Journal of Colloid and Interface Science 415 (2014) 77–84

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Preparation of saline-stable, silica-coated triangular silver nanoplates of use for optical sensing Michael P. Brandon, Deirdre M. Ledwith, John M. Kelly ⇑ School of Chemistry, Trinity College, University of Dublin, Dublin 2, Ireland

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 1 March 2013 Accepted 7 October 2013 Available online 21 October 2013

Triangular silver nanoplates (TSNPs) may find application in next generation optical bio-sensors owing to the high sensitivity of the spectral position of their main plasmon band to changes in local refractive index. Unfortunately, etching of the anisotropic nanoplates to spherical particles occurs upon exposure to chloride ions from salt, with a concomitant decrease in optical sensitivity. Herein are detailed two general methods for the silica coating of TSNPs, with the aim of forming a protective barrier against chloride etching. It has been necessary to modify literature approaches for the coating of spherical Ag nanoparticles, since these are either ineffective for anisotropic nanoplates or lead to their degradation. The first method is a modified Stöber approach using tetraethylorthosilicate (TEOS) as the alkoxide precursor and dimethylamine in low concentration as the basic catalyst, with prior priming of the nanoplate surfaces by diaminopropane. The thickness of the silica layer can be tuned between 7 and 20 nm by varying the primer and alkoxide concentrations. The second method involves deposition of a thin dense layer of silica from sodium silicate solution onto mercaptopropyltriethoxysilane (MPTES) or mercaptopropyltrimethoxysilane (MPTMS) primed TSNPs. This latter method offers protection against anion etching – experiments suggest that the adsorbed MPTES provides much of the barrier to chloride ions, while the silica shell serves to prevent particle aggregation. It was found that the silica coated particles substantially retained the sensitivity to refractive index of the as-grown TSNPs while being able to withstand salt concentrations typical of bio-testing conditions. Ó 2013 Elsevier Inc. All rights reserved.

Keywords: Triangular silver nanoplates Etching Silica shell Salt stable plasmonic nanoparticles

1. Introduction The resonant interaction of light with the collective oscillation of the conduction band electrons in noble metal nanoparticles gives rise to an optical extinction known as a localized surface plasmon resonance (LSPR). It is well documented that the spectral position and intensity of the LSPR depend, not only on the identity of the metal, but also on nanoparticle size and shape as well as surrounding dielectric environoment [1–5]. In particular the LSPR responses of anisotropic Au and Ag nanocrystals are highly versatile and tuneable in comparison to their spherical counterparts [6–8]. Accordingly much effort has been devoted to the synthesis of these materials over the last decade [9–13]. One of the principal motivations behind this research has been the development of refractive index based chemical and biological sensors [14–16]. These are based on the sensitivity of the LSPR peak wavelength to changes in refractive index in the vicinity of a nanoparticle surface, which accompany the binding of a target analyte. While gold nanoparticles generally offer greater chemical stability, silver particles of ⇑ Corresponding author. Fax: +353 1 671 2826. E-mail addresses: [email protected] (D.M. Ledwith), [email protected] (J.M. Kelly).

(M.P.

Brandon),

[email protected]

0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.017

similar size and shape possess superior optical properties, exhibiting sharper resonances and greater refractive index sensitivity [16–18]. In recent years our group has developed protocols for the production of silver nanoplate colloids [19,20]. The latest evolution [21,22] enables the preparation of a sol consisting almost exclusively (over 95%) of monodisperse citrate capped triangular silver nanoplates (TSNPs). The method is rapid and environmentally friendly, being conducted at room temperature in aqueous media. An attractive feature of this preparation is the possibility of readily tuning the edge length of the nanoplates, and accordingly the sol colour and LSPR peak wavelength. By decreasing the amount of starting seed particles, nanoplate edge size can be increased from ca. 10 to ca. 200 nm (thickness increases with edge length from ca. 5 to ca.14 nm) with concomitant shifting of the plasmon band position through the visible and into the near IR. This ability to tune the wavelength of the plasmon maximum to that required for a particular application makes these TSNPs attractive candidate for use in solution phase LSPR sensing. To this end the optical properties of these silver sols were thoroughly investigated [23] and their size dependent refractive index sensitivities were found to exceed those of other reported plasmonic nanostructures [22]. As a proof of concept, our nanoplates when functionalized by phosphocholine, have

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been shown by Brennan-Fournet and coworkers [24] to be effective in detecting low levels of C-reactive protein. In common with other workers [25–27], we have observed that our TSNPs undergo oxidative etching in the presence of halide ions, forming either discal nanoplates and at higher halide concentrations, small spheres or complete destruction of the nanoparticles. This structural change is accompanied by a blue-shift in the position of the main (in-plane dipole) plasmon band and an associated decrease in refractive index sensitivity. Since chloride ions are often a component of biological media, this etching phenomenon poses a difficulty for the widespread adoption of TSNPs in bioassays. It is therefore imperative to devise a means of preserving the morphological integrity of the nanoplates at salt concentrations typical of biological testing conditions. A possible solution involves the treatment of the nanoplates with a protective inorganic oxide layer. In this regard silica is a suitable candidate, since it is chemically inert but optically transparent. Since the pioneering work of Liz-Marzán, Mulvaney and co-workers with spherical Au [28] and Ag [29] nanoparticles, there has been a surge of interest in silica coated metal nanoparticles with two recent reviews on the topic [30,31]. Silica coated silver nanoparticles (Ag@SiO2) have found application in a range of fields including colorimetric sensing [32,33], surface enhanced Raman spectroscopy [34–37], metal enhanced fluorescence [38–42], antibacterials [43], catalysis [44], electronics [45], and laser research [46]. Most commonly, silica shells are deposited on Ag nanoparticles through the Stöber process. This involves the base catalysed hydrolysis and condensation of a silicon alkoxide precursor [47], often tetraethylorthosilicate (TEOS) [32,37,48–52]. The reaction occurs when this is mixed with water in an alcohol solvent. Shell thickness can be controlled through the precursor concentration, with reported values usually in the range of 10– 80 nm [29,32,48,50,53,54]. A primer species is often adsorbed onto the particles prior to Stöber growth. The reasons are twofold – firstly to increase the affinity of the surface for silica, and secondly to provide sufficient colloidal stability to prevent particle aggregation upon transfer to the alcohol solvent. Species utilised as primers include 3-aminopropyltrimethoxysilane [29], polyvinylpyrrolidone [49], and 16-mercaptohexadecanoic acid [55]. Drawbacks of the Stöber approach include the formation of free silica particles and multiple cored Ag@SiO2. Silica coating has also been achieved using microemulsion strategies [56–59], an approach most suited to nanoparticles prepared in organic phases. A third approach involves deposition of the shell from sodium silicate solution. This method was employed by Ung et al. [29] in the original report on Ag@SiO2 preparation. In that case, the nanoparticles were primed with 3-aminopropyltrimethoxysilane, prior to deposition of a nanometer thick silica shell from an aqueous solution of sodium silicate. These particles were sufficiently stable for subsequent shell thickening by the Stöber method. Similar strategies were adopted by Nomura et al. [60], and Hunyadi and Murphy [61]. Reports of Ag@SiO2 are almost exclusively confined to spherical cores. Forming shells around anisotropic crystals such as TSNPs is likely to pose greater challenges. To our knowledge the only account of silica coating of TSNPs is due to the Mirkin group [55]. This involved the adsorption of a monolayer of 16-mercaptohexadecanoic acid (MHA) prior to a standard Stöber growth routine. The thinnest silica shell observed was 15 nm. Herein, we describe two methods for silica coating of TSNPs. In the first, diaminopropane is adsorbed as a primer, onto PVP stabilised nanoplates. Shell growth is then accomplished through Stöber chemistry. In the second method, thin silica shells are deposited onto 3-mercaptopropyltriethoxysilane or mercaptopropyltrimethoxysilane (MPTMS) primed nanoplates from sodium silicate

solution. The use of this latter type of shell as an effective barrier to chloride, is described.

2. Materials and methods 2.1. Materials Silver nitrate (AgNO3, 99.9999%), sodium borohydride (NaBH4 P99%), trisodium citrate (TSC, P99%), polyvinylpyrrolidone (PVP, Mw = 10,000), ascorbic acid (P99%), poly(sodium 4-styrenesulfonate) (PSSS, 1000 kDa), 1,3-diaminopropane (P99%), ethanol (HPLC grade), tetraethylorthosilicate (TEOS, 99.999%), dimethylamine solution (DMA, 40 wt% in H2O), 3-mercaptopropyltriethoxysilane (MPTES, P80%, GC), 3-mercaptopropyltrimethoxysilane (MPTMS, 95%, GC), 3-aminopropyltriethoxysilane (APTES, P98%), sodium silicate solution and sucrose (P99.5%, GC) were purchased from Sigma–Aldrich and used without further purification. All aqueous solutions were prepared using distilled Millipore water (resistivity of 18.2 MO cm). 2.2. Silver nanoplate preparation Silver nanoplate sols, grown by two distinct seeded preparations, are considered. The detailed synthetic procedures have been described elsewhere and the respective products are denoted as either oTSNP [20] or nTSNP sols [21]. In brief, the seed particles for oTSNP sols were produced at room temperature by the dropwise addition of an aqueous solution of NaBH4 (0.01 M, 0.6 mL) to a 20 ml volume of aqueous AgNO3 (0.25 mM) + TSC (0.25 mM) solution. Seed solution (100 lL) and aqueous solutions of PVP (1 wt%, 10 mL), ascorbic acid (0.1 M, 50 lL) and TSC (25 mM, 300 lL) were combined and heated to 50 °C. To the latter solution AgNO3 (0.01 M) was added in 50 lL aliquots at 30 s intervals to a total volume of 250 lL. The final sol, green in colour, was centrifuged and re-dispersed in the original volume of Millipore water. Seeds for nTSNP sols were obtained by the steady addition (2 mL min 1) of aqueous AgNO3 (0.5 mM, 5 mL) to an aqueous solution of TSC (2.5 mM, 5 mL), PSSS (500 mg L 1, 0.25 mL) and NaBH4 (10 mM, 0.3 mL). In the preparation of the nanoplates, a volume of seed solution was combined with water (5 mL) and aqueous ascorbic acid (10 mM, 75 lL). An aqueous solution of AgNO3 (0.5 mM, 3 mL) was then added at a rate of (1 mL min 1). The dimensions of the produced triangular nanoprisms and hence the colour of the sol was dependent on the used volume of seed solution, e.g. 400 lL seeds produced a pink sol with main Plasmon band peak of kmax = 562 nm and nanoplate edge length 21 nm, 100 lL seeds yielded a light blue sol (kmax = 780 nm) with nanoplate edge length 85 nm, while 20 lL seeds gave rise to a pale blue/grey sol (kmax = 859 nm) with edge length 105 nm. Preparation was completed by the addition of aqueous TSC (25 mM, 0.5 mL), before centrifugation and re-dispersion in water. All steps in nTSNP sol production were conducted at room temperature. 2.3. 1,3-Diaminopropane primed silica coating Aqueous diaminopropane solution (10 mM) was added to 10 mL re-dispersed sol in appropriate volumes to give total concentrations in the range of 10–200 lL. After stirring for 30 min, ethanol (40 mL) was added, followed by an ethanolic solution of TEOS (0.1 M) in suitable volume to provide overall concentrations of 0.25, 0.5 or 1.0 mM as desired. Finally, DMA was added to a total concentration of 10 mM and the sol was stirred for 18 h before being centrifuged and the pellets re-dispersed in either ethanol

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or water. The entire coating procedure was performed at room temperature. 2.4. Silica deposition from sodium silicate solution In this alternative method, aqueous MPTES solution (2.5 mM, 60

lL) was added to 10 mL re-dispersed sol and allowed to stir for 30 min. Aqueous sodium silicate solution (silicate ion concentration of 0.53 wt%) was then added in the correct volume to give a total silicate concentration of either 0.02 or 0.035 wt%. The sol was stirred for 2 days before centrifugation and re-dispersion in ethanol or water. In some experiments, nanoplates coated by this method were subjected to additional silica shell growth by the hydrolysis of TEOS, as described in the previous section, but without diaminopropane priming. Again all steps were conducted at room temperature. 2.5. Characterization The preparation of nanoparticles and their silica coating was monitored by ultraviolet–visible (UV–vis) spectroscopy using a Cary 50 spectrophotometer. Transmission electron microscopy (TEM) images were captured with either a JOEL 2100 or a JOEL JEM-2100F microscope both operating at 200 kV. The latter was equipped with an EDAX Genesis EDS detector which facilitated energy dispersive X-ray (EDX) analysis. TEM samples were prepared by depositing a drop of ethanolic suspension onto carbon coated 200 mesh copper grids. Zeta potential measurements were performed with a Malvern Instruments Zetasizer Nano ZS. The method of determining the sensitivity of the localized surface plasmon resonance (LSPR) to changes in local refractive index (RI) has been outlined previously [22]. The technique involves the preparation of aqueous sucrose solutions with concentrations between 10 and 50 wt%. The RIs of these solutions were measured with a temperature-controlled Abbe AR-2008 digital refractometer. Small volumes of nanoplate sol (50 lL) were dispersed in the various sucrose solutions (750 lL) and the composite RIs were calculated using the Lorenz–Lorentz equation. The peak LSPR wavelength (kmax) was plotted against the RI for each nanoplate/ sucrose suspension, with the sensitivity obtained from the slope of these graphs.

Fig. 1. TEM images of oTSNPs: (A) As prepared, and (B) after silica coating by the diaminopropane method – conditions: diaminopropane (40 lM), TEOS (1 mM).

3. Results and discussion 3.1. Coating via a diaminopropane method Our first samples of core–shell materials with silica deposited on silver nanoplates were prepared using particles prepared by the ascorbic acid reduction of silver ions in the presence of polyvinylpyrrolidone and citrate as growth regulators and stabilisers [20]. These oTSNP materials consist mainly of triangular nanoplates but accompanied by a significant population of spherical nanoparticles (Fig. 1A). Initial coating experiments were carried out using the procedures reported by Ung et al. [29], Graf et al. [49], and Kobayashi et al. [50] for spherical silver nanoparticles. However we found that these did not give satisfactory results with TSNPs – see Supplementary Data for more details. The most successful method we devised used diaminopropane as primer and subsequent reaction in ethanol solution with TEOS and dimethylamine as catalyst – see Scheme 1. This gave particles with a well-defined coating of silica, approximately 20 nm thick, not only on the edges, but also on the (1 1 1) faces (see electron-micrograph in Fig. 1B, where particles are viewed both from the edge and onto the face). Halving the TEOS concentration to 0.5 mM reduced the extent of the silica layer to 12 nm, while the thickness could also

Scheme 1. Silica coating of triangular silver nanoplates by diaminopropane priming.

be varied (from 7 nm to 15 nm) by changing the concentration of the diaminopropane primer (Fig. 2 and Supplementary Data Fig. S4). A disadvantage of this method was that the silver nanoparticles varied somewhat in shape, a feature due partly to a variation in the shape of the original TSNPs, but principally because some etching occurs during the coating phase. The spectra however are only slightly changed during the process – Supplementary Data Fig. S5. Additionally we were unable to prepare samples with very thin layers which were of most interest to us for optical sensing applications. 3.2. MPTES/sodium silicate method To tackle these deficiencies we turned next to nTSNP samples which had been shown to be much more homogenous and also

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Fig. 2. Variation in silica shell thickness as diaminopropane primer concentration is altered at constant TEOS concentration (0.5 mM).

Fig. 3. UV–vis spectra of the silica coated TSNP samples detailed in Table 1.

Scheme 2. Silica coating of triangular silver nanoplates by MPTES priming followed by deposition from sodium silicate solution.

which were prepared in the absence of polymer in the growth medium [21]. Our first experience with these particles using the procedure used with the oTSNPs was unsuccessful, presumably indicating an important role for the polymer (PVP) in priming the surface of the nanoparticles. We therefore changed the primer species, substituting 3-mercaptopropyltriethoxysilane (MPTES) for diaminopropane. It was noted by UV/vis that the plasmon band was almost completely damped, suggesting that extensive aggregation had occurred. This was confirmed by TEM which showed that several TSNPs had become coagulated upon formation of silica particles (see Supplementary Data, Fig. S6). Hence it is clear that this direct Stöber method is inappropriate. To circumvent this problem, and inspired by earlier studies of Liz Marzan and Mulvaney [28,29], we decided to treat the surface of the particles with silicate. As depicted in Scheme 2, samples were first treated by stirring in a 15 lM MPTES solution and then with sodium silicate (0.02% and 0.35% by weight) for two days (samples 1 and 2 resp. in Table 1). Further samples (3 and 4 in Table 1) were prepared by taking TSNPs processed as for sample 1 and subsequently applying a modified Stöber procedure using dimethylamine and TEOS in ethanol water solution. UV/vis spectra showed that the plasmon band red-shifted somewhat on treatment with MPTES and further with the silicate and TEOS treatments, although the shape of the plasmon band did not change significantly indicating that aggregation of the

Fig. 4. Representative TEM images of the silica coated TSNP samples detailed in Table 1. (1) Sample 1, (2) Sample 2, (3) Sample 3, and (4) Sample 4.

particles had not occurred – Fig. 3. These conclusions are supported by examination of the TEM images of Fig. 4. The TEOS-treated TSNPs (3 and 4 in Table 1) have been coated with a thick layer of silica (ca. 40 nm for sample 3 and ca. 40–50 nm for sample 4), with the morphology of the TSNPs being largely retained (as again is quite consistent with the plasmon band spectra). They also show

Table 1 Details of the silica coating of various MPTES primed TSNP samplesa. Sample

Na2Si3O7 conc. (wt%) and duration

TEOS conc. (mM) and duration

kmax (nm)

Shell thickness (nm)

RI sensitivity (nm RIU

1 2 3b 4

0.020, 0.035, 0.020, 0.020,

– – 0.25, 15 h 0.50, 15 h

793 802 810 815

Indiscernible Indiscernible ca. 40 and free silica 45–55 and free silica

324 379 104 81

48 h 48 h 24 h 24 h

1

)

a A sample of citrate stabilised sol (kmax = 780 nm) was divided in 5. To each sub sample, 3-mercaptopropyltriethoxysilane was added to a concentration of 15 lM and was stirred in for 20 min. The resulting sols exhibited kmax = 788 nm. b Samples 3 and 4 were suspended in a 4:1 ethanol/water solution, prior to the addition of DMA to a concentration of 0.3 M followed by TEOS addition.

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excellent resistance to etching by chloride (Fig. 5) with the spectral position not changing over several days in the presence of 20 mM NaCl. However the TEM of samples 3 and 4 also reveal the presence of substantial amounts of free silica, making them unsatisfactory for many types of experiment. By contrast with samples 3 and 4, the TEMs of samples 1 and 2 do not show obvious SiO2 layers. However studies in salt solution do indeed show the desired greatly enhanced stability. (Fig. 5) The spectral position remains unchanged for up to 5 days, indicating morphological stability of the TSNPs. We have therefore studied these coated particles in much more detail. Initially we checked whether the stability in salt solution was due to the MPTES. It was observed that a blue shift of the plasmon-band (as expected for etching of the TSNPs) was not observed for particles coated with MPTES only (Fig. 6A) but rather a darkening and red shift consistent with aggregation took place (This behaviour may be contrasted with that for 3-aminopropyltriethoxysilane (APTES), which has also been used a particle primer, where a strong blue shift is found, consistent with etching of the particles – Fig. 6B). This shows that MPTES does convey stability towards etching for the particles but not towards salt-induced aggregation. Samples 1 and 2, by contrast, are found to be stable in 20 mM salt (and indeed for NaCl concentrations up to 100 mM – see Supplementary Data, Fig. S7), suggesting that deposition of a thin silica shell must have occurred and furthermore that this silica inhibits the aggregation phenomenon observed for the MPTES-only treated nanoplates. Zeta potentiometry reveals a much enhanced colloidal stability for MPTES capped TSNPs relative to as-prepared or APTES capped nanoplates – Table 2 and Supplementary Data, Fig. S8. This may partially explain the success of MPTES compared to APTES as a

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surface primer in conferring stability against chloride etching. Since amines form soluble complexes with silver [50], it is perhaps not surprising that APTES is not a very effective surface passivating agent for triangular silver nanoplates with their reactive edge and corner atoms. The zeta potential is essentially unaffected by the deposition of silica onto the MPTES primed particles, indicating that the silica shell offers a physical, rather than additional electrostatic barrier to aggregation in the presence of salt. Although the UV–vis spectra and salt stability of samples 1 and 2 are indicative of the deposition of a silica shell, the TEM images in Fig. 4 are inconclusive. In an attempt to obtain visual evidence for the presence of a silica layer it was decided to repeat the coating procedure with a different batch of nTSNPs (kmax = 690 nm). On this occasion we used 3-mercaptopropyltrimethoxysilane (MPTMS) as the primer. The latter is similar to MPTES except that methoxy, instead of ethoxy, groups are bound to the silicon atom. The TSNP sol was treated with MPTMS at a concentration of 15 lM for 20 min, which induced a red-shift in kmax to 709 nm (Supplementary Data, Fig. S9). Sodium silicate (0.035 wt%) was then added with stirring and the deposition reaction allowed to continue for 1 day (before washing) in the case of half the sol (sample 5, kmax = 724 nm), and 2 days for the remainder (sample 6, kmax = 732 nm). Shells of thickness ca. 2–3 and ca. 5 nm are apparent in the TEM images of Fig. 7A and B for samples 5 and 6 respectively. The dark field scanning TEM image of Fig. 7C reveals that the shells have a somewhat granular morphology near their outer surface. Energydispersive X-ray (EDX) analysis was also attempted. Owing to the thinness of the nanoplates (ca. 7 nm) it was difficult to acquire an X-ray signal of sufficient intensity in the case of an individual

Fig. 5. UV–vis spectra of samples 1–4 at various times after the addition of NaCl to a total concentration of 20 mM. (A) Sample 1, (B) Sample 2, (C) Sample 3, and (D) Sample 4.

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Fig. 7. Representative TEM images of MPTMS primed silica coated TSNPs: (A) sample 5 and (B) sample 6. (C) and (D) Dark field scanning TEM images of particles from sample 6. Inset to (D) is an EDX line profile collected along the length of, and in the direction of the arrow in the image.

Fig. 6. UV–vis spectra recorded at various times after the addition of NaCl (total concentration 10 mM) to (A) MPTES and (B) APTES capped TSNP samples.

Table 2 Zeta potential data for a split nTSNP sample treated either with APTES, MPTES or silicate. Sample nTSNPs as prepared APTES (30 lM) capped MPTES (15 lM) capped Silica coateda

adsorption of the former. Alternatively it could be related to the more facile occurrence with MPTMS of the hydrolysis and condensation reactions required to form Si–O–Si bonds and commence shell growth. In any case, given the chemical likeness of MPTMS and MPTES, and the similar stability conferred upon TSNPs when they are interchangeably used as primers for silica deposition, it is likely that ultrathin silica layers have encased the nanoprisms of samples 1 and 2, even if these shells cannot be readily resolved in the TEM images of Fig. 4. 3.3. Refractive index sensitivity

f Potential (mV) 31.8 ± 0.5 34.7 ± 0.8 55.7 ± 0.6 54.8 ± 0.8

a Silica coated sample prepared as for sample 1, Table 1.

particle lying flat on the TEM grid as in Fig. 7C. However it was possible to acquire useful EDX data where the corner of one nanoprism lay on top of another, as in Fig. 7D. The line profile shows the onset of a silicon signal several nanometers before that of the intense silver signal arising from the overlapping TSNPs. Upon reaching the opposite edge of the lowermost particle, the silver signal drops to zero several nanometers prior to the disappearance of the silicon response. This EDX data is consistent with the presence of a silica shell covering both the faces and edges of the nanoplates. Further TEM images are included in the Supplementary Data, Fig. S10. The MPTMS primed samples 5 and 6 exhibit stability comparable to that of the MPTES primed samples 1 and 2 in the presence of 20 mM NaCl – see Supplementary Data, Fig. S11. The clear observation of thicker silica shells for the MPTMS coated TSNPs in comparison to those treated with MPTES may arise from denser surface

A convenient method for checking the sensitivity of the position of the maximum of the Plasmon band to changes in the medium refractive index is to determine the spectrum in sucrose solutions of varying concentration. As was previously shown, the sensitivity (Dkmax/Dn) for the TSNPs varies with the aspect ratio of the particles [22]. The results for this sucrose sensitivity test of the silica coated samples 1–4 are presented in Fig. 8. In interpreting this data it is important to understand that the sucrose medium does not have to be in direct contact with the silver nanoparticle surface in order to induce a shift in kmax. As predicted and mapped by numerical simulations [2], and observed and mapped experimentally [62], significant in-plane electric field enhancement caused by the main dipole plasmon resonance of a TSNP extends well beyond the metal surface (to distances of the order of 1/3 the particle edge length for an isolated TSNP in aqueous solution [2]). Since the plasmonic enhancement of the electric field will extend far beyond the thin silica shells of samples 1 and 2, it is expected that the shifts in kmax brought about by altering the concentration of sucrose solution (which encapsulates the entire spatial range of the plasmon field) will be significant relative to the red-shift caused by the deposition of the silica layer, despite the fact that the difference in refractive indices of bulk silica and water is much larger than the refractive index changes between the

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sharpness of the nanoplates. and prevents the thermodynamic tendency towards ‘rounding’. Finally, it should be noted that we previously reported on the epitaxial deposition of a thin gold layer (ca. 5 nm) around the edges of triangular silver nanoplates [63]. These gold framed particles proved to be resistant to etching at a chloride concentration of 10 mM for up to five days. A possible drawback with this method is that there is a narrow window of Au/Ag ratios over which the gold framing is effective – if the concentration of gold precursor is too great the silver particles are degraded through galvanic replacement. Nanoplate deterioration is however not an issue during MPTES primed silica coating, while resistance to etching has been observed for NaCl concentrations up to 100 mM. 4. Conclusions Fig. 8. Peak wavelength plotted against refractive index for samples 1–4.

different sucrose solutions. This is borne out by the experimental results. Consider sample 2 – the growth of the silica shell atop the MPTES coated particles results in a red-shift in kmax of 14 nm compared to a shift of 33 nm across the range of sucrose solutions tested. Thus it may be noted that while samples 3 and 4 (with their thick Stöber-grown shells outside the initial silicate mantle) have very low refractive index sensitivity values (104 and 81 nm RIU 1 resp.) the values for samples 1 and 2 (and also samples 5 and 6 – see Supplementary Data, Fig. S12) are comparable to those of the citrate-capped TSNPs [22]. We have thus studied samples 1 and 2 in more detail. In particular we have prepared a set of TSNPs of different sizes with Plasmon band maxima at 562, 613, 670, 742, 859 and 959 and coated a portion of each of these with a thin layer of SiO2 (i.e. similar process as sample 1 in Table 1). The refractive index sensitivity of these particles was then measured (typically two days after the original preparation of the TSNPs). As may be observed in Fig. 9, the silica-coated particles exhibit rather similar performance as their citrate-stabilised precursors, revealing that the SiO2 layer does not adversely affect the behaviour of the Plasmon band. Indeed it may be noted that the Ag@SiO2 particles may even have a slightly greater sensitivity than the citrate capped particles. This could arise from a progressive ‘rounding’ of the vertices of the citrate protected nanoprisms over the course of the first two days. It is possible that the silica coating effectively ‘‘freezes’’ the

Stabilising triangular silver nanoplates with a coating of silica, so that they may be used in media containing halide ions presents considerable challenges. As detailed above, the extension of methods previously reported for the coating of spherical silver particles to TSNP colloids is non-trivial. However we have successfully developed several strategies, which are reported herein. For our purposes the most useful procedure is one based on the priming of the surface with mercaptopropyltriethoxysilane (MPTES) and subsequent treatment with sodium silicate. This conveys excellent stability towards chloride ions, while retaining RI sensitivity (Dkmax/Dn) comparable to that of the original uncoated particles. In this regard it may also be noted that we have found that these particles outperform TSNPs coated with 16-mercaptohexadecanoic acid MHA [64], which, while stable against salt, show significantly reduced Dkmax/Dn values. The ability to produce high quality, monodisperse, silica coated triangular silver nanoplates by a simple colloidal method will be of interest to a range of emerging technologies beyond the colorimetric biosensing application considered here. Silica coated spherical plasmonic nanoparticles (Ag or Au) have recently been employed in metal enhanced fluorescence [65–67], photocatalysis [68], as light harvesting or energy transfer nano-antennae [69,70], and in the enhancement of the photoluminescence efficiency of light emitting diodes [71–73]. The wide tuneability of the plasmonic energies of silica coated TSNPs compared to nanospheres brings obvious advantages in the optimisation of the aforementioned technologies. Acknowledgments This research was financially supported by Enterprise Ireland (Grant 2008/0313). The authors also thank the staff of the Centre for Microscopy and Analysis, Trinity College Dublin and Mr. Tadhg Kennedy, Ms. Shalini Singh and Dr. Emma Mullane of the University of Limerick for TEM and EDX studies. We are also grateful to Mr. David Hinds (University College Dublin) for synthesis of the n-TSNP samples for the preparation of coated samples 5 and 6. 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.jcis.2013.10.017. References [1] [2] [3] [4] [5]

Fig. 9. Refractive index sensitivity data for a batch of TSNPs treated similarly to sample 1.

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