# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Graphene and Graphene Derivatives
#### 3.3.1 Enzyme-Free Immunosensors  
As for classical substrates, most of the graphene-based immunosensors concern detection of antigens; however, some immunosensors were described for antibody detection. All of them proposed transduction architecture strictly similar to those used to detect antigens. In 2012, [PERSON] et al. [49] reported a conventional label-free electrochemical impedimetric immunosensor for detection of IgG antibodies as model, based on chemically modified graphene serving as electrode substrate. Anti-IgG were immobilized on the electrodes and EIS used with FeCN\({}_{6}\)\({}^{3-/4-}\) as diffusing redox molecule for probing Ab/Ag interaction on the electrode surface. The linear range of detection was from 0.3 \(\upmu\)g mL\({}^{-1}\) to 7 \(\upmu\)g mL\({}^{-1}\) (LoD of 0.1 \(\upmu\)g mL\({}^{-1}\)). In 2013, [PERSON] et al. [50] presented a sandwich electrochemical immunosensing strategy with AuNP-functionalized graphene, also used as immobilization substrate for probe antibodies, and 1,1\({}^{\prime}\)-ferrocenedicarboxylic acid as label on the signaling antibodies. Human IgG was detected using DPV with a LoD of 0.4 ng mL\({}^{-1}\) and a linear  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Target** & **Probe** & **LoD** & **Method** & **Reference** \\ \hline _E. coli_ & Ab & \(5\times 10^{3}\) CFU mL\({}^{-1}\) & HRP/CV & [32] \\ Cardiac troponin T & Ab & 0.033 ng mL\({}^{-1}\) & HRP/Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [33] \\ Aflatoxin B\({}_{1}\) & Ab & 3.5 pg mL\({}^{-1}\) & AlkP/a-NP/DPV & [34] \\ TGF protein & Ab & 1.3 pg mL\({}^{-1}\) & HRP/HQ/Amperometry & [35] \\ PSA protein & Ab & 20 pg mL\({}^{-1}\) & HRP/TH/DPV & [36] \\ \(\alpha\)-Fetoprotein & Ab & 0.011 ng mL\({}^{-1}\) & HRP/PB/DPV & [37] \\ Procalcitonin & Ab & 1.23 pg mL\({}^{-1}\) & ChOx/H\({}_{2}\)O\({}_{2}\)-CoPC/DPV & [38] \\ Aflatoxin B\({}_{1}\) & Ab & 0.08 ng mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [39] \\ Endosulphan & Hapten & 0.01 pg mL\({}^{-1}\) & Fe/SWV & [40] \\ Gonadotropin & Ab & 3 \(\upmu\)l mL\({}^{-1}\) & HRP/CV & [41] \\ \(\alpha\)-Transglutaminase & Ab & - & AlkP/Ag\({}^{0}\)-Ag\({}^{+}\) & [42] \\ Zearalenoene & Ab & 1.5 pg mL\({}^{-1}\) & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [43] \\ IgG & Ag & 9 pg mL\({}^{-1}\) & AuNP/AuCl\({}_{4}\)\({}^{-}\)/DPV & [44] \\
8-Isoprostane & Ag & 12 pg mL\({}^{-1}\) & HRP/HQ/Competition/CV & [45] \\ AFP protein & Ag & 0.33 pg mL\({}^{-1}\) & GOx/HRP/Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [46] \\ CRP protein & Ag & - & Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/CV & [47] \\ Vangl1 protein & Ag & 0.03 pg mL\({}^{-1}\) & AuNPs/H\({}_{2}\)O\({}_{2}\)/Amperometry & [48] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Figures of merit of selected immunosensors using carbon nanostructures.  
dynamic range from 1 to 300 ng mL\({}^{-1}\), that is much lower values than those obtained from the previous example, with a sensor that does not need a redox label to be added in solution. More recently, [PERSON] et al. [51] reported an IgG immunosensor based on AuNPs/polydopamine-functionalized RGO as substrate for probe antibodies, and AgNPs/carbon dots (C-dots) as signal probe and catalytic material onto which signaling antibodies were immobilized. The presence of the IgG target was detected by monitoring the electroreduction current of BQ coming from the reaction of H\({}_{2}\)O\({}_{2}\) and HQ (both added in solution) on the AgNPs/C-dots. The current responses were linear between 0.01 and 100 ng mL\({}^{-1}\), with a LoD of 1 pg mL\({}^{-1}\). There are more examples of immunosensors for antigen detection. For this reason, we sorted them between sensors needing diffusing redox probes and the ones, more interesting, which use immobilized redox probes without needing added reactant.