This supplementary material describes a label-free, single-molecule detection method using optical microcavities. The technique employs ultra-high-Q whispering gallery mode (WGM) resonators, which offer long photon lifetimes and high sensitivity. Detection occurs when molecules bind to the resonator surface, interacting with the evanescent tail of the WGM. The signal depends on the molecule's position relative to the intensity profile of the evanescent field, leading to a distribution of signals visible in histograms. A single-mode, tunable external cavity laser at 681.5 nm is coupled to a tapered optical fiber waveguide, which efficiently couples light into the microcavity. The microcavity is placed in a water-filled microaquarium, with the gap between the fiber and microcavity controlling the power coupling. The resonant wavelength and quality factor (Q) are determined by monitoring the power transmission spectrum. The Q factor is calculated using a resonator-waveguide coupling model. The study uses IL-2 antigen and its corresponding polyclonal antibody for detection. The antibody-antigen pair is immobilized on the microcavity surface through a series of steps involving Protein G, which binds to the antibody's Fc region. IL-2 is then introduced into the solution surrounding the microcavity. Single-molecule detection is performed using extremely low concentrations of IL-2 (300 aM), where binding events are rare and can be resolved. The detection mechanism relies on the thermo-optic effect, where changes in the resonant wavelength are caused by the binding of molecules. The Q factor decreases when a highly absorbing molecule binds, which is incorporated into the thermo-optic detection mechanism. The effect is generalized to other lower-Q resonant cavities by increasing the power levels. The study also includes a single molecule photo-bleaching experiment using Cy5-labeled antibodies. The fluorescent dye is photobleached using a 680 nm excitation source, and the resulting changes in the resonant wavelength and Q factor are monitored. The data shows sequential bleaching of two Cy5 molecules, indicating individual molecule detection. The thermal stability of the microcavities is addressed, with the use of water's positive thermo-optic coefficient to neutralize thermal shifts. The neutrality condition is achieved by optimizing the overlap of the WGM with the water, which is modeled using COMSOL Multiphysics. Statistical analysis of the data shows that the resonance position changes with concentration, and the binding events are resolved at the data acquisition rate. The results demonstrate the sensitivity and accuracy of the detection method, with the ability to detect single molecules in complex environments like fetal bovine serum. The method is applicable to a wide range of molecules and optical microcavities, making it a versatile tool for biological and chemical detection.This supplementary material describes a label-free, single-molecule detection method using optical microcavities. The technique employs ultra-high-Q whispering gallery mode (WGM) resonators, which offer long photon lifetimes and high sensitivity. Detection occurs when molecules bind to the resonator surface, interacting with the evanescent tail of the WGM. The signal depends on the molecule's position relative to the intensity profile of the evanescent field, leading to a distribution of signals visible in histograms. A single-mode, tunable external cavity laser at 681.5 nm is coupled to a tapered optical fiber waveguide, which efficiently couples light into the microcavity. The microcavity is placed in a water-filled microaquarium, with the gap between the fiber and microcavity controlling the power coupling. The resonant wavelength and quality factor (Q) are determined by monitoring the power transmission spectrum. The Q factor is calculated using a resonator-waveguide coupling model. The study uses IL-2 antigen and its corresponding polyclonal antibody for detection. The antibody-antigen pair is immobilized on the microcavity surface through a series of steps involving Protein G, which binds to the antibody's Fc region. IL-2 is then introduced into the solution surrounding the microcavity. Single-molecule detection is performed using extremely low concentrations of IL-2 (300 aM), where binding events are rare and can be resolved. The detection mechanism relies on the thermo-optic effect, where changes in the resonant wavelength are caused by the binding of molecules. The Q factor decreases when a highly absorbing molecule binds, which is incorporated into the thermo-optic detection mechanism. The effect is generalized to other lower-Q resonant cavities by increasing the power levels. The study also includes a single molecule photo-bleaching experiment using Cy5-labeled antibodies. The fluorescent dye is photobleached using a 680 nm excitation source, and the resulting changes in the resonant wavelength and Q factor are monitored. The data shows sequential bleaching of two Cy5 molecules, indicating individual molecule detection. The thermal stability of the microcavities is addressed, with the use of water's positive thermo-optic coefficient to neutralize thermal shifts. The neutrality condition is achieved by optimizing the overlap of the WGM with the water, which is modeled using COMSOL Multiphysics. Statistical analysis of the data shows that the resonance position changes with concentration, and the binding events are resolved at the data acquisition rate. The results demonstrate the sensitivity and accuracy of the detection method, with the ability to detect single molecules in complex environments like fetal bovine serum. The method is applicable to a wide range of molecules and optical microcavities, making it a versatile tool for biological and chemical detection.