With the above-mentioned drawbacks,

With the above-mentioned drawbacks, efforts to develop a method for S. typhi determination with increased sensitivity and selectivity and a reduction in analysis time have been proposed. Currently, alternative methods for biological molecular analysis are enzyme immunoassay [9,10], surface plasmon resonance [11], and electrochemical immunoassay [12–14]. In particular, the use of electrochemical immunoassay has attracted considerable interest for S. typhi determination because of its inherent simplicity, high sensitivity, inexpensive instrumentation, and miniaturization. Although this method has a low detection limit and could be used in the diagnosis of typhoid fever, a sensitive and rapid method for analyzing and obtaining important information about the effectiveness of therapy, follow-up treatments, the epidemic disease, and protection is still needed. With the development of nanotechnology, various nanoparticles [15,16] and nano-quantum dots [17,18] have been used as labels to enhance the sensitivity of the electrochemical immunoassay technique. Amplified electrochemical detection of biological molecules was achieved by the dissolution of the metallic nanoparticles and by recording the subsequent electrochemical stripping of the dissolved ions. Gold nanoparticles act as a class of labels with many unique features, such as optical, electronic, and catalytic properties, that have been previously explored for potential applications in biomolecular detection. Based on these advantages, colloidal gold was used as an electrochemical marker or catalytic label for nanoparticle enlargement in order to elevate the sensitivity of the bioassay.