Journal of Luminescence 101 (2003) 141–146 Study on the co-luminescence effect of terbium–gadolinium– nucleic acids–cetylpyridine bromide system Lei Lia, Jinghe Yanga,*, Xia Wua, Changxia Suna, Guangjun Zhoub a Department of Chemistry, Laboratory of Colloid & Interface Chemistry for Educational Ministry, Shandong University, Jinan 250100, People’s Republic of China b Shandong Supervision & Inspection Institute for Product Quality, Jinan 250100, People’s Republic of China Received 2 March 2002; received in revised form 14 June 2002; accepted 11 July 2002 Abstract In this paper, the luminescence mechanism of a new co-luminescence system (Tb–Gd–nucleic acids–cetylpyridine bromide) was studied. The experiment has shown that cetylpyridine bromide (CPB) could combine with nucleic acids 3+ 3+ 3À through both electrostatic forces and hydrophobic forces, then Gd and Tb were bound to PO4 of nucleic acid and formed Tb–nucleic acid–CPB and Gd–nucleic acid–CPB complexes, respectively. When the system was excited, the DNA–CPB absorbed light energy and then transferred it to Tb3+ through both intramolecular energy transfer in Tb– nucleic acid–CPB complex and intermolecular energy transfer between Gd–nucleic acid–CPB and Tb–nucleic acid–CPB complexes. In addition, it was found that the gadolinium complex acted not only as the energy donor but also the energy-insulating sheath, which improved the fluorescence quantum efficiency. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Nucleic acids; Rare earth ions; Co-luminescence effect; Fluorescence mechanism 1. Introduction used for characterization of nucleic acids, such as organic dyes [3–9], metal complexes [10–14], metal Nucleic acid involves deoxyribonucleic acid ions [15–19] and light [20]. (DNA) and ribonucleic acid (RNA), which exist As rare-earth ions have luminescence character- extensively in various biocell and play important istics such as narrow spectral width, long lumines- roles in the synthesis of proteins and inheritance of cence lifetime, large Stokes shift and strong organisms. Thus their qualitative and quantitative binding with biological molecules, they are used analyses are becoming more and more important. as fluorescence probes to study nucleic acids. But the natural fluorescence intensity of nucleic However, the sensitivity is low [15]. acids is weak [1,2], so direct characterization of While studying the Eu3+ multi-complex system, nucleic acids using the fluorescence method is a fluorescence enhancement phenomenon was limited. Therefore, some fluorescence probes are found when Gd3+,La3+,Lu3+,Tb3+ and Y3+ were added, in 1986 [21]. This enhance- *Corresponding author. ment effect was named the co-luminescence E-mail address: [email protected] (J. Yang). effect. This method has been used for the 0022-2313/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-2313(02)00406-4 142 L. Li et al. / Journal of Luminescence 101 (2003) 141–146 characterization of rare earth elements [22–30] and 2.2. Apparatus nucleic acids [31]. Recently, it was found that the fluorescence All fluorescence measurements were made with intensity of Tb–nucleic acids–CPB system was FLS920 spectrofluorimeter (Edinburgh, UK), and considerably enhanced by Gd3+, so Tb–Gd– Hitachi 850 spectrofluorimeter. The surface ten- nucleic acids–CPB system is a new co-lumines- sion was measured with Kruss K12 (Switzerland), cence system. In this paper, the emphasis of surface tensionmeter. All absorption spectra were research is the interaction of CPB with nucleic recorded with an UV-240 spectrophotometer acid and the luminescence mechanism in Tb–Gd– (Shimadzu). All pH measurements were made nucleic acid–CPB system. with a pHS-2 acidity meter (Leici, Shanghai). 2.3. Procedure 2. Experimental To a 25 ml test tube, solutions were added in the 2.1. Chemicals following order: CPB, Gd3+, Tris-HCl, Tb3+, DNA (or RNA). The mixture was diluted to 10 ml Stock solutions of DNA and RNA were with water and allowed to stand for 16 min. The prepared by dissolving commercial yeast RNA fluorescence intensity was measured in a 1 cm (Sigma), fish sperm DNA (Sigma) and calf thymus quartz cell; the excitation and emission slits were DNA (Beijing Baitai Co., China) in 0.05 mol/l both 10 nm. sodium chloride solution. These stocks needed to be stored at 0–41C. Stock standard solutions (0.01 mol/l) of rare-earth ions were prepared by 3. Results and discussion dissolving the corresponding oxides (99.9%) in hydrochloric acid and diluting it, with deionized 3.1. Fluorescence spectra water. A stock solution of cationic surfactant cetylpyridine bromide (CPB, 1.0 Â 10À2 mol/l) was Excitation and emission spectra of: (1) Tb–CPB, prepared. All the chemicals used were of analytical (2) Tb–DNA, (3) Tb–DNA–CPB, (4) Tb–Gd– grade and doubly deionized water was used DNA and (5) Tb–Gd–DNA–CPB systems are throughout. shown in Fig. 1. From this figure, it can be seen 6000 6000 5 5 5000 5000 4000 4000 (%) 3000 (%) 3000 f f I I 2000 4 2000 4 3 1000 1000 3 2 2 1 1 0 0 275 280 285 290 295 300 440 460 480 500 520 540 560 (a) wavelength (nm) (b) wavelength (nm) Fig. 1. Excitation (a) and emission (b) spectra: (1) Tb–CPB; (2) Tb–DNA; (3) Tb–DNA–CPB; (4) Tb–Gd–DNA; (5) Tb–Gd–DNA– CPB. Conditions: Tb3+: 6.0 Â 10À5 mol/l; Gd3+: 1.5 Â 10À4 mol/l; CPB: 1.5 Â 10À5 mol/l; DNA (or RNA): 1.0 Â 10À6 g/ml; Tris-HCl: pH=9.0. L. Li et al. / Journal of Luminescence 101 (2003) 141–146 143 that all the systems have the same excitation peak The effect of CPB concentration on both the at 285 nm and two emission peaks at 490 nm and fluorescence intensity and the surface tension of 3+ 5 7 544 nm of Tb , corresponding to the D4– F6 the system is shown in Fig. 3. It can be seen that at 5 7 3+ À5 and D4– F5 transitions of Tb , respectively. The a concentration of CPB less than 1.5 Â 10 mol/l, fluorescence intensity at 544 nm is strongest. the fluorescence intensity increases sharply with Although the fluorescence of Tb–DNA–CPB increasing CPB concentration, and then tends to system is stronger than that of Tb–DNA system, reach a maximum of 1.5 Â 10À5 mol/l. At further its intensity is very weak. However, the fluores- increase of CPB concentration, the fluorescence cence intensity of the system is much enhanced intensity will begin to gradually decrease. From when Gd3+ is added to the Tb–DNA–CPB the curve of the surface tension, it can be seen that system; this is a newly found co-luminescence it first decreased fast with increasing CPB con- system. In addition, the intensity of system (5) is centration, soon gets to a minimum and then higher than that of system (4), so CPB can enhance remains constant. The concentration 1.5 Â the fluorescence intensity of the system. 10À5 mol/l may be regarded as the apparent critical Except Gd3+, other rare-earth ions, which possess micelle concentration (CMC) of CPB in this empty or full of 4f shell, also show co-luminescence system. It is then concluded that upto this effect, such as La3+,Lu3+,Sc3+ and Y3+.The concentration CPB is the pre-micellar or single- fluorescence intensity of difference enhancing ions molecule domain. has compared in Table 1. The fluorescence intensity So, we presume that in Tb–DNA–CPB system, of Gd3+ is greatest, so Gd3+ was selected as the when the concentration of CPB is 1.5 Â 10À5 mol/l, enhancing ion for further research. CPB is regarded as a nucleus, and DNA was absorbed on its surface through both electrostatic 3.2. Formation of Tb–DNA–CPB complex 0.20 In the Tb–DNA–CPB system, CPB is a cationic surfactant, while nucleic acids are polyanions. So, 0.15 2 they can interact each other through electrostatic 1 force and an association complex of CPB–DNA is 4 0.10 formed. This can be seen from the absorption A 3 spectra of CPB and DNA in Fig. 2. In comparison with the absorption spectra of CPB, the location 0.05 of the absorption peak of DNA–CPB system (vs. DNA) was not changed, but its intensity increased, while in comparison with the absorption 0.00 spectra of DNA, the location of the absorption 240 260 280 300 320 wavelength (nm) peak of DNA–CPB system (vs. DNA) appeared red-shift from 260 to 265 nm, and its intensity also Fig. 2. Absorption spectra of CPB, DNA and their mixture: enhanced. This indicates that CPB can interact (1) CPB (vs. water); (2) CPB–DNA (vs. DNA); (3) DNA (vs. water); (4) CPB–DNA (vs. CPB). Conditions: CPB: with nucleic acid, which causes the change of their 1.5 Â 10À5 mol/l; DNA (or RNA): 1.0 Â 10À6 g/ml; Tris-HCl: absorption spectra. pH=9.0. Table 1 Compared with the fluorescence intensity of different enhancing ions for the co-luminescence Gd La Lu Sc Y Sm Ho Nd Pr Tm Dy DIf (%) 100 26 29 53 66 7 0 5 0 9 7 144 L. Li et al. / Journal of Luminescence 101 (2003) 141–146 3+ 90 90 Tb can bind with some base residues of 3À 80 a 80 DNA, as well as PO4 , and form Tb–DNA–CPB complex. 70 70 When the concentration of Tb3+ is in the range À5 À5 60 60 of 1.0 Â 10 –6.0 Â 10 mol/l, we find that the (%) f tendency of the effect of the concentration of I 50 50 b Gd3+ on the fluorescence intensity of the system is 40 40 the same.