## 6 Discussion  
In recent years, a number of important trends have emerged in the development of electrochemical sandwich assays using aptamers. Given that many biomarker proteins are glycoproteins, the development of a pair of aptamers where the binding of one member \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Analyte** & **Components of Sandwich Assay** & **Method of Detection** & **Linear Range** & **Limit of Detection** & **Ref.** \\ \hline \multirow{6}{*}{Amyloid \(\beta_{1-a2}\) oligomers} & Glassy carbon electrode/Craphene oxide and An nanoparticles/C Chitosan and molecularly imprinted polymer layer/A@ oligomer/ aptamer-modified Ag nanoparticles bound to SiO\({}_{2}\) larger nanoparticle & Differential pulse voltammetry & 5 pg/mL–10 ng mL & 1.22 pg/mL & [90] \\ \hline \multirow{2}{*}{Carcin-embryonic antigen} & Au electrode/Hilolated agtem/CEA/Concanvalain A lectin/horseradishis peroxidase & Differential pulse voltammetry & 5–40 ng/mL & 3.4 pg/mL & [91] \\ \hline \multirow{2}{*}{human epidermal growth factor receptor-2} & Au electrode/HER2 specific peptide/HER2 MnO2 nanosheet with phosphate ions and aptamers, molybdate ions & Square-wave voltammetry & 5 pg/mL–1 ng/mL & 0.05 pg/mL & [92] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Electrochemical Sandwich Assays Combining Aptamers and Lectins or Other Materials. Figure 9: Schematic illustration of (**A**) the preparation of the SiO\({}_{2}\)@Ag-aptamer composite, and (**B**) the fabrication of the MIPs-based antibody-free biosensor and the electrochemical detection of Ag oligomer via a sandwich-type assay. Reprinted with permission from reference [90]. Copyright 2020, Elsevier. of the pair depends on the glycosylation state of the protein is important since abnormal glycosylation is found in many disease states [93]. Given the significance of glycoproteins as assay targets, the incorporation of lectins with aptamers in sandwich assays is likely to develop further. A trend toward more sophisticated architecture making use of a wider variety of nanomaterials is evident in recent years. Within the past five years, the use of carbon nanotubes in assay structures has increased, and highly novel structures such as gold-sputtered coccoliths [72], doped graphene oxide nanosheets [48], hollow CeO\({}_{2}\) particles [54], Mn\({}_{3}\)O\({}_{4}\) and Pd@Pt nanzymes [67], and MnO\({}_{2}\) nanosheets [92] have been introduced. These nanomaterials are used as supports for the detection of aptamers or antibodies and also for signal-producing enzymes with the goal of increasing sensitivity and dynamic range. These materials can also in some cases exhibit improved electron transfer behavior. While sandwich assays for thrombin continue to be explored, efforts are diversifying to a wide range of other biomarkers and targets of diagnostic significance. The development of additional complimentary aptamer pairs that bind to distinct regions of a protein target will be important for the advancement of these assays. Aptamers have proven to be a highly valuable tool for the development of electrochemical assays. The stability of aptamers and their ability to regenerate into their active conformations makes their use in assays especially robust. Aptamers can be used to modify nanoparticles and nanomaterials employed for assay signal amplification. Aptamers also open the possibility for amplification by applying processes that extend the length of the DNA associated with detection for binding of a greater number of redox markers when detection takes place using chronocoulometry. Common nanoparticles such as gold nanoparticles are popular choices for signal amplification but other more unique nanomaterials such as nanodiamonds, templated materials, nanocages and others have proven well worthy of investigation. For these electrochemical sandwich assays, differential pulse voltammetry which provides reduced background current from double-layer charging and its sensitivity stands out as a method of choice.