- Email: [email protected]
turn-off fluorescent hybrid silica nanoparticles. A new promising material for selective anions’ sensing
Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Turn-on/turn-off fluorescent hybrid silica nanoparticles. A new promising material for selective anions’ sensing Matthieu Becuwe a,1 , Francine Cazier a , Patrice Woisel b , Franc¸ois Delattre a,∗ a b
Unité de Chimie Environnementale et Interactions sur le Vivant, Univ. Lille Nord de France, ULCO, 145 Avenue M. Schuman, 59140 Dunkerque, France Unité Matériaux Et Transformations, CNRS UMR 8207, Univ. Lille Nord de France, USTL, Bâtiment C6, 59655 Villeneuve d’Ascq, France
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Preparation of a new fluorescent hybrid material including a pyridinium unit. • The new material exhibits detection properties toward anions in aqueous solution. • Promising fluorescent specific behavior for perchlorate sensing.
a r t i c l e
i n f o
Article history: Received 14 November 2012 Received in revised form 21 March 2013 Accepted 12 April 2013 Available online 30 April 2013 Keywords: Hybrid material Silica nanoparticles Fluorescent Pyridinoindolizin Anion’s sensing
a b s t r a c t This work focused on the elaboration of a new fluorescent hybrid material based on spherical silica nanoparticle generated by sol–gel chemsitry (Stöber process). A fluorescent molecule (quaternized pyridinoindolizin) was introduced on the inorganic framework via an organosilane entities possessing hydrolysable group (methoxy group in the present case) to ensure the covalent linkage between the inorganic host and the organic substrate. The material obtained was fully characterized with FTIR, 29 Si and 13 C Solid state NMR, thermogravimetric and elemental analysis, Atomic force and scanning electron microscopy and the fluorescence properties of these nano-objects were determined with a home-made device. Finally, the influence of the counter-ion of the pyridinium fragment on the fluorescence emission was also investigated. In this case, a specific behavior (turn-on, exaltation of emission) was observed following exchange with perchlorate anions contrary to exchange with other anions such as acetate, chloride and nitrate which provoke a quenching of fluorescence. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Encouraged by the possibility of combining different scientific domain like chemistry, physic, biology, mechanic. . . in a miniaturized device, nanotechnology’s world is nowadays an important area of investment and interest by the scientific community. Consequently, high-added value systems have been created and a wide
∗ Corresponding author. Tel.: +33 328658246; fax: +33 328658231. E-mail address: [email protected] (F. Delattre). 1 Permanent address: Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 7314, Université de Picardie Jules Verne, 33 rue Saint-leu, 80039 Amiens Cedex, France. 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.04.030
range of purpose in energy [1,2], medicine [3], catalysis [4], photonic [5], etc. have been submerged by this phenomena. Thus, clever nano-system controlled by several stimuli such as guest-host complexation [6], temperature [7], pH control [8] or electrochemical control [9] may be related and opened the way to new field of investigation. So, among possible starting materials of these “smart” devices we find nanometric silica’s particles [10], mostly due to the relative low toxicity of these compounds but also for his tailored form, low cost and functionality [11]. On the basis of previous work made by Kolbe [12], one of the first nanometric material drawn up was carried out by Stöber and coworkers [13] based of non-porous spherical silica particles. Initially, designed for biomedical applications, their field of applications has been opened to other areas generally by modifying the original
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
89
Fig. 1. Incorporation of fluorescent molecule onto silica’s nanoparticles.
process established to introduce (directly or by post-grafting method) an organic molecule with specific properties. For example, through the introduction of specific entities, this material has also been used as a carrier for the immobilization of catalyst [14] and as a basis for designing photonic crystals [15]. The incorporation of fluorescent compounds on the surface of spherical nanoparticles also provides a wide range of biomedical applications [16,17] like imaging [18,19], drug delivery [20,21] or detection of biomolecules [22]. Apart biological uses, many papers related the conception of fluorescent nanoparticles with sensing properties toward variation of pH with fluorescein [23] or rhodamine derivatives [24] and ions with, for example, a binary fluorophore-ligand system such as dansyl-picolinamide [25] or anthracen-thiourea [26]. However, there still an attractive domain of search due to the need of pushing back detection limit jointly with the search of selectivity. Here, we report a new fluorescent hybrid material based on silica’s nanoparticles obtained by the Stöber process. The major advantage of this process is the tuning of the particle size provides by variation of synthesis parameters such as alkyl chain length of alcohol solvent, concentration of tetraalkoxysilane or ammonia [13] and also the temperature chose for the synthesis [27]. In the present study we have opt for a room temperature synthesis (more reproducible compared with those performed at higher temperature) inducing an average particle diameter of 210 nm. These nanoparticles were then hybridized (by post-grafting method) using a fluorescent fragment based on pyridinium salt and including an hydrolysable organosilane entities. The selected pathway to synthesize the new fluorescent nanoparticle consists in a direct incorporation onto the surface of nanosized silica, of an alkylpyridinoindolizin unit including a hydrolysable siliceous fragment (Fig. 1). Previously to this latter grafting, a solvent-free quaternization of the free pyridyl group with iodopropyltrimethoxysilane as organosilane entities was achieved with quantitative yield (Fig. 2). The first part of this manuscript will be devoted to the synthesis and characterization of the fluorescent nanomaterial including solid state NMR experiments (29 Si and 13 C), thermal analysis (TGA and EA) and by microscopy (SEM and AFM). The last part of this paper will focused on the first result of fluorescence measurements and counter-ion’s exchange experiments making this original material a promising tool to produce a selective fluorescent anion’s sensor.
2. Materials and methods 2.1. Reagents and chemicals TetraEthylOrthoSilicate (98%), NH4 OH (28%), DiMethylFormamide (DMF) and absolute Ethanol were purchased from across chemicals and were used without any purification step. Iodopropyltrimethoxysilane was obtained from Alfa Aesar and used as received. This compound, particularly sensitive to moisture, was keept in a dissecator. Ethylpyridinoindolizin was synthesized according to the procedure previously described [28]. 2.2. Instrumentation FTIR spectra in transmission mode were recorded at room temperature with a Perkin-Elmer spectrum BX spectrometer over the range of 4000–400 cm−1 with a resolution of 2 cm−1 and 16 scans for each run. The samples were dispersed in KBr (10% in weight) and pressed at 10 bars. ThermoGravimetric Analyses (TGA) were performed on a Netzsh STA 409 equipped with a Differential analysis microbalance. The samples were heated over air until 900 ◦ C with an increments rate of 5 ◦ C/min. Elemental analyses (C, H and N) were recorded on a Thermo Finnigan EA 1112 with a Sartorius MC balance with a precision of ±0.2%. All the analyses were reproduced three times to confirm the organic content of materials. The solid-state NMR spectra were carried out on Bruker AVANCE 400 spectrometer with a 7 mm probe and executive frequencies of 100.62 MHz. The 13 C NMR experiments were performed with a magic angle spinning frequency set to 14 kHz and a contact time of 3 ms. SEM analyses were performed on the 438 VP microscope (LEO, Cambridge, England) equipped with an Energy Dispersive X-ray spectrometer (IXRF, USA). Typical working parameters were an accelerating voltage of 30 kV with a beam current of 10 pA. Surface’s topography over a range of were realized in ambient air with an Atomic Force Microscope in tapping mode with a lateral resolution of 10 nm and a vertical resolution of 0.1 nm. Fluorescence properties were investigated with a homemade device presented previously. Briefly, it is compose of a Xenon light source (ASBXE175), a monochromator (CM110) and a CCD spectrophotometer (BRC641E). This system is also equipped with a coupled-fibrered probe allowing the measurement of different kinds of sample like powder, solution and colloidal suspension. The
Fig. 2. Incorporation of iodopropyltrimethoxysilane in the fluorescent building block.
90
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
acquisition of the measurements were made at 293 K with an excitation angle of 90◦ and a fixed sample excitation source distance (the excitation wavelength was fixed at 471 nm to maximize the emission spectra which was recorded from 490 to 800 nm).
2.3. Synthesis of the pyridinoindolizin derivative The fluorescent molecule was generated by 1,3 dipolar cycloaddition between a bipyridinium salt [29] and alcynic compounds such as ethyl propiolate [30]. Typically, a solution of freshly distilled Et3 N (7.84 mmol) in 5 ml dry DMF was added to a stirred solution of pyridinium salt (5.23 mmol) and ethylpropiolate (5.23 mmol) in dry DMF (15 ml). The dark-brown solution was stirred at 273 K under N2 in the absence of light during 24 h. The solvent was totally removed by distillation under vacuum and the crude product was recristalized in 20 ml of methanol to give the fluorescent molecule with a yield of 40%. 1 H NMR (DMSO-d6 + 5% CFCOOD/TMS/ı ppm): 9.47 (d, 1H, J = 7.5 Hz, H-4); 9.01 (d, 2H, J = 5.9 Hz, H-1); 8.72 (s, 1H, H-5); 8.46 (d, 2H, J = 5.9 Hz, H-2); 7.82 (s, 1H, H-6); 7.74 (d, 1H, J = 7.5 Hz, H-3); 4.36–4.27 (m, 4H, 2 CH2 ); 1.35–1.29 (m, 6H, 2 CH3 ). 13 C NMR (DMSO-d + 5% CFCOOD/TMS/ı ppm): 164.1 (CO); 161.1 6 (CO); 150.6 (C-1); 145.4 (C-b); 138.8 (C-a); 134.7 (C-c); 128.2 (C4); 124.7 (C-d); 121.1 (C-2); 117.3 (C-5); 112.8 (C-3); 106.5 (C-e); 60.55 ( CH2 ); 60.1 ( CH2 ); 14.6 (CH3 ); 14.5 (CH3 ).
2.4. Synthesis of the organosilane spacer (Fluo-sil) The pyridinoindolizin derivative (1 g), previously dried at 100 ◦ C under vacuum to avoid any moisture and solvent residue, was placed in a 10 ml bicol round-bottom nesk flask equipped with refrigerant under nitrogen atmosphere. Then 5 ml of 3iodopropyltrimethoxy silane was added over the solid and the suspension was heated at 90 ◦ C during 2 days. After this time, the fluorescent orange liquid was poured into freshly distilled diethyl ether to precipitate the fluorescent silane. The orange powder obtained was then recrystallized in dichloromethane/diethyl ether (1/9) and filtered to afford quantitatively fluorescent silane (confirmed by the NMR spectra of brut solid). 1 H NMR (DMSO-d6/ı pm): 9.42 (d, 1H, H-4 , J = 7.2 Hz); 9.21 (d, 2H, H-1 , J = 6.4 Hz); 8.70 (s, 1H, H-5 ); 8.59 (d, 2H, H-2 , J = 5.4 Hz); 7.88 (d, 1H, H-3 , J = 6.7 Hz); 7.74 (s, 2H, H-6 ); 4.63 (t, 2H, CH2 -N+ , J = 7.1 Hz); 4.36 (q, 2H, CH2 O , J = 6.3 Hz); 3.51 (s, 9H, CH3 O ); 2.04 (m, 2H, CH2 (ˇ)/N+ ); 1.38 (t, 3H, CH3 , J = 6.9 Hz); 0.68 (t, 2H, CH2 Si , J = 5.4 Hz). 13 C NMR (DMSO-d6 /ı ppm): 160,5 (CO); 160,1 (CO); 153.1 (C-b); 145.3 (C1); 138.2 (C-a); 132,4 (C-c); 129.9 (C-4); 125.1 (C-2); 123.6 (C-d); 119.4 (C-5) 113,9 (C-3); 112.8 (C-6); 109.4 (C-e); 63.1 ( CH2 N+ ); 60.7 ( CH2 O ); 50.9 (CH3 O ); 25.6 ( CH2(ˇ) /N+); 14.6 (CH3 ethoxy); 5.8 ( CH2 Si).
2.5. Preparation of the silica’s nanoparticles (Si-Nps) The silica’s nano-object was obtained by the Stöber process and was as briefly described as follows: to a solution composed of 100 ml of absolute ethanol and 7.5 ml of NH4 OH (28%) stirred at 400 rpm, was added at room temperature (18 ◦ C), 3 ml of TEOS. After 15 min of stirring, the first nanometric object was observed, thus forming a translucid colloidal’s suspension. The suspension was stirred during 12 h, to completely hydrolyze the silane. After this time, the nanoparticles were recovered by centrifugation (6000 rmp) and copiously washed with absolute ethanol and distilled water. Finally, the inorganic particles were calcined at 500 ◦ C with a heating rate of 1 ◦ C/min during 6 h in order to remove residual solvent and organic species.
2.6. Incorporation of fluorescent silane on the nanoparticles (Fluo-Nps) To a suspension of 100 mg of calcined silica’s nanoparticles in freshly distilled DMF (5 ml) was added 33.6 mg fluorescent silane (Fluo-Sil). The mixture was then heated at 90 ◦ C under magnetic stirring during 24 h. After this time, the fluorescent nanoparticles (Fluo-Si-Nps) were recovered by centrifugation (600 rpm) and washed subsequently with DMF (until no more fluorecence was detected in the filtrate), acetone, méthanol and thus with diéthylether. Finally, the fluorescent powder was dried under vacuum at 100 ◦ C during 5 h.
3. Results and discussion 3.1. Characterization of the material In a first time we observed these nanomaterials (both native and fluorescent) by the means of scanning electron microscopy (SEM) in order to verify the shape and also to appreciate the diameter of the particles (Fig. 3a and b). As defined by the temperature of hydrolysis-condensation of TEOS (room temperature), average native nanoparticle’s diameter was 210 nm and remains the same after the incorporation of the fluorescent organosilane and confirmed by atomic force microscopy using a taping mode acquisition (Fig. 3c and d). Although immobilization of the fluorescent molecule was suggested by the color’s evolution of the material (white to yellow and above all fluorescent), we used jointly TGA and EA to accurately estimate the amount of organic moiety present on silica’s nanoparticle. As shown in Fig. 4, TGA experiment was carried out from 20 ◦ C to 850 ◦ C on the fluorescent material (Fig. 4). The thermogravimetric profile of hybrid fluorescent material exhibits two losses of weight attributed to water/solvent removal and to the degradation of the organic part respectively. Thus, the water content of the materials was estimated at 1.64% (0.9 mmol) and the organic content at 2.14%. The conversion of this last mass loss (regarding only fluorescent silane and silicon atom excluded), corrected in considering the water content, indicates that 43 mol/g have been incorporated on the surface of the nanoparticles. The organic content obtained by TGA is consistent with Element Analysis especially with the total content of carbon (1.11%) and nitrogen (0.09%) of the material. In fact, regarding to the carbon content, 39 mol/g of fluorescent content has been incorporated in the silica confirming the TGA data (Table 1). This grafting’s rate is also consistent with the native silica used (low silanol content) since it has been calcined inducing a low silanol content. Despite a not high organic content in the hybrid material, observation of the organic part and of the link between each substance is possible. Additionally, the proof of fixation by a covalent anchoring between the organic substrate and the inorganic matrix was obtained by solid-state NMR experiments. First, 29 Si CPMAS spectra was realized and presented in Fig. 5. Even if the native silica was calcined, inducing low silanols content, slight evolution of the intensity of the signal related to the Qn species is nevertheless distinguished. In fact, we can remark a decrease of the peaks at −92 ppm corresponding to the Q2 species (germinal silanols) indicating that this species have react to form a new siloxane bridge (Q3 ). As consequence, the Q3 /Q4 ratio is affected because we observe a flattening of the Q4 signal and confirmed the covalent anchoring with the surface’s silanol. 13 C CPMAS NMR spectrum also corroborated anchoring of fluorophore since we can observe specific peaks corresponding to the fluorescent unit and also to the hybrid spacer (Fig. 6).
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
91
Fig. 3. SEM micrographs (up) and AFM images (bottom) of native nanoparticle (left, a–c) and corresponding fluorescent materials (right, b–d).
Table 1 Elemental and thermogravimetrical analysis obtained for the hybrid material and subsequent estimation of the fluorophore content. Elemental analysis
Fluo-Sil Fluo-Nps
Thermogravimetric measurements
%C
%H
%N
Total organic content (from %C/%N)
47.7 1.11
5.29 0.28
4.46 0.09
39 mol g−1
–
Loss weight* (%)
Total organic content
– 2.18
– 43 mol g−1
Fig. 4. Thermogravimetrical analysis obtained for the hybrid fluorescent material.
Indeed, characteristic signals of alkylpyridinoindolizine entities are attributed using chemical shift obtained by liquid NMR spectra (presented just before in experimental data) and with 13 C CPMAS NMR spectra previously report on the pristine fluorescent derivative [31]. Thus the signal at 138 ppm and 127 ppm are correlated to the multiple aromatic carbons of the pyridinoindolizin unit and the signals at 62 ppm and 12 ppm are respectively assigned to CH2 O and CH3 of the ethoxy fragment. Moreover, some peaks corresponding to the aliphatic chain of the spacer are distinguish at 60 ppm, 23 ppm and 9 ppm corresponding respectively to CH2 N+ , CH2() N+ and CH2 Si. The presence of this latter signal in conjunction with the 29 Si CP MAS spectra indicated and confirms the formation of the hybrid by condensation of the fluorescent silane with nanoparticle’s surface.
Fig. 5. (a) Solid-state NMR 29 Si CPMAS spectra of native nanoparticles and (b) the corresponding fluorescent nanomaterial Fluo-Nps.
3.2. Counter-ion’s exchange experiments Firstly, fluorescence measurements on native unmodified silica nanoparticles were performed and confirm that no detectable emission of fluorescence was observed for siliceous support. Since anchoring of the fluorophore is achieved by the formation of a pyridinium bond between the spacer and the fragment hybrid pyridyl fluorescent molecules, there is formed a stable pair anion/cation. If one refers to the basic principles of solution chemistry as well as on works of Gushikem [32] concerning exchange properties
92
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
Fig. 6. Solid-state NMR 13 C CPMAS spectra of fluorescent hybrid nanomaterial.
of pyridinium entities, it is feasible to use the pyridinium part to perform ion exchange. Furthermore, this latter is close to the fluorescent unit and so ion exchange could influence the optical properties and induce a variation of fluorescence (exaltation or quenching). Thus, we investigated the influence of the counter-ion associated with the pyridinium on the fluorescence properties of materials in order to generate new fluorescent anions sensors. Formally, the colloidal suspension was prepared by dispersing 10 mg of fluorescent material in 1 ml of absolute ethanol and then placed in a tank that has a narrow space of agitation by magnetic stirrer. Measurements of fluorescence emission were conducted following the successive addition of a calibrated quantity of an amount of a metal salt solution such as nitrate, acetate, chloride or perchlorate (Fig. 7). Anion exchange was achieved by addition of small amounts of copper nitrate solution (Cu(NO3 )2 ) prepared using milliQ water to avoid interaction with other ion. As shown in Fig. 8a, addition of copper nitrate with increasing amount results in a significant
decrease of relative fluorescence emission of the suspension thereby operating in turn-off mode. Using the same methodology, other anions such as chloride and acetate were tested and introduced in new suspension of nanoparticle (Fig. 8a). As observed consequently to addition of nitrate solution, introduction of chloride or acetate also leads to a decrease in fluorescence with however different final level of fluorescence and turns out to be more important in the case of acetates (Fig. 8b). This fact, suggests a stronger interaction/influence of this counter-ion on the pyridinium fragment and thus on the fluorescence properties, comparing to chloride and nitrate species. Aside that and surprisingly, the addition of copper perchlorate not lead to a quenching of fluorescence but involve an exaltation of fluorescence emission (Fig. 9a). The introduction of copper perchlorate lead to an exaltation of fluorescence emission (“turn-on” mode) independent of the cationic metal bound initially to perchlorate (Fig. 9b) since the final level of emission is the same in each case and corresponding to the concentration of pyridinium present at the surface. Many examples
Fig. 7. Representation of the counter-ion exchange of iodide by different anions.
Fig. 8. Evolution of the variation of fluorescence following the introduction of copper nitrate (a), chloride and acetate (b) in the fluorescent colloidal suspension.
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
93
Fig. 9. Evolution of the variation of fluorescence following the introduction of copper perchlorate in the fluorescent colloidal suspension (left) and influence of the cationic counter-ion (right).
Fig. 10. Evolution of the variation of fluorescence following the introduction of copper perchlorate in the fluorescent colloidal suspension (left) and influence of the cationic counter-ion (right).
of variation of fluorescence are clearly observed in the literature (but not on pyridinium structure) with a majority of paper dealing with the quenching of fluorescence by addition of chloride [33,34], acetate [35], nitrate and even perchlorate [36]. In most of the case, this quenching is explain by a decrease of quantum yield induces by a destabilization of the excited state provoke by a close proximity of ion with fluorophore. Nevertheless, one paper reported the same fluorescence variation behavior than we observed here but on 2,2 -pyridinium entities since they obtained an exaltation of fluorescence by addition of lithium perchlorate while addition of lithium chloride causes a quenching of emission [37]. In this case, authors explain that exaltation obtained for the ion-pair pyr-ClO4 − is causes by an increasing of molecular rigidity inhibiting rotation contrary to the ion-pair pyr-Cl− . Taking into account this fact and if we focused on our structure, the strong influence of anions on the fluorescence emission could be explain using resonance structure of pyridinum’s core previously reported by Druta [38] (Fig. 10). Thus and as clearly visible on mesomeric structure IV, anions can directly interacts with fluorescent fragment (see surrounded fragment) and so impacts strongly the electronic distribution of fluorescent center. Consequently, quenching of fluorescence observe for acetate, nitrate and chloride could be explain by a Electronic Energy Transfer (EET) occurring when the substrate has empty or half-filled energy levels between the HOMO and LUMO of the fluorophore. Aside that, an exception is observed for perchlorate anion since we observed an exaltation of fluorescence following introduction of perchlorate in the suspension. As suggested by Szabó [37] and by experimental observation performed on soluble pyridinium iodide,
this turn-on behavior may results of a specific interaction between anion-pyridinium species inducing an increasing of the overall rigidity of the complex [37,39].Complementary experimental and theoretical investigations oriented on the origin of these different behavior are actually in progress to understand the mechanism of fluorescence variation and to finally exploit this phenomena to design new specific fluorescent anions complexing agents. 4. Conclusions In summary, a new fluorescent hybrid material containing a pyridinium fragment was presented. This latter was obtained by grafting an alkylpyridinoindilizin modified with an organosilane fragment and containing a pyridinium part, on silica’s nanoparticle generated by the Stöber process at room temperature. After a full characterization by microscopy (SEM and AFM), spectroscopic and thermal analysis, fluorescence properties of the materials were determined and anion-exchange experiments were conducted using copper salt of chloride, nitrate, acetate and perchlorate. For this latter, we observed a unique response of the material (“turnon” behavior) attributed to a rigidity effect contrary to other tested anions which induce a quenching of fluorescence by an EET process. This interesting results open new way of investigations for the specific detection and anions ‘quantification in water and in the conception of new fluorescent complexing agents. Acknowledgements This work was supported by the “Conseil Général du Nord”, the “Région Nord-Pas de Calais” and the “Institut de Recherche
94
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
en ENvironnement Industriel (IRENI)”. The authors would like to thank Lucie Courcot, Bertrand Revel and Benoit Duponchel for their assistance in SEM, NMR experiments and AFM studies respectively. References [1] S.S. Mao, X.B. Chen, Int. J. Energy Res. 31 (2007) 61. [2] E. Serrano, G. Rus, J. Garcia-Martinez, Renew. Sust. Energy Rev. 13 (2009) 2373. [3] W.H. Suh, K.S. Suslick, G.D. Stucky, Y.H. Suh, Prog. Neurobiol. 87 (2009) 133. [4] H.H. Kung, M.C. Kung, Catal. Today 97 (2004) 219. [5] T.L. Dawson, Color. Technol. 124 (2008) 261. [6] H. Meng, M. Xue, T. Xia, Y.L. Zhao, F. Tamanoi, J.F. Stoddart, J.I. Zink, A.E. Ne, J. Am. Chem. Soc. 132 (2010) 12690. [7] J. Bigot, D. Fournier, J. Lyskawa, T. Marmin, F. Cazaux, G. Cooke, P. Woisel, Polym. Chem. 1 (2010) 1024. [8] S. Angelos, N.M. Khashab, Y.W. Yang, A. Trabolsi, H.A. Khatib, J.F. Stoddart, J.I. Zink, J. Am. Chem. Soc. 131 (2009) 12912. [9] T. Ye, A.S. Kumar, S. Saha, T. Takami, T.J. Huang, J.F. Stoddart, P.S. Weiss, ACS Nano 4 (2010) 3697. [10] R. Klajnwa, J.F. Stoddartb, B.A. Grzybowski, Chem. Soc. Rev. 39 (2010) 2203. [11] H. Goesmann, C. Feldmann, Angew. Chem. Int. Ed. 49 (2010) 2. [12] G. Kolbe, Das komplexchemische Verhalten der Kieselsäure, Jena, 1956 (Dissertation). [13] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [14] A.T. Papacıdero, L.A. Rocha, B.L. Caetano, E. Molina, H.C. Sacco, E.J. Nassar, Y. Martinelli, C. Mello, S. Nakagaki, K.J. Ciuffi, Colloids Surf. A: Physicochem. Eng. Aspects 275 (2006) 27. [15] S. Reculusa, S. Ravaine, Appl. Surf. Sci. 246 (2005) 409. [16] J.E. Smith, L. Wang, W. Tan, Trends Anal. Chem. 25 (2006) 9. [17] J. Lu, M. Liong, Z.X. Li, J.I. Zink, F. Tamanoi, Small 6 (2010) 1794. [18] L. Sun, Y. Zang, M.D. Sun, H.G. Wang, X.J. Zhu, S.F. Xu, Q.B. Yang, Y.X. Li, Y.M. Shan, J. Colloid Interface Sci. 350 (2010) 90. [19] S-W. Ha, C.E. Camalier, G.R. Beck Jr., J.-K. Lee, Chem. Commun. 20 (2009) 2881. [20] D.J. Zhang, X.M. Wang, Z.A. Qiao, D.H. Tang, Y.L. Liu, Q.S. Huo, J. Phys. Chem. C 114 (2010) 12505.
[21] M. Kovochich, M. Liong, H. Meng, S. Kabehie, S. George, J.I. Zink, A.E. Nel, ACS Nano 3 (2009) 3273. [22] F. Gao, F. Luo, X. Chen, W. Yao, J. Yin, Z. Yao, L. Wang, Talanta 80 (2009) 202. [23] F. Gao, L.J. Tang, L. Dai, L. Wang, Spectrochim. Acta A 67 (2007) 517. [24] F. Gao, L. Wang, L.J. Tang, C.Q. Zhu, Microchim. Acta 152 (2005) 131. [25] E. Brasola, F. Mancin, E. Rampazzo, P. Tecilla, U. Tonellat, Chem. Commun. 24 (2003) 3026. ˜ [26] P. Calero, R. Martínez-Mánez, F. Sancenón, J. Soto, Eur. J. Inorg. Chem. 27 (2008) 5649. [27] C.G. Tan, B.D. Bowen, N. Epstein, J. Colloid Interface Sci. 118 (1987) 290. [28] F. Delattre, P. Woisel, G. Surpateanu, F. Cazier, P. Blach, Tetrahedron 61 (2005) 3939. [29] I.I. Druta, M.A. Andrei, P.S. Aburel, Tetrahedron 54 (1998) 2107. [30] I.I. Druta, R.M. Dinica, E. Bacu, L. Humelnicu, Tetrahedron 54 (1998) 10811. [31] M. Becuwe, F. Cazier, F. Delattre, D. Landy, S. Ruellan, P. Woisel, Colloids Surf. A: Physicochem. Eng. Aspects 384 (2011) 248. [32] (a) Y. Gushikem, W.C. Moreira, Colloids Surf. A: Physicochem. Eng. Aspects 25 (1987) 155; (b) E.W. de Menezes, E.C. Lima, B. Royer, F.E. de Souza, B.D. dos Santos, J.R. Gregório, T.M.H. Costa, Y. Gushikem, E.V. Benvenutti, J. Colloid Interface Sci. 378 (10) (2012); (c) N. Fattori, C.M. Maroneze, H.A. Magosso, Y.V. Kholin, Y. Gushikem, J. Colloid Interface Sci. 384 (2012) 137; (d) S.T. Fujiwara, Y. Gushikem, R.V.S. Alfaya, Colloids Surf. A: Physicochem. Eng. Aspects 178 (2001) 135. [33] A. Martin, R. Narayanaswamy, Sensor. Actuat. B 38–39 (1997) 330. [34] M.S. Mehata, H.B. Tripathi, J. Lumin. 99 (2002) 47. [35] B. Quintero Osso, F.M. Carazo Rodrıˇıguez, J.J. Morales Domingo, M.C. Cabeza González, J. Thomas Gómez, Spectrochim. Acta A 55 (1999) 279. [36] M. Mac, P. Kwiatkowski, A.M. Turek, Chem. Phys. Lett. 250 (1996) 104. [37] K. Szabó, N. Marek, J. Biochem. Biophys. Methods 69 (2006) 223. [38] (a) L. Leontiea, I. Druta, B. Furdui, V. Nica, G.I. Rusu, Synthetic Methods 159 (2009) 1831; (b) L.L. Leontie, R. Danac, I. Druta, A. Carlescu, G.I. Rusu, Synthetic Methods 160 (2010) 2526. [39] D. Curiel, E.J. Hayes, P.D. Beer, Top. Fluoresc. Spectrosc. 9 (2005) 59.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Turn-on/turn-off fluorescent hybrid silica nanoparticles. A new promising material for selective anions’ sensing Matthieu Becuwe a,1 , Francine Cazier a , Patrice Woisel b , Franc¸ois Delattre a,∗ a b
Unité de Chimie Environnementale et Interactions sur le Vivant, Univ. Lille Nord de France, ULCO, 145 Avenue M. Schuman, 59140 Dunkerque, France Unité Matériaux Et Transformations, CNRS UMR 8207, Univ. Lille Nord de France, USTL, Bâtiment C6, 59655 Villeneuve d’Ascq, France
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Preparation of a new fluorescent hybrid material including a pyridinium unit. • The new material exhibits detection properties toward anions in aqueous solution. • Promising fluorescent specific behavior for perchlorate sensing.
a r t i c l e
i n f o
Article history: Received 14 November 2012 Received in revised form 21 March 2013 Accepted 12 April 2013 Available online 30 April 2013 Keywords: Hybrid material Silica nanoparticles Fluorescent Pyridinoindolizin Anion’s sensing
a b s t r a c t This work focused on the elaboration of a new fluorescent hybrid material based on spherical silica nanoparticle generated by sol–gel chemsitry (Stöber process). A fluorescent molecule (quaternized pyridinoindolizin) was introduced on the inorganic framework via an organosilane entities possessing hydrolysable group (methoxy group in the present case) to ensure the covalent linkage between the inorganic host and the organic substrate. The material obtained was fully characterized with FTIR, 29 Si and 13 C Solid state NMR, thermogravimetric and elemental analysis, Atomic force and scanning electron microscopy and the fluorescence properties of these nano-objects were determined with a home-made device. Finally, the influence of the counter-ion of the pyridinium fragment on the fluorescence emission was also investigated. In this case, a specific behavior (turn-on, exaltation of emission) was observed following exchange with perchlorate anions contrary to exchange with other anions such as acetate, chloride and nitrate which provoke a quenching of fluorescence. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Encouraged by the possibility of combining different scientific domain like chemistry, physic, biology, mechanic. . . in a miniaturized device, nanotechnology’s world is nowadays an important area of investment and interest by the scientific community. Consequently, high-added value systems have been created and a wide
∗ Corresponding author. Tel.: +33 328658246; fax: +33 328658231. E-mail address: [email protected] (F. Delattre). 1 Permanent address: Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 7314, Université de Picardie Jules Verne, 33 rue Saint-leu, 80039 Amiens Cedex, France. 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.04.030
range of purpose in energy [1,2], medicine [3], catalysis [4], photonic [5], etc. have been submerged by this phenomena. Thus, clever nano-system controlled by several stimuli such as guest-host complexation [6], temperature [7], pH control [8] or electrochemical control [9] may be related and opened the way to new field of investigation. So, among possible starting materials of these “smart” devices we find nanometric silica’s particles [10], mostly due to the relative low toxicity of these compounds but also for his tailored form, low cost and functionality [11]. On the basis of previous work made by Kolbe [12], one of the first nanometric material drawn up was carried out by Stöber and coworkers [13] based of non-porous spherical silica particles. Initially, designed for biomedical applications, their field of applications has been opened to other areas generally by modifying the original
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
89
Fig. 1. Incorporation of fluorescent molecule onto silica’s nanoparticles.
process established to introduce (directly or by post-grafting method) an organic molecule with specific properties. For example, through the introduction of specific entities, this material has also been used as a carrier for the immobilization of catalyst [14] and as a basis for designing photonic crystals [15]. The incorporation of fluorescent compounds on the surface of spherical nanoparticles also provides a wide range of biomedical applications [16,17] like imaging [18,19], drug delivery [20,21] or detection of biomolecules [22]. Apart biological uses, many papers related the conception of fluorescent nanoparticles with sensing properties toward variation of pH with fluorescein [23] or rhodamine derivatives [24] and ions with, for example, a binary fluorophore-ligand system such as dansyl-picolinamide [25] or anthracen-thiourea [26]. However, there still an attractive domain of search due to the need of pushing back detection limit jointly with the search of selectivity. Here, we report a new fluorescent hybrid material based on silica’s nanoparticles obtained by the Stöber process. The major advantage of this process is the tuning of the particle size provides by variation of synthesis parameters such as alkyl chain length of alcohol solvent, concentration of tetraalkoxysilane or ammonia [13] and also the temperature chose for the synthesis [27]. In the present study we have opt for a room temperature synthesis (more reproducible compared with those performed at higher temperature) inducing an average particle diameter of 210 nm. These nanoparticles were then hybridized (by post-grafting method) using a fluorescent fragment based on pyridinium salt and including an hydrolysable organosilane entities. The selected pathway to synthesize the new fluorescent nanoparticle consists in a direct incorporation onto the surface of nanosized silica, of an alkylpyridinoindolizin unit including a hydrolysable siliceous fragment (Fig. 1). Previously to this latter grafting, a solvent-free quaternization of the free pyridyl group with iodopropyltrimethoxysilane as organosilane entities was achieved with quantitative yield (Fig. 2). The first part of this manuscript will be devoted to the synthesis and characterization of the fluorescent nanomaterial including solid state NMR experiments (29 Si and 13 C), thermal analysis (TGA and EA) and by microscopy (SEM and AFM). The last part of this paper will focused on the first result of fluorescence measurements and counter-ion’s exchange experiments making this original material a promising tool to produce a selective fluorescent anion’s sensor.
2. Materials and methods 2.1. Reagents and chemicals TetraEthylOrthoSilicate (98%), NH4 OH (28%), DiMethylFormamide (DMF) and absolute Ethanol were purchased from across chemicals and were used without any purification step. Iodopropyltrimethoxysilane was obtained from Alfa Aesar and used as received. This compound, particularly sensitive to moisture, was keept in a dissecator. Ethylpyridinoindolizin was synthesized according to the procedure previously described [28]. 2.2. Instrumentation FTIR spectra in transmission mode were recorded at room temperature with a Perkin-Elmer spectrum BX spectrometer over the range of 4000–400 cm−1 with a resolution of 2 cm−1 and 16 scans for each run. The samples were dispersed in KBr (10% in weight) and pressed at 10 bars. ThermoGravimetric Analyses (TGA) were performed on a Netzsh STA 409 equipped with a Differential analysis microbalance. The samples were heated over air until 900 ◦ C with an increments rate of 5 ◦ C/min. Elemental analyses (C, H and N) were recorded on a Thermo Finnigan EA 1112 with a Sartorius MC balance with a precision of ±0.2%. All the analyses were reproduced three times to confirm the organic content of materials. The solid-state NMR spectra were carried out on Bruker AVANCE 400 spectrometer with a 7 mm probe and executive frequencies of 100.62 MHz. The 13 C NMR experiments were performed with a magic angle spinning frequency set to 14 kHz and a contact time of 3 ms. SEM analyses were performed on the 438 VP microscope (LEO, Cambridge, England) equipped with an Energy Dispersive X-ray spectrometer (IXRF, USA). Typical working parameters were an accelerating voltage of 30 kV with a beam current of 10 pA. Surface’s topography over a range of were realized in ambient air with an Atomic Force Microscope in tapping mode with a lateral resolution of 10 nm and a vertical resolution of 0.1 nm. Fluorescence properties were investigated with a homemade device presented previously. Briefly, it is compose of a Xenon light source (ASBXE175), a monochromator (CM110) and a CCD spectrophotometer (BRC641E). This system is also equipped with a coupled-fibrered probe allowing the measurement of different kinds of sample like powder, solution and colloidal suspension. The
Fig. 2. Incorporation of iodopropyltrimethoxysilane in the fluorescent building block.
90
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
acquisition of the measurements were made at 293 K with an excitation angle of 90◦ and a fixed sample excitation source distance (the excitation wavelength was fixed at 471 nm to maximize the emission spectra which was recorded from 490 to 800 nm).
2.3. Synthesis of the pyridinoindolizin derivative The fluorescent molecule was generated by 1,3 dipolar cycloaddition between a bipyridinium salt [29] and alcynic compounds such as ethyl propiolate [30]. Typically, a solution of freshly distilled Et3 N (7.84 mmol) in 5 ml dry DMF was added to a stirred solution of pyridinium salt (5.23 mmol) and ethylpropiolate (5.23 mmol) in dry DMF (15 ml). The dark-brown solution was stirred at 273 K under N2 in the absence of light during 24 h. The solvent was totally removed by distillation under vacuum and the crude product was recristalized in 20 ml of methanol to give the fluorescent molecule with a yield of 40%. 1 H NMR (DMSO-d6 + 5% CFCOOD/TMS/ı ppm): 9.47 (d, 1H, J = 7.5 Hz, H-4); 9.01 (d, 2H, J = 5.9 Hz, H-1); 8.72 (s, 1H, H-5); 8.46 (d, 2H, J = 5.9 Hz, H-2); 7.82 (s, 1H, H-6); 7.74 (d, 1H, J = 7.5 Hz, H-3); 4.36–4.27 (m, 4H, 2 CH2 ); 1.35–1.29 (m, 6H, 2 CH3 ). 13 C NMR (DMSO-d + 5% CFCOOD/TMS/ı ppm): 164.1 (CO); 161.1 6 (CO); 150.6 (C-1); 145.4 (C-b); 138.8 (C-a); 134.7 (C-c); 128.2 (C4); 124.7 (C-d); 121.1 (C-2); 117.3 (C-5); 112.8 (C-3); 106.5 (C-e); 60.55 ( CH2 ); 60.1 ( CH2 ); 14.6 (CH3 ); 14.5 (CH3 ).
2.4. Synthesis of the organosilane spacer (Fluo-sil) The pyridinoindolizin derivative (1 g), previously dried at 100 ◦ C under vacuum to avoid any moisture and solvent residue, was placed in a 10 ml bicol round-bottom nesk flask equipped with refrigerant under nitrogen atmosphere. Then 5 ml of 3iodopropyltrimethoxy silane was added over the solid and the suspension was heated at 90 ◦ C during 2 days. After this time, the fluorescent orange liquid was poured into freshly distilled diethyl ether to precipitate the fluorescent silane. The orange powder obtained was then recrystallized in dichloromethane/diethyl ether (1/9) and filtered to afford quantitatively fluorescent silane (confirmed by the NMR spectra of brut solid). 1 H NMR (DMSO-d6/ı pm): 9.42 (d, 1H, H-4 , J = 7.2 Hz); 9.21 (d, 2H, H-1 , J = 6.4 Hz); 8.70 (s, 1H, H-5 ); 8.59 (d, 2H, H-2 , J = 5.4 Hz); 7.88 (d, 1H, H-3 , J = 6.7 Hz); 7.74 (s, 2H, H-6 ); 4.63 (t, 2H, CH2 -N+ , J = 7.1 Hz); 4.36 (q, 2H, CH2 O , J = 6.3 Hz); 3.51 (s, 9H, CH3 O ); 2.04 (m, 2H, CH2 (ˇ)/N+ ); 1.38 (t, 3H, CH3 , J = 6.9 Hz); 0.68 (t, 2H, CH2 Si , J = 5.4 Hz). 13 C NMR (DMSO-d6 /ı ppm): 160,5 (CO); 160,1 (CO); 153.1 (C-b); 145.3 (C1); 138.2 (C-a); 132,4 (C-c); 129.9 (C-4); 125.1 (C-2); 123.6 (C-d); 119.4 (C-5) 113,9 (C-3); 112.8 (C-6); 109.4 (C-e); 63.1 ( CH2 N+ ); 60.7 ( CH2 O ); 50.9 (CH3 O ); 25.6 ( CH2(ˇ) /N+); 14.6 (CH3 ethoxy); 5.8 ( CH2 Si).
2.5. Preparation of the silica’s nanoparticles (Si-Nps) The silica’s nano-object was obtained by the Stöber process and was as briefly described as follows: to a solution composed of 100 ml of absolute ethanol and 7.5 ml of NH4 OH (28%) stirred at 400 rpm, was added at room temperature (18 ◦ C), 3 ml of TEOS. After 15 min of stirring, the first nanometric object was observed, thus forming a translucid colloidal’s suspension. The suspension was stirred during 12 h, to completely hydrolyze the silane. After this time, the nanoparticles were recovered by centrifugation (6000 rmp) and copiously washed with absolute ethanol and distilled water. Finally, the inorganic particles were calcined at 500 ◦ C with a heating rate of 1 ◦ C/min during 6 h in order to remove residual solvent and organic species.
2.6. Incorporation of fluorescent silane on the nanoparticles (Fluo-Nps) To a suspension of 100 mg of calcined silica’s nanoparticles in freshly distilled DMF (5 ml) was added 33.6 mg fluorescent silane (Fluo-Sil). The mixture was then heated at 90 ◦ C under magnetic stirring during 24 h. After this time, the fluorescent nanoparticles (Fluo-Si-Nps) were recovered by centrifugation (600 rpm) and washed subsequently with DMF (until no more fluorecence was detected in the filtrate), acetone, méthanol and thus with diéthylether. Finally, the fluorescent powder was dried under vacuum at 100 ◦ C during 5 h.
3. Results and discussion 3.1. Characterization of the material In a first time we observed these nanomaterials (both native and fluorescent) by the means of scanning electron microscopy (SEM) in order to verify the shape and also to appreciate the diameter of the particles (Fig. 3a and b). As defined by the temperature of hydrolysis-condensation of TEOS (room temperature), average native nanoparticle’s diameter was 210 nm and remains the same after the incorporation of the fluorescent organosilane and confirmed by atomic force microscopy using a taping mode acquisition (Fig. 3c and d). Although immobilization of the fluorescent molecule was suggested by the color’s evolution of the material (white to yellow and above all fluorescent), we used jointly TGA and EA to accurately estimate the amount of organic moiety present on silica’s nanoparticle. As shown in Fig. 4, TGA experiment was carried out from 20 ◦ C to 850 ◦ C on the fluorescent material (Fig. 4). The thermogravimetric profile of hybrid fluorescent material exhibits two losses of weight attributed to water/solvent removal and to the degradation of the organic part respectively. Thus, the water content of the materials was estimated at 1.64% (0.9 mmol) and the organic content at 2.14%. The conversion of this last mass loss (regarding only fluorescent silane and silicon atom excluded), corrected in considering the water content, indicates that 43 mol/g have been incorporated on the surface of the nanoparticles. The organic content obtained by TGA is consistent with Element Analysis especially with the total content of carbon (1.11%) and nitrogen (0.09%) of the material. In fact, regarding to the carbon content, 39 mol/g of fluorescent content has been incorporated in the silica confirming the TGA data (Table 1). This grafting’s rate is also consistent with the native silica used (low silanol content) since it has been calcined inducing a low silanol content. Despite a not high organic content in the hybrid material, observation of the organic part and of the link between each substance is possible. Additionally, the proof of fixation by a covalent anchoring between the organic substrate and the inorganic matrix was obtained by solid-state NMR experiments. First, 29 Si CPMAS spectra was realized and presented in Fig. 5. Even if the native silica was calcined, inducing low silanols content, slight evolution of the intensity of the signal related to the Qn species is nevertheless distinguished. In fact, we can remark a decrease of the peaks at −92 ppm corresponding to the Q2 species (germinal silanols) indicating that this species have react to form a new siloxane bridge (Q3 ). As consequence, the Q3 /Q4 ratio is affected because we observe a flattening of the Q4 signal and confirmed the covalent anchoring with the surface’s silanol. 13 C CPMAS NMR spectrum also corroborated anchoring of fluorophore since we can observe specific peaks corresponding to the fluorescent unit and also to the hybrid spacer (Fig. 6).
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
91
Fig. 3. SEM micrographs (up) and AFM images (bottom) of native nanoparticle (left, a–c) and corresponding fluorescent materials (right, b–d).
Table 1 Elemental and thermogravimetrical analysis obtained for the hybrid material and subsequent estimation of the fluorophore content. Elemental analysis
Fluo-Sil Fluo-Nps
Thermogravimetric measurements
%C
%H
%N
Total organic content (from %C/%N)
47.7 1.11
5.29 0.28
4.46 0.09
39 mol g−1
–
Loss weight* (%)
Total organic content
– 2.18
– 43 mol g−1
Fig. 4. Thermogravimetrical analysis obtained for the hybrid fluorescent material.
Indeed, characteristic signals of alkylpyridinoindolizine entities are attributed using chemical shift obtained by liquid NMR spectra (presented just before in experimental data) and with 13 C CPMAS NMR spectra previously report on the pristine fluorescent derivative [31]. Thus the signal at 138 ppm and 127 ppm are correlated to the multiple aromatic carbons of the pyridinoindolizin unit and the signals at 62 ppm and 12 ppm are respectively assigned to CH2 O and CH3 of the ethoxy fragment. Moreover, some peaks corresponding to the aliphatic chain of the spacer are distinguish at 60 ppm, 23 ppm and 9 ppm corresponding respectively to CH2 N+ , CH2() N+ and CH2 Si. The presence of this latter signal in conjunction with the 29 Si CP MAS spectra indicated and confirms the formation of the hybrid by condensation of the fluorescent silane with nanoparticle’s surface.
Fig. 5. (a) Solid-state NMR 29 Si CPMAS spectra of native nanoparticles and (b) the corresponding fluorescent nanomaterial Fluo-Nps.
3.2. Counter-ion’s exchange experiments Firstly, fluorescence measurements on native unmodified silica nanoparticles were performed and confirm that no detectable emission of fluorescence was observed for siliceous support. Since anchoring of the fluorophore is achieved by the formation of a pyridinium bond between the spacer and the fragment hybrid pyridyl fluorescent molecules, there is formed a stable pair anion/cation. If one refers to the basic principles of solution chemistry as well as on works of Gushikem [32] concerning exchange properties
92
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
Fig. 6. Solid-state NMR 13 C CPMAS spectra of fluorescent hybrid nanomaterial.
of pyridinium entities, it is feasible to use the pyridinium part to perform ion exchange. Furthermore, this latter is close to the fluorescent unit and so ion exchange could influence the optical properties and induce a variation of fluorescence (exaltation or quenching). Thus, we investigated the influence of the counter-ion associated with the pyridinium on the fluorescence properties of materials in order to generate new fluorescent anions sensors. Formally, the colloidal suspension was prepared by dispersing 10 mg of fluorescent material in 1 ml of absolute ethanol and then placed in a tank that has a narrow space of agitation by magnetic stirrer. Measurements of fluorescence emission were conducted following the successive addition of a calibrated quantity of an amount of a metal salt solution such as nitrate, acetate, chloride or perchlorate (Fig. 7). Anion exchange was achieved by addition of small amounts of copper nitrate solution (Cu(NO3 )2 ) prepared using milliQ water to avoid interaction with other ion. As shown in Fig. 8a, addition of copper nitrate with increasing amount results in a significant
decrease of relative fluorescence emission of the suspension thereby operating in turn-off mode. Using the same methodology, other anions such as chloride and acetate were tested and introduced in new suspension of nanoparticle (Fig. 8a). As observed consequently to addition of nitrate solution, introduction of chloride or acetate also leads to a decrease in fluorescence with however different final level of fluorescence and turns out to be more important in the case of acetates (Fig. 8b). This fact, suggests a stronger interaction/influence of this counter-ion on the pyridinium fragment and thus on the fluorescence properties, comparing to chloride and nitrate species. Aside that and surprisingly, the addition of copper perchlorate not lead to a quenching of fluorescence but involve an exaltation of fluorescence emission (Fig. 9a). The introduction of copper perchlorate lead to an exaltation of fluorescence emission (“turn-on” mode) independent of the cationic metal bound initially to perchlorate (Fig. 9b) since the final level of emission is the same in each case and corresponding to the concentration of pyridinium present at the surface. Many examples
Fig. 7. Representation of the counter-ion exchange of iodide by different anions.
Fig. 8. Evolution of the variation of fluorescence following the introduction of copper nitrate (a), chloride and acetate (b) in the fluorescent colloidal suspension.
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
93
Fig. 9. Evolution of the variation of fluorescence following the introduction of copper perchlorate in the fluorescent colloidal suspension (left) and influence of the cationic counter-ion (right).
Fig. 10. Evolution of the variation of fluorescence following the introduction of copper perchlorate in the fluorescent colloidal suspension (left) and influence of the cationic counter-ion (right).
of variation of fluorescence are clearly observed in the literature (but not on pyridinium structure) with a majority of paper dealing with the quenching of fluorescence by addition of chloride [33,34], acetate [35], nitrate and even perchlorate [36]. In most of the case, this quenching is explain by a decrease of quantum yield induces by a destabilization of the excited state provoke by a close proximity of ion with fluorophore. Nevertheless, one paper reported the same fluorescence variation behavior than we observed here but on 2,2 -pyridinium entities since they obtained an exaltation of fluorescence by addition of lithium perchlorate while addition of lithium chloride causes a quenching of emission [37]. In this case, authors explain that exaltation obtained for the ion-pair pyr-ClO4 − is causes by an increasing of molecular rigidity inhibiting rotation contrary to the ion-pair pyr-Cl− . Taking into account this fact and if we focused on our structure, the strong influence of anions on the fluorescence emission could be explain using resonance structure of pyridinum’s core previously reported by Druta [38] (Fig. 10). Thus and as clearly visible on mesomeric structure IV, anions can directly interacts with fluorescent fragment (see surrounded fragment) and so impacts strongly the electronic distribution of fluorescent center. Consequently, quenching of fluorescence observe for acetate, nitrate and chloride could be explain by a Electronic Energy Transfer (EET) occurring when the substrate has empty or half-filled energy levels between the HOMO and LUMO of the fluorophore. Aside that, an exception is observed for perchlorate anion since we observed an exaltation of fluorescence following introduction of perchlorate in the suspension. As suggested by Szabó [37] and by experimental observation performed on soluble pyridinium iodide,
this turn-on behavior may results of a specific interaction between anion-pyridinium species inducing an increasing of the overall rigidity of the complex [37,39].Complementary experimental and theoretical investigations oriented on the origin of these different behavior are actually in progress to understand the mechanism of fluorescence variation and to finally exploit this phenomena to design new specific fluorescent anions complexing agents. 4. Conclusions In summary, a new fluorescent hybrid material containing a pyridinium fragment was presented. This latter was obtained by grafting an alkylpyridinoindilizin modified with an organosilane fragment and containing a pyridinium part, on silica’s nanoparticle generated by the Stöber process at room temperature. After a full characterization by microscopy (SEM and AFM), spectroscopic and thermal analysis, fluorescence properties of the materials were determined and anion-exchange experiments were conducted using copper salt of chloride, nitrate, acetate and perchlorate. For this latter, we observed a unique response of the material (“turnon” behavior) attributed to a rigidity effect contrary to other tested anions which induce a quenching of fluorescence by an EET process. This interesting results open new way of investigations for the specific detection and anions ‘quantification in water and in the conception of new fluorescent complexing agents. Acknowledgements This work was supported by the “Conseil Général du Nord”, the “Région Nord-Pas de Calais” and the “Institut de Recherche
94
M. Becuwe et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 433 (2013) 88–94
en ENvironnement Industriel (IRENI)”. The authors would like to thank Lucie Courcot, Bertrand Revel and Benoit Duponchel for their assistance in SEM, NMR experiments and AFM studies respectively. References [1] S.S. Mao, X.B. Chen, Int. J. Energy Res. 31 (2007) 61. [2] E. Serrano, G. Rus, J. Garcia-Martinez, Renew. Sust. Energy Rev. 13 (2009) 2373. [3] W.H. Suh, K.S. Suslick, G.D. Stucky, Y.H. Suh, Prog. Neurobiol. 87 (2009) 133. [4] H.H. Kung, M.C. Kung, Catal. Today 97 (2004) 219. [5] T.L. Dawson, Color. Technol. 124 (2008) 261. [6] H. Meng, M. Xue, T. Xia, Y.L. Zhao, F. Tamanoi, J.F. Stoddart, J.I. Zink, A.E. Ne, J. Am. Chem. Soc. 132 (2010) 12690. [7] J. Bigot, D. Fournier, J. Lyskawa, T. Marmin, F. Cazaux, G. Cooke, P. Woisel, Polym. Chem. 1 (2010) 1024. [8] S. Angelos, N.M. Khashab, Y.W. Yang, A. Trabolsi, H.A. Khatib, J.F. Stoddart, J.I. Zink, J. Am. Chem. Soc. 131 (2009) 12912. [9] T. Ye, A.S. Kumar, S. Saha, T. Takami, T.J. Huang, J.F. Stoddart, P.S. Weiss, ACS Nano 4 (2010) 3697. [10] R. Klajnwa, J.F. Stoddartb, B.A. Grzybowski, Chem. Soc. Rev. 39 (2010) 2203. [11] H. Goesmann, C. Feldmann, Angew. Chem. Int. Ed. 49 (2010) 2. [12] G. Kolbe, Das komplexchemische Verhalten der Kieselsäure, Jena, 1956 (Dissertation). [13] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [14] A.T. Papacıdero, L.A. Rocha, B.L. Caetano, E. Molina, H.C. Sacco, E.J. Nassar, Y. Martinelli, C. Mello, S. Nakagaki, K.J. Ciuffi, Colloids Surf. A: Physicochem. Eng. Aspects 275 (2006) 27. [15] S. Reculusa, S. Ravaine, Appl. Surf. Sci. 246 (2005) 409. [16] J.E. Smith, L. Wang, W. Tan, Trends Anal. Chem. 25 (2006) 9. [17] J. Lu, M. Liong, Z.X. Li, J.I. Zink, F. Tamanoi, Small 6 (2010) 1794. [18] L. Sun, Y. Zang, M.D. Sun, H.G. Wang, X.J. Zhu, S.F. Xu, Q.B. Yang, Y.X. Li, Y.M. Shan, J. Colloid Interface Sci. 350 (2010) 90. [19] S-W. Ha, C.E. Camalier, G.R. Beck Jr., J.-K. Lee, Chem. Commun. 20 (2009) 2881. [20] D.J. Zhang, X.M. Wang, Z.A. Qiao, D.H. Tang, Y.L. Liu, Q.S. Huo, J. Phys. Chem. C 114 (2010) 12505.
[21] M. Kovochich, M. Liong, H. Meng, S. Kabehie, S. George, J.I. Zink, A.E. Nel, ACS Nano 3 (2009) 3273. [22] F. Gao, F. Luo, X. Chen, W. Yao, J. Yin, Z. Yao, L. Wang, Talanta 80 (2009) 202. [23] F. Gao, L.J. Tang, L. Dai, L. Wang, Spectrochim. Acta A 67 (2007) 517. [24] F. Gao, L. Wang, L.J. Tang, C.Q. Zhu, Microchim. Acta 152 (2005) 131. [25] E. Brasola, F. Mancin, E. Rampazzo, P. Tecilla, U. Tonellat, Chem. Commun. 24 (2003) 3026. ˜ [26] P. Calero, R. Martínez-Mánez, F. Sancenón, J. Soto, Eur. J. Inorg. Chem. 27 (2008) 5649. [27] C.G. Tan, B.D. Bowen, N. Epstein, J. Colloid Interface Sci. 118 (1987) 290. [28] F. Delattre, P. Woisel, G. Surpateanu, F. Cazier, P. Blach, Tetrahedron 61 (2005) 3939. [29] I.I. Druta, M.A. Andrei, P.S. Aburel, Tetrahedron 54 (1998) 2107. [30] I.I. Druta, R.M. Dinica, E. Bacu, L. Humelnicu, Tetrahedron 54 (1998) 10811. [31] M. Becuwe, F. Cazier, F. Delattre, D. Landy, S. Ruellan, P. Woisel, Colloids Surf. A: Physicochem. Eng. Aspects 384 (2011) 248. [32] (a) Y. Gushikem, W.C. Moreira, Colloids Surf. A: Physicochem. Eng. Aspects 25 (1987) 155; (b) E.W. de Menezes, E.C. Lima, B. Royer, F.E. de Souza, B.D. dos Santos, J.R. Gregório, T.M.H. Costa, Y. Gushikem, E.V. Benvenutti, J. Colloid Interface Sci. 378 (10) (2012); (c) N. Fattori, C.M. Maroneze, H.A. Magosso, Y.V. Kholin, Y. Gushikem, J. Colloid Interface Sci. 384 (2012) 137; (d) S.T. Fujiwara, Y. Gushikem, R.V.S. Alfaya, Colloids Surf. A: Physicochem. Eng. Aspects 178 (2001) 135. [33] A. Martin, R. Narayanaswamy, Sensor. Actuat. B 38–39 (1997) 330. [34] M.S. Mehata, H.B. Tripathi, J. Lumin. 99 (2002) 47. [35] B. Quintero Osso, F.M. Carazo Rodrıˇıguez, J.J. Morales Domingo, M.C. Cabeza González, J. Thomas Gómez, Spectrochim. Acta A 55 (1999) 279. [36] M. Mac, P. Kwiatkowski, A.M. Turek, Chem. Phys. Lett. 250 (1996) 104. [37] K. Szabó, N. Marek, J. Biochem. Biophys. Methods 69 (2006) 223. [38] (a) L. Leontiea, I. Druta, B. Furdui, V. Nica, G.I. Rusu, Synthetic Methods 159 (2009) 1831; (b) L.L. Leontie, R. Danac, I. Druta, A. Carlescu, G.I. Rusu, Synthetic Methods 160 (2010) 2526. [39] D. Curiel, E.J. Hayes, P.D. Beer, Top. Fluoresc. Spectrosc. 9 (2005) 59.