Increasing Sensitivity and Selectivity for Heavy Metal Sensing through Microfluidic Electrochemical Sensor
Authors: Ghanbari Mohammad Hossein , Etzold Bastian J. M. 
Published: February 2024

Introduction

Cadmium usually exists in nature as a compound and under normal environmental conditions, its small content will not affect human health. Nevertheless, with human society's development, heavy metal ions pollution has become substantially serious, and they have serious biological toxicity. Hence, it is important to develop simple, rapid, sensitive, and portable devices for Cd2+ sensing [1].
In the present work, we show how paper based microfluidic sensors improve sensitivity and limit of detection (LOD) in comparison to classical screen-printed carbon-based electrode (SPCE). Furthermore, the presented microfluidic electrochemical carbon-based sensor (μCS) do not employ critical metals and rely on graphite foil that is modified by N-doped carbon nanoonions (N-CNOs) N-CNOs.

Experimental

For the synthesis of N-doped CNOs the Kuznetsov method was employed [2]. The alignment of the component for μCS device is sketched below. After providing the analyte solution through the sponge the electrochemical detection is carried out through accumulation of the metal at the working electrode for varying times and cathodic potentials. The final detection is carried out through square wave anodic stripping voltammetry (SWASV) of the accumulated metal.

Results and discussion

To realize a sensitive measurement, some experimental conditions including the optimal amount of the employed N-doped CNOs and Nafion in the nanocomposite, deposition time and potential, and the effect of pH of supporting electrolyte were studied. By increasing the deposition time ranging from 0 to 5 min, the SPCE/N-CNOs signal was increased and after that, it remains roughly constant because the WE surface is being saturated and also the amount of analyte in the droplet was reduced due to the presence of a stagnant flow. However, for μCS/N-CNOs by increasing the accumulation time ranging from 0 to 10 min the signal was increased which shows that the electrode surface is still far from being saturated (because of the live flow). A linear concentration range from 1.0 to 100.0 μg L-
1 was achieved using SWASV. Also, the LOD and sensitivity are calculated to be 0.5 μg L-
1 and 1.02 μA µM-1 cm-2, respectively. Additionally, the method was successfully employed for measurements of Cd2+ in different real samples, which demonstrated the excellent applicability of the device for the adsorption and detection of heavy metal ions.

Conclusion

The μCS/N-CNOs offers several core advantages over SPCE/N-CNOs in terms of cost, simplicity, analysis time, and sensitivity. These unique advantages are due to the combined microfluidic configuration, 3D electrode layout, and a unique modifier. We believe our findings would have significant implications in developing other portable, fast, and cost-effective electrochemical detection platforms, such as clinical diagnosis and security inspection.

References

[1]. Fan, Guojing, Dasun Zhai, and Dongsheng Zou. "Highly sensitive electrochemical determination of cadmium (II) in environmental water based on the electrodeposited bismuth nanoparticles." Int. J. Electrochem. Sci 11 (2016): 4362-4370.
[2]. Krishnamurthy, S., et al. "In situ formation of onion-like carbon from the evaporation of ultra-dispersed nanodiamonds." Carbon 52 (2013): 145-149.

Figure 1