Manipulating micro-objects plays
a crucial role in a wide range
of fundamental and applied research works. Here, we propose an original
strategy based on the chemical modification of a substrate by hydrogen-bonded
films elaborated by layer-by-layer (LbL) assemblies of poly(ethylene
glycol) (PEG) and poly(acrylic acid) (PAA). First, the influence of
the polymer molecular weight on the growth of the PEG/PAA multilayer
was evaluated. Optical reflectometry analysis used to follow in situ the film buildup revealed a strong dependence of
the deposited amount of polymer on the ratio of monomer units of each
polymer (n
PAA/n
PEG). Then, colloidal probe atomic force microscopy (AFM) microscopy
was carried out in an aqueous medium to monitor the adhesion forces
of multilayer surfaces composed of N polymer layers. Pull-off forces
were converted using the Johnson–Kendall–Roberts (JKR)
model to access the thermodynamic work of adhesion. Results indicated
that PEG/PAA multilayer films exhibit weak adhesion forces that are
sensitive to the number of deposited polymer layers at pH 2. In addition,
a progressive increase of the solution pH reduced the adhesion due
to the destruction of the hydrogen-bonded multilayer film. To simulate
the capture and the release of a micro-object, borosilicate particles
acting as spherical micro-objects were adsorbed onto a PEG/PAA film.
Once again, an increase of the solution pH led to desorption of particles,
as shown by optical microscopy. Finally, an AFM tip functionalized
by a PEG/PAA multilayer was used to achieve successful micromanipulation
operations (capture and release) of a 10 μm diameter borosilicate
sphere in an aqueous solution.
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