CORDIS

Plasmonic nanoparticles (PNPs) present unique optoelectronic properties that depend on their size and shape and are not present in larger particles or the bulk material. Such properties arise from their localized surface plasmon resonances (LSPRs) . LSPRs are the light-induced coherent motion of electrons that produce dramatic enhancements of the electromagnetic field close to the surface of the particle (hot spots) as well as large scattering and absorption cross-sections . These properties have motivated the use of PNPs in many applications in the field of nanotechnology including ultra-sensitive sensing, light harvesting, imaging, photonics, catalysis, and medical and pharmaceutical therapies . Very recently, a previously unexplored feature of LSPRs opened a new perspective of these systems. Non-radiative decay of LSPRs can result in the excitation of electron-hole pairs with high, far-from-equilibrium energies known as hot carriers . These carriers can either dissipate their energy to the phonon lattice producing heat or – if they reach the surface fast enough without relaxing – can be injected into a nearby molecule causing its chemical transformation . Manipulating LSPRs allows for the fine control of the reactive properties of hot carriers, in a similar way in which it has enabled control of electromagnetic fields.
The advances in the fundamental understanding of plasmon-mediated chemical reactions could find applications in the design of alternative photocatalytic methods with improved efficiency and/or selectivity. This would contribute to European Union goals for energy and environment policies. For example, the outcome of this plan can lead to new materials for solar energy conversion.

However, determining the role of hot carriers in plasmon-mediated chemistry is a difficult task as it is usually masked by other catalytic properties (heat generation and field enhancement).
The main objective of this proposal is the implementation of an optical method for reactive-spot mapping, which will allow to create a map that highlights areas of low and high photochemical reactivity on single PNPs with high spatial resolution.

The project involved developing optical techniques allowing the study of chemical reactivity on individual metallic nanoparticles with spatial and temporal resolution.
The first part of the work consisted of building the microscope for the super-resolution mapping of hot carriers-driven chemical reactions at the single-particle level. For this, an existent wide-field microscope was modified, by coupling three different lasers, installing a homemade focus-lock system, and developing the software for image analysis.
The fluorescence imaging technique relies on finding a fluorogenic reaction, where a non-fluorescent reactant is converted into a fluorescent product upon interaction with a plasmon-derived energetic carrier. For this purpose, It was shown that Ag nanoparticles (Ag PNPs) illuminated at their plasmon resonance can produce hot-electrons capable of reducing Resazurin into Resorufin. Localizing the turn-over events at the single-molecule level allows the creation of reactivity maps on individual PNPs.
Then, numerical simulations were carried out to design optical nanoantennas so that they resonate at the visible range. The designed nanoantennas were then fabricated through an electron beam lithography procedure and studied using the developed super-resolution technique. The results provided information on the wavelength-dependent spatial distribution of reactivity of the hot-electron induced reduction of Resazurin on Au PNPs.
In addition, a method to measure the energy contribution from a plasmonic nanoparticle to a chemical reaction at the single-particle level under in operando conditions was introduced. The method is based on the combined information between electrochemistry and single-particle dark field microscopy and spectroscopy. Wavelength-dependence studies showed that the overall energy requirements of the electrochemical oxidation aniline on Au NPs can be reduced up to ∼35% when exciting the NP at its plasmon resonance. The measurement of the energy of the carriers was possible because thermal effects were ruled out as the driving photocatalytic mechanism. This was done by applying a photoluminescence thermometry method based on Anti-stokes emission of PNPS.

The developed fluorescence microscope is available for future study of plasmon reactivity on individual particles with high spatial resolution. In addition, other novel tools such as a nanothermometry technique and a combined dark-field microscope with electrochemistry were developed. These single-particle techniques are key to unearthing the underlying mechanisms of hot-carrier generation, transport and injection, as well as to disentangling the role of the temperature increase and the enhanced near-field at the nanoparticle–molecule interface. Gaining nanoscopic insight into these processes and their interplay could aid in the rational design of plasmonic photocatalyst, which could hopefully impact potential industrial applications of photocatalysis