Nowadays, nanotechnology and nanochemistry are very often combined in order to develop nanostructured materials and, also, determine to what extent the manipulation of matter on an atomic, molecular, and supramolecular level may affect the desired nanomaterials properties
[1]. The atomic structure of materials having nanometric sizes promotes the implementation of their physical, chemical, and biological properties
[2]. In particular, the electronic energy levels in nanomaterials are quantized and not continuous as to their corresponding bulk conformation; this effect, known as the quantum confinement effect, demonstrates that material properties are size-dependent
[3]. The modification of surface area and electron delimitation, due to the confinement of electronic wave function in up to three physical dimensions, induces the development and the possibility to customize some properties, such as chemical reactivity, melting point, electrical conductivity, fluorescence, and magnetic permeability as a function of the size of nanoparticles
[4]. The history of AuNPs dates back to remote times when red ruby glass began to be used; however, they received the maximum attention starting from the end of the seventeenth century. The properties of metallic gold, such as optical and thermal, are explained by describing the plasmon resonance, which makes the AuNPs employable as sensors
[5], ultra-small light emitters
[6], nano heaters
[7], or nano antennas
[8]. Gold is an element that has a singular mix of physical and chemical features both in macroscopic and microscopic conditions. While the macroscopic properties concern its unique yellow color, chemical stability, and high redox potential, at the nanometric level, gold features are explained by a combination of the electronic structure with other effects due to the extremely small dimensions. Moreover, this is also due to (i) a high ratio of surface atoms to bulk atoms, (ii) electromagnetic confinement due to a localized plasmon resonance after the interaction with an optical wave, and (iii) the quantum effects, which justify, for instance, the change from metallic to a semiconducting character
[9]. One of the most impressive and useful AuNPs properties is plasmon resonance related to the collective behavior of conduction gold electrons. In fact, when it comes to metals, the conduction electrons behave as free charges, which can be excited by an electromagnetic wave. Thus, plasmon waves result from both charge mechanical oscillations and electromagnetic oscillations of the electric field. When this phenomenon occurs at the nanoscale, it is called Localized Surface Plasmon Resonance (LSPR), and it is the result of the confinement of the electric field within a small metallic sphere. This explains the red–purple color of spherical nanoparticles and its slight change when the shape or the surrounding medium are altered. For example, the LSPR is a powerful technique to input energy in metallic nanoparticles, enhancing the light-to-heat conversion. This study reports different chemical and green synthesis methods for the production of gold nanoparticles (AuNPs), namely, chemical reduction or others, such as electrochemical
[10], thermal
[11], and photochemical reduction techniques. The applications of AuNPs are strictly related to their shape and size; for example, gold nanorods are employed as biosensors, antineoplastic drugs
[12], and as carriers in drug delivery systems
[13]. AuNPs can penetrate cancer cell membranes, preventing their proliferation and growth
[14]. When these nanoparticles interact with light, the oscillating electric field induces the conduction electrons to oscillate with the same frequency of the electromagnetic wave; this is coherent with their plasmon electron cloud and its distribution over the whole nanoparticle volume. The AuNP surface charge is neutralized through undesired aggregation phenomenon. This can be mitigated by using opportune functional capping agents and depositing them on the surface; they can be small molecules, polymers, or biomolecules
[15]. Depending on these surface modifications, AuNPs can be employed in engineering, chemical, biochemical, and medical applications
[16].
This review collects the synthesis and chemical–physical characterization methods of AuNPs with interesting shapes that are requested in common applications (Figure 1). Their use in the biology and medicine fields are discussed, both for drug delivery and therapeutic treatments. This work concludes with an overview of all AuNP technological applications that could become a part of everyday life in the near future.
Most of the aforementioned products are still restricted to the research and development stages, with human tests, delivery systems, and toxicological assessments that have yet to be analyzed and developed.