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Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides
Accepted Manuscript Title: Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides Authors: Heman Burhanalden Abdulrahman, Jan Krajczewski, Andrzej Kudelski PII: DOI: Reference:
S0169-4332(17)32521-7 http://dx.doi.org/10.1016/j.apsusc.2017.08.163 APSUSC 37000
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-7-2017 11-8-2017 24-8-2017
Please cite this article as: Heman Burhanalden Abdulrahman, Jan Krajczewski, Andrzej Kudelski, Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.163 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides
Heman Burhanalden Abdulrahman, Jan Krajczewski, and Andrzej Kudelski*
Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland
*Correspondence to: A.K. (e-mail: [email protected], Fax: +048-225526434, phone: +048-225526401)
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Graphical abstract
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SiO2, MnO2, or TiO2
Raman intensity
Ag
Raman shift
Abstract In this work we show that nanometric-thick layers of SiO2, MnO2, and TiO2 may be effectively deposited on various silver nanoparticles (including cubic Ag nanoparticles) covered by a very thin (below 0.4 nm) layer of silver sulphide. The background in Raman measurements generated by sulphide-protected Ag nanoparticles is significantly smaller than that for analogous Ag nanoparticles protected by a monolayer formed from alkanethiols depositing alkanethiols on a surface of anisotropic silver nanoparticles is the current standard method used for protecting a surface of Ag nanoparticles before depositing a layer of silica. Because of significantly smaller generated Raman background, Ag@SiO2 nanostructures with an Ag2S linkage layer between the silver core and the silica shell are very promising lowbackground electromagnetic nanoresonators for carrying out Raman analysis of various surfaces - especially using what is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Sample SHINERS analyses of various surfaces (including pesticide-contaminated surfaces of tomatoes) using cubic-Ag@SiO2 nanoparticles as electromagnetic nanoresonators are also presented.
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KEYWORDS: Shell-isolated nanoparticle-enhanced Raman spectroscopy; SHINERS; Surface-enhanced Raman spectroscopy; SERS; Ag@SiO2; Cubic Ag nanoparticles
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1. Introduction Plasmonic nanoparticles (e.g. from gold or silver) covered by a very thin layer of SiO2 (or layers from other dielectric materials such as MnO2, Al2O3 or TiO2) are often used for constructing optical sensors. For example, such nanostructures are used for manufacturing plasmonic sensors utilising metal-enhanced fluorescence (MEF) [1-5] or surface-enhanced Raman scattering (SERS) [4-10]. The main role of the plasmonic metal core in these nanostructures is to enhance the electric field of the incident radiation - this enhancement is due to the dislocation of metal electrons in the nanoparticles during irradiation, which leads to the creation of an electric dipole and, consequently, to an additional electric field near the surface of the nanoparticles [4,11,12]. This field enhancement leads to a large increase in the efficiency of many optical processes, such as the above-mentioned fluorescence and Raman scattering, although electromagnetic nanoresonators may be also used for enhancing the efficiency of infrared absorption or of many nonlinear optical processes [4,13,14]. The role of the deposited dielectric layer is to prevent direct interaction between the plasmonic metal structure and the fluorophore (in MEF experiments) [1-4], or the plasmonic metal structure and the molecules being analysed (in SERS experiments) [4-10]. In MEF measurements, the fluorophore cannot be too close to the metal cluster, since fluorescence can be highly enhanced only when the distance between the plasmonic nanoparticles and the fluorophore is larger than what is known as the quenching distance - for very small distances a quenching of fluorescence occurs [15]. In the case of SERS analysis, the dielectric layer prevents any direct interaction between the analyzed surface and the metallic cores, which is especially important when analysing biological samples. Many biochemically important molecules (e.g. peptides) significantly change their structure during direct interaction with metallic nanoparticles [16,17]. This is worthy of mention because many applications, various Au@SiO2 and
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Ag@SiO2 nanoparticles are commercially available (for example, in the year 2017 there are at least two commercial suppliers of such nanoparticles). A very important step in the synthesis of some Ag@SiO2 nanoparticles is to properly protect the surfaces of the silver cores before depositing the SiO2 layers (a similar problem must be solved when depositing other dielectric layers). Although spherical and semispherical silver nanoparticles may be covered by silica without special protection of their surfaces, Mirkin et al. showed that, in the case of some other silver nanostructures, proper preparation of their surfaces is necessary because, without surface passivation, etching and aggregation of such silver nanoparticles (e.g., silver nanoprisms) occur [18]. Before depositing an SiO2 layer, the silver nanoparticles are usually protected by a monolayer formed from long-chain alkanethiols (e.g. 16mercaptohexadecanoic acid) [9,18,19]. The alkanethiols bind strongly to the surface of the silver nanoparticles via formed an Ag–S bond, and at high surface coverage form self-assembled, ordered, quasi-crystalline structures that are very stable [20,21]. Unfortunately, the Raman spectra of alkanethiols adsorbed on the surface of silver nanostructures are relatively intense (for example, see ref. [22] and Section 3 of this work), and therefore, such nanostructures produce a relatively intense background when used as nanoresonators in Raman measurements. In this work, we report a novel strategy for the passivation of surfaces of silver nanoparticles, involving the formation of a layer of Ag2S on the surface of silver nanoparticles. Sulphide-modified silver nanoparticles produce a significantly weaker Raman background, and are therefore very promising candidates as nanoresonators for SHINERS analysis.
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2. Materials and methods 2.1. Materials Silver nitrate, potassium manganate(VII), potassium hydroxide, potassium oxalate, trisodium citrate dihydrate, 37% hydrochloric acid, ethylene glycol, cyclohexane, acetone, and absolute ethanol (99.8%) were purchased from POCH SA. Sodium sulphide, a solution of sodium silicate (26.5% of SiO2), titanium(IV) isopropoxide, 4mercaptobenzoic acid, Larginine,
(3mercaptopropyl)triethoxysilane,
(3aminopropyl)trimethoxysilane,
and
16mercaptohexadecanoic acid were acquired from Sigma-Aldrich. Sodium borohydride, O,O-dimethyl-O-(4-nitrophenyl)phosphorothioate (common name: methyl parathion) and polyvinylpyrrolidone (PVP) with an average molar mass of ca. 4104 g mol−1 were purchased from Fluka. Platinum sheets were acquired from the Polish State Mint. The water used in all the experiments was purified in the Millipore Milli-Q manner. All materials were of high purity, and were used as received, without further purification or treatment.
2.2. Synthesis of silver nanoparticles Semi-spherical silver nanoparticles, which were used for investigation of the efficiency depositing a silica layer on bare and Ag2S-protected silver nanostructures, were synthesised according to a modified Lee and Meisel method [23]. Briefly, 250 ml of a 1 mM AgNO3 solution was placed in a round-bottom flask. This solution was stirred and heated to boiling under reflux. Next, 10 ml of a 40 mM sodium citrate solution was added rapidly, and the mixture was kept boiling for 90 min. The average diameter of the nanoparticles formed was ca. 45 nm, and the position of the plasmon band for the sol obtained was 410 nm. In some cases, to modify the surfaces of the semi-spherical silver nanoparticles by the formation of a surface layer of Ag2S, sols of Ag nanoparticles (10 ml) were mixed with a solution of Na2S (0.010 ml of a 10 mM Na2S solution).
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Silver nanocubes were synthesised by reducing silver nitrate by ethylene glycol in the presence of Na2S and PVP [24]. Briefly, 60 ml of anhydrous ethylene glycol was heated at 160°C for 1.5 h. Then, to the boiling ethylene glycol, the following solutions in the ethylene glycol were added: 0.8 ml of a 3 mM Na2S solution, 0.3 g of PVP dissolved in 15 ml of ethylene glycol, and 5 ml of a 0.3 mM AgNO3 solution. The heating at 160°C was continued for another 20 min. Then, the reaction mixture was cooled very quickly by immersing the reaction flask in an ice bath. The nanoparticles obtained were concentrated by centrifuging and pouring off the supernatant. The nanoparticles were suspended in acetone, and the cleaning procedure was repeated three times. After the final concentration, the nanoparticles were suspended in water. The position of the plasmon band for the sol obtained was 452 nm.
2.3. Deposition of oxides dielectric layers The silica layer was deposited using the procedure developed by Mulvaney et al. [25], which has been further modified by Li et al. [26]. Briefly speaking, 0.13 ml of a 1 mM aqueous solution of (3-aminopropyl)trimethoxysilane was added to 10 ml of the solution of silver nanoparticles and stirred at room temperature for 15 min. Next, 1.07 ml of a freshly prepared diluted sodium silicate solution (0.54% of SiO2), adjusted with HCl to pH 10−11, was added. Then, the reaction mixture was stirred for 3−4 days at room temperature. The Ag@SiO2 nanoparticles were cleaned by centrifuging for 15 min, pouring off the supernatant, and adding water to suspend them again. This cleaning procedure was repeated three times (the final Ag@SiO2 nanoparticles were obtained in concentrated solutions). Coating the silver nanoparticles with MnO2 layers was carried out according to the procedure developed by Tian et al. [7] and modified by Abdulrahman et al. [27]. In the first stage, 5 ml of the Ag sol was alkalized to pH = 9.5 by the addition of the proper amount of KOH solution. Then, the alkalized Ag sol was cooled in an ice bath, and 0.04 ml of a 10 mM
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KMnO4 and 0.2 ml of a 10 mM K2C2O4 aqueous solution were added. After 10 min, the reaction mixture was heated to 60◦C and kept at this temperature for 2 h. The Ag@MnO2 nanoparticles obtained were cleaned by centrifuging, pouring off the supernatant, and again suspending them in water - this cleaning procedure was repeated three times. The TiO2 layers were deposited according to the procedure proposed by Huang et al. [28]. In the first stage, 10 ml of a freshly prepared 0.02 M aqueous solution of Larginine was added to 30 ml of the silver sol, and the mixture obtained was stirred at room temperature for 5 min. 12 ml of cyclohexane and 12 l of (3-mercaptopropyl)triethoxysilane were then added to the aqueous sol of silver nanoparticles, which resulted in the formation of a biphasic system. The reaction mixture was stirred for 6 h in darkness. After 6 hours of stirring, the cyclohexane layer (to which Ag nanoparticles have transferred) was removed, and the Ag nanoparticles were transferred (by several centrifugations and the subsequent addition of ethanol to again suspend the nanoparticles) into ethanol. To grow the TiO2 shell, 1 L of titanium(IV) isopropoxide was added to the alcohol solution of the silver nanoparticles. After 15 min., the reaction was terminated by rinsing the particles with ethanol and centrifuging.
2.4. Formation of a monolayer from 4mercaptobenzoic acid on the Pt surface For model SHINERS measurements, monolayers formed from 4mercaptobenzoic acid (MBA) on platinum surfaces were used. To form such monolayers, flame-annealed Pt substrates were immersed in a 0.5 mM aqueous solution of MBA for 1 day. Then, the MBAmodified platinum surfaces were rinsed with water and allowed to dry in air.
2.5. Experimental techniques Transmission electron microscopy (TEM) analyses were carried out using a Zeiss LIBRA 120 electron microscope equipped with an Incolumn OMEGA filter, working at an 9
accelerating voltage of 120 kV. Before the TEM measurements, samples of the nanoparticles were deposited onto 300-mesh copper grids coated with a Formvar layer, and the deposited solution was allowed to dry. The elemental composition of the sulphide-modified silver nanostructures was determined using a Merlin field emission scanning electron microscope (Zeiss, Germany) equipped with an energy-dispersive X-ray microanalysis (EDS) probe (Bruker). Samples of the sols analysed were deposited on the surface of the graphite substrate, and the deposited solution was allowed to dry. Raman spectra were taken using a Horiba Jobin-Yvon Labram HR800 spectrometer equipped with: a He−Ne laser generating radiation of a wavelength of 633 nm, a Peltier-cooled CCD detector (1024 × 256 pixels), a 600 groove/mm holographic grating, and an Olympus BX40 microscope with a long distance 50× objective. A Shimadzu UV-2401PC spectrophotometer was used for recording the UV–vis extinction spectra.
3. Results and discussion As mentioned in the introduction, depositing a silica layer on some silver nanoparticles often requires increasing the chemical stability of the surface of silver nanostructures. The classical method of passivating the silver surface to form self-assembled monolayers from long-chain alkanethiols (e.g., from 16mercaptohexadecanoic acid) [9,18,19]. In some applications of Ag@SiO2 nanoparticles (actually Ag@thiol@SiO2 nanoparticles), for example, when measuring metal-enhanced fluorescence, alkanethiols deposited directly on the surface of the plasmonic cores cause no disturbance. However, because alkanethiols deposited directly on the plasmonic cores generate a relatively intense Raman signal [22,29], alkanethiols monolayers deposited directly on the silver cores significantly decrease the usefulness of Ag@SiO2 nanostructures as nanoresonators in enhancing Raman scattering efficiency. Figure 1 shows the Raman spectrum generated by
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silver nanoparticles protected by a monolayer of 16-mercaptohexadecanoic acid (a typical compound used for protecting the surface of Ag nanoparticles before depositing SiO2 [9,18,19]). As can be seen, in this case the spectrum measured contains many Raman bands due to the various vibrations of molecules of the chemisorbed alkanethiols. Therefore, for Ag@SiO2 nanoparticles dedicated as nanoresonators for Raman experiments, it is very important to find an alternative method of protecting the surface of the plasmonic cores. Due to the very large affinity of sulphur to silver, we decided to test very thin layers of Ag2S as protection layers. We began our experiments from cubic silver nanoparticles prepared in the presence of Na2S - these nanoparticles contain some sharp apexes and edges (at which the highest electromagnetic field enhancement is induced and which could be preferentially etched) and they were synthesised in sulphide-containing media. Figure 2 shows a TEM image of silver cubic nanoparticles synthesised according to the procedure described in Section 2. An atomic analysis of the cubic nanostructures formed revealed that the ratio of atomic concentrations of S to Ag was in a range of from 1:100 to 1:400. This means that, for these nanoparticles, only between 0.5-2% of the silver is in the form of a sulphide. Taking into account: the size of the Ag@Ag2S nanoparticles (ca. 80 nm), the relative amount of Ag2S formed, and the difference in the density of Ag (10,49 g/cm3) and Ag2S (7,23 g/cm3), it can be estimated that the average thickness of the Ag2S layer on the surface of Ag cubic nanoparticles should be below 0.4 nm. The deposited Ag2S layer produces a significantly weaker Raman signal than the monolayer formed from 16-mercaptohexadecanoic acid (see Figure 1), which means that such Ag2S-protected silver nanoparticles should constitute a significantly better material for electromagnetic nanoresonators for Raman measurements than do silver nanoparticles protected by a monolayer formed from alkanethiols. The Ag2S-modified silver cubic nanoparticles obtained were then covered with a ca. 5 nm-thick protection layer formed from SiO2, MnO2, and TiO2 (for details, see section 2.3).
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The deposition of the dielectric layer occurs with relatively good efficiency, and many wellseparated cubicAg@SiO2 (or cubicAg@MnO2, or cubicAg@TiO2) nanoparticles may be found in the products obtained. Figure 3 shows example TEM images of synthesised cubicAg@SiO2, cubicAg@MnO2, and cubicAg@TiO2, nanoparticles. Such nanoparticles were then tested as nanoresomators for characterising various interfaces, using what is known as shell-isolated nanoparticle-enhanced Raman spectroscopy – SHINERS [5-10]. In this technique, the surface being analysed is covered with a layer of surface-protected plasmonic nanoparticles (e.g. from gold or silver), and the Raman spectrum of the investigated sample is then recorded. As mentioned in the introduction, metal plasmonic nanoparticles act as electromagnetic resonators, significantly enhancing the electric field of the incident electromagnetic radiation, hence leading to a very large increase in the Raman signal from the surface on which nanoparticles have been spread (for Raman bands with a small ‘Raman shift’, to the fourth power of the electric field enhancement [4]). The ultrathin protection layer (in this case, SiO2, MnO2, and TiO2) does not damp surface electromagnetic enhancement, but does prevent the nanoparticles from having direct contact with the probed material and does prevent them from agglomerating [6]. The principles of SHINERS measurements using cubicAg@SiO2, cubicAg@MnO2, or cubicAg@TiO2 nanoparticles presents Figure 4. Using this method, it is possible, for example, to record high-quality Raman spectra of various molecules adsorbed at the surfaces of metal single-crystals [6], or Raman spectra of certain biological samples, such as the walls of living cells [6,9]. Figures 5 and 6 show example SHINERS spectra recorded using nanoresonators produced from Ag2S-covered cubic Ag nanoparticles. Fig. 5 shows SHINERS spectra of a platinum substrate modified with a 4-mercaptobenzoic acid (MBA) monolayer before the deposition of the electromagnetic nanoresonators, and covered with cubicAg@SiO2 and cubicAg@MnO2 nanoparticles (in both cases, the nanoparticles were produced from roughly 12
the same amount of material, and the procedure for depositing the nanoparticles was the same). The Raman spectra recorded are undoubtedly characteristic for MBA molecules: two the strongest Raman bands at 1077 and 1587 cm−1 , are due to the 12 and 8a vibrations of the aromatic ring of MBA, respectively [30–33], and the clearly visible band at 1185 cm−1 is also characteristic for adsorbed MBA molecules (this band is due to δ(C–H) vibrations [30,34,35]). This means that a possible Raman signal from the Ag2S protection layer does not disturb in this type of SHINERS measurements. The other interesting "model analysis" carried out using SHINERS nanoresonators is that of detecting traces of methyl parathion (which is an efficient insecticide) on the surface of various botanical fruits (including culinary vegetables). Figure 6 shows Raman spectra of solid methyl parathion, the skin of a "clean" tomato, the skin of a tomato fruit contaminated by methyl parathion, the skin of the tomato fruit covered with cubicAg@SiO2 nanoparticles, and the skin of the tomato fruit contaminated by methyl parathion and covered with cubicAg@SiO2 nanoparticles. As can be seen in Figure 6, the Ag2S layer between the Ag core and the SiO2 shell causes no disturbance in this type of SHINERS measurements - the Raman spectra of the surface of the tomato is dominated by two bands at 1156 and 1520 cm−1 (see Figure 6), which are due to the vibrations due to carotenoid molecules [6]. Also, the spectral region at which the band characteristic for methyl parathion is observed (at ca. 1350 cm−1) is not disturbed by the Raman signal generated by the SHINERS nanoresonators themselves. Because it would be very difficult to produce "bare" (only stabilised, for example, by citrates) cubic silver nanoparticles (the Ag2S layer is already formed during the synthesis of cubic silver nanoparticles), to compare the efficiency of depositing a silica layer on bare and Ag2S-protected silver nanostructures, we decided to analyse a deposition of silica on bare and Ag2S-protected semi-spherical silver nanoparticles. It is worth mentioning that semi-spherical
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silver nanoparticles usually display significantly lower SERS activity than nanoparticles containing some sharp apexes and edges - in the case of the two types of silver nanoparticles used in this work, the SERS activity of the cubic nanoparticles is one order of magnitude higher than that of the semi-spherical nanoparticles (the methodology of the measurement is given in ref. [9]), and therefore, in this work we did not use semi-spherical Ag@SiO2 nanoparticles for the SHINERS measurements. On the basis of an analysis of TEM images of 500 synthetised nanoparticles (obtained after the deposition of SiO2 on bare and Ag2Sprotected semi-spherical silver nanoparticles), we found that the number of isolated (not aggregated) Ag@SiO2 nanoparticles in the sol product increased by more than 3 times (from ca. 23% to 74%) when the surface of the silver nanoparticles was covered with an Ag2S film before deposition of the SiO2 layer. Therefore, it can be concluded that the passivation of the surface of Ag nanoparticles by the layer of Ag2S may be used to synthesise Ag@SiO2 nanoresonators for SHINERS measurements, which generate a very low Raman background.
4. Conclusions In this work, we propose an improved method of synthesising of Ag@SiO2 nanoparticles (a similar procedure may be also applied for synthesising Ag@MnO2 and Ag@TiO2 nanostructures). Instead of the protecting the surface of the silver core with a monolayer formed from long-chain alkanethiols, we suggest forming a very thin (below 0.4 nm) layer of silver sulphide on the surface of the Ag nanoparticles. The background in Raman measurements generated by sulphide-protected Ag nanoparticles is significantly smaller than that for analogous Ag nanoparticles protected by an organic (thiol) layer. This means that the nanostructures obtained should be especially useful as electromagnetic nanoresonators in Raman experiments,
for
example,
in shell-isolated nanoparticle-enhanced Raman
spectroscopy measurements.
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Acknowledgments This work was financed from funds of the National Science Centre (Poland) allocated on the basis of decision number DEC-2013/11/B/ST5/02224. A.K. thanks the Faculty of Chemistry, University of Warsaw for its financial support.
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Figures captions Figure 1. Raman spectra generated by semi-spherical silver nanoparticles covered with: (a) ca. 0.4 nm thick layer of Ag2S, and (b) monolayer of 16mercaptohexadecanoic acid. Figure 2. TEM image of synthesised cubic silver nanoparticles. Figure 3. TEM images of synthesised nanoparticles: (a) Ag@SiO2, (b) Ag@MnO2, and (c) Ag@TiO2. Figure 4. The principles of SHINERS measurements using cubic Ag@SiO2, Ag@MnO2 or Ag@TiO2 nanoparticles. Figure 5. (a) Raman spectrum of MBA monolayer on Pt before deposition of SHINERS nanoresonators. (b, c) Raman spectra of MBA monolayer on Pt covered with (b) cubicAg@SiO2, and (c) cubicAg@MnO2 nanoparticles. Inset: Raman spectrum of solid polycrystalline MBA. Figure 6. Raman spectra of: (a) solid methyl parathion, (b) skin of tomato fruit, (c) skin of tomato fruit contaminated by methyl parathion, (d) skin of tomato fruit covered with cubicAg@SiO2 nanoparticles, and (e) skin of tomato fruit contaminated by methyl parathion and covered with cubicAg@SiO2 nanoparticles. The strongest band due to the vibration of the molecules of methyl parathion is marked with an asterisk.
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c)
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SiO2, MnO2, or TiO2
Raman intensity
Ag
Raman shift
1595
900
1292
1182
1096
1200
1500
1800
Raman Intensity
1587
1077
1185
c b
a 800
1000
1200
1400
1600
Raman shift / cm 1 23
1800
1156 1520
*
1326
Raman Intensity
e
d
c b
* a
800
1000 1200 1400 1600 1800
Raman shift / cm1
24
S0169-4332(17)32521-7 http://dx.doi.org/10.1016/j.apsusc.2017.08.163 APSUSC 37000
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-7-2017 11-8-2017 24-8-2017
Please cite this article as: Heman Burhanalden Abdulrahman, Jan Krajczewski, Andrzej Kudelski, Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.163 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides
Heman Burhanalden Abdulrahman, Jan Krajczewski, and Andrzej Kudelski*
Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland
*Correspondence to: A.K. (e-mail: [email protected], Fax: +048-225526434, phone: +048-225526401)
1
Graphical abstract
2
SiO2, MnO2, or TiO2
Raman intensity
Ag
Raman shift
Abstract In this work we show that nanometric-thick layers of SiO2, MnO2, and TiO2 may be effectively deposited on various silver nanoparticles (including cubic Ag nanoparticles) covered by a very thin (below 0.4 nm) layer of silver sulphide. The background in Raman measurements generated by sulphide-protected Ag nanoparticles is significantly smaller than that for analogous Ag nanoparticles protected by a monolayer formed from alkanethiols depositing alkanethiols on a surface of anisotropic silver nanoparticles is the current standard method used for protecting a surface of Ag nanoparticles before depositing a layer of silica. Because of significantly smaller generated Raman background, Ag@SiO2 nanostructures with an Ag2S linkage layer between the silver core and the silica shell are very promising lowbackground electromagnetic nanoresonators for carrying out Raman analysis of various surfaces - especially using what is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Sample SHINERS analyses of various surfaces (including pesticide-contaminated surfaces of tomatoes) using cubic-Ag@SiO2 nanoparticles as electromagnetic nanoresonators are also presented.
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KEYWORDS: Shell-isolated nanoparticle-enhanced Raman spectroscopy; SHINERS; Surface-enhanced Raman spectroscopy; SERS; Ag@SiO2; Cubic Ag nanoparticles
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1. Introduction Plasmonic nanoparticles (e.g. from gold or silver) covered by a very thin layer of SiO2 (or layers from other dielectric materials such as MnO2, Al2O3 or TiO2) are often used for constructing optical sensors. For example, such nanostructures are used for manufacturing plasmonic sensors utilising metal-enhanced fluorescence (MEF) [1-5] or surface-enhanced Raman scattering (SERS) [4-10]. The main role of the plasmonic metal core in these nanostructures is to enhance the electric field of the incident radiation - this enhancement is due to the dislocation of metal electrons in the nanoparticles during irradiation, which leads to the creation of an electric dipole and, consequently, to an additional electric field near the surface of the nanoparticles [4,11,12]. This field enhancement leads to a large increase in the efficiency of many optical processes, such as the above-mentioned fluorescence and Raman scattering, although electromagnetic nanoresonators may be also used for enhancing the efficiency of infrared absorption or of many nonlinear optical processes [4,13,14]. The role of the deposited dielectric layer is to prevent direct interaction between the plasmonic metal structure and the fluorophore (in MEF experiments) [1-4], or the plasmonic metal structure and the molecules being analysed (in SERS experiments) [4-10]. In MEF measurements, the fluorophore cannot be too close to the metal cluster, since fluorescence can be highly enhanced only when the distance between the plasmonic nanoparticles and the fluorophore is larger than what is known as the quenching distance - for very small distances a quenching of fluorescence occurs [15]. In the case of SERS analysis, the dielectric layer prevents any direct interaction between the analyzed surface and the metallic cores, which is especially important when analysing biological samples. Many biochemically important molecules (e.g. peptides) significantly change their structure during direct interaction with metallic nanoparticles [16,17]. This is worthy of mention because many applications, various Au@SiO2 and
5
Ag@SiO2 nanoparticles are commercially available (for example, in the year 2017 there are at least two commercial suppliers of such nanoparticles). A very important step in the synthesis of some Ag@SiO2 nanoparticles is to properly protect the surfaces of the silver cores before depositing the SiO2 layers (a similar problem must be solved when depositing other dielectric layers). Although spherical and semispherical silver nanoparticles may be covered by silica without special protection of their surfaces, Mirkin et al. showed that, in the case of some other silver nanostructures, proper preparation of their surfaces is necessary because, without surface passivation, etching and aggregation of such silver nanoparticles (e.g., silver nanoprisms) occur [18]. Before depositing an SiO2 layer, the silver nanoparticles are usually protected by a monolayer formed from long-chain alkanethiols (e.g. 16mercaptohexadecanoic acid) [9,18,19]. The alkanethiols bind strongly to the surface of the silver nanoparticles via formed an Ag–S bond, and at high surface coverage form self-assembled, ordered, quasi-crystalline structures that are very stable [20,21]. Unfortunately, the Raman spectra of alkanethiols adsorbed on the surface of silver nanostructures are relatively intense (for example, see ref. [22] and Section 3 of this work), and therefore, such nanostructures produce a relatively intense background when used as nanoresonators in Raman measurements. In this work, we report a novel strategy for the passivation of surfaces of silver nanoparticles, involving the formation of a layer of Ag2S on the surface of silver nanoparticles. Sulphide-modified silver nanoparticles produce a significantly weaker Raman background, and are therefore very promising candidates as nanoresonators for SHINERS analysis.
6
2. Materials and methods 2.1. Materials Silver nitrate, potassium manganate(VII), potassium hydroxide, potassium oxalate, trisodium citrate dihydrate, 37% hydrochloric acid, ethylene glycol, cyclohexane, acetone, and absolute ethanol (99.8%) were purchased from POCH SA. Sodium sulphide, a solution of sodium silicate (26.5% of SiO2), titanium(IV) isopropoxide, 4mercaptobenzoic acid, Larginine,
(3mercaptopropyl)triethoxysilane,
(3aminopropyl)trimethoxysilane,
and
16mercaptohexadecanoic acid were acquired from Sigma-Aldrich. Sodium borohydride, O,O-dimethyl-O-(4-nitrophenyl)phosphorothioate (common name: methyl parathion) and polyvinylpyrrolidone (PVP) with an average molar mass of ca. 4104 g mol−1 were purchased from Fluka. Platinum sheets were acquired from the Polish State Mint. The water used in all the experiments was purified in the Millipore Milli-Q manner. All materials were of high purity, and were used as received, without further purification or treatment.
2.2. Synthesis of silver nanoparticles Semi-spherical silver nanoparticles, which were used for investigation of the efficiency depositing a silica layer on bare and Ag2S-protected silver nanostructures, were synthesised according to a modified Lee and Meisel method [23]. Briefly, 250 ml of a 1 mM AgNO3 solution was placed in a round-bottom flask. This solution was stirred and heated to boiling under reflux. Next, 10 ml of a 40 mM sodium citrate solution was added rapidly, and the mixture was kept boiling for 90 min. The average diameter of the nanoparticles formed was ca. 45 nm, and the position of the plasmon band for the sol obtained was 410 nm. In some cases, to modify the surfaces of the semi-spherical silver nanoparticles by the formation of a surface layer of Ag2S, sols of Ag nanoparticles (10 ml) were mixed with a solution of Na2S (0.010 ml of a 10 mM Na2S solution).
7
Silver nanocubes were synthesised by reducing silver nitrate by ethylene glycol in the presence of Na2S and PVP [24]. Briefly, 60 ml of anhydrous ethylene glycol was heated at 160°C for 1.5 h. Then, to the boiling ethylene glycol, the following solutions in the ethylene glycol were added: 0.8 ml of a 3 mM Na2S solution, 0.3 g of PVP dissolved in 15 ml of ethylene glycol, and 5 ml of a 0.3 mM AgNO3 solution. The heating at 160°C was continued for another 20 min. Then, the reaction mixture was cooled very quickly by immersing the reaction flask in an ice bath. The nanoparticles obtained were concentrated by centrifuging and pouring off the supernatant. The nanoparticles were suspended in acetone, and the cleaning procedure was repeated three times. After the final concentration, the nanoparticles were suspended in water. The position of the plasmon band for the sol obtained was 452 nm.
2.3. Deposition of oxides dielectric layers The silica layer was deposited using the procedure developed by Mulvaney et al. [25], which has been further modified by Li et al. [26]. Briefly speaking, 0.13 ml of a 1 mM aqueous solution of (3-aminopropyl)trimethoxysilane was added to 10 ml of the solution of silver nanoparticles and stirred at room temperature for 15 min. Next, 1.07 ml of a freshly prepared diluted sodium silicate solution (0.54% of SiO2), adjusted with HCl to pH 10−11, was added. Then, the reaction mixture was stirred for 3−4 days at room temperature. The Ag@SiO2 nanoparticles were cleaned by centrifuging for 15 min, pouring off the supernatant, and adding water to suspend them again. This cleaning procedure was repeated three times (the final Ag@SiO2 nanoparticles were obtained in concentrated solutions). Coating the silver nanoparticles with MnO2 layers was carried out according to the procedure developed by Tian et al. [7] and modified by Abdulrahman et al. [27]. In the first stage, 5 ml of the Ag sol was alkalized to pH = 9.5 by the addition of the proper amount of KOH solution. Then, the alkalized Ag sol was cooled in an ice bath, and 0.04 ml of a 10 mM
8
KMnO4 and 0.2 ml of a 10 mM K2C2O4 aqueous solution were added. After 10 min, the reaction mixture was heated to 60◦C and kept at this temperature for 2 h. The Ag@MnO2 nanoparticles obtained were cleaned by centrifuging, pouring off the supernatant, and again suspending them in water - this cleaning procedure was repeated three times. The TiO2 layers were deposited according to the procedure proposed by Huang et al. [28]. In the first stage, 10 ml of a freshly prepared 0.02 M aqueous solution of Larginine was added to 30 ml of the silver sol, and the mixture obtained was stirred at room temperature for 5 min. 12 ml of cyclohexane and 12 l of (3-mercaptopropyl)triethoxysilane were then added to the aqueous sol of silver nanoparticles, which resulted in the formation of a biphasic system. The reaction mixture was stirred for 6 h in darkness. After 6 hours of stirring, the cyclohexane layer (to which Ag nanoparticles have transferred) was removed, and the Ag nanoparticles were transferred (by several centrifugations and the subsequent addition of ethanol to again suspend the nanoparticles) into ethanol. To grow the TiO2 shell, 1 L of titanium(IV) isopropoxide was added to the alcohol solution of the silver nanoparticles. After 15 min., the reaction was terminated by rinsing the particles with ethanol and centrifuging.
2.4. Formation of a monolayer from 4mercaptobenzoic acid on the Pt surface For model SHINERS measurements, monolayers formed from 4mercaptobenzoic acid (MBA) on platinum surfaces were used. To form such monolayers, flame-annealed Pt substrates were immersed in a 0.5 mM aqueous solution of MBA for 1 day. Then, the MBAmodified platinum surfaces were rinsed with water and allowed to dry in air.
2.5. Experimental techniques Transmission electron microscopy (TEM) analyses were carried out using a Zeiss LIBRA 120 electron microscope equipped with an Incolumn OMEGA filter, working at an 9
accelerating voltage of 120 kV. Before the TEM measurements, samples of the nanoparticles were deposited onto 300-mesh copper grids coated with a Formvar layer, and the deposited solution was allowed to dry. The elemental composition of the sulphide-modified silver nanostructures was determined using a Merlin field emission scanning electron microscope (Zeiss, Germany) equipped with an energy-dispersive X-ray microanalysis (EDS) probe (Bruker). Samples of the sols analysed were deposited on the surface of the graphite substrate, and the deposited solution was allowed to dry. Raman spectra were taken using a Horiba Jobin-Yvon Labram HR800 spectrometer equipped with: a He−Ne laser generating radiation of a wavelength of 633 nm, a Peltier-cooled CCD detector (1024 × 256 pixels), a 600 groove/mm holographic grating, and an Olympus BX40 microscope with a long distance 50× objective. A Shimadzu UV-2401PC spectrophotometer was used for recording the UV–vis extinction spectra.
3. Results and discussion As mentioned in the introduction, depositing a silica layer on some silver nanoparticles often requires increasing the chemical stability of the surface of silver nanostructures. The classical method of passivating the silver surface to form self-assembled monolayers from long-chain alkanethiols (e.g., from 16mercaptohexadecanoic acid) [9,18,19]. In some applications of Ag@SiO2 nanoparticles (actually Ag@thiol@SiO2 nanoparticles), for example, when measuring metal-enhanced fluorescence, alkanethiols deposited directly on the surface of the plasmonic cores cause no disturbance. However, because alkanethiols deposited directly on the plasmonic cores generate a relatively intense Raman signal [22,29], alkanethiols monolayers deposited directly on the silver cores significantly decrease the usefulness of Ag@SiO2 nanostructures as nanoresonators in enhancing Raman scattering efficiency. Figure 1 shows the Raman spectrum generated by
10
silver nanoparticles protected by a monolayer of 16-mercaptohexadecanoic acid (a typical compound used for protecting the surface of Ag nanoparticles before depositing SiO2 [9,18,19]). As can be seen, in this case the spectrum measured contains many Raman bands due to the various vibrations of molecules of the chemisorbed alkanethiols. Therefore, for Ag@SiO2 nanoparticles dedicated as nanoresonators for Raman experiments, it is very important to find an alternative method of protecting the surface of the plasmonic cores. Due to the very large affinity of sulphur to silver, we decided to test very thin layers of Ag2S as protection layers. We began our experiments from cubic silver nanoparticles prepared in the presence of Na2S - these nanoparticles contain some sharp apexes and edges (at which the highest electromagnetic field enhancement is induced and which could be preferentially etched) and they were synthesised in sulphide-containing media. Figure 2 shows a TEM image of silver cubic nanoparticles synthesised according to the procedure described in Section 2. An atomic analysis of the cubic nanostructures formed revealed that the ratio of atomic concentrations of S to Ag was in a range of from 1:100 to 1:400. This means that, for these nanoparticles, only between 0.5-2% of the silver is in the form of a sulphide. Taking into account: the size of the Ag@Ag2S nanoparticles (ca. 80 nm), the relative amount of Ag2S formed, and the difference in the density of Ag (10,49 g/cm3) and Ag2S (7,23 g/cm3), it can be estimated that the average thickness of the Ag2S layer on the surface of Ag cubic nanoparticles should be below 0.4 nm. The deposited Ag2S layer produces a significantly weaker Raman signal than the monolayer formed from 16-mercaptohexadecanoic acid (see Figure 1), which means that such Ag2S-protected silver nanoparticles should constitute a significantly better material for electromagnetic nanoresonators for Raman measurements than do silver nanoparticles protected by a monolayer formed from alkanethiols. The Ag2S-modified silver cubic nanoparticles obtained were then covered with a ca. 5 nm-thick protection layer formed from SiO2, MnO2, and TiO2 (for details, see section 2.3).
11
The deposition of the dielectric layer occurs with relatively good efficiency, and many wellseparated cubicAg@SiO2 (or cubicAg@MnO2, or cubicAg@TiO2) nanoparticles may be found in the products obtained. Figure 3 shows example TEM images of synthesised cubicAg@SiO2, cubicAg@MnO2, and cubicAg@TiO2, nanoparticles. Such nanoparticles were then tested as nanoresomators for characterising various interfaces, using what is known as shell-isolated nanoparticle-enhanced Raman spectroscopy – SHINERS [5-10]. In this technique, the surface being analysed is covered with a layer of surface-protected plasmonic nanoparticles (e.g. from gold or silver), and the Raman spectrum of the investigated sample is then recorded. As mentioned in the introduction, metal plasmonic nanoparticles act as electromagnetic resonators, significantly enhancing the electric field of the incident electromagnetic radiation, hence leading to a very large increase in the Raman signal from the surface on which nanoparticles have been spread (for Raman bands with a small ‘Raman shift’, to the fourth power of the electric field enhancement [4]). The ultrathin protection layer (in this case, SiO2, MnO2, and TiO2) does not damp surface electromagnetic enhancement, but does prevent the nanoparticles from having direct contact with the probed material and does prevent them from agglomerating [6]. The principles of SHINERS measurements using cubicAg@SiO2, cubicAg@MnO2, or cubicAg@TiO2 nanoparticles presents Figure 4. Using this method, it is possible, for example, to record high-quality Raman spectra of various molecules adsorbed at the surfaces of metal single-crystals [6], or Raman spectra of certain biological samples, such as the walls of living cells [6,9]. Figures 5 and 6 show example SHINERS spectra recorded using nanoresonators produced from Ag2S-covered cubic Ag nanoparticles. Fig. 5 shows SHINERS spectra of a platinum substrate modified with a 4-mercaptobenzoic acid (MBA) monolayer before the deposition of the electromagnetic nanoresonators, and covered with cubicAg@SiO2 and cubicAg@MnO2 nanoparticles (in both cases, the nanoparticles were produced from roughly 12
the same amount of material, and the procedure for depositing the nanoparticles was the same). The Raman spectra recorded are undoubtedly characteristic for MBA molecules: two the strongest Raman bands at 1077 and 1587 cm−1 , are due to the 12 and 8a vibrations of the aromatic ring of MBA, respectively [30–33], and the clearly visible band at 1185 cm−1 is also characteristic for adsorbed MBA molecules (this band is due to δ(C–H) vibrations [30,34,35]). This means that a possible Raman signal from the Ag2S protection layer does not disturb in this type of SHINERS measurements. The other interesting "model analysis" carried out using SHINERS nanoresonators is that of detecting traces of methyl parathion (which is an efficient insecticide) on the surface of various botanical fruits (including culinary vegetables). Figure 6 shows Raman spectra of solid methyl parathion, the skin of a "clean" tomato, the skin of a tomato fruit contaminated by methyl parathion, the skin of the tomato fruit covered with cubicAg@SiO2 nanoparticles, and the skin of the tomato fruit contaminated by methyl parathion and covered with cubicAg@SiO2 nanoparticles. As can be seen in Figure 6, the Ag2S layer between the Ag core and the SiO2 shell causes no disturbance in this type of SHINERS measurements - the Raman spectra of the surface of the tomato is dominated by two bands at 1156 and 1520 cm−1 (see Figure 6), which are due to the vibrations due to carotenoid molecules [6]. Also, the spectral region at which the band characteristic for methyl parathion is observed (at ca. 1350 cm−1) is not disturbed by the Raman signal generated by the SHINERS nanoresonators themselves. Because it would be very difficult to produce "bare" (only stabilised, for example, by citrates) cubic silver nanoparticles (the Ag2S layer is already formed during the synthesis of cubic silver nanoparticles), to compare the efficiency of depositing a silica layer on bare and Ag2S-protected silver nanostructures, we decided to analyse a deposition of silica on bare and Ag2S-protected semi-spherical silver nanoparticles. It is worth mentioning that semi-spherical
13
silver nanoparticles usually display significantly lower SERS activity than nanoparticles containing some sharp apexes and edges - in the case of the two types of silver nanoparticles used in this work, the SERS activity of the cubic nanoparticles is one order of magnitude higher than that of the semi-spherical nanoparticles (the methodology of the measurement is given in ref. [9]), and therefore, in this work we did not use semi-spherical Ag@SiO2 nanoparticles for the SHINERS measurements. On the basis of an analysis of TEM images of 500 synthetised nanoparticles (obtained after the deposition of SiO2 on bare and Ag2Sprotected semi-spherical silver nanoparticles), we found that the number of isolated (not aggregated) Ag@SiO2 nanoparticles in the sol product increased by more than 3 times (from ca. 23% to 74%) when the surface of the silver nanoparticles was covered with an Ag2S film before deposition of the SiO2 layer. Therefore, it can be concluded that the passivation of the surface of Ag nanoparticles by the layer of Ag2S may be used to synthesise Ag@SiO2 nanoresonators for SHINERS measurements, which generate a very low Raman background.
4. Conclusions In this work, we propose an improved method of synthesising of Ag@SiO2 nanoparticles (a similar procedure may be also applied for synthesising Ag@MnO2 and Ag@TiO2 nanostructures). Instead of the protecting the surface of the silver core with a monolayer formed from long-chain alkanethiols, we suggest forming a very thin (below 0.4 nm) layer of silver sulphide on the surface of the Ag nanoparticles. The background in Raman measurements generated by sulphide-protected Ag nanoparticles is significantly smaller than that for analogous Ag nanoparticles protected by an organic (thiol) layer. This means that the nanostructures obtained should be especially useful as electromagnetic nanoresonators in Raman experiments,
for
example,
in shell-isolated nanoparticle-enhanced Raman
spectroscopy measurements.
14
Acknowledgments This work was financed from funds of the National Science Centre (Poland) allocated on the basis of decision number DEC-2013/11/B/ST5/02224. A.K. thanks the Faculty of Chemistry, University of Warsaw for its financial support.
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Figures captions Figure 1. Raman spectra generated by semi-spherical silver nanoparticles covered with: (a) ca. 0.4 nm thick layer of Ag2S, and (b) monolayer of 16mercaptohexadecanoic acid. Figure 2. TEM image of synthesised cubic silver nanoparticles. Figure 3. TEM images of synthesised nanoparticles: (a) Ag@SiO2, (b) Ag@MnO2, and (c) Ag@TiO2. Figure 4. The principles of SHINERS measurements using cubic Ag@SiO2, Ag@MnO2 or Ag@TiO2 nanoparticles. Figure 5. (a) Raman spectrum of MBA monolayer on Pt before deposition of SHINERS nanoresonators. (b, c) Raman spectra of MBA monolayer on Pt covered with (b) cubicAg@SiO2, and (c) cubicAg@MnO2 nanoparticles. Inset: Raman spectrum of solid polycrystalline MBA. Figure 6. Raman spectra of: (a) solid methyl parathion, (b) skin of tomato fruit, (c) skin of tomato fruit contaminated by methyl parathion, (d) skin of tomato fruit covered with cubicAg@SiO2 nanoparticles, and (e) skin of tomato fruit contaminated by methyl parathion and covered with cubicAg@SiO2 nanoparticles. The strongest band due to the vibration of the molecules of methyl parathion is marked with an asterisk.
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b 50 cps
Raman Intensity
I)
a 150
350
550
Raman shift / cm
750 1
b 20 cps
Raman Intensity
II)
a
800
1000
1200
1400
Raman shift / cm
21
1600 1
1800
a)
b)
c)
22
SiO2, MnO2, or TiO2
Raman intensity
Ag
Raman shift
1595
900
1292
1182
1096
1200
1500
1800
Raman Intensity
1587
1077
1185
c b
a 800
1000
1200
1400
1600
Raman shift / cm 1 23
1800
1156 1520
*
1326
Raman Intensity
e
d
c b
* a
800
1000 1200 1400 1600 1800
Raman shift / cm1
24