Different interaction of codeine and morphine with DNA: A concept for simultaneous determination

► The interaction between codeine and morphine with dsDNA were assessed. ► A pencil graphite electrode modified with carbon nanotubes and DNA was tested. ► Detection limits of 0.041 for codeine and 0.043 μg mL−1 for morphine were obtained. ► This biosensor was used for the analysis of codeine and morphine in real samples.