Abstract
Lactoferrin in the saliva is recently considered a biomarker for the diagnosis of Alzheimer's disease. In this paper, a solution-based, user-friendly biosensing system has been developed to quickly measure lactoferrin at low concentrations. This aptasensor is applied to the fluorescence resonance energy transfer (FRET) quenching mechanism, in which carbon quantum dots (CDs) act as the FRET donor; the FRET quenching element is made of graphene oxide (GO) nanosheets which show good quenching capability. CDs bioconjugated with a chosen aptamer (CDs-aptamer) have the strongest emission (λem) at 447 nm when excitation (λex) is 365 nm. Due to the interaction of the aptamer and GO through the π-π* interaction, GO can approach CDs, resulting in FRET quenching. In the presence of lactoferrin, the fluorescence intensity of CDs-aptamer is restored as the binding affinity between lactoferrin and the aptamer is stronger than the π-π* interaction between the aptamer and GO. A linear relationship between the restored fluorescence intensity and the concentration of lactoferrin in artificial saliva with a range from 4 to 16 μg/mL is observed. The limit of detection of the solution-based aptasensor is estimated at 2.48 μg/mL. In addition, the sensing performance of the aptasensor made of carbon nanomaterials has been evaluated to test different proteins including major salivary proteins. The results show that this aptasensor has a high selectivity to detect LF with a low concentration, <16 μg/mL.