# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Metal or Metal Oxide Nanoparticles
#### 3.4.2 Enzyme-Free Immunosensors  
There are even more examples of enzyme-less immunosensors using metal of metal-oxide nanoparticles except for antibody detection, for which only one recent work has been reported, where [PERSON] et al. [116] IgG detection using AuNP-modified polypyrrole (PPy). Simply using Fe(CN)\({}_{6}\)\({}^{3-/4-}\) as redox probe, IgG was detected, by measuring changes in charge transfer resistance with EIS, between 0.5 and 125 ng mL\({}^{-1}\) with a LoD of 0.02 ng mL\({}^{-1}\). For cell detection, [PERSON] et al. described in 2012 a quantification assay based on a transduction by electrocatalysis of hydrogen evolution (HER) on AuNPs. The system, where cells are labeled by AuNPs through a specific immunoreaction, is described on Figure 22A; it can detect \(4\times 10^{3}\) cancer cells in suspension [117]. It must be underlined that this amplification approach does need a reactant to be added in solution, conversely to most of the other amplified methodologies. Indeed, the HER reaction only used H\({}_{2}\)O, which is of course freely available in biological media such as those used for biosensing. For antigen detection, [PERSON] et al. [118] described in 2012 a label-free immunosensor for detection of HbA1c, based on AuNP-modified glycosylated pentapeptides (GPP) being analogs to HbA1c. Exposure of this interface to anti-HbA1c IgG resulted in a change in charge transfer resistance. Transduction was achieved using EIS and Ru(NH\({}_{3}\))\({}_{6}\)\({}^{2+/3+}\) as redox probe in solution, in a competitive inhibition assay where the surface bound GPP and HbA1c in solution competed for the anti-HbA1c IgG antibodies (Figure 22B). The LoD was of a few tens of ng mL\({}^{-1}\). More recently, [PERSON] et al. [119] described an electrochemical lateral flow immunosensor for detection of dengue NS1 protein. A conventional lateral flow architecture was adapted to a sandwich-type electrochemical transduction provided by a AuNP-labeled NS1 antibody, with Fc grafted on the AuNPs. From EIS measurements (and more particularly the total resistance of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Target** & **Probe** & **LoD** & **Method** & **Reference** \\ \hline IgG & Ag & 0.01 ng mL\({}^{-1}\) & AuNPs/HRP/H\({}_{2}\)O\({}_{2}\) & [104] \\ CEA protein & Ag & 0.11 pg mL\({}^{-1}\) & AuNPs/HRP/H\({}_{2}\)O\({}_{2}\)/PB & [105] \\ CEA protein & Ag & 1.3 pg mL\({}^{-1}\) & AuNPs/GO\({}_{x}\)/Fe(CN)\({}_{6}\)\({}^{3-/4-}\)/EIS & [106] \\ CEA, PSA proteins & Ag & 1 pg mL\({}^{-1}\) & PPy NPs/HRP & [107] \\ CEA protein & Ag & 1 pg mL\({}^{-1}\) & AuNPs/MnO\({}_{2}\)/GO\({}_{x}\)/TMB & [108] \\ AFP protein & Ag & 0.2 pg mL\({}^{-1}\) & AuNPs/HRP & [109] \\ AFP protein & Ag & 1.5 pg mL\({}^{-1}\) & AuNPs/HRP polyaniline & [110] \\ TNF-\(\alpha\) protein & Ag & 5 pg mL\({}^{-1}\) & Au nanorods/GO\({}_{x}\)/Fc/SWV & [111] \\ Interferon-\(\gamma\) & Ag & 0.05 pg mL\({}^{-1}\) & AuNPs/HRP/H\({}_{2}\)O\({}_{2}\)/HQ & [112] \\ Influenza virus & Ag & 1 pg mL\({}^{-1}\) & ZnO nanorods/HRP/TMB & [113] \\ EpCAM protein & Ag & 0.8 pg L\({}^{-1}\) & AgNPs/HRP/H\({}_{2}\)O\({}_{2}\) & [114] \\ Ara-H1 protein & Ag & 4 ng mL\({}^{-1}\) & AuNPs/AlkP/Ag\({}^{0}\)/ASV & [115] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Figures of merit of selected enzymatic immunosensors using metal or metal oxide nanoparticles. electrode), a linear calibration range was shown between 1 and 25 ng mL\({}^{-1}\), with a LoD of 0.5 ng mL\({}^{-1}\) (Figure 22D). Figure 22: _Cont._  
[PERSON] et al. [120] reported an electrochemical immunosensor for detection of two mycotoxins, fumonisin B1 and deoxynivalenol. A SPE was modified by AuNPs, PPy and RGO. Capture antibodies were immobilized on the AuNPs and free-diffusing Fe(CN)\({}_{6}\)\({}^{3-/4-}\) was used as redox probe. The LoD was of a few ng L\({}^{-1}\) for both targets and the detection range between 50 ng L\({}^{-1}\) and 1 \(\mu\)g L\({}^{-1}\). Very recently, [PERSON] et al. [121] described an immunosensor for detection of amyloid beta 1-42 protein A\(\beta\)(1-42).