Biomolecular Scaffold-controlled Self-assembly Of Hematite NPs And Interactions In The Presence Of Cationic Species

Nature is abundant with aromatic acids,polyphenols,and highly complex molecular ensembles of natural organic matter(NOM).The absence of structural periodicity in complex NOM minimized the possibility to obtain a distinct molecular structure-aggregate geometry relationship,when interacted with minerals particles in environmental media.Therefore,investigations on aggregate geometries produced from mineral's interactions with simple organic acids,and polyphenols provide a necessary framework to obtain a desired correlation between structural makeup and ensemble geometries.These will enable identification of the molecular signatures through pattern recognition even in a relatively complex matrix.In this endeavor,we have studied the self-assembly behavior of hematite NPs in the presence of gallic acid(GA)and tannic acid(TA),abundant in soil environment in the presence of inorganic and organic cations.We have also extended this study with complex NOM fractions including Suwannee River fulvic acid(FA),and Amherst peat humic acid(HA)to identify the spatial distribution of functional moieties within these ensembles.The molecular scaffold-assisted assembly of nanoscale hematite are pertinent specifically in coherence of soil mineral particles in primary soil aggregate formation as well as maintenance of soil organic carbon against microbial degradation.Moreover,these unique ensembles can be applied as novel geomaterials with diverse potential application.The supramolecular scaffold-controlled self-assembly of naturally occurring nanoscale hematite NPs in the presence of GA,TA,and FA exhibited formation of ordered aggregates in the presence of various cationic species.Unlike carboxylate-rich biomolecule-mediated random aggregation,the phenolic components showed significant molecular control on the aggregate growth.GA in the presence of weakly hydrated alkaline earth metal ions produced nanotoroids,which undergoes spontaneous isomerization to produce coiled helical structures.The interlocking of the nanotoroids and topological growth of the assembled structures has shown morphological similarity to DNA topoisomers with catenanes.Bending of the GA molecules adhered to hematite NPs surface due to the Gaussian curvature of the adsorbed organic molecules based on Helfrich's model contributed to such assembly formation in the presence of Mg²⁺.However,weakly hydrated cations-mediated stronger polarizability of the?-electron cloud of the aromatic rings eventually led to the formation of hematite tube structure.1D stacking of the nanotoroids eventually produced coiled-coil hematite nanotube.The interactions of the polyphenolic TA and hematite mixture in the presence of weakly hydrated divalent cations produced superlattice thin film due to screw-dislocation driven2D nucleation.Similar propagation rate of the newly generated steps near the dislocation core,and the outward movement of the initial step away from the spiral core caused 2D propagation instead of 3D growth following the Burton-Cabrera-Frank(BCF)crystal growth model.The interactions of the inorganic cations with FA exhibited a much more complex assembly formation in the presence of hematite NPs.The enhanced proportion of carboxylate ions along with phenols,and likely presence of N-containing Schiff's base species possibly contributed to the neural network-like growth of hematite NPs assembly in the presence of heavily hydrated Mg²⁺.The polyelectrolyte condensation model and asymmetric distribution of surface charge in FA possibly contributed to such assembly growth.The dendrites are produced by the helical assembly of NPs due to topology-assisted growth.Similar assembly formation was detected due to interactions of HA with organic cation.Therefore,an empirical molecular signature and aggregate geometry relationship can be established despite significant complexity of the NOMs especially with nanoscale minerals.Soil aggregate stability is extremely important regarding maintenance of agricultural productivity,prevention of soil erosion,and dissipations of contaminant ions through soil-water interface,and prevention of soil carbon degradation.However,similar surface charge of naturally abundant silica and clay colloids limit the plausibility of coherence of these particles towards stable soil aggregate formation as determined from the force vs.distance(F-D)plots using colloidal probe force microscopy between synthesized silica probe and freshly cleaved mica substrate.However,development of hematite NPs thin film on mica surface(Hm-mica)diminishes the electrostatic repulsions with silica probe,and contribute as an effective cementing agent.The amphoteric nature of the hematite NPs constituting the thin film shows strong p H dependence and limited application as an effective cementing agent.In addition,polyanionic humic acid(HA)-mediated surface modifications and alteration of the surface charge diminishes the possibility of stable soil aggregate formation as determined from the F vs D data.The naturally occurring alkaline earth metal ions although diminished the long-range electrostatic repulsions but unable to contribute substantially in adhesion behavior of the silica probe.The ionic hydration and permeability of the cations through HA-matrix encapsulating Hm-mica and membrane dehydration substantially diminished adhesion.However,a significantly higher adhesion behavior was noticed between silica probe and HA-modified Hm-mica in the presence of H-bond donor organic guanidinium(Gd⁺)cation,abundant in mussel foot proteins.development of porous nano-channels produced upon 1D stacking of the nanotoroids due HA interactions with Gd⁺has enhanced interfacial water holding capacity and subsequent rise in adhesion.This is in line with protein-assisted bridging of DNA segments.Unlike HA,TA modified hm-mica showed much higher adhesion with silica probe in the presence of inorganic cations.Weakly hydrated cation-assisted stronger polarizability of the?-electron cloud and molecular stacking and development of porous network prevented membrane dehydration and enhanced colloidal adhesion with the silica probe.The adhesion mechanism was also supported by the evaluation of thermodynamic parameters determined from the isothermal calorimetric analysis and evaluation of thermodynamic parameters.