The ongoing global pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to active research in its associated diagnostics and medical treatments. While quantitative reverse transcription polymerase chain reaction (qRT-PCR) is the most reliable method to detect viral genes of SARS-CoV-2, serological tests for specific antiviral antibodies are also important as they identify false negative qRT-PCR responses, track how effectively the patient's immune system is fighting the infection, and are potentially helpful for plasma transfusion therapies. In this work, based on the principle of localized surface plasmon resonance (LSPR), we develop an opto-microfluidic sensing platform with gold nanospikes, fabricated by electrodeposition, to detect the presence and amount of antibodies specific to the SARS-CoV-2 spike protein in 1μL of human plasma diluted in 1mL of buffer solution, within ∼30min. The target antibody concentration can be correlated with the LSPR wavelength peak shift of gold nanospikes caused by the local refractive index change due to the antigen-antibody binding. This label-free microfluidic platform achieves a limit of detection of ∼0.08ng/mL (∼0.5pM), falling under the clinical relevant concentration range. We demonstrate that our opto-microfluidic platform offers a promising point-of-care testing tool to complement standard serological assays and make SARS-CoV-2 quantitative diagnostics easier, cheaper, and faster.