Table 1.

Apparent dissociation constants for Fura–2 and other calcium probes determined from intensity (I), phase angle (θ) and modulation (m) measurements. Numbers in square brackets refer to references.

Probe	λexc (nm)	$K_d^I$ (nM)	$K_d^{\theta }$ (nM)	$K_d^m$ (nM)
Fura-2	345	135 [4]	66 ± 3	41 ± 7
	354	156 ± 19	108 ± 15	67 ±9
	365		258 ± 8	160 ± 10
	370		546 ± 34	341 ± 26
	375		1068 ± 52	691 ± 106
	380		1920 ± 171	1393 ± 360
Quin-2 [16]	342a	48, 60 [32]	29	10
Indo-1 [19]	345	250 [4]	1130	953
CaG [10]	514	128, 189 [12]	40	14
CaO [10]	565	269, 328 [12]	250	150
CaC [10]	590	283, 205 [12]	150	100
Fluo-3b	-	400 [5]	-	-
STBTc	-	1500 [6]	-	-
CG5Nd	-	4300 [37]	-	-
Mag-Fura-5	-	6500 [2]	-	-
BTCe	-	7000 [7]	-	-

^aAt longer excitation wavelengths higher values of apparent K_ds are expected from phase angle and modulation measurements.

^bThe quantum yield of calcium-free form is too low for lifetime based sensing.

^cThe lifetimes of the Ca²⁺-free and bound forms are 70 and 100 ps, respectively.

^dThe quantum yield of CG5N increases upon Ca²⁺ binding, similarly to the other Calcium Series probes. The apparent dissociation constants from phase angle and modulation are expected to be shifted towards lower values, as for CaG.

^eDisplays an absorption spectra shift and increase of mean lifetime upon Ca²⁺ binding. Phase-modulation measurement may provide similar properties to Fura–2.