Inorganica Chimica Acta 360 (2007) 721–727 www.elsevier.com/locate/ica
Review
Template assisted self-organized chemosensors Maria Arduini a, Enrico Rampazzo a,c, Fabrizio Mancin a, Paolo Tecilla b,*, Umberto Tonellato a,* a
Dipartimento di Scienze Chimiche, Universita` di Padova, via Marzolo 1, I-35131 Padova, Italy Dipartimento di Scienze Chimiche, Universita` di Trieste, via L. Giorgeri 1, I-34127 Trieste, Italy Dipartimento di Chimica ‘‘G. Ciamician’’, Universita` di Bologna, via Selmi 2, I-40126 Bologna, Italy b
c
Received 25 May 2006; accepted 4 June 2006 Available online 21 June 2006 Dedicated to Prof. Vincenzo Balzani.
Abstract The self-organization of fluorescent dyes and receptors on a proper template to form an organized assembly is a new strategy for the realization of fluorescence chemosensors. In the assembly, the two subunits do not interact directly and the communication between the bound substrate and the dye is only determined by their spatial closeness ensured by the template. The method is simple and the main advantages are related to the minimization of the synthetic work, the ease of modification and optimization of the sensor, the possibility to tune its properties by the simple adjustment of the components ratio. Self-organizing methodologies can open new perspectives to fluorescence chemosensors, both by allowing a simplified preparation and by opening the way to new and more complex functions. This article deals with this new approach and discusses its evolution, applications, and limitations. 2006 Elsevier B.V. All rights reserved. Keywords: Supramolecular chemistry; Chemosensors; Self-organization; Fluorescence; Nanostructured materials
Contents 1. 2. 3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The template assisted self-organized chemosensors strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-organization in surfactant aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-organization on nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Avoiding the ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
* Corresponding authors. Tel.: +39 0405583925; fax: +39 0405583903 (P. Tecilla); tel.: +39 0498275269; fax: + 39 0498275239 (U. Tonellato). E-mail addresses:
[email protected] (P. Tecilla), umberto.
[email protected] (U. Tonellato).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.06.017
Chemosensors are defined as molecular systems that can selectively recognize and signal the presence of a specific analyte [1]. Up to date, they represent one of the main achievements of supramolecular chemistry. Among the chemosensors, particular attention has been devoted to
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those systems that use fluorescence emission as signalling method [2]. In fact, fluorescence offers several advantages, among which high sensitivity and low cost instrumentation are particularly valuable. In addition, the molecular dimensions of chemosensors, combined with the availability of techniques such as confocal microscopy, allow high spatial resolution in the analyte detection and a wide range of application such as, notably, intracellular monitoring of selected species for medical and biochemical studies. The design of fluorescence chemosensors has been continuously evolved since their first appearance in early 1980s [3]. In their typical design, fluorescent chemosensors are molecules composed by one or more substrate binding units (receptors) and by photoactive components (reporters), but their chemical complexity greatly varies from case to case, as the sensor components can be integrated in each other or simply connected by spacers. However, the synthetic efforts involved in the realization and optimization of chemosensors may be demanding. More recently, self-assembling and self-organizing methodologies have attracted an increasing attention in the chemistry of complex systems with functional properties [4]. The building of complex structures, following this strategy, simply requires the design and synthesis of a limited number of relatively simple building blocks which are then allowed to self-organize. As a result of the molecular organization into a supramolecular assembly, novel properties and functions may result and lead to possible important applications. On these bases, self-organization of receptors and fluorescent dyes to form organized assemblies can, at least partially, overcome the synthetic problems connected to the classical systems and provide an efficient strategy for the easy realization and optimization of fluorescence chemosensors. Among the different strategies that can be followed to exploit such principles [5], one of the most successful is the so-called chemosensing ensemble approach, initially proposed by Anslyn and his co-workers and then developed and applied to the detection of a large number of substrates by the same author and by other groups. This approach is based on a competitive assay in which the fluorophore and the substrate compete for the receptor [6]. The displacement of the dye from the complex results in a change in the local environment experienced by the fluorophore and, hence, in its emission properties. We recently proposed a different approach based on the self-organization of the essential sensor subunits on a proper template [7]. This article will briefly describe the principles at the base of this method discussing its evolution, applications, and limitations. 2. The template assisted self-organized chemosensors strategy This approach is based on the self-organization of fluorescent dyes and receptors on a proper template forming an organized assembly (Scheme 1). In the assembly, the two subunits do not interact directly and, as a consequence,
template
substrate template-assisted high self-organization fluorescence recognition
= Reporter Dye
= Binding Unit
low fluorescence
= Substrate
Scheme 1. The template assisted self-organized chemosensors strategy.
the communication between the bound substrate and the dye is ensured only by their spatial closeness. Of course, an appropriate transduction mechanism must be envisaged in the sensor design in order to convert the substrate recognition event into a modification of the emission properties of the dye. Depending on the template, little or no synthetic modifications of the ligand and the dye are needed and this allows the easy formation of the sensor and the rapid screening of a large number of receptors and dyes in order to optimize its properties for a given application. Moreover, due to the spatial proximity of a large number of subunits in the assembly, new collective effects and properties may arise and contribute to the improvement of the sensors performances. In the last few years, different types of templates have been used by us and others to guide the selforganization of the chemosensors spanning from micellar aggregates to monolayers, to glass surfaces and, more recently, to nanoparticles. Each of these templates has its own peculiar properties that are reflected in a characteristic performance of the resulting sensor. 3. Self-organization in surfactant aggregates At first, we reported a novel methodology to self-organize a fluorescent chemosensor for Cu(II) ions which encloses the sensor components into surfactant aggregates [7]. Following this approach, lipophilic ligands and fluorophore molecules, dispersed in an aqueous solution containing micelles, move into the surfactant aggregates to generate a co-micellar assembly. The concentration of the species within the sub-microscopic micellar pseudo-phase, due to their lipophilic character, ensures the proximity between the ligand and the dye. As a result, the complexation of Cu(II) ions by the ligands leads to the quenching of the dye fluorescence emission, as schematically shown in Scheme 2. By employing the lipophilic ligand N-decyl-glycylglycine (C10GlyGly), which strongly and selectively binds Cu(II) ions due to the deprotonation of the amide nitrogen [8], the fluorophore 8-anilino-naphthalensulfonic acid (ANS), and the inert surfactant hexadecyltrimethylammonium bromide (CTABr), a self-organized chemosensor was obtained which can detect metal ion concentrations down to the micromolar range. The several other divalent metal ions tested do not cause any variations of the ANS emission, and competition titrations reveal a remarkable selectivity.
M. Arduini et al. / Inorganica Chimica Acta 360 (2007) 721–727
components
self assembling
low fluorescence
high fluorescence +
+
+
surfactant +
+
Cu(II)
ligand
+
+ fluorescent
O
O R NH HN HO
+
+
Cu(II) - 2H O
R NH
+
N
Cu
2+
O
PhNH SO3
O
fluorescent dye = ANS
ligand = C10 GlyGLy
Scheme 2. Self-organized fluorescence chemosensors based on surfactant aggregates.
The sensitivity of the system can be improved both by increasing the ligand/total surfactant (ligand + CTABr) molar ratio, reaching the best performances at a limiting value of 1:2, and by decreasing the surfactant concentration down to values approaching the c.m.c. value of the resulting co-micelles (Fig. 1). The main advantages of such a system are (a) selectivity, which is mainly due to the ligand choice; (b) simplicity: the sole mixing of the components (two of them, CTABr and ANS are commercially available) in water is required to prepare the sensor; (c) the possibility to tune the detection range just by the modification of the components ratio; (d) modularity, which allows the modification or the optimiza-
100
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tion of the system by simply substituting one of the components. The last point was demonstrated by setting up a combinatorial experiment in which, keeping constant the ligand, 16 combinations of surfactant and dye were tested employing four different surfactants and four different dyes all commercially available (Fig. 2). The combination of the CTABr surfactant and 1-naphthylphosphate dye resulted to be more sensitive than the original system. One of the limits of this system is brought about by the use of the inert surfactant to form the micellar aggregates. The surfactant is needed, in its micellar form, to avoid the precipitation of the neutral and poorly soluble C10GlyGly Æ Cu(II) complex, but its use implies dilution of the ligand in the aggregate and hence a decrease in the sensor sensitivity. In order to improve the system, we designed a family of ligands which are amphiphilic both in the free and in the complexed form [9]. These ligands produce stable homo-aggregates also in the presence of the metal ion thus avoiding the use of any added surfactant and allowing to reduce the system from three to two essential components as indicated in Scheme 3. The ligands were prepared by substituting one of the two glycine residues of the previous example with lysine or glutamic acid, both bearing in the lateral chain an ionizable group: we supposed that this could ensure amphiphilicity to the ligands and hence the capability to form homo-aggregates also when complexed to the Cu(II) ion. Lipophilic fluorophores, like ANS or Rodamine 6G, are effectively bound into the aggregate pseudo-phase and the binding of Cu(II) ions to the dipeptide units causes a strong fluorescence quenching. The sensor system is very sensitive
100 80 80
40 20 0 0.0
40
60
0
-6
5.0x10
-5
1.0x10
DANSA
40
ACA
20
[Cu(II)],M
1-NAFOSF
-4
2.0x10
-4
4.0x10
[Cu(II)], M Fig. 1. Spectrofluorimetric titrations of co-micelles made by CTABr, C10GlyGly and ANS with Cu(NO3)2 in HEPES buffer 0.01 M, pH 7. Conditions: [ANS] = 5.0 · 10 7 M; [CTABr]/[C10GlyGly] = 2; [CTABr] = 9.4 · 10 4 M (h); 4.7 · 10 4 M ( ); 2.4 · 10 4 M (d). Inset: enlargement of the first part of the titrations. I/I0 ratio of fluorescence intensity and original fluorescence intensity.
CTABr
0.0
ANS DMMAPS
0
Brij 35
0
20
Triton X-100
I/I0,%
60
100
60
I/I , %
0
I/I ,%
80
Fig. 2. Residual fluorescence in the presence of 5.0 · 10 5 M Cu(II) measured for different co-micellar systems. Conditions: HEPES buffer 0.01 M, pH 7; [fluorophore] = 5.0 · 10 7 M; [C10GlyGly] = 2.3 · 10 4 M; [surfactant]/[C10GlyGly] = 2 (CTABr); 6 (DMMAPS); 10 (Brij 35); 10 (Triton X-100). DMMAPS: 3-(hexadecyldimethylammonium)propylsulphonate; DANSA: dansylamide; ACA: 9-anthracenecarboxylic acid; 1-NAFOSF: 1-naphthylphosphate.
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components
self assembling
+
The driving force for the self-assembling of the chemosensors based on surfactant aggregates is the hydrophobic interaction of the lipophilic building blocks and this results in a usually simple chemical synthesis of the sensor components and in the easy preparation of the sensor. However, the actual applicability of such systems is limited by several factors. In particular, surfactant aggregates are delicate objects due to their dynamic nature: they form only above the critical micellar concentration, so that the fraction of non-micellized components may be not negligible, and they are very sensitive to environmental conditions, such as temperature and ionic strength. To address these limitations, the use of other templates was explored. The structure of surface functionalized nanoparticles closely reminds that of a frozen co-micelle. Some time after our reports on surfactant aggregates based systems, Montalti and co-workers published an interesting study on the photochemical properties of fluorophores surface functionalized silica nanoparticles, reporting evidences that collective processes were at play in those systems [12]. The advantages of the use of nanoparticle based chemosensors had been highlighted by the work of Kopelman and Rosenzweig and their co-workers [13]. As a matter of fact, silica nanoparticles are well suited for the realization of fluorescence chemosensors: they are transparent to light, photophysically inert, and their surface can be easily modified by reaction with alkoxysilane derivatives. Following the studies of Montalti and co-workers [12], we set up a method for the realization of self-organized fluorescence chemosensors for Cu(II) ions obtained by surface modification of silica nanoparticles [14]. Commercially available particles (20 nm diameter) were functionalized
+
+
+
+
ligand surfactant
Cu(II) +
fluorescent
+
+
+
ligand surfactant: O
O NH2 n-C16 H33 NH HN
R NH HN HO
4. Self-organization on nanoparticles
low fluorescence
high fluorescence
O
HO
COOH O
R = n-C10 H21 ;n-C12 H25 ;
n-C16 H33 ;n-C18 H37 Scheme 3. Second generation of self-organized fluorescence chemosensors based on surfactant aggregates. In this case, the ligand is amphiphilic and forms homo-aggregates making superfluous the use of the inert surfactant.
to Cu(II) (concentrations in the sub-micromolar range are detected), is promptly reversible and no interference is observed due to the presence of many metal ions. The sensitivity of the systems improves by decreasing the ligand concentration and (up to a point) the ligand’s c.m.c by changing the size of the lipophilic alkyl chain. Later on, similar concepts have been exploited by Pallavicini and co-workers, using again micellar aggregates [10], and by Leblanc and his co-workers for the development of self-assembled Cu(II) chemosensors onto Langmuir and Langmuir–Blodgett (LB) films [11].
components
self-organization
high fluorescence
substrate recognition
low fluorescence
EtO EtO Si EtO
EtO EtO Si EtO
fluorescent
O O Si O
Cu(II)
O O Si O OO Si O
SiO2 Nanoparticle
SiO2 Nanoparticle
ligand
O O Si O O O Si O OO Si O
SiO2 Nanoparticle
O R
N H
O
Cu(II) N
- H+
R
N
ligand = picolinamide
N Cu 2+
(EtO)3Si
O HN S O
N
fluorescent dye = dansylamide
Scheme 4. Self-organized fluorescence chemosensors on silica nanoparticles.
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N
H N
S O2
O O
N H
OEt Si OEt OEt
N
O
0.0
O N O
0.2
0.4
0.6
0.8
1.0
χ
Fig. 3. Concentration of Cu(II) necessary to quench 50% of the initial emission ([Cu(II)]50%) as a function of the ligand molar fraction on the nanoparticles surface at constant ligand concentration. Conditions: [ligand] = 2 lM, 10% water/DMSO, HEPES buffer 0.01 M pH 7, 25 C.
1.0
OEt Si OEt OEt
H N
OEt Si OEt OEt
10
5
b
N
O2N
15
OEt Si OEt OEt
I, arbitrary units
a
Also the recognition unit was changed, and using a stronger ligand we were able to exploit collective processes in which one single metal ion is capable of quenching the emission of about 10 surrounding dyes [15]: such result is particularly interesting as it allows the amplification of the signal produced by the sensor, thus lowering the detection limit to the nanomolar range [16]. A similar effect (about 45 fluorescent dyes quenched by a single Cu(II) ion) was previously observed by Larpent and co-workers using cyclam functionalized polystyrene nanoparticles impregnated with the BODIPY fluorophore [17]. Among the template assisted self-organized chemosensors based on surface functionalization, also those investigated by Reinhoudt and Crego-Calama are worth mentioning [18]. They are based on self-assembled monolayers (SAM) of alkoxysilane derivatives, bearing either fluorophoric or ligand groups, on glass surfaces and appear particularly suitable for the realization of sensing devices and arrays.
[Cu(II)]50%,μM
with the triethoxysilane derivatives of the ligand picolinamide, selective for Cu(II), and of the fluorophore dansylamide (Scheme 4). The grafting of the sensor components to the particle surface ensures the spatial proximity required to signal Cu(II) by quenching the fluorescence emission. In 9:1 DMSO/water solution, the coated silica nanoparticles (CSNs) selectively detect copper ions down to micromolar concentrations. Similarly to the micelle-based systems, the operative range of the sensor can be tuned by the simple modification of the components ratio. Beside the practical advantages of the use of such systems, which are non-dynamic and can even be stored as dry powders, the most interesting result was the demonstration of the cooperation of the ligand subunits bound to the particles surfaces to form binding sites with an increased affinity for the substrate. This was highlighted by a series of experiments carried out on solutions of nanoparticles featuring different ligand/fluorophore molar fractions (v) on their surfaces, at constant overall concentration of the picolinamide ligand. The plot of the sensors sensitivity (expressed as the concentration of Cu(II) necessary to quench 50% of the initial emission) against the v value shows that the CSNs with a large amount of ligand have the best efficiency as sensors (Fig. 3). Moreover, the sigmoidal shape of the profile clearly indicates cooperation between the ligands subunits. Quite likely, this can be ascribed to the surfaceorganization of the picolinamide subunits that may lead to the formation of multivalent binding sites (e.g. with 2:1 or 3:1 ligand to metal stoichiometries) with a greater Cu(II) affinity. The versatility of this approach was probed by preparing a small library of CSNs coated with the same ligand and different fluorophores (Fig. 4) [15]. The emission spectra of these CSNs span over a large wavelengths interval from 300 to 600 nm (correspondingly, excitation wavelengths are in the range 285–466 nm), allowing the choice of the more suitable sensor for the desired application.
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0.8
0.6
O O
O
N H
OEt Si OEt OEt
O
N H
OEt Si OEt OEt
300
400
500
600
λ, nm
O
Fig. 4. (a) Library of trialkoxysilane-derivatized dyes for CSNs functionalization and (b) their corrected emission spectra. Conditions: 10% water/DMSO, HEPES buffer 0.01 M pH 7, 25 C.
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5. Avoiding the ligand 100
75 I/I0 (%)
A careful analysis of the properties of silica gels, and particularly of their metal sorption ability [19], led us to imagine that the sensors could be further simplified. We prepared silica particles including the dansylamide dye, by co-polymerization of tetraethoxysilane (TEOS) and the dansyl triethoxysilane derivative (Fig. 5) in ethanol/ water/ammonia mixtures [20]. Addition of Cu(II) ions to solutions of such dye doped silica nanoparticles results in a strong fluorescence quenching, which allows the determination of the metal ion concentration in the micromolar range. In this case, the silica provides the receptor units of the sensor: the network of acidic silanol groups on the particles surface forms the binding sites for metal ions. Once bound to the silica network, the Cu(II) ions are close enough to the fluorescent units entrapped in the particle to quench their emission, thus producing a measurable signal. Hence, the formation of the particles leads, at the same time, to the formation of metal ion binding sites as well as to the linking of a fluorescent reporter in their proximity, and results in a simple conversion of fluorescent dyes into sensors. Surprisingly, the sensor is remarkably selective and addition of Co(II) or Ni(II) to a particle solution results into very small variation of the emission intensity. The sources of such unexpected selectivity are still under investigations. Interestingly, the particle size is an important parameter that influences the efficiency of the sensors (Fig. 6). Smaller particles show an almost complete quenching of the emission, while the larger ones undergo a smaller fluorescence variation upon Cu(II) addition. This was explained by assuming that the particles are accessible to the solvent and the analyte only up to a certain depth [21]. When the particle is small, the solvent permeable layer is deep enough to make the whole body of the particle accessible to the Cu(II) ions so that all the fluorophores are effectively quenched. As the diameters of the colloid increases, an increasing fraction of the dyes, located in the particle core, remains unaffected by the binding of the metal ions to the outer shell. The method is easily transferable from nanoparticles to sol–gel films: in this way, reusable sensing devices for optical detection of Cu(II) were obtained [20].
50
25
0 0.0000
0.0002
0.0004
0.0006
0.0008
2+
[Cu ], M Fig. 6. Spectrofluorimetric titration of dye doped nanoparticles with increasing dimensions (d;: 23 nm; : 70 nm; h: 290 nm) with Cu(NO3)2, in 15% ethanol/water, [dye]tot = 2.5 · 10 6 M, HEPES buffer 0.01 M pH 7, 25 C, kexc = 340 nm, kem = 520 nm.
6. Conclusions Self-organized systems can open new perspectives to a wider application of fluorescence chemosensors. Libraries of receptors and fluorescent dyes could be made available and then easily combined to produce the most suitable system for the desired use. Moreover, the several functional subunits, kept close by the template, can collectively operate and give new properties to the material. As we found in the case of silica nanoparticles systems, the ligand units can cooperate to form binding sites with an improved affinity for the substrate. Even if the sensors are not neatly selective when different recognition sites are randomly assembled, such problem could be overcome by the realization of sensors arrays, where fingerprint responses takes the place of selectivity. Moreover, spatial confinement of many fluorescent units can lead to simultaneous response to the analyte recognition, thus producing an amplified signal. Other collective phenomena between the photoactive units, as energy transfer or antenna effects, could be put at play to improve the sensor features. Unfortunately, at least so far, all this potentialities are counterbalanced by the intrinsic difficulty in the individuation of effective transduction mechanism. As there is no
Fig. 5. TEM images of dye-doped silica nanoparticles: (a) batch 1, bar: 250 nm, (b) batch 2, bar: 250 nm, (c) batch, 3 bar: 500 nm.
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direct interaction between the recognition and the signaling units, the modification of the fluorophore state, as a consequence of analyte binding, must occur by long-range interactions. Cu(II) and other transition metal ions can quench fluorescence emission either by electron and energy transfer, and the latter mechanism can operate even at a relatively long distance. For this reason, almost all the template based systems so far proposed by us or other groups are based on Cu(II) induced fluorescence quenching [5]. Recently, Pallavicini and co-workers have proposed a micelle based chemosensor for pH in which a photoinduced electron transfer (PET) fluorescence quenching is operative [10a]. This mechanism has found large application in the realization of molecular chemosensors for non-quenching substrates [2]. Hopefully, this example would open the way to the sensing of many other substrates than the sole Cu(II) ion and to the realization of template assisted selforganized chemosensors for different analytes, ionic or neutral substrates; this is actively pursued in our laboratories. In conclusion, self-organized fluorescence chemosensors appear to be a promising answer to the need of systems with larger applications in sensing and detection problems. Of course, the great potentialities of such strategy must face some problems that have to be positively solved before obtaining really useful systems. Acknowledgements This research has been partly supported by the Ministry of Instruction, University and Research (MIUR Contracts 2003030309 and 2003037580) and by the University of Padova (University Reasearch Project CPDA034893). References [1] (a) A.W. Czarnik (Ed.), Fluorescent Chemosensors for Ion and Molecule Recognition, ACS Symposium Series, vol. 538, American Chemical Society, Washington, DC, 1993; (b) T.S. Snowden, E.V. Anslyn, Curr. Opin. Chem. Biol. 3 (1999) 740; (c) L. Fabbrizzi, M. Licchelli, P. Pallavicini, L. Parodi, A. Taglietti, in: J.-P. Sauvage (Ed.), Transition Metal in Supramolecular Chemistry, Wiley, Chichester, 1999, pp. 93–134. [2] (a) A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515; (b) A.P. de Silva, P. Tecilla, J. Mat. Chem. 15 (2005) 2637. [3] G. Grynkiewicz, M. Poenie, R.Y. Tsien, J. Biol. Chem. 260 (1985) 3440. [4] (a) S. Mann, Chem. Commun. (2004) 1; (b) S. Hecht, Angew. Chem., Int. Ed. 42 (2003) 24.
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