LUMINESCENT ANALYSIS FOR CADMIUM
MICROQUANTIFICATION
K.Yu. Shunyaev, E.V. Dedyuikhina, N.V. Pechishcheva
Institute of Metallurgy of UB RAS,
101, Amundsen st., Ekaterinburg, Russia
shun@ural.ru
Cadmium compounds are dangerous inorganic toxic substances, influencing on
environment and human health. Maximum permitted concentration of cadmium in
domestic water is 0.001 mg/l [1], in fish industry waters – 0.005 mg/l and 0.01 mg/l
for sea reservoirs [2]. Significant enrichment of environment by heavy metals together
with their low natural level in biosphere and their high toxicity demands the constant
control of their content in various environment objects [3].
Atomic absorption method is commonly applied for of cadmium concentration
in environment [4-7]. In spectrophotometric analysis of cadmium widely use complex
compound of an ion Cd2+ with organic reagents [8-11]. For determination of cadmium
are offered polarographic [12-14], voltamperometric [15-17], coulometric methods of
the analysis [18]; determination with use ionselective electrodes [19]. In table 1
comparative characteristics of physicochemical methods of the analysis of cadmium
are presented. From table 1 one can see, that modern methods not always allow to
carry out direct determination of cadmium because of influence of matrix structure of
test or low concentration of a defined element. In this connection searching and
development of new effective techniques for cadmium determination is actual.
High sensitivity of luminescent methods of the analysis causes their special role
at qualitative and quantitative determination of microimpurity in high-clean
substances, materials of nuclear power, semi-conductor and luminescent technics; at
the analysis of mineral raw material; the control of foodstuff; in toxicology; at the
analysis of pharmaceutical, biological and medical preparations, including the
luminescence can be used at the analysis of cadmium [20].
Use of methods of a molecular luminescence, on sensitivity and cost comparable
with some methods of nuclear spectroscopy, for example, atomic-fluorescent, is not
accompanied by loss of test that is important at the analysis of toxic, radioactive
substances or rare materials. Methods of the molecular luminescent analysis much
more cheaply also demand smaller qualification, than methods neutron-activation
analysis, X-ray spectroscopy and mass-spectrometry which are distinguished also
with high sensitivity.
Methods molecular absorption spectrometry and a luminescence are comparable
on selectivity. However sensitivity and natural selectivity of luminescent methods can
be a little bit above as for qualitative indication use not one is possible, and two, and
sometimes and four spectra (spectra of excitation and radiation as fluorescence, and a
phosphorescence).
The chemical luminescent analysis is based on determination of concentration of
substance or on change of intensity of fluorescence, or on occurrence of fluorescing
products of reaction, or on clearing fluorescence at presence of a defined component
[21].
The main restrictions of methods of the molecular luminescent analysis are
connected with insufficiently universality. Really, if practically all substances absorb
in UV-, visible and near IR-areas all from them is far not find out ability to a
luminescence because of prevalence nonradiative processes.
154
Selectivity of many luminescent reactions is low enough, this lack should be
eliminated, entering into reactionary system masking substances or spending
preliminary division of ions. It is necessary to resort to methods of masking and
division and consequently, that spectra of complex molecules do not high
characteristic. The foolowing techniques are used to eliminate admixtures: extraction,
co precipitation on the collector, chromatography, evaporation, sublimation,
adsorption, electrochemical methods.
155
Table 1
Physicochemical methods of the analysis of cadmium
Method
spectrophotometry
Range of
determination
n·10-1 - n·102 μg
Emission
spectrometry
From n·10-7 % to
several tens of
mass parts
0,001-2 %
mass parts
atomic absorption
spectroscopy
0,001-2 %
mass parts
luminescence
10-4 % mass
parts,
10-10 g
polarography
(voltamperometric)
Advantages
Disadvantages
Cheapness of the equipment,
simplicity, rapidity, accuracy,
selectivity
Formation of the coloured
complex,
maintenance рН a solution,
the account of influence of the
third components
Universality,
sensitivity, rapidity, accuracy
Simultaneous determination of
several tens elements, low
LOD, accuracy, speed,
universality
Small amount of assay
(the graphite electrothermal
analyzer),
low requirements to stability of
a source of excitation,
insufficiently low LOD,
high cost of the device
High sensitivity, simplicity and
cheapness of the equipment
Limit of
detection
up to 10-5 %
Error of the
analysis
3-5 %
[11], [22]
Toxicity of mercury
10-9 %
2-5 %
[11], [22]
Low reproducibility owing to
instability of a source of
excitation
0,1 μg /ml
(10-3-10-4 %)
1-2 %
[11], [22]
Necessity of the lamp for each
element, insufficiently low
limit of detection, high cost of
the device
10-4-10-5 %
1 - 30 μg /ml
(flame source
of
atomisation),
0.00005 - 0.01
μg /ml.
(electrothermal
sprayer)
10-5 %
1-5 %
[22]
5-7 %
[22]
Not each substance is
luminescing, influence of
admixture
156
Reference
Luminescent methods of cadmium determination
Comparison of luminescent properties of complexes of metals to electronic
structure of the same metals has given the basis to state luminescent methods of
determination of cadmium an empirical rule: metals with the blank electronic
environments form not fluorescing complexes; complexes of metals with the filled
electronic environments are fluorescing [23]. The electronic structure of cadmium is
4d105s2 (for Cd2+ 4p64d10), hence at complexation of cadmium with organic reagents
should be observed an increasing of fluorescence intensity.
In the literature there are following versions of luminescent determination of
cadmium: fluorimetry, determination with fluorosensors, electrochemiluminescence
and X-ray fluorescence.
The main part of methods of the analysis of cadmium is based on the
phenomenon of increase of intensity of fluorescence of organic reagents (their
structures are given in table 2) at interaction with metal ion. The given methods are
presented in table 3.
Haloid derivatives of fluorescein - eosine and erythrozyne (reagents 1) form
with cadmium at presence 1,10-phenantroline the threefold complexes, capable to
fluoresce in organic solvents. The brightest fluorescence chloroformic extracts have,
intensity of their luminescence is increased with addition of acetone [11].
Triple complexes are widely applied in luminescent analysis. Use as reagents
acid xanthene dyes having an intensive luminescence and forming complexes with
cadmium, enables to increase sensitivity of determination of microamounts of this
element. Sensitivity of luminescent reaction usually depends on size of a quantum
yield of used dye [24] rather high quantum yield 4,5-dibromofluorescein (DBF) in
comparison with others halogen derivatives of fluorescein (eosine, erythrozyne)
increases sensitivity of luminescent reaction of formation a triple complex of
cadmium with DBF and 1,10-phenantroline (Phe) (reagents 2). Besides stronger acid
properties DBF in comparison with fluorescein are provided with course of reaction in
more sour area where has less an effect competing complexation with extraneous ions.
It is established, that a ratio of metal:Phe:DBF in a complex which extract by
chloroform at рН 9 is 1:2:1, and at sufficient excess DBF - 1:2:2. Linear dependence
of intensity of a luminescence is observed from 0,01 up to 0,2 μg/ml of cadmium, the
limit of detection is 1,5∙10-7 mol/l. Determination of cadmium is interfered by ions of
copper, iron, aluminium, zinc. The method is approved on a standard sample of
bronze and on a mix of sulfate of cadmium and zinc.
2-(o-oxyphenyl)-benzoxazole (a reagent 3) forms with ions of cadmium chelate
compound, which is dissolved in an acetic acid, the solution luminesces in UV-area
[25].
Fluorosensor (probe, chemosensor) is the three-componental system consisting
of a signalling component (fluorophor) and the centre of linkage "guest" (receptor),
the spacer usually divides them.
The new fluorescent probe (4) (DBD-ED-CNEDTA) is synthesized on reaction
N-(3-carboxy-2-naphtyl)-ethylenediamin-N,N',N'-triacetic acid (CNEDTA) with 4(N,N-dimethylaminosulphonyl) 7-(2-aminoethylamino)-2,1,3-benzoxadiazole (DBDED) [26]. Big stockes shift occurs because of fluorescent resonance energy transfer
(FRET) between the donor (CNEDTA) and an acceptor (DBD-ED) of fluorophor. The
probe reacts with metal cations in the water environment with formation chelate.
FRET - this interaction between the electronic-excited states of two fluorescent
molecules in which excitation passes from donor molecules to acceptor molecule
157
without emission of a photon. Structures of naphthalene and benzofurazan are chosen
as donor and acceptor fluorophores. At addition of ions of cadmium intensity of
fluorescence of the probe is increased due to complexation metal with a reagent in the
water environment.
Chemosensor 5 contains anthrylazamacrocycle derivatives [27]. In the free hosts
photoinduced electronic transition (PET) can occur between the lone pair of the
amines in the polyamine chain and the excited benzene moieties, while protonation or
coordination with metal ion by the amines partially prevents the electron transfer to
occur inducing an enhancement of fluorescence. Preliminary concentration on a
sorbent allows reaching a low limit of detection.
The same authors have improved the above-stated method - in quality of
fluorogenic reagent for cadmium also is used anthrylazamacrocycle 5 - they have
offered a combination preliminary concentration on a sorbent and flow-fluorescent
determination. It allows using the given technique [28] for serial analyses.
Fluorophor 5 it is applied for remote determination of cadmium with use fiber
optic device (optode) [29]. Fluorescent spectra wrote down after immersing optode in
an analyzed solution of cadmium. Carry of ions of cadmium through a optode
membrane results in increase of a signal from a complex of cadmium with ligand.
Disadvantage of chemical system is its irreversibility, however low cost of optode
allows using it in the analysis.
Fluorosensor 6 consists from 4-aminophthalimido (AP) as fluorophor,
dimethylene the spacer and a receptor: diazacrown (I) and - monoazacrown (II)
derivatives. Components are chosen so, that connection between a receptor and
fluorophor results in "switching" fluorescence in system. However, fluorescence is
included at presence of "guest" which switches off connection between a receptor and
fluorophor [30]. Т.о. presence of the guest is an attribute of increase of fluorescence
in system. PET is most frequently the used mechanism of connection between
terminal moieties of fluorosensor. A principle of creation "off-on" a fluorescent signal
of presence of ions of cadmium it is schematically submitted on fig. 1.
Fig.1. [30]
It is supposed, that crown compounds because of macrocycle effect show strong
linkage of the guest and show the selectivity of linkage dependent on the size of their
cavity. At replacement of atom of oxygen incrown an ether on atom of nitrogen, last
158
can connect ions of transition metals, than oxygen more strongly. Since AP is
electron-deficient molecule, it ideally approaches as fluorophoric component of a
sensor control. In the given system of 2 various components azacrown are used as
receptors for studying influence of number electron-donating centres on the signaling
efficiency. Monoazacrown compound I shows small increase of intensity of
fluorescence at the presence of cadmium. On the other hand, diazacrown derivative II
gives substantial growth of fluorescence. Hence, compound II shows the best
signaling of systems, than I.
In a water solution occurs complexation N, N '-dibenzylated polyamines (a
reagent 7) to an ion of cadmium. In free reagent PET can occur between the lone pair
of the amines in a chain in the polyamine and the excited components of benzene
while protonation or coordination with metal ion of the amines partially prevents the
electronic transition to occur inducing enhancement of fluorescence. [31].
The sensor control 8 on a basis amidsulphonaphtalyne is received by addition of
three dansyl groups to a molecule tren. Intensive fluorescence arises in a condition of
carry of a charge from an amino group dansyl to a ring of naphthalene [31]. Strong
change of luminescent properties is observed at addition of ions of cadmium. The
increase of a quantum yield of fluorescence is observed. The mechanism
complexation is based on deprotonation of sulfaphamid groups.
3,5'-dis-(dicarboxymethylaminomethyl)-4,4'-dioxy-trans-stylbene (the reagent
9) forms with cadmium fluorescent complex. For selective determination cadmium
preliminary extract by sodium diethylcarbaminat , then reextract by HCl [11].
One of the widespread organic reagents is 8-quinoline for determination of
metals, but methods of determination with its use nonselective and are insensitive
[32]. Selectivity is increased at introduction in position 2 aliphatic substituents,
luminescent properties of a molecule thus practically do not change. Luminescent
properties in similar organic molecules change at introduction of amino groups. In the
given work are investigated 2-aminoderivatives 8-quinoline (10-21) in which
molecules substituents influence spectral-luminescent properties and same time can
create steric obstacles at complexation, that results in increase of selectivity. It is
established, that reagents 10-21 form fluorescing complexes with cadmium in waterethanol solutions.
The most widespread fluorimetric method of determination of cadmium in the
water environments, recommended Ministry of Health, is based on formation of
complex compound with 8-mercaptoquinoline (a reagent 22) in the environment of
the ammoniac buffer, extraction its chloroform and measurement of intensity of
fluorescence [33]. A determined range of concentration - 0,0005-2,0 mg/l at presence
up to 1 g/l of alkaline metals, magnesium, aluminium, chlorides, nitrates, sulfates, up
to 5 mg меди, iron, up to 2 mg /l, up to 1 mg /l of manganese, cobalt, nickel, zinc. For
elimination of preventing influence of other elements use extraction of cadmium as
ditizonate from strong alkaline solution in an organic phase with the subsequent
destruction of ditizonate cadmium in the acid media. The minimal determined amount
of cadmium in test - 0.25 μg.
8-(benzolesulphonylamino)quinoline хинолин (a reagent 23) forms with
cadmium a complex which is taken by many organic solvents, but at extraction by
chloroform intensity of fluorescence is maximal [11].
Tripodal ligands frequently use as components of receptors. On their basis the
fluorescent sensor control 24 on a basis quinoline and pyridine which forms a
complex cadmium is synthesized, thus intensity of fluorescence quinoline component
[31] is increased.
159
Determination of cadmium in salts of zinc is based on ability of compounds of
group pyrazole (a reagent 25) to form fluorescing chelate compound. In absence of
cadmium the reagent in the same solution does not fluoresce. Intensity of fluorescence
of a complex of cadmium is increased repeatedly at extraction of a complex by
chloroform.
1-(8-oxy-2-quinoline)-3,5-dimethylpyrazole (the reagent 26) forms with
cadmium a fluorescing complex, which extract by chloroform in alkaline [11] media.
In comparison with a method of usual fluorescent spectroscopy fiber optic
sensor controls allow to carry out simultaneous determination of various metals, they
cheap, easily adapted and it is easy miniaturize. Chemosensors approach for
monitoring processes and ecological monitoring. The principle of measurement of a
sensor control is based on capture of analyte ions by ligands with formation metal –
ligand complexes. With increase of concentration of an ion static fluorescence
sequestering agent can or be increased or decrease. 5-nitrobenzothiazole coumarin
(BTC-5N) has significant fluorescence [34], but at addition of ions of cadmium at рН
7.0 intensity of fluorescence is increased as a result of formation of a complex with
metal. The limit of detection is 0.5 μg /l, linearity is found out in a range of 10-100 μg
/l. To determination prevent Be, Ni, Cu. The sensor control is applied for the analysis
of superficial and underground waters.
Electrochemiluminescence is the important method of determination in
analytical chemistry due to low limits of detection and selectivity. In work [35]
electrochemical reaction of determination of cadmium at presence co-reactant
tripropylamine (TPA) and organic ligand 1,10-phenantroline (a reagent 2) is offered.
Reaction occurs between a complex metal – ligand and TPA at рН 8.2 Quantitative
determination of cadmium is based that 1,10- phenantroline gives increase of intensity
of fluorescence at the presence of ions of metal. The limit of detection is 5·10 -9 %.
Emission is linear over the (50-1000)10-9 % range. Determination of cadmium is
prevented by ions Zn2 +, Co2+, V5+, Cu2+, Ni2+, Fe2+, Sn2+.
For determination of microamounts of cadmium the combined methods
combining extraction reduction by organic substances and X-ray fluorescent
determination of metal in a firm extract are perspective. The developed technique is
based on extraction cadmium by melt of higher carboxylic acids of fraction C 17-C20
and their mixes with 1,10-phenantroline. The method provides a high degree
preliminary reduction of metal, eliminates influence of a matrix and allows to carry
out X-ray fluorescent determination of cadmium directly in a solid extract at рН 3.57.0 [36]. The limit of detection is 0,01-0,05 mg/l. Linearity is over of 0.002-0.5 %
range. The developed technique is applied for the analysis of waste and natural
waters.
Also in the literature methods of determination of the cadmium, based on
suppression of fluorescence of a reagent are mentioned. At pH 9,1 α,β,γ,δ-tetra (5sulfophenyl)porphine could from an self-ordering ring (SOR) on the surfaces of glass
slides with aid of polyvinyl alcohol and the fluorescence intensity of the SOR was
found to be quantitatively quenched by Cd(II). An SOR method for the determination
of metal in drinking and tap water is establish in the range 1.0·10-14 to 2.0·10-13 mol,
and the limit of detection is 5·10-15 mol [37].
160
Table 2.
Structures of organic reagents
N
1
Structure
Reference
erythrozyne R=I, eosine R=Br
[11]
R
R
HO
O
O
R
R
O
2
[24]
O
HO
OH
Br
Br
4,5-dibromofluorescein
N
N
1,10-phenantroline
3
[25]
O
C
N
OH
[26]
4
Z , where
Y
X
COOH
=X
N
HOOCH2C
(CH2)2
Y=
N
CH2COOH
CH2CHONH
NH
N
O
Z=
N
SO2N(CH3)2
161
N
Structure
5
Reference
N
6
[27,28,29]
NH
n
O
I: X
=O
II: X H2N
=
NН
[30]
O
O
N
N
X
O
O
(
7
NH
O
)n
[31]
NH
NH
[31]
8
9
[11]
H CH COOH
2
N CH COOH
2
CH2
HO
CH
CH
H
OH
HOOCH2C N
CH2COOH
8-oxyquinoline derivatives
2-amino derivatives of 8-oxyquinoline
(NN 10-21)
N
OH
162
R
N
Structure
Reference
10
R = - NH2
[32]
11
- NHCH3
[32]
12
- N(CH3)2
[32]
13
- N(C2H5)2
[32]
14
- NHC4H9
[32]
15
[32]
N
16
[32]
N
O
17
[32]
H
N
18
[32]
H
N
N
19
[32]
CH3
N
N
20
[32]
CH3
N
N
OH
21
[32]
C4H9
N
N
OH
22
[33]
N
SH
23
[11]
N
O3 S
163
N
N
Structure
24
Reference
[31]
N
N
N
( )n
Techniques on the base of pyrazole
25
[21]
H
N
N
26
[11]
H3C
N
O
N
N
CH3
164
Table 3
Cadmium determination methods, based on the enchancement in the luminescence intensity
Method
Reagent
Technique
description
Eosine fluorescing,
extraction of complex
by chloroform
increases intensity of
fluorescence
1
Eosine + 1,10phenantroline
1
Conditions
рН 8.0 in acetone,
extract by
chloroform
530/570
erythrozyne + 1,10phenantroline
similar
рН 8,5
530-570
2
4,5-dibromofluorescein
+ 1,10-phenantroline
Increasing of intensity
of luminescence DBF
and Phe at formation
of a complex with
cadmium
pH 8.5
3
2-(о-oxyphenyl)
benzoxazole
Complex fluorescing
chemosensor
4
DBD-ED-CNEDTA
(DBD-ED)
chemosensor
5
9-(1’,4’,7’,10’,13'pentaazacycleаpentadec
yl)methylanthracene
Increasing of intensity
of fluorescence of a
reagent at formation
of a complex with
cadmium
Increasing of intensity
of fluorescence of a
reagent at formation
of a complex with
cadmium
11.0, precipitation
dilute in
CH3COOH
366/рН 5.2 in
acetonitrile
рН 10,0; 13,0
165
LOD
1.5∙10-7
mol/l
Linearity
range
0.05-1.0
μg
Interfering
ions
Cr, Fe, Ga,
Hg, Mn, Ni,
Pb, Zn
Analysis
object
Pure
solutions,
mineral
assays
Referen
ce
[11],
[21]
0.1-1.5 μg
Mineral
assays
[11]
0.01-0.2
μg/ml
Cr, Fe, Ga,
Hg, Mn, Ni,
Pb, Zn
Cu, Fe, Al,
Zn
bronze,
mixture of
cadmium
and zinc
sulphates
[24]
100-2500
μg
Co, Cu, Ni et
al..
[25]
5∙10-9
mol
[26]
(1-90)∙10-9
mol
Zn2+ (at
рН=13 not
interfere),
Hg2+, Cu2+
See water
[27]
Method
Reagent
flowfluorescent
chemosensor
5
remote
chemosensor
(optode)
5
9-(1’,4’,7’,10’,13'pentaazacycleаpentadec
yl)methylanthracene
(complex 1:1)
9-(1’,4’,7’,10’,13'pentaazacycleаpentadec
yl)methylanthracene
chemosensor
6
fluorophor 4-aminophtalimide,
receptors:
monoazacrown
derivative - I
Diazacrown derivative
– II
N,N'-dibenzylated
polyamines (complex
1:1)
chemosensor
7
chemosensor
8
luminescence
9
10-18
19-21
on a basis
amidsulphonaphtalyne
3,5'-dis(dicarboxymethylamino
methyl)-4,4'-dioxytrans-stylbene
(complex 1:2)
2-amino derivatives of
8-oxyquinoline
Technique
description
similar
Conditions
LOD
Linearity
range
рН 13,0
35·10-12
mol
(35-2000)·
10-12 mol
similar
рН 9.0
4.5 μg/l
The greatest
increasing of intensity
fluorescing of sensor
is observe in AN for II
tetrahydrofuran
(THF) and
acetonitrile (AN)
Increasing of intensity
of fluorescence of a
reagent at formation
of a complex with
cadmium
similar
рН 9.5
formation
fluorescencing
complex
рН 7.9, complex
extract by sodium
diethylcarbaminat
formation
fluorescencing
complex
Water-ethanol
solution
-»-
166
Interfering
ions
Hg2+, Cu2+
Analysis
object
See water
Referen
ce
[28]
Cl-, F-, NO3-,
NH4+, Fe3+,
Fe2+, Co2+,
Ni2+, Pb2+,
Cu2+, Hg2+,
Cr3+ et al.
Drinking
water
[29]
[30]
0,5-10 μg
Co2+, Ni2+,
Pb2+, Cu2+,
Zn2+
[31]
Cu2+, Co2+,
Zn2+
Pb, Tl, Zn,
rare-earth
elements
[31]
[11]
Zn
[32]
Zn, Y, La, Lu
[32]
Method
Reagent
Technique
description
similar
22
8-mercaptoquinoline
23
8(benzolsylfanylamino)quinoline (complex 1:2)
similar
chemosensor
24
on basis of pyridine and
quinoline
25
pyrazole
Increasing of intensity
of fluorescence of a
reagent at formation
of a complex with
cadmium с кадмием
formation
fluorescencing
complex,
in absence of
cadmium the reagent
does not fluoresce
similar
26
1-(8-oxy-2-quinoline)3,5-dimethylpyrazole
extraction of complex
by chloroform
increases intensity of
fluorescence
Conditions
LOD
ammoniac buffer
solution,
cadmium extract
by chloroform as
dithizonat, then
dithizonat is
destroy in acid
media
рН 8.0, extract by
chloroform
280/515
0.25 μg
Linearity
range
0,0005-2.0
mg/l
0,005 μg
Interfering
ions
Al, Cu, Fe,
Mn, Co, Ni,
Zn et al.
Analysis
object
Drinking
water,
surface and
undergroun
d water
sources
Co, Cu, Ni,
Zn, Cr, Fe,
Hg, Sb, Al,
Be et al.
Referen
ce
[33]
[11]
[31]
Complex is
extract by
chloroform
430/600
Alkaline media,
complex is extract
by chloroform
590
3% КОН
366/590
167
0.03 μg
in 5 ml
0.03-2.0 μg
in 5 ml
Mn, Fe, Co,
Ag, Al, Cr,
Mn, Ni, Zn
Zinc salts
[21]
[21]
0.03-2.0
μg
Ag, Al, Ca,
Ce, Co, Fe,
Mg, Mn, Ni,
Pb, Zr
[11]
Conclusion
There is a great number of luminescent methods of cadmium determination, but
a lot of them use extraction and harmful organic solvents, the directly aqueous
fluorimetric techniques are rarely. It is necessary to develop new methods, which are
satisfy to need requirements of availability of equipment, linearity, sensitivity, and
selectivity.
This work was supported by RFBR grant № 07-03-96098 and grant “Leading
scientific schools” NSh-5468.2006.3.
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