mCerulean3-Based Cameleon Sensor to Explore Mitochondrial Ca2+ Dynamics In Vivo.

Genetically Encoded Ca2+ Indicators (GECIs) are extensively used to study organelle Ca2+ homeostasis, although some available probes are still plagued by a number of problems, e.g., low fluorescence intensity, partial mistargeting, and pH sensitivity. Furthermore, in the most commonly used mitochondrial Förster Resonance Energy Transfer based-GECIs, the donor protein ECFP is characterized by a double exponential lifetime that complicates the fluorescence lifetime analysis. We have modified the cytosolic and mitochondria-targeted Cameleon GECIs by (1) substituting the donor ECFP with mCerulean3, a brighter and more stable fluorescent protein with a single exponential lifetime; (2) extensively modifying the constructs to improve targeting efficiency and fluorescence changes caused by Ca2+ binding; and (3) inserting the cDNAs into adeno-associated viral vectors for in vivo expression. The probes have been thoroughly characterized in situ by fluorescence microscopy and Fluorescence Lifetime Imaging Microscopy, and examples of their ex vivo and in vivo applications are described.

Engineering interactions in QDs-PCBM blends: a surface chemistry approach.

Here we present a comprehensive study on the photophysics of QDs-fullerene blends, aiming to elucidate the impact of ligands on the extraction of carriers from QDs. We investigated how three different ligands (oleylamine, octadecanethiol and propanethiol) influence the dynamics of charge generation, separation, and recombination in blends of CdSe/CdS core/shell QDs and PCBM. We accessed each relevant process directly by combining the results from both optical and magnetic spectroscopies. Transient absorption measurements revealed a faster interaction dynamics in thiol-capped ligands. Through phenomenological modeling of the interaction processes, i.e., energy transfer and electron transfer, we estimated the suppression of exciton migration and the enhancement of electron transfer processes when alkyl-thiols are employed as ligands. Contextually, we report the profound impact of the ligands' alkyl chain length, leading to strengthened interactions with PCBM acceptors. Quantitatively, we measured a 10-fold increase in the electron transfer rate when oleylamine ligands were exchanged with propanethiol ligands. EPR spectroscopy gave access to subtle details regarding both the enhanced charge generation and lower binding energy of charge-transfer states in blends compared to PCBM alone. Moreover, through pulsed EPR techniques, we inferred the localization of deep electron traps in localized sites close to QDs in the blends. Therefore, our thorough characterization evidenced the essential role of ligands in determining QD interactions. We believe that these discoveries will contribute to the efficient incorporation of QDs in existing organic PV technologies.

Hybrid Organic/Inorganic Perovskite-Polymer Nanocomposites: Toward the Enhancement of Structural and Electrical Properties.

Hybrid organic/inorganic perovskite nanoparticles (NPs) have garnered remarkable research attention because of their promising photophysical properties. New and interesting properties emerge after combining perovskite NPs with semiconducting materials. Here, we report the synthesis and investigation of a composite material obtained by mixing CH3NH3PbBr3 nanocrystals with the semiconducting polymer poly(3-hexylthiophene) (P3HT). By the combination of structural techniques and optical and magnetic spectroscopies we observed multiple effects of the perovskite NPs on the P3HT: (i) an enlargement of P3HT crystalline domains, (ii) a strong p-doping of the P3HT, and (iii) an enhancement of interchain order typical of H-aggregates. These observations open a new avenue toward innovative perovskite NP-based applications.

Spectroscopic Insights into Carbon Dot Systems.

The controversial nature of the fluorescent properties of carbon dots (CDs), ascribed either to surface states or to small molecules adsorbed onto the carbon nanostructures, is an unresolved issue. To date, an accurate picture of CDs and an exhaustive structure-property correlation are still lacking. Using two unconventional spectroscopic techniques, fluorescence correlation spectroscopy (FCS) and time-resolved electron paramagnetic resonance (TREPR), we contribute to fill this gap. Although electron micrographs indicate the presence of carbon cores, FCS reveals that the emission properties of CDs are based neither on those cores nor on molecular species linked to them, but rather on free molecules. TREPR provides deeper insights into the structure of carbon cores, where C sp2 domains are embedded within C sp3 scaffolds. FCS and TREPR prove to be powerful techniques, characterizing CDs as inherently heterogeneous systems, providing insights into the nature of such systems and paving the way to standardization of these nanomaterials.

Boosting carbon quantum dots/fullerene electron transfer via surface group engineering.

The design of novel nanostructures with tailored opto-electronic properties is a crucial step for third-generation photovoltaics, and the development of cheap and environmentally compatible materials is still a challenge. Carbon quantum dots (CQDs) emerged as promising candidates but usually a low processability and poor electron-donor properties hampered their photovoltaic applications. We tackle these issues through the synthesis and photophysical characterization of N-doped CQDs functionalized with different thiophene-containing groups. Functionalization was aimed at enhancing the electron donating properties of the carbon dots and improving the solubility in nonpolar solvents. The increased solubility in organic solvents allowed us to investigate the photoinduced interactions of the functionalized carbon dots with the fullerene derivative PCBM in solution and in solid blends. The investigation was carried out by cyclic voltammetry, photoluminescence spectroscopy and electron paramagnetic resonance (EPR). The remarkable oxidation potential shift of the functionalized carbon dots with respect to the pristine materials and the HOMO-LUMO energies strongly suggest them as good electron donors towards PCBM. The electron transfer process between CQDs and PCBM resulted in efficient fluorescence quenching in solution and in total quenching in solid blends. By using EPR spectroscopy in the solid blends, we demonstrated the efficient electron transfer by observing the photoinduced formation of a PCBM radical anion in the presence of functionalized CQDs. Time-resolved EPR allowed us to identify differences in the charge transport efficiency for different CQD:PCBM blends. The enhanced processability of CQDs with PCBM and the promising charge-generation and separation properties pave the way to the development of "all-carbon" photovoltaic devices.

Engineering of Semiconductor Nanocrystals for Light Emitting Applications.

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

SERS Properties of Gold Nanorods at Resonance with Molecular, Transverse, and Longitudinal Plasmon Excitations.

The amplification of Raman signals of the heteroaromatic cation 1-(N-methylpyrid-4-yl)-2-(N-methylpyrrol-2-yl)ethylene (PEP+)) bound to Au nanorods (NRs) was investigated at different excitation wavelengths to study the effect of the laser resonance with the absorption band of the PEP+ moiety and with the two plasmon oscillation modes of the NR. Two different PEP+ derivatives, differing in the length of the alkyl chain bearing the anchoring group, were used as target molecules. Raman spectra obtained exciting at 514 or at 785 nm (i.e., exciting the transverse or the longitudinal plasmon band) present a higher intensity than that at 488 nm suggesting a higher Raman amplification when the laser excitation wavelength is resonant with one of the two plasmon modes. Moreover, considering results of Discrete Dipole Approximation (DDA) calculations of the local field generated at the NR surface when either the transverse or the longitudinal plasmon modes are excited, we deduced that the resonance condition of the 514-nm laser excitation with the absorption band of the dye strongly contributes to the amplification of the Raman signal.

Sensitive detection of Ochratoxin A in food and drinks using metal-enhanced fluorescence.

Easy, sensitive, rapid and low cost ochratoxin biosensors are strongly demanded in food analysis since Ochratoxin A (OTA) is a widely diffused food contaminant, highly detrimental for human health. In this work, a novel plasmonic based optical biosensor prototype for ochratoxin A is described. It exploits the metal-enhanced fluorescence phenomenon due to the silver film over nanosphere plasmonic substrate. Since ochratoxin A could be present in different food commodities, sensor performances have been tested on three different matrices (dried milk, juices, and wheat mix). Firstly, a common OTA extraction solvent and a labeling and detection protocol were defined for the analyzed matrices. Then, the efficiency of the Ag-FON surfaces in signal amplification for the detection of low ochratoxin A concentrations was defined. Using samples spiked with OTA-AF 647 or with unlabeled OTA we were able to detect the mycotoxin at concentrations lower than E.U. specifications of 0.5 µg/kg in wheat, milk and apple juice. The test performances are comparable to those of ELISA kits but the platform presented here, once optimized, present some perspective advantages, such as: low cost and time consuming, versatility of the protocol for the investigation of different matrices, employment also in non-qualified laboratories, small dimensions that allow its integration in a compact device for OTA on-site detection.

Investigation into the heterostructure interface of CdSe-based core-shell quantum dots using surface-enhanced Raman spectroscopy.

The structural nature of heterointerfaces in core-shell semiconductor quantum dots (QDs) plays a crucial role in tailoring their optical properties. In this work we have focused on using surface-enhanced Raman spectroscopy as a nondestructive tool to investigate the structural evolution of such interfaces in CdSe/CdS and CdSe/Cd0.5Zn0.5S colloidal QDs. A comparison between the two systems shows significant structural variation across the core-shell interfaces for the two different materials: a smooth interface for the former and an abrupt interface for the latter. This structural difference modifies the electronic structure within the QDs, which directly dictates the confinement behavior of the electrons and holes. The implications of this translate to a better understanding of why graded CdSe/CdS/Cd0.5Zn0.5S/ZnS QDs are so lucrative for linear and nonlinear fluorescence-based applications.

Photophysics and dynamics of surface plasmon polaritons-mediated energy transfer in the presence of an applied electric field.

The possibility to transfer energy between molecular excitons across a metal film up to 150 nm thick represents a very attractive solution to control and improve the performances of thin optoeletronic devices. This process involves the presence of coupled surface plasmon polaritons (SPPs) at the two dielectric-metal interfaces, capable of mediating the interactions between donor and acceptor, located on opposite sides of the metal film. In this Article, the photophysics and the dynamics of an efficient SPP-mediated energy transfer between a suitable dye and a conjugated polymer is characterized by means of steady-state and time-resolved photoluminescence techniques. The process is studied in model multilayer structures (donor/metal/acceptor) as well as in electrically pumped heterostructures (donor/metal cathode/acceptor/anode), to verify the effects of applied electric fields on the efficiency and the dynamics of SPP-mediated energy transfer. A striking enhancement of the overall luminescence was recorded in a particular range of applied bias, suggesting the presence of cooperative effects between optical and electrical stimulations.

Facile production of up-converted quantum dot lasers.

We report a facile production of an up-converted surface-emitting DFB laser, performed by exploiting the versatility of sol-gel chemistry, the intriguing properties of well designed graded CdSe-CdS-Cd(0.5)Zn(0.5)S-ZnS colloidal quantum dots, and the scalability of nanoimprinting. Our laser prototype operates in the visible region following efficient optical pumping by either direct one-photon excitation or through the up-conversion of near infrared (NIR) light. By achieving cavity mode Q-factors in excess of 650 and retaining high lasing stabilities in air, this work highlights the feasibility of creating integrated lasing devices through solution based methods.

Holstein-Peirls-Hubbard trimer as a model for quadrupolar two-photon absorbing dyes.

The linear and nonlinear optical properties of a Donor-Acceptor-Donor system have been investigated by using a two-electron three-point-site model system. Some basic features of electron correlations are included in the model by means of a bi-electronic density matrix. The polarizabilities and second hyperpolarizabilities have been computed with a modified version of the Collective Electronic Oscillators (CEO) method which allowed us to include the electron-phonon coupling. Both singly- and doubly-excited states are taken into account in the computation of (hyper-)polarizabilities. The effects of electron-phonon coupling on the two-photon absorption and on the third harmonic generation in the infrared region are discussed.

Femtosecond nonlinear absorption of gold nanoshells at surface plasmon resonance.

The nonlinear optical absorption of gold nanoshells (Au NSs) of different size, in water, was investigated using open aperture z-scan technique with femtosecond laser pulses at 806 nm. It is found that, in general, NSs behave as saturable absorbers. The level of saturation depends on the Au NSs structure and precisely on the ratio between the core size and shell thickness. The measured values of the nonlinear absorption coefficient show a dependence on both the repetition rate and the pulse energy. An average value of the nonlinear absorption coefficient ß = -4.5 ± 1.0 × 10(-11) cm W(-1) is obtained from z-scan data of core-shell particles of inner and outer radius 95 and 110 nm, respectively, measured at 20 Hz repetition rate in the energy range 120-300 nJ.

A fullerene-distyrylbenzene photosensitizer for two-photon promoted singlet oxygen production.

Novel compounds endowed with a high two-photon absorption (TPA) cross-section and a high singlet oxygen quantum yield, are sought after for their possible application in anti-cancer therapies. In this paper we present a prototype macromolecule bearing a distyrylbenzene dimer as TPA unit and a [60]fullerene moiety for singlet oxygen generation. Linear absorption and emission spectra are measured, to help understanding the interactions between the single molecular units. The TPA absorption properties of the distyrylbenzene alone as well as bound to the methanofullerene unit are recorded with the TPA-induced fluorescence technique. An appreciable enhancement of the TPA cross-section was observed in the molecular conjugate.Singlet oxygen generation has been detected exciting the sample both in the Vis and NIR through one- and two-photon absorption processes, respectively. Although functionalization decreases the overall singlet oxygen quantum yield of fullerene, the presence of the distyrylbenzene antenna allows two-photon generation of singlet oxygen through an energy transfer process.

Dimers of quadrupolar chromophores in solution: electrostatic interactions and optical spectra.

Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized.

Amplified spontaneous emission from opal photonic crystals engineered with structural defects.

In this work, we report on the optical properties and amplified spontaneous emissions (ASE) of polystyrene opals engineered with planar structural defects containing a conjugated polymer emitter. Defects in opals give rise to allowed states inside the photonic stop band, which are probed by transmittance and reflectance spectroscopy. The emission spectrum of the polymer embedded in the defect layer is strongly modified and fingerprints of defect states located inside the stop band are recognized. Amplified spontaneous emission for these engineered photonic crystals is clearly observed.

Enhancing the efficiency of two-photon absorption by metal coordination.

The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D-pi-A chromophore (D = donor and A = acceptor) is enhanced by a factor of about 2 upon coordination to ZnCl(2). Based on an analysis of linear absorption and fluorescence spectra of both the ligand and its Zn(II) complex, we have defined essential-state models for the two species. Linear and TPA spectra of the ligand are well reproduced in terms of a two-state model accounting for the D-pi-A <--> D(+)-pi-A(-) charge resonance. However, the enhancement of the TPA response of its Zn(II) complex can only be understood by extending the model to account for the active role of the "ZnCl(2)" moiety acting as a virtual A(v) acceptor group of a D-pi-AA(v) structure. The virtual D(+)AA(v)(-) state of the relevant three-state model has negligible weight in the ground state but contributes to the first excited state. The resulting increase of the excited-state dipole moment is responsible for the enhancement of the TPA cross section, and also explains the increase of the second order nonlinear optical response upon coordination.

The polarizability in solution of tetra-phenyl-porphyrin derivatives in their excited electronic states: a PCM/TD-DFT study.

Time dependent density functional theory and polarizable continuum model have been used to describe tetra-phenyl-porphyrin derivatives in different solvents and at different pH values. Our results show that last generation functionals are able to describe with good accuracy the photophysics of the title compounds, including different physical-chemical properties of excited electronic states. In particular, the strong modification of polarizabilities induced in some cases by electron excitation could explain a recent hypothesis about NLO characteristics of specific porphyrin adducts.

Non-resonant z-scan characterization of the third-order nonlinear optical properties of conjugated poly(thiophene azines).

The nonlinear optical properties of a functionalized poly(thiophene azine), namely, poly(3,4-didodecylthiophene azine), PAZ, at the optical telecommunication wavelength of 1550 nm are investigated by means of the closed-aperture z-scan technique in both thin films and solutions. Values of chi((3))=(2.4+/-0.4)x10(-13) esu, n(2)=(4.0+/-0.7)x10(-15) cm(2) W(-1), and gamma=(4.5+/-0.7)x10(-34) esu are estimated for the third-order (Kerr) susceptibility, the intensity-dependent refractive index, and the molecular second hyperpolarizability of solution samples, respectively. A very small dependence on the polymer chain length is found. Markedly higher values of (4.4+/-1.1)x10(-11) esu, (6.6+/-1.0)x10(-13) cm(2) W(-1), and (5.0+/-0.8)x10(-33) esu are measured for the corresponding quantities in thick (up to 20 mum) polymer films cast on quartz plates. The enhancement of the NLO responses on going from solution to solid samples is attributed to a partially ordered structure and to the presence of interchain interactions leading to greater pi-electron delocalization in the cast polymer films. The results are compared with those previously obtained by using third-harmonic generation (THG), taking into account that those data were measured under conditions of three-photon resonance, whereas our z-scan measurements are fully off-resonance.