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Nanophotonics

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Volume 6, Issue 1

Issues

From gold nanoparticles to luminescent nano-objects: experimental aspects for better gold-chromophore interactions

Julien R.G. Navarro
  • KTH, School of Chemical Science and Engineering, Polymer division, Teknikringen 42, SE-100 44 Stockholm, Sweden
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Frederic Lerouge
  • Corresponding author
  • ENS Lyon, Laboratoire de chimie, UMR 5182 ENS/CNRS/Université Claude Bernard Lyon 1, Université de Lyon, France
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Published Online: 2016-07-06 | DOI: https://doi.org/10.1515/nanoph-2015-0143

Abstract

Gold nanoparticles have been the center of interest for scientists since many decades. Within the last 20 years, the research in that field has soared with the possibility to design and study nanoparticles with controlled shapes. From spheres to more complex shapes such as stars, or anisotropic architectures like rods or bipyramids, these new systems feature plasmonic properties making them the tools of choice for studies on light-matter interactions. In that context, fluorescence quenching and enhancement by gold nanostructures is a growing field of research. In this review, we report a non-exhaustive summary of the synthetic modes for various shapes and sizes of isotropic and anisotropic nanoparticles. We then focus on fluorescent studies of these gold nano-objects, either considering “bare” particles (without modifications) or hybrid particles (surface interaction with a chromophore). In the latter case, the well-known metal-enhanced fluorescence (MEF) is more particularly developed; the mechanisms of MEF are discussed in terms of the additional radiative and non-radiative decay rates caused by several parameters such as the vicinity of the chromophore to the metal or the size and shape of the nanostructures.

Keywords: gold; nanoparticles; plasmon; fluorescence enhancement

1 Introduction

The design and study of nanostructured materials exhibiting new photophysical properties has emerged as one of the most exciting fields during the last decade. In that context, gold nanoparticles (AuNPs) are probably the class of materials that has provided the largest variety of sizes and shapes [1], [2], [3], [4], [5]. One of the great interests motivating such activity is related to the arising of new optical properties when reaching the nanoscale size regime (<100 nm).

This phenomenon can be described by means of the Mie theory [6]. In 1908, Mie solved Maxwell’s equations for the interaction of the electromagnetic light wave with small metallic spheres (e.g. particle smaller than the wavelength of light). When the conduction electrons in a small particle are subjected to an external driving field, the conduction electrons start oscillating. These oscillating electrons may, in turn, generate electromagnetic radiation. This process is called scattering. The particle may also convert the energy of the exciting light into heat. In this case, the light is absorbed. The total extinction of the light beam by the particle contains contributions of the scattering and absorption processes. The efficiency of extinction is therefore the sum of the absorption and scattering efficiency. This feature can be evidenced by spectroscopy with the presence of absorption bands (surface plasmon bands – SPB) in the visible region. Moreover, the extinction efficiency (scattering and absorption phenomena) of AuNPs can be so intense that their detection, even at extremely low concentrations, becomes possible [7], [8]. The SPB of noble metal nanoparticles is influenced by several factors such as the size, shape, dielectric constant of the environment, and presence of neighboring particles [1], [9], [10], [11], [12]. Moreover, the SPB and the field generated by the particles can strongly interact with a nearby organic dye, enhancing its optical properties such as fluorescence intensity, lifetime, and quantum yield. Nowadays, a wide range of nanostructures are described (spheres [10], [11], [12], rods [13], [14], [15], prisms [16], [17], [18], bipyramids [19], [20], [21], and stars [22], [23], [24]), where it is possible to tune the spectral overlap between the SPB [25], [26], [27], [28], [29] and the optical response of a dye. The main goal being the design of the hybrid nano-object featuring new photophysical properties, offering a wide range of possibilities in diverse areas of photonics, catalysis, and biology [30], [31], [32].

For such purpose, noble metal nanoparticles can be easily functionalized and stabilized with organic molecules bearing sulfur atoms such as thiols, thanks to the strong affinity between gold and sulfur [33], [34], [35]. The loading capacity of these organic molecules at the surface of the particles varies from a few hundreds to thousands [36], [37]. As the interactions of a fluorescent probe with a metallic surface strongly affects its photophysical properties [38], [39], the precise control and understanding of such interactions have been the focus of many scientists, yielding a great number of publications on quenching [40], [41], [42], [43], or enhancement [44], [45], [46], [47], of the photophysical properties of nearby molecules [39].

In the first part of this review, we highlight the synthesis and optical properties of AuNPs with various shapes. Isotropic structures like spheres or nanostars are described, followed by anisotropic shapes such as nanorods and bipyramids. In the second part, the intrinsic fluorescence features of plasmonic nanoparticles are presented. The properties of photoresponsive molecules labeled onto these nanostructures will then be discussed, taking into account the influence of several parameters such as the sizes of the particles, the dye to gold surface distance, and the shape of the nanostructures.

2 AuNP design

2.1 Isotropic nanoparticles

Behind the term isotropic, a great number of shapes can be considered, such as spheres, cubes, polyhedral nanoparticles, etc. It would be necessary to dedicate an entire review to describe them all properly. However, here we will first focus on classical spherical nanoparticles of different sizes, then on spherical hollow structures also known as nanoshells, and finally on more complex architectures described as nanostars.

2.1.1 Spherical nanoparticles

Spherical nanoparticles are the subject of considerable and increasing interest, as evidenced by the great number of papers focusing on such systems [48], [49], [50], [51], [52]. In a typical way, gold colloids are obtained by a reaction between a gold salt [usually a gold(III) derivative] and a reducing agent, in the presence of a capping moiety to ensure the stability of the final particles in suspension.

Considering the smallest systems, the design of AuNP suspension with a size ranging from 1.5 to 5.2 nm was first described by Brust et al. [53] in 1994. This method allows the facile preparation of highly stable AuNPs with a precise control of the size and a low polydispersity. The synthesis is based on a two-phase process in which a water-soluble gold salt (AuCl4-) is transferred in toluene using tetraoctylammonium bromide as a phase-transfer agent. Gold ions are then reduced by NaBH4 in the presence of dodecanethiol acting as a stabilizing agent [54]. During the reaction, the color of the organic phase changes from orange to deep brown upon the addition of NaBH4, evidencing the formation of the nanoparticles. Transmission electron microscopy (TEM) pictures confirmed that the diameters were in the 1–3 nm range. Two structures were observed, either cuboctahedral or icosahedral (Figure 1). By varying the experimental conditions, they proved the possibility to tune the size of the resulting particles: larger thiol/gold mole ratios, fast reductant addition, or cooled solutions lead to smaller and more monodisperse particles. A few years later, Murray and collaborators proposed a way to obtain a higher amount of small core sizes (2 nm) using steric hindrance of bulky ligands. Varying the surface coverage of the nanoparticles opened the way to a new functionalized family of AuNP called monolayer-protected clusters [55], [56].

Brust method description (top). TEM pictures of the thiol derivatized gold nanoparticles at (A) low and (B) high magnification (down). Adapted from Ref. [53] with permission of The Royal Society of Chemistry.
Figure 1:

Brust method description (top). TEM pictures of the thiol derivatized gold nanoparticles at (A) low and (B) high magnification (down). Adapted from Ref. [53] with permission of The Royal Society of Chemistry.

Among the numerous methods for preparing AuNPs, the most widely applied was the one introduced by Turkevitch et al. [57] in 1951. The breakthrough associated with this preparation is based on the fact that large colloids are prepared in aqueous medium. During the reaction, sodium citrate is used as both reducing and capping agent, ensuring the good stability of the particles. The process leads to AuNPs with a diameter of 15 nm and the obtained rubi-red suspensions exhibit an intense absorption band centered at 520 nm when characterized with optical spectroscopy. Years later, the process was revisited by Frens [9] where the ratio between the reducing/stabilizing agent and the gold salt varied in order to obtain AuNPs of different sizes with diameter up to 147 nm. Although the process offers many advantages (reaction in water, reactive AuNPs easily functionalized with the desired ligands), the precise control of the size with a low polydispersity remains difficult for particles >30 nm. In order to provide maximum control on large-sized AuNPs, Navarro and co-workers [37] used a “one-pot” protocol, relying on a modified Turkevitch-Frens procedure. The method consists in the addition of sodium acetylacetonate Na(acac) [co-reducing agent, allowing also the complexation of Au(III) to Au(I)], immediately followed by the addition of sodium citrate [reduction of Au(I) to Au(0)]. The concentration of sodium acetylacetonate directly impacts the nuclei numbers, inducing an increase of the AuNP size (up to 90 nm) with a low polydispersity. Ultraviolet/visible (UV/Vis) analysis showed a rather sharp absorption band that is consistent with a narrow size distribution, and a red shift was observed as the particle diameter increased. The TEM pictures were in good agreement with these results (Figure 2).

Left: Nanoparticle size evolution depending on the acetylacetonate concentration. Right: UV/Vis (middle) and TEM analysis (right).
Figure 2:

Left: Nanoparticle size evolution depending on the acetylacetonate concentration. Right: UV/Vis (middle) and TEM analysis (right).

2.1.2 Hollow spheres

The investigation of gold-based hollow nanospheres was pioneered in 1997 by the Halas research group [58]. These hollow spheres consist in a gold shell covering the surface of spherical silica particles. The preparation of such systems [59] consists in a two-step process: first, the functionalization of silica nanoparticles with gold seeds followed by the growth of a shell by reduction of a gold salt (Figure 3 – left). Interestingly, depending on the silica particle functionalization with different silanes (thiol or amine), the seed covering differs drastically from well-dispersed particles to aggregates. This first step remains very important as it drives the homogeneity of the final gold shell [60]. It was found that gold nanoshells exhibit different optical properties from the usual AuNPs. During the growth of the gold layer [61], [62], [63] on the silica core, the resulting absorption band undergoes very large shifts from 650 to 900 nm. The optical properties revealed that such systems have an SPB that depends on the ratio of the core radius to the total radius. The Halas group evidenced the great potential of these nanoshells for optical applications as the absorption band can be tuned up to the near-infrared (NIR) range (Figure 3 – right). More recently, we proposed a biphasic method for the preparation of gold hollow spheres with a liquid core [64]. The synthesis is derived from the Brust process: it occurs in a biphasic medium consisting in a suspension of water droplets in toluene (Figure 4). A gold complex resulting in the association between Au(I) and dodecanethiol is obtained in toluene after previous dissolution of a gold salt in water. An emulsion is formed using strong mechanical stirring. The gold shells are formed after addition of a solution of NaBH4 in water. Through this step, the Au(I) complexes are reduced to Au(0) at the water/toluene interphase of the droplets, forming a shell of metallic gold stabilized with dodecanethiol at their surface (Figure 5 – right). The obtained shells exhibit absorption bands similar to the ones obtained with the hollow spheres on a silica core (two contributions: dipolar and quadrupolar centered in the NIR area) (Figure 5 – left). Interestingly, this process also allowed the entrapment of organic dyes within the gold shell; in that case, the final hybrid nano-objects exhibit interesting optical properties that will be discussed in the second part of this review.

Left: Strategy used to grow a gold nanoshell around a silica nanoparticle core. Reprinted with permission from Ref. [60]. Copyright (2015) American Chemical Society. Right: Spectra of nanoshells with silica seed solution: gold chloride solution (0.38 mm HAuCl4) ratios of (A) 0.6:6.0 and (B) 0.5:6.0 (solid lines). Dotted lines are calculated best fits with a 210 nm silica core and gold shell thicknesses of (A) 6.0 and (B) 10.5. Reprinted with permission from Ref. [61]. Copyright (2015), AIP Publishing LLC.
Figure 3:

Left: Strategy used to grow a gold nanoshell around a silica nanoparticle core. Reprinted with permission from Ref. [60]. Copyright (2015) American Chemical Society. Right: Spectra of nanoshells with silica seed solution: gold chloride solution (0.38 mm HAuCl4) ratios of (A) 0.6:6.0 and (B) 0.5:6.0 (solid lines). Dotted lines are calculated best fits with a 210 nm silica core and gold shell thicknesses of (A) 6.0 and (B) 10.5. Reprinted with permission from Ref. [61]. Copyright (2015), AIP Publishing LLC.

Emulsion process leading to gold hollow spheres. Reprinted from Ref. [64], Copyright IOP Publishing. Reproduced with permission. All rights reserved.
Figure 4:

Emulsion process leading to gold hollow spheres. Reprinted from Ref. [64], Copyright IOP Publishing. Reproduced with permission. All rights reserved.

Left: Typical absorption spectrum of single hollow sphere. Right: TEM picture of the particles.
Figure 5:

Left: Typical absorption spectrum of single hollow sphere. Right: TEM picture of the particles.

2.1.3 Nanostars

Compared to all other isotropic particles described in that section, nanostars are the newest shapes designed within the last decade. These multipod particles can be considered as isotropic as the average objects are still round shaped. These systems were first described as star polyhedral gold nanocrystals by Burt et al. [65] in 2005. In fact, the preparation of these nanostars was based on fast and easy gold salt reduction with ascorbic acid in water under ambient conditions. The authors outlined two distinct types of star polyhedral nanocrystals, corresponding to icosahedra and cuboctahedra with preferential growth of their exposed {1 1 1} surfaces. The size of these crystals is in the 100–200 nm range; however, despite this broad variation, the geometric proportions of a given type of star nanocrystal are remarkably consistent from one crystal to another, regardless of the size. In 2008, Senthil Kumar et al. [66] described the preparation of multipod Au nanoparticles with single crystalline tips in high yield. The method employed used preformed Au nanoparticle seeds and the reduction of HAuCl4 in different solutions of poly(vinylpyrrolidone) in N,N-dimethylformamide (10, 5, and 2.5 mm). The particle diameter is in the 50–80 nm range with a low polydispersity when gold seeds are used during the preparation (Figure 6 – left). The nanostar suspension displayed a well-defined optical response, which can be theoretically resumed as a main mode confined within the tips and a secondary mode confined in the central core (Figure 6 – right). Theoretical modeling of the surface-enhanced Raman scattering (SERS) response shows that this morphology will be relevant for sensing applications allowing a strong signal enhancement. A few years later, similar shapes were investigated with a seed-mediated growth approach using surfactants [67]. Following these studies, we recently proposed a method to prepare star-shaped gold nanostructures in high yield, through the same process [24]. Ascorbic acid was used as a mild reducing agent and AuNPs (13 nm) as seeds. The process was optimized by studying the influence of several types of surfactants (domiphen bromide, myristil bromide, or cetyl trimethyl ammonium bromide) in the growth solution and stabilizing the seeds. Nanostars with sharps tips were obtained in 100% yield according to TEM pictures, exhibiting two distinct plasmon resonances that could be attributed to the star core (630 nm) and the elongation of the tips for the resonance in the NIR region (800–1100 nm), making them suitable for applications in biotechnologies (Figure 7).

Left: TEM picture of nanostars. Right: Experimental (solid line) and BEM calculated absorption spectra for Au nanostars, averaged over the measured distribution of tip-aperture angles α (broken curves). The morphologies corresponding to the various calculations are graphically depicted along with a TEM image. Adapted from Ref. [66], Copyright IOP Publishing. Reproduced with permission. All rights reserved.
Figure 6:

Left: TEM picture of nanostars. Right: Experimental (solid line) and BEM calculated absorption spectra for Au nanostars, averaged over the measured distribution of tip-aperture angles α (broken curves). The morphologies corresponding to the various calculations are graphically depicted along with a TEM image. Adapted from Ref. [66], Copyright IOP Publishing. Reproduced with permission. All rights reserved.

Left: Extinction spectrum of nanostars in solution. Right: TEM picture of gold nanostars. Reprinted from Ref. [24], Copyright IOP Publishing. Reproduced with permission. All rights reserved.
Figure 7:

Left: Extinction spectrum of nanostars in solution. Right: TEM picture of gold nanostars. Reprinted from Ref. [24], Copyright IOP Publishing. Reproduced with permission. All rights reserved.

2.2 Anisotropic nanoparticles

During the last decade, a large variety of processes and techniques involving stabilizing agents have been used to synthesize many types of anisotropic AuNPs. However, the versatile and more complex seed-mediated method has become dominant.

2.2.1 Nanorods

Within the toolbox of anisotropic AuNPs, nanorods are among the more studied systems. Although various preparation methods [68], [69], [70] for such objects have been described since 1995 (photochemical reduction, growth in hard templates, electrochemical process), it is in the early 2000s that the Murphy research group developed a seminal method for the formation of gold nanorods (AuNRs) in solution [71], [72]. Briefly, a gold salt (HAuCl4) is partially reduced by ascorbic acid in the presence of cetyltributylammonium bromide (CTAB) and AgNO3; the growth of the rods is then triggered by the addition of citrate-capped spherical AuNPs acting as seeds. The mechanism involved is quite complex, although it was shown that these seeds catalyze the reduction of Au(I) to Au(0) with ascorbic acid. In the very first procedure, the AuNR aspect ratio (length/width) could be controlled from 1 to 7 by simply varying the ratio of seed to metal salt in the presence of a rodlike micellar template. A higher aspect ratio could be reached through a three-step procedure in the absence of AgNO3: the AuNRs formed during the first step serve as seeds for a second growth, and the latter are used as seeds for the third growth [15]. With that process, rods with aspect ratios up to 18 were obtained as evidenced by TEM pictures (Figure 8). During the next years, the process was slightly improved [73], [74], [75]; for example, the purity of the samples was increased with the addition of nitric acid to the third seeding growth solution [76]. Murphy and co-workers also pointed out the interesting optical properties of these nanorods. The SPB of these anisotropic particles is split in two bands: the longitudinal plasmon band, corresponding to light absorption and scattering along the longitudinal axis of the rod (LSPR), and the transverse plasmon band, for the short axis of the rod. This longitudinal plasmon band can be tuned up to the NIR for the longest systems. Within the same period of time, the El-Sayed research group proposed the preparation of AuNR based on the seed-mediated growth process with several modifications from the previous method [77]. First, the seeds are stabilized with CTAB instead of citrate ions and AgNO3 is used in the growth solution to control the aspect ratio of the AuNRs. In this approach, the seeds are obtained upon reduction of HAuCl4 with a cooled solution of NaBH4 in the presence of CTAB, followed by the addition of AgNO3. The AuNRs were obtained in high yield (99%) with aspect ratios between 1.5 and 4.5. The group [78] also studied the impact of AgNO3 concentration on the aspect ratio; in that case, longitudinal plasmon resonances could be observed in the NIR (Figure 9). Even higher aspect ratios, up to 10 or 20, could be obtained upon addition of a co-surfactant; a mixture of CTAB and benzyldimethylhexadecylammonium chloride (BDAC) led to rods with LSPR up to 1200 nm.

TEM picture of gold nanorods with various aspect ratios. Adapted with permission from Ref. [15]. Copyright (2015) American Chemical Society.
Figure 8:

TEM picture of gold nanorods with various aspect ratios. Adapted with permission from Ref. [15]. Copyright (2015) American Chemical Society.

Visible spectra of five identical growth solutions in which the silver content increases from sample no. 1–5. By controlling the Ag ion concentration, the length of the nanorods can be adjusted. Adapted with permission from Ref. [78]. Copyright (2015) American Chemical Society.
Figure 9:

Visible spectra of five identical growth solutions in which the silver content increases from sample no. 1–5. By controlling the Ag ion concentration, the length of the nanorods can be adjusted. Adapted with permission from Ref. [78]. Copyright (2015) American Chemical Society.

2.2.2 Bipyramids

One of the great interests with gold nanorods is their optical properties such as high extinction cross sections and local electric field enhancements at their tips. It has been recently shown that such optical properties could be enhanced with sharp nanostructures. In that context, anisotropic nanoparticles described as bipyramids represent a system of choice as they have shaper tips than the rods (stronger electric field) but also because they exhibit longitudinal surface plasmon (LSP) wavelengths close to those of AuNRs. Such nanostructures were first described as ϕ-shaped particles by Murphy’s group [72] in 2001, but mainly seen as a by-product during the preparation of nanorods. It is only in 2005 that Liu and Guyot-Sionnest [20] clearly studied the preparation and optical properties of gold bipyramids (AuBPs). In their work, they evidenced the importance of the seed structure (either single crystalline or multiply twinned) for obtaining either AuNRs or AuBPs with the seed-mediated growth process. The different structures depend on the method of preparation. CTAB-stabilized seeds (HAuCl4 reduction with NaBH4 in a CTAB solution) are mostly single crystalline, while multiply twinned nanoparticles are prepared by reduction of HAuCl4 with NaBH4 in the presence of sodium citrate. The final nanostructures are obtained by addition of the seeds in a growth solution containing CTAB, ascorbic acid, and silver salt. TEM pictures show diamond-like particles together with spheroids as by-products (Figure 10 – right). Bipyramids exhibit plasmon resonances with two contributions (longitudinal and transversal resonances) and a narrow LSPR associated with the sharpness of the tips (Figure 10 – left). Following this work, efforts have been made in the last 10 years to increase the yield of bipyramids and to understand their growth mechanism [20], [21], [79], [80], [81], [82]. Despite these efforts, relatively large amounts of other impurities such as nanospheres or prisms were still observed. Purification protocols have been proposed to prepare highly pure samples using a surfactant (CTAB) with larger head groups [21]. Recently, purification through depletion-induced flocculation has been shown to successfully separate a range of AuBPs using BDAC as surfactant [83]. In that context, pure AuBPs appeared difficult to prepare using synthetic approaches. Recently, we proposed a process allowing the synthesis of AuBPs in high yield (70%) using CTAB-stabilized gold seeds [19]. Briefly, the anisotropic particles were grown in a CTAB solution using hydroquinone as mild reducing agent and silver nitrate. It was possible to tune the size of the particles, thus the resonance of the plasmon, with the amount of seeds added in the solution. In 2015, we proposed a new method [84] based on the previous one, allowing the preparation of bipyramids with LSPR up to 1050 nm in high yield (90%) (Figure 11). The quality of the seeds, i.e. pentatwinned crystallinity, was ensured, thanks to a thermal treatment at 80°C. Anisotropic nanostructures were synthesized in a growth solution where hydroxylamine was used as reducing agent. Finally, by combining two surfactants in the growth solution, we were able to grow elongated bipyramids (nanojavelin) exhibiting longitudinal plasmon resonances in the infrared range up to 1900 nm.

Left: UV/Vis spectra of different gold nanobipyramid solutions prepared with various silver amounts. Right: TEM picture of bipyramids. Adapted with permission from Ref. [20]. Copyright (2015) American Chemical Society.
Figure 10:

Left: UV/Vis spectra of different gold nanobipyramid solutions prepared with various silver amounts. Right: TEM picture of bipyramids. Adapted with permission from Ref. [20]. Copyright (2015) American Chemical Society.

Left: Absorption spectra of the synthesized bipyramids versus the volume of seed solution added. Right: TEM of bipyramids prepared with CTAB and 800, 80, 40, and 6 μl of seed suspension. Reproduced from Ref. [84] with permission from The Royal Society of Chemistry.
Figure 11:

Left: Absorption spectra of the synthesized bipyramids versus the volume of seed solution added. Right: TEM of bipyramids prepared with CTAB and 800, 80, 40, and 6 μl of seed suspension. Reproduced from Ref. [84] with permission from The Royal Society of Chemistry.

3 Fluorescent studies

The interaction of a fluorescent probe with a metallic surface strongly affects its photophysical properties [38], [39]. The precise control of the dye and metallic surface interactions have been the focus of many scientists, yielding a huge range of publications on fluorescence quenching [40], [41], [42], [43] or enhancement [44], [45], [46], [47]. Several parameters may affect this fluorescence response such as the metal SPB-chromophore spectral overlap [85] (absorption-emission of the dye), the surface-dye distance [86], [87], [88], or the molecular dipole orientation on the surface [89], [90]. The close vicinity (<10 nm) of a chromophore with a metallic nanoparticle surface usually leads to a strong fluorescence quenching [91]. Thus, many efforts have been made to modulate the surface-dye distance using spacers like DNA [88], [92], [93], polyelectrolyte [86], [94], [95], or silica shells [96], [97].

One major issue when using fluorescent dyes is their loss of fluorescence intensity within a certain period of irradiation [98], [99] (e.g. bleaching effect). This issue might compromise their further use in biological applications, for example. However, the interaction of a chromophore with a metallic surface may prevent this bleaching process [98]. Nonetheless, the dye optical signature, when labeled onto the AuNPs, is unfortunately affected (e.g. quenching [38], [43], [91], [100], [101] or enhancement [47], [93], [94], [102], [103], [104], [105] of the fluorescence signal).

The possibility to both modulate the optical response of these nanoparticles and functionalize their surface with organic moieties, e.g. chromophores, opens the way to new families of luminescent organic-inorganic nanomaterials. In this part, we will discuss the different parameters that might affect the photophysical response of a dye near metallic nanoparticles.

3.1 Photoluminescence of individual, naked plasmonic nanoparticles

Within the large spectrum of gold nanostructures, numerous reports describe the intrinsic luminescence properties of such systems [106], [107], [108], [109], [110], [111], [112], [113], [114]. This interesting feature is a great alternative with regard to light scattering measurements [7], [115], especially for biological or clinical applications with an easier detection and localization of these particles. However, the photoluminescent quantum yield of these AuNPs remains lower than the one of classical organic dye; however, their resistance against photobleaching and photoblinking make them attractive candidates. These intrinsic luminescent properties can be observed in different conditions: with one photon or multiphoton excitation.

In a recent review, Zheng et al. proposed to consider several aspects influencing the final luminescent properties of gold particles. They postulated that luminescent AuNPs could be divided into two groups [109]: the molecular nanoparticles [116], [117], [118], [119] (from gold cluster to 3 nm AuNPs) and the plasmonic nanoparticles [110], [120]. The luminescent property of plasmonic nanoparticles is explained through different mechanisms. Mohamed et al. [106] assigned the emission property to the electron and hole interband recombination, while Dulkeith et al. [121] attributed the phenomena to an energy transfer between the excited d-band holes and the sp electrons. However, Dulkeith et al. [121] measured the photoluminescence (PL) quantum yield of spherical AuNPs with various sizes and found a PL efficiency close to 10-6. In their very recent work, Rao et al. [110] compared the PL of individual gold nanorods and bipyramids by combining PL and atomic force microscopy measurements with discrete dipole approximation simulations. They were able to obtain the PL quantum yield of both rods and bipyramids. Due to the stronger field intensity surrounding bipyramids, they exhibit higher PL quantum yields compared to rods in the similar surface plasmon resonance range (Figure 12).

PL quantum yield for various aspect ratios of gold nanorods and bipyramids. Reprinted with permission from Ref. [110]. Copyright (2015) American Chemical Society.
Figure 12:

PL quantum yield for various aspect ratios of gold nanorods and bipyramids. Reprinted with permission from Ref. [110]. Copyright (2015) American Chemical Society.

3.2 Chromophore on nanoparticle surface

3.2.1 Influence of the nanoparticle size

Spherical AuNPs with different sizes can be synthesized through different approaches (see before). To ensure a good stability of these nanoparticles in suspension into different media (biological buffer, organic solution) and for further grafting of the desired entities onto the metallic surface, the nanoparticles can be functionalized through the self-assembled monolayer (SAM) process [122], [123], [124], [125], [126], [127], [128]. SAMs are generally produced by simply mixing an AuNP suspension with a solution of the desired molecules. The labeling on the particle surface can be possible through different pathways [129]. The first option uses the binding affinities of molecules with the gold surface to form a monolayer on the AuNPs. The second one is the ligand exchange reaction, allowing the grafting of a desired ligand on already covered nanoparticles. The third one is the chemical reaction between the protecting ligand terminal group and other molecules for introducing new entities on the nanoparticle surface (post-functionalization). Through the mentioned self-assembly approaches, a wide range of multifunctional hybrid nano-objects that incorporate fluorescent probes were described [39], [41], [100], [104], [130], [131]. Among all of them, fluorescein derivative has been extensively used mostly because its precursors present the advantage to be commercially available, water soluble, and reactive toward various chemical functions.

In a series of publications, Werts and co-workers [40], [100], [132] studied the optical properties of dye-based ligands on 13 nm AuNPs. Their main strategy was to produce hybrid-protected AuNPs through the self-assembly process with modified chromophore-bearing thiol units. After characterizing the photophysical properties of the hybrid nanoparticles, the fluorescent moiety was released from the surface through a ligand exchange reaction with the addition of a large excess of mercaptoethanol (Figure 13). Before the ligand exchange reaction, the fluorescent signal of fluorescein-5-thiocarbamoyl chromophore was totally quenched; the signal was fully recovered once the ligand exchange reaction was completed. Using the same procedure but with a reversed strategy, Nerambourg et al. [38] inserted fluorescently labeled thiol molecules on AuNPs stabilized with thiolated ligands. The reaction was followed through the change in the photophysical properties of the dye as the grafting onto the AuNP occurred. During that study, three different chromophores were studied, and all exhibited a fluorescence emission quenching when grafted on the AuNP surface (Figure 14). As a matter of fact, when using this quenching phenomenon, it remained possible to analyze the ligand exchange equilibrium, and study the affinity of the fluorescently labeled thiols for the protected AuNPs. In the early 2000s, a different method was proposed by Dulkeith et al. [133] who investigated the photophysical effect (radiative and non-radiative decay rates) of a thioether-modified lissamine dye grafted around AuNPs of different sizes (1–30 nm), through time-resolved fluorescence spectroscopy. The chromophore was estimated to be at 1 nm distance from the gold surface. To limit the presence of unbound dye in the suspension and to avoid the dye-to-dye proximity on the surface, they decided to cover only 50% of the AuNP surface with the luminescent ligand. The labeling and close proximity of the dye onto the AuNP surface quenched the fluorescence signal, which was evidenced by a fast lifetime emission signal when compared to the signal of the free dye (1.54 ns). The fluorescence lifetimes were respectively estimated to 169, 99, and 72 ps for particle sizes 2, 30, and 60 nm. The quenching effect was attributed to an increase of the non-radiative rate (due to resonant energy transfer) and a decrease of the dye’s radiative rate. More recently, Sutarlie et al. [134] studied the AuNP size effect on the fluorescent property of a protein-DNA fluorescent complex. They used fluorescein-labeled DNA probes with different proteins (FoxA1 and AP2γ) to form the protein-DNA complexes, and used the ability of the proteins (cysteine, histidine units) to functionalize citrate-stabilized AuNPs of various sizes (13 and 100 nm). The 13 nm AuNPs quenched the fluorescence signal of the dye (>30%), while the 100 nm AuNPs enhanced it up to 50%.

Left: Liberation of bound fluorophores by ligand exchange using a large excess of β-mercaptoethanol. Right: Evolution of the fluorescence intensity (λexc=469 nm) of fluorescently tagged MP-AuNPs upon addition of 4 mm β-mercaptoethanol. The fluorescence intensity increases to reach a new constant value after 33 h (spectra taken before; just after; and at 0.5, 18, 23, and 33 h after addition of β-mercaptoethanol). Reprinted from Ref. [40]. Copyright 2008 Inderscience Enterprises Ltd.
Figure 13:

Left: Liberation of bound fluorophores by ligand exchange using a large excess of β-mercaptoethanol. Right: Evolution of the fluorescence intensity (λexc=469 nm) of fluorescently tagged MP-AuNPs upon addition of 4 mm β-mercaptoethanol. The fluorescence intensity increases to reach a new constant value after 33 h (spectra taken before; just after; and at 0.5, 18, 23, and 33 h after addition of β-mercaptoethanol). Reprinted from Ref. [40]. Copyright 2008 Inderscience Enterprises Ltd.

Left: Thiol-modified chromophores. Top right: Ligand exchange reaction. Bottom right: Fluorescence intensity evolution during ligand exchange reaction. Adapted with permission from Ref. [38]. Copyright (2015) American Chemical Society.
Figure 14:

Left: Thiol-modified chromophores. Top right: Ligand exchange reaction. Bottom right: Fluorescence intensity evolution during ligand exchange reaction. Adapted with permission from Ref. [38]. Copyright (2015) American Chemical Society.

According to these works, it appears clearly that the size of spherical particles plays an important role on the photophysical properties of the final hybrid nanoparticles. When increasing the size of the particles, scattering becomes significant and might be an important parameter to take into account. Nevertheless, it remains difficult to see a clear trend (i.e. quenching or enhancement) linked to the increase or diminution of the particles sizes, as long as the dyes are not always located at the same distance from the gold surface in the different studies. It thus seems interesting to focus on that last parameter.

3.2.2 Influence of the chromophore-particle distance

One of the most important factors influencing the final luminescent properties of hybrid AuNPs is without any doubt the chromophore to gold surface distance. In the last decade, several approaches have been considered in order to control such parameter. Among them, Anger et al. [91] studied experimentally and theoretically the chromophore to gold surface influence, through a physical approach. First, a single spherical AuNP was attached to the end of an optical fiber and positioned into the center of a laser focus. In the meantime, a Nile blue solution was spin coated on a glass coverslip and covered with a thin layer of polymethyl methacrylate. Finally, the emission rates were recorded as a function of the distance between the two elements. For distance (z) <5 nm, the fluorescence signal was quenched, while the intensity maximum was reached for a distance of 5 nm. Thus, by tuning the dye-to-particle distance, Anger and co-workers showed the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching.

In one of our studies, we investigated the role of AuNP sizes when functionalized with modified luminescent polymers of various lengths [37]. The polymer shell thickness was monitored by TEM and estimated respectively to 2.5, 6.5, and 12 nm for 13, 50, and 90 nm AuNPs (Figure 15 – left). To study the interaction between the polymer and the AuNPs and further highlight the influence on the photophysical properties of the dye, the gold core was dissolved using cyanide salts. This operation allowed to restore the initial luminescence of the dye and thus estimate the fluorescence quenching (comparison before and after the removal of the gold core). Moreover, it was also possible to estimate the number of polymer chain per nanoparticle. For the different gold particles sizes, 13, 50, and 90 nm, the relative fluorescent brightness was estimated to be 60%, 71%, and 85% (Figure 15 – right).

Left: TEM pictures of the polymer functionalized nanoparticles 50–90 nm. Right: Fluorescence intensity of the hybrid nanoparticles.
Figure 15:

Left: TEM pictures of the polymer functionalized nanoparticles 50–90 nm. Right: Fluorescence intensity of the hybrid nanoparticles.

An efficient chemical strategy allowing great control of the distance between a dye and the gold surface is the layer-by-layer approach [86], [135]. This method consists of successive absorptions of oppositely charged polyelectrolytes on the AuNPs, allowing a perfect control on the final polymer thickness. The polyelectrolyte adsorption can be monitored using UV/Vis spectroscopy. Zeta potential measurements and the polymer shell thickness can also be estimated using TEM. In the case of the UV/Vis analysis, the position of the SPB is affected (red shifted) when the surrounding medium of the AuNPs is modified (e.g. changing the refractive index around the nanoparticles). This red shift can be attributed to an electron transfer from the particles to the polyelectrolyte. Indeed, the SPB will be red shifted as the deposition of the polyelectrolytes consecutively increases. Schneider et al. [86] used this approach to modulate the distance between a chromophore and an AuNP (13 nm). By using different kinds of non-fluorescent polyelectrolytes, they were able to tune the distance between the surface and the final layer, which was modified with a luminescent dye (Figure 16 – left). Through the deposition of up to 20 layers, 50% of the initial fluorescent signal could be recovered (Figure 16 – right). The use of such spacer allowed avoiding a complete quenching of the fluorescence signal.

Left: TEM overview on particles bearing 10 primer pairs of PAH/PSS layers and further coated with PAH-FITC, Au(PAH/PSS)10(PAH-FITC). Right column: TEM of layer-by-layer-coated individual gold nanoparticles ensheathed with n = 1, 5, and 10 pairs of PAH/PSS spacer layers, respectively, with a last layer of PAH-FITC, Au(PAH/PSS)n-(PAH-FITC), before deposition of the terminating PSS layer. Right: Fully corrected emission spectra of core-shell nanoparticles of structure Au(PAH/PSS)n(PAH-FITC/PSS) (exc=490 nm) in aqueous suspension. Adapted with permission from Ref. [86]. Copyright (2015) American Chemical Society.
Figure 16:

Left: TEM overview on particles bearing 10 primer pairs of PAH/PSS layers and further coated with PAH-FITC, Au(PAH/PSS)10(PAH-FITC). Right column: TEM of layer-by-layer-coated individual gold nanoparticles ensheathed with n = 1, 5, and 10 pairs of PAH/PSS spacer layers, respectively, with a last layer of PAH-FITC, Au(PAH/PSS)n-(PAH-FITC), before deposition of the terminating PSS layer. Right: Fully corrected emission spectra of core-shell nanoparticles of structure Au(PAH/PSS)n(PAH-FITC/PSS) (exc=490 nm) in aqueous suspension. Adapted with permission from Ref. [86]. Copyright (2015) American Chemical Society.

A different way to modulate the particle-to-chromophore distance was considered by Reineck et al. [136] who grew silica shells of various thicknesses, from 3.6 to 45 nm, around AuNPs (Figure 17 – left). Different chromophores (Atto488, Atto532, Atto565, and Atto700) were then grafted on the silica shell, which could be excited at different wavelengths. The results showed that whatever the dye, 90% of the fluorescence signals were quenched for the shortest distance (3.6 nm). The full fluorescence signal was recovered in the case of large chromophore-gold distance (45 nm) (Figure 17 – right). However, the signal recovery appears to have an excitation wavelength dependency with the position of the SPB. The 13 nm AuNPs exhibit such resonance around 525 nm, and the most consequent quenching was observed for the Atto532 dye. In that particular case, the quenching was still observed until a distance of 20 nm was reached, while other dyes almost recover 80% of their initial fluorescence. That result clearly pointed out another important question regarding the overlap between plasmon resonance and fluorophore features (extinction and emission bands).

Left: Au@SiO2 core-shell nanoparticles with 12.7±0.3 nm Au cores and 3.6±0.5 nm (up), 17.6±0.7 nm (middle), and 34±1.0 nm thick (down) silica shells. Scale bar in all images: 50 nm. Right: Experimental (black squares) and theoretical (solid and dashed lines) results for the relative fluorescence intensity as a function of the nanoparticle dye separation d. The fluorophores shown are Atto488 (A), Atto532 (B), Atto565 (C), and Alexa 700 (D). Adapted with permission from Ref. [136]. Copyright (2015) American Chemical Society.
Figure 17:

Left: Au@SiO2 core-shell nanoparticles with 12.7±0.3 nm Au cores and 3.6±0.5 nm (up), 17.6±0.7 nm (middle), and 34±1.0 nm thick (down) silica shells. Scale bar in all images: 50 nm. Right: Experimental (black squares) and theoretical (solid and dashed lines) results for the relative fluorescence intensity as a function of the nanoparticle dye separation d. The fluorophores shown are Atto488 (A), Atto532 (B), Atto565 (C), and Alexa 700 (D). Adapted with permission from Ref. [136]. Copyright (2015) American Chemical Society.

Another possibility to tune the distance between the gold nanoparticle surface and the dye is to use fluorescently modified DNA. Acuna et al. [43] studied such dependence of a single chromophore using self-assembled DNA structure (DNA origami). With this approach, a chromophore (ATTO647N) and 10 nm AuNP are incorporated into the DNA origami, at a specific position (Figure 18 – left). Four different DNA origamis were constructed with different nanoparticle-chromophore distance (10–28.6 nm). The attachment of the gold nanoparticle on the origami leads to a decrease of the fluorescence intensity and lifetime for distance <28.6 nm (Figure 18 – right). Other groups also investigated the use of modified DNA on metallic surface [137], [138], [139], [140]. Cheng and co-workers [141] used different sizes of AuNPs (20, 50, and 100 nm) functionalized with fluorescently labeled hairpin DNA probes of different lengths. In all cases, the close proximity of the dye around the surface leads to a strong quenching of the dye. To modulate the distance, they introduced a complementary target DNA, which opened the hairpin and stretched the DNA strands. Through this variation, the distance between the particle and the dye was varied from 8 to 20 nm. For the higher value, it was stated that all double-stranded DNA adopted a straight configuration. As stated before, the increase of the spacing allowed recovering the initial fluorescence signal. However, an increase of the fluorescence signal was noticed for the 100 nm AuNPs with a distance >13 nm.

Left: Sketch of the two-dimensional DNA origami with attached metallic nanoparticle. To illustrate the dye positions, all four positions are shown simultaneously (d1d4), although every DNA origami sample contains only one red fluorophore. The green dots represent single-stranded DNA labeled with Cy3 to visualize the gold nanoparticles Right: (A) Relative change in fluorescence intensity and (B) fluorescence lifetime as a function of the distance between the fluorophore and the MNP. The black squares represent the mean values and standard deviations obtained from the single molecule data. Blue, red, black, and gray solid lines represent the tangential, radial, weighted average, and NSET calculation (with d0=8.38 nm), respectively. Adapted with permission from Ref. [43]. Copyright (2015) American Chemical Society.
Figure 18:

Left: Sketch of the two-dimensional DNA origami with attached metallic nanoparticle. To illustrate the dye positions, all four positions are shown simultaneously (d1d4), although every DNA origami sample contains only one red fluorophore. The green dots represent single-stranded DNA labeled with Cy3 to visualize the gold nanoparticles Right: (A) Relative change in fluorescence intensity and (B) fluorescence lifetime as a function of the distance between the fluorophore and the MNP. The black squares represent the mean values and standard deviations obtained from the single molecule data. Blue, red, black, and gray solid lines represent the tangential, radial, weighted average, and NSET calculation (with d0=8.38 nm), respectively. Adapted with permission from Ref. [43]. Copyright (2015) American Chemical Society.

The dye-to-particle distance is a key parameter in the optimization of optical features of hybrid nanoparticles, much more than the influence of the particle size. Interestingly, all studies agree with the fact that for shortest lengths, quenching occurs all the time and that it is possible to recover the luminescent signal when increasing the distance. In the meantime, no accurate optimal spacing was found to enhance the fluorescence properties of the chromophores. That specific feature (although it has been evidenced) is highly dependent on the system studied. Thus, considering the results presented with spherical AuNPs, it appears that the best possibilities to enhanced fluorescence signal could be obtained by combining both large-sized nanoparticles with an optimal distance between the metallic surface and the dye.

3.2.3 Influence of the nanoparticle shape

Another parameter influencing the photophysical properties of the final object is the morphology of the nanoparticles [105], [131], [142]. In that context, sharp structures are very promising due to the field enhancement they feature thanks to their tips. Thus, one can expect a strong coupling between a dipole and these nanostructures.

Thanks to their shape, nanostars appear to be attractive systems for fluorescence enhancement of surrounding dyes. Although few works are focusing precisely on such studies, organically modified gold nanostars exhibit attractive behavior for SERS and further use in bioapplications [97], [142], [143]. Recently, Li et al. [144] developed fluorescent hybrid gold nanostars to monitor their distribution in mice blood vessels, liver, and spleen. The gold nanostars were functionalized with a PEGylated ligand followed by the labeling of fluorescent molecule (Cy5.5). Even considering the quenching effect of the dye around the gold nanostructure, it remained possible to precisely image the biodistribution of the nano-objects in the liver and the spleen (Figure 19). Thirty minutes after injection of the modified gold nanostars, the fluorescence appear to be intense in the liver part of the mice, while the fluorescence intensity in the spleen constantly increased after 3 h.

Fluorescence images of the biodistribution of GNS-Cy5.5 in the liver (red arrow) and the spleen (black arrow). (A) In vivo fluorescence images of liver and spleen in mouse acquired before (0 h) and after injection (0.5, 1, 1.5, 2, 2.5, and 3 h) of GNS-Cy5.5. (B) Average light intensity of the liver time-activity curve. (D) Average signal of the spleen time-activity curve. Fluorescence signal unit: 109 photons/cm2/s. Reprinted from Ref. [144]. Copyright © 2014 Optical Society of America.
Figure 19:

Fluorescence images of the biodistribution of GNS-Cy5.5 in the liver (red arrow) and the spleen (black arrow). (A) In vivo fluorescence images of liver and spleen in mouse acquired before (0 h) and after injection (0.5, 1, 1.5, 2, 2.5, and 3 h) of GNS-Cy5.5. (B) Average light intensity of the liver time-activity curve. (D) Average signal of the spleen time-activity curve. Fluorescence signal unit: 109 photons/cm2/s. Reprinted from Ref. [144]. Copyright © 2014 Optical Society of America.

In our group, we prepared surface-modified gold nanostars using the layer-by-layer deposition process with the idea to study the plasmonic effects on different dyes at various distances from the gold surface [135]. The precise tuning of organic layer was achieved using a successive deposition of anionic and cationic polyelectrolyte. Thus, the distance between the metallic surface and the dye varied from 4.5 to 15 nm. The fluorescent dyes (rhodamine B, Atto590, and Atto610) were finally deposited on the last polyelectrolyte layer. Through this process, the influence of the spectral overlap (absorption-emission) of the chromophore with the extinction band of the gold nanostars, as well as the dye-particle distance were respectively investigated. The photophysical properties of all the dyes were strongly quenched for shortening the particle-chromophore distance (4.5 nm). However, it is interesting to note that the fluorescence intensities were almost totally recovered with a minimal distance of 10 nm (n=7) (Figure 20).

Emission spectrum of hybrid gold nanostars stabilized with several layers of polyelectrolytes. (A) Rhodamine B, (B) Atto590, and (C) Atto610. “n” corresponds to the number of deposited layer. The chromophores were deposited on the ultimate PSS layer of the structure through electrostatic interactions: AuStar(PSS-PDD)0(PSS)1 and AuStar(PSS-PDD)5(PSS)1. (D) Normalized fluorescence intensity as a function of the estimated dye-to-particle distances. Reprinted with permission from Ref. [135]. Copyright (2015) American Chemical Society.
Figure 20:

Emission spectrum of hybrid gold nanostars stabilized with several layers of polyelectrolytes. (A) Rhodamine B, (B) Atto590, and (C) Atto610. “n” corresponds to the number of deposited layer. The chromophores were deposited on the ultimate PSS layer of the structure through electrostatic interactions: AuStar(PSS-PDD)0(PSS)1 and AuStar(PSS-PDD)5(PSS)1. (D) Normalized fluorescence intensity as a function of the estimated dye-to-particle distances. Reprinted with permission from Ref. [135]. Copyright (2015) American Chemical Society.

To extend the possibilities of surface modification of gold nano-object with photoresponsive organic moieties, hollow spheres appear to be very interesting tools for fluorescence enhancement. In 2007, Tam and co-workers [145] examined the role of the nanoparticle plasmon resonance energy and nanoparticle scattering cross section on the fluorescence enhancement of adjacent indocyanine green (ICG) dye molecules. Several objects were considered with various sizes and gold layer thicknesses, in order to compare the influence of the plasmon resonance with the absorption and emission bands of the dye. The strongest enhancement factor (up to 50) was achieved with the nanoshell, whose plasmon resonance overlaps with the ICG emission wavelength (Figure 21). A few years later, the same group [146] proposed a study comparing the impact of either gold hollow spheres or gold nanorods on the emission properties of a cyanine dye (IR800). For that study, human serum albumin (HSA) was used as a spacer between the gold surface and IR800. The measurements revealed that the quantum yield of IR800 was enhanced from 7% as an isolated fluorophore to 86% in the case of the hollow sphere conjugate and 74% in the case of the nanorods, proving, once again, the necessity of an overlap between the emission band of the dye and the plasmon resonance of the particles for an efficient enhancement. In 2011, we proposed a different approach for studying the luminescent properties of a dye in the vicinity of a gold shell. For this work, instead of surrounding the particle, rhodamine dyes were entrapped within the liquid core of gold hollow spheres [98]. Finite difference time domain calculations revealed that excitation and emission radiations were efficiently transmitted through the gold nanoshells. Measurements showed that encapsulated fluorophores keep their brightness, but exhibit fluorescence lifetimes 1 order of magnitude shorter leading to highly photostable systems (Figure 22).

(A) Schematic of gold nanoparticles used as fluorescence enhancement substrates, arranged from short to long plasmon resonance wavelength. One Au colloid and four nanoshells of various [r1, r2] were used. (B) Normalized extinction measurements from nanoparticle substrates corresponding to (A) in air prior to HSA and ICG deposition. The laser excitation is at 785 nm, and the emission wavelength of ICG attached to HSA is 850 nm. (C) Corresponding fluorescence emission from ICG conjugated to the nanoshell substrates adjusted for surface area available for fluorophore conjugation and normalized to the fluorescence from a control sample with no nanoparticles (black). Inset schematic illustrates experimental geometry. Adapted with permission from Ref. [145]. Copyright (2015) American Chemical Society.
Figure 21:

(A) Schematic of gold nanoparticles used as fluorescence enhancement substrates, arranged from short to long plasmon resonance wavelength. One Au colloid and four nanoshells of various [r1, r2] were used. (B) Normalized extinction measurements from nanoparticle substrates corresponding to (A) in air prior to HSA and ICG deposition. The laser excitation is at 785 nm, and the emission wavelength of ICG attached to HSA is 850 nm. (C) Corresponding fluorescence emission from ICG conjugated to the nanoshell substrates adjusted for surface area available for fluorophore conjugation and normalized to the fluorescence from a control sample with no nanoparticles (black). Inset schematic illustrates experimental geometry. Adapted with permission from Ref. [145]. Copyright (2015) American Chemical Society.

Left: Typical emission spectra of different rhodamine 610@Au nanoshells with water cores (dashed and dot curves) under the same excitation conditions. Black curve corresponds to the emission of rhodamine 610 in liposomes. Right: Fluorescence photoresistance of rhodamine 610 molecules encapsulated in Au nanoshell and liposomes. Adapted with permission from Ref. [98]. Copyright (2015) American Chemical Society.
Figure 22:

Left: Typical emission spectra of different rhodamine 610@Au nanoshells with water cores (dashed and dot curves) under the same excitation conditions. Black curve corresponds to the emission of rhodamine 610 in liposomes. Right: Fluorescence photoresistance of rhodamine 610 molecules encapsulated in Au nanoshell and liposomes. Adapted with permission from Ref. [98]. Copyright (2015) American Chemical Society.

We have seen earlier that nanorods are very interesting tools able to feature strong intrinsic fluorescence. Recently Murphy’s group focused their work on the enhancement properties of two photon absorbers for non-linear optics applications [131]. They proposed to use a layer-by-layer deposition of polyelectrolyte (up to eight layers) to control the dye to gold surface spacing on nanorods of a given aspect ratio. Then, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Interestingly, the strongest enhancements (40-fold) were observed for the chromophores separated only by two layers of polyelectrolyte (5 nm) from the gold surface (Figure 23). These results clearly showed that the non-linear optical properties of molecules can be significantly increased when placed near an active plasmonic surface, the shorter molecule-metal distances correlating with improved non-linear cross sections. Such behavior was confirmed later by the same group [105]. More recently, the same strategy was employed to evaluate the fluorescence enhancement of gold nanorods on infrared dyes [147]. The aspect ratio of the rod was adapted so the longitudinal plasmon band could overlap the absorption band of the dyes. Control of the chromophore-to-particle distance was achieved with different polyelectrolyte layers (0–24). These experiments clearly revealed that the enhancement depends on several parameters such as the excitation wavelength, extinction and emission of the dye, extinction of the AuNR, and the distance between the AuNR and the dye. From this, they postulated three main points: (i) the absorption maximum of the dye and extinction maxima of the metal nanostructures should have maximum overlap; (ii) the emission maximum of the dye should also partially overlap with the extinction band of the AuNRs; and (iii) the optimal distance (about 4 nm in their study) between dye molecules and plasmonic nanostructures is obtained by avoiding non-radiative energy transfer and ensuring a strong electromagnetic field.

Two-photon absorption cross-section enhancement (left axis) and simulated surface plasmon polariton electric field (right axis) versus distance from the gold nanorod surface, in terms of polyelectrolyte layers. The black line is an exponential fit to the data. The red lines are based on calculations of the electric field; the red dashed line is a single exponential fit including the two-layer data, while the red solid line does not include two-layer data. Adapted with permission from Ref. [131]. Copyright (2015) American Chemical Society.
Figure 23:

Two-photon absorption cross-section enhancement (left axis) and simulated surface plasmon polariton electric field (right axis) versus distance from the gold nanorod surface, in terms of polyelectrolyte layers. The black line is an exponential fit to the data. The red lines are based on calculations of the electric field; the red dashed line is a single exponential fit including the two-layer data, while the red solid line does not include two-layer data. Adapted with permission from Ref. [131]. Copyright (2015) American Chemical Society.

We have seen earlier that sharp and edgy structures are of great interest as they present strong localized electric field at the ends of the tips, that can be used for emission enhancement [148], [149], [150]. For that purpose, we studied hybrid systems consisting in bipyramidal AuNPs functionalized with hydrosoluble two-block polymers [151]: the first block acting as a spacer between the gold surface and the chromophore, while the second block was functionalized with an average of five Lucifer yellow dyes along the chain. After studying the photophysical properties of the hybrid gold nano-objects, the bipyramid nanoparticles were dissolved using a cyanide salt, revealing the brightness of the pure ungrafted luminescent polymer (Figure 24). Interestingly, the fluorescence intensity was higher before the dissolution of the gold core, proving its positive effect; it was found that the fluorescence intensity was enhanced by a factor of 1.4 even if no overlap was observed between the optical properties of the polymer and the surface plasmon resonance of the particles.

Left: Extinction spectra of CTAB-protected AuBPs (full line) and Lucifer yellow polymer functionalized PEGylated AuBPs (dotted line). Right: Fluorescence spectra of the LY-PNAM-PEG-Au-BP nanoparticles before (full line) and after (dotted line) dissolution of the gold core with the addition of the cyanide solution (12 h). Adapted from Ref. [151] with permission of The Royal Society of Chemistry.
Figure 24:

Left: Extinction spectra of CTAB-protected AuBPs (full line) and Lucifer yellow polymer functionalized PEGylated AuBPs (dotted line). Right: Fluorescence spectra of the LY-PNAM-PEG-Au-BP nanoparticles before (full line) and after (dotted line) dissolution of the gold core with the addition of the cyanide solution (12 h). Adapted from Ref. [151] with permission of The Royal Society of Chemistry.

4 Conclusion

For years, AuNPs have been the subject of an exponentially increasing number of studies. Such excitement is due to the diversity of their methods of preparations combined with the ability for scientists to design a wide range of morphologies. We have seen that the optical properties of these systems are closely related to their structures at the nanometer scale, which holds great potential for various applications such as in the optical, electronic, magnetic, catalytic, and biomedical fields. More recently, great progress has been made in terms of surface modifications with fluorophores. Thanks to their unique properties, AuNPs can modulate the photophysics of these dyes. In fact, the perfect control and understanding of the interactions between the excited state of a chromophore and the optical properties of an AuNP remain a great challenge for chemists and physicists. The idea of gathering all these efforts is to create new multifunctional organic-inorganic nano-tools featuring optimized luminescent properties.

Applications in biotechnologies are the proof of such interest, with the emergence of imaging agents combining several modalities using, for example, X-rays and fluorescence imaging. Efficient theranostic systems, associating diagnostic tools with treatment, focus also great attention, either through hyperthermia triggered by irradiation of the particles on their plasmon band or with the possibility for a gold nanoplatform to enhance the properties of a chromophore to generate singlet oxygen for photodynamic therapy.

Considering other fields of applications, assembling fluorophore-gold nanohybrids as two- or three-dimensional arrays should promote the design of a new generation of nanophotonic and optoelectronic materials, for applications in lasing or optical limiting, for example.

Finally, it is even realistic to consider over time that fluorescence enhancement is likely to develop into a field in its own right, in a manner analogous to the evolution of SERS with respect to Raman spectroscopy, as suggested by Geddes and Lakowicz in 2002.

Acknowledgments

We gratefully acknowledge the present and past members of our group for their contributions to the work described in this review, whose names are cited in the references. We also thank our collaborators at ENS Lyon and at other institutions. This work was supported by a grant from the French National Research Agency (ANR) P3N project nanoPDT ANR-09-NANO-027-04.

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About the article

Received: 2015-11-20

Revised: 2016-02-27

Accepted: 2016-02-28

Published Online: 2016-07-06


Citation Information: Nanophotonics, Volume 6, Issue 1, Pages 71–92, ISSN (Online) 2192-8614, ISSN (Print) 2192-8606, DOI: https://doi.org/10.1515/nanoph-2015-0143.

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©2016, Frederic Lerouge et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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