Phase Transfer Properties of Nanoparticles - why and how? By Michiel Dokter Undergraduate Research Occidental College, summer 2005 Professor E. M. Spain

Composition Nanoparticles Dodecanethiol (DDT) Tetra-n-octyl-ammonium bromide (TAB) Silver Ag

Composition Nanoparticles

Stabilization by TAB

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 nm –A living cell is about nm –Proteins are about 5 nm –Our nanoparticles are about 5 nm –Atomic Radius of Silver is nm

Electronegativity So: O, Cl > C, H

Polarity ++ -- ++ C, H same electronegativity, not polar O more electronegative than H  water is a polar solvent ++ -- -- -- 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 (CHCl 3 ) over polar solvent (H 2 O/EtOH), because of the apolar tails. Result:

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. Result: - 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). MUA

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 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. excess dodecanethiol reflux

MUA is rejected MUA 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: 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: > > > > > > 438 reflux doesn’t seem to affect wavelength significantly: > > > 448 (toluene)438 --> > > 432 After reflux always big peak around nm (thiol) thiol in dry CHCl 3 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 CH 2 stretch: 2856 cm -1 antisymmetric CH 2 stretch: 2928 cm -1 crystalline alkane chains: symmetric CH 2 stretch: 2850 cm -1 antisymmetric CH 2 stretch: 2920 cm -1 So: less freedom of movement  higher wavenumber

Gauche Effect 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 Refluxed nanoparticle: - Rigid alkane chains? Non-refluxed nanoparticle: - Gauche defects

IR spectroscopy (II) Dodecanethiol: symmetric CH 2 stretch: cm -1 antisymmetric CH 2 stretch: cm -1 Thiol Chains in nanoparticles: symmetric CH 2 stretch: cm -1 antisymmetric CH 2 stretch: cm -1 Thiol Chains in refluxed nanoparticles: symmetric CH 2 stretch: cm -1 antisymmetric CH 2 stretch: 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 Dokter Under Graduate Research Occidental College, summer 2005 Professor E. M. Spain Acknowledgements: Professor E. M. Spain John Vigorita, Don Johnson, William Sohn Dr. C. M. Garland, Caltech URC