Synergistic effect of a “stellate” mesoporous SiO2@Au nanoprobe and coffee-ring-free hydrophilic–hydrophobic substrate assembly in an ultrasensitive SERS-based immunoassay for a tumor marker

Abstract

In the field of surface-enhanced Raman scattering (SERS)-based immunoassays utilizing a sandwich structure, one of the main barriers to overcome is the development of an immunoprobe and substrate with ultrahigh sensitivity and relatively good reproducibility. In this work, a novel gold (Au)-coated “stellate” mesoporous SiO₂ (SMSiO₂) hybrid structure was proposed and applied as an immunoprobe. It was noticed that the SMSiO₂ matrix with an appropriate coating amount of Au nanoparticles (NPs) demonstrated a much higher SERS intensity than solid SiO₂ (SSiO₂)@Au NPs with an enhancement ratio of 2.6. Electromagnetic “hot spots” existed around the sharp edges and ultrasmall nanogaps of the multibranched SiO₂ matrix were verified to be the main reason responsible for the conspicuous enhancement given by the simulation results. Meanwhile, a hydrophilic–hydrophobic SERS-active immunosubstrate was prepared by linking antibodies onto the hydrophilic Ag-deposited sandpaper assembled with hydrophobic-treated filter paper. Such a novel substrate maintained excellent SERS homogeneity with quite a low relative standard deviation (RSD) of 12.85% arising from the elimination of the coffee-ring effect. The constructed sandwich immunoassay structure using the developed immunoprobe and the substrate was successfully employed in a specific assay for the ferritin (FER) antigen and exhibited an extensive dynamic linear range from 1 × 10⁻⁵ to 3 × 10⁻¹³ g mL⁻¹ with an ultralow limit of detection (LOD) of 3.16 × 10⁻¹⁴ g mL⁻¹. The simplicity and versatility of this strategy demonstrated its superior sensitivity for tumor markers and it can be expected to have potential practical application in the clinical diagnoses of cancer.