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Apoptosis is a main type of cell death in which caspase-3 plays a key role. In this work, a simple, fast and sensitive immunoassay for caspase-3 is established by using inductively coupled plasma mass spectrometry (ICP-MS) detection and signal enhancement gold nanoparticle (Au-NP) labelling of the secondary antibody (IgG). After the immunoreaction of caspase-3, primary antibody and Au-NP labeled IgG, the concentration of caspase-3 was determined by ICP-MS analysis of the Au-NPs released from the immunocomplex. Under the optimized conditions, a limit of detection (LOD, 3σ) of 0.42 ng mL(-1) (0.31 nM) was obtained for caspase-3, with a dynamic range of 1-200 ng mL(-1) and a relative standard deviation of 4.1% (c = 5 ng mL(-1), n = 11). The proposed method has good specificity towards caspase-3. To demonstrate the application potential of the proposed method, the cell lysates of Hg(2+)-induced HepG2 cells were analyzed, and it was found that the concentration of caspase-3 increased when increasing the incubation concentration of Hg(2+). This method is easy, sensitive and selective, and has excellent capability of tolerating a complex biological matrix, indicating the potential of the ICP-MS-based assay in the study of apoptosis.

Upconversion nanoparticles (UCNPs) have received increasing attention due to their unique optical properties. Recognizing that UCNPs are lanthanide-doped nanoparticles, we incorporated UCNPs into an immunoassay with inductively coupled plasma mass spectrometry (ICP-MS) detection for the determination of specific proteins, e.g., alpha-fetoprotein (AFP). The sensitivity of the assay was enhanced because of the ICP-MS detection of UCNPs that contained large numbers of lanthanide elemental tags. Conjugates of UCNPs and antibodies were prepared and the morphology of the conjugates was characterized by transmission electron microscopy. After a sandwich immunoreaction, the AFP was determined by the ICP-MS analysis of UCNPs. Under the optimized conditions, a limit of detection (3σ) of 0.31 ng mL−1 based on 89Y signal and 0.22 ng mL−1 based on 174Yb signal was obtained for AFP, with a dynamic range of 0.5–35 ng mL−1 and a relative standard deviation of 4.8% (c = 5 ng mL−1, n = 9). The developed method was applied to the determination of AFP in human serum and the recovery for the spiked sample was in the range of 98.6–123%. The proposed method is simple, rapid, selective and sensitive, and has a good tolerance for the complex biological matrix, indicating great potential for the application of UCNP in biological research as an elemental tag.