Chemiresistive sensing with chemically modified metal and alloy nanoparticles

Abstract

This review describes the use of chemically-modified pure and alloy metal nanoparticles for chemiresistive sensing applications. Chemiresistors are materials that change their resistance in the presence of a particular analyte of interest. Chemically-modified metal nanoparticles consist of a pure or alloy metallic core that has some type of chemical coating, which could be an organic monolayer, polymer, surfactant, biomolecules, inorganic material, or organometallic molecules. Researchers have studied the electronic properties of one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) assemblies of chemically-modified metal nanoparticles and even single individual nanoparticles. These assemblies are well-suited for chemiresistive sensing applications as the metallic core provides a conductive path and the nanoparticle coating provides a means for controlling interactions with an analyte of interest. The interaction with the analyte alters the conductivity of the material, providing a signal to measure the analyte concentration. Much of this review focuses on the use of metal monolayer-protected clusters (MPCs) for chemiresistive sensing applications. This particular class of nanoparticles consists of a pure or alloy metallic core surrounded by a self-assembled monolayer coating, usually an organomercaptan or amine-based ligand. The versatile and well-understood synthesis of MPCs has allowed researchers to tailor the size and composition of the metallic core and coating for chemiresistive sensing of a wide variety of gas- and liquid-phase analytes. This review also describes chemiresistive sensing applications of other types of metal nanoparticles synthesized with different coatings (or stabilizers), such as ions, polymers, surfactants, and biomolecules. Chemiresistive sensing can be performed with these materials assembled as large scale films, micro/nano-patterned films, or as individual nanoparticles. The nanoparticles may be chemically-linked or assembled through weak intermolecular forces. Here we review different strategies used to incorporate chemically-modified nanoparticles into chemiresistive sensing devices, focusing on the different types of metal and alloy compositions, coatings, methods of assembly, analytes (vapors, gases, liquid phase, biological), and other various factors, such as particle size, stability, conditioning steps, and practical considerations. This review also includes a summary and future directions of the field.