Nanoparticle-Assisted NMR Spectroscopy: Enhanced Detection of Analytes by Water-Mediated Saturation Transfer

Abstract

Nanoparticle-assisted "NMR chemosensing" is an experimental protocol that exploits the selective recognition abilities of nanoparticle receptors to detect and identify small molecules in complex mixtures by nuclear Overhauser effect magnetization transfer. Although the intrinsic sensitivity of the first reported protocols was modest, we have now found that water spins in long-lived association at the nanoparticle monolayer constitute an alternative source of magnetization that can deliver a remarkable boost of sensitivity, especially when combined with saturation transfer experiments. The approach is general and can be applied to analyte− nanoreceptor systems of different compositions. In this work, we provide an account of the new method and we propose a generalized procedure based on a joint water−nanoparticle saturation to further upgrade the sensitivity, which ultimately endows selective analyte detection down to the micromolar range on standard instrumentation.

1H NMR Chemosensing of Potassium Ions Enabled by Guest‐Induced Selectivity Switch of a Gold Nanoparticle/Crown Ether Nanoreceptor

“…The limit of detection (defined as the minimum concentration of analyte producing a signal larger than three times the noise) was 0.6 mM, established on the basis of these data. In addition, when we repeated the experiment using STD (Saturation Transfer Difference) in place of NOE pumping, we obtained the same behavior reducing the acquisition time to 20 min (Figure S6). The affinity constant obtained by the fitting of the data of Figure b, which represents the apparent average affinity of the nanoparticle‐bound crown ether moieties for K + in the presence of tyramine and phloretate (each 5 mM), was 165±12 M −1 .…”

“…Indeed, the coating thiols are converted from neutral‐amphiphiles to cationic‐amphiphiles. This modification converts the nanoparticles into receptors for amphiphilic organic anions, as it enables the simultaneous occurrence of ion pairing and hydrophobic partition with this class of substrates (Figure b) …”

Molecular‐Dynamics‐Simulation‐Directed Rational Design of Nanoreceptors with Targeted Affinity

“…Finally, we sought to use the greater affinity of 3 ‐AuNP to achieve the detection of salicylate at micromolar concentrations. To achieve this aim, we used the water‐STD experiment recently developed by some of us . Compared to the NOE pumping, water‐STD delivers a much higher sensitivity through the simultaneous saturation of water and nanoparticle spins at high power.…”

“…Finally,wesought to use the greater affinity of 3-AuNP to achieve the detection of salicylate at micromolar concentrations.T oachieve this aim, we used the water-STD experiment recently developed by some of us. [14] Compared to the NOE pumping, water-STD delivers am uch higher sensitivity through the simultaneous saturation of water and nanoparticle spins at high power. However,d etecting analytes at low concentrations also requires nanoreceptors with sufficient affinity.W eanalyzed samples containing the nanoparticles as well as salicylate and 4-hydroxybenzoate both at 50 mm concentration in 9:1H 2 O/D 2 Os olutions ( Figure 6).…”

Tailoring Sensors and Solvents for Optimal Analysis of Complex Mixtures Via Discriminative 19F NMR Chemosensing

“…NMR-based sensing methods have recently emerged as a powerful way for the differentiation of structurally similar analytes and the precise identification of target analyte in complex mixtures. − Among these, 19 F NMR-based approaches benefit from the low natural occurrence of organofluorine compounds, which minimizes the interfering background signals. , Taking advantage of the high sensitivity of 19 F chemical shifts to the surrounding environment, analytes could produce discrete NMR signals upon reversible binding to 19 F-labeled sensors. This approach proved to be valuable for the detection of a wide range of analytes, including amines, N -heterocycles, and various ions. − Given the analogy between chromatographic peaks and singlet NMR signals, this detecting system behaves like in situ chromatography with a precise correlation between 19 F signals and the identity of analytes.…”