A sulfite oxidase (SOX) (EC 1.8.3.1) purified from Syzygium cumini leaves was immobilized onto carboxylated
gold coated magnetic nanoparticles (Fe3O4@GNPs) electrodeposited onto the surface of a
gold (Au) electrode through N-ethyl-N-(3-dimethylaminopropyl) carbodiimide (EDC)-N-hydroxy succinimide
(NHS) chemistry. An amperometric sulfite biosensor was fabricated using SOX/Fe3O4@GNPs/Au
electrode as working electrode, Ag/AgCl as standard and Pt wire as auxiliary electrode. The working electrode
was characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Cyclic Voltammetry (CV),
Scanning Electron Microscopy (SEM) and Electrochemical Impedance Spectroscopy (EIS) before and after
immobilization of SOX. The biosensor showed optimum response within 2 s when operated at 0.2 V (vs.
Ag/AgCl) in 0.1 M Tris–HCl buffer, pH 8.5 and at 35 ◦C. Linear range and detection limit were 0.50–1000 M
and 0.15 M (S/N = 3) respectively. Biosensor was evaluated with 96.46% recovery of added sulfite in red
wine and 1.7% and 3.3% within and between batch coefficients of variation respectively. Biosensor measured
sulfite level in red and white wines. There was good correlation (r = 0.99) between red wines sulfite
value by standard DTNB (5,5-dithio-bis-(2-nitrobenzoic acid)) method and the present method. Enzyme
electrode was used 300 times over a period of 4 months, when stored at 4 ◦C. Biosensor has advantages
over earlier biosensors that it has excellent electrocatalysis towards sulfite, lower detection limit, higher
storage stability and no interference by ascorbate, cysteine, fructose and ethanol.