Influence of laser parameters and staining on femtosecond laser-based intracellular nanosurgery

Epithelial restitution in 3D - Revealing biomechanical and physiochemical dynamics in intestinal organoids via fs laser nanosurgery

Intestinal organoids represent a three-dimensional cell culture system mimicking the mammalian intestine. The application of single-cell ablation for defined wounding via a femtosecond laser system within the crypt base allowed us to study cell dynamics during epithelial restitution. Neighboring cells formed a contractile actin ring encircling the damaged cell, changed the cellular aspect ratio, and immediately closed the barrier. Using traction force microscopy, we observed major forces at the ablation site and additional forces on the crypt sides. Inhibitors of the actomyosin-based mobility of the cells led to the failure of restoring the barrier. Close to the ablation site, high-frequency calcium flickering and propagation of calcium waves occured that synchronized with the contraction of the epithelial layer. We observed an increased signal and nuclear translocation of YAP-1. In conclusion, our approach enabled, for the first time, to unveil the intricacies of epithelial restitution beyond in vivo models by employing precise laser-induced damage in colonoids.

Investigation of Colonic Regeneration via Precise Damage Application Using Femtosecond Laser-Based Nanosurgery

Organoids represent the cellular composition of natural tissue. So called colonoids, organoids derived from colon tissue, are a good model for understanding regeneration. However, next to the cellular composition, the surrounding matrix, the cell-cell interactions, and environmental factors have to be considered. This requires new approaches for the manipulation of a colonoid. Of key interest is the precise application of localized damage and the following cellular reaction. We have established multiphoton imaging in combination with femtosecond laser-based cellular nanosurgery in colonoids to ablate single cells in the colonoids' crypts, the proliferative zones, and the differentiated zones. We observed that half of the colonoids recovered within six hours after manipulation. An invagination of the damaged cell and closing of the structure was observed. In about a third of the cases of targeted crypt damage, it caused a stop in crypt proliferation. In the majority of colonoids ablated in the crypt, the damage led to an increase in Wnt signalling, indicated via a fluorescent lentiviral biosensor. qRT-PCR analysis showed increased expression of various proliferation and Wnt-associated genes in response to damage. Our new model of probing colonoid regeneration paves the way to better understand organoid dynamics on a single cell level.

Femtosecond laser oocyte enucleation as a low-invasive and effective method of recipient cytoplast preparation

Recipient cytoplast preparation, commonly performed by DNA aspiration with a needle, inevitably leads to the loss of reprogramming factors. As an alternative to the traditional enucleation technique, femtosecond laser enucleation can eliminate DNA effectively without loss of reprogramming factors and without oocyte puncturing. In this work we have performed oocyte enucleation by destructing the metaphase plate using a 795 nm femtosecond laser. The disability of the enucleated oocytes to develop after the parthenogenetic activation, as well as the lack of DNA staining luminescence, strongly confirms the efficiency of the femtosecond laser enucleation. The parthenogenetic development of oocytes after the cytoplasm treatment suggests a low-invasive effect of the laser enucleation technique.

Evaluation of laser induced sarcomere micro-damage: Role of damage extent and location in cardiomyocytes

Whereas it is evident that a well aligned and regular sarcomeric structure in cardiomyocytes is vital for heart function, considerably less is known about the contribution of individual elements to the mechanics of the entire cell. For instance, it is unclear whether altered Z-disc elements are the reason or the outcome of related cardiomyopathies. Therefore, it is crucial to gain more insight into this cellular organization. This study utilizes femtosecond laser-based nanosurgery to better understand sarcomeres and their repair upon damage. We investigated the influence of the extent and the location of the Z-disc damage. A single, three, five or ten Z-disc ablations were performed in neonatal rat cardiomyocytes. We employed image-based analysis using a self-written software together with different already published algorithms. We observed that cardiomyocyte survival associated with the damage extent, but not with the cell area or the total number of Z-discs per cell. The cell survival is independent of the damage position and can be compensated. However, the sarcomere alignment/orientation is changing over time after ablation. The contraction time is also independent of the extent of damage for the tested parameters. Additionally, we observed shortening rates between 6–7% of the initial sarcomere length in laser treated cardiomyocytes. This rate is an important indicator for force generation in myocytes. In conclusion, femtosecond laser-based nanosurgery together with image-based sarcomere tracking is a powerful tool to better understand the Z-disc complex and its force propagation function and role in cellular mechanisms.

Wavelength-Selective Nonlinear Imaging and Photo-Induced Cell Damage by Dielectric Harmonic Nanoparticles

We introduce a nonlinear all-optical theranostics protocol based on the excitation wavelength decoupling between imaging and photoinduced damage of human cancer cells labeled by bismuth ferrite (BFO) harmonic nanoparticles (HNPs). To characterize the damage process, we rely on a scheme for in situ temperature monitoring based on upconversion nanoparticles: by spectrally resolving the emission of silica coated NaGdF4:Yb³⁺/Er³⁺ nanoparticles in close vicinity of a BFO HNP, we show that the photointeraction upon NIR-I excitation at high irradiance is associated with a temperature increase >100 °C. The observed laser-cell interaction implies a permanent change of the BFO nonlinear optical properties, which can be used as a proxy to read out the outcome of a theranostics procedure combining imaging at 980 nm and selective cell damage at 830 nm. The approach has potential applications to monitor and treat lesions within NIR light penetration depth in tissues.

Femtosecond laser-based nanosurgery reveals the endogenous regeneration of single Z-discs including physiological consequences for cardiomyocytes

A highly organized cytoskeleton architecture is the basis for continuous and controlled contraction in cardiomyocytes (CMs). Abnormalities in cytoskeletal elements, like the Z-disc, are linked to several diseases. It is challenging to reveal the mechanisms of CM failure, endogenous repair, or mechanical homeostasis on the scale of single cytoskeletal elements. Here, we used a femtosecond (fs) laser to ablate single Z-discs in human pluripotent stem cells (hPSC) -derived CMs (hPSC-CM) and neonatal rat CMs. We show, that CM viability was unaffected by the loss of a single Z-disc. Furthermore, more than 40% of neonatal rat and 68% of hPSC-CMs recovered the Z-disc loss within 24 h. Significant differences to control cells, after the Z-disc loss, in terms of cell perimeter, x- and y-expansion and calcium homeostasis were not found. Only 14 days in vitro old hPSC-CMs reacted with a significant decrease in cell area, x- and y-expansion 24 h past nanosurgery. This demonstrates that CMs can compensate the loss of a single Z-disc and recover a regular sarcomeric pattern during spontaneous contraction. It also highlights the significant potential of fs laser-based nanosurgery to physically micro manipulate CMs to investigate cytoskeletal functions and organization of single elements.

Novel Polymer-Free Everolimus-Eluting Stent Fabricated using Femtosecond Laser Improves Re-endothelialization and Anti-inflammation

The aim of this study was to fabricate a novel polymer-free everolimus-eluting stent with nanostructure using a femtosecond laser (FSL). The stent were coated with everolimus (EVL) using FSL and electrospinning processes. The surface was rendered hydrophobic, which negatively affected both platelet adhesion (82.1%) and smooth muscle cell response. Animal study was performed using a porcine coronary restenosis model. The study groups were divided into 1) bare metal stent (BMS), 2) poly(L-lactide) (PLA)-based EVL drug eluting stent (DES), 3) commercial EVL-eluting DES, and 4) FSL-EVL-DES. After four weeks of stent implantation, various analyses were performed. Quantitative analysis showed that the amount of in-stent restenosis was higher in the BMS group (BMS; 27.8 ± 2.68%, PLA-based DES; 12.2 ± 0.57%, commercial DES; 9.8 ± 0.28%, and FSL-DES; 9.3 ± 0.25%, n = 10, p < 0.05). Specifically, the inflammation score was reduced in the FSL-DES group (1.9 ± 0.39, n = 10, p < 0.05). The increment in re-endothelialization in the FSL-DES group was confirmed by immunofluorescence analysis. Taken together, the novel polymer-free EVL-eluting stent fabricated using FSL can be an innovative DES with reduced risk of ISR, thrombosis, and inflammation.

Targeted Ablation Using Laser Nanosurgery

Laser-mediated dissection methods have been used for many years to micro-irradiate biological samples, but recent technological progress has rendered this technique more precise, powerful, and easy to use. Today pulsed lasers can be operated with diffraction limited, sub-micrometer precision to ablate intracellular structures. Here, we discuss laser nanosurgery setups and the instrumentation in our laboratory. We describe how to use this technique to ablate cytoskeletal elements in living cells. We also show how this technique can be used in multicellular organisms, to micropuncture and/or ablate cells of interest and finally how to monitor a successful laser nanosurgery.

Effect of Hedyotis diffusa water extract on protecting human hepatocyte cells (LO2) from H2O2-induced cytotoxicity

CONTEXT

Natural products are good sources of natural dietary antioxidants that are believed to protect the body against hepatotoxic effect induced by oxidative stress. Hedyotis diffusa Willd (Rubiaceae) (HDW) is a traditional Chinese medicinal herb that has been shown to possess a variety of antioxidant properties.

OBJECTIVE

The present study examines and explains the cell protective property of HDW water extract (WEHDW).

MATERIALS AND METHODS

2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH) assay was used to measure the free radical scavenging property of WEHDW (0.001-10 mg/mL). The protective effect of WEHDW (0.3-10 mg/mL 2 h pretreatment) against hydrogen peroxide (H2O2, 200 μM for 6 h) induced cytotoxicity in human hepatic cells, LO2, was evaluated using cell viability assay and nuclear staining. The molecular pathway of WEHDW's effect was investigated by using Western blot assay.

RESULTS

WEHDW had a 50% scavenging concentration (SC50) at 0.153 mg/mL in the DPPH assay. Exposure of LO2 cells to H2O2 resulted in apoptosis which could be markedly attenuated by pre-treating WEHDW in a concentration-dependent manner (0.5, 1, 3, 5, or 10 mg/mL) (all with p < 0.001, versus control). Moreover, Hoechst (nuclear) staining showed that 1 mg/mL WEHDW could protect LO2 cells by attenuating apoptotic cell death mediated by H2O2. It was found that WEHDW reversed H2O2-induced activation of MEK/ERK pathway and H2O2-induced inhibition of P13-K/AKT/GSK3β pathway in LO2 cells.

DISCUSSION AND CONCLUSION

WEHDW may help to improve the antioxidant defense system, resulting in prevention of oxidative stress-related fatty liver diseases.

Intravital Imaging Study on Photodamage Produced by Femtosecond Near-infrared Laser Pulses In Vivo

Ultrashort femtosecond pulsed lasers may provide indispensable benefits for medical bioimaging and diagnosis, particularly for noninvasive biopsy. However, the ability of femtosecond laser irradiation to produce biodamage in the living body is still a concern. To solve this biosafety issue, results of theoretical estimations as well as the in vitro and in situ experiments on femtosecond biodamage should be verified by experimental studies conducted in vivo. Here, we analyzed photodamage produced by femtosecond (19, 42 and 100 fs) near-infrared (NIR; ~800 nm) laser pulses with an average power of 5 and 15 mW in living undissected Drosophila larvae (in vivo). These experimental data on photodamage in vivo agree with the results of theoretical modeling of other groups. Femtosecond NIR laser pulses may affect the concentration of fluorescent biomolecules localized in mitochondria of the cells of living undissected Drosophila larva. Our findings confirm that the results of the mathematical models of femtosecond laser ionization process in living tissues may have a practical value for development of noninvasive biopsy based on the use of femtosecond pulses.

Biophysical effects in off-resonant gold nanoparticle mediated (GNOME) laser transfection of cell lines, primary- and stem cells using fs laser pulses

Gold nanoparticle mediated (GNOME) laser transfection is a powerful technique to deliver small biologically relevant molecules into cells. However, the transfection of larger and especially negatively charged DNA remains challenging. The efficiency for pDNA was 0.57% using parameter that does not influence the endo- and exogenous DNA. In order to gain a deeper understanding of the actual molecule uptake process, the uptake efficiency was determined using molecules of different sizes. It was evaluated that uncharged dextran molecules (2000 kDa) were delivered with an efficiency of 68%. The intracellular distribution of injected molecules was visualized and larger molecules were primary found in the cytoplasm. Patch clamp measurements suggested a permeabilization time up to 15 minutes. The uptake efficiency depended on the size and charge of the molecule to deliver as well as the cell size. A minor role for transfection plays the cell type since primary stem cells were successfully transfected. The perforation efficiency of semi-adherent and suspension cells is influenced by the cell and molecule size.

Characterization of nanoparticle mediated laser transfection by femtosecond laser pulses for applications in molecular medicine

BACKGROUND

In molecular medicine, the manipulation of cells is prerequisite to evaluate genes as therapeutic targets or to transfect cells to develop cell therapeutic strategies. To achieve these purposes it is essential that given transfection techniques are capable of handling high cell numbers in reasonable time spans. To fulfill this demand, an alternative nanoparticle mediated laser transfection method is presented herein. The fs-laser excitation of cell-adhered gold nanoparticles evokes localized membrane permeabilization and enables an inflow of extracellular molecules into cells.

RESULTS

The parameters for an efficient and gentle cell manipulation are evaluated in detail. Efficiencies of 90% with a cell viability of 93% were achieved for siRNA transfection. The proof for a molecular medical approach is demonstrated by highly efficient knock down of the oncogene HMGA2 in a rapidly proliferating prostate carcinoma in vitro model using siRNA. Additionally, investigations concerning the initial perforation mechanism are conducted. Next to theoretical simulations, the laser induced effects are experimentally investigated by spectrometric and microscopic analysis. The results indicate that near field effects are the initial mechanism of membrane permeabilization.

CONCLUSION

This methodical approach combined with an automated setup, allows a high throughput targeting of several 100,000 cells within seconds, providing an excellent tool for in vitro applications in molecular medicine. NIR fs lasers are characterized by specific advantages when compared to lasers employing longer (ps/ns) pulses in the visible regime. The NIR fs pulses generate low thermal impact while allowing high penetration depths into tissue. Therefore fs lasers could be used for prospective in vivo applications.

Enhancement of extracellular molecule uptake in plasmonic laser perforation

The use of laser induced surface plasmons on metal nanoparticles has proven to be an excellent tool for the delivery of molecules like siRNA and DNA into cells. However, a detailed understanding of the basic mechanisms of molecular uptake and the influence of parameters like biological environment is missing. In this study we analyzed the uptake of fluorescent dextrans with sizes from 10 to 2000 kDa, which resembles a wide range of biologically relevant molecules in size using a 532 nm picosecond laser system and 200 nm gold nanoparticles. Our results show a strong uptake-dependence on cell medium or buffer, but no dominant dependence on osmotic conditions. The relation between pulse energy and number of pulses for a given perforation efficiency revealed that multiphoton ionization of water might contribute to perforation. Moreover, a seven-fold uptake-enhancement could be reached with optimized parameters, providing a very promising basis for further studies and applications.

Er/Tm:fiber laser system for coherent Raman microscopy

We present a novel architecture for a fiber-based hybrid laser system for coherent Raman microscopy, combining an amplified Er:fiber femtosecond oscillator with a Tm:fiber amplifier boosting the power of the 2-μm portion of a supercontinuum up to 300 mW. This is enough to obtain, by means of nonlinear spectral compression, sub-20-cm(-1) wide pump and Stokes pulses with 2500-3300  cm(-1) frequency detuning and average power at the 100-mW level. Application of this system to stimulated Raman scattering microscopy is discussed.

Photoinduced damage resulting from fluorescence imaging of live cells

The widespread application of fluorescence microscopy to study live cells has led to a greater understanding of numerous biological processes. Many techniques have been developed to uniquely label structures and track metabolic pathways using fluorophores in live cells. However, the photochemistry of nonnative compounds and the deposition of energy into the cell during imaging can result in unexpected and unwanted side effects. Herein, we examine potential live cell damage by first discussing common imaging considerations and modalities in fluorescence microscopy. We then consider several mechanisms by which various photochemical and photophysical phenomena cause cellular damage and introduce techniques that have leveraged these phenomena to intentionally create damage inside cells. Reviewing conditions under which intentional damage occurs can allow one to better predict when unintentional damage may be important. Finally, we delineate ways of checking for and reducing photochemical and photophysical damage.

Gold nanoparticle mediated laser transfection for efficient siRNA mediated gene knock down

Laser based transfection methods have proven to be an efficient and gentle alternative to established molecule delivery methods like lipofection or electroporation. Among the laser based methods, gold nanoparticle mediated laser transfection bears the major advantage of high throughput and easy usability. This approach uses plasmon resonances on gold nanoparticles unspecifically attached to the cell membrane to evoke transient and spatially defined cell membrane permeabilization. In this study, we explore the parameter regime for gold nanoparticle mediated laser transfection for the delivery of molecules into cell lines and prove its suitability for siRNA mediated gene knock down. The developed setup allows easy usage and safe laser operation in a normal lab environment. We applied a 532 nm Nd:YAG microchip laser emitting 850 ps pulses at a repetition rate of 20.25 kHz. Scanning velocities of the laser spot over the sample of up to 200 mm/s were tested without a decline in perforation efficiency. This velocity leads to a process speed of ∼8 s per well of a 96 well plate. The optimal particle density was determined to be ∼6 particles per cell using environmental scanning electron microscopy. Applying the optimized parameters transfection efficiencies of 88% were achieved in canine pleomorphic adenoma ZMTH3 cells using a fluorescent labeled siRNA while maintaining a high cell viability of >90%. Gene knock down of d2-EGFP was demonstrated and validated by fluorescence repression and western blot analysis. On basis of our findings and established mathematical models we suppose a mixed transfection mechanism consisting of thermal and multiphoton near field effects. Our findings emphasize that gold nanoparticle mediated laser transfection provides an excellent tool for molecular delivery for both, high throughput purposes and the transfection of sensitive cells types.

Label-free second harmonic and hyper Rayleigh scattering with high efficiency

We present a method to perform hyper Rayleigh scattering from aqueous solutions and second harmonic scattering measurements from unlabeled interfaces of liposomes and nanoparticles in dilute solutions. The water and interfacial response can be measured on a millisecond timescale, thus opening up the possibility to measure label-free time dependent transport processes in biological (membrane) systems.

Nanosurgery of cells and chromosomes using near-infrared twelve-femtosecond laser pulses

ABSTRACT. Laser-assisted surgery based on multiphoton absorption of near-infrared laser light has great potential for high precision surgery at various depths within the cells and tissues. Clinical applications include refractive surgery (fs-LASIK). The non-contact laser method also supports contamination-free cell nanosurgery. In this paper we describe usage of an ultrashort femtosecond laser scanning microscope for sub-100 nm surgery of human cells and metaphase chromosomes. A mode-locked 85 MHz Ti:Sapphire laser with an M-shaped ultrabroad band spectrum (maxima: 770  nm/830  nm) and an in situ pulse duration at the target ranging from 12 fs up to 3 ps was employed. The effects of laser nanoprocessing in cells and chromosomes have been quantified by atomic force microscopy. These studies demonstrate the potential of extreme ultrashort femtosecond laser pulses at low mean milliwatt powers for sub-100 nm surgery of cells and cellular organelles.

The formation of actin waves during regeneration after axonal lesion is enhanced by BDNF

During development, axons of neurons in the mammalian central nervous system lose their ability to regenerate. To study the regeneration process, axons of mouse hippocampal neurons were partially damaged by an UVA laser dissector system. The possibility to deliver very low average power to the sample reduced the collateral thermal damage and allowed studying axonal regeneration of mouse neurons during early days in vitro. Force spectroscopy measurements were performed during and after axon ablation with a bead attached to the axonal membrane and held in an optical trap. With this approach, we quantified the adhesion of the axon to the substrate and the viscoelastic properties of the membrane during regeneration. The reorganization and regeneration of the axon was documented by long-term live imaging. Here we demonstrate that BDNF regulates neuronal adhesion and favors the formation of actin waves during regeneration after axonal lesion.

DNA multiphoton absorption generates localized damage for studying repair dynamics in live cells

Investigations into the spatiotemporal dynamics of DNA repair using live-cell imaging are aided by the ability to generate well defined regions of ultravioletlike photolesions in an optical microscope. We demonstrate that multiphoton excitation of DNA in live cells with visible femtosecond pulses produces thymine cyclopyrimidine dimers (CPDs), the primary ultraviolet DNA photoproduct. The CPDs are produced with a cubic to supercubic power dependence using pulses in the wavelength range from at least 400 to 525 nm. We show that the CPDs are confined in all three spatial dimensions, making multiphoton excitation of DNA with visible light an ideal technique for generating localized DNA photolesions in a wide variety of samples, from cultured cells to thicker tissues. We demonstrate the utility of this method by applying it to investigate the spatiotemporal recruitment of GFP-tagged topoisomerase I (TopI) to sites of localized DNA damage in polytene chromosomes within live cells of optically thick Drosophila salivary glands.

Two-photon induced collagen cross-linking in bioartificial cardiac tissue

Cardiac tissue engineering is a promising strategy for regenerative therapies to overcome the shortage of donor organs for transplantation. Besides contractile function, the stiffness of tissue engineered constructs is crucial to generate transplantable tissue surrogates with sufficient mechanical stability to withstand the high pressure present in the heart. Although several collagen cross-linking techniques have proven to be efficient in stabilizing biomaterials, they cannot be applied to cardiac tissue engineering, as cell death occurs in the treated area. Here, we present a novel method using femtosecond (fs) laser pulses to increase the stiffness of collagen-based tissue constructs without impairing cell viability. Raster scanning of the fs laser beam over riboflavin-treated tissue induced collagen cross-linking by two-photon photosensitized singlet oxygen production. One day post-irradiation, stress-strain measurements revealed increased tissue stiffness by around 40% being dependent on the fibroblast content in the tissue. At the same time, cells remained viable and fully functional as demonstrated by fluorescence imaging of cardiomyocyte mitochondrial activity and preservation of active contraction force. Our results indicate that two-photon induced collagen cross-linking has great potential for studying and improving artificially engineered tissue for regenerative therapies.

Femtosecond laser-induced fusion of nonadherent cells and two-cell porcine embryos

Cell fusion is a fundamental biological process that can be artificially induced by different methods. Although femtosecond (fs) lasers have been successfully employed for cell fusion over the past few years, the underlying mechanisms are still unknown. In our experimental study, we investigated the correlation between fs laser-induced cell fusion and membrane perforation, and the influence of laser parameters on the fusion efficiency of nonadherent HL-60 cells. We found that shorter exposure times resulted in higher fusion efficiencies with a maximum of 21% at 10 ms and 100 mJ/cm(2) (190 mW). Successful cell fusion was indicated by the formation of a long-lasting vapor bubble in the irradiated area with an average diameter much larger than in cell perforation experiments. With this knowledge, we demonstrated, for the first time, the fusion of very large parthenogenetic two-cell porcine embryos with high efficiencies of 55% at 20 ms and 360 mJ/cm(2) (670 mW). Long-term viability of fused embryos was proven by successful development up to the blastocyst stage in 70% of cases with no significant difference to controls. In contrast to previous studies, our results indicate that fs laser-induced cell fusion occurs when the membrane pore size exceeds a critical value, preventing immediate membrane resealing.

Patterned femtosecond-laser ablation of Xenopus laevis melanocytes for studies of cell migration, wound repair, and developmental processes

Ultrafast (femtosecond) lasers have become an important tool to investigate biological phenomena because of their ability to effect highly localized tissue removal in surgical applications. Here we describe programmable, microscale, femtosecond-laser ablation of melanocytes found on Xenopus laevis tadpoles, a technique that is applicable to biological studies in development, regeneration, and cancer research. We illustrate laser marking of individual melanocytes, and the drawing of patterns on melanocyte clusters to help track their migration and/or regeneration. We also demonstrate that this system can upgrade scratch tests, a technique used widely with cultured cells to study cell migration and wound healing, to the more realistic in vivo realm, by clearing a region of melanocytes and monitoring their return over time. In addition, we show how melanocyte ablation can be used for loss-of-function experiments by damaging neighboring tissue, using the example of abnormal tail regeneration following localized spinal cord damage. Since the size, shape, and depth of melanocytes vary as a function of tadpole age and melanocyte location (head or tail), an ablation threshold chart is given. Mechanisms of laser ablation are also discussed.

Mechanisms of high-order photobleaching and its relationship to intracellular ablation

In two-photon laser-scanning microscopy using femtosecond laser pulses, the dependence of the photobleaching rate on excitation power may have a quadratic, cubic or even biquadratic order. To date, there are still many open questions concerning this so-called high-order photobleaching. We studied the photobleaching kinetics of an intrinsic (enhanced Green Fluorescent Protein (eGFP)) and an extrinsic (Hoechst 33342) fluorophore in a cellular environment in two-photon microscopy. Furthermore, we examined the correlation between bleaching and the formation of reactive oxygen species. We observed bleaching-orders of three and four for eGFP and two and three for Hoechst increasing step-wise at a certain wavelength. An increase of reactive oxygen species correlating with the bleaching over time was recognized. Comparing our results to the mechanisms involved in intracellular ablation with respect to the amount of interacting photons and involved energetic states, we found that a low-density plasma is formed in both cases with a smooth transition in between. Photobleaching, however, is mediated by sequential-absorption and multiphoton-ionization, while ablation is dominated by the latter and cascade-ionization processes.

Mechanisms of high-order photobleaching and its relationship to intracellular ablation

In two-photon laser-scanning microscopy using femtosecond laser pulses, the dependence of the photobleaching rate on excitation power may have a quadratic, cubic or even biquadratic order. To date, there are still many open questions concerning this so-called high-order photobleaching. We studied the photobleaching kinetics of an intrinsic (enhanced Green Fluorescent Protein (eGFP)) and an extrinsic (Hoechst 33342) fluorophore in a cellular environment in two-photon microscopy. Furthermore, we examined the correlation between bleaching and the formation of reactive oxygen species. We observed bleaching-orders of three and four for eGFP and two and three for Hoechst increasing step-wise at a certain wavelength. An increase of reactive oxygen species correlating with the bleaching over time was recognized. Comparing our results to the mechanisms involved in intracellular ablation with respect to the amount of interacting photons and involved energetic states, we found that a low-density plasma is formed in both cases with a smooth transition in between. Photobleaching, however, is mediated by sequential-absorption and multiphoton-ionization, while ablation is dominated by the latter and cascade-ionization processes.

Superresolved femtosecond laser nanosurgery of cells

We report on femtosecond nanosurgery of fluorescently labeled structures in cells with a spatially superresolved laser beam. The focal spot width is reduced using phase filtering applied with a programmable phase modulator. A comprehensive statistical analysis of the resulting cuts demonstrates an achievable average resolution enhancement of 30 %.