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Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission
Journal Pre-proofs Full Length Article Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission Kang Yuan, Ruohan Qin, Jingjing Yu, Xiang Li, Lanlan Li, Xiaojing Yang, Xiaofei Yu, Zunming Lu, Xinghua Zhang, Hui Liu PII: DOI: Reference:
S0169-4332(19)33093-4 https://doi.org/10.1016/j.apsusc.2019.144277 APSUSC 144277
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 July 2019 26 September 2019 3 October 2019
Please cite this article as: K. Yuan, R. Qin, J. Yu, X. Li, L. Li, X. Yang, X. Yu, Z. Lu, X. Zhang, H. Liu, Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144277
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Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission Kang Yuan,a Ruohan Qin,a Jingjing Yu,a Xiang Li,b,* Lanlan Li,a Xiaojing Yang,a Xiaofei Yu,a Zunming Lu,a Xinghua Zhang,a, Hui Liuc a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China b Graduate School, Hebei University of Technology, Tianjin 300130, China c School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
Abstract: We disclose the effects of localized surface plasmon resonance (LSPR) of Ag nanoparticles (NPs) on luminescence properties of carbon dots (CDs) with blue, green and yellow emission, respectively. The Ag NPs without coating layer result in fluorescence quenching of CDs with blue, green and yellow emissions. The Ag NPs coated with SiO2 layer only quench the blue emitting CDs (B-CDs) since the overlap of emission and LSPR absorption of Ag NPs. However, the fluorescence enhancement of CDs with green (G-CDs) and yellow emission (Y-CDs) can be realized by adjusting the size of the Ag NPs and the distance between Ag NPs and CDs via silica coating (Ag@SiO2). The luminescence intensity of G-CDs and Y-CDs can be improved 5-6 times as the coating thickness of SiO2 is 15 nm. Fluorescence quenching and enhancement mechanism can be clarified by the electron transition and energy transfer process between CDs and Ag NPs, which provides guidance principle for LSPR enhanced fluorescence. Keywords: Carbon dots; Photoluminescence; Localized surface plasmon resonance; Fluorescence enhancement; Energy transfer
corresponding author. E-mail: [email protected], [email protected]. 1
1. Introduction Among various fluorescent nanomaterials, CDs have attracted considerable interests due to its high water solubility, easy functionalization, low toxicity, low photobleaching, and upconversion photoluminescence [1-10]. However, compared with fluorescein molecules, the low fluorescence intensity of CDs (especially for the long wavelength emission) has limited its further applications [11-14]. Precious metal nanoparticles (NPs) are extensively used in metal-enhanced fluorescence (MEF) due to the unique localized surface plasmon resonance (LSPR) effect [15-17]. Recent studies have shown that metals can couple with nearby photons and enhance the emission intensity of nearby fluorescent molecules through electromagnetic field enhanced excitation or energy transfer. This may provide an effective solution to solve the problem of low fluorescence intensity of CDs. MEF are mainly affected by the plasma coupling and energy transfer process between the metal and the fluorescent molecules [18]. In recent years, people have found that the size and shape of metal NPs and the thickness of the spacer shell have a certain effect on the plasma resonance peak, which will further affect the spectral superposition and fluorescence enhancement ratio of the fluorescent molecules [19-21]. By modifying the surface active agent which is negatively charged with fluorescent molecules outside the dielectric shell, the fluorescent molecules are attached to the shell surface to achieve the purpose of fluorescence enhancement. The interaction between fluorophore and the plasma resonance of the metal nanoparticles can greatly increase the excitation and the radiation attenuation rate of the fluorophore. 2
In addition, the shortened lifetime of the excited state is beneficial to improve the anti-bleaching properties of photons [22, 23]. Regardless of shape, size and thickness of spacer shell, the ultimate goal is to achieve energy coupling between the fluorescent molecules and the metal for fluorescence enhancement [24-27]. Although MEF is easy to achieve for fluorescein by LSPR of metal NPs, MEF for CDs is still difficult since there are complicated surface structures and different luminescence mechanisms for CDs, which leads to more complicated process for energy coupling. Up to now, there are few reports on LSPR enhanced luminescence for CDs and the enhancement is far from satisfactory compared with fluorescent dyes [28, 29]. Moreover, electron transition process and energy coupling mechanism between the plasmon of the metal NPs and CDs need further investigation. Here we reported the LSPR effects of Ag NPs on the fluorescence properties of CDs with blue, green and yellow emission, respectively. We successfully prepared Ag NPs with different particle sizes and Ag@SiO2 core-shell NPs with various shell thicknesses. The results disclose that Ag NPs without coating layer will quench the fluorescence for the three kinds of CDs and the quenching effect increases with the increase of Ag particle size. In addition, the encapsulated Ag NPs coated with SiO 2 can greatly enhance the intensity of CDs with green and yellow emission while decrease the blue emitting CDs since the overlap with LSPR adsorption of Ag NPs. The optimum excitation range of the fluorescence enhancement is located at 420-440 nm, indicating that the fluorescence enhancement is related to LSPR of Ag NPs. At last, we obtained the possible electron transition process and energy transfer 3
mechanism between Ag NPs and CDs to clarify the fluorescence quenching and fluorescence enhancement of CDs.
2. Experimental section 2.1 Materials Methionine with analytical reagent (AR, 98%,), ethylene glycol (AR, 99.7%), AgNO3 (AR, 99.7%), hydrated hydrazine (AR, 80%), NH3·H2O (AR, 25%), ethyl orthosilicate (TEOS) (AR, 28%), anhydrous ethanol (AR, 99.7%), deionized (DI) water, hexadecyl trimethyl ammonium bromide (CTAB) (AR, 99%) were purchased from Tengyin trading company of Tianjin. 2.2 Synthesis of CDs The fluorescent colorful CDs were prepared through a simple, low-cost, and one-step microwave synthesis route. First, citric acid and methionine were dissolved in ethylene glycol, and the microwave was pulverized at 180 ºC and 560 W for 15 min. The blue CDs (B-CDs), green CDs (G-CDs) and yellow CDs (Y-CDs) were obtained when the mole ratio of citric acid to methionine was 2: 1, 1: 2, and 1: 5, respectively. 2.3 Synthesis of Ag NPs 72.8 mg CTAB was dissolved in 200 mL deionized water, 2.5 mL hydrated hydrazine solution with the concentration of 0.1 M was added to the solution, and the solution was kept stirring for 5 min at room temperature. Then, 0.05 M AgNO3 of 1 mL, 3 mL and 5 mL were added dropwise and the solution changed from colorless to yellow, yellowish brown and reddish brown. At last, different sizes of Ag NPs with 2.2±0.3 nm, 9.6±0.8 nm and 24.3±3.1 nm were obtained, which is defined as Ag1, 4
Ag2 and Ag3, respectively. 2.4 Synthesis of Ag@SiO2 NPs 40 mL Ag NPs solution (the same synthesis condition with Ag3 since larger size has more absorption overlap between CDs and Ag NPs) was combined with 10 mL ethanol and 400 μL aqueous ammonia (NH3∙H2O) with concentration of 25%, then 20 μL TEOS was added to the above solution drop by drop and kept stirring for 24 h to form the core-shell structure of Ag@SiO2 NPs. The coating layer thickness is about 10 nm and the sample is denoted as Ag@SiO2(10). Similarly, when the amount of TEOS is 40 μL, the thickness of coating layer is about 15 nm, which is denoted as Ag@SiO2 (15). As the amount of TEOS is 60 μL, the coating thickness of SiO2 is around 20 nm, which is defined as Ag@SiO2(20) NPs. 2.5. Characterizations A high resolution transmission electron microscope (JEOL 2100) was used to characterize the morphology of the CDs. X‒ray photoelectron spectra (XPS) of CDs were obtained with a PHI1600EXCA photoelectron spectrometer. Fourier transform infrared (FTIR) spectroscopy of CDs was performed on a spectrometer (Bruker, WQF‒410). All fluorescence spectra were obtained by a steady and transient state spectrophotometer (Horiba, FL‒3‒22). The ultraviolet‒visible (UV‒Vis) absorption spectra were obtained by a spectrophotometer (Hitachi, U‒3900H).
3. Results and discussion 3.1. Characterizations of CDs and Ag NPs Fig. 1a-c shows TEM images of B-CDs, G-CDs and Y-CDs, respectively. All CDs 5
have well dispersion in water solution. The particle size distribution of CDs obeys a Gaussian distribution. The most probable size is 3.0±0.3 nm, 4.2±0.4 nm and 5.4±0.25 nm for B-CDs, G-CDs and Y-CDs, respectively. The emission of CDs is red shifted from blue to yellow with slightly increase of particle size. Fig. 1d gives the excitation and emission spectra of B-CDs, G-CDs and Y-CDs. The optimum excitation and emission peaks are located at 380 nm and 460 nm, 480 nm and 540 nm, 560 nm and 595 nm for B-CDs, G-CDs and Y-CDs, respectively. In addition, the luminescence intensity is decreased from B-CDs to G-CDs and Y-CDs. Fig. 1e shows the emission spectra of B-CDs, G-CDs, and Y-CDs under different excitation wavelengths. Emission spectra of B-CDs display strong dependence of excitation wavelength, and the maximum emission intensity is at 460 nm under excitation of 380 nm. It can be seen that from B-CDs to Y-CDs, the excitation-dependent properties gradually decrease, and the fluorescence intensity also gradually decreases. In the UV-Vis spectra (Fig. 1f), the B-CDs show a maximum absorption at 325 nm, which is originated from the π−π* transition of the conjugated C=C bonds [30]. Meanwhile, the G-CDs and Y-CDs display two obvious peaks centered at 302 nm and 350 nm, which is corresponded to the n−π* transition from the carbogenic core and n-p* from the surface/molecule state region, respectively [31, 32]. The FTIR spectra (Fig. S1a-c) display that the peaks at 3430 and 3289 cm–1 are attributed to the stretch vibration of O-H and N-H bonds, and the absorption peaks centered at 3067 and 2930, 2857 cm–1 which are originated from sp2 C-H (C=C-H) and sp3 C-H (C-C-H) vibration, respectively [33, 34]. Comparing the three kinds of 6
CDs, the contents of O-H are gradually decreased while the contents of N-H, sp2 C-H and sp3 C-H are increased from B-CDs, G-CDs to Y-CDs. In addition, the predominant C1s peak at 284 eV, N1s peak at 400 eV, and O1s peak at 532 eV are observed in the full survey XPS spectra (Fig. S1d) for B-CDs, G-CDs and Y-CDs, which indicates the existence of nitrogen and oxygen elements in prepared CDs [35]. XRD patterns of prepared CDs (Fig. S2) exhibit broad diffraction peaks centered at 23º due to their small size or partial graphitization, which is similar to that of the reported CDs [36, 37]. Three distinct Ag nanoparticles with diameters of 2.2±0.3 nm, 9.6±0.8 nm, and 24.3±3.1 nm were synthesized and their TEM images with size distribution are shown in Fig. 2a-c. Because of changes in absorption and scattering efficiencies as well as local intensity of the plasmon-generated electric field, these silver colloids are expected to interact with light differently. Mie theory predicts that particles smaller than 30 nm support plasmonic oscillations of reduced amplitude and show a characteristic extinction dominated by absorptive phenomena, while larger particles are more efficient at scattering light [36]. Fig. 2d-f shows that the shell thickness of Ag@SiO2 is about 10 nm, 15 nm, 20 nm for Ag@SiO2(10), Ag@SiO2(15) and Ag@SiO2(20) NPs, respectively. Ag NPs in Ag@SiO2 have similar particle size distribution (~18±1.0 nm), smaller than Ag3 shown in Fig. 2c, which may be induced by the uncontrollable accidental error in the experiment although the Ag NPs were obtained with same synthesis condition. Fortunately, this deviation has little effect on LSPR of Ag NPs. The extent of the plasmon-enhanced electric field into the 7
surrounding media is evaluated by comparing the wavelength shift of the plasmon band for Ag and Ag@SiO2 NPs. The difference is that the absorption peak is gradually red shifted (Fig. S3c) with the increase of Ag NPs size. However, with increasing thickness of SiO2 shell, the absorption peak remains unchanged and the intensity is decreased (Fig. S3d). These results are helpful for designing multilayer concentric core-shell architectures and controlling the range of LSPR sensing by adjusting the size of the metallic domains and the thickness of the dielectric shells. 3.2. Fluorescence effect of Ag on B-CDs, G-CDs and Y-CDs The effects of Ag NPs size and coating layer thickness on the luminescence properties of B-CDs under various excitation wavelengths are investigated, as shown in Fig. 3. We use the emission intensity change factor (F/F0) to evaluate the quenching or enhancement effects of Ag and Ag@SiO2 NPs on luminescence of CDs. F0 is the emission peak intensity of CDs and F is the emission peak intensity of CDs with Ag or Ag@SiO2 NPs. For Ag NPs, the factor F/F0 of Ag/B-CDs is less than 1, and it increases first and then gently increases with increasing excitation wavelength. The emission quenching effect becomes more obvious with the increase of Ag particle size since the higher probability of CDs touching the Ag NPs. For Ag@SiO2 NPs, the intensity change factor F/F0 for Ag@SiO2 NPs is decreased first and then increased with increasing excitation wavelength, and the maximum quenching is occurred at 420 nm. Fig. S5a-b displays the emission spectra of B-CDs and B-CDs/Ag@SiO2(15), and the emission intensity as a function of excitation wavelength for B-CDs and B-CDs/Ag@SiO2 is also shown in Fig. S6a. The tendency of the quenching is nearly 8
the same for Ag@SiO2 NPs with different coating thicknesses, and the value of F/F0 is larger for Ag@SiO2(15) than that of Ag@SiO2(10) and Ag@SiO2(20) NPs. Both Ag and Ag@SiO2 NPs have the effects of decreasing the luminescence intensity of B-CDs, but the intensity reduction degree for Ag@SiO2 is less than that of Ag NPs, which is induced by the coating layer of SiO2. In addition, the contents of Ag and Ag@SiO2 NPs also influence the luminescence intensity of B-CDs, as shown in Fig. 3b, d. Different volumes fraction (0, 0.1%, 0.3%, 0.5%, 1% and 1.5%) of Ag3 and Ag@SiO2(20) NPs were added to 2.0 mL B-CDs solution, respectively. The emission intensity of B-CDs decreases with the increased contents of Ag3 or Ag@SiO2(20) NPs, which is originated from the LSPR absorption of Ag NPs. Ag@SiO2(15) NPs show similar properties as Ag@SiO2(20) (Fig. S4a). With the high contents of Ag@SiO2(20) (0.03 mL, 1.5% volume fraction), two weak emission peaks (380 nm, 530 nm) appear and the blue emission peak at 450 nm is completely quenched with various excitation wavelengths (Fig. S4b), which is originated from the different quenching effects of Ag@SiO2 on various emission wavelengths of B-CDs. According to the absorption spectra (Fig. S7), since the overlap range between LSPR absorption of Ag NPs and emission of B-CDs is from 360 nm to 560 nm, both Ag and Ag@SiO2 NPs will quench the luminescence of B-CDs. The degree of overlap in the red frame (425-435 nm) is much higher, so the quenching effect is more obvious. The luminescence of G-CDs will also be quenched by Ag NPs, and the quenching effects become more obvious with increasing size of Ag NPs (Fig. S5b), which may be induced by the electrons transfer from G-CDs to Ag NPs. The intensity change 9
factor F/F0 is also related to the excitation wavelength of G-CDs, it is increased first and reached a maximum value at 440 nm, and then it is decreased with increasing excitation wavelength (Fig. 4a). The change of factor F/F0 for Ag NPs is a result of competition between LSPR enhancing effects of Ag NPs and electron transfer process between G-CDs and Ag NPs. For Ag@SiO2 NPs, the electron transfer process will be reduced due to the coating layer of SiO2, which leads to the increase of luminescence intensity of G-CDs by LSPR enhancing effects. As shown in Fig. 4b, the emission peak intensity of G-CDs is increased by Ag@SiO2 with various coating layer thicknesses, and it reaches the maximum enhancement when the coating layer thickness is 15 nm. Fig. S5c-d gives the emission spectra of G-CDs and G-CDs/Ag@SiO2(15), and the emission intensity as a function of excitation wavelength for G-CDs and G-CDs/Ag@SiO2 is also shown in Fig. S6b. In addition, the peak position of intensity change factor F/F0 is located around 420 nm. The tendency curve of F/F0 for G-CDs is opposite to that of B-CDs, which means that the energy transfer process is different between Ag@SiO2 and G-CDs. The maximum intensity change factor F/F0 is reached 4.7 folds by the Ag@SiO2 NPs. In addition, the excitation wavelength results in difference multiples of fluorescence enhancement, so it can be speculated that there is some wavelength dependent energy transfer between Ag@SiO2 NPs and G-CDs. The LSPR effects of Ag and Ag@SiO2 NPs on luminescence of Y-CDs are similar to that of G-CDs. The Ag NPs also reduce the luminescence intensity of Y-CDs with increasing particle size, but the factor F/F0 is increased with the increase of excitation 10
wavelength (Fig. 4c). In addition, there is a little enhancement of emission intensity for Y-CDs by Ag1 NPs, which may be originated from the surface plasmon resonance wavelength of Ag1 NPs coincides with the part of absorption band of G-CDs. Moreover, the Ag@SiO2 NPs can enhance the luminescence intensity in a wide range of excitation wavelength, and the enhancement effects are also sensitive to the thickness of SiO2 layer. The enhancing effects of Ag@SiO2(10) NPs is increased with the excitation wavelength, which is similar to that of Ag NPs since the coating layer is very thin. Previous research [38] pointed out that local electromagnetic field of metal NPs is closely related to the distance between the fluorescent substance and metal NPs. Non-radiative transition will dominate the emission process as the distance between the fluorescent substances is too close. Herein, coating layer with 10 nm thicknesses exactly leads to fierce competition of radiative transitions and non-radiative transitions. However, the factor F/F0 for Ag@SiO2(15) and Ag@SiO2(20) NPs is increased first, and reaches maximum enhancement at 420 nm. Therefore, there should be different extents of energy transfer between Ag NPs and Y-CDs with increasing excitation wavelength, and it reaches maximum energy transfer at LSPR absorption peak of Ag NPs, thus leading to the change of luminescence enhancement. Fig. S5e-f gives the emission spectra of Y-CDs and Y-CDs/Ag@SiO2(15), and emission intensity as a function of excitation wavelength for Y-CDs and Y-CDs/Ag@SiO2 is shown in Fig. S6c. 3.3 Mechanism of fluorescence quenching or enhancement The electron transition process between CDs and Ag or Ag@SiO2 NPs can be
11
elucidated with time-resolved spectra. Fig. 5 shows fluorescence decay curves and fitting results of B-CDs (G-CDs/Y-CDs), Ag3/B-CDs (G-CDs/Y-CDs) and Ag@SiO2(15)/B-CDs (G-CDs/Y-CDs), and the decay emission is fixed at 460, 530, 590 nm for the corresponding B-CDs, G-CDs and Y-CDs, respectively. All the samples can be fitted well by dual exponential function with lifetimes t 1 and t2. For B-CDs, the t1 and t2 is 2.4 ns and 11.8 ns, but the Ag and Ag@SiO2 NPs have no influence on lifetimes of B-CDs. It suggests that the radiation attenuation rate of B-CDs has not been enhanced, which is also the main reason for the lack of enhancing fluorescence. On the other hand, the decrease of luminescence intensity for B-CDs/Ag (or Ag@SiO2) is also induced by the LSPR absorption of Ag metal in Ag or Ag@SiO2 NPs. In addition, Ag3 NPs can decrease the lifetimes of G-CDs and Y-CDs which lead to the increase of radiation attenuation rate, but there is no fluorescence enhancement by LSPR effects of Ag NPs since the electrons transfer from CDs to Ag NPs. However, the enhancement of G-CDs and Y-CDs by Ag@SiO2 NPs is induced by the combination of the enhancement of excitation field and the improvement of radiation attenuation rate. For G-CDs or Y-CDs, the emission will not be absorbed by Ag metal, and the Ag@SiO2 NPs can reduce the lifetimes, which results in the increase of radiation attenuation rate and enhancement of emission intensity. Fig. 6 shows the electrons transition and energy transfer process between CDs and Ag or Ag@SiO2 NPs with fluorescence quenching or enhancement mechanism, respectively. For CDs/Ag NPs, the electrons transfer process between them is similar with the photocatalyst and noble metal transfer mechanism [39], and some electrons 12
are transferred from the conduction band of CDs to the surface of the Ag NPs, which leads to the fluorescence quenching of the CDs (Fig. S8). For B-CDs/Ag@SiO2 NPs, the emission spectra of B-CDs have large overlap to that of LSPR absorption from Ag NPs, and most luminescence of B-CDs is absorbed by Ag NPs, which also results in fluorescence quenching. For G-CDs/Ag@SiO2 and Y-CDs/Ag@SiO2 NPs, the electrons transition from CDs to Ag NPs is prevented by the SiO2 layer, and the excitation filed and energy transfer is increased by LSPR effectsˈthus resulting in much more excited state electrons in G-CDs/Y-CDs, which leads to the enhancement of luminescence of CDs (Fig. S6). In addition, the Ag@SiO2 NPs can increase the radiation decay rate of G-CDs and Y-CDs, which is also helpful to improve the luminescence intensity of CDs.
4. Conclusion In this work, a series of CDs with blue, green, yellow emission and Ag NPs with different sizes and SiO2 coating thicknesses were synthesized, and LSPR effects of Ag NPs on luminescence of CDs were systematically investigated. Ag NPs (regardless of particle size) will quench the fluorescence of CDs with various emissions due to the electrons transfer from CDs to Ag NPs. However, the introduction of dielectric shell SiO2 can prevent the electrons transfer process and enhance the luminescence of CDs, and the maximum luminescence enhancement can achieve 5-6 times for G-CDs and Y-CDs as the coating layer thickness of SiO2 is 15 nm. The fluorescence enhancement is induced by the excitation enhancement and increase of radiation attenuation rate from LSPR effects of Ag NPs. The clarification of electron transition process and 13
fluorescence enhancement mechanism between CDs and LSPR of Ag NPs is beneficial to thoroughly understand the LSPR interaction mechanism of noble metals and promote the application of quantum dots.
Conflicts of interest The authors declare no conflicts of interest.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51771068, 21603052, 51671079, 51771067), Natural Science Foundation of Hebei Province (No. E2018202082), Financial Support for Scientific and Technological Activities of Returnees from Abroad (CL201606), and Innovation Fund for Excellent Youth of Hebei University of Technology (No. 2015005).
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version.
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Fig. 1 TEM images and size distribution of (a) B-CDs, (b) G-CDs, and (c) Y-CDs, (d) optimal excitation and emission spectra of B-CDs, G-CDs and Y-CDs,dotted lines represent excitation spectra of CDs at specific monitoring emission wavelengths (λem=460, 540, 595 nm for B-CDs, G-CDs and Y-CDs, respectively) and the solid lines represent the emission spectra of CDs with specific excitation wavelengths (λex=380, 480 and 560 nm for B-CDs, G-CDs and Y-CDs, respectively), (e) emission spectra of B-CDs (excitation wavelength: 320-440 nm), G-CDs (excitation wavelength: 360-500 nm) and Y-CDs (excitation wavelength: 380-580 nm) under different excitation wavelengths, (f) UV-Vis absorption spectra of B-CDs, G-CDs and Y-CDs.
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Fig. 2 TEM images and size distribution of (a) Ag1, (b) Ag2, (c) Ag3, (d) Ag@SiO2(10), (e) Ag@SiO2(15) and (f) Ag@SiO2(20), respectively.
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Fig. 3 Intensity change factor F/F0 of (a) Ag1/B-CDs, Ag2/B-CDs, Ag2/B-CDs and (c) Ag@SiO2(10)/B-CDs, Ag@SiO2(15)/B-CDs, Ag@SiO2(20)/B-CDs, F and F0 represent the emission peak intensity of B-CDs with and without adding Ag or Ag@SiO2 NPs, respectively. Emission spectra of B-CDs with different concentrations of (b) Ag3 and (d) Ag@SiO2(20) solution.
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Fig. 4 Intensity change factor F/F0 of (a) Ag1/G-CDs, Ag2/G-CDs, Ag2/G-CDs and (b) Ag@SiO2(10)/G-CDs, Ag@SiO2(15)/G-CDs, Ag@SiO2(20)/G-CDs. Intensity change factor F/F0 of (c) Ag1/Y-CDs, Ag2/Y-CDs, Ag2/Y-CDs and (d) Ag@SiO2(10)/Y-CDs, Ag@SiO2(15)/Y-CDs, Ag@SiO2(20)/Y-CDs, respectively.
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Fig. 5 Decay curves and fitting results of (a) B-CDs, B-CDs/Ag3 and B-CDs/Ag@SiO2(15), (b) G-CDs, G-CDs/Ag3, G-CDs/Ag@SiO2(15) and (c) Y-CDs, Y-CDs/Ag3, Y-CDs/Ag@SiO2(15), respectively.
Fig. 6 The scheme of electrons transition and energy transfer process between CDs and Ag or Ag@SiO2 NPs for (a) fluorescence quenching and (b) fluorescence enhancement.
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Declaration of interests ܈The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights • LSPR Effects of Ag NPs on luminescence of CDs are systematically investigated. •Luminescence intensity of CDs can be improved 5-6 times by LSPR of Ag@SiO2 NPs. •Fluorescence quenching and enhancement mechanism is clarified between CDs and Ag NPs.
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S0169-4332(19)33093-4 https://doi.org/10.1016/j.apsusc.2019.144277 APSUSC 144277
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
7 July 2019 26 September 2019 3 October 2019
Please cite this article as: K. Yuan, R. Qin, J. Yu, X. Li, L. Li, X. Yang, X. Yu, Z. Lu, X. Zhang, H. Liu, Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144277
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© 2019 Published by Elsevier B.V.
Effects of localized surface plasmon resonance of Ag nanoparticles on luminescence of carbon dots with blue, green and yellow emission Kang Yuan,a Ruohan Qin,a Jingjing Yu,a Xiang Li,b,* Lanlan Li,a Xiaojing Yang,a Xiaofei Yu,a Zunming Lu,a Xinghua Zhang,a, Hui Liuc a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China b Graduate School, Hebei University of Technology, Tianjin 300130, China c School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
Abstract: We disclose the effects of localized surface plasmon resonance (LSPR) of Ag nanoparticles (NPs) on luminescence properties of carbon dots (CDs) with blue, green and yellow emission, respectively. The Ag NPs without coating layer result in fluorescence quenching of CDs with blue, green and yellow emissions. The Ag NPs coated with SiO2 layer only quench the blue emitting CDs (B-CDs) since the overlap of emission and LSPR absorption of Ag NPs. However, the fluorescence enhancement of CDs with green (G-CDs) and yellow emission (Y-CDs) can be realized by adjusting the size of the Ag NPs and the distance between Ag NPs and CDs via silica coating (Ag@SiO2). The luminescence intensity of G-CDs and Y-CDs can be improved 5-6 times as the coating thickness of SiO2 is 15 nm. Fluorescence quenching and enhancement mechanism can be clarified by the electron transition and energy transfer process between CDs and Ag NPs, which provides guidance principle for LSPR enhanced fluorescence. Keywords: Carbon dots; Photoluminescence; Localized surface plasmon resonance; Fluorescence enhancement; Energy transfer
corresponding author. E-mail: [email protected], [email protected]. 1
1. Introduction Among various fluorescent nanomaterials, CDs have attracted considerable interests due to its high water solubility, easy functionalization, low toxicity, low photobleaching, and upconversion photoluminescence [1-10]. However, compared with fluorescein molecules, the low fluorescence intensity of CDs (especially for the long wavelength emission) has limited its further applications [11-14]. Precious metal nanoparticles (NPs) are extensively used in metal-enhanced fluorescence (MEF) due to the unique localized surface plasmon resonance (LSPR) effect [15-17]. Recent studies have shown that metals can couple with nearby photons and enhance the emission intensity of nearby fluorescent molecules through electromagnetic field enhanced excitation or energy transfer. This may provide an effective solution to solve the problem of low fluorescence intensity of CDs. MEF are mainly affected by the plasma coupling and energy transfer process between the metal and the fluorescent molecules [18]. In recent years, people have found that the size and shape of metal NPs and the thickness of the spacer shell have a certain effect on the plasma resonance peak, which will further affect the spectral superposition and fluorescence enhancement ratio of the fluorescent molecules [19-21]. By modifying the surface active agent which is negatively charged with fluorescent molecules outside the dielectric shell, the fluorescent molecules are attached to the shell surface to achieve the purpose of fluorescence enhancement. The interaction between fluorophore and the plasma resonance of the metal nanoparticles can greatly increase the excitation and the radiation attenuation rate of the fluorophore. 2
In addition, the shortened lifetime of the excited state is beneficial to improve the anti-bleaching properties of photons [22, 23]. Regardless of shape, size and thickness of spacer shell, the ultimate goal is to achieve energy coupling between the fluorescent molecules and the metal for fluorescence enhancement [24-27]. Although MEF is easy to achieve for fluorescein by LSPR of metal NPs, MEF for CDs is still difficult since there are complicated surface structures and different luminescence mechanisms for CDs, which leads to more complicated process for energy coupling. Up to now, there are few reports on LSPR enhanced luminescence for CDs and the enhancement is far from satisfactory compared with fluorescent dyes [28, 29]. Moreover, electron transition process and energy coupling mechanism between the plasmon of the metal NPs and CDs need further investigation. Here we reported the LSPR effects of Ag NPs on the fluorescence properties of CDs with blue, green and yellow emission, respectively. We successfully prepared Ag NPs with different particle sizes and Ag@SiO2 core-shell NPs with various shell thicknesses. The results disclose that Ag NPs without coating layer will quench the fluorescence for the three kinds of CDs and the quenching effect increases with the increase of Ag particle size. In addition, the encapsulated Ag NPs coated with SiO 2 can greatly enhance the intensity of CDs with green and yellow emission while decrease the blue emitting CDs since the overlap with LSPR adsorption of Ag NPs. The optimum excitation range of the fluorescence enhancement is located at 420-440 nm, indicating that the fluorescence enhancement is related to LSPR of Ag NPs. At last, we obtained the possible electron transition process and energy transfer 3
mechanism between Ag NPs and CDs to clarify the fluorescence quenching and fluorescence enhancement of CDs.
2. Experimental section 2.1 Materials Methionine with analytical reagent (AR, 98%,), ethylene glycol (AR, 99.7%), AgNO3 (AR, 99.7%), hydrated hydrazine (AR, 80%), NH3·H2O (AR, 25%), ethyl orthosilicate (TEOS) (AR, 28%), anhydrous ethanol (AR, 99.7%), deionized (DI) water, hexadecyl trimethyl ammonium bromide (CTAB) (AR, 99%) were purchased from Tengyin trading company of Tianjin. 2.2 Synthesis of CDs The fluorescent colorful CDs were prepared through a simple, low-cost, and one-step microwave synthesis route. First, citric acid and methionine were dissolved in ethylene glycol, and the microwave was pulverized at 180 ºC and 560 W for 15 min. The blue CDs (B-CDs), green CDs (G-CDs) and yellow CDs (Y-CDs) were obtained when the mole ratio of citric acid to methionine was 2: 1, 1: 2, and 1: 5, respectively. 2.3 Synthesis of Ag NPs 72.8 mg CTAB was dissolved in 200 mL deionized water, 2.5 mL hydrated hydrazine solution with the concentration of 0.1 M was added to the solution, and the solution was kept stirring for 5 min at room temperature. Then, 0.05 M AgNO3 of 1 mL, 3 mL and 5 mL were added dropwise and the solution changed from colorless to yellow, yellowish brown and reddish brown. At last, different sizes of Ag NPs with 2.2±0.3 nm, 9.6±0.8 nm and 24.3±3.1 nm were obtained, which is defined as Ag1, 4
Ag2 and Ag3, respectively. 2.4 Synthesis of Ag@SiO2 NPs 40 mL Ag NPs solution (the same synthesis condition with Ag3 since larger size has more absorption overlap between CDs and Ag NPs) was combined with 10 mL ethanol and 400 μL aqueous ammonia (NH3∙H2O) with concentration of 25%, then 20 μL TEOS was added to the above solution drop by drop and kept stirring for 24 h to form the core-shell structure of Ag@SiO2 NPs. The coating layer thickness is about 10 nm and the sample is denoted as Ag@SiO2(10). Similarly, when the amount of TEOS is 40 μL, the thickness of coating layer is about 15 nm, which is denoted as Ag@SiO2 (15). As the amount of TEOS is 60 μL, the coating thickness of SiO2 is around 20 nm, which is defined as Ag@SiO2(20) NPs. 2.5. Characterizations A high resolution transmission electron microscope (JEOL 2100) was used to characterize the morphology of the CDs. X‒ray photoelectron spectra (XPS) of CDs were obtained with a PHI1600EXCA photoelectron spectrometer. Fourier transform infrared (FTIR) spectroscopy of CDs was performed on a spectrometer (Bruker, WQF‒410). All fluorescence spectra were obtained by a steady and transient state spectrophotometer (Horiba, FL‒3‒22). The ultraviolet‒visible (UV‒Vis) absorption spectra were obtained by a spectrophotometer (Hitachi, U‒3900H).
3. Results and discussion 3.1. Characterizations of CDs and Ag NPs Fig. 1a-c shows TEM images of B-CDs, G-CDs and Y-CDs, respectively. All CDs 5
have well dispersion in water solution. The particle size distribution of CDs obeys a Gaussian distribution. The most probable size is 3.0±0.3 nm, 4.2±0.4 nm and 5.4±0.25 nm for B-CDs, G-CDs and Y-CDs, respectively. The emission of CDs is red shifted from blue to yellow with slightly increase of particle size. Fig. 1d gives the excitation and emission spectra of B-CDs, G-CDs and Y-CDs. The optimum excitation and emission peaks are located at 380 nm and 460 nm, 480 nm and 540 nm, 560 nm and 595 nm for B-CDs, G-CDs and Y-CDs, respectively. In addition, the luminescence intensity is decreased from B-CDs to G-CDs and Y-CDs. Fig. 1e shows the emission spectra of B-CDs, G-CDs, and Y-CDs under different excitation wavelengths. Emission spectra of B-CDs display strong dependence of excitation wavelength, and the maximum emission intensity is at 460 nm under excitation of 380 nm. It can be seen that from B-CDs to Y-CDs, the excitation-dependent properties gradually decrease, and the fluorescence intensity also gradually decreases. In the UV-Vis spectra (Fig. 1f), the B-CDs show a maximum absorption at 325 nm, which is originated from the π−π* transition of the conjugated C=C bonds [30]. Meanwhile, the G-CDs and Y-CDs display two obvious peaks centered at 302 nm and 350 nm, which is corresponded to the n−π* transition from the carbogenic core and n-p* from the surface/molecule state region, respectively [31, 32]. The FTIR spectra (Fig. S1a-c) display that the peaks at 3430 and 3289 cm–1 are attributed to the stretch vibration of O-H and N-H bonds, and the absorption peaks centered at 3067 and 2930, 2857 cm–1 which are originated from sp2 C-H (C=C-H) and sp3 C-H (C-C-H) vibration, respectively [33, 34]. Comparing the three kinds of 6
CDs, the contents of O-H are gradually decreased while the contents of N-H, sp2 C-H and sp3 C-H are increased from B-CDs, G-CDs to Y-CDs. In addition, the predominant C1s peak at 284 eV, N1s peak at 400 eV, and O1s peak at 532 eV are observed in the full survey XPS spectra (Fig. S1d) for B-CDs, G-CDs and Y-CDs, which indicates the existence of nitrogen and oxygen elements in prepared CDs [35]. XRD patterns of prepared CDs (Fig. S2) exhibit broad diffraction peaks centered at 23º due to their small size or partial graphitization, which is similar to that of the reported CDs [36, 37]. Three distinct Ag nanoparticles with diameters of 2.2±0.3 nm, 9.6±0.8 nm, and 24.3±3.1 nm were synthesized and their TEM images with size distribution are shown in Fig. 2a-c. Because of changes in absorption and scattering efficiencies as well as local intensity of the plasmon-generated electric field, these silver colloids are expected to interact with light differently. Mie theory predicts that particles smaller than 30 nm support plasmonic oscillations of reduced amplitude and show a characteristic extinction dominated by absorptive phenomena, while larger particles are more efficient at scattering light [36]. Fig. 2d-f shows that the shell thickness of Ag@SiO2 is about 10 nm, 15 nm, 20 nm for Ag@SiO2(10), Ag@SiO2(15) and Ag@SiO2(20) NPs, respectively. Ag NPs in Ag@SiO2 have similar particle size distribution (~18±1.0 nm), smaller than Ag3 shown in Fig. 2c, which may be induced by the uncontrollable accidental error in the experiment although the Ag NPs were obtained with same synthesis condition. Fortunately, this deviation has little effect on LSPR of Ag NPs. The extent of the plasmon-enhanced electric field into the 7
surrounding media is evaluated by comparing the wavelength shift of the plasmon band for Ag and Ag@SiO2 NPs. The difference is that the absorption peak is gradually red shifted (Fig. S3c) with the increase of Ag NPs size. However, with increasing thickness of SiO2 shell, the absorption peak remains unchanged and the intensity is decreased (Fig. S3d). These results are helpful for designing multilayer concentric core-shell architectures and controlling the range of LSPR sensing by adjusting the size of the metallic domains and the thickness of the dielectric shells. 3.2. Fluorescence effect of Ag on B-CDs, G-CDs and Y-CDs The effects of Ag NPs size and coating layer thickness on the luminescence properties of B-CDs under various excitation wavelengths are investigated, as shown in Fig. 3. We use the emission intensity change factor (F/F0) to evaluate the quenching or enhancement effects of Ag and Ag@SiO2 NPs on luminescence of CDs. F0 is the emission peak intensity of CDs and F is the emission peak intensity of CDs with Ag or Ag@SiO2 NPs. For Ag NPs, the factor F/F0 of Ag/B-CDs is less than 1, and it increases first and then gently increases with increasing excitation wavelength. The emission quenching effect becomes more obvious with the increase of Ag particle size since the higher probability of CDs touching the Ag NPs. For Ag@SiO2 NPs, the intensity change factor F/F0 for Ag@SiO2 NPs is decreased first and then increased with increasing excitation wavelength, and the maximum quenching is occurred at 420 nm. Fig. S5a-b displays the emission spectra of B-CDs and B-CDs/Ag@SiO2(15), and the emission intensity as a function of excitation wavelength for B-CDs and B-CDs/Ag@SiO2 is also shown in Fig. S6a. The tendency of the quenching is nearly 8
the same for Ag@SiO2 NPs with different coating thicknesses, and the value of F/F0 is larger for Ag@SiO2(15) than that of Ag@SiO2(10) and Ag@SiO2(20) NPs. Both Ag and Ag@SiO2 NPs have the effects of decreasing the luminescence intensity of B-CDs, but the intensity reduction degree for Ag@SiO2 is less than that of Ag NPs, which is induced by the coating layer of SiO2. In addition, the contents of Ag and Ag@SiO2 NPs also influence the luminescence intensity of B-CDs, as shown in Fig. 3b, d. Different volumes fraction (0, 0.1%, 0.3%, 0.5%, 1% and 1.5%) of Ag3 and Ag@SiO2(20) NPs were added to 2.0 mL B-CDs solution, respectively. The emission intensity of B-CDs decreases with the increased contents of Ag3 or Ag@SiO2(20) NPs, which is originated from the LSPR absorption of Ag NPs. Ag@SiO2(15) NPs show similar properties as Ag@SiO2(20) (Fig. S4a). With the high contents of Ag@SiO2(20) (0.03 mL, 1.5% volume fraction), two weak emission peaks (380 nm, 530 nm) appear and the blue emission peak at 450 nm is completely quenched with various excitation wavelengths (Fig. S4b), which is originated from the different quenching effects of Ag@SiO2 on various emission wavelengths of B-CDs. According to the absorption spectra (Fig. S7), since the overlap range between LSPR absorption of Ag NPs and emission of B-CDs is from 360 nm to 560 nm, both Ag and Ag@SiO2 NPs will quench the luminescence of B-CDs. The degree of overlap in the red frame (425-435 nm) is much higher, so the quenching effect is more obvious. The luminescence of G-CDs will also be quenched by Ag NPs, and the quenching effects become more obvious with increasing size of Ag NPs (Fig. S5b), which may be induced by the electrons transfer from G-CDs to Ag NPs. The intensity change 9
factor F/F0 is also related to the excitation wavelength of G-CDs, it is increased first and reached a maximum value at 440 nm, and then it is decreased with increasing excitation wavelength (Fig. 4a). The change of factor F/F0 for Ag NPs is a result of competition between LSPR enhancing effects of Ag NPs and electron transfer process between G-CDs and Ag NPs. For Ag@SiO2 NPs, the electron transfer process will be reduced due to the coating layer of SiO2, which leads to the increase of luminescence intensity of G-CDs by LSPR enhancing effects. As shown in Fig. 4b, the emission peak intensity of G-CDs is increased by Ag@SiO2 with various coating layer thicknesses, and it reaches the maximum enhancement when the coating layer thickness is 15 nm. Fig. S5c-d gives the emission spectra of G-CDs and G-CDs/Ag@SiO2(15), and the emission intensity as a function of excitation wavelength for G-CDs and G-CDs/Ag@SiO2 is also shown in Fig. S6b. In addition, the peak position of intensity change factor F/F0 is located around 420 nm. The tendency curve of F/F0 for G-CDs is opposite to that of B-CDs, which means that the energy transfer process is different between Ag@SiO2 and G-CDs. The maximum intensity change factor F/F0 is reached 4.7 folds by the Ag@SiO2 NPs. In addition, the excitation wavelength results in difference multiples of fluorescence enhancement, so it can be speculated that there is some wavelength dependent energy transfer between Ag@SiO2 NPs and G-CDs. The LSPR effects of Ag and Ag@SiO2 NPs on luminescence of Y-CDs are similar to that of G-CDs. The Ag NPs also reduce the luminescence intensity of Y-CDs with increasing particle size, but the factor F/F0 is increased with the increase of excitation 10
wavelength (Fig. 4c). In addition, there is a little enhancement of emission intensity for Y-CDs by Ag1 NPs, which may be originated from the surface plasmon resonance wavelength of Ag1 NPs coincides with the part of absorption band of G-CDs. Moreover, the Ag@SiO2 NPs can enhance the luminescence intensity in a wide range of excitation wavelength, and the enhancement effects are also sensitive to the thickness of SiO2 layer. The enhancing effects of Ag@SiO2(10) NPs is increased with the excitation wavelength, which is similar to that of Ag NPs since the coating layer is very thin. Previous research [38] pointed out that local electromagnetic field of metal NPs is closely related to the distance between the fluorescent substance and metal NPs. Non-radiative transition will dominate the emission process as the distance between the fluorescent substances is too close. Herein, coating layer with 10 nm thicknesses exactly leads to fierce competition of radiative transitions and non-radiative transitions. However, the factor F/F0 for Ag@SiO2(15) and Ag@SiO2(20) NPs is increased first, and reaches maximum enhancement at 420 nm. Therefore, there should be different extents of energy transfer between Ag NPs and Y-CDs with increasing excitation wavelength, and it reaches maximum energy transfer at LSPR absorption peak of Ag NPs, thus leading to the change of luminescence enhancement. Fig. S5e-f gives the emission spectra of Y-CDs and Y-CDs/Ag@SiO2(15), and emission intensity as a function of excitation wavelength for Y-CDs and Y-CDs/Ag@SiO2 is shown in Fig. S6c. 3.3 Mechanism of fluorescence quenching or enhancement The electron transition process between CDs and Ag or Ag@SiO2 NPs can be
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elucidated with time-resolved spectra. Fig. 5 shows fluorescence decay curves and fitting results of B-CDs (G-CDs/Y-CDs), Ag3/B-CDs (G-CDs/Y-CDs) and Ag@SiO2(15)/B-CDs (G-CDs/Y-CDs), and the decay emission is fixed at 460, 530, 590 nm for the corresponding B-CDs, G-CDs and Y-CDs, respectively. All the samples can be fitted well by dual exponential function with lifetimes t 1 and t2. For B-CDs, the t1 and t2 is 2.4 ns and 11.8 ns, but the Ag and Ag@SiO2 NPs have no influence on lifetimes of B-CDs. It suggests that the radiation attenuation rate of B-CDs has not been enhanced, which is also the main reason for the lack of enhancing fluorescence. On the other hand, the decrease of luminescence intensity for B-CDs/Ag (or Ag@SiO2) is also induced by the LSPR absorption of Ag metal in Ag or Ag@SiO2 NPs. In addition, Ag3 NPs can decrease the lifetimes of G-CDs and Y-CDs which lead to the increase of radiation attenuation rate, but there is no fluorescence enhancement by LSPR effects of Ag NPs since the electrons transfer from CDs to Ag NPs. However, the enhancement of G-CDs and Y-CDs by Ag@SiO2 NPs is induced by the combination of the enhancement of excitation field and the improvement of radiation attenuation rate. For G-CDs or Y-CDs, the emission will not be absorbed by Ag metal, and the Ag@SiO2 NPs can reduce the lifetimes, which results in the increase of radiation attenuation rate and enhancement of emission intensity. Fig. 6 shows the electrons transition and energy transfer process between CDs and Ag or Ag@SiO2 NPs with fluorescence quenching or enhancement mechanism, respectively. For CDs/Ag NPs, the electrons transfer process between them is similar with the photocatalyst and noble metal transfer mechanism [39], and some electrons 12
are transferred from the conduction band of CDs to the surface of the Ag NPs, which leads to the fluorescence quenching of the CDs (Fig. S8). For B-CDs/Ag@SiO2 NPs, the emission spectra of B-CDs have large overlap to that of LSPR absorption from Ag NPs, and most luminescence of B-CDs is absorbed by Ag NPs, which also results in fluorescence quenching. For G-CDs/Ag@SiO2 and Y-CDs/Ag@SiO2 NPs, the electrons transition from CDs to Ag NPs is prevented by the SiO2 layer, and the excitation filed and energy transfer is increased by LSPR effectsˈthus resulting in much more excited state electrons in G-CDs/Y-CDs, which leads to the enhancement of luminescence of CDs (Fig. S6). In addition, the Ag@SiO2 NPs can increase the radiation decay rate of G-CDs and Y-CDs, which is also helpful to improve the luminescence intensity of CDs.
4. Conclusion In this work, a series of CDs with blue, green, yellow emission and Ag NPs with different sizes and SiO2 coating thicknesses were synthesized, and LSPR effects of Ag NPs on luminescence of CDs were systematically investigated. Ag NPs (regardless of particle size) will quench the fluorescence of CDs with various emissions due to the electrons transfer from CDs to Ag NPs. However, the introduction of dielectric shell SiO2 can prevent the electrons transfer process and enhance the luminescence of CDs, and the maximum luminescence enhancement can achieve 5-6 times for G-CDs and Y-CDs as the coating layer thickness of SiO2 is 15 nm. The fluorescence enhancement is induced by the excitation enhancement and increase of radiation attenuation rate from LSPR effects of Ag NPs. The clarification of electron transition process and 13
fluorescence enhancement mechanism between CDs and LSPR of Ag NPs is beneficial to thoroughly understand the LSPR interaction mechanism of noble metals and promote the application of quantum dots.
Conflicts of interest The authors declare no conflicts of interest.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51771068, 21603052, 51671079, 51771067), Natural Science Foundation of Hebei Province (No. E2018202082), Financial Support for Scientific and Technological Activities of Returnees from Abroad (CL201606), and Innovation Fund for Excellent Youth of Hebei University of Technology (No. 2015005).
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version.
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Fig. 1 TEM images and size distribution of (a) B-CDs, (b) G-CDs, and (c) Y-CDs, (d) optimal excitation and emission spectra of B-CDs, G-CDs and Y-CDs,dotted lines represent excitation spectra of CDs at specific monitoring emission wavelengths (λem=460, 540, 595 nm for B-CDs, G-CDs and Y-CDs, respectively) and the solid lines represent the emission spectra of CDs with specific excitation wavelengths (λex=380, 480 and 560 nm for B-CDs, G-CDs and Y-CDs, respectively), (e) emission spectra of B-CDs (excitation wavelength: 320-440 nm), G-CDs (excitation wavelength: 360-500 nm) and Y-CDs (excitation wavelength: 380-580 nm) under different excitation wavelengths, (f) UV-Vis absorption spectra of B-CDs, G-CDs and Y-CDs.
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Fig. 2 TEM images and size distribution of (a) Ag1, (b) Ag2, (c) Ag3, (d) Ag@SiO2(10), (e) Ag@SiO2(15) and (f) Ag@SiO2(20), respectively.
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Fig. 3 Intensity change factor F/F0 of (a) Ag1/B-CDs, Ag2/B-CDs, Ag2/B-CDs and (c) Ag@SiO2(10)/B-CDs, Ag@SiO2(15)/B-CDs, Ag@SiO2(20)/B-CDs, F and F0 represent the emission peak intensity of B-CDs with and without adding Ag or Ag@SiO2 NPs, respectively. Emission spectra of B-CDs with different concentrations of (b) Ag3 and (d) Ag@SiO2(20) solution.
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Fig. 4 Intensity change factor F/F0 of (a) Ag1/G-CDs, Ag2/G-CDs, Ag2/G-CDs and (b) Ag@SiO2(10)/G-CDs, Ag@SiO2(15)/G-CDs, Ag@SiO2(20)/G-CDs. Intensity change factor F/F0 of (c) Ag1/Y-CDs, Ag2/Y-CDs, Ag2/Y-CDs and (d) Ag@SiO2(10)/Y-CDs, Ag@SiO2(15)/Y-CDs, Ag@SiO2(20)/Y-CDs, respectively.
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Fig. 5 Decay curves and fitting results of (a) B-CDs, B-CDs/Ag3 and B-CDs/Ag@SiO2(15), (b) G-CDs, G-CDs/Ag3, G-CDs/Ag@SiO2(15) and (c) Y-CDs, Y-CDs/Ag3, Y-CDs/Ag@SiO2(15), respectively.
Fig. 6 The scheme of electrons transition and energy transfer process between CDs and Ag or Ag@SiO2 NPs for (a) fluorescence quenching and (b) fluorescence enhancement.
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Declaration of interests ܈The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights • LSPR Effects of Ag NPs on luminescence of CDs are systematically investigated. •Luminescence intensity of CDs can be improved 5-6 times by LSPR of Ag@SiO2 NPs. •Fluorescence quenching and enhancement mechanism is clarified between CDs and Ag NPs.
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