Ultrasensitive detection and characterization of biomolecules using superchiral fields

Abstract

The spectroscopic analysis of large biomolecules is important in applications such as biomedical diagnostics and pathogen detection 1,2 , and spectroscopic techniques can detect such molecules at the nanogram level or lower. However, spectroscopic techniques have not been able to probe the structure of large biomolecules with similar levels of sensitivity. Here we show that superchiral electromagnetic fields 3 , generated by the optical excitation of plasmonic planar chiral metamaterials 4,5 , are highly sensitive probes of chiral supramolecular structure. The differences in the effective refractive indices of chiral samples exposed to left-and right-handed superchiral fields are found to be up to 10 6 times greater than that those observed in optical polarimetry measurements, thus allowing picogram quantities of adsorbed molecules to be characterised. The largest differences are observed for biomolecules that possess chiral planar 2 sheets, such as proteins with high β-sheet content, which suggest that this approach could form the basis for assaying technologies capable of detecting amyloid diseases and certain types of viruses.Since the building blocks of life are chiral molecular units such as amino acids and sugars, biomacromolecules formed from these units also exhibit chirality on molecular and supramolecular scales. Chirally sensitive (chiroptical) spectroscopic techniques, such as circular dichroism (CD), optical rotatory dispersion (ORD) and Raman optical activity (ROA), are therefore especially incisive probes of the threedimensional aspects of biomacromolecular structure and are widely used in biomolecular science 1,2 . Chiroptical methods typically measure small differences, or dissymmetries, in the interaction of left-and right-circularly polarised light, the chiral probe, with a chiral material 2 . However, the inherent weakness of these existing chiroptical phenomena usually restricts their application to samples of microgram level. Recently, Tang and Cohen 3 postulated that under certain circumstances superchiral electromagnetic fields could be produced that display greater chiral asymmetry than circularly polarised plane light waves. We have realised such superchiral electromagnetic fields are generated in the near fields of PCMs, and can greatly enhanced the sensitivity of a chiroptical measurement, enabling us to detect and characterise just a few picograms of a chiral material.PCMs were first fabricated, and shown to display large chiroptical effects such as optical rotation, by Schwanecke and co-workers 4 and Gonokami and co-workers 5 .The PCMs used in this study, Fig. 1 (a), are composed of left or right handed (LH / RH) Au gammadions, of length 400 nm and thickness 100 nm (plus a 5 nm Cr adhesion layer) deposited on a glass substrate and arranged in a square lattice with a periodicity of 800 nm. As a control we repeated all experiments using a metamaterial composed of achiral crosses with the same thickness and periodicity as the gammadions: these structures showed no dissymmetry in excita...

Optical Chirality Flux as a Useful Far-Field Probe of Chiral Near Fields

“…If the structure then selectively dissipates optical chirality of one handedness, a non-zero outgoing optical chirality flux must be generated according to Eq. (6). In this case, the only chiral light contained in the scattered field is due to the structure.…”

“…Further, a significant increase in enantioselectivity was found when chiral molecules interact locally with electromagnetic fields with χ greater than in circularly polarized light [5]. Consequently, χ has since been used to study chiral plasmonic nanostructures [6,18,19,27]. It has identified near fields of high optical chirality enhancement, defined as the time-averaged optical chirality density [Eq.…”

“…Due to the conservation law [Eq. (6)], the optical chirality flux can be determined with a surface integral of Eq. (9) over the boundaries of the computational domain, following Gauss' law.…”

Triple-helical nanowires by tomographic rotatory growth for chiral photonics

“…A dvanced chiral photonic components that are able to manipulate the polarization state of light are widely required in several application fields [1][2][3][4][5] . Recently, chiral metamaterials have attracted great interest because of the possibility to achieve negative refraction by means of chirality 6 or to artificially reproduce and enhance the weak chiro-optical effects present in natural chiral molecules (enantiomers).…”

Chirality of matter shows up via spin excitations

“…1 Department of Physics, Budapest University of Technology and Economics and Condensed Matter Research Group of the Hungarian Academy of Sciences, 1111 Budapest, Hungary, 2 Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), Japan c/o The University of Tokyo, Tokyo 113-8656, Japan, 3 Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Chiba 277-8561, Japan, 4 Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan, 5 Department of Physics, The University of Tokyo, Tokyo 113-0033, Japan, 6 National Institute of Chemical Physics and Biophysics, Tallinn 12618, Estonia, 7 Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan, 8 Cross-correlated Materials Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN Advanced Science Institute, Wako 351-0198, Japan. *e-mail: kezsmark@dept.phy.bme.hu.…”