Table 1.

Overview of precise probe formulations examined in this review.

Luminescent Probe Type	Probe Name/Formulation	Cell line/Animal Model	Advantage	Limitation	References
Dye/Small Molecule Based Probes	azo rhodamine (indirect detection by reductase)	- Rat liver microsomes - HeLa cells - Colon26 cells - A 549 cells	- Azo group has excellent properties as a hypoxia-sensitive moiety - Multicolor imaging with both probes allows visualization of oxygen concentration gradient in living cells	- Responds only to very low oxygen concentrations - The quencher is not susceptible to further chemical modification	[30]
	p-azobpy and dmap-azobpy complexes with iridium	- HeLa cells -Rat Microsomes	- Long phosphorescent lifetime	- Difficult for any further chemical modification	[31]
	2-Nitroimidazole-tricarbocyanine conjugate	- Mouse xenograft models - SUIT-2/HRE-Luc pancreatic cancer cells	- High signal-to-noise ratio in vivo - High spatial resolution - Sufficiently high aqueous solubility	- Limited penetration depth	[32]
	7-Azido-4-Methylcoumarin (azide-containing substrate for oxygen-dependent reduction by cytochrome P450)	- HepG2 (hepatic) cancer cell line - OE21 (esophageal) cancer cell line - Spheroids grown using HCT116 cell	- Favorable physicochemical properties	- Possible alternative mechanism that are causing the detected signal in low oxygen conditions	[34]
	4-Nitroimidazole- 3-hydroxyflavone conjugate	- HeLa cells	- High selectivity -“Turn-On” fluorescence response with suitable sensitivity -No cytotoxicity -Large Stokes’ shift -Dual emission	- Indirect detection of hypoxia by assessment of reductase activity	[35]
	semi-CyHP (condensation product of p-nitrobenzaldehyde and 1,2,3-trimethyl-3- methyl-benzo indole iodized salt)	- Lung carcinoma cell line A549 cells	- Good selectively -Minimal endogenous interference	- After 10 uM of probe semi-CyHP it becomes cytotoxic	[36]
	7-[(5-nitrofuran- 2-yl)methoxy]-3H-phenoxazin-3-one	- HeLa cells -A549 cells	- High spatiotemporal resolution -High sensitivity of detection -Good water solubility -Good selectivity	- Weak absorption in the long-wavelength region	[37]
	hemicyanine dye : 2-(2-hydroxyphenyl)- benzothiazole hybrid	- HeLa cells -A549 cells	- Long-wavelength absorption -Dual emission	- Indirect detection of hypoxia by assessment of reductase activity	[38]
	p-nitrobenzyl moiety tethered to Nile Blue fluorophore	- A549 cells	- Minimal interference -Good fluorescence intensity -Highly selective and sensitive	- Indirect detection of hypoxia by assessment of reductase activity	[41]
	dicyanomethylene- 4H-pyran dye with azo bond quencher	- HeLa cells -A549 cells	- High selectivity -Long-wavelength absorption -Anti-interference ability	- Indirect detection of hypoxia by assessment of reductase activity	[43]
	heptamethine carbocyanine dyes	- Human glioblastoma U87 cells -HEK293 cells -U87 tumor xenografts in mice	- Strong near IR fluorescence -Capable of conjugation to drug for dual use theranostic	- Indirect uptake by requiring HIF1alpha/OATP signaling for cellular uptake which could causes a false positive signal	[46]
Polymeric/Chelator based probes	HYPOX fluorescent probes (indirect detection by reductase induced cleavage of azo-bonds)	- C57BL/6J (mice) - Human Retinal Pigment Epithelial Cell Line ARPE-19 - Human Muller Cell Line MIO-M1 - Rat Retinal Neuronal Cell Line R28	- Detection of retinal-hypoxia - No acute toxicity in retinal tissue	- Poor pharmacokinetic properties - Indirect detection of hypoxia by assessment of reductase activity	[56]
	ruthenium(II) anthraquinone complex	- A549 Cancer Cell Line - Zebrafish	- High resolution and deep - Can be used to detect tissue penetration repeated cycles of hypoxia/re-oxygenation	- Accumulation in mitochondria - Potential cytotoxicity above 10uM	[58]
	poly( - vinylpyrrolidone) (PVP)-conjugated N iridium (III) complex	- Human Neuroblastoma SH- SY5Y cells Hepatoma Cell Line H22 - Mice from Murine	- Water soluble - Good tumor retention time	- Limited penetration depth	[59]
	diketopyrrolopyrrole/polystyrene/platinum benzoporphyrin encapsulated in pluronic F-68	- Male BALB/c Nude Mice	- Efficient deep tissue imaging - Prolonged blood circulation - Easy to functionalize	- Uptake by liver	[68]
Inorganic Nanoparticle based probes	Binary nanocluster of cyanovinylene- backbone conjugated polymers and bisthienylethene derivative, 1,2-bis(2,4- dimethyl-5-phenyl-3- thienyl)-hexafluoro-1- cyclopentene	- Rats - Mice	- Bright uorescence in the NIR - Nondestructive and reversible uorescence	- Low stability - Complexity in probe synthesis and delivery formulation	[64]
	AgInS2/ZnS quantum dots coated with perylene	- Male Sprague Dawley Rats	- Rapid, high tumor uptake - Good quantum yield - High photostability - Large absorption cross- section	- Low lipophilicity causes fast excretion - Toxicity of quantum dots	[65]
	NaYF4:Yb/Tm nanoparticles coated with Iridium complex modified SiO2	- HeLa cells	- Good long-term use in vivo - Highly sensitive oxygen sensing and imaging performances - Low cytotoxicity - Good water dispersibility and stability	- Low quantum efficiency coefficient of complex Ir - Weak molar extinction	[66]
	NaYF4:Yb/Tm nanoparticles coated with Ruthenium complex modified SiO2	- U87MG cells - Zebrafish embryos - HeLa cells	- Selective and concentration dependent response -Unaffected by changes - Reversible response in acidic or reducing environments	- Low emission wavelength	[67]