Optical Sensors for Monitoring Metabolic Disorders
Our research consists of the development and optimisation of optical colorimetric sensors for applications in point-of-care diagnostics. The sensing devices consist of metal or dielectric nanostructures periodically organised in a 10 μm thick functionalised hydrogel film. Such hydrogels may be selected from poly(2-hydroxyethyl methacrylate), polyacrylamide or gelatin, and they can be functionalised to be sensitive to a wide range of analytes such as pH, glucose, metal ions and antibodies. These hydrogels can be transformed into colorimetric devices by integrating periodically organised nanoparticles or highly crosslinked density regions within the matrix. We record diffraction gratings in these functionalised polymers by nanosecond pulsed laser writing. The gratings can be formed by photochemistry, laser ablation or photopolymerisation in Denisyuk reflection mode using a pulsed laser (6 ns, 532 nm, 350 mJ). The formed grating has a periodicity of half the wavelength of the laser light used since the grating is an image of the periodicity of the standing wave created during laser exposure. The image and periodicity can be controlled by changing the object and exposure conditions. When the fabricated sensor is illuminated with a white-light source, the grating produces visible-light diffraction and displays a monochromatic colour. This diffraction is governed by Bragg's law:
λpeak = 2 n d sin(θ)
where λpeak is the wavelength of the first order diffracted light at the maximum intensity in vacuo, n is the effective index of refraction of the recording medium, d is the spacing between the two consecutive recorded nanoparticle (or highly crosslinked regions) layers (constant parameter), and θ is the Bragg angle which is determined by the recording geometry.
The mode of action of these sensors involves the modulation of Bragg diffraction gratings and localised refractive index changes. When a target analyte is introduced to the sensor in an aqueous solution, the target analyte binds to a receptor in the polymer matrix, and the binding process produces a change in Donnan osmotic pressure. This change in the osmotic pressure swells or shrinks the polymer matrix, which allows the diffraction grating to change periodicity and/or index of refraction, hence report on the concentration of the target analyte by fine changes in λpeak. Such sensors exhibit reversible wavelength shifts, and diffract the spectrum of narrow-band light over the wavelength range λpeak ≈ 300-1100 nm. λpeak measurements can be obtained by fully-quantitative readouts through spectrophotometry, and semi-quantitative results through visual readouts.
The optical sensor represents a simple and label-free analytical platform for the quantification of clinical and environmental analytes, while showing potential scalability. We anticipate that our sensing platform will lead to many novel applications from printable devices to low-cost colorimetric biosensors.
Diabetes Screening through Nanotechnology
Diabetes is one of the most challenging health problems of the 21st century. Today, 382 million people live with diabetes. This epidemic on the rise all over the world and countries are struggling to keep pace in controlling this disease. The number of people with the disease is estimated to reach 592 million in less than 25 years. In 2035, one in ten people will have diabetes. The number of people with diabetes is rapidly increasing in the Middle East, Western Pacific, sub-Saharan Africa and South-East Asia where economic development has transformed lifestyles. These rapid transitions are bringing high rates of obesity and diabetes; developing countries are facing a healthcare challenge coupled with inadequate resources to protect their population. The new estimates show an increasing trend towards younger people developing diabetes.
Diabetes has been known to be ‘a disease of the wealthy’. But studies showed that this was not the case. 80% of people with diabetes live in low- and middle- income countries, and the socially disadvantaged in any country are the most vulnerable to the disease. The financial burden of diabetes is taking up 548 billion dollars in health spending, which is 11% of the global healthcare expenditure. Yet, it is estimated that 175 million people are undiagnosed today. This is because there are few symptoms during the early years of type 2 diabetes, or those symptoms may not be recognised as being related to diabetes. Type 2 diabetes can go unnoticed and undiagnosed for years. In such cases, those affected are unaware of the long-term damage being caused by this disease. Population-based diabetes studies consistently show that 40% diagnosed people live in low income countries. According to International Diabetes Federation, in sub-Saharan Africa, where resources lack and governments may not prioritise screening for diabetes, this proportion is as high as 90% in some countries.
We can screen for diabetes using glucose meters and urine dipsticks. These technologies might look low cost, but considering that 1 billion people live on less than $1.25 a day, they are not affordable. For example, glucometers, a lancing device, lancets and tests costs up to $115 for testing 100 people in the rural village. On the other hand, a urine dipstick test, which costs about $0.5, have low sensitivity and often provide erroneous results due to subjective reading. If we develop diagnostic tools that are low cost, reusable, user friendly, non-invasive and reliable, we can help deprived communities. We design and develop medical diagnostics that intend to satisfy these criteria.
The principle of operation of these sensors is based on swelling and shrinking of the holographic films, which in turn diffracts light at different wavelengths. These wavelengths are colours that can be read by naked eye. In this case, the smart material is made out of a polymer and boronic acid derivative that can reversibly bind to glucose so that we can see the colour change in the presence or absence of glucose. These sensors can be tuned to diffract light in the entire visible spectrum. It is also possible to pattern these devices to give written messages.
We can use a single sensor for about 100 times by washing with water and it costs 20¢. We recently completed clinical trials of this sensor by testing urine samples of diabetic patients. It has comparable performance with the commercial tests while also showing higher cost effectiveness. These sensors can be read by eye or quantitatively using spectrometers. However, alternative solutions such as generic smartphone applications we developed in our research group can also be used to read sensors semi-quantitatively. Such applications offer connectivity for global diagnostic data management. Such technologies can make a difference for monitoring disease where early diagnosis and the treatment are needed the most.
Contact Lens Sensors
Contact lenses as a minimally-invasive platform for diagnostics and drug delivery have emerged in recent years. Contact lens sensors have been developed for analysing the glucose composition of tears as a surrogate for blood glucose monitoring, and for the diagnosis of glaucoma by measuring intraocular pressure. However, the eye offers a wider diagnostic potential as a sensing site, and therefore contact lens sensors have the potential to improve the diagnosis and treatment of many diseases and conditions. With advances in polymer synthesis, electronics and micro/nanofabrication, contact lens sensors can be produced to quantify the concentrations of many biomolecules in ocular fluids. Non- or minimally-invasive contact lens sensors can be used directly in a clinical or point-of-care setting to monitor a disease state continuously. We develop new approaches for the fabrication, sensing, wireless powering, and readout mechanisms in contact lens sensors. We also explore the possibilities to integrate the contact lens sensors with wearable devices and smartphones.
3. Yetisen, A.K., Butt, H., Vasconcellos F.C., Montelongo, Y., Davidson, C.A.B., Blyth, J., Chan, L., Carmody, J.B., Vignolini, S., Steiner, U., Baumberg, J.J., Wilkinson, T.D., Lowe, C.R., Light-Directed Writing of Chemically Tunable Narrow-Band Holographic Sensors†. Advanced Optical Materials, 2 (3), 250-254 (2014) link
5. Yetisen, A.K., Qasim, M., Nosheen, S., Wilkinson, T.D., Lowe, C.R. Pulsed laser writing of holographic nanosensors. Journal of Materials Chemistry C, 2 (18), 3569-3576 (2014) link
Zawadzka, M., Mikulchyk, T., Cody, D., Martin, S., Mihaylova, E., Yetisen, A.K., Martinez-Hurtado, J.L., Butt, H., Mihaylova, E., Awala, H., Mintova, S., Yun, S.H., Naydenova, I. Ed. Serpe M.J., Kang, Y., Zhang, Q.M., Photonic Materials for Holographic Sensing In Photonic Materials for Biosensing, and Display devices. Springer International Springer International, 2016; Vol. 229, pp. 315-359 (2016)
Yetisen, A.K., “Holographic Sensors”, Springer, 162 pages, ISBN 978-3-319-13583-0 (2015) link