Preparation of hollow silver-polymer microspheres with a hierarchical structure for SERS

Applied Surface Science 490 (2019) 293–301

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Preparation of hollow silver-polymer microspheres with a hierarchical structure for SERS ⁎

Tao Hu, Hanwen Wang, Lijing Zhang , Shengyang Tao

T

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Department of Chemistry, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hollow Porous silver-polymer microspheres Hierarchical structure SERS

The Surface-enhanced Raman scattering (SERS) effect on the surface of metal conductors can significantly improve the sensitivity of molecular detection. Therefore, it is widely used in the field of chemical sensors. The nanostructure of the metal surface strongly influences the strength of the SERS by adsorption and surface electric field. Herein, we combine microfluidic technology with electroless silver plating to produce hollow microspheres with different wall thicknesses. The microspheres have a diameter of 400 μm and a spherical shell thickness of 40–100 μm. The spherical shell has a porous structure formed by phase separation, on which nanosilver particles can be deposited. The silver-polymer hybrid microspheres can adsorb dyes in aqueous solution and produce significant SERS effects, and the Raman signal enhancement factors can be up to 7.11 × 1011. Computational simulations and experimental results show that the Raman enhancement effect is related to the distance between the silver particles on the surface and the hollow structure of the microspheres.

1. Introduction Due to the unique spherical shell and pore structure, porous hollow microspheres have a strong capacity of adsorptive storage and can encapsulate a variety of substances [1–3]. Therefore, they have significant advantages for applications of control release, cell culture, heterogeneous catalysis, separation, and purification [4–10]. During the use, the diameter, uniformity of microsphere size, the thickness of the spherical shell, porosity and other structural parameters will have a significant influence on the properties of microspheres [11–12]. Traditional methods for getting hollow microspheres include emulsion polymerization, template casting, nanoparticle self-assembly, suspension polymerization, spray drying and so on [13–16]. Although these methods can prepare various microspheres, they still have some limitations. For example, the size distribution of the micro-droplets prepared by these methods is difficult to control, and the obtained microspheres are not sufficiently monodisperse. The template method often requires the sacrifice of templates, resulting in the additional consumption of chemicals. The self-assembly method often has unique requirements for the components of the microspheres, and only molecules and materials with specific structure can meet the requirements of the self-assembly system. As a new method for preparing hollow microspheres in recent years, microfluidic chips can precisely control the shape, size and inner structure of micro-droplets by the interaction

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between multiphase fluids, and the obtained microspheres have a spherical structure, moral hollowness, and high monodispersity [17–23]. At the same time, it also has excellent reproducibility and good universality. It can make a variety of organic small molecules, polymers and inorganic materials into hollow microspheres [24–29]. Surface-enhanced Raman scattering (SERS) effect refers to the Raman scattering signal of adsorbed molecules in the excitation region due to the enhancement of the electromagnetic field on the surface or near the surface of the substrate [30–31]. The Raman scattering phenomenon has been greatly enhanced. SERS overcomes the shortcomings of the low sensitivity of Raman spectroscopy and can obtain ultra-low concentration single molecule detection which is difficult to obtain by traditional Raman spectroscopy. It is widely used in the research of the interface. It can effectively analyze the adsorption orientation of the compound at the interface, the change of the adsorption state, and the interface information of the substrate [32–39]. The SERS effect is closely related to the micro/nanostructure of the metal surface or the aggregation state of metal nanoparticles [40–46]. In this study, we prepared porous polyacrylate microspheres with a hollow structure by using a capillary focusing microfluidic chip. Hollow silver-polymer hybrid microspheres with a porous structure were then prepared by electroless plating. The microsphere had excellent Raman enhancement property and was applied to the SERS detection of molecules in an aqueous solution. Through the calcination process, the

Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (S. Tao).

https://doi.org/10.1016/j.apsusc.2019.06.061 Received 31 January 2019; Received in revised form 14 May 2019; Accepted 6 June 2019 Available online 11 June 2019 0169-4332/ © 2019 Published by Elsevier B.V.

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2.4. Preparation of the silver plating solution

aggregation state of silver nanoparticles changed significantly, which affected the SERS effected of the microspheres. The hollow structure was also beneficial for the detection process by enhancing the deposition of silver in the microsphere.

Solution A: 3.5 g of AgNO3 was dissolved in 60 mL of deionized water, then the amount of ammonia (25%) was added until the formed precipitate was all dissolved. 100 mL of NaOH (0.025 g mL−1) was added, and the color of the solution turned black and no longer changed. Finally, ammonia (25%) was added until the solution was transparent. Solution B: 4.5 g of glucose and 0.4 g of seignette salt was added to 100 mL of deionized water, and the solution was boiled for 10 min. After it was cooling, 10 mL of ethanol was added. When we plated silver, the solution of A and B were mixed with a ratio of 1 to 1.

2. Experimental section 2.1. Materials 2H2M(2-Hydroxy-2-methylpropiophenone), undecanol, 18-TMOS trimethoxy(octadecyl) silane, TMPTA (trimethylolpropane triacrylate), GMA (glycidyl methacrylate), PVA (polyvinyl alcohol, Mw = 67,000) and rhodamine 6G (R6G) were bought from Aladdin Chemical Co., Ltd. (Beijing, China). Hydrogen Peroxide (H2O2, 30 wt%), sulfuric acid (H2SO4, 98 wt%), silver nitrate (AgNO3), sodium hydroxide (NaOH) and ammonia (NH3·H2O, 25 wt%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Ethanol and seignette salt were bought from Guangfu Fine Chemical of Tianjin Co., Ltd. (Tianjin, China). Glucose was bought from Damao Fine Chemical of Tianjin Co., Ltd. (Tianjin, China).

2.5. Preparation of silver@polymer microsphere (Ag@PM) and silver microspheres (Ag-M) 20 mL of 0.1 M AgNO3 aqueous solution was poured into the sample vial, and 1.0 g of PM was added. The vial was fixed in a shaking table at a temperature of 25 °C at a vibration frequency of 300 rpm (Revolutions Per Minute) for 2 h. The remaining liquid in the vial was then removed, and the AgNO3-PM core-shell porous microspheres were dried at 60 °C for 24 h. Finally, the above AgNO3-PM was immersed in the silver plating solution at 25 °C for 30 min. The obtained microspheres were filtered after the silver plating was finished and washed 3 times with deionized water. The core-shell porous microspheres with silver particles (Ag@PM) were obtained with the color changed from pale yellow to dark green. The pure Ag microspheres (Ag-M) without polymer were obtained by the calcination of the Ag@PM in O2 at 350 °C for 2 h and in H2 at 200 °C for 1 h with a temperature ramp of 1 °C min−1.

2.2. Preparation of microfluidic device The capillary glass tube was assembled on a glass slide to make a microfluidic device [47,48]. A circular borosilicate glass capillary (Sutter Instrument Co., USA) having an inner diameter of 0.58 mm and an outer diameter of 1.00 mm was gradually applied by using a micropipette puller (P-1000, Sutter Instrument Co., USA). Pulled off to form a tapered glass capillary with a needle tip. The tapered glass capillary was buffed and polished with sandpaper (CC88P, P3000, M.T., Inc.). The inner diameter and outer diameter of the tip were 180 μm and 200 μm, respectively. The resulting conical glass capillary was washed and dried with nitrogen and allowed to dry to remove any residual glass particles. The capillary was immersed in a 30% H2O2 and 98% H2SO4 solution at a volume ratio of 3 to 7, and then purified with ethanol and treated with 18-TMOS for about 1 min to make it hydrophobic. The round borosilicate glass capillary was burned to the melting point on a gas torch and then drawn into an ultrafine capillary having a diameter of 5–10 μm. It is inserted into the above capillary to form a cannula structure. The cannula structure capillary and the unprocessed cylindrical capillary were aligned in a square channel (internal size: 1.05 mm × 1.05 mm, Harvard Borosilicate Square Tubing). The channel was bonded to the middle of the microscope slide (using 5 min epoxy, Devcon). The exposed capillary holes were sealed with a syringe needle using epoxy resin. The liquid will be injected into the microfluidic device through a Teflon liquid tube (Aladdin PTFE 1.6 mm × 1.2 mm) using a syringe pump (Longer Pump, LSP01-2A).

2.6. SERS detections R6G has been fully characterized by Raman spectroscopy and it has been widely used in SERS detection with its good vibration characteristics [49–52]. Therefore, R6G was chosen as a probe molecule to test the performance of microsphere as SERS substrates. The spectral analysis of R6G is in Table S1. The microspheres were separately immersed in different concentrations of R6G aqueous solution for 30 min. The obtained microspheres were washed with deionized water to remove unadsorbed R6G, and dried in an oven at 60 °C for 2 h. In the SERS detection, except for special instructions, all the microdroplets (microspheres) are about 350 μm in size and the flow rates of inner, middle and outer phase are 0.3 mL h−1, 0.5 mL h−1, 5 mL h−1, respectively. The thickness of the shell is about 60 μm. The silver plating was performed for three times and each time is for 2 h. 2.7. Characterization Microscopic features of the samples were taken at 30 kV using a QUANTA 450 scanning electron microscope (SEM) instrument of FEI Company. The surface of the sample was coated with Pt before testing. X-ray diffraction (XRD) measurements were conducted on a Rigaku D/ MAX-2400 X-ray diffractometer instrument, with Cu Kα radiation, operating at 40 kV and 10 mA. The SERS spectra were obtained using a Thermo Fisher XDR system with 532 nm and 785 nm radiation from a 0.1 mW laser excitation source. Transmission electron microscopy (TEM) analysis was conducted with a Thermo Scientific microscope (Tecnai G2 F30 S-Twin, TF30) operated at 300 kV. All the photos and video in the article were taken by the Phantom Miro C210 high-speed camera. Photoluminescence (PL) spectra were measured on a G9800A fluorescence spectrometer (Agilent Technologies).

2.3. Preparation of core-shell porous microspheres (PM) First, 3.98 g of TMPTA, 2.62 g of GMA, 3.00 g of undecanol and 0.40 g of 2H2M were weighed and mixed in a sample bottle to obtain a homogeneous solution. After sonication for 10 min, 5 mL of the liquid was aspirated with a syringe and then used as a middle phase. The innermost and outermost phases were both 5 wt% PVA solutions. Monodisperse core-shell droplets were generated by the microfluidic device and collected with deionized water. The colorless droplets become white polymer microspheres (PM) under UV curing (365 nm, 50 W). Finally, the PM was washed 3 times with deionized water to wash away the PVA thoroughly. After washing 3 times with absolute ethanol, the PM was immersed in anhydrous ethanol for 24 h to exchange the undecanol molecules in the PM. After soaking, the PM was naturally dried at room temperature.

3. Results and discussion 3.1. Synthesis and characterization of hollow PM The structure and generation process of a microfluidic chip for 294

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Fig. 3. The relationship of the flow rate with the morphology of microspheres. The grey part represents the stable co-flow regime, the blue part represents the solid microspheres area, and the pink part is the core-shell microspheres area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 1. Optical microscopy images of the core-shell microspheres. (A) and (B) are the formation of micro-droplets in the microfluidic device; (C) and (D) are the optical microscopy images of the micro-droplets and core-shell microspheres. The scale bar is 100 μm.

The effects of different flow rates on the droplet size and wall thickness were further investigated. As shown in Fig. 2(A–D), the velocity of the inner and middle phases remains constant at 0.3 mL h−1 and 0.5 mL h−1. When the flow velocity of the outer phase gradually increases, from 1 mL h−1 to 7 mL h−1, the outer diameter of the microdroplets becomes smaller. As a result, the droplet size gradually decreases, and the thickness of the shell becomes thinner, changing from 100 μm to 40 μm. Similar changes are observed when other phase flows were adjusted. Therefore, the structure of the droplets can be easily adjusted according to the liquid flow rate, which provides many possibilities for subsequent application of the droplets. When the flow rate of the intermediate term is enlarged, and the flow rates of the inner and outermost phase are fixed, the obtained rule is the opposite. The droplet size gradually increases, and the thickness of the shell becomes larger, as shown in Fig. 2(E–H). In order to further explore the relationship between the hollow microsphere structure and the liquid flow rate, we explored the flow

preparing micro-droplets is shown in Fig. 1(A, B). The hollow structure of the droplets was produced by a flow focusing process produced by microfluidic chips based on capillaries. The inner and outer phases of the droplets were aqueous solutions containing surfactants (PVA) to reduce surface tension. The droplets consist of a polymerizable acrylate and undecanol. The undecanol induces phase separation in the spherical shell when the acrylate is polymerized. The formation of the W/O/ W (water-in-oil-in-water) structure is also related to the flow rate of the three-phase fluid. The obtained micro-droplets can be cured in a short time under the irradiation of a 365 nm ultraviolet lamp to form microspheres. It can be seen in the optical microscope that the color of the droplets changes from transparent (Fig. 1C) to opaque (Fig. 1D) during the curing process.

Fig. 2. The change of droplets sizes at different flow rates. (A)–(D) are the effect of the flow rate of the outer phase. When the flow rates of the inner phase and the middle phase were 0.3 mL h−1 and 0.5 mL h−1, respectively, the flow rates of the outer phase are:(A) 1 mL h−1, (B) 3 mL h−1, (C) 5 mL h−1, (D) 7 mL h−1; (E)–(H) are the effect of the flow rate of the middle phase. When the flow rates of the inner and the outer phases are 0.3 mL h−1 and 5 mL h−1, respectively, the flow rates of the middle phase are: (E) 0.5 mL h−1, (F) 1.5 mL h−1, (G) 2.5 mL h−1, (H) 3 mL h−1. All the scale bars are 100 μm. 295

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Fig. 4. Optical and SEM images of microspheres with different shell thickness: (A)–(D): The optical images of microspheres and the scale bars are 200 μm; (E)–(H) SEM images of microspheres, the scale bar is 50 μm; (I)–(L): SEM images of porous structure diagram on the surface of microspheres, the scale bar is 2 μm.

Fig. 5. Different silver plating time and frequency distribution map: (1) different silver plating time: (A) 0 min (loaded silver as a nucleation point); (B) 10 min; (C) 30 min; (D) 60 min; (2) different silver plating frequency: (E) 1; (F) 2; (G) 3; (H) 4. The scale bar is 10 μm.

of generating the spherical shell is less affected by the flow velocity of the intermediate phase. Therefore, increasing the flow rate of the outermost phase within a specific range (0.5 mL h−1 to 9 mL h−1) is advantageous for obtaining a better droplet having a core-shell structure. By adjusting the flow velocities of the intermediate phase and the

rate conditions of the different phase liquids required to form the coreshell structure. As shown in Fig. 3, the flow rate of the innermost phase is 0.3 mL h−1. It can be seen from Fig. 3 that as the flow velocity of the outermost phase fluid increases, the region where the spherical coreshell micro-droplets are generated gradually increases, and the process 296

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external phase respectively, the variation of the size and thickness of the droplets can be obtained as shown in Fig. S1. As shown in Fig. 4, the photocurable resin and the other components of the middle phase polymerize to form hollow microspheres with different shell thickness under UV lamp curing. Undecanol is used as a pore former in the shell which separates from the polymer during polymerization of the acrylate and results in the formation of a porous structure on the shell. The water in the core and the undecanol in the shell are then released from the microspheres after washing with ethanol to produce a hollow porous sphere structure. As shown in Fig. 4, both the optical microscope and SEM images prove that the microspheres have an average size of about 440 μm and a thickness of 40 μm to 100 μm. A distinct porous structure is visible on the wall of the spherical shell.

Intensity(a.u.)

111

Ag@PM Ag-M

200 220

311 222

JCPSC 04-0783 3.2. Characterization of Ag@PM and Ag-M

40 Electroless deposition is a widely used method for depositing metal coatings on polymer surfaces. This process typically requires metal ions as deposition nucleation sites, such as silver and palladium. The porous hollow microspheres have a fine porous structure so that silver ions can be loaded into the pores of microspheres as a nucleation point by the impregnation method. We investigated the effect of silver plating time and frequency on the content of silver. The results (Fig. 5) show that after silver ions are loaded as a nucleation point, the plating time has little effect on the final silver content. The content of silver does not change obviously after 10 min of plating time. The plating frequency has a more significant impact. The silver content reached the maximum after three times (Fig. S2). The pure hollow silver microspheres were obtained by subsequent calcination and reduction. As shown in Fig. 6, compared with the two kinds of microspheres, the silver-plated microspheres with the polymer substrate also have a porous structure on the surface, indicating that the silver plating process did not completely block the pores. At the same time, a TEM photograph (Fig. S3) shows that the silver particles on the microspheres have a diameter of 150–200 nm. The surface of the calcined hollow silver microspheres is relatively smooth and does not

60 2Theta(Degree)

80

Fig. 7. XRD pattern of silver microspheres and silver-polymer hybrid microspheres.

show any pores, which should be caused by the sintering of silver nanoparticles during the calcination. According to the energy spectrum of SEM (Fig. 6), the silver elements in Ag@PM are uniformly distributed from the outer surface to the inner surface of the sphere, indicating that the silver-polymer hybrid microspheres with the uniform composition can be obtained by the electroless silver plating process. The morphology of the silver elements in Ag@PM and Ag-M was analyzed by X-ray diffraction (XRD). As shown in Fig. 7, the diffraction peaks at 38.1°, 44.2°, 64.4°, 77.39° and 81.46° correspond to the (111), (200), (220), (311) and (222) plane of cubic Ag [53], respectively. The diffraction peaks of the XRD images show that the main characteristic diffraction peaks of both the two kinds of microspheres are the same. However, the half width of the Ag@PM is narrow. It is because during the calcination process, the matrix of the polymer microspheres is removed, and the silver nanoparticles in the microspheres are sintered

Fig. 6. SEM images of Ag@PM and Ag-M at a different scale: (A), (B) and (C):Ag@PM, the scale bar are 100 μm, 5 μm, and 2 μm, respectively; (C), (D) and (E): Ag-M, the scale bar are 50 μm 5 μm, and 2 μm respectively. 297

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Fig. 8. The Raman spectra of the R6G solution at the concentration of 10−5 M absorbed on the substrates of polymer microspheres and Ag@PM: (A) the laser excitation is 532 nm; (B) the laser excitation is 785 nm.

Fig. 9. (A)–(B) The Raman spectra of R6G solution at different concentration absorbed on the substrates of Ag-M and Ag@PM: (A) the concentration of R6G is 10−6, 10−7, 10−8, 10−9 M; (B) the concentration of R6G is 10−5, 10−6, 10−7 M. (C)–(D) The comparison of the R6G Raman intensities centered at 610 cm−1, 1361 cm−1, 1510 cm−1 and 1647 cm−1 bands with different SERS active substrates and different concentration of R6G: (C) the substrate is Ag@PM; (D) the substrate is Ag-M.

with Ag@PM, it can be found that pure polymer microspheres show only fluorescence peaks and no clear Raman signal under 532 nm laser excitation when silver particles are not deposited. After the deposition of silver, the Ag@PM showed visible Raman characteristic peaks of R6G molecule, and the intensity of the fluorescence background decreased. According to the PL spectra of four kinds of microspheres (Fig. S4), when silver particles are loaded on the microspheres, the fluorescence of R6G is weakened. When 785 nm laser was used as the exciting light, the fluorescence background signal of the R6G was suppressed (in

together, which increased the grain size of the silver. In the XRD spectra, only the diffraction peaks of the elemental silver were observed, indicating that the silver element remained stable in the microspheres, and no visible valence change occurred. It facilitates the subsequent surface Raman enhancement process.

3.3. SERS effects of the microspheres As shown in Fig. 8(A), by comparing pure polymer microspheres 298

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Fig. 10. The numerical simulation results about the effect of different silver nanoparticle spacing on Raman Signal: (A) Ag-M; (B) Ag@PM.

Fig. 11. Raman spectra of R6G detected by different thickness microspheres and the numerical simulation results with different thickness microspheres: in the Figure (A): (1) hollow microspheres, the thickness is about 40 μm; (2) solid microspheres; (B) solid microspheres; (C) hollow microspheres, the thickness is about 40 μm.

Fig. 8B). It was more evident that blank polymer microspheres only showed a weak R6G Raman signal, while Ag@PM had strong Raman scattering signals. The above evidence confirms that the blank microspheres have no Raman enhancement effect, and the SERS effect of the composite microspheres is due to the deposition of silver. According to the current research results, the resonance absorption peak of R6G is located at 524 nm [54]. When the excitation wavelength is 532 nm, there is resonance enhancement of R6G. It can get a better Raman signal. So 532 nm was chosen as the excitation source. As shown in Fig. 9, the Raman enhancement effect of Ag@PM is stronger than that of Ag-M, and the detection limit of Ag@PM can reach 10−9 M. The Raman enhancement factor can be up to 7.11 × 109 times. The enhancement factor of Ag-M is only 1.4 × 107 times. Numerical simulations explain the changes in the SERS effect (Fig. 10). The surface electric field strength of the silver particles is related to the distance between the particles. After the calcination, the silver crystal grains fuse, and the interface between them disappears. It results in a sharp drop in the surface electric field strength and lowers the SERS effect. At the same time, after calcination, the porous surface structure of the microspheres also disappears. It is not conducive to the adsorption of the dye by the microspheres and is therefore not conducive to the SERS process. Finally, we explored the SERS effect of Ag@PM with different shell

Fig. 12. Silver distribution of hollow microspheres and solid microspheres: (A) hollow microspheres, the scale bar is 50 μm; (B) solid microspheres, the scale bar is 100 μm.

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Appendix A. Supplementary data

thicknesses. As shown in Fig. 11(A), when the microspheres are solid, the SERS effect is weak. Hybrid microspheres have a better SERS effect when they remain in a hollow structure. The possible reason is that the hollow structure of the microsphere affects the adsorption of silveramino coordination ions. More silver particles can be deposited in the microspheres. So it may induce a better Raman signal. The numerical simulation results in Fig. 11(B), (C) show that when the microspheres are solid, the concentration of the adsorbate decreases from the surface of the microsphere to the core gradually. When the microspheres have a hollow structure, the adsorbate exhibits a higher concentration in the shell than it in the solid microspheres at the same thickness. It means that the hollow structure is beneficial for the diffusion and enrichment of molecules in the microsphere. Therefore, the SERS effect of the solid microspheres is relatively weak to the microspheres having a hollow structure. Similarly, the efficiency of the diffusion of silver ions in their voids is different for hollow microspheres compared to solid microspheres. As shown in Fig. 12, silver ions also will diffuse and be enriched better in the hollow microspheres during the plating process so that it will lead to a different distribution of the silver element in the two kinds of microspheres. According to the element mapping diagram (Fig. 12), in the solid microspheres, most silver is on the out surface of the microsphere, and little silver is in the inner part. In the hollow ones, more silver evenly distributes throughout the shell than it in the solid one, which may result in stronger Raman signals.

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3.4. SERS mechanism Silver nanoparticles are often used in surface enhanced Raman techniques. There are two main types of mechanism research: electromagnetic field enhancement and chemical enhancement [51,55–57]. In this work, silver particles are deposited on the surface of polymer microspheres. A surface plasmon resonance effect occurs on the surface of Ag@PM, causing local electromagnetic field enhancement. Since Ag@ PM has a porous structure, R6G molecules are easily adsorbed and have a certain chemical interaction with each other and with silver particles, which changes the electron cloud density of the molecules and leads to Raman enhancement.

4. Conclusions In summary, this work developed a method for preparing silverpolymer hollow microspheres with a hierarchical structure via microfluidic chips and electroless plating technology and applied them to SERS. The capillary confocal microfluidic chip easily and quickly prepares micro-droplets with a core-shell structure and then convert them into microspheres. The size and thickness of the micro-droplets can be conveniently adjusted by the liquid flow rate. Ag@PM microspheres have a porous structure, and load silver nanoparticles in pores, which can effectively adsorb dye molecules and enhance their Raman signals. At the same time, the numerical simulation proves that the hollow structure of the microsphere improve the diffusion of the mass and increase the content of silver in the porous spherical shell, which will enhance its SERS performance. The experimental and simulated method used in this work provides a useful reference for the preparation of other functional porous microsphere materials with a hollow structure.

Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51703017, No. 21872018), the Financial Grant from the China Postdoctoral Science Foundation (No. 2016M601302, No. 20170520116), and the Fundamental Research Funds for the Central Universities (DUT18RC(4)013). 300

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