Biospintronics

In biological systems spin has a role in long-range electron transfer, reactions involving multiple electrons and biorecognition through a phenomena called chiral-induced spin selectivity (CISS). CISS links electron transfer through chiral molecules to the spin state of the electron. One spin state is preferred when electrons are transferred through chiral molecules. Chiral molecules lack mirror symmetry and cannot be superposed on their mirror image. Chiral molecules have a right-handed enantiomer or left-handed enantiomer.

Biological systems tend to use only one enantiomer of any given molecule. Biological electron transfer processes use proteins, which are chiral, a feature predicted to enhance efficiency of electron transfer. The CISS effect predicts that electrons move elastically through chiral molecules with their spin and linear momentum coupled. According to the CISS effect an electron should be able to propagate farther through a chiral molecule than through an achiral analog. The CISS effect is also thought to constrain backscattering with a fluctuating electronic potential in a biological environment. For peptides, proteins and DNA, spin plays a role in electron transfer. Electrons transferred through Photosystem I, involved in photosynthesis in chloroplasts, are spin-polarized. The CISS effect has been studied in systems involving biological molecules such as Langmuir-Blodgett films of L- or D- stearoyl lysine, self-assembled monolayers of polyalanine, DNA, bacteriorhodopsin and helicene molecules.

In photosynthesis, water is split to generate oxygen and protons. Artificial devices that perform photodriven formation of hydrogen have been shown to be more efficient at splitting water when a chiral anode is used compared to an achiral anode.

In spintronics, spin transistors require the ability to inject spins into a semiconductor selectively and efficiently. The CISS effect can enable technology for the injection of spin polarized current without the need for a permanent magnetic layer. Electron transport through chiral molecules occurs by tunneling and hopping. Chiral molecules have a helical electrostatic potential, from a symmetry perspective and their electrostatic properties can be modelled by studying an electron gas confined to a helix-shaped narrow tube. Unlike a straight cylinder where electrons are separable, the curvature entangles degrees of freedom along the tube with the ones across it. CISS could allow for an alternative approach to giant magnetoresistance effect (GMR) based devices. Organic or inorganic chiral non-magnetic material could replace the permanent magnetic layer to provide a spin-polarized electron tunneling current.

A self-assembled monolayer (SAM) of the amino acid cysteine on a thin gold film electrode has been proposed for the design of a spin-injector. When a layer of Al₂O₃ is grown by atomic layer deposition, it is believed the alumina adopts a chiral structure.

A family of helical metallopeptides called Lanthanide Binding Tags LBTs have applications in biospintronics due to being paramagnetic as well as helical. Self-assembling monolayers based on helical lanthanide binding tag peptide formed on a ferromagnetic substrate showed room temperature spin filtering.

DNA has been proposed as a material for use as molecular conductors or nanowires. DNA has a long spin lifetime which would make it useful in spintronics. Long spin lifetime means that the electron spin can travel for long distances without disturbance. Researchers have shown that DNA damage by free radicals is reduced due to a spin-blockade effect. After spin-injection optical pumping the HOMO (Highest Occupied Molecular Orbital) of both the OH-free radicals and the DNA, they became spin-polarized. The injected spin triplet state provides a barrier that can block OH-DNA reactivity.

A bacteriorhodopsin membrane system can serve as a light-activated spin switch. Bacteriorhodopsin is a molecule found in some archaebacteria that pumps protons across a membrane and is powered by sunlight that it captures. Bacteriorhodopsin is part of the purple membrane of Halobacterium salinarum. A bacteriorhodopsin system is a candidate for bioelectronics due to spin-filtering properties explained by the chiral-induced spin selectivity (CISS) effect.

A research group lead by Ron Naaman at the Weizmann Institute in Rehovot, Isreal demonstrated control of spin filtering using light. Spin filtering is when only electrons with a certain spin can easily pass through a material. The researchers compared purple membranes composed of bacteriorhodopsin with a mutant form called D96N adsorbed on nickel substrates. Light absorption by bacteriorhodopsin triggers a shift in electron density through the chromophore. The shift in charge sets in motion event that lead to photoisomerization of the retinal chromophore. The group observed high spin-dependent electron transmission through the membranes and that 532 nm light had the effect of reducing the spin filtering property of the D96N mutant.

Helical oligopeptide molecules attached to CdSe nanoparticles (NPs) on one end and ferromagnetic substrates on the other were constructed. The CISS effect was used to control photoluminescence intensity from the NPs by controlling the direction of the magnetic field of the substrate. The preferred spin transmitted through the structures were also controllable by light.