Large-Scale Uniform Two-Dimensional Hexagonal Arrays of Gold

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Large Scale Uniform 2-D Hexagonal Arrays of Gold Nanoparticles Templated from Mesoporous Silica Film for Surface Enhanced Raman Spectroscopy Yi-Wen Wang, Kun-Che Kao, Juen-Kai Wang, and Chung-Yuan Mou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08116 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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The Journal of Physical Chemistry

Large Scale Uniform 2-D Hexagonal Arrays of Gold Nanoparticles Templated from Mesoporous Silica Film for Surface Enhanced Raman Spectroscopy Yi-Wen Wang,† Kun-Che Kao,*‡ Juen-Kai Wang,‡,§ and Chung-Yuan Mou*,†,‡ †

Department of Chemistry, 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

ACS Paragon Plus Environment

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ABSTRACT A good surface-enhanced Raman spectroscopy (SERS) substrate requires precise control of the enhancement factor in large area which may be achieved with large-scale hot spot engineering. Here, we present a facile method for synthesizing 2-D hexagonally patterned gold nanoparticle arrays on centimeter-sized substrates of mesoporous silica thin films with vertical nanochannels by chemical reduction. Scanning electron microscopy (SEM) images showed densely packed gold nanoparticles directly anchored on the openings of vertical mesopores (~5 nm) leading to 2 nm nanogaps between the gold nanoparticles. The gold nanoparticle arrays showed red-shifted localized surface plasmon resonance (LSPR) spectra due to strong couplings between closepacked gold. The dense on-substrate 2 nm plasmonic nanogaps lead to highly enhanced local electric field and excellent macroscopic uniformity in surface-enhanced Raman scattering (SERS).


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1. Introduction Surface-enhanced Raman Scattering (SERS) is one of the most powerful analytic tools in label-free biosensing and surface-enhanced spectroscopy.1-5 Its high enhancement in normally weak Raman scattering is associated with local enhanced electromagnetic (EM) field within nanostructured metallic systems. Particularly, forming so-called “hot spots” that bears even higher enhanced Raman scattering6,7 in such systems is critical to the success of transforming SERS from laboratorial study to field applications. A substrate consisting of dense arrays of metal nanoparticles with well-controlled junction spots, large area, and excellent spatial reproducibility would be very desirable for SERS enhancements.8-10 Making such SERS-active substrate still remains a challenge because extending sub-5 nm inter-particle gap uniformly to centimeter scale (precision control of one million) is definitely not trivial. Moreover, studies have shown that the EM field between the metal nanoparticles would increase dramatically with the interparticle gap below 2 nm.7 In this study, we demonstrate a simple method to make a SERS substrate of 2-D arrays of gold nanoparticles with gap of 2 nm while maintaining uniformity over a large area of centimeter size. Mesoporous silica materials have been adopted as hard templates for synthesizing various metallic nanostructures.11,12 3-D densely spaced gold nanoparticles (~2 nm) and silver nanoparticles (28 nm) inside mesostructured silica channels with ultra-small gaps have been reported.13-15 However, the nanoparticles are located within the channels and their densities are often not high enough to create dense hot spots for SERS. By creating dense and small nanogaps between metallic nanoparticles, these metal-mesoporous silica composites could show excellent SERS performance. Nonetheless, these are free-standing particles of micron-size domain(long internal nanochannels). In comparison to powder materials, on-substrate mesoporous thin films


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with accessible nanochannels is particularly suitable for fabricating sensing devices. Recently, we reported a new approach to grow wafer-size continuous mesoporous silica thin film (MSTF) with well-ordered perpendicular nanochannels on a wide range of substrates, from hydrophobic to hydrophilic surfaces.16 Here by using these MSTFs, we developed a method to create 2-D gold nanoparticle arrays on top of mesoporous channels with 2 nm nanogaps via a facile chemical reduction process. With periodic mesopores as templates, an on-substrate centimeter-scale 2-D gold nanoparticles array with ultrahigh density of hot-spots was obtained. Besides MSTF, mesoporous silica nanoparticles (MSNs)17,18 were also used as template to create densely coated periodical Au array on its external surfaces. These on-substrate mesoporous silica templated gold nanoparticle arrays can be directly employed for SERS applications without transferring procedures. Scheme 1 illustrates the experimental design for making 2-D gold nanoparticle arrays on mesoporous silica materials. In the first step, as-extracted MSTF (or MSN) silica surfaces were functionalized with a high density of (3-aminopropyl) trimethoxysilane (APTMS) through postmodification method in ethanol solution. From elemental analysis, the amount of APTMS grafted on mesoporous silica was 2.53 mmol/g of SiO2, equivalent to a high density of 1.43 APTMS (nm-2) which is close to a monolayer coverage (Table S1).19 Then, amine-functionalized MSTF or MSNs were immersed in a HAuCl4 aqueous solution at a pH value of 3.2. The presence of high density of amine groups on silica surfaces enhanced the adsorption of the gold precursor (AuCl4-) into the nanochannels through electrostatic attraction. Subsequently, with the introduction of NaBH4, gold nanoparticles were formed on the openings of mesopores of MSTF (or MSNs). Details of the procedures are given in experimental section.


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Scheme 1. Experimental concept of utilizing mesoporous silica thin film (MSTF) as template to form gold nanoparticle arrays. Silica surfaces were firstly functionalized with APTMS. Then, amine-functionalized MSTF was soaked in HAuCl4 aqueous solution, followed by introduction of NaBH4 to form gold nanoparticles which is deposited on the mouths of MSTF’s perpendicular nanochannels to form MSTF-Au. Mesoporous silica nanoparticles (MSN) spin-coated on Si wafer were also employed to form MSN-Au using the same approach.

2. Experimental

Chemicals: Cetyltrimethylammonium bromide (CTAB, 99+%), tetraethyl orthosilicate (TEOS, 98%), decane (99+%), (3-aminopropyl) trimethoxysilane (APTMS), tetrachloroauric acid trihydrate (HAuCl4．3 H2O, 99%) and sodium borohydride (99%) were obtained from Acros Organics. Ethanol (≥ 99.5%), aqueous ammonium hydroxide solution (35 wt.%), and hydrogen chloride (36 wt.% in water) were purchased from Fisher Scientific. 4-Mercaptobenzoic acid (4MBA, 90%) and Rhodamine 6G, (R6G, dye content 95%) were obtained from Sigma-Aldrich, and methanol was purchased from J.T.Baker. All reagents were used as received without further


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purification. Polished silicon wafers (4”, Summit-Tech Resource Corp.) were pre-cleaned with ethanol and water via sonication for 3 times at room temperature.

Syntheses and functionalization of Mesoporous Silica Thin Films (MSTF): MSTF was synthesized from a procedure we reported previously with little modification.16 In a typical synthesis, an oil-in-water emulsion was prepared by mixing a CTAB (0.193 g)/de-ionized water (80 g) solution with a decane (600 µL)/ethanol (6.0 g) solution. After stirring at 50°C for 1 h, NH3 aqueous solution (1.5 g, 35 wt.%) was added into the emulsion. Then, a polished silicon wafer was immersed into the solution, followed by an introduction of TEOS/ethanol solution (1.67 mL, 20% v/v) under stirring at 50°C overnight. As-synthesized MSTFs were rinsed and sonicated with ethanol to remove precipitated side-products, and then calcination in air at 300°C for 6 h to remove organic surfactants. To modify of APTMS, calcined MSTF was shaken in an APTMS/ethanol solution (1%, v/v) at room temperature for 16 h. Then, APTMS-modified MSTF was rinsed with ethanol several times and was dried in vacuum.

Syntheses and functionalization of Mesoporous Silica Nanoparticle (MSN): For the synthesis of MSN (pore size ~6 nm), the CTAB/H2O/decane/ethanol emulsion was stirred at 50°C for 12 h before the introduction of NH3 solution (1.5 g, 35 wt.%) and TEOS/ethanol solution (1.67 mL, 20% v/v). The mixture was kept stirring at 50°C for 1 h, and then aged at 50°C for 20 h. Assynthesized products were filtered with a filter paper (qualitative type No. 1, Advantec) to remove side products formed on the oil-water interfaces. Filtrate MSN solution was then hydrothermally treated in an autoclave at 80°C for 24 h. To remove organic surfactants, MSNs were treated with an HCl/ethanol (5 mg/ml) solution at 60°C for 2 h twice, followed by centrifugation and sonication with ethanol 5 times. For the modification of APTMS, MSNs were


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suspended in an APTMS/ethanol solution (1%, v/v) and refluxed at 90°C for 16 h. Functionalized MSNs were centrifuged and sonicated with ethanol 5 times, and then stored in ethanol. For spin-coating, 100 µl of APTMS-functionalized MSN/ethanol solution (2.5 mg/ml) was deposited on a silicon wafer (10 × 10 mm2), and spin-coated using a Swienco PM490 spinner at 800 rpm for 60 s. Then, the spin-coated samples were dried in vacuum overnight.

Growth of gold nanoparticles arrays on mesoporous silica: In a 5 ml of HAuCl4 aqueous solution (2.5 x 10-4 M), APTMS-functionalized MSTF or spin-coated MSN were immersed in the solution and shaken at room temperature (~25°C) for 3 h. Then, with an introduction of 600 µl of ice-bath NaBH4 solution (0.02 M), gold nanoparticles were reduced and the mesoporous silicagold nanocomposites were kept aging in the solution for 1 h. Although the chemical reduction is rapid, the aging time is needed for forming closely packed two-dimentional arrays of the nanoparticles. The nanocomposites were rinsed with water and dried in vacuum.

Raman Spectra measurements: MSTF-Au and MSN-Au were immersed in 1 ml of 4MBA/methanol or R6G aqueous solutions with different concentrations (10 µM – 1 nM). After 19 h, samples were rinsed with methanol or water and dried in vacuum prior to Raman measurements. SERS spectra were collected using a Micro-Raman spectrometer (Horiba Jobin Yvon’s HR800) equipped with a CCD (3 Mega Pixel) and a 633-nm laser, with a laser spot size of 0.7 mm and a beam power density of 15 mW cm−2. The integration time was 15 s for each spectrum. SERS enhancement factor (EF) was calculated from the following equation: EF = (ISERS / CSERS) / (Iref / Cref ), where ISERS denotes the intensities of the SERS spectra of MSTF-Au and MSN-Au after soaking in the solution of R6G with a concentration of CSERS, and Iref denotes the Raman signals measured on MSTF and MSN substrates after soaking in the solution of R6G


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with a concentration of Cref. The EFs values were estimated with the same condition of laser power and normalized with acquisition time (15 s for ISERS and 80 s for Iref).

Materials Characterizations: Scanning electron microscopy (SEM) images were taken on a field emission scanning electron microscope (Hitachi S-4800) operated at an accelerating voltages of 5.0 kV. Specimens were dried in vacuum, and then loaded onto a plate holder with conducting carbon tape adhered at the bottom. Transmission electron microscopy (TEM) images were performed on a transmission electron microscope (Hitachi H-7100) operated at an acceleration voltage of 200 kV. Solution UV-Vis absorption spectra were carried out on a Hitachi U-3010 spectrophotometer. A Zeiss Axiovert 200 MAT inverted microscope equipped with a spectrometer (Horiba iHR320) was used for the acquisition of dark-field scattering spectra. Illumination light was generated by a halogen lamp (HAL 100), and the scattered light was collected with a 100× NA 0.75 objective (LD EC Epiplan-Neofluar 100x/0.75 HD DIC). The scattering spectra were calibrated using a white standard (WS-1-SS, Mikropack). Powder X-ray diffraction patterns were obtained on a Scintag X1 diffractometer with Cu Kα radiation at λ= 0.154 nm. Nitrogen adsorption-desorption isotherms were collected on a Micrometric ASAP 2010 apparatus at 77 K. All the samples were degassed at 100°C overnight at 10-3 Torr prior to adsorption analysis. Elemental analyses were carried out on an element analyzer of elementar vario EL cube (Germany). The amounts of CHN on APTMS-functionalized mesoporous silica materials were measured twice for each sample. Hydrodynamic nanoparticle sizes were measured using dynamic light scattering (DLS) on a Nano ZS90 laser particle analyzer (Malvern instrument, UK). Zeta potential of bare gold nanoparticles were collected on the same instrument of DLS with an electrode cell.


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3. Results and Discussion Figure 1 shows the SEM images of MSTF and spin-coated MSN on Si wafers before (Figure 1a,d) and after gold deposition (Figure 1b,e). Here, the gold nanoparticle arrays formed on MSTF and MSN were denoted as MSTF-Au and MSN-Au, respectively. Before gold reduction, both APTMS-functionalized mesoporous materials showed well-ordered mesostructures and fully-accessible perpendicular nanochannels (Figure 1a,d). After gold reduction, in the case of MSTF-Au (Figure 1b), one can clearly see that the periodic mesopores were occupied by gold nanoparticles. The average gold particle diameter (5.1 ± 0.5 nm) deduced from the SEM image matched well to the pore size of MSTF (5.0 ± 0.5 nm) (Figure 2), and the gold nanoparticles were uniformly separated with an ultrasmall distance (2.2 ± 1.0 nm) (Figure 2c), in agreement with the wall thickness of MSTF (~ 2.0 nm).16 In addition, a low-magnification SEM image confirms the homogeneity of MSTF-Au (Figure 1c). More SEM images of MSTF-Au with high density of gold nanoparticles over a large range surface are available in Figure S1. Hexagonal patterned gold nanoparticle arrays over centimeter range can be routinely made on MSTF. It is noted that, in the case of bare MSTF (without amine functionalization), with other synthetic condition being the same, only few gold nanoparticles were randomly attached (Figure S2a), indicating the amine-functionalization was necessary for anchoring the gold nanoparticles on MSTF. Also, in Figure S2 we show that the gold nanoparticles that were deposited by the same method on an APTMS-functionalized flat Si surface (without MSTF) are randomly distributed with rather non-uniform sizes on the surface of Si.


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Figure 1. Representative SEM images of APTMS-functionalized (a) MSTF and (d) spin-coated MSN on silicon wafers. High-magnification and low-magnification SEM images of (b, c) MSTFAu and (e, f) MSN-Au with high density of gold nanoparticles formed on the mesopores.

Figure 2. Statistical analysis of (a) mesopore size of MSTF in Figure 1a, (b) gold nanoparticle diameter, and (c) gap distance between gold nanoparticles on MSTF-Au in Figure 1b.


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In the case of MSN, the silica surfaces were also first functionalized with APTMS and spincoated on Si wafers, then we did the Au reduction with the same method. From SEM image (Figure 1d), one can clearly see the hexagonally arranged pore entrances of MSN on its disc-like morphology with vertically-aligned nanochannels. From powder X-ray diffraction patterns (Figure S3a), excellent mesostructures of MSNs with four 2-D hexagonal reflections could be identified. According to nitrogen adsorption isotherm (Figure S3b and summarized in Table S1), the pore size of amine-functionalized MSN is determined to be 5.3 nm which is a little less than that of bare MSN (5.9 nm), confirming a uniform surface modification within its short nanochannels. In the MSN-Au sample (Figure 1e), similar to MSTF-Au, gold nanoparticles with dense periodic arrangement were directly anchored on top of the nanochannels of MSN. A TEM image of MSN-Au scratched from the Si surface (lying side-way) showed that the gold nanoparticles were mostly located on the entrances of nanochannels (as shown by arrows in Figure 3a) with average particle diameter of 5.5 ± 0.8 nm and inter-particle gap as 1.9 ± 0.4 nm (Figure 3). Gold nanoparticles located inside the nanochannels are much fewer. In a lowmagnification SEM image (Figure 1f),


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Figure 3. (a) A representative TEM image of MSN-Au scratched from Si wafer. The arrows indicates the locations of gold nanoparticles are mainly on the entrances of mesopores. (b) Size distributions of gold nanoparticles and (c) gaps on MSN-Au deduced from Figure 1e. MSNs-Au were densely deposited on a Si wafer with nanochannels aligned mostly perpendicularly to the substrate, a preferred orientation due to the disc-like morphology of MSN. It is noted that in our synthesis, gold nanoparticles were firstly formed in the solution with an average particle size of 4.2 ± 0.2 nm (determined by dynamic light scattering). The bare gold nanoparticles were temporarily stable even without capping reagents because a high pH condition (~9.0) after introduction of NaBH4 led to negatively charged gold surfaces with a zeta potential of -32.6 ± 5.3 mV. UV-Vis absorption spectrum of the gold nanoparticles solution showed a localized surface plasmon resonance (LSPR) absorption peak at ~513 nm (Figure 4, black line). Optical property of the gold nanoparticle arrays formed on mesoporous silica were characterized by dark-field scattering spectra as shown in Figure 4. The LSPR peak of MSTF-Au (Figure 4, red line) and MSN-Au (Figure 4, blue line) were red-shifted to ~600 nm and ~650 nm, respectively. It is known that plasmonic coupling between adjacent nanoparticles results in a redshift in the absorption spectrum.6,20 The spectral resonance peak of MSN-Au is more red-shifted than that of MSTF-Au. This is probably because that Au nanoparticles in MSN-Au display more narrow distribution than that in MSTF-Au (Figure 2), conferring smaller averaged inter-particle gap and greater plasmonic coupling. The collective plasmonic field in MSN-Au is thus stronger.20


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Figure 4. UV-Vis absorption spectrum of gold nanoparticle solution (black) reduced by NaBH4 without capping reagent, and dark-field scattering spectra of MSTF-Au (red) and MSN-Au (blue) on Si wafers. These on-substrate 2-D closely packed gold nanoparticles with small nanogaps created a large number of dense SERS hot spots which would be very suitable for label-free chemical sensing.14,15,21-23 In addition, without introduction of capping reagent during the synthesis, analyte molecules can be efficiently adsorbed on the cap-free gold surfaces. For a demonstration of SERS effect, a laser (λ= 633 nm) with excitation wavelength close to the LSPR of the gold nanoparticles arrays (Figure 4) was used. SERS spectra were collected using a Micro-Raman spectrometer (Horiba Jobin Yvon’s HR800) equipped with a CCD (3 Mega Pixel) and a 633-nm laser, with a laser spot size of 0.7 µm and a beam power density of 15 mW cm−2. The integration


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Figure 5. Raman spectra of R6G on (a) MSTF-Au and (b) MSN-Au with a series of concentrations indicated in the figures. (c) Raman spectra of R6G (100 µM) on MSTF-Au at 8 different positions (distance = 100 µm), and (d) SERS intensity plots of the 8 positions at 612 cm-1, 772 cm-1, and 1360 cm-1 in (c). Relative standard deviations of the SERS signals at 612 cm1

, 772 cm-1, and 1360 cm-1 are 7.1%, 7.0%, and 6.5%, respectively.

time was 15 s for each spectrum. The Raman spectra of rhodamine 6G (R6G) adsorbed on MSTF-Au and MSN-Au are shown in Figure 5. The gold array substrates were soaked in 1 mL of aqueous R6G solutions with a series of concentration dilution and rinsed with water prior to measurements. In the MSTF-Au sample, SERS signal of R6G was detectable even at a concentration as low as 1 nM (Figure 5a). For a control experiment, Raman spectra of R6G deposited at a high concentration of 1 mM on surfaces without any gold depositions were measured (Figure S4a). One cannot detect any Raman spectra peak for the substrates Si wafer and MSTF. Very weak intensities can be seen for R6G deposited on MSNs, perhaps due to the


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stronger adsorption on MSN. On the other hand, MSN-Au showed a detection limit for R6G at 100 nM. By comparing to the Raman spectra of 1 mM of R6G on MSTF and MSN templates (Figure S4a), the analytical SERS enhancement factor (EF)20 for R6G on MSTF-Au and MSNAu are 2.2 × 107 (at 1 nM) and 2.4 × 105 (at 100 nM), respectively. From reported Finite-difference time-domain (FDTD) calculations , electric field intensity between the nanogaps of an excited plasmonic dimer was about an order of magnitude higher than that of a single nanoparticle.24 Approximately, the SERS EF is proportional to the 4th power of the localized electric field, and thus the ultrasensitive SERS detection could be attributed to the strongly enhanced electric fields at hot-spots between hexagonally packed gold nanoparticles.6,24-26 MSTF-Au showed better SERS sensitivity than that of MSN-Au probably because of more “hot spots” on the average (Figure 1). Also, we note that we have been using a rather short integration time (15 s); longer integration time (180 s) can improve the sensitivities (Figure S5). Our substrate is uniform, versatile and macro-sized in area. SERS spectra of other chemicals have been examined and they too show large SERS enhancement, the example of 4mercaptobenzoic acid (4-MBA) is given in Figure S6. For examining the spatial uniformity of spectral intensity of MSTF-Au, the SERS signals at 8 different positions in a line scan (distance = 100 µm, Figure 5c) show rather constant intensities with a relative standard deviation of ~7 % (Figure 5d) indicating great spatial homogeneity over cm range of the gold-silica nanocomposites. The uniformity over macroscopic range is unprecedented compared to previous reports which mostly gave a homogeneous range of tens of microns only.10 Now, we discuss the formation of 2-D gold nanoparticle arrays in the mesopore-templating method. Previously, a method of NaBH4 reduction of Au(III) adsorbed on APTMS


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functionalized MCM-41 have been used to make high density of Au nanoparticle confined within mesopores.27,28 Here, in this work, however, most of the gold nanoparticles are supplied from chemical reduction of Au(III) in solution and they were self-assembled with the guidance of the pore mouths. The pre-adsorbed Au(III) in the pore further supply to chemical reduction around the deposited Au nanoparticles. In order to understand the deposition process, we have designed two other methods (Method 2 and 3 in Figure S7) for making Au nanoparticle arrays in addition to the standard method (Method 1) in the main text. In the alternative methods, the substrate Si wafer was put into the synthesis solution after the chemical reduction of Au has been done (i.e. Au nanoparticles already formed). We found arrays of Au nanoparticles of a little less density and similar sizes could also be formed (Figure 6), but there are some aggregated Au nanoparticles on top of the surface. This indicates that in the standard method Au nanoparticles were formed in solution first and then deposited onto the surface of MSTF as shown in scheme 1.

Figure 6. SEM images of (a, b) MSN-Au fabricated by “method 2” described in Figure S7b and (c, d) MSN-Au fabricated by “method 3” described in Figure S7c.


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The pre-adsorbed Au(III) in mesopores does help the full growth of the array, however. Comparing the products from the three methods, the standard method (method 1) in the main text gave the best order in Au array. One may optimize the SERS enhancement by further controlled growth of gold nanoparticle to make the nanogaps even smaller. There have been several recent reports on large area uniform assembly of dense noble metal nanoparticles exhibiting uniform SERS intensity with about the same level of variation of spotto-spot SERS signals of the analyte molecules.29-32 Our work distinguishes among these however in several aspects: (1) The size of the gold nanoparticles, at about 5 nm, is the smallest as compared to the other reported dense 2-D nanoparticles. Such small size of gold nanoparticle may allow interesting photochemical reactions as enhanced by the dense hot spots. (2) The fact that underneath each gold nanoparticle there is a small mesopore which may allow further design as a nanoreactor that strongly coupled with the strong plasmonic field. (3) The ligand-free synthesis of the gold nanoparticle may avoid many interferences in further application in either SERS or photochemical applications. (4) The hot spots between tightly packing gold nanoparticles are quite accessible from solution. Further exploit of these advantages may lead to many interesting applications. For example, by using mesoporous silica as a size-exclusion material with proper surface functionalization, detection of a certain size of concentrated chemical or biomolecules in mesopore is achievable.33,34 Combining the high density small gold nanoparticles and nanogaps, the hybrid material could be an excellent platform for in situ monitoring of interested photocatalytic reactions via SERS signals.35,36 Finally, we also note that the MSNs with dense coverage of Au nanoparticles on the external surface could be developed as excellent SERS nanoparticles in biomedicine since they can be suspended in solution well for cell-uptakes.


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4. Conclusion: In conclusion, we have developed an efficient method to create large area 2-D gold nanoparticle arrays on well-ordered mesoporous silica (MSTF and MSN) by utilizing a mesopore-templating method. Highly uniform close-packed gold nanoparticles with diameter of 5.1 nm anchored on each individual mesopores lead to ultrasmall gaps of 2 nm and hence strong coupling of plasmonic fields. Dark-field scattering spectra of MSTF-Au and MSN-Au showed red-shifted LSPR signals (λ = 600 – 650 nm) indicating the plasmonic coupling effect between close-packed gold nanoparticles. The strongly enhanced electric fields between the nanogaps make the gold nanoparticle arrays excellent SERS-active substrates. The mesopores in our MSTF and MSN further served as a nanovessel for concentrating analytes to enhance the SERS signal. These facile on-substrate SERS nanocomposites, especially the MSTF-Au which showed exceptional spatial uniformity with an ultrasensitive SERS detection limit down to 1 nM, promise useful applications in label-free chemical sensing and biosensing.

ASSOCIATED CONTENT Supporting Information More SEM images of MSTF-Au and Au nanoparticles growth on bare MSTF or Si wafer, XRD and nitrogen adsorption analysis of MSNs, measured Raman spectra on bare MSTF and MSN, additional Raman spectra using 4-MBA as a molecular probe, and alternative methods for making Au nanoparticles on MSTF. This information is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; phone: +886-233665235 (K.-C. Kao) *E-mail: [email protected]; phone: +886-233668205 (C.-Y. Mou) Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This research was funded by MOST (Ministry of Science and Technology), Taiwan. Valuable discussion with Dr. Peilin Chen is acknowledged. We thank Ching-Yen Lin, Ya-Yun Yang, SuJen Ji, and Chia-Ying Chien at Instrument Center of National Taiwan University for assistance with SEM and TEM, and Drs. Mykhaylo M. Dvoynenko and Yu-Chi Chang for helping in darkfield scattering spectroscopy.


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TOC GRAPHIC


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Large-Scale Uniform Two-Dimensional Hexagonal Arrays of Gold

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