Experimental Methods for Nanofluidics—Focus on Sealing Technology for Delicate Nanomaterials

Water is essential to live organisms and human societies as the primordial solvent of life. The primordial role of water can be related to its unique physical properties particularly due to the structure of its hydrogen bond network or its dipolar moment. In extreme confinement situations, such as inside nanoscale channels, important modifications of these features are expected. Thus, many theoretical works predicted that confined water in extremely small diameter pores exhibits both unusual phase behavior[1] and structure, compared to bulk water. Thanks to their stiffness, hydrophobic smooth surface, and high aspect ratio, Single Wall Carbon Nanotubes (SWCNT) can be considered as a model nanochannel to get insight into the confinement of water molecules. For instance, it was calculated that the phase of confined water in SWCNT whose diameter is smaller than 1.4nm, behaves as ice-like water with structures from single-file water chains to pentagonal or hexagonal structures depending on the SWCNT diameter [2]. On the other hand, it has been shown from experimental [3] and theoretical [4] calculations that water diffusion in SWCNT is significantly enhanced. Expanding our fundamental understanding of water properties when it is confined in SWCNTs can yield substantial progress in desalination, drug delivery, energy harvesting, etc. applications. Although there are many numerical simulations reported on this system, only a few experimental works have been achieved and it remains a challenging task to get experimental proof of water behavior in such a confined environment.<br/>One of the main challenges to experimentally measuring the properties of nanoconfined water is to be able to fabricate fully sealed microchips. Sealing is a central process for micro and nanofluidic chip fabrication, yet only a few materials can be used, as they must be chemically inert, vacuum compatible, resistant to temperature change, and must have no electrical effect.<br/>In this presentation, I will present a new sealing technology based on SU8 epoxy resist. We show that our bonding method is reliable and versatile for microfluidic, as it can be patterned by photolithography down to micrometric dimensions. It is thus found that a 30 µm high and 20 µm thick wall made of SU8 can sustain relatively pressures up to ~5 bars. We also measured ions permeation through the SU8 walls and found that it is similar or even better to PDMS. The electrical test shows no significant perturbation of electrical devices down to the sensitivity of our measurement set-up, i.e. pA, which makes the SU8 wall resistivity several orders of magnitude higher than that of carbon nanotubes. In addition, our sealing technology turned out to be chemically stable even at high temperatures. Importantly, it does not require plasma activation (contrary to PDMS), therefore it is perfectly suited for the fabrication of delicate nanochannels such as carbon nanotubes.<br/>I will discuss ongoing work to distinguish the effect of water confined inside of the individual single-wall NT based on analyzing the change of the conductance shape. Our preliminary data show that water confined inside the NT mainly influences the gate hysteresis (charge transfer or dopping) of NT, rater that its conductance or the carriers mobilities.<br/><br/>1. Takaiwa D, Hatano I, Koga K, Tanaka H. Phase diagram of water in carbon nanotubes. <i>Proc Natl Acad Sci</i>. 2008;105(1):39-43. doi:10.1073/pnas.0707917105<br/>2. Pascal TA, Goddard WA, Jung Y. Entropy and the driving force for the filling of carbon nanotubes with water. <i>Proc Natl Acad Sci</i>. 2011;108(29):11794-11798. doi:10.1073/pnas.1108073108<br/>3. Holt JK, Park HG, Wang Y, et al. Sub – 2-Nanometer Carbon Nanotubes. 2006;312(May):1034-1038.<br/>4. Thomas JA, McGaughey AJH. Water flow in carbon nanotubes: Transition to subcontinuum transport. <i>Phys Rev Lett</i>. 2009;102(18):1-4. doi:10.1103/PhysRevLett.102.184502