Antibody-Conjugated Nanodiamonds as Dual-Functional Immunosensors for In Vitro Diagnostics

Nanodiamonds (NDs) are carbon nanoparticles with a largerefractiveindex, a high density, and exceptional chemical stability. When excitedby green light, they can emit bright red fluorescence from implantednitrogen-vacancy (NV) centers. Taking advantage of these properties,we have developed antibody-conjugated NDs as in vitro diagnostic sensorsfor two complementary assays: particle-enhanced turbidimetric immunoassay(PETIA) and spin-enhanced lateral flow immunoassay (SELFIA). To achievethis goal, monocrystalline diamond powders (& SIM;100 nm in diameter)with or without NV implantation were first treated in molten KNO3 to reduce their size and shape inhomogeneity, followed bysurface carboxylation in strong oxidative acids and non-covalent conjugationwith antibodies in water. PETIA and SELFIA were carried out separatelywith a microplate reader and a magnetically modulated fluorescenceanalyzer. Using C-reactive protein (CRP) as the target antigen, wefound that anti-CRP-conjugated NDs exhibited high colloidal stabilityover 1 month at 4 & DEG;C in buffer solution. The limits of detectionfor 3 & mu;L of CRP sample solution were 0.06 & mu;g/mL and 1ng/mL with variation coefficients of less than 10 and 15% for PETIAand SELFIA, respectively. These two methods together provide a detectionrange of 1 ng/mL-10 & mu;g/mL, potentially useful for clinicalapplications. This work represents the first practical use of roundedmonocrystalline NDs as in vitro diagnostic reagents.

Automated summary

Nanodiamonds (NDs) have been developed as in vitro diagnostic sensors for two complementary assays: particle-enhanced turbidimetric immunoassay (PETIA) and spin-enhanced lateral flow immunoassay (SELFIA). These NDs, with a largerefractiveindex, emit bright red fluorescence when excited by green light. In experiments using CRP as the target antigen, anti-CRP-conjugated NDs displayed high colloidal stability and low limits of detection.