Development of a gold nanoparticle-based colorimetric sensor utilizing cysteine-loaded liposomes in acidic buffer solutions

This study presents the development of a colorimetric sensing system, utilizing gold nanoparticles (AuNPs) and cysteine (Cys)-encapsulated liposomes (CELPs) as sensing probes, that functions in a variety of buffer solutions. Triton X-100 (TX-100), a nonionic surfactant, was used to simulate a biological activity that disrupts the liposome membrane. The Cys released from the CELPs by TX-100 triggered the aggregation of AuNPs, causing a noticeable red-to-blue color change, which was enhanced by Cu²⁺ chelation. The AuNP-CELP/TX-100 system was tested at pH 5-7.4 in various buffers. At neutral pH (7.0-7.4), the system with citrate-capped AuNPs (cit-AuNPs) displayed a distinctive colorimetric response in the presence of Cu²⁺ (0.3 mM), transitioning from red to blue with a UV-vis absorption shift from 525 nm to approximately 660 nm. However, the cit-AuNPs showed a limited stability in acidic buffers, and cetyltrimethylammonium bromide (CTAB)-capped AuNPs (cit-AuNPs-CTAB) were employed as an alternative at acidic pH. Overall, the AuNP-CELP/TX-100 sensing system, utilizing the cit-AuNPs-CTAB and Cu²⁺ chelation to Cys, exhibited the potential to be an effective sensing method working in acidic media. The test results with TX-100, which disrupts the liposome membrane, suggest that this system can be used to visualize diverse biological interactions involving membrane disruption, such as viral attacks, in acidic solutions with diverse ionic compositions.