Substrates for Plasmon-enhanced VCD Spectroscopy
The detection of pure optical isomers via infrared spectroscopy requires both specially equipped measuring instruments and little or undiluted substances. The plasmon-based amplification of the vibrational circular dichroism makes it possible to investigate comparatively small amounts of substances. Such substrates were designed, produced, and characterized for the first time.
By R. Knipper // T. Mayerhöfer
Chirality is a physicochemical property that we encounter daily. Translated, the term means “handiness” and illustrates nicely that two objects behave like mirror images of each other (i.e., they have the same relationship to each other as our left and right hands do). The same phenomenon is also observed in molecules, such as the right and left-turning lactic acid seen in advertising. From a chemical point of view, with regard to their reactions, this means that both molecules show practically no difference. In fact, both forms have the same empirical formula and the same functional groups. Both also have in common that they have a C atom which carries four different functional groups. The only difference is the spatial arrangement of these functional groups. This difference is responsible for the fact that both molecules behave like mirror images of each other. They are, therefore, referred to as “enantiomers.”
While chemically there is practically no difference between enantiomers, biochemically the difference is considerable. The reason for this is that the majority of the building blocks of life, the amino acids, are also chiral and, categorically speaking, only one of the enantiomers occurs in organisms. Accordingly, our enzymes are chirally selective (i.e., they only catalyze the reactions of one enantiomer, and the other is not degraded. The consequences can be fatal, as for example with the drug thalidomide, in which one enantiomer has a calming effect while the other causes severe malformations in unborn children. Since chemical synthesis often produces both enantiomers, or one enantiomer can transform into the other over time, rapid analytical methods are required.
Such a fast analysis method can be carried out with light. The enantiomers are not referred to as left or right turning for nothing. Rather, this is a description of the property that one enantiomer rotates the plane of oscillation of linearly polarized light to the left and the other one to the right. This property is accompanied by the fact that the two enantiomers absorb so-called circularly polarized light (the plane of oscillation of light with this property is permanently rotated, and the tip of the polarization vector describes a circle) to different degrees. However, these differences are very small in circular dichroism. In the visible light range, the difference is one one-thousandth. In the infrared spectral range, in which the molecular vibrations are excited, they are even one to two orders of magnitude smaller. Together with the less brilliant radiation sources and the lower sensitivity of the detectors in this range, it seems to make little sense to measure circular dichroism in this range. However, the bands for biological molecules are usually much more numerous and characteristic of the structure, in contrast to the UV range and the visible range of the spectrum. So characteristic that the absolute structure (i.e., the actual arrangement of the substituents) can even be determined using quantum mechanical calculations.
In addition, the small differences can theoretically be amplified plasmonically. In contrast to the visible range, plasmon structures can have resonance frequencies which correspond to the vibrational excitations. At least in the UV range, this cannot be achieved with gold and silver structures because they are no longer plasmon metals (strictly speaking, both are no longer metals in this range).
In practice, however, this resonant plasmonic amplification in the infrared spectral range has not yet been achieved. One reason for this may be that the theory behind the plasmonic amplification of chiral effects is not yet sufficiently understood. The common doctrine is that plasmon substrates themselves should not be chiral for the amplification of chiral effects. Instead, however, they should produce a so-called “superchiral field” in which the field lines of the electric and magnetic fields should not be perpendicular to each other and which, in addition to great strength, should always have the same sign.
In the substrate we designed and manufactured, we have ignored all these principles: it is itself chiral and the areas with a positive superchiral field are equalized by those with a negative superchiral field. Nevertheless, it is the first substrate which actually works and with which one can clearly distinguish between the enantiomers and the 1:1 mixture, the so-called racemate (see Figure 1). Accordingly, future work will consist not only of optimizing the substrate but also understanding why and how the substrate works and how the existing theory has to be modified accordingly.
Gefördert von: EU, Free State Thuringia, BMBF, DFG, FCI, Carl-Zeiss-Stiftung.