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Sensors

Overview

Sensors have become integrated into our daily lives, for either chemical, biological, mechanical, or optical applications.  Of these different types of sensors, chemical sensors have the widest application in a multitude of areas.  In particular, biosensors and electrochemical sensors have become increasingly important as biological and biochemical applications continue to emerge.  These applications include:

  • medical diagnostic
  • automotive
  • food monitoring
  • home/environmental monitoring
  • biological/chemical warfare (Homeland Security)
  • industrial
  • high throughput screening (HTS)

Electrochemical sensors can be divided into three categories: potentiometric, amperometric and conductometric.  Each of these categories can be divided further into different types of sensors by virtue of their mode of operation or specialized area of application:

 

Figure 1.  Typical chemical sensors: (a) tubular SnO2 gas sensor; (b) planar semiconductor sensor; (c) ion selective electrode (potentiometric); (d) amperometric gas sensor with liquid electrolyte; (e) potentiometric solid electrolyte O2 sensor (concentration cell); (f) amperometric solid electrolyte O2 sensor (current-limit type).

From “Sensors, Chemical Sensors, Electrochemical Sensors and ECS,” J.R. Stetter, W.R. Penrose and S. Yao, JECS, 150 (2) S11-S16, 2003. Reproduced by permission of The Electrochemical Society, Inc.

Potentiometric Sensors  
There are 4 basic types of potentiometric sensors, which all measure a change in potential on their surface relative to a reference electrode:

Ion selective electrodes
  • ion-selective electrode (ISE) 
  • eg. K+, Na+, Li+, Ca+, Cl-,HS-, HPO4-2, pH, etc.
  • coated-wire electrodes
  • similar to ISE, coated with ion-selective polymer membrane
  • eliminates need for separate reference (wire acts as reference)
  • allows for miniaturization
  • e.g. biomedical monitoring (immunosensors)
  • ion-selective field effect transistors (ISFET's)
  • ion-selective membrane on a field effect transistor (FET)
  • FET can measure charge buildup on membrane
  • allows for multi-sensor system microelectronics
  • eg. in vivo sensors, multiple ion sensing
  • gas sensors
  • uses solid electrolyte
  • allows for measurement of oxidizable gases O3, Hydrogen, Humidity NOx, Gaseous sulfur/H2S and exhaust gas hydrocarbons, etc.

Potentiometric sensors typically require for operation a voltmeter with 1 mV resolution over a range of 1 Volt, which allow these sensors to be the easiest to develop and implement.  Researchers working on the development of these type of sensors normally use cyclic voltammetry and linear sweep voltammetry to determine the reduction and oxidation potentials (redox) of the ion adsorbed at the sensor surface.

Amperometric Sensors  
These sensors are found in many forms and application areas and these operate on the principle that adsorbed ions will change the amount of current generated when a potential is appled; by both applying a known potential and measuring the resulting current, or by applying a scanning potential and measuring the resulting current, specific determinations can be made regarding the adsorbed species of interest.  They come in the form of:

  • thin film sensors (amperometric transducers
  • increased microscopic surface area within small geometric area
  • microarrays
  • environmental trace analyisis
  • chemically modified electrodes
  • detection of organic toxins, ionic surfactants
  • gas sensors
  • food quality monitoring (O2 and CO2)
  • electronic “noses”
  • hydrogen sensing
  • NO2 and NO
  • glucose sensors
  • pesticides, toxins
  • genomic (DNA/RNA)
  • proteomic (Proteins, enzymes, drug discovery, antigen/antibody)

These are just a few of the many examples of amperometric sensors that are under development or are commercially available.  These sensors generally utilize chronoamperometry (current measured as a function of time and applied potential) or chronopotentiometry (potential measured as a function of time and applied current), and typically require measured current ranges from 100 nA to 100 mA or + 1Volt with 1 mV resolution, respectively.

Conductometric Sensors  
These sensors rely on changes of electric conductivity of a film or bulk material where the conductivity is affected by the presence of the molecule of interest. The most common method is to measure changes in direct current (DC) values; other methods include measurements of the local electrochemical impedance.  Areas of application include: thin film sensors for O2, H2S, electronic “noses ,”  humidity, hydrocarbons, and biosensors (antigen/antibody and enzymes).  For DC measurements, no reference electrode is required and these sensors are the easiest to develop, usually requiring a conductivity meter to make the measurement.

Electronic "nose"

However, for the development of such a sensor and its film/bulk material, techniques such as chronoamperometry (current measured as a function of time and applied potential) or chronopotentiometry (potential measured as a function of time and applied current) are typically used, requiring a computer-controlled potentiostat/galvanostat.   For AC measurements, admittance data can be determined for specific frequencies, and require more sophisticated instrumentation.  

Instrumentation for Sensors and Sensor Development  
The potentiostats/galvanostats offered by Princeton Applied Research have outstanding performance specifications, whether it be the Versastat 3, PARSTAT 2273, PARSTAT 2263, 263A, or 273A.  Obviously, the model best suited for the sensor of interest that is being studied will be determined by the potential/current range requirements.  For example, if admittance measurements in combination with chronoamperometry are required, then a Versastat 3 potentiostat/galvanostat/FRA is recommended.

 
For sensor development, if throughput is an issue where multiple sensors need to be evaluated, the VMC series multi-channel potentiostat is the best choice. These systems are available with up to 4 independent potentiostats/galvanostats for simultaneous analyses on different cells and samples.

 
Software  
For sensor development and analysis studies, the most common methods involve: cyclic voltammetry and linear sweep voltammetry (contained in the PowerCV software packages), electrochemical impedance spectroscopy  (contained in the PowerSine software package), and  chronopotentiometry and chronoamperometry (contained in the PowerStep and PowerPulse electroanalytical software packages).   

 

 

Accessories

The Model K0264 Micro-Cell Kit is a complete electrochemical cell allowing for the measurement and analysis of various microelectrodes using voltammetric and  amperometric techniques, or the Model K0026 Coulometry cell kit for coulometric techniques for sensor applications.

Links:

Biosensors: http://clu-in.com/download/char/sensr_ec.pdf
Electronic “noses”: http://cns-web.bu.edu/pub/laliden/WWW/Papers/nose.html
Ion selective electrodes: http://www.chem.tufts.edu/courses/chem42/History-ISE.pdf
Admittance data: http://www.princetonappliedresearch.com/products/markets/psine.pdf
Chemical sensors:  http://www.irnsearch.sandia.gov/cgi-bin/techlib/access-control.pl/2001/010643.pdf
                                                        		 
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