprises an important advance towards studies of neurovascular coupling, stroke, and other diseases relevant to neurovascular health in humans.