PluS Nanoparticles as a tool to control the metal complex stoichiometry of a new thio-aza macrocyclic chemosensor for Ag(I) and Hg(II) in water

https://doi.org/10.1016/j.snb.2014.07.107Get rights and content

Highlights

  • We have synthesized a new fluorescent chemosensor for the detection of heavy atoms.

  • We have carefully studied the binding mode of the ligand.

  • We have hosted the chemosensor inside multi-compartment nanoparticles in water.

  • We controlled the stoichiometry of the complexes tuning the number of L per NP.

Abstract

We report here the synthesis of a new thio-aza macrocyclic chemosensor based on the 2,5-diphenyl[1,3,4]oxadiazole in which two thioether groups were inserted in a macrocycle with the aim to make it suitable for the coordination of soft and heavy metal ions. In acetonitrile solution, the fluorescence of the chemosensor changes upon addition of different metal ions, such as Cu(II), Zn(II), Cd(II), Pb(II), Hg(II) and Ag(I), that form a not fluorescent ML species and a fluorescent M2L species characterized also via NMR experiments. The hosting of the chemosensor inside the PluS Nanoparticles leads to a high water solubility, allowing to perform the metal detection without the use of additional solvents and also induced an higher selectivity towards Ag(I) and Hg(II). Moreover, it was demonstrated for the first time the possibility to control the stoichiometry of the formed complex upon changing the number of ligands per nanoparticles. To our opinion, this possibility can give an additional tool for the tuning of the affinity and selectivity of the chemosensor that could be of great interest for the design of more and more efficient systems.

Introduction

The development of fluorescent molecular sensors for the detection of a specific analyte is a growing area of chemistry. Their appeal is due to the fact that they offer many advantages in terms of sensitivity, response time and costs with respect to other detection methods, such as the expensive and time-consuming inductively coupled plasma-mass spectroscopy and atomic absorption spectroscopy [1], [2], [3], [4]. Among the possible substrates, metal ions have a central role since they are almost ubiquitous in the functions governing life. Therefore, their selective detection and quantification rises considerable attention in many fields such as environmental and security monitoring, waste management, nutrition, and clinical toxicology [1], [2], [3], [4], [5], [6], [7]. In particular, soft and heavy metal ions such as Ag+, Hg2+, Cd2+, and Pb2+ presenting a toxic impact on human health [8], [9], [10], [11], catalyze great research efforts worldwide towards the design of sensors for their detection in real matrices.

In particular, silver is widely used in the electrical, photographic and imaging industry, as well as in pharmacy [12], [13], [14], and it is known to inhibit the activity of many bacteria, viruses, and fungi [15], [16], [17]. It is toxic and undergoes bioaccumulation; for example, silver ion inactivate sulfhydryl enzymes and in combination with several metabolites, cause serious environmental and health problems [18], [19]. Mercury is one of the most toxic metals; nevertheless it is always present in the lithosphere and in waters due to its widespread use in industry and agriculture. Because of its high affinity for sulphur atoms it strongly interacts with many proteins and enzymes thus bioaccumulating and producing pathological diseases [20], [21]. Similarly, the presence of cadmium and lead in several industrial processes and devices (for example in batteries) increases the possible exposure to these elements with serious fallouts for human health (inducing memory loss, anaemia, muscle paralysis, and mental retardation) and for the environment [22].

In this context, we have recently reported the synthesis and study of the binding properties of a class of macrocyclic chemosensors based on polyamines as coordinating groups and the 2,5-diphenyl[1,3,4]oxadiazole (PPD) as signalling unit incorporated in the macrocyclic skeleton [23], [24]. The ligand shows four amine functions and responds selectively in water via an increase of the fluorescence intensity to Zn(II) also at physiological pH (7.4) [24].

Here we report the synthesis of a new PPD-based chemosensor L (Chart 1), in which two thioether groups were inserted together with two amine functions in the macrocycle. We inserted the two S atoms to make the chemosensor suitable for the coordination of soft and heavy metal ions both in organic and aqueous media, enlarging the available toolbox for the detection of metal ions. We also report on the effects of the inclusion of L in core–shell water soluble nanoparticles, the so-called PluS Nanoparticles (PluS NPs): silica nanoparticle synthesized with a strategy based on the formation of micelles of Pluronic® F127 in water [25], [26], [27], [28]. We have already proposed this strategy to increase the water solubility, the affinity and the signal-to-noise ratio of some chemosensors [29], [30], [31], [32], [33]. The extension of this approach to other systems can afford, to our opinion, additional information for the design of a new generation of chemosensors able to investigate the toxic effects of metal ions directly in the compromised environment that is usually an aqueous media, giving analytical answers with high selectivity and sensitivity [34].

Section snippets

Synthesis

The synthetic pathway used to obtain the ligand L is presented in Scheme 1. The 2,11-diaza-5,8-dithiadodecane reagent 6 was obtained in good yield starting from 2-aminoethanethiol 1 and 1,2-dibromoethane 2 and following the procedure reported by Zheng et al. [35] to get the 1,10-diaza-4,7-dithiadecane compound 3 which was then isolated as 3·2HCl and characterized. The two amine functions of 3 were nosyl-protected using 2-nitrobenzenesulfonylchloride in pyridine to afford in good yield (54%)

Conclusion

We have reported here the synthesis of a new PPD-based chemosensor presenting two thioether groups together with two amine functions in the macrocycle to make it suitable for the coordination of soft and heavy metal ions. We have performed the photophysical characterization and the study of its metal ion binding properties both in acetonitrile and in buffered water, in the latter case after hosting the chemosensor inside PluS Nanoparticles. NMR spectra obtained in acetonitrile helped us to

General methods

1H NMR and 13C NMR spectra were recorded at 298 K on a Bruker Avance instrument, operating at 200.13 and 50.33 MHz, respectively. For the spectra recorded in D2O, the peak positions are reported with respect to HOD (4.75 ppm) for 1H NMR spectra, while dioxane was used as reference standard in 13C NMR spectra (δ = 67.4 ppm). For the spectra recorded in CDCl3 the peak positions are reported with respect to TMS. All reagents and solvents used were of analytical grade.

Synthesis

Compound L was obtained following

Acknowledgements

We thank Fabrizio Luzi for having performed some photophysical experiments. This work has been financially supported by MIUR (PRIN2009Z9ASCA and PON 01_01078 projects) and the University of Bologna (FARB Project “Advanced Ultrasensitive Multiplex Diagnostic Systems Based on Luminescence Techniques”). MIUR and the University of Bologna have no role in the study design, nor in the collection, analysis and interpretation of data, in the writing of the report, and in the decision to submit the

Gianluca Ambrosi is a technician at the University of Urbino; he received his PhD in Chemical and Pharmaceutical Sciences at the University of Urbino after a degree in Chemistry (Università di Bologna). He is an expert in organic synthesis of new ligands able to bind metal cations and anions.

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    Gianluca Ambrosi is a technician at the University of Urbino; he received his PhD in Chemical and Pharmaceutical Sciences at the University of Urbino after a degree in Chemistry (Università di Bologna). He is an expert in organic synthesis of new ligands able to bind metal cations and anions.

    Elisa Borgogelli obtained his MSc degree in Chemistry at the University of Bologna; she then got her PhD in Chemical and Pharmaceutical Sciences at the University of Urbino in 2014. Her research activity regards the metal complex-formation properties in solution.

    Mauro Formica attended the University of Messina and graduated in chemistry in 1991 with a 110/110 score. He was awarded with a fellowship position of the National Research Council lasting one year. In 1998 he obtained his PhD degree in Chemical and Pharmaceutical Sciences at the University of Urbino. From 2000 he has a permanent position as researcher of Inorganic Chemistry at the University of Urbino. His main research interests are focused on synthetic macrocyclic compounds, metal complexes, and solution equilibrium studies.

    Vieri Fusi is an associate professor of General and Inorganic Chemistry at the University of Urbino; he got his degree in Chemistry and his PhD in Chemical Sciences at the University of Firenze. He is an expert in Supramolecular and Coordination Chemistry, his research activity mainly focussing on the synthesis of new ligands, most of them having a macrocyclic skeleton, and on the study of their metal complex-formation properties. Recent interests center on the synthesis of binding units and metal-complexes behaving as metallo-receptors for the recognition, sensing, signalling, sequestration, transport, dosage, and/or activation of biological and environmental substrates.

    Luca Giorgi graduated in Chemistry at the University of Bologna (Italy) in 1996 with a 110/110 score. In 1998 he moved at the University of Urbino (Italy) with a fellowship grant of Sigma-Tau (Rome, Italy) lasting two years. In 2003 he obtain the PhD in Chemical and Pharmaceutical Sciences at the University of Urbino. In 2005 he was appointed researcher of Inorganic Chemistry at the same University. Dr. Giorgi research interests focus on the synthesis of macrocyclic compounds, of their metal complexes, and of solution equilibrium studies.

    Mauro Micheloni was born in Prato (Italy) in 1949. He attended the University of Florence and graduated in Chemistry in 1973, with the highest score. In 1990 Dr. Micheloni was appointed full professor of Inorganic Chemistry, and moved to the University of Urbino. Prof. Micheloni's research interests focus on synthesis of new ligand molecules, including many macrocyclic compounds and their metal complexes; thermodynamic of protonation and metal complexes formation equilibria; PGM refining and recovery; PGM plating solutions preparation.

    Enrico Rampazzo was born in Verona in 1973. He completed his PhD at the University of Padua under the supervision of Prof. Umberto Tonellato and Fabrizio Mancin. He was a FIRB young researcher and a postdoctoral fellow with Luca Prodi and Marco Montalti at the Photochemical Nanosciences Laboratory of the University of Bologna (Italy). He is now a researcher in the same group. His research activity focus on the synthesis of luminescent dyes, sensors and (electro)luminescent systems based on dye doped silica nanoparticles for the development of sensors and labels.

    Massimo Sgarzi obtained his MSc degree in Photochemistry and Chemistry of Materials at University of Bologna in 2008. He received his PhD degree in 2014 defending a thesis about the characterization of optical transduction-based molecular systems and nanoparticles for the development of chemical sensors, working with Professor Prodi's group in Bologna. He is currently a postdoctoral fellow in the same laboratory, continuing the work on the photophysical characterization of fluorescent chemosensors and silica nanoparticles for biological and environmental applications.

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    Luca Prodi is full professor of General and Inorganic Chemistry at the University of Bologna. His research activity is focussed on the synthesis and characterization of luminescent sensors and labels. In this context, he is also actively working on the synthesis and characterization of metal and silica nanoparticles, in order to obtain more efficient sensors and labels mainly for bio-medical applications. He is also inventor of 4 patents and a co-founder of two spin-off companies.

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