title: High-speed interferometric detection of label-free immunoassays on the biological compact disc.
abstract: BACKGROUND
  We describe a direct-detection immunoassay that uses high-speed optical interferometry on a biological compact disc (BioCD).
  
  
  METHODS
  We fabricated phase-contrast BioCDs from 100-mm diameter 1.1-mm thick borosilicate glass disks coated with a 10-layer dielectric stack of Ta2O5/SiO2 that serves as a mirror with a center wavelength at 635 nm. The final layer is a lambda/4 layer of SiO2 onto which protein patterns are immobilized through several different chemical approaches. Protein on the disc is scanned by a focused laser spot as the disc spins. Interaction of the light with the protein provides both a phase-modulated signal and a local reference that are combined interferometrically to convert phase into intensity. A periodic pattern of protein on the spinning disc produces an intensity modulation as a function of time that is proportional to the surface-bound mass. The binding of antigen or antibodies is detected directly, without labels, by a change in the interferometric intensity. The technique is demonstrated with a reverse assay of immobilized rabbit and mouse IgG antigen incubated against anti-IgG antibody in a casein buffer.
  
  
  RESULTS
  The signal increased with increased concentration of analyte. The current embodiment detected a concentration of 100 ng/L when averaged over approximately 3000 100-micron-diameter protein spots.
  
  
  CONCLUSIONS
  High-speed interferometric detection of label-free protein assays on a rapidly spinning BioCD is a high-sensitivity approach that is amenable to scaling up to many analytes.
authors.affiliation: Department of Physics, Purdue University, West Lafayette, IN 47907, USA.
authors.forename: Ming
authors.initials: M
authors.lastname: Zhao
authors.forename: David
authors.initials: D
authors.lastname: Nolte
authors.forename: Wonryeon
authors.initials: W
authors.lastname: Cho
authors.forename: Fred
authors.initials: F
authors.lastname: Regnier
authors.forename: Manoj
authors.initials: M
authors.lastname: Varma
authors.forename: Greg
authors.initials: G
authors.lastname: Lawrence
authors.forename: John
authors.initials: J
authors.lastname: Pasqua
mesh_terms: D000818:Animals; D002138:Calibration; D007118:Immunoassay / Q000295:instrumentation* / Q000379:methods*; D007074:Immunoglobulin G / Q000032:analysis* / Q000276:immunology; D007368:Interferometry / Q000295:instrumentation; D007834:Lasers; D008027:Light; D051379:Mice; D011506:Proteins / Q000032:analysis*; D011817:Rabbits; D012680:Sensitivity and Specificity; D013997:Time Factors
chemical_list: D007074:Immunoglobulin G; D011506:Proteins