Microbial synthesis of iron-based nanomaterials—A review

Nanoparticles are the materials having dimensions of the order of 100 nm or less. They exhibit a high
surface/volume ratio leading to different properties far different from those of the bulk materials. The development
of uniform nanoparticles has been intensively pursued because of their technological and fundamental scientific
importance. A number of chemical methods are available and are extensively used, but these are often energy inten-
sive and employ toxic chemicals. An alternative approach for the synthesis of uniform nanoparticles is the biological
route that occurs at ambient temperature, pressure and at neutral pH. The main aim of this review is to enlist and
compare various methods of synthesis of iron-based nanoparticles with emphasis on the biological method. Biolo-
gically induced and controlled mineralization mechanisms are the two modes through which the micro-organisms
synthesize iron oxide nanoparticles. In biologically induced mineralization (BIM) mode, the environmental factors
like pH, pO2, pCO2, redox potential, temperature etc govern the synthesis of iron oxide nanoparticles. In contrast,
biologically controlled mineralization (BCM) process initiates the micro-organism itself to control the synthesis.
BIM can be observed in the Fe(III) reducing bacterial species of Shewanella, Geobacter, Thermoanaerobacter, and
sulphate reducing bacterial species of Archaeoglobus fulgidus, Desulfuromonas acetoxidans, whereas BCM mode
can be observed in the magnetotactic bacteria (MTB) like Magnetospirillum magnetotacticum, M. gryphiswaldense
and sulphate-reducing magnetic bacteria (Desulfovibrio magneticus). Magnetite crystals formed by Fe(III)-reducing
bacteria are epicellular, poorly crystalline, irregular in shapes, having a size range of 10–50 nm super-paramagnetic
particles, with a saturation magnetization value ranging from 75–77 emu/g and are not aligned in chains. Mag-
netite crystals produced by MTB have uniform species-specific morphologies and sizes, which are mostly unknown
from inorganic systems. The unusual characteristics of magnetosome particles have attracted a great interdisci-
plinary interest and inspired numerous ideas for their biotechnological applications. The nanoparticles synthesized
through biological method are uniform with size ranging from 5 to 100 nm, which can potentially be used for various
applications.