Metal-Enhanced Fluorescence and Ultrafast Energy Transfer of Dyes

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Chapter 9

Metal-Enhanced Fluorescence and Ultrafast Energy Transfer of Dyes near Silver Nanosurfaces Jaebeom Lee,1 Sebok Lee,2 Myungsam Jen,2 Daedu Lee,2 Junghyun Song,2 and Yoonsoo Pang*,2 1Department

of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea 2Department of Chemistry, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea *E-mail: [email protected]

When dye molecules exist near the plasmonic metal nanostructures, emission intensities of dye molecules are strongly increased by a locally enlarged electric field around the nanostructures or as a result of energy transfer and subsequent emission occurring between dye molecules and metal nanostructures. The excited-state dynamics of several dyes including 4-(dicyanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4H-pyran (DCM) and rhodamine 6G were explored in the presence of a silver island film (SIF) by ultrafast time-resolved electronic spectroscopic methods. Metal-enhanced fluorescence (MEF) of dyes showed strong dependences on local environments including solvents and the excitation wavelength of time-resolved optical measurements. From experimental evidences of ultrafast energy transfer between metal nanostructures and dye molecules and changes in the excited-state lifeteimes of dye molecules, detailed mechanisms of MEF will be thoroughly investigated. Lastly, recent developments in experimental investigations including homogeneous silver nanostructures and dye molecules under various local environments will also be included.

© 2016 American Chemical Society Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Introduction Emission of dye molecules is strongly augmented near plasmonic metal nanoparticles or nanosurfaces due to the increase of local electric field or the energy transfer between dye molecules and the surface plasmons of metal nanosurfaces (1, 2). This phenomenon is called metal-enhanced fluorescence (MEF) and has been extensively investigated and applied to many disciplines, including biological molecule detection, single-molecule spectroscopy and imaging, biophotonics, and etc. (1, 3–5) The details of the MEF mechanisms are still left in question (1, 6). It is generally accepted that the fluorescence of photoexcited molecules is significantly increased in intensity (up to 104 times) by a locally increased electromagnetic field due to plasmonic metal nanoparticles (1). This is called the electric field effect and the enhancements strongly depend on the size and shape of nanoparticles and distance of dye molecules from the nanoparticles (1, 7, 8). The so called induced plasmonic effect based on through-space energy transfer from fluorophores to nanoparticles results in significant increases in the quantum yields and decreases in the fluorescence lifetimes (1, 6). Recently, Geddes and co-workers investigated the wavelength dependence of MEF on silver island film (SIF) and introduced the surface plasmon coupled emission (SPCE) and fluorophore radiation through the scattering mode of the nanoparticles, which has been confirmed by many other following reports (2, 9–14). A number of metal nanostructures from inhomogeneous structures of silver island films (SIFs) and silver colloidal nanoparticles, to homogeneous nanostructures such as gold nanorods and a gold bowtie nano-antenna have been introduced for localized surface plasmons and efficient MEF for many applications (15–23). The SIFs are mainly composed of inhomogeneously distributed islands of different sizes and shapes, but have been numerously applied in many studies especially with biological systems (18, 22, 23). Especially, Mackowski and co-workers investigated the MEF of photosystem 1 of cyanobacteria with SIFs, where the wavelength-dependent enhancement factor of 200 was achieved (23). Inside the PS1 complex, there are two major pigments which might interact with the surface plasmon of the SIF, carotenoids and chlorophylls, and the largest enhancements were observed with 640 nm excitation which is quite off from the plasmonic excitation maximum around 450 nm, but very close to the emission of chlorophyll a. This might be interpreted as the SPCE or fluorophore radiation coupled to the scattering mode of nanoparticles, but any temporal changes in the emission dynamics with the SIF was not observed in a couple of hundred picoseconds to several nanoseconds time sclaes. It might be interesting to study the same system under femtosecond transient absorption to reveal any ultrafast energy transfer between chlorophylls and the SIF. On the other hand, from similar PS1 complexes deposited on the silver colloidal surface composed of 20 nm colloidal nanoparticles, strong enhancements of chlorophyll a emission bands and a much faster decay in the emission lifetime (~15 ps) of PS1 were observed especially from local hot spots composed of multiple nanoparticles (18). The experimental results on PS1 complexes might be a good example to show that the 210 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


mechanisms of MEF are inherently complex and that further investigation in both experiment and theory are necessary. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) is one of laser dyes often used in many experimental and theoretical studies due to its very unique photophysics including intramolecular charge transfer (ICT) state formation and large Stokes shift (24–29). DCM is one of 4,4′-substituted stilbene derivatives and also push-pull-type emitters. Upon photoexcitation, DCM shows an ultrafast (less than a few hundred femtoseconds) transition to the ICT state in the S1 state, especially in polar solvents, and fluoresces from the S1-ICT state with high quantum yields. The molecular structure of DCM in the S1-ICT state, often called the twisted ICT state, and the existence of a transient state between the locally excited (LE) and ICT states have been of great interest, which has been explored recently in time-resolved IR and 2D UV-IR experiments (29, 30). Rhodamine 6G is one of most frequently used dyes in dye lasers and as a fluorescence tracer in many environmental and biological applications (31, 32). Recently, nanostructure or morphology-dependent surface-enhanced fluorescence of rhodamin 6G on various silver nanostructures have been reported (32). The absorption and emission spectra of DCM in several polymer matrices have been investigated, where the absorption and emission wavelengths, and the emission lifetime of DCM showed strong dependence on the matrix polarity (33). A numerical method called finite-difference time-domain (FDTD) method has been introduced to calculate optical properties of metal nanostructures and fluorophore-metal interactions by the Mie theory which is based on the solution of Maxwell equations (34, 35). The FDTD method is one of general tools in estimating the electric field distributions and the fluorophore-metal interactions, and MEF in metal nanosurfaces can be easily studied by several commercial software packages (36, 37). In this chapter, results of recent investigations on time-resolved fluorescence enhancements and ultrafast energy transfers of DCM and rhodamine 6G with the SIF were summarized and some preliminary results on MEF of several dyes on various metal nanostructures were also presented. Excited-state dynamics and ultrafast energy transfer between DCM in thin solution layer and the SIF were investigated by femtosecond transient absorption spectroscopy. MEF and resulting changes in the excited-state dynamics of laser dyes DCM and rhodamine 6G were also investigated with the SIF by steady-state absorption and emission measurements and by time-resolved fluorescence measurements. To minimize emission quenching of dyes, thin polymer films of polyvinyl alcohol (PVA), polyethylene glycol (PEG), and etc. containing dye molecules were spin-coated on the SIF substrate, and strong emission enhancements of dyes were observed near the silver island surface. In order to understand the mechanism of MEF on various silver nanosurfaces, theoretical approaches including the FDTD method and time-dependent density functional theory (TDDFT) method and further experimental results with more homogeneous nanostructures and chromophores facing diverse local environments might be required (28, 38).

211 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Experimental Section Chemicals


Silver nitrate (Junsei Chemical), Rhodamine 6G, 4-(dicyanomethylene)2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (Sigma-Aldrich), and all other chemicals were used without further purification. All glassware and fused silica windows (Spectrosil 200, UQG Optics) were thoroughly cleaned with an aqua regia solution and rinsed many times with triply-distilled water before use.

Silver Nanosurfaces Silver nanosurface synthesized from colloidal nanoparticles and directly formed by the silver mirror reaction were used in this study. Silver colloidal surfaces prepared by immersing fused silica windows in silver colloidal solutions (reduced by borohydride and citrate ions) (39, 40) provided, in general, weak fluorescence enhancements of dyes, due to low particle density observed in SEM image (Figure 1a). Silver island films (SIFs) were formed on the fused silica substrates with minimal fluorescence background by reducing a silver ammonium complex solution with d-glucose (16, 41). Absorbance of the SIFs was optimized to 0.2-0.3 in 400-550 nm range, which was found optimal for fluorescence enhancements with several dyes. The size of silver islands was measured by a Hitachi 4700 FE-SEM and a broad distribution of 80-170 nm in diameter (Figure 1b) was found in the SIFs used.

Figure 1. (a) SEM image of a silver island film (SIF; inset: image of a colloidal silver film), (b) the size distribution of silver islands in the SEM image, (C) the sample flow cell. (Reproduced with permission from reference (43). Copyright 2015 American Chemical Society.) 212 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Half of the SIF coated on the fused silica windows was wiped out, which was used for the measurements of emission enhancements and dynamics changes in time-resolved experiements. A flow sample cell composed of two fused silica windows with the half of one window coated with the SIF, was used in the the steady-state fluorescence and transient absorption measurements (Figure 1c). The thickness of the solution layer in the sample cell was determined from absorption measurements of reference samples and the solution was circulated slowly by a peristaltic pump during the transient absorption measurements.

Dyes Dispersed in Thin Polymer Films Dyes were dissolved in 1 % (w/v) solutions of PVA (avg. m.w. 88,000) in methanol-water mixture (1:1) and PEG (avg. m.w. 2,000-880,000) in tetrahydrofuran. These polymer solutions were spin-coated on the SIFs prepared on the fused silica windows. Thickness of coated polymer films which were determined from absorption measurements was maintained as thin as

Metal-Enhanced Fluorescence and Ultrafast Energy Transfer of Dyes

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