Redox features of hexaammineruthenium(III) on MXene modified interface: Three options for affinity biosensing

In this article we describe construction of a bioreceptive interface for detection of a breast cancer biomarker carbohydrate antigen CA15-3. The conductive interface was patterned by a 2D nanomaterial MXene, to which a mixed layer containing sulfobetaine and carboxybetaine was electrochemically grafted through a diazonium moiety. Such a modified interface was then applied for covalent immobilisation of anti-CA15-3 antibody as a bioreceptive probe for detection of a breast cancer biomarker. Two different strategies were applied for final construction of an immunosensor i.e. an interface finally blocked by bovine serum albumin or an immunosensor without such modification. Finally, electrochemical reading was accomplished using a soluble redox probe Ru(NH₃)₆³⁺ ion for detection of CA15-3 in a clinically relevant range up to 50 U mL^-1. The results indicate that immunosensor based on non-blocked interface can be applied for biosensing using two modes of action: 1. differential pulse voltammetry (a plot of a peak current vs. analyte concentration) and 2. an electrochemical impedance spectroscopy (a plot of a charge transfer resistance vs. analyte concentration). The electrode blocked by bovine serum albumin (BSA) can be used by additional 3. mode of action: through detection of changes in the potential (a plot E_pvs. c). Additionally, we reveal and explain that Ru(NH₃)₆³⁺ is redox probe, which can be applied as interfacial molecular nanoscale ruler to distinguish negatively charged protein molecules present in the close proximity (≤ 6 nm) of the electrode (in our case adsorbed BSA molecules) from the negatively charged protein molecules at a larger distance (>12 nm) from the electrode (i.e. CA15-3 analyte).