Assessing Interparticle J-Aggregation of Two Different Cyanine Dyes

Nov 30, 2015 - Department of Chemistry, State University of New York at Binghamton, Binghamton, New York13902, United States. J. Phys. Chem. C , 2015 ...
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Assessing Interparticle J‑Aggregation of Two Different Cyanine Dyes with Gold Nanoparticles and Their Spectroscopic Characteristics Han-Wen Cheng,*,†,‡ Stephanie I. Lim,‡ Weiqin Fang,‡ Hong Yan,‡,§ Zakiya Skeete,‡ Quang Minh Ngo,‡,∥ Jin Luo,‡ and Chuan-Jian Zhong*,‡ †

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China Department of Chemistry, State University of New York at Binghamton, Binghamton, New York13902, United States

‡

S Supporting Information *

ABSTRACT: While nanoparticle plasmonic coupling is known to be responsible for the enhancement of the local electrical field that determines optical or spectroscopic properties, understanding of the structural details of the interparticle interactions remains elusive. This report describes findings of an investigation of plasmonic coupling of gold nanoparticles via J-aggregation of cyanine dyes to define the interparticle interaction. The adsorption of two cyanine dye molecules with subtle differences in structures on gold nanoparticles of different sizes and their resulting interparticle π−π interactions, or Jaggregation, were examined to assess the interparticle-interactioninduced changes of spectroscopic properties, surface plasmon resonance absorption, and surface-enhanced Raman scattering. The results demonstrate that these two spectroscopic properties work in concert with plasmonic coupling in the kinetic process, as evidenced by the comparable apparent rate constants determined in terms of nanoparticle dimerization or aggregative growth in the solution. This finding is further substantiated by examining the effects of the dye structure, the particle size, and the solution pH on the spectroscopic characteristics. The experimental data are supported by theoretical simulation of the spectroscopic properties in terms of the plasmonic resonance absorption and the electrical field enhancement in terms of nanoparticle dimer models, which have implications in the better design of interparticle structures to harness the nanoscale interparticle molecular π−π interactions for a wide range of technological applications.

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INTRODUCTION Cyanine dyes are one important class of dyes for many applications that take advantage of their optical and spectroscopic properties, including photochemical, photoluminescent, electroluminescent, and nonlinear optical properties.1,2 These characteristics stem largely from the highly conjugated π-structure of cyanine dyes, which is responsible for various supramolecular or aggregate phenomena, especially Jor H-aggregation. The noncovalent molecular π-stacking interaction and arrangement inside an aggregate determine its physical and chemical properties, which constitutes the basis of cyanine dyes being explored as molecular building blocks in well-defined architectures or functional devices. For Jaggregation, the interactions between the neighboring dipoles in terms of stacking geometries and molecular ordering leads to the absorption spectrum shifts. This type of intermolecular interactions functions as a molecular glue.3 It can also function as a probe of protein−ligand interactions, resulting an aggregation-related spectral singling. Indeed, there are examples showing the exploitation of the J-band and fluorescence for noncovalent protein labeling and sensing.4 As a result of the surge of interests in metal nanoparticles and interparticle interactions, the J-aggregate interaction of cyanine dyes was © XXXX American Chemical Society

explored in the presence of gold nanoparticles, as shown in our earlier work, which demonstrated interesting changes in optical and fluorescent properties for both the π−π interaction of indolenine cyanine dyes and the surface plasmon resonance of gold nanoparticles.5 There have been increasing examples showing molecular self-assembly on nanoparticles via π−π interaction of different conjugated molecules,6 π−π stacking and solvophobic interactions of perylene diimides in a J- or Htype configuration,7,8 and J-aggregation of a thiacyanine dye.9 The formation of J-aggregation could occur on the surface nanoparticles9−12 or in between nanoparticles,5,6 so what controls the specific π−π stacking mode remains elusive. Also, while plasmonic coupling among nanoparticles is known to be responsible for electrical field enhancement, how the interparticle molecular interactions determine the optical or spectroscopic properties in real time is not well understood. Answering these questions is fundamental for technological exploration of the unique optical and functional properties of this intriguing class of nanostructures. Received: August 23, 2015 Revised: November 12, 2015

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DOI: 10.1021/acs.jpcc.5b09973 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (Top) UV−vis spectra showing spectral characteristics of molecular electronic band (left panel) and nanoparticle SP band (right panel) as a result of the formation of J-aggregation via π−π stacking of Icds and Tlcs adsorbed on Au NPs (middle panel). (a, b) π−π* Bands and J-band of the dyes before and after addition of Au12nm into the solution of Icd. (a′, b′) Change of SP band of Au nanoparticles before and after addition of Icd into the solution of Au12nm. (c, d) π−π* Bands and J-band of the dyes before and after addition of Au12nm into the solution of Tlc. (c′, d′) Change of SP band of Au nanoparticles before and after addition of Tlc into the solution of Au12 nm. Note that the above spectral data are for illustrating the spectral characteristics, and details are described in the corresponding data sets presented in this report. (Bottom) Molecular stacking structures of Tlc (left panel) and Icd (right panel) molecules.

the plasmonic coupling in nanoparticle solutions. In this report, we describe findings of an investigation of the adsorption of two cyanine dye molecules with subtle differences in structures on gold nanoparticles of different sizes and their resulting interparticle π−π interactions, or J-aggregation, focusing on assessing the interparticle-interaction-induced changes of spectroscopic properties, surface plasmon resonance absorption, and surface-enhanced Raman scattering. This understanding has implications for the design of dye-structured nanoprobes for optical and spectroscopic applications.

Nanoparticle plasmonic coupling is known to be responsible for the enhancement of the local electrical field, which determines the optical or spectroscopic properties, but understanding of the structural details of the interparticle interactions remains elusive. This understanding is particularly useful for exploring surface-enhanced Raman scattering (SERS) detection of dye-containing nanoparticle systems, e.g., dyeembedded core−shell nanoparticles.13,14 The adsorption of indocyanine green dye on colloidal silver and gold was exploited for fluorescence detection in living cells15,16 and multiplexed detection of oligonucleotides with Raman labels.17 The local electric field enhancement (i.e., “hot spot”) for a dimer of NPs and its dependence on interparticle spacing have been demonstrated using the discrete dipole approximation method;18 SERS has been utilized for detecting DNA binding via complementary base pairs and enzyme cutting of ds-DNA at a specific site,19 probing DNAs,20,21 and monitoring interactions of small molecules with gold or silver nanoparticles.22,23 While there have been experimental and theoretical studies of particle size effect on SERS of Ag and Au nanoparticle aggregates,24 little is known on how the SERS effect is correlated with plasmonic coupling via interparticle π−π interactions in solutions in relation to particle size. This understanding requires the ability to delineate the relationship among the interparticle structure and particle sizes under the condition of controllable π−π interaction and nanoparticle assembly. In our recent preliminary work,25 both surface plasmon resonance absorption and surface-enhanced Raman scattering were found to work in concert with plasmonic coupling in the process of π−π interaction and nanoparticle assembly. The kinetic correlation between the two spectroscopic signatures indicated an effective pathway for harnessing

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EXPERIMENTAL SECTION Chemicals. The chemicals included hydrogen tetracholoroaurate (HAuCl4, 99%), sodium citrate (Cit, 99%), DLhomocysteine (Hcys 95%), and ethanol (EtOH, 99.9%). All chemicals were purchased from Sigma-Aldrich and used as received. Water was purified with a Millipore Milli-Q water system. Cyanine dyes examined in this work include 1,1′dibutyl-3,3,3′,3′-tetramethylindocarbocyanine iodide (Tlc) (C31H41IN2, purity ≥98.0%) and 3-butyl-2-[(1E,3E)-3-(3butyl-1,1-dimethyl-1,3-dihydro-2H-benzo[e]indol-2-ylidene)-1propen-1-yl]-1,1-dimethyl-1H-benzo[e]indolium iodide (Icd) (C39H45IN2, purity ≥96.0%), which were obtained from Changzhou Dongnan Pengcheng Chemicals Co., Ltd. Synthesis of Gold Nanoparticles. Citrate-/acrylatecapped gold nanoparticles (Au NPs, Aunm) feature negatively charged surface species. Gold nanoparticles (Aunm) capped with citrate were synthesized according to a reported procedure.26,27 An aqueous solution of AuCl4− (1 mM) was refluxed under vigorous stirring in a round-bottom flask, in which an excess of sodium citrate was added. The resulting citrate-capped Aunm feature sizes of 10−15 nm. Gold nanoparticles with diameters B

DOI: 10.1021/acs.jpcc.5b09973 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

dyes, indicating the formation of J-aggregates by the π−π stacking of Icds adsorbed on Au NPs, which leads to a consequent interparticle plasmonic coupling. For the case of Au12nm in the presence of Tlc (c′), the absorbance or broadening of the band in the longer wavelength region (580−800 nm) (d′) indicates also a strong plasmonic coupling. The spectrum of Tlc exhibits two absorption bands at λ= 523 and 664 nm due to π−π* transitions (c). There is a dip in the spectrum at 590 nm, which is attributed to the J-band due to interband or intraband interactions. The red-shift SP characteristic is due to the formation of J-aggregates via π−π stacking of Tlc adsorbed on different Au NPs, which leads to a consequent interparticle plasmonic coupling. In the above schemes, the interaction between the positively charged dye molecules and the negatively charged nanoparticle surface is electrostatic. The interparticle interaction involves π−π stacking of the dye molecules adsorbed on the nanoparticle surfaces, which leads to a J-band due to interband or intraband interactions of the electrons in the dye molecule,5 and interparticle plasmonic coupling, which leads to a shift of the surface plasmon resonance band of gold nanoparticles. In general, electrostatic effect could have played an important role in driving the aggregation. The aggregation process started from an initial adsorption of dyes on the particles by electrostatic force, which is followed by J-aggregation of the adsorbed dyes. In an earlier report,5 evidence was obtained through experiments using different positively charged molecules that have a strong ion-pairing effect with negatively charged groups and that are also relatively hydrophobic and showed that the electrostatic interaction is not the only factor contributing to the driving force because these molecules did not show any spectral evolution similar to the dyes described in this report. As such, a combination of electrostatic interaction and J-aggregation is likely operative for the interparticle aggregation. It is important to emphasize that the absorption spectra of J-aggregates are due to dyes actually adsorbed onto the Au NPs, which is evidenced by control experiments of the dye molecules in the solution without Au NPs. Note that there are several pieces of evidence supporting that J-aggregation is operative in the aggregation of Au NPs, including our earlier work on the observation of a J-band and fluorescence quenching5 and others’ work.26,27 While ζ-potential measurement could provide information for assessing the electrostatic effect on the aggregation, the explanation of the data would not be straightforward due to concurrent surface adsorption of cationic dyes and interparticle aggregation. Also, as shown in our earlier work,5 TEM does not provide a clear insight either because the propensity of aggregation during the drying process prevented us from assessing the different aggregation stages in the solution. In this regard, dynamic light scattering could be useful for looking at the different stages of aggregation in the solution, as shown in our previous work for similar interparticle aggregations.30,31 Nevertheless, the assessment of the optical and SERS characteristics in the interparticle aggregation process is the focus of the present report. In Figure 2, SERS spectra for Icd (a) and Tlc (b) in the presence of gold nanoparticles (12 nm) in aqueous solutions are compared. The SERS effect is evident since the Raman bands were not detectable for both dyes in the absence of the gold nanoparticles. The bands are largely similar, characteristic of the similar basic structures for the two dye molecules. However, there are subtle differences in some bands and the relative positions of bands which are expected due to the

larger than 25 nm were prepared using a previously reported method.26,27 Gold nanoparticle seeds were first formed, which was then followed by seeded growth. Sodium acrylates were used as reducing agent and capping agent. By controlling concentrations of the reducing and capping agents and reaction time, larger sized Au nanoparticles (up to ∼100 nm) were prepared. Measurements and Instrumentation. The stock concentration of gold nanoparticles was determined according to the absorbance data and the average size of the particles. The molar absorptivity (εAu) for 12 nm sized Aunm particles determined at the surface plasmon resonance band maximum (λ = 520 nm) was 2.01 × 108 M−1 cm−1.26 For the Icds, the value of εdye(Icd) determined at λ = 550 nm was 1.15 × 105 M−1 cm−1. The dye-mediated assembly of Aunm was carried out under ambient conditions. Briefly, upon adding a quantitative amount of dye solution (in ethanol) to the Aunm, the solution was mixed for ∼5 s before measurements using different instruments were taken. UV−visible (UV−vis) spectra were acquired with a HP 8453 spectrophotometer and the data were collected over the range of 200−1100 nm. A quartz cuvette with a path length of 1.0 cm was utilized. SERS spectra were acquired in the range of 200 to 3400 cm−1 using an Advantage 200A Raman spectrometer (DeltaNu). The laser power and wavelength were 5 mW and 632.8 nm, respectively. The spectral resolution of the system was about 10 cm−1. Peak fitting of the SERS spectra was performed using PeakFit 4.12 Demo software. Structural optimization of dye molecules was performed using ChemBio3D Ultra software. The simulation of the surface plasmon resonance band and the electrical field was performed using MNPBEM toolbox,28,29 which involved setting up the nanoparticles’ boundary conditions and the dielectric environment to calculate the optical absorption and the electrical field around the nanoparticles.

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RESULTS AND DISCUSSION General Characteristics of Interparticle J-Aggregation. Two cyanide dyes, Icd and Tlc, both with positive charges, are used as a model system for the study of the nanoparticle−dye interaction. The J-aggregation of these cyanide dyes is characterized by a red-shifted J-band, which features either a sharp peak or a dip9,10 in the spectrum as a result of interband or intraband interactions. The spectroscopic characteristic depends on the specific structure of the dye molecules and the nanoparticle size and composition.9,10 Figure 1 presents a representative set of UV−vis spectra to show the spectral characteristics of molecular electronic band and interparticle plasmonic coupling band as a result of the formation of J-aggregate-linked nanoparticles via the π−π stacking of Icds and Tlcs adsorbed on Au NPs. For a solution of Aunm (a′), the surface plasmon (SP) band peaks at 520 nm. For the case of Au12nm in the presence of Icd, the spectrum shows absorbance or broadening of the band in the longer wavelength region (640−800 nm) (b′), which produces a strong coupling of the surface plasmon resonance of Au NPs with the laser excitation wavelength. The spectrum of Icd exhibits two absorption bands at λ= 545 and 582 nm due to π−π* transitions (a). The appearance of the longer-wavelength band at λ ∼ 630 nm (b) and the red-shift characteristic are clearly associated with the formation of J-aggregates from the adsorbed C

DOI: 10.1021/acs.jpcc.5b09973 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (Left) Typical SERS spectra for Icd (a) and Tlc (b) in the presence of Au NPs. (Right) Schemes showing the Icd and Tlc structures on the nanoparticle surface.

structural differences of the two dye molecules. Note that the above comparisons are only qualitative, providing general spectral characteristics; a more meaningful comparison would be to compare the normal Raman spectrum to the calculated Raman spectrum, which is somewhat difficult due to the strong fluorescence of these dye molecules under the laser wavelength of 633 nm in this work. The use of longer wavelength lasers will be needed in this regard to minimize the fluorescence impact. It is the quenching of the fluorescence of the molecule upon adsorption on Au NPs and subsequent J-aggregation that enable the detection of the Raman spectrum. Note that it is rather difficult to obtain the Raman spectrum of the neat dye due to the strong fluorescence of the molecules in the wavelength range of the laser (632 nm). The theoretical surface coverage of the molecules on Au NPs is based on the 3D molecular modeling of Icd or Tlc molecules in a densely packed monolayer on the surface of the nanoparticle. The number of Icd or Tlc molecules on the surface of gold particles was estimated in three models of dense packing of the molecules in a rectangular shape [Figure S1, Supporting Information (SI)] on the surface Au NP, which corresponds to the coverage of a full monolayer on a single Au NP. It is evident that the number of the dye molecules per NP for a full monolayer increases with the particle size. Different packing modes (e.g., flat on, long side on, or short side on) leads to a different number of dye molecules per particle, with the flat orientation for dye adsorbing on the surface exhibiting the lowest number of dyes molecules per NP. As will be shown in the results, the relative surface coverages (θT) correspond to the formation of either multilayer or submonolayer adsorption. To understand how the interparticle “hot spot” formation is responsible for the observed SERS, correlations of the SERS intensity dependence on these interparticle structural parameters and properties are expected in terms of the spectral intensity vs the dye concentration and particle size for Au NPs under submonolayer/monolayer/above-monolayer coverages. Findings on each of these correlations are discussed in the following subsections. Kinetic Assessment. A representative set of UV−vis spectra obtained for the case of Tlc−Au assemblies is shown in Figure 3A as a function of time for an aqueous solution of 34 nm Au upon addition of Tlc. For the Tlc−Au34nm assemblies, upon addition of Tlcs to the solution, new bands emerge at wavelengths longer than the SP band of Au34nm. A band emerges at 600−760 nm accompanying the decrease of the 521

Figure 3. (A) UV−vis spectra monitoring the Tlc-mediated assembly of Au34nm in solutions from 1 to 12 000 s. [Au34nm] = 8.9 × 10−2 nM, [Tlc] = 2.4 × 10−2 μM. The inset shows the kinetic curve for the longer-wavelength peak (integrated peak area from 600 to 1000 nm); the dashed line represents curve fitting by the Avrami model [y = A1(1 n

− e−kt )] and yields k = 2.6 × 10−5 s−1 and n = 1.3. (B) SERS spectral evolution in the Tlc-mediated assembly of Au34nm ([Au34nm] = 8.9 × 10−2 nM, [Tlc] = 2.4 × 10−2 μM) in an aqueous solution (wavenumber) as a function of time from 1 to 6000 s (from a to e) (integration time: 20 s). The inset shows the peak intensity (at 1589 cm−1) as a function of time (s). The dashed line represents curve n

fitting by the Avrami model [y = A2(1 − e−kt )] and yields k = 3.2 × 10−5 s−1 and n = 1.3.

nm band. This feature is characteristic of the SP band of Au NPs, which shifts as a result of interparticle interaction by π−π stacking of Tlcs adsorbed on the NPs surface, forming small aggregates (e.g., dimers, trimers, etc.). The spectral evolution exhibits a clear isosbestic point at ∼560 nm. This feature is indicative of the presence of two species, i.e., unlinked and Tlclinked Au NPs, at equilibrium. The interparticle aggregation is thus evidenced by the gradual increase for the SP band at the longer wavelength (Figure 3A, inset) accompanied by the decrease of the SP band at the short wavelength (Figure S2, SI). The above SP spectral evolution is also accompanied by a clear SERS spectral evolution, as evidenced in Raman spectra in Figure 3B, characteristic of the Raman bands of Tlc.32 Note that the Raman bands were not detectable in the absence of Au NPs due to the lack of the SERS effect. The data reveal an intriguing similarity for the simultaneous spectral evolutions for both SP bands and SERS bands. The SP are analyzed by considering a first-order reaction, with the rate for the apparent D

DOI: 10.1021/acs.jpcc.5b09973 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (A) SERS spectra for Icd−Au12nm assembly in an aqueous solution of Au12nm upon addition of Icd in different concentrations: [Icd] = 0.03, 0.07, 0.10, 0.13, 0.17, 0.20, 0.23, and 0.27 μM and [Au12nm] = 2.5 nM. (B) Plots of the peak intensity vs concentration of Icd [linear regression slope: 3.65 (R2 = 0.98)].

ensure that the coverage is below a full monolayer. For the Icd−Au12nm assemblies (Figure S3A, SI), there is a gradual decrease in SERS intensity. The decrease of the SERS intensity is believed to reflect the fast assembly rate, which leads to graduate precipitation. In this case, the SERS intensity of the Icd-mediated assembly of Au12nm gradually decreases with time and eventually levels off after 1 h. Note that the Icd solution in the absence of Aunm shows no detectable peaks in the solution. In contrast, for the Icd−Au52 nm assemblies (Figure S3B, SI), the SERS intensity is basically independent of the time. There was no observable precipitation of the assemblies, as observed in the Icd−Au12nm assembly case, indicative not only of a slower assembly rate but also likely smaller aggregates (e.g., dimers or trimers) that can be suspended in the aqueous solution. In order to ensure that the maximum intensity of the assembly is recorded, most of the SERS spectra discussed in this report are taken within the 1 h time frame. This type of kinetics is different from that found using fluorescence spectroscopy. As previously reported,5 the fluorescence quenching of Icd occurs almost immediately (