A piezoelectric plate sensor comprising a piezoelectric layer; two electrodes; and an insulation layer. The insulation layer is produced by soaking the piezoelectric layer and two electrodes in a mercaptopropyltrimethoxysilane solution with an amount of water from 0.1 v/v. % to about 1 v/v % and at pH from about 8 to about 150 for a period from about 8 to about 15 hours, and the mercaptopropyltrimethoxysilane solution has a concentration of mercaptopropyltrimethoxysilane from about 0.01 v/v % to about 0.5 v/v %. A method of detecting a biomolecule in a sample using the piezoelectric plate sensor in particular, that of detecting a genetic marker with PCR sensitivity and specificity without the need of DNA isolation or amplification is also provided. The piezoelectric plate sensor may be used to diagnose various diseases including breast cancer, myocardial infarction, diarrhea, Clostridium difficile infection, and hepatitis B infection.