Investigation of Photophysical and Electrofluorochromic Properties of

Article pubs.acs.org/JPCC

Investigation of Photophysical and Electrofluorochromic Properties of Gold Nanoparticles Functionalized by a Luminescent Electroactive Complex L. Guerret,† J.-F. Audibert,† A. Debarre,†,‡ M. Lepeltier,§ P. Haghi-Ashtiani,∥ G. V. Dubacheva,† and F. Miomandre*,† †

PPSM, CNRS, ENS Cachan - Université Paris-Saclay, 61 Avenue Président Wilson, 94235 Cachan, France LAC, CNRS, Université Paris 11, ENS Cachan, Université Paris-Saclay, Campus d’Orsay, Bât 405, 91405 Orsay, France § ILV − CNRS, Université Versailles Saint Quentin, 45 Avenue des Etats-Unis, 78035 Versailles, France ∥ LMSSMAT − CNRS, Ecole CentraleSupélec- Grande Voie des Vignes, 92290 Chatenay-Malabry, France ‡

S Supporting Information *

ABSTRACT: Gold nanoparticles (6 nm diameter) functionalized by a luminescent electroactive iridium complex have been synthesized, leading to stable colloidal suspensions in o-dichlorobenzene, a high boiling point solvent. The photophysical properties of these functionalized nanoparticles have been investigated showing a partial quenching of iridium emission by the gold core. Fluorescence correlation spectroscopy has been used to demonstrate that all the iridium complex is actually located on the nanoparticle surface, allowing the investigation of the electrofluorochromic behavior to be investigated for the first time in colloidal plasmonic systems. Compared with the free complex in solution, which shows a well-defined reversible electrochemically monitored luminescence (electrofluorochromism), the iridium-functionalized nanoparticles display a much less pronounced and less potential-dependent electrofluorochomic behavior, unless the potential is pushed toward values where gold stripping occurs.

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INTRODUCTION Gold nanoparticles coated with photoactive materials are the subject of numerous studies, as the gold core is likely to modify strongly the photophysical features especially through plasmonic properties.1,2 The same phenomena occur in the case of nanoparticles functionalized with luminescent molecular units. It is now acknowledged that the distance between the luminescent moiety and the gold core as well as the spectral overlap between the plasmon resonance and the emission bands are critical to predict quenching or enhancement effects.3,4 Using rigid conjugated linkers instead of the classical thiol-terminated alkyl chains is a good way to fix this distance while preserving the electronic communication between the metal and the emissive part. This is especially important when the luminophore is also electroactive, thus exhibiting an electrofluorochromic (EF) behavior, that is, an electrochemical control of the photoluminescence through the redox state.5 EF molecules and materials are the subject of recent interest because they can be the active components of new bright displays with tunable properties.6,7 While the role of plasmon on luminescence is rather well-documented8−11 and the influence of electrochemistry on plasmon resonance has been also investigated recently,12,13 conversely there are very few data in the literature concerning the role of plasmon on electrofluorochromism, and these mainly concern metallic gratings.14,15 Among EF compounds, ruthenium and iridium complexes exhibit a © 2016 American Chemical Society

reversible luminescence switch upon oxidation−reduction processes,16,17 and due to their large Stokes shift, the overlap between plasmon resonance and the dye absorption or emission can be independently controlled. While in some cases full luminescence quenching is observed,18 there are also examples where luminescence is partially kept on,19 leaving the possibility to investigate its switch by an electrochemical signal. Moreover, functionalization of colloidal gold nanoparticles by ruthenium or iridium complexes has been performed so far, using the ligand exchange procedure.19−21 On our side, we have previously shown that EF iridium complexes with rigid conjugated ligands could be coated at the outer surface of gold or silver nanoparticles;17 however, one of the remaining issues is the long-term stability that needed to be improved to investigate accurately the EF behavior of the functionalized metallic nanoparticles. Among the experimental conditions required for the investigation of EF on nanoparticles, a long-term colloidal stability is crucial as well as the use of a totally nonvolatile solvent to ensure to keep the concentrations constant upon illumination. Therefore, we tried to optimize these conditions by finding a solvent fulfilling these requirements. In a second step, we focused our efforts to demonstrate that the recorded luminescence in the nanoReceived: October 8, 2015 Revised: December 22, 2015 Published: January 6, 2016 2411

DOI: 10.1021/acs.jpcc.5b09865 J. Phys. Chem. C 2016, 120, 2411−2418

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The Journal of Physical Chemistry C

Finally, 70 μL of a 10−4 M solution of the Ir complex in dichloromethane is added in 5 mL of the previous Au colloidal solution under strong stirring and left overnight. A ligand exchange takes place where the Ir complex partially replaces the dodecylamine. Dichloromethane (DCM) is evaporated to obtain the Ir-functionalized Au nanoparticles, which can be dispersed again in the same volume of o-dichlorobenzene (DCB). Characterization. HRTEM pictures are obtained on a FEI TitanG2 microscope (probe aberration corrected, acceleration voltage in the range 60−300 kV).The samples are deposited on Cu minigrids and dried at 100 °C for 2 h prior to the TEM observation. Solvents (SDS, HPLC grade) and electrolyte salts (Fluka, puriss.) were used without further purification for electrochemical and spectroscopic analyses. Cyclic voltammetry was recorded in a three-electrode cell with a potentiostat (CH Instruments 600) driven by a PC. Platinum disk (1 mm diameter) was used as the working electrode, while a platinum wire and Ag+ (10−2 M in acetonitrile)|Ag were used as counter and reference electrodes, respectively. Electrochemical characterizations are performed in DCB with tetraoctylammonium hexafluorophosphate (TOAPF6) 0.01 M as the supporting electrolyte. Electronic absorption spectra were recorded on a Cary 500 (Varian) spectrophotometer in 1 cm quartz cuvettes. Fluorescence spectra were recorded on a Fluorolog3 (Horiba) spectrofluorimeter in a quartz cell at the right angle beam geometry. The solutions had OD below 0.1 at the excitation wavelength. The luminescence quantum yield was determined by comparison with a standard (Rhodamine 6G, quantum yield 0.95) using the following formula

composite totally comes from the complex on the NP surface without contribution from free species that would lead to erroneous conclusions in the data interpretation. This has been achieved through the use of fluorescence correlation spectroscopy (FCS) measurements. FCS is a very efficient technique to discriminate luminescent species according to their diffusion rates, and it has been recently applied to various systems involving gold nanoparticles.22−26 For studying liquid samples by FCS, the observation volume is limited to a very tiny size (subfemtoliter) and a high boiling point solvent is required. At moderate concentrations, the species diffusing in and out of this volume owing to Brownian motion produce fluctuations of the luminescence intensity, which can be analyzed by the intensity autocorrelation function. The profile of this function reflects the dynamics of the species and thus depends on the hydrodynamic radius of the diffusing species. FCS is therefore appropriate to identify several species in a sample provided that their diffusion dynamics are different. In the present paper, FCS is used to ascertain that the gold nanoparticles are indeed functionalized with Ir complexes by comparing the autocorrelation functions of a sample of free complexes and that of a functionalized sample. Finally, we present the first results of EF behavior recorded on functionalized colloidal nanoparticles and discuss them in comparison with the EF behavior of the free complex in solution.

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EXPERIMENTAL SECTION Synthesis. The synthesis of the iridium complex used in this study is described in ref 17. It includes a pyridine end group, allowing its coating on a gold nanoparticle surface (see Scheme 1) according to the following experimental procedure. Scheme 1. Synthesis of Ir-Functionalized Au NPs by Ligand Exchange (top) and Photographs of Au NP in DCM (a), Ir Complex in DCM (b), and Au−Ir NP in DCB (c) (bottom)

0

ΦFi = ΦFs

n i2 ∫∞ IFi(λE , λF) dλF 1 − 10 As(λE) 0 2 A (λ ) nstand ∫∞ IFs(λE , λF) dλF 1 − 10 i E

where Φ, n, IF, and A are, respectively, the luminescence quantum yield, refractive index, luminescence intensity, and absorbance for either the compound of interest (i) or the standard (s); λE and λF are the emission and luminescence wavelengths, respectively. Fluorescence Correlation Spectroscopy. The FCS setup is based on a confocal microscope. (See ref 26 for details.) A Ti:sapphire pulsed laser (80 MHz repetition rate, 100 fs pulse duration) was focused into a drop of sample deposited on a clean coverslip by a water-immersion high numerical aperture objective (Nikon, ×60, N.A. = 1.2). The Ir-complex luminescence was induced by two-photon excitation at 700 nm. The emission collected by the same objective was directed to a 50% fiber coupler after passing through a two-photon dichroic mirror and a telescope designed to match the beam profile to the fiber core and numerical aperture. The fiber coupler was connected to two avalanche photodiodes (SPCM-AQR-FC PerkinElmer instrument), which allowed us to get rid of afterpulsing artifacts by calculating the cross-correlation function of the signal. The signal was spectrally separated from the laser wavelength with a short pass filter (Semrock FF01-680/SP). The avalanche photodiode signals are sent to a Single Photon Counting module (TimeHarp300, Picoquant). A home-written computer program was used to analyze the histograms of the emitted photons and to calculate the correlation of the data. The correlation function reads as

Chloroauric acid HAuCl4·3H2O (0.010 g, 2.5 × 10−5 mol) and dodecylamine (0.2 g, 8.3 × 10−4 mol) are dissolved in 10 mL of dichloromethane. Then, the solution is diluted to 60 mL and a cooled solution of NaBH4 (400 μL, 1M) is slowly added. After at least 2 h of stirring, the aqueous phase is removed with a micropipette. The organic phase is washed with chlorhydric acid 1 M and separated again from the aqueous phase by decantation. 2412


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Figure 1. HRTEM pictures of Au NP in DCM (a) and Au−Ir NP in DCB (b). Red circles show examples of dimers of individual nanoparticles.

G (τ ) =

performed on a Nikon Ti−U microscope under either direct (wide-field) or TIRF illumination (see ref 29 for details) with a mercury lamp as the excitation source and a large numerical aperture (1.49) ×100 oil immersion objective. The excitation wavelength was selected by an adequate filter (DAPI), and the emission was recorded with a high-pass filter (LP500) coupled to a dichroic mirror (DAPI). (See their spectral features in Figure S1.) The region of interest (ROI) for luminescence imaging is ca. 100 μm2. The setup allows simultaneous recording of the electrochemical current and fluorescence intensity when applying a potential signal to the working electrode. The working electrode is a microscope coverslip (170 μm thin) coated with ITO. A Ag wire is used as the pseudoreference and a Pt wire is used as the counter electrode in a three-electrode electrochemical cell containing the colloidal solution in DCB + TOAPF6 0.01 M. This latter is deaerated thoroughly by argon bubbling to avoid traces of oxygen, which strongly decrease the luminescence yield of the complex. (An argon blanket over the solution is maintained during the experiment.) The fluorescence intensity versus time is recorded through a side port of the microscope and collected by a PCO pixelfly QE USB camera (ADC/14 bit @12 MHz) with time lapses recorded using multidimensional acquisition module on Micro-Manager at 1.96 fps, under electrochemical control (Potentiostat CH Instruments 600).

⟨I(t )I(t + τ )⟩ ⟨I(t )⟩2

τ denotes the lag time. The brackets indicate time average. Assuming a Gaussian−Gaussian observation volume, the correlation function corresponding to a Brownian diffusion is obtained with the following expression27 G (τ ) = 1 +

1 2 2N

1

(

1+

τ τD

)

1+

2

( ) ωr ωz

τ τD

(1)

τD, ωr, and ωz are the characteristic diffusion time of the species, the excitation lateral beam waist, and the excitation axial beam waist, respectively. N is the number of species simultaneously present in the observation volume. τD is related to the diffusion constant of the species D by the following expression: τD = D is given by the Stokes−Einstein relation28 D=

kT 3πηd

ωr2 . 8D

(2)

where η, d, k, and T are the dynamic viscosity of the solution, the hydrodynamic diameter of the species, the Boltzmann constant, and the temperature, respectively. The determination of the hydrodynamic diameter relies on the calibration of the observation volume. To calibrate this observation volume, we have recorded the two-photon FCS of rhodamine 6G molecules in water. The fit according to eq 1 leads to the values of 370 nm and 3 μm for ωr and ωz, respectively. Fluorescence Microscopy Coupled to Electrochemistry. Fluorescence microscopy coupled to electrochemistry is

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RESULTS AND DISCUSSION Synthesis and TEM. The synthesis of the functionalized Au NP in DCM follows a protocol adapted from the Brust−Schiffrin method.30 Using dodecylamine as the stabilizer leads to spherical NPs with a mean diameter of 6.1 ± 0.75 nm (see TEM pictures in 2413


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the particle solution like overall aggregation or flocculation takes place on short time range. The photoluminescence of Ir complex, Au NP, and Au−Ir NP in DCB is shown in Figure 3. Upon excitation at 340 nm, the Ir

Figure 1a). The next step is the introduction of the Ir complex on the NP surface by a ligand exchange reaction, where the pyridine anchoring group replaces the amine (see Scheme 1). After slow evaporation of DCM and replacement by DCB, the Au−Ir NP colloidal suspension is analyzed by TEM and UV−vis spectroscopy to check if plasmonic properties are preserved and if NP size and distribution of sizes are modified. Figure 1b shows no significant increase in the average size compared with the aminestabilized NPs, with only a slight increase in the polydispersity from 12 to 19% while preserving the spherical shape. The colloidal stability and plasmonic properties are also preserved, as evidenced by UV−vis spectroscopy (see later). Photophysical and Electrochemical Properties. Figure 2 shows the absorption spectra of Ir complex, Au NP, and final

Figure 2. UV−vis absorption spectra of Ir complex (blue trace), Au NP (red trace), and Au−Ir NP (green trace) in DCB (Ir concentration: 2 × 10−5 M).

Au−Ir nanocomposite in DCB. Concerning the Ir complex, the absorption spectrum shows the main band at 347 nm ascribed to ILCT and the smaller band near 430 nm ascribed to LMCT. Au NP before ligand exchange, that is, stabilized by dodecylamine alone, exhibits a well-defined band at 520 nm due to plasmon resonance. After ligand exchange, Au−Ir NP absorption spectrum displays as expected the sum of the contributions coming from Ir complex and Au NP, that is, three main absorption bands. The plasmon resonance band is damped and slightly red-shifted compared with Au NPs (see Table 1); however, we have thoroughly checked the stability of the Au−Ir NP colloidal solution in DCB by recording UV−vis spectra after several days. Even 1 month after the synthesis has been performed, the three bands identified above are still visible (see Figure S2). This guarantees that no major destabilization of

Figure 3. Emission (a) and excitation (b) spectra of Ir complex (blue trace), Au NP (red trace), and Au−Ir NP (green trace). Excitation wavelength: 340 nm (a). Emission wavelength (b): 620 nm.

complex displays a bright luminescence peak centered at 620 nm, namely, slightly red-shifted compared with DCM.17 This luminescence is partially quenched in Au−Ir NP, as shown by the lower intensity obtained for the same concentration of emitter and confirmed by the quantum yield measurement (see Table 1). Figure 3b shows the excitation spectra recorded at the maximum emission wavelength. Those are very similar in both cases (Ir and Au−Ir), which is a good indication that luminescence actually comes from the Ir complex emitter in Au−Ir NP. Fluorescence Correlation Spectroscopy. The FCS study aims at controlling the efficiency of the ligand-exchange procedure. To this end, we have studied three different samples. Sample 1 (S1) is composed only of iridium complexes freely

Table 1. Photophysical Data for the Ir Complex and the Au−Ir NP in DCB compound

absorption maximum wavelength/nm

emission maximum wavelength/nma

quantum yieldb

Ir Au−Ir

347 340; 520

620 628

0.052 0.044

a

Excitation wavelength: 340 nm. bMeasured in deaerated dichlorobenzene with Rhodamine 590 as a standard. 2414


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The Journal of Physical Chemistry C diffusing in o-dichlorobenzene (DCB). Sample 2 (S2) contains gold nanoparticles functionalized with iridium complexes according to the previously described procedure in DCB. Sample 3 (S3) is obtained by simply mixing free iridium complexes and gold nanoparticles functionalized by dodecylamine. This sample is studied shortly after mixing (half an hour). The dynamic viscosity of DCB is 1.32 mPa·s at 300 K, more than that of pure water and thus sufficient to observe diffusion times of small particles in the range of a few tens of microseconds suitable for FCS measurements. A concentration of 10−8 M has been chosen for all the samples as a satisfying compromise between the rather weak quantum efficiency of the iridium complex luminescence and the need for a good signal-to-noise ratio in FCS measurements. An example of the correlation profiles obtained for S1 and S2 is displayed in Figure 4. The correlation profile of S1 is satisfyingly described by a single source of fluctuations corresponding to the Brownian diffusion of the complexes. The diffusion time of iridium complexes (S1) is 74 μs (±3 μs). According to eq 2, the hydrodynamic diameter of the iridium complexes freely diffusing in DCB is ≅1.4 nm (±0.15 nm). The photon histogram of S2

displays distinct bursts, which correspond to the crossing of single particles through the excitation volume. The correlation profile can be satisfyingly fitted by adding a rotation contribution to the Brownian diffusion model given in eq 1. The correlation function thus expresses as [12e] G (τ ) = 1 +

1 2 2N

1

(1 + ) τ τD

⎛ ⎛ τ ⎞⎞ × ⎜⎜1 + C exp⎜ − ⎟⎟⎟ ⎝ τr ⎠⎠ ⎝

1+

2

( ) ωr ωz

τ τD

(3)

where τr is the rotation time and C is the contrast of the rotation contribution. The rotation time and the diameter d of the particle are linked by d =

6kT τ πη r

(at first order), where the parameters

are identical to those of eq 2. After filtering the few very large bursts, the correlation profile of the functionalized particles is satisfyingly fitted with a diffusion time of 840 μs. It corresponds to a hydrodynamic diameter of the diffusing species of 15 nm (±2 nm). The additional contribution in the short time domain (τr = 0.7 μs) corresponds to the rotation time of particles with a diameter of 16 nm. The observation of a rotation time can be reasonably attributed to a slightly irregular capping of the Ir complexes around the particle and possibly to the slight departure of the core itself from a perfect sphere. This result first demonstrates that the concentration of possibly free iridium complexes in S2 is far below 10−8 M according to the study of S1, indicating that the functionalization process was successful, which was the main purpose of this FCS study. The gold core mean diameter of the functionalized particles determined by FCS from diffusion times is typically around 12 nm, taking into account the size of the iridium complex. The gold core size value is thus larger than the one deduced from TEM, partly due to the solvation shell but also to possible inhomogeneities in the capping, which could also be at the origin of an apparent increase in the Au core size. Moreover, because the brightness of a single iridium complex is weak (see later), the largest cores that are functionalized with a higher number of iridium complexes can give a large contribution to the signal, even if they are less numerous than particles with a gold core of ∼6 nm. Such large cores can be seen as “dimers” in the TEM pictures (see Figure 1b). The filtered larger bursts have hydrodynamic radii that extend up to 42 nm, which is the signature of small clusters of particles, but in a very low amount. The correlation profile of S3 30 min after mixing the two solutions of iridium complexes and gold nanoparticles is very similar to the profile of freely diffusing iridium complexes in S1 (Figure S3). This profile can be fitted by a simple Brownian diffusion model with a diffusion time of 69 μs (±4 μs) in agreement with the result of S1. The concentration of iridium complexes was increased to 10−7 M before mixing to unequivocally test the origin of the long component of the correlation profile observed in S2. Because the functionalization process is not achieved in this situation, this result excludes the possibility that the longer diffusion time observed for S2 could correspond to the emission of aggregates of iridium complexes on the one hand or to two-photon excited intrinsic luminescence of gold particles on the other hand [12e]. As regards S2, the analysis of the correlation profile gives a mean number N of complexes simultaneously present in the observation volume close to unity, which is somewhat lower than

Figure 4. Experimental correlation profiles of samples S1 (A) and S2 (B) (red dots) and fit according to eqs 1 and 3, respectively (blue lines); on top of each graph, residue of the fit; excitation wavelength and power are equal to 720 nm and 6 mW, respectively. Concentration of Ir = 10−8 M in both cases. 2415


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The Journal of Physical Chemistry C the expected number of particles with a 10−8 M concentration used in the present study. The average counts per functionalized particle and per second, also named brightness, is nearly 1900 CPS for an excitation power of 6 mW at 720 nm, while the brightness of a single iridium complex is 330 CPS under the same conditions. On average, a functionalized particle has a brightness equivalent to that of six iridium complexes. On the basis of the ratio between Ir and Au introduced in the synthesis (3.5 × 10−3 Ir per Au), the average number of Ir complex molecules per particle can be estimated to

too low to detect a peak in CV, but it can be assumed that it is close to the one of the free complex based on reported results in similar configurations.19 Electrofluorochromism. Using the coupling of TIRF microscopy with an electrochemical cell under steady-state excitation and adequate excitation and emission filters, the EF properties of Ir and Au−Ir NP were separately investigated. Figure 6 shows the electrochemical modulation of luminescence intensity when potential steps are applied to switch the redox state of the iridium center. In the case of the free Ir complex in DCB, a typical EF behavior is observed; that is, the luminescence drops upon oxidation and is recovered upon reduction, with an amplitude directly related to the applied potential (Figure 6A). For potentials leading to large conversion ratios (typically above 1.5 V), the luminescence recovery slows down so that the initial level is not reached at the end of the step. This is probably related to the sluggish electron-transfer reaction already evidenced by CV and can be bypassed by using a more negative potential for the backward step. The same reason is responsible for the nonconstant luminescence level reached at the end of the forward step when high potential values are used: The Ir(IV)/Ir(III) ratio does not reach equilibrium values (given by Nernst law), instantaneously causing a delay to reach the equilibrium luminescence value, contrariwise to what is observed at lower potentials (1.4 V and lower). When the same type of experiment is performed with the Au−Ir NP colloidal solution, the results are significantly different (Figure 6B). First of all, the amplitude of the modulation is much lower than in the previous case, and it does not seem to be much affected by the final potential value; however, when looking closely to the modulation for a given potential and expanding the scale for luminescence intensity, it is clear that luminescence drops upon oxidation and is recovered upon reduction in the backward step (Figure S4). Thus, addressing the Ir complex when grafted on the Au NP surface is possible, but the efficiency is much lower than in the case of the free complex. When high oxidation potentials are applied (>1.9 V), the behavior changes and a large amplitude is observed. We believe that gold stripping starts to occur in that potential range, and this leads to the modification of the EF modulation observed. As it was established previously that the electrochemical addressing of metallic coordination complexes on gold nanoparticles of similar sizes was possible but involves a change in the charge stored in the gold core,19 it can be concluded that the gold NP core charge actually plays a role in the EF response. This gold core charge impacts the luminescence probably in an opposite way as the redox potential does, resulting in a lower modulation; however, additional experiments are required to confirm this conclusion, especially by changing the gold core size of the particles.

⎛ dNP ⎞3 3.5 × 10 ⎜ ⎟ = 42 ⎝ dAu ⎠ −3

This result implies that the luminescence of the bound complexes is partly quenched by the gold particle, as it was often reported for such plasmonic objects with a small metallic core4,31 and as it was already noticed in the photophysical study above. Thus, it can be concluded that FCS shows particles with mean diameter of 12 nm and a brightness equivalent to six iridium complexes per particle, much lower than the actual number due to the quenching by the Au core and also because FCS is more sensitive to larger and brighter objects. Electrochemistry. The CV of the Ir complex solution (10−4 M) in DCB with TOAPF6 10−2 M as the supporting electrolyte has been recorded (Figure 5). An oxidation peak at 0.82 V is

Figure 5. CV of Ir complex 10−4 M in DCB + TOAPF6 10−2 M. Scan rate: 100 mV/s.

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CONCLUSIONS Gold nanoparticles functionalized by a luminescent electroactive iridium complex have been synthesized with mean diameter close to 6 nm. Colloidal suspensions of these nanocomposites in odichlorobenzene are stable and can be kept for long times retaining their plasmonic properties. Fluorescence correlation spectroscopy measurements give evidence that the luminescence recorded in these plasmonic systems totally come from the iridium complex grafted on the nanoparticle surface. This has allowed us to record the electrofluorochromic properties for the first time of electroactive luminophores grafted on colloidal nanoparticles. Differences from the electrofluorochromism of the free iridium complex are observed: The modulation

observed on the forward scan with the corresponding reduction peak located at 0.44 V on the backward scan. Thus, the standard potential of the redox couple involved (Ir(III/IV)) can be estimated to be 0.63 V versus AgCl|Ag. The potential difference between the anodic and cathodic peaks is significantly larger than the 60 mV expected for a fully reversible redox couple, which is probably due to a sluggish electron transfer kinetics combined with residual ohmic drop, as the solvent is more viscous and thus more resistive than dichloromethane. This allows one to ascribe the potential around which the conversion between the two redox states of the iridium complex takes place in the nanocomposite. The direct measurement cannot be performed on Au−Ir NP because the concentration is 2416


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Figure 6. Modulation of luminescence versus time variations upon potential steps (potential signal figured as dashed line) from 0 V to the indicated values and back to 0 V: (A) Ir complex in DCB and (B) Ir−Au NP in DCB.

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amplitude is lower and much less potential-dependent in the nanocomposite than in the free molecule. Additional experiments are in progress to better understand the reason for these differences, especially by modifying the gold core size, because in that case plasmonic properties should be directly impacted.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09865. Spectral features of the lamp and filters used (S1), influence of aging on absorption spectra of the Au−Ir NP in dichlorobenzene (S2), FCS of sample S3 corresponding to the simple mixing of Ir complex and Au NPs (S3), and zoom on the electrofluorochromism of Au−Ir NP at 1.5 V (S4). (PDF)

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ACKNOWLEDGMENTS

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REFERENCES

The microscopy work was carried out within the MATMECA consortium supported by the ANR under contract number ANR10-EQUIPEX-37.

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