Bioelectronic Properties of Graphene Oxide Co-Immobilized with Penicillinase in Lipid Langmuir-Blodgett Films

Conjugating bioinspired systems with graphene oxide can provide systems with an active layer that combines materials with different functionalities. Bioinspired systems are particularly an interesting alternative for incorporating bioactive species, enabling a molecular environment favorable to some biomolecule properties, such as enzyme activity. In the same sense, graphene oxide (GO), in addition to presenting biocompatibility, has optical and electrical properties that encourage its application in systems of biological interest. In this work, the interaction of the enzyme penicillinase with the phospholipid di-myristoyl phosphatidic acid (DMPA), conjugated with graphene oxide (GO), was studied as Langmuir and Langmuir-Blodgett (LB) films. The incorporation of the enzyme and GO in the phospholipid floating monolayer was evaluated through measurements of surface pressure-area isotherms, surface elasticity, Brewster angle microscopy (BAM), and Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS). The Langmuir films were stabilized with the presence of GO, as identified by the results obtained with the employed techniques. They showed that the enzyme was incorporated in the DMPA monolayers, with its secondary structure being preserved, as identified by PM-IRRAS. Also, the interaction of the mixed lipid-enzyme films with GO located in the aqueous subphase of the monolayers in the form of colloidal dispersion could be identified, forming homogeneous films as observed by BAM. The monolayer stabilization supported the transfer of these hybrid films onto solid substrates using the LB technique, characterized by fluorescence spectroscopy and transfer ratio. The enzymatic activities of the solid devices were then measured by using UV-visible spectroscopy. The approach was effective in co-immobilizing penicillinase and GO, which were co-transferred to solid supports as an ultrathin film with the phospholipid. The presence of GO allowed to improve the identification of the signals for penicillinase detection by electronic excitation and luminescent emission. Also, films with GO increased the catalytic efficiency of the devices towards the hydrolysis of the beta-lactam ring. The presence of GO in the enzyme/lipid LB film not only tuned the catalytic activity of penicillinase, but also conserved its enzyme activity after weeks. The feasibility of the supramolecular device nanostructured as ultrathin films to detect penicillin was essayed in a capacitive electrolyte−insulator−semiconductor (EIS) sensor device. Viability as a penicillin sensor was demonstrated with capacitance/voltage and constant capacitance measurements, exhibiting regular and distinctive output signals for all concentrations used in this work. Therefore, these results may be related to the nanostructured system as an ultrathin film and the synergism between the compounds on the active layer, leading to a surface morphology that allowed a fast analyte diffusion owing to an adequate molecular accommodation, which preserved the penicillinase activity. Therefore, this work demonstrates that the incorporation of graphene oxide in LB films composed of penicillinase and DMPA boosts the biosensing properties of the hybrid ultrathin film as EIS devices for biosensing applications.