Conjugate of graphene quantum dots and glutaminase for the sensing of L-glutamine: Electrochemical vs. fluorescent sensing approaches

This research describes the synthesis of a covalent bioconjugate of graphene quantum dots (GQDs) and glutaminase (10 units·mg−1) for electrochemical and photoluminescent (PL) detection of L-glutamine (Gln). As synthesized GQDs exhibited uniform size distribution, (average diameter ~5–10 nm, and discrete blue emission characteristics (λex = 391 ± 2 nm and λem = 480 ± 2 nm)). Accordingly, the immobilization of glutaminase over the surface of the GQDs was confirmed by the characteristic Fourier-transform infrared (FTIR) spectroscopy peaks of the amide bond at 1,703 and 1,390 cm−1. The conjugate was used to detect Gln using electrochemical and PL approaches on a parallel basis over a concentration range of 100–1,000 µM. Electrochemical analysis of Gln using cyclic voltammetry revealed a limit of detection (LOD) and limit of quantification (LOQ) values of 360.4 µM and 1,100 µM, respectively. In comparison, quantification of Gln based on the PL approach yielded a Stern-Volmer quenching constant, LOD, and LOQ values of 2.2 × 103 M−1, 9.42 µM, and 28.57 µM, respectively. As such, the PL spectroscopy-based analysis was seen to have better sensitivity for Gln quantification than the electrochemical analysis. The PL-based approach is a more recommendable option for Gln quantification with high sensitivity and rapidity.