Michael Hirtz

Substrate-independent dip-pen nanolithography based on reactive coatings

We report that nanostructuring via dip-pen nanolithography can be used for modification of a broad range of different substrates (polystyrene, Teflon, stainless steel, glass, silicon, rubber, etc.) without the need for reconfiguring the underlying printing technology. This is made possible through the use of vapor-based coatings that can be deposited on these substrates with excellent conformity, while providing functional groups for subsequent spatially directed click chemistry via dip-pen nanolithography. Pattern quality has been compared on six different substrates demonstrating that this approach indeed results in a surface modification protocol with potential use for a wide range of biotechnological applications.

Multiplexed biomimetic lipid membranes on graphene by dip-pen nanolithography.

The application of graphene in sensor devices depends on the ability to appropriately functionalize the pristine graphene. Here we show the direct writing of tailored phospholipid membranes on graphene using dip-pen nanolithography. Phospholipids exhibit higher mobility on graphene compared with the commonly used silicon dioxide substrate, leading to well-spread uniform membranes. Dip-pen nanolithography allows for multiplexed assembly of phospholipid membranes of different functionalities in close proximity to each other. The membranes are stable in aqueous environments and we observe electronic doping of graphene by charged phospholipids. On the basis of these results, we propose phospholipid membranes as a route for non-covalent immobilization of various functional groups on graphene for applications in biosensing and biocatalysis. As a proof of principle, we demonstrate the specific binding of streptavidin to biotin-functionalized membranes. The combination of atomic force microscopy and binding experiments yields a consistent model for the layer organization within phospholipid stacks on graphene.

Patterning of polymer electrodes by nanoscratching.

2010 WILEY-VCH Verlag Gmb Growing scientific effort is being devoted to building electronic circuits entirely or partially of organic materials because of their attractive characteristics such as low cost, light weight, and mechanical flexibility. To realize low-cost and high-performance organic transistor circuits for practical applications, utilization of low-cost electrodes (such as conducting polymers) and downscaling the transistor critical feature to the sub-micro/ nanometer scale are two necessary concepts. However, integration of these two strategies, i.e., patterning polymer electrodes with sub-micro/nanometer resolution, remains a great challenge. Here we demonstrate the patterning of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS) (Fig. 1a), an excellent organic electrode material, with 50 nm resolution on both rigid and flexible substrates by atomic force microscopy (AFM) nanoscratching. The scratched grooves show good stability under solvent immersion, heat treatment, and long-term storage in air. This technique can realize small organic transistor electrode pairs (area of 0.5–0.6mm) and a high density electrode array (about 10 elements cm ). The small linewidth of the patterned PEDOT:PSS electrodes yields a parasitic overlap capacitance as low as 0.09 pF mm . The scratched sub-micro/nanometer channel shows excellent performance in organic transistors with high performance and low voltage. Downscaling the size of a single transistor not only allows the realization of high integration density and miniaturization of circuits, but also facilitates improved circuit performance and/or switching speed, as well as lower power consumption. Up to now, organic transistor-based circuits with a relatively fast switching speed (1 KHz to 20MHz) and large integration density ( 2000 transistors) have been demonstrated with metal electrodes defined by high resolution but complicated and expensive photolithography or e-beam lithography. It is widely recognized that the high cost of gold electrodes will overshadow the practical application of organic circuits, while polymer electrodes have a unique ability to fully embody the advantages of organic circuits such as low cost and mechanical flexibility, as well as the ability to achieve higher circuit performance because of their excellent compatibility with organic semiconductors. However, organic transistors/circuits with polymer electrodes such as PEDOT:PSS generally fabricated by volume printing techniques yield a much lower speed of 1–100Hz and require a relatively high voltage of 20–100V, mainly because of the poor resolution (10–50mm) of the direct-printing technique. In addition to mobility, the switching speed of a transistor is inversely proportional to channel length and the overlap linewidth between gate and source/drain electrodes (see Equation (3) in the Experimental section). Therefore, in order to achieve high-performance organic circuits with polymer electrodes, an effective but certainly challenging route is to define polymer electrode materials with submicro/nanometer resolution through a simple and low cost procedure. AFM lithography is a cost-effective and reliable technique for pattering submicro/nanometer-scale structures without complicated steps in comparison with other patterning techniques. To date, there is no report on the use of AFM nanoscratching to structure conducting polymers such as PEDOT:PSS. Figure 1b–d illustrate the schematic process of nanoscratching with a silicon tapping-mode cantilever (Supporting Information 1) in contact mode on a PEDOT:PSS film. Figure 1e shows an exemplified groove formed by nanoscratching, and the groove width is about 50 nm, which is the narrowest PEDOT:PSS groove reported to date. Although a sub-micrometer PEDOT:PSS groove has been achieved for transistor applications by a modified-printing method, this technique requires a prepatterning process based on an original patterningmethod such as photolithography, which will definitely increase the complexity and cost of the manufacturing process. The surface plot (Fig. 2a) and section analysis (Fig. 2b) clearly show the geometry of the multiple grooves. Figure 2c displays the

Interdigitated multicolored bioink micropatterns by multiplexed polymer pen lithography.

Multiplexing, i.e., the application and integration of more than one ink in an interdigitated microscale pattern, is still a challenge for microcontact printing (μCP) and similar techniques. On the other hand there is a strong demand for interdigitated patterns of more than one protein on subcellular to cellular length scales in the lower micrometer range in biological experiments. Here, a new integrative approach is presented for the fabrication of bioactive microarrays and complex multi-ink patterns by polymer pen lithography (PPL). By taking advantage of the strength of microcontact printing (μCP) combined with the spatial control and capability of precise repetition of PPL in an innovative way, a new inking and writing strategy is introduced for PPL that enables true multiplexing within each repetitive subpattern. Furthermore, a specific ink/substrate platform is demonstrated that can be used to immobilize functional proteins and other bioactive compounds over a biotin-streptavidin approach. This patterning strategy aims specifically at application by cell biologists and biochemists addressing a wide range of relevant pattern sizes, easy pattern generation and adjustment, the use of only biofriendly, nontoxic chemicals, and mild processing conditions during the patterning steps. The retained bioactivity of the fabricated cm(2) area filling multiprotein patterns is demonstrated by showing the interaction of fibroblasts and neurons with multiplexed structures of fibronectin and laminin or laminin and ephrin, respectively.

On-chip microlasers for biomolecular detection via highly localized deposition of a multifunctional phospholipid ink

We report on a novel approach to realize on-chip microlasers, by applying highly localized and material-saving surface functionalization of passive photonic whispering gallery mode microresonators. We apply dip-pen nanolithography on a true three-dimensional structure. We coat solely the light-guiding circumference of pre-fabricated poly(methyl methacrylate) resonators with a multifunctional molecular ink. The functionalization is performed in one single fabrication step and simultaneously provides optical gain as well as molecular binding selectivity. This allows for a direct and flexible realization of on-chip microlasers, which can be utilized as biosensors in optofluidic lab-on-a-chip applications. In a proof-of-concept we show how this highly localized molecule deposition suffices for low-threshold lasing in air and water, and demonstrate the capability of the ink-lasers as biosensors in a biotin-streptavidin binding experiment.

Reactive superhydrophobic surface and its photoinduced disulfide-ene and thiol-ene (bio)functionalization

Reactive superhydrophobic surfaces are highly promising for biotechnological, analytical, sensor, or diagnostic applications but are difficult to realize due to their chemical inertness. In this communication, we report on a photoactive, inscribable, nonwettable, and transparent surface (PAINTS), prepared by polycondensation of trichlorovinylsilane to form thin transparent reactive porous nanofilament on a solid substrate. The PAINTS shows superhydrophobicity and can be conveniently functionalized with the photoclick thiol-ene reaction. In addition, we show for the first time that the PAINTS bearing vinyl groups can be easily modified with disulfides under UV irradiation. The effect of superhydrophobicity of PAINTS on the formation of high-resolution surface patterns has been investigated. The developed reactive superhydrophobic coating can find applications for surface biofunctionalization using abundant thiol or disulfide bearing biomolecules, such as peptides, proteins, or antibodies.

Advances in DNA-directed immobilization

DNA-directed immobilization (DDI) of proteins is a chemically mild and highly efficient method for generating (micro)structured patterns of proteins on surfaces. Twenty years after its initial description, the DDI method has proven its robustness and versatility in numerous applications, ranging from biosensing and biomedical diagnostics, to fundamental studies in biology and medicine on the single-cell level. This review gives a brief summary on technical aspects of the DDI method and illustrates its scope for applications with an emphasis on studies in cell biology.

Structured Polymer Brushes by AFM Lithography

Structured polymer surfaces have gained increased attention in various research fields during the past few years. By structuring of polymer surfaces, in particular of polymer brushes, functional surfaces with defined properties, for example for the study of cell adhesion and of cell alignment, have been prepared. Moreover, patterning of polymer brushes allows the alteration and control of their wetting properties. Patterned polymer brushes can be obtained by different methods including top–down approaches such as electron beam chemical lithography (EBCL), photolithography, dip-pen lithography, and microcontact printing (mCP) and bottom–up methods such as Langmuir–Blodgett (LB) lithography. These methods mainly rely on a ‘‘grafting from’’ approach in which lithography is used to site-selectively initiate a polymerization process or to site-selectively install a polymerization initiator. In the latter case, subsequent polymerization eventually leads to structured polymer brushes. In addition, studies on the mechanical manipulation (nanoscratching/nanoshaving) of polymer films prepared by spin-coating, as well as selfassembled monolayers (SAMs) and the nanowear of different polymer film architectures, have been performed, and even heated atomic force microscopy (AFM) tips have been used for lithographic processes on polymer films. Herein, we present an alternative approach to obtain structured polymer brushes by mechanical nanoscratching

Selective Adsorption of DNA on Chiral Surfaces: Supercoiled or Relaxed Conformation

The right fit: Plasmid DNA molecules show chirality-dependent interaction with gold surfaces modified by L and D N-isobutyrylcysteine. Relaxed DNA molecules have a stronger interaction and adsorption on the L surface, while their counterparts on the D surface maintain a supercoiled conformation, indicating a weak interaction (see picture).

Apertureless cantilever-free pen arrays for scanning photochemical printing.

A novel, apertureless, cantilever-free pen array can be used for dual scanning photochemical and molecular printing. Serial writing with light is enabled by combining self-focusing pyramidal pens with an opaque backing between pens. The elastomeric pens also afford force-tuned illumination and simultaneous delivery of materials and optical energy. These attributes make the technique a promising candidate for maskless high-resolution photopatterning and combinatorial chemistry.