Investigations On The Voltage-current Properties Of DNA Molecules Solution And Buffer Solution In Micro/nanochannels

Micro/nanofluidic technology is an effective tool for manipulation and analysis of biomacromolecule in life science and applications.In micro/nanofluidic devices,electrokinetic（EK）mechanism is widely used in the control of flow and particles and the detection of analytes.Based on the three-electrode microdetection system,the current signal characteristics of different solutions,e.g.buffer solutions,metal ion solutions,λ-DNA solutions with or without metal ion,are systematically studied in micro/nanochannels.The transport phenomena ofλ-DNA and biological macromolecules in microchannels were experimentally studied.In the meanwhile,we experimentally and theoretically investigated the interfacial phenomena in electroosmotic flow in nanochannel.The main contents and conclusions of this article are as following:（1）The electric properties ofλ-DNA solutions with/without metal ions(i.e.,Na⁺,K⁺,Mg²⁺and Ca²⁺)are experimentally studied in a 5μm microcapillary.By studying the conductance of solutions,we found that the electric conductance of metal ions solutions in the microcapillary is different from their counterpart in the bulk solution,although the ion concentrations are the same.In the microcapillary,the conductivity of the sodium ion solution is the highest,and the calcium ion solution which has the lowest conductivity.The conductance relationship can be as σ_Na⁺＞σ_K⁺＞σ_Mg²⁺＞σ_Ca²⁺.However,in the bulk solution,the calcium ion solution has the highest conductivity,and the sodium ion solution.The conductance relationship can be described as σ_Na⁺<σ_K⁺<σ_Mg²⁺<σ_Ca²⁺.By studying the voltage-current（–）characteristic curve,we found when the metal ion solutions were filled in the microcapillary,the–relations were linear or segmentally linear.For K⁺,Mg²⁺and Ca²⁺solution,we found there existed critical voltages where a jump of electric conductance could be found.But for Na⁺,this phenomenon was not observed.The–curves of the salt solutions withoutλ-DNA behaves similarly to that of biased P-N junction diode.In the solution of λ-DNA,regardless of adding metal ions or not,the–curve is always nonlinear.When the metal ions are mixed withλ-DNA solutions,the electric properties ofλ-DNA solutions with monovalent and divalent metal ions show divergent variations.As the concentration of metal ions is increased,the–curves ofλ-DNA solutions with monovalent metal ions become more bending,while those of divalent metal ions become flatter.Theof theλ-DNA solutions with metal ions can sometimes increase faster withthan that without metal ions,and there exists an intersection of the–curves ofλ-DNA solutions with and without metal ions,resulting in a interaction voltage,i.e.₄).In the monovalent ion solution,₄)decreases with ion concentration.While in divalent ion solution,it can be predicted that₄)increases with ion concentration increases.The nonlinear–curves can be attributed to the so-called excess ion polarizability.Here,we developed a power-law model with exponent6)to evaluate the sensitivity of varying polarization to the applied voltage.It was found,in theλ-DNA solution with monovalent ions,6)increased with the ion concentration.In contrast,for theλ-DNA solution with divalent ions,6)decreased with increasing ion concentration.Meanwhile,the reason for the different behavior of–curves in theλ-DNA solutions with metal ions was analyzed.We tend to believe that the flatter–curves in theλ-DNA solutions with divalent metal ions is caused by the ion-hydrate shell around DNA molecules,which decreases the electrophoresis mobility of charged particles.This investigation is particularly helpful for the precise manipulation of individual DNA molecules in micro/nanofluidics and the design of devices and sensors relies on electrophoresis.It will provide indepth insight into the structure and dynamics of DNA molecules in electrokinetic applications.（2）The experimental and theoretical studies on the abnormally rheological phenomena in nanocapillary with a diameter of 200 nm have been carried out.We observed in the DC electroosmotic flows of the 200 nm nanochannel,when the electric field intensity was below a critical value of 6.7 V/m,the TBE buffer solution which is generally considered to be Newtonian fluid in the nanochannel can become non-Newtonian fluid near the wall.And the fluids exhibited dilatant（shear thickening）effects is exhibited.This effect causes the fluid viscosity changed with electric field intensity.Futher,the near-wall shear rate of an electroosmotic flow changes with electric field intensity via a power-law relation represented.In solutions with different concentrations and pH values,the flow behaviour indices of the power-law fluids is different.When the electric field intensity surpasses the critical electric field,the fluid again becomes Newtonian and has a constant viscosity.The investigation shows that in nanocapillaries,fluids commonly believed to be Newtonian can become non-Newtonian near walls,which has important physical meaning for understanding the fluid interface properties.At the same time,it has important practical significance for understanding the nanoscale flow dynamics and revealing the wall slip in nanochannels.Since the fluid transport of nanochannels plays a fundamental role in life phenomena（eg,ion transport,electrophysiology,etc.）,this research will also promote further understanding on the corresponding mechanisms in life science.