# Recent Advances in Electrochemical Immunosensors
## 4 Conclusions and Perspectives  
As shown on Figure 38, conventional electrode substrates (Figure 38A) gave higher LoDs than those featuring carbon nanostructures (Figure 38B). For both, enzymes used as amplifiers allowed to lower the LoD, as expected. This behavior is not general, however. Indeed, for the most studied immunosensors during this period, those based on metal or metal oxide nanoparticles, the best sensitivities were obtained with enzyme-free architectures (Figure 38G). On the contrary, the less studied ones are those featuring ionic liquids (Figure 38J) which, in addition, were not reported these very last years, which tends to indicate that the initial promises of IL in terms of sensitivity were not kept. Dendrimers were also rarely reported (Figure 38I), even if LoDs achieved with these structures are significantly lower than those reported for other architectures. Enzyme-free immunosensors with graphene and immobilized redox probes (Figure 38D) gave better results compared to those with diffusing redox probes (Figure 38C), and one of the lowest LoDs in average. This approach must therefore be emphasized as one of the most promising ones for electrochemical immunosensors.  
Also, enzymatic immunosensors with metal or metal oxide nanoparticles (Figure 38F) gave better results compared to graphene only (Figure 38E). It is also noteworthy that immunosensors based on magnetic nanoparticles (Figure 38H) did not allow to lower detection limits as it could have been expected.  
All this allow to conclude that the way to improve the detection limits of immunosensors is to increase the molecular and redox probe densities, both immobilized on high surface-to-volume nanoparticles. Because the most general transduction pathway implies a sandwich architecture with enzymes or artificial catalytic sites coupled to the signaling antibody, which significantly amplifies currents, one could consider that improvement perspectives rely on catalytically (non-enzymatic) amplified sandwich immunoassay using substrates of specific area as high as possible.  
Figure 37: **(A)** Bode plots for different concentrations of MS2 bacteriophage. (**B**) It is shown that the smaller pores (73 nm) are less efficient due to compromised accessibility of the walls to viruses when the diameter of the pore is smaller than 3 times that of the virus, while larger pores (97 nm) are not affected. R\({}_{\text{sol}}\) is the resistance of solution above the membrane, R\({}_{\text{pore}}\) and C\({}_{\text{pore}}\) are the resistance and capacitance of the pores in the membrane, and R\({}_{\text{ox}}\) and C\({}_{\text{ox}}\) are the resistance and capacitance of the oxide layer. Adapted with permission from [190]. Copyright 2016 American Chemical Society.  
Figure 38: _Cont._  
The ratio between the number of available recognition sites and the number of available target molecules in the analyzed solution must absolutely be considered. It is indeed not consistent to target very low concentrations in a small volume of solution (that is, very small amounts of targets) with large electrodes having so high densities of molecular probes that the probes/targets ratio tends to zero; in such case, the sensor should not be sensitive at all. Finally, even if this point is extremely scarsely pointed out in immunosensors' publications, authors should also consider that the quality of the antibodies, and particularly Ab-Ag affinities, is of the highest importance and may be also a significant factor for improvement.  
The authors thank University Paris Diderot, Sorbonne Paris Cite and CNRS for funding.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data and in the writing of the manuscript.