Two-photon Excitation Research Articles

Efficient Energy Transfer Enabled by Dark States in van der Waals Heterostructures.

Dark exciton states show great potential in condensed matter physics and optoelectronics because of their long lifetime and rich distribution in band structures. Therefore, they can theoretically serve as efficient energy reservoirs, providing a platform for future applications. However, their optical-transition-forbidden nature severely limits their experimental exploration and hinders their current application. Here, we demonstrate a universal dark state nonlinear energy transfer (ET) mechanism in monolayer WS2/CsPbBr3 van der Waals heterostructures under two-photon excitation, which successfully utilizes the enormous energy reserved in the dark exciton state of CsPbBr3 to significantly improve the photoelectric performance of monolayer WS2. We first propose the scenario of resonant ET between the dark state of CsPbBr3 and WS2, and then reveal that this is a typical Förster resonant ET and belongs to the 2D-2D category. Interestingly, the dark state ET in CsPbBr3 is identified as a long-range donor-bridge-acceptor hopping mode, with a potential distance exceeding 200 nm. Finally, we successfully achieve nearly an order of magnitude enhancement in the near-infrared detection performance of monolayer WS2. Our results enrich the theory of dark exciton states and ET, and they provide a way of using dark exciton states for future practical applications.

Dendritic spines of layer 5 pyramidal neurons of the aging somatosensory cortex exhibit reduced volumetric remodeling.

Impairments in synaptic dynamics and stability are observed both in neurodegenerative disorders and in the healthy aging cortex, which exhibits elevated dendritic spine turnover and decreased long-term stability of excitatory connections at baseline, as well as an altered response to plasticity induction. In addition to the discrete gain and loss of synapses, spines also change in size and strength both during learning and in the absence of neural activity, and synaptic volume has been associated with stability and incorporation into memory traces. Furthermore, intrinsic dynamics, an apparently stochastic component of spine volume changes, may serve as a homeostatic mechanism to prevent stabilization of superfluous connections. However, the effects of age on modulation of synaptic weights remain unknown. Using two-photon excitation (2PE) microscopy of spines during chemical plasticity induction in vitro and analyzing longitudinal in vivo 2PE images after a plasticity-inducing manipulation, we characterize the effects of age on volumetric changes of spines of the apical tuft of layer 5 pyramidal neurons of mouse primary somatosensory cortex. Aged mice exhibit decreased volumetric volatility and delayed rearrangement of synaptic weights of persistent connections, as well as greater susceptibility to spine shrinkage in response to chemical long-term depression. These results suggest a deficit in the aging brain's ability to fine-tune synaptic weights to properly incorporate and retain novel memories. This research provides the first evidence of alterations in spine volumetric dynamics in healthy aging and may support a model of impaired processing and learning in the aged somatosensory system.Significance Statement Aging is known to impact cognitive functions and sensory processing, yet the underlying mechanisms at the synaptic level remain unclear. This study investigates dendritic spine dynamics of layer 5 pyramidal neurons in the aging somatosensory cortex. By employing two-photon excitation microscopy and analyzing spine volume changes during plasticity induction, we reveal that aged mice exhibit decreased volumetric volatility and delayed synaptic weight rearrangement. These alterations may impair the brain's ability to fine-tune synaptic weights, which is crucial for incorporating and retaining new memories. This research provides the first evidence of altered spine volumetric dynamics in healthy aging, suggesting a potential mechanism for impaired processing and learning in the aged cortex. Understanding these changes could aid in mitigation of age-related cognitive decline.

Dissipative stabilization of maximal entanglement between nonidentical emitters via two-photon excitation

Two nonidentical quantum emitters, when placed within a cavity and coherently excited at the two-photon resonance, can reach stationary states of nearly maximal entanglement. In Vivas-Viaña, Martín-Cano, and Sánchez Muñoz [], we introduce a frequency-resolved Purcell effect stabilizing entangled W states among strongly interacting quantum emitters embedded in a cavity. Here we delve deeper into a specific configuration with a particularly rich phenomenology: two interacting quantum emitters under coherent excitation at the two-photon resonance. This scenario yields two resonant cavity frequencies where the combination of two-photon driving and Purcell-enhanced decay stabilizes the system into the subradiant and superradiant states, respectively. By considering the case of nondegenerate emitters and exploring the parameter space of the system, we show that this mechanism is merely one among a complex family of phenomena that can generate both stationary and metastable entanglement when driving the emitters at the two-photon resonance. We provide a global perspective of this landscape of mechanisms and contribute analytical characterizations and insights into these phenomena, establishing connections with previous reports in the literature and discussing how some of these effects can be optically detected. Published by the American Physical Society 2024

Application Exploration of the Photon Chip Technology in Real time Processing of Biomedical Images

In the biomedical field, photonic chip technology is gradually becoming the key to improving the speed and accuracy of medical imaging. The development of this technology has given us new hope in early disease diagnosis and cell monitoring. Recent research has not only sorted out the hotspots and future trends of biomedical photonics, but also paid special attention to the application of photon technology in these fields. Specifically, photonic chips have demonstrated their unique capabilities in multi-color two-photon excitation fluorescence microscopy imaging, two-photon fluorescence lifetime imaging, and two-photon fiber endoscopic imaging. These applications not only improve the clarity of imaging, but also accelerate processing speed, which is crucial for rapid diagnosis and treatment. At the end of the article, the author summarized the contributions of photonic chip technology in the field of biomedical imaging and provided prospects for future development directions.

A highly specific two-photon fluorescent probe for real-time monitoring of acetylcholinesterase in neurogenic disorders in vivo

Acetylcholinesterase (AChE) hydrolyses choline into thiocholine, which is essential for cholinergic neurons to revert to their resting state following activation. Abnormal changes in AChE activity can directly affect nervous system function. Thus, the specific detection of AChE activity is urgently needed for elucidating the function of the nervous system and diagnosing AChE-related diseases. Current methods for detecting AChE activity have several limitations, including strong background interference and poor tissue penetration. Thus, we designed and synthesized a two-photon (TP) excited fluorescent probe, WZ-AChE, for the specific detection of AChE. Briefly, a carbamate bond was chosen to specifically recognize AChE, which can also be cleaved by AChE. The product, WZ, released strong deep red fluorescence signal under TP excitation at 800 nm. Our results showed that WZ-AChE can detect AChE activity in PC12 cells with both superior sensitivity and selectivity. In addition, we successfully applied WZ-AChE to a C. elegans Parkinson's disease (PD) model and a mouse model of depression. The findings revealed that AChE activity was greater in both disease models than in the control group. To summarize, a novel tool was created to investigate the mechanisms underlying PD and depression.

Two-photon photodynamic therapy with curcumin nanocomposite

Two-photon photodynamic therapy (TP-PDT) offers an innovative approach to cancer treatment that utilizes near-infrared light to activate photosensitizers and generate reactive oxygen species (ROS) for targeted cancer cell elimination. TiO2-CUR-Sofast (TCS), which uses TiO2 nanoparticles and Sofast cationic polymer to modify curcumin (CUR), has demonstrated potential as a photosensitizer under visible light irradiation, addressing the limitations of CUR's narrow spectral range and low bioavailability. This study explores the utility of the two-photon technique to activate TCS within the infrared spectrum, aiming to enhance ROS production and penetration depth compared to traditional CUR. TCS exhibits a significantly higher ROS production at 900 nm excitation wavelength, approximately 6–7 times that of CUR, signifying a substantial increase in efficiency. In TP-PDT, TCS showed significant phototoxicity against HeLa and T24 cell lines compared to CUR. Furthermore, TCS's photodynamic efficacy is further confirmed by cell apoptosis and necrosis studies, where approximately 89 % of cells treated with TCS under 900 nm light irradiation were observed in an apoptosis/necrosis state. And the TP-PDT effect in deep tissue was simulated using pig skin. It shows that the two-photon excitation has a significant penetration depth advantage over the single-photon excitation. These results indicate that the two-photon PDT scheme of TCS has greater potential than the single-photon PDT scheme in the treatment of cancer, and provides an experimental foundation for the effective treatment of deep lesions.

Excitation energy transfer based naphthalimide-boron-dipyrromethene dyads as two-photon fluorescent probes for palladium(0) detection and live cell imaging

Increasing industrial demand has accelerated the consumption and contamination of palladium (Pd) worldwide. The implementation of fluorescent probes, especially with two-photon excitation, holds great promise for Pd detection in environmental and biological analysis. In this work, two naphthalimide (NPI)-boron-dipyrromethene (BDP) dyads (named NBH and NBN) were developed as two-photon fluorescent probes for Pd(0) detection based on an excitation energy transfer (EET) strategy, employing NPI as the energy donor and BDP as the acceptor. Spectroscopic studies revealed that both probes exhibited a fast response to Pd(0) in a pronounced fluorescence "ON-OFF" manner. The Pd-catalyzed Tsuji-Trost reaction activated an intramolecular photoinduced electron transfer (PET) process between the aniline and BDP moieties, resulting in the fluorescence quenching of the BDP fluorophore. The reaction-based probes exhibited excellent sensitivity and selectivity for Pd(0) species with a detection limit as low as 2.4 nM. These probes were successfully applied to the determination of Pd residues in drug, soil, and water samples. A simple smartphone-based platform was also constructed to determine Pd(0) via an RGB reader. With negligible cytotoxicity, confocal imaging study validated their feasibility of monitoring Pd(0) in living cells through multiple excitation channels.

A Two-Photon-Active Zr-Based Metal-Organic Framework-Based Orthogonal Nanoprobe for Recognition of Cellular Senescence.

A luminescent nanoprobe capable of orthogonal sensing of two independent events is highly significant for unbiased disease-related detection such as the detection of senescent cells. Moreover, it is invaluable that the nanoprobe possesses a two-photon excitable characteristic that is highly suitable for imaging living cells and tissues. Herein, we present a two-photon-excitable multiluminescent orthogonal-sensing nanoprobe (OS nanoprobe) capable of detecting both pH elevation and β-galactosidase (β-gal) overexpression in senescent cells. In the design, Zr-based dual-emissive metal-organic frameworks prepared from two mixed amino linkers, referred to as NH2-MU, were used as the component for the ratiometric sensing of pH; additionally, fluorogenic resorufin-β-d-galactopyranoside, linked to the NH2-MU framework, enables β-gal detection. In the OS nanoprobe, the signals for pH and β-gal sensing remain independent while maintaining high colocalization. The two-photon excitable organic linkers of NH2-MU impart the OS nanoprobe with a bioimaging capability, allowing for the differentiation of senescent human foreskin fibroblast (HFF) cells from younger HFF cells or LacZ positive cells with the 800 nm laser excitation. This study marks the first instance of achieving the multiplexed orthogonal fluorescent sensing of cellular senescence using a two-photon excitation strategy, suggesting the potential of using versatile metal-organic framework (MOFs)-based fluorophores to realize the orthogonal multiplexing of disease-related biomarkers through multiphoton excitation.

Trapping Light, Revealing Properties: Laser Trapping as a Powerful Tool for Photoluminescence Spectroscopy

Laser trapping, a non-contact technique for manipulating microscopic objects, has gained prominence in scientific research. When coupled with fluorescence spectroscopy, it offers a powerful tool for exploring material properties. This study demonstrates the application of laser trapping in the crystallization of MAPbBr3 perovskites and its simultaneous use for photoluminescence imaging. MAPbBr3 perovskites are a class of materials with exceptional optical properties, making them attractive for various optoelectronic applications. However, conventional excitation methods using UV light can lead to phase segregation and mechanical distortion of these materials. Two-photon excitation, on the other hand, offers advantages such as deeper penetration and reduced scattering interference. In this research, we utilize a 1064 nm continuous wave laser for both trapping and excitation purposes. The MAPbBr3 perovskite, with its absorption band ranging from 400 to 540 nm, exhibits two-photon excitation at 532 nm. By focusing the laser beam at the air-solution interface, we successfully crystallize MAPbBr3 perovskites from an unsaturated precursor solution. Simultaneously, the same laser source is used for photoluminescence imaging, allowing for real-time analysis of the crystal's emission properties. This approach eliminates the need for additional excitation sources and simplifies the experimental setup. The combination of laser trapping and two-photon excitation opens up new possibilities for studying perovskite materials. It provides a gentle and non-invasive method for manipulating and characterizing hybrid perovskites materials, paving the way for advancements in various fields such as optoelectronics and energy harvesting.

Photo-induced loss of H2 from H2S, CH4, H2O and SiH4, when and why is it possible

We have explored the possibility of forming H⋅ and H2 from H2S, CH4, H2O and SiH4 in a series of ab initio calculations. The key finding is that H2S can give rise to direct photolytic ultrafast formation of H2 This finding is in agreement with the available experimental data and is a result of a directed pathway that leads toward H2 formation at easily accessible wavelengths. It arises at the (conical) intersection between the two lowest-lying electronic states. This intersection is a result of the valence orbital interaction of the two hydrogen atoms. This interaction is becoming more favorable as the S-H bonds simultaneously becomes longer. The H-S σ-σ∗ bonding interaction cannot compete with the bond formation that takes place between the hydrogens as two bonds stretch simultaneously. For oxygen this interaction it predominant. The reason is that sulfur has a higher principal quantum number and the interaction with the 1s of hydrogen therefore is less favorable. The excited states that are involved in the H2 formation from H2S could be populated by, for example, a two-photon 400 nm excitation. The other hydrides have a more entangled potential energy landscape close to the Franck-Condon region in the direction of the reaction coordinate that leads to H2 loss and as a result the direct loss of a hydrogen atom is favored. A two-photon excitation could be induced by femtosecond laser pulses that at the same time is proposed as a future platform for sensing the H2S molecules with the backward transient absorption scheme. The implication would be that both sensing and photoinduced H2 formation could be in place at the same time which justifies the use of expensive femtosecond light sources getting “two for the price of one”.

Impact of Graphene Oxide Nanoparticles on In Vitro Development of a Mouse Preimplantation Embryo andInteraction With the Zona Pellucida.

The development of assisted reproductive technologies increases the likelihood of nanoparticles' (NPs) direct contact with gametes and embryos in in vitro conditions. Analyzing the influence of nanomaterials on the early mammalian embryo becomes increasingly relevant. This work is devoted to the effect of graphene oxide (GO) NPs on the in vitro development of mammalian embryos. Mouse 2-cell embryos were preincubated with GO NPs. The interaction of GO with the Zona Pellucida (ZP) of the embryo was investigated using fluorescence lifetime imaging with two-photon excitation (2p-FLIM). During embryo development, the NPs penetration into ZP (blastocyst stage) and perivitelline space (blastocyst hatching stage) was observed. Despite this, GO did not affect the embryo's ability to develop till late and hatching blastocysts. The mechanism of the NPs getting into the perivitelline space and the consequences of NP-embryo direct contact are discussed. The 2p-FLIM efficiency for studying NP interaction with mammalian embryos is evaluated.

Interference patterns of two-photon excited fluorescence by spatial beam shaping

We report on two-photon excited fluorescence interference patterns produced by spatial laser beam shaping. Thereto, a focused spatially modulated laser beam component is overlapped with an unfocused Gaussian beam component, and both are directed on a cuvette filled with dye. The phase retardance between the two common path laser components is precisely adjusted by a 2D liquid crystal modulator. With this method it is possible to generate various fluorescence patterns which exhibit sharp features and a high contrast due to constructive and destructive interference of a two-photon excitation process. The developed spatial shaping interference technique with two-photon excitations has a high potential for optical and biophotonic imaging applications.

Modulation of High-Intensity Optical Properties in CdS/CdSe/CdS Spherical Quantum Wells by CdSe Layer Thickness.

Spherical quantum wells (SQWs) have proven to be excellent materials for suppressing Auger recombination due to their expanded confinement volume. However, research on the factors and mechanisms of their high-intensity optical properties, such as multiexciton properties and third-order optical nonlinearities, remains incomplete, limiting further optimization of these properties. Here, a series of CdS/CdSe (xML)/CdS SQWs with varying CdSe layer thicknesses were prepared. The modulation effects of CdSe shell variations on the PL properties, defect distribution, biexciton binding energy, and third-order optical nonlinearities of the SQWs were investigated, and their impact on the material's multiexciton properties was further analyzed. Results showed that the typical CdS/CdSe(3ML)/CdS sample exhibited a large volume-normalized two-photon absorption cross-section (18.17 × 102 GM/nm3) and favorable biexciton characteristics. Optical amplification was observed at 12.4 μJ/cm2 and 1.02 mJ/cm2 under one-photon (400 nm) and two-photon (800 nm) excitation, respectively. Furthermore, different amplified spontaneous emission spectra were observed for the first time under one/two-photon excitation. This phenomenon was attributed to thermal effects overcoming the biexciton binding energy. This study provides valuable insights for further optimizing multiexciton gain characteristics in SQWs and developing optical gain applications.

Perylene-Diimide-Based Supramolecular Radical Anion as a Platform for Highly Effective Photoreduction of Inert Sulfoxide to Sulfide.

Due to the limitations of common photoredox catalysts, unlocking their applications in photoreduction reactions remains an ongoing challenge. We herein present a supramolecular radical anion, PDI(CB[7])2, that formed by the assembly of perylene diimide derivative (PDI) and cucurbit[7]uril (CB[7]) via a host-guest interaction for an effective photoreduction reaction. Studies revealed that it could effectively accomplish a consecutive excitation process by two-photon excitation, enabling a potent photoreductant PDI(CB[7])2• - * that can even reduce the inert feedstocks, such as sulfoxides to sulfides. Mechanistic investigations indicate that, besides exceptional photophysical properties, supramolecular PDI(CB[7])2 also significantly enhances the lifetime and robustness of the in situ generated higher energy photoreductant PDI(CB[7])2• - * upon second quantum photon excitation, leading to the observed highly active photoreducing behavior.

Impact of Classical and Quantum Light on Donor-Acceptor-Donor Molecules.

Investigations of entangled and classical two-photon absorption have been carried out for six donor (D)-acceptor (A)-donor (D) compounds containing the dithieno pyrrole (DTP) unit as donor and acceptors with systematically varied electronic properties. Comparing ETPA (quantum) and TPA (classical) results reveals that the ETPA cross section decreases with increasing TPA cross section for molecules with highly off-resonant excited states for single-photon excitation. Theory (TDDFT) results are in semiquantitative agreement with this anticorrelated behavior due to the dependence of the ETPA cross section but not TPA on the two-photon excited state lifetime. The largest cross section is found for a DTP derivative that has a single photon excitation energy closest to resonance with half the two-photon excitation energy. These results are important for the possible use of quantum light for low-intensity energy-conversion applications.

Insights into the Mechanisms of Single-Photon and Two-Photon Excited Surface Enhanced Fluorescence by Submicrometer Silver Particles.

Surface enhanced fluorescence (SEF) based on noble metal nanoparticles is an effective means to achieve high sensitivity in fluorescence detection. Currently, the physical mechanism behind enhanced fluorescence is not fully understood. This paper measures the fluorescence signals of Dihydroporphyrin f methyl ether (CPD4) under both single-photon and two-photon excitation based on submicrometer silver particles with rough morphologies, achieving enhancement factors of 34 and 45 times, respectively. On this basis, by combining the radiative field characteristics produced by the silver particles, a stimulated radiation model of molecules is established to elucidate the changes in the molecular photophysical process when influenced by silver particles. Moreover, the fluorescence lifetime of the molecules was measured, showing that the presence of silver particles induces an increase in the molecular radiative decay rate, causing the fluorescence lifetime to decay from 3.8 ns to 3 ns. The results indicate that the fluorescence enhancement primarily originates from the submicrometer silver particles' enhancement effect on the excitation light. Additionally, the fluorescence signal emitted by the molecules couples with the silver particles, causing the local surface plasmon resonances generated by the silver particles to also emit light signals of the same frequency. Under the combined effect, the fluorescence of the molecules is significantly enhanced. The findings provide a theoretical foundation for understanding the fluorescence enhancement mechanism of silver particles, adjusting the enhancement effect, and developing enhanced fluorescence detection devices based on submicrometer silver particles, holding significant practical importance.

The ATTO 565 Dye and Its Applications in Microscopy.

ATTO 565, a Rhodamine-type dye, has garnered significant attention due to its remarkable optical properties, such as a high fluorescence quantum yield, and the fact that it is a relatively stable structure and has low biotoxicity. ATTO 565 has found extensive applications in combination with microscopy technology. In this review, the chemical and optical properties of ATTO 565 are introduced, along with the principles behind them. The functionality of ATTO 565 in confocal microscopy, stimulated emission depletion (STED) microscopy, single-molecule tracking (SMT) techniques, two-photon excitation-stimulated emission depletion microscopy (TPE-STED) and fluorescence correlation spectroscopy (FCS) is discussed. These studies demonstrate that ATTO 565 plays a crucial role in areas such as biological imaging and single-molecule localization, thus warranting further in-depth investigations. Finally, we present some prospects and concepts for the future applications of ATTO 565 in the fields of biocompatibility and metal ion detection. This review does not include theoretical calculations for the ATTO 565 molecule.

Photobleaching analysis of fluorescent proteins in two-photon microscopy at high repetition rates

Severe photobleaching in two-photon excitation fluorescence microscopy (TPM) has limited its potential for long-term and quantitative imaging of live cells and tissues. One solution is to excite fluorophores with a high-repetition rate which reduces the peak intensity of irradiance. However, there is a lack of knowledge about the general utility of this strategy for fluorescent proteins. Here, using simple photobleaching assay, we studied the photobleaching of EGFP and tdTomato at 80 MHz and 640 MHz repetition rates. Our analysis shows that the impact of high repetition rate excitation on reducing photobleaching in TPM is highly dependent on the excitation wavelength and fluorophores. Our results are useful for selecting optimal parameters to minimize photobleaching and photodamage in TPM.

Cellular uptake of multi-walled carbon nanotubes is associated to genotoxic and teratogenic effects towards the freshwater diatom Nitzschia linearis

The increase in industrial production of multi-walled carbon nanotubes (MWCNTs) raises concerns about their potential adverse effects associated to environmental releases, especially in aquatic environments where they are likely to accumulate. This study focuses on the environmental impact of MWCNTs, specifically on a benthic freshwater diatom (Nitzschia linearis), which plays a major role in the primary production of water bodies. The obtained results indicate that exposure to MWCNTs in the presence of natural organic matter (NOM) inhibits diatom's growth in a dose-dependent manner after 72 h of exposure. Interestingly, the photosystem II quantum yield (PSIIQY) in diatoms remains unaffected even after exposure to MWCNTs at 10 mg/L. After 48 h of exposure, MWCNTs are found to bind preferentially to extracellular polymeric substances (EPS) produced by diatoms, which could decrease their toxicity by limiting their interaction with this organism. However, measurement of genotoxicity and teratogenicity in diatoms exposed to MWCNTs revealed that the exposure to MWCNTs increased the occurrence of cells with micronuclei and abnormal frustules. Microscopy analyses including two-photon excitation microscopy (TPEM) revealed the internalization of MWCNTs. Investigations of the diatom's frustule structure using Scanning electron microscopy (SEM) indicated that the presence of pore structures constitutes a pathway allowing MWCNTs uptake. The presence in the diatom's cytoplasm of MWCNTs might possibly induce disturbances of the cellular components, leading to the observed genotoxic and teratogenic effects. In view of previous studies, this work underscores the need for further studies on the interaction between nanomaterials and different diatom species, given the species-specific nature of the interactions.

Variational calculus approach to Zernike polynomials with application to FCS

Zernike polynomials are a sequence of orthogonal polynomials that play a crucial role in optics and, in particular, modeling microscopy systems. Introduced by Frits Zernike in 1934, they are particularly useful in expressing wavefront aberrations and, thus, imperfections of imaging systems. However, their origin and properties are rarely discussed and proven. Here, we present a novel approach to Zernike polynomials using variational calculus and apply them to describe aberrations in fluorescence microscopy. In particular, we model the impact of various optical aberrations on the performance of one-photon and two-photon excitation fluorescence microscopy.