Tuning the Optical Coupling between Molecular Dyes and Metal

Article pubs.acs.org/JPCC

Tuning the Optical Coupling between Molecular Dyes and Metal Nanoparticles by the Templated Silica Mineralization of J‑Aggregates Kargal L. Gurunatha and Erik Dujardin* NanoSciences Group, CEMES CNRS UPR 8011, B.P. 94347, 29 r. J. Marvig, 31055 Toulouse Cedex 4, France S Supporting Information *

ABSTRACT: Supramolecular porphyrin aggregates are used as a template for the higherorder assembly of fluorophore−dielectric−metal hybrid nanostructures in which the optical properties of the molecules are modulated by the finely tuned coupling to localized plasmons. First, J-aggregates are encapsulated inside a dielectric silica shell of wellcontrolled thickness, which reinforces mechanically the template and serves as a precise optical coupling spacer. The silicified J-aggregates are then decorated with gold or silver nanoparticles. UV−visible and fluorescence spectroscopies show that the presence of metal nanoparticles induces a marked enhancement of the J-aggregate fluorescence when the silica thickness is tuned to 7−12 nm, whereas a significant quenching is measured when the dielectric thickness is sub-2 nm. Interestingly, the enhancement is maximized when oxidized silver nanoparticles are placed very close to the J-aggregates.

1. INTRODUCTION The design, synthesis and structuring of hybrid multifunctional materials in order to promote new physical phenomena at the nanometer scale has received increasing attention in the past two decades. In particular, the integration of nanoparticles with functional molecules provides a versatile approach to the coupling of colloidal and molecular properties. The resulting multifunctional assembled architectures can display new synergetic electronic,1 magnetic,2 sensing,3,4 or optical5 properties. Molecular plasmonics is one of these very active fields in which the optical properties of noble metals have been coupled to those of molecules.6−11 Plasmons are collective oscillations of surface electrons, which are sensitive to their near-field environment. When the refractive index in the direct vicinity of the metal surface is modified, the spectral features of the plasmon modes are modified, which is now widely used in surface plasmon resonance (SPR) detection.12,13 The interplay between molecular and plasmonic optical properties has been extensively studied from two perspectives. The influence of the molecular optically active environment on a metal surface, for example, in the strong coupling regime between excitonic states and plasmon modes has been exploited to enhance the plasmonic wave guiding of metal films used in subwavelength optical devices.14−17 While this strong coupling regime leads to fluorescence quenching,18−21 a weaker coupling to metal surfaces can significantly enhance the emission rate of molecular fluorophores. This property has been investigated in numerous configurations of metallic films,22 patterned nanostructures,23−25 and colloids26−31 onto which the fluorophores are grafted. To result in the enhancement of the emission, the plasmon−exciton spacing has to be finely tuned © 2013 American Chemical Society

and is usually achieved by coating the metal surface with a dielectric or a sacrificial layer onto which the fluorophore is tethered.28,31 The overall modulation or enhancement of the quantum dot or molecular fluorescence was, therefore, successfully demonstrated in many occasions, but the standard metal-centric strategies do not provide any insight on the role played by the fluorophore/dielectric spacer interface on the fluorescence modulation. Here, we propose to use a reverse f luorophore-centric approach, where a finite number of fluorophores are self-assembled into rod-shaped supramolecular structures, the core of our hybrid architecture, upon which the dielectric spacer shell is formed by templated silica mineralization. The evolution of the fluorescence can thus be assessed upon encapsulation. Finally, metal nanoparticles are conjugated to the silica surface of the fluorophore−dielectric core−shell structure allowing us to monitor the sole effect of plasmondriven modulation of the fluorescence. meso-Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) has been known to form π-stacked, one-dimensional aggregates, classified as J-aggregates.32,33 Depending on the substituting moieties, the self-assembly can be induced by altering the ionic strength, concentration, and/or pH (Figure 1, step 1).32,34,35 The resulting J-aggregates are tapelike structures with a length varying between 0.1 and 6 μm, a height of 11 nm, and a width of 20 nm.33,36 As the self-assembly occurs, the molecular optical properties are markedly modified with the emergence of an excitonic state, the narrow absorbance band of which lies at 490 Received: December 4, 2012 Revised: January 21, 2013 Published: January 24, 2013 3489

dx.doi.org/10.1021/jp311911f | J. Phys. Chem. C 2013, 117, 3489−3496

The Journal of Physical Chemistry C

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of 10% v/v APTES-TEOS to the neutral solution of TPPS4 with vigorous stirring and then by inducing the assembly as well as triggering the silica condensation by acidification at pH 2. The volume of silica precursor solution was varied between 10 and 50 μL, by portions of 10 μL, in order to control the sheath thickness. 2.3. Au and Ag Nanoparticle Synthesis. Gold and silver colloids were prepared by standard chemical reduction methods.39−41 For gold nanoparticles, an aqueous solution of HAuCl4 (1 mM, 100 mL) was refluxed and vigorously stirred before adding rapidly 10 mL of a 38.8 mM trisodium citrate solution. A color change from pale yellow to deep red was observed. The solution was refluxed for an additional 15 min before it was allowed to cool to room temperature. For silver nanoparticles, 50 mL of 1 mM AgNO3 were similarly refluxed and vigorously stirred prior to the dropwise addition of 5 mL of a 34 mM trisodium citrate solution. The reaction was kept boiling until a color change to pale yellow was evident and then allowed to cool to room temperature with stirring. Au and Ag nanoparticle sizes were 12 ± 2 and 10−50 nm, respectively. 2.4. Structural and Optical Characterization. Transmission electron microscopy (TEM) was carried out on carbon-coated copper grids using a Philips CM20 microscope operated at 200 kV. Sample grids were prepared by dropcasting slightly diluted aliquots and were viewed unstained. Atomic force microscopy (AFM) was performed on a Bruker Dimension 3000 microscope operated in tapping mode in air. AFM samples were prepared by drop-casting solutions onto silica substrates with a nominal 1 nm rms roughness. Silica sheath thickness was obtained by subtracting the average value of the width (respectively, height) of uncoated aggregates from the width (respectively, height) of encapsulated aggregates measured from TEM (respectively, AFM) images. UV−vis absorption spectra were recorded in a 1 cm light path quartz cuvette on a Cary-5000 UV−vis-NIR spectrophotometer. Fluorescence emission and excitation spectra were recorded on a Hitachi F-4500 spectrophotometer.

Figure 1. Scheme of the three-step bottom-up synthesis of fluorophore−metallic conjugates with finely tuned coupling by templated growth of silica. (step 1) J-aggregation of TPPS porphyrins into a rod-shaped ensemble of well-defined morphology. (step 2) Templated silica encapsulation of the J-aggregates, which provides a mechanical reinforcement and a dielectric sheathing of controlled thickness. (step 3) Attachment of Au or Ag nanoparticles at a controlled distance from the fluorophore in order to enhance the intensity of the local electromagnetic field.

nm. The spectral proximity of the excitonic band and Au or Ag localized plasmon resonances has made TPPS aggregates a very promising system in molecular plasmonics.7,37,38 Here, we report on an original approach in which mineralization chemistry is used to encapsulate TPPS J-aggregates in a silica shell of controlled thickness in the 1−20 nm regime, relevant to optimal plasmon−exciton coupling (Figure 1, step 2). The silica coating ensures the mechanical robustness of the aggregate core, it provides a dielectric spacer for optimal exciton−plasmon coupling, and it serves as a template for the binding of Au or Ag nanoparticles onto the surface using electrostatic interaction (Figure 1, step 3).

3. RESULT AND DISCUSSION Our multilayer, hybrid architecture is based on the wellestablished supramolecular assembly of tetrakis(4-sulfonatophenyl)porphine molecules, TPPS, into rodlike aggregates upon acidification.42−45 Anionic tetrasulfonated TPPS molecules are highly soluble in neutral water; however, upon protonation at low pH, the partially neutralized aromatic molecules stack by π−π interactions and form elongated aggregates. The self-assembled aggregates consist of tapelike nanostructures of well-defined size and aspect ratio that can be imaged by transmission electron microscopy (TEM) and atomic force microscopy (AFM), as described in the Experimental Section. The individual superstructures sometimes further agglomerate into larger bundles, as shown in Figure 2A,B. The length of individual needles ranges from 500 nm to 5 μm, while their width is more uniformly distributed (15−20 nm). AFM measurements of J-aggregates deposited on silica substrates yielded a uniform thickness approximately of 11 ± 1 nm. The strong intermolecular coupling in the aggregates results in the emergence of an excitonic state, the optical properties of which differ markedly from the monomeric species.32,34,46 In particular, the light pink color of the starting monomer solution at pH 7 changes to a light green color at pH 4 and dark green at pH 2.39 More quantitatively, the UV−vis absorption spectra present a strong suppression of the Soret

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium salt of tetrakis(4-sulfonatophenyl) porphyrin (H2TPPS4), silver nitrate (AgNO3), sodium citrate (C6H5Na3O7), gold(III) chloride trihydrate (HAuCl4·3H2O), and aminopropyltriethoxy-silane (APTES) were obtained from Aldrich, and tetraethoxy-silane (TEOS) was purchased from CASTER. Chemicals were used as received. Ultrapure deionized (18 M·Ω) water was produced from a Veolia Purelab Classic system. All glassware was cleaned with aqua regia (HCl/ HNO3 in 3:1 ratio by volume) and rinsed with copious amounts of ethanol and ultrapure water. 2.2. Synthesis and Encapsulation of J-Aggregates. TPPS J-aggregates were prepared and encapsulated with a silica shell by adapting the method reported previously.36 Typically, 10 mL of a 50 μM (5 × 10−5 M) solution of TPPS4 in deionized water was acidified below pH 4 by addition of aliquots of 1 M hydrochloric acid. A gradual color change from pink to a green was observed as the J-aggregates formed. Jaggregates were encapsulated in silica by adding a small volume 3490



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fluorescence loss results both from the partial disassembly of the original J-aggregates observed in TEM and from nonradiative quenching of particle-bound aggregates due to the close proximity of the plasmonic material. Indeed, both theoretical calculations20 and experimental investigations28,29 have shown that the presence of metallic structures in the close vicinity of luminescent molecules or quantum dots can either suppress or enhance the emission for fluorophore−metal distances varying in the 1−20 nm range. To promote radiative emission, the optical coupling between the self-assembled chromophores and the plasmonic colloids must be finely tuned by adjusting their separation distance beyond the quenching regime. The insertion of a dielectric spacer, such as silica, deposited on the metallic surface has been successfully proposed.28,48,49 However, the quantification of this phenomenon rarely considers the alteration of the molecular fluorescence once adsorbed on the dielectric surface.50,51 In the following, we investigate a reverse, f luorophore-centric, approach, in which the TPPS J-aggregates are encapsulated in a silica shell of controlled thickness, and we monitor the evolution of the fluorescence upon encapsulation. Subsequently, the plasmon-driven modulation of the fluorescence is monitored upon tethering the metal nanoparticles to the silica shell. In an earlier work, we have shown that intact J-aggregates could be encapsulated in a silica shell by templating the condensation of a 10% v/v APTES/TEOS (see the Experimental Section).36 Although the general principles were established, no specific attention was paid to the fine control of the silica shell thickness. Here, we show that this approach can provide a nanometer precision for the silica sheath thickness in the range of 2−10 nm (Figure 2C,D), which is relevant for metal-enhanced fluorescence (MEF). The solution of hydrolyzed silica precursor is mixed with a neutral pH solution of TPPS4, followed by acidification to pH 2 with hydrochloric acid, which triggers simultaneously the porphyrin aggregation and the acid-catalyzed silica condensation. TEM and AFM analyses of silicified samples show the presence of nanoribbons with a morphology similar to that of pristine J-aggregates. At this low pH, silicic acid gels by the slow condensation of silanols. It appears that the finest thickness tuning is achieved by varying the amount of precursor rather than any other parameters, such as reaction time, temperature, or APTES/ TEOS volume ratio. For a typical 10 mL solution of 50 μM TPPS, the silica coating thickness could be adjusted to 2.2, 4.5, 6.7, 9.0, and 11.2 nm (±1 nm) by adding 10, 20, 30, 40, or 50 μL of APTES/TEOS solution, respectively. This strictly linear correlation is observed for both the width and the thickness data (Figure 3). The thickness is defined as the difference between the width (respectively, height) of the encapsulated Jaggregate with the width (respectively, height) of uncoated Jaggregates measured from TEM (respectively, AFM) images (see the Supporting Information, Figure S5). The thickness dispersion of the encapsulated aggregates is mostly accounted for by the original polydispersity of the J-aggregate template itself. The average length of the silica-coated aggregates appears to be shorter (ca. 200 nm to 1 μm), but more monodispersed than the uncoated ones; the smooth and linear increase of the silica coating thickness further indicates that the encapsulation of porphyrin nanotapes occurs without significant disruption to the supramolecular organization. The reliable templating effect of the J-aggregates is promoted by the electrostatic interactions between the anionic TPPS moieties and the cationic APTES

Figure 2. (A) AFM and (B) TEM images of TPPS J-aggregates showing an approximate size of 0.5−5 μm in length, 15−20 nm in width, and 10−12 nm in thickness. (C) AFM and (D) TEM images of silica-coated J-aggregates showing a narrower length distribution (200−1000 nm) and a slightly increased width and thickness.

(413 nm) and Q (516, 552, 580, and 633 nm) bands for monomeric [H2TPPS]4−, accompanied by the emergence of corresponding bands of the diacid, [H4TPPS]2−, at 434, and 592, and 645 nm, respectively (see the Supporting Information, Figure S1 and Table S2).46 Following a slower kinetics, which depends on the amount of acid added, self-aggregation of the [H4TPPS]2− anions produced a darker green solution within 20−30 min, which is observed in the UV−vis spectra by the red shifted conversion of the Soret band from 434 to 490 nm along with the emergence of a broad peak at 706 nm. This red shift of the main absorption band is characteristic of J-aggregates in equilibrium with the [H4TPPS]2− monomer. The direct coupling of metallic nanoparticles to the uncoated J-aggregates results in a complex reorganization of the fluorophores that left virtually no intact rods, as shown by TEM (Figure S3, Supporting Information), where the few observed rodlike objects are ill-shaped. This apparent disassembly systematically induces a net decrease of the emitted intensity at 720 nm (see the Supporting Information, Figure S4).19 Upon addition of the metallic nanoparticles, the intensity of the 434 nm Soret absorption band of the monomer decreases as the 490 nm peak increases, indicating a further depletion of the free TPPS in solution and a reinforcement of the J-aggregate assembly. This suggests that metallic nanoparticles template the assembly of the free TPPS molecules as well as molecules originating from the preformed J-aggregates, which are destroyed, into small surface-bound J-aggregates. The net amount of TPPS molecules engaged in a J-aggregate structure increases. Yet, the conversion from self-standing Jaggregates to particle-adsorbed J-aggregates also accounts for the TEM observations and fluorescence measurements (see the Supporting Information, Figures S3 and S4). Indeed, the nanoparticle-templated J-aggregates are in the direct vicinity of the plasmonic metallic surfaces, which, therefore, suppress the fluorescence by nonradiative dissipation.18,47 Hence, the 3491



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TPPS aggregation, yet the average rod length is shorter after encapsulation. This apparent discrepancy can be explained by the electrostatic synergy between assembly and templating effects in the presence of the silica precursors (see the Supporting Information, Figure S8). At pH 2, the J-aggregates keep a net negative charge, whereas the amine-bearing silica precursors are cationic. Moreover, the rate of condensation of silicic acid is minimal,52 which gives an opportunity to the preformed J-aggregate templates to disassemble. The length distribution of the rod particle is, therefore, skewed toward shorter lengths. Silica nuclei produced from APTES/TEOS can template the nucleation of new J-aggregates by attractive electrostatic interaction, followed by the rapid growth of the organic template, which is never highly negatively charged thanks to the ongoing slow mineralization. New rod-shaped structures are thus produced, although their length never reaches that of uncoated J-aggregates. The interplay between organic template assembly and inorganic precursor adsorption was first evidenced to determine the hybrid structures in the silica mineralization of octapeptide nanotubes.53 Once the condensation of the silica shell around the aggregates is completed, the presence of the amino moieties enables the electrostatic decoration of the encapsulated Jaggregates by negatively charged metallic nanoparticles. The conjugation of citrate-stabilized gold or silver nanoparticles with freshly synthesized silicified J-aggregates is obtained, at room temperature, by stirring the mixed component solutions for 30 min and then leaving them undisturbed for 12 h. Due to the combined effect of surface charge neutralization of the encapsulated J-aggregates and the electrostatic cross-linking of the aggregates by the metal nanoparticles, precipitation is observed for incubation times exceeding 4 days. However, the adjustment of the pH around 2.2−2.4 allows us to produce stable conjugate suspensions. The grafting of the metallic nanoparticles to the silica shell is evidenced by TEM (Figure 5 and Supporting Information, Figure S9), SEM, and energy-dispersive X-ray analysis (EDXA) (see the Supporting Information, Figure S10). TEM images show that all Au or Ag nanoparticles are systematically attached to the silica shells, and the encapsulated aggregate coverage is varied by adjusting the nanoparticle/J-aggregate ratio. Interestingly, in this case, no morphological change of the encapsulated aggregates is observed upon attachment of the nanoparticles. Similarly, the absorption spectrum of the encapsulated Jaggregates is almost not affected by the attachment of metallic nanoparticles. In particular, the intensity of the 490 and 434 nm bands are unchanged, irrespective of the amount of nanoparticles. To investigate the influence of the metal particles on the molecular fluorescence of the J-aggregates, we have first identified the most relevant excitation wavelengths and then systematically monitored the emission at all stages of our templated construction. The fluorescence of TPPS molecules and their aggregates is well-established.34,46 They are usually excited using the Soret and Q-bands (Figure 4, black line). Emission of the TPPS monomer at 640 and 702 nm can be produced with similar spectral features by exciting either the Soret band (413 nm) or the lower Q-band (515 nm), the latter one being more efficient than the former (Figure S6 and Table S7, Supporting Information). Upon acidification, the fluorescence of the anionic TPPS monomer shifts to 675 nm and can be excited at 434, 590, or 645 nm. At last, the specific emission of J-aggregates at 720 nm is most selectively excited at

Figure 3. Variation of the silica coating thickness, t, as a function of the volume of silica precursor, APTES/TEOS, added to the porphyrin solution. Inverted triangles represent height data measured by AFM, and gray circles are derived from width data measured on TEM images.

precursor. Moreover, the encapsulated nanotapes appear to be more numerous in the TEM and AFM samples compared to uncoated samples, which further confirms that the J-aggregates remain stable upon silica templated growth. The changes in the UV−visible spectrum of the J-aggregates are monitored as a function of the silica sheath thickness (Figure 4). The silica mineralization does not induce major

Figure 4. Evolution of the J-aggregates’ absorption spectrum with silica shell encapsulation for thicknesses comprised between 2.2 and 11.2 nm. The anionic precursor absorption peaks (434 and 645 nm) decrease in intensity as the aggregate bands (490 and 708 nm) increase. The thick gray arrow indicates the fluorescence excitation wavelength.

spectral modifications to the four main peaks. However, the respective intensities of the bands are affected. As the silica thickness increases from 2.2 to 11.2 nm, the intensity of the Soret and Q-band at 490 and 706 nm increases with a concomitant diminution in the absorption of the 434 and 645 nm bands (see also Figure S6 and Table S7, Supporting Information). This conversion of the protonated monomer peaks (434 and 645 nm) to the J-aggregate peaks (490 and 706 nm) reflects exactly the evolution of the spectrum upon aggregation by acidification of the TPPS solution below pH 2, although the observed pH of the encapsulated J-aggregate suspension remains constant at 2.06−2.02. The TEM structural and UV−visible spectral data strongly suggest that the templated silica shell formation promotes the 3492



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our system evidences that the silica encapsulation induces both a spectral and an intensity alteration of the native fluorescent spectra. This is in contrast to usual metal-centric approaches, in which the fluorophores are tethered to dielectric-coated metal nanoparticles but do not allow assessing the effect of the dielectric interface on the adsorbed fluorophores. Importantly, the modification of the emission upon initial silica coating is related to the immediate interface and changes of the local dielectric constant and does not vary much with thicker coatings. Thus, the encapsulated J-aggregates are considered as the reference samples in the following metal-conjugation experiments. The attachment of freshly prepared Au or Ag nanoparticles on the outer surface of the silica coating modifies significantly the emission intensity of the encapsulated J-aggregates (Figure 6). For a silica thickness thinner than 6.7 nm, one observes a

Figure 5. TEM micrographs of silica-encapsulated J-aggregates decorated with Au nanoparticles at (A) low and (B) high nanoparticle/aggregate molar ratios. Inset in (B) is an SEM image of a similar sample.

490 nm. This last J-aggregate emission band has been monitored throughout the encapsulation and plasmonic coupling steps sketched in Figure 1. First, we observe that the silica encapsulation process leads to a markedly different behavior of the J-aggregates (Figure S11, Supporting Information). Upon silica encapsulation, the Jaggregate fluorescence at 720 nm decreases by a factor of 35− 40% as soon as a minimal 2.2 nm of silica is formed to reach a roughly constant level for a silica sheath thickness exceeding 6− 8 nm. It is very likely that the loosely condensed silica framework wrapping the J-aggregates provides nonradiative paths for the excitonic state, hence reducing the fluorescence intensity. Moreover, the gradual increase of the silica shell thickness induces a red shift of the emission line for a silica thickness of 2.2 and 9.0 nm (see the Supporting Information, Figure S11). We correlate this spectral red shift to the recent observation that mineralization of J-aggregates can result in the intercalation of thin inorganic sheets in between TPPS stacks.54 This reduces the molecular overlap in the aggregate, which lowers the energy levels of the emitting excitonic state. Hence,

Figure 6. (A) Fluorescence spectra of J-aggregates encapsulated in silica (black curves) and spectra showing enhanced emission with the attachment of freshly prepared, nonoxidized Au (red curve) or Ag (blue curve) nanoparticles and 9.0 nm thick silica shell. (B) Enhanced fluorescence with oxidized Ag colloids and 2.2 nm silica shell.

further quenching of the fluorescence, which tends to vanish as the silica thickness increases (see the Supporting Information, Figure S12). A fluorescence enhancement is even observed for a silica coating thicker than 6.7 nm, which is maximized at 9.0 nm for Au and Ag nanoparticles (Figure 6A). The uniform enhancement of the J-aggregate fluorescence by Au and fresh nonoxidized Ag nanoparticles amounts to ×140% and ×200%, 3493



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respectively. For the thicker coatings, the fluorescence intensity is not affected by the addition of metal nanoparticles. Two major mechanisms account for the emission enhancement and quenching.22,47,55 The resonant excitation of plasmon modes in confined metals results in strong enhancement of the electromagnetic field, which can locally increase the population of the excited state of the fluorophores (mechanism 1). Conversely, the presence of free plasmonic states near an excited fluorophore can reduce the emission lifetime and promote the photon emission rate (mechanism 2). The spectral tuning is done by matching the plasmon resonance with the absorption band (mechanism 1) or emission line (mechanism 2). In our system, the spectral overlap between the 420 nm plasmon band of Ag or the 520 nm plasmon resonance of Au nanoparticles with the narrow 490 nm J-aggregate Soret band is similar (see the Supporting Information, Figure S13) and suggests that the fluorescence quenching or enhancement results from a plasmon-enhanced electromagnetic field for Jaggregates placed in the near-field of the metallic nanoparticles (mechanism 1). The fluorescence intensity has been monitored as a function of the density of adsorbed nanoparticles, which were increased by up to a 4-fold. However, no distinct correlation could be identified, probably because of the superposition of several adverse phenomena. Increasing the isolated nanoparticle coverage will result in a linearly increasing fluorescence signal from a larger number of TPPS molecules located in the enhanced field, but it could also reduce the absorption cross section of the encapsulated aggregates, which would reduce the effective emission. In addition, as the particle density increases on the silica shell, the interparticle plasmonic coupling results in a modified spectrum due to the emergence of a coupled plasmon mode. The variation of the spectral overlap between the plasmon modes and the J-aggregates spectrum can result in either an increase or a decrease of the fluorescence depending on the aggregation topology. Interestingly, when the Ag nanoparticles are left in their aqueous medium for 24−48 h, the surface plasmon absorption band roughly doubles. When these aged nanoparticles are attached to the silica-encapsulated J-aggregates, a strong enhancement (×420%) of the fluorescence is observed as soon as a minimal thickness of silica sheath (2.2 nm) is present (Figure 6B). The increase of the plasmon band intensity is in agreement with the larger enhancement of the J-aggregate fluorescence. Moreover, in agreement with the other data sets, the absence of a silica sheath results in fluorescence quenching, and for the thicker silica coatings, the fluorescence of the encapsulated J-aggregates remains unchanged, although the onset is 4.7 nm, in this case. We interpret this specific behavior as resulting from the combined effect of two dielectric layers, namely, a very thin AgOx shell on the metallic nanoparticles and the silica sheath. Because of the presence of the silver oxide, the optimal enhancement factor is observed for a thinner silica sheath thickness than for pristine unoxidized nanoparticles.

nanoparticles has been investigated in our f luorophore-centric system and the three expected regimes of fluorescence quenching, strong enhancement, as well as decoupling were observed as a function of the silica dielectric spacing thickness.34 The encapsulation of fluorescence nanoparticle or supramolecular aggregates inside a silica shell presents several advantages in terms of mechanical and optical stability as well as biocompatibility, which are of high relevance for biosensing applications. We think that such bottom-up templated constructions could contribute to the design of optical probes for sensing and imaging applications but also to the efficient integration of molecular absorbers and emitters into plasmonic devices for optical information processing, which require a finetuning of the strong coupling regime between fluorophores and metallic nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

Structural and optical characterization of TPPS monomer, Jaggregates at different pHs, with conjugation of metal nanoparticles and silicified J-aggregates with and without conjugation of metal nanoparticles. Detailed AFM and TEM analysis of coated and uncoated J-aggregates to extract the silica sheath thickness. Fluorescence analysis of metal nanoparticle conjugated to silicified J-aggregates at different silica thicknesses. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +335 6225 7838. Fax: +335 6225 7999. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a European Research Council grant, FP7-ERC-2007-StG (Contract 203872 COMOSYEL). The authors thank A. Thete, J. Sharma, J. Dexpert, and S. Joulié for technical assistance.

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REFERENCES

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4. CONCLUSION In conclusion, we have developed a layer-by-layer construction in which a fluorescent supramolecular assembly of dyes is encapsulated in a dielectric shell of well-controlled thickness in the 1−10 nm range prior to conjugation to Ag and Au nanoparticles. The influence of the dye−silica interface on the emission properties could thus be characterized independently from the coupling to surface plasmons. The strong coupling regime between encapsulated J-aggregates and plasmonic 3494



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