## 3 Results
### Analytical Performance of the Rh\({}_{2}\)O\({}_{3}\)-_Fg_o/gce  
As shown in Figure 5, a linear relationship between oxidation current and BPA concentration within the range of 0.6-40 \(\mu\)M could be expressed as:  
\[I=3.1569\times C+0.1002, \tag{4}\]  
where \(I\) is oxidation peak current of BPA (\(\mu\)A), and \(C\) is the concentration of BPA in solution (\(\mu\)M). The limit of detection (LOD) was estimated to be 0.12 \(\mu\)M using the Equation (5)  
\[\mathrm{LOD=(k\times SD)/b}, \tag{5}\]  
where k is a constant, SD is the standard deviation of the intercept, and b is the slope of the calibration plot. In this paper, the values of k, SD, and b are 3, 0.1315 and 3.1569, respectively. Compared with some other electrodes, based on gold and copper oxide-graphene nanoparticles [42; 45], the Rh\({}_{2}\)O\({}_{3}\)-rGO/GCE presented comparable performance in similar linear detection ranges (Table 1).  
Figure 4: (**a**) CVs of Rh\({}_{2}\)O\({}_{3}\)-rGO/GCE detected 20 \(\mu\)M BPA with scan rate from 10 to 270 mV s\({}^{-1}\); (**b**) Dependence of the oxidation peak current on the scan rate; (**c**) The relationship between \(E\) and ln\(\upsilon\).  
The stability, reproducibility, and selectivity of the Rh\({}_{2}\)O\({}_{3}\)-rGO/GCE were also evaluated. To test the stability of prepared electrode, Rh\({}_{2}\)O\({}_{3}\)-rGO/GCE was stored at 4 \({}^{\circ}\)C and examined three times per week by CV. After two weeks, the peak current retained 77.2% of the initial response. The Rh\({}_{2}\)O\({}_{3}\)-GO solution displayed excellent stability, being deposited at 4 \({}^{\circ}\)C within 30 days. Using this Rh\({}_{2}\)O\({}_{3}\)-GO solution to prepare Rh\({}_{2}\)O\({}_{3}\)-rGO/GCE, the response current for detecting BPA still remained within 98% of the initial currents. The good stability and reproducibility indicated the sensor should be appropriate for routine measurements of real samples. The relative standard deviation (RSD) for detecting BPA (n = 7) by one as-prepared sensor was less than 2.73%. The RSD for seven Rh\({}_{2}\)O\({}_{3}\)-rGO/GCEs fabricated in parallel was less than 3.22%.  
The i-t curves of bisphenol A, by the addition of different inorganic ions and BPA analogs, are shown in Figure 6. The concentrations of inorganic ions, BPA, and BPA analogs are 200 \(\upmu\)M, 20 \(\upmu\)M, and 20 \(\upmu\)M, respectively. Common inorganic ions (Ba\({}^{2+}\), Cu\({}^{2+}\), Ca\({}^{2+}\), K\({}^{+}\), Na\({}^{+}\), OH\({}^{-}\), SO\({}_{4}\)\({}^{2-}\) and Cl\({}^{-}\)) have no effect on analyzing BPA, even when their levels are 10 times higher than BPA. Meanwhile, no obvious interferences from BPA analogs have been observed, such as phenol, hydroquinone, bisphenol B, bisphenol F, and bisphenol S. The RSDs for different interferences were all less than 3.53%.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Sensors** & **Linear Range (\(\upmu\)M)** & **LOD (\(\upmu\)M)** & **Reference** \\ \hline Residual metal impurities within SWCNT electrode & 10–100 & 7.3 & [46] \\ Gold nanoparticles supported carbon nanotubes electrode & 0.87–87 & 0.13 & [47] \\ Gold nanoparticles dotted graphene electrode & 0.01–10 & 0.005 & [45] \\ Exfoliated Ni\({}_{2}\)Al layered double hydroxide nanosheets electrode & 0.02–1.51 & 0.007 & [48] \\ Copper oxide and graphene electrode & 0.1–80 & 0.053 & [42] \\ Rhodium oxide and graphene electrode & 0.6–40 & 0.12 & This work \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of the Rh\({}_{2}\)O\({}_{3}\)–rGO and other sensors for BPA determination.  
Figure 5: (a) CVs of Rh\({}_{2}\)O\({}_{3}\)–rGO/GCE with BPA concentration from 0.6 to 40 \(\upmu\)M; (b) Relationship between the oxidation peak current and the concentration of BPA (from 0.6 to 40 \(\upmu\)M).  
Figure 6: The i-t curves of bisphenol A by the addition of (a) different inorganic ions and (b) BPA analogs.