A novel approach for qualitative biosensing of anti-leptospiral IgG antibodies in human sera using functionalised silver nanoparticles

Nanoparticles can interact with biological molecules, thus broadening the applications in the field of medicine. Surface plasmon resonance of nanoparticles in the ultraviolet-visible (UV-visible) region can be applied to identify nanoparticle-protein interactions. This property is used to detect antibodies against leptospirosis, which is an emerging infectious zoonosis in many developing countries and endemic in Sri Lanka. A rapid screening test to detect anti-leptospiral antibodies in human sera by analysing antigen-antibody interactions using UV-visible spectroscopy is reported. The protocol uses hydroxylamine hydrochloride stabilised silver nanoparticles followed by functionalisation with rLigA (recombinant leptospiral immunoglobulin-like protein A) antigens. rLigA-functionalised silver nanoparticles were characterised by UV-visible spectroscopy, Dynamic Light Scattering (DLS), Fourier Transform Infra-Red (FT-IR) spectroscopy and Scanning Electron Microscopy (SEM). A monodispersed solution of LigA-functionalised silver nanoparticles was prepared at 25 °C and 7.0 pH with optimum LigA concentration of 1.5 µg mL^-1. This localised surface plasmon resonance (LSPR) biosensor shows a positive response to anti-leptospiral IgG antibodies at optimum conditions in dilutions lower than 1:25 (SD = ± 0.047; 95 % CI = 1.100–1.126). The dilutions of 1:300 to 1:50 IgG antibodies maintained no response compared to nanoparticle conjugate. Furthermore, the non-responsive UV-visible spectra of nanoparticle conjugate with anti-leptospiral antibody negative serum samples confirmed that the conjugate was specific to anti-leptospiral antibodies (SD = ± 0.012; 95 % CI = 1.177–1.183). This novel technique offers  substantial improvement in terms of screening time, which does not take more than 30 minutes, compared to conventional methods, such as ELISA, which consumes 2-3 hours.