Development of a mass sensitive quartz crystal microbalance (QCM)-based DNA biosensor for the detection of the hybridization of CaMV 35S promoter sequence (P35S) was investigated for the screening of genetically modified organisms (GMOs). Attention was focused on the choice of the coating chemistry that could be used for the immobilization of probe sequences on the gold surface of the quartz crystal. Two immobilization procedures were tested and compared considering the amount of the immobilized P35S probe and the extent of the hybridization reaction with the target oligonucleotide. In wet chemistry procedure, the interaction between the thiol and gold for the immobilization of a thiolated probe was employed. Direct surface functionalization of piezoelectric quartz crystals were achieved in 13.56 MHz plasma polymerization reactor utilising ethylenediamine (EDA) precursors for the immobilization of amined probes. Results indicated that immobilization of a thiolated probe provides better immobilization characteristics and higher sensitivity for the detection of the hybridization reaction. The thiolated probe was used for the detection of P35S sequence in PCR-amplified DNAs and in real samples of pflp (ferrodoxin like protein)-gene inserted tobacco plants. Fragmentation of the genomic DNAs were achieved by digestion with restriction endonucleases and ultrasonication. The results obtained from the fragmented genomic DNAs demonstrated that it is possible to detect the target sequence directly in non-amplified genomic DNAs by using the developed QCM-based DNA biosensor system. The developed QCM-based DNA biosensor represented promising results for a real-time, label-free, direct detection of DNA samples for the screening of GMOs. (C) 2008 Elsevier B.V. All rights reserved.