A straightforward method for quantification of vinyl functionalized water soluble alginates via 13C-NMR spectroscopy

Form-Factor Control of Alginate Aerogels Via Thixotropic Sols: From Monoliths to Fibers to Films

Producing homogeneous alginate aerogel monoliths is challenging due to the uncontrollably fast gelation of sodium alginate (NaALG) solutions, which is typically induced with metal ions. This issue is overcome by decoupling molding from gelation. Aqueous NaALG solutions are first converted into thixotropic liquids via in situ acidification with acetic acid (AcOH) generated gradually through the hydrolysis of acetic anhydride (Ac2O), thus, providing time for casting in molds. The resulting solid-like thixotropic liquids are rigidized into wet gels conforming to the molds by a membraneless dialysis process via nonsolvent-induced phase separation, treatment with strong acids (HCl), or aqueous metal ion solutions (Ca2+, Cu2+, Fe3+, Ni2+, Ag+, Au3+). The thixotropic nature of the NaALG/Ac2O mixtures enables extrusion into fibers or spreading into films, followed by rigidization using the same methods. Wet gels in monolithic, fibrous, or film form were dried into aerogels with supercritical fluid (SCF) CO2. All aerogels were characterized by SEM, N2-sorption porosimetry, IR spectroscopy, solid-state CP MAS 13C NMR, and elemental analysis. Oscillatory rheology tracked the transition of NaALG to thixotropic liquids during Ac2O and AcOH titrations run in parallel. Thixotropic liquids from both titrations were rigidized with the nonsolvent-induced phase separation method and were processed into aerogels establishing that AcOH from the hydrolysis of Ac2O converts up to ∼30% mol/mol of NaALG to alginic acid (ALGH). Conversion of the remaining NaALG in thixotropic liquids to ALGH (>95% mol/mol) requires a strong acid (HCl), while rigidization with metal ions just replaces residual Na. Dynamic mechanical analysis (DMA) showed nearly identical storage moduli (E') for wet gels and aerogels, confirming that the solid network is formed during the rigidization step and is unaffected by subsequent processing. Notably, elemental analysis, porosimetry, and DMA data indicated that gels rigidized with Fe3+ included a secondary network, which, based on literature Mössbauer reports, is assigned to sol-gel-derived iron oxide.

Effect of low aspect ratio one-dimensional nanoparticles on properties of photocrosslinked alginate nanocomposite hydrogels

One-dimensional nanoparticles such as fibers, wires, represent an exciting type of filler for obtaining nanocomposite hydrogels due to their high aspect ratio. Unlike continuous fibers, in composites with short fibers, stress transfer occurs not only at the cylindrical surface of the fiber but also at the fiber ends. This work aimed to study the influence of the length of low aspect ratio one-dimensional nanoparticles on the rheological, swelling and thermal properties of nanocomposite hydrogels. We synthesized nanocomposite hydrogels via photopolymerization of methacrylated alginate macromonomers and aluminum oxide nanoparticles, nanowires and nanofibers as fillers. The main difference between the nanoparticles lies in their length. Longer nanoparticles (nanofibers) provided a glassier structure and a more significant reinforcement to the hydrogel, for example, at the concentration of 0.5 wt% the complex modulus increased approximately 2-fold and 8-fold for nanowires and nanofibers, respectively. Similarly, the incorporation of nanofibers produced hydrogels with a lower swelling capacity (2.5-fold decrease), which was attributed to a more connected network structure due to the higher aspect ratio of the nanofibers. Finally, both nanoparticle types produced an increase in the activation energy of thermal degradation.

13C NMR detection of non-protein nitrogen substance adulteration in animal feed

Illegal adulteration of melamine in animal feed and food has been widely studied. However, the risk of using substitute non-protein nitrogen substances still exists. In this study, we developed the ¹³C NMR method for the detection of non-protein nitrogen substance adulteration in animal feed. Three compounds, i.e., urea, melamine, and biuret, were used for method development. We found that the chemical shifts of the characteristic peaks in the carbon spectra of high-nitrogen adulterants were all between 150 and 170 ppm, whereas the chemical shifts of real protein peptide bonds (-CO-NH-) were between 170 and 180 ppm, demonstrating a good distinction between non-protein nitrogen and authentic protein. The method for analyzing melamine, urea, and biuret was validated. The R² values were all above 0.99 within the calibration range of 0.05-2% (w/w). The limits of quantification of urea, melamine, and biuret were 0.0120%, 0.0660%, and 0.0806%, respectively. This method involves simple sample pretreatment and rapid detection while also providing high accuracy. All the sample information obtained by NMR detection does not require strict impurity removal. Compared with a previously reported ¹H NMR method, the developed ¹³C NMR method does not require strict moisture removal to avoid active hydrogen exchange, and the interfering peak overlap is mitigated.