Ultrasensitive and Simultaneous Detection of Two Cytokines Secreted by Single Cell in Microfluidic Droplets via Magnetic-Field Amplified SERS

A surface-enhanced Raman scattering (SERS)-microfluidic droplet platform for the rapid, ultrasensitive and simultaneous detection of vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) secreted by a single cell is presented. The high-throughput water-in-oil droplets containing an individual cell along with four kinds of immune-particles (antibody-conjugated silver nanoparticles or magnetic beads, AgNPs@Ab₁ and MNs@Ab₂) in each were achieved by a cross-typed microfluidic chip, and then they were captured by a collection channel array for SERS measurements. In the appearance of cytokines secreted by one cell, AgNPs@Ab₁ can be linked onto the surface of MNs@Ab₂ through the immune-recognition to form an immune-sandwich, which makes the "turn on" SERS signal of the Raman reporters previously laid on the surface of MNs due to the adjacent AgNPs. Furthermore, the second SERS signal amplification is from the magnetic field-induced spontaneous collection effect, which brings 75 times enhancement for SERS signal. Additionally, the encapsulation of cytokines in an isolated droplet permits an accumulation effect of targets with time. Owing to the dual signal enhancement and the accumulation effect, such few cytokines secreted by a single cell become detectable and a limit of detection is achieved as 1.0 fg/mL in one droplet. By using this ultrasensitive SERS-microdroplet method, the VEGF and IL-8 secretions from several cells in one droplet were explored and the data show that the cell-cell interaction may promote angiogenesis of cancer cells through the up-regulation of VEGF and IL-8.