The nominal value of suspension was checked via flame atomic absorption spectrometry (AAS, Perkin
Elmer AAnalyst 100, Waltham, Massachusetts, USA) after over-night digestion in a 1.0 M HNO3(suspension/acid ratio 1:1 vol/vol). Then 8 mL of the diluted suspension (25 mg L1 ) was ultra-centrifuged in triplicate at 100000 g for 30 min at 20 C (Beckman Coulter L8-70M class H preparative ultracentrifuge with Type 70.1 Ti rotor and 10 mL thickwall polyallomer tubes)