Green and sustainable synthesis of carbon dots for heavy metal detection in environmental monitoring

This thesis will focus on the latest emerging optical-chemical sensing (Opto-Chem-sensing) analytical techniques for the detection of heavy metal ions in water through easy-to-use and cheap monitoring methods. The exploitation of sensing nanomaterials, with peculiar tuneable optical properties and a large interaction area, has provided further stimulus to this field of research towards a huge number of chemical and biological analytes, from heavy metal (HM) ions to organic molecules such as adenosine triphosphate (ATP). In particular, this work will deal with Carbon Dots (CDs) as a versatile sensing material. CDs are a class of carbon nanoparticles that have attracted considerable attention because of the peculiar characteristics and plentiful preparation methods, including “bottom-up” and “top-down” approaches. Both these strategies for the synthesis of different CDs and their employment in the detection of heavy metal ions [Cr(VI) and Cu(II)] in water by the colorimetric and fluorescence-based response were experimentally carried out during the PhD. The immobilization of the CDs in the Agarose hydrogel and their optical characteristics for the detection of Cu(II) by means of both the sensing techniques was also explored. This investigation demonstrated the ease of production, stability and reproducibility for the onside detection without the need of any specific sophisticated instrument thus allowing handling by unexperienced personal without a fear of spoiling the sample. First, a literature survey was conducted to identify shortcomings in the field. These were summarised as: a limited diversity in materials studied, use of complex structures or expensive manufacturing techniques, and use of complex sensing process along with the involvement of toxic effluents. Then, sensing of hexavalent chromium (Cr(VI)) ions in water was addressed. Water pollution caused by hexavalent chromium (Cr(VI)) ions represents a serious hazard for human health due to the high systemic toxicity and carcinogenic nature of this metal species. In this study, N-doped carbon dots obtained by the cage opening of C60 fullerene in water–THF solutions (N-CDs-W-THF) were investigated for the detection of Cr(VI) . The N-CDs-W-THF selectively showed variations of optical absorbance in the presence of Cr(VI) ions in water through the arising of a distinct absorption band peaking at 550 nm, where the pristine CDs are completely transparent. This enabled visual and ratiometric determination of Cr(VI) concentration with a limit of detection (LOD) of 300 nM and a linearity range of 1-100 µM. without the need for any other reactant or specific sample treatment. In order to explore a different approach to synthesis and sensing, nitrogen and sulfur co-doped carbon dots (NS-CDs) were prepared by a simple one-pot hydrothermal method starting from ophenylenediamine (OPD) and ammonium sulfide. These CDs presented the arising of an absorption band at 660 nm and simultaneous fluorescence enhancement at 564 nm. The first effect was attributed to formation of cuprammonium complexes through coordination with amino functional groups of NSCDs. On the other hand, the fluorescence enhancement can be explained by the oxidation of residual OPD bound to NSCDs. Both absorbance and fluorescence showed a linear increase with an increase of Cu(II) concentration in the range 1–100 µM, with the lowest detection limit of 100 nM and 1 µM, respectively. Moreover, tuning of pH of the NS-CD solution permitted to reduce the interference to sensitivity by the possible presence of Fe(III) ions, thus confirming the different nature of the two interactions. In addition, a concurrent response to Co(II) appeared in a different spectral region thus suggesting the possibility of dual-species multiple sensitivity. As in the case of N-CDs-W-THF, this method neither requires any other reagents nor any previous assay treatment and thus can be a promising candidate for low-cost monitoring of copper onsite and by unskilled personnel. NSCDs were successfully incorporated in a hydrogel agarose matrix for easier handling and application to sensing. The formation of cuprammonium complexes was strongly hampered in an agarose matrix while oxidation of OPD was still effective. As a result, color variations could be perceived both under white light and UV light for concentrations as low as 10 µM. Since these color changes were similarly perceived in tap and lake water samples, the present method could be a promising candidate for simple, cost-effective visual monitoring of copper onsite.