pH-dependent Response Research Articles

A turn-off xanthene-based fluorescent probe for detection of cysteine and its practical application in bioimaging and food samples

BackgroundCys, as an essential amino acid that can be ingested from daily food, plays an important role in maintaining the oxidative balance in cells. Abnormal Cys levels in organisms will lead to various diseases. Therefore, it is of great significance to construct a fluorescent probe that can detect Cys levels in food and biological systems. ResultsHere, a turn-off type probe TA had been successfully synthesized, which attached diethylamine as the strong electron donor, acrylate as the weak electron donor, and xanthene as the π-bridge. TA showed wonderful selectivity, low detection limit, good photostability and well live-cell compatibility for Cys by reducing acrylate group to hydroxyl group of TAOH. The reaction mechanism was demonstrated by 1H NMR, ESI-MS spectra, pH-dependent response experiments, and DFT calculations. Importantly, the reason why TAOH exhibited no fluorescence was the disappearance of the ICT effect in the molecule due to the dominant existence of spirocyclic state of TAOH. In addition, the probe can be used not only for the imaging detection of Cys in A549 cells and zebrafish, but also for the detection of Cys levels in food samples. SignificanceThis work provides a new idea for the design of Cys fluorescent probe, which may be beneficial to the comprehension of the potential mechanism of novel fluorescent probe.

Determination of the interior pH of lipid nanoparticles using a pH-sensitive fluorescent dye-based DNA probe

Lipid nanoparticles (LNPs) containing ionizable cationic lipids are proven delivery systems for therapeutic nucleic acids, such as small interfering RNA (siRNA). It is important to understand the relationship between the interior pH of LNPs and the pH of the external environment to understand LNP formulation and function. Here, we developed a simple and rapid approach for determining the pH of the LNP core using a pH-sensitive fluorescent dye-based DNA probe. LNP siRNA systems containing pH-responsive DNA probes (LNP-siRNA&DNA) were generated by rapid mixing of lipids in ethanol and pH 4 aqueous buffer containing siRNA and DNA probes. We demonstrated that DNA probes were readily encapsulated in LNP systems and were sequestered into an environment at a high concentration as evidenced by an inter-probe FRET signal. It was shown that the pH of LNP encapsulated probes closely follows the pH increase or decrease of the external environment. This indicates that the clinically approved LNP RNA systems with similar lipid compositions (e.g., Onpattro and Comirnaty) are highly permeable to protons and that the pH of the interior environment closely mirrors the external environment. The pH-dependent response of the probe in LNPs was also confirmed under buffer conditions at various pHs. Furthermore, we showed that the pH-sensitive DNA probe can be incorporated into LNP systems at levels that allow the pH response to be monitored at a single LNP level using convex lens-induced confinement (CLiC) confocal microscopy. Direct visualization of the internal pH of single particles with the fluorescent DNA probe was achieved by CLiC for LNP-siRNA&DNA systems formulated under both high and normal ionic strength conditions.

Sustainable utilization of valorized agro-wastes for active and intelligent packaging of processed meats

We propose a novel biodegradable active-intelligent film solely fabricated from valorized agricultural residues and processing wastes, for preserving and monitoring the freshness of pastrami. Cellulose was extracted from honeydew melon rind (MR) by a modified chemical approach, and derivatized into carboxymethyl cellulose (CMC). Compared to commercial CMC and those synthesized from cotton linter/rice husk, CMC-MR displayed superior physicochemical properties in terms of solubility (98.13%), purity (99.66%), molecular weight (760–1050 kDa), degree of substitution (1.25), moisture content (7.80%), and viscosity (38.50 cP). CMC-MR (3% w/v) was used as the supporting matrix, into which anthocyanin-rich onion scale extract (10% weight of polymer; OSE) was incorporated based on excellent pH-dependent response in pHs 1 to 14, potent antimicrobial, and antioxidant activity. The glycerol-sorbitol plasticized halochromic film showed an improved moisture content, UV–vis blocking, water vapor permeability, mechanical strength, and thermal stability. The film applied as a single-layer overwrap, efficiently inhibited the growth of aerobic mesophilic populations by 1.57 log units after 3 days, and extended the shelf life of pastrami by 4 days at 4 °C compared to OSE-free films and inert conventional packaging used in the market. Simultaneously, the film's color changed from reddish-brown to yellowish-brown after 8 days in response to evolving volatile nitrogen compounds, providing evidence for its promising communication prospects that enables consumers to judge freshness and edibility of food products in real-time. The data offers insights for the significance of agro-industrial wastes valorization in fulfilling the needs for reliable smart food packaging technologies with an eye on sustainability.

PH-dependent structural transitions in cationic ionizable lipid mesophases are critical for lipid nanoparticle function.

Lipid nanoparticles (LNPs) are advanced core-shell particles for messenger RNA (mRNA) based therapies that are made of polyethylene glycol (PEG) lipid, distearoylphosphatidylcholine (DSPC), cationic ionizable lipid (CIL), cholesterol (chol), and mRNA. Yet the mechanism of pH-dependent response that is believed to cause endosomal release of LNPs is not well understood. Here, we show that eGFP (enhanced green fluorescent protein) protein expression in the mouse liver mediated by the ionizable lipids DLin-MC3-DMA (MC3), DLin-KC2-DMA (KC2), and DLinDMA (DD) ranks MC3 ≥ KC2 > DD despite similar delivery of mRNA per cell in all cell fractions isolated. We hypothesize that the three CIL-LNPs react differently to pH changes and hence study the structure of CIL/chol bulk phases in water. Using synchrotron X-ray scattering a sequence of ordered CIL/chol mesophases with lowering pH values are observed. These phases show isotropic inverse micellar, cubic Fd3m inverse micellar, inverse hexagonal [Formula: see text] and bicontinuous cubic Pn3m symmetry. If polyadenylic acid, as mRNA surrogate, is added to CIL/chol, excess lipid coexists with a condensed nucleic acid lipid [Formula: see text] phase. The next-neighbor distance in the excess phase shows a discontinuity at the Fd3m inverse micellar to inverse hexagonal [Formula: see text] transition occurring at pH 6 with distinctly larger spacing and hydration for DD vs. MC3 and KC2. In mRNA LNPs, DD showed larger internal spacing, as well as retarded onset and reduced level of DD-LNP-mediated eGFP expression in vitro compared to MC3 and KC2. Our data suggest that the pH-driven Fd3m-[Formula: see text] transition in bulk phases is a hallmark of CIL-specific differences in mRNA LNP efficacy.

Synthesis of multiple-color emitting carbon dots by co-doping of sulfur and nitrogen

Herein, multiple color (blue, green and red) emissive carbon dots (CDs) were synthesized from the main precursor via different protocols, with emission wavelengths of 439 nm, 528 nm and 623 nm, respectively, realizing the synthesis of fluorescent CDs spanning the fluorescence emission from short to long wavelengths. Their relative PLQYs reach up to 28.9%, 31.4% and 12.3%, respectively, exceeding the PLQYs of many similar CDs. The green CDs (G-CDs) and red CDs (R-CDs) show optimal photoluminescence performance in acetone (Me2CO); while the B-CDs have a pH-dependent response in the pH 2 to 12 range. All the three CDs have excellent photostability and salt resistance. Finally, serving as fluorescent anti-counterfeiting ink was taken as an example to briefly introduce the application of multi-color CDs. This work provides a new strategy to develop low-cost multiple color phosphors for display device, biological imaging, analysis and detection and other fields.

Elucidating the Effect of Aliphatic Molecular Plugs on Ion-Rejecting Properties of Polyamide Membranes.

Polyamide RO membranes are widely used for seawater desalination owing to their high salt rejection and water permeability; however, improved selectivity-permeability trade-off is still desired. "Molecular plugs," small molecules immobilized within the polyamide structure, offer an attractive approach; however, their overall effect on polyamide physicochemical properties poses many questions. Here, we analyze the effect of decylamine, a promising plug, and a few charged and uncharged mimics on polyamide films using several in situ techniques. Electrochemical impedance spectroscopy (EIS) reveals a complex pH-dependent response, whereby, upon exposure to amine solution, conductivity first rapidly drops; however, under alkaline conditions, when amine is uncharged, the trend subsequently slowly reverses, and conductivity increases. This slow reversal was observed for noncharged alcohols of similar size as well, but not for larger surfactant molecules. The reversal was assigned to the uptake of plug molecules within polyamide, as opposed to the fast initial drop assigned to surface adsorption. EIS and quartz-crystal microbalance (QCM) results showed that exposure to decylamine under alkaline conditions ultimately led to an irreversible decrease in conductivity, that is, stronger ion rejection, remaining after re-exposure of polyamide to amine-free buffer. This suggests that plug uptake within polyamide resulted in polymer stress, indeed observed in surface stress measurements, and subsequent relaxation. The results indicate that the moderate size of decylamine and conditions minimizing its charge were optimal for irreversible change; however, charge interactions helped maximize its binding within polymer and induce the desired sustained change in selectivity. The results have many potential implications for improving current membrane desalination technology and increasing inherent membrane selectivity toward hard-to-remove species.

A single point mutation converts a proton-pumping rhodopsin into a red-shifted, turn-on fluorescent sensor for chloride.

The visualization of chloride in living cells with fluorescent sensors is linked to our ability to design hosts that can overcome the energetic penalty of desolvation to bind chloride in water. Fluorescent proteins can be used as biological supramolecular hosts to address this fundamental challenge. Here, we showcase the power of protein engineering to convert the fluorescent proton-pumping rhodopsin GR from Gloeobacter violaceus into GR1, a red-shifted, turn-on fluorescent sensor for chloride in detergent micelles and in live Escherichia coli. This non-natural function was unlocked by mutating D121, which serves as the counterion to the protonated retinylidene Schiff base chromophore. Substitution from aspartate to valine at this position (D121V) creates a binding site for chloride. The binding of chloride tunes the pKa of the chromophore towards the protonated, fluorescent state to generate a pH-dependent response. Moreover, ion pumping assays combined with bulk fluorescence and single-cell fluorescence microscopy experiments with E. coli, expressing a GR1 fusion with a cyan fluorescent protein, show that GR1 does not pump ions nor sense membrane potential but instead provides a reversible, ratiometric readout of changes in extracellular chloride at the membrane. This discovery sets the stage to use natural and laboratory-guided evolution to build a family of rhodopsin-based fluorescent chloride sensors with improved properties for cellular applications and learn how proteins can evolve and adapt to bind anions in water.

Multifunctional hybrid nanosystems based on mesoporous silica and hydroxyapatite nanoparticles applied as potential nanocarriers for theranostic applications

In recent years, nanomaterials with properties such as biocompatibility, high specific surface area, and narrow pore distribution have been extensively studied for the treatment of cancer patients. These materials can be loaded with anti-tumor agents to promote a targeted delivery of drugs, reducing the occurrence of side effects common in conventional treatments. However, the ideal characteristics for biomedical applications are not often found combined in a single material, causing the need for the creation of systems composed of more phases to achieve the best material for such applications. Mesoporous silica nanoparticles are promising materials for this purpose, considering the possibility to incorporate organic and inorganic phases into this matrix, such as sensitive polymers, hydroxyapatite (HA) nanoparticles, and rare-earth elements, creating a theranostic material—a single hybrid compound with multifunctionality and capable of performing diagnostic and treatment functions simultaneously. In this study, we report the production of a europium-doped mesoporous silica/hydroxyapatite nanocomposite functionalized with the pH-sensitive polymer poly(methacrylic acid)—P(MAA). The samples were characterized by XRD, FTIR, CHN, TGA, SEM, TEM, XRF, N2 adsorption, and PL techniques. Incorporation and release assays using the antitumor drug methotrexate were performed to evaluate the potential use of these materials as drug carriers in cancer therapy. The results show the photoluminescent potential of these systems, due to the europium doping, and the pH-dependent response in drug release, due to the P(MAA) functionalization. In conclusion, data from this work show that these hybrid nanocomposites exhibit adequate characteristics to be used as a drug delivery platform.

FRET-based colorimetric and ratiometric sensor for visualizing pH change and application for bioimaging in living cells, bacteria and zebrafish

Acid-alkaline balance plays a crucial role in all biological processes. Accordingly, monitoring pH changes will help us to understand the functional status of these physiological and pathological processes. Though fluorescent probes may be a useful tool for detecting pH changes, and there are many limitations to currently available probes, such as background interference, potential cytotoxicity, and poor cell permeability, which call for a solution urgently. In this work, a rhodamine-derived colorimetric and ratiometric sensor (Rh–HN) was fabricated for monitoring pH change via the mechanism of fluorescence resonance energy transfer (FRET). Rh–HN has been shown to possess several advantages over other probes, such as high sensitivity, outstanding permeability, and low toxicity. Besides, the fluorescence intensity ratio (F526/F592) of Rh–HN displays a pH-sensitive response from 2.0 to 7.5 (pKa = 5.05) and linear response from pH 3.8 to 6.4, which was desirable for mapping pH change in the biological systems. Besides, the results indicated that Rh–HN generated a pH-dependent response regulated by switchable forms between closed and opened spirolactam ring. Overall, Rh–HN has accomplished sensing and mapping of pH in living cells, bacteria, and zebrafish. Those results demonstrated that the great potential of Rh–HN in sensing and visualizing pH in the living biosystem.

Facile synthesis of pH-responsive gadolinium(III)-doped carbon nanodots with red fluorescence and magnetic resonance properties for dual-readout logic gate operations

The great challenge still exists in the synthesis of red-emissive carbon nanodots (C-dots) and the development of molecular logic devices with better operation stability for biological applications. In this study, gadolinium (III)-doped C-dots are synthesized by a new solvothermal approach using citric acid, urea and GdCl3 as precursors. The as-prepared Gd3+-doped C-dots exhibit bright red fluorescence (FL) centered at 620 nm in an excitation wavelength-independent manner, and a high T1 relaxivity (∼16.0 mM−1 s−1). More excitingly, the Gd3+-doped C-dots exhibit a pH-dependent response in not only FL behaviour but also magnetic resonance (MR) signal. When triggered by H+, OH−, or Cu2+, the Gd3+-doped C-dots can behave as a switch for FL emission and MR signal, leading to dual-readout and multi-addressable logic systems. Therefore, by employing the Gd3+-doped C-dots as logic gate with varying the chemical inputs, FL/MR dual-readout logic operations including IMP and NOR have been successfully demonstrated not only in all-aqueous media but also within the living HeLa cells. Together with the good biocompatibility and cell-permeability, the Gd3+-doped C-dots hold great potentials for real-time monitoring pH changes both in solution and biological cells, and even future evaluating cellular states via in-cell biocomputation.

Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions.

In this study, the phase transition phenomena of linear poly(acrylic acid) (PAA) and linear or star-shaped poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) in highly concentrated KCl solutions were investigated. The effects of polymer molecular weight, topology, and composition on their phase transition behavior in solution were investigated. The cloud point temperature (TCP) of polymers drastically increased as the KCl concentration (CKCl) and solution pH increased. CKCl strongly influenced the temperature range at which the phase transition of PAA occurred: CKCl of 1.0–2.2 M allowed the phase transition to occur between 30 and 75 °C. Unfortunately, at CKCl above 2.6 M, the TCP of PAA was too high to theoretically trigger the crystallization of KCl. The addition of hydrophobic methyl acrylate moieties decreased the TCP into a temperature region where KCl crystallization could occur. Additionally, the hydrodynamic diameters (Dh) and zeta potentials of commercial PAA samples were examined at room temperature and at their TCP using dynamic light scattering. The salt concentration (from 1 to 3 M) did not impact the hydrodynamic diameter of the molecules. Dh values were 1500 and 15 nm at room temperature and at TCP, respectively.

Light- and pH-Controlled Hierarchical Coassembly of Peptide Amphiphiles.

The coassembly behavior of peptide amphiphiles (PAs) C4-Bhc-EE-NH2 and C14-FKK-NH2 has been investigated by transmission electron microscopy, atomic force microscopy, fluorescence microscopy, circular dichroism, Fourier transform infrared spectroscopy, and 1H nuclear magnetic resonance. These two PAs coassembled into nanofibers by electrostatic and π-π stacking interactions at a low concentration and further aggregated into nanofiber bundles via charge complementation on the surface of nanofibers. As the charge number varied with pH, the bundles could be disassembled/assembled with pH regulation. More interestingly, as C4-Bhc-EE-NH2 was a photodegradable molecule, the bundles could also be responsive to both ultraviolet (UV) and near-infrared (NIR) light. In contrast to the reversible pH-dependent response, the light responses were irreversible as C4-Bhc-EE-NH2 broke under UV or NIR radiation. The highlight of this article is that structural changes were realized for control at the aggregate level, not only at the molecular level. With this inspiration, we hope that we can support the novel biomaterial construction and exploitation of new functions of biomaterials.

Characterization of pH Dependent Growth Response of Agriculturally Important Microbes for Development of Plant Growth Promoting Bacterial Consortium

Characterization of pH Dependent Growth Response of Agriculturally Important Microbes for Development of Plant Growth Promoting Bacterial Consortium

Amino-attapulgite/mesoporous silica composite films generated by electro-assisted self-assembly for the voltammetric determination of diclofenac

Diclofenac (DCF) is a common anti-inflammatory drug. In this work, we have developed two new modified electrodes based on attapulgite clay mineral for the determination of DCF. The first one is made of an amino-functionalized attapulgite modified glassy carbon electrode (GCE/Amino-AT) issued from the grafting of attapulgite particles with [3-(2-aminoethylamino)propyl]trimethoxysilane. Voltammetric data indicate slightly easier oxidation of DCF on such modified electrode (irreversible anodic peak at +0.750 V vs. Ag/AgCl i.e., about 85 mV lower than on bare GCE), and pH-dependent response resulting from interactions between the probe and the electrode material. The second one is an amino-attapulgite–mesoporous silica composite film generated by electro-assisted self-assembly of a surfactant-templated silica layer entrapping the amino-functionalized attapulgite particles on GCE pretreated by electrografting of [3-aminopropyl]triethoxysilane (GCE/APTES-Amino-AT-Silica), which was designed in order to avoid the loss of clay particles in solution over prolonged use of the modified electrode. After characterization of the film morphology, composition and permeability, respectively by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and cyclic voltammetry (CV), the modified electrodes were applied to the detection of DCF by square wave voltammetry (SWV). Both electrodes gave rise to SWV peak currents varying linearly with DCF concentration in the range of 0.3–20 μM (in phosphate buffer at pH 5.7). The detection limits were 0.204 μM on GCE/Amino-AT and 0.053 μM on GCE/APTES-Amino-AT-Silica. The proposed method was successfully applied to the determination of DCF in pharmaceuticals and water samples.

Anaerobic nitrate reduction divergently governs population expansion of the enteropathogen Vibrio cholerae.

To survive and proliferate in the absence of oxygen, many enteric pathogens can undergo anaerobic respiration within the host by using nitrate (NO3-) as electron acceptor 1,2. In these bacteria, NO3- is typically reduced by a nitrate reductase to nitrite (NO2-), a toxic intermediate that is further reduced by a nitrite reductase3. However, Vibrio cholerae, the intestinal pathogen that causes cholera, lacks a nitrite reductase, leading to NO2- accumulation during nitrate reduction4. Thus, V. cholerae is thought to be unable to undergo NO3--dependent anaerobic respiration4. Here, we show that during hypoxic growth, NO3- reduction in V. cholerae divergently impacts bacterial fitness in a manner dependent on environmental pH. Remarkably, in alkaline conditions, V. cholerae can reduce NO3- to support population growth. Conversely, in acidic conditions, accumulation of NO2- from NO3- reduction simultaneously limits population expansion and preserves cell viability by lowering fermentative acid production. Interestingly, other bacterial species such as Salmonella typhimurium, enterohemorrhagic Escherichia coli (EHEC) and Citrobacter rodentium also reproduced this pH-dependent response suggesting that this mechanism might be conserved within enteric pathogens. Our findings explain how a bacterial pathogen can use a single redox reaction to divergently regulate population expansion depending on fluctuating environmental pH.

Synthesis and Characterization of Tunable, pH-Responsive Nanoparticle-Microgel Composites for Surface-Enhanced Raman Scattering Detection.

The synthesis of microgels with pH-tunable swelling leads to adjustable and pH-responsive substrates for surface-enhanced Raman scattering (SERS)-active nanoparticles (NPs). Sterically stabilized and cross-linked latexes were synthesized from random copolymers of styrene (S) and 2-vinylpyridine (2VP). The pH-dependent latex-to-microgel transition and swellability were tuned based on their hydrophobic-to-hydrophilic content established by the S/2VP ratio. The electrostatic loading of polystyrene/poly(2-vinylpyridine) microgels [PSxP2VPy (M)] with anions such as tetrachloroaurate (AuCl4–) and borate-capped Ag NPs was quantified. The PSxP2VPy (M) can load ∼0.3 equiv of AuCl4– and the subsequent photoreduction results in Au NP-loaded PSxP2VPy (M) with NPs located throughout the structure. Loading PSxP2VPy (M) with borate-capped Ag NPs produces PSxP2VPy (M) with NPs located on the surface of the microgels, where the Ag content is set by S/2VP. The pH-responsive SERS activity is also reported for these Ag NP-loaded microgels. Analytical enhancement factors for dissolved crystal violet are high (i.e., 109 to 1010) and are set by S/2VP. The Ag NP-loaded microgels with ∼80 wt % 2VP exhibited the most stable pH dependent response.

Efficient encapsulation and release of RNA molecules from gelatin-based nanoparticles

The design of synthetic carriers for nucleic acid delivery has become a research field of increasing interest. Studies on the delivery of DNA have brought up a variety of gene delivery vehicles. Recent studies in our group have demonstrated the preparation of new gelatin-based nanoparticles for the sustainable intracellular DNA delivery. Furthermore, the more recently emerged strategy by the intracellular delivery of RNA takes benefit from existing expertise in DNA transfer. In this work, the preparation and physicochemical characterization of new nucleic acid-based particles for the sustainable RNA delivery have been demonstrated. Gelatin (either high or low gel strength) and protamine sulfate have been selected to form particles by interaction of oppositely charged compounds. Particles in the absence of RNA (binary system) and in the presence of RNA (ternary system) have been prepared. The physicochemical characterization (particle size, polydispersity index, degree of RNA entrapment and RNA binding efficiency) has been evaluated as a function of the nature of the RNA derivative (acid form, diethylaminoethanol salt or core form) from torula yeast. The pH-dependent response of nanoparticles co-incubated in buffers at defined pHs that mimic late endo-lysosomal environment has demonstrated that the nanoparticles tend to destabilize and RNA can be successfully released as a consequence of changes in the intracellular pHs. Among the different systems, gelatin B (RNA)-PS nanoparticles using RNA acid form from torula yeast has proved to be the best system, from an effective and economic point of view.

PH Dependence of Interfacial Water in the Presence of Amino Acid Side Chains Revealed by Heterodyne-Detected Sum-Frequency Generation Spectroscopy

Proteins are built of amino acid residues that differ from each other only by their side chain. The pH-dependent response of these side chains on protein surface is vital for various biological processes. Here we have investigated the aqueous interface in the presence of different amino acid side chains at varying pH (bulk) using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. It is observed that amine/imidazolic (e.g., lysine/histidine) and alcoholic (e.g., serine and threonine) side chains preferentially orient the interfacial water as “H-down” (i.e., the water hydrogens are pointed toward the aqueous bulk) in acidic solution (pH ∼2). At physiological pH (7.4), the interfacial water takes “H-up” orientation (i.e., the water hydrogens are pointed away from the aqueous bulk) for the alcoholic/imidazolic side chains but remains “H-down” for the amine containing side chain. On further increasing the pH (up to 12.0), the interfacial water becomes increasingly H-up oriented, r...

Functional Poly(N-isopropylacrylamide)/Poly(acrylic acid) Mixed Brushes for Controlled Manipulation of Nanoparticles

Molecular dynamics simulations of a coarse-grained model with soft, nonbonded interactions and implicit solvent are used to study the temperature- and pH-sensitive response of mixed brushes composed of poly(N-isopropylacrylamide) [PNIPAm] and poly(acrylic acid) [PAA] polymers. The model is developed in order to address experimentally relevant, large invariant degrees of polymerization, and nonbonded interactions are expressed via a third-order virial expansion of the equation of state. The choice of interaction parameters for PNIPAm mimics the swelling behavior in water as the temperature increases toward the lower critical solution temperature, TLCST, and the model captures the pH-dependent response of PAA at fixed ionic strength (IS). For this case, the solvent-mediated Flory–Huggins parameter is adapted to reproduce the experimental pH swelling of the homopolymer brush. Mixed brushes incorporating various amounts of PAA are considered, and the effect of mixing polymers on the response of the mixed brus...

PH-Dependent response of a hydrogen peroxide sensing probe

The response of a fiber-optic hydrogen peroxide (H2O2) sensing probe is investigated under a range of pH conditions. The H2O2 optrode is prepared by depositing a thin layer of Prussian blue (PB) onto the tip of a multimode optical fiber, and its detection mechanism is based on the PB/Prussian white (PW) redox reaction. PW is oxidized to PB by H2O2 leading to changes in the absorption of visible light. The sensor can be restored after usage by reducing the PB to PW with ascorbic acid. This optrode was previously shown to have good repeatability and sensitivity to H2O2 concentrations at room temperature and a pH of 4. For practical applications, the sensor must be capable of detecting H2O2 in environments spanning a range of pH conditions, including biological and industrial environments which together span pH values from 7 to less than 3. We present experiments using multiple PB optrodes, at pH values ranging from 7 to 2, and for different concentrations of H2O2. The work demonstrates that the optrode remains functional and provides stable and reproducible measurements of H2O2 under a range of pH conditions that correspond, in particular, to those in an operating PEM fuel cell.