**5. Conclusions**

In the past two decades, cell secretion monitoring has gained increasing attention. To understand the heterogeneity of protein secretion in single cell level, various optic methods have been developed including labelling and label-free techniques. The label-free techniques such as plasmonic sensors, PCR, EOT, are able to recognize the secreted proteins by sensing the changes of refractive index on the sensor interfaces. The limit of detection differs from 20 pg/mL to 50 ng/ mL. The signal transduction during the analyte-ligand interaction does not require *Advanced Biosensing towards Real-Time Imaging of Protein Secretion from Single Cells DOI: http://dx.doi.org/10.5772/intechopen.94248*

the involvement of any additional materials, therefore, the label-free techniques allow simple operation and fast response, suitable for real-time imaging. The labelling techniques such as fluorescence microscopy utilize fluorophore-labeled bioreceptors to recognize the proteins. Different from the label-free techniques, the fluorescence based methods conventionally use two recognition elements to form a sandwich complex, one of them as the capture element, the other as the signal transducer (the fluorophore-labeled bioreceptor). The limit of detection differs from 0.1 pg/mL to 20 ng/mL. Even though the two recognition element approaches involve additional processing and washing steps, which prolong the response time, the extraordinary sensitivity and low background improve the reliability and allow multiplex detection, making it very attractive to some applications.

For instance, TIRFM has demonstrated its possibilities on real-time imaging of protein secretion from single cells by utilizing both high sensitivity from fluorescence microscopy, and wash-free process due to the evanescent field created by total internal reflection. In spite of the achievements mentioned above, intensive work is demanded in future such as understanding of cell secretion dynamics, improving the detection specificity, developing multiplex real-time imaging platform, and investigating the interactions of different cells, and their intercellular communication.
