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Synthesis of a β-cyclodextrin-modified Ag film by the galvanic displacement on copper foil for SERS detection of PCBs
Journal of Colloid and Interface Science 365 (2012) 122–126
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of a b-cyclodextrin-modified Ag film by the galvanic displacement on copper foil for SERS detection of PCBs Jingpeng Yuan, Yongchao Lai, Junling Duan, Quanqin Zhao, Jinhua Zhan ⇑ Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Department of Chemistry, Shandong University, Jinan 250100, Shandong, PR China
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Article history: Received 6 July 2011 Accepted 28 August 2011 Available online 14 September 2011 Keywords: Polychlorinated biphenyls (PCBs) Detection Surface-enhanced Raman spectroscopy (SERS) Cyclodextrins (CDs) Silver films
a b s t r a c t A mono-6-thio-b-cyclodextrin-modified silver film was synthesized via galvanic displacement on copper foil. The prepared silver films could enrich non-polar polychlorinated biphenyls (PCBs) molecules from hydrophilic phase using thiolate b-cyclodextrins (SH-b-CDs) as receptors. The components of as-prepared Ag-coated-Cu (Ag–Cu) film were confirmed by powder X-ray diffraction (XRD). Both surface-enhanced Raman spectroscopy (SERS) and energy dispersive X-ray spectroscopy (EDS) measurements gave strong evidences that the thiolated b-cyclodextrins (SH-b-CDs) had been immobilized on the surface of silver film. Compared to the substrates prepared in the absence of SH-b-CD, the surface morphology of the CD-modified Ag films was obviously changed. The interfacial enrichment and the capability of substrates to form inclusion complexes with PCBs molecules were tested by using PCB-15 (4,40 -dichlorobiphenyl) as the probe molecules via SERS technique. The measured SERS spectra could distinguish the PCB-15 molecules at micro-molar level according to the most intense CCC bending in-plane mode of PCBs. The enhancement factor (EF) of the SERS substrates for PCB-15 was 1.2 105, which was comparable with a number of previous reports. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Polychlorinated biphenyls (PCBs), a group of synthetic organic chemicals with 1–10 chlorine atoms attached to biphenyl, have shown toxic and mutagenic effects by interfering with hormones in the body. Its products have been banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2001 [1]. PCBs can transport on a global scale and do not decompose readily. The extensive distribution and the low-concentration residue in the environment make the detection of these PCBs molecules a challenging task [2–4]. The current ordinary method used for measuring PCBs is highresolution gas chromatography–mass spectrometry (HRGC–MS). In this case, the compounds are separated in time and space via the differential partitioning into the stationary phase [2]. Its wide practical application is limited by the cumbersome pretreatment and analysis. An efficient and sensitive method for detecting PCBs is urgently needed. Raman spectroscopy can provide vibrational structural spectra of the detected molecules with the highly informative character [2]. Surface-enhance Raman spectroscopy (SERS) with an amplification of the Raman signal can be attributed to an enhancement of scattering efficiencies when molecules localized near the nanostructured
⇑ Corresponding author. Fax: +86 0531 8836 6280. E-mail address: [email protected] (J. Zhan). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.08.075
noble-metal surfaces (typically within 10 nm from the surface, at most) [5]. SERS is a powerful technique for detection and characterization of the adsorbed molecules on rough metallic surface [6,7]. Surface plasmons (SPs) are excited and localized with incident light to produce a strong electric field for SERS enhancement [6,8]. Particularly, the nanomorphology of silver particles is paramount to achieving maximum SERS enhancements. Theoretical models predict that curvature and shape of the noble-metal particles are responsible for enhancement of the field intensities, where the active hot spots occur at interstitial spots between adjacent nanoparticles [9,10]. Up to now, a number of preparation methods, including solvothermal synthesis, electron beam deposition, nanosphere lithography (NSL), vacuum evaporation deposition, and electrochemical oxidation/reduction have been developed to fabricate SERS-active substrates with the well distributed ‘‘hot spots’’ [11–13]. It remains a challenge to fabricate SERS-active substrates through simple and low-cost methods [14]. For SERS detection, analyte molecules must be located near the metallic nanostructures, implying that a special chemical moiety should be incorporated on the SERS substrate for enriching trace organic species. Thus, lipophilic and poorly soluble in water are the physical properties of all PCBs. For molecule-binding chemistry, we sought to exploit the binding properties of cyclodextrins (CDs) for special small organic molecule. CDs are a class of cyclic polysaccharide molecules commonly used in host–guest complexation chemistry [15–17]. b-Cyclodextrins (b-CDs) have a cavity whose dimension are depth 7 Å in width and 9 Å in depth, which
J. Yuan et al. / Journal of Colloid and Interface Science 365 (2012) 122–126
would be suitable for accommodating PCBs guest molecules [18,19]. Moreover, a special molecular structure-hydrophobic internal cavity and hydrophilic external surface of b-CDs could be an important driving force in inclusion complexes. In the inclusion complexation, b-CDs can be used as receptors to capture PCBs molecules in solution, which is approved by theoretical simulation [15,19,20]. Based on the theoretical calculation, inclusion complexes could be formed on Ag film between the PCBs and SH-b-CD [18]. The purposes of this paper are to experimentally evaluate the possibility of the above theory and to determine PCBs molecules in solution via SERS technique. In this study, a SH-b-CDmodified Ag film was synthesized via one-pot step, which was used as SERS-active substrate for PCBs detection. After confirmation and characterization of the inclusion complexes formed between SH-bCD and PCB-15, the results showed that the prepared substrates were sensitive to the low concentration of PCB-15 in aqueous solution, which indicated SH-b-CD-modified Ag film was a simple and effective SERS substrate for PCBs analysis. 2. Experimental section 2.1. Materials All the chemicals were analytical reagent grade and used as purchased without further purification. Beta-cyclodextrin (b-CD) was purchased from Aladdin Chemical Co. Silver nitrate (AgNO3), p-toluenesulfonyl chloride, thiourea, and copper foils (Cu, 10 cm 10 cm in size, 0.1 mm in thickness, 99.99%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ammonia solution (analytically pure, 25–28 wt%), acetonitrile, and acetone were purchased from Tianjin Kemeng Chemical Co. (Tianjin, China). The polychlorinated biphenyls compounds, PCB15 (4,40 -dichlorobiphenyl) and PCB-77 (3,30 ,4,40 -tetrachlorobiphenyl) were purchased from J & K Chemical Co. Ltd. Ultrapure water (18.2 MX cm1) was used throughout the experiments.
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the solution was filtered through a 0.22 lm syringe filter. Then, the cleaned Cu foils were dipped into the clear filtrated solution slowly, while the silver atoms were formed on copper foil spontaneously by the galvanic displacement. The loading amount of silver was controlled by reaction time and temperature [27]. In this work, the reaction was kept at 25 °C under constant stirring for 5 min. The Ag–Cu foils were rinsed consecutively with ultrapure water to obtain fresh surface and dried at room temperature for future analysis.
2.4. Characterization of substrates The as-prepared substrate was examined by powder X-ray diffraction (XRD). The chemical adsorption on the fresh substrate was investigated with SERS spectra and an energy dispersive X-ray spectroscopy (EDS). The morphology and microstructure of the silver film were characterized using a cold field emission scanning electron microscope (FE-SEM) (JSM-6700F) operating at an accelerating voltage of 3.0 kV. The Raman spectra were collected using an Ocean Optics QE65000-Raman Spectrometer. The excitation wavelength was 785 nm, and the maximum excitation power was 455 mW. Integration times were 10 s.
2.5. SERS measurements The CD-modified Ag film was used as substrate for SERS detection. PCB-15 as probe was used to confirm the sensor’s ability to enrich target molecules for SERS detection. The SERS substrates were immersed in the adsorbate solution for 24 h with stirring at room temperature. PCB-15 samples were prepared in aqueous solution with concentrations of 4.4 104, 4.4 105, 4.4 106, and 4.4 107 M. Additionally, the mutterlauge of PCB-15 was dissolved in ethanol (4.4 104 M) and diluted to the desired concentration with water.
2.2. Synthesis of mono-6-thio-b-cyclodextrin The synthesis and characterization of SH-b-CD were the same as those reported in the literatures [21–24]. A white powder was obtained. 1H NMR (400 MHz), [D6] DMSO, 25 °C, TMS, d): d = 2.05 (m, SH), 2.49–3.00 (m, 2H), 3.29–3.47 (m, overlapping with HDO), 3.55–3.79 (m, 28H), 4.32–4.48 (m, 6H), 4.82, 4.83 (br d, 7H), 5.60–5.83 (m, 14H) ppm. Anal. Calcd. for C42H70O34S7H2O: C 39.50; H 6.63; S 2.51. Found: C 41.09; H 6.60; S 3.08. The measured NMR shifts for prepared SH-b-CD were in good agreement with literature, which gave strong evidences for the quantitative occurrence of the desired substitutions and the prepared SH-b-CD [22–24]. Moreover, the prepared mono-thiolated derivative of b-cyclodextrin was further confirmed by the Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Figs. S1 and S2). 2.3. Preparation of CD-modified-Ag film The Cu foils were cut into pieces (0.5 cm 0.5 cm) and degreased thoroughly by sonication in acetone for 30 min and then rinsed with ultrapure water. Subsequently, the Cu foils were immersed in 0.1 M HNO3 aqueous solution for 1 min to remove surface oxides and then rinsed with ultrapure water and dried at room temperature for future use [25]. Ammonia solution (4 ml) and AgNO3 (0.17 g) were mixed and diluted with water so as to form 20 mM ([Ag(NH3)2]+) aqueous solution [26]. The prepared SH-b-CD solid (0.05 g) was dissolved in the mixed solution. After stirring for 5 min at room temperature,
3. Results and discussion 3.1. Pathway of the galvanic displacement The CD-modified Ag substrates were fabricated by the galvanic displacement (GD, also known as electroless deposition) reaction on commercial copper foils. This controllable and inexpensive dip-and-rinse GD process enabled the synthesis of silver films. For the synthesis of Ag nanoparticles, the Tollen’s reagent ([Ag(NH3)2]+) was employed in the present work, which was prepared by adding excessive ammonia solution into the AgNO3 solution until a transparent solution was obtained [28]. The Tollen’s reagent as the silver precursor was reduced and the nanostructured Ag films were formed on copper foils [29,30]. In GD reaction, the redox potential of the Ag+/Ag0 couple in solution (+0.7991 V) was decreased by the ammonia down to +0.373 V [31]. The [Ag(NH3)2]+ ions were reduced rapidly and stable coordination compounds were formed between Cu2+ and the ammonia in solution, which resulted in a bright and uniform silver film on the copper foil. In the synthesis, a direct ion-exchange self-metallization technique was carried out without any external current sources or reducing agents in the bath to contaminate the prepared Ag film. In galvanic displacement procedure, the surface distribution and the metallic nanostructures of the silver film on copper foil were influenced by the Ag+ ion concentrations, temperature, and reaction time. The as-obtained results were confirmed to be the suitable experimental parameters to synthesize CD-modified Ag film for SERS detection.
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Fig. 2. Raman spectra of SH-b-CD: (a) SH-b-CD solid served as reference and (b) SHb-CD functionalized Ag–Cu film.
Fig. 1. XRD patterns of the CD-modified Ag–Cu film (top) and the copper foil (bottom).
3.2. Component and morphology analysis of the CD-modified Ag film A uniform and bright silver film formed on copper foils via a wet chemical method, which was analyzed by XRD pattern (Fig. 1). The strong peaks in the pattern could be assigned to cubic copper (JCPDS card 04-0836) and the characteristic diffraction peaks of silver-3C (JCPDS card 04-0783), indicating that a thin layer of Ag was deposited on Cu foil [25]. Moreover, the electron affinity of copper (1.24 eV) is little lower than that of silver (1.30 eV), leading to the copper in electron-deficient state and silver in electron-rich state via the free electron redistribution between the heterogeneous materials [32]. Silver films are more inert to oxidation in electron-rich state than the bottom copper foils, which could protect the upper inactive Ag films from corrosion and enable the SERS substrates to maintain their enhancement capabilities along with preserving their nanomorphologies. As shown in Fig. 2, the background SERS spectrum of the SH-bCD functionalized Ag–Cu film corresponded well with the normal Raman spectrum of the powdered solid SH-b-CD. The clear band at about 483 cm1 appeared in both spectrums, which could be assigned to a certain deformation vibration of SH-b-CD [33]. The band at 665 cm1 of solid SH-b-CD was related to the C–S stretching vibrations, which shifted to 650 cm1 in the SERS spectrum of SH-b-CD functionalized Ag–Cu film. Also, the strong intensity peak at 1137 cm1 and 1336 cm1 (attributable to the C–C stretching and C–H deformation vibration of (CH2)2) in the solid SH-b-CD spectrum (Fig. 2a), shifted to 1130 cm1 and 1328 cm1 in the SERS spectrum of the SH-b-CD functionalized Ag–Cu film (Fig. 2b) [19]. The results suggested that the SH-b-CD bounded to the silver surface via S–Ag bonds and adsorbed on Ag film steadily. As shown in Fig. 3 with energy dispersive X-ray spectroscopy (EDS), the peak of S was extremely weak and the peak of Ag was intensive, which could be vested in thiol groups of SH-b-CD and silver film, respectively. Both SERS spectra and EDS image gave strong evidences that the SH-b-CD had been immobilized on the surface of silver film through one-pot synthesis. The surface morphology and microstructure of the substrates were confirmed by SEM image (Fig. 4b). The CD-modified Ag film displayed irregular particles with an uneven size distribution over
Fig. 3. EDS image of the CD-modified Ag–Cu film.
the surface (Fig. 4c) in contrast to the bare Ag film (Fig. 4b) with spherical silver particles on the copper foils (Fig. 4a) [33]. The SH-b-CD as stabilizer played a role in inducing the crystal nucleus growth during the reduction of silver ions [34]. A self-assembled monolayer (SAM) of SH-b-CD formed on the silver surface due to the bonds between the silver particles reaction and the thiol groups [35]. The resulting irregular particles were randomly accumulated together on the copper foils, which could provide sufficient gaps among the adjacent silver particles [36]. Namely, there were more robust active sites as ‘‘hot spots’’ where the signal of Raman-active molecules could be enhanced [37]. It should be mentioned that the prepared SERS-active substrates commonly relied on a salt-induced aggregation or self-assembly process [38].
3.3. SERS detection of polychlorinated biphenyl compounds Theoretical simulations have proved that SH-b-CD could form inclusion complexes with PCBs and Raman spectra could be used for the identification of CD-modified PCBs [18,19]. Based on those results, schematic representation of inclusion complex of PCBs-CD Ag film for SERS experiments was shown (Fig. S3). [39]. SERS spectra of the PCB-15 molecules at different concentrations were shown in Fig. 5. It could be seen that PCB-15 can be detected even at about 10 ppm level, which should be owe to the fact that PCB-15 were included in the cavity of chemisorbed CDs derivatives on silver film [19]. Some prominent bands of PCB-15 could be observed. Such as C–Cl stretch bands (776 cm1 shift), and the
J. Yuan et al. / Journal of Colloid and Interface Science 365 (2012) 122–126
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Fig. 5. SERS spectra of the PCB-15 CD-modified Ag substrate following immersed in sample solutions. (a) The powdered solid sample. (b) 4.4 104 M. (c) 4.4 105 M. (d) 4.4 106 M. (e) 4.4 107 M.
Fig. 6. The most intense ring deformation in-plane mode for PCB-15 according to the Gaussian calculations.
Fig. 4. SEM images of (a) the bare copper foil and (b) Ag–Cu film and (c) CDmodified Ag–Cu film.
aromatic bands (543, 631 and 1094 cm1 shift), and the biphenyl C–C bridge stretch (1285 cm1 shift), and the most intense CCC bending in-plane mode (1600 cm1 shift) [40,41]. With dilution of the samples, most peaks of PCB-15 were covered by the SH-bCDs except the peak at about 1600 cm1, and the SERS band at about 1600 cm1 could serve as an internal standard in the evaluation of the band intensity of the PCB-15 molecules. The SERS intensity of the vibrational band at about 1600 cm1 was the strongest, which could be assigned to the most intense CCC bending inplane mode for PCB-15 according to the references [18]. The experimental results confirmed the theoretical simulation that SH-b-CDs as host molecules had the ability to accommodate PCB-15 molecules. Meanwhile, the adsorption behavior of the host–guest inclusion complexes on the silver film was elucidated [18,42,43]. To gain a clear understanding of these spectra, Density Functional Theory (DFT) with Gaussian 03 program package were used to simulate the vibration modes (Fig. 6) [13]. On the basis of experiments and computational simulation, the intensity of the Raman mode at 1600 cm1 was quite sensitive to the concentration of PCB-15, which could be quantified as the integrated peak area for the CDmodified PCB-15 on the substrate. For enriching and detecting PCBs, the SERS substrates were incubated in sufficient time for absorbing as much target molecules as possible. The results
demonstrated that the intensity of the band at about 1600 cm1 decreased with diluted samples, which indicated the enrichment capacity of the SERS substrate was not saturated. The coverage of PCBs peaks and the adsorption capacity of the substrates were attributed to sufficient SH-b-CDs molecules on the Ag films. SERS detection of PCB-77 (3,30 ,4,40 -tetrachlorobiphenyl) was carried out and shown in Figs. S4 and S5. Based on the above results, the CD-modified Ag substrate had high sensitivity for detecting PCBs using SERS measurement (Figs. 5 and S4). The SERS-activity of CD-modified Ag film was mainly attributed to two aspects [2,18]. On the one hand, the unique surface morphology of the silver film could be responsible for SERS efficiency. Compared to the substrates synthesized in the absence of SH-b-CD, The CD-modified Ag substrate had more gaps among adjacent particles due to the robust surface morphology (Fig. 4b and c), resulting in a Raman enhancement when PCBs molecules were trapped in those gaps [44]. The enhancement factor (EF) was calculated using (ISERS/INR)(CNR/CSERS), where ISERS represents the SERS intensities of the 1600 cm1 band for PCBs adsorbed on the SERS substrate, and INR represents the normal Raman signal recorded for powdered PCBs adsorbed on the quartz substrate, whereas CSERS and CNE represent the corresponding concentrations of PCBs on these substrates. The enhancement factor (EF) of PCB-15 was 1.2 105, which was acceptable compared to the previous reports [45].
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On the other hand, the great PCBs-binding capacity of the SERS substrates was attributed to the SH-b-CD molecules on the substrates. Cyclodextrins (CDs) have been extensively studied as molecular receptors due to their abilities of absorbing various hydrophobic compounds [18,19]. PCB-15 as SERS probes were enriched from aqueous solution. The theoretical calculation of the host–guest interations between PCBs and SH-b-CD, indicating the intrinsic capability of CDs for including PCBs molecules was more significant than solvent effect on the inclusion complexes [18]. In other words, PCBs molecules prefer forming the inclusion complexes to dispersing in water. In this case, the PCB-15 molecules are extracted from exterior system to enter the SH-b-CD cavities because the dimension of PCB-15 matches with the cavity diameter (7.47 Å) of SH-b-CD [18]. As a result, the PCB-15 molecules were enriched on the surface of the substrate and SERS spectra of host–guest complexes were obtained (Fig. 5). In this work, the adsorption of PCBs were measured with bare Ag–Cu substrates, whereas practically no SERS bands of PCBs were detected at a saturation concentration of about 100 ppm (Fig. S6). The results further confirmed that the CD-modified Ag film had ability to accommodate PCBs molecules and locate them to the rough metal surface for SERS detection. Both the unique surface morphology of the SERS substrates and inclusion behavior of SH-b-CD on the Ag– Cu film were all critical to the remarkable enhancement of Raman spectroscopy for detecting PCBs.
4. Conclusion In summary, a CD-modified Ag film was synthesized for detecting PCBs via SERS technique. SH-b-CDs as a surface modifier and the receptor were incorporated with the silver particles via onepot synthesis. In addition, we have confirmed the CD-modified Ag films had ability to accommodate PCB-15 molecules and recorded the respective Raman-enhanced spectra at different concentrations. The results showed that the hybridized SERS substrates had good sensitivity when the PCBs molecules matched with the cavity size of the SH-b-CDs in SERS experiments, which could be a new class of analytical assay for trace detection of PCB homologues.
Acknowledgments We thank the financial support from National Basic Research Program of China (973 Program 2007CB936602), National Natural Science Foundation of China (NSFC 21075077), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ201004).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.08.075.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of a b-cyclodextrin-modified Ag film by the galvanic displacement on copper foil for SERS detection of PCBs Jingpeng Yuan, Yongchao Lai, Junling Duan, Quanqin Zhao, Jinhua Zhan ⇑ Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Department of Chemistry, Shandong University, Jinan 250100, Shandong, PR China
a r t i c l e
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Article history: Received 6 July 2011 Accepted 28 August 2011 Available online 14 September 2011 Keywords: Polychlorinated biphenyls (PCBs) Detection Surface-enhanced Raman spectroscopy (SERS) Cyclodextrins (CDs) Silver films
a b s t r a c t A mono-6-thio-b-cyclodextrin-modified silver film was synthesized via galvanic displacement on copper foil. The prepared silver films could enrich non-polar polychlorinated biphenyls (PCBs) molecules from hydrophilic phase using thiolate b-cyclodextrins (SH-b-CDs) as receptors. The components of as-prepared Ag-coated-Cu (Ag–Cu) film were confirmed by powder X-ray diffraction (XRD). Both surface-enhanced Raman spectroscopy (SERS) and energy dispersive X-ray spectroscopy (EDS) measurements gave strong evidences that the thiolated b-cyclodextrins (SH-b-CDs) had been immobilized on the surface of silver film. Compared to the substrates prepared in the absence of SH-b-CD, the surface morphology of the CD-modified Ag films was obviously changed. The interfacial enrichment and the capability of substrates to form inclusion complexes with PCBs molecules were tested by using PCB-15 (4,40 -dichlorobiphenyl) as the probe molecules via SERS technique. The measured SERS spectra could distinguish the PCB-15 molecules at micro-molar level according to the most intense CCC bending in-plane mode of PCBs. The enhancement factor (EF) of the SERS substrates for PCB-15 was 1.2 105, which was comparable with a number of previous reports. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Polychlorinated biphenyls (PCBs), a group of synthetic organic chemicals with 1–10 chlorine atoms attached to biphenyl, have shown toxic and mutagenic effects by interfering with hormones in the body. Its products have been banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2001 [1]. PCBs can transport on a global scale and do not decompose readily. The extensive distribution and the low-concentration residue in the environment make the detection of these PCBs molecules a challenging task [2–4]. The current ordinary method used for measuring PCBs is highresolution gas chromatography–mass spectrometry (HRGC–MS). In this case, the compounds are separated in time and space via the differential partitioning into the stationary phase [2]. Its wide practical application is limited by the cumbersome pretreatment and analysis. An efficient and sensitive method for detecting PCBs is urgently needed. Raman spectroscopy can provide vibrational structural spectra of the detected molecules with the highly informative character [2]. Surface-enhance Raman spectroscopy (SERS) with an amplification of the Raman signal can be attributed to an enhancement of scattering efficiencies when molecules localized near the nanostructured
⇑ Corresponding author. Fax: +86 0531 8836 6280. E-mail address: [email protected] (J. Zhan). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.08.075
noble-metal surfaces (typically within 10 nm from the surface, at most) [5]. SERS is a powerful technique for detection and characterization of the adsorbed molecules on rough metallic surface [6,7]. Surface plasmons (SPs) are excited and localized with incident light to produce a strong electric field for SERS enhancement [6,8]. Particularly, the nanomorphology of silver particles is paramount to achieving maximum SERS enhancements. Theoretical models predict that curvature and shape of the noble-metal particles are responsible for enhancement of the field intensities, where the active hot spots occur at interstitial spots between adjacent nanoparticles [9,10]. Up to now, a number of preparation methods, including solvothermal synthesis, electron beam deposition, nanosphere lithography (NSL), vacuum evaporation deposition, and electrochemical oxidation/reduction have been developed to fabricate SERS-active substrates with the well distributed ‘‘hot spots’’ [11–13]. It remains a challenge to fabricate SERS-active substrates through simple and low-cost methods [14]. For SERS detection, analyte molecules must be located near the metallic nanostructures, implying that a special chemical moiety should be incorporated on the SERS substrate for enriching trace organic species. Thus, lipophilic and poorly soluble in water are the physical properties of all PCBs. For molecule-binding chemistry, we sought to exploit the binding properties of cyclodextrins (CDs) for special small organic molecule. CDs are a class of cyclic polysaccharide molecules commonly used in host–guest complexation chemistry [15–17]. b-Cyclodextrins (b-CDs) have a cavity whose dimension are depth 7 Å in width and 9 Å in depth, which
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would be suitable for accommodating PCBs guest molecules [18,19]. Moreover, a special molecular structure-hydrophobic internal cavity and hydrophilic external surface of b-CDs could be an important driving force in inclusion complexes. In the inclusion complexation, b-CDs can be used as receptors to capture PCBs molecules in solution, which is approved by theoretical simulation [15,19,20]. Based on the theoretical calculation, inclusion complexes could be formed on Ag film between the PCBs and SH-b-CD [18]. The purposes of this paper are to experimentally evaluate the possibility of the above theory and to determine PCBs molecules in solution via SERS technique. In this study, a SH-b-CDmodified Ag film was synthesized via one-pot step, which was used as SERS-active substrate for PCBs detection. After confirmation and characterization of the inclusion complexes formed between SH-bCD and PCB-15, the results showed that the prepared substrates were sensitive to the low concentration of PCB-15 in aqueous solution, which indicated SH-b-CD-modified Ag film was a simple and effective SERS substrate for PCBs analysis. 2. Experimental section 2.1. Materials All the chemicals were analytical reagent grade and used as purchased without further purification. Beta-cyclodextrin (b-CD) was purchased from Aladdin Chemical Co. Silver nitrate (AgNO3), p-toluenesulfonyl chloride, thiourea, and copper foils (Cu, 10 cm 10 cm in size, 0.1 mm in thickness, 99.99%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ammonia solution (analytically pure, 25–28 wt%), acetonitrile, and acetone were purchased from Tianjin Kemeng Chemical Co. (Tianjin, China). The polychlorinated biphenyls compounds, PCB15 (4,40 -dichlorobiphenyl) and PCB-77 (3,30 ,4,40 -tetrachlorobiphenyl) were purchased from J & K Chemical Co. Ltd. Ultrapure water (18.2 MX cm1) was used throughout the experiments.
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the solution was filtered through a 0.22 lm syringe filter. Then, the cleaned Cu foils were dipped into the clear filtrated solution slowly, while the silver atoms were formed on copper foil spontaneously by the galvanic displacement. The loading amount of silver was controlled by reaction time and temperature [27]. In this work, the reaction was kept at 25 °C under constant stirring for 5 min. The Ag–Cu foils were rinsed consecutively with ultrapure water to obtain fresh surface and dried at room temperature for future analysis.
2.4. Characterization of substrates The as-prepared substrate was examined by powder X-ray diffraction (XRD). The chemical adsorption on the fresh substrate was investigated with SERS spectra and an energy dispersive X-ray spectroscopy (EDS). The morphology and microstructure of the silver film were characterized using a cold field emission scanning electron microscope (FE-SEM) (JSM-6700F) operating at an accelerating voltage of 3.0 kV. The Raman spectra were collected using an Ocean Optics QE65000-Raman Spectrometer. The excitation wavelength was 785 nm, and the maximum excitation power was 455 mW. Integration times were 10 s.
2.5. SERS measurements The CD-modified Ag film was used as substrate for SERS detection. PCB-15 as probe was used to confirm the sensor’s ability to enrich target molecules for SERS detection. The SERS substrates were immersed in the adsorbate solution for 24 h with stirring at room temperature. PCB-15 samples were prepared in aqueous solution with concentrations of 4.4 104, 4.4 105, 4.4 106, and 4.4 107 M. Additionally, the mutterlauge of PCB-15 was dissolved in ethanol (4.4 104 M) and diluted to the desired concentration with water.
2.2. Synthesis of mono-6-thio-b-cyclodextrin The synthesis and characterization of SH-b-CD were the same as those reported in the literatures [21–24]. A white powder was obtained. 1H NMR (400 MHz), [D6] DMSO, 25 °C, TMS, d): d = 2.05 (m, SH), 2.49–3.00 (m, 2H), 3.29–3.47 (m, overlapping with HDO), 3.55–3.79 (m, 28H), 4.32–4.48 (m, 6H), 4.82, 4.83 (br d, 7H), 5.60–5.83 (m, 14H) ppm. Anal. Calcd. for C42H70O34S7H2O: C 39.50; H 6.63; S 2.51. Found: C 41.09; H 6.60; S 3.08. The measured NMR shifts for prepared SH-b-CD were in good agreement with literature, which gave strong evidences for the quantitative occurrence of the desired substitutions and the prepared SH-b-CD [22–24]. Moreover, the prepared mono-thiolated derivative of b-cyclodextrin was further confirmed by the Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Figs. S1 and S2). 2.3. Preparation of CD-modified-Ag film The Cu foils were cut into pieces (0.5 cm 0.5 cm) and degreased thoroughly by sonication in acetone for 30 min and then rinsed with ultrapure water. Subsequently, the Cu foils were immersed in 0.1 M HNO3 aqueous solution for 1 min to remove surface oxides and then rinsed with ultrapure water and dried at room temperature for future use [25]. Ammonia solution (4 ml) and AgNO3 (0.17 g) were mixed and diluted with water so as to form 20 mM ([Ag(NH3)2]+) aqueous solution [26]. The prepared SH-b-CD solid (0.05 g) was dissolved in the mixed solution. After stirring for 5 min at room temperature,
3. Results and discussion 3.1. Pathway of the galvanic displacement The CD-modified Ag substrates were fabricated by the galvanic displacement (GD, also known as electroless deposition) reaction on commercial copper foils. This controllable and inexpensive dip-and-rinse GD process enabled the synthesis of silver films. For the synthesis of Ag nanoparticles, the Tollen’s reagent ([Ag(NH3)2]+) was employed in the present work, which was prepared by adding excessive ammonia solution into the AgNO3 solution until a transparent solution was obtained [28]. The Tollen’s reagent as the silver precursor was reduced and the nanostructured Ag films were formed on copper foils [29,30]. In GD reaction, the redox potential of the Ag+/Ag0 couple in solution (+0.7991 V) was decreased by the ammonia down to +0.373 V [31]. The [Ag(NH3)2]+ ions were reduced rapidly and stable coordination compounds were formed between Cu2+ and the ammonia in solution, which resulted in a bright and uniform silver film on the copper foil. In the synthesis, a direct ion-exchange self-metallization technique was carried out without any external current sources or reducing agents in the bath to contaminate the prepared Ag film. In galvanic displacement procedure, the surface distribution and the metallic nanostructures of the silver film on copper foil were influenced by the Ag+ ion concentrations, temperature, and reaction time. The as-obtained results were confirmed to be the suitable experimental parameters to synthesize CD-modified Ag film for SERS detection.
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Fig. 2. Raman spectra of SH-b-CD: (a) SH-b-CD solid served as reference and (b) SHb-CD functionalized Ag–Cu film.
Fig. 1. XRD patterns of the CD-modified Ag–Cu film (top) and the copper foil (bottom).
3.2. Component and morphology analysis of the CD-modified Ag film A uniform and bright silver film formed on copper foils via a wet chemical method, which was analyzed by XRD pattern (Fig. 1). The strong peaks in the pattern could be assigned to cubic copper (JCPDS card 04-0836) and the characteristic diffraction peaks of silver-3C (JCPDS card 04-0783), indicating that a thin layer of Ag was deposited on Cu foil [25]. Moreover, the electron affinity of copper (1.24 eV) is little lower than that of silver (1.30 eV), leading to the copper in electron-deficient state and silver in electron-rich state via the free electron redistribution between the heterogeneous materials [32]. Silver films are more inert to oxidation in electron-rich state than the bottom copper foils, which could protect the upper inactive Ag films from corrosion and enable the SERS substrates to maintain their enhancement capabilities along with preserving their nanomorphologies. As shown in Fig. 2, the background SERS spectrum of the SH-bCD functionalized Ag–Cu film corresponded well with the normal Raman spectrum of the powdered solid SH-b-CD. The clear band at about 483 cm1 appeared in both spectrums, which could be assigned to a certain deformation vibration of SH-b-CD [33]. The band at 665 cm1 of solid SH-b-CD was related to the C–S stretching vibrations, which shifted to 650 cm1 in the SERS spectrum of SH-b-CD functionalized Ag–Cu film. Also, the strong intensity peak at 1137 cm1 and 1336 cm1 (attributable to the C–C stretching and C–H deformation vibration of (CH2)2) in the solid SH-b-CD spectrum (Fig. 2a), shifted to 1130 cm1 and 1328 cm1 in the SERS spectrum of the SH-b-CD functionalized Ag–Cu film (Fig. 2b) [19]. The results suggested that the SH-b-CD bounded to the silver surface via S–Ag bonds and adsorbed on Ag film steadily. As shown in Fig. 3 with energy dispersive X-ray spectroscopy (EDS), the peak of S was extremely weak and the peak of Ag was intensive, which could be vested in thiol groups of SH-b-CD and silver film, respectively. Both SERS spectra and EDS image gave strong evidences that the SH-b-CD had been immobilized on the surface of silver film through one-pot synthesis. The surface morphology and microstructure of the substrates were confirmed by SEM image (Fig. 4b). The CD-modified Ag film displayed irregular particles with an uneven size distribution over
Fig. 3. EDS image of the CD-modified Ag–Cu film.
the surface (Fig. 4c) in contrast to the bare Ag film (Fig. 4b) with spherical silver particles on the copper foils (Fig. 4a) [33]. The SH-b-CD as stabilizer played a role in inducing the crystal nucleus growth during the reduction of silver ions [34]. A self-assembled monolayer (SAM) of SH-b-CD formed on the silver surface due to the bonds between the silver particles reaction and the thiol groups [35]. The resulting irregular particles were randomly accumulated together on the copper foils, which could provide sufficient gaps among the adjacent silver particles [36]. Namely, there were more robust active sites as ‘‘hot spots’’ where the signal of Raman-active molecules could be enhanced [37]. It should be mentioned that the prepared SERS-active substrates commonly relied on a salt-induced aggregation or self-assembly process [38].
3.3. SERS detection of polychlorinated biphenyl compounds Theoretical simulations have proved that SH-b-CD could form inclusion complexes with PCBs and Raman spectra could be used for the identification of CD-modified PCBs [18,19]. Based on those results, schematic representation of inclusion complex of PCBs-CD Ag film for SERS experiments was shown (Fig. S3). [39]. SERS spectra of the PCB-15 molecules at different concentrations were shown in Fig. 5. It could be seen that PCB-15 can be detected even at about 10 ppm level, which should be owe to the fact that PCB-15 were included in the cavity of chemisorbed CDs derivatives on silver film [19]. Some prominent bands of PCB-15 could be observed. Such as C–Cl stretch bands (776 cm1 shift), and the
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Fig. 5. SERS spectra of the PCB-15 CD-modified Ag substrate following immersed in sample solutions. (a) The powdered solid sample. (b) 4.4 104 M. (c) 4.4 105 M. (d) 4.4 106 M. (e) 4.4 107 M.
Fig. 6. The most intense ring deformation in-plane mode for PCB-15 according to the Gaussian calculations.
Fig. 4. SEM images of (a) the bare copper foil and (b) Ag–Cu film and (c) CDmodified Ag–Cu film.
aromatic bands (543, 631 and 1094 cm1 shift), and the biphenyl C–C bridge stretch (1285 cm1 shift), and the most intense CCC bending in-plane mode (1600 cm1 shift) [40,41]. With dilution of the samples, most peaks of PCB-15 were covered by the SH-bCDs except the peak at about 1600 cm1, and the SERS band at about 1600 cm1 could serve as an internal standard in the evaluation of the band intensity of the PCB-15 molecules. The SERS intensity of the vibrational band at about 1600 cm1 was the strongest, which could be assigned to the most intense CCC bending inplane mode for PCB-15 according to the references [18]. The experimental results confirmed the theoretical simulation that SH-b-CDs as host molecules had the ability to accommodate PCB-15 molecules. Meanwhile, the adsorption behavior of the host–guest inclusion complexes on the silver film was elucidated [18,42,43]. To gain a clear understanding of these spectra, Density Functional Theory (DFT) with Gaussian 03 program package were used to simulate the vibration modes (Fig. 6) [13]. On the basis of experiments and computational simulation, the intensity of the Raman mode at 1600 cm1 was quite sensitive to the concentration of PCB-15, which could be quantified as the integrated peak area for the CDmodified PCB-15 on the substrate. For enriching and detecting PCBs, the SERS substrates were incubated in sufficient time for absorbing as much target molecules as possible. The results
demonstrated that the intensity of the band at about 1600 cm1 decreased with diluted samples, which indicated the enrichment capacity of the SERS substrate was not saturated. The coverage of PCBs peaks and the adsorption capacity of the substrates were attributed to sufficient SH-b-CDs molecules on the Ag films. SERS detection of PCB-77 (3,30 ,4,40 -tetrachlorobiphenyl) was carried out and shown in Figs. S4 and S5. Based on the above results, the CD-modified Ag substrate had high sensitivity for detecting PCBs using SERS measurement (Figs. 5 and S4). The SERS-activity of CD-modified Ag film was mainly attributed to two aspects [2,18]. On the one hand, the unique surface morphology of the silver film could be responsible for SERS efficiency. Compared to the substrates synthesized in the absence of SH-b-CD, The CD-modified Ag substrate had more gaps among adjacent particles due to the robust surface morphology (Fig. 4b and c), resulting in a Raman enhancement when PCBs molecules were trapped in those gaps [44]. The enhancement factor (EF) was calculated using (ISERS/INR)(CNR/CSERS), where ISERS represents the SERS intensities of the 1600 cm1 band for PCBs adsorbed on the SERS substrate, and INR represents the normal Raman signal recorded for powdered PCBs adsorbed on the quartz substrate, whereas CSERS and CNE represent the corresponding concentrations of PCBs on these substrates. The enhancement factor (EF) of PCB-15 was 1.2 105, which was acceptable compared to the previous reports [45].
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On the other hand, the great PCBs-binding capacity of the SERS substrates was attributed to the SH-b-CD molecules on the substrates. Cyclodextrins (CDs) have been extensively studied as molecular receptors due to their abilities of absorbing various hydrophobic compounds [18,19]. PCB-15 as SERS probes were enriched from aqueous solution. The theoretical calculation of the host–guest interations between PCBs and SH-b-CD, indicating the intrinsic capability of CDs for including PCBs molecules was more significant than solvent effect on the inclusion complexes [18]. In other words, PCBs molecules prefer forming the inclusion complexes to dispersing in water. In this case, the PCB-15 molecules are extracted from exterior system to enter the SH-b-CD cavities because the dimension of PCB-15 matches with the cavity diameter (7.47 Å) of SH-b-CD [18]. As a result, the PCB-15 molecules were enriched on the surface of the substrate and SERS spectra of host–guest complexes were obtained (Fig. 5). In this work, the adsorption of PCBs were measured with bare Ag–Cu substrates, whereas practically no SERS bands of PCBs were detected at a saturation concentration of about 100 ppm (Fig. S6). The results further confirmed that the CD-modified Ag film had ability to accommodate PCBs molecules and locate them to the rough metal surface for SERS detection. Both the unique surface morphology of the SERS substrates and inclusion behavior of SH-b-CD on the Ag– Cu film were all critical to the remarkable enhancement of Raman spectroscopy for detecting PCBs.
4. Conclusion In summary, a CD-modified Ag film was synthesized for detecting PCBs via SERS technique. SH-b-CDs as a surface modifier and the receptor were incorporated with the silver particles via onepot synthesis. In addition, we have confirmed the CD-modified Ag films had ability to accommodate PCB-15 molecules and recorded the respective Raman-enhanced spectra at different concentrations. The results showed that the hybridized SERS substrates had good sensitivity when the PCBs molecules matched with the cavity size of the SH-b-CDs in SERS experiments, which could be a new class of analytical assay for trace detection of PCB homologues.
Acknowledgments We thank the financial support from National Basic Research Program of China (973 Program 2007CB936602), National Natural Science Foundation of China (NSFC 21075077), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ201004).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.08.075.
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