The heater was removed when the solution appears orange color. The solution was then cooled to room temperature and chloroform was added to the solution. The solution was kept in a fridge at 4 ◦C. Before use, the stock solution was diluted by using dichloromethane to a desired concentration. The unit of concentration reported herein is optical density of a solution at 570nm (OD570), which was determined by using a UV–vis spectrometer.