DIY GLUCOSE DETECTOR STRIPS
This idea occurred to me in the shower one day.
The inspiration came from a diagram in a Studies in Biology book 'Enzymes in industry and medicine' by Gordon Bickerstaff. This excellent little book is long out-of-print, and sadly there is now no book on enzymes for school teachers or their students. Even more sadly, when, a couple of years ago, I phoned the Institue of Biology to check on the book's status, their 'education officer' hadn't even heard of it!
Anyway, Edison said that genius was 1% inspiration, 99% perspiration. Well, it clearly wasn't genius, but to my astonishment this practical did work the first time I tried it.
In 1993, biosensors appeared in one or two syllabuses - notably the superb 'Enzyme technology module' of the NEA Modular Science GCSE. Unfortunately electronic glucose test meters ('biosensors') were prohibitively expensive at the time. This activity was designed to provide a practical exercise to support this part of the syllabus.
Highly-specific test strips for detecting glucose in urine or blood are commonly used in doctors' surgeries and in the home. All of these tests utilise the enzymes glucose oxidase and peroxidase, immobilised on a paper pad at the tip of the strip. The pad is covered with a thin cellulose membrane which is permeable only to small molecules such as glucose. In the presence of oxygen and glucose, the products of the enzyme activity react with a chemical (also on the paper pad) to produce a colour change. With some tests the intensity of the colour that develops indicates the glucose concentration.
The principles behind these tests are readily demonstrated, using potassium iodide as the chromagen (colour-change reagent). Different sugars can be used to indicate the specificity of the reaction catalysed by glucose oxidase.
The glucose oxidase/peroxidase mixture is expensive, but is required only in drop-sized volumes, so a small bottle could last a long time.
In 2002, even the most expensive electronic glucose test meter costs under £30. The ones we use at the NCBE cost just £7.50 from SuperDrug in the UK. The consumable test strips that they use are costly however, so we only permit students to use the electronic meters to test the effectiveness of their home-made devices.