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Improved Raman and photoluminescence sensitivity achieved using bifunctional Ag@SiO2 nanocubes† Nguyen Minh Kha,a Ching-Hsiang Chen,*b Wei-Nien Su,b John Rickb and Bing-Joe Hwang*ac SiO2 coated silver nanocubes Ag@SiO2 with enhanced surface-enhanced Raman scattering (SERS) and metal enhanced photoluminescence (MEPL) sensitivity were synthesized and characterized. The silver nanocubes (NCs) were synthesized by the polyol method and modified, first with different coupling agents, such as 3-mercaptopropyltrimethoxysilane (MPTMS) and 3-aminopropyltrimethoxysilane (APS),
Received 10th November 2014, Accepted 13th January 2015
and secondly with tetraethylorthosilicate (TEOS) to improve their SERS and photoluminescence (PL)
DOI: 10.1039/c4cp05217j
nanocube’s SiO2 shell thickness. Modified Ag NCs (with a 2 nm silica layer) were prepared using 1 mM
performances. The SERS and PL intensity of rhodamine 6G (R6G) can be manipulated by tuning the Ag APS and 1 mM TEOS and found to have a SERS intensity 3 fold higher than bare Ag NCs. Additionally, it
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was found that APS modified Ag@SiO2 NCs possessed both enhanced SERS and PL intensities.
1. Introduction Surface-enhanced Raman Scattering (SERS) spectroscopy and metal enhanced photoluminescence (MEPL) spectroscopy are highly sensitive and selective analytical tools useful for biological and chemical characterization.1–4 Both the SERS and MEPL effects, observed with nanostructured metal surfaces, are characterized by a localized surface plasmon resonance (LSPR) intensified electric field during excitation with light.5,6 Recently, SERS has been widely used as a powerful biomedical diagnostic tool with advantages in terms of an increased level of multiplexing, robustness, and the ability to perform in blood and other biological matrices.7–10 Photoluminescence (PL) emission from molecules is commonly described in terms of oscillating electric dipole moments and characterized by efficiency, quantum yield, and PL lifetime, i.e., the average time the molecule stays in its excited state before emitting a photon. The emission resulting from this dipole can be strong when it is surrounded by materials of a different composition. This approach can be used to determine if the probe molecule has been bonded to given target species, based a
NanoElectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. E-mail: [email protected] b NanoElectrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan. E-mail: [email protected] c National Synchrotron Radiation Research Center, Hsin-Chu, Taiwan † Electronic supplementary information (ESI) available: Raman spectra of silicon wafer and R6G on the Ag nanocube substrate. See DOI: 10.1039/c4cp05217j
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on the energy variation. Plasmonic enhancement of luminescence, also known as ‘surface enhanced luminescence’ (SEL), together with MEPL, are recognized as effective methods that enhance the intensity of luminescence from an emitter in the vicinity of metallic nanostructures.4 This approach is also used in a variety of biological applications such as biomolecule labeling.11 Especially, there is great interest in developing a bifunctional MEPL and SERS emitter by combining the two methods. Such an approach would be helpful in deducing the interactions of many types of molecules, including proteins and viruses. Many studies have focused on tuning the electronic and optical properties of metal nanostructures by varying their sizes and shapes, or by combining them with other nanomaterials.12,13 Both gold and silver based materials are of great interest, due to their ease of preparation, good homogeneity, high compatibility with biomolecules, the manifestation of intense absorption band(s) in the visible region, and their significant contribution to SERS spectroscopy as active substrates.14 Moreover, the optical excitation of plasmon resonances in Ag nanoparticle’s (NPs) is the most efficient mechanism by which light interacts with matter. A single Ag NP interacts with light more efficiently than any particle with the same dimensions that possesses any known organic or inorganic chromophore.15 The light interaction cross section for an Ag NP is approximately ten times that of its geometric cross-section, indicating that it captures much more light than is incident on it.16 Silver is the only material whose plasmon resonance can be tuned to any wavelength in the visible spectrum. Notably, silver NPs are extraordinarily efficient at absorbing and scattering light and unlike many dyes
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and pigments have a color that depends upon the size and the shape of the particle.17 Although silver exhibits many advantages over gold, e.g. higher extinction coefficients, sharper extinction bands, a higher ratio of scattering to extinction, and extremely high field enhancements, it has been employed far less in the development of sensors, with the exception of sensors based on surface enhanced spectroscopies. The reason for this is the lower chemical stability of silver NPs when compared to equivalent materials made of gold.18 Nevertheless, recent developments include the efficient protection of silver NPs by silica coating to enhance chemical stability, colloidal stability in solvents, and the ease of further modification in biological systems.19–22 Silica coating also forms a controlled dielectric environment around the silver NPs,23 which improves precision in surface plasmon resonance (SPR) based sensing, while additionally protecting the NPs, by preventing or slowing down the diffusion of environmental oxygen. Additionally, as the silica coating is inert it does not interfere with redox reactions on the core’s surface. Silica coatings can also easily be functionalized with silane coupling agents for further bioconjugation.5,24 Hence, silver NPs have attracted wide attention, due to their application potential in catalysis, optical devices, and as substrates for SERS spectroscopy in biological and medical applications. In SERS, the silica shell not only enhances colloidal stability but also controls the distance between core particles within assemblies. ¨ber based procedures available for the There are several Sto synthesis of Ag@SiO2 core–shell NPs. Generally, coupling agents with special functional groups, such as poly(vinylpyrrolidone) (PVP),22,25 3-mercaptopropyltrimethoxysilane (MPTMS)26 and 3-aminopropyltrimethoxysilane (APS),10,27 are used to make the Ag NP’s surface vitreophilic for coating with a complete silica shell. Normally, the thickness of the silica layer is controlled by the concentration of added tetraethylorthosilicate (TEOS). However, less attention has been given to the effects of different coupling agents with respect to achieving a SiO2 coating on Ag NPs. As far as we know, no work has been dedicated to this subject. On the other hand, MEPL has shown interesting results for potential applications in biological research.6,28,29 Although Ag NP MEPL has been investigated with e.g. thin films,30 colloids,26,31,32 and nanorods,33 Ag@SiO2 core–shell nanocubes (NCs) have rarely been reported. Both MEPL and SERS spectra can be used to study the influence of localized surface plasmons on the optical properties of Ag@SiO2 NCs. Ag NC cores were first prepared by using a polyol method. MPTMS and APS were then used as coupling agents for the coating process to examine their effect on the growth process of silica shells. Additionally, this study presents the characterization, as well as the SERS spectra (532 nm and 632.8 nm laser excitation), and the MEPL spectra at 375 nm of Ag NCs and Ag@SiO2 core–shell NCs.
2. Experimental section 2.1.
Materials
Ethylene glycol (EG), silver trifluoroacetate (CF3COOAg, Z98%), sodium hydrosulfide (NaHSxH2O, Z60% NaHS), tetraethyl
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orthosilicate (TEOS: Si(OC2H5)4, 98%), ethanol (C2H5OH, Z99.5%), acetone (CH3COCH3, Z99.5%) and (3-mercaptopropyl)trimethoxysilane (MPTMS: HS(CH2)3Si(OCH3)3, 85%) were purchased from Acros. Hydrochloric acid (HCl, 37%), poly(vinylpyrrolidone) (PVP, MW E 55 000), rhodamine 6G (R6G, Z 99%) and (3-aminopropyl)trimethoxysilane (APS: H2N(CH2)3Si(OCH3)3, 97%) were obtained from Sigma-Aldrich. Ammonium hydroxide (NH4OH, 35%) and 2-propanol ((CH3)2CHOH, 99.9%), both of analytical grade, were obtained from Fisher BioReagentst. Deionized water, from a MilliQ system, was used for all experiments.
2.2.
Synthesis of Ag nanocubes
Single-crystal sliver NCs were prepared using a recently published polyol method (Xia et al.) with CF3COOAg as a precursor to elemental silver and EG as a solvent.34 The Ag NC scale-up synthesis was carried out by following the procedure for the standard synthesis, except that the amounts of all reagents were 4 times greater. In brief, EG (20 mL) was added into a 100 mL round bottom flask and heated at 150 1C for 40 min under magnetic stirring in an oil bath. After which, NaSH (0.24 mL; 3 mM in EG) was quickly injected into the heated solution. After two minutes, a 3 mM HCl solution (2 mL) was injected into the heated reaction solution, followed by the addition of PVP (5 mL, 20 mg mL1 in EG). After another 2 min, CF3COOAg (1.6 mL, 282 mM in EG) was added into the mixture. During the entire process, except during the addition of reagents, the flask was capped with a glass stopper. After the addition of CF3COOAg, the transparent reaction solution turned a whitish color and then (after 2 min) quickly became slightly yellow, indicating the formation of the Ag seeds. The reaction was allowed to proceed for different periods of time and its color went through three stages, i.e. dark red, reddish grey, and brown as the edge length of the Ag NCs increased. After 1 h, the reaction solution was quenched by placing the reaction flask in an ice-water bath. The sample was collected by centrifugation and then washed with acetone once to remove the remaining precursor and EG, and then with ethanol four times to remove excess PVP, prior to being re-dispersed in 2-propanol.
2.3.
Synthesis of Ag@SiO2 core–shell nanocubes
In a modified synthesis of Ag@SiO2, 20 mL of MPTMS or APS (1 mM in 2-propanol) was added to 2 mL of Ag NCs solution and stirred for 30 min at 600 rpm at room temperature. Next, the mixture was dissolved in 6 mL of 2-propanol and stirred for 2 min, 2 mL of deionized water and 100 mL ammonia aqueous solution (35 wt%) were added to the mixture. After that, TEOS (200 mL; 1 mM in 2-propanol) was added dropwise with continuous stirring, and the reaction was allowed to continue for 6 h. The Ag@SiO2 core–shell NCs were collected by centrifugation and washed with ethanol once, and deionized water 3 times, before being re-dispersed in deionized water. The thickness of the silica coating layer was found to be able to be controlled by adding or omitting MPTMS, APS and TEOS (Fig. 1).
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Fig. 1 Schematic preparation.
2.4.
illustration
of
Ag@SiO2
core–shell
nanocube
Preparation of SERS and MEPL substrates
Droplets (30 mL solution) of Ag NCs or Ag@SiO2 NCs were spread on silicon wafers (B1 cm2). An aqueous solution of R6G (105 M; 5 mL droplets) was spread on the Ag NC and Ag@SiO2 surfaces and kept in the dark for 1 h at room temperature prior to testing. SERS and MEPL spectra for all samples were measured over 4 different areas of the sample at 25 1C. 2.5.
Characterization
UV-Vis absorption spectra were recorded using a Shimadzu UV-3150 spectrometer. The X-ray diffraction (XRD) pattern was obtained with a D2 Phaser diffractometer (D/Max-250, Bruker) using Cu Ka (l = 1.5406 Å, isolated with a Ni foil filter) radiation. The sample was prepared by sonicating Ag NCs and Carbon Black 72R (CB 72R) in ethanol followed by vacuum-drying for 12 hours before use. A scan rate of 0.05 deg per step was used, for
Fig. 2
2y, between 351 and 851. The Ag NC and Ag@SiO2 NC morphologies were characterized by transmission electron microscopy (TEM, CM200; Philips), operating at 200 kV. Raman spectra were collected on a UniRAM micro-Raman spectrometer with laser excitation (532 and 632.8 nm). The data acquisition time was normally 30 s and the peak intensities of the samples were normalized to those of a silicon wafer at 520 cm1. A thermoelectrically cooled charge-coupled device (CCD) with 1024 256 pixels operating at 60 1C was used as the detector with 1 cm1 resolution. The laser line was focused onto the sample in a backscattering geometry, using an Olympus 50 objective, providing scattering areas of ca. 0.25 mm2. The laser spot size was estimated to be B0.8–1 mm. PL spectra were acquired with a dark chamber, Micro/Macro PL mapping system (UniNanoTech Co. Ltd). Steady state PL measurements were performed with the 375 nm picosecond pulse laser and detected with a CCD as above. The spectra were corrected for the optical transfer function of the system. PL measurements were carried out in a 901 configuration with the PL signal detected in the same direction of the reflected excitation beam in order to avoid re-absorption effects. Four spectra were acquired to take averages from different locations for each sample to ensure representative sampling and incorporate spot-to-spot variability in the signal.
3. Results and discussion Ag NP morphology was characterized by TEM (Fig. 2A). The synthesized Ag particles were almost cubic with edge lengths of B50 nm (The inert image of Fig. 2A). This Ag NC edge length was selected because Lakowicz and coworkers proved that the diameter of Ag NPs is optimal from 50 to 70 nm for optimizing the strength distribution of the enhanced LSPR electromagnetic field, while at the same time suppressing competitive quenching.35,36 The wide angle XRD pattern of a Ag NC–CB 72R mixture confirms the presence of highly crystalline silver NPs (Fig. 2B). The peaks at 38.171, 44.281, 64.431, 77.481, and 81.541 can be assigned to diffraction from the (111), (200), (220), (311) and
(A) TEM image of Ag NCs (edge length B50 nm). (B) XRD pattern of the Ag NCs–CB 72R mixture.
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Fig. 3 UV-Vis spectra of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). The inset is the UV-Vis spectra, expanded in the range 454 nm to 466 nm.
(222) planes, which are characteristic of face-centered cubic (fcc) silver according to JCPDS card (no. 04-0783). The results of the powder diffraction match with the database as does the (111) and (200) peak ratio. Consequently, the average crystallite size of Ag NCs which was calculated from XRD pattern is 20.7 0.7 nm. As shown in Fig. 3, well-dispersed Ag and Ag@SiO2 core–shell NCs were obtained. It is known that the LSPR of Ag NPs is sensitive to their shape, size, and surrounding dielectric environment.37 The UV-Vis spectra of Ag NCs (50 nm edge length) showed a main LSPR band with a maximum at 456 nm and two shoulders at 354 nm and 390 nm. Fig. 3 also shows the solution absorbance spectra of Ag@SiO2 NCs synthesized with different coupling agents. During the preparation of Ag@SiO2 core–shell NCs, the position of the LSPR peak was slightly redshifted from 456 nm to 462 nm when the SiO2 layer was formed on the surface. The Drude model indicates that the wavelength of the LSPR peak is dependent on the dielectric function of the surrounding medium.38 In addition, the relationship between the LSPR peak and the refractive index is based on the method39 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi calculated using the equation lmax ¼ lp 2n2 þ 1. In this equation, lmax is the wavelength at the LSPR peak, lp is the wavelength corresponding to the plasma frequency of the bulk metal, and n is the refractive index in the surrounding dielectric environments. It is interesting to note that the wavelength of the first shoulder and the maximum UV-Vis absorbance peak of all the Ag@SiO2 samples are similar, i.e. 355 nm and 462 nm, respectively; suggesting that the variation in the thickness of SiO2 between samples was minimal. However, the position of the second shoulder was blue-shifted (from 398 nm to 394 nm) after using 1 mM APS instead of 1 mM MPTMS. A possible explanation is that the thickness of silica layer on Ag@SiO2, prepared using 1 mM APS (2 nm silica shell, Fig. 4d), is thinner than that formed using 1 mM MPTMS (4 nm silica shell, Fig. 4a). A summary of UV-Vis absorbance is presented in Table 1.
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¨ber method a mixture of alcohol and When using the Sto ammonia is commonly used for the preparation of NPs’ silica shells. If the total surface area of the seed particles per volume is large enough, compared to the added amount of TEOS, the formation of new silica layers can be completely suppressed and therefore the thickness of the silica shell can be precisely controlled by the amount of TEOS added.5,35,40 For silver NPs synthesized by the polyol method, we found it impossible to coat them directly in this way as large aggregates of uncoated silver particles and silica were formed. Moreover, the dependence on the alcoholic solvent was not the same as for TEOS: in 2-propanol and ethanol Ag@SiO2 core–shell particles were obtained; however, in ethanol coreless SiO2 particles were present as a byproduct, while in methanol, the majority of Ag NPs remained uncoated, with a few of them being randomly entrapped in large silica aggregates.41 Hence, in this study, 2-propanol was used as a solvent to dilute TEOS in coating the SiO2 layer on Ag NCs. Typical TEM images of Ag@SiO2 NCs prepared with the addition of varying amounts of MPTMS, APS and TEOS are shown in Fig. 4. It is apparent that the Ag@SiO2 NPs are uniform cubes. However, a few small NPs (solid silica spheres free of Ag NC cores) were formed when 1 mM MPTMS and 1 mM TEOS were used (Fig. 4a). Additionally, Fig. 4b shows that the thickness of the SiO2 layer (about 4 nm) was not uniform when only TEOS used. On the other hand, a very smooth and uniform silica shell (2.5 nm thick) was found after adding 1 mM MPTMS in the absence of TEOS (Fig. 4c). MPTMS contains a silane group, so it can undergo hydrolysis to form silica. A uniformly thin silica shell (2 nm thick) was prepared using APS to replace MPTMS, although 1 mM was TEOS used (Fig. 4d). From these results, it appears that covalent bonds were formed between the Ag NCs surfaces and the thiol group (in MPTMS), or the amine group (in APS). Furthermore, MPTMS and APS were used as the coupling agents to transfer Ag NCs into 2-propanol, where the silica shell can be grown directly on ¨ber process.20 Similar to other the NPs surface through the Sto types of gold or silver NPs, the metal–sulfur bond is known to be stronger than bonds with alternative functional groups (i.e., amines, carboxylic acids, alcohols, and phosphors).42 Hence, with respect to the coupling agents, the amount of MPTMS adsorbed on Ag NC surfaces was higher, and the growth of silica layer was faster than with APS. Thus, the thickness of SiO2 shell can be decreased by replacing MPTMS with APS at the same concentration. Consequently, the thickness of the silica shell can be controlled by using TEOS in the presence or absence of different coupling agents. The SERS sensitivity of these differently synthesized substrates was investigated by using R6G as a model Raman probe. The SERS spectra of R6G adsorbed on Ag NC substrates, with and without SiO2 NP coatings, are shown in Fig. 5. The peak at 611 cm1 is assigned to the C–C–C ring in-plane vibration mode. The peak at 772 cm1 is assigned to the C–H out-ofplane bending mode. The peaks at ca. 1126 and 1181 cm1 are assigned to the C–H in-plane bending, while the vibrations at 1362, 1507 and 1646 cm1 are assigned to C–C stretching of the
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Fig. 4 TEM images of Ag@SiO2 NCs prepared by adding different chemicals: ((a) 1 mM MPTMS and 1 mM TEOS, (b) 1 mM TEOS, (c) 1 mM MPTMS, and (d) 1 mM APS and 1 mM TEOS, respectively).
Table 1
Summary of Ag NCs and Ag@SiO2 core–shell NCs prepared by various combinations of MPTMS, APS and TEOS
Sample
l1st shoulder (nm)
l2nd shoulder (nm)
lmax (nm)
Average shell thickness (nm)
(a) Ag (b) Ag@SiO2–1MPTMS–1TEOS (c) Ag@SiO2–1TEOS (d) Ag@SiO2–1MPTMS (e) Ag@SiO2–1APS–1TEOS
354 355 355 355 355
390 398 396 395 394
456 462 462 462 462
N.A. 4.0 4.0 2.5 2.0
aromatic ring. The peaks at 1311 and 1571 cm1 are assigned to the N–H in-plane bend modes.43 The SERS intensity was found to increase with a decrease in the silica shell’s thickness (Fig. 5A): the Ag@SiO2 NCs prepared by APS and TEOS has the thinnest thickness of 2 nm among all samples. This can be explained by the fact that SERS enhancement in the SiO2 nanoshell is due to surface plasmon excitation of Ag NCs cores. SERS phenomena are known to depend on the analyte molecules being in close proximity to the metal NP surfaces, with
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maximum SERS enhancement expected only when analyte molecules are situated a few nanometers away from the NP’s surface or within the nanoneck of adjacent metal NPs.44,45 It is reported that silica made using these synthetic technique is porous with pore diameters about 1–3 nm,27,46 which allows for the molecular diffusion of R6G (dimensions approximately 0.8 1.6 nm) through the SiO2 shell.47 Thus, the absorption and diffusion of R6G within the confined SiO2 nanoporous structure depends on the thickness of silica shells. In comparison
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Fig. 5 (A) SERS spectra (excitation laser 532 nm) of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). (B) Enhancement factor for R6G on Ag NCs and Ag@SiO2 with the excitation laser 532 nm.
with Ag@SiO2 NCs, the silica layer of the sample coated with SiO2 using 1 mM APS and 1 mM TEOS (Ag@SiO2–1APS–1TEOS sample) is the thinnest, thus its SERS enhancement is the strongest. Therefore, the samples’ SERS intensities are closely related to the amount of R6G absorbed on the surface of the Ag core NCs. The overall enhancement of the Raman signal is of great importance, so the enhancement factor (EF) is applied to evaluate the performance of the SERS substrate using the ISERS Cref following equation:48 EF ¼ . In this equation, Cref Iref CSERS 5 (0.1 M) and CSERS (1 10 M) represent the concentrations of R6G, and ISERS and Iref are the band intensities of the selected band at 611 cm1 obtained by SERS and the corresponding band intensity of R6G on the silicon wafer (Fig. S1 and S2, ESI†). Encouragingly, the signal of R6G on the Ag@SiO2 NCs (2 nm silica shell, 1 mM APS and 1 mM TEOS used) exhibits the highest intensity, and is 3-fold greater compared with that of R6G adsorbed on an unmodified roughened Ag NC substrate as presented in Fig. 5B. As far as we know, SiO2 NPs are SERSinactive; thus, it is interesting that the SERS of R6G adsorbed on the roughened Ag substrates with incorporated SiO2 NPs can be significantly improved. Typically, either APS or MPTMS was used as the functionalized silane because the terminal amine/ thiol has an affinity for the Ag surface that leaves the trimethoxysilane group exposed to the surrounding solution. Ag NCs then assemble in solution as a result of hydrophobic forces to minimize the number of hydrophobic faces in contact with the surrounding 2-propanol. Once the trimethoxysilane groups were hydrolyzed to form silanol-containing species, TEOS was added to produce a thin silica shell. Hence, the spacings between Ag@SiO2 NCs were closer than bare Ag NCs (Fig. 2A and 4). Furthermore, an important additional feature of LSPRs, besides their dependence on shape, size and composition of the metal nanostructure and the surrounding dielectric environment, the plasmon coupling is observed when nanoparticles are in close proximity to each other. The interacting nanoparticles’ plasmon coupling has a dramatic effect on SERS, localizing large local fields at the hot spot that may provide enhancement factors
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sufficient for single-molecule detection.49 As a result the SERS intensity of all Ag@SiO2 NCs was higher than the bare Ag NCs. This enhancement is in good agreement with previous studies showing that Ag nanostructured surfaces have a strong hot-spot when the NPs are brought together and the interparticle spacing is thereby decreased.50 The detection of R6G molecules depends on the site-selective adsorption at these hot spots, which is in the vicinity of the gaps between the plasmonic nanoparticles. The hot spots are caused by the strong coupling interactions between the near fields of closely spaced nanoparticles. The EF for the SERS detection of R6G in our experiment was calculated to be 1.2 106 for Ag@SiO2–1APS–1TEOS under optimal experiment conditions. In many biological applications, it is often desirable to use near IR excitation to avoid damaging the sample or exciting sample autofluorescence, even at the cost of some signal loss due to the fourth power (n4).51 It has been suggested that the selection of less energetic excitation (NIR to red) in Raman spectroscopy is advantageous because it causes less photochemical damage to the biological samples and results in much less fluorescence background in the Raman spectrum. An excitation laser (l = 632.8 nm) was used to confirm the presence of the R6G signal in all samples, this wavelength is distinct from that of silver NCs (l = 455 nm). The signals of R6G were very clear, and the backgrounds were very flat. Because, the power density of the laser has only a small effect on the baseline intensity caused by absorption – PL processes, but no effect on the molecular spectroscopic signature intensities.52 In contrast the intensity and enhancement factors for all the samples were decreased when the excitation laser wavelength was increased (compare Fig. 5B and 6B). There was an B3.5-fold decrease of SERS enhancement when using the excitation wavelength of 632.8 nm, but there was no significant decrease noted such as the 13-fold decrease noted in previous research.53 This low signal is presumably due to the lack of coupling of the LSPR to the excitation, as these structures have the LSPR band closer to 532 nm than 632.8 nm. In addition, the frequency to n4 for the scattering dependence of Raman photons is also a principle
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Fig. 6 (A) SERS spectra with the excitation laser 632.8 nm of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). (B) Enhancement factor of R6G on Ag NCs and Ag@SiO2 with the excitation laser 632.8 nm.
factor that determines the sensitivity of a Raman experiment.54 The comparative data indicate that the results based on 532 nm excitation are better than those found at 632.8 nm. It is believed that the wavelength of 532 nm is closer to the energy of the Ag NCs’ LSPR. One of the reasons that MEPL has not been explored for silver NCs is that during their synthesis complex organic molecules, e.g. PVP, that remain on their surface are not suitable for biological binding protocols without exchange or complex modifications.42 Most importantly a critical element in designing surface-functionalized particles for surface imprinting or immobilization of ions, organic molecules, or biomolecules is achieving the maximum expression of desired functional groups on the particle’s surface. In this work, the Ag NC surfaces were modified by using coupling agents such as MPTMS and APS, and by silica coating. Clear evidence for a direct relationship between the PL intensity fluorophores and Ag@SiO2 NCs is given in Fig. 7A. The typical emission peak of R6G, at B560 nm, was obtained at an excitation wavelength of 340 nm.55 Here, in order to enhance the LSPR effect, the excitation wavelength (375 nm) was chosen close to that of the energy of the LSPR of the Ag nanocubes to provide the absorption enhancement as referred to the results of the SERS. Moreover, Wang et al.,56 have shown that the adsorption of R6G on Ag would otherwise cause significant quenching, or a decreased fluorescence, such as is found on bare Ag NPs under the same experimental conditions. It is also reported that the properties of the Ag NCs can be modified by the porosity of the silica layer, where different thickness and porosity of silica lead to a varying dielectric environment around the silver particles:23 Hence, the rapid increase of PL intensity for R6G at B560 nm, absorbed on the silica shelled Ag NC samples. Furthermore, in the case of R6G, its emission peak is around 560 nm, which is also close to that of the energy of LSPR of the Ag NCs. However, the spontaneous emission not only depends on the emitter, but also is related to the surrounding medium. The magnitude of the emission of R6G will be further enhanced by the LSPR of the Ag NCs (sometimes this is termed the Purcell effect).57
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It is interesting that the SERS intensity of the Ag@SiO2– 1MPTMS–1TEOS sample, in which the SiO2 coating layer (about 4 nm) was formed using 1 mM MPTMS and 1 mM TEOS contained some free silica (Fig. 3a), is lower than for other samples with silica shells (Fig. 5 and 6). However, the PL intensity of this sample is higher than Ag@SiO2–1TEOS (4 nm of SiO2 shell) and Ag@SiO2–1MPTMS (2.5 nm of SiO2 shell) samples (Fig. 7). It is known that MEPL is affected by numerous factors, such as the type of metal, size and morphology of the NPs, as well as the spatial distance between the fluorophore and the NP’s surface, and additionally the quantum yield of the fluorophore.58 Among these factors, the distance dependence is important for fluorescence enhancement.59,60 Thus, the PL intensity decreased with an increase in the silica shell ‘spacer’ distance between the Ag NC cores and R6G from 2.5 to 4 nm or above. On the another hand, it is clear that together with an increase in the intensity of the emission peaks there is a slight red-shift from 560 nm for silicon wafer to 567 nm for Ag@SiO2–1APS–1TEOS sample and the wavelength range of the detected PL becomes broader. The PL intensity reached the greatest value for Ag@SiO2–1APS–1TEOS with a 2 nm silica shell. It is to note that the increase approximates 7.3-fold and 34-fold, compared to bare Ag NCs and silicon wafers respectively. Although the thickness of the silica layers of Ag@SiO2– 1APS–1TEOS were thinner than Ag@SiO2–1MPTMS–1TEOS (in Fig. 4d and a), the PL intensity of Ag@SiO2–1APS–1TEOS was 3 times higher. The results show that the PL intensity depends on not only the thickness of the SiO2 shell but also on the organosilane molecules. It is known that intermediate silica species (from the hydrolysis of TEOS) and organically capped metal NPs carry negative charges at high pH values (in the presence of ammonia)61 in which electrostatic interactions between silica intermediates and organically capped NPs cannot be ignored. Consequently, it is possible to set the repulsion effect (electrostatic interaction) between the species by adjusting the charge with respect to the metal–water interaction to obtain products with desired thicknesses and morphologies.62 The electrostatic
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Fig. 7 (A) PL spectra of R6G on Si wafer, (a) Ag NCs, and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). Inset: the amplified spectra of R6G on Si wafer. (B) PL intensity at 560 nm of R6G on Si wafer, Ag NCs and Ag@SiO2 NCs.
interactions between the negatively charged silica intermediate and the Ag NCs will be considerably reduced in the presence of APS as most of the amine groups from APS will be indirectly attached to Si by (CH2)3 while simultaneously interacting with Ag. Therefore, the mechanism explains that the coating of SiO2 on Ag NCs, when using APS and TEOS, could be more uniform and generates results with higher PL activity. In the case of MPTMS, used as a silane coupling agent, charge repulsion could be enhanced to the extent that it was able to destabilize the microemulsion system by interfering with metal–water interactions resulting from the negative charge on MPTMS at high pH values in the presence of ammonia.62 As a result, the final product of Ag@SiO2–1MPTMS–1TEOS is an irregular structure with some free silica NPs that exhibits low PL. This offers the possibility of using Ag@SiO2 NCs, prepared with APS and TEOS, for fluorescence-based applications in biomedicine and biotechnology. Therefore, all of our observations show that the Ag@SiO2 NCs provide a significant bifunctional enhancement to the SERS and MEPL, indicating their utilities in various applications.
4. Conclusion A method for the preparation of Ag@SiO2 NCs using vitreophilic pretreatment with MPTMS and APS was successfully developed. The presence of coupling agents greatly modified the Ag NC cores and manipulated the thickness and uniformity of the silica shells. The thin SiO2 layer, due to its stability and higher refractive index, facilitates not only enhanced SERS intensity but also PL activity. The SERS intensity reached a maximum with a silica shell ‘spacer’ 2 nm thick, being 3 fold higher than that in bare Ag NCs. Comparison of the PL emission of core–shell and bare core nanostructures with similar morphologies shows that the PL intensity of the surrounding fluorophores can be increased (7.3 fold) by the Ag@SiO2 core–shell NCs. The improved SERS sensitivity and MEPL of Ag@SiO2 NCs offers the potential to form tailored
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bifunctional nanomaterials for plasmonic applications such as biosensors.
Acknowledgements The authors gratefully acknowledge the financial support from the Ministry of Science and Technology (MOST) (103-2221-E011-156-MY3, 103-3113-E-011-001), the Ministry of Economic Affairs (MOEA) (101-EC-17-A-08-S1-183), and the Top University Projects of Ministry of Education (MOE) (100H451401), as well as the facilities support from the National Taiwan University of Science and Technology (NTUST) and National Synchrotron Radiation Research Center (NSRRC).
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Cite this: DOI: 10.1039/c4cp05217j
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Improved Raman and photoluminescence sensitivity achieved using bifunctional Ag@SiO2 nanocubes† Nguyen Minh Kha,a Ching-Hsiang Chen,*b Wei-Nien Su,b John Rickb and Bing-Joe Hwang*ac SiO2 coated silver nanocubes Ag@SiO2 with enhanced surface-enhanced Raman scattering (SERS) and metal enhanced photoluminescence (MEPL) sensitivity were synthesized and characterized. The silver nanocubes (NCs) were synthesized by the polyol method and modified, first with different coupling agents, such as 3-mercaptopropyltrimethoxysilane (MPTMS) and 3-aminopropyltrimethoxysilane (APS),
Received 10th November 2014, Accepted 13th January 2015
and secondly with tetraethylorthosilicate (TEOS) to improve their SERS and photoluminescence (PL)
DOI: 10.1039/c4cp05217j
nanocube’s SiO2 shell thickness. Modified Ag NCs (with a 2 nm silica layer) were prepared using 1 mM
performances. The SERS and PL intensity of rhodamine 6G (R6G) can be manipulated by tuning the Ag APS and 1 mM TEOS and found to have a SERS intensity 3 fold higher than bare Ag NCs. Additionally, it
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was found that APS modified Ag@SiO2 NCs possessed both enhanced SERS and PL intensities.
1. Introduction Surface-enhanced Raman Scattering (SERS) spectroscopy and metal enhanced photoluminescence (MEPL) spectroscopy are highly sensitive and selective analytical tools useful for biological and chemical characterization.1–4 Both the SERS and MEPL effects, observed with nanostructured metal surfaces, are characterized by a localized surface plasmon resonance (LSPR) intensified electric field during excitation with light.5,6 Recently, SERS has been widely used as a powerful biomedical diagnostic tool with advantages in terms of an increased level of multiplexing, robustness, and the ability to perform in blood and other biological matrices.7–10 Photoluminescence (PL) emission from molecules is commonly described in terms of oscillating electric dipole moments and characterized by efficiency, quantum yield, and PL lifetime, i.e., the average time the molecule stays in its excited state before emitting a photon. The emission resulting from this dipole can be strong when it is surrounded by materials of a different composition. This approach can be used to determine if the probe molecule has been bonded to given target species, based a
NanoElectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. E-mail: [email protected] b NanoElectrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan. E-mail: [email protected] c National Synchrotron Radiation Research Center, Hsin-Chu, Taiwan † Electronic supplementary information (ESI) available: Raman spectra of silicon wafer and R6G on the Ag nanocube substrate. See DOI: 10.1039/c4cp05217j
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on the energy variation. Plasmonic enhancement of luminescence, also known as ‘surface enhanced luminescence’ (SEL), together with MEPL, are recognized as effective methods that enhance the intensity of luminescence from an emitter in the vicinity of metallic nanostructures.4 This approach is also used in a variety of biological applications such as biomolecule labeling.11 Especially, there is great interest in developing a bifunctional MEPL and SERS emitter by combining the two methods. Such an approach would be helpful in deducing the interactions of many types of molecules, including proteins and viruses. Many studies have focused on tuning the electronic and optical properties of metal nanostructures by varying their sizes and shapes, or by combining them with other nanomaterials.12,13 Both gold and silver based materials are of great interest, due to their ease of preparation, good homogeneity, high compatibility with biomolecules, the manifestation of intense absorption band(s) in the visible region, and their significant contribution to SERS spectroscopy as active substrates.14 Moreover, the optical excitation of plasmon resonances in Ag nanoparticle’s (NPs) is the most efficient mechanism by which light interacts with matter. A single Ag NP interacts with light more efficiently than any particle with the same dimensions that possesses any known organic or inorganic chromophore.15 The light interaction cross section for an Ag NP is approximately ten times that of its geometric cross-section, indicating that it captures much more light than is incident on it.16 Silver is the only material whose plasmon resonance can be tuned to any wavelength in the visible spectrum. Notably, silver NPs are extraordinarily efficient at absorbing and scattering light and unlike many dyes
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and pigments have a color that depends upon the size and the shape of the particle.17 Although silver exhibits many advantages over gold, e.g. higher extinction coefficients, sharper extinction bands, a higher ratio of scattering to extinction, and extremely high field enhancements, it has been employed far less in the development of sensors, with the exception of sensors based on surface enhanced spectroscopies. The reason for this is the lower chemical stability of silver NPs when compared to equivalent materials made of gold.18 Nevertheless, recent developments include the efficient protection of silver NPs by silica coating to enhance chemical stability, colloidal stability in solvents, and the ease of further modification in biological systems.19–22 Silica coating also forms a controlled dielectric environment around the silver NPs,23 which improves precision in surface plasmon resonance (SPR) based sensing, while additionally protecting the NPs, by preventing or slowing down the diffusion of environmental oxygen. Additionally, as the silica coating is inert it does not interfere with redox reactions on the core’s surface. Silica coatings can also easily be functionalized with silane coupling agents for further bioconjugation.5,24 Hence, silver NPs have attracted wide attention, due to their application potential in catalysis, optical devices, and as substrates for SERS spectroscopy in biological and medical applications. In SERS, the silica shell not only enhances colloidal stability but also controls the distance between core particles within assemblies. ¨ber based procedures available for the There are several Sto synthesis of Ag@SiO2 core–shell NPs. Generally, coupling agents with special functional groups, such as poly(vinylpyrrolidone) (PVP),22,25 3-mercaptopropyltrimethoxysilane (MPTMS)26 and 3-aminopropyltrimethoxysilane (APS),10,27 are used to make the Ag NP’s surface vitreophilic for coating with a complete silica shell. Normally, the thickness of the silica layer is controlled by the concentration of added tetraethylorthosilicate (TEOS). However, less attention has been given to the effects of different coupling agents with respect to achieving a SiO2 coating on Ag NPs. As far as we know, no work has been dedicated to this subject. On the other hand, MEPL has shown interesting results for potential applications in biological research.6,28,29 Although Ag NP MEPL has been investigated with e.g. thin films,30 colloids,26,31,32 and nanorods,33 Ag@SiO2 core–shell nanocubes (NCs) have rarely been reported. Both MEPL and SERS spectra can be used to study the influence of localized surface plasmons on the optical properties of Ag@SiO2 NCs. Ag NC cores were first prepared by using a polyol method. MPTMS and APS were then used as coupling agents for the coating process to examine their effect on the growth process of silica shells. Additionally, this study presents the characterization, as well as the SERS spectra (532 nm and 632.8 nm laser excitation), and the MEPL spectra at 375 nm of Ag NCs and Ag@SiO2 core–shell NCs.
2. Experimental section 2.1.
Materials
Ethylene glycol (EG), silver trifluoroacetate (CF3COOAg, Z98%), sodium hydrosulfide (NaHSxH2O, Z60% NaHS), tetraethyl
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orthosilicate (TEOS: Si(OC2H5)4, 98%), ethanol (C2H5OH, Z99.5%), acetone (CH3COCH3, Z99.5%) and (3-mercaptopropyl)trimethoxysilane (MPTMS: HS(CH2)3Si(OCH3)3, 85%) were purchased from Acros. Hydrochloric acid (HCl, 37%), poly(vinylpyrrolidone) (PVP, MW E 55 000), rhodamine 6G (R6G, Z 99%) and (3-aminopropyl)trimethoxysilane (APS: H2N(CH2)3Si(OCH3)3, 97%) were obtained from Sigma-Aldrich. Ammonium hydroxide (NH4OH, 35%) and 2-propanol ((CH3)2CHOH, 99.9%), both of analytical grade, were obtained from Fisher BioReagentst. Deionized water, from a MilliQ system, was used for all experiments.
2.2.
Synthesis of Ag nanocubes
Single-crystal sliver NCs were prepared using a recently published polyol method (Xia et al.) with CF3COOAg as a precursor to elemental silver and EG as a solvent.34 The Ag NC scale-up synthesis was carried out by following the procedure for the standard synthesis, except that the amounts of all reagents were 4 times greater. In brief, EG (20 mL) was added into a 100 mL round bottom flask and heated at 150 1C for 40 min under magnetic stirring in an oil bath. After which, NaSH (0.24 mL; 3 mM in EG) was quickly injected into the heated solution. After two minutes, a 3 mM HCl solution (2 mL) was injected into the heated reaction solution, followed by the addition of PVP (5 mL, 20 mg mL1 in EG). After another 2 min, CF3COOAg (1.6 mL, 282 mM in EG) was added into the mixture. During the entire process, except during the addition of reagents, the flask was capped with a glass stopper. After the addition of CF3COOAg, the transparent reaction solution turned a whitish color and then (after 2 min) quickly became slightly yellow, indicating the formation of the Ag seeds. The reaction was allowed to proceed for different periods of time and its color went through three stages, i.e. dark red, reddish grey, and brown as the edge length of the Ag NCs increased. After 1 h, the reaction solution was quenched by placing the reaction flask in an ice-water bath. The sample was collected by centrifugation and then washed with acetone once to remove the remaining precursor and EG, and then with ethanol four times to remove excess PVP, prior to being re-dispersed in 2-propanol.
2.3.
Synthesis of Ag@SiO2 core–shell nanocubes
In a modified synthesis of Ag@SiO2, 20 mL of MPTMS or APS (1 mM in 2-propanol) was added to 2 mL of Ag NCs solution and stirred for 30 min at 600 rpm at room temperature. Next, the mixture was dissolved in 6 mL of 2-propanol and stirred for 2 min, 2 mL of deionized water and 100 mL ammonia aqueous solution (35 wt%) were added to the mixture. After that, TEOS (200 mL; 1 mM in 2-propanol) was added dropwise with continuous stirring, and the reaction was allowed to continue for 6 h. The Ag@SiO2 core–shell NCs were collected by centrifugation and washed with ethanol once, and deionized water 3 times, before being re-dispersed in deionized water. The thickness of the silica coating layer was found to be able to be controlled by adding or omitting MPTMS, APS and TEOS (Fig. 1).
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Fig. 1 Schematic preparation.
2.4.
illustration
of
Ag@SiO2
core–shell
nanocube
Preparation of SERS and MEPL substrates
Droplets (30 mL solution) of Ag NCs or Ag@SiO2 NCs were spread on silicon wafers (B1 cm2). An aqueous solution of R6G (105 M; 5 mL droplets) was spread on the Ag NC and Ag@SiO2 surfaces and kept in the dark for 1 h at room temperature prior to testing. SERS and MEPL spectra for all samples were measured over 4 different areas of the sample at 25 1C. 2.5.
Characterization
UV-Vis absorption spectra were recorded using a Shimadzu UV-3150 spectrometer. The X-ray diffraction (XRD) pattern was obtained with a D2 Phaser diffractometer (D/Max-250, Bruker) using Cu Ka (l = 1.5406 Å, isolated with a Ni foil filter) radiation. The sample was prepared by sonicating Ag NCs and Carbon Black 72R (CB 72R) in ethanol followed by vacuum-drying for 12 hours before use. A scan rate of 0.05 deg per step was used, for
Fig. 2
2y, between 351 and 851. The Ag NC and Ag@SiO2 NC morphologies were characterized by transmission electron microscopy (TEM, CM200; Philips), operating at 200 kV. Raman spectra were collected on a UniRAM micro-Raman spectrometer with laser excitation (532 and 632.8 nm). The data acquisition time was normally 30 s and the peak intensities of the samples were normalized to those of a silicon wafer at 520 cm1. A thermoelectrically cooled charge-coupled device (CCD) with 1024 256 pixels operating at 60 1C was used as the detector with 1 cm1 resolution. The laser line was focused onto the sample in a backscattering geometry, using an Olympus 50 objective, providing scattering areas of ca. 0.25 mm2. The laser spot size was estimated to be B0.8–1 mm. PL spectra were acquired with a dark chamber, Micro/Macro PL mapping system (UniNanoTech Co. Ltd). Steady state PL measurements were performed with the 375 nm picosecond pulse laser and detected with a CCD as above. The spectra were corrected for the optical transfer function of the system. PL measurements were carried out in a 901 configuration with the PL signal detected in the same direction of the reflected excitation beam in order to avoid re-absorption effects. Four spectra were acquired to take averages from different locations for each sample to ensure representative sampling and incorporate spot-to-spot variability in the signal.
3. Results and discussion Ag NP morphology was characterized by TEM (Fig. 2A). The synthesized Ag particles were almost cubic with edge lengths of B50 nm (The inert image of Fig. 2A). This Ag NC edge length was selected because Lakowicz and coworkers proved that the diameter of Ag NPs is optimal from 50 to 70 nm for optimizing the strength distribution of the enhanced LSPR electromagnetic field, while at the same time suppressing competitive quenching.35,36 The wide angle XRD pattern of a Ag NC–CB 72R mixture confirms the presence of highly crystalline silver NPs (Fig. 2B). The peaks at 38.171, 44.281, 64.431, 77.481, and 81.541 can be assigned to diffraction from the (111), (200), (220), (311) and
(A) TEM image of Ag NCs (edge length B50 nm). (B) XRD pattern of the Ag NCs–CB 72R mixture.
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Fig. 3 UV-Vis spectra of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). The inset is the UV-Vis spectra, expanded in the range 454 nm to 466 nm.
(222) planes, which are characteristic of face-centered cubic (fcc) silver according to JCPDS card (no. 04-0783). The results of the powder diffraction match with the database as does the (111) and (200) peak ratio. Consequently, the average crystallite size of Ag NCs which was calculated from XRD pattern is 20.7 0.7 nm. As shown in Fig. 3, well-dispersed Ag and Ag@SiO2 core–shell NCs were obtained. It is known that the LSPR of Ag NPs is sensitive to their shape, size, and surrounding dielectric environment.37 The UV-Vis spectra of Ag NCs (50 nm edge length) showed a main LSPR band with a maximum at 456 nm and two shoulders at 354 nm and 390 nm. Fig. 3 also shows the solution absorbance spectra of Ag@SiO2 NCs synthesized with different coupling agents. During the preparation of Ag@SiO2 core–shell NCs, the position of the LSPR peak was slightly redshifted from 456 nm to 462 nm when the SiO2 layer was formed on the surface. The Drude model indicates that the wavelength of the LSPR peak is dependent on the dielectric function of the surrounding medium.38 In addition, the relationship between the LSPR peak and the refractive index is based on the method39 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi calculated using the equation lmax ¼ lp 2n2 þ 1. In this equation, lmax is the wavelength at the LSPR peak, lp is the wavelength corresponding to the plasma frequency of the bulk metal, and n is the refractive index in the surrounding dielectric environments. It is interesting to note that the wavelength of the first shoulder and the maximum UV-Vis absorbance peak of all the Ag@SiO2 samples are similar, i.e. 355 nm and 462 nm, respectively; suggesting that the variation in the thickness of SiO2 between samples was minimal. However, the position of the second shoulder was blue-shifted (from 398 nm to 394 nm) after using 1 mM APS instead of 1 mM MPTMS. A possible explanation is that the thickness of silica layer on Ag@SiO2, prepared using 1 mM APS (2 nm silica shell, Fig. 4d), is thinner than that formed using 1 mM MPTMS (4 nm silica shell, Fig. 4a). A summary of UV-Vis absorbance is presented in Table 1.
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¨ber method a mixture of alcohol and When using the Sto ammonia is commonly used for the preparation of NPs’ silica shells. If the total surface area of the seed particles per volume is large enough, compared to the added amount of TEOS, the formation of new silica layers can be completely suppressed and therefore the thickness of the silica shell can be precisely controlled by the amount of TEOS added.5,35,40 For silver NPs synthesized by the polyol method, we found it impossible to coat them directly in this way as large aggregates of uncoated silver particles and silica were formed. Moreover, the dependence on the alcoholic solvent was not the same as for TEOS: in 2-propanol and ethanol Ag@SiO2 core–shell particles were obtained; however, in ethanol coreless SiO2 particles were present as a byproduct, while in methanol, the majority of Ag NPs remained uncoated, with a few of them being randomly entrapped in large silica aggregates.41 Hence, in this study, 2-propanol was used as a solvent to dilute TEOS in coating the SiO2 layer on Ag NCs. Typical TEM images of Ag@SiO2 NCs prepared with the addition of varying amounts of MPTMS, APS and TEOS are shown in Fig. 4. It is apparent that the Ag@SiO2 NPs are uniform cubes. However, a few small NPs (solid silica spheres free of Ag NC cores) were formed when 1 mM MPTMS and 1 mM TEOS were used (Fig. 4a). Additionally, Fig. 4b shows that the thickness of the SiO2 layer (about 4 nm) was not uniform when only TEOS used. On the other hand, a very smooth and uniform silica shell (2.5 nm thick) was found after adding 1 mM MPTMS in the absence of TEOS (Fig. 4c). MPTMS contains a silane group, so it can undergo hydrolysis to form silica. A uniformly thin silica shell (2 nm thick) was prepared using APS to replace MPTMS, although 1 mM was TEOS used (Fig. 4d). From these results, it appears that covalent bonds were formed between the Ag NCs surfaces and the thiol group (in MPTMS), or the amine group (in APS). Furthermore, MPTMS and APS were used as the coupling agents to transfer Ag NCs into 2-propanol, where the silica shell can be grown directly on ¨ber process.20 Similar to other the NPs surface through the Sto types of gold or silver NPs, the metal–sulfur bond is known to be stronger than bonds with alternative functional groups (i.e., amines, carboxylic acids, alcohols, and phosphors).42 Hence, with respect to the coupling agents, the amount of MPTMS adsorbed on Ag NC surfaces was higher, and the growth of silica layer was faster than with APS. Thus, the thickness of SiO2 shell can be decreased by replacing MPTMS with APS at the same concentration. Consequently, the thickness of the silica shell can be controlled by using TEOS in the presence or absence of different coupling agents. The SERS sensitivity of these differently synthesized substrates was investigated by using R6G as a model Raman probe. The SERS spectra of R6G adsorbed on Ag NC substrates, with and without SiO2 NP coatings, are shown in Fig. 5. The peak at 611 cm1 is assigned to the C–C–C ring in-plane vibration mode. The peak at 772 cm1 is assigned to the C–H out-ofplane bending mode. The peaks at ca. 1126 and 1181 cm1 are assigned to the C–H in-plane bending, while the vibrations at 1362, 1507 and 1646 cm1 are assigned to C–C stretching of the
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Fig. 4 TEM images of Ag@SiO2 NCs prepared by adding different chemicals: ((a) 1 mM MPTMS and 1 mM TEOS, (b) 1 mM TEOS, (c) 1 mM MPTMS, and (d) 1 mM APS and 1 mM TEOS, respectively).
Table 1
Summary of Ag NCs and Ag@SiO2 core–shell NCs prepared by various combinations of MPTMS, APS and TEOS
Sample
l1st shoulder (nm)
l2nd shoulder (nm)
lmax (nm)
Average shell thickness (nm)
(a) Ag (b) Ag@SiO2–1MPTMS–1TEOS (c) Ag@SiO2–1TEOS (d) Ag@SiO2–1MPTMS (e) Ag@SiO2–1APS–1TEOS
354 355 355 355 355
390 398 396 395 394
456 462 462 462 462
N.A. 4.0 4.0 2.5 2.0
aromatic ring. The peaks at 1311 and 1571 cm1 are assigned to the N–H in-plane bend modes.43 The SERS intensity was found to increase with a decrease in the silica shell’s thickness (Fig. 5A): the Ag@SiO2 NCs prepared by APS and TEOS has the thinnest thickness of 2 nm among all samples. This can be explained by the fact that SERS enhancement in the SiO2 nanoshell is due to surface plasmon excitation of Ag NCs cores. SERS phenomena are known to depend on the analyte molecules being in close proximity to the metal NP surfaces, with
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maximum SERS enhancement expected only when analyte molecules are situated a few nanometers away from the NP’s surface or within the nanoneck of adjacent metal NPs.44,45 It is reported that silica made using these synthetic technique is porous with pore diameters about 1–3 nm,27,46 which allows for the molecular diffusion of R6G (dimensions approximately 0.8 1.6 nm) through the SiO2 shell.47 Thus, the absorption and diffusion of R6G within the confined SiO2 nanoporous structure depends on the thickness of silica shells. In comparison
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Fig. 5 (A) SERS spectra (excitation laser 532 nm) of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). (B) Enhancement factor for R6G on Ag NCs and Ag@SiO2 with the excitation laser 532 nm.
with Ag@SiO2 NCs, the silica layer of the sample coated with SiO2 using 1 mM APS and 1 mM TEOS (Ag@SiO2–1APS–1TEOS sample) is the thinnest, thus its SERS enhancement is the strongest. Therefore, the samples’ SERS intensities are closely related to the amount of R6G absorbed on the surface of the Ag core NCs. The overall enhancement of the Raman signal is of great importance, so the enhancement factor (EF) is applied to evaluate the performance of the SERS substrate using the ISERS Cref following equation:48 EF ¼ . In this equation, Cref Iref CSERS 5 (0.1 M) and CSERS (1 10 M) represent the concentrations of R6G, and ISERS and Iref are the band intensities of the selected band at 611 cm1 obtained by SERS and the corresponding band intensity of R6G on the silicon wafer (Fig. S1 and S2, ESI†). Encouragingly, the signal of R6G on the Ag@SiO2 NCs (2 nm silica shell, 1 mM APS and 1 mM TEOS used) exhibits the highest intensity, and is 3-fold greater compared with that of R6G adsorbed on an unmodified roughened Ag NC substrate as presented in Fig. 5B. As far as we know, SiO2 NPs are SERSinactive; thus, it is interesting that the SERS of R6G adsorbed on the roughened Ag substrates with incorporated SiO2 NPs can be significantly improved. Typically, either APS or MPTMS was used as the functionalized silane because the terminal amine/ thiol has an affinity for the Ag surface that leaves the trimethoxysilane group exposed to the surrounding solution. Ag NCs then assemble in solution as a result of hydrophobic forces to minimize the number of hydrophobic faces in contact with the surrounding 2-propanol. Once the trimethoxysilane groups were hydrolyzed to form silanol-containing species, TEOS was added to produce a thin silica shell. Hence, the spacings between Ag@SiO2 NCs were closer than bare Ag NCs (Fig. 2A and 4). Furthermore, an important additional feature of LSPRs, besides their dependence on shape, size and composition of the metal nanostructure and the surrounding dielectric environment, the plasmon coupling is observed when nanoparticles are in close proximity to each other. The interacting nanoparticles’ plasmon coupling has a dramatic effect on SERS, localizing large local fields at the hot spot that may provide enhancement factors
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sufficient for single-molecule detection.49 As a result the SERS intensity of all Ag@SiO2 NCs was higher than the bare Ag NCs. This enhancement is in good agreement with previous studies showing that Ag nanostructured surfaces have a strong hot-spot when the NPs are brought together and the interparticle spacing is thereby decreased.50 The detection of R6G molecules depends on the site-selective adsorption at these hot spots, which is in the vicinity of the gaps between the plasmonic nanoparticles. The hot spots are caused by the strong coupling interactions between the near fields of closely spaced nanoparticles. The EF for the SERS detection of R6G in our experiment was calculated to be 1.2 106 for Ag@SiO2–1APS–1TEOS under optimal experiment conditions. In many biological applications, it is often desirable to use near IR excitation to avoid damaging the sample or exciting sample autofluorescence, even at the cost of some signal loss due to the fourth power (n4).51 It has been suggested that the selection of less energetic excitation (NIR to red) in Raman spectroscopy is advantageous because it causes less photochemical damage to the biological samples and results in much less fluorescence background in the Raman spectrum. An excitation laser (l = 632.8 nm) was used to confirm the presence of the R6G signal in all samples, this wavelength is distinct from that of silver NCs (l = 455 nm). The signals of R6G were very clear, and the backgrounds were very flat. Because, the power density of the laser has only a small effect on the baseline intensity caused by absorption – PL processes, but no effect on the molecular spectroscopic signature intensities.52 In contrast the intensity and enhancement factors for all the samples were decreased when the excitation laser wavelength was increased (compare Fig. 5B and 6B). There was an B3.5-fold decrease of SERS enhancement when using the excitation wavelength of 632.8 nm, but there was no significant decrease noted such as the 13-fold decrease noted in previous research.53 This low signal is presumably due to the lack of coupling of the LSPR to the excitation, as these structures have the LSPR band closer to 532 nm than 632.8 nm. In addition, the frequency to n4 for the scattering dependence of Raman photons is also a principle
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Fig. 6 (A) SERS spectra with the excitation laser 632.8 nm of: (a) Ag NCs and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). (B) Enhancement factor of R6G on Ag NCs and Ag@SiO2 with the excitation laser 632.8 nm.
factor that determines the sensitivity of a Raman experiment.54 The comparative data indicate that the results based on 532 nm excitation are better than those found at 632.8 nm. It is believed that the wavelength of 532 nm is closer to the energy of the Ag NCs’ LSPR. One of the reasons that MEPL has not been explored for silver NCs is that during their synthesis complex organic molecules, e.g. PVP, that remain on their surface are not suitable for biological binding protocols without exchange or complex modifications.42 Most importantly a critical element in designing surface-functionalized particles for surface imprinting or immobilization of ions, organic molecules, or biomolecules is achieving the maximum expression of desired functional groups on the particle’s surface. In this work, the Ag NC surfaces were modified by using coupling agents such as MPTMS and APS, and by silica coating. Clear evidence for a direct relationship between the PL intensity fluorophores and Ag@SiO2 NCs is given in Fig. 7A. The typical emission peak of R6G, at B560 nm, was obtained at an excitation wavelength of 340 nm.55 Here, in order to enhance the LSPR effect, the excitation wavelength (375 nm) was chosen close to that of the energy of the LSPR of the Ag nanocubes to provide the absorption enhancement as referred to the results of the SERS. Moreover, Wang et al.,56 have shown that the adsorption of R6G on Ag would otherwise cause significant quenching, or a decreased fluorescence, such as is found on bare Ag NPs under the same experimental conditions. It is also reported that the properties of the Ag NCs can be modified by the porosity of the silica layer, where different thickness and porosity of silica lead to a varying dielectric environment around the silver particles:23 Hence, the rapid increase of PL intensity for R6G at B560 nm, absorbed on the silica shelled Ag NC samples. Furthermore, in the case of R6G, its emission peak is around 560 nm, which is also close to that of the energy of LSPR of the Ag NCs. However, the spontaneous emission not only depends on the emitter, but also is related to the surrounding medium. The magnitude of the emission of R6G will be further enhanced by the LSPR of the Ag NCs (sometimes this is termed the Purcell effect).57
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It is interesting that the SERS intensity of the Ag@SiO2– 1MPTMS–1TEOS sample, in which the SiO2 coating layer (about 4 nm) was formed using 1 mM MPTMS and 1 mM TEOS contained some free silica (Fig. 3a), is lower than for other samples with silica shells (Fig. 5 and 6). However, the PL intensity of this sample is higher than Ag@SiO2–1TEOS (4 nm of SiO2 shell) and Ag@SiO2–1MPTMS (2.5 nm of SiO2 shell) samples (Fig. 7). It is known that MEPL is affected by numerous factors, such as the type of metal, size and morphology of the NPs, as well as the spatial distance between the fluorophore and the NP’s surface, and additionally the quantum yield of the fluorophore.58 Among these factors, the distance dependence is important for fluorescence enhancement.59,60 Thus, the PL intensity decreased with an increase in the silica shell ‘spacer’ distance between the Ag NC cores and R6G from 2.5 to 4 nm or above. On the another hand, it is clear that together with an increase in the intensity of the emission peaks there is a slight red-shift from 560 nm for silicon wafer to 567 nm for Ag@SiO2–1APS–1TEOS sample and the wavelength range of the detected PL becomes broader. The PL intensity reached the greatest value for Ag@SiO2–1APS–1TEOS with a 2 nm silica shell. It is to note that the increase approximates 7.3-fold and 34-fold, compared to bare Ag NCs and silicon wafers respectively. Although the thickness of the silica layers of Ag@SiO2– 1APS–1TEOS were thinner than Ag@SiO2–1MPTMS–1TEOS (in Fig. 4d and a), the PL intensity of Ag@SiO2–1APS–1TEOS was 3 times higher. The results show that the PL intensity depends on not only the thickness of the SiO2 shell but also on the organosilane molecules. It is known that intermediate silica species (from the hydrolysis of TEOS) and organically capped metal NPs carry negative charges at high pH values (in the presence of ammonia)61 in which electrostatic interactions between silica intermediates and organically capped NPs cannot be ignored. Consequently, it is possible to set the repulsion effect (electrostatic interaction) between the species by adjusting the charge with respect to the metal–water interaction to obtain products with desired thicknesses and morphologies.62 The electrostatic
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Fig. 7 (A) PL spectra of R6G on Si wafer, (a) Ag NCs, and Ag@SiO2 NCs prepared by adding different chemicals ((b) 1 mM MPTMS and 1 mM TEOS, (c) 1 mM TEOS, (d) 1 mM MPTMS, and (e) 1 mM APS and 1 mM TEOS, respectively). Inset: the amplified spectra of R6G on Si wafer. (B) PL intensity at 560 nm of R6G on Si wafer, Ag NCs and Ag@SiO2 NCs.
interactions between the negatively charged silica intermediate and the Ag NCs will be considerably reduced in the presence of APS as most of the amine groups from APS will be indirectly attached to Si by (CH2)3 while simultaneously interacting with Ag. Therefore, the mechanism explains that the coating of SiO2 on Ag NCs, when using APS and TEOS, could be more uniform and generates results with higher PL activity. In the case of MPTMS, used as a silane coupling agent, charge repulsion could be enhanced to the extent that it was able to destabilize the microemulsion system by interfering with metal–water interactions resulting from the negative charge on MPTMS at high pH values in the presence of ammonia.62 As a result, the final product of Ag@SiO2–1MPTMS–1TEOS is an irregular structure with some free silica NPs that exhibits low PL. This offers the possibility of using Ag@SiO2 NCs, prepared with APS and TEOS, for fluorescence-based applications in biomedicine and biotechnology. Therefore, all of our observations show that the Ag@SiO2 NCs provide a significant bifunctional enhancement to the SERS and MEPL, indicating their utilities in various applications.
4. Conclusion A method for the preparation of Ag@SiO2 NCs using vitreophilic pretreatment with MPTMS and APS was successfully developed. The presence of coupling agents greatly modified the Ag NC cores and manipulated the thickness and uniformity of the silica shells. The thin SiO2 layer, due to its stability and higher refractive index, facilitates not only enhanced SERS intensity but also PL activity. The SERS intensity reached a maximum with a silica shell ‘spacer’ 2 nm thick, being 3 fold higher than that in bare Ag NCs. Comparison of the PL emission of core–shell and bare core nanostructures with similar morphologies shows that the PL intensity of the surrounding fluorophores can be increased (7.3 fold) by the Ag@SiO2 core–shell NCs. The improved SERS sensitivity and MEPL of Ag@SiO2 NCs offers the potential to form tailored
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bifunctional nanomaterials for plasmonic applications such as biosensors.
Acknowledgements The authors gratefully acknowledge the financial support from the Ministry of Science and Technology (MOST) (103-2221-E011-156-MY3, 103-3113-E-011-001), the Ministry of Economic Affairs (MOEA) (101-EC-17-A-08-S1-183), and the Top University Projects of Ministry of Education (MOE) (100H451401), as well as the facilities support from the National Taiwan University of Science and Technology (NTUST) and National Synchrotron Radiation Research Center (NSRRC).
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