High sensitivity colorimetric determination of L-cysteine using gold nanoparticles functionalized graphene oxide prepared by photochemical reduction method

•AuNps and AuNps@ GO were prepared by photochemical reduction method.•The colorimetric detection of L-cysteine was demonstrated in a nanomolar order concentration.•4-Hydroxybenzophenone was used as photoreducing agent for Au ions.•AuNps and AuNps@ GO acted as simple and sensitive nano-sensors for L-cysteine.