Detecting Nanoparticles using Microplasmas Jeff Hopwood Professor, ECE Department Tufts University Supported by NSF CCF (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 Plasmas Plasma electrons will rapidly charge nanoparticles (negatively). These particles may then be trapped within the potential well of the plasma.  (x) x ++ n e = n i n e ~ 0 (sheath) x V (x) -qZ Table 1. Approximate charge on a nanoparticle with radius a (nm) trapped in a plasma with electron temperature T e (eV) -excludes photo-ionization T e (eV)Approx. Number of Charges (Z) 11.9a 23.5a 34.9a 46.3a 57.6a charging trapping microplasma

Portable Microplasma System -- the Split Ring Resonator (SRR) -- VCO (900MHz) Power Amp (GSM Band Cell 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) E gap > 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  m melamine particles coaxial line Window pump particle counter 37 mm

Microplasma Particle Trap Experiment 200  m 20 mm HeNe “shaker” Argon microplasma 1  m melamine particles coaxial line pump particle counter Digital SLR Microscope 632 nm filter Window

Particle Trapping and Localization Time (sec) 2 cm  t res = 2 s microplasma 1 um - melamine formaldehyde

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

Conceptual detection and measurement of nanoparticles O2O2 CF 4 optical spectrometer 1. Trap and concentrate gas-borne nanoparticles microplasma trap 2. Pulse reactive gases SiF 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 Hopwood Professor, ECE Department Tufts University