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Performing electron-transfer reactions on metal nanoparticles requires separation of charge carriers at the nanoparticle and their transfer to the reacting molecules. Inducing these reactions using light is challenging due to the exceedingly short lifetimes of energetic charge carriers formed in metal nanoparticles under light illumination. The results described here show that certain conditions must be met to drive these electron-transfer reactions on plasmonic nanoparticles. One critical requirement is that the process of electronic excitation takes place at the nanoparticle/molecule interface. This is accomplished by high plasmonic electric fields at the surface of plasmonic nanoparticles. Furthermore, it is also evident from our study that the electron (or hole)-donating capacity of the hole (or electron) scavengers needs to be high enough to allow for the extraction of holes (or electrons) from the nanoparticle/molecule complex, therefore completing the catalytic cycle. We discuss these findings through a case study of the conversion of methylene blue (MB) into a reduced MB ion radical on the surface of plasmonic Ag and Ag−Pt core−shell nanoparticles. To directly monitor the reduction reaction of MB on the nanoparticle surfaces, we have used time-dependent in situ surface-enhanced Raman scattering measurement, which also informs us about the underlying mechanistic details of plasmon-driven charge transfer.

The photosensitized interfacial electron transfer (ET) dynamics of the zinc(II)–5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP)–TiO2 nanoparticle (NP) system has been studied using single-molecule photon-stamping spectroscopy. The single-molecule fluorescence intensity trajectories of m-ZnTCPP on TiO2 NP surface show fluctuations and blinking between bright and dark states, which are attributed to the variations in the reactivity of interfacial ET, i.e., intermittent interfacial electron transfer dynamics. Comparing the results with that from our earlier studied p-ZnTCPP–TiO2 nanoparticle system, we show the effect of anchoring group binding geometry (meta or para), hence electronic coupling of sensitizer (m-/p-ZnTCPP) and TiO2 substrate, on interfacial ET dynamics. Compared to p-ZnTCPP on TiO2 NP surface, with m-ZnTCPP, dark states are observed to dominate in single-molecule fluorescence intensity trajectories. This observation coupled with the large difference in lifetime derived from bright and dark states of m-ZnTCPP demonstrates higher charge injection efficiency of m-ZnTCPP than p-ZnTCPP. The nonexponential autocorrelation function decay and the power-law distribution of the dark-time probability density provide a detailed characterization of the inhomogeneous interfacial ET dynamics. The distribution of autocorrelation function decay times (τ) and power-law exponents (m dark) for m-ZnTCPP are found to be different from those for p-ZnTCPP, which indicates the sensitivity of τ and m dark on the molecular structure, molecular environment, and molecule–substrate electronic coupling of the interfacial electron transfer dynamics. Overall, our results strongly suggest that the fluctuation and even intermittency of excited-state chemical reactivity are intrinsic and general properties of molecular systems that involve strong molecule–substrate interactions.

The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (K(p) = 8.59 × 10(3)) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto-enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto-enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ~1%) and higher negative value of binding enthalpy of interaction (-ΔH) than total free energy change (-ΔG) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle.

Due to the increasing applicability of ionic liquids (ILs) as different components of microemulsions (as the polar liquid, the oil phase, and the surfactant), it would be advantageous to devise a strategy by which we can formulate a microemulsion of our own interest. In this paper, we have shown how we can replace water from water-in-oil microemulsions by ILs to produce IL-in-oil microemulsions. We have synthesized AOT-derived surface-active ionic liquids (SAILs) which can be used to produce a large number of IL-in-oil microemulsions. In particular, we have characterized the phase diagram of the [C(4)mim][BF(4)]/[C(4)mim][AOT]/benzene ternary system at 298 K. We have shown the formation of IL-in-oil microemulsions using the dynamic light scattering (DLS) technique and using methyl orange (MO), betaine 30, and coumarin-480 (C-480) as probe molecules.

In this work we have reported the controlled synthesis of gold nanoparticles into the surface cavities of P123 micellar assemblies together with the fluorescent dye molecules and investigated nanometal surface energy transfer (NSET) from confined donor dye to metal nanoparticles. The formation of hybrid spherical assemblies of P123 combined with fluorescent dyes and gold nanoparticles has been confirmed from HR-TEM, DLS, UV−vis, and fluorescence spectroscopic studies. The observed steady state as well as time-resolved fluorescence quenching of the confined micellar dyes present in the close proximity of gold nanoparticles which are attached to the surface of micellar assemblies, indicates efficient surface energy transfer from dye to gold (Au) nanoparticles. Since the NSET process is strongly dependent on the distance between donor dye and acceptor nanoparticles, successful applications of NSET require the perfect control over their relative location. Herein, we investigate the utilization of nanoparticles embedded self-assemblies of P123 for controlled NSET by tuning the precise location of donor dyes. Through the nanoencapsulation of the different fluorophore having different location inside P123 micelles, we have shown the corona region of P123 micelles as a perfect place for NSET and the core region as a barrier for NSET. Additionally, we have investigated the microenvironment of the confined micellar probe molecules in presence and absence of nanoparticles. This study further reveals that when the system changes from normal micelles to nanoparticles loaded hybrid micelles, unlike the probes C480 and C153, the anionic probe C343 undergoes a change in its location indicating the modulation of the properties of micelles in presence of nanoparticles.

The effect of different microenvironments inside various biomimicking supramolecular assemblies of ionic (SDS/CTAB) and nonionic (TX100) micelles and nonionic surfactants (Tween-80/PEG-6000) forming vesicles (niosome) on the photophysical and rotational dynamical properties of 1'-hydroxy-2'-acetonaphthone (HAN) have been studied using steady-state and time-resolved fluorescence spectroscopy. Enhanced fluorescence intensity with a significant blue shift and longer emission lifetime of the caged tautomers of HAN indicate modulation of photophysics of HAN upon encapsulation in both micellar assemblies and the niosome system. The binding constant and free energy change for the complexation of HAN with micelles and niosome demonstrate a comparative study on the binding efficiency of the different assemblies depending on the nature of microenvironments toward HAN. The enhancement in the steady-state anisotropy in niosome solutions compared with that in pure aqueous solution indicates that HAN is located inside the motionally restricted bilayer region of niosome. The fluorescence quenching experiment further reveals the probable location of HAN in micelles and niosome. In TX100 micelles, the obtained lifetime values are 417 ps and 1.63 ns for the caged tautomers, whereas in the comparatively more rigid and confined environment provided by niosome those values are 444 ps and 2.5 ns. The rotational relaxation time constants for the caged tautomers in niosome are also found to be higher than those in micelles. The observed difference in binding ability of the different assemblies is due to the difference in the extent of water penetration and different extent of rigidity around the fluorophore.

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