Facile in Situ Synthesis of Silver Nanoparticles on the Surface of Metal

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A Facile in situ Synthesis of Silver Nanoparticles on the Surface of MetalOrganic Framework for Ultrasensitive SERS Detection of Dopamine Zhongwei Jiang, Peng Fei Gao, Lin Yang, Cheng Zhi Huang, and Yuan Fang Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03058 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Analytical Chemistry

A Facile in situ Synthesis of Silver Nanoparticles on the Surface of Metal-Organic Framework for Ultrasensitive SERS Detection of Dopamine Zhongwei Jiang, † Pengfei Gao, ‡ Lin Yang, ‡ Chengzhi Huang,*†, ‡ and Yuanfang Li*† †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China. ‡ Chongqing Key Laboratory of Biomedical Analysis (Southwest University), Chongqing Science & Technology Commission, College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China. ABSTRACT: Surface-enhanced Raman scattering (SERS) signals are intensively dominated by the Raman hot spots and distance between analyte molecules and metallic nanostructures. Herein, an efficient SERS substrate was developed by in situ synthesis of silver nanoparticles (AgNPs) on the surface of MIL-101 (Fe), a typical metal-organic framework (MOF). The as-prepared SERS substrate combines the numerous Raman hot spots between the high-density Ag NPs and the excellent adsorption performance of MOFs, making it to be an excellent SERS substrate for highly sensitive SERS detection by effectively concentrating analytes in close proximity to the Raman hot spots domains between the adjacent AgNPs. The resulting hybrid material was used for ultrasensitive SERS detection of dopamine based on the peroxidase-like activity of MIL-101 (Fe) by utilizing ELISA colorimetric substrate, 2, 2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS) as a SERS marker. This new developed method showed good linearity in the range from 1.054 pM to 210.8 nM for dopamine with the correlation coefficient of 0.992, detection limit of approximately 0.32 pM (S/N=3), and acceptable recoveries ranging from 99.8% to 108.0 % in human urine. These results predict that the proposed SERS system may open up a new opportunity for chemical and biological assay applications.

In the last decades, surface-enhanced Raman scattering (SERS), as an important and powerful analytical technique in chemical and biochemical analysis, has received increasing attention.1-3 It seems how to fabricate good SERS active substrates is one of the important fields in order to get strong enhancement.4 As we know, the SERS effect is dominated by the strong electromagnetic field enhancement, which attributes to the localized surface plasmon resonance (LSPR) of some noble metal nanomaterials.5-7 Thus, plasmon nanoparticles, such as gold and silver nanoparticles (Au and Ag NPs), have been widely employed as SERS substrates primarily due to their large enhancement of normally weak Raman signals.8, 9 Based on the electromagnetic mechanism, the fabrication of efficient SERS substrates should obey two essentials (i) the SERS substrate should possess excellent adsorption performance to adsorb more molecules in close proximity to the surface of metal nanoparticles; (ii) abundant Raman hot spots should have to be formed in the nanogaps between adjacent metal nanoparticles, which usually have much larger electromagnetic enhancement than the single noble nanostructures.8, 10 In such case, a variety of hybrid material have been successfully prepared and used as SERS substrate such as AgNPs/PSA nanospheres,11 AgNPs/GO,12, 13 AgNPs/silica spheres,14 AgNPs/agar/PAN nanofibers,15 and so on. These hybrids indeed improve the stability of nanoparticles and show better signal reproducibility. Even though, these reported nanomaterials with modest adsorbability cannot capture target molecules effectively in close proximity to the surface of metal nanoparticles.

Metal-organic frameworks (MOFs) as a new class of highly porous materials have been widely used in adsorption and separation,16 sensing,17, 18 catalysis19, 20 and drug delivery.21 MOFs have been considered as SERS substrates owing to their efficient adsorption performance. For instance, Lee et al has reported direct observation of SERS spectra of molecules adsorbed on MOFs surfaces.22 On the other hand, MOFs, since having porosity and chemical stability, have been used as a stabilizing matrix for monodispersed Au or Ag NPs embedded in the framework, and they can preconcentrate analytes proximal to the metal surface due to the large specific surface areas and porosity, namely the excellent adsorption performance.2325 However, these features have been failed to fabricate numerous Raman hot spots at the junctions between nanoparticles. Considering that the unique configuration of MOFs provide 3-D structures and ultrahigh surface area to attach plenty of SERS-active metal nanoparticles, it is possible to form abundant Raman hot spots if the metal nanoparticles could be anchored on the outer-surface of MOFs, which contribute to the SERS signal and the detection sensitivity should be tremendously improved. With these in mind, we herein thus developed a novel SERS substrate AgNPs/MIL-101 (Fe) by in situ synthesis of Ag NPs on the outer-surface of MIL-101 (Fe), which combines the numerous Raman hot spots between the high-density Ag NPs and the excellent adsorption performance of MOFs. In our strategy, the tannic acid (TA) as a reductant was first introduced onto MIL-101 (Fe) through complexation of unsaturated Fe (ш) on the surface of MIL-101 (Fe) with hydroxyl

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Scheme 1. (A) Schematic illustration of the preparation process for the AgNPs/MIL-101 (Fe) hybrid structure and (B) the schematic diagram of detection of DA based on SERS. TA, tannic acid; DA, dopamine.

groups in TA, making Ag+ be directly reduced by TA on the surface of MIL-101 (Fe) to engineer AgNPs/MIL-101 (Fe) complex structure. MIL-101 (Fe) serves as a stabilizing host material providing an ultrahigh surface area and adsorptive domains for enrichment of guest molecules in proximity to the Ag NPs surface. Meanwhile, based on the peroxidase-like activity of Fe-MOFs,26, 27 the resulting hybrid AgNPs/MIL-101 (Fe) has been used for ultrasensitive detection of dopamine (DA) by utilizing ELISA colorimetric substrate as a SERS marker. To the best of our knowledge, this is the first example of surface modification of MOFs with Ag NPs by in situ synthesis strategy. EXPERIMENTAL SECTION Materials and Reagents. Iron (Ⅲ) chloride hexahydrate (FeCl3•6H2O) and terephthalic acid (H2BDC) (Aladdin, Shanghai, China) were used to prepare MIL-101 (Fe). 2, 2’azino-bis-(3- ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS), tannic acid (TA), H2O2 and AgNO3 were obtained from Aladdin. Crystal violet (CV), rhodamine 6G (Rh 6G), malachite green (MG) and p-aminothiophenol (pATP) were purchased from Beijing Dingguo Biological Technology Co., Ltd. All reagents were analytical grade and commercial, and used without further purification. All solutions were prepared using ultra-pure water (18.2 MΩ). Apparatus. The infrared spectra of the materials were recorded by an IR Prestige-21 (Shimadzu, Japan) instrument in the range of 500-3500 cm-1 using the KBr pellet technique. The morphology and the size of the as-prepared AgNPs/MIL101 were characterized by an S-4800 scanning electron microscope (SEM) (Hitachi, Japan). Transmission electron microscopic (TEM) characterization was performed on FEI Tecnai G2 F20 TEM instrument (FEI, America). Powder X-ray diffraction (PXRD) patterns were collected on an D8 ADVANCE X-ray diffractometer (Brooker, Germany) with Cu Kα radiation (λ=1.5406 Å) in the range of 3-80 θ at a scan rate of 3.00° min-1. X-ray photoelectron spectrometry (XPS) analyses were carried out on a Thermo escalab 250 Xi X-ray photoelectron spectrometer using an Al Kα source (hν=1486.6 eV). The UV-

vis spectra of the hybrid materials solution were obtained using a Hitachi U-3010 spectrometer. Fabrication of SERS-Active AgNPs/MIL-101 (Fe) Hybrid. MIL-101 (Fe) was synthesized according to previous report.28 The anchored Ag NPs were prepared via in situ synthesis strategy by utilizing TA as a reductant. Typically, 15 mg of activated MIL-101 was suspended in 9.5 mL H2O, and then 0.5 mL TA solution (20 mg TA dissolved in 0.5 mL H2O) was rapidly injected into the mixture under vigorous stirring. For further stirring of 20 s, the mazarine products (TA/MIL-101) isolated by centrifugation and washed with H2O for three times to remove the excess TA. Then, TA/MIL-101 was redispersed in 9.5 mL H2O, then 0.5 M K2CO3 was used to tune pH of the suspension to 7.5. Finally, 0.5 mL AgNO3 (0.05 M, 0.1 M, 0.15 M and 0.2 M, respectively) was added dropwise under a stirring speed of 600 rpm at room temperature. After further stirring for 1 h, the dark products (AgNPs/MIL-101) was isolated by centrifugation and washed with H2O three times, and the obtained AgNPs/MIL-101 was dried in vacuum freezing drying oven for further use. SERS Activity of AgNPs/MIL-101 Composite as Substrate. In this SERS study, common SERS model analytes, such as CV, Rh 6G, MG and p-ATP, were performed to characterize the SERS activity of the AgNPs/MIL-101 composite. The Ag NPs about 40 nm and the bare MIL-101 were used as controls. Typically, 5 µL Ag NPs, MIL-101 and AgNPs/MIL101 solution (1 mg/mL, respectively) mixed with 95 µL SERS model analytes and incubated for 30 minutes at the room temperature, and then the mixtures were used for SERS detection (Figure S4, SI). Parameter setting: Laser wavelength, 532 nm; power, 28 mW; lens, 50× objective; acquisition time, 10 s. Detection of DA in Aqueous Medium and Urine. DA analysis was carried out as follows: 10 µL 0.2 M NaAc buffer solution (pH 4.0), 10 µL AgNPs/MIL-101 aqueous solution (1.0 mg/mL), 10 µL 1.0 mM H2O2, and 10 µL 10 mM ABTS were added into 200 µL EP vial. Then, a certain amount of DA or urine was added into the above reaction solution. The resultant mixture was incubated at 40 oC for 20 min and then used for DA measurement by monitoring the Raman shifts at

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Figure 1. Characterization of the AgNPs/MIL-101. SEM (A) and TEM (B) images of AgNPs/MIL-101; XPS survey spectra of AgNPs/MIL-101 (C) and the enlarged Ag 3d region (D).

1401 cm-1. Urine samples were obtained from healthy volunteers. Parameter setting: Laser wavelength, 532 nm; power, 28 mW; lens, 50× objective; acquisition time, 10 s. RESULTS AND DISCUSSION Characterization of the AgNPs/MIL-101 (Fe) Nanocomposite. FT-IR characterization shows the presence of intense bands at 1593 and 1392 cm-1 corresponding to the symmetric and antisymmetric O-C-O stretching vibrations of carboxylates, which are the typical characteristic bands of MIL-101 framework (Figure S1).29 The stretching vibrations of C-O and C-O-C bending mode at 1203 and 1029 cm-1 in TA were both observed in the infrared spectra of TA/MIL-101 (Fe) and AgNPs/MIL-101 (Fe),30 indicating that TA was successfully anchored on the MIL-101 (Fe). The peak at 1450 cm-1, which corresponds to the C-O-H in plane bending mode of hydroxyl groups in TA, was shifted to 1439 cm-1 and significantly decreased in intensity for TA/MIL-101 (Fe), suggesting that the complexation of unsaturated Fe (ш) with hydroxyl groups in TA has occurred.31 The disappearance of the peak at 1439 cm-1 in the spectra of AgNPs/MIL-101 (Fe) was due to the oxidation of residual hydroxyl groups into quinone, which offered electrons to reduce Ag ions to form Ag NPs. The morphology and structure of AgNPs/MIL-101 (Fe) were examined by SEM and TEM (Figure 1A and B). The SEM image showed the surface of MIL-101 (Fe) had been successfully decorated with Ag NPs without obvious influence on the morphology. The TEM image clearly showed that the Ag NPs were homogeneously distributed on the outer-surface of MIL-101 (Fe) crystals. The size of Ag NPs could be easily controlled by just changing the concentration of AgNO3 and increased along with the increase of the AgNO3 concentration (Figure S2, SI). In order to further describe the process, the UV-vis absorption spectrum was performed to confirm the size changes of Ag NPs with the increase of the AgNO3 concentration (Figure 2). The AgNPs/MIL-101 (Fe) displayed an obvious broad absorption in a wide range of 320-800 nm due to the Mie plasmon resonance excitation from Ag NPs.32 With the increasing of AgNO3 concentration, the SPR absorption peaks become stronger and the maxima had a red-shift from 470 to 488 nm which attributed to the strong dipole-dipole coupling

between neighboring Ag NPs and the inhomogenous size and shapes of Ag NPs.32-34 Characterization of the crystal structures of the assynthesized products by power XRD pattern showed that the main diffraction peaks of MIL-101 (Fe) at low angles were in good agreement with the previous report (Figure S3, SI).28 The diffraction peaks of MIL-101 (Fe) in the PXRD patterns of AgNPs/MIL-101 (Fe) were not observed, probably because the content of Ag NPs was very high, and the peak of MIL101 (Fe) was covered by the Ag NPs peak. The obvious diffraction peaks at 38.0o, 44.2o, 64.4o and 77.5o corresponded to diffractions from the Ag (111), Ag (200), Ag (220), and Ag (311) lattice planes, respectively, and indicated that Ag NPs on the surface of MIL-101 (Fe) were highly crystalline.35 In addition, the X-ray photoelectron spectroscopy (XPS) spectrum of AgNPs/MIL-101 (Fe) showed the presence of a significant amount of iron, oxygen, silver and carbon (Figure 1C). The emergence of intense Ag 3d peaks proved that the abundant deposition of Ag NPs on the surface of MIL-101 (Fe). In the enlarged Ag 3d spectrum, two peaks at 368.4 (3d5/2) and 374.4 (3d3/2) eV could be seen, corresponding to the Ag0 state (Figure 1D).36

Figure 2. The UV-vis absorption spectrum of the aqueous dispersion of MIL-101 and AgNPs/MIL-101 with the increase of AgNO3 concentration (0.05 M (a), 0.1 M (b), 0.15 M (c) and 0.2 M (d) ). Insert shows the red-shift from 470 to 488 nm along with the increase of AgNO3 concentration.

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SERS Activity of AgNPs/MIL-101 Composite as Substrate. The Raman signal of the common SERS model molecules were preliminarily used to evaluate the enhancement of the AgNPs/MIL-101 (Fe) nanocomposites. The bare MIL-101 (Fe) and Ag colloids with size about 40 nm were used as controls. After loading low concentration of SERS model molecules (0.95 µM) on the AgNPs/MIL-101 (Fe) hybrids, intense Raman signal were detected at multiple Raman shifts characteristic respectively, however, no Raman signals were observed on bare MIL-101 (Fe), even when a conspicuously higher SERS model molecules concentrations were used (Figure S4, SI). The Raman signals with lower intensity were also detected with Ag NPs as SERS substrate even though a 20fold higher (19.0 µM) molecules concentration than that of AgNPs/MIL-101 (Fe) was used. These conspicuous differences may result from the high local concentration of the analytes adjacent to the surface of AgNPs/MIL-101 (Fe) which have abundant hot spots and high adsorption performance. The AgNPs/MIL-101 (Fe) serve as a means to trap analytes and get them sufficiently close to the SERS-active sites for providing strong SERS signals owing to the well-known powerful adsorption capability of MOFs. Furthermore, rhodamine 6G (Rh 6G) was used as a model Raman probe to evaluate the enhancement factor (EF). The EF at the peak of 1650 cm-1 of Rh 6G was calculated to be 1.8×105 for the AgNPs/MIL-101 (Fe) hybrids (Figure S5, SI, details were showed in Supporting Information). Reproducible SERS Signals of the AgNPs/MIL-101(Fe) nanocomposite as substrate. As we know, the good reproducibility of SERS signals is precondition for SERS analysis and estimated by the relative standard deviation (RSD) of the characteristic peaks of malachite green (MG) as a probe molecules. Thus, the reproducible Raman signals were obtained from the SERS spectra of MG (2.0×10-6 M) for 15 different batches tested (Figure 3). The RSD values of main characteristic peaks of MG were calculated and presented in Table 1. All the RSD values with less than 15%, and it can be concluded that the preferable reproducibility of the hybrid AgNPs/MIL101(Fe) as SERS substrate.

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SERS Detection of DA in Human Urine. The oxidation products of ELISA colorimetric substrate 2, 2'-azino-bis-(3ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS), which had been reported as a SERS marker,37, 38 was used to verify the versatility of this hybrid material. In our research, the oxidative product of ABTS resulted from the catalytic activity of MIL-101 (Fe) in the presence of H2O2. As shown in Figure 4A, MIL-101 (Fe) exhibited excellent peroxidase-like activity. Meanwhile, low absorbance was obtained from AgNPs/MIL-101(Fe)-H2O2-ABTS system. The difference may arise from the complexation of unsaturated Fe (ш) with hydroxyl groups in TA and the impediment effect from Ag NPs on the surface of MIL-101 (Fe). Otherwise, the residual hydroxyl groups in TA-coated Ag NPs or AgNPs/MIL-101 (Fe) might inhibit the oxidation of ABTS due to the strong reducibility. In addition, no obvious absorption was detected in Ag NPs-H2O2-ABTS system, indicating that the peroxidase-like activity of AgNPs/MIL-101 (Fe) hybrid mainly originated from MIL-101 (Fe). So, the Raman signal of oxidative product of ABTS was only observed with AgNPs/MIL-101 (Fe) (Figure 4B), and the Raman signal increased along with the size of Ag NPs on the surface of MIL-101 (Fe) (Figure S6, SI).

Figure 4. Absorption spectra (A) and SERS spectra (B) of the oxidation product of ABTS with Ag NPs, MIL-101 and AgNPs/MIL-101 respectively. ABTS, 1 mM; H2O2, 200 µM; Ag NPs, MIL-101 and AgNPs/MIL-101, 0.2 mg/mL, respectively; incubation 20 min in pH 4.0 NaAc buffer at 40 oC.

Figure 3. The reproducibility of the as-synthesized AgNPs/MIL101(Fe) as SERS substrate for 15 different batches.

Table 1. RSD Values of the Characteristic Peaks for MG (1.0× ×10-6 M) SERS Spectrum Peak position (cm-1)

799

915

1176

1365

1617

RSD values (%)

13.2

11.0

4.3

8.0

8.9

ABTS is readily oxidized by free radicals and various peroxidases to the cation radical ABTS+•. It is well known that cation radicals represent an intermediate oxidation step in the redox cycle of azines, and upon extended oxidation and abstraction of the second electron, the corresponding dications can be obtained (Figure S7, SI).39, 40 In order to assign the characteristic peak of 1401 cm-1, the Raman signal of ABTS was obtained from AgNPs/MIL-101(Fe) without the presence of H2O2 (Figure S8, SI). By comparing the Raman signals of ABTS and oxidized-ABTS, all the major peaks of oxidizedABTS could be find in the Raman spectrum of ABTS, except

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for 1401 cm-1. Hence, the peak of 1401 cm-1 was used as diagnostic peaks which might be assigned to the stretching vibration of N=N.41-43 For the detection of dopamine (DA), the oxidation process of ABTS was interdicted by DA as an electron donor and resulted in the loss of Raman signal of ABTS2+. As shown in Figure 5, it indicated that the SERS signal steadily decreased (a-f) with the increase of DA concentration. In order to obtain high sensitivity of the assay, the reaction conditions such as pH, reaction time, and the concentrations of H2O2 and ABTS were optimized, respectively (Figure S9, SI). Under the optimal assay conditions, the standard curve for DA was constructed within the wide range of 1.054 pM−210.8 nM and with the detection limit of 0.32 pM (Figure 6). Comparing to fluorometry, colorimetry, HPLC-EC and electrochemistry (Table S1), the proposed SERS method based peroxidase-like activity of MIL-101 (Fe) is simple, sensitive and with wide linear response range for DA detection.

Figure 5. SERS spectra of ABTS2+ obtained in the presence of different of concentrations of DA (the concentration from top to bottom (a-f) are 1.054 pM, 10.54 pM, 105.4 pM, 1.054nM, 10.54 nM and 105.4 nM).

The complexity of urine presents a great challenge to the analytical methods for DA detection. The selective recognition of DA by the developed method was investigated by considering other biomolecules and ions that exist in urine (Figure S10, SI). Although the concentrations of interfering substances were 20-fold higher than that of DA, the obtained Raman signal changed little. It is clear that the proposed method has very high specificity toward DA.

Table 2. The recovery test of DA in diluted urine samples Samples

1 2 3

Spiked amount (pM)

Found amount (pM)

Recovery (%)

RSD (%, n=3)

210.8

223.0

105.8

3.9

527.0

547.0

103.8

4.6

210.8

227.7

108.0

1.5

527.0

534.6

101.4

3.7

210.8

216.9

102.9

3.4

527.0

526.1

99.8

5.3

To confirm the precision and reliability of this method, the spiked recoveries of DA in urine have been investigated. The spiked samples were obtained by adding a certain amount of DA to the diluent of urine (Table 2). Acceptable recovery rates of 99.8-108.0 % were obtained, which demonstrated that the present method could be used for the detection of DA in urine samples. Furthermore, the reproducibility of AgNPs/MIL-101 (Fe) in urine which contained 105.4 pM DA was also investigated. It clearly indicated the overall uniformity and reliability of the hybrid as SERS substrate for the quantitative detection of DA in urine (Figure S11 and Table S2). Besides, the morphology of AgNPs/MIL-101 (Fe) composite after detection of DA in urine has no serious changes and no great influence upon the reproducibility of AgNPs/MIL-101 (Fe) (Figure S12, SI). CONCLUSIONS In summary, we have developed an efficient SERS substrate by in situ synthesis of silver nanoparticles on the surface of TA-functionalized MIL-101 (Fe). The MIL-101 (Fe) used in this work provides large surface areas for effective loading of Ag NPs and promoting the adsorption of analytes close to the SERS-active sites at the metal surface or in the nanogaps between Ag NPs, leading to a dramatic enhancement of Raman intensity. Furthermore, based on the peroxidase-like activity of MIL-101(Fe), the resulting hybrid AgNPS/MIL-101 (Fe) was used for ultrasensitive detection of dopamine by utilizing ELISA colorimetric substrate ABTS as a SERS marker. Considering the strong reducibility of TA, we anticipate that this facile synthetic strategy will be widely used for other noble metals grafting onto the surface of MIL-101 (Fe) and other Fe contained MOFs for a wide range of applications. Besides, the proposed SERS system may open up a new opportunity for chemical and biological sensing applications.

ASSOCIATED CONTENT Supporting Information Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Figure 6. Calibration curve corresponding to the Raman signal at 1401 cm-1 for varying concentrations of DA. Insert shows a linear calibration curve from 1.054 pM−210.8 nM, R2=0.992.

* Tel.: (+86) 23-68254659. Fax: (+86) 23-68367257. E-mail: [email protected]. * Tel.: (+86) 23-68254659. Fax: (+86) 23-68367257. E-mail: [email protected].

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Notes

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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (NSFC, No. 21175109).

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A Facile in situ Synthesis of Silver Nanoparticles on the Surface of Metal-Organic Framework for Ultrasensitive SERS Detection of Dopamine Zhongwei Jiang, † Pengfei Gao, ‡ Lin Yang, ‡ Chengzhi Huang,* †, ‡ and Yuanfang Li*†

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