The alloying of Pt with a cheap metal, tailoring structures and compositions are significantly important to boost the utilization and catalytic performance of Pt catalysts in catalytic reactions. Here, small-size monodispersed octahedral PtxCu1-x nanocrystals (NCs) with adjustable components were successfully prepared by a simple solvothermal method and the reaction mechanism was analyzed. The alloying of Pt and Cu not only reduces the loading of Pt, but also obtains better catalytic activity due to the synergy between heterogeneous atoms. PtxCu1-x NCs are loaded on SnO₂nanoclusters by rapid thermal annealing (RTA), which were first applied to semiconductor metal oxide (SMO) -based gas sensors. By adjusting the composition of the nanocrystals, the sensing material Pt₄₀Cu₆₀-SnO₂with the best gas sensing performance is obtained. More importantly, Pt₄₀Cu₆₀-SnO₂can significantly distinguish the exhaled breath of healthy people and diabetic ketosis (DK) patients, and achieve effective auxiliary detection of DK.