Photophysics at Unusually High Dye Concentrations

The photochemical inheritance of Eduardo Lissi and Juan Grotewold and the intersystem crossings with other inheritances

In 1963, Eduardo Lissi and Juan Grotewold started a chemical kinetics and photochemistry group at the School of Sciences at the University of Buenos Aires (Facultad de Ciencias Exactas y Naturales, FCEN, UBA). Political circumstances in Argentina and in Chile were a great determinant of the evolution, dispersion, and re-encounters of the group members. The initial graduate students in the group developed their own research groups working in various Countries and on a variety of projects. We relate the story of the strong interactions with each other of the original group as well as the cooperations and synergy (Intersystem Crossings) of Lissi and Grotewold and of their descendants with other research groups, mainly (but not only) in Latin America. A strong network of partnerships, friendships, and fruitful collaborations between the kineticists, photochemists, and photobiologists in Argentina, Chile and Brazil has evolved from the initial steps in the 1960s.

Photoactive Red Fluorescent SiO2 Nanoparticles Based on Controlled Methylene Blue Aggregation in Reverse Microemulsions

We present a reverse microemulsion synthesis procedure for incorporating methylene blue (MB), a known FDA-approved type-II red-absorbing photosensitizer and ¹O₂ generator, into the matrix of hydrophobic-core/hydrophilic-shell SiO₂ nanoparticles. Different synthesis conditions were explored with the aim of controlling the entrapped-dye aggregation at high dye loadings in the hydrophobic protective core; minimizing dye aggregation ensured highly efficient photoactive nanoentities for ¹O₂ production. Monitoring the synthesis in real time using UV-vis absorption allowed tracking of the dye aggregation process. In particular, silica nanoparticles (MB@SiO₂ NPs) of ∼50 nm diameter size with a high local entrapped-MB concentration (∼10^-2 M, 1000 MB molecules per NP) and a moderate proportion of dye aggregation were obtained. The as-prepared MB@SiO₂ NPs showed a high singlet oxygen photogeneration efficiency (Φ_Δ = 0.30 ± 0.05), and they can be also considered as red fluorescent probes (Φ_F ∼ 0.02, λ_max ∼ 650 nm). The distinctive photophysical and photochemical characteristics of the synthesized NPs reveal that the reverse microemulsion synthesis procedure offers an interesting strategy for the development of complex theranostic nano-objects for photodynamic therapy.

Ibuprofen molecular aggregation by direct back-face transmission steady-state fluorescence

Direct back-face transmission steady-state fluorescence was successfully applied to the study of aggregation of ibuprofen and ibuprofenate anion in solution taking advantage of its own fluorescence. The analysis of the experimental data involves the use of the differential reabsorption model to account for re-absorption phenomenon and the closed association model to describe aggregation. The fluorescence quantum yield of ibuprofenate increases when it aggregates in the presence of sodium, but it markedly decreases when 1-butyl-3-methylimidazolium is used as counterion. The proposed methodology allows the accurate determination of the critical aggregation concentrations and the mean aggregation numbers. Results were supported by complementary techniques such as time-resolved fluorescence, ¹H-NMR and small-angle neutron and X-ray scattering. The developed technique constitutes a promising strategy to characterize the aggregation of poorly fluorescent surfactants that aggregates at high concentrations and hence at high absorbance values, conditions in which the most common right-angle configuration for fluorescence acquisition is troublesome due to re-absorption.

Indocyanine green dye based bimodal contrast agent tested by photoacoustic/fluorescence tomography setup

Multimodal imaging systems are in high demand for preclinical research, experimental medicine, and clinical practice. Combinations of photoacoustic technology with other modalities including fluorescence, ultrasound, MRI, OCT have been already applied in feasibility studies. Nevertheless, only the combination of photoacoustics with ultrasound in a single setup is commercially available now. A combination of photoacoustics and fluorescence is another compelling approach because those two modalities naturally complement each other. Here, we presented a bimodal contrast agent based on the indocyanine green dye (ICG) as a single signalling compound embedded in the biocompatible and biodegradable polymer shell. We demonstrate its remarkable characteristics by imaging using a commercial photoacoustic/fluorescence tomography system (TriTom, PhotoSound Technologies). It was shown that photoacoustic signal of the particles depends on the amount of dye loaded into the shell, while fluorescence signal depends on the total amount of dye per particle. For the first time to our knowledge, a commercial bimodal photoacoustic/fluorescence setup was used for characterization of ICG doped polymer particles. Additionally, we conducted cell toxicity studies for these particles as well as studied biodistribution over time in vivo and ex vivo using fluorescent imaging. The obtained results suggest a potential for the application of biocompatible and biodegradable bimodal contrast agents as well as the integrated photoacoustic/fluorescence imaging system for preclinical and clinical studies.

Surface-Radical Mobility Test by Self-Sorted Recombination: Symmetrical Product upon Recombination (SPR)

We describe here a study of the mobility of the alkoxy radical on a surface by detection of its recombination product. A novel method called symmetrical product recombination (SRP) uses an unsymmetrical peroxide that upon sensitized homolysis recombines to a symmetrical product [R'OOR → R'O^•↑ + ^•OR → ROOR]. This allows for self-sorting of the radical to enhance the recombination path to a symmetrical product, which has been used to deduce surface migratory aptitude. SPR also provides a new opportunity for mechanistic studies of interfacial radicals, including monitoring competition between radical recombination versus surface hydrogen abstraction. This is an approach that might work for other surface-borne radicals on natural and artificial particles.

Influence of Rose Bengal Dimerization on Photosensitization

Protein crosslinking photosensitized by rose Bengal (RB^2- ) has multiple medical applications and understanding the photosensitization mechanism can improve treatment effectiveness. To this end, we investigated the photochemical efficiencies of monomeric RB^2- (RB_M ^2- ) and dimeric RB^2- (RB_D ^2- ) and the optimal pH for anaerobic RB^2- photosensitization in cornea. Absorption spectra and dynamic light scattering (DLS) measurements were used to estimate the fractions of RB_M ^2- and RB_D ^2- . RB^2- self-photosensitized bleaching was used to evaluate the photoactivity of RB_M ^2- and RB_D ^2- . The pH dependence of anaerobic RB^2- photosensitization was evaluated in ex vivo rabbit corneas. The 549 nm/515 nm absorption ratio indicated that concentrations > 0.10 mm RB contained RB_D ^2- . Results from DLS gave estimated mean diameters for RB_M ^2- and RB_D ^2- of 0.70 ± 0.02 nm and 1.75 ± 0.13 nm, respectively, and indicated that 1 mm RB^2- contained equal fractions of RB_M ^2- and RB_D ^2- . Quantum yields for RB^2- bleaching were not influenced by RB_D ^2- in RB^2- solutions although accounting for RB^2- concentration effects on the reaction kinetics demonstrated that RB_D ^2- is not a photosensitizer. Optimal anaerobic photosensitization occurred at pH 8.5 for solutions containing 200 mm Arg. These results suggest potential approaches to optimizing RB_M ^2- -photosensitized protein crosslinking in tissues.

Determination of Fluorescence Quantum Yields in Scattering Media

The fluorescence quantum yield is a measure of the efficiency of photon emission and quantifies the luminescent performance of a given sample. The determination of fluorescence quantum yields, particularly in scattering media, is relevant in the areas of materials science, technology and photonics. It is equally crucial when studying fluorescent bioanalytical probes and biological systems either for medical applications, physiological analyses or the interpretation of optical signals in nature. This type of determination represents a challenge since light scattering introduces an appreciable complexity in the measurements. Hence, the use of experimentally accurate methods and the understanding of their basis and principles is indispensable for obtaining reliable results. In addition, light re-absorption processes are usually very significant in these systems and the experimental quantum yields normally differ from the true quantum yields of the fluorophore. The first purpose of this work is to provide a clear and comprehensive compilation of the various optical methods that can be used for the determination of quantum yields in scattering media. A second purpose is to present the correction models to account for light re-absorption processes, applicable in each case. The advantages and disadvantages of each methodology are comparatively discussed, the difference between experimental and true quantum yield is clarified and it is explained which should be used depending on the case. Several examples previously published in literature are illustrated. The methods presented here are adequate for the study of very diverse samples such as suspensions, solid powders, films, animal tissues and even plant material.

Influence of Surface Structure, Pigmentation and Particulate Matter on Plant Reflectance and Fluorescence

Optical properties of plant leaves are relevant to evaluate their physiological state and stress effect. The main objective of this work was to study how variegation, pigment composition or reflective features modifies leaves' photophysical behavior. For this purpose, green leaves (Ficus benjamina), purple leaves (Tradescantia pallida), green leaves covered by white trichomes (Cineraria maritima) and variegated leaves (Codiaeum aucubifolium) were analyzed. Firstly, foliar surface morphology was evaluated by scanning electron microscopy. UV-vis and near-IR reflectance and transmittance spectra were obtained to calculate absorption (k) and scattering (s) coefficients. The theoretical approaches of Pile of Plates and Kubelka-Munk's theory resulted still valid for nonstandard leaves with differing surface conditions. However, frequently used spectral indices were not reliable for predicting water content, when leaves differed from conventional ones. The proportionality between the absorption factor and chromophore/pigment concentration was also lost for hairy leaves. A simplified model to describe these facts was presented here. Fluorescence spectra were recorded and corrected, due to light re-absorption. Water-optical parameter connection and pigment-optical parameter connection were thoroughly discussed. Leaf surface morphology and pigmentation have not only influenced the optical features of leaves but also played a role in the effect that particulate matter could cause on leaf photosynthesis.

Indocyanine green modified silica shells for colon tumor marking

Marking colon tumors for surgery is normally done with the use of India ink. However, non-fluorescent dyes such as India ink cannot be imaged below the tissue surface and there is evidence for physiological complications such as abscess, intestinal perforation and inconsistency of dye injection. A novel infrared marker was developed using FDA approved indocyanine green (ICG) dye and ultrathin hollow silica nanoshells (ICG/HSS). Using a positively charged amine linker, ICG was non-covalently adsorbed onto the nanoparticle surface. For ultra-thin wall 100 nm diameter silica shells, a bimodal ICG layer of < 3 nm is was formed. Conversely, for thicker walls on 2 μm diameter silica shells, the ICG layer was only bound to the outer surface and was 6 nm thick. In vitro testing of fluorescent emission showed the particles with the thinner coating were considerably more efficient, which is consistent with self-quenching reducing emission shown in the thicker ICG coatings. Ex-vivo testing showed that ICG bound to the 100 nm hollow silica shells was visible even under 1.5 cm of tissue. In vivo experiments showed that there was no diffusion of the ICG/nanoparticle marker in tissue and it remained imageable for as long as 12 days.

Chiral Microneedles from an Achiral Bis(boron dipyrromethene): Spontaneous Mirror Symmetry Breaking Leading to a Promising Photoluminescent Organic Material

Supramolecular self-assembly of a highly flexible and achiral meso bis(boron dipyrromethene) [bis(BODIPY)] dye straightforwardly yields fluorescent microfibers, exhibiting an intriguing anisotropic photonic behavior. This performance includes the generation of chiroptical activity owing to spontaneous mirror symmetry breaking (SMSB). Repetition of several self-assembly experiments demonstrates that the involved SMSB is not stochastic but quasi deterministic in the direction of the induced chiral asymmetry. The origin of these intriguing (chiro)photonic properties is revealed by fluorescent microspectroscopy studies of individual micrometric objects, combined with X-ray diffraction elucidation of microcrystals. Such a study demonstrates that J-like excitonic coupling between bis(BODIPY) units plays a fundamental role in their supramolecular organization, leading to axial chirality. Interestingly, the photonic behavior of the obtained fibers is ruled by inherent nonradiative pathways from the involved push-pull chromophores, and mainly by the complex excitonic interactions induced by their anisotropic supramolecular organization.