Rational Design and Applications of Ultrasmall Gold Nanoparticles

Controlling the size of gold nanoparticles (AuNPs) has been critical in diagnostics, biomolecular sensing, targeted therapy, wastewater treatment, catalysis, and sensing applications. Ultrasmall AuNPs (uAuNPs), with sizes Ranging from 2 to 5 nm, and gold nanoclusters (AuNCs), with sizes less than 2 nm, are often dealt with interchangeably in the literature, making it challenging to review them separately. Although they are grouped in our discussion, their chemical and physical properties differ significantly, partly due to their electronic properties. The distinct optoelectronic properties of uAuNPs and AuNCs are usually not observed in gold metal and nanoparticles of larger sizes. Since small AuNPs tend to aggregate, several routes have been developed to prevent the formation of larger sizes, such as nucleation within porous materials. Controlling the particle size using synthesis methods is challenging, and uAuNPs and AuNCs can be fabricated simultaneously in the same preparation, necessitating separation and additional laboratory efforts. AuNCs can be stabilized by the prevalent soft ligands, such as phosphine and thiolate, unlike uAuNPs, in which a wide range of ligand sets can be used for stabilization. This review is organized around core sections concerning the synthesis, medical and environmental applications, and calculation studies of uAuNPs. It remains valuable to address the current stimulating market growth and potential market constraints when reviewing the expanding applications of AuNPs in the healthcare sector. A significant proportion of the synthesis processes involve the fabrication of uAuNPs and AuNCs in aqueous solutions. An obvious advantage of this work is that we focus on the medical and environmental applications, which often require water-dispersible nanoparticles. Calculation investigations explain the structural dynamics and importance of fine-tuning the size of uAuNPs to impart distinct properties. A notable control in the HOMO-LUMO energy gap, associated with the number of gold atoms, significantly affects their performance in various applications.