Emiliano Ronzitti

Frequency dependent detection in a STED microscope using modulated excitation light

We present a novel concept adaptable to any kind of STED microscope in order to expand the limited number of compatible dyes for performing super resolution imaging. The approach is based on an intensity modulated excitation beam in combination with a frequency dependent detection in the form of a standard lock-in amplifier. This enables to unmix fluorescence signal originated by the excitation beam from the fluorescence caused by the STED beam. The benefit of this concept is demonstrated by imaging biological samples as well as fluorescent spheres, whose spectrum does not allow STED imaging in the conventional way. Our concept is suitable with CW or pulsed STED microscope and can thereby be seen as a general improvement adaptable to any existing setup.

Two-photon excitation selective plane illumination microscopy (2PE-SPIM) of highly scattering samples: characterization and application

In this work we report the advantages provided by two photon excitation (2PE) implemented in a selective plane illumination microscopy (SPIM) when imaging thick scattering samples. In particular, a detailed analysis of the effects induced on the real light sheet excitation intensity distribution is performed. The comparison between single-photon and two-photon excitation profiles shows the reduction of the scattering effects and sample-induced aberrations provided by 2PE-SPIM. Furthermore, uniformity of the excitation distribution and the consequent improved image contrast is shown when imaging scattering phantom samples in depth by 2PE-SPIM. These results show the advantages of 2PE-SPIM and suggest how this combination can further enhance the SPIM performance. Phantom samples have been designed with optical properties compatible with biological applications of interest.

Studying the illumination puzzle towards an isotropic increase of optical resolution

The aim of this work is to propose and analyze optical schemes to obtain an improvement of resolution in optical fluorescence microscopy. This goal can be achieved by implementing interfering illumination beams. We start from the insertion, on the illumination arm of the confocal microscope, of appropriately phase plates inducing laterally interfering beams, and then we propose to exploit two-photon excitation, too. We plan to implement solutions for shaping also the axial component of the point spread function by use of phase-only pupil filters and binary filters. In order to implement such schemes we use a computational simulation mainly based on a vectorial approach coupled to experimental procedures utilizing ultra-thin fluorescent layers and thick gels containing immobile fluorescent molecules as 2D and 3D phantoms, respectively. As well, image processing and successive views can be recombined to get a final isotropic improvement of resolution.

6 – Multiphoton Microscopy Advances Toward Super Resolution

Multiphoton microscopy has emerged as a key technique for a variety of applications in the life sciences. Its distinct optical sectioning property, due to its nonlinear dependence on the illumination intensity, makes it a method of choice for in vivo and in vitro imaging of deep-tissue structures and for live animal imaging. This further improves the visualization of in vivo complex and dynamic biological processes with minimal invasion and photodamage. Multiphoton imaging integrated with superresolution techniques such as 4pi, PALM, and STED has opened up various possibilities including nanoscale biological imaging. In this chapter we review the fundamental multiphoton excitation process and the advances made in this exciting field within the nanoscale biophysics framework. Multiphoton microscopy is and will continue to have remarkable advances in times to come, undoubtedly fostering new applications.

Improving Image Formation by Pushing the Signal-to-Noise Ratio

Two-photon and confocal laser scanning microscopes represent fundamental optical techniques for biological investigations, thanks to their optical sectioning capability. However, in several experimental situations, low-intensity signals are detected, and the imaging quality can be remarkably limited by noise contribution. Such a detriment effect is particularly significant in the transmission of the high spatial frequencies of the sample which are weakly transferred by the optical systems. The main effect is an effective reduction of the optical system transfer function bandwidth and thus an effective reduction of the system capability to distinguish fine details of the sample. In order to improve the signal-to-noise ratio in the high spatial frequency range, the interference effect obtained by the insertion of a pupil-plane ring filter juxtaposed to the objective lens in the illumination light pathway can be exploited.

Improving image formation from the illumination side: linear and non-linear excitation cases

Diffraction imposes for each optical system a resolution limit which could be described by using the vectorial theory of Richards and Wolf. This theory defines the intensity distribution of a point like source imaged by a lens assuming ideal imaging conditions. Unfortunately, these conditions can not be completely achieved in practical situations as a recorded microscope image is always affected by noise which makes the resolution limit worse. In this work we propose and analyze optical set-up schemes towards an image quality improvement in terms of Signal to Noise Ratio (SNR) in linear and non-linear fluorescence microscopy. In order to reach this purpose we insert, on the illumination arm of the microscope, a proper amplitude ring filter inducing laterally interfering beams. The effect induced by the filter results in a shape engineering of the 3D-PSF and in a redistribution of the spatial frequencies of the OTF. In particular, the high frequencies information are collected at improved SNR. In order to implement such schemes we use a computational simulation mainly based on a vectorial approach analyzing the results in both space and frequency domain to characterize the optical system response. Analysis reveals that, although the theoretical resolution of the system is unchanged, when we impose a certain noise level the practical imaging quality could be improved in the ring filtering scheme. The results suggest that further improvement can be reached by the usage of the proposed annular filers in combination with image restoration. A comparison between linear and non-linear excitation cases is presented.

Nanostructured Polyelectrolyte-based System as a Toolbox for Metal Ions Detection

The capability of certain heavy metal ions to induce fluorescence decrease by a quenching mechanism suggested us to design and build a sensor potentially tunable for different ions at different concentrations. We propose a quenching-based sensor exploiting a nanostructured architecture in which fluorescent molecules (the sensing probe) are entrapped to recognize a specific analyte (heavy metal ions) through an optical transduction. The polyelectrolyte nanostructured system, named nanocapsule, improves the fluorophore-ion quenching sensitivity allowing a micromolar detection. Furthermore we couple our sensor with an electrical device in order to refine the sensing procedure: the electric field created allows a metal ions spatial gradient, necessary to detect a specific element on a single sample solution, avoiding a comparative analysis with an intensity reference value. Results obtained will show the advantages and the potentialities of our system as a smart toolbox for metal ions detection.