Analytica Chimica Acta 556 (2006) 157–163

Extraction mechanisms of charged organic dye molecules intosilica-surfactant nanochannels in a porous alumina membrane

Akira Yamaguchia,b,∗, Jun Watanabea, Mekawy M. Mahmouda,Rise Fujiwaraa, Kotaro Moritaa,c, Tomohisa Yamashitac,

Yosuke Aminoa, Yong Chena, Logudurai Radhakrishnana, Norio Teramaea,c,∗∗a Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

b PRESTO-JST, Aoba-ku, Sendai 980-8578, Japanc CREST-JST, Aoba-ku, Sendai 980-8578, Japan

Received 28 March 2005; received in revised form 7 June 2005; accepted 8 June 2005Available online 14 July 2005

Abstract

th a channeld ntal resultsc rhodamine6 e extractiono -surfactantn©

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Extraction mechanisms of charged organic dye molecules are examined for an assembly of silica-surfactant nanochannels wiiameter of 3.4 nm, which is formed inside the pores of an anodic alumina membrane by a surfactant-template method. Experimeonfirm that the extraction mechanism depends on the sign of a charge of the dye molecules. The extraction of the cationicG is predominantly caused by an ion-pair extraction process, whereas an anion-exchange process is mainly responsible for thf the anionic sulforhodamine B. These extraction mechanisms are discussed by considering the microstructures of the silicaanochannels.2005 Elsevier B.V. All rights reserved.

eywords: Silica-surfactant nanochannel; Alumina membrane; Extraction

. Introduction

Surfactant-templated mesoporous silica has a potential usen the fields of catalysis, organic synthesis, optoelectronics,nd separation of molecules, because the mesoporous sil-

ca has a uniform pore diameter of molecular dimensions,igh surface area, and high adsorption capacity[1–10]. Theurfactant-templated mesoporous silica was first developedy a hydrothermal synthesis method and was obtained as aowder of which size was hundreds nm-order[1,11]. From

he first development of the surfactant-templated mesoporousilica, many efforts have been made to control the mesoscopictructure. Nowadays, the pore diameter of the mesoporous

∗ Corresponding author. Tel.: +81 22 795 6551; fax: +81 22 795 6551.∗∗ Corresponding author.

E-mail addresses: akira@anal.chem.tohoku.ac.jp (A. Yamaguchi),era@anal.chem.tohoku.ac.jp (N. Teramae).

silica can be tuned from a few to 40 nm by varying the tplate surfactant[11,12]. The mesoporous silica with differemesostructures, such as hexagonal, lamellar, and cubibe synthesized by optimizing the composition of the presor solution[13,14]. On the other hand, morphology of tsurfactant-templated mesoporous silica has also been sand there have been various successive reports onporous silica materials having well-controlled morphologmesoporous silica with a spherical shape[15], a mesoporousilica thin film [16], and a mesoporous membrane[10].

In general, there are two strategies for the use osurfactant-templated mesoporous silica. One is the uthe mesoporous silica after removing the template sutant by calcination[11], elution [17], or irradiation of anultraviolet light [18]. After removing the template surfatant, an ordered mesostructure of a silica-framework hasused as a reaction field and some functional mesoporouica have been developed by an incorporation of functi

003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2005.06.029

158 A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163

Fig. 1. Schematic illustration of: (A) the anodic alumina membrane used in this work; (B) the assembly of silica-surfactant nanochannels (channel diame-ter = 3.4 nm) formed inside the columnar alumina pore (pore diameter = 200 nm); (C) the TEM side-view of the silica-surfactant nanochannels oriented alongthe columnar alumina wall.

compounds inside the ordered mesostructure of the silica-framework[1–5,7–9]. On the other hand, an as-synthesizedmesoporous silica has been used without removing the sur-factant in some attempts[7]. Since the mesoporous silica hasbeen synthesized by utilizing a rod-like surfactant micelleas a template, there are surfactant micellar phases withinthe ordered mesostructure of the silica-framework. Herein,we call the as-synthesized mesoporous silica as an assem-bly of silica-surfactant nanochannels. The surfactant micel-lar phases can be regarded as one-dimensional hydropho-bic nanochannels surrounded by the silica-framework. Thehydrophobic nature can be utilized for solubilization oforganic compounds and the one-dimensionality enables us todesign a well-aligned molecular system inside the hydropho-bic nanochannels. Based on these considerations, there havebeen some reports on the extraction of organic moleculesusing the as-synthesized mesoporous silica[7,19–22]. Forexample, Miyake et al. reported that a maximum amountof extracts of some organic molecules was directly propor-tional to a distribution coefficient between the octanol andwater phase by observing extraction behavior of aryl deriva-tives[21]. This relationship between the extraction efficiencyand the distribution coefficient was also reported by Hann etal. [22]. Although, the extraction behavior of hydrophobicaryl derivatives has been studied, the extraction behavior ofcharged organic molecules has still not been cleared. From as nder-s alsoc

is ofs nodica m-b dredsn porei ecur-s ide( a sil-i , thes me-t porea g the

Fig. 2. Molecular structures of rhodamine dyes used in this study.

alumina wall as shown inFig. 1. We call herein the aluminamembrane with the perpendicularly oriented silica-surfactantnanochannels assembly as a nanochannels-incorporated alu-mina membrane (NAM).

The aim of the present work is to reveal the extractionmechanisms of positively and negatively charged organic dyemolecules into the as-synthesized mesoporous silica. For thispurpose, we used the NAM as the as-synthesized mesoporoussilica material; rhodamine 6G (R6G) and sulforhodamineB (SRB) were used as a cationic and an anionic organicmolecules, respectively (Fig. 2). Our experimental resultsclearly confirm that the extraction mechanism of R6G is dif-ferent from that of SRB. The extraction of the cationic R6Gis predominantly caused by an ion-pair extraction process,whereas an anion-exchange process is mainly responsible forthe extraction of the anionic SRB. These extraction mecha-nisms are discussed by considering the microstructures of thesilica-surfactant nanochannel and usage of an as-synthesizedmesoporous silica material for the solid-phase extraction ofcharged organic molecules is demonstrated.

2. Experimental

2.1. Materials

werep em-i tries,L eri-m wicht ntra-

tandpoint view of separation science, it is essential to utand the extraction behaviors of not only nonionic butharged molecules.

In our previous work, we demonstrated the synthesurfactant-templated mesoporous silica in a porous alumina membrane[10]. The porous anodic alumina merane has a packed array of columnar pores with hunanometers in diameter and the direction of the columnar

s perpendicular to the membrane surface. When a pror solution, containing cetyltrimethylammonium bromCTAB) as a surfactant and tetraethoxysilane (TEOS) asca source, is introduced into the columnar alumina poresurfactant-silica nanochannels with a uniform channel diaer of 3.4 nm are assembled inside the columnar aluminand the channel direction is predominantly oriented alon

R6G (as a chloride salt) and SRB (as an inner salt)urchased from Sigma–Aldrich, Japan (Tokyo). Other ch

cals were purchased from Wako Pure Chemical Industd. (Osaka, Japan). Milli-Q water was used for all expents. It is well known that rhodamine dyes make a sand

ype dimer in an aqueous solution when the dye conce

A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163 159

tion is above 10�M [23,24]. To avoid the dimer formation,we used 1.0�M rhodamine dye solutions for the extractionexperiments.

The details of the preparation of the NAM were describedin our previous report[10]. In brief, a precursor solutioncontaining CTAB and TEOS was dropped onto an alu-mina membrane (average pore diameter = 200 nm, membranediameter = 2 cm) purchased from Whatman International Ltd.(England), which was set in an ordinary membrane filtrationapparatus. Moderate aspiration was applied so that the precur-sor solution penetrated into the columnar alumina pores. Thealumina membrane including the precursor solution was thendried in air at room temperature. The NAM was rinsed withwater carefully and was used for the extraction experiments.

2.2. Measurements

The extraction experiments were performed by immers-ing the NAM in a sample cuvette filled with 10 ml of a dyesolution. The sample cuvette was shaken continuously usinga mild mixer (TAITEC XR-36) to stir the dye solution. Asmall portion of dye solution was taken from the cuvette andwas used to measure an absorption spectrum. After measure-ments, the portion of the dye solution was put back to thesample cuvette. The absorption spectrum of the dye solutionwas recorded on a UV–vis spectrophotometer (JASCOV y thed in thes 5

rgy-d werep S-4

3

3

nicR t

time t:

Rext = At

A0(1)

where A0 is the initial absorbance at 527 nm of the R6Gaqueous solution before immersing the NAM andAt is theabsorbance at timet after immersing the NAM. The pres-ence of inorganic anions facilitates the extraction of R6Gand the extraction efficiency significantly depends on theanion species (Fig. 3(a)). To discuss these extraction behav-iors of R6G, the time-course extraction profiles are analyzedto obtain the apparent extraction rate constant (kext) and theextraction ratio at the extraction equilibrium (x). Based onthe first-order rate constant for the extraction of R6G at theNAM, the time-course extraction profiles shown inFig. 3(a)can be expressed as[25]:

Rext = 1 − x + xe−kextyt (2)

In this equation,x andy are defined as follows:

x = Cw,0 − Cw,eq

Cw,0(3)

y = 1

x(4)

whereCw,0 is the R6G concentration in the aqueous phaseb -t Thet lfi efi

m-p ilica-f smallh intot trac-t . Ass hep ctionr -t e

F n the pa (b) Eff G.T lO4] + [Na )a

-570) and the amount of extracted dye was analyzed becrease in the light absorbance of the rhodamine dyeolution phase. All measurements were performed at 2◦C.

Scanning electron microscopy (SEM) and eneispersive X-ray spectroscopy (EDS) measurementserformed by using a Field-Emission SEM (HITACHI300) equipped with an EDS (EDAX GENESIS 7000).

. Results and discussion

.1. Extraction mechanism of cationic R6G

Fig. 3 shows time-course extraction profiles of catio6G, obtained by plottingRext defined by Eq.(1) agains

ig. 3. (a) Time-course extraction profiles of R6G in the absence (©) and ind ClO4

− (�). The concentration of each inorganic anion is 100 mM.he concentration of ClO4− is varied from 0.0 to 100 mM, where [NaCnd (b) are the curves obtained by fitting plots using Eq.(2).

efore immersing the NAM andCw,eq is the R6G concentraion in the aqueous phase at the extraction equilibrium.ime-course extraction profiles shown inFig. 3(a) are weltted to Eq.(2), and thekext andx values obtained by thtting are summarized inTable 1.

The silica-surfactant nanochannel in the NAM is coosed of the surfactant micellar phase and the s

ramework, and the micellar phase can be regarded as aydrophobic medium. For the extraction of cationic R6G

he hydrophobic micellar phase, it is expected that the exion of R6G is caused by an ion-pair extraction processhown inFig. 3(a), the extraction of R6G is facilitated in tresence of inorganic anions and the order of the extraatio (x) is ClO4

− � NO3− > Br− > Cl−. This order is consis

ent with the Hofmeister series[26,27], which represents th

resence of inorganic anions (as sodium salts): Cl− (�), Br− (�), NO3− (�),

ect of the concentration of ClO4− on the time-course extraction profile of R6

Cl] = 100 mM. The concentration of R6G is 1.0�M. The solid lines in (a

160 A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163

Table 1Extraction ratios at the extraction equilibrium and apparent rate constants for R6G and SRB

Anion (as sodium salts) R6G SRB

Extraction ratio (x) Rate constant (kext/h) Extraction ratio (x) Rate constant (kext/h)

No salt 0.19 0.65 0.95 2.01Cl− (100 mM) 0.24 0.56 0.92 1.42Br− (100 mM) 0.39 0.58 0.91 0.99NO3

− (100 mM) 0.43 0.74 0.90 0.89ClO4

− (100 mM) 0.80 1.41 0.69 0.75aClO4

− (10 mM) 0.61 0.87 0.85 1.04aClO4

− (1.0 mM) 0.39 0.87 0.89 1.20aClO4

− (0.1 mM) 0.26 0.73 0.92 1.12a Mixed solution of NaClO4 and NaCl, and [NaCl] + [NaClO4] = 100 mM.

dehydration energy of inorganic anions. It is well known thatan anion with lower dehydration energy is favored for ion-pair extraction of cations at liquid–liquid and solid–liquidsystems. Hence, the anion dependent facilitation of the R6Gextraction indicates the existence of an ion-pair extractionprocess. InFig. 3(b), the ionic strength is kept at a constantvalue of 100 mM, while the concentration ClO4

− is variedfrom 0 to 100 mM. It can be easily seen that the extractionratio (x) of the R6G becomes large by increasing the concen-tration of ClO4

−. Accordingly, it is concluded that R6G ispredominantly extracted into the hydrophobic micellar phasethrough the ion-pair extraction process as illustrated schemat-ically in Fig. 6(a).

3.2. Extraction mechanism of anionic SRB

In contrast to the extraction of R6G, the extraction behav-ior of SRB cannot be explained by the ion-pair extractionprocess.Fig. 4 shows the time-course extraction profiles ofanionic SRB in the absence and presence of sodium salts.These extraction profiles are well fitted by Eq.(2) and thekext andx values obtained by the fitting are summarized inTable 1. As shown inFig. 4, the presence of sodium saltssuppresses the SRB extraction. Since the SRB solution was

Ft( are1 inedb

prepared by dissolving the SRB as an inner salt, the SRB solu-tion contained no counter cations. The two sulfonate groupsof SRB are regarded to be fully dissociated and SRB is neg-atively charged, because the solution pH used in the presentexperiment is 5.5. Therefore, observed suppression of theSRB extraction upon addition of sodium salts suggests thatthe ion-pair extraction process is not a main mechanism forthe SRB extraction.

The assembly of silica-surfactant nanochannels is formedinside the columnar alumina pore by using an acidic pre-cursor solution containing HCl, CTAB, and TEOS. In thisacidic solution, there are counter anions, such as chlorideand bromide, of a cationic cetyltrimethylammonium (CTA)and/or a protonated surface silanol group at the inner sur-face of a silica-framework[14,28]. It is expected that theextraction of SRB is caused by an anion-exchange processbetween anionic SRB and counter anions inside the silica-surfactant nanochannel, that is, the counter-anions inside thesilica-surfactant nanochannel are replaced by extracted SRB.To confirm the replacement of the counter-anions, followingexperiments were performed. After immersing the NAM inthe SRB solution without sodium salts for 5 h, 1.0 ml of 1.0 MAgNO3 solution was added into the SRB solution. By addingAgNO3, white precipitates were rapidly formed in the SRBsolution, and the EDS analysis reveals that the white precip-itate is composed of AgBr (Fig. 5). When pure water wasu hitep SRBs ionsi aniona h thee

xam-i ncee ime-c ftert tedw ouss .T lu-t reast .

ig. 4. Time-course extraction profiles of SRB in the absence (©) and inhe presence of inorganic anions (as sodium salts): Cl− (�), Br− (�), NO3

−�), and ClO4

− (�). The concentrations of SRB and inorganic anions.0�M and 100 mM, respectively. The solid lines are the curves obtay fitting plots using Eq.(2).

sed instead of the SRB solution, the amount of the wrecipitate was too much smaller than that found in theolution. These results indicate that there are bromidenside the silica-surfactant nanochannel as a counternd the elution of bromide takes place in accordance witxtraction of SRB as schematically illustrated inFig. 6(b).

This anion-exchange extraction process is further ened by measuring an elution of SRB, which was oxtracted into the silica-surfactant nanochannel. The tourse elution profile of SRB was measured as follows. Ahe extraction of SRB, the NAM containing SRB extracas rinsed with water carefully, and immersed in aqueolutions with and without sodium salts (NaClO4 and NaCl)he results are shown inFig. 7. In the pure aqueous so

ion, no significant elution of SRB was observed, whehe presence of ClO4− and Cl− induces the elution of SRB

A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163 161

Fig. 5. (a) EDS spectrum and (b) SEM image of white precipitates. In EDSspectrum, peaks of F and C are arise from the membrane filter used for thecollection of the precipitates by filtration.

The elution of SRB is remarkable for the case of ClO4−

than for Cl−, and the elution becomes larger with increasingthe concentration of ClO4−. These elution behaviors indi-cate that SRB inside the silica-surfactant nanochannels canbe replaced by those anions, that is, the anion-exchange pro-cess is mainly responsible for the extraction and elution ofanionic SRB.

The effect of inorganic anions on the extraction of SRBis discussed as follows. As shown inFig. 4, the suppres-sion of the SRB extraction is observed in the presence ofany inorganic anion and the order of the suppression isClO4

− � NO3− > Br− > Cl− (Table 1). In the presence of

these inorganic anions in the SRB solutions, the anion-exchange process of SRB is considered to be suppressedby a competitive anion-exchange process of these anions,and as a result, the extraction efficiency of SRB decreases.

Fig. 7. Time-course elution profiles of SRB in the absence (©) and presenceof ClO4

− and Cl− (�). The concentrations of ClO4− and Cl− are variedfrom 0.0 to 100 mM, where [NaClO4] + [NaCl] = 100 mM.

This consideration is supported by the ClO4− concentration

dependency of the SRB extraction (Fig. 8). The suppressionbecomes larger as the concentration of ClO4

− increases, indi-cating that the competitive anion-exchange process of ClO4

−becomes more pronounced with increasing its concentration.

The suppression order of anions for the SRB extractionagrees well with the inversion order of the Hofmeister series;the presence of an anion with lower dehydration energycauses larger suppression of the SRB extraction. Since thesuppression of the SRB extraction is ascribed to the com-petitive anion-exchange process of inorganic anions, theobserved suppression order of the SRB extraction representsthat an anion with lower dehydration energy has larger affinitywith the cationic head group of CTA located at the inner wallof the silica-surfactant nanochannel. In general, if an electro-static ion-pair formation is dominant, an inorganic anion witha smaller ionic radius can associate with a cationic ammo-nium group more preferably than that with a larger ionicradius, as demonstrated in a study of anion-exchange chro-matography using aprotic organic solvents[29]. On the other

annel a .

Fig. 6. Schematic illustration of the silica-surfactant nanoch nd extraction mechanisms for (a) cationic R6G and (b) anionic SRB

162 A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163

Fig. 8. Effect of the concentration of ClO4− on the time-course extractionprofiles of SRB. The concentration of ClO4

− is varied from 0.0 to 100 mM,where [NaClO4] + [NaCl] = 100 mM. The concentration of SRB is 1.0�M.The solid lines are the curves obtained by fitting plots using Eq.(2).

hand, a hydrophobicity of an anion is crucial for an ion-pair formation with an ammonium cation in protic media,such as water and alcohol[29,30]. In the silica-surfactantnanochannel, the electrostatic ion-pair formation might bean important factor because of the hydrophobicity insidethe silica-surfactant nanochannel. However, there are watermolecules inside the silica-surfactant nanochannel when theNAM is immersed in water, and a hydration state of an anionas well as the cationic head group of CTA would affect thestability of the ion-pair formation[29,31]. For the anion-exchange process within the silica-surfactant nanochannel,in addition, dehydration of anions would be crucial becauseanions must penetrate into the hydrophobic interior of thesilica-surfactant nanochannel. Thus, it is considered that thehydration and the dehydration properties of anions are moreimportant for the anion-exchange process inside the silica-surfactant nanochannel rather than the electrostatic ion-pairformation.

4. Conclusion

This work demonstrates the extraction of charged organicdye molecules at the NAM and the experimental results con-firm that the predominant extraction mechanism depends ont trac-t ctiono ionicS ect ofa silicam elec-t medi ns,s gedo pho-b , thes pabiliS AM

possesses a uniform channel diameter of 3.4 nm, a small bio-logical compound such as Vitamin B12 can penetrate intothe silica-surfactant nanochannel, whereas the NAM rejectsthe penetration of proteins whose sizes are more than thechannel diameter of the silica-surfactant nanochannel. Bycombining the charge selectivity and the size selectivity,hence, it is expected that the surfactant-templated meso-porous silica would be useful for pre-concentration of smallbiological compounds, such as metabolites, substances, anddrugs, based on a solid extraction system. Furthermore, theNAM will be used for a charge and size selective membraneseparation of biological compounds because of its desiredorientation of the silica-surfactant nanochannel.

Acknowledgements

This work was supported in part by a Grant in Aid forScientific Research (No. 14204074, No. 15750062) from theMinistry of Education, Culture, Sports, Science and Technol-ogy, Japan, and the Asahi Glass Foundation.

References

eck,

kaya,

3.124

, Y.

ad,ture

r,

m.

[ T.

[ sge,.B.114

[ B.F.

[ W.ture

[ 99)

[ ) 14.[ 89.[[ em.

[

he sign of the charge of dye molecules. The ion-pair exion process is predominantly responsible for the extraf cationic R6G and the anion-exchange process is for anRB. These extraction mechanisms provide a new asppplications of the surfactant-templated mesoporousaterials as a charge selective solid extraction system. S

ive extraction of anionic organic molecules can be perforn aqueous solutions with and without hydrophilic aniouch as chloride and bromide. Extraction ability for charrganic molecules can be tuned by varying the hydroicity of anions. As demonstrated in our previous studyurfactant-templated mesoporous silica possesses a caty of precise size selective separation of molecules[10].ince the silica-surfactant nanochannels within the N

-

[1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. BNature 359 (1992) 710.

[2] W. Zhou, J.M. Thomas, D.S. Shephard, B.F.G. Johnson, D. OzyT. Maschmeyer, R.G. Bell, Q. Ge, Science 280 (1998) 705.

[3] K. Kageyama, J. Mamazawa, T. Aida, Science 285 (1999) 211[4] T. Itoh, K. Yano, Y. Inada, Y. Fukushima, J. Am. Chem. Soc.

(2002) 13437.[5] Y. Tanamura, T. Uchida, N. Teramae, M. Kikuchi, K. Kusaba

Onodera, Nano Lett. 1 (2001) 387.[6] Y. Lu, Y. Yang, A. Sellinger, M. Lu, J. Huang, H. Fan, R. Hadd

G. Lopez, A.R. Burns, D.Y. Sasaki, J. Shlnutt, C.J. Brinker, Na410 (2001) 913.

[7] M. Ogawa, J. Photochem. Photobiol. C 3 (2002) 129.[8] X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, J. Liu, K.M. Kemne

Science 276 (1997) 923.[9] H.H.P. Yiu, C.H. Botting, N.P. Botting, P.A. Wright, Phys. Che

Chem. Phys. 3 (2001) 2893.10] A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura,

Yamashita, N. Teramae, Nature Mater. 3 (2004) 337.11] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kre

K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, SMcCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc.(1992) 10834.

12] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson,Chmelka, G.D. Stucky, Science 279 (1998) 548.

13] Y. Liu, R. Ganguli, C.A. Drewien, M.T. Anderson, C.J. Brinker,Gong, Y. Guo, H. Soyez, B. Bunn, M.H. Huang, J.I. Zink, Na389 (1997) 264.

14] C.J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (19579.

15] Q. Huo, J. Feng, F. Schuth, G.D. Stucky, Chem. Mater. 9 (199716] H. Yang, N. Coombs, I. Sokolov, G.A. Ozin, Nature 381 (1996) 517] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865.18] A. Hozumi, H. Sugimura, K. Hiraku, T. Kameyama, O. Takai, Ch

Mater. 12 (2000) 3842.19] R. Denoyel, E.S. Rey, Langmuir 14 (1998) 7321.

A. Yamaguchi et al. / Analytica Chimica Acta 556 (2006) 157–163 163

[20] H. Zhao, K.L. Nagy, J.S. Waples, G.F. Vance, Environ. Sci. Technol.24 (2000) 4822.

[21] Y. Miyake, T. Yumoto, H. Kitamura, T. Sugimoto, Phys. Chem.Chem. Phys. 4 (2002) 2680.

[22] K. Hanna, I. Beurroies, R. Denoyel, D.D. Giscard, A. Galarneau, F.Di Renzo, J. Colloid Interface Sci. 252 (2002) 252.

[23] J.E. Selwyn, J.I. Steinfeld, J. Phys. Chem. 76 (1972) 762.[24] T. Uchida, A. Yamaguchi, T. Ina, N. Teramae, J. Phys. Chem. B 104

(2000) 12091.[25] C.E. Green, M.H. Abraham, J. Chromatogr. A 885 (2000) 41.

[26] D. Wegmann, H. Weiss, D. Ammann, W.E. Morf, E. Pretsch, K.Sugahara, W. Simon, Microchim. Acta 3 (1984) 1.

[27] F. Hofmeister, Arch. Exp. Pathol. Pharmakol. 24 (1888) 247.[28] Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier,

P. Sieger, A. Firouzi, B.F. Chmelka, F. Schuth, G.D. Stucky, Chem.Mater. 6 (1994) 1176.

[29] T. Okada, J. Chromatogr. A 758 (1997) 19.[30] T. Hattori, S. Tanaka, Electroanalysis 15 (2003) 1522.[31] S. Motomizu, Bunseki Kagaku 48 (1999) 151.