Development of silver-magnetite nanocomposites for cysteine sensing and dye degradation

The development of easily recoverable and re-usable catalytic nanostructures exhibiting high

performances using simple and cheap procedures for applications in bio-sensing and

wastewater treatment technologies is currently a prominent area of research. Noble-metal

inclusive magnetic nano-catalysts in particular, have of recent gathered interest owing to their

ease of separation from reaction products, environmental benignity and improved activities

through synergistic qualities and photo-thermal effects. Herein, silver-magnetite core-shell

nanocomposites (Fe3O4@AgNPs) displaying enhanced peroxidase-like catalytic properties

were successfully synthesised via a facile one step solvo-thermal procedure. Governed by

Michaelis-Menten kinetics, the nanocomposites’ capacity to oxidize the colourless o phenylenediamine to the yellow coloured 2, 3-diaminophenazine in the presence of hydrogen

peroxide progressively improved relative to Fe3O4 alone with increasing precursor AgNO3 to

an optimum of 50 mM. Following a study and reaction optimisation conditions on the

inhibitory behaviour of cysteine; a biomarker for Alzheimer and other neurological

conditions, towards the peroxidase-like catalytic activity of these nanocomposites, a

considerably cheap, selective and highly sensitive detection method for this amino acid was

developed. With a low limit of detection of 87 nM, the formed cysteine sensor proved to be

superior to some of the existing sensors.

A simple silver mirror reaction was also employed to fabricate nano-silver decorated

magnetite (Ag@ Fe3O4) free of the often activity limiting and environmentally unfriendly

organic reagents. The nano-sized magnetite supports were synthesised using the precipitation

method. Following synthesis optimisation procedures, these composites were found to

effectively catalyse the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride

more than 8 times repeatedly. With a normalised activity parameter k of 1.44 ×10−3 L. s−1

mg−1, the synthesised catalyst performed better than some reported nanostructures.

Furthermore, the catalytic degradation methylene blue and Rhodamine 6G which are some of

the many water pollutant dyes in textile industry effluents was achieved by these

nanocomposites. Also, owing to their SERS property, Ag@ Fe3O4 particles were efficient in

real-time monitoring the catalytic degradation of p-Nitrothiophenol by NaBH4.

Characterisation of all the synthesised nanocomposites in this work was carried out via

transmission electron microscopy, scanning electron microscopy, X-ray diffraction,

Thermogravimetric analysis and surface-enhanced infrared absorption spectroscopy.