Gold nanoparticle mixture retention test with single particle detection: A fast and sensitive probe for functional pore sizes of ultrafiltration membranes

Ultrafiltration is often one of the most critical steps in the production of high-purity water, chemicals and biopharmaceutical products. Pore size characterization in ultrafiltration membranes is an important yet challenging task. This work presents a long sought-after solution to measure ultrafiltration membrane pore size in a fast, sensitive and reliable way. Ultrafiltration membranes made of different polymeric materials were challenged with a mixture of 20, 30, 40 and 60 nm gold particles, at a concentration of approximately 7 × 107 particles/mL for each nominal size or 2, 8, 19, 66 μg/L gold, respectively, to determine the filtration performance over a wide size range. The low particle loading in the feed suspension resembles the environment where these membranes are practically used, for example, in ultrapure water. In the retention test both sizes and counts of gold nanoparticles (GNPs) were quantitatively measured in feed and permeate suspensions by single particle inductively coupled plasma mass spectrometry (spICP-MS). The GNP retention by membrane adsorption was minimized by adding anionic surfactant sodium dodecyl sulfate (SDS) in the feed suspension. As a result, the cut-off size of fully retained particles was in good agreement with the membranes' functional pore sizes or bubble point sizes measured by gas-liquid displacement porosimetry. The retention test showed that two tightest membranes out of the five tested ones reached full retention of particles between 40 and 60 nm in diameter. This study revealed significant discrepancies between the functional pore sizes and the manufacturers’ removal ratings. It was demonstrated that spICP-MS quantitatively and reliably measures 20 nm GNPs in permeate suspensions at concentrations as low as 50 particles/mL, equivalent to 0.003 ng/L. This is over three orders of magnitude lower than GNP limit of quantification (LOQ) by traditional ICP-MS analysis, over five orders of magnitude lower than the LOQ by liquid nanoparticle sizer (LNS), or over eight orders of magnitude lower than the LOQ of dextran by refractive index measurement. Unlike light-based particle techniques, spICP-MS is highly selective and element specific for gold, so dissolved gas or other particulates like degradants of polymeric membranes do not interfere with the analysis. The filtration part of the retention test was more straightforward and faster since no pre-test rinsing was required. The size-resolved particle retention data for