A comparative study on detection of melamine with different sized citrate capped AuNPs namely 15 nm (AuNPs-I), 30 nm (AuNPs-II), and 40 nm (AuNPs-III) was carried out by fluorescence spectroscopy. The AuNPs emitted strong fluorescence at 421 nm with different intensity at 116.122, 220.511 and 253.665 for AuNPs-I, AuNPs-II and AuNPs-III respectively on excitation with 308 nm. On interaction with melamine, the AuNPs aggregated resulting in the enhancement of the fluorescent intensity of AuNPs. The sensitivity of melamine detection was studied for three different sizes of AuNPs by drawing a calibration curve between the concentration of melamine and fluorescence intensity. A good sensitivity was observed for AuNPs-II having the detection limit as low as 0.66 nM (3σ) which was lower in comparison to the detection limit of AuNPs-I (2.78 nM) and AuNPs-III (7.74 nM). The cost of synthesis was low as the usage of HAuCl4 in the synthesis of AuNPs-II was lower compared to the other sizes of AuNPs resulting in low cost of chemicals. The AuNPs-II was further chosen for carrying out selectivity study and for detecting the concentration of melamine. The recovery percentage of melamine in raw milk, liquid milk and milk powder after pre-treatment was found to be 100 %, 97 % to 100 % and 94 % to 99 % respectively. © 2016, Springer Science+Business Media New York.