Engaging a Battle on Two Fronts: Dual Role of Polyphosphates as Potent Inhibitors of Struvite Nucleation and Crystal Growth

Polyprotic acids often serve as effective regulators of inorganic crystallization in natural, biological, and synthetic systems. These molecules or macromolecules can function as efficient modifiers that either inhibit or promote crystal growth. Few studies report potent inhibitors of crystal nucleation owing to the presumed challenges of hindering these stochastic events; however, in this study, we have identified polyphosphates that operate as potent inhibitors of both nucleation and crystal growth of the natural mineral struvite (MgNH4PO4·6H2O), which is a common component of scale in commercial and physiological processes (e.g., water purification and kidney disease). Systematic investigation of linear and cyclic polyphosphates of varying molecular weight reveal that micromolar concentrations of three modifiers, hexametaphosphate, polyphosphate type 45, and phytic acid, lead to suppressed struvite nucleation. Combined studies of struvite growth at the macroscopic level using microfluidics to track anisotropic rates of crystallization and at the molecular level using in situ atomic force microscopy (AFM) to monitor layered growth of crystal surfaces collectively reveal that these modifiers are also capable of fully suppressing crystal growth via their affinity to interact with all principal crystal facets. Time-resolved AFM images of the (101) surface at varying polyphosphate concentrations reveal a unique mode of crystal growth inhibition, wherein surfaces become laden with an amorphous layer that leads to roughened interfaces and growth succession through dynamic sequences that are not commonly witnessed for other minerals. It is also observed under certain conditions that inhibitors can induce significant changes in crystal morphology, leading to exquisite leaflike hierarchical structures. The discovery of potent inhibitors of struvite formation that operate by unparalleled dual modes of action capable of suppressing both nucleation and crystal growth has potential implications for their use in the prevention of commercial scale in water purification as well as in the treatment of kidney stones. Moreover, the fundamental mechanisms of polyphosphates identified in this study may prove to be relevant for other inorganic crystals given the ubiquitous use of inhibitors in processes spanning from biomineralization to crystal engineering.