Optical Detection of Single Nanoparticles and Viruses

We have developed two different optical techniques for the detection of nanoscale particles. One of the methods is based on measuring the optical gradient force exerted on a nanoparticle as it passes through a confined optical field, and the other method uses a background-free interferometric scheme to detect the scattered field amplitude from a laser-irradiated particle. In both cases, the measured signal depends on the third power of the particle size <formula formulatype="inline"><tex>$(R^3)$</tex></formula> as opposed to the <formula formulatype="inline"><tex>$R^6$</tex></formula> dependence inherent to traditional scattering-based detection methods. The weaker size dependence in our schemes leads to a better signal-to-noise ratio (SNR) for small particles. Similar to mass spectrometry, the first detection method influences the trajectory of a particle as it passes through a tightly focused laser beam. On the other hand, the second detection method combines an interferometer with a split detector that yields no signal in the absence of a particle. For both systems, we demonstrate real-time (1 ms) detection of single nanoparticles in a microfluidic system and discuss the limits of each detection approach.