**3. Detection of nerve agents and pesticides**

The current rise in international concern over the use of chemical warfare (CW) agents in different conflictive sceneries has resulted in an increasing interest in the detection of these lethal chemicals. Among CW species, nerve agents are greatly dangerous and their high toxicity and easy production underscore the need to detect these deadly chemicals via quick and reliable procedures. AuNPs have been used as sensors for some nerve agent simulants with good results. Different sensing mechanisms have been used for detecting these compounds, for example, compensation of charges has been applied for this process [20]. The designed sensing protocol takes advantage of the nucleophilic reactivity of pyridine moieties toward nerve gases [21]. This reactivity generates positive charges on the surface of the gold nanoparticles, diminishing their colloidal stability and inducing aggregation (**Figure 6**).

Several pyridine derivatives were studied, and their ability to act as probes for DCNP detection was evaluated by UV-vis spectroscopy. After addition of increasing amounts of DCNP, the intensity of the surface plasmon peak of the monodispersed AuNPs at 526 nm decreased and a new peak at c.a. 660 nm appeared as the AuNP clusters were formed. The best results were obtained with compound **1** (**Figure 6c**) that showed a limit of detection of 76 ppm under the experimental conditions used.

Following the same approach, compound **2** (**Figure 7**) was bound to gold nanoparticles. In this case, the positive charge appears as a consequence of the reaction described in **Figure 7**. The limit of detection determined in this case was 81 ppm.

Also, triarylcarbinols have been used as recognition motifs. These compounds can be converted into their corresponding carbocations in the presence of nerve agent simulants through phosphorylation of the OH group followed by SN1 elimination. Consequently, AuNPs functionalized with this type of compounds have been used in detecting simulants of these dangerous compounds (**Figure 8**, X = F, CN) [22].

An approach based on enzymatic reactions has also been described for detecting nerve agents GB, GD, and VX and the highly toxic pesticide paraoxon (**Figure 9**). The prepared sensor showed high sensitivity [23].

**Figure 6.**

*(a) Nerve agent Tabun and its mimic DCNP (b) Proposed mechanism for the colorimetric detection of DCNP. (c) Pyridine derivative used as recognition unit.*

**Figure 7.** *Mechanism of positive charge generation upon reaction of DCNP with AuNPs functionalized with compound 2.*

#### **Figure 8.**

*(Left) AuNPs functionalized with triarylcabinols for detecting nerve agent simulants. (Right) Color change observed on the solution in DMF upon addition of the simulants. UV-vis spectra of the triarylcarbinol functionalized AuNPs on addition of increasing amounts of DCNP expressed mg/m3 . Insets: Plots A640/A526 vs. DCNP and DFP concentration, respectively.*

#### **Figure 9.**

*Structures of nerve agents GB, GD, and VX, and pesticide paraoxon.*

In this method, thiocholine (TCh), which is generated from S-acetylthiocholine (ATCh) through acetylcholinesterase (AChE) enzymatic hydrolysis, induces aggregation of AuNPs stabilized with lipoic acid. When the analytes are present in the solution, the production of TCh is suppressed, and a disaggregation of the particles is observed with a change of color from blue to red (**Figure 10**).

**19**

**Figure 10.**

**Figure 11.**

*Sensing mechanism using aptamers.*

*Sensing mechanism based on AChE inhibition.*

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

Neither AChE nor ATCh can induce aggregation of the AuNPs stabilized with lipoic acid, whereas TCh is capable of producing this process. The analytes inactivate AChE via nonreversible phosphorylation even with very low concentrations. In consequence, ATCh cannot be hydrolyzed which inhibits the aggregation. Limits of detection are in the pM range and the system can be used in complex matrices such as in apple juice. Other dangerous pesticides have been detected using AuNP-based colorimetric aptasensors [24]. The sensing mechanism is summarized in **Figure 11**. Aptamers (organophosphorous pesticide aptamers in this case) are able to be adsorbed on the surface of gold nanoparticles through coordination between Au and N atoms in DNA bases. When salt is added, this adsorption gives rise to stable dispersions of gold nanoparticles with the characteristic red color. However, in the presence of the analytes, the aptamers desorb from the surface of the AuNPs, giving rise to the aggregation process with the concomitant change of color. Because double-stranded DNA is not adsorbed by gold, single-stranded DNA was used in these experiments. Although the sensitivity of these assays is not high enough, this design shows a new

approach for preparing sensors based on gold nanoparticles.

**4. Detection of biologically important carboxylic acids**

Detection of dicarboxylates is very important, since some of them have critical roles in the most important metabolic cycles of living organisms. Specifically, succinate plays a fundamental role in processes, such as the Krebs cycle and oxidative phosphorylation. It is an inhibitor of mitochondrial lipid peroxidation, preventing or delaying most of the damage caused by the peroxidation on different mitochondrial structures and functions. For all these reasons, it is of interest to detect this compound. Among the different strategies used in designing sensors *Red or Blue? Gold Nanoparticles in Colorimetric Sensing DOI: http://dx.doi.org/10.5772/intechopen.80052*

*Gold Nanoparticles - Reaching New Heights*

*(c) Pyridine derivative used as recognition unit.*

**18**

**Figure 8.**

**Figure 6.**

**Figure 7.**

**Figure 9.**

*DCNP and DFP concentration, respectively.*

In this method, thiocholine (TCh), which is generated from S-acetylthiocholine (ATCh) through acetylcholinesterase (AChE) enzymatic hydrolysis, induces aggregation of AuNPs stabilized with lipoic acid. When the analytes are present in the solution, the production of TCh is suppressed, and a disaggregation of the particles

*(Left) AuNPs functionalized with triarylcabinols for detecting nerve agent simulants. (Right) Color change observed on the solution in DMF upon addition of the simulants. UV-vis spectra of the triarylcarbinol* 

*(a) Nerve agent Tabun and its mimic DCNP (b) Proposed mechanism for the colorimetric detection of DCNP.* 

*Mechanism of positive charge generation upon reaction of DCNP with AuNPs functionalized with compound 2.*

*. Insets: Plots A640/A526 vs.* 

is observed with a change of color from blue to red (**Figure 10**).

*Structures of nerve agents GB, GD, and VX, and pesticide paraoxon.*

*functionalized AuNPs on addition of increasing amounts of DCNP expressed mg/m3*

**Figure 10.** *Sensing mechanism based on AChE inhibition.*

**Figure 11.** *Sensing mechanism using aptamers.*

Neither AChE nor ATCh can induce aggregation of the AuNPs stabilized with lipoic acid, whereas TCh is capable of producing this process. The analytes inactivate AChE via nonreversible phosphorylation even with very low concentrations. In consequence, ATCh cannot be hydrolyzed which inhibits the aggregation. Limits of detection are in the pM range and the system can be used in complex matrices such as in apple juice.

Other dangerous pesticides have been detected using AuNP-based colorimetric aptasensors [24]. The sensing mechanism is summarized in **Figure 11**. Aptamers (organophosphorous pesticide aptamers in this case) are able to be adsorbed on the surface of gold nanoparticles through coordination between Au and N atoms in DNA bases. When salt is added, this adsorption gives rise to stable dispersions of gold nanoparticles with the characteristic red color. However, in the presence of the analytes, the aptamers desorb from the surface of the AuNPs, giving rise to the aggregation process with the concomitant change of color. Because double-stranded DNA is not adsorbed by gold, single-stranded DNA was used in these experiments. Although the sensitivity of these assays is not high enough, this design shows a new approach for preparing sensors based on gold nanoparticles.
