Roles of Near and Far Fields in Plasmon-Enhanced Singlet

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3654−3660

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Roles of Near and Far Fields in Plasmon-Enhanced Singlet Oxygen Production Nicolas Macia, Vladimir Kabanov, Meĺ anie Côte-́ Cyr, and Belinda Heyne* Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta, Canada T2N 1N4

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S Supporting Information *

ABSTRACT: In plasmon-enhanced singlet oxygen (1O2) production, irradiation of a hybrid photosensitizer−metal nanoparticle leads to a significant alteration of the photosensitizer’s 1O2 yield. The quest for a more rational design of these nanomaterials calls for a better understanding of the enhancement mechanism that, to this day, remains largely unexplored. Herein, we introduce a new methodology to distinguish the near- and far-field contributions to the plasmon-enhanced 1O2 production using a tunable model nanoplatform, Rose Bengal-decorated silicacoated metal nanoparticles. By correlating 1O2 production to the experimental and simulated optical properties of our nanoparticles, we effectively discriminate how the near- and far-field effects contribute to the plasmonic interactions. We show that these effects work in synergy; i.e., for nanoparticles with a similar local field, the production of 1O2 correlates with maximized scattering yields. Our results expound the critical plasmonic aspects in terms of near and far fields for the design of an efficient hybrid plasmonic nanoparticle photosensitizer. a highly reactive oxygen species used in a variety of fields, from organic synthesis to the photodynamic therapy of cancer and the inactivation of bacteria.17 Singlet oxygen is commonly produced upon illumination of a photosensitizer molecule. It has been shown that the production of 1O2 by a photosensitizer can be dramatically altered when interacting with a plasmonic field, from being fully quenched to highly enhanced, depending on several parameters, including but not limited to the nanostructure morphology and composition and the photosensitizer−metal separation distance.18−20 The mechanism of plasmon-enhanced 1O2 production is generally simply thought to result from enhanced absorption due to near-field interactions between a photosensitizer and the enhanced local electric field at the surface of a metal NP.21−24 However, LSPR damping also leads to far-field effects, i.e., scattering, which remains to be discriminated from the near-field contribution to the overall plasmon-enhanced 1O2 production mechanism. Herein, we report a more comprehensive experimental and theoretical investigation of the extent of the near- and far-field contributions to the plasmon-enhanced production of 1O2 by a photosensitizer molecule. To achieve this goal, we report the synthesis of hybrid NPs made of a spherical metal core, either silver (Ag), gold (Au), or their 1:1 alloy (AuAg), coated with a silica shell decorated with an organic 1O2 photosensitizer molecule. This system is inspired by our previous metalenhanced nanoplatforms.19,20,25 Their simple design, such as their spherical shape, and their tunable metal core and silica shell thickness, makes these NPs a versatile model for

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lasmonics is at the core of many technologies, from its practical uses in photonics1 and solar energy2,3 to numerous biomedical applications,4−6 such as in the design of novel sensors and therapeutics. This field emerged from the distinct and unique optical properties of certain nanomaterials providing the extraordinary ability to tightly confine and control light at the nanoscale.7,8 Localized surface plasmon resonance (LSPR) arises from the collective oscillations of the conduction band electrons in metallic nanoparticles (NPs) upon coupling with the electric component of an incident electromagnetic field.9 The plasmon decay, or damping, produces strong near- and far-field effects. On one hand, incident light can be concentrated in the near field, producing a strong local enhanced electric field that extends from the NP’s surface, often termed the plasmonic nanoantenna or nanolens effect.8 On the other hand, LSPR can radiatively decay by the scattering of photons from the incident radiation into the far field.10,11 Plasmonic scattering accounts for the radiating light from the oscillating dipole created by particles interacting with an electromagnetic field.9 The farand near-field effects mainly depend on the morphology and composition of the NP, as well as its local environment, while the balance between these effects will impact their use to enhance a variety of optical phenomena. The mechanisms of most of these plasmon-enhanced phenomena, such as surfaceenhanced Raman scattering (SERS),12 metal-enhanced fluorescence,13 plasmonic energy conversion,14 and photocatalysis,15 are typically explained in terms of an intricate interplay between the distance-dependent near-field and far-field interactions. Plasmon-enhanced singlet oxygen (1O2) production and emission are emerging applications of plasmonics.16 Singlet oxygen is the most stable excited state of molecular oxygen and © 2019 American Chemical Society

Received: April 23, 2019 Accepted: June 13, 2019 Published: June 13, 2019 3654

DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

Letter

The Journal of Physical Chemistry Letters investigating the origins of the plasmonic effects on a photosensitizer’s 1O2 production ability. Moreover, because the photosensitizer is covalently bound at a controlled distance from the surface of the metal core, it is located, unambiguously and at all times, within the plasmonic field decay length of the core−shell NPs developed and used in this study. This design aspect is critical considering the short lifetime of the LSPR, approximately 20−100 fs,11 and is a significant advantage over other reported systems,26−29 which makes our model a more robust one for investigating the fundamentals of the plasmonenhanced phenomenon. In this study, we have experimentally and computationally explored the optical properties of these core−shell NPs and found a clear trend between the scattering yield of the metal core and its plasmon-enhanced 1O2 production ability, the latter monitored using two different detection techniques, namely, direct and indirect. Our results show that for a similar enhanced local electric field, the metal displaying an extinction dominated by its scattering component at the photosensitizer absorption wavelength provides the greatest 1O2 production enhancement. Hybrid metal nanoparticles (NPs) were synthesized in a multistep process inspired by previous reports.19,20,25,30 Briefly, it consists of conjugating a 1O2 photosensitizer on aminated silica-coated metal NPs (Scheme 1). First, a library of ∼55 nm Scheme 1. Rose Bengal-Conjugated Silica-Coated Metal Nanoparticle Synthesis Strategy

Figure 1. (a) Extinction spectra of Ag, AuAg, and Au NPs. (b) Representative ζ potentials of Ag NPs, silica-coated Ag NPs, aminated silica-coated Ag NPs, and RB-decorated silica-coated Ag NPs. TEM images of Ag@SiO2 (left), AuAg@SiO2 (middle), and Au@SiO2 (right) coated with approximately (c−e) 10 nm, (f−h) 20 nm, and (i−k) 30 nm silica shell thicknesses. TEM images of reference SiO2 NPs with diameters of (l) 80 nm, (m) 100 nm, and (n) 120 nm. Scale bars are 100 nm.

diameter metal NPs used as cores made of Ag, Au, and their 1:1 alloy, AuAg, were all synthesized by seeded-growth approaches (additional details can be found in Figures S1− S6). As shown in Figure 1a, the metal NPs have strong and narrow surface plasmon bands with extinction maxima located at 426 nm (Ag), 469 nm (AuAg), and 537 nm (Au) and form stable colloidal suspensions as confirmed by their large negative ζ-potential values (Figure 1b and Figure S7). Energy-dispersive X-ray microscopy (EDX) elemental analysis of the AuAg NPs revealed the alloy character of these NPs with a relative atomic composition of 46% Ag and 54% Au (Figure S4). The metal NPs were coated with mesoporous silica using the base-catalyzed Stöber process. Although silica could be directly deposited on the surface of the Ag and AuAg NPs, the highly vitreophobic character of the Au NPs makes them prone to irreversible aggregation during the coating step. Therefore, to improve their colloidal stability, the surface of the Au NPs had to be primed with a monolayer of (3-aminopropyl)trimethoxysilane further coated with a thin (4 ± 1 nm) and dense SiO2 layer from activated sodium silicate prior to the growth of larger silica shells (Figure S8 and Scheme S1).31 By finely tuning the concentration of the silica precursor, tetraethyl o-silicate (TEOS) (Table S1), we added coatings of three silica shell thicknesses, approximately 10, 20, and 30

nm, to each of the three metal NPs used. These particular shell thicknesses were chosen as previous studies, including ours, have shown that the perfect range of dye−metal separation distances is typically 10−20 nm.19 Transmission electron microscopy (TEM) imaging of the silica-coated metal [M@ SiO2 (M = Ag, AuAg, or Au)] revealed their monodispersed core and silica shell size distributions (Figure 1c−k and Figure S9). After amination with (3-aminopropyl)triethoxysilane (APTES) (Scheme 1), the surface ζ-potential of the M@ SiO2−NH2 NPs changed from a negative (−30 mV) to a positive value (15 mV) (Figure 1b and Figure S7), indicating the presence of the amine groups on the surface of the NPs. The last step of the synthesis was the conjugation of the photosensitizer to the M@SiO2−NH2 NPs by carbodiimide conjugation using N-[3-(dimethylamino)propyl]-N′-ethylcarbodiimide hydrochloride (EDC-HCl) under buffered conditions (pH 6.00) (Scheme 1). Rose Bengal (RB) was chosen 3655

DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

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The Journal of Physical Chemistry Letters

Figure 2. (a) Singlet oxygen time-resolved phosphorescence signals detected at 1270 nm upon excitation at 532 nm for 10 nm shell M@SiO2-RB NPs and 80 nm SiO2-RB NPs in air-equilibrated D2O [containing ∼2% (v/v) H2O] and their corresponding biexponential fittings. (b) Photobleaching kinetics of ABDA followed at 380 nm upon visible irradiation (λ > 435 nm) for M@SiO2-RB NPs and 80 nm SiO2-RB NPs in airequilibrated H2O and their corresponding linear fittings. (c and d) Enhancement factors of 1O2 production obtained by direct (a) and indirect (b) 1 O2 detection methods.

as a model photosensitizer for its high 1O2 quantum yield (ΦΔ = 0.75 in water) and absorbance profile (λmax = 550 nm), which overlaps to different extents with the surface plasmon bands of all of the metal NPs used in this study.32 The success of the synthesis of RB-conjugated silica-coated metal NPs (M@SiO2-RB) was assessed by the change in ζ-potential back to a negative value (approximately −10 mV) (Figure 1b and Figure S7) due to the dye being negatively charged in water at pH 6,33 and the RB absorbance profile clearly visible in the extinction spectra of the purified hybrid NPs (Figure S10). The final RB concentrations were quantitatively evaluated from the unreacted RB after several cycles of centrifugation and washing and were determined to be in the same range for all of the samples (approximately 1.5−1.7 μM) (Table S2). The differences in RB concentrations were considered in the remainder of this study and adjusted by appropriate dilution as explained in the Supporting Information. To investigate the differences between the photosensitizer’s 1O2 production with and without plasmonic coupling, we also prepared RBconjugated silica (SiO2-RB) NPs as control samples using a similar strategy. By controlling the SiO2 NPs formation reaction parameters, we produced three different sizes of NPs, i.e., 78 ± 5, 99 ± 4, and 118 ± 6 nm (Figure 1l−n and Figure S11), corresponding to the overall sizes of the M@ SiO2-RB NPs coated with a 10, 20, and 30 nm silica shell, respectively. The M@SiO2-RB NPs were then used to study their plasmon-enhanced 1O2 production by both direct and indirect detection methods. The direct method relies on the timeresolved near-infrared (TRNIR) detection of the 1 O 2 characteristic phosphorescence at 1270 nm corresponding to the 1Δg → 3Σg− transition, whereas the indirect method is based on analyzing the photobleaching kinetics of a 1O2selective molecular probe.34 TRNIR kinetic traces of 1O2 phosphorescence were recorded for all of the M@SiO2-RB and their corresponding SiO2-RB references (Figure 2a and Figure S12). The phosphorescence signals were fitted to the

rise and decay biexponential function of 1O2 production in a homogeneous environment (eq S1). Important kinetic information concerning the 1O2 production and decay can be quantitatively extracted from these fittings, such as the lifetimes of the photosensitizer (τT) and of 1O2 (τΔ) as well as the signal intensity, S0 (more details can be found in Figures S13 and S14). The fitted τT (2.0 ± 0.3 μs) and τΔ (51 ± 2 μs) values are in good agreement with the expected ones for RB and 1O2 in the solvent system used [deuterium oxide (D2O) containing 2% (v/v) H2O (Table S4 and Figure S14)]. The kinetic traces obtained using the M@SiO2-RB NPs coated with a 10 nm silica shell all displayed signal intensities stronger than the one obtained from the reference SiO2-RB NPs, illustrating the plasmonic effect on the photosensitizer’s ability to produce more 1 O 2 . This plasmonic effect was also indirectly investigated by using 9,10-anthracenediyl-bi(methylene)dimalonic acid (ABDA) as a chemical probe that, in the presence of 1O2, undergoes a photobleaching reaction that can be followed by ultraviolet−visible (UV−vis) spectroscopy (Figure S15). The pseudo-first-order kinetic rates of ABDA photobleaching in the presence of the M@SiO2-RB NPs are all faster than in the presence of the reference SiO2-RB NPs (Figure 2a and Figure S16). Care was taken to perform a series of controls using both detection methods to prove that the 1O2 being detected is produced by only the RB bound to the surface of the NPs and not by the metal NPs themselves (Figures S17 and S18).35 The plasmonic effect of the metal core on the 1O2 production by RB was quantified by calculating the enhancement factor, EF Δ . For the direct detection of 1 O 2 phosphorescence, EFΔdirect is defined as the ratio of the signal intensity, S0, obtained from the kinetic traces of the M@SiO2RB NPs to the S0 of their corresponding references, SiO2-RB NPs. For the indirect detection of 1O2, EFΔindirect is defined as the ratio of the pseudo-first-order rate constant of ABDA photobleaching with M@SiO2-RB NPs to the rate obtained with their corresponding SiO2-RB NP references. The EFΔ 3656

DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

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being closer to the wavelength monitored in the FDTD simulations than the λSPR of Ag NPs (426 nm) and AuAg NPs (469 nm) of the same size. As shown in Figure 3d, the evanescent local fields decay exponentially. At 10 nm from the surface, |E|/|E0| follows similar trends and yields values for Ag (2.86), AuAg (2.97), and Au (3.00). Considering that the |E|/| E0| values are roughly the same for the three NPs used, being slightly lower for Ag, the near-field effect cannot solely account for the observed differences in EFΔ, which calls for the investigation of the far-field contribution to the plasmonenhanced mechanism. The far-field properties are summarized by the extinction of the light interacting with the localized surface plasmon of the NPs, which results from contributions of two processes, namely, absorption and scattering.10 The radiative damping of the LSPR leads to scattering, i.e., re-radiation of light into the far-field that can increase the level of light trapping in the whole sample and act as a secondary light source to enhance the photosensitizer’s net absorption, even at large distances (hundreds of nanometers) from the metal surface.11 This unique intense light trapping ability of plasmonic NPs is possible due to their scattering cross section being much larger than their physical one. The scattering yield (ϕS = Csca/Cext) is defined as the ratio of the scattering contribution (Csca) to the extinction (Cext) at a certain wavelength and quantifies the fraction of light removed from an incident electromagnetic field that is re-emitted as scattered light by LSPR.36 The ϕS values of the M@SiO2 NPs were calculated both theoretically (ϕSth) and experimentally (ϕ Sex) [see the Supporting Information for additional details (Figures S19−S24)]. Briefly, the ϕSth values were calculated from the computed scattering, absorption, and extinction spectra, whereas the ϕSex values were calculated from the spectra obtained using a UV−vis spectrophotometer and an emission spectrophotometer equipped with an integrating sphere (Figure S24). The computed spectra of both metal NPs, used to test the convergence of the FDTD model, and the 10 nm silica shell M@SiO2 NPs are found to corroborate the experimental ones, based on their maximum wavelengths (Table S4), band shapes, and spectral profiles (Figures S22−S24) and their ϕS (Figure 4). As shown in Figure 4, there is a clear correlation between the ϕS and the EFΔ, where a higher EFΔ, such as for Ag@SiO2RB, correlates to a higher scattering yield (ϕSex = 0.77; ϕSth = 0.71) compared to those of AuAg (ϕSex = 0.23; ϕSth = 0.34) and Au (ϕSex = ϕSth = 0.18). The differences in ϕS can be rationalized from the higher imaginary part of AuAg and Au’s wavelength-dependent dielectric functions compared to that for Ag at 550 nm, which translates into an extinction dominated by absorption rather than scattering.37,38 Furthermore, the scattering efficiency (Qsca), which is defined as the scattering cross section (σ sca , in square nanometers) normalized to the geometric area (πr2, where r is the radius), quantifies how far beyond its physical cross section a NP interacts with light and could be calculated from the FDTD results for the three metals studied.39 It is worth highlighting that at 550 nm, far from its λSPR, the Qsca of Ag is 2.49, much larger than the values calculated for AuAg (0.70) and Au (1.01). This larger Qsca of Ag means that it interacts with light, i.e., absorbs and scatters, more strongly and is an overall better plasmonic material than AuAg or Au for NPs of similar shape and size at the wavelength of interest. It is important to note that controls by means of integrated absorption calculations were performed to verify that the

values calculated using both detection techniques confirm the known distance-dependent behavior of plasmon-enhanced 1O2 production (Figure 2c,d).19,21 A trend was observed for all of the metals, with the maximum EFΔ reached at a 10 nm silica spacer thickness, which quickly decreased as the metal−RB separation distance increased. The differences in magnitude between EFΔdirect and EFΔindirect were not surprising considering the fundamental differences between the detection methods, one assessing the fraction of 1O2 available to react with a molecular probe (indirect) and the other depending on the total 1O2 being produced and its radiative decay probability (direct), thus indicating a plasmonic enhancement of the production and emission signal of 1O2.20,34 Interestingly, we found a significant maximum 7.5-fold EFΔdirect for the 10 nm silica shell Ag@SiO2-RB, almost twice the values calculated for AuAg (4.0) and Au (3.5), demonstrating the superiority of Ag as a plasmonic core in nanostructures for applications such as enhancing the production of 1O2 by RB. A similar effect was observed for the values obtained by the indirect detection method, with a maximum EFΔindirect reached for Ag (2.1) at a 10 nm metal−RB separation distance, and values that quickly decreased as this distance increased. To understand the origin of the differences in EFΔ between the library of metals studied, we further investigated the mechanism of the plasmonic enhancement of the production of 1O2 from an electromagnetic perspective. Finite-difference time-domain (FDTD) simulations were performed to ascertain the near- and far-field contributions to the plasmon-enhanced 1 O2 production phenomenon. First, the electric field enhancements (|E|/|E0|) of Ag, AuAg, and Au NPs were evaluated at the RB maximum absorption wavelength (λmax = 550 nm) (Figure 3a−c). The maximum |E|/|E0| values calculated decrease in the following order: Au (5.8) > AuAg (5.5) > Ag (5.2). This trend is explained by the maximum SPR wavelength (λSPR) for 55 nm diameter Au NPs (537 nm)

Figure 3. Theoretical near-field distributions showing the enhanced electric field expressed as the ratio of the moduli of the local electric field to the incident electric field (|E|/|E0|) at λ = 550 nm in a totalfield scattered field of 55 nm: (a) Ag, (b) AuAg, and (c) Au NPs. (d) Electric-field enhancement as a function of the distance from the surface of the NP. 3657

DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

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AuAg, in model spherical core−shell NPs. By using a combination of direct and indirect 1O2 detection methods, we found that Ag is a superior metal for NPs of similar size and shape compared to Au and AuAg, boosting by 7.5-fold the production of 1O2 by RB, due to its high scattering yield and high enhanced local electric field. Indeed, in addition to the quest for ever greater local field enhancements, for instance, by using more exotic NP shapes (e.g., cubes, stars, rods, and prisms, to name but a few), we demonstrated that the hybrid nanostructures should be designed not only to optimize the near field but also to maximize the scattering ability of the metal NP to achieve effective plasmonic effects for enhanced 1 O2 production. Overall, our results should be beneficial as a guide for more rational synthesis of nanomaterials for applications where enhanced 1O2 production is desired, such as for theranostics and sensors.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. Enhancement factors (top) of 1O2 production obtained by direct (blue) and indirect (red) 1O2 detection methods. The inset shows the details of the enhancement factors obtained by the indirect 1 O2 detection method. Experimental (black) and theoretical (gray) scattering yields of M@SiO2 NPs (bottom).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01165. Detailed synthetic procedures and characterization of nanoparticles (extinction spectra, TEM images, size distribution plots, EDX elemental analysis of the AuAg NPs, and ζ-potentials), singlet oxygen direct and indirect experimental procedures and results for all of the samples (direct detection signals and their corresponding fittings, fitting parameters and residuals, ABDA photobleaching kinetics, and controls), finite-domain time-difference (FDTD) computational details, methods for obtaining the experimental and computed extinction, scattering, and absorption spectra of the NPs studied, and integrated absorbance for the density scattering control (PDF)

scattering described here is not just due to the increase in the photon path length resulting from a higher particle density within the sample (Table S5), as described for other systems.29 Also, if the plasmonic effect was simply due to the NPs passively scattering the light, higher EFΔ values should have been obtained at longer metal−photosensitizer separation distances [20 and 30 nm (Figure 2c,d)], because these larger NPs still have significant ϕS (Table S6). Similar conclusions were recently reached by Brolo et al.,40 who showed the improved performance of solar cells using an array of Ag NPs over a similar device but made with an array replaced by SiO2NPs, a difference rationalized by the absence of plasmonic light trapping in the latter. Our results can be explained by the known alteration of the photosensitizer’s absorption cross section (σabs) when coupled with plasmonic nanostructures; i.e., σabs is significantly increased up to 10 nm from the metal core surface and decreases in magnitude at distances beyond that.37,41 In other words, the presence of both a strong local electric field in the near field and the scattering-induced light trapping within a sample is concomitant to an absorption reinforcement by a dye having a plasmon-increased σabs, ultimately resulting in a higher 1 O2 yield. Therefore, our results indicate that the synergy of near- and far-field effects on the photosensitizer is sine qua non for plasmon-enhanced 1O2 production. From deconvoluting the near- and far-field effects, one can consider the following parameters to be critical for the design of effective plasmonenhanced 1O2 nanoplatforms: an optimal metal−photosensitizer separation distance (∼10 nm) and a metal core combining both a high local enhanced electric field and a high scattering yield at the wavelength of interest. In conclusion, we showed that the enhancement of 1O2 production by a hybrid photosensitizer NP is dependent on the near- and far-field properties of the plasmonic nanostructure. Importantly, for the first time, a correlation was made between the 1O2 enhancement factors and the scattering yields of two widely used model metals, i.e., Ag and Au, as well as their alloy,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vladimir Kabanov: 0000-0002-7748-861X Belinda Heyne: 0000-0003-1655-6719 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Natural Sciences and Engineering Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and the University of Calgary. The authors gratefully acknowledge the generous support of NSERC for Alexander Graham Bell Post-Graduate Doctoral Scholarships (N.M. and V.K.) and Alberta Innovates for Doctoral Scholarships in Nanotechnologies (N.M.). M.C.-C. is thankful for the support of NSERC through an Undergraduate Summer Research Award and the Reactive Intermediate Student Exchange (RISE) program. Antoine Macia is also acknowledged for his help with the dielectric function calculations on MATLAB used for the FDTD simulations. 3658

DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

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DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660

Letter

The Journal of Physical Chemistry Letters (40) Brady, B.; Steenhof, V.; Nickel, B.; Blackburn, A. M.; Vehse, M.; Brolo, A. G. Plasmonic Light-Trapping Concept for Nanoabsorber Photovoltaics. ACS Appl. Energy Mater. 2019, 2, 2255−2262. (41) Sun, J.; Li, G.; Liang, W. How Does the Plasmonic Enhancement of Molecular Absorption Depend on the Energy Gap between Molecular Excitation and Plasmon Modes: A Mixed TDDFT/FDTD Investigation. Phys. Chem. Chem. Phys. 2015, 17, 16835−16845.

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DOI: 10.1021/acs.jpclett.9b01165 J. Phys. Chem. Lett. 2019, 10, 3654−3660