Fluorescence imaging has become a fundamental tool for biomedical applications; nevertheless, its intravital imaging capacity in the conventional wavelength range (400-950 nm) has been restricted by its extremely limited tissue penetration. To tackle this challenge, a novel imaging approach using the fluorescence in the second near-infrared window (NIR-II, 1000-1700 nm) has been developed in the past decade to achieve deep penetration and high-fidelity imaging, and thus significant biomedical applications have begun to emerge. In this Perspective, we first examine recent discoveries and challenges in the development of novel NIR-II fluorophores and compatible imaging apparatuses. Subsequently, the recent advances in bioimaging, biosensing, and therapy using such a cutting-edge imaging technique are highlighted. Finally, based on the achievement in the representative studies, we elucidate the main concerns regarding this imaging technique and give some advice and prospects for the development of NIR-II imaging for future biomedical applications.

Advanced Fluorescence Imaging Technology in the Near-Infrared-II Window for Biomedical Applications.

The development of fluorophores for the second near-infrared window (NIR-II, 1000-1700 nm) represents an emerging, significant, and vibrant field in analytic chemistry, chemical biology, and biomedical engineering. The wavelength, brightness, and stability are three crucial factors that determine the performance of an NIR-II fluorophore. Up to now, significant progress has been made in the development of NIR-II fluorescence molecular probes, including the synthesis of D-A-D and D-π-A fluorophores with improved NIR-II fluorescence imaging performance and the construction of off-on probes and ratiometric probes via energy transfer or molecular structure modification. In this review, we summarize the most recent advances in molecular engineering design strategies of NIR-II fluorophores and probes, then highlight a selection of bioimaging and biosensing applications. We also provide perspectives on potential challenges and opportunities in this emerging field.

Molecular Engineering of NIR-II Fluorophores for Improved Biomedical Detection

Persistent luminescence is not affected by background autofluorescence, and thus holds the promise of high-contrast bioimaging. However, at present, persistent luminescent materials for in vivo imaging are mainly bulk crystals characterized by a non-uniform size and morphology, inaccessible core-shell structures and short emission wavelengths. Here we report a series of X-ray-activated, lanthanide-doped nanoparticles with an extended emission lifetime in the second near-infrared window (NIR-II, 1,000-1,700 nm). Core-shell engineering enables a tunable NIR-II persistent luminescence, which outperforms NIR-II fluorescence in signal-to-noise ratios and the accuracy of in vivo multiplexed encoding and multilevel encryption, as well as in resolving mouse abdominal vessels, tumours and ureters in deep tissue (~2-4 mm), with up to fourfold higher signal-to-noise ratios and a threefold greater sharpness. These rationally designed nanoparticles also allow the high-contrast multiplexed imaging of viscera and multimodal NIR-II persistent luminescence-magnetic resonance-positron emission tomography imaging of murine tumours.

X-ray-activated persistent luminescence nanomaterials for NIR-II imaging.

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics.

Bioluminescence imaging has been widely used in life sciences and biomedical applications. However, conventional bioluminescence imaging usually operates in the visible region, which hampers the high-performance in vivo optical imaging due to the strong tissue absorption and scattering. To address this challenge, here we present bioluminescence probes (BPs) with emission in the second near infrared (NIR-II) region at 1029 nm by employing bioluminescence resonance energy transfer (BRET) and two-step fluorescence resonance energy transfer (FRET) with a specially designed cyanine dye FD-1029. The biocompatible NIR-II-BPs are successfully applied to vessels and lymphatics imaging in mice, which gives ~5 times higher signal-to-noise ratios and ~1.5 times higher spatial resolution than those obtained by NIR-II fluorescence imaging and conventional bioluminescence imaging. Their capability of multiplexed imaging is also well displayed. Taking advantage of the ATP-responding character, the NIR-II-BPs are able to recognize tumor metastasis with a high tumor-to-normal tissue ratio at 83.4. Conventional bioluminescence imaging usually operates in the visible region and its performance is limited by strong tissue absorption and scattering. Here, the authors present bioluminescence probes (BPs) with emission in the second near infrared (NIR-II) region, and show the NIR-II-BPs could sensitively recognize tumor metastasis with a high tumor-to-normal tissue ratio.

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NIR-II bioluminescence for in vivo high contrast imaging and in situ ATP-mediated metastases tracing.

The second near-infrared wavelength window (NIR-II, 1,000–1,700 nm) enables fluorescence imaging of tissue with enhanced contrast at depths of millimetres and at micrometre-scale resolution. However, the lack of clinically viable NIR-II equipment has hindered the clinical translation of NIR-II imaging. Here, we describe an optical-imaging instrument that integrates a visible multispectral imaging system with the detection of NIR-II and NIR-I (700–900 nm in wavelength) fluorescence (by using the dye indocyanine green) for aiding the fluorescence-guided surgical resection of primary and metastatic liver tumours in 23 patients. We found that, compared with NIR-I imaging, intraoperative NIR-II imaging provided a higher tumour-detection sensitivity (100% versus 90.6%; with 95% confidence intervals of 89.1%–100% and 75.0%–98.0%, respectively), a higher tumour-to-normal-liver-tissue signal ratio (5.33 versus 1.45) and an enhanced tumour-detection rate (56.41% versus 46.15%). We infer that combining the NIR-I/II spectral windows and suitable fluorescence probes might improve image-guided surgery in the clinic. An optical-imaging instrument that integrates a visible multispectral imaging system with the detection of near-infrared fluorescence in the first and second windows aids the fluorescence-guided surgical resection of liver tumours in patients.

First-in-human liver-tumour surgery guided by multispectral fluorescence imaging in the visible and near-infrared-I/II windows

Functionalized Nano-SiO2 for Improving the cycling stability of 4.6V High Voltage LiCoO2 Cathodes

Stabilizing Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode by generating ion-conductive polymer coating layer with 1, 3-Dioxolane introducing electrolyte for lithium-ion batteries

Yuanxue Zhang

Papers

First-in-human liver-tumour surgery guided by multispectral fluorescence imaging in the visible and near-infrared-I/II windows

Functionalized Nano-SiO2 for Improving the cycling stability of 4.6V High Voltage LiCoO2 Cathodes

Stabilizing Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode by generating ion-conductive polymer coating layer with 1, 3-Dioxolane introducing electrolyte for lithium-ion batteries