Resonance light scattering (RLS) spectroscopy, is a simple and sensitive analytical tool, which has been extensively used for various (bio)analytical tasks. Since RLS intensity depends on the molar absorptivity as well as the size of the scatter, it is particularly useful in studying the aggregation of chromophores in various chemical and biological systems [1, 2]. Morphine and thebaine are naturally occurring alkaloids in poppy seeds, have long been used as a drug, and is also abused .As drug, morphine is used as an effective analgesic in the treatment of severe pain and codeine is widely used as a cough suppressant and moderate pain reliever too. But excessive or habitual use of these drugs causes toxic symptoms. In this work, a new method was investigeted for the determination of morphine in presence of thebaine, using Au nanoparticles coated on the surface of Fe 3 O 4 (AuNPs@Fe 3 O 4 ) -based resonance light scattering (RLS) technique. Light scattering properties of the synthesized nanoparticles in the presence or absence of drugs have been selected as the detection signal. In this regard, resonance light scattering has been used as the detection method. RLS intensity was ex em ( = 0 nm). The addition of these drugs to nanoparticles solution in acidic medium leads to the aggregation of nanoparticles, which, in turn, causes a marked enhancement in RLS intensity at 400 nm.The effects of important experimental factors such as pH, concentration of nanoparticles and ultrasonic time on RLS signal were investigated and optimized. Under the optimum conditions, the calibration graphs were linear in the range of 0.05 – 5.00 and 0.1 – 8.0 mg L -1 , for morphine and thebaine, respectively with corresponding limits of detection of 0.037 mg L -1 and 0.12 mg L -1
