Optical Devices Constructed from Ferrocene-Modified Microgels for H2O2 Sensing

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Ionic Microgel Colloidal Crystals: Responsive Chromism in Dual Physical and Chemical Colors for High-End Information Security and Encryption

Chromic materials play a decisive and escalating role in information security. However, it is challenging to develop chromic materials for encryption technologies that can hardly be imitated. Inspired by versatile metachrosis in nature, a series of coumarin-based 7-(6-bromohexyloxy)-coumarin microgel colloidal crystals (BrHC MGCC) with multiresponsive chromism are able to be assembled by ionic microgels in poly(vinyl alcohol) (PVA) solution followed by two cycles of freezing-thawing. The ionic microgels can be finely tailored by in situ quaternization with tunable size under varied temperatures and hydration energies of counterions as well as quenched luminescence under UV irradiation, which endows BrHC MGCC with intriguing chromism in the dual-channel coloration of physical structural color and chemical fluorescent color. Three types of BrHC MGCC exhibit various change ranges in structural coloration and similar quenching in fluorescence emission, which can be utilized for the development of the static-dynamic combined anticounterfeiting system with dual coloration. The information conveyed by the BrHC MGCC array presents dynamic variation versus temperature, while the static information can be only integrally read in both sunlight and a 365 nm UV lamp. The fabrication of a microgel colloidal crystal with dual coloration opens a facile and ecofriendly window for multilevel information security, camouflage, and a cumbersome authentication process.

Fast Responsive, Reversible Colorimetric Nanoparticle-Hydrogel Complexes for pH Monitoring

Hydrogels containing redox-sensitive colorimetric nanoparticles (NPs) have been used to sense ambient pH in many fields owing to their simple and fast visualization capabilities. However, real-time pH monitoring still has limitations due to its poor response rate and irreversibility. Herein, we developed a fast responsive colorimetric hydrogel called ferrocene adsorption colorimetric hydrogel (FACH). Ferrocene, an organometallic compound, plays a vital role as an electron transfer mediator (i.e., redox catalyst) within the hydrogel network. FACH shows fast color change performance with high reactivity and penetrability to ambient pH changes. In detail, FACH shows distinct color change within 2 min under various pH conditions from four to eight, with good reliability. The speed for color change of FACH is approximately six times faster than that of previously developed colorimetric hydrogels, suggesting the fastest hydrogel-based colorimetric pH sensor. Furthermore, FACH shows reversibility and repeatability of the redox process, indicating scalable utility as a sustainable pH monitoring platform.

Two-dimensional colloidal crystal of soft microgel spheres: Development, preparation and applications

Two-dimensional (2D) colloidal crystals are ordered monolayer arrays of colloidal sphere particles assembled on the substrates or at phase interfaces. Owing to their unique periodic structure and fascinating properties, 2D colloidal crystals have aroused considerable interest because of their potential applications. Among them, 2D colloidal crystals self-assembled from soft microgel spheres stand out particularly. The 2D colloidal crystals of soft microgel spheres combine the advantages of monolayer colloidal crystals and sensitive microgels, which have a good application prospect in biomedical area. In this article, we provide a systematic overview of 2D colloidal crystals of soft microgel spheres related to their development, preparation and applications. First, various preparation methods of 2D colloidal crystal of microgels are introduced, including dip-coating, drop-coating, spin-coating, interface assembly, surface reaction-assisted assembly, and so forth. Second, representative biomedical applications consisting of optical sensor, drug delivery, antibacterial coating, cell culture, and colloidal template are also exemplified to show the high performance of 2D colloidal crystals of soft microgel spheres. In addition, we also present prospects of future developments of 2D microgel colloidal crystals.

Stimuli-responsive polymer-based systems for diagnostic applications

Stimuli-responsive polymers exhibit properties that make them ideal candidates for biosensing and molecular diagnostics. Through rational design of polymer composition combined with new polymer functionalization and synthetic strategies, polymers with myriad responsivities, e.g., responses to temperature, pH, biomolecules, CO2, light, and electricity can be achieved. When these polymers are specifically designed to respond to biomarkers, stimuli-responsive devices/probes, capable of recognizing and transducing analyte signals, can be used to diagnose and treat disease. In this review, we highlight recent state-of-the-art examples of stimuli-responsive polymer-based systems for biosensing and bioimaging.

Optical Devices Constructed From Responsive Microgels for Polyphenols Detection

Polyphenols are used as antioxidants in various foods and beverages, which are considered to be a health benefit. The measurement of polyphenols contents is of great interest in food chemistry and health science. This work reported a microgels based photonic device (etalon) to detect polyphenols. Dopamine was used as a model compound of polyphenols. Herein, we proposed a “block” concept for dopamine detection. The dopamine was oxidized and formed dopamine films catalyzed by tyrosinase on the surface of etalon. As the etalon was immersed in ZnCl₂, the dopamine films blocked the ZnCl₂ diffusion into etalon that caused optical property changes. The film thickness is associated with the concentration of dopamine which can be readout via optical signals.

Copper-based etalon filter using antioxidant graphene layer

Copper is a low-cost material compared to silver and gold, having high reflectivity in the near infrared spectral range as well as good electrical and thermal conductivity. Its properties make it a good candidate for metal-based low-cost multilayer thin-film devices and optical components. However, its high reflectance in the devices is reduced because copper is easily oxidized. Here, we suggest a copper-based Fabry-Perot optical filter consisting of a thin dielectric layer stacked between two copper films, which can realize low-cost production compared to a conventional silver-based etalon filter. The reduced performance due to the inherent oxidation of the copper surface can be overcome by passivating the copper films with monolayer graphene. The anti-oxidation of copper film is investigated by optical microscopy, x-ray photoelectron spectroscopy, and transmission measurement in UV-vi spectral ranges. Our results show that the graphene coating can be expanded for various metal-based optical devices in terms of anti-corrosion.

Model-based design and synthesis of ferrocene containing microgels

Modelling and synthesis go hand in hand to efficiently engineer copolymer microgels with various architectures: core–shell structures (with ferrocene mainly in the core or in the shell) and also microgels with homogeneous comonomer distribution.

Alkanethiol Molecular Barriers for Controlling Small Molecule Release Kinetics from a Microgel-Based Reservoir Device

Poly(N-isopropylacrylamide)-co-acrylic acid microgel-based reservoir devices were constructed by "sandwiching" a single layer of microgels between two thin Au layers (all on a glass support). The microgels were loaded with the model drug crystal violet (CV) utilizing the electrostatic interactions between deprotonated acrylic acid (AAc) and the positively charged CV; release can be triggered from the microgels by neutralizing the deprotonated AAc groups at acidic conditions. Alkanethiols of different alkyl chain lengths and polarities were immobilized on the upper Au layer of the device, and the release rate of the model drug CV from the microgel layer, after acid neutralization, was assessed. We found that the CV release rate was the highest when the alkyl chain length was short and contained a hydrophilic moiety. Conversely, the release rate was hindered by the presence of thiols with long alkyl chain lengths and with no hydrophilic moiety. We explain this phenomenon by quantifying the thiol's ability to hinder acid penetration into the microgel layer, and the ability of free CV to pass through the upper Au layer and into the solution. Utilizing various thiols and mixed thiol layers, we are able to tune release profiles from these reservoir devices to potentially achieve array devices with precisely tuned small molecule release profiles.

Triggering the Shrinking/Swelling Process in Thin Gel Layers on Conducting Surfaces by Applying an Appropriate Potential

Negatively charged, pH-sensitive, very thin gel layers with accumulated hexaammineruthenium (II)/(III) were deposited on conducting surfaces. The gel was synthesized by applying an electrochemically induced free-radical polymerization method. This method allowed covering the electrode surface with an uniform and compact layer. The modified electrodes exhibited excellent current switch on/off behavior in response to changes in pH. However, the main goal of this study was to achieve the control of the layer thickness by changing the oxidation state of hexaammineruthenium. The layers could be reversibly swollen/shrinked by applying appropriate potentials.

Cargo shuttling by electrochemical switching of core-shell microgels obtained by a facile one-shot polymerization

Controlling and understanding the electrochemical properties of electroactive polymeric colloids is a highly topical but still a rather unexplored field of research. This is especially true when considering more complex particle architectures like stimuli-responsive microgels, which would entail different kinetic constraints for charge transport within one particle. We synthesize and electrochemically address dual stimuli responsive core-shell microgels, where the temperature-responsiveness modulates not only the internal structure, but also the microgel electroactivity both on an internal and on a global scale. In detail, a facile one-step precipitation polymerization results in architecturally advanced poly(N-isopropylacrylamide-co-vinylferrocene) P(NIPAM-co-VFc) microgels with a ferrocene (Fc)-enriched (collapsed/hard) core and a NIPAM-rich shell. While the remaining Fc units in the shell are electrochemically accessible, the electrochemical activity of Fc in the core is limited due to the restricted mobility of redox active sites and therefore restricted electron transfer in the compact core domain. Still, prolonged electrochemical action and/or chemical oxidation enable a reversible adjustment of the internal microgel structure from core-shell microgels with a dense core to completely oxidized microgels with a highly swollen core and a denser corona. The combination of thermo-sensitive and redox-responsive units being part of the network allows for efficient amplification of the redox response on the overall microgel dimension, which is mainly governed by the shell. Further, it allows for an electrochemical switching of polarity (hydrophilicity/hydrophobicity) of the microgel, enabling an electrochemically triggered uptake and release of active guest molecules. Hence, bactericidal drugs can be released to effectively kill bacteria. In addition, good biocompatibility of the microgels in cell tests suggests suitability of the new microgel system for future biomedical applications.

Chemical Design of Functional Polymer Structures for Biosensors: From Nanoscale to Macroscale

Over the past decades, biosensors, a class of physicochemical detectors sensitive to biological analytes, have drawn increasing interest, particularly in light of growing concerns about human health. Functional polymeric materials have been widely researched for sensing applications because of their structural versatility and significant progress that has been made concerning their chemistry, as well as in the field of nanotechnology. Polymeric nanoparticles are conventionally used in sensing applications due to large surface area, which allows rapid and sensitive detection. On the macroscale, hydrogels are crucial materials for biosensing applications, being used in many wearable or implantable devices as a biocompatible platform. The performance of both hydrogels and nanoparticles, including sensitivity, response time, or reversibility, can be significantly altered and optimized by changing their chemical structures; this has encouraged us to overview and classify chemical design strategies. Here, we have organized this review into two main sections concerning the use of nanoparticles and hydrogels (as polymeric structures) for biosensors and described chemical approaches in relevant subcategories, which act as a guide for general synthetic strategies.

Silver nanoparticle-loaded microgel-based etalons for H2O2sensing

Silver nanoparticles (AgNPs) were generated inside the network structure of poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels that were sandwiched between two thin Au layers (15 nm) of an etalon. This was done by introducing Ag⁺ to the etalons composed of deprotonated microgels, followed by its subsequent reduction with NaBH₄. The resultant microgels were collected and then characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), verifying the loading of AgNPs with relatively uniform diameter (5–7 nm) within the microgels. Furthermore, the optical properties of the resultant etalons and their response to H₂O₂ were evaluated by reflectance spectroscopy. Specifically, upon the addition of H₂O₂, the AgNP-loaded etalons exhibited both a red shift in the position of the reflectance peaks and an increase in reflected wavelength intensity. We hypothesize that the dual signal response of the devices was a result of oxidative decomposition of the AgNPs, enabling the microgels to swell and for more light to be reflected (due to the loss of the light absorbing AgNPs). Finally, we showed that the AgNPs could be regenerated in the used etalons multiple times without a loss in performance. This work provides a cost-effective means to detect H₂O₂, which could be modified to sense a variety of other species of physiological and environmental importance through rationally loading other functional nanomaterials.

Silver nanoparticle (AgNP)-loaded poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc)-based microgels were generated and used to make etalons. The etalons were shown to exhibit optical properties that depended on the concentration of H₂O₂ in solution.

Comparison of the Responsivity of Solution-Suspended and Surface-Bound Poly(N-isopropylacrylamide)-Based Microgels for Sensing Applications

In this submission, the phase transition behavior for poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and their assemblies was investigated as a function of temperature and pH using UV-vis spectroscopy (to probe light scattering behavior) and quartz crystal microbalance with dissipation (QCM-D) measurements. PNIPAm-co-AAc microgels were "painted" onto Au-coated glass substrates (for UV-vis) and the Au electrode of a QCM crystal to generate monolayers. The subsequent deposition of another Au layer on top of the pNIPAm-co-AAc microgel layer yields what is known as an etalon. UV-vis/QCM-D measurements revealed that the temperature and pH responsivities for the microgel assemblies match well with their solution behavior. UV-vis spectroscopy shows that the transmittance of the microgel monolayers decreased with increasing solution temperature at pH 3.0. At pH 6.5, the AAc groups in the microgels were deprotonated, leading to strong Coulombic repulsive forces inside the microgels that prevented their collapse and lead to minimal change in the transmitted light intensity. However, QCM-D analysis reveals more complex behavior as it is sensitive to the viscosity/viscoelasticity and thickness changes of the microgel layer, which ultimately depends on the microgel chemical composition and the interaction of the etalon's Au layer with the crystal. The maximum sensitivity to temperature is 0.8 × 10^-3 °C·Hz^-1, which is the most sensitive pNIPAm microgel-based QCM temperature sensor thus far reported in the literature. Finally, we exploit this new understanding to characterize the pH and ionic strength of a solution using pNIPAm-co-XAAc microgel-based etalon coated crystals. The research results and the sensing demonstration can inspire new and improved sensor designs for a variety of analytes.