Visual pH Sensors: From a Chemical Perspective to New Bioengineered Materials

Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

Advances in pH Sensing: From Traditional Approaches to Next-Generation Sensors in Biological Contexts

pH has been considered one of the paramount factors in bodily functions because most cellular tasks exclusively rely on precise pH values. In this context, the current techniques for pH sensing provide us with the futuristic insight to further design therapeutic and diagnostic tools. Thus, pH-sensing (electrochemically and optically) is rapidly evolving toward exciting new applications and expanding researchers' interests in many chemical contexts, especially in biomedical applications. The adaptation of cutting-edge technology is subsequently producing the modest form of these biosensors as wearable devices, which are providing us the opportunity to target the real-time collection of vital parameters, including pH for improved healthcare systems. The motif of this review is to provide insight into trending tech-based systems employed in real-time or in-vivo pH-responsive monitoring. Herein, we briefly go through the pH regulation in the human body to help the beginners and scientific community with quick background knowledge, recent advances in the field, and pH detection in real-time biological applications. In the end, we summarize our review by providing an outlook; challenges that need to be addressed, and prospective integration of various pH in vivo platforms with modern electronics that can open new avenues of cutting-edge techniques for disease diagnostics and prevention.

Sensing Mechanism and Excited-State Dynamics of a Widely Used Intracellular Fluorescent pH Probe: pHrodo

The pHrodo with an "off-on" response to the changes of pH has been widely used as a fluorescent pH probe for bioimaging. The fluorescence off-on mechanism is fundamentally important for its application and further development. Herein, the sensing mechanism, especially the relevant excited-state dynamics, of pHrodo is investigated by steady-state and time-resolved spectroscopy as well as quantum chemical calculations, showing that pHrodo is best understood using the bichromophore model. Its first excited state (S1) is a charge transfer state between two chromophores. From S1, pHrodo relaxes to its ground state (S0) via an ultrafast nonradiative process (∼0.5 ps), which causes its fluorescence to be "off". After protonation, S1 becomes a localized excited state, which accounts for the fluorescence being turned "on". Our work provides photophysical insight into the sensing mechanism of pHrodo and indicates the bichromophore model might be relevant to a wide range of fluorescent probes.

Imagining the future of optical microscopy: everything, everywhere, all at once

The optical microscope has revolutionized biology since at least the 17th Century. Since then, it has progressed from a largely observational tool to a powerful bioanalytical platform. However, realizing its full potential to study live specimens is hindered by a daunting array of technical challenges. Here, we delve into the current state of live imaging to explore the barriers that must be overcome and the possibilities that lie ahead. We venture to envision a future where we can visualize and study everything, everywhere, all at once - from the intricate inner workings of a single cell to the dynamic interplay across entire organisms, and a world where scientists could access the necessary microscopy technologies anywhere.

Physical-Vapor-Deposited Metal Oxide Thin Films for pH Sensing Applications: Last Decade of Research Progress

In the last several decades, metal oxide thin films have attracted significant attention for the development of various existing and emerging technological applications, including pH sensors. The mandate for consistent and precise pH sensing techniques has been increasing across various fields, including environmental monitoring, biotechnology, food and agricultural industries, and medical diagnostics. Metal oxide thin films grown using physical vapor deposition (PVD) with precise control over film thickness, composition, and morphology are beneficial for pH sensing applications such as enhancing pH sensitivity and stability, quicker response, repeatability, and compatibility with miniaturization. Various PVD techniques, including sputtering, evaporation, and ion beam deposition, used to fabricate thin films for tailoring materials' properties for the advanced design and development of high-performing pH sensors, have been explored worldwide by many research groups. In addition, various thin film materials have also been investigated, including metal oxides, nitrides, and nanostructured films, to make very robust pH sensing electrodes with higher pH sensing performance. The development of novel materials and structures has enabled higher sensitivity, improved selectivity, and enhanced durability in harsh pH environments. The last decade has witnessed significant advancements in PVD thin films for pH sensing applications. The combination of precise film deposition techniques, novel materials, and surface functionalization strategies has led to improved pH sensing performance, making PVD thin films a promising choice for future pH sensing technologies.

Reliable and accurate prediction of basic pK[Formula: see text] values in nitrogen compounds: the pK[Formula: see text] shift in supramolecular systems as a case study

This article presents a quantitative structure-activity relationship (QSAR) approach for predicting the acid dissociation constant (pK[Formula: see text]) of nitrogenous compounds, including those within supramolecular complexes based on cucurbiturils. The model combines low-cost quantum mechanical calculations with QSAR methodology and linear regressions to achieve accurate predictions for a broad range of nitrogen-containing compounds. The model was developed using a diverse dataset of 130 nitrogenous compounds and exhibits excellent predictive performance, with a high coefficient of determination (R[Formula: see text]) of 0.9905, low standard error (s) of 0.3066, and high Fisher statistic (F) of 2142. The model outperforms existing methods, such as Chemaxon software and previous studies, in terms of accuracy and its ability to handle heterogeneous datasets. External validation on pharmaceutical ingredients, dyes, and supramolecular complexes based on cucurbiturils confirms the reliability of the model. To enhance usability, a script-like tool has been developed, providing a streamlined process for users to access the model. This study represents a significant advancement in pK[Formula: see text] prediction, offering valuable insights for drug design and supramolecular system optimization.

Robust Proton Conduction against Mechanical Stress in Flexible Free-Standing Membrane Composed of Two-Dimensional Coordination Polymer

Introduction of mechanical flexibility into proton-conducting coordination polymers (CPs) is in high demand for future protonic applications such as fuel cells and hydrogen sensors. Although such mechanical properties have been primarily investigated in one-dimensional (1D) CPs, in this study, we successfully fabricated highly flexible free-standing CP membranes with a high surface-to-volume ratio, which is beneficial for enhanced performance in the aforementioned applications. We fabricated a layered CP, Cu2 (NiTCPP) (H4 (H2 TCPP); 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin), in which a two-dimensional (2D) square grid sheet composed of tetradentate nickel porphyrins and paddlewheel-type copper dimers was connected to each other by weak van der Waals forces. The mechanical flexibility was evaluated by bending and tensile tests. The flexural and Young's moduli of the membrane were significantly higher than those of conventional Nafion membranes. Electrochemical impedance spectroscopy analysis revealed that the in-plane proton conductivity of the membrane was maintained even under applied bending stress. Because the X-ray diffraction analysis indicates that the proton-conducting pathway through the hydrogen bonding network remains intact during the bending operation, our present study provides a promising strategy for the fabrication of new and advanced 2D CPs without using substrates or additional polymers for protonic devices.

Sensitive Current Sensor Based on a Lanthanide Framework with Lewis Basic Bipyridyl Sites for Cu2+ Detection

A new Yb-based three-dimensional metal-organic framework with free Lewis basic sites, [Yb₂(ddbpdc)₃(CH₃OH)₂] (referred to as ACBP-6), from YbCl₃ and (6R,8R)-6,8-dimethyl-7,8-dihydro-6H-[1,5]dioxonino[7,6-b:8,9-b']dipyridine-3,11-dicarboxylic acid (H₂ddbpdc) was synthesized by a conventional solvothermal method. Two Yb³⁺ are connected by three carboxyl groups to form the [Yb₂(CO₂)₅] binuclear unit, which is further bridged by two carboxyl moieties to produce a tetranuclear secondary building unit. With further ligation of the ligand ddbpdc^2-, a 3-D MOF with helical channels is constructed. In the MOF, Yb³⁺ only coordinates with O atoms, leaving the bipyridyl N atoms of ddbpdc^2- unoccupied. The unsaturated Lewis basic sites make this framework possible to coordinate with other metal ions. After growing the ACBP-6 in situ into a glass micropipette, a novel current sensor is formed. This sensor shows high selectivity and a high signal-to-noise ratio toward Cu²⁺ detection with a detection limit of 1 μM, due to the stronger coordination ability between the Cu²⁺ and the bipyridyl N atoms.

Chemometric-Assisted Litmus Test: One Single Sensing Platform Adapted from 1-13 to Narrow pH Ranges

A novel 3 × 4 colorimetric sensing platform, named the chemometric-assisted litmus test (CLT), has been developed by covalently anchoring commercial pH indicators to ethylene vinyl alcohol (EVOH). The proposed device can be exploited for pH determinations in a wide range from 1 to 13 and in specific narrow ranges, achieving sufficient accuracy and errors below 0.5 pH units. The experimental procedure is simple, quick and reliable; equilibration is reached in less than 2 h, CLT pictures are acquired by a camera, and data treatment is performed applying chemometric techniques such as principal component analysis (PCA) and partial least square regression (PLS) to RGB indices.

A novel aggregation-induced emission fluorescent probe with large Stokes shift for sensitive detection of pH changes in live cells

The detection of intracellular pH is crucial for elucidating the pathological process of cancers, as well as for medical diagnostic applications. Here, we developed an aggregation-induced emission active pH-responsive fluorescent probe (TPE-DCP) for sensitively detecting cell pH changes. The probe shows obvious pH-sensing properties at ~615 nm, with a pKa value of 6.82 and a good linear pH response ranging from 8.5 to 4.5. TPE-DCP holds advantages such as excellent anti-interference performance, good photostability, and low cytotoxicity, and has been successfully used to image intracellular pH changes in cells.

A Single Fluorescent Protein-Based Indicator with a Time-Resolved Fluorescence Readout for Precise pH Measurements in the Alkaline Range

The real-time monitoring of the intracellular pH in live cells with high precision represents an important methodological challenge. Although genetically encoded fluorescent indicators can be considered as a probe of choice for such measurements, they are hindered mostly by the inability to determine an absolute pH value and/or a narrow dynamic range of the signal, making them inefficient for recording the small pH changes that typically occur within cellular organelles. Here, we study the pH sensitivity of a green-fluorescence-protein (GFP)-based emitter (EGFP-Y145L/S205V) with the alkaline-shifted chromophore's pKa and demonstrate that, in the pH range of 7.5-9.0, its fluorescence lifetime changes by a factor of ~3.5 in a quasi-linear manner in mammalian cells. Considering the relatively strong lifetime response in a narrow pH range, we proposed the mitochondria, which are known to have a weakly alkaline milieu, as a target for live-cell pH measurements. Using fluorescence lifetime imaging microscopy (FLIM) to visualize the HEK293T cells expressing mitochondrially targeted EGFP-Y145L/S205V, we succeeded in determining the absolute pH value of the mitochondria and recorded the ETC-uncoupler-stimulated pH shift with a precision of 0.1 unit. We thus show that a single GFP with alkaline-shifted pKa can act as a high-precision indicator that can be used in a specific pH range.

A library of Rhodamine6G-based pH-sensitive fluorescent probes with versatile in vivo and in vitro applications

Acidic pH is critical to the function of the gastrointestinal system, bone-resorbing osteoclasts, and the endolysosomal compartment of nearly every cell in the body. Non-invasive, real-time fluorescence imaging of acidic microenvironments represents a powerful tool for understanding normal cellular biology, defining mechanisms of disease, and monitoring for therapeutic response. While commercially available pH-sensitive fluorescent probes exist, several limitations hinder their widespread use and potential for biologic application. To address this need, we developed a novel library of pH-sensitive probes based on the highly photostable and water-soluble fluorescent molecule, Rhodamine 6G. We demonstrate versatility in terms of both pH sensitivity (i.e., pK a) and chemical functionality, allowing conjugation to small molecules, proteins, nanoparticles, and regenerative biomaterial scaffold matrices. Furthermore, we show preserved pH-sensitive fluorescence following a variety of forms of covalent functionalization and demonstrate three potential applications, both in vitro and in vivo, for intracellular and extracellular pH sensing. Finally, we develop a computation approach for predicting the pH sensitivity of R6G derivatives, which could be used to expand our library and generate probes with novel properties.

Imaging and Measuring Vesicular Acidification with a Plasma Membrane-Targeted Ratiometric pH Probe

Tracking the pH variation of intracellular vesicles throughout the endocytosis pathway is of prior importance to better assess the cell trafficking and metabolism of cells. Small molecular fluorescent pH probes are valuable tools in bioimaging but are generally not targeted to intracellular vesicles or are directly targeted to acidic lysosomes, thus not allowing the dynamic observation of the vesicular acidification. Herein, we designed Mem-pH, a fluorogenic ratiometric pH probe based on chromenoquinoline with appealing photophysical properties, which targets the plasma membrane (PM) of cells and further accumulates in the intracellular vesicles by endocytosis. The exposition of Mem-pH toward the vesicle's lumen allowed to monitor the acidification of the vesicles throughout the endocytic pathway and enabled the measurement of their pH via ratiometric imaging.

A Water Soluble 2-Phenyl-5-(pyridin-3-yl)-1,3,4-oxadiazole Based Probe: Antimicrobial Activity and Colorimetric/Fluorescence pH Response

The growing demand of responsive tools for biological and biomedical applications pushes towards new low-cost probes easy to synthesize and versatile. Current optical probes are theranostic tools simultaneously responsive to biological parameters/analyte and therapeutically operating. Among the optical methods for pH monitoring, simple small organic molecules including multifunctional probes for simultaneous biological activity being highly desired by scientists and technicians. Here, we present a novel pH-responsive probe with a three-ring heteroaromatic pattern and a flexible cationic chain. The novel molecule shows real-time naked-eye colorimetric and fluorescence response in the slightly acidic pH range besides its excellent solubility both in the organic phase and in water. In addition, the small probe shows significant antibacterial activity, particularly against Escherichia coli. Single-crystal X-ray study and density functional theory (DFT) calculations rationalize the molecule spectroscopic response. Finally, molecular dynamics (MD) elucidate the interactions between the probe and a model cell membrane.

Ratiometric pH-Responsive 19F Magnetic Resonance Imaging Contrast Agents Based on Hydrazone Switches

Hydrazone-based molecular switches serve as efficient ratiometric pH-sensitive agents that can be tracked with 19F NMR/MRI and 1H NMR. Structural changes induced between pH 3 and 4 lead to signal appearance and disappearance at 1H and 19F NMR spectra allowing ratiometric pH measurements. The most pronounced are resonances of the CF3 group shifted by 1.8 ppm with 19F NMR and a hydrazone proton shifted by 2 ppm with 1H NMR.

Instant pH sensor based on the functionalized cellulose for detecting strong acid leaks

Acid spills cause large-scale environmental damage and casualties. To respond to such incidents, a sensor capable of detecting acid leaks is required. Cellulose is a useful substrate material for the fast detection of acid leaks because it has high hydrophilicity and porosity. On the other hand, methods of manufacturing cellulose-based sensors are still complicated or time-consuming. Thus, in this study, a simple and rapid synthesis method for a cellulose-based pH sensor was proposed. The functionalization of α-cellulose was achieved via chloroacetyl chloride, and Congo red was covalently immobilized to the functionalized cellulose for detecting strong acids. The manufacturing process was composed of two steps as above and finished within 8 h. The developed sensor exhibited absorbance changes in the pH range of 0.2 to 3.0, and response time was shorter than 1 s. A prototype system using this sensor was manufactured and tested, and it detected acid leaks easily and quickly.

pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

Herein we report four [Ir(N^C)2(L^L)]n+, n = 0,1 complexes (1-4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2'-bipyridine (2), [2,2'-bipyridine]-4,4'-dicarboxylic acid (3), and sodium 4,4',4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1-4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550-670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1-4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.

Plasmonic Au@Ag@mSiO2 Nanorattles for In Situ Imaging of Bacterial Metabolism by Surface-Enhanced Raman Scattering Spectroscopy

It is well known that microbial populations and their interactions are largely influenced by their secreted metabolites. Noninvasive and spatiotemporal monitoring and imaging of such extracellular metabolic byproducts can be correlated with biological phenotypes of interest and provide new insights into the structure and development of microbial communities. Herein, we report a surface-enhanced Raman scattering (SERS) hybrid substrate consisting of plasmonic Au@Ag@mSiO₂ nanorattles for optophysiological monitoring of extracellular metabolism in microbial populations. A key element of the SERS substrate is the mesoporous silica shell encapsulating single plasmonic nanoparticles, which furnishes colloidal stability and molecular sieving capabilities to the engineered nanostructures, thereby realizing robust, sensitive, and reliable measurements. The reported SERS-based approach may be used as a powerful tool for deciphering the role of extracellular metabolites and physicochemical factors in microbial community dynamics and interactions.

Stimuli-Responsive Zinc (II) Coordination Polymers: A Novel Platform for Supramolecular Chromic Smart Tools

The unique role of the zinc (II) cation prompted us to cut a cross-section of the large and complex topic of the stimuli-responsive coordination polymers (CPs). Due to its flexible coordination environment and geometries, easiness of coordination-decoordination equilibria, "optically innocent" ability to "clip" the ligands in emissive architectures, non-toxicity and sustainability, the zinc (II) cation is a good candidate for building supramolecular smart tools. The review summarizes the recent achievements of zinc-based CPs as stimuli-responsive materials able to provide a chromic response. An overview of the past five years has been organised, encompassing 1, 2 and 3D responsive zinc-based CPs; specifically zinc-based metallorganic frameworks and zinc-based nanosized polymeric probes. The most relevant examples were collected following a consequential and progressive approach, referring to the structure-responsiveness relationship, the sensing mechanisms, the analytes and/or parameters detected. Finally, applications of highly bioengineered Zn-CPs for advanced imaging technique have been discussed.