There is a growing interest to replace conventional organicdyes with stable semiconductor or carbon-based photoluminescentnanomaterials in fluorescence-based sensing applications .Photoluminescent carbon dots (CDs) are better compared to semi-conductor nanomaterials in terms of their high biocompatibilityand low toxicity and cost . To date, fluorescent CDs have beensuccessfully exploited in a wide range of applications such asbioimaging energy conversion and storage [7,8], and sensing[9,10].Detection and quantification of ammonia is crucial for envi-ronmental, industrial, and biomedical purposes [11,12]. Severalmethods, including amperometric fluorimetric andcolourimetric have been used for detecting ammonia insolution and in vapour phases.Early amperometry-based methods for the detection of ammo-nia employed metal oxides, more commonly tin oxide-basedsensors, in which the output resistance varied with ammoniaexposure [19]. Recently, more attention has been paid to thedevelopment of amperometric sensors using carbon-based nano-materials, such as graphene and carbon nanotubes, as well asconducting polymers such as polyaniline [20–22]. For example, an amperometric sensor comprising a multi-walled carbon nanotubeelectrostatically bonded to silver nanocrystals shows enhancedsensitivity compared with sensors comprising carbon nanotubealone [23]. Although amperometric-based ammonia sensors arethe most studied and offer high sensitivity, but suffers with lowselectivity. Furthermore, moisture and volatile organic compoundsstrongly interfere. The requirement of high operating temperaturesand cumbersome sensor design are other disadvantages of thesemethods [19,24,25].Fluorescence sensors offer relatively higher sensitivity, easierdesign strategies, lower cost, and higher portability [26]. Fluoro-metric systems have been successfully used for sensing gaseousanalyte molecules. A selective fluorescence ‘turn-on’ sensing strat-egy for detecting ammonia has been demonstrated with rigid metalorganic frameworks and some fluorophores incorporated as ligand[27]. In another study, fluorophores, such as fluorescein and acri-dine orange, over cross-linked acrylic ester micro-particles wereused for detecting ammonia vapour [15].When the proximity between a donor and an acceptor, havingsufficient spectral overlap, is of <10 nm, the Förster (fluorescence)resonance energy transfer (FRET) process can be achieved. TheFRET-based sensing strategy is applied in diverse fields, includ-ing cellular imaging [28], single molecule spectroscopy [29], DNAhybridisation [30], and small molecule detection [31]. Recently, fewgroups have used the FRET-based fluorescence strategy for detec-ting ammonia in the gaseous as well as liquid phases. For instance,in a fluorescence sensor, the addition of ammonia enhanced theFRET between coumarin (donor) and fluorescein (acceptor) becauseof the deprotonation of fluorescein, thereby leading to an improved spectral overlap [16]. Mader et al. designed a FRET-based sensor,in which upconverted luminescence intensity of the nanoparticledecreased because of energy transfer between the nanoparticle andphenol red in the presence of ammonia [32].Here, we report a novel, highly sensitive and selective fluorescencesensing platform for detecting ammonia in the solutionand vapour phases by using sodium rhodizonate as an analytespecificmolecule and CDs as the signal transducer. The mechanismof sensing scaffold depends on the fact that excited state energytransfer (FRET) from CDs to sodium rhodizonate will be triggeredonce ammonia is introduced into the sensor solution (sensor solutionrefers to a solution containing 3 mL of CDs and 600 L of1 mM sodium rhodizonate), thereby the fluorescence of CDs willbe efficiently quenched; the quenching can be used for sensingammonia.Furthermore, the sensor solution adsorbed over cotton fibresis used for detecting ammonia from the vapour phase. We foundthis platform to be highly sensitive, respond linearly to ammoniaconcentration, and reversible after passing HCl gas. The sensorsystem was highly selective towards ammonia over the oxidisinggases such as NOx, allowing its use for biological and industrialpurposes, where discriminating ammonia from oxidising gases ishighly desirable. Low cost, fast response, high selectivity, linearresponse, and good reversibility make the sensor extremely appealing.
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