Luca Prodi

Luminescent chemosensors for transition metal ions

Abstract The need for selective and sensitive sensors able to monitor in real-time and real-space the concentration of analytes of biological, clinical, and environmental interest is nowadays generally accepted. Of the various kinds of chemosensors, luminescence-based chemosensors present many advantages, since they can take profit of the high sensitivity and versatility typical of photoluminescence spectroscopy. In this contest, special interest has been devoted to the development of luminescent chemosensors for transition metal ions, since some of them are present in biological systems in trace amounts and, at the same time, they all can represent an environmental concern when present in uncontrolled amounts. In this contribution we review the chemical systems able to act as luminescent chemosensors for this class of metal ions.

Luminescent Silica Nanoparticles: Extending the Frontiers of Brightness

Silica nanoparticles are versatile platforms with many intrinsic features, such as low toxicity. Proper design and derivatization yields particularly stable colloids, even in physiological conditions, and provides them with multiple functions. A suitable choice of dyes and synthetic strategy may, in particular, yield a very bright nanosystem. Silica nanoparticles thus offer unique potential in the nanotechnology arena, and further improvement and optimization could substantially increase their application in fields of high social and economic impact, such as medical diagnostics and therapy, environmental and food analysis, and security. This paper describes silica-based, multicomponent nanosystems with intrinsic directional energy- and electron-transfer processes, on which highly valued functions like light harvesting and signal amplification are based.

Luminescent chemosensors: from molecules to nanoparticles

The need to develop sensors for different target analytes is well-recognised and confirmed by the considerable research efforts spent for the preparation of more and more efficient sensory devices. In our laboratories we have been working on this subject for the past few years and in this contribution some selected examples of the systems we have studied will be presented. Through the discussed compounds, the reader will be able to follow our step-by-step approach towards this fascinating research field. The first essential move for a fruitful strategy in the development of efficient macroscopic sensory devices is a thorough understanding of the systems responsible for the binding event and of its consequent transduction into a detectable signal. These species are often molecular or supramolecular structures that behave as molecular-level sensory devices, which are usually referred to as chemosensors. Among them, fluorescence-based chemosensors present many interesting features, such as high sensitivity and versatility. Starting from this first and fundamental step our aims and future perspectives will be presented. There is still a great demand for ever more efficient chemosensors; nanostructured systems such as nanoparticles, in which signal amplification can be obtained, will certainly play a very important role in this field. For this reason this contribution concludes with a section on the results obtained on nanoparticles, which are the natural carry-over of our research work.

Recent developments in transition metal ion detection by luminescent chemosensors

Abstract Chemosensors find wide applications in many disciplines such as biochemistry, clinical and medical sciences, cell biology, analytical chemistry and environmental sciences. Aiming to develop new chemosensors for future applications, we have synthesised and studied many different species. We present here the latest luminescent chemosensors for transition metal ions studied in our laboratory. In the various systems active units and receptors are different as are the target ions: Zn 2+ , Ni 2+ , Cd 2+ , Hg 2+ , Co 2+ , Ag + and Cu 2+ . In solution a good selectivity and affinity was observed and the binding is signalled in all cases by pronounced changes in the photophysical properties such as emission wavelength and intensity, and excited state lifetime of the inserted luminophore. The mechanism for signal transduction depends strongly on the chosen receptor and luminophore moieties, and has been investigated in detail by means of steady state and time resolved spectroscopy.

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.

Iridium Doped Silica−PEG Nanoparticles: Enabling Electrochemiluminescence of Neutral Complexes in Aqueous Media

We report for the first time the stable electrochemiluminescence of a completely insoluble neutral Ir(III) complex in aqueous media. The strategy adopted is the encapsulation of emitting dye into silica-PEG nanoparticles. This nanoassembly by limiting water and oxygen quenching and allowing solubilization makes the electrogeneration of the excited state feasible under typical bioassay conditions.

Novel routes to substituted 5,10,15-triarylcorroles

A one-pot procedure that allows the preparation of 5,10,15-triarylcorroles directly from the condensation of pyrrole and benzaldehydes, avoiding the concomitant formation of the corresponding tetraarylporphyrins, has been developed. This approach is general and allows the preparation of sterically hindered corroles from 2,6-disubstituted benzaldehydes, where the Rothemund approach failed. The first examples of fully substituted corrole free bases were prepared following this approach, but in this case corroles were obtained as a mixture with the corresponding dodecasubstituted porphyrins. Unsymmetrically substituted ABC corroles were obtained from the condensation of two different dipyrromethanes. A [2+12] biladiene-like approach afforded 10-(4-methylphenyl)-5,15-diphenylcorrole, along with a significant amount of tetraphenylporphyrin from the self-tetramerization of the starting pyrryl carbinol. An X-ray structure of 5,10,15-triphenylcorrole shows deviations from planarity that are attributed to the ster...

Characterization of 5-chloro-8-methoxyquinoline appended diaza-18-crown-6 as a chemosensor for cadmium

Abstract 5-Chloro-8-methoxyquinoline appended diaza-18-crown-6 ( 1 ) selectively responds to Cd 2+ over other tested metal ions via a large increase in fluorescence. In addition, 1 selectively binds Cd 2+ over Zn 2+ . Compound 1 may be useful in measuring Cd 2+ concentrations in waste effluent streams and in food products.

Green and blue electrochemically generated chemiluminescence from click chemistry-customizable iridium complexes

Cationic cyclometalated iridium complexes containing two anionic phenylpyridine (ppy) ligands and the neutral bidentate triazole-pyridine ligand, 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine (pytl), were investigated. The complexes display a rich and reversible electrochemical behavior, upon investigations by cyclic voltammetry in strictly aprotic conditions, that couples with excellent emission quantum yields and long lifetimes of the excited states. Therefore, in organic media, all complexes have generated intense green electrochemiluminescence (ECL) through the so-called annihilation procedure and, importantly, a modulation of the emission energy (to blue) has been easily obtained by simple fluorination of the ppy ligand. Finally, taking advantage of their remarkable solubility in water, intense ECL was also obtained from aqueous buffer solutions using the co-reactant method, thus making all the investigated complexes highly promising for their effective use as ECL labels in bioanalytical applications.

From observed to corrected luminescence intensity of solution systems: an easy-to-apply correction method for standard spectrofluorimeters

Abstract Although spectrofluorimetry is a very sensitive analytical technique, its use is limited by the non linear relationship between the concentration of the analyte of interest and the generated electric signal. Here we discuss a very easy correction method which takes into account all the instrumental factors and transforms the observed intensity value in a quantity, the corrected luminescence intensity, that is directly proportional to the concentration of the observed luminophore. At the end of the discussion, a general formula is presented together with two examples illustrating the method and its application.