1. M. Meyyappan Director, Center for Nanotechnology NASA Ames Research Center Moffett Field, CA 94035 http://www.ipt.arc.nasa.gov

2. Jun Li Cattien Nguyen Lance Delzeit Hou Tee Ng Kris Matthews Bin Chen Ramsey Stevens Jing Li Martin Cinke Alan Cassell Jie Han

3. HOGPGlassy Carbon Carbon Fiber Carbon Black Edge Plane: electron transfer rate ~ 0.1 cm/s Basal Plane electron transfer rate < 10 -7 cm/s R. L. McCreery, in Electroanalytical Chemistry, Ed. A. J. Bard, 17, 221-374 (1991). HOPG

4. Macro-, Micro-, and Nano- Electrodes GC 2mm dia. Carbon fiber 7  m dia. Semi-infinite planar linear diffusion Semi-infinite hemispherical diffusion: Current exhibits a steady state I total = I planar + I radial Nanoelectrodes ?

5. Carbon Nanotube Electrodes Previous Work P. M. Ajayan et al, Nano Lett., 1(2), 87(2001). MWCNT bundle Dia. 600  m, length ~1.5 mm MWCNT bundle Dia. ~200 nm, length ~30  m R. M. Crooks et al, J. Am. Chem. Soc., 121, 3779 (1999).

6. The Fabrication of CNT Nanoelectrode Array (1) Growth of Vertically Aligned CNT Array (2) Dielectric Encapsulation (3) Planarization (4) Electrical Property Characterization By Current-sensing AFM (5) Electrochemical Characterization we re ce Potentiosta t

7. Growth of CNT Array with Varied Density (a) (b) Criteria: (1)Good conductivity in metal underlayer, contact points, and CNTs (2)Good alignment (3)Controlled density

8. Dielectric (SiO 2 ) Encapsulation By TEOS CVD Top View Side View (30º tilt) SiO 2 CNTs

9. Fabrication of CNT Nanoelectrodes 45 degree perspective view Top view Side view after encapsulation Top view after planarization J. Li et al, Appl. Phys. Lett., 81(5), 910 (2002)

10. Characterization of Electrical Properties nA Topography Current Image Cross-section Profile

11. Electrical Properties of CNTs -5V 0 +5V +1mA -1mA 0 HP analyzer Current Sensing AFMFour-probe station And HP parameter analyzer

12. Electrochemical Properties of CNT Nanoelectrode Array CV of 1 mM K 4 Fe(CN) 6 in 1.0 M KCl at 20 mV/s As-prepared electrode  Ep > 600 mV After pretreatment  Ep = ~100 mV

13. Nanoelectrode Array at Different Densities CNT coverage: ~ 20% Average nearest-neighbor distance: ~200-300 nm CNT coverage: < 5% Average nearest-neighbor distance: > 1000 nm

14. Nanoelectrode Array at Different Densities High CNT coverage: ~ 20% Similar to a macro- planar electrode CNT coverage: < 1 % Approaching nanoelectrode CV of 1 mM K 4 Fe(CN) 6 in 1.0 M KCl at 20 mV/s

15. Chemical Functionalization Highly selective reaction of primary amine with surface –COOH group

16. Chemically Attached Fc Derivatives Cyclic voltammetry at 20 mV/s  Ep = ~30 mV  surface adsorbed species  quasi-reversible due to defects in CNTs AC voltammetry  enhanced sensitivity

17. Functionalization of DNA Cy3 image Cy5 image

18. 3+ 2+ e 3+ 2+ CNT DNA Sensor Using Electrochemical Detection  MWNT array electrode functionalized with DNA/PNA probe as an ultrasensitive sensor for detecting the hybridization of target DNA/RNA from the sample. Signal from redox bases in the excess DNA single strands  The signal can be amplified with metal ion mediator oxidation catalyzed by Guanine.

19. Electrochemical Detection of DNA Hybridization 1 st, 2 nd, and 3 rd cycle in cyclic voltammetry 1 st – 2 nd scan: mainly DNA signal 2 nd – 3 rd scan: Background

20. Technical Platform Top View Side View Each individual electrode is electronically addressable with an array of CNTs with d ~ 10 to 100 nm d nn ~ 500 nm to 5000 nm Each Electrode Immobilized with A Specific PNA or DNA Probes 10 to 200  m

21. Coupling to Cy3-labeled DNA Nanotubes with spin-on glass film 1. EDC/Sulfo-NHSH 2 O Washed 60 o C/1 hr. *DNA = H 2 N(CH 2 ) 6 -ACACGAGTCAGCGCAGCCATCGC-Cy3 2. DNA*

22. Hybridization Test EDC/Sulfo-NHS PNA* H 2 O Washed 60 o C/1 hr. c-DNA-(Cy3-labeled) # *PNA = H 2 N(CH 2 ) 6 -GCCGATGCACC # c-DNA = CGGTACGTGG-Cy3 Hybridization Washes Nanotubes with Spin-on Glass

26. Conclusions Vertically aligned CNTs can be used to fabricate nanoelectrode array. The electrical and electrochemical properties of such nanoelectrode array have been thoroughly characterized. Chemical functionalization has been demonstrated to be highly selective at CNT ends. CNT nanoelectrode array has potential applications as highly sensitive DNA sensors