2.1. Colorimetric assay for detection of Brucella using gold nanoparticles
Gold nanoparticles are one of the highly used nanoparticles in the development of biosensors. These nanoparticles possess useful optical properties such as large surface area to volume ratio and stability at high temperatures. The optical properties of gold nanoparticles depend on the size of the particles or the distance between them. So, aggregation of gold nanoparticles will alter their optical properties (Amini, Kamali, Salouti, & Yaghmaei, 2018; Emameian, Ahangari, Salouti, & Amirmozafari, 2020; Shahbazi et al., 2018). The proper optical attributes make these nanoparticles excellent optical labels to increase sensitivity of optical biosensors (Eltzov & Marks, 2016). Based on the physico-chemical properties of gold nanoparticles (AuNP), Sattarahmady et al. developed a colorimetric method to detect genomic DNA ofBrucella melitensis in clinical samples. To do this, AuNP-DNA probes were used to investigate a specific DNA sequence of B. melitensis , BMEII0466, which resulted in the color change of the solution from red to purple after hybridization in samples containingB. melitensis genomic DNA. One advantage of this method was the visibility of color shift by the naked eye (Sattarahmady et al., 2015).
In another research, Pal et al. reported a new diagnostic test forBrucella , on the basis of hybridization assay using gold nanoparticles, in which the color of the solution was changed following successful detection of a specific DNA sequence of the pathogen by the nanobiosensors. They designed a thiol-modified probe which was specific for an outer membrane protein. Their results showed that the AuNP-oligo probe could be utilized for the rapid detection of Brucella in samples of bovine origin. The detection scheme is given in figure 1 (Pal et al., 2017).