Self-Assembled Monolayers of Thiolates on Metals as - Whitesides ...

1146 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
complex and dynamic behaviors. 14 They consist <strong>of</strong>
supermolecular assemblies <strong>of</strong> proteins, glycoproteins,
and large oligosaccharides anchored to or embedded
in a fluid lipid bilayer or protein coat. The assemblies
can have dimensions ranging from a few to hundreds
<strong>of</strong> nanometers; for example, integrins are cylindrical
transmembrane proteins (8-12 nm in diameter) that
assemble into small clusters when cells attach to
extracellular matrixes <strong>of</strong> protein. 648
The broad range <strong>of</strong> membrane-bound assemblies
present in biological membranes control many processes
in living organisms (from bacteria to complex,
multicellular organisms). The interactions between
single ligand-receptor pairs (or <strong>of</strong>ten groups or
clusters <strong>of</strong> molecules) 425 enable the cell to sense its
environment, communicate with other cells, and
regulate intracellular functions such as migration,
adhesion, growth, division, differentiation, and
apoptosis.
The compositional complexity and dynamic nature
<strong>of</strong> biological surfaces make it difficult to study certain
fundamental aspects <strong>of</strong> biological systems in detail.
Model surfaces with well-defined compositions provide
useful tools for studying the physical-organic
chemistry <strong>of</strong> biomolecular recognition (for example,
the thermodynamics and kinetics <strong>of</strong> the association/
dissociation <strong>of</strong> proteins or other biomolecules with
ligands) for determining the effect <strong>of</strong> individual
recognition events on the functional behavior <strong>of</strong> cells
and investigating the structural factors that enable
surfaces to resist the adsorption <strong>of</strong> proteins. The
development <strong>of</strong> biotechnological applications, such as
cell culture, tissue engineering, and biosensors, 649
also can benefit from simplified systems that allow
only one or a few types <strong>of</strong> interactions between
species in solution (cells, biomolecules, analytes) and
the surfaces <strong>of</strong> the engineered system. One primary
challenge in developing model surfaces ex vivo is
developing methods that allow precise control <strong>of</strong> the
composition and structure <strong>of</strong> the surface while permitting
natural biological interactions to occur in
such a way that the results can be interpreted clearly
and related to biology in vivo.
SAMs are useful as model surfaces for studying
biological and biochemical processes. First, like the
biological surfaces they mimic, they are nanostructured
materials that form by self-assembly.
Second, they can present a wide range <strong>of</strong> organic
functionality rationally, including functionality that
can resist the adsorption <strong>of</strong> proteins, at positions
away from the plane <strong>of</strong> the substrate with nearly
atomic-level precision. Third, it is easy to prepare
SAMs functionalized with the large, delicate ligands
needed for biological studies by either synthesizing
molecules with the ligand attached to form the SAM
or, more commonly, attaching the ligands to the
surface <strong>of</strong> a preformed, reactive SAM (see section 5).
Fourth, SAMs are directly compatible with a number
<strong>of</strong> techniques (surface plasmon resonance (SPR)
spectroscopy, 650,651 optical ellipsometry, 408,409 RAI-
RS, 409 QCM, 652 mass spectroscopy 653 ) for analyzing
the composition and mass coverage <strong>of</strong> surfaces as well
as the thermodynamics and kinetics <strong>of</strong> binding
events. Fifth, SAMs are less influenced by effects <strong>of</strong>
mass transport than the thick gel layers sometimes
used to immobilize ligands on surfaces for SPR.
One disadvantage <strong>of</strong> SAMs as model surfaces is
that the structure <strong>of</strong> the SAM is essentially static.
This characteristic differs from that <strong>of</strong> biological
membranes, which are fluid and rearrange dynamically.
Langmuir-Blodgett (LB) films 654 and reconstituted
bilayers <strong>of</strong> lipids on solid supports 655 present
two alternative technologies for creating dynamic
models <strong>of</strong> biological surfaces. The utility <strong>of</strong> LB films
in this area <strong>of</strong> study remains limited due to instrumental
complexity and difficulties with reproducibility;
LB films (at the air-water interface) do,
however, generate systems in which in-plane diffusion
and transmembrane experiments are possible.
Advances in techniques for patterning regions <strong>of</strong> lipid
bilayers on solid supports and defining their compositions
also are beginning to emerge and could <strong>of</strong>fer
an important complementary tool for generating
model biological systems. 656
8.4.1. Designing SAMs To Be Model Biological Surfaces
To test a range <strong>of</strong> experimental conditions and
facilitate the interpretation <strong>of</strong> the interactions observed,
SAMs should have, at a minimum, three
characteristics: (1) they should be able to prevent
nonspecific adsorption <strong>of</strong> proteins or other biomolecules
on the surface, that is, they should only allow
interactions between the molecules and ligands <strong>of</strong>
interest and thus permit meaningful analysis <strong>of</strong>
observations made to determine the sensitivity or
kinetics <strong>of</strong> these processes; (2) they should allow
modifications to the composition and density <strong>of</strong> the
immobilized ligands or biomolecules (proteins, sugars,
antigens); and (3) they should present the ligands
<strong>of</strong> interest in a structurally well-defined manner that
minimizes the influences <strong>of</strong> the surface, e.g., limited
mass transport, blocked binding sites, or induced
conformational changes. It is also helpful if the model
surfaces can be used easily with common analytical
methods without modifying the existing instrumentation
or without subjecting the samples to unnatural
(for biology) conditions, e.g., in dehydrated form in
UHV.
Protein-Resistant Surfaces. Surfaces that resist
the nonspecific adsorption <strong>of</strong> other biomolecules and
cells commonly are called “inert” surfaces. The best
protein-resistant surfaces presently known are ones
composed <strong>of</strong> oligo- or poly(ethylene glycol) (OEG or
PEG). 657 Alkanethiols terminated with tri- or hexa-
(ethylene glycol) groups are a standard component
<strong>of</strong> SAMs used to study biology and biochemistry. 658,659
On gold the alkane chains form a dense, ordered
monolayer with the same molecular conformation
found for n-alkanethiols, i.e., all-trans chains with a
30° tilt; the terminal ethylene glycol end-group
adopts either a helical conformation aligned perpendicular
to the surface or an amorphous conformation.
660,661 The helical structures yield a quasicrystalline
surface phase, but the amorphous chains
produce a liquidlike phase. NEXAFS measurements
indicate that both conformations exist on the surface
simultaneously in a vacuum. 662 There is also evidence
that these end groups can undergo order-order