# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Magnetic Nanoparticles
#### 3.5.2 Enzyme-Free Immunosensors
coli_ concentrations (from top to bottom: 0, 102, 5 102, 103, 104 and 105 CFU mL\({}^{-1}\)); **(C)** Cyclic voltammograms in 1 M HCl for 0 (red curve) and 10\({}^{5}\) (blue curve) CFU mL\({}^{-1}\); scan rate: 50 mV/s. **(D)** SEM image for heat-killed _E. coli_ (left) and antibody modified MBs before (center) and after (right) incubation with 10\({}^{5}\) CFU mL\({}^{-1}\) of _E. coli_ (bacteria are pointed by red arrows). Reprinted from [156], Copyright 2015, with permission from Elsevier. Figure 29: **(A)** GCE/chitosan/AuNP/anti-AFB1/AFB1/anti-AFB1/MB/catechol immunoensor; **(B)** DPV recorded for GCE/chitosan/AuNP/anti-AFB\({}_{1}\)/BSA/AFB\({}_{1}\)/anti-AFB\({}_{1}\)/catechol–Au–Fe\({}_{3}\)O\({}_{4}\) immunoelectroda as a function of AFB\({}_{1}\) concentration from 0 ng mL\({}^{-1}\) to 110 ng mL\({}^{-1}\) in pH 7.0 (scan rate 50 mV s\({}^{-1}\)). Reprinted from [154], Copyright 2013, with permission from Elsevier;  
electrochemical immunosensor for clenbuterol (CLB) detection using magnetic gold-coated Fe\({}_{3}\)O\({}_{4}\) nanoparticles in a competitive immunoassay. The carbon SPE working electrode was modified with graphene and Nafion. Au/Fe\({}_{3}\)O\({}_{4}\) MBs were modified with CLB then magnetically collected on the electrode. A competitive immunoassay was used with CLB and anti-CLB added simultaneously in solution. CV and DPV were used to evaluate antibody binding on the electrode, using diffusing Fe(CN)\({}_{6}\)\({}^{3-/4-}\) as redox probe. The signal was linear with clenbuterol between 0.5 and 200.0 ng mL\({}^{-1}\), with a LoD of 0.22 ng mL\({}^{-1}\). As for the previous article, even if using magnetic MBs, it seems that they were not used to collect the analyte in solution, which is probably the cause of their relatively high detection limits. [PERSON] et al. [159] also reported an electrochemical immunosensor using Au/Fe\({}_{3}\)O\({}_{4}\) MPs as labels, for detection of digoxin. Anti-digoxin antibodies were immobilized on a PVA-modified SPE and MBs were decorated with digoxin and capture anti-digoxin. In a competitive assay were digoxin and AuMB/digoxin/anti-digoxin complex were present in solution, AuCl\({}_{4}\)\({}^{-}\) was electrochemically reduced into Au\({}^{0}\) on the Au/MBs using DPV. The current depending on the surface concentration of Au/MBs, digoxin was detected in the range 0.5-5 ng mL\({}^{-1}\), with a LoD of 50 pg mL\({}^{-1}\). [PERSON] et al. reported an alzheimer disease biomarker detection using IrO\({}_{2}\)NPs [160]. IrO\({}_{2}\) were used as tags for their electrocatalytic activity towards water oxidation reaction (WOR). Citrate-capped IrO\({}_{2}\)NPs were modified with anti-Apolipoprotein E antibodies (\(\alpha\)ApoE) for the detection of the ApoE alzheimer disease biomarker, with a detection limit of 68 ng mL\({}^{-1}\). Using IrO\({}_{2}\) NPs allowed signal generation in the same medium where the immunoassay takes place (PBS, pH 7.4). [PERSON] et al. reported in 2015 also Alzheimer's disease biomarkers detection but using porous magnetic microspheres and labelling with AuNPs [161]. The magnetic microparticles were functionalized by carboxylic groups for covalent grafting of antibodies, allowing an efficient capturing of the Alzheimer biomarkers directly from serum samples. AuNP were used to electrocalayze the hydrogen evolution reaction. The LoD was estimated between 20 and 80 pg mL\({}^{-1}\). In 2015, [PERSON] et al. [162] reported the detection of EGFR using an original electrochemical aptamer/antibody sandwich immunosensor. The capture aptamer was immobilized on streptavidin-coated MBs and the anti-human EGFR antibody was conjugated to AuNPs and, after magnetic collection on the electrode, was used as electrochemical signaling probe by dissolution of the AuNPs in HCl (Figure 31). The detection range was from 1 to 40 ng mL\({}^{-1}\), with a LoD of 50 pg mL\({}^{-1}\). At last, very recently, [PERSON] et al. [163] described interferon-\(\gamma\) detection using antibody-functionalized MBs labeled with AuNPs.