Conference-ICSFS-14-Optical Studies of the Methylene Blue-Semiconductor Nanocrystals Hybrid System ∗

The methylene blue–semiconductor nanocrystals system was studied by absorption and fluorescence correlation spectroscopy, as well as integrated and time-resolved photoluminescence measurements. These studies point to the adsorption of methylene blue molecules on NC surface, with partial dimerization of the dye. Quantum dot luminescence quenching was observed with increasing dye concentration. Examination of band offsets for this system and lifetime measurements suggest that efficient charge transfer is responsible for the quenching. Surface functionalization of nanocrystals may reduce the extent of dye dimerization, thereby improving the charge transfer between CdTe nanocrystals and methylene blue. [DOI: 10.1380/ejssnt.2009.349]


I. INTRODUCTION
Nanoscale materials have become increasingly more important in current research due to their unique physical and chemical properties.In this work we report on the development of a hybrid material with enhanced properties for photochemical and light-harvesting applications.We chose methylene blue (MB) dye and CdTe nanocrystals (NCs) as a model system for our hybrid material.
Methylene Blue (MB) is a dye that has been extensively used for a variety of photochemical [1] and medical [2] applications.It is also a very good electron acceptor [1].Common problems with this dye include its dimerization in the presence of interfaces [3] and its reduction to a photochemically inactive leuco-MB form under some conditions [4].
Semiconductor nanocrystals (NCs), otherwise known as quantum dots (QDs), are promising for such applications because of their size-dependent optical properties [5].They have wide absorption bands and relatively narrow, tunable emissions, which makes them ideal candidates for solar energy conversion applications-by carefully selecting NCs of appropriate size, maximum spectral overlap can be achieved between the suns' emission and QD absorption spectra.The shell of ligand molecules surrounding the NCs allows their chemical properties to be adjusted through relatively straightforward solutionbased surface chemistry [6].

A. Materials and sample preparation
Methylene Blue dye powder was acquired from Sigma-Aldrich.Thioglycolic acid (TGA)-stabilised CdTe NC samples were prepared by an aqueous method as reported previously [6].Doubly purified deionised water from an 18 MΩ Millipore system was used for all dilutions.Two CdTe samples were used, with emissions centred at 545 nm and 645 nm (Fig. 1(a)).These emission maxima correspond to NC sizes of 2.8 nm and 3.3 nm respectively, as calculated according to Peng [7].Both of these samples were diluted to give 10 −5 M stock solutions and then mixed with a 10 −5 M MB stock solution to give two sets of mixtures of increasing MB concentrations.To aid the reference to the samples, each set was labelled with NC size (2.8 or 3.3 nm) and then letters "a" to "g" corresponding to decreasing QD:MB ratios (Table I).QD solutions were sonicated for approximately 5 minutes prior to mixing.

B. Measurements
Absorbance spectra were recorded on a Varian Cary50Conc UV-visible spectrophotometer.Steady-state photoluminescence measurements were carried out using a Varian CaryEclipse Fluorescence Spectrophotometer (λ ex = 400 nm).A Malvern NanoZS was used for zeta potential measurements.PL decays were measured using time-correlated single photon counting utilising a PicoQuant MicroTime200 set-up.Measurements were performed in ambient conditions at room temperature on solutions diluted to yield reasonable signal intensity.Samples were excited by a 480 nm picosecond laser pulse (PicoQuant LDH-480 laser head controlled by PDL-800B driver).The overall resolution of the set-up was ∼150 ps.The measured PL decays were reconvoluted using nonlinear least-squares analysis.This was done with FluoFit software (PicoQuant) using an equation of the form where τ i are the PL decay lifetimes and α i are the corresponding pre-exponential factors, taking into account the normalisation of the initial point in the decay to unity.Weighted residuals and χ 2 values were used to judge the quality of the fit.A fit with an χ 2 value of less than 1.1 was considered to be good.The τ i and α i values obtained from the fit were then used to calculate the average lifetime τ av using: Fluorescence Correlation Spectroscopy (FCS) measurements were performed on a MicroTime 200 confocal microscope (PicoQuant) fitted with an oil immersion objective.Appropriately diluted samples were excited with a 480 nm pulsed laser diode (LDH-480) with a repetition rate of 80 MHz.Laser intensity was adjusted to give a reasonable signal.Measurements were performed at room temperature with total acquisition times of about 1 minute.The data was stored in the Time-Tagged Time-Resolved mode (TTTR) and then analysed using Sym-PhoTime software (PicoQuant).A pure diffusion fitting model was employed.In this case, only the diffusion of fluorophores contributes towards the correlation curve, and the FCS intensity (G(t)) is given by where t is the lag time, τ i the diffusion time of the ith diffusing species and ρ i the contribution of ith species.κ is the length to diameter ratio of the focal volume (V eff = π 3/2 w 2 0 z 0 ), where w 0 and z 0 are the effective lateral focal radius and focal radius along the optical axis at e −2 intensity.Once τ i and w 0 were determined, the diffusion constant, D i , of ith species were calculated using Figures 1(b) and 1(c) show the absorption spectra of 2.8 nm and 3.3 nm sample sets respectively.An increase in the absorption of solutions in the 600 nm to 700 nm region was observed for samples a to e in each set, which corresponds to an increase in dye concentration in these samples.The absence of any new absorption peaks in QD-MB mixtures indicates that no major chemical changes occur upon the addition of the dye to QD solutions.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology  Samples f and g in each sample set have equal MB concentrations (see Table II).Nonetheless, when compared to samples g, a decrease in the absorption of samples f at 664 nm and an increase in the absorption at 633 nm were observed.The absorption at 644 nm is due to MB monomers, and absorption at 613 nm is due to the MB dimers [3].This data suggests that some dimerization of the dye occurs in QDs solutions.This is consistent with previous findings that presence of an interface causes partial dimerization of the dye [3] and indicated that MB dye molecules adsorb onto nanocrystals' surface.This is not surprising, since there is an electrostatic attraction between the positively-charged MB and negatively-charged TGA-capped CdTe NCs.

B. Adsorption of methylene blue on QD surface
As already mentioned above, it is likely that MB molecules adsorb onto the NCs surface.Non-linear Stern-Volmer plots, derived from the PL data at 480nm excitation (Figs.2(c) and 2(d)), are in support of this since such concave upward curves have been shown to indicate that both static and dynamic quenching take place [9].
Several additional measurements were performed to validate MB adsorption.First, the dependence of PL intensity for every sample in the 3.3 nm NC set was measured as a function of pH (Fig. 3).By increasing the pH of the http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)  sample, the ionic strength of solution is altered.For sample a (pure QD solution), this results in a better solubility and stability of nanocrystals up to pH of about 11.Consequentially, the PL intensity of CdTe NCs increases.This effect was also seen once the MB is introduced.However, complete quenching was not achieved for pH above 11.This may be attributed to a lesser association/attraction between MB and QDs due to increased ionic strength.
A more direct confirmation for the adsorption was obtained from the average hydrodynamic size measurements and FCS data.Zeta measurements showed increasing nanocrystal size and zeta potential with increasing dye concentration (Table II).The values for the hydrodynamic size of the QDs are the average size of the NCs in a sample with a typical Gaussian size distribution.An increase in the size is consistent with adsorption of dye molecules on QD surface.Figure 4 QDs-the spikes in this curve correspond to QDs emission as they pass through the focal volume.This data was fitted to a pure diffusion model to yield the correlation curves shows in Fig. 4(b).From these fits, the diffusion coefficients were calculated, revealing decreasing diffusion coefficients, that are in agreement with increasing particle size (Fig. 4 and Table II).

C. Quenching mechanism
Steady-state photoluminescence measurements showed that QD luminescence quenched upon addition of MB (Figs. 2(a) and 2(b)).Complete quenching was achieved for 10:1 MB to QD ratio for both QDs sets.There was, however, a difference in quenching for the 2 sets (Fig. 2(b)), with more efficient quenching for the 3.3 nm CdTe NCs for the same MB to QDs ratio.
In this system, quenching can occur either by Forster resonance energy transfer (FRET) or charge transfer.If it is assumed that FRET is primarily responsible for the observed quenching of luminescence, then the difference in the quenching curves for the two samples must be explicable by the differences in the spectral overlaps between the emission bands of the two QDs samples and the absorption band of the MB.However, as it can be seen qualitatively from Fig. 1(a), the difference in the spectral overlap is expected to be rather large for the two samples.Therefore, we would expect a much more drastic difference in quenching behaviour of QDs' emission.Also, as in this case NCs act as FRET donors, an enhanced emission from the acceptor, the methylene blue dye, would be expected [10].MB fluoresces at approximately 680 nm, but no enhancement in its fluorescence was observed for either of NC sets (curves f and g, inset of Fig. 2(a)).In fact, its fluorescence was found to also be completely quenched in the presence of QDs.
Another signature of FRET is a decrease of the average emission lifetime of the donor [11].However, no change in the lifetime of 2.8 nm NCs was detected (Fig. 5(a)), and a very small decrease in the lifetime of 3.3 nm NCs was detected (Fig. 5(b)).This implies that FRET cannot be responsible for the quenching of 2.8 nm NCs, but may contribute to the quenching of 3.3 nm QDs.This conclusion is in agreement with the differences in the spectral overlap between MB absorption and NC emission bands (Fig. 1(a)).It follows that QD luminescence quenching occurs primarily via charge transfer-the inspection of energy band off-sets for MB and QDs (Fig. 6) concurs with this.
The comparison of PL spectra for 2.8 nm QD set at 480nm and at 633 nm excitation wavelengths provides evidence for the photoinduction of charge transfer in this system.At 480 nm the QDs are excited, leading to an efficient charge transfer and a subsequent quenching of the dye luminescence (inset of Fig. 2(a)), most likely due to the formation of the MB triplet state [8].2.8 nm CdTe NCs do not absorb at 633 nm, there is thus no charge transfer and a full, unquenched emission from the dye can be measured (Fig. 7, inset).The data presented here suggest that this QDs-MB system could be utilised for solar energy conversion and electrochemical applications, as efficient charge transfer is induced between the components upon absorption of photons by the hybrid nanostructures.

IV. CONCLUSIONS
We have demonstrated that QD luminescence is quenched by MB via efficient charge transfer, implying that charge separation can be achieved in the general case of semiconductor nanocrystals coupled with a good electron-accepting dye.Such a system has the potential to be beneficial for solar energy conversion applications.Partial dimerization of MB upon its adsorption to nanocrystal surface was shown to occur, but perhaps this can be diminished by appropriate functionalization of the NC surface.

Volume 7 (
FIG. 1: (a) Normalised absorption spectrum of methylene blue dye (black) and photoluminescence spectra of 2.8 nm (blue) and 3.3 nm (red) colloidal CdTe samples.(b) and (c) show the change in the absorption spectrum of (b) 2.8 nm and (c) 3.3 nm CdTe NCs as the concentration of the dye is increased (curves a-g).

FIG. 2 :
FIG. 2: (a) PL changes of 3.3 nm CdTe NCs as the concentration of increasing dye concentration (curves a-g).Inset: PL spectra of sample f (black) and sample g (red).(b) Photoluminescence intensity against the number of MB molecules per NC in solution for both 2.8 nm (black squares) and 3.3 nm (red triangles) QDs.Stern-Volmer plots for (c) 2.8 nm and (d) 3.3 nm NC series.

FIG. 3 :
FIG. 3: The pH dependence of PL intensity for 3.3 nm QD set.
FIG. 7: Photoluminescence spectra of 2.8 nm CdTe NCs/MB mixtures at 633 nm excitation.The inset shows last two samples in the series normalised based on absorption at excitation wavelength.

TABLE I :
A representative table detailing the volumes of QD and MB stock solutions used to make up solutions with fixed QD:MB ratios.The total volume of all solutions was 2 mL and the concentration of CdTe was kept constant for all ratios (with exception of the last solution).

TABLE II :
FCS and zeta measurements results for 2.8 nm NC set.