Ocular glucose biosensing using boronic acid fluorophores

Abstract

Boronic acids (BAs) are well-known for their interactions with diol-containing compounds like glucose. Fluorescent moieties are commonly incorporated into a BA derivative’s framework to monitor the effect of varying glucose concentrations in a given environment. Hence, a novel carboxylic acid fluorescent BA derivative, o-COOHBA, has been investigated for glucose sensing, in solution and when immobilised onto a polydimethylsiloxane (PDMS) “lens”-like surface. This approach aims to develop smart-contact lenses that will allow people suffering from diabetes to track their condition continuously and non-invasively in real-time. 1. Introduction Diabetes is a worldwide incurable disease known to have acute and chronic health effects1-2. This disease affects the cardiovascular and peripheral nervous systems, kidneys, and can also be fatal in some cases1-2. Blindness, heart or kidney failures are among the most common life-threatening effects of diabetes2. Monitoring physiological blood-glucose concentrations is a means of managing the disease, however few non-invasive continuous monitoring methods currently exist1-2. Consequently, there is considerable interested in using aqueous ocular fluid as a sample medium for tracking the disease marker glucose. 2. Sensing Mechanism The Lewis acidic BA moiety of the sensor is known for its strong interaction with electron-rich diols, like sugars1-2. On interaction with sugars, e.g. glucose, the fluorescent BA form is transformed into the anionic boronate form, which is non-fluorescent, leading to a decrease in the fluorescence intensity with increasing sugar concentrations1-2. Scheme 1: Sensing mechanism for BA derivatives. 3. Synthesis of o-COOHBA Sensor A novel BA sensor, o-COOHBA, was synthesized via a one-step nucleophilic substitution reaction that required equimolar quantities of BA and quinoline derivatives, as seen in Scheme 2. The successful formation of o-COOHBA was confirmed by 1H NMR. Scheme 2: Synthesis of o-COOHBA; (i) anhydrous dimethylsulfoxide, N2, 70 0C for 48h. 4. Fluorescence of o-COOHBA Fluorescence measurements were performed on a Jasco FP-8300 Spectrophotometer using a precision cell made from quartz with 10 mm path length. The excitation wavelength required was 380 nm with a corresponding emission wavelength of 485 nm. On increased glucose concentrations, a decrease in fluorescence intensity was observed in the range of 0-10 mM in solution and similarly, in the range of 0-5 mM when anchored to a PDMS surface, corresponding to the ocular-glucose concentrations for diabetics, ~50 μM – 5 mM2. All tests were carried out at an ambient temperature using a pH 7.4 phosphate buffer. 5. Conclusion In both solution studies and when anchored on to the PDMS surface, a decrease in fluorescence intensity was observed on increased glucose concentrations. The excitation wavelength of 380 nm is also advantageous, as it lies close to the visible-region of the electromagnetic spectrum, which allows for the use of cheap, readily available LEDs as excitation sources. The carboxylic acid substituent was desirable for immobilizing the BA sensor onto a wide variety of polymeric substrates