Determination method of trace Cd(II) in rice using a luminescent Cd(II)-Tb(III)-Thiacalix[4]arene ternary complex

Determination methods for Cd in a food stock have attracted much attention, since it is a major source of Cd exposure. In the present work, we developed a luminescence determination method for Cd(II) at nM levels in aqueous solution, which has been successfully applied to rice samples to determine sub-ppm levels of Cd. At pH>8, Terbium(III) and thiacalix[4]arene-p-tetrasulfonate (TCAS) formed a 1:1 complex Tb(TCAS), in which excited energy was transferred from TCAS to Tb(III), leading to luminescence. In contrast, Tb(TCAS) barely formed at pH around 6.5. In the presence of Cd(II), the components self-assembled to form a 2:2:2 ternary complex, Cd₂Tb₂(TCAS)₂, which exhibited the Tb(III)-centered luminescence. At pH 6.5, where the difference in the luminescence intensities between Cd-Tb-TCAS and Tb-TCAS systems was largest, the calibration curve for Cd(II) was linear to give a detection limit (S/N = 3) of 2.08 nM. The luminescent determination method was applied to rice sample solutions, which were prepared by digestion with nitric acid and hydrogen peroxide under heating. Any interference by Cu(II) and phosphate in the acid digest was removed by the masking of Cu(II) with iminodiacetate and the separation of phosphate using a chelate disk, respectively. The detection limit for 1 g of a rice sample was 12.2 ppb (S/N = 3), which cleared the standard of cadmium in rice based on the Food Sanitation Act, Japan (0.4 ppm). The recoveries for 1.007 g brown rice spiked with 3.56 nmol Cd(II) and 2.002 g brown rice spiked with 7.12 nmol Cd(II) were 101.0% and 100.3%, respectively. Also, in the standard rice sample (certified value of Cd: 0.548 ppm) measurement, the result was 0.564 ppm.