Dual-Electrode Glutamine Sensor
Ammonium ISE Method (Approach 2)
Approach 2: Ammonium ISE Method

Enzyme Immobilization Protocol

Procedures for immobilizing Pseudomonas sp. glutaminase on the active ISE for ammonium-based glutamine detection

pH Compatibility - Resolved
This protocol uses Pseudomonas sp. glutaminase which maintains optimal activity at pH 7.0-7.5, making it fully compatible with whole blood pH (~7.4) and room temperature operation (25°C). This is a significant advantage over E. coli glutaminase which requires pH 5.0-6.0.

1. Overview

In this approach, Pseudomonas sp. glutaminase is immobilized on a porous membrane placed directly over the active ISE (Electrode 2). The enzyme converts glutamine to glutamate + NH₄⁺ locally at the electrode surface. The released NH₄⁺ is detected by the ISE, and the differential signal (Electrode 2 - Electrode 1) represents glutamine concentration.

2. Solution Preparation

2.1 Buffer Solution

  1. Prepare 50 mM phosphate buffer (pH 7.4): Dissolve 0.68 g KH₂PO₄ and 0.87 g K₂HPO₄ in 100 mL deionized water.
  2. Adjust pH: Use 0.1 M NaOH or 0.1 M HCl to adjust to pH 7.4.
  3. Store: Keep at 4°C for up to 1 week.

2.2 Enzyme Solution

  1. Prepare glutaminase solution: Dissolve Pseudomonas sp. glutaminase powder in 50 mM phosphate buffer (pH 7.4) to achieve 10 U/mL.
  2. Filter: Pass through 0.22 μm sterile filter.
  3. Aliquot: Divide into small volumes (50-100 μL) and store at -20°C until use.

2.3 Cross-linking Solution

  1. 5% BSA solution: Dissolve 0.25 g BSA in 5 mL 50 mM phosphate buffer (pH 7.4).
  2. 2.5% glutaraldehyde: Add 0.5 mL of 25% glutaraldehyde to 4.5 mL phosphate buffer. Prepare fresh and use within 2 hours.

3. Enzyme Membrane Preparation

  1. Prepare support membrane: Cut porous cellulose acetate membrane (0.2-0.45 μm pore size) into 5 mm diameter circles.
  2. Prepare enzyme mixture: In a microcentrifuge tube, combine:
    • 50 μL glutaminase solution (10 U/mL)
    • 25 μL 5% BSA solution
  3. Add cross-linker: Add 10 μL of 2.5% glutaraldehyde. Mix gently.
  4. Apply to membrane: Place the cellulose acetate disc on a clean surface. Apply 5 μL of the enzyme mixture onto the center.
  5. Cross-link: Allow to dry at room temperature for 1 hour in a humidity chamber (~60% RH).
Important
At room temperature (25°C), the reaction will proceed at roughly half the speed compared to 30°C (optimal). Consider using 3-5× higher enzyme loading to compensate for the reduced activity at room temperature.

4. Immobilization on Active ISE

  1. Prepare the active ISE: Ensure the NH₄⁺ ISE (Electrode 2) is clean and dry.
  2. Attach enzyme membrane: Carefully place the enzyme-loaded cellulose acetate membrane onto the tip of the active ISE, directly over the ion-selective membrane.
  3. Secure in place: Use a small O-ring or silicone glue to secure the enzyme membrane to the electrode body. Ensure good contact but don't compress the membrane.
  4. Allow the assembly to equilibrate in 0.1 M PBS (pH 7.4) for 30 minutes before testing.

5. Reference Electrode

Electrode 1 (Reference) should remain without enzyme immobilization. This electrode measures the baseline NH₄⁺ and K⁺ in the blood sample. The differential signal (Electrode 2 - Electrode 1) eliminates background interference.

6. Storage

  1. Short-term: Store completed sensors at 4°C in PBS (pH 7.4) when not in use. Use within 1 week.
  2. Long-term: Store dry at -20°C in a sealed container with desiccant. Use within 1 month.

7. Next Steps

After enzyme immobilization, proceed to Assembly & QC to verify the differential measurement setup and quality control.