Mesoporous Silica Nanospheres Decorated by Ag ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Mesoporous Silica Nanospheres Decorated by Ag−Nanoparticle Arrays with 5 nm Interparticle Gap Exhibit Insignificant Hot-Spot Raman Enhancing Effect Published as part of The Journal of Physical Chemistry virtual special issue “Hai-Lung Dai Festschrift”. Wan-Tzu Chen,†,‡,# Yu-Wei Cheng,‡,§,# Ming-Chien Yang,*,† Ru-Jong Jeng,§ Ting-Yu Liu,*,‡ Juen-Kai Wang,*,∥,⊥ and Yuh-Lin Wang*,⊥ Downloaded via UNIV OF SOUTHERN INDIANA on July 25, 2019 at 02:22:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Department of Materials Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan § Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ∥ Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan ⊥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Arrays of 20 nm silver nanoparticles (AgNPs) with tunable interparticle gaps have been grown on mesoporous silica (MPS) nanospheres to form AgNPs@MPS nanohybrids for surface-enhanced Raman scattering (SERS). Adenine was employed as the probing molecule to study the hot-spot SERS effect as a function of interparticle gap. The SERS signal intensity increased by 8−10fold when the gaps were reduced to 5 nm, appearing to suggest that the hot-spot Raman enhancing effect was derived from adjacent AgNP pairs. However, after rescaling the signal intensity by the number of AgNPs on each MPS corresponding to different interparticle gaps, the renormalized signal exhibits an insignificant dependence on the gap size, invalidating the existence of the hot-spot effect on AgNPs@MPS even for a gap as small as 5 nm. The observation is in sharp contrast to the large hot-spot effect clearly demonstrated by narrowing the gaps in arrays of AgNPs grown on a flat anodic alumina with vertical nanochannels. The unexpected lack of a hot-spot effect from AgNPs@MPS could be understood in terms of the misalignment between the directions of the field for the impinging laser with those of the axes between most adjacent AgNP pairs due to the spherical shape of MPS.

1. INTRODUCTION

Among all of the highly sensitive SERS substrates, mesoporous silica (MPS) materials, bearing the unique characteristics of high porosity, chemical durability, biocompatibility, monodispersion, and tunable nanoparticle size, have been used in a wide variety of practical applications.14−18 The MPS nanoparticles of uniform porous structures have been fabricated with the self-template method,19 the soft-template method,20 and surface-protected etching.21 Owing to the large surface area of spherical particles, nanoparticles decorated on their surfaces are believed to provide more SERS-active area than their counterparts on a flat template.9,22−24 A variety of metal−SiO2 nanocomposites have been successfully fabricated with self-assembly,25 a seeded growth method,26 and surface functionalization deposition.27

Surface-enhanced Raman scattering (SERS) has provided a rapid, sensitive, and powerful analytical technique for the detection of molecules at low concentrations.1,2 Typically, nanoparticles (NPs) of gold (Au) or silver (Ag) are arranged into arrays with different packing configurations on templates of various materials to create highly SERS-active substrates.3 In particular, AgNPs provide a higher SERS enhancement because of their better affinity to many analytes and higher plasmon-enhanced local field in the wavelength regions of practical interest.4−6 Moreover, template materials such as dendritic polymer,7 honeycomb-like materials,8 porous materials,9 and AAO (anodic aluminum oxide) templates10 have offered tunability of the separation between the nanoparticles to create an enhanced local field at the interparticle gap, dubbed a “hot spot”, owing to electromagnetic coupling between the local plasmons of the neighboring nanoparticles.11−13 © XXXX American Chemical Society

Received: April 30, 2019 Revised: July 9, 2019 Published: July 10, 2019 A

DOI: 10.1021/acs.jpcc.9b04074 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Process for Preparing AgNPs@MPS Nanohybrids

in Scheme 1.31 First, octane was dispersed in deionized water and then stirred at 70 °C for 30 min. To produce the micelles, proper amounts of CTAB were then added to the solution with stirring at 70 °C under a N2 atmosphere. Subsequently, AIBA, styrene, L-lysine, and TEOS were added to the solution and stirred for 4 h, and then the suspension was washed three times with methanol and centrifuged at 5000 rpm for 10 min. The products were dried in a vacuum oven at 80 °C for 24 h, followed by heat treatment at 400 °C for 4 h to obtain the MPS nanospheres. 2.3. Preparation of AgNPs@MPS. The procedure for preparing the AgNPs@MPS nanohybrids is illustrated in Scheme 1. The 1 mL solution of MPS in ethanol (2.5 mg/mL) was added to 5 mL of a deionized water solution of AgNO3 with different concentrations (0.5, 1, 2, 4, and 8 mM) to form different kinds of AgNPs@MPS nanohybrid solutions, abbreviated as AgNPs1@MPS, AgNPs2@MPS, AgNPs3@ MPS, AgNPs4@MPS, and AgNPs5@MPS, respectively. Further, this was stirred overnight at room temperature before 0.6 mL of NaBH4 in deionized (DI) water (0.02 M) was added to the above solution and their pH values were adjusted to ∼10.5 with NaOH aqueous solution (0.1 M). Finally, these solutions were stirred and heated at 90 °C for 20 min to form the resultant AgNPs@MPS nanohybrids. 2.4. Measurement of the SERS Enhancement of Nanohybrids. Raman spectra were measured by a Raman spectrometer (HR800, Horiba) equipped with a 632.8 nm He−Ne laser. Its spectral resolution and error are 7 and 0.1 cm−1, respectively. (The 632.8 nm He−Ne laser was selected because its wavelength is within the plasmonic resonance wavelength range of AgNPs@MPS nanohybrids and it has much lower fluorescence excitation in comparison with that of the shorter wavelength laser.) The laser beam spot size is about 1.2 μm2 (40× of objective lens). The 10 μL solution of AgNPs@MPS nanohybrids (3 mg/mL) was mixed with 10 μL of 10−5 M of adenine, and the mixtures were sonicated for 30 min. After that, the 20 μL mixture solution was dropped on an aluminum coated glass slide for Raman measurement. After the material dried in air, the typical diameter of the dried mixtures was ∼0.5 cm. Therefore, the typical average areal densities of the AgNPs@MPS and adenine were, respectively, ∼30/μm2

As demonstrated earlier, a large portion of the SERS signal is contributed by the analytes residing at a few hot-spots in between pairs of nanoparticles for making the SERS enhancers because the local field enhancement there is much higher than that at other positions.28 As a consequence, producing uniform hot spots in a large quantity via nanometer-scaled control in material growth is crucial to optimize the ultimate performance of the SERS enhancer.12,29,30 In this study, homogeneous decorations of AgNPs with a uniform size on MPS were demonstrated, and the interparticle gaps of AgNPs were tuned by simply changing the concentration of the silver nitrate (AgNO3) in the chemical reduction process to grow arrays of AgNPs. Adenine was employed as the probing molecule to study the hot-spot SERS effect as a function of the gap size. Decorating arrays of AgNPs with narrow gap sizes to form SERS hot-spots on a spherical MPS surface has an intrinsic drawback because it is well-known that the direction of the excitation laser field must match the axes between AgNP pairs to maximize the hot-spot effect. Such an alignment is obviously harder to achieve for AgNP arrays on a sphere than on a plane. However, it is not clear how much this purely geometric misalignment would reduce the hot-spot effect on average, and how small the gap size between adjacent AgNP pairs needs to be in order to render the effect of misalignment much smaller than that of the SERS hot-spot enhancement itself. This study hopes to shed some light on these fundamental issues and provide insights that would help us take better advantage of exploiting mesostructures as templates for supporting nanoparticle SERS enhancers. After all, such nanohybrids have their intrinsic merits in certain types of SERS applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Cetyltrimethylammonium bromide (CTAB), L-lysine, n-octane, styrene, tetraethylorthosilicate (TEOS), AgNO3, 2,2′-azobis(2-amidinopropane) dihydrochloride (AIBA), sodium borohydride (NaBH4), and adenine were all purchased from Sigma-Aldrich and Acros and used without further purification. 2.2. Preparation of MPS. MPS was synthesized by modifying the method proposed by Chen et al., as illustrated B


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The Journal of Physical Chemistry C and ∼3 × 105/μm2 for the Raman measurements, during which the irradiated laser power was carefully adjusted to avoid laser-induced damage. The recorded SERS spectra underwent removal of baseline with a sensitive nonlinear iterative peak clipping algorithm and then averaging. 2.5. Characterization. The morphology of MPS and AgNPs@MPS nanohybrids was examined by transmission electron microscope (TEM, JEOL LEM-2100) and scanning electron microscope (SEM, JEOL JSM 6701F). The average AgNP diameter (DSP) and interparticle gap (gSP) of AgNPs were calculated from SEM images of AgNPs decorated on the MPS using commercial ImageJ software. To avoid unnecessarily large systematic error in the extraction of DSP and gSP due to the geometric projection effect, only AgNPs located within 45° latitude on the MPS nanospheres were included in the statistical counting. X-ray diffraction (XRD, Panalytical X’PERT PRO) with a Cu Kα radiation at 1.5406 Å, operated at a scanning rate of 2°/min, was employed to study the atomic structures of both the MPS and AgNPs@MPS nanohybrids. The chemical binding conditions of MPS and AgNPs@MPS nanohybrids were examined by X-ray photoelectron spectrometer (XPS) (Thermo VG-Scientific). UV−vis absorption spectra of AgNPs@MPS solutions were measured by a UV−vis spectrometer (Vasco V650) in the wavelength range from 200 to 800 nm.

surface of MPS have similar sizes and their average diameter derived from the statistical analysis (ImageJ software) is 20 nm (Figure S1). Note that the AgNPs appear separated and uniformly distributed on the MPS in the case of AgNPs4@ MPS while they appear aggregated in the case of AgNPs5@ MPS. The chemical binding of the MPS and AgNPs@MPS nanohybrids was examined with XPS, as shown in Figure 3. The XPS spectrum of MPS exhibits characteristic peaks of oxygen (O-1s), carbon (C-1s), and silicon (Si-2s, Si-2p) in the XPS spectra. On the other hand, the XPS spectrum of AgNPs@MPS displays additional peaks at 374 and 368 eV, corresponding to Ag 3d3/2 and Ag 3d5/2, which again confirm the decoration of AgNPs onto MPS. The combination of XRD, TEM, SEM, and XPS results presented above shows that AgNPs with an average diameter of 20 nm were decorated relatively uniformly onto the MPS nanospheres using a simple reduction reaction of AgNO3 with concentration ranging from 0.5 to 4 mM. These AgNPs@MPSs in water were also characterized with UV−vis absorption spectroscopy. Figure 4 shows the obtained spectra. All the absorption spectra exhibit an absorption peak at around 400 nm. Notice that all the spectra from AgNPs1@ MPS to AgNPs4@MPS are almost identical, indicating that the expected spectral shifting due to the plasmonic coupling between neighboring AgNPs on MPS32 is absent in different kinds of AgNPs@MPS. The implications of this observation will be discussed later. Finally, the absorption peak in the absorption spectrum of AgNPs5@MPS is narrower, which could originate from the fact that the AgNPs on MPS have aggregated and are in contact with each other (Figure 2f) in that case, thus altering their plasmonic propensity. Since their ultimate form is not well-controlled and no apparent interparticle gap exists (Figure 2f), no further discussion will be made with regard to this aspect. 3.2. SERS Measurements of Adenine on AgNPs@MPS. Adenine was chosen as the probing molecule to examine the dependence of the Raman enhancing power of the geometric arrangement of the AgNP arrays on different AgNPs@MPS nanohybrids because it does not have any appreciable onephoton absorption at the excitation wavelength and, therefore, has a very low fluorescence background, causing minimal interference with the SERS measurement. The SERS spectra of aqueous adenine solution (10−5 M) after mixing with five different AgNPsj@MPS (j = 1∼5) enhancers are shown in Figure 5a. The prominent peaks observed at 733 cm−1 correspond to the ring-breathing vibration.33 Notice that that peak signal remains roughly invariant as the enhancer is changed from AgNPs1@MPS to AgNPs2@MPS, increases drastically as the enhancer is changed from AgNPs2@MPS to AgNPs4@MPS, and, finally, for AgNPs5@MPS, drops to the same signal level of AgNPs1@MPS. The index j indicates the concentration of AgNO3 (CSN) in the reduction process, CSN = 0.5 × 2j−1 mM, and thus reflects the density of AgNPs on the surface of MPs. Figure 2 shows that, for j = 1−4, the AgNPs appear to be separated and their diameters (DSP) remains approximately constant at 20 nm while their density increases with CSN; for j = 5, the AgNPs form aggregates. For the cases of well-separated AgNPs on the surface of MPS, the number of AgNPs (NSP) on each MPS is therefore approximately proportional to CSN, namely, NSP ∝ CSN, assuming that the silver reduction reaction is an equilibrium process to achieve the same diameter for each MPS sample. Since the interparticle

3. RESULTS AND DISCUSSION 3.1. Characterizations of AgNPs@MPS SERS Nanohybrids. Figure 1 shows typical X-ray diffraction patterns of

Figure 1. XRD diffraction patterns of (a) MPS and (b) AgNPs@ MPS.

MPS and AgNPs@MPS. For MPS, only a broad peak was observed at 2θ = 22°, indicating the MPS nanospheres are made of amorphous silica. In contrast, the pattern of AgNPs@ MPS clearly exhibits prominent peaks at 2θ = 38.2°, 44.4°, 64.6°, 77.4°, and 81.7°, which can be assigned to (111), (200), (220), (311), and (222) planes of Ag, respectively, confirming that crystalline AgNPs were formed in AgNPs@MPS. The morphologies of MPS and AgNPs@MPS nanohybrids were examined by SEM and TEM (Figure 2). The average diameter of MPS is 360 nm, and the pore size of the nanochannels inside MPS is ∼8 nm (Figure 2a). For AgNPs@ MPS nanohybrids, as shown in Figure 2b−f, AgNPs on the C


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The Journal of Physical Chemistry C

Figure 2. (a) TEM image of pristine MPS; SEM image of (b) AgNPs1@MPS, (c) AgNPs2@MPS, (d) AgNPs3@MPS, (e) AgNPs4@MPS, and (f) AgNPs5@MPS nanohybrids.

spacing (dSP) is inversely proportional to the square root of NS, dSP ∝ CSN−1/2. Indeed, the extracted interparticle distance, dSP (=gSP + DSP), as shown in Figure 2 for j = 1−4 follows the above relationship within 10% experimental uncertainty. Therefore, this good agreement between the experimental findings and the prediction shows that this AgNPs@MPS can serve as a good template to tune the interparticle gap by adjusting CSN. In comparison, the interparticle gap of the AgNP array reported in our previous work12 was controlled by the diameter and the spacing of the nanochannels of anodic aluminum oxide which are adjusted by the anodizing voltage during the nanochannel formation process and the etching time in the subsequent pore opening process. In Figure 5b, the Raman intensity at 733 cm−1 (ISERS) increases drastically as gSP becomes smaller than 30 nm, or gSP/DSP is below 1.5, as shown in the inset. In particular, ISERS is increased by 9-fold as gSP is changed from 30 to 5 nm. Since the SERS signal is attributed to the molecules adsorbed on the surface of AgNPs, the firstlayer theory of SERS,34 it is more sensible to consider how the average SERS signal of an individual AgNP (ISERS * ) varies with the interparticle gap, where ISERS * = ISERS/(NSP × nMPS) ∝ ISERS/ CSN, assuming a constant number density of MPS (nMPS). On the basis of the data shown in Figure 4b, the renormalized I*SERS remains essentially invariant as gSP is decreased from 61 to 5 nm, which corresponds to an 8-fold increase in CSN from 0.5 to 4 mM. In other words, the apparent 8-fold increase in the ISERS derived from AgNPs@MPS with the smallest gSP of 5 nm is mainly due to the 8-fold increase in CSN. This observation is in sharp contrast to that for the case of AgNP/AAO,12 where the ISERS * jumps by 5-fold as gSP/DSP is reduced from 1 to 0.2, clearly demonstrating the Raman enhancing effect of hot spots. The distinct dependence of I*SERS on gSP between AgNPs@MPS and AgNP/AAO will be discussed later to reveal its origin. The dependence of the SERS signal per Ag nanoparticle * ) in the case of AgNPs@MPS can be discussed in the (ISERS following three-step process. For the sake of simplicity, the center of an AgNPs@MPS is located at the origin of a spherical

coordinate O. The position of any Ag nanoparticle on the surface of an MPS sample is described by its coordinate (DMPS/2, θ, φ), where DMPS is the diameter of the MPS, θ is the polar angle, and φ is the azimuthal angle. Furthermore, the Ag nanoparticles decorated on the surface of the MPS are assumed to be a series of circular nanoparticle chains with their normal axes aligned along the z axis at different polar angles (as shown in Figure 6). Let us also suppose that the Ag nanoparticles are aligned along longitude and latitude directions and the distances between the adjacent Ag nanoparticles along the polar direction, êθ, and the azimuthal direction, êφ, are equal. This means there are two coupled directions (êθ and êφ) of each pair of adjacent Ag nanoparticles. The light wave is incident along the positive z direction with its electric field oriented along the x axis. In the first step of the discussion, the light wave passes through the dielectric MPS without generating a scattered field. Thus, the projection of the incident electric field on the coupling axis between an adjacent Ag nanoparticle on the circular chain (along êφ) is equal to sin φ and that on the polar direction êθ is cos θ·cos φ. Since the SERS signal is proportional to the fourth power of the local electric field, the additional contribution in the SERS signal caused by the field coupling between adjacent Ag nanoparticles will be approximately proportional to sin4 φ + cos4 θ·cos4 φ. The integration over the MPS surface thus gives the plasmonic-coupling contribution to I*SERS without the scattered field from the MPS: 0 M pc |MPS =

∫0

=

∫0

π

dθ sin θ

∫0

dθ sin θ

∫0

π

= 9π /5

2π

2π

[(E0⃗ ·eφ̂ )4 + (E0⃗ ·eθ̂ )4 dφ /(E0)4 ] (sin 4 φ + cos 4 θ· cos 4 φ) dφ (1)

where E0 is the amplitude of the incidence light wave. In eq 1, the Raman radiation generated from individual pairs of adjacent Ag nanoparticles is considered incoherent in phase. This assumption is made on the basis of ultrafast dephasing D


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Figure 4. Normalized absorption spectra of AgNPsj@MPS, j = 1−5, in water.

ε − εm zyz ji zz = jjj1 − MPS j 2εm z{ ε + MPS k + cos 4 θ·cos 4 φ) dφ = 2.55

4

s M pc |MPS

mechanisms of free electrons within Ag nanoparticles involved in the SERS process.35 In comparison, the corresponding contribution for the same number of Ag nanoparticles arranged on a plane with the same surface area while maintaining its interparticle distance is M0pc|plane = 4π instead. Accordingly, the additional contribution to I*SERS by plasmonic coupling between adjacent Ag nanoparticles in the case of AgNPs@MPS is reduced by (4π)/(9π/5) = 20/9. The next step of the discussion is to consider the effect of the scattered field due to the dielectric MPS. Under a quasielectrostatic approximation, the scattered field is εMPS − εm ji DMPS zy3 jj zz [(ex̂ ·er̂ )er̂ − ex̂ ]E0 εMPS + 2εm k 2r {

π

dθ sin θ

∫0

2π

(sin 4 φ (3)

where εMPS is assumed to be the mean of the dielectric constants of silica and vacuum: εMPS = 1.66. On the other hand, the corresponding contribution in the case of Ag nanoparticles lying on a plane of the same surface area is still 4π. As a consequence, Mspc|plane/Mspc|MPS ≈ 4.9. Compared with Ag nanoparticles on a plane, the spherical shape of MPS reduces the contribution of ISERS * by a factor of ∼5, which essentially cancels out the ∼5-fold increase of the SERS signal per AgNP observed in the case of AgNP arrays on a plane AAO template as the interparticle gap approaches 5 nm.12 The comparison semiquantitatively explains why the SERS hot-spot effect appears to be insignificant in the case of spherical MPS decorated by AgNP arrays with a 5 nm interparticle gap. Although some approximations are made in the estimation above, we believe that it captures the essential part of the physics. The absence of the plasmonic-coupling effect in the case of AgNPs@MPS is also evident from their UV−vis absorption spectra. Figure 4 shows that the resonant absorption feature at around 400 nm is insensitive to the density of Ag nanoparticles decorated on the MPS surface, also indicating that the expected spectral shifting due to the plasmonic coupling between adjacent Ag nanoparticles is insignificant. Of course, the estimation above is based on a quasi-electrostatic approximation which is not entirely correct because the diameter of the MPS is comparable to the excitation wavelength in the SERS measurement. Namely, the field amplitude around the MPS surface is not uniform owing to the phase-retardation effect. Most importantly, some resonance effects may occur and will be discussed next. The third step of the discussion is the resonance effect of the incident light interacting with the dielectric MPS36 that might

Figure 3. (a) XPS spectra of pristine MPS and AgNPs@MPS nanohybrids, (b) Ag-3d spectrum of MPS and AgNPs@MPS nanohybrids.

Es⃗ =

∫0

(2)

where εMPS and εm are the dielectric functions of the MPS and the medium around it, respectively, r is the distance from the center of the MPS to the observation point, and êr and êx are the unit vectors in the radial direction and along the x axis, respectively. Similar to eq 1, the plasmonic-coupling contribution to I*SERS induced by the total field (E⃗ 0 + E⃗ s) is E


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Figure 6. Schematic layout in spherical coordinates of AgNPs@MPS. MPS is the shaded sphere, and the small gradient spheres are Ag nanoparticles. The light wave is incident along the positive z direction with its electric field pointing along the x axis. θ and φ are the polar and azimuthal angles, respectively.

which is supported by the numerical calculation of the MPS decorated with sparse Ag nanoparticles reported in ref 9. Namely, the possible resonance and phase-retardation effects of the MPS do not increase the plasmonic-coupling effect effectively. The discussion above therefore shows a semiquantitative analysis of the suppressed contribution of the plasmonic coupling to the SERS signal of AgNPs@MPS. The observation of the insignificant hot-spot Raman enhancing effect from NP arrays with narrow gaps on spherical substrates bears important implications. This prompts us to take a closer look at the “conventional wisdom” for creating a SERS-active substrate with larger Raman enhancing power by simply reducing the gaps between adjacent NP pairs as much as possible. This “conventional wisdom” originated from experiments in which the axes of adjacent NP pairs lie on a same plane whose surface normal is mostly aligned with the propagating direction of the excitation light source. This oversimplified geometry is not applicable even if NPs are decorated on flat nanopallets to form arrays with narrow gaps unless a certain method could be invented to align the surface normal of the nanopallets. Without such an alignment, the axes of adjacent NP pairs on an ensemble of randomly oriented nanopallets will also be randomly oriented with respect to field direction of the excitation light source, rendering the hot-spot Raman enhancing effect insignificant.

Figure 5. (a) SERS spectra of adenine (10−5 M) measured by AgNPs1@MPS (0.5 mM of AgNO3), AgNPs2@MPS (1 mM of AgNO3), AgNPs3@MPS (2 mM of AgNO3), AgNPs4@MPS (4 mM of AgNO3), and AgNPs5@MPS (8 mM of AgNO3). (b) Intensity of the 733 cm−1 peak as a function of the interparticle gap on AgNPs3@ MPS. (Inset: intensity as a function of the ratio between gap and diameter of AgNPs.)

increase the surface field. Since the diameter of the MPS sphere is DMPS ≈ 360 nm, the wavelengths of the resonant whispering gallery modes of a dielectric sphere37 are located at approximately π εMPS DMPS /l , l = 1, 2, 3... The resonance occurs at around 1440, 720, 480 nm, etc. That is, the excitation wavelength (632.8 nm) in the SERS measurement is off the resonance. Furthermore, the absorption spectra of the AgNPs@MPS (Figure 4) do not manifest other resonant features at the above stated wavelengths, also suggesting a negligible resonant effect. The absent resonant feature in the absorption spectra may be due to the fact that the surface roughness is caused by the decorated Ag nanoparticles (Figure 2). While the resonance effect is expectedly small, the possible enhanced surface field mostly resides at (DMPS/2, π/2, 0) and (DMPS/2, π/2, π) in the spherical coordinate just outside the sphere and is normal to the sphere surface. Accordingly, the field direction is not favorable to the plasmonic coupling between adjacent Ag nanoparticles around the two regions,

4. CONCLUSIONS We have successfully decorated 20 nm AgNPs onto MPS nanospheres to form AgNPs3@MPS nanohybrid Raman enhancers based on arrays of AgNPs with tunable interparticle gaps. By measuring the intensity of the characteristic 733 cm−1 peak as a function of the gap size between 5 and 60 nm, we conclude that the hot-spot Raman enhancing effect that is expected to originate from AgNP pairs with a gap as small as 5 nm is insignificant. This observation is in sharp contrast to the large hot-spot effect clearly demonstrated by narrowing the F


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The Journal of Physical Chemistry C gaps in arrays of AgNPs grown on a flat anodic alumina sample with vertical nanochannels. We believe that the absence of a hot-spot enhancing effect on AgNPs3@MPS can be attributed to the misalignment between the directions of the field for the impinging laser with those of the axes between most of adjacent AgNP pairs due to the spherical shape of MPS. Furthermore, the effect of the scattered field due to the dielectric MPS also plays an important role in reducing the hot-spot effect. Our observation suggests the necessity to pay much more attention to the relative orientation between the field of the excitation source and the axes of adjacent NP-pairs in arrays of NPs on certain template materials with favorable dielectric constants, if the hot-spot Raman enhancing effect of such nanohybrids is to be fully exploited. Furthermore, the polarization direction of the excitation source with respect to the axes of adjacent NP-pairs also need to be carefully arranged.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04074.

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Detail of the statistical analysis with the ImageJ software giving an average diameter of AgNPs on AgNPs@MPS nanohybrids (PDF)

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Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (M.-C.Y.). [email protected] (T.-Y.L.). [email protected] (J.-K.W.). [email protected] (Y.-L.W.).

ORCID

Ru-Jong Jeng: 0000-0002-0913-4975 Yuh-Lin Wang: 0000-0001-5920-3468 Author Contributions #

W.-T.Z. and Y.-W.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Mykhaylo Dvoynenko for the derivation of the equations and assitance with the preparation of Figure 6,. The support from the Ministry of Science and Technology of Taiwan (MOST 106-2221-E-131-006-MY3, MOST 108-2623E-011-002-D, and MOST 107-2745-M-001-004) is acknowledged.

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REFERENCES

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