Extra- and Intracellular Monitoring of TGF-β Using Single Immunoplasmonic Nanoprobes

Transforming growth factor-β (TGF-β) is a well-known disease-related biomarker associated with fibrotic diseases, and initiation and progression of cancer in many organs. Therefore, quantitative and sensitive detection of TGF-β and similar biomarkers is crucial for patient treatment in the early stages of diagnosis. In many studies, the detection of TGF-β, an important profibrotic and cancer promoting cytokine, has been generally conducted by fluorescence or absorbance-based immunoassays. However, conventional methods for detecting TGF-β have problems including use of time-consuming sample pretreatment steps and multiple reagents for signal amplification and difficulty in real-time detection from living cells. Herein, we present a plasmon-based immunoassay for TGF-β using antibody-conjugated single gold nanoparticles that act as optically excellent intracellular and extracellular detection probes that do not require additional signal amplification. To detect TGF-β sensitively and selectively, we exploited the localized surface plasmon resonance (LSPR) property of antibody-conjugated plasmonic gold nanoparticles at a single particle level. By measuring the LSPR spectral shifts of the single plasmonic nanoprobes, TGF-β can be detected down to the picomolar level, which is comparable with the conventional methods but without significant interference from other proteins. The optimized plasmonic nanoprobes were applied to quantify and monitor the extracellular TGF-β level secreted from the cells under stress conditions, such as cancer, and exposure to toxic environments. Owing to the ease of cellular internalization of the nanoprobes, we directly image and detect increases in intracellular TGF-β levels in living cells under the given stress conditions without cell lysis. We envision that this strategy of using individual nanoparticles as sensors to monitor protein biomarkers in living cells could be applied for various biological assays and diagnosis.