1. Revolutionary Technology in pH Measurement
Elicia Wong, PhD
Mar 17-19, 2009

2. pH Control Wide Applications: Why is it Important in bioprocessing?
Pharmaceutical/biotechnology (medicine)
Cosmetic (perfume)
Food and beverage (wine, sugar)
Water purification & wastewater treatment
Petroleum/deep sea/outer space exploration
Why is it Important in bioprocessing?
Critical quality parameter for cell and bacterial growth and metabolic activity, productivity and consistency.
2 L
80 L
400 L
2,000 L
10,000 L
Upstream Process
Downstream Process
Why is it a problem in bioprocessing?
Drifting
Fouling
Calibration

3. Outline Potentiometric pH measurement Glass electrode
Existing pH measurement technologies
Potentiometric pH measurement
Glass electrode
Ion-selective field effect transistor (ISFET)
Optical pH measurement
New technology
Amperometric pH measurement

4. Potentiometric Approach – Glass electrode
External reference electrode
Measuring electrode
High impedance pH meter
Glass frit
Glass membrane
Inner solution
Metal wire
Glass frit
Inner solution
External reference electrode
High impedance pH meter
Glass membrane
Glass electrode
Voltage difference builds up on the sides of the glass membrane due to difference between H+ activities on both sides
This voltage difference is measured with the help of an external reference electrode
Often reference and measuring electrodes are combined in one single structure
Fragile, wet storage, long term drift, requires frequent calibration

5. Ion-Selective Field Effect Transistor
Vgs
External reference electrode
Analyte solution
Solid state device
Insulator
Ion-selective interface
Insulator
Source
Drain
Electron path
Vds
Current flows
Four major components: Source (s), Drain (d), Ion-selective interface and Gate (g).
Hydrogen ions at or near the ion-selective interface will cause a variable voltage potential (Vgs – Vds)
Cathode (-)
No current flows
Anode (+) H+ - - - - - +

6. Optical pH Measurement System
Photo-detector
Photodetector, optical fiber, sensor
Sensor = fluorophore on solid support
An incident beam of light is passed to the sensor, where it interacts with the fluorophore. The emitted light intensity is pH dependent
Keep in dark (photo-bleaching)
Sensor
Optical light guide
Incident beam
Emitted light

7. Comparison Feature Glass Electrode ISFET Optical Robustness
Glass (fragile)
Solid state
Calibration
Required (frequently)
Required (longer calibration interval)
Required (factory+on site)
Stability
Drift
Maintenance
Intensive (wet storage)
Easy (dry storage)
Intensive (kept in dark (photo-bleaching)
Temperature Range
Wide
Narrow
pH range
0-14
5-9 (in ranges)
Biological Fouling
Occur
Sensitivity
 Low
 Medium
Good 

8. Amperometric pH Sensor
Power supply
Reference electrode
Counter electrode V i
Counter electrode
Analyte solution
pH-sensitive interface
Internal reference
pH-sensitive interface + Internal reference
External reference electrode V V V V i i i i H+ H+ H+ H+
pH-sensitive interface
semiconductor H+ H+ H+ H+
pH-sensitive interface
semiconductor

9. Self-Calibrating Internal reference = pH insensitive molecule pH 10
pH sensitive molecules
Internal reference

10. Stability
Both the proprietary pH sensitive and insensitive molecules are chemically stable (won’t leach) and thermally stable (able to withstand high temperature up to 250 C).
No degradation in performance with continuous operation and after heat sterilization.
No degradation in signal with 2-day continuous operation after autoclaving
Responses were observed after multiple autoclaving cycles (measurement performed after each autoclaving cycle)
Continuous operation
Multiple Autoclaving

11. Free from Biological Fouling
pH detection relies purely on chemical interaction between hydrogen ion and sensing interface rather than membrane diffusion effect (glass electrode)
- H+
Clotting of glass membrane H+
Continuous measurement in medium containing living cells
pH sensitive molecules
Detect change in pH from pH 7.82* to 7.01*
Without pH control
pH insensitive molecules
*pH verified by off-line pH measurements

12. Ideal for bioprocess pH measurement
Summary Chart
Ideal for bioprocess pH measurement
Feature
Glass Electrode
ISFET
Optical
Amperometric Sensor
Robustness
Glass (fragile)
Solid state
Calibration
Required (frequently)
Required (longer calibration interval)
Required (factory)
Self calibrated
Stability
Drift
No drift
Maintenance
Intensive (wet storage)
Easy (dry storage)
Intensive (Kept in dark (photo-bleaching)
Temperature Range
Wide
Narrow
pH range
0-14
5-9 (in ranges)
Biological Fouling
Occur NO
Sensitivity
 Low
Medium
Good 
Good