Cantilever-like micromechanical sensors - Artek

Rep. Prog. Phys. 74 (2011) 036101 A Boisen et al
2.2.1. Origin of surface stress—experimental focus.
Although the equation for measuring the surface stress from
a cantilever deflection has been available for more than 100
years, the origin of the surface stress and the relation to the
force transduction (chemical recognition event to mechanical
deflection of the cantilever) has been under dispute and is still
not clarified within the research community. There exist a few
theoretical papers that try to decipher the fundamental physics
of the origin of surface stress [67–69]. These papers are not
linked to experimental studies. In this section, we will focus
on the most relevant experimental findings existing within the
community. For clarity, we only consider studies of DNA
hybridization and alkanethiol layers.
DNA hybridization is studied in two steps. First, singlestranded
DNA (ss-DNA) is immobilized on a cantilever. The
actual hybridization event takes place when a complementary
ss-DNA strand is introduced, resulting in a hybridized doublestranded
DNA (ds-DNA) bound to the cantilever. The first
proposal on the physics of surface stress came in 2000
from Fritz et al, where it was suggested that the surface
stress originates from electrostatic, steric and hydrophobic
interactions [19] between the hybridized DNA strands on
the cantilever. It was observed that the DNA hybridization
event causes a compressive surface stress. DNA strands hold
a net negative charge. It is argued that the electrostatic
interaction changes since ds-DNA has a larger number of
charges compared with ss-DNA. The steric contribution is
believed to arise from a denser chain packing as a result
of the hybridization. A year later, Wu et al [70] showed
on-line measurements of a cantilever first being functionalized
with ss-DNA and then exposed to complementary DNA for
hybridization. The immobilization step resulted in a large
compressive stress whereas the hybridization event resulted
in a small tensile stress. Their findings indicated that the
cantilever deflects in the opposite direction as to what Fritz
et al reported. Furthermore, the cantilever was observed to
bend in opposing directions for the ss-DNA immobilization
and the subsequent ds-DNA hybridization. Intuitively this is
rather surprising as one might simply expect the cantilever
to bend more (in the same direction) as more molecules are
added to the surface. To explain the experimental results,
Wu et al argued that the surface stress cannot be a result
of electrostatic and steric interactions alone. They proposed
to also consider configurational entropy, which is related to
the order of the immobilized molecules. The ss-DNA has
a significantly lower persistence length than ds-DNA (under
the same buffer conditions), which means that the ds-DNA
has a higher stiffness and is more rod like. As a result, the
cantilever is required to bend more to ensure sufficient space
for the ss-DNA strands to form a SAM. The ds-DNA in turn will
not force the cantilever to deflect as much since it requires less
space to form a SAM due to its more rigid structure. Therefore,
when Wu et al started with a blank cantilever and introduced
ss-DNA for immobilization they observed a compressive stress
on the cantilever. When the complementary strands were
introduced to form the ds-DNA this stress was released (since
a more ordered SAM could be formed). They then observed a
tensile stress/release of compressive stress.
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However, this does not explain why their findings
contradict the findings of Fritz et al with respect to
DNA hybridization. To further support their theory on
the significance of the configurational entropy, Wu et al
immobilized ss-DNA on two different cantilevers in a 1M
buffer. These cantilevers were then exposed to two different
hybridization conditions. In the first situation, DNA was
hybridized under the same buffer conditions (1M) and a tensile
stress was observed. In the second situation, the buffer
concentration was reduced to 0.1M before the complementary
ss-DNA strand was introduced. The change in buffer
concentration forced the ss-DNA to form a more ordered
monolayer. In turn, a compressive stress was observed during
hybridization. This clearly indicates that configurational
entropy plays a role in signal generation and it stresses
the importance of controlling all aspects of a cantilever
measurement (buffer conditions, pre-treatments, temperature
variations, etc) since several effects are involved in generating
the surface stress signal.
The group of Rachel McKendry has studied the origin
of surface stress in a methodical manner. In 2002 [71]
the influence of the physical steric crowding of molecules
on a cantilever was pinpointed as the main contributor to
the cantilever deflection for DNA hybridization. It was
seen that the hybridization signal is close to non-existent
when the density of probe ss-DNA is tailored for maximized
hybridization efficiency. Again, this indicates that molecules
with ‘sufficient space’ do not generate a cantilever deflection.
In 2007, the effects of electrostatics [72] were investigated.
Here, simple SAMs of thiolated n-alkane chains with
different end-groups were used as model system. The
active SAM had carboxyl-groups at the end, which could
be protonated/deprotonated upon pH variations. The main
physical effect of the cantilever deflection in this situation was
assigned to electrostatic repulsion and ionic hydrogen bond
formation. Emphasis was also put on the counter ions present
in the aqueous solution; these can affect both the magnitude
and direction of the cantilever deflection. Figure 6 shows
the contribution from each individual effect (electrostatic
repulsion and ionic hydrogen bond formation) as well as the
effective interactions. Experimental data are also plotted and
seen to agree well with the proposed theory.
In 2008, McKendry, Sushko and co-authors proposed
the ‘first quantitative model to rationalize the physics of
nanomechanical sensors’ [73]. Here, a theoretical model is
developed that predicts the cantilever deflection direction and
magnitude upon pH variations of the buffer solution and for
various chain lengths of SAMs. The model considers the
effects of chemical, elastic and entropic properties of both
cantilever material and sensing layer.
Recently, the group of Grütter [74] analysed the effects of
intermolecular electrostatics, Lennard-Jones interactions and
adsorption-induced changes in the electronic density of a metal
film when thiolated alkane chains are immobilized on a goldcoated
silicon cantilever. Their findings indicate that it is the
latter effect combined with the associated electronic charge
transfer from the gold to the thiol bond that account for the
largest contribution to the measured surface stress. These