Fluorescence enhancement based on cooperative effects of a photonic nanojet and plasmon resonance

Weina Zhang and Hongxiang Lei *
School of Materials Science and Engineering, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, China. E-mail: leihx@mail.sysu.edu.cn

Received 24th January 2020 , Accepted 7th February 2020

First published on 7th February 2020


Developing a universal and simple structure with an excellent fluorescence enhancement is a highly desirable goal for practical applications in optical detection and imaging. Herein, a hybrid structure composed of melamine-formaldehyde (MF) microspheres covering an Au nanorod (AuNR) film (MS/AuNR for short) is reported to enhance fluorescence, which is based on the cooperative effects of a photonic nanojet and plasmon resonance. Moreover, to obtain an excellent plasmonic property, an additional poly(methyl methacrylate) (PMMA) spacing layer with an optimal thickness of 8 nm is added to prevent the fluorescence from directly coming in contact with the AuNR film. Using the proposed hybrid structure and taking the quantum dots (QDs) as fluorescent materials, a maximum enhancement of fluorescence of up to 260 fold is measured. Besides, the hybrid structure is also applied in fluorescence imaging. Utilizing the fluorescence enhancement and pattern magnification effects of the hybrid structure, clear imaging of the 100 nm fluorescent particles is achieved. The above results have important academic value and application prospects in many fields such as weak fluorescence detection and nano-fluorescence imaging.


Due to its low cost, high selectivity, sensitivity and wide versatility, fluorescence-based optical detection and imaging technologies are becoming increasingly important for applications ranging from photonics and biomedicine to materials science and cell biology.1–7 However, the weak fluorescence intensity, which generally decreases the detecting sensitivity and the imaging brightness, limits its further application and development. Therefore, it is extremely essential to enhance fluorescence.8,9 To this end, many substances or structures such as resonant waveguide gratings,10 photonic crystals,11,12 and multilayer metamaterial nanostructures13,14 have been introduced to enhance the fluorescence by adjusting the excitation and fluorescence fields. The above structures can achieve high enhancement factors, but they usually require fine designs and complex nanofabrication processes.15,16 In addition to the high enhancement factors, an ideal fluorescence enhancement structure should also have universality, simplicity and flexibility for practical industrial applications. Metal nanostructure substrates and transparent dielectric microspheres are two promising candidates for fluorescence enhancement. The former can adjust the incident light or the radiation and non-radiative decay rate of the fluorescent molecules based on their unique plasmon resonance effect,17,18 and they can be prepared by simple methods like spacing and spin-coating with good uniformity and repeatability.19–21 The latter, which are known as photonic nanojets22,23 and can be prepared by a simple self-assembly method, can concentrate the incident light into the sub-wavelength range, forming a narrow high-energy beam on the back side of all-dielectric spherical structures and thus increase the energy density and enhance the fluorescence.24–26 However, the above metal-enhanced fluorescent substrates and the microsphere-based fluorescence enhancement structures, especially for the single-photon fluorescence enhancement, have non-ideal enhancement factors of a few to several tens.

Thus, a structure that is simply fabricated and can enhance fluorescence effectively is urgently needed. Compared with the limitations of a single structure, the hybrid structure not only has the original physicochemical properties of the components but also exhibits optimized optical properties and special photo-responses through the interaction between the structural components, so it is widely used in various fields.27–29 Hence, to overcome the limitations of a single structure and further enhance fluorescence, we introduce a hybrid structure composed of melamine–formaldehyde (MF) microspheres covering an Au nanorod (AuNR) film (MS/AuNR, for short) by a simple transfer method and a spin-coating process. Moreover, to obtain an excellent chemical stability and plasmonic property, an additional poly(methyl methacrylate) (PMMA) spacing layer with an optimal thickness of 8 nm is added to prevent the fluorescence from directly coming in contact with the Au nanorod (AuNR) film. Using the proposed hybrid structure and taking the quantum dots (QDs) as fluorescent materials, a maximum enhancement of fluorescence of up to 260 fold is measured. Besides, the hybrid structure is also applied in fluorescence imaging and clear imaging of the 100 nm fluorescent particles is achieved.

Fig. 1a shows the transmission electron microscopy (TEM) image of the Au nanorods (AuNRs) used in our experiment, which are provided by Nanjing Henna Biotech Pte. Ltd, with average sizes of 60 nm in the major axis and 20 nm in the minor axis. The fluorescent material chosen for our experiment is CdSe/ZnS QDs, which are provided by Xingzi New Material Technology Development Co., Ltd, with an average size of 10 nm, as shown in Fig. 1b. Both the AuNRs and QDs are oil-soluble and dispersed in acetone solution. The transparent dielectric microspheres used in our experiment are the melamine-formaldehyde (MF) microspheres with a refractive index of 1.90, provided by Wuhan Huake Weike Technology Co., Ltd. Fig. 1c shows the scanning electron microscopy (SEM) image of the microspheres with a diameter of 10 μm, which shows that they are uniform in size. Fig. 1d shows the photoluminescence (PL) spectrum of the QDs (excited by a 365 nm laser) and the absorption spectrum of AuNRs in the film. It can be seen that the QDs have one emission peak centered at 610 nm and the AuNRs have two absorbance peaks centered at 517 and 616 nm. These two spectra have a large overlap from 550 to 670 nm, which is beneficial for the interaction of the QDs and AuNRs. It is known that matching the localized surface plasmon resonance (LSPR) wavelength with the excitation wavelength of the fluorophores is much more efficient compared to matching with the emission wavelength. However, the commercially available AuNRs usually have a LSPR wavelength range from 500 to 1300 nm. The absorption spectrum of QDs in Fig. S1 exhibits an obviously decreasing absorption with the wavelength changing from 350 to 600 nm. Thus, matching the LSPR wavelength with the excitation wavelength of the fluorophores (500–600 nm) will lead to weak excitation efficiency for the QDs. Hence, in our work, we chose to use AuNRs with a longitudinal LSPR wavelength of 610 nm as plasmonic enhancers for QDs with an excitation wavelength of ∼365 nm. Here, the longitudinal LSPR is always stronger than the transversal one.


image file: d0nr00675k-f1.tif
Fig. 1 The TEM images of the AuNRs (a) and QDs (b). (c) The SEM image of the MF microspheres with a diameter of 10 μm. (d) PL spectrum of QDs (red) and absorption spectrum of AuNRs (black). The PL spectrum is recorded under 365 nm excitation. (e) The schematic illustration of the MS/QD/AuNR hybrid structure. (f) The top-view SEM image of the MS/AuNR hybrid structure.

In our experiment, the QDs are placed between the microspheres and the AuNR film and the MS/AuNR hybrid structure with the QDs is abbreviated as MS/QD/AuNR. Considering that the AuNR film can both enhance the fluorophore quantum yield and also quench the fluorophore quantum yield, an additional 8 nm thick PMMA spacing layer is added to prevent the fluorescence from directly coming in contact with the AuNR film.

A schematic illustration of the MS/QD/AuNR hybrid structure is shown in Fig. 1e. From bottom to top, it can be seen that the hybrid structure contains six portions: a substrate (Si), a plasmonic layer (AuNRs), a spacing layer (PMMA), a fluorescent material layer (QDs), a protective layer (PMMA) and microspheres. Here, the microspheres can be precisely moved by the tapered fiber fabricated through a flame-heating method.30 The protective layer is used to prevent photobleaching to protect the fluorescence. The hybrid structure was fabricated mainly by a spin-coating method and the detailed process can be seen in the Experimental section. Fig. 1f shows the SEM image of the MS/AuNR hybrid structure, including the Si substrate, AuNR (indicated by red arrows) layer and microspheres (marked with a yellow dotted line), indicating the feasibility of the hybrid structure. To further demonstrate the layered structure, the cross-sectional FE-SEM image of the multilayer film is obtained together with elemental mapping, as shown in Fig. S2 in the ESI. It can be seen that a multilayer film with a thickness of ∼70 nm can be faintly observed. The Au and Zn elements have layered distribution at different heights, indicating the separated layered structure of the AuNRs and QD film. The PMMA protective layer and spacing layer were still separated by the QD film layer. The thickness of each layer has been measured and is shown in Fig. S3 in the ESI. It can be seen that the thickness of the plasmonic, spacing, fluorescent material and protective layers is 20.2, 8.2, 22.0 and 19.9 nm, respectively. For comparison, the QD (substrate, fluorescent material layer and protective layer), MS/QD (substrate, fluorescent material layer, protective layer and microspheres), QD/AuNR (substrate, plasmonic layer, fluorescent material layer and protective layer) samples were also fabricated.

To investigate the effect of the fluorescence enhancement mechanism on the MS/AuNR hybrid structure, the field distribution (|E|2/|E0|2) was obtained by a three-dimensional finite-different time-domain (FDTD) method for the different samples and the result is shown in Fig. 2. Fig. 2a shows the field distribution of the MS/AuNR hybrid structure in x, z plane, which is under 365 nm excitation. The diameter and refractive index of the microsphere (marked by the yellow dotted line) are set as 10 μm and 1.9, respectively. The thickness of the PMMA and AuNR layer (marked by the white lines) is set as 50 and 20 nm, respectively. The details of the AuNR layer model are shown in Fig. S4 in the ESI. It can be seen that the incident light passing through the microsphere is concentrated, which forms a photonic nanojet, and thus the intensity is enhanced obviously between the microsphere and the AuNR layer. At 42 nm below the bottom of the microsphere (or 8 nm above the AuNR layer), where the fluorescent material should be located, the incident light was enhanced up to 40 fold. The thickness of 42 nm was chosen to make the simulated results closer to the experimental ones. The thickness of 42 nm in FDTD simulations refers to the distance from the bottom of the microsphere to the upper surface of the PMMA spacing layer, which indicates the thickness of the QD-protective PMMA layer (∼19.9 nm) and the fluorescent material layer. The simulation shows that reducing the thickness from 42 to 20 nm leads to a slight variation, indicating that the impact of the thickness can be ignored in our work (Fig. S5, ESI). In order to explore the source of this field enhancement, the field distribution simulations of the microsphere and the AuNR layer were also obtained under the same excitation conditions, as shown in Fig. 2b and c, respectively. It can be seen that the incident light is concentrated and enhanced at the bottom of the microsphere and the enhancement factor at 42 nm below is 26, which is smaller than that for the MS/AuNR hybrid structure. The field distribution shown in Fig. 2c is obtained at 8 nm above the AuNR layer in the x, y plane. The average enhancement factor of the incident light was 1.73 (maximum ∼2.5). The result shows that the enhancement of excitation light in the MS/AuNR hybrid structure is attributed to the enhancement effect of the microsphere and AuNR layer on the incident light, and the photonic nanojet effect of the former is dominating. It should be noted that the sampling area in our experiment is 1 × 1 μm2 (the minimum sampling area of the microspectrophotometer). Therefore, the enhancement factor of the MS/AuNR hybrid structure for excitation light that can be obtained in simulations is 13.1 under our experimental conditions, and is calculated as an average value in the sampling area.


image file: d0nr00675k-f2.tif
Fig. 2 Simulated field distribution in the central cross section of the MS/AuNR hybrid structure (a) and the microsphere (b) in x, z plane. (c) Simulated field distribution obtained at 8 nm above the AuNR layer, which is in the x, y plane. The excitation wavelength is 365 nm. Simulated field distribution of the fluorescence without (d) and with the microsphere (e). (f) Simulated field distribution obtained at 8 nm above the AuNR layer under 610 nm excitation.

Furthermore, the field distribution of different samples under the fluorescence was obtained by the FDTD method to explore the effect of the MS/AuNR hybrid structure on the fluorescence field. The fluorescence source is set as numerous dipole sources with a wavelength of 610 nm, located in the fluorescent material layer (42 nm below the bottom of microsphere or 8 nm above the AuNR layer). As shown in Fig. 2d, the fluorescence in free space is divergent. The microsphere can concentrate the diverging fluorescent energy, as shown in Fig. 2e, resulting in the increase of the fluorescence detection efficiency, thereby enhancing the fluorescence intensity. To further quantify the enhancement of detection efficiency, a far-field profile for the pure QD sample and MS/AuNR hybrid structure is obtained as shown in Fig. S6 in the ESI. The detection efficiency, defined as the ratio between the collected energy and the total energy of the fluorescence, can be calculated as 17.0 and 41.6% for the pure QD sample and MS/AuNR hybrid structure, respectively, meaning a 2.45 fold enhancement for the detection efficiency. Fig. 2f shows the field distribution under 610 nm excitation, which was obtained at 8 nm above the AuNR layer. Here, the fluorescence field was obviously enhanced with an average enhancement factor of 7.97 (maximum ∼31). Combining the spectra overlap shown in Fig. 1d, this field enhancement here is attributed to the plasmon resonance effect of the AuNRs.

The simulated results above show that the MS/AuNR hybrid structure can enhance fluorescence through the enhancement of the incident light, fluorescence detection efficiency and the fluorescence, which are attributed to the cooperative effects of the photonic nanojet and surface plasmon resonance.

In the experiment, the QD, MS/QD, QD/AuNR and MS/QD/AuNR samples were fabricated and all the QD concentrations are the same as 0.3 mg mL−1. The wavelength of the excitation light is 365 nm. The PL spectra of different samples under the same excitation conditions are shown in Fig. 3a, wherein the inset shows the dark-field optical microscopy image of the MS/QD/AuNR sample. Taking the fluorescence intensity of the pure QD sample as a reference, the fluorescence intensity of the MS/QD/AuNR sample has a 260-fold enhancement, in which 12 fold enhancement results from the presence of the AuNRs (QD/AuNR sample) and 22 fold enhancement results from the presence of the microspheres (MS/QD sample). These fluorescence enhancement results are in agreement with the above simulated results and can be mainly attributed to the photonic nanojet and plasmon resonance effects in the hybrid structure. Besides, the reproducibility of the fluorescence intensities is tested. Three MS/QD/AuNR samples were fabricated under the same conditions and the PL spectra of the samples were measured. The results (Fig. S7) show that a deviation value of less than 5% for the fluorescence intensity of different samples is obtained, indicating the good reproducibility of the fluorescence intensities.


image file: d0nr00675k-f3.tif
Fig. 3 (a) PL spectra of QD, QD/AuNR, MS/QD and MS/QD/AuNR samples. The inset shows the dark-field optical microscopy image of the MS/QD/AuNR sample. (b) Decay curves of 610 nm emission in QD, QD/AuNR, MS/QD and MS/QD/AuNR samples.

To further investigate the effect of dynamics of the fluorescence enhancement on the hybrid structure, the lifetime decay curves of the QDs in different samples were collected and are shown in Fig. 3b. All decay curves can be well fitted by the double exponential decay equation I = A1[thin space (1/6-em)]exp(−t/τ1) + A2[thin space (1/6-em)]exp(−t/τ2), in which τ1 is the fast decay lifetime, resulting from the radiative decay of the QDs and τ2 is the slow decay lifetime, caused by the energy transfer with the quencher or other non-radiative decays of the QDs. For the pure QD sample, the average fluorescence lifetime is 13.2 ns, with fast and slow lifetimes as 11 and 24 ns, respectively. The presence of microspheres makes the average lifetime increase to 17.6 ns, with fast and slow lifetimes increasing as 12 and 38 ns, respectively. According to the theory, the internal quantum efficiency (Q) here can be calculated by the fast and slow decay lifetime as Q = τ2/(τ1 + τ2).24,31,32 The Q of QDs in the pure QD sample is calculated as 68.5% while it is increased as 76% by the reduced non-radiative decay rate of QDs when the microspheres are present. The result shows that the fluorescence enhancement of the microspheres on the QDs is because of not only the enhancement of the incident light and detection efficiency, but also the improvement of the internal quantum efficiency of QDs. For the AuNR/QD sample, the presence of the AuNRs makes the average lifetime decrease to 10.8 ns, with fast and slow lifetimes decreasing as 9 and 20 ns, respectively. The Q of QDs in the AuNR/QD sample is calculated to be 69%, which is consistent with that in the pure QD sample. The change in fluorescence lifetime shows that the presence of AuNRs can increase both radiative and non-radiative decay rates, verifying the occurrence of the plasmon resonance effects. In the MS/QD/AuNR sample, the average fluorescence lifetime increases as 15.6 ns with fast and slow lifetimes of 12 and 33 ns, respectively, and Q has an obvious improvement (73.3%). These results show that the MS/AuNR hybrid structure can also enhance the fluorescence through changing the radiative and non-radiative decay rates of the fluorophore and thus increase the internal quantum efficiency.

Besides, the size effect of the MF microspheres on fluorescence enhancement is further investigated. Fig. 4a shows the dark-field optical microscopy images of the MS/QD samples with microsphere diameters of 1, 5, 8, 10 and 13 μm under 365 nm excitation and the corresponding PL spectra are measured and shown in Fig. 4b. Taking the fluorescence intensity of the pure QD sample as a reference, the enhancement factor as a function of the microsphere size can be obtained as shown in Fig. 4c. In all cases, the QD emission at the place with microspheres is brighter than others because of the fluorescence enhancement effect of the microsphere on the fluorophore. The enhancement factor for the MS/QD samples with a microsphere diameter of 1, 5, 8, 10 and 13 μm is 3.0, 10.7, 22.3, 26.0, and 20.5, respectively. It can be seen that the best enhancement effect is obtained with a 10 μm MF microsphere in our experiment and it can be explained as follows. The fluorescence enhancement by microspheres is mainly due to the convergence of incident light, which can enhance the optical field density. However, the microspheres also cause a certain degree of occlusion of the incident light, which is why the actual enhancement factor is smaller than the simulated one. When the size of the microsphere is small, the convergence of the beam is weak; hence the fluorescence enhancement is not obvious. With the increase of size, both the convergence and occlusion effect for the incident light are enhanced (Fig. S8, ESI) and this competition leads to an optimal size of microspheres for fluorescence enhancement.23


image file: d0nr00675k-f4.tif
Fig. 4 (a) Dark-field optical microscopy images of the MS/QD samples with a microsphere diameter of 1, 5, 8, 10 and 13 μm under 365 nm excitation and the corresponding PL spectra (b). (c) Enhancement factor as a function of the microsphere diameter.

It is well known that the plasmon-based fluorescence regulation is closely related to the distance between the metal nanostructure and the fluorophore.33 Therefore, by changing the concentration of PMMA solution, and controlling the speed and time of spin-coating, the PMMA spacing layers with a thickness of 0, 4, 8, 12 nm were prepared to control the distance between the AuNR layer and QDs. The experimental results are shown in Fig. 5. Fig. 5a shows the dark-field optical microscopy images of the QD/AuNR samples with a PMMA thickness of 0, 4, 8, 12 nm (from left to right) under 365 nm excitation. In particular, to minimize the experimental error, the area without AuNRs on the QD/AuNR sample is engraved, which is marked by the yellow dotted lines and arrows, as a comparison. The schematic illustration of QD/AuNR sample fabrication is shown in Fig. S9 in the ESI. For the QD/AuNR sample without a spacing layer (thickness T = 0), the emission without AuNRs is brighter than that with AuNRs, indicating the fluorescence quenching effect of the AuNRs on QDs. For the QD/AuNR sample with spacing layers T = 4, 8 and 12 nm, the emissions with AuNRs are brighter than those without AuNRs. When the thickness of the spacing layer is T = 8 nm, this brightness contrast is most obvious, meaning the best enhancement effect of AuNRs on QDs. The PL spectra of the QDs on different QD/AuNR samples were collected and are shown in Fig. 5b and the enhancement factor as a function of the PMMA thickness is shown in Fig. 5c. It can be seen that when the AuNRs are directly in contact with the QDs, the fluorescence intensity is decreased and an enhancement factor of 0.46 is obtained, resulting from the quenching effect of the AuNRs. With the increase of thickness, the fluorescence enhancement effect of AuNRs gradually dominates. The fluorescence intensity is enhanced for the QD/AuNR samples and the enhancement factor of 4.1 and 12.0 is obtained when the thickness is 4 and 8 nm, respectively. Then, when the thickness is increased to be 12 nm, the fluorescence enhancement factor is 5, indicating that the fluorescence enhancement was weakened due to the increase of distance. Hence, the 8 nm thick PMMA spacing layer between the AuNRs and QDs is suitable.


image file: d0nr00675k-f5.tif
Fig. 5 (a) Dark-field optical microscopy images of the QD/AuNR samples with a PMMA thickness of 0, 4, 8, and 12 μm (from left to right) under 365 nm excitation and the corresponding PL spectra (b). (c) Enhancement factor as a function of the PMMA thickness.

On the other hand, transparent dielectric microspheres are always used for super-resolution imaging because of their pattern magnification effect.23,34–36 In order to explore this effect of the MF microspheres used in our experiment, an Ag nanopillar array with a side length of 100 nm and a spacing of 100 nm was engraved as shown in Fig. 6a. A MF microsphere with a diameter of 10 μm was placed on the nanopillar array, as shown in Fig. 6b, in which the focal plane is on the array surface. Due to the existence of the diffraction limit, the array cannot be distinguished. By adjusting the focus, the nanopillar array can be seen under the assistance of the microsphere, as shown in Fig. 6c. Fig. 6d shows the intensity distribution extracted along the dashed line in Fig. 6c. It can be seen that the distance between the adjacent peaks is 560 nm, indicating that a 2.8 fold amplification under white light was obtained with the assistance of the 10 μm MF microsphere. This pattern magnification effect allows the MS/AuNR hybrid structure to be used in visual detection of nanoscale materials.


image file: d0nr00675k-f6.tif
Fig. 6 (a) SEM image of the Ag nanopillar array. Optical microscopy images of the array when the focus is on the array surface (b) or on the MS (c). (d) Intensity distribution extracted along the dashed line in (c).

Based on the pattern magnification effect and the fluorescence enhancement, the hybrid structure can also be applied in fluorescence imaging. For example, the nano-fluorescent particles (NFPs) with diameters of 100 nm (Fig. 7a) were used as the fluorescent material layer and the MS/NFP/AuNR sample was fabricated. Fig. 7b and c show bright-field optical microscopy images of the MS/NFP/AuNR sample with the focal plane on the fluorescent material layer surface and on the microsphere, respectively. It can be seen that the NFPs can be resolved under the assistance of the microsphere, but still somewhat blurred. Fig. 7d and e show the corresponding dark-field optical microscopy images of Fig. 7b and c, respectively, which are under 365 nm excitation. It can be seen that the fluorescence under the microsphere is brighter than that in other places. In addition, by adjusting the focus plane, the individual NFP can be resolved clearly. The result shows that the visual detection of a 100 nm fluorescent substance can be realized on the MS/AuNR hybrid structure.


image file: d0nr00675k-f7.tif
Fig. 7 (a) SEM image of the fluorescent nanoparticles. Bright-field optical microscopy image of the MS/AuNR hybrid structure with a focal plane on the fluorescent material layer surface (b) and on the MS (c). Corresponding dark-field optical microscope images of b (d) and c (e).

Besides, the MS/AuNR hybrid structure can be further harnessed in bioimaging. To demonstrate this, an imaging experiment on labeled Staphylococcus aureus (S. aureus) at a single-cell resolution was performed. As shown in Fig. S10 (ESI), the fluorescence and size of the labeled S. aureus on the MS/AuNR hybrid structure were obviously enhanced and amplified, and single S. aureus can be observed clearly. The result shows that the combined effects of the plasmon resonance and photonic nanojet are beneficial for a clear bioimaging and thus have great potential application for life science.

In summary, an easily fabricated MS/AuNR hybrid structure composed of MF microspheres covering an AuNR film is reported to enhance the fluorescence. The QD fluorescence is enhanced by 260 fold in the hybrid structure as a result of the cooperative effects of a photonic nanojet and surface plasmon resonance, which increase the intensity of incident light, the fluorescence detection efficiency and the internal quantum efficiency of QDs. Besides, the size effect of the MF microspheres and the thickness influence of the PMMA spacing layer on fluorescence enhancement are investigated. The result shows that the MF/AuNR hybrid structure has the best fluorescence enhancement effect when the diameter of the microsphere is 10 μm and the thickness of the PMMA spacing layer is 8 nm. In addition, the fluorescence-based super-resolution imaging is also demonstrated using the hybrid structure, indicating the achievement of the visual detection of the nanoscale fluorescent substance on the MS/AuNR hybrid structure. The MS/AuNR hybrid structure shows great promise in fluorescence-based optical detection and imaging.

Experimental section

The fabrication process of the MS/QD/AuNR hybrid structure

First, the substrate was prepared by cleaning silica wafer with acetone, ethanol and distilled water consecutively, and then drying under a stream of nitrogen. Second is the fabrication of the plasmonic layer. The AuNRs and PMMA acetone solutions were mixed at a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and then diluted to a concentration of 2.5 mg mL−1. Afterward, the mixed solution was kept under magnetic stirring at 25 °C for 5 hours to ensure its uniformity. Then, a few drops of the mixture were spread on the Si substrate and then spin-coated at 2000 rpm for 20 s to form a uniform AuNR film. Third, by adjusting the concentration of the PMMA acetone solutions, the PMMA films with different thicknesses can be obtained by spin-coating at 4000 rpm for 20 s. The measurement for the thickness of samples and the PMMA film thickness under different spin-coating conditions is shown in Fig. S11 and S12 in the ESI, respectively. Then, the PMMA-coated sample was annealed at 200 °C for 3 min. The fluorescent material and protective layers were then fabricated using the same spin-coating method consecutively. Afterwards, the microspheres, which were dispersed in water, were transferred on the sample with the injector assistance and then covered on the sample surface through the self-assembly. Finally, the sample was kept in an drying oven at 60 °C for 24 hours to evaporate water and then the MS/QD/AuNR hybrid structure was fabricated.

Apparatus and measurements

The excitation and spectral measurements have been performed under a microspectrophotometer (CRAIC 20, 20/20PV). The optical microscopy images were obtained with the same microspectrophotometer. The specifications of the objective in the microscope, including magnification, numerical aperture (NA), and working distance (WD), are ×10 (×40), 0.2 (0.6), and 7.3 (1.8) mm, respectively. The decay curves of the samples were recorded using a phosphorescence spectrometer (FLS980, Edinburgh) equipped with a 365 nm laser as the excitation source. Transmission and scanning electron microscopes (TEM, JEM-200CX; SEM, Zeiss Gemini Ultra-55) were used to observe the surface morphologies of the AuNRs, QDs, MSs and the hybrid structures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (11974435) and the Guangdong Natural Science Foundation (2018A030313498).

References

  1. I. Rasnik, S. A. McKinney and T. Ha, Nat. Methods, 2006, 3, 891–893 CrossRef CAS.
  2. F. Vetrone, R. Naccache, A. Zamarron, A. J. Fuente, F. Sanz-Rodriguez, L. M. Maestro, E. M. Rodriguez, D. Jaque, J. G. Sole and J. A. Capobianco, ACS Nano, 2010, 4, 3254–3258 CrossRef CAS PubMed.
  3. W. Zhang, J. N. Yao and Y. S. Zhao, Acc. Chem. Res., 2016, 49, 1691–1700 CrossRef CAS PubMed.
  4. X. Feng, Y. Li, X. He, H. Liu, Z. Zhao, R. T. Kwok, M. R. J. Elsegood, J. W. Y. Lam and B. Z. Tang, Adv. Funct. Mater., 2018, 28, 1802833 CrossRef.
  5. Y. H. Tang, D. Y. Lee, J. L. Wang, G. H. Li, J. H. Yu, W. Y. Lin and J. Y. Yoon, Chem. Soc. Rev., 2015, 44, 5003–5015 RSC.
  6. B. J. Beliveau, A. N. Boettiger, M. S. Avendano, R. Jungmann, R. B. McCole, E. F. Joyce, C. K. Kiselak, F. Bantignies, C. Y. Fonseka, J. Erceg, M. A. Hannan, H. G. Hoang, D. Colognori, J. T. Lee, W. M. Shih, P. Yin, X. W. Zhuang and C. T. Wu, Nat. Commun., 2015, 6, 7147 CrossRef CAS PubMed.
  7. S. J. Sahl, S. W. Hell and S. Jakobs, Nat. Rev. Mol. Cell Biol., 2017, 18, 685–701 CrossRef CAS.
  8. A. Y. Ammar, D. Sierra, F. Merola, N. Hildebrandt and X. L. Guevel, ACS Nano, 2016, 10, 2591–2599 CrossRef.
  9. B. Zhou, B. Y. Shi, D. Y. Jin and X. G. Liu, Nat. Nanotechnol., 2015, 10, 924–936 CrossRef CAS PubMed.
  10. J. H. Lin, H. Y. Liou, C. D. Wang, C. Y. Tseng, C. T. Lee, C. C. Ting, H. C. Kan and C. C. Hsu, ACS Photonics, 2015, 2, 530–536 CrossRef CAS.
  11. D. Y. Li, D. L. Zhou, W. Xu, X. Chen, G. C. Pan, X. Y. Zhou, N. Ding and H. W. Song, Adv. Funct. Mater., 2018, 28, 1804429 CrossRef.
  12. N. Ganesh, W. Zhang, P. C. Mathias, E. Chow, J. A. N. T. Soares, V. Malyarchuk, A. D. Smith and B. T. Cunningham, Nat. Nanotechnol., 2007, 2, 515–520 CrossRef.
  13. L. Li, W. Wang, T. S. Luk, X. D. Yang and J. Gao, ACS Photonics, 2017, 4, 501–508 CrossRef CAS.
  14. Y. F. Shen, Y. X. Yan, A. N. Brigeman, H. Kim and N. C. Giebink, Nano Lett., 2018, 18, 1693–1698 CrossRef CAS PubMed.
  15. G. Quaranta, G. Basset, O. J. F. Martin and B. Gallinet, Laser Photonics Rev., 2018, 12, 1800017 CrossRef.
  16. S. Walia, C. M. Shah, P. Gutruf, H. Nili, D. R. Chowdhury, W. Withayachumnankul, M. Bhaskaran and S. Sriram, Appl. Phys. Rev., 2015, 2, 011303 Search PubMed.
  17. J. F. Li, C. Y. Li and R. F. Aroca, Chem. Soc. Rev., 2017, 46, 3962–3979 RSC.
  18. M. Bauch, K. Toma, M. Toma, Q. W. Zhang and J. Dostalek, Plasmonics, 2014, 9, 781–799 CrossRef CAS PubMed.
  19. O. Kedem, W. Wohlleben and I. Rubinstein, Nanoscale, 2014, 6, 15134–15143 RSC.
  20. W. N. Zhang, J. Li, H. X. Lei and B. J. Li, ACS Appl. Mater. Interfaces, 2017, 9, 42935–42942 CrossRef CAS PubMed.
  21. J. Sun, Z. Y. Li, Y. H. Sun, L. B. Zhong, J. Huang, J. C. Zhang, Z. Q. Liang, J. M. Chen and L. Jiang, Nano Res., 2018, 11, 953–965 CrossRef CAS.
  22. B. S. Luk'yanchuk, R. Paniagua-Domínguez, I. Minin, O. Minin and Z. B. Wang, Opt. Mater. Express, 2017, 7, 1820–1847 CrossRef.
  23. J. L. Zhu and L. L. Goddard, Nanoscale Adv., 2019, 1, 4615–4643 RSC.
  24. W. W. Liu, X. H. Li, Y. L. Song, C. Zhang, X. B. Han, H. Long, B. Wang, K. Wang and P. X. Lu, Adv. Funct. Mater., 2018, 28, 1707550 CrossRef.
  25. H. Yang and M. A. M. Gijs, Anal. Chem., 2013, 85, 2064–2071 CrossRef CAS.
  26. Y. Z. Yan, Y. Zeng, Y. Wu, Y. Zhao, L. F. Ji, Y. J. Jiang and L. Li, Opt. Express, 2014, 22, 23552 CrossRef PubMed.
  27. D. H. Kang, S. R. Pae, J. Shim, G. Yoo, J. Jeon, J. W. Leem, J. S. Yu, S. J. Lee, B. Shin and J. H. Park, Adv. Mater., 2016, 28, 7799–7806 CrossRef CAS.
  28. J. Kang, Y. H. Jang, Y. Kim, S. H. Cho, J. Suhr, B. H. Hong, J. B. Cho and D. Byun, Nanoscale, 2015, 7, 6567–6573 RSC.
  29. Z. Yin, H. Li, W. Xu, S. B. Cui, D. L. Zhou, X. Chen, Y. S. Zhu, G. S. Qin and H. W. Song, Adv. Mater., 2016, 28, 2518–2525 CrossRef CAS PubMed.
  30. H. B. Xin, D. H. Bao, F. Zhong and B. J. Li, Laser Phys. Lett., 2013, 10, 036004 CrossRef.
  31. N. Kawano, M. Koshimizu, Y. Sun, N. Yahaba, Y. Fujimoto, T. Yanagida and K. Asai, J. Phys. Chem. C, 2014, 118, 9101–9106 CrossRef CAS.
  32. M. X. Li, W. Zhao, G. S. Qian, Q. M. Feng, J. J. Xu and H. Y. Chen, Chem. Commun., 2016, 52, 14230–14233 RSC.
  33. A. I. Dragan, E. S. Bishop, J. R. Casas-Finet, R. J. Strouse, J. McGivney, M. A. Schenerman and C. D. Geddes, Plasmonics, 2012, 7, 739–744 CrossRef CAS.
  34. Z. B. Wang, W. Guo, L. Li, B. Luk'yanchuk, A. Khan, Z. Lin, Z. C. Chen and M. H. Hong, Nat. Commun., 2011, 2, 218 CrossRef PubMed.
  35. H. Yang, N. Moullan, J. Auwerx and M. A. M. Gijs, Small, 2014, 10, 1712–1718 CrossRef CAS PubMed.
  36. H. Yang, R. Trouillon, G. Huszka and M. A. M. Gijs, Nano Lett., 2016, 16, 4862–4870 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr00675k

This journal is © The Royal Society of Chemistry 2020