## 2 Application of NPs in the Diagnosis of CKD
### Detection of Kidney Injury Biomarkers  
A dipstick contains a thin plastic stick and chemical-based strips, which can detect the abnormalities in urine. The presence or increased levels of distinct substances in urine result in changes of the stick color [27]. The incidence and progression of CKD are related to the increment of albuminuria as an important risk factor [28; 29]. Only a concentration over 30 mg/dL of urinary albumin be detected via routine urinalysis dipstick. Specific dipsticks or specific antibody techniques are required to detect microalbuminuria [30]. Nonetheless, these strategies are inconvenient and insensitive. Surface-enhanced Raman scattering (SERS) with silver/copper/gold NP surfaces has been applied in this context by employing laser energy [31]. Easy sample processing steps, high sensitivity, rapid analysis, and the availability of portables apparatus are the advantages of the SERS method [32]. The application of silver NPs has significantly increased the Raman signal [31]. It has been reported that the SERS spectroscopy method reveals a strong connection between the reference and predicted albumin concentration, proposing that this technique is appropriate for the determination of absolute albuminuria. SERS as a sensitive tool has been able to detect extremely low levels (3 mg/mL) of albumin [33]. Currently, the available commercial devices have provided point-of-care monitoring of microalbuminuria [32]. A recent investigation developed a disposable electrochemical immunosensors for a point-of-care detection of microalbuminuria in which gold NPs were used on the electrodes via dielectrophoresis. This sensor contained a polystyrene NP core with HAS antibodies that were covalently conjugated to silver nanoshells. The developed nanosensor exhibited high conductivity, biocompatibility, and sensitivity which can be used in the early detection and monitoring of CKD in point-of-care settings [34] (Figure 1). Serum and urine creatinine are considered important biomarkers of CKD [35]. Peptides are among the suitable detectors of creatinine due to their simple synthesis and modification properties and low molecular weight [36; 37]. Phage display technology has been utilized to fabricate peptides that have a specific affinity to creatinine [38]. In this approach, gold NPs, due to their distance-dependent optical features, have been used in combination with peptide probes to detect creatinine. The synergistic effect between these NPs and peptides has shown high selectivity and specificity for detecting creatinine in human urine [39]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Section** & **Main Features** & **Ref.** \\ \hline Mesangium & Gold NPs (80–100 nm) & [20] \\ \hline Cortex, nephritis & Iron oxide core NPs (4–6 nm) & [21] \\ \hline Medulla & MHC-II antibody & [22] \\ \hline Unilateral obstruction & Anti-CDIIb antibody & [23] \\ \hline Tumor vasculature & Integrin \(\alpha\)v\(\beta\)3 & [24] \\ \hline Vascular endothelium & PLGA coated CoQ10 & [25] \\ \hline Medulla during ischemia & Dendrimer & [26] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of NPs for CKD targeting. Other promising biomarkers such as N-acetyl-\(\beta\)-d-glucosamineaminidase (NAG), kidney injury molecule-1 (KIM-1), and cystatin C (CysC) have been introduced as efficient tools for CKD prediction [40]. Mounting evidence demonstrated that NPs could significantly enhance the response of immunosensors in recognition of these biomarkers [41; 42]. Serum CysC was detected by a disposable amperometric immunosensor through a sandwich-type assay in which gold NPs were used to generate the augmented response with high sensitivity [43]. In another study, a new \"light-switch\" molecule of the Ru (II) complex ([Ru(dcpy)2 dppz]2+-DPEA) with self-enhanced electrochemicaluminescence (ECL) specifications was developed [42]. The combination of this ECL with DNA nanotechnology has been used as an effective signal amplifier in NAG detection high sensitivity excellent linear range (0.1 pg/mL to 1 mg/mL) [42].