Self-assembling, structure and nonlinear optical properties of fluorescent organic nanoparticles in water.

Owing to their intense emission, low toxicity and solubility in aqueous medium, fluorescent organic nanoparticles (FONs) have emerged as promising alternatives to inorganic ones for the realization of exogenous probes for bioimaging applications. However, the intimate structure of FONs in solution, as well as the role played by intermolecular interactions on their optical properties, remains challenging to study. Following a recent Second-Harmonic Scattering (SHS) investigation led by two of us [Daniel et al., ACS Photonics, 2015, 2, 1209], we report herein a computational study of the structural organization and second-order nonlinear optical (NLO) properties of FONs based on dipolar chromophores incorporating a hydrophobic triphenylamine electron-donating unit and a slightly hydrophilic aldehyde electron-withdrawing unit at their extremities. Molecular dynamics simulations of the FON formation in water are associated with quantum chemical calculations, to provide insight into the molecular aggregation process, the molecular orientation of the dipolar dyes within the nanoparticles, and the dynamical behavior of their NLO properties. Moreover, the impact of intermolecular interactions on the NLO responses of the FONs is investigated by employing the tight-binding version of the recently developed simplified time-dependent density functional theory (sTD-DFT) approach, allowing the all-atom quantum mechanics treatment of nanoparticles.

Performance of DFT functionals for calculating the second-order nonlinear optical properties of dipolar merocyanines.

The second-order nonlinear optical responses of a series of recently designed dipolar merocyanines are investigated using the 2006 Minnesota family of hybrid exchange-correlation functionals (XCFs), as well as the LC-BLYP, ωB97XD and CAM-B3LYP long-range (LR) corrected XCFs. The performance of these different levels of approximation is discussed in regard to reference second-order Møller-Plesset calculations and experimental data obtained from Hyper-Rayleigh Scattering (HRS) measurements. Particular focus is given to the influence of the amount of exact Hartree-Fock exchange included in the XCF on the magnitude of the static HRS responses, as well as to the impact of tuning the range-separation parameter in LR-XCFs, according to a system-specific nonempirical procedure. Frequency dispersion effects are also investigated, as well as their crucial role in the comparison between theoretical and experimental data.

PIP2 Phospholipid-Induced Aggregation of Tau Filaments Probed by Tip-Enhanced Raman Spectroscopy.

The morphology and secondary structure of peptide fibers formed by aggregation of tubulin-associated unit (Tau) fragments (K18), in the presence of the inner cytoplasmic membrane phosphatidylinositol component (PIP2 ) or heparin sodium (HS) as cofactors, are determined with nanoscale (<10 nm) spatial resolution. By means of tip-enhanced Raman spectroscopy (TERS), the inclusion of PIP2 lipids in fibers is determined based on the observation of specific C=O ester vibration modes. Moreover, analysis of amide I and amide III bands suggests that the parallel ß-sheet secondary structure content is lower and the random coil content is higher for fibers grown from the PIP2 cofactor instead of HS. These observations highlight the occurrence of some local structural differences between these fibers. This study constitutes the first nanoscale structural characterization of Tau/phospholipid aggregates, which are implicated in deleterious mechanisms on neural membranes in Alzheimer's disease.