Surfactant-mediated colorimetric assay assisted with in-situ rolling circle amplification on magnetic beads

Gold nanoparticles (AuNPs) with localized surface plasmon resonance effect have been widely used for colorimetric detection based on the interparticle plasmon coupling during AuNPs aggregation. However, it is still challenging to develop portable and quantitative methods with good sensitivity and excellent selectivity. In this study, a smartphone-based colorimetric assay is developed on the principle of surfactant-mediated AuNPs aggregation assisted with rolling circle amplification (RCA) on magnetic beads (MBs). The detection of adenosine is demonstrated as an example. The cetyl trimethyl ammonium bromide (CTAB) causes the negatively charged AuNPs to aggregate, which results in the color change from red to blue. When adenosine is in solution, the RCA process is triggered on the MBs because of specific adenosine-aptamer recognition, resulting in prolongation of single-stranded nucleic acid (ssDNA). The solution color remains red due to the electrostatic interaction between CTAB and ssDNA. Using this method, the limit of detection (LOD) for adenosine can be as low as 16 pM. Besides, it also works well in human serum. In addition, a portable device integrated with in-situ RGB analysis software is developed for the detection with a smartphone. This study offers a new strategy to improve the sensitivity and selectivity for the AuNPs-based colorimetric assay, taking advantages of specific aptamer recognition, in-situ RCA on MBs, magnetic separation, and smartphone-based portable device.