Real-Time and Background-Free Detection of Nanoscale Particles

We introduce a background-free real-time detection scheme capable of recognizing low-index nanoparticles such as single viruses in water. The method is based on interferometrically measuring the electromagnetic field amplitude of the scattered light. A split detector is used to generate a background-free signal that renders unprecedented sensitivity for small particles. In its current configuration the sensor is capable of detecting low-index particles in water down to 10 nm in radius or single gold particles as small as 5 nm. We demonstrate the detection of such small particles in a microfluidic system with a time resolution of 1 ms and we discuss the theoretical limits of this novel detection scheme.

Figure 1

Schematic rendering of the single particle detection experiment. (a) A laser beam is split by a beam splitter (BS) into a reference beam and a beam that is focused by a high numerical aperture (NA) objective ($\mathrm{NA}=1.4$) into a nanoscale channel which is part of a microfluidic system. A particle solution is transported through the channel using electro-osmosis. Scattered light from a passing particle is recombined with the optionally attenuated reference beam and directed onto a split photodetector which renders a background-free signal. A pinhole ($\mathrm{\text{diameter}}=500\mathrm{\text{microns}}$) is used to reduce the amount of ambient light incident on the photodetector. (b) Photograph of a sequence of nanoscale channels. (c) Typical time trace of the detector signal. Individual detection events are represented by peaks with a width of a few milliseconds.

Figure 2

Histogram of signal amplitudes for (a) a mixture of 15 nm and 50 nm polystyrene particles, (b) a mixture of 7 nm and 20 nm gold particles, and (c) a mixture of Influenza X31 virus (left peak) and 100 nm polystyrene beads. All data sets have been acquired in water with each individual detection event lasting $\approx 1\mathrm{ms}$.

Figure 3

Experimental analysis of detection limits using 50 nm polystyrene particles. (a) Dependence of the signal amplitude $S(t)$ on the reference beam power $P_r$. The red line is a fit according to Eq. (3). (b) Dependence of the signal-to-noise ratio (SNR) on the reference beam power. The red line is a fit according to Eq. (5). The measured detector noise equivalent power is $P_v=0.7\mathrm{nW}$ (rms), and the laser pointing instability is $\theta =4.5\times {10}^{-4}$ (rms). These values predict a maximum at $P_r=1.6\mu \mathrm{W}$ which is in agreement with the fitted curve in (b). (a),(b) Several hundreds of particles are used for each data point.

Figure 4

Dependence of the signal amplitude on particle radius $r_0$ for (a) gold particles and (b) polystyrene particles in water. The red line is a fit according to $r_0^n$ with (a) $n=2.9\pm 0.3$ and (b) $n=2.6\pm 0.3$.