Rapid switch-on fluorescent detection of nanomolar-level hydrazine in water by a diacetoxy-functionalized MOF: application in paper strips and environmental samples

Article abstract of DOI:10.1039/d0dt02491k

Here, we present a new diacetoxy-functionalized UiO-66 metal-organic framework (MOF) for the trace level detection of hydrazine in water. The MOF material (1) was solvothermally prepared by the reaction between ZrOCl₂·8H₂O and 2,5-di

Full text of DOI:10.1039/d0dt02491k

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Rapid Switch-On Fluorescent Detection of Nanomolar Level
Hydrazine in Water by a Diacetoxy Functionalized MOF:
Application in Paper Strips and Environmental Samples
Received 00th January 20xx,
Accepted 00th January 20xx
Soutick Nandi, Mostakim SK and Shyam Biswas*
DOI: 10.1039/x0xx00000x
Here, we present a new diacetoxy functionalized UiO-66 metal-organic framework (MOF) for trace level detection of
hydrazine in water. The MOF material (1) was solvothermally prepared by the reaction between ZrOCl2∙8H2O and 2,5-
diacetoxy-1,4-benzenedicarboxylic acid (H2BDC-(OCOCH3)2) ligand. The desolvated material (1ʹ) showed highly selective
fluorescent turn-on signal towards hydrazine in water, which can be visualized in naked eye under UV lamp. Within 1 min of
hydrazine addition, there was 14-fold fluorescence enhancemment. The probe can detect hydrazine up to nanomolar level
(detection limit = 78.8 nM) in water. This detection limit is the lowest among MOF based fluorescent probes for hydrazine.
The material was also utilized for the sensing of hydrazine in paper strips and environmental water samples. Hydrazine
selective deprotection of ester groups anchored with ligand is the principal reason behind the switch-on nature of sensing.
www.rsc.org/
electrophoresis,17 electrochemical analysis,18 flow detection,19
coulometry,20
potentiometry,21
titrimetry,22
Introduction
spectrophotometry,23 etc. But, these traditional methods have
lots of shortcomings including poor sensitivity, slow response
time, complex process, etc., which limit their application in
practical purpose. In comparison with these methods, the
fluorometric method are highly preferable due to its high
sensitivity, fast response, low detection limit and real-time
detection.24 Moreover, fluorescence “turn-on” probes which
have larger signal-to-noise ratio are much advantageous for
practical detection purpose.25 In this decade, various chemical
reactions have been implemented in order to develop
Hydrazine is a strong reducing agent with two single-bonded
nucleophilic nitrogen atoms which exhibits ambivalent chemical
behavior.1 For the great reducing and nucleophilic property,
hydrazine has widely been used in industry as a catalyst,
corrosion inhibitor,
a pharmaceutical intermediate, an
emulsifier, a blowing agent, etc.2, 3 Furthermore, hydrazine and
its derivatives are utilized for manufacturing explosives,
antioxidants, photographic chemicals, insecticides, and plant
growth regulators.4-7 For its high combustion enthalpy and
flammable characteristic, hydrazine is a well-known fuel for
rocket and space craft.8 However, hydrazine has been classified
as a toxic and carcinogenic chemical by the U.S. Environmental
Protection Agency (EPA).9 Due to its low vapor pressure, easy
uptake of hydrazine via inhalation and dermal routes is possible.
Even very low concentration of hydrazine in vapor phase may
cause sever skin irritation and dermatitis.10, 11 Acute exposure
causes serious damage to the liver, kidney, and central nervous
system.12, 13 In view of the toxicity of the hydrazine, US EPA has
set its threshold exposure limit to a low value of only 10 ppb.9
Thus, recently invention of sensitive and reliable analytical
detection techniques for hydrazine has found great attention.
Till date, a wide variety of methods have been utilized to
fluorescent probes for hydrazine.26
A huge variety of
fluorescent probes have been designed based on the following
reaction mechanisms: condensation reaction of aldehyde or
ketone with hydrazine,27-30 hydrazinolysis of phthalimide or
ester,31-35 nucleophilic substitution reaction by hydrazine in
halogenated compound,36-39 reaction of hydrazine with
arylidene malononitrile, etc.40-45 However, these reported
chemical probes still suffer from various shortcomings such as
long reaction time, low sensitivity, less fold increment, etc.
Hence, development of new fluorescent probes with great
detection capability is highly desirable.
In recent times, metal-organic frameworks (MOFs) are
regarded as a promising and unique kind of porous crystalline
materials.46 MOFs are constructed from inorganic metal ions
and organic ligands.47 Based on characteristics of used metal
ions and functionalization of ligand moiety, the properties of
the MOFs can be tuned.48 Owing to the unique structural and
functional features of the MOF materials, they have been
applied in several fields such as gas storage and separation,49, 50
catalysis,51, 52 drug delivery,53 proton conduction54 and sensing
detect hydrazine such as gas chormatography,14 high
16
performance
liquid
chromatography,15,
capillary
Department of Chemistry, Indian Institute of Technology Guwahati, 781039
Assam, India. Phone: 91-3612583309, Fax: 91-3612582349, E-mail:
sbiswas@iitg.ac.in
† Electronic Supplementary Information (ESI) available: NMR spectra, FE-SEM
images, XRPD patterns, FT-IR spectra, TG curves, sorption isotherms and
fluorescence spectra. See DOI: 10.1039/x0xx00000x
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study.55 Rational design of a MOF material is possible due to dissolved in 3 mL DMF (DMF = N,N-dimethylformamide) in a
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DOI: 10.1039/D0DT02491K
tunable pore size, adjustable surface area, functionalized ligand sealed tube. The resulting mixture was sonicated until the metal
moiety with specific analyte interaction sites and high physico- salt and ligand dissolved. Afterward, it was kept in a preheated
chemical stabilities of MOF, which make it a promising sensory block heater for 24 h at 100 °C. The off-white colored precipitate
material. Recently, MOF materials have been extensively was filtered through membrane filtration set-up after cooling
utilized for the efficient detection of environmental down to ambient temperature. The obtained precipitate was
contaminants including heavy metal ions, toxic oxo-anions, washed with acetone (2 × 10 mL). After that the material was
organic dyes and toxic gases, etc.56-59 In continuation of our dried in a conventional oven at 70 °C for half an hour. Yield was
current research work in the development of various 36 mg (0.01 mmol, 85% based on Zr salt). FT-IR (KBr, cm−1): 3449
functionalized MOF materials for the detection of toxic (br), 3268 (br), 3017 (w), 2959 (w), 1747 (s), 1657 (m), 1586 (vs),
analytes, herein, we describe the design and synthesis of a 1502 (m), 1457 (vs), 1380 (vs), 1225 (s), 1115 (w), 1006 (w), 903
diacetoxy functionalized UiO-66 type MOF material (UiO: (w), 870 (w), 787 (m), 677 (m), 574 (w), 477 (w).
University of Oslo) for selective “turn-on” fluorometric
Occluded solvent molecules were removed by exchanging the
recognition of toxic hydrazine in aqueous phase (Scheme 1).
as-prepared material (30 mg) with dichloromethane (DCM) (100
Selective deprotection of acetoxy functionality of the present
mL) for 1 day and after filtration, the slightly remaining volatile
probe in presence of hydrazine and formation of its
DCM was removed under vacuum at 90 °C. The thermally
treated compound is called as 1ʹ hereafter. Homogeneous small
octahedral shaped particles were found from FE-SEM images of
1ʹ (Fig. S4, ESI†). Moreover, the presence of Zr, C and O atoms
in 1ʹ were confirmed from EDX spectrum (Fig. S5, ESI†) and EDX
elemental mapping (Fig. S6, ESI†).
corresponding auxo chromic hydroxy species is the principal
cause behind the drastic “turn-on” response.35 Though few
MOF materials are reported for hydrazine detection but very
few of them could show great response towards hydrazine with
respect to response time and LOD (LOD = limit of detection)
value (Table S1, ESI†). Interestingly, till date, only one MOF
material has been reported for hydrazine detection (UiO-66-
Fluorescence detection of hydrazine in portable paper strips
phmd).60 But, the presented material showed better LOD value
Whatman grade I (qualitative) filter paper was immersed in an
(78.8 nM) than the previously reported MOF (0.87 µM). In
addition, the presented material showed faster response time
aqueous suspension of 1ʹ for 20 min. Afterward, the filter paper
was dried well in an oven at 80 °C and cut into 2 cm × 2 cm size
(within seconds) than the reported MOF (20 min). The fold
to get MOF coated paper strips. The as-prepared paper strips
increment (15-fold) of the presented material after addition of
were utilized for the hydrazine detection.
hydrazine is also much higher than the UiO-66-phmd MOF (4.4-
fold). Moreover, UiO-66-phmd was not applied for the paper
Fluorescence detection of hydrazine in environmental samples
strip device-based sensing and detection of hydrazine in
In-order to detect hydrazine from environmental water
samples, three types of water samples were collected (i.e. tap
water, lake water and river water). Tap water was collected
from IIT Guwahati campus, lake water was collected from Tihor
lake of IIT Guwahati and water from Brahmaputra river was
used as river water. Known amount of hydrazine (5 µM, 10 µM
and 20 µM) were spiked in different water samples containing
1ʹ and emission spectra were collected immediately after
preparation of samples.
environmental water samples. But, for the presented material
(1ʹ), MOF-coated paper test strips were developed for the on-
site recognition of hydrazine. In addition, the MOF sensor was
effectively utilized to detect hydrazine in environmental water
samples.
Results and discussion
Characterization of Zr-UiO-66-(OCOCH3)2
Scheme 1. Chemoselective fluorogenic “turn-on” detection of hydrazine by
activated Zr-UiO-66-(OCOCH3)2 (1ʹ) material in aqueous medium.
Compound 1 was synthesized by a single step solvothermal
procedure by mixing ZrOCl2∙8H2O, H2BDC-(OCOCH3)2 ligand and
formic acid in 1:1:100 molar ratio and heating the mixture at
100 °C in DMF solvent. The XRPD pattern of the as-synthesized
compound and the simulated XRPD pattern of Zr-UiO-66 is
greatly similar (Fig. 1). Hence, we can conclude that the present
material has similar structural framework construction with the
material of UiO-66 family.
Experimental
Preparation
and
activation
of
[Zr6O4(OH)4(BDC-
(OCOCH3)2)6]∙6H2O∙1DMF (Zr-UiO-66-(OCOCH3)2) (1)
H2BDC-(OCOCH3)2 ligand (28.2 mg, 0.1 mmol), ZrOCl2∙8H2O (32.2
mg, 0.1 mmol) and formic acid (380 µL, 10 mmol) were
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The obtained unit cell parameters (Table S2, ESI†) from the In FT-IR spectra (Fig. S8, ESI†) of as-synthesized 1 and
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XRPD pattern of the as-prepared material by Dicvol refinement desolvated sample 1ʹ, strong absorption bands near 1586 and
method is very similar to the reported Zr-UiO-66 material. The 1457 cm-1 are observed. These bands are observed for the
obtained lattice parameter values unveil that the as- asymmetrical and symmetrical –CO2 stretching vibrations of
synthesized compound has cubic crystal system like UiO-66 H2BDC-(OCOCH3)2 ligands bonded with the metal (Zr),
MOF.61 Furthermore, a Pawley fit of the XRPD pattern of the as- respectively. The peak near 1747 cm-1 suggests the carbonyl
prepared compound was accomplished which showed great stretching frequency of acetoxy group anchored with the
similarity with the theoretically obtained peak pattern of Zr- incorporated ligand molecule.62 In the spectrum of as-prepared
UiO-66 MOF (Fig. S7, ESI†). The obtained values of Rwp and Rp compound, a strong band near about 1657 cm-1 arises which
from the Pawley fit are 6.64% and 4.39%, respectively. All these denotes carbonyl stretching vibration of the occluded DMF
data suggest that the framework of the present material is molecules.62, 63 This band is absent in the de-solvated sample 1ʹ,
isostructural with Zr-UiO-66. Hence, the framework of 1 which signifies the removal of DMF molecules from the pores.
contains [Zr6O4(OH)4]12+ clusters as secondary building units
The thermal stability of both activated (1ʹ) and as-prepared (1)
(SBUs) and these SBUs are interconnected with the carboxylate material was checked by thermogravimetric (TG) analyses.
ends of the employed linker (H2BDC-(OCOCH3)2). The From the TG curves (Fig. S9, ESI†), it is confirmed that the
framework of the material contains both octahedral as well as compound shows thermal stability up to 230 °C. In the TG trace
tetrahedral cages (Fig. 2). Each zirconium atom resides in a of as-prepared compound, three weight loss steps are observed
square anti-prismatic coordination environment having a which signify the escape of water, DMF and ligand molecules
coordination number of 8.
consecutively with increasing the temperature. The first weight
loss (4.3 wt.%) up to 120 °C is in good agreement with the
elimination of 6 guest water molecules per formula unit (calcd.:
4.2 wt.%). The second weight loss of 2.9 wt.% in between 120
and 220 °C can be attributed to one DMF molecule per formula
unit (calcd.: 2.8 wt.%). After 230 °C, framework of the material
collapsed for the destruction linkage between metal ion and
ligand molecule.
In order to inspect the chemical stability of the compound, it
was soaked in different kinds of liquids (i.e. DCM, MeOH, EtOH,
acetone, water, 1M HCl, DMF and CH3COOH) for 6 h. Then, the
samples were filtered and XRPD patterns were collected (Fig.
S10, ESI†). The XRPD patterns of the material remained almost
unaltered after soaking with the above-mentioned solvents and
liquids. Hence, in presence of different liquids, the material
could retain its structural integrity.
To demonstrate the permanent microporous nature of the
UiO-66-(OCOCH3)2 compound, N2 adsorption experiment was
conducted with the activated material. The compound showed
type-I adsorption isotherms (Fig. S11, ESI†), which revealed its
microporosity. As obtained from the N2 sorption isotherms, 1′
has a specific BET surface area of 917 m2 g-1. The micropore
volume possess of the material at p/p0 = 0.5 is 0.50 cm3 g-1. As
expected, 1ʹ has lower micropore volume and specific BET
surface area than the un-functionalized UiO-66 MOF. Actually,
grafting of two bulky acetoxy groups at 2- and 5-position of
terephthalic acid moiety is the main cause behind the lowering
of surface area and pore volume.
Fig. 1. Theoretical (black) XRPD pattern of the Zr-UiO-66 material and experimental (red)
XRPD pattern of the as-synthesized Zr-UiO-66-(OCOCH3)2 compound.
Fig. 2. (a) 3D cubic framework of 1 revealing (b) tetrahedral and (c) octahedral cages
present within the framework structure of 1. Color codes: Zr, green polyhedra; C, gray;
O, red. The structure of 1 was modeled and drawn by using the Materials Studio (version
5.0, Accelrys Inc., San Diego, 2009) software package.
Fluorescent based detection of hydrazine in aqueous medium
For the widespread applications and toxic effects on living
beings, development of reliable and sensitive analytical
methods for the selective detection of hydrazine is highly
desirable. Great efforts have been noticed for the development
of sensitive detection method for hydrazine for in last few
decades. A lot of small molecule fluorescent probes have been
The incorporation of the ligand moiety in the framework of the
material and the escape of trapped guest molecules from the
pores after de-solvation process were confirmed by FT-IR study.
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explored for the detection purpose of hydrazine.1, 28, 29, 32, 33, 35,
clearly suggest that the co-existence of congeners could hardly
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38, 45, 64-70
bring any effect in the emission responseDoOfI:11ʹ0a.1f0t3e9r/Dad0DdTit0io24n9o1Kf
hydrazine. Hence, for practical application purpose, the present
probe is a supreme candidate for hydrazine detection.
Most of them suffered from slow response, poor
sensitivity, less fold increment, etc. Though MOFs have been
employed as smart crystalline materials for the detection of
several analytes, the detection of hydrazine by MOF materials
has remained unexplored. Recently, our group reported a
phthalimide functionalized UiO-66 MOF for selective detection
of hydrazine.60 But, for fast and trace level on-site detection of
hydrazine by naked eye, much improvement in the sensory
material is required. In this context, we have designed and
synthesized
a Zr-based diacetoxy functionalized UiO-66
material and examined the sensing performance of the MOF
towards hydrazine.
The time dependent detection study was explored with the
addition of 1 mM hydrazine solution (500 µL) in the aqueous
suspension of 1ʹ. The emission signal of 1ʹ enhanced around 15-
fold (I535/I450) within 1 min of addition of the solution and the
fluorescence signal acquired saturation within only 2 min with
a drastic color change in naked eye under UV lamp (Fig. 3).
Furthermore, a significant bathochromic shift in emission signal
(from 450 nm to 535 nm) was observed after addition of
hydrazine solution. This event suggests the selective
deprotection of acetoxy functionality and formation of auxo-
chromic hydroxy functionality in presence of hydrazine. It is
worthy to note that the response time and fold increment of the
present probe is better than previously reported MOF-based
luminescent probe.60
Fig. 4. Comparative emission increment behavior of 1ʹ in aqueous medium
towards 1 mM of different analytes denoted as: b (NaCN), c (arginine), d (NaHCO3),
e (NaF), f (NaHSO3), g (urea), h (NaOAc), i (NaN3), j (Na2SO3), k (NaSCN), l (aspartic
acid), m (cysteine), n (serine), o (glucose), p (Na2S2O3), q (Na2SO4), r (NaI), s (NaCl),
t (NaNO3), u (glutathione), v (thiourea), w (NaBr), x (NaNO2), y (NH2OH) and z
(NaHSO4). The error bars indicate the standard deviations of three measurements.
Fig. 5. Fluorescence responses of aqueous suspension of 1ʹ towards hydrazine (a)
solution (500 μL, 1 mM) in the presence of other analytes (b-z). The error bars
indicate the standard deviations of three measurements.
Fig. 3. (a) Fluorescence “turn-on” response of aqueous suspension of 1ʹ towards the
addition of hydrazine solution up to 10 min. Inset: naked eye response of hydrazine
under UV lamp. (b) Time-dependency of the emission intensity monitored at 535 nm.
In order to quantify the detection process, incremental
addition of hydrazine solution (1 mM) was conducted in the
aqueous suspension of 1ʹ. Gradual enhancement in emission
response of the probe was observed with the stepwise addition
of hydrazine solution (50-500 µL) (Fig. 6).
A significant red-shift (450 nm to 535 nm) in emission band
was visualized after the introduction of hydrazine. This result
indicated that acetoxy group was selectively deprotected by
hydrazine and transformed to highly fluorescent hydroxy
functionality.71
The selectivity of the probe towards hydrazine was
examined in order to ensure its applicability in real field. It has
been observed from Fig. 4 and Figs S12-S36 (ESI†) that out of 25
number of competitive analytes (1 mM, 500 µL), only hydrazine
could enhance the emission intensity around 15 folds (I535/I450
)
whereas other congeners showed almost negligible
enhancement in emission response of aqueous suspension of
1'. This experiment strongly suggests the selectivity of the
emission response of the material towards 1ʹ over other
analytes.
To evaluate the limit of detection for the present probe
towards hydrazine, titration studies with low concentrations of
hydrazine were conducted, which revealed a good linearity in
between the fluorescence intensity and added hydrazine
concentration (Fig. S37, ESI†). The calculated LOD value is 78.8
Furthermore, for practical application purpose, the
detection behavior of 1ʹ towards hydrazine was checked with
the co-existence of 25 analytes. The emission signal of 1ʹ in
presence of hydrazine with the co-existence of other analytes
(1 mM, 500 µL) was monitored. Fig. 5 and Figs S12-S36 (ESI†)
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nM (3.9 ppb) based on the equation 3σ/k (k = slope of the
ARTICLE
The limit of detection value was also evaluated for the paper
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curve, σ = standard deviation of several blank experiments) strip device-based sensing. MOF coated paper strips were
(Table S3, ESI†). The evaluated LOD value is lower than the TLV soaked with different concentrations of hydrazine solution (10-
4
(threshold limit value) set by the U.S. EPA (10 ppb). Till date,
M - 10-10 M). After proper drying, those paper strips were
nanomolar level fluorescent detection of hydrazine using MOFs observed under UV lamp. Fig. 39 (ESI†) reveals that the intensity
and organic molecules is very rear. The LOD of 1ʹ is the lowest of bright yellow color gradually diminished with lowering the
among reported60 MOF type luminescent sensors. The LOD concentrations of treated hydrazine solution. It has been
value and response time of 1ʹ for hydrazine are better or observed that the material has the ability to detect up to 10-8 M
comparable with existing small organic molecule based concentration of hydrazine in paper strip. After 10-8
M
luminescent probes tabulated in Table S1 (ESI†).1, 60, 64-68, 71-84 concentration, the color of the MOF-coated paper strip
Fig. S38 (ESI†) indicates that framework topology of the probe remained almost unchanged. Hence, the LOD value in paper
remains almost the same even after the sensing experiment. strip sensing is 10-8 M. Interestingly, this value (10-8 M) is close
Though the XRPD pattern of the material after the detection to the LOD value evaluated for the material towards hydrazine
experiment is almost similar with the XRPD pattern of the as- in aqueous medium (78.8 nM).
synthesized material, a slight decrease in the crystallinity of the
material was observed after the detection experiment. This
observation suggests that partial decomposition of the
structure of the parent material might have occurred during
hydrazine detection event. However, complete decomposition
of the structure of the MOF material was not observed. Though
the crystallinity decreased slightly after hydrazine detection,
the XRPD pattern remained almost similar.
Fig. 7. Photographs of 1ʹ-coated paper test strips (a) before and (b) after soaking
with 1 µM solution of hydrazine under UV lamp. Images of un-coated paper strip
(c) before and (d) after hydrazine treatment under UV lamp are also shown.
Detection of hydrazine in environmental samples
Industrial processes involving hydrazine may contaminate
water bodies and severely affect the ecological systems. Hence,
detection of hydrazine in real environmental samples is an
Fig. 6. Emission response of aqueous suspension of 1ʹ upon gradual addition of 1
interesting task. Practical applications have been extended for
the detection of hydrazine in tap water, lake water and
environmental water samples by our material. Different water
samples containing 1ʹ were spiked with various concentrations
of hydrazine (5 µM, 10 µM and 20 µM). Afterward, emission
spectra of the respective samples were collected. Fig. 8 suggests
that different real water samples can detect hydrazine
efficiently. Moreover, with increasing the concentration of
spiked hydrazine, emission intensity was also enhanced. All
these results clearly suggest that our probe has great efficiency
for the recognition of hydrazine in environmental water
samples.
mM hydrazine solution.
Detection of hydrazine in portable paper strips
For on-site detection purpose of hydrazine by the present MOF
material without applying any sophisticated instrument, a MOF-
coated paper test strip was developed. The test strip showed a
blue fluorescence signal under the UV lamp. When the same
test strip was exposed to 1 µM aqueous solution of hydrazine
for few seconds, a drastic fluorescence change from blue to
bright yellow was noticed under UV lamp (Fig. 7). Hence, the
developed probe can be utilized for the on-site detection of
hydrazine. For control experiment, un-coated paper strip was
exposed with hydrazine and images of empty paper strips were
collected before and after hydrazine (1 µM aqueous solution)
treatment. Fig. 7c-d shows that no observable change was
detected in the empty paper strip after hydrazine treatment.
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Fig. 9. Plausible mechanism for the conversion of diacetoxy-anchored 1ʹ to
corresponding dihydroxy functionalized framework by hydrazine in aqueous
medium.
To prove the function of acetoxy group for the interaction and
enhancement of the emission response of the probe in
presence of hydrazine, similar detection experiments were
carried out with un-functionalized Zr-UiO-66 MOF and H2BDC-
(OCOCH3)2 ligand. It can be observed from Fig. S42 (ESI†) that no
observable “turn on” response was obtained from un-
functionalized Zr-UiO-66 MOF in presence of hydrazine,
whereas, H2BDC-(OCOCH3)2 ligand showed around two-fold
fluorescence increment in presence of hydrazine which is much
lesser than the emission response of 1ʹ in presence of hydrazine.
We can conclude that the acetoxy group is strongly responsible
for the “turn on” response in presence of hydrazine due to
conversion of acetoxy functionality to auxo-chromic hydroxy
functionality. Furthermore, hydrazine can easily diffuse through
the porous cages of the present MOF material and react with
the acetoxy functionality which is not possible in discreet ligand
molecules.57 This may be the possible reason behind the less
fluorescence fold increment of H2BDC-(OCOCH3)2 ligand in
presence of hydrazine. Hence, not only acetoxy functionality
but also porous nature of the material enhances the capability
of the material for the ultrasensitive detection of hydrazine.
The recyclability experiment was carried out up to five cycles
of sensing experiments for the reaction-based fluorescent
probe. Fig. S43 (ESI†) suggests that after the first cycle, the
emission response is diminished greatly. The poor recyclability
of the MOF material towards hydrazine can be attributed to the
fact that the acetoxy functionality is converted to hydroxy
functionality. For this reason, 1ʹ showed negligible fluorescence
turn-on responses in the subsequent cycles of detection
experiment. Hence, the poor recyclability of the present probe
for hydrazine detection is in well agreement with the proposed
reaction-based detection mechanism.
Fig. 8. Fluorescence detection of hydrazine (0-20 µM) in distilled water, tap water, lake
water and river water by 1ʹ (λex = 365 nm and λem = 535 nm). The error bars indicate the
standard deviations of three measurements.
Possible detection mechanism
During sensing experiment, we expect that selective
deprotection of acetoxy functionality of the present probe in
presence of hydrazine and formation of its corresponding auxo
chromic hydroxy species is the primary reason behind the
drastic “turn-on” change in the emission response of the
material (Fig. 9). The successful conversion of H2BDC-(OCOCH3)2
to 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H2BDC-(OH)2) in
1
presence of hydrazine solution can be proved by FT-IR and H
NMR spectroscopy. The material was recovered after the
sensing experiment and its FT-IR spectrum was recorded. Fig.
S40 (ESI†) showed that for hydrazine treated material, the peak
at 1747 cm-1 which is due to carbonyl frequency of acetoxy
functionality, is totally disappeared. This result suggests that in
presence of hydrazine, selective deprotection of acetoxy group
has occurred. In addition, broadening of the spectrum near
about 3400 cm-1 indicates the formation of hydroxy
functionality after treatment with hydrazine.85 For 1H NMR
spectroscopy, hydrazine treated H2BDC-(OCOCH3)2 ligand was
1
dried well and H NMR spectrum was recorded. Fig. S41 (ESI†)
suggests that after hydrazine treatment, the proton peak for
methyl functionality at 2.26 ppm vanished and a shift of
aromatic proton peak (7.69 ppm to 7.11 ppm) was observed.
These events clearly suggest the deprotection of acetoxy
functionality and subsequent formation of H2BDC-(OH)2 species.
1
It is worth to mention that we have performed the H NMR
Time-resolved photoluminescence (TRPL) decay experiments
were carried out with the present material to verify its excited-
state behavior under sensing condition. TRPL decay profile of 1ʹ
(Fig. S44, ESI†) suggests that before addition of hydrazine
solution, the material showed a bi-exponential decay profile
having two species with an average excited-state lifetime of
0.16 ns (Table S4, ESI†). After introduction of hydrazine solution
(1 mM, 500 µL), a drastic enhancement in average lifetime (4.47
ns) of the component was visualised, which suggests the
formation of highly emissive hydroxy species (H2BDC-(OH)2)
from non-emissive acetoxy species (H2BDC-(OCOCH3)2) by the
deprotection of acetoxy functionality in presence of hydrazine.
Hence, the dramatic change in average lifetime value of the
experiment with the ligand instead of MOF material, because
during MOF digestion harsh condition is used by employing
either HF, K3PO4 or CsF. Under these conditions, the labile ester
linkage undergoes deprotection before addition of hydrazine.
Hence, we have employed pure ligand in DMSO-d6 solvent for
studying the sensing mechanism. It is also noticeable from Fig.
S1 (ESI†) that the hydrazine treated MOF has same color as 2,5-
dihydroxyterephthalic acid (H2BDC-(OH)2). This incident again
suggests the transformation of 2,5-diacetoxy terephthalic acid
(H2BDC-(OCOCH3)2) to 2,5-dihydroxyterephthalic acid (H2BDC-
(OH)2) anchored in MOF in presence of hydrazine.
6 | J. Name., 2012, 00, 1-3
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National Center for Environmental AssessmeVnietw, AOrtifcfliecOenlionef
present compound is in well agreement with the formation of
strong emissive species.86, 87
Research and Development, Washington, DC, 1999.
DOI: 10.1039/D0DT02491K
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Conclusions
The diacetoxy functionalized UiO-66 MOF was characterized
thoroughly with multiple analytical techniques. The activated
form of the material (1ʹ) showed fluorometric turn-on detection
capability towards hydrazine in aqueous environment. The
sensing event can be observed under UV lamp with naked eye.
The detection ability towards hydrazine is retained even in the
co-existence of potentially interfering analytes. Moreover, the
MOF showed rapid response for hydrazine. There was 14-fold
fluorescence increment within 1 min of hydrazine addition.
Probe 1ʹ also exhibited ultra-sensitive detection capability
towards hydrazine. The detection limit towards hydrazine was
78.8 nM, which is the lowest among reported MOF probes for
hydrazine. For on-site detection of hydrazine, the MOF-coated
paper strip device was also developed. The capability of the
material for the detection of hydrazine in environmental
samples was also demonstrated. The super selectivity of the
material for hydrazine is attributed to the chemo-selective
transformation of acetoxy group of the material to hydroxyl
group in presence of hydrazine. The detail mechanistic pathway
for the fluorometric sensing ability of hydrazine by 1ʹ was
successfully explored with the help multiple spectroscopic
techniques.
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Acknowledgements
We acknowledge the Science and Engineering Research Board
(SERB), New Delhi (grant no. EEQ/2016/000012) for financial
support. Support for instrumental facility from the Central
Instruments Facility (CIF) of IIT Guwahati is gratefully
acknowledged.
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