Due to proportional relationship between vibrational frequency and surface mass, the quartz crystal microbalance (QCM) has been used to detect the accumulation of small quantities of chemical and biological compounds on electrode-covered quartz crystals. Classically, these devices have been used to detect gases and vapor which adhere to chemically selective coatings. Recently, however, increasing attention has been focused on the use of QCMs in aqueous environments. Herein reported is the development of an antibody-based QCM biosensor for measurement of (2,4-dichlorophenoxy)acetic acid (2,4-D) in aqueous systems. The dynamic range of measurement of 2,4-D is 0.1 to 10 ppm.