On-Chip Functionalization of Carbon Nanotubes by the Photosystem I

Prof. Dr. Alexander Holleitner

Photosynthesis in plants and bacteria is driven by molecular electronic complexes, such as the photosystem I (PSI). Photoexcitation of PSI causes an electron transfer through a series of redox reactions from the chlorophyll special pair P700, as a primary electron donor, to a final electron acceptor, ∼6 nm away from the oxidant P700+. Here, we used carbon nanotubes (CNTs) as nanoscale electrical wires to electrically contact the PSI [1,2]. We compared the optoelectronic properties of the CNT-PSI hybrids for three different on-chip functionalization strategies [3]. The PSI was bound to the CNTs via covalent, hydrogen, or electrostatic bonds.

For device fabrication, individual carboxylated CNTs are first deposited on an insulating SiO2 substrate and then contacted with Pd source-drain electrodes by e-beam lithography. In the first chemical route, selectively generated cysteins on the lumenal side of the PSI reacted with maleimide functionalized CNTs. AFM images before and after functionalization of the CNTs with PSI provide a first indication that PSI proteins are attached to the sidewalls of the CNTs due to covalent bonding between the CNT and the PSI. Hence, the electron transfer path in PSI is expected to be perpendicular to the CNT (sketch in Figure 2a). Filled squares in Figure 2a depict the wavelength dependence of the photoconductance of sample A, which was fabricated by the first chemical route. The photoconductance showed an enhanced signal at 680 and 360 nm. The findings are consistent with the absorbance data of the PSI (solid line in Figure 2a). Importantly, the enhanced GPH below 700 nm can be attributed to an effect of the PSI. This was proven by measurements on sample B (Figure 2b). Here, the photoconductance of a CNT bundle before and after on-chip functionalization with PSI by covalentbonding was compared. The maxima at 680 nm and at ∼360 nm only appear when the PSI is attached to the CNTs. Above 700 nm, the PSI does not absorb light, and in turn, the magnitude of the photoconductance before and after functionalization with PSI is the same within the experimental error. Consequently, the two photoconductance maxima around 760 and 845 nm and also the maximum at 825 nm in Figure 2a are interpreted to result from electron-hole dynamics only within the contacted CNTs [2,3]. These findings are consistent with resonances assigned to the E22 transition in semiconducting CNTs with a diameter in the range 1-2 nm [4]. The origin of the enhanced photoconductance at the resonances of the PSI can be attributed to an electron or energy transfer between the excited PSI and the CNTs.