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Fluorescence from Molecular Silver Nanoparticles Brian A. Ashenfelter, Anil Desireddy, Sung-Hei Yau, Theodore Goodson III, and Terry P. Bigioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05735 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 17, 2015

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The Journal of Physical Chemistry

Fluorescence from Molecular Silver Nanoparticles Brian A. Ashenfelter,1 Anil Desireddy,1† Sung Hei Yau,2 Theodore Goodson III2 and Terry P. Bigioni1,3* 1

Department of Chemistry, University of Toledo, Toledo, Ohio 43606, 2Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, and 3The School of Solar and Advanced Renewable Energy, University of Toledo, Toledo, Ohio 43606.

Supporting Information Placeholder ABSTRACT: Small metal nanoparticles are known to be

much more fluorescent than larger plasmonic nanoparticles, however the nature and origin of their fluorescence are not fully understood. Fluorescence is thought to originate from either the quantum states within the metal core or mixed ligand states at the inorganic-organic interface. Here we show that fluorescence from molecular silver glutathionate nanoparticles has its origin in interfacial electronic states. Fluorescence spectra were found to be independent of size, with very similar wavelength and bandwidth, although the quantum yield was not. Excitation spectra indicated that the strongest fluorescence had its origin in that part of the spectrum that is dominated by ligand-related states. Further, excitations to strictly core states and to higher lying d-band states had no apparent contribution to the fluorescence. Timeresolved spectroscopic measurements showed that Ag32(SG)19 and Ag15(SG)11 have a common emissive state, with the same emission wavelength and dynamic, which can be assigned to the metal-ligand state.

INTRODUCTION Molecular nanoparticles are fundamentally different than their larger nanoparticle counterparts in that they are molecular species with precisely defined structures.1-5 The optical properties of molecular metal nanoparticles are very different than those of bulk metals due to fundamental differences in their electronic structures. They are also significantly different than those of semiconductor quantum dots. Quantum confinement is the dominant effect in spherical quantum dots since they are isoelectronic,6 with the details of each structure not playing an important role. In contrast, molecular metal nanoparticles are generally not isoelectronic.7,8 As a result, their optical properties widely vary to produce a variety of colors that do not vary systematically (see Fig. 1),9,10 in contrast to quantum dots.11 Molecular metal nanoparticles have been shown to have fluorescence quantum yields that are orders of magnitude higher than their bulk counterparts.12-15 Fluorescence has

Figure 1. Photographs of a PAGE gel with positions of bands 1, 2 and 6 indicated, shown under visible and UV illumination.

been observed in different spectral regions, which may be characteristic of different operative mechanisms. Visible fluorescence from small bare clusters of gold and silver atoms has a strong size dependence that has been successfully modeled using the electronic states in the metal cluster core.14,16-20 Infrared fluorescence from ligated molecular metal nanoparticles does not exhibit a strong size dependence and has not been modeled successfully. Possible origins of IR emission include hot electrons,12 surface or ligand states,15,21,22 ligandto-metal charge transfer,23-25 and core states.26 Visible fluorescence from ligated molecular Au and Ag nanoparticles also does not exhibit a strong size dependence.27-29 The origin of visible emission from molecular gold nanoparticles has been attributed both to states related to the ligand-metal interface,28-30 which is analogous to work done on aggregates of gold thiolates,31 as well as to states in the metal core.15,22 Silver is an attractive metal for optical studies due to the position of its d-electrons compared to gold.32 Fluorescence from molecular silver nanoparticles is particularly interesting, and visible emission is now beginning to be studied.33-35 Although the mechanistic details are not yet known, ligandrelated states may be involved.36 Here, we show that emission from three of the most fluorescent species from the family of Ag-glutathione (Ag:SG) molecular nanoparticles does not depend on size. Fluorescence and excitation spectra for all three species were very similar, indicating that the thiolate

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ligand shell played a key role in the fluorescence mechanisms. Steady-state absorption and fluorescence measurements showed that the transitions contributing most strongly to emission originated from states that likely have significant ligand character and were found to couple strongly into a single emissive state. We therefore attribute the fluorescence to states related to the ligand-metal interface. EXPERIMENTA L Chemicals. All reagents were obtained commercially and used without additional purification. The following reagents were received from Fisher Chemicals: sodium borohydride, acrylamide, bis-acrylamide, glycine, tetramethylethylenediamine (TEMED). Silver nitrate and glutathione (GSH) were purchased from Acros Organics and ammonium persulfate was purchased from GE Healthcare. Synthesis. Silver nanoparticles were formed by reducing silver nitrate in the presence of excess glutathione, using a method that was similar to a previously reported method.10 The reaction was carried out at pH 8.8 using a tris/glycine buffer consisting of 0.025 M tris base and 1.92 M glycine. In a typical synthesis, 42.4 mg (0.25 mmol) of AgNO3 and 307 mg (1 mmol) of glutathione were dissolved in the solution, resulting in the formation of a solid silver glutathionate precursor, before being put in an ice bath for 30 min. This icecold precursor was reduced by adding 12.5 mL of ice-cold aqueous 0.2 M NaBH4 drop-wise into the mixture with constant stirring. This mixture was allowed to react for 1 hr, during which time the molecular Ag nanoparticles formed to produce a deep brown solution. The Ag nanoparticles were separated from the reaction mixture by precipitating with ethanol. The precipitate was then collected and washed 3 times with excess ethanol to remove unreacted material before finally being dried under vacuum. Separation. The synthesis produced several different sizes of Ag nanoparticles, which were separated using polyacrylamide gel electrophoresis. The stacking and resolving gels were prepared according that reported previously.10 The same tris/glycine buffer used for the synthesis was used as the running buffer. The separation was run on a Thermo Scientific P10DS vertical electrophoresis system at a constant potential of 300 V for 900 min while chilling the gel with 0°C coolant. Silver nanoparticles were dissolved in 10% (v/v) glycerol-water at a concentration of 20 mg/mL. Each plate was cast with a single prep-scale well that was loaded with 1000 μL of this solution. For analysis, separated bands were excised from the gel and crushed. Nanoparticles were extracted by soaking the crushed gel in water for several minutes. Residual gel pieces were removed by centrifugation and filtering through a 0.22 μm syringe filter. Absorbance and luminescence were measured immediately following extraction to minimize decay of the nanoparticles.

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Optical Absorption Spectroscopy. Absorption measurements were carried out in a 10 mm path-length quartz cuvette using a PerkinElmer LAMDA 950 UV-vis-NIR spectrophotometer. Nanoparticles were diluted with deionized water until an absorbance of ~0.1 was achieved at 420 nm. The slit width was set to 2.00 nm and the detector response was 0.20 s. The PbS detector gain was set to 1.00. Fluorescence Spectroscopy. Fluorescence and excitation measurements were made using a Thermo Scientific Aminco-Bowman Series II Luminescence Spectrometer in a 10 mm path-length quartz cuvette. The bandwidth was set to 4 nm and the scan speed was 5 nm/min. Detection was made at 90 degrees to the incident beam using a photomultiplier tube. The detector high-voltage was fixed at 860 V. Relative fluorescence quantum yields were calculated using fluorescein as a reference. Femtosecond Time-resolved Fluorescence Spectroscopy. The time-resolved fluorescence measurements were made with a FOG-100 system produced by CDP Systems. The samples were excited with frequency doubled light from a mode-locked Ti-sapphire laser (Tsunami, Spectra Physics). All samples were held in a 1 mm thick rotating sample cuvette. Fluorescence emitted from the sample was upconverted in a nonlinear crystal of barium borate, passed through a variable delay line before up conversion. The instrument response function (IRF) had a duration of ~200 fs for visible excitation. The energy per excitation pulse did not exceed 600 pJ for any experiment. Lifetimes of the fluorescence decay were obtained by fitting the experimental profile with multi-exponential decay functions convoluted with the IRF. Spectral resolution was achieved using a monochromator. The up-converted signal was detected by a photomultiplier tube. RESULTS AND DISCUSSION Three of the smallest Ag:SG nanoparticles in the crude mixtures, previously denoted as bands 1, 2 and 6, were separated using polyacrylamide gel electrophoresis (PAGE), extracted, and purified for analysis.10 Two of these species have been formally identified using electrospray-ionization mass spectrometry as Ag32(SG)19 (band 6)37 and Ag15(SG)11 (band 2).27 The smallest size (band 1) has yet to be identified definitively due to the susceptibility of these species to fragmentation during mass analysis,37 however it is expected to be very similar to Ag15(SG)11 in size based on PAGE results.10 It is most similar to a recently reported Ag11(SG)7 species,38 so we adopt that assignment for the present work. Although the smallest species are known to decay most slowly, transformation of these nanoparticles over short time periods produces samples with decay products (including smaller particles) that can be more fluorescent than the original material, thereby producing misleading measurements and misassignments.39 Samples were therefore measured immediately following separation and purification and this


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The molar absorptivities of Ag32(SG)19 and Ag15(SG)11 were calculated using their molecular formulae, as reported in Fig. 2. Although the spectra of these two species are quite distinct and easily differentiable, it is important to point out that the molar absorptivity of Ag32(SG)19 is an order of magnitude larger than that of Ag15(SG)11. The strong spectral features of Ag32(SG)19 can therefore easily dominate spectra of mixtures of these materials, meaning a solution of Ag15(SG)11 containing only a small amount of Ag32(SG)19 can be misidentified as Ag32(SG)19 by absorption spectroscopy. In contrast, it is extremely difficult to identify the presence of smaller species in solutions of Ag32(SG)19 by using the absorption spectrum alone due to the difference in molar absorptivities. The fluorescence spectra of these purified nanoparticles were also recorded, as shown in Fig. 2. The solutions were all excited at 420 nm, which is close to the maximum in the excitation spectra of each particle (vida infra). Although the absorption spectra are quite distinct from one another, the fluorescence spectra of these nanoparticles were found to have very similar peak widths and only slightly different peak positions. This demonstrates again the extreme difficulty in identifying the presence of smaller species in solutions of Ag32(SG)19 by using fluorescence spectroscopy, due to the similarity of the spectra of the three species. The fluorescence spectra were transformed to the natural units of energy41 for further analysis since wide peaks can become significantly distorted when spectra are displayed as a function of wavelength (see Fig. 2). Once transformed, it was possible to fit each spectrum with a single Gaussian lineshape (see Fig. 3). This indicated that only a single emissive state existed for each of the Ag:SG species. The observed emission energies did not vary systematically with particle size and were not found to agree with a model for emission from a metal core.14 In fact, large differences in

Figure 2. Optical absorption (black), excitation (blue), and fluorescence (red) spectra of Ag32(SG)19, Ag15(SG)11, Ag11(SG)7, and a silver glutathionate polymer for comparison.

processing time was kept to a minimum. Purity was also ensured by carefully avoiding regions of overlap between PAGE bands when excising the nanoparticles from the gel. Optical absorption, fluorescence, and excitation spectra for the three pure materials were measured, as shown in Fig. 2. The spectrum of Ag32(SG)19 is the most distinct as it has the most prominent features, with a large characteristic peak at 490 nm and two smaller peaks at 335 nm and 635 nm, with an absorption onset near 800 nm . The spectra of Ag15(SG)11 and Ag11(SG)7 are quite similar in that they contain no prominent features, however their absorption onsets of ~600 nm and ~550 nm are easily distinguished. Above the onsets, there are only subtle absorbance features until the onset of d-

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core size had no significant effect on emission energies or peak widths. We therefore conclude that the observed emission was not related to excited states in the metal core. Further, glutathione is not known to be fluorescent in the visible, therefore we can also conclude that the observed emission was not related to states that reside only on the ligands.12 The only remaining source for fluorescence is therefore the metal-thiolate interface between the core and the ligands. To better understand the origin of the observed fluorescence, excitation spectra were measured for each particle while detecting at the wavelength of the emission maximum (see Fig. 2). First, we note that these species are complex and have a diverse set of excitations. Only some of these excitations contribute to fluorescence, therefore the steady-state absorption spectrum of each particle will necessarily be different from its corresponding excitation spectrum. Second, although the absorption spectra of the three particles are quite distinct, their excitation spectra are very similar. Again this indicates that the states involved in the excitation spectra are not those of the metal core. This is consistent with emission originating from the metal-thiolate interface, however, since the states related to the metal-thiolate capping units ought to be localized and therefore would be decoupled from the rest of the nanoparticle. A more detailed analysis of the relationship between excitation and emission was done by making qualitative comparisons between least-squares Gaussian fits of the absorption and excitation spectra for each particle, as shown in Fig. 4. Fitting was done systematically and self consistently to avoid the common pitfalls of fitting with many free parameters. The spectrum of Ag32(SG)19 was fit first due to its many distinct and prominent features, which served to limit the fitting parameters (see Fig. 4a). Fits were performed using the fewest possible peaks. Analysis of the two remaining particle sizes was similar. The absorption spectrum of Ag32(SG)19 was used as a start-

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ing point to fit the absorption spectrum of Ag15(SG)11, which in turn served as the starting point for fitting the absorption spectrum of Ag11(SG)7. For example, the peak positions and widths from the Ag32(SG)19 fit were initially fixed for fitting the absorption spectrum of Ag15(SG)11, such that initially only peak heights were allowed to vary (see Fig. 4c). Peak positions and widths were allowed to vary in the final fitting step. This empirical methodology assumed an underlying commonality in the electronic structure of these three closely related species. Based on the success of the fitting, this appears to be a reasonable assumption. We note and caution, however, that although this is a useful method of analysis, no unique fits were found to exist for these complex spectra due to the large number of parameters. The fits should therefore be treated as a qualitative analysis only. Note that the difference between the Ag15(SG)11 and Ag11(SG)7 spectra is primarily the absorption onset, which can be modeled as the presence or absence of a peak at 2.39 eV (the leading peak in Fig. 4c). Likewise the difference between the Ag32(SG)19 and Ag15(SG)11 spectra is primarily the presence or absence of two peaks: (i) the peak in Fig. 4a at 1.96 eV corresponds to the absorption onset and possibly the HOMO-LUMO gap if it is an allowed transition, and (ii) the prominent peak at 2.56 eV in Fig. 4a could in principle be fit as the sum of two peaks at the same energy (2.56 eV) such that only one would be present in the Ag15(SG)11 spectrum in Fig. 4c. Since the peaks at 1.96 eV and 2.56 eV in Fig. 4a and the peak at 2.39 eV in Fig. 4c depend on core size, we can conclude that these transitions involve superatomic orbitals.7,42 Delocalized superatomic (molecular) orbitals arise from the Ag 5s atomic orbitals and normally include the HOMO and LUMO and other frontier orbitals.5,24,26,43-45 It is expected that there are superatomic states related to the Ag11(SG)7 core, since it ought to have 4 delocalized electrons in the core,7,42 however we are unable to distinguish transitions

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Figure 4. Scaled optical absorption (top) and excitation (bottom) spectra for Ag32(SG)19 (a and b), Ag15(SG)11 (c and d), and Ag11(SG)7 (e and f) nanoparticles. Fits shaded in red indicate the regions of the spectra with the largest contribution to the fluorescence. Gaps in the excitation data near 4 eV are instrumental artifacts.


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delocalized core electrons at 2.56 eV does not couple to the fluorescent interfacial states.

Figure 5. Excitation (dotted lines) and fluorescence (thick solid lines) spectra of Ag11(SG)7 (red) and a silver glutathionate polymer (blue) for comparison. Fluorescence spectra were fit with Gaussians (thin solid line). The emission peak widths are 0.37 eV and 0.50 eV for the nanoparticle and polymer, respectively. The Stokes shifts were 0.67 eV in both cases.

involving these states using this methodology without a smaller nanoparticle for comparison. The remaining peaks appear in all three spectra and can be attributed primarily to transitions involving localized states related to the silverthiolate capping units, the d-electrons from Ag, and the glutathione ligands. Excitation spectra were analyzed by a similar methodology. The absorption spectrum fit for each particle was used as the starting point for fitting the excitation spectrum of that particle; peak positions and widths were fixed and only peak heights were allowed to vary (see Fig. 4). Comparing the excitation spectra and their corresponding absorption spectra, it is clear that some of the most pronounced differences occurred in the short wavelength region for all three species. This region of the spectrum is typically dominated by excitations of the Ag 4d electrons, which for bulk silver occurs at 3.2 eV.32 One can therefore conclude that excited Ag 4d electrons do not couple strongly to the fluorescent state. This coupling tended toward zero as the energy difference between the excited 4d electrons and the emissive state increased, as expected. Other pronounced differences are apparent when comparing Ag32(SG)19 absorption and excitation spectra. The two excitations at 1.96 eV and 2.56 eV, which were attributed to superatomic states on the metal core, do not couple to the emissive state. The lowest energy transition centered at 1.96 eV may not have involved enough energy to populate the fluorescent state if the internal conversion involved a significant Stokes shift. The prominent absorption feature at 2.56 eV did involve enough energy to populate the fluorescent state, however. This feature has been assigned to a transition between 1D- to 1F-orbitals on the metal core based on a Ag31(SCH3)19 model.27 This assignment is consistent with our measurements. We can conclude that the component of the absorption spectrum that corresponds to excitation of

All three excitation spectra show that optical excitations in the range of ~2-3.5 eV couple very strongly to the emissive state and make the largest contribution to fluorescence (red peaks in Fig. 4). Density functional theory (DFT) calculations have shown that when thiolate ligands bind to a silver core, localized states arising from mixing Ag 4d and S 3p atomic orbitals sit ~2 eV or more below the Fermi level of the molecule.5,24,26,43-45 The size independence of the excitation spectra and the position of the interfacial metal-thiolate states, as determined by DFT calculations, both indicate that a significant fraction of the excitations between 2-3.5 eV likely involve these interfacial metal-thiolate states. This is also consistent with recent reports that have attributed visible fluorescence of thiolated gold nanoparticles to the thiolate ligand shell.28,29,31,46 Comparing fluorescence and excitation spectra of the nanoparticles with the silver glutathionate polymer (see Fig. 5) reveals several similarities and differences. Here, Ag11(SG)7 was used for the comparison but all species were similar. Both species have a Stokes shift of 0.67 eV but with a significant redshift of the nanoparticle spectra compared to the polymer spectra. This is consistent with the damping expected due to coupling to the metal core, which would lead to a redshift. It is worth noting that this is significantly larger than typical Stokes shifts for semiconductor quantum dots and most organic dyes47 and that there is very little overlap between the excitation and emission spectra in both cases. There is a significant difference between the excitation spectra, however. The nanoparticle excitation spectra are rather complex in shape and span a wide spectral region whereas the polymer spectrum is a narrow single Gaussian peak. The nanoparticle excitation spectra appear to consist of the polymer excitation spectrum Stokes shifted by 0.67 eV in addition to a number of other transitions. Finally, the emission peak width is broader for the polymer than for the nanoparticle despite the fact that damping due to coupling to the metal core would normally lead to broadening of the nanoparticle emission. The polymer can be thought of as a system of identical coupled oscillators, which would also lead to broadening. The capping units on the nanoparticles can be thought of as discrete fragments of the polymer,4,5,43,48,49 however, in which case the emission band might be expected to be more narrow. Further, the motion of the discrete capping units would be constrained when attached to the metal core of a nanoparticle, which again might be expected to narrow the emission band due to the reduced degrees of freedom. Comparing the quantum yields (QY) of each nanoparticle can also provide useful insights. The QYs were found to be a monotonic function of size, where the QY of Ag32(SG)19 was 0.86%, Ag15(SG)11 was 2.9%, and Ag11(SG)7 was 3.2%; the QY of silver glutathionate was only 0.84%. This is considerably higher than the QYs for emission attributed to core states



in Au nanoparticles.12,15,23 This size dependence of the nanoparticle fluorescence efficiency can be rationalized by considering that coupling to a smaller system normally provides fewer non-radiative decay channels, therefore higher QYs are expected for decreasing core size. It is curious, however, that the QYs of Ag15(SG)11 and Ag11(SG)7 were higher than that of the silver glutathionate polymer, indicating a fundamental difference between the optical properties of the capping units and the polymer. Like the emission peaks widths, this might also be related to discrete nature of the capping units and their constrained motion due to their attachment to the metal core, both of which would be expected to reduce the number of non-radiative decay channels compared to the polymer. It is worth noting that the photographs in Fig. 1 make it appear as though the QY of Ag15(SG)11 might be considerably larger than that of Ag32(SG)19 since the relative concentration of Ag15(SG)11 appears to be much smaller than that of Ag32(SG)19. This is due to the large difference in their molar absorptivities. While this large difference in absorptivity also makes it difficult to identify Ag15(SG)11 (and Ag11(SG)7) as decay products39 (i.e. contaminants) in solutions of Ag32(SG)19, differences in quantum yield could reveal their presence when solutions of Ag32(SG)19 exhibit stronger than expected fluorescence. The general picture of emission from interfacial silverthiolate states was supported by time-resolved spectroscopic measurements. Femtosecond time-scale emission dynamics of Ag32(SG)19 and Ag15(SG)11 were resolved with fluorescence up-conversion at 630 nm and 650 nm, respectively (see Fig. 6 and Figs. S1 and S2). The emission dynamics can be fit with bi-exponential decays convoluted with the instrument response function. The fitted lifetimes were 3.0 ± 0.2 ps and 130 ± 10 ps for Ag15(SG)11 and 480 ± 30 fs and 130 ± 10 ps for Ag32(SG)19. The first lifetime for each Ag nanoparticle suggests a fast relaxation process that is likely related to the metal core. Large plasmonic gold nanoparticles have a very fast lifetime of 50 fs50 whereas small molecular Au nanoparticles with discrete states, such as Au25(SG)18, have a single emission lifetime around 250 fs measured in the visible.15 The bi-exponential lifetime of Ag32(SG)19 and Ag15(SG)11 suggests a more complex emission mechanism. The first lifetime suggests a fast relaxation process, very much similar to the core emissive states of gold nanoparticles.50 The longer lifetimes for Ag are likely due to weak coupling to the Ag 4d electrons, which are considerably lower in energy compared with the Au 5d electrons.32 It is worth noting that the 3 ps lifetime for Ag15(SG)11 was much longer than the 480 fs lifetime for Ag32(SG)19, which was consistent with the expectation that the number of non-radiative decay channels ought to scale with the system size. The second fitted lifetime for Ag32(SG)19 and Ag15(SG)11 were both about 130 ps and constituted almost all of the measured emission (Fig. 6, blue).

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The similar lifetimes for the second decay component support the finding that the two species have a common emissive state, with the same emission wavelength and dynamic, which can be assigned to the metal-ligand state. A better theoretical understanding of the optical properties of these materials is needed to advance their design for optical or optoelectronic applications. For example, subtle differences in emission energies are likely due to differences in the details of the structures of the ligand shell for each nanoparticle. One might then expect that if the nature of the ligand shell and its coupling to the metal core is understood, then these interactions could be controlled in such a way as to rationally design and tune the color and brightness of the fluorescent metal nanoparticles. For example, choosing the smallest nanoparticles ought to give the highest QY and choosing the core material may provide color tunability. Such an understanding is of fundamental interest and could also be used to develop fluorophores for multiplexing in bioimaging34,51 or explain optical sensitivities to specific metal ions for detection.38,52 CONCLUSIONS Our results show that the interfacial silver-thiolate shell is the primary origin of fluorescence; weak metal core emission was also detected. They also show that the metal core improved energy capture by increasing the spectral range for excitation from ~340-360 nm for silver thiolates to ~300-550 nm for silver nanoparticles (compare the excitation spectra in Figs. 2 and 5). It remains to be seen, however, precisely how the different nanoparticle excited states couple to the same fluorescent state. Likewise, a detailed understanding of why superatomic excited states did not couple to the emissive state will require further investigation. Calculations based upon experimentally determined structural models would facilitate this, however aliphatic ligands such as glutathione have thus far failed to yield an x-ray crystal structure. AUTHOR INFORMATION


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Corresponding Author

[email protected] Present Addresses

†Current address: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 Funding Sources

TPB gratefully acknowledges support from the National Science Foundation, under award number CBET-0955148, and the School of Solar and Advanced Renewable Energy. TGIII would like to thank the Army Research Office (Materials Program) for support of this research. Notes

The authors declare no competing financial interests. Supporting Information

Additional details regarding the time-resolved measurements of the fast decay components are supplied as supporting information. This material is available free of charge via the internet at http://pubs.acs.org.

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