Recent Advances in Energy Transfer Processes in Gold-Nanoparticle

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Recent Advances in Energy Transfer Processes in Gold Nanoparticle Based Assemblies Tapasi Sen, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp302615d • Publication Date (Web): 03 Jul 2012 Downloaded from http://pubs.acs.org on July 11, 2012

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The Journal of Physical Chemistry

Recent Advances in Energy Transfer Processes in Gold Nanoparticle Based Assemblies Tapasi Sen, and Amitava Patra* Department of Materials Science Indian Association for the Cultivation of Science, Kolkata 700 032, India

Abstract This feature article highlights the recent developments of energy transfer processes of Au nanoparticle based assemblies. Many recent studies reveal that the energy transfer from dye to Au nanoparticle is a surface energy transfer (SET) process as established from 1/d4 distance dependence. Such distance dependence energy transfer phenomenon serves as spectroscopic ruler for long distance measurement. Recently, energy transfer processes in Au nanoparticle assemblies have been used to understand specific binding site and conformational changes of protein, DNA hybridization, RNA folding/unfolding, metal ion detection, and designing of new optical based materials using porous materials. Here, we highlight various aspects of energy transfer between dye molecule and Au nanoparticle, particularly focusing on the size and shape dependent energy transfer, understanding the interactions between bio-molecules (protein, DNA and RNA) with Au nanoparticle and the energy transfer between confined dye and Au nanoparticle. The designing of nanostructures materials with efficient energy transfer between confined dye in porous materials (mesoporous silica, zeolites and cyclodextrin) with Au nanoparticle for developing new photonic devices has also been highlighted.

Interesting

findings reveal that Au nanoparticle based energy transfer offers an exciting opportunity to overcome many obstacles and help to solve the challenging problems for future applications. Finally, a tentative outlook on future developments of this research field is given.

*

Author to whom correspondence should be addressed; electronic mail: [email protected]

Phone: (91)-33-2473-4971, Fax: (91)-33-2473-2805

ACS Paragon Plus Environment

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1. INTRODUCTION Gold nanoparticles have recently gained significant interest in the areas of luminescence tagging, imaging, medical diagnostics, multiplexing and most recently as biosensors.1-3 This interest stems from their electrodynamic properties which result in characteristic plasmon resonance. The strong surface plasmon absorption bands observed for metal nanoparticles, originating from the collective oscillation of free conduction electrons on the nanoparticle surface.4,

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This surface plasmon absorption band makes a metal nanoparticle potentially an

extremely useful probe for optical based molecular ruler. The tunability of the highly organized materials offers fascinating new possibilities for exploring energy transfer phenomena for developing new challenging photonic devices. The fluorescence resonance energy transfer (FRET) is useful to determine submicroscopic distances between two interactive organic molecules,6,

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and identifying active sites in macromolecular assemblies (e. g., biological

membranes8 and proteins9). In FRET process, an excited donor molecule transfers the excitation energy non-radiatively to an acceptor molecule. This process is driven by dipole-dipole interaction between an excited donor (D) molecule and an acceptor (A) molecule. The essential requirement for effective energy transfer is that the emission spectrum of donor must overlap adequately with the absorbance spectrum of acceptor. The efficiency of FRET depends strongly on the distance of separation between donor and acceptor molecules. According to the Förster theory,6, 10 the rate of energy transfer is given by

1  R0  k T (r ) =   τD  r 

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(1)

Where τD is the lifetime of the donor in the absence of the acceptor, r is the distance between the donor and acceptor, and R0 is known as the Förster distance, the distance at which the transfer rate kT(r) is equal to the decay rate of the donor in absence of the acceptor. The Förster distance (R0) is defined as

[

]

R0 = 9.78 × 10 3 κ 2 n − 4 Q D J (λ )

1/ 6

(In angstroms)

(2)

where φD is the quantum yield of donor in absence of acceptor, n is the refractive index of the medium, k2 is the orientation factor of 2 dipoles interacting and is usually assumed to be equal to 2/3, and J(λ) is the overlap integral between donor emission and acceptor absorption spectra. FRET rate depends strongly on the degree of spectral overlap and the spectral overlap is constant


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for a given donor/acceptor system. However, this FRET method has restriction on the upper limit of separation of only 80 Å. The energy transfer becomes too weak to be useful beyond this distance.1 Many recent studies reveal that the energy transfer from dye to Au nanoparticle is a distance dependent energy transfer process. Such distance dependence energy transfer phenomenon serves as spectroscopic ruler for long distance measurement. The spectral properties of fluorophores are dramatically changed in the vicinity of metallic surfaces.11 The metal nanoparticles can either quench12 or enhance13, 14 the fluorescence of organic molecules that lie in their immediate vicinity. The behavior of fluorophore near metals has been well described by the theoretical models and experimental validation. The quenching mechanism of donor dipole in presence of metal surface is described by Chance and co-workers.15 Persson and Lang16 further calculated the rate of energy transfer by using a Fermi Golden Rule. The Golden Rule approximation relates the energy transfer rate (kET) to a product of the interaction elements of the donor (FD) and acceptor (FA), kET = FDFA. For single dipoles, F = 1/d3; for a 2D dipole array, F = 1/d; and for a 3D dipole array, F = constant where d is the separation distance.16 For dipole-dipole interaction, i.e. FRET, kFRET = FDFA = (1/d3) (1/d3) = 1/d6. During the interaction of dipole with surface, kET = FDFA = (1/d3) (1/d) = 1/d4 which is known as surface energy transfer (SET). According to their model, the exact form of dipole-surface energy transfer rate is given by

k SET =

1  d0    τD  d 

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(3)

Where τD is the lifetime of the donor in the absence of the acceptor, d is the distance between the donor and acceptor. The characteristic distance length d0 can be calculated by using this model

 0.225c 3 Φ dye d0 =  2  ω dyeω k F F 

   

1/ 4

(4)

where d0 is the distance at which a dye will display equal probabilities for the energy transfer and spontaneous emission, φdye is the quantum efficiency of dye, ω is the frequency of the donor electronic transition, ωF is Fermi frequency and κF is Fermi wave vector of the metal.16 The d0 value is calculated using ω =3.6×1015 s−1, ωF = 8.4 ×1015 s−1 κF =1.2×108 cm−1, and c = 3×1010 cm s−1. At very close distances (99%) of lissamine dye molecules in presence of gold nanoparticles and they found a monotonic decrease of fluorescence lifetime of lissamine dye with increasing size of Au


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nanoparticles. The PL quenching of the dye molecules seen in this study has been attributed to increase of nonradiative decay rate. They compared their experimental findings with theoretical calculations based on the Gersten and Nitzan’s model.22 Bagchi et al.23 have revisited the problem of EET between donor dye and acceptor metal nanoparticles by employing theoretical formulation, considering size, shape, and orientation of nanoparticle. They found that at large separations compared to the radius of the nanoparticle (a), the rate of EET is proportional to 1/d6. However, at separations approximately between d = a, and d = 4a, the rate of energy transfer varies as 1/dσ, where σ lied between 3 and 4. Swati et al. reported that the rate of energy transfer from a dye molecule to a metal nanoparticle follows conventional r−6 dependence at large distances while small deviation is observed at shorter distances.24 Interesting finding recently reported is that the energy transfer efficiency changes with varying the concentrations of both Au nanoparticles and dye.24 The pronounced quenching (4085%) on the photoluminescence (PL) intensity and the shortening of lifetime of rhodamine 6G dye in presence of Au nanoparticles confirm the energy transfer from dye to Au nanoparticles. The shortening of decay time of dye is due to change in nonradiative rate. The change in radiative rate of the dye (3-4%) is very less compared to quenching of PL intensity. Analysis reveals that the PL quenching is due to energy transfer from dye to Au nanoparticles, not due to re-absorption.25 The measured distance (d) between the donor and acceptor varies from 86.06 Å to 102.47 Å, using FRET equation. The length scale for detection in FRET based method is restricted only on the upper limit of 80 Å. Therefore, it is suggested that the energy transfer from dye to metal surface is a surface energy transfer (SET) process which follows 1/d4 distance dependence (Scheme 1). The PL quenching efficiency depends on the size of particles and the efficiency values are 59%, 69%, 81% and 86% for 14 nm, 20 nm, 33 nm and 48 nm sized Au nanoparticles, respectively. It reveals that the energy transfer depends on size of particle. The shape of the particle also influences the surface energy transfer (SET) between dye molecules and Au nanoparticles. Scheme 2 summarizes the synthetic approach used for the preparation of spherical and shaped Au nanoparticles. Spherical Au nanoparticles of small size have been prepared by using the sodium borohydride reduction method and the anisotropic (shaped) Au have been prepared by using the seed mediated growth method.26,

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The energy transfer

efficiencies from dye to Au nanoparticles are found to be 53.1% and 30.6% for spherical and shaped Au nanoparticles, respectively (shown in Figure 1).28 The surface energy transfer (SET)


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process is found to be a very powerful technique to measure the molecular coverage on the nanoparticle surface29 and shell thickness of core-shell nanoparticles.30 Ray et al. reported the rapid and ultrasensitive detection of mercury in soil, water and fish by using Au nanoparticle based surface energy transfer process.31 They showed drastic fluorescence quenching efficiency (100%) when the rhodamine B dye is statically adsorbed on the Au nanoparticle surface. However, the addition of 130 ppb of Hg (II) ions, rhodamine B molecules are released from the gold particle surface and thus fluorescence signal enhancement by a factor of 1100 is observed within a second. It is because that the binding energy of dye onto gold (8-16 kcal/mol) is smaller than the binding energy of dye and Hg (80-100 kcal/mol). Such detection of heavy transition metal ions is of paramount interest due to the high toxicity of these metal ions toward human health and the environment.

Thus, gold nanoparticles-based surface energy transfer probe

provides a useful method for the development of practical nanosensors.

3. ENERGY TRANSFER BETWEEN CONFINED DYE AND Au NANOPARTICLES Investigations on dye molecules confined in porous materials, such as in sol-gel matrix, mesoporous silica, zeolites, and cyclodextrin etc., have opened up new possibilities for the use of porous materials for light harvesting applications.32-41 Confinement of the dye molecules into cavity not only prevent the dyes to form aggregates but also improve their photostability. Therefore, cyclodextrin, mesoporous silica and zeolites are the novel hosts to confine the dye inside the cavity.

3.1. CONFINED DYE IN MESOPOROUS SILICA AND Au NANOPARTICLES A large number of studies exist on the energy transfer between confined dye molecules in mesoporous silica.33, 37, 41-43 The enhancement of the energy transfer efficiency is obtained at low concentration of dye. For example, control of the energy transfer in conjugated polymer immobilized in the mesoporous silica has been reported by Tolbert et al.33 The doping of multicolor quantum dots in mesoporous materials has been demonstrated by Gao et al.42 The influence of mesoporous host on the energy transfer process has been discussed by Wang et al.43 Nanoscale architectures have been designed by entrapping rhodamine 6G dye molecules into the channels of mesoporous silica (MCM-41) and Au nanoparticles anchored onto the surface of the mesoporous matrix (Figure 2A, B).44 Figure 2A shows the anchoring of (3-


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mercaptopropyl)-trimethoxysilane (3-MPTS) onto MCM-41 through silicon alkoxide route and the attachment of Au nanoparticles onto the surface of mercapto-MCM-41 through Au-S bond formation. The surface energy transfer between confined dye and Au nanoparticles has been studied by steady state and time resolved spectroscopy. The PL quenching (27.4% to 76.3%) and the energy transfer efficiency (17.4% to 51.8%) have been varied with changing the parameters.44 To unraveling the rotation dynamics of dye molecules inside the MCM-41 and the direction of the energy transfer, time-resolved anisotropy study has been performed. This anisotropy decay curve suggests the restriction motion of dye molecules inside the channels of mesoporous silica (Figure 2C). As the size of the dye is about only 5 Å and the radius of the channels of MCM-41 is 4 nm, therefore large number of dye molecules may reside inside the cavity. The fluorescence anisotropy decay of R6G shows bi-exponential with the correlation time constants of 235 ps (69%) and 1.51 ns (31%), when dye molecule is confined inside MCM-41 mesoporous. The motion of the central dye molecule will be free and it gives the fast component of 235 ps. The motion of the dyes residing around the periphery inside the cavity is restricted and it gives the slow component of the anisotropy decay. Tolbert et al.33 have seen the unidirectional energy transfer in oriented conjugated polymer-mesoporous silica composites from anisotropy study. The initial larger value of the anisotropy suggests the unidirectional energy transfer in the present hybrid system. The efficient energy transfer from dye to Au nanoparticles could pave the way for developing new challenging photonic devices.

3.2. CONFINED DYE IN ZEOLITE CAGE AND Au NANOPARTICLES Zeolite is another interesting host because it is optically transparent and rigid structure with three dimensional framework forming channels and/or cages with molecular dimension.45, 46 Many groups have studied the energy transfer between confined dye molecules in zeolite.32, 34, 35, 45, 46 The storage of solar energy by photoelectron transfer in zeolite structure has been demonstrated by Dutta et al.32 Calzaferri et al.34, 35 have done extensive work on the energy transfer in dye-zeolite system for light-harvesting antenna materials. A new optical based nanostructured material has been designed where coumarin 480 dye is confined within Y-zeolite and Au nanoparticles are attached onto the surface of Y-zeolite.47 The enhanced fluorescence and large blue shift (9 nm) in the emission maximum of C480 in zeolite confirm the confinement of the probe dye molecules inside the zeolite cavity. Figure 3A is the schematic representation of


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the energy transfer between NaY-zeolite functionalized Au nanoparticles and the confined coumarin 480 dyes within Y-zeolite.47 To avoid incorporation of Au nanoparticle inside the cage of zeolite (the diameter of the cage of Y-zeolite is 13 Å) (Figure 3A), the size of Au nanoparticle is being used 14 nm (Figure 3B). The immobilization of Au nanoparticles onto the surface of Y zeolite has been confirmed by TEM study (Figure 3C). The unprecedented PL quenching (94%) of confined dye and the efficient energy transfer (70%) between confined dye and Au nanoparticles have been demonstrated (Figure 3D).47 However, less photoluminescence quenching efficiency

(70%) is observed without confinement of dye (Figure 3D).47 This

efficient energy transfer due to confinement of dye provides for fabrication of new challenging photonic devices. The rotation dynamics of dye molecules inside the zeolite has been studied by time resolved anisotropy and it is interesting to note that the fluorescence anisotropy decay of C480 in zeolite is fitted to a bi-exponential function47 r (t ) = r0 [a1R exp(t / φ1R ) + a 2 R exp(t / φ 2 R )]

(6)

Here, r(t) is the rotational time and φR represents the rotational correlation time constant. The anisotropy decay of C480 in zeolite is bi-exponential with a fast time constant of 221 ps (68%) and a slow component of 1.93 ns (32%), leading to an average correlation time of 768 ps. This higher correlation time implies the restricted rotation of the dye molecules due to the confinement of the dyes in zeolite cavity.47 Such energy transfer between confined dye and Au nanoparticles could pave the way for designing new optical based materials for the application in chemical sensing or for developing new challenging photonic devices.

3.3. CONFINED DYE IN γ- CYCLODEXTRIN AND Au NANOPARTCLES γ -Cyclodextrins (CD) are cyclic oligosaccharides compounds in which eight (γ-CD) glucose units are linked to form a truncated conical structure. The interior cavity of γ-CD is hydrophobic in nature having dimension of 7.5-8.5 Å and the height of ~ 8 Å.48 Therefore, large number of organic molecules can be encapsulated in its hydrophobic cavity and form host-guest supramolecular structures. A large numbers of studies exist on dynamics of organic fluorophores confined in a CD cavity.

49-51

Freeman52 demonstrated the FRET–based

competitive assay using β–cyclodextrin modified CdSe/ZnS QDs as sensors and chiroselective sensors. Tang et al.53 have designed Au NPs-β–CDs-FL assembly for sensing cholesterol by using energy transfer between fluorescein and Au nanoparticles. The designing of new optical


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based materials having nanotubular γ-CD aggregates linked by coumarin 480 dye: Au nanoparticle assemblies has been reported (Scheme 3).54 A schematic representation for the attachment of glutathione (GSH) capped Au nanoparticle with γ-cyclodextrin is given in Scheme 4. A drastic PL quenching (99%) of the confined coumarin 480 dye inside γ-CD nanocavity in presence of Au nanoparticle is reported.54 The nanotubular aggregates of at least 70 γ-CD units of cyclodextrin has been confirmed by fluorescence time resolved anisotropy study (shown in Figure 4) and TEM measurement (inset of Figure 4).54 The time resolved decay studies (Figure 5a) of C480 dye solution in γ-CD without Au nanoparticle and in γ-CD attached Au nanoparticle clearly demonstrate the shortening of lifetime of C480 dye in presence of Au nanoparticle. The average decay times of C480 dye inside γ-CD in absence and presence of attached Au nanoparticle are 4.77 ns and 1.91 ns, respectively. The PL quenching and the shortening of decay time confirm the energy transfer between confined dyes inside γ-CD with attached Au nanoparticle. Ultrafast spectroscopic study (Figure 5b) showed that there is a rise of decay time of the confined dye in γ-CD (inset of Figure 5b) which generally originates from the salvation dynamics of C480 dye. However, the shortening of ultrafast decay time is observed in case of γCD attached gold nanoparticle. The absence of the rise component and shortening of ultrafast decay time reveal the electron transfer process. Thus, the shortening of the longer part of the decay as well as the ultrafast part reveals that the energy transfer may be associated with very fast electron transfer process.54 Further investigations using ultrafast spectroscopy are necessary for in-depth understanding of the phenomenon.

4. INTERACTIONS OF BIO-MOLECULES WITH Au NANOPARTICLES The research in bio-conjugated nanoparticles has been paid a great attention for various applications.55-58 Despite the remarkable advancement of nanoscience, relatively little is known about the effects of nanoparticle in biological systems.

4.1. DYNAMICS OF PROTEIN MOLECULES USING ENERGY TRANSFER Recently, gold nanoparticle is used as a model for understanding nanoparticle-protein interactions because it is inert, and it possesses facile bio conjugation ability, which is desired for quantitative analyses.59 The interaction of nanoparticles with proteins will change protein conformation, expose novel epitopes on the protein surface, or perturb the normal protein


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function, which could induce unexpected biological reactions and lead to toxicity.55,

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The

interaction between gold nanoparticles with blood proteins has been reported by Douglas et al.60 In most of the studies, the information about the structural changes of the bio-molecules upon binding to nanoparticles and the identification of the protein binding site has been based mainly on infrared spectroscopy, circular dichroism, fluorescence, and other methods that can monitor changes in the secondary and tertiary structure of proteins. The identification of the specific location of Au nanoparticles in human ubiquitin (hUbq) using NMR study has been reported.61 However, the major drawback of the NMR technique lies on the limitation of size of proteins that can be measured.61 The understanding of the conformational changes of protein and the identification of the specific binding site of protein with nanoparticles in Human

Serum

Albumin protein conjugated nanoparticles using the surface energy transfer method has been discussed recently.62 Human serum albumin (HSA)63 is the most abundant protein in plasma and it contains 18 tyrosines, six methionines, one tryptophan (Trp 214), 17 disulfide bridges, and only one free thiol (Cys 34). HSA comprises three homologous domains (I, II and III) that assemble to form a heart-shaped molecule. Each domain contains two sub-domains (A and B) that possess common structural motifs.63 The single tryptophan of HSA at residue 214 has been used extensively as a fluorescent reporter group for ligand binding and conformational studies.64 Dynamic light scattering (DLS) study of HSA conjugated Au nanoparticles of different sizes (1.5 nm, 2.0 nm and 2.9 nm sized Au) show mono-disperse particle size distribution with hydrodynamic diameters of 9.4 nm, 10.2 nm and 11.2 nm whereas hydrodynamic diameter of HSA protein is 8.0 nm at pH 7.4. Thus, the hydrodynamic diameter increases from HSA to AuHSA, and the increments are 1.4 nm, 2.2 nm and 3.2 nm. These increments nicely match with the average sizes of the HSA conjugated Au nanoparticles obtained from TEM images of Au nanoparticles which are 1.5 nm, 2.0 nm and 2.9 nm sized Au-HSA at pH 7.0, respectively. It reveals that the Au nanoparticle is strongly interacting with HSA protein and the ratio of HSA and Au nanoparticle in the HSA-AuNP complex is 1:1.62 The far UV- CD spectra show that the extent of unfolding of HSA increases with increasing the size of the Au nanoparticle.62 Reverse phase HPLC elution profile confirms that all HSA proteins are tagged with Au nanoparticle and no free-HSA is left in the solution.62 The energy transfer efficiency from tryptophan to Au nanoparticle varies from 73 to 92% with varying Au nanoparticle size.62 Using SET equation, the measured distances between the Trp molecule of HSA and Au nanoparticle are 42.5 Å, 41.9


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Å and 48.1 Å for 1.5 nm, 2.0 nm and 2.9 nm HSA-conjugated Au nanoparticle, respectively. Au nanoparticle may reside either in domain I or II or III of HSA because of the presence of thiolate (-SH) or disulphide (SS) linkage.62 Three body interactions have been used to understand the exact domain of HSA protein in which Au nanoparticle is attached.62 For this purpose, ANS dye has been used as a probe because the binding site (sub domain IIIA) of this dye in HSA protein is known.65 Time resolved anisotropy study is performed to understand the rotational dynamics of ANS dye molecule inside HSA protein. In pure HSA, the anisotropy decay of ANS dye is single exponential with time constant of 39.6 ns due to tight binding inside the protein.62

The

correlation time constant of ANS dye increases to 62.9 ns for Au nanoparticles conjugated HAS because the hydrodynamic volume of the whole rotating moiety becomes larger. Thus, the anisotropy of the whole protein is seen to decay slowly with longer rotational correlation time constant which implies the rigidity of the protein. Using surface energy transfer process, the calculated distances (d) between the donor ANS and acceptor Au nanoparticles are 35.9 Å, 31.5 Å and 25.7 Å for 1.5 nm, 2.0 nm and 2.9 nm HSA-conjugated Au nanoparticles, respectively.62 The preferable binding of Au nanoparticles to HSA protein may be either through free Cys 34 residue or through Cys-Cys disulphide (SS) linkage because Au-S bond formation is energetically favorable. From the structural data of HSA protein using PyMol software, the distances from Trp to free Cys 34 (domain I), Cys53-Cys62 SS bond (domain I), Cys316-Cys361 SS bond (domain II), Cys360-Cys369 SS bond (domain II), and Cys558-Cys567 SS bond (domain III) are 34.1 Å, 39.1 Å, 27.3 Å, 29.0 Å and 53.1 Å, respectively (shown in Figure 6 A, B).62 These distances are close to the distances between Trp residue and the binding site of HSA with Au nanoparticles, obtained from SET. The distances between ANS dye and Au nanoparticles are 51 Å, 51.5 Å and 54.7 Å for 1.5 nm, 2.0 nm and 2.9 nm HSA-conjugated Au nanoparticles, respectively. From the protein structural data, the distances from the centre of sub domain IIIA to Cys 34, Cys53-Cys62 SS bond (domain I), Cys316-Cys361 SS bond (domain II), Cys360-Cys369 SS bond (domain II), and Cys558-Cys567 SS bond (domain III) are found to be 35.2 Å, 51.5 Å, 37.9 Å, 34.9 Å and 35.0 Å, respectively (shown in Figure 6 A, B).62 Thus, all the conditions (Trp to Au binding site distance and ANS to Au binding site distance) would be satisfied only if Au nanoparticle binds to HSA through Cys53-Cys62 disulfide bond (sub domain IA). Analysis of the results suggests that Au nanoparticle interacts with the binding site located at sub domain IA of HSA for all the three different sizes of Au nanoparticles (shown in Figure


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7).62 It reveals that the surface energy transfer method is an effective tool to identify the specific binding site of protein with nanoparticle. Similarly, the understanding of the conformational changes of BSA conjugated Au nanoparticles with varying pH values by using the surface energy transfer has also been reported (Figure 8A).66 It is well known that the conformation of BSA undergoes structural changes very easily with changing the pH values.67,

68

All of these

conformational states have their own molecular dimensions and shape. The quenching of Tryptophan emission in presence of Au nanoparticle is found to be a static quenching process. Static quenching arises only from the formation of complex between BSA and Au nanoparticles. The binding constant and the bound/unbound ratio vary with changing the conformation of protein.66 The observed quenching of PL intensities of tryptophan are 91.7%, 97.1%, and 94.1% for E, N, and B forms of BSA protein, respectively. Using surface energy transfer, the measured distances (d) between Trp and Au nanoparticles are 19.1 Å, 14.6 Å, and 17.7 Å for E, N, and B forms of BSA protein, respectively. Rhodamine 6G dyes is used because it is a hydrophilic dye and it would be located in the polar region of the protein, i.e. on the surface of the protein exposed to the polar solvent.66 Figure 8B shows the PL spectra of dye in presence of BSAconjugated Au nanoparticles prepared at pH 3.0, 7.0, and 9.0, respectively. The calculated energy transfer efficiencies from dye to Au nanoparticles are 19.4%, 56.6%, and 44.2% for BSAconjugated Au nanoparticles prepared at pH 3.0, 7.0, and 9.0, respectively, indicating the energy transfer is fastest in BSA-conjugated Au nanoparticles prepared at pH 7.0.66 The distance between donor (dye) and acceptor (Au nanoparticle) is estimated by using FRET and SET methods.66 Using the efficiency of SET, the calculated distances (d) between the donor and the acceptor are 116.5 Å, 76.1 Å, and 86.4 Å for E, N, and B forms of BSA protein, respectively. The study reveals that Au nanoparticle-based surface energy transfer can be used for understanding the conformational changes in protein molecules.

4.2. FOLDING MECHANISM OF RNA MOLECULES USING ENERGY TRANSFER An interesting finding recently reported is that the surface energy transfer method is used to track the folding of RNA.69 The conformational changes of two-helix junction RNA molecules induced by the binding of Mg2+ ions has been studied by measuring the time-dependent fluorescence signal. Experimentally, it is observed that there are four separate states involved during docking to undocking transition (Figure 9A). RNAs are found to switch very slowly


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between the four states. In the folded state (docked state), the fluorescence from the Cy3 dye attached to one end of RNA is drastically quenched (quenching efficiency of nearly 98%) by the attached Au nanoparticle (as shown in Figure 9B).69 Thus, the lowest emission intensity configuration is docked configuration (D), where the two domains are in contact because of the nearest distance between Cy3 dye and Au nanoparticle. The transition from the folded to an open configuration changes the distance between the Au nanoparticle and the dye molecule, and as a result, PL intensity increases 120 times (as shown in Figure 9B). Using distance-dependent SET, the structure of these two transition states from D to U state can be understood. The time dependent fluorescence data indicate that the quenching efficiency is about 98% in the D state, 85% in the first transition state, 45% in the second transition state, and 0% in the U state.69 From the distance-dependent curve, the obtained distance between the Au nanoparticle and Cy3 are ~3 nm in the D state, 5 nm in the first transition state, and 12 nm in the second transition state.69 Time-dependent SET can therefore clearly distinguish structural transitions between unfolded to folded states even for RNA with 130 nucleotides where donor and acceptors are more than 10 nm apart that measurement is not possible with FRET probes.69

4.3. DETECTION OF DNA HYBRIDIZATION USING ENERGY TRANSFER Dubertret et al. described the single-mismatch detection using Au nanoparticles based quenching of fluorescent oligonucleotides.70 In competitive hybridization assays, the ability to detect single mismatch is eightfold greater with this probe than with other molecular beacons.70 This type of hybrid material can also be used to gain insight the distance-dependent quenching of PL of dye by composing different lengths of dye-functionalized DNA oligonucleotides conjugated Au nanoparticles.71 Strouse and co-workers72 found that the rate of energy transfer from a dye to Au nanoparticle attached DNA is 1/d4 distance dependent. The quenching efficiencies are found to be 68.2%, 44.0%, 16.7 %, and 2.6 %, for 62 Å, 96 Å, 130 Å and 232 Å, respectively. Finally, the energy transfer efficiency values obtained from experiment have been compared with the energy transfer efficiency values from theory, using both dipole-dipole (FRET) and dipole-surface (SET) energy transfer processes (Figure 10). It is important to mention that experimental data’s are fitted well with the (1/R4) i.e. surface energy transfer. The average value of n is 4.0 with an average r0 of 92 Å, which nicely matches (< 2% error) with the theoretically calculated values for a SET mechanism.72,

73

Chhabra et al. reported the energy


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transfer efficiencies of two dyes Cy3 and Cy5, with two different sizes of Au nanoparticles without spectral overlap between dye and Au nanoparticle.71 They also observed that the rate of energy transfer is proportional to 1/d4 (SET). Thus, surface energy transfer process is being used to understand the nature of interactions between dyes and a biomolecule (DNA).74 Interesting findings reveal that SET method is suitable for detection of DNA hybridization.

5. CONCLUSIONS AND OUTLOOK This study undoubtedly demonstrates that the energy transfer processes in Au nanoparticle based assemblies could pave the way for developing new challenging photonic devices. Energy transfer processes in Au nanoparticle - dye assemblies are realized from photoluminescence (PL) quenching and the shortening of decay time of dye without remarkable change in radiative rate. Analysis reveals that the energy transfer from dye to Au nanoparticles is a surface energy transfer (SET) process as established from the 1/d4 distance dependence. Interesting findings recently reported are that this process is a very useful in detection of metal ions, virus, specific binding affinity, proteins, shell thickness of core-shell nanoparticles, and fluorescence imaging.75-80 The design of nanostructured materials with unidirectional energy transfer would be the emerging field of research for the application of energy storage system. The development of Au nanoparticle- semiconductor or Au nanoparticle – graphene composites provides an important milestone for the applications in light energy harvesting, and solar cells.81 The design of new optical based materials based on conjugated polymer nanoparticle - Au nanoparticle has opened up new possibilities for optoelectronic applications.82,

83

Though many

issues are to be addressed, the general interest in Au nanoparticle based energy transfer is expected to grow in the coming years because applications are still in the embryonic stage. Therefore, further investigations in this field are necessary for in-depth understanding of the phenomenon.

Acknowledgement Authors thank to CSIR and DST (Indo-Spanish research project) for their financial support. Special thanks to Mr. Gopal Krishna Manna, IACS for graphic design.


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Figures Caption Scheme 1 Surface energy transfer from rhodamine 6G dye to Au nanoparticle. Scheme 2 Flowchart for the synthesis of spherical and shaped Au nanoparticles. Figure 1 (A) Absorption spectra of (a) spherical and (b) shaped gold nanoparticles and photoluminescence (PL) spectra of (c) rhodamine 6G (R6G) dye solution, (d) shaped Au and 1 µM R6G, (e) spherical Au and 1 µM R6G; (B) Decay curves of (a) rhodamine 6G (R6G) dye solution, (b) spherical Au and 1 µM R6G, and (c) shaped Au and 1 µM R6G. Reproduced with permission from ref. 28, Copyright 2008 ACS. Figure 2 (A) Schematic representation of thiol functionalized MCM-41 and its attachment with Au nanoparticles; (B) Schematic representation of SET from confined dye to MCM-41 surface attached Au nanoparticle; and (C) Fluorescence anisotropy decay curves of rhodamine 6G (R6G) dye in the presence of citrate stabilized Au NPs [curve (i)] and MCM-41 attached Au nanoparticles (Au: S = 1:0.2) [curve (ii)]. Reproduced with permission from ref. 44, Copyright 2010 ACS. Figure 3 (A) Schematic representation of the energy transfer between confined dye and Yzeolite functionalized Au nanoparticles; (B) TEM images of pure citrate stabilized Au nanoparticles, (C) TEM images Au nanoparticles attached with zeolite; and (D) Emission spectra of C480 dye [curve (i)] in water, in zeolite dispersion [curve (ii)], in presence of citrate stabilized Au NPs without zeolite [curve (iii)], and in presence of immobilized Au NPs on the surface of zeolite [curve (iv)] (λex= 405 nm). Reproduced with permission from ref.47, Copyright 2010 ACS. Scheme 3 Schematic representation of the energy/electron transfer from Coumarin 480 dye confined in γ-cyclodextrin cavity to γ-cyclodextrin attached Au NPs. Reproduced with permission from ref.54, Copyright 2010 ACS. Scheme 4 Schematic presentation for the fabrication of γ-cyclodextrin attached Au NPs. Reproduced with permission from ref.54, Copyright 2010 ACS. Figure 4 Fluorescence anisotropy decay of C480 dye in 10 mM γ-CD. (λex = 405 nm). Inset shows the TEM image of γ-cyclodextrin attached Au nanoparticles. Reproduced with permission from ref.54, Copyright 2010 ACS. Figure 5 (a) Picosecond transients of C480 dye (i) in 10 mM γ-CD and (ii) in Au nanoparticles attached γ-CD solution; (b) Femtosecond transients of C480 (λem = 470 nm) (i) in 10 mM γ-CD, and (ii) in Au nanoparticles attached γ-CD solution. (λex = 405 nm). The initial part of decay (i)


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is shown in the inset of Figure 5b. Reproduced with permission from ref.54, Copyright 2010 ACS.

Figure 6 Schematic representations of different residues in HSA protein and their distances from Tryptophan using Pymol software. Reproduced with permission from ref. 62, Copyright 2011 ACS. Figure 7 Schematic representation of binding of ANS dye and Au nanoparticle to HSA protein. Reproduced with permission from ref. 62, Copyright 2011 ACS. Figure 8 (A) Schematic respresentation of surface energy transfer from R6G dye to Au and trytophan to Au nanoparticle. (B) Photoluminescence (PL) spectra of 1 µM Rhodamine 6G (R6G) dye solution (a) at pH 3, (b) at pH 7.0, (c) at pH 9.0, and in presence of Au nanoparticles (d) at pH 3.0, (e) at pH 7.0, and (f) at pH 9.0. Reproduced with permission from ref. 66, Copyright 2008 ACS. Figure 9 (A) Schematic of the docking and undocking transitions of the hairpin ribozyme is shown. (B) NSET intensity change in presence of 15 mM Mg2+ at 25 °C due to transition from docked to undocked form. Reproduced with permission from ref. 69, Copyright 2008 ACS. Figure 10 (a), (b) Graphic representation of a donor dye-nanometal acceptor pair separated by dsDNA [Ref 81]; and (c) Energy transfer efficiency plotted versus separation distance between FAM and Au(NM). Filled circles (b) represent DNA lengths of 15bp, 20bp, 30bp, and 60bp. The measured efficiencies of these strands with the addition of M.EcoRI are represented by the open circles (O). The error bars reflect the standard error in repeated measurements of the fluorescence as well as the systematic error related to the flexibility of the C6 linker. The dashed line is the theoretical FRET efficiency, while the solid line is the theoretical SET efficiency. Reproduced with permission from ref. 72, Copyright 2005 ACS.


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S

Scheme 1 Surface energy transfer from rhodamine 6G dye to Au nanoparticle.

Synthesis of spherical Au NPs HAuCl4 (aq.) + Tri-sodium citrate

Stirring Ice cool NaBH4 aq. solution

(Seed Au soln)

Seed mediated Synthesis of anisotropic Au NPs HAuCl4 (aq.) + CTAB (Growth solution)

Ascorbic acid soln

Mixture Seed soln solution

Anisotropic Au NPs

Centrifuged Washed

+

+

Shaped Au NPs

Scheme 2 Flow chart for the synthesis of spherical and shaped Au nanoparticles.


21


528 nm 552 nm (c) (a)

2500

6

(b)

4

0.4 (d)

2

0.2 (e)

(a)

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(b)

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0 300

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8

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Absorbance

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400 500 600 Wavelength (nm)

700

0

5

10

15

20

Time (ns)

Figure 1 (A) Absorption spectra of (a) spherical and (b) shaped gold nanoparticles and photoluminescence (PL) spectra of (c) rhodamine 6G (R6G) dye solution, (d) shaped Au and 1 µM R6G, (e) spherical Au and 1 µM R6G; (B) Decay curves of (a) rhodamine 6G (R6G) dye solution, (b) spherical Au and 1 µM R6G, and (c) shaped Au and 1 µM R6G. Reproduced with permission from ref. 28, Copyright 2008 ACS.


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(A)

(B)

(C) (i) Rh6G in pure citrate reduced Au

0.2

r(t)

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(ii) Rh6G in MCM-41 attached Au assembly

0.1 (ii) (i)

0.0

0

1

2

3

Time (ns)

Figure 2 (A) Schematic representation of thiol functionalized MCM-41 and its attachment with Au nanoparticles; (B) Schematic representation of SET from confined dye to MCM-41 surface attached Au nanoparticle; and (C) Fluorescence anisotropy decay curves of rhodamine 6G (R6G) dye in the presence of citrate stabilized Au NPs [curve (i)] and MCM-41 attached Au nanoparticles (Au: S = 1:0.2) [curve (ii)]. Reproduced with permission from ref. 44, Copyright 2010 ACS.


23


(B)

(A)

180

(ii)

150 PL Intensity (a.u.)

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(C)

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(D)

(i)

120 90 60

(iii)

30

(iv)

0 450 500 550 600 650 700 Wavlength (nm)

Figure 3 (A) Schematic representation of the energy transfer between confined dye and Y-zeolite functionalized Au nanoparticles; (B) TEM images of pure citrate stabilized Au nanoparticles, (C) TEM images Au nanoparticles attached with zeolite; and (D) Emission spectra of C480 dye [curve (i)] in water, in zeolite dispersion [curve (ii)], in presence of citrate stabilized Au NPs without zeolite [curve (iii)], and in presence of immobilized Au NPs on the surface of zeolite [curve (iv)] (λex= 405 nm). Reproduced with permission from ref. 47, Copyright 2010 ACS.

ET

hυ υ O C

Au

HN

CH 3

O B O

N

O

O

C480 dye: γ-CD

Scheme 3 Schematic representation of the energy/electron transfer from Coumarin 480 dye confined in γ-cyclodextrin cavity to γ-cyclodextrin attached Au NPs. Reproduced with permission from ref.54, Copyright 2010 ACS.


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O

O

Au

C

NHS

C

HN OH

Au

OH

B

H2N

OH

OH B OH

Glutathione capped Au NPs

3-Aminophenyl boronic acid

HO HO

O C

HN

γ -Cyclodextrin

O

Au

B O

γ -Cyclodextrin attached Au NPs

Scheme 4 Schematic presentation for the fabrication of γ-cyclodextrin attached Au NPs. Reproduced with permission from ref.54, Copyright 2010 ACS.

0.4

Anisotropy of C480 in γ-CD

0.2 r(t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

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15

20

25

30

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Figure 4 Fluorescence anisotropy decay of C480 dye in 10 mM γ-CD. (λex = 405 nm). Inset shows the TEM image of γ-cyclodextrin attached Au nanoparticles. Reproduced with permission from ref.54, Copyright 2010 ACS.


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0.4 0.2

0.8 0.6 0.4 0.2 0

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10 20 Tim e (ps)

30

60 90 120 150 180 Time (ps)

Figure 5 (a) Picosecond transients of C480 dye (i) in 10 mM γ-CD and (ii) in Au nanoparticles attached γ-CD solution; (b) Femtosecond transients of C480 (λem = 470 nm) (i) in 10 mM γ-CD, and (ii) in Au nanoparticles attached γ-CD solution. (λex = 405 nm). The initial part of decay (i) is shown in the inset of Figure 5b. Reproduced with permission from ref.54, Copyright 2010 ACS.


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(A)

Cys 558-Cys567 SS bond Cys 34 35.0 53.1

Cys 53-Cys62 SS bond

ANS

Trp

(B)

ANS

Trp Cys 360-Cys369 SS bond Cys 316-Cys361 SS bond

Figure 6 Schematic representations of different residues in HSA protein and their distances from Tryptophan using Pymol software. Reproduced with permission from ref. 62, Copyright 2011 ACS.


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Figure 7 Schematic representation of binding of ANS dye and Au nanoparticle to HSA protein. Reproduced with permission from ref. 62, Copyright 2011 ACS.

180

(A)

(B)

a

150

b

c

120

PL-Intensity

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90

d

60 30

f e

0 500

550

600

650

700

750

Wavelength (nm)

Figure 8 (A) Schematic respresentation of surface energy transfer from R6G dye to Au and trytophan to Au nanoparticle. (B) Photoluminescence (PL) spectra of 1 µM Rhodamine 6G (R6G) dye solution (a) at pH 3, (b) at pH 7.0, (c) at pH 9.0, and in presence of Au nanoparticles (d) at pH 3.0, (e) at pH 7.0, and (f) at pH 9.0. Reproduced with permission from ref. 66, Copyright 2008 ACS.


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Figure 9 (A) Schematic of the docking and undocking transitions of the hairpin ribozyme is shown. (B) NSET intensity change in presence of 15 mM Mg2+ at 25 °C due to transition from docked to undocked form. Reproduced with permission from ref. 69, Copyright 2008 ACS.

(c)

Figure 10 (a), (b) Graphic representation of a donor dye-nanometal acceptor pair separated by dsDNA [Ref 81]; and (c) Energy transfer efficiency plotted versus separation distance between FAM and Au (NM). Filled circles (b) represent DNA lengths of 15bp, 20bp, 30bp, and 60bp. The measured efficiencies of these strands with the addition of M.EcoRI are represented by the open circles (O). The error bars reflect the standard error in repeated measurements of the fluorescence as well as the systematic error related to the flexibility of the C6 linker. The dashed line is the theoretical FRET efficiency, while the solid line is the theoretical SET efficiency. Reproduced with permission from ref. 72, Copyright 2005 ACS.


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TOC This article highlights Au nanoparticles based surface energy transfer process in different systems.


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Amitava Patra (born 1965), received Ph. D (1993) from Jadavpur University, India. He is now Professor at Indian Association for the Cultivation of Science, India. He is the recipient of DAE-SRC Outstanding Investigator Award, A.V. Rama Rao Foundation Prize in Chemistry, AsiaNANO 2010 Award, CRSI Bronze Medal, Ramanujan Fellowship, and MRSI Medal. He is an Advisory board member of Nanoscale. He is author or co-author of more than 124 scientific papers, 3 book chapters and 2 Indian patents. His research interests include the designing on functional nanoparticles with novel optical properties, quantum dot based resonance energy transfer, photonic materials using up- and down- conversion luminescence.

Tapasi Sen has completed her Bachelors and Masters degree in Chemistry from Visva-Bharati University, in 2004 and 2006, respectively. She received Ph. D (2011) from Jadavpur University, India under the guidance of Prof. Amitava Patra. Her research interests include synthesis and spectroscopy of colloidal semiconductor, metal nanoparticles with particular focus on the surface energy transfer.


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Recent Advances in Energy Transfer Processes in Gold-Nanoparticle

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