Evolution of average diameter during a ten-day incubation period for suspensions of (a) ◆ 40 ng/L 100 nm AgNPs spiked into Xiangjiang River, ◇ 40 ng/L 100 nm AgNPs spiked into HuNan University effluent, ● 10 ng/L 50 nm AgNPs spiked into Xiangjiang River, ○ 10 ng/L 50 nm AgNPs spiked into HuNan University effluent (b) ▲ 50 ng/L 100 nm AuNPs spiked into HuNan University effluent, △ 50 ng/L 100 nm AuNPs spiked into Xiangjiang River,▼ 25 ng/L 60 nm AuNPs spiked into HuNan University effluent, ▽ 25 ng/L 60 nm AuNPs spiked into Xiangjiang River. Error bars represent the standard deviation of triplicate experiments.