Using interferometric lithography (IL), macroscopically extended (ca. 1 cm2) arrays of gold nanostructures have been formed using a double exposure process in combination with an alkylthiolate self-assembled monolayer resist formed on polycrystalline gold on Cr-primed silica. Rotation of the sample between exposures enables the fabrication of a wide range of morphologies, from dots to needles with controllable periods and geometric organisation. Annealing of these structures yields arrays that are highly crystalline and exhibit strong plasmon bands. The samples are robust and may be re-used after cleaning in cold piranha solution. According to the previous literature, the adsorption of proteins onto plasmonic structures yields a small red shift of the plasmon band but no other significant changes in extinction spectra. However, when wild-type light harvesting complex 2 (LH2) from the purple bacterium Rhodobacter sphaeroides was attached to our arrays of gold nanostructures, dramatic changes were observed in the extinction spectrum. New peaks were observed at ca. 430 nm, between 500 and 600 nm and at ca. 850 nm. Spectra of mutant proteins in which the carotenoid had been changed from sphaeroidene to lycopene yielded significant changes between 500 and 600 nm, indicating remarkable sensitivity to variation in protein structure. Similar strong new features were observed in extinction spectra acquired for light harvesting complex 1 (LH1) from R. sphaeroides and from light harvesting complex II (LHCII) from chloroplasts. Changing the array geometry from square to hexagonal also caused significant spectral changes. These features were absent in spectra of films of adsorbed bacteriochlorophyll (BChl) and in spectra of chlorosomes from green sulphur bacteria. We conclude that observation of these unexpected spectral features requires the presence of both a periodic array of gold nanostructures and also the presence of BChl in a specific spatial presentation – as found in LH1, LH2 and LHCII. We believe that they are signatures for strong coupling between lattice plasmons and the excitonic states found in LH1, LH2 and LHCII, and measurements on such systems could prove to be a valuable aid to the investigation of mechanisms of photosynthetic light harvesting

Strong coupling between periodic arrays of gold nanostructures and excitonic states in light-harvesting complexes

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.