**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 for these compounds, functionalized gold nanoparticles have shown interesting applications [25].

The sensing protocol, in this case, is related to the well-known interaction between the carboxylate and thioureas groups. For this reason, AuNPs containing thiourea groups were prepared, and their behavior in front of different dicarboxylates was evaluated. Among the different dicarboxylates studied (oxalate (**C2**), malonate (**C3**), succinate (**C4**), glutarate (**C5**), and adipate (**C6**); all of them as TBA salts) only with succinate, a color change of the solution from red to blue was observed (**Figure 12**, right, bottom). The limit of detection determined for this dianion was 0.5 μM.

Following a similar approach, maleate and fumarate were distinguished [26, 27]. Compound **3** was used to cap gold nanoparticles (**Figure 13**). The prepared AuNPs were able to recognize trans-dicarboxylates such as fumarate, one of the key components generated in the Krebs cycle, over its cis-isomer, maleate. The trans-isomer, fumarate, seems to have the appropriate geometry to induce the nanoparticle aggregation, whereas the cis compound presents a similar behavior to that of flexible dicarboxylates like oxalate, malonate, succinate, glutarate, propionate, and 4-pentenoic acid.

Pyruvic acid (2-oxopropanoic acid, PA) is the simplest alpha-keto acid. It plays important roles in several biochemical pathways. For example, it supplies energy to cells through the citric acid cycle when oxygen is present and under hypoxic conditions produces lactate. It also appears as an intermediate in several metabolic processes such as the glycolysis of glucose or the synthesis of carbohydrates or fatty acids. Gold nanoparticles have proved to be useful for detecting this acid [28]. The approach, in this case, is also based on the use of unmodified AuNPs and Cytidine-rich oligonucleotides (C-rich DNA). C-rich DNA can fold into one closely packed four-stranded structure called i-motif [29] through protonated cytosine-cytosine (C-C+) base-pair formation under slightly acidic conditions. The principle of the designed PA sensor is similar to that previously indicated in **Figure 11**. The rigid i-motif structure

**Figure 12.** *Detection of succinate.*

#### **Figure 13.**

*(Left) Ligand used to cap gold nanoparticles for detection of fumarate. (Right) Color changes of an aqueous solution of AuNPs capped with* **3** *upon addition of various analytes (as sodium salt). From the left: fumarate, maleate, oxalate, malonate, succinate, glutarate, propionate, and 4-pentenoic acid.*

**21**

**Figure 14.**

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

determined was 3.0 mM.

2–, or Cr2O7

studies were carried out with PO4

and separation of enantiomers.

detection determined for this method was 0.15 μM.

HCrO4 – , CrO4

NO3 –

is unable to stabilize gold nanoparticles, but when a change of pH is induced in the medium from acid to neutral, the C-rich DNA changes its structure and prevents gold nanoparticles aggregation. The change in pH is induced by the addition of PA and pyruvate decarboxylase (PDC). The enzyme transforms PA into acetaldehyde and CO with the corresponding change of pH from acid to neutral. At neutral pH, the C-rich DNA presents its extended single-stranded structure and effectively binds to AuNPs, stabilizing them against NaCl-induced aggregation. Based on this principle, PA can be

UV-Vis spectra were recorded to demonstrate the proposed sensing mechanism. Solutions of AuNPs (3 nM) containing C-rich DNA showed a maximum absorption peak centered at 520 nm. After addition of PA, the changes induced in the absorption band depend on the acid concentration. At 5.6 mM, a hypochromic effect was observed, but at 16.8 mM, a bathochromic shift can be observed from 520 to over 600 nm with the corresponding change of color. After addition of PCD, the disaggregation is produced and the solution recovers its red color. Selectivity against lactic acid, ascorbic acid, and glucose was established, and the limit of detection

Ascorbic acid (AA, also known as vitamin C), an antioxidant compound, is present not only in biological fluids but also in foodstuffs and pharmaceuticals. Taking into account the red-ox properties of ascorbic acid, an approach using gold nanoparticles and Cr(VI) has been described for its detection [30]. Gold nanoparticles were stabilized with sodium tripolyphosphate (Na5P3O10), and the sensor was prepared using this AuNPs and Cr (VI) salts. As Cr(VI) exists in the form of

that precludes the nanoparticles aggregation. By contrast, Cr(III), a hard lewis acid strongly coordinates to the polyphosphate ligand, giving rise to the charges compensation and in consequence the nanoparticles aggregation (**Figure 15**). Ascorbic acid is able to trigger the process by reducing Cr(VI) to Cr(III). Selectivity

3–, Zn2+, Cu2+, SO4

Finally, also amino acids have been detected using functionalized gold nanopaticles. Thus, tyrosine (Tyr) was detected using N-acetyl-L-cysteine modified gold nanoparticles [31]. In this case, the chiral ligand N-acetyl-L-cysteine (NALC) was chosen to include chirality in the sensors and study its use in selective recognition

The chiral selectivity is attributed to a chemical interaction between chiral

*(Left) Mechanism for pyruvic acid detection. (Right) UV-Vis spectra of the AuNP suspensions (3 nM) containing C-rich DNA after incubation with (b) 5.6 mM PA, (c) 16.8 mM PA, (d) 5.6 mM PA + PDC, (e) 16.8 mM PA + PDC, and (f) PDC only; curve (a) is the background signal (reproduced with license number Reprinted with permission from Li et al. [28]. Copyright 2014 Royal Society of Chemistry).*

NALC–Au NPs and L-Tyr at the molecular level as is shown in **Figure 16**.

, glucose (Glu), citric acid (CA), and oxalic acid (OA) as interferences, and no appreciable changes were observed with any of the studied compounds. The limit of

2–, there is an electrical repulsion with tripolyphosphate

<sup>2</sup>−, Ni2+, Li+

, Na+

, Mg2+, Br–

,

selectively detected by the color change of the AuNPs (**Figure 14**).

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

*Gold Nanoparticles - Reaching New Heights*

applications [25].

dianion was 0.5 μM.

for these compounds, functionalized gold nanoparticles have shown interesting

The sensing protocol, in this case, is related to the well-known interaction between the carboxylate and thioureas groups. For this reason, AuNPs containing thiourea groups were prepared, and their behavior in front of different dicarboxylates was evaluated. Among the different dicarboxylates studied (oxalate (**C2**), malonate (**C3**), succinate (**C4**), glutarate (**C5**), and adipate (**C6**); all of them as TBA salts) only with succinate, a color change of the solution from red to blue was observed (**Figure 12**, right, bottom). The limit of detection determined for this

Following a similar approach, maleate and fumarate were distinguished [26, 27]. Compound **3** was used to cap gold nanoparticles (**Figure 13**). The prepared AuNPs were able to recognize trans-dicarboxylates such as fumarate, one of the key components generated in the Krebs cycle, over its cis-isomer, maleate. The trans-isomer, fumarate, seems to have the appropriate geometry to induce the nanoparticle aggregation, whereas the cis compound presents a similar behavior to that of flexible dicarboxylates like oxalate, malonate, succinate, glutarate, propionate, and 4-pentenoic acid. Pyruvic acid (2-oxopropanoic acid, PA) is the simplest alpha-keto acid. It plays important roles in several biochemical pathways. For example, it supplies energy to cells through the citric acid cycle when oxygen is present and under hypoxic conditions produces lactate. It also appears as an intermediate in several metabolic processes such as the glycolysis of glucose or the synthesis of carbohydrates or fatty acids. Gold nanoparticles have proved to be useful for detecting this acid [28]. The approach, in this case, is also based on the use of unmodified AuNPs and Cytidine-rich oligonucleotides (C-rich DNA). C-rich DNA can fold into one closely packed four-stranded structure called i-motif [29] through protonated cytosine-cytosine (C-C+) base-pair formation under slightly acidic conditions. The principle of the designed PA sensor is similar to that previously indicated in **Figure 11**. The rigid i-motif structure

*(Left) Ligand used to cap gold nanoparticles for detection of fumarate. (Right) Color changes of an aqueous solution of AuNPs capped with* **3** *upon addition of various analytes (as sodium salt). From the left: fumarate,* 

*maleate, oxalate, malonate, succinate, glutarate, propionate, and 4-pentenoic acid.*

**20**

**Figure 12.**

**Figure 13.**

*Detection of succinate.*

is unable to stabilize gold nanoparticles, but when a change of pH is induced in the medium from acid to neutral, the C-rich DNA changes its structure and prevents gold nanoparticles aggregation. The change in pH is induced by the addition of PA and pyruvate decarboxylase (PDC). The enzyme transforms PA into acetaldehyde and CO with the corresponding change of pH from acid to neutral. At neutral pH, the C-rich DNA presents its extended single-stranded structure and effectively binds to AuNPs, stabilizing them against NaCl-induced aggregation. Based on this principle, PA can be selectively detected by the color change of the AuNPs (**Figure 14**).

UV-Vis spectra were recorded to demonstrate the proposed sensing mechanism. Solutions of AuNPs (3 nM) containing C-rich DNA showed a maximum absorption peak centered at 520 nm. After addition of PA, the changes induced in the absorption band depend on the acid concentration. At 5.6 mM, a hypochromic effect was observed, but at 16.8 mM, a bathochromic shift can be observed from 520 to over 600 nm with the corresponding change of color. After addition of PCD, the disaggregation is produced and the solution recovers its red color. Selectivity against lactic acid, ascorbic acid, and glucose was established, and the limit of detection determined was 3.0 mM.

Ascorbic acid (AA, also known as vitamin C), an antioxidant compound, is present not only in biological fluids but also in foodstuffs and pharmaceuticals. Taking into account the red-ox properties of ascorbic acid, an approach using gold nanoparticles and Cr(VI) has been described for its detection [30]. Gold nanoparticles were stabilized with sodium tripolyphosphate (Na5P3O10), and the sensor was prepared using this AuNPs and Cr (VI) salts. As Cr(VI) exists in the form of HCrO4 – , CrO4 2–, or Cr2O7 2–, there is an electrical repulsion with tripolyphosphate that precludes the nanoparticles aggregation. By contrast, Cr(III), a hard lewis acid strongly coordinates to the polyphosphate ligand, giving rise to the charges compensation and in consequence the nanoparticles aggregation (**Figure 15**). Ascorbic acid is able to trigger the process by reducing Cr(VI) to Cr(III). Selectivity studies were carried out with PO4 3–, Zn2+, Cu2+, SO4 <sup>2</sup>−, Ni2+, Li+ , Na+ , Mg2+, Br– , NO3 – , glucose (Glu), citric acid (CA), and oxalic acid (OA) as interferences, and no appreciable changes were observed with any of the studied compounds. The limit of detection determined for this method was 0.15 μM.

Finally, also amino acids have been detected using functionalized gold nanopaticles. Thus, tyrosine (Tyr) was detected using N-acetyl-L-cysteine modified gold nanoparticles [31]. In this case, the chiral ligand N-acetyl-L-cysteine (NALC) was chosen to include chirality in the sensors and study its use in selective recognition and separation of enantiomers.

The chiral selectivity is attributed to a chemical interaction between chiral NALC–Au NPs and L-Tyr at the molecular level as is shown in **Figure 16**.

#### **Figure 14.**

*(Left) Mechanism for pyruvic acid detection. (Right) UV-Vis spectra of the AuNP suspensions (3 nM) containing C-rich DNA after incubation with (b) 5.6 mM PA, (c) 16.8 mM PA, (d) 5.6 mM PA + PDC, (e) 16.8 mM PA + PDC, and (f) PDC only; curve (a) is the background signal (reproduced with license number Reprinted with permission from Li et al. [28]. Copyright 2014 Royal Society of Chemistry).*

#### **Figure 16.**

*(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 Chemistry.*

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 with NALC.
