Steven Charles Wasserman

Weighing nanoparticles in solution at the attogram scale

Significance Naturally occurring and engineered nanoparticles (e.g., exosomes, viruses, protein aggregates, and self-assembled nanostructures) have size- and concentration-dependent functionality, yet existing characterization methods in solution are limited for diameters below ∼50 nm. In this study, we developed a nanomechanical resonator that can directly measure the mass of individual nanoparticles down to 10 nm with single-attogram (10−18 g) precision, enabling access to previously difficult-to-characterize natural and synthetic nanoparticles. Physical characterization of nanoparticles is required for a wide range of applications. Nanomechanical resonators can quantify the mass of individual particles with detection limits down to a single atom in vacuum. However, applications are limited because performance is severely degraded in solution. Suspended micro- and nanochannel resonators have opened up the possibility of achieving vacuum-level precision for samples in the aqueous environment and a noise equivalent mass resolution of 27 attograms in 1-kHz bandwidth was previously achieved by Lee et al. [(2010) Nano Lett 10(7):2537–2542]. Here, we report on a series of advancements that have improved the resolution by more than 30-fold, to 0.85 attograms in the same bandwidth, approaching the thermomechanical noise limit and enabling precise quantification of particles down to 10 nm with a throughput of more than 18,000 particles per hour. We demonstrate the potential of this capability by comparing the mass distributions of exosomes produced by different cell types and by characterizing the yield of self-assembled DNA nanoparticle structures.

The Bessel-beam random access trap

We report on the investigation of crossed-Bessel-beam and hybrid Bessel-Gauss configurations for optical trapping of microscopic particles. The non-diffractive nature of the Bessel beam removes the need for high-NA optics. Crossed beam configurations allow creating trapping volumes with small aspect ratio, in comparison to single-beam Bessel traps that create wave-guide like structures. We present numerical simulations of said geometries and present experimental data of in-situ Bessel beam forces on polystyrene beads as precursor to the realization of a random access Bessel trap.