Analysis of Structural Changes of pH–Thermo-Responsive Nanoparticles in Polymeric Hydrogels

The pH- and thermo-responsive behavior of polymeric hydrogels <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><msub><mrow><mi mathvariant="normal">M</mi></mrow><mrow><mi mathvariant="normal">C</mi></mrow></msub><mo>−</mo><mi mathvariant="normal">c</mi><mi mathvariant="normal">o</mi><mo>−</mo><msub><mrow><mi mathvariant="normal">M</mi></mrow><mrow><mi mathvariant="normal">A</mi></mrow></msub></mrow></mfenced></mrow></semantics></math></inline-formula> have been studied in detail using dynamic light scattering <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><mi mathvariant="normal">D</mi><mi mathvariant="normal">L</mi><mi mathvariant="normal">S</mi></mrow></mfenced></mrow></semantics></math></inline-formula>, scanning electron microscopy<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><mfenced separators="|"><mrow><mi mathvariant="normal">S</mi><mi mathvariant="normal">E</mi><mi mathvariant="normal">M</mi></mrow></mfenced></mrow></semantics></math></inline-formula>, nuclear magnetic resonance (¹H NMR) and rheology to evaluate the conformational changes, swelling–shrinkage, stability, the ability to flow and the diffusion process of nanoparticles at several temperatures. Furthermore, polymeric systems functionalized with acrylic acid <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><msub><mrow><mi mathvariant="normal">M</mi></mrow><mrow><mi mathvariant="normal">C</mi></mrow></msub></mrow></mfenced></mrow></semantics></math></inline-formula> and acrylamide <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><msub><mrow><mi mathvariant="normal">M</mi></mrow><mrow><mi mathvariant="normal">A</mi></mrow></msub></mrow></mfenced></mrow></semantics></math></inline-formula> were subjected to a titration process with a calcium chloride <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><msub><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">a</mi><mi mathvariant="normal">C</mi><mi mathvariant="normal">l</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></mfenced></mrow></semantics></math></inline-formula> solution to analyze its effect on the average particle diameter<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><mfenced separators="|"><mrow><msub><mrow><mi mathvariant="normal">D</mi></mrow><mrow><mi mathvariant="normal">z</mi></mrow></msub></mrow></mfenced></mrow></semantics></math></inline-formula>, polymer structure and the intra- and intermolecular interactions in order to provide a responsive polymer network that can be used as a possible nanocarrier for drug delivery with several benefits. The results confirmed that the structural changes in the sensitive hydrogels are highly dependent on the corresponding critical solution temperature <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">S</mi><mi mathvariant="normal">T</mi></mrow></mfenced></mrow></semantics></math></inline-formula> of the carboxylic (–COOH) and amide (–CONH₂) functional groups and the influence of calcium ions <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><msup><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">a</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></mfenced></mrow></semantics></math></inline-formula> on the formation or breaking of hydrogen bonds, as well as the decrease in electrostatic repulsions generated between the polymer chains contributing to a particle agglomeration phenomenon. The temperature leads to a re-arrangement of the polymer chains, affecting the viscoelastic properties of the hydrogels. In addition, the diffusion coefficients <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfenced separators="|"><mrow><mi>D</mi></mrow></mfenced></mrow></semantics></math></inline-formula> of nanoparticles were evaluated, showing a closeness among with the morphology, shape, size and temperature, resulting in slower diffusions for larger particles size and, conversely, the diffusion in the medium increasing as the polymer size is reduced. Therefore, the hydrogels exhibited a remarkable response to pH and temperature variations in the environment. During this research, the functionality and behavior of the polymeric nanoparticles were observed under different analysis conditions, which revealed notable structural changes and further demonstrated the nanoparticles promising high potential for drug delivery applications. Hence, these results have sparked significant interest in various scientific, industrial and technological fields.