- Email: [email protected]
Determination of aluminium(III) in water samples in a microemulsion system by spectrofluorimetry
Analytica Chimica Acta 523 (2004) 43–48
Determination of aluminium(III) in water samples in a microemulsion system by spectrofluorimetry Xiashi Zhu∗ , Li Bao, Rong Guo, Jun Wu Department of Chemistry, Yangzhou University, Yangzhou 225002, PR China Received 17 May 2004; received in revised form 18 June 2004; accepted 18 June 2004 Available online 7 August 2004
Abstract The sensitizing effect of cetyltrimetrylammonium bromide (CTAB) microemulsion media on the determination of aluminium(III) by spectrofluorimetry was developed. The main factors affecting the determination were investigated in detail. The results showed that 8hydroxyquinoline (HQ) react with aluminium(III) forming a complex with fluorescence in the system of potassium acid phthalate–NaOH buffer solution at pH 6.0, the maximum excitation and emission wavelengths are at 380.0 and 502.6 nm, the sensitizing effect of CTAB microemulsion is higher than that of CTAB micelle. The proposed method has been applied to the determination of aluminium(III) in tap water and lake water samples with satisfactory results. © 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium; 8-Hydroxyquinoline; Microemulsion; Spectrofluorimetry
1. Introduction Although aluminium is the most abundant metal in the earth’s crust, it shows a low bioavailability and it is still questionable if aluminium has biological functions. Acid rains cause partial dissolution of soil aluminium leading to an increase in the aluminium concentration in natural waters and biological systems. Recent report associated aluminium with several skeletal (osteomalacia) and neurological disorders (encephalopathy, Alzheimer’s disease) in humans [1]. It was suggested that exposure to aluminium might present a hazard to health. Hence, the determination of trace aluminium(III) is very significant. At present, for the determination of aluminium(III), the methods employed were spectrophotometry [1,2], spectrofluorimetry [3–5], atomic absorption spectrometry (AAS) [6–8] and atomic emission spectrometry (AES) [9]. Among them, spectrofluorimetry was more applied because of its high sensitivity and simplicity. 8-Hydroxyquinoline has been ex∗
Corresponding author. Tel.: +86 514 7975568; fax: +86 514 7975244. E-mail addresses: [email protected], [email protected] (X. Zhu).
0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.06.067
tensively used as chelating reagent and it could react with metal ions and forms complex with five-members ring structure. Aluminium(III) can be bound to 8-hydroxyquinoline and form the steady complex with fluorescence under certain experimental conditions. Ning [10] have reported that the determination of aluminium(III) in natural water samples by ion exchange/chloroform extracting 8-hydroxyquinoline spectrofluorimetry, but the reproducibility and simplicity of this method were lacking, and there was also the limitation of sample volume, which cannot exceed 100 mL each time. Microemulsion, which are made from water, an organic solvent, and a surfactant and occasionally an alcohol as a cosurfactant, have unique properties as media. In the single phase water-in-oil (W/O) and oil-in-water (O/W) microemulsions, the partition and the interfacial adsorption of a solute in the microheterogeneous systems are responsible for the chemical reactivity [11]. In our previous publications, the sensitizing effect of microemulsion on the determination of metal ion by spectrophotometry was developed [12–16]. We have reported the determination of trace nickel in nonionic microemulsion medium by the fluorescence quenching method with satisfactory results [17]. But cationic mi-
44
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
croemulsion application to spectrofluorimetry seems to be lacking. In this paper, O/W cationic microemulsion has been applied as the medium for the determination of Al3+ . The proposed method showed higher sensitivity. The precision and accuracy of the proposed approach were evaluated by analysis of water samples with satisfactory results.
2. Experimental 2.1. Apparatus A model BF-540 fluorophotometer (Shimadzu, Japan), a model UV2501 spectrophotometer (Shimadzu, Japan). Conventional 1 cm quartz cells were used in all above instruments. The pH of solutions was measured using a pHS-29A pH meter (Shanghai, China). 2.2. Reagents Unless otherwise stated, all reagents used were of analytical grade and their solutions were prepared by serial dilutions with distilled water. The concentration of each reagent was as follows: CTAB micelle (CTAB-m), 5.0 × 10−3 mol L−1 ; (CTAB from Shanghai Lingfeng Chemistry Reagents Co., Ltd.); CTAB microemulsion(CTAB-M), CTAB:nC5 H11 OH:n-C7 H16 :water = 1.0:0.9:0.1:98 (mass); SDS microemulsion (SDS-M), SDS (from China Medicine (Group) Shanghai Chemical Reagent Corporation):n-C4 H9 OH:nC7 H16 :water = 0.23:0.67:0.10:4.26(mass); Triton X-100 microemulsion (Triton X-100-M), Triton X-100 (from China Medicine (Group) Shanghai Chemical Reagent Corporation):n-C5 H11 OH:n-C7 H16 :water = 5.0:3.3:0.8:90.9 (mass) (n-C5 H11 OH, n-C7 H16 , n-C4 H9 OH, from Shanghai Lingfeng Chemical Reagent Co., Ltd.). A standard aluminium(III) stock solution of 1000 g mL−1 was prepared The solution was diluted as needed to concentration being used. Series of potassium acid phthalate–NaOH buffer solutions, pH 4–8. A 1.0 × 10−3 mol L−1 solution of HQ (from China Medicine (Group) Shanghai Chemical Reagent Corporation) was prepared. A standard quinine sulfate (Fluka Chemie GmbH) stock solution of 1000 g mL−1 was prepared by dissolving 0.1000 g quinine sulfate in 0.5 M H2 SO4 . The solution was diluted to 3 g mL−1 being used for. The determination of relative fluorescence quantum yield.
luted to the mark with distilled water, shaken and allowed to stand for 30 min at 25 ◦ C. The fluorescence intensity of the solution was measured at emission wavelength 502.6 nm in a 1 cm quartz cell by BF-540 fluorophotometer, keeping the excitation wavelength 380.0 nm and the blank solution was measured at the same time. 2.3.2. Determination of relative fluorescence quantum yield Fluorescence quantum yields of Al(III)–HQ complex were measured by ration value of integrated area under corrected fluorescence spectra [18,19] in the various media. Quinine sulfate was applied to the standard. According to the equation Φf = nf 2 Df ΦS /ns 2 DS . Briefly, ΦS and Φf are corresponding the standard and unknown fluorescence quantum yield, and DS and Df are the integral areas of two calibration fluorescence emission curves, nf and ns are the refractive index of the standard and unknown, and ΦS = 0.546 (25 ◦ C) is known. 3. Results and discussion 3.1. Fluorescence spectra The fluorescence spectra of Al(III)–HQ complex in the different media (water, micelle, various microemulsions) are shown in Fig. 1. The excitation wavelength was 380.0 nm, the λem and fluorescence intensity F were listed in Table 1. From Fig. 1 and Table 1, comparing with aqueous solution, the sensitivity was improved in the O/W microemulsion medium. 3.2. Effect of microemulsion media In the experiment, various media were chosen for studying their effects on F, and all results were shown in Fig. 2. From Fig. 2, CTAB-M had highest sensitivity in correspond-
2.3. Procedure 2.3.1. Determination of fluorescence intensity In a 25 mL volumetric flask, 5.0 mL aluminium(III) solution, 3.0 mL HQ solution, 5.0 mL CTAB-M and 2.0 mL buffer solution (pH 6.0) were added. The solution was di-
Fig. 1. Fluorescence spectrum of Al(III)–HQ in different media (1) CTABM; (2) CTAB-m; (3) SDS-M; (4) Triton X-100-M; (5) Water.
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
45
Table 1 Fluorescence intensity in different media Medium
λem (nm)
Fa
Fb
CTAB-M CTAB-m SDS-M Triton X-100-M Water
502.6 505.4 503.4 524.6 521.2
157.6 143.2 49.2 30.7 6.2
4.5 1.8 – 1.6 0.4
a b
Fluorescence intensity of Al(III)-HQ in different media. Fluorescence intensity of HQ in different media.
Fig. 4. Effect of the amount of HQ (1.0 × 10−3 mol L−1 ) on fluorescence intensity: (1) CTAB-M; (2) CTAB-m.
3.3. Effect of amounts of HQ
Fig. 2. Effects of various media on F.
ing CTAB-m and in aqueous solution. From microemulsion media, the order of their sensitivities was as follows: CTAB > SDS > Triton X-100 Therefore, CTAB-M was chosen for further study. In the case of CTAB-M medium, adjusting the volume of CTAB, the effect of amounts of CTAB-M on fluorescent intensity F was investigated. When the volume of CTABM varied from 2.0 to 7.0 mL, F reached a maximum. Thus, 5.0 mL was optimum (Fig. 3).
Fig. 3. Effect of the amount of CTAB-M on fluorescent intensity, CTABM:CTAB:n-C5 H11 OH:n-C7 H16 :water = 1.0:0.9:0.1:98 (mass).
Amounts of HQ solution were added variously to investigate the effects on F in the CTAB-M and corresponding CTAB-m, the results are shown in Fig. 4. The maximum and stable F gained in the range of 3.0–5.0 mL in the CTAB-M. Optimum volume of HQ solution chosen for this work was 3.0 mL. 3.4. Effect of pH According to Section 2.3.1, the effects of pH on F were studied. The results are summarized in Fig. 5. In the CTAB micromulsion, when the pH varied in the range of 5.2–6.4, F reached the maximum, while in the CTAB micelle, F reached the maximum in the pH range of 5.8–6.2, which was smaller than in the microemulsion Hence, pH 6.0 was selected.
Fig. 5. Effect of pH on fluorescence intensity: (1) CTAB-M; (2) CTAB-m.
46
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
3.5. Determination of developing time
Table 2 Determination of standard substance
In the optimum conditions described above, the developing time of complex was measured. It was found that F reached the maximum instantly and the maximum of F was steady for 7 days.
Sample
3.6. Effect of co-existing ions Determination of 25 g Al3+ in the presence of foreign ions was investigated. With a relative error of less than ±5%, the tolerance limits for various foreign ions were as follows: Fe2+ (1); Cr3+ (8); Ni2+ (12); Cu2+ (15); Cr(15); Bi3+ (20); Pb2+ (22); Ca2+ (23); Mg2+ (50); Co2+ (50); Zn2+ (80); Ag+ (600); Cd2+ (1200); Sodium citrate(30); Sodium tartrate(180). 3.7. Working curve, detecting limit and precision According to procedure Section 2.3.1, 1.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0 and 35.0 g of Al3+ standard solutions were prepared and determined, respectively. The linear relationship of F with alumium(III) amounts was displayed in Fig. 6. In Fig. 6, the working curve was linear over the range of 0∼1400 ng mL−1 Al3+ . The corresponding regression equation and coefficient of correlation was calculated as follows: C(g/25 mL) = 0.16652F + 0.06658,
r = 0.9995
The 3 limits of detection for alminium(III) was 3.95 ng mL−1 The relative standard deviations (R.S.D.) was 2.4% (n = 9). 3.8. Sample determination The standard sample was determined according to procedure Section 2.3.1. The results are shown in Table 2. 220 200 180 160 140
F
120 100 80 60 40
B Data1B
20 0 0
5
10
15
20
25
30
C(µg/25ml) Fig. 6. Calibration graph of Al(III)–HQ.
35
40
GB a
602-88a
Measured (g/mL)
Certified (g/mL)
99.2 ± 0.8
100.0 ± 0.5
National reference materials (China).
The proposed method was applied for the determination of Al3+ in environmental reference materials and in tap water and lake water samples. The standard addition method was used and the analytical results and the recovery are presented in Tables 2 and 3. The good quantitative results could be obtained. The analytical content of Al3+ in environmental reference materials is in good agreement with the certified value. 3.9. Discussion of mechanism of sensitizing effect The sensitizing effect of the surfactants on spectroanalysis rested with two factors: (1) the solubilization capacity; (2) the microenvironment of medium [20,21]. The sensitivity of Al(III)–HQ complex in various O/W microemulsion was different, which was mainly related to surfactants, complexant and complex. The nanometer-sized structure brings about an extraordinarily large specific interfacial area to the microemulsion system. Thus, the interfacial adsorption at the micro-droplet surface becomes a significant factor. In the CTAB-M, the micro-droplet interface of CTAB micromulsion charged with positive charge, while the nitrogen atom of HQ molecule could afford electron and show electronegativity. So HQ and/or Al(III)–HQ complex could be adsorbed to the micro-droplet interface and solubilized to the mono-layer simultaneously, sequentially, the amounts of solubilization was increased. In the SDS-M, the microdroplet interface charged with negative charge, which was the same with the electronegativity of the nitrogen atom of HQ, it is disadvantage for the solubilizaton of HQ and/or Al(III)–HQ complex in SDS micromulsion. In the Triton X100-M, the micro-droplet interface was no charge and adverse to HQ and/or Al(III)–HQ complex enter into microemulsion micro-droplet. Al(III)–HQ complex had higher sensitivity in the CTAB micromulsion than corresponding CTAB micelle, which was mainly related to the different composition between microemulsion and micelle. Microemulsion contain finely divided domains of oil and water topologically ordered by surfactant at the internal interfaces. Therefore, the solute can distribute among the microscopic water phase, oil phase and interface. This microscopic partitioning greatly affect the solubility and reactivity. The solubilization capacity of CTAB-M were highly dependent on the charge types of the surfactant and the volume fraction of the organic component. The influence factors were as follows: (1) the micro-droplet interface of CTAB micromulsion charged with positive charge, it is propitious to the solubilizaton of HQ and/or Al(III)–HQ complex; (2) the volume and the internal nuclear of micelle
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
47
Table 3 Determined results and the percent recovery of this method Samples
Found (g/25 mL)
Added (g/25 mL)
Determined (g/25 mL)
Average (g/25 mL)
R.S.D. (/%)
Recovery (/%)
Tap water
0.4 ± 0.01
0.5
0.91, 0.98 0.92, 0.94 0.99, 0.97 1.49, 1.45 1.29, 1.40 1.36, 1.47 2.08, 1.94 2.07, 2.05 1.98, 1.93 0.99, 0.81 0.78, 0.85 0.96, 0.88 1.19, 1.27 1.27, 1.39 1.42, 1.43 1.82, 1.75 1.75, 1.85 1.80, 1.85
0.95
3.9
101.1
1.41
2.5
97.9
2.00
2.9
103.1
0.88
3.1
106
1.33
2.9
100
1.81
3.4
1.0
1.5
Lake water
0.3 ± 0.03
0.5
1.0
1.5
was increased because of the adding of co-surfactants, and HQ and/or Al(III)–HQ complex could more solubilize in the internal nuclear; (3) the volume of micelle was increased and the space of the mono-layer enlarged due to the existence of oil, and 8-Ox and/or Al(III)–HQ complex was easily absorbed on the micellar interface and accommodated to the micellar mono-layer. The total solubilization amounts and the enrichment ability of HQ and/or Al(III)–HQ complex in the O/W CTAB microemulsion were increased. Therefore, the fluorescence intensity was increased and the sensitivity was higher in the O/W microemulsion than in the micelle system. On the other hand, the chemical kinetics are largely affected by the characteristic properties of media which are used. The marked acceleration of the reaction in the microemulsion was interpreted by an adsorption of the complex ion at the interface of micro-droplet. The complexations of Al3+ and HQ in microemulsions is gaining a continuing interest in relation to the located of the Al–HQ reaction. The reaction can occur both on the macroscopic and microscopic interface. The organic droplets in O/W microemulsion can be thought analogous to an organic phase in solvent extraction. The formation of the adduct complexes of Al–HQ in the oil droplets was concluded from an enhancement of the fluorescence intensity characteristic for the fluorescent adduct. The fluorescence quantum yields of Al(III)–HQ complex (Φf ) in various media were determined respectively in order to discuss the influence of the microenvironment on the fluo-
98.9
rescence intensity of Al(III)–HQ complex. The results were listed in Table 4. The fluorescence quantum yields was one of the mostly basic and significant parameters in all the characters of fluorescence substance [18,22,23]. It was represented the ability of translating absorption energy to fluorescence. The Φf value was tightly related to chemical structure and the microenvironment of the system, and the fluorescence quantum yield would change with the inflect of the microenvironment. Φf could be seem to the function of molecular structure. The order of Φf was as follows: CTAB-M > CTAB-m > SDS-M > Triton X-100-M > water This order was the same with the order of their sensitivities. The Φf in surfactant mediums was obvious enhancement, There were three possibilities: (1) the microenvironment character was changed in the presence of surfactant; (2) this microenvironment could offer the protective environment to the excited single state; (3) the Al(III)–HQ complex were dispersed in the droplet of microemulsion or micelle, and their fluidity was smaller, therefore, they were effectively shielded. Summarize above, the protective microenvironment decreased the concentration quenching of fluorescent particles and quenching effect of external quencher, which decreased obviously the rate of the excited single state nonraditive and deactivation process The fluorescence
Table 4 The results of relative fluorescence quantum yield Medium
λex (nm)
Φf
CTAB microemulsion CTAB micelle SDS microemulsion Triton X-100 microemulsion Water
343 341 345 342 336
0.1739 0.1187 0.0544 0.0280 0.0077
Φf (average) 0.1729 0.1164 0.0546 0.0270 0.0077
0.1723 0.1181 0.0524 0.0270 0.0078
0.1730 0.1177 0.0538 0.0273 0.0077
48
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
intensity was higher in the CTAB microemulsion than that in the other medium, because the CTAB microemulsion droplet could better accommodate the microenvironment.
4. Conclusion The sensitizing effect of the surfactants on the determination of Al(III)–HQ was developed in this paper. The main factors affecting the determination were investigated in detail. The fluorescence quantum yields of Al(III)–HQ complex in various mediums were measured and the mechanism of sensitizing effect was primarily discussed. The results showed that the sensitivity of determining Al(III)–HQ complex was highest in the CTAB microemulsion. The proposed method has been applied satisfactorily to determine aluminium(III) in environmental reference materials and in tap water and lake water samples.
References [1] R. Ranau, J. Oehlenschlager, H. Steinhart, Fresenius J. Anal. Chem. 364 (1999) 599. [2] M.J. Ahmed, J. Hossan, Talanta. 42 (1995) 1135. [3] S. Ma, Phys. Testing Chem. Anal. (Chinese) 34 (2) (1998) 68.
[4] S.M.Z. Al-Kindy, F.O. Suliman, S.B. Salama, Microchem. J. 74 (2) (2003) 173. [5] D. De Armas, M. Miro, A. Cladera, J.M. Estela, V. Cerda Anal. Chim. Acta 455 (1) (2002) 149. [6] J. You, Q. Zhu, L. Zhou, Phys. Testing Chem. Anal. (Chinese) 34 (3) (1998) 121. [7] S. Knezevic, R. Milacic, M. Veber, Fresenius J. Anal. Chem. 362 (1998) 162. [8] L.G. Danielsson, A. Sparen, Anal. Chim. Acta. 306 (1995) 173. [9] T. Bantan, R. Milacic, B. Pihlar, Talanta 46 (1998) 227. [10] N. Gan, S. Bi, G. Zou, Chin. J. Anal. Chem. 28 (11) (2000) 1375. [11] I. Danielsson, B. Lindman, Colliod. Surf. 3 (1981) 391. [12] R. Guo, X. Zhu, Chem. J. Chin. Univ. 8 (6) (1987) 508. [13] X. Zhu, R. Guo, X. Liu, Chin. J. Anal. Chem. 22 (8) (1994) 860. [14] X. Zhu, R. Guo, Phys. Testing Chem. Anal. (Chinese) 27 (1) (1991) 31. [15] X. Zhu, R. Guo, Spectrosc. Spectral Anal. (Chinese) 19 (3) (1999) 456. [16] X. Zhu, X. Zhang, R. Guo, Spectrosc. Spectral Anal. (Chinese) 19 (5) (1999) 747. [17] X. Zhu, R. Guo, X. Zhang, Spectrosc. Spectral Anal. (Chinese) 24 (4) (2001) 517. [18] L. Li, M. Zhang, Chin. J. Anal. Chem. 16 (8) (1988) 732. [19] J.N. Demas, G.A. Crosky, J. Phys. Chem. 75 (8) (1971) 991. [20] X. Zhu, R. Guo, W. Qi, Chin. J. Anal. Chem. 23 (9) (1995) 989. [21] X. Zhu, R. Guo, C. Kang, M. Shen, Acta Chimica Sinica (Chinese) 53 (1995) 716. [22] A.S. Klymchenko, V.G. Pivovarenko, A.P. Demchenko, Spectrochimica Acta A 59 (2003) 787. [23] P.R. Callis, J.T. Vivian, Chem. Phy. Lett. 369 (2003) 409.
Determination of aluminium(III) in water samples in a microemulsion system by spectrofluorimetry Xiashi Zhu∗ , Li Bao, Rong Guo, Jun Wu Department of Chemistry, Yangzhou University, Yangzhou 225002, PR China Received 17 May 2004; received in revised form 18 June 2004; accepted 18 June 2004 Available online 7 August 2004
Abstract The sensitizing effect of cetyltrimetrylammonium bromide (CTAB) microemulsion media on the determination of aluminium(III) by spectrofluorimetry was developed. The main factors affecting the determination were investigated in detail. The results showed that 8hydroxyquinoline (HQ) react with aluminium(III) forming a complex with fluorescence in the system of potassium acid phthalate–NaOH buffer solution at pH 6.0, the maximum excitation and emission wavelengths are at 380.0 and 502.6 nm, the sensitizing effect of CTAB microemulsion is higher than that of CTAB micelle. The proposed method has been applied to the determination of aluminium(III) in tap water and lake water samples with satisfactory results. © 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium; 8-Hydroxyquinoline; Microemulsion; Spectrofluorimetry
1. Introduction Although aluminium is the most abundant metal in the earth’s crust, it shows a low bioavailability and it is still questionable if aluminium has biological functions. Acid rains cause partial dissolution of soil aluminium leading to an increase in the aluminium concentration in natural waters and biological systems. Recent report associated aluminium with several skeletal (osteomalacia) and neurological disorders (encephalopathy, Alzheimer’s disease) in humans [1]. It was suggested that exposure to aluminium might present a hazard to health. Hence, the determination of trace aluminium(III) is very significant. At present, for the determination of aluminium(III), the methods employed were spectrophotometry [1,2], spectrofluorimetry [3–5], atomic absorption spectrometry (AAS) [6–8] and atomic emission spectrometry (AES) [9]. Among them, spectrofluorimetry was more applied because of its high sensitivity and simplicity. 8-Hydroxyquinoline has been ex∗
Corresponding author. Tel.: +86 514 7975568; fax: +86 514 7975244. E-mail addresses: [email protected], [email protected] (X. Zhu).
0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.06.067
tensively used as chelating reagent and it could react with metal ions and forms complex with five-members ring structure. Aluminium(III) can be bound to 8-hydroxyquinoline and form the steady complex with fluorescence under certain experimental conditions. Ning [10] have reported that the determination of aluminium(III) in natural water samples by ion exchange/chloroform extracting 8-hydroxyquinoline spectrofluorimetry, but the reproducibility and simplicity of this method were lacking, and there was also the limitation of sample volume, which cannot exceed 100 mL each time. Microemulsion, which are made from water, an organic solvent, and a surfactant and occasionally an alcohol as a cosurfactant, have unique properties as media. In the single phase water-in-oil (W/O) and oil-in-water (O/W) microemulsions, the partition and the interfacial adsorption of a solute in the microheterogeneous systems are responsible for the chemical reactivity [11]. In our previous publications, the sensitizing effect of microemulsion on the determination of metal ion by spectrophotometry was developed [12–16]. We have reported the determination of trace nickel in nonionic microemulsion medium by the fluorescence quenching method with satisfactory results [17]. But cationic mi-
44
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
croemulsion application to spectrofluorimetry seems to be lacking. In this paper, O/W cationic microemulsion has been applied as the medium for the determination of Al3+ . The proposed method showed higher sensitivity. The precision and accuracy of the proposed approach were evaluated by analysis of water samples with satisfactory results.
2. Experimental 2.1. Apparatus A model BF-540 fluorophotometer (Shimadzu, Japan), a model UV2501 spectrophotometer (Shimadzu, Japan). Conventional 1 cm quartz cells were used in all above instruments. The pH of solutions was measured using a pHS-29A pH meter (Shanghai, China). 2.2. Reagents Unless otherwise stated, all reagents used were of analytical grade and their solutions were prepared by serial dilutions with distilled water. The concentration of each reagent was as follows: CTAB micelle (CTAB-m), 5.0 × 10−3 mol L−1 ; (CTAB from Shanghai Lingfeng Chemistry Reagents Co., Ltd.); CTAB microemulsion(CTAB-M), CTAB:nC5 H11 OH:n-C7 H16 :water = 1.0:0.9:0.1:98 (mass); SDS microemulsion (SDS-M), SDS (from China Medicine (Group) Shanghai Chemical Reagent Corporation):n-C4 H9 OH:nC7 H16 :water = 0.23:0.67:0.10:4.26(mass); Triton X-100 microemulsion (Triton X-100-M), Triton X-100 (from China Medicine (Group) Shanghai Chemical Reagent Corporation):n-C5 H11 OH:n-C7 H16 :water = 5.0:3.3:0.8:90.9 (mass) (n-C5 H11 OH, n-C7 H16 , n-C4 H9 OH, from Shanghai Lingfeng Chemical Reagent Co., Ltd.). A standard aluminium(III) stock solution of 1000 g mL−1 was prepared The solution was diluted as needed to concentration being used. Series of potassium acid phthalate–NaOH buffer solutions, pH 4–8. A 1.0 × 10−3 mol L−1 solution of HQ (from China Medicine (Group) Shanghai Chemical Reagent Corporation) was prepared. A standard quinine sulfate (Fluka Chemie GmbH) stock solution of 1000 g mL−1 was prepared by dissolving 0.1000 g quinine sulfate in 0.5 M H2 SO4 . The solution was diluted to 3 g mL−1 being used for. The determination of relative fluorescence quantum yield.
luted to the mark with distilled water, shaken and allowed to stand for 30 min at 25 ◦ C. The fluorescence intensity of the solution was measured at emission wavelength 502.6 nm in a 1 cm quartz cell by BF-540 fluorophotometer, keeping the excitation wavelength 380.0 nm and the blank solution was measured at the same time. 2.3.2. Determination of relative fluorescence quantum yield Fluorescence quantum yields of Al(III)–HQ complex were measured by ration value of integrated area under corrected fluorescence spectra [18,19] in the various media. Quinine sulfate was applied to the standard. According to the equation Φf = nf 2 Df ΦS /ns 2 DS . Briefly, ΦS and Φf are corresponding the standard and unknown fluorescence quantum yield, and DS and Df are the integral areas of two calibration fluorescence emission curves, nf and ns are the refractive index of the standard and unknown, and ΦS = 0.546 (25 ◦ C) is known. 3. Results and discussion 3.1. Fluorescence spectra The fluorescence spectra of Al(III)–HQ complex in the different media (water, micelle, various microemulsions) are shown in Fig. 1. The excitation wavelength was 380.0 nm, the λem and fluorescence intensity F were listed in Table 1. From Fig. 1 and Table 1, comparing with aqueous solution, the sensitivity was improved in the O/W microemulsion medium. 3.2. Effect of microemulsion media In the experiment, various media were chosen for studying their effects on F, and all results were shown in Fig. 2. From Fig. 2, CTAB-M had highest sensitivity in correspond-
2.3. Procedure 2.3.1. Determination of fluorescence intensity In a 25 mL volumetric flask, 5.0 mL aluminium(III) solution, 3.0 mL HQ solution, 5.0 mL CTAB-M and 2.0 mL buffer solution (pH 6.0) were added. The solution was di-
Fig. 1. Fluorescence spectrum of Al(III)–HQ in different media (1) CTABM; (2) CTAB-m; (3) SDS-M; (4) Triton X-100-M; (5) Water.
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
45
Table 1 Fluorescence intensity in different media Medium
λem (nm)
Fa
Fb
CTAB-M CTAB-m SDS-M Triton X-100-M Water
502.6 505.4 503.4 524.6 521.2
157.6 143.2 49.2 30.7 6.2
4.5 1.8 – 1.6 0.4
a b
Fluorescence intensity of Al(III)-HQ in different media. Fluorescence intensity of HQ in different media.
Fig. 4. Effect of the amount of HQ (1.0 × 10−3 mol L−1 ) on fluorescence intensity: (1) CTAB-M; (2) CTAB-m.
3.3. Effect of amounts of HQ
Fig. 2. Effects of various media on F.
ing CTAB-m and in aqueous solution. From microemulsion media, the order of their sensitivities was as follows: CTAB > SDS > Triton X-100 Therefore, CTAB-M was chosen for further study. In the case of CTAB-M medium, adjusting the volume of CTAB, the effect of amounts of CTAB-M on fluorescent intensity F was investigated. When the volume of CTABM varied from 2.0 to 7.0 mL, F reached a maximum. Thus, 5.0 mL was optimum (Fig. 3).
Fig. 3. Effect of the amount of CTAB-M on fluorescent intensity, CTABM:CTAB:n-C5 H11 OH:n-C7 H16 :water = 1.0:0.9:0.1:98 (mass).
Amounts of HQ solution were added variously to investigate the effects on F in the CTAB-M and corresponding CTAB-m, the results are shown in Fig. 4. The maximum and stable F gained in the range of 3.0–5.0 mL in the CTAB-M. Optimum volume of HQ solution chosen for this work was 3.0 mL. 3.4. Effect of pH According to Section 2.3.1, the effects of pH on F were studied. The results are summarized in Fig. 5. In the CTAB micromulsion, when the pH varied in the range of 5.2–6.4, F reached the maximum, while in the CTAB micelle, F reached the maximum in the pH range of 5.8–6.2, which was smaller than in the microemulsion Hence, pH 6.0 was selected.
Fig. 5. Effect of pH on fluorescence intensity: (1) CTAB-M; (2) CTAB-m.
46
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
3.5. Determination of developing time
Table 2 Determination of standard substance
In the optimum conditions described above, the developing time of complex was measured. It was found that F reached the maximum instantly and the maximum of F was steady for 7 days.
Sample
3.6. Effect of co-existing ions Determination of 25 g Al3+ in the presence of foreign ions was investigated. With a relative error of less than ±5%, the tolerance limits for various foreign ions were as follows: Fe2+ (1); Cr3+ (8); Ni2+ (12); Cu2+ (15); Cr(15); Bi3+ (20); Pb2+ (22); Ca2+ (23); Mg2+ (50); Co2+ (50); Zn2+ (80); Ag+ (600); Cd2+ (1200); Sodium citrate(30); Sodium tartrate(180). 3.7. Working curve, detecting limit and precision According to procedure Section 2.3.1, 1.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0 and 35.0 g of Al3+ standard solutions were prepared and determined, respectively. The linear relationship of F with alumium(III) amounts was displayed in Fig. 6. In Fig. 6, the working curve was linear over the range of 0∼1400 ng mL−1 Al3+ . The corresponding regression equation and coefficient of correlation was calculated as follows: C(g/25 mL) = 0.16652F + 0.06658,
r = 0.9995
The 3 limits of detection for alminium(III) was 3.95 ng mL−1 The relative standard deviations (R.S.D.) was 2.4% (n = 9). 3.8. Sample determination The standard sample was determined according to procedure Section 2.3.1. The results are shown in Table 2. 220 200 180 160 140
F
120 100 80 60 40
B Data1B
20 0 0
5
10
15
20
25
30
C(µg/25ml) Fig. 6. Calibration graph of Al(III)–HQ.
35
40
GB a
602-88a
Measured (g/mL)
Certified (g/mL)
99.2 ± 0.8
100.0 ± 0.5
National reference materials (China).
The proposed method was applied for the determination of Al3+ in environmental reference materials and in tap water and lake water samples. The standard addition method was used and the analytical results and the recovery are presented in Tables 2 and 3. The good quantitative results could be obtained. The analytical content of Al3+ in environmental reference materials is in good agreement with the certified value. 3.9. Discussion of mechanism of sensitizing effect The sensitizing effect of the surfactants on spectroanalysis rested with two factors: (1) the solubilization capacity; (2) the microenvironment of medium [20,21]. The sensitivity of Al(III)–HQ complex in various O/W microemulsion was different, which was mainly related to surfactants, complexant and complex. The nanometer-sized structure brings about an extraordinarily large specific interfacial area to the microemulsion system. Thus, the interfacial adsorption at the micro-droplet surface becomes a significant factor. In the CTAB-M, the micro-droplet interface of CTAB micromulsion charged with positive charge, while the nitrogen atom of HQ molecule could afford electron and show electronegativity. So HQ and/or Al(III)–HQ complex could be adsorbed to the micro-droplet interface and solubilized to the mono-layer simultaneously, sequentially, the amounts of solubilization was increased. In the SDS-M, the microdroplet interface charged with negative charge, which was the same with the electronegativity of the nitrogen atom of HQ, it is disadvantage for the solubilizaton of HQ and/or Al(III)–HQ complex in SDS micromulsion. In the Triton X100-M, the micro-droplet interface was no charge and adverse to HQ and/or Al(III)–HQ complex enter into microemulsion micro-droplet. Al(III)–HQ complex had higher sensitivity in the CTAB micromulsion than corresponding CTAB micelle, which was mainly related to the different composition between microemulsion and micelle. Microemulsion contain finely divided domains of oil and water topologically ordered by surfactant at the internal interfaces. Therefore, the solute can distribute among the microscopic water phase, oil phase and interface. This microscopic partitioning greatly affect the solubility and reactivity. The solubilization capacity of CTAB-M were highly dependent on the charge types of the surfactant and the volume fraction of the organic component. The influence factors were as follows: (1) the micro-droplet interface of CTAB micromulsion charged with positive charge, it is propitious to the solubilizaton of HQ and/or Al(III)–HQ complex; (2) the volume and the internal nuclear of micelle
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
47
Table 3 Determined results and the percent recovery of this method Samples
Found (g/25 mL)
Added (g/25 mL)
Determined (g/25 mL)
Average (g/25 mL)
R.S.D. (/%)
Recovery (/%)
Tap water
0.4 ± 0.01
0.5
0.91, 0.98 0.92, 0.94 0.99, 0.97 1.49, 1.45 1.29, 1.40 1.36, 1.47 2.08, 1.94 2.07, 2.05 1.98, 1.93 0.99, 0.81 0.78, 0.85 0.96, 0.88 1.19, 1.27 1.27, 1.39 1.42, 1.43 1.82, 1.75 1.75, 1.85 1.80, 1.85
0.95
3.9
101.1
1.41
2.5
97.9
2.00
2.9
103.1
0.88
3.1
106
1.33
2.9
100
1.81
3.4
1.0
1.5
Lake water
0.3 ± 0.03
0.5
1.0
1.5
was increased because of the adding of co-surfactants, and HQ and/or Al(III)–HQ complex could more solubilize in the internal nuclear; (3) the volume of micelle was increased and the space of the mono-layer enlarged due to the existence of oil, and 8-Ox and/or Al(III)–HQ complex was easily absorbed on the micellar interface and accommodated to the micellar mono-layer. The total solubilization amounts and the enrichment ability of HQ and/or Al(III)–HQ complex in the O/W CTAB microemulsion were increased. Therefore, the fluorescence intensity was increased and the sensitivity was higher in the O/W microemulsion than in the micelle system. On the other hand, the chemical kinetics are largely affected by the characteristic properties of media which are used. The marked acceleration of the reaction in the microemulsion was interpreted by an adsorption of the complex ion at the interface of micro-droplet. The complexations of Al3+ and HQ in microemulsions is gaining a continuing interest in relation to the located of the Al–HQ reaction. The reaction can occur both on the macroscopic and microscopic interface. The organic droplets in O/W microemulsion can be thought analogous to an organic phase in solvent extraction. The formation of the adduct complexes of Al–HQ in the oil droplets was concluded from an enhancement of the fluorescence intensity characteristic for the fluorescent adduct. The fluorescence quantum yields of Al(III)–HQ complex (Φf ) in various media were determined respectively in order to discuss the influence of the microenvironment on the fluo-
98.9
rescence intensity of Al(III)–HQ complex. The results were listed in Table 4. The fluorescence quantum yields was one of the mostly basic and significant parameters in all the characters of fluorescence substance [18,22,23]. It was represented the ability of translating absorption energy to fluorescence. The Φf value was tightly related to chemical structure and the microenvironment of the system, and the fluorescence quantum yield would change with the inflect of the microenvironment. Φf could be seem to the function of molecular structure. The order of Φf was as follows: CTAB-M > CTAB-m > SDS-M > Triton X-100-M > water This order was the same with the order of their sensitivities. The Φf in surfactant mediums was obvious enhancement, There were three possibilities: (1) the microenvironment character was changed in the presence of surfactant; (2) this microenvironment could offer the protective environment to the excited single state; (3) the Al(III)–HQ complex were dispersed in the droplet of microemulsion or micelle, and their fluidity was smaller, therefore, they were effectively shielded. Summarize above, the protective microenvironment decreased the concentration quenching of fluorescent particles and quenching effect of external quencher, which decreased obviously the rate of the excited single state nonraditive and deactivation process The fluorescence
Table 4 The results of relative fluorescence quantum yield Medium
λex (nm)
Φf
CTAB microemulsion CTAB micelle SDS microemulsion Triton X-100 microemulsion Water
343 341 345 342 336
0.1739 0.1187 0.0544 0.0280 0.0077
Φf (average) 0.1729 0.1164 0.0546 0.0270 0.0077
0.1723 0.1181 0.0524 0.0270 0.0078
0.1730 0.1177 0.0538 0.0273 0.0077
48
X. Zhu et al. / Analytica Chimica Acta 523 (2004) 43–48
intensity was higher in the CTAB microemulsion than that in the other medium, because the CTAB microemulsion droplet could better accommodate the microenvironment.
4. Conclusion The sensitizing effect of the surfactants on the determination of Al(III)–HQ was developed in this paper. The main factors affecting the determination were investigated in detail. The fluorescence quantum yields of Al(III)–HQ complex in various mediums were measured and the mechanism of sensitizing effect was primarily discussed. The results showed that the sensitivity of determining Al(III)–HQ complex was highest in the CTAB microemulsion. The proposed method has been applied satisfactorily to determine aluminium(III) in environmental reference materials and in tap water and lake water samples.
References [1] R. Ranau, J. Oehlenschlager, H. Steinhart, Fresenius J. Anal. Chem. 364 (1999) 599. [2] M.J. Ahmed, J. Hossan, Talanta. 42 (1995) 1135. [3] S. Ma, Phys. Testing Chem. Anal. (Chinese) 34 (2) (1998) 68.
[4] S.M.Z. Al-Kindy, F.O. Suliman, S.B. Salama, Microchem. J. 74 (2) (2003) 173. [5] D. De Armas, M. Miro, A. Cladera, J.M. Estela, V. Cerda Anal. Chim. Acta 455 (1) (2002) 149. [6] J. You, Q. Zhu, L. Zhou, Phys. Testing Chem. Anal. (Chinese) 34 (3) (1998) 121. [7] S. Knezevic, R. Milacic, M. Veber, Fresenius J. Anal. Chem. 362 (1998) 162. [8] L.G. Danielsson, A. Sparen, Anal. Chim. Acta. 306 (1995) 173. [9] T. Bantan, R. Milacic, B. Pihlar, Talanta 46 (1998) 227. [10] N. Gan, S. Bi, G. Zou, Chin. J. Anal. Chem. 28 (11) (2000) 1375. [11] I. Danielsson, B. Lindman, Colliod. Surf. 3 (1981) 391. [12] R. Guo, X. Zhu, Chem. J. Chin. Univ. 8 (6) (1987) 508. [13] X. Zhu, R. Guo, X. Liu, Chin. J. Anal. Chem. 22 (8) (1994) 860. [14] X. Zhu, R. Guo, Phys. Testing Chem. Anal. (Chinese) 27 (1) (1991) 31. [15] X. Zhu, R. Guo, Spectrosc. Spectral Anal. (Chinese) 19 (3) (1999) 456. [16] X. Zhu, X. Zhang, R. Guo, Spectrosc. Spectral Anal. (Chinese) 19 (5) (1999) 747. [17] X. Zhu, R. Guo, X. Zhang, Spectrosc. Spectral Anal. (Chinese) 24 (4) (2001) 517. [18] L. Li, M. Zhang, Chin. J. Anal. Chem. 16 (8) (1988) 732. [19] J.N. Demas, G.A. Crosky, J. Phys. Chem. 75 (8) (1971) 991. [20] X. Zhu, R. Guo, W. Qi, Chin. J. Anal. Chem. 23 (9) (1995) 989. [21] X. Zhu, R. Guo, C. Kang, M. Shen, Acta Chimica Sinica (Chinese) 53 (1995) 716. [22] A.S. Klymchenko, V.G. Pivovarenko, A.P. Demchenko, Spectrochimica Acta A 59 (2003) 787. [23] P.R. Callis, J.T. Vivian, Chem. Phy. Lett. 369 (2003) 409.