There is a growing interest to replace conventional organic dyes with stable semi-conductor or carbon-based photoluminescent nanomaterials in fluorescence-based sensing applications. Photoluminescent carbon dots (CDs) are better compared to semiconductor nanomaterials in terms of their high biocompatibility and low toxicity and cost. To date , fluorescent CDs have been successfully exploited in a wide range of applications such as bioimaging , energy conversion and storage , and sensing.
Detection and quantification of ammonia is crucial for environmental , industrial , and biomedical purposes . Several methods , including amperometric , fluorimetric , and colourimentric , have been used for detecting ammonia in solution and in vapour phases .
Early amperometry-based methods for the detection of ammonia employed metal oxides , more commonly tin oxide-based sensors , in which the output resistance varied with ammonia exposure. Recently , more attention has been paid to the development of amperometric sensors using carbon-based nano-materials , such as grapheme and carbon nanotubes , as well as conducting polymers such as polyaniline. For example , an amperometric sensor comprising a multi-walled carbon nanotube electrostatically bonded to silver nanocrystals shows enhanced sensitivity compared with sensors comprising carbon nanotube alone. Although amperometric-based ammonia sensor are the most studied and offer high sensitivity , but suffers with low selectivity . Furthermore , moisture and volatile organic compounds strongly interfere. The requirement of high operating temperatures and cumbersome sensor design are other disadvantages of these methods.
Fluorescence sensors offer relatively higher sensitivity , easier design strategies , lower cost , and higher portability. Fluorometric systems have been successfully used for sensing gaseous analyte molecules. A selective fluorescence ‘turn-on’ sensing strategy for detecting ammonia has been demonstrated with rigid metal organic frameworks and some fluorophores incorporated as ligand. In another study , fluorophores , such as fluorescein and acridine orange , over cross-linked acrylic ester micro-particles were used for detecting ammonia vapour.
When theproxomity between a donor and an acceptor , having sufficient spectral overlap , is of < 10 nm , the Forster (fluorescence) resonance energy transfer (FRET) process can be achieved. The FRET-based sensing strategy is applied in diverse field , including cellular imaging , single molecule spectroscopy , DNA hybridization , and small molecule detection. Recently , few groups have used the FRET-based fluorescence strategy for detecting ammonia in the gaseous as well as liquid phases. For instance , in a fluorescence sensor , the addition of ammonia enhanced the FERT between coumarin (donor) and fluorescein (acceptor) because of the deprotonation of fluorescein , thereby leading to an improved spectral overlap. Mader et al. designed a FRET-based sensor , in which upconverted luminescence intensity of the nanoparticle decreased because of energy transfer between the nanoparticle and phenol red in the presence of ammonia.
Here , we report a novel , highly sensitive and selective fluorescence sensing platform for detecting ammonia in the solution and vapour phases by using sodium rhodizonate as an analyte specific molecule and CDs as the signal transducer. The mechanism of sensing scaffold depends on the the fact that excited state energy transfer (FRET) from CDs to sodium rhodizonate will be triggered once ammonia is introduced into the sensor solution (sensor solution refers to a solution containing 3 mL of CDs and 600 uL of 1mM sodium rhodizonate) , there by the fluorescence of CDs will be efficiently quenched ; the quenching can be used for sensing ammonia .
Furthermore , the sensor solution adsorbed over cotton fibres is used for detecting ammonia from the vapour phase. We found this platform to be highly sensitive , respond linearly to ammonia concentration , and reversible after passing HCI gas. The sensor system was highly selective towards ammonia over the oxidizing gases such as Nox , allowing its use for biological and industrial purpose , where discriminating ammonia from oxidizing gases is highly desirable. Low cost , fast response , high selectivity , linear response , and good reversibility make the sensor extremely appealing .