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DOI: 10.1039/C6RA06584H (Communication) RSC Adv., 2016, 6, 67204-67211

Core/shell Ag@silicate nanoplatelets and poly(vinyl alcohol) spherical nanohybrids fabricated by coaxial electrospraying as highly sensitive SERS substrates†

Chih-Wei Chiu* and Po-Hsien Lin
Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan. E-mail: cwchiu@mail.ntust.edu.tw; Fax: +886-2-2737-6544; Tel: +886-2-2737-6521

Received 12th March 2016 , Accepted 9th July 2016

First published on 11th July 2016

Abstract

A novel, flexible, freestanding, and large-scale substrate for surface-enhanced Raman spectroscopy (SERS) was successfully prepared by coaxial electrospraying. Nanohybrids of silver nanoparticles (AgNPs)/triblock copolymer surfactant (copolymer)/silicate nanoplatelets (Ag@silicate) were prepared by the in situ reduction of AgNO3 in the presence of silicate platelets and a polymeric surfactant. Nonwoven mats of the hybrids were prepared via coaxial electrospraying, assembling Ag@silicate hybrids outside a poly(vinyl alcohol) (PVA) surface to form core–shell microstructures. Characterization showed that the core–shell Ag@silicate/PVA nanosphere substrate significantly enhanced the SERS signal intensity, with enhancement values approaching 5.1 × 105 for adenine molecules from DNA. These core–shell nanosphere hybrids fabricated by electrospraying have great potential as SERS substrates in biosensor technology.

Introduction

Nanotechnology has demonstrated that materials on the nanometer scale (from 1 to 100 nm) can exhibit physical and chemical properties distinct from their bulk behaviors.1 Metal nanoparticles (NPs) have shown many specific electrical,2 magnetic,3 and optical4 properties that are controlled by morphology and size.5,6 Common bile salt detergents or amphiphilic polymeric dispersants are typically used to stabilize the generated metal NPs;7–11 identifying such stabilizing materials is an important research issue.

Silver nanoparticles (AgNPs) are especially interesting because they possess unique properties for biosensing,12,13 catalysis,14,15 and conducting applications.16,17 Among current biosensing techniques, surface-enhanced Raman spectroscopy (SERS) is an extensively developed spectroscopic technique that uses the signal enhancement of Raman scattering via chemical effects18,19 and localized surface plasmon resonance (LSPR).20,21 SERS detects strong structural messages with high sensitivity from the molecules attached to noble-metal NPs.22,23 The SERS enhancement depends on nanostructure-based SERS-active substrates and optical field polarizations.24,25 For example, the electromagnetic enhancement provided by SERS substrates was substantially improved by inter-particle distances of less than 10 nm, which produced two-dimensional hot-junctions by generating AgNP orders on anodic aluminum oxide nano-passageways.26–28 However, the design and fabrication of SERS substrates with metal NPs remains challenging for several reasons, including the susceptibility of silver to oxidation in air, which hinders large-scale production, increases the price, and affects the sensitivity of the substrate.29,30

In this study, we developed a novel hybrid surfactant as a support for AgNPs formed from an amphiphilic copolymeric dispersant and silicate nanoplatelets. Previously, we reported an exfoliation process that permitted the randomization of smectite clay-layered structures using polyamine quaternary salts and the subsequent isolation of the randomized silicate nanoplatelets in an aqueous suspension.31 By controlling the ratios of AgNPs, copolymer, and silicate, AgNPs with a narrow distribution of particle sizes within 20–40 nm was obtained. Furthermore, core/shell Ag@silicate nanoplatelets and poly(vinyl alcohol) (PVA) micro-spherical hybrid nonwoven mats were fabricated by coaxial electrospraying. The resulting flexible, freestanding, and disposable nonwoven SERS substrate exhibited advantages such as superior sensitivity, high stability, large-scale production ability, and low cost because of the simplicity of the manufacturing process compared to that of other SERS substrates. This new class of SERS substrates based on Ag@silicate spherical nanohybrids could be applied in biomedical devices.

Experimental

Materials

The silicate nanoplatelets in this study were single-layered structures with 720 m2 g−1 surface area, ∼18[thin space (1/6-em)]000 ionic charges per platelet, and high aspect ratios (100 × 100 × 1 nm).31 Polyisobutylene-g-succinic anhydride (PIB-SA) with a molecular weight Mw = 1335 was obtained from Chevron Corp. Jeffamine® ED2003 (poly(oxyethylene)-diamine or POE-diamine, abbreviated POE-2000) with Mw = 2000 was obtained from Huntsman Chemical Co. Silver nitrate (AgNO3, >99.9%), N,N-dimethylformamide (DMF), and PVA (87–89% hydrolyzed) with Mw = 89[thin space (1/6-em)]000–98[thin space (1/6-em)]000 were obtained from Aldrich Chemical Co. Ethanol (>99.5%, SHOWA) was used to reduce AgNO3. Adenine powder (>99.9%) was obtained from Sigma-Aldrich.

Synthesis of tri-block copolymers (copolymer) as an organic dispersant

The tri-block dispersant was synthesized by reacting POE-2000 with PIB-SA at a 1[thin space (1/6-em)]:[thin space (1/6-em)]2 molar ratio (forming the product “copolymer”) by the following processes (see ESI Scheme S1†). PIB-SA (100.0 g, 0.076 mol) in tetrahydrofuran (THF, 400.0 g) was added to a 1 L three-necked round-bottomed flask equipped with a thermocouple and mechanical stirrer. POE-2000 (76.0 g, 0.038 mol) in THF (300.0 g) was then added. The mixture was stirred continuously at 30 °C for 3 h. The collected mixture was analyzed by Fourier-transform infrared spectroscopy (FT-IR). Characteristic absorption peaks at 1562 cm−1 and 1642 cm−1 corresponded to amido acid functionalities (see ESI Fig. S1†). The reaction temperature was increased to 150 °C and maintained at this value for 3 h. Absorption peaks at 1700 cm−1 and 1770 cm−1 appeared, indicating cyclized imide functionalities. This change in peaks indicated a significant conversion of the functional groups from amido acids to imides. The product was recovered by rotary evaporation under vacuum and water extraction to remove the solvents and unreacted amines. The solubility of the amphiphilic copolymer in organic solvents and water depended on the ratio of the hydrophobic PIB chain to the hydrophilic POE blocks (see ESI Table S1†).

Synthesis of AgNPs in the presence of copolymer/silicate nanohybrid surfactants

The typical procedures for preparing the AgNPs stabilized on the hybrid surfactants are described here. A silicate solution (2 g of silicate solid; 1 wt%) was dispersed by stirring in 200 g 1[thin space (1/6-em)]:[thin space (1/6-em)]1 water/DMF at 25 °C for 1 h. A solution of copolymer (2 g) and AgNO3 (2 g, 0.012 mol) was dissolved in 400 mL of deionized water and added to the silicate solution. The designed precursor weight ratios of AgNO3/copolymer/silicate were 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1. These mixtures were continuously stirred for 8–10 h at 80 °C, while the apparent color changes from white to dark yellow were monitored. UV-visible (UV-vis) absorption spectra indicated the reduction of Ag+ to Ag0.

Preparation of Ag@Silicate spherical nanohybrids as SERS substrates by coaxial electrospraying

In a typical electrospraying process, the Ag@silicate hybrid suspension and PVA solution were electrospun with an applied voltage, working distance between needle tip and grounded target, and flow rate of 25 kV, 15 cm, and 0.3 mL h−1, respectively, at 25 °C. Nonwoven mats of the composite nanohybrids were collected from the target. Large-scale 10 × 10 cm nonwoven mats were immersed in 5 mL of 2 mM adenine solution in ethanol for specified times and then dried at 25 °C to evaporate the remaining ethanol. The mats were monitored by field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and Raman spectromicroscopy measurements.

Characterization and instruments

FT-IR spectroscopy was performed on a Perkin-Elmer Spectrum One FT-IR Spectrometer from 4000 to 400 cm−1. The copolymers were dissolved in THF and evaporated to form thin films on KBr plates. The AgNP suspension was characterized using a Shimadzu UV-2450 UV-vis spectrophotometer. TEM and FE-SEM were performed by a Zeiss EM 902A operated at 80 kV, using samples (1 wt% in deionized water) deposited onto carbon-coated copper grids and fixed to FE-SEM holders using conductive carbon paste with thin gold coatings, respectively. Raman spectra were recorded and integrated with a ProMaker confocal Raman microscopy system (Protrustech Corp., Ltd., Taiwan). A 532 nm laser was used as the excitation source with a 10 s exposure time and a power of 20 mW. The laser line was focused on an excitation area of ∼4 μm2 on the substrate using a 50× objective lens.

Results and discussion

Synthesis of well-stabilized AgNPs by novel copolymer/silicate hybrid dispersants

In our previously study, a layered sodium montmorillonite (Na+-MMT) was directly exfoliated by polyamine into silicate nanoplatelets with high surface ionic charges distributed on thin units.31 In this study, the silicate nanoplatelets with ionic charges distributed on their surfaces (from [triple bond, length as m-dash]SiO−Na+ functionalities) were hydrophilic, dispersible in water, and estimated as ∼100 nm in length and ∼1 nm in thickness. The uniform distribution of the AgNPs was stabilized via ionic charge interactions between the silver ions and silicate layers.32 Simultaneously, the copolymers had polar functionalities, including the hydrophilic POE block, hydrophobic PIB block, and imide (–C(O)–N–C(O)–) group. These copolymers became non-covalently tethered to the silicate and the AgNPs.

The optimally dispersed performance of the two types of non-covalent bonding forces of ionic charge interaction and ion-dipole interaction among the AgNPs, silicate layers, and copolymers are conceptually illustrated in Fig. 1a. For the stabilization of AgNPs on the silicate surface, DMF was used as a reducing agent.33 The mechanism for Ag+ reduction may involve an initial binding of the Ag+ ions to the silicate; the copolymer, possessing oxygen ions and ethylene oxide, may consequently induce the redox reaction, with DMF as the oxidized species.


image file: c6ra06584h-f1.tif
Fig. 1 (a) Schematic of the nanohybrid dispersion mechanism by various non-covalent bonding forces among the AgNPs, copolymer, and silicate nanoplatelets. (b) UV-vis absorption spectra of the colloidal AgNO3/copolymer/silicate with weight ratios of (1) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, (2) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, (3) 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and (4) 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1. Inset: photograph of AgNP solutions stabilized by the hybrid dispersants. (c and d) TEM micrographs at different magnifications. Insets of (d) depict particle size histograms of the AgNO3/copolymer/silicate nanohybrids with weight ratios of (1) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, (2) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, (3) 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and (4) 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1.

In order to identify the optimal Ag@silicate level for stabilization, the synthesis of AgNPs was performed with aqueous solutions of AgNO3, copolymer, and silicate in various weight ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1 at 80 °C. In the presence of the stabilizing copolymer, the solution appeared dark yellow in color (Fig. 1b). Simultaneously, the UV-vis spectra showed the presence of nanometer-sized silver particles by the characteristic absorption peak at 404–421 nm after 8–10 h of stirring (ESI Fig. S2–S5† and Table 1). Through a reduction reaction, the DMF effectively converted the AgNO3 to metallic silver and stabilized the formation of AgNPs by monitoring the characteristic UV-vis absorption peak at approximately 410 nm.

Table 1 UV-vis absorption, silver particle size, inter-particle distance, and SERS intensity values of the core–shell (Ag/copolymer/silicate)/PVA spherical nanohybrids at different weight ratios
(Ag/copolymer/silicate)/PVA weight ratio (w/w/w) UV-vis absorption (nm) Average silver particle size (by TEM)a (nm) Inter-particle distance (by TEM)a (nm) SERS intensity valueb
a Average size and inter-particle distance of the AgNPs as measured by TEM.b Integrated area of Raman intensity (intensity value is the average Raman signal per silver particle) from 700 to 770 cm−1 of adenine for the band at 733 cm−1.c Ag@silicate/PVA composite nanofibers.
1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 415 19.1 ± 2.5 50.6 ± 2.5 9.3 × 104
1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 421 20.3 ± 2.5 23.8 ± 2.5 1.6 × 105
10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1 404 28.8 ± 2.5 16.3 ± 2.5 2.6 × 105
20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1 405 43.2 ± 2.5 7.8 ± 2.5 5.1 × 105, (4.8 × 104)c

The oxygen ion-functionalized silicate and the ethylene oxide-functionalized copolymer formed complexes with the silver ions and stabilized the generated AgNPs from this state. The presence of the hybrid dispersants was proven to effectively link the silver particles and control particle growth. Further, the nanohybrids were successfully well dispersed in water and observed in TEM micrographs at different magnifications (Fig. 1c and d). On a TEM grid, the size distribution of the AgNPs was centered at the diameter of ∼20 nm. The sizes of the supported AgNPs appear to depend on the presence of the hybrid dispersant. The average size of the AgNPs apparently decreases from 43.2 to 19.1 nm when the concentration of copolymer is increased.

Preparation of core/shell spherical Ag@silicate/PVA nanohybrids by coaxial electrospraying

Spherical nanohybrids with core/shell microstructures were prepared by coaxial electrospraying, as shown in Scheme 1. The Taylor cone and jetting from the hybrid charged liquid droplets were observed microscopically during the electrospraying process (see ESI Video S1†). The liquid body formed by the PVA solution and the Ag@silicate hybrids dispensed from the inner and outer needles, respectively, showed a conical shape with a half angle of 49.8°, referred to as the Taylor cone, at a threshold voltage of 25 kV. The hybrid substrates were formed by the assembly of the Ag@silicate sheet hybrids on the PVA droplet surfaces to create core–shell microstructures with external AgNPs immobilized for use as SERS substrates.
image file: c6ra06584h-s1.tif
Scheme 1 Schematic of the fabrication of core/shell Ag@silicate nanoplatelets and PVA spherical nanohybrids by coaxial electrospraying.

The high surface areas of nanoscale materials render suspensions of AgNPs generally unstable in organic media; in typical static-state nonpolar suspension media, AgNPs are only stable as large aggregates of particles. This aggregation prevents the exhibition and application of the nanoscale properties of the materials. In this experiment, the silicate sheets contain many oxygen functional groups, which provide versatile non-covalent interaction sites that improve the compatibility of the synthesized AgNPs with the PVA solvent, thereby stabilizing the suspension of the AgNP-decorated silicate sheets.34,35 Here, the optimally dispersed performance of the three types of non-covalent bonding forces of ion-dipole interactions, hydrogen bonding, and ionic charge interactions among the AgNPs, copolymer, silicate sheets, and PVA are conceptually illustrated in Scheme 2.


image file: c6ra06584h-s2.tif
Scheme 2 Schematic of dispersion mechanisms of nanohybrids by various non-covalent bonding forces among the AgNPs, silicate nanoplatelets, and PVA.

The FE-SEM analyses shown in Fig. 2a display spherical microstructures formed from the (Ag/copolymer/silicate)/PVA nanohybrid precursor solutions at all tested weight ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1. The core/shell microstructures are observed in the highly magnified TEM micrographs of Fig. 2b–d, which depict Ag@silicate sheet hybrids surrounding the spherical PVA structures. The core/shell Ag@silicate/PVA spherical hybrids have dimensions of ∼1 μm.


image file: c6ra06584h-f2.tif
Fig. 2 (a) FE-SEM and (b) TEM micrographs of the core/shell spherical nanohybrids of Ag/copolymer/silicate/PVA nanohybrids with weight ratios of (1) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, (2) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, (3) 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and (4) 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1 (inset: corresponding 10 cm × 10 cm Ag@silicate nonwoven mat). (c and d) TEM micrographs at high magnification of Ag@silicate nanohybrids formed from the precursor with the weight ratio of 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1.

Relationship between SERS intensity and Ag@silicate/PVA microstructures

Fig. 3 shows a sequence of Raman spectra of adenine molecules adsorbed on various microstructured Ag@silicate/PVA substrates. An adenine solution of 2 mM concentration was drop-coated onto the SERS substrates; the typical ring breathing and stretching modes of the adenine molecules are clearly observed with high reproducibility by the appearance of Raman bands at 733 cm−1. These bands can be attributed to adenine based on previous SERS measurements in the literature.36 The SERS intensities of the Ag@silicate hybrid substrates, obtained from the Raman scattering spectra, are summarized in Fig. 3a and b and Table 1. In the control experiment, no significant signal appears in the SERS spectrum in the presence of the PVA substrate. With increases in the Ag/copolymer/silicate/PVA nanohybrid weight ratio from 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 to 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1, the SERS intensity value is increased from 9.3 × 104 to 5.1 × 105. High-magnification TEM images and a corresponding schematic depict the inter-particle distances of the Ag/copolymer and Ag@silicate hybrids (Fig. 4 and Table 1). The TEM images of the Ag@silicate hybrid dispersions show that colloidal AgNPs are adsorbed on both sides of the silicate sheets. These images demonstrate the development of a novel and highly efficient method for fabricating nanohybrid SERS substrates by stabilizing AgNPs on both sides of ∼1 nm-thick silicate sheets, which provide inter-particle distances of less than 10 nm between the AgNPs. The Ag@silicate hybrid provides highly sensitive SERS substrates capable of detecting adenine molecules because of the exhibited 3D hot-junction behavior of the hybrid.
image file: c6ra06584h-f3.tif
Fig. 3 (a) Relationship between SERS intensity from adenine molecules adsorbed on Ag/copolymer/silicate/PVA hybrid substrates and substrate weight ratios, (b) integrated area of Raman intensity from 700 to 770 cm−1 of adenine for the band at 733 cm−1. (c) Raman intensity of adenine as a function of the different surface areas related to different microstructures and (d) integrated area of Raman intensity from 700 to 770 cm−1 of adenine for the band at 733 cm−1. SERS spectra of 1 mM adenine molecules collected with a (20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1)/PVA of Ag/copolymer/silicate/PVA hybrid substrates with (e) different electrospray times and (f) different immersion times. (A laser operating at λ = 532 nm was used as the excitation source at 20 mW power and 10 s exposure time.)

image file: c6ra06584h-f4.tif
Fig. 4 (a) TEM micrographs at high magnification of colloidal AgNPs adsorbed on both sides of silicate sheets (dark contrast: topside AgNPs; light contrast: bottom-side AgNPs) in Ag/copolymer/silicate hybrid solutions prepared with different precursor weight fractions: (1) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0, (2) 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, (3) 10[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]1, and (4) 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1. (b) Particle size, inter-particle distance, and composition of AgNPs dispersed on copolymer and copolymer/silicate hybrid. Inset: schematic corresponding to the TEM images, showing interactions of the Ag/copolymer and Ag/copolymer/silicate hybrid.

The relationship of SERS intensity for the adenine molecules with the Ag@silicate/PVA microstructures is discussed in Fig. 3c and d. The SERS signal of the core/shell Ag@silicate/PVA spherical nanohybrids is observed with greater intensity than that of the composite nanofibers because of the higher surface area of the core/shell nanohybrids. Fig. 3e and f show a series of SERS spectra from silver microstructured substrates fabricated with different electrospraying times and adenine adsorption times. The results indicate an optimized electrospraying time of 2 h and a maximum adenine adsorption time of 90 s. As the silver loading and adenine concentrations on the substrate are increased, both the density of the AgNPs per unit area and the number of adsorbed adenine molecules per unit area on the nanohybrid are increased; the spectra illustrate that the intensity of the Raman peaks increases correspondingly. This indicates the successful development of a novel and high-significance SERS substrate by employing core/shell nanohybrids. The hybrid SERS substrates allow the high-efficiency detection of adenine molecules because of the hybrid arrangement of silver and stabilizers in a unique and high-surface-area microstructure. The novel morphologies of the SERS bio-device were demonstrated and could be extended to applications in sensing microorganisms and larger biological cells, such as fungi and cancer cells.

Conclusions

Novel substrates based on core/shell Ag@silicate nanoplatelets and PVA nanosphere hybrids were fabricated by coaxial electrospraying and shown to exhibit excellent SERS enhancement. By controlling the weight ratios of Ag/copolymer/silicate, the AgNPs were synthesized with a size distribution within 20–40 nm. In addition, the microscale core/shell Ag@silicate SERS substrate had a high contact area for microorganisms. A flexible and freestanding SERS substrate with an enhancement intensity reaching 5.1 × 105 was prepared by electrospraying using the Ag/copolymer/silicate precursor with a 20[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]1 weight ratio. Such core/shell Ag@silicate nanosheet hybrids may find potential applications in anti-bacterial devices, catalysts, and gas sensors.

Acknowledgements

We acknowledge financial support from the Aim for the Top University Plan of the National Taiwan University of Science and Technology and the Ministry of Science and Technology (MOST 104-2221-E-011-155-) of Taiwan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, UV-vis spectra, TEM micrographs of the Ag@silicate nanohybrids, and a movie depicting the Taylor cone formed by hybrid liquid droplets in the electrospray process. See DOI: 10.1039/c6ra06584h
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