Site-specific conjugated antibody-element tags for single-cell ICP-MS.

Inductively coupled plasma mass spectrometry (ICP-MS) has demonstrated high sensitivity, accuracy, and multi-element detection capacity for biomolecule analysis. However, traditional ICP-MS-used antibody-elemental tags (AETs) are typically constructed through random antibody modification, resulting in structural heterogeneity that compromises binding affinity, specificity, and analytical performance. To overcome this limitation, we developed a traceless, site-specific conjugation strategy that enables precise attachment of a single-stranded DNA (ssDNA) to the K248 residue of IgG antibodies. The "traceless" feature ensures that no residual peptide remains on the antibody after labeling, thereby preserving its native structure and binding activity. The ssDNA serves as a bridge for hybridization with europium (Eu)-loaded MS2 capsid nanoparticles, which act as signal amplifiers by delivering a large payload of metal atoms. Through the integration of precise antibody modification and high-capacity MS2 elemental carriers, the resulting site-specific AETs (ssAETs) exhibit exceptional structural uniformity, stability, and sensitivity. Using human epidermal growth factor receptor 2 (HER2) as a model, the AETs synthesized through our strategy demonstrated exceptional specificity and quantitative performance, achieving 1.8-fold stronger single-cell signals and labeling 1.5 times more HER2-positive cells compared with randomly conjugated antibodies. This traceless, site-specific ssAET platform offers a versatile and precise approach for highly sensitive and quantitative single-cell bioanalysis using ICP-MS.