Nickel oxide (NiO) thin films prepared by glancing angle
deposition
(GLAD) were investigated as electrodes for enzymatic electrochemical
quantification of xanthine, a noted indicator of meat freshness. The
large surface area of the macroporous GLAD NiO electrodes provided
a suitable scaffold to successfully immobilize the enzyme xanthine
oxidase (XO), and this XO immobilization was characterized by cyclic
voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron
spectroscopy. The XO-modified GLAD NiO electrodes electrochemically
oxidize xanthine, with electron transfer from this adsorbed XO to
the (Ni2+/Ni3+) redox species resulting in a
strong amperometric response to xanthine in a reagent-free alkaline
medium. Under optimal conditions, the fabricated xanthine biosensor
exhibited a rapid response (∼7 s), wide dynamic range (0.1–650
μM), good reproducibility (relative standard deviation of ∼4%, n = 18), superior limit of detection (37 nM), and very high
sensitivity (1.1 μA·μM–1·cm–2 in the low concentration range from 0.1–5
μM and 0.3 μA·μM–1·cm–2 in the higher concentration range from 5–650
μM). The biosensor was also evaluated against a selection of
potential interferents commonly found in fish samples (hypoxanthine,
uric acid, glucose, and sodium benzoate), demonstrating good selectivity
toward xanthine. A low Michaelis–Menten constant (K
m) of 0.4 mM for xanthine signifies the high affinity
of the enzymatic sensor toward the target analyte. Measurements in
real fish samples were also successfully performed, revealing strongly
increased xanthine sensitivity in the presence of fish matrices (from
0.085 μA μM–1 without fish extract to
as much as 0.27 μA μM–1).