Reinforcing pomelo peel pectin beads with l-glutamate-modified graphene oxide for high-stability BOD biosensing

Abstract

In this study, we report the green modification of graphene oxide (GO) with 2-aminopentanedioic acid (L-glutamic acid, Glu) to enhance the mechanical stability and biocompatibility of pectin-based hydrogel beads for biosensor applications. The highest modification yield of 91.76% was achieved at 70 °C using 100 ppm of L-glutamic acid. Structural analyses via FT-IR spectroscopy, Raman spectroscopy, and XPS confirmed the successful functionalization, with the Glu moieties attaching to and replacing reactive epoxy groups on GO, thereby reducing cytotoxicity and microbial inhibition (5.35–6.12%) compared to the results achieved with the pristine GO. Incorporating GluGO into the pectin matrix significantly improved the mechanical stability, swelling behavior, thermal stability, and microbial compatibility of the composite beads. BOD measurements in packed-bed bioreactors showed markedly higher microbial activity with GluGO than with GO (15.49 and 4.92 in GGA 5 and 24.92 and 9.76 in GGA 10, respectively), highlighting the improved biocompatibility critical for biosensing. The structural integrity of the pectin beads reinforced with GO or GluGO remained above 80% when exposed to Ni²⁺, Cr⁶⁺, Zn²⁺, oxytetracycline, and simulated wastewater media, whereas the pristine pectin degraded rapidly. These improvements are attributed to the strong interfacial interactions, including hydrogen bonding and electrostatic interactions, between GluGO and the pectin matrix. In general, the results demonstrate that Pec–GluGO composite beads combine high stability, reduced cytotoxicity, and enhanced microbial compatibility, establishing them as promising, eco-friendly materials for wastewater treatment and sensitive BOD biosensing systems.