ANALYST, SEPTEMBER 1988, VOL. 113 1453

Fluorescence Enhancement of the EuroDium(ll1) - Thenoyltrifluoroacetone - Trioctylphosphine Oxide Ternary Complex by Gadolinium(lll) and its Application to the Determination of Europium (I1 I) * Yun-Xiang Cit and Zhang-Hua tan Department of Chemistry, Peking University, Beijing, China

A study of the enhancement of the fluorescence intensity of the Eulll - thenoyltrifluoroacetone (-A) - trioctylphosphine oxide (TOPO) system by Lalll, Gdlll, Lull1 and Ylll has been carried out in a colloidal-like suspension. Gadolinium(ll1) caused the largest enhancement of the fluorescence intensity. In the presence of this element the weak fluorescence of the Eulli- TTA - TOPO system was enhanced by a factor of about 18 at a pH of between 5.5 and 6.5. The excitation and emission wavelengths were 343 and 61 5 nm, respectively. The fluorescence intensity was linear up to 10-8 M of europium; the detection limit was found to be 5 x I O - I ~ M and the precision at 8.0 x 1 0 - 1 1 ~ of europium was 1.4%. By stabilising the fluorescence with cetyltrimethylammonium bromide (CTAB), the Eulll - TTA - TOPO - Gd"1 system together with three other enhancing systems could be applied to the determination of trace amounts of europium in lanthanide oxides and yttrium oxide. The fluorescence intensity of the Srnlll - TTA - TOPO system was also enhanced by La"1, Gdlll, Lull1 and Ylll. The factors influencing the fluorescence enhancement are discussed and a general mechanism involving an intermolecular transfer of energy from the enhancing complex to the fluorescing complex is proposed.

Keywords: Intermolecular energy transfer; europium determination; lanthanides; yttrium; fluorimetry

In solution, the ternary complexes formed by the reaction of europium or samarium with thenoyltrifluoroacetone (TTA) and certain substituted ammonium cations or neutral ligands, such as nitrogen and phosphorus oxides (L), will usually emit an intrinsic fluorescence for the metal ion when exposed to ultraviolet radiation.'-3 However, the fluorescence is seriously interfered with by other lanthanide ions and europium and samarium mutually interfere. In the past, the solvent extrac- tion method was commonly employed for the elimination of interferences, whereas nowadays an organic solvent or a surfactant is added to the system in order to increase selectivity and sensitivity."-7 Compared with the solvent extraction method, the latter procedure is a simple and efficient means of improving the fluorimetric determination of europium and samarium. Over the last few years interest in the application of micellar systems has increased. So far, the micellar-enhanced fluorimetric method seems to be the only type of enhanced fluorimetric method available for the determination of trace amounts of lanthanide ions.

Recently, it has been shown that some of the lanthanides, yttrium and the alkaline earth metal ions can enhance the intrinsic fluorescence of a lanthanide comp1ex.x-1" For exam- ple, the sensitivity and selectivity of the Eu"' - TTA - L and Sm"1 - TTA - L systems were increased by the addition of trivalent lanthanum, gadolinium, lutetium or yttrium ions; in all instances Gd"' caused the largest enhancement of the fluorescence intensity. Early in 1967, during a study of the fluorescence characteristics of the ion associate of Eu"I (or SmIrr) with TTA - H-collidine, Melenteva et al. I 1 observed that the fluorescence intensity of the system was enhanced dramatically by the addition of GdI" or T P . Moreover, this enhancement was only observed when Gd"1 or TbIIl formed co-precipitates with Eu"] or SmIII. At the time, this phen- omenon was regarded as an interference in the presence of other lanthanide ions and was eliminated by employing an extraction procedure. However, further investigations in our

* This study was supported by the National Nature Science

i To whom correspondence should be addressed. Foundation of China.

laboratories have shown that the fluorescence enhancement effect is a general phenomenon and that this "co-fluorescence effect" can be used as the basis of a simple and rapid method for the determination of trace amounts of europium and samarium in pure lanthanide oxides.

Yang et al.'O have reported a Lensitive system for the enhanced fluorimetric determination of europium as its TTA and phenanthroline complex using terbium in Triton X- 100 micelles; a detection limit of 1.0 x 10-13111 of europium was achieved. However, these workers did not propose a general enhancement mechanism. We have carried out a systematic study of the enhancement of the fluorescence intensity of the Eu"1 - TTA - L and SmIII - TTA - L systems by La"', GdIII, Lu"1 and YIII ions, and have investigated the fluorescence characteristics of 18 systems (using nine different ligands) to establish the generality of this tluorescence enhancement. Some of these systems were subsequently applied to the determination of trace amounts of europium or samarium in pure lanthanide oxides and yttrium oxide; satisfactory results were obtained.

In the Eu"1- @-diketone - trioctylphosphine oxide (TOPO) ternary system used for the fluorimetric determination of europium, the 6-diketones most commonly employed include hexafluoroacetylacetone (HFA), 2-naphthoyltrifluoroacet- one and benzoyltrifluoroacetone.'.12." Of these ternary systems, the Eu"] - HFA - TOPO system gave the highest sensitivity; the detection limit for europium was 1 .O X 10-9 M when the system was used for extracting this element into methylcyclohexane. In contrast, the EuI" - TTA - TOPO system exhibits only weak fluorescence and so cannot be used for the fluorimetric determination of europium.

In this paper, the fluorescence characteristics of the complex formed by the reaction of europium(II1) with TTA and TOPO in the presence of gadolinium are reported, and a spectrofluorimetric method for the determination of euro- pium in the suspension is described. The detection limit of the EulII - TTA - TOPO - GdI" suspension system for europium is two orders of magnitude lower than that of the Eu"1- HFA - TOPO extraction system. Further, in the presence of gado- linium, europium can be determined directly without the need for a complicated extraction procedure.

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1454 ANALYST, SEPTEMBER 1988, VOL. 113

Experiment a1 Apparatus All fluorescence intensity measurements were made on a Hitachi M850 fluorescence spectrophotometer equipped with a high-pressure xenon lamp and 1 .O-cm quartz cells. The pH was measured with an Orion Model 811 pH meter.

Reagents Analytical-reagent grade chemicals were used, and distilled, de-ionised water was used to prepare the solutions.

Yttrium oxide and the lanthanide oxides were obtained in purities of 99.9% or better; special high-purity gadolinium oxide was used (Yuelong Chemical Plant, Shanghai, China). Stock solutions of the lanthanides and yttrium were prepared to be 0.05 or 0 . 0 0 1 ~ by dissolving a known amount of the appropriate rare earth oxide in hydrochloric acid. The solutions were standardised by titration with EDTA. Working solutions were prepared by appropriate dilution with water; these solutions should have a final pH of about 3.

A 0.01 M solution of thenoyltrifluoroacetone (TTA) was prepared by dissolving the appropriate amount of the reagent in several millitres of ethanol and then making up to volume with water. A 0.01 M solution of trioctylphosphine oxide (TOPO) was prepared by dissolving the appropriate amount of the reagent in 100 ml of ethanol.

Stock solutions of the following surfactants were prepared by dissolving each surfactant in water: Triton X-100 (TX-loo), 2% m/V; sodium dodecyl sulphate (SDS), 0.0125 M; cetyl- trimethylammonium bromide (CTAB), 0.001 M; and poly- (vinyl alcohol) (PVA), 0.2% m/V.

A 10% m/V solution of hexamine was used as a buffer; the pH was adjusted to 6.0 with hydrochloric acid.

General Procedure To each of a series of 25-ml calibrated flasks add an aliquot of a solution containing 0.13-38 ng of europium. Then add 0.4 ml of 0.001 M gadolinium, 0.2 ml of 0.01 M TTA, 0.2 ml of 0.01 M TOPO, 0.25 ml of 0.001 M CTAB and 1 ml of buffer solution. If the sample solution is strongly acidic, then the pH of the mixture should be checked and adjusted to 5.5-6.5 with hydrochloric acid before diluting to volume with water. Measure the fluorescence intensity immediately after prepara- tion of the solution at an excitation and emission wavelength of 343 and 615 nm, respectively. To study the effect of other surfactants on the fluorescence intensity, a known amount of each surfactant to be tested is added (under the optimum reaction conditions) in place of CTAB. To establish the optimum experimental conditions, the fluorescence intensities were measured only when they reached a maximum, because under different experimental conditions the time required for the maximum fluorescence intensity to develop will be different. In such instances, the excitation and emission wavelengths were found by wavelength scanning.

Results and Discussion Spectral Characteristics Fig. 1 shows the excitation and emission spectra of both the EuIII - 'ITA - TOPO and Ed11 - TTA - TOPO - Gd"1 suspension systems at pH 6.0. It can be seen that the intrinsic EuIII emission of the EuIII - TTA - TOPO complex is weak, whereas the addition of gadolinium causes a large increase in the fluorescence intensity (about 18-fold at 615 nm), but with no shift in either the excitation or emission wavelength. In the presence of gadolinium the spectral emission lines of the Eu3+ ion were observed ai: 593, 615 and 655 nm; these values were assigned to the transitions from the 5Do level to the 7F1, 7F2 and 7F3 levels, respectively. The emission line at 615 nm was

,C

0

I c

B \ A P L

320 420 550 650 Wavelengthhm

Fig. 1. (a) Excitation and (b) emission spectra. (A) GdTXx - TTA - TOPO; (€3) EuIII - ?TA - TOPO; and (C) Eu"1- TTA - TOPO - GdIX1. [Eu] = 4 x 10-9~; Gd] = 1.2 x l o - S M ; [TTA] = [TOPO] = 8 X 10-5 M; [CTAB] = 1 6 -5 M; pH = 6.0

r I

I I I

4 0 5.0 6 0 7 0 PH

Fig. 2. Effect of pH on the fluorescence intensity. [Eu = 8 X 1 0 - 1 0 ~ ; [Gd] = 1.6 x 1 0 - 5 ~ ; [TTA] = 1 0 - 4 ~ ; [TOPO] = 8 x 10-5 M; and [CTAB] = 10-5 M

the most intense; hence, this wavelength was selected for further experiments. Under the described experimental condi- tions, the Gd"1 - TTA - TOPO system did not exhibit any fluorescence.

It is imperative that high-purity gadolinium oxide be used. In our experiments several commercial gadolinium oxides were tested, and some were found to emit a strong fluores- cence caused by the presence of europium as an impurity. In this work the high-purity gadolinium oxide produced by the Yuelong Chemical Plant was used throughout. This ensured that any enhancement effect observed was actually caused by gadolinium rather than by europium present as an impurity in the gadolinium oxide.

Effect of pH

The fluorescence intensity of an 8.0 x 1 0 - 1 O ~ europium solution containing a 20 000-fold molar excess of gadolinium was measured over the pH range 4.0-7.5, by using hexamine as the buffer solution and adjusting the pH with hydrochloric acid or sodium hydroxide solution. The maximum fluor- escence intensity was obtained at a pH of between 5.5 and 6.5 (Fig. 2). The fluorescence intensity is independent of the buffer concentration at this pH.

It was observed that the pH of the solution not only influenced the fluorescence intensity, but also other fluor- escence characteristics of the EuI" - TTA - TOPO - GdI" system, for example, its fluorescence stability. Generally, the fluorescence intensity increased with time (over a short period). The lower is the pH, the longer the system takes to reach its maximum fluorescence intensity. On the other hand, the pH of the solution also influenced the excitation wavelength. Hence, at pH 6 . 3 , the maximum excitation wavelength remained at 343 nm, whereas at pH >6.35, it

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ANALYST, SEPTEMBER 1988, VOL. 113 1455

.- L. (I)

$ 130 E .-

- 0 0.4 0.8 1.2

[TTA]/l 0-4 M

Fig, 3. Effect of TTA concentration on the fluorescence intensity. Eu] = 8 x 10-10 M; [Gd] = 1.6 x 10-5 M; [TOPO] = 8 X 10-5 M; and t CTAB] = 1 0 - s ~

I

1 .o 2.0 90

0 [CTABl/10-5 M

Fig. 6. Effect of CTAB concentration on the fluorescence intensity and the fluorescence stability. A, Determined immediately after preparation of the solution; and B, after UV (343 nm) irradiation for 1 min. [Eu] = 4 x 1 0 - " J ~ ; [Gd] = 1.6 x l o - S M ; [=A] = 8 X l o - 5 ~ ; and [TOPO] = 8 x l o - S M

0.4 0.8 1.2 [TOPO]/1 w4 M

Fig. 4. Effect of TOPO concentration on the fluorescence intensity. Eu] = 8 x 10-10~; [Gd] = 1.6 x l o - S M ; [TTA] = 8 x lo-SM; and t CTAB] = 10-SM

I 1

0 0.5 1.0 1.5 2.0 2.5 [Gdl/10-5 M

Fig. 5. Effect of Gd concentration on the fluorescence intensity. A, Eu] = 4 x 10-10~; B, [Eu] = 8 x 10-'OM; and C, Eu] = 8 X 1 0 - 9 ~ . t TTA] = [TOPO] = 8 x l 0 - J ~ and [CTAB] = 1 6 - s ~

shifted to a longer wavelength, e.g., from 345 nm at pH 6.35 to 371 nm at pH 7.30.

Effect of TTA and TOPO Concentrations The effect of varying the TTA concentration (g1 .2 X l o - 4 ~ ) at fixed concentrations of europium (8.0 X 10-10~), gadolin- ium (1.6 x 10-5 M) and TOPO (8.0 X 10-5 M) was studied. A maximum and constant fluorescence intensity was obtained between 4.0 x 10-5 and 1.0 x l o - 4 ~ of TTA (Fig. 3). In the absence of TTA the intrinsic europium emission was not observed. It was also found that higher concentrations of TTA ( i .e . , outside the optimum range) caused a red shift in the maximum excitation wavelength (cf., the effect of pH). When the TTA concentration in the solution became higher than 1.0 x l O - 4 ~ , the maximum excitation wavelength shifted from 343 to 371 nm. This may be due to the fact that at high pH or high concentrations of TTA, TOPO is unable to participate in the complexing reaction between the Eu3+ ion and TTA. Consequently, the fluorescing species present in solution is the

- TTA binary complex. This hypothesis is supported by

Table 1. Effect of time of W irradiation (343 nm) on the fluorescence intensity in the presence and absence of 10-5 M of CTAB

Relative intensity

Time/min 0 2 4 6 8 10 12

CTAB 100 100.9 96.6 94.1 84.7 83.9 83.5

No CTAB 100 87.7 86.4 85.8 85.2 84.4 84.4

the experimental results, viz. , the fluorescence intensity of the Ed11 - TTA binary complex is enhanced dramatically by the addition of gadolinium, and the maximum excitation wavelength of the EuUI - TTA - GdIII system is 370 nm, which is very close to that of the EuIJI - TTA - TOPO - GdIII system at a higher pH or larger concentrations of TT'A.

The effect of varying the TOPO concentration at fixed concentrations of europium (8.0 x 10-10 M), gadolinium (1.6 X 1 0 - 5 ~ ) and TTA (8.0 x l o - 5 ~ ) was also studied. It was observed that when the TOPO concentration was increased, the fluorescence intensity also increased up to a TOPO concentration of 8.0 x 10-SM; above this concentration a sharp decrease in the fluoresence intensity was observed (Fig. 4).

Effect of Gadolinium Concentration The variation in the fluorescence intensity was investigated as a fundon of the concentration of gadolinium in the presence of a fixed amount of europium (Fig. 5) . The europium concentrations tested were 4.0 X 10-10,8.0 X 10-10 and 8.0 X 10-9 M. When the gadolinium concentration was increased the fluorescence intensity of the system also increased, and in all instances the maximum intensities were reached at the same concentration of gadolinium, namely, about 1.6 X 10-5 M, i.e., the concentration of gadolinium at which the enhanced fluorescence of the - ?TA - TOPO system reached its maximum intensity was independent of the concentration of the EuIII - TTA - TOPO ternary complex itself (in the concentration range between 4.0 X 10-10 and 8.0 X l o - 9 ~ of europium). From Fig. 5 it can be seen that at different concentrations of europium the degree of enhancement of the fluorescence by gadolinium is the same, the fluorescence intensities being increased by a factor of about 18. These results indicate that for efficient enhancement of the fluor- escence intensity a specific concentration of gadolinium should be maintained and that the efficiency of this enhance- ment is independent of the Concentration of the fluorescing species.

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1456 ANALYST, SEPTEMBER 1988, VOL. 113

Table 2. Analysis of various samples containing trace amounts of europium

Synthetic samples- Europium addedng

Foreign ions addedlpg

0.30 Gd (62.8) 1.51 Gd (62.6); La (0.14)

3.02 La (6.90); Dy (0.65) 4.53

15.1 Gd (62.6); Sm (0.60); Tb (0.48)

Y (34.3); Ce (0.70); Yb (0.69) Lu (35.0); Srn (1.20); Tm (1.69) 30.2

Commercialgadolinium omdes- No. 1 No. 2 No. 3

Europium foundlng Errorlng Error, '30

0.32 +0.02 +6.3 1 S O -0.01 -0.7

15.4 +0.03 +2.0 3.05 +0.03 +1.0 4.50 -0.03 -0.7

31.0 +0.8 +2.2

Europium found, % mlm 0.00460 0.00331 0.000106

Fluorescence Stability The system studied was in the form of a suspension and was unstable, the fluorescence intensity decreasing considerably with time by the aggregation and deposition of minute particles of the fluorescent precipitate. In an attempt to stabilise the fluorescence of the EurII - 7TA - TOPO - Gd"' system, we investigated the effect of several surfactants. It was found that the anionic surfactant SDS had no effect on the fluorescence intensity, whereas the non-ionic surfactant TX- 100 decreased the intensity. PVA slightly increased the fluorescence intensity, but none of these surfactants improved the fluorescence stability of the system. It was found that the cationic surfactant CTAB was effective in this respect. The effect of CTAB on the fluorescence intensity and fluorescence stability of the system is shown in Fig. 6 (A and B, respectively). At CTAB concentrations 4 . 0 x ~ O - S M , the fluorescence intensity increased slightly but then decreased with time; at CTAB concentrations >1.0 x ~ O - S M , the fluorescence intensity decreased slightly but subsequently increased with time. However, at about 1.0 X l O - S M of CTAB, stabilisation of the system was observed.

As can be seen from Table 1, in the presence of 1 .O x 10-5 M of CTAB, the - TTA - TOPO - Gd"1 suspension system is stable for at least 2 min.

Analytical Characteristics The calibration graph was linear up to 1.0 x 1 0 - 8 ~ of europium, and the straight line passed through the origin. The detection limit (signal to noise ratio = 2) was calculated to be 5 X 10-12~ of europium. The relative standard deviation for 11 replicate analyses of a sample containing 8.0 x 10-*IM of europium was 1.4%.

The effect of foreign ions on the fluorescence intensity of the Ed1' - TTA - TOPO - GdII1 system was studied for 4.0 X 10-1O M of europium. The tolerance allowed in the variation of the fluorescence intensity was +5%. All the other trivalent lanthanides and the Y3+ ion were examined and it was found that the following molar excesses of these ions caused no interference: La (100-fold), Ce (64), Pr (38), Nd (40),, Sm (65), Tb (6S), Dy (60), Ho (48), Er (88), Tm (63), Yb (76), Lu (75) and Y (86). The following metal ions were also examined in a 100-fold molar excess and none was found to interfere: Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+, Cd2+, AP+, Mg2+, Zn2+, Sc3+, Ag+ and Ti4+, The following anions were examined in a 500-fold molar excess over 4.0 x 10- OM of europium and again none was found to cause any interference: chloride, bromide, perchlorate, nitrate, acetate, carbonate and sulphate.

The EuII* - TTA - TOPO - GdIII system was applied to the determination of trace amounts of europium in the lanthanide oxides and yttrium oxide. The results obtained for various prepared mixtures and commercial gadolinium oxides are

given in Table 2. The analysis of each sample was repeated three times. For the determination of europium in La203, Lu2O3 or Y2O3, the addition of GdIII as described in the recommended procedure was replaced by the addition of lanthanum, lutetium or yttrium ions, respectively.

Nature of the Enhanced Fluorescence In solution, where the gadolinium ion co-exists with the europium ion, the addition of TTA and TOPO could give either a coprecipitate of the Ed11 - TTA - TOPO and GdIrI - TTA - TOPO complexes, or a suspension of the Ed11 - TTA - TOPO - GdIII mixed-metal complex. From the structure of the ligands, it can be seen that formation of the latter is most unlikely. Melby et al. 14 and Bauer et al. 15 have demonstrated independently that trivalent lanthanide (Ln) ions form a Ln(lITA)3(TOP0)2 complex with TTA and TOPO. There- fore, it seems likely that the suspension system obtainedin this work consists solely of a coprecipitate of the europium and gadolinium complexes, and that the enhancement of the intrinsic Ed11 emission of the Ed11 - TTA - TOPO complex is due to an intermolecular transfer of energy from Gd( TTA)3( TOP0)2( donor) to Eu ( TTA)3 ( TOP0)2 (accep- tor).

The molar ratio, equilibrium-promotion and Asmus methods were employed to determine the stoicheiometric composition of the fluorescent species formed in the presence of gadolinium. The composition of the LnI" - TTA - TOPO ternary complex was found to be 1 : 3 : 2, which is in agreement with the results of both Melby etal.14 and Bauer et al. 15 Also, a series of ratios, viz., 1: 1, 1 :2 and 1 : 6 for Eu(TTA)~- (TOP0)2 : Gd(TTA)3(TOP0)2 were studied, of which the ratio 1 : 6 gave the maximum fluorescence intensity. Similar results were obtained for other systems; it should be noted that only in suspension was enhancement of the fluorescence intensity observed. In such instances, the donor and acceptor complex molecules must be arranged in a highly ordered way; the minute particles of the coprecipitated europium and gadolinium complexes suspended in the solution can be regarded as microcrystals containing trace amounts of impuri- ties (vit., the europium complex). Obviously, the intermole- cular transfer of energy is affected by the microcrystalline structure. As shown in Fig. 6, UV radiation strongly influ- ences the fluorescence characteristics of the system. This result seems to indicate that the aggregation of donor and acceptor molecules, which determines the structure of the microcrystals, plays an important role in the enhancement of the fluorescence intensity. We believe that UV radiation can facilitate this aggregation. When exposed to UV radiation, the donor and acceptor molecules are excited, with the result that the electronic dipole moments of these neutrally charged complex molecules reinforce each other in such a way that an intense fluorescence occurs. 16

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ANALYST, SEPTEMBER 1988, VOL. 113 1457

Table 3. Characteristics of the enhanced fluorescence intensity

Ionic Eu”’ - TTA - TOPO Sm”I - TTA - TOPO Enhancing radius/

ion A AR* E.F.1. h,/h,/nm C/M$ AR* E.F.? A, /Aem /nm C/M$ La(4P) . . 1.06 +0.11 7.36 340/615 2.0 X 10-5 +o. 10 2.77 343/644 2.0 X 10-5 Gd(4fl) . . 0.94 -0.01 17.83 343/615 1.6 X 10-5 -0.02 6.23 350/644 1.6 X 10-5 Lu(4f14) . . 0.85 -0.10 3.48 343/615 1.2 X 10-5 -0.11 3.18 343/644 1.2 x 10-5 Y(4do) . . 0.88 -0.07 7.88 337/615 1.6 X 10-5 -0.08 2.74 3441644 1.6 X 10-5

Fluorescing ion Eu(4f6) . . 0.95 Sm(4fs) . . 0.96

* R is obtained by subtracting the ionic radius of the enhancing ion from that of the fluorescing ion. t E.F. (enhancement factor) is defined as the ratio of the fluorescence intensity in the presence of an enhancing ion to that in the absence of

$ C is the concentration of an enhancing ion at which the largest enhancement of the fluorescence intensity is observed. an enhancing ion at the maximum excitation wavelength.

It was also observed that the fluorescence intensity of the SmIII - TTA - TOPO suspension was enhanced dramatically by the addition of lanthanum, gadolinium, lutetium and yttrium ions, and that GdIII gave the largest enhancement of the fluorescence intensity. The results obtained, together with various properties of the La, Gd, Lu and Y ions, are presented in Table 3. It was found that all the lanthanide ions that were able to cause an enhancement of the fluorescence intensity possess a stable 4f orbital, which is empty, half-filled or full. These ions have no low-lying 4f energy levels, hence the energy absorbed by their complex molecules cannot be dissipated through these energy levels, as with other lanthan- ide ions, but instead it is transferred to the europium (or samarium) complex. A similar situation exists for the Y3+ ion, which has an empty 4d orbital. On the other hand, as can be seen from Table 3, another factor affecting the efficiency of the intermolecular transfer of energy is the ionic radius of both the enhancing (La, Gd, Lu or Y) and fluorescing (Eu or Sm) ions. Generally, the closer is the ionic radius of the enhancing ion to that of the fluorescing ion, the greater is the enhancement of the fluorescence intensity. These results show that those donor and acceptor complexes that have the most similar structures cause the largest enhancement of the fluorescence intensity.

The intermolecular transfer of energy can occur by one of two acceptor concentration-dependent processes, which include resonance excitation transfer and exchange transfer, or by an exciton - migration process. From the above results, we have shown that the efficiency of the transfer of energy from the gadolinium to the europium complex is independent of the acceptor concentration.16 Hence, the transfer of energy would appear to be a result of an exciton - migration process, i.e., the energy of the exciting radiation absorbed by the donor molecule is rapidly delocalised throughout the Gd(lTA)3(T- OPO)z matrix and is eventually localised on the acceptor species, Eu(TTA)~(TOPO)~, which then emits the observed intrinsic fluorescence of the Eu3+ ion.

In conclusion, the transfer process can be written as

[EU”(TTA)~(TOPO)~] % [Eu(TTA)~(TOPO)~] + hv. [Gd(TTA)3(TOPO),]* [Eu(TT’A)~(TOFO)~]*

1.

2.

3.

4.

5. 6.

7. 8.

9. 10.

11.

12.

13.

14.

15.

16.

References Kononenko, L. I., Polyektov, N. C., and Nikonova, M. P., Zavod. Lab., 1964,30,779. Kononenko, L. I., and Tishchenko, M. A., Zh. Anal. Khim., 1969,24,1823. Fisher, R. P., and Winefordner, J. D., Anal. Chem., 1971,43, 454. Ci, Y.-X., Hu, K.-R., Liu, J.-R., and Ma, H.-J., Fenxi H w u e , 1982,10,232. Shi, H.-M., and C h i , W.-C., Fenxi Huaxue, 1982, 10,561. Aihara, M., Arai, M., and Taketatsu, T., Analyst, 1986,111, 641. Huang, H., and Geng, X., Bunseki Kagaku, 1986,35,584. Ci, Y.-X., and Lan, Z.-H., “Proceedings of the 1st Chinese Symposium on Luminescence Analysis, Xian, 1986,” Paper 16. Ci, Y.-X., Lan, Z.-H., and Liu, W., Analyst, 1988, 113, 933. Yang, J.-H., Zhu, G.-Y., and Wu, B., Anal. Chim. Acta, 1987, 198,287. Melenteva, E. B., Polyektov, N. C., and Kononeko, L. I., Zh. Anal. Khim., 1967,22,187. Shigematsu, T., Matsui, M., and Wake, R., Anal. Chim. Acta, 1969,46, 101. Shigematsu, T., Matsui, M., and Wake, R., Bull. Insf. Chem. Res. Kyoto Univ., 1968, 46, 248. Melby, L. R., Rose, N. J., Abramson, E., and Caris, J. C., J. Am. Chem. SOC., 1964,86,5117. Bauer, H., Blanc, J., and Ross, D. L., J. Am. Chem. Soc., 1964,86,5125. Schulman, S. G., Editor, “Molecular Luminescence Spectro- scopy, Methods and Applications: Part I,” Wiley, New York, 1985, pp. 13-16.

Paper &/01128A Received March 21st, 1988

Accepted May 5th, 1988

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