A typical experiment in a "H-structure" is to pump two different solutions in Hamilton syringes, which are connected to the chip inlets: one solution contained just buffer solution and the other Rhodamine-6G-labeled single stranded DNA of length 21nt (Rh6G-GATGGTCACAGCATGTCTGTS). A pump rate chosen between 0 and 2 µl/hr was maintained stably in the supply and drain channels. The buffer solution employed was a freshly prepared histidine buffer (50 mM, 59 µS/cm) to minimize the electrostatic screening effects (Debye length [1]).
The figure series a-k showing a complete cycle of filling and draining on electronically programmable cell filling using the R6G-labelled DNA flowing through the upper horizontal channel of the "H-structure" and excited with laser-light at 488nm.
Four samples (a-d) at different times of a micro-compartment filling experiment are shown. At the beginning, no fluorescence could be observed in the reaction chamber.Starting at a concentration of 5x10-7 M, the filling process was stopped at an approximately tenfold concentration and the washed out very quickly, compared to channel and chamber dimensions.
Two different physical effects are contributing to filling of the reaction-chamber: electrophoresis (positive electrode, 3.3V) attracts the negatively charged DNA and probably electroosmotic-flow (EOF) which drags fluid from the upper supply-channel against the weak background flow from the lower drain-channel upwards. The two field effects, EOF and electrophoresis, help each other in this case. The negatively charge DNA is attracted from the supply-channel and in addition a net EOF flow is generated which pumps fluid from the supply-channel towards the drain-channel. On the other hand, the electrical field is polarized in such a way that the DNA is repelled from the lower electrode and attracted by the upper electrode. This results in the observed increase of concentration of DNA, (see also figure series a-c and the corresponding movie). Note also the times involved with this filling procedure. Even after more the 200 seconds no electrolysis was observed.
The second task was to drain the reaction chamber again. Two possible draining directions are available – transport the material back into the supply-channel (see the related movie) or into the drain channel (figure series h-k). [2]
REFERENCES
[1] C.-Y. Lin, L.-M. Fu, K.-H. Lee, R.-J. Yang and G.-B. Lee. "Novel surface modification methods and surface property analysis for separation of DNA biomolecules using capillary electrophoresis" in micro-TAS, pp. 1081-1084, Squaw Valley, California USA, 2003.