Enhanced solid-state plasmon catalyzed oxidation and SERS signal in the presence of transition metal cations at the surface of gold nanostructures

Distinction of Plasmonic Intrananoparticle and Internanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Molecular reactions on the surface of a plasmonic nanoparticle (intrananoparticle) and between nanoparticles (internanoparticle) may differ in kinetics, dynamics, and product selectivity. We report that the difficulty in distinguishing the kinetics in a dispersion medium could be overcome by probing the reactions at the three-phase contact line (TPCL) of an evaporating sessile droplet using surface-enhanced Raman spectroscopy (SERS). Thus, when an evaporating aqueous droplet on glass containing 4-aminothiophenol-stabilized Ag nanoparticles was monitored by SERS at the TPCL, dimerization into 4,4'-dimercaptoazobenzene followed two steps, each preceded by the loss of H-bonded water accordingly. On the basis of the results, we assigned the first step with a higher relative kinetic rate (∼3 times) to be an intrananoparticle reaction and the second one as an internanoparticle reaction. In D2O medium, the ratio of the rates was ∼1.8. The observed vibrational signatures of the losses of water molecules before reactions and product formations were accounted for by using density functional theoretical calculations.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

DNA-Condensed Plasmonic Supraballs Transparent to Molecules

DNA nanotechnology offers an unrivaled programmability of plasmonic nanoassemblies based on encodable Watson-Crick basepairing. However, it is very challenging to build rigidified three-dimensional supracolloidal assemblies with strong electromagnetic coupling and a self-confined exterior shape. We herein report an alternative strategy based on a DNA condensation reaction to make such structures. Using DNA-grafted gold nanoparticles as building blocks and metal ions with suitable phosphate affinities as abiological DNA-bonding agents, a seedless growth of spheroidal supraparticles is realized via metal-ion-induced DNA condensation. Some governing rules are disclosed in this process, including kinetic and thermodynamic effects stemming from electrostatic and coordinative forces with different interaction ranges. The supraballs are tailorable by adjusting the volumetric ratio between DNA grafts and gold cores and by overgrowing extra gold layers toward tunable plasmon coupling. Various appealing and highly desirable properties are achieved for the resulting metaballs, including (i) chemical reversibility and fixation ability, (ii) stability against denaturant, salt, and molecular adsorbates, (iii) enriched and continuously tunable plasmonic hotspots, (iv) permeability to small guest molecules and antifoulingness against protein contaminates, and (v) Raman-enhancing and photocatalytic activities. Innovative applications are thus foreseeable for this emerging class of meta-assemblies in contrast to what is achieved by DNA-basepaired ones.

A gold nanoparticle doped flexible substrate for microplastics SERS detection

Due to overuse of plastic products, decomposed microplastics (MPs) are widely spread in aquatic ecosystems, and will cause irreparable harm to the human body through the food chain. Traditional MP detection methods require cumbersome sample pre-processing procedures and complex instruments, so there is an urgent demand to develop methods to achieve simple on-site detection. Herein, a simple, sensitive, accurate, and stable MP detection method based on surface-enhanced Raman scattering (SERS) is investigated. Considering the hydrophobic problems of MPs, gold nanoparticle (AuNP) doped filter paper as a flexible SERS substrate is applied to capture MPs in the fiber pores. Benefitting from the electromagnetic (EM) hot spots generated by AuNPs, the Raman signal of MPs can be effectively enhanced. Meanwhile, the flexible SERS substrate has good sensitivity to a minimum detectable concentration of 0.1 g L^-1 for polyethylene terephthalate (PET) in water, and the maximum enhancement factor (EF) can reach 360.5. Furthermore, the practicability of the developed method has been proved by the successful detection of MPs in tap water and pond water. This research provides an easy process, high sensitivity, and good reproducibility method for MP detection.