Sara Bonacchi

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.

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.

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.

Energy transfer from silica core-surfactant shell nanoparticles to hosted molecular fluorophores.

Very monodisperse water-soluble silica core-surfactant shell nanoparticles (SCSS NPs) doped with a rhodamine B derivative were prepared using micelles of F127 as nanoreactors for the hydrolysis and condensation of the silica precursor tetraethoxysilane (TEOS). The functionalization of the rhodamines with a triethoxysilane group allowed the covalent binding of the fluorophores to the silica core: no leaking of the dye was observed when the NPs were purified either by ultrafiltration (UF) or dialysis. The diameter of the core (d(c) = 10 ± 1 nm) was determined by TEM and subtracted from the hydrodynamic diameter, measured by DLS, (d(H) = 24 nm, PdI = 0.1) to calculate the shell thickness (∼7 nm). The presence of a single population of NPs with a radius compatible with the one measured by DLS after UF was confirmed by AF4-MALS-RI measurements. The concentration of the NPs was measured by MALS-RI. This allowed us to determine the average number of rhodamine molecules per NP (10). The ability of the NPs to host hydrophobic species as cyanines in the SS was confirmed by fluorescence anisotropy measurements. Steady-state and time-resolved fluorescence measurements allowed us to observe the occurrence of a very efficient Förster resonance energy transfer process from the covalently linked rhodamines to the hosted cyanines. In particular, the analysis of the TCSPC data and steady-state measurements revealed that the adsorption of a single cyanine molecule causes an almost complete quenching of the fluorescence of the NP. Thanks to these observations, it was possible to easily determine the concentration of the NPs by fluorescence titration experiments. Results are in good agreement with the concentration values obtained by MALS-RI. Finally, the hosted cyanine molecule could be extracted with (±)-2-octanol, demonstrating the reversibility of the adsorption process.

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.

Temperature‐Dependent Fluorescence of Cu5 Metal Clusters: A Molecular Thermometer

The accurate measurement of temperature is of increasing importance as it is required for widespread applications (electronic devices, biology, medical diagnostics). In this context, fluorescence thermometry has already shown great potential, and a variety of molecules have been proposed as luminescent molecular thermometers. Herein, we describe Cu5 metal cluster 1 (Figure 1) that presents remarkable photophysical properties, both in solution and as the solid, characterized by temperature-dependent emission intensity and lifetime that change significantly in the range between 45 and + 80 8C. These properties allow for an unprecedented accuracy in temperature determination by fluorescence measurements, with the high sensitivity and the high temporal (sub-millisecond) and spatial (sub-micrometer) resolution typical of photoluminescence spectroscopy. Complex 1 can be seen as a metal nanoparticle composed of five copper atoms bound to three highly conjugated dianionic cationic ligands (EtNC(S)PPh2NPPh2C(S)NEt) ; Figure 1A). 14] Its absorption spectrum presents a broad and unstructured band below 450 nm (Figure 2A). The system is luminescent in all phases, both at room temperature and at 77 K (Figure 2B) and no dependence on the solvent was observed. A summary of the photophysical properties is shown in Table 1.

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.

Drug-Loaded Fluorescent Cubosomes: Versatile Nanoparticles for Potential Theranostic Applications

In this work, monoolein-based cubosomes were doped with two fluorescent probes, namely, fluorescein and dansyl, properly modified with a hydrocarbon chain to increase their encapsulation efficiency within the monoolein palisade. The same nanocarriers were also loaded with quercetin, a hydrophobic molecule with potential anticancer activity. Particularly, the cubosomes doped with the modified fluorescein probe were successfully exploited for single living cell imaging. The physicochemical and photophysical characterizations reported here, along with the well-known ability of cubosomes in hosting molecules with pharmaceutical interest, strongly encourage the use of these innovative fluorescent nanocarriers for theranostic purposes.

Prevention of Self‐Quenching in Fluorescent Silica Nanoparticles by Efficient Energy Transfer

Stars that shine bright: A high local dye concentration in doped silica-based core–shell nanoparticles causes self-quenching and spectral broadening (top images). This phenomenon jeopardizes the potential advantages of heavily doped systems. Förster resonance energy transfer (FRET) to an acceptor co-included in the silica led to ultrabright nanoparticles (bottom images) with a preselected narrow-band emission and a pseudo-Stokes shift of 129 nm.

Luminescent Chemosensors Based on Silica Nanoparticles

The field of nanoparticles is amazingly many-sided and consequently their applications range between many different areas from industry to bio-analysis and catalysis. In particular, luminescent nanoparticles attract close attention in the areas of biology, medical diagnosis and therapy, where they already find many applications. In this so fascinating and wide framework we have focussed our attention on luminescent silica nanoparticles able to act as sensing materials. We highlight here the importance, especially with the aim of sensing, of gaining precise knowledge and control of their structures; the performance of a chemosensor is, in fact, totally dependent on its design. We then briefly present the state of the art and the progress both in the synthetic protocols and in the application of luminescent silica nanoparticles as chemosensors. We present many recent examples, organized into two main sections, the first dealing with systems presenting the signalling units on the surface (dye coated silica nanoparticles, DCSNs) and the second with systems entrapping the dyes inside the silica matrix (dye doped silica nanoparticles, DDSNs).