Todays, metal nanoparticles (MNPs)-based colorimetric sensors have been extensively applied to detect toxic metal ions, small organic molecules, proteins, and oligonucleotides by using the “aggregation strategy” [1–3].
MNPs solutions show unique optical features depending on the level of target induced aggregation that can be tracked on their surface plasmon resonance (SPR) spectra. The aggregation leads to change of the color of AuNPs solution from red to blue/purple and that of AgNPs from vivid yellow to orange-red that are accompanied by SPR peak shifting at higher wavelengths. MNPs color change can also be monitored with a UV-Vis spectrophotometer to determine the amounts of target quantitatively [3].
However, the aggregation of MNPs is a dynamic process which is fast at the beginning and slows down then. At low concentration of the target, the small aggregates with long-term stable are formed and with increasing its concentration big clusters are formed which settles down due to the gravitational force effects [4, 5].
In addition, NaCl is sometimes used for assisting aggregation reactions to adjust the detection sensitivity and dynamic range to a desired concentration range [6, 7]. As well as, unmodified MNPs-based colorimetric biosensors use NaCl at relatively high concentration to distinguish folded and unfolded ssDNA, [1, 8, 9]. Since at high ionic strengths, the time course of the MNPs aggregation is much faster, usually data with large errors is obtained. Therefore, it is so important to define the well-repeatable signal capture time with high discrimination between different concentrations to resolve the problem brought by the dynamic aggregation process of MNPs.
Till now, a paper-based readout has been reported to achieve accurate results of AuNPs aggregation-based colorimetric biosensors [10]. In this method, a spectrophotometer equipped with an integrating sphere accessory must be used and the optimum volume of AuNPs solution dropped on paper must be determined before analysis.
Herein, we report for the first time that hydrophobic surfactants can act as stop solution for MNPs aggregation based colorimetric sensors. The hydrophobic surfactants that have been already reported for stabilizing well-dispersed MNPs can also stabilize the aggregates and stop or decrease the rate of the chemical induced aggregation reaction. A suitable surface coating could not only keep the small aggregates apart from each other, but could also ensure the charge neutrality and steric stability of aggregates in their media [11].