Photoswitchable NIR-Emitting Gold Nanoparticles.

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.

Dye Encapsulation in Polynorbornene Micelles.

The encapsulation efficiency of high-Tg polynorbornene micelles was probed with a hydrophobic dye 2,6-diiodoboron-dipyrromethene (BODIPY). Changes in the visible absorption spectra of aggregated versus monomeric dye molecules provided a probe for assessing encapsulation. Polynorbornene micelles are found to be capable of loading up to one BODIPY dye per ten polymers. As the hydrophilic block size increased in the polymeric amphiphiles, more of the dye was incorporated within the micelles. This result is consistent with the dye associating with the polymer backbone in the shell of the micelles. The encapsulation rate varied significantly with temperature, and a slight dependence on micellar morphology was also noted. Additionally, we report a 740 µs triplet lifetime for the encapsulated BODIPY dye. The lifetime is the longest ever recorded for a BODIPY triplet excited state at room temperature and is attributed to hindered triplet-triplet annihilation in the high-viscosity micellar shell.

Local detection of pH-induced disaggregation of biocompatible micelles by fluorescence switch ON.

Fluorogenic nanoparticles (NPs) able to sense different physiological environments and respond with disaggregation and fluorescence switching OFF/ON are powerful tools in nanomedicine as they can combine diagnostics with therapeutic action. pH-responsive NPs are particularly interesting as they can differentiate cancer tissues from healthy ones, they can drive selective intracellular drug release and they can act as pH biosensors. Controlled polymerization techniques are the basis of such materials as they provide solid routes towards the synthesis of pH-responsive block copolymers that are able to assemble/disassemble following protonation/deprotonation. Ring opening metathesis polymerization (ROMP), in particular, has been recently exploited for the development of experimental nanomedicines owing to the efficient direct polymerization of both natural and synthetic functionalities. Here, we capitalize on these features and provide synthetic routes for the design of pH-responsive fluorogenic micelles via the assembly of ROMP block-copolymers. While detailed photophysical characterization validates the pH response, a proof of concept experiment in a model cancer cell line confirmed the activity of the biocompatible micelles in relevant biological environments, therefore pointing out the potential of this approach in the development of novel nano-theranostic agents.

Ultra-bright and stimuli-responsive fluorescent nanoparticles for bioimaging.

Fluorescent nanoparticles (NPs) are unique contrast agents for bioimaging. Examples of molecular-based fluorescent NPs with brightness similar or superior to semiconductor quantum dots have been reported. These ultra-bright NPs consist of a silica or polymeric matrix that incorporate the emitting dyes as individual moieties or aggregates and promise to be more biocompatible than semiconductor quantum dots. Ultra-bright materials result from heavy doping of the structural matrix, a condition that entails a close mutual proximity of the doping dyes. Ground state and excited state interactions between the molecular emitters yield aggregation-caused quenching (ACQ) and proximity-caused quenching (PCQ). In combination with Föster resonance energy transfer (FRET) ACQ and PCQ originate collective phenomena that produce amplified quenching of the nanoprobes. In this focus article, we discuss strategies to achieve ultra-bright nanoprobes avoiding ACQ and PCQ also exploiting aggregation-induced emission (AIE). Amplified quenching, on the other hand, is also proposed as a strategy to design stimuli-responsive fluorogenic probes through disaggregation-induced emission (DIE) in alternative to AIE. As an advantage, DIE consents to design stimuli-responsive materials starting from a large variety of precursors. On the contrary, AIE is characteristic of a limited number of species. Examples of stimuli-responsive fluorogenic probes based on DIE are discussed.

Benchmarking TD-DFT against Vibrationally Resolved Absorption Spectra at Room Temperature: 7-Aminocoumarins as Test Cases.

Time-dependent density functional theory (TD-DFT) is usually benchmarked by evaluating how the vertical excitation energies computed by using different exchange-correlation (XC) functionals compare with the maximum of the absorption spectra. However, the latter does not necessarily coincide with the vertical energies because it is affected by the vibronic band structure that has to be properly taken into account. In this work, we have evaluated the performance of several functionals belonging to different families in reproducing the vibronic structure (band shape) of four 7-aminocoumarin molecules of technological interest, whose spectra have been recorded in methylcyclohexane and acetonitrile solvents. In order to compare the computed vibronic spectra with the experimental ones in the most consistent way, the effect of temperature, often neglected, was also taken into account. We have found that no single functional provides simultaneously accurate band positions and shapes, but the combination of ωB97X vibronic couplings with PBE0 vertical energies can lead to very satisfactory results. In addition to the assessment of XC functionals, several adiabatic and vertical models proposed in the literature to compute vibrationally resolved electronic spectra have been tested and validated with respect to experiments. On these grounds, the adiabatic Hessian model has been used to perform a complete analysis of the ωB97X/PBE0 vibronic transitions contributing to the final band shapes of the investigated aminocoumarin molecules.

Photo-tunable multicolour fluorescence imaging based on self-assembled fluorogenic nanoparticles.

Non-fluorescent nanoparticles resulting from the self-assembly of a new perylene diimide behave as fluorogenic probes for biological cells under physiological conditions giving a dosage-dependent green or red fluorescence and showing very low cytotoxicity. The emission colour can be tuned by photo-irradiation to achieve multicolour labelling.