recent developments in rhodamine salicylidene hydrazone chemosensors

8/19/2019 Recent Developments in Rhodamine Salicylidene Hydrazone Chemosensors

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standards and chemosensors.13,14 Rhodamine spirocyclic che-

mosensors have attracted a lot of attention due to their

simplicity, high sensitivity, and real-time detection for various

analytes in vivo and in vitro.15–18 The preferential binding sites

are spiro-carbonyl O, imide N and other N or O sites such as

ortho-phenol O. However, their selectivity properties to analytes

are strongly inuenced by the structure, substituents, solvent

and temperature.19 Hence a subtle change in the rhodamine

binding sites might aff ect the “spirolactam ring-opening ”process which could modify their selectivity and sensitivity

(Fig. 1).20

2. Detection of analytes based onrhodamine salicylidene hydrazone2.1 Sensors for detecting Cu2+

The copper ion is an essential trace element in biological

systems, and plays as a catalytic cofactor for a variety of met-

alloenzymes in the lifecycle.21 Therefore, a convenient and fast

method to detect Cu2+ existing in environmental and biological

resources is of considerable importance. However, it was not

until 2006 that Tong's group reported the rst salicylidene

rhodamine hydrazone chemsensor RhB-Sal (1) and applied it to

the detection of Cu2+ ions in neutral buff ered media.22 In their

study, the rhodamine chemsensor 1 displayed a reversible

absorption and uorescence enhancement response to Cu2+ via

a 1 : 1 binding mode. Furthermore, the sensitivity of 1 for Cu2+

can be lower than 25 nM in 50% (v/v) buff ered water/CH3CN by

the absorption spectra method and the 0.1 mM level for the

uorescence method under these conditions. Even in neutral

buff ered aqueous solutions, the uorescence method was

successful in sensing Cu2+ at a micromolar level.

Ma et al. have designed and synthesized a rhodamine-baseduorescent probe 2 for copper ions.23 Probe 2 exhibited high

sensitivity toward Cu2+ and about a 37-fold increase in uo-

rescence emission intensity, which was observed upon the

addition of 10 equiv. of Cu2+ in 50% water–ethanol buff ered at

pH 7.10. Besides, upon binding Cu2+, a remarkable color

change from colorless to pink was easily observed by the naked

eye. The linear response range covered a concentration range of

Cu2+ from 8.0 107 to 1.0 104 mol L1 and the detection

limit was 3.0 107 mol L1. Except for Co2+, the probe

exhibited high selectivity for Cu2+ over a large number of

coexisting ions. It has been used for direct measurement of Cu2+

content in environmental and biological systems.

Tang's group designed a rhodamine B based derivative 3 as

a colorimetric and uorescent dual mode sensor for recognition

of Cu2+

in CH3CN/H2O (1 : 1, v/v, HEPES 10 mM, pH ¼ 7.0)solution.24 Sensor 3 displayed highly selective, sensitive and

rapid recognition behavior toward Cu2+ among a range of bio-

logically and environmentally important metal ions. The 1 : 1

binding stoichiometry of 3 and Cu2+ was proved by nonlinear

least-squares tting of titration proles and Job's plot; the

association constant and uorescence detection limit were

calculated to be 1.92 106 M1 and 7.96 108 M, respectively.

The Cu2+ recognition process was reversible and showed little

interference from other coexisting metal ions.

Chen and co-workers synthesized a salicylidene rhodamine

chemosensor 4 through the reaction of rhodamine hydrazide

and salicylaldehyde receptor.

25

It exhibited a reversible andsensitive “turn-on” response of absorption and uorescence

toward Cu2+ in aqueous acetonitrile solution. An approximate

65 and 6-fold enhancement in the absorbance at 556 nm and

uorescence intensity at 573 nm were estimated when the

concentration of Cu2+ reached 10 mM. 4 displayed more sensi-

tivity than the known compound RhB-Sal for Cu2+ (ca. 30 and

2-fold, respectively) under the same conditions. The competi-

tion experiments for Cu2+ mixed with common metal ions

exhibited no obvious change in absorption and emission except

for Cr3+ ion, which could induce uorescence quenching to

a certain extent.

Yang's group developed a colorimetric and uorescent

sensor 5 by the condensation reaction of rhodamine B hydra-zide and 2,4-dihydroxybenzaldehyde, which showed reversible

and highly selective and sensitive recognition toward Cu2+ over

other examined metal ions.26 Upon addition of Cu2+, sensor 5

exhibited remarkably enhanced absorbance intensity and color

change from colorless to pink in a DMSO and MeCN aqueous

Jinglin Liu received his Ph.D.

degree in 2006 under the guid-

ance of Prof. Xu Bai and Prof.

Hengbin Zhang from Jilin

University. He began his inde-

pendent research career at

NENU in 2007. He was

promoted to a full professor in

the college of chemistry and

chemical engineering of IMUN in

2012. His current research

interests involve combinatorial

chemistry and organic synthesis

methodology.

Dewen Dong received his PhD in

1999 from Changchun Institute

of Applied Chemistry of CAS,

and then he became a Lecturer

in Shizuoka University (1999–

2001) and a postdoctoral fellow

in the University of Hull (2001–

2003). He joined the faculty of

NENU as a full professor in 2003

and moved to Changchun Insti-

tute of Applied Chemistry of CAS

as a professor in 2006. His

research includes synthetic

chemistry and functional

organic/polymeric materials.

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buff er solution or pure MeCN, and showed signicant off –on

uorescence accompanied by color changes from colorless to

orange in MeCN. In DMSO/Tris–HCl buff er (1 : 9, v/v, pH 7.0)

solutions, the quantication of Cu2+ by 5 was satisfactory in

a linear working range of 10–300 mM, with an absorbance

detection limit of 3.42 106 M. The sensor 5 was also

successfully applied to the determination of Cu2+ in water

samples.

Gupta and co-workers also synthesized the rhodamine-

derived Schiff base 6 and investigated its sensing behavior by

UV-vis and uorescence spectroscopic techniques.27 The sensor

exhibited a highly selective and sensitive colorimetric response

Table 1 The comparison of the

uorescence probes for Cu

2+

based on salicylidene

Structure Media AnalyteDetectionlimit

Working range

Associationconstant

Detectionmode Probe ref.

CH3CN/H2O,Tris–HCl, pH ¼ 7.0

Cu2+ 25 nM, 0.1 mM 0–10 mM 6.91 104 Abs, FL 1 (ref. 22)

EtOH/water,Tris–HCl, pH ¼ 7.1

Cu2+ 0.3 mM 0.8–100 mM — FL 2 (ref. 23)

CH3CN/H2O,HEPES, pH ¼ 7.0

Cu2+ 7.96 108 0–10 mM 1.92 106 Abs, FL 3 (ref. 24)

CH3CN/Tris–HCl,pH ¼ 7.0

Cu2+ 10 mM,

naked eye 0–20 mM 3.09 104 Abs, FL 4 (ref. 25)

DMSO/Tris–HCl,pH ¼ 7.0

Cu2+ 3.42 106 10–300 mM 2.83 104 Abs, FL 5 (ref. 26)

CH3OH/H2O Cu2+, Al3+,

Fe3+ 0.99 108 0–20 mM 1.1 106 Abs 6 (ref. 27)

CH3OH/HEPES,pH ¼ 7.0

Cu2+, VO2+ 106 to 105,

naked eye 0–80 mM — Abs 7 (ref. 28)

CH3CN/H2O Cu2+, Hg 2+ 105 10–100 mM — Abs, FL 8 (ref. 29)


Cu2+ 3.7 108 0–5 mM — Abs 9 (ref. 30)


Cu2+ 1.2 109 0–20 mM — Abs, FL 10 (ref. 30)

Dry CH3CN Cu2+ 0.49, 14.98 mM 0–20 mM

6.72 104,4.23 104

Abs, FL 11 (ref. 31)

CH3CN/HEPES,pH ¼ 7.04

Cu2+ 0.20 mM 0–20 mM 3.7 104 Abs, FL 12 (ref. 32)

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to Cu2+ and Al3+, and an “off –on” uorescence response toward

Fe3+ in semi-aqueous media. The spectral changes obtained are

large enough in the visible region of the spectrum and thus

enable naked-eye detection. The 1 : 1 stoichiometric ratio

between probe and metal ions was proposed based on a Job's

plot, which was further conrmed by ESI mass analysis. Paper

strips were also used for the detection of Cu2+, Al3+ and Fe3+

ions.

Huo et al. synthesized a rhodamine-based colorimetric che-mosensor by incorporating rhodamine hydrazide and 5-chlor-

osalicylaldehyde in ethanol. The sensor 7 exhibited specic

absorbance responses to Cu2+ and turned from colourless to

purple red, which allowed naked-eye detection of Cu2+ ions in

50% CH3OH–H2O solution.28 The low detection threshold for

Cu2+ in UV-vis spectrum was 106 to 105 M and at this level the

color change was very obvious. In contrast, the selectivity

towards VO2+ was determined from changes in the emission

spectra in the nanomolar range. This represents the rst re-

ported rhodamine-based sensor capable of detecting both Cu2+

and VO2+ using two diff erent modes.

Yang and co-workers reported a rhodamine salicylidenehydrazone derivative 8 by condensation of 3,5-dichlor-

osalicylaldehyde and rhodamine B hydrazide. The sensor 8 was

utilized as a colorimetric and reversible chemosensor for Cu2+

and Hg 2+ in aqueous CH3CN media.29 Among the various metal

ions, the sensor 8 exhibited remarkably enhanced absorbance

intensity and color change for Cu2+, and showed signicant

“off –on” uorescence accompanied with red emission upon

binding with Hg 2+. The absorbance and uorescence signals of

8 could be restored with addition of EDTA into solutions of 8-

Cu2+ and 8-Hg 2+, indicating that the binding process is chemi-

cally reversible.

Huo's group synthesized two salicylidene-based rhodamine

derivatives 9 and 10.30 Because of their diff erent salicylidene

groups, 9 exhibited particular selectivity towards Cu2+ with color

changes from colorless to yellow, which can be used as a UV-vischemosensor for Cu2+. Due to the strong intramolecular charge

transfer, 9 was very weakly uorescent and the 9-Cu2+ metal

complex remains non-uorescent. The absorbance intensity of

9 was linearly proportional to Cu2+ concentrations of 0–5 mmol L1

with a detection limit of 3.7 108 M. Diff erent from the

process of 9, the sensor 10 was strongly uorescent, which can

be used as a dual-channel colorimetric and uorescent

compound for Cu2+. The uorescence detection limit of 10 to

Cu2+ is 1.2 109 M, which was more sensitive than the UV-vis

behavior of 9. The cell experiments show the good cell-

membrane permeability of compound 10, and it can thus be

used to mark Cu

2+

within living cells. A new rhodamine derivative 11 bearing an electron with-

drawing group –NO2 at the 5-position of the 2-hydroxyphenyl

moiety was synthesized by Chen et al.31 The sensor displayed

similar high selectivity for Cu2+ over coexisting metal ions

except that Fe3+ brought about some absorption interference

and Bi3+ led to a little uorescence interference. The detection

Table 2 The comparison of the uorescence probes for Cu2+ from naphthaldehyde and rhodamine 6G


Working range

Associationconstant



Cu2+ 0.2 mM 0–20 mM 5.0 104 Abs, FL 13 (ref. 32)

CH3CN/H2O Cu2+ 0.32 ppb 0–5 mM 5.4 105 Abs 13 (ref. 33)


Cu2+ 0.156 mM 0–4 mM — FL 15 (ref. 34)


Cu2+ 0.2 mM 0–20 mM 5.6 105 Abs, FL 14 (ref. 32)


Cu2+ — 0–200 mM 2.5 104 Abs, FL 16 (ref. 35)

EtOH/H2O, NaAc–HAc,pH ¼ 7.0

Cu2+ 10 nM,

25 nM 0–5 mM 1 106 Abs, FL 17 (ref. 36)

CH3CN/H2O,pH ¼ 7.0

Cu2+ 5 mM,

naked eye 0–50 mM 4.877 104 Abs, FL 18 (ref. 37)



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limit for Cu2+ calculated from the uorescence titration data

was 0.49 mM. The sensitivity of 11 for Cu2+ in pure CH3CN is

higher than aqueous solutions. The sensor 11 was expected to

reversibly bind to Cu2+ in each medium, forming a 1 : 1 stoi-

chiometric 12-Cu2+ complex with an association constant of

6.72 104 M1 and 4.23 104 M1, respectively (Table 1).

Wang and co-workers reported three rhodamine-salicylidene

derivative 12, rhodamine-naphthalene derivative 13 and

rhodamine-binaphthol derivative 14 as specic uorescent andcolorimetric chemosensors for Cu2+.32 These probes exhibited

selective “off –on” type changes in both absorption and emis-

sion spectra toward Cu2+ ions compared to other metal ions.

The detection limits of 12, 13 and 14 toward Cu2+ were all 0.20

mM (12.7 ppb) by plotting the uorescence intensity at 571 nm

versus the concentration of Cu2+. The Job's plots indicated a 1 : 1

stoichiometry between 12, 13 or 14 and Cu2+. The association

constants were 3.7 104 M1, 5.0 104 M1 and 5.6 105 M1

respectively, which suggested that the complex of 14-Cu2+ was

more stable than that of 12-Cu2+ and 13-Cu2+.

Wu's group reported a rhodamine-based 2-hydroxy-1-naph-

thaldehyde hydrazone chemosensor 13, which showeda reversible, selective, and sensitive absorbance enhancement

response to Cu2+ in a buff ered CH3CN–H2O media.33 The probe

13 had a linear response to increasing amounts of low Cu2+

concentration (between 5 and 50 nM), establishing that the

system has a limit of quantication down to 0.32 ppb. The

association constant of molecule 13 with Cu2+ was 5.4 105

M1, and the stoichiometry for 13-Cu2+ was calculated to be 1 : 1

by the Job's plot method and assumed by the nonlinear tting

of the titration curve.

Yin and co-workers synthesized a sugar-rhodamine salicyli-

dene uorescent sensor 15, and investigated its properties for

Cu2+ according to the Cu2+ triggered spirolactam ring-opening

mechanism.34 The introduction of a sugar residue into the sal-icylidene part, combined with the solvent eff ect, signicantly

improved its selectivity and sensitivity. The synthesized probe

15 exhibited high selectivity and excellent sensitivity to Cu2+ in

acetonitrile media. The detection limit was 0.15 mM, about 200

times lower than the recommended water quality standard of

Cu2+ ions in drinking water. The probe 15 could be simply,

rapidly, and satisfactorily used to detect the concentration of

Cu2+.

Yoon and co-workers reported a rhodamine-pyrene deriva-

tive 16 as a ratiometric and “naked-eye” sensor for the detection

of Cu2+ ion in neutral buff ered media.35 It displayed a highly

selective and ratiometric

uorescence change and a colori-metric change upon the addition of Cu2+, utilizing the spi-

rolactam (nonuorescent) to ring opened amide (uorescent)

process. The nonlinear tting of the titration curve and the data

of Job's plot from absorption spectra assumed a 1 : 1 stoichi-

ometry for the 16-Cu2+ complex with an association constant of

2.5 104 M1.

Tong ’s group designed another rhodamine chemosensor 17

using salicylaldehyde and rhodamine 6G hydrazine as copper-

chelating and signal-reporting groups.36 The sensor exhibited

selective absorbance enhancement to Cu2+ over other metal

ions at 529 nm, with a dynamic working range of 0.05–5.00 mM

and a detection limit of 10 nM. The linear working range using

17 for Cu2+ was 0.1–3.6 mM and the detection limit was 25 nM.

Both absorptiometric and uorometric methods were applied

for the detection of Cu2+ in three water samples.

A rhodamine 6G based 4-(diethylamino) salicylidene hydra-

zone chemosensor 18 was reported by Liu et al.37 The sensor

exhibited a high selectivity for Cu2+ and could serve as a good

selective naked-eye chemosensor for Cu2+ in CH3CN. The

association constant of 18 with Cu2+

ion was found to be 4.877 104 M1, and a 1 : 1 stoichiometric complexation was

conrmed by the Job's plot. Upon the addition of Cu 2+, the

spirolactam ring of 18 was opened and the solution changed

from colorless to red. Strangely, an unexpected uorescence

quenching was observed upon the addition of 5 equiv. Cu2+,

which is contrary to the uorescence turn-on of most rhoda-

mine based chemosensors (Table 2).

2.2 Sensors for detecting Hg 2+

Hg 2+ is one of the most severe environmental contaminants

because it can cause serious health problems, damaging thecentral nervous and endocrine systems, leading to many

cognitive and motion disorders.38 There are many rhodamine

spirocyclic chemosensors that have been applied for sensing

Hg 2+ in environmental and biological resources.39–45 However,

rhodamine sensors modulated with a salicylidene structure for

real-time monitoring of Hg 2+ in environmental samples are still

greatly needed.46,47

Yoon's group synthesized two rhodamine hydrazone deriv-

atives 19 bearing a thiol group as selective uorescent and

colorimetric chemosensor to Hg 2+.48 The ring-opening process

of spirolactam enables large uorescent enhancement and

colorimetric change upon the addition of Hg 2+. The 2 : 1 stoi-

chiometry between 19 and Hg 2+ was conrmed by Job's plots. A plot of the uorescent intensities of 19 versus the log concen-

tration of Hg 2+ exhibited a linear response in the range of 1 nM

to 1 mM, and the detection limit was 1 nM. The sensor 19 can be

employed for the in vivo imaging of nanomolar concentrations

of Hg 2+.

Zhao et al. also reported that 13 can achieve double-channel

detection of Hg 2+ and Mg 2+ by diff erent binding modes and can

detect Hg 2+ through a visible color change.49 The detection limit

for the sensor was estimated to be 80 nM. The association

constant between 13 and Hg 2+ was determined to be 1.0 105

M1. Selective binding sites of the compound 13 to Hg 2+ and

Mg 2+

caused immediate and remarkable

uorescenceenhancement at 589 nm and 523 nm. Furthermore, this probe

can indirectly detect glutathione and cysteine with good linear

relationships.

Jiang and co-workers found the sensor 13 exhibits extremely

high sensitivity (as low as 2 ppb) and selectivity to Hg 2+ in

methanol solution.50 The association constant of 13 with Hg 2+

was calculated to be 3.9 105 ( R ¼ 0.9966) by using nonlinear

least-squares analysis. Further characterization conrmed

a 1 : 1 complex, which restores the ICT eff ect to show uores-

cence. The excellent biological value of 13 was demonstrated by

the uorescence imaging in living yeast and HeLa cells.



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Jiang et al. developed a highly sensitive and selective Hg 2+

probe 20 by connecting a water soluble receptor group

(sulfonated b-naphthol) and rhodamine B together through

hydrazine hydrate.51 The results illustrated that 20 has excellent

sensitivity and selectivity abilities toward Hg 2+ and reaches as

low as 4 ppb of detection limit in methanol. The 1 : 1 binding

stoichiometry was conrmed by the Job's plot method, and the

association constant for Hg 2+ was estimated to be 4.6 106

M1. These outstanding cell permeablility and compatibility

characteristics conrm that this kind of Hg 2+ probe has great

potential in biological and pharmacological systems.

Ma and co-workers designed a novel functionalized SBA-15

nanosensor with salicylidene based rhodamine hydrazone asthe binding site for Hg 2+.52 The sensor showed uorescence

enhancement selectivity as well as selective coloration toward

Hg 2+. Covalently gra ed 21 onto the inner surface of SBA-15

makes the nanosensor easy to recover and recycle. Job's plot

indicated that 21 coordinated to Hg 2+ in a 1 : 1 binding stoi-

chiometry; the detection limit of 21 for Hg 2+ is about 1.5

108 M. The addition of S2 led to both color and uorescence

fading, indicating the reversibility of the binding between 21

and Hg 2+. Using Hg 2+ and S2 as chemical inputs and the

uorescence intensity signal as output, 21-SBA-15 can be

utilized as an INHIBIT logic gate at the nanoscale level

(Table 3).

2.3 Sensors for detecting other metal ions

Zheng et al. have found that the “old” chemsensor RhB-Sal (1)

could be used for the selective and sensitive detection of CrO42

in acidic conditions.53 Based on the special oxidation reaction

with potassium dichromate to produce a highly uorescent

rhodamine B, the uorescence enhancement at 591 nm was

linearly well related to the concentration of CrO42 from 1.0

108 to 3.0 107 M (0.42–12.6 ng mL1) with a detection limit

of 1.5 109 M (0.063 ng mL1). The proposed method could

act as a simple “naked-eye” probe for selective detection of Cr6+,

and be explored to indicate Cr6+ from Cr2O72 and CrO4

2

anions.

Guchhait and Kar et al. developed a novel turn-on uores-

cent chemosensor based on a rhodamine–dihydroxy-

benzaldehyde conjugate.54 The sensor 22 displayed an excellent

selectivity and high sensitivity toward Al3+ with remarkably

enhanced uorescent intensity by a chelation-enhanced uo-

rescence (CHEF) process and also shows a clear color change

from colorless to deep magenta. Job's plot and TOF-MS analysis

conrmed the 1 : 1 binding stoichiometry between 22 and Al3+

ions, and the association constant calculated from the absorp-

tion titration result was found to be 2.56 103 M1. Under UV light illumination, one can visually detect even 2 108 M Al3+

in aqueous-acetonitrile buff er solution without the aid of any

sophisticated instruments.

Tong et al. found that 4- N , N -diethylamino-salicylidene

rhodamine hydrazone 23 exhibited selective and ratiometric

uorescent response toward Zn2+ over other metal ions in

aqueous ethanol.55 Upon the addition of Zn2+, there was an

obvious color change of the uorescence from dark cyan to

greenish yellow which could be monitored easily by the naked

eye. An association constant of log K a ¼ 5.22 was calculated for

the 1 : 1 metal-to-ligand complex according to the absorption

spectral titration data. The 23-Zn

2+

complex showed its revers-ibility in the presence of EDTA.

Li et al. synthesized a colorimetric and uorescence turn on

chemosensor 24 for the detection of Pb2+ ions.56 The sensor

displayed a highly selective uorescence enhancement (about

550 fold) and colorimetric change from colorless to pink in the

presence of Pb2+ in chloroform–THF (7 : 3). The absorbance at

557 nm was saturated a er 2 equiv. of Pb2+ was added. The

uorescence intensity at 577 nm increased continuously with

the Pb2+ concentration in the range of 106 to 105 mol L1. The

binding stoichiometry between 24 and Pb2+ was estimated to be

1 : 1 by Job's plot. The reversibility of the sensor was further

Table 3 The comparison of the uorescence sensors for Hg2+


Working range

Associationconstant


CH3CN/H2O Hg 2+ 1 nM 109 to 106 — FL 19 (ref. 48)

CH3OH Hg 2+ 2 ppb 0–300 mM 3.9 105 Abs, FL 13 (ref. 49)

CH3CN Hg 2+

, Mg 2+

80 nM 0–

100 mM 1 105

FL 13 (ref. 50)

CH3OH Hg 2+ 4 ppb 0–8 104 1 106 Abs, FL 20 (ref. 51)

CH3CN/H2O Hg 2+ 15 nM 0–200 mM — Abs, FL 21 (ref. 52)



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conrmed by EDTA experiments. Sensor 24 could be used

potentially for the detection Pb2+ in the environment.

Chen et al. have designed and synthesized a new targetable

uorescent probe 25 by linking a conjugated naphthalene

chromophore to a rhodamine platform and a lipophilic tri-

phenylphosphonium cation.57 The probe could sensitively and

selectively detect mitochondrial Fe3+ in living cells. It exhibited

a pseudo-large Stokes shi on the basis of the FRET mechanism

and excellent selectivity for Fe3+

excluding the interference of other metal ions, especially Cr3+. The stability constant of the

25-Fe3+ complex was calculated to be 2.0 104 M1. The

detection limit of 25 responded to Fe3+ linearly in the

micromolar concentration range, was reasonably estimated to

be 6.93 106 M. Furthermore, a er treatment with EDTA, the

color and uorescent emission intensity changed back, indi-

cating that 25 can be classied as a reversible chemosensor for

Fe3+ (Table 4).

2.4 Detection of CN and amino acids

Many well-designed rhodamine hydrazone cation chemo-sensors could be successfully developed as anionic chemo-

sensors by utilizing the indirect method. Using the ensemble

salicylidene rhodamine hydrazone RhB-Sal and Cu2+ ions, Li's

Table 4 The comparison of the salicylidene probes for other metal ions


Working range

Associationconstant


H2SO4 buff er CrO42 1.5 nM 0–3 mM — FL 1 (ref. 53)

EtOH/H2O, HEPES,pH ¼ 7.0

Zn2+ 0.05 mM 0–10 mM 1.66 105 Abs, FL 22 (ref. 54)

CH3CN/H2O, HEPES,pH ¼ 7.2

Al3+ 20 nM 0–100 mM 2.56 103 FL 23 (ref. 55)

CHCl3/THF Pb2+

— 0–10 mM — Abs, FL 24 (ref. 56)

EtOH/H2O Fe3+ 6.93 mM 0–50 mM 2 104 FL 25 (ref. 57)

Table 5 The comparison of the metal complex sensors for CN and amino acids

Structure Media Analyte Detection limit Working range Detection mode Probe ref.

CH3CN/H2O, Tris–HCl,pH ¼ 7.0

CN 0.013 ppm 0.1–7 mM Abs 26 (ref. 58)ClO 0.81 mM 0–70 mM Abs 26 (ref. 59)Histidine,protease

— 0–5 mg mL Abs 26 (ref. 60)

EtOH/H2O, Tris–

HCl,pH ¼ 7.1 Cysteine 0.14 mM 0

–28 mM Abs, FL 27 (ref. 61)

CH3CN/H2O CN 0.72 nM 0–10 mM Abs, FL 28 (ref. 62)

THF–CH2Cl2 solid state UV — — Abs 29 (ref. 63)



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group designed an indirect anionic chemosensor for

cyanide.58 Upon the addition of trace CN, the magenta color

faded to colorless immediately, with a detection limit as low

as 0.013 ppm, much lower than the maximum contaminant

level for cyanide in drinking water (0.2 ppm). Later they

developed the rhodamine chemosensor 1 as a new type of

probe for the detection of ClO based on the oxidation

property of hypochlorites and diff erent coordinating prop-

erties of Cu+

and Cu2+

.59 Upon the addition of trace ClO

, thecolorless solution turned magenta to report the concentra-

tion of the present hypochlorite ions, with the detection limit

as low as 8.1 107 M in real water samples. They also

proposed an indirect approach to utilize sensitive colori-

metric sensor 26-BSA to detec sensitivelyt a-amino acids.60 As

the hydrolysis of bovine serum albumin (BSA) with the aid of

trypsin produces a-amino acids, the complex of 26-BSA could

act as a label-free, sensitive, selective sensor toward trypsin.

The detection process could be visually observed by the

naked eye.

Using the chemosensing ensemble method, Yang et al.

developed a

uorescent chemosensing ensemble 27 for thedetection of cysteine based on the uorescence inner lter

eff ect.61 Upon adding cysteine to the above solution, the

complexation of Cu2+ and cysteine led to the dissociation of 27,

which thus decreases the uorescence inner lter eff ect of the

solution, and leading to the uorescence increase of the che-

mosensing system. The uorescence increase is linear with

cysteine concentration up to 10.0 mM, with a detection limit of

1.4 107 M.

Hu and co-workers reported a selective and sensitive method

to detect aqueous CN based on a rhodamine B hydrazide and

2-tertbutyldimethyl silyloxy benzaldehyde conjugate (RTSB)

and Fe3+ derivative.62 RTSB displayed highly selectivity and

sensitivity to Fe3+ with uorescence emission enhancement at 581 nm accompanied by a color change from colorless to pink.

In response to CN, the system 28 provides a remarkable

uorescence intensity change, blue shi and also a clear color

change from pink to colorless. The background anions show

small or no interference with the detection of CN. The detec-

tion limit of the system for CN was around 7.2 108 M.

Treatment of 28 with CN aff orded a similar NMR spectrum

with that of RTSB alone, which indicated that addition of CN

prompted the dissociation of 28 and the release of free probe

RTSB.

Recently, Tong and Tang et al. developed a new photo-

chromic system based on rhodamine B salicylidene hydrazonemetal complex 29.63 The molecules showed absorption “turn-

on” and uorescence “turn-off ” response upon UV irradiation

both in solution and in solid matrix. UV light promoted the

isomerization of the salicylaldehyde hydrazone moiety from the

enol-form to the keto-form, and subsequently induced the spi-

rolactam ring-opening in the rhodamine B part and caused the

photochromic reaction. Owing to the good fatigue resistance,

and the tunable lifetime of the ring-open state, 29 was applied

in photo printing and UV strength measurement in the solid

state (Table 5).

3. Conclusions

In this review, we have covered the development and applica-

tions of rhodamine salicylidene hydrazone chemosensors based

on spiroring-opening of the xanthene platform. Considering the

uorophores and structure–activity of the salicylidene group,

most of them display strong selectivity and sensitivity to Cu2+ in

neutral buff ered aqueous solution. From the type of substitu-

ents on the salicylidene group, we found that the electron-donating groups have superior uorescence detection limits to

the electron-withdrawing substituent, in the order of –CH3 >

–OCH3 > –H > –NO2. The detection limits based on the absor-

bance intensity are similar: –OCH3 > H > –F > –Cl > –OH > –NO2.

However, for both ligands and complexes, the presence of an

electron-withdrawing substituent, compared with no substit-

uent on the salicylidene ring, will greatly improve its emission

band or absorption band enhancements and binding capac-

ities. It is noteworthy that the structural modulation of salicy-

lidene is a very powerful approach for other cations, and

application of the indirect sensing strategy is a good idea for the

sensitive detection of anions and other species.Rhodamine salicylidene hydrazone probes play a major role

in pure organic solvent and aqueous organic media, the

combination of rhodamine probes with SiO2 or Fe3O4 nano-

materials and ber polymers, pave a fast and efficient way to the

detection and separation of heavy metal ions in the environ-

ment. Furthermore, the design and implementation of hydro-

philic groups in rhodamine derivatives develops their sensing

abilities in biological imaging and drug delivery. In short, the

development of high selectivity, sensitivity, photostability and

good biocompatibility rhodamine probes will be of great

importance for the environmental and life sciences.

Acknowledgements

The authors gratefully thank the nancial supports of the

National Natural Science Foundation of China (21172211,

21542006, 21362020), the Natural Science Foundation of Inner

Mongolia Autonomous Region, China (2014BS0205) and the

Scientic Research Foundation of Inner Mongolia University for

the Nationalities (NMD1311, NMDGP1403).

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recent developments in rhodamine salicylidene hydrazone chemosensors

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