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2.2.3.1.1 Uni-molecular Sensors

Bio-sensors utilise biomolecules to detect targets.  However, the format of bio-sensors varies, from free molecules (molecular sensors) to those conjugated to a substrate such as nanoparticles, nanowires, nanotubes, and thin-films.  Interaction of the target with the biosensor can be measured either directly or indirectly with readouts taking the form of changes in colour, fluorescence and electrical potential.  In array technologies, multiple biomolecules are fixed to a substrate allowing multiple analytes to be measured simultaneously.

A number of different sensors, based on a nanotechnology platform and incorporating single biomolecular species have been studied, including acetylcholinesterase (AChE)[i]^,[ii]m[iii], glucose oxidase[iv]^,[v], glucose dehydrogenase[vi], tyrosinase[vii]^,[viii].  For the agricultural and environmental industries it is the development of technologies based on AChE and tyrosinase which have gathered greatest interest.  AChE is an enzyme that is involved in nerve signalling in many different species, and is inhibited by organophosphate and carbamate pesticides, and heavy metals.  This inhibition can be measured by the failure of AChE to catalyse the conversion of substrate, or an analogue, that would normally result in a pH decrease.  This pH decrease can be measured electrochemically, or by using a dye molecule that is sensitive to pH changes and exhibits a change in colour or fluorescence.  Tyrosinase can catalyse the oxidation of phenolic compounds, which are present in many industrial wastewaters and are also used as pesticides.

Uni-molecular species can be either attached to an electrode or encapsulated in some form of matrix or other capsule, the nanostructured materials include liposomes[ix], self-assembled monolayers[x], carbon nanotubes[xi]^,[xii], nanoparticles^7,9,10. Each provides increased sensitivity through the greater surface area of the nanoparticle; allowing either more of the biomolecule to be present, or greater access to the analyte.  In the case of electronic detection the nanostructured material coated with biomolecule can either form the electrode itself or be used to coat the electrode (e.g. nanoparticles and self-assembled monolayers).  While electrode based systems offer the convenience of an electrical readout, encapsulation affords greater stability (for example AChE is stable for at least 50 days at 4C when enclosed within liposomes)[xiii].  Whichever approach is used, sensitivity is better than that required to detect minimum legal safe limits, capable of detecting pesticide down to levels of 10^-10 to 10^-11 M ^1,2.  While AChE sensors do not show specificity towards individual pesticides, they are cheap to manufacture and useful for an overall measurement, and therefore could be a tool for rapid assessment in the field, with follow-up as required in an analytical lab.  However, to date there have been no field trials using such sensors with non-purified samples, a point which will need to be addressed.