Enhancing nonlinear optical imaging through porphyrin-based dyes

Over the past several decades, advances in cell imaging have dramatically transformed biology and medicine. Fluorescence spectroscopy and microscopy are currently the most popular imaging techniques however, there are intrinsic limitations; many substrates are non-fluorescent or weakly fluorescent, fluorescent labels are often perturbative for small molecules and peptides and, perhaps most importantly, labelling or staining with fluorophores are not recommended for in vivo medicinal applications in humans. Hence, the search for highly sensitive optical imaging methods is increasingly desirable in biomedical and material sciences.

Nonlinear optical imaging is an emerging technology that encompasses a range of optical phenomena. In a recent study by Prof. Koen Clays of the University of Leuven and Prof. Harry Anderson of Oxford University, a new group of chromophores based on pyropheophorbide-a methyl ester (PPa-OMe) was developed for the linear and nonlinear optical imaging of membrane potentials as well as biological imaging of structures through two-photon excited fluorescence (TPEF) and second harmonic generation (SGH) microscopy.

In TPEF, a fluorophore is excited by the simultaneous absorption of two photons in the infrared spectral range. In conventional one-photon fluorescence, the same transition to higher energy levels requires photons in the ultraviolet or visible range. The longer incident wavelength in TPEF leads to improved depth penetration in tissues, with reduced potential for photolytic damage. SHG, on the other hand, is a nonlinear process where two photons interact with a nonlinear material and are effectively combined to generate new photons with twice the energy. This process does not involve absorption of photons but relies on virtual energy states and can only occur in materials that exhibit a non-centrosymmetric structure.

Unlike incoherent processes such as fluorescence, coherent nonlinear optical spectroscopies generate different optical signals depending on the underlying processes. They have broad utility as biomedical tools, offering contrasting mechanisms to fluorescence emissions and provide a useful alternative to label-based imaging.

In this OBC publication, the electronic structure of PPa-OMe (1a) was altered to tune it’s linear and nonlinear optical properties. Porphyrins and related porphyrinoid chromophores inherently possess excellent linear and nonlinear optical properties due to their large, conjugated π-system. By incorporating both electron-donating and –accepting groups, a push-pull type system was generated in which greater SHG intensity was observed due to the increased polarization of its π-system. A hydrophilic group, bis-triethyleneglycol (TEG) amide, was attached to make PPa-OMe amphiphilic and was then suspended in lipid-based water in oil monolayer droplets—a simple model system used to probe potentials across cellular membranes. TPEF and SHG images of the bis-TEG amide attached dyes revealed that the TPEF and SHG involve transition dipole moments in different orientations. While TPEF is detectable in all directions around the sample, SHG is detected in the forward direction of the incident light meaning there is an overall cancelling of the SHG signal from anti-parallel dyes. In order to improve these systems, control over orientation within cell membranes is crucial however, chromophores based on these PPa-OMe derivatives are promising prototypes for future cell imaging studies.

To find out more see:

Push-pull pyropheophorbides for nonlinear optical imaging
Anjul Khadria, Yovan de Coene, Przemyslaw Gawel, Cécile Roche, Koen Clays and Harry L. Anderson
DOI: 10.1039/C6OB02319C

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

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