Using a Functional Nanogold Membrane Coupled with Laser Desorption/Ionization Mass Spectrometry to Detect Lead Ions in Biofluids

Abstract

AbstractIn this paper, a matrix‐free strategy based on the analysis of nitrocellulose membranes (NCMs) modified with gold nanoparticles (AuNPs) is described, using pulsed‐laser desorption ionization mass spectrometry (LDI‐MS) for comprehensive quantification of lead ions (Pb) with a sub‐nanomolar sensitivity in complicated biofluids. The strong hydrophobic interactions between the NCM and bovine serum albumin (BSA) lead to trapping of BSA‐modified AuNPs (BSA‐AuNPs), resulting in the formation of a nanocomposite film of BSA‐AuNPs on the membrane (BSA‐AuNP/NCM). When the AuNPs interact with thiosulfate (S2O32−) ions in solution, Au+·S2O32− complexes form on the AuNP surfaces, facilitating the deposition of Pb atoms in the form of PbAu alloys in the presence of Pb2+ ions. The BSA‐AuNP/NCM nanocomposite is a useful LDI‐MS matrix because it allows: i) the soft and enhanced ionization of Pb−Au alloys from the AuNP surfaces; ii) accurate mass measurements (precision: 5 ppm) of Au, Pb, and Au–Pb ions; iii) the extraction of Pb2+ ions from very‐dilute aqueous solutions (1.0 × 10−9 M); and iv) analyses to be performed directly after the introduction of the substrate into the mass‐analysis LDI spectrometer (i.e., without the need for an elution process). In contrast to the noisy spectra typically obtained when using other AuNP‐assisted LDI approaches, our homogeneous BSA‐AuNP/NCM nanocomposite provides clean mass spectra with fewer and weaker signals from AuNP‐associated interfering species. As a result, the BSA‐AuNP/NCM substrates allow sensitive LDI‐MS detection of analytes with low mass‐to‐charge ratios. Under optimal conditions, this LDI‐MS approach provides high sensitivity, a wide dynamic detection range (1.0 × 10−9–5.0 × 10−6 M), and a high selectivity toward Pb2+ ions (with at least a 100‐fold concentration tolerance relative to other metal ions). The BSA‐AuNP/NCM nanocomposite also provides excellent shot‐to‐shot (<5%) and sample‐to‐sample (<5%) reproducibilities of ion production because of its homogeneous substrate surface, thereby enabling LDI‐based measurements to a consistent quantification of Pb2+ ions in real samples (e.g., urine, whole blood).