# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Diffusing Redox Probes
Pd/V\({}_{2}\)O\({}_{5}\) was immobilized on MWCNT-labelled secondary antibodies. A nanocomposite of SnO\({}_{2}\)/RGO was used as high specific surface area substrate for immobilizing AuNPs coupled to capture antibodies. With this architecture, the authors obtained a linear range between 0.5 and 25 ng mL\({}^{-1}\) and a LoD of 0.17 pg mL\({}^{-1}\). Still with a sandwich-type architecture, [PERSON] et al. [67] described a CEA immunosensor based on signal amplification using Fc-functionalized Fe\({}_{3}\)O/SiO\({}_{2}\) as labels and AuNP/GO as substrate. Anti-CEA were immobilized on both the substrate and the signaling NPs, the latter having catalytic properties towards H\({}_{2}\)O\({}_{2}\) oxidation (Figure 12). DPV was used to reoxidize Fc. The linear range was found between 1 pg mL\({}^{-1}\) and 80 ng mL\({}^{-1}\), with a LoD of 0.2 pg mL\({}^{-1}\). Figure 10: **(A)** Covalent attachment of 3-aminopropyltriethoxysilane (APTES) to a hydroxyl-terminated graphene surface; **(B)** Resistance across a 100 μm \(\times\) 4 mm graphene channel as a function of the hCG concentration. Resistance across the channel as a function of the urea and cortisol concentration (inset, top left). I–V curves are plotted (inset, bottom right) for the unmodified device (black curve), amine-terminated (red curve), and Ab-modified (blue curve) graphene surface. Reprinted from [59], Copyright 2014, with permission from Elsevier. **(C)** Sandwich-type electrochemical immunosensor based of Fc-modified Cu\({}_{2}\)ONPs, for PSA detection. **(D)** SWVs for detection of 0.05 pg mL\({}^{-1}\) (a), 0.1 pg mL\({}^{-1}\) (b), 0.5 pg mL\({}^{-1}\) (c), 1 pg mL\({}^{-1}\) (d), 5 pg mL\({}^{-1}\) (e), 10 pg mL\({}^{-1}\) (f), 50 pg mL\({}^{-1}\) (g) and 100 pg mL\({}^{-1}\) (h) PSA. Reprinted from [62], Copyright 2016, with permission from Elsevier. Very recently (2017), [PERSON] et al. [68] also reported a sandwich-type assay for CEA, based on IrNPs acting as electrochemical signal amplifier. Pylodyopamine-reduced graphene oxide (PDA-RGO) was employed to immobilize anti-CEA capture antibodies, and IrNPs for coupling on signaling antibodies. The large surface area of PDA-RGO and the electrocatalytic properties of IrNPs for H\({}_{2}\)O\({}_{2}\) reduction allowed a working potential of \(-0.6\) V (vs. SCE) without the use of HRP. The sensor presented a linear range from \(0.5\) pg mL\({}^{-1}\) to \(5\) ng mL\({}^{-1}\) and a LoD of \(0.2\) pg mL\({}^{-1}\). Finally, still for its catalytic property  
Figure 11: (**A**) RGO and RGO/AgNPs for PSA detection, with Fe(CN)\({}_{6}\)\({}^{3-/4-}\) as diffusing redox probe. Reprinted from [63], Copyright 2017, with permission from Elsevier; (**B**) Cyclodextrin-modified graphene (**a**); functionalization by Pt/PdCuNPs and Ab\({}_{2}\) (**b**); and (**c**) global immunosensing scheme. Reprinted from [65], Copyright 2016, with permission from Elsevier. Figure 12: (**A**) Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)–Fc–Ab\({}_{2}\)/HRP particles for enzyme-less catalytic immunodetection of CEA. (**B**) DPV curves of the above-described sensor for various CEA concentrations in PBS + 4 mM H\({}_{2}\)O\({}_{2}\); (**C**) _Calibration plots._ Adapted from [67], Copyright 2016, with permission from Elsevier. towards H\({}_{2}\)O\({}_{2}\), AgPtNPs-functionalized GR was reported by [PERSON] et al. [69]. The detection range was from 0.5 ng mL\({}^{-1}\) to 140 ng mL\({}^{-1}\) with a LoD of 0.2 ng mL\({}^{-1}\). These works demonstrated that that novel metallic nanoparticles sch ag AgPt, AuPt or Ir NPs could efficiently replace an enzyme such as HRP for the catalytic oxidation of a redox probe through H\({}_{2}\)O\({}_{2}\) reduction, which ensures a more robust sensor (HRP, as any other enzymes, is fragile and works properly only in given conditions such at mild temperature, mild pH, etc.). The other way to get rid of enzymes is to use classical diffusing redox probes but, without amplification procedure, sensitivities are generally lower.