**1. Introduction**

A molecular sensor (or chemosensor) is a molecule or molecular ensemble designed to indicate the presence of a specific analyte through a detectable change in a macroscopic measurable signal. Molecular sensors are generally composed of two main elements: a recognition unit, which selectively interacts with the molecule or ion to be detected, and a signaling unit that is responsible of generating a macroscopically measurable signal (optical, electrochemical, mechanical…) upon the molecular recognition event. In general, in a chemosensor, the receptor binds the target analyte in a reversible manner. If the recognition event takes place through an irreversible chemical reaction, the chemosensor is better described as "probe." The transduction mechanism, which is the mechanism by which the chemical interaction between the analyte and the recognition unit is converted into a change in the macroscopic signal of the signaling unit, will depend on the type of signaling unit and the structure of the receptor.

The use of chemosensors or probes for the colorimetric detection of small biologically active molecules offers some advantages over traditional instrumental analytical methods. Chromogenic probes are usually cheap, easy to use, and do not require of expensive instrumentation, and very often, the presence of the analyte can be detected by the naked eye, which allows for rapid *in-situ* detection.

**Figure 1.**

*Analyte-induced aggregation of gold nanoparticles with concomitant changes in the color of the colloidal suspensions.*

Among the different approaches for colorimetric sensing, the use of gold nanoparticles (AuNPs) as scaffolds and signaling units for the construction of molecular sensors has attracted enormous interest for several reasons: They can be easily synthesized from Au(III) salts in various sizes and shapes. Their surface can be functionalized with a wide range of thiol- or disulfide-terminated organic ligands, by ligand exchange reactions, through the formation of strong Au–S bonds, leading to stable colloidal suspensions in water or organic solvents (depending on the ligand). Finally, gold nanoparticles have remarkable optoelectronic properties. In particular, the localized surface plasmon resonance (SPR) gives rise to a strong absorption band in the visible region. This SPR band is influenced not only by the size and shape of the nanoparticles but also by the dielectric properties of the environment and the proximity of other nanoparticles [1, 2]. This last property is the basis for colorimetric assays using AuNPs. Thus, analyte-triggered aggregation of AuNPs results in an important bathochromic shift of the SPR band (from ca. 520 to ca. 650 nm) and a change in the color of the colloidal solution from red (dispersed) to blue (aggregated) due to interparticle surface plasmon coupling (**Figure 1**). Moreover, AuNPs have very high molar extinction coefficients (ε) (ca. 108 –109 M<sup>−</sup><sup>1</sup> cm<sup>−</sup><sup>1</sup> for AuNPs with diameters between 10 and 50 nm), which confers this sensing method a great sensitivity. In fact, the color change associated to the aggregation process can be observed by naked eye even at nanomolar concentration [3, 4].

The main challenge in the design of colorimetric sensors with AuNPs is the election of the recognition units to be attached onto the surface of the nanoparticles and the nature of the molecular interaction leading to the aggregation process.

A large number of target analytes (metal ions, anions, small organic molecules, or large biomolecules) have been detected using functionalized AuNPs as colorimetric probes. However, in this chapter, the discussion has been limited to the use of functionalized spherical gold nanoparticles for the detection of small molecules with biological activity, such as neurotransmitters, nerve agents, pesticides, and biologically relevant carboxylates.
