Gold Nanoparticles in CoreâPolyelectrolyteâShell Assemblies

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Gold Nanoparticles in Core-Polyelectrolyte-Shell Assemblies Promote Large Enhancements of Phthalocyanine Fluorescence Raquel Teixeira, Pedro Miguel Ribeiro Paulo, and Prof. Dr. Silvia M. B. Costa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04667 • Publication Date (Web): 21 Jul 2015 Downloaded from http://pubs.acs.org on August 23, 2015

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The Journal of Physical Chemistry

Gold Nanoparticles in Core-Polyelectrolyte-Shell Assemblies Promote Large Enhancements of Phthalocyanine Fluorescence Raquel Teixeira * †, Pedro M. R. Paulo * and Sílvia M. B. Costa Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal.

ACS Paragon Plus Environment

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Abstract

We have incorporated gold nanoparticles into a core-polyelectrolyte-shell type of assembly with a coating lipid vesicle encapsulating a phthalocyanine dye. The supramolecular construct having polyelectrolyte layers as spacers between the particle´s surface and the phthalocyanine was designed to control the emission enhancement through the plasmonic antenna effect of gold nanoparticles. We have observed large emission enhancements, of about three orders of magnitude, for an optimum number of 13 to 15 polyelectrolyte bilayers. Such large emission enhancements were attributed to hot-spots formed by clustering of gold nanoparticles during the process of layer-by-layer deposition of polyelectrolytes. Fluorescence lifetime imaging microscopy allowed to correlate those large emission enhancements with shortening of emission lifetimes, as expected from plasmonic enhancement of radiative and non-radiative rates. It has also shown that these polyelectrolyte-assembled clusters of gold nanoparticles are of submicrometric size, which makes these nano-objects promising for enhanced imaging or biosensing applications.

Keywords: gold nanoparticles; plasmon-enhanced fluorescence; polyelectrolytes and colloids; fluorescence lifetime imaging microscopy; DDA simulations


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Introduction The application of fluorescence techniques to probe phenomena at the nanoscale often depends on finding an emitter with suitable spectral and photophysical properties. For this purpose, organic dyes are a privileged choice of emitters due to their versatility and chemical diversity. However, it is not uncommon for organic dyes to suffer from low emission quantum yield or poor photostability. This becomes a strong limitation for fluorescence applications, in particular, for novel microscopy techniques working at single-molecule detection level.1,2 One way around this limitation has been sought by coupling metallic nanostructures to organic dyes in order to enhance their brightness, while preserving other features of the dye, such as spectral range or chemical functionality. This effect is commonly termed plasmon- or metalenhanced fluorescence.3-7 There is a close analogy with earlier known surface-enhanced Raman spectroscopy (SERS), in which a strong increase of the detected Raman signal is observed from molecules adsorbed on nanostructured metal surfaces.8 In both phenomena, SERS or emission enhancement, the effect is somehow related to the strong local enhancement of the electric field of light induced by surface plasmon modes of metal nanostructures. Surface plasmons are collective oscillation modes of the free conduction electrons in the metal that can be excited by the electric field of light, and that in metal nanoparticles are confined by their surface. These localized surface plasmon modes (LSP) give rise to the well-known optical properties of metal nanoparticles, such as strong light absorption and scattering. Also, LSP’s are responsible for the strong nearfield enhancement referred to above, which is sometimes described as nanofocusing of light. These phenomena are more pronounced at the plasmon


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resonance frequency, which for noble metal nanoparticles typically occurs in the visible to infrared wavelengths. The photophysical behavior of an organic dye is modified by the interaction with a metallic nanoparticle through its combined effects on excitation, radiative and non-radiative rates, as briefly explained hereafter. The excitation rate is proportional to the intensity of light, and thus is accelerated by the local electric field enhancement near the particle. The effect on the radiative rate can be explained within classical electrodynamics by a description that approximates the emitting dye to a radiating dipole, which excites a plasmon mode of the particle that jointly radiates, thus accelerating the dye’s radiative rate relatively to that in free space. However, the particle’s proximity also opens up additional non-radiative decay channels that have quenching effects on the dye’s emission. The most common can be described as energy transfer from the excited-state of the dye to the plasmon modes of the particle, which is then dissipated as heat instead of being radiated. This effect is more pronounced at close distances to the particle, but it usually decays more rapidly with distance than the acceleration of excitation and radiative rates, thus leading to an effective emission enhancement only a few nanometers away from the particle’s surface. Therefore, the distance between metal nanoparticle and dye molecule is critical for observing plasmonic emission enhancement. Other conditions, such as spatial orientation and spectral dependence, also play an important role, and have been clearly established from both theoretical and experimental reports in the literature.9-16 Single molecule experiments on dye molecules immobilized on surfaces using mechanical probes to approach a single gold nanosphere have much contributed to understand plasmonic effects on fluorescence emission.9,10 The maximum enhancement observed in one of those


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studies was about 9-fold increase of the photon emission rate for a particle with a size of 80 nm at a distance of ~5 nm from the surface.9 These studies have inspired others to develop supramolecular and surface chemistry approaches to obtain fluorescent nano-objects with plasmonic emission enhancement in colloidal solution. There are several examples of nanoobjects composed of a core metallic nanoparticle and a polymeric or silica coating that serves as a spacing layer for dye molecules attached in the outer shell.17-23 The reported emission enhancements are not much greater than 10-fold increase of brightness, which are in agreement with the weak nearfield enhancement for spherical particles; and in some examples only emission quenching was observed.22,23 It is possible to reach large enhancements of fluorescence emission with more efficient plasmonic nanoantennas. For instance, enhancements of three orders of magnitude have been reported for dye molecules in the gap of bowtie nanoantennas or at the tip of single gold nanorods.24,25 Both examples rely on the intense plasmon nearfield obtained at specific regions in these nanostructures known as hot-spots. The design of plasmonic nanoantennas with complex architectures requires nanolithography methods to specifically control the size and arrangement of metal nanostructures.3,6,7 However, these methods are more expensive and limited in the number of copies that can be produced. Also, the fabrication of nanostructures with narrow gaps is not so easily achieved for distances below 20 nm, which are the most interesting for producing hot-spots. Alternatively, the preparation of nanoparticle assemblies by supramolecular and colloidal chemistry methods provides a way to obtain narrow interparticle gaps and better particle crystallinities, which are essential for large plasmonic enhancement effects. Several examples of gold nanoantennas produced by DNA assisted self-assembly have been recently reported.26-28


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We have previously reported plasmon-enhanced emission of a phthalocyanine in polyelectrolyte films induced by gold nanoparticles.29 The spatial heterogeneity in the number and interparticle distances for the immobilized gold nanoparticles within the polyelectrolyte organic/inorganic hybrid films originated different plasmonic enhancements of phthalocyanine fluorescence, and the observation of hot-spots where local emission was more pronounced. In this contribution, we report on the preparation of polyelectrolyte modified gold nanoparticles that were designed for plasmonic enhancement of fluorescence emission from a phthalocyanine dye. The approach described here relies exclusively on supramolecular and colloidal chemistry methods, with the advantages referred to above. The large emission enhancements observed, of about three orders of magnitude, were attributed to hot-spots formed by clustering of gold nanoparticles during the process of layer-by-layer deposition of polyelectrolytes. The emission enhancements are accompanied by shortening of decay times, as measured by fluorescence lifetime imaging microscopy. The extent of these effects depends on the number of polyelectrolyte spacing layers in qualitative agreement with the distance dependence of plasmonic nanoantenna effects. The clusters of gold nanoparticles are of submicrometric size, which makes these nano-objects promising for enhanced imaging or biosensing applications.


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Experimental Section Materials and Instruments Sodium tetrachloroaurate (III) dihydrate (NaAuCl4.2H2O, 98%), sodium citrate tribasic dihydrate

(C6H5Na3O7.2H2O,

puriss.

p.a.),

L-ascorbic

acid

(≥

99%),

Hexadecyltrimethylammonium bromide (CTAB, ≥ 99%), poly(sodium 4-styrenesulfonate) (PSS, MW~75000, 18% wt.% in water), poly(allylamine hydrochloride) (PAH, MW~15000) and 1,2dimyristoyl-sn-glycero-3-phosphocholine (DMPC, >99% purity) were obtained from SigmaAldrich. Monosulfonated aluminium phthalocyanine (AlPcS1) was synthesized according to Ambroz et al.30 All chemicals were used as obtained. Double distilled water was employed throughout. Glass microscope slides (0.13-0.17 µm thickness) were obtained from Normax. Fluorescence lifetime imaging microscopy (FLIM) measurements were performed with a timeresolved confocal microscope (MicroTime 200, PicoQuant GmbH). The excitation at 639 nm is carried out by a pulsed diode laser at a repetition rate of 20 MHz, through a water immersion objective (60×, N.A. 1.2). Samples were placed in the microscope sample holder perpendicularly to the excitation light that propagates through the bottom of the sample (glass - AuNPs - PE multilayers - lipid bilayer with phthalocyanine). The emitted fluorescence is collected in the reverse pathway and is sent through a dichroic mirror, an emission filter (bandwidth of 55 nm centered at 695 nm), and a 50 µm pinhole that rejects out-of-focus light providing confocal detection. The fluorescence is then detected with a single-photon counting avalanche diode (SPAD) (PerkinElmer) whose signal is processed by TimeHarp 200 TC-SPC PC board (PicoQuant) working in the time-tagged time-resolved (TTTR) operation mode. Each sample was imaged several times, and the fluorescence decays of a number of fixed points were


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collected from different regions of the slides. Decay fitting was performed by reconvolution with the IRF using a nonlinear least-squares procedure based on the Marquardt algorithm. The quality of the fits was evaluated by the usual criteria for the χ2 parameter and by visual inspection of the weighted residuals. TEM characterization was performed on a HITACHI H-8100 electron microscope operating at 200 kV. Synthesis of AuNPs Citrate-capped gold nanoparticles (AuNPs) were obtained by chemical reduction of the metal precursor salt with sodium citrate.31 Briefly, 1.9 mL of 0.1 M sodium citrate solution was added to 48.1 mL of a boiling aqueous solution of sodium tetrachloroaurate (5×10-5 mol), and the mixture was left under stirring at 100ºC for 15 min. These AuNPs were used as seeds to produce larger particles through the seed-mediated growth described by Fernández et al.32 Briefly, the seeds suspension was diluted twice with a solution of CTAB (0.03 M). L-ascorbic acid (5×10-4 M) was added to a solution of NaAuCl4.2H2O (2.5×10-4 M, acidified to pH= 2) and CTAB (0.015 M), at 35-40 ºC. The orange gold solution (due to chloride counter-ion replacement with bromine from CTAB) becomes colorless due to the reduction of Au (III) to Au (I). The solution of seeds (1/3 of the final volume) was added immediately after the reducing agent, and the mixture was left under stirring for about 2 hrs. These AuNPs were used as-prepared as seeds for the next step, and this procedure was repeated three or four times in order to obtain AuNPs with diameters between 60-100 nm. Polyelectrolyte-coated AuNPs


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The large excess of CTAB in the AuNPs was removed by centrifugation before the coating with PEs. AuNPs were coated by resuspending the pellet (obtained after centrifugation) in solutions of the oppositely charged PEs, PAH and PSS (2 mg/mL, 0.5 M NaCl), alternately. Each adsorption step was carried out at room temperature for 6h, under vigorous stirring. To remove the excess polyelectrolyte, the suspension was centrifuged (5000 rpm, 10 min) and washed 3 times after each PE layer adsorption. Coating with AlPcS1-labelled DMPC vesicles The appropriate amounts of DMPC and AlPcS1 were co-solubilized in 200 µL of chloroform/methanol 2:1. The solvent was evaporated under a nitrogen stream to obtain a lipid film. The film was hydrated with bi-distilled water at 25-30ºC (above the main phase transition of the phospholipid, Tm= 24ºC),33 while sonicated for 10 minutes. The final concentration of DMPC and AlPcS1 was 1 mg/mL and 0.1 µM, respectively. The lipid vesicles were mixed with a suspension of AuNPs coated with (PAH/PSS)n, and incubated for 20 min under vigorous stirring. The samples were centrifuged for 15 min at 4000 rpm and the supernatant was removed. Centrifugation and re-suspension with bi-distilled water was repeated until no vesicles or free phthalocyanine molecules remained in the bulk (checked by confocal fluorescence microscopy). Computational methods We have used the discrete dipole approximation34 (DDA) method to evaluate the enhancement in excitation, radiative and non-radiative rates of a phthalocyanine dye in the gap between gold nanoparticles of 80 nm in diameter. The spherical nanoparticles are described as an array of point dipoles centered in cubic volume elements with size of 0.5 nm. In particle configurations with wider gaps, it was necessary to increase dipole size to 1 nm, in order to keep computational time


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affordable. The dielectric function of gold, as reported by Johnson and Christy, was used in these calculations.35 The phthalocyanine dye was described as a point-like dipole with emission characteristics of this molecule, i.e., a transition dipole moment of |p p | = 13.8 D radiating at an emission wavelength of 683 nm. The details on the calculated gap widths and on the dye’s position within the gap are given in the section of Results and Discussion. The excitation rate enhancement was assumed as the ratio of local field intensities with and without metal nanoparticles, exc = |E E rr | ⁄|E E rr | , at the molecule’s position r for an excitation wavelength of 639 nm. The enhanced radiative rate r

and the additional non-

radiative rate nr were calculated following the approach of D'Agostino et al.36 The enhancement of the spontaneous decay rate, Γ = r + nr , is calculated from the scattered electric field at the molecule’s position, E rr , which is obtained from DDA simulations, Γ 6 B = 1+ Im p∗ ∙E rr # r |p p |

(1)

here r is the dye’s radiative rate in free space, and B are the dielectric constants of vacuum and background medium, and is the wavenumber of emitted light. The additional non-radiative rate is derived from the time-averaged power absorbed by the metal nanoparticles: 6

$abs

nr = ≈ Im ,-.local,2 - × 45 ℏ) 2ℏ

(2)

278

where is the complex dielectric permittivity of gold at the emitted frequency, .local,2 is the internal electric field calculated at the position of the ith dipole element, and 45 is the volume of each dipole element. The apparent emission quantum-yield of the dye within the gap, 9 gap , is calculated from that in free space, 9 0 , and from the enhanced radiative and non-radiative rates,


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9 gap =

r + r

r

1⁄9 0

− 1 + nr

(3)

The near-field intensity maps around the gold nanoparticles were calculated using the subroutine implemented in the A-DDA software package.37

Results and Discussion The procedure developed in this work for the modification of gold nanoparticles begins with the deposition of polyelectrolyte PAH/PSS bilayers around the particles (Figure 1A). The layerby-layer deposition creates a polyelectrolyte shell with a thickness proportional to the number of bilayers that have been deposited (Figure 2A-C and Figure S1 from SI). This approach makes it possible to gradually increase the distance between the particle’s surface and the last coating layer, which is prepared by encapsulation in a DPMC lipid vesicle. The phthalocyanine dye is embedded within the lipid bilayer of the vesicle. This allows to control the concentration of dye at the particle’s surface by fixing the ratio of lipid:dye in the vesicle formulation. Moreover, we have presupposed that the intercalation of the macrocyclic plane of the phthalocyanine in the lipid bilayer would impose, to a certain extent, a perpendicular orientation of its transition dipoles relatively to the particle’s surface. This particle-dye configuration seemed apriori more favorable for plasmon-enhancement of fluorescence emission, than a completely random orientation of the dye’s transition dipoles relatively to the particle.


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Figure 1. (A) Sequence used for coating gold nanoparticles with a polyelectrolyte shell of PAH/PSS bilayers, and an outer DMPC lipid bilayer embedding a phthalocyanine dye. (B) Scheme depicting the clustering of gold nanoparticles as the number of polyelectrolyte bilayers is increased.

The size of gold nanoparticles with a diameter of 80 nm was chosen here for the purpose of achieving at least one order of magnitude in fluorescence enhancement. This possibility was theoretically evaluated from a dipolar electrostatic model of the emission behavior of a dye molecule in the vicinity of a spherical metal particle, as proposed by Bharadwaj and Novotny.14 The model afforded a top enhancement factor of about 8-fold for a separation of 12 nm between a phthalocyanine dye and a gold nanoparticle with 80 nm (Figure S2 in SI). For a smaller particle size of 40 nm, the emission enhancement is estimated to be less than 3-fold, and for an even smaller size of 20 nm only emission quenching is predicted. Thus, the choice of gold


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nanoparticles with 80 nm was crucial to detect emission enhancement. Also, the enlarged surface area of the particles with 80 nm facilitates the wrapping of polyelectrolyte chains in the layer-bylayer deposition. Further increasing the particle size above 80 nm could become detrimental to emission enhancement due to the increasing role of multipolar non-radiative plasmon modes.38

Figure 2. Electron microscopy (TEM) images of gold nanoparticles modified with a polyelectrolyte shell for different number of PAH/PSS bilayers 5, 9 and 21 respectively from A to C. Fluorescence lifetime images of gold nanoparticles double-coated with a polyelectrolyte shell, and an outer lipid bilayer embedding a phthalocyanine, for several number PAH/PSS bilayers of 5, 9 and 25 respectively from D to F.

The fluorescence lifetime images of gold nanoparticles double-coated with a polyelectrolyte shell and a lipid bilayer containing a phthalocyanine dye show a clear effect of shell thickness on


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the emission lifetime (Figure 2E-F). The gradual increase of emission lifetime is illustrated here for particles with a number of 5, 9 and 25 PAH/PSS bilayers, which gave average lifetimes of approximately sub-ns, 1 ns and 3 ns, respectively. Both enhancements of the radiative and nonradiative rates decrease with the distance between metal particle and emitting dye over a few tens of nm. Eventually, the emission lifetime of the dye approaches its value in free space. The confocal fluorescence images also show that for a small number of polyelectrolyte bilayers the emission is from spots of diffraction-limited size (Figure 2D and 2E). This result shows that emission originates from submicrometric objects, which is consistent with single particles or very small aggregates, e.g. dimers or trimers. As the number of polyelectrolyte bilayers is increased, the fluorescence images show objects larger than the diffraction-limit size (Figure 2F), which suggest the formation of particle aggregates. The aggregation of gold nanoparticles is likely to occur with an increasing number of polyelectrolyte layers due to incomplete surface coating after many deposition steps, which may lead to surface patches of opposite charge acting as adhesion points between particles. The clustering of gold nanoparticles is already visible in fluorescence images of the sample with 9 PAH/PSS bilayers, and it becomes clear for a larger number of PAH/PSS bilayers. The observation of large emission enhancements is most likely related to the particle clustering giving rise to hot-spots for plasmon effects, as it is further developed below. Fluorescence decays were measured on several of the bright spots observed in fluorescence images of gold nanoparticles modified with increasing number of polyelectrolyte bilayers. Some selected examples are shown in Figure 3. These individual measurements allowed us to characterize the dispersion of emission lifetimes and intensity on each sample and, thus, to have a more comprehensive insight of plasmonic effects on phthalocyanine’s emission and their


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dependence on the polyelectrolyte shell thickness. Before discussing the full set of results, some remarks about individual fluorescence decays are in order. The decays are not single exponential, which probably reflects the contribution from several phthalocyanine molecules embedded in the lipid vesicle. In terms of plasmonic effects, the local environment changes rapidly with the decay of enhanced near field over a nanometric distance away from the particle’s surface. Even if emission is collected from a single spot, and only one particle is present within the confocal volume, the possibility of slightly different local environments for the phthalocyanine dyes is a plausible explanation for a multi-exponential decay. In order to compare emission lifetimes, the intensity-averaged values extracted from multi-exponential decay fits will be used hereafter. Moreover, the increase in decay time with the separation distance between phthalocyanine and particle’s surface can be visually perceived from the comparison of the fluorescence decays for the assemblies with 9, 15 and 25 PAH/PSS bilayers, as exemplified in Figure 3D-F.


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Figure 3. Fluorescence lifetime images of gold nanoparticles double-coated with a polyelectrolyte shell, and an outer lipid bilayer embedding a phthalocyanine, for a number PAH/PSS bilayers of 5, 15 and 25 respectively from A to C. The labels a to h show the emission spots on which the fluorescence decays shown in parts D to F were measured. For comparison purposes, the decay curves were normalized between 10 and 104 counts.

The emission intensity and average lifetimes of the individual spots measured for the several fluorescence images of each sample are correlated in the scatter plot of Figure 4. For assemblies with thinner polyelectrolyte shells, i.e. between 3 and 9 PAH/PSS bilayers, most of the decay


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times are very short, close to the time resolution limit of our setup. However, the emission intensity increases by more than one order of magnitude in the same range. This trend reaches a turning point for assemblies with 13 and 15 PAH/PSS bilayers that show the maximum emission enhancement. At this point, the average lifetimes begin to shift toward longer values. For assemblies with thicker polyelectrolyte shells, i.e. with 25 and 31 PAH/PSS bilayers, the emission intensity gradually decreases to minimum values and decay times become longer, but without reaching the lifetime of free phthalocyanine. For comparison purposes, a control measurement was included in Figure 4 (open symbols). It consists of hollow microcapsules of PAH/PSS polyelectrolytes coated with a DMPC lipid vesicle embedding phthalocyanine dyes in a concentration equivalent to that in the modified gold nanoparticles. In the microcapsule system, the fluorescence decay of phthalocyanine is single exponential with a lifetime close to 5 ns, in agreement with the value measured for this dye, e.g. in aqueous solution. The emission intensity in hollow microcapsules is fairly close to that of the gold nanoparticles with 31 PAH/PSS bilayers. In the microcapsule system there are no plasmonic effects, while in the gold nanoparticles with 31 PAH/PSS bilayers the large separation of more than 40 nm between metal surface and phthalocyanine dye should bring plasmon effects down to a minimum. This seems to be confirmed by the similar values of emission intensity in these two systems, even though, the emission intensities are affected by the surface coverage with lipid-vesicle embedding the phthalocyanine, which most likely differs between the microcapsule system and the gold nanoparticles with 31 PAH/PSS bilayers due to size difference between these colloidal objects. Interestingly, there is still some effect in the emission lifetime of the assemblies of gold nanoparticles even with 31 PAH/PSS bilayers.


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er

N. bilayers 3 5 9 13 15 25 31

4

10

Intensity (counts/ms)

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3

10

2

10

1

10

0

10

0

1

2

3

4

5

6

Decay time (ns)

Figure 4. Correlation plot of emission intensity and average lifetime from the collection of points measured for each sample of gold nanoparticles double-coated with a polyelectrolyte shell, and an outer lipid-vesicle embedding a phthalocyanine, for a number PAH/PSS bilayers between 3 and 31 (closed symbols). For comparison purposes, a control measurement on hollow polyelectrolyte microcapsules coated with the same lipid-vesicle bilayer encapsulating a phthalocyanine was also included (open symbols). The bar plots in the top and right sides show the average values of emission lifetime or intensity, respectively, for each set of data in the scatter plot using the same color code.

The maximum emission enhancements observed are of about three orders of magnitude, when comparing the maximum intensity values for gold nanoparticles with 13 or 15 PAH/PSS bilayers and the minimum values for those with 31 PAH/PSS bilayers, or hollow microcapsules. Such large enhancements are not achievable with a single spherical gold nanoparticle, as previously


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noted. We attribute these enhancements to hot-spots generated by clustering of gold nanoparticles when the number of polyelectrolyte layers is increased (Figure 1B). The hot-spots occur within the interparticle gap due to the large near field enhancement in this region (Figure 5A). The spatial distribution of near field causes the effect of plasmon-enhanced emission to depend significantly on the exact position of phthalocyanine dye relative to the particles’ surface. This probably explains the dispersion of emission intensity and lifetime observed in each sample with a particular number of polyelectrolyte bilayers. In agreement, the gold nanoparticles with 31 PAH/PSS layers, or hollow microcapsules, are the systems that show less dispersion of emission intensity and lifetime, since in these cases the plasmon effects are minimal or not occurring. In order to evaluate the possibility of having emission enhancements of three orders of magnitude in our systems, we used DDA calculations to simulate hot-spots created within a gap between spherical gold nanoparticles of 80 nm diameter. The DDA method has been extensively used to simulate the optical properties of metal nanoparticles.34 More recently, a theoretical framework and its implementation using DDA method was proposed for describing an optical antenna composed of an emitter, e.g. a fluorescent molecule, and a metal nanostructure.36 In this framework, the fluorescent molecule is approximated as an oscillating electric dipole with a spontaneous decay rate. In the vicinity of a metal nanostructure, the dipole’s decay rate is modified by the scattered field from its environment (eq 1). The change in decay rate can be translated into an apparent emission quantum-yield of the fluorescent molecule in the vicinity of the metal nanostructure (eq 3). The DDA method provides numerical values of the local fields then used to estimate modified decay rates and emission quantum-yields (Figure 5A).


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Figure 5. (A) Near-field plot calculated with DDA method for a dimer of gold nanoparticles with diameter of 80 nm and a gap of 16 nm. (B) Scheme of the simulated particle-dye configurations; the dye was positioned in the middle plane of the gap at several displacements of 0, 10, 20 and 40 nm away from the axis connecting particle centers – respectively, red, orange, green and blue arrows. (C) Excitation rate enhancements and (D) relative emission quantum yield calculated from eqs 1 to 3 for DDA simulations of the particle-dye configurations illustrated above – the same color code used in scheme B is applied here for plotting enhancement factors. (E) Overall emission enhancements calculated with DDA for a phthalocyanine dye in gaps between gold nanoparticles of 80 nm.


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The acceleration of the excitation, radiative and non-radiative rates were calculated separately for the particle-dye configurations in Figure 5B (Table S1 in the SI). The excitation rate enhancement, exc , combined with the relative emission quantum yield of the phthalocyanine within the gap, 9 gap ⁄9 0 , gives the overall emission enhancement, = = exc × 9 gap ⁄9 0 , i.e. the increase in brightness of emission from the dye. The interparticle gap distance was varied to simulate a range of assembled particle configurations. The polarization of excitation light and the orientation of dye’s transition dipoles was considered parallel to the axis joining the particle centres, which gives the strongest plasmon enhancement effects. The dye was positioned in the middle position of the interparticle axis at different displacement heights from the same axis (Figure 5B), to evaluate its effect on the calculated enhancement factors. The calculation results are shown in Figure 5C-E. Our simulations show that for a narrow gap, i.e. below 8 nm, the overall emission enhancement is calculated to be larger than three orders of magnitude for a dye on the interparticle axis. We do not observe such large enhancements, probably, because this situation would occur for gold nanoparticles modified with a small number of polyelectrolyte bilayers, and in these conditions there is almost no particle clustering that may lead to the formation of hotspots from narrow gaps. Besides, it would also require an exact alignment of the dye’s dipole moment with the interparticle axis, which is difficult to obtain in self-assembled systems. In the gap range from 8 to 32 nm, the overall enhancements calculated are two to three orders of magnitude depending on the exact position of the dye within the gap (Figure 5E). This situation seems to be more comparable with our experimental results, as particle clustering becomes evident in fluorescence images of gold nanoparticles modified with 9 to 11 PAH/PSS bilayers. The dependence of calculated enhancements on the dye’s position gives valuable insight into the


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dispersion of emission enhancements observed in the experimental results, which can also span about one order of magnitude in emission intensity. For wider interparticle gaps or for dye’s positions away from the center, the calculated enhancements drop considerably. This would occur for gold nanoparticles modified with a larger number of polyelectrolyte bilayers, e.g. 25 or 31 PAH/PSS bilayers, in which case there is a decrease of plasmon effects on emission intensity and lifetime. The top emission enhancement calculated for a single particle configuration is about one order of magnitude, as recalculated with DDA method that accounts for the contribution of multipolar plasmon modes and retardation effects (Figure S3 in the SI). The large emission enhancements observed here cannot be achieved by a single-particle, as previously known. On the other hand, the formation of small nanoparticle clusters giving rise to hot-spots from interparticle gaps seems to provide a plausible explanation. Our calculations show that top emission enhancements of more than three orders of magnitude are possible in the gap between two gold particles with diameter of 80 nm. This is a very simple model when compared to the actual gold nanoparticle assemblies, which may be composed of clusters of more than two particles. It is also not straightforward to correlate the polyelectrolyte shell thickness with the gap distance, as clustering of particles probably occurs between defects on the polyelectrolyte shells at different stages of the LbL process. Moreover, the phthalocyanine dyes in the lipid vesicle may assume positions quite different from those in model configurations of Figure 5B, which would then correspond to lower enhancement factors. The measured emission enhancements are an average over the occurring particle-dye configurations within the confocal volume of our fluorescence microscope, which may represent a more heterogeneous situation than modeled in our simulations. Nevertheless, these simulations are useful for the purpose of comparing top


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enhancement factors, and support that emission enhancements of more than three orders of magnitude are possible in our particle assemblies. By analyzing separately the enhancement factors on the excitation and relative emission quantum-yield shown in Figure 5C and D, it is clear that the major contribution for the overall enhancement is from the excitation rate enhancement. Nevertheless, these results show an effective enhancement of the emission quantum-yield up to an increase of 2-fold relatively to the intrinsic quantum-yield of the phthalocyanine, which has a value around 30% in water. In the simulations for a single particle, the results show only emission quenching, i.e. values of relative quantum-yield below one (Figure S3B in the SI), which means that the overall emission enhancement of about 10-fold is derived exclusively from the excitation rate enhancement. In a previous work, we have used polyelectrolyte layer-by-layer assembly to create a molecular spacer between gold nanoparticles dispersed on a glass surface and a phthalocyanine dye immobilized on the last layer.29 In that case, it was also possible to observe emission intensity and lifetime enhancements that depended non-monotonically on the number of polyelectrolyte layers. However, the top enhancement factors were rather modest reaching only 4- to 8-fold in emission increase. This was partially due to the random distribution of gold nanoparticles on the surface that generated some hot-spots, but without control over the gap width which was probably far from optimal to obtain plasmon-enhanced emission. The use of self-assembly approaches to control the interparticle gap distances in aggregates of metal nanoparticles has proved successful to produce optical nanoantennas that enhance fluorescence emission from organic dyes. For instance, the assembly of particle dimers using DNA spacers, or more elaborate origami motifs, have afforded emission enhancements of up to two orders of magnitude.26-28 The assembly of larger particle clusters with narrow gaps to produce plasmon


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hot-spots has also been extensively reported, particularly, in the context of surface-enhanced Raman spectroscopy.39-44 Conclusions Our approach to modify gold nanoparticles for plasmon-enhanced emission has resulted in a sub-micrometric particle assembly that displays emission enhancements up to three orders of magnitude. These conditions were obtained for polyelectrolyte-assembled clusters of gold nanoparticles with an optimal number of 13 to 15 PAH/PSS bilayers. The modified gold nanoparticles are fully prepared using supramolecular self-assembly and colloidal chemistry approaches, which is advantageous for inexpensive production of a large number of copies. Some dispersion is observed in the emission intensity and lifetime enhancements within each sample, as it would be expected for this type of nanofabrication approach. However, the trend of emission enhancements is clearly perceived from the comparison of samples with a different number of polyelectrolyte bilayers. The large enhancements achieved with these gold nanoparticle assemblies, their sub-micrometric size, and their preparation in colloidal solution makes these systems very promising for fluorescent labeling or biosensing. These applications would require to further functionalize the nanoparticle assemblies with a targeting or receptor unit. The large nearfield enhancement of the particle’s plasmon provides a way to selectively achieve this purpose by using photocrosslinking reactions, an approach that is currently under development.

Corresponding Authors * E-mail: [email protected] , [email protected]


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Present Addresses † 3Bs Research Group in Biomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, 4806-909 Taipas, Guimarães, Portugal.

Acknowledgements Authors gratefully acknowledge financial support from Fundação para a Ciência e a Tecnologia, FCT (Pest-OE/QUI/UI0100/2013/2014 and PTDC/CTM-NAN/2700/2012). R.T. acknowledges a Ph.D. grant SFRH/BD/39006/2007 from FCT. P.M.R.P. acknowledges a research grant from FCT (Ciência 2008).

Supporting Information. Polyelectrolyte shell thicknesses; Fluorescence enhancements calculated for a single particle-dye; Enhancement factors of decay rates calculated from DDA; Enhancement factors of a single particle-dye calculated from DDA. This material is available free of charge via the Internet at http://pubs.acs.org.

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PAH/PSS

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Gold Nanoparticles in CoreâPolyelectrolyteâShell Assemblies

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