We report our latest progresses in the design and synthesis of fluorescent silica nanoparticles. Two different approaches are proposed: the first one is adopted for the inclusion of the dye to prepare doped silica nanoparticles (DSN). The second strategy allows the grafting of the surface of nanoparticles with the dye molecules and is suitable for the synthesis of covered silica nanoparticles (CSN). The two families of nanoparticles are radically different. While DSN are water soluble, surface coverage can dramatically lower the solubility in aqueous solvents. From the point of view of inter-chromophoric interactions, inclusion in DSN allows a good control of the average distance between the dye molcecules but, on the other hand, by surface modification a higher density of fluorophores can be reached making effective short range interactions (in particular electron transfer processes). Finally because of segregation, the interection of dye molecules with the external environment and macromolecules is less effective in the case of DSN. Three different examples are reported. In the first one energy transfer between fluorescein molecules in DSN is demonstrated though fluorescence anisotropy studies. The average distance between the fluorophores was tuned by controlling the degree of loading in order to have energy transfer inside the nanoparticles and in the mean time avoid a too large quenching because of self quenching processes. In the second example extended quenching via electron transfer processes on the surface of CSN is reported showing a simple case of 'amplified' quenching of the fluorescence upon protonation. Finally this last concept was applied to increase the sensibility in the field of metal ion sensing: a case of a water soluble nanosensor for metal ion with amplified response is, in fact, reported.
Although magnesium ions play a key role in many fundamental biological processes, information about its intracellular regulation is still scarce, due to the lack of appropriate detection methods. Here, we report the spectroscopic characterization of two diaza-18-crown-6 hydroxyquinoline derivatives (DCHQ) and we propose their application for the determination of total Mg2+ concentration and in confocal imaging as effective Mg2+ indicators. DCHQ derivatives 1 and 2 bind Mg2+ with much higher affinity than other available probes (Kd = 44 and 73 mM, respectively) with a concomitant strong fluorescence increase. On the other hand, the fluorescence intensity is not significantly affected by other divalent cations, most importantly Ca2+, or by pH changes within the physiological range. Evidence is provided on the use of fluorometric data to derive totalcellular Mg2+content, which is in agreement with atomic absorption data. Furthermore, we show that DCHQ compounds can be effectively employed to map intracellular ion distribution and movements in live cells by confocal microscopy. These findings suggest that DCHQ derivatives may serve as new probes for the study of Mg2+ regulation, allowing sensitive and straightforward detection of both static and dynamic signals.
Aiming to develop new fluorescent chemosensors for biological and environmental applications, we have designed and synthesized new chemical species able to reversibly bind alkali, earth-alkali, and transition metal ions. For signaling the binding of the target analyte, we have inserted in the structure of the chemosensors different luminophores, such as dioxyxanthone derivatives, dansyl derivatives, ruthenium complexes, and hydroxyquinoline derivatives. In solution, the binding is always signaled by pronounced changes in the photophysical properties of the inserted luminophore such as emission wavelength and intensity, and excited state lifetime. The mechanism for the signal transduction strongly depends on the chosen receptor and luminophore moieties, and has been investigated in detail by means of steady state and time resolved spectroscopy. In all cases, the synthesized chemosensors have proved to be chemically and photochemically stable. Good selectivity and affinity has been obtained with different sensors for K+, Mg2+, Ba2+, Zn2+, Ni2+ and Cu2+, even in physiological pH conditions. Moreover the use of an array of these sensors in optodes could lead to the construction of the so called electronic tongues. All these features make these sensors promising candidates for analytical applications.