# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Diffusing Redox Probes
immunoassay for CEA and squamous cell carcinoma antigen (SCCA) for the diagnosis of cervical cancer. Tetreathylene pentaamine-modified RGO was used to immobilize the primary antibody, and Au@mesoporous carbon CMK-3 were used to couple secondary antibodies (Ab\({}_{2}\)) with Neutral Red or Thi as redox probes. A conventional sandwich assay followed by DPV measurements showed that the immunosensor presented a linear range up to 20 ng mL\({}^{-1}\) and a LoD of 10 pg mL\({}^{-1}\). [PERSON] et al. [75] also reported a CEA sensor, but based on graphene decorated with AuNPs and Thi as redox probe; anti-CEA were immobilized on the AuNPs. The immunosensor showed a very low LoD of 0.05 fg mL\({}^{-1}\) and a linear response from 0.1 fg mL\({}^{-1}\) to 1 ug mL\({}^{-1}\). [PERSON] et al. [76] described a AuNP-functionalized mesoporous carbon foam (MCF) coupled with carbon-Au synergetic silver enhancement for immunosensing of CEA. Antibodies were grafted on RGO/Chi-modified electrodes. Through a sandwich-type immunoreaction, Au/MCF tags were captured on the immunoconjugate to induce a silver deposition process. Using anodic stripping voltammetry (ASV) for silver redissolution, the authors found a linear range from 0.05 pg mL\({}^{-1}\) to 1 ng mL\({}^{-1}\) and a LoD of 0.024 pg mL\({}^{-1}\). This technique of deposition/redissolution of metallic NPs is particularly efficient because it provides a very large amplification compared to other approaches. The other approaches which were published later did not feature such strategy and showed less satisfactory limits of detection. [PERSON] et al. [77] also reported CEA detection, but with a sandwich-format, using graphene sheets modified by toluidine blue for labeling anti-CEA, and Prussian Blue (PB) for labeling anti-AFP. The capture antibodies were immobilized onto chitosan-AuNPs immobilized on the electrode (Figure 13B). Using the electroactivity of the two different immobilized redox probes, a linear range of 0.5-60 ng mL\({}^{-1}\) was obtained for both targets, and LoDs were of 0.1 ng mL\({}^{-1}\) for CEA and 0.05 ng mL\({}^{-1}\) for AFP. [PERSON] et al. [78] described a CEA immunosensor using AuNPs-decorated mesoporous silica KIT-6. AuNPs were used to immobilize both the secondary antibodies and toluidine blue. For the immobilization of primary antibodies (Ab\({}_{1}\)), APTES-functionalized graphene sheets decorated with AuNPs were used as substrate on glassy carbon electrodes (GCE). Using DPV to sense toluidine blue electroactivity, this immunosensor exhibited a LoD of 3 fg mL\({}^{-1}\) and a linear range from 10\({}^{-5}\) ng mL\({}^{-1}\) to 10\({}^{2}\)ng mL\({}^{-1}\). More recently, in 2016, [PERSON] et al. [79] reported an electrochemical immunosensor for CEA, based on GR/Chi/Fc used to immobilize the primary antibodies. Fe\({}_{3}\)O\({}_{4}\)/AuNPs were also functionalized with antibodies, for signaling through a sandwich pathway. \(p\)-Aminophenol was used as redox probe, and Fc as catalyst of p-aminophened electrooxidation. This immunosensor offered a linear signal between 1 pg mL\({}^{-1}\) and 30 ng mL\({}^{-1}\), with a LoD of 0.4 pg mL\({}^{-1}\). Last example for CEA detection, [PERSON] et al. [80] described a Nile blue-modified GO which was electrochemically reduce in the presence of AuCl\({}_{4}\)\({}^{-}\), to led to a AuNPs/NB/RGO-modified electrode. CEA antibodies were immobilized on this nanocomposite. Upon formation of the Ab/Ag immunocomplex, Nile Blue electroactivity decreased proportionally to the CEA concentration. A linear response was obtained between 1 pg mL\({}^{-1}\) and 40 ng mL\({}^{-1}\), with a LoD of 0.5 pg mL\({}^{-1}\). No more for CEA but for AFP detection, [PERSON] et al. [81] reported a PdNPs-functionalized RGO used as substrate for anti-AFP immobilization. H\({}_{2}\)O\({}_{2}\) was used as redox probe added in solution. Upon formation of the Ab/Ag immunocomplex, the amperometric current of H\({}_{2}\)O\({}_{2}\) reduction was proportional to the concentrations of AFP.