Article pubs.acs.org/ac
Femtoliter Droplet Handling in Nanofluidic Channels: A Laplace Nanovalve Kazuma Mawatari,† Shogo Kubota,† Yan Xu,† Craig Priest,‡ Rossen Sedev,‡ John Ralston,‡ and Takehiko Kitamori*,† †
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia
‡
ABSTRACT: Analytical technologies of ultrasmall volume liquid, in particular femtoliter to attoliter liquid, is essential for single-cell and single-molecule analysis, which is becoming highly important in biology and medical diagnosis. Nanofluidic chips will be a powerful tool to realize chemical processes for such a small volume sample. However, a technical challenge exists in fluidic control, which is femtoliter to attoliter liquid generation in air and handling for further chemical analysis. Integrating mechanical valves fabricated by MEMS (microelectric mechanical systems) technology into nanofluidic channels is difficult. Here, we propose a nonmechanical valve, which is a Laplace nanovalve. For this purpose, a nanopillar array was embedded in a nanochannel using a twostep electron beam lithography and dry-etching process. The nanostructure allowed precise wettability patterning with a resolution below 100 nm, which was difficult by photochemical wettability patterning due to the optical diffraction. The basic principle of the Laplace nanovalve was verified, and a 1.7 fL droplet (water in air) was successfully generated and handled for the first time.
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approach provides the platform for the next generation of analytical tools in cell biology and medical diagnosis. In order to develop such analytical devices, highly precise fluidic control is one of the key tools. More specifically, the generation and handling of attoliter to femtoliter amounts of liquid is essential in the extended nanospace. The conventional approach mainly uses a two-phase water/organic flow to make small droplets, as demonstrated in both microspace and extended nanospace.8−11 However, with the very high surface-to-volume ratio and the ultrasmall volume for femtoliter to attoliter droplets, a multiphase flow (e.g., water droplets in an organic solvent) will not be appropriate due to the potential cross-contamination of the respective liquids involved. An array of femtoliter chambers was proposed as a detection device for single molecules. However, the array must be sealed with a cover plate,12 and further chemical operations (mixing, reaction, detection, etc.) are usually difficult. Other than the micro/nanofluidic chip technology, a piezoelectric injection system has been developed and can eject single femtoliter droplets in air.13 However, the droplet evaporates soon after the ejection due to the high surface-to-volume ratio, and further chemical operations are again difficult. Moreover, the size of the droplet distributes in the range of 0−50 fL, and precise size
ver the last few decades, microfluidic systems known as lab-on-a-chip or micro-total analysis systems (μ-TAS), have been developed as tools for chemical analysis, synthesis, biology and other applications, and technologies. They take advantage of the small volume of liquid involved and the related rapid processing, high functionalization, device miniaturization, and ease of operation.1 Recently, microchemical systems on a chip are being further downscaled to 10−1000 nm, which we designate as “extended nanospace” in order to emphasize the difference from conventional nanotechnology dealing with scales in the 0.1−10 nm range. In this extended nanospace, new analytical devices have been developed involving ultrasmall volumes, extremely high surface-to-volume ratios, and minimum sizes accessible by top-down fabrication technology.2−4 For example, liquid chromatography was integrated in the extended nanospace, where femtoliter to attoliter volume samples were injected and successfully separated. In comparison with conventional liquid chromatography, much shorter separation times (down to several seconds) and higher separation resolution (7 000 000 plates/m) have been achieved.5,6 For the injection, a flow-control by air pressure system was developed, which allowed MPa pressure control with a 10 ms time resolution.7 Because the typical volume of a nanochannel is between an attoliter and a femtoliter [i.e., several orders of magnitude smaller than the volume of a single cell (∼1 pL)], various ultrasensitive analyses for single-cell fluids and single molecules are possible. The nanofluidic © 2012 American Chemical Society
Received: October 4, 2012 Accepted: November 29, 2012 Published: December 7, 2012 10812
dx.doi.org/10.1021/ac3028905 | Anal. Chem. 2012, 84, 10812−10816
Analytical Chemistry
Article
control at the femtoliter level is currently difficult. A nanofluidic chip provides a closed system and has lower vapor pressure than the bulk liquid due to capillary condensation in the nanoscale space, when the droplets are surrounded by wetting surfaces (e.g., silica).14 Furthermore, various chemical operations can be included by suitably designing the channel, and the droplet can be processed chemically. Therefore, nanofluidic devices are suitable for generating femtoliter to attoliter droplets and also for their chemical processing. To design a valve (either mechanical or nonmechanical) that would work in a nanochannel is a major challenge. A number of miniaturized mechanical valves were fabricated using microelectric mechanical systems (MEMS) technology in the microspace,15 but conventional mechanical valves are difficult to integrate into the nanospace due to the small size. A nonmechanical valve based on a wettability boundary is a better option. Such a Laplace valve was previously used in the microspace using a localized photochemical reaction.16 The extended nanospace, however, is several orders of magnitude smaller and actually smaller than the optical diffraction limit. Thus, creating the wettability boundary with a photochemical reaction could not be done with the resolution needed. In this study, we propose a new wettability patterning method which combines nanofabrication and surface modification (Figure 1). Nanopillars are fabricated at the bottom of
an extended nanochannel, and then the whole channel is made hydrophobic by chemisorption. A sharp wettability boundary (spatial resolution
