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A novel method for systematic variation of chemical selectivity is introduced using mixed-ligand gold nanoparticles. We have shown that altering the composition of ligand molecules attached on the nanoparticle surface affected interaction of the nanoparticle sensors with the analyte vapors. A gold nanoparticle flinctionalized with 4-methylbenzenethiol (4-MBT, -SC6H4CH3) was synthesized and used to produce a series of mixed-ligand nanoparticles of 4-MBT and 4-mercatophenol (4-MP, -SC6H4OH) with varying ligand composition. High resolution transmission microscopy (HREM) analysis of the nanoparticles confirmed that the ligand-exchange reaction to produce binary-ligand nanoparticles did not affect the size of the resulting nanoparticle diameter (∼2.3 nm). Thermal gravimetric analysis (TGA) revealed that the amount organic ligand comprising the nanoparticles ranged from 10.4 to 11.3 weight % of the mixed ligand nanoparticles. Especially the two-step mode in weight loss observed in the binary-ligand particles was correlated with the ligand composition. The compositional variation of the ligands in the nanoparticles was well reflected on the selectivity variation as was confirmed by diversified amplitude and response time in the signals. Fairly impressive signal reproducibility was observed at least for ethanol vapor. At least 10 ppm detection limit was observed toward ethanol. S3 showed reasonable linearity ranging from 10 ppm to 100 ppm of ethanol.