The
Preparation and Characterisation of Anisotropic
Particles and Microstructures
Olivier Cayre (PhD Student.) and Vesselin
N. Paunov
Sponsors: University of Hull
Duration: Oct 2001 to Oct
2004
Colloid particles of
asymmetric or dipolar charge distribution would have special properties based
on additional orientational interactions in aqueous suspensions and may find a
number of applications, including preparation of photonic crystals with novel
symmetries, colloidal liquid crystals and water-based electrorheological
fluids.
In this project we develop a
novel method for preparation of dipolar colloid particles which is based on
microcontact printing of water-insoluble ionic surfactants onto colloid
monolayers of opposite surface charge. The idea is presented schematically in
Fig.1. Our method involves the following three steps:
(i)
A monolayer of monodisperse polystyrene (PS) latex microparticles is prepared
by evaporating latex particle suspension on a glass substrate.
(ii) A film of water-insoluble
surfactant of opposite charge to that of the particles is deposited on the
surface of a flat sheet of elastomer (PDMS).
(iii) The surfactant film is printed onto the
colloid monolayer and then the colloid particles are redispersed in an aqueous
solution.
We use the natural flexibility
of the PDMS to deposit the surfactant film over the exposed surface of the
microparticles in a supported monolayer. The PDMS deforms locally when pressed
against the particles in the monolayer and the surfactant film is transferred
by direct microcontact due to adhesion to the exposed hydrophobic surface of
the PS particle. Different fractions of the particle surface can be exposed to
the surfactant film by partial embedding of the particle monolayer into a
protective film of glucose on a solid substrate. Using of water-insoluble surfactants
guarantees that the produced surfactant deposit on the particle surface would
not desorb and dissolve upon redispersion in water
Fig. 1
Scheme
of the preparation of dipolar colloid particles by microcontact printing of
water-insoluble cationic surfactant (ODTAB) on a monolayer of sulphate latex particles.
We use different
types of fluorescent water-insoluble cationic and anionic dyes, which allows
direct observation of the outcome of the microcontact printing on the surface
of the microspheres.
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Fig. 2 (A) A fluorescent microscopy image of 9.6 mm sulphate PS latex particles stamped with a film of lissamine-rhodamine (fluorescently-tagged lipid, negatively
charged at high pH) deposited on an elastomer stamp, and redispersed in milliQ water. (B) An individual particle partially coated
with lissamine-rhodamine at high magnification. (C)
and (D) High magnification fluorescent microscopy images of 9.6 mm sulphate PS latex particles stamped with a film of Neuro Dio (hydrophobic
fluorescent cationic dye) deposited on an elastomer stamp, and redispersed in milliQ water. The anionic and cationic amphiphilic
molecules stick equally well on the PS latex particles predominantly due to
hydrophobic interaction with the PS surface
In addition, we have extended the method to print with
one colloid monolayer onto another colloid monolayer. This allowed us to
produce a number of anisotropic colloidal structures as a result of directed
assembly aided by microcontact printing.
Fig.3. Asymmetrically coated colloid particles have been produced by
microcontact printing of colloid monolayers of 1.5 mm
sulfate latex particles onto a monolayer of 9.8 mm
amino-latex particles.
More details of this project can be found in the
recently published papers:
1. Cayre, O., Paunov, V.N., Velev, O.D., “Fabrication of Dipolar Colloid Particles
by Microcontact Printing“, Chem. Comm., 18 (2003) 2296 -
2297.
2. Cayre, O., Paunov, V.N., Velev, O.D., “Fabrication of Asymmetrically Coated
Colloid Particles by Microcontact Printing Techniques“, J. Materials
Chemistry, 10 (2003)
2445-2450.
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