2.2. Preparation of PBNPs.
PB NPs were synthesized according to the previous method developed by our group. In brief, 264 mg K3[Fe(CN)6]•3H2O and 3 g PVP were dissolved in 40 mL of 0.01 M HCl solution under stirring for 1 h at room temperature. Then, the mixture solution was heated at 80 °C for 20 h with oil bath. After centrifugation and water washing for 3 times, the precipitate was dried under vacuum for the preparation of PB NPs.
2.3 Preparation of HMPB NPs.
HMPB NPs were prepared following the previous method with little modification32. 100 mg PVP and 20 mg PB NPs were dissolved in 20 mL of 1 M HCl solution under magnetic stirring for 3 h. Then, the mixture solution was transferred into a stainless autoclave and heated at 140 °C for 4 h. After acid etching, HMPB NPs is collected by centrifugation and washed three times with deionized water.