Tuning Metamaterials Nanostructure of Janus Gold ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Tuning Metamaterials Nanostructure of Janus Gold Nanoparticle Film for Surface-Enhanced Raman Scattering Yixuan Du, Wei Wei, Xiaowei Zhang, and Yunbo Li* School of Materials Science & Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: Large-area metamaterial nanostructures of a Janus gold nanoparticle (AuNP) film decorated with thiolterminated polymers have been fabricated and tuned at an oil/ water interface by a facile and innovative process. The results show that AuNPs had been arrayed with ultrasmall gaps between neighboring particles laminated with polymer which is called a Janus film. This film exhibits a sensitive property for surface-enhanced Raman scattering (SERS) for determination of both hydrophilic methylene blue and hydrophobic thiram. The structure-dependent SERS of the Janus AuNP film has also been confirmed by a finite-difference time domain (FDTD) method.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) has received intense attention for the detection of pesticides, viruses, pollutants, and even a single molecule due to ultrasensitivity, facility, and ultrafast interaction.1,2 The large SERS enhancements are related to the enormously intensified electromagnetic (EM) fields at “hotspots”, which are attributed to the strong coupling of localized surface plasmon resonances (LSPR) of noble metal nanoparticles (NPs).3−5 To gold nanoparticles (AuNPs), the fabrication of the sharp shape and large-area nanostructure is focused on the large Raman enhancement factor (EF).6−8 Therefore, many wet chemical methods were carried out to prepare novel shape and metamaterials nanostructure of AuNPs to obtain SERS hotspots.9,10 However, there are few studies on obtaining sufficient hotspots for SERS under the economizing AuNPs. Recently, Janus materials have attracted tremendous attention for various applications such as functional surfactants,11 display technologies,12 and catalytic agents13,14 due to their two distinct faces with anisotropic properties. Particularly, Janus membranes or nanofilms have been intensely studied according to their highly anisotropic shape and asymmetric properties.15 To SERS, Janus films would supply sufficient hotspots under the economizing AuNPs because the polymer on one side of AuNPs as the gap-directing molecule can produce ordered array nanostructures with reproducibility in a small gap. In addition, due to the anisotropic properties, the Janus film is also suitable for detection of the target molecules whether they are hydrophilic or hydrophobic or not. There are many methods to prepare the Janus film and the AuNP film at the interface. However, the majority of AuNPs in aqueous solution cannot be transferred to the interface by a traditional self-assembly method, which results in waste of AuNPs (Figure 1a). For example, the AuNP Janus film was © XXXX American Chemical Society

Figure 1. Schematic depiction of (a) the traditional self-assembly method for obtaining AuNPs film, (b) the interface method for obtaining the AuNP Janus film, (c) the improved self-assembly method for obtaining the AuNP film, and (d) the improved selfassembly method for obtaining the Janus AuNP film at the oil/water interface.

obtained by an interface method by wasting AuNPs in a previous study (Figure 1b).16 Recently, Girault and his collaborators got a metallic mirror-like film without superfluous AuNPs by slowly injecting the AuNPs (ethanol solution) at the water/oil interface (Figure 1c).17 Accordingly, AuNP colloids were injected at the water/oil interface which could economize AuNPs and tune the metamaterial nanostructure by thiolpolymers. In this work, AuNP colloids (dissolved in ethanol) were injected at the water/oil interface which was formed by thiolpolymers in water and oil. AuNPs had been arrayed with ultrasmall gaps between neighboring particles laminating with Received: January 20, 2018 Revised: March 21, 2018

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DOI: 10.1021/acs.jpcc.8b00676 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

FDTD Calculations. FDTD calculations were performed using FDTD simulation software (Lumerical Solutions, Inc.) in order to study the plasmonic properties of a Janus gold nanoparticle film via tuning the metamaterial’s nanostructure with polymer. The spatial FDTD domain containing unit cell was discretized on a rectangular grid set to 0.2 nm. The periodic boundary condition was applied to x- and y-axes (symmetric boundary for x-axis while antisymmetric for y-axis), and a perfectly matched layer (PML) boundary condition was applied to the z-axis. A plane wave source (633 nm wavelength) with polarization vector (E) and propagation vector (K) was incident on the AuNPs nanofilms. Dielectric dispersion of gold was performed using plasma and a Drude approximation of experimental dependencies from the literature.22 Spatial maps of the field intensity were extracted from the calculations. In this calculation, E0 was defined with 1 as the initial field amplitude.

polymer without a waste of AuNPs (Figure 1d). The combination with thiol-terminated polymers on both sides of the Janus AuNP film would decrease the gap distance between the AuNPs, which further enhanced the detection sensitivity. The structure-dependent SERS of the Janus AuNP film has also been confirmed by the finite-difference time domain (FDTD) method.

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EXPERIMENTAL SECTION Chemicals and Materials. Sodium citrate (Na3C6O7), tetrachloroauric acid hydrate (HAuCl4·4H2O), acetone, isoamyl acetate, heptane, and ethanol absolute were obtained from Sinopharm Chemical Reagent Co., Ltd. Methylene blue and thiram were purchased from Aladdin. Thiol-terminated polystyrene (PS-SH, Mn = 11000) and thiol-terminated poly(ethylene glycol) (PEG-SH, Mn = 6000) were purchased from Sigma-Aldrich. All solutions and dilutions were prepared using deionized water. Synthesis of AuNPs. Citrate-stabilized AuNPs were synthesized according to our previous work.18 A solution of HAuCl4 (0.243 mM) in deionized water (100 mL) was heated to the boiling temperature in a 250 mL three-necked roundbottomed flask under continuous stirring. A 2.0 mL portion of sodium citrate (38.8 mM) was injected, and the mixture was boiled for 30 min resulting in a wine-red solution. Fabrication of Janus AuNP Film. The Janus AuNP film had been fabricated using a self-assembly strategy at the oil/ water interface in a rectangular glass cell (4.0 cm × 3.0 cm × 3.0 cm). The oil was a mixture of heptane and isoamyl acetate (Vheptane/Visoamyl acetate = 3:2). Already-prepared solutions of citrate-stabilized AuNPs (20 mL, 0.008 wt % HAuCl4) were centrifuged; then the supernatant solution was removed, and finally the material was redissolved in ethanol (1.5 mL). PS-SH (0.1 mg/cm2) was dissolved into mixed oil (3 mL), and PEG-SH (0.1 mg/cm2) was dissolved into water (6 mL). Then, the oil/water interface was obtained. The AuNPs in ethanol were injected into the oil/water interface which leads AuNPs to arrange into a film by decreasing their surface charge.7 Janus AuNP films were achieved (Figure S1). These nanofilms showed gold-reflection and colorful appearance in transmission when they were transferred on quartz (Figure S1). The top face (facing to the oil phase) is attached with hydrophobic PS-SH, while the bottom face (facing to the water phase) is linked with hydrophilic PEG-SH.15 The prepared Janus films had been transferred onto substrates by horizontal lifting from an oil/water interface according to previous reports.19 Quartz (76 mm × 26 mm) and silicon wafers were immersed with acetone, ethanol, and deionized water in an ultrasonic bath for 15 min and dried before used.20 Characterizations. Samples for scanning electron microscopy (SEM) images were deposited on silicon wafers. The morphologies of the Janus films were studied using a JEOL JSM-6700F microscope at 5.0 kV. Transmission electron microscope (TEM) images were obtained by a JEOL 200CX electron microscope at 120.0 kV, which was equipped with the model GATAN782 CCD camera. The SERS substrates of sample were immobilized on silicon wafers. SERS spectra were obtained from a Renishaw INVIA Raman spectrometer with a laser (2 mW) at excitation wavelength of 633 nm.21 The spot size was approximately 1.5 μm2, and optical focus was adjusted manually to probe several different positions of substrates.

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RESULTS AND DISCUSSION Figure 2a−c shows the detailed procedure for fabricating a AuNPs film, a Janus PS-AuNP film, and a Janus PS-AuNP-PEG

Figure 2. Schematic representation of (a) AuNPs film, (b) Janus PSAuNPs film, and (c) Janus PS-AuNP-PEG film formatted at the liquid−liquid interface. SEM images of (d) AuNPs film, (e) Janus PSAuNPs film, and (f) Janus PS-AuNP-PEG film deposited on silicon wafers.

film as SERS substrates. The average diameter of AuNPs is 26 nm in the assembled film (Figure S2, the magnified TEM image). The AuNPs were arrayed into a film in a worm-like aggregate which exhibited a broader interparticle space distribution (Figure 2d). In contrast, the Janus AuNP film was obtained in an ordered array when thiol-terminated polymers were involved. As shown in Figure 2e,f, the AuNPs were assembled in a compact array. There are alkanethiolcoated metallic AuNPs reported on the ordered monolayer film.23 However, the short alkyl chain did not provide sufficient steric repulsion to tune the interparticle space. For a polymer, the interparticle space could be tuned due to the strong sizedependent van der Waals attractive forces between thiolpolymers and AuNPs.24 Therefore, the metamaterials nanostructure of the Janus gold nanoparticle film could be tuned with polymer. B


Article


Figure 3. Comparison of the SERS: (a) MB and (b) thiram spectra using blank, AuNP film, Janus PS-AuNP film, and Janus PS-AuNP-PEG film substrates (λex = 633 nm). SERS spectra of Janus PS-AuNP-PEG film at different concentrations of (c) MB and (d) thiram solutions (λex = 633 nm). Plot showing the analytic concentration vs peak integration: (e) 1625 cm−1 Raman band of MB and (f) 1370 cm−1 Raman band of thiram. (g) The linear correlation between Raman intensity of MB on the Janus PS-AuNP-PEG substrate at 1625 cm−1 and the concentration from 10−9 to 10−5 M. (h) The linear correlation between Raman intensity of thiram at 1370 cm−1 and the concentration from 10−9 to 10−5 M.

In a comparison with the AuNP films obtained at the interface without thiol-polymer, the void areas on the Janus AuNP films dwindled significantly.25,26 According to the previous reports, the interaction energy among AuNPs, φ, can be calculated by26,27 φ = φvdW (dc − c) + φelec(dc − c) + φster(dc − c , l)

The Raman signal with enhancement is several orders of magnitude times that without enhancement, due to the increase in cross section of the normal Raman scattering.29 Accordingly, the AuNP nanofilms obtained in this work have excellent SERS activity, and the Janus PS-AuNP-PEG film exhibits the strongest Raman intensity for MB and thiram. The characteristic peaks of MB at around 1447 cm−1 have been assigned to C−N−C skeletal bending, which exhibits obvious signals in the SERS spectra.30 The bands at around 1395 and 1625 cm−1 can be assigned to the asymmetric stretching of the C−C ring and in-plane ring deformation of C−H, respectively.31 Similarly, the strongest peak of thiram is at 1370 cm−1, which is caused by the C−N stretching mode and symmetric CH3 deformation mode. The peak of 1490 cm−1 is attributed to the antisymmetric n(CH3) stretch, and the C−N and S−S stretching vibrations occur at 1142 and 552 cm−1.32 In order to elucidate SERS spectra in more detail, the signal intensities at peaks 1625 cm−1 (MB) and 1370 cm−1 (thiram) were chosen to obtain EF which is used to evaluate the SERS quantitative activity of the AuNPs nanofilms. The EF of the AuNP films can be calculated by33

(1)

Here, φvdW(dc−c), φelec(dc−c), and φster(dc−c, l) are the van der Waals attraction potential, electrostatic repulsion potential, and steric elastic repulsive energy, respectively. The AuNPs’ surface charge density can be significantly tuned by the addition of thiol-polymer, and then φvdW(dc−c), φelec(dc−c), and φster(dc−c, l) reach a new balance.28 Without thiol-polymers (i.e., φster(dc−c, l) ≈ 0) in the system, the balance between φelec(dc−c) and φvdW(dc−c) is broken which resulted in plenty of gap areas in the AuNP film.26 It is noteworthy that the AuNPs aggregate is hard to form with an increase of PS-SH after a certain value. There is a large φster(dc−c, l) with too much PS-SH, and it is hard to get a balance in the system. Therefore, the addition of PEG-SH is a better choice to get a smaller gap between adjacent AuNPs when a certain amount of PS-SH exists. The thiol-polymers replace sodium citrate on the bottom of AuNP surfaces, and AuNPs packed in a compact array to get the larger φelec(dc−c) and φvdW(dc−c) to reach a new balance. As shown in Figure 3a,b, SERS spectra were collected at the wavelength of 633 nm for methylene blue (MB) and thiram on a bare silicon wafer (Blank), AuNP film, Janus PS-AuNP film, and Janus PS-AuNP-PEG film, respectively. The sharp signal enhancement of the Janus PS-AuNP-PEG film is 100 times that of the AuNP film and the Janus PS-AuNP film. Raman bands are barely seen without the enhancement of AuNP nanofilms.

EF =

ISERS/NSERS IRaman/NRaman

(2)

where ISERS is the peak integration of the enhanced Raman scattering, and IRaman is the integration of the corresponding normal Raman scattering on silicon without enhancement. NSERS and NRaman represent the number of MB (thiram) excited by a laser beam in SERS and normal Raman scattering, respectively. EF of the Janus PS-AuNP-PEG film for MB is 2.86 × 106 (see details in the Supporting Information), which is C


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Figure 4. TEM images of (a) AuNP film, (b) Janus PS-AuNP film, and (c) Janus PS-AuNP-PEG film (the regions in the red squares are the selected typical areas to use for FDTD simulations). Finite-difference time domain (FDTD)-simulated electrical field |E| distribution at 633 nm of (d) AuNP film, (e) Janus PS-AuNP film, and (f) Janus PS-AuNP-PEG film under the XY-view (the color bar scales versus |E|2 and the blue balls represent the AuNPs). FDTD-simulated electrical field |E| distribution images at 633 nm for (g) AuNP film, (h) Janus PS-AuNP film, and (i) Janus PS-AuNP-PEG film under the 3D view.

almost 100 times that of the Janus PS-AuNP film (3.13 × 104) and AuNP film (7.69 × 103). Similarly, EF of the Janus PSAuNP-PEG film for thiram is 4.06 × 106, which is also higher than that of the Janus PS-AuNP film (1.43 × 105) and the AuNP film (1.55 × 104). The intensities of average EM field enhancements depend greatly on both incident wavelength and diameter-spacing ratio (D/dgap).34 The incident wavelength of Raman laser light is a constant (633 nm) in this work. Besides, the decrement of interparticle gap among AuNPs results in a strong EM coupling and a large EM field.35 According to a previous report,34 the maximum EM enhancement, according to the Drude freeelectron response assumption, can be estimated in AuNP film by 4 −2 −3 −1 EFmax Drude = (3p /2)ωp ϵd ωt ωres

ϵd ≫ −(D/dgap)/em′

where D is the particle diameter, and dgap is the interparticle 3 edge-to-edge separation. Therefore, EFmax Drude ∝ (D/dgap) is obtained according to eqs 3−5. For the AuNP film without polymer, the aggregation of AuNPs leads to the large interparticle gap, whereas when it is capped with PEG-SH on the other side of the PS-AuNP film, dgap of PS-AuNP-PEG films dwindled which results in a significant enhancement of EFmax Drude. SERS spectra of PS-AuNP-PEG films with different concentrations of MB and thiram were measured as shown in Figure 3c,d. The SERS peak of MB is clearly shown when the concentration decreases to 10 nM. Similarly, the SERS peak of thiram is obviously identified even when the concentration of thiram decreases to 1 nM. Accordingly, SERS of MB and thiram adsorbed on Janus PS-AuNP-PEG film could be obtained at their detection limits. The detection limit for MB on the Janus PS-AuNP-PEG film is 10−9 M; thus, it can be used as a “fingerprint” to detect MB. By monitoring the intensity of the strong band of 1370 cm−1, 10−9 M thiram could be clearly detected on the Janus nanofilm. The detecting limit is lower than the maximal residue limit (MRL) of 7 ppm (∼2.91 × 10−5

(3)

where ωτ is the relaxation frequency of one AuNP (ωτ = 7.254 × 1012 Hz) and ωp is the plasma frequency of the AuNP film (ωp = 2.25 × 1015 Hz).35 The resonance frequency (ωres) and dielectric constant (ϵd) are defined as ωres ≫ ωp(D/dgapϵd)−1/2

(5)

(4) D



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CONCLUSIONS A Janus AuNP film is fabricated by a modified self-assembly strategy. This film exhibits highly active SERS and is suitable to detect hydrophilic and hydrophobic targeted molecules. The SERS EF of the Janus PS-AuNP-PEG film reaches 4 × 106, which is higher than that of the Janus PS-AuNP film and AuNP film. The metamaterial’s nanostructure of the Janus gold nanoparticle film has been tuned by the polymer. The excellent SERS property is due to the compact array which can be observed by SEM. The extraordinarily low limit detection of thiram was achieved on the basis of the PS-AuNP-PEG Janus film at 633 nm. In addition, the EFs evaluated by the experiments were confirmed by the FDTD-simulated method.

M) for fruit prescribed by the U.S. Environmental Protection Agency (EPA).32 The plot of SERS intensity versus −log (concentration) (Figure 3e,f) shows that the characteristic peak integration gradually decreases with the target concentration decreases. The plot of SERS integration does not increase linearly with the target molecule concentration. However, a linear dependence is found between the logarithmic concentrations of MB/thiram and the logarithmic integrations of the fingerprint peak (1625 cm−1/1370 cm−1) as shown in Figure 3g,h and the linear correlation equations as follows log I1625 = 8.78 + 0.45 log CMB

(6)

log I1370 = 8.08 + 0.20 log C thiram

(7)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00676. Optical images, TEM images, gap histograms, UV−vis spectra, and enhancement factor calculations (PDF)

where I is the peak integration of the SERS spectra, and C is the concentration of target molecule. This highlights the potential application of the Janus SERS substrate for the quantitative detection of target molecules. The excellent SERS activity of the Janus PS-AuNP-PEG film is mainly attributed to the small gap between two adjacent AuNPs which generates strong electromagnetic field coupling. To clarify that the plasmonic effect of these AuNPs nanofilms, a further study is conducted by simulation. The finite-difference time domain (FDTD) method (Lumerical Solutions, Inc.) was applied to estimate the electromagnetic field distribution of plasmonic nanostructures. In order to match the experimental system, the models of the structures were established by extraction from TEM images (Figure 4a−c). The dielectric layers (PS-SH layer and PEG-SH layer) were considered in the simulation models, and the corresponding parameters of polymers are obtained from E.F. Schubert’s research.36 The corresponding gap histograms with fitted Gaussian distributions are shown in Figure S3. When capped with thiol-polymers, the gaps of adjacent AuNPs in films dwindle and have a lower distribution. In the meantime, the gap distribution is also shown in the extinction spectra of three varying structures of AuNP nanofilms (Figure S4). The fwhm (the full width at half-maximum) of the SPR peak of the PS-AuNP-PEG Janus film is smaller than that of the PS-AuNP film as well as that of the AuNP film, suggesting that the former has the narrowest interparticle distance distributions among these AuNP nanofilms.37 Figure 4d−f shows the simulated local electric field distribution of AuNP films using the XY-view by the FDTD method. Two adjacent AuNPs could create the strong electric fields, especially when they are located in a narrow gap.6 The Janus PS-AuNP-PEG film has the strongest electric fields among these three nanofilms. Figure 4g−i shows images of FDTD-simulated electrical field |E| distribution at 633 nm in 3D view. The strongest EM field coupling is also obtained from the PS-AuNP-PEG Janus film (Figure 4i) in comparison with the other two AuNPs nanofilms (Figure 4g,h). This phenomenon is also attributed to the narrow and welldistributed gaps among AuNPs as mentioned above.24,38 In other words, the PS-AuNP-PEG Janus film exhibits a narrow EM field distribution, which results in the accuracy for SERS detection. Therefore, the simulation results are consistent with the trend in EF values for the three different structures of the AuNP nanofilms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone and fax: +86-21-66137105. ORCID

Yunbo Li: 0000-0002-8231-6580 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51203088). REFERENCES

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