TOPICS IN (NANO) BIOTECHNOLOGY

Self-assembly

19th January, 2007

Self-AssemblySelf-Assembly• Carries out many of the difficult steps in nanofabrication - atomic-level modification of structure, using highly developed techniques of synthetic chemistry

• Inspiration from a wealth of examples in biology - Proteins, DNA, cell-membrane etc.

•Target structure is thermodynamically stable - structures are relatively defect-free and self-healing

• Understanding is still at a very elementary level - ”molecular shape” - Enthalpy vs. Entropy - nature of non-covalent forces

Self-AssemblySelf-Assembly• the classic ’bottom-up’ approaches

• idea could be to throw everything together and wait for the structures to self assemble

• still very much a research topic and true application is a long way off

• self assembled monolayers on gold and silicon, nanoparticle self assembly, supported lipid bilayers, nanoparticle films, ligand directed assembly etc.

Self-AssemblySelf-Assembly

Self-Assembled MonolayersSelf-Assembled Monolayers

Langmuir Blodgett Films of Lipids

Amphiphiles on WaterAmphiphiles on Water

Micelles, liposomes and other self-assembled structures

WATER

Hydrophobic tailHydrophilic head

S

S

O

O

O

S

S

O

O

O

S

S

O

O

O

S

S

O

O

O

n

Water

Air

O

O

O

O

O

O

Hydrophobic groups

Conjugated -electron system

Hydrophilic groups

-stacking of adjacent polymers

Air

WaterWater

Air

Space filling model

A.

B. C.

J. Am. Chem.Soc. 120, P. 7643,(1998)

Langmuir-BlodgettLangmuir-Blodgett

0 1000 2000

0

10

20

30

40

50

(m

N/m

)

Area (Å2/cluster)

Compression isotherm

1. Spreading

3. Transfer

2. Compression

Langmuir-BlodgettLangmuir-Blodgett

Langmuir-BlodgettLangmuir-Blodgett

Langmuir-BlodgettLangmuir-Blodgett

Self-assembled monolayers on gold

Gold Self-Assembled Monolayers (SAMs)Gold Self-Assembled Monolayers (SAMs)

Gold Self-Assembled Monolayers (SAMs)Gold Self-Assembled Monolayers (SAMs)

Gold Self-Assembled Monolayers (SAMs)Gold Self-Assembled Monolayers (SAMs)

Self-assembled monolayers on silicon

Si Self-Assembled Monolayers (SAMs)Si Self-Assembled Monolayers (SAMs)

Si Self-Assembled Monolayers (SAMs)Si Self-Assembled Monolayers (SAMs)

Thermal Stability of SAMsThermal Stability of SAMs

Self-Assembled Monolayers (SAMs)Self-Assembled Monolayers (SAMs)

Polycation/polyanion self assembly

Electrostatic self assemblyElectrostatic self assembly

Electrostatic self assemblyElectrostatic self assembly

Electrostatic self assemblyElectrostatic self assembly

Electrostatic self assemblyElectrostatic self assembly

Electrostatic self assembly – protein multilayersElectrostatic self assembly – protein multilayers

Electrostatic self assembly – protein multilayersElectrostatic self assembly – protein multilayers

Electrostatic self assembly – protein multilayersElectrostatic self assembly – protein multilayers

Electrostatic self assembly – nanoparticlesElectrostatic self assembly – nanoparticles

Electrostatic self assembly – nanoparticlesElectrostatic self assembly – nanoparticles

Nanoparticle self assembly

3-7 nm

SS

AuS

S

SSS

S

SS

S = CnH2n+1S

x

x X = OH, DNA, OPV etc.

Ligand Stabilized Gold NanoparticlesLigand Stabilized Gold Nanoparticles

Nanoparticle Films

Ligand Directed Assembly

Bifunctional ligand

nanoparticle

substrate +

+

Natan, M. J.; et. al. Chem. Mater. 2000, 12, 2869-2881

Tapping mode AFM (1mm x 1mm) of HSCH2CH2OH linked Au colloid multilayers: (A) monolayer; (B) 3 Au treatments; (C) 5 Au treatments; (D) 7 Au treatments; (E) 11 Au treatments.

• Monolayer formed by adsorption of Au particles on 3-mercaptopropyltrimethoxysilane derivatized SiO2 surface

• Multilayers constructed by immersion in a 5mM solution of 2-mercaptoethanol for 10 min. followed by immersion in Au particle solution for 40 – 60 min.

Ligand Directed Assembly

Electrostatic Assembly

• Polycationic polymer

• Very stable in most solvents

• Control inter-layer spacing

• Conductive, semiconductive, or insulating

- --- ---- --

+ ++

+- --- --

Shipway, A.N.; Katz, E.; Willner, I. CHEMPHYSCHM. 2000, 1, 18-52.

Convective Self Assembly

• Definition: Particles are allowed to freely diffuse. As the solvent evaporates, particles crystallize in a hexagonally close-packed array.

• Optimize: Particle concentration Particle/Substrate charge Evaporation

Top View

Colvin, V.L.; et. al. J. Am. Chem. Soc. 1999, 121, 11630-11637.

Photolithography Patterning• Typically pattern the capture monolayer followed by

particle adsorption• Few examples of patterning after nanoparticle

deposition

SEM images showing lithographically defined patterned nanoparticle films with combination of spin-coating driven self-assembly of nanoparticles, interferometric lithography (IL) and reactive ion etching (RIE):

(a) photoresist pattern above blanket nanoparticle layer;

(b) nanoparticle pattern after etching and photoresist removal;

(c) photoresist pattern; (d) nanoparticle pattern after etching and

photoresist removal; (e)-(f) 2D isolated discs.

Photolithography Patterned Nanoparticles

SEM image of Au nanoparticles adsorbed onto a patterned (3-mercaptopropyl)-trimethoxysilane monolayer on SiO2 coated Silicon wafer.

AFM image (80 mm x 80 mm) of a three-layer coating of nanoparticles followed by photopatterning.

Electron Beam Lithography

• Typically: – coat substrate with polymer film – write pattern with e- beam– dissolve exposed polymer– evaporate metal into “holes”

Somorjai, G. A.; et. al. J. Chem. Phys. 2000, 113(13), 5432-5438.

Images of Nanoparticle Arrays formed by Electron Beam Lithography

AFM and SEM of Pt nanoparticle array. Particles are 40nm in diameter and spaced 150nm apart.

Spin-coat PMMA on Si(100) wafer with 5nm thick SiO2 on surface.

Beam current: 600pA

Accelerating Voltage: 100dV

Beam diameter: 8nm

Exposure time: 0.6s at each site

Pt deposition: 15 nm by e- beam evaporation

Nanosphere Lithography

Hulteen, J.C.; Van Duyne, R.P. J. Vac. Sci. Technol. A 1995, 13(3), 1553-1558.

(A) Representation of a single-layer nanopshere mask formed by convective self assembly.

(B) Illustration of the exposed sites on the substrate with single-layer mask

(C) AFM image (1.7mm x 1.7mm) of Ag deposited on mica with a mask of 264nm diameter nanoparticles.

Mask preparation: Spin coat 267 nm polystyrene nanoparticles at 3600 rpm.

Deposition: Ag vapor deposition

Mask removal: sonicate 1-4 min. in CH2Cl2

Microcontact Printing• PDMS stamp to “ink” a capture monolayer on a

substrate followed by nanoparticle adsorption• PDMS stamp to “ink” the nanoparticles directly

onto the substrate

Shipway, A.N.; Katz, E.; Willner, I. CHEMPHYSCHM. 2000, 1, 18-52.

Side View

Top View

AFM of Microcontact Patterned Nanoparticle Array

Natan, M. J.; et. al. Chem. Mater. 2000, 12, 2869-2881

AFM scan (10m x 10m) of microcontact printed Au surfaces. HOOC(CH2)15SH is initially stamped on substrate. The surface is then exposed to 1.0 mM 2-mercaptoethylamie followed by exposure to a 17nM solution of 12nm Au nanoparticles.

SuperstructuresSuperstructures

Collective properties Site energies, interparticle coupling strength, lattice dimensionsControl of superstructure, 2D nanoarrays(Nanoalloys)