Room-temperature mechanochemical synthesis of silver nanoparticle homojunction assemblies for the surface-enhanced Raman scattering substrate

Abstract

A room-temperature mechanochemical synthesis technique, featuring convenience and environmental friendliness, has been successfully developed to fabricate silver hierarchical architectures. The polyethylene glycol 400 was employed to assist the fabrication of silver nanoparticle assemblies with homojunctions. The result indicated that the formation of homojunction was sensitive to the feed ratio of the volume of polyethylene glycol 400 to the moles of AgNO₃. Based on the induction effect of polyethylene glycol molecules, the formation mechanism of Ag/Ag-homojunctions was proposed. The Raman scattering substrate based on these silver nanoparticle homojunction hierarchies possesses stronger Raman scattering responses for rhodamine 6G than isolated nanoparticles, which probably arise from the huge and extra enhancement of the electromagnetic field in homojunctions. These hierarchies have high stability endowing them with excellent surface-enhanced Raman scattering reproducibility. This work opens a valuable way to fabricate silver nanostructures with surface-enhanced Raman scattering activity.

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1. Introduction

As an important surface-enhanced Raman scattering (SERS) substrate, silver nanostructures, especially hierarchical architectures have motivated the worldwide enthusiasm of scientists in recent years. Many theoretical and experimental studies have shown that individual silver nanocrystals display distinct optical scattering spectra, and they can be assembled into coupled hierarchical structures with an improvement of the optical properties.^1–14 Hierarchical structures can be divided into three types: one-dimensional (1-D) nanochains or polymers fabricated with isolated silver nanoparticles (Ag-NPs) through oriented attachment growth or aggregated with isolated Ag NPs by van der Waals,^15–22 two-dimensional (2-D) hierarchical architectures formed by arrangement of subunits with certain gaps between neighboring subunits,^8,23–29 and three-dimensional (3-D) hierarchical objects formed through spatial interconnection of subunits with many joints.^{1,2,13,30–32} These structures were endowed with better excellent SERS capability than that of isolated nanoparticles, which may arise from additional amplification of electromagnetic (EM) field in the gap region or junction of coupled subunits. Based on the study of the quantitative theory and practice, the strong dependence of interparticle-coupling-induced enhancement factors or electromagnetic field factors on interparticle gaps were illustrated.^{1–3,8–16,21,24,26,27,31,33} The results indicated that the SERS action of hierarchy depends on the size, shape, and crystallinity of building units, especially, the microstructure of hierarchy including the arrangement of building blocks and the spacing between adjacent blocks. Hereinto, the “hot spots” were decisive for SERS enhancement.^{15–32,34–36} Firstly, they were commonly performed by the rough sites on particle surfaces such as straight edge, tip, corner, or rip, in where the continuity of structure was broken, generally exposing high-energy crystal planes (we can call them “structural mutation” region). Furthermore, the gaps and junctions in hierarchical nanostructures are even more remarkable in this respect.

Up to now, many groups have exploited some artificial techniques to fabricate the silver hierarchical architectures with nanoscale building units including nanorods/nanowires, nanocubes/nanopolyhedra, nanopyramids, nanosphere, etc.^15–32 In general, they are prepared through Langmuir–Blodgett technique,^5,29,32 nanolithography and nanoimprint,^5,8,10,25 anodic aluminum oxide (AAO) or porous aluminum oxide (PAO) template-induced growth,^5,13,26–28 or other artificial techniques.^4,5,24 However, these methods are mostly solution-based process, and commonly require a complicated manipulation with high time/energy consumption. The low-temperature solid-state chemical reaction technique (mechanochemical synthesis), which has the advantages of convenient operation, low cost, and less pollution, has provided a relatively simple and powerful method for synthesis of nanomaterials.^37–39 Herein, we employed it to explore the controllable synthesis of the silver hierarchical nanostructure. The silver nanoparticle homojunction assemblies (Ag/Ag-HJAs) consisted of rich silver/silver homojunctions were synthesized through this method under the assistant of polyethylene glycol 400 (PEG-400). These structures can effectively enhance the Raman signal of R6G (Rhodamine 6G) adsorbing on them.

2. Experimental section

All of the reagents are pure of analytical-grade and used without further purification. The synthesis was carried out by the room-temperature solid-state chemical synthesis method with silver nitrates (AgNO₃), sodium hypophosphite hydrates (NaH₂PO₂·H₂O), and PEG-400. In a typical procedure, a measured volume of PEG-400 (2 mL, the ratio of the volume of PEG-400 (mL) to the moles of AgNO₃ (mmol) was designated as VM_PEG–AgNO3, here, VM_PEG–AgNO3 = 1) was firstly poured into powdery AgNO₃ (2 mmol), and then ground together in an agate mortar for 10 min at room temperature. Subsequently, the NaH₂PO₂·H₂O (4 mmol) were added at 1 : 2 molar ratio between AgNO₃ and NaH₂PO₂·H₂O. The mixture was ground for about 30 min. Finally, the resulting mixture was centrifugation washed and dried naturally. And here the Ag hierarchical architectures were successfully synthesized and named as S_PEG-1.

The crystalline structures of the products were analyzed by a powder X-ray diffractometer (XRD, MXP18AHF, MAC) with Cu-Kα radiation (λ = 0.154056 nm). The morphologies and microstructure resolution of samples were carried out on a transmission electron microscopy (TEM JEM-2010F, at 200 kV), coupled with the field emission scanning electron microscopy (FESEM, JSM-6700F, at 15 kV).

The R6G was used as target molecule to evaluate the SERS enhancement effect of as-prepared Ag nanostructures. Typically, 0.1 mg of sample was added in 2 mL of 10⁻⁶ M R6G aqueous solution (prepared with Milli-Q water (>18 MΩ cm)) and stirred for 0.5 h at room temperature, and the dispersion was centrifuged 3 times with water to remove excess analyte. Next, the analyte-modified Ag-NPs were redispersed in 1 mL of Milli-Q water, and 10 μL of suspension was dropped and dried on silicon wafer. The reference sample was prepared by drop casting 10 μL of 10 mM R6G aqueous solution onto a silicon wafer and allowing the solvent to evaporate. The Raman scattering experiment was performed with a Renishaw invia plus Raman spectrometer and Leica confocal microscope. The excitation light source was a He–Ne laser with a wavelength of 632.8 nm and a power of 2 mW. A 20× objective lens was used to focus the beam onto the sample. The grating of spectrometer has 1800 lines per mm. The spectrum's scattering detection region ranges from 100 to 2000 cm⁻¹ with a 2 s exposure time and CCD detector. The scattered radiation was measured at an angle of 180° from the incident laser beam. All experiments were carried out at room temperature. The Raman band of a silicon wafer at 520 cm⁻¹ was used to calibrate the spectrometer.

3. Results and discussion

Fig. 1a and b and S1† illustrate typical TEM and SEM images of the as-prepared silver nanostructure in S_PEG-1. They visually reveal that the products hold three-dimensional (3-D) coral-shaped HJAs architecture. It was assembled by quasi-spherical and irregular polyhedral nanoparticles through silver/silver (Ag/Ag) homojunctions. The diameters of quasi-spheres ranged in 35–50 nm, while the polyhedron have a slightly bigger size. In fact, according to the growth habit of silver (FCC structure), it can be speculated that these quasi-spherical nanoparticles are also polyhedral objects, such as truncated polyhedrons. Because the particles have random shapes, the exposure of all silver crystal planes is possible, which agrees well with Prywer's theory of crystal growth, such as {111}, {100}, {110}, and {311} crystal planes.⁴⁰ Moreover, the high-energy crystal faces with high Miller index (such as {110}, and {311} crystal faces) are generally exposed on edges, corners, tips, and etc.⁴⁰ On the other hand, the Ag/Ag homojunction widths are around 25–45 nm, obtained from the statistical average of 40 junctions. Fig. 1c displays the HRTEM and electron diffraction pattern (EDP) resolutions of one pair of neighboring particles. It is clearly showed that the nanoparticles have high crystallinity. And the coupled nanoparticles with homojunction form a twinned crystalline structure through sharing [110] crystalline planes (Fig. 1c′). HRTEM resolution indicated that the lattice distances are estimated to be about of 0.2046 nm, which is in good agreement with the d-spacing of {200} plane of cubic Ag. Different from the Ag chains aggregated with isolated Ag NPs by van der Waals, the Ag/Ag-HJAs are very stable. In addition, some dislocations (indicated by white arrow in Fig. 1c) and stacking faults at joint were observed in their crystal lattice.

Fig. 1 Electron microscopy images of S_PEG-1 Ag nanostructures prepared with present method under the assistant of PEG-400 at VM_PEG–AgNO3 = 1: (a) TEM, (b) SEM images, (c and c′) HRTEM images of Ag/Ag homojunction displaying twinned crystalline structure.

Based on the present study, it can be predicted that the surfactant PEG-400 and the value of VM_PEG–AgNO3 play decisive roles in the formation of Ag/Ag-HJAs. The effect of PEG-400 was investigated by varying the value of VM_PEG–AgNO3. Firstly, in the absence of PEG-400, the product S_free presents the isolated quasi-spherical Ag-NPs with diameters in the range of five to several hundred nanometers, accompanied by some large agglomerates (Fig. 2A). Compared to the nanoparticles in HJAs, the nanoparticles in S_free have more structural mutants, such as defect, tip, or rip. HRTEM images of small nanoparticles reveal that they have multiply twinned particle (MTP) structure, such as common decahedral nanoparticle displaying five-fold twinned structure (Fig. 2a, a nanoparticle in marked area a in Fig. 2A with a diameter of about 27 nm). Decahedron can be considered as an assembly of five single-crystal tetrahedrons sharing a common {111} twin planes, and each decahedron is enclosed by ten low-energy {111} planes.⁴¹ Fig. 2a′ is a five-fold symmetry EDP of decahedron which is created by superimposing the [110] direction EDP of five tetrahedrons rotated with respect to each other. Due to the space mismatch in which a gap of 7.35° will leave when five tetrahedrons joined with {111} twin planes rotating on an axis (two {111} planes of a tetrahedron is 70.53°), the separation, defected arrange (indicated by white arrow in Fig. 2a), even groove (red arrow) between adjacent planes formed. Moreover, there is some smaller particle with the size of a few nanometers, and they also have the MTP structure (Fig. 2b, nanoparticles in marked area b in Fig. 2A). Furthermore, the surfaces of some particle are rough, including protuberance tips (Fig. 2c, a nanoparticle in marked area c in Fig. 2A) and rip (Fig. S2,† a nanoparticle in marked area Sa in Fig. 2A).

Fig. 2 Microstructure resolutions of S_free Ag nanostructures prepared according to the same procedure for preparing Ag/Ag-HJAs except for not using PEG-400: (A) TEM image of sample S_free, (a and a′) HRTEM and corresponding Fast Fourier Transform (FFT) resolution of a decahedron, (b) HRTEM resolution of smaller nanoparticles, (c) HRTEM images of surface a nanoparticle indicating the formation of tips.

Secondly, when the value of VM_PEG–AgNO3 is less than 1, for example VM_PEG–AgNO3 = 0.5 (S_PEG-0.5), the mainstream nanostructures are the short junction nanochains, including dimer, trimer, and etc. (Fig. 3a). Finally, if the VM_PEG–AgNO3 is increased to 2 (S_PEG-2, Fig. 3b), the reaction generates irregular Ag-NPs with a broad size distribution, coexisted with aggregate of nanoparticle, small quasi-spherical nanoparticles with smaller size (marked by dot circles), and some Ag/Ag homojunction assemblies (indicated by red arrows). Fig. S3a† is a HRTEM resolution with a stochastic zone axis for a homojunction joint S3a (marked with red dot square in Fig. 3b). Moreover, HRTEM resolution of a small nanoparticle marked with red dot circle S3b in Fig. 3b indicates that it also holds MTP structure (Fig. S3b†).

Fig. 3 TEM images of Ag nanostructures prepared with different VM_PEG–AgNO3: (a) S_PEG-0.5 obtained with VM_PEG–AgNO3 = 0.5, (b) S_PEG-2 fabricated with VM_PEG–AgNO3 = 2.

The above results suggest that the morphologies of Ag-NPs are sensitive to the value of VM_PEG–AgNO3. This probably associates with the template induction of chainlike PEG-400 molecule under appropriate VM_PEG–AgNO3 (the skeleton symbol of PEG-400 is illustrated in Fig. S4†). Now, the possible mechanism for the formation of Ag/Ag-HJAs is proposed and shown with schematic diagram in Fig. 4. Here, it is worth noting that the oxygen atoms in PEG can provide the isolated electron pairs to coordinate with silver ions.⁴² Firstly, at the beginning stage, if the amount of PEG-400 is appropriate, well-proportioned silver ions will be bonded to the linear chain of the PEG-400 molecule while others are in the unbound state. Subsequently, Ag⁺ is reduced to metallic silver by low-valent phophorus atom of NaH₂PO₂·H₂O. Then, the silver monomers aligning along PEG molecular chain were formed by the nucleation and growth. Meanwhile, adjacent nanoparticles join each other along the PEG molecule to form homojunction. On the other hand, calculation based on the ‘ideal surface’ model shows that the surface free energy of the FCC crystal is on the order of γ{110} > γ{100} > γ{111}.⁴¹ And the growth kinetic energy barrier of the crystal facet is inversely proportional to the surface energy. This indicates that the bonding ability and chemical reactivity of the (110) facet are greater than the (100) and (111) facets. Therefore, the fresh monomers will assemble along PEG-400 molecule by sharing (110) crystalline plane to form Ag/Ag homojunction structures. When the PEG-400 is excessive, reaction environment becomes a semi-liquid state. The diffusion of silver atoms quickened, resulting in the formation of some thermodynamically stable polyhedral particles, such as decahedron. This shows that the PEG-400 molecules play a key role to induce the formation of Ag/Ag-HJAs. The experiments for the influence of different molecular weight of PEG (PEG-200, 600, etc.) on final products were carried out with VM_PEG–AgNO3 = 1, respectively. TEM data indicated that the PEG-200 and PEG-600 also can play a role in inducing the formation of junction structure (Fig. S5†). The difference between them is that the one-dimensional growth of the junction structure with PEG-200 or PEG-600 is worse than that with PEG-400. The HJAs structure induced by PEG-200 or PEG-600 displayed more agglomerate-like junction structure than that with PEG-400. These differences may be due to the length of the PEG molecular chain and the entanglement of the molecular chains. On the other hand, in the absence of surfactant, the reaction rate is fast which will facilitate some thermodynamically stable nanoparticles, such as decahedron. However, due to the lack of template induction of surfactant, the fresh nanoparticles tend to agglomerate to reduce surface free energy.

Fig. 4 Diagram of proposed formation mechanism for Ag/Ag-HJAs.

The UV-vis spectra of S_PEG-1 and S_free suspended in water are compared in Fig. 5. It is clearly shown that the shape of Ag-NPs and their assembly patterns have great effect on the absorption location. The S_free exhibits broad absorbance band at about 414 nm containing some scraggly peaks, which is attributed to the surface plasmon resonance (SPR) band of Ag-NPs.^11,12,20 The broadening and multi-peaks of the plasmon absorbance bands can be contributed to the broad size distribution of Ag-NPs in S_free. For the S_PEG-1, it shows two broad absorbance bands, one is near 404 nm and another is located at about 578 nm. According to the point-dipole model,⁴³ the nanochain of Ag-NPs can present two SPR modes: a longitudinal and a transverse plasmon resonance along and perpendicular to the chain-axis, respectively. It is universally accepted that the transverse modes are located around the SPR position of a single-particle dipole mode, and the longitudinal modes arised from the SPR coupling band of linear-aggregated Ag-NPs and their positions depend on the number of aggregated particles.^20,21 Here, it can be understood that the transverse plasmon resonance of Ag/Ag-HJAs is superimposed or submerged in the ensemble of absorption peaks near 404 nm, and the broad absorbance bands around 578 nm are indexed to longitudinal resonance of HJAs. The X-ray photoelectron spectroscopy (XPS) measurement focusing on Ag 3d was carried out for S_PEG-1 and reported in Fig. 5b. The peak at 374.18 eV corresponds to Ag 3d_3/2 and the peak at 368.13 eV corresponds to Ag 3d_5/2, which are characteristic peaks of metallic Ag,¹⁶ indicating that the present synthesis strategy is effective for preparing the pure Ag phase.

Fig. 5 (a) UV-vis absorbance spectra of S_PEG-1 and S_free, (b) XPS patterns of S_PEG-1.

Fig. 6a shows XRD patterns of as-prepared samples S_PEG-1 and S_free. The diffraction peaks at 38.1°, 44.3°, 64.4°, and 77.5° (2θ) can be indexed to (111), (200), (220), and (311) planes of the face-centered cubic (FCC) structure of Ag (JCPDS card, no. 65-2871), respectively. No obvious impurity was observed by XRD, indicating that the sample is pure Ag. Moreover, the sharp diffraction peaks reveal that the as-prepared sample has a good crystallinity. After refinement, the cell constants were calculated from (111) peak by reference to the reported data (a = b = c = 0.23591 nm), revealing that the lattice contractions of Δa = 0.33% and 0.36% occurred to S_PEG-1 and S_free, respectively.

Fig. 6 (a) XRD patterns of S_PEG-1 and S_free; the bottom is the standard patterns of Ag (JCPDS file: 65-2871). (b) Comparison of SERS spectra of R6G (10⁻⁶ M) adsorbed on S_PEG-1, S_PEG-0.5, S_PEG-2, and S_free substrates. (c) Repeatability tests of SERS for the same batch of S_PEG-1 (d_r: relative deviation). (d and e) TEM image of S_PEG-1 stored in the bottle naturally for two years and HRTEM image of one Ag/Ag homojunction in it.

Fig. 6b shows the SERS spectra of 10⁻⁶ M R6G on S_PEG-1, S_PEG-0.5, S_PEG-2, and S_free substrates. Strong SERS peaks of R6G located at 1650, 1575, 1511, 1363, 1313, and 1180 cm⁻¹, respectively assigned to xanthene ring stretching (XRS) + in plane C–H bending (ip C–H bend) (1650 cm⁻¹), XRS + ip N–H bend (1575 cm⁻¹), XRS + ip C–N str + C–H bend + N–H bend (1511 cm⁻¹), XRS + ip C–H bend (1363 cm⁻¹), in xanthene ring breach (XRB) N–H bend + CH₂ wag (1313 cm⁻¹), and ip xanthene ring deformation (XRD) + C–H bend + N–H bend (1180 cm⁻¹) based on the DFT calculations in literature.⁴⁴

The surface enhancement factors (EFs) of the S_free and S_PEG-1 were calculated from (I_sig/C_sig)/(I_ref/C_ref), where I_sig and I_ref represent the intensities of the 1511 cm⁻¹ band for the R6G adsorbed on the Ag nanostructure-modified substrate and the silicon substrate, respectively, whereas C_sig and C_ref represent the corresponding concentrations of R6G on corresponding substrates. It is worth notice that these EFs are response of an ensemble-averaged measurement.^2,8,31 The EFs of the S_PEG-1, S_PEG-0.5, S_PEG-2, and S_free were roughly estimated by comparing the peak intensity at 1511 cm⁻¹ to 2.10 × 10⁷, 1.82 × 10⁷, 1.46 × 10⁷, and 1.68 × 10⁵, respectively. More succinctly, the EFs of the S_PEG-1, S_PEG-0.5, and S_PEG-2 substrate are about 125, 108, and 87 times stronger than that on the S_free substrate, respectively, which can be perhaps attributed to the junction structure on the Ag/Ag-HJAs. Finally, although a huge SERS signal was clearly observed for Ag/Ag-HJAs, the enhancement factor of one Ag/Ag-HJA unit cannot be estimated here because the number of probe molecules per unit or the number of units in the sampled volume cannot be measured properly and precisely. The differences in the enhancement effects of several HJAs nanostructures may be mainly derived from the different density of the joint structures.

To evaluate the stability of Ag/Ag-HJAs, the repeatability tests of SERS for the same batch of S_PEG-1 (sealed in the bottle naturally, but not vacuum) were carried out following above-mentioned experimental parameters at two month intervals within two years. The SERS enhancements were evaluated with the Raman intensities of the 1511 cm⁻¹ band of R6G. The results expounded that the intensities of Raman signals don't present significant reduction. And the relative deviation (d_r) is less than ±3% in twelve parallel tests (Fig. 6c). To further study the stability of Ag/Ag-HJAs prepared by room-temperature solid-state reaction, we characterized them with TEM at the end of twelfth experiment. The results indicated that the morphologies of them are almost identical to that of the as-prepared sample (Fig. 6d, refers to Fig. 1a and S1†). HRTEM resolution showed that the sample maintained perfect Ag/Ag homojunction structure in two years (Fig. 6e). It is beyond doubt that the samples obtained with this method are highly stable in stock packing condition in where is no air flow. However, when the sample was deliberately exposed in the atmosphere for 1.5 years, the X-ray energy dispersive spectroscopy (EDS) characterization showed that some silver sulfide (Ag₂S) formed on the surface of the sample (Fig. S6†), arising from that the silver is one sulphophile element.

Two mechanisms, now, electromagnetic (EM) and chemical (CHEM) effects, are currently accepted for SERS enhancement.^{1,3,6,36,45,46} EM is the enhancement of the electric field around the molecule due to the interaction of light with the surface plasmon of the nanoparticles. CHEM occurs when a charge-transfer complex is formed between the molecule and the nanoparticle. In this present work, the SERS enhancements of samples were believed to be due to a combination of EM and CHEM effects. Firstly, as above-mentioned analysis, whether in the S_PEG or in the S_free, there are some nanoparticles which expose high-energy crystal faces with high Miller index (such as {110}, and {311} crystal faces) on edges, corners, tips, and etc.⁴⁰ They can act as “hot spots” for the surface plasmon resonance (SPR). In addition, some narrow gaps (<2 nm) between closely adjacent Ag-NPs were respectively constructed on nanostructure substrate, also forming the so-called hot spot region.^15,17,18,21 Of specially note here, the Ag/Ag-HJAs have rich homojunctions which additionally offered effective “hot spots” for EM enhancement. Yang et al. have simulated the SERS behaviors of Ag/Ag homojunction by discrete dipole approximation (DDA) and three-dimensional finite-difference time domain (3D-FDTD) methods.¹⁶ Theoretical calculation results indicated that the Ag/Ag homojunctions had different SPR property from the isolated silver nanoparticles, and they exhibited higher SERS enhancement than the latter because of the near-field coupling effect. Moreover, the results showed that the SERS enhancement of the nanochain mainly depends on the numbers of Ag nanoparticles in it. In the present work, the stable homojunction with twinned crystalline structure provided intense and persistent near-field coupling effect, which will generate the strong localized surface plasmon resonance, thereby inducing giant surface electric field enhancement for SERS. On the other hand, the nanochain in the S_PEG-1, S_PEG-0.5, and S_PEG-2 HJAs contain different quantities of nanoparticles, including two, three, four, and more. So, the electric field enhancements of above three sorts of HJAs are different.

Secondly, the homojunctions and above-mentioned high-energy sites can coordinate to nitrogen atom in the analyte molecule R6G to form charge-transfer complex (Ag–N bond).⁴⁷ The Ag–N stretching vibrations at 234 cm⁻¹ confirmed the R6G molecule is coordinated to the Ag surface through the nonbonding electrons of the nitrogen atom (Fig. S7†). And then the fluorescence energy of R6G can transfer from the molecules to the metal surface leading to the reduction of the fluorescence intensity and amplification of the Raman enhancement factors.⁴⁸ To sum up, the Ag/Ag-HJAs can be employed as more excellent SERS substrate because they have richer “hot spots” than isolated nanoparticles (Fig. S8†).

4. Conclusions

In summary, the Ag/Ag-HJAs with rich “hot spots” have been successfully synthesized through surfactant-assistant mechanochemical technique at room temperature. The quantity of Ag/Ag homojunction can be controlled by adjusting the VM_PEG–AgNO3 in a narrow range. An appropriate VM_PEG–AgNO3 will guide the formation of Ag/Ag homojunction, while the lower VM_PEG–AgNO3 will be apt to produce the isolated nanoparticle coexisted with agglomerate. The evaluation of SERS indicated that the Ag/Ag-HJAs are more effective to enhance the Raman signal of R6G than isolated silver nanoparticle. Note, the Ag/Ag-HJAs are highly stable, and can give giant and steady SERS signals during two years. The SERS enhancement mechanism was discussed from the EM and CHEM effects of “hot spots” which were acted by the Ag/Ag homojunction and structural mutation region of silver nanoparticles.

Acknowledgements

This work was funded by the National Nature Science Foundation of China (No. 21361024, 21271151, 51471101, 51472154, 51272154), and China Postdoctoral Science Foundation (No. 2014T70955).

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Footnote

† Electronic supplementary information (ESI) available: The TEM of Ag/Ag-HJAs in S_PEG-1, HRTEM images of marked areas in TEM images of S_free and S_PEG-2, the skeleton symbol of PEG-400, and EDS spectrum of the sample which was deliberately exposed in the atmosphere for 1.5 years. See DOI: 10.1039/c6ra14603a

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