A transition metal borylene
complex stabilized by a non-
carbonyl ligand set: formation
by spontaneous halide loss to
give an extremely short metal-
-boron bond (video).
(work by David Addy)
Sensors for Anions and Neutral Molecules
The binding of anions by receptor molecules is an area of enormous research interest, which is relevant to biological systems, and has widespread applications, e.g. in catalysis and sensors. From the viewpoint of sensor design, key features are selectivity (i.e. the recognition of the target anion over possible contaminants) and signalling (i.e. the triggering of a measurable response on anion binding). A wide variety of chemical strategies have been employed to selectively bind anions, and we have been using group 13 based Lewis acids in this area – with the selectivity for the target anion based either on the strength of the donor/acceptor bond formed (e.g. for fluoride) or on the complementary geometry of the binding sites and target anion (e.g. cyanide or acetate).
Ongoing work is centred on the design of novel multifunctional Lewis acids and mixed Lewis acid/base systems for the selective detection of fluoride (and its conjugate acid HF), together with the exploitation of such receptors in the sensing of fluorinated chemical warfare agents (CWAs). Key receptor design principles are (i) the known selectivity of fluoride binding by boronic esters; and (ii) electrochemical, colorimetric or fluorescence based reporting (e.g. utilizing ferrocene units, Fc). A further target is the selective sensing of cyanide (or hydrogen cyanide) in the presence of potentially competitive anions (e.g. halides).
Key future targets with a view to device construction are improvements in sensitivity (i.e. lower detection limits) and kinetics of response. Two approaches are currently being appraised: (i) the incorporation of suitable redox-matched dyes for fast, sensitive colorimetric sensing and (ii) the development of catalytic sensors. The latter approach relies on identifying host/guest complexes formed between the receptor and the target analyte which will catalyze an orthogonal reaction. Our approach utilizes electron transfer chemistry as the basis for catalysis, e.g. of a dye bleaching reaction.
(left) Crystal structure of a cyanide
receptor based on a tolan backbone.
(centre) Ferrocene-functionalized Lewis
acids offer electrochemical and/or
colorimetric sensing platforms. See:
(right) Electrochemical response to
anion binding (here a ca. 500 mV shift
on addition of cyanide). See:
New chemistry (video)
A 1,2-difunctionalized ferrocene
based Lewis acid. Click here.