A QDs Nanocomposites-Based Ratiometric Fluorescence Sensor for Detection of Trace Cadmium Ions
Authors: Wang Xinyi , Gan Ying , Liang Tao , Zhou Shuqi , Sun Jiadi , Wan Hao , Wang Ping 
Published: May 2020

Introduction
Heavy metal contamination in water environment is increasingly more serious to human health and environment. In particular, cadmium ions (Cd2+), as an extremely toxic heavy metal, has been defined as a human carcinogen by The International Agency for Research on Cancer (IARC) [1]. In addition, hypertension, renal dysfunction, rickets, liver damage and “itai-itai disease” can be caused by Cd2+. However, the traditional analytical techniques of Cd2+ such as atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, and electrochemical methods have the characteristics of large volume, high cost, complicated operation, and time-consuming. Thus it is urgent to achieve a simple, rapid, reliable, and selective probe to detect Cd2+ on site. Quantum dots (QDs) are a kind of inorganic fluorophores with unique optical properties and electrical properties. However, due to its large specific surface area, the fluorescent properties are highly susceptible to the surrounding environment, including the reagent concentrations and intrinsic background PL in sample water. Thus ratiometric PL sensor based on reference fluorophore was developed to achieve accurate measurement. The combination of two fluorophores is still known to be challenging because of the complicated reaction and purification. Niu et al. established a carbon quantum dots/gold nanoclusters (CQDs/AuNCs) nanohybrid based ratiometric fluorescent probe for detection of Cd2+. But it still needs complex synthetic conditions and ligand reactions which will affect the photophysical properties of the QDs. Jin et al. developed a ratiometric pH sensor using FITC conjugated glutathione(GSH)-capped CdSe/CdZnS nanocrystals. However, the combination of QDs-FITC needs complicated purification. Gui et al. created FITC-CdTe QDs conjugates using polyethyleneimine (PEI) to detect Cd2+. Nevertheless, amines like PEI will cause apparently fluorescence quenching of QDs, and the single layer QDs are highly susceptible to the surrounding environment.
In this study, PVA combined with CdTe/ZnS/CdS QDs and FITC fluorophores to form stable nanocomposites (FPQ) was creatively proposed (Fig. 1). PVA, a biodegradable hydrophilic polymer, can interact with QDs easily without unnecessary ligand exchange. Then with the FPQ added to ethylenediaminetetraacetic acid (EDTA) solution, PL quenching was caused by the generation of specific Cd2+ recognition sites. These sites can then be identified by the added Cd2+ with the PL recovering. In the process, the PL of FITC remains essentially unchanged to achieve the reference effect.
Characterization of QDs nanocomposites
The FPQ nanohybrid was formed via the following two steps: (1) Dropping 100 μL of 100 mM PVA aqueous solution into 650 μL of 10 mM Tris-HCl buffer. Then 50 μL of 1 mg/mL QDs was added to acquire red colored PVA-QDs nanohybrid with 30 min continuously stirring. In addition, the 10 mM of Tris-HCl buffer was prepared by dissolving 0.6057 g of Tris powder in 500 mL ultrapure water with quantitative HCl. (2) 200 μL of 50 μM FITC was dispersed in the PVA-QDs solution. After another 30 min stirring, the yellow colored FPQ solution was formed. In particular, the above reactions were kept in the dark and room temperature. The morphology (size and shape) of CdTe/ZnS/CdS QDs and the combination of as-prepared FPQ nanocomposites were characterized by TEM (200 keV) and Microplate Reader (Fig.2).

Method

To evaluate the effect of EDTA etching on the PL of FPQ (Fig. 3(a)), the as-prepared FPQ was injected into 10 mM of Tris-HCl (pH = 8.0), and the concentration of FPQ was 0.005 mg/mL. Then the mixtures were mixed thoroughly with different concentrations of EDTA solutions for 10 min in the dark. To detect Cd2+, the 65 μΜ EDTA was chosen to react with FPQ. With the gradual increase of Cd2+ concentration, the PL of FPQ gradually recovers (Fig.3(b) and (c)). In addition, we optimized the pH, EDTA concentration, reaction time, temperature, etc.

Results and Conclusions

In this paper, an improved ratiometric PL sensor based on the "off-on" mode for cadmium ions (Cd2+) detection was developed. Stable nanocomposites based on CdTe/ZnS/CdS QDs are synthesized on the basis of polyvinyl alcohol (PVA). Then combined with fluorescein isothiocyanate (FITC) to acquire a conjugate (FPQ) with two fluorophores. The FPQ was verified by the transmission electron microscopy (TEM) and ultraviolet–visible spectroscopy (UV–vis). The added EDTA in the FPQ conjugates induce fluorescence quenching due to chemical etching. In the presence of Cd2+, EDTA-etched FPQ probe will capture the Cd2+ thus inducing the fluorescence recovery. In the process, the existed EDTA and Cd2+ hardly affect the PL of FITC. Under optimal conditions, the proposed system could detect Cd2+ with a low detection limit of 2.97 μg/L in the range of 10-1200 μg/L. Furthermore, compared with conventional fluorescence sensors, this purposed ratiometric sensor has highly sensitivity, selectivity and rapid detection of trace Cd2+ in complex water samples.

References

[1] Li, L., L. Liao, Y. Ding and H. Zeng (2017). "Dithizone-etched CdTe nanoparticles-based fluorescence sensor for the off–on detection of cadmium ion in aqueous media." RSC Advances 7(17): 10361-10368.doi:10.1039/c6ra24971

Figure 1