## 2 Electrochemical Sandwich Assays for Thrombin Using Aptamers
Figure 2: **(A)** Differential pulse voltammograms of an AuMNPs-Apt1/thrombin/APt2-AuNPs-HRP-modified gold electrode in solution of pH 7.0 PBS containing (a) no H\({}_{2}\)O\({}_{2}\) and hydroquinone (HQ), (b) 2 mM HQ, and (c) 1.5 mM H\({}_{2}\)O\({}_{2}\) + 2 mM HQ. **(B)** The current response of the aptasensor in PBS containing 2 mM HQ and 1.5 mM H\({}_{2}\)O\({}_{2}\) at different pH values. **(C)** The current response of the aptasensor in PBS (pH 7.0) containing different HQ concentrations. **(D)** The current response of the aptasensors in PBS (pH 7.0) at the potential of \(-0.2\) V while H\({}_{2}\)O\({}_{2}\) was stepwise added into the system. The final concentration of H\({}_{2}\)O\({}_{2}\) reaches to (a) 0.25, (b) 0.5, (c) 0.75, (d) 1.0, (e) 1.25, (f) 1.5, (g) 1.75, (h) 2.0, (i) 2.25, and (j) 2.5 mM. Reprinted with permission from reference [52]. Copyright Elsevier 2011. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Components of Sandwich Assay for Thrombin (Th = Thrombin, SAM = Self-Assembled Monolayer)** & **Method of Detection** & **Linear Range** & **Limit of Detection** & **Ref.** \\ \hline Au/SAM/ Appearance covalently attached to methylene blue (not a sandwich assay, for comparison only) & AC voltammetry & 6.4 nM-768 nM & 6.4 nM & [35] \\ \hline Au/thiolated aptamer/Th/apatmer-modified gold nanoparticles with adsorbed rhodamine 6G & Electrochemical Impedance Spectroscopy & 0.05-18 nM & 0.02 nM & [38] \\ \hline Au/thiolated aptamer/Th/glucose dehydrogenase (GDH)-labeled aptamer (glucose substrate) & Amperometry & nonlinear & 1 nM & [39] \\ \hline Au/mixed SAM with thiolated aptamer/Th/apatmer labeled CdS quantum dots & Potentionetry & 100 pM-1 nM & 0.14 nM & [40] \\ \hline Au electrode/thiolated aptamer/Th/graphene modified Au nanoparticles with reporter aptamer & Electrochemical Impedance Spectroscopy & 0.3–50 nM & 0.01 nM & [41] \\ \hline Nanoporous Au/thiolated aptamer in mixed SAM/Th/apatmer-modified Au nanoparticles & Chromocoulometry & 0.01–22 nM & 30 nM & [42] \\ \hline Screen-printed carbon electrode/streptavidin coated magnetic beads/Stojto-liylated primary aptamer/Th/5-bio-liylated secondary aptamer/streptavidin-all-all-alanine phosphatase conjugate & Differential pulse voltammetry & 0.1–100 nM (non-linear) & 0.45 nM & [44] \\ \hline Screen-printed carbon electrode, streptavidin-coated magnetic beads, thrombin, biotinylated thrombin, streptavidin-all-alanine phosphatase conjugate, & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrochemical Sandwich Assays for Thrombin using Aptamers. Figure 3: Schematic of an electrochemical assay for thrombin (TB) based on indium tin oxide electrodes (ITO) first modified with Ag nanowires (AgNW) onto which the capture aptamer is bound along with mercaptohexanol (MCH). The Cu\({}_{2}\)O particle is a template for formation of hollow CeO\({}_{2}\) particles that are decorated with Au nanoparticles. Detection of catalysis of reduction of H\({}_{2}\)O\({}_{2}\) by CeO\({}_{2}\) is done using differential pulse voltammetry. Reprinted with the permission from reference [54]. Copyright Elsevier 2020.