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.