Interactions of Plasmonic Nanoparticles with Photosynthetic Complexes

Abstract

Tailored metallic nanoparticles (NPs) coupled to specific proteins (bio-nanohybrids) can provide tunable functional materials at the nanoscale. Malleable characteristics of the NPs and diversity of the protein functions widen the horizons of conceivable applications of such bio-nanohybrids. In the presented work the interplay between Photosystem I (PSI) from Thermosynechococcus elongatus (T. elongatus) and different plasmonic NPs was investigated. PSI, an abundant photoactive pigment-protein complex is the essential component of photosynthesis. It binds 300 chlorophyll molecules (Chl a) per trimer within its protein scaffolding to facilitate the conversion of solar energy into chemical energy.

PSI complexes were coupled to different mono- and bimetallic NPs and corresponding plasmonic interaction effects on the fluorescence properties of the PSI were investigated using single-molecule spectroscopy (SMS). The used metallic NPs were fabricated by thermal annealing of thin metallic films. Measurements were performed at different temperatures in a range 1.6 - 250 K and variations in the emission properties of isolated and NP-coupled PSI complexes were analyzed comprehensively.

Interactions with bimetallic NPs exhibiting various metallic compositions were studied at cryogenic temperature (1.6 K). The fluorescence emission of the PSI was intensified upon coupling with NPs. Different compositions of the bimetallic NPs resulted in different enhancement factors (EFs). NPs with Au/Au composition led the maximum enhancement followed by the Ag/Au and Au/Ag. On average the fluorescence was enhanced up to 4.4 (Au/Au), 2.3 (Ag/Au), and 1.1 (Au/Ag)-fold. For individual complexes, however, EFs even up to 22.9 (Au/Au), 5.1 (Ag/Au), and 1.7 (Au/Ag)-fold were observed.The enhancement of the fluorescence demonstrated strong wavelength dependence for all compositions. This was explained considering the largely extended multichromophoric composition of PSI.

To further study the impact of temperature on metal-enhanced fluorescence (MEF) of PSI, monometallic NPs composed of gold only were employed. Measurements were performed at 1.6, 90, 190 and 250 K. A strong temperature dependence of the shape and intensity of the emission spectra was noticed. On average the fluorescence was enhanced 4.3 (1.6 K), 19.4 (90 K), 57.6 (190 K) and 84.0 (250 K)-fold. However, for individual complexes, the EFs up to 230, 250 and even 441-fold were observed. The remarkable increase in EFs at higher temperatures was discussed taking into account the low initial fluorescence yield of PSI, an increase in excitation rate, appearance of new emission channels due to altered excitation energy transfer (EET) pathways and increased spectral overlap between absorption spectrum of gold nanoparticles (AuNPs) and emission spectrum of PSI.

In the last part, photopatterned self-assembled monolayers (SAMs) of the thiols were applied to achieve a precise and controlled attachment of the proteins to the gold substrate. Two thiols, i.e., perfluorinated and carboxylic acid-terminated were applied to prevent the unwanted and encourage the wanted adsorption, respectively. Different surface analysis techniques were used to examine the formation and photopatterning of the SAMs. It was noticed that to ensure the complete photooxidation of the perfluorinated thiol, it is essential to use a specific wavelength of UV light. Perfluorinated thiol did not prove a strong resister against unwanted attachment, as an adequate amount of the PSI was found at nonspecific sites. Carboxylic acidterminated thiol, however, fulfilled the objective and appeared as a suitable candidate to encourage the desired attachment of the PSI.