Development of gold nanoparticles biosensor for ultrasensitive diagnosis of foot and mouth disease virus

Fig. 1. High resolution transmission electron microscope of the naked AuNPs and AuNPs-FMDV biosensor: A the electron microscopy image showing 13 nm naked AuNPs. B, C The electron microscopy image showing slight increase in AuNPs size 17–20 nm with grey conjugation zone and forming of AuNPs-FMDV biosensor

Fig. 2. UV–Vis–NIR spectrophotometer of the naked AuNPs and AuNPs-FMDV biosensor: curve A the UV–Vis–NIR spectrophotometer in the visible region image showing one absorption peak of 520 nm representing 13 nm of naked AuNPs. Curve B the UV–Vis–NIR spectrophotometer in the visible region image showing one absorption peak of 260 nm representing free oligonucleotides. Curve C the UV–Vis–NIR spectrophotometer in the visible region image showing two absorption peaks one for AuNPs at 520 nm and the other was for conjugated oligonucleotides at 260 nm forming of AuNPs-FMDV biosensor

Fig. 3. The dynamic light scattering of the naked AuNPs and AuNPs-FMDV biosensor: curve a demonstrates a peak of size distribution by number representing 13 nm of naked AuNPs. Curve b demonstrates a peak of size distribution by number showing slight increase in size due to the conjugation process representing 17–20 nm AuNPs-FMDV biosensor

Fig. 4. The zeta potential of the naked AuNPs and AuNPs-FMDV biosensor: curve a illustrates a peak of zeta potential distribution representing surface charges (− 38.9 m V) of naked AuNPs. Curve b illustrates a peak of zeta potential distribution representing slight decrease in AuNPs surface charge (from − 38.9 to − 18.3 m V) due to the conjugation process and forming of AuNPs-FMDV biosensor (b)

Fig. 5. The optical density of AuNPs-FMDV biosensor before and after PCR using UV–Vis spectrum for validation of AuNPs-FMDV biosensor PCR condition: The peaks of absorbance–wavelength of AuNPs after PCR (blue curve) have little variety from the peaks of absorbance–wavelength of AuNPs before PCR (yellow curve), both have the same peak at 520 nm of absorbance–wavelength with slight disappearance of immobilized primer peak (260 nm) due to consuming of primers in PCR reaction and AuNPs peak not affected before or after PCR

Fig. 6. The amplification plot of FMDV isolates with classical rRT-PCR reagents and AuNPs-FMDV biosensor: the amplification plot is showing amplification of FMDV isolates with classical rRT-PCR reagents (yellow and violet) curves with CT values 7.03 and 15.9 and with AuNPs-FMDV biosensor (red and blue) curves with CT values 4.09 and 12.93

Fig. 7. The standard and amplification curve of rRT-PCR assay using AuNPs-FMDV biosensor (400 nM primer): analytical sensitivity, dynamic range and limit of detection (LOD) of rRT-PCR assay using AuNPs-FMDV biosensor (400 nM primer) with serial dilution RNA standard of FMDV 3D gene show Eff. % = 94.5% and R2 = 0.989 and slope = − 3.544 standard curve (a) and amplification plot (b)

Fig. 8. The amplification plot with CT values of the field samples with AuNPs-FMDV biosensor: Red curves represent field samples with AuNPs-FMDV biosensor with CT value 12.29, 13.54, 26.48, 27.44, 14.26, 21.02, 15.89, 24.74. Green curves represent field samples with classical rRT-PCT reagent with CT values 14.1, 17, 29.66, 29.82, 19.32, 24.03, 17.6, 26.82