Morphology evolution of functionalized styrene and methyl methacrylate copolymer latex nanoparticles by one-step emulsifier-free emulsion polymerization

Smart Polymer Microspheres: Preparation, Microstructures, Stimuli-Responsive Properties, and Applications

Smart polymer microspheres (SPMs) are a class of stimulus-responsive materials that undergo physical, chemical, or property changes in response to external stimuli, such as temperature, pH, light, and magnetic fields. In recent years, their diverse responsiveness and tunable structures have enabled broad applications in biomedicine, environmental protection, information encryption, and other fields. This study provides a detailed review of recent preparation methods of SPMs, focusing on physical methods such as emulsification-solvent evaporation, microfluidics, and electrostatic spraying as well as chemical approaches such as emulsion and precipitation polymerization. Meanwhile, different types of stimulus-responsive behaviors, such as temperature-, pH-, light-, and magnetic-responsiveness, are thoroughly examined. This study also explores the applications of SPMs in drug delivery, tissue engineering, and environmental monitoring, while discussing future technological challenges and development directions in this field.

Stimuli-Chromic Oxazolidine Latex Nanoparticles for Dual-Responsive pH-Sensors and Rewritable Halochromic Papers: A Physicochemical Study on Colorimetric and Fluorimetric Signals

Oxazolidine is a new category of stimuli-chromic compounds that has unique intelligent behaviors such as halochromism, hydrochromism, solvatochromism, and ionochromism, all of which have potential applications for designing and constructing chemosensors by using functionalized-polymer nanocarriers. Here, the poly(MMA-co-HEMA) based nanoparticles were synthesized by emulsion copolymerizing methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) in different copolymer compositions. The poly(MMA-co-HEMA) based nanoparticles were modified physically with tertiary amine-functionalized oxazolidine (as an intelligent pH-responsive organic dye) to prepare halochromic latex nanoparticles. Investigation of optical properties including absorbance and emission by spectroscopic methods indicates that the halochromic behavior of the oxazolidine in nanoparticles is influenced significantly by the particle size, morphology, and concentration of hydroxyl groups. To develop optical chemosensors for the detection of pH, the pH-responsivity of halochromic latex nanoparticles in aqueous solutions with different pHs in the wide range of 1-14 were studied by UV-vis and fluorescence spectroscopies, and these results confirmed the successful photodetection of pH in a fast and facile manner. The investigation of the solid-state optical properties and pH-responsivity of halochromic nanoparticles by impregnation of latex-coated cellulosic papers with solutions having different pHs indicates the halochromic nanoparticles maintained their optical properties after incorporation to cellulose matrix and displayed notable colorimetric and fluorimetric pH-responsivity. Hence, the paper-based pH-sensors were prepared from halochromic papers and the investigation of their pH-responsivities showed the halochromic papers constructed from halochromic nanoparticles with HEMA concentration above 20 wt % have the best pH-responsivity with high resolution, high contrast, and high-intensity color change and fluorescence emission change. In addition, the halochromic papers were used as rewritable hydrochromic papers for hand-writing and stamp-printing by using acid and base solutions as inks, in which papers based on halochromic nanoparticles with a HEMA concentration above 20 wt % have maximum printability, resolution, and intensity. This study proposed the significant effects of polarity and concentration of functional groups on the halochromic properties of oxazolidine molecules and the unique role of functionalized polymer nanoparticles as a carrier of oxazolidine for its protection toward environmental degradations. These parameters should be considered in future studies on the development of halochromic papers for intelligent pH-sensor and rewritable papers.

Multivariate Analysis of Cellular Uptake Characteristics for a (Co)polymer Particle Library

Controlling cellular responses to nanoparticles so far is predominantly empirical, typically requiring multiple rounds of optimization of particulate carriers. In this study, a systematic model-assisted approach should lead to the identification of key parameters that account for particle properties and their cellular recognition. A copolymer particle library was synthesized by a combinatorial approach in soap free emulsion copolymerization of styrene and methyl methacrylate, leading to a broad compositional as well as constitutional spectrum. The proposed structure-property relationships could be elucidated by multivariate analysis of the obtained experimental data, including physicochemical characteristics such as molar composition, molecular weight, particle diameter, and particle charge as well as the cellular uptake pattern of nanoparticles. It was found that the main contributors for particle size were the polymers' molecular weight and the zeta potential, while particle uptake is mainly directed by the particles' composition. This knowledge and the reported model-assisted procedure to identify relevant parameters affecting particle engulfment of particulate carriers by nonphagocytic and phagocytic cells can be of high relevance for the rational design of pharmaceutical nanocarriers and assessment of biodistribution and nanotoxicity, respectively.

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.

Chameleon-like Photoluminescent Janus Nanoparticles as Full-Color Multicomponent Organic Nanoinks: Combination of Förster Resonance Energy Transfer and Photochromism for Encryption and Anticounterfeiting with Multilevel Authentication

Increasing the security by the multilevel authentication mechanism was the most significant challenge in recent years for the development of anticounterfeiting inks based on photoluminescent nanomaterials. For this purpose, the greatest strategy is the use of multicomponent organic materials and a combination of Förster resonance energy transfer (FRET) with the intelligent behavior of photochromic compounds like spiropyran. Here, the hydroxyl-functionalized polymer nanoparticles were synthesized by emulsion copolymerization of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) in different compositions (0-30 wt % of HEMA). Results illustrated that the size of the nanoparticles changed from 64 to 204 nm, and a morphology evolution from spherical to Janus shape was observed by increasing the concentration of HEMA. Photoluminescent inks with red, green, and blue (RGB) fluorescence emissions were prepared by modification of nanoparticles containing 15 wt % of HEMA with spiropyran, fluorescein, and coumarin, respectively. To develop dual-color and multicolor photoluminescent inks that display static and dynamic emission, RGB latex samples were mixed together in different ratios and printed on cellulosic paper. Results display that the fluorescence emission of developed inks can be photoswitched between different statuses, including white to blue, green to blue, green to red/orange, purple to pink, and white to pink, utilizing the FRET phenomenon, photochromism, and a combination of both phenomena. Samples containing spiropyran displayed dynamic color changes in the emission to red, orange, and pink depending on the composition. Hence, developed dual-color and multicolor photoluminescent inks were used for printing of security tags and also painting of some hand-drawn artworks, which obtained results indicating high printability, maximum fluorescence intensity, high resolution, and fast responsivity upon UV-light irradiations of 254 nm (for static mode) and 365 nm (for dynamic mode). In addition, the multilevel authentication mechanism by a static emission under UV-light irradiation of 254 nm, a dynamic emission under UV-light irradiation of 365 nm, and photochromic color change was observed, resulting in increasing the security of developed inks. Actually, developed multicolor photoluminescent inks are the most efficient candidates for developing a new category of chameleon-like high-security anticounterfeiting inks that have tunable optical properties and complex multilevel authentication mechanisms.

Dual-Color Photoluminescent Functionalized Nanoparticles for Static-Dynamic Anticounterfeiting and Encryption: First Collaboration of Spiropyran and Coumarin

Increasing the security of anticounterfeiting materials has been the most important challenge in recent years, and the development of dual-color photoluminescent inks with multi-level security, static/dynamic emission, and dynamic color change is an important solution to overcome this problem. In this study, the multi-functionalized copolymer nanoparticles containing different functional groups (with a concentration of 20 wt %), including ester, carboxylic acid, hydroxyl, epoxide, amide, and amine groups were synthesized successfully by the emulsion polymerization method. The results showed that the particle size and morphology of nanoparticles are affected by the polarity of functional groups. The prepared multi-functionalized copolymer nanoparticles were modified physically with spiropyran (photochromic and red fluorescence emission) and coumarin (cyan emission) derivatives to develop dual-color photoluminescent polymer nanoparticles with application in static-dynamic photoluminescent anticounterfeiting inks, which have multi-level security. The investigation of optical properties indicates that the kinetics of photochromism and photoluminescence properties of samples containing spiropyran is dependent on the local polarity on the surface of polymer nanoparticles. Hence, an increase in the polarity (functionalization with amide, carboxylic acid, and hydroxyl groups) has resulted in fast photochromism, high-intensity photoluminescence emission and increased the efficiency of the photoswitchable color change of emission from cyan to pink. Dual-color photoluminescent anticounterfeiting inks were prepared by mixing polymer nanoparticles containing spiropyran with polymer nanoparticles containing coumarin, in different ratios (1:1, 1:3, 1:5, 1:8, and 1:10). Obtained results showed that prepared samples have cyan emission under UV light of 254 nm (static mode), and a dynamic photoswitching of fluorescence emission from cyan to pink (as a function of irradiation time) was also observed under UV-light irradiation of 365 nm, which is well known as a dynamic mode of emission. The responsivity and intensity of dynamic photoluminescence emission are dependent on the local polarity of the surface functional groups, in which the samples based on amide functionalized copolymer nanoparticles displayed high-intensity emission in the static mode and high-intensity photoswitchable dual-color emission in the dynamic mode, in the case of all ratios of colloid solution mixtures. Printing security tags on cellulose paper by dual-color photoluminescent inks indicates advantages such as maximum printability, resolution, brightness, and static-dynamic photoluminescence emission with high intensity for inks based on amide functionalized nanoparticles. The static-dynamic dual-color photoluminescent anticounterfeiting ink with unique properties and multi-level security was reported for the first time by the collaboration of spiropyran and coumarin. This study can open a new approach and window to the future of advanced and high-security anticounterfeiting technologies.

Photoluminescent Polymer Nanoparticles Based on Oxazolidine Derivatives for Authentication and Security Marking of Confidential Notes

Colloidal materials have widely been used to develop innovative anticounterfeiting nanoinks for information encryption. Latex nanoparticles based on methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) bearing hydroxyl functional groups were synthesized via semicontinuous miniemulsion polymerization. The size determination of the nanoparticles and microscopic results showed mostly spherical nanoparticles with a narrow size distribution and a mean size of about 80 nm. Two oxazolidine derivatives were physically doped at the surface of the nanoparticles to prepare photoluminescent polymer nanoparticles. Hydroxyl functional groups at the surface of the nanoparticles led to their hydrogen bonding interactions with the doped luminescent compounds. Optical analysis of the photoluminescent nanoparticles displayed different fluorescence emission and UV-vis absorbance intensities based on the amount of polar groups located at the surface of colloidal nanoparticles. Reducing the particle size to below 100 nm along with increasing the surface area can assist the decrease of the light reflectance and improvement of the latex nanoparticles' efficiency in the anticounterfeiting industry. This preparation methodology can efficiently provide remarkable photoreversible anticounterfeiting nanoinks used in different applications, such as print marking, security encoded tags, labeling, probing, and handwriting.

Synthetic Strategies for Polymer Particles with Surface Concavities

Over the past decade or so, there has been increasing interest in the synthesis of polymer particles with surface concavities, which mainly include golf ball-like, dimpled and surface-wrinkled polymer particles. Such syntheses generally can be classified into direct polymerization and post-treatment on preformed polymer particles. This review aims to provide an overview of the synthetic strategies of such particles. Some selected examples are given to present the formation mechanisms of the surface concavities. The applications and future development of these concave polymer particles are also briefly discussed. This article is protected by copyright. All rights reserved.

Layer-by-Layer Assembly of Polystyrene/Ag for a Highly Reproducible SERS Substrate and Its Use for the Detection of Food Contaminants

Polystyrene (PS) spheres were prepared through an emulsifier-free emulsion polymerization method, in which the reaction time, ionic strength, concentrations of copolymer were studied in detail. The resulting PS microspheres and Ag nanoparticles were used to construct a surface enhanced Raman scattering (SERS) substrate by a layer-by-layer assembly method. A relatively uniform distribution of PS/Ag in the films was obtained, and the multilayer substrate presented excellent SERS reproducibility and a tunable enhancement effect. The SERS substrate was used for detecting harmful pesticides (malachite green and dimetridazole) in food samples, with a limit of detection as low as 3.5 ppb. The obtained plasmonic composite has a promising future in the field of SERS sensing.

Photoluminescent Nanoinks with Multilevel Security for Quick Authentication of Encoded Optical Tags by Sunlight: Effective Physicochemical Parameters on Responsivity, Printability, and Brightness

Investigation of developed photoactive security inks and anticounterfeiting technologies in recent years indicates significant challenges for future of this research area, such as increase of security, fast responsivity, and facile authentication. Here, amine-functionalized latex nanoparticles were synthesized by emulsion copolymerization of methyl methacrylate (MMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA). Size of the latex nanoparticles was increased as a function of poly(dimethylaminoethyl acrylate) (PDMAEMA) contents, and also a decrease of particle size was obtained in response to an increase of temperature from 25 to 70 °C, above the lower critical solution temperature (LCST) of PDMAEMA. Surface physical modification of the functional latex nanoparticle with spiropyran photoswitches led to the development of anticounterfeiting nanoinks that have multilevel security and photochromic/fluorescence properties with a higher intensity and also brightness. The photoluminescent nanoinks were made of spiropyran latex nanoparticles and used for printing of the encoded optical security tags on cellulosic papers and banknotes. The results displayed that an increase of the particle size above 100 nm and an increase of the PDMAEMA contents led to a remarkable decrease of printability, fluorescent emission, brightness, intensity of photochromism, and also resolution of the printed security tags. As a significant advantage of the developed security inks, the printed security tags could be authenticated easily and fast upon sunlight irradiation by means of photochromism. The responsivity of encoded tags from the invisible to visible state is immediate upon sunlight irradiation for some seconds, whose intensity of coloration is appropriate and detectable clearly by naked eyes. The security anticounterfeiting inks based on spiropyran with multilevel security have been reported for the first time for applying in printing of encoded security tags on cellulosic papers, banknotes, and other documents, where the printed marks are detectable on sunlight exposure.

Waterborne Cross-Linkable Polyacrylate Latex Coatings with Good Water Resistance and Strength Stabilized by Modified Hectorite

Polyacrylate emulsions were prepared by Pickering emulsion polymerization with multi-modified hectorite as a modifier. The proper wettability of modified hectorite and the stability of O/W emulsions prove that modified hectorite has good emulsification properties. The stability of polyacrylate latexes and the morphology of polymer latex particles were then investigated to explain the role of multi-modified hectorite in stabilizing polyacrylate latex. In addition, the improved mechanical properties and water resistance of the latex make it a potentially excellent coating. Multi-modified hectorite as an alternative modifier to conventional surfactants offers a potential application of nanosolid particles that meet the partial wetting conditions for water and oil as stabilizers for the production of latexes for coatings.

Alignment Layer of Liquid Crystal Using Plant-Based Isoeugenol-Substituted Polystyrene

We synthesized a series of renewable and plant-based isoeugenol-substituted polystyrenes (PIEU#, # = 100, 80, 60, 40, and 20, where # is the molar percent content of isoeugenol moiety), using polymer modification reactions to study their liquid crystal (LC) alignment behavior. In general, the LC cells fabricated using polymer film with a higher molar content of isoeugenol side groups showed vertical LC alignment behavior. This alignment behavior was well related to the surface energy value of the polymer layer. For example, vertical alignments were observed when the polar surface energy value of the polymer was smaller than approximately 3.59 mJ/m2, generated by the nonpolar isoeugenol moiety with long and bulky carbon groups. Good alignment stability at 100 °C and under ultraviolet (UV) irradiation of 15 J/cm2 was observed for the LC cells fabricated using PIEU100 as a LC alignment layer. Therefore, renewable isoeugenol-based materials can be used to produce an eco-friendly vertical LC alignment system.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Encryption and authentication of security patterns by ecofriendly multi-color photoluminescent inks containing oxazolidine-functionalized nanoparticles

Counterfeiting of confidential documents has been a costly challenge for banks, companies, and customers. Encryption of invisible security marks, such as barcodes, quick response codes, and logos, in national or international confidential documents by high-security anticounterfeiting inks is the most significant solution for counterfeiting problems. Ecofriendly multi-color photoluminescent anticounterfeiting inks based on highly-fluorescent polymer nanoparticles functionalized with new oxazolidine derivatives were developed for the fast and facile encryption of security labels on cellulosic documents, such as paper currency, passport, and certificate. Depending on the polarity of functionalized polymer nanoparticles, a wide range of colors and fluorescence emissions were observed as a result of polar-polar interactions between the oxazolidine molecules and surface functional groups of the nanoparticles. The fluorescent polymer nanoparticles showed spherical, vesicular, and cauliflower-like morphologies resulted from different surface functional groups. Functional polymer nanoparticles displayed high stability and printability on cellulosic substrates due to hydrogen bonding interactions. The highly-fluorescent polymer nanoparticles were also used to prepare anticounterfeiting inks with different colors and fluorescence emissions. All the ecofriendly polymeric anticounterfeiting inks were loaded to stamps with specific marks, and then applied to different confidential documents. Printed labels displayed highly intense fluorescence emission in different colors (green, orange, pink, and purple depending on the matrix polarity) under UV irradiation (365 nm). These water-based multi-color fluorescent anticounterfeiting inks with highly intense, bright, and sensitive fluorescence emission have potential applications in encryption and authentication of security patterns.