Diketopyrrolopyrrole-Based Semiconducting Polymer Nanoparticles for In Vivo Photoacoustic Imaging

Highly Stable and Red-Emitting Nanovesicles Incorporating Lipophilic Diketopyrrolopyrroles for Cell Imaging

Diketopyrrolopyrroles (DPPs) have recently attracted much interest as very bright and photostable red-emitting molecules. However, their tendency to form nonfluorescent aggregates in water through the aggregation-caused quenching (ACQ) effect is a major issue that limits their application under the microscope. Herein, two DPP molecules have been incorporated into the membrane of highly stable and water-soluble quatsomes (QS; nanovesicles composed of surfactants and sterols), which allow their nanostructuration in water and, at the same time, limits the ACQ effect. The obtained fluorescent organic nanoparticles showed superior structural homogeneity, along with long-term colloidal and optical stability. A thorough one- (1P) and two-photon (2P) fluorescence characterization revealed the promising photophysical features of these fluorescent nanovesicles, which showed a high 1P and 2P brightness. Finally, the fluorescent QSs were used for the in vitro bioimaging of Saos-2 osteosarcoma cell lines; this demonstrates their potential as nanomaterials for bioimaging applications.

Organic Dye Based Nanoparticles for Cancer Phototheranostics

Phototheranostics, which simultaneously combines photodynamic and/or photothermal therapy with deep-tissue diagnostic imaging, is a promising strategy for the diagnosis and treatment of cancers. Organic dyes with the merits of strong near-infrared absorbance, high photo-to-radical and/or photothermal conversion efficiency, great biocompatibility, ready chemical structure fine-tuning capability, and easy metabolism, have been demonstrated as attractive candidates for clinical phototheranostics. These organic dyes can be further designed and fabricated into nanoparticles (NPs) using various strategies. Compared to free molecules, these NPs can be equipped with multiple synergistic functions and show longer lifetime in blood circulation and passive tumor-targeting property via the enhanced permeability and retention effect. In this article, the recent progress of organic dye-based NPs for cancer phototheranostic applications is summarized, which extends the anticancer arsenal and holds promise for clinical uses in the near future.

Bright Aggregation-Induced-Emission Dots for Targeted Synergetic NIR-II Fluorescence and NIR-I Photoacoustic Imaging of Orthotopic Brain Tumors

Precise diagnostics are of significant importance to the optimal treatment outcomes of patients bearing brain tumors. NIR-II fluorescence imaging holds great promise for brain-tumor diagnostics with deep penetration and high sensitivity. This requires the development of organic NIR-II fluorescent agents with high quantum yield (QY), which is difficult to achieve. Herein, the design and synthesis of a new NIR-II fluorescent molecule with aggregation-induced-emission (AIE) characteristics is reported for orthotopic brain-tumor imaging. Encapsulation of the molecule in a polymer matrix yields AIE dots showing a very high QY of 6.2% with a large absorptivity of 10.2 L g-1 cm-1 at 740 nm and an emission maximum near 1000 nm. Further decoration of the AIE dots with c-RGD yields targeted AIE dots, which afford specific and selective tumor uptake, with a high signal/background ratio of 4.4 and resolution up to 38 µm. The large NIR absorptivity of the AIE dots facilitates NIR-I photoacoustic imaging with intrinsically deeper penetration than NIR-II fluorescence imaging and, more importantly, precise tumor-depth detection through intact scalp and skull. This research demonstrates the promise of NIR-II AIE molecules and their dots in dual NIR-II fluorescence and NIR-I photoacoustic imaging for precise brain cancer diagnostics.

A water-soluble PEGylated RGD-functionalized bisbithiophenyl diketopyrrolopyrrole as a photoacoustic sonophore

Photoacoustic imaging presents an innocuous imaging modality with good penetration depth and resolution. To use this modality for detection and imaging of pathological sites, new imaging probes need to be developed to enhance the contrast over endogenous sonophores. These contrast agents should specifically bind to the site of interest, be non-toxic and be cleared renally if applied intravenously. Small organic dyes with absorption in the near infrared spectrum often exhibit good photoacoustic response. However, such dyes are often not water soluble or they are cytotoxic. Here, we present a novel PEGylated sonophore based on diketopyrrolopyrrole (DPP), which overcomes these limitations and can be functionalized with desired biological recognition motifs using thiol-yne click chemistry. Proof of concept is demonstrated by functionalizing the DPP-based probe with an RGD peptide, resulting in specific binding to endothelial (HUVEC) cells and an efficient photoacoustic response.

Light-driven transformable optical agent with adaptive functions for boosting cancer surgery outcomes

Fluorescence and photoacoustic imaging have different advantages in cancer diagnosis; however, combining effects in one agent normally requires a trade-off as the mechanisms interfere. Here, based on rational molecular design, we introduce a smart organic nanoparticle whose absorbed excitation energy can be photo-switched to the pathway of thermal deactivation for photoacoustic imaging, or to allow opposed routes for fluorescence imaging and photodynamic therapy. The molecule is made of a dithienylethene (DTE) core with two surrounding 2-(1-(4-(1,2,2-triphenylvinyl)phenyl)ethylidene)malononitrile (TPECM) units (DTE-TPECM). The photosensitive molecule changes from a ring-closed, for photoacoustic imaging, to a ring-opened state for fluorescence and photodynamic effects upon an external light trigger. The nanoparticles’ photoacoustic and fluorescence imaging properties demonstrate the advantage of the switch. The use of the nanoparticles improves the outcomes of in vivo cancer surgery using preoperative photoacoustic imaging and intraoperative fluorescent visualization/photodynamic therapy of residual tumours to ensure total tumour removal.

The combination of imaging techniques in cancer treatment often involves a trade-off in properties due to the opposite working mechanisms. Here, the authors report on a material that avoids the trade-off by switching from photoacoustic imaging to fluorescence imaging upon an external light trigger

Self-Assembled Peptide-Based Nanomaterials for Biomedical Imaging and Therapy

Peptide-based materials are one of the most important biomaterials, with diverse structures and functionalities. Over the past few decades, a self-assembly strategy is introduced to construct peptide-based nanomaterials, which can form well-controlled superstructures with high stability and multivalent effect. More recently, peptide-based functional biomaterials are widely utilized in clinical applications. However, there is no comprehensive review article that summarizes this growing area, from fundamental research to clinic translation. In this review, the recent progress of peptide-based materials, from molecular building block peptides and self-assembly driving forces, to biomedical and clinical applications is systematically summarized. Ex situ and in situ constructed nanomaterials based on functional peptides are presented. The advantages of intelligent in situ construction of peptide-based nanomaterials in vivo are emphasized, including construction strategy, nanostructure modulation, and biomedical effects. This review highlights the importance of self-assembled peptide nanostructures for nanomedicine and can facilitate further knowledge and understanding of these nanosystems toward clinical translation.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Purification of Semiconducting Polymer Dots by Size Exclusion Chromatography Prior to Cytotoxicity Assay and Stem Cell Labeling

Semiconducting polymer dots (Pdots) as fluorescent probes have shown promising applications because of their excellent optical properties. However, apparent differences were observed in cytotoxicity assays, which might originate from impurities introduced in polymer synthesis or nanoparticle preparation. A simple gel-filtration-based purification method was used to address this issue. Purified Pdots displayed obviously decreased cytotoxicity as compared with the same batch of unpurified Pdots. The purified Pdots were further examined in a cytotoxicity study on mesenchymal stem cells (MSCs), which are very sensitive to exogenous probes. The results indicated that purified Pdots did not affect the proliferation ability of MSCs, while unpurified Pdots could have obvious cytotoxicity. In addition, the purified Pdots did not show cytotoxicity even after 6 months of storage. Our results demonstrated that gel filtration is an effective method for obtaining Pdots with minimal cytotoxicity, which are more suitable for biological applications.

Organic molecule-based photothermal agents: an expanding photothermal therapy universe

Over the last decade, organic photothermal therapy (PTT) agents have attracted increasing attention as a potential complement for, or alternative to, classical drugs and sensitizers involving inorganic nanomaterials. In this tutorial review, we provide a structured description of the main classes of organic photothermal agents and their characteristics. Representative agents that have been studied in the context of photothermal therapy since 2000 are summarized and recent advances in using PTT agents to address various cancers indications are highlighted.

Dual-Peak Absorbing Semiconducting Copolymer Nanoparticles for First and Second Near-Infrared Window Photothermal Therapy: A Comparative Study

Near-infrared (NIR) light is widely used for noninvasive optical diagnosis and phototherapy. However, current research focuses on the first NIR window (NIR-I, 650-950 nm), while the second NIR window (NIR-II, 1000-1700 nm) is far less exploited. The development of the first organic photothermal nanoagent (SPN_I-II ) with dual-peak absorption in both NIR windows and its utilization in photothermal therapy (PTT) are reported herein. Such a nanoagent comprises a semiconducting copolymer with two distinct segments that respectively and identically absorb NIR light at 808 and 1064 nm. With the photothermal conversion efficiency of 43.4% at 1064 nm generally higher than other inorganic nanomaterials, SPN_I-II enables superior deep-tissue heating at 1064 nm over that at 808 nm at their respective safety limits. Model deep-tissue cancer PTT at a tissue depth of 5 mm validates the enhanced antitumor effect of SPN_I-II when shifting laser irradiation from the NIR-I to the NIR-II window. The good biodistribution and facile synthesis of SPN_I-II also allow it to be doped with an NIR dye for fluorescence-imaging-guided NIR-II PTT through systemic administration. Thus, this study paves the way for the development of new polymeric nanomaterials to advance phototherapy.

Diketopyrrolopyrrole-based semiconducting polymer nanoparticles for in vivo second near-infrared window imaging and image-guided tumor surgery

A diketopyrrolopyrrole-based semiconducting polymer nanoparticle (PDFT1032) has been developed as a NIR-II (near infrared window II, 1000-1700 nm) fluorescent probe. It shows high photostability, a favorable absorption peak at 809 nm, a large Stokes shift of 223 nm, outstanding biocompatibility and minimal in vivo toxicity. More importantly, the versatile use of PDFT1032 for several important biomedical applications in the NIR-II window has been demonstrated, including the NIR-II optical imaging of tumors on a subcutaneous osteosarcoma model, assessing the vascular embolization therapy of tumors, and NIR-II image-guided orthotopic tumor surgery and sentinel lymph node biopsy (SLNB) with high spatial and temporal resolution. Overall, excellent biocompatibility, favorable hydrophilicity, and desirable chemical and optical properties make the semiconducting polymer nanoparticle PDFT1032 a highly promising NIR-II imaging probe with the potential to be widely applicable in clinical imaging and the surgical treatment of malignancy.

Big Potential from Small Agents: Nanoparticles for Imaging-Based Companion Diagnostics

The importance of medical imaging in the diagnosis and monitoring of cancer cannot be overstated. As personalized cancer treatments are gaining popularity, a need for more advanced imaging techniques has grown significantly. Nanoparticles are uniquely suited to fill this void, not only as imaging contrast agents but also as companion diagnostics. This review provides an overview of many ways nanoparticle imaging agents have contributed to cancer imaging, both preclinically and in the clinic, as well as charting future directions in companion diagnostics. We conclude that, while nanoparticle-based imaging agents are not without considerable scientific and developmental challenges, they enable enhanced imaging in nearly every modality, hold potential as in vivo companion diagnostics, and offer precise cancer treatment and maximize intervention efficacy.

Organic Semiconducting Photoacoustic Nanodroplets for Laser-Activatable Ultrasound Imaging and Combinational Cancer Therapy

Combination of photoacoustic (PA) and ultrasound (US) imaging offers high spatial resolution images with deep tissue penetration, which shows great potential in applications in medical imaging. Development of PA/US dual-contrast agents with high contrast and excellent biocompatibility is of great interest. Herein, an organic semiconducting photoacoustic nanodroplet, PS-PDI-PAnD, is developed by stabilizing low-boiling-point perfluorocarbon (PFC) droplet with a photoabsorber and photoacoustic agent of perylene diimide (PDI) molecules and coencapsulating the droplet with photosensitizers of ZnF₁₆Pc molecules. Upon irradiation, the PDI acts as an efficient photoabsorber to trigger the liquid-to-gas phase transition of the PFC, resulting in dual-modal PA/US imaging contrast as well as photothermal heating. On the other hand, PFC can serve as an O₂ reservoir to overcome the hypoxia-associated resistance in cancer therapies, especially in photodynamic therapy. The encapsulated photosensitizers will benefit from the sustained oxygen release from the PFC, leading to promoted photodynamic efficacy regardless of pre-existing hypoxia in the tumors. When intravenously injected into tumor-bearing mice, the PS-PDI-PAnDs show a high tumor accumulation via EPR effect. With a single 671 nm laser irradiation, the PS-PDI-PAnDs exhibit a dual-modal PA/US imaging-guided synergistic photothermal and oxygen self-enriched photodynamic treatment, resulting in complete tumor eradication and minimal side effects. The PS-PDI-PAnDs represents a type of PFC nanodroplets for synergistic PDT/PTT treatment upon a single laser irradiation, which is expected to hold great potential in the clinical translation in dual-modal PA/US imaging-guided combinational cancer therapy.

Molecular Engineering of Photoacoustic Performance by Chalcogenide Variation in Conjugated Polymer Nanoparticles for Brain Vascular Imaging

As conjugated polymer nanoparticles (CPNs) have attracted growing interest as photoacoustic (PA) imaging contrast agents, revelation of the relationship between the molecular structure of conjugated polymers and PA property is highly in demand. Here, three donor-acceptor-structured conjugated polymer analogs are designed, where only a single heteroatom of acceptor units changes from oxygen to sulfur to selenium, allowing for systematic investigation of the molecular structure-PA property relationship. The absorption and PA spectra of these CPNs can be facilely tuned by changing the heteroatoms of the acceptor units. Moreover, the absorption coefficient, and in turn the PA signal intensity, decreases when the heteroatom changes from oxygen to sulfur to selenium. As these CPNs exhibit weak fluorescence and similar photothermal conversion efficiency (≈70%), their PA intensities are approximately proportional to their absorption coefficients. The in vivo brain vasculature imaging in this study also demonstrates this trend. This study provides a simple but efficient strategy to manipulate the PA properties of CPNs through changing the heteroatom at key positions.

Enhancing Both Biodegradability and Efficacy of Semiconducting Polymer Nanoparticles for Photoacoustic Imaging and Photothermal Therapy

Theranostic nanoagents are promising for precision medicine. However, biodegradable nanoagents with the ability for photoacoustic (PA) imaging guided photothermal therapy (PTT) are rare. We herein report the development of biodegradable semiconducting polymer nanoparticles (SPNs) with enhanced PA and PTT efficacy for cancer therapy. The design capitalizes on the enzymatically oxidizable nature of vinylene bonds in conjunction with polymer chemistry to synthesize a biodegradable semiconducting polymer (DPPV) and transform it into water-soluble nanoparticles (SPNV). As compared with its counterpart SPN (SPNT), the presence of vinylene bonds within the polymer backbone also endows SPNV with a significantly enhanced mass absorption coefficient (1.3-fold) and photothermal conversion efficacy (2.4-fold). As such, SPNV provides the PA signals and the photothermal maximum temperature higher than SPNT, allowing detection and photothermal ablation of tumors in living mice in a more sensitive and effective way. Our study thus reveals a general molecular design to enhance the biodegradability of optically active polymer nanoparticles while dramatically elevating their imaging and therapeutic capabilities.

Photothermal-Responsive Conjugated Polymer Nanoparticles for Remote Control of Gene Expression in Living Cells

Remote control and noninvasive manipulation of cellular bioprocess has received intensive attention as a powerful technology to control cell functions. Here, a strategy is developed to remotely control intracellular gene expression with high spatial and temporal resolutions by using photothermal-responsive conjugated polymer nanoparticles (CPNs) as the transducer under near-infrared light irradiation. After being modified with positive charged peptide, the CPNs with superior photothermal conversion capacity could effectively coat on the surface of living cells and generate localized heat to trigger target gene expression. The heat-inducible heat shock protein-70 promoter starts transcription of downstream EGFP gene in response to heat shock, thus producing green fluorescent protein in the living cells. The combination of heat-inducible gene promoter and photothermal-responsive CPNs provides a method for the development of thermogenetics.

Recent progress on semiconducting polymer nanoparticles for molecular imaging and cancer phototherapy

As a new class of organic optical nanomaterials, semiconducting polymer nanoparticles (SPNs) have the advantages of excellent optical properties, high photostability, facile surface functionalization, and are considered to possess good biocompatibility for biomedical applications. This review surveys recent progress made on the design and synthesis of SPNs for molecular imaging and cancer phototherapy. A variety of novel polymer design, chemical modification and nanoengineering strategies have been developed to precisely tune up optoelectronic properties of SPNs to enable fluorescence, chemiluminescence and photoacoustic (PA) imaging in living animals. With these imaging modalities, SPNs have been demonstrated not only to image tissues such as lymph nodes, vascular structure and tumors, but also to detect disease biomarkers such as reactive oxygen species (ROS) and protein sulfenic acid as well as physiological indexes such as pH and blood glucose concentration. The potentials of SPNs in cancer phototherapy including photodynamic and photothermal therapy are also highlighted with recent examples. Future efforts should further expand the use of SPNs in biomedical research and may even move them beyond pre-clinical studies.

Oxygenic Hybrid Semiconducting Nanoparticles for Enhanced Photodynamic Therapy

Photodynamic nanotheranostics has shown great promise for cancer therapy; however, its therapeutic efficacy is limited due to the hypoxia of tumor microenvironment and the unfavorable bioavailability of existing photodynamic agents. We herein develop hybrid core-shell semiconducting nanoparticles (SPN-Ms) that can undergo O₂ evolution in hypoxic solid tumor to promote photodynamic process. Such oxygenic nanoparticles are synthesized through a one-pot surface growth reaction and have a unique multilayer structure cored and coated with semiconducting polymer nanoparticles (SPNs) and manganese dioxide (MnO₂) nanosheets, respectively. The SPN core serves as both NIR fluorescence imaging and photodynamic agent, while the MnO₂ nanosheets act as a sacrificing component to convert H₂O₂ to O₂ under hypoxic and acidic tumor microenvironment. As compared with the uncoated SPN (SPN-0), the oxygenic nanoparticles (SPN-M1) generate 2.68-fold more ¹O₂ at hypoxic and acidic conditions under NIR laser irradiation at 808 nm. Because of such an oxygen-evolution property, SPN-M1 can effectively eradicate cancer cells both in vitro and in vivo. Our study thus not only reports an in situ synthetic method to coat organic nanoparticles but also develops a tumor-microenvironment-sensitive theranostic nanoagent to overcome hypoxia for amplified therapy.

Photoacoustic Imaging of Embryonic Stem Cell-Derived Cardiomyocytes in Living Hearts with Ultrasensitive Semiconducting Polymer Nanoparticles

The last decade has seen impressive progress in human embryonic stem cell-derived cardiomyocytes (hESC-CMs) that makes them ideal tools to repair injured hearts. To achieve an optimal outcome, advanced molecular imaging methods are essential to accurately track these transplanted cells in the heart. Herein, we demonstrate for the first time that a class of photoacoustic nanoparticles (PANPs) incorporating semiconducting polymers (SPs) as contrast agents can be used in the photoacoustic imaging (PAI) of transplanted hESC-CMs in living mouse hearts. This is achieved by virtue of two benefits of PANPs. First, strong PA signals and specific spectral features of SPs allow PAI to sensitively detect and distinguish a small number of PANP-labeled cells (2,000) from background tissues in vivo. Second, the PANPs show a high efficiency for hESC-CM labeling without adverse effects on cell structure, function, and gene expression. Assisted by ultrasound imaging, the delivery and engraftment of hESC-CMs in living mouse hearts can be assessed by PANP-based PAI with high spatial resolution (~100 μm). In summary, this study explores and validates a novel application of SPs as a PA contrast agent to track labeled cells with high sensitivity and accuracy in vivo, highlighting the advantages of integrating PAI and PANPs to advance cardiac regenerative therapies.

Hydrogen peroxide degradable conjugated polymer nanoparticles for fluorescence and photoacoustic bimodal imaging

A hydrogen peroxide degradable fluorescence/photoacoustic dual-modality contrast agent is prepared via in situ Sonogashira polymerization for cellular imaging.

Mesoporous Carbon Nanospheres as a Multifunctional Carrier for Cancer Theranostics

Optical nanomaterials with intense absorption in near-infrared (NIR) region hold great promise for biomedical applications such as photothermal therapy (PTT) and photoacoustic imaging (PAI). In this work, we report mesoporous carbon nanospheres (Meso-CNs) with broadband and intense absorption in the UV-Vis-NIR region (300-1400 nm) and explore their potential as a multifunctional platform for photoacoustic imaging and chemo-photothermal therapy. Methods: Meso-CNs were prepared by a "silica-assisted" synthesis strategy and characterized by transmission electron microscope and optical spectroscopy. We investigated the photothermal conversion and photoacoustic imaging of Meso-CNs in comparison with single-walled carbon nanotubes (SWCNTs), graphene and gold nanorods (GNRs). In vitro cellular assays and in vivo chemo-photothermal combination therapy were performed. Results: The absorption coefficients of Meso-CNs are 1.5-2 times higher than those of SWCNTs and graphene and are comparable to those of GNRs in both the first and the second near-infrared optical windows (NIR-I and NIR-II) of tissues. When exposed to an NIR laser, the photothermal and photoacoustic signal generation of Meso-CNs are also stronger than those of SWCNTs, graphene, and GNRs. DOX was loaded into Meso-CNs with a high efficiency (35 wt%) owing to the unique mesoporous structure. Particularly, the drug release from Meso-CNs is sensitive to both pH and NIR light stimulation. In vivo chemo-photothermal combination therapy demonstrates a remarkable inhibition effect on tumor growth under NIR laser treatment. Conclusions: We have developed Meso-CNs for photothermal conversion and photoacoustic imaging. The porous structure also serves as a drug carrier and the drug release can be controlled by pH and external light. The high drug loading capacity, superior photothermal and photoacoustic generation, together with the apparent chemo-photothermal therapeutic effect, make Meso-CNs a promising platform for cancer theranostics.

Zwitterionic diketopyrrolopyrrole for fluorescence/photoacoustic imaging guided photodynamic/photothermal therapy

Water-soluble zwitterionic diketopyrrolopyrrole (DPP-SPMA) for fluorescence/photoacoustic imaging guided photodynamic/photothermal therapy with favorable renal excretion and ultralow cytotoxicity.

Therapeutic Considerations and Conjugated Polymer-Based Photosensitizers for Photodynamic Therapy

Conjugated polymers have recently attracted a great deal of attention for applications in photodynamic therapy (PDT) because of their light-harvesting capability, efficient energy transfer, and singlet oxygen generation properties. This review describes recent advances in PDT development, including therapeutic mechanisms of PDT in cancer treatments, light excitation methods, and especially recent advances of conjugated polyelectrolytes and conjugated polymer nanoparticles as photosensitizers. The future direction on PDT and further development of conjugated polymer photosensitizers are discussed. The aim of this review is to stimulate innovative ideas to synthesize a new generation of conjugated polymer photosensitizers and promote their translation to clinical applications of PDT.

Semiconducting Polymer Nanoparticles for Centimeters-Deep Photoacoustic Imaging in the Second Near-Infrared Window

Thienoisoindigo-based semiconducting polymer with a strong near-infrared absorbance is synthesized and its water-dispersed nanoparticles (TSPNs) are investigated as a contrast agent for photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (1000-1350 nm). The TSPNs generate a strong PA signal in the NIR-II optical window, where background signals from endogenous contrast agents, including blood and lipid, are at the local minima. By embedding a TSPN-containing tube in chicken-breast tissue, an imaging depth of more than 5 cm at 1064 nm excitation is achieved with a contrast-agent concentration as low as 40 µg mL^-1 . The TSPNs under the skin or in the tumor are clearly visualized at 1100 and 1300 nm, with negligible interference from the tissue background. TSPN as a PA contrast in the NIR-II window opens new opportunities for biomedical imaging of deep tissues with improved contrast.

Molecular Engineering of Conjugated Polymers for Biocompatible Organic Nanoparticles with Highly Efficient Photoacoustic and Photothermal Performance in Cancer Theranostics

Conjugated polymer nanoparticles (CP NPs) are emerging candidates of "all-in-one" theranostic nanoplatforms with dual photoacoustic imaging (PA) and photothermal therapy (PTT) functions. So far, very limited molecular design guidelines have been developed for achieving CPs with highly efficient PA and PTT performance. Herein, by designing CP1, CP2, and CP3 using different electron acceptors (A) and a planar electron donor (D), we demonstrate how the D-A strength affects their absorption, emission, extinction coefficient, and ultimately PA and PTT performance. The resultant CP NPs have strong PA signals with high photothermal conversion efficiencies and excellent biocompatibility in vitro and in vivo. The CP3 NPs show a high PA signal to background ratio of 47 in U87 tumor-bearing mice, which is superior to other reported PA/PTT theranostic agents. A very small IC₅₀ value of 0.88 μg/mL (CP3 NPs) was obtained for U87 glioma cell ablation under laser irradiation (808 nm, 0.8 W/cm², 5 min). This study shows that CP NP based theranostic platforms are promising for future personalized nanomedicine.

Multilayered upconversion nanocomposites with dual photosensitizing functions for enhanced photodynamic therapy

The energy transfer from upconversion nanoparticles (UCNPs) to photosensitizers has been widely used for generating reactive oxygen species (ROS) in photodynamic therapy (PDT) by near infrared (NIR) excitation. However, the poor spectral overlap of lanthanide ions and conventional photosensitizers leads to low PDT efficiency. In this study, we construct a multilayered upconversion nanoplatform with dual photosensitizers to efficiently use the UV and visible upconversion emissions in NIR-responsive PDT. The nanoplatform consists of three functional layers, which are the upconversion nanoparticle as a core, and light-sensitive conjugated polymer and apo-transferrin-titanocene (Tf@Tc) as shells. Under NIR irradiation, apparent energy transfer occurs from the core to the polymer and Tc components in the shell, producing reactive oxygen species and free radicals for cancer cell killing. In vitro cellular assays show the synergistic therapeutic effect of the conjugated polymer and Tc as photosensitizers. In vivo animal studies show that tumor growth is significantly inhibited in the mice receiving the theranostic platform and NIR irradiation. Based on these observations, the multilayered upconversion nanocomposites can find potential applications in NIR-mediated anti-tumor therapy.

Single Near-Infrared Emissive Polymer Nanoparticles as Versatile Phototheranostics

Attaining consistently high performance of diagnostic and therapeutic functions in one single nanoplatform is of great significance for nanomedicine. This study demonstrates the use of donor-acceptor (D-A) structured polymer (TBT) to develop a smart "all-five-in-one" theranostic that conveniently integrates fluorescence/photoacoustic/thermal imaging and photodynamic/photothermal therapy into single nanoparticle. The prepared nanoparticles (TBTPNPs) exhibit near-infrared emission, high water solubility, excellent light resistance, good pH stability, and negligible toxicity. Additionally, the TBTPNPs exhibit an excellent singlet oxygen (1O2) quantum yield (40%) and high photothermal conversion efficiency (37.1%) under single-laser irradiation (635 nm). Apart from their two phototherapeutic modalities, fluorescence, photoacoustic signals, and thermal imaging in vivo can be simultaneously achieved because of their enhanced permeability and retention effects. This work demonstrates that the prepared TBTPNPs are "all-five-in-one" phototheranostic agents that can exhibit properties to satisfy the "one-fits-all" requirement for future phototheranostic applications. Thus, the prepared TBTPNPs can provide fundamental insights into the development of PNP-based nanoagents for cancer therapy.

Monodisperse Au-Fe2C Janus Nanoparticles: An Attractive Multifunctional Material for Triple-Modal Imaging-Guided Tumor Photothermal Therapy

Imaging-guided photothermal therapy (PTT) by combination of imaging and PTT has been emerging as a promising therapeutic method for precision therapy. However, the development of multicomponent nanoplatforms with stable structures for both PTT and multiple-model imaging remains a great challenge. Herein, we synthesized monodisperse Au-Fe₂C Janus nanoparticles (JNPs) of 12 nm, which are multifunctional entities for cancer theranostics. Due to the broad absorption in the near-infrared range, Au-Fe₂C JNPs showed a significant photothermal effect with a 30.2% calculated photothermal transduction efficiency under 808 nm laser irradiation in vitro. Owing to their excellent optical and magnetic properties, Au-Fe₂C JNPs were demonstrated to be advantageous agents for triple-modal magnetic resonance imaging (MRI)/multispectral photoacoustic tomography (MSOT)/computed tomography (CT) both in vitro and in vivo. We found that Au-Fe₂C JNPs conjugated with the affibody (Au-Fe₂C-Z_HER2:342) have more accumulation and deeper penetration in tumor sites than nontargeting JNPs (Au-Fe₂C-PEG) in vivo. Meanwhile, our results verified that Au-Fe₂C-Z_HER2:342 JNPs can selectively target tumor cells with low cytotoxicity and ablate tumor tissues effectively in a mouse model. In summary, monodisperse Au-Fe₂C JNPs, used as a multifunctional nanoplatform, allow the combination of multiple-model imaging techniques and high therapeutic efficacy and have great potential for precision theranostic nanomedicines.

Regulating Near-Infrared Photodynamic Properties of Semiconducting Polymer Nanotheranostics for Optimized Cancer Therapy

Development of optical nanotheranostics for the capability of photodynamic therapy (PDT) provides opportunities for advanced cancer therapy. However, most nanotheranostic systems fail to regulate their generation levels of reactive oxygen species (ROS) according to the disease microenvironment, which can potentially limit their therapeutic selectivity and increase the risk of damage to normal tissues. We herein report the development of hybrid semiconducting polymer nanoparticles (SPNs) with self-regulated near-infrared (NIR) photodynamic properties for optimized cancer therapy. The SPNs comprise a binary component nanostructure: a NIR-absorbing semiconducting polymer acts as the NIR fluorescent PDT agent, while nanoceria serves as the smart intraparticle regular to decrease and increase ROS generation at physiologically neutral and pathologically acidic environments, respectively. As compared with nondoped SPNs, the NIR fluorescence imaging ability of nanoceria-doped SPNs is similar due to the optically inactive nature of nanoceria; however, the self-regulated photodynamic properties of nanoceria-doped SPN not only result in dramatically reduced nonspecific damage to normal tissue under NIR laser irradiation but also lead to significantly enhanced photodynamic efficacy for cancer therapy in a murine mouse model. This study thus provides a simple yet effective hybrid approach to modulate the phototherapeutic performance of organic photosensitizers.

Single-walled carbon nanotube: One specific inhibitor of cancer stem cells in osteosarcoma upon downregulation of the TGFβ1 signaling

Cancer stem cells (CSCs) are believed to have a critical role in tumorigenesis, metastasis, therapeutic resistance or recurrence. Therefore, strategies designed to specifically target and eliminate CSCs have become one of the most promising and desirable ways for tumor treatment. Osteosarcoma stem cells (OSCs), the CSCs in osteosarcoma (OS), are critically associated with OS progression. Here, we show that single-walled carbon nanotubes (SWCNTs), including unmodified SWCNT (SWCNT-Raw) and SWCNT-COOH, have the ability to specifically inhibit the process of TGFβ1-induced OS cells dedifferentiation, prevent the stem cell phenotypes acquisition in OS cells and reduce the OSC viability under conditions which mimic the OS microenvironment. Concurrently, SWCNT treatment significantly down-regulates the expression of OSC markers in OS, and markedly reduces the tumor microvessel density and tumor growth. Furthermore, we found that SWCNT could suppress the TGFβ1-induced activation of TGFβ type I receptor and downstream signaling, which are key for the OSC formation and maintenance. Our results reveal an unexpected function of SWCNT in negative modulation of OSCs, and provide significant implications for the potential CSCs-targeted therapeutic applications of SWCNT.

Investigating the Effect of Chemical Structure of Semiconducting Polymer Nanoparticle on Photothermal Therapy and Photoacoustic Imaging

The donor-acceptor semiconducting polymers (SPs) have robust absorbance in near-infrared (NIR) region, great photostability, high photothermal conversion efficiency, and good biocompatibility. Thus, the SPs exhibit great potentials for photothermal therapy (PTT) and photoacoustic imaging (PAI). However, poor understanding of the underlying mechanisms and the correlation between the SP polymer chemical structures and their performances of PTT and PAI have significantly hindered their biomedical application. Herein, a series of acceptor-π-acceptor type (A1-π-A2) type SPs were synthesized. The diketopyrrolopyrrole (DPP) and thiophene are used as A1 electron accepting block and π-bridge, and the chemical structure of A2 unit was variable. The SPs were formulated into PEGylated nanoparticles, which ensured these SP-based nanoparticles (SP@NPs) exhibited similar size, shape, and physiological stability. Thus, the chemical structure of A2 unit was the only variable. The effects of the SP chemical structures are carefully and comprehensively evaluated through both in vitro and in vivo experiments. Our results demonstrated the chemical structure of A2 unit simultaneously impact their absorption spectra and photothermal (PT) conversion efficiency, which finally determined their PTT and PAI performances. Among these A2 acceptors, thieno[3,2-b]thiophene (TT) unit exhibited the best in vitro and in vivo anticancer efficacies and PAI performances. This study not only provides molecular insights into the design of efficient SPs for PTT and PAI but also highlights the flexibility and potential of SP@NPs for biomedical application. Thus, SP@NPs can act as a versatile nanoplatform for the development of novel light intensive imaging and therapeutic approaches.

Perylene Diimide-Grafted Polymeric Nanoparticles Chelated with Gd3+ for Photoacoustic/T1-Weighted Magnetic Resonance Imaging-Guided Photothermal Therapy

Developing versatile and easily prepared nanomaterials with both imaging and therapeutic properties have received significant attention in cancer diagnostics and therapeutics. Here, we facilely fabricated Gd³⁺-chelated poly(isobutylene-alt-maleic anhydride) (PMA) framework pendent with perylene-3,4,9,10-tetracarboxylic diimide (PDI) derivatives and poly(ethylene glycol) (PEG) as an efficient theranostic platform for dual-modal photoacoustic imaging (PAI) and magnetic resonance imaging (MRI)-guided photothermal therapy. The obtained polymeric nanoparticles (NPs) chelated with Gd³⁺ (PMA-PDI-PEG-Gd NPs) exhibited a high T₁ relaxivity coefficient (13.95 mM^-1 s^-1) even at the higher magnetic fields. After 3.5 h of tail vein injection of PMA-PDI-PEG-Gd NPs, the tumor areas showed conspicuous enhancement in both photoacoustic signal and T₁-weighted MRI intensity, indicating the efficient accumulation of PMA-PDI-PEG-Gd NPs owing to the enhanced permeation and retention effect. In addition, the excellent tumor ablation therapeutic effect in vivo was demonstrated with living mice. Overall, our work illustrated a straightforward synthetic strategy for engineering multifunctional polymeric nanoparticles for dual-modal imaging to obtain more accurate information for efficient diagnosis and therapy.

Ultralong Phosphorescence of Water-Soluble Organic Nanoparticles for In Vivo Afterglow Imaging

Afterglow or persistent luminescence eliminates the need for light excitation and thus circumvents the issue of autofluorescence, holding promise for molecular imaging. However, current persistent luminescence agents are rare and limited to inorganic nanoparticles. This study reports the design principle, synthesis, and proof-of-concept application of organic semiconducting nanoparticles (OSNs) with ultralong phosphorescence for in vivo afterglow imaging. The design principle leverages the formation of aggregates through a top-down nanoparticle formulation to greatly stabilize the triplet excited states of a phosphorescent molecule. This prolongs the particle luminesce to the timescale that can be detected by the commercial whole-animal imaging system after removal of external light source. Such ultralong phosphorescent of OSNs is inert to oxygen and can be repeatedly activated, permitting imaging of lymph nodes in living mice with a high signal-to-noise ratio. This study not only introduces the first category of water-soluble ultralong phosphorescence organic nanoparticles but also reveals a universal design principle to prolong the lifetime of phosphorescent molecules to the level that can be effective for molecular imaging.

Amphiphilic semiconducting polymer as multifunctional nanocarrier for fluorescence/photoacoustic imaging guided chemo-photothermal therapy

Chemo-photothermal nanotheranostics has the advantage of synergistic therapeutic effect, providing opportunities for optimized cancer therapy. However, current chemo-photothermal nanotheranostic systems generally comprise more than three components, encountering the potential issues of unstable nanostructures and unexpected conflicts in optical and biophysical properties among different components. We herein synthesize an amphiphilic semiconducting polymer (PEG-PCB) and utilize it as a multifunctional nanocarrier to simplify chemo-photothermal nanotheranostics. PEG-PCB has a semiconducting backbone that not only serves as the diagnostic component for near-infrared (NIR) fluorescence and photoacoustic (PA) imaging, but also acts as the therapeutic agent for photothermal therapy. In addition, the hydrophobic backbone of PEG-PCB provides strong hydrophobic and π-π interactions with the aromatic anticancer drug such as doxorubicin for drug encapsulation and delivery. Such a trifunctionality of PEG-PCB eventually results in a greatly simplified nanotheranostic system with only two components but multimodal imaging and therapeutic capacities, permitting effective NIR fluorescence/PA imaging guided chemo-photothermal therapy of cancer in living mice. Our study thus provides a molecular engineering approach to integrate essential properties into one polymer for multimodal nanotheranostics.

Broadband Absorbing Semiconducting Polymer Nanoparticles for Photoacoustic Imaging in Second Near-Infrared Window

Photoacoustic (PA) imaging holds great promise for preclinical research and clinical practice. However, most studies rely on the laser wavelength in the first near-infrared (NIR) window (NIR-I, 650-950 nm), while few studies have been exploited in the second NIR window (NIR-II, 1000-1700 nm), mainly due to the lack of NIR-II absorbing contrast agents. We herein report the synthesis of a broadband absorbing PA contrast agent based on semiconducting polymer nanoparticles (SPN-II) and apply it for PA imaging in NIR-II window. SPN-II can absorb in both NIR-I and NIR-II regions, providing the feasibility to directly compare PA imaging at 750 nm with that at 1064 nm. Because of the weaker background PA signals from biological tissues in NIR-II window, the signal-to-noise ratio (SNR) of SPN-II resulted PA images at 1064 nm can be 1.4-times higher than that at 750 nm when comparing at the imaging depth of 3 cm. The proof-of-concept application of NIR-II PA imaging is demonstrated in in vivo imaging of brain vasculature in living rats, which showed 1.5-times higher SNR as compared with NIR-I PA imaging. Our study not only introduces the first broadband absorbing organic contrast agent that is applicable for PA imaging in both NIR-I and NIR-II windows but also reveals the advantages of NIR-II over NIR-I in PA imaging.

Advanced Photoacoustic Imaging Applications of Near-Infrared Absorbing Organic Nanoparticles

Progress of nanotechnology in recent years has stimulated fast development of nanoparticles in biomedical research. Photoacoustic (PA) imaging as an emerging non-invasive technique in molecular imaging has improved imaging depth relative to conventional optical imaging, demonstrating great potential in clinical applications. The convergence of nanotechnology and PA imaging has enabled a broad spectrum of new opportunities in fundamental biology and translation medicine. This review focuses on the recent advances of organic nanoparticles in PA imaging applications. Near-infrared absorbing organic nanoparticles are classified and discussed according to their different imaging applications, which include tumor imaging, gastrointestinal imaging, sentinel lymph node imaging, disease microenvironment imaging and real-time drug imaging. The chemistry and PA properties of organic nanoparticles are discussed in details to highlight their own merits, and their challenges and perspectives in PA imaging are also discussed.

Highly Stable Organic Small Molecular Nanoparticles as an Advanced and Biocompatible Phototheranostic Agent of Tumor in Living Mice

Near-infrared (NIR)-absorbing organic small molecules hold great promise as the phototheranostic agents for clinical translation by virtue of their intrinsic advantages such as well-defined chemical structure, high purity, and good reproducibility. However, most of the currently available ones face the challenges in varying degrees in terms of photothermal instability, and photobleaching/reactive oxygen nitrogen species (RONS) inresistance, which indeed impair their practical applications in precise diagnosis and treatment of diseases. Herein, we developed highly stable and biocompatible organic nanoparticles (ONPs) for effective phototheranostic application by design and synthesis of an organic small molecule (namely TPA-T-TQ) with intensive absorption in the NIR window. The TPA-T-TQ ONPs with no noticeable in vivo toxicity possess better capacities in photothermal conversion and photoacoustic imaging (PAI), as well as show far higher stabilities including thermal/photothermal stabilities, and photobleaching/RONS resistances, when compared with the clinically popularly used indocyanine green. Thanks to the combined merits, the ONPs can serve as an efficient probe for in vivo PAI in a high-contrast manner, which also significantly causes the stoppage of tumor growth in living mice through PAI-guided photothermal therapy. This study thus provides an insight into the development of advanced NIR-absorbing small molecules for practical phototheranostic applications.

Enhanced Phototherapy by Nanoparticle-Enzyme via Generation and Photolysis of Hydrogen Peroxide

Light has been widely used for cancer therapeutics such as photodynamic therapy (PDT) and photothermal therapy. This paper describes a strategy called enzyme-enhanced phototherapy (EEPT) for cancer treatment. We constructed a nanoparticle platform by covalent conjugation of glucose oxidase (GOx) to small polymer dots, which could be persistently immobilized into a tumor. While the malignant tumors have high glucose uptake, the GOx efficiently catalyzes the glucose oxidation with simultaneous generation of H₂O₂. Under light irradiation, the in situ generated H₂O₂ was photolyzed to produce hydroxyl radical, the most reactive oxygen species, for killing cancer cells. In vitro assays indicated that the cancer cells were destroyed by using a nanoparticle concentration at 0.2 μg/mL and a light dose of ∼120 J/cm², indicating the significantly enhanced efficiency of the EEPT method when compared to typical PDT that requires a photosensitizer of >10 μg/mL for effective cell killing under the same light dose. Furthermore, remarkable inhibition of tumor growth was observed in xenograft-bearing mice, indicating the promise of the EEPT approach for cancer therapeutics.