The perceived utility of biosensors analytical devices based on molecules that recognise specific biological substances within the consumer health, biotechnology and biomedical industries is well established.
However, the commercial realisation of such systems has been slow, as early biosensor concepts were largely impractical, expensive and not suited to large scale manufacture.
Biorecogntion systems
These issues are now being addressed through the development of novel biorecognition systems coupled with transducer technologies amenable to the mass-production techniques developed for the microelectronics, printing and photography industries.
One such approach utilises a simple reflection hologram as the interactive element in a (bio)chemical sensor.
The holographic image is stored in a thin polymeric film that is chemically sensitised to react with a specific substance in, for example, a bioassay or biological fluid.
Target substance
During the test, the target substance reacts with the polymer leading to an alteration in the hologram either a change in brightness, image, colour or position.
For example, a sensor hologram has been constructed that can be used to indicate the presence of alcohol on human breath. In Fig. 1. the left sensor shows no alcohol vapour and indicates asafe to drive', while the right sensor indicates the presence of alcohol vapour and indicates aunsafe to drive'.
The advantage of those sensors is their simplicity, inexpensiveness and the fact that they require no external power other than the availability of a light source.
For applications where quantitative measurements are desirable a holographic diffraction grating is used.
The hologram is incorporated into the analyte-specific responsive polymer film using diffusion systems involving immersion of the film in a solution of silver salt, followed by agitation in a solution of a bromide.
Photosensitive silver bromide
In this way, ultra-fine grains of photosensitive silver bromide (<20nm in diameter) are precipitated within the matrix of the polymer film.
The holographic image is recorded by passing a single collimated laser beam through the film backed by a mirror.
Interference between the incident and reflected beams, followed by a conventional photographic development step, creates a modulated refractive index in the form of silver fringes lying in planes parallel to the polymer surface and approximately half a wavelength apart within the 10mm thickness of the polymeric film.
Under white light illumination, the developed hologram acts as a reflector of the light for a specific narrow band of wavelengths and holographically recreates the monochromatic image from the original mirror used in its construction.
The constructive interference between partial reflections from each fringe plane gives a characteristic spectral peak with a wavelength governed by the Bragg equation (lmax=2ndcosq), where d is the fringe separation distance, n is the average refractive index and q is the angle of illumination to the normal.
Changing physical dimensions
The polymer matrix supporting the holographic image is chemically sensitised to contain appropriate areceptor' molecules and when complementary ligands bind these they cause the polymer film to change its physical dimensions by swelling or contracting by either absorbing or expelling water.
This change in volume alters the spacing between the embedded holographic silver fringes and thus generates observable changes in the reflection hologram.
For example, swelling the polymeric layer increases the holographic fringe separation and causes a longer wavelength of light to be reflected whilst if the polymer contracts a blue-shift is observed and these colour changes are quantified using a spectrometer.
Diagnostic applications
Sensor holograms can be manufactured in a wide range of polymer matrices, including natural, synthetic or rationally designed systems containing appropriate receptor systems, in order to create inexpensive optical sensors that will respond to a range of targets including pH, specific ions, solvents, drugs, metabolites and enzymes.
For example, by incorporating glucose binding ligands into the hologram that then become capable of monitoring the glucose concentration in real-time (Fig.2).
The holograms are comprised of crosslinked films of polyacrylamide containing boronic acid derivatives. When glucose is then introduced to the monitored media it diffuses into the hologram and binds to these boronic acid groups and causes the supporting polymer film to swell.
The hologram initially appears green but upon the addition of glucose the holographic image shifts towards the red-end of the spectrum. Sensor holograms have also been used to monitor the metabolic products of microbial growth.
For example, sensors holograms for pH were used to monitor the fermentation of lactose in whole milk to lactic acid by a culture of Lactobacillus casei (Fig.3).
Receptor systems
Current work is focused on development of new receptor systems for incorporation into advanced systems for Disease Diagnosis and Management.
This strategy incorporates hand-held sensors containing nanocartridges (disposable sensor holograms and nanofluidic achips') that will measure specific metabolites for various disease states.
The devices also have wireless connections to on-line patient databases and are being seen as the next generation devices for individualised electronic- or
E-medicine.
Dr Alexander J Marshall, Senior Scientist, Dr Satyamoorthy Kabilan, Principal Scientist and Dr Frank F Craig PhD MBA, Chief Executive Officer, are with Smart Holograms, Institute of Biotechnology, University of Cambridge, Cambridge, UK. www.smartholograms.com
