Detecting Nanoparticles with
Bradley Deutsch, Ryan Beams and Lukas Novotny
As we learn more about the effect of atmospheric nanoparticles on
climate change1 and human health, 2
careful monitoring of these particles
becomes crucial. Nanoparticles appear as
pathogens in bioterrorism3 and as contaminants in manufacturing processes.
In medicine, metallic nanoparticles can
be used as cancer-fighting agents. 4 An
effective measurement system needs to
be sensitive to small, single particles. If
real-time detection is needed, the scheme
should not require labeling the target
particles with fluorescent molecules.
Elastic light scattering is convenient
for label-free detection. Small particles
scatter light only weakly, but phase-sensitive interferometry can greatly
increase measurement accuracy. Amplitude and phase decoupling usually relies
on heterodyne interferometry, in which
a lock-in amplifier demodulates at a beat
frequency between the reference and signal beams. Such detection systems tend
to be large or electronically complicated
due to their reliance on active optical
elements, so deployment in the field or
clinic is challenging.
We have recently developed an
interferometric detection system that
decouples amplitude and phase using
two orthogonal, simultaneous measurements using passive optical elements. We
have shown sensitivity to single 30-nm
Au particles. 5 Light is focused on a
nanometric channel etched in glass and
filled with the particle solution. A particle takes about 1 ms to pass through
the focus, scattering light as it goes. The
light is collected with a dual-phase interferometer, as shown in (a) of the figure. The optical signal is combined with
a circularly polarized reference beam,
and a polarizing beamsplitter directs
the two orthogonal polarizations on to
two detectors. Since the relative phase
between reference and signal differs by
PBS NPBS Focus Polarizer, 45° QWP, 45° Mirror l=532 nm Polarizer, 0° Signal [a.u.] Detector 1 (a) (b) 30 nm
Dual-phase apparatus and histograms of particle-detection events. (a) A microscope
objective focuses light onto a nanometric channel, and particles flow past. Scattered light
is collected by a dual-phase interferometer. (b) Histogram showing results from three dif-
ferent immobilized particles.
90° at the two detectors, amplitude and
phase can be decoupled.
The amplitude of the collected signal
indicates the size of the particle and
contains material information. By collecting signals from many particles, we
can construct histograms that represent
the population of particles in a sample,
as shown in (b) of the figure. In this
experiment, three immobilized gold
particles of different sizes are moved
through the focus on a microscope
coverslip. Decoupling amplitude and
phase improves measurement precision
drastically and effectively separates the
peaks. The remaining width is due to
The microscope objective used to fo-
cus light on the channels can be replaced
with an approximately hemispherical
lens called a numerical aperture increas-
ing lens (NAIL) along with a small
aspheric lens. Since the NAIL is only
a few hundred microns in diameter, it
provides even more scalability opportu-
nities. The dual-phase system is useful
wherever label-free detection is necessary
at a single-particle level. That includes
samples with very low concentrations
of contaminants, like ultrapure water
in semiconductor manufacturing, or in
medical studies in which species popula-
tions must be characterized. The simplic-
ity and scalable nature of the apparatus
makes field deployment feasible for
biodefense or atmospheric monitoring. t
Bradley Deutsch ( email@example.com),
Ryan Beams and Lukas Novotny are with the Institute
of Optics at the University of Rochester, N. Y., U.S.A.
1. S. Menon et al. “Climate effects of black carbon aerosols
in China and India,” Science 297, 2250–3 (2002).
2. S. T. Holgate et al., eds., Air Pollution and Health, Academic Press, San Diego, 1999.
3. M. R. Hillman. “Overview: cause and prevention in biowarfare and bioterrorism,” Vaccine 20, 3055–67 (2002).
4. M.V. Yezhelyev et al. “Emerging use of nanoparticles in
diagnosis and treatment of breast cancer,” Lancet Oncol.
7, 657–67 (2006).
5. B. Deutsch et al. “Nanoparticle detection using dual-phase interferometry,” Appl. Opt., in press.