Recent progress in the development of fluorescent probes for hydrazine

Chang and co‐workers developed two phenylacetate‐ based fluorescent probes 1 and 2 for hydrazine detection by incorpo-rating acetate group onto dichlorofluorescein and resorufin fluorop

R E V I E W

Recent progress in the development of fluorescent probes for hydrazine

1

College of Chemistry and Chemical

Engineering, Central South University,

Changsha, Hunan Province, P R China

2

Henan Key Laboratory of Biomolecular

Recognition and Sensing, College of Chemistry

and Chemical Engineering, Shangqiu Normal

University, Shangqiu, Henan Province, P R

China

Correspondence

Yuanqiang Hao, Henan Key Laboratory of

Biomolecular Recognition and Sensing,

College of Chemistry and Chemical

Engineering, Shangqiu Normal University,

Shangqiu, Henan Province, 476000, P R

China

Email: hao0736@163.com

You‐Nian Liu, College of Chemistry and

Chemical Engineering, Central South

University, Changsha, Hunan Province,

410083, P R China

Email: liuyounian@csu.edu.cn

Funding information

National Natural Science Foundation of China,

Grant/Award Numbers: 21476266,

21475084, 21505091, U1404215 and

B061201; Innovation Scientists and

Techni-cians Troop Construction Projects of Henan

Province, Grant/Award Number: 41

Abstract

Hydrazine (N2H4) is an important and commonly used chemical reagent for the prep-aration of textile dyes, pharmaceuticals, pesticides and so on Despite its widespread industrial applications, hydrazine is highly toxic and exposure to this chemical can cause many symptoms and severe damage to the liver, kidneys, and central nervous system As a consequence, many efforts have been devoted to the development of fluorescent probes for the selective sensing and/or imaging of N2H4 Although great efforts have been devoted in this area, the large number of important recent studies have not yet been systematically discussed in a review format so far In this review,

we have summarized the recently reported fluorescent N2H4probes, which are clas-sified into several categories on the basis of the recognition moieties Moreover, the sensing mechanism and probes designing strategy are also comprehensively discussed

on aspects of the unique chemical characteristics of N2H4 and the structures and spectral properties of fluorophores.

K E Y W O R D S

fluorescent probes, hydrazine, review

1 | I N T R O D U C T I O N

Hydrazine, coined by Emil Fischer in 1875, is an inorganic compound

with the chemical formula N2H4.[1]Hydrazine can also be written as

H2NNH2, called diamidogen, therefore it has basic (alkali) chemical

properties (Kb = 1.3 × 10−6) like ammonia At ambient conditions,

hydrazine is a colourless fuming liquid with a faint ammonia‐like

odour Since the by‐products are typically nitrogen gas and water,

hydrazine often acts as a convenient reductant such as antioxidant,

oxygen scavenger and corrosion inhibitor Additionally, hydrazine is

also used as a propellant in space vehicles or used as a component

in rocket fuel due to its high heat of combustion and since large vol-umes of hot gas are generated during its decomposition

Ascribed to its other unique properties, including nucleophility, reductibility and double nucleophilic character, hydrazine also can be utilized as an important reactant for many chemical products, including textile dyes, pharmaceuticals and pesticides.[2–5] Despite its wide-spread industrial applications, hydrazine is highly toxic Exposure to hydrazine may cause symptoms of irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, coma

in humans, as well as damage to the liver, kidneys and the central ner-vous system.[6,7]The US Environmental Protection Agency (EPA) iden-tified hydrazine as a potential carcinogen with a threshold limit of

10 ppb.[8]Thus, it is highly desirable to develop selective and sensitive assays for the detection of trace hydrazine Several traditional analyt-ical techniques, including titrimetry,[9] voltammetry,[10–12]

Abbreviations used: AIE, aggregation‐induced emission; DMSO, dimethyl

sulfoxide; LOD, limit of detection; N2H4, hydrazine; NIR, near‐infrared; PBS,

phosphate‐buffered saline; TICT, twisted‐intramolecular charge transfer; TPE,

tetraphenylethylene

Received: 29 January 2018 Revised: 8 April 2018 Accepted: 26 April 2018

DOI: 10.1002/bio.3505

Luminescence 2018;1–21 wileyonlinelibrary.com/journal/bio Copyright © 2018 John Wiley & Sons, Ltd 1

chromatography,[13,14] and chemiluminescence[15]have been widely

used for hydrazine detection However, most of these approaches

have major disadvantages associated with the need for sophisticated

instrumentation and time‐consuming manipulations, and the inability

to be miniaturized for in situ and in vivo studies.

Alternatively, analytical techniques based on fluorescence sensor

systems are very popular because fluorescence measurements are

usually easy to perform, inexpensive, very sensitive (parts per billion/

trillion) with detection limits as low as sub‐parts‐per million, and able

to be employed for in situ and in vivo monitoring.[16–22] Hydrazine

can act as a good nucleophile for a variety of transformations in

syn-thetic chemistry, such as hydrazone formation, Wolff–Kishner

reduc-tion, heterocyclic chemistry, deprotection of phthalimides and so on

Recently, these characters of hydrazine have provide a starting point

for the development of a large number of efficient fluorescent

hydra-zine probes (Figure 1) Although great efforts have been devoted in

this area, a large number of important recent studies have not yet

been systematically discussed in a review format to the best of our

knowledge Herein, we make such an effort to summarize the rapid

progress in the development of fluorescent hydrazine probes and

highlight a variety of inventive strategies to achieve good reactivity

and selectivity The topics of this review are classified into several cat-egories based on the different sensing mechanisms and recognizing moieties of these probes for hydrazine, including probes based on ace-tyl, 4‐bromobutyryl, vinyl malononitrile, phthalimide, β‐diketone, levulinate and other moieties

2 | P R O B E S B A S E D O N A C E T Y L M O I E T Y

Phenol acetate can be readily hydrazinolyzed by hydrazine to generate its phenolic analogue (Figure 2) Based on this reaction, several probes containing phenol acetate moieties have been developed for sensing

of hydrazine Chang and co‐workers developed two phenylacetate‐

based fluorescent probes (1 and 2) for hydrazine detection by

incorpo-rating acetate group onto dichlorofluorescein and resorufin fluorophore scaffolds, respectively (Figure 3).[23] In a mixture of dimethyl sulfoxide (DMSO) and Tris buffer solution (pH 8.0, 10 mM,

1:1, v/v), probe 1 is colourless and non‐fluorescent Treating the probe solution with 100 equivalents of hydrazine creates a strong absorption band at 512 nm with a corresponding colour change from colourless

to greenish yellow and a prominent green emission at 534 nm, which

FIGURE 1 Fluorescent hydrazine (N2H4) probes based on different reaction mechanisms

FIGURE 2 Proposed sensing mechanism of probe for hydrazine (N2H4) based on hydrazinolysis of phenol acetate

are the characteristic spectral features of free dichlorofluorescein.

Hydrazinolysis of probe 2 also causes evident chromogenic and

fluo-rescent turn‐on type signals Both 1 and 2 exhibit excellent

selectiv-ities for hydrazine with limits of detection (LODs) of 9.0 × 10−8 M

and 8.2 × 10−8M, respectively, which is sensitive enough for industrial

chemical detection

Peng and co‐workers reported a NIR (near‐infared region)

ratiometric fluorescent probe (3) for hydrazine based on a

heptamethine cyanine dye derivative (Figure 4).[24]In the presence

of hydrazine in a mixture of acetate buffer (pH 4.5, 10 mM) and

DMSO (1:9, v/v), 3 undergoes a hydrazinolysis process to release

enol form, which further transforms it into its corresponding ketone

form, leading to large hypsochromic shifts in both absorption

and emission maxima Specifically, the colour of the solution

changes from cyan (784 nm) to pink (520 nm), and the emission

band shifts from 810 nm to 582 nm The fluorescence intensity

ratio at 582 and 810 nm (I582/I810) was found to linearly

increase with the concentration of hydrazine in the range

10–80 μM And the LOD of 3 for hydrazine was determined to

be 2.5 × 10−8 M Moreover, the probe was successfully utilized for imaging hydrazine in living MCF‐7 cell line and visualizing hydrazine in mice (Figure 4B)

Pang and co‐workers designed a ESIPT (excited state

intramo-lecular proton transfer) probe 4 by masking the phenol group of

flavonoid with the ethyl ester (Figure 5).[25] Hydrazine can selec-tively remove the ester protection group, leading to the recovery

of flavonoid ESIPT Addition of 20 equivalents of hydrazine to the probe solution causes a large fluorescence enhancement, giving intense green fluorescence, which increases by about eight‐fold Under optimized conditions, the fluorescence intensity of the probe solution was nearly proportional to the hydrazine concentration range from 0 to 50 μM with a calculated LOD of 1.0 × 10−5 M The probe was also successfully used for monitoring hydrazine in live cells and zebrafish

Sun et al developed a ratiometric fluorescent hydrazine probe 5

(Figure 6)[26]by incorporating an acetate moiety onto naphthalimide,

a widely used scaffold for the construction of fluorescent probes.[27,28]

Probe 5 displayed a fluorescence maximum at 432 nm Upon addition

FIGURE 3 Structures and reactions of probes 1 and 2 with

hydrazine

FIGURE 4 (A) Structure and reaction of probe 3 with hydrazine (B) In vivo images of a mouse given a skin‐popping injection of probe 3 and a

subsequent skin‐popping injection of hydrazine with the effect over different time intervals The top images were taken with an excitation laser of

740 nm and an emission filter of 820 ± 20 nm, and the bottom ones were taken with an excitation laser of 480 nm and an emission filter of

600 ± 20 nm (Reprinted from ref 24)

FIGURE 5 Structure and reaction of probe 4 with hydrazine

of hydrazine, the emission intensity at 432 nm decreased gradually

with the simultaneous appearance of a new red‐shifted emission band

centred at 543 nm, affording the ratiometric detection The emission

intensity ratio (I543/I432) showed a good linearity against the hydrazine

concentration in the range 0–10 μM, with a LOD of 2.1 × 10−8M

Probe 5 has also been applied to image hydrazine in living cells

(Figure 6B)

Compound 6 was reported as a NIR and turn‐on fluorescent

probe for hydrazine detection (Figure 7).[29]Reaction of the probe

with hydrazine removes the acetate moiety, producing the highly

fluorescent NIR hemicyanine fluorophore In vitro experiments

showed that a linear correlation existed between the fluorescence

response and the concentration of the hydrazine in the range

0–50 μM, with a LOD of 1.9 × 10−7 M Furthermore, the probe is

capable of imaging hydrazine not only in living cells but also in living

mice due to its efficient NIR emission, a critical feature for application

in bioimaging.[30,31]

Peng and co‐workers developed a two‐photon NIR fluorescent

probe 7 for the detection of hydrazine (Figure 8).[32]The probe has

an acetate moiety as the reaction site for hydrazine and a 2‐(2‐(4‐

hydroxystyryl)‐4H‐chromen‐4‐ylidene) malononitrile complex as the

fluorescent reporter unit The non‐fluorescent 7 reacts with hydrazine

leading to the removal of an acetate group and the release of the

highly fluorescent moiety The fluorescence increase at 680 nm is directly proportional to the hydrazine concentration from 0 to

40μM with a LOD of 5.7 × 10−7M Obviously, the response time of

7 toward hydrazine is about 1 min, and the probe is also capable

of visualizing hydrazine in MCF‐7 cells by two‐photon microscopy (TPM) imaging (Figure 8B)

Yin and co‐workers recently reported a ratiometric fluorescent

hydrazine probe 8 by incorporating an acetate moiety to a coumarin

derivative (Figure 9).[33]Noticeably, this probe displayed a different rec-ognition mechanism for hydrazine, in which the carbanyl group of the probe reacts with hydrazine affording a Schiff‐base intermediate and further forming a stable heterocyclic structure The probe exhibited a high sensitivity for hydrazine with a linear response range 0–10 μM

Cell imaging experiments also demonstrated the capacity of probe 8

for monitoring hydrazine in live samples

Incorporation of the acetate moiety onto a variety of other fluorophore scaffolds has afforded probes in a range of colour Reports of hydrazine based on coumarin and its derivatives

(9 –11),[34–36]fluorescein (12),[37]1,4‐dihydroxyanthraquinone (13),[38] 1,8‐naphthalimide (14),[39] benzthiazole (15),[40] the

dicyanomethylenedihydrofuran scaffold (16)[41,42] and rhodamine

derivative (17)[43]have been described The structures of these fluo-rescent hydrazine probes are summarized in Figure 10 However, it

FIGURE 6 (A) Structure and reaction of probe 5 with hydrazine (B) Fluorescence images of 7860 cells, (a–c) cells incubated with 5; (d–f) cells treated with 5 and hydrazine (a, d) Bright‐field images, (b, e) blue channel, (c, f) green channel (Reprinted from ref 26)

FIGURE 7 Structure and reaction of probe 6

with hydrazine

should be pointed out that, the acetyl group located on the aromatic

phenol is also a reaction site for BO3 −anion, and several fluorescent

probes have been develop for BO3 − ions based on the acetyl

recognition moiety,[44–47] indicating that BO3 − may interfere with

the hydrazine detection by using this type of probe

3 | P R O B E S B A S E D O N 4 ‐BROMOBUTYRYL

M O I E T Y

Hydrazine, also written as H2NNH2, can actually be regarded as a

simple molecule consisted of two amino groups, which implies that it

can perform two consecutive nucleophilic reactions This double

nucleophilic character is unique to hydrazine over other amines and

anions Thus, fluorescent probes with excellent selectivity for hydrazine would be afforded by taking advantage of this special reac-tivity For exploiting the double nucleophilic ability of hydrazine, a

4‐bromo butyrate group has been employed as the reaction moiety for the design of hydrazine probes This type of fluorescent probe is normally prepared via the incorporation of 4‐bromo butyrate onto a phenolic‐containing fluorophore The sensing process involves two steps (Figure 11), hydrazine first nucleophilically substitutes bromine atom and then performs a nucleophilic attack on the ester carbonyl, followed by intramolecular cyclization to release the fluorophore

Goswami et al firstly developed a fluorescent hydrazine probe (18)

employing 4‐bromo butyrate as the reaction moiety (Figure 12).[48] The probe is designed in such a way that ESIPT of the HBT

FIGURE 8 (A) Structure and reaction of probe 7 with hydrazine (B) Confocal microscope images of MCF‐7 cells (a–c) cells treated with 7; (d–i) cells treated with hydrazine and subsequent treatment of the cells with 7; (d–f) OPM image of cells upon excitation at 560 nm, emission window

650–750 nm; (g–i), TPM image of cells upon excitation at 820 nm, emission window 575–630 nm (Reprinted from ref 32)

FIGURE 9 Structure and reaction of probe 8

with hydrazine

(2‐(2'‐hydroxyphenyl)benzothiazole) moiety gets blocked by the

substituted 4‐bromo butyrate group The presence of hydrazine can

result in the release of the HBT moiety as well as the recovery of the

ESIPT of fluorophore through subsequent substitution, cyclization

and elimination processes Moreover, live‐cell imaging experiments

establish the utility of this probe for tracking hydrazine in live cells

Incorporation of a 4‐bromo butyrate moiety onto a resorufin

fluorophore afforded a turn‐on fluorescent probe (19) for N2H4

(Figure 13).[49]Reaction of the probe with hydrazine in a HEPES buffer

(10 mM, pH 7.4, containing 10% acetonitrile (CH3CN)) leads to the

release of fluorescent resorufin The fluorescence increase is directly

proportional to the hydrazine concentration in the range 10–200 μM

with a LOD of about 2 × 10−6 M The dramatic colour change of

the probe solution from colourless to red upon the treatment with

hydrazine demonstrated that 19 can serve as a‘naked‐eye’ probe for

hydrazine Probe 19 also has been applied to image hydrazine in living

cells (Figure 13B)

Recently, our group reported a ratiometric fluorescent hydrazine

probe (20) based on the 1,8‐naphthalimide fluorophore (Figure 14) [50] The probe operates by hydrazine‐mediated removal of the

4‐bromo butyrate moiety via a substitution‐cyclization‐elimination process to liberate the 1,8‐naphthalimide moiety Upon the treatment with hydrazine, the probe solution displayed a bathochromic shift in

emission from 420 to 550 nm The emission intensity ratio (I550/I420)

is found to be proportional to the concentration of hydrazine in the range 1.0–30.0 μM with a LOD of 2.7 × 10−7M Moreover, the probe has been utilized for practical detection of gaseous hydrazine, as well

as imaging hydrazine in live cells

By anchoring a 4‐bromo butyrate moiety onto a cyanine scaffold,

Lu and co‐workers developed a NIR ratiometric fluorescent probe (21)

(Figure 15) for hydrazine detection.[51] Addition of hydrazine to a

solution of 21 in DMSO–H2O (1:4, v/v, phosphate‐buffered saline (PBS) 20 mM, pH 7.4) induced a significant hypsochromic shift of the emission maximum from 810 to 627 nm The probe displayed high sensitivity (LOD = 1.2 × 10−8M) and excellent selectivity over other interfering analytes Furthermore, the probe is capable of imaging exogenous hydrazine not only in living cells but also in living mice (Figure 15B)

Installation of a 4‐bromo butyrate moiety onto different fluorophores has afforded a series of fluorescent hydrazine probes in

a variety of colours (Figure 16) Based‐on fluorescein, Goswami et al reported a ‘turn on’ fluorescent probe (22).[52] By utilizing

FIGURE 10 Structures of fluorescent hydrazine probes 9–17 with an acetate moiety

FIGURE 11 Proposed sensing mechanism of 4‐bromobutyryl‐based probes for hydrazine

FIGURE 12 Structure and reaction of probe 18 with hydrazine

dicyanomethylenedihydrofuran scaffold, Li and co‐workers prepared a

far‐red fluorescent hydrazine probe (23).[53] Zhu and co‐workers

developed two flavonoid‐based fluorescent hydrazine sensors (24

and 25),[54,55]and both of them have been applied to the detection

of hydrazine in living cells Chen et al reported a highly sensitive fluo-rescent turn‐on probe (26) for hydrazine based on a coumarin

fluorophore.[56] Using the similar strategy, a new ESIPT hydrazine

probe (27) was also developed It displayed good water solubility and

FIGURE 13 (A) Structure and reaction of probe 19 with hydrazine (B) Confocal fluorescence images of Chinese hamster ovary (CHO) cells: cells incubated with 19 (a –c); image of cells after treatment with 19 and subsequent treatment of the cells with hydrazine for (e–g) (a and e) Bright‐field

images; (b and f) red channel; (c and g) merged images (Reprinted from ref 49)

FIGURE 14 Structure and reaction of probe

20 with hydrazine

FIGURE 15 (A) Structure and reaction of probe 21 with hydrazine (B) Representative fluorescence images of the mice that were pre‐treated with 21 and subsequently incubated with hydrazine Images were taken after incubation of hydrazine for 0, 3, 6, and 10 min (Reprinted from

ref 51)

can be performed in a PBS buffer (pH 7.4) solution with 1%

etha-nol.[57]Lu et al reported a NIR fluorescent probe (28) for hydrazine

by using a hemicyanine dye.[58]

4 | P R O B E S B A S E D O N V I N Y L

M A L O N O N I T R I L E

Previous studies have demonstrated that arylidene malononitrile can

selectively react with hydrazine to yield a product of hydrazone

(Figure 17).[59] This specific reactivity of hydrazine combined with

the synthetic ease of incorporating malononitrile onto fluorophores

possessing a vinyl aldehyde or benzaldehyde moiety has led to rapid

progress in the development of fluorescent hydrazine probes The first

fluorescent probe (29) based on this approach was developed by Peng

and co‐workers (Figure 18).[60]

Probe 29 displays a strong emission

with a maximum in the red region around 640 nm due to the

intramo-lecular charge transfer (ICT) process from the 7‐N,N‐diethyl group to

the electron‐withdrawing vinyl malononitrile through a π‐conjugated

system Upon reacting with hydrazine, the vinyl malononitrile can be

converted to hydrazone, which inhibits the ICT process within the

probe and thus leads to ratiometric responses both in absorption

and fluorescence signals In addition, this ICT‐based ratiometric probe

is exploited to image hydrazine in living cells (Figure 18B)

The malononitrile trigger has been incorporated onto a

phenothi-azine dye by Yang and co‐workers to give a fluorescent hydrazine

probe (30) (Figure 19).[61]Upon reaction with hydrazine in DMF–Tris

buffer (10 mM, pH 7.4, 7:3, v/v), the probe exhibits a distinct turn‐

on fluorescence response at 490 nm, which can be ascribed to the

change in electronic structure of the probe due to the formation of

hydrazone The probe displays a dynamic range of 5.0 to 20.0μM for hydrazine with a LOD of 1.2 × 10−8M Moreover, the probe has

an excellent biocompatibility, and has been successfully applied to visualize hydrazine in live cells and zabrafish

Kumar et al reported a N,N‐dimethylaminocinnamaldehyde‐based

ICT fluorescent probe (31) (Figure 20) for the ratiometirc detection of

hydrazine.[62] Due to the efficient ICT from electron‐donating dimethylamino group to the electron‐withdrawing cyano groups,

probe 31 exhibits an emission in the red region The addition of hydra-zine to the solution of 31 in HEPES buffer–CH3CN (10 mM, pH 7.2,

99.5/0.5, v/v), the emission band at 582 nm shifts to 480 nm due to

the conversion of cyano groups to hydrazone and the consequent inhibition of the ICT process within the probe molecule The probe displays an ultralow LOD of 8.87 × 10−9 M Furthermore, the probe has been applied for intracellular imaging of hydrazine and the preparation of fluorescent test strips to detecting trace level of hydrazine in water

Incorporating dicyanovinyl group to derivated tetraphenylethylene (TPE) moieties, Liu and co‐workers devised a series of aggregation‐

induced emission (AIE) probes (32 –34)[63]

(Figure 21) for both fluores-cence and colourimetric detection of hydrazine in solution as well as in solid state based on the probe‐stained paper strips These probes were designed on the basis of the different electron‐donating abilities of the substituent groups Introducing the electron‐donating groups, such as methoxyl and N,N‐dimethylamino, into the TPE structure, the yielded

probes (33 and 34) feature a more red‐shifted absorption and emission

in the visible region due to the enhanced ICT system Thus, probe 34 gives the best response to hydrazine, and 34 ‐stained paper strip can

achieve sensing low‐level hydrazine vapour

The vinyl malononitrile as the recognition moiety has also expanded

to develop family fluorescent hydrazine probes by using various

fluorophore scaffolds or their derivatives, including benzothiazole (35),[64] carbazole (36),[65] acenaphthequinone (37),[66] anthraldehyde (38),[67]

pydazoline (39),[68] naphthaoxazole (40),[69] formylated benzothiazole

(41)[70]and dicyanomethylene‐4H‐chromene (42)[71](Figure 22) Besides vinyl malononitrile, some other electron‐deficient alkene structures also can react with hydrazine to form the hydrazone via a similar mechanism Based on this type of reaction, several new

FIGURE 16 Structures of fluorescent hydrazine probes 22–28 with a 4‐bromo butyrate moiety

FIGURE 17 Proposed sensing mechanism of vinyl malononitrile‐

based probe for hydrazine

fluorescent hydrazine probes have been reported recently (Figure 23).

Two probes (43 and 44)[72,73]based on a recognition unit of 2

‐cyano-acrylate have been designed by using two different signalling

moieties, pyridomethene and phenanthroimidazole Compound

(45)[74]was synthesized as a colorimetric and fluorogenic probe for

hydrazine detection based on the degradation of π‐conjugated

system of the probe triggered by hydrazine By utilizing

2‐benzothiazoleacetonitrile as a new recognition site, Lin and

co‐workers reported a turn‐on two‐photon fluorescent hydrazine

probe (46).[75]By conjugating hemicyanine to a coumarin fluorophore,

Ni and co‐workers developed a NIR‐emissive (λex = 580 nm,

λem= 660 nm) hydrazine selective probe (47).[76]Reaction of 47 with

hydrazine gives a coumarin hydrazone derivative and a corresponding

blue‐shift emission, and thus a ratiometric fluorescence response is

achieved Based on a similar hemicyanine linked electron‐deficient

alkene structure, Ban et al synthesized a mitochondria‐targeted

ratiometric fluorescent hydrazine probe (48).[77]

5 | P R O B E S B A S E D O N P H T H A L I M I D E

M O I E T Y

Gabriel synthesis, named after the German chemist Siegmund Gabriel,

is a classical approach for the preparation of primary amines, specifi-cally transforming alkyl halides into primary amines Traditionally, this reaction involves the N‐alkylation of phthalimide by a target primary alkyl halide, followed by hydrazine‐mediated cleavage of the phthaloyl group to liberate the primary amines This strategies involved in Gabriel synthesis has been successfully adapted for the development

of fluorescent hydrazine probes, typically by incorporating phthalimide into amine‐containing fluorophores (Figure 24) The first two phthalimide‐based fluorescent hydrazine probes (49 and 50) were

reported simultaneously by Lin, Cui and their co‐workers by using

4‐aminonaphthalimide as the fluorescent reporters (Figure 25).[78,79]

In a mixture of PBS buffer (10 mM, pH = 7.2) and ethanol (1:9, v/v),

probe 49 exhibits a UV–vis absorption band and a fluorescence emission band at 344 and 467 nm, respectively Upon reaction with hydrazine, the phthalimide group was cleaved, the released

4‐aminonaphthalimide displays a yellow colour (λabs= 467) and emits yellowish‐green fluorescence (λem = 528) The probe demonstrates

an ultralow LOD of 4.2 × 10−9M, and is capable of imaging

intracellu-lar hydrazine Probe 50 exhibits simiintracellu-lar highly specific ratiometric

response for hydrazine over other primary amines

FIGURE 19 Structure and reaction of probe 30 with hydrazine

FIGURE 20 Structure and reaction of probe

31 with hydrazine

FIGURE 18 (A) Structure and reaction of probe 29 with hydrazine (B) Confocal fluorescence images of HeLa cells Cells incubated with 29 (top); image of cells after treatment with 29 and subsequent treatment of the cells with hydrazine (a, d) Bright‐field images; (b, e) green emission (540 ± 20 nm); and (c, f) red emission (640 ± 20 nm) (Reprinted from ref 60)

By installing phthalimide onto the dansyl fluorophore, Zhao and

co‐workers synthesized a turn‐on fluorescent hydrazine probe (51)

(Figure 26).[80]In a solution of HEPES buffer (pH 7.0, 20 mM) and

DMSO (1/9, v/v), the probe only shows extremely weak fluorescence

at 475 nm (ɸ = 0.093), and addition of hydrazine leads to a ‘switched

on’ emission (ɸ = 0.4983) with a bathochromic shift to 512 nm.

The probe has also successfully exploited to detect gaseous and intra-cellular hydrazine

Notably, Cui et al reported a multi‐responsive optical probe (52)

for the specific detection of hydrazine (Figure 27).[81]On the basis

of a Gabriel‐type reaction, hydrazinolysis of 52 can produce 7‐

amino‐4‐methylcoumarin as a chromogenic and fluorogenic reporter, and luminol as a chemiluminescence probe The ratiometric

fluores-cence response of the probe 52 toward hydrazine is highly selective

over other interfering substances, with a linear dynamic range of 0.1

to 1.0μM and a LOD of 1 × 10−7M The probe is also used to detect hydrazine in vapour state Furthermore, the probe has also been applied for the detection of hydrazine in HeLa cells (Figure 27B)

FIGURE 22 Structures of fluorescent hydrazine probes 35–42 with a malononitrile moiety

FIGURE 23 Structures of fluorescent hydrazine probes 43–48 possessing electron‐deficient alkene structure

FIGURE 24 Proposed sensing mechanism of phthalimide‐based probe for hydrazine

FIGURE 21 Structures and reactions of probes 32–34 with

hydrazine