Au@SnO2-vertical graphene-based microneedle sensor for in-situ determination of abscisic acid in plants

Highlights: • Graphene was grown vertically on Ta wire by CVD with SnO₂ as an Au catalyst carrier. • Vertical graphene (VG) sheets were decorated with core-shell Au@SnO₂ nanoparticles. • Microneedle array sensors composed of Au@SnO₂-VG microelectrodes were constructed. • The sensor has a wide pH range and low detection limit for abscisic acid. • The microneedle array sensor is suitable for in situ detection of abscisic acid in plants. For developing electrochemical plant sensors, in-situ detection of hormone levels in living plants is worth attempting. A microneedle array sensor based on Au@SnO₂-vertical graphene (VG)/Ta microelectrodes was constructed for analyzing abscisic acid (ABA) in plants. Graphene was vertically grown on Ta wires with a diameter of 0.6 mm by direct current arc plasma jet chemical vapor deposition with SnO₂ as the Au catalyst carrier. These VG nanosheets were embedded with core-shell Au@SnO₂ nanoparticles, and the formation mechanism of the sensing layer was investigated. Three Au@SnO₂-VG microelectrodes, one Ti wire, and one Pt wire were packed into a microneedle array sensor with a three-electrode system. ABA was then quantitatively detected by direct electrocatalytic oxidation, which involves the synergistic catalytic effects of the abundant catalytic active sites of the Au@SnO₂ nanoparticles and the excellent conductivity of the VG nanosheets. The microneedle array sensor responds to ABA in the pH range 4–7, the response concentration range was 0.012 (or 0.024)–495.2 μM, and the detection limit varied between 0.002 and 0.005 μM. The small size, wide pH range, low detection limit, and wide linear concentration range allow the microneedle array sensor to be inserted into plants for in-situ detection of ABA.