Simultaneous binding of carboxyl and amino groups to liquid metal surface for biosensing

Eutectic gallium indium (EGaIn) nanoparticles can be modified with aniline derivatives to introduce versatile functional groups toward bioapplications beyond materials science. p-Aminobenzoic acid (PABA) modified EGaIn (EGaIn-PABA) demonstrated high wettability due to the presence of carboxyl groups, but the priority of binding of carboxyl and amino groups on the EGaIn surface remains unclear. To study the binding nature of PABA to EGaIn, the anti-mouse IgG antibody labeled with fluorescein isothiocyanate (FITC) (IgG-FITC) was covalently modified to EGaIn-PABA to verify the presence of terminal carboxyl groups on the EGaIn surface. The binding of gold nanoparticles (AuNPs) to EGaIn-PABA nanoparticles suggested the presence of terminal amino groups on the EGaIn surface. Then, taking advantage of the reductive nature of amino groups, the Almar blue fluorescence experiment was designed to determine the co-existence of carboxyl and amino groups on the EGaIn-PABA surface with an approximate ratio of 3 : 7, suggesting that carboxyl groups had a higher probability of binding with the EGaIn surface than that of amino groups. Then, an aptasensor was fabricated on the EGaIn-PABA surface with AuNPs for electrochemical detection of interleukin-6 with a sensitivity of 1 pg mL^-1.