- Nano Express
- Open Access
Distance-Dependent Plasmon-Enhanced Fluorescence of Submonolayer Rhodamine 6G by Gold Nanoparticles
Nanoscale Research Letters volume 16, Article number: 90 (2021)
We investigate the fluorescence from submonolayer rhodamine 6G molecules near gold nanoparticles (NPs) at a well-controlled poly (methyl methacrylate) (PMMA) interval thickness from 1.5 to 21 nm. The plasmonic resonance peaks of gold NPs are tuned from 530 to 580 nm by the PMMA spacer of different thicknesses. Then, due to the plasmonic resonant excitation enhancement, the emission intensity of rhodamine 6G molecules at 562 nm is found to be enhanced and shows a decline as the PMMA spacer thickness increases. The variation of spectral intensity simulated by finite-difference time-domain method is consistent with the experimental results. Moreover, the lifetime results show the combined effects to rhodamine 6G fluorescence, which include the quenching effect, the barrier effect of PMMA as spacer layer and the attenuation effect of PMMA films.
Fluorescence quenching [1,2,3,4] and enhancement [5,6,7] are two contradictory phenomena caused by the interaction between optical molecules and metals or metallic nanoparticles. In the past few decades, a considerable amount of reports published are focusing on the fluorophores emission properties in the near field of metal nanoparticles [8, 9]. These studies indicate that the suppression results from damped molecule dipole oscillation or orbital hybridization by the interface interaction [10,11,12,13,14], while the amplification is due to the highly enhanced incident field by local surface plasmon resonance [14,15,16].
Rhodamine 6G (R6G) is widely used as a fluorescent marker and laser dye for its stability, high fluorescence quantum efficiency and low cost. Most researches about R6G molecules have mostly focused on their solutions [17,18,19,20], while R6G molecules in solid state have been less studied [21, 22]. Meanwhile, although extensive researches have been carried out on the plasmon-assisted fluorescence, it is still too complicated to fully understand the interplay between the plasmonic resonance of metallic NPs and the intrinsic optical properties of molecules. In particular, matching the plasmonic resonance peak position with the emission peak of the fluorophore has been emphasized by many groups [23,24,25,26]. This is of great significance to understand the nature of plasmonic effects and for the development of molecular fluorescence-based measuring devices [27,28,29,30,31], such as organic light-emitting diodes (OLEDs) [32, 33], optical sensors [34, 35] and molecular electronic devices [36,37,38].
In our previous work, the single nanocrystal upconversion luminescence enhancement could be obtained by controlling surface plasmon resonance wavelength and NPs sizes . The thickness of poly (methyl methacrylate) (PMMA) separating layer could be precisely controlled to tune the emission properties of single quantum dots . Tetraphenyl porphyrin (TPP) molecules have been demonstrated to be affected dramatically by the localized plasmon mode [41, 42].
In this work, the plasmonic resonance peaks are tuned well overlapped with the molecular emission peak. The photoluminescence (PL) spectra and fluorescent lifetimes of submonolayer R6G molecules on the surface of gold NPs strengthen the evidence of a plasmon-enhanced dominance over nonradiative decay. This study provides an important opportunity to advance the understanding of single or submonolayer R6G molecules in solid state.
Fabrication of Substrate
To obtain the clean glass substrate with negative charges on its surface, the glass substrate was soaked in piranha solution for 30 min and rinsed by deionized water. Then, the glass substrate was put into 3 ml of 140-nm gold nanoparticle (Au NPs) solution (Crystano™) with a PH of 3.0 for over 12 h. Following this treatment, the Au NPs were absorbed firmly on the substrate based on electrostatic adsorption. After being washed and dried, the density of Au NPs on glass substrate was characterized by atomic force microscope (AFM).
Poly (methyl methacrylate) (PMMA) film was prepared by spin-coating at 3000 rpm for 60 s as a spacer between Au NPs and rhodamine 6G (R6G) molecules. For the purpose of controlling thicknesses of the spacer, PMMA methylbenzene solution with different concentrations of 0.03wt%, 0.1wt% and 0.4wt% was spin-coated on the glass surface.
Submonolayer Rhodamine 6G Molecules Preparation
The submonolayer R6G molecules were sublimated to the surface of gold or glass substrate in 10–6 mbar vacuum at room temperature by thermal evaporation. The evaporation rate and the molecular coverage are controlled by continuous heating voltage, current and deposition time. The process was repeated several times using a scanning tunneling microscope (STM) in order to ascertain an appropriate preparation condition. The distribution of submonolayer R6G molecules on the substrates was characterized by STM and AFM.
The photoluminescence (PL) spectra and fluorescence lifetime were obtained in 10–5 mbar vacuum at room temperature. Steady-state PL spectra were measured by a liquid nitrogen-cooled charge-coupled device (CCD) spectrometer (Princeton Instruments), while photon counting and lifetime measurements were completed with a microchannel plate photomultiplier tube (Hamamatsu) combined with time-correlated single photon counting technique (Edinburgh Instruments). A pulse picosecond semiconductor laser at 375 nm (Advance Laser System) was used to excite the samples.
The finite-difference time-domain (FDTD) method was used to perform the numerical simulation with the software FDTD Solutions (Lumerical Solution, Inc., Canada). Au nanoparticles with a diameter of 140 nm and different PMMA spacer thicknesses are placed on glass substrates. The dielectric constant of gold is taken from Ref. , and the dielectric constants of CTAB and PMMA are taken as 1.40 and1.49, respectively. The refractive index of the surrounding matrix is set to 1.0 for air. A plane-wave total field-scattered field source ranging from 400 to 700 nm is utilized as the incident light. The electric field distribution near Au nanoparticle is evaluated using the frequency-domain field profile monitors. A three-dimensional nonuniform meshing is used, and a grid size of 0.5 nm is chosen for the inside and vicinity of Au nanoparticle. We use perfectly matched layer absorption boundary conditions as well as symmetric boundary conditions to reduce the memory requirement and computational time. The numerical results pass prior convergence testing.
Results and Discussion
The size and shape of 140 nm Au NPs were firstly characterized by a transmission electron microscope (TEM). As shown in Fig. 1a, most particles are well dispersed with an average diameter of 140 ± 10 nm. The gold NPs coated with ultrathin cetyltrimethylammonium bromide (CTAB) surfactant layers are also identified in Fig. 1b. The histogram shows that the shell thicknesses are 2.5 ± 1 nm (Fig. 1c), corresponding to monolayer or bilayer CTAB .
Typical AFM images of the Au NPs adsorbed on the glass substrate without and with a layer of PMMA are shown in Fig. 2a, b. Comparing Fig. 2a and b, we can find that the density of Au NPs in the same range is similar, but the spin-coated PMMA film cannot be distinguished by AFM image. Therefore, a benchtop stylus profilometer (Bruker) was used to assess the thickness of PMMA films with different concentrations. Figure 2c–e shows three samples with concentrations of 0.03wt% (~ 1.5 nm), 0.1wt% (~ 6.5 nm) and 0.4wt% (~ 21 nm), respectively. Figure 2f shows the absorption spectra of four different substrates. A clear redshift of surface plasmon resonance (SPR) peak can be found with the increasing thickness of PMMA layers. This might be due to the increase in the local refractive index around Au NPs, which has good consistency with the previous literature .
Submonolayer R6G molecules were achieved by the following steps. Firstly, the Au (111) surface reconstructed after ion sputtering and high temperature annealing, which can be confirmed by its regular ‘herringbone’ stripes (Fig. 3a). Then, after 60 s of thermal evaporation at a voltage of 0.8 V and a current of 0.6 A, R6G molecules were steamed onto the treated Au (111) surface and cooled to 80 K using liquid nitrogen. The distribution state of molecular submonolayer can be characterized by STM (Fig. 3b). When narrowing the range, single isolated molecules, 3 nm in diameter and 0.4 nm in height, which are similar to poached eggs, can be observed stably and repeatedly (Fig. 3c, d). This can be attributed to weak molecular–substrate interaction.
In the same condition, R6G molecules were deposited on the glass and PMMA/glass substrates, respectively. However, the room temperature is much higher than the temperature in the operation cavity of STM. So, the decrease of molecular–substrate interaction and the acceleration of molecular mobility make R6G molecules on the surface of the glass molecular clusters. But their coverage is still less than a single layer (Fig. 4a), which can also be verified on PMMA film (Fig. 4b). Comparing the insets of Fig. 4a and b, we can find that the size of the molecular clusters on the PMMA film is larger at room temperature in air, while the quantity decreases. This result may be explained by the fact that the migration rate and adsorption capacity of R6G molecules on different substrate surfaces are different.
Figure 4c shows the AFM image of R6G molecules on PMMA/Au NPs/glass. Due to the large difference in size between R6G molecular clusters and gold nanoparticles, it is difficult to observe both gold nanoparticles and molecular clusters at the same time. But the molecular clusters can be observed in the profile line (inset of Fig. 4c) compared with that on PMMA/glass.
Fluorescent spectra of R6G/PMMA/Au NPs/glass with different thicknesses of PMMA layers, R6G/Au NPs/glass and R6G/glass are plotted in Fig. 4d. The luminescent intensity of R6G/(PMMA)/Au NPs/glass is found to be enhanced compared with that of R6G/glass. From the luminescent enhancement factor of Table 1, we can see that R6G/Au NPs/glass has the largest enhancement with a factor of about 3.78. And its intensity decreases with the increasing thickness of the PMMA film.
Considering the absorption peaks of Au NPs coated with different thicknesses of PMMA (530–580 nm) and the emission peak of R6G molecules (562 nm), the fluorescence enhancement mechanism is related to the spectral overlap extent and the separation distance between molecules and nanoparticles. Both PMMA and CTAB on the surface of the gold sphere as the separation layers play key roles in reducing the nonradiative energy transfer between R6G molecules and Au NPs. Since plasmon resonance is a strong local near-field effect, the thickening of the interval layer makes R6G gradually get away from the range of strong local field. This leads to the weakening of the enhancement effect with the increase in PMMA thickness. On the other hand, the shift of the plasmon resonant peak also makes a contribution to the emission intensity. The absorption spectral linewidth of the four samples is very wide. All of them cover the emission peak of R6G. Although there is the best match between the plasmon resonant peak of Au nanospheres and the emission peak of R6G when the separation thickness is about 9 nm, the emission enhancement of R6G is not stronger than that without PMMA layer due to the decrease in near-field enhancement effect of Au nanospheres in large separation distance. Hence, the separation distance between Au nanospheres and R6G molecules plays a key role in the emission enhancement of R6G molecules.
In addition to the intensity change, the fluorescence lifetimes of R6G molecules are also detected, as shown in Fig. 4e. A tri-exponential function can well fit the decay process of excited R6G molecules, which are shown in Table 1. As the diagram shows, when the R6G molecule is directly evaporated onto the gold nanoparticles, the fluorescence lifetime is the shortest due to the quenching effect of the metal. With the thickening of the spacer layer, the quenching effect decreases, and the plasmonic enhancement effect also weakens, leading to the increase in the lifetime. However, the test results show that the fluorescence lifetime is not prolonged with the increasing thickness of PMMA, but gets shortened. Although, they are still longer than the molecular lifetime directly on the gold nanoparticles. In order to find out the reason, we tested the fluorescence lifetime of the R6G molecules on the PMMA/glass substrate (Fig. 4f and Table 2). It is also found that PMMA can affect the lifetime of the molecules, which decreases with increasing thickness. This is consistent with the phenomena of PMMA/Au NPs/glass. Therefore, the lifetime of the molecules in Fig. 4f is extended and then shortened, which results from the existence of PMMA film. When R6G molecules are close to Au NPs, the lifetime shows an obvious quenching effect. As the thickness of PMMA film increases, the barrier and the attenuation effects of PMMA film are observed.
To explain the observed distance-dependent fluorescence intensity of R6G molecules, the near-field distributions of Au NPs with different spacer thicknesses were simulated with the FDTD method. As shown in Fig. 5a–d, strong electric field enhancements (|E/E0|2) are observed around the surfaces of PMMA/CTAB/Au NPs nanostructures. In Fig. 5e, the enhancement factors of the experimental fluorescence spectra and the simulated electric field with the increasing spacer thickness display a good agreement. The simulated near-field enhancement factor is much larger than that obtained in the experiment. This reason can be mainly ascribed to the ideal models of theoretical simulations and the fluorescence quenching effect of Au NPs.
In summary, we evaporated submonolayer R6G molecules on the gold NPs with the controlled PMMA spacer thickness (1.5–21 nm). The PL spectra and decay curves were studied. The molecular fluorescence intensity is enhanced by the resonant excitation enhancement and shows a decline as the thickness of PMMA film increased. The experimental enhancement factor is far below the theoretical one obtained by FDTD simulation mainly due to the quenching effect induced by the charge transfer and nonradiative energy transfer between the excited molecules and the Au NPs. Furthermore, it is interesting to note that the PMMA films with different thicknesses contain both barrier and lifetime attenuation effects, which is confirmed by the fluorescence lifetime measurements. This study may pave the way to the practical metal-enhanced fluorescence applications in optical imaging, biotechnology and material detection fields.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
- Au NPs:
Poly (methyl methacrylate)
Organic light-emitting diodes
Atomic force microscope
Scanning tunneling microscope
Finite difference time domain
Transmission electron microscope
Surface plasmon resonance
Xie XS, Dunn RC (1994) Probing single molecule dynamics. Science 265:361–364
Avouris P, Persson BNJ (1984) Excited states at metal surfaces and their non-radiative relaxation. J Phys Chem 88:837–848
Huang CW, Lin HY, Huang CH, Lo KH, Chang YC, Liu CY, Wu CH, Tzeng Y, Chui HC (2013) Fluorescence quenching due to sliver nanoparticles covered by graphene and hydrogen-terminated graphene. Appl Phys Lett 102:053113
Kittler S, Klemmed B, Wolff T, Eychmüller A (2018) Quenching of R6G fluorescence by gold nanoparticles of various particle geometries. Z Phys Chem 232:1–11
Shimizu KT, Woo WK, Fisher BR, Eisler HJ, Bawendi MG (2002) Surface-enhanced emission from single semiconductor nanocrystals. Phys Rev Lett 89:117401
Mo J, Jiang J, Zhai Z, Shi F, Jia Z (2017) Enhancement of R6G fluorescence by N-type porous silicon deposited with gold nanoparticles. Optoelectron Lett 13:10–12
Ziegler J, Djiango M, Vidal C, Hrelescu C, Klar TA (2015) Gold nanostars for random lasing enhancement. Opt Express 23:15152–15159
Jiang T, Du Y, Ma Y, Zhou J, Gu C, Tang S (2017) Decrease of amplified spontaneous emission threshold achieved by core-shell Ag nanocube@SiO2 with ultrasmall shell thicknesses. Mater Res Express 4:115030
Zhang Y, Yang C, Zhang G, Peng Z, Yao L, Wang Q, Cao Z, Mu Q, Xuan L (2017) Distance-dependent metal enhanced fluorescence by flowerlike silver nanostructures fabricated in liquid crystalline phase. Opt Mater 72:289–294
Dulkeith E, Morteani AC, Niedereichholz T, Klar TA, Feldmann J (2002) Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects. Phys Rev Lett 89:203002
Gebauer W, Langner A, Schneider M, Sokolowski M, Umbach E (2004) Luminescence quenching of ordered π-conjugated molecules near a metal surface: quaterthiophene and PTCDA on Ag(111). Phys Rev B 69:155431
Anger P, Bharadwaj P, Novotny L (2006) Enhancement and quenching of single-molecule fluorescence. Phys Rev Lett 96:113002
Kühn S, Håkanson U, Rogobete L, Sandoghdar V (2006) Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna. Phys Rev Lett 97:017402
Chen Y, Munechika K, Ginger DS (2007) Dependence of fluorescence intensity on the spectral overlap between fluorophores and plasmon resonant single silver nanoparticles. Nano Lett 7:690–696
Tam F, Goodrich GP, Johnson BR, Halas NJ (2007) Plasmonic enhancement of molecular fluorescence. Nano Lett 7:496–501
Kern AM, Martin OJF (2011) Excitation and reemission of molecules near realistic plasmonic nanostructures. Nano Lett 11:482–487
Guo X, Zafar A, Nan H, Yu Y, Zhao W, Liang Z, Zhang X, Ni Z (2016) Manipulating fluorescence quenching efficiency of graphene by defect engineering. Appl Phys Express 9:055502
Zhang X, Li F (2012) Interaction of graphene with excited and ground state rhodamine revealed by steady state and time resolved fluorescence. J Photochem Photobiol A 246:8–15
Zehentbauer FM, Moretto C, Stephen R, Thevar T, Gilchrist JR, Pokrajac D, Richard KL, Kiefer J (2014) Fluorescence spectroscopy of Rhodamine 6G: concentration and solvent effects. Spectrochim Acta A 121:147–151
Hu F, Gao S, Zhu L, Liao F, Yang L, Shao M (2016) Tunable fluorescence enhancement based on bandgap-adjustable 3D Fe3O4 nanoparticles. Nanotechnology 27:245709
Uehara Y, Ushioda S (2005) Single molecule spectrum of rhodamine 6G on highly oriented pyrolytic graphite. Appl Phys Lett 86:181905
Klingsporn JM, Jiang N, Pozzi EA, Sonntag MD, Chulhai D, Seideman T, Jensen L, Hersam MC, Duyne RPV (2014) Intramolecular insight into adsorbate−substrate interactions via low-temperature, ultrahigh-vacuum tip-enhanced Raman spectroscopy. J Am Chem Soc 136:3881–3887
Heiderscheit TS, Gallagher MJ, Baiyasi R, Collins SSE, Jebeli SAH, Scarabelli L, Al-Zubeidi A, Flatebo C, Chang W, Landes CF, Link S (2019) Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence. J Chem Phys 151:144712
Munechika K, Chen Y, Tillack AF, Kulkarni AP, Plante IJ, Munro AM, Ginger DS (2010) Spectral control of plasmonic emission enhancement from quantum dots near single silver nanoprisms. Nano Lett 10:2598–2603
Lee SA, Biteen JS (2019) Spectral reshaping of single dye molecules coupled to single plasmonic nanoparticles. J Phys Chem Lett 10:5764–5769
Wertz EA, Isaacoff BP, Biteen JS (2016) Wavelength-dependent super-resolution images of dye molecules coupled to plasmonic nanotriangles. ACS Photonics 3:1733–1740
Conklin D, Nanayakkara S, Park T, Lagadec MF, Stecher JT, Chen X, Therien MJ, Bonnell DA (2013) Exploiting plasmon-induced hot electrons in molecular electronic devices. ACS Nano 7:4479–4486
Banerjee P, Conklin D, Nanayakkara S, Park T, Therien MJ, Bonnell DA (2010) Plasmon-induced electrical conduction in molecular devices. ACS Nano 4:1019–1025
Chen M, Yang Y, Chen S, Li J, Aklilu M, Tai Y (2013) Self-assembled monolayer immobilized gold nanoparticles for plasmonic effects in small molecule organic photovoltaic. ACS Appl Mater Interfaces 5:511–517
Pozzi EA, Sonntag MD, Jiang N, Chiang N, Seideman T, Hersam MC, Duyne RPV (2014) Ultrahigh vacuum tip-enhanced Raman spectroscopy with picosecond excitation. J Phys Chem Lett 5:2657–2661
Ni Y, Kan C, He L, Zhu X, Jiang M, Shi D (2019) Alloyed Au–Ag nanorods with desired plasmonic properties and stability in harsh environments. Photonics Res 7:558–565
He X, Wang W, Li S, Wang Q, Zheng W, Shi Q, Liu Y (2015) Localized surface plasmon-enhanced electroluminescence in OLEDs by self-assembly Ag nanoparticle film. Nanoscale Res Lett 10:468
Fukuura T (2015) Plasmons excited in a large dense silver nanoparticle layer enhance the luminescence intensity of organic light emitting diodes. Appl Surf Sci 346:451–457
Jain PK, Huang X, El-sayed IH, El-sayed MA (2008) Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine. Acc Chem Res 41:1578–1586
Deng H, Yu H (2018) Self-assembly of rhodamine 6G on silver nanoparticles. Chem Phys Lett 692:75–80
Yu H, Luo Y, Beverly K, Stoddart JF, Tseng H, Heath JR (2003) The molecule–electrode interface in single molecule transistors. Angew Chem Int Ed 42:5706–5711
Pal PP, Jiang N, Sonntag MD, Chiang N, Foley ET, Hersam MC, Duyne RPV, Seideman T (2015) Plasmon-mediated electron transport in tip-enhanced Raman spectroscopic junctions. J Phys Chem Lett 6:4210–4218
James DK, Tour JM (2004) Electrical measurements in molecular electronics. Chem Mater 16:4423–4435
Xue Y, Ding C, Rong Y, Ma Q, Pan C, Wu E, Wu B, Zeng H (2017) Tuning plasmonic enhancement of single nanocrystal upconversion luminescence by varying gold nanorod diameter. Small 13:1701155
Song M, Wu B, Chen G, Liu Y, Ci X, Wu E, Zeng H (2014) Photoluminescence plasmonic enhancement of single quantum dots coupled to gold microplates. J Phys Chem C 118:8514–8520
Dong ZC, Zhang XL, Gao HY, Luo Y, Zhang C, Chen LG, Zhang R, Tao X, Zhang Y, Yang JL, Hou JG (2010) Generation of molecular hot electroluminescence by resonant nanocavity plasmons. Nat Photonics 4:50–54
Niu J, Pan C, Liu Y, Lou S, Wu E, Wu B, Zhang X, Jin Q (2018) Plasmon-enhanced fluorescence of submonolayer porphyrins by silver-polymer core-shell nanoparticles. Opt Express 26:3489–3496
Carper J (1999) The CRC handbook of chemistry and physics. Libr J 124:192
Sui ZM, Chen X, Wang LY, Xu LM, Zhuang WC, Chai YC, Yang CJ (2006) Capping effect of CTAB on positively charged Ag nanoparticles. Phys E 33:308–314
Liz-Marzán LM, Giersig M, Mulvaney P (1996) Synthesis of nanosized gold-silica core-shell particles. Langmuir 12:4329–4335
This work was supported by the National Key R&D Program of China (2017YFA0303403), the National Natural Science Foundation of China (Grant Nos. 11674095, 11874015, 11674099, 11722431), Shanghai International Cooperation Project (16520710600) and Program of Introducing Talents of Discipline to Universities (B12024).
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Bian, Y., Liu, S., Zhang, Y. et al. Distance-Dependent Plasmon-Enhanced Fluorescence of Submonolayer Rhodamine 6G by Gold Nanoparticles. Nanoscale Res Lett 16, 90 (2021). https://doi.org/10.1186/s11671-021-03546-7
- Rhodamine 6G molecule
- Gold nanoparticles
- Poly (methyl methacrylate)
- Plasmon-enhanced fluorescence
- Quenching effect
- Atomic force microscope
- Scanning tunneling microscope
- Finite-difference time-domain method