Solid-State Molecular Protonics Devices of Solid-Supported Biological Membranes Reveal the Mechanism of Long-Range Lateral Proton Transport.

Lateral proton transport (PT) on the surface of biological membranes is a fundamental biochemical process in the bioenergetics of living cells, but a lack of available experimental techniques has resulted in a limited understanding of its mechanism. Here, we present a molecular protonics experimental approach to investigate lateral PT across membranes by measuring long-range (70 µm) lateral proton conduction via a few layers of lipid bilayers in a solid-state-like environment, i.e., without having bulk water surrounding the membrane. This configuration enables us to focus on lateral proton conduction across the surface of the membrane while decoupling it from bulk water. Hence, by controlling the relative humidity of the environment, we can directly explore the role of water in the lateral PT process. We show that proton conduction is dependent on the number of water molecules and their structure and on membrane composition, where we explore the role of the headgroup, the tail saturation, the membrane phase, and membrane fluidity. The measured PT as a function of temperature shows an inverse temperature dependency, which we explain by the desorption and adsorption of water molecules into the solid membrane platform. We explain our findings by discussing the role of percolating hydrogen bonding within the membrane structure in a Grotthuss-like mechanism.

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.

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.

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.

Proton Transport Across Collagen Fibrils and Scaffolds: The Role of Hydroxyproline.

Collagen is one of the most studied proteins due to its fundamental role in creating fibrillar structures and supporting tissues in our bodies. Accordingly, collagen is also one of the most used proteins for making tissue-engineered scaffolds for various types of tissues. To date, the high abundance of hydroxyproline (Hyp) within collagen is commonly ascribed to the structure and stability of collagen. Here, we hypothesize a new role for the presence of Hyp within collagen, which is to support proton transport (PT) across collagen fibrils. For this purpose, we explore here three different collagen-based hydrogels: the first is prepared by the self-assembly of natural collagen fibrils, and the second and third are based on covalently linking between collagen via either a self-coupling method or with an additional cross-linker. Following the formation of the hydrogel, we introduce here a two-step reaction, involving (1) attaching methanesulfonyl to the -OH group of Hyp, followed by (2) removing the methanesulfonyl, thus reverting Hyp to proline (Pro). We explore the PT efficiency at each step of the reaction using electrical measurements and show that adding the methanesulfonyl group vastly enhances PT, while reverting Hyp to Pro significantly reduces PT efficiency (compared with the initial point) with different efficiencies for the various collagen-based hydrogels. The role of Hyp in supporting the PT can assist in our understanding of the physiological roles of collagen. Furthermore, the capacity to modulate conductivity across collagen is very important to the use of collagen in regenerative medicine.

Excitation-Dependent Fluorescence with Excitation-Selective Circularly Polarized Luminescence from Hierarchically Organized Atomic Nanoclusters.

Nanometer-scaled objects are known to have dimension-related properties, but sometimes the assembly of such objects can lead to the emergence of other properties. Here, we show the assembly of atomically precise gold nanoclusters into large fibrillar structures that are featuring excitation-dependent luminescence with an excitation-selective circularly polarized luminescence (CPL), even though all components are achiral. The origin of CPL in the assembly of atomic clusters has been attributed to the hierarchical organization of atomic clusters into fibrillar structures, mediated via a hydrogen bonding interaction with a surfactant. We follow the assembly process both experimentally and computationally showing the advance in the structural formation along with its chiroptical electronic properties, i.e., circular dichroism (CD) and CPL. Our study here can assist in the rational design of materials featuring chiroptical properties, thus leading to a controlled CPL activity.

Robust Biosensor Based on Carbon Nanotubes/Protein Hybrid Electrolyte Gated Transistors.

Semiconducting single walled carbon nanotubes (SWCNTs) are promising materials for biosensing applications with electrolyte-gated transistors (EGT). However, to be employed in EGT devices, SWCNTs often require lengthy solution-processing fabrication techniques. Here, we introduce a simple solution-based method that allows fabricating EGT devices from stable dispersions of SWCNTs/bovine serum albumin (BSA) hybrids in water. The dispersion is then deposited on a substrate allowing the formation of a SWCNTs random network as the semiconducting channel. We demonstrate that this methodology allows the fabrication of EGT devices with electric performances that allow their use in biosensing applications. We demonstrate their application for the detection of cortisol in solution, upon gate electrode functionalization with anti-cortisol antibodies. This is a robust and cost-effective methodology that sets the ground for a SWCNT/BSA-based biosensing platform that allows overcoming many limitations of standard SWCNTs biosensor fabrications.

Molecular-Doped Protein-Based Elastomers as a Versatile Platform for Energy-Transfer Studies and Emissive Down-Converting Polymers for Light-Emitting Applications.

Much effort is being employed for designing "green" environmental emissive materials that are capable of color-tuning, i.e., down-converting the emission, and white-light generation (WLG). Here, we introduce a protein-based elastomer that can noncovalently bind a variety of chromophores while preventing their aggregation. Such binding capabilities are unique to the albumin-based materials that we use here in a process we refer to as "molecular doping". In the first part of this study, we explore the energy transfer across five different chromophores within the protein matrix, where the closely packed chromophore organization enables high energy-transfer efficiencies among them. In the second part, we show the easy control of blue, green, and red chromophores within the biopolymer, resulting in tunable emission properties of the film and WLG. The highly affordable chosen protein and the straightforward molecular doping strategy make our protein elastomers an attractive choice for an emissive material, as either a scaffold for investigating energy transfer in proteins or possible integration in light-emitting applications.

Photocurrent Generation and Polarity Switching in Electrochemical Cells through Light-induced Excited State Proton Transfer of Photoacids and Photobases.

Light is a common source of energy in sustainable technologies for photocurrent generation. To date, in such light-harvesting applications, the excited electrons generate the photocurrent. Here, we introduce a new mechanism for photocurrent generation that is based on excited state proton transfer (ESPT) of photoacids and photobases that can donate or accept a proton, respectively, but only after excitation. We show that the formed ions following ESPT can either serve as electron donors or acceptors with the electrodes, or modify the kinetics of mass transport across the diffuse layer, both resulting in photocurrent generation. We further show that control of the current polarity is obtained by switching the irradiation between the photoacid and the photobase. Our study represents a new approach in photoelectrochemistry by introducing ESPT processes, which can be further utilized in light-responsive energy production or energy storage.

Solid-State Electron Transport Through Carbon Dots Junctions: The Role of Boron and Phosphorus Doping.

Carbon dots (CDs) are a new class of nanoparticles that gained widespread attention recently because of their easy preparation, water solubility, biocompatibility, and bright luminescence, leading to their integration in various applications. Despite their nm-scale and proven electron transfer capabilities, the solid-state electron transport (ETp) across single CDs was never explored. Here, a molecular junction configuration is used to explore the ETp across CDs as a function of their chemical structure using both DC-bias current-voltage and AC-bias impedance measurements. CDs are used with Nitrogen and Sulfur as exogenous atoms and doped with small amounts of Boron and Phosphorous. It is shown that the presence of P and B highly improves the ETp efficiency across the CDs, yet without an indication of a change in the dominant charge carrier. Instead, structural characterizations reveal significant changes in the chemical species across the CDs: the formation of sulfonates and graphitic Nitrogen. Temperature-dependent measurements and normalized differential conductance analysis reveal that the ETp mechanism across the CDs behaves as tunneling, which is common to all CDs used here. The study shows that the conductivity of CDs is on par with that of sophisticated molecular wires, suggesting CDs as new 'green' candidates for molecular electronics applications.

Glycoproteins as a Platform for Making Proton-Conductive Free-Standing Biopolymers.

Biopolymers are an attractive environmentally friendly alternative to common synthetic polymers, whereas primarily proteins and polysaccharides are the biomacromolecules that are used for making the biopolymer. Due to the breadth of side chains of such biomacromolecules capable of participating in hydrogen bonding, proteins and polysaccharide biopolymers were also used for the making of proton-conductive biopolymers. Here, we introduce a new platform for combining the merits of both proteins and polysaccharides while using a glycosylated protein for making the biopolymer. We use mucin as our starting point, whereas being a waste of the food industry, it is a highly available and low-cost glycoprotein. We show how we can use different chemical strategies to target either the glycan part or specific amino acids for both crosslinking between the different glycoproteins, thus making a free-standing biopolymer, as well as for introducing superior proton conductivity properties to the formed biopolymer. The resultant proton-conductive soft biopolymer is an appealing candidate for any soft bioelectronic application.

The Role of Surface Groups in Dictating the Chiral-Solvent-Induced Assembly of Carbon Dots into Structures Exhibiting Circularly Polarized Luminescence.

Here, the use of achiral nanoparticles and solvent-induced chirality transfer is combined for the making of large structures exhibiting chiroptical properties in the form of circularly polarized luminescence (CPL). The nanoparticles that the authors use are carbon dots (C-Dots) that are known for their bright luminescence and the ability to tune their surface moieties by using different precursors in their synthesis. Here, the result of adding the chiral solvent limonene into an aqueous solution of various C-Dots is explored, differentiated by their surface group. It is shown that only nitrogen-containing C-Dots with amine functional groups see the emergence of a CPL signal and the formation of a large fibrillar assembled structure. The various forces happening in the interface between the C-Dots and the limonene phase and the role of the amine groups in both the chirality transfer interactions and the interactions between C-Dots in the assembly process are discussed, whereas these two processes intertwine with each other. The ability to form fluorescent chiral structures exhibiting CPL from achiral nanoparticles and the understanding of the various interactions in this process are both important to the rationale design of any supramolecular chiral assemblies.

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

Molecular fluorescent probes are an essential experimental tool in many fields, ranging from biology to chemistry and materials science, to study the localization and other environmental properties surrounding the fluorescent probe. Thousands of different molecular fluorescent probes can be grouped into different families according to their photophysical properties. This Account focuses on a unique class of fluorescent probes that distinguishes itself from all other probes. This class is termed photoacids, which are molecules exhibiting a change in their acid-base transition between the ground and excited states, resulting in a large change in their pKa values between these two states, which is thermodynamically described using the Förster cycle. While there are many different photoacids, we focus only on pyranine, which is the most used photoacid, with pKa values of ∼7.4 and ∼0.4 for its ground and excited states, respectively. Such a difference between the pKa values is the basis for the dual use of the pyranine fluorescent probe. Furthermore, the protonated and deprotonated states of pyranine absorb and emit at different wavelengths, making it easy to focus on a specific state. Pyranine has been used for decades as a fluorescent pH indicator for physiological pH values, which is based on its acid-base equilibrium in the ground state. While the unique excited-state proton transfer (ESPT) properties of photoacids have been explored for more than a half-century, it is only recently that photoacids and especially pyranine have been used as fluorescent probes for the local environment of the probe, especially the hydration layer surrounding it and related proton diffusion properties. Such use of photoacids is based on their capability for ESPT from the photoacid to a nearby proton acceptor, which is usually, but not necessarily, water. In this Account, we detail the photophysical properties of pyranine, distinguishing between the processes in the ground state and the ones in the excited state. We further review the different utilization of pyranine for probing different properties of the environment. Our main perspective is on the emerging use of the ESPT process for deciphering the hydration layer around the probe and other parameters related to proton diffusion taking place while the molecule is in the excited state, focusing primarily on bio-related materials. Special attention is given to how to perform the experiments and, most importantly, how to interpret their results. We also briefly discuss the breadth of possibilities in making pyranine derivatives and the use of pyranine for controlling dynamic reactions.

Using Proton Geminate Recombination as a Probe of Proton Migration on Biological Membranes.

Proton migration on biological membranes plays a major role in cellular respiration and photosynthesis, but it is not yet fully understood. Here we show that proton dissociation kinetics and related geminate recombination can be used as a probe of such proton migration mechanisms. We develop a simple model for the process and apply it to analyze the results obtained using a photo-induced proton release probe (chemically modified photoacid) tethered to phosphatidylcholine membranes. In our theoretical model, we apply approximate treatment for the diffusional cloud of the geminate proton around the dissociated photoacid and consider arbitrary dimension of the system, 1 < d < 3. We observe that in d > 2, there is a kinetic phase transition between an exponential and a power-law kinetic phases. The existence of an exponential decay phase at the beginning of the proton dissociation is a signature of d > 2 systems. In most other cases, the exponential decay phase is not present, and the kinetics follows a diffusional power-law P(t) ∼ t-d/2 that develops after a short initiation time. Specifically, in a 1D case, which corresponds to the desorption of a proton from the surface, the dissociation occurs by the slow power-law ∼1/t and explains the abnormally slow desorption rate reported recently in experiments.

Application of Super Photoacids in Controlling Dynamic Processes: Light-Triggering the Self-Propulsion of Oil Droplets.

The dynamic control of pH-responsive systems is at the heart of many natural and artificial processes. Here, we use photoacids, molecules that dissociate only in their excited state and transfer their proton to nearby proton acceptors, for the dynamic control of processes. A problem arises when there is a need to protonate highly acidic acceptors. We solve this problem using super photoacids that have an excited-state pKa of -8, thus enabling them to protonate very weak proton acceptors. The process that we target is the light-triggered self-propulsion of droplets, initiated by an excited-state proton transfer (ESPT) from a super photoacid donor to a surfactant acceptor situated on the surface of the droplet with a pKa of ∼0. We first confirm using steady-state and time-resolved spectroscopy that a super photoacid can undergo ESPT to the acidic surfactant, whereas a "regular" photoacid cannot. Next, we show self-propulsion of the droplet upon irradiating the solvated super photoacid. We further confirm the protonation of the surfactant on the surface of the droplet using transient surface tension measurements. Our system is the first example of the application of super photoacids to control dynamic processes and opens new possibilities in the field of light-triggered dynamic systems.

Tailoring Quantum Dot Sizes for Optimal Photoinduced Catalytic Activation of Nitrogenase.

Many efforts have been directed towards elucidating the nitrogenase structure, its biocatalytic activity, and methods to artificially activate it by external stimuli. Here, we investigated how semiconductor nanoparticles (NPs) with sizes ranging between 2.3-3.5 nm form nano-biohybrids with the nitrogenase enzyme and enable its photoinduced biocatalytic activity. We examined two homogenously synthesized quantum dots (QDs), CdS, CdSe, and two nitrogenase variants, the wild-type and a cysteine-mutated. We show that the cysteine-mutated variant does not enhance the hydrogen generation amounts, as compared with the wild type. Nevertheless, we show that the 2.3 nm-sized CdSe NPs facilitate an eightfold increase compared with larger CdSe NPs. The obtained results were investigated using electrochemical techniques, transmission electron microscopy, and further confirmed by time-resolved spectroscopic measurements, which allow us to determine the electron tranfer rate constant (kET ) of the different configurations.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*.

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

A Protein-Based Free-Standing Proton-Conducting Transparent Elastomer for Large-Scale Sensing Applications.

A most important endeavor in modern materials' research is the current shift toward green environmental and sustainable materials. Natural resources are one of the attractive building blocks for making environmentally friendly materials. In most cases, however, the performance of nature-derived materials is inferior to the performance of carefully designed synthetic materials. This is especially true for conductive polymers, which is the topic here. Inspired by the natural role of proteins in mediating protons, their utilization in the creation of a free-standing transparent polymer with a highly elastic nature and proton conductivity comparable to that of synthetic polymers, is demonstrated. Importantly, the polymerization process relies on natural protein crosslinkers and is spontaneous and energy-efficient. The protein used, bovine serum albumin, is one of the most affordable proteins, resulting in the ability to create large-scale materials at a low cost. Due to the inherent biodegradability and biocompatibility of the elastomer, it is promising for biomedical applications. Here, its immediate utilization as a solid-state interface for sensing of electrophysiological signals, is shown.

Long-range light-modulated charge transport across the molecular heterostructure doped protein biopolymers.

Biological electron transfer (ET) across proteins is ubiquitous, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via intramolecular cofactors within the protein. Far from biology, carbon dots (C-Dots) with their unique optoelectronic properties can be considered as game-changers for next-generation advanced technologies. Here, we use C-Dots for making heterostructure (HS) configurations by conjugating them to a natural ET mediator, the hemin molecule, thus making an electron donor-acceptor system. We show by transient absorption and emission spectroscopy that the rapid intramolecular charge separation happens following light excitation, which can be ascribed to an ultrafast electron and hole transfer (HT) from the C-Dot donor to the hemin acceptor. Upon integrating the HS into a protein matrix, we show that this HT within the HS configuration is 3.3 times faster compared to the same process in solution, indicating the active role of the protein in supporting the rapid light-induced long-range intermolecular charge separation. We further use impedance, electrochemical, and transient photocurrent measurements to show that the light-induced transient charge separation results in an enhanced ET and HT efficiency across the protein biopolymer. The charge conduction across our protein biopolymers, reaching nearly 0.01 S cm-1, along with the simplicity and low-cost of their formation promotes their use in a variety of optoelectronic devices, such as artificial photosynthesis, photo-responsive protonic-electronic transistors, and photodetectors.

The porphyrin ring rather than the metal ion dictates long-range electron transport across proteins suggesting coherence-assisted mechanism.

The fundamental biological process of electron transfer (ET) takes place across proteins with common ET pathways of several nanometers. Recent discoveries push this limit and show long-range extracellular ET over several micrometers. Here, we aim in deciphering how protein-bound intramolecular cofactors can facilitate such long-range ET. In contrast to natural systems, our protein-based platform enables us to modulate important factors associated with ET in a facile manner, such as the type of the cofactor and its quantity within the protein. We choose here the biologically relevant protoporphyrin molecule as the electron mediator. Unlike natural systems having only Fe-containing protoporphyrins, i.e., heme, as electron mediators, we use here porphyrins with different metal centers, or lacking a metal center. We show that the metal redox center has no role in ET and that ET is mediated solely by the conjugated backbone of the molecule. We further discuss several ET mechanisms, accounting to our observations with possible contribution of coherent processes. Our findings contribute to our understanding of the participation of heme molecules in long-range biological ET.