A biosensor is an analytical device, which combines a bioreceptor — a biological recognition element — and a transducer. The bioreceptor can be organisms, tissues, cells, enzymes, antibodies, nucleic acids, etc and detects the target analyte while the transducer can be electrochemical, optical, thermal or mechanical in nature and converts the recognition event into a measurable signal. This is where the interaction between disciplines, like materials, electrical engineering and physics, with biologists/biochemists becomes meaningful. Electrochemical sensors are widely used in biosensor systems since they rely on chemical reactions for the biological recognition and measure the ensuing results in terms of electrical parameters making it easy to interface to an electronic system for further signal processing. There is a huge market for biosensors in the health as well as food industries with applications including diagnostics, and testing of food, soil, water, environment, etc.
Before talking about our work, I would like to briefly discuss ongoing biosensors work at a few other laboratories in India, cautioning that this is far from being comprehensive. National Physical Laboratory had developed an ion-sensitive field-effect transistor (ISFET) which has been fabricated at CEERI Pilani as a Glucose biosensor. The ISFET platform, among other sensors, is also being investigated by researchers at IISc. Work on impedance biosensors for the detection of pathogens and food toxins is ongoing at the Bengal Engineering and Science University. Using low cost screen printing methods, they have developed a prototype of a macroporous silicon device, interfaced with electronic readout, for sensitive detection of e.coli in blood.
There is a close interaction between engineering and school of biosciences in IIT Bombay and they have earlier reported on a wearable silicon based cardiac marker detector called the Silicon Locket. Concentration of recent work is on optical sensors due the inherent robustness of such sensors to electrical noise. They have developed fiber optic based biosensors for proteins and bacteria that rely on optical absorption of antibody and antigen pairs and have achieved a sensitivity of sub nanomolar concentrations. A low cost sensor and instrumentation system has been developed for the detection of e. coli as well, which can sense about 100 cfu/ml. They are also replacing the fibres with microfabricated waveguides and exploring the coupling of gold and silver nanoparticles with the optical waveguides to develop quantitative sensors. Advent of microelectromechanical sytems (MEMS) technology has helped in miniaturinsing the sensors, with a possibility of integration into point of care systems. A very sensitive MEMS sensor, working in the resonant mode, is a microcantilever and work on this is ongoing at the IITs in Bombay and Madras.
AT IIT Madras, in a collaborative effort between the Electrical and the Biotechnology departments, we have developed enzymatic reaction based silicon potentiometric sensors to detect bioanalytes like triglycerides (TG) and urea. The device is a silicon Electrolyte – Insulator – Semiconductor capacitor (EISCAP), basically a pH sensor, which exhibits a shift in the capacitance-voltage (CV) characteristics when the pH of the electrolyte (containing the bioanalyte) changes. The EISCAP becomes a specific biosensor because of the enzyme. For eg. Tributyrin, a short chained TG, on hydrolysis in the presence of the enzyme lipase, produces butyric acid and glycerol making the electrolyte more acidic. Similarly hydrolysis of Urea, in the presence of the enzyme urease, produces ammonia and carbon-di-oxide with the electrolyte becoming more basic. We first used a planar sensor with a teflon cell on top to hold 2 ml of the electrolyte. After calibrating the EISCAP with standard pH solutions, and optimising the enzyme quantity and the hydrolysis time, the electrolyte containing the bioanalyte, the enzyme and a buffer was introduced into the teflon cell for the CV measurements. We got a good match, within 10%, between our measured values with those from a clinical laboratory.
The next step was to miniaturise the sensor, by etching a microreactor into the silicon to accommodate a sample size of 10 micro litres or less, with a future possibility of integrating many sensors on the same chip. For ease in handling and measurement, the enzyme was immobilised on the sensor surface and the sensor was packaged with the counter electrode embedded. A direct readout circuit was designed to complete the biosensor system that included the calibration and measurements protocols. The measurement takes five minutes and agreement with clinical results, though not as good as in the case of the larger sensor, was still within 20%. The smaller sample volumes now require a tighter control on sample preparation and can be addressed.
When we started working on this sensor, the idea was to understand how it works and what would be the applications. Though our final aim was to make a simple and rapid system to measure the TG concentration in blood serum, along the way, we have used the sensor for other applications, for eg. to measure the TG content in edible oils, the rancidity of butter with ageing, etc.
So what next? And this applies to the field of biosensors in general. Is this research viable for the industry? If the answer is yes, then is an industry going to take it up? If it is no, does the work lose its value? I think it would be great if an industry took up laboratory research and transformed them into products. Or if we, or our students, could start industries to do that — IIT Bombay has initiated some activity in this. And if it is not taken up, it is still not a loss as there is a tremendous amount of learning involved in this kind of work, and a change in the mindset of researchers and students which is most welcome and will bear fruit somewhere some day. It is not a cakewalk for industries to step in either, as there is a huge gap between a laboratory device and shaping it into a product. But more importantly, there appears to be a disconnect between what is being done in the laboratories and what the industry wants and a meaningful dialogue with all parties including clinicians will immensely benefit the field. A few directed workshops to understand each other’s language, capabilities and limitations will go a long way towards this.
Wish to thank Chirasree Roy Chaudhuri from BESU and Soumyo Mukherji of IIT Bombay for their inputs. The work at IIT Madras reported here was done with Anju Chadha of the Department of Biotechnology. Shanthi Pavan of EE did the circuits part and a large number of students have been involved over the years with funding from IITM, DST, DBT and NPMASS.