**3. Biosensing for environmental monitoring**

The increased industrial and agricultural activities have increased the levels of chemical and biological substances being release into the environment. Thus, environmental monitoring is mandatory. This drives the need for real-life detection with rapid measurements that allows environmental monitoring in various real-life situations. Plasmonic biosensors have shown the potential to detect pollutants directly and reliably in the environment. The most common analytes detected by biosensors for environmental monitoring include heavy metals in water, pesticides, and potentially toxic and dangerous chemicals such as explosives [19, 23, 47–49]. Conventional sensing substrates have been improved with plasmonic materials for enhanced performance. The design of specific plasmonic structures made it possible for easy binding of analytes, bringing the pollutants close to the surface of the plasmonic sites [47].

### **3.1 Heavy metals**

Heavy metals such as Cu, Hg, Pb, and Cd are among the harmful inorganic pollutants released into the environment from industrial and agricultural activities [50–52]. They do not decompose naturally and thus become persistent in water streams and agricultural land. Since their presence is regulated, their detection is mandatory to prevent diseases, protect the environment and strategize viable treatment plans to meet regulation requirements [53].

One of the most common plasmonic biosensors are colorimetric sensors. They offer convenient, rapid in field pollutant response that can be observed with the naked eye. This type of sensing is possible because the local surface Plasmon resonance peaks of the biosensors fall within the visible spectrum and the aggregation that occurs in the presence of the pollutant/analytes cause changes in color [47]. For instance, detection of Hg2+ ions were demonstrated using mercaptopropionic acid (MPA) capped Au NPs, where Hg ions complexed with carboxylic acid groups of MPA revealing a color change from red to colorless. The selectivity of Hg2+ ions among other metals was improved in the presence of 2.6-pyridinedicarboxylic acid (PDCA) due to increased complexation coefficient. As a result, a quantitative detection range of 250–500 nM with a limit of detection of 100 nM was established [54].

In another development, a rapid color change from blue to red was observed in the detection of Pb2+ions with DNA zyme assembly of Au NPs (**Figure 4**) [55]. The DNA zyme assembly forms a hybrid with Au NPs, which resulted in aggregation. Furthermore, the presence of lead catalyzes the hydrolytic cleavage, which dissembles the hybrid into dispersed Au NPs bringing the color, back to red (**Figure 4**) [55].

### **3.2 Pesticide and explosives**

The release of organic pollutants such as pesticides (used in agriculture) and polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, phenol, dioxins (by-products of combustion, incineration, and chemical manufacturing processes)

*Application of Plasmonic Nanostructures in Molecular Diagnostics and Biosensor Technology… DOI: http://dx.doi.org/10.5772/intechopen.108319*

#### **Figure 4.**

*DNA zyme-directed assembly formation and cleavage of Au NPs during Pb2+ ion colorimetric sensing. Reprinted with permission from Reference [55].*

into the environment presents a concern for environmental protection. For example, pesticides and PAHs can accumulate in soil and water and they pose endocrinedisrupting activity, which can be a threat to human health and local ecosystems [56, 57]. Many SPR-based immunosensors have shown potential in the detection of these environmental pollutants such as atrazine, Dichloro-Diphenyl Trichloroethane (DDT), 2,3,7,8-tetrachlorodibenzo-p-dioxin, carbaryl, 2,4-D, benzo[a]pyrene (BaP), biphenyl derivatives, and trinitrotoluene (TNT) [22, 58, 59]. Recently, signal amplification for the detection of TNT was demonstrated using LSPR-based AuNPs. TNT detection occurred through the formation of a Meisenheimer complex with

#### **Figure 5.**

*The formation of Meisenheimer complex between L-cysteine capped Au NPs and TNT. Reprinted with permission from Reference [60].*

L-Cysteine capped AuNPs (**Figure 5**). The electrostatic attraction between the TNT and L-cysteine capped Au NPs resulted in aggregation of the NPs. The material could detect TNT in shampoo solution demonstrating good selectivity [61]. SERS substrates based on AuNRs were applied for the detection of low levels of three dithiocarbamate fungicides. Their interaction between the pesticides and the Au-NRs resulted in the formation of Au-S covalent bonds on the surface between the pesticides and the Au-NRs. Detection limits of 34, 26, and 13 nM were discovered for thiram, firbam, zeram, respectively [60].
