Interleukin-6 (IL-6) is a critical biomarker in various cancers and inflammatory diseases, requiring precise and rapid detection for early diagnosis and monitoring. Conventional antibody-based biosensors face limitations due to challenges in antibody orientation, immobilization methods, and production, impacting their performance. To address these issues, we propose a two-fold solution: Nanobody engineering via CDR grafting and Solution-Immersed Silicon (SIS) sensor technology. CDR grafting allows for the direct conversion of well-characterized antibodies into nanobodies, eliminating the need for immunization and providing improved stability, surface immobilization, and reproducibility. These CDR-grafted nanobodies were integrated into an SIS-based biosensor platform, utilizing single-wavelength ellipsometry for label-free, highly sensitive IL-6 detection. The SIS sensor's high sensitivity, combined with the unique properties of nanobodies, enabled detection of IL-6 at concentrations as low as 4.5 fg/mL in serum. The platform distinguished between healthy individuals and cancer patients, including those with renal cell carcinoma and pancreatic cancer, with exceptional sensitivity. Compared to antibody and scFv variants, which exhibit higher binding affinities, the CDR-grafted nanobodies performed better despite having moderate binding affinity. This highlights the significance of capturing molecule orientation and the enhanced sensitivity of the SIS sensor. This combined approach of CDR grafting and SIS detection offers a scalable, cost-effective solution for IL-6 detection in clinical diagnostics.