Stimuli-responsive block copolymers as pH chemosensors by fluorescence emission intensification mechanism

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

Colloidal Polymer Nanoparticles as Smart Inks for Authentication and Indication of Latent Fingerprints and Scratch

An environmentally friendly smart ink was developed by incorporating fluorescein into functionalized poly(methyl methacrylate) (PMMA) nanoparticles synthesized using an emulsifier-free emulsion copolymerization approach. The functional comonomers of 2-(dimethylamino)ethyl methacrylate (DMAEMA), acrylamide, hydroxyethyl methacrylate, and glycidyl methacrylate in 10 wt % with respect to methyl methacrylate were used to obtain the functionalized colloidal PMMA nanoparticles. Functional groups of the latex nanoparticles were characterized by Fourier-transform infrared spectroscopy. Field emission scanning electron microscopy results showed that all of the latex nanoparticles have nearly spherical morphologies with variations in size and surface smoothness due to the presence of different comonomers. Ultraviolet-visible and fluorescence spectra indicated that the fluorescein-doped latex nanoparticles containing the DMAEMA comonomer had the highest absorbance and fluorescence intensity. In the alkaline media, fluorescein turns to a dianion, showing a red shift and increased absorbance in the UV-vis spectroscopy. In addition, the electron inductive characteristics of the tertiary amine groups result in enhancing the conjugation of fluorescein molecules and increasing the fluorescence intensities. Therefore, the colloidal nanoparticles with amine functional groups were used in the formulation of a smart ink with applications in securing documents and fingerprints, encrypting banknotes and money, detecting latent fingerprints, crafting anticounterfeiting paper, and eventually providing optical detection and indication of surface scratches.

Fabrication of Polymer Membrane Enzyme Reactors with a Dual-Responsive Feature for Cascade Enzyme Reaction Regulation

Enzymes are known to drive biological processes efficiently and uniquely. However, the integration of multienzymes and immobilization technology to improve the stability of enzymes and boost the catalytic efficiency in cascade reactions by controlling the biocatalysis process in complex conditions via an external stimulus is still facing a significant challenge. Herein, we demonstrate a concept for cascade enzyme reaction regulation by controlling the catalytic efficiency step by step based on a dual-stimuli-responsive porous polymer membrane enzyme reactor (DPMER). Poly(styrene-maleicanhydride-acrylic acid-4-[(4-methacryloyloxy)phenylazo] benzoic acid) was designed for the fabrication of the DPMER with glutaminase and alanine aminotransferase as the model modified enzymes. Under UV light irradiation and by regulating the solution pH, the cascade enzyme reaction was carried out step by step. The polymer membrane surface demonstrated a configuration change while improving the enzymolysis efficiency of the DPMER. The enzymatic kinetics of the DPMER was investigated by a chiral capillary electrophoresis technique, and the cascade enzyme reaction regulation capability was evaluated. Under pH of 4.9 and 365 nm UV irradiation, the poly(4-[(4-methacryloyloxy)phenylazo] benzoic acid) and poly(acrylic acid) moieties changed from a "stretched state" to a "curled state" to form surface nanopores, which embedded the enzymes into the surface nanopores while causing the spatial confinement effect and enhancing the enzymolysis efficiency of the DPMER by 9.9-fold in comparison with the free enzymes. This concept provided a potential platform for cascade enzyme reaction regulation and highlighted the perspective of stimuli-responsive polymers.