Cysteamine-coated gold nanoparticles (cysAuNPs) are positively charged as-synthesised and hence can interact with negatively charged DNA with ease. We have investigated the dependency of the particles' dispersion stage on different concentrations of lysozyme-binding aptamer (LBA). On top of the commonly reported phenomenon where cysAuNPs aggregate as the concentration of LBA increases, we observed that cysAuNPs redispersed after the amount of LBA achieved a certain threshold, dubbed as the critical redispersion concentration (CRC). By harnessing the aggregation and dispersion behaviour of cysAuNPs at LBA below and above the CRC, respectively, we have demonstrated a bimodal colorimetric aptasensor to detect lysozyme as a proof-of-concept study. Apart from being able to quantify the lysozyme in different ranges of concentrations with a visual change in colour, this aptasensor also demonstrated a novel concept of inverse sensitivity (i.e. higher signal with less analyte), leading to a 24-fold higher of signal-to-noise ratio (SNR), in comparison to the conventional sensors. The aptasensor can also selectively distinguish lysozyme and eliminate false results from other control proteins via both modes. The generalisability, as well as potential of cysAuNPs for bimodal colorimetric detection and inverse sensitivity behaviour have made this material an interesting alternative to citrate-coated AuNPs.
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