Fabrication of electric field-regulated electrochemical sensing device with laser-induced graphene for heavy metal detection

Heavy metal pollution is one of the most serious risks to sustainable agriculture in China. Among them, mercury (Hg) and lead (Pb) are two of the most hazardous heavy metals. Typically, Hg²⁺ and Pb²⁺ are believed to threaten the growth of crops and the quality of agricultural products, when entering the farmland via atmospheric sedimentation and irrigation. As a result, it is urgent to develop efficient devices for the accurate, rapid and on-site detection of Hg²⁺ and Pb²⁺ in farmland. In this work, a functional nucleic acid (FNA)-based ratiometric and electrochemical device was developed to couple the microcontroller unit (MCU) voltage application and laser-induced graphene (LIG) electrode array. A series of experiments were carried out to verify the feasibility of this device for the Hg²⁺ and Pb²⁺ detection in farmland. The electric field was designed to improve the analytical performance. Briefly, the 3.0V electric field was applied to push the Hg²⁺ and Pb²⁺ approaching FNA, while retaining the orientation of FNA at the surface of the LIG electrode; The 2.5 V electric field was also used to collect the signals, where the FNA orientation was remained to provide a constant current as a reference signal. The single-chip microcomputer (AT89S52) was selected as the main control chip on the applied voltage, while the DAC0832 was the output port of voltage. The electric field generator was developed to control the output voltage using a single-chip microcomputer. The programmable output of voltage was realized to verify the stability of voltage signal output by oscilloscope. The electric field was utilized to enhance the preconcentration of heavy metals. The output voltage and time were directly displayed on the LCM display. LIG electrode array was applied to fabricate the FNA-based biosensor. The CO₂ laser direct writing was utilized to prepare the LIG electrode. One step free of any treatment was realized to prepare the working, reference, and counter electrodes. The large surface area and high conductivity of graphene allowed for the high performance of the as-prepared sensor. The epoxy resin transfer was also developed to efficiently fabricate the LIG electrode array. The LIG-based reference electrode showed higher stability and reliability, compared with the commercial Ag/AgCl ones. The ferrocene (Fc)-labeled FNA for Hg²⁺, and methylene blue (MB)-labeled FNA for Pb²⁺ were bound to the anthraquinone-2-carboxylic acid (AQ)-labeled complementary DNA (cDNA), in order to assemble the sensing interface at the LIG electrode array. Double-stranded DNA (dsDNA) was obtained to assemble on the LIG electrode array. Then, the MCU voltage module and LIG electrode array were combined to assemble the detection device. And the currents of MB (I_MB), Fc (I_Fc), and AQ (I_AQ) were recorded by the electrochemical workstation. The ratios of I_AQ/I_Fc and I_AQ/I_MB increased at the higher concentration of Hg²⁺ and Pb²⁺, thus representing the response signal for the detection of Hg²⁺ and Pb²⁺, respectively. Consequently, the higher limits of detection were achieved in 8.5×10⁻¹² mol/L (4.25×10⁻¹¹ mg/L) and 4.6×10⁻¹³ mol/L (2.22×10⁻¹² mg/L) (S/N=3) for Hg²⁺ and Pb²⁺, respectively. The linear ranges of this device were 1.0×10⁻¹¹-1.0×10⁻⁸ mol/L (5×10⁻¹¹-5×10⁻⁸ mg/L), 1.0×10⁻¹²-1.0×10⁻⁹ mol/L (4.8×10⁻¹²-4.8×10⁻⁹ mg/L) for the detection of Hg²⁺ and Pb²⁺, respectively. Moreover, the high selectivity was realized in the field of accurate detection of Hg²⁺ and Pb²⁺. This device was successfully used to analyze the water for agricultural irrigation. At last, the reliability of the detection was also validated by the gold standard of inductively coupled plasma mass spectrometry (ICP-MS). The findings can offer scientific information to monitor and remediate heavy metal pollution.