The following statements describe some features of a Quartz Crystal Microbalance (QCM) biosensor:
A. A QCM biosensor is built with a thin disc-shaped piezoelectric material connected to metal electrodes on opposite faces.
B. When an AC voltage is applied to the quartz crystal, it produces oscillation at a stable resonant frequency that is not determined by the applied AC voltage.
C. The surface of the quartz crystal can be functionalized with a specific receptor that binds to the target analyte.
D. As the mass on the surface of the crystal increases, the resonant frequency of the crystal increases in a predictable, quantitative way following the Sauerbrey equation.
Which one of the following options represents a combination of all correct statements?

Correct option is A

Correct Answer:
(a) A, B and C
Explanation:
A is correct: QCM sensors use a thin quartz crystal with metal electrodes on both sides to exploit the piezoelectric effect.
B is correct: The resonant frequency is an intrinsic property of the crystal and is largely independent of the applied AC voltage amplitude.
C is correct: Functionalization of the crystal surface with receptors enables selective biosensing.
D is incorrect: According to the Sauerbrey equation, an increase in surface mass causes a decrease, not an increase, in resonant frequency.
Information Booster :
· QCM detects mass changes at the nanogram level by monitoring frequency shifts.
· The Sauerbrey equation quantitatively links mass increase to frequency decrease.
· QCM is widely used in biomolecular interaction studies.
· Selectivity is achieved through surface chemistry, not the crystal itself.
Additional Information (Incorrect Options):
(b) and (c): Include statement D, which incorrectly describes the frequency–mass relationship.
(d): Incomplete, as statement B is also a core characteristic of QCM operation.