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Abstract
The accurate detection of nanoplastics is crucial due to their harmful effects on the environment and human beings. However, there is a lack of detection methods for nanoplastics smaller than 50 nm. In this research, we successfully constructed an Ag/CuO nanowire (NW)/BaTiO3@Polyvinylidene fluoride (PVDF) Bowl-shaped substrate with a nanowire-in-Bowl-shaped piezoelectric cavity structure that can modulate surface-enhanced Raman scattering (SERS) by the piezoelectric effect by the virtue of the tip effect of the CuO NW and light focusing effect of the Bowl-shaped cavity. Due to its unique nanowire-in-Bowl-shaped structure and piezoelectrically modifiable ability, nanoplastics less than 50 nm were successfully detected and quantitatively analyzed. We believe that the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate can provide an efficient, accurate, and feasible way to achieve qualitative and quantitative detection of nanoplastics.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the increasing progress of science and technology, microplastic as a pollutant is increasingly coming into the attention of people and considered to be a serious biological health hazard, which can easily enter the blood, liver and lymphatic tissues through the digestive system, skin, and so on [1–4]. However, the current detection of microplastic is mainly focused on the range of 100 nm to 5 mm, but there is a lack of detection methods for nanoplastic less than 50 nm. Therefore, there is an urgent need to design a detection method for the qualitative and quantitative analysis of nanoplastic in the environment.
Surface-enhanced Raman scattering (SERS) has shown promising applications as an ultra-sensitive and non-destructive detection method in many fields such as biological sensing, [5–9], food security [10–12], chemical analysis [13,14], environmental testing [15–18] and medicine [19–21]. The enhancement mechanism of SERS is mainly based on the resonance of surface plasmon of noble metal nanoparticles (NPs) on the substrate surface [22–24], which can create the local electromagnetic field on the substrate surface and thus enhance the Raman signal. Since the surface plasmon resonance (SPR) of adjacent noble metal nanostructures can be coupled [25,26], the array of metal NPs with periodic nanostructures is formed as an effective way to enhance SERS [27–29]. Various types of noble metal nanostructures such as nanosheet [30], nanorod [31], and nanoflower [32] have been investigated with improved SERS performance. And some SERS substrates, such as AuNSs@Ag@AAO substrate [33], Au NPs substrate [34], and Au-nanoparticle-decorated sponge (SA) substrate [35], have designed and used for the detection of microplastics, although the above substrates have achieved accurate detection of microplastics, the size of microplastics detected by these substrates was still large and the concentration of detected microplastics was still high. Bowl-shaped structures are increasingly recognized by virtue of the strong optical focusing performance and larger SERS “hot spot” area compared with other nanostructures [36,37]. What’s more, the special shape of the Bowl-shaped structure is easy to enrich the detection material, which has special applications in certain scenarios, such as the detection of microplastic [38]. Nevertheless, the detection of nanoplastics smaller than 100 nm in a simple Bowl-shaped structure is quite difficult due to the weak Raman signal.
The tip of the metallic nanowire (NW) structure can serve as the nanoantenna and excite the local surface plasmon (LSP) near the tip, which can further enhance the electromagnetic field [39,40], at the same time the dense NW structure of the substrate facilitates the adsorption of nanoplastics in solution. Combining NW structures with Bowl-shaped periodic arrays may be a feasible strategy to adsorb the nanoplastics and recognize the near-trace amounts of nanoplastics. In addition to constructing noble metal nanostructures on the substrate surface, the presence of the extra electric field can further enhance the SERS by modulating the electron density of the noble metal. Although electric-field can effectively modulate the SERS activity and is beneficial for the investigation of the enhanced mechanism, the traditional electric-field modulated methods require the use of both Raman spectrometer and electrochemical workstation, which is accompanied by complex operational procedures and equipment requirements. Compared to the electrochemical workstations, it is more feasible to introduce an extra electric field with the help of the piezoelectric or pyroelectric effect, which can greatly reduce the environmental and operational limitations. In the piezoelectric material, the generated polarized charge can effectively modulate the local electromagnetic field thus enhancing the SERS signal [41–43].
Based on the above principles, the nanowire-in-Bowl-shaped piezoelectric cavity was constructed with Ag/CuO NW/BaTiO3@Polyvinylidene fluoride (PVDF) Bowl-shaped substrate. The piezoelectric BaTiO3@PVDF film can cause a change in the surface charge distribution when subjected to external forces. The Bowl-shaped structure was constructed on the BaTiO3@PVDF film and then CuO NW was formed inside the BaTiO3@PVDF Bowl-shaped structure to synthesize the CuO NW/BaTiO3@PVDF Bowl-shaped structure that acts as (1) an optical field amplifier, as the Bowl-shaped structure with its large specific surface area and light focusing properties will enhance the intensity of the SPR around the noble metal nanoparticles, (2) a molecular gripper for nanoplastics, the dense CuO NW in the Bowl-shaped structure can not only transfer the local electric field from the surface to space, creating multi-dimensional SERS “hot spot” in the Bowl-shaped structure, but also grab nanoplastics, endowing the substrate the function of directional detection of nanoplastics, (3) a nanogenerator, with the piezoelectric effect of the BaTiO3@PVDF layer enabling the substrate to generate a polarized charge when subjected to external forces, inducing an enhancement of the SPR. In theory, we employed the COMSOL Multiphysics software to simulate the local electric field in the nanowire-in-Bowl-shaped structure, the results showed that stronger “hot spot” were generated between neighboring Ag NPs when they were deposited on the nanowire-in-Bowl-shaped structures, which can be attributed to the enhanced local electric field caused by the light focusing effect of the Bowl-shaped structure and the tip effect of the NW structure. In the experiment, by detecting rhodamine 6 G (R6G), we demonstrated the enhancement of the substrate SERS signal can be attributed to the synergistic effect of the piezoelectric effects and the SPR. Through the detection of different sizes of nanoplastics (PS (Polystyrene) sphere), we found that the substrate has the ability to detect microplastics smaller than 50 nm in a directional manner. The detection limit of 20 nm PS sphere solution can reach 10−4 mg/ml by piezoelectric modulation of the substrate. This work achieved the directional and quantitative detection of nanoplastic, which can effectively promote the application of SERS in environmental protection.
2. Experimental section
2.1 Material
PVDF powder and BaTiO3 NPs were purchased from Aladdin. High purity Copper (Cu) particles were purchased from Lijia metal materials. Acetone (CH3COCH3), ethanol (C2H6O), Sodium dodecyl sulfate (SDS, C12H25NaO4S), and sodium hypochlorite solutions (NaClO) were purchased from Sinopharm Chemical Reagent Co. 300 and 900 nm diameters Monodisperse PS sphere suspension (5 wt% in ultrapure water) was purchased from Maxinne dareen. 20 and 50 nm PS sphere suspension (2.5 wt% in ultrapure water) purchased from Yiyuan Biology. R6G was purchased from Melen Biotech Co.
2.2 Preparation of Cu-PS sphere array template
The gas-water interface self-assembly technique was used to prepare PS sphere array template with periodic structure: the slides were washed by deionized water (DI) and anhydrous ethanol, then treated with oxygen plasma treatment for 180 s to obtain the highly hydrophilic surface. The 900 nm PS sphere suspension was mixed 1:1 with anhydrous ethanol, and a homogeneous mixture was formed by sonicating for 10 min. After that, 35 µl of the mixture was added dropwise to the slides, and the solution containing 900 nm PS sphere will cover the surface of the slides uniformly due to the high hydrophilicity of the surface of the slides after treatment by plasma. Subsequently, the slides containing PS sphere were dried at room temperature for 1 h to form a large-scale layer of PS sphere on the slides. After that, 0.3 ml of 2 wt% SDS as surfactant was added dropwise to a beaker containing 300 ml of DI to change the tension of the water surface to facilitate the overall shedding of the PS sphere layer. After that, the slide was slowly immersed into the beaker where the PS sphere layer would separate from the slide and form a floating monolayer of PS sphere in the beaker. The monolayer of PS sphere were then transferred to oxygen plasma-treated coverslips and heated to 40 °C on the heating table for drying, subsequently heated at 90 °C for 30 min to make the PS spheres adhere to each other for immobilization. Subsequently, to prevent the PS sphere from falling off the template during the peeling process, a 30 nm Cu layer was deposited on the PS sphere by physical vapor deposition (PVD).
2.3 Preparation of Ag/CuO NW/BaTiO3@PVDF bowl-shaped substrate
2.1 g PVDF powder and 0.38 g BaTiO3 NPs powder were dissolved in 5 ml mixed solution of acetone and dimethylformamide (DMF), the homogeneous colloidal was obtained by magnetic stirring at 70 °C for 6 h. Thereafter, the BaTiO3@PVDF colloida was spin coated on the Cu-PS sphere array template by spin coater at 2000 r/s rotating for 30 s and placed at room temperature environment for 5 min, so that the BaTiO3@PVDF colloida was fully contacted with the template and fixed into the Bowl-shaped structure, after which the Cu-PS sphere array template was stripped to obtain the BaTiO3@PVDF Bowl-shaped substrate. To make the nanowire-in-Bowl-shaped structure, we first deposited a 100 nm Cu layer on the surface of the BaTiO3@PVDF Bowl-shaped substrate by PVD to provide sufficient reactants, and then immersed the Cu/BaTiO3@PVDF Bowl-shaped substrate in 20 wt% NaClO solution for 20 s to grow CuO NW. Finally, the SERS “hot spot” were provided by depositing a 30 nm Ag layer on the surface of the prepared substrate using PVD.
2.4 Apparatus and characterization
We used the scanning electron microscopy (SEM, ZEISS Sigma 500) at 3.0 kV to characterize the surface morphology of the prepared substrates. Meanwhile, we used the X-ray diffraction (XRD, Rigaku Ultima IV) to characterize the phase structure of the substrate surface materials. The chemical state of the prepared substrate surface materials was analyzed by the X-ray photoelectron spectroscopy (XPS, Thermo Fisher). The ultraviolet-visible (UV-vis) absorption spectra of the prepared substrates were measured using UV-vis spectrophotometer (UV, Shimadzu Solid Spec-3700i). The SERS spectra were measured using the high-resolution Raman spectrometer (Horiba HR Evolution 800): 532 nm laser, diffraction grid = 600 gr/mm and the integration time was set as 8 s; 50× objective. The laser excitation energy and spot were 0.48 mW and 1 µm, respectively. Using electrochemical workstation (CHI-760e) to examine piezoelectric properties of prepared substrate, the pressure is provided by a self-made instrument. The electromagnetic field distribution of Ag NPs on BaTiO3@PVDF substrates with different morphologies and the piezoelectric intensity of PVDF films containing different amounts of BaTiO3 NPs were simulated using COMSOL Multiphysics software.
3. Results and discussion
The fabrication process of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate was illustrated schematically in Fig. 1, which has been described in detail in the experimental section. In the process of preparing the template, the 30 nm Cu layer was crucial for the successful achievement of the BaTiO3@PVDF Bowl-shaped substrate. Figure 2(a) presents an SEM image of the PS sphere template, revealing a neat and smooth array structure. But as shown in Fig. 2(b), we observed that the BaTiO3@PVDF Bowl-shaped substrate obtained using PS sphere array template would lead to many PS sphere adhering to the surface, and thus it needed to be soaked in toluene to etch off the PS sphere to form the Bowl-shaped structure. The BaTiO3@PVDF Bowl-shaped substrate obtained using the PS sphere array template after immersion in toluene is shown in Fig. 2(c), and we found that it was not well formed with blurred boundaries between the Bowl-shaped structure, which was caused by organic solvents in the BaTiO3@PVDF colloid corroding the PS sphere array template. By depositing Cu film on the PS sphere array template, we obtained the Cu-PS sphere array template shown in Fig. 2(d). The deposited Cu film can prevent the PS spheres from falling off the template and prevent the organic solvent in the colloid from corroding the PS sphere. Figure 2(e) shows the SEM image of the BaTiO3@PVDF Bowl-shaped substrate obtained from the Cu-PS sphere array template. Compared with the Bowl-shaped structure obtained using the PS sphere array template, we observe that the Bowl-shaped structure is more neatly arranged, the boundaries between adjacent bowls are clearer, and the Bowl-shaped structure has a more complete morphology. To observe the morphology of this Bowl-shaped structure more clearly, we performed a larger magnification SEM characterization. As shown in Fig. S1(a), it was further demonstrated that the Bowl-shaped structure obtained using the Cu-PS sphere array template has a more complete morphology. To better demonstrate the Bowl-shaped structure on the substrate surface, we show the tilted SEM image of the Bowl-shaped structure in Fig. S1(b), in which we can clearly observe the sidewalls of the Bowl-shaped structure. To further understand the difference in SERS activity between arrayed Bowl-shaped structure fabricated using PS sphere array templates and Cu-PS sphere array templates, we characterized the substrates using UV-vis absorption spectra. As shown in Fig. 2(f), the UV-vis absorption spectrum intensity of the Bowl-shaped substrate prepared using the Cu-PS sphere array template is slightly higher than that of the substrate prepared using the PS sphere array template, and more importantly, the absorption spectrum intensity at 532 nm is 0.833 for the substrate prepared using the Cu-PS sphere array template, while that of the substrate prepared using the PS sphere array template is 0.672, which indicates that the intensity of the substrate prepared with Cu-PS sphere array template will utilize the laser energy to a greater extent, and the main reason for this phenomenon is the influence of the substrate surface morphology. The above results indicate that the Bowl-shaped structures obtained using Cu-PS sphere array templates have a more complete morphology, which would help to obtain larger SERS “hot spot” regions and a significant increase in the Raman signal.
Fig. 1. Illustration diagram of the fabrication process for the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate.
Fig. 2. (a) SEM image of the PS sphere array template. (b) SEM image of BaTiO3@PVDF film using PS sphere array template. (c) Bowl-shaped structure of BaTiO3@PVDF film using PS sphere array template after the PS sphere was eroded off. (d) SEM image of the Cu-PS sphere array template. (e) SEM image of Bowl-shaped formed on BaTiO3@PVDF film using Cu-PS sphere array template. (f) UV-vis absorption spectra of the films obtained using PS and Cu-PS sphere array template.
During the fabrication of CuO NW/BaTiO3@PVDF Bowl-shaped substrates, we found that different morphologies could be obtained by soaking Cu/BaTiO3@PVDF Bowl-shaped substrates in NaClO solutions for different times. As shown in Figs. 3(a-c) and Fig. S1(c), with increasing soaking time, the Bowl-shaped substrate surface was progressively covered by CuO NW and formed comparatively dense nanowire structures inside the BaTiO3@PVDF Bowl-shaped structure. To confer the nanowire-in-Bowl-shaped structures with SERS activity, we deposited 30 nm Ag on all the above CuO NW/BaTiO3@PVDF Bowl-shaped substrate. Meanwhile, to better research the diversity induced by the density change of the CuO NW, we collected the SERS spectra of R6G at a concentration of 10−6 M on Cu/BaTiO3@PVDF Bowl-shaped substrates immersed in NaClO solutions for various times (0-30 s). As illustrated in Fig. 3(d) and Fig. S6(a), it is obvious that the Raman intensity enhances with an increasing soaking time from 0 to 20 s because the formation of the CuO NW inside the Bowl-shaped structure gradually increased. The tip effect of the CuO NW combined with the light-focusing function of the Bowl-shaped structure greatly enhanced the Raman intensity [44]. With the further increasing the time from 20 to 30 s, there is a significant decrease in Raman intensity, because excessive growth of CuO NW would cover the morphology of the Bowl-shaped structure, which would reduce the intensity of SPR. As a result, we concluded that the best Raman performance was obtained at a socking time of 20 s, which was maintained to further research throughout the subsequent experiments. Due to the surface micro-nanostructure of the substrate will have an effect on the UV-vis absorption spectra, we characterized the Cu/BaTiO3@PVDF Bowl-shaped substrates immersed in NaClO solution for different times using UV-vis absorption spectra. The absorbance of the substrates gradually increased with the gradual increase in soaking time, the absorbance of the substrates was strongest at 20 s of soaking and then decreased, as shown in Fig. 3(e). This further reflected that the CuO NW growth after 20 s of soaking combined with the Bowl-shaped structure formed the most favourable morphology for plasmonic coupling, and was consistent with the regularity obtained from Raman testing.
Fig. 3. SEM images of Cu/BaTiO3@PVDF Bowl-shaped substrates after soaking in NaClO solution for different times. (a) 10 s; (b) 20 s; (c) 30 s. (d) Raman spectra of R6G (10−6 M) on the substrates after soaking in NaClO solution for different times (unsoked, 10 s, 20 s and 30 s). (e) UV-vis absorption spectra of Cu/BaTiO3@PVDF Bowl-shaped substrates after soaking in NaClO solution for 10-30 s. (f) Raman spectra of R6G as probe molecule on the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate with different thicknesses of Ag layer.
To research the effect of the Ag layer thickness on the Raman scattering of CuO NW/BaTiO3@PVDF Bowl-shaped substrate, we have compared the enhancement effects of 0 nm to 50 nm Ag layer on the CuO NW/BaTiO3@PVDF Bowl-shaped substrate. Figure 3(f) presents the Raman spectra of the 10−6 M R6G solution detected on fabricated Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates with different thicknesses. The change of the Raman signal intensity with increasing the deposition thickness from 0 nm to 50 nm was exhibited, where we can observe that the SERS activity increased with the Ag thickness increasing until 30 nm, followed by a decrease as the thickness continues to increase. It is evidently that the optimal enhancement effect on Raman signal can be achieved when the Ag layer thickness reached 30 nm. The SERS activity dependence on the deposition thickness can be understood from the coupling strength of the Ag layer with the nanowire-in-Bowl-shaped structure. When too little Ag was deposited, the distribution of the Ag on the substrate surface was inhomogeneous, the plasmonic coupling formed between the CuO NW structure was weak. When the thickness was appropriate, the tip effect of CuO NW would be more obvious, and all the Ag layers attached to the Bowl-shaped structure formed the light-focusing effect, which would substantially increase the SERS activity of the substrate. When excessive Ag was deposited, CuO NW would adhere to each other, the tip effect would be weakened. From the conclusions drawn from the above experiments, we conclude that the substrate deposited with a 30 nm Ag layer have the best SERS performance and will be used for subsequent experiments. The exact atomic valence states of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate were characterized using XRD and XPS in Fig S2.
In order to estimate the advantages of the prepared Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate, the Raman spectra of R6G with a concentration of 10−6 M on four substrates (Ag/CuO NW/BaTiO3@PVDF Bowl-shaped, Ag/BaTiO3@PVDF Bowl-shaped, Ag/BaTiO3@PVDF and Ag/CuO NW/BaTiO3@PVDF substrates) were all collected in the same condition. As expected, R6G on the base of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate has the strongest Raman characteristic peaks, as shown in Fig. 4(a). The peak intensity of 613 cm-1 on the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates was 3.9 times stronger than that on the Ag/BaTiO3@PVDF Bowl-shaped substrates, 4.9 times stronger than that on the Ag/BaTiO3@PVDF substrate and 12.8 times stronger than that on Ag/CuO NW/BaTiO3@PVDF substrate, which proves that the existence of Bowl-shaped structure can significantly enhance the Raman properties of the substrate. The difference of the enhancement for Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates were contributed to the presence of the Bowl-shaped structure and CuO NW that can further enhance the Raman signal due to a large surface area and the effective magnified SPR effect around the Ag. The UV-vis absorption spectra are extremely sensitive to the surface morphology of the substrate. Figure 4(b) shows the UV-vis absorption spectra of the four substrates mentioned above. We found that the absorption spectral intensity of UV-vis absorption spectra increased with the gradual improvement of the substrate surface nanostructure. The presence of the CuO NW caused the absorption peak at 417 nm, while the presence of the Bowl-shaped structure led to higher absorption intensity of Ag/ BaTiO3@PVDF Bowl-shaped substrate after 922 nm. Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate has the UV-vis absorption spectra of Bowl-shaped and NW structure, and Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate has the highest UV-vis absorption spectra intensity at 532 nm compared with the other three substrates. These means that the substrate can make better use of the energy of the incident laser. In conclusion, the nanowire-in-Bowl-shaped structure enhances the absorption of incident light and thus enhances the “hot spot” intensity of the electric field. The Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate obtained the UV-vis absorption spectra characteristics of both Bowl-shaped and NW structures. In conclusion, the nanowire-in-Bowl-shaped structure enhances the increases the absorption of incident light, and thus enhances the “hot spot” intensity of the electric fields. Therefore, the Raman signal of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate are stronger than Ag/BaTiO3@PVDF Bowl-shaped, Ag/BaTiO3@PVDF and Ag/CuO NW/BaTiO3@PVDF substrates.
Fig. 4. (a) Raman spectra of R6G as the probe molecule on four substrates (Ag/CuO NW/BaTiO3@PVDF Bowl-shaped, Ag/BaTiO3@PVDF Bowl-shaped, Ag/BaTiO3@PVDF and Ag/CuO NW/BaTiO3@PVDF substrates). (b) UV-vis absorption spectra of the four substrates mentioned above. COMSOL Multiphysics software simulations of local electric field distributions on the (c-e) Functional schematic of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate.
To further investigate the changes in the intensity and distribution of the electromagnetic field on the substrate surface caused by changes in the nanostructure of the substrate surface. We used COMSOL Multiphysics software to simulate the electromagnetic field distribution and intensity of Ag NPs on different morphological structural substrates. The dimensions of the established model were obtained by measuring the size distribution of the CuO NW and Ag NPs in SEM, as shown in Figs. S3 and S4, with an average value of 38 nm for the Ag NPs size, 460 nm for the CuO NW length, and 54 nm for the bottom width. Just as expected, the intensity of the local electric field on the Ag NPs-CuO NW-PVDF Bowl-shaped model in Fig. S5(d) was ∼ 11.18 times stronger than that on the Ag NPs-PVDF model (Fig. S5(a)), ∼ 59.5 times stronger than that on the Ag NPs-CuO NW-PVDF model (Fig. S5(b)) and ∼ 7.02 times stronger than on the Ag NPs-PVDF Bowl-shaped model (Fig. S5(c)). Figures 4(c-e) describes the action mechanism of the Ag NPs-CuO NW-PVDF Bowl-shaped structure in enhancing Raman. By combining the light-focusing effect of the Bow-shaped structure and the tip effect of the CuO NW structure, the plasmon coupling of Ag NPs on the substrate surface was further enhanced, mainly due to the light-focusing function of the Bowl-shaped substrate, where the incident light oscillates back and forth within the nanoscale cavity so that the laser energy was confined within the cavity and favours the Raman signal excitation. At the same time, the presence of CuO NW shifted the SERS “hot spot” distributed at the bottom of the Bowl-shaped structure into the Bowl-shaped space, which would greatly increase the number of SERS “hot spot”. The large surface area of the Bowl-shaped structure would provide more attachment points for CuO NW, and the laser energy confined in the cavity would be fully utilized by the CuO NW due to the sufficient contact between the CuO NW and the Bowl-shaped structure, and there would have highly active SERS “hot spot” at the top of the intertwined CuO NW.
A SERS substrate need to possess the fundamental feature of good reproducibility. Therefore, we used the 10−6 M solution of R6G as the probe molecule on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate for testing at arbitrarily selected 20 points, the obtained data are shown in Fig. S6(b). It was apparent that for the case of R6G, all the data exhibited a slight fluctuation around the average intensity. Fig. S6(c) shows the Raman peaks (613, 774 and 1365 cm-1) extracted from the above Raman spectra to compare the peak fluctuations. The relative standard deviations (RSD) of the substrate at 613, 774 and 1365 cm-1 were 10.13%, 10.42% and 13.98% with relative standard deviation values of 1280.54, 622.32 and 1300.19, respectively, which can be ascribed to the well-ordered and uni-form Bowl-shaped structure as well as CuO NW. R6G was selected to further research the SERS activity of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates. We detected the Raman peaks of R6G with concentrations ranging from 10−7 to 10−12 M in Fig. S6(d). Although the Raman intensity of R6G decreased with decreasing R6G concentration, the Raman characteristic peaks would still be detected at the concentrations down to 10−12 M, which proved that the substrate has ultra-sensitivity. To research the capability of this quantitative detection for molecules, we performed a linear fit of the Raman peak intensity versus concentration for the R6G molecule at 613 cm-1 and obtained a linear fit curve with a high coefficient of determination (R2: 0.95263), as shown in Fig. S6(e), suggesting a well linear response within the concentration.
To quantize the SERS performances of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates, R6G was chosen as a test probe for the calculation of analytical enhancement factor (EF) using the following formula: EF = (ISERS / IRS) / (CSERS / CRS), where ISERS and IRS are the Raman signal intensities of R6G from the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate and BaTiO3@PVDF substrate, respectively, CSERS and CRS represent the corresponding concentration of R6G on the above substrate. Thus we obtained EF = (546 / 2025) / (10−11 M / 10−2 M) = 2.696 × 108. Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate has the high EF, which can be attributed to the nanowire-in-Bowl-shaped structure supporting the large surface area and high intensity SERS “hot spot”. The stability of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates was also a critical parameter for SERS analysis. To test the stability of the above substrate, we recorded SERS spectra of the substrate over a period of 25 days (Fig. S6(f)). As displayed, there was no appreciably change in the profiles of the Raman spectrum of R6G after storage for 25 days, and the substrate exhibited good stability. Such a remarkable SERS substrate with stability and sensitivity can meet the requirements for practical SERS application.
In order to obtain Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates with optimal piezoelectric modulation Raman functionality, we investigated the piezoelectric properties of the PVDF films containing different mass fractions of BaTiO3 NPs. Integrating the fabricated PVDF films containing different mass fractions of BaTiO3 NPs in a closed circuit, reduplicated and stable piezoelectric output voltage generated under the 2 Hz frequency and 15 mm displacement mechanical conditions (The sketch of the piezoelectric test setup is shown in Fig. S7), as shown in Fig. 5(a). We observed that when introduced into an open circuit, the piezoelectric output voltage were ∼1.1 V, ∼2.1 V, ∼2.5 V, 3.9 V and ∼2.0 V for the PVDF film containing different mass fractions of BaTiO3 NPs from 0 wt% to 20 wt%, respectively. The piezoelectric output voltage reached up to 3.9 V when the mass fraction of BaTiO3 NPs in the PVDF film reached 15 wt%. This was attributed to the fact that BaTiO3 NPs can increase the polarization intensity of PVDF, and the BaTiO3 NPs had its piezoelectric effect, which provided additional piezoelectric output. In contrast, when the mass fraction of doped BaTiO3 NPs reached 20 wt%, the piezoelectric output voltage decreased to 2.2 V, which was mainly because excessive doping of BaTiO3 NPs would lead to their agglomeration in the film, thus diminishing the macromolecular structure within the substrate and thus reducing the piezoelectric effect of the substrate. To reveal the piezoelectric effect mechanism, piezoelectric potential simulations were performed on pure PVDF film, and BaTiO3@PVDF film, respectively, using COMSOL Multiphysics software. As shown in Fig. S8, we measured the average diameter of BaTiO3 NPs by SEM to accurately model. The radius of BaTiO3 NPs was chosen to be 65 nm, which was consistent with the distribution of the particle size analysis. The films were set to a thickness of 0.025 µm, a height of 2 µm and a width of 0.4 µm, as well as the bottom of the model was fixed and grounded. The piezoelectric output voltage of PVDF film containing different amounts of BaTiO3 NPs was studied using surface stress of 0.8 MPa. As shown in Figs. 5(c-e), the piezoelectric output voltage of the PVDF film only had a piezoelectric potential of 2.05 V under pressure. After adding BaTiO3 NPs to PVDF film, the piezoelectric potential of PVDF film was improved to 2.25 V. However, when the arrangement of BaTiO3 NPs on the PVDF film was too dense, the piezoelectric output voltage decreased to 1.85 V. Obviously, the addition of the appropriate amount of BaTiO3 NPs facilitates the piezoelectric output voltage increase, which will generate a stronger piezoelectric potential and cause the substrate to develop a stronger piezoelectric potential on the substrate surface when performing piezoelectric modulation of SERS, thus enhancing the Raman signal intensity by modulating the strength of the electromagnetic field on the substrate surface. The ability to prolong the time of the piezoelectric output voltage is an indispensable function in boosting signals during SERS measurements. As shown in Fig. 5(b), we examined the piezoelectric retention ability of the substrate. For the 0 wt%-20 wt% BaTiO3@PVDF substrate, the piezoelectric output signal was reduced by more than 90% within 20 s after pressing, however, when CuO NW was grown on the surface of 15 wt% BaTiO3@PVDF substrate, the piezoelectric output voltage was preserved by 50% at 20 s. This is mainly due to the high dielectric constant of CuO, therefore the CuO NW grown on the substrate can maintain the piezoelectric potential well and mitigated the decrease of the piezoelectric potential, which provides us enough time for the SERS detection.
Fig. 5. (a) Piezoelectric output voltage-time profile of PVDF, 5 wt% BaTiO3@PVDF, 10 wt% BaTiO3@PVDF, 15 wt% BaTiO3@PVDF and 20 wt% BaTiO3@PVDF substrates. (b) Piezoelectric output voltage-time profiles of PVDF, 5 wt% BaTiO3@PVDF, 10 wt% BaTiO3@PVDF, 15 wt% BaTiO3@PVDF, 20 wt% BaTiO3@PVDF and Ag/CuO NW/BaTiO3@PVDF substrates after press. (c-e) COMSOL Multiphysics software simulated of the piezoelectric voltages generated by PVDF films containing different amounts of BaTiO3 NPs.
To demonstrate that the SERS signal can be modulate according to the piezoelectric effect of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate, we chose R6G as the probe molecule for the SERS test. To ensure that the deformation of the substrate during pressing does not affect the laser focusing, we first pressed the substrate using a weight to allow the piezoelectric effect of the substrate, and then performed the Raman test afterwards. Figure 6(a) shows the Raman spectra of R6G every ten seconds after pressing the substrate. We distinctly observed the decline in relative intensities with time, which was basically consistent with the decreasing trend of piezoelectric potential. This result demonstrated that declining electric field leads to a reduction of Raman signals, which confirmed our assumption that the piezoelectric effect modulates the Raman signals. After that, we compared the Raman intensity of the 10−7 M R6G solution before and after pressing the substrate. As shown in Fig. 6(b), where the piezoelectric field formed on the substrate surface amplified the intensity of the Raman signal by a factor of 2.5. This result demonstrated that our substrates exhibit significantly higher SERS activity after pressing than before pressing. In the previous experiment, the Raman characteristic peak was observed when the substrate was detected with 10−12 M concentration of R6G solution, but it was blurred. We detected 10−12 M of R6G after pressing the substrate, where the intensity of Raman signal after pressing was not only enhanced, but also the characteristic peak was more obvious, as shown in Fig. 6(c). Based on the above conclusions, the enhancement of the SERS by the substrate is due to a synergistic effect of the piezoelectric effect and the SPR. Figure 6(d) presents the working mechanism of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate on piezoelectric modulated Raman. When the substrate was not subjected to external pressure, the electrons and holes within the BaTiO3@PVDF film were randomly arranged, when the substrate was subjected to external force, the BaTiO3@PVDF film would be deformed due to the impact, the film would be polarized internally and the electrons and holes would be rearranged, generating positive and negative piezoelectric potentials at the top and bottom of the substrate. The electrical potential generated by the piezoelectric effect would be stored between the CuO NW prolonging the existence of the piezoelectric potential. Since the work functions of CuO and Ag being 4.2 eV and 2.5 eV, the electrons would be transferred to the Ag layer. This would first affect the electron distribution on the Ag layer and then the electromagnetic field on the substrate surface, and ultimately affected the coupling of the incident laser to the electromagnetic field and modulating the SERS [45]. The above results demonstrated that the proposed Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates can further enhance the intensity and clarity of the SERS signal through piezoelectric effects based on SPR to modulate Raman.
Fig. 6. (a) Raman spectra of 10−7 M R6G collected within 0-40 s after pressing Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate. (b) Raman spectra of 10−7 M R6G before and after pressing. (c) Raman spectra of 10−12 M R6G before and after pressing. (d) Illustration of the principle of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate for piezoelectric modulation of SERS.
In addition to the detection for dye molecules, we found that Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate has excellent detection ability for nanoplastic smaller than 50 nm in size. The SEM images Figs. 7(a-d) demonstrated that both larger and smaller PS spheres can be found on the substrate surface, with the smaller PS spheres mainly adsorbed on CuO NW. Then, we examined the Raman spectra of PS spheres at 20 nm, 50 nm, 100 nm and 300 nm on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate. The characteristic peaks of PS were located at 1003 and 1032 cm-1. As shown in Fig. 7(e) the Raman intensity of 20 nm and 50 nm PS spheres was much higher than that of 100 nm and 300 nm. This demonstrated that the substrate has directional detection ability for smaller-sized nanoplastics. The EF of the above substrate of PS sphere of different sizes was calculated to evaluate the enhancement effect. The characteristic peak of the PS at 1003 cm-1 was chosen to calculate the EF. Raman spectra of the PS sphere collected from the BaTiO3@PVDF substrate were used as a reference. As shown in Fig. 7(f), the EF of PS sphere increases with decreasing particle size, where the substrate has the highest EF value for 20 nm PS sphere, which can reach 1107.7. This was mainly due to the fact that smaller-sized PS sphere can more easily get between the intersecting CuO NW [46], however larger sized PS sphere would be blocked out of the nanowire-in-Bowl-shaped structure, which leaded to the ability of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate to provide directional detection for nanoplastics smaller than 50 nm. From the above data, we can find that the EF of the substrate to 20 nm PS sphere is smaller than that of the dye molecule, which is mainly due to the lower Raman scattering cross section of PS compared with that of the dye molecule [49].
Fig. 7. (a) SEM image of the 300 nm PS sphere. (b) SEM image of the 100 nm PS sphere. (c) SEM image of the 50 nm PS sphere. (d) SEM image of the 20 nm PS sphere. Orange circles in (a-d) represent nanoplastics in the nanowire-in-Bowl-shaped structure. (e) Raman spectra of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate containing PS sphere of different sizes. (f) EF vs the size of PS sphere.
To demonstrate more specifically the ability of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates to directionally detect nanoplastics, the SERS spectra of 20 nm PS sphere were collected at a concentration of 100 mg/ml on the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped, Ag/CuO NW/BaTiO3@PVDF, Ag/Cu/Si, Ag/BaTiO3@PVDF Bowl-shaped and Ag/CuO NW/Si substrates. The intensities on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate were larger than those on the other substrates, as shown in Fig. 8(a). The peak intensity of 1003 cm-1 on the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates was 1.6 times stronger than that on the Ag/CuO NW/BaTiO3@PVDF substrates, 5.6 times stronger than that on the Ag/Cu/Si Bowl-shaped substrates, 18.9 times stronger than that on the Ag/BaTiO3@PVDF Bowl-shaped substrate and 84.74 times stronger than that on Ag/CuO NW/Si substrates. These results demonstrated the natural advantages of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate for the detection of nanoplastics. Subsequently, we tested the Raman detection limit for the 20 nm PS sphere. We observed that the Raman characteristic peaks of the PS were still clearly visible until the concentration of the PS sphere was reduced to 10−3 mg/ml, as shown in Fig. 8(b). To investigate this ability to quantitatively detect the nanoplastic, we performed a linear fit of the Raman peak intensity versus concentration for the PS at 1003 cm-1 and obtained a linear fit curve with a high coefficient of determination (R2: 0.95051), as shown in Fig. 8(c). The results obtained above demonstrated that our prepared substrates have significant potential to quantitatively and qualitatively recognize nanoplastic. In the piezoelectric enhanced Raman test, we found that the SERS signal of the dye molecule can be enhanced by pressing the substrate. In further experiments, we found that substrate can even identify nanoplastic with extremely low concentration by simple pressing. Consistent with the previous piezoelectric modulated Raman experiments, we first pressed the substrate with a weight, and then quickly focused and performed Raman testing. This operation of pressing first and focusing later avoids the effect on Raman testing due to substrate deformation. The characteristic peak of 10−4 mg/ml PS sphere hardly appeared before pressing the substrate, but after pressing, the Raman peak of PS at 1003 cm-1 was clearly visible and matched well with the standard Raman spectrum of PS, as shown in Fig. 8(d). In order to ensure that the homogeneity of the substrate does not show large different when testing different substances, we again tested the homogeneity of the substrate using 1 mg/ml of PS sphere solution, and the results are shown in Fig. S9. By calculating the Raman characteristic peak of PS at 1003 cm-1, the relative standard variance of the substrate was obtained as 13.21%, which is basically consistent with the results tested with R6G. Table 1 compared the detection limits of different substrates for different sizes of PS sphere. The results showed that our fabricated substrates have lower detection limits compared to the substrates described in the literature.
Fig. 8. (a) Raman spectra of 20 nm PS sphere on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped, Ag/CuO NW/BaTiO3@PVDF, Ag/Cu/Si, Ag/BaTiO3@PVDF Bowl-shaped and Ag/CuO NW/Si substrate. (b) SERS spectra of different concentrations of 20 nm PS sphere on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates. (c) Correlation between the relative SERS intensity of the Raman spectrum of PS sphere at 1003 cm-1 and the concentration of the solution. (d) Raman spectra of 10−4 M PS sphere before and after pressing. (e-h) Simulated of electric field distribution of 300 nm, 100 nm, 50 nm, 20 nm PS spheres on Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate. (i) Schematic of the Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate for detection of nanoplastic of different sizes.
Table 1. Research progress in the detection of microplastic using Raman.
In order to more accurately understand the substrate's ability to directionally detect nanoplastics, we performed COMSOL Multiphysics software simulations to identify the electric-field distributions in Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate with PS nanospheres inside. We reached the same conclusion as in the experiment (SEM images of PS spheres with 300 nm, 100 nm, 50 nm and 20 nm on the substrate surface are shown in Figs. 7(a-d)). As shown in Figs. 8(e-h), the electromagnetic field intensity of the substrate gradually increased with the gradual decrease of the PS sphere size. In Fig. 8(i) we explain the reasons for this phenomenon. We considered that for Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate, when the diameter of the PS sphere was too large, it would obscure the nanowire-in-Bowl-shaped structure, which would affect the SPR of the substrate. As the diameter of the PS sphere gradually decreased, the PS sphere adsorbed on CuO NW can enhance the scattering effect, which leaded to a clear Raman signal [35]. At the same time, the piezoelectric effect can be adjusted to detect lower concentrations of the PS sphere. In conclusion, Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate shows great potential in the detection of nanoplastic.
4. Conclusion
In summary, we successfully prepared Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrate. In addition, the substrate's directional detection function for nanoplastics was experimentally and theoretically demonstrated. The detection of the R6G molecule demonstrated that the enhancement of the SERS signal by the substrate is based on the synergistic effects of SPR and piezoelectric effect. Due to the nanowire-in-Bowl-shaped structure on the substrate surface, it can grab nanoplastics as well as prevent the entry of microplastics. As a result, we successfully performed Raman detection of the 20 nm PS sphere, when using piezoelectric modulated Raman the detection limit can be increased to 10−4 M. The successful preparation of Ag/CuO NW/BaTiO3@PVDF Bowl-shaped substrates provides a new idea for the detection of nanoplastics smaller than 50 nm and shows enormous potential in promoting environmental protection and monitoring.
Funding
National Natural Science Foundation of China (12174229, 11974222, 11904214, 12004226); Qingchuang Science and Technology Plan of Shandong Province (2021KJ006, 2019KJJ014, 2019KJJ017); Shandong Provincial Natural Science Foundation (ZR2022YQ02, ZR2020QA075); Taishan Scholars Program of Shandong Province (tsqn201812104); China Postdoctoral Science Foundation (2019M662423).
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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