The carbon-coated copper grids (400-mesh, 3.05 mm O.D., Ted Pella Inc.) were pretreated with glow-discharge plasma at 15 mA on the negative mode using PELCO easiGlow (Ted Pella Inc.), with no glow discharge for mixture A grid, 60s glow discharge time for mixture B grid, and 30s for mixture C grid. Around 1 mg powder from each set was transferred to a 10 mL scintillation vial and separately mixed with the grid. After a gentle shaking of the vial, the grids were taken out and clipped at room temperature.