Roy et al. (ACS Catalysis 2020) demonstrated that electrostatic attraction between charged ligands on gold nanoparticles and the NAD+^+ reactant funnels the reactant to the photocatalyst surface and enables mediator-free photocatalytic regeneration of the NADH cofactor, with intensity-dependent kinetics indicating the transfer of multiple electrons per photoexcited particle. Three linked issues remain open for this system: the relative contributions of photothermal heating versus non-thermal hot-electron transfer to the observed activity; the microscopic route by which successive hot electrons cross the charged organic ligand shell, including how interband and plasmonic excitation partition into reactive carriers; and whether the interaction-driven design generalizes to demanding multielectron targets such as CO2_2 reduction to hydrocarbons, N2_2 reduction, or water oxidation. Partial progress exists on each strand: the interband/intraband partition has been established for gold nanorods, thermoplasmonic quantification methods have been developed, and interaction-driven multielectron chemistry has been validated on quantum dots for nitrate-to-ammonia. No published work, however, delivers a quantitative thermal/non-thermal partition for the electrostatic-funneling system, a microscopic ligand-shell multielectron-delivery mechanism, or a plasmonic-gold demonstration on a demanding multielectron target beyond cofactor regeneration. The problem remains open.

Background

Nicotinamide adenine dinucleotide (NAD+^+/NADH) is the most widely used biological redox cofactor: enzymes called oxidoreductases draw on NADH to deliver a hydride equivalent (two electrons and one proton) to their substrates. Because NADH is too expensive to consume stoichiometrically, any large-scale biocatalytic process needs its continuous regeneration, and photocatalytic regeneration - using light to reduce NAD+^+ back to the enzymatically active 1,4-NADH isomer, as opposed to the inactive 1,6- and 2-isomers - is a leading route. Most artificial systems require soluble electron mediators (typically rhodium complexes) to shuttle charge between a photosensitizer and NAD+^+.

Plasmonic gold nanoparticles offer a chemically simpler alternative. Their localized surface plasmon resonance (LSPR) - a collective oscillation of conduction electrons driven by visible light - provides strong absorption and, upon decay, generates non-equilibrium ('hot') electrons and holes that can in principle be injected into adsorbed reactants before they give up their energy as heat within picoseconds. Competing with this non-thermal channel, illuminated metal nanoparticles also act as nanoscale heat sources (thermoplasmonics), so an observed light-driven rate can combine photothermal and hot-carrier contributions; separating the two is experimentally subtle because the exponential (Arrhenius) temperature sensitivity of reaction rates amplifies small unmeasured temperature differences into apparent 'hot-electron' effects. In gold, excitation above the interband threshold (electron transitions from the filled dd bands to the spsp conduction band, near 2.4 eV) can be probed separately from purely plasmonic (intraband) excitation, most cleanly in nanorods, which carry two spectrally distinct plasmon bands.

Colloidal gold nanoparticles are capped by an organic ligand shell. Roy, Jain, Kashyap, Rao and Pillai (ACS Catalysis 10, 5522-5528, 2020) showed that endowing this shell with net charge creates electrostatic attraction to the oppositely charged NAD+^+ reactant, raising its local concentration at the particle surface and enabling mediator-free photocatalytic NADH regeneration with performance comparable to or exceeding more complex plasmonic systems. Kinetics- and intensity-dependent measurements indicated that multiple electrons are transferred from a single photoexcited particle to one reactant - the accumulative multielectron chemistry that demanding transformations (CO2_2-to-hydrocarbons, N2_2 fixation, water splitting) also require.

The demonstration left the mechanism and the generality of the design open: photothermal contributions were not excluded, so a quantitative discrimination between thermal and non-thermal mechanisms is still needed; the microscopic path by which successive hot electrons cross the charged ligand shell, and how interband versus plasmonic excitation partitions into reactive carriers, remains to be elucidated; and the interaction-driven design still awaits experimental validation and mechanistic study on targets beyond cofactor regeneration - the source work's own forward-looking statement (ref 1) is that the use of electrostatics "can serve as a generic approach for other important artificial multielectron photocatalytic reactions as well". Parts of this agenda have since been clarified - most notably the study of interband versus intraband excitation in nanorods for this same reaction, methodological work on quantifying thermoplasmonic heating, and field-wide studies and reviews showing that the thermal/non-thermal discrimination problem remains unsettled and system-specific - while the general thermal-versus-non-thermal debate in plasmonic catalysis is itself unresolved. These threads define the surviving open problem stated below.

Problem Statement

The problem left open by Roy et al. (2020) and by the subsequent literature is to convert the electrostatic-funneling demonstration into a mechanistically understood and validated design for multielectron photocatalysis, by resolving three jointly linked questions:

(1) Mechanism partition. Quantify, for the electrostatically ligand-capped gold-nanoparticle/NAD+^+ system of Roy et al. (2020), the relative contributions of photothermal heating and non-thermal hot-electron transfer to the observed photocatalytic activity.

(2) Charge-delivery mechanism. Elucidate the microscopic mechanism by which multiple photoexcited electrons traverse the charged organic ligand shell and accumulate on a single reactant - including how interband versus intraband (plasmonic) excitation contributes to reactive carriers in that system, a partition substantially advanced for gold nanorods by Jain et al. 2023 (whose two spectrally distinct plasmon bands separate the two excitation regimes) but not settled for the original charged-ligand AuNP system, where interband and plasmonic excitation are not separated as cleanly, and by what route (direct tunneling, hopping through ligand orbitals, or transport through shell defects) the charged shell mediates rather than blocks accumulative multielectron chemistry.

(3) Generality. Determine whether - and either demonstrate or refute - that the electrostatic, interaction-driven multielectron-transfer design generalizes to at least one demanding photocatalytic target beyond cofactor regeneration (for example CO2_2 reduction to hydrocarbons, N2_2 reduction, or water oxidation), and give a mechanistic account of how the design and optimization parameters - ligand charge and density, reactant pairing, excitation regime - control the activity observed on that target.

These three questions are linked and serve one overarching objective - a quantitative, mechanism-grounded account of whether and how electrostatically organized plasmonic nanoparticles drive multielectron chemistry, and whether that design principle extends beyond cofactor regeneration. Progress on one strand (as already achieved for the interband/intraband partition on gold nanorods) narrows the remaining questions.

Each strand is an independently refutable sub-question: a quantitative thermal/non-thermal partition (1), a microscopic account of how successive electrons traverse the charged shell and how interband versus intraband excitation partitions into reactive carriers (2), and a validated, mechanistically rationalized determination - affirming or refuting generalization - on a demanding multielectron target (3). A resolution of any single strand is a partial advance that narrows the remaining questions; a complete resolution of the open problem requires all three strands, with the sub-asks named within each strand forming part of that strand.

References

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  2. Vanshika Jain, Indra Narayan Chakraborty, Rohit B. Raj, Pramod P. Pillai. Deciphering the Role of Light Excitation Attributes in Plasmonic Photocatalysis: The Case of Nicotinamide Cofactor Regeneration. The Journal of Physical Chemistry C 127(10), 5153-5161 (2023). DOI: 10.1021/acs.jpcc.2c08678. https://doi.org/10.1021/acs.jpcc.2c08678
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