Modulation of Excited-State Proton Transfer Dynamics in a Model

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Modulation of Excited-State Proton Transfer Dynamics in a Model Lactim−Lactam Tautomeric System by Anisotropic Gold Nanoparticles Debarati Ray,† Arghyadeep Bhattacharyya,‡ Subhash Chandra Bhattacharya,*,† and Nikhil Guchhait*,‡ †

Department of Chemistry, Jadavpur University, 188, Raja S. C. Mallick Road, Kolkata 700032, India Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India

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ABSTRACT: The past few decades have witnessed significant advances in the development of functionalized gold nanoparticles (GNPs) for diverse applications in various fields such as chemistry, biology, pharmacy, and physics. However, a recent in vivo toxicity study of GNPs in Drosophila melanogaster demonstrated that GNPs are capable of inducing mutagenesis. Considering mutagenicity and tautomerism (lactim−lactam and amine−imine) of DNA base pairs are correlated with each other, a recent theoretical study reveal the modification of doubleproton-transfer process of guanine−cytosine DNA base pairs upon interaction with the gold surface (Au(111)). However, there is a dearth of experimental data about the proton-transfer (PT) process in isolated base pairs on gold nanosurface. In this particular work, we have chosen a model lactim−lactam tautomeric system, namely, 1-(2-hydroxy-5-chloro-phenyl)-3,5-dioxo1H-imidazo-[3,4-b] isoindole (ADCL), and carried out for the first time a comprehensive study of the modification of excited-state PT dynamics in the presence of synthesized isotropic (spherical) and anisotropic (trianglular, rod, and trigonal bipyramidal) GNPs by means of steady-state and time-resolved fluorescence spectroscopy. The PT process operative in ADCL was experimentally found to be accelerated on the anisotropic gold nanosurface, whereas spherical-shaped GNPs showed little impact on the dynamics of the above-mentioned photophysical phenomenon. Therefore, our experimental results regarding the modification of lactim−lactam tautomerism of a model compound on the gold nanosurface are an indirect evidence of mutation caused by GNPs. reasons behind spontaneous mutation.16 Inspired by the aforesaid work by Vecchio et al., Freitas et al. have investigated the impact of gold surface (Au(111)) on the simultaneous double-proton-transfer (SDPT) process of guanine−cytosine (GC) DNA base pairs by means of first-principles computational simulations.17 They observed significant differences in the electronic structure, energetics, and kinetics of the tautomeric process of the GC pair during the contact with an inert gold surface which can influence the SDPT mechanism.17 They have also determined a reduction of about 31% in the activation energy barrier of the protontransfer (PT) process in the GC/Au(111) system which points toward the faster PT process on the gold surface. However, there is dearth of experimental data about the PT process in isolated base pairs on gold nanosurface till date. In this particular work, we have strategically chosen an imidazolebased heterocyclic molecule, namely, 1-(2-hydroxy-5-chlorophenyl)-3,5-dioxo-1H-imidazo-[3,4-b]isoindole (ADCL) where lactim−lactam tautomerism happens,18 and carried out for the first time a comprehensive study of the modification of excited-state PT (ESIPT) dynamics in the presence of synthesized isotropic (spherical) and anisotropic (trianglular,

1. INTRODUCTION Over the past few years, there have been many studies devoted to the application of functionalized nanomaterials in biochemistry, microbiology, immunology, cytology, plant physiology, morphology, and so forth.1−8 Among the various nanomaterials, gold nanoparticles (GNPs) have been widely used in modern medical and biological sciences; in particular, it comprises genomics, biosensors, immune analysis, clinical chemistry, detection, and photothermolysis of microorganisms and cancer cells; the targeted delivery of drugs, DNA, and antigens; optical bioimaging and monitoring of cells and tissues using modern registration systems; and so forth.1−8 The growing interest in colloidal GNPs in various fields lends a progressive impetus to the development of their novel preparation methods for different shapes and sizes with surface modification.9−14 However, before exploring the wide potentiality of GNPs, it is necessary to assess their effects upon interaction with living systems. Recently, in vivo toxicity study of GNPs in Drosophila melanogaster by Vecchio et al. revealed a phenotype modification in the subsequent generations of Drosophila which lent them to conclude that the GNPs are capable of inducing mutagenesis.15 It is proposed that the tautomeric equilibrium, that is, lactim (enol)−lactam (keto) tautomerism (in thymine and guanine) and amine−imine tautomerism (in adenine and cytosine), occurring in the nitrogen bases of DNA is one of the main © XXXX American Chemical Society

Received: June 8, 2018 Revised: June 30, 2018 Published: July 2, 2018 A

DOI: 10.1021/acs.jpcc.8b05481 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

other chemicals such as trisodium citrate (TSC), sodium borohydride dehydrate (NaBH4), ascorbic acid, concentrated hydrochloric acid (HCl), and silver nitrate (AgNO3) were purchased from Emarck, Germany, and were used as received. The water used was treated in a Milli-Q water purification system. The spectroscopic grade solvents acetonitrile (ACN) and methanol (MeOH) were purchased from Spectrochem (India) and were used after proper distillation whenever required. 2.2. Synthesis of Different Sized and Shaped GNPs. 2.2.1. Spherical-Shaped GNPs. Spherical-shaped GNPs were synthesized through a normal citrate reduction method. Nineteen milliliters of aqueous 0.526 mM HAuCl4·3H2O (hydrogen tetrachloroaurate(III) trihydrate) was brought to boil at ∼65−70 °C while stirring, and then 1.0 mL of 30 mM aqueous TSC was added. The boiling and stirring were continued for 25 min. 2.2.2. Synthesis of Gold Nanorods. 2.2.2.1. Preparation of Seed Solution. The seed solution was prepared according to the literature method:9 5 mL aqueous solution of 0.5 mM HAuCl4 was mixed with 5 mL of 200 mM CTAB solution, and the mixture was stirred for 2 min to obtain an amber colored solution. Then, 600 μL ice-cold aqueous solution of NaBH4 (10 mM) was added, and the mixture was stirred vigorously and was allowed to stand for few minutes and then used for the subsequent synthesis of gold nanorods. 2.2.2.2. Preparation of Gold Nanorods. For the preparation of the growth solution, 5 mL of 1 mM HAuCl4 and 5 mL of 200 mM CTAB were mixed with proper stirring. To the mixture of 350 μL of AgNO3 and 90 μL of ascorbic acid were added to obtain a colorless solution. Then, 18 μL of seed solution was injected to the growth solution and left for 24 h for gold nanorod at room temperature. 2.2.3. Synthesis of Gold Nanotriangles. TSC (7.5 mL, 1.7 mM) was refluxed at 100 °C under stirring. Aqueous solution (5 mL) containing 1.25 mM HAuCl4 and 7.5 mM CTAB was heated to 50 °C and then injected to the hot TSC. After 5 min, the flask was separated from heating mantle and cooled to room temperature.11 2.2.4. Synthesis of Gold Trigonalbipyramids. The trigonal bipyramids were grown in a solution containing 10 mL of 0.1 M CTAB, 0.5 mL of 0.01 M HAuCl4, 0.1 mL of 0.01 M AgNO3, 0.2 mL of 1 N HCl, 0.08 mL of 0.1 M ascorbic acid, and 120 μL of seed solution. The solution was gently stirred at 400 rpm, 30 °C for 2 h.10 2.3. Preparation of Nanoparticle Solutions. Four different synthesized nanoparticle (spherical-, trianglular-, rod-, and TBP-shaped) reaction mixtures were centrifuged at 7000 rpm for 20 min and washed with millipore water, and the washing process was performed thrice. Then, the resulted solids (nanoparticles) were repeatedly washed with MeOH solvent twice to remove water. After that MeOH solvent was added in the dried nanoparticle and redispersed in the methanolic medium by ultrasound sonification. The same procedure was followed in the case of nanoparticle solution in the ACN medium. However, it is found that the triangularshaped nanoparticles were extremely unstable in the ACN medium as it cannot be redispersed in this medium. Therefore, in our present study, we have used four different nanoparticle solutions (spherical-, triangular-, rod-, and TBP-shaped) in the MeOH medium and three different nanoparticle solutions (spherical-, rod-, and TBP-shaped) in the ACN medium. The concentrations of gold nanorod and spherical-shaped nano-

rod, and trigonal bipyramidal) GNPs by means of steady-state and time-resolved fluorescence spectroscopy. The very molecule ADCL contains a four-member intramolecular hydrogen-bonding network which closely resembles the basic building block of DNA base, thymine and guanine. The previous photophysical study of ADCL in various solvents reveals that the molecule remains both in lactim (enol) and lactam (keto) forms in the ground state, and upon photoexcitation of the lactim form, it exhibits dual emissions attributed to the excited normal (or lactim emission, N*) and tautomer (or, lactam emission, T*) forms that originate from an ESIPT (N* → T*, Scheme 1) reaction.18 The ESIPT rate Scheme 1. Schematic Presentation of ADCL Molecule (Both Lactim and Lactam Forms)

of this molecule is quite slower in organic solvents in comparison to the other PT system because of the high lactim* → lactam* energy barrier through the four-member intramolecular hydrogen-bonding unit.18 Therefore, any modification in the tautomeric energy barrier in the presence of the GNPs must influence the dynamics of the PT process. Recently, Kyrychenko et al. have studied the interaction of 3hydroxychromone (an ESIPT probe) with dodecanethiolprotected GNP (spherical shaped) in order to understand the extent of hydrophobicity of the surface of the nanoparticle, but they concluded nothing about the modification of PT dynamics of the probe.19 Nag et al. have also investigated the effect of nanopores in porous GNPs on excited-state double PT (DPT) in [2,2′-bipyridyl]-3,3′-diol (BP(OH)2) in an aqueous environment, but their main focus is the understanding of the mechanism of the DPT process.20 Till date, there is no such experimental evidence present regarding the influence of GNPs on the kinetics of the PT process of any molecule. On the other hand, the title compound has biological interest also because this kind of molecule is widely used as a drug template in medicinal chemistry.21,22 As GNPs are widely used as drug carriers, the interaction of ADCL with various kinds of GNPs is also an essential task for the future application in medicinal or biological chemistry.

2. EXPERIMENTAL SECTION 2.1. Materials. The title compound 1-(2-hydroxy-5-chlorophenyl)-3,5-dioxo-1H-imidazo-[3,4-b] isoindole (ADCL) was synthesized according to the reported literature procedure.22 The structure of ADCL is presented in Scheme 1. Tetrachloro aurate hydrate (HAuCl4.3H2O) and cetyltrimethyl ammonium bromide (CTAB) were purchased from Sigma-Aldrich, and the B

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investigations were prepared by evaporation of MeOH of its dilute nanoparticle solution on the surface of carbon films.

particles are calculated by considering the available molar extinction coefficient value of the corresponding wavelength,23,24 whereas the concentrations of triangle- and TBPshaped GNPs in a typical solution were estimated by presuming that all of the HAuCl4 used in the preparation were converted to nanoparticle of exactly the same size. 2.4. Instrumentation and Procedure. 2.4.1. SteadyState Spectral Measurements. The steady-state UV−vis absorption spectra and fluorescence emission spectra were recorded in a Hitachi UV−Vis U-3501 spectrophotometer and PerkinElmer LS-55 fluorimeter, respectively. For spectral measurement, only freshly prepared solutions are used and every time proper background corrections have been done. All experiments were carried out at room temperature (303 K). 2.4.2. Time-Resolved Fluorescence Decay. Fluorescence lifetimes of all solutions were carried out with a Horiba Jobin Yvon Fluorocube-01-NL time-correlated single-photon counting setup. A nanolight-emitting diode operating at λex = 340 nm with a repetition rate of 1 MHz is used as the excitation source. The width of the instrument function, which was limited by the full width at half-maximum of the excitation source, was 1 ns. The lamp profile was recorded by placing a scatterer in place of the sample. The signals were collected at the magic angle of 54.7°. The decays were analyzed using DAS-6 decay analysis software, and the acceptability of the fits was judged by χ2 criteria. The time-resolved fluorescence decay (I(t)) is described by the following expression25 I (t ) =

∑ αi exp(−t /τi) i

3. RESULTS AND DISCUSSION 3.1. Characterizations of Synthesized GNPs. 3.1.1. UV−Vis Study. The GNPs of different shapes (spherical, triangular, rod, and trigonalbipyramid) were synthesized according to the available literature procedures,9−12 and the formations of isotropic and anisotropic GNPs were confirmed from UV−visible spectroscopic analysis (Figure 1). The

Figure 1. Absorption spectra of spherical-, rod-, triangular-, and TBPshaped GNPs in the MeOH medium.

plasmon band of spherical-shaped GNPs is centered at ∼510 nm, and triangular nanoplates show a strong surface plasmon resonance band at ∼565 nm and a broad but weak band in the near-infrared region. In the case of rod-shaped GNP characteristics, transverse and longitudinal plasmon peaks appear at ∼600 and ∼715 nm, respectively. The extinction spectrum of TBP-shaped GNPs shows absorption maxima at ∼550 nm (transverse peak) and ∼827 nm (longitudinal peak). These results are in good agreement with the available literature reports.9−12 3.1.2. TEM Study. We have also performed TEM for further confirmation regarding the shape and the size of the synthesized nanoparticles. Figure 2 represents the TEM images

(1a)

where τi is the characteristic decay time constant of the ith component with relative amplitude of αi. For the time-resolved anisotropy measurement, the fluorescence decay curves were recorded at vertical and horizontal positions of the polarizer and analyzed by the following equations25 I (t ) = I (t )

[1 + 2r(t )] 3

(2a)

I⊥(t ) = I(t )

[1 − r (t )] 3

(2b)

where I∥ and I⊥ are the intensities collected at emission polarizations parallel and perpendicular, respectively, to the polarization axis of the excitation beam. I(t) and r(t) are the time-dependent intensity (at the magic angle) and anisotropy decay, respectively. Thus, the anisotropy decay function r(t) has been constructed from these I∥(t) and I⊥(t) decays25 r (t ) =

I (t ) − G ·I⊥(t ) I (t ) + 2·G ·I⊥(t )

(3)

G is the correction factor for the detector sensitivity to the polarization detection of the emission, and it is defined by G=

I⊥(t ) I (t )

(3a)

2.4.3. Transmission Electron Microscopy. The transmission electron microscopy (TEM) images were obtained using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 200 kV. Samples for microscopic

Figure 2. TEM images of (a) spherical-, (b) triangular-, (c) rod-, and (d) TBP-shaped GNPs. C

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between donor and acceptor, up to only 80 Å, mostly when they are covalently labeled at the two specific sites of a rigid biological macromolecule, whereas NSET study allows determining the distance nearly twice than that obtained from FRET mechanism.26−31 The necessary condition for the operation of this long-range energy-transfer process is that there should be an considerable amount of overlapping between the absorption profile of the acceptor (here, GNP) and the emission profile of the donor (here, ADCL).26−31 It is quite evident from the emission profile of ADCL that apart from the spherical-shaped GNPs, other shaped nanoparticles would not have considerable amount of overlapping, which was very helpful to understand long-range energy transfer. Therefore, with this analysis except spherical GNPs, we can negate the possibility of operation of NSET mechanism between anisotropic GNPs and ADCL molecule for the quenching of the emission of the molecule. From the fluorescence intensity data of ADCL in the presence of varying concentration of GNPs of different shapes, we have determined the Stern−Volmer quenching constant (KSV) using the following Stern−Volmer relationship25 between the fluorescence intensity and concentration of metal nanoparticles as a quencher

of synthesized spherical GNPs, nanorods, nanotriangular, and nanotrigonal bipyramids. Figure 2a,b represents spherical- and triangular-shaped GNPs of size range 2−4 and 60−65 nm, respectively. Gold nanorods of size (longitudinal length) in the range of 20−22 nm are given in Figure 2c. Figure 2d represents trigonal bipyramidal-shaped GNPs. 3.2. Interaction of ADCL with synthesized GNPs. 3.2.1. Steady-State Absorption and Emission Spectroscopic Studies. The absorption profile of ADCL in MeOH and ACN shows two bands centered at λabs ∼320 and ∼400 nm, characteristic absorption maxima of the lactim and lactam forms of ADCL, respectively.18 In the presence of GNPs (both isotropic and anisotropic) in methanolic and acetonitrilic media, no change in the absorption profile of ADCL is observed (figure not given). The emission profile of ADCL in MeOH and ACN is characterized by a large Stokes’ shifted, broad band at λem ≈ 490 nm upon excitation at λex ≈ 320 nm, which is attributed to the phototautomer (lactam form) of ADCL because of the operation of ESIPT phenomenon (lactim* → lactam*).18 In the presence of various shapes of GNPs (spherical, triangle, rod, and TBP) in the MeOH medium, the fluorescence intensity of ADCL is found to be quenched without any shifting of emission maxima (Figure 3). Similar results were

F0 = 1 + KSV[Q ] (4) F where F0 and F are the steady-state fluorescence intensity in the absence and presence of quencher, respectively, and [Q] is the concentration of the quencher, that is, GNPs. The calculated Stern−Volmer quenching constants from Figure 4

Figure 4. Stern−Volmer plots for fluorescence quenching of ADCL in the presence GNPs of varying shape.

Figure 3. Steady-state fluorescence (λex = 320 nm) quenching profiles of ADCL (6 μM) in the presence of (a) spherical where (i) → (xii) [GNP] = 0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, and 30 μM, (b) triangular where (i) → (xi) [GNP] = 0, 3, 5, 8, 11, 13, 15, 18, 21, 24, and 27 μM, (c) rod where (i) → (xi) [GNP] = 0, 2, 4, 6, 8,10, 12, 15, 17, 20, 22, and 25 μM, and (d) TBP where (i) → (xi) [GNP] = 0, 2, 4, 6, 8,10, 12, 15, 17, 20, 22, and 25 μM shaped GNPs in the MeOH medium.

in the presence of spherical-, triangular-, rod-, and TBP-shaped GNPs are 22.95, 14.62, 89.27, and 51.56 μM−1, respectively at 303 K. The nature of quenching mechanism, that is, static or dynamic, of ADCL with anisotropic GNPs can be confirmly obtained from the time-resolved fluorescence study25 that will be discussed in the following section. One question may arise that whether the modification of the fluorescence intensity of ADCL in the presence of GNP is due to the GNP surface-stabilizing agent, CTAB (for anisotropic) and citrate ion (isotropic) or the gold nanosurface itself. In our previous work,32 we have studied the modulation of lactim− lactam tautomeric process of an isoindole-fused imidazole system, namely, 1-(2-hydroxy-5-methyl-phenyl)-3,5-dioxo-1Himidazo-[3,4-b] isoindole (ADII) in three different micellar assemblies, SDS, CTAB, and TX 100. We have observed that upon interaction with CTAB in aqueous medium, the lactim emission at ∼430 nm of ADII dramatically increases, whereas the lactam emission quenches up to a certain concentration of

also obtained in the ACN medium (figure not given). Generally, quenching of emission intensity of any probe upon interaction with the metal nanoparticles is a result of nanometal surface energy-transfer (NSET) mechanism from molecular donor to metal nanomaterial (acceptor).26−31 Basically, NSET is a fluorescence resonance energy-transfer (FRET) type process which originates from the interaction of the electromagnetic field of the donor dipole with the conduction band’s delocalized electrons of the acceptor.26−31 The fundamental difference is that unlike FRET, NSET does not require resonant electronic transitions.26−31 FRET is used as a powerful optical tool for the determination of the distances D

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the ESIPT time constant is found to be quite faster (∼0.307 ns) than that obtained in ACN (∼0.401 ns) which is probably due to the involvement of the molecule in hydrogen-bonding interaction with the solvent molecules (MeOH). Figures 5 and 6 present the time-resolved decay profile of ADCL (at λex = 340 nm and λem = 490 nm) in the presence of

CTAB (1.0 mM) of CTAB. Basically, ADII and ADCL (our present system) are structurally similar to a little dissimilarity in the phenolic unit; therefore, similar type of results as obtained in the case of ADII is also observed in the case of ADCL molecule. However, in our present work, we found no such increase of lactim emission of ADCL (vide Figure 3) in the presence of anisotropic GNPs. On the other hand, the emission profile of ADCL is practically unchanged in the presence of tri sodium citrate (figure not given). Therefore, we can conclude that the coated CTAB molecule on the surface of anisotropic GNPs and the citrate anion of isotropic GNPs do not affect the ESIPT operation of ADCL. 3.2.2. Time-Resolved Fluorescence Study. Time-resolved fluorescence study is believed to be the most efficient technique to understand various excited-state processes.25 When the molecule ADCL in ACN was excited at 340 nm and λem is monitored at 385 nm (lactim emission), the resulting decays are well fitted by a sum of triexponential decay components, that is, 0.315 ns (τ1, 59%), 6.53 ns (τ2, 12%), and 1.82 ns (τ3, 29%). On the other hand, when lactam emission maxima at 490 nm (T* or lactam emission) have been monitored by 340 nm excitation, the tautomeric emission consists of an initial fast growth features of time constant 0.401 ns (29%), followed by a decay component of 6.99 ns (71%) (Table 1).18 It is found that the fast decay component (τ1) of

Figure 5. Typical time-resolved fluorescence decay profile of ADCL in the presence of different shaped GNPs in MeOH medium at λex = 340 nm and λmon = 490 nm.

Table 1. Fluorescence Lifetime of ADCL in the Presence of Different Shaped GNPs

ADCL (ACN) ADCL (MeOH) ADCL (ACN) ADCL (MeOH) spherical (ACN) spherical (MeOH) rod (ACN)

λex (nm)

λem (nm)

τ1 (ns) (α1)

τ2 (ns) (α2)

τ3 (ns) (α3)

340

385

0.315 (0.59) 0.303 (0.23) 0.401 (−0.29) 0.307 (−0.25) 0.393 (−0.29) 0.316 (−0.26) 0.316 (−0.23) 0.203 (−0.26) 0.251 (−0.27) 0.221 (−0.20) 0.194 (−0.22)

6.53 (0.12) 6.84 (0.03) 6.99 (0.71) 6.51 (0.75) 7.01 (0.71) 5.93 (0.74) 7.02 (0.77) 5.67 (0.74) 7.10 (0.73) 6.36 (0.80) 5.30 (0.78)

1.82 (0.29) 1.49 (0.74)

340

340

340

490

490

490

rod (MeOH) TBP (ACN) TBP (MeOH) triangular (MeOH)

340

340

490

490

χ2 1.06 1.05 1.02 1.07 0.98

Figure 6. Typical time-resolved fluorescence decay profile of ADCL in the presence of different shaped GNPs in ACN medium at λex = 340 nm and λmon = 490 nm.

1.04 0.98 1.2

GNPs of different shapes in MeOH and ACN media, respectively, and the corresponding data are summarized in Table 1. The time-resolved data clearly show significant modification of ESIPT time constant of ADCL molecule in the presence of anisotropic GNP (rod, triangular, and TBP) in both organic media. In the presence of triangular (only in methanol)-, rod-, and TBP-shaped GNP, the ESIPT time constant (τPT) of the molecule decreases but the τPT value is practically unchanged in the presence of spherical nanoparticles in both organic media. The decreasing time constant of the ESIPT reaction is a clear indication of the faster PT dynamics of the molecule. In a theoretical study, Freitas et al. suggested that the adsorption of a molecule (here, ADCL) on the surface of any abiogenic minerals (here, gold nanosurface) is governed by the weak van der Waals interaction between the adsorbed molecule and surface atom of the minerals which induces some changes in the electronic structure of the adsorbate.17 Considering the weak van der Waals interaction and SDPT mechanism, they obtained a high reduction (of

1.02 1.06 1.15

the normal emission is identical, within experimental error, to the rise time of the tautomer emission, demonstrating that ESIPT dynamics must connect these two species. Therefore, the growth component appeared in the lactam emission corresponds to the time constant of the ESIPT (τPT) reaction of ADCL in ACN and the long-lived component (τ2) is the excited-state lifetime of the lactam form. We have also assigned the other two short-lived components obtained in the lactim emission, τ1 and τ3, to the excited state lifetime of the closed and open forms of the lactim form, respectively.18 Similar result was also obtained in the case of ADCL in MeOH, but E

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The Journal of Physical Chemistry C around 31%) in the activation energy barrier of the GC → G*C* tautomeric process which actually triggers the occurrence of a spontaneous point mutation during DNA replication.17 Taking into account the above-mentioned fact, we can conclude that in our case also, the high activation energy barrier of the lactim* → lactam* tautomeric process through four-member intramolecular hydrogen-bonding unit of ADCL also decreases upon adsorption on the anisotropic gold surface by weak van der Waals interaction which results the faster ESIPT dynamics of the probe. On the other hand, in the presence of spherical-shaped GNP in both organic media, no change in fluorescence decays of ADCL is observed and from the steady-state fluorescence study, we found a quenching of emission maxima. Similar results were also observed by Kyrychenko et al. during their study of interaction between 3hydroxychromone (an ESIPT probe) with thiol-coated spherical GNP.19 As in the earlier section, we have mentioned about the possibility of NSET mechanism between ADCL and spherical GNP but from the fluorescence decays of ADCL in the presence of GNP, no change in the lifetime of the molecule is found out. This results completely negate the possibility of NSET mechanism between these two pair as NSET is an excited-state process. Therefore, we can conclude that the interaction between ADCL and spherical GNP is of ground state type, that is, a static quenching mechanism is operative between them. The nature of quenching mechanism of ADCL fluorescence in the presence of different anisotropic GNP can also be stated from the data tabulated in Table 1. As seen in Table 1, the individual time constant of ADCL (specially ESIPT time constant) is found to be modified in the presence of triangular-, rod-, and TBP-shaped GNPs that suggest the dynamic nature of quenching mechanism among ADCL and other anisotropic GNPs. 3.2.3. Time-Resolved Fluorescence Anisotropy Study. Time-resolved fluorescence anisotropy measurements are widely used to probe the molecular rotational motion that can lead to depolarization of emission, following photoselection by polarized excitation.25 This technique is widely applied for the investigation of adsorption of a dye to nanoparticles, polymer matrix, biological macromolecules, and so forth.33,34 The anisotropy decays of ADCL in MeOH and in the presence of different shaped GNPs are given in Figure 7. Similar kind of results is also obtained in the ACN medium (the data not shown here). In the pure MeOH medium (absence of GNPs), the fluorescence anisotropy signal of ADCL decays rapidly to zero, as expected for a simple (spherical) emitting rotor in bulk homogeneous solution. A single exponential fit of this decay gives a rotational correlation time of ∼510 ps which is typical of the fast depolarizing rotation of free ADCL in bulk solution. Similar single exponential fit of the time-resolved anisotropy decay of ADCL of rotational correlation time ∼520 ps was obtained in the presence of spherical-shaped GNP, as shown in Figure 7i. However, the situation is quite different in the presence of anisotropic GNPs (triangular-, rod-, and TBP-shaped) (Figure 7ii−iv). In each cases, the r(t) profiles attain a minimum; thereafter, over a timescale of few nanoseconds, the fluorescence anisotropy again increases with time. This type of complex time-resolved anisotropy was previously found in the study of interaction of pyrene with a polymer poly(Nisopropylacrylamide).35 In our case also, this complex timeresolved fluorescence anisotropy of ADCL in the presence of different anisotropic GNPs (triangular, rod, and TBP) can only

Figure 7. Typical time-resolved fluorescence anisotropy decay profile of ADCL in the presence of (i) spherical-, (ii) triangular-, (iii) rod-, and (iv) TBP-shaped GNPs in the MeOH medium at λex = 340 nm and λmon = 490 nm.

be explained by considering the fact that the probe (ADCL) must experience two distinct environments; one is free in bulk solution and other is adsorbed to the GNPs. When the molecule ADCL is located in the bulk solution, rapid rotational mobility happens that corresponds to the fact that r(t) profile attains a minimum. On the other hand, when ADCL molecules are adsorbed on the GNPs, slower rotational mobility leads to the regaining of the fluorescence anisotropy with time. So, the time resolved anisotropy study suggest that a fraction of ADCL molecule is bound on the anisotropic gold nanosurface that results in the reduction of the high activation energy barrier of Lactim* → Lactam* process of the molecule. So this result supports our previous findings that upon adsorption on the anisotropic GNPs the ESIPT time constant of ADCL molecule becomes faster. However, in the presence of spherical-shaped GNPs, the interaction of ADCL with GNPs is of ground state type (as found from the time-resolved fluorescence decay study); therefore, the as-obtained single-exponential timeresolved fluorescence anisotropy decay is due to the free ADCL in the bulk ACN or MeOH medium.

4. CONCLUSIONS The present study deals with the interaction of an biologically important fluorescent molecule ADCL with GNPs of different shapes (spherical, triangular, rod, and TBP) in ACN and MeOH media by means of TEM, steady-state absorption, and fluorescence (both steady-state and time-resolved) spectroscopic techniques. From the steady-state absorption study of ADCL in the presence of different shaped GNPs, no such significant results are obtained that can help us to find out the interaction process. In the steady-state fluorescence study, the large Stokes shifted emission band of ADCL due to the PT process is found to be quenched in the presence of GNPs of different shapes. Time-resolved fluorescence data reveal that the quenching between ADCL with spherical-shaped GNPs is of static type, whereas that with other anisotropic GNPs is dynamic in nature. Time-resolved fluorescence study also reveals the faster PT rate of the molecule in the presence of anisotropic GNPs (triangular, rod, and TBP), whereas PT rate F

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Article

The Journal of Physical Chemistry C

(10) Lee, J.-H.; Gibson, K. J.; Chen, G.; Weizmann, Y. BipyramidTemplated Synthesis of Monodisperse Anisotropic Gold Nanocrystals. Nat. Commun. 2015, 6, 7571. (11) Chu, H.-C.; Kuo, C.-H.; Huang, M. H. Thermal Aqueous Solution Approach for the Synthesis of Triangular and Hexagonal Gold Nanoplates with Three Different Size Ranges. Inorg. Chem. 2006, 45, 808−813. (12) Li, N.; Zhao, P.; Astruc, D. Anisotropic Gold Nanoparticles: Synthesis, Properties, Applications, and Toxicity. Angew. Chem., Int. Ed. 2014, 53, 1756−1789. (13) Fazal, S.; Jayasree, A.; Sasidharan, S.; Koyakutty, M.; Nair, S. V.; Menon, D. Green Synthesis of Anisotropic Gold Nanoparticles for Photothermal Therapy of Cancer. ACS Appl. Mater. Interfaces 2014, 6, 8080−8089. (14) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Shape Control in Gold Nanoparticle Synthesis. Chem. Soc. Rev. 2008, 37, 1783−1791. (15) Vecchio, G.; Galeone, A.; Brunetti, V.; Maiorano, G.; Rizzello, L.; Sabella, S.; Cingolani, R.; Pompa, P. P. Mutagenic Effects of GoldNanoparticles Induce Aberrant Phenotypes in Drosophila melanogaster. Nanomed. Nanotechnol. Biol. Med. 2012, 8, 1−7. (16) Griffiths, A. J. F.; Wessler, S. R.; Lewontin, R. C.; Gelbart, W. M.; Suzuki, D. T.; Miller, J. H. An Introduction to Genetic Analysis, 8th ed.; W. H. Freeman: New York, 2000. (17) Freitas, R. R. Q.; Rivelino, R.; de B. Mota, F.; Gueorguiev, G. K.; de Castilho, C. M. C. Energy Barrier Reduction for the Double Proton-Transfer Reaction in Guanine-Cytosine DNA Base Pair on a Gold Surface. J. Phys. Chem. C 2015, 119, 15735−15741. (18) Ray, D.; Pramanik, A.; Guchhait, N. Slow Proton Transfer Dynamics of a Four Member Intramolecular Hydrogen Bonded Isoindole Fused Imidazole System: A Spectroscopic Approach to Photophysical Properties. J. Photochem. Photobiol., A 2015, 302, 42− 50. (19) Kyrychenko, A.; Karpushina, G. V.; Svechkarev, D.; Kolodezny, D.; Bogatyrenko, S. I.; Kryshtal, A. P.; Doroshenko, A. O. Fluorescence Probing of Thiol-Functionalized Gold Nanoparticles: Is Alkylthiol Coating of a Nanoparticle as Hydrophobic as Expected? J. Phys. Chem. C 2012, 116, 21059−21068. (20) Nag, T. N.; Das, T.; Mondal, S.; Maity, A.; Purkayastha, P. Promoting the ‘‘Water-Wire’’ Mechanism of Double Proton Transfer in [2,20-bipyridyl]-3,3’-diol by Porous Gold Nanoparticles. Phys. Chem. Chem. Phys. 2015, 17, 6572−6576. (21) Boiani, M.; Gonzalez, M. Imidazole and Benzimidazole Derivatives as Chemotherapeutic Agents. Mini-Rev. Med. Chem. 2005, 5, 409−424. (22) Das, S.; Fröhlich, R.; Pramanik, A. Synthesis and Fluorescent Properties of a New Class of Heterocycles of Isoindole Fused Imidazoles with Phenolic Subunits. Org. Lett. 2006, 8, 4263−4266. (23) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Determination of Size and Concentration of Gold Nanoparticles from UV-Vis Spectra. Anal. Chem. 2007, 79, 4215−4221. (24) Orendorff, C. J.; Murphy, C. J. Quantitation of Metal Content in the Silver- Assisted Growth of Gold Nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (25) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1999. (26) Rakshit, S.; Moulik, S. P.; Bhattacharya, S. C. Understanding the Effect of Size and Shape of Gold Nanomaterials on Nanometal Surface Energy Transfer. J. Colloid Interface Sci. 2017, 491, 349−357. (27) Mandal, S.; Ghatak, C.; Rao, V. G.; Ghosh, S.; Sarkar, N. Pluronic Micellar Aggregates Loaded with Gold Nanoparticles (AuNPs) and Fluorescent Dyes: A Study of Controlled Nanometal Surface Energy Transfer. J. Phys. Chem. C 2012, 116, 5585−5597. (28) Rakshit, S.; Moulik, S. P.; Bhattacharya, S. C. Deciphering the Role of the Length of the Corona in Controlled NSET within Triblock Copolymers. J. Phys. Chem. B 2015, 119, 8457−8467. (29) Lakowicz, J. R. Radiative Decay Engineering 5: MetalEnhanced Fluorescence and Plasmon Emission. Anal. Biochem. 2005, 337, 171−194.

does not alter in the presence of spherical GNPs. The adsorption of ADCL on the anisotropic GNPs is evidenced from the time-resolved fluorescence anisotropic study. Upon adsorption of ADCL on the anisotropic GNPs, the PT rate becomes faster because of the probable reduction in high activation energy barrier of the lactim* → lactam* process upon interaction (van der Waals’ type) with the gold nanosurface. As mutagenicity and tautomerism (lactim−lactam and amine−imine) of DNA base pair are correlated with each other therefore we can infer that our work is an indirect evidence of the occurrence of a spontaneous mutation by GNPs during DNA replication.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], scbhattacharyya@chemistry. jdvu.ac.in. Phone: +91-33-2350-8386. Fax: +91-33-2351-9755 (S.C.B.). *E-mail: [email protected] (N.G.). ORCID

Nikhil Guchhait: 0000-0001-7576-6930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.R. acknowledges University Grant Commission, India, for D. S. Kothari Post Doctoral fellowship. A.B. would like to acknowledge CSIR, India, for senior research fellowship. N.G. would like to acknowledge DST, India (project no. EMR/ 2016/004788) for financial support.

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REFERENCES

(1) Dykman, L. A.; Khlebtsov, N. G. Gold Nanoparticles in Biology and Medicine: Recent Advances and Prospects. Acta Nat. 2011, 3, 34−55. (2) Yeh, Y.-C.; Creran, B.; Rotello, V. M. Gold Nanoparticles: Preparation, Properties, and Applications in Bionanotechnology. Nanoscale 2012, 4, 1871−1880. (3) Dykman, L. A.; Bogatyrev, V. A.; Shchegolev, S. Y.; Khle-btsov, N. G. Zolotye Nanochastitsy: Sintez, Svoystva, Bio-meditsinskoe Primenenie (Gold Nanoparticles: Synthesis, Properties, and Biomedical Applications); Nauka: Moscow, 2008; p 319. (4) Boisselier, E.; Astruc, D. Gold Nanoparticles in Nanomedicine: Preparations, Imaging, Diagnostics, Therapies and Toxicity. Chem. Soc. Rev. 2009, 38, 1759−1782. (5) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Surface Plasmon Resonance Scattering and Absorption of anti-EGFR Antibody Conjugated Gold Nanoparticles in Cancer Diagnostics: Applications in Oral Cancer. Nano Lett. 2005, 5, 829−834. (6) Sperling, R. A.; Gil, P. R.; Zhang, F.; Zanella, M.; Parak, W. J. Biological Applications of Gold Nanoparticles. Chem. Soc. Rev. 2008, 37, 1896−1908. (7) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. J. Phys. Chem. B 2006, 110, 7238−7248. (8) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (9) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. G

DOI: 10.1021/acs.jpcc.8b05481 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (30) Lakowicz, J. R. Radiative Decay Engineering: Biophysical and Biomedical Applications. Anal. Biochem. 2001, 298, 1−24. (31) Persson, B. N. J.; Lang, N. D. Electron-Hole-Pair Quenching of Excited States Near a Metal. Phys. Rev. B: Condens. Matter Mater. Phys. 1982, 26, 5409−5415. (32) Ray, D.; Pramanik, A.; Guchhait, N. Differential Modulation of Lactim−Llactam Tautomerism Process of an Isoindole Fused Imidazole System in Three Different Micellar Assemblies of Varying Surface Charge: A Spectroscopic Approach to Various Photophysical Properties. RSC Adv. 2014, 4, 13256−13265. (33) Smith, T. A.; Ghiggino, K. P. A Review of the Analysis of Complex Time-Resolved Fluorescence Anisotropy Data, Methods. Appl. Fluoresc. 2015, 3, 022001. (34) Smith, T. A.; Irwanto, M.; Haines, D. J.; Ghiggino, K. P.; Millar, D. P. Time-Resolved Fluorescence Anisotropy Measurements of the Adsorption of Rhodamine-B and a Labelled Polyelectrolyte onto Colloidal Silica. Colloid Polym. Sci. 1998, 276, 1032−1037. (35) Chee, C. K.; Ghiggino, K. P.; Smith, T. A.; Rimmer, S.; Soutar, I.; Swanson, L. Time-Resolved Fluorescence Studies of the Interactions between the Thermoresponsive Polymer Host, Poly(Nisopropylacrylamide) and a Hydrophobic Guest, Pyrene. Polymer 2001, 42, 2235−2240.

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DOI: 10.1021/acs.jpcc.8b05481 J. Phys. Chem. C XXXX, XXX, XXX−XXX