Inexpensive Alizarin Red S-based optical device for the simultaneous detection of Fe(III) and Al(III)
Introduction
Many metal ions play significant roles in living systems and affect human health; among them stand out Fe(III) and Al(III), since their overconsumption and the Fe(III) deficit, are associated with several diseases [1].
Fe(III) is an essential cation, playing a crucial role in physiological processes, such as oxygen uptake, oxygen metabolism, and electron-transfer [2,3]. Its deficiency may results in low oxygen delivery, drop in blood pressure, anaemia and lowering of immune system [4]. On the other hand, reactive oxygen species (ROS) can be produced by excess iron, causing severe organism dysfunctions [5].
Aluminium is one of the most abundant metals in the Earth's crust. It is of everyday use in several fields from textile, paper, cosmetics, pharmaceutical, and mechanic industries; thanks to its lightweight, it is applied in aviation to produce different parts of aeroplanes, for tools manufacturing and also food wrapping foil [1].
An excess of aluminium intake in the human body can cause damage to some cells and tissues. It is also well-established that aluminium is a neurotoxic agent. However, the link to the aetiology of some neurodegenerative diseases, like Parkinson's and Alzheimer, is still unclear, and future study has to be done [[6], [7], [8]].
Colorimetric sensors are promising methods for qualitative and quantitative detection of ionic or neutral analytes, of biomedical and environmental relevance, without sophisticated instruments employment. They can also be used as a test kit for on-site detection, since their simple preparation, fast response and low-cost realization [1]. Methods using these devices, usually, do not need any sample pre-treatment and manual expertise, providing an excellent alternative to classic and expensive instrumental analysis. Polymers, paper support, and thin layers metals are commonly used as substrates for the sensor's preparation.
Recent literature shows the development of selective and sensitive chemosensors of Fe(III) and Al(III). Colorimetric and fluorescent devices are particularly attractive, due to their simple realization, affordability, in-situ and real-time monitoring, ease usage and, in some cases, a naked-eye determinations [[9], [10], [11], [12], [13], [14], [15]]. Particularly for Al(III), some luminescent and fluorescent chemosensors were developed [[16], [17], [18], [19], [20]]. These devices, despite their selectivity, have several drawbacks, since their required several synthetic steps, using several toxic and expensive reagents and solvents. For most of them, the analysis can be performed only in non-aqueous solvents and, the application to real samples or certified materials is not reported.
Recently we developed solid optical sensors for metal ions, using different supports, as chelating resins [20] or triacetylcellulose membrane [21] and, specifically for Fe(III), we studied different strategy, synthesising polymers or linking each different selective ligands to natural or commercially available supports [[22], [23], [24], [25]].
In this study, we decided to develop a colorimetric sensor using a simple preparation strategy, standard reagents, and cheap items. The device is obtained by sorption of a common sulfonated dye, Alizarin Red S, on a commercial and easy-to-use material, as the Colour Catcher®. This support is more similar to a tissue, or a soft sheet of paper than to a solid; it is insoluble in water, and it lasts in water solution for days, under apparently stable conditions. It is a commercial product of the washing powder market, distributed in Italy by Grey, a partner of the Henkel company, and in England by Dylon [26]. According to product advertisement [26]: “The Colour Catcher® is a leading brand in colour run prevention, that helps over a million households to wash smarter. Colour Catcher® uses its revolutionary anti-transfer technology to help save time and money, and make life easier, by allowing mixed colour wash loads and reducing the need to sort laundry. The proof is on the sheet”.
To achieve this goal the “sheet”, as they name it, must exhibit sequestration properties towards molecules and ions when they are released by clothes, even in the presence of surfactants, since it was designed to be used together with washing powder substances during laundry. For its properties, ease to supply, and low-cost (It costs around € 0.2 per sheet of 11 × 25 cm) it was chosen as excellent and cheap support for the Alizarin Red S dye, that provides the detection unit of the colorimetric sensor. The device obtained is called Aliz-CC@.
We performed an in-depth characterisation of the solid phase and a study of the sorption mechanism before applying the new sorbing material as an extractant and as a sensor.
The sensing properties of the Alizarin Red S-based chemosensor towards Fe(III) and Al(III), is tested by registering UV–vis spectra of the device after equilibration with different Fe(III) or/and Al(III) solutions.
The final purpose of this device is the simultaneous determination, in a water medium, of Fe(III) and Al(III), both able to form stable complexes with Alizarin Red S.
We achieved this goal by applying a chemometric tool: the multivariate regression PLS, (Partial Least Square Regression) to the entire UV–vis spectra of the Aliz-CC@ registered after equilibration with aqueous solutions of both cations.
Section snippets
Chemicals and apparatus
All reagents were of analytical reagent grade, MilliQ water was used throughout. Iron and Aluminium standard solutions were obtained by dilution of 1000 mg/L Standard for ICP (Sigma-Aldrich, Italy). Alizarin Red-S was obtained by Carlo Erba (Italy). Sewage Sludge, ERM (EMR CC136a/0291) was employed as reference material for validation. The Colour Catcher®, (hereafter CC) was bought in a common supermarket. For the certified material digestion, a MarsXpress microwave system, supplied by CEM (CEM
Maximum sorption capacity of CC towards anionic dyes
As the first step, we determined the maximum sorption capacity of CC towards anionic ligands. In an early stage, we examined different anionic molecules. As an example, we reported the results for Alizarin Red S, which present just one negative charge, and another sulfonated azo-ligands, the 2-(tetrazolyl azo)-1,8 dihydroxy naphthalene-3,6,-sulphonic acid (TazoC), that exhibits two negative charges.
The graph presented in Fig. 1 represents the CC sorption profiles towards these two ligands. The
Conclusions
We developed an optical sensor by immobilisation of Alizarin Red S on an inert solid support, i.e., a product of the washing powder market known as “Colour Catcher®.” The device obtained is named Aliz-CC@, and it acted as an excellent receptor for Fe(III) and Al(III).
The Aliz-CC@ sensor is easily prepared by immersion of a portion of CC in a solution of the dye for about 3 h, and when kept in water solutions, it is stable for at least 3 days. The concentration of active sites on the solid,
Acknowledgements
We acknowledge Chiara Milanese (Hydrogen Lab, University of Pavia) for IR, EDX and SEM measurements; Simone Marchetti and lab 3D@UniPV (Virtual Modelling and Additive Manufacturing for Advanced Materials, University of Pavia), for the film holder realization, and funds provided from FAR (Fondi Ateneo Ricerca) of the University of Pavia and from PRIN 2015: 2015MP34H3_004.
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