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α-Fe2O3 nanocomposite for surface-enhanced Raman spectroscopy
Sensors & Actuators: B. Chemical 286 (2019) 94–100
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of nanoporous silver film by dealloying Ag/α-Fe2O3 nanocomposite for surface-enhanced Raman spectroscopy Deribachew Bekanaa,b, Rui Liua, Shasha Lia,b, Jing-Fu Liua,b,
T
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a State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P. O. Box 2871, Beijing, 100085, China b University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous silver film Iron oxide Nanocomposite Dealloying Surface-enhanced Raman spectroscopy Sudan III
A facile and reliable method for fabrication of nanoporous Ag film as surface-enhanced Raman spectroscopy (SERS) substrates was developed by dealloying the low-cost Ag/α-Fe2O3 nanocomposite in dilute acid solution. Influencing parameters such as different dealloying solutions, electrolyte concentration and dealloying time were optimized, and a uniform nanoporous Ag film of tunable and smaller ligament size was synthesized by etching the nanocomposite in sulfuric acid as a dealloying solution, while hydrochloric acid produced coarsened Ag nanoporous structures. Besides, the ligament size of the synthesized nanoporous Ag film increased with the electrolyte concentration and dealloying time. SERS activity of the as-synthesized nanoporous Ag film was evaluated using R6G as Raman probe, and the nanoporous Ag film with smaller ligament size exhibited strong SERS activity with a SERS enhancement factor of 7.47 × 106. The substrate exhibited good sensitivity and uniformity (RSD, 13.3%), and practical applicability for SERS detection of Sudan III, a synthetic organic dye banned as food additives, and achieved a detection limit of 0.5 μM. More importantly, the developed method ensures low-cost and reliable method of nanoporous materials fabrication from low-cost semiconductor based nanomaterials for applications in wide variety of fields.
1. Introduction The noble metal nanostructures (Ag, Au and Cu) has been topic of research interest due to their unique electronic, physical and chemical properties [1,2]. The fast-growing chemical nanoscience and nanotechnology have made an intriguing breakthrough in engineering different morphology of metal nanostructures for various applications such as surface-enhanced Raman spectroscopy (SERS) and catalysis [3–6]. Nanoporous metal structures are among the most fascinating morphology of metallic nanostructures and become one of the fundamental research topic in various fields [7–13]. There are several reports on potential applications of nanoporous materials for sensing and catalytic applications [14–16]. In this regard, numerous studies have been reported on the synthesis of nanoporous materials for catalysis, electrocatalysis, and electrocatalytic sensor applications [7,8,14,17]. In addition, different nanoporous materials reported in the literature demonstrated potential applications for electrochemical, biosensing and SERS [18–22]. The nanoporous metal structures are characterized by intriguing networks of interconnected structures with a morphology of tunable open pores and ligament sizes ranging from nanometer to
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micrometers giving rise to a large specific surface area [7,20,23,24]. The pore and ligament size can be tuned by altering dealloying conditions such as electrolyte concentration, dealloying time and temperature [25,26]. Several synthesis strategies have been explored for fabrication of nanoporous structures such as electrochemical dealloying [24,27], galvanic replacement [28,29], vapor-phase dealloying [30] and sacrificial template method [31,32]. So far, remarkable progress has been made in the fabrication of nanoporous metal structures using a dealloying approach. Dealloying is a process in which less noble elements selectively leached from the precursor alloy and the noble component undergoes surface diffusion leaving behind interconnected nanoporous structures [33–35]. Although several dealloying methods for fabrication of nanoporous metal structures have been reported, the methods have some limitations. This includes laborious multi-step electrochemical dealloying procedures such as corrosion-coarsening-corrosion and use of highly corrosive dealloying conditions (e.g 70% HNO3) [24]. Besides, the precursor material/master alloy used for dealloying are very expensive binary, ternary or multicomponent noble metal precursor like Au-Ag, Ag-Au-Pt, AuAgAl, Au20Cu48Ag7Pd5Si20 [36–38] and
Corresponding author. E-mail address: jfl[email protected] (J.-F. Liu).
https://doi.org/10.1016/j.snb.2019.01.114 Received 26 July 2018; Received in revised form 23 January 2019; Accepted 24 January 2019 Available online 25 January 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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method [44]. Then, the nanocomposite was etched in a dilute sulfuric acid (0.01 M) for 2.0 h at room temperature. The produced white product was rinsed successively in ultrapure water and ethanol to remove residual chemicals and dried under argon gas. To investigate effects of different electrolyte solution on the morphology of nanoporous structure, the nanocomposite was dealloyed in aqueous solution of hydrochloric acid and sulfuric acid. In addition, effect of different concentration of sulfuric acid (0.001, 0.01 and 0.02 M) was evaluated. The effect of dealloying time was also investigated by etching the nanocomposite at a different time (5 min, 30 min, 1 h, 1.5 h, 2.0 h, and 2.5 h). The obtained nanoporous Ag film was rinsed with ultrapure water and ethanol, and then dried under argon.
Al-Ag alloys that are prepared from pure metals (99.9%) [39]. This hampers the use of nanoporous metal films for real-life applications such as for SERS sensing. Therefore, there is a critical need to develop a facile and inexpensive approach for synthesizing a low-cost precursor material for the fabrication of metal nanoporous films with a capability to tailor the pore and ligament size to the desired nano- or microstructure [40]. In this regard, metal/metal oxide nanocomposites are the best candidates. Although several nanocomposites have been reported in the literature [41–43], the Ag/α-Fe2O3 nanocomposite synthesized in our previous work can be a good precursor material for the synthesis of nanoporous silver. In this study, we developed a novel, simple and effective approach for the fabrication of metal nanoporous films by dealloying a low-cost noble metal/metal oxide nanocomposites in dilute sulfuric acid. The motive of this study is to use low-cost materials such as metal oxides as less noble component that can be leach out in very dilute acid and hence, decrease the costs and toxic waste. To this end, Ag/α-Fe2O3 nanocomposite was synthesized as precursor alloy and etched in dilute acid solution. This dissolves the less-noble metal oxide nanomaterial, αFe2O3, leaving behind bicontinuous nanoporous structure of the noble metal, Ag. The current method has the following merits: (i) low-cost metal/metal oxide nanocomposite has been used as precursor alloy; (ii) the precursor alloy was prepared from non-toxic precursor, Fe metal and electroless deposition of Ag on the metal oxide nanostructure using simple wet chemistry method; (iii) dealloying process was conducted in a very dilute acidic solution; (iv) unlike previous methods, the dealloying process requires a very short dealloying time. Therefore, owed to the aforementioned advantages, the fabricated the nanoporous Ag films has been used effectively as low-cost and efficient SERS substrates for the detection of a synthetic organic dye banned as food additives.
2.3. Characterization The surface morphologies of as-fabricated nanoporous Ag film was characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan) and field-emission scanning electron microscope (FESEM, Su-8020, Hitachi, Japan). The elemental compositions of the produced nanoporous film was characterized by built-in energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectrometer (XPS, ESCALAB 250XI, Thermo Fisher, USA) with monochromatic Al-Kα X-ray source was used to characterize surface composition and chemical states.
2.4. SERS measurement SERS signal collection was performed by a Renishaw Invia Raman microscope (New Mills, U.K.) using 100x objective lens, 532 nm excitation wavelengths and laser power of 5 μW. The data acquisition time was 1 s with 10 s accumulation time. A series concentration of (10−6–10−9 M) of R6G was prepared and detected using the synthesized nanoporous silver films as SERS-active substrates.
2. Experimental section 2.1. Materials and reagents Iron foil, 0.1 mm (99.99%) and potassium hydroxide (KOH, ≥85%) were obtained from Alfa Aesar Co., Ltd. Silver nitrate (AgNO3, ≥99.8%), Rhodamine 6 G (R6 G, ≥95%) were purchased from the Sigma-Aldrich Chemical Co. (UK). Ammonia solution (NH3, 25–28%), and glucose (C6H12O6) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Hydrochloric acid (HCl, 37%) and sulfuric acid (H2SO4, 78%) were purchased from Merck (Darmstadt, Germany).
2.5. SERS analysis of Sudan III The samples for SERS measurement of Sudan III was prepared by loading 10 μL of different concentration (0–50 μM) of Sudan III acetone solution on the synthesized nanoporous Ag film and then left at room temperature for about 5 min to evaporate the solvent. Then the SERS spectrum were collected.
2.2. Fabrication of nanoporous silver film First, Ag/α-Fe2O3 nanocomposite was fabricated using our previous
Scheme 1. Schematic illustration of fabrication of the nanoporous silver film. 95
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Fig. 1. (a & b) SEM and TEM images of as-synthesized nanoporous Ag film; (c) HRTEM images showing the lattice spacing associated to the as-synthesized nanoporous Ag film; (d) STEM-EDS elemental mapping images of the nanoporous Ag film; (e) SEM-EDS elemental mapping images of nanoporous Ag film; (f) EDS spectra of as-synthesized nanoporous Ag films.
3. Results and discussion
with binding energy of 531.7 eV is attributed to surface adsorbed oxygen, while the 530.3 eV is associated to oxygen ions in the crystal lattice (Olatt). However, according to previous report, this oxygen species may not be associated with oxygen bonded to silver, AgXO, which should be in the range of 529.2–529.5 eV [45]. Therefore, it is attributed to the lattice oxygen in residual iron oxide.
3.1. Characterization of the synthesized nanoporous Ag film The nanoporous Ag film was synthesized by dealloying Ag/α-Fe2O3 nanocomposite in very dilute acid as shown in Scheme 1. First, the precursor nanocomposite (i.e AgNPs decorated α-Fe2O3 nanostructure) was synthesized by in situ reduction of Ag[(NH3)2]+ ions into metallic Ag on the surface of α-Fe2O3 nanoflakes (NFs), which was presented in our previous report [44]. Then, the Ag/α-Fe2O3 nanocomposite was etched in dilute H2SO4, which leached the α-Fe2O3 nanomaterial as a less noble component leaving behind the nanoporous Ag films. The SEM and TEM images of the nanoporous Ag films are shown in Fig. 1a and b, respectively, which reveals formation of bi-continuous nanoporous structures. Fig. 1c shows HRTEM image of the produced nanoporous structure, which clearly shows the lattice fringe of the nanoporous Ag film. The interplanar spacing was estimated to be 0.235 nm, which can be indexed to the (111) plane of Ag (PDF No. 99-0094). In addition, the STEM-EDS (Fig. 1d) and SEM-EDS (Fig. 1e) elemental mapping of the produced interconnected nanoporous Ag films revealed homogenous distribution of Ag on the surface of the nanoporous structure with a trace amount of Fe and O. Furthermore, Fig. 1f shows EDS spectrum of as-synthesized nanoporous structure and reveals presence of Ag, Fe and O. The EDS elemental mapping revealed that the wt% of Ag, O and Fe in the synthesized nanoporous Ag are 92.8%, 6.2% and 1.0%, respectively. The element Fe and O comes from residual iron oxide that has not been leached during dealloying process. The synthesized nanoporous material was further investigated using X-ray photoelectron spectroscopy (XPS) technique to confirm the surface composition and chemical states. As shown in Fig. 2a, high-resolution XPS spectra of Ag 3d consists two peaks at 368.4 and 374.4 eV separated by 6 eV, attributed to Ag3d3/2 and Ag3d5/2, which confirms presence of metallic Ag. In addition, the O1 s spectra (Fig. 2b) was fitted to two gaussian peaks with a binding energy of 530.3 eV and 531.7 eV, which confirm existence of two different kinds of O species. The peak
3.2. Optimization of dealloying conditions It is well known that the properties of nanoporous metals depend on their ligament and pore size, and thus, the precise control of the pore and ligament sizes is crucial to optimize the performance of nanoporous metals for certain applications [46]. Here, the synthesis conditions, which affect the morphology of the nanoporous structures such as different dealloying electrolyte solutions, electrolyte concentration and dealloying time were evaluated. First, the effect of different dealloying solutions viz. hydrochloric acid (HCl) and sulfuric acid (H2SO4) on the morphology evolution of nanoporous Ag film was evaluated. Fig. 3b and c shows the SEM images of the as-dealloyed nanocomposites in 0.01 M HCl and 0.01 M H2SO4, respectively. As depicted in Fig. 3b and c, the nanocomposites etched in HCl and H2SO4 solution has produced nanoporous structure with average ligament size of 73.4 ± 15 nm and 51.0 ± 9 nm, respectively. The coarsening of the nanoporous structure in HCl is due to the Cl−, which accelerated surface diffusion of Ag along the alloy/electrolyte interfaces during dealloying [47]. It is also ascribed to the dissolution rate of the less noble component, α-Fe2O3 nanostructure, which was reported to be more rapid in dilute concentration of H2SO4 than in HCl solution of equivalent concentration [48,49]. Thus, the rapid dissolution of α-Fe2O3 in H2SO4 probably increases formation of more nucleation sites, which lead to formation of small ligament size. The effect of concentration of electrolyte on morphology of the nanoporous Ag films were evaluated by etching the nanocomposite in different concentration of H2SO4 (0.001, 0.01, and 0.02 M). As shown in Fig. 3d–f, the ligament size of the nanoporous Ag films slightly 96
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Fig. 2. (a) Ag (3d) and (b) O (1 s) XPS spectra of the as-synthesized nanoporous Ag films.
3.3. Mechanism of nanoporous structure formation
increased with the concentration of H2SO4. When the nanocomposite etched in 0.001 M H2SO4, only few ligaments are formed. However, dealloying in higher concentration of H2SO4 (0.01 M and 0.02 M) produced nanoporous Ag films with an average ligament size of 51.5 ± 8 nm to 52.4 ± 11 nm, respectively. However, uniformity of the nanoporous structures decrease as concentration of H2SO4 increase and hence, relatively a uniform nanoporous Ag films were synthesized using 0.01 M H2SO4. To further investigate the influence of dealloying time on the evolution of nanoporous structure, the nanocomposite was etched at different time keeping the electrolyte concentration (0.01 M H2SO4) constant. The SEM images (Fig. 4) of as-synthesized nanoporous Ag films was examined at different dealloying time. Initially, after dealloying the nanocomposite precursor in 0.01 M H2SO4 for 5 min, the α-Fe2O3 has partially dissolved and the Ag begins to diffuse across the alloy/electrolye interface forming few nanoparticle clusters (Fig. 4a). After further 30 min dealloying, nanoporous structures with ligaments in the range of nanometer were formed, and further etching resulted in complete nanoporous structures with average ligament size ranging from 48 ± 8.7 to 56 ± 11 as the dealloying time increase from 0.5 to 2.5 h (Fig. 4). The ligament size tends to coarsen with increasing dealloying time resulting complete and a uniform nanoporous Ag films. However, a long time etching produces nanoporous structures with surface irregularities.
The mechanism of the nanoporous structure formation in current method can be explained as follows. It is well known that the dealloying method is selective dissolution of one or more less noble or active components from a precursor alloy in appropriate electrolyte to produce nanoporous metals with nanoporosity and bi-continous structure. Here, when Ag/α-Fe2O3 nanocomposite was etched in the dilute acid solution, the α-Fe2O3 will dissolve because of its high chemical activity in acidic solutions. During dealloying process, as the α-Fe2O3 start dissolving the Ag nanoparticles diffuse across the alloy/electrolye interface to form Ag nanoparticles cluster. As the dealloying proceed, αFe2O3 continue to leach out resulting Ag clusters, which grow to form ligaments through further surface diffusion of Ag atoms with different orientations, and self-assemble to form a uniform nanoporous structure. The formation of different ligament size in different electrolyte solution was attributed to impact of the acid solution counter anions on the diffusion rate of more noble metal across the interface and dissolution rate of the less-noble component, metal oxide nanomaterial. In both cases, the counter anions play an important role in accelerating the surface diffusion of the noble metal, Ag and increasing the dissolution rate of the metal oxide, α-Fe2O3. The dissolution of the iron oxide in acid involves the breakdown of Fe-O bonds, which depend on the hydrogen ion (H+) activity, and counter anions of the corresponding acid
Fig. 3. (a) SEM images of as synthesized Ag/α-Fe2O3 precursor; SEM images of the nanoporous Ag film produced using different electrolytes solutions (b) 0.01 M HCl and (c) 0.01 M H2SO4; SEM images of the nanoporous Ag film produced in different concentrations of H2SO4 (d) 0.001 M, (e) 0.01 M, (f) 0.02 M. 97
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Fig. 4. SEM images of the nanoporous Ag film synthesized by varying dealloying time in 0.01 M H2SO4. (a) 5 min, (b) 0.5 h, (c) 1 h, (d) 1.5 h, (e) 2.0 h and 2.5 h.
adsorbed on the surface. Here, the dissolution of α-Fe2O3 in HCl and H2SO4 involves formation of Fe(III)-chloro and Fe(III)-sulphato complexes on the oxide surfaces, respectively. This weakens the attractive force between Fe(III) and neighboring O2−, and reduces the repulsion between the oxide surface and protons in solution, which facilitates the dissolution rate. Therefore, rapid dissolution of α-Fe2O3 in dilute H2SO4 than equivalent concentration of HCl solution increases the number of nucleation sites leading to smaller ligament size [48,49].
signal enhancement as the dealloying time increase from 0.5 h to 2.0 h but no significant SERS enhancement was observed with further dealloying time. The potential of the synthesized nanoporous Ag films as SERS-active substrate was further investigated using R6 G. Fig. 6a shows the series concentrations of R6 G collected from the synthesized nanoporous Ag films. As can be seen, the intensity of the SERS signal of R6 G was enhanced as concentration of analyte increases with a minimum detectable concentration of 1 nM. Then, the SERS enhancement was quantitatively evaluated and the SERS enhancement factor (EF) was estimated to be 7.47 × 106 based on Eq. 1:
3.4. SERS performance of the synthesized nanoporous Ag films The SERS performance of the as-dealloyed nanocomposites for the optimized conditions were investigated using R6 G as Raman probe and the SERS enhancements are compared as given in Fig. 5. The nanoporous Ag films synthesized using H2SO4 produced small ligament size and exhibited strongest SERS enhancement (Fig. 5a), which is in good agreement with the literature [50]. In addition, SERS performance of the nanoporous Ag films synthesized at varying concentration of H2SO4 were also investigated, and the nanoporous structure with ligament size of 51.5 ± 8 nm that produced in 0.01 M H2SO4 exhibited strongest SERS enhancement (Fig. 5a). Furthermore, SERS performance of assynthesized nanoporous Ag films with varying dealloying time was compared as shown in Fig. 5b. As can be seen, there is increase in SERS
EF =
(ISERS / NSERS ) (INR/ NNR)
(1)
where ISERS and INR denotes the intensity of SERS and normal Raman (non-SERS) intensity of the analyte. NSERS and NNR stand for the number of probe molecules, R6 G, in the scattering volume for SERS and the normal Raman experiment, respectively. Moreover, to demonstrate the homogeneity of the synthesized nanoporous Ag films as a uniform SERS substrate, spot-to-spot Raman mapping was conducted on randomly selected area and SERS spectra were collected. Fig. 6b shows the 30 representative SERS spectra of R6 G collected from the randomly selected area of as-synthesized
Fig. 5. SERS spectra of R6 G (1 μM) collected on the synthesized nanoporous Ag film; (a) synthesized in different dealloying solutions (0.01 M HCl and 0.01 M H2SO4) and different concentrations of H2SO4 (0.001, 0.01 and 0.02 M); (b) synthesized in 0.01 M H2SO4 at varying dealloying time (0.5 h–2.5 h). 98
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Fig. 6. (a) SERS spectrum of series concentration of R6 G collected from nanoporous Ag films; (b) SERS spectrum of R6 G (1 μM) collected from 30 different spots via spot-to-spot Raman mapping.
nanoporous Ag films. The RSD values of relative intensity of the 30 SERS spectrum at 1650 cm−1 was 13.3%, which reveals good uniformity of nanoporous Ag films as SERS-active substrate.
Importantly, the present SERS platform fabrication method is simple, reliable, and inexpensive, which is of interest for further use in various environmental analysis applications. Furthermore, the reproducibility (spot-to-spot and sample-tosample reproducibility) of the SERS platform for Sudan III detection was evaluated. First, the point-to-point reproducibility of SERS signal enhancement over a large area of a single substrate was investigated by collecting the SERS spectrum of Sudan III from randomly selected 30 spots on the surface of the synthesized nanoporous Ag film. Figure S1a and b depicts the SERS spectrum of Sudan III collected from 30 spots and shows a consistent signal enhancement with RSD of 13.1% at a peak of 1598 cm−1. On the other hand, sample-to-sample reproducibility was conducted by collecting SERS spectra of Sudan III from three independent nanoporous Ag film substrates and five acquisition for each sample. As depicted in Figure S1c and d reproducible SERS signal enhancement with RSD of 14.6% were obtained, which is of paramount importance for quantitative analysis.
3.5. Application of the SERS substrate for analysis of Sudan III Adulteration of synthetic dyes in foodstuffs has raised concerns about food safety. Sudan dyes are among the synthetic dyes that are banned as food additives such as chili powder and other spices. Here, the suitability of the nanoporous Ag film as a SERS-active substrate for practical application was demonstrated for detection of Sudan III. Fig. 7a shows representative SERS spectra of Sudan III collected using the synthesized nanoporous Ag films. The major vibrational bands observed were attributed to Raman band of Sudan III and, according to the reported literature, [51] the peak at 1137 cm−1 assigned to δ(C–H), δ(O–H), σ(CeH)ph1, ph2, ν(CeN); 1232 cm−1 to ν(CeO), σ(CCH); 1389 cm−1 to ν (C]N), ν(N]N), δ(C–H); 1498 cm−1 to ν(C]N), ν(NeN), δ(N–H), and 1598 cm-1 to σ(CeC)ph1,ph2, ν(N]N) (Fig. 7a). For quantitative detection of Sudan III, a series of concentrations of Sudan III ranging from 0 to 50 μM were dropped on the synthesized nanoporous Ag film and their corresponding SERS spectra were collected (Fig. 7b). As can be seen, the signal intensity increases with increasing analyte concentration and the characteristic Raman bands of Sudan III can still be distinguished at a concentration down to 0.5 μM. This reveals the applicability of the nanoporous Ag film as a SERS-active substrate to detect Sudan III, which is crucial for monitoring synthetic dyes and others colorants adulteration in food foodstuffs.
4. Conclusion In summary, we report a novel, simple, affordable and effective approach for the fabrication of nanoporous Ag films by dealloying lowcost Ag/α-Fe2O3 nanocomposite. The dealloying conditions were optimized to obtain a uniform nanoporous film with bi-continuous structure. The nanoporous Ag films of different ligament sizes were synthesized by varying dealloying solution, electrolyte concentration and dealloying time. The nanoporous Ag films synthesized using H2SO4
Fig. 7. (a) SERS spectra of Sudan III (50 μM) collected from nanoporous Ag films; Inset: Structural formula of Sudan III; (b) SERS spectrum of series concentration of Sudan III collected from a hot-spot located on nanoporous Ag film. 99
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produced smaller ligament size and exhibited strong SERS enhancement with good uniformity and sensitivity. The potential of utilizing the nanoporous Ag film as SERS substrates for practical applications was demonstrated detecting Sudan III, and exhibited good sensitivity and reproducibility (13.1%). Interestingly, the presented strategy will be highly useful for further fabrication of various nanoporous metal for varieties of applications, such as catalysis and environmental analysis. Acknowledgments This work was supported by the National Key R&D Program of China (2016YFA0203102), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB14020101), and the National Natural Science Foundation of China (21620102008, 21527901). Deribachew Bekana acknowledges the support of CAS-TWAS President’s Fellowship for his PhD study. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.01.114. References [1] T.K. Sau, A.L. Rogach, Nonspherical noble metal nanoparticles: colloid-chemical synthesis and morphology control, Adv. Mater. 22 (2010) 1781–1804. [2] S. Guo, E. Wang, Noble metal nanomaterials: controllable synthesis and application in fuel cells and analytical sensors, Nano Today 6 (2011) 240–264. [3] L. Scarabelli, M. Coronado-Puchau, J.J. Giner-Casares, J. Langer, L.M. Liz-Marzán, Monodisperse gold nanotriangles: size control, large-scale self-assembly, and performance in surface-enhanced Raman scattering, ACS Nano 8 (2014) 5833–5842. [4] W. Niu, Y.A.A. Chua, W. Zhang, H. Huang, X. Lu, Highly symmetric gold nanostars: crystallographic control and surface-enhanced Raman scattering property, J. Am. Chem. Soc. 137 (2015) 10460–10463. [5] J. Biener, M.M. Biener, R.J. Madix, C.M. Friend, Nanoporous gold: understanding the origin of the reactivity of a 21st century catalyst made by pre-columbian technology, ACS Catal. 5 (2015) 6263–6270. [6] W. Li, X. Zhao, Z. Yi, A.M. Glushenkov, L. Kong, Plasmonic substrates for surface enhanced Raman scattering, Anal. Chim. Acta 984 (2017) 19–41. [7] S. Pedireddy, H.K. Lee, W.W. Tjiu, I.Y. Phang, H.R. Tan, S.Q. Chua, C. Troadec, X.Y. Ling, One-step synthesis of zero-dimensional hollow nanoporous gold nanoparticles with enhanced methanol electrooxidation performance, Nat. Commun. 5 (2014) 4947. [8] C. Xu, J. Su, X. Xu, P. Liu, H. Zhao, F. Tian, Y. Ding, Low temperature CO oxidation over unsupported nanoporous gold, J. Am. Chem. Soc. 129 (2007) 42–43. [9] R. Liu, J.F. Sun, D. Cao, L.Q. Zhang, Jf. Liu, G.B. Jiang, Fabrication of highly-specific SERS substrates by co-precipitation of functional nanomaterials during the selfsedimentation of silver nanowires into a nanoporous film, Chem. Commun. 51 (2015) 1309–1312. [10] L.Y. Chen, J.S. Yu, T. Fujita, M.W. Chen, Nanoporous copper with tunable nanoporosity for SERS applications, Adv. Funct. Mater. 19 (2009) 1221–1226. [11] L. Qian, B. Das, Y. Li, Z. Yang, Giant Raman enhancement on nanoporous gold film by conjugating with nanoparticles for single-molecule detection, J. Mater. Chem. 20 (2010) 6891–6895. [12] K. Wang, C. Stenner, J. Weissmüller, A nanoporous gold-polypyrrole hybrid nanomaterial for actuation, Sens. Actuators B Chem. 248 (2017) 622–629. [13] E. Wierzbicka, G.D. Sulka, Fabrication of highly ordered nanoporous thin Au films and their application for electrochemical determination of epinephrine, Sens. Actuators B Chem. 222 (2016) 270–279. [14] A. Wittstock, A. Wichmann, M. Bäumer, Nanoporous gold as a platform for a building block catalyst, ACS Catal. 2 (2012) 2199–2215. [15] A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Bäumer, Nanoporous gold catalysts for selective gas-phase oxidative coupling of methanol at low temperature, Science 327 (2010) 319–322. [16] A. Wittstock, J. Biener, M. Bäumer, Nanoporous gold: a new material for catalytic and sensor applications, PCCP 12 (2010) 12919–12930. [17] W. Luc, F. Jiao, Nanoporous metals as electrocatalysts: state-of-the-art, opportunities, and challenges, ACS Catal. 7 (2017) 5856–5861. [18] J. Huang, Y. Liu, X. He, C. Tang, K. Du, Z. He, Gradient nanoporous gold: a novel surface-enhanced Raman scattering substrate, RSC Adv. 7 (2017) 15747–15753. [19] C. Ma, M.J. Trujillo, J.P. Camden, Nanoporous silver film fabricated by oxygen plasma: a facile approach for SERS substrates, ACS Appl. Mater. Interfaces 8 (2016) 23978–23984. [20] L. Zhang, X. Lang, A. Hirata, M. Chen, Wrinkled nanoporous gold films with ultrahigh surface-enhanced Raman scattering enhancement, ACS Nano 5 (2011) 4407–4413. [21] J. Patel, L. Radhakrishnan, B. Zhao, B. Uppalapati, R.C. Daniels, K.R. Ward, M.M. Collinson, Electrochemical properties of nanostructured porous gold electrodes in biofouling solutions, Anal. Chem. 85 (2013) 11610–11618.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of nanoporous silver film by dealloying Ag/α-Fe2O3 nanocomposite for surface-enhanced Raman spectroscopy Deribachew Bekanaa,b, Rui Liua, Shasha Lia,b, Jing-Fu Liua,b,
T
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a State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P. O. Box 2871, Beijing, 100085, China b University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous silver film Iron oxide Nanocomposite Dealloying Surface-enhanced Raman spectroscopy Sudan III
A facile and reliable method for fabrication of nanoporous Ag film as surface-enhanced Raman spectroscopy (SERS) substrates was developed by dealloying the low-cost Ag/α-Fe2O3 nanocomposite in dilute acid solution. Influencing parameters such as different dealloying solutions, electrolyte concentration and dealloying time were optimized, and a uniform nanoporous Ag film of tunable and smaller ligament size was synthesized by etching the nanocomposite in sulfuric acid as a dealloying solution, while hydrochloric acid produced coarsened Ag nanoporous structures. Besides, the ligament size of the synthesized nanoporous Ag film increased with the electrolyte concentration and dealloying time. SERS activity of the as-synthesized nanoporous Ag film was evaluated using R6G as Raman probe, and the nanoporous Ag film with smaller ligament size exhibited strong SERS activity with a SERS enhancement factor of 7.47 × 106. The substrate exhibited good sensitivity and uniformity (RSD, 13.3%), and practical applicability for SERS detection of Sudan III, a synthetic organic dye banned as food additives, and achieved a detection limit of 0.5 μM. More importantly, the developed method ensures low-cost and reliable method of nanoporous materials fabrication from low-cost semiconductor based nanomaterials for applications in wide variety of fields.
1. Introduction The noble metal nanostructures (Ag, Au and Cu) has been topic of research interest due to their unique electronic, physical and chemical properties [1,2]. The fast-growing chemical nanoscience and nanotechnology have made an intriguing breakthrough in engineering different morphology of metal nanostructures for various applications such as surface-enhanced Raman spectroscopy (SERS) and catalysis [3–6]. Nanoporous metal structures are among the most fascinating morphology of metallic nanostructures and become one of the fundamental research topic in various fields [7–13]. There are several reports on potential applications of nanoporous materials for sensing and catalytic applications [14–16]. In this regard, numerous studies have been reported on the synthesis of nanoporous materials for catalysis, electrocatalysis, and electrocatalytic sensor applications [7,8,14,17]. In addition, different nanoporous materials reported in the literature demonstrated potential applications for electrochemical, biosensing and SERS [18–22]. The nanoporous metal structures are characterized by intriguing networks of interconnected structures with a morphology of tunable open pores and ligament sizes ranging from nanometer to
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micrometers giving rise to a large specific surface area [7,20,23,24]. The pore and ligament size can be tuned by altering dealloying conditions such as electrolyte concentration, dealloying time and temperature [25,26]. Several synthesis strategies have been explored for fabrication of nanoporous structures such as electrochemical dealloying [24,27], galvanic replacement [28,29], vapor-phase dealloying [30] and sacrificial template method [31,32]. So far, remarkable progress has been made in the fabrication of nanoporous metal structures using a dealloying approach. Dealloying is a process in which less noble elements selectively leached from the precursor alloy and the noble component undergoes surface diffusion leaving behind interconnected nanoporous structures [33–35]. Although several dealloying methods for fabrication of nanoporous metal structures have been reported, the methods have some limitations. This includes laborious multi-step electrochemical dealloying procedures such as corrosion-coarsening-corrosion and use of highly corrosive dealloying conditions (e.g 70% HNO3) [24]. Besides, the precursor material/master alloy used for dealloying are very expensive binary, ternary or multicomponent noble metal precursor like Au-Ag, Ag-Au-Pt, AuAgAl, Au20Cu48Ag7Pd5Si20 [36–38] and
Corresponding author. E-mail address: jfl[email protected] (J.-F. Liu).
https://doi.org/10.1016/j.snb.2019.01.114 Received 26 July 2018; Received in revised form 23 January 2019; Accepted 24 January 2019 Available online 25 January 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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method [44]. Then, the nanocomposite was etched in a dilute sulfuric acid (0.01 M) for 2.0 h at room temperature. The produced white product was rinsed successively in ultrapure water and ethanol to remove residual chemicals and dried under argon gas. To investigate effects of different electrolyte solution on the morphology of nanoporous structure, the nanocomposite was dealloyed in aqueous solution of hydrochloric acid and sulfuric acid. In addition, effect of different concentration of sulfuric acid (0.001, 0.01 and 0.02 M) was evaluated. The effect of dealloying time was also investigated by etching the nanocomposite at a different time (5 min, 30 min, 1 h, 1.5 h, 2.0 h, and 2.5 h). The obtained nanoporous Ag film was rinsed with ultrapure water and ethanol, and then dried under argon.
Al-Ag alloys that are prepared from pure metals (99.9%) [39]. This hampers the use of nanoporous metal films for real-life applications such as for SERS sensing. Therefore, there is a critical need to develop a facile and inexpensive approach for synthesizing a low-cost precursor material for the fabrication of metal nanoporous films with a capability to tailor the pore and ligament size to the desired nano- or microstructure [40]. In this regard, metal/metal oxide nanocomposites are the best candidates. Although several nanocomposites have been reported in the literature [41–43], the Ag/α-Fe2O3 nanocomposite synthesized in our previous work can be a good precursor material for the synthesis of nanoporous silver. In this study, we developed a novel, simple and effective approach for the fabrication of metal nanoporous films by dealloying a low-cost noble metal/metal oxide nanocomposites in dilute sulfuric acid. The motive of this study is to use low-cost materials such as metal oxides as less noble component that can be leach out in very dilute acid and hence, decrease the costs and toxic waste. To this end, Ag/α-Fe2O3 nanocomposite was synthesized as precursor alloy and etched in dilute acid solution. This dissolves the less-noble metal oxide nanomaterial, αFe2O3, leaving behind bicontinuous nanoporous structure of the noble metal, Ag. The current method has the following merits: (i) low-cost metal/metal oxide nanocomposite has been used as precursor alloy; (ii) the precursor alloy was prepared from non-toxic precursor, Fe metal and electroless deposition of Ag on the metal oxide nanostructure using simple wet chemistry method; (iii) dealloying process was conducted in a very dilute acidic solution; (iv) unlike previous methods, the dealloying process requires a very short dealloying time. Therefore, owed to the aforementioned advantages, the fabricated the nanoporous Ag films has been used effectively as low-cost and efficient SERS substrates for the detection of a synthetic organic dye banned as food additives.
2.3. Characterization The surface morphologies of as-fabricated nanoporous Ag film was characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan) and field-emission scanning electron microscope (FESEM, Su-8020, Hitachi, Japan). The elemental compositions of the produced nanoporous film was characterized by built-in energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectrometer (XPS, ESCALAB 250XI, Thermo Fisher, USA) with monochromatic Al-Kα X-ray source was used to characterize surface composition and chemical states.
2.4. SERS measurement SERS signal collection was performed by a Renishaw Invia Raman microscope (New Mills, U.K.) using 100x objective lens, 532 nm excitation wavelengths and laser power of 5 μW. The data acquisition time was 1 s with 10 s accumulation time. A series concentration of (10−6–10−9 M) of R6G was prepared and detected using the synthesized nanoporous silver films as SERS-active substrates.
2. Experimental section 2.1. Materials and reagents Iron foil, 0.1 mm (99.99%) and potassium hydroxide (KOH, ≥85%) were obtained from Alfa Aesar Co., Ltd. Silver nitrate (AgNO3, ≥99.8%), Rhodamine 6 G (R6 G, ≥95%) were purchased from the Sigma-Aldrich Chemical Co. (UK). Ammonia solution (NH3, 25–28%), and glucose (C6H12O6) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Hydrochloric acid (HCl, 37%) and sulfuric acid (H2SO4, 78%) were purchased from Merck (Darmstadt, Germany).
2.5. SERS analysis of Sudan III The samples for SERS measurement of Sudan III was prepared by loading 10 μL of different concentration (0–50 μM) of Sudan III acetone solution on the synthesized nanoporous Ag film and then left at room temperature for about 5 min to evaporate the solvent. Then the SERS spectrum were collected.
2.2. Fabrication of nanoporous silver film First, Ag/α-Fe2O3 nanocomposite was fabricated using our previous
Scheme 1. Schematic illustration of fabrication of the nanoporous silver film. 95
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Fig. 1. (a & b) SEM and TEM images of as-synthesized nanoporous Ag film; (c) HRTEM images showing the lattice spacing associated to the as-synthesized nanoporous Ag film; (d) STEM-EDS elemental mapping images of the nanoporous Ag film; (e) SEM-EDS elemental mapping images of nanoporous Ag film; (f) EDS spectra of as-synthesized nanoporous Ag films.
3. Results and discussion
with binding energy of 531.7 eV is attributed to surface adsorbed oxygen, while the 530.3 eV is associated to oxygen ions in the crystal lattice (Olatt). However, according to previous report, this oxygen species may not be associated with oxygen bonded to silver, AgXO, which should be in the range of 529.2–529.5 eV [45]. Therefore, it is attributed to the lattice oxygen in residual iron oxide.
3.1. Characterization of the synthesized nanoporous Ag film The nanoporous Ag film was synthesized by dealloying Ag/α-Fe2O3 nanocomposite in very dilute acid as shown in Scheme 1. First, the precursor nanocomposite (i.e AgNPs decorated α-Fe2O3 nanostructure) was synthesized by in situ reduction of Ag[(NH3)2]+ ions into metallic Ag on the surface of α-Fe2O3 nanoflakes (NFs), which was presented in our previous report [44]. Then, the Ag/α-Fe2O3 nanocomposite was etched in dilute H2SO4, which leached the α-Fe2O3 nanomaterial as a less noble component leaving behind the nanoporous Ag films. The SEM and TEM images of the nanoporous Ag films are shown in Fig. 1a and b, respectively, which reveals formation of bi-continuous nanoporous structures. Fig. 1c shows HRTEM image of the produced nanoporous structure, which clearly shows the lattice fringe of the nanoporous Ag film. The interplanar spacing was estimated to be 0.235 nm, which can be indexed to the (111) plane of Ag (PDF No. 99-0094). In addition, the STEM-EDS (Fig. 1d) and SEM-EDS (Fig. 1e) elemental mapping of the produced interconnected nanoporous Ag films revealed homogenous distribution of Ag on the surface of the nanoporous structure with a trace amount of Fe and O. Furthermore, Fig. 1f shows EDS spectrum of as-synthesized nanoporous structure and reveals presence of Ag, Fe and O. The EDS elemental mapping revealed that the wt% of Ag, O and Fe in the synthesized nanoporous Ag are 92.8%, 6.2% and 1.0%, respectively. The element Fe and O comes from residual iron oxide that has not been leached during dealloying process. The synthesized nanoporous material was further investigated using X-ray photoelectron spectroscopy (XPS) technique to confirm the surface composition and chemical states. As shown in Fig. 2a, high-resolution XPS spectra of Ag 3d consists two peaks at 368.4 and 374.4 eV separated by 6 eV, attributed to Ag3d3/2 and Ag3d5/2, which confirms presence of metallic Ag. In addition, the O1 s spectra (Fig. 2b) was fitted to two gaussian peaks with a binding energy of 530.3 eV and 531.7 eV, which confirm existence of two different kinds of O species. The peak
3.2. Optimization of dealloying conditions It is well known that the properties of nanoporous metals depend on their ligament and pore size, and thus, the precise control of the pore and ligament sizes is crucial to optimize the performance of nanoporous metals for certain applications [46]. Here, the synthesis conditions, which affect the morphology of the nanoporous structures such as different dealloying electrolyte solutions, electrolyte concentration and dealloying time were evaluated. First, the effect of different dealloying solutions viz. hydrochloric acid (HCl) and sulfuric acid (H2SO4) on the morphology evolution of nanoporous Ag film was evaluated. Fig. 3b and c shows the SEM images of the as-dealloyed nanocomposites in 0.01 M HCl and 0.01 M H2SO4, respectively. As depicted in Fig. 3b and c, the nanocomposites etched in HCl and H2SO4 solution has produced nanoporous structure with average ligament size of 73.4 ± 15 nm and 51.0 ± 9 nm, respectively. The coarsening of the nanoporous structure in HCl is due to the Cl−, which accelerated surface diffusion of Ag along the alloy/electrolyte interfaces during dealloying [47]. It is also ascribed to the dissolution rate of the less noble component, α-Fe2O3 nanostructure, which was reported to be more rapid in dilute concentration of H2SO4 than in HCl solution of equivalent concentration [48,49]. Thus, the rapid dissolution of α-Fe2O3 in H2SO4 probably increases formation of more nucleation sites, which lead to formation of small ligament size. The effect of concentration of electrolyte on morphology of the nanoporous Ag films were evaluated by etching the nanocomposite in different concentration of H2SO4 (0.001, 0.01, and 0.02 M). As shown in Fig. 3d–f, the ligament size of the nanoporous Ag films slightly 96
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Fig. 2. (a) Ag (3d) and (b) O (1 s) XPS spectra of the as-synthesized nanoporous Ag films.
3.3. Mechanism of nanoporous structure formation
increased with the concentration of H2SO4. When the nanocomposite etched in 0.001 M H2SO4, only few ligaments are formed. However, dealloying in higher concentration of H2SO4 (0.01 M and 0.02 M) produced nanoporous Ag films with an average ligament size of 51.5 ± 8 nm to 52.4 ± 11 nm, respectively. However, uniformity of the nanoporous structures decrease as concentration of H2SO4 increase and hence, relatively a uniform nanoporous Ag films were synthesized using 0.01 M H2SO4. To further investigate the influence of dealloying time on the evolution of nanoporous structure, the nanocomposite was etched at different time keeping the electrolyte concentration (0.01 M H2SO4) constant. The SEM images (Fig. 4) of as-synthesized nanoporous Ag films was examined at different dealloying time. Initially, after dealloying the nanocomposite precursor in 0.01 M H2SO4 for 5 min, the α-Fe2O3 has partially dissolved and the Ag begins to diffuse across the alloy/electrolye interface forming few nanoparticle clusters (Fig. 4a). After further 30 min dealloying, nanoporous structures with ligaments in the range of nanometer were formed, and further etching resulted in complete nanoporous structures with average ligament size ranging from 48 ± 8.7 to 56 ± 11 as the dealloying time increase from 0.5 to 2.5 h (Fig. 4). The ligament size tends to coarsen with increasing dealloying time resulting complete and a uniform nanoporous Ag films. However, a long time etching produces nanoporous structures with surface irregularities.
The mechanism of the nanoporous structure formation in current method can be explained as follows. It is well known that the dealloying method is selective dissolution of one or more less noble or active components from a precursor alloy in appropriate electrolyte to produce nanoporous metals with nanoporosity and bi-continous structure. Here, when Ag/α-Fe2O3 nanocomposite was etched in the dilute acid solution, the α-Fe2O3 will dissolve because of its high chemical activity in acidic solutions. During dealloying process, as the α-Fe2O3 start dissolving the Ag nanoparticles diffuse across the alloy/electrolye interface to form Ag nanoparticles cluster. As the dealloying proceed, αFe2O3 continue to leach out resulting Ag clusters, which grow to form ligaments through further surface diffusion of Ag atoms with different orientations, and self-assemble to form a uniform nanoporous structure. The formation of different ligament size in different electrolyte solution was attributed to impact of the acid solution counter anions on the diffusion rate of more noble metal across the interface and dissolution rate of the less-noble component, metal oxide nanomaterial. In both cases, the counter anions play an important role in accelerating the surface diffusion of the noble metal, Ag and increasing the dissolution rate of the metal oxide, α-Fe2O3. The dissolution of the iron oxide in acid involves the breakdown of Fe-O bonds, which depend on the hydrogen ion (H+) activity, and counter anions of the corresponding acid
Fig. 3. (a) SEM images of as synthesized Ag/α-Fe2O3 precursor; SEM images of the nanoporous Ag film produced using different electrolytes solutions (b) 0.01 M HCl and (c) 0.01 M H2SO4; SEM images of the nanoporous Ag film produced in different concentrations of H2SO4 (d) 0.001 M, (e) 0.01 M, (f) 0.02 M. 97
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Fig. 4. SEM images of the nanoporous Ag film synthesized by varying dealloying time in 0.01 M H2SO4. (a) 5 min, (b) 0.5 h, (c) 1 h, (d) 1.5 h, (e) 2.0 h and 2.5 h.
adsorbed on the surface. Here, the dissolution of α-Fe2O3 in HCl and H2SO4 involves formation of Fe(III)-chloro and Fe(III)-sulphato complexes on the oxide surfaces, respectively. This weakens the attractive force between Fe(III) and neighboring O2−, and reduces the repulsion between the oxide surface and protons in solution, which facilitates the dissolution rate. Therefore, rapid dissolution of α-Fe2O3 in dilute H2SO4 than equivalent concentration of HCl solution increases the number of nucleation sites leading to smaller ligament size [48,49].
signal enhancement as the dealloying time increase from 0.5 h to 2.0 h but no significant SERS enhancement was observed with further dealloying time. The potential of the synthesized nanoporous Ag films as SERS-active substrate was further investigated using R6 G. Fig. 6a shows the series concentrations of R6 G collected from the synthesized nanoporous Ag films. As can be seen, the intensity of the SERS signal of R6 G was enhanced as concentration of analyte increases with a minimum detectable concentration of 1 nM. Then, the SERS enhancement was quantitatively evaluated and the SERS enhancement factor (EF) was estimated to be 7.47 × 106 based on Eq. 1:
3.4. SERS performance of the synthesized nanoporous Ag films The SERS performance of the as-dealloyed nanocomposites for the optimized conditions were investigated using R6 G as Raman probe and the SERS enhancements are compared as given in Fig. 5. The nanoporous Ag films synthesized using H2SO4 produced small ligament size and exhibited strongest SERS enhancement (Fig. 5a), which is in good agreement with the literature [50]. In addition, SERS performance of the nanoporous Ag films synthesized at varying concentration of H2SO4 were also investigated, and the nanoporous structure with ligament size of 51.5 ± 8 nm that produced in 0.01 M H2SO4 exhibited strongest SERS enhancement (Fig. 5a). Furthermore, SERS performance of assynthesized nanoporous Ag films with varying dealloying time was compared as shown in Fig. 5b. As can be seen, there is increase in SERS
EF =
(ISERS / NSERS ) (INR/ NNR)
(1)
where ISERS and INR denotes the intensity of SERS and normal Raman (non-SERS) intensity of the analyte. NSERS and NNR stand for the number of probe molecules, R6 G, in the scattering volume for SERS and the normal Raman experiment, respectively. Moreover, to demonstrate the homogeneity of the synthesized nanoporous Ag films as a uniform SERS substrate, spot-to-spot Raman mapping was conducted on randomly selected area and SERS spectra were collected. Fig. 6b shows the 30 representative SERS spectra of R6 G collected from the randomly selected area of as-synthesized
Fig. 5. SERS spectra of R6 G (1 μM) collected on the synthesized nanoporous Ag film; (a) synthesized in different dealloying solutions (0.01 M HCl and 0.01 M H2SO4) and different concentrations of H2SO4 (0.001, 0.01 and 0.02 M); (b) synthesized in 0.01 M H2SO4 at varying dealloying time (0.5 h–2.5 h). 98
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Fig. 6. (a) SERS spectrum of series concentration of R6 G collected from nanoporous Ag films; (b) SERS spectrum of R6 G (1 μM) collected from 30 different spots via spot-to-spot Raman mapping.
nanoporous Ag films. The RSD values of relative intensity of the 30 SERS spectrum at 1650 cm−1 was 13.3%, which reveals good uniformity of nanoporous Ag films as SERS-active substrate.
Importantly, the present SERS platform fabrication method is simple, reliable, and inexpensive, which is of interest for further use in various environmental analysis applications. Furthermore, the reproducibility (spot-to-spot and sample-tosample reproducibility) of the SERS platform for Sudan III detection was evaluated. First, the point-to-point reproducibility of SERS signal enhancement over a large area of a single substrate was investigated by collecting the SERS spectrum of Sudan III from randomly selected 30 spots on the surface of the synthesized nanoporous Ag film. Figure S1a and b depicts the SERS spectrum of Sudan III collected from 30 spots and shows a consistent signal enhancement with RSD of 13.1% at a peak of 1598 cm−1. On the other hand, sample-to-sample reproducibility was conducted by collecting SERS spectra of Sudan III from three independent nanoporous Ag film substrates and five acquisition for each sample. As depicted in Figure S1c and d reproducible SERS signal enhancement with RSD of 14.6% were obtained, which is of paramount importance for quantitative analysis.
3.5. Application of the SERS substrate for analysis of Sudan III Adulteration of synthetic dyes in foodstuffs has raised concerns about food safety. Sudan dyes are among the synthetic dyes that are banned as food additives such as chili powder and other spices. Here, the suitability of the nanoporous Ag film as a SERS-active substrate for practical application was demonstrated for detection of Sudan III. Fig. 7a shows representative SERS spectra of Sudan III collected using the synthesized nanoporous Ag films. The major vibrational bands observed were attributed to Raman band of Sudan III and, according to the reported literature, [51] the peak at 1137 cm−1 assigned to δ(C–H), δ(O–H), σ(CeH)ph1, ph2, ν(CeN); 1232 cm−1 to ν(CeO), σ(CCH); 1389 cm−1 to ν (C]N), ν(N]N), δ(C–H); 1498 cm−1 to ν(C]N), ν(NeN), δ(N–H), and 1598 cm-1 to σ(CeC)ph1,ph2, ν(N]N) (Fig. 7a). For quantitative detection of Sudan III, a series of concentrations of Sudan III ranging from 0 to 50 μM were dropped on the synthesized nanoporous Ag film and their corresponding SERS spectra were collected (Fig. 7b). As can be seen, the signal intensity increases with increasing analyte concentration and the characteristic Raman bands of Sudan III can still be distinguished at a concentration down to 0.5 μM. This reveals the applicability of the nanoporous Ag film as a SERS-active substrate to detect Sudan III, which is crucial for monitoring synthetic dyes and others colorants adulteration in food foodstuffs.
4. Conclusion In summary, we report a novel, simple, affordable and effective approach for the fabrication of nanoporous Ag films by dealloying lowcost Ag/α-Fe2O3 nanocomposite. The dealloying conditions were optimized to obtain a uniform nanoporous film with bi-continuous structure. The nanoporous Ag films of different ligament sizes were synthesized by varying dealloying solution, electrolyte concentration and dealloying time. The nanoporous Ag films synthesized using H2SO4
Fig. 7. (a) SERS spectra of Sudan III (50 μM) collected from nanoporous Ag films; Inset: Structural formula of Sudan III; (b) SERS spectrum of series concentration of Sudan III collected from a hot-spot located on nanoporous Ag film. 99
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produced smaller ligament size and exhibited strong SERS enhancement with good uniformity and sensitivity. The potential of utilizing the nanoporous Ag film as SERS substrates for practical applications was demonstrated detecting Sudan III, and exhibited good sensitivity and reproducibility (13.1%). Interestingly, the presented strategy will be highly useful for further fabrication of various nanoporous metal for varieties of applications, such as catalysis and environmental analysis. Acknowledgments This work was supported by the National Key R&D Program of China (2016YFA0203102), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB14020101), and the National Natural Science Foundation of China (21620102008, 21527901). Deribachew Bekana acknowledges the support of CAS-TWAS President’s Fellowship for his PhD study. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.01.114. References [1] T.K. Sau, A.L. Rogach, Nonspherical noble metal nanoparticles: colloid-chemical synthesis and morphology control, Adv. Mater. 22 (2010) 1781–1804. [2] S. Guo, E. Wang, Noble metal nanomaterials: controllable synthesis and application in fuel cells and analytical sensors, Nano Today 6 (2011) 240–264. [3] L. Scarabelli, M. Coronado-Puchau, J.J. Giner-Casares, J. Langer, L.M. Liz-Marzán, Monodisperse gold nanotriangles: size control, large-scale self-assembly, and performance in surface-enhanced Raman scattering, ACS Nano 8 (2014) 5833–5842. [4] W. Niu, Y.A.A. Chua, W. Zhang, H. Huang, X. Lu, Highly symmetric gold nanostars: crystallographic control and surface-enhanced Raman scattering property, J. Am. Chem. Soc. 137 (2015) 10460–10463. [5] J. Biener, M.M. Biener, R.J. Madix, C.M. Friend, Nanoporous gold: understanding the origin of the reactivity of a 21st century catalyst made by pre-columbian technology, ACS Catal. 5 (2015) 6263–6270. [6] W. Li, X. Zhao, Z. Yi, A.M. Glushenkov, L. Kong, Plasmonic substrates for surface enhanced Raman scattering, Anal. Chim. Acta 984 (2017) 19–41. [7] S. Pedireddy, H.K. Lee, W.W. Tjiu, I.Y. Phang, H.R. Tan, S.Q. Chua, C. Troadec, X.Y. Ling, One-step synthesis of zero-dimensional hollow nanoporous gold nanoparticles with enhanced methanol electrooxidation performance, Nat. Commun. 5 (2014) 4947. [8] C. Xu, J. Su, X. Xu, P. Liu, H. Zhao, F. Tian, Y. Ding, Low temperature CO oxidation over unsupported nanoporous gold, J. Am. Chem. Soc. 129 (2007) 42–43. [9] R. Liu, J.F. Sun, D. Cao, L.Q. Zhang, Jf. Liu, G.B. Jiang, Fabrication of highly-specific SERS substrates by co-precipitation of functional nanomaterials during the selfsedimentation of silver nanowires into a nanoporous film, Chem. Commun. 51 (2015) 1309–1312. [10] L.Y. Chen, J.S. Yu, T. Fujita, M.W. Chen, Nanoporous copper with tunable nanoporosity for SERS applications, Adv. Funct. Mater. 19 (2009) 1221–1226. [11] L. Qian, B. Das, Y. Li, Z. Yang, Giant Raman enhancement on nanoporous gold film by conjugating with nanoparticles for single-molecule detection, J. Mater. Chem. 20 (2010) 6891–6895. [12] K. Wang, C. Stenner, J. Weissmüller, A nanoporous gold-polypyrrole hybrid nanomaterial for actuation, Sens. Actuators B Chem. 248 (2017) 622–629. [13] E. Wierzbicka, G.D. Sulka, Fabrication of highly ordered nanoporous thin Au films and their application for electrochemical determination of epinephrine, Sens. Actuators B Chem. 222 (2016) 270–279. [14] A. Wittstock, A. Wichmann, M. Bäumer, Nanoporous gold as a platform for a building block catalyst, ACS Catal. 2 (2012) 2199–2215. [15] A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Bäumer, Nanoporous gold catalysts for selective gas-phase oxidative coupling of methanol at low temperature, Science 327 (2010) 319–322. [16] A. Wittstock, J. Biener, M. Bäumer, Nanoporous gold: a new material for catalytic and sensor applications, PCCP 12 (2010) 12919–12930. [17] W. Luc, F. Jiao, Nanoporous metals as electrocatalysts: state-of-the-art, opportunities, and challenges, ACS Catal. 7 (2017) 5856–5861. [18] J. Huang, Y. Liu, X. He, C. Tang, K. Du, Z. He, Gradient nanoporous gold: a novel surface-enhanced Raman scattering substrate, RSC Adv. 7 (2017) 15747–15753. [19] C. Ma, M.J. Trujillo, J.P. Camden, Nanoporous silver film fabricated by oxygen plasma: a facile approach for SERS substrates, ACS Appl. Mater. Interfaces 8 (2016) 23978–23984. [20] L. Zhang, X. Lang, A. Hirata, M. Chen, Wrinkled nanoporous gold films with ultrahigh surface-enhanced Raman scattering enhancement, ACS Nano 5 (2011) 4407–4413. [21] J. Patel, L. Radhakrishnan, B. Zhao, B. Uppalapati, R.C. Daniels, K.R. Ward, M.M. Collinson, Electrochemical properties of nanostructured porous gold electrodes in biofouling solutions, Anal. Chem. 85 (2013) 11610–11618.
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