Heavy metals are ubiquitous in nature, Heavy metal ions of lead (Pb(Ⅱ)) and cadmium (Cd(Ⅱ)) have highly toxic even at trace levels. They mainly enter the human body through water sources. At present, simple water samples such as drinking water and tap water, and other domestic water are restricted by major international organizations such as the World Health Organization, the U.S. Environmental Protection Agency, and the European Union. The content of heavy metals is basically within the legal range. However, the metal content in some complex water samples such as groundwater and factory wastewater is not optimistic. The detection is susceptible to interference from a variety of substances. Therefore, it is urgent to detect the content of Pb(Ⅱ) and Cd(Ⅱ) in complex water samples accurately and efficiently.
With the development of material chemistry and nanotechnology, a sensing method that integrates two signal transduction channels, that is, dual-modal sensing technology, has emerged. It can make up for the inherent limitations of each sensing mode and integrate its unique advantages, thereby showing a certain synergistic effect on the analysis performance.
The optical analysis method has high sensitivity, strong anti-interference ability, and low detection limit, and can be used for accurate detection of trace amounts of lead and cadmium. However, the detection steps of the spectroscopic method are complicated, costly, long-period, professional operation, and bulky instruments are required, and cannot be used for rapid on-site detection of heavy metals. As an electrochemical (EC) detection technique, stripping voltammetry has the advantages of high sensitivity, good selectivity, convenient operation, portable detection equipment, and low detection cost. Moreover, it has good advantages for detecting turbid samples and is often used for on-site and rapid detection of heavy metals of lead and cadmium in water. However, the environment of the solution to be tested easily affects the detection performance of the electrode, thereby interfering with the stripping voltammetric signal of the target heavy metal, and reducing the detection accuracy of the target heavy metal.
In summary, both optical and electrochemical methods can be used for the detection of Pb(Ⅱ)(Ⅱ) and Cd(Ⅱ)(Ⅱ) in water, but the detection is restricted by many interference factors. Therefore, this research proposes a dual-mode sensor based on optics and electrochemistry to solve the problems encountered in the actual detection of Cd(Ⅱ) and Pb(Ⅱ) (Fig. 1).
Preperation of the dual model sensing
FeOOH modified gold was utilized for obtaining EC signals. In brief, through the electrodeposition method in the solution of (NH4)2Fe(SO4)2 and sodium acetate, FeOOH was modified on the activated gold working electrodes. The electrodeposition condition is under the constant potential of 0.7 V for 30 minutes. The EC sensing signal could be obtained in the presence of Pb(Ⅱ) due to the special electrode surface structure.
Etched Cd(Ⅱ)Te/Cd(Ⅱ)S QDs were utilized as the sensitive material of fluorescence(FL) sensing. The preparation process is as follows: 100 uM EDTA was mixed with 0.5mg/mL QDs with an equal volume ratio. After reacting for 15 minutes in the dark, it is used to directly detect Cd(Ⅱ) in water samples. EDTA in the sensitive material was used as the chemical etching agent, which could quench the FL of QDs. The Cd(Ⅱ) site was created during the process. In the presence of Cd(Ⅱ), the site would be recovered thus inducing the FL recovery. Also, the degree of fluorescence recovery can be used to measure the level of heavy metal content.
Results and Discussion
As shown in Fig. 2, it is difficult to detect low concentrations of Cd(Ⅱ) in complex water samples. And Pb(Ⅱ) will have an opposing influence on the detection of Cd(Ⅱ). The low sensitivity of the optical sensor will cause the overlapping phenomenon of the FL spectrum of the low concentration solutions, and the precise identification of the trace complex solution cannot be realized. In view of the limitations of optical detection for low-concentration substances in complex samples and the applicability and high sensitivity of electrochemistry to turbid samples, electrochemical sensing method that is sensitive to Pb(Ⅱ) are introduced construct dual model sensor. Machine learning is utilized to compensate for mutual interference between Pb(Ⅱ) and Cd(Ⅱ). Also, the detection of turbid samples without pretreatment is achieved, which saves time, reduces costs, and provides the possibility for rapid simultaneous detection of heavy metals on site. Eigenvalue extraction and merchant learning are utilized to eliminate the influence of low detection stability and improve repeatability.