Factors affecting the stability of chitosan/tripolyphosphate micro- and nanogels: resolving the opposing findings

Abstract

Micro-and nanogels prepared by ionically crosslinking chitosan with tripolyphosphate (TPP) attract keen interest as potential drug carriers. To achieve their optimal performance, it is essential to control their stability to aggregation and dissolution. Yet, literature on this subject (especially at physiological ionic strength and pH) remains filled with opposing reports. Recently, one of us hypothesized that these conflicting findings might stem from: (1) variations in the chitosan molecular structure;(2) the dissimilar particle concentrations used by the various groups; and (3) occasional overreliance on dynamic light scattering (DLS) as the sole analytical tool. To explore this hypothesis, here we use isothermal titration calorimetry, light scattering and UV-Vis spectroscopy to analyze the effects of chitosan degree of deacetylation (DD) and particle concentration on chitosan/TPP particle stability. Their dissolution stability increases with the chitosan DD (due to the stronger chitosan/TPP binding) and when the particles are used at higher concentrations, and evidently depends on the chitosan molecular weight.Conversely, their aggregation stability decreases with the DD, because the chitosan is more prone to TPP bridging and becomes more lyophobic at near-neutral (physiological) pH. We also show how using DLS-derived size distributions as the sole tool for characterizing particle stability can lead to erroneous conclusions. Comparison of these findings to literature experimental conditions reconciles many of the opposing reports and provides essential guidelines for tuning chitosan/TPP particle stability.

Sodium bicarbonate‐gelled chitosan beads as mechanically stable carriers for the covalent immobilization of enzymes

“…In a similar situation, the increased ionic strength of the 150 mM NaCl solution, in which TPP‐chitosan micro‐ and nanogel particles were dispersed, caused the monovalent ions of NaCl to displace TPP. The same authors also reported the dissolution of the chitosan‐TPP particles upon increasing the pH to 7.2 …”

“…The high concentration of the NaHCO 3 (0.6M) increased the ionic strength of the gelling solution. This increased ionic strength weakened the chitosan‐TPP ionic cross‐linking . The chitosan‐TPP ionic cross‐linking could have also been weakened as a result of the increased pH of the 0.6 M NaHCO 3 ‐supplemented gelling solution.…”

“…Such high pH values would lead to the deprotonation of the chitosan's amino groups. This would reduce the chitosan's linear charge density and would weaken the chitosan‐TPP ionic interactions . Eventually, the combined actions of the increased ionic strength and elevated pH of the 0.6 M NaHCO 3 ‐supplemented gelling solution would weaken the chitosan‐TPP ionic interactions to the extent that the chitosan's amino groups would be released from their ionic interaction with TPP.…”

“…This increased ionic strength weakened the chitosan-TPP ionic cross-linking. 23 The chitosan-TPP ionic cross-linking could have also been weakened as a result of the increased pH of the 0.6 M NaHCO 3 -supplemented gelling solution. The pH of this solution was well above the pK a of chitosan (6.3) 11 throughout the entire reaction time ( Table 1).…”

Adsorptive removal of Ni(ii) ions from aqueous solution and the synthesis of a Ni-doped ceramic: an efficient enzyme carrier exhibiting enhanced activity of immobilized lipase

“…Over the past few decades, a variety of strategies have been developed for removing heavy metals from wastewater and soil; however, some of these are limited due to issues with subsequent applications. 12 These nanoparticles often show wide biocompatibility and stability in a number of applications, such as the separation of heavy metals. 5,6 For the purpose of Ni 2+ removal, multiple technologies have been employed, such as chemical precipitation, 7 adsorption, 8 coagulation, 9 electrolysis, 10 and ion-exchange, 11 although these processes require complicated devices, and the carriers cannot be recycled for further applications.…”