Colorimetric and fluorometric detection of protamine by using a dual-mode probe consisting of carbon quantum dots and gold nanoparticles

The paper describes an optical probe for colorimetric and fluorometric determination of protamine. The probe consists of a mixture of carbon quantum dots (CQDs) and gold nanoparticles (AuNPs) where the CQDs (with excitation/emission peaks at 350/440 nm) serve as fluorescent reporter and the AuNPs serve as a colorimetric reporter and a quencher of the fluorescene of CQDs. The nanoparticles applied here are characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), time correlated single photon counting (TCSPC), UV-vis and fluorescence spectroscopy, dynamic lights scattering (DLS) and zeta potentials. The green fluorescence of the CQDs overlaps the red absorption of AuNPs (520 nm) and therefore is quenched after fluorescence resonance energy transfer (FRET) between CQDs and AuNPs. Protamine is rich in basic arginine residues which are positively charged at physiological pH values. Protamine therefore can induce the aggregation of AuNPs which is accompanied by a color change from red to blue. Hence, the fluorescence of CQDs no longer overlaps the absorption of (aggregated) AuNPs (650 nm) and is not quenched as a result. These findings form the basis for a fluorometric assay that has linear response in the 10–220 ng·mL−1 protamine concentration range, with a 1.2 ng·mL−1 lower detection limit. The respective values for the colorimetric assay are 20–160 ng·mL−1 and 2 ng·mL−1. This dual-signal probe also possesses excellent selectivity for protamine and in our perception has a large potential for the determination of protamine in serum.