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Original article
9 (
6
); 854-864
doi:
10.1016/j.arabjc.2015.12.014

“One-pot” synthesis of fluorescent Au@SiO2 and SiO2@Au nanoparticles

, , , , ,
a University Politehnica of Bucharest, Research Centre for Environmental Protection and Eco-friendly Technologies, Polizu 1, RO-011061 Bucharest, Romania
b University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Inorganic Chemistry, Physical Chemistry and Electrochemistry Department, Polizu 1, RO-011061 Bucharest, Romania

⁎Corresponding authors. Tel./fax: +40 213154193. adinarogozea@gmail.com (Elena Adina Rogozea), maria.mihaly@upb.ro (Maria Mihaly)

Received: , Accepted: ,
© The Author(s) 2015
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
1 Tel./fax: +40 213154193

Abstract

A class of fluorescent Au@SiO2 and SiO2@Au core–shell fluorescent nanoparticles has been synthesized using microemulsion assisted sol–gel method coupled with photoreduction reaction as an easy, rapid and versatile “one-pot” procedure. In the case of Au@SiO2-RhB (gold nanoparticles core coated with silica-Rhodamine B shell) different silica shell sizes were obtained ranging from 3 to 116 nm, while for RhB-SiO2@Au (Rhodamine B-silica core coated with gold nanoparticles shell) the size of gold shell falls within 3–38 nm interval. The synthesized core–shell nanoparticles have been characterized by a variety of techniques including Dynamic Light Scattering (DLS), High-Resolution Transmission Electron Microscopy (HR-TEM) coupled with Energy-Dispersive X-ray spectroscopy (EDX), Scanning Electron Microscopy (SEM), X-ray Diffractometry (XRD) and UV–VIS–NIR spectroscopy to confirm the size, shape, stability and topographic structure of the both core–shell nanostructures. The fluorescence enhancement of Rhodamine B (RhB) dye in the presence of gold nanoparticles (AuNPs) was demonstrated for the core–shell nanostructures synthesized in the present study, for different silica or gold shells. These interesting fluorescent core–shell nanoparticles provide ideal candidates for a variety of applications in materials science, biology, catalysis and optical fields.

Keywords

Core–shell plasmonic nanostructure
Microemulsion technique
“One-pot” procedure
Dye fluorescence enhancement
1

1 Introduction

A promising and important approach in the development of fluorescent nanoparticles for technological and biomedical applications is represented by the metal-enhanced fluorescence (MEF) nanoparticles. The interaction between the excited-state of fluorophores and free electrons (surface plasmon electrons) in the metal nanoparticles can result in enhanced optical properties, such as: fluorescence enhancement, potentiated fluorophore photostability, increased quantum yield, and reduced lifetime of the fluorophores (Geddes and Lakowicz, 2002; Wang et al., 2011, 2008a,b). Fluorescent dye molecules in close proximity with metallic nanoparticles have shown modified optical properties which are found to be dependent on: (i) size, shape and capping layer of the metallic nanoparticles; (ii) the extent of spectral overlap of localized surface plasmon resonance (LSPR) band with the absorption and emission bands of the dye and (iii) their cationic or anionic nature (Amjadi and Farzampour, 2014; Narband et al., 2009; Sen and Patra, 2009, 2008). For example, gold nanoparticles (AuNP) and Rhodamine 6G (Rh 6G) dyes are metallic nanoparticle dye composite systems in which the LSPR band of Au nanoparticles (530 nm) significantly overlaps with absorption (550 nm) and emission (579 nm) bands of the dye. Such a synergistic combination is very useful for many applications like: surface enhanced Raman scattering (SERS) (Chen et al., 2007a; Rigo et al., 2011), photostability (Dong et al., 2011) and enhancement of fluorescent dye (Sathiyamoorthy et al., 2011), random lasing (Dong et al., 2012; Popov et al., 2006), photoinduced morphological changes (Chandrasekharan et al., 2000), resolution imaging (Larson et al., 2003), single molecule detection and biological labelling (Kinkhabwala et al., 2009).

Different concentrations of AuNPs lead to modified dye fluorescence intensity, excited state lifetime and Raman scattering of the dye molecules. The enhancement or quenching of the fluorescence dye depends on the distance between the metallic surface and the fluorescent molecules. Therefore, coating the metal nanoparticles with a suitable silica shell is a useful method to keep the fluorescent molecules at a certain distance from the metal. The silica shell surrounding the gold core has several important purposes: limits the effect of the outer environment (such as oxygen, certain solvents, and soluble species in buffer solutions) on the fluorescent dye contained in the nanoparticles, protects the metal core against corrosion when the particles are subjected to electrolyte solution, acts as an insulator to avoid charge recombination within the metal, gives a better thermal stability preventing the sintering of the gold cores and adjusts the plasmon dye separation distance to minimize quenching (Törngren et al., 2014). More importantly, the entrapment enhances the hydrophilicity, biocompatibility and stability of the metal nanoparticles and fluorescent dye under physiological conditions (Knopp et al., 2009; Rossi et al., 2005; Yan et al., 2007). The nanoparticles exhibit little or no cytotoxicity, which makes them promising for in vivo observation of cell trafficking and tumour targeting as well as for disease diagnosis and treatment (Luo et al., 2004; Zhu et al., 2004).

Fluorescent nanoparticles as metal-enhanced fluorescence can be synthesized by using different methods including sol–gel condensation (Popov et al., 2006), chemical deposition (Sen and Patra, 2009), layer-by-layer synthesis (Amjadi and Farzampour, 2014), seed induced deposition (Sen and Patra, 2008), sonochemical process (Dong et al., 2012), electroless plating (Chandrasekharan et al., 2000; Chen et al., 2007b) and reverse microemulsion method (Lu et al., 2013; Pak and Yoo, 2013; Ren et al., 2015), etc. In all these approaches the fabrication of nanoshells is often a multi-step, time consuming process which requires a strict control of each step in order to obtain structures with desired properties. Therefore, the monitoring of their thickness and homogeneity still remains a very difficult task. Herein is proposed an easy, rapid, “one-pot” method for the synthesis of Au@SiO2 and SiO2@Au core–shell fluorescent nanoparticles with a homogeneous structure.

The present paper reports, for the first time, a convenient and versatile “one-pot” procedure by water-in-oil microemulsion assisted sol–gel method combined with photoreduction reaction which allows the obtaining of both Au@SiO2-RhB (gold nanoparticles core coated with silica-Rhodamine B shell) and RhB-SiO2@Au (Rhodamine B-silica core coated with gold nanoparticles shell) core–shell fluorescent nanoparticles with a homogeneous structure. The thickness of the silica and gold shell can be controlled by changing the conditions of the synthesis process such as the volume of TEOS or the gold precursor. The Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HR-TEM), Selected Area Diffraction (SAED), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Energy-Dispersive X-ray Spectroscopy (EDX) were used to confirm the size, shape, structure and stability of the both core–shell nanostructures. The effects of either silica or gold shell on the fluorescence of Rhodamine B dye were investigated. This study is important as fluorescent Au@SiO2 nanoparticles are widely used in imaging and drug delivery (Hayashi et al., 2013), while fluorescent SiO2@Au nanoparticles found applications in fluorescence sensor as valuable tools in biochemistry and medicine, mainly because of its sensitivity and simplicity. In particular, the sensors based on fluorescence resonance energy transfer between donor and acceptor have been used to bioanalysis, such as micro RNA, protein and ion (Miao et al., 2016).

2

2 Materials and methods

2.1

2.1 Materials

Tetrachloroauric acid trihydrate (HAuCl4·3H2O, 99.9%), polyoxyethylene-4-lauryl ether (Brij 30), sodium 3-sulfonate mercaptopropane (3-MS), tetraethoxysilane (TEOS), (3-aminopropyl) trimethoxysilane (APTMOS), 2,2,4-trimethypentan (i-C8), ammonium hydroxide and Rhodamine B (RhB) dye were provided by SIGMA ALDRICH. Ammonia solution 25% (NH4OH) was purchased from Merck. Ultra-pure water (Millipore Corporation) was used. All reagents were used without further purification.

2.2

2.2 Methods

2.2.1

2.2.1 Synthesis of fluorescent silica-coated Au nanoparticles

Fluorescent silica-coated Au nanoparticles were synthesized using microemulsion assisted sol–gel procedure combined with photoreduction reaction (Mihaly et al., 2012). For the synthesis, the ternary system consisting in Brij 30/i-C8/aqueous solution as water-in-oil (W/O) microemulsion was used (Scheme 1). The aqueous solution was obtained by mixing 3-MS (0.1 M) with HAuCl4 (1 g/L) in a 1/3 ratio, and an aqueous solution of RhB dye (3.15 · 10−11 mol L−1). The thiol precursor, MS, was used for AuNPs stabilization. After 45 min of mixing, the microemulsion was irradiated with Ultraviolet C (UVC) light (254 nm) for one hour, until the color changes from transparent to yellow showing the formation of Au–MS NPs. Since the Au–silica bond does not occur without a silane coupling agent, various volumes of APTMOS (7.50 · 10−6–7.60 · 10−5 mol L−1) were used. Then a solution of NH4OH (3.50 · 10−5–1.41 · 10−3 mol L−1) and the corresponding TEOS volume in a 2:1 (NH4OH:TEOS) molar ratio was added and mixed for 30 min in order to obtain different silica shells. The shell thickness was modified from 3 to 116 nm by adjusting the concentration of TEOS in the range 1.61 · 10−5–6.40 · 10−4 mol L−1. The Au@SiO2-RhB NPs were separated by centrifugation (15,000 r.p.m.) from the microemulsion destabilised with acetone and washed five times with acetone (Scheme 1).

Au@SiO2-RhB NP and RhB-SiO2@Au NP synthesis in W/O microemulsion systems.
Scheme 1
Au@SiO2-RhB NP and RhB-SiO2@Au NP synthesis in W/O microemulsion systems.

RhB dye was physically linked inside the silica shell since the dye was simply dispersed in the initial water-in-oil microemulsion. The encapsulation of RhB dye inside the silica shell has been observed in the washing procedure of core–shell nanoparticles when the supernatant was colourless and the fluorescence signal was absent.

2.2.2

2.2.2 Synthesis of fluorescent Au-coated silica nanoparticles

Fluorescent Au-coated silica nanoparticles were obtained by mixing two W/O microemulsions (with the same composition used for silica-coated Au nanoparticles), one containing the pre-formed silica network and the other HAuCl4 and MS aqueous solutions. For the synthesis of Au-coated silica nanoparticles a Brij30/i-C8/RhB aqueous solution as W/O microemulsion was used. After 45 min stirring, APTMOS (2.62 · 10−5 mol L−1) was added. In this microemulsion, NH4OH and TEOS (1.83 · 10−4 mol L−1) in a 2:1 M ratio were subsequently added. In order to obtain Au-coated silica nanoparticles with different gold shell sizes various concentrations of HAuCl4 (8.3 · 10−4–1.4 · 10−3 mol L−1) were used. After mixing the two microemulsions for 45 min, the final system was irradiated in a photoreactor with a UVC light for three hours. The recuperation of the RhB-SiO2@Au NP was realized in the same way as for Au@SiO2-RhB NP.

2.2.3

2.2.3 Irradiation equipment

A Luzchem photoreactor, LZC-4X, equipped with eight lamps, each of 12 W (using UVC – 254 nm), was used for the synthesis of gold nanoparticles. The intensity per each lamp was 0.024 W/m2. Constant stirring of the solution was ensured by using a magnetic stirrer. The temperature was kept constant throughout the reaction time by the ventilation system in the main chamber.

2.2.4

2.2.4 Characterization techniques

The size measurements of nanoparticles in pristine microemulsion were performed on a Malvern Nanosizer Equipment (NANO-ZS) based on Dynamic Light Scattering (DLS) principle, at 90° scattered light. All the measurements were done in triplicate at 25 °C constant temperature. The spectral characterization of Au@SiO2-RhB NP and RhB-SiO2@Au NP as powders was evaluated using a Jasco V-670 spectrophotometer. The X-ray diffraction pattern of fluorescent nanoparticles as powders was obtained using a Shimadzu XRD 6000 diffractometer. Transmission Electron Microscopy (TEM) and High-Resolution Transmission Electron Microscopy (HR-TEM) micrographs coupled with selected area diffraction (SAED) and Energy-Dispersive X-ray (EDX) microanalysis were performed on a Tecnai G2F30 S – TWIN model. The scanning electron images were obtained by using a Quanta Inspect F microscope, with a field emission gun (FEG) which has a resolution of 1.2 nm and equipped with an EDAX spectrometer with a resolution at MnK of 133 eV. A Jasco FP-6500 spectrofluorometer was used for fluorescence spectra registration of Au@SiO2-RhB and RhB-SiO2@Au NPs. The excitation wavelength in experiments was fixed at 450 nm in accordance with the RhB absorbance peak.

3

3 Results and discussion

3.1

3.1 The characterization of the fluorescent core–shell nanoparticles

3.1.1

3.1.1 DLS measurements

Fig. 1a shows that the microemulsion template applied for Au@SiO2-RhB NPs, before the addition of TEOS and NH4OH, contains structural units similar in size and shape. The initial size of AuNP stabilized with MS thiol in microemulsion was around 8 nm. Au@SiO2-RhB NPs with sizes in the range 14–240 nm corresponding to silica shells thickness between 3–116 nm were obtained (Table 1). RhB-SiO2@Au NPs with sizes from 26 to 96 nm were also synthesized (Fig. 1b, Table 1).

Size measurements for: (a) Au@SiO2-RhB NP and (b) RhB-SiO2@Au NP core–shell nanostructures.
Figure 1
Size measurements for: (a) Au@SiO2-RhB NP and (b) RhB-SiO2@Au NP core–shell nanostructures.
Table 1 Experimental condition for obtaining Au@SiO2-RhB NP and RhB-SiO2@Au NP core–shell nanoparticles with different silica and Au shells.
Au@SiO2-RhB NP RhB-SiO2@Au NP
Sample Silica shell (nm) APTMOS concentration (105 mol L−1) TEOS concentration (105 mol L−1) NH4OH concentration (105 mol L−1) Sample Au shell, nm HAuCl4 concentration (103 mol L−1)
S1 3 0.75 1.61 3.50 S7 3 0.83
S2 13 1.52 4.00 8.80 S8 5 1.00
S3 26 3.05 8.00 17.60 S9 8.5 1.18
S4 46 4.57 16.00 35.20 S10 12.5 1.30
S5 58.5 6.10 32.00 70.40 S11 38 1.40
S6 116 7.60 64.00 140.80

Both core–shell structures, Au@SiO2-RhB and RhB-SiO2@Au NPs synthesized in the W/O microemulsion showed a narrow pore size distribution (with polydispersity values smaller than 0.250) thus confirming the high degree of homogeneity of these nano-dispersed systems.

This size evolution of the two types of core–shell nanostructures can be interpreted in tight connection with their electronic spectra.

3.1.2

3.1.2 UV–VIS absorbance spectra

For core–shell nanoparticles synthesis, the thiol (3-MS) was added to functionalize the surfaces of AuNPs by thiol molecules. By adding APTMOS, some 3-MS molecules adsorbed on the surface of AuNP were replaced by APTMOS molecules as the N–Au bonds are stronger than S–Au. Then the UV–VIS spectra of core–shell nanostructures were registered. In Fig. 2a are shown the UV–VIS spectra of pure AuNPs with and without the outer protecting SiO2 shell. No obvious peak was evidenced within the investigated wavelength range (λ = 400–700 nm) for SiO2 nanoparticles. For Au@SiO2 nanoparticles a peak around 530 nm can be seen distinctly originating from the AuNPs inside the core–shell structures. With the increase of TEOS volume (Table 1), the plasmon resonance bands appear at the same wavelength, behaviour which is similar with that of Au nanoparticles observed in ethanol (Daniel and Astruc, 2004). As the silica shells are optically transparent, UV–VIS measurements detect mainly the electronic bands of the inner AuNPs. Therefore, the plasmon resonance bands of the samples synthesized with constant HAuCl4 and different TEOS and NH4OH volumes appears at the same wavelength (525 nm) (Erni and Browning, 2007). The narrow absorption band of Au@SiO2 NPs also indicates that the nanoparticles are well dispersed as a result of electrostatic repulsion arising from the electrical double layer around the Au NPs (Chegel et al., 2012).

UV–VIS absorbance spectra of Au@SiO2 (a) and SiO2@Au (b) core–shell nanostructures.
Figure 2
UV–VIS absorbance spectra of Au@SiO2 (a) and SiO2@Au (b) core–shell nanostructures.

Au nanoparticles with an average diameter of about 8 nm have a surface plasmon resonance (SPR) at wavelength of approximately 525 nm (Link and El Sayed, 2000) (Fig. 2b). With SiO2 nanoparticles inside, the SiO2@Au core–shell structures exhibit a well-defined absorption peak between 546 and 605 nm with increasing the gold shell by modifying HAuCl4 from 0.83 mmol L−1 to 1.4 mmol L−1. When the concentration of HAuCl4 is 0.83 mmol L−1, it produces a plasmon resonance peak at 546 nm. By increasing the HAuCl4 concentration (1 mmol L−1), the plasmon resonance band broadens and displays a red shift (566 nm), indicating the formation of SiO2@Au with higher gold shell. When the concentration of HAuCl4 is increasing to 1.4 mmol L−1, the VIS absorption curve becomes very broad and the maximum absorbance peak shifts to 605 nm with a very low intensity due to the formation of larger gold shells.

3.1.3

3.1.3 XRD analysis

The final Au@SiO2-RhB NPs, as powder, were further subjected to XRD measurements. In Fig. 3 the XRD data for Au@SiO2-RhB NPs with sizes of 60 nm are represented. In general, SiO2 shows a broad amorphous feature at 2θ (44°–77.74°). Au peaks appear at 2θ 38°, 44°, 65°, and 78°, corresponding to gold (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively (Quyen et al., 2014). These peaks are consistent with the Joint Committee on Powder Diffraction Standard (JCPDS 04-0784). On the other hand, SiO2 maintains its amorphous phase and the formation of new phases is not detected. For RhB-SiO2@Au NPs a similar crystalline structure was obtained and therefore, the corresponding XRD pattern was not presented.

XRD pattern of RhB-SiO2@Au NPs.
Figure 3
XRD pattern of RhB-SiO2@Au NPs.

3.1.4

3.1.4 Topographic structure

The morphology and size of as-prepared Au@SiO2-RhB and RhB-SiO2@Au core–shell NPs were measured by TEM, HR-TEM and SEM analyses (Fig. 4a–h). In the case of Au@SiO2-RhB NPs the average diameter of AuNP (stabilized with 3-MS thiol) core and the thickness of SiO2 shell are approximately 3 nm and 12 nm, respectively (Fig. 4b). TEM images showed smaller sizes for the nanoparticles than DLS measurements due to the hydration water that was eliminated during the air drying process. The Au@SiO2-RhB NPs were spherical with smooth surface.

TEM (a), HR-TEM (b, d, and e), SAED (c and f) and SEM (g and h) micrographs of: (a) RhB-Au@SiO2 NP (S3); (b) Au@SiO2NP (S2); (c) the inset shows the corresponding SAED pattern of Au@SiO2 NP (S2); (d) SiO2@Au NP (S7); (e) RhB-SiO2@Au NP (S8); (f) the inset shows the corresponding SAED pattern of RhB-SiO2@Au NP (S8); (g) RhB-Au@SiO2 NP (S3)and (h) RhB-SiO2@Au NP (S8).
Figure 4
TEM (a), HR-TEM (b, d, and e), SAED (c and f) and SEM (g and h) micrographs of: (a) RhB-Au@SiO2 NP (S3); (b) Au@SiO2NP (S2); (c) the inset shows the corresponding SAED pattern of Au@SiO2 NP (S2); (d) SiO2@Au NP (S7); (e) RhB-SiO2@Au NP (S8); (f) the inset shows the corresponding SAED pattern of RhB-SiO2@Au NP (S8); (g) RhB-Au@SiO2 NP (S3)and (h) RhB-SiO2@Au NP (S8).

In the HR-TEM images of SiO2@Au NP and RhB-SiO2@Au NP, the SiO2 NPs encapsulating RhB dye have sizes around 20 nm, whereas small Au NP, around 5 nm are attached to the SiO2 NP surface. The morphology of RhB-SiO2@Au NP revealed by HR-TEM microscopy was confirmed by the SEM analysis (Fig. 4h).

SAED analysis indicated the existence of the crystallinity planes corresponding to Au (1 1 1) for both Au@SiO2-RhB and RhB-SiO2@Au NPs.

SEM images of the prepared samples (Au@SiO2-RhB and RhB-SiO2@Au NPs) have been performed in order to determine the surface morphology and are shown in Fig. 4g and h. The core–shell structure of prepared samples can easily be noticed as well as their spherical and uniform shapes. For RhB-SiO2@Au NP may also be observed the brightest spots due to the AuNP attached to the SiO2 surface.

More SiO2 or Au NPs shells are presented in supporting information (Fig. 1). It can be noticed that for SiO2@Au NP the silica NPs are decorated with more or less Au NPs (Figs. 4d and e and 1c and d from supporting information), results that are in good agreement with other studies regarding the SiO2@Au NPs core–shell nanostructures (Choma et al., 2011; Luo et al., 2013; Saini et al., 2015).

In the EDX spectra of the core–shell nanoparticles (Fig. 5), all characteristic peaks were well matched with their unique elements of Au, O, C and Si. The Cu peaks occurred due to the supporting substrate used for TEM sampling. Furthermore, quantitative elemental analyses of the core–shell nanoparticles are listed in Table 2. This was in good agreement with the elemental peaks presented in Fig. 5.

EDX spectra of Au@SiO2-RhB NP (upper side-corresponding to Fig. 4b) and RhB-SiO2@Au NP (bottom side-corresponding to Fig. 4e).
Figure 5
EDX spectra of Au@SiO2-RhB NP (upper side-corresponding to Fig. 4b) and RhB-SiO2@Au NP (bottom side-corresponding to Fig. 4e).
Table 2 The percentage elemental composition of core–shell nanoparticles from EDX analyses.
NP Element Atom%
Au@SiO2-RhB Cu K 18.00
C K 12.17
O K 31.08
Si K 38.28
Au K 0.47
RhB-SiO2@Au Cu K 7.69
C K 9.34
O K 35.71
Si K 46.71
Au K 0.56

3.2

3.2 Fluorescence enhancement of Au@SiO2-RhB NP and RhB-SiO2@Au NP

3.2.1

3.2.1 Characterization of Au@SiO2-RhB NP with different silica shells

In order to observe the fluorescence enhancement of Au@SiO2NP over RhB dye, several silica-coated Au nanoparticles with different silica shell sizes were synthesized by modification of silica precursor (TEOS) concentration into the initial reaction media. Then Au@SiO2-RhB NPs were characterized by fluorescence analysis.

In Fig. 6 are presented the fluorescence spectra of Au@SiO2-RhB NPs with different silica shells measured in microemulsion and in solid phase. As the concentration of TEOS in reaction media increases from 1.61 · 10−5 to 6.40 · 10−4 mol L−1, the peak fluorescence intensity and nanoparticles size are raising consequently (Fig. 6a). The fluorescence intensity of Au@SiO2-RhB NPs was raised up to 5 times compared to simple AuNPs. Thus, by raising the silica shell size the fluorescence signal increases. Likewise, since the fluorescence intensity increased proportionally with TEOS amount in reaction media one may affirm that the RhB dye is oriented inside the silica shell thus acting like a spacer between AuNP and RhB dye. These results have been also confirmed by the fluorescence measurements in solid phase (Au@SiO2-RhB powder) obtained after breaking the microemulsion with acetone (Fig. 6b). Similar results were obtained by Huang et al. (2013). They observed that the fluorescence quenching evolution of FITC (fluorescein isothiocyanate) dye inside Au@SiO2@FITC nanoparticles is no longer carried out as the silica shell is growing.

Fluorescence spectra of Au@SiO2-RhB NPs with different silica shell sizes measured in: (a) microemulsion and (b) solid phase.
Figure 6
Fluorescence spectra of Au@SiO2-RhB NPs with different silica shell sizes measured in: (a) microemulsion and (b) solid phase.

When comparing the fluorescence enhancement of two types of NPs the confinement of RhB inside SiO2 matrix is more efficient whether it is in the core than in the particle shell, fluorescence values being one magnitude order higher for RhB-SiO2@Au than for Au@SiO2-RhB NPs.

3.2.2

3.2.2 Characterization of RhB-SiO2@AuNP with different AuNP shells

Fluorescence spectra of RhB-SiO2@Au NP with different AuNP shells in microemulsion and solid phase are presented in Fig. 7a and b. It can be noticed that, as the AuNP shell increases covering the silica core the fluorescence signal increases as well.

Fluorescence spectra of RhB-SiO2@Au NPs with different AuNPs shell measured in: (a) microemulsion phase and (b) solid phase.
Figure 7
Fluorescence spectra of RhB-SiO2@Au NPs with different AuNPs shell measured in: (a) microemulsion phase and (b) solid phase.

As the gold concentration increases, the relative amount of the AuNPs entering the plasmonic enhancement region of RhB-SiO2@Au NP is growing, and thus the fluorescence signal grows accordingly. As the gold concentration increases further, the fluorescence signal begins to show a relatively slow growing process. This fluorescence quenching can be ascribed to the dominant fluorescence effect caused by superfluous SiO2@Au NPs with respect to the number of free AuNPs. One can also see from the fluorescence spectra of RhB-SiO2@Au NP that the peak was red-shifted from 575 to 585 nm as the gold coating increases. Initially, less number of gold nanoparticles was attached to silica core and the number of coatings increases the gold particles which smoothly cover almost the whole surface of silica, which shows large measurable red shift in peak. As the shell thickness increased, the fluorescence peak started to shift toward lower wavelength (582 nm).

3.2.3

3.2.3 Quantitative parameters of fluorescence enhancement

The fluorescence enhancement by Au nanoparticles was described by fluorescence enhancement factor, F (F = I1/I0), where I1 is the fluorescence intensity of Au@SiO2-RhB core–shell NPs per unit SiO2 volume (I1 is increased intensity/increased volume and I0 is the fluorescence intensity of RhB-SiO2 spheres without Au core per unit SiO2 volume (I0 = 8.57 · 10−15 nm−3)). This calculus was also done for RhB-SiO2@Au NPs. The enhancement factors for Au@SiO2-RhB and RhB-SiO2@Au NPs were plotted against silica shell thickness, based on the calculation reported in Supporting Information (Tables 1 and 2). The values of F decreased sharply with increasing silica shell thickness, as shown in Fig. 8a. This is also valid for RhB-SiO2@Au NPs with increasing gold shell. Thus these results indicate that the extent of influence by Au nanoparticles on the fluorescence of RhB is significant for a shell thickness less than 50 nm in the case of Au@SiO2-RhB and 15 nm for SiO2@Au NPs. This was also observed by Li and Zhu (2013) who instead of RhB and 8 nm of Au NP, they used OG-488 fluorophores and bigger Au nanoparticles (40, 60, 80 nm) and Chen et al. (2013) who developed nanocomposites with bigger Au cores (13.7 nm) covered by a thin PVP coating layer a silica spacer and a fluorescent dye (TAMRA) layer in the silica matrix. The explanation of such behaviour can be described as follows: in the case of Au@SiO2-RhB NPs the absorption of light by AuNPs which favours the enhancement fluorescence is hindered by RhB dye which in the case of SiO2@Au NPs is not valid. Also, the decoration of AuNPs onto the silica surface enables light to reach the Au and RhB dye with enhanced fluorescence and the compact silica-RhB shell covering the AuNPs surface obtained by polymerization prevents the light to reach AuNPs.

The enhancement factor for Au@SiO2-RhB and RhB-SiO2@Au nanoparticles compared with RhB-SiO2 and RhB-Au without Au core or SiO2 core.
Figure 8
The enhancement factor for Au@SiO2-RhB and RhB-SiO2@Au nanoparticles compared with RhB-SiO2 and RhB-Au without Au core or SiO2 core.

The increasing thickness of the SiO2 shell layer reduces the material surface activity of the core surface; as a result, quantum yield of Au@SiO2 also reduces (Fig. 9). This is due to the quenching effect of metal nanoparticles on nearby fluorescent dye through a non-radiative energy transfer manner, because of surface plasmon resonance of gold (Dubertret et al., 2001). The manner of quantum yield calculation is presented in Supporting Information.

Quantum yield dependence on the shell thickness for Au@SiO2 and SiO2@Au.
Figure 9
Quantum yield dependence on the shell thickness for Au@SiO2 and SiO2@Au.

As for SiO2@Au as the light-emitter is placed close to the plasmons, the light-emitter excitation rate will be enhanced when the wavelength of the plasmon resonance is close to the excitation wavelength of the light-emitter and as such the quantum yield will increase (Fig. 9). When Au gets close to the RhB molecule, the fluorescence intensity continuously increases and reaches a maximum at a distance of z = 5 nm. During this process, the excitation rate is enhanced. At the same time, the overall quantum yield also increases because of the faster increasing speed of the radiative decay rate as compared to that of the non-radiative decay. However, the non-radiative decay rate will increase rapidly if the distance z becomes even smaller, moment at which fluorescence quenching occurs (Ji, 2014).

4

4 Conclusions

A detailed analysis on correlation between size measurements and spectral characteristics allows highlighting the following aspects:

  • versatility of microemulsion assisted sol–gel method coupled with photoreduction reaction able to produce complex noble metal/SiO2 NPs when the roles of core and shell are interchangeable;

  • possibility to conduct the synthesis in a “one-pot” procedure, making thus the route very advantageous over other synthesis procedures, being less time consuming, and reducing the amount of chemicals;

  • ability to enhance the dye fluorescence by tuning silica and gold shell thickness and thus to obtain materials with tailored properties adequate to purpose.

HR-TEM micrographs confirmed the structure of both core–shell nanostructures and the results were in good agreement with DLS measurements. It was noticed that the fluorescence enhancement of RhB dye depends on Au and silica shell thickness, the fluorescence signal being one magnitude order higher in the former case.

Further studies will be performed to complete these linear optical properties (UV–VIS, fluorescence) with non-linear properties (NLO), such as third harmonics generation (THG) (Bazaru Rujoiu et al., 2015).

Acknowledgement

This work was done in the frame of the Program: Cooperation in Priority Fields – PNII, developed with the support of ANCS, CNDI-UEFISCDI, Romania, in the project SimFotoAd, PN-II-PT-PCCA-2013-4-0726, contract no. 217/2014.

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Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2015.12.014.

Appendix A

Supplementary material

Supplementary data 1


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