PCR reactions amplify DNA in minute samples to help researchers study pathogens in detail. Controlling the temperature at which these reactions occur is critical. However, measuring temperature using traditional thermometers that rely on contact with the sample faces several challenges. The equipment must allow for the rapid preparation of new samples to maximize utilization, which is not feasible with direct-contact methods and prevents accurate measurement. Furthermore, direct contact poses a risk of cross-contamination between samples. In addition, the temperature sensor must be precisely calibrated.
To help laboratory equipment manufacturers overcome these challenges, Melexis has developed far-infrared sensors such as the through-hole MLX90614 and the surface-mount MLX90632. These sensors are factory-calibrated and designed to maintain high accuracy in noisy and thermally dynamic environments.
Temperature Control in PCR Analysis:
PCR analysis comprises three processes, beginning with a high-temperature step called denaturation. This step is performed at 94–98°C to cut the double helix structure of DNA into two single-stranded DNA template molecules. In the next step, annealing, DNA primer molecules are forced to selectively bind to the two DNA templates. The temperature must be precisely controlled to ensure the primers bind correctly. The final stage is called elongation and is typically performed at 72°C. Here, nucleotides (the building blocks of DNA/RNA) in solution react with the template and primer to create double-stranded molecules.
Together, these three steps convert one DNA molecule into two double-stranded molecules (Figure 1). Repeating the cycle, doubling the number of molecules each time, quickly creates a large number of molecules (after, say, 40 cycles there will be 2^40 = 1,099,511,627,776 molecules), thus facilitating subsequent detection by other means.
Contactless temperature detection
Thermocyclers are a key component of apparatus used to facilitate temperature-sensitive biochemical reactions. Each thermocycler contains one or more heating blocks with openings into which tubes containing reagents can be inserted. The thermocycler exposes the tubes to a predetermined temperature program and is designed to allow for rapid and accurate temperature sweeps. Some models allow for a controlled temperature gradient across the heating block, exposing different samples to different temperatures. This feature is primarily used in the research phase to optimize certain critical steps in the temperature cycle.
Tight control loops depend on the accuracy of the sensor inputs. When samples are frequently replaced, it can be very difficult for manufacturers to reliably measure the tubes using a direct contact method. In this case, infrared sensors offer a significant advantage by enabling contactless temperature measurement. Furthermore, the risk of cross-contamination between samples is considerably reduced by avoiding direct contact.
Factory-calibrated sensors, such as the MLX90614, allow for easy implementation in the thermocycler. The sensor offers plug-and-play convenience, allowing the user to begin working immediately without going through a calibration process. The MLX90614 is offered with several field-of-view (FoV) options, including 90°, 35°, 12°, 10°, and 5°. Narrowing the FoV allows for a greater maximum distance between the sensor and the sample.
With the trend toward making equipment more portable and easier to use in various scenarios, the surface-mount MLX90632 offers the advantage of a very small form factor, resulting in significant size and weight savings. Encapsulated as a 3 mm x 3 mm x 1 mm QFN device, the sensor responds to temperature changes while maintaining accuracy and stability. The miniaturization of these sensors also facilitates integration into a sensing array, enabling multi-point temperature control.
By improving the speed and accuracy of thermocyclers, infrared sensors like the MLX90632 and MLX90614 play a critical role in enhancing the accuracy and productivity of PCR testing across numerous medical fields, beyond just combating COVID-19. The small size of the MLX90632 increases the prospects for affordable and portable PCR testing near the point of care.
Notably, medical-grade versions of both sensors offer even greater accuracy, within ±0.2°C, in the human body temperature range of 35–42°C. These sensors are well-suited for use in mass screening systems, where a narrow field of view allows for greater detection distance, and in personal temperature monitors, such as wearable devices and patches, which require highly miniaturized and sensitive, yet stable and reproducible, sensors.
Conclusion:
Recent events have highlighted the need for accurate and rapid diagnostics. Small infrared sensors play a crucial role in enabling laboratory teams to meet the need for rapid analysis of biological samples.
The latest infrared temperature sensors combine small size and immunity to thermal interference to overcome the main technical challenges of contactless detection. These advancements offer numerous advantages, including increased accuracy, greater ease of use, and precise temperature control for biochemical processes such as PCR analysis, providing rapid and reliable diagnostic capabilities.
