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WIREs Nanomed Nanobiotechnol
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Unlocking the power of optical imaging in the second biological window: Structuring near‐infrared II materials from organic molecules to nanoparticles

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Abstract Biomedical imaging techniques play a crucial role in clinical diagnosis, surgical intervention, and prognosis. Fluorescence imaging in the second biological window (second near‐infrared [NIR‐II]; 1000–1700 nm) has attracted attention recently. NIR‐II fluorescence imaging offers unique advantages in terms of reduced photon scattering, deep tissue penetration, high sensitivity, and many others. A host of materials, including small organic molecules, single‐walled carbon nanotubes, polymeric and rare‐earth‐doped nanoparticles, have been explored as NIR‐II emitting fluorescent probes. Efficient and viable approaches to design and develop fluorescence probes with tunable photophysical properties without compromising other key features are of paramount importance. Various chemical strategies are explored to increase the quantum yield of these imaging agents without compromising their spatiotemporal resolution, specificity, and tissue penetration capabilities. This review summarizes the strategies implemented to design and synthesize NIR‐II emitting nanoparticles and small organic molecule‐based fluorescent probes for applications in the biomedical field. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Implantable Materials and Surgical Technologies > Nanoscale Tools and Techniques in Surgery
Classification of NIR‐II probes based on their structures and applications. NIR‐II, second near‐infrared; QDs, quantum dots; SMDs, small‐molecule dyes; SMDCs, small‐molecule dye conjugates; SMDNPs, small‐molecule dye‐based organic nanoparticles; SPNPs, semiconducting polymer‐based nanoparticles; SWCNTS, single‐walled carbon nanotubes
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(a) Wavelength range used for fluorescence imaging, (b) schematic representation of tissue penetration for incident beams at different wavelengths, (c) schematic representation of light when entering a tissue, and (d) fluorescence imaging of the cerebrovasculature of mice without craniotomy in the NIR‐I; NIR‐II and NIR‐IIb regions, with the corresponding SBR analysis. (e) Schematic illustration showing general strategies for tuning the absorption wavelength of cyanines, including lengthening polymethine chain (n denotes the number of methine units) and enhancing donor strength; (b) and (c) are redrawn from He et al., (2018); (d) is reprinted with permission from S. Diao et al., (2015), and (e) is reproduced with permission from S. Wang et al., (2019). NIR‐I, first near‐infrared; NIR‐II, second near‐infrared
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Dynamic contrast‐enhanced images of 4T1 tumor‐bearing mice at different time points (using 6PEG‐Ag2S QDs) and the PCA overlaid image based on the continuous NIR‐II fluorescence images. Figures reprinted with permission from Y. Zhang, Hong, et al. (2012). NIR‐II, second near‐infrared; PCA, principal component analysis; PEG, polyethylene glycol; QDs, quantum dots
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(a) Affibody‐DAP used for NIR‐II/PAI dual‐modal imaging, (b) strategy for the construction of NIR‐II dual‐modal imaging probes, (c) PET/CT images of U87MG tumor‐bearing mice (white arrows indicate the location of the tumor, n = 3 per group) acquired at 0.5, 1, and 2 hr after tail vein injection of 68Ga‐CHS2 with and without the blocking agent RGD. (d) NIR‐II images of the U87MG tumor at 1, 3, 9, 12, and 24 h after tail vein injection of CHS2 with and without the blocking agent RGD under 808 nm excitation (82 mW cm2), 1000 LP, and 40 ms; (a) is reprinted with permission from Cheng et al. (2017) and (b)–(d) is reprinted with permission from C. Li et al. (2015). NIR‐II, second near‐infrared; PAI, photoacoustic imaging
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(a) Conjugation of Q4 with NH2‐PEG8‐RM26 peptide to prepare SCH1100. (b) NIR‐II images of PC3 tumor mice at different time points (2, 4, 8, 12, and 36 hr) after tail vein injection of SCH1100 with or without blocking agent RM26 under 808 nm excitation. (c) Synthetic route of LZ‐1105 and noninvasive NIR‐II fluorescence images of brain (d) hindlimb (e) lymphatic system (f) in nude mice (brain) and shaved ICR mice (n = 3) (lymphatic system and hindlimb), intravenously injected with LZ‐1105 (1400 long‐pass filter, λex = 1064 nm, 300 ms) or ICG (1300 long‐pass filter, λex = 808 nm, 300 ms) (inset in (j) shows a magnified view of the red grid, the contrast was calculated in the yellow box). (g), (h), and (i) are the fluorescence intensity profiles (dots) and Gaussian fit (lines) along the red dashed line in the brain, hind limb, and lymphatic system, respectively. Scale bars in (d), (e), and (f) represent 3 mm; (a) and (b) are reprinted with permission from Y. Sun, Guo, et al. (2016); (c)–(i) are reprinted with permission from D. Li, Qu, et al. (2020). ICG, indocyanine green; NIR‐II, second near‐infrared; PEG, polyethylene glycol
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Lowering bandgap (HOMO/LUMO gap) with (a) extension of π‐conjugation, (b) donor–acceptor–donor (D–A–D) interaction, and (c) strategy for designing molecular structures for NIR‐II emission. (d) Chemical structures of currently available NIR‐II emitting fluorophores based on D–A–D, and (e) conjugated cyanines and FDA‐approved ICG. (f) Photophysical pathways of small organic molecule‐based fluorophores with D–A–D scaffold and the corresponding mechanism for NIR‐II emission and (g) structure of D–A–D probe CH1055, (h) structure of PEGylated CH1055; CH1055‐PEG, and Affibody‐conjugated CH1055, (i) structure of follicle‐stimulating hormone‐linked CH1055, FSH‐CH, (j) renal excretion of CH1055‐PEG, (k) brain vessel imaging, (l) glioblastoma brain tumor detection, (m) imaging of a mouse 24‐hr postinjection of CH1055‐anti‐EGFR before fluorescence image‐guided surgery; (j)–(m) are reproduced with permission from Antaris et al. (2016). HOMO, highest occupied molecular orbital; ICG, indocyanine green; LUMO, lowest unoccupied molecular orbital; NIR‐II, second near‐infrared; PEG, polyethylene glycol
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(a) Synthesis of conjugated semiconducting polymer (SP) and encapsulation of SPs into NPs by using surfactant F127 and (b) design of SPPF‐Dex nanoparticles (NPs) for CRISPR/Cas9 delivery, and illustration of the intracellular genome editing process upon 808 nm laser irradiation; (a) is reprinted with permission from L. Feng et al. (2013), and (b) is incorporated with permission from L. Li et al. (2019)
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(a) Structure and fluorescence spectra of rare‐earth‐doped NPs with NIR‐II emission, showing tunable fluorescence depending on the choice of the dopant. (b) Corresponding multicolor NIR‐II fluorescence images in a mouse model. Reprinted with permission from Naczynski et al. (2013). NIR‐II, second near‐infrared; NPs, nanoparticles
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Implantable Materials and Surgical Technologies > Nanoscale Tools and Techniques in Surgery
Diagnostic Tools > In Vivo Nanodiagnostics and Imaging

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