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Mesoporous silicon (PSi) nanoparticles have been widely studied in different biomedical imaging modalities due to their several beneficial material properties. However, they have not been found to be suitable for photoacoustic imaging due to their poor photothermal conversion performance. In the present study, biodegradable black mesoporous silicon (BPSi) nanoparticles with strong light absorbance were developed as superior image contrast agents for photoacoustic tomography (PAT), which was realized with a light-emitting diode (LED) instead of the commonly used laser. LED-based PAT offers the advantages of low cost, compactness, good mobility, and easy operation as compared to the traditional laser-based PAT modality. Nevertheless, the poor imaging sensitivity of the LED-PAT systems has been the main barrier to prevent their wide biomedical application because the LED light has low optical energy. The present study demonstrated that the imaging sensitivity of the LED-PAT system was significantly enhanced with the PEGylated BPSi (PEG–BPSi) nanoparticles. The PEG–BPSi nanoparticles were clearly detectable with a low concentration of 0.05 mg/mL in vitro and with an LED radiation energy of 5.2 μJ. The required concentration of the PEG–BPSi nanoparticles was 10 times lesser than that of the reference gold nanoparticles to reach the corresponding level of the imaging contrast. The ex vivo studies demonstrated that the submillimeter BPSi nanoparticle-based absorbers were distinguishable in chicken breast tissues. The strong contrast provided by the BPSi particles indicated that these particles can be utilized as novel contrast agents in PAT, especially in LED-based systems with low light intensity.

Inorganic photothermal agents (PTAs) have attracted considerable attention in cancer theranostics due to their unique features such as high photothermal conversion efficacy, excellent photothermal stability, and straightforward functionalization. The first part of this Review summarizes progress in methods for synthesizing PTAs, then considers in vitro photothermal evaluations, as well as in vivo photothermal-based applications that attempt to overcome different barriers in cancer theranostics. Next, a clinical trial with an inorganic PTA is described. The final part of the Review examines the challenges and possibilities for successful transfer of inorganic PTAs from the laboratory to the clinic.

Complex experimental design is a common problem in the preparation of theranostic nanoparticles, resulting in poor reaction control, expensive production cost, and low experiment success rate. The present study aims to develop PEGylated bismuth (PEG-Bi) nanoparticles with a precisely controlled one-pot approach, which contains only methoxy[(poly-(ethylene glycol)]trimethoxy-silane (PEG-silane) and bismuth oxide (Bi 2 O 3 ). A targeted pyrolysis of PEG-silane was achieved to realize its roles as both the reduction and PEGylation agents. The unwanted methoxy groups of PEG-silane were selectively pyrolyzed to form reductive agents, while the useful PEG-chain was fully preserved to enhance the biocompatibility of Bi nanoparticles. Moreover, Bi 2 O 3 not only acted as the raw material of the Bi source but also presented a self-promotion in the production of Bi nanoparticles via catalyzing the pyrolysis of PEG-silane. The reaction mechanism was systematically validated with different methods such as nuclear magnetic resonance spectroscopy. The PEG-Bi nanoparticles showed better compatibility and photothermal conversion than those prepared by the complex multiple step approaches in literature studies. In addition, the PEG-Bi nanoparticles possessed prominent performance in X-ray computed tomography imaging and photothermal cancer therapy in vivo. The present study highlights the art of precise reaction control in the synthesis of PEGylated nanoparticles for biomedical applications.

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