Combine 50 mL of conc. nitric acid and 15 g of anhydrous copper(II) nitrate, stir vigorously for 24 h. If the copper nitrate doesn’t dissolve add some water. Heat the solution to 80°C and add 60 g of 95% ethanol while stirring the mixture. After the addition remove from the heat source and cool down to room temperature. Then filter and wash with 100 mL of water, dry in a desiccator, store in a freezer.