Biosensing with Photonic Force Microscopy

Photonic force microscopy (PFM) is a type of scanning probe microscopy to measure topography of surface, in which an optically trapped particle is used as a scanning probe, and has many applications, particularly in the physics of soft- and bio-materials. We demonstrated how this technique can be used to investigate interactions of DNA oligonucleotides at the contact point, with consideration for biological applications, as well as how to simultaneously map multi-information, such as the topography of surface and distribution and binding strength of bio-molecules. We also demonstrated the detection of biomolecules on the surface in target molecule concentrations varying from 10 nM to 0.1 pM, by scanning the probe bead vertically at thousands of different lateral positions. Biosensing using PFM has several advantages: variable probes can be used for multiplex detection when different target molecules are mixed in one sample, and the power of the trapping laser can be controlled to expand the observable strength range of the molecular interactions.

Sub-nanometer Resolution Magnetic Tweezers

The magnetic tweezers is a single-molecule manipulation instrument that has been widely used in the study of nucleic acid structures and protein-nucleic acid interactions. The magnetic tweezer utilizes a magnetic field to apply force to a biomolecule-tethered magnetic bead while using optical bead tracking to measure the biomolecule’s extension.

Compared to optical tweezers, magnetic tweezers are a much simpler, stable, and less expensive single-molecule instrument; yet, previous magnetic tweezers had lacked the high-resolution demonstrated in certain optical tweezers experiments. Indeed, some review articles pointed out that resolution is the key limiting factor in magnetic tweezers experiments.

We developed a high-resolution magnetic tweezers method based on interferometry and nano-fabricated optical coatings. We showed that the interference-based magnetic tweezers system is well suited to precision measurements of configuration-changing bio-molecular activity. The technique directly addressed the resolution issue described above, and achieved a resolution comparable to the other methods. Thus, we solved an important technical barrier in the field.

Nano-Optical Trapping via Plasmonic Nanoantennas

Nano-optical trapping techniques based on the use of near-field photonics have been developed to overcome the limitations imposed by free-space diffraction. In particular, plasmonic nanoantennas, which significantly enhance and confine light in a nano-scale volume, allow not only trapping of nano-scale particles smaller than 100 nm with relatively low incident power, but also suppression of Brownian motion to the few tens of nanometre scale.

We demonstrated the first experimental implementation of low-power nano-optical vortex trapping using plasmonic resonance in rationally-designed gold diabolo nanoantennas. The vortex optical trapping potential was formed with a minimum at 170 nm distant from the central local maximum. A polystyrene nanoparticle with a diameter of 300 nm was trapped strongly at the boundary of the nanoantenna with a low incident power of ~5.5 mW. Our subwavelength-scale nanoantenna system will be highly useful for biomedical applications that require reduced heating and damage in biological particles.