# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Metal or Metal Oxide Nanoparticles
#### 3.4.2 Enzyme-Free Immunosensors
A gold electrode was modified with a monolayer of mercaptopropionic acid, electrodeposited AuNPs and thiolated capture antibodies. Using Fe(CN)\({}_{6}\)\({}^{3-/4-}\) as redox diffusing probe, A\(\beta\)(1-42) was detected within a linear range of 10 to 1000 pg mL\({}^{-1}\) and a LoD of 5 pg mL\({}^{-1}\). Also very recently, [PERSON] et al. [122] reported an enzyme-free electrochemical immunosensor based on a competitive detection scheme using MB, hydrazine and PtNPs in an immunosandwich format (Figure 23A). In the presence of the target antigen, surface-immobilized MB consumes interfacial hydrazine thereby diminishing the electro-oxidation of hydrazine on PtNPs. This sensor was used to detect _Plasmodium falciparum_ histidine-rich protein 2 (_Pf_/HRP2). Chronocoulometric measurements allowed a LoD in the pM range (ca. 70 ng L\({}^{-1}\)). In the same spirit, [PERSON] et al. [123] proposed a multiplexed detection of _Escherichia coli_, campylobacter and salmonella. A mixture of anti-_E. coli_, anti-campylobacter and anti-salmonella antibodies were immobilized on a MWCNT polyallylamine-modified carbon SPE, then incubated in a bacteria suspension. The sandwich immunoassay was performed with CdS, PbS or CuS-conjugated signaling antibodies, used to release the corresponding metal ions upon SWASV. Calibration curves were obtained in the range 10\({}^{3}\)-5.10\({}^{5}\) cells mL\({}^{-1}\), with a LoD of 400 cells mL\({}^{-1}\). Direct detection of carbohydrate antigens (CA72-4) was also reported by [PERSON] et al. [124] using nanoporous gold as  
Figure 22: **(A)** (**a**) Scheme of the cells biorecognition with AuNPs-conjugated antibodies and detection through the electrocatalytic HER; **(b**) CVs performed from +1.35 to \(-\)1.40 V at a scan rate of 50 mV/s in 1 M HCl in the absence (a curve) and in the presence (b curve) of AuNPs-conjugated antibodies; **(c)**_Left_: Chronoamperograms registered in 1 M HCl, during the HER applying a constant voltage of \(-\)1.0 V, for AuNPs-labeled cells (a: control with BSA; b: 3.5 \(\times\) 10\({}^{4}\) cells). _Right_: Comparison of the corresponding analytical signals for the blank and the positive sample. Reprinted from [117], Copyright\({}^{\copyright}\) 2012 WILEY-VCH Verlag GmbH & Co; **(B)** EIS immunosensor based on AuNPs for detection of HbA1c; **(C)** Impedance change with the concentration of target anti-HbA1c IgG (triangle dot), anti-biotin antibody (square dot), and anti-pig IgG (circle dot). Adapted from [118] with permissions. Copyright\({}^{\copyright}\) 2012 WILEY-VCH Verlag GmbH & Co; **(D)** Scheme of the electrochemical lateral flow immunosensor for dengue detection, with AuNPs/Fc-labeled antibodies; **(E)** CVs of NS1 protein detection over the lateral flow immunosensor, for concentrations from 0.5 ng mL\({}^{-1}\) to 400 ng mL\({}^{-1}\) of NS1 protein. Reprinted from [119], Copyright 2016, with permission from Elsevier. substrate and polyaniline-AuNPs (PANi-AuPt) as label. The signaling antibody (Ab\({}_{2}\)) was adsorbed onto the PANi-AuNPs and served as catalyst for H\({}_{2}\)O\({}_{2}\) oxidation (added in solution). The sensor exhibited a linear range from 2 to 200 U mL\({}^{-1}\), with a LoD of 0.10 U mL\({}^{-1}\). At last, detection of small organic molecules was also reported. [PERSON] et al. [125] described a label-free electrochemical immunosensor for atrazine (ATZ) detection using AuNPs-labeled antibodies. Fe(CN)\({}_{6}\)\({}^{3-/4-}\) was used as electrochemical redox indicator in solution. Binding of anti-ATZ and ATZ was monitored with DPV. A LoD of 0.02 ng mL\({}^{-1}\) was obtained, with a linear range between 0.05 ng mL\({}^{-1}\) and 0.5 ng mL\({}^{-1}\). Similarly, [PERSON] et al. [126] reported cortisol detection based on immobilized antibodies on ZnO nanorods or ZnO nanoflakes. Fe(CN)\({}_{6}\)\({}^{3-/4-}\) was added in solution for probing the presence of the Ab/Ag complex. The use of ZnO nanoparticles were justified not for their catalytic properties but for an increase in surface area. Using CV and EIS, the found a LoD of 1 pM (ca.