My question is that does sodium azide in blocking solution or PBS is allowed for Lateral Flow Test development? Does it interact with colloidal gold particles or not?
Sodium azide is typically used as a preservative and anti-bacterial agent in different buffers for lateral flow immunoassays. At the normal concentrations used (~0.02%) it should have very little impact on the LFA results. It could be an issue if it is in your antibody solution and you are trying to perform EDC-NHS covalent conjugation to make your gold particle-antibody conjugates. PBS is also very commonly used in running/blocking buffers for the lateral-flow assay. Neither of these alone are sufficient for to form an effective blocking buffer but are common components. Blocking buffers should include some combination of proteins, polymers, and/or surfactants which will help prevent nonspecific binding on the assay. In the end however, there is no one size fits all solution to lateral flow assay buffers and all components should be tested at different concentrations to find the optimal conditions for your particular assay.
Sodium azide is a potential anti-bacterial agent in blocking buffer. Especially AuNPs are modified with binding covalently antibody, later antibody modified AuNPs are dissolved in blocking buffer before applying LFIA.
Zhang, Ming-Zhou, Min-Zi Wang, Zong-Lun Chen, Jie-Hong Fang, Mei-Ming Fang, Jun Liu, and Xiao-Ping Yu. "Development of a colloidal gold-based lateral-flow immunoassay for the rapid simultaneous detection of clenbuterol and ractopamine in swine urine." Analytical and bioanalytical chemistry 395, no. 8 (2009): 2591.