8.4 Area-Selective Functionalization of Block Copolymer-Based …
159
features with pronounced fluorescence emission was clearly observed. The FV image
(Fig. 8.8d) provides evidence for a non-uniform surface functionality of the films
on the nanometer scale, which is consistent with the notion of domain-selective
protein functionalization. On the other hand, BSA can physisorb to a limited extent
on PS. This effect was quantified by fluorescence microscopy (Fig. 8.8e, f). As can
be concluded from the fluorescence emission intensity histograms, the coverage
on the activated nanostructured areas is by a factor of 4–5 higher compared to
PS. Hence, since BSA was found to adsorb only little via physisorption onto PS,
the covalent coupling was concluded to occur predominantly selectively inside the
50 nm-sized nanodomains. In principle, the non-specific adsorption on the PS areas
can be suppressed by masking the exposed PS using PEO-based block copolymer
(Pluronics) treatments [24].
The fabrication of smaller patterns, in particular on the length scales defined by
the domain size and spacing, could not be accomplished by reactive μCP due to the
lack of suitable stamps. In addition, it is unlikely that diffusion can be neglected
at these length scales (comparing with Chap. 7). To demonstrate the feasibility
of a combined scanning probe lithography [25] (top-down)-nanopatterned block
copolymer films (bottom-up) approach to realize nanoscale platforms for screening of
(bio)molecular interactions, surface reactions, etc., dip pen nanolithography (DPN)
[26] was explored on hydrolyzed and NHS-activated PS 690 -b-PtBA 1210 films.
DPN is a method for directly depositing molecules from an “ink”-coated AFM
tip onto a substrate of interest. In analogy to the inked-nib classical writing, this
technique uses the AFM tip as a “nib”, a chemically well-defined surface as “paper”
(usually a solid-state substrate), and molecules with a chemical affinity for the surface
as the “ink”. The patterning process in a DPN experiment comprises two main
processes. The first step is molecular transport from the tip to substrate, involving
dissolution of ink molecules into the meniscus that spontaneously forms between the
tip and substrate. The second step is ink adsorption (physical or chemical) onto the
surface, and consequent (mono)layer formation. Both the transport and adsorption
of ink molecules often depend on several variables, including temperature, humidity,
writing speed of the AFM tip, the physicochemical properties of the ink and surface,
among others.
To facilitate the independent analysis and to control the diffusion of the molecular
ink [27], generation 5 polyamidoamine (G5 PAMAM) dendrimers were deposited.
Each of these dendrimer molecules possesses 128 primary amino groups at its
periphery. Similar to previously reported work on NHS-functionalized SAMs [28],
the deposited dendrimers were detected by friction mode AFM as low friction areas
compared to the unfunctionalized matrix (Fig. 8.9).
Fluorescence labeling of the dendrimers and the subsequent observation of fluorescent patterns helped to ensure that dendrimers were deposited. In addition, for
densely covered areas on NHS SAMs, the dendrimers could be directly visualized
by AFM. The dendrimers were observed to be bound tightly to the block copolymer
matrix and did not diffuse across the surface, suggesting that the coupling is covalent
in nature. Using DPN under controlled humidity conditions, structures as small as
159
features with pronounced fluorescence emission was clearly observed. The FV image
(Fig. 8.8d) provides evidence for a non-uniform surface functionality of the films
on the nanometer scale, which is consistent with the notion of domain-selective
protein functionalization. On the other hand, BSA can physisorb to a limited extent
on PS. This effect was quantified by fluorescence microscopy (Fig. 8.8e, f). As can
be concluded from the fluorescence emission intensity histograms, the coverage
on the activated nanostructured areas is by a factor of 4–5 higher compared to
PS. Hence, since BSA was found to adsorb only little via physisorption onto PS,
the covalent coupling was concluded to occur predominantly selectively inside the
50 nm-sized nanodomains. In principle, the non-specific adsorption on the PS areas
can be suppressed by masking the exposed PS using PEO-based block copolymer
(Pluronics) treatments [24].
The fabrication of smaller patterns, in particular on the length scales defined by
the domain size and spacing, could not be accomplished by reactive μCP due to the
lack of suitable stamps. In addition, it is unlikely that diffusion can be neglected
at these length scales (comparing with Chap. 7). To demonstrate the feasibility
of a combined scanning probe lithography [25] (top-down)-nanopatterned block
copolymer films (bottom-up) approach to realize nanoscale platforms for screening of
(bio)molecular interactions, surface reactions, etc., dip pen nanolithography (DPN)
[26] was explored on hydrolyzed and NHS-activated PS 690 -b-PtBA 1210 films.
DPN is a method for directly depositing molecules from an “ink”-coated AFM
tip onto a substrate of interest. In analogy to the inked-nib classical writing, this
technique uses the AFM tip as a “nib”, a chemically well-defined surface as “paper”
(usually a solid-state substrate), and molecules with a chemical affinity for the surface
as the “ink”. The patterning process in a DPN experiment comprises two main
processes. The first step is molecular transport from the tip to substrate, involving
dissolution of ink molecules into the meniscus that spontaneously forms between the
tip and substrate. The second step is ink adsorption (physical or chemical) onto the
surface, and consequent (mono)layer formation. Both the transport and adsorption
of ink molecules often depend on several variables, including temperature, humidity,
writing speed of the AFM tip, the physicochemical properties of the ink and surface,
among others.
To facilitate the independent analysis and to control the diffusion of the molecular
ink [27], generation 5 polyamidoamine (G5 PAMAM) dendrimers were deposited.
Each of these dendrimer molecules possesses 128 primary amino groups at its
periphery. Similar to previously reported work on NHS-functionalized SAMs [28],
the deposited dendrimers were detected by friction mode AFM as low friction areas
compared to the unfunctionalized matrix (Fig. 8.9).
Fluorescence labeling of the dendrimers and the subsequent observation of fluorescent patterns helped to ensure that dendrimers were deposited. In addition, for
densely covered areas on NHS SAMs, the dendrimers could be directly visualized
by AFM. The dendrimers were observed to be bound tightly to the block copolymer
matrix and did not diffuse across the surface, suggesting that the coupling is covalent
in nature. Using DPN under controlled humidity conditions, structures as small as