This work describes the formation of silver-coated gold nanoparticles (Ag-AuNPs) using the seeding-growth method. 15 nm AuNPs seeds were synthesized using the citrate reduction method. In the growth stage, the adding sequence of seeds, ascorbic acid (AA), gold chloride (HAuCl4) and silver nitrate (AgNO3) was explored. The effect AgNO3volume (20 mM) was varied at 0.10, 0.25, 0.50, 1.00, 1.25 and 1.50 ml. Morphology of Ag-AuNPs was observed using a Transmission Electron Microscope (TEM) while UV-Vis Spectrophotometer was used to study the concentration and absorption spectra of colloidal Ag-AuNPs. Zeta-sizer analysis was used to study the particle size distribution of Ag-AuNPs in colloidal form. In the growth stage, the optimum adding sequence was found by adding AA as a reducing agent into AuNPs seeds followed by the addition of HAuCl4and AgNO3. This reshuffle sequence is chosen due to the presence of two absorbance peaks at 398 nm and 501 nm compared to others. From TEM images, increasing concentration of silver ions affected the optical properties, end size of Ag-AuNPs as well as raises the concentration of the colloids suspension. The biological properties of samples Ag-AuNPs (0.1 ml to 0.5 ml AgNO3) were studied by performing the conjugation process between samples and antibody; goat anti-human IgA (GaHIgA). An additional washing process was required to perform conjugation for Ag-AuNPs in order to remove by-product or unreacted chemical produced during synthesis process. The Ag-AuNPs was successfully conjugated with GaHIgA and required a minimum concentration of antibody (9µg/ml) after washing process. In addition, the washed Ag-AuNPs also improved its binding capability with GaHIgA by giving higher intense signal when tested with lined Human IgA on the lateral flow immunoassay (LFI) compared to un-washed Ag-AuNPs.
SERS-based lateral flow immunoassay for sensitive and simultaneous detection of anti-SARS-CoV-2 IgM and IgG antibodies by using gap-enhanced Raman nanotags
