Diketopyrrolopyrrole-Based Semiconducting Polymer Nanoparticles for In Vivo Photoacoustic Imaging

1300 nm absorption two-acceptor semiconducting polymer nanoparticles for NIR-II photoacoustic imaging system guided NIR-II photothermal therapy

We designed 1300 nm absorption semiconducting polymer nanoparticles (SPNs3) based on a two-acceptor semiconducting polymer to realize in vivo NIR-II photothermal therapy (PTT) treatment guided by commercial small-animal NIR-II photoacoustic imaging (PAI) systems.

A Gold Nanocage/Cluster Hybrid Structure for Whole-Body Multispectral Optoacoustic Tomography Imaging, EGFR Inhibitor Delivery, and Photothermal Therapy

Gold nanocages (AuNCs) and gold nanoclusters (AuClusters) are two classes of advantageous nanostructures with special optical properties, and many other attractive properties. Integrating them into one nanosystem may achieve greater and smarter performance. Herein, a hybrid gold nanostructure for fluorescent and optoacoustic tomography imaging, controlled release of drugs, and photothermal therapy (PTT) is demonstrated. For this nanodrug (EA-AB), an epidermal growth factor receptor (EGFR) inhibitor erlotinib (EB) is loaded into AuNCs, which are then capped and functionalized by biocompatible AuCluster@BSA (BSA = bovine serum albumin) conjugates via electrostatic interaction. Upon cell internalization, the lysosomal proteases and low pH cause the release of EB from EA-AB, and also induce fluorescence restoration of the AuCluster for imaging. Irradiation with near-infrared light further promotes the drug release and affords a PTT effect as well. The AuNC-based nanodrug is optoacoustically active, and its biodistribution and metabolic process have been successfully monitored by whole-body and 3D multispectral optoacoustic tomography imaging. Owing to the combined actions of PTT and EGFR pathway blockage, EA-AB exhibits marked tumor inhibition efficacy in vivo.

Boron Dipyrromethene Nano-Photosensitizers for Anticancer Phototherapies

As traditional phototherapy agents, boron dipyrromethene (BODIPY) photosensitizers have attracted increasing attention due to their high molar extinction coefficients, high phototherapy efficacy, and excellent photostability. After being formed into nanostructures, BODIPY-containing nano-photosensitizers show enhanced water solubility and biocompatibility as well as efficient tumor accumulation compared to BODIPY molecules. Hence, BODIPY nano-photosensitizers demonstrate a promising potential for fighting cancer. This review contains three sections, classifying photodynamic therapy (PDT), photothermal therapy (PTT), and the combination of PDT and PTT based on BODIPY nano-photosensitizers. It summarizes various BODIPY nano-photosensitizers, which are prepared via different approaches including molecular precipitation, supramolecular interactions, and polymer encapsulation. In each section, the design strategies and working principles of these BODIPY nano-photosensitizers are highlighted. In addition, the detailed in vitro and in vivo applications of these recently developed nano-photosensitizers are discussed together with future challenges in this field, highlighting the potential of these promising nanoagents for new tumor phototherapies.

Precise Deciphering of Brain Vasculatures and Microscopic Tumors with Dual NIR-II Fluorescence and Photoacoustic Imaging

Diagnostics of cerebrovascular structures and microscopic tumors with intact blood-brain barrier (BBB) significantly contributes to timely treatment of patients bearing neurological diseases. Dual NIR-II fluorescence and photoacoustic imaging (PAI) is expected to offer powerful strength, including good spatiotemporal resolution, deep penetration, and large signal-to-background ratio (SBR) for precise brain diagnostics. Herein, biocompatible and photostable conjugated polymer nanoparticles (CP NPs) are reported for dual-modality brain imaging in the NIR-II window. Uniform CP NPs with a size of 50 nm are fabricated from microfluidics devices, which show an emission peak at 1156 nm with a large absorptivity of 35.2 L g^-1 cm^-1 at 1000 nm. The NIR-II fluorescence imaging resolves hemodynamics and cerebral vasculatures with a spatial resolution of 23 µm at a depth of 600 µm. The NIR-II PAI enables successful noninvasive mapping of deep microscopic brain tumors (<2 mm at a depth of 2.4 mm beneath dense skull and scalp) with an SBR of 7.2 after focused ultrasound-induced BBB opening. This study demonstrates that CP NPs are promising contrast agents for brain diagnostics.

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

Exogenous contrast-agent-assisted NIR-II optical-resolution photoacoustic microscopy imaging (ORPAMI) holds promise to decipher wide-field 3D biological structures with deep penetration, large signal-to-background ratio (SBR), and high maximum imaging depth to depth resolution ratio. Herein, NIR-II conjugated polymer nanoparticle (CP NP) assisted ORPAMI is reported for pinpointing cerebral and tumor vasculatures. The CP NPs exhibit a large extinction coefficient of 48.1 L g^-1 at the absorption maximum of 1161 nm, with an ultrahigh PA sensitivity up to 2 µg mL^-1 . 3D ORPAMI of wide-field mice ear allows clear visualization of regular vasculatures with a resolution of 19.2 µm and an SBR of 29.3 dB at the maximal imaging depth of 539 µm. The margin of ear tumor composed of torsional dense vessels among surrounding normal regular vessels can be clearly delineated via 3D angiography. In addition, 3D whole-cortex cerebral vasculatures with large imaging area (48 mm² ), good resolution (25.4 µm), and high SBR (22.3 dB) at a depth up to 1001 µm are clearly resolved through the intact skull. These results are superior to the recently reported 3D NIR-II fluorescence confocal vascular imaging, which opens up new opportunities for NIR-II CP-NP-assisted ORPAMI in various biomedical applications.

Gadolinium-Chelated Conjugated Polymer-Based Nanotheranostics for Photoacoustic/Magnetic Resonance/NIR-II Fluorescence Imaging-Guided Cancer Photothermal Therapy

Our exploiting versatile multimodal theranostic agent aims to integrate the complementary superiorities of photoacoustic imaging (PAI), second near-infrared (NIR-II, 1000-1700) fluorescence and T₁-weighted magnetic resonance imaging (MRI) with an ultimate objective of perfecting cancer diagnosis, thus improving cancer therapy efficacy. Herein, we engineered and prepared a water-soluble gadolinium-chelated conjugated polymer-based theranostic nanomedicine (PFTQ-PEG-Gd NPs) for in vivo tri-mode PA/MR/NIR-II imaging-guided tumor photothermal therapy (PTT). Methods: We firstly constructed a semiconducting polymer composed of low-bandgap donor-acceptor (D-A) which afforded the strong NIR absorption for PAI/PTT and long fluorescence emission to NIR-II region for in vivo imaging. Then, the remaining carboxyl groups of the polymeric NPs could effectively chelate with Gd³⁺ ions for MRI. The in vitro characteristics of the PFTQ-PEG-Gd NPs were studied and the in vivo multimode imaging as well as anti-tumor efficacy of the NPs was evaluated using 4T1 tumor-bearing mice. Results: The obtained theranostic agent showed excellent chemical and optical stability as well as low biotoxicity. After 24 h of systemic administration using PQTF-PEG-Gd NPs, the tumor sites of living mice exhibited obvious enhancement in PA, NIR-II fluorescence and positive MR signal intensities. Better still, a conspicuous tumor growth restraint was detected under NIR light irradiation after administration of PQTF-PEG-Gd NPs, indicating the efficient photothermal potency of the nano-agent. Conclusion: we triumphantly designed and synthesized a novel and omnipotent semiconducting polymer nanoparticles-based theranostic platform for PAI, NIR-II fluorescence imaging as well as positive MRI-guided tumor PTT in living mice. We expect that such a novel organic nano-platform manifests a great promise for high spatial resolution and deep penetration cancer theranostics.

Synthesis and Optoelectronic Properties of New Methoxy-Substituted Diketopyrrolopyrrole Polymers

The introduction of functional groups with varying electron-donating/-withdrawing properties at the β-position of diketopyrrolopyrrole (DPP) has been shown to affect the optoelectronic properties of the polymers. We report the synthesis of a new diketopyrrolopyrrole monomer wherein a strong electron-donating substituent, a methoxy group, was incorporated at the β-position in an effort to modulate polymer properties. Homopolymers and co-polymers of the new β-methoxy DPP and nonderivatized DPP were synthesized, and their properties were measured by cyclic voltammetry and UV-vis-near-infrared. Density functional theory computations also were employed to predict the degree of planarity of β-methoxy oligomers to probe the significance of the newly introduced S-O conformational lock. The combined experimental and computational results showed a reduction in the gap between highest occupied molecular orbital/lowest unoccupied molecular orbital levels, a redshift toward the near-infrared region, and an increased planarity in the β-methoxy polymers.

A BODIPY-Based Donor/Donor-Acceptor System: Towards Highly Efficient Long-Wavelength-Excitable Near-IR Polymer Dots with Narrow and Strong Absorption Features

Bright long-wavelength-excitable semiconducting polymer dots (LWE-Pdots) are highly desirable for in vivo imaging and multiplexed in vitro bioassays. LWE-Pdots have been obtained by incorporating a near-infrared (NIR) emitter into the backbone of a polymer host to develop a binary donor-acceptor (D-A) system. However, they usually suffer from severe concentration quenching and a trade-off between fluorescence quantum yield (Φf ) and absorption cross-section (σ). Herein, we describe a ternary component (D1 /D2 -A) strategy to achieve ultrabright, green laser-excitable Pdots with narrow-band NIR emission by introducing a BODIPY-based assistant polymer donor as D1 . The D1 /D2 -A Pdots possess improved Φf and σ compared to corresponding binary D2 -A Pdots. Their Φf is as high as 40.2 %, one of the most efficient NIR Pdots reported. The D1 /D2 -A Pdots show ultrahigh single-particle brightness, 83-fold brighter than Qdot 705 when excited by a 532 nm laser. When injected into mice, higher contrast in vivo tumor imaging was achieved using the ternary Pdots versus the binary D-A Pdots.

Facial Control Intramolecular Charge Transfer of Quinoid Conjugated Polymers for Efficient in Vivo NIR-II Imaging

Low-band gap conjugated polymers with donor-acceptor (D-A) structures have emerged as  second near-infrared (NIR-II) fluorescence probes for biological imaging. However, how to control the intramolecular charge transfer (ICT) to maintain the low band gap and improve the NIR-II fluorescence intensity is an urgent issue. Here, the quinoid polymers have been developed to effectively regulate the ICT for brighter NIR-II fluorescence signals. Thiophene repeat chain units of different lengths (T, 2T, and 3T) were utilized to link with electron-withdrawing ester-substituted thieno[3,4- b]thiophene (TT) to alter the density of the electron-withdrawing side groups for controlling the ICT. By increasing the thiophene chain length from TT-T to TT-3T, the density of the electron-withdrawing groups decreased and the ICT was weakened. In the case of NIR absorption and NIR-II emission, weakened ICT leads to brighter NIR-II fluorescence. After the preparation of the water-soluble quinoid polymer probes (CPs), TT-3T CPs with weak ICT exhibited the brightest NIR-II fluorescent signals among the three quinoid polymer probes. Several NIR-II biomedical imaging applications, including in vivo cell tracking, blood vascular system images, and lymphatic drainage mapping, show that the TT-3T CP-based nanoprobe had excellent characteristics of long-term stability and high NIR-II spatial resolutions in vivo.

Intelligent polymer-MnO2 nanoparticles for dual-activatable photoacoustic and magnetic resonance bimodal imaging in living mice

Herein, we report a dual-activatable MRI/PAI strategy for bimodal tumor imaging using an intelligent platform based on degradable MnO₂ and a near-infrared absorptive polymer conjugated with BODIPY molecules. We believe the smart platform could promote the advance of numerous dual-activatable bimodal imaging techniques for biological applications.

Photostable Iridium(III)-Cyanine Complex Nanoparticles for Photoacoustic Imaging Guided Near-Infrared Photodynamic Therapy in Vivo

The iridium(III)-cyanine complex (IrCy) was fabricated by conjugating an iridium(III) complex to a cyanine dye with an intense near-infrared (NIR) absorption. IrCy complex nanoparticles (NPs) with high water solubility and photostability were prepared by a solvent evaporation-induced self-assembly strategy. Considering their effective photacoustic (PA) imaging and generation of ¹O₂ property with 808 nm laser irradiation in aqueous solution, PA imaging guided NIR-driven photodynamic therapy in vivo was effectively conducted in the 4T1 xenograft model. We developed a real-time PA imaging methodology to investigate the pharmacokinetics, tumor targeting, and biodistribution of IrCy NPs. Taking advantage of the analysis of the PA signal of the common iliac vein, the blood circulation half-life of IrCy NPs in mice was calculated to be ∼18 h, and the enhanced permeability and retention effect of IrCy NPs offered the maximum targeting property in the tumor at about 24 h. The obvious change of PA imaging signal in kidney and bladder confirmed IrCy NPs should be excreted partially from the urine system, and the PA signal decreased from 12.5× to 2.8× in the liver, and from 28.8× to 9.4× in the spleen also confirmed the hepatic metabolic pathway.

State-of-the-Art Preclinical Photoacoustic Imaging in Oncology: Recent Advances in Cancer Theranostics

The optical imaging plays an increasing role in preclinical studies, particularly in cancer biology. The combined ultrasound and optical imaging, named photoacoustic imaging (PAI), is an emerging hybrid technique for real-time molecular imaging in preclinical research and recently expanding into clinical setting. PAI can be performed using endogenous contrast, particularly from oxygenated and deoxygenated hemoglobin and melanin, or exogenous contrast agents, sometimes targeted for specific biomarkers, providing comprehensive morphofunctional and molecular information on tumor microenvironment. Overall, PAI has revealed notable opportunities to improve knowledge on tumor pathophysiology and on the biological mechanisms underlying therapy. The aim of this review is to introduce the principles of PAI and to provide a brief overview of current PAI applications in preclinical research, highlighting also on recent advances in clinical translation for cancer diagnosis, staging, and therapy.

A 1064 nm excitable semiconducting polymer nanoparticle for photoacoustic imaging of gliomas

Photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (especially at 1064 nm) has the benefits of low background signal, high spatial resolution and deep tissue penetration. Here we report a semiconducting polymer nanoparticle (PDPPTBZ NP) and demonstrate its potential as a contrast agent for PA imaging of orthotopic brain tumors, using a 1064 nm pulsed laser as a light source. PDPPTBZ NPs have maximum absorption at 1064 nm with a mass extinction coefficient of 43 mL mg-1 cm-1, which is the highest value reported so far in this region. The high photothermal conversion efficiency (67%) and near non-fluorescence impart PDPPTBZ NPs with excellent PA properties. We used PDPPTBZ NP-containing agar gel phantoms even at a low concentration (50 μg mL-1) to successfully image to a depth of 4 cm (of chicken-breast tissue), with an ultralow power fluence (4 mJ cm-2). Furthermore, we could clearly visualize a glioma tumor in a mouse at a depth of 3.8 mm below the skull. This study demonstrates that PDPPTBZ NPs display great potential as a NIR-II PA contrast agent for high quality deep tissue imaging.

Endoplasmic reticulum-targeted phototherapy using one-step synthesized trace metal-doped carbon-dominated nanoparticles: Laser-triggered nucleolar delivery and increased tumor accumulation

Lysosomal entrapment and liver accumulation are the two main obstacles faced by many anticancer drugs for achieving satisfactory therapeutic outcomes. Here, we develop a facile one-step hydrothermal synthetic route to prepare trace metal (M)-, N-, and O-doped carbon-dominated nanoparticles (termed as MNOCNPs, M = Ni, Pd, or Cu, metal content: <0.1 mol%) with exceptional photothermal properties (e.g., the ultrahigh extinction coefficient of 32.7 L g^-1 cm^-1), which can simultaneously realize preferable endoplasmic reticulum (ER) targeting and specific tumor enrichment without noticeable liver accumulation after poly(ethylene glycol) (PEG) conjugation. More interestingly, the PEG-modified MNOCNPs with nanoscale lengths exhibit considerable nucleolar delivery and increased tumor accumulation upon laser irradiation. After fluorescence labeling, these PEG-modified MNOCNPs are suitable for fluorescence/photoacoustic/thermal triple-modal imaging-guided photothermal cancer treatment. Additionally, the ultralow metal content ensures the exceptional biosafety of the nanoagents. The present work provides a novel, facile, and general synthetic method of carbon-dominated nanoparticles with superior photothermal properties for highly efficient tumor ablation, and the large-organelle (ER and nucleus)-targeted cancer therapeutic strategy may represent an alternative solution for optimizing the anticancer efficacy of nanomaterials. STATEMENT OF SIGNIFICANCE: Limited wire-like nanomaterials have been used for biomedical applications due to their lack of intrinsic photothermal properties, poor cellular uptake and tumor accumulation, and potential biotoxicity arising from their micrometer lengths and/or massive heavy metal doping. Besides, the clinical applications of many nanoagents are hindered by their tendency to accumulate in liver, which may cause severe liver toxicity. Herein, we develop for the first time a one-step hydrothermal method to prepare wire-like trace metal-, N-, and O-doped carbon-dominated nanoparticles with excellent photothermal properties, massive cellular uptake, preferable ER localization, selective tumor targeting with negligible liver deposition, laser irradiation-enhanced nucleolar delivery and tumor accumulation, and multimodal imaging-guided cancer therapy. This work opens a new window for simultaneously overcoming lysosomal entrapment and liver accumulation in cancer therapy.

Photothermal therapy and photoacoustic imaging via nanotheranostics in fighting cancer

The nonradiative conversion of light energy into heat (photothermal therapy, PTT) or sound energy (photoacoustic imaging, PAI) has been intensively investigated for the treatment and diagnosis of cancer, respectively. By taking advantage of nanocarriers, both imaging and therapeutic functions together with enhanced tumour accumulation have been thoroughly studied to improve the pre-clinical efficiency of PAI and PTT. In this review, we first summarize the development of inorganic and organic nano photothermal transduction agents (PTAs) and strategies for improving the PTT outcomes, including applying appropriate laser dosage, guiding the treatment via imaging techniques, developing PTAs with absorption in the second NIR window, increasing photothermal conversion efficiency (PCE), and also increasing the accumulation of PTAs in tumours. Second, we introduce the advantages of combining PTT with other therapies in cancer treatment. Third, the emerging applications of PAI in cancer-related research are exemplified. Finally, the perspectives and challenges of PTT and PAI for combating cancer, especially regarding their clinical translation, are discussed. We believe that PTT and PAI having noteworthy features would become promising next-generation non-invasive cancer theranostic techniques and improve our ability to combat cancers.

Thieno[3,2-b]thiophene-DPP based near-infrared nanotheranostic agent for dual imaging-guided photothermal/photodynamic synergistic therapy

Diketopyrrolopyrrole (DPP) based organic molecules have drawn significant research attention as phototheranostic agents. Herein, based on thieno[3,2-b]thienyl-DPP (TT-DPP), a near-infrared small molecule photosensitizer diethyl 3,3'-((((2,5-bis(2-decyltetradecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thieno[3,2-b]thiophene-5,2-diyl))bis-(4,1-phenylene))bis(7-bromo-10H-phenothiazine-10,3-diyl))(2E,2'E)-diacrylate (PDBr), with a high singlet oxygen (¹O₂) quantum yield of 67%, was developed. After nano-precipitation, the hydrophilic PDBr NPs present an encouraging photothermal conversion efficiency of 35.7% and excellent fluorescence/infrared-thermal imaging performance. In vitro studies disclosed the high phototoxicity but low dark cytotoxicity of PDBr NPs to tumor cells. Furthermore, PDBr NPs can effectively impede the tumor growth without noticeable side effects in living mice through imaging-guided synergistic photothermal/photodynamic therapy. Therefore, PDBr NPs could be a promising nanotheranostic agent for imaging-guided synergistic photothermal and photodynamic therapy in the clinic.

Facile syntheses of conjugated polymers for photothermal tumour therapy

Development of photothermal materials which are able to harness sunlight and convert it to thermal energy seems attractive. Besides carbon-based nanomaterials, conjugated polymers are emerging promising photothermal materials but their facile syntheses remain challenging. In this work, by modification of a CBT-Cys click condensation reaction and rational design of the starting materials, we facilely synthesize conjugated polymers poly-2-phenyl-benzobisthiazole (PPBBT) and its dihexyl derivative with good photothermal properties. Under the irradiation of either sunlight-mimicking Xe light or near-infrared laser, we verify that PPBBT has comparable photothermal heating-up speed to that of star material single-wall carbon nanotube. Moreover, PPBBT is used to fabricate water-soluble NPPPBBT nanoparticles which maintain excellent photothermal properties in vitro and photothermal therapy effect on the tumours exposed to laser irradiation. We envision that our synthetic method provides a facile approach to fabricate conjugated polymers for more promising applications in biomedicine or photovoltaics in the near future.

Highly efficient photothermal nanoagent achieved by harvesting energy via excited-state intramolecular motion within nanoparticles

The exciting applications of molecular motion are still limited and are in urgent pursuit, although some fascinating concepts such as molecular motors and molecular machines have been proposed for years. Utilizing molecular motion in a nanoplatform for practical application has been scarcely explored due to some unconquered challenges such as how to achieve effective molecular motion in the aggregate state within nanoparticles. Here, we introduce a class of near infrared-absorbing organic molecules with intramolecular motion-induced photothermy inside nanoparticles, which enables most absorbed light energy to dissipate as heat. Such a property makes the nanoparticles a superior photoacoustic imaging agent compared to widely used methylene blue and semiconducting polymer nanoparticles and allow them for high-contrast photoacoustic imaging of tumours in live mice. This study not only provides a strategy for developing advanced photothermal/photoacoustic imaging nanoagents, but also enables molecular motion in a nanoplatform to find a way for practical application.

Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

Photoacoustic (PA) imaging as a fast-developing imaging technique has great potential in biomedical and clinical applications. It is a noninvasive imaging modality that depends on the light-absorption coefficient of the imaged tissue and the injected PA-imaging contrast agents. Furthermore, PA imaging provides superb contrast, super spatial resolution, and high penetrability and sensitivity to tissue functional characteristics by detecting the acoustic wave to construct PA images. In recent years, a series of PA-imaging contrast agents are developed to improve the PA-imaging performance in biomedical applications. Here, recent progress of PA contrast agents and their biomedical applications are outlined. PA contrast agents are classified according to their components and function, and gold nanocrystals, gold-nanocrystal assembly, transition-metal chalcogenides/MXene-based nanomaterials, carbon-based nanomaterials, other inorganic imaging agents, small organic molecules, semiconducting polymer nanoparticles, and nonlinear PA-imaging contrast agents are discussed. The applications of PA contrast agents as biosensors (in the sensing of metal ions, pH, enzymes, temperature, hypoxia, reactive oxygen species, and reactive nitrogen species) and in bioimaging (lymph nodes, vasculature, tumors, and brain tissue) are discussed in detail. Finally, an outlook on the future research and investigation of PA-imaging contrast agents and their significance in biomedical research is presented.

Functional Organic Semiconductors Based on Bay-Annulated Indigo (BAI)

The advancement of organic electronics has been continually pushed by the need for stable and high performance acceptor materials. By utilizing inexpensive and stable indigo dye as a starting material, Bay-Annulated Indigo (BAI) provides a new motif for the development of semiconducting materials. Modular and straightforward synthesis makes BAI an outstanding platform for molecular design, while excellent stability, strong absorption, and high ambipolar mobility render BAI-based materials excellent candidates for organic electronics. BAI-based polymers and small molecules have taken advantage of these properties to show promising results in a variety of applications.

Ratiometric Photoacoustic Nanoprobe for Bioimaging of Cu2

Aberrant copper contents implicate numerous diseases including Alzheimer's disease and Wilson's disease. Conventional copper detection technologies are difficult to offer non-invasive and accurate deep tissue detection of copper. Here, we report a photoacoustic (PA) nanoprobe (NRh-IR-NMs) for ratiometric PA imaging of Cu²⁺. The nanoprobe consists of a selective Cu²⁺-responsive probe (NRh) as the indicator and a nonresponsive dye (IR) as the internal reference. In the presence of Cu²⁺, a selective Cu²⁺-induced structure change of NRh would take place, resulting in the increase of PA signal intensity increment at 716 nm (ΔPA716). However, the ΔPA834 which attributes to IR shows negligible change. Therefore, the ratiometric PA signal (ΔPA716/ΔPA834) could be used as an indicator for Cu²⁺ detection. This ratiometric PA detection method offers a noninvasive technology with high selectivity and tissue penetration depth, which is a promising tool for deep-tissue detection of Cu²⁺ in living organisms.

2-Dicyanomethylenethiazole based NIR absorbing organic nanoparticles for photothermal therapy and photoacoustic imaging

TPTHM NPs are candidates for PAI/PTT agents due to their NIR absorption, good biocompatibility, high photostability and photothermal conversion efficiency (38%).

A targeted biocompatible organic nanoprobe for photoacoustic and near-infrared-II fluorescence imaging in living mice

Multimodal molecular imaging probes have attracted much attention, and they possess great potential to accurately diagnose diseases due to the synergistic superiorities of multiple complementary imaging. Herein, a targeted biocompatible organic nanoplatform (IR-PEG-FA) with a strong optical absorption in the near-infrared window (NIR-I) for photoacoustic imaging (PAI) and excellent second near-infrared (NIR-II) fluorescence imaging property for NIR-II imaging is fabricated. The dual-modal nanoprobe is composed of the small organic dye molecule IR-1061, water-soluble poly(ethylene glycol) (PEG) and folic acid (FA) as the targeted ligands. Depending on the strength of high temporal resolution and preeminent spatial resolution, the targeted biocompatible dual-mode nanoprobe for PAI and NIR-II imaging can provide more detailed date of cancers and diseases, and enables us to specifically diagnose them through quite a precise way.

We developed a targeted organic nanoprobe for both photoacoustic imaging and near-infrared-II fluorescence imaging in living mice.

Current concepts in nanostructured contrast media development for in vivo photoacoustic imaging

To overcome the endogenous photoacoustic contrast arising from endogenous species, specific contrast agents need to be developed, allowing PAI to successfully identify targeted contrast in the range of wavelength in which the interference from the biomatrix is minimized.

Development of organic semiconducting materials for deep-tissue optical imaging, phototherapy and photoactivation

Recent progress in developing organic semiconducting materials (OSMs) for deep-tissue optical imaging, cancer phototherapy and biological photoactivation is summarized.

Bandgap Engineered Polypyrrole-Polydopamine Hybrid with Intrinsic Raman and Photoacoustic Imaging Contrasts

Intrinsically multimodal nanomaterials have revealed their great potential as a new class of contrast agents. We herein report a bandgap engineering strategy to develop an intrinsically Raman-photoacoustic (PA) active probe that is based on semiconducting conjugated polymers. This dual modal probe is prepared by doping a semiconducting conjugated polymer with polydopamine (PDA) through a one-pot reaction. When applied in the polypyrrole (PPy), this strategy can enhance Raman scattering and the PA amplitude of PPy-PDA hybrid by 3.2 and 2.4 times, respectively, so that both signals can be further applied in bioimaging. In the hybrid, such a dual-enhancement effect is achieved by infusing these two macromolecules at the nanoscale to reduce the optical bandgap energy. This work not only introduces a dual modal contrast agent but also provides a new method of manipulating semiconducting polymer's inherent optical features for bioimaging.

Organic Semiconducting Agents for Deep-Tissue Molecular Imaging: Second Near-Infrared Fluorescence, Self-Luminescence, and Photoacoustics

Optical imaging has played a pivotal role in biology and medicine, but it faces challenges of relatively low tissue penetration and poor signal-to-background ratio due to light scattering and tissue autofluorescence. To overcome these issues, second near-infrared fluorescence, self-luminescence, and photoacoustic imaging have recently emerged, which utilize an optical region with reduced light-tissue interactions, eliminate real-time light excitation, and detect acoustic signals with negligible attenuation, respectively. Because there are only a few endogenous molecules absorbing or emitting above the visible region, development of contrast agents is essential for those deep-tissue optical imaging modalities. Organic semiconducting agents with π-conjugated frameworks can be synthesized to meet different optical imaging requirements due to their easy chemical modification and legible structure-property relation. Herein, the deep-tissue optical imaging applications of organic semiconducting agents including small-molecule agents and nanoparticle derivatives are summarized. In particular, the molecular engineering and nanoformulation approaches to further improve the tissue penetration and detection sensitivity of these optical imaging modalities are highlighted. Finally, current challenges and potential opportunities in this emerging subfield of biomedical imaging are discussed.

A light-induced nitric oxide controllable release nano-platform based on diketopyrrolopyrrole derivatives for pH-responsive photodynamic/photothermal synergistic cancer therapy

Emerging treatment approaches, such as gas therapy (GT), photodynamic therapy (PDT) and photothermal therapy (PTT), have received widespread attention. The development of an intelligent multifunctional nano-platform responding to tumor microenvironments for multimodal therapy is highly desirable. Herein, a near-infrared (NIR) light-responsive nitric oxide (NO) photodonor (4-nitro-3-trifluoromethylaniline, NF) and a pH-sensitive group (dimethylaminophenyl) have been introduced into a diketopyrrolopyrrole core (denoted as DPP-NF). The DPP-NF nanoparticles (NPs) can be activated under weakly acidic conditions of lysosomes (pH 4.5-5.0) to generate reactive oxygen species (ROS) and enhance photothermal efficiency. The fluorescence detection demonstrated that NO controllable release can be realized by "on-off" switching of the NF unit under NIR light irradiation or dark conditions. The controllable NO release of DPP-NF NPs can not only trigger tumor cell death by DNA damage, but also overcome PDT inefficiencies caused by hypoxia in tumors. Additionally, DPP-NF NPs displayed 45.6% photothermal conversion efficiency, making them superior to other reported DPP derivatives. In vitro studies showed that DPP-NF NPs possessed low dark toxicity and high phototoxicity with a half-maximal inhibitory concentration of about 38 μg mL-1. In vivo phototherapy indicated that DPP-NF NPs exhibited excellent tumor phototherapeutic efficacy with passive targeting of the tumor site via the enhanced permeability and retention (EPR) effect. These results highlight that the nano-platform has promising potential for NO-mediated multimodal synergistic phototherapy in clinical settings.

NIR-Absorbing water-soluble conjugated polymer dots for photoacoustic imaging-guided photothermal/photodynamic synergetic cancer therapy

"Theranostics" become increasingly significant in current personalized precision medicine. Herein, we developed a new NIR-absorbing photo-theranostic agent based on water-soluble diketopyrrolopyrrole (DPP) conjugated polymer (WSCP) dots. The WSCPs can be easily self-assembled into WSCP dots under ultrasonication only, instead of any other nano-technology. Compared to the monomers of WSCPs, WSCP dots have no fluorescence emission but produce photoacoustic (PA) signal detected upon laser irradiation due to the reduced energy loss from excited state. PA imaging in vivo indicated that WSCP dots can accumulate at tumor site within 4 h post-injection. More importantly, WSCP dots not only generate heat with a photothermal conversion efficiency of ∼54%, but also produce reactive oxygen species (ROS, QY ∼13%). Furthermore, in vitro and in vivo experiments confirmed effective inhibition of tumor growth by WSCP dots via synergetic photothermal/photodynamic therapy. All results indicate a great potential of WSCP dots as highly efficient theranostic agents in PA imaging-guided synergetic cancer treatment.

Photoacoustic and Magnetic Resonance Imaging Bimodal Contrast Agent Displaying Amplified Photoacoustic Signal

Progress in photoacoustic (PA) and magnetic resonance imaging (MRI) bimodal contrast agents has been achieved mainly by utilizing the imaging capability of single or multiple components and consequently realizing the desired application for both imaging modalities. However, the mechanism of the mutual influence between components within a single nanoformulation, which is the key to developing high-performance multimodal contrast agents, has yet to be fully understood. Herein, by integrating conjugated polymers (CPs) with iron oxide (IO) nanoparticles using an amphiphilic polymer, a bimodal contrast agent named CP-IO is developed, displaying 45% amplified PA signal intensity as compared to bare CP nanoparticle, while the performance of MRI is not affected. Further experimental and theoretical simulation results reveal that the addition of IO nanoparticles in CP-IO nanocomposites contributes to this PA signal amplification through a synergistic effect of additional heat generation and faster heat dissipation. Besides, the feasibility of CP-IO nanocomposites acting as PA-MRI bimodal contrast agents is validated through in vivo tumor imaging using mice models. From this study, it is demonstrated that a delicately designed structural arrangement of various components in a contrast agent could potentially lead to a superior performance in the imaging capability.

Aggregation-Induced Emission Luminogens: Union Is Strength, Gathering Illuminates Healthcare

The rapid development of healthcare techniques encourages the emergence of new molecular imaging agents and modalities. Fluorescence imaging that enables precise monitoring and detection of biological processes/diseases is extensively investigated as this imaging technique has strengths in terms of high sensitivity, excellent temporal resolution, low cost, and good safety. Aggregation-induced emission luminogens (AIEgens) have recently emerged as a new class of emitters that possess several notable features, such as high brightness, large Stokes shift, marked photostability, good biocompatibility, and so on. So far, AIEgens are widely explored and exhibit superb performance in the area of biomedicine and life sciences. Herein, this review summarizes and discusses the recent investigations of AIEgens for in vivo diagnosis and therapy including long-term tracking, 3D angiography, multimodality imaging, disease theranostics, and activatable sensing. Collectively, these results reveal that AIEgens are of great promise for in vivo biomedical applications. It is hoped that this review will lead to new insights into the development of advanced healthcare materials.

Highly monodisperse beta-cyclodextrin-covellite nanoparticles for efficient photothermal and chemotherapy

High quality covellite phase copper sulfide nanoparticles with excellent photothermal performance were obtained using an easy, non-hot injection method. Then, colloidal stable drug delivery vehicles were fabricated with an amphiphilic polymer and covalently linked with beta-cyclodextrin, which allows the integration of multiple target ligands and drugs, thus providing a versatile nano-platform for efficient photothermal and chemotherapy.

"Dual Lock-and-Key"-Controlled Nanoprobes for Ultrahigh Specific Fluorescence Imaging in the Second Near-Infrared Window

Fluorescence imaging in the second near-infrared window (NIR-II) is a new technique that permits visualization of deep anatomical features with unprecedented spatial resolution. Although attractive, effectively suppressing the interference signal of the background is still an enormous challenge for obtaining target-specific NIR-II imaging in the complex and dynamic physiological environment. Herein, dual-pathological-parameter cooperatively activatable NIR-II fluorescence nanoprobes (HISSNPs) are developed whereby hyaluronic acid chains and disulfide bonds act as the "double locks" to lock the fluorescence-quenched aggregation state of the NIR-II fluorescence dyes for performing ultrahigh specific imaging of tumors in vivo. The fluorescence can be lit up only when the "double locks" are opened by reacting with the "dual smart keys" (overexpressed hyaluronidase and thiols in tumor) simultaneously. In vivo NIR-II imaging shows that they reduce nonspecific activitation and achieve ultralow background fluorescence, which is 10.6-fold lower than single-parameter activatable probes (HINPs) in the liver at 15 h postinjection. Consequently, these "dual lock-and-key"-controlled HISSNPs exhibit fivefold higher tumor-to-normal tissue ratio than "single lock-and-key"-controlled HINPs at 24 h postinjection, attractively realizing ultrahigh specificity of tumor imaging. This is thought to be the first attempt at implementing ultralow background interference with the participation of multiple pathological parameters in NIR-II fluorescence imaging.