Zn-Modified MgO Nanoparticle-Decorated Carboxyl-Functionalized SWCNTs for Attomolar-Level Electrochemical DNA Biosensing

This study aimed to develop a rapid, ultrasensitive, and selective electrochemical DNA biosensor for ultra-trace detection of Salmonella typhimurium by engineering a nanocomposite interface capable of strong signal amplification and fast hybridization kinetics. A zinc-modified magnesium oxide nanoparticle-decorated carboxyl-functionalized single-walled carbon nanotubes (Zn-MgO-cSWCNTs) nanocomposite was synthesized using a sol–gel auto-combustion process and integrated with carboxylated SWCNTs to generate a conductive, electroactive, and high-surface-area sensing matrix. The composite was electrophoretically deposited onto indium tin oxide (ITO) to produce a stable nanostructured platform that enabled efficient immobilization of amine-modified probe DNA. Hybridization with target DNA was monitored using methylene blue (MB) as a redox reporter, and changes in electrochemical response were evaluated through cyclic voltammetry (CV) and chronoamperometry (CA) to assess analytical performance. The biosensor exhibited a concentration-dependent increase in MB oxidation current following DNA hybridization, providing a wide linear detection range from 0 to 150 attomolar (aM) with a high sensitivity of 283 nA·aM–1. An ultralow limit of detection of 0.34 aM and a limit of quantification of 1.45 aM confirmed the strong signal amplification imparted by the Zn-MgO/cSWCNT hybrid architecture. Chronoamperometric measurements revealed a rapid response time of approximately 0.92 s, indicating efficient electron transfer and fast duplex formation at the electrode interface. The sensor demonstrated excellent selectivity toward complementary DNA over mismatched and noncomplementary sequences and retained nearly 70% of its signal after 75 days, highlighting robust long-term stability. Real-sample validation using milk-spiked samples confirmed accurate detection of extracted Salmonella genomic DNA in a complex food matrix, establishing the Zn-MgO/cSWCNT platform as a powerful and reliable tool for food-borne pathogen monitoring and bio-surveillance applications.