Fluorescence imaging of hybrid nanostructures composed of natural photosynthetic complexes and reduced graphene oxide

Abstract

rates, this technique in its many facets provides insight into various processes such as energy transfer, electron transfer, excited state reactions, molecular rearrangements, plasmonic interactions, and complex formation [1]. These are undoubtedly the key effects determining the properties of materials at the nanoscale, which can lead to changes in fluorescence intensity. While enhancement of fluorescence intensity can be most frequently induced by coupling fluorophores with metallic nanoparticles [2], either through absorption or fluorescence rate increase, fluorescence quenching is often the result of energy transfer to neighbouring acceptors. If these acceptors can emit fluorescence, the energy transfer leads to an increase of acceptor fluorescence intensity [1]; its fluorescence dynamics can also be affected. Alternatively, in the case of non-emitting acceptors, the energy is dissipated non-radiatively, predominantly as heat. Such a scenario appears in hybrid nanostructures that involve metallic nanoparticles [3] or carbon-based materials [4][5][6][7]. Following the first reports on obtaining graphene [8,9], a two-dimensional honeycomb carbon lattice known for its unique optoelectronic, thermal, and mechanical properties, several groups have applied this material as an acceptor in studying fluorescence of emitters placed in its vicinity [10][11][12][13]. The results indicate that incorporating graphene in such structures provides unexplored opportunities in devising novel architectures possibly applicable in photonics, optoelectronics, and biosensing [12][13][14][15].In addition to graphene, other carbon-based compounds, namely graphene oxide (GO) and reduced graphene oxide (rGO), have been suggested as components of functional hybrid nanostructures. At first, they were considered disadvantageous, described often as distorted forms of highly crystalline graphene. Nevertheless, easy processing, solubility in various solvents, e.g. water, and the presence of oxygen containing functional groups [16], mean both GO and rGO are attractive materials for coupling them with other nanostructures. Particularly Abstract: Herein, we describe the results of fluorescence microscopy imaging of peridinin-chlorophyll-protein (PCP) photosynthetic complex mixed with reduced graphene oxide (rGO). Upon incorporation of rGO the fluorescence image of PCP changes substantially from one characterized by uniform intensity towards a more complex pattern. The isolated bright spots feature up to ten times higher emission intensity compared to the fluorescence of PCP in the reference sample, where no rGO was added. The number of the bright spots increases with increasing rGO concentration. At the same time the fluorescence intensity away from the bright spots in the PCP/rGO hybrid system is quenched in comparison to the PCP -only reference.

Spectral Dependence of the Energy Transfer from Photosynthetic Complexes to Monolayer Graphene

“…However, the wavelength dependence differed considerably between the PCP complexes on graphene and on glass. First of all, the intensities measured for PCP on graphene were lower than those measured for the reference sample, which fact may be attributed to the energy transfer between both components of the assembly [ 3 , 4 , 6 , 7 , 58 ]. The direct comparison of actual emission intensities would have been questionable, as we did not know the precise concentration of PCP complexes at a given position of the assembly.…”

Energy Transfer in Graphene-Based Hybrid Photosynthetic Nanostructures

“…Briefly, the process usually starts with oxidizing graphite using one of the many popular oxidation methods: Hummers, Brodie or Staudenmaier [25,26]. In the next step, due to the presence of oxygen-containing functional groups (hydroxyls, carbonyls, carboxyls or oxygen epoxides), graphite oxide can be easily exfoliated into GO via ultrasonication or mechanical stirring.…”

“…Interactions between various emitters and rGO were studied so far only in solution, where ensemble averaging can smear out subtle effects associated with the interaction between emitters and rGO. Our idea was to prepare mixtures with controlled concentration of both PCP and rGO, deposit solutions on glass coverslips and image fluorescence with high spatial resolution and high sensitivity [26]. In all experiments, the concentration of PCP was maintained constant.…”

“…In this way, we strongly reduce any possible influence of particular sample preparation or a way the experiment was carried out. Next, the fluorescence images are analysed by plotting a histogram of all the intensities measured [26]. This procedure can be applied for any single map, but also to all the maps measured for a given excitation wavelength and rGO concentration.…”