Neonicotinoid (NN) pesticides have emerged globally as
one of the
most widely used agricultural tools for protecting crops from pest
damage and boosting food production. Unfortunately, some NN compounds,
such as extensively employed imidacloprid-based pesticides, have also
been identified as likely endangering critical pollinating insects
like honey bees. To this end, NN pesticides pose a potential threat
to world food supplies. As more countries restrict or prohibit the
use of NN pesticides, tools are needed to effectively and quickly
identify the presence of NN compounds like imidacloprid on site (e.g.,
in storage areas on farms or pesticide distribution warehouses). This
study represents a proof-of-concept where the colloidal properties
of specifically modified gold nanoparticles (Au-NPs) able to engage
in the rare intermolecular interaction of halogen bonding (XB) can
result in the detection of certain NN compounds. Density functional
theory and diffusion-ordered NMR spectroscopy (DOSY NMR) are used
to explore the fundamental XB interactions between strong XB-donor
structures and NN compounds, with the latter found to possess multiple
XB-acceptor binding sites. A fundamental understanding of these XB
interactions allows for the functionalization of alkanethiolate-stabilized
Au-NPs, known as monolayer-protected gold clusters (MPCs), with XB-donor
capability (f-MPCs). In the presence of certain NN
compounds such as imidacloprid, thef-MPCs subsequently
exhibit visual XB-induced aggregation that is also measured with absorption
(UV–vis) spectroscopy and verified with transmission electron
microscopy (TEM) imaging. The demonstratedf-MPC-aggregation
detection scheme has a number of favorable attributes, including quickly
reporting the presence of the NN target, requiring only micrograms
of suspect material, and being highly selective for imidacloprid,
the most prevalent and most important NN insecticide compound. Requiring
no instrumentation, the presented methodology can be envisioned as
a simple screening test in which dipping a cotton swab of an unknown
powder from a surface in af-MPC solution causesf-MPCs to aggregate and yield a preliminary indication of
imidacloprid presence.