The invention relates to a method for preparing metal nanoparticle-modified boron-doped diamond the method comprising generating a strong oxidising agent by acid treating a front surface of the boron-doped diamond prior to deposition of the metal nanoparticles onto the front surface of the boron-doped diamond. The metal nanoparticle-modified boron-doped diamond resulting from the acid wash has a front surface which is oxygen terminated. The metal nanoparticle-modified boron-doped diamond may be used in electrodes as an oxygen sensor, the electrode may be made by preparing a boron-doped diamond column; insulating the column so that only a front surface of the column is exposed; polishing the front surface of the column; acid-treating the front surface of the column; and depositing metal nanoparticles onto the front surface of the column.