Goutam
Pramanik
a,
Jana
Humpolickova
a,
Jan
Valenta
b,
Paromita
Kundu
c,
Sara
Bals
c,
Petr
Bour
a,
Martin
Dracinsky
a and
Petr
Cigler
*a
aInstitute of Organic Chemistry and Biochemistry of the CAS, Flemingovo nam. 2, 166 10 Prague 6, Czech Republic. E-mail: cigler@uochb.cas.cz
bDepartment of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic
cEMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
First published on 8th January 2018
The increase in nonradiative pathways with decreasing emission energy reduces the luminescence quantum yield (QY) of near-infrared photoluminescent (NIR PL) metal nanoclusters. Efficient surface ligand chemistry can significantly improve the luminescence QY of NIR PL metal nanoclusters. In contrast to the widely reported but modestly effective thiolate ligand-to-metal core charge transfer, we show that metal-to-ligand charge transfer (MLCT) can be used to greatly enhance the luminescence QY of NIR PL gold nanoclusters (AuNCs). We synthesized water-soluble and colloidally stable NIR PL AuNCs with unprecedentedly high QY (∼25%) upon introduction of triphenylphosphonium moieties into the surface capping layer. By using a combination of spectroscopic and theoretical methods, we provide evidence for gold core-to-ligand charge transfer occurring in AuNCs. We envision that this work can stimulate the development of these unusually bright AuNCs for promising optoelectronic, bioimaging, and other applications.
The increase in nonradiative pathways with decreasing energy of emitted light (i.e., the “energy gap law”)37 makes it extremely challenging to prepare AuNCs that emit in the NIR region with high quantum yields (QYs). The photoluminescence QY of AuNCs can be enhanced via charge transfer from the ligands to the metal core (i.e., LMCT) through the Au–S bonds and is parallel with the ligand's capability of donating electron density to the metal core through the S–Au bond (i.e., charge transfer capability of the ligand).38 The luminescence of AuNCs can also be “turned on” upon introduction of sulfur-containing ligands39 or enhanced by either sulfur oxidation at the Au–ligand interface40 or electronic polarization of the bonds between the Au core and thiolate ligands.41 Nevertheless, the currently known NIR-emitting AuNCs achieve relatively low QYs.
One promising approach to a great increase in QY is based on an efficient but experimentally laborious doping of the Au core with a precise number of Ag atoms.3 In contrast, ligand-based PL enhancement approaches are more general and synthetically straightforward and can directly produce water-soluble AuNCs. In this report, we present results from our study of ligand-mediated improvement of the NIR luminescence efficiency of AuNCs. We found that the weak emission of AuNCs stabilized with thioctic acid (TA) and polyethylene glycol (PEG) can be enhanced dramatically by introducing triphenylphosphonium cations in the capping layer. Motivated by this finding, we investigated in detail the photophysical properties of these NIR PL AuNCs. We present experimental evidence suggesting the enhancement of the luminescence QY via charge transfer interactions from the gold core to the triphenylphosphonium cations, i.e. by metal-to-ligand charge transfer (MLCT).
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Fig. 1 (a) Schematic representation of the structure and preparation of NIR PL AuNCs 1 and 2. X− = Br−, I−, Cl−, or BF4−. (b) Normalized absorption spectra of 200 μg mL−1 aqueous solutions of 1 and 2 (X− = BF4−). Inset: Photograph of the solutions under white light. (c) Normalized emission spectra of 200 μg mL−1 aqueous solutions of 1 and 2 (X− = BF4−) collected upon excitation at 365 nm. Inset: Photograph of the solutions upon UV illumination (365 nm). (d) Time-resolved photoluminescence decay curves for 1 and 2. Inset: Plot of the emission rates (k) of 1 and 2 (X− = BF4−) as a function of reciprocal temperature (1000/T). Fits are based on eqn (2) (note the rate scale is logarithmic with base e). |
The zeta potentials of 1 and 2 in phosphate buffer (10 mM, pH 7.4) were −10.0 ± 1.8 mV and +1.6 ± 0.3 mV, respectively. The higher zeta potential of 2 compared to 1 is consistent with a charge switch from a part of negatively charged carboxylic groups to positively charged TPP.
As expected, unlike the UV-vis absorption spectra of larger Au nanoparticles, the spectra of the AuNCs did not display a strong surface plasmon resonance around 520 nm (Fig. 1b). The AuNC solutions are therefore only light yellow in color (inset Fig. 1b). However, the enhanced absorption of 2 compared to 1 in the UV region revealed a strong interaction between the gold core and TPP (Fig. 1b). This enhancement of absorption can be attributed to the charge transfer from the metallic core to the ligands, as described by Sementa et al. for other highly delocalized sterically hindered ligands present on AuNCs.43
Both 1 and 2 showed broad emission with the maxima around 750 nm (Fig. 1c). However, upon introduction of TPP, the PL intensity dramatically increased (Fig. 1c). The observed change in the PL intensity corresponds to the increase in PL QYs from 10% to 25% estimated for 1 and 2 (X− = BF4−), respectively (Fig. S1 and Table S1 in the ESI†). The PL increase is visible to the unaided eye, as documented by photographs of the AuNC solutions upon UV illumination (λex = 365 nm) (Fig. 1c inset). We observed that the PL intensity depends on the Au:
P ratio (estimated using ICP OES), with a maximum enhancement at Au
:
P ≈ 8
:
1 (Fig. S2 in the ESI†). Considering the approximate size of the nanocluster Au25 (based on the size obtained from HRTEM and on the PL emission maxima), this value suggests ∼3 TPP moieties per one AuNC. This result is fairly consistent with the 2 TPPs per AuNC estimated from 1H NMR (see above).
The time-resolved PL decay profiles of the two sets of AuNCs are shown in Fig. 1d. The PL lifetimes were extracted using a three exponential fit from the PL decay curves using the equation
I(t) = A1e−t/τ1 + A2e−t/τ2 + A3e−t/τ3 + A4, | (1) |
Set of τ used for the fit | τ 1 (ns) | τ 2 (ns) | τ 3 (μs) | τ 4 (μs) |
---|---|---|---|---|
∼10 | ∼457 | ∼2.09 | >10 | |
A 1 | A 2 | A 3 | A 4 | |
AuNC 1 | 0.79 | 0.12 | 0.03 | 0.06 |
AuNC 2 | 0.57 | 0.23 | 0.17 | 0.03 |
To further investigate the origin of luminescence enhancement in 2, we carried out temperature-dependent luminescence measurements. The inset of Fig. 1d presents the emission rate k (inverse luminescence decay time, k = τPL−1) as a function of 1000/T. We fit the observed temperature dependence with the equation
k(T) = k0+k′e−ΔE/(kBT), | (2) |
k 0 (MHz) | k′ (MHz) | ΔE (meV) | |
---|---|---|---|
AuNC 1 | 0.07 | 22 | 92 |
AuNC 2 | 0.25 | 61 | 144 |
To further investigate the Au core to phosphonium cation charge transfer, we employed X-ray photoelectron spectroscopy (XPS). The binding energy (BE) shift of Au 4f band is influenced by the oxidation state of gold. The BE of Au 4f7/2 transition of both 1 and 2 (Fig. 3) falls between the energies of Au(0) (84 eV) and Au(I) (86 eV) of gold thiolate, suggesting the coexistence of Au(0) and Au(I) in both AuNCs.44,50 However, upon conjugation of TPP, the BE of both Au 4f7/2 and 4f5/2 shifts towards higher values. Because both the AuNCs have a similar size and were stored under similar conditions without exposure to oxidizing agents, we attribute the BE shift of Au 4f band to charge transfer from the Au core to phosphonium cations. The charge transfer makes Au atoms in the cluster more positively charged, which in turn increases their core level binding energy. The XPS result of the Au 4f binding energy shift provides direct experimental evidence for charge transfer from the Au core to the phosphonium cation (Fig. 3).
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Fig. 3 Recorded XPS spectra of the Au 4f peaks (black line) of (a) 1 and (b) 2. The fitted curve (blue) shows that the spectra comprise two doublets of Au(0) (dark red) and Au(I) (green). |
Because charge transfer involving the triphenylphosphonium cation is also influenced by the counteranion,51 we investigated the effect of phosphonium salt counteranions on the emission properties of 2. We prepared AuNC 2 variants bearing a set of four different counteranions (BF4−, Cl−, Br−, and I−) by reactions of the corresponding triphenylphosphonium salts. The anions differ in oxidizability,51 forming a series (BF4− > Cl− > Br− > I−) ranging from the nonoxidizable BF4− anion to the most oxidizable I−. As Fig. 4b shows, the PL intensity depends on the counteranion. Decreases in the emission intensity of 2 follow the increasing oxidizability of the counteranions. This trend can be interpreted according to the proposed mechanism of PL enhancement (Fig. 4a). The charge transfer from the counteranion to the phosphonium cation decreases in the order I− > Br− > Cl− > BF4− and competes with the gold core-to-phosphonium ion charge transfer (i.e., MLCT). The more readily oxidizable the counteranion, the more charge transfer takes place from the counteranion to phosphonium cation and the less gold core-to-phosphonium cation charge transfer occurs (and vice versa). These observations further support the involvement of the MLCT mechanism in the enhancement of the PL emission of 2.
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Fig. 4 (a) Proposed luminescence enhancement mechanism in 2. (b) The relative photoluminescence spectra of 2 with different counteranions collected upon excitation at 365 nm. |
Because the intrinsic hydrophobicity of the TPP moiety containing three phenyl groups can be (partially) responsible for the enhancement of QY due to the increased local hydrophobicity in 2, we performed two different experiments investigating this potential environmental effect. First, we introduced a model hydrophobic moiety 2-aminoanthracene to 1, providing 3 (Scheme S1 in the ESI†). The characteristic intrinsic fluorescence of 2-aminoanthracene in the blue region of the spectra indicates the presence of the moiety on 3. However, in contrast to 1 and 2, the AuNC 3 did not show any PL peak in the wavelength range of 600–850 nm (same concentration of all the AuNCs; Fig. S4 in the ESI†), which would be expected upon an increase of local hydrophobicity. Second, we recorded the PL spectra of 2 in binary mixtures of water and dimethyl sulfoxide (DMSO) with increasing volume fractions of DMSO. DMSO is a known disruptor of hydrogen-bond network in water52 and its presence can strongly modulate the local polarity around fluorophores and influence their QYs.53 Nevertheless, we did not observe any change of PL spectra of 2 with an increasing concentration of DMSO (either in intensity or as a solvatochromic shift; Fig. S5 in the ESI†). Our results suggest that the polarity effects of the TPP do not contribute to the enhancement of the PL of AuNCs.
To analyze further the chemical structure and electronic behaviour of the AuNCs we used 1H NMR spectroscopy. The 1H NMR spectrum of 1 (Fig. S6†) shows that the signals corresponding to the TA are missing. However, after conjugation of TPP with the carboxylic acid group on the surface of 1, the TA signals reappear. The disappearance of TA signals in 1 may be explained by the interaction of TA with a paramagnetic gold core, leading to extreme signal broadening due to the paramagnetic relaxation enhancement. It is likely that the carboxylate group also interacts with the nanoparticle surface, because even the signals of hydrogen atoms close to the carboxylate group are missing in the spectrum (Fig. S6†). The reappearance of the signals after TPP addition indicates that the structure of the interacting radical (gold core) is different. Signals of hydrogen atoms distant from the sulphur atoms are broad but in the same positions as for free TA and signals of hydrogen atoms close to sulphur (and AuNC) are significantly broadened and shifted from their original positions. These spectral patterns indicate that TA interacts with a paramagnetic centre (gold core), but the nature of the radical (gold core) has changed upon TPP introduction leading to a suppression of paramagnetic relaxation and relative line narrowing, most likely because of the MLCT effect.
We have also simulated the possibility of electronic charge transfer from AuNC to TPP under electronic excitations by the time dependent density functional theory (TDDFT). From the emission spectra (Fig. 1c), we suppose that our AuNCs are closely related to the Au25 cluster. The Au25 cluster is based on a centered icosahedral Au13 core, which is capped by an exterior shell composed of twelve Au atoms.54 For simplicity, we approximated our particles with the Au13 core bearing one TPP molecule. The Au13 AuNC interacted strongly with TPP and a substantial amount of electronic charge was transferred from the Au13 nanocluster to the TPP. We found the frontier orbitals (HOMO/LUMO) centered at the phosphorus atom and gold cluster, respectively (Fig. 5). This implies an extensive charge transfer during the excitation, large transition dipole moments and hence strong fluorescence intensities.
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Fig. 5 Examples of orbitals in the Au13-TPP model calculated at the BP86/6-31G**/MWB60 level. The red and green colors correspond to the positive and negative lobes. |
Finally, to demonstrate a broader applicability of our PL enhancement method, we prepared another NIR PL AuNC 4 stabilized with mercaptosuccinic acid and PEG. Then we conjugated it with TPP salt via amidic coupling to obtain 5 (Scheme S2 in ESI†). We observed a similar enhancement of absorption and PL upon conjugation of TPP to the case of 2 (Fig. S8†). This experimental result shows that the method developed in this study can be extended to other thiolate-protected NIR emitting AuNCs for PL enhancement.
By using a simple synthetic procedure, we were able to prepare high QY colloidally stable AuNCs emitting in the NIR region. In comparison with other reported methods3 of substitution our two step synthetic method in water (without the need for an organic solvent) at room temperature uses very simple chemistry, which could be appealing for broader research community interested in long term in vitro and in vivo imaging of cells and tissues with a high signal-to-background ratio by avoiding auto fluorescence, reduced light scattering, and high tissue penetration. Most importantly, our method produces water soluble AuNCs, which does not require ligand exchange for biological applications in an aqueous environment. By using state-of-the-art characterization techniques such as HRTEM, XPS, UV-vis absorption, temperature dependent fluorescence lifetime, NMR spectroscopy, counteranion dependent emission enhancement, in combination with DFT modelling, we were able to provide further insight into the mechanism of PL enhancement. Our results indicate a gold-to-phosphonium ion charge transfer, i.e., metal-to-ligand (MLCT) responsible for enhancement of PL. In contrast to commonly observed PL enhancement by ligand-to-metal charge transfer (LMCT) via the S–Au bond from the electron-rich groups (e.g., carboxylic, and amino groups) present in the ligand (e.g., glutathione),38,55 our findings of MLCT mediated that PL enhancement offers an alternative route to achieve high QY NIR PL AuNCs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr06050e |
This journal is © The Royal Society of Chemistry 2018 |