## 2 Application of BRBS in the Detection of Diabetic Nephropathy Biomarkers
### Homocysteine (Hcy)  
Hcy, cysteine (Cys), and glutathione (GSH) are the three most abundant small-molecule amino acids containing thiol groups, playing important roles in biological processes [41]. The normal level of Cys in human blood plasma is 135.8-266.5 umol/L, the concentration of Hcy is in the range of 5-15 umol L\({}^{-1}\), and the GSH concentration in most cells is at mmol/L level [42; 43]. Among them, Hcy is one of the important biomarkers of diabetic nephropathy. Therefore, it is particularly important to establish sensitive, accurate, and highly selective Hcy sensors. Currently, electrochemical sensors(ECS) are commonly modified with noble metal nanoparticles on their surfaces. For example, Au nanoparticles (AuNPs) [44] possess good electron density, dielectric performance, and catalytic activity, as well as excellent conductivity, stability, and surface adsorption [45]. In recent years, silver nanoparticles [46] and alloy particles [47; 48] have also been widely employed in ECS. In addition, MXene, with a chemical formula generally represented as M\({}_{\text{n+1}}\)X\({}_{\text{n}}\)T\({}_{\text{x}}\)[49], is an emerging class of two-dimensional inorganic composite materials. MXene demonstrates immense advantages in areas such as medicine and sensing due to its large surface area, excellent energy storage performance, good bioconductivity, and biocompatibility [50; 51]. Moreover, MXene possesses a strong reducing ability, providing new strategies for synthesizing multifunctional nano-hybrids [52; 53]. Its exceptional performance offers new possibilities for improving the conductivity, electrocatalytic activity, and sensitivity of the target analyte's signal response in biomimetic recognition bio-ECS interfaces. [PERSON] [54] and others developed an unmarked molecularly imprinted electrochemical sensor (MIECS) for rapid and highly sensitive detection of Hcy. In this strategy, AuNPs were first prepared using chloroauric acid as a raw material in the presence of a reducing agent, and then AuNPs were introduced into the multilayer nanostructure of MXene to form MXene@AuNPs nanocomposites. Finally, the MIP with Hcy-specific recognition was formed by electropolymerization of DA monomer on the MXene@AuNPs-modified GCE surface. The MXene in the sensing interface has strong reducibility and excellent conductivity, and its large surface area accommodates more AuNPs and active sites, significantly enhancing the electrochemical performance of the sensor. The Hcy in the sample was detected by cyclic voltammetry, and the current signal decreased with the increase of Hcy concentration, with an LOD of 11.81 fmol L (Table 2). It is worth noting that due to the inherent two-dimensional structure of MXene with stack ability, the target molecules cannot approach the active region [55], which greatly reduces the sensitivity of detecting target molecules, greatly limiting its electrochemical performance. Therefore, the improvement of MXene function and performance is still the focus of current researchers. [PERSON] [56] and others prepared an Aptasensor using a graphene sponge (GS) as a material (Figure 3). The GS and AuNPs were separately modified on the glassy carbon electrode (GCE). Then, an Hcy APT with 66 bases of hairpin deoxyribonucleic acid (DNA) was fixed on the GCE by a Au-S covalent bond. Non-linking sites were blocked by bovine serum albumin (BSA) using mechanical filling and adsorption coverage methods. Finally, MB was specifically adsorbed on the APT due to its high affinity with guanine [57; 58]. In this study, MB had good biocompatibility and could generate electrochemical signals at a specific voltage in DPV, acting as an electrochemical hybridization indicator. In the presence of Hcy, the conformation of the nucleic acid APT changed after specifically binding with Hcy, causing MB release from the APT or moving away from the GCE surface, and thus changing the relative redox current (\(\Delta\)I) of MB.