Nanoparticle Size Measurement with Microfluidic Channel and Dielectrophoresis

Transcription

1 Nanoparticle Size Measurement with Microfluidic Channel and Dielectrophoresis Yi Qiao Measurement and Inspection Lab, Corporate Research, 3M Company, St Paul, MN, 55144

2 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

3 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

4 Nanoparticle Size Measurement In Manufacturing Environment 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 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

5 Microfluidic Y-Cell for Nanoparticle Size Measurement Nanoparticle Concentration proflie y Nano z Buffer Laser Beam x Deflection y y y x x x 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. Optical method of measuring nanoparticle diffusion coefficients in a microfluidic Y-Cell.

6 Microfluidic Y-Cell for Nanoparticle Size Measurement 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

7 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.

8 Dielectrophoresis of Nanoparticles Dielectrophoresis is the manipulation of particles using non-uniform electric field. The particles don t have to be charged. F DEP 3 πr ε p ε r r m = ε m Re{ } E E 3 ε m 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.

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

10 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.

11 Trap and Release Nanoparticles by Dielectrophoresis 10 µm Trapped Nanoparticles By using a laser beam to probe how fast these nanoparticle escape from their trapped location by diffusion, we can measure their size.

12 Size Measurement from Nanoparticle Dielectrophoresis 2.0 NIST-Traceable Standards Equation y = A1*exp(-x/t1 ) + y0 Adj. R-Square Value Standard Error nm y E nm A nm t nm y nm A nm t nm y nm A nm t nm 100nm nm ExpDec1 of 60nm ExpDec1 of 100nm ExpDec1 of nm By measuring the curve decay rate, the mean size of nanoparticle dispersion can be measured within seconds 500nm nm 100nm Time (second)

13 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. u t + Duxx u(0, t) = 0 u( Λ, t) = 0 = 0 πx u( x,0) = U sin( ) Λ u 2 πx π Dt x, t) = U sin( )exp( ) Λ Λ ( 2 r K BT 6πηD 2 π D α = 2 Λ = Stoke-Einstein Relationship

14 Size Calculation from Dielectrophoresis Data α 1 20nm 2.46s, α 75nm = = s 1 D nm = cm s, D75nm = cm s r nm = 35nm, r75nm 129nm 20 = 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.

15 Summary 1. Fast measurement techniques for required for realtime 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.