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We unveil the competitive mechanisms driving the FRET between quantum dots, leading to unexpectedly high enhancements of acceptors’ fluorescence, especially when their local environment is initially unfavourable for their radiative relaxation.

International audience; We present a general formalism to model and calculate linear and nonlinear optical processes in composite systems,based on a graphical representation of light-matter interactions by loop diagrams associated with Feynmanrules. Through this formalism, we recover the usual second-order response of a simple system by drawing fourtimes fewer loop diagrams than doubled-sided ones. For composite systems, we introduce coupling Hamiltoniansbetween subsystems (for example, a molecule and a substrate), graphically represented by virtual bosons. In thisway, we enumerate all the diagrams describing the second-order response of the system and show how to selectthose relevant for the calculation of the molecular second-order hyperpolarizabilities under the influence of thesubstrate, including effective second-order contributions from the molecular third-order response. As it appliesto all nonlinear processes and an arbitrary number of interacting partners, this representation provides a generalframe for the calculation of the nonlinear response of arbitrarily complex systems.

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Colloidal semiconductor quantum dots (QDs) constitute zero-dimension excitonic materials characterized by quantized states and able to emit fluorescence. QDs are promising materials for catalysis, molecular recognition and biosensing. In this context, our work consists in the design of a new class of biochemical sensors based on QD-grafted chips to benefit from their high opto-electronic activity in the visible range. In order to achieve this, we functionalize silica and CaF2 susbtrates with 4 nm-diameter CdTe QDs and organic molecules (e.g. phenylamine). The originality of our work then lies in two-colour sum-frequency generation nonlinear optical spectroscopy, mixing Raman and IR spectroscopies. This technique enables to probe and to quantify the coupling between the excitonic properties of the QDs and the vibrational response of their molecular environment: two tunable visible and IR laser beams are mixed on the QD-grafted chips to electronically excite the QDs while the vibrational spectroscopy of the surrounding organic species is performed. Through this approach, we clearly demonstrated a correlation between QDs and molecules. Especially, the vibrational response of the molecules is maximized when the first excitonic state of the CdTe QDs is pumped by the visible beam, which means it is possible to enhance the detection of given biomolecules thanks to QDs. Considering that confined excitons transfer their energy to molecules through dipolar interaction, the model we developed accounts indeed for such a behaviour.

Given the tunability of their optical properties over the UV–Visible–Near IR spectral range, ligand-capped quantum dots (QDs) are employed for the design of optical biosensors with low detection threshold. Thanks to non-linear optical spectroscopies, the absorption properties of QDs are indeed used to selectively enhance the local vibrational response of molecules located in their vicinity. Previous studies led to assume the existence of a vibroelectronic QD–molecule coupling based on dipolar interaction. However, no systematic study on the strength of this coupling has been performed to date. In order to address this issue, we use non-linear optical Two-Color Sum-Frequency Generation (2C-SFG) spectroscopy to probe thick QD layers deposited on calcium fluoride (CaF2) prisms previously functionalized by a self-assembled monolayer of phenyltriethoxysilane (PhTES) molecules. Here, 2C-SFG is performed in Attenuated Total Reflection (ATR) configuration. By comparing the molecular vibrational enhancement measured for QD–ligand coupling and QD–PhTES coupling, we show that the spatial dependence of the QD–molecule interactions (∼1/r3, with r the QD–molecule distance) is in agreement with the hypothesis of a dipole–dipole interaction.

Hypothesis The fluorescence emission of water-soluble CdTe quantum dots (QDs) capped with mercaptocarboxylic acids (MCAs) is known to be pH-dependent. However, this behaviour is quite different from a study to another, so that literature suffers from a lack of coherence. Here we assume that the QD fluorescence efficiency is actually driven by the acid-base equilibrium of MCA thiol groups, and that light-excited QDs open a non-radiative relaxation path through photoinduced protonation. Experiments We address this issue by examining colloidal CdTe QDs with (time-resolved) fluorescence spectroscopy under various conditions of acidity and light excitation. Findings It appears that the emission of QDs is quenched below a critical pH value of 6.87, and that light excitation power strengthens this quenching. We thus demonstrate the existence of an additional photochemical process and developed a mathematical modeling accounting for all our experimental results. With only three parameters, it is possible to accurately predict the fluorescence decay of QDs over time, at any pH. Further, we also related the critical pH value of 6.87 to QD surface properties, explaining why observations may differ from a study to another and making the literature much more coherent.

In this paper, we report on the study of a novel type of substrate based on a highly crystalline ZnO film photo-irradiated using UV for enhancing the Raman signal. This effect is called photo-induced enhanced Raman spectroscopy (PIERS). This PIERS substrate is composed of a photo-irradiated thin ZnO film on which gold nanoparticles are deposited and allows large photo-induced SERS enhancement to be obtained for the chemical detection of small molecules compared to normal SERS signals. This photo-induced SERS enhancement is due to increasing electron density of the gold nanoparticles and charge transfer mechanisms. Here, we achieve a high quality PIERS substrate, the signal of which exhibits weaker fluctuations and a similar or greater gain (up to 7.52) than those reported in the current literature. Henceforth, these PIERS substrates can be of great potential for industrial applications.

International audience; As luminescent quantum dots (QDs) are known to aggregate themselves through their chemical activation by carbodiimide chemistry and their functionalization with biotin molecules, we investigate both effects on the fluorescence properties of CdTe QDs and their impact on Förster Resonant Energy Transfer (FRET) occurring with fluorescent streptavidin molecules (FA). First, the QDs fluorescence spectrum undergoes significant changes during the activation step which are explained thanks to an original analytical model based on QDs intra-aggregate screening and inter-QDs FRET. We also highlight the strong influence of biotin in solution on FRET efficiency, and define the experimental conditions maximizing the FRET. Finally, a free-QD-based system and an aggregated-QD-based system are studied in order to compare their detection threshold. The results show a minimum concentration limit of 80 nM in FA for the former while it is equal to 5 nM for the latter, favouring monitored aggregation in the design of QDs-based biosensors.

In this Special Issue, invited researchers elaborate on ‘Recent Advances in Linear and Nonlinear Optics’, demonstrating how sensitive light–matter interactions are concerning symmetry [...]

International audience; We report on the recent scientific research contribution of non-linear optics based on Sum-Frequency Generation (SFG) spectroscopy as a surface probe of the plasmonic properties of materials. In this review, we present a general introduction to the fundamentals of SFG spectroscopy, a well-established optical surface probe used in various domains of physical chemistry, when applied to plasmonic materials. The interest of using SFG spectroscopy as a complementary tool to surface-enhanced Raman spectroscopy in order to probe the surface chemistry of metallic nanoparticles is illustrated by taking advantage of the optical amplification induced by the coupling to the localized surface plasmon resonance. A short review of the first developments of SFG applications in nanomaterials is presented to span the previous emergent literature on the subject. Afterwards, the emphasis is put on the recent developments and applications of the technique over the five last years in order to illustrate that SFG spectroscopy coupled to plasmonic nanomaterials is now mature enough to be considered a promising research field of non-linear plasmonics.

Gold nanotriangles structured as honeycombs and fabricated by nanosphere lithography on a gold film are functionalised by thiophenol molecules in order to be used as plasmonic sensors in nonlinear optical sum frequency generation (SFG) spectroscopy. The monitoring and the characterisation of the surface optical properties are performed by UV–visible differential reflectance spectroscopy showing an absorbance maximum located at 540 nm for p- and s-polarisation beams. SFG spectroscopy proves to be effective for thiophenol detection in ssp-polarisation scheme, while the molecular SFG signal disappears in ppp-configuration due to the strong s–d interband contribution of gold. However, in ssp-configuration, the vibration modes of thiophenol molecules at 3050 and 3071 $${\hbox {cm}}^{-1}$$ are yet observed thanks to the excitation of a transversal plasmon mode by the incident visible laser beam, whereas they are usually very difficult to distinguish by surface-enhanced Raman scattering and other vibrational optical probes.

Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we analyse and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis. Exciton/recombination events in semiconductor quantum dots are highly dependent on surface coordination environments and these processes are well studied. Here, conversely, the authors use sum-frequency generation spectroscopy to probe the effect of the quantum dot on the vibrational structure of the ligands.

International audience; A global method for handling fluorescence spectra at high concentration derived from the competition between emission and absorption of colloidal CdTe quantum dots This paper demonstrates that the concentration dependent attenuation and redshift of quantum dots' fluorescence is due to primary and secondary inner filter effects and not to collisional quenching as commonly stated. We present a global method to predict and process such altered fluorescence spectra that can be applied to any fluorophore. We investigate the effects of the concentration of CdTe quantum dots (QDs) on their fluorescence in water. The emission spectra, acquired in right angle geometry, exhibit highly variable shapes. The measurements evidence a critical value of the concentration beyond which the intensity and the spectral bandwidth decrease and the fluorescence maximum is redshifted. To account for these observations, we develop a model based on the primary and secondary inner filter effects. The accuracy of the model ensures that the concentration dependent behaviour of QD fluorescence is completely due to inner filter effects. This result is all the more interesting because it precludes the assumption of dynamic quenching. As a matter of fact, the decrease of the emission intensity is not a consequence of collisional quenching but an effect of competition between fluorescence and absorption. We even show that this phenomenon is linked not only to the QD concentration but also to the geometric configuration of the setup. Hence, our analytical model can be easily adapted to every fluorescence spectroscopy configuration to quantitatively predict or correct inner filter effects in the case of QDs or any fluorophore, and thus improve the handling of fluorescence spectroscopy for highly concentrated solutions.