<i>In Situ</i> Plasmonic Gold Staining-Boosted Paper Test Kits of Human Albumin for Urinary Healthcare Monitoring

Inspired by the distinct circular coffee-ring stains left by coffee droplets, we explore another type of circular stain formed by colorimetric nanomaterials on hydrophilic and porous nitrocellulose (NC) papers, which are termed diffusion coffee rings (DCRs). Layered MoS₂ nanosheets, utilized as colorimetric nanomaterials, exhibit an intriguing MoS₂-marked DCR (MoS₂/DCR) boundary on NC substrates. When MoS₂-loaded NC was immersed into HAuCl₄ solutions, a spontaneous reaction between MoS₂ nanosheets and Au ions results in notable color changes at the MoS₂/DCR due to the formation of <i>in situ</i>-synthesized gold nanoparticles (issAu), and this phenomenon is termed issAu staining in our protocol. Herein, issAu staining is highly stable and repeatable due to the stabilization effect of NC fibers on the attached Au nanomaterials, whereas the presence of proteins effectively inhibits the growth of issAu on MoS₂, responsively weakening the staining effect. Under the inspiration of paper-based kits, we developed a simple and rapid protein analysis method based on the issAu-stained MoS₂/DCR assay (issAu-MoS₂/DCR). The MoS₂/DCR enables the desired recognition effect of proteins in the presence of coexisting compounds, ensuring high anti-interference performance. Our issAu-MoS₂/DCR analysis protocol works via the synchronous formation of Au nanoparticles, which overcomes challenges related to the drawbacks of sensing materials during their storage. The concept of issAu staining enhances point-of-care and ultrafast <i>on-site</i> monitoring techniques for proteins, practically addressing the emergency needs of early urinary disease diagnosis.