Fabrication of Janus Particles

Fabrication
of Janus Particles

Baseemah
Rucker,^1,2 Zohreh Jalilvanh,¹ and Ilona Kretzschmar¹

¹
Dept. of Chemical Engineering, City College of New York

²
Dept. of Chemical Engineering, Hampton University

Abstract

Janus
particles are spherical particles that have two faces with equal surface areas
and different properties.  Janus
particles were fabricated by semi coating monolayers of silica particles with
gold.  Janus particles can be coated with
a variety of different materials.  When a
Janus particle coated with platinum is submerged in a hydrogen peroxide
solution one face of the Janus particle reacts with the hydrogen peroxide
solution, which propels the particle forward. The side of the particle that is
coated with platinum reacts with the hydrogen peroxide and give off water and
oxygen which acts as the fuel for the propulsions. The purpose of this research
is to use different laboratory techniques to fabricate 4 micron semi coated Janus
Particles.

Introduction

Janus particles are spherical
particles with asymmetrical surface properties due to differences in the
chemical compositions of their surface.¹ Janus particles are named
after the ancient Roman god Janus, who had two faces with one looking in the
past and one looking in the future. Recently, Janus particles have gained a lot
of attentions for their potential industrial applications such as use in
display technology, magnetic storage, and drug delivery. Janus Particles are
also used in academic research settings as molecular surfactants, building
blocks of complex superstructures, chemical and biological sensors and
self-motile colloidal particles.³ Recently, Janus particles have
been considered as potential drug carriers because of their unique propulsion
behavior.

  
In some solutions, Janus particles have been observed to
show self-propulsion behavior owing to a region of their surface giving off a
fuel resulting in a forward propulsion of the particle. This property could be
very useful in many different settings, if the direction of the particle motion
could be controlled. However, Brownian rotation, i.e., the tendency of a
microscopic particle to rotate randomly, causes Janus particles to be propelled
in random directions hampering their applicability. However, recent work has
shown that interactions with walls and cracks have a guiding effect on the
particles’ motion because of hydrodynamic interactions.

   The purpose of this research project is to use
the convective assembly technique to Fabricate Janus Particles of 4 micron
silica semi coated in gold.

Results and
Discussion

Close-packed
monolayers of silica particles have been made using the monolayer technique
describe above. To make the solution of silica particles, dry silica powder was
measured out on a scale and water was added resulting in a weight fraction of
50%. The 50 wt% solution was sonicated for 30 minutes and centrifuged at 4.4
rpm for 8 minutes. Afterwards, the water above of the particles was removed and
3 drops of deionized water were added to the solution. The solution was added
into the wedge and the infusion rate of the syringe pump was set to 10 mL/min.
At this rate, multilayers formed. Reducing the infusion rate to 6 mL/min
resulted in a mix of monolayer and multilayers. Next, the infusion rate was
kept the same, but two more drops of water were added to the solution reducing
the weight fraction to ~30%. At 30wt%, the submonolayers were formed with
particles broadly disperse on the substrate. At this point, the infusion rate
was increased to 7.5 mL/min and monolayers were successfully produced (see Figure 1).

Figure 1. Close-Packed Monolayer
of SiO₂ particles.

Some areas of the particle layers
showed non-close packing and were wiped of the silicon wafer with a cotton
swab. Once cleaned, the wafers with close-packed regions were transferred to
the PVD system (Figure 2).

Figure
2. Close-packed Monolayer of SiO₂
in PVD machine before coating

A 5 nm layer of titanium was first
evaporated followed by the evaporation of 15 nm of gold yielding particles with
a 50% gold coating. Both layers are rather thin and thus the gold layer appears
transparent to the naked eye (see Figure
3).   However, inspection of the
monolayer sample under the optical microscope reveals the gold layer (see Figure 4).

Methods and Materials

Janus Particle Fabrication

  
Janus particle fabrication occurs in two steps; (i) monolayer formation
and (ii) physical vapor deposition to create the cap. Closed packed monolayers
are made using a syringe pump. Two glass slides are placed at an angle and one
glass slide is moved using the syringe pump mechanism while the other is helps
steady. A solution of particles with a specific weight percentage of particles
in water is then added in the wedge between the two glass slides and the top
glass slide is moved across the bottom glass slide spreading the particle
solution across the silicon oxide wafer. A microscope is used to identify the
regions of the silicon wafer that contain close-packed monolayers and the
regions with multilayers and dispersed particles are wiped off. Close-packed
monolayers are needed to prepare patchy particles, i.e., particles with less
than 50% coating.

   The wafers containing the close-packed
monolayers are then transferred to a physical vapor deposition (PVD) system and
one side of the particles will be coated with platinum at a reduced pressure of
10^-4 mbar. Once coated, the Janus particles will be sonicated off
the substrate and stored in solution in a glass vial.

Conclusions

Close-packed monolayers of silica
particles have been made with convective assembly technique. Those particles
were then sequentially coated with titanium and gold in a PVD machine making
Janus particles. Next steps include the evaporation of platinum, which requires
higher temperatures and the testing of the Janus particles in a 3 wt% solution
of aqueous hydrogen peroxide to test if they will show propulsion behavior as a
result of hydrogen peroxide decomposition. Once particle activity has been
established, the substrate will be modified with polymer brushes that that arte
intended to create a molecular step to direct the movements of the Janus
particles.  Next, the particle solution
will be put on the modified silicon oxide surface in order to determine if the
Janus particles in the solution are able to sense the molecular properties of
the polymer brushes on the modified surface and be guided along the surface of
the substrate.

Developing
techniques to guide Janus particles can lead to development in drug delivery
and chemical sensors.

Reference and Notes

1.     
Bradly, Melanie; Rowe, Joanne. The Royal Society of Chemistry 2009, 5

2.     
Axel H. E. Müller; Andreas Walther Royal Society of Chemistry 2008, 4

3.     
Okubo,
Masayoshi ;Tanaka, Takuya; Okayama, Masaru; Kitayama, Yukiya; Kagawa, Yasuyuki Langmuir 2010, 26(11)

4.     
Vyas,
Mukesh; Schneider, Konrad; Nandan, Bhau; Stamn, Manfred Royal Society of Chemistry 2008,
4, 1024-1032