February 16, 2009Chung-Ja Yang
| Chung-Ja Yang - Mesoscopic Concentration Fluctuations in a Fluidic Nanocavity Detected by Redox Cycling
M.A.G. Zevenbergen, D. Krapf, M.R.Zuiddam, and S.G. Lemay
Presented by C.J. Yang
| | | | Summary:
They have developed a nanofluidic device consisting of two planar electrodes separated about 282nm that achieves highly sensitive electrical detection of electroactive ions. Redox cycling permits a ~400 amplification of the electrochemical current. The concentration dependence of the fluctuations in this electrical signal, their high frequency f-3/2 behavior and their low frequency amplitude demonstrate that the device acts as a direct probe of the diffusive motion of independent molecules.
Possibility (motivation)
Electrochemically active molecules are amenable to electrical detection because they can transfer charge to appropriately biased
Obstacle of the application electric detection
Involved only one or a few electrons per reacting molecule, rendering the direct detection of a single molecule in solution virtually impossible.
Be able to Overcome with redox cycling- in which multiple electrodes are used to repeatedly flip the charge state of target molecules and thus allow each molecule to contribute multiple electrons to the measured current.
Ex) Dual-band or interdigitated electrode microfluidic sensors have employed this concept.-1
An amplification factor of 40 was reported for an interdigitated nanoelectrode array with 30nm spacing. 3-4
Clue of application of the electrochemical rxn:
They detected fluctuations of as few as ~70 molecules by considering the diffusive motion of independent molecules in and out of the cavity. Redox-active molecules diffusing in the cavity between the two electrodes sequentially shuttle multiple electrons, leading to a dramatic amplification of the electrochemical current.
Comparison with C.J.’s project
Positive: they demonstrated electrochemical rxn method’s redox molecules by a factor of 400 via redox cycling
Distinguished aspects: bio-oil/18Mohm cm Milli-Q water with 250 mM added KCl (extent of the electroactivity), metal electrodes: Iron, Ni, Al/ Pt, electric charge(both metal and solution), current-voltage¤t-time traces are recorded(bipotentiostat).
Interpretation of figures
Fig.1 shows the device and one electrode serves as reactant at the other.
Fig. 2a- c.The device fabrication process: A 30nm thick Pt layer(leading to macroscopic contact pads)& holes. NO oxygen added, isotropic.
Fig. 3 current-voltage : No redox cycling under a condition that potential applied to bottom is varied and top was electrically floating(dotted line), while a potential of 0.1V was applied, bottom electrode increase by a factor of ~400(solid line), and simultaneous measured current (dashed line).
Q. Where did they use Ferrocenedimethanol (Fc(CH2OH2) aqueous solution?
Q. The reason they got the cavity first wetted with ethanol, which was gradually replaced with aqueous solution?
S.Sdiffusion-limited current:
Where LA2= the area of overlap between the two electrodes (100μm2)
Av= Avogadro’s number
C0= the bulk redox-active ion concentration
Z= the distance between the electrodes
= the average number of particles in the active region of the device (zLA2Avc0)
τ= the mean time to diffuse from one electrode to the other (z2/2D)
Fig. 4a: S.S. diffusion-limited current vs electroactive ion concentration c0 –linear fit
i(t) provides a direct probe of the number of redox ions N(t) in the cavity, and fluctuations in i(t) reflect number fluctuations due to diffusive motion along the length of the channel.
4b: Three concentrations are shown. This means the noise amplitude increases with increasing number of ions in the cavity.
4c: shows the corresponding power spectral densities S(f), a faster decay is observed above 10Hz due to the finite bandwidth of the measurement electronics.- fit to the eq.(2)
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