Angelo Taglietti

The design of luminescent sensors for anions and ionisable analytes

Abstract A molecular luminescent sensor for anions can be built through a modular approach, i.e. by covalently linking an appropriate photoactive fragment to the receptor displaying a satisfactory affinity towards the desired substrate. Following the receptor-anion interaction, an intercomponent process must take place, e.g. an electron transfer (eT) or an energy transfer (ET) process, that distinctly modifies the emission of the luminophore, thus signalling the occurrence of the recognition event. In this article, specific molecular sensors are classified according to the type of receptor-anion interaction, whether hydrogen bonding or metal–ligand interactions. Receptors of the latter class are based on a Zn II polyamine platform, which leaves at least a vacant coordination site for the incoming anion. Substrates include natural amino acids, NH 3 + CH( R )COO − , for which the highest selectivity is observed when the receptor subunit specifically interacts with the R portion. An eT process involving R and the nearby excited luminophore may provide the signal transduction mechanism.

Antibacterial Activity of Glutathione-Coated Silver Nanoparticles against Gram Positive and Gram Negative Bacteria

In the present paper, we study the mechanism of antibacterial activity of glutathione (GSH) coated silver nanoparticles (Ag NPs) on model Gram negative and Gram positive bacterial strains. Interference in bacterial cell replication is observed for both cellular strains when exposed to GSH stabilized colloidal silver in solution, and microbicidal activity was studied when GSH coated Ag NPs are (i) dispersed in colloidal suspensions or (ii) grafted on thiol-functionalized glass surfaces. The obtained results confirm that the effect of dispersed GSH capped Ag NPs (GSH Ag NPs) on Escherichia coli is more intense because it can be associated with the penetration of the colloid into the cytoplasm, with the subsequent local interaction of silver with cell components causing damages to the cells. Conversely, for Staphylococcus aureus, since the thick peptidoglycan layer of the cell wall prevents the penetration of the NPs inside the cytoplasm, the antimicrobial effect is limited and seems related to the interaction with the bacterial surfaces. Experiments on GSH Ag NPs grafted on glass allowed us to elucidate more precisely the antibacterial mechanism, showing that the action is reduced because of GSH coating and the limitation of the translational freedom of NPs.

Synthesis, Characterization and Antibacterial Activity against Gram Positive and Gram Negative Bacteria of Biomimetically Coated Silver Nanoparticles

In the present work, we describe a simple procedure to produce biomimetically coated silver nanoparticles (Ag NPs), based on the postfunctionalization and purification of colloidal silver stabilized by citrate. Two biological capping agents have been used (cysteine Cys and glutathione GSH). The composition of the capped colloids has been ascertained by different techniques and antibacterial tests on GSH-capped Ag NPs have been conducted under physiological conditions, obtaining values of Minimum Inhibitory Concentration (MIC) of 180 and 15 μg/mL for Staphylococcus aureus and Escherichia coli, respectively. The antibacterial activity of these GSH capped NPs can be ascribed to the direct action of metallic silver NPs, rather than to the bulk release of Ag(+).

A chemosensing ensemble for selective carbonate detection in water based on metal-ligand interactions.

Displacement of the loosely encapsulated fluorescent indicator coumarine 343 (F) from a dicopper(II) cryptate enables the selective fluorimetric detection of the HCO3 (-) ion (A) in water (see scheme, •○=Cu(2+) ). The fluorophore is quenched when encapsulated, but displays its full emission when released to the solution.

Anion recognition by dimetallic cryptates

Abstract Bis-tren cryptands (i.e. octamine cages consisting of two tripodal tetramine subunits covalently linked by given spacers) are able to incorporate first two metal ions, then an ambidentate anion, according to a cascade mechanism. In particular, dicopper(II) cryptates behave as effective receptors for anions, which fill the empty cavity of the cage and place their donor atoms in the two axial sites left available by each Cu(II) centre (which adopts a trigonal bipyramidal stereochemistry). Anion encapsulation by dicopper(II) cryptates often induces the development of a rather intense anion-to-metal charge transfer absorption band in the visible region, so that the recognition process is signalled by the appearance of a bright colour. Two examples are considered in detail: (i) that of a rigid bis-tren cryptate containing 1,3-xylyl spacers, which does not recognise the shape, but the bite of the polyatomic anion (i.e. the distance between two consecutive donor atoms); and (ii) that of the flexible cryptate containing 2,5-furanyl spacers, which is able to include also monoatomic anions, in particular halides, displaying peak selectivity in favour of Cl − .

Sensing of transition metals through fluorescence quenching or enhancement. A review

A series of fluorescent sensors for transition metal ions were synthesized by linking a light-emitting subunit, anthracene, to a polyaza chelating subunit, either a dioxotetraamine or a tetraamine. Sensing of the divalent cations CuII, NiII and ZnII was investigated through spectrofluorimetric titrations in acetonitrile–N water (4:1) solutions. The selective recognition of CuII and NiII among other transition and non-transition metal ions is signalled through full quenching of fluorescence; discrimination between the two ions can be achieved by performing titrations at controlled pH. The system containing the tetraamine fragment, whose interaction with CuII and NiII induces fluorescence quenching, is also sensitive to ZnII, but in this case the recognition is signalled through a fluorescence enhancement.

Self-assembled monolayers of silver nanoparticles firmly grafted on glass surfaces: Low Ag + release for an efficient antibacterial activity

A two-step, easy synthetic strategy in solution has been optimized to prepare authentic monolayers of silver nanoparticles (NP) on MPTS-modified glass surfaces, that were investigated by AFM imaging and by quantitative silver determination techniques. NP in the monolayers remain firmly grafted (i.e. not released) when the surfaces are exposed to air, water or in the physiological conditions mimicked by phosphate saline buffer, as UV-Vis spectroscopy and AFM studies demonstrate. About 15% silver release as Ag(+) ions has been found after 15days when the surfaces are exposed to water. The released silver cations are responsible of an efficient local microbicidal activity against Escherichia coli and Staphylococcus aureus bacterial strains.

Antibiofilm activity of a monolayer of silver nanoparticles anchored to an amino-silanized glass surface

Biofilm production is the crucial pathogenic mechanism of the implant-associated infection and a primary target for new anti-infective strategies. Silver nanoparticles (AgNPs) are attracting interest for their multifaceted potential biomedical applications. As endowed with highest surface/mass ratio and potent antibacterial activity, they can profitably be applied as monolayers at biomaterial surfaces. Desirably, in order to minimize the risks of toxic effects from freely circulating detached nanoparticles, AgNPs should firmly be anchored to the modified biomaterial surfaces. Here we focus on a newly designed glass surface modified with AgNPs and on its antibiofilm properties. Link of a self-assembled monolayer of AgNPs to glass was obtained through preliminary amino-silanization of the glass followed by immersion in an AgNPs colloidal suspension. Static contact angle measure, AFM, TEM, UV-Vis spectroscopy, ICP atomic emission spectroscopy were used for characterization. Antibiofilm activity against the biofilm-producer Staphylococcus epidermidis RP62A was assayed by both CFU method and CLSM. Performances of AgNPs-glasses were: i) excellent stability in aqueous medium; ii) prolonged release and high local concentration of Ag(+) without any detaching of AgNPs; iii) strong antibiofilm activity against S. epidermidis RP62A. This AgNPs surface-modification can be applied to a large variety of biomaterials by simply depositing glass-like SiO2 films on their surfaces.

The design of fluorescent sensors for anions: taking profit from the metal–ligand interaction and exploiting two distinct paradigms

Fluorescent sensors are molecular systems consisting of a receptor moiety and of a fluorogenic fragment, which are capable of recognising a given analyte and signalling recognition through a variation of the emission intensity. The fluorogenic fragment responsible of the signal can be associated to the receptor either covalently or non-covalently, giving rise to two well distinct classes of fluorosensors and sensing paradigms. The design of fluorescent sensors is described, with a special attention to the sensing of anionic groups (including those of amino acids). In any case, it seems convenient that the receptor moiety contains one or more metal centres, which establish strong coordinative interactions with the envisaged anionic substrate. Selectivity is related to the energy of the metal–analyte interaction and can be achieved by taking profit of the concepts developed in more than one hundred years of coordination chemistry. As an example, recognition and sensing of the amino acid histidine is considered in detail, which is based on the attitude of the imidazole residue to deprotonate and bridge two MII ions prepositioned at the right distance, within a defined coordinative framework (M = Cu, Zn).

Fluorescence sensing of ionic analytes in water: from transition metal ions to vitamin B13.

The fluorescence chemosensor ATMCA has been realised by appending an anthrylmethyl group to an amino nitrogen of TMCA (2,4,6-triamino-1,3,5-trimethoxycyclohexane), a tripodal ligand selective for divalent first-row transition metal ions in water. The ATMCA ligand can act as a versatile sensor for ZnII and CuII ions. Its sensing ability can be switched by simply tuning the operating conditions. At pH 5, ATMCA detects copper(II) ions in aqueous solutions by the complexation-induced quenching of the anthracene emission. Metal ion concentrations < 1 microM can be readily detected and very little interference is exerted by other metal ions. At pH 7, ATMCA signals the presence of ZnII ions at concentrations < 1 microM by a complexation-induced enhancement of the fluorescence. Again the sensor is selective for ZnII over several divalent metal ions, with the exception of CuII, CoII and HgII. Most interestingly, the [ZnII(atmca)]2+ complex can act as a fluorescence sensor for specific organic species, notably selected dicarboxylic acids and nucleotides, by the formation of ternary ligand/zinc/substrate complexes. The oxalate anion is detected in concentrations <0.1 mM; however, no effects on the systems fluorescence is observed in the presence of monocarboxylic acids and long-chain dicarboxylic acids. Among the nucleotides, those containing an imide or amide function are readily detected and an unprecedented high sensitivity for guanine derivatives allows the determination of this nucleotide for 0.05-0.5 mM solutions. Moreover, [ZnII(atmca)]2+ is a very effective and selective sensor in the case of vitamin B13 (orotic acid) in sub-micromolar concentrations. The operative features of the systems investigated are also clearly suitable for intracellular analyses. The factors at the source of organic substrate recognition, here briefly discussed, are of paramount importance for further developments in the applicability of these sensing systems.