Near-Infrared Optical Nanosensors for Continuous Detection of Glucose

Abstract

Background: Continuous glucose monitors (CGMs) enable people with diabetes to proactively manage their blood glucose and reduce the risk of hypoglycemia. Commercially available CGMs utilize percutaneous electrodes that, after days to weeks of implantation, are subjected to the foreign body response that severely reduces sensor accuracy. The previous work demonstrated the use of hydrogels containing a glucose-responsive viologen that quenches a nearby fluorophore. Here, we investigate the immobilization of this sensing motif onto a nanoparticle surface and optimize local surface concentrations for optical glucose sensing.

Methods: A viologen quencher-fluorescent dye system was incorporated into poly(2-hydroethyl methacrylate) hydrogels in varying quantities to assess the effect of quencher-fluorophore concentration on glucose responsiveness. The sensing motif was then immobilized onto silica nanoparticles by carbodiimide chemistry. Nanosensors with a range of dye and quencher concentrations were challenged for glucose responsiveness to determine the optimal sensor formulation.

Results: When incorporated into a hydrogel, high concentrations of viologen quencher and fluorophore were required to permit electron transfer between the two components and yield a detectable glucose response. Immobilization of this glucose-responsive system onto a silica nanoparticle facilitated this electron transfer to yield detectable responses at even low concentrations. Increasing quencher concentration on the nanoparticle, relative to the fluorophore, resulted in the greatest apparent glucose response.

Conclusion: The nanoparticle sensors demonstrated excellent glucose response in the physiological range and are a promising tool for real-time glucose tracking.

Keywords: CGM; fluorescence; glucose sensors; implantable; subcutaneous.

Scheme 1.

A dual component system for fluorescence-based glucose sensing. A boronic-acid modified viologen molecule associates with an anionic fluorophore to quench fluorescence. Upon binding of glucose with the boronic acid moieties, the viologen is neutralized to increase the effective fluorescence of a near-infrared dye.

Scheme 2.

Illustration of subcutaneous optical detection of blood analytes via an implantable nanosensor-hydrogel composite. Microporous hydrogel scaffolds resist fibrotic encapsulation and facilitate tissue integration, yielding shorter diffusion distances between blood analytes and optical nanosensors.

Figure 1.

Scattering electron microscopy image of microporous hydrogels fabricated by microbead templating. Sintering at 140°C for 18 hours fuses microbeads to serve as a template for fabricating microporous hydrogel. Crosslinking of 2-hydroxyethyl methacrylate around sintered microbeads yields hydrogels with large interconnected pores throughout the scaffold (scale bars = 100 μm).

Figure 2.

Two-component glucose sensing chemistry is concentration dependent. (a) 2-Hydroxyethyl methacrylate hydrogels containing IRDye680 and viologen boronic acid at a fixed 10:1 ratio in a range of concentrations were challenged with the addition of glucose (i, 1:0.1; ii, 5:0.5; iii, 10:1; and iv, 15:1.5). (b) Intensity ratios of glucose sensitive hydrogels reveal that higher total concentrations of IRDye680 and viologen boronic acid improve glucose sensitivity.

Figure 3.

Detection of viologen boronic acid quencher on glucose nanosensors. (a) Treatment of viologen boronic acid with NaOH results in the appearance of an absorbance peak at 400 nm. (b) Absorbance of viologen boronic acid at all concentrations plateau after treatment with 10 mM NaOH. (c) Standard curves of viologen boronic acid are indistinguishable after treatment with at least 10 mM NaOH.

Figure 4.

Optimization of glucose nanosensor fabrication. (a) Synthesis scheme of glucose nanosensors with IRDye680 and viologen boronic acid quencher. (b) Comparison of silica nanoparticles reacted with IRDye680/viologen boronic acid (left) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride/N-hydroxysulfosuccinimide activated IRDye680/viologen boronic acid (right). (c) Glucose modulation of glucose nanosensors containing a range of IRDye680:viologen boronic acid ratios show that higher ratios provide better modulation. (d) Table of intensity ratios for particles treated with glucose and particles treated with control (saline) indicate that higher IRDye680:viologen boronic acid ratios have better modulation within physiological ranges (I₂₀₀/I₅₀) and limited drift in fluorescence upon addition of saline.