Bulk acoustic waves have been applied as affinity sensors. In particular, a nucleic acid sensor for hybridization studies has been developed and applied for detecting DNA target sequences in solution. A DNA probe is immobilized on the sensor surface while the target sequence is free in solution; the interaction between the two complementary strands (hybridization) is followed in real-time, without the use of any label. The system has been applied to analytical problems, i.e., genetically modified organisms (GMOs) detection. The probe was complementary to characteristic DNA sequences present in GMOs. The probe sequences were internal to the sequence of 35S promoter and Nos terminator that are inserted sequences in the genome of the GMO regulating the transgene expression. Two different probe immobilization procedures were characterized to improve the performances of a piezoelectric crystal DNA sensor for GMOs detection: 1) thiol-dextran-streptavidin-biotin procedure and 2) thiol-derivatized probe and blocking thiol procedure. The system has been optimized using synthetic oligonucleotides. The probe immobilization step was monitored by a surface plasmon resonance system.