Peptide–Au Cluster Probe: Precisely Detecting Epidermal Growth Factor Receptor of Three Tumor Cell Lines at a Single-Cell Level

[Image: see text] Alterations in protein (e.g., biomarkers) expression levels have a significant correlation with tumor development and prognosis; therefore, it is desired to develop precise methods to differentiate the expression level of proteins in tumor cell lines, especially at the single-cell level. Here, we report a precise and versatile approach of quantifying the protein expression levels of three tumor cell lines in situ using a peptide–Au cluster probe. The probe (Au(5)Peptide(3)) consists of a peptide with a specific cell membrane epidermal growth factor receptor (EGFR) targeting ability and an Au cluster for both cell membrane EGFR imaging using confocal microscopy and cell membrane EGFR counting by laser ablation inductively coupled plasma mass spectrometry. Utilizing the peptide–Au cluster probe, we successfully quantify the EGFR expression levels of SMMC-7721, KB, and HeLa cells at a single-cell level and differentiate the EGFR expression levels among these cell lines. The peptide–Au cluster probe, with the ability to differentiate the protein expression level of different cell lines, shows exceptional promise for providing reliable predictive and prognostic information of tumors at a single-cell level.