A single molecule is a minuscule object. Many molecules have diameters of less than 1 nm, making it a challenging task to consider what experimental techniques could be used to visualize them. For spectroscopic techniques, a single molecule only absorbs or emits a vanishingly small number of photons, meaning experiments with an extreme degree of sensitivity would be needed to detect them.
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However, there are certain tricks that can be used to enhance signal levels for spectroscopic measurements of few or single molecules and that is through the use of plasmonic nanocavities.1 A plasmonic nanocavity is a structure engineered to trap light and enhance the intensities of the interactions between the molecule or materials and the local fields. This results in a subsequent signal enhancement making it possible to perform measurements such as single-molecule imaging.
To confine light efficiently, the plasmonic nanocavity needs to be structured so that its refractive index is a suitable match for the wavelength of the incident light. One of the most important developments in enhancing the experimental possibilities created by plasmonic nanocavities was the ability to carefully control and engineer the dimensions and the shapes and structures of the cavities.
A nanoparticle-on-mirror cavity builds upon a typical nanocavity design, where two nanostructures are joined with a nanometer or sub-nanometer gap between them to form a dimeric structure. For a nanoparticle-on-mirror arrangement, the nanostructure is placed above the metallic layer and a thin dielectric spacer is used to separate the two components. The advantage of the nanoparticle-on-mirror configuration over simpler designs is that it can support multiple resonances and exhibit deep sub-diffraction mode volumes.1
Other cavity designs have involved the use of different shapes and structures for the nanocavity to try and enhance the degree of optical trapping and, in turn, boost the observed signal enhancements.2 Different materials can be used to tune the cavity properties for particular applications and to try and maximize the spontaneous emission enhancement for techniques such as single-molecule imaging.3
In the life sciences, imaging techniques such as confocal microscopy have revolutionized our understanding of biological structures such as cells. Fluorescence microscopy, where strongly emitting dyes are bound to the biological target of interest for imaging, is an alternative, highly sensitive technique and can be used to detect even small amounts of a target molecule.
With the use of plasmonic nanocavities, it is possible to extend the sensitivity of fluorescence imaging down to the single-molecule level. It is also possible to extend these spectroscopic and imaging schemes to perform Raman measurements for non-fluorescent molecules.
Raman spectroscopy comes with the ability to recover information on the vibrational modes of the molecule, which can be used for chemical identification. As certain vibrational modes are sensitive to local bonding environments, this information can be used to build up a detailed picture of the molecule and its local interactions with its environment. It can also offer detailed information about other molecular species and recover important information for monitoring molecular biochemistry.
While fluorescence-based DNA sequencing is challenging due to the large number of amino acids that need to be distinguished and the relatively broad and spectrally indistinct nature of their emission signals, the rich chemical information provided by Raman techniques can help detect single nucleotides and DNA oligonucleotides, provided they are sufficiently close to the metallic nanopore.4 This is known as surface-enhanced Raman scattering (SERS) and can also be used in a quantitative, as well as qualitative, manner.5
Given the length scales involved, creating plasmonic nanocavities is a non-trivial problem. Lithography, either with electrons or ions, is a top-down assembly process for the nanocavities but challenging to control in terms of reproducibility. The current state-of-the-art approach for creating these nanophotonic structures is to make use of self-assembly processes with DNA origami.1
In DNA origami, a long, single strand of DNA is folded using shorter, staple pieces of single-stranded DNA. The long strand, or scaffold, is folded into a breadboard with a construction that is well-defined using different scaffolds. Other particles such as quantum dots or nanoparticles can then be stacked on the breadboard for use.
The fine level of control and ability to make billions of structures at a time has made DNA origami a very popular approach to fabrication. The high quality of structures with limited numbers of defects is also beneficial to the amount of field enhancement it is possible to achieve in the cavities. Tuning the cavity thickness is now also possible through temperature control during the fabrication process.5
The extreme sensitivity offered by plasmonic nanocavities has opened up new possibilities for both fundamental and applied sciences. This includes exploring physical and chemical phenomena at the few molecule level which is a very different regime to the large ensembles of hundreds, or thousands, of molecules that are usually studied.
Greater degrees of control over the fabrication process are making it possible to combine metallic and biological structures for the creation of hybrid devices which may offer a route to efficient, highly sensitive bioassays as a new diagnostics tool.
References and Further Reading
- Maccaferri, N., Barbillon, G., Koya, A. N., Lu, G., Acuna, G. P., & Garoli, D. (2021). Recent advances in plasmonic nanocavities for single-molecule spectroscopy. Nanoscale Advances, 3(3), 633–642. https://doi.org/10.1039/d0na00715c
- Chen, C., Juan, M. L., Li, Y., Maes, G., Borghs, G., Van Dorpe, P., & Quidant, R. (2012). Enhanced optical trapping and arrangement of nano-objects in a plasmonic nanocavity. Nano Letters, 12(1), 125–132. https://doi.org/10.1021/nl2031458
- Russell, K. J., Liu, T. L., Cui, S., & Hu, E. L. (2012). Large spontaneous emission enhancement in plasmonic nanocavities. Nature Photonics, 6(7), 459–462. https://doi.org/10.1038/nphoton.2012.112
- Chen, C., Li, Y., Kerman, S., Neutens, P., Willems, K., Cornelissen, S., … Van Dorpe, P. (2018). High spatial resolution nanoslit SERS for single-molecule nucleobase sensing. Nature Communications, 9(1), 1–9. https://doi.org/10.1038/s41467-018-04118-7
- Simoncelli, S., Roller, E. M., Urban, P., Schreiber, R., Turberfield, A. J., Liedl, T., & Lohmüller, T. (2016). Quantitative Single-Molecule Surface-Enhanced Raman Scattering by Optothermal Tuning of DNA Origami-Assembled Plasmonic Nanoantennas. ACS Nano, 10(11), 9809–9815. https://doi.org/10.1021/acsnano.6b05276