Development of colorimetric lateral flow assays with gold nanostructures for Cystatin C detection

Cystatin C (CysC) is a biomarker indicative of renal function, and its comorbidities, including heart failure. Generally, enzyme-linked immunosorbent assay (ELISA) can detect CysC in hours but requires skilled personnel, thorough reagent preparation, and a laboratory setting. Here, we report quantitative lateral flow immunoassays (LFIAs) with two gold nanostructures (AuNSs): gold nanoparticles (AuNPs) and gold nanorods (AnNRs). UV–Vis suggests 358 μg/ml mAbs in AuNPs conjugation and 45 μg/ml mAbs in AuNRs conjugation. With dynamic light scattering (DLS) measuring the hydrodynamic radius (R_h) of monoclonal antibodies (mAbs) – AuNSs conjugates, AuNPs conjugate maximum R_h is 125.9 ± 11.6 nm (1074 µg/ml conjugated mAbs). 895 µg/ml mAbs – AuNRs conjugate gives maximum R_h (55.9 ± 4.5 nm), but 45 µg/ml AuNRs conjugate displays optimum LFIAs performance. Wash-step and bridged structures were introduced for a wider linear range of CysC quantification (up to 12.5 mg/L), with AuNPs’ limit of detection (LoD) as 0.42 mg/L (wash-step), 0.87 mg/L (bridged structure) and AuNRs LoD as 0.35 mg/L. AuNRs are therefore more sensitive than AuNPs however the test line signal intensity suggests AuNPs are visually stronger. Furthermore, with the drawbacks of the bridged structure's complex fabrication and wash-step dipstick format's multi-steps, conventional LFIAs with AuNPs (89.5 μg/ml mAbs) conjugate in 10 min turnaround time were developed (LoD: 1.04 mg/L). Intra- assay CV% and inter- assay CV% are 13.69% 21.75%, accordingly. Human samples were then tested to address the matrix effect, with an average recovery rate of 99.37 ± 2.23% and an average CV% of 2.81%.