Direct pyrolysis of citric acid (CA) has been proved to be a facile bottom-up technique for making pristine carbon dots (CD) with homogenous size distribution. However, limited reports are available on systematic optimization of carbonization degree. In this investigation, pyrolysis temperatures between 160 °C and 220 °C were studied, based on CA thermal decomposition path, using various heating durations. The effect of the formation of more carbonized carbon particles (MCCPs), as the major byproduct of this method, on photoluminescence properties of CDs was also considered. The NaOH amount that neutralizes the solution and the effect of dilution on the emission intensity, were introduced as simple and accessible factors for monitoring carbonization degree, and an estimate of MCCP/CD ratio, respectively. The results show that the CDs fabricated at 160 °C, 50 minutes attain almost twice higher quantum yield (QY) of 29% than highest QY reported based on pyrolysis of CA. The so-prepared CDs can be employed as excellent candidates for turn-off sensing. As a proof of concept, detection limit of 50 nM for Hg2+ was achieved using a facile and inexpensive smartphone set-up that is able to quantify and compare fluorescent intensity in several samples simultaneously.