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Abstract
A cavity coupled optical fiber decorated by Ag nanoparticles (AgNPs) as the surface-enhanced Raman scattering (SERS) substrate is discussed, and the localized surface plasmon resonance (LSPR) of AgNPs and a cavity enhancement of a silver capillary simultaneously contribute to Raman enhancement. AgNPs were coated on the tapered fiber surface using multiple cycle light induced deposition technology. During the process, two key parameters including the evaporation time and the deposition recycling times were optimized to find an effective coverage of AgNPs. In addition, the forming mechanism based on different distributions of the temperature field and velocity field was discussed. The effect of the cavity length was analyzed. Experiments with R6G (rhodamine 6G) as analyte show that the limit of detection can be down to 10−11 mol/L, with a total enhancement factor (EF) of ∼109, while an additional cavity coupled EF could be ∼7.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As an enhanced Raman spectroscopy technology, SERS (surface-enhanced Raman scattering) is widely used in biology, medicine, environmental protection, food detection and other fields [1–5].At present, it is generally believed that SERS effect is due to electromagnetic enhancement (EM) and chemical enhancement (CM) [6], in which the electromagnetic enhancement of strong Raman scattering generated by localized surface plasmon resonance (LSPR) of metal nanostructures is dominant [7].
The traditional SERS substrates are various types of noble metal nano structures, especially Au, Ag and Cu [8–11]. In order to be used in some specially applications, such as remote, on-line detection requirements, optical fiber SERS probes are required, due to the long interaction distance between the incident light and analytes, and the flexible property of optical fibers. A D-type optical fiber SERS probe can detect 10−7 mol/L R6G [12]. A gold nanorods decorated the optical fiber using a laser-induced dynamic dip coating method can detect 10−6 mol/L melamine [13]. A layer of gold nanoparticles in the hollow photonic crystal fiber can detect 10−5 mol/L Rhodamine B [14]. A biconical taper fiber coated with silver colloid can detect 10−9 mol/L R6G [15]. A double tapered fiber probe modified with gold nanoparticles can detect 10−9 mol/L R6G [16]. A tapered fiber SERS probe was fabricated by a chemical deposition method, and the limit of detection for R6G can be down to 10−10 mol/L [17,18].
We noticed that the preparation processing of a tapered fiber SERS probe can be further improved for a better Raman enhancement performance. In this paper, we analyzed two key parameters, laser evaporation time and deposition cycling times. The mechanism of AgNPs forming, the characterization, simulations and experiments are carried on in details.
2. Preparation
2.1 Materials and instruments
Multimode fibers (50/125 µm, NA = 0.22), hydrofluoric acid solution (49 wt.%, Shanghai Aladdin Co., Ltd.), 532 nm laser (Beijing Xinglinruiguang Technology Co., Ltd.), JY-11L laser power meter (Shanghai Qiaoyi).
The surface morphology of the prepared samples was characterized by JSM-7800F (JEOL) field emission scanning electron microscope (SEM). Raman signals were recorded by Horiba’s LabRAM HR Evolution Confocal Microscope Raman Spectrometer, with the excitation wavelength of 532 nm, the power of 5 mW, and the integration time of 2 s. LapSpec software was used to remove the background information of Raman spectra.
2.2 Preparation of the silver sol
Synthesis of AgNPs solution: AgNPs were prepared as described by Lee and Meisel [19]. Briefly, 34 mg AgNO3 in 100 ml deionized water was boiled under continuous stirring. Then, 20 mg sodium citrate was added. The mixture was boiled with stirring for about 35 min. After the solution cooled down to room temperature, in order to remove impurity, Ag solution was further processed with additional centrifugal (2000 rpm, 60 min) and ultrasonic vibration once. Finally, the solution was stored at 4 ℃ for further use.
2.3 Preparation of the optical fiber
Preparation of flat end optical fiber: Firstly, a 3 cm length of the coating layer is removed by mechanical stripping, and then its end surface is perpendicularly cut and clean with ethanol in order to remove impurities.
Preparation of tapered end optical fiber: Firstly, the prepared flat end optical fiber is vertically immersed into the mixed solution of HF acid and sunflower seed oil. The seed oil layer at the upper part is used to prevent the optical fiber part from being corroded. After twenty minutes corrosion, the tapered optical fiber with 17 degrees is obtained [18]. Finally, the tapered optical fiber is washed several times with ethanol and vacuum dried for storage.
2.4 Preparation of the optical fiber SERS probe by light induction
Figure 1 illustrates the two-step preparation of optical fiber SERS sensor, including Ag seed deposition and Ag aggregation. Firstly, the fiber substrate was treated by a “dip and dry” procedure. The “dip and dry” procedure means that the fiber was put in Ag sol gel aqueous solution for time of Td, with a 532 nm laser at the power of 1 mW excitation (Figs. 1(a1, a2)). Secondly, under the conditions with laser excitation, the fiber substrate is taken out and dried by laser heating for time of Te (Fig. 1(a3). Then the procedure of AgNPs seed forming is completed (Fig. 1(a4). In order to gain more AgNPs on the fiber surface, the procedure of “dip and dry” is cycled several times (Figs. 1(b1)∼(b4)).
Fig. 1. A two-step process of optical fiber SERS probes with a multiple cycle light induced deposition technology: (a1)∼(a4) the process of light induced seed deposition; (b1)∼(b4) the process of light induced AgNPs aggregation deposition.
In order to carry on some comparative analysis, we prepare some flat/tapered ended optical fiber SERS sensors, named as “F/P-Te-c”, while “F/P” means flat/tapered fiber, “Te” means the laser heating time with a unit of second, and “c” means cycle times, respectively.
2.5 Preparation of the cavity enhanced optical fiber SERS probe samples
Figures 2(a) and 2(b) illustrate the cavity enhanced flat and tapered optical fiber SERS sensors using additional Ag capillary and Ag rod, respectively. The inner and outer diameter Ag capillary is 1mm and 2 mm, and the polished solid silver rod with diameter of 0.8 mm, in order to keep the flow of the analyte. A Cu tube is used to fix the fiber. The cavity length is L.
3. Results and discussion
3.1 SEM characterization
Figures 3(a1-c1) and 3(a2-c2) show the SEM images and particle size distributions of samples F-60-5, F-60-15 and F-60-30 respectively. The calculated surface coverage of AgNPs is 2.6%, 13.8% and 10.8% respectively. Within the same area, the calculated AgNPs number is ∼10, 50 and 80 respectively. Figures 3(d1,e1) and 3(d2,e2) are SEM images and particle size distribution of samples F-30-15 and F-90-15, and the corresponding surface coverage of AgNPs is 2.5% and 5.9% respectively. Among our samples, the parameter Te of 60 s and cycles of 15 could be the optimized parameter values. So we prepared the tapered fiber SERS sensors with Te of 60 s and cycles of 15. Figures 3(f1, f2) are SEM images and particle size distribution of a tapered fiber SERS sensor (sample P-60-15). The coverage of silver particles is 32%, which is an increase compared with 26.67% using a chemical deposition method [18]. Higher Ag coverage can lead to higher LSPR effect, which results in larger Raman intensity.
Fig. 3. (a1)∼(e1)SEM images and (a2)∼(e2)particle size distribution of samples prepared with different Te and “dip and dry” cycles; (f1) SEM image and (f2) particle size distribution of a tapered fiber SERS sensor.
In order to investigate the physical mechanism of AgNPs forming, the COMSOL Multiphysics software was used to analyze the distribution of the temperature and liquid flow. Shown in Fig. 4(a), the laser wavelength was 532 nm, travelled at -z direction and polarized at x direction. A AgNP is set on the SiO2 surface with a diameter of 100 nm. The environmental condition is water. In order to see clearly the internal distributions, we focus one quarter of the structure (part 1) in Fig. 4(a) to analyze.
Fig. 4. (a) Schematic diagram of simulation model, an incident light of 532 nm, Ag diameter of 100 nm. Distributions of (b1), (c1), (d1) temperature field of AgNP; (b2), (c2), (d2) 3D (three- dimension) velocity field of AgNP; (b3), (c3), (d3) 2D (two-dimension) velocity field of AgNP under different irradiation power.
It is noted that the temperature distribution on Ag surface is nonuniform because the thermal conductivity of Ag is higher than that of water solution. In addition, heat is dissipated from Ag surface to the border along three-dimension (3D). Strong temperature gradient around AgNP would produce a density gradient in liquid based on heat-to-hydrodynamic energy conversion property, shown in Figs. 4(b1), 4(c1) and 4(d1). Ag particles would be induced to absorb on the surface with higher temperature. The flow velocity is increasing as the laser power rises, shown in Figs. 4(b2-d2). Evaporation flow is vertically flowing away from Ag, characterized by a 3D internal flow. With the increase of laser incidence time, water near Ag, is evaporated and flows upward, while water in other directions flows to Ag shown in Figs. 4(b3-d3). Therefore, under the synergistic effect of photothermal heating and evaporation flow, AgNPs would be gathered around Ag seeds.
3.2 Raman measurements
Raman intensity of R6G with a concentration of 10−6 mol/L absorbed on different samples is shown in Fig. 5(a). For flat fiber samples, Raman intensity at 611 cm−1 of F-60-15 is the largest one, due to the largest Ag NPs coverage, and it is enhanced by 3.9×, 5.3×, 2.4× and 2.3×, compared with samples F-60-5, F-90-15, F-30-15 and F-60-30, respectively. Raman intensity of R6G with different concentration using F-60-15 as SERS substrate is shown in Fig. 5(b). Shown in Fig. 5(c), for tapered fiber samples, the detection of limit can be down to 10−11 mol/L, one an order of magnitude higher than our previous reports [17,18]. Actually, we compared the performance with some other fiber SERS probes, shown in Table 1.
Fig. 5. (a) Raman spectra of 10−6 mol/L R6G on flat fiber SERS probes in 10−6 mol/L R6G; Raman spectra of R6G with different concentrations on sample (b) F-60-15, and (c) P-60-15.
Table 1. Detection limits of different fiber optic probes
In order to investigate the cavity effect to Raman enhancement, we measured Raman intensity of R6G with concentration of 10−8 mol/L using cavity coupled flat fiber SERS sensors. Note that an integration time is 1 s in Raman measurements. Shown in Fig. 6(a), compared with SERS sensors without a cavity function, the intensity at 611 cm−1 is increased by 7.3×, 4× and 2.8× with the cavity length of 3 mm, 5 mm and 7 mm, respectively.
Fig. 6. Raman spectra of R6G with different cavity lengths on (a) F-60-15 (10−8 mol/L R6G) and (b) P-60-15 (10−11 mol/L R6G).
For cavity coupled tapered fiber SERS sensors, we set an integration time of 2 s. Raman intensity of R6G with concentration of 10−11 mol/L is shown in Fig. 6(b). The intensity at 611 cm-1 is increased by 2.9×, 1.6× and 1.2× with the cavity length of 3 mm, 5 mm and 7 mm, respectively, compared with SERS sensors without a cavity function. We analyzed as follows. (a) The reflection of incident laser on the surface of Ag capillary and Ag rod would enhance Raman scattering. (b) Larger cavity length could lead to a weaker reflection of the incident laser, leading to a smaller Raman intensity.
Analytical enhancement factor (AEF) is calculated by
In our calculation, where CSERS is 10−8 mol/L and 10−11 mol/L R6G on flat and tapered fiber SERS probe, respectively. CRS is unified as 10−2 mol/L R6G on SiO2/Si substrate. ISERS is the corresponding SERS intensity of R6G at 611 cm−1 (CSERS condition). IRS is Raman intensity in 10−2 mol/L R6G at 611 cm−1. The calculated AEF of cavity coupled flat and tapered SERS sensors is 1.87×106 and 1.16×109, respectively.
3.3 Electromagnetic field distributions
Shown in Fig. 7(a), the light is reflected backward by the cavity. COMSOL Multiphysics software was used to calculate the distribution of the electric field (E-field). Based on SEM images and experimental condition, simulation parameters are the same with that in Fig. 4(a). Because the fiber surface is much larger than that of AgNPs, we considered a model of particles deposited on a flat SiO2 substrate, which can model both flat and tapered fiber surface. In order to be understood easily, a simplified model is shown in Fig. 7(b). The theoretical cavity enhancement factor ξ is calculated by the ratio of maximal E-field with the cavity and without the cavity. The calculated theoretical and experimental cavity enhancement factor ξ with the cavity length of 3, 5, 7 mm is shown in Fig. 7(c).
Fig. 7. (a) Schematic diagram of light path in cavity enhanced flat fiber SERS probe; (b) schematic diagram of a simplified simulation model; (c) comparison of theoretical and experimental ξ values at 611 cm−1. The distribution of E-field at length of 3, 5, 7 mm is inserted.
The effect of cavity enhancement mainly comes from the increase of the irradiation light power on AgNPs due to the cavity reflectivity, leading to more LSPR [23–25]. Note that shorter cavity length leads to stronger reflection of incident light, but a shorter cavity is more easily broken during the experiments.
3.4 Raman measurements of the 3D printing cavity coupled SERS sensor
Shown in Fig. 8(a), a 3D printing Ag cavity was prepared by losing wax casting method, with a total length of 30 mm, an internal diameter of 3.2 mm and an outer diameter of 5.2 mm. A multimode optical fiber polymer protective layer has an outer diameter of 3 mm. Two holes of 3×3 mm2 are located on the end face of the cavity as the inlet and outlet of the solution. The bottom thick is 1 mm as the reflecting end.
Fig. 8. (a) Schematic diagram of 3D printing cavity; (b) Raman spectra of R6G on samples with different cavity lengths.
Figure 8(b) shows Raman spectra of 10−7 mol/L R6G on samples with different L. We still noticed that larger cavity length induced smaller cavity enhancement factor. The highest cavity is 5.1× (at 611 cm−1) when the cavity length is 1 mm among our samples. Note that the fiber has a low loss within the wavelength width used in this paper, so the length of the total fiber has a weak effect on Raman enhancement, which indicates that this fiber SERS probe can be used in remote detection applications.
4. Conclusion
Preparation optimized cavity coupled fiber SERS samples were studied in details. The best samples can detect 10−11 mol/L R6G with a high enhancement factor. The cavity EF compared with samples without a cavity can be ∼7×. The sample with a 3D printing cavity show a good performance, which is potential in remote applications.
Funding
National Natural Science Foundation of China (61875024, 62175023); Chongqing Outstanding Youth Fund (cstc2019jcyjjqX0018).
Acknowledgments
We would like to thank Dr. Gong Xiangnan at Analytical and Testing Centre of Chongqing University for his help in Raman measurement.
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.
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