We present a nanozyme-based biosensor fabricated from nanostructured Ni films deposited onto a silicon wafer by glancing angle deposition (GLAD) for enzyme-free colorimetric monitoring of uric acid (UA), a biomarker for gout, high blood pressure, heart disease, and kidney disease. The helically structured Ni GLAD nanozymes exhibit excellent peroxidase-like activity to accelerate the oxidation reaction of colorless 3,3′,5,5′tetramethylbenzidine (TMB) to a blue product, oxidized TMB (oxTMB), mediated by H 2 O 2 . In the presence of UA, oxTMB is reduced, decreasing the optical absorbance by an amount determined by the concentration of UA in the solution. The nanozyme not only mimics peroxidase but also possesses the notable qualities of reusability, simple operation, and reliability, making it environment-friendly and suitable for on-demand analysis. We optimized essential working parameters (pH, TMB concentration, and H 2 O 2 concentration) to maximize the initial color change of the TMB solution. The catalytic activity of this nanozyme was compared with conventional nanofilms using the Michaelis−Menten theory. Based on this, enzyme-free biosensors were developed for colorimetric detection of UA, providing a wide detection range and a limit of detection (3.3 μM) suitable for measurements of UA concentration in sweat. Furthermore, interference from glucose and urea was studied so as to explore the potential of the biosensor for use in the clinical diagnosis of UA biomarkers.

Nitrogen-functionalization is an effective means of improving the catalytic performances of nanozymes. In the present work, plasma-assisted nitrogen modification of nanocolumnar Ni GLAD films was performed using an ammonia plasma, resulting in an improvement in the peroxidase-like catalytic performance of the porous, nanostructured Ni films. The plasma-treated nanozymes were characterized by TEM, SEM, XRD, and XPS, revealing a nitrogen-rich surface composition. Increased surface wettability was observed after ammonia plasma treatment, and the resulting nitrogen-functionalized Ni GLAD films presented dramatically enhanced peroxidase-like catalytic activity. The optimal time for plasma treatment was determined to be 120 s; when used to catalyze the oxidation of the colorimetric substrate TMB in the presence of H2O2, Ni films subjected to 120 s of plasma treatment yielded a much higher maximum reaction velocity (3.7⊆10−8 M/s vs. 2.3⊆10−8 M/s) and lower Michaelis-Menten coefficient (0.17 mM vs. 0.23 mM) than pristine Ni films with the same morphology. Additionally, we demonstrate the application of the nanozyme in a gravity-driven, continuous catalytic reaction device. Such a controllable plasma treatment strategy may open a new door toward surface-functionalized nanozymes with improved catalytic performance and potential applications in flow-driven point-of-care devices.

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).