Fluorescence energy transfer in quantum dot/azo dye complexes in polymer track membranes
© Gromova et al.; licensee Springer. 2013
Received: 11 September 2013
Accepted: 12 October 2013
Published: 31 October 2013
Fluorescence resonance energy transfer in complexes of semiconductor CdSe/ZnS quantum dots with molecules of heterocyclic azo dyes, 1-(2-pyridylazo)-2-naphthol and 4-(2-pyridylazo) resorcinol, formed at high quantum dot concentration in the polymer pore track membranes were studied by steady-state and transient PL spectroscopy. The effect of interaction between the complexes and free quantum dots on the efficiency of the fluorescence energy transfer and quantum dot luminescence quenching was found and discussed.
For two decades, photophysical properties of complexes of semiconductor nanocrystals, or quantum dots (QDs), with organic molecules (OM) are of great interest. Creation of different structures based on quantum dots and organic molecules allows expanding significantly the area of quantum dot applications. These QD complexes are employed in sensors, catalysis, electronic devices, biology and medical studies [1–4].
In QD/OM complexes, quantum dots are commonly used as energy donors [2–5]. QDs have high extinction coefficient in a broad spectral range and a high luminescence quantum yield. Moreover, their optical transition wavelengths depend on the nanocrystal size. Therefore, it is very easy to match the QD luminescence and absorption spectrum of an appropriate energy acceptor needed for efficient resonance energy transfer. Additionally, QDs have high photostability and chemical resistance compared with organic molecules .
A lot of studies elucidate formation of complexes of quantum dots and organic molecules [7, 8] that demonstrate effective QD/molecule energy transfer. The fluorescence resonance energy transfer (FRET) is common for such systems. As a rule, FRET in individual complexes was under consideration, but effects of interaction between the complexes and free donors on the efficiency of energy transfer, which may take place at a high donor concentration, were not well analyzed. The high donor (QDs) concentration can be realized, e.g. in porous matrices like the poly-(ethylene terephthalate) pore track membrane (PET TM) . This matrix allows embedding the QDs in the loosened layer of the track pore walls. Another remarkable feature of the track membrane is the possibility to create the complexes by subsequent incorporation of QDs and molecules into the track membrane .
In the present work, FRET in complexes of semiconductor CdSe/ZnS quantum dots and molecules of heterocyclic azo dyes 1-(2-pyridylazo)-2-naphthol (PAN) and 4-(2-pyridylazo) resorcinol (PAR) formed at high concentration in the PET track membranes were studied. We demonstrate the evident effect of free QDs on the efficiency of FRET from QDs to the acceptor molecules.
PET TM were obtained from the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research (Dubna, Russia). The membrane characteristics are as follows: pore diameter d is 0.5 μ m, thickness l is 12 μ m and pore density n is 2.9×107 cm -2.
Hydrophobic colloidal CdSe/ZnS QDs with 2.5-nm core diameter with luminescence at 530 nm were used. The membranes were immersed in the QD colloidal toluene solution with a QD concentration of 10-6 mol/L for 5 days. The membranes with embedded QDs were removed from the solution, rinsed thoroughly by toluene and dried before measurements. The concentration of QDs embedded in the wall layer along pores was estimated from their absorption spectrum taking into account the TM parameters with the assumption that the thickness of the layer with quantum dots does not exceed 200 nm, as was shown in .
PAN and PAR were purchased from Sigma Aldrich (St. Louis, MO, USA) and used without further purification. QD/azo dye complexes were produced both in solutions and in track membranes. QD/PAN complexes in the toluene solutions were prepared by adding PAN solutions with various concentrations (CPAN) to the QD solution with a concentration (CQD) of about 5×10-7 mol/L. The molar ratio (n=CQD:CPAN) in the solutions varied from 5:1 to 1:10. The QD/PAR complexes were created similarly. For creation of QD/azo dye complexes in the track membranes, the TMs with embedded QDs were immersed into toluene PAN or aqueous PAR solutions with different concentrations (10-8÷10-6 mol/L) of molecules for a week. This period is enough for the establishment of chemical equilibrium between QD/azo dye complexes and free compounds. After impregnation of azo dyes, the membranes were thoroughly rinsed in toluene and dried under the ambient condition.
where A i and τ i are the amplitude and decay time of the i th component, respectively.
Results and discussion
Investigation of quantum dot interaction in the track membranes
QD/azo dye complex formation
In both cases, the formation of QD/PAN and QD/PAR complexes is accompanied by QD luminescence quenching (Figure 3a,b). The QD luminescence quenching due to the efficient intracomplex FRET from QDs to these azo dye molecules has been reported in .
The number of azo dye molecules bounded to the quantum dot surface obeys a Poisson statistic .
- 2.The probability of binding x azo dye molecules with the QD surface is(2)
The PL of QD totally quenched in the complex. Therefore, only luminescence of free QDs is observed.
- 4.While only free QDs luminesce, the PL intensity of QDs is proportional to the number of free QDs. Then, x=0 in Equation 2, and the QD PL quenching is described by(3)
where I0 and I are the PL intensity of QDs before and after QD/azo dye complex formation, respectively; CAD and CQD are the molar concentration of azo dye molecules and QDs, respectively.
where n = CAD/CQD.
The adjustable parameters of fitting of QD PL quenching curves presented in Figure 4
QD/PAN in solution
QD/PAN in TM
QD/PAR in solution
QD/PAR in TM
The fitting gives a B of about 1 for QD/azo dye complexes in solutions which means formation mainly of one azo dye molecule to one QD complex. In TMs, however, the B values are more than unity which indicates that one azo dye molecule totally quenches the PL of QD in the complex and at the same time partially quenches some neighboring free QDs. The B values of approximately 3 and approximately 5 for QD/PAN or QD/PAR complexes in track membranes show quenching of luminescence of the host quantum dot and two and four neighboring QDs, respectively.
where Φ is the orientation factor, q0D is the quantum yield of the donor in the absence of the quencher, n is the refractive index of the environment and NA is the Avogadro number.
FRET parameters of QD/azo dye donor-acceptor pairs
J, M-1 cm4
We show that in the polymer track membranes with high local QD concentration, an efficient FRET between neighboring QDs takes place. The strong QD PL quenching is observed at additional embedding of azo dye molecules into the membranes due to formation of QD/azo dye complexes with FRET from QD to molecules. A detailed analysis of steady-state and transient QD PL responses as a function of azo dye concentration shows that one azo dye molecule bound with Zn ions on the surface of QD quenches not only PL of the QD in the complex but also PL of the closest neighboring free QDs. This becomes possible because of high local concentration of QDs in the membranes. Then, the quenching of luminescence of the QD ensemble occurs even at a relatively low complex concentration. Using different azo dyes allows to control the value of the overlap integral and to manage the efficiency of QD luminescence quenching. The present result could be used for designing different types of micro fluidic devices . Dissociative sensors developing based on track membranes and QD/azo dye complexes are promising. In this system, elimination of one quencher molecule will lead to recovering luminescence signal from several QDs at once which will significantly increase sensor sensitivity.
This work was supported by the RFBR (grants 12-02-01263 and 12-02-00938) and the Ministry of Education and Science of the Russian Federation (projects 11.519.11.3026, 14.B37.21.0741, 14.B25.31.0002 and 14.B37.21.1954).
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