# Recent Advances in Electrochemical Immunosensors
## 3 Discussion
### Metal or Metal Oxide Nanoparticles
#### 3.4.2 Enzyme-Free Immunosensors
[140] proposed in 2012 a HSA electrochemical immunosensor using HSA-conjugated AuNPs as electrochemical label and a hexagonal mesoporous silica (MCM-41)-PVA composite as immobilization substrate. DPV was used to electrooxidize AuNPs. HSA was detected with a LoD of 1 ng mL\({}^{-1}\), with a linear range between 0.5 and 200 \(\upmu\)g mL\({}^{-1}\). [PERSON] et al. [141] described an immunosensor to detect PSA using capture antibodies immobilized on mesoporous SiO\({}_{2}\)NPs and AgNPs. HQ added in solution was used as redox probe and its electroactivity measured by CV after Ab-Ag interaction. The peak current was reported to vary linearly with PSA concentration from 0.05 to 50.0 ng mL\({}^{-1}\), with a LoD of 15 pg mL\({}^{-1}\). [PERSON] et al. [142] proposed a PSA sensor using a redox and catalytically active infinite coordination polymer made of Fc-modified PtNP deposited on polyamidoamine dendrimers (Figure 25A). Using a sandwich-type immunoassay format and  
Figure 24: (**A**) GC/Ph/AuNP competitive immunosensor for CEA detection; (**B**) CVs of the modified electrode in 10 mM [Fe(CN)\({}_{6}\)]\({}^{4-/3-}\) + 0.1 M KCl, for (a) a bare electrode; (b) NH\({}_{2}\)-Ph-NH\({}_{2}\)-modified electrode; (c) NH\({}_{2}\)-Ph-NH\({}_{2}\) + AuNPs and (d) NH\({}_{2}\)-Ph-NH\({}_{2}\) + AuNPs + Anti-CEA. Reprinted from [130]. Copyright 2012, with permission from Elsevier; (**C**) Ag/AuNPs coated on GR as CEA immunosensor; (**D**) CVs obtained in PBS form a Ag/AuNPs/GR electrode, showing the oxidation/reduction of the Ag\({}^{0}\)/Ag\({}_{2}\)O couple for different concentrations of CEA. Reprinted from [139], Copyright 2015, with permission from Elsevier. H\({}_{2}\)O\({}_{2}\) added in solution, they showed that the reduction current on the PtNPs was proportional to the logarithm of PSA concentration between 1 pg mL\({}^{-1}\) and 60 ng mL\({}^{-1}\), with a LoD of 0.3 pg mL\({}^{-1}\). As for other infinite coordination polymer-based immunosensors, this one demonstrated an excellent LoD. Also using AuNPs for their catalytic properties toward H\({}_{2}\)O\({}_{2}\) reduction, [PERSON] et al. [143] very recently (2017) described an immunosensor for PSA detection with amino-functionalized Cu\({}_{2}\)O@CeO\({}_{2}\) core-shell NPs able to bind AuNPs through Au-N bonds. These NPs brought a better electrocatalytic activity towards the reduction of H\({}_{2}\)O\({}_{2}\) (added as redox probe in solution) than single Cu\({}_{2}\)O or AuNPs alone. The immunosensor exhibited a linear range from 0.1 pg mL\({}^{-1}\) to 100 ng mL\({}^{-1}\) with a LoD of 0.03 pg mL\({}^{-1}\). Also in 2017, [PERSON] et al. [144] described spiky AuNPs for PSA detection. Such spiky AuNPs were used for silver deposition. After immunodetection following a sandwich format, with a capture antibody attached on the electrode surface and a second antibody attached on the signaling NPs, they proceeded to use Linear Scan Voltammetry (LSV) to dissolve Ag\({}^{0}\). From their own comparison measurements, spiky AuNPs led to over 260-fold of signal increase in comparison with silver enhancement on conventional AuNPs. Two linear relationships between stripping current and PSA concentration were established, in the range of 2-125 pg mL\({}^{-1}\) and 0.1-10 ng mL\({}^{-1}\), with a LoD of ca. 1 pg mL\({}^{-1}\). At last, still in 2017, using the same metal dissolution strategy, [PERSON] et al. [145] described a PSA sandwich assay with a double signal amplification using copper nanoclusters onto which antibodies were immobilized. The double amplification came from dissolution of Cu at +0.06 V, the resulting Cu\({}^{2+}\) ions being used to catalyze the oxidation of ascorbic acid (added in solution as redox marker) (Figure 25C). The immunosensor gave a linear range from 0.5 pg mL\({}^{-1}\) to 100 ng mL\({}^{-1}\) and a LoD of 150 fg mL\({}^{-1}\).