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The ability to image solid surfaces in a
liquid medium makes atomic force microscopy (AFM) an attractive tool in the
study of liquid-solid interfacial phenomena. One such area of research where AFM
has proven useful is in the study of small particles and biological molecules
that adsorb from an aqueous liquid onto a solid surface.
AFM provides magnifications great enough to
resolve single, deep sub-micrometer particles at the surface, while the presence
of the liquid keeps the adsorbed particles in their native, hydrated states. For
these reasons, AFM is being widely used in the study of adsorption of colloidal
particles, including polymer latexes, mineral colloids, and protein
molecules.
Conventional
Contact AFM Images
Conventional contact mode AFM images
particles that are attached firmly to a surface, such as with covalent bonds.
However, when colloidal particles adsorb to a solid, they associate
non-covalently with the surface through electrostatic, van der Waals, or
hydrophobic interactions, so they can have high lateral mobilities.
In contact mode AFM, the dragging motion of
the probe tip exerts a lateral force on the surface that can push around loosely
adsorbed particles, preventing the imaging of particles in their natural
arrangement on the surface. This problem is avoided by imaging in liquid with TappingMode™
AFM.
TappingMode AFM
Images
With this patented technique, the probe tip
rapidly oscillates and taps the surface lightly at the bottom of each
oscillation cycle. The intermittent contact eliminates lateral forces on the
surface by the scanning tip, and allows imaging of adsorbed particles without
moving them on the surface.
Figure 1 shows a liquid TappingMode image
of positively-charged polymer latex particles (amidine latex) adsorbed to mica
in water. The layer of adsorbed particles can be imaged repeatedly without any
movement of the particles.
Figure 1. TappingMode in liquid image of positively charged polystyrene latex
particles adsorbed to mica (in water). The average particle diameter is 120nm.
3μm scan
Figure 2 shows the same 3μm x 3μm area,
imaged moments later in water with contact mode. The blurred streaks in the
image suggest that the adsorbed particles do not remain stationary as the probe
tip scans over them.
Figure 2. Contact mode image in water of the same area in Figure 1. 3μm
scan.
Figure 3 shows a broader area of the sample
(7μm x 7μm) imaged in water with TappingMode. The damage to the layer of
adsorbed particles caused by the contact mode scan is clear. The adsorbed
particles appear to have been pushed into clusters, mostly near the sides of the
previously scanned region, and the bare mica substrate is exposed.
Figure 3. TappingMode image in water obtained after Figure 2. Damage to the
adsorbed layer from the contact mode scan is seen where particles have been
pushed into clusters, exposing bare areas of the mica substrate. 7μm
scan.
Since the adsorbed particles are unaffected
by the oscillating tip in TappingMode, it is
possible to observe how the arrangement of particles at the surface is affected
by system properties, such as the ionic strength of the surrounding liquid. This
information is relevant to the processing of colloidal materials and the
purification of protein products. With TappingMode in liquid, it is also
possible to study how the layer of adsorbed particles grows with time, and to
see how the structure of the adsorbed layer at the liquid-solid interface
differs from the structure at air-solid interface.
Surface
Modification of Probe Tips
For certain experimental systems, adsorbed
particles may tend to adhere to the probe tip, even with TappingMode
AFM. For example, positively charged latex particles will have an
electrostatic attraction to the silicon nitride probe, which has a slightly
negative surface charge in water. Such particles may become attached to the tip,
resulting in artefacts in subsequent images. This problem is countered by
modifying the probe surface with a silane coupling agent,
4-aminobutyldimethylmethoxy silane.
The monoethoxysilane deposits with
sub-monolayer coverage on the probe, and the amino group on the attached silane
confers a positive surface charge to the modified tip.
The positive charge on the tip inhibits
adhesion of positively charged particles and greatly extends its lifetime for
imaging such adsorbed colloids with TappingMode in
water. This tip modification technique also works well when imaging dried
adsorbed layers with TappingMode in air.
Summary
TappingMode
imaging in liquids is an ideal experimental technique in the study of the
adsorption of colloidal particles to solid surfaces. The elimination of lateral
forces between the AFM tip, and the surface in TappingMode, allows particles at
the liquid-solid interface to be imaged without changing their natural
positions. This enables the study of the effects of solvent conditions on the
structure of the adsorbed layer. |