Fluorescence Chemosensors

Molecular sensors, supramolecular systems able to signal the presence of specific substrates, have been recently attracting much interest. These sensors combine the properties of supramolecular receptors, as they specifically recognize a proper guest, with the ability to produce a measurable signal. Optical signals based on changes of absorbance or fluorescence are the most frequently exploited because of their simple applications using non expensive instruments. On one hand, changes of absorbance, particularly of color, can immediately reveal the presence of a given analyte, on the other hand, fluorescence emission allow to realize a very high sensitivity of the sensor.

In our reasearch group, we are addressing our activity to the design and study of two different categories of fluorescence chemosensors. The first class, which we name as “classical fluorescence chemosensors”, is made by molecules in which a supramolecular receptor and a fluorescence dye are part of the same molecule. The second class is that of “self-organized fluorescence chemosensors”, which are realized by the spontaneous self-organizion of the sensor components.

As an examples of the “classical chemosensor” category, Figure 1 shows the binding of vitamin B13 (orotic acid) to the ATMCA·Zn(II) complex. The interaction between the aromatic rings of the anthracenyl residue of ATMCA and of the vitamin B13 leads to a strong fluorescence quenching. Concentration of vitamin B13 in the micromolar range can be detected in water at pH 7, with a high selectivity against other biologically relevant molecules such as nucleotides and aminoacids.

Figure 1. MM simulation of the ternary complex between vitamin B13, Zn(II) and ATMCA. The formation of the complex leads to the quenching of the fluorescence emission of anthracenyl residue

Another example of "classical chemosensor" is reported in Figure 2. In this case, binding of Al(III) to the 3,5-bis(ortho-hydroxyphenyl)-1,2,4-triazole group produces a "chelation induced fluorescence enhancement" (CHEF) which is, however, too weak to allow the detection of the metal ion with high sensitivity. Conjugation of the metal binding subunit with Coumarine 343 allows the amplification of the fluorescence signal via a "fluorescence resonance energy transfer" (FRET) process. Nanomolar concetrations of Al(III) can be measured with this chemosensor.

Figure 2. Schematic rapresentation of the mode of action of a FRET amplified chemosensor for Al(III) ions

Figure 3 reports a schematic representation of a self-assembled chemosensor for Cu(II) ions: inclusion of a Cu(II) selective receptor (decylglycylglycine) and a chromophore (ANS) in CTABr surfactant aggregates allows such non-covalently bound species to be in such a close proximity as to produce fluorescence quenching after Cu(II) addition in concentrations below the micromolar range. This simple method allows easy optimizations by simply changing the subunits and avoids the considerable synthetic efforts required for the realization of the “classical” systems.

Figure 3. Schematic rapresentation of a self-assembled Cu(II) fluorescence chemosensor in surfactant aggragates

The concept of proximal but spatially separated receptor-fluorophore communication can be transferred from solution to suitable surfaces. Commercially available particles (20 nm diameter) were functionalized with the triethoxysilane derivatives of selective Cu(II) ligands and fluorophores. In this case, it is the grafting of the sensor components to the particle surface that ensures the spatial proximity required to signal Cu(II) by quenching of the fluorescence emission. In 9:1 DMSO/water solution, the coated silica nanoparticles (CSNs) selectively detect copper ions down to nanomolar concentrations and the operative range of the sensor can be tuned by the simple modification of the components ratio. Moreover, cooperation of the ligand subunits bound to the particles surfaces to form binding sites with an increased affinity for the substrate was demonstrated.

Figure 4. TEM image (the bar corresponds to 100 nm) and schematic rapresentation of a self-organized Cu(II) fluorescence chemosensor on silica nanoparticles

P. Teolato, E. Rampazzo, M. Arduini, F. Mancin, P. Tecilla, U. Tonellato “Silica nanoparticles for fluorescence sensing of Zn(II): exploring the covalent strategy” Chem Eur. J., 2007, 8, 2238-2245

F. Mancin, E. Rampazzo, P. Tecilla, U. Tonellato Self-Assembled Fluorescent Chemosensors Chem. Eur. J., 2006, 12, 1844-1854 .

M. Arduini, S. Marcuz, M. Montolli, E. Rampazzo, F. Mancin, S. Gross, L. Armelao, P. Tecilla, U. Tonellato Turning Fluorescent Dyes into Cu(II) Nanosensors Langmuir, 2005, 21, 9314-9321.

E. Rampazzo, E. Brasola, S. Marcuz, F. Mancin, P. Tecilla, U. Tonellato “Surface modification of silica nanoparticles: a new strategy for the realization of self-organized fluorescence chemosensors ” J. Mat. Chem. 2005, 15, 2687-2696.

E. Brasola, F. Mancin, E. Rampazzo, P. Tecilla, U. Tonellato “A fluorescence nanosensor for Cu²⁺ on silica particles” Chem. Commun. 2003, 3026-3027.

M. Arduini, F. Felluga, F. Mancin, P. Rossi, P. Tecilla, U. Tonellato, N. Valentinuzzi "Aluminium fluorescence detection with a FRET amplified chemosensor" Chem. Commun. 2003, 1606 -1607.

L. Fabbrizzi, M. Licchelli, F. Mancin, M. Pizzeghello, G. Rabaioli., A. Taglietti., P. Tecilla, U. Tonellato "Fluorescence sensing of ionic analytes in water: From transition metal ions to Vitamin B13" Chem. Eur. J. 2002, 8, 94-101.

P. Grandini, F. Mancin, P. Scrimin, P. Tecilla, U. Tonellato "Exploiting the self assembly strategy for the design of selective Cu(II) ion chemosensors" Angew. Chem. Int. Ed. 1999, 38, 3061-3064.