High Affinity Mannotetraose as an Alternative to Dextran in ConA Based Fluorescent Affinity Glucose Assay Due to Improved FRET Efficiency

A Polarity-Sensitive Far-Red Fluorescent Probe for Glucose Sensing through Skin

The field of glucose biosensors for diabetes management has been of great interest over the past 60 years. Continuous glucose monitoring (CGM) is important to continuously track the glucose level to provide better management of the disease. Concanavalin A (ConA) can reversibly bind to glucose and mannose molecules and form a glucose biosensor via competitive binding. Here, we developed a glucose biosensor using ConA and a fluorescent probe, which generated a fluorescent intensity change based on solvatochromism, the reversible change in the emission spectrum dependent on the polarity of the solvent. The direction in which the wavelength shifts as the solvent polarity increases can be defined as positive (red-shift), negative (blue-shift), or a combination of the two, referred to as reverse. To translate this biosensor to a subcutaneously implanted format, Cyanine 5.5 (Cy5.5)-labeled small mannose molecules were used, which allows for the far-red excitation wavelength range to increase the skin penetration depth of the light source and returned emission. Three Cy5.5-labeled small mannose molecules were synthesized and compared when used as the competing ligand in the competitive binding biosensor. We explored the polarity-sensitive nature of the competing ligands and examined the biosensor's glucose response. Cy5.5-mannotetraose performed best as a biosensor, allowing for the detection of glucose from 25 to 400 mg/dL. Thus, this assay is responsive to glucose within the physiologic range when its concentration is increased to levels needed for an implantable design. The biosensor response is not statistically different when placed under different skin pigmentations when comparing the percent increase in fluorescence intensity. This shows the ability of the biosensor to produce a repeatable signal across the physiologic range for subcutaneous glucose monitoring under various skin tones.

Facile one-step fabrication of glucose oxidase loaded polymeric nanoparticles decorating MWCNTs for constructing glucose biosensing platform: Structure matters

A novel and robust enzymatic biosensing platform with high sensitivity is developed based on facile one-step assembled bio-nanocomposites with enzymes-loaded polymeric nanoparticles decorating multi-walled carbon nanotubes (MWCNTs). An amphiphilic copolymer PAVE containing photo-cross-linkable coumarin segments and carboxylic groups was co-assembled with MWCNTs in aqueous solution while encapsulating the model enzyme namely glucose oxidase (GOx) simultaneously, generating necklace-like bio-nanocomposites (GOx@PAVE-CNTs) with GOx-loading polymeric nanoparticles as nanobeads and MWCNTs as conducting micron-string. Then the GOx@PAVE-CNTs bio-nanocomposites were electro-deposited onto electrode surface and a robust biosensing complex film with porous network structure was formed after following photo-cross-linking. Consequently, an enzymatic glucose biosensor was successfully constructed. The biosensor exhibited ultrafast response (<3 s) to glucose with a considerably wide linear range (1.0 μM ∼ 5 mM) and a low detection limit (0.36 μM) for glucose detection. High sensitivity and selectivity of the biosensor toward glucose were also well demonstrated. Furthermore, the biosensor showed exceptionally good stability and reproducibility. More importantly, the glucose biosensor was practically used for glucose detection from human urine and serum samples with satisfactory results. As a proof-of-concept strategy, this facile and effective strategy for biosensor fabrication is of considerable interest because of its versatility to be generalized to many other enzymatic biosensor systems, exhibiting promising and practical potential in bio-medical and life health applications.

A Layer-by-Layer Approach To Retain a Fluorescent Glucose Sensing Assay within the Cavity of a Hydrogel Membrane

A continuous glucose monitoring device that resides fully in the subcutaneous tissue has the potential to greatly improve the management of diabetes. Toward this goal, we have developed a competitive binding glucose sensing assay based on fluorescently labeled PEGylated concanavalin-A (PEGylated-TRITC-ConA) and mannotetraose (APTS-MT). In the present work, we sought to contain this assay within the hollow central cavity of a cylindrical hydrogel membrane, permitting eventual subcutaneous implantation and optical probing through the skin. A "self-cleaning" hydrogel was utilized because of its ability to cyclically deswell/reswell in vivo, which is expected to reduce biofouling and therefore extend the sensor lifetime. Thus, we prepared a hollow, cylindrical hydrogel based on a thermoresponsive electrostatic double network design composed of N-isopropylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid. Next, a layer-by-layer (LbL) coating was applied to the inner wall of the central cavity of the cylindrical membrane. It consisted of 5, 10, 15, 30, or 40 alternating bilayers of positively charged poly(diallyldimethylammonium chloride) and negatively charged poly(sodium 4-styrenesulfonate). With 30 bilayers, the leaching of the smaller-sized component of the assay (APTS-MT) from the membrane cavity was substantially reduced. Moreover, this LbL coating maintained glucose diffusion across the hydrogel membrane. In terms of sensor functionality, the assay housed in the hydrogel membrane cavity tracked changes in glucose concentration (0 to 600 mg/dL) with a mean absolute relative difference of ∼11%.

Quaternized Polymer-Single-Walled Carbon Nanotube Scaffolds for a Chemiresistive Glucose Sensor

A chemiresistive glucose sensor based on poly(4-vinylpyridine) (P4VP) and single-walled carbon nanotube (SWCNT) composites was reported. To fabricate this glucose sensor, a glass substrate containing gold electrodes was treated with 3-bromopropyltrichlorosilane to obtain a covalent bonding between the polymer-SWCNT composite and the glass substrate. Some of the pyridyl moieties in P4VP react with the surface, and the remainders were quaternized using 2-bromoethanol to achieve highly charged hydrophilic surface with improved biocompatibility with enzyme molecules. The resulting biomimetic surface was functionalized with glucose oxidase (GOx) by electrostatic assembly with the quaternized P4VP-SWCNT composite. This material displays a decrease in electrical resistance as a result of enzymatically liberated hydrogen peroxide produced in response to glucose, which increases the p-doping of the SWCNT. The sensor exhibited high selectivity for glucose and showed an instant response (within 3 s) to glucose.