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FRET based ammonia sensor using carbon dots
Accepted Manuscript Title: FRET based Ammonia Sensor using Carbon Dots Author: Manjunatha Ganiga Jobin Cyriac PII: DOI: Reference:
S0925-4005(15)30657-2 http://dx.doi.org/doi:10.1016/j.snb.2015.11.074 SNB 19332
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-7-2015 12-11-2015 17-11-2015
Please cite this article as: M. Ganiga, J. Cyriac, FRET based Ammonia Sensor using Carbon Dots, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
FRET based Ammonia Sensor using Carbon Dots Manjunatha Ganiga and Jobin Cyriac* Department of Chemistry, Indian Institute of Space Science and Technology
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Thiruvananthapuram – 695 547, INDIA
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*To whom correspondence should be addressed, e-mail: [email protected]
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Prepared for publication in Sensors and Actuators B: Chemical
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Correspondence to:
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Jobin Cyriac Department of Chemistry Indian Institute of Space Science and Technology (IIST) Valiamala P.O. Thiruvananthapuram Kerala – 695 547, INDIA
Abstract
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We present a Förster (fluorescence) resonance energy transfer (FRET)-based sensing platform for the selective detection of ammonia by using carbon dots (CDs) as a signal transducer and sodium rhodizonate as an analyte-specific molecule. A solution containing CDs and sodium rhodizonate was used as the sensor system. Both solution and vapour phase detection of ammonia was performed. Excited state energy transfer (FRET) from CDs to sodium rhodizonate was activated when ammonia was present in the sensor solution. Thus, the FRET process efficiently quenches the fluorescence of CDs and can be used for detecting ammonia. The detection limit of the present sensor system was up to 3 ppm. Furthermore, we exploited cotton fibers coated with the sensor solution to selectively detect ammonia vapours. Selectivity was demonstrated using oxides of nitrogen and common organic solvents. The present FRET-based sensor demonstrates high sensitivity and selectivity as well as good reversibility. Key Words: Förster resonance energy transfer (FRET), ammonia sensor, carbon dots, fluorescence sensor
___________________________________________________________________________ 1. Introduction
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There is a growing interest to replace conventional organic dyes with stable semiconductor or carbon-based photoluminescent nanomaterials in fluorescence-based sensing applications.[1-3] Photoluminescent carbon dots (CDs) are better compared to semiconductor nanomaterials in terms of their high biocompatibility and low toxicity and
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cost.[4] To date, fluorescent CDs have been successfully exploited in a wide range of applications such as bioimaging,[5,6] energy conversion and storage,[7,8] and sensing.[9,10]
Detection and quantification of ammonia is crucial for environmental, industrial, and purposes.[11,12]
Several
methods,
including
amperometric,[13,14]
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biomedical
fluorimetric,[15,16] and colourimetric,[17, 18] have been used for detecting ammonia in
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solution and in vapour phases.
Early amperometry-based methods for the detection of ammonia employed metal oxides,
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more commonly tin oxide-based sensors, in which the output resistance varied with ammonia exposure.[19] Recently, more attention has been paid to the development of amperometric
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sensors using carbon-based nanomaterials, such as graphene and carbon nanotubes, as well as conducting polymers such as polyaniline.[20-22] For example, an amperometric sensor comprising a multi-walled carbon nanotube electrostatically bonded to silver nanocrystals
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shows enhanced sensitivity compared with sensors comprising carbon nanotube alone.[23]
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Although amperometric-based ammonia sensors are the most studied and offer high sensitivity, but suffers with low selectivity. Furthermore, moisture and volatile organic
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compounds strongly interfere. The requirement of high operating temperatures and cumbersome sensor design are other disadvantages of these methods.[19,24,25] Fluorescence sensors offer relatively higher sensitivity, easier design strategies, lower
cost, and higher portability.[26] 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.[27] In another study, fluorophores, such as fluorescein and acridine orange, over cross-linked acrylic ester micro-particles were used for detecting ammonia vapour.[15] When the proximity between a donor and an acceptor, having sufficient spectral overlap, is of <10 nm, the Förster (fluorescence) resonance energy transfer (FRET) process can be achieved. The FRET-based sensing strategy is applied in diverse fields, including cellular imaging,[28] single molecule spectroscopy,[29] DNA hybridisation,[30] and small molecule
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detection.[31] 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 FRET between coumarin (donor) and fluorescein (acceptor) because of the deprotonation of fluorescein, thereby leading to an
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improved spectral overlap.[16] 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.[32]
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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
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analyte-specific molecule and CDs as the signal transducer. The mechanism of sensing scaffold depends on the fact that excited state energy transfer (FRET) from CDs to sodium
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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 µL of 1 mM sodium rhodizonate), thereby the fluorescence of CDs will be efficiently quenched; the quenching
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can be used for sensing ammonia.
Furthermore, the sensor solution adsorbed over cotton fibers is used for detecting
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ammonia from the vapour phase. We found this platform to be highly sensitive, respond
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linearly to ammonia concentration, and reversible after passing HCl gas. The sensor system was highly selective towards ammonia over the oxidising gases such as NOx, allowing its use
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for biological and industrial purposes, where discriminating ammonia from oxidising gases is highly desirable. Low cost, fast response, high selectivity, linear response, and good reversibility make the sensor extremely appealing. 2.
Experimental section
Reagents and Materials: Phosphorous pentoxide, copper turnings, and quinine sulfate
were purchased from Sigma Aldrich, India. Acetic acid, nitric acid, and ammonia solution were supplied by Merck Specialties, India, and sodium rhodizonate was obtained from Otto Chemicals, India. All aqueous solutions were prepared using double distilled water. UV-visible absorption spectra were recorded using a Carry-100 double beam UVvisible spectrophotometer. Infrared (IR) spectroscopy studies were conducted using a Spectrum 100T Perkin-Elmer FT-IR spectrometer. For collecting the FTIR spectra, samples were lyophilised for 2 days at −30 °C, and IR spectra were recorded in an attenuated total internal reflection mode with 32 average scans. The dynamic light scattering (DLS)
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experiments and zeta potential measurements were carried out using a Zetasizer Nano ZS (Malvern Instruments). Transmission electron microscopy (TEM) images were obtained using a JEOL 3010 UHR instrument. All fluorescence measurements were performed using the Fluoro Max-4C Spectrofluorometer (Horiba Instruments, USA). Excitation wavelength
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was fixed at 400 nm with an integration time of 0.1 s. Both excitation and emission slit widths were kept at 5 nm. According to a previous report, the quantum yield of the CDs was calculated using quinine sulphate (φ = 0.54) as a standard.[5] Time-resolved fluorescence
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measurements were performed using time-correlated single-photon counting (360 nm excitation wavelength by using a picosecond diode laser, with 10.5 nm excitation band pass
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and 9.5 nm emission band pass; Horiba Instruments). Thermogravimetric analysis (TGA) was conducted using TA instruments Q50. Heating rate was maintained at 10 °C/min in an inert
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atmosphere. Photostability experiments were performed using a UV light source (360 nm, power: 8 W).
Preparation of CDs: Carbon nanoparticles having a green fluorescence emission were
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prepared according to a previous report.[5] In short, a solution containing glacial acetic acid (1 mL) and water (80 µL) was rapidly added to 2.5 g of P2O5 in a 25 mL beaker without
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stirring, and the reaction mixture was cooled to room temperature. The dark brown solution containing CDs was then collected by dispersing in distilled water and centrifuged, and the
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clear supernatant solution was collected for further use. The pH of the purified solution was 3. The sensor solution was then prepared by mixing 3 mL of CDs and 600 µL of 1 mM
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sodium rhodizonate.
Oxides of nitrogen (NO2 and NO) were generated by slowly adding concentrated HNO3
into a conical flask containing copper turnings. The evolved reddish brown gas was then introduced onto the cotton fibers coated with the sensor solution. 3.
Results and Discussion
3.1 Characterisation of CDs
TEM and DLS studies showed that the average size distribution of the CDs is 3−4 nm (Fig. S1, Supporting Information). UV-visible spectrum (Fig. 1) of the CDs confirmed the presence of C=C and C=O groups as electronic absorption bands were observed at 250 nm and 300 nm, which are ascribed to the π→π* and n→π* transitions of the C=C core and C=O surface groups, respectively.[5,33] CDs showed fluorescence emissions at excitation from 350 to 450 nm.[5] Excitation at 400 nm showed a strong photoluminescence (PL) emission
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band centred at 490 nm (Fig. 1). The quantum yield of purified CDs, calculated using quinine sulfate as a standard (QYref = 0.54), was 5.28% (Fig. S6, Supporting Information).[34] The large full width at half maximum (FWHM) in the PL spectrum was attributed to the presence of multi-emissive states arising from the emission of the core and surface groups or due to the
confirmed by fluorescence lifetime measurements (discussed later).
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presence of surface defects.[4,35] The presence of multi-emissive states was further
The IR spectrum of the purified CDs showed characteristic vibrations at 1720 and 1630 cm , supporting the presence of surface carboxyl groups and sp2-hybridised carbon core,
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−1
respectively (Fig. S2, Supporting Information). Compared with that of acetic acid alone, a
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strong band appeared at 1630 cm−1, which corresponded to the sp2 C=C stretching vibration; reflected the successful carbonisation of acetic acid by P2O5 and water. This was further
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confirmed by the presence of aromatic C-H stretching at 3021 cm−1. The band centred around 3200 cm−1 was due to the stretching vibrations of surface −OH groups.[36] A zeta potential value of +10.3 mV in water suggested dispersion stability because of the presence of
1.0 0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
Normalized emission (a.u.)
0.8
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Absorption Emission
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Normalized absorbance (a.u.)
1.0
d
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numerous surface −COOH groups in the CDs. [37,38]
200 250 300 350 400 450 500 550 600 650 700
Wavelength (nm) Fig. 1. Absorption and emission spectra of CDs. Excitation wavelength was fixed at 400 nm. Inset: Photograph of the CD solution under UV light.
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Na+
O
O-
OH
NH3
HO
O-
O
OH HO
O
OH
O O
(I)
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O
(II)
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Na+
OH
Scheme 1. Schematic illustration of the detection of ammonia. ‘CD’ represents carbon dots with functional groups such as −COOH and −OH.
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3.2 Interaction between CDs and sodium rhodizonate
UV-visible spectra of the aqueous solution of sodium rhodizonate (I) with and without
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CDs are shown in Fig. 2(A). Pure sodium rhodizonate exhibited characteristic visible absorption bands at 445 nm and 485 nm (red trace).[39] However, the sensor solution, composed of CDs and sodium rhodizonate, lacked these features. The addition of sodium
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rhodizonate (I) to CDs having plenty of surface −COOH groups (at pH = 3) resulted in the
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formation of a colourless dihydrate form of rhodizonate (rhodizonate (II); Scheme 1). The protonation and subsequent dehydration of rhodizonate (I) resulted in the formation of
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rhodizonate (II) structure with absorption at 365 nm (blue trace). A similar observation has been reported previously in which rhodizonate (II) was found at a lower pH.[40] These observations confirm that sodium rhodizonate is present as rhodizonate dihydrate form (II) in our sensor solution. Also n-π* absorption band at 290 nm corresponding to the surface – COOH group of CDs was red shifted to 300 nm after the introduction of rhodizonate (I). This is attributed to the conversion of –COOH group of CDs to carboxylate anion (scheme 1), which lead to the stabilization of π* orbital due to the weakening of π bond.However, no appreciable change was observed in the emission behaviour of CDs with the addition of sodium rhodizonate, implying the absence of excited state energy transfer between CDs and rhodizonate (II). This can be ascribed to the lack of spectral overlap between the two molecular species (absorption spectrum of rhodizonate (II) and emission spectrum of CDs) as shown in Figure S3, Supporting Information. The steady-state fluorescence spectra of CDs
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with and without sodium rhodizonate excited at 400 nm are shown in Fig. 2(B). The PL spectra were recorded at an excitation wavelength of 400 nm to avoid the inner filter effect. CD
290
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(ii)
300
50 40 30
Rhodizonate
20 10
0.0
0 400 500 Wavelength (nm)
(iii)
450
600
(iv)
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0.5
500
550
600
650
700
750
Wavelength (nm)
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300
CDs at 400 nm Excitation CDs + 100 µL Rhodizonate (1 mM) CDs + 200 µL Rhodizonate (1 mM) CDs + 300 µL Rhodizonate (1 mM) CDs + 400 µL Rhodizonate (1 mM) CDs + 500 µL Rhodizonate (1 mM) CDs + 600 µL Rhodizonate (1 mM)
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CD (i)
(B)
60
Intensity (a.u.)
Absorbance
70
CD-Rhodizonate
1.5
1.0
(A)
Rhodizonate
2.0
Fig. 2. UV-visible spectra of (A) CDs (black), sodium rhodizonate (red), and CDs with sodium rhodizonate (green). Inset: Photograph of sodium rhodizonate solution before (i) and
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after (ii) the addition of CDs. (B) The PL spectra of CDs with and without sodium rhodizonate recorded at an excitation wavelength of 400 nm. Inset: Photographs of CDs
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under UV light in the absence (iii) and presence (iv) of sodium rhodizonate.
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3.3 Ammonia sensor: solution phase
Aqueous ammonia was introduced at various concentrations to the sensor solution,
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comprising CDs and rhodizonate (II). The fluorescence of CDs decreased concomitantly, as clearly seen in Fig. 3(A). The reduction in the fluorescence intensity of CDs was proportional to the amount of ammonia being introduced, allowing the quantification of ammonia present in the solution. For this purpose, a calibration graph was drawn with F0−F (where F0 and F are the fluorescence intensity at 470 nm before and after the addition of ammonia, respectively) as the ordinate and ammonia concentration along abscissa (Fig. S4, Supporting Information). A linear relationship between the extent of PL quenching (F0–F) and ammonia concentration was observed (R2 = 0.9641). The detection limit calculated using 3*sbl/k was 3 ppm, where sbl was the standard deviation for the blank signal and k was the slope of the plot (Fig. S4, Supporting Information). The fluorescence quenching of CDs was associated with a red shift and decreased FWHM (Fig. 3). This can be ascribed to the selective quenching of blue emitting states of CDs by resonance energy transfer. Only the blue region of the CDs emission spectrum had a spectral overlap with the absorption spectrum of the acceptor (inset of Fig. 3). This provides
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an indirect evidence for the presence of FRET between the donor (CDs) and acceptor (sodium rhodizonate). Efficiency of the FRET process calculated using steady state fluorescence data was 40% (see Supporting Information S9 for the calculation). The photograph shown in Fig. 3 depicts a clear change in the fluorescence of the sensor solution
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with the addition of ammonia, allowing visual detection, under a UV light. As shown in Scheme 1, the addition of ammonia deprotonates rhodizonate (II) to rhodizonate (I). This argument is supported by the UV-visible spectrum of the sensor solution
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treated with various concentration of ammonia (Fig. 4(A)). The absorption bands at 445 and 485 nm, which were attributed to rhodizonate (I), were appeared after the addition of
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ammonia to the sensor solution. These absorption bands had a strong spectral overlap with the emission spectrum of CDs (inset of Fig. 3). Hence, FRET can be presumed between the
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CDs (donor) and the in situ generated sodium rhodizonate (acceptor), which is responsible for
1
60
Absorbance (Acceptor) 1.0 Emission (Donor)
0.8
0.8
0.6
0.6
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80
0.4
0.4
0.2
0.2
0.0
40
3
20
4
5
6
0
450
0.0 450
2
Ac ce p
Intensity (CPS)
100
1.0
Norrmalized emission (a.u.)
120
Normalized absorbance (a.u.)
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quenching the CDs emission when ammonia is introduced.
500
550
600
650
700
Wavelength (nm)
NH3
(i)
(ii)
7 8
500
550
600
650
700
750
Wavelength (nm)
Fig. 3. The PL spectra of the sensor solution with the addition of ammonia at various concentrations recorded at an excitation wavelength of 400 nm. The numbers 1–8 depict 0, 50, 100, 150, 200, 250, 300, and 350 ppm of ammonia, respectively. Photograph no. 1 and 8 taken under UV light are denoted as (i) and (ii), respectively. The spectral overlap between donor’s emission and acceptor’s absorption is shown in inset.
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In addition to spectral overlap, FRET demands the participating molecules to be at close proximity, essentially <10 nm. The absorption bands at 445 and 485 nm corresponding to sodium rhodizonate (structure I) restored with the addition of ammonia (Fig. 4(A), red trace). Here, the structure I containing C=O as well as enolate anion interact with −COOH
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and −OH groups of CDs through hydrogen bonding.[41] As discussed in the preceding section, absorption of CDs at 300 nm was ascribed to the n→π* transition of surface carbonyl groups. When ammonia was introduced to the sensor solution, this peak was strongly blue
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shifted to 290 nm (Fig. 4(A)). Blue shift in the n-π* transition of the carbonyl group was due to the stabilisation of the non-bonding ‘n’ orbital because of strong hydrogen bonding
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interaction.[42] This resulted in an increase in the energy gap between ‘n’ and π* orbitals of the carbonyl group and hence blue shift in absorption, which corroborates the presence of the
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hydrogen bonding interaction between the CDs and in situ generated sodium rhodizonate. The lifetime of the donor’s excitation decreases when the donor molecule is engaged in
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the energy transfer process. Hence, fluorescence lifetime measurement can provide a strong, direct evidence for the presence of the excited state energy transfer process. The timeresolved fluorescence measurements presented in Fig. 4(B) and the data given in Table 1
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clearly depict the presence of the FRET process between the CDs (donor) and rhodizonate
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(acceptor). Lifetime of the CDs solution alone and in sensor solution is triexponential with an average lifetime of 6.8 ns. Instead, lifetime of CDs after the addition of ammonia to the
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sensor solution is a 4-exponential fit with an average lifetime of 5.9 ns. The substantial decrease in lifetime of the donor (CDs) after the addition of the analyte strongly supports the presence of an excited state energy transfer process. From Table 1, we can see the emergence of an ultrafast component having a lifetime of 0.03 ns. This short-lived component majorly originates from the energy transfer from CDs to rhodizonate (I) because the exchange of the virtual photon for energy transfer is a kinetically faster process.[7]
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CDs-Rhodizonate CDs-Rhodizonate-NH3
(A) NH3
300
CDs-Rhodizonate-NH3
0.8
8k
CDs-Rhodizonate-NH3
6k
0.6
Decay
Absorbance
CDs CDs-Rhodizonate CDs-Rhodizonate-NH3
(ii)
(i)
NH3
0.4
(B)
10k
CDs-Rhodizonate-NH3
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1.0
4k
290
2k
cr
0.2
0 0.0 350
400
450
500
550
0
10
20
30
40
50
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300
Wavelength (nm)
Time (ns)
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Fig. 4. (A) UV-visible spectra of the sensor solution with and without ammonia. Corresponding visible light photographs are given as insets (i) and (ii). (B) Time-resolved fluorescence spectra of CDs (green trace), sensor solution (CDs–rhodizonate) (black trace),
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and CDs–rhodizonate–ammonia (red trace). The excitation wavelength was 400 nm and emission collection was at 500 nm using the picosecond diode laser.
τ1 (ns)
a1
CDs
2.41
0.34
τ2 (ns)
τ3 (ns)
a3
τ4 (ns)
a4
χ2
< τ> = (a1 τ12 + a2 τ22 + a3 τ32)/(a1 τ1 + a2 τ2 + a3 τ3) (ns)
0.60
0.32
0.06
-
-
1.35
6.8
1.25
6.8
1.011
5.9
a2
te
System
d
Table 1: Time-resolved fluorescence data of the FRET donor and donor–acceptor system
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7.54
CDs−Rhodizonate
2.43
0.34
7.6
0.59
0.34
0.06
CDs−Rhodizonate −Ammonia
2.62
0.436
0.56
0.116
7.45
0.34
0.03
10.3
3.4 Ammonia sensor: gaseous phase Gaseous phase detection of ammonia is crucial in many industries. Here, we
demonstrated an efficient method for rapidly detecting ammonia vapours by using our sensor solution adsorbed on a cotton substrate (we will refer to the sensor solution-coated cotton substrate as ‘sensor wool’). For this purpose, we prepared a cylindrical-shaped cotton wool with approximately 3 cm height and 1 cm diameter (Fig. 5). The shape was selected to ease fluorescence measurements. Low cost, easy availability, and importantly, lack of fluorescent background are the added advantages of using cotton in the present study. The cotton wool was immersed in the sensor solution for 1 min, followed by vacuum drying to remove the
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excess solvent. The sensor wool was then exposed to various ammonia concentrations, and the fluorescence spectra were eventually recorded. Similar to the solution phase sensor, this gaseous phase sensor showed a detection limit of 3 ppm. The fluorescence emission of CDs decreased as ammonia concentration increased (Fig. 5). A rough conclusion could be
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achieved by observing the sensor wool under UV lamp immediately after the ammonia exposure. The probe has a quick response towards ammonia and the detection can be realized in 5 sec. Alternatively, the sensor-coated silica plate can be used for sensing. There is a clear
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colour contrast in the silica-coated plate adsorbed with the sensing mixture before and after ammonia exposure under the UV light source, facilitating a facile visual detection of
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ammonia (inset of Fig. 5). The fluorescence intensity of the CDs was completely regained when the sensor wool was exposed to HCl gas, and the wool could be reused for detecting
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ammonia. Because the materials and chemicals are inexpensive, detailed studies regarding the reversibility of the sensor were not conducted. The easy design strategy, high sensitivity, and quick response, makes the sensor wool superior to many of the previously described
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ammonia sensors.[16, 19]
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60
NH3
(iii) (i)
(ii)
before NH3 exposure
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Intensity (a.u.)
80
after NH3 exposure
40
20
0
450
500
550
600
650
700
750
Wavelength (nm) Fig. 5. Fluorescence spectra of the sensor wool before and after ammonia vapour exposure. The ammonia concentration was 300 ppm. (i) and (ii) show the photographs of the sensor solution dried over the silica plate before and after ammonia exposure, respectively. Visible
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light photographs of the sensor wool before and after ammonia exposure are given alongside of the corresponding spectra. 3.5 Selectivity towards NOx Most industrial or biological processes require ammonia as the precursor gas for
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producing oxides of nitrogen and vice versa. Therefore, selective sensing of one over the other is critical. Several reports have emphasised the detection of reducing gases, such as
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ammonia, and oxidising gases, such as NOx, but the selectivity of one over the other is rarely discussed in literature.[21,43] Most metal oxide-based amperometric sensors, which change
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the output current when the analyte molecule is adsorbed, respond equally well to the electron-donating molecules, such as ammonia, and electron-accepting gases, such as NOx; this prevents selective detection. For example, a sensing device comprising a reduced
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graphene oxide (RGO) suspension deposited over gold interdigitated electrodes changed its electrical conductivity after NO2 exposure, but the selectivity of the device towards ammonia
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remained unaddressed.[44] In another report, a gas sensor comprising RGO sheets responded to both ammonia and NOx in a similar manner.[45]
By contrast, our sensor wool showed a highly selective response towards ammonia over
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NOx. As shown in Fig. 6, a negligible change in the fluorescence emission of the sensor wool
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was observed after exposure to 350 ppm NOx. The extent of fluorescence quenching with an equal concentration of ammonia clearly dictated the selectivity (inset of Fig. 6). NOx cannot
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change rhodizonate the dihydrate form (II) to rhodizonate (I), thus excluding the possibility of energy transfer from CDs. Hence, the emission from CDs was unaffected after NOx exposure.
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100 CD-Rhodizonate CD-Rhodizonate-NOx
80
CD-Rhodizonate-NH3 70 60 50 40 30 20
40
10 0
Before NOx
After NOx
0 450
500
550
600
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20
NOx
cr
NH3
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60 F0-F
Intensity (a.u.)
80
650
700
750
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Wavelength (nm)
Fig. 6. Fluorescence spectra of the sensor wool before and after exposure to NOx gas.
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Fluorescence spectrum of the sensor wool exposed to ammonia is also shown for comparison (blue trace) with the corresponding photographs. Comparison of the extent of fluorescence quenching is given inset.
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Ammonia is a precursor, solvent, or sometimes, a byproduct in many organic syntheses.
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Therefore, the selectivity of the present sensor system towards ammonia over common organic solvents was an added advantage. Hence, we further extended the selectivity studies
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to solvents such as acetone, ethanol, toluene, diethyl ether, chloroform, and ethyl acetate. The sensor wool was exposed to the vapours of these solvents (350 ppm), and negligible changes were observed in the fluorescence emission profile (Fig. S5, Supporting Information). The results showed that the present sensor solution is highly selective towards ammonia over oxidising gases, such as NOx, and other organic solvents. Hence, the present sensor system can be exploited for industrial and laboratory applications. 3.6 Stability
Several organic dyes studied for fluorescence sensing of ammonia, such as fluorescein and coumarin, are prone to photo-degradation or thermal degradation. Hence, such sensors lack reproducibility because of the fluctuations in fluorescence intensity with time or environment. Photostability experiments of the present sensor solution were performed by irradiating the sensor solution under a 360 nm UV light (lamp power: 8W) for up to 24 h. Emission spectra were recorded at different time intervals. A plot of fluorescence intensity as
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a function of irradiation time is shown in Figure S6. Approximately 21% decrease in emission intensity was observed after 24 h of irradiation. The sensor solution was maintained at ambient condition for 15 days and checked for temporal stability. The fluorescence emission spectrum collected after 15 days is given in
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Figure S7. No considerable variation in emission intensity was observed in this time frame, indicating good stability of the present sensor solution. TGA of CDs, sodium rhodizonate and the sensor solution is showed in Figure S8, which indicates that sensor materials are stable up
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to 100 °C. All these observations reflect the high stability of the present sensor material and
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utility for laboratory and industrial applications. 4. Conclusions
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We successfully demonstrated a FRET-based sensing strategy for the highly selective detection of ammonia in the solution as well as vapour phases. Mechanism of the sensing process is based on the fact that resonance energy transfer between CDs and sodium
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rhodizonate is initiated once ammonia is introduced into the sensor solution. This leads to the quenching of the fluorescence emission of CDs; this can indirectly be used to detect and quantify ammonia. Furthermore, the sensor wool can be effectively used for the vapour phase
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detection of ammonia, with high selectivity over oxides of nitrogen and common organic
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solvents. The detection limit of the present sensor system was 3 ppm in both the solution and vapour phases. Low cost, good reversibility, high selectivity and stability are the appealing
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features of the present sensor system. Acknowledgements
Authors deeply acknowledge IIST Thiruvananthapuram for funding and IIT Madras for characterization studies. References
[1]. D. Bera, L. Qian, T.-K. Tseng, P.H. Holloway, Quantum dots and their multimodal applications: A Review, Materials, 3 (2010) 2260. [2]. S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem Soc Rev, 44 (2015) 362-81. [3]. M. Ganiga, J. Cyriac, Detection of PETN and RDX using a FRET-based fluorescence sensor system, Anal Methods, 7 (2015) 5412-8. [4]. H. Li, Z. Kang, Y. Liu, S.-T. Lee, Carbon nanodots: Synthesis, properties and applications, J Mater Chem, 22 (2012) 24230-53. [5]. Y. Fang, S. Guo, D. Li, C. Zhu, W. Ren, S. Dong, et al., Easy synthesis and imaging applications of cross-linked green fluorescent hollow carbon nanoparticles, ACS Nano, 6 (2012) 400-9.
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[6]. Q. Qu, A. Zhu, X. Shao, G. Shi, Y. Tian, Development of a carbon quantum dots-based fluorescent Cu2+ probe suitable for living cell imaging, Chem Commun, 48 (2012) 5473-5. [7]. R. Narayanan, M. Deepa, A.K. Srivastava, Forster resonance energy transfer and carbon dots enhance light harvesting in a solid-state quantum dot solar cell, J Mater Chem A, 1 (2013) 3907-18. [8]. H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu, et al., Carbon quantum dots/Ag3PO4 complex photocatalysts with enhanced photocatalytic activity and stability under visible light, J Mater Chem, 22 (2012) 10501-6. [9]. W. Shi, Q. Wang, Y. Long, Z. Cheng, S. Chen, H. Zheng, et al., Carbon nanodots as peroxidase mimetics and their applications to glucose detection, Chem Commun, 47 (2011) 6695-7. [10]. H.X. Zhao, L.Q. Liu, Z.D. Liu, Y. Wang, X.J. Zhao, C.Z. Huang, Highly selective detection of phosphate in very complicated matrixes with an off-on fluorescent probe of europium-adjusted carbon dots, Chem Commun, 47 (2011) 2604-6. [11]. M.-Z. Dai, Y.-L. Lin, H.-C. Lin, H.-W. Zan, K.-T. Chang, H.-F. Meng, et al., Highly sensitive ammonia sensor with organic vertical nanojunctions for noninvasive detection of hepatic injury, Anal Chem, 85 (2013) 3110-7. [12]. B. Timmer, W. Olthuis, A.v.d. Berg, Ammonia sensors and their applications: A review, Sens Actuators, B, 107 (2005) 666-77. [13]. S. Cui, S. Mao, Z. Wen, J. Chang, Y. Zhang, J. Chen, Controllable synthesis of silver nanoparticle-decorated reduced graphene oxide hybrids for ammonia detection, Analyst, 138 (2013) 2877-82. [14]. K. Crowley, E. O'Malley, A. Morrin, M.R. Smyth, A.J. Killard, An aqueous ammonia sensor based on an inkjet-printed polyaniline nanoparticle-modified electrode, Analyst, 133 (2008) 391-9. [15]. Y. Takagai, Y. Nojiri, T. Takase, W.L. Hinze, M. Butsugan, S. Igarashi, "Turn-on" fluorescent polymeric microparticle sensors for the determination of ammonia and amines in the vapor state, Analyst, 135 (2010) 1417-25. [16]. S. Widmer, M. Dorrestijn, A. Camerlo, S.K. Urek, A. Lobnik, C.E. Housecroft, et al., Coumarin meets fluorescein: a Forster resonance energy transfer enhanced optical ammonia gas sensor, Analyst, 139 (2014) 4335-42. [17]. J. Courbat, D. Briand, J. Damon-Lacoste, J. Wöllenstein, N.F. de Rooij, Evaluation of pH indicator-based colorimetric films for ammonia detection using optical waveguides, Sens Actuators, B, 143 (2009) 62-70. [18]. S. Pandey, G.K. Goswami, K.K. Nanda, Green synthesis of polysaccharide/gold nanoparticle nanocomposite: An efficient ammonia sensor, Carbohydr Polym, 94 (2013) 229-34. [19]. R. Chandra Sekhar, H. Manu, A. Govindaraj, C.N.R. Rao, Ammonia sensors based on metal oxide nanostructures, Nanotechnology, 18 (2007) 205504. [20]. S. Chen, G. Sun, High Sensitivity Ammonia sensor using a hierarchical polyaniline/poly(ethylene-co-glycidyl methacrylate) nanofibrous composite membrane, ACS Appl Mater Interfaces, 5 (2013) 6473-7. [21]. E. Bekyarova, M. Davis, T. Burch, M.E. Itkis, B. Zhao, S. Sunshine, et al., Chemically functionalized single-walled carbon nanotubes as ammonia sensors, J Phys Chem B, 108 (2004) 19717-20. [22]. E. Danesh, F. Molina-Lopez, M. Camara, A. Bontempi, A.V. Quintero, D. Teyssieux, et al., Development of a new generation of ammonia sensors on printed polymeric hotplates, Anal Chem, 86 (2014) 8951-8.
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Manjunatha Ganiga received his Master’s degree in Organic Chemistry from Mangalore University, India. Presently, he is a PhD candidate in Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram, India. His research interest includes; fluorescent sensors, surface enhanced Raman spectroscopy and functional nanomaterials.
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Jobin Cyriac is an Assistant Professor in Department of Chemistry at Indian Institute of Space Science and Technology, India. He received his Ph.D. degree from the Indian Institute of Technology (IIT) Madras, India. He is interested in developing chemical sensors, exploring applications of surface enhanced Raman spectroscopy (SERS) and ion/surface collision phenomena.
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S0925-4005(15)30657-2 http://dx.doi.org/doi:10.1016/j.snb.2015.11.074 SNB 19332
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-7-2015 12-11-2015 17-11-2015
Please cite this article as: M. Ganiga, J. Cyriac, FRET based Ammonia Sensor using Carbon Dots, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
FRET based Ammonia Sensor using Carbon Dots Manjunatha Ganiga and Jobin Cyriac* Department of Chemistry, Indian Institute of Space Science and Technology
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Thiruvananthapuram – 695 547, INDIA
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*To whom correspondence should be addressed, e-mail: [email protected]
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Prepared for publication in Sensors and Actuators B: Chemical
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Correspondence to:
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Jobin Cyriac Department of Chemistry Indian Institute of Space Science and Technology (IIST) Valiamala P.O. Thiruvananthapuram Kerala – 695 547, INDIA
Abstract
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We present a Förster (fluorescence) resonance energy transfer (FRET)-based sensing platform for the selective detection of ammonia by using carbon dots (CDs) as a signal transducer and sodium rhodizonate as an analyte-specific molecule. A solution containing CDs and sodium rhodizonate was used as the sensor system. Both solution and vapour phase detection of ammonia was performed. Excited state energy transfer (FRET) from CDs to sodium rhodizonate was activated when ammonia was present in the sensor solution. Thus, the FRET process efficiently quenches the fluorescence of CDs and can be used for detecting ammonia. The detection limit of the present sensor system was up to 3 ppm. Furthermore, we exploited cotton fibers coated with the sensor solution to selectively detect ammonia vapours. Selectivity was demonstrated using oxides of nitrogen and common organic solvents. The present FRET-based sensor demonstrates high sensitivity and selectivity as well as good reversibility. Key Words: Förster resonance energy transfer (FRET), ammonia sensor, carbon dots, fluorescence sensor
___________________________________________________________________________ 1. Introduction
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There is a growing interest to replace conventional organic dyes with stable semiconductor or carbon-based photoluminescent nanomaterials in fluorescence-based sensing applications.[1-3] Photoluminescent carbon dots (CDs) are better compared to semiconductor nanomaterials in terms of their high biocompatibility and low toxicity and
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cost.[4] To date, fluorescent CDs have been successfully exploited in a wide range of applications such as bioimaging,[5,6] energy conversion and storage,[7,8] and sensing.[9,10]
Detection and quantification of ammonia is crucial for environmental, industrial, and purposes.[11,12]
Several
methods,
including
amperometric,[13,14]
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biomedical
fluorimetric,[15,16] and colourimetric,[17, 18] have been used for detecting ammonia in
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solution and in vapour phases.
Early amperometry-based methods for the detection of ammonia employed metal oxides,
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more commonly tin oxide-based sensors, in which the output resistance varied with ammonia exposure.[19] Recently, more attention has been paid to the development of amperometric
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sensors using carbon-based nanomaterials, such as graphene and carbon nanotubes, as well as conducting polymers such as polyaniline.[20-22] For example, an amperometric sensor comprising a multi-walled carbon nanotube electrostatically bonded to silver nanocrystals
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shows enhanced sensitivity compared with sensors comprising carbon nanotube alone.[23]
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Although amperometric-based ammonia sensors are the most studied and offer high sensitivity, but suffers with low selectivity. Furthermore, moisture and volatile organic
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compounds strongly interfere. The requirement of high operating temperatures and cumbersome sensor design are other disadvantages of these methods.[19,24,25] Fluorescence sensors offer relatively higher sensitivity, easier design strategies, lower
cost, and higher portability.[26] 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.[27] In another study, fluorophores, such as fluorescein and acridine orange, over cross-linked acrylic ester micro-particles were used for detecting ammonia vapour.[15] When the proximity between a donor and an acceptor, having sufficient spectral overlap, is of <10 nm, the Förster (fluorescence) resonance energy transfer (FRET) process can be achieved. The FRET-based sensing strategy is applied in diverse fields, including cellular imaging,[28] single molecule spectroscopy,[29] DNA hybridisation,[30] and small molecule
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detection.[31] 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 FRET between coumarin (donor) and fluorescein (acceptor) because of the deprotonation of fluorescein, thereby leading to an
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improved spectral overlap.[16] 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.[32]
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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
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analyte-specific molecule and CDs as the signal transducer. The mechanism of sensing scaffold depends on the fact that excited state energy transfer (FRET) from CDs to sodium
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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 µL of 1 mM sodium rhodizonate), thereby the fluorescence of CDs will be efficiently quenched; the quenching
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can be used for sensing ammonia.
Furthermore, the sensor solution adsorbed over cotton fibers is used for detecting
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ammonia from the vapour phase. We found this platform to be highly sensitive, respond
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linearly to ammonia concentration, and reversible after passing HCl gas. The sensor system was highly selective towards ammonia over the oxidising gases such as NOx, allowing its use
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for biological and industrial purposes, where discriminating ammonia from oxidising gases is highly desirable. Low cost, fast response, high selectivity, linear response, and good reversibility make the sensor extremely appealing. 2.
Experimental section
Reagents and Materials: Phosphorous pentoxide, copper turnings, and quinine sulfate
were purchased from Sigma Aldrich, India. Acetic acid, nitric acid, and ammonia solution were supplied by Merck Specialties, India, and sodium rhodizonate was obtained from Otto Chemicals, India. All aqueous solutions were prepared using double distilled water. UV-visible absorption spectra were recorded using a Carry-100 double beam UVvisible spectrophotometer. Infrared (IR) spectroscopy studies were conducted using a Spectrum 100T Perkin-Elmer FT-IR spectrometer. For collecting the FTIR spectra, samples were lyophilised for 2 days at −30 °C, and IR spectra were recorded in an attenuated total internal reflection mode with 32 average scans. The dynamic light scattering (DLS)
3 Page 3 of 17
experiments and zeta potential measurements were carried out using a Zetasizer Nano ZS (Malvern Instruments). Transmission electron microscopy (TEM) images were obtained using a JEOL 3010 UHR instrument. All fluorescence measurements were performed using the Fluoro Max-4C Spectrofluorometer (Horiba Instruments, USA). Excitation wavelength
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was fixed at 400 nm with an integration time of 0.1 s. Both excitation and emission slit widths were kept at 5 nm. According to a previous report, the quantum yield of the CDs was calculated using quinine sulphate (φ = 0.54) as a standard.[5] Time-resolved fluorescence
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measurements were performed using time-correlated single-photon counting (360 nm excitation wavelength by using a picosecond diode laser, with 10.5 nm excitation band pass
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and 9.5 nm emission band pass; Horiba Instruments). Thermogravimetric analysis (TGA) was conducted using TA instruments Q50. Heating rate was maintained at 10 °C/min in an inert
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atmosphere. Photostability experiments were performed using a UV light source (360 nm, power: 8 W).
Preparation of CDs: Carbon nanoparticles having a green fluorescence emission were
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prepared according to a previous report.[5] In short, a solution containing glacial acetic acid (1 mL) and water (80 µL) was rapidly added to 2.5 g of P2O5 in a 25 mL beaker without
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stirring, and the reaction mixture was cooled to room temperature. The dark brown solution containing CDs was then collected by dispersing in distilled water and centrifuged, and the
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clear supernatant solution was collected for further use. The pH of the purified solution was 3. The sensor solution was then prepared by mixing 3 mL of CDs and 600 µL of 1 mM
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sodium rhodizonate.
Oxides of nitrogen (NO2 and NO) were generated by slowly adding concentrated HNO3
into a conical flask containing copper turnings. The evolved reddish brown gas was then introduced onto the cotton fibers coated with the sensor solution. 3.
Results and Discussion
3.1 Characterisation of CDs
TEM and DLS studies showed that the average size distribution of the CDs is 3−4 nm (Fig. S1, Supporting Information). UV-visible spectrum (Fig. 1) of the CDs confirmed the presence of C=C and C=O groups as electronic absorption bands were observed at 250 nm and 300 nm, which are ascribed to the π→π* and n→π* transitions of the C=C core and C=O surface groups, respectively.[5,33] CDs showed fluorescence emissions at excitation from 350 to 450 nm.[5] Excitation at 400 nm showed a strong photoluminescence (PL) emission
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band centred at 490 nm (Fig. 1). The quantum yield of purified CDs, calculated using quinine sulfate as a standard (QYref = 0.54), was 5.28% (Fig. S6, Supporting Information).[34] The large full width at half maximum (FWHM) in the PL spectrum was attributed to the presence of multi-emissive states arising from the emission of the core and surface groups or due to the
confirmed by fluorescence lifetime measurements (discussed later).
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presence of surface defects.[4,35] The presence of multi-emissive states was further
The IR spectrum of the purified CDs showed characteristic vibrations at 1720 and 1630 cm , supporting the presence of surface carboxyl groups and sp2-hybridised carbon core,
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−1
respectively (Fig. S2, Supporting Information). Compared with that of acetic acid alone, a
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strong band appeared at 1630 cm−1, which corresponded to the sp2 C=C stretching vibration; reflected the successful carbonisation of acetic acid by P2O5 and water. This was further
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confirmed by the presence of aromatic C-H stretching at 3021 cm−1. The band centred around 3200 cm−1 was due to the stretching vibrations of surface −OH groups.[36] A zeta potential value of +10.3 mV in water suggested dispersion stability because of the presence of
1.0 0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
Normalized emission (a.u.)
0.8
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Absorption Emission
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Normalized absorbance (a.u.)
1.0
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numerous surface −COOH groups in the CDs. [37,38]
200 250 300 350 400 450 500 550 600 650 700
Wavelength (nm) Fig. 1. Absorption and emission spectra of CDs. Excitation wavelength was fixed at 400 nm. Inset: Photograph of the CD solution under UV light.
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Na+
O
O-
OH
NH3
HO
O-
O
OH HO
O
OH
O O
(I)
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O
(II)
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Na+
OH
Scheme 1. Schematic illustration of the detection of ammonia. ‘CD’ represents carbon dots with functional groups such as −COOH and −OH.
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3.2 Interaction between CDs and sodium rhodizonate
UV-visible spectra of the aqueous solution of sodium rhodizonate (I) with and without
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CDs are shown in Fig. 2(A). Pure sodium rhodizonate exhibited characteristic visible absorption bands at 445 nm and 485 nm (red trace).[39] However, the sensor solution, composed of CDs and sodium rhodizonate, lacked these features. The addition of sodium
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rhodizonate (I) to CDs having plenty of surface −COOH groups (at pH = 3) resulted in the
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formation of a colourless dihydrate form of rhodizonate (rhodizonate (II); Scheme 1). The protonation and subsequent dehydration of rhodizonate (I) resulted in the formation of
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rhodizonate (II) structure with absorption at 365 nm (blue trace). A similar observation has been reported previously in which rhodizonate (II) was found at a lower pH.[40] These observations confirm that sodium rhodizonate is present as rhodizonate dihydrate form (II) in our sensor solution. Also n-π* absorption band at 290 nm corresponding to the surface – COOH group of CDs was red shifted to 300 nm after the introduction of rhodizonate (I). This is attributed to the conversion of –COOH group of CDs to carboxylate anion (scheme 1), which lead to the stabilization of π* orbital due to the weakening of π bond.However, no appreciable change was observed in the emission behaviour of CDs with the addition of sodium rhodizonate, implying the absence of excited state energy transfer between CDs and rhodizonate (II). This can be ascribed to the lack of spectral overlap between the two molecular species (absorption spectrum of rhodizonate (II) and emission spectrum of CDs) as shown in Figure S3, Supporting Information. The steady-state fluorescence spectra of CDs
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with and without sodium rhodizonate excited at 400 nm are shown in Fig. 2(B). The PL spectra were recorded at an excitation wavelength of 400 nm to avoid the inner filter effect. CD
290
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(ii)
300
50 40 30
Rhodizonate
20 10
0.0
0 400 500 Wavelength (nm)
(iii)
450
600
(iv)
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0.5
500
550
600
650
700
750
Wavelength (nm)
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300
CDs at 400 nm Excitation CDs + 100 µL Rhodizonate (1 mM) CDs + 200 µL Rhodizonate (1 mM) CDs + 300 µL Rhodizonate (1 mM) CDs + 400 µL Rhodizonate (1 mM) CDs + 500 µL Rhodizonate (1 mM) CDs + 600 µL Rhodizonate (1 mM)
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CD (i)
(B)
60
Intensity (a.u.)
Absorbance
70
CD-Rhodizonate
1.5
1.0
(A)
Rhodizonate
2.0
Fig. 2. UV-visible spectra of (A) CDs (black), sodium rhodizonate (red), and CDs with sodium rhodizonate (green). Inset: Photograph of sodium rhodizonate solution before (i) and
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after (ii) the addition of CDs. (B) The PL spectra of CDs with and without sodium rhodizonate recorded at an excitation wavelength of 400 nm. Inset: Photographs of CDs
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under UV light in the absence (iii) and presence (iv) of sodium rhodizonate.
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3.3 Ammonia sensor: solution phase
Aqueous ammonia was introduced at various concentrations to the sensor solution,
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comprising CDs and rhodizonate (II). The fluorescence of CDs decreased concomitantly, as clearly seen in Fig. 3(A). The reduction in the fluorescence intensity of CDs was proportional to the amount of ammonia being introduced, allowing the quantification of ammonia present in the solution. For this purpose, a calibration graph was drawn with F0−F (where F0 and F are the fluorescence intensity at 470 nm before and after the addition of ammonia, respectively) as the ordinate and ammonia concentration along abscissa (Fig. S4, Supporting Information). A linear relationship between the extent of PL quenching (F0–F) and ammonia concentration was observed (R2 = 0.9641). The detection limit calculated using 3*sbl/k was 3 ppm, where sbl was the standard deviation for the blank signal and k was the slope of the plot (Fig. S4, Supporting Information). The fluorescence quenching of CDs was associated with a red shift and decreased FWHM (Fig. 3). This can be ascribed to the selective quenching of blue emitting states of CDs by resonance energy transfer. Only the blue region of the CDs emission spectrum had a spectral overlap with the absorption spectrum of the acceptor (inset of Fig. 3). This provides
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an indirect evidence for the presence of FRET between the donor (CDs) and acceptor (sodium rhodizonate). Efficiency of the FRET process calculated using steady state fluorescence data was 40% (see Supporting Information S9 for the calculation). The photograph shown in Fig. 3 depicts a clear change in the fluorescence of the sensor solution
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with the addition of ammonia, allowing visual detection, under a UV light. As shown in Scheme 1, the addition of ammonia deprotonates rhodizonate (II) to rhodizonate (I). This argument is supported by the UV-visible spectrum of the sensor solution
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treated with various concentration of ammonia (Fig. 4(A)). The absorption bands at 445 and 485 nm, which were attributed to rhodizonate (I), were appeared after the addition of
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ammonia to the sensor solution. These absorption bands had a strong spectral overlap with the emission spectrum of CDs (inset of Fig. 3). Hence, FRET can be presumed between the
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CDs (donor) and the in situ generated sodium rhodizonate (acceptor), which is responsible for
1
60
Absorbance (Acceptor) 1.0 Emission (Donor)
0.8
0.8
0.6
0.6
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80
0.4
0.4
0.2
0.2
0.0
40
3
20
4
5
6
0
450
0.0 450
2
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Intensity (CPS)
100
1.0
Norrmalized emission (a.u.)
120
Normalized absorbance (a.u.)
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quenching the CDs emission when ammonia is introduced.
500
550
600
650
700
Wavelength (nm)
NH3
(i)
(ii)
7 8
500
550
600
650
700
750
Wavelength (nm)
Fig. 3. The PL spectra of the sensor solution with the addition of ammonia at various concentrations recorded at an excitation wavelength of 400 nm. The numbers 1–8 depict 0, 50, 100, 150, 200, 250, 300, and 350 ppm of ammonia, respectively. Photograph no. 1 and 8 taken under UV light are denoted as (i) and (ii), respectively. The spectral overlap between donor’s emission and acceptor’s absorption is shown in inset.
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In addition to spectral overlap, FRET demands the participating molecules to be at close proximity, essentially <10 nm. The absorption bands at 445 and 485 nm corresponding to sodium rhodizonate (structure I) restored with the addition of ammonia (Fig. 4(A), red trace). Here, the structure I containing C=O as well as enolate anion interact with −COOH
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and −OH groups of CDs through hydrogen bonding.[41] As discussed in the preceding section, absorption of CDs at 300 nm was ascribed to the n→π* transition of surface carbonyl groups. When ammonia was introduced to the sensor solution, this peak was strongly blue
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shifted to 290 nm (Fig. 4(A)). Blue shift in the n-π* transition of the carbonyl group was due to the stabilisation of the non-bonding ‘n’ orbital because of strong hydrogen bonding
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interaction.[42] This resulted in an increase in the energy gap between ‘n’ and π* orbitals of the carbonyl group and hence blue shift in absorption, which corroborates the presence of the
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hydrogen bonding interaction between the CDs and in situ generated sodium rhodizonate. The lifetime of the donor’s excitation decreases when the donor molecule is engaged in
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the energy transfer process. Hence, fluorescence lifetime measurement can provide a strong, direct evidence for the presence of the excited state energy transfer process. The timeresolved fluorescence measurements presented in Fig. 4(B) and the data given in Table 1
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clearly depict the presence of the FRET process between the CDs (donor) and rhodizonate
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(acceptor). Lifetime of the CDs solution alone and in sensor solution is triexponential with an average lifetime of 6.8 ns. Instead, lifetime of CDs after the addition of ammonia to the
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sensor solution is a 4-exponential fit with an average lifetime of 5.9 ns. The substantial decrease in lifetime of the donor (CDs) after the addition of the analyte strongly supports the presence of an excited state energy transfer process. From Table 1, we can see the emergence of an ultrafast component having a lifetime of 0.03 ns. This short-lived component majorly originates from the energy transfer from CDs to rhodizonate (I) because the exchange of the virtual photon for energy transfer is a kinetically faster process.[7]
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CDs-Rhodizonate CDs-Rhodizonate-NH3
(A) NH3
300
CDs-Rhodizonate-NH3
0.8
8k
CDs-Rhodizonate-NH3
6k
0.6
Decay
Absorbance
CDs CDs-Rhodizonate CDs-Rhodizonate-NH3
(ii)
(i)
NH3
0.4
(B)
10k
CDs-Rhodizonate-NH3
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1.0
4k
290
2k
cr
0.2
0 0.0 350
400
450
500
550
0
10
20
30
40
50
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300
Wavelength (nm)
Time (ns)
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Fig. 4. (A) UV-visible spectra of the sensor solution with and without ammonia. Corresponding visible light photographs are given as insets (i) and (ii). (B) Time-resolved fluorescence spectra of CDs (green trace), sensor solution (CDs–rhodizonate) (black trace),
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and CDs–rhodizonate–ammonia (red trace). The excitation wavelength was 400 nm and emission collection was at 500 nm using the picosecond diode laser.
τ1 (ns)
a1
CDs
2.41
0.34
τ2 (ns)
τ3 (ns)
a3
τ4 (ns)
a4
χ2
< τ> = (a1 τ12 + a2 τ22 + a3 τ32)/(a1 τ1 + a2 τ2 + a3 τ3) (ns)
0.60
0.32
0.06
-
-
1.35
6.8
1.25
6.8
1.011
5.9
a2
te
System
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Table 1: Time-resolved fluorescence data of the FRET donor and donor–acceptor system
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7.54
CDs−Rhodizonate
2.43
0.34
7.6
0.59
0.34
0.06
CDs−Rhodizonate −Ammonia
2.62
0.436
0.56
0.116
7.45
0.34
0.03
10.3
3.4 Ammonia sensor: gaseous phase Gaseous phase detection of ammonia is crucial in many industries. Here, we
demonstrated an efficient method for rapidly detecting ammonia vapours by using our sensor solution adsorbed on a cotton substrate (we will refer to the sensor solution-coated cotton substrate as ‘sensor wool’). For this purpose, we prepared a cylindrical-shaped cotton wool with approximately 3 cm height and 1 cm diameter (Fig. 5). The shape was selected to ease fluorescence measurements. Low cost, easy availability, and importantly, lack of fluorescent background are the added advantages of using cotton in the present study. The cotton wool was immersed in the sensor solution for 1 min, followed by vacuum drying to remove the
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excess solvent. The sensor wool was then exposed to various ammonia concentrations, and the fluorescence spectra were eventually recorded. Similar to the solution phase sensor, this gaseous phase sensor showed a detection limit of 3 ppm. The fluorescence emission of CDs decreased as ammonia concentration increased (Fig. 5). A rough conclusion could be
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achieved by observing the sensor wool under UV lamp immediately after the ammonia exposure. The probe has a quick response towards ammonia and the detection can be realized in 5 sec. Alternatively, the sensor-coated silica plate can be used for sensing. There is a clear
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colour contrast in the silica-coated plate adsorbed with the sensing mixture before and after ammonia exposure under the UV light source, facilitating a facile visual detection of
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ammonia (inset of Fig. 5). The fluorescence intensity of the CDs was completely regained when the sensor wool was exposed to HCl gas, and the wool could be reused for detecting
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ammonia. Because the materials and chemicals are inexpensive, detailed studies regarding the reversibility of the sensor were not conducted. The easy design strategy, high sensitivity, and quick response, makes the sensor wool superior to many of the previously described
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ammonia sensors.[16, 19]
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60
NH3
(iii) (i)
(ii)
before NH3 exposure
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Intensity (a.u.)
80
after NH3 exposure
40
20
0
450
500
550
600
650
700
750
Wavelength (nm) Fig. 5. Fluorescence spectra of the sensor wool before and after ammonia vapour exposure. The ammonia concentration was 300 ppm. (i) and (ii) show the photographs of the sensor solution dried over the silica plate before and after ammonia exposure, respectively. Visible
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light photographs of the sensor wool before and after ammonia exposure are given alongside of the corresponding spectra. 3.5 Selectivity towards NOx Most industrial or biological processes require ammonia as the precursor gas for
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producing oxides of nitrogen and vice versa. Therefore, selective sensing of one over the other is critical. Several reports have emphasised the detection of reducing gases, such as
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ammonia, and oxidising gases, such as NOx, but the selectivity of one over the other is rarely discussed in literature.[21,43] Most metal oxide-based amperometric sensors, which change
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the output current when the analyte molecule is adsorbed, respond equally well to the electron-donating molecules, such as ammonia, and electron-accepting gases, such as NOx; this prevents selective detection. For example, a sensing device comprising a reduced
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graphene oxide (RGO) suspension deposited over gold interdigitated electrodes changed its electrical conductivity after NO2 exposure, but the selectivity of the device towards ammonia
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remained unaddressed.[44] In another report, a gas sensor comprising RGO sheets responded to both ammonia and NOx in a similar manner.[45]
By contrast, our sensor wool showed a highly selective response towards ammonia over
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NOx. As shown in Fig. 6, a negligible change in the fluorescence emission of the sensor wool
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was observed after exposure to 350 ppm NOx. The extent of fluorescence quenching with an equal concentration of ammonia clearly dictated the selectivity (inset of Fig. 6). NOx cannot
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change rhodizonate the dihydrate form (II) to rhodizonate (I), thus excluding the possibility of energy transfer from CDs. Hence, the emission from CDs was unaffected after NOx exposure.
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100 CD-Rhodizonate CD-Rhodizonate-NOx
80
CD-Rhodizonate-NH3 70 60 50 40 30 20
40
10 0
Before NOx
After NOx
0 450
500
550
600
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20
NOx
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NH3
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60 F0-F
Intensity (a.u.)
80
650
700
750
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Wavelength (nm)
Fig. 6. Fluorescence spectra of the sensor wool before and after exposure to NOx gas.
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Fluorescence spectrum of the sensor wool exposed to ammonia is also shown for comparison (blue trace) with the corresponding photographs. Comparison of the extent of fluorescence quenching is given inset.
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Ammonia is a precursor, solvent, or sometimes, a byproduct in many organic syntheses.
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Therefore, the selectivity of the present sensor system towards ammonia over common organic solvents was an added advantage. Hence, we further extended the selectivity studies
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to solvents such as acetone, ethanol, toluene, diethyl ether, chloroform, and ethyl acetate. The sensor wool was exposed to the vapours of these solvents (350 ppm), and negligible changes were observed in the fluorescence emission profile (Fig. S5, Supporting Information). The results showed that the present sensor solution is highly selective towards ammonia over oxidising gases, such as NOx, and other organic solvents. Hence, the present sensor system can be exploited for industrial and laboratory applications. 3.6 Stability
Several organic dyes studied for fluorescence sensing of ammonia, such as fluorescein and coumarin, are prone to photo-degradation or thermal degradation. Hence, such sensors lack reproducibility because of the fluctuations in fluorescence intensity with time or environment. Photostability experiments of the present sensor solution were performed by irradiating the sensor solution under a 360 nm UV light (lamp power: 8W) for up to 24 h. Emission spectra were recorded at different time intervals. A plot of fluorescence intensity as
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a function of irradiation time is shown in Figure S6. Approximately 21% decrease in emission intensity was observed after 24 h of irradiation. The sensor solution was maintained at ambient condition for 15 days and checked for temporal stability. The fluorescence emission spectrum collected after 15 days is given in
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Figure S7. No considerable variation in emission intensity was observed in this time frame, indicating good stability of the present sensor solution. TGA of CDs, sodium rhodizonate and the sensor solution is showed in Figure S8, which indicates that sensor materials are stable up
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to 100 °C. All these observations reflect the high stability of the present sensor material and
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utility for laboratory and industrial applications. 4. Conclusions
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We successfully demonstrated a FRET-based sensing strategy for the highly selective detection of ammonia in the solution as well as vapour phases. Mechanism of the sensing process is based on the fact that resonance energy transfer between CDs and sodium
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rhodizonate is initiated once ammonia is introduced into the sensor solution. This leads to the quenching of the fluorescence emission of CDs; this can indirectly be used to detect and quantify ammonia. Furthermore, the sensor wool can be effectively used for the vapour phase
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detection of ammonia, with high selectivity over oxides of nitrogen and common organic
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solvents. The detection limit of the present sensor system was 3 ppm in both the solution and vapour phases. Low cost, good reversibility, high selectivity and stability are the appealing
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features of the present sensor system. Acknowledgements
Authors deeply acknowledge IIST Thiruvananthapuram for funding and IIT Madras for characterization studies. References
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Manjunatha Ganiga received his Master’s degree in Organic Chemistry from Mangalore University, India. Presently, he is a PhD candidate in Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram, India. His research interest includes; fluorescent sensors, surface enhanced Raman spectroscopy and functional nanomaterials.
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Jobin Cyriac is an Assistant Professor in Department of Chemistry at Indian Institute of Space Science and Technology, India. He received his Ph.D. degree from the Indian Institute of Technology (IIT) Madras, India. He is interested in developing chemical sensors, exploring applications of surface enhanced Raman spectroscopy (SERS) and ion/surface collision phenomena.
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