Nanocatalysis under Nanoconfinement: A Metal-Free Hybrid Coacervate Nanodroplet as a Catalytic Nanoreactor for Efficient Redox and Photocatalytic Reactions

Abstract

Nature utilizes cellular and subcellular compartmentalization to efficiently drive various complex enzymatic transformations via spatiotemporal control. In this context, designing of artificial nanoreactors for efficient catalytic transformations finds tremendous importance in recent times. One key challenge remains the design of multiple catalytic centers within the confined space of a nanoreactor without unwanted agglomeration and accessibility barrier for reactants. Herein, we report a unique blend of nanoscience and chemical catalysis using a metal-free hybrid synthetic protocell as a catalytic nanoreactor for redox and photocatalytic transformations, which are otherwise incompatible in bulk aqueous medium. Hybrid coacervate nanodroplets (NDs) fabricated from 2.5 nm-sized carbon dots (CDs) and poly­(diallyldimethyl)­ammonium chloride have been utilized toward reductive hydrogenation of nitroarenes in the presence of sodium borohydride (NaBH4). It has been found that the reduction mechanism follows the classical Langmuir–Hinshelwood (LH) model at the surface of embedded CDs inside the NDs via the generation of reactive surface hydroxyl groups. These NDs show excellent recyclability without any compromise on reaction kinetics and conversion yield. Importantly, spatiotemporal control over the hydrogenation reaction has been achieved using two mixed populations of coacervates. Moreover, efficient visible light-induced photoredox conversion of ferricyanide to ferrocyanide and artificial peroxidase-like activity have also been demonstrated inside these catalytic NDs. Our findings indicate that the individual polymer-bound CD inside the NDs acts as the catalytic center for both the redox and photocatalytic reactions. The present study highlights the unprecedented catalytic activity of the metal-free CD-based coacervate NDs and paves the way for next-generation catalytic nanoreactors for a wide range of chemical and enzymatic transformations.

Kinetic and Mechanistic Insight into the Surfactant-Induced Aggregation of Gold Nanoparticles and Their Catalytic Efficacy: Importance of Surface Restructuring

“…The catalytic conversion was monitored via time-dependent changes in the UV–vis absorption spectra of 4-NP. All of the solutions were purged with N 2 gas for 15 min prior to any kinetic experiment to get rid of the dissolved oxygen, which is known to interfere with the reaction mechanism. , An absorption band centered at 400 nm appears in the UV–vis spectrum upon addition of 10 mM NaBH 4 to the aqueous solution of 20 μM 4-NP because of the formation of 4-nitrophenolate ions (Figure A). − The spectral changes were monitored after the addition of 6.6 pM Au NPs to the aqueous binary mixture of 4-NP and NaBH 4 . Here it should be noted that during the course of the redox reaction, hydrogen gas evolves from the reaction mixture and visible air bubbles can be observed at the cuvette wall.…”

“…All of the solutions were purged with N 2 gas for 15 min prior to any kinetic experiment to get rid of the dissolved oxygen, which is known to interfere with the reaction mechanism. 36,45 An absorption band centered at 400 nm appears in the UV−vis spectrum upon addition of 10 mM NaBH 4 to the aqueous solution of 20 μM 4-NP because of the formation of 4-nitrophenolate ions (Figure 7A). 36−39 The spectral changes were monitored after the addition of 6.6 pM Au NPs to the aqueous binary mixture of 4-NP and NaBH 4 .…”

Quantum Dot-Embedded Hybrid Photocatalytic Nanoreactors for Visible Light Photocatalysis and Dye Degradation

“…While significant advancement has been made to understand the mechanism and kinetics of various biocatalytic transformations and intracellular trafficking of functional coacervates, very less is known about their role in photocatalytic reactions. Recently, our group developed a new class of organic–inorganic hybrid coacervates using oppositely charged ligand-capped nanoparticles (NPs) and polyelectrolytes. , These optically active and structurally robust droplets have been utilized toward environmental remediation, theranostic applications, , and catalytic nanoreactors . Very recently, using nanoscale optically active carbon dots (CDs) as building units, we demonstrated redox and photoredox conversions inside these CD-embedded NDs .…”

Conjugated Polyelectrolyte‐Based Complex Fluids as Aqueous Exciton Transport Networks

“…There is revived interest in aqueous associative phase separation of polyelectrolytes. Specifically, liquid‐liquid phase separation of inter‐polyelectrolyte complexes, in which oppositely charged polyelectrolytes electrostatically self‐assemble and partition into polyelectrolyte‐rich and dilute phases, has been studied thoroughly to aid understanding in early cell development, drug delivery, catalysis, and underwater adhesives [18–25] . Research efforts have largely centered on polyelectrolytes with saturated, or non‐conjugated, backbones.…”

“…Specifically, liquid-liquid phase separation of inter-polyelectrolyte complexes, in which oppositely charged polyelectrolytes electrostatically self-assemble and partition into polyelectrolyte-rich and dilute phases, has been studied thoroughly to aid understanding in early cell development, drug delivery, catalysis, and underwater adhesives. [18][19][20][21][22][23][24][25] Research efforts have largely centered on polyelectrolytes with saturated, or non-conjugated, backbones. However, associative phase separation of conjugated polyelectrolytes (CPEs) capable of visible light absorption has received significantly less attention.…”