Methods The carbon, hydrogen, and nitrogen analyses were obtained using an Elementar Vario ELIII elemental analyzer and 1 H-NMR spectrum was measured on a Varian 400 NMR (400 MHz) instrument (both from Agilent Technologies Co. Ltd., Beijing, China) in CDCl 3 solution with tetramethylsilane (TMS) as internal standard. The europium ion was determined by ethylenediaminetetraacetic acid (EDTA) titration using xylenol orange as ...