Competitive surface-enhanced Raman scattering assay for the 1,25-dihydroxy metabolite of vitamin D₃

This paper describes the development and preliminary testing of a competitive surface-enhanced Raman scattering (SERS) immunoassay for calcitriol, the 1,25-dihydroxy metabolite (1,25-(OH)₂-D₃) of vitamin D₃. Deficiencies in 1,25-(OH)₂-D have been linked to renal disease, while elevations are linked to hypercalcemia. Thus, there has been a sharp increase in the clinical demand for measurements of this metabolite. The work herein extends the many attributes of SERS-based sandwich immunoassays that have been exploited extensively in the detection of large biolytes (e.g., DNA, proteins, viruses, and microorganisms) into a competitive immunoassay for the low level determination of a small biolyte, 1,25-(OH)₂-D₃ (M_w = 416 g mol^-1). The assay uses surface modified gold nanoparticles as SERS labels, and has a dynamic range of 10-200 pg mL ^-1 and a limit of detection of 8.4 ± 1.8 pg mL^-1. These analytical performance metrics match those of tests for 1,25-(OH) ₂-D₃ that rely on radio- or enzyme-labels, while using a much smaller sample volume and eliminating the disposal of radioactive wastes. Moreover, the SERS-based data from pooled-patient sera show strong agreement with that from radioimmunoassays. The merits and potential utility of this new assay are briefly discussed.