Transferring complementary target DNA from aqueous solutions onto solid surfaces by using affinity microcontact printing

Modification of surfaces by chemical transfer printing using chemically patterned stamps

The preparation of well-defined molecular monolayers and their patterning on the microscale and nanoscale are key aspects of surface science and chemical nanotechnology. In this article, we describe the modification of amine-functionalized surfaces using a new type of contact printing based on chemically patterned, flat PDMS stamps. The stamps have discrete areas with surface-bond tetrafluorophenol (TFP) groups, which allow the attachment of carboxylic acids in the presence of coupling agents such as diisopropylcarbodiimide (DIC). The generated active esters can be reacted by placing the stamps in contact with amine-functionalized surfaces. The process leads to the transfer of acyl residues from the stamp to the substrate and therefore to a covalent attachment. Patterning occurs because of the fact that reaction and transfer take place only in areas with TFP groups present on the stamp surface. Different types of amine-decorated surfaces were successfully modified, and the transfer was visualized by fluorescence microscopy. To the best of our knowledge, the covalent transfer printing (CTP) of an immobilized molecular monolayer from one surface to another surface is unprecedented.

Transfer printing techniques for materials assembly and micro/nanodevice fabrication

Transfer printing represents a set of techniques for deterministic assembly of micro-and nanomaterials into spatially organized, functional arrangements with two and three-dimensional layouts. Such processes provide versatile routes not only to test structures and vehicles for scientific studies but also to high-performance, heterogeneously integrated functional systems, including those in flexible electronics, three-dimensional and/or curvilinear optoelectronics, and bio-integrated sensing and therapeutic devices. This article summarizes recent advances in a variety of transfer printing techniques, ranging from the mechanics and materials aspects that govern their operation to engineering features of their use in systems with varying levels of complexity. A concluding section presents perspectives on opportunities for basic and applied research, and on emerging use of these methods in high throughput, industrial-scale manufacturing.

Simplest method for creating micropatterned nanostructures on PDMS with UV light

The fabrication of micropatterned structures on PDMS is a critical step in soft lithography, microfluidics, and many other PDMS-based applications. To substitute traditional mold-casting methods, we develop a simple method to create micropatterned nanostructures on PDMS in one step. After exposing a flat PDMS surface to a UV pen lamp through a photomask (such as a TEM grid), micropatterned nanostructures can be formed readily on the PDMS surface. We also demonstrate that fabricated PDMS can be used for the microcontact printing of protein immunoglobulin (IgG) on solid surfaces. This method is probably the simplest method of creating micropatterned nanostructures on PDMS reported so far because it does not need casting, surface coating, or chemical reagents. Only a UV pen lamp and a photomask are required, and this method can be performed under ambient conditions without vacuum. We expect that this method will greatly benefit researchers who use PDMS regularly in various applications such as soft lithography and microfluidics.

Effect of surface free energy on PDMS transfer in microcontact printing and its application to ToF-SIMS to probe surface energies

Although polydimethylsiloxane (PDMS) transfer during microcontact printing (microCP) has been observed in previous reports, which generally focused on only one or a few different substrates, in this work we investigate the extent of PDMS transfer onto a series of surfaces with a wide range of hydrophobicities using an uninked, unpatterned PDMS stamp. These surfaces include clean silicon, clean titanium, clean gold, "dirty" silicon, polystyrene, Teflon, surfaces modified with PEG, amino, dodecyl, and hexadecyl monolayers, and also two loose molecular materials. The PDMS transferred onto planar surfaces is, in general, easily detected by wetting and spectroscopic ellipsometry. More importantly, it is detected by time-of-flight secondary ion mass spectrometry (ToF-SIMS) because of the sensitivity of this technique to PDMS. The effect of surface free energy on PDMS transfer in microcontact printing is investigated, and the relationship between the amount of PDMS in ToF-SIMS spectra and the surface tensions of initial surfaces is revealed. We show that PDMS transfer can be applied as a probe of surface free energies using ToF-SIMS, where PDMS preferentially transfers onto more hydrophilic surface features during stamping, with little being transferred onto very hydrophobic surface features. Multivariate curve resolution (MCR) analysis of the ToF-SIMS image data further confirms and clarifies these results. Our data lend themselves to the hypothesis that it is the free energy of the surface that plays a major role in determining the degree of PDMS transfer during microCP.

Engineering of PDMS surfaces for use in microsystems for capture and isolation of complex and biomedically important proteins: epidermal growth factor receptor as a model system

Elastomers based on poly(dimethylsiloxane) (PDMS) are promising materials for fabrication of a wide range of microanalytical systems due to their mechanical and optical properties and ease of processing. To date, however, quantitative studies that demonstrate reliable and reproducible methods for attachment of binding groups that capture complex receptor proteins of relevance to biomedical applications of PDMS microsystems have not been reported. Herein we describe methods that lead to the reproducible capture of a transmembrane protein, the human epidermal growth factor (EGF) receptor, onto PDMS surfaces presenting covalently immobilized antibodies for EGF receptor, and subsequent isolation of the captured receptor by mechanical transfer of the receptor onto a chemically functionalized surface of a gold film for detection. This result is particularly significant because the physical properties of transmembrane proteins make this class of proteins a difficult one to analyze. We benchmark the performance of antibodies to the human EGF receptor covalently immobilized on PDMS against the performance of the same antibodies physisorbed to conventional surfaces utilized in ELISA assays through the use of EGF receptor that was (32)P-radiolabeled in its autophosphorylation domain. These results reveal that two pan-reactive antibodies for the EGF receptor (clones H11 and 111.6) and one phosphospecific EGF receptor antibody (clone pY1068) capture the receptor on both PDMS and ELISA plates. When using H11 antibody to capture EGF receptor and subsequent treatment with a stripping buffer (NaOH and sodium dodecylsulfate) to isolate the receptor, the signal-to-background obtained using the PDMS surface was 82 : 1, exceeding the signal-to-background measured on the ELISA plate (<48 : 1). We also characterized the isolation of captured EGF receptor by mechanical contact of the PDMS surface with a chemically functionalized gold film. The efficiency of mechanical transfer of the transmembrane protein from the PDMS surface was found to be 75-81%. However, the transfer of non-specifically bound protein was substantially less than 75%, thus leading to the important finding that mechanical transfer of the EGF receptor leads to an approximately four-fold increase in signal-to-background from 20 : 1 to 88 : 1. The signal-to-background obtained following mechanical transfer is also better than that obtained using ELISA plates and stripping buffer (<48 : 1). The EGF receptor is a clinically important protein and the target of numerous anticancer agents and thus these results, when combined, provide guidance for the design of PDMS-based microanalytical systems for the capture and isolation of complex and clinically important transmembrane proteins.

Magnetic assembly of high-density DNA arrays for genomic analyses

A method for rapidly assembling high-density DNA arrays with near-perfect order is described. Photolithography is used to generate a wafer-scale array of microwells in a layer of photoresist on a chemically functionalized glass coverslip. The array is enclosed within a microfluidic device, and a suspension of superparamagnetic microbeads conjugated to DNA molecules is introduced into the chamber. A permanent magnet is used to direct the rapid assembly of the beads into the wells, with each well containing a single bead. These beads are immobilized on the glass surface via affinity binding, and excess beads can be recycled or washed away. Nonspecifically bound beads are removed by dissolving the photoresist. The result is a high-density array of beads with virtually no background. This method can be used to produce protein arrays for chip-based assays and DNA arrays for genotyping or genome sequencing.