DPN patterning

In order to characterize the DPN patterns by an independent method, we have employed the Nanospeclroscopy... 

For direct patterning on the nanometer scale, scanning probe microscopy (SPM) based techniques such as dip-pen-nanolithography (DPN), [112-114] nanograftingf, nanoshaving or scanning tunneling microscopy (STM) based techniques such as electron induced diffusion or evaporation have recently been developed (Fig. 9.14) [115, 116]. The SPM based methods, allows the deposition of as-sembhes into restricted areas with 15 nm linewidths and 5 nm spatial resolution. Current capabihties and future applications of DPN are discussed in Ref. [117]. 

DNA arrays have been also generated by Dip-Pen Nanolithography (DPN) [80]. DPN involves the transfer of NAs directly from a coated Atomic Force Microscope (AFM) tip to the substrate of interest by virtue of direct molecular diffusion. Using this technique, thiol-modified ONDs have been patterned onto gold substrates and acrylamide-modified ONDs onto glass sHdes that were previously modified with mercaptopropyltrimethoxysilane. Feature sizes ranging from many micrometers to less than 100 nanometers could be obtained. The deposition of two different OND sequences onto the same substrate has also been reported [80], but the appHcation of this principle to the fabrication of high-density arrays remains to be addressed. 

Controlled delivery of collections of molecules onto a substrate with nanometre resolution can be achieved with the tip of an AFM. This positive printing mode technique is called dip-pen nanolithography (DPN) and its working principle is illustrated in Fig. 3.27. DPN uses an AFM tip as a nanopencil, a substrate as the paper and molecules with a chemical affinity for the substrate as the ink. Capillary transport of molecules from the AFM tip to the solid substrate is used in DPN to directly write patterns consisting of a relatively small collection of molecules in submicrometre dimensions. The hrst example introducing the technique was the transfer of octadecanethiol onto gold surfaces (Piner et al, 1999). 

Dip-pen nanolithography (DPN) is a variety of scanning probe lithography (direct-write) developed by Mirkin and coworkers, where components of interest are transferred from an AFM tip to a substrate.201 DPN has been used to pattern a wide variety of materials on surfaces, including small organic molecules (most commonly n-alkanethiols), DNA, nanoparticles, proteins, viruses, and precursors for inorganic thin films. 

Fig. 13 Comparative study of symmetric and asymmetric electroactive nanoarrays for the study of cell adhesion and polarization (a) DPN was used to pattern a SAM nanospot of hydroquinone-terminated alkanethiolates for subsequent RGD immobilization and cell adhesion, (b) Lateral force microscopy image of a symmetric nanoarmy (left) and fluorescent cell having a diffusive nucleus-centrosome-Golgi vector that indicates no preferential migratory direction (right), (c) Cell polarity vectors orient toward the direction of higher RDG density on asymmetric nanoarrays, (d) Higher magnification of the cell polarization vector (above) and its schematic (below). Reproduced from [37, 38] with permission. Copyright The American Chemical Society, 2008...

Figure 13.18 The formation of chemically patterned substrates by DPN, passivation of the unpattemed surface and subsequent attachment of nanoparticles via electrostatic interactions.

The application of scanning probe lithography (SPL) has been widespread owing to its ability to modify substrates with very high resolution and ultimate pattern flexibility.96 Dip-pen nanolithography (DPN),97 high contact force AFM,98 and constructive nanolithography99 are some of the most commonly employed techniques, all of which aim to control the position and directed assembly of molecules and nanoparticles. 

Figure 14.19 Schematic representation of DPN of protein IgG. The pattern formation can be conducted by two methods, using negatively charged Si02 or aldehyde-modified Si02.

Considering the affinity between metal ions (i.e., Zn2+, Cu2+, Ni2+, and Co2+) and proteins another method to pattern proteins on gold surfaces by DPN was developed.130 This method utilizes Zn2+ ions (Zn(N03)2 6H20) that can be coordinated with MHDA, which can be patterned on gold by an AFM tip after passivation of the remaining gold areas with PEG-SH. The Zn-modified features can be subsequently reacted with the desired antibody. Biorecognition properties can be further studied by interaction with other proteins. 

To decrease the size of the pattern on the surface, DPN can offer ample possibilities. The newest technology introduced 55,000 tip pens working in parallel, which can create 88 million features on an area of 1 cm2. In this field, the biggest challenge still remains to introduce a corresponding 55,000 different inks to the array of tips. 

In conclusion, we have demonstrated a versatile DPN method for nanopatteming y-FejOj nanocrystals on different substrates. The patterns are well defined with minimal lateral diffusion of the ink. The patterns are also magnetic and yields well-resolved LEEM and XPEEM images. 

A general benefit of DPN over other soft lithographic techniques is the ability to pattern nanostructures (including biological materials) by a single step without crosscontamination, since the desired chemistry occurs only in a specifically defined location of the substrate. 

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