Nanomaterials for NIR-II Photoacoustic Imaging.

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.

Gold nanorods (GNRs) have attracted great interest for photo-mediated biomedicines due to their tunable and high optical absorption, high photothermal conversion efficiency and facile surface modifiability. GNRs that have efficient absorption in second near-infrared (NIR-II) window hold further promise in bio-applications due to low background signal from tissue and deep tissue penetration. However, bare GNRs readily undergo shape deformation (termed as 'melting effect') during the laser illumination losing their unique localized surface plasmon resonance (LSPR) properties, which subsequently leads to PA signal attenuation and decreased photothermal efficiency. Polydopamine (PDA) is a robust synthetic melanin that has broad absorption and high photothermal conversion. Herein, we coated GNRs with PDA to prepare photothermally robust GNR@PDA hybrids for enhanced photo-mediated theranostic agents. Ultrasmall GNRs (SGNRs) and conventional large GNRs (LGNRs) that possess similar LSPR characteristics as well as GNR@PDA hybrids were compared side-by-side in terms of the size-dependent photoacoustic (PA) imaging, photothermal therapy (PTT), and structural stability. In vitro experiments further demonstrated that SGNR@PDA showed 95% ablation of SKOV3 ovarian cancer cells, which is significantly higher than that of LGNRs (66%) and SGNRs (74%). Collectively, our PDA coating strategy represents a rational design for enhanced PA imaging and efficient PTT via a nanoparticle, i.e., nanotheranostics.

Recently, photoacoustic (PA) imaging has made a significant impact on biomedical imaging, providing detailed information on tissue structure and function by integrating optical and acoustic techniques. PA imaging can provide functional information at the cellular (e.g., oxygen saturation, hemoglobin concentration, metabolic rate) and molecular levels, owing to its substantial advantages over conventional imaging techniques. PA imaging is particularly useful for neuroimaging, cancer detection, and cardiovascular studies. Over the last decade, there has been a tremendous amount of research and development dedicated to nanomaterials that are ideal for PA imaging. Examples of nanomaterials include carbon-based and gold nanorods, both of which demonstrate greatly enhanced light absorption capabilities in the near-infrared range. Therefore, the properties of these materials make them perfect for achieving deep penetration into tissues. In addition, they exhibit biocompatibility, tunable optical properties, and enhance the acoustic signal for PA imaging, resulting in greater accuracy and precision in PA results. Researchers working in this area have focused on developing nanomaterials with fabrication capabilities, enabling real-time visualization of therapeutic events and enhancing light absorption. This review critically examines recent advances in nanomaterials for PA imaging, emphasizing strategies for signal enhancement and evaluating their impact on imaging performance, including imaging depth, photostability, and signal intensity, as well as their suitability for biomedical applications. Furthermore, complementary approaches for PA signal enhancement are discussed to provide a broader perspective and guide the selection and design of effective contrast agents for clinical and preclinical use.

Photoacoustic imaging is a new imaging technology in recent years, which combines the advantages of high resolution and rich contrast of optical imaging with the advantages of high penetration depth of acoustic imaging. Photoacoustic imaging has been widely used in biomedical fields, such as brain imaging, tumor detection and so on. The signal-tonoise ratio (SNR) of image signals in photoacoustic imaging is generally low due to the limitation of laser pulse energy, electromagnetic interference in the external environment and system noise. In order to solve the problem of low SNR of photoacoustic images, we use feedforward denoising convolutional neural network to further process the obtained images, so as to obtain higher SNR images and improve image quality. We use Python language to manage the referenced Python external library through Anaconda, and build a feedforward noise-reducing convolutional neural network on Pycharm platform. We first processed and segmated a training set containing 400 images, and then used it for network training. Finally, we tested it with a series of cerebrovascular photoacoustic microscopy images. The results show that the peak signal-to-noise ratio (PSNR) of the image increases significantly before and after denoising. The experimental results verify that the feed-forward noise reduction convolutional neural network can effectively improve the quality of photoacoustic microscopic images, which provides a good foundation for the subsequent biomedical research.

Over the past few years, significant efforts have been made to create new fluorescent probes operating at longer wavelengths, particularly in the second near-infrared (NIR-II) window from 1000 to 1700 nm, offering enhanced tissue penetration compared to light in the visible and first near-infrared window (700-900 nm). However, most of the reported NIR-II fluorophores meet such dilemmas; they are excreted slowly and largely retained within the reticuloendothelial system. Here, we report a rapidly excreted NIR-II lanthanide complex Nd-DOTA (over 50% excreted through the kidneys within 3 h postinjection) with a molecular mass only 0.54 kDa. The NIR-II imaging quality of Nd-DOTA was far superior to that of clinically approved ICG with good photostability and deep tissue penetration (7 mm). Superior tumor-to-normal tissue ratio was successfully achieved to facilitate the abdominal ovarian metastases surgical delineation. Metastases with ≤1 mm can be completely excised under NIR-II bioimaging guidance. Significantly, since the Nd-DOTA structure is same to the clinically approved magnetic resonance imaging (MRI) contrast Gd-DOTA, it will speed up the clinical translation for this novel kind of NIR-II probes in the future.

Construction of an active composite as a biomarker with deeper tissue penetration and higher signal-to-noise ratio (SNR) is of great importance for the application in bioimaging. Here, we report a strategy for tuning the emission bandwidth and intensity via crystal field control in long persistent phosphors (LPPs). Ni2+-doped Zn1+ySnyGa2−x−2yO4 phosphors, with a tunable emission band peaking from 1270 to 1430 nm in the second near-infrared (NIR) window, have been successfully prepared. Such featured materials have the advantages of low absorption and scattering as well as more efficient tissue penetration. The emission spectra can be controlled by tailoring the local crystal field around the activator precisely via substitution of Zn and Sn for Ga. Moreover, with high resolution and weak light disturbance, these developed multi-band afterglow phosphors exhibit great application potential in advanced optical imaging.

Special Issue on the 40th Anniversary of University of Macau

Development of Second Near-Infrared Photoacoustic Imaging Agents

Photoacoustic (PA) imaging has received extensive attention in the recent decade due to its ultra-high resolution in deep tissue penetration.

Luminescent quantum dots: Synthesis, optical properties, bioimaging and toxicity

Photoacoustic (PA) imaging is a non-invasive modality that combines high spatial resolution with real-time molecular visualization. The development of contrast agents with high sensitivity, deep tissue penetration, and specificity remains a critical challenge. Water-Soluble chlorophyll-binding proteins (WSCPs) stand out for their tunable optical properties and biocompatibility. Among them, the Class I WSCP from Chenopodium album (CaWSCP) exhibits unique photoconversion-induced red-shifts in absorption, enabling enhanced PA imaging through complete background subtraction and improved deep tissue imaging. To further expand its functionality, CaWSCP was engineered with a SpyTag-SpyCatcher system, enabling site-specific conjugation with HER2-specific affibodies for precise molecular targeting. Experimental findings demonstrate CaWSCP's biocompatibility, photoconversion capabilities, and potential for targeted imaging, making it a promising platform for advanced biomedical applications.Clinical Relevance- The engineered CaWSCP contrast agent, with its targeting capability via the SpyTag-SpyCatcher system, improves PA imaging for non-invasive detection and monitoring of cancer and vascular disorders by enhancing resolution and minimizing background interference.

Ribonuclease-A (RNase-A) encapsulated PbS quantum dots (RNase-A@PbS Qdots) which emit in the second near-infrared biological window (NIR-II, ca. 1000–1400 nm) are rapidly synthesized under microwave heating. Photoluminescence (PL) spectra of the Qdots can be tuned across the entire NIR-II range by simply controlling synthesis temperature. The size and morphology of the Qdots are examined by transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). Quantum yield (Φf) measurement confirms that the prepared Qdots are one of the brightest water-soluble NIR-II emitters for in vivo imaging. Their high Φf (∼17.3%) and peak emission at ∼1300 nm ensure deep optical penetration to muscle tissues (up to 1.5 cm) and excellent imaging contrast at an extremely low threshold dose of ∼5.2 pmol (∼1 μg) per mouse. Importantly, this protein coated Qdot displays no signs of toxicity toward model neuron, normal, and cancer cells in vitro. In addition, the animal’s metabolism results in thorough elimination of intravenously injected Qdots from the body within several days via the reticuloendothelial system (RES), which minimizes potential long-term toxicity in vivo from possible release of lead content. With a combination of attractive properties of high brightness, robust photostability, and excellent biocompatibility, this new NIR-II emitting Qdot is highly promising in accurate disease screening and diagnostic applications.

Fluorescence imaging in the near-infrared (NIR) window (750–900 nm) is one of the most popular work modes in biomedical sciences and clinical procedures for visualization of cells and tissues. Up to date, only indocyanine green (ICG) and methylene blue have been approved by the FDA due to their low toxicity and rapid excretion from the body. Although their imaging quality is much more superior than that by visible wavelengths in accurate real-time tumor delineation, the penetration depth of emitted light in biological tissues is still limited. It has recently come to light that even better images can be achieved using fluorophores emitting within the second near-infrared window (NIR-II; 1000–1700 nm). The benefits of NIR-II fluorophores include diminished tissue autofluorescence, reduced photon scattering, and low levels of photon absorption which allows for micron-scale resolution of deep anatomic features. The problem is, however, that most existing NIR-II fluorophores are made from inorganic nanomaterials, such as single-walled carbon nanotubes (SWNTs) and quantum dots. These materials are excreted slowly from the body, and raise concerns about long-term toxicity because they can be retained within the reticuloendothelial system like the liver and spleen. Very recently, a team of researchers led by Xuechuan Hong at Wuhan University, and Zhen Cheng and Hongjie Dai at Stanford University have reported the first smallmolecule fluorescent dye CH1055 for NIR-II imaging (Figure 1) [1]. CH1055 dye (969 Da) having four carboxyl groups absorbs light at ~750 nm and fluoresces at ~1055 nm which can be integrated with a variety of biomolecules for blood and lymphatic vasculatures, tumors and lymph nodes imaging in mice and rats. PEGylation and anti-EGFR affibody conjugation of the dye within the size limit of 40 kDa can be efficiently excreted by kidneys. Indeed, 90% of

Small-Molecule Absorber A1094 as a Stable and Fast-Clearing NIR-II Imaging Agent.

Photoacoustic (PA) imaging provides excellent optical contrast with decent penetration and high spatial resolution, making it attractive for a variety of neural applications. We evaluated optical contrast agents with high absorption in the near infrared (NIR) as potential enhancers for PA neuroimaging: optical dyes, gold nanorods (GNRs) and PEBBLEs loaded with indocyanine green. Two PA systems were developed to test these agents in excised neural tissue and in vivo mouse brain. Lobster nerves were stained with the agents for 30 minutes and placed in a hybrid nerve chamber capable of electrical stimulation and recording, optical spectroscopy and PA imaging. Contrast agents boosted the PA signal by at least 30 dB using NIR illumination from a tunable pulsed laser. Photobleaching may be a limiting factor for optical dyes-the PA signal decreased steadily with laser illumination. The second setup enabled in vivo transcranial imaging of the mouse brain. A custom clinical ultrasound scanner and a 10-MHz linear array provided near real-time images during and after an injection of 2 nM gold nanorods into the tail vein. The peak PA signal from the brain vasculature was enhanced by up to 2 dB at 710 nm. Temporal dynamics of the PA signal were also consistent with mixing of the GNRs in the blood. These studies provide a baseline for enhanced PA imaging in neural tissue. The smart contrast agents employed in this study can be further engineered for molecular targeting and controlled drug delivery with potential treatment for a myriad of neural disorders.