Signal Amplification by Spatial Concentration for Immunoassay on Cellulose Media

Immunoassay is one of the most common bioanalytical techniques from lab‐based to point‐of‐care settings. Over time, various approaches have been developed to amplify signals for greater sensitivity. However, the need for effective, versatile, and simple signal amplification methods persists yet. This paper presents a novel signal amplification method for immunoassay that utilizes spatial concentration of a cellulose‐based plate possessing sensor transducers, specifically gold nanoparticles. By modifying the dimensions of the plate, the density of nanoparticles increased, resulting in intensified color signals. The coating material, polydopamine, which is utilized to protect the gold nanoparticles. Chemical changes in nanocomposites are characterized using scanning electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The application of this method to colorimetric quantification demonstrated great consistency across various concentrations of nanoparticles, with better reliability at lower concentration ranges. A model immunoassay is designed to evaluate the analytical performance. As a result, this method successfully corrected a false‐negative result with a lowered Kd of 0.509 pmol per zone. This method shows strong signal enhancement capability that can correct false‐negative signals in the immunoassays, with potential benefits including versatility, simplicity, low cost, and the ability to operate multiple plates simultaneously. In this research, the Malaprade reaction plays a key role in the formation of hemiacetal interfibrous crosslinks, allowing for the shrinkage of the fabric network in cellulose‐based immunoassay. By reducing the volume of the plate, the spatial concentration of gold nanoparticles significantly amplifies colorimetric signals from a sensor. This approach has the potential to revolutionize cellulose‐based immunoassays, such as paper‐based ELISA.