CdS QDs surface for the selective recognition of formaldehyde gas via ratiometric contrivance

Journal Pre-proof Crafting CdTe/CdS QDs Surface for the Selective Recognition of Formaldehyde Gas Via Ratiometric Contrivance Imtiaz Ahmad, Zhan Zhou, Hai-Yang Li, Shuang-Quan Zang

PII:

S0925-4005(19)31578-3

DOI:

https://doi.org/10.1016/j.snb.2019.127379

Reference:

SNB 127379

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

17 September 2019

Revised Date:

2 November 2019

Accepted Date:

4 November 2019

Please cite this article as: Ahmad I, Zhou Z, Li H-Yang, Zang S-Quan, Crafting CdTe/CdS QDs Surface for the Selective Recognition of Formaldehyde Gas Via Ratiometric Contrivance, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127379

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Crafting CdTe/CdS QDs Surface for the Selective Recognition of Formaldehyde Gas Via Ratiometric Contrivance

Imtiaz Ahmad a, Zhan Zhou a,b*, Hai-Yang Li a*, Shuang-Quan Zang a

a.

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China. College of Chemistry and Chemical Engineering, Henan Key Laboratory of FunctionOriented Porous Materials, Luoyang Normal University, Luoyang 471934, P. R. China

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b.

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* Corresponding Author

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Tel : +86 371 67780136. E-mail: [email protected]; [email protected]

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Graphical abstract

The multi-emissive CdTe/CdS-HBQP Quantum dots were made by the surfactant displacement method. The new crafted QDs materials exhibit highly selectivity to formaldehyde and the calorimetric change red to green was visually observed.

Highlights 

Surface ligand crafting of CdTe/CdS QDS via HBQP



Formaldehyde sensitivity investigation in liquid phase



Detection of formaldehyde gas via QDs-HBQP entrapped cellulose paper

Abstract The ON-OFF ratiometric fluorescence formaldehyde (FA) reassuring system was introduced

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based on the surface ligand crafting of CdTe/CdS Quantum dots (QDs) by 3-(6-hydrazinyl-1,3dioxo-benzoisoquinolinyl) propanoic acid (HBQP) for the sensitive and selective determination of formaldehyde (FA) gas. The FA react with HBQP capped CdTe/CdS QDs, produced an

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optically active species at emitting fluorescence at 540 nm and ultimately disruption QDs

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surface leads naught fluorescence at 666 nm. The HBQP-QDs sense FA in liquid (phosphate buffer) and in gaseous phase by a solid sensor embedding on cellulose paper. The dual

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fluorescent change was linearly related to FA concentrations at the range of 1 to 28 µM with a

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lower detection limit of 0.49 µM as well the color change was observed with necked eye.

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Keywords: ON-OFF, Ratiometric, Quantum Dots, Formaldehyde, Sensor

1. Introduction Formaldehyde is the smallest aldehyde and a reactive carbonyl species is a well-known carcinogen and toxin. It tardily releases in the environments form various natural or manufactured sources i.e. vehicle exhaust, photooxidation of hydrocarbons, fumigations are outdoor and building materials, paint, textile, furniture are indoor sources [1, 2]. FA is also produced in the body by numerous enzymes and vital for many metabolic pathways [3]. The

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FA normal value in the body 100 μM (blood) and 400 μM (intracellular). The elevated FA level in the body leads to serious physiological consequences and pathogenesis including liver and

heart failure [4], diabetics, cancers [5] and other neurological disorders [6]. FA is a priority

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indoor pollutant and exposure to FA in an indoor environment lead to skin, eyes, throat and

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nose irritations and the level of irritation can be serious when it exceeded above 0.1 mg/m3 in a healthy individual. The elevated FA is accompanied by the body uneasiness sneezing, nausea,

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coughing, with lachrymation [7].

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Quantum dots (QDs) are promising optical transducers and has been widely explored for the biochemical sensor development, bio-labeling, biocatalytic and as an optical probe for

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bioimaging [8]. The surface of QDs are closely related to the nature of the optical properties that respond to a variety of chemical/biological species, henceforth reactivity to a variety of

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species has been explored for analytical determinations [9]. In most cases, it responds precisely to the analyte in corresponding interference [10], but reactivity is generalized for a group of species in the cause of less selective response to the analyte. Anchoring desired surface ligand during preparation or crafting QDs surface with the suitable surface candidate after synthesis made QDs as a selective and sensitive transducer for sensors fabrication or biolabeling. The

QDs with appropriate surface ligand gives a versatile approach to develop sensors for small molecules with improved selectivity in comparison with classical organic dye or organicinorganic molecules [11,12]. The QDs surfactant with carboxylic moiety is more often used for the surface anchoring with QDs during synthesis or at post-synthesis via ligand displacement for the target molecular binding or reaction. The versatility of RCOO- ligand for the QDs depend on the coordination

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ability of the binding mode of metal. Carboxylate ligands can bind to a single metal center via bidentate, anisobidentate, or unidentate coordination modes that, by analogy to nitrate ligands

[13]. The single of double coordination with metals on nanocrystal surface is feasible [14].

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Relating deviation of Cd with other metals, the Cd availing the maximum deviation from the

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carboxylate plan, henceforth it is more favorable to coordinate with RHCOO- [15-16]. The carboxylate coordination with nanocrystal surface atoms undergoes exchange reactions are

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significant. In recent investigation confirming that RCOO- form carboxylate on CdSe quantum

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dots via series of exchange reaction i.e oleate to oleic acid [17]. The typical example of carboxylate ligand exchange in a Tris(2-mercapto-1-t-butylimidazolyl) hydroborate cadmium

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complex has been recently carried out [13]. Concluding that carboxylate surfactant can be used as an effective ligand displacement species to craft the surface functionality of the

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nanomaterials for the target applications. In this report, we have crafted of CdTe/CdS QDs with analyte sensitive/selective fluorescence prob containing carboxylate moiety for ON-OFF dual ratiometric fluorescence change to FA. The selection of water-soluble QDs are favorable in a gas sensing, these QDs are stable in a humid environment and disabling the hindrance of humidity intrusion, which is a challenge

to design a gas sensor for the air quality monitoring [18]. The core QDs without shell are naive to the environment and the reactivity to the surface reactive species are debauched and uncontrolled [19]. Therefore, the core protection for QDs as an optical transducer is crucial to obtain the optimum response to the analyte in the corresponding interfere. Herein we made CdS shell protected CdTe quantum dots which bring the maximum stability and great optical transducing ability.

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The current directed screening of the chemistry work explore and predict the discovery of

the novel chemo-selective sensors[20], which are highly valuable to meet the contemporary

challenges of technology advancements and bring and environmental/health safety. An

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extensive effort has been done recently to produce selective chemical species for the detections

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and measurements of various important analyte [21, 22]. The chemo-selective gas sensors is still being challenging that need gas-phase but it has subsequently developed via click

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chemistry [23]. The chemo-selective species have high affinity to the analyte due the reaction

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specificity and concomitances with nanostructure produced a new generation sensors [24]. The dual detection of a single or a singular detection of two different species by

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fluorescence ON-OFF has not been widely reported [9,25,26]. The fluorescence ON-OFF change gives a response term of color change as well reassuring the selective measurement of

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the detecting species or it can be used to track multiple speciese [27]. The QDs were mixed with non-responsive organic dye for the calorimetric necked eye detections of FA has been reported before but that system does not improve the selectivity [28]. In this article, the FA responsive organic dye (HBQP) was prepared and crafted to QDs via surface ligand

displacement introduce a new model of ON-OFF dual detection that reassuring the FA tracking selectively both in solution and test paper. 2. Results & Discussions Crafting the QDs surface ligands is a key step to convert them into active fluorescent materials for the optical sensors, optical devices and fluorescent labelling for biological detection [2932]. The surface modification involved the ligand exchange to eradicate organic ligand and

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permit charge carriage in nanocrystal and to make them active for the detecting analyte. The

QDs surface manipulations also affect surface energies and hence fluorescent quantum yield (PLQY) has been controlled [33-37]. The CdTe-MSA 600 nm emitting wavelength was

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prepared with reported methods [38] and was further shelled with CdS to increase the

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stability and achieved the redshift to 666 nm that visualize the ratiometric fluorescence ONOFF change. (Figure 1, a). The redshift on the additional layers CdS was obtained. The

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prepared QDs CdTe/CdS-COOH was manipulated with the FA sensitive ligand BHQP that

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could track formaldehyde (Figure S2). A slight redshift was also observed on the ligand modification with BHQP, which is due to the size of the surface ligand. In comparison to

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COOH, BHQP is bigger and slight enhancement in QDs size. During the surface ligand manipulation, the core-shell of CdTe/CdS was stable and no significant alteration was

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observed. The slight energy transfer from the QDs crystals to the crafted ligand was observed, the phenomena was reported in CdSe QDs with polyaromatic acceptors as a surfactant [39]. Figure 1 b shows the PXRD of CdTe and CdTe/CdS with COOH and BHQP ligand. The CdTe QDs core peaks of 111, 220 and 311 lattice planes (JCPDS card No. 150770) was

observed and the shell CdS growth was confirmed on the shifting peaks 24.5, 40.9, and 48.1, shows the transition between CdTe and the cubic CdS phase (JCPDS card No. 210829) which is the characteristics of the CdTe/CdS Qds and reported before [40]. The morphological feature of the crafted CdTe/CdS-BHQP QDs are shown in Figure 1 (d-f). The TEM photographs show that the QDs are highly dispersible and equally distributed. The average diameter of 3.3 ± 0.5 nm was calculated. The crystal lattices of the QDs are clearly

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shown in the TEM pictures with the interplanar distance of 0.344 nm.

The FT-IR spectra of CdTe/CdS-COOH and CdTe/CdS-HBQP shows the significant

difference and the HBQP IR fingerprint (1710, 1594, 1425, 1226, 1058, and 939 cm−1 ) with

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slight shift has been observed in the surface manipulated CdTe/CdS QD by HBQP (1658, 1590,

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1400, 1226, 1030, and 987 cm−1). The IR broad band at∼3455 cm−1, assigned to water and OH. The bands and the shoulders at the range of 1610-1360 cm−1 correspond to Raman bands of

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RCO2− and CO2, but it overlaps with the vibration of the naphthalenic reduces the detailed

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identification of functionalities. Hence carboxylate groups (COO-) and Cd-carboxylate group are expected in this region, but the peak at 1710 (-COOH) is progressively displaced to lower

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1658 frequencies in high carboxylic and cadmium coordination [41]. FA is small, simplest aldehyde molecules with high reactivity to many chemical species

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in a different way and it can change the chemical nature of the number of chemical species. Several organic, inorganic, fluorescence species, that can react less or more specifically with formaldehyde, but the natural interference of FA analogs makes those methods less precise. Our method is superior as it is reassuring the specificity of FA with ON-OFF fluorescence change. The fluorescence spectra presented in Figure 2 b, shows the FA reactivity with

CdTe/CdS nanocrystals as well with its sensitive ligands. The formaldehyde sensitive functionality the ligand L-NH-NH2 that actively and specifically react with FA and produce −N=CH2 [42], which effectually leave the localized system in the HBQP and turn ON the fluorescence signal. The degree of turn-on is quantized by the amount of FA titrated in the system. The sensing system is relieved by the quenching of CdTe/CdS QDs emission at 666 nm, which is attributed by the energy transfer from the QDs crystals to the HBQP, cause there

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was no spectral overlapping with QDs emission and the HBQP absorption spectra or the thiol

displacement by the FA leads to dynamic quenching. The CdSe and CdTe have selective excitation of green or near the activated emission of HBQP fluorophore, the sensitizers triplet

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exciton in CdTe crystals migrate to the surface anchored HBPQ molecular acceptor. It has been

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envisioned and confirms the interfacial Dexter-like triplet-triplet energy transfer (TTET) between CdSe QDs and the surface displaced organic fluorophore [39].

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The reactivity of FA with small thiol molecules has been investigated in many reports,

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cysteine reacts with FA at pH 7 from the thiazolidine [43]. Xia, H et al made a QDs-Fluorescein based RGB sensors with less selective since, fluorescein is not responsive to FA and the CdTe

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QDs were naive with extremely reactive to FA and fast reaction kinetics that could bring a hindrance to use it for the analytical determination therefore the linearity of fluorescence

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response to the FA was not achieved in Xia H et al report. They explained the nucleophilic addition reaction on FA introduction to the QDs. The core protected CdTe/CdS normalized the extreme reactivity and make the materials feasible for analytical determinations. Figure 2 (b) the fluorescence intensity of QDs diminishing gradually, and it has been ratiometrically associated to the fluorescence advent by HBQP Figure 2 (c). The sensor

selectivity and performance were reassured by estimating ON-OFF fluorescence transformation for the respective FA aliquots added into the system shown in Figure 2 (d). The linear estimated range was 1 to 28 µM with a lower detecting concentration of 0.49 µM was achieved by measuring the analytical sensitivity error (LD = 3.3*σ/S) [44]. The kinetics fingerprint of the sensors is important to visualize the sensitivity and the feasibility of the sensor to perform in real sample interferences. The kinetics of reaction shows

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the time domain of the sensor performance and the nature of the fluorescence change [45]. The Figure 3 (b) shows that reaction kinetics of the 50 µM FA to the sensor solution (QDs-HBPQ) at the fluorescence signal at 666 and 540 nm overlapping on each other’s and equilibrize after

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7.2 Sec. The reaction is not unstable or reversible that lead to fluctuating the signal

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measurement. The first order kinetics was observed for both QDs and QDs surfactant HBPQ reaction with FA. The sensitivity of ON and OFF system was 216777, and 214886 CPS S-1 and

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for an ON-OFF change it was 215831 CPS S-1. Our results show that it was also feasible to

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initial rates of reaction (analytical regions) for the quantification of the FA concentrations. The sensitivity of the sensor achieved was risible to use the system in the sold state for the FA

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gas sensing.

The sensor performance was further investigated in the most common interferences i.e.

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ammonia, methanol, water, ethanol, acetic acid, L-cysteine, hydrogen peroxide, dichloromethane, acetaldehyde, benzaldehyde with FA. The equal concentrations of interfering agents mentioned above were added into CdTe/CdS-HBQP. The ON and OFF fluorescence change at the specific wavelength including the submission of ON and OFF (ratiometric) fluorescence change was compared with FA and its analogue. The results were Figured in 3 (a),

which shows the selectivity of the sensor in respect to total ON-OFF ratiometric change (coloured in blue figure 3 a) as well the fluorescence enhanced or quenched in CPS individually on the addition of the interfering agents (black and red respectively for 666 & 540 nm). It was observed that the sensor responds to aldehyde species 40 % in comparison to FA and the response to other intrusive species was negligible. Formaldehyde gas has been released from textile, building materials, paints, and furniture

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and increase the risk of indoor toxicity lives and the consequences (above 0.1 mg/m3) several health complications, and even prolong exposure lead to various cancer developments. It has

been listed as an important indoor pollutant [1]. Thereof is vital to construct a solid-state sensor

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that can detect FA gas at the vital range. The method of gaseous FA measurements is gas

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chromatography [46], high-performance liquid chromatography, which are expensive and complicated in operations and can’t be implemented for in-house FA monitoring [47]. Several

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electrochemical [48], optical, piezoelectric [49] and potentiometric [15] methods reported

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recently but they still need an optimization for long term stabilized selectivity. The QDs based optical solid-state chips are of low cost, easy to fabricate and easy in operation. For the efficient

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FA gas measurement, the appropriate solid substrate that allows maximum gas permeability and with large surface area are significant. Cellulose paper made of glucose units linked by beta

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1, 4-glycosidic linked. They contained a number of hydroxyl groups that facilitates the crosslinking to other species [50]. The dip-casting method of fabrication was used to decorate evenly the fluorescent QDs materials on the cellulose paper and the surface area evenly responded as an optical transducer. The solid sensor was placed in closed 20 mL container with aliquots of FA that produced FA gas on slow evaporation. The FA gas slowly diffused into the cellulose

pores that already decorated with CdTe/CdS-HBPQ. The data shown in Figure 4 represent the fluorescence response of the CdTe/CdS-HBPQ functionalized paper to the 1.5, 3, 4.5, 6, 7.5, 9, 11.2, 13.5 µM FA. The fluorescence was quenched at 666 nm and appeared at 540 nm, which is the function of FA concentration. The fluorescence Turn ON and Turn OFF at 540 and 666 nm respectively was linearly related to the FA concentration in Figure 4 (b) with the sensitivity (slope) for ON was 54228 CPS/Sec and of Turn, OFF was 56427 CPS/Sec. The total of ON-

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OFF fluorescence change was further plotted in Figure 4 (c) and a linear range of 1 to 13.5 µM

was achieved with the lower limit of detection 0.1 µM. It is been clearly demonstrated that the proposed model detect formaldehyde with high selectivity and sensitivity and the range of

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detection anconoid lower limit of exposure authorized by the health and environmental

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authorities [1,7].

The colour identification and vision of human eyes depend on the photoreceptors that

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respond/stimulate relative to another photoreceptor the range of visible light wavelength [51].

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It has been experimentally proven that the human eye responds to the range of wavelength in a different way. Among the range of visible wavelengths, the human eyes respond 10 times lower

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to the high energy wavelength of 440 nm (blue) relative to red and green ranging from 535 to 665 nm [52]. In Figure 4 (d-f) the photograph of QDs-HBQP paper (analysed by Image J) with

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different concentration of FA shows red to green shift. The range of colour change gives visual identification of the formaldehyde presence. The colour coordination obtained from the solid sensor response is shown in figure 4 (e) and was also represented in RGB histogram that can be visualized with necked eye. 3. Conclusion

In this work, we have made FA sensitive CdTe/CdS QDs and the surface ligand of the QDs was exchange with another FA sensitive HBQP ligand. The new system responds to FA selectively and the sensitivity achieved was at the recommended range of the health and environmental authorities. The sensing system responds to FA with ON-OFF change with bringing assurance to the measurement. The sensor will is the road map to the new generation of optical Nano-

4. Experimental section

4.1 Synthesis Carboxylic Acid Capped CdTe/CdS QDs

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chemo selective sensor developments.

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The QDs were prepared, briefly, 0.256 g (0.002 M) cadmium chloride, was diluted to

100 mL in one necked conical flask, and tri-sodium citrate dehydrates of 0.825 g (0.0008 M),

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under continues stirring. Sodium tellurite of 0.019 g (0.0003 M), mercaptosuccinic acid of

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0.875 g (0.0007 M) was added in the reaction mixture and was mixed until the complete solubilization of all reactants. Sodium borohydride of 0.435 g was added finally, and the

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reaction mixture was vigorously stirred at room temperature for 3 mins followed by direct heating in the open air at 100 °C for two hours. The core-shell CdTe/CdS QDs were obtained

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by the injecting 3 mL of 0.002 M CdCl2 solution in water to the red emitting CdTe QDs at 100

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°C. Aliquots of CdTe and CdTe/CdS QDs were collected and characterized. The obtained CdTe/CdS QDs were precipitated by an equal volume of ethanol and water followed by centrifugation for 15 minutes at the rate of 2000 RPM per minute. The first fraction of QDs precipitate was re-dissolved in 20 mL of water and re-precipitated with an equal amount of 1propanol followed by centrifugation for 25 minutes at 2000 RPM per minute. The QDs precipitate was re-dissolved in 35 mL water and was kept in the fridge at 4 oC for further use.

4.2 Synthesis of the Ligand (HBQP) As shown in Figure S1, 4-bromo-1,8-naphthalic anhydride (1.0 g, 3.6 mmol) and 3Aminopropanoic (0.32 g, 3.6 mmol) were dissolved in 20 mL ethanol. The mixture was stirred at room temperature for 1 h and then heated at 80℃ for overnight. After cooling the reaction mixture to room temperature, the mixture was slowly poured into iced water (100 mL). The resulting precipitate was collected by suction filtration and dried in vacuo to give compound 1

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as a brown solid. A mixture of compound 1 (694 mg, 2 mmol) and hydrazine hydrate (80%, 5

mL) in 10 mL ethanol was heated under reflux for 4 h. After cooling to room temperature, the precipitated product was filtered, washed with cold-EtOH, and the crude product was purified

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by flash chromatography on silica gel (MeOH/DCM = 1:30). 1H NMR (400 MHz, DMSO-d6)

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δ 8.61 (d, J = 8.0 Hz, 1H), 8.40 (d, J = 7.2 Hz, 1H), 8.29 (d, J = 8.8 Hz, 1H), 7.65 (t, J = 8.0 Hz, 1H), 7.24 (d, J = 8.8 Hz, 1H), 4.17 (t, J = 7.02 Hz, 2H), 2.17 (m, 2H). HR-MS calculated for

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C15H13N3O4 [M-H]- m/z 298.0827, found 298.0828.

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4.3 Surface Ligand Exchange of CdTe/CdS QDs

The surfaces of CdTe/CdS-COOH QDs were modified with FA sensitive ligand.

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Typically, 0.140 mg/mL CdTe QDs were dispersed in the mixture of 300 mL ethanol and 2 mL water. Then the solution was treated by sonication for 30 min followed by the addition of 10

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mg of ligand and the mixture was mechanically agitated for 3 mins at room temperature. Finally, the products were isolated with centrifugation at 8000 RPM for 5 min and then redispersed in water by 0.1 min sonication. 4.4 Formaldehyde Measurements in an Aqueous Phase

The CdTe/CdS capped HQPB was used to measured FA concentration in aqueous buffer media. Briefly, the FA stock solution of 3700 µM was made in phosphate buffer of pH 7.2. The gradual spiking method of analysis was used. An equal amount of CdTe/CdS- HBQP solution in phosphate buffer of pH 7.2 were spiked into 5 mL of the volumetric flasks followed by the addition of 5, 10, 20, 30, and 40 µL aliquots of the 3700 µM stock FA in flask 0 to 5. The volume was marked-up to 5 mL precisely with phosphate buffer of pH 7.2. The final

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concentration of the FA in the system was 3.6, 7.4, 14.7, 22.1, and 29.3 µM in flask 1 to 5. The fluorescence spectra were immediately measured exciting by 385 nm with a slit width of 2 nm.

the FA concentration.

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4.5 Fabrication of Solid-state Formaldehyde Sensor

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All the measurements were repeated thrice, and the spectra were further analyzed for measuring

The sensor was further used for the FA gas analysis and a solid paper substrate were used

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to fabrication. Briefly, a cellulose paper of 2 mm diameter was used (was produced from the

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chromatography paper) to make circular paper sensor strip. A dip casted method was used to fabricate the sensor. The paper was placed (completely merged) in QDs- HQPB solution for 3

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minutes in a closed system. The sensing paper was then dried in a 10 mL vial with nitrogen flush and air. After drying the sensor was exposed to 1 to 13.5 µg/cm3 in a closed glass vial of

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15 mL and the fluorescence spectra were recorded immediately at excitation of 385 nm. The photograph of the sensor strip was collected under UV irradiation of 365 nm in dark after exposure to the FA concentration range from 10 to 130 µg/cm3. Conflict of interest There are no conflicts to declare in present study.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21801227 and 21671175), the Program for Science & Technology Innovation Talents in Universities of Henan Province (164100510005), the Program for Innovative Research Team (in Science and Technology) in Universities of Henan Province (19IRTSTHN022) and Zhengzhou University.

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Author Biographies

Imtiaz Ahmad is a postdoc research associate in the College of Chemistry and Molecular Engineering of Zhengzhou University, China. He received his Ph.D. from the Department of Chemistry University of Liverpool, United Kingdom. He is working on functional nanomaterials for the optical healthcare and environmental sensors developments.

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Zhan Zhou is an associate professor in College of Chemistry and Chemical Engineering,

Luoyang Normal University, China. He received his Ph.D. degree in School of Chemistry and Environment from South China Normal University in 2015. His current research interests

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cover small-molecule fluorescent sensors and analytical chemistry.

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Haiyang Li received his Ph.D. with Prof. Shuang-Quan Zang at Zhengzhou University, Henan, China (July, 2017). Then he joined Prof. Zang’s group as an assistant professor. His

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metal-organic frameworks.

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current research interests include the synthesis, adsorption and fluorescence properties of

Shuangquan Zang is a professor in college of chemistry and molecular engineering,

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Zhengzhou University, Zhengzhou, China. He received his Ph.D. from Nanjing University, China in 2006. His current research interests include the precise assemble of metal clusters,

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and their applications in biological anticancer, biosensing, and environmental science, etc.

Figure Captions

Figure 1. The characterization of CdTe/CdS-HBQP quantum dots (a) the fluorescence spectra of CdTe QDs, CdTe/CdS and CdTe/CdS after crafting HBQP (b) The PXRD pattern of the CdTe QDs and CdTe/CdS formations and CdTe/CdS after modification with HBQP (c) The Infra-red spectra of the CdTe/CdS-COOH, CdTe/CdS-HBQP and HBQP. (d-f) the HR TEM photograph of the CdTe/CdS-HBQP QDs.

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Figure 2. The QDs-HBQP ON-FF response to FA (a) The fluorescence Quenching of QDsHBQP and fluorescence appearance of HBPQ of 0 to 26 µM of FA addition into the phosphate buffer of pH 7.5 (b) the fluorescence response at 666 and 540 nm of the sensor to the FA concentration (c) calibration curve of the ON-OFF change on the addition of the FA to the sensing system.

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Figure 3. The fluorescence ON-OFF kinetics of the CdTe/CdS-HBQP on 50 µM FA injection and sensor selective response in the interference for FA measurement. Where A to K representing ammonia, methanol, water, ethanol, acetic acid, L-cysteine, hydrogen peroxide, dichloromethane, acetaldehyde, benzaldehyde, and formaldehyde respectively.

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Figure 4. The QDs-HBQP ON-OFF response to FA (a) The fluorescence Quenching of QDsHBQP and fluorescence appearance of HBPQ of 0 to 13 µM of FA addition into 20 mL of closed glass vial that produced FA vapours (b) the fluorescence response at 666 and 540 nm of the sensor to the FA concentration (c) calibration curve of the ON-OFF change due to 0 to 13 µM FA gas in a closed container. (d) The fluorescence response (image at 365 nm under UV) of the paper exposed to different concentration of FA gas. (e) The colour coordination of fluorescence response of the FA gas to CdTe/CdS-BHQP in figure 4 a. (f) The histogram representing RGB coordination of the necked eye detection in a picture Figured 4 d.

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Scheme 1. The schematic of the QDs preparation, crafting by HBQP and the sensing mechanism.

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Figure 1.

Scheme 1.

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Figure 2.

Figure 3.

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