Quin-2 is a high-affinity fluorescent calcium indicator (Kd = 115 nM for calcium). It displays high selectivity for calcium, as it is not affected by sodium gradients, membrane potential, or intracellular pH. High affinity probes like quin-2 are ideal for monitoring low levels of calcium, as are found in resting cells. Loadings of up to 2 mM quin-2 are without serious toxic effects, so quin-2 may be used to buffer intracellular calcium transients. Excitation/emission maxima for quin-2 are 339 and 492 nm, respectively.
ICG-OSu, also known as ICG NHS ester, is a near-infrared fluorescent agent commonly utilized for the development of in vivo imaging probes due to its amine-reactive properties.
HKOCl-4 (BXY2142) is a rhodol-based yellow fluorescent probe that demonstrates exceptional sensitivity and selectivity in detecting hypochlorous acid. Its absorption wavelength is significantly longer than that of fluorescein-based probes, and it exhibits better pH stability. Notably, HKOCl-4 exhibits excitation at 530 nm and emission at 557 nm.