Molecular Transport and Fluidic Manipulation in Three Dimensional Integrated Nanofluidic Networks - Nanofluidics: Nanoscience and Nanotechnology - page 68

4.2 EXPERIMENTAL CHARACTERIZATION OF NANOFLUIDIC

FLOW

4.2.1 Surface Charge

The large surface area-to-volume ratio of nanofluidic channels allows fixed
surface charges to exert a large influence on flow behaviour. A clear demonstration of this
is contained in work by Chun and Stroeve, who measured the diffusive transport of two
charged proteins through nanoporous polycarbonate track etched (PCTE) membranes (with
a diameter of 10 nm) modified with charged alkanethiol surface assembled monolayers
(SAMs). 26 PCTE membranes were coated with Au by electroless deposition 27 and
mercaptoundecanoic acid (HS(CH 2 ) 10 COOH) monolayers were assembled on the pore
surfaces. The diffusive fluxes of bovine serum albumin (BSA, pI = 4.7) and bovine
hemoglobin (BHb, pI = 7.0) were then measured as a function of pH. The transmembrane
diffusion of each protein was maximal at their respective pIs, an observation attributed to
the minimization of electrostatic repulsion between the negatively charged pore walls and
the proteins (Figure 4.1). Interestingly, the flux of both proteins is reduced at pH values
below their pI as well as above. The authors hypothesize that while the proteins are
attracted to the nanopore in this regime due to their positive charge, the electrostatic
exclusion of associated negative counterions results in diminished fluxes. Analogous
results were obtained for diffusive transport of tyrosine and phenylalanine, 28 and diffusion
of a series of lectins was also observed to exhibit the same behaviour. 29 Further
experiments were performed in which the membrane surface charge was controlled by
applying electrical potentials between the Au nanocapillary array membrane (NCAM) and
a Ag/AgCl reference. These experiments show that the diffusive flux for highly charged
species (BHb at pH 4.7, z ≈ 14) is reduced in rough proportion to the magnitude of the
applied potential, while fluxes of less charged species (BSA at pH 4.7, z ≈ 2) are
unaffected, 30 providing further evidence for the role of surface charge in controlling fluid
flow on the nanoscale.

Figure 4.1 Flux of bovine hemoglobin (BHb) ( a ) and bovine serum albumin (BSA) ( b ) across a

PCTE/Au/mercaptodecanoic acid membrane as a function of solution pH. Adapted from Chun,
K.Y. and P. Stroeve, Protein transport in nanoporous membranes modified with self-assembled
monolayers of functionalized thiols. Langmuir, 2002, 18 , 4653 and reproduced with permission.