were added and diluted to 10.0 mL. Then, 200 lL of ethanol (as disperser solvent) and 100 lL of carbon tetrachloride (as extraction solvent) were mixed and injected into the sample solution by using a syringe. When shaking for 3 min, a cloudy solution was quickly formed. Now, the phase separation can be achieved by centrifuging the cloudy solution at 3500 rpm for 3 min. The bulk aqueous phase solution was removed by a syringe, while the sedimented phase was diluted to 0.5 mL with ethanol. Lastly, the absorbance intensity was measured using microcuvette at 740 nm against a reagent blank.