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Detection of Heavy Metal Ions with Nanojunctions

Detection of Heavy Metal Ions with Nanojunctions. N.J. Tao Arizona State University. - Nanojunction Approach. Why Heavy Metal Ion Sensors?. Heavy metals are toxic dangerous pollutants second only to pesticides in environmental importance Heavy metals are not biodegradable

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Detection of Heavy Metal Ions with Nanojunctions

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  1. Detection of Heavy Metal Ions with Nanojunctions N.J. Tao Arizona State University

  2. - Nanojunction Approach Why Heavy Metal Ion Sensors? • Heavy metals are toxic • dangerous pollutants second only to pesticides in environmental importance • Heavy metals are not biodegradable -accumulate and persist for many years • Current methods fall in two categories: Ex situ: Sensitive but expensive and complicate • In situ:Portable device but less sensitive & reliable • High sensitivity is necessary for early warning

  3. Atomic Scale! The electrodes are separated with an atomic-scale gap, so a few ions can be detected. Nanocontact Sensor Sensitivity:

  4. Spectroscopy: • - Current vs. Bias Voltage • Stripping potential - Similar to Anodic Stripping Technique Specificity: • Deposition potential

  5. Atomic Scale! How to Fabricate Electrodes with Atomic-Scale Separations?

  6. E - - + + Fabrication Method • Anode:Etching delocalized, but • Cathode:Deposition localized at sharpest point, • due to: • Self-focusing – directional growth • Decreasing Gap!

  7. Voltage Divider: • Initially, Rgap >> Rext, Vgap ~ V0full speed deposition. • Finally, Rgap << Rext, Vgap ~ 0deposition terminates. Self-Termination • The gap resistance is determined by Rext.

  8. G0=2e2/h Rext=10 MW Gap width=0.6 nm Rext=1 MW Gap width=0.4 nm Fabrication of an atomic-scale gap

  9. An Array of Atomic-Scale Gaps before after Si3N4 / Au / SiO2 / Si SEM images

  10. Conductance Quantized!Number of metal atoms ~ Conductance in quantum unit (G0=2e2/h). Conductance (2e2/h) Deposition of Metal Ions

  11. EPA limit Detection of Cu ions • Deposition time needed to bridge the gap decreases as the concentration -determining the concentration from the deposition time • Wide dynamic range -from mM to at least nM Electrodes potential: -0.55V In CuSO4 electrolytes

  12. EPA limit Detection of Pb ions • Deposition time decreases as the concentration -similar to Cu ions • 0.1 nM (10 ppt) concentration is reached -further improvement can be made to lower the limit Electrodes potential: -0.85V in Pb(NO3)2 electrolytes

  13. Specificity Cu2+ Pb2+ Conductance (2e2/h) Conductance (2e2/h) -0.90 V -0.65 V -0.55 V -0.15 V Potential Cycling Potential Cycling • Different metals have different deposition & stripping potentials.

  14. Summary A Polymer Nanojunction Sensor: A Nanocontact Sensor: Si

  15. Acknowledgements • Vasant Rajagopalan • Salah Boussaad • John Zhang • Win Ly • Larry Nagahara • Islamshah Almani • Ray Tsui Florida International Univ: Yong Cai

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