Liquid dispensing of ultrasmall volumes using miniaturized probes

Small-scale, high-precision liquid delivery technologies are a welcome innovation in a number of areas. For example, the increasing miniaturization of microarrays for use in genomics and proteomics makes impractical conventional dispensing systems based on pipettes, pin and ring arrangements, and inkjet printing. Microfabricated probes developed for scanning probe microscopy (SPM), on the other hand, represent unique, versatile tools for working at the nanometer scale.

Several techniques exist for liquid dispensing based on SPM-like nanoprobes. The first of these, dip-pen nanolithography (DPN), is based on immersing an SPM probe into molecular solution (the ‘ink’), and then writing molecular patterns with feature sizes as small as several tens of nanometers.¹ An alternative is bioplume technology, which allows larger amounts of ink to be deposited by charging the probe with ink along the probe shaft. Surface structures created in this way, however, have larger dimensions.²

Here we describe a technology called nanoscale dispensing (NADIS) that is based on apertured nanoprobes.^3,4 Hollow probes are loaded with ink, either manually or using a fluidic channel system. When the probe tip makes contact with the substrate, a small amount of liquid is transferred through the aperture to the surface: see Figure 1(left). To control the deposition process, the surface energies of both probe and substrate must be sufficiently controlled.

Figure 1(right and inset) shows a scanning electron micrograph of a NADIS probe with an integrated channel system.⁵ This allows liquid delivery from distant macroscopic reservoirs, thus opening the way to high-volume deposition applications without having to repeatedly reload the probe. Similar developments have been reported for DPN-like systems.⁶

NADIS probes can be incorporated into standard force microscope instruments. Figure 2 shows the result of NADIS deposition of a suspension of nanoparticles. After the solvent has been evaporated, small assemblies of nanoparticles remain on the surface. Whereas the liquid droplets deposited can have volumes as small as attoliters, the resulting nanoparticle structures are even smaller, with dimensions n the 50nm range and below.

Proof-of-concept experiments can be carried out conveniently using single probes, but real-world applications often require greater throughout. Operation of large arrays of probes in parallel has been reported for various technologies. In some cases individual actuation of each probe is possible.⁷ A recent development is the combination of liquid deposition and templated substrate surfaces. Indeed, prestructuring substrates at the nanometer scale, say, via molecular assembly, offers a level of control and optimization of great use in microtechnology and biosensing.⁸

This work was partly supported by the Swiss Federal Office for Education and Science in the framework of the EU-funded project Emerging Nanopatterning Methods NaPa (contract no. NMP4-CT-2003-500120) and the Swiss National Competence Center in Nanosciences. Special thanks go to André Meister as well as the CSEM (Swiss Center for Electronics and Microtechnology) microfabrication team for their valuable contribution.