Quantification of the Local Protein Content in Hydrogels Undergoing Swelling and Dissolution at Alkaline pH Using Fluorescence Microscopy

Optical tracking of the heterogeneous solvent diffusion dynamics and swelling kinetics of single polymer microspheres

The diffusion dynamics of small molecules into polymer entities is crucial for driving their morphology and function, which can be applied to research fields such as optical identification, medical implantation and intelligent sensing platforms. Herein, we demonstrate a nondestructive bright-field imaging strategy to monitor and control the morphology of polymer microspheres by varying the interfacial interaction and diffusion in a penetrant bath. The nanoscale interface movement of single polymer microspheres was tracked and converted into the diameter variation during the swelling event with sub-pixel accuracy, which is consistent with the calculation using Li-Tanaka's kinetic equations. More interestingly, the solvent diffusion dynamics along different directions of one particle are heterogeneous, indicating the non-uniform internal structure of a soft confined assembly. The swelling characteristics of single polymer microspheres can be quantified by this simple imaging strategy, and the transient intermediate swelling states are captured. To model the lifetime and stabilization times of microplastic entities, solvent selectivity and thermodynamic regulation were introduced to obtain the activation energy down to the single micro-entity level. This optical methodology shows capability for decoding the complex diffusion mechanism in polymer entities and provides guidance for the design of drug delivery systems, sensor platforms, and optical responsive materials.

Recent studies on proteins and polysaccharides-based pH-responsive fluorescent materials

Polymer-based pH-responsive fluorescent materials have the characteristics of fast response, real-time monitoring, visualisation, and easy forming. Consequently, they have attracted widespread attention in wound healing, sweat monitoring, security and anti-counterfeiting, freshness detection of aquatic products, metal-ion sensing and bioimaging. This paper analyses the preparation principles and characteristics of pH-responsive fluorescent materials based on cellulose, chitosan and proteins. It then outlines the fluorescence properties, environmental response mechanisms and applications of various luminescent materials. Next, the research indicates that amines, N-heterocyclic rings, carboxyl groups and amino plasmonic groups on the fluorescent molecule structure and polymer skeleton appear to change the degree of ionisation under acid or alkali stimulation, which affects the light absorption ability of chromophore electrons, thus producing fluorescence changes in fluorescent materials under different pH stimuli. On this basis, the challenges and growth encountered in the development of proteins and polysaccharides-based pH-responsive fluorescent materials were prospected to provide theoretical references and technical support for constructing pH-responsive fluorescent materials with high stability, high sensitivity, long-lasting pH-response and wide detection range.

Real-time spatial quantification of gastric acid diffusion in whey protein gels with different NaCl concentrations by wide-field fluorescence microscopy

Gastric acid diffusion and penetration constitute an essential process in the structural breakdown and enzymatic hydrolysis of solid food during digestion. This study aimed to quantify the real-time diffusion and spatial distribution of gastric acids in whey protein isolate (WPI) gels in the presence of 0-0.05 M NaCl during simulated digestion using a wide-field fluorescence microscope. For all the gels regardless of NaCl concentration, the outer surface rapidly developed a near-saturated layer, resulting in a higher normalized gastric acid concentration in the outer layer than in the inner layer. The pH decrease was more significant for the gels at a higher NaCl concentration (i.e., 0.05 M) due to the formation of a more discontinuous and looser network structure that would facilitate acid diffusion into the gel matrix and decrease the gel buffering capacity. Consistently, the effective diffusion coefficient (D_A) estimated via the Fick diffusion model was 6.19 × 10^-10 m²/s for 0.05 M WPI-RITC gels, significantly higher than 0.015 M (4.46 × 10^-10 m²/s) and 0 M (3.54 × 10^-10 m²/s) gels. The present study has provided a quantitative understanding of the diffusion process and spatial distribution of gastric acids within the WPI gel matrix in real-time during in vitro gastric digestion as influenced by NaCl concentration.

Fluorescent Submicron-Sized Poly(heptafluoro-n-butyl methacrylate) Particles with Long-Term Stability

Fluorescent submicron particles of fluorinated methacrylate (HFMBA) with long-term stability have been synthesized and characterized with regard to their potential applications. Rhodamine B (RBITC) isothiocyanate was used as the fluorescent component. The core–shell structure of the particles effectively protected the dye against bleaching. HFBMA nanoparticle (NP) stability was confirmed after seven years of storage. Only slight differences were found in the polydispersity index (pdi) from 0.002 to 0.010. Particle size measurements were carried out using dynamic light scattering (DLS), nanoparticle tracking (NTA), and fluorescence correlation spectroscopy (FCS). The hydrodynamic diameter evaluated by different methods were in good agreement, respectively: 184–550 nm, 218–579 nm, and 236–508 nm. Particle and core morphology was estimated by using scanning and transmission electron microscopy (SEM and TEM). The ability to recognize particles in 3D as a reference sample in biological media has been confirmed by epifluorescence optical microscopy, confocal laser scanning microscopy, and super-resolution confocal microscopy (STED).