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.

Single- or few-layered h-BN nanosheets (BNNSs) are analogous to graphene and possess unique properties. However, their technological applications were severely hindered by the low production efficiency of BNNSs. We reported here a study in which BNNSs were efficiently produced by exfoliating bulk h-BN powder in thionyl chloride without using any dispersion agents. The BNNSs yield was as high as 20%, and it could be doubled through the second round of exfoliation of the h-BN precipitate. Microscopic results revealed that the BNNSs generally consisted of 3-20 layers. Pd nanoparticles were successfully immobilized and uniformly distributed on BNNS surfaces through the deposition-precipitation method. The resultant Pd-BNNS catalyst exhibited high catalytic activity and recyclability for the hydrogenation of nitro aromatics, demonstrating that BNNSs served as a promising platform to fabricate heterogeneous catalysts.

Janus nanoparticles (JNPs) refer to the integration of two or more chemically discrepant composites into one structure system. Studies into JNPs have been of significant interest due to their interesting characteristics stemming from their asymmetric structures, which can integrate different functional properties and perform more synergetic functions simultaneously. Herein, we present recent progress of Janus particles, comprehensively detailing fabrication strategies and applications. First, the classification of JNPs is divided into three blocks, consisting of polymeric composites, inorganic composites, and hybrid polymeric/inorganic JNPs composites. Then, the fabrication strategies are alternately summarized, examining self-assembly strategy, phase separation strategy, seed-mediated polymerization, microfluidic preparation strategy, nucleation growth methods, and masking methods. Finally, various intriguing applications of JNPs are presented, including solid surfactants agents, micro/nanomotors, and biomedical applications such as biosensing, controlled drug delivery, bioimaging, cancer therapy, and combined theranostics. Furthermore, challenges and future works in this field are provided.

A promising theranostic platform for solid tumors would deliver and release anticancer nanomedicine effectively in tumor cells. However, diverse biological barriers, especially related to the tumor microenvironment, impede these theranostic agents from reaching the tumor cell. Herein, a sequential pH and reduction-responsive polymer and gold nanorod (AuNR) core-shell assembly to overcome these barriers via a two-stage size decrease and disassembly of the nano platform responding to the specified tumor microenvironment are reported. The tumor uptake of the hybrid nanoparticle (NP) is 14.2% ID g −1 , which is two and four times higher than the noneresponsive hybrid NPs and small AuNR@PEG, respectively. After tumor uptake of the hybrid NPs, the disassembled ultrasmall AuNRs coated with a polymer of polymerized reduction-responsive doxorubicin (DOX) prodrug monomers penetrate into the solid tumor and lead to localized DOX release in the tumor cell. A linear increase in photoacustic (PA) effects from the PA activating polymer on an AuNR cluster surface indicates a critical role of electromagnetic fields in the AuNR assembly, which is consistent with the theoretical calculation results. Furthermore, the hybrid NP can serve as a promising deep-tissue PA and surfaceenhanced Raman scattering imaging agent for real-time in vivo investigation of physiological behaviors and deep tumor penetrating nanotherapy effects.

This review summarizes the recent progress of PA imaging and PTT agents in the second NIR window.

Abstract2D black phosphorus (BP) nanosheets and BP quantum dots (BPQD), as two main material styles of BP, are widely used in the biomedical filed. However, few stimuli‐responsive BP nanocarriers are reported to meet the need of nanomedicine. Herein, near‐infrared region/reactive oxygen species (NIR/ROS) sensitive BPQD vesicles (BPNVs) are prepared by self‐assembly of amphiphilic BPQDs grafted with polyethylene glycol and ROS sensitive poly(propylene sulfide) (PPS). BPNVs exhibit enhanced photo‐absorption in the NIR region, and have high loading efficiency of immunoadjuvant CpG oligodeoxynucleotides (CpG ODNs) in the cavity of the BPNVs. Upon NIR laser irradiation, high levels of ROS are generated from BPNVs to trigger the change of hydrophobic PPS to hydrophilic polymers, leading to disassembly of the vesicles to BPQDs. In this manner, the BPNVs show synergistic photodynamic therapy combined with immunotherapy, due to simultaneous release of small BPQDs with deep tumor penetration and CpG with enhanced immunotherapy. The BPNVs‐CpG achieves potent photodynamic immunotherapy in vivo, in addition to block distant tumor growth and metastasis.

Excessive production of reactive oxygen species such as H2O2 is the pathological basis of chronic inflammatory diseases, as well as bacterial infection‐induced inflammation. Therefore, the in situ H2O2 level is a reliable biomarker of inflammatory responses, and its real‐time measurement can monitor disease progression and improve therapeutic outcomes in inflammation‐linked diseases. However, the currently used strategies for the diagnosis of inflammation is mainly through routine blood test, which are limited in determining the inflammation status and cannot provide comprehensive quantitative information. In this work, a novel H2O2‐responsive theranostic nanoplatform comprising Ag shell coated Pd‐tipped gold nanorods (Au–Pd@Ag NR) is developed. The etching and oxidation of the Ag shell by H2O2 release the Ag ions, which effectively kill bacteria in vivo and trigger their absorption variation at 700 and 1260 nm. The ratiometric photoacoustic (PA) imaging at 1260 and 700 nm (PA1260/PA700) accurately quantifies H2O2 in a mice model of bacterial infection and abdomen inflammation, even in a rabbit model of osteoarthritis. The H2O2 activated second near‐infrared (NIR‐II) PA images of the probe can further precisely differentiate the inflammation region and normal tissue. This nanoplatform not only can quantify H2O2 during inflammation but also act as a potent antibacterial agent.

Asilver-ion-coupled black phosphorus (BP) vesicle (BP Ve-Ag +)w ith as econd near infrared (NIR-II) window photoacoustic (PA) imaging capability was firstly constructed to maximizethe potential of BP quantum dot (QD) in deeper bioimaging and diversified therapy. The embedded Ag + could improve the relatively large band gap of BP QD via intense charge coupling based on theoretical simulation results,s ubsequently leading to the enhanced optical absorption capability,a ccompanied with the occurrence of the strong NIR-II PA signal. Guiding by NIR-II PA bioimaging,the hidden Ag + could be precisely released with the disassembly of Ve during photodynamic therapyp rocess and captured by macrophages located in lesion region for arousing synergistic cancer photodynamic/Ag + immunotherapy. BP Ve-Ag + can contrapuntally kill pathogenic bacteria and accelerate wound healing monitored by NIR-II PA imaging.