1. Introduction

Due to the high sensitive and selective detection, surface-enhanced Raman scattering (SERS) has been utilized as a potential analytical method to detect molecules, down to singular limit. Over the past two decades, SERS has caused wide attention, consequently, achieving the widespread application in chemical, biological and optical fields. It is generally accepted that SERS mechanisms mainly consist of electromagnetic mechanism (EM) and chemical mechanism (CM) [1–3]. EM which can make the enhancement up to 10¹⁴ attributed to the enhancement in the local electromagnetic field [4, 5]. On the contrary, CM only makes the enhancement reach to 10-100 [6, 7], which can be attributed to the charge transfer between the target molecules and the substrate [8–10]. It has been demonstrated that SERS depends on “hot spots”, namely the spots with strong electromagnetic intensity, to stimulate the molecule Raman signals [11–13]. Among the multiple SERS-based materials, it has been investigated that noble metal nanostructures possess excellent activity. The hot spots originate from the gap of noble metal nanostructures where generates extremely strong electromagnetic field under the excitation of the localized surface plasmon resonance (LSPR) [14–17], so that the Raman enhancement factor can increase several orders of magnitude and even single molecule detection can be achieved. Among these noble metals, Ag nanoparticles (AgNPs) stand out with the excellence of low cost, easy fabrication process and superior property despite with poor chemistry stability.

Besides, 3D nanostructures with large specific area and high porosity also play a critical role in enhancing SERS activities, which enables significant coupling of the LSPR and form abundant hotspots [18, 19]. It has been investigated that pyramid silicon (PSi) can serve as a highly sensitive SERS active substrate due to its large specific area and the porosity structure [20, 21], which can make it easy to fix more probe molecules and capture target molecules in low concentration solution. While great efforts have been made to prepare suitable 3D SERS substrates with high sensitivity, well-distributed uniformity and long-term stability using a low-cost method, it is still a challenge, especially to obtain the 3D flexible SERS substrates that can perform rapid detection and identification of surface analytes.

As a prominent representative of two-dimensional nanomaterials, graphene is chemically inert, highly resistant to oxygen, and strongly transparent to light [22–25]. It has been proposed that a proper laser can forge a graphene sheet into controlled 3D shapes based on laser-induced local expansion of graphene [26]. Compared with graphene, graphene oxide (GO) has superior chemical stability and bio-compatibility, endowing it with the advantageous properties such as chemical enhancement, molecular enrichment, passive protection and fluorescence quenching [25]. What’s more, the hybrid structures associated graphene oxide with other nanomaterials have also presented broad application prospects, especially in SERS. Therefore, to achieve the ideal 3D flexible SERS substrates, combining noble metal nanostructures and flexible SERS substrates with 3D structure has been proposed. To design 3D flexible SERS substrates, assembling noble metal nanostructures onto multifarious nano-frameworks such as polyamide-nanofibers [19] or SiO₂ nanohelices [27] have been reported. Furthermore, replicating nanostructures onto various template surfaces including covering an elastomeric stamp onto a plastic template surface [28] or coating Ag on the polydimethylsiloxane (PDMS) substrate employing textured Taro leaf as template [29] have also been adopted. But these hybrid structures have some weaknesses with limited spatial range or inferior periodicity, which is inappropriate to obtain intense and dense hot spots.

In this work, we prepared the GO/AgNPs/pyramid PMMA 3D flexible substrates with a simple and low-cost method. The PMMA with a low Raman cross-section [30, 31] was applied to form the pyramid microstructure with pyramid Si as a stamp and acted as a supporting layer for deposition of AgNPs. The thin pyramidal film with large specific area can attach dense colloidal plasmonic metal nanoparticles, which will be beneficial for establishing high-density hot spots. Besides, the pyramid structure can make target molecules assemble, and localized plasmon polaritons can be excited by the AgNPs, thus enhancing the Raman scattering of target molecules in close proximity [32]. This GO/AgNPs/pyramid PMMA 3D flexible substrate is capable of providing the excellent adhesion and flexibility, as a consequence, this flexible substrate can directly contact with surface analytes and achieve highly sensitive detection.

2. Experimental section

2.1 Materials

Acetone (CH₃COCH₃, 99.5%), alcohol (C₂H₆O, 99.7%), ethylene glycol (C₂H₆O₂, 99.0%), sodium hydroxide (NaOH), Rhodamine 6G (R6G), crystal violet (CV), and silver nitrate(AgNO₃) were purchased from local chemical plant. Malachite green (MG) and Polymethyl methacrylate (PMMA) were purchased from official website of Aladdin. Polyvinylpyrrolidone (PVP, Mw = 55000) was purchased from Sigma-Alorich. The shrimps were purchased from local supermarket.

2.2 Synthesis of Ag nanoparticles

Ag nanoparticles were synthesized by the means of employing PVP to reduce AgNO₃, according to the method described by Zhang et al [33]. 0.25 g PVP and 0.05 g AgNO₃ were dissolved in 20 ml ethylene glycol with continuous stirring. After that, the mixture was heated to 135 °C and hold for one hour at this temperature to totally react. Then, adding a great quantity of acetone to establish precipitation, the silver can be well separated from the solution. Finally pure Ag nanoparticles were collected by centrifugation with 12000 rpm for five minutes and dispersed in deionized water (DI water).

2.3 Fabrication of GO/AgNPs/pyramid PMMA 3D SERS substrates

The flexible GO/AgNPs/pyramid PMMA substrates were fabricated as illustrated in Fig. 1. Every substrate fabricated the pyramid structure was pretreated through sonication cleaning with acetone, alcohol, DI water. Then the thin film of PMMA was spread over the pyramid Si surface according to the method described by Li et al [34]. To prevent the microstructure of pyramid from being hidden by the PMMA, it is quite essential to guarantee the thickness of the covered PMMA layer thin enough. After the preparation of supporting layer, Ag nanoparticles were deposited on the spread PMMA layer with a dip-coating method. Next, the pyramid Si was etched away using NaOH solution with concentration of 35% to form the flexible pyramid PMMA substrates. Then, 0.1 mg/ml GO synthesized on the basis of the Hummers method [35] was dispersed on the flexible substrate surface by dip-coating method. Herein, the GO/AgNPs/pyramid PMMA substrate was successfully fabricated and ready for the SERS. As a contrast, the AgNPs/PMMA/PSi, and AgNPs/pyramid PMMA substrates were also prepared using the same method.

 

Fig. 1 A diagram illustrating the synthesis procedure of GO/AgNPs/pyramid PMMA 3D flexible SERS substrate.

Download Full Size | PDF

2.4 Characterization

SEM images were characterized by a field-emission scanning electron microscope (ZEISS Gemini Sigma 500). SERS spectra were recorded by a Horiba HR Evolution 800 Raman microscope system using a 50 × objective. 532 nm laser was chose to focus on the sample with the integration time of 8 s and laser power of 0.48 mW throughout the experiment, where the diffraction grid was selected as 1800 gr/mm.

3. Results and discussion

The SEM image in Fig. 2(a) illustrates the surface feature of PSi substrate, which clearly exhibits that the pyramid arrays are well-distributed and have relatively regular size with only minor deviations. The SEM observation in Fig. 2(b) confirms that the PSi substrate is covered uniformly and continuously by a flimsy PMMA layer which still presents the pyramid microstructure with some small cracks perhaps caused by the electron beam in the measuring process. The uniform and continuous PMMA film is greatly propitious for the nanoparticle deposition, further increasing the electromagnetic couple of metal nanoparticles. To give the visual exhibition of AgNPs, the obtained AgNPs were transferred to the silicon wafer and characterized with SEM as shown in Fig. 2(c), where we can detect that a mass of AgNPs relatively evenly disperse on the surface of silicon wafer and present as a single layer. As shown in Fig. 2(d), the average diameter of AgNPs synthesized here is 78.5 nm calculated by the software named Nano Measurer based on the statistics of the size of 700 AgNPs. It is distinct to observe from the Fig. 2(e) and the Fig. 2(f) that the flexible AgNPs/pyramid PMMA substrate and GO/AgNPs/pyramid PMMA substrate still inherit the periodic 3D pyramidal nanostructure, where the well-distributed AgNPs layer can be observed on the PMMA thin film. And the area of every pyramidal nanostructure is approximately 4.5 × 4.5 μm² in the Fig. 2(e). To confirm the existence of GO film, ten active hot spots were randomly selected on the same GO/AgNPs/pyramid PMMA substrate as shown in the insert of (f), where the distinct intensities of D (1350 cm⁻¹) and G (1582 cm⁻¹) bands of GO show generally consistent respectively, revealing the uniform distribution of GO film. Moreover, ten active hot spots selected on the silicon wafer randomly can be shown in Fig. 9(a) in the Appendices, where the two distinct peaks of GO become slightly visible and the peak intensities experience a greatly drop compared with the former, which can be ascribed to the absence of the coupling of GO film and the pyramidal AgNPs layer and indicates the excellent SERS activity of the proposed GO/AgNPs/ pyramid PMMA substrate.

 

Fig. 2 (a) and (b) are the SEM image of PSi substrate and PMMA/PSi substrate respectively. (c) SEM image of Ag nanoparticles deposited on the silicon wafer. (d) The size distribution of AgNPs. (e) and (f) are respectively the SEM of the AgNPs/pyramid PMMA substrate and GO/AgNPs/pyramid PMMA substrate. The insert of (f) is SERS spectrum of the GO film on the GO/AgNPs/ pyramid PMMA substrate.

Download Full Size | PDF

It is well-known that the great enhancement of optical response can be modulated by many parameters such as size, shape and density. Consequently, the distributions of the AgNPs on the PMMA/PSi substrate are investigated by controlling the concentration of the colloid AgNPs to optimize the establishment of hot spots in the gap between the AgNPs. Figure 3(a-f) obviously displays the distributions of AgNPs with concentration of 0.2, 0.8, 1.0, 1.1, 1.15 g/ml, respectively, where we confirm the shape regulation for all nanoparticles on the PMMA/PSi substrate. To ensure the synthesis of colloid AgNPs without visible agglomerates, we prepare the highest concentration at 1.15 g/ml relating to personal craft. The colloids are all bright yellow, presenting a thicker colloid at an elevated concentration. We can also see clearly, with the concentration of the colloid AgNPs increasing from 0.2 to 1.0 g/ml, the region covered by the AgNPs increases and the gap between AgNPs decreases, where the AgNPs almost arrange in one layer. When the concentration reaches to 1.0 g/ml, the surface of the pyramid PMMA almost is fully covered by the uniform AgNPs, which is much beneficial for the creation of the hot spots and high enhancement. However, further increase the concentration, such as 1.1 and 1.15 g/ml, the AgNPs cease to be a uniformly single layer arrangement, instead, some AgNPs aggregate and the gap increases accompanied with the AgNPs covered region decreases. Therefore, we can conclude that the arrangement of the AgNPs during the dip-coating method will be more compact and uniform at an appropriate concentration, while inhomogeneous arrangements may appear at a higher concentration.

 

Fig. 3 (a-f) SEM image of AgNPs with different concentrations at 0.2, 0.8, 1.0, 1.1, 1.15 g/ml respectively on the PMMA/PSi substrate. The insert of (a-f) is respectively the photo of the corresponding colloid concentration.

Download Full Size | PDF

Moreover, to investigate the SERS activity, the SERS spectra of R6G (10⁻⁷ M) on the AgNPs/PMMA/PSi substrates concerning different colloidal concentrations were collected and exhibited in Fig. 4(a). We can clearly observe the relation between the concentration and the SERS activity, which emphasizes the significant role of the colloidal concentration in providing optimal SERS activity. Figure 4(b) shows the Raman intensity of R6G peak at 613 cm⁻¹ as a function of the molecular concentration on the AgNPs/PMMA/PSi substrates fabricated with different colloidal concentrations. The SERS activity gradually enhances with the increase of the concentration from 0.2 to 1.0 g/ml, which may be because that the AgNPs distribute more intensively and the gap between AgNPs decreases with the increase of the concentration, further making it easy to establish high-density hot spots. With the higher concentration, a decreased trend of the Raman peaks at 613 cm⁻¹ was observed. In the preparation process, we need to disperse the silver into DI water by ultrasound. In order to ensure high concentration, we can only add a small amount of water, a small number of who will cause difficult to ultrasound. This results in the agglomeration of the particles, which will lead to the decrease of the hot spots and further weaken the enhancement. With the concentration of 1.0 g/ml, the SEM result shows the best even and dense AgNPs layer compared with other concentrations, therefore, we acquired the highest SERS enhancement.

 

Fig. 4 (a) SERS spectra of R6G (10⁻⁷ M) on the AgNPs/PMMA/PSi substrates fabricated with different colloidal concentrations. (b) The intensity of R6G peak at 613 cm⁻¹ changes as a function of colloidal concentration.

Download Full Size | PDF

To further investigate the SERS activity, we implemented more complete experiments to detect R6G molecules on the AgNPs/PMMA/PSi substrates. The obtained SERS spectra of R6G with various concentrations from 10⁻⁸ to 10⁻¹⁶ M are shown in the Fig. 5(a). And the Raman spectra measured at low concentration as 10⁻¹⁶ M in the Fig. 9(b) in the Appendices is individually illustrated, where we can still obviously observe the main characteristic peaks of R6G molecules including 613, 774, 1185, 1315, 1365, 1508 and 1650 cm⁻¹ despite the detection at a such low concentration. In addition, we also analyzed the intensity of R6G peak at 613 cm⁻¹ changing as a function of concentrations from 10⁻⁸ to 10⁻¹⁶ M in log scale as shown in the Fig. 5(b), where the value of R² reaching 0.966 indicates the excellent linearity. There is no doubt that these demonstrations confirm the high sensitivity and the prominent SERS activity of our prepared substrates. To cater to the social need in the improvement of highly active flexible SERS substrates, the flexible AgNPs/pyramid PMMA substrates were fabricated. Similarly, the Fig. 5(c) shows the SERS spectra of R6G with various concentrations from 10⁻⁵ to 10⁻¹³ M indicating the high SERS activity, and the Raman spectra measured at 10⁻¹³ M in Fig. 9(c) in the Appendices is individually illustrated. And the characteristic peak at 613 cm⁻¹ is observed obviously. Perhaps due to the remnants of PVP and alkali, some miscellaneous peaks are inevitable to emerge in the low concentration detection but their low intensities have little impact on the overall test. Although PMMA has low Raman cross-section, if it happens to be situated in a hotspot its peaks can still appear in the Raman spectrum. The obvious PMMA peaks have not been seen whether rigid or flexible SERS substrates. It may be ascribed that R6G peaks are quite obvious to cover up obtainable PMMA peaks. In the meantime, the intensity of R6G peak at 613 cm⁻¹ changing as a function of concentrations from 10⁻⁵ to 10⁻¹³ M in log scale in the Fig. 5(d) shows the value of R² reaching 0.963. As can be seen from the results, we find out that the SERS activity has fallen compared with the former substrate including Raman intensity and sensitivity. Just as shown in Fig. 5(e), we can obviously observe the decrease of the Raman intensity from the Raman spectra of R6G molecules with 10⁻⁸ M obtained on two different kinds of substrates. It may be attributed that the strong alkali solution can cause damnification to the nanoparticles. Even so the AgNPs/pyramid PMMA substrate still exhibits excellent SERS activity, which can be ascribed to the facts that the well-distributed AgNPs layer presented on the pyramid structure can establish high-density hot spots, and greatly reduce the distance between hot spots and probe molecules. Therefore, the hot spots can be easily accessible to target molecules, and thus enhancing the Raman scattering of target molecules in close proximity.

 

Fig. 5 (a) SERS spectra of R6G with concentrations from 10⁻⁸ to 10⁻¹⁶ M on the AgNPs/PMMA/PSi substrates. (b) The intensity of R6G peak at 613 cm⁻¹ changes as a function of R6G molecule concentration on the AgNPs/PMMA/PSi substrates. (c) SERS spectra of R6G with concentrations from 10⁻⁵ to 10⁻¹³ M on the AgNPs/ pyramid PMMA substrates. (d) The intensity of R6G peak at 613 cm⁻¹ changes as a function of R6G molecule concentration on the AgNPs/pyramid PMMA substrates. (e) The Raman spectra of R6G molecule with 10⁻⁸ M on the AgNPs/PMMA/PSi substrate and the AgNPs/pyramid PMMA substrate. (f) The average value of the intensity of R6G peaks at 613 cm⁻¹ on these two kinds of substrates.

Download Full Size | PDF

Furthermore, to endow the flexible AgNPs/pyramid PMMA substrate with well homogeneity and long-term stability, we proposed the flexible GO/AgNPs/pyramid PMMA substrates to prevent the AgNPs from being oxidized. The Fig. 6(a) exhibits the SERS spectra of R6G with various concentrations from 10⁻⁵ to 10⁻¹³ M, which strikingly shows the greatly enhancement of the Raman intensity compared with the former. It can be ascribed to the charge transfer between the R6G molecules and the GO film [36]. The Fig. 9(d) in the Appendices expresses the SERS spectra of R6G with low concentrations from 10⁻¹⁰ to 10⁻¹³ M distinctly, where we can still detect the typical peaks of the R6G even under such ultralow concentrations. And the characteristic peak of GO at 1582 (G band) cm⁻¹ [37], are also observed as marked in Fig. 9(d). Meanwhile, the Fig. 6(b) shows the value of R² reaching 0.992 on the GO/AgNPs/pyramid PMMA substrates which is much higher than the value of R² on the AgNPs/pyramid PMMA substrates. This is because that the GO as exceptional adsorbent can lead to the distribution of probe molecules becoming more uniform. Meanwhile, we selected three AgNPs/pyramid PMMA substrates and three GO/AgNPs/pyramid PMMA substrates to carry out more evaluation. Five active hot spots were also randomly selected on every AgNPs/pyramid PMMA substrate or GO/AgNPs/pyramid PMMA substrate. Thus fifteen active hot spots on AgNPs/pyramid PMMA substrates and GO/AgNPs/pyramid PMMA substrates respectively shows in the Fig. 6(c) and Fig. 6(e), further suggesting the excellent SERS enhancement of the proposed GO/AgNPs/pyramid PMMA substrates. Owing to the existence of the GO film, the GO/AgNPs/pyramid PMMA substrates display better uniformity, where the intensities of R6G peaks at 613, 774, 1185, 1315, 1365, 1508 and 1650 cm⁻¹ present basically equal respectively. While for the AgNPs/pyramid PMMA substrates, the distribution of probe molecules will be uneven since the absence of GO film, leading to the weak uniformity of Raman signals as shown in the Fig. 6(d). With the combination of homogeneous AgNPs layer and the GO film, one not only can make the hot spots distribute uniformly resulting in the active coupling of electromagnetic field, but also promotes the hot spots effectively accessible to probe molecules to stimulate strong SERS signals. All the evidences demonstrate the existence of GO film can lead to the more homogeneous SERS substrates [Fig. 6(f)] and the enhancement of SERS signals due to its unique chemical properties. At the same time, we also implemented experiments to detect CV molecules with concentrations from 10⁻⁵ to 10⁻⁹ M on the GO/AgNPs/pyramid PMMA substrates to further deomonstrate the excellent SERS performance of this substrate. The recorded Raman spectra in Fig. 10(a) in the Appendices shows the characteristic peaks of CV molecules at 223, 422, 523, 730, 915, 1178, 1372, 1533, 1588 and 1621 cm⁻¹ clearly. We can see the characteristic peak intensity of the CV become indistinct while the characteristic peak intensities of the GO become obvious with the low concentration at 10⁻⁹ M. The linearity exhibited in Fig. 10(b) in the Appendices, also suggests the excellent linear response and the good capability in quantitative detection.

 

Fig. 6 (a) SERS spectra of R6G with concentrations from 10⁻⁵ to 10⁻¹³ M on the GO/AgNPs/pyramid PMMA substrates. (b) The intensity of R6G peak at 613 cm⁻¹ changes as a function of R6G molecule concentration on the GO/AgNPs/pyramid PMMA substrates. (c) and (e) are respectively the SERS spectra of R6G (10⁻⁶ M) from 15 spots dispensed on three AgNPs/pyramid PMMA substrates and three GO/AgNPs/pyramid PMMA substrates. (d) and (f) are respectively illustrate the intensity distribution of R6G peaks from 15 spots. The red lines represent the average intensity of R6G peaks at 613 cm⁻¹ from 15 spots.

Download Full Size | PDF

In addition, the enhancement factor (EF) for flexible GO/AgNPs/pyramid PMMA substrate is calculated based on the follow standard equation:

Besides the high sensitivity and well homogeneity, the temporal stability of the Raman signals from the SERS substrates is another vital parameter in detection. Thus the stability of SERS signals from the GO/AgNPs/pyramid PMMA substrate was investigated as shown in the Fig. 7(a) and Fig. 7(b). The GO/AgNPs/pyramid PMMA substrate and the AgNPs/pyramid PMMA substrate were exposed to the ambient air for a month as a comparison. We randomly selected 20 spots on these samples respectively, and estimated the average intensity of R6G peaks at 613 cm⁻¹ illustrated in the Fig. 7(c). For the AgNPs/pyramid PMMA substrate, the intensity of R6G peaks at the same position exhibits a drastic drop, which reveals the weak stability of AgNPs/pyramid PMMA substrate due to the oxidation of AgNPs as the absence of the protection from GO. In contrast, the Raman signal intensity on the GO/AgNPs/pyramid PMMA substrate was almost invariant. This phenomenon can be owe to the existence of the GO film, which can be served as an atomic diffusion barrier to effectively prevent oxygen from entering the AgNPs layer surface and protect the AgNPs/pyramid PMMA substrate from being oxidized. Based on these results, we can conclude that, besides the high sensitivity and well homogeneity, the proposed GO/AgNPs/pyramid PMMA substrate also possesses excellent stability.

 

Fig. 7 Measured Raman spectra from freshly fabricated (a) AgNPs/pyramid PMMA substrate and (b) GO/AgNPs/pyramid PMMA substrate and those exposed to ambient air for a month. (c) Average value of the intensity of R6G peaks at 613 cm⁻¹ from freshly fabricated two samples and those exposed to ambient air for a month.

Download Full Size | PDF

To demonstrate the feasibility of the GO/AgNPs/pyramid PMMA substrates in practical applications, we carried out in situ detection of the MG on the shrimp surface [Fig. 8(a) and Fig. 8(b)]. MG is a synthetic industrial dye, which can be used to treat parasites, fungal or bacterial infections in fish body to improve the survival rate of aquatic products. But MG of environmental residues can be sustained for a long time, and the chemical functional groups of triphenylmethane have been confirmed with poisonous effects of high toxic and high residue [44]. Consequently, achieving the effective detection of the MG is greatly beneficial for human health. Figure 8(c) shows no evident Raman signals can be detected when 532 nm laser directly irradiates onto the clean shrimp surface or the shrimp surface immersed in 10⁻⁵ M MG solution for 12 hours. On the contrast, the two characteristic peaks of MG can be recorded when the flexible GO/AgNPs/pyramid PMMA substrate was stuck onto the clean shrimp surface. With the flexible substrate coating on the shrimp surface immersed in 10⁻⁷ M MG solution for 12 hours, an obvious Raman signal for MG appeared at 1174 cm⁻¹. On this occasion, in situ lowest detection of the MG on the shrimp surface was acquired with the concentration as low as 10⁻⁷ M. In this experiment, it is indispensable to use flexible substrate with the side of positive pyramid to touch the shrimp surface. Due to the outstanding transparency of the PMMA, the laser can easily penetrate through the PMMA layer to access the AgNPs layer, resulting in motivating the plasmon resonance of the AgNPs layer near the pyramid tip region [45], and then the hot spots are effectively amplification of the Raman signals. A series of detection strongly proves the practical significance of the flexible GO/AgNPs/pyramid PMMA substrates in detecting surface analytes rapidly.

 

Fig. 8 (a) and (b) are the photos of the in situ detection of the MG on shrimp surface. (c) SERS spectra of MG on shrimp surface.

Download Full Size | PDF

4. Conclusions

In summary, we have succeeded in developing a simple method to fabricate the 3D flexible GO/AgNPs/pyramid PMMA substrate. All the demonstrations have confirmed this SERS substrate with high sensitivity, homogeneity and stability due to the high-density hot spots excited by the intense and uniform plasmonic AgNPs, the strong field enhancement benefited from the pyramid structure and the rewarding properties of GO film for the SERS, which enables the EF up to 8.1 × 10⁹. In order to evaluate the ability for the real practical application, the in situ detection of the MG on the shrimp surface was achieved. These results reveal that this flexible SERS substrate can be expected to apply in food safety and extend to the other applications.

Appendices

 

Fig. 9 (a) SERS spectrum of the GO film on the silicon wafer. (b) SERS spectra of R6G (10⁻¹⁶ M) on the AgNPs/PMMA/PSi substrate. (c) SERS spectra of R6G (10⁻¹³ M) on the AgNPs/pyramid PMMA substrate. (d) SERS spectra of R6G with concentrations from 10⁻¹⁰ to 10⁻¹³ M on the GO/AgNPs/pyramid PMMA substrate.

Download Full Size | PDF

 

Fig. 10 (a) Raman spectra of CV with concentrations from 10⁻⁵ to 10⁻⁹ M on the GO/AgNPs/pyramid PMMA substrates. (b) The intensity of CV peak at 915 cm⁻¹ changes as a function of concentration on the GO/AgNPs/pyramid PMMA substrates.

Download Full Size | PDF

Funding

Shandong Province Natural Science Foundation (ZR2013AQ012, ZR2017BA004, ZR2016AM19); National Natural Science Foundation of China (NSFC) (11774208, 11747072, 11474187, 11674199, 11604040); China Postdoctoral Science Foundation (2016M602716); Undergraduate research fund project of Shandong Normal University (2017BKSKY43).

References and links

1. E. C. Le Ru, E. Blackie, M. Meyer, and P. G. Etchegoin, “Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study,” J. Phys. Chem. C 111(37), 13794–13803 (2007). [CrossRef]  

2. M. Banik, A. Nag, P. Z. El-Khoury, A. Rodriguez Perez, N. Guarrotxena, G. C. Bazan, and V. A. Apkarian, “Surface-Enhanced Raman Scattering of a Single Nanodumbbell: Dibenzyldithio-Linked Silver Nanospheres,” J. Phys. Chem. C 116(18), 10415–10423 (2012). [CrossRef]  

3. M. Moskovits, “Persistent misconceptions regarding SERS,” Phys. Chem. Chem. Phys. 15(15), 5301–5311 (2013). [CrossRef]   [PubMed]  

4. S. Nie and S. R. Emory, “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science 275(5303), 1102–1106 (1997). [CrossRef]   [PubMed]  

5. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, “Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS),” Phys. Rev. Lett. 78(9), 1667–1670 (1997). [CrossRef]  

6. D. Paria, K. Roy, H. J. Singh, S. Kumar, S. Raghavan, A. Ghosh, and A. Ghosh, “Ultrahigh Field Enhancement and Photoresponse in Atomically Separated Arrays of Plasmonic Dimers,” Adv. Mater. 27(10), 1751–1758 (2015). [CrossRef]   [PubMed]  

7. P. Wang, O. Liang, W. Zhang, T. Schroeder, and Y. H. Xie, “Ultrasensitive Graphene-Plasmonic Hybrid Platform for Label-Free Detection,” Adv. Mater. 25(35), 4918–4924 (2013). [CrossRef]   [PubMed]  

8. R. Esteban, A. G. Borisov, P. Nordlander, and J. Aizpurua, “Bridging quantum and classical plasmonics with a quantum-corrected model,” Nat. Commun. 3(825), 825 (2012). [CrossRef]   [PubMed]  

9. M. Banik, P. Z. El-Khoury, A. Nag, A. Rodriguez-Perez, N. Guarrottxena, G. C. Bazan, and V. A. Apkarian, “Surface-Enhanced Raman Trajectories on a Nano-Dumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse,” ACS Nano 6(11), 10343–10354 (2012). [CrossRef]   [PubMed]  

10. M. Banik, V. A. Apkarian, T.-H. Park, and M. Galperin, “Raman Staircase in Charge Transfer SERS at the Junction of Fusing Nanospheres,” J. Phys. Chem. Lett. 4(1), 88–92 (2013). [CrossRef]   [PubMed]  

11. C. Gao, Y. Hu, M. Wang, M. Chi, and Y. Yin, “Fully alloyed Ag/Au nanospheres: combining the plasmonic property of Ag with the stability of Au,” J. Am. Chem. Soc. 136(20), 7474–7479 (2014). [CrossRef]   [PubMed]  

12. S. Yampolsky, D. A. Fishman, S. Dey, E. Hulkko, M. Banik, E. O. Potma, and V. A. Apkarian, “Seeing a single molecule vibrate through time-resolved coherent anti-Stokes Raman scattering,” Nat. Photonics 8(8), 650–656 (2014). [CrossRef]  

13. K. T. Crampton, A. Zeytunyan, A. S. Fast, F. T. Ladani, A. Alfonso-Garcia, M. Banik, S. Yampolsky, D. A. Fishman, E. O. Potma, and V. A. Apkarian, “Ultrafast Coherent Raman Scattering at Plasmonic Nanojunctions,” J. Phys. Chem. C 120(37), 20943–20953 (2016). [CrossRef]  

14. K. A. Willets and R. P. Van Duyne, “Localized Surface Plasmon Resonance Spectroscopy and Sensing,” Annu. Rev. Phys. Chem. 58(1), 267–297 (2007). [CrossRef]   [PubMed]  

15. K. Liu, Y. Bai, L. Zhang, Z. Yang, Q. Fan, H. Zheng, Y. Yin, and C. Gao, “Porous Au-Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis,” Nano Lett. 16(6), 3675–3681 (2016). [CrossRef]   [PubMed]  

16. D. Z. Lin, Y. P. Chen, P. J. Jhuang, J. Y. Chu, J. T. Yeh, and J. K. Wang, “Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates,” Opt. Express 19(5), 4337–4345 (2011). [CrossRef]   [PubMed]  

17. C. Zhang, C. H. Li, J. Yu, S. Z. Jiang, S. C. Xu, C. Yang, Y. J. Liu, X. G. Gao, and B. Y. Man, “SERS activated platform with three-dimensional hot spots and tunable nanometer gap,” Sensor. Actuat. Biol. Chem. 258, 163–171 (2018).

18. X. Zhang, X. Xiao, Z. Dai, W. Wu, X. Zhang, L. Fu, and C. Jiang, “Ultrasensitive SERS performance in 3D “sunflower-like” nanoarrays decorated with Ag nanoparticles,” Nanoscale 9(9), 3114–3120 (2017). [CrossRef]   [PubMed]  

19. Z. G. Dai, F. Mei, X. H. Xiao, L. Liao, L. Fu, J. Wang, W. Wu, S. S. Guo, X. Y. Zhao, W. Li, F. Ren, and C. Jiang, “Rings of saturn-like” nanoarrays with high number density of hot spots for surface-enhanced Raman scattering,” Appl. Phys. Lett. 105(3), 033515 (2014). [CrossRef]  

20. C. Zhang, S. Z. Jiang, Y. Y. Huo, A. H. Liu, S. C. Xu, X. Y. Liu, Z. C. Sun, Y. Y. Xu, Z. Li, and B. Y. Man, “SERS detection of R6G based on a novel graphene oxide/silver nanoparticles/silicon pyramid arrays structure,” Opt. Express 23(19), 24811–24821 (2015). [CrossRef]   [PubMed]  

21. C. Zhang, B. Y. Man, S. Z. Jiang, C. Yang, M. Liu, C. S. Chen, S. C. Xu, H. W. Qiu, and Z. Li, “SERS detection of low-concentration adenosine by silver nanoparticles on silicon nanoporous pyramid arrays structure,” Appl. Surf. Sci. 347, 668–672 (2015). [CrossRef]  

22. Z. G. Dai, X. H. Xiao, W. Wu, Y. P. Zhang, L. Liao, S. S. Guo, J. J. Ying, C. X. Shan, M. T. Sun, and C. Z. Jiang, “Plasmon-driven reaction controlled by the number of graphene layers and localized surface plasmon distribution during optical excitation,” Light Sci. Appl. 4(10), e342 (2015). [CrossRef]  

23. B. Man, S. Xu, S. Jiang, A. Liu, S. Gao, C. Zhang, H. Qiu, and Z. Li, “Graphene-Based Flexible and Transparent Tunable Capacitors,” Nanoscale Res. Lett. 10(1), 279 (2015). [CrossRef]   [PubMed]  

24. S. Borini, R. White, D. Wei, M. Astley, S. Haque, E. Spigone, N. Harris, J. Kivioja, and T. Ryhänen, “Ultrafast graphene oxide humidity sensors,” ACS Nano 7(12), 11166–11173 (2013). [CrossRef]   [PubMed]  

25. A. M. Dimiev and J. M. Tour, “Mechanism of graphene oxide formation,” ACS Nano 8(3), 3060–3068 (2014). [CrossRef]   [PubMed]  

26. A. Johansson, P. Myllyperkiö, P. Koskinen, J. Aumanen, J. Koivistoinen, H. C. Tsai, C. H. Chen, L. Y. Chang, V. M. Hiltunen, J. J. Manninen, W. Y. Woon, and M. Pettersson, “Optical Forging of Graphene into Three-Dimensional Shapes,” Nano Lett. 17(10), 6469–6474 (2017). [CrossRef]   [PubMed]  

27. R. Tamoto, S. Lecomte, S. Si, S. Moldovan, O. Ersen, M. Delville, and R. Oda, “Gold Nanoparticle Deposition on Silica Nanohelices: A New Controllable 3D Substrate in Aqueous Suspension for Optical Sensing,” J. Phys. Chem. C 116(43), 23143–23152 (2012). [CrossRef]  

28. K. Sivashanmugan, J. D. Liao, B. H. Liu, C. K. Yao, and S. C. Luo, “Ag nanoclusters on ZnO nanodome array as hybrid SERS-active substrate for trace detection of malachite green,” Sensor. Actuat. Biol. Chem. 207, 430–436 (2015).

29. P. Kumar, R. Khosla, M. Soni, D. Deva, and S. K. Sharma, “A highly sensitive, flexible SERS sensor for malachite green detection based on Ag decorated microstructured PDMS substrate fabricated from Taro leaf as template,” Sensor. Actuat. Biol. Chem. 246, 477–486 (2017).

30. C. P. Hagan, J. F. Orr, C. A. Mitchell, and N. J. Dunne, “Real time monitoring of the polymerisation of PMMA bone cement using Raman spectroscopy,” J. Mater. Sci. Mater. Med. 20(12), 2427–2431 (2009). [CrossRef]   [PubMed]  

31. L. Jiao, B. Fan, X. Xian, Z. Wu, J. Zhang, and Z. Liu, “Creation of Nanostructures with Poly(methyl methacrylate)-Mediated Nanotransfer Printing,” J. Am. Chem. Soc. 130(38), 12612–12613 (2008). [CrossRef]   [PubMed]  

32. M. D. Sonntag, E. A. Pozzi, N. Jiang, M. C. Hersam, and R. P. Van Duyne, “Recent Advances in Tip-Enhanced Raman Spectroscopy,” J. Phys. Chem. Lett. 5(18), 3125–3130 (2014). [CrossRef]   [PubMed]  

33. F. Zhang, G. B. Braun, Y. Shi, Y. Zhang, X. Sun, N. O. Reich, D. Zhao, and G. Stucky, “Fabrication of Ag@SiO(2)@Y(2)O(3):Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles,” J. Am. Chem. Soc. 132(9), 2850–2851 (2010). [CrossRef]   [PubMed]  

34. C. H. Li, C. Yang, S. C. Xu, C. Zhang, Z. Li, X. Y. Liu, S. Z. Jiang, Y. Y. Huo, A. H. Liu, and B. Y. Man, “Ag₂O@Ag core-shell structure on PMMA as low-cost and ultra-sensitive flexible surface-enhanced Raman scattering substrate,” J. Alloys Compd. 695, 1677–1684 (2017). [CrossRef]  

35. C. Zhang, Z. Li, S. Z. Jiang, C. H. Li, S. C. Xu, J. Yu, Z. Li, M. H. Wang, A. H. Liu, and B. Y. Man, “U-bent fiber optic SPR sensor based on graphene/AgNPs,” Sensor. Actuat. Biol. Chem. 251, 127–133 (2017).

36. S. Xu, B. Man, S. Jiang, J. Wang, J. Wei, S. Xu, H. Liu, S. Gao, H. Liu, Z. Li, H. Li, and H. Qiu, “Graphene/Cu Nanoparticle Hybrids Fabricated by Chemical Vapor Deposition As Surface-Enhanced Raman Scattering Substrate for Label-Free Detection of Adenosine,” ACS Appl. Mater. Interfaces 7(20), 10977–10987 (2015). [CrossRef]   [PubMed]  

37. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene layers,” Phys. Rev. Lett. 97(18), 187401 (2006). [CrossRef]   [PubMed]  

38. C. Zhang, S. Z. Jiang, C. Yang, C. H. Li, Y. Y. Huo, X. Y. Liu, A. H. Liu, Q. Wei, S. S. Gao, X. G. Gao, and B. Y. Man, “Gold@silver bimetal nanoparticles/pyramidal silicon 3D substrate with high reproducibility for high-performance SERS,” Sci. Rep. 6(1), 25243 (2016). [CrossRef]   [PubMed]  

39. O. Guselnikova, D. P. Postnikov, Y. Kalachyova, D. Z. Kolska, M. Libansky, P. J. Zima, P. V. Svorcik, and D. O. Lyutakov, “Large-Scale, Ultrasensitive, Highly Reproducible and Reusable Smart SERS Platform Based on PNIPAm-Grafted Gold Grating,” Chem. Nano Mat. 3(2), 135–144 (2017).

40. D. He, B. Hu, Q. F. Yao, K. Wang, and S. H. Yu, “Large-Scale Synthesis of Flexible Free-Standing SERS Substrates with High Sensitivity: Electrospun PVA Nanofibers Embedded with Controlled Alignment of Silver Nanoparticles,” ACS Nano 3(12), 3993–4002 (2009). [CrossRef]   [PubMed]  

41. Y. Kalachyova, D. Mares, V. Jerabek, P. Ulbrich, L. Lapcak, V. Svorcik, and O. Lyutakov, “Ultrasensitive and reproducible SERS platform of coupled Ag grating with multibranched Au nanoparticles,” Phys. Chem. Chem. Phys. 19(22), 14761–14769 (2017). [CrossRef]   [PubMed]  

42. C. Xu and X. Wang, “Fabrication of Flexible Metal-Nanoparticle Films Using Graphene Oxide Sheets as Substrates,” Small 5(19), 2212–2217 (2009). [CrossRef]   [PubMed]  

43. W. Xu, X. Ling, J. Xiao, M. S. Dresselhaus, J. Kong, H. Xu, Z. Liu, and J. Zhang, “Surface enhanced Raman spectroscopy on a flat graphene surface,” Proc. Natl. Acad. Sci. U.S.A. 109(24), 9281–9286 (2012). [CrossRef]   [PubMed]  

44. R. Ahmad and R. Kumar, “Adsorption studies of hazardous malachite green onto treated ginger waste,” J. Environ. Manage. 91(4), 1032–1038 (2010). [CrossRef]   [PubMed]  

45. L. B. Zhong, J. Yin, Y. M. Zheng, Q. Liu, X. X. Cheng, and F. H. Luo, “Self-assembly of Au nanoparticles on PMMA template as flexible, transparent, and highly active SERS substrates,” Anal. Chem. 86(13), 6262–6267 (2014). [CrossRef]   [PubMed]