Thickness and viscosity of organic thin ﬁlms probed by combined surface acoustic Love wave and surface plasmon resonance

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organic thin films probed by combined surface

acoustic Love wave and surface

plasmon resonance J.-M Friedt& al

Introduction Experimental setup Experimental results Data modelling Conclusion

Thickness and viscosity of organic thin films probed by combined surface acoustic Love wave

and surface plasmon resonance

J.-M Friedt¹, L.A. Francis², S. Ballandras¹

1FEMTO-ST/LPMO (Besan¸con, France),

2IMEC MCP/TOP (Leuven, Belgium) slides available athttp://jmfriedt.free.fr

16 septembre 2005

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Direct detection biosensor

Aim : detect DNA hybridation, antibody/antigen interaction without labeling (fluorescence, radiolabeling ...)

• in-situmeasurement (not in dry state) : microfluidics ...

• time resolution'1 s for kinetics measurements

⇒detect mass bound to the surface : acoustic method (QCM, SAW)

⇒detect optical index change (ellipsometry, SPR)

⇒detect surface topography change due to adsorption (SPM)

• Acousticmethods beyond Sauerbrey : density, thickness, viscosity

• Opticalmethods : thickness, optical index

• Scanning probe microscopies : molecule conformation, density of molecules, thickness

Modelling is required for extracting quantitative information on the physical properties of the adsorbed layer

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Various solvent concentrations

From QCM-D measurement we know that collagen and fibrinogen provide challenging properties :

• S-layer, IgG : behaves as a rigidly bound mass (Sauerbrey)

• collagen : displays a behaviour consistent with a predominantly viscous interaction

• fibrinogen : intermediate situation ...

Analyte (bulk ∆fn/√

n ∆fn/n ∆D(×10⁻⁶)

concentration,µg/ml) (Hz) QCM (Hz) QCM QCM

S-layer NO 45=900 3-5

CTAB NO 8=160 0.2-0.5

collagen (30µg/ml) 1000 NO 100

collagen (300µg/ml) 1200 NO >120

fibrinogen (46µg/ml) 110±5 55±5'1110 4-10 fibrinogen (460µg/ml) NO 100=1700 8-10 J.-M. Friedtet al., J. Appl. Phys.95(4) 1677-1680 (2004)

C. Zhouet al., Langmuir20(14) 5870-5878 (2004)

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SAW/SPR combination

• Objective : identify physical properties of protein films.

• Acoustic methods are sensitive to layer thickness, density and viscosity, but provide only insertion loss and phase variations.

• Combine with optical method : optical index and thickness.

• Assume that density and optical index vary linearly with solvent content⇒3 unknowns and 3 measurements.

Cu RE Pt CE

PDMS or

ST quartz substrate SU8+glass

(670 nm) incoming laser

laser reflected glass prism Al IDT

Au WE

CCD array linear + temperature

sensor rough optical

allignement screw RF cables to network analyzer 670 nm

laser diode SAW

rotating mirror: +/−2.5^o

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Introduction Experimental setup Experimental results

Data modelling Conclusion

Data collected with combined SAW/SPR setup

• Globular protein (IgG, BSA, S-layer), fibrilar protein (collagen, fibrinogen) and polymers (pNIPAM) adsorption have been investigated.

• Globular proteins display a mostly rigid behaviour

• pNIPAM displays a behaviour dependent on temperature

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Data modelling approaches (acoustics)

Two complementary approches for modelling the acoustic data :

• transmission line model

dr G C

L/2 v L/2

T

• harmonic admittance computation based on the Bl¨otekja¨er-model extended to include the viscous contribution of a newtonian fluid (linear response)

T_ij =−

P+jω

2

3η−ζ

S_kk

δ_ij+ 2ωηS_ij

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Data modelling predictions (acoustics : Bl¨ otekja¨ er)

Parameters are : the layer thickness, its viscosity and density

0 5 10 15 20 25 30 35 40 45 50 55

−3

−2.5

−2

−1.5

−1

−0.5

0x 10⁵ ρ=1.40

viscosity (cP)

∆f (Hz)

0 5 10 15 20 25 30 35 40 45 50 55

0 0.2 0.4 0.6 0.8 1 1.2

viscosity (cP)

∆IL (dB)

0 50 100 150 200 250 300 350

−0.14

−0.12

−0.1

−0.08

−0.06

−0.04

−0.02 0

∆f0 (MHz)

layer thickness (nm)

0 50 100 150 200 250 300 350

0 0.5 1 1.5 2

∆IL (dB)

η=1.00 η=1.60 η=2.30 η=3.00

Varying the viscosity Varying the thickness An insertion loss drop of-6 dB cannot be modelledby a newtonian fluid+rigid mass interactions.

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Data modelling predictions (acoustics : TLM)

0 20 40 60 80 100 120 140 160 180

−30

−20

−10 0

layer thicknesss (nm)

∆φ (degrees)

0 20 40 60 80 100 120 140 160 180

0 1 2 3 4

layer thicknesss (nm)

∆ IL (dB)

0 20 40 60 80 100 120 140 160 180

0 0.5 1 1.5 2

∆IL (dB)

0 20 40 60 80 100 120 140 160 180

−0.14

−0.12

−0.1

−0.08

−0.06

−0.04

−0.02 0

∆f0 (MHz)

δ

dotted: ρ=1.40 solid: ρ=1.10

η=1.0 η=1.7 η=2.4 η=3.1 η=3.1 η=2.4 η=1.7 η=1.0

experimental value for 460 µg/ml fibrinogen adsorption

Transmission line model (η=[1, 1.5, 2, 3]) Bl¨otekja¨er

(harmonic admittance computation)

Here (left) we find a possible set of parameters for the fibrinogen data : a viscosity around 2.6 cP and a thickness around 22.4 nmassuming a density of 1.4.

Additional work requires sweeping the density parameter to extract water content.

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Data modelling results (SPR)

2D model of reflected intensity of a laser by a stack of planar interfaces : requires optical index of all layers at a given wavelength+incident angle

0 5 10 15 20 25

0 100 200 300 400 500 600 700 800 900 1000

layer thickness (nm) SPR ∆θ (mo)

<θ>=70.2297^o

x=1 x=0.7 x=0.5 x=0.4 x=0.3

x=0.2

x=0.1

x=0.0 x=0.05

0 50 100 150 200 250 300 350

0 100 200 300 400 500 600 700 800 900 1000

layer thickness (nm) SPR ∆θ (mo)

x=0.1 x=0.08 x=0.07

x=0.06

x=0.05

x=0.04

x=0.03

x=0.02

x=0.01

x=0.00

<θ>=70.2297^o

Acoustic frequency shift+insertion loss and SPR angle shift

→densityρ, optical indexn, viscosity and water content x assuming ρ=x×ρprotein+ (1−x)×ρwater & n=x×nprotein+ (1−x)×nwater

→with a thickness of 22.4 nm, SPR (755 m^o) tells us x'0.25i.e.

ρ'1.10and iterate ...

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Conclusion and perspectives

• Experimental measurement of thin film adsorption by acoustic and optical means

• Simultaneously monitor using both techniques thesamearea in liquid with time resolution

• Implementation of models for the predictions of SPR angle shift and acoustic frequency shift and insertion loss as a function of adsorbed layer properties

• Models are incomplete : cannot justify large insertion loss decrease upon collagen adsorption

• Possible extension : add Maxwellian liquid behaviour (complex : non-linear, delayed effects, incompatible with transmission line model)

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