**5. Conclusions**

The use of gold nanoparticles for the preparation of colorimetric sensors is a very active field. The changes in the color of colloidal gold nanoparticles in solution because of the change in the surface plasmon absorption band upon aggregation or disaggregation processes can be easily used to transform the molecular recognition event into a macroscopic measurable signal. This change from red to blue can be observed by the nacked eye, allowing in this way cheap and easy detection of the target analytes. In this chapter, different mechanisms for the direct or indirect analyte-triggered aggregation of the AuNPs have been considered, including chemical reactions, supramolecular interactions, or changes in the pH of the medium. The selectivity observed in the sensing response in some cases depends on the conformation or configuration of the analyte, but also it can be achieved by using enzymes that catalyze specific reactions or aptamers able to interact with an analyte. Reactions induced by the analyte have also been

**23**

**Author details**

provided the original work is properly cited.

\*Address all correspondence to: pablo.gavina@uv.es

*Red or Blue? Gold Nanoparticles in Colorimetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.80052*

molecular sensing field.

explored, for example based on redox transformations. Taking into account the interesting photophysical properties of the gold nanoparticles, their easy functionalization, the use of aqueous solutions, and the detection by naked eye, we can conclude that the red or blue question will continue to be very present in the

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Instituto Interuniversitario de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Universitat de València—Universitat Politècnica de València, Valencia, Spain

Pablo Gaviña\*, Margarita Parra, Salvador Gil and Ana M. Costero

*Red or Blue? Gold Nanoparticles in Colorimetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.80052*

*Gold Nanoparticles - Reaching New Heights*

Tyr interacts with the ligand NALC through hydrogen bonds that involve carboxylic, amino, and hydroxyl groups. The selectivity response may be attributed to the conformation of L-Tyr that seems to be more appropriate for forming the complex

*(Left) Interaction between tyrosine and N-acetyl-L-cystein capped gold nanoparticles. (Right) Chyral selectivity showed by the sensor. Reprinted with permission from Su et al. [31]. Copyright 2014 Royal Society of* 

The use of gold nanoparticles for the preparation of colorimetric sensors is a very active field. The changes in the color of colloidal gold nanoparticles in solution because of the change in the surface plasmon absorption band upon aggregation or disaggregation processes can be easily used to transform the molecular recognition event into a macroscopic measurable signal. This change from red to blue can be observed by the nacked eye, allowing in this way cheap and easy detection of the target analytes. In this chapter, different mechanisms for the direct or indirect analyte-triggered aggregation of the AuNPs have been considered, including chemical reactions, supramolecular interactions, or changes in the pH of the medium. The selectivity observed in the sensing response in some cases depends on the conformation or configuration of the analyte, but also it can be achieved by using enzymes that catalyze specific reactions or aptamers able to interact with an analyte. Reactions induced by the analyte have also been

**22**

with NALC.

**Figure 16.**

**Figure 15.**

*Detection of ascorbic acid (AA).*

*Chemistry.*

**5. Conclusions**

explored, for example based on redox transformations. Taking into account the interesting photophysical properties of the gold nanoparticles, their easy functionalization, the use of aqueous solutions, and the detection by naked eye, we can conclude that the red or blue question will continue to be very present in the molecular sensing field.
