Detecting Nanoparticles using Microplasmas

Jeff HopwoodProfessor, ECE Department

Tufts Universityhopwood@ece.tufts.edu

617-627-4358

Supported by NSF CCF-0403460 (in progress)

Problem Statement

• nanoparticles are too small to detect by scattered laser light (r<100nm).• nanoparticles may be too widely dispersed to sense and count accurately.• radio isotopes used for charging particles in DMA’s or IMS’s must be

tracked.• current systems are not portable.

Goals:To use low power, portable microplasma generators to charge particles.

To use ‘potential wells’ to trap and concentrate charged particles.To investigate novel modes of detecting particles by charge, mobility, or

chemical reactivity in microplasmas.

Charging Particles with PlasmasPlasma electrons will rapidly charge nanoparticles (negatively).

These particles may then be trapped within the potential well of the plasma.

(x)

x

+ +

ne = ni

ne~ 0(sheath)

x

V(x)

-qZ

-qZ

Table 1. Approximate charge on a nanoparticle with radius a (nm) trapped in a plasma with electron temperature Te (eV) -excludes photo-ionization

Te (eV) Approx. Number of Charges (Z)

1 1.9a

2 3.5a

3 4.9a

4 6.3a

5 7.6a

charging trapping

microplasma

Portable Microplasma System-- the Split Ring Resonator (SRR) --

VCO(900MHz)

Power Amp(GSM BandCell Phone,

4 watts, ~$1)

Split Ring Resonator: SRR

not shown: 6 v battery, power level control

Prototype SRR operating in air(3 watts)

Split Ring Resonator(SRR)Electric Field Intensity (@ 900 MHz)

Egap > 10 MV/m

25 m discharge gap

This device concentrates power from a cell phone into

a volume of ~ 1 nanoliter

Microplasma Particle Trap Experiment

200 m

20 mm

HeNe

Digital SLR

Microscope

632 nm filter

“shaker”

Argon microplasma (SRR)1 mmelamineparticles-

-

--

-- -

--

-coaxialline

Window

pump

particlecounter

37 mm

Microplasma Particle Trap Experiment

200 m

20 mm

HeNe

“shaker”

Argon microplasma1 mmelamineparticles-

-

--

-- -

--

-coaxialline

pump

particlecounter

Digital SLR

Microscope

632 nm filterWindow

Particle Trapping and Localization

Time (sec)

0 20 40 60 80 100

Par

ticle

Cou

nt

0

100

200

300

400

plasma "on"

shaker

lost particles

trapped particles

Time (sec)

2 cm tres = 2 s

microplasma

1 um - melamine formaldehyde

Particles Trapped by a Microplasma(observed through a 632nm filter to block plasma emissions)

200 um

/4 electrode

/4 electrode

SRRSRR

200 um

/4 electrode

/4 electrode

SRRSRR

Conceptualdetection and measurement of nanoparticles

O2

CF4

opticalspectrometer

1. Trap and concentrategas-borne nanoparticles

microplasmatrap

2. Pulse reactive gasesSiF

4. Detect emission of light from the etch reaction products

3. Etch the nanoparticles

Other concepts

• Use a voltage pulse to ‘push’ the charged nanoparticles from the trap, and detect particle size distribution using time-of-flight

• Use a miniature Ion Mobility Spectrometer to sort and detect charged nanoparticles– See sionex.com, for example

• Use a microplasma to charge the particles prior to entering a commercial DMA

Contact Information

Jeff HopwoodProfessor, ECE Department

Tufts Universityhopwood@ece.tufts.edu

617-627-4358