Temperature non-uniformity detection on dPCR chips and temperature sensor calibration

A microfluidic-based digital polymerase chain reaction (dPCR) chip requires precise temperature control as well as uniform temperature distribution to ensure PCR efficiency. However, measuring local temperature and its distribution over thousands of mu L/nL-volume samples with minimum disturbance is challenging. Here, we present a method of non-contact localized temperature measurement for determination of the non-uniformity of temperature distribution over a dPCR chip. We filled the dPCR chip with a PCR solution containing amplified DNA fragments with a known melting temperature (T-M). We then captured fluorescent images of the chip when it was heated from 70 to 99 degrees C, plotted the fluorescence intensity of each partition as a function of temperature, and calculated measured T-M values from each partition. Finally, we created a 3-D map of the dPCR chip with the measured T-M as the parameter. Even when the actual T-M of the PCR solution was constant, the measured T-M value varied between locations due to temperature non-uniformity in the dPCR chip. The method described here thereby characterized the distribution of temperature non-uniformity using a PCR solution with known T-M as a temperature sensor. Among the non-contact temperature measurement methods, the proposed T-M-based method can determine the temperature distribution within the chip, instead of only at the chip surface. The method also does not suffer from the undesirable photobleaching effect of fluorescein-based temperature measurement method. Temperature determination over the dPCR chip based on T-M allowed us to calibrate the temperature sensor and improve the dPCR configuration and precision. This method is also suitable for determining the temperature uniformity of other microarray systems where there is no physical access to the system and thus direct temperature measurement is not possible.