Quartz Crystal Microbalance

1 Biosensor

Bio Recognition Element Transducer

Signal Output

Enzymes; Electrochemical Antibodies; Requires: Optical Receptors; Simple read out and data interpretation; Requires: Whole Sample cells... Easy to use; Immobilization Quick response.

2 Quartz resonators with front and back electrodes

„ http://en.wikipedia.org/wiki/Image:Quartz_resonators_with_front_and_back_electrodes.jpg

3 Theory

„ Thin quartz disk with electrodes plated on it „ Piezoelectric „ An oscillating electric field applied across the device -> acoustic wave propagates through the crystal „ Thickness of the device is a multiple of a half- wavelength of the acoustic wave -> minimum impedance „ Deposition of thin film -> decrease the frequency (mass of the film)

4 Piezoelectric effect

„ Pressure -> electricity „ Mechanical strain/stress variation -> separate the center of gravity of the positive charges from the center of gravity of the negative charges -> dipole moment -> Polarization change „ Generated voltage between two electrodes „ Insulating materials -> charges on the surface „ Depend on the symmetry of the distributions of the positive and negative charges -> material

5 Single-crystal

„ 32 classes „ 11 -> center of symmetry -> nonpolar ->symmetric ionic displacements -> no net change in dipole moment

„ Quartz

6 Converse effect

„ Electric filed -> strain mechanically „ One-to-one correspondence „ Decays due to the charge dissipation „ Increase with applied force -> drops to zero when force remains constant „ Pressure removed -> negative voltage -> decays to zero

7 Resonant oscillation

„ Electric and mechanical oscillations are close to the fundamental frequency of the crystal „ Depend on: thickness, chemical structure, shape, density, shear modulus of the quartz, mass, physical properties of the adjacent mediums (density, viscosity of air/liquid).

8 Resonant frequency

„ Sauerbrey: changes in the resonant frequency relates to the mass: 2 η Δf = −2Δmnf0 / q ρ q

„ ρq η q are the density and viscosity of the quartz (2.648g/cm3 and 2.947*10-11 g/cm s) „ f0: basic oscillator frequency of the quartz „ Δm: material adsorbed on the surface per unit area „ n: Overtone number

9 Corrections

„ Thick overlayer -> nonlinear relation between Δ f and Δ m „ Liquid -> shear motion on the surface generates motion in the liquid near the interface -> liquid density and viscosity

10 Typical setup

„ 4-6 MHz fundamental resonant frequency „ Resolution down to 1Hz „ Water cooling tubes, oscillation source, frequency sensing equipment, measurement and recording device

11 Classification

„ BAW (Bulk acoustic wave): thickness-shear mode (TSM)

‰ Small quartz crystal disk: 10-15mm diameter

‰ 0.1-0.2 mm thickness

‰ Resonance frequency: 6-20MHz 2 ‰ For a 10 MHz crystal, detection limit: 0.1 ng/mm

‰ Sensitivity is limited by the mass of the whole crystal

12 Classification (cont.)

„ SAW (Surface acoustic wave)

‰ Acoustic energy confined to the surface

‰ Wave propagates along the solid medium surface

‰ Rayleigh wave

„ Displacement of the particles near the surface has: longitudinal component and a shear vertical component

13 SAW

„ IDT (interdigital transducer) electrode „ Time-varying voltage -> synchronously varying deformation of the piezoelectric substrate -> propagating surface wave „ SAW -> alternating voltage in another IDT (receiver) „ Delay line: two IDTs and a propagation path (sensitive area) „ Environmental change -> resonance frequency change

14 SAW

„ High frequencies up to GHz range „ Sensitivity increases as the square of the fundamental frequency -> higher sensitivity potential „ Dual delay configuration -> sensing delay line coated with reactive film -> measure