Enhanced Photoluminescence Property for Quantum Dot-Gold Nanoparticle Hybrid
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In this paper, we have synthesized ZnCdSeS quantum dots (QDs)-gold nanoparticle (Au NPs) hybrids in aqueous solution via bi-functional linker mercaptoacetic acid (MPA). The absorption peaks of ZnCdSeS QDs and Au are both located at 520 nm. It is investigated that PL intensity of QD-Au hybrid can be affected by the amounts of Au and pH value of hybrid solution. The located surface plasmon resonance (LSPR) effect of QD-Au NPs has been demonstrated by increased fluorescence intensity. The phenomenon of fluorescence enhancement can be maximized under the optimized pH value of 8.5. LSPR-enhanced photoluminescence property of QD-Au hybrid will be beneficial for the potential applications in the area of biological imaging and detection.
KeywordsQuantum dot Metal–semiconductor system LSPR
Photoluminescence (PL) properties of CdSe quantum dots (QDs) have been extensively investigated due to its potential application such as solar cell [1, 2], light emitting devices , biological imaging  and so on. With the improvement in material nature, trying composite structure is another way to promote QDs’ properties. Recently, metal–semiconductor system has become research focus because of the complex interplay of enhancing and quenching physicochemical processes due to the metal surface plasmons (SPs). Both the theoretical and experimental study have been done for the promising applications in nanotechnology and biotechnology [5, 6, 7, 8, 9, 10, 11]. To enhance the luminescence, the metal SPs-enhanced fluorescence has been one of the most effective methods . In the isolated metallic nanostructures, due to the collective oscillations of free electrons, the localized surface plasmon resonance (LSPR) is excited accompanied with the enhancement of optical near field . Due to the existence of local electromagnetic field, the distance between the metal and QDs is demonstrated to be an influential factor to the fluorescence enhancement. For example, by using a layer-by-layer polyelectrolyte deposition technique, a distance-dependent enhancement and quenching of CdSe/ZnS core/shell QDs coupling with gold colloids has been observed . The enhancement and quenching of single-molecule fluorescence has also been studied. In the process of gold particle closing to the fluorescent molecule, it is observed that the phenomenon of fluorescence has been enhanced first and then weakened with varied distance between the two particles . Moreover, the coupling of a single molecule to a single spherical gold nanoparticle has been acted as a nanoantenna, and the evidence for the role of LSPR in the excitation and emission processes has been presented . Due to the resonance energy transfer (RET) between donor and acceptor, researchers control the format of the metal NPs similar to the absorption of the acceptor to achieve greatly enhanced luminescence . Additionally, not only the gold NPs, the metal nanoshell can also be used for molecular fluorescence experiments with better adjustability .
However, exploration of the molecular fluorescence enhancement is still so limited, especially in aqueous solution. Up to now, most studies on optical properties of metal–semiconductor system are based on solid state [12, 13, 14, 15]. It is because highly control of the uniformity and dispersibility of the nanoparticles is required in aqueous solution. Different from the solid state, the QDs-metal hybrid presents special properties that can be available for nanotechnology and biotechnology in water phase. On the other hand, the PL-quenching phenomenon indicates the existence of other process in the metal–semiconductor system. Li et al.  investigated carrier dynamics of luminescence quenching in CdSe/ZnS core/shell QDs and emphasized the quenching phenomenon mainly caused by the fluorescence resonance energy transfer. And Yin et al.  proposed that one reason of the PL quenching in metal–semiconductor system is the reverse charge transfer from semiconductor to the metal at the inference. Therefore, to get the strongest enhancement and the weakest quenching is the most important thing to improve the PL properties of the metal–semiconductor system.
In this work, we shall present a novel technique for the synthesis of ZnCdSeS QDs-Au NPs hybrid in aqueous solution. LSPR enhancement for QD-Au hybrid can also be demonstrated.
Synthesis of OA-capped ZnCdSeS QDs
We firstly synthesized oleic acid (OA)-capped ZnCdSeS QDs in chloroform solution . As a typical synthetic procedure, 0.2 mmol of CdO, 4 mmol of zinc acetate, 15.5 mmol of oleic acid (OA), and 30 mL of 1-octadecene were placed in a 100-mL round flask. The mixture was heated to 150 mol in the flowing high-purity N2 for 30 min and further heated to 300 °C to form a clear solution of Cd(OA)2 and Zn(OA)2. At this temperature, a stock solution containing 5 ml of trioctylphosphine, 0.2 mmol of Se, and 4 mmol of S was quickly injected into the reaction flask. After the injection, the reaction temperature was kept for 3 min for promoting the growth of QDs and then cooled down to room temperature to stop the growth. QDs were washed with acetone for three times and finally dispersed in chloroform at the concentration of 10 mg/ml.
Preparation of MPA-capped ZnCdSeS QDs
MPA was chosen as a bi-functional linker to synthesize MPA-capped ZnCdSeS QDs . A 7.5-mL purified OA-capped ZnCdSeS QDs in chloroform solution containing approximately 75 mg of ZnCdSeS QDs in powder form was firstly mixed with 15 mL mercaptoacetic acid (MPA) and 75 mL chloroform in a flask, which was stirred continuously for about an hour. After centrifugation at 6500 rpm for 5 min and the supernatant fluid being discarded, the resulting precipitate containing MPA-capped ZnCdSeS QDs was dissolved in distilled (DI) water and some tetramethylammonium hydroxide (TMAOH) was added to make it clear. The solution was then centrifuged again in the same way. The QDs are stable in the solution which the pH values range from 8 to 12. We controlled the concentrations of the QDs of 10 mg/ml.
Preparation of Au NPs
The water soluble Au NPs were synthesized through the reduction of gold chloride with sodium citrate in aqueous solution . Solutions were prepared of HAuCl4 (0.01 % by weight in DI water, solution A) and of Na3-citrate (0.5 % by weight in DI water, solution B). One hundred millilitre of solution A was heated to boiling, and 4 mL of solution B was then added. The colour of the boiling solution changed from blue to a brilliant red, which indicated the formation of spherical particles. After the boiling state maintaining for about 5 min, the reduction of gold chloride had almost completed.
Preparation of QD-capped Au NPs Hybrid
The QD-Au hybrid was fabricated using the above QDs and Au NPs solution . The QDs solution was firstly diluted to one ninth of the original concentration by using DI water and then taken 1 mL to add certain quantities of Au NPs. The quantity of Au NPs was varied from 0 to 120 μL. After about 5 h standing, the sample solution was completed. The QDs solution and Au NPs solution with the same concentration were prepared to compare with the sample solution. Here, we kept the quantities of QDs constant and only changed the volume of Au NPs solution.
Sample and Device Characterization
The absorption and photoluminescence (PL) spectra were measured by U-4100 UV-visible and NIR-300 spectrophotometer, respectively. The TEM was equipped with a multiscan charge-coupled device (CCD) camera system (Model 894, Gatan, USA) to record the HRTEM images.
Results and Discussion
To investigate the amount of Au NPs on the LSPR effect, we prepared the ZnCdSeS QDs-Au hybrid with the quantity of Au NPs varied from 0 to 120 μL. Figure 2c shows the absorption spectra of bare Au NPs and QDs-Au hybrids. The spectra peak of Au NPs and QDs-Au NPs were both located at 520 nm. It is worth noting that the spectra peak of Au NPs is consistent to ZnCdSeS QDs (Fig. 2a), and the absorption peak intensity is increased with unchanged peak position as the amount of Au NPs increased.
The PL spectra of ZnCdSeS QDs with and without Au NPs excited at 365 nm are shown in Fig. 2d. The emission wavelengths were located at 540 nm regardless of the content of Au NPs. Additionally, the phenomenon of fluorescence enhancement and quenching was clearly observed. The peak value increased until 60 μL of Au NPs solution was added into QDs and then declined. The phenomenon described above indicates that the PL intensity of ZnCdSeS QD-Au hybrid is mainly determined by two processes [4, 12, 13]: (1) PL quenching due to the energy and the charge transfer between Au and the QDs or other nonradiative processes and (2) PL enhancement due to the enhancement of absorption and radiative rate induced by LSPR. The PL intensity of the QDs/Au composite system is the competing result of the two processes above. Obviously, as the amount of Au NPs reached to 60 μL, the second process was playing a major role, as a result of PL enhancement; as the amount of Au NP exceeded 60 μL, the first process dominated, leading to the PL quenching phenomenon.
Figure 5b shows peak fluorescence intensity of QD-Au hybrid solutions in aqueous solution (pH value = 8.5, 9.5, 10.8, 11.7). The fluorescence intensity of the hybrid can be enhanced by decreasing the pH value of the solution. The highest PL value occurs under pH value of 8.5. It can be deduced that a secondary coordination at the CdSe particle surface exists, which in return provides better surface passivation and consequently higher PL efficiency . After adding Au solution, the enhancement phenomenon can also be observed, suggesting an interaction between the QDs and Au NPs exists due to the LSPR from Au NPs. However, the enhancement phenomenon cannot be observed anymore with the increased pH value. It can be explained by that the combination between Au and QDs is affected due to the changing pH value, which contains the LSPR from Au NPs to QDs. Under the condition of pH value of 8.5, the highest PL intensity can be achieved. Meanwhile, it is observed that the emission peak has red-shifted, which was attributed to an additional reaction taking place on the QD surface during the increase of the pH value [20, 21].
Highly efficient ZnCdSeS QDs-Au hybrids have been synthesized via connecting with bi-functional linker. It is demonstrated that Au NPs can result in PL enhancement of ZnCdSeS QDs due to metal LSPR, which depends on the amount of Au NPs and pH value of solution. As the amount of Au NPs increased, the PL intensity reached the maximum value, then decreased, which was reverent with the morphology of QD-Au hybrid. In addition, PL intensity was also affected by pH value of the hybrids. The phenomenon of PL enhancement can be maximized under the pH value of 8.5.
This work was supported partially by the National Key Basic Research Program 973 (2013CB328804, 2013CB328803), the National High-Tech R&D Program 863 of China (2012AA03A302, 2013AA011004), National Natural Science Foundation Project (51120125001, 61271053, 61306140, 61405033, 91333118, 61372030, 61307077 and 51202028, 51372039), Beijing Natural Science Foundation (4144076) and Natural Science Foundation Project of Jiangsu Province (BK20141390, BK20130629, and BK20130618).
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