Ion Permeation through Silica Coating of Silver Nanoparticles

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Ion Permeation Through Silica Coating of Silver Nanoparticles Functionalized with 2-Mercaptoethanesulfonate Anions: SiO-Encapsulated SERS Probes for Metal Cations 2

Piotr Piotrowski, and Jolanta Bukowska J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03462 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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The Journal of Physical Chemistry

Ion Permeation through Silica Coating of Silver Nanoparticles Functionalized with 2-Mercaptoethanesulfonate Anions: SiO2-Encapsulated SERS Probes for Metal Cations

Piotr Piotrowski and Jolanta Bukowska* Department of Chemistry, University of Warsaw, Pasteur Street 1, 02-093 Warsaw, Poland *corresponding author e-mail: [email protected] phone: +48 22 55 26 402

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Abstract In this work, the first use of thick-silica-protected Ag nanocomposites (Ag-MES@SiO2) in SERS (surface enhanced Raman scattering) detection of metal cations is presented. Several factors which are important for the successful synthesis of silica-encapsulated citrate-free Ag nanoparticles functionalized with sodium 2mercaptoethanesulfonate (MES) are elucidated. It is shown that negatively charged MES anions adsorbed on Ag surface effectively protect Ag NPs from both aggregation and dissolution in the presence of ammonia. Simultaneously, MES acts as a primer in the modified Stöber reaction. Owing to the MES functionalization of the silver core, Ag-MES@SiO2 nanoparticles exhibit intense and stable SERS spectra that are sensitive to metal cations. Based on cation-sensitive SERS spectra of silica-encapsulated MES tags it is proved that 30-nm thick silica shell is permeable for metal cations in short spans of time. Differences between several metal cations in transport through the SiO2 layer are observed, with smaller cations reaching the Ag core more efficiently. The Ag-MES@SiO2 SERS nanosensor exhibits better sensitivity than unprotected Ag-MES sensor.

Introduction Silica-encapsulated metal nanoparticles are a subject of extensive studies since the first pioneering work of Mulvaney et al.1 The silica shells have many competitive advantages over other materials, e.g. chemical inertness, optical transparency, robustness, and controllable permeability.2


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These properties turned out to be very important in designing nanoparticle substrates for surface enhanced Raman scattering (SERS) spectroscopy. Covering of SERS nanoprobes (usually Ag or Au nanoparticles) with a silica shell is strongly desired when the probes are applied in biosensing, e.g. in cells or tissues.3-5 In core-shell systems, the inner metallic core, which may be biotoxic, is well isolated from the biological environment. On the other hand, a dense and relatively thick amorphous silica layer effectively prevents molecules such as proteins and other macromolecules from penetrating the shell.3,6 Thus, silica encapsulation of a metal SERS nanoprobe reduces possible interference of proteins when SERS is used for sensing other species in the sample. A successful use of silica-encapsulated Ag nanoparticles functionalized with 4-mercaptobenzoic acid (MBA) was recently demonstrated in SERS-based pH sensing in artificial samples and inside macrophage cells.6 Wang et al. proved that encapsulation of Ag-MBA nanoparticles resulted in improved stability of the sensor and helped in avoiding interference of bovine serum albumin, whose diameter (about 7 nm) is considerably greater than the pore size of the silica shell (assumed diameter ~ 1 nm).6 Porosity of the silica has been a subject of many studies.7-17 Several years ago, van Blaaderen et al. demonstrated the presence of nanopores of diameter ranging from 2 to 50 nm in silica particles synthesized using tetraethylorthosilicate (TEOS).7,8 In a more recent paper, porosity of the silica layer of Au@SiO2 was determined to be of order of several Å.9 The blocking effect of SiO2 on halocarbon reaction between N3 dyes (size of about 1.6 nm) and Au cores indicated that the pores in silica are not greater than 1.6 nm.9 UV-Vis and transmission electron microscopy (TEM) measurements during the etching process of Au and Ag cores by cyanide and sulphide ions clearly show that silica



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shells prepared using sodium silicate are porous.10,11 Lower porosity is suggested for SiO2 shells prepared with TEOS.7,12 The nanoporous structure of silica-coated Au nanoparticles was also demonstrated by SERS experiments.12 However, the SERS signal of 2-naphtalenethiol placed outside the Au@SiO2 nanoparticles (with 3-4 nm thick silicon membrane) was measurable only for internally etched nanoparticles and was ascribed to molecules that diffuse through the silica shell and subsequently bind to the Au core. Permeation of ionic species in nanoporous silica membranes has been an active area of research in recent years because of practical applications of synthetic nanopores as protein or DNA sensors13 or as desalination membranes14-16.

Ab initio molecular

dynamics calculations show that hydroxylated silica nanopores (1.16 nm) strongly attract sodium cations and repel Cl- anions.17 Porosity of a silica shell covering SERS active metal cores is a very important issue in constructing SERS-based ion sensors. As shown by Tian et al.18, cyanide ions could be detected with the SERS spectra only if so called pinholes have been created in ultrathin silica shells that cover Au nanoparticles. In the previous paper, SERS sensor for detection of alkaline and alkaline earth metal

cations,

utilizing

silver

nanoparticles

functionalized

with

sodium

2-mercaptoethanesulfonate (MES) was presented.19 Potential applications of the MES-based sensor in monitoring these cations in biological samples (e.g. in blood serum) encouraged us to study silica encapsulated Ag-MES nanoparticles (NPs) as the SERS probes for alkaline and alkaline earth metal cations. Such NPs can be introduced to biological samples and used in vivo with no or hindered interaction with biological molecules such as proteins. The question is whether a relatively thick SiO2 layer


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(several nm), which completely blocks penetration of larger entities would sufficiently transport cations to the NPs interior, which is indispensable for the effective action of the Ag-MES@SiO2 probes as a SERS sensor. In this work, we demonstrate a simple procedure of the encapsulation of MES-functionalized silver NPs in a silica shell. In the obtained system Raman reporter molecule (MES) simultaneously acts as a primer in modified Stöber synthesis, while silica is used as a permeable membrane for SERS detection of metal cations.

Experimental

2.1 Instrumentation Raman spectra were collected on LabRAM HR800 (Horiba Jobin Yvon) Raman spectrometer with a charge-coupled device detector cooled by Peltier modulus. All the spectra were excited with a 532 nm Nd:YAG laser second harmonic line of a maximum light beam power (at the laser head) of about 100 mW. Holographic grating with 600 grooves/mm was used. The spectrometer was coupled with Olympus BX61 confocal microscope with a pinhole set to 200 µm. Backscattered light was collected through a 50x objective. Calibration of the system was performed with respect to 520 cm-1 silicon band. Transmission electron microscopy images were collected with a Zeiss Libra 120 microscope, with a LaB6 cathode, equipped with OMEGA internal columnar filters and a CCD camera. The samples were prepared by deposition of the suspension on 400-mesh nickel grids coated by the Formvar layer and left to dry.



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2.2. Chemicals 2-propanol (99.7%), magnesium chloride (99%), potassium chloride (99.5%) and silver nitrate (99.9%) were purchased from POCh, the rest of the metal chlorides (99%) as well as hydroxylamine hydrochloride (99.9%), sodium 2-mercaptoethanesulfonate (98%) and tetraethyl orthosilicate (98%) were purchased from Sigma-Aldrich. Ammonia solution (25%) was provided by Chempur. Ultrapure water (18 MΩ·cm-1) was used to prepare all solutions.

2.3. Sample preparation Full procedure for synthesis of MES-functionalized cation nanosensor is described in our previous article.19 In brief, silver nanoparticles synthesized according to Leopold and

Lendl

procedure20

were

functionalized

with

MESNa

(sodium

2-mercaptoethanesulfonate) by mixing the solutions in a ratio of 1:9 (1x10-2 M MESNa aqueous solution to Ag colloid). After centrifugation, the nanosensors were redispersed in water up to the initial value. The final concentration of Ag-MES NP is estimated to be 1.12 nM. Next, the cation nanosensors were covered with a layer of silica. 10 ml of the nanosensor suspension (sample 1 or 2) was added to 40 ml of isopropanol under stirring. Subsequently, 960 µl of ammonia and 24 µl of tetraethyl orthosilicate were added. Mixture was kept for 15 min at room temperature and then at 4ºC. After 2 hours, the mixture was taken out of refrigerator and centrifuged 3 times. Finally, the sample was redispersed in water up to the initial volume of the used nanosensor suspension.


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SiO2 encapsulation on bare AgNPs was carried out analogously. In order to keep the AgNP concentration constant in both experiments, 1 ml of water was added to 9 ml of the AgNPs suspension. After centrifugation, NPs were redispersed in water up to 10 ml. Then, silica encapsulation procedure was applied. Metal cations were introduced to the Ag-MES@SiO2 samples as chloride salts. Stock solutions of salts (100 µl, 0.5 M) were added to 400 µl of the Ag-MES@SiO2 suspension. For concentration-dependent series, KCl stock solution was used to obtain concentrations in the range 5x10-7 M – 5x10-1 M differing by one order of magnitude and those were added to the Ag-MES@SiO2 suspension in volumes as described above.

2.4. Raman measurements and data processing All spectra were collected for 5 accumulations, 50 seconds each. Spectra were baseline-corrected before further processing. In order to calculate intensity ratio of the chosen bands, intensities in corresponding peak positions were read out.

Results and discussion Characterization of silica-encapsulated 2-mercaptoethanesulfonate – functionalized Ag nanoparticles The synthesis of silica-coated silver or gold nanoparticles using a modified Stöber method

involves

ammonium

hydroxide–catalysed

hydrolysis

and

subsequent

condensation of alkoxysilanes, such as tetraethylorthosilicate (TEOS) used in this work. It is generally accepted that the growth of the silica shell directly on the surfaces of Ag 7 ACS Paragon Plus Environment


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and Au nanoparticles is hindered because of their vitreophobic properties. This is usually overcome by using surface primer molecules such as amino functionalized silane coupling

agents,

e.g.

(3-aminopropyl)trimethoxysilane

(APTMS)

or

(3-aminopropyl)triethoxysilane (APTES).21 However, it was demonstrated that direct silica coating with no primer molecules is possible.22-24 Successful coating of Ag nanoparticles stabilized with citrate anions was reported when less aggressive dimethylamine (DMA) instead of ammonium hydroxide was used as a catalyst in the standard Stöber method.25 Mirkin et al. prepared triangular Ag@SiO2 nanoprisms using 16-mercaptohexadecanoic acid (MHA) which prevented both aggregation and etching of nanoparticles by amine catalyst.26 It was confirmed that 11-mercaptoundecanoic acid (MUA) acts as an effective stabilizing and silica growth promoting agent as well.27,28 Most recently Tian et al. reported the use of 4-mercaptobenzoic acid (MBA) as a primer molecule for covering Au nanoparticles with ultrathin silica shells (below 2 nm).29 However, this procedure was not effective for further growth of silica layer and thicker shells have been produced with APTES as a primer. Here we report a very simple and effective procedure of primer-free silica coating of Ag nanoparticles functionalized with 2-mercaptoethanesulfonate (MES) anions, where MES may at the same time be used as a surface tag in SERS-based cation sensing. The MES-functionalized Ag NPs (Ag-MES) prepared by self-assembling of sodium 2-mercaptoethanesulfonate on hydroxylamine-reduced Ag colloid were covered with a silica layer (see experimental) via a modified Stöber reaction. Growth of SiO2 on Ag-MES NPs was controlled with UV-Vis spectroscopy. Reaction was carried out through 20 h and changes in the extinction spectra were monitored throughout this time.


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Initial Ag colloid exhibits a spectrum with a peak at 404 nm, which shifts to 408 nm after modification with MES monolayer (Fig. 1). During the reaction, one can see a gradual red-shift of the maximum. Such a change in extinction spectra of Ag-MES@SiO2 NPs is in agreement with previous reports and proves encapsulation with a relatively thick layer of silica.30,31 The Ag-MES@SiO2 NPs used in subsequent experiments were synthesized for 2 hours. They exhibit maximum of surface plasmon resonance at 422 nm (Fig. 1). Position of the maximum for silica-coated nanoprobes synthesized for longer times remained approximately the same (see inset in Fig.1).

Figure 1. Extinction spectra of Ag colloid (orange), Ag-MES colloid (purple) and AgMES@SiO2 (blue). λmax Ag = 404 nm, λmax Ag-MES = 408 nm, λmax Ag-MES@SiO2 = 422 nm. Spectra were normalized to 1.0 with respect to the maximum extinction. Inset: changes in UV-Vis spectra in the span of 20 hours, the shift of the peak maximum suggests a gradual growth of the silica layer. Numbers in the legend indicate the time of synthesis in hours.

TEM images confirmed that single-core Ag-MES@SiO2 hybrid nanostructures were obtained (Fig. 2). Satisfactory efficiency of silica coverage was achieved. Few pure silica nanospheres devoid of Ag core could be seen as well as there were hardly any 9 ACS Paragon Plus Environment


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uncovered Ag nanoparticles. Mean diameter of the Ag core was 29.7 ± 5.8 nm and the mean thickness of SiO2 was 31.9 ± 3.9 nm. The initial size of Ag-MES NPs did not change upon addition of reagents which indicates that no etching of the metal in the presence of added ammonia occurred. This proves a protective role of compact MES monolayer on the NPs’ surface, which prevented from direct contact between Ag and ammonia.

Figure 2. TEM images of Ag-MES@SiO2 nanoparticles in different magnifications.

In order to check the protective properties of the adsorbed MES monolayer in the synthesis of Ag-MES@SiO2, silica encapsulation was also performed on MES-uncoated Ag nanoparticles. After the reaction and centrifugation, there was substantially less precipitate at the bottom of the tube. Moreover, even though the precipitate was dispersed in 2 ml instead of the initial 10 ml (so it should be 5 times more concentrated than analogous Ag-MES@SiO2 samples), UV-Vis spectrum of this sample was substantially less intensive than the spectrum of the Ag-MES@SiO2 samples (see Supplementary Fig. S1). As previously observed, chemical instability of silver nanoparticles in the presence of ammonia results in relatively fast damping of the plasmon resonance band.25 TEM images confirm that the reaction between Ag and NH3 occurred and NPs were dissolved in this case (Supplementary Fig. S2). Such extensive etching of Ag proves no initial 10 ACS Paragon Plus Environment

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functionalization of hydroxylamine-reduced Ag nanoparticles which would provide effective protection against ammonia. Thereby, it confirms that covering of silver NPs with a MES monolayer not only effectively protects Ag NPs from the aggressive action of ammonium but also that MES molecules enable the silica layer to grow without using any silica-promoting agent molecules (primer molecules). Additionally, adsorbed MES molecules play another important role: they stabilize the metal colloid and protect the NPs from aggregating in the alcohol medium.

SERS detection of metal cations with Ag-MES@SiO2 nanoprobes Silica-encapsulated silver nanoparticles functionalized with MES anions were further used for sensing alkaline and alkaline earth metal cations. The working principle of the MES-based ion sensor was described in our previous paper.19 In brief, the idea of our SERS sensor was based on a high sensitivity of the band assigned to the symmetric stretching vibrations of the SO3- terminal groups of the surface-bound MES to cations (observed in the range of 1035-1070 cm-1). Electrostatic attraction of cations to negatively charged MES anions immobilized on Ag NPs resulted in the splitting of the marker band into two components. The position of the higher frequency component varied for different metal cations while the relative intensity of both components changed upon cation concentration. SERS spectrum of MES molecules encapsulated inside the Ag-MES@SiO2 NPs (Fig. 3) is generally similar to the SERS spectrum of MES from silica-uncovered Ag support.19,32 Most importantly, marker band of the SO3- symmetric stretching vibrations at 1035 cm-1 is of relatively high intensity which will help quantifying changes in the



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spectrum after changing the cation concentration. There is, however, one interesting difference in this frequency range between the SERS spectra of MES in silica-free Ag-MES NPs and that of silica-encapsulated MES molecules adsorbed on Ag NPs (Ag-MES@SiO2). The SERS spectrum of the MES monolayer formed from MESNa on Ag NPs consists of two clearly resolved components of the 1035 cm-1 band (Fig. 3), of which the one at the higher frequency corresponds to the MES anions interacting with sodium cations. Thus, in order to let the detected cations reach the MES anions on the surface, sodium cations have to be removed from the structure of the monolayer. In the system without silica, that is for Ag-MES NPs, we achieved it by exposing the nanosensors to 0.045 M HCl solution while centrifuging. Successful displacement of sodium cation by protons was confirmed by the SERS spectra in which higher frequency component of the 1035 cm-1 band was no longer observed (see Fig. 3). Unexpectedly, in the case of the Ag-MES@SiO2 NPs, the component corresponding to MES- Na+ ion-pairs disappeared with no need to acidifying the medium. We assume it is most probably due to stronger affinity of Na+ to silica layer than to adsorbed MES which in turn promotes incorporating sodium into silica. That would result in a glass-like structure with sodium cations. The SERS spectra presented in Fig. 3 need an additional comment. As may be seen, relative intensities of the band at 795 cm-1, assigned to the C-S stretching vibration and the band at 1035 cm-1 (I795 / I1035 ) differ between the spectra shown in the figure. Thorough analysis of many collected spectra indicates a strong correlation between the I795 / I1035 ratio and the relative intensity of the bands around 630 and 705 cm-1, which are ascribed to gauche and trans conformers of MES molecules, respectively (Supplementary


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Fig. S3) .32 An increase of the I795 / I1035 ratio is always accompanied by the rise of the Itrans / Igauche ratio. Thus, observed changes in the spectral pattern may be assigned to the structural changes of adsorbed MES molecules in varying environment, which in turn are controlled by intermolecular interactions inside the MES monolayer. It is obvious that the formation of MES-cation contact ion pairs will diminish electrostatic repulsion between SO3- groups of MES anion, resulting in a more “upright” position that corresponds to the trans conformation of the -S-C-C-SO3- chain. This is manifested in all the SERS spectra collected for MES monolayer in contact with various salt solutions.32

Figure 3. SERS spectra of: MES molecules adsorbed on the silver core of AgMES@SiO2 (bottom: Ag-MES@SiO2), MESNa adsorbed on Ag NPs before the exposure to 0.045 M HCl solution with two clearly visible components of the SO3- stretching band (middle: Ag-MES before H+) and MES molecules adsorbed on Ag NPs after the exposure to 0.045 M HCl solution with only one component of SO3- stretching band left (top: AgMES after H+). Middle spectrum of Ag-MES before H+ was normalized to the 795 cm-1 band intensity with the scaling factor of 0.25 to illustrate differences in relative intensities of the most prominent band at 795 cm-1 and the SO3- band.



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Obtained hybrid nanostructures (Ag-MES@SiO2) exhibit good reproducibility of the SERS signal. Spectra recorded over 2 hours show only small fluctuations of the integrated intensity for two most intensive bands (795 and 1035 cm-1). However, a intensity drop occurs in the spectra collected from the samples of the same batch 5 days after the synthesis (Fig. 4). In order to characterize long-time stability of the AgMES@SiO2 nanoparticles, we monitored their surface plasmon resonance spectrum over time. As shown in Supplementary Fig. S4, optical properties of the nanoparticles are stable within the first 2 hours but after a few days blue-shift of the extinction band is clearly visible. This may indicate that the local dielectric constant near the Ag core decreases due to progressive etching of the silica shell which covers Ag-MES nanoparticles.12

Figure 4. Integrated intensities of the two most intensive SERS bands of MES on AgMES@SiO2, that is 795 and 1035 cm-1 (blue squares and red triangles, respectively), plotted against the time after the synthesis. Notice a break between 150 and 6800 min (over 4 days) on the time axis.


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Relatively small intensity changes of the SERS spectrum of MES encapsulated in silica shells indicate no significant loss of MES due to desorption followed by diffusion through the SiO2 layer. In order to check a possibility of diffusion of MES anion through the silica layer, additional sodium 2-mercaptosulphonate molecules (2x10-3 M) were introduced into the Ag-MES@SiO2 suspension. The addition of MESNa to the suspension of Ag-MES@SiO2 caused significant changes in the SERS spectra. (Fig. 5). First, we observed a change of relative intensities of the bands at 795 cm-1 and at 1035 cm-1. Initially, the band at 1035 cm-1 was relatively strong, comparable to the C-S band at 795 cm-1. After the addition of MESNa, an increase of the 795 cm-1 band intensity and a small high-frequency shift of the band position were detected. At the same time, one could observe changes in the relative intensities of the bands at about 630 and 705 cm-1, assigned to the gauche and trans conformers of the MES molecules, respectively. Upon the addition of MESNa to Ag-MES@SiO2 colloid, the intensity of the band corresponding to the trans conformer increased at the expense of the band of the gauche form. This was accompanied by the appearance of a band at 1060 cm-1, which is assigned to the SO3- groups interacting directly with sodium cations. This is a strong evidence of the penetration of metal cations through a thick silica shell, which is studied thoroughly in the following section. As explained above, the observed changes in the intensity pattern caused by the addition of MESNa can be unambiguously ascribed to conformational changes within the MES adlayer, provoked by the formation of contact ion pairs with sodium cations. The almost unchanged intensity of the conformationally independent MES band around 1300 cm-1 proves that the amount of MES adsorbed on Ag



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surface does not considerably increases and thus it may be concluded that large and negatively charged MES anions do not penetrate the silica layer.

Figure 5. SERS spectra of MES encapsulated in Ag-MES@SiO2: before (solid line) and after the addition of 2x10-3 M MESNa molecules to the Ag-MES@SiO2 colloid (dashed line). Further experiments confirmed that SERS spectra of MES molecules encapsulated underneath the silica layer on the surface of Ag core are sensitive to cations. Upon the exposure of the Ag-MES@SiO2 nanoprobes to various solutions of metal chlorides, a new component on the higher frequency side of the 1035 cm-1 band arose. Its position depended on the cation and its intensity increased at the higher concentration of the metal chloride at the expense of the 1035 cm-1 band, similar to the previous results for the uncoated Ag-MES NPs.19 Owing to these features, we were able to determine permeability of the silica layer to cations of different radius and charges by detecting their presence near the silver core surface. Indeed, MES-modified Ag@SiO2 NPs in the 10-1 M K+ solution exhibited a different SERS pattern in the region of the SO3- symmetric stretching vibration as compared to the spectra of MES on bare Ag in corresponding concentrations of the ion (Fig. 6). What is more, the intensity ratio of the two ion 16 ACS Paragon Plus Environment

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sensitive bands (I1/I2 – where I1 corresponds to the lower frequency component and I2 to the higher frequency one) was higher for Ag-MES@SiO2 which implies that the effective concentration of K+ in the vicinity of MES adlayer was lower than the corresponding concentration in the case of silica-free NPs. This phenomenon is not limited to potassium, though. In Fig. 6 you can also find analogous effect observed for another three metal cations: Na+, Mg2+ and Ca2+. This demonstrates that a thick silica shell, which is very often used to isolate metal nanoparticles from the medium, is easily penetrated by small inorganic moieties like metal cations. Most probably, negative charge of the silica surface introduced by the deprotonated silanol groups14 facilitates the flow of cations through the nanopores of the silica layer.

Figure 6. Differences in the behaviour of the ion-sensitive SERS bands of MES in the 10-1 M solutions of four metal cations: K+, Na+, Mg2+ and Ca2+ for monolayers adsorbed on: bare Ag NPs (a: red spectra) and on the Ag core of Ag@SiO2 (b: blue spectra). The 17 ACS Paragon Plus Environment


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position of the higher-frequency marker band characteristic for the respective ion is indicated in the figures.

Spectra shown in Fig. 6 can be used for a rough estimation of the cation effective concentration at the Ag surface. The response of Ag-MES monitored by the SERS spectroscopy was already studied in the previous work.19 Based on the calibration curves obtained there, I1/I2 intensity ratio can be ascribed to the concentration of the ion interacting with the MES monolayer. We assume that the MES monolayer responds to the cation in the same way in the Ag-MES@SiO2 system. In this case, however, the response will not depend on the total cation concentration outside the silica layer but on the concentration at the Ag surface, underneath the SiO2 shell. Using the calibration curves from the previous work, effective concentrations of the ion near the Ag-MES surface were calculated and collected in Table 1. It appears that the difference between the effective concentrations at the vicinity of the MES monolayer in Ag-MES@SiO2 and in silica-free Ag-MES is dependent on the cation and increases in the following order: Ca2+ ≈ Na+ < K+ (where the values for Ca2+ and Na+ are of the same order). This strongly correlates with ionic radii of the cations (0.100 nm for Ca2+, 0.102 nm for Na+ and 0.138 nm for K+)33. The only exception is Mg2+ cation (0.072 nm), despite a relatively small radius. This observation could be ascribed to a very high hydration energy of magnesium cation as compared to the rest of the studied cations. The strongly bound hydration shell is most probably the reason why magnesium ions find it hard to penetrate the silica layer.


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Table. 1. I1/I2 intensity ratios for Ag-MES@SiO2 exposed to 10-1 M of the respective ion and the estimation of the effective concentration of the ion at the surface of the Ag core. Concentration was determined based on the calibration curves obtained for Ag-MES NPs.19 cation

I1/I2

on effective

ionic

radius33 hydration

Ag@SiO2

concentration /M

/nm

enthalpy34 /kJ/mol

Ca2+

0.86

1.5x10-2

0.100

-1577

Na+

0.84

3.5x10-2

0.102

-409

K+

0.67

4.6x10-3

0.138

-322

0.072

-1921

Mg2+

Ion Permeation through Silica Coating of Silver Nanoparticles

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