Phase Transfer Properties of

Nanoparticles - why and how?

By Michiel DokterUndergraduate Research

Occidental College, summer 2005

Professor E. M. Spain

Ag

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Ag

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Composition Nanoparticles

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Dodecanethiol (DDT)

Tetra-n-octyl-ammonium bromide (TAB) Silver

Ag

N+

Br-

Composition Nanoparticles

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N+

Br-

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Br-

Stabilization by TAB

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Ag

N+

Br-

N+

Br-

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Ag

N+

Br-

Silver Nanoparticles

Nanoparticle: 3.9 nm ± 0.36

5 nm… I guess that’s…. small?

But let’s compare to get a general idea:– I am 1.820.000.000 nm– A living cell is about 10.000 nm– Proteins are about 5 nm– Our nanoparticles are about 5 nm– Atomic Radius of Silver is 0.144 nm

Electronegativity

So: O, Cl > C, H

Polarity

O

H H+

-

+

O

H H

O

H H

C, H same electronegativity, not polar

O more electronegative than H water is a polar solvent

C ClCl

Cl

H

C ClCl

Cl

H

C ClCl

Cl

H

+- -

-

Cl more electronegative than C chloroform is a polar solvent

Phase Transition: Experimental

water / ethanol (50/50 v/v)

Nanoparticles in chloroform solution

Phase Transition: polar / apolar

• Nanoparticles prefer apolar solvent (CHCl3) over polar solvent (H2O/EtOH), because of the apolar tails.

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Result:

H

O

OHS

Phase Transition: Experimental

water / ethanol (50/50 v/v)

MUA in ethanol solution

Nanoparticles in chloroform solution

Phase Transition: polar / apolar

• MUA is used to form nanofilms.• Exchange of MUA with dodecanethiol makes the

nanoparticles more polar.

H

O

OHS

Result:

Ag

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S SHH

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O

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SS H

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S H

S H

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SHSH

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OO

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- I

Influence TAB

• Nanoparticles with TAB are stable.• Nanoparticles with TAB can form nanofilms• TAB makes the nanoparticles stay in chloroform

layer, even when the particles are made polar by addition of 11-mercaptoundecanoic acid (MUA).

H

O

OHSMUA

Phase Transition: influence TAB

• TAB is a phase-stabilizing molecule.

• Get rid of TAB and extraction by MUA should be possible.

It worked!!

Phase Transition: reversed

• Adding HCl should make particles apolar again and the nanoparticles should sink back to the chloroform layer.

Phase Transition: reversed

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OHO

OHO

OHO

OH

OHO

OHO

OHO

OHO

OHO O

OO

O O

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O

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OHO

OH

O

OHO

OH

O

OHO

OHO

OHO

OH

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HO

O

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OHO

OHO

OHO

OHOO

HOO

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HCl

Phase Transition: reversed

HCl+ H

+

Phase Transition: reversed

• Adding HCl should make particles apolar again and the nanoparticles should sink back to the chloroform layer.

After two hours:

Phase Transition: Reflux

• After washing TAB out, reflux nanoparticles with excess dodecanethiol

Hypothesis:

• refluxed particles will be monodisperse and smaller

(S. Stoeva and K. J. Klabunde)

• The nanoparticles will be covered with thiols, leaving no room for MUA to bind.

Refluxed Nanoparticles

Refluxed Nanoparticle: 3.9 nm ± 0.41

Phase Transition with refluxed particles

• No space for MUA to bind, means no polar particles.

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SHAg

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excess dodecanethiol

reflux

MUA is rejectedMUA and regular nanoparticle:

MUA and refluxed particle:

Phase Transition with refluxed particles

• No space for MUA to bind, means no polar particles.

• Particles should not be extracted by MUA to ethanol/water layer:

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SHAg

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excess dodecanethiol

reflux

Phase Transition: Experimental

water / ethanol (50/50 v/v)

MUA in ethanol solution

Refluxed Nanoparticles in chloroform solution

Characterization Nanoparticles

• Transmission Electron Microscopy (TEM)

• UV VIS Spectroscopy

• Infrared Spectroscopy

UV: washing and reflux effects

• washing doesn’t seem to affect wavelength:424 --> 428 438 --> 444 --> 440440 --> 436 --> 440 438 --> 438

• reflux doesn’t seem to affect wavelength significantly:434 --> 438 434 --> 436440 --> 448 (toluene) 438 --> 440438 --> 438 432 --> 432

• After reflux always big peak around 244-248 nm (thiol)• thiol in dry CHCl3 absorbs at 256 nm, TAB at 242 nm

Given are the wavelength of the maximum intensity in the UV spectra, corresponding to the nanoparticles.

IR spectroscopy by Hostetler (I)

• Dodecanethiol:

symmetric CH2 stretch: 2856 cm-1

antisymmetric CH2 stretch: 2928 cm-1

• crystalline alkane chains:

symmetric CH2 stretch: 2850 cm-1

antisymmetric CH2 stretch: 2920 cm-1

So: less freedom of movement higher wavenumber

Gauche Effect

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Blue shift can be ascribed to the freedom of movement the alkane chains have in solution. Rigid alkane chains won’t have gauche defects.

Nanoparticle in solution

Reflux and Gauche defects

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Refluxed nanoparticle:

- Rigid alkane chains?

Non-refluxed nanoparticle:

- Gauche defects

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IR spectroscopy (II)• Dodecanethiol:

symmetric CH2 stretch: 2853.0 cm-1

antisymmetric CH2 stretch: 2923.7 cm-

1

• Thiol Chains in nanoparticles:

symmetric CH2 stretch: 2850.3 cm-1

antisymmetric CH2 stretch: 2919.5 cm-

1

• Thiol Chains in refluxed nanoparticles:

symmetric CH2 stretch: 2848.8 cm-1

antisymmetric CH2 stretch: 2918.2 cm-

1

But…no extraction when:

water / ethanol (50/50 v/v)

MUA in ethanol solution

Non-refluxed nanoparticles in chloroform solution

dodecanethiol

Phase Transfer Properties of

Nanoparticles - why and how?

By Michiel DokterUnder Graduate Research

Occidental College, summer 2005

Professor E. M. SpainAcknowledgements:

Professor E. M. Spain

John Vigorita, Don Johnson, William Sohn

Dr. C. M. Garland, Caltech

URC