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Abstract
In the present study, a nanoparticle-multilayer metal film substrate was presented with silver nanoparticles (Ag NPs) assembled on a multilayer gold (Au) film by employing alumina (Al2O3) as a spacer. The SERS performance of the proposed structures was determined. It was suggested that the SERS effect was improved with the increase in the number of layers, which was saturated at four layers. The SERS performance of the structures resulted from the mutual coupling of multiple plasmon modes [localized surface plasmons (LSPs), surface plasmon polaritons (SPPs), as well as bulk plasmon polaritons (BPPs)] generated by the Ag NP-multilayer Au film structure. Furthermore, the electric field distribution of the hybrid system was studied with COMSOL Multiphysics software, which changed in almost consistency with the experimentally achieved results. For this substrate, the limit of detection (LOD) was down to 10−13 M for the rhodamine 6G (R6G), and the proposed SERS substrate was exhibited prominently quantitatively detected capability and high reproducibility. Moreover, a highly sensitive detection was conducted on toluidine blue (TB) molecules. As revealed from the present study, the Ag NP-multilayer Au film structure can act as a dependable SERS substrate for its sensitive molecular sensing applications in the medical field.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) has aroused rising concern since its introduction in 1970s [1–3]. SERS, a sensitive and reliable detection method, can be extensively employed in biosensors [4–6], food security [7–9], environment safety [10] and catalysis [11,12]. Considerable efforts have been made to develop the SERS substrate, including fabricating and using a range of structures and materials. Subsequently, the noble metals were demonstrated to exhibit eminent SERS capacity for their surface plasmon resonances [13,14]. Surface plasmons primarily have two types: one is coherent electrons oscillating on the surface of nanoparticles or between nanogap, termed as localized surface plasmons (LSP); the other is surface plasmon polaritons (SPPs) where the coherent electrons oscillate as a longitudinal wave on the extended metal surface [14,15]. Furthermore, in the presence of periodic dielectric/metal stack, the electromagnetic fields that bounded to the individual plasmonic interfaces are coupled with each other, thereby generating a collective response, which is termed as bulk plasmon polaritons (BPPs) resonance [16]. BPPs refer to propagating waves inside the multilayer, whereas they exponentially decay outside [16–18]. Such a unique optical response has been extensively applied in sensors, optical device manufacturing, as well as surface enhancement spectroscopy. The single excitation mode restricts the development of SERS substrates, so the design of nanostructures under high-efficiency excitation modes has constantly been a vital important research direction in the manufacture of expected SERS substrates.
As highlighted by recent reports, the coupling of mutual polariton modes can be excited by the particle-film complex structure [19–22]. In terms of conventional two-dimensional (2D) SERS substrates, the enhanced Raman signal usually relies on the “hot spots” generated by the LSP resonance between nanoparticles [23–25]. As impacted by existing technology limitations, it is considered challenging to control the distance between nanoparticles at a nanometer level. As SERS substrates advanced from 2D to three-dimensional (3D), the concept of multilayer particles was proposed [26]. By exploiting the coupling of the LSP in the vertical direction, prominent electric field enhancing effects can be exerted, whereas the production of such a 3D substrate is troublesome and not highly repeatable. Relatively, the SPP-based SERS substrates are easier to prepare and capable of generating a more effective uniform signal. Likewise, such substrates are flawed, so the enhancing effect of the mentioned substrates is generally weak [27]. To address this problem, composite substrates under multiple excitation modes were proposed by combining the advantages of LSP and SPP [28]. Though the horizontal spacing of nanoparticles is difficult to reduce, the distance between the particles and the film can be regulated in the vertical direction on a nanoscale by exploiting nanomaterials of controllable thickness as the spacer layer [29]. Accordingly, the detection sensitivity of SERS can be further improved by the coupling of LSP and SPP, allowing SERS to have broad application prospects in large-scale devices design and manufacturing. For instance, Wang et al. reported an Ag nanocubes/polyelectrolyte/Au sandwich-structure and subsequently adopted it as an efficient SERS substrate through LSP and SPP coupling. The relationship between the spatial distribution of the coupling material and the electric field enhancing effect was clarified by modulating the thickness of the spacer layer [20]. In addition, Li et al. presented a system in which Al particles separated by alumina (Al2O3) were prepared with a simple annealing method [30]. In such a system, the LSP from the upper Al NPs and the SPP is coupled with the high-confined BPP of the lower Al film, thereby exerting a significant enhancing effect. However, Al cannot act as an ideal SERS substrate as impacted by the limited electric field enhancing effect. On the whole, the mentioned efforts provide the critical guidance for designing more specialized and efficient SERS substrates.
The present study presented a convenient and efficient method to fabricate an Ag NP-multilayer Al2O3/Au film substrate. Here, Al2O3 was taken as the spacer layer since it exhibits high light permeability and is capable of eliminating the charge disorder in the dielectric environment, thereby significantly reducing the spectral diffusion of excitons. As excited by incident laser light, the LSP generated by Ag NPs will couple with SPP and BPP from the underlying multilayer Au films, thereby leading to an excellent behavior of SERS. By employing R6G with a 10−7 M concentration as the probe molecule, the Raman spectra were obtained from Ag NP-different layers of Au film (0-6 layers) and single layer Al2O3/Au film. As revealed from the experimentally achieved results, by comparing with Ag NP-blank substrates and single layer Al2O3/Au film substrates, the Raman signal intensity of R6G was suggested to be noticeably enhanced after Ag NPs were combined with Au film. Besides, the R6G signal on the composite substrate was improved with the rise of the number of Au layers, while nearly no change was identified above four layers. The detection limit of the SERS substrate for R6G reaches 10−13 M. The enhancement mechanism for this substrate was proposed by combining simulated results and Raman detection. By drawing upon the prepared SERS substrate, the detection limit of TB was 10−9 M. Such a low concentration detection of biological dye molecules can demonstrate that the proposed SERS platform has an excellent potential to be applied in the medical field.
2. Experiments
2.1. Materials
Acetone (CH3COCH3, 99.5%), alcohol (C2H6O, 99.7%), ethylene glycol (C2H6O2, 99.0%), silver nitrate (AgNO3) and hexane [CH3(CH2)4CH3, 80%] were purchased from the local chemical plant. In addition, polyvinylpyrrolidone (PVP, Mw=55,000) was offered by Sigma-Alorich. Rhodamine 6G (R6G, MW=479.02) was provided by Meilun Biological Technology Co., Ltd (Dalian). Moreover, toluidine blue (TB, Mw=373.97) was purchased from Yuanye Biotechnology Co., Ltd (Shanghai).
2.2 Preparation of multilayer film structure of Al2O3/Au
All silicon substrates were washed in an ultrasonic cleaner for 30 min in sequence with acetone, alcohol and deionized water (DI water) to remove surface contaminants. Thermal evaporation (VZB-400) was employed to continuously deposit uniform layers of Au and Al. The Au film exhibiting a thickness of 16 nm was deposited on the silicon substrate (current 115 A and deposition rate ∼1 Å/s). The Al film exhibiting a thickness of around 3 nm was deposited on the Au film (current 72 A and deposition rate ∼0.8 Å/s). All deposition process were performed in a low-pressure environment of 7×10−5 Pa, and the substrate was rotated at 30 circle/min. To obtain a high-quality, densified oxide layer of Al film, the samples were transferred while being protected by nitrogen to a high vacuum chamber to perform oxidation treatment by exposing the Al film to high-purity oxygen under low pressures (5×10−6 Torr) for 10 min. By repeating the mentioned steps, the multilayer structure of the Au film could be obtained with a densified Al2O3 film as a nanoscale gap.
2.3 Synthesis of Ag NPs
Ag NPs were synthesized by using the method expressed by Guo et al [31]. 20 ml ethylene glycol was heated to 70 °C while being continuously stirred with a magnetic stirring apparatus. Subsequently, 0.25 g PVP was dissolved in the ethylene glycol solution. Next, 0.05 g AgNO3 was introduced into the heating solution when the mixture was heated to 135 °C with the reaction for 1.5 h. Afterwards, considerable acetone was added into the cooled colloid for precipitation, thereby contributing to well separating the pure Ag NPs from the solution. Lastly, pure Ag NPs were gathered by the centrifugation at 12000rpm for five min and then dispersed in a spot of DI water.
2.4 Preparation of Ag NP-multilayer metal film substrate
Interfacial self-assembly of Ag NPs was conducted by using the recently reported approaches with slight modifications [32]. In brief, two milliliters of aqueous Ag NPs colloid was transferred to a petri dish (inner dimension, 2.5×2.5 cm2), and 1 mL of hexane was added onto the colloid solution surface to produce an immiscible water/hexane interface. Subsequently, 1.5 mL of ethanol was added dropwise onto the surface of the water/hexane layers (0.3 mL/min, with an injector for medical purpose), thereby leading to Ag NPs trapping at the interface. After the hexane evaporated, Ag NPs were transferred to the multilayer Au film. The complete experimental procedure is shown in Fig. 1.
2.5 Sample characterization and SERS measurement
The surface and cross-section of the Ag NPs on the Al2O3/Au multilayer films were obtained with SEM (Zeiss GeminiUltra-55). To determine the SERS effect of Ag NP-different layers Al2O3/Au substrate, the probe molecule R6G was employed. The R6G was dissolved in ethanol to generate the solution from 10−7∼10−13 M. 2 µL probe molecules were dripped onto the samples, and the samples were dried entirely prior to the Raman test. The Raman signals were acquired with Raman spectrometer (Horiba HR Evolution) under the identical conditions (532 nm laser, 0.048 mW laser power, gratings with 600 grooves per 1 mm, ×50 objective lens, 1 µm laser spot, as well as the acquisition time of 4 s).
3. Results and discussion
To verify the morphology of the deposited multilayer film, SEM images were adopted to characterize the cross-section of the multilayer film. According to Fig. 2(a), four pairs of Au/Al2O3 films on the silicon with flat surfaces could be observed. In the figure, the irregularity of the cross-section was attributed to the different ductility of metal and silicon wafers when cutting samples. Based on the corresponding energy dispersive spectrometer (EDS) spectrum illustrated in Fig. 2(b), the compositions of the elements including Au, Al, and O were confirmed. The elements (Al and O) originated from the Al2O3 film. In addition, Fig. 2(c) shows the EDS mapping obtained on the surface of the multilayer film structure, and the uniformity of the Al2O3 and Au films can be seen by observing the color distribution of the different elements. The morphologies of Ag NPs characterized by self-assembly on the liquid surface are illustrated in Fig. 2(d). By calculating the diameter distribution histogram, the normal distribution of the synthesized silver nanoparticles complies with the typical Gaussian curve in the inset of Fig. 2(d). The diameter distribution of Ag NPs was primarily located at 50-70 nm, and the average diameter was suggested to be nearly 60 nm. Obviously, one layer of Ag NPs with a relatively uniform distribution was obtained.
Fig. 2. (a) SEM cross-section image of the 4 layers gold film substrate without Ag NPs. (b) and (c) EDS data of the identical sample. (d) SEM images and size distribution histograms of Ag NPs.
With the COMSOL Multiphysics software, this study simulated the electric field distribution of the substrate. The model parameters were set by complying with the previously characterized results. The thicknesses of Au and Al2O3 films were 16 and 3 nm, respectively. The Ag NPs was 60 nm in diameter, and the spacing between the particles was 20 nm [ Fig. 3(a)]. The wavelength of the incident laser was 532 nm, complying with that in the Raman test, and the direction of incidence was from the top of the particle downward to ensure the simulated results to be scientifically valid. As suggested from the electric field distributions in Fig. 3(c) and (d), the structure of the Au film did not generate a strong electric field, and a higher electric field can be generated by the Ag NPs. E and E0 denote the electric field strength and the background electric field strength in the structure, respectively. However, when they were combined, the electric field could increase considerably, as shown in Fig. 3(e) and (b). Using R6G with a concentration of 10−7 M as the probe molecule, the identical trend to the simulated result was achieved. It was preliminarily proved that the composite substrate exerts an excellent electric field and Raman enhancing effect. To clearly demonstrate the role of the alumina spacer layer, the electric field distributions of Ag NP-Au bulk (the thickness of Au bulk is 6 × 16 nm) and Ag NP-6 layers Au film structures were simulated, respectively (Supplement 1, Fig. S1). By comparing the colors, and the calculation of the electric field enhancement results. We can conclude that for the same thickness of Au film, the Ag NP-6 layers Al2O3/Au film structure produces better electric field enhancement (E/E0=21.64) than Ag NP-Au bulk (E/E0=17.61). This means that for the same thickness of Au film, the Al2O3 spaced multilayer structure has a better electric field enhancement effect compared to the bulk structure.
Fig. 3. (a) Simulation set-up of the SERS substrate. (b) The variation of electric field enhancement (E/E0) and the Raman intensity of R6G at 613 cm-1 for the different structures. The electric field distribution for (c) one layer Al2O3/Au film, (d) Ag NPs and (e) Ag NP-one layer Al2O3/Au film at 532 nm wavelength.
Simulations and experiments were performed to explain the Raman enhancement mechanism and effect produced by the couple of LSP with SPP and BPP. The electric distributions of the Ag NP-multilayer Au films with different numbers of layers: 0, 1, 2, 3, 4, 5, and 6 are presented in Fig. 4(a)-(g). It can be observed that after adding the Au film with Al2O3 as a spacer, a strong electric field was generated between the bottom of Ag NPs and the gold film. With the increase in the number of Au films, the electric field between Ag NPs was enhanced significantly. Besides, the electric field distribution could also be observed in the Al2O3 spacer between the Au films, resulting from the SPP resonance excited on the Au film surface. The red line in Fig. 4(h) represents the electric field enhancement variation (E/E0) of the electric field that occurred in the nanogap with the increase in the number of Au films. It is noteworthy that when the number of the Au film layers increased from 0 to 2, the electric field was enhanced vastly, whereas the trend of enhancement turned slow with the number of gold film layers exceeding two. Such a phenomenon could be obtained in the electric field distribution diagram as shown in Fig. 4(g) Specific to the Au film of six layers structure, the color map of the electric field intensity gradually darkens from top to bottom, indicating that the electric field intensity was reduced.
Fig. 4. Electric field distribution for Ag NP-different layers of Au film at 532 nm: (a) zero layer, (b) one layer, (c) two layers, (d) three layers, (e) four layers, (f) five layers, (g) six layers, (h) variation of electric field enhancement (E/E0) and the signal of R6G at 613 cm-1 for the Ag NP-multilayer Au films substrate with different numbers of Au film layers, (i) SERS spectra of R6G (10−7 M) collected from different Au film layers. Absorption spectrum obtained from the (j) Ag NP-Al2O3/Au film systems, and (k) Al2O3/Au film with different Au film layers.
The simulated results were verified by measuring the SERS activity of the Ag NP-multilayer Au film substrate with the use of R6G exhibiting a concentration of 10−7 M as the probe molecule, as well as by comparing the signal intensities of R6G obtained from Ag NP-different layers of Au film (0-6 layers) [Fig. 4(i)]. The black line in Fig. 4(h) represents the average intensity of the SERS signal of R6G at 613 cm-1 versus the different numbers of Au film layers based on 15 sets of spectra randomly collected from the respective substrate. The increasing trends of the experimental and simulated results were compared and found to be consistent. However, the optimal Raman enhancement obtained from the experiment was in four layers, slightly different from the simulated results. Such a phenomenon could be explained as the ideal model was used in the simulation, while the gold film in the experiment was not completely flat.
It is generally known that SPP is the coupling of electromagnetic waves and electrons on the metal surface. The collective oscillation of electrons at the metal/dielectric interface refers to a surface wave whose energy propagates along the metal surface. The energy decays exponentially in the direction perpendicular to the metal surface. As impacted by the field enhancement properties of SPP, it can act as a physical enhancement mechanism to prepare SERS substrates. It is subject to momentum mismatch under certain conditions since SPP is propagating, dispersive electromagnetic waves. Furthermore, the laser in the medium or the air cannot directly excite the SPP on the metal surface. For the mentioned reason, an incident laser was used to excite LSP generated by Ag NPs as a coupling mechanism, as an attempt to excite SPP on the Au film surface. This could be explained as the LSP is non-propagating excitations of the conduction electrons of metallic nanostructures coupled to the electromagnetic field that can be excited by direct light illumination. In this study, the Ag NPs and the Au film were separated by Al2O3 to form a cavity-like structure, and the LSP and SPP coupling from both led to the generation of a strong electric field. Moreover, a strong electric field distribution was found in the Al2O3 spacer layer in Fig. 4(b), demonstrating that the SPP on the surface of the metal film is successfully excited. In the multilayer film system, each metal/dielectric interface supported a specific SPP, and the 3 nm Al2O3 spacing led to stronger SPP coupling on adjacent Au film surface [33]. The attenuation distance in the direction perpendicular to the metal surface was narrowed, and then the field could be localized. The SPP confined in each periodic dielectric layer would resonate, giving rise to BPP. The mutual coupling of the mentioned multiple excitation modes promoted the electric field to be significantly enhanced.
In the present study, the Au film exhibiting a thickness of 16 nm and the Al2O3 film with a 3 nm thickness was periodically stacked. With the increase in the number of layers, an increase in the BPP resonance was formed by the mutual coupling of the SPP on the surface of the respective Au film layer. However, with the increase in the number of Au film layers, the energy transmitted downward by the incident laser decreased layer by layer. As a result, the laser could not penetrate more films and effectively excite the SPP on the surface of the underlying gold film. According to Fig. 4(g), when the number of gold layers increased to six, only the top layers of gold film could produce sufficient SPP resonance for the laser limited penetration depth, primarily explaining why the Raman enhancing effect stopped increasing significantly when the gold film reached a certain number of layers.
To study the effect of nanoparticle-multilayer film systems on the plasma resonance position, we calculated the optical absorption spectra of Ag NP-mutilayer film system and multilayer film system with different number of Au film layers. As shown in Fig. 4(j), there is an absorption peak at 428 nm for Ag NPs, which is due to the LSPR of Ag NPs. When Ag NPs were combined with the multilayer film system, the optical absorption was enhanced due to the plasmonic coupling effect between Ag NPs and Au film, and two absorption peaks were generated around 350 nm and 500 nm. Meanwhile, increasing the number of layers of gold film absorption is also enhanced, and the absorption peak located at 500 nm is red shift. This means that the plasma resonance position of the whole system is red shift and the resonance has been enhanced. When the number of Au film layers reaches 4-6, the absorption peak is located around 532 nm, which is the same wavelength as the Raman laser we used. However, for individual multilayer systems the absorption is weak as shown in Fig. 4(k), and the absorption enhancement is limited with increasing number of layers. This demonstrates that the enhancement mechanism of the Ag NP-multilayer film system proposed in this paper mainly relies on the plasmonic coupling effect generated between Ag NPs and multilayer Au film system.
To study the SERS activity of the Ag NP-multilayer Au film structure in depth, the Ag NP-four layers Au film were selected as the substrate, and the Raman spectra of R6G molecules with different concentrations (from 10−7 to 10−13 M) were investigated. As shown in Fig. 5(a), the Raman signal intensity decreased with decreased concentration of R6G. When the concentration decreased to 10−13 M, the signal intensity was tiny; only the peaks of 613, 774, and 1651 cm-1 were slightly convex, demonstrating that the detection limit of R6G molecules was reached. Figure 5(b) illustrates the linear relationship between the intensity and concentration of the R6G spectrum at 613 cm-1. To ensure the reliability of the data, ten sampling points were randomly selected for each to determine the average intensity. The correlation coefficient (R2: 0.993) of R6G molecules in logarithmic coordinates was remarkable, indicating that the signal intensity and concentration of R6G molecules had a significant linear response. The Ag NP-four layers of Au film substrate can be adopted for quantitative detection.
Fig. 5. (a) Raman spectra of R6G on the Ag NP-four layers Au film substrate with different concentrations from 10−7 to 10−13 M. (b) SERS intensity at 613 cm-1 for R6G as a function of the molecular concentration.
The SERS performance of the substrate was assessed by calculating the enhancement factor (EF) [34,35]:
Fig. 6. (a) Raman spectra of R6G with concentrations of 10−11 and 10−2 M collected from the Ag NP-four layers Au film substrate and Si substrate. (b) Raman spectra of R6G with a concentration of 10−7 M were randomly collected from the identical substrate, and (c) the peak (613 cm-1) relative intensities were collected from the mentioned Raman spectra. (d) Intensity distribution of the peak at 613 cm-1 for R6G with a concentration of 10−7 M from 10 different batches of the SERS substrate.
For practical applications, TB as a basic thiazine metachromatic dye is applied as probes in biology and biochemistry for its luminous nature. It has been employed in vivo to identify dysplasia and carcinoma of the oral cavity for its metachromatic property [36,37]. Accordingly, sensitive and credible SERS sensing should be exploited for TB at low concentrations. The TB water solutions with concentrations ranging from 10−4 to 10−9 M were generated, and 2 µL solutions were dropped on the Ag NP-multilayer gold film substrate for SERS analysis. The SERS spectra of TB at different concentrations are illustrated in Fig. 7(a), indicating that the intensity of the characteristic peak of TB [38] (i.e., 448, 1385, 1431, and 1630 cm-1) decreased with decreasing concentration. The linear relationship between the intensity and concentration of TB spectra at 1630 cm-1 is presented in Fig. 7(b). The quantitative detection could be conducted in this concentration range, and the variation of intensity was expressed quantitatively by the empirical equation: Log I = 0.37 Log C + 5.46. As expected, the mentioned results demonstrate that the proposed Ag NP-multilayer Au film substrate may be employed to detect stained biological cells.
Fig. 7. (a) Raman spectra of TB on the Ag NP-four layers Au film substrate with different concentrations from 10−4 to 10−9 M. (b) SERS intensity at 1630 cm-1 for TB as a function of the molecular concentration.
4. Conclusion
In brief, the Ag NP-multilayer gold film plasmonic structure exhibiting excellent SERS performance was proposed by the coupling of multiple excitation modes. In theory, the simulation of the electric field distribution was presented to illustrate the enhancement mechanism and superiority of the Ag NP-multilayer gold film plasmonic structure. In addition, the excellent performances including SERS activity, high sensitivity, SERS signals uniformity and outstanding reproducibility were proved experimentally. Moreover, the low concentration detection of biological dye molecule such as TB molecule (LOD: 10−9 M) was conducted. This study considers that the mentioned promising results are capable of effectively promoting the application of the SERS substrate to detect stained biological cells.
Funding
Natural Science Foundation of Shandong Province (ZR2018BF026); National Natural Science Foundation of China (11674199, 12004226, 12074226).
Acknowledgments
Great thanks to Qianqian Peng for her helpful suggestion in our SEM characterization. Thanks to Jinjuan Gao and Shouzhen Jiang for great helps in the manuscript writing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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