Approach 2: Ammonium ISE Method
Materials and Equipment
Complete list of required materials, chemicals, and equipment for developing the differential ammonium ISE glutamine sensor
pH Compatibility Note
This approach uses Pseudomonas sp. glutaminase which maintains optimal activity at pH 7.0-7.5, making it fully compatible with whole blood pH (~7.4). The ammonium ISE method measures NH₄⁺ released when glutaminase converts glutamine to glutamate.
1. Ion-Selective Electrode Components
NH₄⁺ Ion-Selective Membrane
- Ammonium ionophore I Nonactin or similar NH₄⁺ selective carrier
- PVC matrix High molecular weight PVC for membrane
- Plasticizer DOS (dioctyl sebacate) or o-NPOE
- Tetrahydrofuran (THF) Anhydrous, for membrane casting
K⁺ Reference Ionophore
- Potassium ionophore I Valinomycin or similar K⁺ carrier
- PVC matrix For reference electrode membrane
- Plasticizer DOS or dibutyl phthalate
2. Electrode Hardware
Electrode Bodies
- Glass or plastic electrode bodies 2 mm diameter, 5-10 cm length
- Internal reference electrode Silver/silver chloride wire
- Internal filling solution 0.1 M KCl for Ag/AgCl
Connection Hardware
- Coaxial cable Shielded, for low-noise measurement
- BNC connectors For potentiometer connection
- Electrode holder For stable mounting during measurement
3. Enzyme and Reagents
Enzyme
- Glutaminase (Pseudomonas sp.) Recombinant, ≥40 U/mg, optimal pH 7.0-7.5
- Recommended suppliers Sigma-Aldrich, Megazyme, Creative Enzymes
- Quantity 100-250 units
Cross-linking
- Bovine Serum Albumin (BSA) ≥96% purity, lyophilized
- Glutaraldehyde 25% solution in water
4. Membrane and Support Materials
Enzyme Membrane Support
- Porous cellulose acetate 0.2-0.45 μm pore size
- Nylon mesh 20-50 μm pore size, for support
- Porous silica membrane Alternative support material
Protective Membranes
- Nafion solution 5% w/w in alcohol/water
- Polyurethane Medical-grade, aliphatic polyether
5. Buffers and Solutions
Buffer Solutions
- Phosphate Buffered Saline (PBS) 0.1 M, pH 7.4, for measurements
- Phosphate buffer 50 mM, pH 7.4, for enzyme solution
- KCl electrolyte 0.1 M, for internal filling
Standards
- Ammonium chloride standard 1 M stock solution
- L-Glutamine ≥99% purity
- L-Glutamic acid ≥99% purity
- Potassium chloride For interference testing
6. Equipment
Measurement
- High-impedance voltmeter/pH meter ≥10¹² Ω input impedance
- Data acquisition system For potential vs. time measurement
- Temperature controller 25°C ± 0.5°C
Fabrication
- Analytical balance 0.1 mg readability
- Micropipettes 0.5-10 μL, 10-100 μL, 100-1000 μL
- pH meter ±0.01 pH accuracy
- Fume hood For THF handling
- Vortex mixer For solution preparation
7. Key Differences from Approach 1
| Component | Approach 1 (Glutamate Oxidase) | Approach 2 (Ammonium ISE) |
|---|---|---|
| Enzymes | Glutaminase + Glutamate oxidase | Glutaminase only |
| Electrodes | Dual working electrodes (amperometric) | Dual ISEs (potentiometric) |
| Signal | Current (H₂O₂ oxidation) | Potential (NH₄⁺ concentration) |
| Equipment | Potentiostat | High-impedance voltmeter |