© SENSORIN 2009

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Elicia Wong, PhDElicia Wong, PhD

Revolutionary Revolutionary Technology in pH Technology in pH

MeasurementMeasurement

Mar 17-19, 2009Mar 17-19, 2009

© SENSORIN 2009

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2 L 80 L400 L

2,000 L10,000 L

Upstream Process Downstream Process

pH ControlpH Control

Why is it a problem in bioprocessing?Why is it a problem in bioprocessing?

Wide Applications:Wide Applications:

Why is it Important in bioprocessing?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 explorationCritical quality parameter for cell and bacterial growth and metabolic activity, productivity and consistency.

Drifting

Fouling

Calibration

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Potentiometric pH measurement

OutlineOutline

Existing pH measurement technologies

Optical pH measurement

Glass electrodeIon-selective field effect transistor (ISFET)

New technology

Amperometric pH measurement

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External reference electrode

Measuring electrode

High impedance pH meter

Glass fritGlass membrane

Inner solution

Metal wire

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

Potentiometric Approach – Glass Potentiometric Approach – Glass electrodeelectrode

Glass frit

Inner solution

External reference electrode

High impedance pH meter

Glass membrane

Glass electrode

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Ion-Selective Field Effect Ion-Selective Field Effect TransistorTransistor

Vgs

Electron path

Vds

DrainSource

Ion-selective interface

Insulator Insulator

External reference electrodeAnalyte

solution

Cathode (-)

Anode (+)

--

- --

-

++

+ ++ + +

No current flows

- - -

Current flows

H+H+H+

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)

Solid state device

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Photo-detector

Optical pH Measurement Optical pH Measurement SystemSystem

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

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Comparison Comparison

FeatureFeatureGlass Glass

ElectrodeElectrode ISFETISFET OpticalOptical

RobustnesRobustnesss

Glass (fragile)

Solid state Solid state

CalibrationCalibrationRequired (frequently)

Required (longer calibration interval)

Required (factory+on site)

StabilityStability Drift Drift Drift

MaintenanMaintenancece

Intensive (wet storage)

Easy (dry storage)

Intensive (kept in dark (photo-bleaching)

TemperatuTemperature Rangere Range

Wide Wide Narrow

pH rangepH range 0-14 0-14 5-9 (in ranges)

Biological Biological FoulingFouling

Occur Occur Occur

SensitivitySensitivity  Low  Medium Good 

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pH-sensitive interfacesemiconductor

Amperometric pH SensorAmperometric pH Sensor

Voltage (V)

Cu

rre

nt

(i)

Counter electrode

Internal reference

Power supply

iV

H+

H+H+

H+

VV ii

pH-sensitive interfacesemiconductor

H+ H+ H+ H+

VV ii

Voltage (V)

Cu

rre

nt

(i)

pH-sensitive interface + Internal reference

External reference electrode

Counter electrode

Reference electrodeAnalyt

e solution

pH-sensitive interface

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pH (unit)

Vo

lta

ge

(V

)slope

tconsVoltagediffpH

tan)(

Self-CalibratingSelf-Calibrating

Voltage (V)

Cu

rren

t (i

)

pH 4

pH 7

pH 10

Internal reference = pH insensitive molecule

pH sensitive pH sensitive moleculesmolecules

Internal Internal referencereference

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StabilityStability

Voltage (V)

Cu

rre

nt

(i)

Both the proprietary pH sensitive and insensitive molecules are chemicallychemically stable (won’t leach) and thermally stablethermally stable (able to withstand high temperature up to 250 C).

No degradation in performance with continuous operation and after heat sterilization.

Voltage (V)

Cu

rre

nt

(i)

Responses were Responses were observed after observed after multiple multiple autoclaving cycles autoclaving cycles (measurement (measurement performed after performed after each autoclaving each autoclaving cycle)cycle)

Continuous Continuous operationoperation

No degradation in No degradation in signal with 2-day signal with 2-day continuous continuous operation after operation after autoclavingautoclaving

Multiple Multiple AutoclavingAutoclaving

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Free from Biological FoulingFree from Biological FoulingpH detection relies purely on chemical interaction between hydrogen ion and sensing interface rather than membrane diffusion effect (glass electrode)

Voltage (V)

Cu

rren

t (i)

Voltage (V)

Cu

rren

t (i)

Continuous measurement in medium containing Continuous measurement in medium containing living cellsliving cells

pH insensitive pH insensitive moleculesmolecules

pH sensitive pH sensitive moleculesmolecules

Without pH Without pH controlcontrol

Detect change in pH from pH 7.82* to 7.01*

*pH verified by off-line pH measurements

pH-sensitive interfacesemiconductor

H+ H+ H+ H+

pH-sensitive interfacesemiconductor

H+ H+ H+ H+

pH-sensitive interfacesemiconductor

H+ H+ H+ H+

pH-sensitive interfacesemiconductor

pH-sensitive interfacesemiconductor

H+ H+ H+ H+

- H+

H+

Clotting of glass membrane

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Summary ChartSummary Chart

FeatureFeatureGlass Glass

ElectrodeElectrode ISFETISFET OpticalOpticalAmperometric Amperometric

SensorSensor

RobustnessRobustnessGlass (fragile)

Solid state Solid state Solid state

CalibrationCalibration

Required (frequently)

Required (longer calibration interval)

Required (factory)

Self calibrated

StabilityStability Drift Drift Drift No drift

MaintenanceMaintenance

Intensive (wet storage)

Easy (dry storage)

Intensive (Kept in dark (photo-bleaching)

Easy (dry storage)

Temperature Temperature RangeRange

Wide Wide Narrow Wide

pH rangepH range 0-14 0-14 5-9 (in ranges) 0-14

Biological Biological FoulingFouling

Occur Occur Occur NO

SensitivitySensitivity  Low Medium Good  Good

Ideal for Ideal for bioprocess pH bioprocess pH measurementmeasurement