Highly Sensitive Colorimetric Detection of a Variety of Analytes via the Tyndall Effect

This work initially reports the use of a quite familiar optical phenomenon of colloidal solutions, namely, the Tyndall Effect (TE) as signal readout for highly sensitive colorimetric chemical and biological analysis. Taking gold nanoparticles (GNPs) as a model colloid, the TE-inspired assay (TEA) is developed based on the conversion of a specific recognition event (e.g., the aptamer–analyte binding) into the aggregation of GNPs, leading to a significant TE enhancement. In the TEA, a cheap laser pointer pen is used as a hand-held light source, while a smartphone serves as a portable quantitative reader. The results show that the TE signaling strategy achieves a ∼1000-fold sensitivity improvement compared with the most common surface plasmon resonance signaling method using GNPs. The utility of the TEA is well demonstrated with the inexpensive, rapid, and portable detection of trace levels of analytes ranging from an important small-molecule drug (cocaine, ∼1.5 pM detection limit) to a protein biomarker (interferon-γ, ∼2.2 fM detection limit) and a toxic metal ion (Ag+, ∼1.4 nM detection limit). In addition, as the TE enhancement simply stems from the aggregation of either bare (unmodified) or modified GNPs, the TEA is universally applicable to almost all of the existing GNP-based liquid-phase colorimetric assays. The TEA method developed herein lights a new way for equipment-free point-of-care analysis in various fields including medical diagnosis, food safety evaluation, and environmental monitoring, especially in the resource-poor areas of the world.