Ferrocenyl Compounds as Alternative Redox Labels for Robust and Versatile Electrochemical Aptamer-Based Sensors

This study explores the potential of seven ferrocenyl (Fc) compounds with cross-linking groups as alternative redox labels to methylene blue (MB) for electrochemical aptamer-based (E-AB) sensors. The cross-linking efficiency, formal potential (<i>E</i>⁰'), and electrochemical durability of these compounds were evaluated. Compound <b>Fc1a-X</b> exhibited superior performance, characterized by efficient cross-linking, a moderate and pH-insensitive <i>E</i>⁰', and enhanced durability during repeated potential scans. The attachment of <b>Fc1a-X</b>, which includes a 3-carbon chain spacer and an <i>N</i>-hydroxysuccinimide-ester cross-linking group, to an amine-terminated monolayer on a Au electrode demonstrated high cross-linking efficiency, which is critical for achieving high sensitivity. The <i>E</i>⁰' of <b>Fc1a-X</b> attached to the aptamer monolayer was 0.14 V, which is within the optimal range of -0.2 to 0.2 V vs Ag/AgCl. Square wave voltammetry showed that the peak potential and current of <b>Fc1a-X</b> are pH-insensitive, which is critical for versatile use. In serum, <b>Fc1a-X</b> maintained stable peak current levels without a gradual decrease after an initial rapid decrease during the first 2 h with considerably less reduction over 12 h compared to MB. Using <b>Fc1a-X</b> as the redox label, an E-AB sensor effectively detected doxorubicin in serum, covering the clinical range. These findings suggest <b>Fc1a-X</b> as a promising candidate for developing robust, versatile, and sensitive E-AB sensors.