Photon-induced Electron Transfer Research Articles

Enhanced nonlinear optical properties of MXene (Ti3C2Tx) via surface-covalent functionalization with porphyrin

The surface terminations (=O, -OH, and -F) play a key role in determining the physical and chemical properties of MXenes, which have been demonstrated with significant potential in field-effect transistors, humidity sensors, energy storage, and photocatalysis, etc. It is therefore crucial to modify these active functional groups on the surface of MXenes in order to optimize the applicability of these materials. In this study, we introduce a covalent modification strategy to successfully construct a porphyrin-functionalized Ti3C2Tx organic-inorganic nanohybrid (TPP-Ti3C2Tx) by covalently attaching porphyrin molecules to the surface groups on Ti3C2Tx nanosheets for the first time. As revealed by steady-state fluorescence spectra, transient fluorescence spectra, and DFT calculations, the robust covalent bonds between TPP and Ti3C2Tx can effectively promote the photon-induced electron and/or energy transfer within the TPP-Ti3C2Tx nanohybrid. The investigation on the nonlinear optical (NLO) properties of TPP-Ti3C2Tx nanohybrid as well as its precursors, reveals that the TPP-Ti3C2Tx nanohybrid exhibits the highest nonlinear absorption coefficient and the lowest optical limiting threshold among the tested samples at both 532 and 1064 nm, indicating its great potential as a broadband optical limiter for visible and near-infrared wavelengths. This work not only demonstrates the significant promise of covalently-linked TPP-Ti3C2Tx nanohybrid in optical limiting applications but also provides a paradigm for engineering high-performance NLO MXenes-based materials through the covalent modification strategy.

Fluorescent-Based Neurotransmitter Sensors: Present and Future Perspectives.

Neurotransmitters (NTs) are endogenous low-molecular-weight chemical compounds that transmit synaptic signals in the central nervous system. These NTs play a crucial role in facilitating signal communication, motor control, and processes related to memory and learning. Abnormalities in the levels of NTs lead to chronic mental health disorders and heart diseases. Therefore, detecting imbalances in the levels of NTs is important for diagnosing early stages of diseases associated with NTs. Sensing technologies detect NTs rapidly, specifically, and selectively, overcoming the limitations of conventional diagnostic methods. In this review, we focus on the fluorescence-based biosensors that use nanomaterials such as metal clusters, carbon dots, and quantum dots. Additionally, we review biomaterial-based, including aptamer- and enzyme-based, and genetically encoded biosensors. Furthermore, we elaborate on the fluorescence mechanisms, including fluorescence resonance energy transfer, photon-induced electron transfer, intramolecular charge transfer, and excited-state intramolecular proton transfer, in the context of their applications for the detection of NTs. We also discuss the significance of NTs in human physiological functions, address the current challenges in designing fluorescence-based biosensors for the detection of NTs, and explore their future development.

An AIE active imidazole conjugated α-cyanostilbene based sensor for the selective and sensitive detection of picric acid in an aqueous medium

Due to increasing pollution threats, terrorism-sensitive rapid sensing of aromatic nitro explosives (picric acid) has gained predominant significance in environmental safety. Herein, imidazole-derived monofunctional fluorescent sensor 2-(4-aminophenyl)-3-(3-(4,5-diphenyl-1H-imidazol-2-yl)-2-hydroxyphenyl) acrylonitrile (ADHA) was successfully developed for the selective recognition of explosive picric acid. The D-π-A configuration of ADHA facilitates extraordinary photophysical properties with remarkable aggregation-induced emission (AIE). The detection process is induced by the photon-induced electron transfer (PET) and resonance energy transfer (RET) and results in the generation of ADHA+PA complex. The structural relationship and the photophysical properties of ADHA were extensively studied by DFT (Density Functional Theory) methods and spectroscopic analysis. It has been calculated that the detection limit of the formed complex is 6.26 nM. In addition, 1H NMR titrations DFT calculations, and HRMS analysis were performed to understand the detection mechanism better. Test strip-aided detection and invisible ink applications confirmed that ADHA is a versatile sensor for sensitively detecting picric acid without sophisticated instruments. In addition, ADHA was implemented to detect PA in real water samples with remarkable recovery.

Azocalixarene-Based Supramolecular System for the Detection of Paraquat via an Improved Indicator Displacement Assay

In view of the lethal toxicity of paraquat (PQ) on human health, herein, a simple indicator displacement assay (IDA) based on an azo-modified calixarene host (azoCX[4]) and a fluorophore guest (p-DPD) were elaborately constructed for PQ detection in environmental water samples and plant surfaces. The fluorescent signal of p-DPD in the probe can be quenched by azoCX[4] through a photon-induced electron transfer process and recovered upon the addition of PQ within 10 s. The detection range of the p-DPD@azoCX[4] probe was calculated to be 0.35-8 μM in the Tris-HCl buffer solutions (pH = 7.4). Moreover, this probe exhibited excellent detection selectivity toward PQ over five herbicides (glyphosate, bispyribac, atrazine, ametryn, and bensulfuron methyl), together with anti-interference abilities in the presence of inorganic ions (K+, Na+, Zn2+, Ni2+, Li+, F-, Cl-, Br-, CO32-, HCO3-, and NO3-) and amino acids (Asp, Arg, Glu, Ala, and Cys). Particularly, the probe was successfully used to detect PQ in real water samples with acceptable accuracy and showed potential applications for on-site detection with paper-based test strips and on the leaf surface. We believe that this simplified IDA-based probe provided an effective detecting tool for PQ, and the design strategy would guide the further development of new IDA sensing systems.

Single-crystal UV/Vis absorption spectroscopy of aluminosilicate garnet: Part III. {Fe2+} + [Fe3+] → {Fe3+} + [Fe2+] intervalence charge transfer

AbstractThe various intervalence charge transfer (IVCT) mechanisms that can occur in silicate garnet, general crystal-chemical formula {X3}[Y2](Z3)O12, are not fully understood. The single-crystal UV/Vis/NIR absorption spectra of two different almandine-rich, spessartine-rich and grossular-rich garnets, as well as an intermediate almandine-pyrope garnet, were measured. Absorption was observed from roughly 15 000 to 30 000 cm–1. The spectra were deconvoluted and a very broad band with FWHM values ranging from 5000 to 7000 cm–1 (except in the case of one grossular where the FWHM is 8700 cm–1) and having an intensity maximum located between about 20 000 and 22 000 cm–1 in the visible region could be fit. Small weaker features located on this broad band were fit as well. The broad band is strongest in a nearly end-member composition almandine and weakest in a very grossular-rich iron-poor crystal. It is assigned to {Fe2+} + [Fe3+] → {Fe3+} + [Fe2+] IVCT. This is the first recognition of this type of electronic transition mechanism in different aluminosilicate garnet species. Photon-induced electron transfer probably occurs through an overlap of the d orbitals of Fe2+ and Fe3+ in their edge-shared triangular dodecahedral and octahedral coordination polyhedra, respectively. The two Fe cations with different formal charges should have markedly different energy potentials giving rise to asymmetric IVCT behavior. This, together with the relatively long Fe2+-Fe3+ distances (greater than 3.2 Å), could explain the higher energy of the IVCT in garnet compared to Fe2+ + Fe3+ → Fe3+ + Fe2+ IVCT mechanisms observed in other minerals. The latter typically have iron cations in octahedral or quasi-octahedral coordination. The IVCT in aluminosilicate garnet can occur in different species that grew under dissimilar P-T-X conditions. The resulting electronic absorption band affects color markedly, because it is centered at higher energies in the blue visible region. It remains to be determined why IVCT is observed in the spectra of some garnets but not others. The various proposed IVCT mechanisms in Ca-Ti-bearing and aluminosilicate garnets are reviewed and analyzed.

Photon-Induced Electron Transfer in Ligand-Stabilized Monoclinic CsPbBr3 and Alanine-Functionalized Graphene Heterostructures

Photon-Induced Electron Transfer in Ligand-Stabilized Monoclinic CsPbBr₃ and Alanine-Functionalized Graphene Heterostructures

Peptide-RNA complexation-induced fluorescence “turn on” displacement assay for the recognition of small ligands targeting HIV-1 RNA

The regulator of expression of virion (Rev) protein binds specifically to the Rev-responsive element (RRE) RNA in order to regulate the expression of the human immunodeficiency virus (HIV)-1 genes. Fluorescence indicator displacement assays have been used to identify ligands that can inhibit the Rev–RRE interaction; however, the small fluorescence indicators cannot fully replace the Rev peptide or protein. As a result, a single rhodamine B labeled Rev (RB-Rev) model peptide was utilized in this study to develop a direct and efficient Rev–RRE inhibitor screening model. Due to photon-induced electron transfer quenching of the tryptophan residue on the RB fluorophore, the fluorescence of RB in Rev was weakened and could be dramatically reactivated by interaction with RRE RNA in ammonium acetate buffer (approximately six times). The interaction could reduce the electron transfer between tryptophan and RB, and RRE could also increase RB fluorescence. The inhibitor screening model was evaluated using three known positive Rev–RRE inhibitors, namely, proflavin, 6-chloro-9-[3-(2-chloroethylamino)propylamino]-2-methoxyacridine (ICR 191), and neomycin, as well as a negative drug, arginine. With the addition of the positive drugs, the fluorescence of the Rev–RRE decreased, indicating the displacement of RB-Rev. This was confirmed using atomic force microscopy (AFM) and the fluorescence was essentially unaffected by the addition of arginine. The results demonstrated that RB-Rev can be used as a fluorescent probe for recognizing small ligands that target RRE RNA. The Rev–RRE inhibitor screening model offers a novel approach to evaluating and identifying long-acting Rev inhibitors.

Interaction of 4-nitrothiophenol with low energy electrons: Implications for plasmon mediated reactions.

The reduction of 4-nitrothiophenol (NTP) to 4-4'-dimercaptoazobenzene (DMAB) on laser illuminated noble metal nanoparticles is one of the most widely studied plasmon mediated reactions. The reaction is most likely triggered by a transfer of low energy electrons from the nanoparticle to the adsorbed molecules. Besides the formation of DMAB, dissociative side reactions of NTP have also been observed. Here, we present a crossed electron-molecular beam study of free electron attachment to isolated NTP in the gas-phase. Negative ion yields are recorded as a function of the electron energy, which helps to assess the accessibility of single electron reduction pathways after photon induced electron transfer from nanoparticles. The dominant process observed with isolated NTP is associative electron attachment leading to the formation of the parent anion of NTP. Dissociative electron attachment pathways could be revealed with much lower intensities, leading mainly to the loss of functional groups. The energy gained by one electron reduction of NTP may also enhance the desorption of NTP from nanoparticles. Our supporting experiments with small clusters, then, show that further reaction steps are necessary after electron attachment to produce DMAB on the surfaces.

Synthesis of Isofagomine Derivatives as New Fluorescence pH Indicators/Glycosidase Inhibitors

Inhibitor protonation of azasugars of the isofagomine type when bound to enzyme can be investigated using photon induced electron transfer (PET) quenching of an attached fluorophore. For this purpose, Isofagomine, iso‐d‐galacto‐fagomine, and iso‐l‐gulo‐fagomine were converted to N‐(10‐chloroanthracenenyl‐9‐alkyl) derivatives where the alkyl group contained one, two, or three methylene groups. The new derivatives displayed pH dependent fluorescence; as the ammonium forms they were fluorescent, while 90–99 % of the fluorescence was quenched in the amine forms. The 3 isofagomine derivatives were competitive inhibitors of T. Maritima Ι‐glucosidase with Ki values from 0.37–4.6ΤM. Similarly, the iso‐d‐galacto‐fagomines inhibited A. Niger Ι‐galactosidase with Ki values from 63–2000 nm. When bound to the enzymes the inhibitors displayed between 1–15 % fluorescence.

The Role of the Hydrogen Bond between Piperazine and Fullerene Molecules in Stabilizing Polymer:Fullerene Solar Cell Performance.

Piperazine has been recently reported as a stabilizer for polymer:fullerene solar cells that can minimize the "burn-in" degradation of the cell. In this paper, the influence of N-substituents on the stabilization effect of piperazine in P3HT:PC61BM cells was investigated. Results confirmed that only piperazine derivatives (PZs) with N-H bonds showed the stabilization effect, whereas the bis-alkyl-substituted piperazine compounds cannot improve the stability. An efficient photon-induced electron transfer (PET) process between PZ and PC61BM was only detected for the N-H-containing PZ:PC61BM blends, corresponding very well to the stabilization effect of the PZs, which indicates that the PET process between PZ and PC61BM stabilizes the cell performance, and the N-H bond plays a critical role ensuring the PET process and the consequent stabilization effect. Both 1H-NMR spectroscopy and theoretical calculations confirmed the formation of N-H···O-C and N-H···π bonds for the PC61BM:piperazine adduct, which was considered as the driving force that promotes the PET process between these two components. In addition, comparison of the calculated electron affinity energy (EA) and excitation energy (EEx) of PC61BM with/without piperazine confirmed that piperazine doping is able to promote the electron transfer (which leads to the formation of PC61BM anions) than the energy transfer (leads to the formation of PC61BM excitons) between P3HT and PC61BM, which is beneficial for the performance and stability improvement.

Switching sides—Reengineered primary charge separation in the bacterial photosynthetic reaction center

We report 90% yield of electron transfer (ET) from the singlet excited state P* of the primary electron-donor P (a bacteriochlorophyll dimer) to the B-side bacteriopheophytin (HB) in the bacterial photosynthetic reaction center (RC). Starting from a platform Rhodobacter sphaeroides RC bearing several amino acid changes, an Arg in place of the native Leu at L185-positioned over one face of HB and only ∼4 Å from the 4 central nitrogens of the HB macrocycle-is the key additional mutation providing 90% yield of P+HB - This all but matches the near-unity yield of A-side P+HA - charge separation in the native RC. The 90% yield of ET to HB derives from (minimally) 3 P* populations with distinct means of P* decay. In an ∼40% population, P* decays in ∼4 ps via a 2-step process involving a short-lived P+BB - intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P+HB - conversion takes place in ∼20 ps by a superexchange mechanism mediated by BB An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of A- versus B-side initial charge separation in native RCs and have implications for the mechanism(s) and timescale of initial ET that are required to achieve a near-quantitative yield of unidirectional charge separation.

"Dual-Key-and-Lock" Ruthenium Complex Probe for Lysosomal Formaldehyde in Cancer Cells and Tumors.

Biomedical investigations reveal that excessive formaldehyde generation is possibly a critical factor for tissue cancerization, cancer progression, and metastasis. Responsive molecular probes that can detect lysosomal formaldehyde in live cells and tumors and monitor drug-triggered formaldehyde scavenging contribute potentially to future cancer diagnosis and treatment monitoring. Herein, a novel "dual-key-and-lock" strategy-based ruthenium(II) complex probe, Ru-FA, is reported as an effective tool for formaldehyde detection in vitro and in vivo. Ru-FA shows weak luminescence due to photon-induced electron transfer (PET) process from Ru(II) center to electron withdrawing group 2,4-dinitrobenzene (DNB). Triggered by the specific reaction with formaldehyde (first "key") in an acidic microenvironment (second "key"), DNB is cleaved from Ru-FA, affording an emissive Ru(II) complex derivative, Ru-NR. Spectrometric analysis including steady-state and time-gated luminescence indicates that Ru-FA is favorable to be used as the probe for quantification of formaldehyde in human sera and mouse organs. Ru-FA is biocompatible and cell membrane permeable. Together with its smart "dual-key-and-lock" response to formaldehyde, luminescence imaging of lysosomal formaldehyde in live cells, visualization of tumor-derived endogenous formaldehyde, and monitoring of formaldehyde scavenging in mice were achieved, followed by the successful demonstration on detection of formaldehyde in tumors and other organs. These in vivo and in vitro detection confirm not only the excessive formaldehyde generation in tumors, but also the efficient drug administration to scavenge formaldehyde, demonstrating the potential application of Ru-FA in cancer diagnosis and treatment monitoring through lysosomal formaldehyde detection.

Effects of metal free side chain on the optical properties of the probe for hydrogen peroxide detecting and responsive mechanism

Optical properties and responsive mechanisms of the two fluorescent probes Mol1 and Mol2 for Hydrogen peroxide (H2O2) are investigated by using time-dependent density functional theory. The result shows that the absorption and emission wavelengths of the probe Mol2 appear red shift comparing with the probe Mol1, which is due to the influence of the two side chains of the probe Mol2. The Mol2 has better probing properties. Moreover, responsive mechanisms of the probes are studied by analyzing the distributions of molecular orbitals and charge transfer, which are shown as the photon-induced electron transfer (PET) for them. Specially, the energy gaps of the two product molecules are calculated with optimal Hartree-Fock (OHF) method. We find that the S-T energy gap (ΔEst) of the product of probe Mol2 is 0.31 eV, which is narrower than 0.39 eV of the product of probe Mol1. Also, it is less than 0.42 eV which is the generation threshold of delayed fluorescence. From the computational results, one can see that the probe Mol2 is a thermally activated delayed fluorescence (TADF) probe that has excellent performance, according to the probing behavior. Our computational results have a guiding significance for further experimental works.

The effects of polybenzimidazole and polyacrylic acid modified carbon black on the anti-UV-weathering and thermal properties of polyvinyl chloride composites

In this study, the effects of polybenzimidazole (PBI) and polyacrylic acid (PAA) modified carbon black (MCB) on the anti-UV-weathering and thermal properties of PVC composites were investigated. The optimal mass ratios of PBI and MCB to PVC are 0.1 wt% and 0.2 wt%, respectively, and the resultant 1PBI/2MCB/PVC composite membrane with a thickness of about 60 μm can block over 99% of UV light below 380 nm. The UV absorption mechanism was investigated by the optional band gap (Eg) and the fluorescence spectroscopy. The incorporation of PBI and MCB results in a decrease in Eg, from 4.96 eV for PVC membrane to 2.90 eV for 1PBI/2MCB/PVC composite membrane, and MCB can quench the fluorescence of PBI by photon-induced electron transfer to further protect PVC. The results of accelerated UV-weathering experiment indicate that the incorporation of PBI and MCB can improve the anti-UV-weathering property of PVC. The thermal degradation behaviors of PVC and its composite membranes in air and N2 atmosphere were also investigated. The highest char residue (13.7 wt%) is obtained in 1PBI/2MCB/PVC composite membrane at 800 °C in N2 atmosphere, with an increase of 73.4% compared with that of PVC membrane, which may be because PBI and MCB can synergistically accelerate the carbonization of PVC molecules to rapidly form stable char residue.

Theoretical Design of a Two-Photon Fluorescent Probe for Nitric Oxide with Enhanced Emission Induced by Photoninduced Electron Transfer.

In the present work, we systematically investigate the sensing abilities of two recently literature-reported two-photon fluorescent NO probes, i.e., the o-phenylenediamine derivative of Nile Red and the p-phenylenediamine derivative of coumarin. The recognition mechanisms of these probes are studied by using the molecular orbital classifying method, which demonstrates the photoinduced electron transfer process. In addition, we have designed two new probes by swapping receptor units present on fluorophores, i.e., the p-phenylenediamine derivative of Nile Red and the o-phenylenediamine derivative of coumarin. However, it illustrates that only the latter has ability to function as off-on typed fluorescent probe for NO. More importantly, calculations on the two-photon absorption properties of the probes demonstrate that both receptor derivatives of coumarin possess larger TPA cross-sections than Nile Red derivatives, which makes a better two photon fluorescent probe. Our theoretical investigations reveal that the underlying mechanism satisfactorily explain the experimental results, providing a theoretical basis on the structure-property relationships which is beneficial to developing new two-photon fluorescent probes for NO.

Responsive mechanism of three novel hypochlorous acid fluorescent probes and solvent effect on their sensing performance**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374195 and 11404193) and the Taishan Scholar Program of Shandong Province, China.

Optical properties and responsive mechanisms of three newly synthesized fluorescent probes for hypochlorous acid (HOCl) are investigated by employing time-dependent density functional theory. The computational results show that the absorption and emission properties of these probes change obviously when they react with hypochlorous acid. It is found that the probe FHZ has the best performance according to the probing behavior. Moreover, the responsive mechanisms of the probes are studied by analyzing the distributions of molecular orbitals and charge transfer, which are shown as the photon-induced electron transfer (PET) for FHZ and the intramolecular charge transfer (ICT) for the other two probes. Specially, solvent effect on optical properties of the probe FHZ before and after reaction is studied within the polarizable continuum model (PCM). It is shown that performance of the probe depends crucially on the solvent polarity. Our computational results agree well with the experimental measurement, and provide information for design of efficient two-photon fluorescent probes.

Gold nanoparticles modified by new conjugated S=C=N terminal and its biological imaging application

Gold nanostructures had recently become the most interesting areas of scientific endeavor. Herein, a novel nanostructure material was designed by S1 (2, 2': 6′, 2″-terpyridyl derivative Zn(SCN)2 complex) modified gold nanoparticles with obvious improving two photon absorption (TPA) cross section (σ), the fluorescence intensity, lifetime, fluorescence quantum yield due to the enhancement of intramolecular charge transfer (ICT) process. Compared with R1 (3-([2, 2': 6′, 2″-terpyridin]-4'-yl) -9-hexyl-9H-carbazole), a conjugated ligand of carbazolyl styryl terpyridine derivatives, capping with Ag nanoparticles, photon induced electron transfer (PET) process led to the fluorescence quenching. The 1H NMR study on the coordination reaction mechanism provided a new insight into the great mystery of nanoscience. Other related characterizations, such as IR and Far IR, could also prove the interaction of Au NPs and S1. The composite material with good nonlinear optical properties had potential applications in vitro cellular imaging.

Near-infrared fluorescence probe: BSA-protected gold nanoclusters for the detection of metronidazole and related nitroimidazole derivatives

BSA-protected gold nanoclusters (AuNCs@BSA), as near infrared fluorescence probes, are synthesized to detect metronidazole (MTZ) and nitroimidazole derivatives, based on a photon-induced electron transfer (PET) process.

Monitoring Dynamic Cellular Redox Homeostasis Using Fluorescence-Switchable Graphene Quantum Dots.

Monitoring cellular redox homeostasis is critical to the understanding of many physiological functions ranging from immune reactions to metabolism, as well as to the understanding of pathological development ranging from tumorigenesis to aging. Nevertheless, there is currently a lack of appropriate probes for this ambition, which should be reversibly, sensitively, and promptly responsive to a wide range of physiological oxidants and reductants. In this work, a redox-sensitive fluorescence-switchable probe is designed based on graphene quantum dots (GQDs) functionalized with a chelated redox Fe2+/Fe3+ couple. The underlying mechanism is investigated and discussed. The high sensitivity and fast response are attributable to the fact that the GQD's photoluminescence is highly sensitive to photon-induced electron transfer because of its ultrasmall size and associated prominent quantum confinement effect. Also taking advantages of GQDs' excellent photostability, biocompatibility, and readiness for cell uptake, our reversibly tunable fluorescence probe is employed to monitor in real time the triggered dynamic change of the intracellular redox state. This addition to the limited arsenal of available redox probes shall be useful to the still poorly understood redox biology, as well as for monitoring environment or chemical processes involving redox reactions.

Graphene-Based Fluorescence-Quenching-Related Fermi Level Elevation and Electron-Concentration Surge.

Intermolecular p-orbital overlaps in unsaturated π-conjugated systems, such as graphene and fluorescent molecules with aromatic structure, serve as the electron-exchanged path. Using Raman-mapping measurements, we observe that the fluorescence intensity of fluorescein isothiocyanate (FITC) is quenched by graphene, whereas it persists in graphene-absent substrates (SiO2). After identifying a mechanism related to photon-induced electron transfer (PET) that contributes to this fluorescence quenching phenomenon, we validate this mechanism by conducting analyses on Dirac point shifts of FITC-coated graphene. From these shifts, Fermi level elevation and the electron-concentration surge in graphene upon visible-light impingements are acquired. Finally, according to this mechanism, graphene-based biosensors are fabricated to show the sensing capability of measuring fluorescently labeled-biomolecule concentrations.