Modifying the Fluorescence of CdTe Quantum Dots by Silica-Coated Gold Nanorods

1. Introduction

Luminescent semiconductor quantum dots (QDs), especially those emitting near-infrared light, such as CdTe, PdS, are considered to be very interesting material for photovoltaic [1], photodetection [2] and biological sensing [3, 4, 5]. However, the photoluminescence (PL) efficiency of QDs prepared in aqueous phase is not very high. It is important to investigate increasing the PL intensity of QDs.

Noble metal nanostructures that support the collective oscillations of conduction-band electrons, which are known as localized surface plasmon resonances (LSPR), can concentrate optical electromagnetic field into subwavelength regions close to the surface of the nanostructures. They enhance inherently weak physical processes such as fluorescence and Raman scattering [6]. The processes of light emission in QDs can be significantly affected by proximal metal nanoparticles (NPs) through coupling between QD excitons and LSP of metal nanostructures. This is due to the joint effect of the excitation enhancement and quantum yield modification. Thus, metal/QDs hybrid nanostructures with excitonic and plasmonic resonances that are independently tunable are a promising way of enhancing the PL efficiency of QDs [7, 8, 9, 10].

The LSPR properties of metal nanostructures can be tailored by synthetically tuning their sizes, shapes components and other topological aspects. The plasmon wavelength of the longitudinal mode of Au nanorods can be synthetically tuned from visible to near-infrared regions by tailoring their aspect ratios, that is, the ratio between the length and diameter [11]. Thus, it is easily achievable for Au nanorods/CdTe QDs hybrid nanostructures to realize exciton and LSP coupling, and further obtain PL efficiency enhancement.

In this paper, Au nanorods were used to modify the PL of CdTe QDs. A thin silica layer on the Au nanorods was prepared to investigate the influence of the distance between particles and QDs on the PL properties of CdTe QDs. We found that the quenching effect of the Au nanorods was decreased by the isolation of the silica layer.

2. Experiment

Gold nanorods were synthesized according to Jana's report [12]. Au nanorod precipitates were collected and dispersed in water. To coat a silica layer onto the surface of the Au nanorods, 100 μL 1%(v/v) 3-mercaptopropyltrimethoxysilane (MPS) ethanol solution was added into 10 mL Au nanorod solution. The mixture was stirred for 20 min. Then, 0.2 mL 1.25 wt% Na₂SiO₃·9H₂O aqueous solution was added and the pH value of the solution was adjusted to about 8.5 using a diluted HCl solution. The resulting dispersion was then kept for two days, so that the silica layer with a thickness of about 5 nm formed onto the gold nanorod surface. As the -SH group of MPS has a stronger affinity than CTAB, MPS displaced CTAB on the surface of the gold nanorods. Moreover, the Si(OH)₃ groups acted as the growth sites of active silica supplied by the hydrolysis of SiO₃²⁺. CdTe QDs were prepared according to previous literature [13].

To connect CdTe QDs to the surface of silica layers of Au@SiO₂ nanorods, Au@SiO₂ nanorods were dispersed in 20ml ethanol. Then, 1 mL 3-aminopropyltriethoxysilane (APS) was added. The mixture was stirred for 12 h before it was centrifuged and re-dispersed in 10 mL water. 1 mL CdTe was centrifuged and re-dispersed in 10 mL water. 10 μL EDC·HCL and 14 mg N-hydroxysuccinimide (NHS) were added into the CdTe solution and stirred for 15 min. Adding the Au@SiO₂ solution to the CdTe solution, the pH value of the solution was adjusted to about 7.5 using 1 M NaOH. The mixture was stirred for 4 h. The sample was collected by centrifugation and re-dispersed in water.

The extinction spectra of particle colloids were carried on an UV-vis-NIR spectrophotometer (U4100, Hitachi). The morphology of the particles was characterized by a transmission electron microscope (TEM, Philips CM200). PL and time-resolved PL decay spectra were measured by a multichannel photon counting system (Edinburg Photonics, Livingston, UK).

3. Results and Discussion

The TEM image of the Au nanorods is shown in Fig. 1. It can be seen that the length of the Au nanorods is about 40 nm and the diameter is about 10 nm. The surface of the Au nanorods carries positive charges, while that of the CdTe QDs carries negative charges. As such, when mixing these two kinds of aqueous solution, CdTeQDs and Au nanorods should aggregate, as displayed in Fig. 2a. In this condition, the CdTe QDs are directly attached to the Au nanorods. This is suggested in Fig. 2b, which shows the CdTe QDs with obvious crystal lattices adsorbing on the surface of the Au nanorods. After adding the Au nanorods, the PL of CdTe QDs was obviously quenched (Fig. 2c), while the bare CdTe QDs had a strong PL peak at ∼720 nm. There are two reasons for the quenching. First, as the distance between the CdTe QDs and Au surface is very short, the nonradiative energy transfer of CdTe QDs to the Au nanorods is very strong [14, 15]. Second, the light emitted by the CdTe QDs is absorbed by the Au nanorods in solution. As we know, the energy transfer depends on the distance between the donor and the acceptor [16]. Furthermore, the extremely high absorption in close proximity to the metallic surface is seriously detrimental to fluorescence. Thus, in our experiment, a thin silica layer was deposited on the Au nanorod surface to reduce the quenching effect.

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Figure 1.

TEM image of the Au nanorods

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Figure 2.

TEM images of the Au nanorods/CdTe aggregates with (a) low and (b) high magnifications. (c) The PL spectra of CdTe QDs, which directly attached to the Au nanorods.

The TEM image of Au@SiO₂ is shown in Fig. 3a. The gold particles were coated with a 5 nm silica layer. The extinction spectra of the Au nanorods and Au@SiO₂ nanorods are shown in Fig. 3c. The longitudinal extinction peak of the Au nanorods was at ∼720 nm. Due to the higher refractive index of the coated SiO₂ layer, the peak shifted to ∼765 nm. There was no obvious broadening of the extinction peak, which suggests that there were no severe aggregation during the formation of the Au@SiO₂ particles.

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Figure 3.

TEM images of (a) Au@SiO₂ nanorods and (b)Au@SiO₂-CdTe complex. (c) The UV-vis-NIR extinction spectra of the Au and Au@SiO₂ nanorods.

To attach CdTe QDs to the surface of silica layers of the Au@SiO₂ nanorods, APS was added to modify the surface of the Au@SiO₂ particles, resulting in NH₂⁻ group on the silica surfaces. Meanwhile, EDC were added into the CdTe solution to activate the COO⁻ group of MPA on the surface of the CdTe QDs. As reported, at pH 7.5, the NH₂⁻ group and COO⁻ group react and form amide bonds [17, 18]. Thus, the CdTe QDs were connected onto the surface of the Au@SiO₂ nanorods after mixing the two solutions. As shown in Fig. 3b, the CdTe QDs with recognizable crystal lattices in red circles stayed at a distance of more than 5 nm away from the surface of gold particles due to the silica space layer. This clearly demonstrates that CdTe QDs and Au nanorods are separated by the SiO₂ layer.

The normalized PL spectra of Au@SiO₂-CdTe solution and the reference of bare CdTe QDs were obtained, as shown in Fig. 4a. The pure CdTe QDs had an emission peak at ∼720 nm. After attaching to Au@SiO₂, there was a 20 nm red shift in emission, which indicates the coupling between QDs and LSPs of metal NPs. Meanwhile, the PL intensity of the QDs was still very strong. Therefore, the isolation of the silica layer prevents the quenching effect of Au nanorods on QDs. Moreover, the coupling between QDs and metal NPs can alter the total decay rate, which is the sum of the radiative, and nonradiative decay rates owing to the localized density of photonic states [7]. This was confirmed by the time-resolved PL decay spectra. As shown in Fig. 4b (the black and blue lines are experimental results and the red and green lines are fitted curves), CdTe QDs on Au@SiO₂ nanorods decreased faster than pure CdTe QDs. From the fitted curves, the average lifetime of CdTe QDs was calculated as 37 ns, while that of the CdTe QDs on Au@SiO₂ NPs was decreased to 23 ns. Through further tuning the extinction peak of the Au nanorods, the thickness of the SiO₂ layer and the excitation conditions as in the study of Au NP@SiO₂-CdTe [14], it is possible to enhance the PL intensity of CdTe QDs.

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Figure 4.

(a) Normalized PL spectra and (b) time-resolved PL decay spectra of CdTe QDs, which attached to the Au@SiO₂ nanorods

4. Conclusion

In conclusion, Au nanorod @SiO₂-CdTe QDs hybrid nanostructures were synthesized and the influence of Au nanorods on the PL of CdTe QDs was examined. Through coating a silica layer on the surface of the Au nanorods, the quenching effect was reduced. The red-shift of CdTe QDs emission wavelength and the enhanced decay rate corroborated the coupling between the CdTe QDs and Au nanorods, which resulted in PL quenching reduction. Considering the convenience and flexibility of functionalizing the surface of the Au@SiO₂-CdTe complex, this hybrid nanostructure has great potential for biological sensing.