thiol monolayers by means of high-rate dynamic force spectroscopy

Transcription

1 1) Max Planck Institute for Polymer Research 2) Poznan University of Technology Adhesion on self-assembled thiol monolayers by means of high-rate dynamic force spectroscopy Hubert Gojżewski 1,2, Arkadiusz Ptak 2, Michael Kappl 1 July 17,, 2008, MST Conference 1

2 HAMBURG BERLI COLOG E MAI Z FRA KFURT MU ICH Max Planck Institute for Polymer Research Mainz mainz.mpg.de More than 500 international employees ca publications / day 2

3 Johannes Gutenberg ( ) 1468) Gutenberg Bible from 1455 Martin s s cathedral 3

4 Rosenmontag in Mainz 4

5 Outline Motivation Dynamic force spectroscopy Bell-Evans single bond model Modified AFM setup Samples preparation and examination Results Conclusions & Outlook 5

6 Motivation Cells in culture Tree frog Tokay gecko K. Autumn, American Sci. 94 (2006) 124 6

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10 Force vs tip-sample distance Force Spectroscopy (FS) can provide: - unbinding force - elasticity of samples Interaction potential But, does not provide: - information about whole interaction potential - bond length - bond lifetime DS 10

11 Dynamic force spectroscopy (DFS) F Force-Extension Curve F Force vs. Time 0 F ad 0 Adhesion Z 0 F ad Loading Rate t Adhesion 11

12 Dynamic force spectroscopy (DFS) Time series of adhesion forces Histogram of adhesion forces RATE DEPENDENT ADHESION Adhesion force Loading rate r f = df dt Can provide information of: adhesion strength molecular scale energy landscape 12

13 Bell-Evans single bond model 0 E(x) Energy E(x) - Fx Fx Applied force F x 0 x β x 13

14 Bell-Evans single bond model Rupture force depends on the loading rate F ad adhesion force F β thermal force r f - loading rate 0ff - kinetic off-rate coeff. x β activation barrier k B Boltzmann constant T - temperature k 0 0ff Adhesion F F ad β = = F β k BT x r f ln( 0 F k β β off ) r 0 f = k 0 off F β Loading Rate E. Evans,, K. Ritchie, Biophys.. J. 72 (1997) 1541 G.I. Bell, Science 200 (1978) 618 E. Evans, Annu.. Rev. Biophys. Biomol. Struct. 30 (2001)

15 Realistic Energy Landscape 0Energy 0 X β (2) X β (1) x F ad E 2 0 E 1 ln (r f ) Bond breaking of a complex systems may involve overcoming multiple energy barriers 15

16 Modified AFM setup A. Ptak, M. Kappl, H.-J. Butt, Appl. Phys. Lett. 88 (2006) Standard AFMs: Tip-sample separation rates < 50 µm/s 16

17 Modified AFM setup Standard AFMs: Tip-sample separation rates < 50 µm/s 17

18 Self-assembled monolayer (SAM) Advantages: easy preparation 2D crystallization variable properties stability F Au-S = 1.4 n > F Au-Au 18

19 Self-assembled thiol monolayers -CH 3 hydrophobic tail group hydrophilic -OH spacer S S S S S head group S S S S S HS(CH 2 ) x CH 3 HS(CH 2 ) x OH immersion time: 16-24h solution concentration: 1mM 19

20 SAM of thiols / structure 3 x 3 1-dodecanthiol HS(CH 2 ) 11 CH Å HS(CH 2 ) x CH Å 2 / molecule STM 20

21 Contact angle SAMs of thiols SAM Contact angle [ 0 ] CH 3 (CH 2 ) 15 SH 1-butanethiol CH 3 (CH 2 ) 3 SH 94.9 ± heptanethiol CH 3 (CH 2 ) 6 SH 95.3 ± nonanethiol CH 3 (CH 2 ) 8 SH 97.6 ± HS(CH 2 ) 4 OH 1-dodecanethiol CH 3 (CH 2 ) 11 SH 1-hexadecanethiol CH 3 (CH 2 ) 15 SH 2-methyl-2-propanthiol (CH 3 ) 3 CSH 2-mercapto-1-ethanolethanol HS(CH 2 ) 2 OH 4-mercapto-1-butanol HS(CH 2 ) 4 OH 6-mercapto-1-hexanol HS(CH 2 ) 6 OH 99.7 ± ± ± ± ± ±

22 Dynamic force spectroscopy Results for bare Au(111) surface 22

23 Au(111) Au(111) Loading Piezo rate velocity 10 0 nn/s 1 nm/s 10 7 nn/s 1 cm/s weak dependence for Au(111) substrate F ad ( r F ) const The rupture of the adhesive contact is not a single bond event but b is expected to involve many tens of bonds simultaneously ONE EFFECTIVE BOND 23

24 Metals, semiconductors, isolators UHV~ Tr 1.33x10-8 Pa 10 Tr Air Air UHV~ Tr 1.33x10-8 Pa 10 Tr 24

25 Dynamic force spectroscopy Results for SAM of thiols coated gold 25

26 1-hexadecanthiol RH 25% a) b) a) t 0 off F ad ( r ) = aln( r ) F a= (0.9± 0.4) n b= (60± 2) n b) F ( r ) = aln( r ) F F ad b= ( 14± 12) n F a= (10.3± 1.2) n + + b off = years x β = Å b t 0 off = 2.77 s x β = Å CH 3 (CH 2 ) 15 SH The Universe is ONLY ~ years old! 26

27 2-methyl-2-propanthiol (CH 3 ) 3 CSH RH 25% a) b) a) t 0 off F ad ( r ) = aln( r ) F F a= (0.2± 0.1) n b= (17.4± 0.6) n b) F ( r ) = aln( r ) F F ad + + a= (3.4± 0.2) n b= ( 10.9± 2.3) n b off = years x β = Å b t 0 off = 0.13 s x β = Å 27

28 Dynamic force spectroscopy Results for SAM of thiols coated gold Influence of Humidity on Adhesion 28

29 1-heptanthiol CH 3 (CH 2 ) 6 SH RH: 29

30 1-dodecanthiol CH 3 (CH 2 ) 11 SH RH: 30

31 6-mercapto-1-hexanol HS(CH 2 ) 6 OH RH: t c/min /MIN~500 ns 31

32 11-mercapto mercapto-1-undecanol HS(CH 2 ) 11 OH RH: 32

33 CONCLUSIONS We modified a standard AFM to extend the range of accessible loading rates by up to four orders of magnitude. We applied Bell-Evans model (for single bond) for nanoadhesion and extracted information about the effective bond (strength and bond lifetime). Only weak dependence between adhesion and unloading rate was found for metals, semiconductors, isolators anda a Si 3 N 4 tip. 33

34 CONCLUSIONS There are at least two activation barriers in energy landscapes of the adhesion interactions between single or multi CH 3 terminated or OH terminated thiols and a Si 3 N 4 tip. For CH 3 terminated thiols a little dependence of the adhesion vs loading rate curves on humidity is seen (no significant meniscus formation). For OH terminated thiols, humidity has a clear influence on the dependence of adhesion vs loading rate. This can give information about the contribution of the meniscus force. 34

35 Outlook We plan to investigate other SAMs of thiols, especially with hydrophilic end groups e.g. NH 2 -terminated We plan to vary contact times down to the milli- and microsecond range what will allow us to get information about the kinetics of meniscus formation responsible for capillary force 5 Approaching the tip Retracting the tip A Voltage [V] α B 20 Deflection [V] 0 Load -1 Adhesion Contact time Time [ms] Force [nn] 35

36 Thank you for your attention 36