SWCNT Trap/Release/Enrich via 3-Electrode, HPLC Experiments Rick Rajter & Steve Lustig

SWCNT Trap/Release/Enrichvia 3-Electrode, HPLC Experiments

Rick Rajter & Steve Lustig

Historical Background• Dupont-MIT Alliance (DMA) had many concurrent experiments for the

design, placement, and creation of SWCNT devices.

• Anion IEC experiments (Ming Zheng) were perhaps most significant. They helped solve the nagging issues of metal/semi separation.

• DMA program cut in summer 05 for variety of economic reasons.

• Ming continued his experiments on the side. I attempted a last ditch effort to resurrect a portion of the program by working on enrichment experiments with funding from Yet Chiang.

• A very favorable meeting with Steve Lustig resulted in an ongoing series of experiments that eventually continued under the NIRT grant. What follows is the end-results of this endeavor.

Rajter, Lustig — NIRT 05/06/08

Experiment Design Considerations• Anion IEC experiments were Ming’s domain and we certainly weren’t

going to out do him there! Wanted to try something different.

• Once DMA funding was cut, we needed a method which cheap, has a short run-time, and would require very minimal quantities of sample.

• Ideally we wanted to be able to explore/control more parameters instead of just salt concentration.

• Eventual automation/scalability was considered.

• Immediate goal was to prove the concept before spending too much time, money, and effort on more advanced experimental setups.


Our Strategy• Move from a packed array of charged beads to a charged surface.

This would prevent clogging; expand the allowable SWCNT length.

• Control the surface potential via 3-electrode system vs having a fixed surface charge.

• Variations we can now play:– Surface charge density (vs time)

– Salt Concentration (vs time)

– Flow rate (vs time)

– Surface coatings on working electrode

– Salt concentration (vs time)

• Potentially have a lot more control and parameter space to explore.


Comparison of Methods


Anion IEC 3 Electrode, HPLC

SWCNT Length Restrictions

500-1000 nm HPLC tube diameter

(~0.1 mm)

Controllable Parameters Salt, Flow Rate, Bead composition

Salt, Surface Potential, Flow Rate, Surface Coatings, Geometry

Costs IEC Chambers, lost sample in clogging.

Initial flow cell design, wafer replacement, reference electrode.

Enrichment Excellent Signs of enrichment

System


System Description• Modified ICMFG QCM flow cell to attach 3 electrodes to potentiostat.

• HPLC controls injection time/size, flow rate.

• Diode array detector (DAD) records the optical absorption spectra of the outflow line to determine when the sample ejects.

• Fractionator collects ejected sample based on time, DAD, or both.

• Computer controls the HPLC + Poteniostat + QCM and logs data.


Typical Experimental Parameters• 1-5 uL of A4 HiPCo SWCNTs injected

• 10-25 uL/min flow rate.

• Surface potential held -900/900mV vs Ag/AgCl for 4/4 minutes. (That is, at the 4 minute mark, potential is stepped from -900 to 900 mV and held an additional 4 minutes).

• Working electrode is either a gold QCM or gold coated glass slide.

• Total experiment run time can vary from 7-15 minutes.


Sanity Checks• You may have noticed that a typical experiment begins with a

NEGATIVE surface potential to trap and ejects the SWCNTs with a POSTIVIVE surface potential.

• For this and many other reasons, we had to continually and methodically test every stage and counter examples to make sure the system was behaving and the results were legitimate.

• Or findings thus far show that the results are in fact working as we describe, but there are still some conceptual things to figure out in explaining why they occur in such a manner.


Benchmarking• Benchmarking the HiPCo, CoMoCat, and other samples is a

straightforward process to determine the typical absorption peak heights of the chiralities in a given sample.

• DAD detects from 200-950 nm in up to 0.25 second intervals. Here we see it between 400-950 (ignoring ssDNA signals)


Trap Release• -900/900 mV @ 25 uL/min for 4/4 minute holds.


Trap Release Sanity Checking• To ensure we were not getting plating/trapping on the platinum

counter electrode, we ran a series of experiments with CuSO4.

• When run at -900/900mV vs Ag/AgCl, we clearly get electroplating of Cu+ on the working electrode and this is detectable via a slowing of the QCM. We can also see the CuSO4 leaving via DAD signals.

• When we try the reverse (+900/-900) we never see a detectable release of Cu ions back into solution… implying no deposition is occuring on the counter electrode.

• The counter electrode is recessed outside of the flow cell volume, so it is less likely to be participating in any near field trapping.


Fast Flow• The next variation is to see if we can improve resolution/separation

by speeding up the flow rate after SWCNTs trap.

• By increasing the flow rate, we decrease experiment run time while increasing the spacing between released chiralities.


Fast Flow Rates Continued…• A fast flow rate can also be used to strip off weakly held SWCNTs. • Note in the figure below how once we turned up the speed, we saw

another blip of sample making it to the DAD.• It was only upon the step voltage that that remaining SWCNTs left.


Ramping Voltage• Releasing all at once really limits our ability to try any sort of

enrichment because it assumes enrichment can only occur due to selective trapping instead of selective releasing.

• If we linearly vary the voltage from attractive to repulsive, we should mimic the working mechanism from the Zheng experiments by sequentually releasing based on binding energy.


Ramping Voltage - Caveats• Ramping was initially the hardest experiment because it had to be

done manually. Recently we obtained software to automate this.

• The biggest 2 issues with ramping are currently:– Reference electrode stability/drift (may need more salt in solvent)

– Ejection of SWCNT appears to require rapid changes in potential or contain transient effects.

• Nevertheless, our previous slide showed an example where the material clearly began to elute/eject at a surface potential of ~ +500 to +600mV vs Ag/AgCl.

• Ideally we could quickly ramp to ~ 400 mV and then slowly go through the sensitive area (from +500 to +700) and have a finer resolution…


Signs of Enrichment• Although ramp experiments appear to be the most likely candidate for

enrichment, our best results have occurred during voltage steps.

• Here we see an example in which the “missed” sample and ejected sample have a very pronounced differences!


Signs of Enrichment 2• We do have examples involving ramp experiments, but we haven’t

played enough to really find the “sweet spot”.• While most experiments have identical relative peak heights, we do

get cases where we see a notable shift in a particular area.• Let us not forget that the Zheng experiments really succeeded once

the correct ssDNA sequence was found… this can be equally true here when the right parameters are found.


Other Variations Tried• It is hard to go into detail on every variation tried, but here are some

global/generalized themes.

– NaSCN concentration variation while surface charge held.

– Voltage magnitude variation

– Ramp rate variation

– Surface cleaning procedures

– Surface functionalization (SAMs, glass w/amine groups instead, etc)

– Flow rate variation

– Chitosan wrapped tubes

– ITO metal working electrodes

– Au w/polyethylene coatings

• This simple flow cell design offers a wealth of possibilities… and would benefit greatly from a more exploration now that a path has been cleared.


Misc Discussion• Most of the heavy lifting has been done as the system is ready to go,

the flow cell is there, and the software can automate a lot.

• Parameter optimization and/or trial/error will be what determines if the matches or exceeds the results of the initial Zheng experiments.

• Charge reversal is still a puzzling phenomenon. Later experiments showed that the same ITO wafer could trap and release in both directions during a continuous run. That is, -900/900 and 900/-900 both trapped and released.

• Standard electrochemical texts/professors do not really help in explaining this system. Some discussion/theory is needed to determing what the controlling mechanism is (counter ions, fluid mech boundary layer, transients, etc).


Misc Discussion• New flow cell designs may be able to greatly improve upon our

current system, particularly by increasing surface area to volume.

• There is a balance though, a new flow cell design may take some more time to setup and run, but that may be what’s needed to take things to the next level.

• Monolayer coatings still seems like an exciting area to me, in which one could put a competing “repulsive” layer down at near contact and make the tubes “hover” much like the Zheng alignment experiments.


Conclusions• A new variation upon the Zheng experiments has been successfully

setup, tested, and to be patented by Dupont.

• First known usage of HPLC, potentiostat, QCM, and fractionator. Lots of control and possibilities are there for this and other experiments.

• Signs of enrichment are clearly present.

• Need further help in parameter optimization to ensure the highest enrichment possible and experiment repeatability.

• There is plenty of parameter space to explore, both in new cell design, surface preparation, surface potentials, etc.


Acknowledgements• Steve Lustig: Providing me inspiration, lab space, a bed, and a new

family to stay with during what would otherwise be an isolated time.

• Dupont for allowing me on board as a visiting scientist when the DMA project was clearly over.

• Ming Zheng for graciously providing sample that made these experiments a possibility.

• NIRT for continued support (financially and intellectually)


Questions?


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SWCNT Trap/Release/Enrich via 3-Electrode, HPLC Experiments Rick Rajter & Steve Lustig