Plasmonic nanopores

 

Plasmonic nanopores are solid-state nanopores equipped with a plasmonic nanoantenna. These gold nanoantennas control visible light at the nanoscale through excitation of collective electron oscillations known as plasmons (or localized surface plasmons), concentrating into small subdiffraction-limited “hot spots” in the feed-gap of the antenna. These electron oscillations are a resonant effect, only efficiently excited at one particular light frequency. This resonance wavelength of the antenna is very sensitive to its geometry and electric permittivity of the environment, especially in the dielectric material present in the feed-gap.

By fabricating a nanopore right at the feed-gap, biomolecules translocating through the nanopore are ensured to interact with the extremely confined light. This provides exciting functionality to the nanopore. For example, the high intensity localized fields can be used for surface-enhanced Raman scattering of single translocating biomolecules, providing optical readout with chemical selectivity for nanopore sensors. Moreover, the strong light localization creates extreme electromagnetic field gradients that can be used for gradient force optical trapping of small particles, like protein and DNA, allowing speed control over the translocating molecules. Finally, the resonance change of the antenna induced by the presence of the biomolecule in the feed-gap can be monitored and allows for all optical read-out of the trapped molecules.

Plasmonic nanopores

Figure 1. Illustration of a plasmonic nanopore and its use for optical profiling. The graph to the right shows an experimentally obtained intensity map of the focal plane of a focused 10mW laser beam.

 

 

RAMAN SPECTROSCOPY

Raman spectroscopy could provide molecular fingerprints by detecting the energy shift of inelastically scattered photons after interacting with molecules. By combining Raman spectroscopy with solid-state nanopores, it is possible to identify the species and chemical states of the analyte during its translocating through the nanopore. In principle, Raman signals can be used to distinguish the chemical structure of a translocating DNA polymer and allow for optical single-molecule DNA sequencing. However, the Raman signal is usually much weaker than the Rayleigh/elastic scattering, making it impractical to directly detect Raman spectra from single molecules in solid-state nanopores.

In this project, we are integrating plasmonic nanostructures with a solid-state nanopores to significantly enhance the localised electromagnetic field. By doing so we attempt to boost the Raman signal by a factor of 108 – 109. Such extreme signal enhancement opens the possibility of single molecule detection in the nanopores with chemical selectivity. To achieve this notable high local electromagnetic field tiny feed-gaps (<5 nm) in the plasmonic antenna are required and we have developed a new nanofabrication protocol to  achieve this.

By integrating the voltage control and ionic current measurement of the nanopore with Raman spectroscopy, we can effectively deliver the biomolecule into the plasmonic hotspots. The presence of single DNA polymer inside the nanopore and the gap between the antennas could be recorded by the changes of ionic current, while the species of molecules could be identified by the Raman spectroscopy.

RAMAN

Figure adapted from: Maxim Belkin et al. ACS Nano, 2015, 9 (11), 10598–10611

 

 

OPTICAL TRAPPING

A plasmonic nanoantenna focusses light down to sub-diffraction limited hotspots, creating strong electromagnetic field gradients that can be used to act optical forces on biomolecules. To optimize the trapping forces, we create a nanoaperture in a freely suspended gold film. When light is impinged on this antenna, it is focused down to the smallest constriction of it, the feed-gap. Monitoring the transmitted light, we can see a shift in the resonance of the antenna when a molecule gets trapped in it, giving us information about the physical properties on the nanoantenna. Moreover, the thin film allows us to actively attract molecules towards the sensor and act electrokinetic forces on them to calibrate our nanoplasmonic trap and perform force spectroscopy on trapped molecules.

TRAPPING