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Layer-by-layer self-assembly of plexcitonic 

nanoparticles 

Brendan G. DeLacy, 1 ' Wenjun Qiu 2 , Marin Soljacic , 2 Chia Wei Hsu , 2 Owen D. Miller , 3 
Steven G. Johnson , 3 and John D. Joannopoulos 2 

1 U.S. Army Edgewood Chemical Biological Center, Research & Technology Directorate 5183 Blackhawk Rd.. 
Aberdeen Proving Ground, MD 21010-5424, USA 

2 Department of Physics, Massachusetts Institute of Technology 77 Massachusetts Ave, Cambridge, MA 02139, USA 
2 Department ofMathematics, Massachusetts Institute of Technology 77 Massachusetts Ave, Cambridge, MA 02139, 

USA 

* brendan.g.delacy.civ@mail.mil 


Abstract: Colloidal suspensions of multilayer nanoparticles composed of a 
silver core, a polyelectrolyte spacer layer (inner shell), and a J-aggregate 
cyanine dye outer shell have been prepared for the first time. Absorption 
properties of the colloid were measured in the visible region. This 
multilayer architecture served as a framework for examining the coupling 
of the localized surface plasmon resonance exhibited by the silver core with 
the molecular exciton exhibited by the J-aggregate outer shell. The 
polyelectrolyte spacer layer promotes the formation of an excitonic J- 
aggregate while serving as a means of controlling the plasmon-exciton (i.e. 
plexciton) coupling strength through changing the distance between the 
core and the shell. An analytical expression based on Mie Theory and the 
Transfer Matrix Method was obtained for describing the optical response of 
these multilayered nanostructures. Computational and experimental results 
indicate that the absorption wavelength of the J-aggregate form of the dye is 
dependent on both the distance of the dye layer from the silver core and the 
degree of dye aggregation. 

©2013 Optical Society of America 

OCIS codes: (160.4236) Nanomaterials; (290.5825) Scattering theory; (290.2200) Extinction. 


References and links 

1. M. Kerker, The Scattering of Light and Other Electromagnetic Radiation (Academic Press, 1969). 

2. S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, 2010). 

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supramolecular engineering of functional dye materials,” Angew. Chem. Int. Ed. Engl. 50(15), 3376-3410 
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#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19103 



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Layer-by-layer self-assembly of plexcitonic nanoparticles 


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14. ABSTRACT 

Colloidal suspensions of multilayer nanoparticles composed of a silver core, a polyelectrolyte spacer layer 
(inner shell), and a J-aggregate cyanine dye outer shell have been prepared for the first time. Absorption 
properties of the colloid were measured in the visible region. This multilayer architecture served as a 
framework for examining the coupling of the localized surface plasmon resonance exhibited by the silver 
core with the molecular exciton exhibited by the J-aggregate outer shell. The polyelectrolyte spacer layer 
promotes the formation of an excitonic Jaggregate while serving as a means of controlling the 
plasmon-exciton (i.e. plexciton) coupling strength through changing the distance between the core and the 
shell. An analytical expression based on Mie Theory and the Transfer Matrix Method was obtained for 
describing the optical response of these multilayered nanostructures. Computational and experimental 
results indicate that the absorption wavelength of the J-aggregate form of the dye is dependent on both the 
distance of the dye layer from the silver core and the degree of dye aggregation. 

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13. G. A. Wurtz, P. R. Evans, W. Hendren, R. Atkinson, W. Dickson, R. J. Pollard, A. V. Zayats, W. Harrison, and 
C. Bower, “Molecular plasmonics with tunable exciton-plasmon coupling strength in J-aggregate hybridized Au 
nanorod assemblies,” Nano Lett. 7(5), 1297-1303 (2007). 

14. N. Kometani, M. Tsubonishi, T. Fujita, K. Asami, and Y. Yonezawa, “Preparation and optical absorption spectra 
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Langmuir 17(3), 578-580 (2001). 

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Plasmon-exciton coupling in nanoshell-j-aggregate complexes,” Nano Lett. 8(10), 3481-3487 (2008). 

16. A. Yoshida, Y. Yonezawa, and N. Kometani, “Tuning of the spectroscopic properties of composite nanoparticles 
by the insertion of a spacer layer: effect of exciton-plasmon coupling,” Langmuir 25(12), 6683-6689 (2009). 

17. P. C. Lee and D. Meisel, “Adsorption and surface-enhanced Raman dyes on silver and gold sols,” J. Phys. Chem. 
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Citrate a “Magic” Reagent?” J. Am. Chem. Soc. 133(46), 18931-18939 (2011). 

20. N. R. Jana, L. Gearheart, and C. J. Murphy, “Wet chemical synthesis of silver nanorods and nanowires of 
controllable aspect ratio,” Chem. Commun. (Camb.) 0(7), 617-618 (2001). 

21. G. B. Sukhorukov, E. Donath, H. Lichtenfel, E. Knippel, M. Knippel, A. Budde, and H. Mohwald, “Layer-by- 
layer self assembly of polyelectrolytes on colloidal particles,” Coll, and Surf. A: Phys. and Eng. Aspects 137(1 - 
3), 253-266 (1998). 

22. G. B. Sukhorukov, E. Donath, S. Davis, H. Lichtenfeld, F. Caruso, V. I. Popov, and H. Mohwald, “Stepwise 
polyelectrolyte assembly on particle surfaces: a novel approach to colloid design,” Polym. Adv. Technol. 9(10- 
11), 759-767 (1998). 

23. W. Qiu, B. G. DeLacy, S. G. Johnson, J. D. Joannopoulos, and M. Soljacic, “Optimization of broadband optical 
response of multilayer nanospheres,” Opt. Express 20(16), 18494-18504 (2012). 

24. H. C. van de Hulst, Light Scattering by Small Particles, (Dover, 1981). 

25. C. Bohren and D. Huffman, Absorption and Scattering of Light by Small Particles, (John Wiley & Sons, 1983). 

26. R. E. Hamam, A. Karalis, J. D. Joannopoulos, and M. Soljacic, “Coupled-mode theory for general free-space 
resonant scattering of waves,” Phys. Rev. A 75(5), 053801 (2007). 

27. Z. Ruan and S. Fan, “Superscattering of light from subwavelength nanostructures,” Phys. Rev. Lett. 105(1), 
013901 (2010). 

28. Z. Ruan and S. Fan, “Temporal coupled-mode theory for fano resonance in light scattering by a single obstacle,” 
J. Phys. Chem. C 114(16), 7324-7329 (2010). 

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( 2011 ). 

30. T. Kobayashi, J-Aggregates, (World Scientific Publishing Co. Pte. Ltd., 1996). 

31. A. Dixon, C. Duncan, and H. Samha, “Self assembly of cyanine dye on clay nanoparticles,” Am. J. Undergrad. 
Res. 3, 29-34 (2005). 

32. H. Kuhn and H. D. Foresterling, Principles of Physical Chemistry (Wiley, 2000). 

33. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1985). 

34. Y. Zong, F. Xu, X. Su, and W. Knoll, “Quartz crystal microbalance with integrated surface plasmon grating 
coupler,” Anal. Chem. 80(13), 5246-5250 (2008). 


1. Introduction 

Silver and gold nanoparticles have been studied extensively for their unique optical properties 
in the UV, visible, and infrared regions of the electromagnetic spectrum [1-4]. These 
properties arise from the collective oscillation of conduction band electrons throughout the 
particle in response to optical excitation, a process commonly termed as localized surface 
plasmon resonance. Resonance occurs when the frequency of incident radiation is at or near 
the frequency of the electronic oscillation. This resonance results in a strong enhancement of 
the local electric field, useful for surface-enhanced Raman scattering (SERS), surface- 
enhanced fluorescence, and the study of nonlinear optical response [5-7]. Additionally, the 
size and morphology of the silver and gold nanoparticles have a significant impact on surface 
plasmon resonance frequency and therefore have a great significance in such fields as sensors 
and photonic devices. 

Cyanine dyes are commonly used in spectral sensitization, and have potential application 
in novel optoelectronic materials [8]. Structurally, these dyes consist of two heterocyclic units 
that are connected by an odd number of methine groups (CH) n (n = 1,3,5, etc.). The color of 
the dye is mainly determined by the length of the polymethine chain [9]. A particularly 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OS A 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19104 



intriguing property of cyanine dyes is their tendency to aggregate under certain conditions in 
solution. These so called J-aggregates exhibit a narrow absorption band that is red shifted 
with respect to the monomer absorption band [9]. The shift in absorption of the aggregate has 
been described by a Frenkel exciton model, in which excited states are formed by the 
coherent coupling of molecular transition dipoles [10,11]. 

Multi-layered nanoparticles, composed of both a noble metal and a J-aggregate dye, 
provide a unique framework for studying plasmon-exciton interactions. Numerous structures 
exhibiting these plasmon-exciton interactions have been fabricated and studied in recent 
years. Some nanostructures have involved the direct adsorption of J-aggregate dyes onto the 
surface of silver nanoparticles with varying geometries [12-14]. Other efforts have focused 
on the aggregation of cyanine dyes onto complex geometries. For example, cyanine dyes 
were adsorbed onto silica core/gold shell nanoparticles [15]. In these studies, it was observed 
that the absorption spectra of these composite nanoparticles were not the simple sum of the 
absorption of the metal nanoparticle and the J-band of the aggregate. Rather, strong plasmon- 
exciton interactions (plexcitons) were observed, resulting in a shift in the absorption bands of 
the individual plasmon and exciton resonances. A more recent study by Yoshida et al. 
explored the impact of inserting a spacer layer (inner shell) between a metallic core particle 
(Au) and an outer J-aggregate dye layer (outer shell) [16]. The spacer layer used in the study 
was a cationic thiol which promoted the J-aggregation of anionic cyanine dyes onto the 
surface of the nanocomposite. The spectral line shape of these composites was dependent on 
the strength of the plasmon-exciton coupling between the metal core and the J-aggregate 
shell. 

The metallic core composition and size, excitonic shell composition, degree of J- 
aggregate formation within the excitonic shell, and distance between the plasmonic core and 
excitonic shell are among the factors that play the most significant roles in controlling optical 
response. Metallic core particles and excitonic dyes of varying composition are readily 
available and selection is based on the intrinsic optical properties of the material. In terms of 
particle shape, a range of metallic core shapes such as spheres, rods, and platelets may be 
fabricated using well documented solution-based techniques [17-20]. However, spacer layer 
thickness and its impact on plasmon-exciton coupling in these nanoparticle systems have not 
been studied systematically. The spacer layer must serve the two-fold purpose of promoting 
the formation of a J-aggregate shell while also creating a distance, with specified length, 
between the metallic core and excitonic shell. To that end, we report the use of a variably 
thick polyelectrolyte spacer layer as a means of building multilayered plexcitonic 
nanoparticles and controlling plasmon-exciton distance. Specifically, we report the use of 
alternating cationic poly(diallyldimethyl ammonium chloride) (PDADMAC) and anionic 
poly(styrene sulfonate) (PSS) as a spacer layer between a silver core and a J-aggregate outer 
shell. The layer-by-layer self-assembly of oppositely charged polyelectrolytes has been used 
extensively as a means of coating and functionalizing colloidal particles [21,22]. The 
thickness of the polyelectrolyte spacer layer was controlled in this study by varying the 
number of PDADMAC/PSS layers that were adsorbed. The outer J-aggregate shell was 
formed by adsorbing the cationic form of PIC onto the outermost anionic PSS layer. A 
schematic of the double-shell structure is provided in Fig. 1(a). Figure 1(b) provides a 
schematic of the electrostatic adsorption of 1,1 ’-diethyl-2,2-cyanine (PIC) J-aggregates at the 
surface of a PDADMAC/PSS coated silver nanoparticle. The use of a polyelectrolyte spacer 
layer may be incorporated into a wide range of core-shell systems, regardless of geometry and 
composition. Therefore, this approach may be used as a means of studying plasmon-exciton 
coupling in a wide range of multilayered plexcitonic structures. 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OS A 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19105 




Fig. 1. Schematic of (A) a double-shell structure composed of a silver metallic core, a spacer 
layer consisting of alternating layers PDADMAC/PSS polyelectrolytes, and a 1,1 ’-diethyl- 
2,2’-cyanine iodide (PIC) exterior shell. The outer radius and dielectric function of individual 
layers are (R i? 8j, i = 1,2,..., n). The dielectric function of the medium is 8 m . (B) The 
electrostatic adsorption of (PIC) J-aggregates onto a silver core/polyelectrolyte spacer. Na + , Cl' 
, and I' ions are not drawn for clarity. Figures are not drawn to scale. 


In order to explain the measured absorption features of the Ag/polyelectrolyte/PIC 
composite nanoparticles, Mie Theory and the Transfer Matrix Method were used to calculate 
the optical response of these multilayered nanospheres. Numerical simulations yielded good 
agreement with experimental results. Specifically, the impact of polyelectrolyte spacer layer 
thickness on the shift of the exciton frequency was elucidated. These results indicate that a 
polyelectrolyte spacer layer is a plausible means for tuning plasmon-exciton coupling in 
colloidal suspensions of multilayered plexcitonic nanoparticles. 


2. Experimental methods 

Silver nitrate, trisodium citrate tribasic dihydrate, sodium poly(styrene sulfonate) (PSS, MW 
-70,000), poly(diallyldimethyl ammonium chloride) (PD AD MAC, medium molecular weight 
26062-79-3), sodium phosphate monobasic dihydrate, sodium phosphate dibasic dihydrate, 
and l,l’-diethyl-2,2’-cyanine iodide were purchased from Sigma Aldrich (St. Louis, MO). 

Silver nanoparticles were prepared using a modified procedure by Lee and Meisel in 
which Ag + ions are chemically reduced in aqueous solution [17]. Approximately 110 mg of 
AgN0 3 were dissolved in 400 mL deionized H 2 0 (18MQ) and placed in a 1L 3-neck flask. A 
condenser tube (250 mm jacket length) was placed on the center neck of the flask to minimize 
evaporation losses. An addition funnel, containing 10 mL of 1% (wt/wt) trisodium citrate 
solution, was attached to a second neck of the flask. The solution was magnetically stirred 
and heated to 100°C using a heating jacket and temperature controller (Glass-Col, Terre 
Haute, IN) while refluxing. Once the boiling point was reached, the trisodium citrate solution 
was slowly added drop-wise. The mixture was heated and stirred for an additional hour at 
100°C. The solution was then cooled to room temperature and stirring continued for an 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19106 





additional hour. Assuming a 100% yield, the silver colloid had an approximate concentration 
of 0.3 mg/mL. 

A standard solution of l,l-diethyl-2,2’cyanine iodide (PIC) was prepared by dissolving 25 
mg PIC in 100 mL phosphate buffer (pH = 6.0). 

Silver core - polyelectrolyte shell nanoparticles were prepared by exposing the silver sol 
to supersaturated concentrations of polyelectrolytes. First, 10 mL Ag sol were transferred to a 
50 mL polypropylene centrifuge tube. The sample was centrifuged at 4000 RPM for 15 
minutes and the supernatant containing excess citrate was removed. 5 mL H 2 0 were then 
added to the tube followed by sonication. Next, 5 mL PDADMAC (1 mg/mL, 0.01 M NaCl) 
were added, and the suspension was vigorously mixed for 1 minute. The solution was allowed 
to stand for 30 minutes. Centrifugation (4000 RPM, 15 minutes) was used to remove the 
supernatant and excess polymer. Two cycles of a 10 mL water addition, followed by 
sonication, centrifugation, and supernatant removal were performed to ensure the removal of 
excess polymer. The same solution concentration and procedures used for the adsorption of 
PDADMAC were used for adsorption of additional polyelectrolyte layers (i.e. PSS, 1 mg/mL, 
0.01 M NaCl). Combinations of 2, 4, and 6 polyelectrolyte layers, i.e. (PDADMAC/PSS)i 2,3, 
were adsorbed onto the silver particles. The Ag core - polyelectrolyte shell nanoparticles 
were resuspended in 5 mL water with sonication. 

For the in situ generation of a J-aggregate shell on the silver core - polyelectrolyte shell 
nanoparticles, the concentration of the Ag core - polyelectrolyte shell nanoparticle suspension 
was adjusted until absorbance measurements yielded an approximate absorbance of 0.5 to 1.5 
absorbance units. This involved diluting 1 mL of the suspension to 25 mL with water. 3 mL 
of the diluted suspension were then placed in a 10 mm optical path quartz cuvette. 100 pL 
aliquots of PIC standard (0.25 mg/mL) were added to the cuvette, followed by mixing and 
immediate absorbance measurement. 

Ag/polyelectrolyte/PIC composite nanoparticles were also prepared by exposing Ag core - 
polyelectrolyte shell nanoparticles to PIC for 24 hours. This involved placing 5 mL Ag core - 
polyelectrolyte shell nanoparticles in a 50 mL polypropylene centrifuge tube, follow by the 
addition of 5 mL of PIC standard. The samples sat for 24 hours to ensure adequate adsorption 
and aggregation of the dye onto the surface of the particles. The following day, the colloidal 
suspensions were centrifuged at 4000 RPM for 15 minutes, and the supernatant discarded for 
removal of excess PIC standard. The solid nanoparticles were washed with 10 mL water 
followed by sonication, centrifugation, and supernatant removal to ensure the removal of 
excess PIC. The particles were then resuspended in 5 mL water and sonicated. Finally, 
excessively large NPs were removed using a 0.22 pm Millex GP syringe filter. This 
procedure was repeated for each of the Ag/polyelectrolyte/PIC NP combinations. 

All UV/Vis absorption spectra were measured in a quartz cuvette (10 mm optical path 
length) using a V-670 spectrophotometer (JASCO Co.). Spectra were measured from 190 nm 
to 800 nm. Transmission electron microscopy (TEM) images were observed in a FEI 
(Hillsboro, OR) Tecnai 12 TWIN TEM operating at lOOkV. Samples were prepared by 
placing 5.0 pL of the particle solution onto a freshly ionized 300 copper mesh carbon 
coated/formvar grid. The samples were allowed to dry completely. Images were collected 
using an Olympus Soft Imaging System (Lakeland, CO) Megaview III digital camera. Figures 
were assembled in Adobe Photoshop using only linear adjustments in brightness and contrast. 
For the determination of zeta-potential, a Zetasizer Nano ZS (Malvern Instruments) was used 
to measure the electrophoretic mobilities of colloidal particles. The mobility p was converted 
into a (^-potential using the Smoluchowski relation (£ = pq/s, where q is the viscosity of the 
solvent and £ is the permittivity of the solvent). The hydrodynamic radius was also 
determined using the Zetasizer Nano ZS instrument. This measurement is a dynamic light 
scattering technique. 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OS A 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19107 



3. Theoretical methods 

In order to calculate the optical response of the Ag/polyelectrolyte/PIC nanocomposites, we 
generalize Mie theory to multilayer nanospheres via the Transfer Matrix Method [23-25]. For 
multilayer nanospheres, the electric field can be decomposed into orthogonal channels labeled 
by where a takes TE or TM, and / = 1, 2, 3,... [26-29]. The scattering and absorption 
cross-sections are the sum of contributions from each channel, and the extinction cross- 
section is the sum of scattering and absorption cross-sections: 

(1) 

<7 /=1 ®7l 

°sca= ZZ4l( 2/ + 1 )| 1-r ^f +<7 abs ( 2 ) 

a 1=1 

In order to determine r ah we decompose the electric field inside each shell into incoming 
and outgoing spherical waves with coefficients (A„, B„). The coefficients of adjacent shells are 
connected by the transfer matrix of that interface, whose elements are determined by the 
boundary condition of TE or TM modes: 


xr 

= M l+ , , 

~ A~ 

A,_ 

l+l,l 

A. 


The transfer matrix of the whole system can be calculated by combining the transfer matrices 
of individual interfaces: 


IX*, 1 



[A. 1 


\A, 1 

An, 1. 

= M n+l ,M nn _ l 

~M 3a ,M 2A 

A. 

= M 

A. 


(4) 


Since the second kind of spherical Bessel function is singular at the origin, we can set Aj = B t 
= 1. Therefore, 


_ Bn+\ _ 3 T 21 + M22 
A n+l M u + M n 


(5) 


Incorporation of the appropriate dielectric constant for the aggregated form of PIC is required 
for the total cross-section calculations. We use a model that describes the red shifted energy 
of aggregate forms of cyanine dyes, in which [30]: 


e = e„—2V cos 


n 


N +1 


( 6 ) 


Here V is the nearest neighbor coupling energy, N c is the number of coherently coupled 
molecules that make up the aggregate, and e 0 is the frequency dependent dielectric constant of 
the monomer. Using this model for PIC, a red-shifted J-aggregate absorption peak at 575 nm 
is expected for aggregates composed of 14 or more dye molecules. This prediction is 
consistent with numerous experimental observations in which aggregated forms of PIC, both 
in solution and adsorbed onto surfaces, exhibit absorption at 575 nm [16,31]. Hence, 575 nm 
was taken as the excitation wavelength of the J-aggregate for the complex frequency- 
dependent dielectric function of the PIC J-aggregate, which was approximated using: 


£(co) = £ 0 + 


M 2 


co 0 2 - or -ico 0 yco 


(7) 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OS A 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19108 



Here, s 0 = s me dmm — 1-85, co 0 corresponds to the excitation frequency of the J-aggregate 
(corresponding to 575 nm), y is the relaxation rate, and / represents the oscillator strength. 
Literature values for y range from 0.01 to 0.02, and for/range from 0.01 to 1 [32]. 

4. Results and discussion 


The citrate reduction method used in this study yielded silver core nanoparticles with an 
average particle size of 54.5 nm and a standard deviation of 9.8 nm. Figure 2(a) displays a 
TEM image of the silver nanoparticles. Most of the particles have a spherical shape with a 
relatively narrow size distribution, although a small amount of non-spherical particles are 
observed. Figure 2(b) displays the absorbance spectra of the silver colloid (blue curve) and 
the PIC standard (red curve). The silver colloid and PIC standard solutions are characterized 
by an absorption L max at 442 nm and 523 nm, respectively. Of particular note is the absence of 
the PIC J-aggregate peak, which is commonly observed at or near 575 nm. This observation is 
consistent with published data, as the concentration of the PIC standard in this solution is too 
low for the formation of the J-aggregate [16,31]. 



Fig. 2. (A) TEM image of silver nanoparticles fabricated using the citrate reduction method. 

(B) Experimental absorbance spectra of solutions containing 0.022 mg/mL silver sol (blue 
curve) and 0.023 mg/mL PIC (red curve). Jtmax = 442 nm for the silver sol. A^ax = 523 nm for 
the PIC solution. (C) Zeta-potential as a function of layer number for silver particles coated 
with alternating layers of PDADMAC and PSS. Polyelectrolyte deposition of PDADMAC and 
PSS was performed in 0.01M NaCl. 

Insertion of a polyelectrolyte layer between the Ag core and the cyanine dye outer shell 
was a central theme in this study. Specifically, our goal was to demonstrate the formation of 
the PIC J-aggregate on a silver core - polyelectrolyte shell composite nanoparticle. The layer- 
by-layer self assembly of polyelectrolytes on colloidal particles is an established technique for 
building stable and functionalized multilayers on nanoparticles. The irreversible electrostatic 
interaction between cationic and anionic polyelectrolyte layers aid in stabilizing these films. 
In our study, the sequential addition of PDADMAC and PSS, with PSS forming the outermost 
polyelectrolyte layer, was used to control the thickness of the polyelectrolyte layer and to 
render the outer surface of the particles with a negative charge. The negative surface charge 
promotes the adsorption and J-aggregation of the cationic PIC dye onto the surface of the 
particles. Figure 2(c) provides a summary of the ^-potential measurements of Ag particles 
coated with sequential layers of PDADMAC and PSS. The ^-potential alternates between 
positive and negative values for the sequential additions of PDADMAC and PSS, 
respectively. 

Silver core - polyelectrolyte shell nanoparticles were exposed to PIC in a series of 
experiments. First, UV/Visible absorbance measurements of Ag core - polyelectrolyte shell 
colloids (0.013 mg/mL) were taken immediately after the addition of 100 pL increments of 
PIC standard, in order to determine if the PIC J-aggregate would form in situ. All in situ 
experiments were performed on Ag particles coated with one PDADMAC/PSS bi-layer. 
Figure 3 displays the absorbance spectra of these particles. As a control, the absorbance 
spectrum of Ag/PDADMAC/PSS was determined prior to the addition of PIC. The 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19109 







absorbance spectrum of Ag/PDADMAC/PSS is characterized by a /, max at 435 nm; compared 
to the absorbance spectra of the silver sol displayed in Fig. 2, it has a somewhat broader 
absorbance from 600 nm to 700 nm, possibly due to the slight agglomeration of silver core- 
polyelectrolyte shell nanoparticles. As PIC standard was incrementally added, a sharp peak 
near 570 nm is observed, indicative of J-aggregate fonnation. We note that the concentration 
of the PIC standard is too low to form J-aggregates in solution; therefore the observed J- 
aggregation of PIC must be promoted by the presence of the polyelectrolyte shell. J-aggregate 
peaks at 570 nm, 571 nm, and 571 nm were observed after the sequential addition of 100 pL, 
200 pL, and 300 pL of PIC standard, respectively. Typically, when fully aggregated, the J- 
aggregate of PIC is characterized by an absorbance at 575 nm. Based on the theoretical model 
of the J-aggregate absorbance wavelength, we conclude that the PIC shell in this experiment 
is not in a fully aggregated form. 



Fig. 3. Experimental absorbance spectra immediately following the sequential addition of PIC 
standard to a 3 mL colloidal suspension containing silver core-PDADMAC/PSS nanoparticles 
(0.013 mg/mL). 100 pL portions of a 0.23 mg/mL PIC standard solution were added to 3 mL 
of the suspension. The formation of the red-shifted J-aggregate peak was observed at 570 nm, 

571 nm, and 571 nm upon addition of 100 pL, 200 pL, and 300 pL quantities of the PIC 
standard, respectively. For comparison, the absorbance spectra of the silver core/polyectrolyte 
nanoparticles and PIC standard (0.023 mg/mL) are provided. 

Next, we studied the impact of spacer layer thickness. This was achieved by adsorbing 
PIC to Ag/polyelectrolyte composite nanoparticles composed of 1 PDADMAC/PSS bi-layer, 
2 PDADMAC/PSS bi-layers, and 3 PDADMAC/PSS bi-layers. In this set of experiments, 
formation of a fully aggregated PIC shell was promoted by exposing Ag/polyelectrolyte 
composite nanoparticle suspensions to PIC dye for 24 hours. A TEM image of the 
silver/polyelectrolyte/PIC nanoparticles is provided in Fig. 4(a). Figure 4(c) also displays the 
absorbance spectra of these experiments. The spectra is characterized by the formation of a 
red-shifted J-aggregate peak at 581 nm, 581 nm, and 579 nm for Ag particles coated with 1 
(PDADMAC/PSS), 2 (PDADMAC/PSS), and 3 (PDADMAC/PSS) shells, respectively. 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19110 




570 


580 590 

Wavelength (nni) 


580 590 

Wavelength (nm) 


600 



B 


— N = 1 
— N = 2 
—N = 3 


600 


0.9 

0.8 

0.7 

0.6 

j=0.5 

0.4 

0.3 

0.2 


c 


— N = I 
—N = 2 
—N = 3 



0.1 

0 


570 


Fig. 4. (A) TEM image of silver/polyelectrolyte soaked in PIC for 24 hours. (B) Experimental 
absorbance spectra of PIC coated Ag core-polyelectrolyte shell nanoparticles, with varying 
polyelectrolyte layer thickness. 1 (PDADMAC/PSS) (blue curve), 2 (PDADMAC/PSS) (red 
curve), and 3 (PDADMAC/PSS) (green curve) correspond to 2, 4, and 6 alternating layers of 
PDADMAC and PSS, respectively. Formation of the red-shifted J-aggregate peak was 
observed at 581 nm, 581 nm, and 579 nm for the 
Ag/1 (PDADMAC/PSS)/PIC, Ag/2(PDADMAC/PSS)/PIC, and Ag/3(PDADMAC/PSS)/PIC 
nanoparticles, respectively. (C) Simulated absorbance spectra for the impact of spacer layer 
thickness on the absorption wavelength of the J-aggregate peak. An oscillator strength if) of 
0.35 and a relaxation rate (y) of 0.015 were assumed in the calculations. 

The J-aggregate peaks were observed at wavelengths that were comparatively longer than 
what is commonly reported in the literature for PIC, i.e. 575 nm. Additionally, the J-aggregate 
peak wavelength was found to increase with decreasing spacer thickness. These results 
suggest that the PIC was fully, or close to fully aggregated, and that the plasmon-exciton 
interaction further red shifted the J-aggregate peak. This observation is consistent with the 
fact that plasmon-exciton interactions are enhanced as the distance between the J-aggregate 
shell and the Ag core particles is diminished. 

Simulated absorbance spectra of Ag/(PDADMAC/PSS)/PIC nanoparticle composites 
were calculated in Fig. 4(c) in order to explain the main absorption features of the 
experimental data. For the dielectric constants, we used experimental frequency-dependent 
complex dielectric constant for the silver [33], literature values of 2.25 for PDADMAC and 
2.56 for PSS [34], and Eq. (8) (with/= 0.35 and y = 0.015) for the PIC J-aggregate. We 
assume a silver core of 54.5 nm (with SD = 9.8 nm), a J-aggregate dye outer shell of 5 nm, 
and a polyelectrolyte spacer of thickness 1 nm per bi-layer. This calculation is consistent for 
similar structures, in which the thickness of the polyelectrolyte bi-layers was determined to be 
1.7 to 3.0 nm [21,34]. The simulation result in Fig. 4(c) was in qualitative agreement with the 
experimental absorbance spectra in Fig. 4 (b). Namely, the experimental and simulation 
results demonstrate the dependence of coupling strength and shift of the exciton frequency on 
spacer layer thickness. The red-shifted energy of the J-aggregate peak increases as the spacer 
layer thickness is minimized, i.e. the plasmon-exciton interaction is maximized. 
Discrepancies between the experimental and simulated results, in terms of J-aggregate peak 
shape, may be attributed to the fact that non-spherical and/or aggregated particles were not 
included in the simulated results. 

5. Conclusion 

Multilayered plexcitonic nanoparticles composed of an Ag core, polyelectrolyte spacer layer, 
and a PIC outer shell have been synthesized. The optical properties of the composite 
nanoparticles in aqueous solution were measured in the UV/Visible regions. Absorbance 
spectra of the colloidal nanoparticles indicate that PDADMAC/PSS spacer layers successfully 
promoted the J-aggregation of PIC. The in situ generation of the J-aggregate was observed 
when PIC standard was added directly to a colloidal suspension containing silver core - 
polyelectrolyte shell composite nanoparticles. The formation of the fully J-aggregated form 
was observed when PIC was exposed to a colloidal suspension of silver core - polyelectrolyte 
shell nanoparticles for 24 hours. The 24 hour study yielded a J-aggregate absorbance at 579- 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19111 



















581 nm, indicating the formation of a fully aggregated dye on the surface of the composites. 
The fully aggregated form of PIC is typically observed at 575nm. The additional red-shift in 
energy that was experimentally observed may be explained by the plasmon-exciton 
interaction between the silver core and the J-aggregate exterior. It is concluded that the use of 
oppositely charged polyelectrolytes is a valid approach for inducing the formation of J- 
aggregates on silver core-polyelectrolyte shell particles. Furthermore, polyelectrolyte spacer 
layers are a means of controlling the plasmon-exciton distance, which can ultimately be used 
to control the shift in exciton frequency. Finally, the ease with which polyelectrolyte layers 
are assembled onto colloidal nanoparticles of varying shape and composition, makes this 
technique suitable for building a wide range of plexcitonic particles. 

Acknowledgments 

This research was funded by the Department of the Army Basic Research Program and 
sponsored by the Edgewood Chemical Biological Center. Support was also provided by the 
U.S. Army Research Office under contract W911NF-13-D-0001. Special thanks to Dr. 
Michael McCaffery of the Johns Hopkins University Integrated Imaging Center for his 
assistance with the TEM images. 


#187530 - $15.00 USD Received 27 Mar 2013; accepted 18 Jun2013; published 5 Aug 2013 

(C) 2013 OS A 12 August 2013 | Vol. 21, No. 16 | DOI: 10.1364/OE.21.019103 | OPTICS EXPRESS 19112