Atomic Chromium Coordinated Graphitic Carbon Nitride for Bioinspired Antibiofouling in Seawater

Abstract Artificial nanozymes exerting enzyme functionality are recognized as promising alternatives of natural enzymes in biomimetic chemistry. Natural haloperoxidases that utilize hydrogen peroxide (H2O2) to catalytically convert halide into strong biocidal hypohalous acid hold great promise for thwarting biofouling, while their practical application remains highly questionable as instability of natural enzymes and inadequate H2O2. Herein a semiconducting nanozyme consisting of chromium single atoms coordinated on carbon nitride (Cr‐SA‐CN) that performs bifunctional roles of nonsacrificial H2O2 photosynthesis and haloperoxidase‐mimicking activity for antibiofouling is constructed. Such nanozyme is capable of generating H2O2 from water and O2 upon visible‐light illumination, and then sustainably self‐supplying H2O2 for haloperoxidase‐mimicking reaction in a sequential manner. This dual‐activity Cr‐SA‐CN overcomes H2O2 dilemma and yields hypobromous acid continuously, inducing remarkable bactericidal capability. When used as an eco‐friendly coating additive, it is successfully demonstrated that Cr‐SA‐CN enables an inert surface against marine biofouling. Thereby, this study not only illustrates an attractive strategy for antibiofouling but also opens an avenue to construct valuable nanoplatform with multifunctionality for future applications.


Experimental Section
Preparation of Cr-SA-CN: For the synthesis of Cr-SA-CN, 5 mmol melamine was dissolved in 80 mL dimethyl sulfoxide followed by stirring for approximately 30 min at room temperature, then chromium (III) nitrate nonahydrate dissolved in 5 mL dimethyl sulfoxide was slowly added into melamine solution. The resulting mixture was then heated at 60 °C under continuous magnetic stirring. Subsequently, 5 mmol cyanuric acid dissolved in 50 mL dimethyl sulfoxide was dropwise slowly added to the heat solution using a syringe with the total volume approaching about 60 mL. The white precipitate was formed immediately upon the addition of cyanuric acid solution. The resulting precipitate was collected and washed with ethanol several times. The solid precursor was then calcined at 550 °C for 8 h under N 2 atmosphere. For the preparation of pristine carbon nitride, the similar procedure was used but without the addition of Cr precursor.
Photocatalytic production of H 2 O 2 : 50 mg of nanozyme was ultrasonically dispersed in 50 mL deionized water or real seawater in a 100 mL borosilicate glass bottle without the addition of sacrificial agent. Under mild magnetic stirring, O 2 was bubbled into the aqueous solution for 30 min to achieve equilibrium. The photocatalytic reaction was performed by exposing the sealed reaction system to visible light using a solar simulator (AM 1.5 G, 100 mW cm -2 , Oriel, USA) with a 420 nm cutoff filter. The reaction was maintained at 25±0.5 o C. During the photoirradiation, 2 mL solution was taken at different time intervals and filtered to remove the photocatalyst. The yield of H 2 O 2 was measured by HPLC in combination with an electrochemical analyzer or using a standard iodimetry analysis method. [29,40] For stability test, the nanozyme was recovered after each reaction by centrifugalizing and washed with deionized water, then the sample was reused for photocatalytic reaction (8 h for each cycle).
For photoelectrochemical measurements, electrochemical impedance spectroscopy (EIS) was conducted in 0.1 M KCl solution under light illumination (λ≥420 nm) in a three-electrode configuration on a Zahner electrochemical system (Zahner-Electrik GmbH&Co. KG, S-4 Germany). Linear sweep voltammetry on a rotating disk electrode (RDE) was also performed with the similar method. The Nyquist plots were obtained at a bias of 0.7 V in the frequency range of 1Hz to 100 kHz. The catalysts deposited on FTO substrates by spin coating were applied as the working electrode. Ag/AgCl and Pt wire were used as reference electrode and counter electrode, respectively. The average electron transfer number for O 2 reduction reaction was determined according to the slopes of Koutecky-Levich plots.
The Mott-Schottky plots were also investigated by using a conventional three-electrode cell. The working electrodes were prepared by depositing Cr-SA-CN photocatalysts on fluorine doped tin oxide coated glass. Ag/AgCl and Pt wire were used as the reference electrode and reference electrode, respectively. The measurements were carried out in 0.2 M Na 2 SO4 aqueous solution electrolyte (pH=7.1, 25 o C). In situ marine field tests: The real field test was performed on July-September at Haikou bay in Haikou, China. The stainless-steel plates (2 cm × 2 cm) were ultrasonically washed with detergent, deionized water, and acetone successively. The Cr-SA-CN with a dry weight of about 3.5 wt.% was added into the abrasive rosin-based formulation paint. After homogenization, the paint was applied onto the clean stainless-steel plates. The painted plates without additional additives were also used for comparison. The painted substrates were fixed to a stationary experimental raft and were immersed into seawater at a depth of ~1 m. UV-vis diffuse reflectance spectra were recorded on a Shimadzu UV-2550 spectrophotometer.

Haloperoxidase-like activity mimic and reaction kinetics:
Fourier transform infrared (FTIR) spectra of the as-prepared catalysts were obtained on an FTIR-650 spectrometer. XPS were characterized using a ESCALAB 250Xi spectrometer (Thermo Fisher Scientific Inc., USA) with an aluminum anode X-ray source. The XAFS spectra were obtained on the beamline BL01C1 in NSRRC (Shanghai, China) with a Silicon