Evaporative preconcentration and cryopreservation of fluorescent analytes using superhydrophobic surfaces

Abstract

The preconcentration of analytes is important in biochemical analysis as it offers the ability to detect trace species, and increase signal-to-noise ratios when using optical sensing. A strong advantage of the evaporation technique lies in its ability to operate without any external energy source. We have found that the evaporation of droplets on typical surfaces is susceptible to the coffee staining effect that leaves remnant fluorescence on the surface. This is less pronounced with hydrophobic surfaces but shown here to be still significant. The use of a superhydrophobic surface is demonstrated to engender minuscule fluorescent material losses. The difficulty of locating a droplet on this surface is addressed here using wells in which a depth of 0.51 mm with 7 μl analyte volume was found to accommodate a comfortable tilt working range of up to 19°. Experiments revealed preconcentration rates to be faster with lower humidity but maximal concentrations were achieved using 50% relative humidity. We have also shown the ability to attain solid–liquid state changes of the spherical shaped droplets of analytes using sub-zero temperatures. This allows for tandem preconcentration and cryopreservation without any analyte transfer, leading to the advantages of minimal sample handling loss and contamination. With EGFP used as a model protein, we found no deterioration in fluorescence for up to 8 freeze–thaw cycles.