# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Dendrimers
#### 3.6.2 Enzyme-Less Immunosensors  
[PERSON] et al. [167] reported PAMAM/carbon dots modified by AuNPs for immunostaining of AFP. PAMAM/carbon dots/AuNPs were immobilized on the electrode surface, followed by anti-AFP immobilization. Upon complexation of AFP on these capture antibodies, the redox process of a diffusion probe Fe(CN)\({}_{6}\)\({}^{3-/4-}\) was slowed down. Changes in current as well as in impedance were used to quantify AFP within a wide range from 100 fg mL\({}^{-1}\) to 100 ng mL\({}^{-1}\), with a LoD of 0.025 pg mL\({}^{-1}\). [PERSON] et al. [168] also described an electrochemical immunoassay for detection of AFP. This immunosensor was constructed by covalent immobilization of PAMAM dendrimer-encapsulated AuNPs on a Au electrode, followed by covalent immobilization of ethyleneamineviologen (Vio) as electrochemical redox marker and AFP monoclonal antibody. The Au/PAMAM nanocomposite was used to increase the electrode surface area and accelerate the electron transfer kinetics. Upon immunoprecipitation of the immobilized AFP to its antibody, the Vio peak current decreased due to the hindered electron transfer reaction on the electrode surface. Through DPV, AFP was detected over a wide linear range (0.001-45 ng mL\({}^{-1}\)) and a LoD of 130 fg mL\({}^{-1}\). This result underline that dendrimers afford a much better sensitivity than planar electrodes when considering immobilized redox probes.  
Figure 32: **(A)** PAMAM-based electrochemical immunosensor for detection of \(\alpha\)-SYN. **(B)** Calibration curves obtained with [HRP-Ab\({}_{2}\)-GNPs]/Ab\({}_{1}\)/\(\alpha\)-SYN/Th/PAMAM-Au/\(\alpha\)-ABA/GCE immunosensor for different concentrations of \(\alpha\)-SYN. Inset: (a) amperometric responses of the immunosensor in PBS (pH 6.5) to \(\alpha\)-SYN at various concentrations of (a) 0, (b) 0.05, (c) 0.1, (d) 0.2, (e) 0.5 ng mL\({}^{-1}\); (b) amplification of calibration curve in the low \(\alpha\)-SYN concentration. Reprinted from [164], Copyright 2012, with permission from Elsevier. **(C)** Detection principle of a CEA immunosensor based on a sandwich capture and a HRP-GOx bienzymatic cascade. **(D)** SWV responses recorded for Au/Cys/Den/AuNP/Th-based CEA immunosensors at (i) blank noise and at various CEA concentrations: (ii) 10, 50, 100 pg mL\({}^{-1}\), 0.5, 1.0, 5.0, 10, 50 ng mL\({}^{-1}\), 0.1, 0.5 and 1.0 ug mL\({}^{-1}\). Reprinted with permission from [165]. Copyright 2013 American Chemical Society.  
Using stripping instead of a diffusing redox probe, [PERSON] et al. [169] reported detection of three protein biomarkers (CA 125, CA 15-3, and CA 19-9) in one single immunoassay. They used antibody-decorated magnetic beads as immunosening probes, as reported in the previous section, and PAMAM dendrimer-metal sulfide quantum dot (QD) as nanolabels. The MBs were functionalized by co-immobilization of the three capture antibodies (anti-CA 125, anti-CA 15-3 and anti-CA 19-9) and PAMAM/QDs containing CdS, ZnS and PbS were used for their labeling (Figure 33). ASV of cadmium, zinc, and lead led to a dynamic range of 0.01-50 U mL\({}^{-1}\) and a LoD of 5 mu mL\({}^{-1}\).  
Finally, [PERSON] et al. [170] reported an IgG electrochemical immunosensor using a redox-active ferrocenyl G2 dendrimer (G\({}_{2}\)Fc) as redox marker, onto which the capture antibody was immobilized. The presence of the captured IgG generates steric hindrance which was evidenced by a change in electroactivity of the immobilized Fc. The LoD was 2 ng mL\({}^{-1}\).  
As shown, dendrimers generally bring lower detection limits than conventionally grafted redox probes; one of the obvious drawbacks, however, looks to be the complexity of the sensor's architecture, which impedes reproducibility and make these sensors hardly applicable. Figures of merit discussed in this section are summarized in Table 10.