Carbon nanodots (CDs) are a promising nanomaterial due to their solubility in water, facile functionality and good photochemical stability. However, the development of novel carbon nanodots (CDs) with excellent optical properties still remains challenging.

A research group led by Dr. JIN Zongwen from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences demonstrated that the fluorescence properties of CDs can be optimized by simple modification of the molecular structure of the carbon source, providing a novel strategy for the manipulation of CDs luminescence. 

Their findings were published in Chemistry of Materials on July 23.  

The researchers found that the finally optimized CDs exhibited a rare extreme acid (pH 1 to 3) sensitivity. A distinguishable red/brown/green fluorescence color change could be achieved for a visual readout by simple tuning of the pH 1/2/3.  

The distinguishable color changes combined with the ratiometric fluorescence changes can improve the naked eye visual resolution to higher than 0.2 pH units. 

"To the best of our knowledge, this is higher than the commercial precision pH test trips and maybe is the best resolution to-date by the naked eye," said Dr. JIN, the corresponding author of this study. 

"Development of emerging technologies, such as smartphone-based sensing technology is likely to lead to the next generation of point-of-care testing (POCT) devices," said Dr. ZHAO Jianglin, the first author of this study. Given the colorful changes with the high correlation coefficient for the calibration curve of CDs under extreme acidic conditions, the researchers obtained precise color signals by the utilization of the smartphone-integrated CDs sensing platform and developed a portable, precise, rapid, visual pH measurement method. 

Besides, the researchers have designed a simple but efficient molecular "signal traffic light" logic circuit inspired by the reversible color changes of red/brown/green tri-fluorescence by tuning of the pH to 1/2/3. And given the unique pH dependent luminescence properties, the CDs have been further utilized in the anti-counterfeiting and information encryption field. 

Fig. 1 The schematic of tuning CDs fluorescence properties by modified the molecular structure of carbon sources (Image by SIAT)