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Facile fabrication of SERS substrate based on food residue eggshell membrane
Chemical Physics Letters xxx (2016) xxx–xxx
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Research paper
Facile fabrication of SERS substrate based on food residue eggshell membrane Na Wang ⇑, Zhennan Ma, Shutao Zhou, Guo Liang College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, Sichuan, PR China
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Article history: Received 19 September 2016 In final form 28 October 2016 Available online xxxx Keywords: Eggshell membrane (ESM) Sonochemical synthesis Surface-enhanced Raman scattering (SERS) Solid substrate
a b s t r a c t This work reports a facile and in situ method for synthesis of silver nanoparticles on the surface of eggshell membrane, which contains 3D interconnected porous networks and can be used for immobilization of metal nanoparticles. The as-prepared composites exhibit excellent SERS activities as substrates for 4mercaptopyridine and Rhodamine 6G. The SERS measurements show detection limits of 5.0 109 M for 4-mercaptopyridine and 1.0 106 M for Rhodamine 6G on the prepared substrates, respectively. Furthermore, the uniformity and stability of this substrate have been studied and the results are satisfactory. The as-prepared substrate is potentially useful in practical SERS detection applications. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) technique has been widely utilized in sensitive detection of low concentration analytes owing to its high amplification of electromagnetic fields produced by the excitation of localized surface plasmons [1–3]. The interaction between adsorbed molecules and the surface of plasmonic nanostructures determines SERS activity, thus plasmonic nanostructure is the decisive role in success of SERS. It is well known the natural world has abundant, functional and almost perfect mesoscopic and macroscopic structures such as DNA [4], butterfly wings [5,6], lotus leaves [7,8] and enzymes [9], etc. These natural creatures have been manipulated or modified to extended structures for application in various fields including SERS. Xu et al. developed a highly efficient SERS substrate through a facile physical vapor deposition coating of silver nanolayer on white rose petals [10]. The SERS detection limit for Rhodamine 6G (R6G) was as low as 109 M. Likewise, Chou et al. synthesized ecofriendly substrates based on common rose petals due to their quasi-threedimensional nanofold structure for ultrasensitive SERS [11]. The SERS measurements had a detection limit for R6G below 1015 M. Mu et al. in situ prepared gold nanoparticles (AuNPs) in natural 3D photonic architectures of butterfly wings for SERS. The substrate could detect 109 M of 4-amionthiophenol [12]. However, as far as we know, low attention has been given to the solid SERS substrates based on the natural materials despite some nature nanostructures may hold great promise for achieving localized surface plasmon ⇑ Corresponding author. E-mail address: [email protected] (N. Wang).
resonance. It is vital to explore other inexpensive, environmentalfriendly and readily available biomaterials for synthesizing and incorporating NPs for SERS applications. The eggshell membrane (ESM) is a portion of hen egg situated between the eggshell and the egg white, which is generally considered as waste and can be readily obtainable from kitchens and industry. The ESM mainly consists of two parts: the outer ESM and the inner ESM, which are composed mainly of proteins and glycoproteins containing lysine-derived cross-links [13]. Today, ESM has been widely studied [14] because of its fascinating interwoven fibrous 3D structure and protein-rich component. Number of papers dealing with ESM have been published including matrices for immobilization of AuNPs and enzyme for biosensing application [15,16], adsorption of heavy metal [17–19], platform for the growth of nanocrystals [20,21] and substrate for metal NPs as catalyst [22,23]. Recently, Lin et al. reported the fabrication of SERSactive substrate using a porous ESM decorated with silver nanoparticles (AgNPs) [24]. However, their approach has several drawbacks. First, it is time-consuming that the ESMs have to be immersed in 35% H2O2 for 24 h and vacuum dried for 48 h prior to further treatments. Second, researchers firstly prepared AgNPs then dropped solution onto ESMs. In other words, the preparation and deposition of the AgNPs on ESMs were not one-pot process. Third, hydrogen peroxide is a strong oxidant which can be toxic in high concentrations. Exposure of skin or eye to concentrations above 30% H2O2 may lead to severe irritation or burns, while ingestion may cause worse consequences [25]. In this study, we present a newly modified sonochemical strategy to in situ synthesize AgNPs on ESM at ambient conditions and room temperature. Sonochemical synthesis of nanomaterials is a
http://dx.doi.org/10.1016/j.cplett.2016.10.077 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
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facile choice for combining the formation of various nanostructures and deposition of them on solid substrates in a single operation. There are a large number of reports about coating of nanoparticles on solid substrates by sonochemical methods [26,27]. However, to the best of our knowledge, the coating of metal nanoparticles on ESM by sonication route has never been reported. The ESMs are used without any treatment of chemicals and the preparation process includes only hydroxylamine hydrochloride and sodium hydroxide for reduction of silver nitrate without any surfactants and/or organics. Compared to other syntheses of AgNPs on solid substrates, the proposed method is facile, green and efficient. The as-prepared AgNPs/ESMs composites are utilized as SERS substrates for detecting 4-mercaptopyridine (4MPY) and R6G. The samples are characterized by a variety of analytical methods to confirm the existence of AgNPs. Furthermore, the AgNPs in solid phase are more stable than that in liquid phase thus leading to good stability of the as-prepared substrate. As a result, the ESM-based SERS substrate provides high sensitivity, with good reproducibility and stability which make it potential as practical SERS application. 2. Materials and methods 2.1. Materials All chemicals used were of analytical grade. Silver nitrate (AgNO3, 99.8%), hydroxylamine hydrochloride (NH2OHHCl, 98.5%), sodium hydroxide (NaOH, 98.0%) and hydrochloric acid (HCl) were obtained from Kelong (Chengdu, China). 4mercaptopyridine (4-MPY, 95%) was from Saen (Shanghai, China) and Rhodamine 6G (R6G) was from Marckin (Shanghai, China). Fresh eggshell was obtained from the canteen of Southwest Petroleum University. The water used in all the experiments was ultrapure water. 2.2. Synthesis of AgNPs on ESM An ESM was carefully peeled off from eggshell and was then washed with copious amounts of water to remove the albumen. The cleaned ESM was immersed in a 10%HCl for 15 min to remove the residual impurities. And the air cell was used to separate the inner membrane and outer membrane. The inner ESM was washed with plenty of water. Then clean inner ESM was dried in air at ambient conditions and cut into small rectangular pieces (ca. 10 10 mm) for future use. The inner ESM was used throughout our study and shall be referred to as ESM throughout the manuscript. In a typical procedure, the ESM was immersed in 18.0 mL of 1.0 103 M AgNO3 solution and stirred gently. After agitation for 2 h, the mixed hydroxylamine hydrochloride (1.8 mL, 1.67 102 M)/sodium hydroxide (0.2 ml, 3 101 M) solution was added rapidly to AgNO3 solution containing ESM and the mixture was synchronously sonicated with a PS-10AD ultrasonic cleaner (Kejie, China). The ultrasonic power was 100 W and ultrasonic frequency was 28 kHz. After 20 min, the AgNPs/ESM samples were removed from the solution and rinsed with copious water to remove impurities. Finally, the sample was placed onto microscope slides and air-dried at ambient conditions before further analysis. 2.3. Characterization of AgNPs/ESM The diffuse reflectance UV–vis spectra (UV–vis–DRS) were measured with a Lambda 850 UV–vis spectrophotometer (PerkinElmer, USA) equipped with an intergrating sphere accessory using BaSO4 as reference. The morphological features were observed by field-emission scanning electron microscopy FESEM (Zeiss EVO
MA15, Germany) coupled with energy-dispersive X-ray spectroscopy (EDS) detectors (Oxford AzetX Max 80, Britain). The samples, like non-conductive surfaces, were sputter coated with gold. The detailed morphology and size distribution of the AgNPs were determined using a transmission electron microscope (TEM) (Libra200, Carl zeiss irts, Germany). The AgNPs/ESM composite was ultrasonicated in water for 30 min, and then a drop containing the released AgNPs and fragments of ESM was deposited onto the copper grids. The sample was dried in air at room temperature for TEM analysis. The Powder XRD patterns of samples were recorded on an X’Pert PRO MPD powder X-ray diffractometer (Panalytical, Nertherlands) with Cu Kɑ radiation. XRD patterns were recorded from 30° to 80° (2h) with a scanning step of 0.017°. The X-ray photoelectron spectroscopy (XPS) experiment was performed on an ESCALAB 250Xi XPS spectrometer (Thermo SCIENTIFIC, USA) using Al Kɑ source. Fourier transform infrared (FTIR) spectra were recorded on a WQF520 FTIR spectrometer (Ruili, China) within the range 400–4000 cm1. The ESMs were ground into powder and then mixed with KBr (spectroscopic grade) in an agate mortar. Discs were prepared in a manual hydraulic press for FTIR analysis. The thermo-gravimetric (TGA) analyses of natural ESM and AgNPs/ ESM composites were performed a STA449F3 simultaneous thermal-gravimetric and differential thermal (TGA-DTA) apparatus (NETZSCH, Germany), from 40 °C to 700 °C at a heating rate of 10 °C/min in Air. 2.4. SERS measurements A 2-uL droplet of 4-MPY or R6G was placed on the as-prepared AgNPs/ESM substrate and allowed to dry for Raman analysis. All the Raman measurements were performed with a confocal ID Raman microscope (Ocean Optics, USA) equipped with a polarizer at an excitation wavelength of 785 nm and a power of 6 mW at the sample. All Raman spectra were recorded by fine-focusing a microscope objective (40 /0.5) under data acquisition time of 1 s. For all quantitative analyses, 10 SERS measurements were acquired on different locations of the substrates. 3. Results and discussion 3.1. Formation of AgNPs on ESM ESM comprises interwoven fibrous structure, which can be easily observed by FESEM. As shown in Fig. 1-(a)-(b), ESM contains compact network-like structure with smooth protein fibers whose diameters range within 0.5–1.5 lm. It has been reported the silver colloids were prepared by reduction of silver nitrate with hydroxylamine hydrochloride at alkaline pH [28]. In our study, the AgNPs were synthesized on the ESM in this way by ultrasonic cleaner instead of stirrer. Fig. 1(c)-(d) evidently shows the irregular spherical AgNPs are immobilized and dispersed on the surface of ESM fibers. Most of the AgNPs are well separated while large aggregates can also be observed. The fibers of ESM are rich in proteins, collagens (types Ⅰ, Ⅴ and Ⅹ) and glycoproteins [29,30], of which the amino acid could offer many reactive sites for anchoring Ag+ ions by immersing. The coordinated Ag+ ions are reduced to AgNPs by hydroxylamine hydrochloride. The color of the ESM changed from white to brown after ultrasonic reaction which also suggested the formation of AgNPs. The additional support of AgNPs, as confirmed by EDS analysis (Fig. S1), shows the characteristic Ag0 peaks at 3.0 and 3.2 keV due to surface plasmon resonance [22,31]. The TEM measurements (Fig. 2(a)) show that the silver particle size distributions are relatively broad; the particle diameters for most of the particles are almost 3.0–6.0 nm, which are welldispersed on the ESM while the particle diameters for the larger
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Fig. 1. FESEM photographs of eggshell membrane: (a)-(b) natural ESM for comparison and (c)-(d) immobilization of AgNPs on ESM.
Fig. 2. (a) TEM images of AgNPs immobilized on ESM. (b) Released AgNPs in the aqueous solution. The inset photo is local TEM image of AgNPs with high magnification.
particles range from 20 to 100 nm, some of which are made up of an agglomeration of smaller particles. Thus it is difficult to determine an average particle size of AgNPs immobilized on ESM. As shown in Fig. 2(b), the released AgNPs are homogeneously distributed in the aqueous solution without agglomeration. It is calcu-
lated from more than 200 particles in several TEM images that the average size of AgNPs released in aqueous solution is 4.6 ± 0.9 nm. The crystalline structure of AgNPs on ESM was studied by the powder XRD technique. Fig. 3(a) shows the XRD patterns of natural ESM and AgNPs on ESM. Natural ESM is almost amorphous due to its ingredients containing mainly amides, amines, and carboxylic compounds. As a result, there are no evident peaks observed for natural ESM. After immobilization of AgNPs, the Bragg reflection peaks of the sample exhibited at 2h of 38.1°, 44.3°, 64.5°, and 77.4° are assigned to diffraction from the (111), (200), (220), and (311) planes of face center cubic (fcc) crystalline silver (JCPDS 36-1451). The result confirms the formation of typical fcc structure of polycrystalline AgNPs on ESM. Fig. 3(b) is the UV–vis–DRS spectra of natural ESM and AgNPs/ESM composites. Compared to natural ESM, the AgNPs/ESM composites show intense adsorption band with the maximum at 428 nm, which is a characteristic peak of surface plasmon resonance of AgNPs [23]. Despite of the different adsorption value, both of the samples show the same two adsorption bands at around 210 and 280 nm, which are ascribed to the polypeptide chains and the aromatic groups of proteins, respectively [9]. Moreover, the XPS measurements were performed to confirm the existence of AgNPs. As depicted in Fig. 4, two peaks at 368.4 and 374.4 eV belong to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. The splitting of the 3d doublet of Ag is 6.0 eV illustrating the presence of AgNPs (Ag0) on the surface of ESM [32]. The result is in consistent with XRD, EDS and UV–vis–DRS data illustrating the metallic silver nanocrystals form on the surface of ESM. In order to investigate the structure of AgNPs/ESM composites comprehensively, FTIR and TGA analysis were performed on natural ESM and AgNPs/ESM samples (Fig. S2). For natural ESM and AgNPs/ESM, the characteristic bands of FTIR spectra are nearly the same which illustrate the AgNPs immobilized on ESM may be derived from physical interactions. While TGA analyses on these two samples demonstrate the AgNPs/ESM hybrid materials possess higher thermal decomposition temperatures. The improved thermal stability of AgNPs/ESM composites in comparison to natural ESM may be a result of the interactions between AgNPs and ESM. The interactions between AgNPs and groups of ESM fibers such as ACOOH, ANH2, and ASH are supposed to reduced mobility of
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Fig. 4. (a) XPS survey scan of a silver nanoparticles coated ESM. (b) Ag 3d XPS spectrum of AgNPs on the surface of ESM. The dash line is the background intensity.
Fig. 3. (a) XRD patterns and (b) UV–vis–DRS spectra of (A) natural eggshell membrane and (B) silver nanoparticles-immobilized eggshell membrane.
the protein chains of ESM [33,34]. Consequently, the thermal decomposition rate of AgNPs/ESM is slower than that of natural ESM. 3.2. SERS performances of the AgNPs/ESM substrates To investigate the detection capabilities of our AgNPs/ESM substrate, the SERS spectra of 4-MPY and R6G as probe molecules with different concentrations were collected as displayed in Fig. 5. In Fig. 5(a), there are many spectra features that are characteristic of 4-MPY, such as those at 712, 1005, 1061, 1091, 1220, 1575, and 1604 cm1. The strongly enhanced band at 1091 cm1 is indicative of covalent binding of sulfur to the AgNPs due to effects of the altered CAS bond on the ring vibrations [35,36]. The peak around 712 cm1 is assigned to in-plane ring deformation with C@S, which also supports the formation of AgAS bond. The band at 1575 and 1604 cm1 correspond to the ring stretch with nitrogen vibrations and that at 1061 and 1220 cm1 are assigned to in-plane CAH vibrations [35,36]. Furthermore, the band at around 1005 cm1 is ascribed to the ring-breathing vibrations. The peak around 1091 cm1 is the most intensive one which is selected as
instructive peak for the quantitative analysis. In Fig. 5(c), some characteristic bands of R6G are also observed. Vibrations at 1363, 1505, 1575, and 1651 cm1 are ascribed to CAC stretching of the aromatic ring. The peak at 773 cm1 is assigned to the out-ofplane motion of the hydrogen atoms of the xanthenes skeleton [37,38]. The peak intensities of the R6G at 1505 cm1 were collected for quantitative analysis. As can be seen in Fig. 5, the substrates show the detection limits of concentration of 5.0 109 M for 4-MPY and 1.0 106 M for R6G, respectively. The enhancement factors (EF) for different concentrations of 4MPY and R6G are calculated as following equation [39,40]:
EF ¼
ISERS C NR INR C SERS
where ISERS is the SERS signal intensity of representative bands for 4-MPY or R6G, INR is the normal Raman signal for powdered 4MPY or R6G on the glass slide, and CSERS and CNR represent the corresponding concentrations of 4-MPY or R6G on AgNPs/ESM substrates and neat 4-MPY or R6G on the glass slide. The specific calculation process is demonstrated in Supplementary material. The estimated EF values for different concentrations of 4-MPY and R6G have been shown in Table 1. For 4-MPY, the EF values range from 6.1 105 to 1.1 109, while those for R6G range from 2.4 103 to 1.2 105. The origins
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Fig. 5. (a) SERS spectra of 4-MPY on AgNPs/ESM substrate with concentrations of (A) 5.0 104 M, (B) 5.0 105 M, (C) 5.0 106 M, (D) 5.0 107 M, (E) 5.0 108 M, (F) 5.0 109 M. The inset displays magnified SERS spectra changes of 4-MPY at lower concentration of (C)-(F). (b) Intensity-log plot for 4-MPY in plane (a). The inset is the chemical formula of 4-MPY. (c) SERS spectra of R6G on AgNPs/ESM substrates with concentrations of (A) 1.0 103 M, (B) 1.0 104 M, (C) 1.0 105 M, (D) 1.0 106 M. The inset displays magnified SERS spectra changes of R6G at lower concentration of (C)-(D). (d) Intensity-log plot for R6G in plane (c). The inset is the chemical formula of R6G.
Table 1 EF values for different concentrations of 4-MPY and R6G. Concentration of 4-MPY (mol/L)
EF for 4MPY
Concentration of R6G (mol/L)
EF for R6G
5.0 104 5.0 105 5.0 106 5.0 107 5.0 108 5.0 109
6.1 105 3.5 106 9.1 106 1.8 107 1.4 108 1.1 109
1.0 103 1.0 104 1.0 105 1.0 106 _ _
2.4 103 1.1 104 2.1 104 1.2 105 _ _
of the SERS effect are mainly owing to electromagnetic (EM) and chemical or charge transfer (CT) mechanisms. The much higher EF values of 4-MPY than that of R6G may be attributed to a chemical effect caused by the high affinity between mercapto group of the 4-MPY and silver nanoparticles [41,42]. In order to study the uniformity of the as-prepared substrate, we compared the intensities of the most obvious band of 1091 cm1 for 4-MPY (5.0 106 M) at 15 random spots of the
same substrates, the relative standard deviation (RSD) of signal was 7.29%. In Fig. 6(a), the result suggests the AgNPs/ESM substrate provides uniform SERS enhancement upon its entire surface. Furthermore, the stability of the substrate was also studied. The substrate based on the ESM was prepared according to above method. Then the substrate was cut into two pieces with the same size. The SERS intensity of 4-MPY (5.0 106 M) was collected on the half of the freshly prepared substrate. And the left half of the membrane was stored at ambient temperature for further analysis. Twenty days later, 4-MPY also with the concentration of 5.0 106 M was dropped onto the substrate to record its SERS spectrum. In Fig. 6(b), Raman spectra of 4-MPY adsorbed onto the two substrates show close signal intensities at its characteristic peaks. The intensity of the 4-MPY at 1091 cm1 on the substrate after long time storage decreases 11.5% compared to that on freshly prepared substrate, which illustrates the AgNPs/ESM has outstanding stability and the solid substrate like ESM could protect the AgNPs from accumulation. These results suggest that the AgNPs/ESM can be used as a promising SERS-active material for analysis.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.10. 077. References
Fig. 6. (a) Raman signal intensity of the 1091 cm1 line for 4-MPY (5.0 106 M) recorded at 15 randomly chosen spots on the same AgNPs/ESM substrate. (b) Raman spectra for 4-MPY collected on AgNPs/ESM substrate (A) of freshly prepared and (B) after 20 days storage.
4. Conclusions In summary, we have developed a facile, environmentalfriendly strategy to fabricate the SERS-active substrate based on the eggshell membrane. Natural ESM has interwoven 3D nanostructure and the functional groups, which can be used as immobilization of silver ions. Then silver ions anchored on the surface of ESM can be rapidly reduced to AgNPs by addition of reducing agent with the assistance of ultrasonic radiation. The 4-MPY and R6G are used as probe molecules to research the SERS performance of the AgNPs/ESM substrates. The detection limits of these two probes are 5.0 109 M and 1.0 106 M, respectively. Additionally, the uniformity and stability of the substrates are studied, the little signal variation illustrates the substrates have the potential for SERS application. More importantly, the method for fabricating nanocomposites based-on ESM are very promising for application in other fields such as biosensor and catalysis, etc. Acknowledgments
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This work was supported by the scientific research starting project of SWPU (No. 2015QHZ016).
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Research paper
Facile fabrication of SERS substrate based on food residue eggshell membrane Na Wang ⇑, Zhennan Ma, Shutao Zhou, Guo Liang College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, Sichuan, PR China
a r t i c l e
i n f o
Article history: Received 19 September 2016 In final form 28 October 2016 Available online xxxx Keywords: Eggshell membrane (ESM) Sonochemical synthesis Surface-enhanced Raman scattering (SERS) Solid substrate
a b s t r a c t This work reports a facile and in situ method for synthesis of silver nanoparticles on the surface of eggshell membrane, which contains 3D interconnected porous networks and can be used for immobilization of metal nanoparticles. The as-prepared composites exhibit excellent SERS activities as substrates for 4mercaptopyridine and Rhodamine 6G. The SERS measurements show detection limits of 5.0 109 M for 4-mercaptopyridine and 1.0 106 M for Rhodamine 6G on the prepared substrates, respectively. Furthermore, the uniformity and stability of this substrate have been studied and the results are satisfactory. The as-prepared substrate is potentially useful in practical SERS detection applications. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) technique has been widely utilized in sensitive detection of low concentration analytes owing to its high amplification of electromagnetic fields produced by the excitation of localized surface plasmons [1–3]. The interaction between adsorbed molecules and the surface of plasmonic nanostructures determines SERS activity, thus plasmonic nanostructure is the decisive role in success of SERS. It is well known the natural world has abundant, functional and almost perfect mesoscopic and macroscopic structures such as DNA [4], butterfly wings [5,6], lotus leaves [7,8] and enzymes [9], etc. These natural creatures have been manipulated or modified to extended structures for application in various fields including SERS. Xu et al. developed a highly efficient SERS substrate through a facile physical vapor deposition coating of silver nanolayer on white rose petals [10]. The SERS detection limit for Rhodamine 6G (R6G) was as low as 109 M. Likewise, Chou et al. synthesized ecofriendly substrates based on common rose petals due to their quasi-threedimensional nanofold structure for ultrasensitive SERS [11]. The SERS measurements had a detection limit for R6G below 1015 M. Mu et al. in situ prepared gold nanoparticles (AuNPs) in natural 3D photonic architectures of butterfly wings for SERS. The substrate could detect 109 M of 4-amionthiophenol [12]. However, as far as we know, low attention has been given to the solid SERS substrates based on the natural materials despite some nature nanostructures may hold great promise for achieving localized surface plasmon ⇑ Corresponding author. E-mail address: [email protected] (N. Wang).
resonance. It is vital to explore other inexpensive, environmentalfriendly and readily available biomaterials for synthesizing and incorporating NPs for SERS applications. The eggshell membrane (ESM) is a portion of hen egg situated between the eggshell and the egg white, which is generally considered as waste and can be readily obtainable from kitchens and industry. The ESM mainly consists of two parts: the outer ESM and the inner ESM, which are composed mainly of proteins and glycoproteins containing lysine-derived cross-links [13]. Today, ESM has been widely studied [14] because of its fascinating interwoven fibrous 3D structure and protein-rich component. Number of papers dealing with ESM have been published including matrices for immobilization of AuNPs and enzyme for biosensing application [15,16], adsorption of heavy metal [17–19], platform for the growth of nanocrystals [20,21] and substrate for metal NPs as catalyst [22,23]. Recently, Lin et al. reported the fabrication of SERSactive substrate using a porous ESM decorated with silver nanoparticles (AgNPs) [24]. However, their approach has several drawbacks. First, it is time-consuming that the ESMs have to be immersed in 35% H2O2 for 24 h and vacuum dried for 48 h prior to further treatments. Second, researchers firstly prepared AgNPs then dropped solution onto ESMs. In other words, the preparation and deposition of the AgNPs on ESMs were not one-pot process. Third, hydrogen peroxide is a strong oxidant which can be toxic in high concentrations. Exposure of skin or eye to concentrations above 30% H2O2 may lead to severe irritation or burns, while ingestion may cause worse consequences [25]. In this study, we present a newly modified sonochemical strategy to in situ synthesize AgNPs on ESM at ambient conditions and room temperature. Sonochemical synthesis of nanomaterials is a
http://dx.doi.org/10.1016/j.cplett.2016.10.077 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
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facile choice for combining the formation of various nanostructures and deposition of them on solid substrates in a single operation. There are a large number of reports about coating of nanoparticles on solid substrates by sonochemical methods [26,27]. However, to the best of our knowledge, the coating of metal nanoparticles on ESM by sonication route has never been reported. The ESMs are used without any treatment of chemicals and the preparation process includes only hydroxylamine hydrochloride and sodium hydroxide for reduction of silver nitrate without any surfactants and/or organics. Compared to other syntheses of AgNPs on solid substrates, the proposed method is facile, green and efficient. The as-prepared AgNPs/ESMs composites are utilized as SERS substrates for detecting 4-mercaptopyridine (4MPY) and R6G. The samples are characterized by a variety of analytical methods to confirm the existence of AgNPs. Furthermore, the AgNPs in solid phase are more stable than that in liquid phase thus leading to good stability of the as-prepared substrate. As a result, the ESM-based SERS substrate provides high sensitivity, with good reproducibility and stability which make it potential as practical SERS application. 2. Materials and methods 2.1. Materials All chemicals used were of analytical grade. Silver nitrate (AgNO3, 99.8%), hydroxylamine hydrochloride (NH2OHHCl, 98.5%), sodium hydroxide (NaOH, 98.0%) and hydrochloric acid (HCl) were obtained from Kelong (Chengdu, China). 4mercaptopyridine (4-MPY, 95%) was from Saen (Shanghai, China) and Rhodamine 6G (R6G) was from Marckin (Shanghai, China). Fresh eggshell was obtained from the canteen of Southwest Petroleum University. The water used in all the experiments was ultrapure water. 2.2. Synthesis of AgNPs on ESM An ESM was carefully peeled off from eggshell and was then washed with copious amounts of water to remove the albumen. The cleaned ESM was immersed in a 10%HCl for 15 min to remove the residual impurities. And the air cell was used to separate the inner membrane and outer membrane. The inner ESM was washed with plenty of water. Then clean inner ESM was dried in air at ambient conditions and cut into small rectangular pieces (ca. 10 10 mm) for future use. The inner ESM was used throughout our study and shall be referred to as ESM throughout the manuscript. In a typical procedure, the ESM was immersed in 18.0 mL of 1.0 103 M AgNO3 solution and stirred gently. After agitation for 2 h, the mixed hydroxylamine hydrochloride (1.8 mL, 1.67 102 M)/sodium hydroxide (0.2 ml, 3 101 M) solution was added rapidly to AgNO3 solution containing ESM and the mixture was synchronously sonicated with a PS-10AD ultrasonic cleaner (Kejie, China). The ultrasonic power was 100 W and ultrasonic frequency was 28 kHz. After 20 min, the AgNPs/ESM samples were removed from the solution and rinsed with copious water to remove impurities. Finally, the sample was placed onto microscope slides and air-dried at ambient conditions before further analysis. 2.3. Characterization of AgNPs/ESM The diffuse reflectance UV–vis spectra (UV–vis–DRS) were measured with a Lambda 850 UV–vis spectrophotometer (PerkinElmer, USA) equipped with an intergrating sphere accessory using BaSO4 as reference. The morphological features were observed by field-emission scanning electron microscopy FESEM (Zeiss EVO
MA15, Germany) coupled with energy-dispersive X-ray spectroscopy (EDS) detectors (Oxford AzetX Max 80, Britain). The samples, like non-conductive surfaces, were sputter coated with gold. The detailed morphology and size distribution of the AgNPs were determined using a transmission electron microscope (TEM) (Libra200, Carl zeiss irts, Germany). The AgNPs/ESM composite was ultrasonicated in water for 30 min, and then a drop containing the released AgNPs and fragments of ESM was deposited onto the copper grids. The sample was dried in air at room temperature for TEM analysis. The Powder XRD patterns of samples were recorded on an X’Pert PRO MPD powder X-ray diffractometer (Panalytical, Nertherlands) with Cu Kɑ radiation. XRD patterns were recorded from 30° to 80° (2h) with a scanning step of 0.017°. The X-ray photoelectron spectroscopy (XPS) experiment was performed on an ESCALAB 250Xi XPS spectrometer (Thermo SCIENTIFIC, USA) using Al Kɑ source. Fourier transform infrared (FTIR) spectra were recorded on a WQF520 FTIR spectrometer (Ruili, China) within the range 400–4000 cm1. The ESMs were ground into powder and then mixed with KBr (spectroscopic grade) in an agate mortar. Discs were prepared in a manual hydraulic press for FTIR analysis. The thermo-gravimetric (TGA) analyses of natural ESM and AgNPs/ ESM composites were performed a STA449F3 simultaneous thermal-gravimetric and differential thermal (TGA-DTA) apparatus (NETZSCH, Germany), from 40 °C to 700 °C at a heating rate of 10 °C/min in Air. 2.4. SERS measurements A 2-uL droplet of 4-MPY or R6G was placed on the as-prepared AgNPs/ESM substrate and allowed to dry for Raman analysis. All the Raman measurements were performed with a confocal ID Raman microscope (Ocean Optics, USA) equipped with a polarizer at an excitation wavelength of 785 nm and a power of 6 mW at the sample. All Raman spectra were recorded by fine-focusing a microscope objective (40 /0.5) under data acquisition time of 1 s. For all quantitative analyses, 10 SERS measurements were acquired on different locations of the substrates. 3. Results and discussion 3.1. Formation of AgNPs on ESM ESM comprises interwoven fibrous structure, which can be easily observed by FESEM. As shown in Fig. 1-(a)-(b), ESM contains compact network-like structure with smooth protein fibers whose diameters range within 0.5–1.5 lm. It has been reported the silver colloids were prepared by reduction of silver nitrate with hydroxylamine hydrochloride at alkaline pH [28]. In our study, the AgNPs were synthesized on the ESM in this way by ultrasonic cleaner instead of stirrer. Fig. 1(c)-(d) evidently shows the irregular spherical AgNPs are immobilized and dispersed on the surface of ESM fibers. Most of the AgNPs are well separated while large aggregates can also be observed. The fibers of ESM are rich in proteins, collagens (types Ⅰ, Ⅴ and Ⅹ) and glycoproteins [29,30], of which the amino acid could offer many reactive sites for anchoring Ag+ ions by immersing. The coordinated Ag+ ions are reduced to AgNPs by hydroxylamine hydrochloride. The color of the ESM changed from white to brown after ultrasonic reaction which also suggested the formation of AgNPs. The additional support of AgNPs, as confirmed by EDS analysis (Fig. S1), shows the characteristic Ag0 peaks at 3.0 and 3.2 keV due to surface plasmon resonance [22,31]. The TEM measurements (Fig. 2(a)) show that the silver particle size distributions are relatively broad; the particle diameters for most of the particles are almost 3.0–6.0 nm, which are welldispersed on the ESM while the particle diameters for the larger
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Fig. 1. FESEM photographs of eggshell membrane: (a)-(b) natural ESM for comparison and (c)-(d) immobilization of AgNPs on ESM.
Fig. 2. (a) TEM images of AgNPs immobilized on ESM. (b) Released AgNPs in the aqueous solution. The inset photo is local TEM image of AgNPs with high magnification.
particles range from 20 to 100 nm, some of which are made up of an agglomeration of smaller particles. Thus it is difficult to determine an average particle size of AgNPs immobilized on ESM. As shown in Fig. 2(b), the released AgNPs are homogeneously distributed in the aqueous solution without agglomeration. It is calcu-
lated from more than 200 particles in several TEM images that the average size of AgNPs released in aqueous solution is 4.6 ± 0.9 nm. The crystalline structure of AgNPs on ESM was studied by the powder XRD technique. Fig. 3(a) shows the XRD patterns of natural ESM and AgNPs on ESM. Natural ESM is almost amorphous due to its ingredients containing mainly amides, amines, and carboxylic compounds. As a result, there are no evident peaks observed for natural ESM. After immobilization of AgNPs, the Bragg reflection peaks of the sample exhibited at 2h of 38.1°, 44.3°, 64.5°, and 77.4° are assigned to diffraction from the (111), (200), (220), and (311) planes of face center cubic (fcc) crystalline silver (JCPDS 36-1451). The result confirms the formation of typical fcc structure of polycrystalline AgNPs on ESM. Fig. 3(b) is the UV–vis–DRS spectra of natural ESM and AgNPs/ESM composites. Compared to natural ESM, the AgNPs/ESM composites show intense adsorption band with the maximum at 428 nm, which is a characteristic peak of surface plasmon resonance of AgNPs [23]. Despite of the different adsorption value, both of the samples show the same two adsorption bands at around 210 and 280 nm, which are ascribed to the polypeptide chains and the aromatic groups of proteins, respectively [9]. Moreover, the XPS measurements were performed to confirm the existence of AgNPs. As depicted in Fig. 4, two peaks at 368.4 and 374.4 eV belong to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. The splitting of the 3d doublet of Ag is 6.0 eV illustrating the presence of AgNPs (Ag0) on the surface of ESM [32]. The result is in consistent with XRD, EDS and UV–vis–DRS data illustrating the metallic silver nanocrystals form on the surface of ESM. In order to investigate the structure of AgNPs/ESM composites comprehensively, FTIR and TGA analysis were performed on natural ESM and AgNPs/ESM samples (Fig. S2). For natural ESM and AgNPs/ESM, the characteristic bands of FTIR spectra are nearly the same which illustrate the AgNPs immobilized on ESM may be derived from physical interactions. While TGA analyses on these two samples demonstrate the AgNPs/ESM hybrid materials possess higher thermal decomposition temperatures. The improved thermal stability of AgNPs/ESM composites in comparison to natural ESM may be a result of the interactions between AgNPs and ESM. The interactions between AgNPs and groups of ESM fibers such as ACOOH, ANH2, and ASH are supposed to reduced mobility of
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Fig. 4. (a) XPS survey scan of a silver nanoparticles coated ESM. (b) Ag 3d XPS spectrum of AgNPs on the surface of ESM. The dash line is the background intensity.
Fig. 3. (a) XRD patterns and (b) UV–vis–DRS spectra of (A) natural eggshell membrane and (B) silver nanoparticles-immobilized eggshell membrane.
the protein chains of ESM [33,34]. Consequently, the thermal decomposition rate of AgNPs/ESM is slower than that of natural ESM. 3.2. SERS performances of the AgNPs/ESM substrates To investigate the detection capabilities of our AgNPs/ESM substrate, the SERS spectra of 4-MPY and R6G as probe molecules with different concentrations were collected as displayed in Fig. 5. In Fig. 5(a), there are many spectra features that are characteristic of 4-MPY, such as those at 712, 1005, 1061, 1091, 1220, 1575, and 1604 cm1. The strongly enhanced band at 1091 cm1 is indicative of covalent binding of sulfur to the AgNPs due to effects of the altered CAS bond on the ring vibrations [35,36]. The peak around 712 cm1 is assigned to in-plane ring deformation with C@S, which also supports the formation of AgAS bond. The band at 1575 and 1604 cm1 correspond to the ring stretch with nitrogen vibrations and that at 1061 and 1220 cm1 are assigned to in-plane CAH vibrations [35,36]. Furthermore, the band at around 1005 cm1 is ascribed to the ring-breathing vibrations. The peak around 1091 cm1 is the most intensive one which is selected as
instructive peak for the quantitative analysis. In Fig. 5(c), some characteristic bands of R6G are also observed. Vibrations at 1363, 1505, 1575, and 1651 cm1 are ascribed to CAC stretching of the aromatic ring. The peak at 773 cm1 is assigned to the out-ofplane motion of the hydrogen atoms of the xanthenes skeleton [37,38]. The peak intensities of the R6G at 1505 cm1 were collected for quantitative analysis. As can be seen in Fig. 5, the substrates show the detection limits of concentration of 5.0 109 M for 4-MPY and 1.0 106 M for R6G, respectively. The enhancement factors (EF) for different concentrations of 4MPY and R6G are calculated as following equation [39,40]:
EF ¼
ISERS C NR INR C SERS
where ISERS is the SERS signal intensity of representative bands for 4-MPY or R6G, INR is the normal Raman signal for powdered 4MPY or R6G on the glass slide, and CSERS and CNR represent the corresponding concentrations of 4-MPY or R6G on AgNPs/ESM substrates and neat 4-MPY or R6G on the glass slide. The specific calculation process is demonstrated in Supplementary material. The estimated EF values for different concentrations of 4-MPY and R6G have been shown in Table 1. For 4-MPY, the EF values range from 6.1 105 to 1.1 109, while those for R6G range from 2.4 103 to 1.2 105. The origins
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Fig. 5. (a) SERS spectra of 4-MPY on AgNPs/ESM substrate with concentrations of (A) 5.0 104 M, (B) 5.0 105 M, (C) 5.0 106 M, (D) 5.0 107 M, (E) 5.0 108 M, (F) 5.0 109 M. The inset displays magnified SERS spectra changes of 4-MPY at lower concentration of (C)-(F). (b) Intensity-log plot for 4-MPY in plane (a). The inset is the chemical formula of 4-MPY. (c) SERS spectra of R6G on AgNPs/ESM substrates with concentrations of (A) 1.0 103 M, (B) 1.0 104 M, (C) 1.0 105 M, (D) 1.0 106 M. The inset displays magnified SERS spectra changes of R6G at lower concentration of (C)-(D). (d) Intensity-log plot for R6G in plane (c). The inset is the chemical formula of R6G.
Table 1 EF values for different concentrations of 4-MPY and R6G. Concentration of 4-MPY (mol/L)
EF for 4MPY
Concentration of R6G (mol/L)
EF for R6G
5.0 104 5.0 105 5.0 106 5.0 107 5.0 108 5.0 109
6.1 105 3.5 106 9.1 106 1.8 107 1.4 108 1.1 109
1.0 103 1.0 104 1.0 105 1.0 106 _ _
2.4 103 1.1 104 2.1 104 1.2 105 _ _
of the SERS effect are mainly owing to electromagnetic (EM) and chemical or charge transfer (CT) mechanisms. The much higher EF values of 4-MPY than that of R6G may be attributed to a chemical effect caused by the high affinity between mercapto group of the 4-MPY and silver nanoparticles [41,42]. In order to study the uniformity of the as-prepared substrate, we compared the intensities of the most obvious band of 1091 cm1 for 4-MPY (5.0 106 M) at 15 random spots of the
same substrates, the relative standard deviation (RSD) of signal was 7.29%. In Fig. 6(a), the result suggests the AgNPs/ESM substrate provides uniform SERS enhancement upon its entire surface. Furthermore, the stability of the substrate was also studied. The substrate based on the ESM was prepared according to above method. Then the substrate was cut into two pieces with the same size. The SERS intensity of 4-MPY (5.0 106 M) was collected on the half of the freshly prepared substrate. And the left half of the membrane was stored at ambient temperature for further analysis. Twenty days later, 4-MPY also with the concentration of 5.0 106 M was dropped onto the substrate to record its SERS spectrum. In Fig. 6(b), Raman spectra of 4-MPY adsorbed onto the two substrates show close signal intensities at its characteristic peaks. The intensity of the 4-MPY at 1091 cm1 on the substrate after long time storage decreases 11.5% compared to that on freshly prepared substrate, which illustrates the AgNPs/ESM has outstanding stability and the solid substrate like ESM could protect the AgNPs from accumulation. These results suggest that the AgNPs/ESM can be used as a promising SERS-active material for analysis.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.10. 077. References
Fig. 6. (a) Raman signal intensity of the 1091 cm1 line for 4-MPY (5.0 106 M) recorded at 15 randomly chosen spots on the same AgNPs/ESM substrate. (b) Raman spectra for 4-MPY collected on AgNPs/ESM substrate (A) of freshly prepared and (B) after 20 days storage.
4. Conclusions In summary, we have developed a facile, environmentalfriendly strategy to fabricate the SERS-active substrate based on the eggshell membrane. Natural ESM has interwoven 3D nanostructure and the functional groups, which can be used as immobilization of silver ions. Then silver ions anchored on the surface of ESM can be rapidly reduced to AgNPs by addition of reducing agent with the assistance of ultrasonic radiation. The 4-MPY and R6G are used as probe molecules to research the SERS performance of the AgNPs/ESM substrates. The detection limits of these two probes are 5.0 109 M and 1.0 106 M, respectively. Additionally, the uniformity and stability of the substrates are studied, the little signal variation illustrates the substrates have the potential for SERS application. More importantly, the method for fabricating nanocomposites based-on ESM are very promising for application in other fields such as biosensor and catalysis, etc. Acknowledgments
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This work was supported by the scientific research starting project of SWPU (No. 2015QHZ016).
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