Pseudo-Chiral Resonators for the Optical Detection of Chiral Molecules – ChiROptMol

With the aim of understanding the interaction between chiral molecules and pseudochiral resonators, the Chiroptmol project was divided into 3 working groups (WG). Two theses were funded, one of which was co-directed by INSP and LRS. INSP was in charge of polarimetric measurements and effective medium models adapted to the analysis of polarimetric measurements, LRS was in charge of grafting development, Institut Fresnel of numerical modeling (finite element method) and plasmonic metasurfaces realization.
GT 'Calculations': lead Institut Fresnel
A scattered-field formulation well-suited to the finite-element method (FEM) has been used to calculate the complete optical responses of any type of periodic metasurface. We have developed a code capable of modeling the polarimetric properties of metasurfaces by calculating the 16 elements of the Stokes-Mueller matrix. The main advantage of the finite element method lies in its versatility and its ability to calculate the optical properties of structures of both arbitrary and realistic shapes. Various achiral, pseudo-chiral and chiral metasurfaces with specific polarimetric properties have been studied, with the aim of maximizing near-field chirality density.
GT 'Grafting': lead LRS
Various biomolecules were tested, but we mainly studied temporins, which are peptides with anti-bacterial properties. These peptides were functionalized with a thiol group, enabling them to be absorbed onto gold. Characterization of grafting and its kinetics on flat gold was obtained by combining different measurements.
Polarimetry WG: INSP lead
This workgroup involved the development of a micropolarimeter that would enable the complete Mueller matrix to be measured with microscopic resolution, so that measurements could be taken on lithographed areas. A complete Mueller matrix can be acquired in 2 minutes, given that the expected absorption kinetics are of the order of 3 hours to obtain a monolayer. Measurements were taken in reflection mode, during the absorption of peptide (temporine-Sh) and various proteins. Absorption kinetics were monitored at several wavelengths characteristic of the resonant modes of plasmonic resonators: magnetic, electrical and quadrupole modes. The symmetry properties of Mueller arrays were used to enable measurements to be carried out at a fixed angle, while compensating for the natural CD of the resonators.

The polarimeter was set up in transmission mode. In this configuration, Mueller matrix measurements are obtained with state-of-the-art accuracy (5 10-3). Lateral resolution is better than 6µm.
The sensitivity of pseudo-chiral metasurfaces enables ultra-sensitive detection of very small molecules (temporine-Sh: 1.4 kDa) with a limit of detection (LoD) of 0.6% of resonator overlap.
Analysis of the symmetries between the elements of the Mueller matrices enables us to propose a measurement where the CD of the metasurfaces is compensated not by changing the direction of measurement propagation, but by using all the elements of the Mueller matrix.
Numerical simulations show excellent agreement between measured and calculated Mueller matrix values. New metasurfaces are proposed to enhance the local chiral field, while cancelling the far-field circular dichroism inherent in bare metasurfaces: this makes it possible to envisage measurements that are more sensitive to chirality. Anisotropic media have been introduced into the calculation code.
The detection of chiral objects and chiral fields remains a topical issue, as evidenced by recent publications in the literature addressing the distribution of chiral fields around resonators, and the sensitivity of measurements in relation to instrumentation. The generation of chiral fields and CDs by plasmonic nanostructures also remains a topical issue.
The results of the CHirOptMol project have been exploited mainly through scientific publications (11) and conference papers (8). Two theses have been defended. The calculation code developed during the project has been made freely available in the ONELAB suite.

The Chiroptmol project offers a new perspective on the use of plasmonic metasurfaces for the detection of chiral objects. It appears that certain achiral resonators are particularly interesting because they can generate an exacerbated chiral local field, either by inducing no intrinsic far-field optical activity, or by offering the possibility of completely compensating for their far-field optical activity. This second point stems from the use of full polarimetric measurements. It is certainly possible to reduce the number of polarimetric parameters while compensating for the optical activity of resonators: in this case, simplified polarimeters are conceivable. With this in mind, we still need to understand the sensitivity of polarimetric parameters to the presence of chiral objects in the near field of resonators, in order to target the most relevant polarimetric parameters. It is likely that these polarimetric parameters will depend on the resonator under consideration.
An open-access code has been produced that will enable other researchers to approach the calculation of the polarimetric response of resonators in an anisotropic environment. A version that will take into account a chiral environment is currently under development.
These issues are partly addressed in a subsequent ANR-funded project involving the same partners.

“Gold Nanorods for LSPR Biosensing: Synthesis, Coating by Silica, and Bioanalytical Applications” Vincent Pellas, David Hu, Yacine Mazouzi, Yoan Mimoun, Juliette Blanchard, Clément Guibert, Michèle Salmain, Souhir Boujday, Biosensors 10, 146 (2020)
“Antibody-Gold Nanoparticle Bioconjugates for Biosensors: Synthesis” L. Zhang, Y. Mazouzi, M. Salmain, B. Liedberg, S. Boujday., Characterization and Selected Applications. Biosensors and Bioelectronics 165, 112370 (2020)
“Mueller micropolarimeter for color imaging of aluminium metasurfaces”, M. Nicolas, I. Soumahoro, L. Zhang, G. Guida, W. Daney de Marcillac, D. Demaille, C. Schwob, S. Boujday, B. Gallas, J. Opt. Soc. Am. B 38, 1184-1191 (2021)
“Strategies for Antimicrobial Peptides Immobilization on Surfaces to Prevent Biofilm Growth on Biomedical Devices”, M. Nicolas, B Beito, M. Oliveira, M. T. Martins, B. Gallas, M. Salmain, S. Boujday, V. Humblot, Antibiotics 11, 13 (2022) – issue cover
“Biosensing Extracellular Vesicle Subpopulations in Neurodegenerative Disease Conditions” Y. Mazouzi, F. Sallem, F. Farina, A. Loiseau, N. Rocha Tartaglia, M Fontaine, A. Parikh, M. Salmain, C. Neri, S. Boujday, ACS Sensors 7, 1657-1665 (2022)
“Design and Optimization of a Magneto-Plasmonic Sandwich Biosensor for Integration within Microfluidic Devices” M. Soroush, W. Ait Mammar, A. Wilson, H. Ghourchian, M. Salmain, S. Boujday, Biosensors, 12 799 (2022)
“True circular dichroism in optically active achiral metasurfaces and its relation to chiral nearfields”, M. Nicolas, P. M. Walmsness, J. Amboli, L. Zhang, G. Demesy, N. Bonod, S. Boujday, M. Kildemo, B. Gallas, ACS Appl. Opt. Mater. 1, 1360-1366 (2023)
«Revisiting Alkoxysilane Assembly on Silica Surfaces: Grafting versus Homo-Condensation in Solution” Y. Millot, A. Hervier, J. Ayari, N. Hmili, J. Blanchard, S. Boujday, J. Am. Chem. Soc., 2023, 145, 6671-6681 (2023)
“Achiral magnetic photonic antenna as a tunable nanosource of superchiral light” Cui, L., Yang, X., Reynier, B., Schwob, C., Bidault, S., Gallas, B., Mivelle, M. ACS Photonics 10, 3850 (2023)
“Design and analysis of chiral and achiral metasurfaces with the finite element method”, J. Amboli, B. Gallas, G. Demésy, N. Bonod, Op. Express 31, 4317 (2023)
“Hollow Gold Nanoshells for Sensitive 2D Plasmonic Sensors”, D. Sun, F. Ben Romdhane, A. Wilson, M. Salmain, S. Boujday, ACS Appl. Nano Mater. xx, xxxx (2024)

Photonic nanoresonators feature the ability to resonantly interact with light and are characterized by moderate quality factors and small effective volumes. This resonant interaction leads to an increase of the light scattering (far field) and of the field intensity in the vicinity of the resonators (near field). Metallic nanostructures hosting localized surface plasmons have been widely investigated to enhance light matter interactions.
Near field: The enhancement of the near electric field in the vicinity of the photonic resonators is used in sensors to allow for more efficient coupling with light and increase the excitation rate of molecules. This light-matter enhancement is the key to probe molecules with high sensitivity (toward the single molecule resolution). The engineering of the near-field symmetries is a promising emerging topic in the enhancement of the detection sensitivity.
Far field: the symmetries of the near-field interaction with chiral molecules are transferred to the angular repartition, spectral dependence and polarimetric properties of the field scattered in the radiation region. The understanding of the relations between the near-field and chiral molecules allow proposing a new class of plasmonic resonators for the detection of biomolecules.
The CHirOptMol aims at exploiting the resonant optical responses of metallic nanostructures to enhance light matter interactions via the helicity of light. Circular polarization has always played a key role in light matter interactions, in particular to detect chiral molecules with respect to the helicity of light. Symmetric planar structures called pseudo-chiral nanostructures, such as U-shaped scatterers, are very interesting since they can be organized onto planar metasurfaces. They present the great advantage over chiral nanostructures to boost a different circular dichroism for unpolarized light depending on the direction of propagation. The complex interaction between chiral emitters and pseudo-chiral nanostructures has not yet been described analytically. One of the biggest challenges is to calculate each element of the polarizability tensor of (i) resonant pseudo-chiral nanostructures and of (ii) interacting pseudo-chiral and chiral nanostructures or molecules.
The CHirOptMol project aims at exploiting the optical activity of photonic nanoscatterers to develop groundbreaking techniques in the detection of chiral biological molecules.
This objective consists of using pseudo-chiral nanostructures to realize highly sensitive compact biosensors with sensitivity depending on the enantiomer detected. The same detector could thereby detect both the presence and the handedness of biomolecules.Different biomolecules known for their importance in biology will be tested.