# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Diffusing Redox Probes
Figures of merit discussed in this section are summarized in Table 4.  
* Immobilized Redox Probes To avoid some drawbacks generated by the use of diffusing redox probes, Among those who used immobilized redox probes, [PERSON] et al. [70] described in 2012 an immunosensor for PSA using GR/MB/Chi as electrode material, onto which capture antibodies were immobilized. GR provided high specific area for increasing both the redox probe and the capture probe surface concentration. Using current changes of the CV peaks, induced by specific Ag-Ab interactions, the signal decreased linearly with PSA concentration (0.05-5.00 ng mL\({}^{-1}\)), with a LoD of 13 pg mL\({}^{-1}\). [PERSON] et al. [71] described a similar idea where the redox probe (here Thi) was immobilized on graphene/PtNPs, for detection of kanamycin, an antibiotic used to treat severe bacterial infections. The anti-kanamycin Ab was immobilized onto the modified electrode through electrostatic adsorption (Figure 13A). The authors claimed that Thi electroactivity was improved not only by graphene but also by PtNPs (due to an increase in surface area and conductivity). They reported a LoD of 6 pg mL\({}^{-1}\) and a linear range from 0.01 to 12 ng mL\({}^{-1}\). In 2013, [PERSON] et al. [72] also described a kanamycin sensor with capture antibodies labelled with Ag/Fe\({}_{3}\)O\({}_{4}\)NPs and immobilized on a thionine/graphene-modified electrode. CV and SWV were used to characterize the recognition of kanamycin through change in Thi electroreduction currents, which decreased upon increase of kanamycin concentration. The LoD was 15 pg mL\({}^{-1}\), and the linear range was from 0.050 and 16 ng mL\({}^{-1}\). [PERSON] et al. [73] described in 2012 a nanotubular mesoporous PdCu alloy for a label-free electrochemical immunosensor for CEA. It operated through physisorption of anti-CEA on NM-PdCu sulfonated graphene sheets modified with Thi as immobilized redox probe. From CV peak currents, a linear response was observed between 0.01 and 12 ng mL\({}^{-1}\) with a LoD of 5 pg mL\({}^{-1}\). In 2014, [PERSON] et al. [74] also reported an \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Target** & **Probe** & **LoD** & **Method** & **Reference** \\ \hline \(\beta\)-Lactoglobulin & Ab & 0.85 pg mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/DPV & [52] \\ AFP protein & Ab & 5 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/AuPdNPs/CV & [53] \\ AFP protein & Ab & 0.03 ng mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/TiO\({}_{2}\)/GR/Chi/AuNPs/DPV & [54] \\ AFP protein & Ab & 0.06 ng mL\({}^{-1}\) & Fe/DPV & [55] \\ AFP protein & Ab & 0.01 ng mL\({}^{-1}\) & Gr/SnO\({}_{2}\)/Au Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/DPV & [56] \\ AFP protein & Ab & 12 mL mL\({}^{-1}\) & Fe(CN)\({}_{3}\)\({}^{3-/4-}\)/DPV & [57] \\ CA 15-3 & Ab & 0.033 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/CeO\({}_{2}\)NPs & [58] \\ hCG & Ab & 0.62 ng mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [59] \\ PSA protein & Ab & 0.6 ng mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [60] \\ PSA protein & Ab & 1 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/ZnO/AgNPs/CV & [61] \\ PSA protein & Ab & 0.05 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/Fc/DPV & [62] \\ PSA protein & Ab & 0.01 ng mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/AgNPs/CV & [63] \\ _E. Coli_ & Ab & 3.8 CFU mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CuONPs/EIS & [64] \\ SCCA protein & Ab & 25 fg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/PtPdCuNPs & [65] \\ CEA protein & Ab & 0.17 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/PtV\({}_{2}\)C\({}_{5}\) & [66] \\ CEA protein & Ab & 0.2 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/Fe\({}_{3}\)ONPs & [67] \\ CEA protein & Ab & 0.2 pg mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/InNPs & [68] \\ CRP protein & Ab & 0.17 ng mL\({}^{-1}\) & H\({}_{2}\)O\({}_{2}\)/AgPtNPs & [69] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Figures of merit of selected enzyme-less immunosensors using graphene and diffusing redox probe.