The Dual Use of the Pyranine (HPTS) Fluorescent Probe: A Ground-State pH Indicator and an Excited-State Proton Transfer Probe

Utilizing excited-state proton transfer fluorescence quenching mechanism, layered rare earth hydroxides enable ultra-sensitive detection of nitroaromatic

Convenient, rapid, and accurate detection of nitroaromatic organic toxins and harmful substances is of great significance in research. In the present study, two-dimensional layered rare-earth hydroxides (LYH) were used as ion-exchange matrix materials, and the anionic fluorescent dye molecules (HPTS) were successfully introduced into the LYH structures in situ via a simple and effective "plug-and-play" strategy, which gave the compounds ultra-sensitive fluorescence sensing detection of nitrobenzene, p-nitrotoluene and p-nitrophenol (Fluorescence response time < 1 sec, and the LOD for nitrobenzene, p-nitrophenol and p-nitrotoluene reached an impressive 349 ppb, 22 ppb and 98 ppb, respectively). Combined with theoretical calculations, we elucidated in detail the fluorescence quenching response mechanism of the LYH-HPTS towards nitroaromatic. Additionally, we also constructed fluorescent paper sensor, which effectively transformed the LYH-HPTS from theoretical detection to device application. The LYH-HPTS material is not only simple to synthesize, cost-effective and stable, but also has the features of fast response, excellent sensitivity and selectivity, and good reproducibility, which provides a new approach for the rapid and accurate detection of nitroaromatic.

Visible Light-Driven CO2 Capture and Release Using Photoactive Pyranine in Water in Continuous Flow

The urgent need to address climate change and its environmental consequences demands innovative carbon capture technologies, given the relationship between rising global temperatures and increased atmospheric CO2 levels. Here, we present a reversible photochemical carbon capture and release strategy and system utilizing photoactive pyranine in an aqueous bicarbonate buffer system. Control experiments suggested that the photoacid effect occurs at the surface which contributes to CO2 release, complemented by the photothermal effect at the surface and in the bulk. A continuous flow setup employing a tube-in-tube configuration with a hollow fiber membrane demonstrates the efficiency and reliability of the visible light-driven carbon capture system, with the release of CO2 captured from a 15% CO2 feed in the dark, at a rate of 0.48 mmol CO2 per hour to a nitrogen sweep stream under light irradiation at 200 W/m2, a level comparable to solar intensity of visible light (150 W/m2 of blue light -250 W/m2 of blue and green light). The robustness and scalability of the system has been demonstrated, with long-term operation over 7 days yielding 60 mmol (1.34 L CO2 at STP) of cumulated CO2 separation. Additionally, we explored the potential for direct air capture, yielding 3 μmol of CO2 separation over 2 h of operation from a bicarbonate buffer solution saturated with ambient air (420 ppm). This work introduces the prospect of photoswing of carbon capture systems, which can avoid external energy input beyond solar irradiation, offering promising avenues for addressing the challenges associated with climate change.

Assessing the mechanism of facilitated proton transport across GUVs trapped in a microfluidic device

Proton transport across lipid membranes is one of the most fundamental reactions that make up living organisms. In vitro, however, the study of proton transport reactions can be very challenging due to limitations imposed by proton concentrations, compartment size, and unstirred layers as well as buffer exchange and buffer capacity. In this study, we have developed a proton permeation assay based on the microfluidic trapping of giant vesicles enclosing the pH-sensitive dye pyranine to address some of these challenges. Time-resolved fluorescence imaging upon a rapid pH shift enabled us to investigate the facilitated H+ permeation mediated by either a channel or a carrier. Specifically, we compared the proton transport rates as a function of different proton gradients of the channel gramicidin D and the proton carrier carbonyl cyanide-m-chlorophenyl hydrazone. Our results demonstrate the efficacy of the assay in monitoring proton transport reactions and distinguishing between a channel-like and a carrier-like mechanism. This groundbreaking result enabled us to elucidate the enigmatic mode of the proton permeation mechanism of the recently discovered natural fibupeptide lugdunin.

Proton diffusion on the surface of mixed lipid membranes highlights the role of membrane composition

Proton circuits within biological membranes, the foundation of natural bioenergetic systems, are significantly influenced by the lipid compositions of different biological membranes. In this study, we investigate the influence of mixed lipid membrane composition on the proton transfer (PT) properties on the surface of the membrane. We track the excited-state PT (ESPT) process from a tethered probe to the membrane with timescales and length scales of PT relevant to bioenergetic systems. Two processes can happen during ESPT: the initial PT from the probe to the membrane at short timescales, followed by diffusion of dissociated protons around the probe on the membrane, and the possible geminate recombination with the probe at longer timescales. Here, we use membranes composed of mixtures of phosphatidylcholine (PC) and phosphatidic acid (PA). We show that the changes in the ESPT properties are not monotonous with the concentration of the lipid mixture; at a low concentration of PA in PC, we find that the membrane is a poor proton acceptor. Molecular dynamics simulations indicate that the membrane is more structured at this specific lipid mixture, with the least number of defects. Accordingly, we suggest that the structure of the membrane is an important factor in facilitating PT. We further show that the composition of the membrane affects the geminate proton diffusion around the probe, whereas, on a timescale of tens of nanoseconds, the dissociated proton is mostly lateral restricted to the membrane plane in PA membranes, while in PC, the diffusion is less restricted by the membrane.

Universal Approach to Direct Spatiotemporal Dynamic In Situ Optical Visualization of On-Catalyst Water Splitting Electrochemical Processes

Electrochemical reactions are the unrivaled backbone of next-generation energy storage, energy conversion, and healthcare devices. However, the real-time visualization of electrochemical reactions remains the bottleneck for fully exploiting their intrinsic potential. Herein, for the first time, a universal approach to direct spatiotemporal-dynamic in situ optical visualization of pH-based as well as specific byproduct-based electrochemical reactions is performed. As a highly relevant and impactful example, in-operando optical visualization of on-catalyst water splitting processes is performed in neutral water/seawater. HPTS (8-hydroxypyrene-1,3,6-trisulfonicacid), known for its exceptional optical capability of detecting even the tiniest pH changes allows the unprecedented "spatiotemporal" real-time visualization at the electrodes. As a result, it is unprecedentedly revealed that at a critical cathode-to-anode distance, the bulk-electrolyte "self-neutralization" phenomenon can be achieved during the water splitting process, leading to the practical realization of enhanced additive-free neutral water splitting. Furthermore, it is experimentally unveiled that at increasing electrolyte flow rates, a swift and severe inhibition of the concomitantly forming acidic and basic 'fronts', developed at anode and cathode compartments are observed, thus acting as a "buffering" mechanism. To demonstrate the universal applicability of this elegant strategy which is not limited to pH changes, the technique is extended to visualization of hypochlorite/ chlorine at the anode during electrolysis of sea water using N-(4-butanoic acid) dansylsulfonamide (BADS). Thus, a unique experimental tool that allows real-time spatiotemporal visualization and simultaneous mechanistic investigation of complex electrochemical processes is developed that can be universally extended to various fields of research.

Anomalous proton transfer of a photoacid HPTS in nonaqueous reverse micelles

The proton transfer reaction is one of the fundamental chemical reactions where the reaction dynamics strongly depend on solvent properties such as acidity or basicity. A photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) shows a sharp decrease of pKa (7.7 → 0.5) upon photoexcitation, and the excited-state proton transfer (ESPT) occurs with ultrafast time constants of 2.5 and 89 ps in bulk aqueous solution. However, the two-step proton transfers via the contact ion pair formation and the proton diffusion are strongly limited inside the nanopools of reverse micelles (RMs). The confinement in small RMs strongly impeded the proton transfer reactions. In this work, we report the ESPT of HPTS confined in methanol-in-oil RMs by steady-state and time-resolved electronic spectroscopy. Interestingly, HPTS shows substantial deprotonation in the excited state only in small RMs, while the ESPT of HPTS does not occur in bulk methanol solution due to the low basicity of aliphatic alcohols. The kinetic analysis of time-resolved fluorescence and transient absorption measurements will compare the proton transfer dynamics of HPTS in the water-in-oil and methanol-in-oil RMs. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes.

Light-Triggered Reversible Change in the Electronic Structure of MoO3 Nanosheets via an Excited-State Proton Transfer Mechanism

Light is an attractive source of energy for regulating stimulus-responsive chemical systems. Here, we use light as a gating source to control the redox state, the localized surface plasmonic resonance (LSPR) peak, and the structure of molybdenum oxide (MoO3) nanosheets, which are important for various applications. However, the light excitation is not that of the MoO3 nanosheets but rather that of pyranine (HPTS) photoacids, which in turn undergo an excited-state proton transfer (ESPT) process. We show that the ESPT process from HPTS to the nanosheets and the intercalation of protons within the MoO3 nanosheets trigger the reduction of the nanosheets and the broadening of the LSPR peak, a process that is reversible, meaning that in the absence of light, the LSPR peak diminishes and the nanosheets return to their oxidized form. We further show that this reversible process is accompanied by a change in the nanosheet size and morphology.

Excited-State Dynamics of a Photoacid: A Potential Probe for Recognizing Transition from Lamellar to Nonlamellar Inverted Structures of Liposome based Nanocarriers

Liposomes of a cationic lipid dioctadecyldimethylammonium bromide (DODAB) are efficient nanocarriers of nucleic acids. Incorporation of a neutral lipid monoolein (MO) in excess (xMO >0.5) changes the lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes facilitating nucleic acid delivery to cells. Photoexcitation of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), a photoacid, initiates an excited state proton transfer (ESPT) reaction in its protonated form (ROH*) generating the deprotonated anionic form (RO- *). The fluorescence intensity ratio (IROH* /IRO-* ) of these two forms is governed by the ESPT dynamics, and increases with increasing MO content (xMO ) in the cationic liposomes of DODAB. Transition from lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes, due to incorporation of MO (xMO ~0.7), is manifested by a significant increase of ESPT time (τPT ) and the time constant of wobbling motion (τW ) of HPTS. Thus, the lamellar organizations of DODAB or DODAB-rich (xMO 0.2) liposomes and the non-lamellar organizations of MO-rich (xMO ~0.7) liposomes are recognized by significantly different excited state dynamics of the photoacid.

Light-Triggered Enhancement of Fluorescence Efficiency in Organic Cages

The fluorescence efficiency of excited molecules can be enhanced by many external factors. Here, we showcase a surprising phenomenon whereby light is used as a gating source to increase the fluorescence efficiency of organic cages composed of biphenyl subunits. We show that the enhancement of fluorescence is not due to structural changes or ground-state events. Cryo-fluorescence measurements and kinetic studies suggest a restriction of the phenyl-based structures in the excited state, leading to increased fluorescence, which is also supported by time-resolved measurements. Through computational calculations, we propose that the planarization of the biphenyl units within the cages contributes to emission enhancement. This phenomenon offers insights into the design of optoelectronic structures with improved fluorescence properties.

Controlling pH-Sensitive Chemical Reactions Pathways with Light - a Tale of Two Photobases: an Arrhenius and a Brønsted

Light-gated chemical reactions allow spatial and temporal control of chemical processes. Here, we suggest a new system for controlling pH-sensitive processes with light using two photobases of Arrhenius and Brønsted types. Only after light excitation do Arrhenius photobases undergo hydroxide ion dissociation, while Brønsted photobases capture a proton. However, none can be used alone to reversibly control pH due to the limitations arising from excessively fast or overly slow photoreaction timescales. We show here that combining the two types of photobases allows light-triggered and reversible pH control. We show an application of this method in directing the pH-dependent reaction pathways of the organic dye Alizarin Red S simply by switching between different wavelengths of light, i. e., irradiating each photobase separately. The concept of a light-controlled system shown here of a sophisticated interplay between two photobases can be integrated into various smart functional and dynamic systems.

Effect of salt addition on a triblock copolymer-zwitterionic surfactant assembly: insight from excited-state proton transfer

Copolymer-surfactant assemblies are frequently utilized across various fields, from medicine to nanotechnology. Understanding the organization of the mixed assemblies in a saline environment will further expand their application horizons, especially under physiological conditions. Excited-state proton transfer (ESPT) can provide insight into the hydration nature and organization of the non-toxic assembly of a triblock copolymer F127 (poly-(ethylene oxide)101 (PEO101)-poly(propylene oxide)56 (PPO56)-PEO101)) and a zwitterionic sulfobetaine surfactant N-dodecyl-N,N-dimethyl-3-ammoniopropane sulfonate (SB12). Here, we present a comprehensive investigation of the compactness and hydration nature of the F127-SB12 mixed assemblies at different salt concentrations using the ESPT of 8-hydroxy pyrene-1,3,6-trisulfonate (HPTS). In the absence of salts, gradual SB12 addition to a premicellar (0.4 mM) or a post-micellar (4 mM) F127 solution leads to an anomalous modulation of the protonated and deprotonated emission bands. The emission intensity ratio (protonated/deprotonated) first increases to a maximum at a particular SB12 concentration (6 mM and 35 mM for the premicellar and post-micellar F127 assemblies, respectively), and then the ratio decreases with a further increase in the surfactant concentration. Since the intensity ratio is an indicator of the retardation of the ESPT process, the mixed micellar configuration displaying a maximum intensity ratio represents the most compact and least hydrated state. Salt addition to this configuration lowers the intensity ratio, signifying an enhanced ESPT process. Dynamic light scattering (DLS) results indicate that the size of the mixed assembly remains almost unaltered with the addition of salts. Thus, salinity enhances the ESPT process inside the F127-SB12 mixed assemblies without significantly altering the hydrodynamic radius.

Excited-State Proton Transfer in a Photoacid-Based Crystalline Coordination Compound: Reversible Photochromism, Near-Infrared Photothermal Conversion, and Conductivity

By harnessing the power of coordination self-assembly, crystalline materials can act as carriers for photoacids. Unlike their solution-based counterparts, these photoacids are capable of altering the properties of the crystalline material under light and can even generate proton transfer in a solid-state environment. Due to the photoinduced proton transfer and charge transfer processes within this functional material, this crystal exhibits powerful absorption spanning the visible to near-infrared spectrum upon light irradiation. This feature enables reproducible, significant chromatic variation, near-infrared photothermal conversion, and photocontrollable conductivity for this photoresponsive material. The findings suggest that the synthesis of pyranine photoacid-based crystalline materials via coordination self-assembly can not only enhance light-harvesting efficiency but also enable excited-state proton transfer processes within solid crystalline materials, thereby maintaining and even improving the properties of photoacids.

Proton- versus Cation-Selective Transport of Saccharide Rim-Appended Pillar[5]arene Artificial Water Channels

Transport of water across cell membranes is a fundamental process for important biological functions. Herein, we focused our research on a new type of symmetrical saccharide rim-functionalized pillar[5]arene (PA-S) artificial water channels with variable pore structures. To point out the versatility of PA-S channels, we systematically varied the nature of anchoring/gate keepers d-mannoside, d-mannuronic acid, or sialic acid H-bonding groups on lateral pillar[5]arene (PA) arms, known as good membrane adhesives, to best describe the influence of the chemical structure on their transport activity. The control of hydrophobic membrane binding-hydrophilic water binding balance is an important feature influencing the channels' structuration and efficiency for a proper insertion into bilayer membranes. The glycosylated PA channels' transport performances were assessed in lipid bilayer membranes, and the channels were able to transport water at high rates (∼106-107 waters/s/channel within 1 order of magnitude as for aquaporins), serving as selective proton railways with total Na+ and K+ rejection. Molecular simulation substantiates the idea that the PAs can generate supramolecular pores, featuring hydrophilic carbohydrate gate-keepers that serve as water-sponge relays at the channel entrance, effectively absorbing and redirecting water within the channel. The present channels may be regarded as a rare biomimetic example of artificial channels presenting proton vs cation transport selectivity performances.

Sensitive and specific detection of saccharide species based on fluorescence: update from 2016

Increasing evidence supports the critical role of saccharides in various pathophysiological steps of tumor progression, where they regulate tumor proliferation, invasion, hematogenic metastasis, and angiogenesis. The identification and recognition of these saccharides provide a solid foundation for the development of targeted drug preparations, which are however not fully understood due to their complex and similar structures. In order to achieve fluorescence sensing of saccharides, extensive research has been conducted to design molecular probes and nanoparticles made of different materials. This paper aims to provide in-depth discussion of three main topics that cover the current status of the carbohydrate sensing based on the fluorescence sensing mechanism, including a phenylboronic acid-based sensing platform, non-boronic acid entities, as well as an enzyme-based sensing platform. It also highlights efforts made to understand the recognition mechanisms and improve the sensing properties of these systems. Finally, we present the challenge of achieving high selectivity and sensitivity recognition of saccharides, and suggest possible future avenues for exploration.

A regenerable electrochemical sensor for electro-inactive cyclovirobuxine D detection in biological samples

Based on the pK_a determination of cyclovirobuxine D (CVB-D) using the method of potentiometry, we predicted the ionization state of CVB-D at physiological pH. Thus, by taking advantage of the ionization state and consequent non-covalent interactions between protonated CVB-D and deprotonated polymerized bromothymol blue (poly-BTB) under physiological conditions, we developed a simple and reusable electrochemical sensor that contains a poly-BTB/SWNT-modified electrode for electro-inactive CVB-D detection in biological fluids using poly-BTB as both the recognition unit and the electrochemical probe. Upon being immersed in the solution of CVB-D, the poly BTB-based electrode shows a current decrease due to the interaction-driven binding of CVB-D on the electrode surface. The current decrease in the electrochemical sensor toward CVB-D concentration shows a linear relationship in the dynamic ranges of 0.01-1 μM and 1-50 μM with a detection limit of 1.65 nM based on 3σ. The sensor can be easily regenerated through the removal of the binding of CVB-D from the electrode surface by highly negatively charged heparin, and it presents high repeatability with an RSD of less than 4.0% for seven measurements. In animal experiments, the electrochemical sensor was selective and sensitive for CVB-D determination in plasma and liver homogenates. The electrochemical sensor is readily accessible, robust, and cost-effective and holds good promise for more applications in biological and clinical fields associated with CVB-D using less technically demanding and simple operating procedures.