# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Diffusing Redox Probes
The LoD was 5 pg mL\({}^{-1}\) and the response linear from 0.01 to 12 ng mL\({}^{-1}\). Very recently (2017), and for AFP detection, [PERSON] et al. [82] used Cu\({}_{2}\)O-decorated GO as substrate for immobilization of AFP-antibodies. Toluidine Blue (TB) was used as redox probe, adsorbed on the GO. Detection was performed using SWV to monitor the electroactivity of TB upon immunoreaction. The linear range was between 1 fg mL\({}^{-1}\) and 100 ng mL\({}^{-1}\), with a LoD of 0.1 fg mL\({}^{-1}\). Other antigens were also reported. For example, [PERSON] et al. [83] described a sandwich electrochemical immunoassay for avian leukosis virus, using Fe\({}_{3}\)O\({}_{4}\) NPs modified with graphene quantum dots (GRD), apoferritin-encapsulated CuNPs and signaling antibodies (Ab\({}_{2}\)). After a sandwich-type assembly with capture antibodies immobilized on the working electrode, CuNPs were released from the apoferritin cavity under anodic stripping voltammetry (ASV). Detection was achievedbetween 120 and 30,000 TCID\({}_{50}\) mL\({}^{-1}\) (50% Tissue Culture Infective Dose) with a detection limit of 115 TCID\({}_{50}\) mL\({}^{-1}\). The same year, [PERSON] et al. [84] reported a dual signal amplification strategy for tumor cells detection. Graphene was used to increase the electrode area, covalently immobilize capture antibodies and accelerate electron transfer, and ZnSe or CdTe-coated SiO\({}_{2}\)NPs were used to label signaling antibodies. A classical sandwich-type immunoreaction was followed by dissolution of the tracing NPs into HNO\({}_{3}\); this solution was then analyzed by SWV, which gave signals proportional to the quantity of capture tags, therefore proportional to the quantity of detected cell, from 10 to 10\({}^{6}\) cells mL\({}^{-1}\). [PERSON] et al. [85] reported an electrochemical immunoassay for interleukin-6 (IL-6) and matrix metallopeptidase-9 (MMP-9) using polystyrene/polydopamine/silverNPs (PS/PDA/AgNPs) and graphene nanoribbon (GNR), in a sandwich assay format. Capture antibodies were immobilized on graphene and PS@PDA/AgNPs were used to label signaling antibodies. The detection range was reported between 10\({}^{-5}\) and 10\({}^{3}\) ng mL\({}^{-1}\) and the LoD was 5 fg mL\({}^{-1}\) for IL-6 and 0.1 pg mL\({}^{-1}\) for MMP-9. [PERSON] et al. [86] described an immunosensor for simultaneous detection of estradiol and diethylstilbestrol. Amino-functionalized mesoporous Fe\({}_{3}\)O\({}_{4}\) NPs was loaded with Pb\({}^{2+}\) or Cd\({}^{2+}\), and then incubated with anti-estradiol and anti-diethylstilbestrol antibodies, respectively. Estradiol and diethylstilbestrol antigens were adsorbed on graphene sheets deposited on GC electrodes. Using SWASV, two well-separated peaks were generated by the redox reaction of Pb\({}^{2+}\) or Cd\({}^{2+}\) on the electrode, making the simultaneous detection of the two antigens possible (Figure 14A). Peak currents were proportional to the concentrations of estradiol and diethylstilbestrol in the range from 0.050 pg mL\({}^{-1}\) to 100 ng mL\({}^{-1}\) and 1.0 pg mL\({}^{-1}\) to 100 ng mL\({}^{-1}\), respectively. The detection limits were of 0.015 pg mL\({}^{-1}\) and 0.38 pg mL\({}^{-1}\), respectively. Again, one can see form these works that strategies using the electroactivity of metallic NPs generally provide lower detection limits than more classical approaches. [PERSON] et al. [87] reported an amperometric immunosensor based on AuNPs and GO for detection of cardiac troponin-I. GO and AuNPs were anchored on the electrode surface by covalent bonding using aryldiazonium coupling. Antibodies were immobilized on GO along with Fc as signal reporter (Figure 14B). Using SWV, they demonstrated a LoD of 0.05 ng mL\({}^{-1}\). [PERSON] et al. [88] described a  
Figure 13: (**A**) Ab/PtNPs/Thi/Nafion/GR/GC electrode for detection of kanamycin.