CdTe Quantum Dot/Dye Hybrid System as Photosensitizer for Photodynamic Therapy
We have studied the photodynamic properties of novel CdTe quantum dots—methylene blue hybrid photosensitizer. Absorption spectroscopy, photoluminescence spectroscopy, and fluorescence lifetime imaging of this system reveal efficient charge transfer between nanocrystals and the methylene blue dye. Near-infrared photoluminescence measurements provide evidence for an increased efficiency of singlet oxygen production by the methylene blue dye. In vitro studies on the growth of HepG2 and HeLa cancerous cells were also performed, they point toward an improvement in the cell kill efficiency for the methylene blue-semiconductor nanocrystals hybrid system.
KeywordsQuantum dots Nanocrystals Photosensitiser Electron transfer Singlet oxygen
Methylene blue (MB) is a dye that has been extensively used for a variety of photochemical  and medical applications , including photodynamic therapy (PDT). In PDT, it acts as a photosensitizer for the production of singlet oxygen  (O2) through energy transfer between its excited triplet state and the ground triplet state of molecular oxygen. It has been suggested that singlet oxygen may be linked to apoptosis of cancerous cells, while oxygen free-radicals are more likely to cause necrosis . Apoptosis is a more favorable cell death mechanism, since it includes a safe disposal of cellular debris. Necrosis, on the other hand, is a form of traumatic cell death and often leads to inflammation and other complications. The use of MB in PDT has been limited by the formation of MB dimers  and the reduction in MB to the photochemically inactive leuco-MB under physiological conditions , both of which result in a decreased efficiency of singlet oxygen production. In this work, we attempt to increase this efficiency by addition of CdTe nanocrystals (NCs).
Semiconductor nanocrystals (NCs), otherwise known as quantum dots (QDs), are promising for such applications because of their size-dependent optical properties . They have wide absorption bands and relatively narrow, tunable emissions, which makes them ideal candidates as Förster resonance energy transfer (FRET) pairs. The shell of ligand molecules surrounding the NCs allows their chemical properties to be adjusted through relatively straightforward solution-based surface chemistry . On the basis of their versatility and unique optical properties, semiconductor nanocrystals are impacting the areas of photonics [7, 8], electronics , and bio-imaging [8, 9].
Materials and Sample Preparation
Methylene blue dye powder was bought from Sigma–Aldrich. Thioglycolic acid (TGA)-stabilized CdTe NC samples were prepared by an aqueous method as reported previously [10, 11]. Both the dye and the NC samples were diluted in water to give 1 × 10−5 M stock solutions. Doubly purified deionized water from an 18 MΩ Millipore system was used for all dilutions.
Experimental setup used to study the effect of NC–MB mixtures on the growth of cancerous cells
Control (no QD/MB)
QD only control
MB only (1:1)
MB only (5:1)
MB only (10:1)
MB only (20:1)
MB only (30:1)
MB and QD (1:1)
MB and QD (5:1)
MB and QD (10:1)
MB and QD (20:1)
MB and QD (30:1)
MB concentration (M)
5 × 10−6
2.5 × 10−5
5 × 10−5
1 × 10−4
1.5 × 10−4
Results and Discussion
Figure 1b and 1c show absorption spectra for 2.8 nm and 3.3 nm sets, respectively. The absorption spectra of 2.8-nm NC/MB samples appear to be linear combinations of the NC and dye absorptions, suggesting that no major chemical changes occur upon mixing. At 664 nm, the absorption for samples “f” for both NC sizes was less than those of samples “g”, despite the fact that the concentration of the dye in these samples was the same. This is equivalent to saying that the amount of methylene blue monomers (absorb at 664 nm) in NC solutions is less than if NCs were not present. This suggests that some dimerization of dye molecules occurs, with a subsequent increase in absorption at 613 nm (dimer absorption). This result is consistent with previous findings that the presence of interfaces (surfaces) causes the partial dimerization of methylene blue ; it also suggests that MB molecules adsorb onto the nanocrystal surface. This is not surprising since there is an electrostatic attraction between the positively charged MB and negatively charged TGA-capped CdTe NCs.
Steady-state and Time-resolved Photoluminescence Measurements
Adsorption of Methylene Blue on NC Surface
FCS and zeta measurements results for 2.8-nm nanocrystals’ set
Diffusion constant (μm2/s)
Hydrodynamic size (nm)
Zeta potential (mV)
Fluorescence too low: Zeta potential and size measurements were taken instead
Direct Spectroscopic Observation of O2 by NIR PL
HepG2 and HeLa Cell Growth
In solution, methylene blue adsorbs onto the nanocrystal surface with partial dimerization. The dye quenches the NC luminescence primarily via charge transfer, but Förster resonance energy transfer also occurs if there is sufficient overlap between NC emission and MB absorption bands. Both energy and charge transfer imply increased excitation of dye molecules, which suggests that the efficiency of MB to produce reactive oxygen species is also increased. Near-infra red photoluminescence measurements confirmed the increase in the production of singlet oxygen by MB in D2O, while cell growth studies demonstrated an increase in cell kill efficiency for MB–NC mixtures. These results hint toward the possibility of improving the efficiency of any general photosensitizer utilizing semiconductor nanocrystals.
1 Before the subtraction of “f” and “g” PL spectra, both were corrected for the contribution from the tail of NCs’ emission, which had a Gaussian profile in the spectral region in question.
This project was partly funded by the Embark Postgraduate Research Scholarship Scheme of the Irish Research Council for Science, Engineering and Technology (IRCSET). Dr. Yuri Gun’ko and Dr. Nikolai Gaponik are acknowledged for their academic assistance.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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