ORIGINAL PAPER Fluorescence Spectral Properties of Rhodamine 6G at the Silica/Water Interface Zhe Chen & Yao-Ji Tang & Tang-Tang Xie & Ying Chen & Yao-Qun Li Received: 24 May 2007 /Accepted: 14 August 2007 /Published online: 25 September 2007 # Springer Science + Business Media, LLC 2007 Abstract In this work, total internal reflection synchronous fluorescence spectroscopy (TIRSF) is applied successfully to investigate rhodamine 6G (R6G) at the silica/water interface. In comparison with the bulk spectra, 5 nm red shift is observed in the interface spectra, which is mainly due to the limitation of freedom of rotational movement of R6G molecules at the interface. The increase of R6G concentration induces the self-quenching of adsorbate at the interface. The dependence of interfacial fluorescence on the acidity and ionic strength was studied. Both the acidity and ionic strength affect the adsorptive behaviors of R6G at the silica/water interface. Keywords TIRSF. R6G . Silica/water interface Introduction Of particular interest in interface analysis is the optical characteristic of molecules affected by their physical and chemical environment [1���4]. In last few decades, many efforts were focused on photophysics and photochemistry of interfaces [5���11]. On the other hand, many species, especially dyes, were examined on materials [12], ordered supramolecular assemblies [13] in attempt to understand the phenomena that happen at interfaces. However, the behavior of the asymmetric forces acting on molecules is more complex and inhomogenous at the interface than in the bulk phase, which is both qualitatively and quantitatively well known. One of the key problems in the investigation of the interface is the adsorption of molecules, which reflects the weak interaction between environment and molecules [14, 15]. As a common cationic dye, rhodamine 6G (R6G, Fig. 1) has been employed to observe phenomena at the interface on the physical and chemical aspects for a long time [1, 2, 4���7]. L��pez Arbeloa et al. [16] probed the orientation of R6G in the clay by using florescence polarization. Brennan et al. [17] applied time-resolved fluorescence anisotropy approach to measure the surface coverage of SiO2 in the bulk solution. However, there are only a few tools available for probing surface without disturbance from the bulk phase or probing interface origins directly. In order to study molecules at silica/water interface with fluorescence approaches, it is necessary to distinguish the interface-originated signals with those from a large over- whelming number of molecules in the bulk solution. Total internal reflection fluorescence (TIRF) spectroscopy is chosen in this study because it is of interface-specific and can be easily applied to in situ observation at an interface [11, 18���20]. Synchronous fluorescence spectroscopy has been shown to give narrow and simple spectra [21���24]. Unlike conventional fluorescence excitation and emission spectra, a synchronous fluorescence spectrum is taken as a combined function of excitation and emission wavelengths. The simplest synchronous scanning technique maintains a constant-wavelength difference (��l) between emission and excitation wavelengths during the acquisition of a spec- trum. The combination of TIRF and synchronous scanning technique has been suggested in a previous paper [25]. The goal of this work is to investigate the adsorption of R6G on silica/water interface, to discuss the causes for the interfacial fluorescence behaviors, and to provide a novel perspective to recognize surface-active molecules. In this J Fluoresc (2008) 18:93���100 DOI 10.1007/s10895-007-0241-x Z. Chen : Y.-J. Tang : T.-T. Xie : Y. Chen : Y.-Q. Li (*) Department of Chemistry and The Key Laboratory of Analytical Sciences of the Ministry of Education, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China e-mail: yqlig@xmu.edu.cn

work, total internal reflection synchronous fluorescence spectroscopy (TIRSF) is applied successfully to measure rhodamine 6G (R6G) at the silica/water interface. The mechanism of the spectral red shift of R6G at the interface is proposed. The effects of concentrations, solution acidity and ionic strength on fluorescent intensity are studied. Experimental section Materials Rhodamine 6G (Shanghai Reagent Inc.) was of analytical grade and used without further purification. For the experiments on acidity effects, 1.0 M HCl was used to prepare serial solutions of R6G with different pH. The ionic strength of the solution was adjusted with 2.0 M NaCl. Other reagents were of analytical grade. All solutions were prepared with deionized water. Hydrophobic silica slice was processed by dichlorodi- methylsilane ((CH3)2SiCl2). Apparatus TIRF measurement and bulk phase measure- ments were carried out by a home-made multifunctional spectrofluorimeter as previously described [22, 23, 26]. It was equipped with a 150 W xenon lamp and the slit bandpasses of excitation and emission monochromators were set at 5 nm. The bulk observation was performed with the spectrofluorimeter at regular detection manner (10 mm��10 mm cell). For TIRF measurement, a cylindrical prism was attached to a sample cell consisting with two sandwiched silica slices (20 mm��20 mm) and gaskets used as spacer to make TIRF sample cell (Fig. 2). Glycerol was used to agglutinate the slice and prism. Passing a cylindrical prism, a beam of light went from optically denser medium (silica, refractive index, n1 =1.46) to the optically thinner medium (water, refractive index, n2=1.33) at a fixed incidence angle of 70��, which is greater than the critical angle (66��) in silica/water interface, the light undergoes total internal reflection. The evanescent wave excites the fluorophores of molecules at the interface region. Combining synchronous fluorescence technique with a ��l of 24 nm, thus, interfacial synchronous fluorescence spectra are obtained. The measurement at toluene/water interface refers to Watarai et al. [39]. Results and discussion Interface-originated fluorescence of R6G The fluorescence spectra of R6G obtained from silica/water and bulk phase at 1.0��10���7 M are shown on Fig. 3. The overlap between the excitation and emission spectra is attributed to the low energy transitions between the ground and the first excited states (S0���S1) of R6G. Researchers usually select an excitation wavelength that can avoid the interference of Rayleigh scattering on the spectra at the sacrifice of fluorescence intensity or obtain incomplete Fluorescence Incident Light Reflection Light Gasket Silica Plates Fig. 2 Schematic drawing of TIRF cell 400 450 500 550 600 650 700 0 50 100 150 200 250 Reletive Fluorescence Intensity Wavelength(nm) a b c d x5 Fig. 3 The fluorescence spectra of R6G in different host environ- ments, a synchronous spectrum (��l=24 nm) at silica/water interface (the intensity amplified fivefold), b synchronous spectrum (��l= 24 nm) in aqueous solution, c excitation and d emission spectra in aqueous solution. [R6G]=1.0��10���7 M, PBS 7.4 O N H C2H5 CH3 N C 2 H 5 H C O OC 2 H 5 CH3 .. 13.8 �� 11.5 �� 9.0 �� Fig. 1 Sketch plot of the structure of Rhodamine 6G 94 J Fluoresc (2008) 18:93���100