The understanding of carrier dynamics and the electromagnetic interaction between emerging quantum-confined nanostructures and plasmonic structures is crucial for future biological applications. In this study, plasmonic gold monolayer protected clusters (AuMPCs) were fabricated. We demonstrate enhanced light absorption and emission of the AuMPCs using initial alkali concentration parameters. The absorption is enhanced up to 9 times as a broad and distinct feature centered at 3.33 eV, while the fluoresce emission have been measured up to 3.9 times higher. We also show that the initial alkali concentrations increase caused the change of the charge transfer capability of the surface thiolate ligands through sulfur-gold bonds, which in turn enhance/reduce the fluorescence intensity of the AuMPCs.
Keywords: Gold nanoclusters, carrier dynamics, quantum yield, decay time.
1. Introduction
Optical processes such as absorption and emission, particularly in quantum-confined nanostructures such as platinum nanoclusters, gold nanoparticles (AuNCs), and silver nanoclusters [1-5], can greatly benefit from localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS) for new applications in areas such as photonics [5], sensing [2, 3], imaging [6, 7], and medicine [8-10]. They are considered ‘artificial atoms’ because the wave functions of their charge carriers resemble those of atomic orbitals. Understanding carrier dynamics and the electromagnetic interaction between emerging quantum-confined nanostructures with plasmonic structures is crucial for future optoelectronic devices. However, carrier dynamics are more complex and still not fully understood. The key characteristics of Au monolayer protected clusters (AuMPCs) are heavily dependent on nanostructure morphology and capping agents. They require parameters tuning for optimal performance.
The preparation of gold monolayer-protected clusters which emit high fluorescence intensity have been reported elsewhere[3, 13, 14]. The number of the atoms that form the AuMPCs can be precisely controlled[15, 16]. Compared to the conventional fluorophores such as organic dyes and fluorescent proteins, AuMPCs show advantageous photophysical properties including the photostability, the water-solubility, the large Stoke shift and easy synthesizing with different ligands [14]. The quantum efficiency of AuMPCs depends on both the atom numbers in the cluster and the nature of ligands that have been reported in a number of studies, which in turn, influence optical applications [3, 5, 15]. Owing to the strong binding between thiols and noble metals, thiol-containing molecules are the most commonly used as stabilizers on the surface of metal nanoparticles or metal nanoclusters.
In this article, we report a direct one-pot approach using mercaptoundecanoic acid (MuA) as a thiol ligand to protect and tetrakis (hydroxymethyl) phosphonium (THPC) as a reducing agent, for the preparation of the fluorescent AuMPCs from HAuCl4 in basic aqueous solution at room temperature. The initial alkali concentration is key to precisely control small AuMPCs and the capability of surface ligands. We have concentrated attention to a plasmon absorption band that appears as a broad and distinct feature centered at 3.33 eV (371.5 nm), where enhanced fluorescent emissions of up to 3.9 times have been observed experimentally. We also show that the increase of the initial alkali concentration is accompanied by the strong red shift of the dominant peak and the fluorescence intensity enhancement. The AuMPCs exhibit a quantum yield of a few percent and a maximum fluorescence decay time of about 1.66 ns.
2. Experimental details
Tetrachloroauric acid trihydrate (HAuCl4.3H2O, 99.9 %), ethanol, tetrakis(hydroxymethyl)phosphonium (THPC) (80% solution in water), sodium hydroxide (NaOH, 99%) were purchased from Merck. 11-Mecaptoundecanoic acid (MuA) was purchased from Sigma Aldrich. Deionized (DI) water was used in all experiments.
The aqueous AuNCs solutions were prepared by using a reduction of chloroaric acid in the presence of the MuA. Seven aqueous solutions with various NaOH concentrations ranging from 0.83 mM to 1.1 mM in 2 mL DI water were prepared under magnetic stirring. followed by adding MuA (100 µL, 25 mM) to 1.1 mM. HAuCl4 was also added to the concentration of 1.32 mM. After stirring for 5 minutes, THPC (70 µL, 68 mM ) was added to 1.96 mM for all these samples. The reactions stopped 3 hours later. The AuMPCs solutions were precipitated with 10 µL HCl 1M, the precipitation was then collected and re-dissolved in the 2 mL ID water with 3 µL NaOH 1M. These fluorescent AuMPCs solutions were stored in the dark at 40C. Consequently, we observed reference sample A as having the “classic” NaOH concentration of 0.83 mM to a sample B and sample C having the NaOH concentration including 1.01 mM and 1.1 mM, respectively.
High resolution transmission electron microscopy (HR-TEM) images of the AuMPCs were obtained on a high resolution Tecnai G2 20 S-TWIN / FEI microscope at an acceleration voltage of 200 kV. The absorption spectra of AuMPCs samples were collected by using a Shimazu UV2600 spectrophotometer. Their fluorescence spectra were measured by Varian Carry Eclipse Fluorescence Spectrophotometer. Fluorescence lifetime measurements were performed by using a time-correlated single photon counting (TCSPC) system. Accordingly, a semiconductor laser at 405 nm, 200 ps pulse width was used as the reactive source. A photomultiplier tube (Hamamatsu 5783P) with the response time of 300 ps operating on photon-counting regime was used to obtain the fluorescence decay.
3. Results and Discussion
The TEM images collected from dispersions of the nanoclusters grown in the presence of the AuMPCs. As shown in Figure 1, the TEM image reseals that the nanoclusters are well dispersed, while aggregates are absent, and uniform AuMPCs are formed. The narrow size-distributions of the samples are clearly demonstrated by the corresponding histograms. An analysis of the average diameters, dave, yield 1.6 (b), 1.8(d), and 1.93 (f) nm for sample A, B, and C, respectively. The capping agents are adsorbed onto the particle surfaces due to significant fragmentation energies, the AuMPCs tend to dissociate into smaller Au and Au–thiolate clusters [17,18] These ultra small sizes promise to offer great opportunities for biological labels, energy transfer pairs, and light emitting sources in nanoscale optoelectronics.
For optical properties of the gold nanoslusters, Figure 2 shows typical electronic absorption spectra for the AuMPCs, showing multiple distinct electronic transitions. The lowest four resonances can be reproduced by the superposition of four Gaussian bands. According to the literature, the absorption bands of the AuMPCs originate from metal-centered (Au 5d10 to 6sp interband transitions) and ligand-metal charge-transfer transitions [19, 20]. Interestingly, the best fit-peak energies clearly show the first absorption peak position remains the same, which exhibited energy of 3.33 eV. In sample B, a plasmon band appears as a broad and distinct feature centered at this peak position. This transition is also known as the 1st exciton peak, as it arises from light absorption by an electron in the highest occupied molecular orbital (HOMO) state to transition to the lowest unoccupied molecular orbital (LUMO) state [15, 21]. The first four absorption peaks may be assigned to (1) au(3)-ag(2)[sp-sp], (2) au(3)-ag(3)[d-sp], (3) au(3)-ag(1)[d-sp], and (4) au(3)-ag(2) [S(3p)-sp].
On the basis of these results, maximum absorption peak, which is enhanced about 9 times, is achieved only when the concentration of NaOH solution is 1.01 mM in 2 mL, and increasing/decreasing the concentration reduces the absorption intensity. While the 4th absorption peak, au(3)-ag(2)[S(3p)-sp], increases having increasing the concentration. Such concentration-dependent of the light absorption intensity suggests that localized surface Plasmon resonance (LSPR) of the small AuMPCs and poly-nuclear gold(I) complexes due to the thiol capping agents form very strong covalent, distinctly directional Au-S bonds on the AuNPs surfaces. They can be assumed to be the similar to different capping agents [3, 16, 22], each of which corresponds to the maximum absorption peak within the UV and visible absorption bands.
To clarify whether the enhancement is due to the LSPR of the AuMPCS and poly-nuclear gold(I) complexes, the results for the normalized fluorescence spectra of the all samples measured at room temperatures are shown in Fig. 3a. Sample A possessed the highest-energy ground state of 2.35 eV; sample B and sample C had ground states of 2.27 eV and 2.23 eV, respectively. The full width at half maximum (FWHM) is 350 meV, 340 meV and 324 meV for sample A, sample B and sample C, respectively (Fig. 3b). Compared to sample A, the fluorescence intensity enhancement of sample B is as much as 3.8 times brighter, while sample C exhibited the weakest level of intensity, having a fluorescence emission which was approximately 9 times darker due to saturation effects of the capping ligands. Furthermore, larger Stokes shifts for the sample B and sample C, relative to those for the sample A, are attributed to the former pair’s narrower size distributions and lower degree of distortion in the excited state [14]. The increase in the NaOH concentration in the synthesis process is accompanied by the strong red shift of the dominant peak and the fluorescence intensity enhancement.
Let us focus on the quantum yield (QY). For the experiments described in this section, the photoluminescence quantum yield was calculated referring to Rhodamine6G in ethanol (QY=94%) when the absorbance of all samples is taken into account. Once the three samples, A, B, and C, were diluted in distilled water; their fluorescence quantum yields were found at 0.7, 1.17 and 0.05, respectively. In PBS 1x buffer solution, the values of QY are 0.65, 1.15, and 0.13 for sample A, B, and C, respectively. The fluorescence quantum yield of the AuMPCs follows the order sample B > sample A > sample C. Interestingly, a similar trend was observed in analysis of absorption spectra and fluorescence emissions. The above observation implies that fluorescence is not only due to quantum effects of the small Au monolayer-protected clusters, but more precisely, the surface ligands of the thiol capping agents also play an important role. Increasing NaOH concentration causes the variation of the charge transfer capability of the surface ligands via S-Au bonds, which in turn enhance/reduce the fluorescence intensity of the AuMPCs.
Finally, we have recently studied photophysical processes in the AuMPCs using time-resolved spectroscopy techniques. The time-resolved emission spectra of all samples are studied with 3405 nm light from the excitation source further to understand the luminescence properties of the AuMPCs as shown in Figure 4. From these qualitative observations, the following fitting function is used: I(t) = A1 exp(−t/1) + A2 exp(−t/2) + A3 exp(−t/3), where I(t) is the time-dependent fluorescence intensity, A is the amplitude (noted in parentheses as the normalized percentage, i.e., [Ai/(A1 + A2 + A3 )] × 100), and is the fitted decay time. c The averaged decay times (avg) are obtained using (A11 + A22 + A33)/(A1 + A2 + A3) . The χ2 values for the deconvolution fitting are 1.0-1.2. The values of avg were found at 0.67 ns, 1.66 ns, and 0.25 ns for sample A, B, and C, respectively. The similar trends were discussed about the absorption and fluorescence spectra. Suggesting that S–Au charge-transfer states are longest involved in the sample B. These results demonstrated that the fluorescent decay time of these AuMPCs solutions strongly depends on the alkali concentrations in environment.
4. Conclusion
We synthesized a series of water-soluble green fluorescent AuMPCs whose light emission is tunable through modification of the initial alkali concentrations. As the result, the enhanced absorption band of up to 9 times as a broad and distinct feature centered at 3.33 eV, enhanced fluoresce emission of up to 3.9 times have been measured. We also showed that increasing alkali concentration causes the variation of the charge transfer capability of the surface ligands via S-Au bonds, which in turn enhance/reduce the fluorescence intensity of the AuMPCs. The AuMPCs exhibit a quantum yield of a few percent and a fluorescence decay time maximum of about 1.66 ns.