Preparation of Protein-like Silver–Cysteine
Hybrid Nanowires and Application in Ultrasensitive Immunoassay of
Cancer Biomarker
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Abstract
Novel
protein-like silver–cysteine hybrid nanowires (<i>p</i>-SCNWs) have been synthesized by a green, simple, nontemplate,
seedless, and one-step aqueous-phase approach. AgNO<sub>3</sub> and l-cysteine were dissolved in distilled water, forming Ag–cysteine
precipitates and HNO<sub>3</sub>. Under vigorous stirring, the pH
of the solution was rapidly adjusted to 9.0 by addition of concentrated
sodium hydroxide solution, leading to quick dissolution of the Ag-cysteine
precipitates and sudden appearance of white precipitates of <i>p</i>-SCNWs. The <i>p</i>-SCNWs are monodispersed
nanowires with diameter of 100 nm and length of tens of micrometers,
and have abundant carboxyl (−COOH) and amine (−NH<sub>2</sub>) groups at their surfaces, large amounts of peptide-linkages
and S-bonding silver ions (Ag<sup>+</sup>) inside, making them look
and act like Ag-hybrid protein nanostructures. The abundant −COOH
and −NH<sub>2</sub> groups at the surfaces of <i>p</i>-SCNWs have been found to facilitate the reactions between the <i>p</i>-SCNWs and proteins including antibodies. Furthermore,
the fact that the <i>p</i>-SCNWs contain large amounts of
silver ions enables biofunctionalized <i>p</i>-SCNWs to
be excellent signal amplifying chemiluminescence labels for ultrasensitive
and highly selective detection of important antigens, such as cancer
biomarkers. In this work, the immunoassay of carcinoembryonic antigen
(CEA) in human serum was taken as an example to demonstrate the immunoassay
applications of antibody-functionalized <i>p</i>-SCNWs.
By the novel <i>p</i>-SCNW labels, CEA can be detected in
the linear range from 5 to 400 fg/mL with a limit of detection (LOD)
of 2.2 fg/mL (at signal-to-noise ratio of 3), which is much lower
than that obtained by commercially available enzyme-linked immunosorbent
assay (ELISA). Therefore, the synthesized <i>p</i>-SCNWs
are envisioned to be an excellent carrier for proteins and related
immunoassay strategy would have promising applications in ultrasensitive
clinical screening of cancer biomarkers for early diagnostics of cancers