1) Experimental measurement of the dispersion curve
• Thick PECVD SiO2(> 5 µm) wet etching in fluorhydric acid
• Continuous monitoring of the transfer function during the etching
• Mass sensitivity obtained by derivation of the dispersion curve
—V - -Vg
—α 9Mechanical transmission lines model
• Block approachfor each layer of the structure with density ρ, shear stiffness µ , viscosity η
• Parallel/series connection along x/z directions
• Complex coupling angle θ
• Transverse resonance principle
• Dispersion curve Î - phase velocity V (t) - group velocity Vg (t) - attenuation α (t)
9Biosensing = viscous sensing
•µ/ηfrom ~1 to ~ 10 MHz < f0
•h from ~1 to ~20 nm < 3δwater(150 nm)
• Calibration by viscous solutions not enough to evaluate and
SENSING AREA
L W
D SUBSTRATE
INPUT IDT OUTPUT IDT
λ
PIEZOELECTRIC SUBSTRATE GUIDING LAYER(S)
t
LIQUID CELL
9Sensitivity evaluation
• Mass sensitivity:
• Sensor response model:
INTERFERENCES LINEAR
• Theoretical value for
• Insensitive to SiO2thickness
Techniques to evaluate the mass sensitivity of Love mode surface acoustic wave biosensors
1 Department of Physical Chemistry and Physics of Materials, Université catholique de Louvain, Louvain-la-Neuve, Belgium Phone: +32 16 28 85 64, fax: +32 16 28 10 97, E-mail: francis@pcpm.ucl.ac.be
2Biosensors Group, IMEC, Leuven, Belgium; 3LMN, Université de Franche-Comté, Besançon, France;
4Department of Chemistry, Physical and Analytical Chemistry, Katholiek Universiteit Leuven, Leuven, Belgium
INTRODUCTION
EXPERIMENTAL TECHNIQUES
CONCLUSION
L. A. Francis
1,2, J.-M. Friedt
3, R. De Palma
2,4, C. Zhou
2,4, C. Bartic
2, P. Bertrand
1and A. Campitelli
2•Love mode: shear-horizontal surface acoustic wave guided in single or multiple layer coatings on a piezoelectric substrate
•Biosensing: acoustic signal (delay phase angle and insertion loss) shifts caused by the adsorption of biomolecules from a liquid medium (e.g. antibodies in blood sample)
•Purpose of the research: experimental and theoretical investigation of the sensing characteristics of the Love mode device for its application as biosensor
THEORETICAL TECHNIQUE LOVE MODE SAW BIOSENSOR
• Structure ST-cut quartz substrate t= 1.2 µm PECVD SiO2 coating
• IDT: 100 split fingers pairs of 200 nm Al
• Center frequency f0= 123.5 MHz
• Wavelength λ= 40 µm, L = 9 mm
• Sensing area: D = 4.7 mm, W = 3.2 mm covered with 50 nm Au
• Static liquid cell above sensing area
ωρ
=i Z
ωη
µ ω
i i
= + Y
∆x Shear
stress Tin
+ +
- Shear stress Tout
- Shear velocity vin
Shear velocity vout
• Theoretical value for
• Sensitive to SiO2thickness 0
limh→ h>3δ
-17.05 -5.114*10--4
3.729*105 742.5
δ 3
πρf δ≅ η V
V V t V
S V g−
≅
= σ ρ
σ φ
1 d d 1
,
θ θ cos sin k k
k k
z x
=
=
θ θ cos sin
0 0
0 0
Z Z
Z Z
z x
=
=
ρη α ρ
α ρη
ρ d
d d d
d d 1 d d 1
t V V t V V
Intrinsic S
=
α φ d d
ρη α σ α
ρη φ σ φ
, ,
, ,
S S
S
S
ρη σ d
d
D kD
0 0 Structure
φ α
φ α
kS
=S d d
RIGID f0
η>
µ
f0
η<
µ VISCOUS
• Ratio attenuation shiftto phase shift:
ÎBIOSENSING = VISCOUS SENSING with unknown parameters: ρ, ηand h
• Decay length:
x z
x y SENSING
AREA
9Rigid or viscous sensing ? 2) Sensitivity evaluated by modifying the surface density ∆σ
9Gold thin film etching:∆σ = ρ∆h
9Copper electrodeposition: ∆σ ∼ current
• Sensing area = working electrode
• Rough surface:
- hydrodynamic drag effect - overestimated mass sensitivity:
9CTAB adsorption: ∆σ ∼ surface coverage
• Small molecule (Mw= 365 g/mol)
• Monolayer formation:∆σ= 137 ng/cm2
• Attenuation < noise level (0.05 dB)
• Surface coverage ~ solution concentration
•Surface density or viscous thin layer ?Î - similar phase and attenuation shifts - distinction density/viscosity impossible
kg m 74 . 1 43 .
27
2,σ =− ±
Sφ
)
, (h Sφρη
0 dσ=
0 d ρη=
)
, (h Sαρη
~
N.A.
•∆h= 50 nm Auetching (linear part)
•ρ= 19300 kg/m3Æ ∆σ = 96500 ng/cm2
• Parasitic interferences enhanced by Au
• 200 nm gold etchingÏin KI/I2
• Mixed contribution of density and viscosity interactions for biomolecules • Modeling is needed to extract both contributions to the acoustic signal
