# Role of Forster Resonance Energy Transfer in Graphene-Based Nanomaterials for Sensing
## 3 Application of Graphene-Based Nanomaterials in Sensing
### Metal Ion Sensor  
Heavy metal ion contamination in environmental and biological systems causes adverse effects to mankind, even at low concentrations, including cardiovascular, mental defects, neurological, and renal disorders [53]. Keeping this in mind, [PERSON] et al. [54] developed a paper-based aptasensor **G1** combining a graphene oxide (GO) quencher via \(\pi\)-\(\pi\) stacking, which quenched the aptamer fluorescence through the FRET process (Figure 3). The synthesized aptasensor **G1** could selectively detect Pb\({}^{2+}\) ions without interference from other metal ions. Briefly, the addition of the Pb\({}^{2+}\) ion into the aptasensor enhanced the fluorescence through the conformational switching of the aptasensor from a random coil to a G-quadruplex structure. The Pb\({}^{2+}\) ion sensing mechanism of **G1** was further applied in various real samples, such as tap water, lake water, milk, and human blood serum, and the ultra-low limit of detection (LOD) was calculated to be 0.5 pM. Figure 2: Schematic representation of the FRET process in the donor and acceptor containing system. **Scheme 1. ** Graphical representation of the graphene-based nanomaterial design strategy and its sensing of various target analytes. **Figure 3. Schematic illustration of the preparation of **G1** and its Pb\({}^{2+}\) ion sensing. Reprinted from [54] with permission from Elsevier. Similarly, the same research group [55] reported a low-cost paper-based aptasensor **G2** for the detection of Hg\({}^{2+}\) and Ag\({}^{+}\) ions (Figure 4). In detail, the **G2** zones were utilized for dual metal ion detection using a T12 aptamer for the Hg\({}^{2+}\) zone and a C12 aptamer for the Ag\({}^{+}\) zone. For each zone, Hg\({}^{2+}\)/Ag\({}^{+}\) ions, separately, were injected, while the center part was left out for the sample zone. The detection zone was injected with GO, and the other two zones were already injected with metal ions. After the incubation of solutions to particular zones, the detection zone was monitored using a microscope and the images were captured through the digital camera. The basic mechanism involves the conformational change of a specific aptamer with Hg\({}^{2+}\)/Ag\({}^{+}\), followed by aptamer release from the GO surface through the addition of target metal ions on paper. Finally, the **G2** metal ion sensing approach was applied in milk, water, and human serum samples (LOD: 1.33 (Hg\({}^{2+}\)) and 1.01 pM (Ag\({}^{+}\))). [PERSON] and co-workers [56] prepared a GQD/nitrogen-doped GQD (NGQD) modified with a DNA **G3** probe for the detection of Ag\({}^{+}\) ions (Figure 5). Here, GQDs acted as the fluorophore and Ag\({}^{+}\) acted as the quencher. Upon the addition of Ag\({}^{+}\) ions into **G3**, the fluorescence intensity was heavily quenched by increasing the concentrations of Ag\({}^{+}\) ions and inhibited the FRET process. The LOD of **G3** was decreased to 0.05 to 0.02 nM, which was lower than the concentration allowed by the U.S. Environmental Protection Agency (US-EPA). In 2019, [PERSON] et al. [57] synthesized a fluorescent nanosensor (HRGO-AuNCs) **G4** for the detection of Cd\({}^{2+}\) based on glutathione-gold nanoclusters (GSH-AuNCs) and a holey reduced graphene oxide (HRGO) quencher. First, HRGO was prepared through the hydrothermal method in the presence of GO, an etchant (H\({}_{2}\)O\({}_{2}\)), and a reducing agent (ascorbic acid (AA)) (Figure 6). The FRET process between GSH-AuNCs and HRGO was confirmed through lifetime analysis. After the addition of Cd\({}^{2+}\) with **G4**, the fluorescence  
Figure 4: Schematic diagrams of the preparation of **G2** and its dual Hg\({}^{2+}\) and Ag\({}^{+}\) sensing approaches. Reprinted from [55] with permission from Elsevier. intensity was recovered through in situ Cd-GSH complex formation at the AuNC surface, and blocked the FRET process. Furthermore, **G4** was applied to real water analysis for the detection of Cd\({}^{2+}\) ions (LOD = 42 nM).