Diketopyrrolopyrrole-Based Semiconducting Polymer Nanoparticles for In Vivo Photoacoustic Imaging

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.

Tunable Semiconducting Polymer Nanoparticles with INDT-Based Conjugated Polymers for Photoacoustic Molecular Imaging

Photoacoustic imaging combines both excellent spatial resolution with high contrast and specificity, without the need for patients to be exposed to ionizing radiation. This makes it ideal for the study of physiological changes occurring during tumorigenesis and cardiovascular disease. In order to fully exploit the potential of this technique, new exogenous contrast agents with strong absorbance in the near-infrared range, good stability and biocompatibility, are required. In this paper, we report the formulation and characterization of a novel series of endogenous contrast agents for photoacoustic imaging in vivo. These contrast agents are based on a recently reported series of indigoid π-conjugated organic semiconductors, coformulated with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, to give semiconducting polymer nanoparticles of about 150 nm diameter. These nanoparticles exhibited excellent absorption in the near-infrared region, with good photoacoustic signal generation efficiencies, high photostability, and extinction coefficients of up to three times higher than those previously reported. The absorption maximum is conveniently located in the spectral region of low absorption of chromophores within human tissue. Using the most promising semiconducting polymer nanoparticle, we have demonstrated wavelength-dependent differential contrast between vasculature and the nanoparticles, which can be used to unambiguously discriminate the presence of the contrast agent in vivo.

Molecular Imaging in Nanotechnology and Theranostics

The fields of biomedical nanotechnology and theranostics have enjoyed exponential growth in recent years. The "Molecular Imaging in Nanotechnology and Theranostics" (MINT) Interest Group of the World Molecular Imaging Society (WMIS) was created in order to provide a more organized and focused forum on these topics within the WMIS and at the World Molecular Imaging Conference (WMIC). The interest group was founded in 2015 and was officially inaugurated during the 2016 WMIC. The overarching goal of MINT is to bring together the many scientists who work on molecular imaging approaches using nanotechnology and those that work on theranostic agents. MINT therefore represents scientists, labs, and institutes that are very diverse in their scientific backgrounds and areas of expertise, reflecting the wide array of materials and approaches that drive these fields. In this short review, we attempt to provide a condensed overview over some of the key areas covered by MINT. Given the breadth of the fields and the given space constraints, we have limited the coverage to the realm of nanoconstructs, although theranostics is certainly not limited to this domain. We will also focus only on the most recent developments of the last 3-5 years, in order to provide the reader with an intuition of what is "in the pipeline" and has potential for clinical translation in the near future.

Self-Assembly of Semiconducting-Plasmonic Gold Nanoparticles with Enhanced Optical Property for Photoacoustic Imaging and Photothermal Therapy

Although various noble metal and semiconducting molecules have been developed as photoacoustic (PA) agents, the use of semiconducting polymer-metal nanoparticle hybrid materials to enhance PA signal has not been explored. A novel semiconducting-plasmonic nanovesicle was fabricated by self-assembly of semiconducting poly(perylene diimide) (PPDI) and poly(ethylene glycol (PEG) tethered gold nanoparticles (Au@PPDI/PEG). A highly localized and strongly enhanced electromagnetic (EM) field is distributed between adjacent gold nanoparticles in the vesicular shell, where the absorbing collapsed PPDI is present. Significantly, the EM field in turn enhances the light absorption efficiency of PPDI, leading to a much greater photothermal effect and a stronger photoacoustic signal compared to PDI nanoparticle or gold nanovesicle alone. The optical property of the hybrid vesicle can be further tailored by controlling the ratio of PPDI and gold nanoparticle as well as the adjustable interparticle distance of gold nanoparticles localized in the vesicular shell. In vivo imaging and therapeutic evaluation demonstrated that the hybrid vesicle is an excellent probe for cancer theranostics.

Near-infrared light-activatable polymeric nanoformulations for combined therapy and imaging of cancer

Near infrared (NIR) light allows deep tissue penetration and high spatial resolution due to the reduced scattering of long-wavelength photons. NIR light-activatable polymer nanoparticles are widely exploited for enhanced cancer imaging (diagnosis) and therapy owing to their superior photostability, photothermal conversion efficiency (or high emission rate), and minimal toxicity to cells and tissues. This review surveys the most recent advances in the synthesis of different NIR-absorbing and emissive polymer nanoformulations, and their applications for cancer imaging, photothermal therapy, theranostics and combination therapy by delivering multiple small molecule chemotherapeutics. Photo-responsive drug delivery systems for NIR light-triggered drug release are also discussed with particular emphasis on their molecular designs and formulations as well as photo-reaction mechanisms. Finally, outlook and challenges are presented regarding potential clinical applications of NIR light-activatable nanoformulations.

Versatile Polymer Nanoparticles as Two-Photon-Triggered Photosensitizers for Simultaneous Cellular, Deep-Tissue Imaging, and Photodynamic Therapy

Clinical applications of current photodynamic therapy (PDT) photosensitizers (PSs) are often limited by their absorption in the UV-vis range that possesses limited tissue penetration ability, leading to ineffective therapeutic response for deep-seated tumors. Alternatively, two-photon excited PS (TPE-PS) using NIR light triggered is one the most promising candidates for PDT improvement. Herein, multimodal polymer nanoparticles (PNPs) from polythiophene derivative as two-photon fluorescence imaging as well as two-photon-excited PDT agent are developed. The prepared PNPs exhibit excellent water dispersibility, high photostability and pH stability, strong fluorescence brightness, and low dark toxicity. More importantly, the PNPs also possess other outstanding features including: (1) the high ¹ O₂ quantum yield; (2) the strong two-photon-induced fluorescence and efficient ¹ O₂ generation; (3) the specific accumulation in lysosomes of HeLa cells; and (4) the imaging detection depth up to 2100 µm in the mock tissue under two-photon. The multifunctional PNPs are promising candidates as TPE-PDT agent for simultaneous cellular, deep-tissue imaging, and highly efficient in vivo PDT of cancer.

Semiconducting polymer dots with bright narrow-band emission at 800 nm for biological applications

The development of near-infrared (NIR) fluorescent probes is critical for in vivo exploration of the fundamental and complex processes in living systems by noninvasive fluorescence imaging techniques. Semiconducting polymer dots (Pdots) are emerging as important probes that exhibit several advantages, such as high brightness and biocompatibility. However, NIR-emitting Pdots are very rare, particularly at the center (∼800 nm) of the first optical window of biological tissues (between 650 nm and 950 nm). In this paper, we describe the synthesis of a semiconducting polymer with bright and narrow-band emission at 800 nm. The polymer was designed by incorporating a NIR porphyrin unit into a conjugated backbone; the polymer used a cascade energy transfer to produce the signal. The resulting Pdots possessed a narrow emission bandwidth (FWHM ∼ 23 nm) and good fluorescence quantum yield (QY = 8%), which is high for a near-IR emitter. The Pdots were bioconjugated with streptavidin for specific labeling of cellular targets, yielding higher staining index when compared with commercial NIR probes, such as PE-Cy7. Moreover, the NIR polymer was combined with a long-wavelength absorbing polymer to make bright Pdots (QY = 15%) for in vivo noninvasive imaging. These NIR Pdots with surface PEGylation led to high-contrast imaging of lymph nodes and tumors in a mouse model. This work highlights the great potential of the NIR Pdots for cellular and in vivo imaging applications.

Degradable Semiconducting Oligomer Amphiphile for Ratiometric Photoacoustic Imaging of Hypochlorite

Upregulation of highly reactive oxygen species (ROS) such as hypochlorite (ClO^-) is associated with many pathological conditions including cardiovascular diseases, neuron degeneration, lung injury, and cancer. However, real-time imaging of ClO^- is limited to the probes generally relying on fluorescence with shallow tissue-penetration depth. We here propose a self-assembly approach to develop activatable and degradable photoacoustic (PA) nanoprobes for in vivo imaging of ClO^-. A near-infrared absorbing amphiphilic oligomer is synthesized to undergo degradation in the presence of a specific ROS (ClO^-), which integrates a π-conjugated but ClO^- oxidizable backbone with hydrophilic PEG side chains. This molecular architecture allows the oligomer to serve as a degradable nanocarrier to encapsulate the ROS-inert dye and self-assemble into structurally stable nanoparticles through both π-π stacking and hydrophobic interactions. The self-assembled nanoprobe exhibits sensitive and specific ratiometric PA signals toward ClO^-, permitting ratiometric PA imaging of ClO^- in the tumor of living mice.

Terrylenediimide-Based Intrinsic Theranostic Nanomedicines with High Photothermal Conversion Efficiency for Photoacoustic Imaging-Guided Cancer Therapy

Activatable theranostic nanomedicines involved in photothermal therapy (PTT) have received constant attention as promising alternatives to traditional therapies in clinic. However, the theranostic nanomedicines widely suffer from instability and complicated nanostructures, which hamper potential clinical applications. Herein, we demonstrated a terrylenediimide (TDI)-poly(acrylic acid) (TPA)-based nanomedicine (TNM) platform used as an intrinsic theranostic agent. As an exploratory paradigm in seeking biomedical applications, TDI was modified with poly(acrylic acid)s (PAAs), resulting in eight-armed, star-like TPAs composed of an outside hydrophilic PAA corona and an inner hydrophobic TDI core. TNMs were readily fabricated via spontaneous self-assembly. Without additional vehicle and cargo, the as-prepared TNMs possessed a robust nanostructure and high photothermal conversion efficiency up to approximately 41%. The intrinsic theranostic properties of TNMs for use in photoacoustic (PA) imaging by a multispectral optoacoustic tomography system and in mediating photoinduced tumor ablation were intensely explored. Our results suggested that the TNMs could be successfully exploited as intrinsic theranostic agents for PA imaging-guided efficient tumor PTT. Thus, these TNMs hold great potential for (pre)clinical translational development.

Near-Infrared Light-Responsive Semiconductor Polymer Composite Hydrogels: Spatial/Temporal-Controlled Release via a Photothermal "Sponge" Effect

Near-infrared (NIR) light-responsive hydrogels are important for biomedical applications, such as remote-controlled release, but the NIR agents previously used were largely limited to heavy-metal inorganic materials such as gold nanoparticles. In this article, we report a new type of NIR photothermal-responsive hydrogel that can undergo structural changes in response to NIR light for biomedical applications in drug delivery and controlled release. The hydrogels synthesized by integrating a narrow-bandgap semiconductor polymer poly(diketopyrrolopyrrole-alt-3,4-ethylenedioxythiophene) with the polymerization of N-isopropylacrylamide show rapid and reversible mechanical shrinkage upon NIR light irradiation and can serve as carriers for anticancer drug loading and spatial/temporal control of drug release. These stimuli-responsive hydrogels, which can be prepared in different sizes and shapes, integrate photothermal properties and hydrogel characteristics and can provide on-demand, repeated, remote-controlled drug delivery for biomedical applications such as cancer treatment.

Amphiphilic Semiconducting Oligomer for Near-Infrared Photoacoustic and Fluorescence Imaging

Semiconducting polymer nanoparticles (SPNs) have emerged as an alternative class of optical nanoagents for imaging applications. However, the general preparation method of SPNs is nanoprecipitation, which is likely to encounter the issue of nanoparticle dissociation. We herein report nondissociable near-infrared (NIR)-absorbing organic semiconducting nanoparticles for in vivo photoacoustic (PA) and fluorescence imaging. The nanoparticles are self-assembled from an amphiphilic semiconducting oligomer (ASO) that has a hydrophobic semiconducting oligomer backbone attached by hydrophilic poly(ethylene glycol) (PEG) side chains. The ASO has a higher structural stability and brighter PA signals compared to those of its counterpart nanoparticles synthesized by nanoprecipitation. The small size and the PEG-passivated surface of the ASO allow it to passively target to and efficiently accumulate in the tumor of living mice, permitting tumor imaging with high signal-to-background ratios. Our study provides new NIR-absorbing organic nanoparticles for PA and fluorescence imaging, which also have the potential to be used as a drug carrier for theranostics.

Development of Semiconducting Polymer Nanoparticles for Photoacoustic Imaging

Semiconducting polymer nanoparticles (SPNs) have evolved into a new class of photonic materials with great potential for biomedical applications. Depending on the polymer structures, SPNs can be developed into optical agents for fluorescence and chemiluminescence imaging, photosensitizers for photodynamic therapy, and heat converters for photothermal therapy. In this feature article, recent work is summarized on the development of SPNs for in vivo photoacoustic (PA) imaging, a state-of-the-art imaging modality that converts light energy into mechanical acoustic waves to provide deep tissue penetration. The structure-property relationship and doping approaches are discussed to reveal the importance of promoting nonradiative decay in amplifying the PA brightness of SPNs. Moreover, their imaging applications, including lymph node mapping, tumor imaging, and monitoring of pathological indexes, are highlighted. These studies demonstrate that SPNs can serve as versatile PA agents for advanced molecular imaging applications.

Biocompatible D-A Semiconducting Polymer Nanoparticle with Light-Harvesting Unit for Highly Effective Photoacoustic Imaging Guided Photothermal Therapy

The development of nanotheranostic agents that integrate diagnosis and therapy for effective personalized precision medicine has obtained tremendous attention in the past few decades. In this report, biocompatible electron donor-acceptor conjugated semiconducting polymer nanoparticles (PPor-PEG NPs) with light-harvesting unit is prepared and developed for highly effective photoacoustic imaging guided photothermal therapy. To the best of our knowledge, it is the first time that the concept of light-harvesting unit is exploited for enhancing the photoacoustic signal and photothermal energy conversion in polymer-based theranostic agent. Combined with additional merits including donor-acceptor pair to favor electron transfer and fluorescence quenching effect after NP formation, the photothermal conversion efficiency of the PPor-PEG NPs is determined to be 62.3%, which is the highest value among reported polymer NPs. Moreover, the as-prepared PPor-PEG NP not only exhibits a remarkable cell-killing ability but also achieves 100% tumor elimination, demonstrating its excellent photothermal therapeutic efficacy. Finally, the as-prepared water-dispersible PPor-PEG NPs show good biocompatibility and biosafety, making them a promising candidate for future clinical applications in cancer theranostics.

Ternary Chalcogenide Nanosheets with Ultrahigh Photothermal Conversion Efficiency for Photoacoustic Theranostics

2D materials (TDMs) have been explored for photonic theranostics. To achieve deep-tissue penetration, near-infrared (NIR) light is essential for photoacoustic (PA) theranostics. However, because the absorption profiles of existing TDMs are generally featureless with no obvious NIR absorption peaks, their PA signals and therapeutic efficacies are limited. This paper herein reports the synthesis and application of ternary chalcogenide nanosheets (Ta₂ NiS₅ -P) for PA theranostics. In contrast to the current TDMs for such application, Ta₂ NiS₅ -P has a ternary instead of binary composition. This difference brings in the strong and featured NIR for Ta₂ NiS₅ -P. To the best of the knowledge, this is the first example using ternary chalcogenide nanosheets for such application; moreover, the photothermal conversion efficiency of Ta₂ NiS₅ -P is the highest (35%) among all the reported TDMs based on the same calculation method. These advantages allow Ta₂ NiS₅ -P to passively target, effectively delineate, and completely eradicate the tumor of living mice after systemic administration.

Polymer nanoparticles with high photothermal conversion efficiency as robust photoacoustic and thermal theranostics

Synthesis of photothermal agents with absorption in the near-infrared (NIR) region and featuring excellent photostability, high photothermal conversion efficiency, and good biocompatibility is necessary for the application of photothermal therapy (PTT). In this work, a low band gap thiophene-benzene-diketopyrrolopyrrole (TBD)-based polymer was synthesized and used to fabricate TBD polymer nanoparticles (TBDPNPs) through a one-step nanoprecipitation method. The obtained near-infrared-absorbing TBDPNPs presented good water dispersibility, high photothermal stability, and low toxicity. Significantly, the TBDPNPs exhibited an ultrahigh photothermal conversion efficiency of approximately 68.1% under 671 nm laser irradiation. In addition, photoacoustic (PA) imaging, with high spatial resolution and deep tissue penetration, showed that the TBDPNPs targeted tumor sites through the enhanced permeability and retention effect. Therefore, the robust TBDPNPs with a photothermal conversion efficiency of 68.1% can serve as an excellent therapeutic agent for PA-imaging-guided PTT.

Protein-Stabilized Gadolinium Oxide-Gold Nanoclusters Hybrid for Multimodal Imaging and Drug Delivery

A protein-stabilized multifunctional theranostic nanoplatform, gadolinium oxide-gold nanoclusters hybrid (Gd₂O₃-AuNCs), is constructed for multimodal imaging and drug delivery. The Gd₂O₃-AuNCs nanohybrid is developed by integrating Gd₂O₃ nanocrystals and gold nanoclusters into bovine serum albumin scaffold as a stabilizer. The nanohybrid exhibits favorable biocompatibility and is capable of enhancing the contrast in magnetic resonance and X-ray computed tomography imaging. Meanwhile, the integrated AuNCs component not only endows the nanohybrid to produce red fluorescence, but also sensitizes the generation of singlet oxygen (¹O₂) upon near-infrared laser stimulation at 808 nm. Bovine serum albumin surrounding the nanoparticles makes Gd₂O₃-AuNCs a brilliant carrier for the delivery of indocyanine green (ICG). ICG loading endows the Gd₂O₃-AuNCs-ICG nanocomposite with a near-infrared fluorescence imaging capability, and improves its photodynamic property and photothermal capability. Ultimately, further experiments have demonstrated that Gd₂O₃-AuNCs-ICG nanocomposite is a promising theranostic agent for image guided cancer therapy.

The development and characterization of a novel yet simple 3D printed tool to facilitate phantom imaging of photoacoustic contrast agents

We report a new approach to preparing phantoms using 3D printing. This device supports plastic tubing containing the contrast agent and is immersed in a solution with absorption or scattering properties that mimic tissue. Up to 12 tubing samples could be placed in the device with sample-to-sample spacing as low as 0.3 mm and at a constant distance from the transducer (±0.16 mm), which is critical in validating photoacoustic contrast agents. We also studied different types of tubing and found that tubing with a larger outside diameter has more inherent signal. Both 40% India Ink and lipids in the immersion media modulated the signal. Finally, we created a depth phantom and found that signal decayed following a linear relationship (R² = 0.997) with respect to distance from the focal point. We include computer-assisted drafting code the community can use to print this phantom or customized versions of this phantom.

Near-infrared absorbing amphiphilic semiconducting polymers for photoacoustic imaging

Development of photoacoustic (PA) imaging agents is crucial to advancing PA imaging in biology and medicine. In this study, we report the design and synthesis of near-infrared (NIR) absorbing amphiphilic semiconducting polymers that can spontaneously self-assemble into homogeneous water-soluble nanoparticles for PA imaging of tumor in living mice.

Activatable Photoacoustic Nanoprobes for In Vivo Ratiometric Imaging of Peroxynitrite

Organic semiconducting nanoprobes doped with bulky borane can undergo specific activation by ONOO^- even at tumor-relevant acidic pH (6.8), permitting in vivo ratiometric photoacoustic imaging of ONOO^- in the tumor environment of living mice.

Water-Gated n-Type Organic Field-Effect Transistors for Complementary Integrated Circuits Operating in an Aqueous Environment

The first demonstration of an n-type water-gated organic field-effect transistor (WGOFET) is here reported, along with simple water-gated complementary integrated circuits, in the form of inverting logic gates. For the n-type WGOFET active layer, high-electron-affinity organic semiconductors, including naphthalene diimide co-polymers and a soluble fullerene derivative, have been compared, with the latter enabling a high electric double layer capacitance in the range of 1 μF cm-2 in full accumulation and a mobility-capacitance product of 7 × 10-3 μF/V s. Short-term stability measurements indicate promising cycling robustness, despite operating the device in an environment typically considered harsh, especially for electron-transporting organic molecules. This work paves the way toward advanced circuitry design for signal conditioning and actuation in an aqueous environment and opens new perspectives in the implementation of active bio-organic interfaces for biosensing and neuromodulation.

Water Soluble Melanin Derivatives for Dynamic Contrast Enhanced Photoacoustic Imaging of Tumor Vasculature and Response to Antiangiogenic Therapy

A dynamic contrast enhanced (DCE) approach for tumor photoacoustic (PA) imaging is described. Novel water soluble melanin-based derivatives are synthesized that exhibit good PA properties, stability, safety and accumulation in tumor bearing mice. This melanin derivative is capable to characterize tumor vasculature and to monitor vessel permeability changes upon antiangiogenic treatment. DCE-PA imaging can assess functional response to cancer treatments.

Near-infrared conjugated polymers for photoacoustic imaging-guided photothermal/chemo combination therapy

Conjugated polymers with intensive near-infrared absorption and high photothermal conversion efficiency have emerged as a new generation of photothermal therapy and photoacoustic imaging agents for cancer therapy.

Hydrophilic graphene oxide/bismuth selenide nanocomposites for CT imaging, photoacoustic imaging, and photothermal therapy

GO/Bi₂Se₃/PVP nanocomposites fabricated by a one-pot solvothermal method show good performance in CT/photoacoustic imaging and photothermal therapy.

Self-quenched semiconducting polymer nanoparticles for amplified in vivo photoacoustic imaging

Development of photoacoustic (PA) imaging agents provides opportunities for advancing PA imaging in fundamental biology and medicine. Despite the promise of semiconducting polymer nanoparticles (SPNs) for PA imaging, the molecular guidelines to enhance their imaging performance are limited. In this study, semiconducting polymers (SPs) with self-quenched fluorescence are synthesized and transformed into SPNs for amplified PA imaging in living mice. The self-quenched process is induced by the incorporation of an electron-deficient structure unit into the backbone of SPs, which in turn promotes the nonradiative decay and enhances the heat generation. Such a simple chemical alteration of SP eventually leads to 1.7-fold PA amplification for the corresponding SPN. By virtue of the targeting capability of cyclic-RGD, the amplified SPN can effectively delineate tumor in living mice and increase the PA intensity of tumor by 4.7-fold after systemic administration. Our study thus provides an effective molecular guideline to amplify the PA brightness of organic imaging agents for in vivo PA imaging.

Photoacoustic probes for real-time tracking of endogenous H2S in living mice

H₂S is a key chemical mediator that exerts a vital role in diverse physiological and pathological processes. However, in vivo tracking of endogenous H₂S generation still remains difficult due to the lack of reliable analytical methods. Herein, we present the first example of activatable photoacoustic probes for real-time imaging of H₂S in living mice through the full utilization of the superiority of photoacoustic imaging modality at fine spatial resolution during deep tissue penetration. The designed probe can generate high NIR absorption at 780 nm in the presence of H₂S, thus producing a strong photoacoustic signal output in the NIR region. Furthermore, this probe exhibits extremely fast and highly selective responsiveness, good water-solubility and excellent biocompatibility. In light of these outstanding features, this probe realizes the direct photoacoustic trapping of endogenous H₂S generation in a HCT116 tumor-bearing mouse model. These preliminary imaging studies show that HCT116 colon tumors exhibit CBS upregulation activity, resulting in an increased rate of H₂S generation.

Nanocomposites of Molybdenum Disulfide/Methoxy Polyethylene Glycol-co-Polypyrrole for Amplified Photoacoustic Signal

Photoacoustic activity is the generation of an ultrasonic signal via thermal expansion or bubble formation, stimulated by laser irradiation. Photoacoustic nanoplatforms have recently gained focus for application in bioelectric interfaces. Various photoacoustic material types have been evaluated, including gold nanoparticles, semiconductive π-conjugating polymers (SP), etc. In this study, surfactant-free methoxy-polyethylene glycol-co-polypyrrole copolymer (mPEG-co-PPyr) nanoparticles (NPs) and mPEG-co-PPyr NP/molybdenum disulfide (mPEG-co-PPyr/MoS₂) nanocomposites (NCs) were prepared and their photoacoustic activity was demonstrated. The mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS₂ NCs both showed photoacoustic signal activity. The mPEG-co-PPyr/MoS₂ NCs presented a higher photoacoustic signal amplitude at 700 nm than the mPEG-co-PPyr NPs. The enhanced photoacoustic activity of the mPEG-co-PPyr/MoS₂ NCs might be attributed to heterogeneous interfacial contact between mPEG-co-PPyr and the MoS₂ nanosheets due to complex formation. Laser ablation of MoS₂ might elevate the local temperature and facilitate the thermal conductive transfer in the mPEG-co-PPyr/MoS₂ NCs, amplifying PA signal. Our study, for the first time, demonstrates enhanced PA activity in SP/transition metal disulfide (TMD) composites as photoacoustic nanoplatforms.

Recent Advances in Photoacoustic Imaging for Deep-Tissue Biomedical Applications

Photoacoustic imaging (PAI), a novel imaging modality based on photoacoustic effect, shows great promise in biomedical applications. By converting pulsed laser excitation into ultrasonic emission, PAI combines the advantages of optical imaging and ultrasound imaging, which benefits rich contrast, high resolution and deep tissue penetration. In this paper, we introduced recent advances of contrast agents, applications, and signal enhancement strategies for PAI. The PA contrast agents were categorized by their components, mainly including inorganic and organic PA contrast agents. The applications of PAI were summarized as follows: deep tumor imaging, therapeutic responses monitoring, metabolic imaging, pH detection, enzyme detection, reactive oxygen species (ROS) detection, metal ions detection, and so on. The enhancement strategies of PA signals were highlighted. In the end, we elaborated on the challenges and provided perspectives of PAI for deep-tissue biomedical applications.

Surfactant-Stripped Frozen Pheophytin Micelles for Multimodal Gut Imaging

Edible, surfactant-stripped, frozen micelles are formed from pheophytin (demetallated chlorophyll), a pigment that is naturally consumed in human diets. Pheophytin nanoparticles pass completely and safely through the gastrointestinal tract and enable trimodal gut contrast imaging via photoacoustic, fluorescence, and positron emission tomography techniques.

Intraparticle Energy Level Alignment of Semiconducting Polymer Nanoparticles to Amplify Chemiluminescence for Ultrasensitive In Vivo Imaging of Reactive Oxygen Species

Detection of reactive oxygen species (ROS), a hallmark of many pathological processes, is imperative to understanding, detection and treatment of many life-threatening diseases. However, methods capable of real-time in situ imaging of ROS in living animals are still very limited. We herein report the development and optimization of chemiluminescent semiconducting polymer nanoparticles (SPNs) for ultrasensitive in vivo imaging of hydrogen peroxide (H2O2). The chemiluminescence is amplified by adjusting the energy levels between the luminescence reporter and the chemiluminescence substrate to facilitate intermolecular electron transfer in the process of H2O2-activated luminescence. The optimized SPN can emit chemiluminescence with the quantum yield up to 2.30 × 10(-2) einsteins/mol and detect H2O2 down to 5 nM, which substantially outperforms the previous probes. Further doping of this SPN with a naphthalocyanine dye creates intraparticle chemiluminescence resonance energy transfer (CRET), leading to the near-infrared (NIR) luminescence responding to H2O2. By virtue of high brightness and ideal NIR optical window, SPN-NIR permits ultrasensitive imaging of H2O2 in the mouse models of peritonitis and neuroinflammation with the minute administration quantity. Thus, this study not only provides a category of optical probes that eliminates the need of external light excitation for imaging of H2O2, but also reveals the underlying principle to enhance the brightness of chemiluminescence systems.

Semiconducting polymer nanoparticles as photoacoustic molecular imaging probes

As an emerging class of optical nanomaterials, semiconducting polymer nanoparticles (SPNs) are highly photostable, optically active and versatile in chemistry; these properties make them attractive as molecular imaging agents to enable imaging of biological events and functionalities at multiple scales. More recently, a variety of SPNs have been found to exhibit high photoacoustic properties, and further empowered photoacoustic imaging for contrast enhanced in vivo molecular imaging. Target-sensitive components can be incorporated in the SPNs to create activatable imaging probes to sense and monitor the target dynamics in living objects. Intrinsically biophotonic and biocompatible, SPNs can be further engineered for multimodal imaging and for real-time imaging of drug delivery. WIREs Nanomed Nanobiotechnol 2017, 9:e1418. doi: 10.1002/wnan.1418 For further resources related to this article, please visit the WIREs website.

In vivo photoacoustic tomography of EGFR overexpressed in hepatocellular carcinoma mouse xenograft

EGFR is a promising cell surface target for in vivo imaging that is highly overexpressed in hepatocellular carcinoma (HCC), a common cancer worldwide. Peptides penetrate easily into tumors for deep imaging, and clear rapidly from the circulation to minimize background. We aim to demonstrate use of an EGFR specific peptide to detect HCC xenograft tumors in mice with photoacoustic imaging. Nude mice implanted with human HCC cells that overexpress EGFR were injected intravenously with Cy5.5-labeled EGFR and scrambled control peptides respectively. Photoacoustic images collected from 0 to 24 h. Photoacoustic signal peaked in tumors at 3 h post-injection. Images from 0 to 1.8 cm beneath the skin revealed increased target-to-background (T/B) ratio from tumors. The T/B ratio was significantly greater for the EGFR versus control peptide. Clearance of signal was observed by ∼24 h. EGFR overexpression was validated with immunofluorescence and immunohistochemistry. A peptide specific for EGFR delivered systemically can detect HCC xenograft tumors in vivo with photoacoustic imaging.

Engineering Lysosome-Targeting BODIPY Nanoparticles for Photoacoustic Imaging and Photodynamic Therapy under Near-Infrared Light

Developing lysosome-targeting organic nanoparticles combined with photoacoustic imaging (PAI) and photodynamic therapy (PDT) functions toward personalized medicine are highly desired yet challenging. Here, for the first time, lysosome-targeting BODIPY nanoparticles were engineered by encapsulating near-infrared (NIR) absorbed BODIPY dye within amphiphilic DSPE-mPEG5000 for high-performing lysosomal PAI and acid-activatable PDT against cancer cells under NIR light.

Semiconducting Oligomer Nanoparticles as an Activatable Photoacoustic Probe with Amplified Brightness for In Vivo Imaging of pH

An activatable photoacoustic nanoprobe based on a semiconducting oligomer with amplified brightness and pH-sensing capability is developed by taking advantage of nanodoping to simultaneously create both intraparticle photoinduced electron transfer and intramolecular protonation within a single particle. This organic nanoprobe permits noninvasive real-time ratiometric photoacoustic imaging of pH in tumors in living mice through systemic administration at a relatively low dosage.

Intraparticle Molecular Orbital Engineering of Semiconducting Polymer Nanoparticles as Amplified Theranostics for in Vivo Photoacoustic Imaging and Photothermal Therapy

Optical theranostic nanoagents that seamlessly and synergistically integrate light-generated signals with photothermal or photodynamic therapy can provide opportunities for cost-effective precision medicine, while the potential for clinical translation requires them to have good biocompatibility and high imaging/therapy performance. We herein report an intraparticle molecular orbital engineering approach to simultaneously enhance photoacoustic brightness and photothermal therapy efficacy of semiconducting polymer nanoparticles (SPNs) for in vivo imaging and treatment of cancer. The theranostic SPNs have a binary optical component nanostructure, wherein a near-infrared absorbing semiconducting polymer and an ultrasmall carbon dot (fullerene) interact with each other to induce photoinduced electron transfer upon light irradiation. Such an intraparticle optoelectronic interaction augments heat generation and consequently enhances the photoacoustic signal and maximum photothermal temperature of SPNs by 2.6- and 1.3-fold, respectively. With the use of the amplified SPN as the theranostic nanoagent, it permits enhanced photoacoustic imaging and photothermal ablation of tumor in living mice. Our study thus not only introduces a category of purely organic optical theranostics but also highlights a molecular guideline to amplify the effectiveness of light-intensive imaging and therapeutic nanosystems.

Perfluorocarbon-Loaded Hollow Bi2Se3 Nanoparticles for Timely Supply of Oxygen under Near-Infrared Light to Enhance the Radiotherapy of Cancer

Hollow Bi2 Se3 nanoparticles prepared by a cation exchange method are loaded with perfluorocarbon as an oxygen carrier. With these nanoparticles, a promising concept is demonstrated to enhance radiotherapy by not only using their X-ray-absorbing ability to locally concentrate radiation energy in the tumor, but also employing near-infrared light to trigger burst release of oxygen from the nanoparticles to overcome hypoxia-associated radio-resistance.

Photoacoustic Molecular Imaging: From Multiscale Biomedical Applications Towards Early-Stage Theranostics

Photoacoustic imaging (PAI) has ushered in a new era of observational biotechnology and has facilitated the exploration of fundamental biological mechanisms and clinical translational applications, which has attracted tremendous attention in recent years. By converting laser into ultrasound emission, PAI combines rich optical contrast, high ultrasonic spatial resolution, and deep penetration depth in a single modality. This evolutional technique enables multiscale and multicontrast visualization from cells to organs, anatomy to function, and molecules to metabolism with high sensitivity and specificity. The state-of-the-art developments and applications of PAI are described in this review. Future prospects for clinical use are also highlighted. Collectively, PAI holds great promise to drive biomedical applications towards early-stage theranostics.