A platinum precursor Pt(acac)2 was dissolved in ethanol to give a solution of 10 -3 M. The pH was measured and controlled by adding droplets of aqueous solutions of NaOH and HCl since the pH control is essential to achieve the desired target molecules . It is known that a strong basic environment (pH ~ 13) results in smaller clusters (n = 4, whereas a neutral pH (pH = 7) favours the formation of larger clusters (n = 7 or 8). The obtained solutions were deaerated with argon gas for 20 min and subsequently saturated with CO gas (1 atm). Next, the solutions were irradiated with a 60 Co panoramic gamma source for 20-40 min (the total dose 835 Gy, low-dose irradiation). UV-Vis absorption spectra were recorded using a single-beam Hewlett-Packard 8453 spectrophotometer and 1 cm optical path cuvettes.