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Ag microspheres as recyclable and ultra-sensitive SERS substrates
Accepted Manuscript Title: Template-free Synthesis of Porous ZnO/Ag Microspheres as Recyclable and Ultra-sensitive SERS Substrates Authors: Yanjun Liu, Chunxiang Xu, Junfeng Lu, Zhu Zhu, Qiuxiang Zhu, A.Gowri Manohari, Zengliang Shi PII: DOI: Reference:
S0169-4332(17)32229-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.229 APSUSC 36763
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Please cite this article as: Yanjun Liu, Chunxiang Xu, Junfeng Lu, Zhu Zhu, Qiuxiang Zhu, A.Gowri Manohari, Zengliang Shi, Template-free Synthesis of Porous ZnO/Ag Microspheres as Recyclable and Ultra-sensitive SERS Substrates, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.229 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free Synthesis of Porous ZnO/Ag Microspheres as Recyclable and Ultra-sensitive SERS Substrates
Yanjun Liu, Chunxiang Xu*, Junfeng Lu, Zhu Zhu, Qiuxiang Zhu, A. Gowri Manohari and Zengliang Shi
State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, 210096, China E-mail:[email protected]
Graphical abstract
Highlights
Porous ZnO microspheres were fabricated without any template.
Porous structure improved the light trapping for the sensitive response.
Ag nanoparticles were decorated in situ on ZnO by photochemical reaction.
Metal/semiconductor hybrid SERS substrate could be self-cleaned for recycling.
Synergic enhancement mechanism of the multi-effects to the SERS was analyzed.
Abstract: The porous structured zinc oxide (ZnO) microspheres decorated with silver nanoparticles (Ag NPs) have been fabricated as surface-enhanced Raman scattering (SERS)
substrate
for
ultra-sensitive, 1
highly
reproducible
and
stable
biological/chemical sensing of various organic molecules. The ZnO microspheres were hydrothermally synthesized without any template, and the Ag NPs decorated on microspheres via photochemical reaction in situ, which provided stable Ag/ZnO contact to achieve a sensitive SERS response. It demonstrates a higher enhancement factor (EF) of 2.44 × 1011 and a lower detection limit of 10-11 M 10-12 M. This porous SERS substrate could also be self-cleaned through a photocatalytic process and then further recycled for the detection of same or different molecules, such as phenol red (PhR), dopamine (DA) and glucose (GLU) with ultra-low concentration and it possessed a sensitive response. The excellent performances are attributed to morphology of porous microspheres, hybrid structure of semiconductor/metal and corresponding localized field enhancement of surface plasmons. Therefore, it is expected to design the recyclable ultra-sensitive SERS sensors for the detection of biological molecules and organic pollutant monitoring.
Key words: porous, ZnO/Ag, SERS, ultra-sensitive, photocatalyst
1. Introduction Surface enhanced Raman scattering (SERS) has been developed as an effective analytical method for the detection of organic molecules as well as environmental pollutants through interfacial analysis, intermolecular interaction monitoring and surface modification characterization.[1-4] In recent years, various sensitive SERS substrates have been developed based on Ag, Au and their composites with different morphologies, such as nanoparticles, nanorods, nanocubes, and core-shell nanoparticles.[2, 5-8] However, a crucial strategy is to develop a sensitive, reusable and stable SERS substrate with high enhancement factor through simple fabrication approaches both in technological and cost view. Moreover, a suitable energy level configuration of metal/semiconductor system can construct a proper charge transfer path and also create a localized electric field for an enhancement of Raman scattering. The metal/semiconductor composites provide a candidate approach to recyclable SERS substrates based on photocatalytic self-cleaning function of the semiconductors. 2
For example, ZnO and TiO2 have been used as typical semiconductors. Besides the wide applications of ZnO in photoelectronics, it has also been employed as a propitious material in biosensors because of its high isoelectronic point (IEP, 9.5), which is beneficial for the adsorption and immobilization of the organic molecules with low IEP through electrostatic attraction.[9] Thus, the various ZnO nanostructures have been employed as SERS substrates in the past decades and combined with metal nanoparticles, such as ZnO/Ag nanosheets[10] and nanoflowers,[11] branched ZnO/Au 3D architecture[12] and 3D Ag/ZnO hybrids[13]. Nevertheless, the constituent units of above ZnO are one-dimensional or two-dimensional nanomaterials including sheet [10] and rod.[12, 13] On one hand, nanoparticles are more conducive to molecular adsorption than that of sheets and rods due to their higher surface-to-volume ratio. On the other hand, a sphere can provide more active sites for the reaction because of its largest specific surface area. In addition, it is expected that a better configuration for molecular immobilization and sensing when assembling the nanoparticles into porous spheres. However, it is not easy to self-assemble the microspheres without template, especially the porous structured ZnO microspheres. Furthermore, the decoration of metal nanoparticles is generally processed in colloidal solution[10] [11]or ion sputtering[13], so the presence of weak bonding between metal and semiconductor further affects the stability and reusability of the substrate in SERS detection. The photochemical deposition as an in-situ growth method can provide a stable semiconductor/metal contact and homogeneous hot-spot distribution, which further improves the performances of biosensing such as stability, responsibility and sensitivity. In this work, a novel, recyclable and highly sensitive SERS substrate was constructed based on porous ZnO/Ag microspheres. The ZnO microspheres were synthesized by template-free low temperature hydrothermal reaction at 90°C and Ag NPs decorated on microspheres in situ via photochemical deposition. Based on synergetic effect between Ag and ZnO, the microspheres exhibited a significantly enhanced Raman signal for Rhodamine 6G (R6G), phenol red (PhR), dopamine (DA) and glucose (GLU). The performances of SERS substrate including high sensitivity, 3
stability, and recyclability were analyzed and the mechanism was discussed in detail.
2. Experimental 2.1 Synthesis of ZnO microspheres. ZnO microspheres were synthesized by using a low temperature hydrothermal technique. In a typical process, the 0.05M of ZnCl2 and hexamethylenetetramine (HMT) solution were mixed with same molar ratio and stirred until the complete dissolution of the chemicals in deionized water. Then, the as-prepared 0.038M of Na3C6H5O7·2H2O in deionized water was added to the above solution with volume ratio of 1:2. After that, the mixed solution was put into a tightly closed glass bottle with quartz films and placed in an oven at 90°C for 90 min. The ZnO microspheres were grown on quartz substrates. After 90 min, the collected product was washed several times with ethanol and deionized water and then dried in the oven at 45°C for 1 h. 2.2 Synthesis of porous ZnO/Ag microspheres. Photochemical deposition was employed to obtain close bonded porous ZnO/Ag microspheres. During the process, the as-prepared ZnO microspheres on quartz films were put into 0.05M of AgNO3 and NaNO3 mixed solution with same molar ratio and irradiated under UV light (18W mercury lamp, 254nm) for 20min. The dried ZnO/Ag samples were further annealed at 400 °C in an ambient atmosphere for 4 h to improve the crystallinity of them. The Ag NPs were also decorated on ZnO microspheres by using ion sputtering method for comparison. 2.3 Characterization The morphology of samples was observed by field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra Plus). The energy dispersive X-ray spectroscopy (EDX, EX-250) was used to reveal elemental distribution of the composites which equipped with FESEM. The crystal structure was characterized by X-ray diffraction (XRD, Siemens D5005). The Raman scattering measurements were conducted by using confocal micro-Raman system (Lab RAM HR 800) under laser excitation of 514.5 nm and power of 1mW 1% with an integration time of 10 s. the 4
theoretical calculation and simulations were carried out by using finite difference time domain (FDTD) method. A photocatalytic reaction experiment was conducted by using photocatalytic reaction device. The UV-Vis spectrophotometer (UV-vis, UV-2600PC spectrometer) was used to measure the absorbance of powder samples as well as solutions after photocatalytic degradation.
3. Results and Discussion 3.1 Nanostructure formation of porous ZnO/Ag microspheres FESEM, EDX and XRD have been used to analyze the morphology, composition and elemental distribution of the nanocomposites (Fig. 1). The as-grown ZnO exhibits spheres with smooth surface and diameter of about 4µm as shown in Fig. 1(A). Fig. 1(B) depicts the decoration of Ag NPs on the smooth surface of ZnO spheres after the photochemical deposition. After annealing, the products are changed into porous structure with rough surfaces and the size of Ag NPs and ZnO particles are observed as 10-20nm as shown in Fig. 1(C) and 1(D). The XRD patterns in Fig. 1(F) confirm the hexagonal phase of ZnO (JCPDS NO. 89-1397) with diffraction angles of 2θ=31.029°, 33.697°, 35.506°, 46.812°, 55.811°, 62.278°, 67.298° and 68.609°, which corresponding to the crystallographic planes of (100), (002), (101), (102), (110), (103), (112) and (201) respectively. Concomitantly, the identified diffraction peaks at 28.133°, 38.587° and 44.613° with crystallographic planes of (220), (111), and (200) are related to Ag NPs. Moreover, the diffraction peaks of ZnO/Ag become sharper after the process of annealing (Fig. 1(F)) which denotes an improved crystalline nature. The EDX spectrum and the elemental mappings in Fig. 1(G) depict the elemental distributions of O, Zn and Ag within the structure of ZnO/Ag clearly and the Ag peaks are observed clearly in the EDX spectrum of ZnO/Ag. The above results can be indicated the successful formation of ZnO/Ag composite. .
The schematic diagrams for morphological changes and reaction mechanism of porous ZnO/Ag microspheres are illustrated in Fig. 2. A large amount of Ag+ is adsorbed on the surface of ZnO microsphere and then reduced into Ag NPs by the photo-generated electrons from ZnO under the irradiation of UV light. The detailed 5
photochemical process is displayed in Fig. 2(B). In addition, the crystallization of ZnO/Ag microspheres is increased and the porous structure was formed after the process of annealing at 400 °C for 4h. 3.2 SERS measurement for R6G and its enhancement mechanism In order to examine an enhancement effect of ZnO in SERS, Ag NPs was sputtered on the quartz film and compared with annealed ZnO. Also, R6G solution was diluted successively in the range of concentrations from 10-2 M to 10−12 M by factors of 10 for SERS investigation. The SERS substrates of Ag NPs, ZnO microspheres and ZnO/Ag microspheres were immersed into R6G solution and dried at room temperature naturally. From Fig. 3(A), it can be found that the porous ZnO/Ag microspheres (curve 1) exhibit the strongest enhancement followed by pure Ag NPs as substrate (curve 2), whereas the pure ZnO as substrate presents a very weak response (curve 3) for the same concentration of R6G. Consider Fig. 1 and Fig. 3(B), it can be noticed that the size of sputtered Ag NPs is similar to that of those obtained from photochemical deposition, but the latter method offers a better distribution of Ag on ZnO surface when compared to former one because of in situ synthesis method which stabilizes the bonding between ZnO and Ag. The porous ZnO/Ag microspheres mentioned later in the article are all fabricated by photochemical deposition. Fig. 4(A) and (B) represents the Raman spectra of R6G with different concentrations (from 10-4 M to 10-12 M) under the same experimental conditions. The spectral peaks of R6G are observed clearly at 1361, 1505 and 1645 cm-1, which assigned to symmetric modes of in-plane C-C stretching vibrations.[14] In addition, the R6G peaks are identified even in a lower concentration of it (10-12M). The SERS enhancement factor (EF) of porous ZnO/Ag microspheres as substrate was calculated according to the following equation:[15]
EF
ISERS Nbulk Ibulk NSERS
.
(1)
Where, ISERS and Ibulk denote the integrated intensities for 10-12M of R6G on ZnO/Ag microspheres as substrate and 10-2M of R6G on quartz substrate at 1361 cm-1 band, 6
whereas NSERS and Nbulk represent the corresponding number of R6G molecules adsorbed onto them. The calculated EF value for R6G is as high as 2.44 × 1011 which indicates an excellent sensitivity of the substrate. Furthermore, an outstanding linear relation is obtained between Raman intensity and concentrations of R6G in the ranging from 10-5 M to 10-12 M at 1361 cm-1 as shown in Fig. 4(C). The yielded regression equation is Y=0.36096X+6.07207 (Y is a logarithmic Raman intensity and X is a logarithmic concentration of R6G) and the correlation coefficient is r=0.99541. The results can be proposed that the porous ZnO/Ag microsphere as substrate is an effective tool for qualitative and quantitative detections of Raman signal. Table 1 displays the comparison of detection limit and enhancement factor for porous ZnO/Ag microspheres with previously reported works. The types and morphologies of semiconductors affect the sensitivity of detection, [2, 8, 10-13, 15-20] and the porous ZnO/Ag microspheres as SERS substrates can significantly improve the intensity of Raman signal. The reasons behind the prominent enhancement of EF for porous ZnO/Ag microspheres as SERS substrates are as follows. Firstly, the sizes of Ag NPs and ZnO NPs composed of ZnO/Ag microspheres are 10-20 nm, and there is an occurrence of positive proportional relationship between adsorption capacity and specific surface area. The porous structured microspheres can be improved the light trapping for the sensitive response of SERS besides higher surface-to-volume ratio which is beneficial for the immobilization of target molecules. Moreover, as a high IEP material, ZnO nanoparticles can assist the Ag NPs to adsorb and immobilize the organic molecules around them [9] and shown in Fig. 5(A). Secondly, as shown in Fig. 5(B), a strong absorption of sputtered Ag NPs on quartz film is observed in the ranging between 400nm and 550 nm, which is closed to photochemically deposited Ag NPs (inset of Fig. 5(B)). Under an excitation wavelength of 514.5 nm laser, the Ag NPs can be excited resonantly which leads to the generation of localized electromagnetic oscillation. Thus, the Raman signal is enhanced sufficiently through localized surface plasma resonance (LSPR). [21] Initially, the adequate hot spots are produced by 7
plenty of Ag NPs when exciting by incident light which contributes to greater enhancement of Raman signal.[22, 23] Fig. 5(C) and (D) illustrate the simulated optical field distributions based on the effects of nanoparticles. During the simulation, the refractive index of ZnO was set as 2.43 and the monitor wavelength was 514.5 nm, respectively. A clear enhancement of optical field is displayed in Fig. 5(C), and the hot spots appear at the nanoparticles nearby. The ambient dielectric function of Ag NPs is changed after the introduction of ZnO NPs, and the electric field is further enhanced, as shown in Fig. 5(D). Furthermore, the sputtered Ag NPs on porous ZnO microspheres, ZnO film and smooth ZnO microspheres under same experimental conditions reveal the effect of morphology in SERS substrate and their morphologies are depicted in Fig. 5(E) and (F). Fig. 5(G) shows the Raman signals of R6G collected on the substrates of porous ZnO/Ag microspheres (sputtering), ZnO/Ag film and smooth ZnO/Ag microspheres, respectively. It can be noted that the porous ZnO/Ag microspheres (sputtering) exhibit the strongest enhancement. The spherical structure and completely rough surface of ZnO can promote scattering of light, which is conducive to SERS detection. Thirdly, the Fermi level of Ag is higher than that of ZnO, so the electrons of Ag can easily transfer into ZnO at Ag/ZnO interface. Also, the accumulation of electrons can induce a potential barrier at the interface thus resulting in different charge distributions: the negative charge presented at the side nearest to the ZnO and the positive charge located at the other side. Subsequently, the formed local electrical fields around the Ag NPs further enhance the LSPR effect and improve the Raman scattering of target molecules significantly. [24] In addition, the chemical mechanism has also been involved in the enhancement of Raman signal via directional charge transfer from ZnO surface into target molecules. Most of the electrons in Ag/ZnO can be escaped from the potential barrier and transferred into target molecule thus resulting in enhanced Raman signal for the molecule. [25] The above reasons are significantly contributed to an enhancement of Raman signal, and the two most important reasons are particle size effect and light scattering effect. The peculiar structure is the advantage of porous ZnO/Ag microspheres different from other semiconductor/metal materials. 8
3.3 Recyclability, stability and homogeneity of ZnO/Ag substrates The above results can be proposed that the SERS substrate of porous ZnO/Ag microspheres is the suitable candidates for the potential applications in chemical analysis and biosensing. Moreover, the three factors such as recyclability, stability and homogeneity are crucial for the applications of porous ZnO/Ag microspheres. To analyze the recyclability, the used substrate of porous ZnO/Ag microspheres was irradiated with an 18 W mercury lamp for 1.5 h after SERS detection. Comparing curves 1 and 2 in Fig. 6(A), it can be demonstrated that the intensity of R6G is decreased dramatically after UV irradiation. After treatment, the reused substrate almost recovered the SERS activity, and exhibited a strong enhanced Raman signal in curve 3 of Fig. 6(A). Fig. 6(A) demonstrates the Raman signals of self-cleaned SERS substrate, hence, could be reused by many times through photocatalytic treatment as discussed below. In order to further verify the self-cleaning effect of SERS substrate, the photocatalytic experiments were performed. The ZnO/Ag samples were immersed into culture dishes with R6G solution (10−6 M, 30mL) and placed in a dark for 30 minutes to reach equilibrium in adsorption. Then, the culture dishes were exposed under UV light and absorbance of R6G solutions were measured every 10 min to investigate the photocatalytic process. Fig. 6(B) represents the UV-visible absorption spectra of R6G in photocatalysis process under UV irradiation. The characteristic peak intensity of R6G is gradually decreased when prolonging the reaction time. The concentration reduction of R6G is obtained due to photo-degradation which induced by catalyzing of ZnO/Ag. However, it is essential to wash out the impurities from the substrate for further usage in practical applications. In addition, the Ag NPs may be fall off after the process of retreating or reusing by many times and be oxidized when exposing in air for a long time. Therefore, it is indispensable to ensure the stability of substrate. In order to test the stability of porous ZnO/Ag microspheres, the substrates were immersed in the deionized water for 1 h or placed in an ambient atmosphere for 2 weeks before being reused for SERS detection. As shown in Fig. 6(C), it can be 9
noticed that the treated samples are still maintained their very strong Raman signal but the intensities are slightly decreased in comparison with that of untreated samples. The homogeneity has been examined by comparing the Raman signals of R6G at different locations on the same ZnO/Ag substrate as shown in Fig. 6(D). The Raman spectra from different spots on the same ZnO/Ag substrate possess the similar profiles and intensities. For example, the intensity values of Raman signal at 1361 cm-1 for R6G are 6342, 6500, 6442, 6732 and 6411. The similar spectral behaviors can be suggested a better homogeneity of SERS substrate. 3.4 Ultra-sensitive SERS detection for other organic molecules The enhanced SERS performances of porous ZnO/Ag microspheres for R6G predicted a possible ultra-sensitive sensing to other organic molecules. The solutions of DA, PhR and GLU with different concentrations in deionized water were prepared separately. The conditions of their SERS measurements are similar to the detection of R6G. Fig. 7 displays the SERS responses to DA, PhR and GLU molecules with ultra-low concentration. The spectral feature characteristics of DA are still identified clearly at 441, 463, 670, 1180, 1314 and 1417 cm-1 even in a lower concentration of 10-12 M. Similarly, the sharp peaks for 10-11 M of PhR are distinctly observed at 327, 400, 446, 479, 908, 1154 and 1440 cm-1. The Raman peaks of GLU with concentration of 10-11 M are also presented obviously at 606, 1029, 1057, 1157, 1266, 1386 and 1402 cm-1. All the above ultra-sensitive detections are based only on the porous ZnO/Ag microspheres as SERS substrate, whereas the pure Ag NPs as substrate did not achieve this effect. The significant enhancement of Raman signal is profiting from the synergetic effect between Ag and ZnO which already discussed in the Section 3.2.
3. Conclusions In this work, a novel method was introduced in the fabrication of porous ZnO/Ag microspheres and employed as highly sensitive, stable and recyclable SERS substrates. The Ag NPs decorated ZnO microspheres manifested the ultra-sensitive Raman 10
detection for R6G, PhR, DA and GLU with ultra-low concentration. The obtained detection limits of four organic compounds were 10-12 M, 10-11 M, 10-12 M, and 10-11 M, respectively. The peculiar structure of porous ZnO/Ag microspheres exhibited a significant effect on enhancement of Raman signal in SERS measurement. The porous ZnO/Ag microspheres as SERS substrates possess the self-cleaning behavior through photocatalytic process. The unique porous, recyclable, stable and ultra-sensitive properties of ZnO/Ag offer a new opportunity for
the development of SERS
substrates and lead to a promising application in biological sensors and organic pollutant monitoring.
Acknowledgements This work was supported by NSFC (61475035, 61275054) and Science & Technology Project of Jiangsu Province (BE2016177). Also, we thank the help of collaborative Innovation Center of Suzhou Nano Science and Technology.
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References [1] D. Kurouski, R.P. Van Duyne, In situ detection and identification of hair dyes using surface-enhanced Raman spectroscopy (SERS), Anal. Chem. 87 (2015) 2901-2906. [2] L. Tang, S. Li, F. Han, L. Liu, L. Xu, W. Ma, H. Kuang, A. Li, L. Wang, C. Xu, SERS-active Au@Ag nanorod dimers for ultrasensitive dopamine detection, Biosen. Bioelectron. 71 (2015) 7-12. [3] H.Y. Wu, B.T. Cunningham, Point-of-care detection and real-time monitoring of intravenously delivered drugs via tubing with an integrated SERS sensor, Nanoscale. 6 (2014) 5162-5171. [4] G. Zito, G. Rusciano, G. Pesce, A. Dochshanov, A. Sasso, Surface-enhanced Raman imaging of cell membrane by a highly homogeneous and isotropic silver nanostructure, Nanoscale. 7 (2015) 8593-8606. [5] M. Fan, F. J. Lai, H. L. Chou, W. T. Lu, B. J. Hwang, A.G. Brolo, Surface-enhanced Raman scattering (SERS) from Au:Ag bimetallic nanoparticles: the effect of the molecular probe, Chem. Sci. 4 (2013) 509-515. [6] H. Wei, H. Xu, Hot spots in different metal nanostructures for plasmon-enhanced Raman spectroscopy, Nanoscale. 5 (2013) 10794-10805. [7] Y. Liu, Z. Huang, F. Zhou, X. Lei, B. Yao, G. Meng, Q. Mao, Highly sensitive fibre surface-enhanced Raman scattering probes fabricated using laser-induced self-assembly in a meniscus, Nanoscale. 8 (2016) 10607-10614. [8] T.J. Dill, M.J. Rozin, E.R. Brown, S. Palani, A.R. Tao, Investigating the effect of Ag nanocube polydispersity on gap-mode SERS enhancement factors, Analyst. 141 (2016) 3916-3924. [9] B. Gu, C. Xu, C. Yang, S. Liu, M. Wang, ZnO quantum dot labeled immunosensor for carbohydrate antigen 19-9, Biosen. Bioelectron. 26 (2011) 2720-2723. [10] Z. Wang, G. Meng, Z. Huang, Z. Li, Q. Zhou, Ag-nanoparticle-decorated porous ZnO-nanosheets grafted on a carbon fiber cloth as effective SERS substrates, Nanoscale. 6 (2014) 15280-15285. [11] Q. Tao, S. Li, C. Ma, K. Liu, Q.Y. Zhang, A highly sensitive and recyclable SERS substrate based on Ag-nanoparticle-decorated ZnO nanoflowers in ordered arrays, Dalton. T. 44 (2015) 3447-3453. [12] S. Picciolini, N. Castagnetti, R. Vanna, D. Mehn, M. Bedoni, F. Gramatica, M. Villani, D. Calestani, M. Pavesi, L. Lazzarini, A. Zappettini, C. Morasso, Branched gold nanoparticles on ZnO 3D architecture as biomedical SERS sensors, RSC Adv. 5 (2015) 93644-93651. [13] C. Huang, C. Xu, J. Lu, Z. Li, Z. Tian, 3D Ag/ZnO hybrids for sensitive surface-enhanced Raman scattering detection, Appl. Surf. Sci. 365 (2016) 291-295. 12
[14] Z. Huang, G. Meng, Q. Huang, Y. Yang, C. Zhu, C. Tang, Improved SERS performance from Au nanopillar arrays by abridging the pillar tip spacing by Ag sputtering, Adv. Mater. 22 (2010) 4136-4139. [15] J. Lu, C. Xu, H. Nan, Q. Zhu, F. Qin, A.G. Manohari, M. Wei, Z. Zhu, Z. Shi, Z. Ni, SERS-active ZnO/Ag hybrid WGM microcavity for ultrasensitive dopamine detection, Appl. Phys. Lett.109 (2016) 073701. [16] Y. Xie, Y. Meng, SERS performance of graphene oxide decorated silver nanoparticle/titania nanotube array, RSC Adv. 4 (2014) 41734-41743. [17] X. Li, H. Hu, D. Li, Z. Shen, Q. Xiong, S. Li, H.J. Fan, Ordered array of gold semishells on TiO2 spheres: an ultrasensitive and recyclable SERS substrate, ACS Appl. Mater. Inter. 4 (2012) 2180-2185. [18] Q. Huang, S. Liu, W. Wei, Q. Yan, C. Wu, Selective synthesis of different ZnO/Ag nanocomposites as surface enhanced Raman scattering substrates and highly efficient photocatalytic catalysts, RSC Adv. 5 (2015) 27075-27081. [19] X. He, H. Wang, Z. Li, D. Chen, J. Liu, Q. Zhang, Ultrasensitive SERS detection of trinitrotoluene through capillarity-constructed reversible hot spots based on ZnO-Ag nanorod hybrids, Nanoscale. 7 (2015) 8619-8626. [20] J. Yin, Y. Zang, C. Yue, Z. Wu, S. Wu, J. Li, Z. Wu, Ag nanoparticle/ZnO hollow nanosphere arrays: large scale synthesis and surface plasmon resonance effect induced Raman scattering enhancement, J. Mater. Chem. 22 (2012) 7902. [21] S. Chen, Z. Yang, L. Meng, J. Li, C.T. Williams, Z. Tian, Electromagnetic enhancement in shell-isolated nanoparticle-enhanced Raman scattering from gold Flat surfaces, J. Phys. Chem. C. 119 (2015) 5246-5251. [22] X. Li, G. Chen, L. Yang, Z. Jin, J. Liu, Multifunctional Au-coated TiO2 nanotube arrays as recyclable SERS substrates for multifold organic pollutants detection, Adv. Funct. Mater. 20 (2010) 2815-2824. [23] W. Y. Li, X. M. Lu, Y. N. Xia, Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering, Nano Lett. 9 (2009) 485-490. [24] S.K. Anna Rumyantseva, P. M. Adam, S. V. Gaponenko, S. V. Vaschenko, O. S. Kulakovich, A. A. Ramanenka, D. V. Guzatov,, V. D. D. Korbutyak, A. Stroyuk, V. Shvalagin, Nonresonant surface-enhanced Raman scattering of ZnO quantum dots with Au and Ag nanoparticles, ACS Nano. 7 (2013) 3420-3426. [25] N. Valley, N. Greeneltch, R.P. V. Duyne, G. C. Schatz, A look at the origin and magnitude of the chemical contribution to the enhancement mechanism of surface-enhanced Raman spectroscopy (SERS): theory and experiment, J. Phys. Chem. Lett. 4 (2013) 2599-2604.
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Fig. 1 SEM images of (A) ZnO (B) unannealed ZnO/Ag (C) annealed ZnO/Ag (inserts are enlarged SEM images) (D) the surface (E) and cross section of annealed ZnO/Ag, (F) XRD patterns of ZnO/Ag with and without annealing (G) EDX elemental mapping and EDX spectrrum of ZnO/Ag
Fig. 2 (A) Schematic diagram of changes in morphology and (B) mechanism of photochemical deposition
14
Fig.3 (A) Raman spectra of R6G with different substrates (B) SEM image of Ag NPs (sputtering time of 30s).
Fig.4 (A) (B) Raman spectra of R6G with different concentrations ranging from 10-12 M to 10-4 M (C) The linear relation between Raman peak intensity and concentrations of R6G at 1361 cm-1 (logarithmic concentration and intensity).
15
Fig.5 (A) Schematic diagram of ZnO/Ag and adsorption of molecules (B) UV-visible absorption spectra of ZnO and ZnO/Ag and insert Ag NPs (sputtering time of 30s), the optical field distributions of (C) Ag NPs (D) ZnO/Ag simulated by FDTD method, SEM images of (E) ZnO/Ag film (F) smooth ZnO/Ag microspheres (G) Raman spectra of R6G with different substrates.
16
Fig.6 (A) Raman spectra of R6G after detection or photocatalysis (B) UV-visible absorption spectra of R6G with ZnO/Ag as photocatalyst, Raman spectra of R6G (10-6 M) on ZnO/ Ag substrates(C) placed in different environments and (D) with different locations of the same substrate.
17
Fig.7 Raman spectra of (A) DA (10-8 M, 10-10 M, 10-12 M, 10-10 M) (B) PhR (10-8 M, 10-11 M, 10-11 M ) (C) GLU ( 10-10 M, 10-11 M, 10-10 M) with ZnO/Ag and Ag NPs as substrates.
18
Table 1 Comparison of detection limits or enhancement factors for the molecules on different substrates.
NO.
Substrate
Probe
Detection limit(mol/L)
Enhancement factor
Ref.
1
Ag nanocube
Thiophenol
-
107
8
2
Au@Ag nanorod
Dopamine
10-12
-
2
3
GO-Ag/TiO2 nanotube
Methyl blue
10-9
104
16
4
Au/TiO2 spheres
Rhodamine 6G
-
105
17
5
ZnO/Au 3D architecture
Apomorphine
-
106
12
6
3D Ag/ZnO hybrids
Phenol red
-
109
13
7
ZnO/Ag nanocomposites
Rhodamine B
-
107
18
8
ZnO/Ag nanorod
Rhodamine 6G
-
108
19
9
Ag/ZnO nanoflowers
Rhodamine 6G
-
1010
11
10
ZnO/Ag hybrid
Rhodamine 6G
-
1010
15
11
Ag /ZnO nanosphere
Rhodamine 6G
-
108
20
12
ZnO/Ag nanosheets
Rhodamine 6G
10-10
-
10
13
Porous ZnO/Ag microspheres
Rhodamine 6G
10-12
1011
This work
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S0169-4332(17)32229-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.229 APSUSC 36763
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-5-2017 14-7-2017 24-7-2017
Please cite this article as: Yanjun Liu, Chunxiang Xu, Junfeng Lu, Zhu Zhu, Qiuxiang Zhu, A.Gowri Manohari, Zengliang Shi, Template-free Synthesis of Porous ZnO/Ag Microspheres as Recyclable and Ultra-sensitive SERS Substrates, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.229 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free Synthesis of Porous ZnO/Ag Microspheres as Recyclable and Ultra-sensitive SERS Substrates
Yanjun Liu, Chunxiang Xu*, Junfeng Lu, Zhu Zhu, Qiuxiang Zhu, A. Gowri Manohari and Zengliang Shi
State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, 210096, China E-mail:[email protected]
Graphical abstract
Highlights
Porous ZnO microspheres were fabricated without any template.
Porous structure improved the light trapping for the sensitive response.
Ag nanoparticles were decorated in situ on ZnO by photochemical reaction.
Metal/semiconductor hybrid SERS substrate could be self-cleaned for recycling.
Synergic enhancement mechanism of the multi-effects to the SERS was analyzed.
Abstract: The porous structured zinc oxide (ZnO) microspheres decorated with silver nanoparticles (Ag NPs) have been fabricated as surface-enhanced Raman scattering (SERS)
substrate
for
ultra-sensitive, 1
highly
reproducible
and
stable
biological/chemical sensing of various organic molecules. The ZnO microspheres were hydrothermally synthesized without any template, and the Ag NPs decorated on microspheres via photochemical reaction in situ, which provided stable Ag/ZnO contact to achieve a sensitive SERS response. It demonstrates a higher enhancement factor (EF) of 2.44 × 1011 and a lower detection limit of 10-11 M 10-12 M. This porous SERS substrate could also be self-cleaned through a photocatalytic process and then further recycled for the detection of same or different molecules, such as phenol red (PhR), dopamine (DA) and glucose (GLU) with ultra-low concentration and it possessed a sensitive response. The excellent performances are attributed to morphology of porous microspheres, hybrid structure of semiconductor/metal and corresponding localized field enhancement of surface plasmons. Therefore, it is expected to design the recyclable ultra-sensitive SERS sensors for the detection of biological molecules and organic pollutant monitoring.
Key words: porous, ZnO/Ag, SERS, ultra-sensitive, photocatalyst
1. Introduction Surface enhanced Raman scattering (SERS) has been developed as an effective analytical method for the detection of organic molecules as well as environmental pollutants through interfacial analysis, intermolecular interaction monitoring and surface modification characterization.[1-4] In recent years, various sensitive SERS substrates have been developed based on Ag, Au and their composites with different morphologies, such as nanoparticles, nanorods, nanocubes, and core-shell nanoparticles.[2, 5-8] However, a crucial strategy is to develop a sensitive, reusable and stable SERS substrate with high enhancement factor through simple fabrication approaches both in technological and cost view. Moreover, a suitable energy level configuration of metal/semiconductor system can construct a proper charge transfer path and also create a localized electric field for an enhancement of Raman scattering. The metal/semiconductor composites provide a candidate approach to recyclable SERS substrates based on photocatalytic self-cleaning function of the semiconductors. 2
For example, ZnO and TiO2 have been used as typical semiconductors. Besides the wide applications of ZnO in photoelectronics, it has also been employed as a propitious material in biosensors because of its high isoelectronic point (IEP, 9.5), which is beneficial for the adsorption and immobilization of the organic molecules with low IEP through electrostatic attraction.[9] Thus, the various ZnO nanostructures have been employed as SERS substrates in the past decades and combined with metal nanoparticles, such as ZnO/Ag nanosheets[10] and nanoflowers,[11] branched ZnO/Au 3D architecture[12] and 3D Ag/ZnO hybrids[13]. Nevertheless, the constituent units of above ZnO are one-dimensional or two-dimensional nanomaterials including sheet [10] and rod.[12, 13] On one hand, nanoparticles are more conducive to molecular adsorption than that of sheets and rods due to their higher surface-to-volume ratio. On the other hand, a sphere can provide more active sites for the reaction because of its largest specific surface area. In addition, it is expected that a better configuration for molecular immobilization and sensing when assembling the nanoparticles into porous spheres. However, it is not easy to self-assemble the microspheres without template, especially the porous structured ZnO microspheres. Furthermore, the decoration of metal nanoparticles is generally processed in colloidal solution[10] [11]or ion sputtering[13], so the presence of weak bonding between metal and semiconductor further affects the stability and reusability of the substrate in SERS detection. The photochemical deposition as an in-situ growth method can provide a stable semiconductor/metal contact and homogeneous hot-spot distribution, which further improves the performances of biosensing such as stability, responsibility and sensitivity. In this work, a novel, recyclable and highly sensitive SERS substrate was constructed based on porous ZnO/Ag microspheres. The ZnO microspheres were synthesized by template-free low temperature hydrothermal reaction at 90°C and Ag NPs decorated on microspheres in situ via photochemical deposition. Based on synergetic effect between Ag and ZnO, the microspheres exhibited a significantly enhanced Raman signal for Rhodamine 6G (R6G), phenol red (PhR), dopamine (DA) and glucose (GLU). The performances of SERS substrate including high sensitivity, 3
stability, and recyclability were analyzed and the mechanism was discussed in detail.
2. Experimental 2.1 Synthesis of ZnO microspheres. ZnO microspheres were synthesized by using a low temperature hydrothermal technique. In a typical process, the 0.05M of ZnCl2 and hexamethylenetetramine (HMT) solution were mixed with same molar ratio and stirred until the complete dissolution of the chemicals in deionized water. Then, the as-prepared 0.038M of Na3C6H5O7·2H2O in deionized water was added to the above solution with volume ratio of 1:2. After that, the mixed solution was put into a tightly closed glass bottle with quartz films and placed in an oven at 90°C for 90 min. The ZnO microspheres were grown on quartz substrates. After 90 min, the collected product was washed several times with ethanol and deionized water and then dried in the oven at 45°C for 1 h. 2.2 Synthesis of porous ZnO/Ag microspheres. Photochemical deposition was employed to obtain close bonded porous ZnO/Ag microspheres. During the process, the as-prepared ZnO microspheres on quartz films were put into 0.05M of AgNO3 and NaNO3 mixed solution with same molar ratio and irradiated under UV light (18W mercury lamp, 254nm) for 20min. The dried ZnO/Ag samples were further annealed at 400 °C in an ambient atmosphere for 4 h to improve the crystallinity of them. The Ag NPs were also decorated on ZnO microspheres by using ion sputtering method for comparison. 2.3 Characterization The morphology of samples was observed by field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra Plus). The energy dispersive X-ray spectroscopy (EDX, EX-250) was used to reveal elemental distribution of the composites which equipped with FESEM. The crystal structure was characterized by X-ray diffraction (XRD, Siemens D5005). The Raman scattering measurements were conducted by using confocal micro-Raman system (Lab RAM HR 800) under laser excitation of 514.5 nm and power of 1mW 1% with an integration time of 10 s. the 4
theoretical calculation and simulations were carried out by using finite difference time domain (FDTD) method. A photocatalytic reaction experiment was conducted by using photocatalytic reaction device. The UV-Vis spectrophotometer (UV-vis, UV-2600PC spectrometer) was used to measure the absorbance of powder samples as well as solutions after photocatalytic degradation.
3. Results and Discussion 3.1 Nanostructure formation of porous ZnO/Ag microspheres FESEM, EDX and XRD have been used to analyze the morphology, composition and elemental distribution of the nanocomposites (Fig. 1). The as-grown ZnO exhibits spheres with smooth surface and diameter of about 4µm as shown in Fig. 1(A). Fig. 1(B) depicts the decoration of Ag NPs on the smooth surface of ZnO spheres after the photochemical deposition. After annealing, the products are changed into porous structure with rough surfaces and the size of Ag NPs and ZnO particles are observed as 10-20nm as shown in Fig. 1(C) and 1(D). The XRD patterns in Fig. 1(F) confirm the hexagonal phase of ZnO (JCPDS NO. 89-1397) with diffraction angles of 2θ=31.029°, 33.697°, 35.506°, 46.812°, 55.811°, 62.278°, 67.298° and 68.609°, which corresponding to the crystallographic planes of (100), (002), (101), (102), (110), (103), (112) and (201) respectively. Concomitantly, the identified diffraction peaks at 28.133°, 38.587° and 44.613° with crystallographic planes of (220), (111), and (200) are related to Ag NPs. Moreover, the diffraction peaks of ZnO/Ag become sharper after the process of annealing (Fig. 1(F)) which denotes an improved crystalline nature. The EDX spectrum and the elemental mappings in Fig. 1(G) depict the elemental distributions of O, Zn and Ag within the structure of ZnO/Ag clearly and the Ag peaks are observed clearly in the EDX spectrum of ZnO/Ag. The above results can be indicated the successful formation of ZnO/Ag composite. .
The schematic diagrams for morphological changes and reaction mechanism of porous ZnO/Ag microspheres are illustrated in Fig. 2. A large amount of Ag+ is adsorbed on the surface of ZnO microsphere and then reduced into Ag NPs by the photo-generated electrons from ZnO under the irradiation of UV light. The detailed 5
photochemical process is displayed in Fig. 2(B). In addition, the crystallization of ZnO/Ag microspheres is increased and the porous structure was formed after the process of annealing at 400 °C for 4h. 3.2 SERS measurement for R6G and its enhancement mechanism In order to examine an enhancement effect of ZnO in SERS, Ag NPs was sputtered on the quartz film and compared with annealed ZnO. Also, R6G solution was diluted successively in the range of concentrations from 10-2 M to 10−12 M by factors of 10 for SERS investigation. The SERS substrates of Ag NPs, ZnO microspheres and ZnO/Ag microspheres were immersed into R6G solution and dried at room temperature naturally. From Fig. 3(A), it can be found that the porous ZnO/Ag microspheres (curve 1) exhibit the strongest enhancement followed by pure Ag NPs as substrate (curve 2), whereas the pure ZnO as substrate presents a very weak response (curve 3) for the same concentration of R6G. Consider Fig. 1 and Fig. 3(B), it can be noticed that the size of sputtered Ag NPs is similar to that of those obtained from photochemical deposition, but the latter method offers a better distribution of Ag on ZnO surface when compared to former one because of in situ synthesis method which stabilizes the bonding between ZnO and Ag. The porous ZnO/Ag microspheres mentioned later in the article are all fabricated by photochemical deposition. Fig. 4(A) and (B) represents the Raman spectra of R6G with different concentrations (from 10-4 M to 10-12 M) under the same experimental conditions. The spectral peaks of R6G are observed clearly at 1361, 1505 and 1645 cm-1, which assigned to symmetric modes of in-plane C-C stretching vibrations.[14] In addition, the R6G peaks are identified even in a lower concentration of it (10-12M). The SERS enhancement factor (EF) of porous ZnO/Ag microspheres as substrate was calculated according to the following equation:[15]
EF
ISERS Nbulk Ibulk NSERS
.
(1)
Where, ISERS and Ibulk denote the integrated intensities for 10-12M of R6G on ZnO/Ag microspheres as substrate and 10-2M of R6G on quartz substrate at 1361 cm-1 band, 6
whereas NSERS and Nbulk represent the corresponding number of R6G molecules adsorbed onto them. The calculated EF value for R6G is as high as 2.44 × 1011 which indicates an excellent sensitivity of the substrate. Furthermore, an outstanding linear relation is obtained between Raman intensity and concentrations of R6G in the ranging from 10-5 M to 10-12 M at 1361 cm-1 as shown in Fig. 4(C). The yielded regression equation is Y=0.36096X+6.07207 (Y is a logarithmic Raman intensity and X is a logarithmic concentration of R6G) and the correlation coefficient is r=0.99541. The results can be proposed that the porous ZnO/Ag microsphere as substrate is an effective tool for qualitative and quantitative detections of Raman signal. Table 1 displays the comparison of detection limit and enhancement factor for porous ZnO/Ag microspheres with previously reported works. The types and morphologies of semiconductors affect the sensitivity of detection, [2, 8, 10-13, 15-20] and the porous ZnO/Ag microspheres as SERS substrates can significantly improve the intensity of Raman signal. The reasons behind the prominent enhancement of EF for porous ZnO/Ag microspheres as SERS substrates are as follows. Firstly, the sizes of Ag NPs and ZnO NPs composed of ZnO/Ag microspheres are 10-20 nm, and there is an occurrence of positive proportional relationship between adsorption capacity and specific surface area. The porous structured microspheres can be improved the light trapping for the sensitive response of SERS besides higher surface-to-volume ratio which is beneficial for the immobilization of target molecules. Moreover, as a high IEP material, ZnO nanoparticles can assist the Ag NPs to adsorb and immobilize the organic molecules around them [9] and shown in Fig. 5(A). Secondly, as shown in Fig. 5(B), a strong absorption of sputtered Ag NPs on quartz film is observed in the ranging between 400nm and 550 nm, which is closed to photochemically deposited Ag NPs (inset of Fig. 5(B)). Under an excitation wavelength of 514.5 nm laser, the Ag NPs can be excited resonantly which leads to the generation of localized electromagnetic oscillation. Thus, the Raman signal is enhanced sufficiently through localized surface plasma resonance (LSPR). [21] Initially, the adequate hot spots are produced by 7
plenty of Ag NPs when exciting by incident light which contributes to greater enhancement of Raman signal.[22, 23] Fig. 5(C) and (D) illustrate the simulated optical field distributions based on the effects of nanoparticles. During the simulation, the refractive index of ZnO was set as 2.43 and the monitor wavelength was 514.5 nm, respectively. A clear enhancement of optical field is displayed in Fig. 5(C), and the hot spots appear at the nanoparticles nearby. The ambient dielectric function of Ag NPs is changed after the introduction of ZnO NPs, and the electric field is further enhanced, as shown in Fig. 5(D). Furthermore, the sputtered Ag NPs on porous ZnO microspheres, ZnO film and smooth ZnO microspheres under same experimental conditions reveal the effect of morphology in SERS substrate and their morphologies are depicted in Fig. 5(E) and (F). Fig. 5(G) shows the Raman signals of R6G collected on the substrates of porous ZnO/Ag microspheres (sputtering), ZnO/Ag film and smooth ZnO/Ag microspheres, respectively. It can be noted that the porous ZnO/Ag microspheres (sputtering) exhibit the strongest enhancement. The spherical structure and completely rough surface of ZnO can promote scattering of light, which is conducive to SERS detection. Thirdly, the Fermi level of Ag is higher than that of ZnO, so the electrons of Ag can easily transfer into ZnO at Ag/ZnO interface. Also, the accumulation of electrons can induce a potential barrier at the interface thus resulting in different charge distributions: the negative charge presented at the side nearest to the ZnO and the positive charge located at the other side. Subsequently, the formed local electrical fields around the Ag NPs further enhance the LSPR effect and improve the Raman scattering of target molecules significantly. [24] In addition, the chemical mechanism has also been involved in the enhancement of Raman signal via directional charge transfer from ZnO surface into target molecules. Most of the electrons in Ag/ZnO can be escaped from the potential barrier and transferred into target molecule thus resulting in enhanced Raman signal for the molecule. [25] The above reasons are significantly contributed to an enhancement of Raman signal, and the two most important reasons are particle size effect and light scattering effect. The peculiar structure is the advantage of porous ZnO/Ag microspheres different from other semiconductor/metal materials. 8
3.3 Recyclability, stability and homogeneity of ZnO/Ag substrates The above results can be proposed that the SERS substrate of porous ZnO/Ag microspheres is the suitable candidates for the potential applications in chemical analysis and biosensing. Moreover, the three factors such as recyclability, stability and homogeneity are crucial for the applications of porous ZnO/Ag microspheres. To analyze the recyclability, the used substrate of porous ZnO/Ag microspheres was irradiated with an 18 W mercury lamp for 1.5 h after SERS detection. Comparing curves 1 and 2 in Fig. 6(A), it can be demonstrated that the intensity of R6G is decreased dramatically after UV irradiation. After treatment, the reused substrate almost recovered the SERS activity, and exhibited a strong enhanced Raman signal in curve 3 of Fig. 6(A). Fig. 6(A) demonstrates the Raman signals of self-cleaned SERS substrate, hence, could be reused by many times through photocatalytic treatment as discussed below. In order to further verify the self-cleaning effect of SERS substrate, the photocatalytic experiments were performed. The ZnO/Ag samples were immersed into culture dishes with R6G solution (10−6 M, 30mL) and placed in a dark for 30 minutes to reach equilibrium in adsorption. Then, the culture dishes were exposed under UV light and absorbance of R6G solutions were measured every 10 min to investigate the photocatalytic process. Fig. 6(B) represents the UV-visible absorption spectra of R6G in photocatalysis process under UV irradiation. The characteristic peak intensity of R6G is gradually decreased when prolonging the reaction time. The concentration reduction of R6G is obtained due to photo-degradation which induced by catalyzing of ZnO/Ag. However, it is essential to wash out the impurities from the substrate for further usage in practical applications. In addition, the Ag NPs may be fall off after the process of retreating or reusing by many times and be oxidized when exposing in air for a long time. Therefore, it is indispensable to ensure the stability of substrate. In order to test the stability of porous ZnO/Ag microspheres, the substrates were immersed in the deionized water for 1 h or placed in an ambient atmosphere for 2 weeks before being reused for SERS detection. As shown in Fig. 6(C), it can be 9
noticed that the treated samples are still maintained their very strong Raman signal but the intensities are slightly decreased in comparison with that of untreated samples. The homogeneity has been examined by comparing the Raman signals of R6G at different locations on the same ZnO/Ag substrate as shown in Fig. 6(D). The Raman spectra from different spots on the same ZnO/Ag substrate possess the similar profiles and intensities. For example, the intensity values of Raman signal at 1361 cm-1 for R6G are 6342, 6500, 6442, 6732 and 6411. The similar spectral behaviors can be suggested a better homogeneity of SERS substrate. 3.4 Ultra-sensitive SERS detection for other organic molecules The enhanced SERS performances of porous ZnO/Ag microspheres for R6G predicted a possible ultra-sensitive sensing to other organic molecules. The solutions of DA, PhR and GLU with different concentrations in deionized water were prepared separately. The conditions of their SERS measurements are similar to the detection of R6G. Fig. 7 displays the SERS responses to DA, PhR and GLU molecules with ultra-low concentration. The spectral feature characteristics of DA are still identified clearly at 441, 463, 670, 1180, 1314 and 1417 cm-1 even in a lower concentration of 10-12 M. Similarly, the sharp peaks for 10-11 M of PhR are distinctly observed at 327, 400, 446, 479, 908, 1154 and 1440 cm-1. The Raman peaks of GLU with concentration of 10-11 M are also presented obviously at 606, 1029, 1057, 1157, 1266, 1386 and 1402 cm-1. All the above ultra-sensitive detections are based only on the porous ZnO/Ag microspheres as SERS substrate, whereas the pure Ag NPs as substrate did not achieve this effect. The significant enhancement of Raman signal is profiting from the synergetic effect between Ag and ZnO which already discussed in the Section 3.2.
3. Conclusions In this work, a novel method was introduced in the fabrication of porous ZnO/Ag microspheres and employed as highly sensitive, stable and recyclable SERS substrates. The Ag NPs decorated ZnO microspheres manifested the ultra-sensitive Raman 10
detection for R6G, PhR, DA and GLU with ultra-low concentration. The obtained detection limits of four organic compounds were 10-12 M, 10-11 M, 10-12 M, and 10-11 M, respectively. The peculiar structure of porous ZnO/Ag microspheres exhibited a significant effect on enhancement of Raman signal in SERS measurement. The porous ZnO/Ag microspheres as SERS substrates possess the self-cleaning behavior through photocatalytic process. The unique porous, recyclable, stable and ultra-sensitive properties of ZnO/Ag offer a new opportunity for
the development of SERS
substrates and lead to a promising application in biological sensors and organic pollutant monitoring.
Acknowledgements This work was supported by NSFC (61475035, 61275054) and Science & Technology Project of Jiangsu Province (BE2016177). Also, we thank the help of collaborative Innovation Center of Suzhou Nano Science and Technology.
11
References [1] D. Kurouski, R.P. Van Duyne, In situ detection and identification of hair dyes using surface-enhanced Raman spectroscopy (SERS), Anal. Chem. 87 (2015) 2901-2906. [2] L. Tang, S. Li, F. Han, L. Liu, L. Xu, W. Ma, H. Kuang, A. Li, L. Wang, C. Xu, SERS-active Au@Ag nanorod dimers for ultrasensitive dopamine detection, Biosen. Bioelectron. 71 (2015) 7-12. [3] H.Y. Wu, B.T. Cunningham, Point-of-care detection and real-time monitoring of intravenously delivered drugs via tubing with an integrated SERS sensor, Nanoscale. 6 (2014) 5162-5171. [4] G. Zito, G. Rusciano, G. Pesce, A. Dochshanov, A. Sasso, Surface-enhanced Raman imaging of cell membrane by a highly homogeneous and isotropic silver nanostructure, Nanoscale. 7 (2015) 8593-8606. [5] M. Fan, F. J. Lai, H. L. Chou, W. T. Lu, B. J. Hwang, A.G. Brolo, Surface-enhanced Raman scattering (SERS) from Au:Ag bimetallic nanoparticles: the effect of the molecular probe, Chem. Sci. 4 (2013) 509-515. [6] H. Wei, H. Xu, Hot spots in different metal nanostructures for plasmon-enhanced Raman spectroscopy, Nanoscale. 5 (2013) 10794-10805. [7] Y. Liu, Z. Huang, F. Zhou, X. Lei, B. Yao, G. Meng, Q. Mao, Highly sensitive fibre surface-enhanced Raman scattering probes fabricated using laser-induced self-assembly in a meniscus, Nanoscale. 8 (2016) 10607-10614. [8] T.J. Dill, M.J. Rozin, E.R. Brown, S. Palani, A.R. Tao, Investigating the effect of Ag nanocube polydispersity on gap-mode SERS enhancement factors, Analyst. 141 (2016) 3916-3924. [9] B. Gu, C. Xu, C. Yang, S. Liu, M. Wang, ZnO quantum dot labeled immunosensor for carbohydrate antigen 19-9, Biosen. Bioelectron. 26 (2011) 2720-2723. [10] Z. Wang, G. Meng, Z. Huang, Z. Li, Q. Zhou, Ag-nanoparticle-decorated porous ZnO-nanosheets grafted on a carbon fiber cloth as effective SERS substrates, Nanoscale. 6 (2014) 15280-15285. [11] Q. Tao, S. Li, C. Ma, K. Liu, Q.Y. Zhang, A highly sensitive and recyclable SERS substrate based on Ag-nanoparticle-decorated ZnO nanoflowers in ordered arrays, Dalton. T. 44 (2015) 3447-3453. [12] S. Picciolini, N. Castagnetti, R. Vanna, D. Mehn, M. Bedoni, F. Gramatica, M. Villani, D. Calestani, M. Pavesi, L. Lazzarini, A. Zappettini, C. Morasso, Branched gold nanoparticles on ZnO 3D architecture as biomedical SERS sensors, RSC Adv. 5 (2015) 93644-93651. [13] C. Huang, C. Xu, J. Lu, Z. Li, Z. Tian, 3D Ag/ZnO hybrids for sensitive surface-enhanced Raman scattering detection, Appl. Surf. Sci. 365 (2016) 291-295. 12
[14] Z. Huang, G. Meng, Q. Huang, Y. Yang, C. Zhu, C. Tang, Improved SERS performance from Au nanopillar arrays by abridging the pillar tip spacing by Ag sputtering, Adv. Mater. 22 (2010) 4136-4139. [15] J. Lu, C. Xu, H. Nan, Q. Zhu, F. Qin, A.G. Manohari, M. Wei, Z. Zhu, Z. Shi, Z. Ni, SERS-active ZnO/Ag hybrid WGM microcavity for ultrasensitive dopamine detection, Appl. Phys. Lett.109 (2016) 073701. [16] Y. Xie, Y. Meng, SERS performance of graphene oxide decorated silver nanoparticle/titania nanotube array, RSC Adv. 4 (2014) 41734-41743. [17] X. Li, H. Hu, D. Li, Z. Shen, Q. Xiong, S. Li, H.J. Fan, Ordered array of gold semishells on TiO2 spheres: an ultrasensitive and recyclable SERS substrate, ACS Appl. Mater. Inter. 4 (2012) 2180-2185. [18] Q. Huang, S. Liu, W. Wei, Q. Yan, C. Wu, Selective synthesis of different ZnO/Ag nanocomposites as surface enhanced Raman scattering substrates and highly efficient photocatalytic catalysts, RSC Adv. 5 (2015) 27075-27081. [19] X. He, H. Wang, Z. Li, D. Chen, J. Liu, Q. Zhang, Ultrasensitive SERS detection of trinitrotoluene through capillarity-constructed reversible hot spots based on ZnO-Ag nanorod hybrids, Nanoscale. 7 (2015) 8619-8626. [20] J. Yin, Y. Zang, C. Yue, Z. Wu, S. Wu, J. Li, Z. Wu, Ag nanoparticle/ZnO hollow nanosphere arrays: large scale synthesis and surface plasmon resonance effect induced Raman scattering enhancement, J. Mater. Chem. 22 (2012) 7902. [21] S. Chen, Z. Yang, L. Meng, J. Li, C.T. Williams, Z. Tian, Electromagnetic enhancement in shell-isolated nanoparticle-enhanced Raman scattering from gold Flat surfaces, J. Phys. Chem. C. 119 (2015) 5246-5251. [22] X. Li, G. Chen, L. Yang, Z. Jin, J. Liu, Multifunctional Au-coated TiO2 nanotube arrays as recyclable SERS substrates for multifold organic pollutants detection, Adv. Funct. Mater. 20 (2010) 2815-2824. [23] W. Y. Li, X. M. Lu, Y. N. Xia, Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering, Nano Lett. 9 (2009) 485-490. [24] S.K. Anna Rumyantseva, P. M. Adam, S. V. Gaponenko, S. V. Vaschenko, O. S. Kulakovich, A. A. Ramanenka, D. V. Guzatov,, V. D. D. Korbutyak, A. Stroyuk, V. Shvalagin, Nonresonant surface-enhanced Raman scattering of ZnO quantum dots with Au and Ag nanoparticles, ACS Nano. 7 (2013) 3420-3426. [25] N. Valley, N. Greeneltch, R.P. V. Duyne, G. C. Schatz, A look at the origin and magnitude of the chemical contribution to the enhancement mechanism of surface-enhanced Raman spectroscopy (SERS): theory and experiment, J. Phys. Chem. Lett. 4 (2013) 2599-2604.
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Fig. 1 SEM images of (A) ZnO (B) unannealed ZnO/Ag (C) annealed ZnO/Ag (inserts are enlarged SEM images) (D) the surface (E) and cross section of annealed ZnO/Ag, (F) XRD patterns of ZnO/Ag with and without annealing (G) EDX elemental mapping and EDX spectrrum of ZnO/Ag
Fig. 2 (A) Schematic diagram of changes in morphology and (B) mechanism of photochemical deposition
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Fig.3 (A) Raman spectra of R6G with different substrates (B) SEM image of Ag NPs (sputtering time of 30s).
Fig.4 (A) (B) Raman spectra of R6G with different concentrations ranging from 10-12 M to 10-4 M (C) The linear relation between Raman peak intensity and concentrations of R6G at 1361 cm-1 (logarithmic concentration and intensity).
15
Fig.5 (A) Schematic diagram of ZnO/Ag and adsorption of molecules (B) UV-visible absorption spectra of ZnO and ZnO/Ag and insert Ag NPs (sputtering time of 30s), the optical field distributions of (C) Ag NPs (D) ZnO/Ag simulated by FDTD method, SEM images of (E) ZnO/Ag film (F) smooth ZnO/Ag microspheres (G) Raman spectra of R6G with different substrates.
16
Fig.6 (A) Raman spectra of R6G after detection or photocatalysis (B) UV-visible absorption spectra of R6G with ZnO/Ag as photocatalyst, Raman spectra of R6G (10-6 M) on ZnO/ Ag substrates(C) placed in different environments and (D) with different locations of the same substrate.
17
Fig.7 Raman spectra of (A) DA (10-8 M, 10-10 M, 10-12 M, 10-10 M) (B) PhR (10-8 M, 10-11 M, 10-11 M ) (C) GLU ( 10-10 M, 10-11 M, 10-10 M) with ZnO/Ag and Ag NPs as substrates.
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Table 1 Comparison of detection limits or enhancement factors for the molecules on different substrates.
NO.
Substrate
Probe
Detection limit(mol/L)
Enhancement factor
Ref.
1
Ag nanocube
Thiophenol
-
107
8
2
Au@Ag nanorod
Dopamine
10-12
-
2
3
GO-Ag/TiO2 nanotube
Methyl blue
10-9
104
16
4
Au/TiO2 spheres
Rhodamine 6G
-
105
17
5
ZnO/Au 3D architecture
Apomorphine
-
106
12
6
3D Ag/ZnO hybrids
Phenol red
-
109
13
7
ZnO/Ag nanocomposites
Rhodamine B
-
107
18
8
ZnO/Ag nanorod
Rhodamine 6G
-
108
19
9
Ag/ZnO nanoflowers
Rhodamine 6G
-
1010
11
10
ZnO/Ag hybrid
Rhodamine 6G
-
1010
15
11
Ag /ZnO nanosphere
Rhodamine 6G
-
108
20
12
ZnO/Ag nanosheets
Rhodamine 6G
10-10
-
10
13
Porous ZnO/Ag microspheres
Rhodamine 6G
10-12
1011
This work
19