The simple and effective two-step procedure (assembly/separation) outlined
here was also successfully applied to other types of QDs (CdSe, CdSe/ZnS) and
seems to be applicable to a wide range of ligand-stabilized colloidal nanoparticles.
Moreover, it opens the way to a detailed study of electronic coupling in, e.g.,
quantum dot molecules.
2.3 Biological–Chemical Hybrids Built from
Light-Harvesting Complexes
2.3.1 Hybrid Constructs of Light-Harvesting Complex II (LHCII)
and Quantum Dots
Biological photosynthesis as well as artificial constructs for solar energy conversion
such as photovoltaic devices are dependent on the efficient absorption of solar light.
In natural systems, this is accomplished by the presence of light-harvesting
complexes (i.e., protein complexes containing a number of pigments that are able
to absorb the incident light, ideally over the entire spectrum) and then to conduct the
excitation energy towards reaction centers where energy conversion takes place.
QDs have widely been used as artificial light harvesters in chemical constructs,
due to their efficient absorption and their exceptional photochemical stability
[63]. Numerous attempts have been made to combine QDs with biological systems
to improve their performance. Thus, QDs have been used in combination
with isolated reaction centers to replace their biological light-harvesting units
[64–66]. Alternatively, QDs have been added to improve the light-harvesting
capacity of biological complexes such as cyanobacterial phycobilins [67, 68],
cyanobacterial phycoerythrin [69, 70] and purple-bacterial light-harvesting proteins
LH1 and LH2 [70]. In these combinations with biological light-harvesting
complexes, the QDs were intended to serve as energy donors to one or several
biological pigment(s). The efficiency of this energy transfer was difficult to
compare with the expected one because the interactions between QDs and protein
complex were not defined enough to predict distances between the components.
We have been working with a recombinant version of the major light-harvesting
complex (LHCII) in green plants. The advantage of the recombinant origin of this
complex was the possibility to modify the apoprotein structure such that it
contained defined anchors for the interaction with QDs, such as hexahistidine
tags tightly interacting with Zn
2+ in the outer ZnS shell [71]. The main pigments
in LHCII, chlorophylls a and b, absorb very well in the blue and red spectral range
but only poorly in the green one. To try and fill this “green gap”, we used CdSe/ZnS
QDs emitting at 600 nm, binding via the C-terminal hexahistidine tag in trimeric
recombinant LHCII. When LHCII was added to the QDs at various ratios between
0.25 and 3, the fluorescence emission of the QD donor concomitantly decreased up
to about 70% quenching (Fig. 8). At the same time, only a small amount of
Optical Properties of Assemblies of Molecules and Nanoparticles
81
here was also successfully applied to other types of QDs (CdSe, CdSe/ZnS) and
seems to be applicable to a wide range of ligand-stabilized colloidal nanoparticles.
Moreover, it opens the way to a detailed study of electronic coupling in, e.g.,
quantum dot molecules.
2.3 Biological–Chemical Hybrids Built from
Light-Harvesting Complexes
2.3.1 Hybrid Constructs of Light-Harvesting Complex II (LHCII)
and Quantum Dots
Biological photosynthesis as well as artificial constructs for solar energy conversion
such as photovoltaic devices are dependent on the efficient absorption of solar light.
In natural systems, this is accomplished by the presence of light-harvesting
complexes (i.e., protein complexes containing a number of pigments that are able
to absorb the incident light, ideally over the entire spectrum) and then to conduct the
excitation energy towards reaction centers where energy conversion takes place.
QDs have widely been used as artificial light harvesters in chemical constructs,
due to their efficient absorption and their exceptional photochemical stability
[63]. Numerous attempts have been made to combine QDs with biological systems
to improve their performance. Thus, QDs have been used in combination
with isolated reaction centers to replace their biological light-harvesting units
[64–66]. Alternatively, QDs have been added to improve the light-harvesting
capacity of biological complexes such as cyanobacterial phycobilins [67, 68],
cyanobacterial phycoerythrin [69, 70] and purple-bacterial light-harvesting proteins
LH1 and LH2 [70]. In these combinations with biological light-harvesting
complexes, the QDs were intended to serve as energy donors to one or several
biological pigment(s). The efficiency of this energy transfer was difficult to
compare with the expected one because the interactions between QDs and protein
complex were not defined enough to predict distances between the components.
We have been working with a recombinant version of the major light-harvesting
complex (LHCII) in green plants. The advantage of the recombinant origin of this
complex was the possibility to modify the apoprotein structure such that it
contained defined anchors for the interaction with QDs, such as hexahistidine
tags tightly interacting with Zn
2+ in the outer ZnS shell [71]. The main pigments
in LHCII, chlorophylls a and b, absorb very well in the blue and red spectral range
but only poorly in the green one. To try and fill this “green gap”, we used CdSe/ZnS
QDs emitting at 600 nm, binding via the C-terminal hexahistidine tag in trimeric
recombinant LHCII. When LHCII was added to the QDs at various ratios between
0.25 and 3, the fluorescence emission of the QD donor concomitantly decreased up
to about 70% quenching (Fig. 8). At the same time, only a small amount of
Optical Properties of Assemblies of Molecules and Nanoparticles
81