Carboxylated/Oxidized Diamond Nanoparticles for Quantifying Immunoglobulin G Antibodies Using Mass Spectrometry

Antibody-conjugated nanodiamonds (NDs) have recently received considerable attention from researchers in the biomedical fields. Accurately quantifying antibody (Ab) surface coverage on ND surfaces is critical to improve sensitivity in drug delivery and immunotherapy applications. We present for the first time that high affinity carboxylated/oxidized ND particles are useful to directly quantify antibodies, i.e., immunoglobulin G (IgG), in the nanomolar (nM) concentration range using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The adsorption of IgG molecules with two different NDs, i.e., carboxylated high-pressure high-temperature ND (HPHT ND) and air-oxidized detonation ND (oxDND), is shown and is quantitatively analyzed using the Langmuir–Freundlich isotherm model. The lowest detectable concentrations of immobilized IgG are ∼10 nM for HPHT ND and ∼4 nM for oxDND. The lowest detectable surface coverages of IgG molecules per single HPHT ND particle and oxDND particle are ∼1140 and ∼700 IgG, respectively. The IgG monolayer is immobilized on the oxDND surface, and the maximum adsorption capacity of oxDND is higher than that of HPHT ND. Moreover, the orientation of most hCG (human chorionic gonadotropin)-β Abs on the oxDND surface is probably tail-on, as verified by analyzing the ion signal of the hCG antigen recognized by the hCG-β Ab-oxDND conjugate and by the hCG-β Ab-Mag-Beads Protein G conjugate. Compared to the commercial Ab-Mag-Beads Protein G conjugate, the hCG-β Ab-oxDND conjugate does not require Protein G as a linker to the antibody and can be used as a simple antibody–antigen platform in immunodiagnostics.