Oligonucleotide (T30695) modified gold nanoparticles (T30695-Au NPs) have been prepared and employed for quantification of lead ions (Pb(2+)) in blood. The detection of Pb(2+) ions is through the formation of Au-Pb alloys and oligonucleotide-Pb(2+) complexes that catalyze the H(2)O(2)-mediated oxidation of non-fluorescent Amplex UltraRed (AUR) to form a highly fluorescent oxidized AUR product. Surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) and inductively coupled plasma mass spectrometry (ICP-MS) revealed the formation of Au-Pb alloys on the surfaces of the 40T30695-Au NPs (i.e., the system featuring 40 molecules of T30695 per Au NP) in the presence of Pb(2+) ions, leading to increased catalytic activity for the H(2)O(2)-mediated oxidation of AUR. The fluorescence intensity (excitation/emission maxima: ca. 540/584 nm) of the oxidized AUR product is proportional to the concentration of Pb(2+) ions over the range 0.1-100 nM, with a linear correlation (R(2) = 0.99). The 40T30695-Au NP/AUR probe is highly selective toward Pb(2+) ions (by at least 200-fold over other tested metal ions). The 40T30695-Au NPs/AUR probe provided limits of detection (LOD, at a signal-to-noise ratio 3) for Pb(2+) ions of 0.05 and 0.1 nM, in Tris-acetate solution (5 mM, pH 8.0) without and with salt (150 mM NaCl, 5 mM KCl, 1 mM MgCl(2), and 1 mM CaCl(2)), respectively. Without conducting tedious sample pretreatment, the approach allows detection of Pb(2+) ions in blood samples, showing the potential of the 40T30695-Au NPs/AUR assay for on-site and real-time detection of Pb(2+) ions in biological samples.