Near-infrared (NIR) region within the wavelength range of 700–1000 nm has attracted much attention in the biomedical field, offering the advantages of decreased photon attenuation, autofluorescence, and scattering. In optical imaging technology, near-infrared fluorescence imaging has attracted much attention due to its low absorption and autofluorescence from organisms and tissues. The NIR probes applied during imaging generally include inorganic and organic molecules. Concerning the spectral characteristics, organic NIR fluorescent dyes have recently attracted a lot of interest, they are capable of covalent or non-covalent conjugation with cancer-targeting organic and biomolecules. Therefore, NIR dyes can be used for specific and sensitive imaging of cancer, enabling early detection of tumor cells in patients to improve patients' survival rates.
Fig.1 The ﬂuorescence imaging instruments and ﬂuorescent molecular probes for cancer imaging
According to the different structures, near-infrared dyes are divided into many categories, such as cyanine dyes and squaraine dyes.
Cyanide dyes have a complex conjugation system in which two aromatic nitrogen-containing heterocycles are connected by an odd-numbered chain of linked methane units. As the excellent near-infrared dyes, cyanide dyes have high molar absorption coefficient and fluorescence quantum yield in the visible-to-near-infrared region. They are used as probes to label a variety of biological macromolecules for various fluorescent detection techniques. Cyanine dyes can be classified as open or closed chain cyanines and hemocyanines, which are generally easy to synthesize and have adjustable wavelengths and good biocompatibility. Some cyanine dyes and their derivatives are among most common NIR fluorescent dyes. For instance, the pentamethine (Cy5) and its derivatives can reach a near-infrared region (>700 nm) while heptamethine cyanines (Cy7) may show absorption beyond 1000 nm.
Among cyanine dyes, Indocyanine green (ICG) is a zark green and NIR fluorescent polymethine dye, absorbing light from 790 to 805 nm and remitting it with an excitation wavelength of 835 nm. It has a maximum absorption wavelength of 800-820 nm in water. As the only FDA-approved cyanine dye, it has been used as a non-targeting contrast agent for optical imaging to measure cardiac output and assess liver function.
Fig.2 Representative cyanine derivatives
Squaraine dyes are squaric acid derivatives that consist of a central acceptor squaryl ring with donor aromatic or heterocyclic rings on each side, and these compounds exhibited the excellent photophysical properties such as intense absorption bands in the red-NIR region, high molar coefficients, and excellent photoconductivity. Among them, a series of substituted squaraines such as 2,3,3-trimethylindolenine-based squaraine dyes are synthesized rapidly and efficiently by microwave methods. And the maximum absorbance of these dyes is between 625 and 700 nm, and the fluorescence emission is between 635 and 800 nm. These microwave-synthesized squaraine dyes exhibit improved quantum yields and reduced reaction time when compared with conventional synthesis methods.
Fig.3 General chemical structures of squaraine dyes, 
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