Table 3.

Summary of optical spectroscopy based methods for detecting aqueous Ln(III) ions

	Gd-sensor	Xylenol orange [15]	Calcein blue [14]
Desired limit of detection	As low as possible to ensure higher purity of synthesized GBCAs
Limit of detection of Gd(III) (S/N = ~3)	~80 nM	~10 – 15 µM	~100 nM
Reactivity toward other common metal cations, anions and neutral small molecules	Significantly less reactive than toward the Ln(III) ions	Not tested in this study. However, it is a commonly used complexometric agent for metal cations	React with various metal cations including Ca(II), Mg(II), Fe(II), Mn(II) and Ni(II)
Method of detection and quantification	Increase in fluorescence signal intensity	Change in absorbance ratio at two different wavelengths	Quenching of fluorescence