Composite Structure Based on Gold-Nanoparticle Layer and HMM for Surface-Enhanced Raman Spectroscopy Analysis

nanomaterials

Article Composite Structure Based on Gold-Nanoparticle Layer and HMM for Surface-Enhanced Raman Spectroscopy Analysis

Zirui Wang 1, Yanyan Huo 1,2, Tingyin Ning 1,2, Runcheng Liu 1, Zhipeng Zha 1, Muhammad Shaﬁ 1, Can Li 1, Shuanglu Li 1, Kunyu Xing 1, Ran Zhang 1, Shicai Xu 3, Zhen Li 1,* and Shouzhen Jiang 1,2,*

1 Collaborative Innovation Center of Light Manipulations and Applications in Universities of Shandong School of Physics and Electronics, Shandong Normal University, Jinan 250014, China; [email protected] (Z.W.); [email protected] (Y.H.); [email protected] (T.N.); [email protected] (R.L.); [email protected] (Z.Z.); [email protected] (M.S.); [email protected] (C.L.); [email protected] (S.L.); [email protected] (K.X.); [email protected] (R.Z.) 2 Shandong Key Laboratory of Medical Physics and Image Processing & Shandong Provincial Engineering and Technical Center of Light Manipulations, Jinan 250014, China 3 Shandong Key Laboratory of Biophysics, Institute of Biophysics, College of Physics and Electronic Information, Dezhou University, Dezhou 253023, China; [email protected] * Correspondence: [email protected] (Z.L.); [email protected] (S.J.)

Abstract: Hyperbolic metamaterials (HMMs), supporting surface plasmon polaritons (SPPs), and highly confined bulk plasmon polaritons (BPPs) possess promising potential for application as surface- enhanced Raman scattering (SERS) substrates. In the present study, a composite SERS substrate based on a multilayer HMM and gold-nanoparticle (Au-NP) layer was fabricated. A strong electromagnetic field was generated at the nanogaps of the Au NPs under the coupling between localized surface Citation: Wang, Z.; Huo, Y.; Ning, T.; plasmon resonance (LSPR) and a BPP. Additionally, a simulation of the composite structure was Liu, R.; Zha, Z.; Shafi, M.; Li, C.; Li, S.; assessed using COMSOL; the results complied with those achieved through experiments: the SERS Xing, K.; Zhang, R.; et al. Composite Structure Based on Gold-Nanoparticle performance was enhanced, while the enhancing rate was downregulated, with the extension of Layer and HMM for Surface-Enhanced the HMM periods. Furthermore, this structure exhibited high detection performance. During the Raman Spectroscopy Analysis. experiments, rhodamine 6G (R6G) and malachite green (MG) acted as the probe molecules, and the −10 −8 Nanomaterials 2021, 11, 587. https:// limits of detection of the SERS substrate reached 10 and 10 M for R6G and MG, respectively. doi.org/10.3390/nano11030587 Moreover, the composite structure demonstrated prominent reproducibility and stability. The mentioned promising results reveal that the composite structure could have extensive applications, Academic Editor: such as in biosensors and food safety inspection. Maurizio Muniz-Miranda Keywords: hyperbolic metamaterials (HMMs); gold-nanoparticle (Au-NP) layer; SERS Received: 8 February 2021 Accepted: 23 February 2021 Published: 26 February 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Surface-enhanced Raman spectroscopy (SERS), as a molecular detection technology published maps and institutional affil- exhibiting high sensitivity, has been broadly employed for environmental monitoring, iations. chemical monitoring, and biosensors [1–5]. The electromagnetic mechanism (EM) is of critical significance for SERS enhancement mechanisms. It is implemented using the surface plasmon of the metals to improve the electromagnetic field, elevating the SERS signal to 108 or more [6–12]. The localized surface plasmon resonance (LSPR) is considered a typical EM. The excitation of the LSPR Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of metallic surfaces (such as a noble metal nanoparticle layer) results in electromagnetic This article is an open access article field enhancement. This field enhancement has a significant effect on the probe molecules distributed under the terms and adsorbed on the metal surface, enhancing their Raman signal effectively [13]. On this basis, conditions of the Creative Commons the researchers fabricated several different substrates [4,14]. Attribution (CC BY) license (https:// A surface plasmon polariton (SPP) indicates a surface electromagnetic wave prop- creativecommons.org/licenses/by/ agating along the planar interface between a metal and a dielectric medium. Since its 4.0/). electromagnetic field is confined close to the metal/dielectric interface, the electromagnetic

Nanomaterials 2021, 11, 587. https://doi.org/10.3390/nano11030587 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 587 2 of 11

field is improved at the interface [15]. Yinzhou Chu and Fei Zhou proposed a substrate based on nanoparticles and a metal/dielectric bilayer, demonstrating that the coupling between LSPR and a SPP can enhance the Raman signal [16,17]. Nevertheless, the enhancement for the signal is relatively weak because only a metal/dielectric bilayer supports the SPP. Adjacent SPPs are effectively coupled when the metal/dielectric bilayer is replaced with a multilayer metal/dielectric bilayer structure, leading to the generation of a bulk plasmon polariton (BPP) within the multilayer structure. A BPP is a wave capable of propagating in the bulk of the material and maintaining the properties of a propagating wave [18]. A BPP is generated by the interaction of SPPs, enabling it to be more effectively coupled with LSPR than an SPP. Afterwards, Zhen Li et al. developed a substrate with an Al nanoparticle−film system, contributing to the generation of BPPs in the ultraviolet region [19]. However, the material properties are subject to several limitations, and the gaps between the dispersed metal particles in this structure are extremely large. Conse- quently, electromagnetic fields are not exposed between the metal particles, reducing the effectiveness of the coupling between the LSPR and the BPP. Thus, a novel Raman substrate should be proposed to more effectively exploit the coupling between LSPR and a BPP. In the present study, a composite structure combining a gold-nanoparticle layer with a hyperbolic metamaterial (HMM) was formed. An HMM refers to an artificial optical metamaterial composed of a certain thickness of metal films and medium films periodically stacked [20–22]. It supports SPPs and BPPs and also exhibits unique dispersion characteristics [18,23,24]. In such a composite structure, an HMM consisting of a multistack of Au/Al2O3 bilayers was deposited on a PET (polyethylene terephthalate) film, and a Au-NP layer was formed and then deposited on the topmost part of the HMM. Given the momentum mismatch between an SPP and incident light [25,26], the Au-NP layer acted as a 2D grating, which could help to match the momentum conditions and successfully excite the SPP mode and the BPP mode at optical frequencies [20,27,28]. With the increase in the number of Au/Al2O3 bilayers, the BPP could facilitate the LSPR excitation of the Au-NP layer [29]; a great interaction (coupling) was identified between the BPP and LSPR, and the coupling would generate a strong local electromagnetic field (hot spots) at the nanogaps of the Au NPs, so as to greatly enhance the Raman signal for analysis. In the experiments, rhodamine 6G (R6G) acted as the probe molecule. The intensity of the Raman signal was suggested to increase, whereas the rate of increase was downregulated as the number of Au/Al2O3 bilayers increased. Next, different concentrations of R6G and malachite green (MG) were used to examine the performance of the composite structure, with the R6G and the MG concentrations ranging from 10−6 to 10−10 M and 10−4 to 10−8 M, respectively, as obtained in six periods of the composite structure under 532 nm incident light. According to the results of COMSOL simulation and SERS experiments, the composite structure exhibited excellent sensitivity and practicability.

2. Materials and Methods 2.1. Fabrication of the Au Nanoparticles The fabrication of the composite structure substrate was illustrated in Figure1. The Au nanoparticles were prepared with the method described in [30]. A 0.2 wt% (5 mL) chloroau- ric acid solution and 75 mL of deionized water were mixed and then added into a ﬂask. The mixture was heated at 60 ◦C for 5 min under magnetic stirring. Subsequently, a reducing mixture of 0.0010 g of tannic acid, 0.040 g of trisodium citrate and 0.0007 g of potassium carbonate dispersed in deionized water (20 mL) was added into the ﬂask. After the reaction was allowed to occur for one hour, the obtained solution was centrifugated at 1000 rpm for 15 min. Afterwards, the centrifugated deposit was dispersed in deionized water again for use.

2.2. Fabrication for the Composite Structure Substrate First, a PET substrate with the dimensions of 10 × 10 cm2 was ultrasonically washed with acetone, alcohol and deionized water in a consecutive manner to annihilate the surface Nanomaterials 2021, 11, 587 3 of 11 Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 12

impurities.2.2. Fabrication Subsequently, for the Composite vacuum Structure thermal Substrate evaporation was employed to deposit an 8 nm Au filmFirst, and a PET a 3substrate nm Al with film the on dimensions the PET film of 10 at × the10 cm rates2 was ofultrasonically 0.1 and 0.5 washed Å/s, respectively, underwith acetone, a constant alcohol pressure and deionized of 7 × water10−5 inPa. a consecutive Additionally, manner the to samples annihilate were the oxidizedsur- in pure face impurities. Subsequently, vacuum thermal evaporation was employed to deposit an oxygen8 nm Au forfilm 10and min a 3 nm to completelyAl film on the oxidizePET film at the the Al rate films of 0.1 and and form 0.5 Å a /s, high-quality respectively, oxide layer. Theunder mentioned a constant pressure steps were of 7 × repeated, 10−5 Pa. Additionally and the multilayer, the samples structure were oxidized in the in differentpure periods couldoxygen be for successfully10 min to completel fabricated.y oxidize Next,the Al thefilm preparedand form a high gold-quality nanoparticles, oxide layer. with hexane andThe mentioned ethyl alcohol steps atwere a volumetricrepeated, and ratiothe multilayer of 2:1:1, structure were dropped in the different onto periods the substrate and thencould air-dried.be successfully Finally, fabricated. near-uniformly Next, the prepared distributed gold nanoparticles gold nanoparticles, with hexane wereand fabricated successfully.ethyl alcohol at a volumetric ratio of 2:1:1, were dropped onto the substrate and then air- dried. Finally, near-uniformly distributed gold nanoparticles were fabricated successfully. 2.3. Characterization and SERS Experiments 2.3. Characterization and SERS Experiments A scanning electron electron microscope microscope (SEM, (SEM, Zeiss Zeiss Gemini Gemini Ultra Ultra-55,-55, Oberkochen, Oberkochen, Ger- Germany) andmany atomic) and atomic force force microscopy microscopy (AFM, (AFM, Park Park XE100 XE100,, France France)) were were employed employed to test the to test the mor- phologymorphology of theof the multilayer multilayer structure structure andand the surface surface topographies topographies of the of thegold gold nano- nanoparticles. Theparticles. Raman The spectroscopy Raman spectroscopy was performed was performed using using a Raman a Raman spectrometer spectrometer ( (HoribaHoriba HR Evolu- tion,HR Evolution Kyoto, Japan)., Kyoto, Furthermore,Japan). Furthermore, the Raman the Raman spectra spectra were were collected collected under under the the conditions of aconditions 532 nmlaser; of a 532 the nm laser laser; power the laser was power 0.48 was mW, 0.48 the mW, gratings the gratings had 600had grooves600 grooves per nanometer, per nanometer, a ×50 objective lens was used, and a 1 μm laser spot was used; the acqui- a ×50 objective lens was used, and a 1 µm laser spot was used; the acquisition time was 4 s. sition time was 4 s.

FigureFigure 1. 1. TheThe fabrication fabrication of the of composite the composite structure structure substrate. substrate.

3.3. Results and and Discussion Discussion After the the gold gold nanoparticles nanoparticles were weredropped dropped and dried and naturally, dried naturally, an SEM image an SEMof the image of the Au NPs with a near-uniform distribution was taken and is illustrated in Figure 2a. It re- Auveals NPs that with a particle a near-uniform layer was deposited distribution on the was top taken of the and substrate, is illustrated and the insizeFigure of the2 a . It reveals thatgaps a was particle nearly layer 4 nm. was The depositedparticle size on statistical the top distribution of the substrate, is presented and thein Figure size of2b, the gaps was nearlywhere the 4 nm. Au NPs The exhibited particle a size diameter statistical of about distribution 16 nm, conforming is presented to a Gaussian in Figure distri-2b , where the Aubution. NPs For exhibited instance, a a cross diameter-sectional of image about of 16five nm, Au/Al conforming2O3 bilayers is to exhibited a Gaussian in Fig- distribution. ure 2c. The actual thickness reached 56.2 nm, close to the expected thickness of 55 nm. For instance, a cross-sectional image of five Au/Al2O3 bilayers is exhibited in Figure2c. TheBesides, actual an 8 thickness nm Au film reached (light area), 56.2 a nm, 3 nm close Al2O to3 film the (dark expected area) thicknessand the boundary of 55 nm. Besides, could be observed. In the mentioned structure, the topmost Al2O3 film in the composite an 8 nm Au film (light area), a 3 nm Al O film (dark area) and the boundary could be structure exhibited a root-mean-square (RMS)2 roughness3 of 0.25 nm (Figure 2d). As re- observed. In the mentioned structure, the topmost Al O film in the composite structure vealed from the mentioned result, the topmost Al2O3 film was smooth2 3 and could act as an exhibited a root-mean-square (RMS) roughness of 0.25 nm (Figure2d). As revealed from the mentioned result, the topmost Al2O3 film was smooth and could act as an effective platform for making Au NPs distribute uniformly on the top of the substrate. Meanwhile,

the obtained smooth surface prevented the losses of the SPP from being scattered and exerted a more signiﬁcant effect in supporting plasmonic resonance coupling [19,31]. Moreover, it was simulated with COMSOL to verify that the composite structure exhibited a great coupling. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 12

effective platform for making Au NPs distribute uniformly on the top of the substrate. Nanomaterials 2021, 11, 587 Meanwhile, the obtained smooth surface prevented the losses of the SPP from being scat- 4 of 11 tered and exerted a more significant effect in supporting plasmonic resonance coupling [19,31]. Moreover, it was simulated with COMSOL to verify that the composite structure exhibited a great coupling.

Figure 2. (a). SEM image of the Au-nanoparticle (NP) layer after drying naturally. (b) Table 2. O3- bilayerFigure structure. 2. (a). SEMThe inset image shows of a the local Au-nanoparticle-magnification image. (NP) (d) The layer root after-mean drying-square (RMS) naturally. (b) The distri- roughness of the Al film can be seen from the white line drawn in the inset. bution of the diameters of the Au NPs. (c) Cross-sectional SEM image of five-Au/Al2O3-bilayer structure. The inset shows a local-magnification image. (d) The root-mean-square (RMS) roughness In this study, the electric field distributions of different structures were simulated usingof the the Al COMSOL film can be simulation seen from software. the white All linethe simulations drawn in the were inset. under 532 nm incident wave polarities along the y-direction. The electric field distribution of the Au-NP layer is illustratedIn this in Figure study, 3a. the The electricdiameter fieldof the distributionsAu NP was 16 nm, of and different the gap structuresreached 4 nm. were simulated Figureusing 3b the presents COMSOL the HMM simulation structure software.in six periods All with the an simulations 8 nm Au film wereand 3 nm under Al2O 5323 nm incident wavefilm. According polarities to the along two theimages, y-direction. both the Au The-NP electric layer and field HMM distribution exhibited extremely of the Au-NP layer is weakillustrated hot-spot in intensity. Figure3 Asa. indicated The diameter by Figure of the3c, when Au NP the wasAu-NP 16 layer nm, was and combined the gap reached 4 nm. with the HMM, the hot spots at the gaps of the Au-NP layer exhibited strong intensity, whileFigure a 3greatb presents coupling the was HMM achieved structure inside the in HMM. six periods The results with of an the 8 nmcomparison Au film are and 3 nm Al 2O3 providedfilm. According in Figure 3d to. theAdditionally two images,, the substrate both the model Au-NP is presented layer andin Figure HMM 3e. In exhibited this extremely weakstudy, a hot-spot 3 nm Al2O intensity.3 film as the As interval indicated layer bywas Figure selected3c, since when it isthe convenient Au-NP to layer fabri- was combined withcate. Additionally, the HMM, the the Al hot2O3 film spots was at applied the gaps as the of interval the Au-NP layer as layerit can separate exhibited adja- strong intensity, whilecent Au a films great and coupling prevent the was interaction achieved of Au inside films the [32]. HMM. An SPP The was resultsgenerated of on the the comparison are providedAu film surface in Figure when 3thed. HMM Additionally, was excited the. The substrate SPPs of the model adjacent is presented films should in also Figure 3e. In this be well coupled to generate a strong BPP. study,The a skin 3 nm depth Al2 ofO 3thefilm SPP as was the limited interval when layer it was was propagated selected in since the metal it is film, convenient and to fabricate. Additionally,the limitation was the about Al2 30O 3nmfilm for wasAu [33,34 applied]. Thus, as the the 8 intervalnm Au film layer could as generate it can separatean adjacent AuSPP filmsand be andpenetrated prevent by the SPP, interaction resulting ofin AuSPP filmscoupling [32 with]. An at SPPleast two was adjacent generated on the Au films.film surfaceAccording when to Figure the 3c, HMM the coupling was excited. inside the The HMM SPPs could of be the clearly adjacent noticed, films and should also be welleach film coupled was excited to generate to varying a strong degrees. BPP. Furthermore, the enhancement of the Au-NP layer was suggested to be relatively higher than that of the HMM since there was a weak The skin depth of the SPP was limited when it was propagated in the metal film, LSPR at the gap of the Au NPs, as demonstrated by comparing the electromagnetic field and the limitation was about 30 nm for Au [33,34]. Thus, the 8 nm Au film could generate an SPP and be penetrated by the SPP, resulting in SPP coupling with at least two adjacent films. According to Figure3c, the coupling inside the HMM could be clearly noticed, and each film was excited to varying degrees. Furthermore, the enhancement of the Au- NP layer was suggested to be relatively higher than that of the HMM since there was a weak LSPR at the gap of the Au NPs, as demonstrated by comparing the electromagnetic field intensities of the different structures. Moreover, the enhancement of the composite structure (Au NPs + HMM) was significantly higher than that of the Au-NP layer and HMM. This result can be explained, as the Au-NP layer was employed to be a 2D grating and matched the momentum conditions of the SPP; the excited SPPs propagated inside the HMM and coupled with each other, and the synergistic effect of the BPP would have significantly facilitated the LSPR excitation and the generation of strong electromagnetic fields at the gap of the Au NPs. Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 12

intensities of the different structures. Moreover, the enhancement of the composite structure (Au NPs + HMM) was significantly higher than that of the Au-NP layer and HMM. This result can be explained, as the Au-NP layer was employed to be a 2D grating and matched the momentum conditions of the SPP; the excited SPPs propagated inside the Nanomaterials 2021, 11, 587 5 of 11 HMM and coupled with each other, and the synergistic effect of the BPP would have significantly facilitated the LSPR excitation and the generation of strong electromagnetic fields at the gap of the Au NPs.

Figure 3. 3.TheThe simulation simulation of y–z views of y–z of electricviews field of distribution electric fields for ( distributionsa) single Au-NP forlayer, (a ()b) single Au-NP layer, the hyperbolic metamaterial (HMM) consisting of six bilayers with 8 nm Au and 3 nm Al2O3, and (cb) )the the combination hyperbolic of the metamaterial Au-NP layer and (HMM) HMM. consisting(d) A histogram of sixdrawing bilayers a comparison with 8 nmof the Au and 3 nm Al2O3, electricand (c field) the enhancement combination (E/E of0) among the Au-NP the Au layerNPs, HMM and HMM.and Au NPs (d) + A HMM histogram composite drawing struc- a comparison of ture. (e) The simulation setup of the composite structure substrate. the electric field enhancement (E/E0) among the Au NPs, HMM and Au NPs + HMM composite structure.Figure (4ae)– Thef present simulation the simulations setup of of the the composite electric field structure distribution substrate.s of the composite structures with 1–6-bilayer Au/Al2O3 films, respectively. It can be observed that the prominent electricFigure fields4a–f presentwere all generated the simulations at the gaps of of the the electricgold nanoparticles. field distributions Meanwhile, of the composite thestructures intensity with of the 1–6-bilayer hot spots turned Au/Al stronger2O3 films, with respectively. increasing per Itiods can of be the observed HMM. As that the prominent revealedelectric fieldsfrom the were mentioned all generated results, ata synergistic the gaps of effect the was gold exerted nanoparticles. for hot spots Meanwhile, along the intensity theof thevertical hot direction, spots turned inducing stronger a strong with coupling increasing at the nanogap periods region. of the Such HMM. an effect As revealed from the was closely related to the coupling between the LSPR and BPP, which would have en- hancementionedd the localized results, electromagnetic a synergistic field effect intensity was exertedon the top for of the hot multilayer spots along structure. the vertical direction, Notably,inducing Figure a strong 4g was couplinggenerated by at collecting the nanogap the information region. from Such the an simulation effect wasim- closely related ages,to the indicating coupling that betweenthe intensity the of LSPRthe hot spots and BPP,increased which while would the rate haveof increase enhanced was the localized loweredelectromagnetic as the number field of Au/Al intensity2O3 bilayers on the was top elevated. of the multilayerThis phenomenon structure. can be ex- Notably, Figure4g wasplained generated as follows by: the collecting LSPR of the the Au information NPs would have from absorb theed simulation the energy partially images, indicating that when the substrate was under the incident light condition [17], and part of the energy was absorbedthe intensity by the multilayer of the hot structure spots HMM. increased When whilethe SPP themode rate was ofexcited, increase part of was the lowered as the number of Au/Al2O3 bilayers was elevated. This phenomenon can be explained as follows: the LSPR of the Au NPs would have absorbed the energy partially when the substrate was under the incident light condition [17], and part of the energy was absorbed by the multilayer structure HMM. When the SPP mode was excited, part of the incident light energy was converted to an evanescent field appearing on each surface of the Au film, and the energy was partially transferred to the SPP [35], making the SPP propagated on the Au surface. Since the SPP was decaying in the propagation direction, there was an energy loss in this process [25,34]. Accordingly, the deeper the incident light penetrated, the stronger the energy losses. Additionally, the limited energy of the incident light excited a limited number of Au/Al2O3 bilayers to support the SPP because each Au/Al2O3-bilayer- supporting SPP mode would have absorbed energy, causing the intensity of the BPP to be impacted. Moreover, the excited SPP achieved little energy when the incident light was penetrated to the bottom film, hindering it from being coupled well with adjacent SPPs. Furthermore, it was difficult for the bottom SPP to interact directly with the top SPP. Thus, the bottom excited SPP would have exerted less influence on the top SPP and the enhancing effect with the increase in the number of the Au/Al2O3 bilayers. Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 12

incident light energy was converted to an evanescent field appearing on each surface of the Au film, and the energy was partially transferred to the SPP [35], making the SPP propagated on the Au surface. Since the SPP was decaying in the propagation direction, there was an energy loss in this process [25,34]. Accordingly, the deeper the incident light penetrated, the stronger the energy losses. Additionally, the limited energy of the incident light excited a limited number of Au/Al2O3 bilayers to support the SPP because each Nanomaterials 2021, 11, 587 Au/Al2O3-bilayer-supporting SPP mode would have absorbed energy, causing the inten- 6 of 11 sity of the BPP to be impacted. Moreover, the excited SPP achieved little energy when the incident light was penetrated to the bottom film, hindering it from being coupled well with adjacent SPPs. Furthermore, it was difficult for the bottom SPP to interact directly with the top SPP. Thus, the bottom excited SPP would have exerted less influence on the Figure4h presents the electric field intensity of each Au/Al 2O3 bilayer in different top SPP and the enhancing effect with the increase in the number of the Au/Al2O3 bilayers. periods. It can be observed that the electric field intensity of the topmost Au/Al2O3 bilayer Figure 4h presents the electric field intensity of each Au/Al2O3 bilayer in different interlayer was always the strongest, and the other bilayers’ intensities were diminishing periods. It can be observed that the electric field intensity of the topmost Au/Al2O3 bilayer withinterlayer increasing was always depth. the strongest, The mentioned and the other result bilayers can be’ intensit explainedies were by thediminishing coupling between thewith SPPs. increasing Specifically, depth. The thementioned number result of SPPscan be participating explained by the in coupling coupling between increased as the numberthe SPPs. ofSpecifically, Au/Al2 Othe3 bilayersnumber of increased. SPPs participating Such interaction in coupling increased in the vertical as the num- direction led to theber strongestof Au/Al2O SPP3 bilayers in the increased. topmost bilayer.Such interaction Moreover, in the the vertical intensity direction of the led topmost to the SPP tended tostrongest be enhanced, SPP in the suggesting topmost bilayer. that it Moreover, absorbed the more intensity energy, of the resulting topmost inSPP less tended and less energy transferredto be enhanced, to thesuggesting lower layers.that it absorbed Thus,for more the energy, SPP, theresulting closer in to less the and bottom, less energy the less energy willtransferred be available. to the lower Furthermore, layers. Thus, the for increase the SPP, the rate closer for theto the SPP bottom, intensity the less on energy the topmost part will be available. Furthermore, the increase rate for the SPP intensity on the topmost part was lowered as the periods increased because the absorption of energy causing the weak was lowered as the periods increased because the absorption of energy causing the weak SPPSPP at the the bottom bottom could could not notsignificantly significantly improve improve the topmost the topmost SPP. SPP.

Figure 4. (a–f) The simulations of the electric field distribution from one to six Au/Al2O3 bilayers. Figure 4. (a–f) The simulations of the electric field distribution from one to six Au/Al2O3 bilayers. (g) The electric field enhancement (E/E0) in different periods. (h) The electric field intensity of each (g) The electric field enhancement (E/E0) in different periods. (h) The electric field intensity of each Au/Al2O3 bilayer in different periods. Au/Al2O3 bilayer in different periods. R6G as a probe molecule was chosen to measure the SERS performance of the com- positeR6G structure as a probesubstrate molecule more specifically. was chosen In the to measureexperiment the shown SERS in performance Figure 5a, 10−6 of M the compos- − iteR6G structure was dropped substrate on the more top of specifically. the substrate In and the air experiment-dried to evaluate shown the in SERS Figure perfor-5a, 10 6 M R6G wasmance dropped of the composite on the top structure of the in substrate different andperiods. air-dried Figure to5b evaluate shows the the Raman SERS signal performance of theintensity composite of the characteristic structure in peak different 612 cm periods.−1 in the different Figure 5periods.b shows The the peak Raman at 612 signal cm−1 intensity of the characteristic peak 612 cm−1 in the different periods. The peak at 612 cm−1 is a Raman-characteristic peak with high enhancement, which can reflect a good interaction between the probe molecule and the substrate [36]. It can be observed that the increase rate for the Raman signal slowed down as the number of Au/Al2O3 bilayers increased. The mentioned results are consistent with the theoretical simulation, and the difference can be attributed to the fact that the simulated figure (Figure4g) is an idealized model, while our substrate had a small number of nanoparticles with a nonuniform distribution. Figure5c presents the Raman spectra of the 10 −6 M R6G on the HMM, single Au-NP layer, and composite structure with six Au/Al2O3 bilayers. The characteristic Raman peaks of R6G at 612, 773, 1184, 1311, 1362, 1508 and 1649 cm−1 were clearly observed for the composite structure, and the signal from the composite structure exhibited a noticeably higher intensity compared to those for the Au-NP layer and HMM. The signal intensity of the Raman peaks at 612 and 773 cm−1 of R6G in an identical scenario is illustrated in Figure5d to compare the SERS properties of the different structures. As revealed from the calculation of the intensity of the characteristic peak, the intensity of the peak at 612 cm−1 measured for the composite structure was approximately 10.8 times that measured for the Au-NP layer and nearly 13.2 times that measured for the HMM. Furthermore, the peak intensity at Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 12

is a Raman-characteristic peak with high enhancement, which can reflect a good interaction between the probe molecule and the substrate [36]. It can be observed that the increase rate for the Raman signal slowed down as the number of Au/Al2O3 bilayers increased. The mentioned results are consistent with the theoretical simulation, and the difference can be attributed to the fact that the simulated figure (Figure 4g) is an idealized model, while our substrate had a small number of nanoparticles with a nonuniform distribution. Figure 5c presents the Raman spectra of the 10−6 M R6G on the HMM, single Au-NP layer, and composite structure with six Au/Al2O3 bilayers. The characteristic Raman peaks of R6G at 612, 773, 1184, 1311, 1362, 1508 and 1649 cm−1 were clearly observed for the composite structure, and the signal from the composite structure exhibited a noticeably higher intensity compared to those for the Au-NP layer and HMM. The signal intensity of the Raman Nanomaterials 2021, 11, 587 peaks at 612 and 773 cm−1 of R6G in an identical scenario is illustrated in Figure 5d to 7 of 11 compare the SERS properties of the different structures. As revealed from the calculation of the intensity of the characteristic peak, the intensity of the peak at 612 cm−1 measured for the composite structure was approximately 10.8 times that measured for the Au-NP −1 773layer cm and nearlymeasured 13.2 times for thethat compositemeasured for structure the HMM. was Furthermore, about 8.6 timesthe peak that intensity measured for the Au-NPat 773 cm layer−1 measured and nearly for the 12.4 composite times thatstructure measured was about from 8.6 the times HMM, that measured demonstrating for that the couplingthe Au-NP betweenlayer and thenearly Au-NP 12.4 times layer that and measured HMM from exerted the HMM, a signiﬁcant demonstrating enhancing that effect. the coupling between the Au-NP layer and HMM exerted a significant enhancing effect.

Figure 5. (a) The Raman signal of 10−6 M rhodamine 6G (R6G) on HMM, Au-NP layer, and compo- −6 Figuresite structure 5. (a) with The six Raman Au/Al signal2O3 bilayers. of 10 (b)M The rhodamine Raman signal 6G intensity (R6G) on of HMM, characteristic Au-NP peak layer, at and composite −1 structure612 cm in withdifferent six periods. Au/Al (2c)O The3 bilayers. relationship (b diagram) The Raman of the peak signal intensity intensity of the ofR6G characteristic Raman peak at −1 −6 612spectra cm at− 1612in and different 773 cm periods.. (d) The ( Ramanc) The spectra relationship of 10 M diagram R6G from of the the different peak intensity structures. of the R6G Raman spectra at 612 and 773 cm−1.(d) The Raman spectra of 10−6 M R6G from the different structures. Additionally, reproducibility was considered to be an essential indicator for examin- ing the SERS substrate performance. As shown in Figure 6a, 15 spots from composite structuresAdditionally, were taken reproducibilityrandomly, and the was Raman considered signal was to collected be an essentialfor 10−6 M indicatorR6G. No for exam- iningsignificant the SERSfluctuations substrate for each performance. characteristic Aspeak shown of R6G in were Figure observed6a, 15 from spots the fromdata. composite −6 structuresThe distribution were histogram taken randomly, of the intensity and of the theRaman characteristic signal peak was at collected612 cm−1 for for R6G 10 M R6G. Nois provided significant in Figure fluctuations 6b, where forthe black each horizontal characteristic line repres peakents of the R6G average were intensity observed from the data. The distribution histogram of the intensity of the characteristic peak at 612 cm−1 for R6G is provided in Figure6b, where the black horizontal line represents the average −1 intensity of the 612 cm peak for SERS from the mentioned 15 positions, and the gray area indicates the fluctuation of the SERS signal. The relative standard deviation (RSD) of the peak intensity for R6G at 612 cm−1 was 4.7%, confirming that the composite structure had excellent reproducibility. Next, R6G, as the probe molecule, was employed to measure the detection performance of the composite structure. When the probe molecules were dropped on the composite structure, they would have been gathered at the nanogaps of the Au NPs spontaneously, and the Raman signal was enhanced effectively. The SERS spectra of R6G at concentrations ranging from 10−6 to 10−10 M are illustrated in Figure6c, where the characteristic peaks could still be clearly observed at 10−10 M. In the mentioned concentration range, quantita- tive detection could be conducted, and the variation of the intensity could be quantitatively expressed using an empirical equation for the concentration (Log I = 0.31Log C + 6.06, for R6G), where I denotes the intensity of the SERS signal, C represents the concentration of R6G, and the coefficients of determination (R2) are 0.993. The LOD of R6G was calculated as LOD = 3.3(SD/S), where SD indicates the standard deviation of the response, and S is the slope of the calibration curve; the result was nearly 7.9 × 10−11 M, satisfying practical Nanomaterials 2021, 11, 587 8 of 11

detection use. Furthermore, the electromagnetic enhancement factors (EFs) were calculated by [37,38] I /N EF = SERS SERS (1) IRS/NRS

where ISERS and IRS represent the peak intensities of the SERS spectrum and normal Raman spectrum, respectively; NSERS and NRS denote the numbers of the molecules in the laser spot regions [39]. The intensity of the peak at 612 cm−1 for the composite structure reached 11,725, and the intensities for 10−6 M R6G for the single Au-NP layer and HMM were 997 and 810, respectively. Additionally, the intensity of the identical peak on the blank PET substrate was 750, and the concentration reached 10−2 M. The EF of the composite structure substrate was calculated as 1.56 × 105. Likewise, the EFs of the single Au-NP layer substrate and HMM substrate were determined as 1.33 × 104 and 1.08 × 104, respectively, which were signiﬁcantly lower than the former. The mentioned result was attributed to the

Nanomaterials 2021, 11, x FOR PEER couplingREVIEW between the Au-NP layer and HMM. Speciﬁcally, the enhancing9effect of 12 was very strong when they were combined, and the effect was weak when they were separated.

FigureFigure 6. ((aa).). The The Raman Raman signal signal of the of 10 the−6 M 10 −R6G6 M collected R6G collected randomly randomly from 15 spots from in 15 15 spotscompo- in 15 composite −1 substratessite substrates with with six six Au/Al Au/Al2OO3 bilayers.bilayers. (b) (Theb) The intensity intensity of 612 of cm 612 peak cm −for1 peakthe composite for the composite struc- structures. (c) The SERS spectra2 of3 R6G at concentrations ranging from 10−6 to 10−10 M. (d) The cali- −6 −10 tures.bration (c curve) The of SERS the normalized spectra of R6GRaman at intensity concentrations at 612 cm ranging−1 versus from R6G of 10 differentto 10 concentrM. (da-) The calibration curvetions. of the normalized Raman intensity at 612 cm−1 versus R6G of different concentrations.

The detection detection capability capability of the of thecomposite composite structure structure at a low at concentration a low concentration was inves- was investi- gatedtigated with with MGMG as as the the probe probe molecule. molecule. The SERS The SERSspectra spectra of MG at of concentration MG at concentrationss ranging ranging fromfrom 10 −−4 to4 to10− 108 M− are8 M presented are presented in Figure in 7a. Figure Similarly,7a. Similarly,the 1617 cm the−1 pea 1617k of MG cm could−1 peak of MG still be observed at the concentration of 10−8 M, demonstrating−8 that the detection limit of could still be observed at the concentration−8 of 10 M, demonstrating that the detection the composite structure of MG was 10 M in magnitude.−8 Figure 7b shows the standard limitcalibration of the plot composite of the SERS structure signal at of1617 MG cm was−1, while 10 theM peak in magnitude.at 1617 cm−1 wa Figures also 7theb shows the −1 −1 standardpeak with calibrationhigher enhancement, plot of thewhich SERS is indicative signal at of 1617 an interaction cm , while between the the peak probe at 1617 cm wasmolecule also theand peakthe substrate with higher [36]. The enhancement, intensity waswhich quantitatively is indicative changed of using an interaction the em- between thepirical probe equation molecule for the and con thecentration substrate (Log [ 36I =]. 0. The59Log intensityC + 6.94). was The quantitativelycoefficients of deter- changed using themination empirical (R2) reached equation 0.986 for for the 1617 concentration cm−1, exhibiting (Log a greatI = linear 0.59Log responseC +6.94). in the range The coefﬁcients ofof determinationconcentrations. Additionally (R2) reached, the 0.986 Raman for spectra 1617 cmof −the1, 10 exhibiting−2 and 10−8 aM great MG acquired linear response in thefrom range the blank of concentrations. PET substrate and Additionally, composite structure the Raman substrate spectra in six of periods the 10 are−2 exhib-and 10−8 M MG ited in Figure 7c. The EF was calculated as 1.7 × 106. Moreover, the long-term stability, acquired from the blank PET substrate and composite structure substrate in six periods where the intensity of the SERS signal was stable, showed an advantage of the composite × 6 arestructure. exhibited In this in study, Figure different7c. The positions EF was were calculated randomly as 1.7selected10 from. Moreover, the composite the long-term structure; then, the SERS spectra were immediately collected. Subsequently, the substrate was exposed to the air, and the SERS spectra were obtained again two weeks and one month later, respectively (Figure 7d). According to this figure, the SERS signal barely decreased even after one month. This is due to the fact that the gold nanoparticles were stable in the air, and the Al2O3 film exhibited a high antioxidant property and kept the substrate stable for a long time.

Nanomaterials 2021, 11, 587 9 of 11

stability, where the intensity of the SERS signal was stable, showed an advantage of the composite structure. In this study, different positions were randomly selected from the composite structure; then, the SERS spectra were immediately collected. Subsequently, the substrate was exposed to the air, and the SERS spectra were obtained again two weeks and one month later, respectively (Figure7d). According to this ﬁgure, the SERS signal barely decreased even after one month. This is due to the fact that the gold nanoparticles Nanomaterials 2021, 11, x FOR PEERwere REVIEW stable in the air, and the Al2O3 ﬁlm exhibited a high antioxidant property10 of 12 and kept

the substrate stable for a long time.

Figure 7. (a) SERS spectra acquired from 10−4 to 10−8 M malachite green (MG) deposited on the Figure 7. (a) SERS spectra acquired from 10−4 to 10−8 M malachite green (MG) deposited on composite structure substrate displaying six Au/Al2O3 bilayers. (b) The calibration curve of the thenormalized composite Raman structure intensity substrate at 1617 cm displaying−1 versus MG six of Au/Al different2O 3concentrations.bilayers. (b) Error The calibrationbars indi- curve of − thecate normalized standard deviations Raman from intensity 10 spectra. at 1617 (c) cmThe Raman1 versus spectra MG for of different10−8 M MG concentrations. collected for the Error bars indicatecomposite standard structure, deviations and 10−2 M from obtained 10 spectra. for PET (c for) The comparison. Raman spectra (d) The for long 10-term−8 M stability MG collected for for the compositethe MG SERS structure, spectra andcollected 10−2 fromM obtained the composite for PET structure for comparison. substrate stored (d) The for long-term the different stability peri- for the ods. MG SERS spectra collected from the composite structure substrate stored for the different periods.

4.4. Conclusions Conclusions In this study, a Au-NP/HMM composite structure was successfully formed. The Au In this study, a Au-NP/HMM composite structure was successfully formed. The Au NPs acted as a 2D grating, capable of efficiently exciting the SPP mode and the BPP mode. NPsThe actedLSPR excitation as a 2D grating, was facilitated capable by of the efﬁciently excited BPP. exciting Accordingly, the SPP a modestrong andelectromag- the BPP mode. Thenetic LSPR field excitationwas generated was at facilitated the nanogaps by of the the excited Au NPs. BPP. Throughout Accordingly, the simulations, a strong electromag- the neticresults ﬁeld displayed was generated a tendency at consistent the nanogaps with the of experimentally the Au NPs. achieved Throughout results. the The simulations, Ra- theman results signal acquired displayed from a tendencythe composite consistent structure with was noticeably the experimentally stronger than achieved that col- results. Thelected Raman from signalthe HMM acquired and Au from-NP layer the compositeand exhibited structure great sensitivity. was noticeably Moreover, stronger the than thatintensity collected of the from hot spots the HMM and the and increase Au-NP rate layer for the and Raman exhibited signal greatwere suggested sensitivity. to Moreover,be thelowered intensity with of an the increase hot spots in the and Au/Al the2O increase3 bilayer rates. The for mentioned the Raman factors signal could were be suggestedat- Commented [CD1]: Please check intended mean- tributed to energy absorption and losses. Additionally, low concentrations of R6G and to be lowered with an increase in the Au/Al2O3 bilayers. The mentioned factors coulding is retained. beMG attributed were employed to energy to examine absorption the detection and losses. capacity Additionally, of the composite low concentrations structure, with of R6G −10 −8 andlower MG-limit were concentration employeds of to 10 examine and 10 the M, detection respectively. capacity Furthermore, of the compositethe reproduci- structure, bility and long-term stability performance of the composite structure were determined, with lower-limit concentrations of 10−10 and 10−8 M, respectively. Furthermore, the repro- generating excellent results. As revealed from the mentioned promising results, the com- ducibility and long-term stability performance of the composite structure were determined, posite structure could exploit the value of SERS and contribute to the in-depth develop- generatingment of biosensors excellent and results. food safety As revealed inspection. from the mentioned promising results, the composite structure could exploit the value of SERS and contribute to the in-depth development ofAuthor biosensors Contribution and foods: Conceptualization, safety inspection. Z.W., S.J., Y.H., Z.L. and T.N.; investigation, Z.W., R.L., Z.Z., M.S. and C.L.; data curation, Z.W., R.L. and S.L.; writing—original draft preparation, Z.W.; writing—review and editing, Z.W., K.X. and R.Z.; funding acquisition, Z.L., S.X. and S.J. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 12074226, 11674199 and 12004226), Taishan Scholars Program of Shandong Province (Grant No.

Nanomaterials 2021, 11, 587 10 of 11

Author Contributions: Conceptualization, Z.W., S.J., Y.H., Z.L. and T.N.; investigation, Z.W., R.L., Z.Z., M.S. and C.L.; data curation, Z.W., R.L. and S.L.; writing—original draft preparation, Z.W.; writing—review and editing, Z.W., K.X. and R.Z.; funding acquisition, Z.L., S.X. and S.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 12074226, 11674199 and 12004226), Taishan Scholars Program of Shandong Province (Grant No. tsqn201812104), and Qingchuang Science and Technology Plan of Shandong Province (Grant No. 2019KJJ017). Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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