Biomedical Engineering Reference
In-Depth Information
[338-340] and several bioactive proteins have been studied with regard to their potential applications
in biosensing devices [341-343].
There are many advantages of LB fi lms for making biosensors, such as orderly fi lm on a molec-
ular scale, short response time, one-step membrane fabrication, and biocompatible environment.
14.2.4.2 Self-AssembledMonolayers
As discussed in Section 14.2.1.1, on one hand, LB method is capable of transferring the mono-
layer from the air-water interface onto a solid support, and the multilayer could be formed
by repetitive transfer, but those LB fi lms are often thermodynamically unstable. On the other
hand, self-assembled membrane (SAM) is not only thermodynamically stable but also capable
of imprinting a desired function when assembling individual molecules into highly ordered
architecture.
Molecular self-assembly is a key link among physics, chemistry, and biology, and it can be used
to create novel nano- and microstructures, materials, and devices used in biosensors following the
bottom-up procedure [344,345].
The term self-assembly involves the arrangement of atoms and molecules in an ordered form
or even aggregation of functional entities without the intervention of humankind toward an ener-
getically stable form [346]. Formation of an SAM is essentially an organization of molecules
at solid-liquid interfaces induced by strong chemisorption between the substrate and the head
group. The SAM offers several attractive features for its application of biosensors, such as molecu-
lar dimensions, high stability, good organization, and compatibility with metal substrates and
biomolecules.
Many systems are capable of undergoing the process of self-assembly [347,348], for example,
long chains of carboxylic acids (C
n
H
2
n
+
1
COOH) at metal oxide substrates, organosilane species
(RSiX
3
, R
2
SiX
2
, or R
3
SiX, where R is an alkyl chain and X a chloro- or alkoxygroup) at hydroxyl-
ated substrates such as glass, silicon, and aluminum oxide, and organosulfur-based species at noble
metal surfaces. The last system has been most extensively used in biosensors because of its stability
and physicochemical properties. Sulfur-containing compounds, for example, alkanethiols, dialkyl
disulfi des, and dialkyl sulfi des, have a strong affi nity for noble metal surfaces.
For Au electrodes, these fi lms commonly consist of a methylene chain that has thiol function-
ality on one end that binds to the electrode and an organic functionality on the other end. SAM
fi lms have been used to immobilize proteins through covalent linkages [349], coordinative bonding
[350], electrostatic attraction, [351] and hydrophobic interactions [352]. Electroactive end groups
(for mediator or electron transfer function) of SAMs can be used as electrical wiring or for com-
munication between the redox-active enzymes and the electrode surface.
The SAM fi lms can also be used in the SPR sensors. Knoll et al. [353] used a variety of sulfur-
containing compounds to create a wide range of biotinylated SA monolayers on Au surfaces, and
the modifi ed monolayers were then employed to bind streptavidin. Modifi cation of biotin deriva-
tives with long-chain hydroxythiols and dilution of the monolayer with short-chain alkanethiols are
found to increase the effectiveness of binding to streptavidin.
14.2.5 D
IAMOND
The biosensor development based on conductive diamond (
sp
3
-bonded) thin fi lms is one of the
frontiers of the integration of microelectronics technology and biotechnology in the very recent year
[354,355]. Diamond possesses unparalleled properties including outstanding hardness, chemical
robustness, and inertness with high thermal conductivity [356], as well as excellent electrical and
optical properties [357]; moreover, diamond is also well-known for its biocompatibility and bioin-
ertness [358,359]. These properties make it an ideal platform material for biointerfaces.
The diamond fi lms on different substrates are obtained by CVD at reasonable cost, and the
fi lms can be made electrically conductive by selective incorporation of nitrogen or boron [360].
