A glucose-sensing contact lens: a new approach to noninvasive continuous physiological glucose monitoring

Ramachandram Badugu, Joseph R. Lakowicz, Chris D. Geddes, Israel Gannot
2004 Optical Fibers and Sensors for Medical Applications IV  
We have developed a new technology for the non-invasive continuous monitoring of tear glucose using a daily use, disposable contact lens, embedded with sugar-sensing boronic acid containing fluorophores. Our findings show that our approach may be suitable for the continuous monitoring of tear glucose levels in the range 50 -500 µM, which track blood glucose levels that are typically ≈ 5-10 fold higher. We initially tested the sensing concept with well-established, previously published, boronic
more » ... cid probes and the results could conclude the used probes, with higher pK a values, are almost insensitive towards glucose within the contact lens, attributed to the low pH and polarity inside the lens. Subsequently, we have developed a range of probes based on the quinolinium backbone, having considerably lower pK a values, which enables them to be suitable to sense the physiological glucose in the acidic pH contact lens. Herein we describe the results based on our findings towards the development of glucose sensing contact lens and therefore an approach to non-invasive continuous monitoring of tear glucose using a contact lens. contact lens. 20-22 By incorporating monosaccharide sensitive fluorescent probes within such a lens, we can indeed make progress towards this non-invasive approach for glucose monitoring. As with any sensors, there are several issues that have to be addressed. The first is to identify suitable transduction elements, which in the presence of glucose, can report / produce suitable signals. The second is the design of the matrix to incorporate the transduction elements. For this, we have chosen an off-the-shelf disposable plastic contact lens, primarily because its physiological compatibility has already been assessed, and finally, the optimization of the sensor, with regard to sensitivity, response time, reversibility and shelf-life etc. The later two issues will be discussed throughout much of this paper. For the identification of suitable transduction elements, boronic acid has been known to have high affinity for diol-containing compounds such as carbohydrates, 23-25 where the strong complexation has been used for the construction of carbohydrate sensors, 26-33 transporters 34 and chromatographic materials. 35 Naturally, boronic acid compounds have been used for the synthesis of glucose sensors, 36-42 where we note the work of Shinkai, 36,37 Norrild, 38 Lakowicz 39-42 and Drueckhammer, 29 to name but just a few. Boronic acids are weak Lewis Acids composed of an electron deficient boron atom and two hydroxyl groups, (1 in Figure 1 ), which can interact with strong bases like OHto from the anionic boronate form (2 in Figure 1) , showing typically high pK a around 9. 43,44 Boronic acids couple with diols to form a boronic acid diester group (3 in Figure 1 ). The diol is linked covalently, and the reaction is fast and completely reversible. 44 In comparison to the boronic acid group, the boronic acid diester group shows higher acidity (pK a ≈ 6) due to a more electrophilic boron atom. The monophenylboronic acid group shows higher affinity for D-fructose with a smaller affinity for D-glucose with dissociation constants of ≈ 0.5 and 10 mM respectively. 44 The use of the boronic acid groups for sensing sugars is strongly dependent on the molecular geometry and the aromatic species where the boronic acid group is present, hence glucose sensitive probes can be made with a variety of affinities, in the mM range for blood glucose, 40-42 and in the µM range for tear glucose. In this paper we report on Boronic Acid containing Fluorophores (BAFs, Chart 1) which employ different mechanisms to induce spectral changes in the presence of sugar in particular excited-state charge transfer (CT), 44 photoinduced electron transfer (PET) 37 and charge neutralization mechanism. 20-22 CT is a versatile mechanism that can be applied to a large number of fluorophores, where the boronic acid group and an electron donor group are present on the same fluorophore, in which the BA group [-B(OH) 2 ] acts as an electron withdrawing group. However, in the presence of sugar and at an appropriate pH, the boronic acid group is present in its anionic form, namely [-B(OH)(Sugar)]and is no longer an electron withdrawing group. Hence spectral changes can be observed due to the perturbation of the charge transfer nature of the excited state. Here we employ this mechanism with a range of probes developed in our laboratory (Chart 1), [39] [40] [41] [42] [44] [45] [46] [47] [48] [49] for glucose sensing within a plastic contact lens polymer. Photoinduced electron transfer mechanism is another extensively utilized mechanism in the monosaccharide sensing. 37 The change in the boronic acid conformation from neutral to anionic form with sugar binding alters the PET mechanism in the system and thereby the fluorescence spectrum. Compound ANDBA is a known PET probe selective for glucose with its geometrically preferred structure. The compatibility of the probe in contact lens is assessed. Before developing a new set of probes based on quinolinium nucleus (BMOQBA and BMQBA, Chart 1) we have made an attempt to understand the data obtained on CT and PET probes in contact lens along with the local pH and polarity of the contact lens. These new compounds with positively charged nitrogen center are highly fluorescent in aqueous solution. However, in the presence of monosaccharides the negatively charged boronate ester form resulted from the sugar complexation, partially neutralizes the charge density at
doi:10.1117/12.530060 fatcat:vgvkcgzdqre4jey6i226qsnfli