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The cross-linked chitosan is synthesized by homogeneous reaction of medium molecular weight chitosan in aqueous acetic acid solution with glutaraldehyde as cross-linking agent. Using surface plasmon resonance technique, the optical properties of cross-linked chitosan thin film before and after contacting with different concentrations of copper ion in a range of 0 to 100 ppm had been obtained by fitting. The imaginary part of refractive index increased while the thickness of the film decreased as copper ion concentration increased from 0 (deionised water) to 100 ppm. The resonance angle shifted to lower value as the copper ion concentration increased. By introducing the cross-linked chitosan film, copper ion detection can be obtained for concentration as low as 0.5 ppm using surface plasmon resonance technique.