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Abstract
With the development of surface enhanced fluorescence (SEF) spectroscopy technology, uniform and low-cost SEF substrate is urgently needed. In this paper, the nanocomposite films of poly (vinyl alcohol) (PVA) embedded with in-situ Au particles, their localized surface plasmon resonance (LSPR) bands locate at different wavelengths from 525 nm to 569 nm, were used as substrates to enhance the fluorescence of rhodamine 6 G (R6G). The results shows that the uniform light emission in large area can be measured, and the maximum enhancement factor (EF) is about 13 folds. With increasing concentration of R6G films, the EF first increases and then slowly decreases. It is demonstrated that the EF greatly depends on the matching degree of the emission/excitation of R6G and the LSPR band of PVA-Au substrate. All the results further suggests that the PVA-Au substrate not only realize the fluorescence enhancement but also attenuates the fluorescence quenching at higher concentration. In addition, the local electric distribution of the substrate is simulated by using three-dimensional finite different time-domain (FDTD) to further demonstrate the mechanism of the SEF. This substrate has good development prospects in the fields of fluorescent probes and fluorescence imaging, which can be beneficial to the development of uniform and low-cost SEF substrate.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fluorescence can be used in many fields, including optoelectronic devices [1], biological labeling [2–5] and fluorescence detection [6–8]. Surface enhanced fluorescence (SEF) is mostly realized by the localized surface plasmon resonance (LSPR) effect of plasmonic nanoparticles (NPs). LSPR of plasmonic NPs can improve fluorescence by excitation enhancement and emission enhancement. Meanwhile, fluorescence quenching is dominated as the distance between fluorophore molecules and the adjacent plasmonic NPs is reduced to a few nanometers. Therefore, the fluorescence enhancement of fluorophore molecules is the result of a competition between plasmonic enhancement and quenching. The obviously fluorescence enhancement would be obtained for the well designed plasmonic nanostructures.
Rhodamine 6 G (R6G) is a strong fluorescent rhodamine family dye, which belongs to common cationic dyes with strong visible light absorption. It is one of the most commonly used probes in experimental research due to its fluorescence, absorption, and emission in the visible range of 500–600 nm, as well as favorable lasing properties. Because of the excellent optical properties of R6G, it is widely used in fluorescence probes [9], solar cells [10–12] and other fields [13,14]. Meanwhile, R6G also has disadvantages such as low luminous efficiency and fluorescence yield, so there are many reports on the realization of SEF of R6G. Rachael Knoblauch et al. prepared a new silver cone-bottom platform for improved detection of R6G at micromolar concentrations [15]. Akhilesh Kumar Gupta et al. prepared defect-free ZnO nanohexagonal rods (ZnO-NH) structures decorated with Au nanoparticles (Au NPs), due to the large surface area of the uniaxial rod shape and the powerful plasmonic between metal and ZnO coupled, the fluorescence signal of R6G can be effectively detected [16]. Andrea Stefancu et al. enhanced the fluorescence of R6G and led to an increase in the total decay rate of excited R6G by changing the chemical interface of Ag nanoparticles (Ag NPs) with halide ions [17]. Hongwen Cao et al. prepared a low-cost, environmentally friendly and stable SEF substrate, namely Ag@razor clam (Ag@RC), for enhanced fluorescence by magnetron sputtering technology. R6G has maximum enhancement factor (EF) of 1574 with a small relative standard deviation (RSD) [18]. Yulia Borodaenko et al. employed direct fs-laser processing to generate light-absorbing surface textures on silicon locally functionalized with photoluminescent R6G nanolayers. The surface texture enhances the photoluminescence (PL) signal of the R6G nanolayer, which can be used to realize various sensing devices [19]. However, much effort has been devoted to maximizing enhancement rather than addressing the uniformity enhancement. The uniform and low-cost SEF substrate used for fluorescence enhancement is crucial for the practical application and urgently needed. Meanwhile, as the development of science and technology, the demand on wearable and transparent equipment covering all domains for comfortable material life is growing. The flexibility of SEF substrates should be a developing trend.
In this paper, the nanocomposite films of poly (vinyl alcohol) (PVA) embedded with in situ generated Au NPs were conveniently prepared, which can be used as flexible substrates to enhance the fluorescence of R6G. The experiments demonstrated that this substrate can realize uniform fluorescence in large area, and the EF can reach to 13. With the increasing concentration of R6G films, EF first increases and then slowly decreases. The results show that EF largely depends on the matching degree between the emission/excitation degree of R6G and the LSPR band of PVA-Au substrate. All the results further show that PVA-Au substrate not only enhances the fluorescence, but also weakens the fluorescence quenching at a higher concentration. In addition, three-dimensional finite different time-domain (FDTD) is used to simulate the local power distribution of substrate, which further proves the action mechanism of SEF. This uniform, flexible and low-cost SEF substrate is expected to be a powerful tool for large-scale fabrication of fluorescence-enhanced detection platforms.
2. Materials and methods
2.1 Materials
Polyvinyl alcohol (PVA) was purchased from Kermel. Tetrachloroauric (III) acid trihydrate (HAuCl4, 99.9% Au 50%) and rhodamine 6 G (R6G, 95%) was purchased from Innochem.
2.2 Synthesis of SEF substrates
Silicon wafers were pre-treated with ultrapure water, acetone, and ethanol in an ultrasonic bath, and then dried with nitrogen. Figure 1 shows the specific preparation process of the samples. The Au NPs were prepared by in-situ growth method, and the HAuCl4 was adjusted to prepare Au NPs with suitable resonance positions from 525 nm to 569 nm. To get pure 2 wt.% of pure PVA solution, 1 g of the PVA was added to the 50 ml of distilled water then stirred at 100 °C. The mixture was stirred until the solution became clear. Subsequently, the solution was cooled to 50 °C, and the HAuCl4 was added. The content of HAuCl4 in PVA were 0.1 wt.%, 0.2 wt.%, 0.5 wt.%, 0.8 wt.% and 1.0 wt.% respectively. The mixture was stirred in a 50 °C water bath for 48 hours. In the process, the formation of Au NPs occurred by reducing Au3+ ions to Au° with PVA polymer at 50 °C [20,21]. In order to obtain the PVA-Au nanocomposite film, 1 ml of the prepared PVA-Au solution was spin-coated on the cleaned silicon wafer and heated in a mixed box tube furnace at 400 °C for 1 h. The nanocomposite films were obtained as SEF substrates.
2.3 Synthesis of R6G films
To get the R6G films, different concentrations (2.0 × 10−8 mol/L, 1.0 × 10−7 mol/L, 2.0 × 10−7 mol/L, 2.0 × 10−6 mol/L, 1.0 × 10−5 mol/L, 2.0 × 10−5 mol/L, 4.2 × 10−5 mol/L, 6.3 × 10−5 mol/L and 8.4 × 10−5 mol/L) of R6G and 2 wt.% PVA were mixed in volume ratio of 1:1 and stirred for 1 h. 1 ml of the prepared mixture solution was spin coating at 3000 rpm for 60 s, and then baked at 100 °C for 30 min to get the R6G films, the thickness of the film was about 24 nm.
2.4 Characterization
The film thickness was measured by a M-2000VI Ellipsometer. The distribution of PVA-Au nanocomposites film was estimated by Bruker Dimension Icon Atomic Force Microscopy (AFM) and Scanning Electron Microscope (SEM) images on a ZISS sigma 300 electron microscope. The absorption spectra of the SEF substrate were recorded by U-3310 UV-visible spectrophotometer produced by Hitachi high tech company. Fourier Transform Infrared Spectroscopy (FTIR) spectra of the substrates were recorded by using IRTracer-100 in the range 500–4000 cm-1 with a resolution of 4 cm-1. The PL spectra and the fluorescence decay curves of samples were measured by Edinburgh FLS920 spectrophotometer system. The fluorescence mapping of samples was measured by High-Resolution Spatial Current-Spectral Measurements.
3. Result and discussion
Figure 2 shows the optical properties of R6G. Figure 2(a) shows the molecular structure of R6G. The molecule consists of two chromophores, a dibenzopyrene chromophore (xanthene) and an R6G carboxyphenyl tilted about 90 ° relative to the xanthene ring. Therefore, the π-system of the two chromophores of R6G is not conjugated, resulting the excitation wavelength to be closer to the emission wavelength [22]. Figure 2(b) shows the absorption and the emission of R6G under excitation of 532 nm. As shown of the blue line in Fig. 2(b), R6G has a main absorption band located at 527 nm and a weak absorption band located at 347 nm, respectively. And the red line shows the emission band which peak located at 560 nm under excitation of 532 nm.
Fig. 2. (a) Molecular structure diagram of R6G. (b) The absorption spectrum of R6G (blue line) and the emission spectrum of R6G (red line) under the excitation of 532 nm.
The main properties of SEF substrate PVA-Au film used in this work are shown in Fig. 3. Figure 3(a) and (b) are the AFM images of the pure PVA film and the PVA-Au film located at 560 nm respectively. It can be seen that the surface of the pure PVA film is relatively smooth, while many bumps are generated when the PVA-Au film is formed. To get further information about these bumps, the SEM image of PVA-Au film is shown in Fig. 3(c). And the SEM image of pure PVA film is shown in Supplement 1, Fig. S1. Compared with the pure PVA film, it can be seen that the bumps on the surface of the PVA-Au nanocomposite film are composed by several Au NPs, and the distribution of the bumps is relatively uniform. The size distribution of PVA-Au film is shown in Fig. 3(d). The average diameter of Au NPs is about 28.2 ± 0.85 nm. Figure 3(e) shows the absorption of pure PVA and PVA-Au films with different content of HAuCl4 in PVA, respectively. The absorption of pure PVA film is relatively weak and has no resonant absorption band. While the PVA-Au film has a wide plasmon resonance absorption band, further indicating the formation of Au NPs. And when the mass ratio of HAuCl4 to PVA is 0.1 wt.%, 0.2 wt.%, 0.5 wt.%, 0.8 wt.% and 1.0 wt.%, the resonance absorption peaks are located at 525 nm, 536 nm, 545 nm, 560 nm and 569 nm, respectively. With the increase of HAuCl4, the peak of the resonance absorption band of Au NPs is red-shifted. The AFM images of other PVA-Au films with different resonance positions at 525 nm, 536 nm, 545 nm and 569 nm are also obtained, which are shown in Fig. S2. As shown, similar with the PVA-Au film located at 560 nm, there are also many bumps generated on the other PVA-Au films.
Fig. 3. The main properties of PVA-Au film. (a) AFM image of PVA film. (b) AFM image of PVA-Au film located at 560 nm. (c) SEM image of the corresponding PVA-Au film. (d) Size distribution of Au NPs in PVA-Au film. (e) The absorption of PVA and PVA-Au film.
To further demonstrate the properties of PVA-Au film, FTIR spectra is performed to present the intermolecular interaction between the Au NPs and PVA matrix. Figure 4 shows the FTIR analysis of pure PVA and PVA-Au films. The broadband at 3323 cm-1 corresponds to the O–H stretching vibrations of the pure PVA film. Compared with the pure PVA film, it is smoother and wider for the PVA-Au film due to the interaction between Au NPs and the matrix at the O–H group [23]. The band at 1409 cm-1 is due to the C–H wagging vibrations and the band at 1232 cm-1 is attributed to the in-plane vibrations of O–H group. The absorption band at 850 cm-1 is due to out-of-plane vibrations of C–H group [24]. For the bands at 1409 cm-1 and 850 cm-1, the peak positions of the PVA-Au film are slightly shifted compared with the pure PVA film. The inset shows the slightly shifts in the peak position at 850 cm-1 for different mass percentages of PVA-Au films. All of these results demonstrate that the PVA-Au films show some irregular shifts in peak positions and intensities implying the strong interactions between Au NPs and O–H groups of PVA through intra/intermolecular hydrogen bonding. Therefore, the change in the infrared spectrum further indicates the presence of Au NPs.
Fig. 4. FTIR spectra of pure PVA and PVA-Au films located at 525 nm, 536 nm, 545 nm, 560 nm and 569 nm, respectively.
To demonstrate the SEF effect of the PVA-Au substrate, R6G films were deposited on the PVA-Au nanocomposite substrates, and the fluorescence properties of R6G films with different concentrations with and without the PVA-Au substrate were measured. For pure R6G films, almost no fluorescence can be detected when the concentration is lower than 2.0 × 10−8 mol/L. Therefore, the lowest concentration of R6G film used is 2.0 × 10−8 mol/L, and the highest concentration of R6G could reach 8.4 × 10−5 mol/L. The corresponding fluorescence spectra are shown in Fig. 5(a). For pure R6G films, when the concentration increases from 2.0 × 10−8 mol/L to 8.4 × 10−5 mol/L, the fluorescence intensity first increases and then decreases with the increasing of concentration. For the trend of fluorescence intensity with concentration, we attribute it to the aggregation of R6G at higher concentrations. It is well known that R6G begin to form dimers, which is consisted of two dye molecules bridged by water molecules, at concentrations as low as 1 × 10−5 mol/L in aqueous solution [25]. Dimers are known to have low quantum yields and thus effectively quench emission due to excitation energy transfer between monomers and aggregates. Therefore, the fluorescence intensity increases sharply and then gradually decreases with increasing of concentration.
Fig. 5. (a) The emission intensities of the R6G at different concentrations with pure silicon wafers. (b) The emission intensities of the R6G at different concentrations with PVA-Au substrates located at 560 nm. (c) The corresponding fluorescence EF for the R6G with PVA-Au substrate.
Figure 5(b) is the fluorescence spectra of the R6G films with the PVA-Au substrate located at 560 nm. Compared with the pure R6G films, the fluorescence intensity of sample with the PVA-Au substrate is significantly enhanced and it has similar variation trend with concentration. Figure 5(c) shows the corresponding fluorescence EF of the SEF substrate. The EF is calculated by EF = Is/I0, where Is and I0 are the fluorescence intensities of the samples with and without the PVA-Au substrate on the basis of the integrated area from 550 nm to 610 nm [26]. As shown in Fig. 5(c), the fluorescence EF shows a trend of first increasing and then decreasing with the increase of concentration of R6G, and the EF is up to 13 at concentration of 2.0 × 10−5 mol/L. In presence of PVA-Au substrate, there also appears to be concentration dependence for SEF. Samples with higher concentrations seem to produce higher enhancements. The change trend of the fluorescence enhancement effect of R6G molecules is related to the agglomeration of R6G molecules. As mentioned above, when R6G reaches certain concentration, R6G molecules begin to approach each other and form dimers, which leads to the fluorescence quenching effect [27]. According to the results, the presence of the PVA-Au substrate attenuates the fluorescence quenching of R6G. However, as the concentration of R6G further increases, the fluorescence quenching effect is more obvious, the fluorescence quenching effect at higher concentration could limit the enhancement, and the fluorescence EF began to slowly decrease.
To further present the mechanism of the enhanced fluorescence, the emission spectra and EF based on other PVA-Au substrates located at 525 nm, 536 nm, 545 nm and 569 nm were also obtained. The results are shown in Supplement 1, Fig. S3 and Fig. S4. It is indicated that all the EF have the similar trend of first increasing and then decreasing with the increasing concentrations of R6G in the presence of different PVA-Au substrates. The biggest EF is about 5.3 (525 nm), 2.9 (536 nm), 6.0 (545 nm) and 10.3 (569 nm) respectively. Considering the resonance peaks (525-569 nm) of the PVA-Au substrates and the excitation/emission peaks of the R6G, the fluorescence enhancement mechanism is related to the degree of spectral overlap between the LSPR band and the emission/excitation band of R6G. For the PVA-Au substrates located at 525 nm and 536 nm, their peaks of LSPR bands are near to the excitation band of R6G, the probability of the R6G molecule transitioning from the ground state to the excited state increases, and more ions are excited to the excited state, which improves the excitation efficiency of the R6G molecule and realizes the enhancement of fluorescence. The 525 nm substrate matches with the excitation better and therefore it provides greater fluorescence enhancement than that for the 536 nm substrate [28]. For the PVA-Au substrate located at 545 nm, 560 nm and 569 nm, the emission process of R6G can be modulated due the spectra match between the LSPR bands and the emission band of R6G, which improve their fluorescence intensity, radiative decay rate and the fluorescence efficiency. The PVA-Au substrate located at 560 nm exhibits the best enhancement, which is attributed to the best overlap between the LSPR band of the PVA-Au substrate and the emission band of R6G [26].
To demonstrate the modulation of the PVA-Au substrate on the emission process, especially on the radiative and non-radiative decay rates of R6G films. The lifetime decay curves of R6G with PVA-Au substrate were measured. Figure 6(a) and (b) are the decay curves of pure R6G films and the R6G films with PVA-Au film located at 560 nm. Supplement 1, Fig. S5, is the decay curves of pure R6G films and the R6G films with PVA-Au film located at 525 nm, 536 nm, 545 nm and 569 nm. The decay time was fitted from the fluorescence decay curves, the fitting curves of the R6G without and with PVA-Au substrate located at 560 nm are shown in Fig. 6(a) and (b). The fitting values of all decay time are shown in Supplement 1, Table S1. For our samples, the lifetime decay curve can be well fitted by the following double exponential decay equation [29]:
Fig. 6. (a) The lifetime decay curves of the R6G at different concentrations with pure silicon wafers. (b) The lifetime decay curves of the R6G at different concentrations with PVA-Au substrate located at 560 nm. (c) The corresponding average lifetime of R6G without and with the PVA-Au substrate and the relatively change of the average lifetime.
The lifetime and the quantum yield of R6G film can be expressed as [31]:
To demonstrate the properties of the local field and gain deeper understanding of the enhanced fluorescence mechanism of the R6G film on the PVA-Au substrate, FDTD simulations were performed. It can be seen from the SEM image that the agglomeration on PVA-Au substrate is formed by several small spherical particles. Therefore, in the simulation, three different Au NPs were used to demonstrate their near local electric field distribution. In the simulation, the diameters of the three Au NPs were chosen to be 28 nm, 25 nm, and 24 nm respectively. The refractive index was chosen to be 1.52, which is the refractive index of PVA. The grid order was set to be 0.7 nm. The local electric fields of PVA-Au were simulated at wavelength of 532 nm and 560 nm respectively to present the influence of this substrate on the excitation and emission processes. The simulation results are shown in Fig. 7. It can be seen that the electric field distribution at 560 nm is stronger than that at 532 nm. As shown in Fig. 3(e) and Fig. 2(b), comparing the resonance peak (560 nm) of the PVA-Au film and the excitation/emission peak (532 nm/560 nm) of the R6G film, due to the matching of the LSPR of the PVA-Au substrate with emission wavelength of the R6G, indicating that the PVA-Au substrate mainly affects the emission process [32].
Fig. 7. The electromagnetic field intensity distribution in the x-z plane of the platform at (a) 532 nm, (b) 560 nm.
To explore the uniformly fluorescence enhancement of the PVA-Au substrate, the fluorescence of R6G films with the concentration of 2 × 10−5 mol/L was further measured at 20 random locations on the sample. Figure 8(a) is schematic explanation of confocal microscopy setup. The spot size of the 532 nm laser is about 1 μm, which enables the system to realize the micro-area measurements. Figure 8(b) and Supplement 1, Fig. S6 shows the fluorescence intensity of R6G films without and with PVA-Au substrate at these 20 random locations. It can be shown that the fluorescence intensity is both relatively uniform for R6G films with and without PVA-Au substrate. And the fluorescence intensity is obviously enhanced in presence of the PVA-Au substrate. The EF is nearly 14.5 folds. The maximum enhancement effect is larger than the mentioned above, because the micro-area measurements are excited with laser light source.
Fig. 8. (a) Schematic explanation of confocal microscopy setup. (b) Fluorescence intensities of the R6G films without and with the PVA-Au substrate on the basis of the integrated area from 550 nm to 610 nm.
In order to further observe the large area fluorescence enhancement of the PVA-Au substrate, the fluorescence mapping image of R6G was further demonstrated, which is shown in Fig. 9. The size of the fluorescence area we chose was 20 × 20 μm2. The fluorescence integrated intensity within this area is shown in different color. The corresponding relations between the color and the intensity shown in the right scalar. It can be seen from the Fig. 9 that the fluorescence mapping image of R6G in the measuring area is relatively uniform. And compared with the reference, the obviously fluorescence enhancement can be observed, and the EF is about 14.5 folds. The result further demonstrates that the PVA-Au substrate prepared by in situ growth method can achieve large-area uniform fluorescence enhancement of R6G.
Fig. 9. Fluorescence mapping of 2 x10−5 mol/L R6G film without (a) and with (b) PVA-Au substrate located at 560 nm.
4. Conclusion
In this paper, PVA-Au substrate was prepared by in situ growth method for large-area uniform fluorescence enhancement of R6G. The achievable fluorescence enhancement effect of PVA-Au substrate is about 13 folds. And the EF has great relationship with the matching degree of the LSPR band of PVA-Au substrates and the excitation/emission band of R6G film. The results further suggests that the PVA-Au substrate not only realize the fluorescence enhancement but also attenuates the fluorescence quenching at higher concentration. The fluorescence spectra and fluorescence mapping images of R6G film obtained by the confocal system shows that the flexible substrate achieves large-area relatively uniform fluorescence enhancement of R6G film. This uniform, flexible and low-cost SEF substrate is expected to be a powerful tool for large-scale fabrication of fluorescence-enhanced detection platforms, which can be widely used in fluorescent probes, fluorescence imaging, and other fields.
Funding
Natural Science Foundation of Shandong Province (ZR2019MF068); Science and Technology Support Plan for Youth Innovation of Colleges and Universities of Shandong Province of China (2019KJJ019).
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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