Combined Detectionof Serum Brain-Derived NeurotrophicFactor and Interleukin‑6 for Evaluating Therapeutic Efficacyin Major Depressive Disorder

The assessment of major depressive disorder (MDD) treatment efficacy typically relies on clinician-rated scales of depressive symptoms with a notable absence of predictive biomarkers. In this study, a concentration ratio of serum brain-derived neurotrophic factor (BDNF) to interleukin-6 (IL-6) was identified as a promising predictive biomarker. To overcome the significant disparity in blood levels between BDNF (ng·mL^–1 range) and IL-6 (pg·mL^–1 range), a dual-pathway signal amplification method involving small carbon dots (Fe-CDs and Pb-CDs) was employed. Fe-CDs exhibited a nanozyme-like behavior, effectively enhancing the signal through solution reactions, rendering it ideal for IL-6 detection at low concentrations. Meanwhile, Pb-CDs, laden with numerous signal molecules, amplified signals via the surface pathway and were suitable for BDNF detection. This dual signal output approach met the distinct sensitivity needs for quantifying IL-6 and BDNF, achieving detection limits of 0.1 pg·mL^–1 and 0.02 ng·mL^–1, respectively. Analysis of clinical samples showed that the concentration ratio of BDNF to IL-6 not only effectively differentiates MDD patients from healthy controls but also strongly correlates with individual treatment responses, affirming its value as a biomarker for MDD diagnosis and treatment efficacy evaluation.