An Introduction to Nanosensors

Imagine having to measure the temperature of separate living cells. With a cell being around 0.010 mm (0.00004") in size, the thermocouple wires (if that was the method chosen) will have to be less than a micron in diameter. This is the realm of nanosensors.

This OMEGA Engineering article defines nanotechnology and explains how it is enabling new types of sensors. It talks about the new measurement applications these sensors are opening up and points out the benefits of using compact sensors. The article places specific emphasis on the measurement of temperature with compact thermocouples. While few Engineers have to work at the nano scale, the lessons are applicable across several fields.

Fine gage bare wire thermocouples

Individual Sections Address:

• Nanotechnology and nanosensors
• Federal research support
• Examples of nanosensing applications
• Advantages of compact sensors
• Small-scale temperature measurement

Nanotechnology and Nanosensors

“Nano” refers to objects measured in nanometers, or billionths of a meter. To put that in perspective, a sheet of paper is about 100,000 nm thick and a strand of blonde hair measures around 30,000 nm. At this scale, surface area has a bigger effect on material behavior than it does for bigger objects. Consequently, properties like reflectivity, conductivity and magnetism change when compared to larger bodies.

Examples of Nanosensing Applications

Nanosensors have applications in the medical, defense and healthcare world, and consumer products. Here are a few examples:

Cleanroom

Laboratory setting

Detecting airborne chemicals: These sensors harness the change in electrical conductivity that occurs when molecules bond to nanowires developed from semiconducting materials such as zinc oxide. One application is detecting excess levels of carbon monoxide.

Detecting viruses and bacteria: These sensors also utilize variations in electrical conductivity, in this case that of carbon nanotubes to which an antibody is bonded. When a matching virus or bacteria attaches to the antibody, it is possible to measure a change in conductivity.

Measuring the temperature of living cells: At the Universities of Princeton and California-Berkeley, Researchers created “nano thermometers” capable of being inserted into separate cells. Instead of using conventional thermocouple wires, their technique makes use of semiconductor crystals that change color in response to temperature changes. On a larger scale, fine gage thermocouples are often used by Scientists for measuring temperatures in ex vivo tissues, as when investigating the heating effects of ultrasound.

Measuring temperature of nanofluids: Heat management is a growing issue, specifically in electronics, and research is ongoing in order to develop nanofluids with superior thermal conductivity characteristics. In this case, sensors are required for measuring these “nano” effects.

Advantages of Compact Sensors

Reducing the size of a sensor has a many benefits:

• Better signal-to-noise
• Faster response
• Increased data density
• More accurate data
• Less impact on the phenomenon being measured

To illustrate these points, the benefits of using fine gage bare wire thermocouples for temperature measurement should be considered.

Small-Scale Temperature Measurement

Response time correlates with wire gage. For example, OMEGA figures show while a thermocouple using 0.75 mm (0.03") diameter wire requires 40 seconds to respond to a given change in air temperature, one of 0.025 mm (0.0010") requires only 0.05 seconds.

Therefore, using a fine gage bare wire leads to significantly improved time-based resolution. This permits faster control responses, possibly enhancing quality in temperature-critical processes, and creates a higher data density which is valuable when trying to capture transient effects.

Attempting to measure a small phenomenon with a comparatively large tool results in a poor signal-to-noise ratio [imagine measuring the diameter of a fine wire with a 30. 5 cm (12") ruler]. Data quality is improved by scaling the sensor to the feature being measured.

Data quality is also improved via more precise placement of the sensor. With regard to a thermocouple, one developed from fine gage wire can usually be put closer to the heat source or desired location.

Shrinking the size of the measurement sensor (in this case, a thermocouple) means more can be employed in a given area. As a result, this increases the spatial density of data attained, enabling more precise tracking of effects such as heat flow.

In many situations, particularly when extremely small quantities are being measured or precise measurements are carried out, the influence of the sensor on the phenomenon becomes an issue. An accelerometer adds mass to a system of movement, possibly changing the results, and the same can be done by a thermocouple by conducting heat away from the measurement location. The same applies when temperature of a moving fluid is being measured; a bigger thermocouple produces a bigger disturbance in the flow. All these examples illustrate the benefits of minimizing the size and mass of sensors.

Thinking Small

Laboratory setting

Nanotechnology is considered to be an extremely active research field, and one with particular implications for sensor technology. Nano-materials, both fluids and solids, are enabling the development of new products, including compact sensors capable of being incorporated into a wide range of devices. However, this also drives a need for the ability to sense at the nano-scale, as when measuring changes in temperature.

Fine gage bare wire thermocouples are compact measurement devices that offer the potential to enhance both the quality and density of data harvested. Shrinking the thermocouple reduces the impact on the phenomenon being studied, provides faster response and allows more such devices to be integrated in a given area. Continued growth in nanoscale measurement applications is guaranteed since nanotechnology work is being supported by Federal funds.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.