Polyaniline-coated nanoelectrospray emitters treated with hydrophobic polymers at the tip

In nanoelectrospray ionization (nanoESI) techniques, the hydrophilic character of the emitters generally produces large bases for the Taylor cones, thereby generating relatively large droplet sizes and consequently reduced sensitivity. In order to minimize this 'wetting' effect in nanoESI, a model hydrophobic polymer (an acrylic paint) was coated at the tip of commercial polyaniline (PANI)-coated emitters, and their performance was compared with that of unmodified PANI emitters using oxytocin and neuropeptide Y (NPY) solutions. In experiments with oxytocin, the hydrophobic emitter produced higher signal intensities (up to 3.6 times) as well as higher signal-to-noise ratios (33% increase) than those from the unmodified PANI emitter. In addition, the hydrophobic emitter showed reusability and a slightly wider linear dynamic range (10 nM to 50 μM, r² = 0.9938) than that from the unmodified PANI emitter (10 nM to 10 μM, r² = 0.9904). In the case of NPY, the hydrophobic emitter also enabled an approximately 350-fold overall increase in sensitivity than the unmodified PANI emitter (70 zmol vs. 25 amol). The enhanced performance of the hydrophobic emitter clearly indicates potential for further increases in nanoESI sensitivity using emitters with tailored hydrophobic overcoatings.