The reusability and stability of the sorbent was also evaluated for successive cycles using the same batch of Fe3O4@PDA NPs. Before each cycle, the sorbents were rinsed one time with pH=2 hydrochloric acid water solution to remove the residual eluent. The results (Fig. 6) indicate that the recoveries of the analytes were almost no change in the first three cycles. A slight loss in the recoveries was observed when further using the same batch of Fe3O4@PDA NPs, but the recovery values were still higher than 70% after five cycles. These data demonstrate that the Fe3O4@PDA NPs have good stability and great potential for cycling use.