Marco Montalti

Ru(bpy)3 Covalently Doped Silica Nanoparticles as Multicenter Tunable Structures for Electrochemiluminescence Amplification

The electrochemiluminescence (ECL) of doped silica nanoparticles (DSNPs), prepared by a reverse microemulsion method that leads to covalent incorporation of the Ru(bpy)(3)(2+), was investigated in acetonitrile and aqueous buffers. The emission was produced for the first time by cation-anion direct annihilation, and the position of ECL maxima indirectly allowed estimation of the E(1/2,IOx) and E(1/2,IRed) potentials for Ru(bpy)(3) inside DSNPs. The weak ECL emission is most likely generated by an intrananoparticle ruthenium unit annihilation rather than by the electron transfer between a reduced and oxidized DSNP due to the very low diffusivities of the nanoparticles. Thiol-terminated DSNPs were self-assembled on gold substrates, forming compact and stable monolayers which mimic probe-target assays with DSNPs as labels. The ECL intensity obtained by such functionalized substrates in aqueous media, using tripropylamine (TPrA) as coreactant, was surprisingly increased with respect to direct electrochemical oxidation because of the ability of oxidized TPrA to diffuse within the DSNPs structure and reach a higher number of emitting units with respect to direct electron tunneling. The experimental results have been explained by proposing a basic physical-chemical model which supports evaluation of the number of redox-active centers per nanoparticle. In the model the contrasting effects of increased luminescence quantum yield and decreased diffusion coefficient with respect to free (i.e., not bound within the silica structure) emitting molecules were taken into account. This allows, in principle, optimizing the ECL emission intensity as a function of DSNP size, doping material, charge, doping level, supporting electrolyte, electrode material, and solvent. Finally, it is worth noting that this study has provided a more than 1000-fold increase of the ECL signal of a chemically and electrochemically stable DSNP compared to that of a single dye, suggesting that use of this kind of nanostructures as luminescent labels represents a very promising system for ultrasensitive bioanalysis.

Fluorescence quenching amplification in silica nanosensors for metal ions

We report here the synthesis, photophysical characterization and ion binding ability of new silica nanoparticles bearing covalently linked luminescent chemosensors. These moieties possess a dansyl unit as a fluorophore and a polyamine chain as a receptor. Interestingly, the addition of copper, cobalt and nickel ions induces a strong quenching of the fluorescence intensity even at nanomolar concentrations. The results obtained suggest that each ion can quench up to 13 dansyl units, leading to strong signal amplification. This is possible because the nanoparticle structure, in which a high density of chemosensor units is present, allows the occurrence of multicomponent cooperative photophysical processes. In our opinion, the versatility of this approach opens up new perspectives for the design of a new class of photonic devices at the nanometric level.

Induced Fit Interanion Discrimination by Binding-Induced Excimer Formation

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a 2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change, giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6, which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host complexes with a range of anions.

Molecular recognition on a cavitand-functionalized silicon surface.

A Si(100) surface featuring molecular recognition properties was obtained by covalent functionalization with a tetraphosphonate cavitand (Tiiii), able to complex positively charged species. Tiiii cavitand was grafted onto the Si by photochemical hydrosilylation together with 1-octene as a spatial spectator. The recognition properties of the Si-Tiiii surface were demonstrated through two independent analytical techniques, namely XPS and fluorescence spectroscopy, during the course of reversible complexation-guest exchange-decomplexation cycles with specifically designed ammonium and pyridinium salts. Control experiments employing a Si(100) surface functionalized with a structurally similar, but complexation inactive, tetrathiophosphonate cavitand (TSiiii) demonstrated no recognition events. This provides evidence for the complexation properties of the Si-Tiiii surface, ruling out the possibility of nonspecific interactions between the substrate and the guests. The residual Si-O(-) terminations on the surface replace the guests original counterions, thus stabilizing the complex ion pairs. These results represent a further step toward the control of self-assembly of complex supramolecular architectures on surfaces.

Fully reversible guest exchange in tetraphosphonate cavitand complexes probed by fluorescence spectroscopy

We report here the monitoring of reversible guest inclusion in phosphonate cavitands through a large increase in luminescence intensity caused by the modulation of the exoergonicity of an electron-transfer reaction.

Amplified fluorescence response of chemosensors grafted onto silica nanoparticles.

In conventional fluorescent chemosensors, the recognition of the target by the receptor unit affects the fluorescence properties of a single covalently coupled fluorescent moiety. Here we show for the first time that when a suitable TSQ derivative is densely grafted onto the surface of preformed silica nanoparticles electronic interactions between the individual chemosensor units enable the free units to recognize the state of the surrounding complexed ones. As a result, the fluorescence transduction is not limited to the local site where binding occurs, but it involves a wider region of the fluorophore network that is able to transfer its excitation energy to the complexed units. Such behavior leads to an amplification of the fluorescence signal. What we report here is the first example of amplification in the case an off-on chemosensor due to its organization onto the surface of silica nanoparticles. We also describe a simple general model to approach amplification in multifluorophoric systems based on the localization of the excited states, which is valid for assemblies such as the supramolecular ones where molecular interactions are weak and do not significantly perturb the individual electronic states. The introduction of an amplification factor f in particular allows for a simple quantitative estimation of the amplification effects.

The erratic emission of pyrene on gold nanoparticles.

Gold nanoparticles functionalized with chromophores are known to present unpredictable fluorescence as a function of their structure. Odd-even effects, based on the number of methylene units of the chain to which the fluorophore is attached, and the nature of the anchoring group on the gold surface have, in the past, been suggested to be responsible for the behavior. Here we investigate the fluorescence processes of two newly synthesized pyrene derivatives bound to gold nanoparticles. Two structurally identical ligands, differing only in the nature of the anchoring group (a thiolate in one case and an amine in the other), were newly synthetized and attached to the gold nanoparticles. The same changes in the fluorescence properties, namely, a red spectral shift with a moderate increase of the quantum yield and a shortening of the excited-state lifetime, are observed in the two cases and ascribed to the proximity of the gold core. By comparison with the results reported for other pyrene derivatives, it has been possible to draw the conclusions that (i) the nature of the binding group does not affect the fluorescence properties of the fluorophores attached to the nanoparticle surface and (ii) much stronger fluorescence is observed in the case of pyrene separated from the gold by short alkyl chain. The unusual behavior is explained in simple terms of competing chain-chain and chromophore-chromophore interactions and by means of proper energy diagrams.

Stabilization of terpyridine covered gold nanoparticles by metal ions complexation

The introduction of 1, a thiol presenting a terpyridine ligand, on the surface of weakly stabilized nanoparticles leads, at high concentration of this capping agent, to the aggregation of the gold clusters. This behaviour is due to the weak, but not negligible interaction of the terpyridine moiety with the metal surface, an interaction that becomes relevant at high concentrations of 1 on the surface, when several units can cooperate in interlinking the nanoparticles. To avoid this effect, we have investigated the complexation of zinc by the terpyridine derivative bound to gold nanoclusters, in excess of metal ion and in the presence of coordinating anions, as a powerful tool for the stabilization of gold nanoparticles. In this context, anions, such as bromide, showed to play an important role in this processes; the use of anions containing desired functions can than be a very versatile strategy to obtain new enriched nanoparticles.

Make sense of nanochemistry and nanotechnology

A class in a Scientific-Technological Lyceum (age 17) decided to produce a PowerPoint presentation to introduce nanochemistry and nanotechnology to the students in lower grades. Because the subject is very new, there was nothing in the School textbooks and, therefore, the students had to cooperate in order to find materials, to use ICT sources and to take decisions, such as selecting information and choosing slide layouts. Furthermore, the Cooperative Learning methodology was employed to solve the problem of setting up the presentation. To make nanochemistry and nanotechnology a reality for the students, they used a link between these new frontiers of Chemistry and subjects currently tackled at the Secondary School level. This was the quantitative determination of Ca2+ ions by using calcein, a luminescent chemosensor, in which well known concepts, such as chemical equilibrium and stability constants of coordination compounds, are involved. The educational aims of the project were to promote both content knowledge and social skills in Secondary School students. The activity created a good class atmosphere and also led to the retention of content knowledge.

New fluorescent chemosensors for magnesium ions in living cells

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.