110
6 Fabrication of Robust Biomolecular Patterns …
6.1 Introduction
Controlling the selective immobilization of (bio)molecules in defined positions
of a surface is very important for the development of biosensors and highthroughput biomolecule screening assays [1]. In the development of rapid, robust
screening microarrays, several strategies have been successfully applied. These
comprise, among others, robot-based high-precision contact-printing [2], multistep photolithography [3], selective molecular assembly patterning [4], and soft
lithographic approaches, such as the microcontact printing (μCP) method [5]. As
discussed in Chap. 2, the μCP technique developed by Whitesides et al. [6] is
widely used for the fabrication of monolayer-based micrometer and sub-micrometer
scale patterns. In applications, such as the development of certain biosensors [7], the
simplicity of the method, as well as the low cost, the flexibility, and the possibility
to pattern curved substrates make μCP very attractive.
Using μCP one can transfer a variety of molecules with sub-micrometer resolution
to reactive substrates without the need for dust-free environments or harsh chemical
treatments [8]. Molecules can be patterned normally through physical interaction
(including electrostatic and hydrophobic interactions) [9], or covalently coupling
[10] with the substrate. However, the coverage or activity quantitation of molecules
immobilized in patterns through mere physical interactions are often difficult to
achieve because the molecules can be washed away during the processing conditions
[11]. Covalent linkages, on the other hand, are thought to provide more specific and
stronger attachment [12]. In general, covalent attachment utilizes bifunctional linker
chemistry that involves, e.g., amide or imine bond formation, gold-thiol interactions,
or silane chemistry. These approaches afford efficient and secure immobilization of
(bio)molecules on suitable substrates.
Although μCP is a simple and flexible method for covalent coupling of
biomolecules onto monolayers, these reactions require highly activated species,
because no or little solvent is involved in this procedure. Recently, it has been shown
that the methodology can be extended to perform local chemical reactions, including
catalytic reactions, coupling, and protection/deprotection reactions.
For the mentioned screening-type applications, polymeric thin films have been
shown to possess a number of important advantages compared to SAM-based
systems. These advantages comprise robustness and stability, high molecular loading,
high reactivity, and the inherent possibility to introduce simultaneously chemical
(compositional), as well as topographical patterns and structures. Hence it is not
surprising that patterned thin films of polymers have a wide range of applications,
for example, as etch resists [13], in biological [14] and chemical sensors [15],
and in tissue engineering [16]. Thin films of patterned polymers that incorporate
reactive functional groups provide a surface that can be further modified by chemical reactions [17]. To increase the molecular loading, hydrogels [18], dendrimers
[19], hyperbranched polymers [20], chemical vapor deposition approaches [21],
self-assembled polyelectrolyte multilayers [22], plasma polymers [23], and polymer
brushes obtained by grafting-from approaches have been investigated.
6 Fabrication of Robust Biomolecular Patterns …
6.1 Introduction
Controlling the selective immobilization of (bio)molecules in defined positions
of a surface is very important for the development of biosensors and highthroughput biomolecule screening assays [1]. In the development of rapid, robust
screening microarrays, several strategies have been successfully applied. These
comprise, among others, robot-based high-precision contact-printing [2], multistep photolithography [3], selective molecular assembly patterning [4], and soft
lithographic approaches, such as the microcontact printing (μCP) method [5]. As
discussed in Chap. 2, the μCP technique developed by Whitesides et al. [6] is
widely used for the fabrication of monolayer-based micrometer and sub-micrometer
scale patterns. In applications, such as the development of certain biosensors [7], the
simplicity of the method, as well as the low cost, the flexibility, and the possibility
to pattern curved substrates make μCP very attractive.
Using μCP one can transfer a variety of molecules with sub-micrometer resolution
to reactive substrates without the need for dust-free environments or harsh chemical
treatments [8]. Molecules can be patterned normally through physical interaction
(including electrostatic and hydrophobic interactions) [9], or covalently coupling
[10] with the substrate. However, the coverage or activity quantitation of molecules
immobilized in patterns through mere physical interactions are often difficult to
achieve because the molecules can be washed away during the processing conditions
[11]. Covalent linkages, on the other hand, are thought to provide more specific and
stronger attachment [12]. In general, covalent attachment utilizes bifunctional linker
chemistry that involves, e.g., amide or imine bond formation, gold-thiol interactions,
or silane chemistry. These approaches afford efficient and secure immobilization of
(bio)molecules on suitable substrates.
Although μCP is a simple and flexible method for covalent coupling of
biomolecules onto monolayers, these reactions require highly activated species,
because no or little solvent is involved in this procedure. Recently, it has been shown
that the methodology can be extended to perform local chemical reactions, including
catalytic reactions, coupling, and protection/deprotection reactions.
For the mentioned screening-type applications, polymeric thin films have been
shown to possess a number of important advantages compared to SAM-based
systems. These advantages comprise robustness and stability, high molecular loading,
high reactivity, and the inherent possibility to introduce simultaneously chemical
(compositional), as well as topographical patterns and structures. Hence it is not
surprising that patterned thin films of polymers have a wide range of applications,
for example, as etch resists [13], in biological [14] and chemical sensors [15],
and in tissue engineering [16]. Thin films of patterned polymers that incorporate
reactive functional groups provide a surface that can be further modified by chemical reactions [17]. To increase the molecular loading, hydrogels [18], dendrimers
[19], hyperbranched polymers [20], chemical vapor deposition approaches [21],
self-assembled polyelectrolyte multilayers [22], plasma polymers [23], and polymer
brushes obtained by grafting-from approaches have been investigated.