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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Efficient Energy Transfer from Near-Infrared Emitting Gold Nanoparticles to Pendant Ytterbium(III) Scott E. Crawford,† Christopher M. Andolina,† Derrick C. Kaseman,† Bo Hyung Ryoo,† Ashley M. Smith,† Kathryn A. Johnston,† and Jill E. Millstone*,†,‡,§ †
Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States Department of Chemical and Petroleum Engineering and §Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States
‡
S Supporting Information *
First, we examined the emission profile of oMBA-AuNPs exposed to progressively higher concentrations of YbCl3. In the absence of the YbCl3, oMBA-capped AuNPs showed a broad emission peak centered at 890 nm, with a full width at halfmaximum (fwhm) of ∼220 nm. After the introduction of Yb3+, the characteristic Yb3+ 2F5/2→2F7/2 emission is observed,27 with a maximum (λEM) of 981 nm at the most intense AuNP excitation wavelengths (Figure 1). With increasing concentration of Yb3+, the emission peak of the AuNPs decreased in intensity while the Yb3+ emission peak increased in intensity at the same excitation energies, indicating an energy transfer process (Figure 2). Further, at the highest Yb3+ concentration studied the emission peak exhibits a fwhm of ∼60 nm, a narrowing that is consistent with emission from a lanthanide species. In addition, the emission lifetime and quantum yield of the AuNP-Yb3+ conjugates are both sensitive to solvent deuteration, which is also consistent with emission from a lanthanide species (Figures S2−S3, Table 1).28−31 Remarkably, regardless of Yb3+ concentration, there is no change observed in the excitation profile of the AuNPs (λEM = 980 nm), despite dramatic changes in the emission profile. The insensitivity of the excitation peak to Yb3+ concentration is a strong indication that, in all cases, the AuNPs are directly excited and that the Yb3+ PL originates from on-particle processes (Figure S4). As a control, the PL properties of a solution containing oMBA and Yb3+ were also measured. Although oMBA is capable of sensitizing Yb3+ emission, the Yb3+-oMBA mixture exhibits marked differences in both its quantum yield and excitation profile, indicating that off-particle processes are not significantly contributing to the observed PL features of the AuNP-Yb3+ conjugates (Figure S5, Table S1). Further, experiments in which we deliberately removed the Yb3+ from the particles via competitive chelation (using ethylenediaminetetraacetic acid, EDTA) led to a systematic decrease of the Yb3+ emission and recovery of AuNP NIR emission profiles. These results suggest that removal of the Yb3+ from the AuNP surface prevents the energy transfer process from occurring (Figures S6−S7) and that the AuNP structure and optoelectronic features are preserved throughout the conjugation. Characterization of the AuNPs showed that the particles did not exhibit changes in either size or morphology over the course
ABSTRACT: Here, we demonstrate efficient energy transfer from near-infrared-emitting ortho-mercaptobenzoic acid-capped gold nanoparticles (AuNPs) to pendant ytterbium(III) cations. These functional materials combine the high molar absorptivity (1.21 × 106 M−1 cm−1) and broad excitation features (throughout the UV and visible regions) of AuNPs with the narrow emissive properties of lanthanides. Interaction between the AuNP ligand shell and ytterbium is determined using both nuclear magnetic resonance and electron microscopy measurements. In order to identify the mechanism of this energy transfer process, the distance of the ytterbium(III) from the surface of the AuNPs is systematically modulated by changing the size of the ligand appended to the AuNP. By studying the energy transfer efficiency from the various AuNP conjugates to pendant ytterbium(III) cations, a Dextertype energy transfer mechanism is suggested, which is an important consideration for applications ranging from catalysis to energy harvesting. Taken together, these experiments lay a foundation for the incorporation of emissive AuNPs in energy transfer systems. mall gold nanoparticles (AuNPs, d = ∼1−3 nm) are known to exhibit near-infrared (NIR) photoluminescence (PL) upon excitation with ultraviolet (UV) and/or visible light.1,2 These particles have shown promise in applications including bioimaging3−5 and sensing6−11 and exhibit intriguing optoelectronic properties at the boundary between bulk and molecular electronic structures.12−15 Mechanistic studies of these emissive processes indicate that the NIR emission originates from surface states on the particle, which can be altered via changes to the surface metal composition16,17 and/or the stabilizing ligand shell.1,12,15,18−24 Here, we illustrate how these AuNP emissions can be channeled for energy transfer applications.25,26 Specifically, we demonstrate efficient energy transfer from NIR-emitting ortho-mercaptobenzoic acid (oMBA)-capped AuNPs to pendant ytterbium(III) cations (Yb3+, 78.6% preservation of the initial quantum yield). Using a combination of PL, nuclear magnetic resonance spectroscopy (NMR), and electron microscopy techniques, we show that the Yb3+ is conjugated to the ligand shell and that this architecture facilitates energy transfer via a Dexter mechanism.
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© XXXX American Chemical Society
Received: October 20, 2017
A
DOI: 10.1021/jacs.7b11220 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 1. Excitation/emission contour plots of AuNP-oMBA conjugates exposed to increasing Yb3+ concentration in DMSO.
between the quantum yield of the initial AuNP samples and that of the corresponding AuNP-Yb3+ conjugates (Figure 3). However, the emission profiles from which the quantum yields originate are markedly different before and after Yb3+ addition, where the spectra after Yb3+ addition are consistent with Yb3+ emission and contain little to no contribution from the AuNPs themselves. A linear regression of the data passes through the origin with a slope of 0.786, indicating that the AuNP-Yb3+ conjugates show a quantum yield that is 78.6% of that produced by the AuNPs in the absence of the lanthanide. Analogous studies on nonemissive AuNPs of a similar size, such as PVP (polyvinylpyrrolidone)-capped AuNPs, do not efficiently excite Yb3+ despite the fact that the PVP itself can act as a sensitizer (Figure S9). Taken together, these experiments provide strong evidence that the PL properties of the initial AuNPs correlate strongly with the PL properties of the corresponding AuNPlanthanide conjugates and are hence an essential component of the energy transfer mechanism. There are three general mechanisms by which the AuNPs may be responsible for exciting Yb3+ emission: (i) Förster-type energy transfer, (ii) Dexter-type energy transfer, and/or (iii) photoinduced, redox-mediated energy transfer.32 A Förster-type mechanism is a through-space interaction that primarily depends on dipole−dipole interactions between the donor and acceptor species, with an energy transfer efficiency that decays with distance (d) as d−6.33 Conversely, a Dexter-type energy transfer is through-bond, during which a concerted electron exchange occurs between the donor and acceptor.34,35 Often, this process involves an energy transfer from the excited triplet state of the donor to the acceptor, leading to the excitation of the acceptor into a triplet state. For this mechanism, the energy transfer efficiency decays with distance as e−2d. For both Förster and Dexter mechanisms, spectral overlap between the donor emission and the acceptor absorption is required. A third energy transfer mechanism involves a photoinduced electron transfer from the donor to the acceptor, followed by a back-electron transfer that leaves the lanthanide cation in its excited state. This mechanism has been reported for both Yb3+ and Eu3+ and does not require spectral overlap.36
Figure 2. (A) 2D emission slices of the AuNPs as a function of Yb3+ concentration (λEX = 360 nm) and (B) a plot of emission intensity of the AuNP emission peak at 850 nm versus the emission intensity of the AuNP-Yb 3+ emission peak at 980 nm as a function of Yb 3+ concentration. The asterisk in (A) denotes a solvent absorption artifact.
of these experiments. Electron micrographs show discrete particles with average core diameters of 2.2 ± 0.5 nm (Figure S8), consistent with their preconjugation diameters (as measured by both transmission electron microscopy (TEM) and pulsed field gradient stimulated echo 1H NMR).12,16,17 The efficiency of energy transfer from the AuNPs to pendant Yb3+ can be assessed by comparing the quantum yield of the AuNPs before and after conjugation to Yb3+ as a function of wavelength. A plot of the quantum yields of AuNPs before and after Yb3+ addition shows a clear correlation (R2 = 0.9979)
Table 1. Optical Properties of AuNPs and AuNP-Yb Conjugates in DMSO and d6-DMSO Sample
Solvent
Φ (×10−3, a.u.)a
τobs (μs)
τrad (μs)
λEM (nm)
fwhm (nm)
ε (×106, M−1cm−1)a
Brightness (×103, M−1 cm−1)a
AuNPs AuNPs AuNP-Yb AuNP-Yb
DMSO d6-DMSO DMSO d6-DMSO
1.8 ± 0.1 6.9 ± 0.4 1.6 ± 0.1 5.4 ± 0.2
1.48 ± 0.03 3.06 ± 0.05 3.1 ± 0.4 12 ± 1
850 ± 30 450 ± 30 2000 ± 400 2300 ± 300
890 ± 10 900 ± 10 981 ± 0 982 ± 0
220 ± 20 260 ± 10 62 ± 2 56 ± 2
1.21 ± 0.04 0.97 ± 0.06 1.19 ± 0.04 1.22 ± 0.06
2.1 ± 0.1 6.8 ± 0.6 1.9 ± 0.1 6.6 ± 0.4
Quantum yield (Φ) reported for an excitation wavelength (λEX) of 360 nm, and both molar absorptivity (ε) and brightness (ε × Φ) were determined at an extinction wavelength of 360 nm (Figure S1). PL values are reported as the average ± the standard error of five independent trials. For each trial, the same particles were measured in both DMSO and d6-DMSO. a
B
DOI: 10.1021/jacs.7b11220 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 4. Plot of the peak intensity ratio (980 nm:875 nm) as a function of AuNP-Yb3+ separation distance, following incubation of the AuNPs in 20 μM Yb3+. For comparison, the data are fit to both a Dexter function (red trace) and a Förster function (dashed blue trace).
Figure 3. Scatter plot of the oMBA-AuNP conjugate quantum yield before and after Yb3+ addition for particles measured in DMSO and d6DMSO (N.B. additional PL figures of merit for these particles are listed in Table 1). Inset: Normalized emission profile of the AuNPs before Yb3+ addition (black dashed line) and after Yb3+ addition (red solid line); (λEX = 360 nm).
escent light emitting diodes (LEDs).44,45 Interestingly, because a Dexter-type mechanism often includes a donor with a triplet excited state, these experiments provide further evidence that the AuNP PL mechanism also involves a triplet state, which has been previously postulated.7,13,20,46,47 To support the assignment of a Dexter mechanism, the location of Yb3+ in these experiments was analyzed via both electron microscopy and NMR techniques. Scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS) maps illustrate the Yb3+ association with the AuNP surface, as opposed to a random distribution of Yb3+ throughout the solution (Figures S12−S17). NMR experiments indicate that Yb3+ is directly bound to the NP ligand shell, providing a more detailed picture of this surface architecture compared to STEM-EDS analysis. Specifically, the NMR spectrum of oMBA-AuNPs shows expected features. The oMBA peaks are observed at 6.925, 6.75, 6.65, and 7.65 corresponding to the protons on the 3, 4, 5, and 6 carbon positions, respectively, relative to the carboxylic acid (Figure S21). The shifts of the resonances relative to free ligand (Figure S22) are explained by the attachment of the ligand to the AuNPs and occur primarily via dipolar coupling and chemical shift anisotropies that are incompletely averaged by the rotation of the NP and have been described in detail elsewhere.48 Upon addition of Yb3+, all proton peaks are further broadened and also shifted upfield. We attribute these spectral changes at all proton sites to coordination of the ligand shell to the paramagnetic Yb3+ cation.49 To control for potential line broadening effects due to cation-induced aggregation, analogous NMR experiments were conducted using lanthanum(III) chloride, which has a similar ionic radius to Yb3+ but is not paramagnetic. When the oMBA-AuNPs are exposed to La3+, no additional spectral broadening or upfield chemical shifts are observed in the 1H NMR spectrum (Figure S23). To simulate potential concentration-related aggregation effects, 1H NMR spectra were taken at high AuNP concentrations (5× greater than those used in the orignal NMR experiments) and also do not match the spectra of AuNPs following Yb3+ addition. Taken together, these data support the assignment that paramagnetic ion effects are responsible for the shifts and broadening of the ligand proton peaks due to nearby Yb3+ cations, generating a ligand shell model that places the carboxylic acid and coordinated Yb3+ near the nanoparticle surface (Figure S24). In summary, we have demonstrated NIR-emitting AuNP participation in energy transfer events using AuNP-Yb3+ conjugates. Mechanistic studies indicate that the energy transfer
The photoinduced, redox-mediated energy transfer mechanism is unlikely, as our data indicates that spectral overlap is a requirement for the excitation of Yb3+ by AuNPs (Figure 3, Figure S9). The importance of spectral overlap to the energy transfer mechanism is demonstrated by comparing the emission of AuNPs terminated by three different capping ligands: oMBA (discussed above), meta-mercaptobenzoic acid (mMBA), and para-mercaptobenzoic acid (pMBA). Both mMBA- and pMBAcapped AuNPs exhibit lower energy emission profiles than the oMBA-capped AuNPs, which overlap poorly with the Yb3+ transition at 980 nm. As a result, limited excitation of Yb3+ is observed for mMBA- and pMBA-capped AuNPs, occurring at the very narrow range of excitation/emission energies that overlap with the Yb3+ 2F5/2→2F7/2 transition (Figure S10). Due to the importance of spectral overlap to the energy transfer mechanism, excitation of Yb3+ by AuNPs is then more likely to occur by Dexter or Förster processes. The donor−acceptor distance-dependent energy transfer efficiency may be used to further distinguish between Dexter and Förster mechanisms. To test this distance dependence, AuNPs were synthesized with n-mercaptoalkanoic acids of varying substituent chain lengths (2 to 11 carbon atoms, ∼4.1 to 14.9 Å, Figure S11). Here, we assume that the AuNP emission occurs at the surface of the AuNPs, that the Yb3+ binds to the terminal carboxylic acid group and that the ligands are fully extended on the AuNP surface, such that the donor−acceptor distance is equal to the ligand length.12,37,38 The energy transfer efficiency following Yb3+ introduction showed a clear correlation with the AuNP capping ligand length. For example, the emission profile of AuNPs capped by thioglycolic acid, the shortest ligand tested (∼4.1 Å), showed a pronounced peak at 980 nm and little to no emission from the AuNPs at 875 nm. Conversely, the emission profile from AuNPs capped with 11-mercaptoundecanoic acid, the longest ligand studied (∼14.1 Å), was not significantly altered regardless of the Yb3+ concentration introduced (Figures S12−S17). A plot of the 980 nm:875 nm emission intensity ratio versus the estimated distance between the AuNP surface and Yb3+ indicates that the emission decays as roughly e−2d, consistent with a Dexter-type mechanism (Figure 4).32 Dexter-type energy transfer is important for many applications, including energy harvesting,35,39,40 photocatalysis,41,42 biological probes,43 and phosphorC
DOI: 10.1021/jacs.7b11220 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
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pathway is sensitive to both donor−acceptor spectral overlap and separation distance, suggesting a Dexter-type mechanism. The Dexter mechanism has important implications for the integration of luminescent AuNPs into a variety of applications including energy harvesting, LEDs, and photocatalysis. These materials also combine the strong light harvesting properties of AuNPs with the narrow emission lines of Yb3+, a critical step in developing of bright, high-performance NIR-emitting materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b11220. Details of syntheses, methods, and additional data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Christopher M. Andolina: 0000-0003-2157-9114 Ashley M. Smith: 0000-0002-0225-1618 Jill E. Millstone: 0000-0002-9499-5744 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the National Science Foundation (CHE-1253143), Research Corporation for Science Advancement, and the University of Pittsburgh. S.E.C. thanks the University of Pittsburgh and Pittsburgh Quantum Institute for financial support. We thank Michael Hartmann for ligand modeling, Prof. Alexander Star for use of his fluorometer, Prof. Daniel Bain for use of his ICP-MS, and Arda Genc from FEI, Inc. for the STEM-EDS maps.
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REFERENCES
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DOI: 10.1021/jacs.7b11220 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
