The density functional B3LYP was utilised for scrutinising the effect of Pt-decoration on the sensing performance of an AlP nanosheet (AlPNS) in detecting mercaptopurine (MP). Pure AlPNS has a weak interaction with MP, and the sensing response is ∼ 2.4. There was an increase in the energy of adsorption (Ead) for MP from −6.2 to −24.1 kcal/mol when the surface of AlPNS was decorated with a Pt atom. There was also a dramatic increase in the sensing response to 77.4. The recovery time (RT) for the desorption of MP from the AlPNS surface that was decorated with Pt at 298 K was 36.4 s. The solvent H$_2$O decreased E$_{ad}$ for MP to −19.9 kcal/mol. These values suggest that AlPNS decorated with Pt can be used as an MP sensor because of its high sensitivity and shorter RT.