Apparatus and method for detecting small changes in attached mass of piezoelectric devices used as sensors

Apparatus for indicating the amount of material bound to the surface of a piezoelectric device, comprising

• a piezoelectric device carrying a reagent for selectively binding an analyte to a surface of the device, the device having first and second electrocde means mounted on the surface thereof for launching and receiving, respectively, acoustic waves on the said surface,
• first and second contact means for making electrical contact with the first and second electrode means, respectively,
• a voltage-controlled oscillator having a control terminal, the frequency of the said oscillator being variable over a range in dependence on the magnitude of a d.c. control signal applied to the said control terminal,
• phase-comparator means, having first and second inputs and an output, for generating a d.c. output signal of magnitude representaive of any phase difference in signals applied to the said first and second phase-comparator inputs, the first input being coupled to the said oscillator and the second input being coupled to the second contact means, and the said phase comparator output being coupled to the said control terminal.

A method of obtaining an indication of the amount of analyte bound to a piezoelectric device, comprising the steps of

• applying a reagent to the device which selectively binds an analyte to said surface,
• obtaining an output signal from an electrical oscillator having a frequency which is variable over a range in dependence upon the value of a control signal applied to the said oscillator,
• using the said oscillator output signal to generate a launch signal to launch an acoustic wave across the device,
• receiving the said acoustic wave,
• deriving, from the said oscillator output and the received acoustic wave, a delay signal representative of the time required by an acoustic wave to travel across the said device between launch and reception, and
• deriving the said control signal for the said oscillator from the said delay signal,
• whereby the frequency of the said oscillator provides an indication of the said speed of propagation and therefore the amount of analyte bound to said surface.

A piezoelectric device carrying a reagent for selectively binding an analyte to the surface of the said device,

• the said device having
• first and second electrodes in close proximity for launching acoustic waves on the surface of the crystal,
• third and fourth electrodes in close proximity but spaced from the said first and second electrodes for receiving acoustic waves on the said surface,
• a first connection for use in coupling said first electrode to an oscillator, the second electrode having no low conductivity connection for coupling the said second electrode to the said oscillator,
• a second connection for use in coupling the said third electrode to phase comparison means, and
• a third connection for coupling the said fourth electrode to a ground connection between the said oscillator and the said measurement means.