We report on the observation of chirality induced spin selectivity for electrons transmitted through monolayers of oligopeptides, both for energies above the vacuum level as well as for bound electrons and for electrons conducted through a single molecule. The dependence of the spin selectivity on the molecular length is measured in an electrochemical cell for bound electrons and in a photoemission spectrometer for photoelectrons. The length dependence and the absolute spin polarization are similar for both energy regimes. Single molecule conductance studies provide an effective charge transport barrier between the two spin channels and it is found to be on the order of 0.5 eV

Spin-dependent photoelectron transmission and spin-dependent electrochemical studies were conducted on purple membrane containing bacteriorhodopsin (bR) deposited on gold, aluminum/ aluminum-oxide, and nickel substrates. The result indicates spin selectivity in electron transmission through the membrane. Although the chiral bR occupies only about 10% of the volume of the membrane, the spin polarization found is on the order of 15%. The electrochemical studies indicate a strong dependence of the conduction on the protein's structure. Denaturation of the protein causes a sharp drop in the conduction through the membrane.electron transfer | electrochemistry | magnetic effect | chirality T he role of the electron spin in chemistry and biology has been receiving much attention because of a plausible relation to electromagnetic field effects on living organisms (1), and due to the seemingly importance of the earth's magnetic field on birds and fish navigation (2). Part of the difficulty in studying the subject arises from the lack of a physical model that can rationalize these phenomena. Recently, the chiral-induced spin selectivity (CISS) effect was observed in electron transmission and conduction through organic molecules (3). The spin selectivity was observed for photoelectron transmission through monolayers of double-stranded DNA adsorbed on gold (4). Another study discovered a spin dependence in the conduction through single molecules of double-stranded DNA. In this configuration, one end of the molecule was adsorbed on a Ni substrate, whereas the other was attached to a gold nanoparticle (5).The CISS effect may provide a novel approach for better understanding the role of electron spin in biological systems. The studies mentioned above led to several questions, including the actual role played by the gold substrate in the overall spinfiltering process. Gold exhibits a very large spin orbit coupling; hence, one may wonder whether gold itself affects the CISS phenomenon. In addition, the interface between gold and the thiol group, through which the molecules are attached to the gold, may play a role. Because many of the past studies were performed with DNA, an important question arises whether CISS is a general effect or possibly a special property of DNA. CISS was only observed for double-stranded DNA, whereas for single-stranded molecules, no spin selectivity was found. On the one hand, this was attributed to the lack of ordered monolayers (4, 6); on the other hand, a theoretical model, proposed to rationalize the CISS effect, predicted that a double-helix structure (7) was needed for CISS to occur, whereas other approaches do not emphasize this need (8). Finally, because many of the past studies were performed in vacuum or in ambient air, it is of importance to probe to what extent the effect persists in solutions, which are more relevant to biology. The present study aims at answering the above questions in an attempt to establish CISS as a general phenomenon.For the present study, we chose bacteriorhodopsin (...

The role of the electron spin in chemistry and biology has received much attention recently owing to to the possible electromagnetic field effects on living organisms and the prospect of using molecules in the emerging field of spintronics. Recently the chiral-induced spin selectivity effect was observed by electron transmission through organic molecules. In the present study, we demonstrated the ability to control the spin filtering of electrons by light transmitted through purple membranes containing bacteriorhodopsin (bR) and its D96N mutant. The spin-dependent electrochemical cyclic voltammetry (CV) and chronoamperometric measurements were performed with the membranes deposited on nickel substrates. High spin-dependent electron transmission through the membranes was observed; however, after the samples were illuminated by 532 nm light, the spin filtering in the D96N mutant was dramatically reduced whereas the light did not have any effect on the wild-type bR. Beyond demonstrating spin-dependent electron transmission, this work also provides an interesting insight into the relationship between the structure of proteins and spin filtering by conducting electrons.

The search to understand the origin of homochirality in nature has been ongoing since the time of Pasteur. Previous work has shown that DNA can act as a spin filter for low-energy electrons and that spin-polarized secondary electrons produced by X-ray irradiation of a magnetic substrate can induce chiral selective chemistry. In the present work it is demonstrated that secondary electrons from a substrate that are transmitted through a chiral overlayer cause enantiomeric selective chemistry in an adsorbed adlayer. We determine the quantum yields (QYs) for dissociation of (R)- or (S)-epichlorohydrin adsorbed on a chiral self-assembled layer of DNA on gold and on bare gold (for control). The results show that there is a significant difference in the QYs between the two enantiomers when adsorbed on DNA, but none when they are adsorbed on bare Au. We propose that the effect results from natural spin filtering effects cause by the chiral monolayer.

Spin-dependent photoluminescence (PL) quenching of CdSe nanoparticles (NPs) has been explored in the hybrid system of CdSe NP purple membrane, wild-type bacteriorhodopsin (bR) thin film on a ferromagnetic (Ni-alloy) substrate. A significant change in the PL intensity from the CdSe NPs has been observed when spin-specific charge transfer occurs between the retinal and the magnetic substrate. This feature completely disappears in a bR apo membrane (wild-type bacteriorhodopsin in which the retinal protein covalent bond was cleaved), a bacteriorhodopsin mutant (D96N), and a bacteriorhodopsin bearing a locked retinal chromophore (isomerization of the crucial C13=C14 retinal double bond was prevented by inserting a ring spanning this bond). The extent of spin-dependent PL quenching of the CdSe NPs depends on the absorption of the retinal, embedded in wild-type bacteriorhodopsin. Our result suggests that spin-dependent charge transfer between the retinal and the substrate controls the PL intensity from the NPs.

We have investigated the room temperature ferromagnetic order that develops in Al-substituted magnesium oxide, Mg(Al)O, nanoparticles with Al fractions of up to 5 at.%. All samples, including undoped MgO nanoparticles, exhibit room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at.% of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization for 2 at.% Aldoped MgO. Our results suggest that the oxygen vacancies are largely responsible for room temperature ferromagnetism in MgO.

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A combined sonochemical co-precipitaion method has been developed for the synthesis of a CoFe(2)O(4)-Cr(2)O(3)-SiO(2) magnetic nanocomposite. The synthesis involved the pre-synthesis of CoFe(2)O(4)-Cr(2)O(3) nanoparticles, which were subsequently coated with SiO(2) by treatment with tetraethyl orthosilicate. It was observed that the as-prepared CoFe(2)O(4)-Cr(2)O(3)-SiO(2) nanocomposite exhibits photoluminescence properties without the addition of any external fluorescent marker. The fluorescent magnetic nanoparticles (FMNPs) had a typical diameter of 30 ± 5 nm and a saturation magnetization of 5.1 emu g(-1) at room temperature. This as-prepared nanocomposite was used for staining cultured HeLa cells for fluorescence imaging.