Structure and Dynamics of a N-Methylfulleropyrrolidine-Mediated Gold

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Structure and Dynamics of a N‑Methylfulleropyrrolidine-Mediated Gold Nanocomposite: A Spectroscopic Ruler Sanjeeb Sutradhar and Archita Patnaik* Colloid and Interface Chemistry Laboratory, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: A mechanistic understanding of the structure and dynamics of a chemically tunable N-methylfulleropyrrolidine (8NMFP)-assisted gold nanocomposite and its aggregation via a controllable interparticle interaction is reported as a function of the molar ratio and pH of the medium. Electronic structure calculations adopting density functional theory methods implied electrostatic interactions to play a dominant role between 8-NMFP and citratecapped gold nanoparticles. MM+ molecular mechanics force field computations revealed intermolecular gold−gold interactions, contributing toward the formation of spherical composite aggregates. Corroborating these, optical absorption spectra showed the usual surface plasmon band along with a higher-wavelength feature at ∼600− 650 nm, indicative of the aggregated nanocomposite. pH-controlled reversible tuning of the plasmonic features in the composite was evident in a pH interval ∼5−6.8, revealing prevalent interparticle electrostatic interactions. In addition, photoluminescence (PL) and time-correlated single-photon counting studies revealed a strong nanocomposite interaction with a pure fluorescent dye, Rhodamine B, indicating excitation energy transfer from the dye to the composite. The dye upon interaction with the nanocomposite showed a significant quenching of its PL intensity and shortening of lifetime. Energy coupling between the metal nanoparticle composite and the emitting molecular dipole resulted in a long-range surface energy transfer (SET) from the donor dye to the surface plasmon modes of the nanoparticle following a donor−acceptor distance dependence of 1/r4. This molecular beacon with correlation between the nanoscale structure and the nonradiative nanometal SET can be used as a spectroscopic/ molecular ruler in probing advanced functional materials. KEYWORDS: N-methylfulleropyrrolidine, gold nanoparticles, nanocomposite, charge transfer, aggregation, excitation energy transfer, molecular dynamics

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INTRODUCTON Fabrication of noble metal nanoparticles (NPs) protected by multifunctional organic molecules like fullerene-C60 is emerging as fascinating and challenging scientific areas. 1,2 The interparticle coupling between the surface plasmons of two nearest particles results in new optical properties when the metal NPs are aggregated into one-dimensional (1D), twodimensional (2D), and three-dimensional (3D) structures, and the extent of coupling interactions depends on the nanostructure. Macromolecule-mediated noble metal NPs in 3D assemblies are important for potential applications in sensing, electrocatalysis, and surface-enhanced Raman spectroscopy.3−6 The prime necessity for creating and understanding 3D NP assemblies is due to their ability to chemically control the spatial arrangement of individual particles in the composite structure.7−9 Fullerene-C60 is used as a mediator for nanosized particles and fabricating new functional materials due to its multifaceted physical, chemical, photophysical, electronic, and electrochemical properties.10−13 The potential of fullerene-C60 lies in the immense scope of the chemical modification to its © XXXX American Chemical Society

basic structure. Fullerene-C60 is highly hydrophobic in nature. Resourceful methodologies to functionalize the basic structure of fullerene-C60 and the protocols to tether a wide range of polar or hydrophilic groups to fullerene-C60 have led to its enhanced solubility in an aqueous medium. 14−17 The incorporation of functionalized fullerene-C60 with polar or hydrophilic groups into a nanosized structure has become an important component to encourage and modulate supramolecular aggregation,18−21 a process where growth, arrangement, and dimension features are controlled. Gold NPs are widely used nanomaterials in the nanotechnology field due to their easy synthesis, increased stability, low toxicity, and remarkable uses in electronics, photonics, biology, sensing, and catalysis.22−24 Incorporation of polymers and DNA into gold NPs creates 3D assembled architectures, fully organized with narrow size dispersion in the nanocomposite.25−28 The Received: February 22, 2017 Accepted: June 8, 2017 Published: June 8, 2017 A

DOI: 10.1021/acsami.7b02640 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

are reported as δ ppm and referenced to Me4Si. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum was recorded on a Voyager DE PRO Biospectrometry Workstation (Applied Biosystems) instrument with 2,5-hydroxybenzoic acid as the matrix. UV−visible absorption spectra were recorded on a JASCO UV-650 spectrometer in a 1 cm path length quartz cuvette with a resolution of 2 nm. Fourier transform infrared (FT-IR) spectra were recorded with a JASCO FTIR 4000 spectrometer with 2 nm resolution using thin films deposited on freshly prepared KBr pellets. High-resolution transmission electron microscopy (HRTEM) micrographs were acquired on TECHNI with an operating voltage of 200 kV. Samples for HRTEM were prepared by placing a drop on a carbon-coated copper grid (200 mesh) followed by drying in a vacuum desiccator overnight. The HRTEM micrographs were analyzed by ImageJ software. The hydrodynamic diameter of the nanocomposite along with the temporal effect on its size was measured by dynamic light scattering (DLS) using a Brookhaven 90Plus particle size analyzer. Surface-enhanced Raman scattering (SERS) spectra were recorded on a Raman spectrometer with 632 nm excitation for an integration time of 50 s in an aqueous medium. The zeta potential measurements were carried out by photon correlation spectroscopy with a nanoparticle analyzer SZ-100 (HORIBA scientific nano partica) in an aqueous medium. The fluorescence emission spectra of all of the samples were acquired with a JASCO FP-6300 spectrofluorimeter with high sensitivity. For time-correlated single-photon counting (TCSPC) measurements, the samples were excited at 405 nm (pulse repetition rate ∼1 MHz) using a picosecond diode laser (IBH NanoLED-07) on HORIBA Jobin Yvon’s FluoroCube instrument. The liquid scatter, LUDOX AS-40 colloidal silica (Sigma-Aldrich) was used for collecting the instrument response function. The typical instrumental full width at half-maximum was about 250 ps for the 405 nm light-emitting diode light source. The microchannel-plate photomultiplier tube as a timeresolved fluorescence decay detector was used for acquiring the excited state decays of the samples by fixing the emission wavelength at 580 nm, and the decay profiles were analyzed using IBH (DAS6) software on the basis of the equation P(t) = b + ∑in αi exp(−t/τi), where P(t) is the time-resolved fluorescence decay, b is the baseline correction term, n is the number of discrete species, and αi and τi are the preexponential factor and the excited state lifetime of the ith component, respectively. Computational Methodology. DFT calculations were carried out using the Gaussian 09 set of programs with the B3LYP functional.40 The ground-state geometries of the citrate ligand and N-methylfulleropyrrolidine were optimized with the 3-21G basis set, whereas the LANL2DZ basis set was adopted for the gold trimer (Au3). In this basis set, the [Xe] 4f inner electrons of gold as core electrons and 11 outermost electrons were explicitly described using a double-ζ basis set.41,42 The B3LYP functional with a mixed basis set “SDD” was used for studying the interaction between the protonated nitrogen of N-methylfulleropyrrolidine and the citrate-capped gold trimer. This effective core pseudopotential on the gold atoms allowed efficient computations, yielding a good correlation with experimental averages.43 MD simulations were performed using the MM+ molecular mechanics force field under NVE conditions and at 300 K. Geometry optimization/minimum-energy configuration of the composite was obtained from the Polak−Ribiere (PR) conjugate gradient and was further used as the structural element. Molecular interactions between the composite units were unraveled from fs/ps MD simulations with HyperChem version 8.0 (Hypercube Inc.). Synthesis of 8-NMFP. The N-methylfulleropyrrolidine derivative was synthesized by modifying the Prato method.44 Fullerene-C60 (0.14 mmol, 0.108 g), sarcosine (4.2 mmol, 0.356 g), and paraformaldehyde (4.2 mmol, 0.126 g) in 1:30:30 ratio were dissolved in toluene, and the reaction mixture was refluxed for 24 h under stirring conditions. The color of the reaction mixture changed from violet to wine red, the mixture was cooled and filtered, and the excess solvent was removed by a rotavapor. Finally, the product was purified by column chromatography using methanol and toluene as eluents (yield: 32%). The product was characterized by 1H NMR and MALDI-

other way to create 3D assembled structures is based on using multidentate organic ligands, either by direct synthesis or by coupling reactions in functionalized gold NPs.29−31 Currently, aggregation of NPs has been utilized in the development of 2D and 3D nanostructures due to their attractive electronic, optical, and spectroscopic properties. The interface of metallic NPs and organic dye molecules has been found to be a prospective candidate because of its higher rate of electron transfer from the excited organic dye molecule to the NPs toward waste water treatment, solar cells, biophotonics, and electronic device applications.32−34 NPs play a crucial role in changing the fluorescence intensity with organic dye molecules and fluorophores, where the delocalized conduction band electrons (Fermi gas) interact very strongly with the oscillating dipole of the organic dye/fluorophore.35 Darbha et al. reported a surface energy transfer (SET) probe based on 3-mercaptopropanoic acid and homocystine-functionalized gold NPs in the presence of Rhodamine B (RB) for rapid and ultrasensitive detection of mercury in contaminated soil, water, and fish. A drastic fluorescence quenching efficiency was observed when the RB dye was statically bound to the gold NP surface.36 Patel et al. demonstrated the DNA-functionalized gold NPs to act as an acceptor of fluorescence intensity from RB molecules. They observed SET from RB to gold NPs where the energy transfer efficiency decreased with an increase in the concentration of DNA molecules.37 Fu et al. also established that cetyltrimethylammonium bromide-assisted gold nanorods could increase the optical signal of dye Cy5. The gold nanorod−oligonucleotide hybrid nanocomplexes with dye revealed a 40 times higher fluorescence emission rate and 7 times reduced decay rate in comparison to those of the oligonucleotide-labeled dye alone.38 The gold NPs are considerably used as acceptors in biophysical experiments due to their extraordinary stability, nontoxicity, and strong quenching ability.39 To our knowledge, no report exists on the characteristic energy transfer from organic dyes, such as RB, to functionalized fullerene-C60-mediated gold nanocomposites. Owing to the importance of interfacing an organic dye with NP composites, first, we report the formation of 8-N-methylfulleropyrrolidine (8-NMFP)-mediated gold nanocomposites via noncovalent interactions. Here, the effect of electrolyte (NaCl) and pH on the surface plasmon resonance (SPR) band was studied for an in-depth understanding of the formation of the 3D structural assembly of the composite. The structure and dynamics of the composite were unraveled following density functional theory (DFT) calculations along with molecular dynamics (MD) simulations. Subsequently, the preferred mode of energy transfer from dye RB to the nanocomposite aggregate was unraveled from steady-state and time-resolved emission spectral characteristics.

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MATERIALS AND METHODS

Chemicals. Fullerene-C60 (Sigma-Aldrich, 99.5%), chloroauric acid (HAuCl4·xH2O) (Sigma-Aldrich, 99.5%), trisodium citrate dihydrate (Merck), sarcosine (Acros Organics), paraformaldehyde (Rankem), RB (Sigma-Aldrich, 97%), high-performance liquid chromatography (HPLC) grade toluene (Merck), dimethyl sulfoxide (DMSO) (Merck), HPLC grade methanol (Memba Chem Industries Pvt. Ltd.), and sodium chloride (Merck) were purchased and used without further purification. The ultrapure water with resistivity 18.2 MΩ cm from a Millipore Elix A3-Milli Q system (Milli Q, Germany) was used throughout the experiments. Characterization. 1H NMR spectra were recorded on a Bruker 500 spectrophotometer, operating at 500 MHz. Chemical shift values B


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Scheme 1. (a) Schematic Route for the Synthesis of 8-NMFP; (b) Optimized Geometry of 8-NMFP with and without Atom Labeling; (c) Mechanism of Formation of 8-NMFP via the Nucleophilic Addition Reaction

Table 1. Tabulated Positions for NMP Attachment on Fullerene-C60a

a

NMP addition

position on fullerene-C60

NMP addition

position on fullerene-C60

first addition second addition third addition fourth addition

C9(f)-C10(f)-C105(r)-N107-C106(r) C12(f)-C13(f)-C61(r)-N63-C62(r) C14(f)-C15(f)-C138(r)-N140-C139(r) C17(f)-C18(f)-C83(r)-N85-C84(r)

fifth addition sixth addition seventh addition eighth addition

C32(f)-C33(f)-C94(r)-N96-C95(r) C49(f)-C36(f)-C116(r)-N118-C117(r) C45(f)-C55(f)-C127(r)-N129-C128(r) C53(f)-C54(f)-C72(r)-N74-C73(r)

f: fullerene carbon; r: ring carbon. mM, 0.005 g) was heated to boiling, followed by dropwise addition of aqueous trisodium citrate (34 mM, 0.025 g) solution to the boiling solution. The color change was noted from light yellow to black and to ruby red upon continued heating for 30 min and finally with continued heating for 2 h at room temperature. The characteristic SPR peak at 522 nm (Figure S4) confirmed the formation of pristine Ct@Au NPs with their estimated size of ∼18 nm from the transmission electron microscopy (TEM) image (vide Scheme S1). Subsequently, the concentration of Ct@Au NPs was estimated to be 1.46 nM (see the SI). Synthesis of N-Methylfulleropyrrolidine-Assisted Gold Nanocomposites. As a function of varied molar ratios (MRs), the NMFP-assisted gold nanocomposites were prepared by mixing the respective components. The MRs, that is, the ratio of the concentration of NMFP to the concentration of Ct@AuNPs, were 200, 300, 400 (as 5.84 μM/1.46 nM), and 900. A calculated amount of NaCl (5 mM) was used to reflect the effect of electrolyte on

TOF mass spectrometry and FT-IR and UV−visible spectroscopy (Figures S1−S4 in the Supporting Information (SI)). The synthetic route for the preparation of 8-NMFP is shown in Scheme 1a. 8-NMFP was dissolved in 1% DMSO−water for further experiments. For geometry optimization of 8-NMFP via the DFT method, the input geometry was taken with the first six pyrrolidine units attached to six pyracylene units of fullerene-C60 based on the octahedral geometry. The rest two pyrrolidine moieties were attached next to opposite of any two pyrrolidine moieties at 6:6 position of fullerene-C60. Thus, the ground-state geometry of 8-NMFP was optimized with an estimated energy of −3652.23 au, as depicted in Scheme 1b. The tabulated positions of N-methylpyrrolidine (NMP) via 1,3-dipolar additions on the optimized fullerene-C60 geometry are also depicted in Table 1. Scheme 1c lists the plausible mechanism of formation of NMFP.45 Synthesis of Citrate-Capped Gold Nanoparticles (Ct@ AuNPs). The synthesis of Ct@AuNPs was carried out by adopting the Turkevich method.46 A 47.5 mL aqueous solution of HAuCl4 (0.30 C


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Figure 1. Comparative UV−visible absorption spectra of (a) the aqueous dispersions and the pH-dependent spectral evolution of the 8-NMFP− Ct@AuNPs nanocomposite at (b) MR = 200 and (c) MR = 300.

Scheme 2. Plausible Interaction Scheme Depicting 8-NMFP−Ct@AuNPs Nanocomposite Formation as a Function of pH at the MRs under Consideration

obtained from the temporal evolution of the ∼600 nm peak, as shown in Figure S6. A pseudo-first-order reaction for the nanocomposite assembly was revealed from the fitted experimental data as y = y0 + a(1 − e−kt). Table S1 summarizes the apparent rate constants, k (s−1), and their increase as a function of MR and upon addition of the electrolyte, NaCl, indicating the overall rate of nanocomposite formation. The assembly accompanied a gradual color change from ruby red to blue-purple, as seen in Figure S7. Upon overnight stay, the nanocomposite solution precipitated out and could be subsequently redispersed again after a brief sonication, indicating its reversible aggregation and disaggregation in aqueous solution (Figure S7). Effect of pH on Interparticle Charge Transfer-Induced Nanocomposite Aggregation. The gold NPs assembly directly or indirectly depended on pH. Si et al. reported the reversible aggregation of peptide-functionalized gold NPs to be regulated by the pH of the medium.49 Figure 1 depicts the effect of pH on the aggregation of the nanocomposite; at a more acidic pH ∼3 and at an alkaline pH ∼9.5, no clear spectral difference was observed, indicating the absence of aggregation. On the other hand, a bathochromically shifted SPR peak along with a new higher-wavelength broad absorption feature at 630 nm appeared in the pH range between 5.0 and 6.8 irrespective of the MRs used. These absorption spectral changes could be

nanocomposite aggregation. The effect of pH on the nanocomposite aggregation was studied for the chosen MRs by maintaining it for an interval of 45 min. Photoluminescence (PL) and time-resolved fluorescence spectral measurements were performed using a freshly prepared aqueous solution of 1 mL, 1 μM RB as the donor, which was added to 3.0 mL each of the aqueous citrate-capped gold NPs (1.46 nM) and the aqueous dispersions of the nanocomposite with different MRs (900, 20 000, and 60 000). Spectral acquisition was done after an equilibration time of 45 min.

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RESULTS AND DISCUSSION Composite Aggregation and Disaggregation. The comparative UV−visible spectra in Figure 1a depict the characteristic absorption features of 8-NMFP (the featureless absorption in the range ∼400−500 nm) and the SPR band of Ct@Au NPs at 522 nm for the composite spectra. The aggregation behavior of the nanocomposite with MRs 400 and 900 was investigated using UV−visible spectroscopy with and without the electrolyte (NaCl),47 where NaCl could reduce the interaction energy barrier between two neighboring NPs. The 8-NMFP−Ct@Au nanocomposite spectra in Figure S5 showed the SPR absorption peak with a temporal red shift along with a newly evolved broad feature at ∼630 nm (600−650 nm), arising as a result of aggregation of the 8-NMFP−Ct@AuNPs nanocomposite.48 The kinetics of the composite assembly was D


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Figure 2. TEM micrographs of the nanocomposites at pH 3.0 (a), 6.8 (b), and 9.5 (c) with MR 300. The images clearly depict the composite aggregation in (b) and disaggregation in (c).

Figure 3. (a) Optimized geometry of −COO− binding to Au3. (b) Optimized geometry for the interaction between NMFP and Ct@Au3, showing 3.23 Å as the distance between the protonated nitrogen of NMFP and the −COO− charge cloud. (c) ESP map of the composite resulting from the individual ESP maps of NMFP and Ct@Au3. (d−g) HOMO and LUMO isosurfaces of the composite upon interaction, depicting a gradual charge transfer from the HOMO of Ct@Au3 to the higher-order LUMOs of NMFP.

explained following interaction Scheme 2 that depicts intermolecular electrostatic interactions to be dominant in the nanocomposite between at least one protonated NMP of NMFP and the delocalized negative electron cloud arising from the two deprotonated −COOH groups of the NP capping citrate ligand. This attribution resonates with the pKa values of NMFP (pKa: 6.3)50 and citric acid ligand [pKa:18 3.1 (a1), 4.8 (a2), 6.4 (a3)] in the pH range 5−6.8. The predominant charge transfer between 8-NMFP and citrate-capped gold NPs in the above pH

range ultimately plays a crucial role in the nanocomposite formation and aggregation. The TEM micrographs at various pHs in Figure 2 show the aggregation and disaggregation of the nanocomposite to be pH-controllable. The DLS of the nanocomposite in Figure S8 provided the hydrodynamic radii of the nanocomposites at different MRs, showing their enhancement in the presence of the electrolyte and with the progress of time. Structure of and Interaction in the 8-NMFP−Ct@Au Nanocomposite Aggregation. To understand the existing E


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Scheme 3. (a) Plausible 8-NMFP−Ct@Au Nanocomposite Aggregate Structure via Electrostatic Interaction (the Structures Drawn are Not to Scale); (b) TEM Micrograph of the 8-NMFP−Ct@Au Nanocomposite Aggregate at MR 900

spherical cluster aggregates, according to the TEM image in Scheme 3b. This observation prompted us to explore the dynamic assembly and the mechanism of aggregation adopting molecular mechanics computations. Electrokinetic potentials of the aqueous colloidal dispersions of 8-NMFP, Ct@Au NPs, and 8-NMFP−Ct@Au NPs as their respective zeta potentials were measured via the electrophoretic light scattering method. Table 2 and Figure 4 display the

intermolecular interaction in the nanocomposite, the lowestenergy optimized ground-state structure of the nanocomposite was computed following density functional calculations. The prevalent mode of interaction was electrostatic between the citrate ligand’s −COO electron cloud and the Au core, as shown in Figure 3a, with a minimum-energy stable structure at E = −1164.63 au. Here, the gold trimer, Au3, was used as a model cluster42,51 for computational simplicity. The input model structure for the 8-NMFP−Ct@Au nanocomposite was obtained by substituting one unit of NMP with one pyracyclene unit of fullerene-C60 at the 6:6 position for computational simplicity. Accordingly, in Figure 3b, the free −COO− group of the citrate ligand interacted with NMFP via electrostatic interactions, where the protonated nitrogen of NMFP remained positively charged with a closest noncovalent distance approaching ∼3.23 Å from the COO− charge cloud of the citrate ligand. Figure 3c−g shows the electrostatic potential map (ESP) and the frontier molecular orbital isosurfaces of the nanocomposite, where the maximum localized electron density on the citrate ligand −COO− is transported to higher lowest unoccupied molecular orbitals (LUMOs) of NMFP, ultimately concentrating on the fullerene-C60 moiety. The isosurfaces of the highest occupied molecular orbital (HOMO), LUMO, and higher-order LUMOs of the nanocomposite evidence the composite to be essentially built and stabilized by charge transfer from the capping ligand of the NP to the fullerene-C60 surface. The perimeter of the 18 nm citrate-capped gold NPs was estimated to be 56.41 nm (2πr = 2 × 3.14 × 18/2), and the projected length of the capping citrate ligand (vide Figure S9) was estimated from geometry optimization to be 0.50 nm. Thus, ∼113 ligand moieties per gold NP could be estimated from the geometry-optimized structure of the citrate ligand. Therefore, the interaction of protonated 8-NMFP with isotropically available capping ligands, attached on the spherical gold NPs, could be electrostatically favorable, leading to the formation of a 3D nanoarchitecture, as depicted in Scheme 3a. However, the number of available NMP groups on fullereneC60 is much less than the available citrate ligands on the gold NP surface, implying a relatively greater negative charge cloud on the gold NP surface than the positive charge on the NMFP surface. Therefore, it is unlikely that the nanocomposite assembly is governed solely by electrostatics, as observed in the above computational model (cf. Figure 3b), resulting in

Table 2. Measured Zeta Potentials for the Aqueous Dispersions of Ct@Au, 8-NMFP, and the Composites with Varied MRs aqueous dispersions Ct@Au 8-NMFP composite (MR 900) composite (MR 20 000) composite (MR 60 000)

zeta potential (mV) −69.0 1.7 −8.5 −4.7 0.0

−69.5 5.2 −9.4 −4.2 −3.4

−69.5 0.0 −7.4 −8.7 −2.2

average (mV) −69.3 2.3 −8.9 −5.9 −1.9

Figure 4. Zeta potential graph of aqueous dispersions of 8-NMFP, Ct@Au NPs, and the composites with varied MRs. The inset shows the data for the composite only. F


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MD simulation in a 52 × 10 × 56 Å3 periodic boundary box on a pre-equilibrated water slab of 962 water molecules (cf. Figure 6a). Evidence for a rhombic gold tetramer was succinctly reported by Ray54 via ab initio calculations. Here, the global minimum-energy structures of the neutral, cationic, and anionic gold tetramer clusters were found with a rhombic geometry. Subsequently, the initial geometry was optimized using the MM+ force field with the PR (conjugate gradient) algorithm. The dynamics was followed with the optimized geometry in the microcanonical ensemble under NVE conditions with a constant energy. The equations of molecular motion were integrated for different simulation times at a constant temperature (300 K) with a time step of 1 fs. With predominant electrostatic and intermolecular Au−Au interactions in the composite aggregates (cf. Figure 6c−f), a minimum-energy configuration was observed for the 100 fs simulated structure, as in Figure 6c. This lucidly suggested the aqueous assembly of the gold nanocomposite to be chargeinduced and therefore could be tunable with the pH of the medium, as illustrated in Scheme 2. Consequently, a regular arrangement of the nanocomposite leading to a 3D nanoarchitecture is evident from Figure 7. Characteristic PL and Excitation Energy Transfer between Fluorescent RB and the Nanocomposite. Excitation energy transfer from a photoexcited donor to a dark acceptor (quencher) provides the basis for a variety of sensors and molecular switches.55 In this context, the PL study was performed by taking the organic dye RB as the donor and the 8-NMFP−Ct@Au NP nanocomposite as the acceptor to investigate the nature of excitation energy transfer between the organic dye and the nanocomposite. The PL spectra of the pure RB dye (1 μM in water) and the dye in presence of the nanocomposite with different MRs are shown in Figure 8a. The PL peak at 575 nm for the pure RB dye undergoes no change in its peak position in the presence of the aqueous nanocomposite. With a highly aggregated nanocomposite at a higher MR, the PL quenching of the RB dye emission is significant. The PL quenching efficiency (in percentage) was estimated using the relative fluorescence intensity of the donor in the absence of the acceptor (ID) and in the presence of the acceptor (IDA) as follows:35 EPL = [1 − (IDA/ID)] × 100. The PL quenching efficiency was estimated to be 99% in the presence of the isolated Ct@Au NPs, whereas the extent of quenching decreased from lower to higher MRs of the nanocomposite. The overlap of the emission spectrum of the donor (pure RB dye) and absorption spectrum of the acceptor (8-NMFP−Ct@ Au) as an essential criterion for SET or Förster resonance energy transfer (FRET) is shown in Figure S10, where the energy transfer efficiency is closely related to the degree of spectral overlap. The characteristic overlap suggests PL quenching to be due to an energy transfer process. Time-resolved fluorescence spectroscopy was performed by taking aqueous solutions of pure RB with varied MRs of the nanocomposite solutions for an accurate understanding of energy transfer between dye RB and the 8-NMFP−Ct@Au NP nanocomposite. The PL decay profiles for the respective dye and the nanocomposite shown in Figure 8b depict that the pure dye solution (1 μM) without the gold nanocomposite shows a monoexponential behavior with a lifetime estimated to be 1.494 ns, similar to the earlier reported value.56 However, all of the nanocomposite samples showed a biexponential behavior and the decay times are listed in Table 3. The RB dye in the presence of the gold nanocomposite showed a decrease in

magnitudes of zeta potential as a key indicator for the stability of the dispersions. The zeta potential for the most stable Ct@ Au NPs was found to be −69.3 mV due to the presence of the stabilizing citrate ligands anchored onto the gold core. Going from 8-NMFP to the composite, the magnitude of the zeta potential became more negative, indicating the degree of electrostatic repulsion between the neighboring, similarly charged particles in the dispersion. A more negative zeta potential conferred a relatively greater electrical stability to the lower MR composites. SERS spectra were recorded on a Raman spectrometer with a 5 mW laser at 632 nm excitation for an integration time of 50 s. The Raman active modes of C60 have Ag and Hg symmetry among its irreducible representations. In NMFP, of the 10 Raman active modes, two prominent bands of Ag(2) (1448 vs 1469 cm−1 in pristine C60) and Hg(8) (1549 vs 1561 cm−1 in pristine C60) modes are expected.52 However, in Figure 5, the

Figure 5. SERS spectra of aqueous dispersions of 8-NMFP, Ct@Au, and the 8-NMFP−Ct@Au NP composite (MR = 900). In the pristine 8-NMFP spectrum, the concentration of 8-NMFP (1.31 μm) corresponded to that used for the composite with MR = 900.

Hg(8) feature for 8-NMFP is indistinguishable in view of its low concentration (MR = 900) in the 1% DMSO/H2O dispersion medium. Nonetheless, the detection of SERS for these two bands in the nanocomposite Ct@AuNP−8-NMFP is evident due to the surface effect of Ct@AuNPs, implying adsorption of 8-NMFP on their surface. Mechanism of the 3D Nanocomposite Aggregation. MD simulation was carried out with the HyperChem Professional version 8.0 (Hypercube Inc.) program package toward understanding the mechanism of nanocomposite aggregation. Intermolecular Au−Au aurophilic interactions as a result of the propensity of gold centers to aggregate via electron correlations of the closed-shell components have been reported.53 Furthermore, gold complexes have been reported to aggregate via formation of weak gold−gold bonds, where evidence for aurophilicity has been obtained from the crystallographic analysis of Au(I) complexes. Therefore, the input composite model, for simulating its dynamics in the aqueous medium and obtaining the minimum-energy structure, was carefully chosen. A four-unit (4-NMFP−Ct@Au4) model with 4-NMFP and a rhombic gold tetramer (interconnected gold trimers from Ct@Au3) was taken as the input model for G


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Figure 6. Snapshots of the input model with (a) its optimized geometry by the PR optimization and (b) the PR-optimized ensemble without water molecules for clarity. (c−f) Snapshots of the simulated structures after 100, 200, 300, and 900 fs simulations, respectively. The distances between the protonated N of NMFP and C of the COO− charge cloud of Ct@Au3NP for various nanocomposite moieties are depicted.

transfer to decrease with higher MRs of the aggregated nanocomposite. The excitation energy transfer to the surface plasmon modes of the nanoparticle was commended because the absorption spectra of NPs are dominated by SPR.58 The degree of decreasing spectral overlap between the pure RB dye and the 8-NMFP−Ct@Au NP nanocomposite due to bathochromic shifting of the SPR peak has influenced the energy transfer process. Additionally, the rate of energy transfer (k) was estimated using k = ϕET × τD−1/(1 − ϕET).59 Here, τD, the experimental lifetime of the donor, was taken as 1.494 ns and efficiency ϕET = 0.208 was used corresponding to the MR of 900, the nanocomposite experiencing the most efficient energy transfer (vide Table 3). The rate of energy transfer was estimated to be 1.153 × 108 s−1.

lifetime, evidencing energy transfer from the dye to the 8NMFP−Ct@Au NP nanocomposite. Dulkeith et al.57 observed a similar pronounced fluorescence quenching with lissamine dye molecules with different sizes of the gold nanoparticles. Sen et al.58 also observed a similar significant fluorescence quenching with the Rhodamine 6G dye with different capping agents (3-mercaptopropanoic acid and 2-mercaptoethanol) and different concentrations of self-assembled gold nanoparticles. Consequently, ϕET, the energy transfer efficiency from the pure RB dye to the 8-NMFP−Ct@Au NP nanocomposite, was estimated using the following relation: ϕET = [1 − (τDA/τD)] × 100, where τD and τDA are the decay times in the absence and presence of the nanocomposite and are enlisted in Table 3. The analysis of the data from Table 3 revealed the degree of energy H


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Figure 7. (a) Snapshot of the optimized extended simulated structure after the 100 fs simulation, (b) cross section of the simulated structure, and (c) TEM micrograph of the composite.

Figure 8. (a) PL spectra of the pure RB dye (1 μM) and in the presence of different MRs of the 8-NMFP−Ct@Au NP nanocomposite. (b) Timeresolved fluorescence spectra of the pure dye (1 μM) and in the presence of different MRs of the 8-NMFP−Ct@Au NP nanocomposite.

Table 3. Half-life time (⟨τ⟩) and Energy Transfer Efficiency for the Pure RB Dye and in the Presence of the 8-NMFP−Ct@Au NP Nanocomposite with Different MRs b1

τ1 (ns)

b2

τ2 (ns)

⟨τ⟩ = (b1τ1 + b2τ2) (ps)

efficiency (%)

0.077 0.059 0.050 0.039

1.494 1.359 1.306 1.015

0.007 0.011 0.017

3.799 3.118 3.048

115.038 106.774 99.598 91.140

7.18 13.42 20.77

sample pure pure pure pure

RB RB with MR 60 000 RB with MR 20 000 RB with MR 900

Table 4. Estimation of the Donor-to-Acceptor Distance between the Pure RB Dye and 8-NMFP−Ct@Au NPs Nanocomposites with Different MRs sample pure pure pure pure

RB RB with MR 60 000 RB with MR 20 000 RB with MR 900

quantum yield of the dye

distance R0 (Å)

donor-to-acceptor distance r (Å) (distance dependence: 1/r4)

donor-to-acceptor distance r (Å) (distance dependence: 1/r6)

0.31 0.31 0.31 0.31

61.62 61.62 61.62

116.84 98.21 86.12

309.46 275.61 252.50

Yun et al. reported SET to play a pivotal role for the metallic and fluorophore systems compared with FRET.60 The interaction of the electromagnetic field of the donor dipole with the delocalized electrons of the acceptor conduction band is the principal reason for the physical origin of SET. These delocalized electrons are hugely adequate in metallic nanoparticles to act as an acceptor. Furthermore, the rate of energy transfer is reported to be equal to 1/r4, instead of 1/r6 (r, the

donor−acceptor distance), obtained from FRET when the electronic continuum of metallic systems interacts with the dipole of the fluorophore.60 A large number of experimental and theoretical studies in the literature are based on the rate of nonradiative energy transfer to NPs from a dye or fluorophore.59−62 Also, few studies exist in the literature based on the distance dependence of the nonradiative energy transfer between the dye and NPs.63,64 We analyzed the I


Research Article

ACS Applied Materials & Interfaces distance dependence of the energy transfer rate between the donor dye and the acceptor nanocomposite in the present investigation; the distance, “r”, between the acceptor and the donor was estimated by correlating it with the efficiency of SET as ϕSET = 1/[1 + (r/R0)4],63 where R0, defined as the distance at which the donor displays equal probabilities for energy transfer and spontaneous emission,65 was estimated using the Persson model as R0 = (0.225c3QD/ωD2ωFκF)1/4. Here, QD is the quantum yield of the donor (0.31),56 ωD is the frequency of the donor electronic transition (3.6 × 1015 s−1), ωF is the Fermi frequency (8.4 × 1015 s−1), and κF is the wave factor for the gold metal (1.2 × 108 cm−1).61 Accordingly, the R0 value was estimated to be 61.62 Å. The calculated distances with 1/r4 (SET) and 1/r6 (FRET) are enlisted in Table 4, which reveals that the dye−nanocomposite distance depends on the MR value and increases with the increasing MR. In the present context, FRET-based energy transfer would become too weak over the estimated distances because the FRET-based process of energy transfer is restricted to the upper limit of ∼80 Å. Therefore, it is established that SET is the prominent mode of energy transfer from the pure RB dye to the nanocomposite, following a distance dependence of 1/r4. It is worth mentioning here that the gold nanocomposite could be used as an acceptor in a biophysical experiment where SET would be useful for long-distance measurements to understand the large-scale conformation of complex biomolecules in macroscopic details. Furthermore, measuring distances >100 Å could be important for diverse material applications where large-scale conformational changes are observed. Moreover, fluorescence quenching by metal NPs is applied in surface-enhanced Raman spectroscopy and fluorescence quenching by the Au nanocomposite could be highly useful for sensing applications as molecular beacons as well as biomolecular recognition.

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CONCLUSIONS

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ASSOCIATED CONTENT

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Spectroscopic characterization of the nanocomposite, DLS profiles for the nanocomposite assembly, and emission spectral details (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 044-2257-4217. Fax: 044-2257-4202. ORCID

Archita Patnaik: 0000-0002-0754-7055 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. acknowledges the Senior Research Fellowship from Indian Institute Technology (IIT) Madras. P.G. Senapathy Center for Computing Resources and Metallurgical and Materials Engineering (MME) Department are gratefully acknowledged for providing computational and TEM facilities, respectively. The authors are thankful to Dr. M.G. Basavaraj and Anjali for zeta potential measurements. Department of Science and Technology funded Sophisticated Analytical Instrument Facility for TCSPC measurement is highly acknowledged. The authors thank CSIR, New Delhi, India (grant no. 01(2830)/15/EMRII) for financial support.

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REFERENCES

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A newly synthesized 8-NMFP facilitated the citrate-capped gold nanocomposite self-aggregation into a 3D nanostructure via intermolecular electrostatic and gold−gold aurophilic interactions. The aggregation was instantaneous with higher MRs and was regulated by the pH of the medium. Computational modeling via DFT showed the ground-state geometry of the nanocomposite and its probable intermolecular interactions. MD simulations evidenced aurophilic gold−gold interactions along with electrostatic interactions as governing driving forces for the composite aggregation, leading to equilibrium spherical geometries. A concise study on excitation energy transfer from the photoexcited RB donor to the nanocomposite as the acceptor/quencher revealed that the RB dye shows a significant PL quenching as well as shortening of lifetime for different MRs of the nanocomposite, further revealing the efficiency of excitation energy transfer. The mode of energy transfer was established as SET from the estimated acceptor−donor distances. This investigation correlated the nanocomposite structure and the extent of nonradiative SET from an emissive dipole, which can be used as a molecular ruler in probing advanced functional materials.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02640. J


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