IOP PUBLISHING NANOTECHNOLOGY
Nanotechnology 18 (2007) 424028 (6pp) doi:10.1088/0957-4484/18/42/424028
Effect of chain length on nanomechanics of
alkanethiol self-assembly
Ramya Desikan1,2, Sarah Armel1, Harry M Meyer III1 and
Thomas Thundat1,2
1 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2 Department of Physics, The University of Tennessee, Knoxville, TN, USA
Received 17 March 2007, in final form 11 June 2007
Published 21 September 2007
Online at stacks.iop.org/Nano/18/424028
Abstract
The ability to generate nanomechanical cantilever motion from molecular
interactions between analytes and immobilized receptors offers a unique
platform for chemical and biological sensor development. A fundamental
understanding of the origins of nanomechanical motion, however, is essential
for developing reliable and reproducible sensors. We have investigated the
nanomechanical bending of microfabricated cantilevers during the
immobilization of alkanethiols of different chain lengths in the liquid phase.
The bending of the cantilevers has been monitored using both piezoresistive
and optical readout approaches. Our results suggest that the surface packing
density in a liquid medium is largely affected by the length of the chains,
which will have a profound influence on sensor sensitivity.
1. Introduction
Recently, microcantilevers have been increasingly investi-
gated as a sensing platform for chemical and biological
molecules [1–14]. Molecular adsorption on one side of a can-
tilever results in cantilever bending due to adsorption-induced
forces. The selectivity and sensitivity of a microcantilever sen-
sor relies heavily on the proper formation of a functional layer
on one surface of the cantilever. Transducing adsorption forces
to the cantilever requires developing efficient ways of immo-
bilizing selective receptors on the cantilever surface. Often,
selective receptors are immobilized on the cantilever surface
using alkanethiol linkers, which form self-assembled mono-
layers (SAMs) on the gold-coated surface of the cantilever.
These thiolated molecules are chosen due to the strong affin-
ity of sulfur head groups with the gold surface of the micro-
cantilever. The cantilever bending changes due to interaction
forces when analyte molecules adsorb on immobilized recep-
tors on the cantilever. Sensitive monitoring of the cantilever
bending enables detection of target molecules binding to the
receptors. The selectivity of detection is entirely due to the
uniqueness of analyte–receptor binding. It has been speculated
that the reproducibility of sensor responses is directly related
to the quality of the SAM receptor layer.
The physics and chemistry of alkanethiol self-assembled
monolayers have been the target of continuous and exten-
sive research [15–21]. These self-assembled monolayers have
numerous applications in surface modification, solid lubrica-
tion, surface coating, and biosensing [22, 23]. Previous stud-
ies have revealed many characteristics of the monolayers re-
garding formation, kinetics and thermodynamics [24]. Self-
assembled monolayers of alkanethiols on surfaces alter the
surface free energy and generate a surface stress. However,
not much information is available on the mechanical proper-
ties of SAMs, especially regarding the nature of surface stress
during the film formation process. The difference in surface
stress response during vapour-phase alkanethiol adsorption on
large- and small-grained gold has been shown to correlate well
with differences in alkanethiol coverage and SAM structure on
those substrates [25]. Though it is more difficult to observe
the structural evolution of the monolayer self-assembly in the
liquid phase than in the vapour phase, the formation of self-
assembled monolayers in liquid is important for the develop-
ment of biosensors, such as those functionalized with thiolated
oligonucleotides.
Here we report on the effect of alkanethiol chain length
on the cantilever bending response during self-assembly under
solution. Three different alkanethiols with various lengths,
1-octanethiol, 1-dodecanethiol, and 1-octadecanethiol, were
used to functionalize cantilevers with a thin layer of gold on
one side. These thiol molecules have no functional groups
and the only difference is their chain length. The cantilever
bending was monitored by using piezoresistive as well as laser
beam deflection approaches. We have also carried out an x-
ray photoelectron spectroscopy (XPS) examination of probe
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