Preparation of Gap-Controlled Monodispersed Ag Nanoparticles by

DOI: 10.1021/acs.iecr.8b00717. Publication Date (Web): May 21, 2018 ..... The inset is fast Fourier transform (FFT) image. (f) The corresponding parti...
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Preparation of Gap-Controlled Monodispersed Ag Nanoparticles by Amino Groups Grafted on Silica Microspheres as a SERS Substrate for the Detection of Low Concentrations of Organic Compounds Wen-Jing Chen, Xiao Qing Liu, Shun Zhang, and Hong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00717 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Industrial & Engineering Chemistry Research

Preparation of Gap-Controlled Monodispersed Ag Nanoparticles by Amino Groups Grafted on Silica Microspheres as a SERS Substrate for the Detection of Low Concentrations of Organic Compounds Wen-Jing Chen, Xiao-Qing Liu, Shun Zhang, Hong Jiang*

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

*Corresponding author: Dr. Hong Jiang, Fax: +86-551-63607482; E-mail: [email protected] 1

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ABSTRACT

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The synthesis of new surface-enhanced Raman scattering (SERS) substrates for the

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detection of trace poisonous organic pollutants is important for environmental

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monitoring. Herein, we designed and prepared monodispersed and uniform Ag NPs

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anchored on silica microspheres (Ag@ASMs) by the orientation of grafted amino

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groups, and applied it as a SERS substrate for the detection of ultra-low

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concentrations of poisonous organic pollutants (Rhodamine 6G (R6G), crystal violet

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(CV), and melamine) with high sensitivity and reproducibility in aqueous solution. A

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ratio of 2:1 Ag@ASMs exhibited optimum SERS performance, allowing the detection

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of 10–12 M of R6G with an enhancement factor of 6.36×107 (collect the normal Raman

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signal based on 10-4 M R6G), as well as 10–8 M of CV and melamine. The density

13

functional

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3-triethoxysilylpropylamine molecules are interspaced, and the optimum Ag dosage

15

further tuned the gap between Ag NPs as well as the size of Ag NPs. This approach

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can suggest new ideas for the design and synthesis of other improved SERS active

17

substrates.

theory

calculation

also

provided

evidences

that

modified

18 19

Keywords: surface-enhanced Raman scattering; trace pollutants; amino-modification;

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silica microspheres; Ag nanoparticles; DFT.

21

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INTRODUCTION

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Trace poisonous organic pollutants (TPOPs) are widespread in the air, water, and

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on the surfaces of crops.1 Eating food and drinking water may contain TPOPs and the

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long-term exposure to TPOPs can increase the risk of cancers.2 Traditional methods to

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detect TPOPs include gas chromatography coupled to mass spectrometry (GC-MS),

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high-performance liquid chromatography (HPLC), fluorescence spectroscopy,

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electrochemical methods, and immunoassays.3-5 Although these analytical methods

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offer good specificity, the sensitivity and precision is not sufficient for the effective

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detection of ultra-trace concentrations of pollutants. In addition, most of these

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analytical methods require sophisticated sampling, or prior separation/extraction and

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pre-concentration procedures, making these approaches high in cost and time. Thus, it

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is urgent to develop simple, rapid, cost-effective, and ultrasensitive detection

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techniques for TPOPs.

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Since its discovery by Fleischmann and Van Duyne in 1970s, surface-enhanced

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Raman scattering (SERS) has been applied as a powerful and attractive spectroscopic

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technique that offers non-destructive and ultrasensitive characterization down to the

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single molecule level by identification of the specific information for particular

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analytes. This method has wide potential applications in environmental monitoring,

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biochemistry, chemical synthesis, and food safety and offers rapidity, high sensitivity,

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and selectivity.6-8 Two mechanisms have been proposed to account for the SERS

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effect, the chemical effect depending on electronic coupling by a charge-transfer

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mechanism between the metal and the analyte molecules adsorbed on the surface and

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electromagnetic enhancement generated by the excitation of localized surface

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plasmon resonance (LSPR) near appropriately nanostructured metal systems.9-11

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Hot-spots that are formed at nanoscale junctions and at the interstices of the

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SERS substrates can greatly enhance the normally weak Raman signal, providing

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each molecule a unique vibrational fingerprint. Therefore, tremendous efforts have

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been made for the preparation of SERS active substrates, particularly the use of noble

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metal nanoparticles (NPs) in colloids or films.12, 13 For instance, Wang et al. reported

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that “flower-like” silver composite microspheres with concentrated hot-spots on the

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rough surface could greatly enhance Raman signals.12 Although this type of SERS

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substrates shows satisfactory sensitivity, it is difficult to control the locations of the

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hot-spots because bottom-up fabrication methods create random nanostructures,

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resulting in low reproducibility. Zhao et al. synthesized an Ag/Si nanopillar array

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SERS substrate with microfluidic channels and homogeneous hot-spots using a

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combined photolithography and nanosphere-lithography technique, which can

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increase the reproducibility of the material and signals.14 However, synthesis of these

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structures often requires expensive and complicated electron beam lithography,

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limiting more extensive application. Moreover, the moderate sensitivity (enhancement

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factor ≤ 106) is not sufficient for the detection of TPOPs.15 Niu et al. developed

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magnetic Fe3O4@SiO2@Ag microspheres as an active SERS substrate. In addition to

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the magnetism, the SiO2 microspheres improved the dispersity of Ag and endowed the

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substrate with excellent sensitivity and reproducibility.16

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In our previous work, we found that amino groups grafted on the pyrolytic

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biochar and hydrothermal char surface can efficiently trap the Cu and Pb ions by

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complexation.17, 18 The interaction based on the chemical bonds allowed the even

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dispersal of metal ions on the surface or pores of carbonaceous materials. Inspired by

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this finding, we hypothesized that if amino groups were grafted onto the surface of

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silica and the distance among amino groups was controlled, the distance of the

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combined Ag ions and amino groups should be also controlled. Consequently, after

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the reduction reaction, the distance between Ag NPs can be tuned and the

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homogeneity and immobility of Ag NPs can be significantly improved.

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Thus, the main objective of this study was to design and synthesize moderately

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segregated Ag NPs anchored on amino-modified silica microspheres, and evaluate the

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SERS reproducibility and sensitivity of these materials for TPOPs. To this end, we 1)

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prepared silica microspheres (SMs) with moderate amino modification (ASMs); 2)

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synthesized Ag@ASMs by in situ loading of Ag NPs onto the surface of ASMs; 3)

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demonstrated the homogeneity and immobility of Ag NPs onto silica microspheres by

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detailed characterization using a scanning electron microscope (SEM), field-emission

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transmission electron microscope (FE-TEM), X-ray diffractometer (XRD), X-ray

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photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy

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(FTIR); 4) tested the improved SERS performance toward several model TPOPs, such

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as rhodamine 6G (R6G), melamine, and crystal violet (CV); and 5) explored the

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formation mechanism of Ag@ASMs by calculating the possible structures of the

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chemisorbed APTES molecules on silica surface and the interaction between Ag atom

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and APTES molecules using density functional theory (DFT). This approach can

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suggest new ideas for the design and synthesis of other improved SERS active

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substrates.

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EXPERIMENTAL SECTION

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Materials. Anhydrous ethanol (99.7%), 3-triethoxysilylpropylamine (APTES,

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98%), ammonium hydroxide (NH3•H2O, analytical standard), tetraethyl orthosilicate

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(TEOS, analytical standard), silver nitrate (AgNO3, 99.8%), trisodium citrate

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dehydrate (C6H5Na3O7•2H2O, 99.5%), crystal violet (CV, analytical standard), and

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melamine (99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd,

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China. Rhodamine 6G (R6G, 95%) was purchased from the Aladdin Chemical

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Reagent Co., Ltd, China.

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Synthesis Procedure. SMs and ASMs were produced using the modified Stöber

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method (Text S1). 5 mg ASM (or SM) powder was dispersed into 50 mL pure water

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and sonicated for 10 min. Next, 6 mL of 10 mM AgNO3 solution (10.2 mg AgNO3)

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was added to the above solution and stirred on a magnetic stirrer for 30 min at room

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temperature. The mixed solution was heated in a boil bath to 100 oC, and then 1.5 mL

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1% wt solution of sodium citrate was added rapidly. The reaction was refluxed for 1 h

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at 100 oC, and then precipitated by centrifugation and redispersed in pure water to

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give the final product (denoted 2:1 Ag@ASMs or 2:1 Ag@SMs). To obtain 1:1

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Ag@ASMs and 4:1 Ag@ASMs, the concentrations of AgNO3 (5 mM, 20 mM) and

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sodium citrate (0.5% wt, 2% wt) were varied as the dosage of the ASM and other

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experimental conditions were kept constant (Experimental details are given in Figure

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S1 and Text S1 of supporting information).

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SERS Measurements. R6G, CV, and melamine were used as model Raman

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probes for SERS measurements. For the preparation of SERS substrates, 50 µL

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different concentrations of R6G ranging from 10–6 M to 10–12 M (10–4 M to 10–9 M for

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CV and melamine) were dropped onto the substrates and dried at room temperature

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(Figure S3), and then SERS signals were recorded using a Raman spectrometer

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(Renishaw inVia, UK) equipped with a power of 0.25 mW of 532 nm He-Ne laser.

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The integral time was 1 s with five scans and a 50× microscopic lens (1 µm2 spot) was

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used.

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Instruments. The morphologies of the samples were observed by SEM (Sirion

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200, FEI Electron Optics Company, U. S. A) and FE-TEM (JEM-2100F, JEOL,

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Japan). STEM images were obtained using a high-angle annular dark-field detector

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(HAADF). The crystallographic phase of the sample was determined by

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multifunctional target XRD (TTR-III, Rigaku, Japan) with Cu Kα radiation (30

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kV/160 mA, λ=1.54056 Å) in a scan range of 20° to 80°. The chemical state and

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surface composition of the samples were analyzed by XPS (ESCALAB250,

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Thermo-VG Scientific, U. K.) using monochromatized Al Kα radiation (1486.92 eV).

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The surface functional groups of the sample were determined by FTIR (Nicolet 8700,

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Thermo Nicolet Corporation, U. S. A). Ultraviolet-visible (UV-vis) diffuse reflection

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spectras

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spectrophotometer (DUV-3700, SHIMADZU, Japan) with a wavelength range from

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200 nm to 800 nm.

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(DRS)

were

recorded

on

a

deep

ultraviolet-visible-near-infrared

DFT Computation. All the calculations were based on density functional theory

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(DFT) and performed with Material Studio modeling Dmol3.19, 20 Given that the silica

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produced by modified Stöber method was amorphous silica, the (100) and (111)

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surfaces of β-cristobalite were used as two possible models of hydroxylated

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amorphous surface.21 The adsorption geometries of APTES molecules at the

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hydroxylated (111) facets of β-cristobalite (Figure S2) were investigated using a

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double-numerical-quality basis set with polarization functions

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GGA-PBE.19, 22 The same functional and basis set were used to APTES-modified

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silica nanoparticles bonded to Ag. The selection of the models and computational

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method were discussed in greater detail in Text S2.

(DNP) and

141 142

RESULTS AND DISCUSSION

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Characterization. Figure 1a displays the FT-IR spectra of the SMs, ASMs, and

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2:1 Ag@ASMs. The peak at 1097 cm–1 was attributed to Si–O–Si asymmetric

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stretching vibrations.23 The presence of bands at around 1449 cm-1 (Figure 1, SM)

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may due to CH2 scissor vibration and CH3 asymmetric bending vibration come from

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the remained TEOS on the surface of silica. The peak at 1635 cm–1 was displayed in

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all FT-IR spectra and can be attributed to adsorbed water molecules. The broad band

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at 3434 cm–1 can be ascribed to –OH or –NH2 stretching vibration.24 The N-H peak at

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1400 cm–1 was observed in the spectra of both the ASMs and Ag@ASMs, indicating

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the successful graft of the amino groups.25 Moreover, two slight peaks at 2925 and

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2853 cm–1, which can be assigned to C–H stretching vibrations in the aminopropyl

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groups, were observed in the spectra of ASMs and Ag@ASMs,26 further verifying the

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successful grafting of the amino groups onto ASMs and Ag@ASMs.

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SEM images showed that SMs were prepared with monodisperse, uniform

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morphology and were about 250 nm in size (Figure 1b). After amino modification, the

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size and morphology of ASMs did not change (Figure 1c), indicating that amino

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modification does not damage the monodispersity and surface morphology of SMs.

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With the in-situ reduction of Ag ions, we found that many NPs were deposited on the

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surface of ASMs, suggesting the successfully formation and distribution of Ag NPs on

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the surface of the silica microspheres (Figure 1d and e).

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SEM images of 1:1 Ag@ASMs, 2:1 Ag@ASMs, 4:1 Ag@ASMs, and 2:1

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Ag@SMs are shown in Figure S4, and indicate that the dispersity of Ag NPs is mainly

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influenced by the amino modification and dosage of Ag ions. As shown in Figure

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S4a-c, the average gap size of Ag NPs in 1:1, 2:1, and 4:1 Ag@ASMs are 52.5, 10.4,

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and 8.2 nm, respectively, which indicates that it is possible to obtain gap-controlled

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Ag NPs on silica microspheres by controlling the concentration of AgNO3. Though

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the average gap size of Ag NPs of 2:1 Ag@SM is slightly greater than that of 4:1

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Ag@ASM, the former exhibited higher enhancement of Raman signal than the latter,

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which may be caused by the aggregation of Ag NPs of 4:1 Ag@ASM. TEM images

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more clearly showed that NPs are distributed on the surface of ASMs (Figure 2a-c and

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Figure S5-8), but cannot form on SMs (Figure 2d), indicating grafted amino groups

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play an important role in the formation of monodispered NPs. Notably, uniform NPs

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are formed on the surface of 2:1 Ag@ASMs (Figure 2b), but only few NPs were

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formed on the surfaces of 1:1 Ag@ASMs and NPs with large sizes aggregated on the

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surface of the 4:1 Ag@ASMs (Figure 2a and c). This demonstrates that the

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morphology and size of Ag NPs are controllable by the amino modification of SMs

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and tuning of the AgNO3 dosage. APTES modification resulted in the replacement by

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amino groups of the abundant hydroxyl groups on the surface of SMs that originated

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during Stöber method synthesis. The Ag ions were adsorbed on the surface of ASMs

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via strong chemical bonding with amino groups, and were then converted to Ag NPs

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by the reduction of sodium citrate. The HR-TEM shows that Ag NPs were embedded

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in the surface of ASMs, suggesting that some Ag ions bonded with the amino groups

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that were grafted into the pores of SMs and then reduced as initial Ag seeds. The Ag

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seeds act as growing nuclei to form Ag NPs.27 The typical HR-TEM image (Figure 2e)

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shows the spherical shape of the Ag NPs of 2:1 Ag@ASM with 20 nm diameter. The

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FFT image presented in Figure 2f shows that the lattice distance is 0.232 nm, which is

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the typical (111) crystal phase of Ag, demonstrating that the NPs are Ag NPs. The

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corresponding particle size distribution histogram of 2:1 Ag@ASMs are displayed in

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Figure 2f, and the average size of Ag NPs is 24.5 nm. The EDS spectrum of 2:1

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Ag@ASM shows that the materials contain Ag element, which is consistent with the

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XRD and XPS results. EDS-mapping in a scanning transmission electron microscope

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revealed the even distribution of Ag on the surface of silica microspheres (Figure 3).

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The XRD patterns of the 2:1 Ag@ASMs and ASMs are shown in Figure S9. A

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broad reflection peak at around 23o can be assigned to the amorphous SiO2.28 The

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crystalline reflection peaks centered at 38.2 o, 44.5 o, 64.5 o and 77.4o correspond to the

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(111), (200), (220), and (311) crystal planes of the face-centered cubic structure of Ag

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NPs. No evidence of Ag oxide formation was observed from the XRD pattern,

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suggesting that the Ag NPs on the surface of ASMs reduced by sodium citrate are

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cheical stable.

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Figure S10 and S11 show the color and UV-vis spectra of SMs colloids (2:1

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Ag@SMs, 1:1 Ag@ASMs, 2:1 Ag@ASMs, and 4:1 Ag@ASMs). The SMs and ASMs

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did not show any absorption in the UV-vis absorption spectra (from 250 nm to 750

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nm), but after the decoration with Ag NPs, an obvious absorption peak was present at

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around 406 nm due to the Mie plasmon resonance excitation from the attached Ag

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NPs.29 As shown in UV-vis spectra, much larger Ag NPs and higher coverage of Ag

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NPs forming on the surfaces of SMs as indicated in Figure 2 caused the position of

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the plasmon resonance peak to gradually red-shift and become more broad.

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Figure S12 shows the chemical analysis of the elements in the 2:1 Ag@ASMs as

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determined by XPS. As shown in Figure S12b, the two peaks at 368.4 and 374.4 eV

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with a spin-orbit separation of 6.0 eV, correspond to the binding energies of Ag 3d5/2

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and Ag 3d3/2. These two characteristic peaks of Ag 3d5/2 and Ag 3d3/2 are attributed

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to the Ag0 species,30 which clearly indicates the presence of zero-valent metallic Ag

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NPs and is consistent with the XRD results. The energy spectrum of electron Si 2p

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reveals two kinds of peaks (Figure S12c). The peak at 102.6 and 103.4 eV are

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ascribed to the binding energy value of Si–C and Si–O, respectively.31 The peak of O

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1s (Figure S12d) could be deconvoluted into two peaks with binding energies of 532.7

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and 533.4 eV, similar to those of Si–O and Si–OH, respectively.31,

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spectrum showing peaks (Figure S12e) assigned to hydrogen bonded NH2 (402.4 eV)

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The N 1s

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and C-NH2 (NH2 terminal groups on APTES) at 399.6 eV.33 The peak at 401.6 eV is

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likely due to the interactions between nitrogen atoms and Ag ions.24 The relative

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content of major elements is listed in Table S1, and about 10.4% of Ag NPs are on the

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surface of Ag@ASMs.

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Ag@ASMs as a Sensitive SERS Substrate. An aqueous solution of R6G at a

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concentration of 10–7 M was chosen to evaluate the SERS performance of Ag@ASMs

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(Figure 4a). The peaks from 500 to 1700 cm–1 are attributed to R6G signals. The

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peaks at 609 cm–1 is assigned to the C–C–C ring in-plane bending, the peak at 1126

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and 1180 cm–1 are attributed to C–H in-plane bending, C–O–C stretching show peak

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at 1311 cm–1, vibrations 1369, 1507, 1574, and 1647 cm–1 are assigned to C–C

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stretching of the aromatic ring, and the peak at 772 cm–1 is due to out-of-plane

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bending motion of the hydrogen atoms of the xanthene skeleton.34 Clearly, all four

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samples show the R6G characteristic bands, and the 2:1 Ag@ASMs sample exhibited

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the best performance among all the samples.

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A series of SERS spectra of R6G at concentrations ranging from 10–6 M to 10–12

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M were obtained to demonstrate the sensitivity of the 2:1 Ag@ASMs. As Figure 4b

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shows, the Raman peaks from 500 to 1700 cm–1 are in good agreement with the above

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description of R6G characteristic peaks, and the Raman spectral intensity of R6G

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decreased with decreasing concentration. Even at a concentration as low as 10–12 M,

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all enhancement peaks could be observed clearly, as shown in the inset of Figure 4b.

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Therefore, the 2:1 Ag@ASMs can achieve the detection of analytes. The correlations

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between SERS intensity and R6G concentration (negative logarithmic relationship)

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are listed in Figure 4c. The linear relationship exists in the concentration ranging from

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10–6 M to 10–12 M, with a correlation coefficient of 0.965. We calculated the limit of

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detection (LOD) with the signal-to-noise ratio of 3,45 and the LOD of R6G is 3.2×

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10-14 M (details shown in Text S3).

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The homogeneity of Raman signals was investigated by considering randomly

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sites on the SERS substrate. As seen in Figure 4d, the SERS spectra of 10–7 M R6G at

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10 random sites on the substrate showed good uniformity in intensity. The Raman

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peak intensities of R6G at 1369 cm–1 from 10 random sites were recorded as shown in

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Figure 4e, and the average relative standard deviation (RSD) of intensities at 1369

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cm–1 was about 7.61%. The sensitivity, reproducibility, and reliability of the 2:1

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Ag@ASMs exceed that of most previously reported SERS substrates (Table 1). The

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Raman enhancement factor (EF) was used to quantitatively evaluate the SERS

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activity of the prepared substrate, and the detailed calculation can be found in Text S4.

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The calculated EF of the 2:1 Ag@ASMs substrate is about 6.36×107 (collect the

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normal Raman signal based on 10-4 M R6G).

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SERS Substrate for CV and Melatine. The prepared 2:1 Ag@ASMs were next

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applied for the detection of other trace chemicals harmful to humans and the

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environment. Crystal violet (CV), is a typical triphenylmethane dye, and is widely

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used in various textile industries, as well as human and veterinary medicine as a

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biological stain.46 CV is carcinogenic and has been classified as a recalcitrant

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molecule since it is poorly metabolized by microbes, is non-biodegradable, and can

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persist in a variety of environments.47 Figure 5a shows the SERS spectra of CV with

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concentrations from 10–4 to 10–9 M. The Raman peaks of CV located at 422, 909,

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1177, 1376, 1588 and 1617 cm–1 are attributed to CV signals based on previous

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reports, the peak at 422 cm–1 is due to phenyl-C-phenyl out-of-plane antisymmentric

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vibrations, the peak at 909 cm–1 is attributed to phenyl ring breathing mode, 1171,

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1588, and 1617 cm–1 are assigned to C-phenyl and C-H in-plane antisymmetric

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stretching, and 1376 cm–1 is related to C-N and phenyl-C-phenyl antisymmetric

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stretching.48 The signals of CV can be recognized clearly, even at the 10-9 M level.

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The relationship between SERS intensity at 1171 cm-1 and concentrations of CV is

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also shown in Figure 5b, shows good linear correlation.

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Melamine is a commonly TPOP, and long-term accumulation may cause the

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formation of kidney stones, acute renal failure, and even infant death.49 The US Food

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and Drug Administration (FDA) set a safety limit (1 ppm) for melamine in infant

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formula.50 Figure 5c shows the SERS spectra of melamine at concentrations ranging

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from 10–4 to 10–9 M on the substrate. The peak at 701 cm–1 is attributed to ring

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breathing mode II and involves in-plane deformation of the triazine ring in melamine

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molecules.49 The SERS signals of 10–8 M melamine can be seen clearly, the LOD of

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melamine (1.5×10-9 M) using the 2:1 Ag@ASMs substrate the is lower than the FDA

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standards (1 ppm).51 A good linear relationship (correlation coefficient of 0.998) was

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observed between peak intensity and the concentration, over the range of 10–4 to 10–8

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M (Figure 5d).

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The Mechanisms of SERS Enhancement. The SERS enhancement was mainly

285

attributed to the occurrence of an electromagnetic field in the nanostructures on the

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metal surface associated with LSPR (Figure 6). The gap between two closely spaced

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metallic NPs (at a distance < 10 nm) is called hot-spot, and a Raman signal

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enhancement by electromagnetic coupling occurs when a SERS active molecule is

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positioned within the hot-spots.8, 52 The silver content and the number of Ag NPs

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increase with Ag ions concentration, leading to the formation of a large number of

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active sites that may form high density hot-spots to enhance the Raman signal.36 In

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addition, the particle size of metal NPs may affect the SERS enhancement, and

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smaller metallic NPs give higher enhancement. However, for particle size less than 15

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nm, this enhancement effect reaches saturation.53 Comparison of the HR-TEM images

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and the corresponding particle size distribution histogram (Figure 2f and Figure S7d)

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show that although the size of the Ag NPs of the 1:1 Ag@ASMs (17.8 nm) was

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smaller than that of the 2:1 Ag@ASMs (24.5 nm), the Ag NP coverage density of the

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2:1 Ag@ASMs was much greater than that of the 1:1 Ag@ASMs, thus offering more

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hot-spots to position R6G molecules. Form SEM image (Figure S5), the average

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interparticle gaps of Ag NPs on 2:1 Ag@ASM (11.3 ± 3.2 nm) is near the hot-spot

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distance. Furthermore, the clusters of Ag NPs grown out of the silica surface can also

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generate hot-spots. In the 4:1 Ag@ASMs, although the average interparticle gaps of

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Ag NPs satisfies the condition of less than 10 nm, the aggregated Ag NPs with large

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sizes were not conducive to the formation of hot-spots, and resulted in inferior SERS

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performance.

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DFT computation provided another evidence of the formation of suitable gaps

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between Ag NPs. As shown in Figure 7, five possible orientations of APTES molecule

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attached to silanol-terminated silica were proposed, the most likely and fundamental

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orientation (Figure 7e) was selected as computational model, see the Text S2 for full

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details. The single APTES molecule was chemisorbed with one Si-O-Si covalent

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bonds (Figure 8a and b), implying the loss of one ethoxy groups. When only one

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Si-O-Si bond forms, the APTES molecule lie horizontally on the surface because of

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the formation of a hydrogen bond between the N of the APTES and a H of the surface

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hydroxyl group, and the distance of OH … N is 1.38 Å. The distance of a

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donor-acceptor N…O can comparable with the length of charge-assisted bonds in

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charged groups of metalorganic complexes.54 The binding energy is positive (1.45 eV),

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indicating that APTES absorption is favorable on β-cristobalite (111) surface.55 To

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better simulate the adsorption, a second APTES molecule was attached to

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β-cristobalite (111) surface. In the most stable structure (Figure 8c and d), another

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APTES molecule also lies on surface, and binding energy is 2.49 eV. A similar

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hydrogen bond was found between the N atom of the second APTES molecule and an

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H atom of the adjacent hydroxyl group, and the distance of hydrogen bond (N…H)

323

was 1.53 Å. The two APTES molecules were interleaved with space on the

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β-cristobalite (111) surface, and the distance between two N atoms was 10.84 Å.

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These results implied that adjacent APTES molecules modified on the silica surface

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were interspaced.

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The Ag ions were anchored on silica surface by complexation between Ag ions

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and amino groups of APTES molecule. For simplicity, we assumed that silica

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nanoparticle only slightly affected the interaction of Ag-APTES, more details were

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discussed in Text S2. In order to investigate the growth patterns of Ag atoms on

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APTES molecule, the geometric structure of Agn-APTES (n=2, 3) were further

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calculated. The first Ag atom was attached to N atom and located in the opposite side

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of two H atoms which due to the resistance of H atoms, as shown in Figure 8e. The

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distance of Ag atom and N atom is 2.57 Å, and the adsorption energy is -0.34 eV,

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which suggests a stable adsorption. By geometry optimization, the structure that the

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second Ag atom attached to the first Ag atom is most stable when the second Ag atom

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was put into the adsorption system. The third Ag atom combined with previous two

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Ag atoms to form a triangle. Similar results can be found in other studies.56 Ead (Table

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S2) becomes more negative with the increasing of the number of Ag atoms, and the

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distance of Ag atom and N atom grows shorter, which means that subsequent Ag

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atoms prefer to adsorption on the previous Ag atoms to form cluster rather than

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combined with N atom.

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The gap between two adjacent Ag NPs embedded in silica surface (Figure 2b and

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e) was far greater than the distance between two N atoms from two adjacent APTES

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molecules, but the contrast between experiment and theoretical results is

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understandable. The theoretical model is based on the assumption that each active –

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O-Si site (one half of the total –O-Si sites based on the DFT configuration) was

348

occupied by APTES molecule, but the actual concentration of APTES is limited,

349

resulting in the unoccupied –O-Si sites. Therefore, the gap between the APTES

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molecules is much higher than the minimum distance (10.84 Å). Similarly, not every

351

modified APTES can absorb Ag ions, and further extends the distance between Ag

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NPs. Actually, the density and size of Ag NPs were controlled by the Ag ions

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concentration, as shown in the Figure 2. Although the deviation exists in theoretical

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and experimental results, the DFT calculation still provide important information,

355

such as the adsorption mechanism between APTES and silica surface and the growth

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patterns of Ag atoms on APTES molecule.

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Besides the suitable gaps, a contribution is also made by the chemical effect

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involving a charge-transfer between the analytes and the metal surface (Figure 6). The

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charge transfer model of the “bridge” system may be applicable to the present

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nanosystem.9, 57 In this case, a dynamic charge-transfer between Ag NPs could occur

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by coupling with the vibrations of the bridging molecules, since the energy levels of

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the Ag NPs match the HOMO and LUMO energy levels of the molecules under

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excitation by light.57 Compared with the 1:1 and 4:1 Ag@ASMs, the Ag NPs of 2:1

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Ag@ASMs appeared to be more uniform and were tightly adhered on the surface of

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the SMs, allowing increased contact with R6G molecules and facilitating

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charge-transfer.9 In addition, the graft of amino groups on SMs significantly improves

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the monodispersity of Ag NPs, and also facilitates the charge-transferand formation of

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hot-spots. Thus, the SERS signals of 2:1 Ag@ASMs are stronger than those of 2:1

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Ag@SMs.

370 371

CONCLUSIONS

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In summary, we successfully synthesized Ag@ASMs as an active SERS

373

detection substrate by in situ loading Ag nanoparticles onto the surface of ASMs. The

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gaps in or between evenly distributed Ag NPs provide sufficient hot-spots when used

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as a SERS substrate. The 2:1 Ag@ASMs were used as a SERS substrate to detect

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R6G with concentration down to 10–12 M with an EF of 6.36×107 (collect the normal

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Raman signal based on 10-4 M R6G). The SERS substrate can also produce highly

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enhanced Raman signals with good uniformity and reproducibility for the detection of

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CV and melamine at below nanomolar concentrations.

380 381

382

ASSOCIATED CONTENTS

383

Supporting Information Available. Text S1-S4, Tables S1 and S2, and Figures

384

S1-S14 are provided in the supporting information. These materials are available free

385

of charge via the Internet at http://pubs.acs.org.

386 387

AUTHOR INFORMATION

388

Corresponding Author

389

*Fax: +86-551-63607482; e-mail: [email protected].

390

Notes

391

The authors declare no competing financial interest.

392

ACKNOWLEDGEMENTS

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The authors gratefully acknowledge financial support from National Natural Science

394

Foundation of China (21677138), Program for Changjiang Scholars and Innovative

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Research Team in University “PCSIRT, the Key Special Program on the S&T for the

396

Pollution Control, and Treatment of Water Bodies (No.2012ZX07103-001).

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REFERENCES

399

(1) Alsbaiee, A.; Smith, B. J.; Xiao, L. L.; Ling, Y. H.; Helbling, D. E.; Dichtel, W. R.

400

Rapid removal of organic micropollutants from water by a porous β-cyclodextrin

401

polymer. Nature 2016, 529, 190-194.

402

(2) Neale, P. A.; Ait-Aissa, S.; Brack, W.; Creusot, N.; Denison, M. S.; Deutschmann,

403

B.; Hilscherová, K.; Hollert, H.; Krauss, M.; Novák, J.; Schulze, T.; Seiler, T. B.;

404

Serra, H.; Shao, Y.; Escher, B. I. Linking in Vitro Effects and Detected Organic

405

Micropollutants in Surface Water Using Mixture-Toxicity Modeling. Environ. Sci.

406

Technol. 2015, 49, 14614–14624.

407

(3) Zhang, D. X.; Xu, N.; Li, H. D.; Yao, Q. C.; Xu, F.; Fan, J. L.; Du, J. J.; Peng, X. J.

408

Probing Thiophenol Pollutant in Solutions and Cells with BODIPY-Based Fluorescent

409

Probe. Ind. Eng. Chem. Res. 2017, 56, 9303–9309.

410

(4) Hu, L. S.; Fong, C. C.; Zhang, X. M.; Chan, L. L.; Lam, P. K. S.; Chu, P. K.; Wong,

411

K. Y.; Yang, M. S. Au Nanoparticles Decorated TiO2 Nanotube Arrays as a Recyclable

412

Sensor for Photoenhanced Electrochemical Detection of Bisphenol A. Environ. Sci.

413

Technol. 2016, 50, 4430-4438.

414

(5) He, Y. Z.; Priestley, R. D.; Liu, R. A One-Step and Scalable Continuous-Flow

415

Nanoprecipitation for Catalytic Reduction of Organic Pollutants in Water. Ind. Eng.

416

Chem. Res. 2016, 55, 9851–9856.

417

(6) Yu, F.; Kuppe, C.; Valev, V. K.; Fu, H. B.; Zhang, L. W.; Chen, J. M. Surface

418

Enhanced Raman Spectroscopy: a Facile and Rapid Method for the Chemical

419

Components Study of Individual Atmospheric Aerosol. Environ. Sci. Technol. 2017,

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

420

51, 6260-6267.

421

(7) Anantharaj, S.; Nithiyanantham, U.; Ede, S. R.; Kundu, S. Osmium Organosol on

422

DNA: Application in Catalytic Hydrogenation Reaction and in SERS Studies. Ind.

423

Eng. Chem. Res. 2014, 53, 19228-19238.

424

(8) Zhu Y.; Jiang, X. X.; Wang H. Y.; Wang S. Y.; Wang H.; Sun B.; Su Y. Y.; He Y. A

425

Poly Adenine-Mediated Assembly Strategy for Designing Surface-Enhanced

426

Resonance Raman Scattering Substrates in Controllable Manners. Anal. Chem. 2015,

427

87, 6631-6638.

428

(9) Lin, J.; Shang, Y.; Li, X. X.; Yu, J.; Wang, X. T.; Guo, L. Ultrasensitive SERS

429

Detection by Defect Engineering on Single Cu2O Superstructure Particle. Adv. Mater.

430

2017, 29, 1604797-1604804.

431

(10) Liu, H. L.; Yang, Z. L.; Meng, L. Y.; Sun, Y. D.; Wang, J.; Yang, L. B.; Liu, J. H.;

432

Tian, Z. Q. Three-Dimensional and Time-Ordered Surface-Enhanced Raman

433

Scattering Hotspot Matrix. J. Am. Chem. Soc. 2014, 136, 5332-5341.

434

(11) Braun G.; Pavel L.; Morrill A. R.; Seferos D. S.; Bazan G. C.; Reich N. O.;

435

Moskovits M. Chemically Patterned Microspheres for Controlled Nanoparticle

436

Assembly in the Construction of SERS Hot Spots. J. Am. Chem. Soc. 2007, 129,

437

7760-7761.

438

(12) Wang, C. W.; Wang, J. F.; Li, P.; Rong, Z.; Jia, X. F.; Ma, Q. L.; Xiao, R.; Wang,

439

S. Q. Sonochemical synthesis of highly branched flower-like Fe3O4@SiO2@Ag

440

microcomposites and their application as versatile SERS substrates. Nanoscale 2016,

441

8, 19816-19828.

ACS Paragon Plus Environment

Page 22 of 41

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

442

(13) Dai, Z. G.; Mei, F.; Xiao, X. H.; Liao, L.; Wang, J.; Wu, W.; Guo, S. S.; Zhao, X.

443

Y.; Ren, F.; Jiang, C. Z. “Rings of saturn-like” nanoarrays with high number density

444

of hot spots for surface-enhanced Raman scattering. Appl. Phys. Lett. 2014, 105,

445

033515.

446

(14) Zhao, Y. Q.; Zhang, Y. L.; Huang, J. A.; Zhang, Z. Y.; Chen, X. F.; Zhang, W. J.

447

Plasmonic nanopillar array embedded microfluidic chips: An in situ SERS monitoring

448

platform. J. Mater. Chem. A 2015, 3, 6408-6413.

449

(15) Chen, J.; Qin, G. W.; Shen, W.; Li, Y. Y.; Das, B. Fabrication of long-range

450

ordered, broccoli-like SERS arrays and application in detecting endocrine disrupting

451

chemicals. J. Mater. Chem. C 2014, 3, 1309-1318.

452

(16) Niu, C. Y.; Zou, B. F.; Wang, Y. Q.; Cheng, L.; Zheng, H. L.; Zhou, S. M. Highly

453

Sensitive and Reproducible SERS Performance from Uniform Film Assembled by

454

Magnetic Noble Metal Composite Microspheres. Langmuir 2016, 32, 858-863.

455

(17) Yang, G. X.; Jiang, H. Amino modification of biochar for enhanced adsorption of

456

copper ions from synthetic wastewater. Water Res. 2014, 48, 396-405.

457

(18) Chen, Y. L.; Chen, S. Q.; Tian, K.; Chen, J. J.; Jiang, H. Ultra-high capacity and

458

selective immobilization of Pb through crystal growth of hydroxypyromorphite on

459

amino-functionalized hydrochar. J. Mater. Chem. A 2015, 3, 9843-9850.

460

(19) Becke, A. D. Density-functional exchange-energy approximation with correct

461

asymptotic behavior. Phys.Rev. A 1988, 38, 3098-3100.

462

(20) Delley, B. From molecules to solids with the DMol3 approach. J. Chem. Phys.

463

2000, 113, 7756-7764.

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

464

(21) Iarlori, S.; Ceresoli, D.; Bernasconi, M.; Donadio, D.; Parrinello, M.

465

Dehydroxylation and silanization of the surfaces of β-cristobalite silica: an ab initio

466

simulation. J. Phys. Chem. B 2001, 105, 8007-8013.

467

(22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation

468

made simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

469

(23) Li, L.; Gu, W. Y.; Liu, J.; Yan, S. Y.; Xu, Z. P. Amine-functionalized SiO2

470

nanodot-coated layered double hydroxide nanocomposites for enhanced gene delivery.

471

Nano Res. 2015, 8, 682-694.

472

(24) Liu, X. Q.; Chen, W. J.; Jiang, H. Facile synthesis of Ag/Ag3PO4/AMB

473

composite with improved photocatalytic performance. Chem. Eng. J. 2017, 308,

474

889-896.

475

(25) Feng, H.; Wang, N. Y.; Thanhthuy, T. T.; Yuan, L. J.; Li, J. Z.; Cai, Q. Y. Surface

476

molecular imprinting on dye–(NH2)–SiO2 NPs for specific recognition and direct

477

fluorescent quantification of perfluorooctane sulfonate. Sensors Actuat. B Chem. 2014,

478

195, 266-273.

479

(26) Oh, J. M.; Choi, S. J.; Lee, G. E.; Han, S. H.; Choy, J. H. Inorganic Drug‐

480

Delivery Nanovehicle Conjugated with Cancer‐Cell‐Specific Ligand. Adv.Funct.

481

Mater. 2009, 19, 1617-1624.

482

(27) Chen, K. H.; Pu, Y. C.; Chang, K. D.; Liang, Y. F.; Liu, C. M.; Yeh, J. W.; Shih,

483

H.; Hsu, Y. J. Ag-Nanoparticle-Decorated SiO2 Nanospheres Exhibiting Remarkable

484

Plasmon-Mediated Photocatalytic Properties. J. Phys. Chem. C 2012, 116,

485

19039-19045.

ACS Paragon Plus Environment

Page 24 of 41

Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

486

(28) Xu, C.; Li, W. J.; Wei, Y. M.; Cui, X. Y. Characterization of SiO2/Ag composite

487

particles synthesized by in situ reduction and its application in electrically conductive

488

adhesives. Mater. Design 2015, 83, 745-752.

489

(29) Kim,

490

Surface-Enhanced-Raman-Scattering-Active Ag Nanostructures on Silica Spheres.

491

Langmuir 2006, 22, 8083-8088.

492

(30) Ding, J; Zhu, J. W.; Yao, P. C.; Li, J.; Bi, H. P.; Wang, X. Synthesis of ZnO-Ag

493

Hybrids and Their Gas-Sensing Performance toward Ethanol. Ind. Eng. Chem. Res.

494

2015, 54, 8947–8953.

495

(31) Binner, J.; Zhang, Y. Characterization of silicon carbide and silicon powders by

496

XPS and zeta potential measurement. J. Mater. Sci. Lett. 2001, 20, 123-126.

497

(32) Jeong, S. J.; Xia, G. D.; Kim, B. H.; Dong, O. S.; Kwon, S. H.; Kang, S. W.;

498

Sang, O. K. Universal Block Copolymer Lithography for Metals, Semiconductors,

499

Ceramics, and Polymers. Adv. Mater. 2008, 20, 1898-1904.

500

(33) Talavera-Pech, W. A.; Esparza-Ruiz, A.; Quintana-Owen, P.; Vilchis-Nestor, A.

501

R.; Carrera-Figueiras, C.; Ávila-Ortega, A. Effects of different amounts of APTES on

502

physicochemical and structural properties of amino-functionalized MCM-41-MSNs. J.

503

Sol-Gel Sci. Techn. 2016, 80, 697-708.

504

(34) Zhang, C.; Jiang, S. Z.; Huo, Y. Y.; Liu, A. H.; Xu, S. C.; Liu, X. Y.; Sun, Z. C.;

505

Xu, Y. Y.; Li, Z.; Man, B. Y. SERS detection of R6G based on a novel graphene

506

oxide/silver nanoparticles/silicon pyramid arrays structure. Opt. Express 2015, 19,

507

24811-24821.

K.; Kim, H.

S.; Park,

H. K.

Facile Method To

ACS Paragon Plus Environment

Prepare

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

508

(35) Zhang, K. B.; Zeng, T. X.; Tan, X. L.; Wu, W. D.; Tang, Y. J.; Zhang, H. B. A

509

facile surface-enhanced Raman scattering (SERS) detection of rhodamine 6G and

510

crystal violet using Au nanoparticle substrates. Appl. Surf. Sci. 2015, 347, 569-573.

511

(36) Zhang, M. F.; Zhao, A. W.; Wang, D. P.; Sun, H. H. Hierarchically assembled

512

NiCo@SiO2@Ag magnetic core-shell microspheres as highly efficient and recyclable

513

3D SERS substrates. Analyst 2014, 140, 440-448.

514

(37) Huang, J. A.; Zhang, Y. L.; Zhao, Y. Q.; Zhang, X. L.; Sun, M. L.; Zhang, W. J.

515

Superhydrophobic SERS chip based on a Ag coated natural taro-leaf. Nanoscale 2016,

516

8, 11487-11493.

517

(38) Liu, B. H.; Han, G. M.; Zhang, Z. P.; Liu, R. Y.; Jiang, C. L.; Wang, S. H.; Han,

518

M. Y. Shell Thickness-Dependent Raman Enhancement for Rapid Identification and

519

Detection of Pesticide Residues at Fruit Peels. Anal. Chem. 2011, 84, 255-261.

520

(39) Liu, J.; Meng, G. W.; Li, X. D.; Huang, Z. L. Ag-Nanoparticle-Decorated Ge

521

Nanocap Arrays Protruding from Porous Anodic Aluminum Oxide as Sensitive and

522

Reproducible Surface-Enhanced Raman Scattering Substrates. Langmuir 2014, 30,

523

13964-13969.

524

(40) Zhang, C.; Jiang, S. Z.; Yang, C.; Li, C. H.; Huo, Y. Y.; Liu, X. Y.; Liu, A. H.;

525

Wei, Q.; Gao, S. S.; Gao, X. G.; Man, B. Y. Gold@silver bimetal

526

nanoparticles/pyramidal silicon 3D substrate with high reproducibility for

527

high-performance SERS. Sci. Rep. 2016, 6, 25243-25251.

528

(41) Zhong, L. B.; Yin, J.; Zheng, Y. M.; Liu, Q.; Cheng, X. X.; Luo, F. H.

529

Self-Assembly of Au Nanoparticles on PMMA Template as Flexible, Transparent, and

ACS Paragon Plus Environment

Page 26 of 41

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

530

Highly Active SERS Substrates. Anal. Chem. 2014, 86, 6262-6267.

531

(42) Mondal, S.; Rana, U.; Malik, S. Facile Decoration of Polyaniline Fiber with Ag

532

Nanoparticles for Recyclable SERS Substrate. ACS Appl. Mater. Interfaces 2015, 7,

533

10457-10465.

534

(43) Fan, W.; Lee, Y. H.; Pedireddy, S.; Zhang, Q.; Liu, T. X.; Ling, X. Y. Graphene

535

oxide

536

single-particle surface-enhanced Raman scattering (SERS) sensing. Nanoscale 2014,

537

6, 4843-4851.

538

(44) Xu, S. C.; Jiang, S. Z.; Wang, J. H.; Wei, J.; Yue, W. W.; Ma, Y. Graphene

539

isolated Au nanoparticle arrays with high reproducibility for high-performance

540

surface-enhanced Raman scattering. Sens. Actuators B Chem. 2016, 222, 1175-1183.

541

(45) Zhu, C. H.; Meng, G. W.; Zheng, P.; Huang, Q.; Li, Z. H.; Hu, X. Y.; Wang, X. J.;

542

Huang, Z. L.; Li, F. D.; Wu, N. Q. A Hierarchically Ordered Array of Silver-Nanorod

543

Bundles for Surface-Enhanced Raman Scattering Detection of Phenolic Pollutants.

544

Adv. Mater. 2016, 28, 4871-4876.

545

(46) Singh, K. P.; Gupta, S.; Singh, A. K.; Sinha, S. Optimizing adsorption of crystal

546

violet dye from water by magnetic nanocomposite using response surface modeling

547

approach. J. Hazard. Mater. 2011, 186, 1462-1473.

548

(47) Liu, X. Y.; Zhang, T. T.; Xu, D.; Zhang, L. Microwave-Assisted Catalytic

549

Degradation of Crystal Violet with

550

Res. 2016, 55, 11869-11877.

551

(48) Zhang, C.; Li, C. H.; Yu, J.; Jiang, S. Z.; Xu, S. C.; Yang, C.; Liu, Y. J.; Gao, X.

and

shape-controlled

silver

nanoparticle

hybrids

for

ultrasensitive

Barium Ferrite Nanomaterial. Ind. Eng. Chem.

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G.; Liu, A. H.; Man, B. Y. SERS activated platform with three-dimensional hot spots

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and tunable nanometer gap. Sens. Actuators B 2018, 258, 163-171.

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(49) Wu, M. T.; Wu, C. F.; Chen, B. H. Behavioral Intervention and Decreased Daily

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Melamine Exposure from Melamine Tableware. Environ. Sci. Technol. 2015, 49,

556

9964-70.

557

(50) Hu, H. B.; Wang, Z. H.; Pan, L.; Zhao, S. P.; Zhu, S. Y. Ag-Coated Fe3O4@SiO2

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Three-Ply Composite Microspheres: Synthesis, Characterization, and Application in

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Detecting Melamine with Their Surface-Enhanced Raman Scattering. J. Phys. Chem.

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C 2010, 114, 7738-7742.

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(51) Hakonen, A.; Svedendahl, M.; Ogier, R.; Yang, Z. J.; Lodewijks, K.; Verre, R.;

562

Shegai, T.; Andersson, P. O.; Käll, M., Dimer-on-mirror SERS substrates with

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attogram sensitivity fabricated by colloidal lithography. Nanoscale 2015, 7,

564

9405-9410.

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(52) Radziuk D.; Schuetz R.; Masic A.; Moehwald H. Chemical imaging of live

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fibroblasts by SERS effective nanofilm. Phys. Chem. Chem. Phys. 2014, 16,

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24621-25634.

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(53) García-Vidal, F. J.; Pendry, J. B., Collective Theory for Surface Enhanced Raman

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Scattering. Phys. Rev. Lett. 1996, 77, 1163-1166.

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(54) Macetti, G.; Rizzato, S.; Beghi, F.; Silvestrini, L.; Presti, L. L. On the molecular

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basis of the activity of the antimalarial drug chloroquine: EXAFS-assisted DFT

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evidence of a direct Fe–N bond with free heme in solution. Phys. Scr. 2016, 91,

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023001-023013.

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(55) Dalod, A. R.; Grendal, O. G.; Skjærvø, S. L.; Inzani, K.; Selbach, S. M.;

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Henriksen, L.; Beek, W. V.; Grande, T.; Einarsrud, M. A. Controlling Oriented

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Attachment and in Situ Functionalization of TiO2 Nanoparticles During Hydrothermal

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Synthesis with APTES. J. Phys. Chem. C 2017, 121, 11897-11906.

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(56) Sun, Y. Y.; Yanagisawa, M.; Kunimoto, M.; Nakamura, M.; Homma, T. Depth

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profiling of APTES self-assembled monolayers using surface-enhanced confocal

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Raman microspectroscopy. Spectrochim. Acta A. 2017, 184, 1-6.

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(57) Zhou, Q.; Li, X. W.; Fan, Q.; Zhang, X. X.; Zheng, J. W. Charge Transfer

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between Metal Nanoparticles Interconnected with a Functionalized Molecule Probed

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by Surface‐Enhanced Raman Spectroscopy. Angew. Chem. Int. Edit. 2006, 118,

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4074-4077.

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Figure captions

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Figure 1. (a) FT-IR spectra of the SMs, ASMs, and 2:1 [email protected] image of (b)

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SMs; (c) ASMs; (d and e) 2:1 Ag@ASMs.

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Figure 2. TEM image of (a) 1:1 Ag@ASMs; (b) 2:1 Ag@ASMs; (c) 4:1 Ag@ASMs;

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(d) 2:1 Ag@SMs; (e) HR-TEM image of 2:1 Ag@ASM. The inset is fast Fourier

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transform (FFT) image; (f) the corresponding particle size distribution histogram of

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2:1 Ag@ASMs.

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Figure 3. (a) Energy dispersive X-ray Spectroscopy (EDS) of corresponding area in

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inset TEM image; (b) Scanning transmission electron modes-High angle annular dark

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field (STEM-HADDF) image of 2:1 Ag@ASM and the corresponding elemental

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mapping of Si (c), O (d), and Ag (e).

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Figure 4. (a) SERS spectra of R6G adsorbed on the as-prepared samples with

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different morphologies shown in Fig. 2 at R6G concentrations of 10-7 M. (b) SERS

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spectra of R6G adsorbed on the 2:1 Ag@ASMs with different concentrations. The

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inset is a magnified curve of the SERS spectrs at 10–12 M. (c) The relationship

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between the intensity at 1369 cm–1 and R6G concentrations. (d) A series of SERS

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spectra of R6G (10–7 M) collected from 10 random sites by using the 2:1 Ag@ASMs

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as SERS substrate. (e) The intensity at 1369 cm–1 of R6G (10–7 M) collected from 10

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random sites by using the 2:1 Ag@ASMs as SERS substrate.

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Figure 5. (a) SERS spectra of CV adsorbed on the 2:1 Ag@ASMs with different

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concentrations; (b) The relationship between the intensity at 1171 cm–1 and CV

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concentrations; (c) SERS spectra of melamine adsorbed on the 2:1 Ag@ASMs with

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different concentrations; (d) The relationship between the intensity at 701 cm–1 and

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melamine concentrations.

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Figure 6. Schematic diagram illustrating the enhancement mechanism of Ag@ASM.

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Figure 7. Possible orientation of the APTES molecules attached to the

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silanol-terminated silica through the silanization reaction: (a) the NH2 group of the

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APTES molecule attachment to the silica surface; (b) reaction of all three ethoxy

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groups with the silanol-terminated silica; (c) cross-linking of the APTES molecules;

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(d) reaction of two of the ethoxy groups with the silanol-terminated silica; (e) reaction

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of only one of the ethoxy groups with the silanol-terminated silica.

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Figure 8. Side (a) and top (b) views of one APTES molecule on β-cristobalite (111)

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surface; Side (c) and bottom (d) views of two APTES molecules on β-cristobalite (111)

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surface. (e) The optimized structure of Agn-APTES (n=1-3) Color code: Si (yellow),

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O (red), N (deep bule), H (white), C (gray) and Ag (light blue). The unit of length is

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Å.

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Table List

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Page 32 of 41

Table 1. The SERS performance of reported SERS substrates. Samples

Detection limit –6

RSD

EF

Reference

Au colloid deposited PS beads

10 M R6G aqueous solution

Not available

Not available

35

NiCo@SiO2@Ag magnetic core–shell microspheres

10–7 M p-ATP ethanol solution

11.3%

3.0×106

36

Ag coated natural taro-leaf

10–8 M R6G aqueous solution

9.7%

106

37

Silver-coated gold nanoparticles (Au@Ag NPs)

10–9 M thiam aqueous solution

Not available

Not available

38

Ag-Nanoparticle-Decorate d Ge Nanocap Arrays

10–10 M p-ATP ethanol solution

Not available

3.2×106

39

Au@Ag/3D-pyramidal Si substrate

10–8 M R6G aqueous solution

6%

1.2×109

40

Au nanoparticles on PMMA template

10–10 M MGITC ethanol solution

Not available

2.4×107

41

Polyaniline fiber@Ag composite

10–10 M R6G aqueous solution

Not available

1.2×109

42

Ag octahedron @Graphene oxide

10–10 M R6G aqueous solution

Not available

Not available

43

Graphene oxide /Au NP arrays

10–11 M R6G aqueous solution

6.9%

4.8×107

44

Ag@ASMs composite microspheres

10–12 M R6G aqueous solution

7.6%

6.36×107

This work

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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TOC

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