Surface Functionalization for Advanced Sample Preparation in TXRF

Abstract

In total reflection X-Ray fluorescence spectroscopy (TXRF), careful and reproducible sample preparation is crucial to satisfy the thin film approximation. [IJ Typically, a dilute analyte is pipetted on a reflector and dried; ideally into a small residual spot that is well aligned with the detector axis and smaller than the field of view (FOV) of the detector. However, droplet drying typically suffers from the so-called coffee stain effect, which makes control over position and spread of the analyte difficult. To avoid the coffee stain effect, the surface of the reflector can be hydrophobized to reduce the size of the residue. [2] However, for higher concentrated samples this can violate the thin film approximation. Previous work has tackled this challenge by picoliter droplet printing. (3] Here, we present a simple and low cost approach creating a well-defined hydrophilic area on a hydrophobized reflector, providing control over position and pattern of the residue. Functionalized quartz reflectors were prepared using a photomasking technique (Fig IA). [41 First, a self-assembled monolayer (SAM) was prepared creating a hydrophobic surface layer. Next, UV ozone cleaning, combined with a photomask was used to remove the SAM in a well-defined region, creating a clean, hydrophilic spot. The photomask was designed in such a manner that the hydrophilic area matches the FOV of the detector, maximizing the spread of the analyte and avoiding violation of the thin-film approximation for concentrated samples (Fig 1B). In Fig IC, we show the confinement of fluorescent (visible light) nanoparticles within the hydrophilic area on the reflector after drying from solution. The developed method is easy to implement, low cost, versatile and applicable to various reflector materials and analytes, providing enhanced control over sample preparation for TXRF.