Probing Low-Copy-Number Proteins in a Single Living Cell

Single-cell analysis techniques are essential for understanding the microheterogeneity and functions of cells. Low-copy-number proteins play important roles in cell functioning, but their measurement in single cells remains challenging. Herein, we report an approach, called plasmonic immunosandwich assay (PISA), for probing low-copy-number proteins in single cells. This approach combined in vivo immunoaffinity extraction and plasmon-enhanced Raman scattering (PERS). Target proteins were specifically extracted from the cells by microprobes modified with monoclonal antibody or molecularly-imprinted polymer (MIP), followed by labeling with Raman-active nanotags. The PERS detection, with Raman intensity enhanced by 9 orders of magnitude, provided ultrasensitive detection at the single-molecule level. Using this approach, we found that alkaline phosphatase and survivin were expressed in distinct levels in cancer and normal cells, and that extended culture passage resulted in reduced expression of survivin. We further developed acupuncture needle-based PISA for probing low-copy-number proteins in living bodies.