Sulphur and nitrogen doped carbon dots synthesis by microwave assisted method as quantitative analytical nano-tool for mercury ion sensing

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Highlights

  • N,S-CDs were synthesized via a fast microwave method.

  • As the main source of carbon, citric acid was used and glutathione or thiourea as precursors of doping elements.

  • As-prepared carbon dots have optical properties as fluorescence.

  • Sensor quite well suited for mercury (II) determination was obtained with citric acid and thiourea.

  • The real sample as river water and wastewater were used to evaluate potential application.

Abstract

Quantum dots prepared from carbon precursors are a rising star among novel trends in the field of carbon materials. The subject of this paper is the synthesis of carbon dots that can serve as a sensor for determination of heavy metals such as mercury. Carbon dots were obtained in this study with a fast method of microwave synthesis and application of citric acid with glutathione or thiourea. Both versions of carbon dots have optical properties, i.e. fluorescence; however, when thiourea was used as a doping element precursor, the dots made better sensors as – due to quenching by mercury ions – the carbon dots synthesized from citric acid and thiourea have more significant optical effect than those made form citric acid and glutathione. As-synthesized carbon dots had relative quantum yield (QY) as 26% and the limit of quantification when these dots were used was 5.4 μM. In order to test the sensor efficiency in real samples, river water and wastewater were used.

Introduction

In order to look for new analytical methods, efforts are being made to create new analytical tools that will ensure rapid and accurate determination. Nowadays, the “nano” prefix denotes new trends in science, and the concept of nanosensors is becoming increasingly popular [1]. Such nano-tools undoubtedly include carbon dots, which in recent years have been employed in the area of analytical chemistry, for example as selective sensors. Carbon dots (CDs) are promising materials which can be used as a tool in quantification methods. The properties of carbon dots are dependent on many factors, starting with the choice of the carbon source (precursor), and the synthesis path [2]. These nanotools have such properties as excellent solubility in water, good biocompatibility, chemical inertness, high resistance to photo bleaching, and high-stability structure; they can be easily functionalized while their toxicity is negligible [3,4]. However, this type of carbon material has also demonstrated such unique optical properties as luminescence [5]. For this reason, carbon dots can be regarded as a new generation of fluorescent nanomaterials which can enhance analytical performance. They can be used as a sensor for quantification analysis using the interaction between CDs and analytes (e.g. heavy metal) after calibration with a chemical standard. CDs can work in turn-off and turn-on mode. After interaction with the investigated analyte, photoluminescence of sensor is quenched (in most cases) or enhanced [3,6]. Citric acid is a typical reagent used in carbon dot synthesis as a result of its capacity to form carbonaceous materials through bottom-up thermal approach [7]. In order to enhance quantum efficiency and specificity of carbon dots, various methods of surface modification are used. One of the methods is heteroatom elemental doping, using the precursor elements such as nitrogen, sulphur or boron during the synthesis of carbon dots [[8], [9], [10]]. This is a common method to improve their photoluminescence properties by inserting atoms other than carbon, oxygen and hydrogen. A number of experiments have been carried out so far to obtain a selective sensor for mercury ions. Table 1 presents an illustrative example of the results of these experiments, including the preparation methods which are mostly used is microwave and hydrothermal synthesis [11,12]. However, two things must be noted regarding the limits of detection and the range of linearity in this set: the detection limit is often presented in a different unit than the linearity range, which in our opinion poses an obstacle to correct reception. In addition, the linearity range is given from point zero while from an analytical point of view it should not be lower than the limit of quantification.

Considering that carbon dots are a highly intriguing subject and that literature reports suggest that they can be used as a sensor for determination of mercury, pilot experiments have been undertaken to obtain such a sensor and to compare the results obtained with other methods – an important task considering that carbon dots can be a good alternative to such instrumental methods as atomic absorption spectroscopy (AAS) or inductively coupled plasma-mass spectroscopy (ICP-MS), which are mostly used for metal detection. However, this technique requires first of all, expensive equipment and secondly, skilled operators, which limits its applicability [13]. Compared to this, fluorescence methods are easier to perform and provide quick response while maintaining high selectivity. Moreover, CDs have a number of benefits over other conventional fluorescent substances such as organic dyes, which are susceptible to photobleaching, while quantum dots form highly toxic metals. Furthermore, synthesis of carbon dots uses easily available and inexpensive carbon precursors and is easy to execute, while limiting the release of harmful by-products [14]. Thus the aim of the current study was to synthesize a sensor dedicated for mercury ion detection. Preparation of carbon dots is carried out by a microwave-assisted method. As mercury ions changed the native fluorescence of as-prepared carbon dots, a quenching mechanism was used. In addition, the method was also used for the analysis of environmental samples such as river water and wastewater. Validation parameters and recoveries were determined for both artificial samples in the buffer and environmental samples. The paper highlights the impact of the matrix which is rich in interfering compounds.

Section snippets

Materials and reagents

The materials and reagents used in this study included: anhydrous citric acid (Merck, Germany), l-glutathione (reduced), thiourea (Sigma-Aldrich, USA), sodium hydroxide (POCH, Gliwice), acetic acid (J.T. Baker), orthophosphoric acid and boric acid (POCH, Gliwice) for preparation of the Britton-Robinson buffer, quinine sulfate (suitable for fluorescence, Sigma-Aldrich, USA), potassium chloride (KCl), iron (III) chloride hexahydrate (FeCl3·6H2O), calcium chloride (CaCl2), magnesium sulfate (MgSO4

Results and discussion

In this study N,S-CDs were prepared by a fast, microwave-assisted method. Anhydrous citric acid (CA) was used as the main carbon source. However, to improve fluorescence effect, a doping element such as nitrogen and sulphur was introduced.

Conclusion

To sum up, microwave synthesis method is a simple, fast and cheap method for preparation of carbon dots. Both types of N,S-carbon dots obtained in the study display fluorescence properties. However, when citric acid was used as the main carbon source and thiourea as the doping element, it was possible to obtain a sensor quite well suited for mercury determination. However, if the resulting sensor is to be used for real samples, further research is needed to reduce the limit of quantification of

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was financially supported by the National Science Centre within the framework of the project OPUS No. 2018/29/B/ST4/01681.

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