Nanoparticle Size Measurement with Microfluidic Channel and Dielectrophoresis

Yi Qiao

Measurement and Inspection Lab, Corporate Research,

3M Company, St Paul, MN, 55144

Outline

1. Background of Nanoparticle Size Measurement

2. Industrial Needs on Nanoparticle Size Measurement

3. Nanoparticle Size Measurement Using Microfluidic Y-Cell

4. Nanoparticle Size Measurement Using Dielectrophoresis

5. Summary

Nanoparticle Size Measurement Techniques

1. Imaging methods: Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Atomic Force Microscope (AFM) and etc.

2. Light Scattering Methods: Static Light Scattering (microparticles), Dynamic Light Scattering (nanoparticles)

3. Separation Methods: Capillary Chromatography, Field Flow Fractionation combined with UV absorption methods

Nanoparticle Size Measurement In Manufacturing Environment

Most existing measurement techniques are for lab environment only.

Several requirements for manufacturing needs:

1. Fast enough for manufacturing process feedback purpose.

2. Less strict sample requirements such as concentration, purity

3. In some cases, qualitative information is needed to tell “good” from “bad”

Nanoparticles are widely used in industry. A few examples include:

1. Surface coatings for optical, mechanical properties

2. Nanoparticle composites for better mechanical strength

3. Drug in dry nanoparticle powder form for faster absorption

Microfluidic Y-Cell for Nanoparticle Size Measurement

Nano Buffer Laser BeamDeflectionx

z

y

NanoparticleConcentration proflie

x

y

x

y

x

y

Optical method of measuring nanoparticle diffusion coefficients in a microfluidic Y-Cell.

1. Measure nanoparticle size from their diffusion speed.

2. Drawback of this technique: diffusion can take pretty long for larger size nanoparticles, so fluidic flow stability becomes an issue.

The images recorded by the CCD camera before and after the nanoparticle dispersion was pumped into the microfluidic channel.

(a) The laser beam shows no deflection before the nanoparticle dispersion is pumped.

(b) The laser beam shows largest beam deflection at the position A and little deflection down stream at position C.

(c) Larger slope means faster diffusion and smaller particles

Microfluidic Y-Cell for Nanoparticle Size Measurement

Nanoparticle Size Measurement using Dielectrophoresis and Diffusion

Basic Measurement Principle:

1. A microfabricated electrode array is used to create a nanoparticle density grating via dielectrophoresis effect.

2. A laser is used to probe the nanoparticle density grating (essentially a refractive index grating).

3. Once the dielectrophoresis is turned off, nanoparticles diffuse and density grating is washed out. Diffusion speed can be measured from the density grating decay speed.

Dielectrophoresis of Nanoparticles

Dielectrophoresis is the manipulation of particles using non-uniform electric field. The particles don’t have to be charged.

EErFm

mpmDEP

rr∇⋅

−= }Re{

3

3

εεε

επ

For nanoparticles, since the radius is so small, the electric field and its gradient need to be large enough for the dielectrophoresis force to be dominant over Brownian motion.

An electrode array with small pitch is needed to create large electric field intensity and gradient.

Finite element analysis of the electric field distribution of a 10 micron pitch electrode array

Dielectrophoresis of Nanoparticles

Experimental Setup

The first order diffracted beam is used as an indicator of the nanoparticle refractive index grating. The diffracted beam from the electrode array itself is minimized and is a baseline of the measurement.

10 µm

Trap and Release Nanoparticles by Dielectrophoresis

By using a laser beam to probe how fast these nanoparticle “escape”from their trapped location by diffusion, we can measure their size.

Trapped Nanoparticles

Size Measurement from Nanoparticle Dielectrophoresis

NIST-Traceable Standards

-2 0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.0

2.5-2 0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.0

2.5

60nm

100nm

Time (second)

60nm100nm500nmExpDec1 of 60nmExpDec1 of 100nmExpDec1 of 500nm

Equation y = A1*exp(-x/t1) + y0

Adj. R-Square 0.73093 0.90507 0.99063Value Standard Error

60nm y0 0.03103 5.62244E-460nm A1 0.12584 0.0023560nm t1 1.67048 0.04946100nm y0 0.06434 0.00125100nm A1 0.21217 0.0015100nm t1 3.04275 0.06529500nm y0 0.74475 0.00463500nm A1 0.99467 0.00356500nm t1 5.50705 0.05686

500nm

By measuring the curve decay rate,the mean size of nanoparticle dispersioncan be measured within seconds

Size Calculation from Dielectrophoresis Data

1. The decay rate of the nanoparticle grating can be obtained from fitting the decay curve in previous slide.

2. The nanoparticle diffusion coefficient can be calculated from the decay rate.

3. Nanoparticle size can be obtained from the diffusion coefficient from the Stokes-Einstein relationship.

0=+ xxt Duu

)sin()0,(

0),(0),0(

Λ=

=Λ=

xUxu

tutu

π

)exp()sin(),( 2

2

Λ−

Λ=

DtxUtxu ππ2

2

Λ=

Dπα

DTKr B

πη6= Stoke-Einstein Relationship

12675

12620 1070.1,1023.6 −−−− ×=×= scmDscmD nmnm

175

120 67.0,46.2 −− == ss nmnm αα

nmrnmr nmnm 129,35 7520 ==

Measurement error could be from using Stoke-Einstein relationship at relative high concentration sample. Nonetheless, it is a fast qualitative measurement, and calibration could be used to improve accuracy.

Size Calculation from Dielectrophoresis Data

Summary

1. Fast measurement techniques for required for real-time inline measurement of small particles in industrial applications.

2. Less stringent requirement on sample concentration and purity.

3. Good qualitative measurement for industrial process monitoring.

4. The microfluidic Y cell and dielectrophoresis measurement methods presented here are viable candidates.