Quantum Dots Assembled with Photosynthetic Antennae on a Carbon Nanotube Platform: A Nanohybrid for the Enhancement of Light Energy Harvesting

Abstract

The construction of artificial systems for solar energy harvesting is still a challenge. There needs to be a light-harvesting antenna with a broad absorption spectrum and then the possibility to transfer harvested energy to the reaction center, converting photons into a storable form of energy. Bioinspired and bioderivative elements may help in achieving this aim. Here, we present an option for light harvesting: a nanobiohybrid of colloidal, semiconductor quantum dots (QDs) and natural photosynthetic antennae assembled on the surface of a carbon nanotube. For that, we used QDs of cadmium telluride and cyanobacterial phycobilisome rods (PBSr) or light-harvesting complex II (LHCII) of higher plants. For this nanobiohybrid, we confirmed composition and organization using infrared spectroscopy, X-ray photoelectron spectroscopy, and high-resolution confocal microscopy. Then, we proved that within such an assembly, there is a resonance energy transfer from QD to PBSr or LHCII. When such a nanobiohybrid was further combined with thylakoids, the energy was transferred to photosynthetic reaction centers and efficiently powered the photosystem I reaction center. The presented construct is proof of a general concept, combining interacting elements on a platform of a nanotube, allowing further variation within assembled elements.

Figure 1

Quenching of PBSr (A) and LHCII (B) fluorescence by increasing the concentration of MWCNTs. For LHCII, excitation at 405 nm was followed by emission at 675 nm. For PBSr, emission was 650 nm with excitation at 575 nm. F0 is the fluorescence intensity without a quencher, and F—fluorescence intensities with increasing MWCNTs. Titration was done at ∼1 μM LHCII and 0.2 μM PBSr, respectively, in a final 1 mL of 25 mM HEPES/NaOH, pH 7.5, and MWCNTs were added in 1–2 μM aliquots from concentrated stock solution. The transmittance effect shows the hypothetical fluorescence intensity change if caused by MWCNT light screening only (see the text and Figure S1 for an explanation).

Figure 2

FT-IR analysis of the CNT conjugation process. (A) The OH and CH vibration range and (B) amide vibration range recorded for original MWCNTs, their oxidized version, as well as MWCNTs decorated with BSA and BSA-PBSr-QD530. BSA, PBSr, and QD530 spectra are shown for reference. No normalization was applied.

Figure 3

XPS analysis of MWCNT-BSA-QD preparation. (A) The region of O 1s with a clearly present COOH bond and (B) the region of N 1s with visible Cd bonds.

Figure 4

CLSM images of MWCNT-QD530-PBSr hybrids and their mixture with Synechocystis PCC 6803 thylakoids. The samples contained MWCNT-QD530-PBSr (∼300 μg/mL) with or without thylakoids (the amount corresponding to 100 μg/mL of chlorophyll). The excitation wavelength was 500 nm, and the emission ranges for each channel are indicated.

Figure 5

Representative example of the FLIM analysis for MWCNT-QD530-PBSr (∼300 μg/mL) and thylakoid mixtures (100 μg/mL of chlorophyll). (A–C) FLIM images of QD530, PBSr, and thylakoid, respectively. Averaged lifetimes [ns] for selected ROIs are indicated. The excitation wavelength was 500 nm, and the emission ranges for each channel were listed. (D) The overlap image of ROIs extracted from FLIM images. The contours of the individual ROIs are colored according to their lifetime. (E, F) The dependence of QD530 and the chlorophyll lifetime on the spatial overlap with the other components of the FRET system. The points represent individual ROIs, colored according to the lifetime. For the details of the analysis procedure, see the Supporting Information, Figures S5 and S6.

Figure 6

Green channel (A, E)—SIM₂ image of QD530 fluorescence (excitation 405 nm, emission 490–570 BP), red channel (B, F)—SIM₂ image of PBSr fluorescence (excitation 630 nm, emission with FS filter), transmission channel (C, G) normal resolution, no processing) and overlay (D, G) of channels. Scale bar = 2 μm.

Figure 7

Light-dependent oxygen consumption by thylakoids is greatly enhanced in the presence of the MWCNT-BSA-QD530-PBSr nanohybrid. The samples contained the number of thylakoids corresponding to 20 μg/mL chlorophyll and ∼400 μg/mL nanohybrid in the presence of a redox system (1 mM sodium ascorbate +0.1 mM DCPIP + 0.5 mM methyl viologen). The rate of oxygen consumption was calculated according to the slopes of oxygen concentration curves measured by the Clark-type electrode during the first 2–3 min after turning on the illumination.

Figure 8

Schematic representation of the possible routes of energy transfer in a system composed of CNT-QD-PBSr and thylakoids (A) and differentiation of the thylakoid photosystems according to the spatial distance to a nanohybrid and access to energy transfer donors. Note that the scheme does not preserve the size ratio of combined elements and may not represent exactly the actual arrangement of elements.