Piersandro Pallavicini

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(+).

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.

Molecular events switched by transition metals

Transition metals can typically give rise to two (or more) distinct states of comparable stability (two consecutive oxidation states; two different stereochemical arrangements). In a multicomponent system, the conversion of one state to the other can modify a given property of a nearby subunit or can induce drastic changes in the system topology. In this sense, the metal behaves as a switch, which can be operated through an external input (the variation of the pH or of the redox potential). Recent examples of molecular switching by transition metals are reviewed: (i) the quenching/enhancement of the emission of a luminescent fragment effected by a nearby metal centred redox couple (e.g. NiII/NiIII); (ii) the pH driven motion of an aminoalkyl side chain in a NiII scorpionate complex, which is signalled by the variation of the light emission intensity of an appended anthracenyl fragment; (iii) the pH controlled translocation of a NiII ion within a multidentate ligand containing two compartments of different coordinating tendencies and (iv) the intramolecular translocation of a Cl− anion between two pre-positioned metal centres CuII and NiII, within a ditopic receptor, electrochemically driven through the NiII/NiIII redox change.

Controllable Intramolecular Motions That Generate Fluorescent Signals for a Metal Scorpionate Complex

Simply by changing the pH value, the side chain of complex 1 can be reversibly moved between two positions. Coordination to the metal center through the nitrogen atom of the side chain at moderate pH values is accompanied by a decrease in fluorescence intensity (from IF =100% to IF =60%). A further decrease is observed upon deprotonation of the bound water molecule at higher pH (IF ≤2%). Therefore, 1 can be seen as a molecular three-position switch.

A Molecular Thermometer for Nanoparticles for Optical Hyperthermia

We developed an all-optical method to measure the temperature on gold (nanorods and nanostars) and magnetite nanoparticles under near-infrared and radiofrequency excitation by monitoring the excited state lifetime of Rhodamine B that lies within =/~20 nm from the nanoparticle surface. We reached high temperature sensitivity (0.029 ± 0.001 ns/°C) and low uncertainty (±0.3 °C). Gold nanostars are =/~3 and =/~100 times more efficient than gold nanorods and magnetite nanoparticles in inducing localized hyperthermia.

Synthesis of branched Au nanoparticles with tunable near-infrared LSPR using a zwitterionic surfactant

Asymmetric branched gold nanoparticles are obtained using for the first time in the seed-growth approach a zwitterionic surfactant, laurylsulfobetaine, whose concentration in the growth solution allows to control both the length to base-width ratio of the branches and the LSPR position, that can be tuned in the 700-1100 nm near infrared range.