The bacterial loading of liquid media, either environmental or biological ones, could be detected through various methods, based on microorganism features as well as on the interactions developed with chemical or physical factors. Since in previous studies the magnetite nanoparticle ability of changing the intensity of the fluorescent signal released by Pseudomonas aeruginosa cells was revealed, the influence of magnetite nanoparticle surface on the green-bluish emission of bacterial cells was chosen as the target of the present investigation.

Magnetite nanoparticles were prepared by co-precipitation method being further stabilized in aqueous suspension by coating with organic shells. Two types of magnetite core stabilization were accomplished: electrostatic – based on iron ions interaction with tetramethyl ammonium hydroxide (TMA-OH) and steric – by means of sodium oleate coating. Average crystallite diameter was evaluated (by X-ray diffractometry) at 13.6 nm (for magnetite core/TMA-OH shell) and respectively 12.13 nm (for magnetite core/sodium oleate shell), while saturation magnetization was found of about 24 emu/g, and respectively, 49 emu/g.

The P. aeruginosa bacteria were proved to present increased fluorescent signal for magnetite suspension in the culture medium of the order of magnitude of microl/l, so that the surface modified magnetite particles could be taken as probes in the basic structure of a chemical sensor for bacterial loading detection. Our experiments consisted in the recording of fluorescent emission as well as of the turbidity of P. aeruginosa samples with cell density ranging from 10⁸ cell/ml up to 10² cell/ml - the latest representing the level of the possible microbial contamination of injectable antibiotic drugs. The same range of inoculum cell density was spectrofluorimetrically investigated in the presence of coated magnetite suspension – the influence of the coating shell on the fluorescent signal being discussed. The sensitivity of the proposed chemical sensing system was discussed related to the slope of the fluorescent signal intensity versus inoculums density.

The molecular background of the interaction between the colloidal magnetite and the microbial cells is given by the biosynthesis of the fluorescent siderophore named pyoverdine –stimulated for low iron loading of the environment due to the bacteria behavior as iron scavenger. Though the cellular mechanisms of iron internalization could not be elucidated from the data acquired in the frame of this experimental study, however it was evidenced the possibility of designing of a chemical sensing device able to detect small contamination level with the fluorescent bacteria P. aeruginosa of antibiotic products, biological specimens and environmental samples. The chemically modification of the magnetite nanoparticle surface could provide the means of sensor sensitivity monitoring according to the contamination level of the tested bacterial samples.

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