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Research Summary* Chemoselective Nanowire Fuses: Chemically Induced Cleavage and Electrical Detection of Carbon Nanofiber Bridges

* Nanowire Fuses for Biological Detection: An Enzyme-Based Cleavage and Real-time Electrical Detection of Carbon Nanofiber Bridges

* Non-specific Adsorption of Nanowires to Surface: the Influence of Solution Compositon

* Photochemical grafting on TiO2 thin film

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• Ohmic contact is needed• Analog signal - Signal drift• Only effective in deionized water• Salt solution is necessary for

biomolecular interaction

Electrical Addressable Nanoscale Biosensors

Electrical detection: label-free, direct real-timeNanoscale biosensor: miniaturization, dense array, high sensitivity

Hahm, J., et al, Nano Letters, 2004(4), 51-54

•Measure conductance change

•Sensitive and specificity

•Tens of femtomolar DNA

•Single base specificity

Can we develop a biosensor working in salt solution?

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Research GoalsOur goals are (1) to develop a new form of inherently "digital" nanowire sensor based on making/breaking an electrical connection of a nanowire bridging between two electrodes, and (2) to understand how to manipulate and control nanoscale materials for novel sensing applications.

Challenges:1) Biomolecular recognition --> To develop biomolecular functionalization chemistry2) Choice of nanoscale materials --> To impact nonspecific binding, chemical stability3) Sensitivity--> Need to use high saline solutions, resulting in high background current

Cleavable Chemical/ Biological Group

nanowirenanowire

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Nanowire Fuses for Biological Detection

DNA and endonuclease are used to demonstrate the ability of biosensor fuse

Modified NWs

Amorphous Carbon Microelectrodes

Complementaryss-DNAs

AluI

I(f)

AG

CT T

CG

A

“0”“1”

AluI: restriction nuclease to cleave ds-DNA5'-A G^C T-3'3'-T C^G A-5' 

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Synthesis and Functionalization of Carbon Nanofiber (CNF)

300nm

Carbon-based Materials as biological interface- Highly controllable surface chemistry- Excellent stability and biological activity

M. Endo et al., Appl. Phys. Lett. 2002, 80, 1267.

CarbonCarbon

H HHH

254 nm

HNF3C

OHN

F3CO

Carbon

H2N

NO

NO O

O

OO

SO3-

Carbon

HN

NO O

O

S

SH

SSMCC

DN

A

DN

A

NaBH4

MeOH

Yang, W. S., Hamers, R. J., et al., Nature Materials 2002, 1, 253.

Sun, B., Hamers, R. J., et al., LANGMUIR 2006, 22: 9598

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Manipulation of Nanofibers

~

Dielectrophoresis combined with fluid flow is used to assemble nanowire to form circuit

1 µm

Carbon nanofiber was manipulated under 0.2 Vpp at 1 MHz in deionized water

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PDMS cell

Quartz cover slip

Instrument Setup

Vmeas. = 20 mVrms at 20 kHz

FunctionGenerator

Meas. I/V converter

Lock-inAmplifier

10mM solution

Ref.

All-pass filter

“Internal Nulling” “External Nulling”

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Flow Flow

320

300

280

Cu

rren

t (p

A)

12080400

Time (s)

Current signal change (~21 pA) due to the nanowire unbridging are detected in 10 mM buffer.

Real-time Detection of Nanowire Unbridging

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Frequency-dependent Current Change

Change in current is directly proportional to frequency

3

4

6

8

10

2

Cu

rren

t ch

ang

e, p

A

3 4 6 8

104

2 4

Frequency, Hz

2

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Summary and Future Work

We have developed a new type of biological detection element that is based on the direct digital detection of the of binding/release of individual nanowires across electrodes in saline solutions. To achieve this requires, we need to understand how to optimize the biochemical, mechanical, and electrical properties of nanoscale materials in saline environments.

As a proof-of-concept, we have demonstrated the ability to combine these element to achieve direct real-time detection of enzymatic cleavage of double-stranded DNA molecules.

To summarize…

“0”

“1”

RNA Aptamer

target

I(f) ~

When RNA aptamer binds a specific target, the aptamer is cleaved, and the bio-switch is opened

Future work…

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Photochemical Grafting onto TiO2 Thin Film

•TiO2 exhibits high stability over a wide range of pH and has good

optical and electronic properties, making it of interest for applications in sensing and renewable energy.•TiO2 is of importance as a naturally forming surface coating on Ti and

Ti alloys, which are widely used in biomedical applications.

Motivation

Previous Methods for Surface Functionalization

• Silane• Carboxylic acid group• Phosphonic acid group

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Our Approach…

How does it work?

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FTIR & XPS Analysis of TiO2 after TFAAD Grafting and Deprotection

TiO2Si or glass

TFAADFused Silica Window

UV/254nm

2000 1600 1200

10

8

6

4

2

0Ab

sorb

ance

(10

-3)

3600 3200 2800Wavenumbers (cm-1)

deprotected

TFAAD/TiO2(a)

(b)

-CF3-C=O

TFAAD can be grafted to TiO2 surface and deprotected to form free amine terminated surface

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Are DNA on TiO2 surface stable?F2

Denaturate, then F1

S1 S2

Denaturate, then F1+F2

S1 S2

S1 S2

Inte

ns

ity

86420

Inte

nsi

ty

86420

Inte

nsi

ty

86420Distance (mm)

Day1

Day2

Day3

Day4

1.0

0.8

0.6

0.4

0.2

0.0

No

rmal

ized

Inte

nsi

ty

2520151050Cycle Number

TiO2

HN

N OO

O

S

TiO2

HN

N OO

O

S

Denatured in 8.3 M urea

DNAs tethered on TiO2 surface show excellent specificity and good stability

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Can We Photopattern TiO2 Surface?

50 μm

5 μm

240

200

160

120

Rel

ati

ve

inte

nsi

ty

250200150100500Distance (um)

TiO2

Si or glass

TFAADMask

Mask

SEM

TiO2

RR R RR

h

2 mm

NaBH4

65 ºC

NH

2

NH

2

NH

2

NH

2

NH

2

NH

2

TiO2

TiO2 TiO2

TiO2

SH

SS

MC

C

hF3C

HNO

TiO2 thin film can be photopatterned by TFAAD and then DNA molecules.

TiO2

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•Organic alkenes can graft to the surface of TiO2 when illuminated with ultraviolet light at 254 nm;

• The TFAAD-grafted surfaces can serve as a starting point for preparation of DNA-modified TiO2 thin films exhibiting excellent stability and selectivity;

• Chemical / biological molecules can be grafted to surface selectively;

• Our results suggest that the use of TiO2 as a thin transparent coating may provide enhanced control over the surface chemistry and yield (bio)molecular layers that are more reproducible and/or more stable than layers produced on glass and other transparent materials.

Summary

To summarize…