Free-Standing Networks of Core–Shell Metal and Metal Oxide Nanotubes for Glucose Sensing

Abstract

Nanotube assemblies represent an emerging class of advanced functional materials, whose utility is however hampered by intricate production processes. In this work, three classes of nanotube networks (monometallic, bimetallic, and metal oxide) are synthesized solely using facile redox reactions and commercially available ion track membranes. First, the disordered pores of an ion track membrane are widened by chemical etching, resulting in the formation of a strongly interconnected pore network. Replicating this template structure with electroless copper plating yields a monolithic film composed of crossing metal nanotubes. We show that the parent material can be easily transformed into bimetallic or oxidic derivatives by applying a second electroless plating or thermal oxidation step. These treatments retain the monolithic network structure but result in the formation of core-shell nanotubes of altered composition (thermal oxidation: CuO-CuO; electroless nickel coating: Cu-Ni). The obtained nanomaterials are applied in the enzyme-free electrochemical detection of glucose, showing very high sensitivities between 2.27 and 2.83 A M cm. Depending on the material composition, varying reactivities were observed: While copper oxidation reduces the response to glucose, it is increased in the case of nickel modification, albeit at the cost of decreased selectivity. The performance of the materials is explained by the network architecture, which combines the advantages of one-dimensional nano-objects (continuous conduction pathways, high surface area) with those of a self-supporting, open-porous superstructure (binder-free catalyst layer, efficient diffusion). In summary, this novel synthetic approach provides a fast, scalable, and flexible route toward free-standing nanotube arrays of high compositional complexity.

Metal Nanotube/Nanowire-Based Unsupported Network Electrocatalysts

“…It should be considered that even in the case of replacement with a single metal, the commonly occurring incompleteness of the exchange reaction-and/or accompanying purification steps intended to remove the residual educt metal-will result in bimetallic systems, in which a considerable impact of the composition on the catalytic performance is to be expected [61]. By applying galvanic replacement reactions or Kirkendall transformations on metal nanotubes instead of nanowires, double-walled or porous nanotubes can be created [61][62][63].…”

“…Also, parallel arrays of metal nanotubes deposited in porous membranes with different methods (e.g., electroless plating [63][64][65][66][67] or chemical vapor deposition [68]) can be released from the template, and then drop-coated to form unsupported catalyst networks. Such template membranes also can be combined with secondary templates like polymer spheres [69] or surfactants [70] to modify the internal morphology and porosity of the deposited nanostructures.…”

Nucleation‐Controlled Solution Deposition of Silver Nanoplate Architectures for Facile Derivatization and Catalytic Applications

“…Free‐standing metal nanostructure assemblies display a privileged class of functional materials, which simultaneously provide numerous desired properties (e.g., large surface area, high conductivity, tunable reactivity and selectivity, efficient diffusive access, and robustness) . In addition, they represent excellent supports for fabricating 3D hybrid nanomaterials …”

“…However, efficiently constructing such materials still poses a challenge. Lithographic, template‐assisted, and direct nanostructure writing techniques rely on multistep procedures and deposit metal by evaporation, plating or electron beam decomposition, yielding products with impurities and unspecific nanostructure, which restricts property optimization. In nanoparticle syntheses, adjusting the reaction conditions allows precise control over the nanoparticle shape, defects, terminating facets, and degree of crystallinity .…”

“…Based on its flexibility, facility, and scalability, autocatalytic deposition is of high interest for preparing nanomaterials . However, due to its conformal growth mode, it requires hard templates for molding the deposit into complex designs, and the products typically lack a defined internal nanostructure …”