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Micronit has set a new mark for nanochannel etching. It has now fabricated nanochannels with a depth of only 10nm to 30nm connected to microscale reservoirs in a sealed device.
To study the behaviour of molecules in confinement Micronit has made a series of microfluidic chips with nanochannels. The nanochannel chips were commisioned by the Interdisciplinary Research Centre on Biomaterials (CRIB) and the University Federico II.
subdiffusive molecular motion in nanochannels observed by fluorescence correlation spectroscopy
Ilaria De Santo, Filippo Causa and Paolo A. Netti
Interdisciplinary Research Centre on Biomaterials (CRIB), University Federico II, Piazzale Tecchio 80, 80125, Naples, Italy, Italian Institute of Technology (IIT), Via Morego, 30 Genoa, Italy, and Department of Experimental and Clinical Medicine, University Magna Graecia, Germaneto, 88100, Catanzaro, Italy
Analytical Chemistry, 2010, 82 (3), pp 997-1005
Abstract
The influence of confinement on biomolecule motion in glass channels of nanometric height has been investigated with fluorescence correlation spectroscopy (FCS). We measured intrachannel molecule diffusion time and concentration based on a single-component diffusion model as a function of molecule size to channel height (rg/h). Poly(ethylene glycol) (PEG) of 20 kDa and dextran of 40 kDa showed a reduction of their diffusion coefficients of almost 1 order of magnitude when nanochannel height approached probe diameter, whereas rhodamine 6G (Rh6G) was shown to be almost unaffected from confinement. Subdiffusive motion has been proven for flexible molecules in nanochannels, and deviations toward a square root dependence of mobility with time for confinement up to molecule size rg/h 0.5 were registered. Diffusion coefficient time dependence has been evaluated and described with a model that accounts for diffusion time increase due to molecule rearrangements related to molecule flexibility and surface interactions dynamics. The evaluation of the subdiffusive mode and the key parameters extracted at the single-molecule level of partitioning, intrachannel diffusion time, desorption time, and binding probability at surfaces can be exploited for the engineering of bioanalytic nanodevices.
In a previous project Micronit developed chips with nanofluidic channels (depth down to 100 nm) for Delft University. These chips were used to study the effect of charge inversion in such small channel structures.
A pressure-driven flow inside a nanofluidic channel carries with it the electrical counter charges that are induced by the charged channel walls. The electrical current that is generated in this way is called a streaming current, and can be used to deduce the effective charge density of a surface [1].
Charge Inversion
We used the method of streaming currents to study the effect of charge inversion in rectangular silica nanochannels [2]. Charge inversion is the phenomenon that the effective surface charge flips sign due to excessive accumulation of counter ions in the Stern layer close to a charged surface. This effect has been suggested to facilitate the attraction between like-charged objects, and as such can be biologically relevant in, e.g., DNA condensation, viral packaging and drug delivery. Charge inversion is known to occur for tri- and tetravalent ions.
We found conclusive evidence that charge inversion also occurs at high concentrations of divalent ions. Moreover, charge inversion by trivalent ions was found to disappear when large concentrations of monovalent salt were added. This observation was explained by a theoretical model.
References
[1] F.H.J. van der Heyden, D. Stein and C. Dekker, Phys. Rev. Lett. 95, 116104 2005)
[2] F.H.J. van der Heyden, D. Stein, K. Besteman, S.G. Lemay and C. Dekker, Phys. Rev. Lett. 96, 224502 (2006)