Electrostatically Driven FÓ§rster Resonance Energy Transfer between

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C: Physical Processes in Nanomaterials and Nanostructures

Electrostatically Driven F#rster Resonance Energy Transfer between a Fluorescent Metal Nanoparticle and JAggregate in an Inorganic-Organic Nanohybrid Material Aman Kumar Agrawal, Prabhat Kumar Sahu, Sudipta Seth, and Moloy Sarkar J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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The Journal of Physical Chemistry

Electrostatically Driven Fӧrster Resonance Energy Transfer between a Fluorescent Metal Nanoparticle and J-Aggregate in an Inorganic-Organic Nanohybrid Material

Aman Kumar Agrawal,a Prabhat Kumar Sahu,a † Sudipta Seth,b and Moloy Sarkar a 

_________________________________________________________________________ a

Aman Kumar Agrawal, Prabhat Kumar Sahu, Dr. Moloy Sarkar School of Chemical Sciences, National Institute of Science Education and research, Bhubaneswar, HBNI, Bhimpur-Padanpur, Jatni, Khorda-752050, Odisha, India, Email: [email protected] b

Sudipta Seth Department of chemistry, University of Hyderabad, Hyderabad – 500046, India. †

Present Address: Department of Chemistry, University of Michigan, Ann-Arbor, MI 48109, USA

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Abstract With an objective to understand the interaction between inorganic and organic components of inorganic-organic hybrid nanomaterials, we have fabricated and studied the photonic properties of a nanocomposite system consisting of silver capped gold nanoparticle as inorganic and J-aggregate of cyanine based dye (S2165) as organic component. The present hybrid construct has been fabricated by adopting electrostatically driven self-assembly of organic (cyanine dye) and inorganic silver capped gold nanoparticle (AgCAu). In contrast to the previously developed hybrid systems where fluorescent inorganic semiconductor quantum dots are integrated with Jaggregates, the formation of hybrid system in the present work is carried out by exploiting fluorescent silver capped gold nanoparticle and J-aggregate of cyanine dye. The hybrid system has been characterized by spectroscopic and microscopic techniques. Steady state and time resolved fluorescence measurements have been performed on this hybrid system to understand the interaction of metal nanoparticles with the J-aggregate. Additionally, fluorescence lifetime imaging microscopy (FLIM) studies have also been done on the hybrid system to examine the heterogeneity in the fluorescence lifetime of the system. The composite system displays a broad absorption in the UV part of the spectrum typically associated with the inorganic nanoparticles, and shows a peak in visible region corresponding to J- aggregate. The quenching of fluorescence of AgCAu and increase in the intensity of J-aggregate on excitation of AgCAu reveal the process of energy transfer process from AgCAu to J-aggregate. Analysis of the data in light of Fӧrster theory reveals that inorganic AgCAu and organic J-aggregate in the hybrid system are electronically coupled. The zeta potential measurements on AgCAu and the hybrid system reveal that the interaction between AgCAu (donor) and J-aggregate (acceptor) and consequently, the energy transfer process between them is electrostatically driven. The observation of highly


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efficient energy transfer between inorganic and organic units of the hybrid material indicates that the present hybrid material can be quite useful in various optoelectronic applications. 1. Introduction Designing of nanomaterials with specific structure and desired properties is one of the important theme of current research in nanomaterial science.1-3 In this regard inorganic-organic hybrid nanoparticles offer a new possibility of developing functional materials as they are expected to have unique optical and electrical properties which can be very different from those of the individual constituents.1, 4-7 In this context, in the recent times, few studies have been carried out by focusing on the nanohybrid structures having semiconductor quantum dots or metal nanoparticles and J-aggregate of organic dye molecules.8-13 It is anticipated that J-aggregates can serve as an important component in the designing of inorganic-organic hybrid materials for its favorable optical properties like high molar extinction coefficient, narrow absorption band, etc.1415

By virtue of these interesting optical properties, J-aggregates can undergo strong excitonic

coupling with the excitons of QDs and plasmons of metal nanoparticles and thus can help in developing interesting optoelectronic materials.6, 14 However, studies that focus in understanding the kinship among interparticle interaction (within inorganic (QDs, metal NPs) and organic nanoparticles (J-aggregates)), and the optical properties of these hybrid heterostructured systems are rather limited. Majority of the studies on this aspect are carried out by involving inorganic nanoparticles and organic molecules.16-20 Therefore, designing and development of hybrid system comprising of inorganic and organic nanomaterials and understanding the fundamentals of their action are expected to be quite helpful for the development of efficient optical devices at nanometer length scale.



J-aggregates (J for Jelly) are usually formed through self- assembly of individual molecules in head to tail arrangement.21-22 Due to this arrangement in J-aggregates, the transition dipoles of individual molecules are aligned parallel to each other. Interaction of transition dipoles results in splitting of the excitonic state of the monomer upon aggregation. As a result of this, Jaggregates exhibit red-shifted and a very sharp absorption band with high extinction coefficient as compared to the monomeric species.14-15 Metal nanoparticles having size in the range of 2 nm-100 nm have been exploited immensely in the field of material chemistry research for their unique optical properties which arise due to the surface plasmon phenomena.23-25 These properties are dependent on the composition, size or shape of the nanoparticles.26-30 Nanoparticles in this size regime are mostly non-fluorescent. Inorganic-organic hybrid nanoparticles comprising of inorganic non-fluorescent metal cores and organic molecular shell are known to exhibit interesting optical properties like rabi splitting, fano resonance, non-linear properties, and photoinduced charge separation.6, 11-12, 31-37 These studies have demonstrated the strong coupling limit that can be achieved in such systems which could pave the way towards development of active photonic devices operating at room temperature. Please also note that in recent time, fluorescent semiconductor QDs have been exploited to study the interaction with dyes or J-aggregates. Such studies of semiconductor hybrid nanoparticles are primarily focused on the investigation of energy transfer process between the semiconductor quantum dots and J-aggregate.8-10, 38 Among these, the independent studies carried out by Bawendi and coworkers,8, 10, 38 and Rakovich and coworkers9 are noteworthy. They have demonstrated that QD/ J-aggregate FRET pairs can be interesting candidates for optoelectronic application as the electronic coupling between the constituents is observed to be quite high. However, it may also be noted here that as heavy metals are used in the formation of semiconductor QDs, the toxicity of these systems can hamper the real life applications


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of such systems.39-41 In this context, fluorescent metal nanoclusters are expected to be better alternatives to semiconductor quantum dots for designing such systems because of their ultrasmall size, high photostability and low toxicity.42-43 Metal nanoclusters having size less than 2 nm, comparable to the fermi wavelength of electrons, are known to exhibit molecule like optical properties along with the characteristic features of nanoparticles.43-49 While non-fluorescent metal nanoparticles have been exploited in fabricating the inorganic-organic hybrid systems,6,

11-12

studies on the hybrid systems consisting of a fluorescent metal nanoclusters and J-aggregate have not been explored. Studies on such systems can be helpful in developing and designing new hybrid materials which would be helpful in energy related applications, photonic devices and bio-imaging studies. Keeping the above facts in mind, in the present work, we have studied the resonance energy transfer process from fluorescent silver capped gold nanoparticle (AgCAu) to J-aggregate of cyanine dye (S2165 dye), to basically understand the electronic interaction (coupling) between these two units. The hybrid structure, consisting of AgCAu and J-aggregate, has been fabricated by utilizing the electrostatic interaction between the positively charged surface of AgCAu and anionic site on the dye molecule. The choice of silver capped gold nanoparticle as the inorganic component is primarily due to its highly fluorescent nature and optical stability.50 It may be noted here that Pal and coworkers50 have demonstrated that AgCAu nanoparticle is essentially composed of highly fluorescent silver clusters which is stabilized by the synergistic interaction with positive gold (I) core. They have also depicted the versatility and robustness of this nanoparticle through various studies.51-57 In the current study cyanine dye has been chosen as the organic component due to the fact that it has high tendency to form J-aggregates, which possess interesting optical properties.15, 58-59 The present studies show that the cyanine based dye (S2165 dye) spontaneously 5 ACS Paragon Plus Environment


forms J-aggregate in the presence of AgCAu. The absorption spectrum of S2165 dye aggregate shows large overlap with the emission spectrum of AgCAu, making it suitable for energy transfer studies. Furthermore, the zeta potential measurements have indicated that negatively charged dye interacts electrostatically with the positively charged nanoparticle, thereby allowing the FRET pair to approach close to each other and have efficient FRET interaction. The study demonstrates that the efficiency of energy transfer process from AgCAu nanoparticle to J-aggregate is quite high, and thus existence of an electronic coupling between the inorganic and organic moieties in the hybrid nanomaterial. The chemical structure of cyanine dye (S2165 dye) is shown in Figure 1.

Figure 1. Chemical structure of S2165 dye. 2. Experimental section 2.1. Materials Chloroauric acid trihydrate (HAuCl4.3H2O), Silver Nitrate (AgNO3), and L-Glutathione were purchased from Sigma Aldrich. KCl and S2165 dye were bought from Himedia, India and Few


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chemicals, Germany respectively. All the reagents were used without further purification. Milli-Q water was used throughout the experiment. 2.2. Synthesis of Fluorescent Silver Capped Gold core shell nanoparticle (AgCAu) Giant core shell particle with glutathione as both reducing agent and ligand has been synthesized using method reported by Pal and coworkers.50 It has been synthesized by photoreduction of 10 mM HAuCl4 and 10 mM AgNO3 in aqueous solution using glutathione as reducing and stabilizing agent. In a 15 ml vial, 2 mL of 5 mM aqueous GSH solution, 4 mL of Milli-Q water, 200 μL of 10 mM HAuCl4 and 340 μL of 10 mM AgNO3 are mixed. A white turbid solution immediately appears. This is then kept under 365 nm UV light under vigorous stirring condition. A faint yellow solution is obtained after 12 hrs of irradiation. The solution is stored at 4°C.The material has been characterized by optical absorption, emission measurements and SEM imaging. Elemental analysis has been carried out by recording EDAX spectrum. 2.3. Preparation of J-aggregate: A stock solution (concentration 5×10-4 M) of S2165 dye is prepared in water. A specified amount of this is added to 1.5 ml water. This diluted solution is used for further studies. J-aggregate of this dye is prepared by adding suitable amount of KCl to the diluted solution. 2.4. Preparation of Silver Capped Gold/J-aggregate composite: To 1.5 mL of AgCAu solution, 60 μL of stock solution of S2165 is added. The AgCAu particle facilitates the formation of J-aggregate on its surface without the need of any external salt.

2.5. Instrumentation and methods 7 ACS Paragon Plus Environment


The steady state absorption spectra are recorded on a Cary 100 Bio UV-VIS spectrophotometer and fluorescence spectra are recorded on a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies). Time-resolved fluorescence measurements are carried out using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920). A light emitting diode (LED) (λexc = 405 nm) is used to excite the probe, and a MCP photomultiplier (Hamamatsu R3809U-50) is used as the detector (response time 40 ps). The lamp profile is recorded by scatterer (dilute ludox solution in water) in place of the sample. Decay curves are analyzed by a nonlinear least squares iteration procedure using F900 decay analysis software. The quality of the fit is judged by the chi square (χ2) values, and weighted deviation is obtained by fitting. Malvern Zetasizer particle analyzer with a helium neon laser at 632 nm is used for the zeta potential measurement. Fieldemission scanning electron microscope (FESEM) system (Carl Zeiss, Germany make, Model: ∑igma) is used for taking FESEM images and calculating the size of particles. FESEM samples are prepared by drop casting a drop of aqueous solution of the samples on Si wafer and dried at air. Compositional analysis of the sample is done by using an energy-dispersive X-Ray microanalyzer attached to the FESEM system. Fluorescence lifetime images are recorded using a time-resolved confocal fluorescence setup (MicroTime 200, PicoQuant), which is equipped with an inverted microscope (Olympus IX 71) containing a water immersion objective (Olympus UPlansApo NA 1.2 60×). The samples are excited by a pulsed diode laser (PicoQuant) at 405 nm with a stable repetition rate of 4 MHz (fwhm: 176 ps). The laser output is coupled with the main optical unit through a polarization maintaining single-mode optical fiber, guided through a dichroic mirror, and then the collimated laser beam is directed into the entrance port of the inverted microscope. The sample is placed on a coverslip, and the laser beam is focused onto the sample through the water immersion objective. 8 ACS Paragon Plus Environment

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Fluorescence from the sample is collected by the same objective and is passed through the dichroic mirror, and filtered, by using a 430 nm long-pass filter to cut off any exciting light. The signal is then focused onto a 50 μm diameter pinhole to remove the out-of-focus signal, recollimated, and directed onto single-photon avalanche photodiode (SPAD). The data acquisition is performed with a SymPhoTime software controlled PicoHarp 300 time-correlated single-photon counting (TCSPC) module in a time-tagged time-resolved mode. The electronic interaction between AgCAu and J-aggregate has been investigated by employing FRET studies and the data has been analyzed in light of Fӧrster theory. Importantly, the electronic interaction between the donor and acceptor depends strongly on the spectral overlap between the donor’s emission and the acceptor’s absorption spectra. Overlap integral (𝐽(𝜆)) gives the magnitude of spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. The overlap integral and the Fӧrster distance (𝑅0 ) are calculated using equation (1) and equation (2), respectively. ∞

𝐽(𝜆) =

∫0 𝐹𝐷 (𝜆)𝜀(𝜆)𝜆4 𝑑𝜆 ∞

∫0 𝐹𝐷 (𝜆)𝑑𝜆

𝑅0 = 0.211[𝜅 2 𝜂 −4 𝜑𝐷 𝐽(𝜆)]1/6

(1)

(2)

where 𝐹𝐷 (𝜆) is the normalized emission spectrum of the donor, 𝜀(𝜆) (M-1cm-1) is the molar extinction coefficient of the acceptor at each wavelength 𝜆 (𝜆 is in nm), 𝜅 2 (=2/3) is the orientation factor, 𝜑𝐷 is the fluorescence quantum yield of the donor and 𝜂 is the refractive index of the medium. 𝑅0 (in Å) is the distance between the donor and the acceptor molecule at which the rate of deactivation of donor’s excited state through energy transfer is equal to the rate of its excited



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state deactivation through other processes. The fluorescence quantum yield of AgCAu is estimated using Rhodamine 6G (R6G) dissolved in ethanol as a standard reference and equation (3), 𝐼

𝜂2

𝑄𝑌𝑠 = 𝐼𝑠 × 𝜂𝑠2 × 𝑄𝑌𝑟 𝑟

(3)

𝑟

where 𝐼𝑠 and 𝐼𝑟 are the integrated fluorescence emission of the sample and the reference, respectively, determined by integrating the emission spectra over the whole spectral range. 𝑄𝑌𝑟 is the quantum yield of the reference (for R6G in ethanol, 𝑄𝑌𝑟 = 95%). The refractive index of water (𝜂𝑠 ) and ethanol (𝜂𝑟 ) are 1.33 and 1.36, respectively. The sample and the reference are excited at the wavelength corresponding to same OD. The quantum yield of AgCAu is estimated to be 4%. 2.6. Characterization of Silver Capped Gold particle The size and morphology of the particle has been characterized by field emission scanning electron microscopy (FESEM). FESEM image of the particles are shown in Figure 2 (a). As can be seen from the FESEM images, the particles are spherical in shape. The average size of the particles, from the FESEM data, is calculated to be 480 nm. EDAX spectrum confirms the presence of silver and

gold

in

the

particle

(Figure

2(b)

and

Table

(1)).

Figure 2. (a) FESEM image of AgCAu nanoparticle. (b) EDAX spectrum of AgCAu nanoparticle. 10 ACS Paragon Plus Environment

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Table 1. Elemental composition as determined from EDAX spectrum. Element

Weight (%)

Atomic (%)

C

32.78

63.99

N

9.52

15.93

O

7.51

11.00

S

4.38

3.20

Ag

4.29

0.93

Au

41.52

4.94

3. Results and discussion 3.1. Steady State Absorption and Fluorescence Measurements The UV-VIS absorption spectrum of AgCAu is shown in Figure 3. As can be seen, a broad absorption band is observed for AgCAu. The characteristic plasmon bands of silver and gold nanoparticles are not observed in the UV-VIS spectrum.60 The absence of plasmon band for nanoparticle might be due to the plasmon damping phenomenon because of the coating of silver nanoclusters on the surface of gold. Similar observation of plasmon damping in Au nanorods by metal clusters has been reported previously by Whehai and coworkers.61 Moreover, it is also evident from Figure 3 that the particles are highly fluorescent with an emission maximum at 564 nm. It has been reported that AgCAu nanoparticles consist of large core of Au(I) with a shell of Ag2/Ag3clusters.50 We would like to note here that while explaining the fluorescence behavior of silver nanoclusters, Dickson et. al.62 have shown that the fluorescence of 2-8 atom silver clusters can originate due to the interband transition. Later on, in light of this work, the fluorescence behavior of AgCAu has been explained by considering the fact that AgCAu are composed of 11 ACS Paragon Plus Environment


fluorescent silver nanoclusters, which are synergistically stabilized by electron acceptor Au(I) core.50-51

Figure 3. The normalized absorption (blue curve) and emission spectra (red curve) of aqueous solution of AgCAu (λex=365nm). The absorption and emission spectra of S2165 dye in molecular and aggregated form are shown in Figure 4. The absorption spectrum of molecular form of S2165 dye in aqueous solution shows two bands with maxima at 545 nm and 583 nm (Figure 4(a)). It has been depicted that cyanine dye can exist in monomer- dimer equilibrium.15, 58, 63-64 Therefore, the present cyanine dye is also expected to exhibit monomer- dimer equilibrium. Please note that the short wavelength maximum at 545 nm has been assigned to the absorption band of dimer while the long wavelength maximum at 583 nm has been assigned to the absorption band of monomeric form.64 It is well known that a concentrated aqueous solution of dye can form aggregate on the addition of a salt.15, 59

Particularly, cyanine dyes have a high tendency to form J-aggregate.15, 58-59 It is pertinent to

mention here that the spontaneous aggregation of cyanine molecules results in the formation of Frenkel exciton with a narrow and red-shifted absorption band relative to the monomeric species 12 ACS Paragon Plus Environment

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and high extinction coefficient.15 In the present work, on addition of KCl salt to the concentrated aqueous solution of S2165, the absorption spectrum shows an additional narrow absorption band (full width at half maxima (fwhm) =11 nm) with a maximum at 636 nm (Figure 4(a)). The appearance of new absorption band at higher wavelength region (red region) indicates that the band is due to the formation of J-aggregate.14, 21 The fwhm value corresponding to the absorption band of J-aggregate is estimated to be 11 nm, while the corresponding value for the monomer absorption band is 26 nm. Using these values of linewidths and by applying equation (4),65 𝑁𝑐 = 1.5 × (∆𝑀/∆𝐽)2

(4)

where ∆𝑀 and ∆𝐽 are the fwhm of the absorption peaks of the monomer and J-aggregate, respectively, the number of molecules in the aggregate across which the exciton is delocalized is estimated to be ~8. This narrow absorption band also indicates the formation of J-aggregate of S2165 dye on addition of KCl salt. Emission spectra of S2165 dye in its molecular and aggregated form are shown in Figure 4b. As can be seen from Figure 4b, S2165 dye in its molecular form is found to be emissive with emission maximum at 605 nm, on the other hand J-aggregate of S2165 dye shows a redshifted and narrow emission band as compared to that of the monomer, with an emission maximum at 641 nm.



Figure 4. (a) Absorption and (b) emission spectra of S2165 dye in molecular (black) and aggregated form (red). The interaction between fluorescent metal particle and J-aggregate has been investigated by employing Fӧrster resonance energy transfer (FRET) studies. It has been already established that in order to have an efficient FRET communication, the emission spectrum of donor should have sufficient overlap with the absorption spectrum of the acceptor and the distance between the donor and the acceptor moieties should be very close (usually less than 100Å).66 Figure 5 depicts that a significant overlap exists between the emission spectrum of AgCAu (donor) and the absorption spectrum of J- aggregate(acceptor) and thus also indicating they form a suitable FRET pair. The overlap integral J(λ) and the Fӧrster distance 𝑅0 have been calculated by employing equation (1) and equation (2), respectively. The overlap integral J(λ) and the Fӧrster distance 𝑅0 are estimated to be 5.05×1015 M-1cm-1nm4 and 39.4 Å, respectively. Please note that the close association of the inorganic and organic constituents of the hybrid system have been achieved by utilizing the electrostatically driven self-assembly of AgCAu particle, which has positively charged surface, and S2165 molecules, which has anionic sites (vide infra). It has been shown through zeta potential measurements that negative charge on dye molecule interacts with the positively charged surface of AgCAu particle (vide infra). 14 ACS Paragon Plus Environment

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Figure 5. Graph depicting the spectral overlap between the emission spectrum of AgCAu (donor) (red) and absorption spectrum of S2165 dye in aggregated form (acceptor) (blue). The absorption spectra of hybrid material along with its individual components AgCAu, and S2165 dye (in molecular and aggregated form) are shown in Figure 6. As can be seen from Figure 6, the absorption spectrum of hybrid system shows broad absorption in the UV region and a narrow band with peak at 644 nm and two other bands with peaks at 548 nm and 583 nm. On the basis of comparison between the absorption spectra of hybrid system and its individual inorganic and organic components, it can be realized that the band at 642 nm is associated with the formation of J-aggregate. Interestingly, one can also see from Figure 6 that the absorption profile of J-band in the presence of AgCAu is red- shifted and broadened as compared to what has been observed in the absorption spectrum of free J-aggregate (in the absence of AgCAu). This change in absorption profile of J-aggregate, in presence of AgCAu, is suggestive of the ground state interaction between AgCAu and the J-aggregate in the hybrid system. Moreover, the broadening of the spectrum also indicates higher heterogeneity in the structure of the J-aggregate which is formed on the surface of AgCAu. The appearance of strong J-band in the current study 15 ACS Paragon Plus Environment


essentially indicates that the dye spontaneously forms aggregate in the presence of AgCAu due to the interaction between dye monomers and the particles. Similar observations have been observed by other groups while studying the J-aggregate formation of cyanine dyes in presence of QDs and metal nanoparticles.8-9, 67-69 It has been shown that the formation of J-aggregate over the surface of AgCAu is the result of electrostatic interaction between the anionic dye and the positively charged, glutathione stabilized, surface of AgCAu (vide infra).

Figure 6. Absorption spectra of S2165 dye aggregated in the presence of AgCAu (green) along with that of AgCAu (blue dotted), and S2165 dye in molecular (black dotted) and aggregated form (red). For energy transfer studies, donor is excited at 420 nm. The choice of excitation wavelength is primarily governed by the fact that absorption of the acceptor at that wavelength should be minimum so that direct excitation of acceptor is avoided and donor is preferentially excited.70 The emission spectra of AgCAu in the absence and presence of dye aggregate are shown in Figure 7. As can be seen from Figure 7, the emission intensity of AgCAu in the presence of dye aggregate is quenched considerably with simultaneous appearance of an intense narrow emission 16 ACS Paragon Plus Environment

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band at the wavelength corresponding to J-aggregate of S2165 dye. The observation of quenching of AgCAu fluorescence (donor) by the J-aggregate (acceptor) indicates energy transfer process between them. Increase in the emission intensity of J-aggregate (acceptor) in the presence of AgCAu (Figure 7) also demonstrates the FRET process in the hybrid system.66 Therefore, both lowering of the donor intensity and simultaneous increase of the acceptor intensity clearly demonstrate that energy transfer is occurring from AgCAu to J-aggregate in this hybrid structure. It may be noted here that similar type of argument has been provided while explaining the FRET process between QDs and J-aggregates.8-9 Please note that on further addition of dye to the hybrid system, more amount of J-aggregate is formed and the emission of AgCAu is almost completely quenched and only emission bands corresponding to J-aggregate are seen, as depicted in Figure 8.

Figure 7. Emission spectra of only AgCAu (red), AgCAu in the presence of dye aggregate (blue) and only dye aggregate formed by addition of KCl salt (green dotted). (λex = 420 nm) The excitation wavelength is chosen so that the dye aggregate has minimum absorbance.



Figure 8. Emission spectrum of AgCAu in the presence of dye aggregate (blue) obtained by further addition of dye to the hybrid system, recorded using excitation at 420 nm. 3.2. Zeta Potential Measurements and Thermodynamics parameter calculations The surface charge of silver capped gold nanoparticle and the hybrid system have been determined by zeta potential measurements. Plots of zeta potential measurements of the above mentioned systems are shown in Figure 9 and the corresponding zeta potential values are collected in Table 2. As shown in Figure 9 and Table 2, the AgCAu particles are found to be positively charged with zeta potential value of +6.7 mV while AgCAu/J-aggregate hybrid structures are also positively charged but with zeta potential of +3.5 mV. The decrease in zeta potential upon formation of Jaggregate indicates the involvement of weak electrostatic interactions between the negatively charged sulphonate group present in the dye and positively charged surface of AgCAu. A similar observation has been reported by Sarkar and coworkers17 where they have demonstrated the lowering of the zeta potential of negatively charged nanoparticles upon addition of positively charged dye. It may be noted here that Vasic and coworkers67, 71-73 have performed numerous studies on the mechanism of adsorption of TC dye and formation of J-aggregate on the surface of 18 ACS Paragon Plus Environment

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silver and gold nanoparticles. In one such study, they have shown that the interaction between AgNP surface and TC dye occurs through the oxygen atom from the sulphonate group of TC dye.71 Since the nanoparticle in the current study is composed of silver atoms in the outer shell, it is reasonable to believe that in the present hybrid system the interaction occurs through the oxygen atom of the sulphonate group. The hybrid structure is thus achieved by utilizing the electrostatically driven self-assembly. This electrostatic interaction brings the FRET pair close to each other to have efficient FRET communication. The role of electrostatic interaction in driving the energy transfer process between nanoparticle and organic dye moiety is an interesting issue and thus has attracted considerable attention in recent times. 8-9, 74-76

a)

b)

Figure 9. Zeta potential graphs for (a) Silver capped Gold (AgCAu) nanoparticle and (b) hybrid system of AgCAu and S2165 dye. Table 2. DLS measurement of zeta potential of the AgCAu nanoparticle solution in the absence and presence of S2165 dye, at 25°C. Sample

Zeta Potential (mV)

AgCAu

+6.7 ± 2.83

AgCAu+ S2165

+3.5 ± 3.89



Figure 10. (a) Fluorescence spectra of AgCAu in presence of increasing concentration of S2165 dye. (λex = 420 nm) (b) Stern-Volmer plots for AgCAu at different temperatures. (c) Van’t Hoff graph for hybrid system of AgCAu and S2165 dye. To further confirm the presence of electrostatic interaction between the negatively charged dye and positively charged surface of AgCAu, we have estimated the relevant thermodynamic parameters for the interaction between AgCAu and S2165 dye (in monomeric form) through fluorescence quenching experiments. We would like to state here that similar type of studies have been performed earlier by other researchers on different inorganic-organic hybrid materials.77-79 Since the fluorescence of AgCAu is quenched in presence of S2165 dye (Figure


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10(a)), Stern-Volmer equation can be used to study the variation in F0/F against the dye concentration.66 The Stern-Volmer equation is given by:66 𝐹0 𝐹

= 1 + 𝐾𝑆𝑉 [𝑄]

(5)

where 𝐹0 and 𝐹 are the fluorescence intensities of AgCAu in the absence and presence of dye, respectively, and 𝐾𝑆𝑉 is the Stern-Volmer constant. Plots of 𝐹0 /𝐹 v/s dye concentration ([Q]) at different temperatures are shown in Figure 10(b), and the values of 𝐾𝑆𝑉 and other thermodynamic parameters are collected in Table 3. It can be seen from the graph (Figure 10(b)) that the variation in F0/F against [Q] follows linear Stern-Volmer relationship (R2 is nearly 0.99 in the temperature range examined). The estimated KSV values are close to 0.1106 M-1 (Table 3), and the bimolecular quenching rate constant obtained using this value and the lifetime of AgCAu (18.6 ns) is estimated to be 5.38 1012 M-1s-1. Therefore, the estimated quenching rate constant is observed to be higher than the value typically associated with diffusion- controlled rate constant (1010 M-1s-1). In addition to this, temperature dependent fluorescence studies have revealed that the slope of the SternVolmer line is decreased appreciably as the temperature of the system is increased. Consequently, 𝐾𝑆𝑉 is found to decrease with increase in temperature (Table 3). This behavior indicates that the decrease in fluorescence intensity of AgCAu on addition of S2165 dye is predominantly static in nature. Please note that while explaining the static nature of quenching process similar arguments have also been provided by other researchers in case of separate works on inorganic-organic hybrid systems.16, 18-19, 80 In these cases, the slope of the plot of F0/F against [Q] is considered to predict the binding constant66 value corresponding to the binding event and thereby, can be used to determine the thermodynamic parameters using van’t Hoff equation (6), and equation (7):77, 79 ∆𝐻

ln(𝐾𝑎 ) = − 𝑅𝑇 +

∆𝑆 𝑅


(6)


∆𝐺 = ∆𝐻 − 𝑇∆𝑆

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(7)

where 𝑅 is the gas constant, T is the temperature, ∆𝐻 is the enthalpy change, ∆𝑆 is the entropy change and ∆𝐺 is the free energy change on adsorption of the dye on the surface of AgCAu nanoparticle. It can be seen from Table 3 that ∆𝐻 is negative which indicates that the interaction between AgCAu and the dye is exothermic. Interestingly, from Table 3 one can also see that ∆𝑆 is positive which indicates that the entropy of the system increases during this process. It has been reported in previous studies81 that electrostatic interaction may be the cause for the negative enthalpy change and positive entropy change. Therefore, this result and the fact that AgCAu and the dye molecules are oppositely charged particles corroborate our conclusion that the formation of hybrid system is electrostatically driven. The negative value of ∆𝐺 (Table 3) demonstrates that the interaction between AgCAu and the dye is a spontaneous process. Table 3. Stern-Volmer quenching constant and thermodynamic parameters for the interaction between AgCAu and S2165 dye (in monomeric form) at different temperatures. T (K)

KSV (106 L mol-1)

R2

ΔH (kJ mol-1)

ΔS (J mol-1 K-1)

ΔG (kJ mol-1)

293

0.16± 0.02

0.996

-13.40 ±0.83

53.69±2.74

-29.13± 0.83

297

0.14± 0.03

0.992

-29.34± 0.83

301

0.13± 0.02

0.993

-29.56± 0.83

305

0.12± 0.02

0.994

-29.78± 0.83


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3.3. Time-Resolved Fluorescence Measurements To get further insights into the FRET process we have carried out time resolved fluorescence intensity decay measurements of the AgCAu solution and the hybrid structure. Fluorescence decay curves for AgCAu and the hybrid structure are shown in Figure 11 and the corresponding decay parameters are collected in Table 4. Data in Table 4 reveals that in absence of J-aggregate, the short lifetime component for AgCAu nanoparticles is estimated to be 1.5 ns while the long lifetime component is found to be 28.2 ns. Metal nanoclusters are known to exhibit biexponential decay behavior with one short lifetime component and the other long lifetime component.82 The fluorescence intensity decay curves corresponding to AgCAu nanoparticles clearly show a significant quenching of fluorescence lifetime in presence of J-aggregates. The reduction in the average fluorescence lifetime of AgCAu from 12.6 ns to 1.0 ns, in the presence of J-aggregate, further indicates the energy transfer process in the hybrid structure. This large reduction in lifetime is also a reflection of highly efficient FRET process from AgCAu to J-aggregates. It has been demonstrated previously that Fӧrster theory can be employed successfully to explain the energy transfer process involving metal NCs.80,

83

On the other hand, surface energy transfer (SET)

mechanism occurs when the dipole of the molecule interacts with the continuum of states present in large-sized metal nanoparticles.84 Since the silver nanoclusters, which interact with Jaggregates, have discrete energy states44, they are less likely to provide such coupling interactions.80 For further analysis of the energy transfer process between AgCAu and J-aggregate, we have also used stretched exponential function to determine average fluorescence lifetime. The decay curve of AgCAu in presence of J-aggregate has been fitted using equation (8) and the average fluorescence lifetime is determined from equation (9):85-86



𝑡 𝛽

𝐼(𝑡) = 𝐴 ∗ exp⁡[− (τ) ] τ

1

〈τ〉 = Γ ( ) 𝛽 𝛽

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(8)

(9)

where 𝐴 is the pre exponential factor, 𝛽 is the non- exponential parameter (0

Electrostatically Driven FÓ§rster Resonance Energy Transfer between

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