Preparation of Gap-Controlled Monodispersed Ag Nanoparticles by

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Materials and Interfaces

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

ACS Paragon Plus Environment

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2

ABSTRACT

3

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

4

detection of trace poisonous organic pollutants is important for environmental

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

functional

14

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

16

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;

20

silica microspheres; Ag nanoparticles; DFT.

21


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22

INTRODUCTION

23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

techniques for TPOPs.

35

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

36

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

37

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

38

single molecule level by identification of the specific information for particular

39

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

40

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

41

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

42

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

43

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



44

electromagnetic enhancement generated by the excitation of localized surface

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

combined photolithography and nanosphere-lithography technique, which can

58

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

59

structures often requires expensive and complicated electron beam lithography,

60

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

61

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

62

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

63

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

64

substrate with excellent sensitivity and reproducibility.16

65

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


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66

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

67

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

68

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

69

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

70

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

71

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

72

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

73

homogeneity and immobility of Ag NPs can be significantly improved.

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy

83

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

84

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

85

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

86

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

87

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



88

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

89

substrates.

90

EXPERIMENTAL SECTION

91

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

92

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

93

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

94

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

95

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

96

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

97

Reagent Co., Ltd, China.

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

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

106

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

107

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

108

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

109

S1 and Text S1 of supporting information).


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110

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

111

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

112

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

113

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

114

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

115

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

116

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

117

used.

118

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

119

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

120

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

121

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

122

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

123

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

124

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

125

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

126

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

127

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

128

spectras

129

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

130

200 nm to 800 nm.

131

(DRS)

were

recorded

on

a

deep

ultraviolet-visible-near-infrared

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



Page 8 of 41

132

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

133

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

134

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

135

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

136

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

137

double-numerical-quality basis set with polarization functions

138

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

139

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

140

method were discussed in greater detail in Text S2.

(DNP) and

141 142

RESULTS AND DISCUSSION

143

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

144

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

145

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

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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


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154

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

155

SEM images showed that SMs were prepared with monodisperse, uniform

156

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

157

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

158

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

159

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

160

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

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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



176

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

177

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

178

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

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

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

190

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

191

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

192

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

193

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

194

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

195

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

196

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

197

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


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198

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

199

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

200

cheical stable.

201

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

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

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

213

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

214

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

215

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

216

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

217

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

218

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

219

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


32

The N 1s


220

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

221

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

222

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

223

surface of Ag@ASMs.

224

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

225

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

226

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

227

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

228

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

229

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

230

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

231

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

232

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

233

the best performance among all the samples.

234

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

235

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

236

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

237

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

238

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

239

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

240

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

241

between SERS intensity and R6G concentration (negative logarithmic relationship)


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242

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

243

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

244

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

245

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

246

The homogeneity of Raman signals was investigated by considering randomly

247

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

248

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

249

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

250

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

251

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

252

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

253

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

254

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

255

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

256

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

257

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

258

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

259

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

260

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

261

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

262

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

263

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



264

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

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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

282

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

283

M (Figure 5d).

284

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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286

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

287

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

288

enhancement by electromagnetic coupling occurs when a SERS active molecule is

289

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

290

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

291

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

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

302

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

303

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

304

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

305

performance.

306

DFT computation provided another evidence of the formation of suitable gaps

307

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



308

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

309

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

310

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

311

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

312

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

313

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

314

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

315

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

316

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

317

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

318

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

319

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

320

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

321

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

322

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

324

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

325

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

326

were interspaced.

327

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

328

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

329

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


Page 16 of 41

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330

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

331

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

332

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

333

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

334

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

335

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

336

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

337

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

338

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

339

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

340

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

341

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

342

combined with N atom.

343

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

344

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

345

molecules, but the contrast between experiment and theoretical results is

346

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

347

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

350

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



352

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

353

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

354

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

355

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

356

patterns of Ag atoms on APTES molecule.

357

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

358

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

359

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

360

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

361

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

362

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

363

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

364

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

365

the SMs, allowing increased contact with R6G molecules and facilitating

366

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

367

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

368

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

369

Ag@SMs.

370 371

CONCLUSIONS

372

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


Page 18 of 41

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374

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

375

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

376

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

377

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

378

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

379

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

393

The authors gratefully acknowledge financial support from National Natural Science

394

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

395

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).



397


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Figure captions

587

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

588

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

589 590

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

591

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

592

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

593

2:1 Ag@ASMs.

594 595

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

596

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

597

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

598

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

599 600

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

601

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

602

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

603

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

604

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

605

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

606

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

607

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


Page 30 of 41

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608 609

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

610

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

611

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

612

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

613

melamine concentrations.

614 615

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

616 617

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

618

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

619

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

620

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

621

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

622

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

623 624

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

625

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

626

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

627

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

628

Å.

629 630



Table List

631 632

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


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


Not available

1.2×109

42

Ag octahedron @Graphene oxide


Not available

Not available

43

Graphene oxide /Au NP arrays


6.9%

4.8×107

44

Ag@ASMs composite microspheres


7.6%

6.36×107

This work


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633

Figure 1

634 635



636

Figure 2

637 638


Page 34 of 41

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639

Figure 3

640 641



642 643 644

Figure 4


Page 36 of 41

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645 646 647

Figure 5



648

Figure 6

649


Page 38 of 41

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650

Figure 7

651



652

Figure 8

653 654


Page 40 of 41

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655

TOC

656


Preparation of Gap-Controlled Monodispersed Ag Nanoparticles by

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