Continuous monitoring of bisulfide variation in microdialysis effluents by on-line droplet-based microfluidic fluorescent sensor

• A gold nanoparticle-based probe was synthesized for rapid and sensitive detection of bisulfide.
• A droplet-based microfluidic fluorescent sensor was constructed by incorporation of the probe.
• Continuous monitoring of the bisulfide variation in microdialysis effluents was realized.

We demonstrate a novel fluorescent sensor for real-time and continuous monitoring of the variation of bisulfide in microdialysis effluents by using a nanoparticle–glutathione-fluorescein isothiocyanate (AuNP–GSH-FITC) probe coupled with on-line droplet-based microfluidic chip. The AuNP–GSH-FITC fluorescent probe was firstly developed and used for bisulfide detection in bulk solution by quantitative real-time PCR, which achieved a linear working range from 0.1 μM to 5.0 μM and a limit of detection of ~50 nM. The response time was less than 2 min. With the aid of co-immobilized thiol-polyethylene glycol, the probe exhibited excellent stability and reproducibility in high salinity solutions, including artificial cerebrospinal fluids (aCSF). By adding 0.1% glyoxal to the probe solution, the assay allowed quantification of bisulfide in the presence of cysteine at the micro-molarity level. Using the AuNP–GSH-FITC probe, a droplet-based microfluidic fluorescent sensor was further constructed for online monitoring of bisulfide variation in the effluent of microdialysis. By using fluorescence microscope-charge-coupled device camera as the detector, the integrated microdialysis/microfluidic chip device achieved a detection limit of 2.0 μM and a linear response from 5.0 μM to 50 μM for bisulfide in the tested sample. The method was successfully applied for the on-line measurement of bisulfide variation in aCSF and serum samples. It will be a very useful tool for tracking the variation of bisulfide or hydrogen sulfide in extracellular fluids.