Highly Sensitive and Reproducible SERS Performance from Uniform

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Highly Sensitive and Reproducible SERS Performance from Uniform Film Assembled by Magnetic Noble Metal Composite Microspheres Chunyu Niu, Bingfang Zou, Yongqiang Wang, Lin Cheng, Haihong Zheng, and Shaomin Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03802 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Highly Sensitive and Reproducible SERS Performance from Uniform Film Assembled by Magnetic Noble Metal Composite Microspheres Chunyu Niua, Bingfang Zoua,b∗, Yongqiang Wanga∗, Lin Chenga, Haihong Zhenga, Shaomin Zhoua a

Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University,

Kaifeng 475004, P. R. China b

School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China.

∗Corresponding

author. Email: [email protected]; [email protected]

ABSTRACT: To realize highly sensitive and reproducible SERS performance, a new route was put forward to construct uniform SERS film by using magnetic noble composite microspheres. In the experiment, monodisperse Fe3O4@SiO2@Ag microspheres with hierarchical surface were developed and used as building block of SERS substrate, which not only realized fast capturing analyte through dispersion and collection under external magnet, but also could be built into uniform film through magnetically induced self-assembly. By using R6G as probe molecule, the as-obtained uniform film exhibited great improvement on SERS performance both in sensitivity and reproducibility when compared with non-uniform film, demonstrating the perfect integration of high sensitivity of hierarchal noble metal microspheres and high reproducibility of ordered microspheres array. Furthermore, the as-obtained product were used to detect pesticide thiram, and also exhibited excellent SERS performance for trace detection. KEYWORDS: SERS, magnetic separation, microsphere, assembly, detection

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1. Introduction Due to the unexampled advantages of integration of unique spectroscopic fingerprint, high sensitivity, and non-destructive data acquisition, the surface-enhanced Raman scattering (SERS) spectroscopy has been intensely explored as a powerful and extremely sensitive analytical technique with wide potential applications in biochemistry, chemical synthesis, food safety, and environmental monitoring.1-7 Since it was first discovered that a rough silver metal surface could greatly enhance the Raman scattering spectroscopy of adsorbed molecules, the SERS substrate has always been the research focus for its strong relation with Raman signal.8,

9

Especially the subsequent seminal discovery of single-molecule SERS

(SMSERS) aroused the researchers' attention greatly on the features of SERS substrate once again.10, 11 Brus et al investigated the structure closely by using scanning probe microscopy (SPM) and found the gaps or junctions in the aggregates to be the origin of the above phenomenon, which were called "hot spots" later.12 Thereafter, the design of substrates with hot spot-containing structures has been a major research interest in SERS field.13-19 Both top-down and bottom-up nanofabrication strategies have been explored for the generation of SERS substrate with hot spots in recent years.20 So far, three main classes of SERS substrates have been developed including metallic rough surfaces, colloidal micro/nanoparticles, and periodic nanostructures. Obviously, periodic nanostructures was attractive for the excellent homogeneity of "hot spots", which meant high reproducibility of SERS signals. However, these structures were usually produced by advanced top-down techniques such as electron beam lithography, which are associated with limitations regarding throughput, cost, and processable materials. Moreover, it is still difficult to fabricate well-controlled small gaps or complex geometries on the scale of a few nanometers for efficient and abundant “hot spots”.21 As a result, only a moderate enhancement has typically been achieved. To obtain both high sensitivity, some progresses have been made in 2


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the fabrication of SERS substrates based on colloidal micro/nanoparticles, which were usually produced in a large scale through down-top methods. Controllable hierarchical micro/nanostructure particles like “urchin-like” Au or "flower-like" Ag microspheres could provide SERS-active substrates with superior SERS sensitivity, because their large surface area could absorb more probe molecules actively and their concentrated "hot spots" on the rough surface could enhance Raman signals greatly.22-25 However, their separation from the analyte solution or the following-up assembly for SERS detection was rather time-consuming and complex.26-30 To simplify the collection of hierarchical micro/nanostructure particles from solution, Fe3O4 particles were naturally introduced to endow them with magnetic separable property,31-33 but the reproducibility of the magnetic noble metal microspheres became worse due to three aspects:

poor uniformity of Fe3O4 cores, magnet-induced

aggregation during the synthesis and irregular assembly as SERS substrate. Therefore, it is a great challenge to realize fast separation and high reproducibility simultaneously for hierarchical noble microspheres. Previous reports showed that periodic nanostructures like ordered array of polystyrene microspheres could be used as template to deposit noble metal nanoparticles and exhibited highly reproducibility for their uniformity across the whole SERS substrate, however their synthesis procedure was rather complex and expensive, and the sensitivity was also not high. Therefore, an ideal SERS substrate should combine the advantages of hierarchical microspheres with order microspheres array. Here, a new route was put forward to construct uniform film by using monodisperse magnetic noble composite microspheres as building block through magnetically induced self-assembly, which are expected to realize highly sensitive and reproducible SERS substrate. In our experiment, monodisperse Fe3O4@SiO2@Ag composite microspheres with hierarchical surface were synthesized and used as building block of effective SERS substrate as illustrated in Scheme 1. (a) The monodisperse Fe3O4@SiO2@Ag 3


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composite microspheres were firstly dispersed in solution to capture the analyte actively; (b) they were fast separated after capture and washed under external magnet; (c) the solution containing Fe3O4@SiO2@Ag composite microspheres was dropped on cleaned glass slide and assembled into an uniform film under external magnet; (d) the as-obtained uniform film were finally used as SERS substrate. The experimental results exhibited higher SERS sensitivity and reproducibility of the assembled uniform film than non-uniform film without external magnet. Further investigation indicated that the as-obtained monodisperse Fe3O4@SiO2@Ag composite microspheres could be used as an effective SERS substrate for residual pesticide detection. Obviously, the as-obtained product, which combined high performance with simple manipulation and low cost, is a promising candidate for SERS substrate.

Scheme 1 The manipulation process of Fe3O4@SiO2@Ag composite microspheres into uniform SERS film. 2. Experimental Section 2.1 Synthesis of Fe3O4 and Fe3O4@SiO2 Microspheres. The monodisperse Fe3O4 microspheres were prepared through hydrothermal method. The distilled water was deoxygenated before use. Typically, FeCl3·6H2O (0.27 g), FeCl2·4H2O (0.6 g), sodium citrate (2.4 g) and urea (0.72 g) were dissolved in 80 ml distilled water under nitrogen atmosphere to avoid oxidization, and then sodium polyacrylate (0.6 g) was added 4


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under continuous stirring until it was dissolved totally. The solution was transferred to a 100 mL autoclave and heated at 180 °C for 4 h. After repeated magnetic centrifugation and washing, polyacrylate-modified Fe3O4 microspheres were obtained. The water-dispersible Fe3O4 microspheres used as magnetic cores could be easily coated with uniform silica shell by a sol-gel method.34 2.2 Synthesis of Fe3O4@SiO2@Ag Microspheres. The silver shell coating was conducted by template-activated strategy with slight modification.35 The above Fe3O4@SiO2 microspheres (about 20 mg) were dispersed in a water/ammonia/ethanol mixed solution (2 mL/0.2 mL/13 mL) containing AgNO3 (0.1 g) and PVP (1 g), and then the whole solution was dispersed with the aid of ultrasound for 30 min at 40 °C. The above solution (15 mL) was transferred into a Teflon-lined stainless steel autoclave (20 mL) and heated at 110 °C for 10 h in an electric oven. After the autoclave was cooled naturally, and the product was taken out and separated by external magnet, and then washed by ethanol and tetrahydrofuan for several times. The final product was saved in ethanol for further characterizations and usages. 2.3 Characterization. The products were analyzed by X-ray diffraction (XRD), in a 2θ range from 10° to 80°, using CuKα radiation (Philips X’pert diffractometer). Scanning electron microscopy (SEM; Hitachi S-4800), and transmission electron microscopy (TEM; JEOL-2010). Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS XL). 2.4 SERS measurements. R6G were used as Raman probes to test the reproducibility of the SERS substrate. The R6G solutions (20 mL) with different concentrations were firstly prepared, and the above as-prepared Fe3O4@SiO2@Ag microspheres stocked in ethanol were added and placed on the 5


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shaking bed for 2 hours, and then the product were extracted by external magnet and washed by ethanol. The remained solution was dropped on cleaned glass slides with circular magnet under it, and then they were covered by a petri dish and left alone until all ethanol evaporated. The whole processes were conducted on anti-shock platform. Finally, the above dried film on glass slide was measured under the Raman instrument. The test of thiram was conducted in a similar procedure. Here a confocal microscopy Raman Spectrometer (LABRAM-HR) was used, with its laser at an excitation wavelength of 633 nm in this study. The laser spot focused on the sample surface was about 3 µm in diameter, and the acquisition time was 3 s for each spectrum. 3. Results and discussion

Figure 1 SEM images of (a) Fe3O4, (b) Fe3O4@SiO2, (d) and (e) Fe3O4@SiO2@Ag microspheres; TEM images of (c) Fe3O4@SiO2 and (f) Fe3O4@SiO2@Ag microspheres. Monodisperse Fe3O4 microspheres are necessary in order to realize monodisperse magnetic noble metal composite microspheres. As shown in Figure 1a, uniform Fe3O4 spherical particles with a diameter of about 220 nm was synthesized by a hydrothermal method, where the sodium polyacrylate was used especially to improve the water-dispersibility.36 The TEM image showed the Fe3O4 microspheres exhibited excellent dispersibility and the close observation of a single microsphere revealed that it was consisted of a large amount of nanoparticles with sizes of less than 15 nm in Figure S1. Although the as-obtained Fe3O4 6


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microspheres exhibited good water-dispersible, the silica shell was still coated around the Fe3O4 microspheres to improve their dispersibility due to its highly hydrophilic property, which is a critical factor in our experiment to avoid magnetic aggregation or heterogeneous aggregation in the following silver coating process. As a matter of fact, after uniform silica shell (about 100 nm) was coated around Fe3O4 microspheres through sol-gel method, the as-obtained Fe3O4@SiO2 composite microspheres indeed exhibited better dispersibility than bare Fe3O4 microspheres by direct comparison of SEM images as seen in Figure 1b and 1c. The silver coating was realized through template-activated coating method after several modifications, based on one-pot hydrothermal treatment of Fe3O4@SiO2 microspheres in silver nitrate, ammonia and PVP solution.35 And uniform silver shell was successfully coated around the Fe3O4@SiO2 composite microspheres (growth process seen in Figure S2), and the magnetic composite microspheres still kept well-dispersibility in Figure 1d. From the magnified SEM image in Figure 1e, the final product are monodisperse even after coating silver shell. The particle size distribution in Figure S3 were characterized by Malvern laser particle analyzer, and one narrow peak was observed, indicating the excellent monodispersity of the Fe3O4@SiO2@Ag composite microspheres. Further TEM characterization in Figure 1f showed the textured surface of silver shell with thickness of about 60 nm, and core shell structure was still distinguished from different contrast in Figure 1f. The XRD patterns in Figure S4 showed the composition of three different products mentioned above, including original cubic-phase of Fe3O4 (curve a), amorphous silica coating with peak at 23o (curve b) and cubic-phase of Ag (curve c), which revealed that the Fe3O4@SiO2@Ag microspheres were obtained through the successive coating procedures. Additionally, the UV-Vis adsorption spectra in Figure S5 was used to investigate the silver shell, and a broad hump peaks around 600 nm indicated that complete silver shell was coated around the Fe3O4@SiO2 microspheres.37 7


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The magnetic properties of the Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@Ag composite microspheres were investigated as shown in Figure 2. The saturated magnetization values of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@Ag composite microspheres are 73.3 emu/g, 20.2 emu/g and 3.83 emu/g, respectively. The magnetic properties of Fe3O4@SiO2@Ag composite microspheres, which inherited from the magnetic Fe3O4 particles, decreased obviously when compared with Fe3O4 microspheres, which may be ascribed to the increase of silica and silver coating layer. The zero coercivity and the reversible hysteresis behaviors observed from curve c in Figure 2 indicated the superparamagnetic nature of the Fe3O4@SiO2@Ag composite microspheres. Although saturated magnetization value decreased greatly after successive silica and silver coating, the Fe3O4@SiO2@Ag composite microspheres still could be separated from a suspension system quickly and redispersed into solution again by simple shaking as seen in the inset of Figure 2.

Figure 2 Room temperature magnetic hysteresis curves of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2@Ag microspheres (The inset showed the water-dispersibility and magnetic separation of Fe3O4@SiO2@Ag composite microspheres). Magnetically induced self-assembly was reported to be a very powerful method that can assemble monodisperse magnetic colloidal particles into ordered structures quickly and efficiently.38-40 According to the above characterizations, the Fe3O4@SiO2@Ag composite microspheres showed superparamagnetic and well-dispersible with narrow size distribution, 8


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which were adequate to be building block for magnetically induced self-assembly. The experimental results proved that the as-obtained monodisperse Fe3O4@SiO2@Ag composite microspheres could be successfully assembled into film under external magnet, and large-area and uniform film could be realized easily as seen in Figure S6a and 6b. Meanwhile, the Fe3O4@SiO2@Ag composite microspheres synthesized without using optimized reaction condition showed poor shell structures even conjunctions as marked by arrows in Figure S6d, and poor film with many cavities as marked by red circles in Figure S6c was obtained through magnetically induced self-assembly. The above control experiments indicated the importance of monodispersity and the shell structure on the subsequent film assembly under external magnet. More importantly, it could be seen that the surface was bestrewed with lots of silver nanoparticles from Figure S2d, and many gaps generated in the textured surface from the inset of Figure S2d, which were very useful as SERS substrate.

Figure 3 (a) SERS spectra of the assembled film (I) with or (II) without external magnet and (b) the corresponding Raman intensity distribution at R6G concentrations of 10-8 M. To quantitatively estimate the improvement owing to magnetically induced self-assembly, the SERS performance of the film assembled with or without external magnet were compared using R6G aqueous solutions with different concentrations (10-6, 10-8, 10-10 and 10-12 M ) as analyte. The manipulation processes were conducted according to Scheme 1. The control experiment were shown in Figure 3a. The peaks from 500 to 1750 cm-1 are attributed to R6G 9


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signals with 1363, 1509, and 1650 cm-1 assigned to C-H in-plane bending, C-O-C stretching, and C-C stretching of the aromatic ring.41 Clearly, the Raman intensity of the film (I) assembled with external magnet was stronger than film (II) without external magnet, especially at low concentrations of 10-10 or 10-12 M. The average enhancement factor was roughly estimated by comparing the SERS band intensity (for the vibration peak at 1363 cm-1) and the normal Raman band intensities of the R6G molecule.42 The enhancement factor of film (I) with external magnet was estimated to be 0.72 × 108 (five times larger than that of film (II) without external magnet). Besides, the reproducibility of SERS signal from film (I) and film (II) were investigated by choosing twenty spots across the substrate as seen in Figure S7, and the corresponding Raman intensity (1363 cm-1) were recorded as shown in Figure 3b. The average relative standard deviation (RSD) of film (I) was about 0.076, which was much lower than that of film (II) with the value about 0.179. Therefore, the above experimental results indicated that magnetically induced self-assembly rendered the monodisperse Fe3O4@SiO2@Ag composite microspheres a great increase both on the average Raman enhancement factor and reproducibility.

Figure 4 SEM images of film (a) with and (b) without external magnet, (c) the magnified SEM image of film (a) and (d) particle numbers in ten different circles marked in (c). The difference in the sensitivity, especially the reproducibility between the above two kinds of films, could be explained by surface structural difference between the film (I) and 10


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film (II). As shown in Figure 4a and 4b, the film (I) assembled with external magnet obviously showed flatter and more orderly than the film (II) without external magnet. Under the same conditions, the surface structure of film exposed in laser spot mainly influenced the Raman signal. It has been shown that the SERS intensity directly correlates with the strength of the localized electromagnetic fields that are enhanced by "hot spots".43, 44 Obviously, the density of microspheres in uniform film (I) was much higher than that of the film (II) in same area, meanwhile the gaps formed between microspheres were also denser than that in film (II) as seen in Figure 4a and 4b. Thus, film (I) provided more "hot spots" which played a great role on the enhanced sensitivity. As to the reproducibility, which mainly determined by the uniformity of the film, is also the main purpose of our experiments. It was found that the distribution of the as-obtained monodisperse composite microspheres became more uniform through magnetically induced self-assembly as seen in Figure 4c. For example, ten random circles with diameter of 3 µm was studied statistically as shown in Figure 4d, and the RSD value of the number of particles was as low as 0.05 through rough calculation, which was lower than the RSD value of film (I) above. Although the reproducibility was influenced by other factors like surface texture, here the reproducibility was proposed to be mainly influenced by the uniformity of the film for using the same composite microspheres as building block. From the above experiments, the monodisperse Fe3O4@SiO2@Ag composite microspheres were shown to be an effective SERS detection platform through magnetically separated and induced self-assembly.

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Figure 5 (a) The SERS spectra of thiram with different concentrations, (b) the relationship between the intensity (peak at 1386 cm-1) and concentrations, the signal reproducibility with concentrations of (c) 10-6 M, (d) 10-7 M and (e) 10-8 M. Analytical applications of monodisperse Fe3O4@SiO2@Ag composite microspheres film through magnetically induced self-assembly were tested toward the detection of trace pesticide thiram, which is a dithiocarbamate fungicide largely used worldwide in agriculture as a fungicide and animal repellent. Conventional analyses are performed with colorimetric, chromatographic, spectrophotometer and other electrochemical methods, but these techniques

often

require

complex

sampling

including

separation/extraction

and

pre-concentration procedures, which are high cost and time-consuming. The as-obtained monodisperse Fe3O4@SiO2@Ag composite microspheres not only could capture analyte actively but also could be assembled into uniform film as SERS substrate under external magnet, which are expected to detect poisonous or harmful substances sensitively and rapidly. The manipulation of Fe3O4@SiO2@Ag composite microspheres as SERS substrate was similar with the R6G experiment after capture of thiram in solution. Figure 5a is the Raman spectra of thiram in ethanol with concentrations increasing from 10-9 to 10-5 M. All the peaks from 400 to 1800 cm-1 in curve a-d are attributed to thiram signals according to previous reports, where the main Raman bands could be assigned as following, 560 cm-1 attributed to 12


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υs(CSS), 1150 cm-1 to ρ(CH3) or υ(C-N), and 1386 and 1514 cm-1 to ρ(CH3) or υ(C-N), respectively. The intensity of the Raman peaks of thiram decreased with the thiram concentrations, and signals of thiram can be noticed clearly even at the 10-8 M (0.01 ppm) level. The excellent detection limit may relate to the structure of thiram, since the S-S bond of thiram cleavage gave rise to two dimethyl residues that are strongly adsorbed on the silver shell.45 The relationship between SERS intensity at 1386 cm-1 and the concentrations of the thiram was also shown in Figure 5b, and good linear correlation between the peak intensity and concentrations was found. The reproducibility was further investigated with thiram concentrations of 10-6, 10-7 and 10-8 M in Figure S8 and the RSD values were calculated to be 0.082, 0.133 and 0.199 in Figure 5c-e, respectively. In this case, the RSD value of signal intensities of major SERS peaks were below 0.2 even at low concentration of 10-8 M, revealing a good reproducibility of Fe3O4@SiO2@Ag composite microspheres film.46 3. Conclusion The monodisperse Fe3O4@SiO2@Ag composite microspheres were synthesized and demonstrated to be assembled into uniform film as SERS substrate under external magnet, and the whole process was rather simple and fast because all the manipulation could be conducted under external magnet. The assembled film exhibited higher signal reproducibility than non-uniform film assembled without magnet. Through combining high performance with simple manipulation and low cost, monodisperse Fe3O4@SiO2@Ag composite microspheres are a promising candidate for SERS substrates. Acknowledgments This work was supported by the Natural Science Foundation of China (No. 51372070), the Research Foundation of Henan University (No. 0000A40409), Program for Science & Technology Innovation Talents in University of Henan Province (No. 16HASTIT009), Research Foundation for Young Scholar of Henan Province (No. 2014GGJS-022) and 13


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Changjiang Scholars and Innovative Research Team in University (PCS IRT15R18).

Supporting Information Available: TEM images of the Fe3O4 microspheres, SEM images of the products during the growth procedure, particle size distribution of the Fe3O4@SiO2@Ag composite microspheres, XRD patterns and UV-Vis adsorption spectra of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@Ag microspheres, SERS spectra of R6G collected on the selected 20 spots of the film with and without magnetically induced self-assembly, and SERS spectra of thiram on the selected 20 spots of the magnetically assembled film with different concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

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ToC Title: Highly Sensitive and Reproducible SERS Performance from Uniform Film Assembled by Magnetic Noble Metal Composite Microspheres

A new route was put forward to construct highly sensitive and reproducible SERS substrate by using monodisperse magnetic noble composite microspheres, where uniform SERS film was realized through magnetically induced self-assembly.

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Highly Sensitive and Reproducible SERS Performance from Uniform

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