Yolk–Shell-Structured SnO₂–C and Poly-Tyrosine Composite Films as an Impedimetric “Signal-Off” Sensing Platform for Transgenic Soybean Screening

The yolk–shell-structured SnO₂–C nanospheres have been prepared by a hydrothermal reaction of SnCl₂·2H₂O and glucose, followed by carbonization under 500 °C in air conditions. Then, an inorganic/organic hybrid film bearing SnO₂–C and polytyrosine (pTyr) is fabricated by electropolymerization of the SnO₂–C-modified electrode in tyrosine solution. The modified electrode is utilized as a supporting platform for covalent immobilization of cauliflower mosaic virus 35s (CaMV35s) promoter gene fragments to construct an electrochemical DNA sensor. Chronocoulometric experiments show that the loading density of probe DNA (pDNA) and hybridization efficiency are determined to be as high as 4.54 × 10¹³ strands cm^–2 and 83.2%, respectively. Upon hybridization with target DNA (tDNA), the probe DNA that lay flat on the electrode surface through hydrogen bonding with pTyr is erected, reducing the charge repulsion and steric hindrance for [Fe­(CN)₆]^3–/4– diffusion. Therefore, a “signal-off” response strictly dependent on the hybridization reaction is achieved in electrochemical impedance spectroscopy. The response mechanism is predicted by theoretical calculation. Owing to the high probe density and hybridization efficiency of the sensor, a wide kinetic linear range of 1.0 aM to 100 pM and an ultralow detection limit of 0.53 aM for the target sequence are obtained. The biosensor also presents high recognition ability toward the DNA samples extracted from real transgenic and nontransgenic soybeans, showing great promise for facile monitoring of the transgenic product.