Real-time visualization of colorimetric probe for pH-sensitive based on poly-(γ-glutamic acid)-functionalized gold nanoparticles

A rapid and simple real-time visualization of colorimetric method for pH-sensitive was development based upon the aggregation of poly-(γ-glutamic acid)-stabilized gold nanoparticles (PGA-AuNPs). The gold nanocrystal seed-mediated approach using cetyltrimethylammonium bromide (CTAB) as a stabilizing surfactant was commonly used to make large quantities of AuNPs. Electrostatic self-assembly technique was used to assemble negatively charged PGA on the surface of positively charged CTAB-capped AuNPs. Results showed that decrease in pH near 5–6 and increase in pH near 8–9 led to the aggregation of PGA-AuNPs. The colorimetric probe showed a sensitive response to pH variation between 1 and 9. The change in aggregation behavior with pH occurred within a narrow range of pH from 5 to 6 (pKa ∼ 4.5–5.5). The decrease in the solution pH gradually converted the side chains of PGA from deprotonate form to protonate form. Such change switching could weaken electrostatic interactions with the modified AuNPs and thus accelerate their release and aggregation. The helix–coil transition of PGA indicated that the aggregation of PGA-AuNPs corresponded to the transition in the secondary structure of the polyacid.