The synthesis, spectroscopic characterization and complexing properties of calixarene-based fluorescent
sensors are reported. The calixarene bearing four dansyl fluorophores (Calix-DANS4) exhibits a very high affinity for
the detection of lead. A fluorimetric micro-device based on the use of a Y-shape microchannel was developed and
allows lead detection with a 5 ppb detection limit. For mercury detection, a fluorescent molecular sensor containing a
calixarene anchored with four 8-quinolinoloxy groups (Calix-Q) has been synthesized. The absorption and fluorescence
spectra of this sensor are sensitive to the presence of metal cations. An efficient fluorescence quenching is observed
upon mercury complexation because of a photoinduced electron transfer from the fluorophore to the bound mercury.
Calix-Q shows a high selectivity towards Hg<sup>2+</sup> over interfering cations (Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup> and Pb<sup>2+</sup>) and a
70 ppb sensitivity.
In the first part of this paper, we use a specially developed sensitive polymer (PLG) which belongs to the polysiloxane family. Thin layers of this polymer are deposited onto the surface of the optical transducers. Results will be presented on the response of diffraction-based optical transducers such as gratings and also on interferometric transducers and especially integrated Mach-Zehnder (MZ) interferometers. In the first case, a relief grating is coated with the sensitive polymer. A small variation of the refractive index of this layer, due to the presence of pollutant, induces a variation of the intensity of the diffracted orders which can be measured. In the second case, one arm of the integrated MZ interferometers is coated with the polymer. The variation of the refractive index of the polymer causes a phase shift in the measuring arm which can be measured by the modification of the output intensity. Assessment of sensitivity for the detection of nitro-aromatic compounds using a PLG sensitive layer on both sensors are presented and are also compared to the response of a SAW-based sensor coated with the same polymer.
In the second part of this paper, synthesis, spectroscopy and fluorescence quenching behaviour of a N-(2,5-ditertio-butylphenyl)-1,8-naphthalimide functionalised polystyrene (PST-NI) are reported. PST-NI was synthesized by free radical polymerisation of the corresponding monomer. The molecular weight (Mn) is 43 000 g.mol-1. Introduction of a bulky moiety on the naphthalimide chromophores avoids P-stacking of the polymer side chains as well as excimer formation and hence leads to very high fluorescence quantum yields in thin solid films (up to 60%). Upon 1 minute exposure to DNT vapour, it was shown that a 5.5 nm thick film of PST-NI exhibits a 45% drop in its fluorescence intensity, which makes this polymer very attractive for sensing applications.
The synthesis, spectroscopic characterization and fluorescence quenching efficiency of a polymer (PSt-NI) and a low molecular weight molecule (NI) containing the 4-(N, N disubstituted)amino-N-2,5ditertiobutylphenyl-1,8-naphthalimide chromophore are reported. Similar spectroscopic properties of thin films and solutions are observed. This is consistent with the absence of interactions between polymer side chains. The absorption and fluorescence spectra of PSt-NI studied in various solvents of different polarity are compared to the corresponding spectra of NI. The longest wavelength absorption of PSt-NI and NI is characterized by a band with a maximum wavelength around 410 nm. The peak position is sensitive to the polarity of the solvent, which is in agreement with the charge transfer character of the transition. The fluorescence spectrum of PSt-NI shows a maximum emission in chloroform at 515 nm and is red shifted compared to those of NI. Fluorescence lifetimes of PSt-NI and NI are measured in presence and absence of 2,4-dinitrotoluene (DNT) and the results are interpreted via the Stern-Volmer analysis. In solution, the fluorescence quenching of NI is purely collisional, whereas both dynamic and static quenching are observed with PSt-NI Upon 1 minute exposure to DNT vapor, it was shown that a 5 nm thick film of PSt-NI exhibited a 45% drop in its fluorescence intensity, which makes this polymer very attractive for sensing applications.